29. STRUCTURE AND SEDIMENT DISTRIBUTION IN THE NORTHWEST CORNEROF THE WEST PHILIPPINE BASIN

D. E. Karig, Department of Geological Sciences, Cornell University, Ithaca, New Yorkand

J. M. Wageman, Gulf Research and Development Company, Pittsburgh, Pennsylvania

INTRODUCTION

It is becoming increasingly clear that the West Philip-pine Basin displays abnormally complex tectonism,resulting from the effects of several types and ages ofdeformation. Reconstruction of the basin history re-quires that these deformations and their effects beseparated and understood individually. One of thelogical ways to approach this problem is to investigatethose areas where the deformational zones intersect oroverlap.

The northwest corner of the basin is one such area,where the Central Basin Fault appears to intersect theLuzon arc and is in turn covered by a large sedimentapron which appears to postdate a period of subductionoccurring along the eastern flank of the arc (Karig,1973). Although no deformation of the apron sedimentsis observed, the insular slope to the south is the site of arecent subduction zone which may be extending itselfnorthward (Fitch, 1972; Karig, 1973).

The recognition of the region's tectonic importanceled to a rather intensive survey program during theWestern Pacific geotraverse of the NOAA ShipOceanographer. Seismic reflection, magnetic, and gravi-ty profiles were collected along satellite-controlled tracklines (Figure 1). Also available since the earlier recon-naisance study (Karig, 1973) are satellite-navigatedprofiles from V-21 and V-24 cruises of LDGO, Tasaday5 of SIO and the DSDP Leg 31. The previously availabledata were incorporated and reanalyzed after being ad-justed to the satellite-controlled tracklines.

The study began as an attempt to locate the north-western end of the Central Basin Fault and to in-vestigate its behavior at the basin margin. A majorobstacle confronting such an endeavor is that thedelineation of basement ridges in marginal basins isnearly impossible without detailed surveys, because theusual track spacing is several times the topographicwavelength. However, in the turbidite-fed apron adja-cent to Luzon, the sediments were ponded and dammedbetween ridges, and because these ponds lie at differentdepths, the ridge correlation becomes relatively easy.Therefore, rather reliable contours were drawn at smallintervals over the apron, but no new contouring wasattempted elsewhere. These new data also permitted thebetter delineation of channels feeding the sedimentapron, which in turn had significant implications for thelate Tertiary tectonism of northern Luzon.

DATA AND DISCUSSIONThe youngest major geologic feature in the region is

the large sediment apron which extends northeastward

from sources in northeast Luzon. Controlled in largepart by basement topography, the apron represents sub-marine sediment transport exceeding 500 km, and sub-stantially more along the trough which lies at the foot ofthe Palaui Ridge (Figure 1). A second major sedimentbody lies west of the Palaui Ridge but this area was notstudied in any detail.

The main apron is presently fed by the Cagayan Riverand submarine canyon, which drains most of northernLuzon, and probably also receives much smaller con-tributions from short streams on the east coast. Sedi-ment thickness in the apron exceeds 2 km near the baseof the island slope and along much of the trough next tothe Palaui Ridge (Figure 2).

Internal geometries of the sediment bodies, as inter-preted from seismic reflection profiles, are almost com-pletely those of laterally transported, turbidity-typedeposits. Beneath the turbidites near Site 293 indicationsof a basal pelagic section can be seen on reflectionprofile (Site 293, this volume), and it is likely that basalpelagic sediments exist beneath most of the basin. Se-quences of reflective and transparent units are observedon most profiles, but cannot be easily correlated becausethe basement troughs which comprise the apron havequite different acoustic sections.

Limited available piston core data (Lair and Sanko,1969) and both 3.5 and low-frequency reflection profilesindicate that coarse-grained turbidites are beingdeposited in the northern area and in the trough flank-ing the Palaui Ridge, and that much finer grained,almost hemipelagic-appearing turbidites are collectingelsewhere. For several hundred kilometers beyond thedistal edges of the apron, the pelagic brown clay shows ahigh silt content (Lair and Sanko, 1969) and developsthicknesses over 200 meters, or nearly twice the norm inthe West Philippine Basin.

