the makran accretionary wedge off pakistan: tectonic evolution and fluid migration. hannover 1997

BUNDESANSTALT FÛR GEOWISSENSCHAFTEN IJND ROHSTOFFEHANNOVER

MAKRAN 1

Tektonische Entwicklung und Fluidtransportim Makran-AkkretionskeiVPakistan' Teil 1

The Makran Accretionary Wedge off Pakistan:Tectonic Evolution and Fluid Migration - Part L

Fôrderungsvorhaben 03 G Ot22 A

SONNE cruise SO-122 (7 August - 6 September 1997)

Operational Report and Preliminary Results

Author:

Funding Agency:

Date:

Archiv-Nr.:

H.A. Roeser with contributions by J. Adam, H.-O. Bargeloh, M.Block, V. Damm, H. Dohmann, J. Fritsch, P. Kewitsch, K. Puskeppe-leit, U. von Rad, C. Reichert, U. Schrader, B. Schreckenberger, J.Sievers, D. Steinmann,'W. VoB (all BGR), T. Schillhorn (Geomar), A.Inam, M. Tahir (both NIO) and A.H. Cheema (HDIP)

Bundesministerium flir Bildung, Wissenschaft, Forschung und Tech-nologie

November 1997

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Contents

List of Tab1es.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3List of Fi9ures............. ......................3Summary ........................ 8Zusammenfassung ........ l01 Geoscientif ic objectives ............ .................... l2

1.1 Introduct ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2 Source, migration and discharge of fluids. ............... 131.3 Geophysics and morphology of the Makran accretionary wedge....... ..........141.4 Stnrcture of the crust off the Makran accretionary wedge ......... 151.5 Targets of cruise SO-122...... .........,........17

2 Participants ................ .................. 182.1 Scientif ic crew..... ................. 182.2 Ship's crew.......... ................. 19

3 Cruise diary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .204 Geophysical instrumentation...... ...................25

4.1 Airgun system....... ................254.2 Streamer system.... .................; ................274.3 Seismic recording equipment .................274.4 Gravimetric and magnetic equipment ......................274.5 Computer systems for navigation, data acquisition, data processing and interpretation 284.6Util ized equipment of the ship........... ......................28

5 Operational reports ......................295.1 Profi le plan and list of profi1es............. ....................295.2 Fishing off Pakistan ..............295.3 Navigation and acquisition of non-seismic data.... ..................... 34

5.3.1 Navigation and positioning .............345.3.2Data acquisit ion .............345.3.3 Data processing ............. 35

5.4 Reflection seismics................ .................365.5 Gravity ................44

5.5. 1 Gravity connections............5.5.2 Gravity measurements at sea......... ....................47

5.5.2.I Short description of the seagravimeter system. ............475.5.2.2 Processing of the gravity data........... ..........4g

5.6 Magnet ics. . . . . . . . . . . . . . . . . . . . . . . . . . . . .495.6.1 The gradient magnetometer............. ..................495.6.2 Processing of the data of the magnetic gradiometer................. .............50

5.7 Hydroacoustics .....................535.7.1 Hydroswoop;....... ...........535.7.2Parasound.. . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . 56

6 Scientific results ........566.1 Composite line drawings ...... 566.2 Reflection seismics................ .................77

6.2.1 Introduction ...................776.2.2Mafuan accretionary complex.............. .............776.2.3 Oman Abyssal Plain.........r......;.......... ............... 806.2.4Munay Ridge.... .............80

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6.2.5lndus She1f......... ............836.2.6 Indus Fan........... ............ 83

6.3 Gravity and magnetics............ ......,......... 876.3.1 Age and nature of the crust of the Indus Fan............ ........... 876.3.2T\e Murray Ridge area........... .........906.3.3 The Oman Abyssal Plain and the Makran shelf .......... ........ 906.3.4The relation of the Murray Ridge to structures onshore Pakistan .........91

6.4 Hydroacoustics .....................916.4.1Data recording and quality .............. 916.4.2 Preliminary results

Acknowledgements .... 105References .................. 106Press c1ippin9s............................. ................... 110

List of Tables

Table 1. List of the profiles surveyed during cruise SO-122

Table 2. Summarized seismic data volume recorded during cruise so-rzzTable 3. Observation report on gravity connections in Djibouti

Table 4. Parameters for magnetic lines of cruise SO-122

List of Figures

Fig. 1. Tectonic setting of the Arabian Sea and its surroundings (Coumes & Kolla,1984)

Fig.2. The Makran accretionary wedge with the areas where during SO-90 detailedsurveys were carried out, and the profiles that were planned for cruise SO-122;the lines that were actually surveyed are shown in Fig. 4 on p. 30.

Fig. 3. Configuration of the airgun system of BGR

Fig. 4. Map in mercator projection showing the profiles surveyed during cruise SO-r22

Fig. 5. Streamer configuration for profiles SOl22-03 andSOl22-04

Fig. 6. Streamer configuration for profile SOL22-04A

Fig.7. Streamer configuration for profiles SO122-08 - SOl22-23

Fig. 8. Streamer configuration for profiles SOl22-14 - SOl22-23

Fig. 9. Streamer configuration for profiles SOI22-24 - SOl22-027

Fig. 10. Port of Djibouti (from Admiralty Chart 262); (a) mooring site of RVSONNE at quay 13 from August 6 to 9, 1997; (b) reference station BGI

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#798 at quay 13. The inset shows details of the gravity observation station atbollard 74 alongside R/V SONNE.

Fig, I 1. Gravity Sensor GSS30 of the seagravimeter system KSS31

Fig. 12. The configuration of the tow system of the magnetic gradiometer duringcruise SO-122

Fig. 13. Mean gradient on SO-122lines plotted against the course. (a) Lines wherethe sensor no. 805 was used and (b) lines without sensor no. 805.

Fig. 14. Example for the reconstruction of the magnetic anomaly from the measuredgradient (a) on line SOl22-04A. (b) shows the reconstructed anomaly(continuous curve) compared with the total intensity measured by the mastersensor (dotted curve) when the linear trend (stippled line in (a)) is removed.(c) is the result of the reconstruction if only the mean gradient is removed.(d) shows the difference between the two curves in (c).

Fig. 15. Planetary magnetic three-hour-range indices (Kp) for the interval June 21-Oct3l, L997

Fig. 16. Line SO122-01 across the Indus Fan, without reflection seismics. Uppermostpanel: Gravimetric and magnetic anomalies; central panel: Hydrosweepswath; lowermost panel: Bathymebry

Fig. 17. Line SO122-02 across the Murray Ridge, without reflection seismics. Up-permost panel: Gravimetric and magnetic anomalies; central panel: Hy-drosweep swath; lowermost panel: Bathymetry

Fig. 18. Line SO122-03 across the Murray Ridge, with reflection seismics. Upper-most panel: Gravimetric and magnetic anomalies; central panel: Hy-drosweep swath; lowermost panel: Line drawing of one channel of the re-flection seismic record

Fig. 19. Line Sol2z-Ml04{across Murray Ridge, Dalrymple Trough, Little MunayRidge, oman Abyssal Plain and the Makran accretionary wedge, with re-flection seismics. Uppermost panel: Gravimetric and magnetic anomalies;central panel: Hydrosweep swath; lowermost panel: Line drawing of onechannel of the reflection seismic record

Fig. 20. Line SO122-05 on the Makran accretionary wedge,,without reflection seis-mics. Uppermost panel: Gravimetric and magnetic anomalies; central panel:Hydrosweep swath; lowermost panel: Bathymetry

Fig.2l. Line SO122-05A across the Oman Abyssal Plain, without reflection seis-mics. Uppermost panel: Gravimetric and magnetic anomalies; central panel:Hydrosweep swath; lowermost panel: Bathymetry

Fig.22. Line 50122-06 across the Oman Abyssal Plain and the Makran accretionarywedle, without reflection seismics. Uppermost panel: Gravimetric andmagnetic anomalies; central panel: Hydrosweep swath; lowermost panel:Bathymetry

Fig.23. Line SOl22-07 along the Makran accretionary wedge, without reflectionseismics. Uppermost panel: Gravimetric and magnetic anomalies; centralpanel: Hydrosweep swath; lowermost panel: Bathymetry

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Fig. 24. Line SO122-08 from the Indus Shelf to the eastern,end of the Murray Ridge,with reflection seismics. Uppermost panel: Gravimetric and magneticanomalies; central panel: Hydrosweep swath; lowermost panel: Line draw-ing of one channel of the reflection seismic record

Fig.25. Line SO122-09 from the eastern end of the Murray Rdige across the Makranaccretionary wedge, with reflection seismics. Uppermost panel: Gravimetricand magnetic anomalies; central panel: Hydrosweep swath; lowermostpanel: Line drawing of one channel of the reflection seismic record

Fig.26. Line SO122-10 across the eastern end of the Makran accretionary wedge andthe Indus Shell with reflection seismics. Uppermost panel: Gravimetric andmagnetic anomalies; central panel: Hydrosweep swath; lowermost panel:Line drawing of one channel of the reflection seismic record

Fig.27 . Line SO122-11 from the Indus Shelf to the eastern end of the Murray Ridge,with reflection seismics. Uppermost panel: Gravimetric and magneticanomalies; central panel: Hydrosweep swath; lowermost panel: Line draw-ing of one channel of the reflection seismic record

Fig. 28. Line SO122-L2 aqoss the Oman Abyssal Plain and the Makran accretionarywedge, with reflection seismics. Uppermost panel: Gravimetric and mag-netic anomalies; central panel: Hydrosweep swath; lowermost panel: Linedrawing of one channel of the reflection seismic record

Fig.29. Line SO122-I3/L3A across the Makran accretionary wedge, with reflectionseismics. Uppermost panel: Gravimetric and magnetic anomalies; centralpanel: Hydrosweep swath; lowermost panel: Line drawing of one channel ofthe reflection seismic record

Fig. 30. Line SO122-14 along the slope of the Indus Shelf and across the Indus Can-yon, with reflection seismics. Uppermost panel: Gravimetric and magneticanomalies; central panel: Hydrosweep swath; lowermost panel: Line draw-ing of one channel of the reflection seismic record

Fig. 31. Line SOl22-15 across the Indus Canyon and on the Indus Shell with reflec-tion seismics. Uppermost panel: Gravimetric and magnetic anomalies; cen-tral panel: Hydrosweep swath; lowermost panel: Line drawing of one chan-nel of the reflection seismic record

Fig.32. Line 50122-16 from the Indus Shelf to the Indus Fan, with reflection seis-mics. Uppermost panel: Gravimetric and magnetic anomalies; central panel:Hydrosweep swath; lowermost panel: Line drawing of one channêl of thereflection seismic record

Fig. 33. Line SO122-17 fromthe Indus Fan across the eastern Murray Ridge, withreflection seismics. Uppermost panel: Gravimetric and magnetic anomalies;central panel: Hydrosweep swath; lowermost panel: Line drawing of onechannel of the reflection seismic record

Fig. 34. Line SO122-18 across the Murray Ridge, with reflection seismics. Upper-most panel: Gravimetric and magnetic anomalies; central panel: Hy-drosweep swath; lowermost panel: Line drawing of one channel of the re-flection seismic record

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Fig. 35. Line SOl22-l9Bll9 across the south-eastern Murray Ridge, with reflectionseismics. Uppermost panel: Gravimetric and magnetic anomalies; centralpanel: Hydrosweep swath; lowermostpanel: Line drawing of one channel ofthe reflection seismic record

Fig. 36. Line SO122-20 across the Murray Trough, without reflection seismics. Up-permost panel: Gravimetric and magnetic anomalies; central panel: Hy-drosweep swath; lowermost panel: Bathymetry

Fig.37. Line SO122-21 across the south-eastern Munay Ridge and to the Indus Fan,with reflection seismics. uppermost panel: Gravimetric and magneticanomalies; central panel: Hydrosweep swath; lowermost panel: Line draw-ing of one channel of the reflection seismic record

Fig. 38. Line SO122-22ftomthe Indus Fan across the Murray Ridge, with reflectionseismics. Uppermost panel: Gravimetric and magnetic anomalies; centralpanel: Hydrosweep swath; lowermost panel: Line drawing of one channel ofthe reflection seismic record

Fig. 39. Line SO122-23 across the Murray Ridge and the Indus Fan, with reflectionseismics. Uppermost panel: Gravimetric and magnetic anomalies; centralpanel: Hydrosweep swath; lowermost panel: Line drawing of one channel ofthe reflection seismic record

Fig. 40. Line SO122-24 onthe Indus Fan, with reflection seismics. Uppermost panel:Gravimetric and magnetic anomalies; central panel: Hydrosweep s*uth;lowermost panel: Line drawing of one channel of the reflection seismic rec-ord

Fig. 41. Line SO122-25 on the Indus Fan, with reflection seismics. Uppermost panel:Gravimetric and magnetic anomalies; central panel: Hydrosweep swath;lowermost panel: Line drawing of one channel of the reflection seismic rec-ord

Fig.42. Line 50122-26 on the Indus Fan, with reflection seismics. Uppermost panel:Gravimetric and magnetic anomalies; central panel: Hydrosweep swath;lowermost panel: Line drawing of one channel of the reflection seismic rec-ord

Fig. 43. Line SO122-27 onthe Indus Fan, with reflection seismics. Uppermost panel:Gravimetric and magnetic anomalies; central panel: Hydrosweep swath;lowermost panel: Line drawing of one channel of the reflection seismic rec-ord

Fig.44. Line SO122-28 across the Murray Ridge, without reflection seismics. Up-permost panel: Gravimetric and magnetic anomalies; central panel: Hy-drosweep swath; lowermost panel: Bathymetry

Fig. 45. Structural map of the survey area based mainly on data of the SONNE cruiseso-r22

Fig. 46. Part of the single-channel monitor record of the seismic reflection profileSO122-04A showing the BSR at the front of the Makran accretionary com-plex

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Fig.47. Part of the single-channel monitor record of the reflection seismic lineSO122-10 showing a 9 km long and 600 m thick sheet which has slumpeddown from the Indus Shelf

Fig. 48. Part of the single-channel monitor record of the reflection seismic lineSOl22-t4 showing the Indus Canyon near the slope of the Indus Shelf

Fig.49. Part of the single-channel monitor record of the reflection seismic lineSO I22- 14 showing channel-levees

Fig. 50. Part of the single-channel monitor record of the reflection seismic lineSA9}-I4 showing some structures which we interpret as mud diapirs

Fig. 51. Magnetic anomalies (master sensor) observed on cruise S}-I22plottedalong the ship's tracks. Positive anomalies are red, negative green.

Fig.52. Magnetic anomalies plotted along the ship tracks. Positive anomalies in darkgray, negative anomalies in light gray. Lines in the north-western part of themap are from this cruise (Fig. 51), lines in the south-eastern part are fromCHARLES DARWIN cruise 20 (Miles & Roest, 1993). The harched areaindicates the location of the continuation of the Laxmi Ridge to the east in-ferred from gravity anomalies (Miles & Roest, 1993).

Fig. 53. Structural map of a pa$ of the Bela-Whaziristan ophiolite zone (Bannert etal.,1992) with the free-air gravity anomalies observed in the north-easternpart of our survey. A gravity high indicates that on the shelf the ophiolitezone bends south-westwards and possibly extends into the Murray Ridge.

Fig; 54. Line SOl22-M:Puallel packages of subbottom reflectors intervened byacoustically transparent layers of varying thickness in the Dalrymple Trough

Fig. 55. Line SO122-MA: Grabenlike structures with multiple and parallel subbot-tom reflectors between the Dalrymple Trough andihe Little Munay Ridge

Fig. 56. Line SO122-O4A: Unconformity with underlying dipping sediments at thenorthern edge of the Little Murray Ridge

Fig. 57. Line SO122-04A: Small-scale folding and faulting just south of the frontalfold of the Makran accretionary complex

Fig. 58. Line SO122-MA: Basin on the Makran accretionary complex with gentlydipping reflectors

Fig. 59. Line SO122-10: Highty deformed sedimentary sequence underneath anacoustically stratified section at the uppermost continental slope

Fig. 60. Line SO122-10: Continental shelf with a strong subbottom reflector that in-dicates an erosional unconformity

Fig. 61. Line SO122-14: Channel in the Indus Fan area (printed by the Paradigmasystem)

Fig.62. Line SO122-14:Upper slope region of the Indus Canyon with acoustic faciesULc,h

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Summary

The Makran accretionary wedge is located in the southern part of western Pakistan and off thesouth coast of this area. It has formed by the subduction of oceanic crust under the EurasianPlate which lasts since 70 - 80 mill. years. Two features make this accretionary wedge espe-cially interesting: Firstly, the sediment thickness on the oceanic crust is extremely high, sec-ondly the angle of subduction is extremely small. On this cruise with R/V SONNE réflectionseismic, gravimetric, magnetic, sediment echographic and bathymetric me{rsurements wereplanned to investigate the structure of the submarine part of the accretionary wedge and todcfine targets for sampling fluid discharge areas in early 1998. The cruise was financially sup-ported by the Federal Ministry of Education, Science, Research and Technology.

The cruise was severely affected by the problem that - at least at the end of the SW monsoon -fishery with several km long gill nets was very intense off the Makran coast. Already at thebegin of our cruise we lost about 213 of our streamer when we crossed one of the numerousgroups of small fishing-boats. Furthermore, the fishery forced us to change the survey linesconsiderably in comparison to the initial plan. Altogether, 4082 km were surveyed with gra-vimetry, magnetometry, swath bathymetry and sediment echography,2927 km of them weresurveyed additionally with multichannel reflection seismics (Fig. 4).

The main structural units can be described best with line SOL2}-M1O4A (Fig. 19, p. 59):

The line starts at the southem margin of the Murray Ridge, which at least partly is covered byvolcanic rocks which in the summit area partly outcrop out of the sediments. Largepans areclearly not volcanic.

On this line, the Little Murray Ridge divides the oceanic crust between the Murray Ridge andthe front of the Makran accretionary wedge into two very different units. The sedimentarycover of the south-eastern unit shows numerous, mostly recent faults which indicate activeextensional tectonics. The Dalrymple Trough with a depth of more than 4000 m at the north-western flank of the Murray Ridge has formed by this extension. It is an asymmetric grabenwith a steep north-west facing main fault at its south-eastern margin. To the north-west of theLittle Murray Ridge is the trench area of the Makran subduction zone. The basement surfacedips here towards the accretionary wedge. The sedimentary cover with a total thickness of upto 5.6 s (twt) (roughly 7 km) is divided by a distinct unconformity which subparallel to thebasement surface dips northward. A wide drift structure lies immediately below the uncon-formity. The sediments above the unconformity show a horizontal layering. They mask thetrench which therefore is not a topographic trench.

The frontal area of the Makran accretionary complex shows intensively folded and over-thrusted sedimentary thrust slices and a well developed BSR (Bottom Simulating Reflector).The subducting oceanic crust is nowhere visible on the single-channel monitor ràcord.

Off Karachi, in the more than 100 km wide shelf with water depths of less than 200 m and inthe easternmost part of the Makran accretionary complex, lines SO122-07 - SOl22-13 weresurveyed. The most important new result concerns the extension of prominent tectonic ele-ments from Pakistan into the Arabian Sea:

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In the central part of Pakistan, along distinct strike-slip faults (Chaman Fault, GhazabandFault, Ornach-Nal Fault) the Bela-Waziristan ophiolite zone separates the Makran Flyschcomplex in the west from the Khuzdar block in the east. It consists of marine limestone andclaystone, basaltic and andesitic lavas, gabbros, serpentinites and their conglomerates and isconsidered as the boundary between the Eurasian Plate and the subducted Arabian Plate in thewest and the Indo-Pakistanian Plate in the east. This mainly N-S strikingzone is bent eastwardin the Karachi Arch north of Karachi. Near Karachi, the layers of this zone are covered byalluvial sediments.

