Polarforschung 69,3 -15,1999 (erschienen 2001)

Eurasian Arctic Margin:Earth Science Problems and Research Challenges

By Igor S. Gramberg, VladimirYu. Glebovsky, Garrik E. Grikurov, Vladimir L. Ivanov, Evgeny A. Korago,Mikhail K. Kos'ko, Sergey P. Maschenkov, Alexey L. Piskarev, Yulian E. Pogrebitsky,

Yuri V Shipelkevitch and Oleg 1. Suprunenko

Invited Plenary Lecture

Summary: The fundamental features of evolution and hydrocarbon potentialof the Eurasian continental margin are considered in the light of structure andgeological history of major shelf basins whose level of exploration and geo­dynamic position vary from west to east. The best studied Barents-North KaraBasin is a typical passive margin which borders the Eurasian oceanic openingand displays a prolonged pre-breakup depositional history spanning almostthe entire Paleozoic and Mesozoic eras and resulting in a great thickness ofsediments with positively proven oil and gas potential. Unique gas condensatefields have also been discovered in the South Kara Basin, although theaccumulation of cover sequences did not begin here until about the Paleo­zoic/Mesozoic time boundary and was probably more influenced by the eventsin the West Siberian province than in the central Arctic. The location of theLaptev Basin at a unique structural 'I-junction between continental margin andthe Eurasian spreading axis accounts for a specific geodynamic environmentleading to post-breakup extension and associated formation ofunusually stret­ched and thinned continental crust beneath a substantial thickness ofpredomi­nantly latest Mesozoic and Cenozoic sediments. Although the least exploredEast Siberian and Chukchi Basins represent an apparent morphostructuraltransition from north-eastern continental Asia to the central Arctic Ocean,their designation as a typical passive margin may, perhaps, be questioned untilevolutionary links between the formation of the Amerasian oceanic deepseabed and late Mesozoic - Cenozoic processes of subsidence and sedimen­tation in eastern basins are established with greater confidence.

Because of an outstanding oil and gas potential of the Eurasian continentalmargin, the fundamental and applied dimensions of its earth science explora­tion have always been closely interrelated. Of particular interest in a funda­mental context are- the examination of the structural and evolutionary continuity between the

Eurasian Arctic margin and its neighboring crustal assemblages on both themainland and the oceanic sides;

- unraveling the formation of different parts of this margin in relation toCenozoic geodynamic processes in the Arctic Ocean;

- the issues related to expansion of continental crust in the course of itsextensional stretching;

- palinspastic reconstructions accounting for the changes in dimensions ofcontinental masses in the course oftheir pre- and post-breakup evolution,

- and studies of deep interior processes causing reorganizations at the upperlevel of the lithosphere.

Among more specific research objectives are- the recognition of syn-oceanic structural elements and tectonic events as

opposed to features inherited from the pre-oceanic evolution;- determining the lithostratigraphic composition of the sedimentary cover

and developing tectono-stratigraphic concepts for basin modeling;identifying the factors influencing generation and preservation of hydro­carbons and the criteria for discriminating between predominantly oil- andgas-bearing basins.

I All-Russia Research Institute for Geology and Mineral Resourees of the World Oeean(VNIIOkeangeologia), I Angliisky pr., 190121 SI. Petersburg, Russia

Manuseriptreeeived 15 Deeember 1999, aeeepted21 November 2000

INTRODUCTION

The Eurasian Arctic margin faces the Arctic Ocean between 10"E and 170 0w. By far the largest part of this margin pertainsto the Russian exclusive economic zone. Consequently, earthscience studies on the Eurasian Arctic margin have beenpredominantly performed by Russian research institutions andindustrial groups, except for the western part of the BarentsSea which is covered mainly by Norwegian surveys. Majorcontributions to the exploration of the Laptev, East Siberianand Chukchi Seas were also made by German institutions(BGR) and North American companies (Western Geophysical)by contracting Russian vessels for seismic reconnaissancesurveys; in the East Siberian and Chukchi Seas these surveysprovided the bulk of all existing seismic data.

The current level of exploration of the Eurasian Arctic margindecreases dramaticaIly from west to east. The Barents Sea isrelatively well surveyed by seismic investigations accompa­nied by stratigraphie and exploration drilling. In the SouthKara Sea both seismic and weIl data are less abundant,whereas the existing knowledge in the North Kara and LaptevSeas is mainly based on as yet limited seismic evidence notverified by drilling results. Aeromagnetic and gravity surveysof varying scales cover the entire Eurasian Arctic margin; inthe East Siberian and Chukchi Seas, where Russian seismicdata available to the authors are very scarce, the potential fieldevidence provided the main source for geological interpreta­tions (e.g. GRAMBERG et al 1997).

The Eurasian Arctic margin is one of the largest continentalmargins in the world and is also leading in the amount and sizeof shelf/slope sedimentary basins whose outstanding oil andgas potential was predicted by VNIIOkeangeologia specialistsmore than 30 years ago. Since then it has been positivelyconfirmed by discoveries of giant offshore fields (GRAMBERG& SUPRUNENKO 1995, GRAMBERG et al. 1983, OSTISTY &FEDOROVSKY 1993). The main basins traditionaIly recognizedon the Eurasian Arctic margin are the Barents - North KaraBasin, the South Kara Basin, the Laptev Basin, and the easternbasins underlying the shelves ofthe East Siberian and ChukchiSeas; the first two are the apparent offshore continuations ofwell known highly productive onshore provinces. This paperattempts to highlight the key geological features of each ofthese subdivisions with a view to identify fundamentalconcerns relevant to better understanding of crustal evolutionof the entire Eurasian Arctic margin and improved assessmentof its hydrocarbon potential.