Despite the variation in stratigraphic character in theapron, there does seem to be a widespread developmentof very reflective unit in the uppermost few hundredmeters and an equally widespread underlyingtransparent layer which also has a turbidite geometry.This pair of units has been interpreted (Site 293, this vol-ume) as reflecting an early deposition of fine ter-rigenous material following the demise of a subduc-tion zone, in turn followed by more rapid collection ofcoarser material caused by the late Pliocene uplift ofLuzon (Durkee and Pederson, 1961; Christian, 1964).

Surficial variability of sediment type seems closelycoupled with the development of canyon-fan channelsystems. Presently active and dominating the feeders isthe Cagayan system (Figure 1). After travelling acrossthe shelf north of Luzon in a number of small channels

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Figure 1. Revised bathymetric map of the sediment apron area northeast of Luzon in uncoπected fathoms. Stippled borderindicates the edge of the turbidite apron. Channels are shown by heavy dots and ridge trends by heavy anticline symbols.The location of profiles shown in Figures 2 and 3 are shown by heavier track lines.

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Figure 2. Seismic reflection profile from NOAA ship Oceanographer across trough forming the northend of the sediment apron. This profile shows trough sedimentslapping onto the Palaui Ridge and gives an indication of more than 2 km of trough sediments.

D. E. KARIG, J. M.WAGEMAN

(Figure 1), the system cuts through the Palaui Ridge in adeep canyon and can then be traced northward as a fanchannel until it develops a large suprafan complex (Nor-mark, 1974) which primarily feeds the trough along thebase of the Palaui Ridge. Because of the damming effectof intervening basement ridges, little of this sedi-ment would appear to reach the area where Site 293 wasdrilled.

Another well-developed fan channel can be observedto the east (Karig, 1973, and Figure 1) which can betraced with reasonable confidence from near the base ofthe island slope to about 19°30'N. Despite the large sizeof the channel near 19°N (Karig, 1973), there is no signof a distributary system nor a correlatively large riversystem and canyon. This channel may connect to one ofseveral poorly delineated but small canyons along theLuzon coast between 17°20' and 18°N, but there arepresently no significant streams draining into this coast.Thus, the large channel, without associated large sourcedeviates from the general correlation of these twoparameters (Normark, 1974). One solution is that thissystem is now inactive, that the sediment supply sourcehas been shifted to the presently active system, and thatthe distributary system of the eastern system has beenburied by sediment from the Cagayan system.

Still other, less persistent and less distinct channels oc-cur southeast of the two larger systems. These cannot betraced toward Luzon, partly because of lack of data andpartly because they cannot be distinguished from othermorphologic features. If the degree of preservation can,in this situation, be correlated with age, then the channelsystems would be progressively older toward the southand east.

The change in apron levels across basement ridges,observed on several profiles (Figure 3) initially sug-gested recent faulting, but more closely spaced linesindicate instead that the different sediment levels are a

result of confinement of turbidites to troughs betweenthe ridges. Evidence of recent faulting on the apron islacking, but cannot be entirely dismissed because to thesouth along the east coast of Luzon there is very goodevidence of a developing subduction zone (Fitch, 1972;Karig, 1973; Karig and Sharman, in press). An activetrench can be traced to about 17°N, but cannot be dif-ferentiated from the ridge-trough topography abuttingthe insular slope on the more northerly crossings. Howthe subduction attenuates northward remains to bedetermined.

The strongly lineated basement beneath the apronmust have originated before the apron formed, in themid-Tertiary or earlier, and could represent eitheroriginal basin topography or the effects of later tectonicdisruption. All the ridges and troughs trends betweenN15°E and N30°E except for the trough bordering thePalaui Ridge, which is probably or a different, sub-duction-related origin. Relief of the north-northeast-trending structures averages near lkm and ranges togreater than 2 km. Some of the larger ridges along thesoutheast border of the apron can be traced for 175 to250 km.