On our lines, we found zones of positive gravity anomalies related to areas with elevatedseismic reflectors which show characteristics of melange zones. It seems reasonable to parallelthese high-density subbottom structures to the Bela-Waziristan ophiolite zone. This impliesthat south of the Karachi Arch the ophiolite zone is deflected WSW in direction to the MurrayRidge. The Murray Ridge may even be a direct prolongation of the Bela-Waziristan ophiolitezone. This finding throws new light onto the development of the triple junction between theEurasian, the Arabian and the Indo-Pakistanian Plates which is also documented in the re-gional disfribution of the ophiolite belts.

Another focus concerned margin and slope of the Indus Shelf . Especially, lines SO122-14and 50122-16 cross at a point where, based on data from SONNE cruise SO-90, an ODP drillsite will be proposed for investigation of the oxygen minimum zone.

The basement of the Indus Fan consists mainly of oceanic crust. The sediment thickness de-creases southward. Our lines lie partly outside the present EEZ (Exclusive Economic Zone) ofPakistan in an area into which Pakistan could extend its EEZ in case of sufficient sedimentthickness. (For a part of our lines this could be case.)

The magnetic anomalies in the area of the Indus Fan are conspicuously weak. An anempt toextend the identification of the seafloor spreading anomalies which has been made south of20'N to the north, has not been successful.

The elevated parts of the Murray Ridge show strong magnetic anomalies only in a few places.Therefore, basic volcanism has not played a dominating role. North-west of the Murray Ridge,in the area of the presumed Little Murray Ridge, extensive magnetic anomalies with ampli-tudes of 200 nT and more are observed which continue into the region below the Makranaccretionary complex. They strike predominantly SW-NE. These anomalies are stronger thanthose observed on our Indus Fan lines. We cannot yet say whether they are at least partlyseafloor spreading lineations which might allow determination of age and spreading rate. Al-together, magnetically the crust north-west of the Murray Ridge differs considerably from thecrust of the northern Indus Fan.

The nature of the Murray Ridge remains unknown. Surely, it is a part of the boundary betweenthe Arabian and the Indo-Pakistanian Plates. The present extension in this part of the bound-ary is responsible for the forrnation of the Dalrymple Trough and an additional basin lying inits north-eastern prolongation. The ridge which parallels the basins to the south-east, mayhave formed by overthrusting of two oceanic crustal plates. However, it could also be a vol-canic ridge which has formed at the plate boundary which presumably is a weakness zone ofthe lithosphere.

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Zusammenfassung

Im Sûden des westlichen Pakistans und vor der Sûdkûste dieses Gebietes liegt der Makran-Akkretionskeil, der durch die seit ca.70 - 80 Millionen Jahren andauernde Subduktion ozea-nischer Kruste unter die Eurasische Platte entstanden ist. Aus zwei Grûnden verdient dieserAkkretionskeil unser besonderes Interesse: Zum einen ist die Mâchtigkeit der Sedimente aufder ozeanischen Kruste extrem hoch, zum anderen ist der Abtauchwinkel der subduzierendenPlatte extrem gering. Die auf dieser vom BMBF gefôrderten Fahrt mit FS SONNE geplantenreflexionsseismischen, gravimetrischen, magnetischen, sedimentechographischen und bathy-metrischen Messungen sollten die Struktur im seewâirtigen Bereich des Makran-Akkretions-keils erkunden rrnd Untersuchungsgebiete fiir die im Fnihjahr 1998 geplante Beprobung vonFluidaustritten ausweisen.

Die Fahrt war durch das Problem geprâgt, daB zumindest am Ende des SW-Monsuns die Fi-scherei mit mehrere km langen Stellnetzen vor der Makranktiste Pakistans auBerordentlichrege ist. Beim Durchqueren einer der zahlreichen Gruppen von kleinen Fischerbooten gingenbereits zu Beginn der Fahrt etwa2l3 des Streamers verloren. Ferner fthrte die Fischerei zueinem gegenûber der urspriinglichen Planung stark geânderten Me8netz (Fig. 4). Insgesamtwurden 4082 km mit Gravimetrie, Magnetik, Hydrosweep und Parasound vermessen, 2927km davon wurden zusâtzlich mit Mehrspurreflexionsseismik vermessen.

Die wichtigsten tektonischen Einheiten lassen sich am besten an Hand von Profil SOI22-04.I0/.A(Fig. 19, S. 59) beschreiben:

Das Profil beginnt im Stiden am Siidrand des Murray-Rûckens, der zumindest teilweise vonVulkaniten bedeckt ist, die im Gipfelbereich stellenweise aus der Sedimentbedeckung ragen.Gro8e Teile des Rûckens bestehen jedoch nicht aus Vulkaniten.

Der Kleine Murray-Rûcken unterteilt auf diesem Profil die ozeanische Kruste zwischen demMunay-Riicken und der Front des Makran-Akkretionskomplexes in zwei sehr unterschiedli-che Einheiten. Im Sûdosten zeigt die Sedimentbedeckung zahlreiche, meist rezente Stôrungen,die auf aktive Dehnungstektonik hinweisen. Der tiber 4000 m tiefe Dalrymple-Trog an derNW-Flanke des Murray-Rûckens ist durch diese Dehnung entstanden und Uitaet einen asym-metrischen Graben mit der Hauptabschiebungsflâche im SE. Nordwestlich des Kleinen Mur-ray-Rùckens befindet sich der Trenchbereich der Makran-Subduktionszone. Das Basementfiillt dort zum Akkretionskomplex hin ein. Die bis zu 5,6 s (twt) (grob geschâtzt7 h,rrl mâch-tige Sedimentbedeckung wird von einer sehr markanten Unkonformitat unterteilt, die sub-parallel zur Basementoberflâche ebenfalls nach Norden einftillt. Direkt unter der Unkonformi-tât liegt eine ausgedehnte Driftstruktur. Dariiber verlaufen die Sedimentschichten horizontalund decken den Trench za, der deshalb topographisch nicht als Tiefseerinne erkennbar ist.

Der Makran-Akkretionskomplex zeigt im Frontbereich einige gefaltete und iibereinanderge-schobene Sedimentschuppen und einen deutlichen BSR (Bottom simulating reflector). Diesubduzierende ozeanische Kruste ist im gesamten Bereich des Akkretionskomplexes auf derEinspur-Monitoraufzeichnung nicht erkennbar.

In dem ûber 100 km weiten Schelfbereich mit Wassertiefen unter 200 m vor Karachi und imôstlichen Bereich des Makran-Akkretionskeils sind die Profile SOl22-07 bis SO122-13 ver-

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messen worden. Der wichtigste neue Befund in diesem Gebiet betrifft die Fortsetzung bekann-ter tektonischer Elemente Pakistans in das Arabische Meer:

Im zentralen Teil von Pakistan treûnt die Bela-Waziristan-Ophiolith-Zone den Makran-Flysch-Komplex im'Westen vom Khuzdar-Block im Osten

"ïU*g ausgeprâgter Blattver-

schiebungen (Chaman-Stôrun9, Ghazaband-Stôrung, Ornach-Nal-Stôrung). Sie wird aus ma-rinen Kalk- und Tonsteinen, basaltischen und andesitischen Laven, Gabbrogesteinen, Serpen-tiniten und Konglomeraten dieser Gesteine gebildet und als Grenzbereich zwischen der Eura-sischen Platte im Westen und der Indo-Pakistanischen Platte im Osten betrachtet. Die im we-sentlichen Nord-Sûd ausgerichtete Erstreckung dieser Zone ist nôrdlich von Karachi im sog.Karachi-Bogen nach Osten gebogen.ImBereich von Karachi werden die zu dieser Tnne gehô-renden Schichten von alluvialen Sedimenten iiberdeckt.

Im Schelfbereich wurden Tnnen positiver Schwereanomalien kartiert, denen sich Hochlagenseismischer Reflektoren zuordnen lassen. Es liegt nahe, diese Untergrundstrukturen hôhererDichte mit der Bela-Waziristan-Ophiolith-Zone zu parallelisieren. Demriach wtirde dieOphiolith-Zone stidlich vom Karachi-Bogen nach rWSW in Richtung auf den Murray-Rtickenabgelenkt. Môglichenreise geht sie direkt in diesen ûber. Dieser Befund wirft neues Licht aufdie Tektonik im Bereich des Tripelpunktes zwischen Eurasischer, Arabischer und Indo-Pakistanischer Platte, tiber die bisher bemerkenswert wenig bekannt ist.

Weitere Schwerpunkte waren der Site Survey fiir eine Bohrung im Bereich der Sauerstoff-Minimum-Zone in der Niihe des Indus-Canyons und die Profile ûber den Murray-Rticken biszum Indus-Sediment-Fâcher.

Im Bereich des Indus-Fâchers ist die Kruste ozeanisch, die Mâchtigkeit der Sedimente nimmtnach Sûden ab.ZumTeil liegen die Profile auBerhalb des EEZ Pakistans in einem Bereich, inden Pakistan bei ausreichender Sedimentmâchtigkeit seine EEZ ausdehnen kônnte. (Auf ei-nem Teil der Profile kônnte das der Fall sein.)

Die magnetischen Anomalien im Bereich des Indus-Fâchers sind auffallend schwach. EineFortsetzung der Identifizierung der sûdlich von 20oN beobachteten ost-west-streichenden ma-gnetischen Lineationen nach Norden hin ist uns nicht gelungen.

Ûber den Hochlagen des Murray-Riickens finden sich nur an wenigen Stellen auffallend starkemagnetische Anomalien. In weiten Bereichen hat hier also basischer Vulkanismus keine do-minierende Rolle gespielt. Nordwestlich vom Munay-Riicken, im Bereich des vermutetenKleinen Murray-Rûckens, fallen groBrâumige Anomalien mit Amplituden von 200 nT undmehr auf, die sich bis unter den Makran-Akkretionskomplex fortsetzen. Ihre Streichrichtungist vorwiegend SW-NO. Die magnetischen Anomalien sind in diesem Gebiet stiirker als aufden Indus-Fâcher-Profilen. Ob sie wenigstens teilweise durch Seafloor-Spreading entstandensind und ob man aus ihnen Alter und Spreadingrate ableiten kann,lâBt sich beim gegenwiirti-gen Stand der Datensammlung und -bearbeitung nicht sagen. Insgesamt unterscheidet sich dieKruste nordwestlich vom Murray-Rûcken in magnetischer Hinsicht deutlich von der Krusteim Bereich des Indus-Fâchers.

Die Natur des Murray-Rûckens selbst bleibt noch ungekllirt. Er ist sicherlich ein Teil derGrenze zwischen der Arabischen und der Indo-Pakistanischen Platte, zwischen denen im Be-reich des Munay-Rûckens derzeit Extension stattfindet, die zur Bildung des Dalrymple-Grabens und eines weiteren, nordôstlich von diesem liegenden Becken gefûhrt hat. Der siid-

BGR 116&3 - 1 2 -

ôstlich an die Grabenzone anschlieBende Rûcken kônnte in einer frtiheren Kompressionsphasedurch Ûberschiebung zweier ozeanischer Krustenplatten entstanden sein. Es kônnte sich aberauch um einen vulkanischen Rticken handeln, der an der Plattengrenze als einer Schwâchezo-ne der Lithosphiire entstanden ist.

1 Geoscientific objectives(H.A.Roeser, U. von Rad & C. Reichert)

l.L fntroductionAs shown by the COSOD tr priority list and the "White Papers" of the Tectonic Panel and theSedimentary and Geochemical Processes Panel of the Ocean Drilling Program, the investiga-tion of accretionary wedges (particularly the Nankai Trough off Japan, the Cascadia subduc-tion zone off Oregon and British Columbia, the continental margin of Peru and Chile, and theBarbados accretionary wedge) are in the center of current scientific interest. A considerableamount of research has been focussed on this field within the framework of the ODP drillingcampaigns and ODP presite surveys (Suess, von Huene et al. 1990; Suess t992,1994).

A main topic concerns the seepage of reduced vent gases and interstitial waters from the seafloor associated with very specialized biota of sulfur-oxidizing or methanotrophic bacteria, thechemo-autotrophic "cold vent faunas", and the formation of authigenic minerals which arefrequently observed at accretionary wedges. Indeed, these cold seeps belong to the most excit-ing discoveries of the marine sciences within the past decade (see Ritger et al., 1987;Schmaljohann, 1993).

The characteristics of subduction zones vary in a wide range, depending on a wealth of pa-rameters. Among these are:- age of the oceanic crust- sediment thickness, age and properties of the sediments- rate ofconvergence- sediment supply during the subduction- angle of subduction- angle between the direction of convergence and the direction of the subduction zone

The Makran accretionary eomplex, which has developed by the subduction of the ArabianPlate beneath the Eurasian Plate (Fig. l), is characterized by a very thick column of incomingsediment on the oceanic crust. This creates an accretionary wedge which to a large part is ele-vated above the sea level where it is intensely eroded and deposited back onto the lower partsof the wedge. Reflection seismic data indicate that the wedge consists mainly of imbricatethrust slices which often show a regular appearance. Bottom Simulating Reflectors (BSR)indicate the presence of gas hydrates at a large part of the complex.

We have selected the Makran accretionary wedge for a thorough and multidisciplinary inves-tigation because due to its extreme situation it may give particularly clear answers to some ofthe existing questions.

BGR 116&3 - 1 3 -

Fig. 1. Tectonic setting of the Arabian Sea and its surroundings (Coumes & Kolla, Lg84)

Some of the other parameters are only poorly known for the Makran subduction zone, espe-cially the age of the oceanic crust, the direction of fracture zones within it and the conver-gence rate. As these features influence considerably the evolution of the wedge, it is most im-portant to study also the oceanic crust south of the wedge.

1.2 Source, migration and discharge of fluidsIncreasing compaction of the sediments with increasing tectonic and lithostatic pressurewithin the accretionary wedge is the most important driving force for expelling fluids. Thecompaction can be estimated on the basis of the relationship between the sonic velocity andthe porosity (von Huene et al., 1993; Fruehn etal.,1994). For this purpose it is necessary totrace back the deformation chronologically using tectonic balancing methods. This allowscalculation of a lower limit to the present flow rate. The calculated flow rate can then be com-pared with the directly measured fluid discharge from the accretionary sediments into the deeprwater of the Arabian Sea.

The upward migration of methane and other gaseous hydrocarbons from the deeper part of thesedimentary wedge may be strongly affected by gas hydrates in the upper 400 rn-of the sedi-

EURASIAN

PLATE

ARABIAN KARACHI

PLATE

ù

BGR I 16g3 - t 4 -

ments. The presence of these gas hydrates has been demonstrated by identification of the BSRin the Makran accretionary wedge down to water depths of 1500 m. The gaseous hydrocar-bons from below the BSR can penetrate the gas hydrate zone only by fluid migration. Duringtheir migration through the hydrate zone, the fluids may mobilize additional methane.

The distribution of vents is very variable. In most cases it is related to local tectonic structures,such as the outcrops of reverse faults or thrusts, transform faults, back thrusts, and steepslump escary)ments and canyon walls over the entire continental slope area (Minshull &White, 1989). A detailed knowledge of the tectonic structure of the accretionary wedge is nec-essary to assess the spatial distribution of structures which favour the occurrence of vents sothat the mean fluid discharge over a large area can be estimated. This requires investigation ofthe active dewatering area on a large scale along the strike of the accretionary structures. Inaddition, information on the escape channels in unconsolidated sediment is necessary: At whatdepth and under which pressure/temperature regime is the catchment area of the fluids in thechannels? Do the channels originate at the foot of faults or in another kind of tectonic struc-ture? Do they penetrate the BSR horizon, which can be expected to occur in the study area?All this would help to determine the best sampling sites.

1.3 Geophysics and morphology of the Makran accretionary \iledgeThe active Makran continental margin and its dewatering was investigated geochemicalty bythe working group of J. Leggett, London, and R. rwhite, cambridge, England, during the1980s. A few single-channel seismic profiles were obtained by RRS Shackleton (Leg 1/S0)and Atlantis tr (Leg 96113) during an ODP Site Survey in the area south of Gwadar. A multi-channel (MC) seismic profile was published by Minshull & V/hite (1989). In addition to acommercial MC line (Harms et al., 1983 ,Fig.2), several unpublished commercial single-channel and multichannel seismic lines exist (e.g. Raza et al., 1990; Wintershall from the1960s).

Gnos, Immenhauser & Peters (1997) have investigated the ages of the emplacement of theophiolites in the Vy'estern Ophiolite Belt in Pakistan and the sediments related to that event.They found that the youngest obducted oceanic crust is 65 - 70 million years old. They sawclear evidence that subduction along this belt did not start earlier than 70 - 75 mill. years ago.

The youngest accretionary ridge presently forms at the foot of the steep continental slope at awater depth of about 3000 m. On the continent and on the upper part of the slope, there arefive to seven E-W striking, folded, and elevated accretionary ridges which have been thrustnorthward; these ridges are separated by ponded slope basins filled with turbidites andhemipelagic sediments, which are horizontal or dip downslope (White & Louden, 1982;White, 1983; von Rad et al.,1994). The subduction zone has an unusually small inclination(about 2o) and the temperature gradient is only l8"C per km (Harms et al., 1984).

A large part of the accretionary wedge is well exposed on the continent because the area isfree of vegetation (Bannert et al., 1992: Leggett & Platt, 1984). The sedimentation rate of theslope sediments is extremely high (> 1-5 mmla; von Rad et a7., 1994) because of rapid tec-tonic uplift, the narrow shelf, and the lack of vegetation in the hinterland. As a result, thehemipelagic sediments have a very high rate of consolidation and compaction, and the accre-tionary sediments undergo rapid dewatering due to tectonic deformation (Fowler et al., 1985).

BGR 116&3

According to these and other data, the Makran accretionary wedge has formed by subductionof the Arabian Plate below the continental Eurasian Plate (Lut Block) which may have started70 - 80 million years ago and is still active. Some of the sediments of the oceanic plate, whichare up to 9 km thick, have been folded, sheared, and repeatedly thrust, increasing the thicknessto up to 15 km (Harms et al., 1984; Leggett & Platt, 1984; Farah et al., 1984; Raza et al.,1990). The Makran fore-arc is an especially interesting type of convergent continental marginin so far as (1) a morphologically distinct deep sea trench does not exist (probably because ofthe enormous supply of sediment from the continent), and (2) the accretionary wedge is ex-tremely wide (about 500 km, only 70 km of which are submarine) and characterized by largethicknesses of sediment @annert et al., 1992). The Arabian Plate is being subducted belowthe Eurasian Plate along a very gently inclined, northward dipping subduction zone at about 5cmla. This causes uplift of the Makran coast at a rate of 1.5 mmla and 1 cm/a seaward move-ment of the coastline (White & Louden, 1982; White, 1983).

During SONNE cruise SO-90 PAKOMIN (1993), four areas of the continental slope offPakistan were surveyed in detail in water depths between 100 and 3000 m using Hydrosweepswath bathymetry and Parasound 3.5 kHz profiling (von Rad & Shipboard Scientific Party,1994; von Rad et al., 1995; von Rad & Tahir, 1997). Two of these areas (A and B) are in theregion of the Makran accretionary wedge (Fig. 2). The morphological structure of these areas,which is strongly influenced by deformation during the subduction process, is very complex.A 90 km long, N-S transect (area A) shows five or six E-W trending, folded, faulted, imbri-cated, and uplifted anticlinal structures (accretionary ridges) separated by ponded turbiditeslope basins in water depths between 1300 and 3000 m (White & Louden, l9S2). Area Bshows a complex system of submarine E-V/ running canyons which probably are related totectonic zones of weakness.Near "oil seeps" often weak local magnetic anomalies are found (Donovan etal.,1979, e.g.)which may be due to the reduction of minerals by hydrocarbons. It is not yet known whethersuch anomalies can exist also at places of fluid discharges out of accretionary wedges. Aero-magnetic measurements at a flight altitude of 300 m within the frame of Project MAGNETshow a quiet magnetic field over most of the Makran accretionary wedge (Taylor, 1968). Withthe magnetic gradiometer of BGR it may be possible to detect weak local anomalies if theyexist at places of fluid discharge.