There are adequate data to conclude that nonorthwest-trending features which might be attributedto the Central Basin Fault extend into the apron area.Areas of smooth basement in the vicinity of 21°N to22°N and 124°E, and 19°N, 125°E to 126°E precludethe tracing of a major fault zone to either side of theapron. This, coupled with the recognition that the Cen-tral Basin Fault consists of north-south-trending ridgesand troughs in several areas along its trace (Karig, thisvolume), suggests that the north-northeast trendingbasement topography beneath the apron is a splayedcontinuation of that fault zone. Rather than represent-ing a spreading rise or major transcurrent fault, thiszone of en echelon character seems to mark a rather

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modest amount of right lateral, distributed shear oreast-west extension. If this is correct, then the problemof why the Central Basin Fault has not displaced thePalau-Kyushu Ridge disappears. The timing and tec-tonic significance of the fault zone are as yet unknown.

Several indicators suggest that this relief representsdisruption of a basin crust well after its creation. Per-haps the most compelling is the occurrence of amphi-bolitic grade highly sheared igneous rocks beneath theapron sediments at Site 293. Either these rocks were up-lifted by normal faulting or lie on transform faults. Theshort wave length of the north-northeast-trendingtopography, its persistence over much of the westernpart of the basin and the apparent exclusion of othertrends rules against a transform interpretation.

Basal pelagic sections over several ridges can be ob-served but occur only on one flank, suggesting that theother flank is a normal fault system (Site 293, thisvolume) and that the ridges are tilted fault blocks. Theseridges are larger and more persistent along strike thanthose generally observed in actively spreading marginalbasins, which might suggest a different origin.

The north-northeast basement trend can be tracedsouthward past the Benham Rise and eastward to theCentral Basin Fault. On the Benham Rise, at least, thelargest ridges are pre-late Eocene in age, and there is noevidence for significant faulting of the pelagic cover (Site292, this volume). There is a chance therefore that anoriginal north-northeast lineation was generated withthe crust and reactivated by later tectonism in someareas.

Miocene to Pliocene discoasters mixed into theslumped tectonic basement breccias and Miocene(?)apron sediment overlying basement in Hole 293 place anupper bound on the age of deformation. If motion onthe Central Basin Fault, which also consists of north-northeast to north trending subunits, is a related event,then displacement is post Eocene (Site 290, this vol-ume). The lack of tilting in the deeper turbidites over theridge flanks argues that faulting terminated before muchof the apron developed.

The northeast-trending basement lineations appear tobe truncated by the eastern edge of the Palaui Ridgewhich trends north-south. Either the development of thePalaui Ridge postdated the faulting in the basin or elsethere was some type of plate boundary between them.There is no displacement along the boundary at presentor during the growth of most of the apron (Figure 2).

At the boundary between the ridge and the basin,there is a linear basement trough, masked by more than2 km of sediment, but over 7.5 km deep. Undoubtedly apart of the anomalous depth is attributable to sedimentloading, but the sharp break into the trough on the eastflank, and the still greater than 7 km basement depth atthe narrower, fractionally filled northern end demon-strate the trench-like nature of this feature.

A change in the character of the sediment reflectors inthe trough occurs at about 1.5 sec at the south end and0.5 sec at the north end. The sediments above this breakare contiguous with and assumed to be part of the sedi-ment apron. The deeper sediments form a more reflec-tive unit showing slight deformation. These are possibly

the last trench wedge sediments, deposited in a dyingsubduction zone along this boundary (Karig, 1973).

Both the gravity and magnetic profiles of theOceanographer support the idea that the Palaui Ridgeconsists of deformed sediments and forms a trench slopebreak of an inactive subduction zone. The nearlycharacterless magnetic profiles across the ridge require avery low remnant magnetism and susceptibility. In com-parison, profiles across the Palau Kyushu Ridge, a rem-nant arc in a similar geometric and magnetic field situa-tion, have anomaly amplitudes of several hundred gam-mas, which can be attributed to volcanics of andesiticand basaltic composition.