1.4 Structure of the crust off the Makran accretionary wedgeThe nature of the Murray Ridge south of the Makran accretionary wedge is only poortyknown. In 1961162 the British Admiralty had carried out magnetic and bathymetric surveys ofthe area (Barker, 1966). The quality of the prepared maps is remarkably high. They show anelongated ridge with water depths of partly less than 200 m in the south-rffest that is paralleledby a more than 4000 m deep trough on its north-eastern flank. This structure is offset at 64oEwhere we observe several prominent circular highs. The crust of the Murray Ridge and thetroughs do not show a strikingly high magnetization. Only the circular highs at the offset mustconsist at least partly of higtrly magnetized basic lava. To the north-west of the troughs, weobserve several local magnetic anomalies that partly are not related to topographic expres-sions.

1 5 -

BGR 1t6&3 - 1 6 -

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BGR 116&3 - 1 7 -

Mainly on the base of reflection seismic measurements, Minshull et al. (1992) have developeda picture of the plate tectonic evolution. Using additional information by Bannert et al. (1992),it can be reviewed as follows (Fig. 1):

Three plates must be considered: The Indo-Pakistanian Plate moves northwards relative to theEurabian Plate. Main expression of the collision is the emergence of the Himalaya. In the cen-tral part of Pakistan, along distinct strike-slip faults (Chaman Fault, Ghazaband Fault, Ornach-Nal Fault), the Bela-Waziristan Ophiolite Zone separates the Makran Flysch complex in thewest from the Khuzdar Block in the east. This zone consists of marine.limestone and clay-stone, basaltic and andesitic lavas, gabbros and serpentinites and their conglomerates. Thismainly NS striking zone bends eastward in the Karachi Arch north of Karachi. Near Karachiand south of it, alluvial sediments cover the layers. The seaward extension of this zone is notknown. It is assumed that the Bela-Waziristan Ophiolite Zone, the Murray Ridge and theOwen Fracture Zone separate the Indo-Pakistanian Plate in the SW from the Arabian Plate inthe NW (Minshull et al., 1992).

Between the Murray Ridge and the Makran accretionary wedge, the Arabian Plate consists ofoceanic crust which subducts to the north. The nature of the triple junction between the threeplates mentioned is completely unknown.

It is assumed that most of the oceanic crust of the Arabian Plate, namely the Oman Basin andthe Owen Basin, bel.ongs to the old Tethys Ocean. The crust between the Sharbithat Ridge andthe Sheba Ridge has formed much later, namely in parallel with the Gulf of Aden (Stein &Cochran, 1985). Age determinations on the basis of magnetic anomalies exist neither for theOwen Basin nor the Oman Basin. From a synthesis of the existing geophysical evidence, Min-shull et al. (1992) conclude that the crust north of the Murray Ridge possibly has formed dur-ing the Cretaceous Magnetic Quiet Zone.

Between the axis of the Murray Ridge and the Oman Abyssal Plain, extensional featuresdominate. Their most prominent expression is the Dalrymple Trough. Fig. 8 of Minshull et al.(1992) shows that the extension is still active. The nature of the Murray Ridge itself is un-known. It may be continental or a compressional feature where oceanic crust of the Indo-Pakistanian Plate is thrust over oceanic crust of the Arabian Plate.

South of the Murray Ridge, ages of the oceanic crust are well defined south of 19"N whereanomaly 28 is observed (Miles & Roest, 1993). Further to the north, some east-west strikinganomalies are observed but not identified. The buried Laxmi Ridge is the boundary betweenthese two regions.

1.5 Targets of cruise SO.l22SO-I22 has been the first of four geoscientific research cruises with R/V SONNE in the areaof the Makran accretionary wedge. It comprises multichannel reflection seismics, gravimetry,magnetics, swath echosounding and sediment echosounding. The main questions approachedby this survey are:

- Does a trench exist below the thick sediments at the foot of the continental slope?- Is it possible to estimate the amount of sediment that has been subducted?

BGR 1t6&3 - 1 8 -

- Does a sliver-shaped, high-velocity, allochthonous splinter of crust exist off Makran, ashas been observed at many active continental margins ?.

- What is the relationship between the oceanic crustal material in the Bela-WaziristanOphiolite Zone of central Pakistan and the submarine Murray Ridge (Minshull et al.,r992)?

- Does the oceanic crust show any specific features and are they related to the subductionpatûern?

- Is the particular sfyle of subduction related to the hydrocarbon potential, the dewatering ofthe sediments and the fluid transfer?

- How does the study area compare with other types of collision zone between oceanic andcontinental plates (e.g., Barbados, Costa Rica, Peru, Oregon ) with respect to structure?

- V[hat is the nature of the Murray Ridge?- How differs the oceanic crust north-west of the Murray Ridge from that south-east of it?

We had planned to- concentrate during cruise SO-122 on a geophysical survey of an area of about 400 km x

150 km on the Makran continental margin offPakistan (Fig. 2),- to convert the velocity data into a porosity model to derive fluid losses (Fowler et al., 1985;

Minshull & lVhite, 1989; von Huene et al., 1993) and- on this basis to select key regions for investigation by geological sampling during cruise

so-130.

Intense fishery in the area of the Makran accretionary wedge prevented a large part of theseinvestigations. We had to shift the emphasis to more southerly regions (Fig. a). This provideddata for an improved understanding of the regional tectonic evolution and to questions con-cerning the development of the crust on a larger scale which previously had not been in thecenter of interest.

2 Participants

2.1 Scientific crewAdam, Ernst-JûrgenBargeloh, Hans-OttoBlock, MartinCheema, Amjed HussainDamm, VolkmarDohmann, HansFritsch, JûrgenInam, AsifKewitsch, PeterPuskeppeleit, KlausRoeser, Hans AlbertSchillhorn, ThiesSchreckenberger, BerndSievers, JoachimSchrader, UweSteinmann. Dieter

BGRBGRBGRHDIPBGRBGRBGRNIOBGRBGRBGRGeomarBGRBGRBGRBGR

BGR 116&3

Tahir, MuhammadVoB, Wolfgang

BGR -

Geomar -

HDIP -

NIO -

- 1 9 -

NIOBGR

2.2 Ship's crewAndresen, Hartmut A.Angermann, Rudolfvon Arronet, Johannes GeorgBethke, Hans G.Bochnik, EberhardBronn, JohannDracopoulos, LazarosEvers, WolfgangHartig, VolkerHartwig, Karl-HeinzHoffmann, HilmarKlein, AndreasKraft, JûrgenKrause, StefanLude, GtinterMeyer, HelmutNaeve,IngoPaul, GerhardPrechtl, Hans-JûrgenSchade, Peter UweScholz, Uwe Rudi GerhardSchramme, HeinrichSchrappel, AndreasStiingl, GtinterSzymanski, LeszekTscharnke, RudolfVor, Hans-JiirgenV/itkowski,IngoZieten, Wolfgang

Bundesanstalt fiir Geowissenschaften und Rohstoffe,Hannover, GermanyGeomar Forschungszentrum fiir marine Geowissen-schaften, Kiel, GermanyHydrocarbon Development Institute, Islamabad,PakistanNational Institute of Oceanography, Karachi, Paki-stan

KapitiinElektronikerMotorenwiirterMotorenwiirter2.Ingenieurl. StewardKochsmaatKochLtd.IngenieurBootsmannLtd. ElektronikerSystem-ManagerMatroseSystem-ManagerMatroseMotorenwiirterArztMotorenwiirter2. Steward2. Ingenieur1. Offizier2. StewardMatroseMatrose1. OffizierDeckschlosserMatroseMatroseFt.-Offizier

BGR 1T6æ3 -20 -

3 Cruise diary(H.A.Roeser)

All times are in in ship time. This is UTC + 3h until August 12 andUTC + 4h afterwards.

Cruise SO-122 started at Djibouti, a town which recalls unfortunate reminiscences to theGerman marine science community because here on March 18, 1987 four German students ofmarine biology: Annette Barthelt, Marco Buchalla, Hans-Wilhelm Halbeisen and Daniel Rein-schmidt, lost their lives during a terrorist attack. We have often remembered them during ourstay in Djibouti. Relatives and friends of the victims have established the Annette-BartheltFoundation. It is the intention of this foundation to inform about the problem of terrorism andits consequences for the persons affected and the community, and to honour outstanding in-vestigations of young German oceanography scientists.

FS SONNE arrived in schedule at Djibouti on August 6,1997. Cruise SO-122 started onAugust 7 with the loading of three 20'containers and the winch with the 3000 m long digitalstreamer. Installation of the heavy equipment was finished on August 9. One of the pakistaniguests had arrived in schedule on August 7, the others were delayed due to flight problems.After arrival of the missing two guests in the evening of August 9, the ship sailed at22hlocattime.

On August l0 at 12.45 h, we received an emergency call from RCC (rescue coordinating cen-ter) Stavanger. On the ship MV RESOURCEFUL, a navy ship which was on its last cruise tobeing scrapped in India, a motor man had been heavily burned by hot steam. At 18 h we arriv-ed at the ship. The swain and our doctor I. Naeve went aboard the ship. The doctor treated thewounds and gave the necessary injections to the man. Later, a boat from land took the man toa hospital at Aden. At 19.25 h we continued our journey.

On August 12we came into monsoonal winds with wind force 7. A short test of the gradientmagnetometer on August 13 revealed problems with the acquisition of the data from the depthmeters and the fluxgate compass. These were remedied on August 14.

On the occasion of the celebration of the 50th Anniversary of the Independence of Pakistanwe congratulated on August 14 our partner institutes to this highly important day in the historyof their country. Captain Bhatti, hydrographer of Pakistan navy, and Dr. Shahid Amjad, Direc-tor General of NIO, conveyed their regards to us and thanked for our interest.

In the afternoon of August 14, we started line SO122-01 without seismics because the sea wastoo rough for deployment of the streamer. On August 15, the weather had sufficiently improv-ed and we started deployment of the streamer. It turned out that the birds were hardly ablè tobring the streamer down to the required depth. Therefore we added 2kgleadper section toadapt it to the calculated density 1.024 glcm3. After adding a weight of 50 kg to the end of thetow lead, the birds had wing angles of about 5 - 7 degrees downward, showing that the strea-mer had the desired buoyancy of about 1 kg per section.

In the morning of August 16, we started line SO122-03, the first one with reflection seismics.This line was only 30 km long. At 05.50 h we started line SOI22-04 which was intended to

BGR 1t6&3

cross Mruray Ridge, Dalrymple Trough, Oman Abyssal Plain and the Makran accretionarywedge.

In the evening, the number of fishing-boats increased, they were all drifting. 4 of them werepassed at distances between 0.8 and 1.0 nm. At2339 h the last three birds failed. As we sup-posed a collision with parts of drifting nets, we decided to intemrpt the line and to pick up theequipment. Indeed, great pieces of nets were picked up with the airgun arrays. On August 17at 00.35 h we observed the end buoy of the streamer the last time. Shortly after that it becameclear that the sûeamer was broken. After picking up the remaining sections, we waited fordaylight. Very soon we observed the end buoy and began to pick up the streamer from its end.Parallel to this action, a fishing-boat approached which, as we observed, picked up the strea-mer with its net. We contacted them by megaphone and agreed to separate streamer and net incooperation. After several hours it turned out that the fishermen had already cut the streamerat several places, without knowing its value.

Because of the danger to the propeller, it was not possible to search for the streamer with theship itself. Therefore the speedboat went in several hundred meter distance before the shipsearching for the streamer. Because of the rough sea, this had to be stopped in the evening.During the night, we tried to observe more reliable values for the water currents. The new ob-servations essentially confirmed our previous values. Altogether it followed that the chance tofind the remainder of the sûeamer was very small. Also, it had to be taken into account that apassing ship may have dragged with it streamer or that it had already sunk due to insufficientbuoyancy. Therefore we did not continue the search.

Instead, we began in the morning to reconfigure the streamer from the parts that were left, andfrom the spare parts. On August 18 at 18.08 h we had finished the new configuration with all17 sections and a weight of 100 kg at the end of the tow lead, and started line SO122-04Awith an overlapping over SOIZ2-U of about 5 nm.

During this line we observed ftequently fishing-boats which were in more than 2 nm distancefrom our line. This limit seemed safe enough for us because we could not imagine that theobserved small boats could work with nets of this length. On August 19 at 14.10 h we observ-ed several fishing-boats 5 nm ahead on our survey line. We changed course from 0o to 45". At17.10 h we saw numerous fishing-boats in front of us and on both sides. Although the distan-ces from us were still around 5 nm, from their movements it became clear that it was neces-sary to take as fast as possible the airguns, the magnetometer and the streamer on board.

At 18.45 h, we started our way back. At 20.00 h we crossed two fishing-nets which mean-while were put out.

'We were not able to return to the end of line SO-122-04A because the

area was occupied by many fishermen.

From our observations we concluded, that it was much to dangerous to work with our com-plete equipment. Instead, at?I.s0 h we started line SO122-05 with the magnetometer alonein direction 148' to the southern end of the planned next seismic line to survey that line firstlywith the magnetometer alone and then with reflection seismics from as far north as possible.

Only a few hours later, at 00.35 h on August 20, we had to finish the line because there weremany fishermen around us and one exactly in front of us. Only 15 nm later, we could start lineSO122-05A in direction 120'. On this line, until5.20 h we observed many fishermen. As af-

-21

BGR 1T6&3 -22 -

terwards no fishermen were observed, we concluded that we could try to go northwards atlongitude 63o50'E.

At 8.00 h on August 20, Mr. Inam, one of the participants from NIO, got information from hisinstitute that his wife had died during the night. Therefore we decided to go to Karachi. Onthis way we surveyed line 50122-06 at longitude 63'50'E until 24"50'N and then line SOl22-07 eastwards. On line 06 we observed at 13.30 h a group of 7 boats at 3 nm distance from ourtrack line.

On August 21 at 8.00 h, we arrived at Karachi. Three hours delayed, at 10.50 h a boat fromland picked up Mr. Inam.

As we had no fishing observed in the eastern part of the Makran accretionary wedge, we wentfrom Karachi only 60 nm in SE direction and started there with our easternmost lines. Becauseof the high danger to loose the streamer again, we used only 12 of the 17 sections that hadremained. The 62 km long line SO122-08 was surveyed without fishing problems. During theline, the seismic data acquisition system displayed an error message that it was unable to writethe data for the monitor record to disk. Consequently, it was not possible to prepare the usualmonitor records at the end of these lines. The same problem occurred later also on lines -09and -10. It turned out that this was caused by the high temperature in the laboratory and conse-quently in the housings of the seismic data acquisition system. We solved the problem by theinstallation of fans in front of the warmest units. At the end of cruise SO-122, the refrigerantof the air conditioning of the ship was cleaned.

On line SO122-09 we observed on August22 at 7 .00 h two fishing-boats in front of us whichforced us to shift our line 2 nm to the east. In all these cases we changed the course by 3"every 3 minutes. Obviously, this slow change did not disturb the records excessively. Fur-thermore, it did not cause problems of collision between the towed equipment. We did not goback to the old line because then we would have had to make additional course changes.

Similarly, we had to change line SO122-10 by 2 nm because of two fishing-boats. In this case,we did not shift the line parallel. Instead, we kept the same end point because we wanted tocross there the Drill Site Indus Marine A-1.

On line SOl22-12 we observed on August23 at 19.00 h one boat near our survey line whichwe passed by shifting our line 2nm. Some time later, we lost the signal of bird (depth control-ler) no. 6, the last one. The signal of that channel where the bird was positioned seemed tohave a slightly increased noise. As bird no. 5 continuously controlled the streamer upward, incontrast to its usual behaviour, we suspected that a heavy object was caught by the bird. How-ever, as no additional damage was expected, we did not pick up the streamer. Some time later,channel T disappeared. We suspected an intemrption of the line from the channel to the mod-ule which did not require any immediate action.

On the connection between lines SO122-12 and-13 we observed several boats at a distance ofabout 4 nm. They became unvisible when at the first beginning of dawn they extinguishedtheir lights. On August24 at 6.40 h we observed at a distance of 1.5 nm a boat with an I nmlong net which extended parallel to our course. 5 nm in front of us two more boats appeared.However, they did not affect us because we were just changing course for line SOI22-13which we started at 7.10 h.

BGR 116&3 -23 -

In the morning, the signal of bird no. 6 reappeared. Possibly, the range of the signal is notlarge enough so that it disappears under some conditions.

At 9.00 h we approached two boats with unclear behaviour. One picked up a net and approa-ched a red flag whereas the second boat tied to keep it away from the flag. We finished theline at 9.24h, changed slightly our course and shortened magnetometer and airguns to enablerapid reactions. Finally the first boat passed the red flag which bbviously belonged to the netof the other boat. In safe distance of the two boats we started line SO122-13A at 10.00 h.Some time later, the signal of bird no.4 disappeared.

The problems with fîshing-boats continued on this line. At 14.00 h we approached a group ofboats which started to move in different, mostly souther$, directions at speeds of 4 - 6 knots.All this could be observed very well on the excellent radar of R/V SONNE. One boat in frontof us was only drifting, we changed our course in the usual way by 3'13 minutes. At sufficientdistance we were able to observe the red end buoy of the net. Then in the radar several drift-ing boats appeared in front of us. As we suspected that they were fishing, we finished our lineand began to pick up the equipment as fast as possible. When the magnetometer was on board,we had only 300 m in front of us the red buoy of a net. We changed our course sharply andpicked up the streamer. Then we moved carefully southward out of this extensive group offishermen which covered a 15 nm long area.

At the start of line SOl22-14 we o6served that the last bird continuously headed upward. Wehad t9 pick up the whole equipment and to replace the bird. The new bird gave wrong com-pass values, but that was tolerablé. The defective bird was ok after removing and reinstallingthe batteries. Therefore we suspect that a bad pulse had disturbed the electronic board withinthe bird. After an intemrption of more than 12 hours we were able to start line SOL22-I4 at04.58 h.

August 25 has been the 60 birthday of the chief scientist. The most suitable present was asmall package of "Fishermen's Friend".

The main purpose of lines SOI22-I4 - 3U^122-16 was a site survey for an ODP hole whichwill be proposed on the base of data obtained on cruise SO-90 (PAKOMIN). These lines weresurveyed without any disturbances by fishing. There were only some intemrptions in the re-ception of the DGPS signals due to an unfavourable course. The officers tried to keep the in-temrptions as short as possible.

Lines SOl22-17 and SO122-18 should return us to the Makran accretionary wedge. However,20 minutes after the end of line SOl22-17 in the Oman Abyssal Plain, on August 28 at2 .00hthe lights of five boats became visible in the noctoviser. We decided to turn tô direction 270"to go northward further in the west. However, then we saw another boat exactly west of us, wehad to turn further southwards. When after short time also there a boat appeared, we turned to180". Finally, we had to turn to 167" to pass another boat at a distance of 2 nm which ransouthward at a speed of 2 knots. Supposedly it was paying out his net, however, we did notsee anything of it. Other boats forced us then to change direction to south-west. Only at 5.45 hon August 28 we were able to start line SO122-18 south-eastward across the Murray Ridge.

On line SOt22-19 we encountered fishing-boats much earlier than expected, namely at23'15'N on August 29 at 5.50 h. First we tried to pass between the two boats, however, at 6.10h we detected there some weak reflections in the radar, indicating that there might be a net.

BGR 116643 -24 -

We finished the line at6.14 h and began to turn to the west. Meanwhile, we shortened airgunsand magnetometer, to be more reactive in case of additional problems. This was necessarybecause during the dawn the optical visibility is very poor. At 7.48h we continued the line,only shifted by 9 km to the west and called 50122-194.