The gravity field over the Palaui Ridge area is similarin shape and position to those over active subductionzones, but with slightly subdued amplitudes (Figure 4;Watts and Talwani, 1974). A very large high, which is inpart attributable to edge effects, occurs over the BatánesIsland Ridge, where uplifted Tertiary limestone andvolcanics and recent volcanism are displayed. A lowover the basins to the east is bisected by a relative highover the Palaui Ridge. Simple calculations and com-parison with gravity profiles (Caputo et al., 1964) overthe Palau-Kyushu Ridge demonstrate that these anoma-lies are not primarily topographic or deeper crustal edgeeffects along the insular slope. Lack of constraining dataprevents useful gravity modelling, but if the PalauiRidge is approximated by a two-dimensional triangularprism superimposed, together with the flanking sedi-ment fill, on a downbowed ocean crust, the 100-150mgal crestal anomaly attributed to this ridge (Figure 4)implies an average ridge density significantly less than2.7 g/cm3. Despite its low accuracy, this exercise doesimplement the argument that the ridge is composed ofdeformed sedimentary material.

CONCLUSIONSPerhaps the biggest surprise evolving from this study

is that there does not appear to be any northwest-trending topography associated with the Central BasinFault in the northwestern corner of the West PhilippineBasin. It is most likely that the north-south trends in thesurveyed parts of the fault zone resulted from the sameactivity which produced the large basement ridges eastof Luzon. This deformation occurred between the lateEocene and late Miocene.

Because the basement lineations do not affect thePalaui Ridge, and because that feature is identified aspart of an inactive subduction zone, the basement defor-mation seems to have occurred before cessation of sub-duction. The subduction episode remains poorly bound-ed by geology on Luzon as pre-mid-Miocene (Karig,1973). Crust as old as Late Cretaceous was possibly sub-ducted because pelagic foraminifera of that age were fedto the apron and recovered at Site 293 (Site Chapter, thisvolume).

Migration of the apron-feeding channels northwardwith time appears to be related to the uplift history ofthe Sierra Madre Range. The large abandoned channelwhich heads into the structural low in the range mostlikely once drained the Cayagan River through that gap.

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Figure 4. Bathymetric and gravity profiles across the Batanes-Palaui ridge-trough couple showing island arc -like character.Depth of sediment in the two troughs flanking the Palaui Ridge is estimated from Oceanographer reflection profiles. Gravityis from Lusiad cruise.

Continuing uplift of that section of the range probablyforced the river northward, over the Sicallao basementridge. This uplift was part of the general, large-scale up-lift of northern Luzon operative since the late Pliocene,but its cause is not clear. No collision during that agespan has occurred along this section of the arc. Subduc-tion has been occurring along the western side of theisland since at least the late Miocene so that uplift of theSierra Madre seems to reflect a different process. Onepossibility is that, associated with preceding thenorthward propagation of the new subduction zonealong the east coast, there is uplift of the future upperplate region.

REFERENCESCaputo, M., Masada, R., Heifer, M., and Hoger, C. L., 1964.

Gravity measurementsin the Atlantic, Pacific, and Indianoceans: May 1962-Aug. 1963 (R/V Argo): Interim Rept.,Inst. Geophys. Plan. Phys. Univ. Calif., no. 7.

Christian, L. B., 1964. Post Oligocene tectonic history of theCagayan basin, Philippines: Philippine Geol., v. 18, p. 114-147.

Durkee, E. F. and Pederson, S. L., 1961. Geology of NorthernLuzon, Philippines. Am. Assoc. Petrol. Geol. Bull., v. 45, p.137-168.

Fitch, T. J., 1972. Plate convergence, transcurrent faults andinternal deformation adjacent to southeast Asia and thewestern Pacific: J Geophys. Res., v. 77, p. 4432-4460.

Karig, D. E., 1973. Plate convergence between the Philippinesand the Ryakya Islands: Marine Geol., v. 14, p. 153-168.

Karig, D. E. and Sharman, G. F., Ill, in press. Subduction andaccretion in trenches: Geol. Soc. Am. Bull.

Lair, C. and Sanko, P., 1969. Core analysis and bottomphotographs: Marine Geophysical survey program 65-67,Area 13, v. 5: Washington (U.S. Naval Oceanographic Of-fice).

Normark, W. R., 1974. Submarine canyons and fans valleys:factors affecting growth processes of deep sea fans. InModern and ancient geosynclinal sedimentation, Proc.Conf. U. Wise. Nov. 1972.

Watts, A. B. and Talwani, M., 1974. Gravity anomaliesseaward of deep-sea trenches and their tectonic im-plications: Geophys. J. Astron. Soc, v. 36, p. 57-90.

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