During these manoeuvres, we saw hundreds of tunas (bonitos?) jumping out of the water.During the day, we observed the same several times. This indicates the presence not only ofmany tunas but also of larger fishes feeding on them.

At 8.30 h on August 29 intermittent error messages started at the Syntron system by which welost 12 shots duringz00 shot points. It was necessary to finish line SOI22-19A. We made aloop and started line SO122-20 without seismics in direction 150o, at a distance of 6 km anti-parallel to line -19. These lines seemed to us important because we had observed there atrough which was similar to the Dalrymple Trough and had a depth of more than 4000 m. At12.45 h we identified the cause for loosing shots: The PC for triggering the shots was defec-tive: It triggered sometimes between the shots, and then the Syntron system was unable tostart the next record. After exchange of the PC and adapting everything, the problem was re-moved. We ended line -20 at 14.37 h, made a loop and started line 50122-198 at 16.2lh.After 12nmwe reached the starting point of line SO122-19A, and began repeating it. At18.55 h we observed optically the lights of 6 fishing-boats which do not appear in the radar.At 19.10 h, they appeared at 6 nm distance in the radar.'We found that obviously the boatsconsisted of two groups and that there was a 4 nm wide gap between them through which wemight be able to pass.

In the VFIF radio we heard very intense talking, it sounded Portuguese, which nobody on thebridge could understand. At 20.00 h the two boats through which we intended to pass and athird boat started blinking with strong searchlight beams.

'We understood that they tried to

inform us that there were nets between them and that we should not pass. As the techniciansand the deck crew were already informed that a rapid reaction might become necessary, westopped the line and started immediately with picking up the airguns and the magnetometer.During that time we had reduced already the speed as much as possible. At20.14 h we beganto turn over starboard to direction south. After some minutes we passed the end buoy of ourstreamer which still moved to the north. At secure distance distance from the boats, we chang-ed course to south-west and then to west to find a place to get through northward. However,observation with the noctoviser showed fishing-boats everywhere and also in larger distancesto the north. After 10 nm we observed boats not only to the north but also to the west of us.We concluded that any attempts to come more northward were unreasonable, and turnedsouthward.

On August 30 at 00.46 we crossed a net which supposedly belonged to a boat that was 3.5 nmaway from us. The signals of bird 1 of the streamer disappeared 2 minutes later. At 01.04 hthe birds of the streamer showed the following depths: Bird I nothing, bird229 m, bird 3 20m, bird 4 10 m, bird 5 1.5 m. Otherwise the streamer was ok. We picked up the magnetometerand found that the fin of one of the sensors was broken. Then we began to pick up thestreamer partly.

'We found that the 50 kg weight at the end of the tow lead was lost. Further

b i rd lwasshi f tedbylm,onecol larwasbroken.Apieceofs ize l0mx3mofaf ish ing-netwas still attached to it. We removed the net and refixed the bird to the right place. Then weput out the streamer again, replacing the weight at the end of the tow lead. At 03.20 h thestreamer and the magnetometer were ready for continuation of the survey. However, there

BGR 1t6&3 -25 -

were still several fishing-boats around which we had to circumvent. At last, 25 nm more to thesouth we started line SO122-2t at5.56 h.

Line SO12 2-22 wasintended to bring us again to the north. However, already at 4.00 h onAugust 31 we observed 5 fishing-boats and finished the line. Without problems we went to anappropriate point on line SOtr22-23 in direction 145'. It ended at 8.28 h on September I onthe Indus Fan. For the next lines in the area of the Indus Fan we extended the length of thestreamer to 16 sections because firstly we did not expect problems with fishermen and second-ly the additional moveout would be very important for the determination of the velocity ofsound within the thick sedimentary sequence.

The next three lines were partly outside of the present EEZ of Pakistan. Line 5U-122-26 endedat the SW end of line SOI22-L6. The last line SO122-27 was intended to enable connection oflines SOl22-17 - SOl22-23. However, at 15.40 h we detected two fishing-boats and withinthe next 40 minutes two additional.ones which forced us to break off the line. During pickingup the equipment with careful cleaning of the streamer two fishing-boats laid out their nets inour immediate neighbourhood. After the end of this work we started at sufficient distancefrom the fishing-boats line SO122-27 without seismics in direction to Muscat. At longitude61'50''W we picked up the magnetometer and ended our investigations on September 5,1997at 11.00 h.

On September 6 at 7.30 h the ship moored at Mutrah, near Muscat, the capital of Oman. Laterin the morning, the streamer winch was removed and a part of the scientific crew disem-barked. This ended cruise SO-I22.

On September 7, four scientists from the University of Oman visited the ship. 'We

informedthem about the equipment of the ship, the geophysical equipment installed especially for thiscruise and the first scientific results. Afterwards, a group of scientists from the ship wasguided through the impressive geoscientific facilities of the University of Oman which hadbeen built up by a group of very engaged scientists within only a few years.

4 Geophysical instrumentation

4.1 Airgun systemThe seismic signals were generated by two tuned linear:urays consisting of together 20 arr-guns (type VLF Prakla-Seismos) in 6 groups with a total volume of Sl.Zlifte (3,124 cu.in.),total length of each anay 19.6 m, operating pressure 135 bar (1,920 psi), operating depth 7 m(Fig. 3). Exact timing of the shooting of the airguns was ensured by a microprocessor-con-trolled airgun-synchronization unit type Prakla-Seismos VZAD andYZAC2 with storage os-cilloscope.

BGR r t6æ3 -26 -

Fig. 3. Configuration of the airgun system of BGR

,EE EI! l * 9 !Ë Ë E s rH H 3

* E* ËË =J , g E Ë E Eg i V â âAA

Ee

=E

'il;:

:ii

'ï1::

,orj

Ê 5q,O )

- l

. r U )\-./ oC\ôl u)r t

f - L U )^ c ! $ È i1 1 - - o I - J@ r r l g V(r)

Ë : ; #- c à - ( L( U ÉÈ 5 , *r P ; ; fo È Ë =C È v ' F J- ! ! ô à oq i t a Ë É@ o o

BGR 116643 -27

4.2 Streamer systemFor acquiring the multichannel seismic data a digital 3,000 m Syntron streamer manufacturedby Syntron Inc., Houston, Texas was used. It consisted of:- one tow lead,length 150 m- three stretch sections, length 50 m each- 4ORDS active sections, eactr wittr three seismic channels, length 75 m each- ten active 24bit electronic modules- one passive electronic module to repeat (amplify) the data signals- 11 MultiTRAK remote units (birds) to control interactively streamer depth and position- 22O m end rope with Prakla-Seismos tail buoy

4.3 Seismic recording equipmentThe equipment for recording the multichannel seismic dataconsisted of:- digital seismic recording system capable of recording up to 480 channels per streamer,

type Syntron SYNTRAK 480 MSRS (Multiple streamer Recording sysrem)The settings were:- Recording time: 15 s- sampling rgte:- Filter LC:

4 ms | 1 ms (cf. Table 2 onp. 43)3Hz

- two 4280 StorageTek 3480IBM compatible dual tape units, for 3480 cartridges with208 m tape length, using SEG-D demultiplexed2.Sbyte format

- Syntron MultiTRAK System Controller to control streamer depth and position- two OYO GEOSPACE GS-624 thermal plotters. Near resp. single-channel paper plors

were made on one of the thermal plotters. Shot gathers were plotted every 200 shots (lhour) on the second thermal plotter.

- master control system including trigger control, shot point counter and additional equip-ment developed by BGR (Adam & Sievers, 1996) for communication between the record-ing system, the airgun control system, the streamer control system and the navigationsystem to create, transfer and store an external data header containing

navigation data of the navigation system,- air pressure data of the airgun anays,- YZAD data,- MultiTRAK data,- GPS time.

- one satellite-controlled clock Meinberg GPS 166

4.4 Gravimetric and magnetic equipment- land gravimeter LaCoste-Romberg model G no. 480- manne gravrmeter KSS31 no.Z2,Bodenseewerk Geosystem GmbH- high sensitivity proton precession gradient magnetometer Geometrics G-81lG- Hatlapa gradient magnetometer winch

BGR 1t6&3 -28 -

4.5 Computer systems for navigation, data acquisition, data processingand interpretation

- workstation for data acquisition: VAXstation 3200,32\\tBmemory, 32 serial lines,2 GBhard disk, MO optical drive

- workstation for data processing and interpretation: VAXstation 3100/NI76,32 MB mem-ory,3 GB hard disk, MO optical drive, DAT tape drive

- 2 PC systems- 3 plotters (1 drum plotter Hewlett-Packard Draftmaster II, I desk-top plotter Mutoh iP

220, I ink-jet printer/plotter DEC LJ250)- 3 printers (l laser printer DEClaser 3500, | 24-dot matrix printer Fujitsu DL-2400, 1

9-dot matrix printer DEC LA50)- 3 graphic terminals, 1 alphanumeric terminal- 1 precision digitizer Kontron Summagraphics (DIN Al)- I scanner Hewlett-Packard ScanJet 4c- 1 satellite-controlled clock Meinberg GPS 166.

All computers were integrated into the LAN of the ship.In addition, the VAXstation 3100worked as a network server (file and print services using Digital PCSA) for the PC systems.

4.6 Utilized equipment of the ship

The list contains only the most important systems.

- 2 LMF compressors, manufactured by I-eobersdorfer Maschinenfabrik, capacity25 m'/h each

- 1 Elliot V/hite Gill azimuth thruster- I Hydrosweep DS swath echosounder system, manufactured by Atlas Elektronik- I HMS 300 system (processing of Hydrosweep data)- 1 Parasound sediment echograph, manufactured by Atlas Elektronik- I Paradigma recording system- 2 GPS receivers Ashtec LD-Xtr- 1 GPS receiver Trimble 4000 DS- I DGPS system Racal Skyfix with MultiFix 2 software- 1 interface processor belonging to the ANP2000 navigation system- I general purpose minicomputer MicroVax 3100 ('WISVAX')- 4 terminal seryers (DEC)- I plotter Graphtec MP 4300 (DIN A3)- I ink-jet plotter Hewlett-Packard DesignJet 650C- 1 precision digitizer Kontron Summagraphics (DIN A0)- alphanumeric terminals- PC systems (text processing and graphic data processing)

BGR 116&3 -29 -

5 Operational reports

5.1 Profile plan and list of profÏles

Fig. 4 shows the profiles obtained during cruise SO-L22. The positions of the lines are greatlydetermined by the intense fishing off Pakistan which did not allow to survey the lines whichhad been planned originally (Fig. 2). Table 1 is a list of the profiles surveyed during cruiseSO-122. The positions are start and end points. The profile lengths are not the distances be-tween start and end points, but measured along the lines.

5.2 Fishing off Pakistan(H.A.Roeser & M. Tahir)

Unfortunately, during this geophysical cruise we have learned a lot about the fishing off theMakran coast of Pakistan. The following section desciibes the main observations. We supposethat the fishing activity is very variable during the year. Thus our experience refers only to themonths August and September.

We encountered mainly fishing-boats with Pakistanis. Only in one case we encountered agroup of Portuguese fishermen with larger boats. When rwe approached them, they showed uswith their searchlight beams that their nets, which extended between neighbouring boats,where about 4 nm long.

Usually, the fishermen work in groups of 5 - 10 fishing boats which during the night lie sev-eral miles apart and use separate nets. The nets which we have observed are 0.8 nm long, theyhave a red and a black end flag, the distance between the flags is about 30 m. Several of thesenets can be connected to one another, with the end flags attached to each net. The nets are putout in the evening and picked up in the morning. The fishing-boats remain several days in thefishing area. Usually, the boats move with speeds around 6 - 7 kn, the nets are put out at 2 kn.During the night, the boats lie mostly in lee of their nets.

The nets are several (5?) m high. Their material is nylon thread or something similar with adiameter of 2 mm, consisting of fibres with a diameter of 0.01 mm. The meshes are 70 mmwide. The nets have a weight of 30 glrrÊ. Occasionally, black spheres or pieces of styropor areused.

During the night the boats bear lights. Then they become visible at a distance of 6 nm. Duringday time, on clear days, they are visible at5 - 6 nm distance. During dawn, the boats arenearly invisible because they do not use their lights. The radar detects most boats at a distanceof 4 nm, some, however, only at 1 - 3 nm distance.

In response to a fax request, we received on August 21,1997 from Captain Bhatti, Hydrogra-pher of the Pakistanian Navy, the following additional information:

"The fishing in Pakistani waters during summer months is generally restricted to northof latitude 24'.It is, however, more concentrated up to the 200 m isobath along the

BGR T166/.3

with seismics without seismics without seismics, without magnetics

Fig.4. Map in mercator projection showing the profiles surveyed during cruise SO-122

BGR 116&3 - 3 1 -

Table 1. List of the profiles surveyed during cruise SO-LZ2Part I

so122-01

so122-02

so122-03

so122-04

SOL22-MA

sol22-05

so122-05A

sol22-06

so'22-a7

so122-08

so122-09

sol22-10

sot22-11

so122-12

sol22-13

sor22-13A

sor22-14

It245

I1980

It940

I2260

I198s

I450

I1260

14220

14.8.9714.8.97

t4.8.9715.8.97

16.8.9716.8.97

16.8.97 .16.8.97

18.8.9719.8.97

19.8.9719.8.97

t9.8.9720.8.97

20.8.9720.8.97

20.8.9720.8.97

2t.8.972r.8.97

21.8:9722.8.97

22.8.9722.8.97

22.8.9723.8.97

23.8.9723.8.97

24.8.9724.8.97

24.8.9724.8.97

25.8.9725.8.97

lI:252O:M

20:0403:00

00:4003:28

05:5019:52

14:08l0:12

17:48

20 25.3 N22 08.5 N

22 08.5 N23 10.1 N

22 16.5N220/..7N

22 00.5 N22 58.7 N

22 53.6N24 38.1 N

24 35.7 N24 t3.7 N

24 06.8 N23 37.0 N

23 37.5 N24 50.5 N

24 51.9 N24 44.4N

24 09.5 N24 02.8 N

24 02.4N24 55.6 N

24 55.9 N2424.7 N

2 4 2 5 . 1 N24 09.3 N

24 09.4 N25 02.7 N

25 01.0 N24 48.7 N

24 45.s N2412.2N

23 59.5 N22 57.rN

63 20.0863 33.28

63 33.286237.68

63 r7.986328.28

63 21.586229.38

6234.886221.28

6224.286239.s8

6253.6863 48.0 E

63 48.3863 48.08

63 5r.7 E66 06.6 E

66A7.2865 3 l . t E

65 3r.9865 34.38

65 31 .3 E6616.68

66 16.4865 11 .9 E

65 r2.5865 15.0 E

64 54.5864 53.98

64 53.2864 5r.48

65 11.7 E66 55.6 E

150

320

130"

320'

3200/ 00

l4go

1200

00

93"

259'

00

127. / l2l"

t < 5 0

360.

lg0.

lg0.

123.

G M P192.63 km

G M P148.46 km

S G M P29.68[<îL

S G M Pl4O.42kurî

S G M P201.63 km

G M P48.35 km

G M P107.34 km

G M P135.34 km

G M P227.59lçn

S G M P62.53 km

S G M P99.51km

S G M P97.48 km

S G M P1 1 2 . 8 1 k m

S G M P99.75 km

S G M P22.67 km

S G M P62.79h.nl

S G M P211.15 km

Methods used:G - graviryM - magneticsP - Hydrosweep,parasoundS - reflection seismics

BGR 116æ3 -32 -

Table 1. List of the profiles surveyed during cnrise SO-122Part 2

so122-1s

sot22-t6

sor22-17

so122-18

sol22-19

so122-19A

so122-20

so122-198

sot22-21

so122-22

so122-23

sot22-24

so122-25

so122-26

sot22-27

sot22-28

I1300

12165

I4670

12745

It430

2350

I733

11696

I2227

I5005

I2Lr0

I1695

16212

13529

25.8.9726.8.97

26.8.9726.8.97

26.8.9727.8.97

28.8.9728.8.97

28.8.9729.8.97

29.8.9729.8.97

29.8.9729.8.97

29.8.9729.8.97

30.8.9730.8.97

30.8.9731.8.97

3t.8.97t.9.97

r.9.9'7r.9.97

1.9.972.9.97

2.9.973.9.97

3.9.974.9.97

4.9.975.9.97

23:5506:24

09:2620:15

22;192l:40

01:4515:28

l9:0502:14

03:4805:32

07:10IO:37

L2:2L16:01

01:5610:25

l2:5600:04

03:2704:28

09:2920:01

22:2106:49

08:3615:39

18:5112:30

l 6 : 1 8

2254.8N2329.6N

23 34.0 N22 51.3 N

22 51 .1 N24 09.8 N

23 54.2N2248.4N

2241.3N23 15.2 N

23 17.9 N2326.2N

2 3 2 4 . 8 N23 08.5 N

23 08.0 N23 25.7 N

22 59.8 N2 2 2 r . 7 N

22 r5.2N23 04.9 N

23 00.0 N2t 04.5 N

21 04.8 N2024.4N

2024.7 N20 57.4 N

20 56.3 N22 5r .8 N

22 5r.4N22 42.3N

2 2 3 9 . 0 N23 08.3 N

66 5s.3 E66 50.9 E

66 56.1 E66 12.68

66 13.8 E6426.48

6422.9865 00.2 E

æ46.486426.28

64 18.5 E6413.58

64 10.0 E& 1 9 . 6 8

6424.486414.5Ê,

6419.5864 47.48

64 40.1864 03.1 E

63 56.3865 18.1 E

65 18.8 E6435.98

64 36.8 E6402.58

6402.r866 13.48

66 16.9 E6435.58

6422.8861 48.68

3530

223"

30go

1520

331'

331o

15 10

33lo

1460

325"

145"

225.

315 '

47"

27001249"

280"

S G M P64.98 krrr

S G M P108.32 km

S G M P233.63km

S G M P137.53 km

S G M P71.77lsl

S G M P17.55 km

G M P34.28 km

S G M P36.75 km

S G M P85.31km

S G M P111.63 km

S G M P251.20km

S G M P105.64 km

S G M P84.93 km

S G M P311.05 km

S G M P176.39 km

G M P261.34krrl

Methods used:G - gravityM - magneticbP - Hydrosweep,ParasoundS - reflection seismics

total survey length 4OBZ.43kf,nwith reflection seismics 29Zi.lOkm

BGR 116643 - 3 3 -

entire coast. Extreme caution is, therefore, advised while operating underwater sen-sors."

From Dr. Amjad, Director General of the National Institute of Oceanography, Karachi, Dr.Tahir received fuither information by telephone on August27, 1997:

"During the summer, fishing is very active on tuna, especially bonitos. The tunas mi-grate around the whole Indian Ocean. At the end of the SE monsoon, they iue espe-cially frequent off Pakistan because here at this time nutrition is excellent. They are themost important part of the catch. Fishing with long nets is done mainly during thenight when the fish can't see the nets. The nets are 500 m long, several nets can be at-tached to another. During the day, the main feeding time of tunas, use of fish-rods ismore effective. Most fishermen are not alphabetized. It is difficult to talk with thembecause they have a traditional slang in which the words often have unusual meanings.The knowledge of the positions of the most promising fishing grounds in dependenceofthe season is handed down over generations.

Often, the fishing-boats belong to larger companies and the fishermen have to delivermost of the catch. For being able to fish several days, the fishing-boats have ice onboard into which the fishes are laid. The main home port is Karachi."

There are two differences to our observations:- The net lengths that we have observed were much greater. This may be due to technical

development.- We have never seen fishermen using fish-rods.

Typical for the observed fishery is that they use only surface gear. For future geophysical workin this area it is important to find out whether at other seasons this type of fishery is lesspromising and thus perhaps less intensive. The most comprehensive study on fishery in theIndian Ocean has been prepared by Stéquert & Marsac (1989) on behalf of FAO . They statethat the Baluchistan fishing ground are exploited more intensively during the months April toSeptember and that most tuna schools were sighted between 30 and 180 nm offshore. Kingfishmake up about two third of the total catches.

According to the information by Dr. Amjad, fishery concentrates on several kinds of tuna.They belong to the category "tropical surface tunas" which pose a riddle to the biologists(Roger, 1994): Although they have high metabolic demands, they live in a poor environment,namely in the uppermost 200 m of tropical oceans. Moreover, they are day-feeders. Thus theirprey-fishes must stay in the depth range 0 - 200 m during the day. However, the base of thefood chain, the micronekton migrates depending on the illumination: During the day it isdeeper than 400 m, during the night it comes up to 0 - 200 m. Only a small proportion remainsat 0 - 200 m during the day. Investigation of the stomach content of tunas showed that theprey-fishes of tunas are predominantly species which prey on this small proportion ofzooplankton that remains at shallow depth during the day. It was found that in those areaswhere fishing for surface tunas is successful, not only the total biomass of zooplankton ishigher, but also the proportion of "useful" zooplankton.

The fact that the nets are put out at night demonstrates that during the night fishing with gillnets is more successful than during the day. This indicates that the day-feeding tunas can

BGR 116æ3 -34 -

avoid the nets during the day whereas during the night, in spite of their reduced activity, theyget caught sufficiently frequent in the nets.

Sharp (1979) emphasizes another effect. Tuna are near the surface where the deeper layers ofwater are hostile to them. One cause may be that there is less oxygen, possibly due to highproductivity in the upper'layers of water. Thus, well expressed oxygen minimum zones maybe indicative ofhigh concentration oftunas in the surface layers.

5.3 Navigation and acquisition of non-seismic data(8. Schreckenberger & H.-O. Bargeloh)

5.3.1 Navigation and positioning

Since 1996, SONNE uses the differential GPS (DGPS) system SkyFix by Racal Survey. Darafrom several reference stations are collected for SkyFix and correction values are broadcastedvia INMARSAT satellites to the users. On board of SONNE the special software packageMultiFix 2 utilizes the the signals from a special decoder and from the new GpS receiverTrimble 4000 DS for the calculation of the DGPS positions. The reference stations Bahrein,Baku, Bombay, and Abu Dhabi were used by MultiFix on this cruise. The system calculated amean RMS error of a few meters, in general below 10 meters, for the final position solution.

Navigation by SkyFix overcomes all former problems with Selective Availability givingpositions that are accurate within approximately ten meters. Therefore, for the first time onour cruises, it was possible to use the GPS position without further postprocessing.

The only problem on cruise SO-122 were short gaps of one or two minutes on a few linesbecause of problems with the rotation of the INMARSAT antenna beyond 360" into the rightangle and because of its possible shadowing by the mast of the ship.Under these conditionsthe SkyFix system did not use the unconected GPS position but simply stopped the dataoutput. Mostly, the ship's officers remedied this problem within very short time.

5.3.2 Dataacquisition

The BGR's marine gravity/magnetics group operates two VAXstation computers that rununder the operating system VMS. On board of R/V SONNE they are connected with the ship'scomputers and terminal servers via thinwire Ethernet. One of the computers (VAXstation3200) is equipped with 32 serial interface connectors and is used for data acquisition andrealtime data display. Data processing, interpretation, and text processing is done on aVAXstation 3 I 00/lvl76.

All data are read into the computer via serial interfaces or over the Ethernet network. There isa number of real time programs that write the data into the memory as soon as they areavailable. The main data acquisition program checks, reformats, and collects the data items toone data set each 20 seconds and writes it to direct access files on magnetic disk.

BGR 1t6&3 - 3 5 -

The navigation data come via the ship's WISVAX computer over an ethernet link once persecond. On this cruise the following data were received from the ship's navigation system:

- position from GPS- heading fromthe gyro- speed from the doppler-sonar (DO-Log)- waterdepth values from HYDROSWEEP (the central beam only) and PARASOIJND- weather data, water temperature, and salinity.

The following data are read over serial lines:

- precise time marks (UTC) from a GPS controlled clock once per second- shot point numbers and shot point times from the seismic system- magnetic total intensity and depth below the sea surface for both sensors from the

gradient magnetometer- heading of the magnetometer array from the compass between the sensors- raw gravity values from the marine gravimeter

The data aequisition progrzrm provides online navigation data for the following systems:

- once per second to the marine Gravity meter where they are used to support the gyrosystem

- for every shotpoint to the seismic system

Analog records are produced for the magnetic total intensity, the gradient, and the gravity.Moreover, we use a small navigation program (Roeser et al., 1992) for plotting the plannedprofile lines and a continuous online track plot on a DIN A3 plotter.

5.3.3 Data processing

Processing of the positions used to take much time before the installaton of DGPS but now itis reduced to filling the (rare) gaps mentioned in 5.3.1.

Standard processing of the magnetic data includes the elimination of obviously erroneousreadings of the magnetometers, time shift of the measurements due to the cable length of thesensors and subtraction of the geomagnetic reference field. Vy'e use a two-dimensional poly-nomial of degree 4thatapproximates the IGRF 1995 reference field (IAGA, 1996) with anaccuracy of better than I nT. Obviously, the IGRF does not reproduce the magnetic field inthe survey area accurate enough because the magnetic anomalies calculated in this way are toohigh by about 80 nT. This is caused by the fact that the IGRF is only an approximation of themain field of the Earth up to degree and order 10. The observed deviation is in the usualrange. Therefore we subtract this value from the residual field whenever we display the ma-gnetic anomalies on maps or diagrams.

A first reconstruction of the magnetic field from its longitudinal gradient was done routinelyduring the survey. The procedure and the results obtained on board are discussed in section5.6.2. Final processing will be done after the cruise when the records from the magneticobservatory in Karachi are available.

BGR I 16æ3 -36 -

The processing of the gravity values includes a time shift due to the relaxation time of theinternal filters of the instrument, scaling, the connection to the gravity values in the port,subtraction of the normal gravity according to the international gravity formula and calcu-lation of the Eôtvôs correction under application of the processed navigation data (cf. sections.s.2.2).

5.4 Reflection seismics(V. Damm, J. Adam, H. Dohmann, K. Puskeppeleit, U. Schrader, J. Sievers, D. Steinmann &W. VoB)

The Leobersdorfer compressor and the airgun iurays were very reliable. There was no loss ofshooting during the whole survey. Maintenance was done during the transit between succes-sive profiles.

The shots were triggered in time intervals of 18 seconds depending on the instantaneous speedof the ship. The intended shot distance was 50 m at a speed of 5.4 knots. In general, the dis-tances were very constant due to the use of DGPS.

The time triggering interval was superposed with a random time function of 300 ms for sup-pressing multiples from previous shots through stacking after CDP sorting. The shot time in-terval with the random function, which was exactly evenly distributed, was generated on theMaster PC with an interface card for tiggering the airgun array via the SYNTRAK 480 sys-tem and the VZAD.

Time triggering involves that the CDP sorting has to be done by coordinates. To correlatebetween shot numbers and positions the FFID (Field File Identification) numbers written oncartridge were transmitted to the positioning system.

The streamer configuration had to be changed during the survey compared to the initial one(Fig. 6). On August 17 the streamer was broken due to collision with a fîsher net and only 6sections were recovered. Another 6 sections were picked up next day, but 28 sections werelost. V/ith these and the spare sections the survey was continued on August 18, 14:08 UTCwith 17 active sections (each with three data channels) (Fig. 5). Because of a group of fisherboats in the area seistnic data acquisition was finished at Aug. 19, 10: 12UTC. At Aug. 21,14:39 UTC we continued the reflection seismic investigations in the eastern part of the surveyarea were fishery was less intensive. Because the risk of loosing the streamer was consideredto be very high, as a precaution the streamer was reduced to 12 active sections (Fig. 7). Thesurvey was continued in this configuration until Alg.24,12:17IJTC. Then, in view of thegreat problems with fishery, the streamer was further shortened to 8 active sections (Fig. 8).The survey was restarted with that configuration on Aug. 25,00:58 UTC. On Sep. I thestreamer was reconfigured to 16 sections to get enough move-out on the lines for investigationof the Indus Fan where less intense fishery was expected (Fig. 9). This configuration wal useduntil the end of the cruise. The streamer configurations for all profiles measured during thecruise are shown in Fig. 5 - Fig. 9. During the whole time of the cruise the survey was se-verely disturbed by the intense fishery. Therefore a number of survey lines had to be changed,intemrpted, or cancelled.

-37 -

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BGR 116&3

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- 4 0 -BGR 116&3

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BGR 116643

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BGR 1166/'3 - 4 2 -

The seismic signals are amplified, filtered, analogue-digital converted by up to ten activeelectronic acquisition modules, each one capable to acquire twelve data channels. One addi-tional passive electronic module is used in front of the streamer (within the winch) to repeat(amplify) the data before recording with the SYNTRAK 480 system.

A tail buoy (constructed by hakla-Seismos) with Radar reflector and blinking light was usedduring the whole survey.

For controlling the streamer position, both depth and heading, a Syntron MultiTRAK systemwas used with up to l1 MultiTRAK Remote Units (MTU's or birds) and a PC-based digitalcontrol system which allows separate interactive control for each bird. For our measurementswe chose 9 m which is an optimum depth to reduce reverberations between water surface andstreamer. To reduce the relative high buoyancy of the streamer according to water temperatureand salinity we applied an additional weight of 2 kg lead to each section. In addition, a weightof 100 kg was mounted at the seaward end of the tow lead. During the measurements the av-erage streamer depth variations as indicated by the MTU-values were between I-2m. The birdangles were mostly around 7o downward.

Recording of the seismic data is performed by the SYNTRAK 480 Multiple Streamer Teleme-try System. The main components are the System Controller, the Multiple Streamer TelemetryProcessor (MSTP), and the Multiple Streamer Recording System (MSRS).

The MSTP is the shipboard interface between the MSRS and the digital streamer, it can ac-quire seismic data at 1 ms sample rate. Usually, the sampling rate was set to 4 ms. To get ahigher resolution of thin layers (in particular free gas beneath a gas hydrate zone) along lineswhere we expected bottom simulating reflectors (BSR) the samplingrate was set to 1 ms(Table 2). The MSRS is the actual recording system performing data collection, recording,and plotting. The MSRS supports two IBM 3480 compatible dual tape units, one 4 GB harddisk for the single-channel, and two OYO GEOSPACE thermal plotters. The recording systemis interfaced by the Master PC with the MultiTRAK data acquisition streamer control system.

The seismic data are received via the data collector boards in the MSRS chassis. Then the dataare converted into SEG-D demultiplexed format and written into the VME memory. The tapeheaders are created using the MultiTRAK and the navigational data and together with theformatted seismic channels written to the tapes. One to twelve single channels can be gatheredand written to the hard disk as well. These channels can be accessed at the end of the line ei-ther for storing to tape or for plotting. During the survey, one plotter plotted online a singlechannel resp. near-trace and the other plotter plotted a shot gather every 200 shots (that is onceper hour). After ending the line, up to 9 additional single-channel plots were produced fromthe single-channel data written to the hard disk. For most of these plots the plotting parame-ters were 10 s record length and 25 channels per inch (TPI), in addition we produced for de-tailed studies three plots with 4 s record length (time windows: 0-4 s,4-8 s, and 8-12 s) and,25channels per inch (TPI).

On lines SO122-08 until SO122-10, high room temperature and humidity caused sometimesfailures in the SCSI controller of the hard disk. Therefore it was not possible to plot copies ofthe single channels after the end of these lines. In these cases only one online plot with l0 srecord length was used for interpretation. For plotting the shot gathers and auxiliary channelsa time window of up to 10 s was used depending on the water depth. An AGC with a 1000 ms

BGR 116&3

Table 2. Summarized seismic data volume recorded during cruise SO-I22

Profile Shots Channels Sampling interval

so122-03 560 r20 4 m s

SOL22.M 2805 r20 4 m s

sor22-MA 4018 60 | 5l active 4 m s

so122-08 t245 36 4 m s

sot22-09 1980 36 l m s

so122-10 1940 36 4 m s

sot22-rr 2260 36 4 m s

sor22-12 1985 36 4 m s

sot22-13 450 36 l m s

so122-13A 1260 36 1 m s

soL22-14 4220 24 4 m s

so122-15 r300 24 4 m s

sot22-16 2165 24 4 m s

sol22-17 4670 24 4 m s

so122-18 2745 24 4 m s

sor22-t9 1429 24 4 m s

sot22-I9A 350 24 4 ms Testso122-198 733 24 l m s

sot22-21 1696 24 4 m s

so122-22 2227 24 4 m s

sot22-23 5005 24 4 m s

soL22-24 2rt0 48 4 m s

so122-25 r695 48 4 m s

so122-26 6212 48 4 m s

soL22-27 48 4 m s

s.s.r

BGR 116643 - 4 4 -

window length was applied to the single-channel plots, whereas a fixed gain of 60 dB wasapplied to the shot gathers and auxiliary channels.

Quality control during acquisition consisted of:- continuously controlling the airgun pressure,- controlling the airgun functions,- observing the signals of the hydrophones within the arrays and adjusting the trigger delays

for an optmum signal,- checking and recording the streamer depth and position (heading) of each shot via the

control screen of the MultiTRAK system. These data were also stored in the header andwritten to field tape.

- continuously checking, whether all sections of the streamer were free of abnormal noiseand yielded about the same signal amplitude. This was done for every shot via the QCgraphics display of the SYNTRAK system.

- continuously observing the single-channel resp. neartrace records.

5.5 Gravity(Jiirgen Fritsch, Peter Kewitsch & Bernd Schreckenberger)

Gravity connectionsThe results of different surveys of gravity at sea are only comparable, if they are related to theInternational Gravity Standardization Net IGSN71 (Morelli, 1974). Therefore, gravity meas-urements at land have to be carried out to connect the gravity measurements at sea with theworld gravity net. The marine geophysics group of BGR uses for the gravity connections aLaCoste-Romberg gravimeter model G, no. 480.

The International Gravity Standardization Net IGSN71 was established in 197lby the Inter-national Union of Geodesy and Geophysics IUGG as a set of world-wide distributed locationswith known gravity values better than a few tenths of mGal. It replaced the formerly PotsdamSystem which did not conform any more to the necessities of modern geodesy and geophysics.According to the recommendations of the ruGG, all gravity surveys, marine or land, must berelated to the datum and to the scale of the IGSN7l . The data of the IGSN71 reference sta-tions is provided on requebt by the Bureau Gravimetrique International BGI, ToulouseÆrance.

Of course, for gravity measurements at sea one cannot rely in every case on the existence of areference station near to the harbour site. Therefore, the marine gravity group of the BGR per-forms always a connection of the harbour sites to the gravity reference point of BGR situatedin the storage and test building VB11.

In Djibouti, the only IGSN71 gravity reference station at the airport is now obsolete due to areconstruction of the airport buildings. Instead we received from BGI a description of twogravity stations in the dock area of Djibouti Harbour. The corresponding gravity values weredetermined in l97I from Woods Hole Oceanographic Institution in the Potsdam system, andlater, in l9T4,transformed into IGSN7l values. According to Gilles Balma from BGI, aFrench group reassessed the stations in 1995 and recomputed the gravity values into theIGSN7l svstem.

BGR 1t6&3 - 4 5 -

The gravity connections in Djibouti were made between an auxitiary point at the entrance ofthe Sheraton Hotel, the mooring site of RA/ SONNE at quay tg GiÉ.-101, and the referencestation no.798 on the small fuel pier at the north-west comer of the mole du Fontainebleau(Jetée du Large), adjacent to shed 8.

11" 37,5' N

:\ffi

M ô l c S u dContrimr Têrminsl

Fig. 10. Port of Djibouti (from Admiralty Chart 262); (a) mooring site of RV SONNE at quay13 from August 6 to 9, 1997; (b) reference station BGI #798 at quay 13. The inseishows details ôf ttre gravity observation station at bollard Z+ aongsiae R/V SONNE.

Table 3 gives the observation report of the gravity connections. From 3 consecutive readingsbetween days 4 and 6 of August at the entrance of the Sheraton Hotel it can be deduced, thàtthe instrumental drift is about +0.07 mGaVday or +2 mGaVmonth, respectively, which indi-cates a typical value for the (positive) drift according to the manufacturer's manual: The grav-ity difference between the mooring site of R/V SONNE and the reference station results to+0.8 mGal; absolute gravity at the mooring site is97824L.4 mGal while the water level is 2.0m below the instrument.

BGR tt6643

Table 3. Observation report on gravity connections in Djibouti

r(,ç1

Reference station:DJ j.bouÈi harbour, nole du FoaËaiaebleau, guay 10, BGI no.798IcS![71 gravity: 978240.6 mGal (waÈer leve1 2.0n)Gravity stations:1. ojibouti, IIoteI Sheraton (HS)2. Djibouti harbour, quay 13, R/V SONNE site at boIlard,7tL (13)Obserrrers: F = Fritsch, K = Kewitsch, S = Schreckenberger

station/ seaobserver level daÈe

4 . 8 . 9 74 . 8 . 9 74 . 8 . 9 75 . 8 . 9 75 . 8 . 9 76 . 8 . 9 76 . 8 . 9 76 . 8 . 9 77 . 8 . 9 77 . 8 . 9 7

' t . 8 . 9 77 . 8 . 9 77 . 8 . 9 7

urc

U O : J . J

0 6 : 1 - 80 6 : 2 00 5 : 5 1 -0 5 : 5 50 6 : 5 90 7 : 0 20 7 : 0 50 3 : 5 00 3 : 5 3

read,ingunits

t 8 3 3 . 7 7L 8 3 3 . 7 81 8 3 3 . 7 81 _ 8 3 3 . 8 8L 8 3 3 . 8 7L 8 3 3 . 9 21 8 3 3 . 9 41 8 3 3 . 9 4l _ 8 3 3 . 9 71_83 3 . 97

L834 .491 8 3 4 . 5 0L 8 3 4 . s 0

1 8 3 3 . 9 11 8 3 3 . 9 21 8 3 3 . 9 01 8 3 3 . 9 11 _ 8 3 3 . 9 1

] -834 .541 8 3 4 . s 3

1 _ 8 3 3 . 7 31"833 .77] -833 .7 4

L 8 3 4 . 4 91 -834 .49

tidalcorrected

1 8 3 3 . 7 91-83 3 . 80L 8 3 3 . 8 11 _ 8 3 3 . 8 51 8 3 3 . 8 51 8 3 3 . 9 31_83 3 . 9sL 8 3 3 . 9 5r - 8 3 3 . 9 3i -83 3 . 93

L 8 3 4 . 5 2l _ 8 3 4 . 5 3L 8 3 3 . 5 2

1 8 3 3 . 8 91 8 3 3 . 8 9L 8 3 3 . 9 21_83 3 . 931 8 3 3 . 9 3

1_834 . s1 -1 _ 8 3 4 . 5 0

1 8 3 3 . 7 0L833 .7 4l - 8 3 3 . 7 1

L 8 3 4 . 4 7L 8 3 4 . 4 7

average( - 2 . 0 m )

L 8 3 3 . 8 0

1 8 3 3 . 8 5

1 _ 8 3 3 . 9 4

1 8 3 3 . 9 3

+ 0 . 5 9

1 -834 .52

- 0 . 6 31 8 3 3 . 8 9

1 8 3 3 . 9 3

+ 0 . 5 4L 8 3 4 . 5 L

( re34 .47 )- 0 . 7 9

1 8 3 3 . 7 2( 1 _ 8 3 3 . 6 8 )

+ 0 . 7 5L 8 3 4 : 4 7

( i - 834 . 43 )

(Hs) /s( H S ) / K( H S ) / K( H s ) / K( H S ) / S( H S ) / S( H S ) / F( H S ) / s( H S ) / F( H S ) / F

d i f fe rence( L 3( 1 3( 1 3

/ F/ r - 2 . 0 m/ F

1-4L4t 4

2 83 23 5

4 04 54 8

di f ference( H s ) / s( H S ) / F( H S ) / F( H S ) / F( H s ) / Fdi f ference( 1 3 ) / F - t . 2 m( 1 3 ) / sd i f ference7 9 8 ) / F7 9 8 ) / S - 1 . r - s7 9 8 ) / Fdi f ference( 1 3 ) / S - 1 . ] - s( L 3 ) / F

7 .8.e7 J+-,31_7 . 8 . 9 7 [ 0 5 : 5 9 \8 . 8 . e7 Ëi4T-8 . 8 . 9 7 0 3 : 4 48 . 8 . 9 7 0 3 : 4 7

8 . 8 . 9 78 . 8 . 9 7

8 . 8 . 9 78 . 8 . 9 78 . 8 . 9 7

8 . 8 . 9 78 . 8 . 9 7

n A . ? n

0 6 : 3 3

0 6 :

0 6

0 60 7

5 70 0

Instnr.mental drift from Àugrust 4 to 5:Àverage differences (LCR factor at 1800( H S ) - ( 1 3 ) = - 0 . 5 9 c . u . - > - 0 . 6 0 n G a L( 1 3 ) - ( 7 9 8 ) = 0 . 7 7 c . u .

0 .14 c .u . l 2 days- > 1 . 0 1 5 6 3 ) :

BGR I t6643 - 4 7 -

Using the very first observation on August I,1997 at the BGR storage and test building VB11and without considering an instrumental drift, the gravity on the BGI reference site no. 798 isdeduced to978239.3 mGal, i.e. more than I mGal less than the value of 97840.6 mGal knownfrom BGI files. Either a negative (!) drift of -1.3 mGaVl0 days, i.e. about 4 mGal/month, or awrong gravity value for the BGI station no.798 can explain this discrepancy. Until completionof the observation loop Hannover-Djibouti-Muskat-Hannover it will not be possible to deducethe exact value of absolute gravity for the BGI station no.798. Thus, the value of 978240.0mGal for the absolute gravity at the mooring site of R/V SONNE related to VB11 was usedfor the preliminary processing of the marine gravity data on board R/V SONNE. When re-duced to sea level, it gives978240.6 mGal.

5.5.2 Gravity measurements at sea

5.5.2.1 Short description of the seagravimeter systemThe BGR owned gravimeter system KSS31 is a high performance instrument for marinegravity measurements, manufactured by BODENSEEWERK GEOSYSTEM. The sensor isbased on the ASKANIA sensor GSS3 designed by Prof. Graf in the sixties. The gyro-stabilized platform and its electronic control devices were developed by the BODEN-SEEWERK GEOSYSTEM in the second half of the seventies.

The seagravimeter system KSS3l consists of two main assemblies: the gyro-stabilized plat-form with the gravity sensor and the data handling subsystem.

The gravity sensor is a tube-shaped mass guided by 5 threads in frictionless manner (Fig. l l).It is non-astatized and particularly designed to be insensitive to horizontal accelerations. Ac-cordingly, the motion of the mass is limited to one degree of freedom in vertical direction. Themain part of the gravity acceleration is compensated by a mechanical spring. Changes of thegravity are detected by an electromagnetic system. A displacement of the spring-mass assem-bly with respect to the outer casing of the instrument is measured with a capacitance trans-ducer. The output from the transducer is fed back into an electromagnetic moving coil systemused for feedback control. A P-I feedback (P - proportional, I - integration) suppresses theaccelerations of sea motion. The I-acting feedback provides an integral signal which drives thesystem to zero; it determines the (overcritical) damping of the system. The current flowingthrough the moving coil is the measure for the gravity change.

A voltage to frequency converter is used to provide highly accurate output to the rack-mounted data handling subsystem. The power supply of the gravity sensor contains a sealedbuffered battery unit with sufficient capacity to maintain the internal temperature stabilizationof the sensor lor 24 hours in case of main power loss. The sensor caging electronics whichactivates the sensor caging mechanics in case of failure, is included in the data handling sub-system.

The levelling subsystem consists of the platform and a vertical electrically erected two-axesgyro. The platform stabilizes the gravity sensor in pitch and roll. Also the control electronicsand the power supply of the platform are located in the data handling subsystem. All logicfunctions of the gyro run-up and -down sequence as well as the automatic platform caging ar"performed in the system controller are located in the data handling subsystem.

The data handling subsystem provides all equipment necessary for filtering, logging, pre-processing and self-testing of gravity measurements. It also provides the control electronics of

BGR 1166/.3 - 4 8 -

the platform, the power supply for the sensor/platform and monitor record facilities. The sys-tem controller is the cental part of the data handling subsystem consisting of a central proces-sor and the interfaces to the peripheral equipment such as gyro, platform, gravity sensor, ex-ternally derived navigation data and computer for data processing, and analog monitor re-corder for control pulposes in real-time.

Fig. 11. Gravity Sensor GSS30 of the seagravimeter system KSS3I

The seagravimeter system KSS31 is installed entirely in the so-called gravimeter room of R/VSONNE which is close to the ship's center of gravity. Gravity data are transmitted to the BGRdata acquisition and processing system in the gravitylmagnetic laboratory, and position,course and speed are returned from there to the gravimeter system.

5.5.2.2 Processing of the gravity dataProcessing of the gravity data consists essentially of the following steps:- a time shift of 175 seconds due to the overcritical damping of the sensor,- conversion of the output from reading units (r.u.) to mGal by applying a conversion factor

of 0.94542 mGaVr.u..

BGR I L6&3 -49

connection of the harbour value to the world gravity net; the gravity at the pier reduced tosea level gives 978240.6 mGal which has to be compared with the KSS3t harbour readingof -1685.0 r.u. or -1593.0 mGal using the conversion factor 0.94542 mGaUr.u.,

- correction for Eôtvôs effect using the navigation data,- correction for the instrumental drift (not performed until completion of the cruise),- subtraction of the normal gravity (WGS67).As a result, we get the so-called free-air anomaly (FAA) which in the case of gravity at sea issimply observed gravity minus normal gravity. According to the selectable time intèrval of thedata acquisition system, gravity values are available every 20 seconds, normally.

5.6 Magnetics

5.6.1 The gradient magnetometer

@. Schreckenberger & P. Kewitsch)

Generally all marine magnetometer measurements suffer from the lack of base stations thatcan be used in order to reduce the temporal magnetic variations, as it is routine for land andairborne surveys. In the past, only on a few cruises we were able to install a base station onadjacent islands or coasts. Unfortunately, permanently operating magnetic observatories arerare in many areas of the world and therefore mostly they are too fargway from our surveyareas in remote oceanic regions. In particular, the short period variations éan not be reducèd inthis way.

In order to avoid these problems and to obtain variation-free magnetic measurements on theship we use the gradient magnetometer Geometrics G-81 lG. The instrument consists of twoproton magnetometer sensors which are towed 150 meters apart as a longitudinal aray about600 meters astern of the ship (Fig. 12). We have largely modified the instnrment, espjciallythe towing system including all connections of the sensors and the splitter box to the cables.We use plugs from seismic streamer technology instead of the original fixed connectors at allthese connections in order to enable fast replacement of defective cables and sensors andeasier handling.

<- 600m __+ + 1 5 0 m - - )

Fig.12. The configuration of the tow system of the magnetic gradiometer during cruiseso-122

Both sensors simultaneously measure the total intensity of the magnetic field. The differencebetween the two values appioximates the longitudinal gradient of the field in tne airectionofthe profile line. It is free from temporal variations and its integration along the survey linerestores the variation;free total intensity or magnetic anomaly (apart from a constant value).The reconstruction of the anomaly from the gradient is not trivial because several kinds ofmeasuring errors have severe effects during the integration.

BGR 116&3 - 5 0 -

The remanent, viscous, and induced magnetizations of the ship are much more critical forgradiometer than for normal marine magnetic measurements. Because the two sensors havedifferent distances to the ship, the gradient may contain a systematic error depending on thecourse. Due to the viscous part of the ship's mtlgnetization, the error may depend on the timesince the last change ofcourse. These long-period errors can have a disastrous effect on thereconstruction of the magnetic field from the gradient. Because mostly mathematical modelsfor the magnetic effect of the ship are not precise enough, it is necessary to tow the sensors atgreater distances behind the ship than it is standard for normal marine magnetic surveys.

Other sources of error are the deviations of the magnetometer sensors from the profile line andtheir different depths. In order to obtain information about the location and orientation of thearray behind the ship we installed a fluxgate compass in the middle of the cable between thetwo sensors. Furthermore, both magnetometers contain pressure sensors that providecontinuously the depths ofthe sensors.

Eilers et al. (L994) present a more detailed discussion of these and other errors. For example,magnetic fields are induced by the movement of sea water within the Earth's magnetic field.This results in an increased noise level of the records during times of rough seas. However,due to the statistical character of the disnubing fields and the smoothing effect of theintegration this reduces the qualify of the reconstructed anomaly only slightly.

5.6.2 Processing of the data of the magnetic gradiometer(B. Schreckenberger)

Magnetic measurements carried out during the cruise SO-122 cover about 3500 profilekilometers on27 lines. Nearly all of them are gradiometer measurements except of a few linesover the shelf offshore Pakistan (lines 50122-10,-11,-15,-16, and -17) where the water wastoo shallow (below 150 m) to deploy the gradient magnetometer to its entire length of 750 m.At the normal speed during seismic mqasurements of 5.4 knots the depth of the master sensorvaries between about 85 and more than 100 meters while the slave always lies 20 meters moreshallow.'Where we shortened the cable in order to reduce the depth of the master sensor toless than 50 m only the total intensity from the master sensor could be used.

For all lines with magnetic measurements, Table 4 documents the parameters that are relevantfor the reconstruction process. In general, there is a non-zero value of the mean gradient that isstrongly dependent from the course. Because we always calculate the gradient of the magneticanomaly it should be expected that its mean is near to zero. Fig. 13 shows the mean gradientsfrom Table 4 plotted against the course for those lines where it could be distinguished fromtrue gradients due to magnetic anomalies. It turned out that there are two cases: When thesensor no. 805 was used then a clear sinusoidal dependency of the mean gradient from thecourse is obvious (Fig. 13a) while without sensor no. 805 a more or less constant o.ffset ofabout 1 nT is observed (Fig. 13b). We concluded that sensor no. 805 had a problem andexchanged it with an other sensor (no. 833). In principle, this kind of error is well known buthere with sensor no. 805 its amplitude is.unusually high. Nevertheless, when the offset isfairly constant on a line with constant course it can be corrected during the reconstructionprocess.

BGR 1t6643 - 5 1 -

nT/150 m nT/l502.2.01-1.7

Table 4. Parameters for magnetic lines of cruise SO-122

For each line are shown the course, the length of the line, the arithmetic mean of thegradient, a "corrected gtadient", and the serial numbers of the sensors. The "cor-rected gradient" takes into account the estimated true gradient due to magneticanomalies. For some of the lines it was not possible to determine a mean gradient.

Mean gradient Corrected gradLengthIkml

No.master

No.slave

010203040/'a-lO4a-20505a06070809l01 1T21 313at415T6171 8t919a19b202 l222324252627a27b28

20.0.

48.

3

II

93.258.

95II2.

0.180.180.00123.353.223308r523 3 1 .

t92.148.28.

t3935

16547.

LO7.135.

97.TT2,99.2T

2 Il 76 T

23rr37.69.16.30.34.84

1 1 1 .

1 .0 .132.

-1

0 .15-1.572.

2.4-2.-2.3-1.4

t.45-12. t22.212.51

-2.2.

-2.3- 1 . 3 1-0.31-0.2

32.

- 1 .)

2.-2.1-2- 1 .2.

-1l .

-1.80

-2.2.r

- 1 .-0.

83383380580583383384184184184r84184184r841841841841841841841841841841841841

8383

805805805805805805805

805805805805805805

805805805833833833833833

0 99

3313311 5 1I326.r45.225.3 15 .47.

t3r32.280.

a

-0.-0.

84.10.

I

3

270.216.

84184184184184184184184r841

BGR 1166/'3 - 5 2 -

5.00

4.00

3.00

-3.00

-4.00

-5.00

Course [deg]

Fig. 13. Mean gradient on SO-122lines plotted against the course. (a) Lines where the mag-netically polluted sensor no. 805 was used and (b) lines without sensor no. 805.

The most simple method for the reconstruction of the magnetic field is the integration of thegradient in the space domain along the profile line. Another approach is the formulation of theproblem in the frequency resp. in the wavenumber domain (Eilers et al., L994). In this methodthe integration corresponds to a filter operation on the fourier transform of the gradient. Itprovides a better insight into the problems of the reconstruction and can be combined withadditional filter methods.

The-simple integration in the space domain gave a first impression of the quality of thegradient measurements. A constant value must always be removed from the gradient recordbecause an offset of varying magnitude is present on nearly every line. In thJreconstruction

2.00

1.00

0.00

-1.000

-2.00

EctoÈ-t

tr.gEEg

E 2cr o 1

È9 0 ,c

Ë -1 .

l!

ô -2.

a

. o l

)0 e0.00 180.00 vo.ooa aaa

t r

361

(a)

Gourse [deg]

BGR 116&3 - 5 3

process this would result in a strong linear deviation from the true anomaly.Fig. 14 shows anexample where the gradient (Fig. l4a),in addition to a constant offset, obviously has a lineartrend. This causes a strong parabolic deviation of the reconstructed anomaly from themeasured total intensity that is unlikely to represent time variations of the earth's magneticfield (Fig. l4c and d). If the linear trend is removed before the integration, then thereconstructed anomaly deviates only slightly from the measured total intensity of the mastersensor (Fig. lab). The disadvantage of this ad-hoc procedure is that any constant offset of thegradient (equivalent to a linear slope in the anomaly) or a linear slope of the gradient(equivalent to a parabolic function in the anomaly) will be removed even it is real.

Because of mathematical reasons as demonstrated above, the gradient method will alwayshave its limitations for long-wavelength anomalies and land-based records of the variationswill be higttly desirable even in the future. For this cruise, the Space and Upper AtmosphereResearch Commission (SUPARCO) of Pakistan agreed to provide after the ôruise the valuesof the magnetic total intensity from the magnetic observatory in Karachi to BGR via the NIO.Therefore, the final processing of the magnetic data and the reconstruction of the magneticanomaly from the gradient can only be done after the cruise. The Kp values for the time of thecruises SO-122 and SO-123 shows that the variations were not exceptionally high duringthese cruises @g. l5).

5.7 Hydroacoustics(T. Schillhorn, M. Tahir, A. Cheema, M. Block, V. Damm & A. Inam)

5.7.1 llydrosweep

On R/V Sonne, the bathymetric image of the ocean bottom can be continuously recorded us-ing the swathmapping system HYDROSWEEP (HYDRQgraphic multi-beam STyEEping rur-vey echosounder, Atlas Elektronik GmbH, Bremen).

HYDROSWEEP uses two narrow beam transducer arrays. The sound frequency is 15.5 kHz.While the transmitter system is situated perpendicular to the longitudinal axis of the ship, thereceiver system is parallel to the axis. With 59 preformed beams and an opening angle of 90.,a swath about twice as wide as the water depth is surveyed. Precision is about lqo it tne waterdepth if the roll angle is less then 10" and the pitch angle less then about 5". The central beamhas a range of 10 to >10000 m, the outermost ones of at least 7000 m. There are several waysto deal with the velocity of sound in water: Firstly, one can use a mechanism of self-calibration during data acquisition, or, secondly, one can assume a constant average velocity,or, thirdly, one can use a depth profile for the water velocity as obtained e.g. from a CTD logÇonductivity lemperature !epth). Position, time, and water depth are continuously writteritomagnetic tapes and optical discs. The observations can be plotted online as contour map dur-ing acquisition. This has not been done on this cruise as the quality of this plot is roo poor.

For postprocessing of HYDROSWEEP data onboard the Hydro Map System 300 (Arlas Elek-tronik GmbH, Bremen) is used to store data on magneto-optical disc and to produce contourcharts of the profile for printing. Furthermore, the Hydro Map 300 softwareian producebathymetric maps and coloured 3D perspective views. HYDROSWEEP data cari also belisted, edited, processed and displayed with the MB-System, a software package which hasbeen developed at the Lamont.Doherty Earth Observatory.

BGR I t6&3 - 5 4 -

Reeidual: Observed - conputed Ernonaly

2 5 . 5

Dotted curve: Obsetived anomalySolid curve : Cotputed anornly

.j '1

a",/----\

Dotted curire: Observed anomalySo1id curve : Conputed anornaly

(d)

nT

1 0 0 .

0 .

- 1 0 0 .

F r

2 0 0 .

1 0 0 .

(c) o.

- r u u .

nf

2 0 0 .

' l ^ n

(b ) o .

1 7 5 . k n

r r t t t t t l l

o o ç i e e e ? s e e ? o o o i o o o ? o: ! ! F î ! ! l i i i É i i î t ; i i Ë i s u r c: 3 R * * r R * 3 g â { 3 3 3 i 3 È s i â i

7 5 . 1 5 0 . 1 7 5 . k n

- 1 ô n

nT

4 ,

z .ô

(a ) o .- 2 .

- 4 .

-â,

Fig. 14. Example for the reconstruction of the magnetic anomaly from the measured gradient(a) on line SO122-MA. (b) shows the reconstructed anomaly (continuous curve)compared with the total intensity measured by the master sensor (dotted curve) whenthe linear trend (stippled line in (a)) is removed. (c) is the result of the reconstructionif only the mean gradient is removed. (d) shows the difference between the twocurves in (c).

BGR 116643

Fig. 15. Planetary magnetic three-hour-range indices (Kp) for the interval June 2l- Oct 31.1997

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BGR 116&3 - 5 6 -

5.7.2 Parasound

The parametric sediment echosounder PARASOUND @ARAmetric sediment survey echo-SOUNDeT, Atlas Elektronik GmbH, Bremen) can image the near ocean floor sedimentarylayers down to a maximum penetration depth of about 100 m. In contrast to conventional 3.5kHz echosounders, the interference of two signals with adjacent high frequencies (I8 - 23kHz) which by a paramenic effect creates a nflrow beam with the difference of the two fre-quencies. The depth of penetration of this signal with frequencies selectable between 2.5 to5.5 kHz is comparable to 3.5 kHz systems. The nzlrrower beam width and the considerablyshorter pulse length enable a much clearer and better resolved acoustic image of multi-layerstructures. The beam width of about 4o corresponds to 7Vo of the water depth. The data qualitydepends largely on the morphology and the nature of the sediment of the ocean bottom. Atsteep slopes, data quality often is poor.

A colour display unit is incorporated within the system to permit monitoring of the operation.The display screen can be split to allow a general overview ofthe bottom and a high resolu-tion display of the sediment layers. The raw data are recorded in analog form on a black andwhite thermal graphic plotter DESO 25. Using the software package PARADIGMA(PABAsound DlGitalisierungs- und lv[ehrkanal {uswertesystem, V. Spie8, University ofBremen), data can also be printed on a colour printer and digitally recorded on disc or ma-gnetic tape in the SEG-Y format, which allows subsequent seismic processing.

6 Scientific results

6.1 Composite line drawings(H.-O.Bargeloh, M. Block, V. Damm, J. Fritsch, H.A. Roeser & B. Schreckenberger)

Fig. 4 on p. 30 shows the geophysical lines surveyed during cruise SO-122, Table I on p. 31 isa list of the lines. Fig. 16 - Fig.44 document systematically the observations on these lines.Line SO122-l9A is omitted because due to the fishery problems it is very short and com-pletely included in line 50122-198. For all lines with reflection seismics, line drawings of theone-channel monitor records, the hydrosweep swaths, the magnetic anomalies observed by themaster sensor and the gravimetric anomalies are shown. For the lines without reflection seis-mics the line drawings of the monitor records are replaced by the bathymetry (central beam ofHydrosweep).

-57 -BGR 116643

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BGR 1t6643 - 5 9 -

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Line SO1224A I S,o122 M - Makrân mconfonrity

Fig. 19. Line SOI2}-MIMA across Munay Ridge, Dalrymple Trough, Little Munay Ridge,Oman Abyssal Plain and the Makran accretionary wedge, with reflection seismici.Uppermost panel: Gravimetric and magnetic anomalies; central panel: Hydrosweepswath; lowermost panel: Line drawing of one channel of the reflection seismic record

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6.2 Reflection seismics(M. Block & V. Damm)

6.2.L lntroduction

Five main structural units can be identified in the area of our investigations (Fig. 45): TheMakran accretionary complex, the Oman Abyssal Plain, the Munay Ridge, the Indus Shelf,and the Indus Fan. Our main observations concerning these units are presented and discussedin the following sections.

6.2.2 l$'{akran accretionary complex

During Sonne cruise SO-122 five lines were measured over the frontal area of the accretionarywedge. Line SO122-0/'A is located in the west of the area of our investigations near theboundary to kan and crosses the accretionary complex perpendicular to the strike of thestructures along 62"20'8. Four lines, SOL22-09, SOl22-10, SOl22-12, and SO122-13 coverthe eastern area of the accretionary complex. Between these lines and line SO122-04A a biggap of about 250 km without any new seismic measurements exists because fishering pre-vented seismic investigations.

The base of the Makran accretionary complex is clearly recognizable on the reflection seismicmonitor records of our easternmost lines SOI22-09 and SO122-10. Line SOl22-12 showsonly some questionable reflection elements from this boundary. On line SOI22-13 and on ourwesternmost line SO122-04A such a base or decollementzone cannot be identified as well ason a multichannel reflection seismic profile (Minshull and White, 1989) running at a distanceof about 40 km parallel to line SO122-04A. This suggests that east of about 65'E the base ofthe Makran accretionary complex is imaged'on the reflection seismic lines and west of thislongitude this base is not recognizable. But the published line N1804 (Lehner et al., 1983)Gig. a5) shows very clearly this base. Therefore the reflector from this boundary could bemasked by multiples. Possibly it becomes visible after processing of the data.

The frontal area of the Makran accretionary complex consists of intensively folded and over-thrusted sedimentary thrust slices scraped off the subducting Arabian Plate. The thickness ofthe accreted sediments varies between 5 s (twt) to 6 s (twt) in the dast and probably more than7 s (twt) in the west. Between individual thrust slices small sedimentary basins with a land-ward dipping divergent reflection pattern are existing, which have been filled probably duringthe latest phase of deformations. These sediments mainly do not originate from the subductingoceanic crust.

Well developed BSRs (Bottom Simulating Reflectors) occur on our lines SO122-04A (Fig. 4,Fig. aO and SO122-13 between 0.5 s (twt) and 0.7 s (twt) below the seafloor. Such reflectorsimage the base of gas hydrate layers. In the area between these two seismic lines some pub-lished profiles show BSRs, too (Lehner et al., 1982; White and Louden,1982; Minshull et al.,1992). But on our profiles SOI22-I2,SOI22-09, and SO122-10 BSRs could not be identi-fied. Therefore we summarize that west, but probably not east, of about 65'E gas hydratesmay exist in the frontal 50 km of the Makran accretionary complex.

We see seismic bright spots on line N1804 (Lehner et al, 1982) below the summits of the twomost seaward anticlinal ridges of the Makran accretionary complex (Fig. a). They are directindications for accumulations of hydrocarbons.

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axis of the central Munay Ridge grabenaxis of the southern Munay Ridge

r normalfault (large offset)

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axis of the central Munay Ridge grabenaxis of the southern Munay Ridge

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normal fault (large ofiset)

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mainly on data of the SONNE cruise SO-122

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Fig. 46. Part of the single-channel monitor record of the seismic reflection profile SOI2Z-04A showing the BSR at the front of the Makran accretionary complex

BGR 116643 8 0 -

6.2.3 Oman Abyssal Plain

Like the Makran accretionary complex the Oman Abyssal Plain is covered by the reflectionseismic grid of SONNE cruise SO-122 only in the west along line SOI22-04A and in the eastalong lines SO122-l7,SOL22-l3lt3A, and SO122-12 @g. at.

The water depth exceeds 3000 m nearly in the whole Oman Abyssal Plain. Towards the east,the Oman Abyssal Plain narrows due to convergence of the Murray Ridge and the Makranaccretionary complex leading to its disappfiIrance near line 5o-122-09.

The Oman Abyssal Plain is part of the Arabian Plate and consists of oceanic crust. The basal-tic top of the crust is clearly recognizable along our westemmost line 5O122-O4A, but in theeastern area we are not able to identify the basement everywhere. Along lines SOl22-l3lI3Aand SO122-12 it seems to be formed by lava flow series.

The trench of the Makran subduction zone is located in the area of the Oman Abyssal Plain.There the basement descends in direction to the accretionary complex. Near the front of theMakran accretionary complex the basement is overlain by about 5.6 s (twt) thick sediments inthe west and about 3.6 s (twt) thick deposits in the east (rough estimate: 7 km and 4.5 km).This sedimentary cover is subdivided by a distinct unconformity, which dips subparallel to thebasement surface in direction to the continent, and which we have named M (Makran).Un-conformity M outcrops near the southern boundary of the Omarr Abyssal Plain. The sedimentsdeposited on M are horizontally layered and mask the trench. On line SOL22-O4A a wide driftstructure lies directly beneath unconformity M.

Line SO122-0/,A shows a volcanic structure at shotpôint (SP) 1000, which rises from thebasement through the deposits forming a morphological high. The volcanics outcrop at theflanks of the high. The foot of this structure overlaps the basement, indicating that it isyounger than the oceanic crust beneath. Influenced by the subduction process this volcanicstructure is tilted to the north like the basement.'White (1983) published a seismic line withsuch a structure further in the east and a bathymetric map with morphological highs formed bysuch structures. He assumed that all these structures belong to a linear ridge which he named'Little Murray Ridge'. The Little Murray Ridge runs through the Oman Abyssal Plain fromsouth-west to north-east and probably is incorporated into the accretionary complex at lineSOl22-I3l13A and east of it.

Indications for several seismic bright spots are observed on line SOl22-04{between shot-points 1500 and 2200 above unconformity M and in the drift structure. These indications re-quire verification by processing of our seismic data. Seismic bright spots were discovered inthis area also by White (1979) and by Minshull et aL. (1992).

6.2.4 Murray Ridge

The Murray Ridge is a well defined bathymetric structure that extends south of the OmanAbyssal Plain from south-west to north-east. In the south-west it borders at the Owen FractureZone and in the north-east at the collision zone between the Indo-Pakistanian Plate and theEurasian Plate.

During SONNE cruise SO-122 the Murray Ridge has been surveyed by several reflectionseismic lines (Fig. 4). The observations indicate that the Murray Ridge may be a volcanicstructure or an anticlinal structure covered by volcanics. The volcanic cover may overlie con-

BGR 116&3 - 8 1 -

tinental crust or intact oceanic crust. Indications for overthrusting were not observed on oursingle-channel reflection seismic records.

On our structural map (Fig. 45) the boundaries of the Murray Ridge are defined by the north-western foot and the south-eastern foot of the volcanic structure. A deep graben running paral-lel to the strike from south-west to north-east divides the Munay Ridge into a shallower north-western ridge and an upstanding south-eastern ridge.

The Dalrymple Trough is part of the graben. Line SO122-04 shows that the Dalrymple Troughis an asymmetric graben with a steep north-west facing main normal fault at its south-easternmargin forming the footwall and several minor antithetic faults descending in steps south-eastward forming the hanging wall. The seafloor is influenced by this normal faulting indicat-ing active extension. Around 2.5 s (twt) thick sediments are deposited in this rift graben. Lo-cally, we observe at the flanks of the graben a chaotic reflection pattern above the basement.The layering of the sediments could be destroyed there by wrenching along the strike of thegraben.

Another asymmetric graben north-east of the Dalrymple Trough is crossed by our linesSOL22-I4, SOl22-17, and SO122-18. Also here the foonvall of the rift graben is located at itssouth-eastern flank. Normal faults at the seafloor indicate recent extension, locations with achaotic reflection pattern and strongly folded sedimentary layers indicate wrenching. The de-posits in this graben are up to 4.0 s (twt) thick.

V/e propose that this graben is the north-eastward prolongation of the Dalrymple Trough.Between 64'E and &"40'E the iraben is displaced;'We assume that the graben along theMurray Ridge is formed by rifting related to the subduction of the Arabian Plate below theEurasian Plate and by wrenching due to relative motions between the Arabian and Indo-Pakistanian Plates.

The ridge running in the north-west of the graben is covered by sediments intersected by nu-merous normal faults which also influence the seafloor. This indicates presently active exten-sion. The sedimentary layers rise from the flanks of the ridge in direction to its summit whichcould be caused by uplift during the formation of the magmatic ridge or by tilting of the layersas a consequence of the subduction of the Arabian Plate.

A huge volcanic structure occurs on line SO122-18 and some indications for sills were rec-ognized on line SOI22-17.

On line SOl22-10 where the Makran accretionary complex is thrusted on the northern MunayRidge, a huge sheet has slumped down from the Indus Shelf into the trench (Fig. 47). Thissheet is 9 km long and about 600 m thick and is not destroyed. But chaotic masses are existingat the upper slope above the sheet and around its base and front.

The ridge running along the south-eastern side of the Munay Ridge graben has a higher topo-graphic level than the ridge extending at the north-western side of the graben. The southernMurray Ridge bounds the Indus Fan to the north-west. Most of our reflection seismic linesshow at the south-eastern flank of the Munay Ridge a southward inclined unconformity withsouthward dipping layering of the lower sequences and onlapping of the upper layers. Thisunconformity marks the uplift of the Murray Ridge.

BGR 116643 -82 -

s u! outll uollco[aH

Fig. 47 . Part of the single-channel monitor record of the reflection seismic line SO 122- 10showing a 9 km long and 600 m thick sheet which has slumped down from the IndusShelf

' E=: ott-o to o

.C CLo E5o

BGR 116&3 - 8 3 -

Several outcrops of the basement are located along the top of the southern Murray Ridge (Fig.45). The southern flank of the Munay Ridge shows some indications for sills and for muddiapirs which need confirmation by processing of our reflection seismic data.

6.2.5 Indus Shelf

The Indus Shelf is covered by our grid of reflection seismic profiles only near its edge andonly in the north-western prolongation of the Murray Ridge and in the area of the Indus Can-yon (Fig. 4,Fi9.45).

On lines SO122-10, SO122-11, and SOl22-08 an anticlinal structure is identified below theshelf sediments by updomed sedimentary layering at its flanks (Fig. a5). These lines showvery little reflectivity from the surface of the structure. Therefore the structure may be a me-lange which continues onshore into the collisional belt between the Indo-Pakistanian Plate andEurasia. The melange could consist of Eocene, Paleocene, and probably Cretaceous highlydeformed sediments. At the eastern flank of the assumed melange the well INDUS MARINECl had been drilled by V/intershall in 1975. This site was abandoned due to high pressure inthe Lower Eocene (Raza et al., 1990; Quadri, 1984).

The Indus Canyon is crossed by our lines SO122-14 and SOl22-15 (Frg. 45). In the area ofthese profiles the canyon is about 10 km wide and up to 900 m deep (Frg.48). The active cur-rents have cut a I km wide and 300 m deep channel into the canyon's bottom. Another chan-nel which was active in former times and which is now filled by sediments is recognizable atthe bottom of the Indus Canyon. Current controlled sediments and slumped sediment massesoccur in the canyon.

The continental basement of the outer Indus Shelf and of the slope is imaged on our linesSAl22-14, SO122-15, 501,22-16, and SOl22-27 by a reflector with high amplitudes and lowfrequencies. This reflector is not masked by the strong multiples and can be observed between4.9 s (twt) below the shelf and 8.5 s (twt) under the slope and under the south-eastern flank ofthe Murray Ridge (Fig. a5).

Near the shelf edge a strong reflector onlaps on the basement. This reflector merges along lineSOL22-14 into the volcanic top of the Murray Ridge. So this reflector can be interpreted toimage the top of the magmatic unit which covers the Murray Ridge.

The thickness of the sediments varies between 4.5 s (twt) on the shelf and 6.7 s (twt) at theslope. In the area of the shelf and the upper slope strong multiples largely mask the reflectionpattern of the deposits. Some channels exist at the upper slope which often are characterizedby channel-levees (Fig. a9). Remnants of such channel-levees now buried by younger sedi-ments are recognizable. Some indications for mud diapirs were observed along line SOl22-14at the upper slope (Fig. 50).

6.2.6 Indus Fan

The Indus Fan extends from the southern flank of the Murray Ridge southward to about 10'N.Our grid of reflection seismic lines covers only the northernmost part of this huge sedimentarystructure.

The magmatic basement of the Indus Fan shows a smooth to moderate relief. At several loca-tions it is overlain by a magmatic unit consisting of a mixture of sills or lava-flows and sedi-

BGR 1166/'3

u t l( / ) ô -,

u! ourll uo[cauo'

Fig. 48. Part of the single-channel monitor record of the reflection seismic line SO122-14showing the Indus Canyon near the slope of the Indus Shelf

BGR 116643

]UO ç

s ul oLull uollcollou

Fig. 49. Part of the single-channel monitor record of the reflection seismic line SOl22-14showing channel-levees

- 8 5 -

==

BGR I 16&3 - 8 6 -

==

S u! oLu!l uollcollou

Fig. 50. Part of the single-channel monitor record of the reflection seismic line SO122-14showing some structures which we interpret as mud diapirs

BGR 11,6643 -87 -

ments. Along line SO122-25 an area with a divergent reflection pattern and an area with asubparallel reflection pattern occurs beneath the basement and extends up to 1.3 s (twt) intothe crust.

The thickness of the sediments varies between 0.8 s (twt) and 3.8 s (twt). In the upper half ofthe sedimentary unit numerous channel-levees or buried remnants of such structures are ob-served indicating that the upper Indus Fan.consists mainly of such channellevee complexes.According to Coumes & Kolla (1984) the sediments of the Indus Fan are deposited since thelate Oligocene or early Miocene uplift of the Himalayas.

Series of sills occur along our lines at many locations mainly in the upper half of the IndusFan sediments indicating a relatively young magmatic event.

6.3 Gravity and magnetics(8. Schreckenberger & J. Fritsch)

Fig. 16 -Fig.44 show the gravimetric free-air anomalies and the magnetic total intensityanomalies together with the other geophysical data obtained along our lineS. Fig. 51 shows themagnetic anomalies of the entire survey plotted perpendicular to the ship's track.

The discussion of the gravimetric and magnetie data concentrates on four topics.

6.3.1 Age and nature of the crustof the Indus Fan

The nature of the crust of the Indus Fan area north of 19oN is not yet understood. Naini &Talwani (1983) state that the direction of the SE-NW trending Laxmi Ridge (Fig. 1) changesto E-V/ at about 19'N/66"E. They suppose that this ridge marks the northern edge of oceaniccrust in the Arabian Sea. This ridge lies more than 100 km south of our lines SOl22-24 andSOL22-25 which would thus lie on thinned continental crust.

Magnetic seafloor spreading lineations in the northern Arabian Sea have been investigated inseveral studies (Norton & Sclater, 1979, Schlich,1979, Nauka, 1993, Miles & Roest (1993).The northernmost data set is by Miles & Roest (1993) who investigated regularly spacedprofiles perpendicular to the lineations in the area between 15'N and 21oN and between 63"Eand 66'E (Fig. 52). Their northernmost correlatable lineation is anomaly 28 at l8'30'Nindicating a paleocene crustal age (64 My). If this anomaly represents the continental marginthen break-up would have been contemporaneous to the emplacement of the Deccan Trapbasalts (Vandamme et al., 1991) and the oldest lination north of the Seychelles Bank/IV1asca-rene Plateau.

Magnetic anomalies over the inferred continuation of the Laxmi Ridge to the East at l9'N(Naini & Talwani, 1983) are strong and can be conelated to form E-IV lineations but they cannot be correlated with magnetic time scales. Two lines north of the Laxmi Ridge and east oflines SO122-24 and3o-122-26 (Miles & Roest, 1993) show strong anomalies at21'N/65'40'E that are not correlatable with time scales either and that decrease in amplitudeto the north-east (Fig. 52).

BGR 116643 - 8 8 -

Fig. 51. Magnetic anomalies (master sensor) observed on cruise S}-Izzplotted along theship's tracks. Positive anomalies are red, negative green.

BGR 116643 - 8 9 -

Fig.52. Magnetic anomalies plotted along the ship tracks. Positive anomalies in dark gray,negative anomalies in light gray. Lines in the north-western part of the map are fromthis cruise (Fig. 51),lines in the south-eastern part are from CHARLES DARWINcruise 20 (Miles & Roest, 1993). The hatched area indicates the location of the con-tinuation of the Laxmi Ridge to the east inferred from gravity anomalies (Miles &Roest, 1993).

BGR 116&3 - 9 0 -

The magnetic anomalies observed on our lines SO122-23 to 50122-26 are much weaker than

the ones shown by Miles & Roest (1993). Also the large and partly linear anomalies observed

by Miles & Roest (1993) north of 19oN cannot be identified as seafloor spreading lineations

with any reasonable spreading rates.

The seismic records show sills within the sedimentary layers on lines SOl22-23 (Fig. 39) and50122-26 (Fig. D and an overprinting of the basement by volcanics that may rest on oceanicas well as on continental crust (cf. 6.2.6). Although obviously the crust is thin, recent ODPresults off Iberia have shown that small thickness of the crust alone is not sufficient as proofof oceanic nature (Whitmarsh et al., in press).

6.3.2 The Murray Ridge area

Magnetic anomalies over the Murray Ridge between 63'E and 64'E (Fig. 51) have sur-prisingly low amplitudes (lines SOl22-02, -03 and -04 and Barker, 1966) and do not support avolcanic origin of the ridge (cf. 6.2.4). Short-wavelength anomalies over the top of the ridgeon line SOIzz-M (Fig. 19) partly correlate with bathymetric features, suggesting that theycontain igneous rocks. However, the ridge as a whole does obviously not cause an anomaly.

One possible explanation might be that the ridge represents oceanic crust of normal sea-floorspreading origin with uniform magnetic polarity and therefore causes virtually no magneticanomaly. Another explanation would be that continental crust extends from the Laxmi Ridgeto the Murray Ridge as it may be concluded from Naini & Talwani (1983) and Miles & Roest(1993). Local occurences of volcanic rocks would be responsible for the short-wavelengthmagnetic anomalies. A seamount at 23'N/63o50'E is without any doubt volcanic because ithas a strong magnetic anomaly (line SOl22-23 andBarker, 1966). Further, our reflectionseismic sections show volcanic structures on all lines across the boundary between MurrayRidge and Indus Fan. Magnetic anomalies seem associated with that boundary on linesSOl22-14, SOI22-17 and SO122- 1 8.

6.3.3 The Oman Abyssal Plain and the Makran shelf

Magnetic anomalies over the Oman Abyssal Plain north of the Murray Ridge/DalrympleTrough have longer wavelengths and higher amplitudes than those in the Indus Fan area. Mostof the anomalies belong to an anomaly chain trending south-west to north-east with a high inthe south-east and a low in the north-west (lines SO122-04104A, -05, -06, -07 , -09 and -12).

The chain is intemrpted on line SOt22-l3l13A where no positive anomaly can be observed(Fie. s1).

This anomaly chain may be associated with the Little Murray Ridge, a basement ridge that ismostly concealed by sediments but pierces through the seafloor on line SO122-044 (SP 1000,Fig. 19) where its sub-seafloor structure can be seen in the reflection seismic section. On line50122-06 we also observe a tiny expression on the seafloor (at kilometer 50 in Fig.22)whereas on line SO122-05 (without reflection seismic measurements) we do not find anybathymetric expression of the basement high. A local high in the gravity data that can be seenon all three lines strongly suggests that also on line SO122-05 the basement high is present(kilometer 70 in Fig. 20) and that it may represent the continuation of the Little Munay Ridge.The crest of the ridge coincides mostly with the minimum or the flank to the positive part ofthe magnetic anomaly. It is not yet clear what causes the anomaly. It is very wide (Fig. 51) andthe topography of the basement on line SO122-MA cannot explain the shape of the anomaly.The seismic section (Fig. 19) suggests that a 100 km wide volcanic structure partly overlies

BGR 116&3 - 9 1 -

oceanic crust of the same or older age. If the strong magnetic anomaly trending south-west tonorth-east could be explained by the volcanic structure then there remains hardly any indica-tion of magnetic seafloor-spreading lineations in the Oman Abyssal Plain.

6.3.4 The relation of the Murray Ridge to structures onshore Pakistan

Off Karachi, on the more than 100 km wide shelf with water depths of less than 200 m, linesSOL22-O7 - SOl22-13 were surveyed. The most important ne\ry résult concerns the extensionof prominent tectonic elements from Pakistan into the Arabian Sea @ig. 53):

In the cenûal part of Pakistan, along distinct strike-slip faults (Chaman Fault, GhazabandFault, Ornach-Nal Fault) the Bela-Waziristan ophiolite zone separates the Makran Flyschcomplex in the west from the Khuzdar block in the east. It consists of marine limestone andclaystone, basaltic and andesitic lavas, gabbros, serpentinites and their conglomerates and isconsidered as the boundary between the Eurasian Plate and the subducted Arabian Plate in thewest and the Indo-Pakistanian Plate in the east. This mainly N-S strikingzoîe is bent eastwardin the Karachi Arch north of Karachi. Near Karachi, the layers of this zone are covered byalluvial sediments.

On our lines, we found zones of positive gravity anomalies related to areas with elevatedseismic reflectors which show characteristics of melange zones. It seems reasonable to parallelthese high-density subbottom structures to the Bela-Waziristan ophiolite zone. This impliesthat south of the Karachi Arch the ophiolite zone is deflected WSW in direction to the MurrayRidge. The Murray Ridge may even be a direct prolongation of the Bela-Waziristan ophiolitezone. This finding throws new light onto the development of the triple junction between theEurasian, the Arabian and the Indo-Pakistanian Plates which is also documented in the re-gional distribution of the ophiolite belts.

6.4 Hydroacoustics(M. Tahb, T. Schillhoffi, A. Cheema, M. Block, V. Damm & A. Inam)

For high-resolution bathymetry and sediment echography, three acoustic systems were usedsimultaneously and continuously along all profiles: a) PARASOUND subbottom profilingsystem, b) HYDROSWEEP multibeam swath bathymetry system, and c) ELAC single-beamechosounder. The main objective was to obtain detailed information on the seafloor topogra-phy and the structure of the uppermost sediments. This information will be helpful also for thecruises SO-123, SO-124 and SO-130 in this area. This report briefly describes preliminaryresults obtained along the selected profiles during the cruise.

6,4.! Data recording and quality

The PARASOUND data is recorded in analog form on paper charts and beginning with profileSO122-10 on computer tapes as well. HYDROSWEEP bathymetric data was completely re-corded digitally. Later maps of the data were produced, while the Elac soundings were re-corded only on paper chart.

The analog subbottom profiling data of PARASOTIND was analysed onboard. Over flat orgently sloping seafloor good data were obtained with penetrations of up to 60 meters or more

BGR 1t6&3

Fig. 53. Structural map of a part of the Bela-Whaziristan ophiolite zone (Bannert et al., 1992)with the free-air gravity anomalies observed in the north-eastern part of our survey. Agravity high indicates that on the shelf the ophiolite zone bends south-westwards andpossibly extends into the Murray Ridge.

BGR I 16&3 -93 -

and good resolution of about 50 cm. As the slopes increase the data quality decreases ac-cordingly. On seabed slopes greater than 3o only a very poor, noisy or no record is available.

During the first part of the cruise, rôugh sea affected the HYDROSWEEP data quality. Later,the quality improved. The water sound velocity profile measured on cruise SO-90 with a CTDlog was used for sound beam calibration.

6.4.2 Preliminary results

In the following, we will describe some of the observed features and present preliminary re-sults inferred from the PARASOUND records along selected reflection seismic lines.

Profiles SOl22-04 and -044These lines represent a typical transect in the Makran offshore survey area, extending from thesouthern flank of the Munay ridge and passing over the Dalrymple Trough, the Little MurrayRidge and the Oman Abyssal Plain. They terminate at about 1800 m water depth after cross-ing a part of the submarine Makran accretionary complex. At the start of the profile SOI22-04prolonged and diffracted echoes and no subbottom reflectors are observed because the slopesare steeper than 3o. No signal is obtained in the upper parts of the slope which are steeper than8'. At the top of the ridge, indistinct to distinct uneven seafloor with prolonged reflections,occassionally with few parallel subbottom reflectors at certain places continue till the northernflank of the ridge. Acoustic penetrations are less than 18 m. The sediments drape the underly-ing ridge morphology along with overtopping and slumping of sediments at various places.The acoustic character of the northern flank is more or less similar to the southern flank.

The Dalrymple Trough borders the Murray Ridge on the northem side and is considered to beproduced by extensional tectonic processes. It is characterized by distinct and strong echoesfrom a flat seafloor with prominent and parallel packages of subbottom reflectors intervenedby acoustically transparent layers of varying thicknesses from few to 10 meters (Fig. 54).These acoustic layers represent very homogeneous sediments having strong acoustic imped-ance contrasts to overlying and underlying sediments. Maximum acoustic penetrations reach60 m. This sequence is suddenly disturbed and covered by a sediment slump and ultimatelydissappears underneath. This sediment slump has its origin probably from the northern flankof the trough which marks a big fault.

North of the Dalrymple Trough a well-stratified acoustic sequence consisting of multiple andclosely spaced parallel subbottom reflectors with transparent layers below them extends to theLittle Murray Ridge. This sequence is more or less regularly folded and shows many graben-like structures (Fig. 55). Most of these folds appear to be the surface expression of deep-seated faults which have been prominently marked on the seismic section. This reveals thatthe area is tectonically active at present. As expected, there is poor record from the flanks ofthe Little Munay Ridge. The top of the ridge shows layered sediments with about 30 macoustic penetration in the areas where topography is not very steep.

The Oman Abyssal Plain begins north of the Little Munay Ridge, where an unconformitywith underlying dipping sediments is very prominent (Fig. 56). The abyssal plain sedimentsshow very high-amplitude prolonged reflections with generally indistinct strong subbottomreflectors up to a considerable distance along the profile after which the strength of the ampli-tude decreases slightly and acoustic penetration decreases from about 60 m to 30 m. From

BGR 1166/.3 -94 -

Fig. 54. Line SO12}-04:ParaIlel packages of subbottom reflectors intervened by acousticallytransparent layers of varying thickness in the Dalrymple Trough

BGR 116&3 - 9 5 -

Fig. 55. Line SO122-O4A: Grabenlike structures with multiple and parallel subbottom reflec-tors between the Dalrymple Trough and the Little Murray Ridge

BGR 116&3 -96 -

Fig. 56. Line SO122-O4A: Unconformity with underlying dipping sediments at the northernedge of the Little Murray Ridge

BGR 116643 -97 -

SP 2710 on appear very small scale folding and faulting in PARASOTIND records, whichprogressively increase in size, both in amplitude and throw respectively up to about the top of

the presumed frontal fold (Fig. 57). This probably reveals the initiation of the compressionalregime of the subduction zone about 14 km south of the top of the frontal fold. This smallscale folding and faulting do not seem to extend landward below the peak of the frontal fold.These features are relatively small and therefore not visible on the seismic monitor record. '

The maximum height of the frontal fold, relative to the adjacent abyssal plain seafloor at thisplace, is about 160 meter.

Northward of the frontal fold due to steep slopes of accretionary ridges poor or no PARA-SOUND record could be obtained. However, prolonged and strong reflections with indistinctsubbottom reflector(s) of high amplitude appear in the inter-ridge or inter-slope basin areaswhere penetrations up to 50 m are observed (Fig. 58).

Profile SO122-10Relatively less steep accretionary ridge(s) with fault scarps and uneven topography appear inthe first part of the profile. The acoustic penetration varies from almost nil to about 50 m nearthe northward base of one of the accretionary ridges, where some strong subsurface reflectorsare visible. A sediment slump is prominent here. Further up, to the upper slope, no respec-tively poor record is available.

The uppermost part of the continental slope (between SP 1180 to t290 on the seismic profile)shows a very complex sedimentary structure where an acoustically stratified section at the topunconformably overlies a highly deformed sedimentary sequence (Fig. 59). Penetrations of upto 60 m were obtained in this deformed area. These structures disappear abruptly along a scarp(fault?) where the continental shelf starts. The continental shelf has a strong and distinctseafloor echo with a closely spaced, strong subbottom reflector which further eastward be-comes deeper with intervening reflectors of varying acoustic character (Fig. 60). This strongsubbottom reflector at the base marks a prominent erosional unconformity which is probablywidespread along the entire outer shelf region and outcrops at places. Also the seafloor be-comes uneven and rugged up to a steplike feature, which might be the shelf break. Until theend of profile the seafloor generally remains flat and relatively smooth.

Profiles SOl22-12 and -131134These lines cross folded and faulted accretionary ridges with steep slopes and rough morphol-ogy which do not allow meaningful PARASOLJND data recording. Locally, especially nearthe base of these ridges and in the interslope basins, slumped sediments are observed.

Profile SOl22-14This profile starts in the tectonic region of the Murray Ridge NW of a faulted trough and runsalong the continental slope of the Indus passive continental margin up to the eastern bank ofthe Indus Canyon. Gently upsloping seafloor with some continuous and parallel subsurfacereflectors of varying echo strength and almost uniform penetration of about 20 m continuesuntil near the trough. The hummocky features, prolonged reflections with occasional diffrac-tion patterns, are indicative of the irregular, rough topography and sediment slumps withinthis trough. From the SE bank of the faulted trough, the sedimentary sequence is characterizedby multiple subbottom reflectors grouped into few distinct respectively indistinct packagesparallel to seafloor. In the lower part one or two semi-transparent layers, not very distinct,follow the sequence. The seafloor here is slightly uneven and wavy. The acoustic penetrationis around 30 m. It increases rapidly to about 80 m near the edge of the levee where the

BGR 1r6U3

Fig. 57. Line SO122-O4A: Small-scale folding and faulting just south of the frontal fold of theMakran accretionary complex

- 9 8 -

BGR 1T6&3 -99 -

Fig. 58. Line SO122-O4A: Basin on the Makran accretionary complex with gently dippingreflectors

BGR 116&3 - r00-

Fig. 59. Line SO122-10: Highly deformed sedimentary sequence underneath an acousticallystratified section at the uppermost continental slope

BGR 116&3 - 1 0 1 -

Fig. 60. Line SO122-10: Continental shelf with a strong subbottom reflector that indicates anerosional unconformiw

BGR 116&3 -102-

sequence is disrupted by a deep channel of approximately 100 m depth (SP 1725). Furtheralong the profile another deep channel of slightly larger dimension cuts the seafloor (Fig. 61).These channels have lower eastern levees. The sedimentary sequence between them and alsoSE of the second channel has less coherent and weaker acoustic expression. Few minorslumps and debris flows are also visible at some places in this part of the profile.

Further on line SOI22-I4, seafloor and subsurface strata suddenly become very irregular andridge-like features appear with varying amplitudes from few tens of meters to about 80 m.These features most probably correspond to the superficial expression of the deeper subsur-face mud diapirs as inferred on the respective seismic line drawing (SP 234O to SP 2850).Further east at the upper slope region an acoustic facies Lfi-ch (Fig. 62), as described by vonRad &Tatrir (L997),continues up to the Indus Canyon. V/ithin the canyon, only scattered re-flections are observed. The echofacies LII-c,h, though less prominent, is also observed on theSE side of the Indus Canyon.

BGR rt66/.3

Fig. 61. Line SO12 2-14: ChanHel in the Indus Fan area (printed by the Paradigma system)

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Fig.62. Line SO122-14:Upper slope region of the Indus Canyon with acoustic facies L[*h

BGR 116æ3 - 1 0 5 -

AcknowledgementsFunding for cruise SO-122 by the Federal Ministry of Education, Science, Research andTechnology @undesministerium fiir Bildung, Wissenschaft, Forschung und Technologie) isgratefu lly acknowledged.

Cruise SO-122 has been carried out in cooperation with the National Institute of Oceanogra-phy in Karachi (NIO), the Hydrocarbon Development Institute of Pakistan in Islamabad(IDIP) and Geomar in Kiel. V/e acknowledge especially the support by Director General Dr.Shahid Amjad (NIO), Mr. Gilles Balma (Bureau Gravimetrique International, Toulouse,France), Captain C.D. Bhatti (Hydrographic Department of the Pakistan Navy), Director Z.M.Khan (Pakistan Space and Upper Atnosphere Research Comrnission), Director General HilalA. Raza (HDIP), Dr. W.E.K. Warsi (Sultan Qaboos University, Oman) and Mr. M.'Wegener,Dt. Botschaft Islamabad).

The results described herein would not have been obtained without the great experience, theexcellent work and the tireless commitment of Captain Andresen and the crew of R/VSONNE. Further assistance came from the Reedereigemeinschaft Forschungsschiffahrt(Bremen) and BEO (Warnemiinde) during the cruise and particularly its preparation.

Bundesanstalt fiir Geowissenschaftenund Rohstoffe For the shipboard party:

7;4Z.ztu

(Dr. H.A. Roeser)- Chief Scientist -

(Dr. K. Hinz)- Direktor und Professor -

BGR 116643 - 1 0 6 -

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BGR 116643 - t07

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Whitmarsh, R.B.,Beslier, M.-O. & Wallace, P. (1997): I-e,gl73 Preliminary Report.

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Times of Oman, l2-Oct-1997

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Franldurter Allgemeine Zeitung, 19 -Oct-1997

Methaneis auch in tiefer SeeVulkane als Quelle / Forschungsfahrt der ,,Sonne"

Wâhrend der kiirztièh zu EndE gegang€-nen 123. Fahrt des: Forschunssschiffes,iSonne" wurde erstmals ein tMithaneis-Vorkommen i_rf"ôff1iàfsee entdeckr.Gleichzeitig fanden die an der vier Wo-chen dauernden Fahrt im nôrdlichen Ara-bis.chen Meer ùnd im Golf.von Oman'be-teiligten Wissenschaftler ,êin Gebiet amMeeresboden, das auf âhnliche Weise ent-standen ist wie der Oberrheintal-Craben..An der Fahrt nahmen Forscher des Geo-mar-Instituts in Kiel ùnd der Bundesan-stalt fùr Geowissenschaftén und Rohstoffeiri Hannover sowie aus GroBbritannien

.und Pakistan teil. .Unter dem Meeresgebiet, das von der

StraBe von Hôrnius, dem.Mûndungsgebietdes. Indus und dem ôstlichen Zipfél .derarabischen Halbinsel (,,Rad el-Hàd") be-grepzt.wird, stoBen die indische,.die arabi-sche und die eurasiséhe Kontinentalplattézusamrhen. Die arabische Platte wandartin diesem Gebiet mit einer Geschwindig-keit von etwa 4:Zentimeteln pro Jahr naçhNorden uûd schiebt sich dabei unter dieeurasische Platte.

Bei .diesem Vorgang.àpielt sich ein ein-zigaftiger geologischer'. ProzeB ab. Uberden Indus und die zahireichen FlûsseWestasiens gelàngt' hâmlich einé groBe'Menge an Sedimenten aus dem Himalaiaund aus dem iranischen Zagros-Gebirge indieses Meeresgebiet. Diê Sedimente wer-den auf dem Meeresboden des Golfes.vonfuggbgelagert. Sie landen daËiffitâerarabischen Platte und kônnen an manchenStellen eine Mâchtiekeit von bis zu sieben- Klgrygpan erreichén. Wegen-flrechtsffilGn-Orift der arabiscËen Platte ver-weilen die Sedimente nur wenige MillionenJahre auf dem Meeresgrund. Diese recht

.kurze Zeit reicht bei weitem nichf aus, denSchlamm vollstândig zu hartem Gesteirt zuverfestigen. Die Sedimenté enthalten alsonoch sehr viel Wasser, wenn sie zwischendie beiden Kontinentalplatten gelangen. ..

Das Wasser verringert den Reibungswi-derstand und IâBt deshalb die beiden.PIat-ten leichter aneiirander vorbeigleiten. Daserklârt, weshalb es in ôiesrim Gebiet imVergleich zu anderen Abtauihzonen aufder Erde wenig Erdbeben gibt; Bei derKollision der Platten wird aber auch einTeil der Sedimente von der Oberflâche derarabischen Platte ,,abgehobelt" und aufge-tûrmt. Auf diese Weise'ist im Laufe derZeit der sogenannte Makran-Akkretions-

keil entstanden - ein bis zu 300 Kilometerbreites Kiistengebirge im Grenzgebiet zwi-schen lran und Pakistan. Bei ihrem Zu-,sammensto8 wirken'die beiden platten aufdie Sedimente' wie die. Backen einæSchraubstocks..Sie pressen das Wasser ausdem Gestein heraus. Charakteristisch liirdieses Gebiet sind deshalb zahlreicheSchlammwlkane, die vôn deir urspriine-lich in den Sedimenten erit[raltenen Wassérgespeist werden.

Wâhrend der Forschungsfahrt wùrdennun auch Schlammvulkane injdem unterder Meeresoberflâche liegenôen Teil desKûstengebirges entdectt. Das aus dieænVulkanen austretende'Wasser ênthâlt ee-ringe Mengen Methan, das bich unter démgroBen Druck und der niedrigen Ternpefa.:tur am Meeresgrund zu Gashydraten verjfestigt. Dieses Methaneis komirit an vielênStellen in den Vy'eltmeerbn.tor.-Allerdings lwurde es bisher immer nur in Kiistennâfieentdeckt. Im Oma'ri-Becken des ârabischcnMeeres kommen solche Gdshydratê aberauch in mehr als 3200 Meter ,Wassêrtiefevor.

Nach Sûden wird der Golf yon Omandurch den Murray-Rûbken 'begrenzt'

dieNahtstelle zwischen der arabisôhen'undder inilischen Platte. Bisher war ân!ênom;meh worden, da8 es sich dabei um ein mitdem Mittelatlantischen Rûckeri vereleich-bares Spreizungszentrum handelt, ai demstândig neue Erdkru'ste entsteht.-Die G9-schwindigkeit, mit der die Platten im'Mur-ray-Rùcken auseinanderdriften, betrâgtaber nur etwa ein fùnftel Zentimeter proJahr. Das ist weniger als,ein Zehntel derGeschwindigkeit, mit der sich'die Plattenin den anderen Weltmeeren voneinanderentfernen. Au8erdem fehlt atr Murrav-Rùcken der typische untermeeiische Vul-kanismus. ., Vom Forschiingsschiff Sonne aus wur-

de nun eine markante Stelle innerhalbdieses Rùckens, der sogenannte Dalrym.ple-Gtaben, mjt verschiedenen geophysi-'kalischen Me8verfahren genauer untef:sucht. Die Ergebnisse legen den'SchluBnahe, da8 dpr Dalrymple:Graben nicht

. mit. anderen .Gebieten am Meere$grund,sondern eher mit dem Oberrheintal-Gri-ben vergleichbar ist. Wie in dem Gebietzwischen Basel und Mainz dehnt sich hierdie Erdkruste ausgesprochen langsam,was zur allmâhlichen.Absenkung des Un-tergrundes fùhrt.

'- . hra.

the makran accretionary wedge off pakistan: tectonic evolution and fluid migration. hannover 1997

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