3

Fig. 1: Russian seismic and weIl data coverage in the Barents and Kara Seas. Thin lines = MCS lines and refraction lines; bold lines = wide-angle seismic reflec­tion lines; line A - B geological profile shown in Figure 4; triangles = main stratigraphie and exploration wells.

SUMMARY OF EXISTING KNOWLEDGE

The Barents - North Kara Basin

Within the Barents - North Kara Basin, earth science activitieswere concentrated mainly in the Barents Sea shelf, whereasthe North Kara Sea shelf remains relatively poorly studied. Inthe course of several decades the Russian seismic surveysconcentrated predominantly in the eastern Barents Sea where afew hundred thousand kilometers of reflection and refractionlines were acquired mainly by SMNG Trust and MAGE. Thedistribution of these surveys is shown on Figure 1. Only thoselines that were shot during the last 20-25 years using modernCDP methods (ca. 250,000 km) are depicted. Special techno­logies, such as wide angle seismic reflection profiling, wereapplied along several of these lines. In the deepest parts of thebasins, they enabled the recognition of ancient coversequences previously not distinguished from the basement(PAVLENKIN et. al 1998). Aeromagnetic surveys (Fig. 2) wereanother major tool in earth science investigations. Togetherwith systematic gravity observations at 1 : 1,000,000 and moredetailed scales, they constituted an important contribution toverifying the position of the basement, developing models ofthe Earth's crust and identifying the nature of its individuallayers. Finally, the ground truth control of geophysical datawas provided by direct geological evidence derived from

4

numerous onshore and offshore wells and bedrock outcrops onthe surrounding archipelagoes and mainland.

Based on the presence of ancient crystalline complexes, latePrecambrian low grade sequences and late Vendian to Cam­brian undeformed strata on the surrounding archipelagoes andmainlancl, the entire Barents - North Kara Basin is commonlybelieved to be underlain by a heterogeneous Baikalian base­ment whose cratonization is attributed mainly to the latest Pre­cambrian orogeny caused by convergence of older continentalblocks; this event was, perhaps, locally superseded by a Cale­donian reactivation. The basement structure is dominated by amajor trough running parallel to Novaya Zemlya in the easternBarents Sea shelf (Figs. 3, 4). This feature is in markedcontrast to the western Barents Sea where the basementsurface generally lies at much shallower depths.

Despite apparent variations in total thickness, the lithostrati­graphic composition of the basin fill is rather uniform andcharacterized by the presence of four principal sequencesthought to extend more or less continuously over the entireBarents-North Kara Basin (GRAMBERG & SUPRUNENKO 1998).The lowermost basin sequence has an Ordovician to earlyDevonian age and commonly rests directly on the basement,though it may locally be underlain by older cover strata oflimited thickness and distribution. The unit consists of terrige-

o 20W 40W 60W 80W 100W 120W 140W

20E

60E 80E

North Poleo

100E 120E 140E

60

80

160E

Fig. 2: Russian aeromagnetic tracklines on the Eurasian Arctic margin,

nous-carbonate deposits with a conspicuous proportion ofcoarse clastic rocks; numerous internal disconformitiesaccount for strong variations in its composition and thickness.The mid-Devonian to early Permian sequence is much morepersistent in lateral distribution and lithology. It is dominatedby carbonate platform strata with prominent reef formationsand intercalations of salt-bearing rocks. The Late Pennian toTriassie clastic sequence in the East Barents trough reaches10-12 km in thickness and constitutes here more than a half ofthe total sediment fill which in the deepest depocenters mayexceed 20 km; the depositional features indicate very highsedimentation rates in shallow marine to lacustrine, locallycontinental environments. Jurassic-Cretaceous marine,shallow marine and lacustrine sediments form the uppermost,relatively uniform regional sequence which, however, stillexhibits gentle thickness gradients consistent with the under­lying major structural features. Only a minor late Cenozoicveneer is observed at the top of the section.

At least three upper sequences with their great thickness ofsediments are apparently favourable for generating and trap­ping hydrocarbons. This is confirmed by a very wide stratigra­phie interval of oil and gas occurrences currently known in theBarents - North Kara Basin and ranging from oil accumula­tions of variable size in the Carboniferous to lower Permianbeds to the giant gas condensate fields Shtockmanskoye andLedovoye in the Jurassie deposits. On the whole, the greatmajority of proven oil and gas fields and prospects is concen­trated predominantly in the eastern Barents Sea, but it remainsuncertain to what extent the present-day position ofhydrocar­bons (in both vertical and lateral dimensions) reflects thepattern of their generation and original distribution (see

discussion seetion). Despite the unquestionable great resourcepotential of the eastern Barents Sea, only the Prirazlomnoyeoil field in the Pechora Sea is currently approaching produc­tion stage, whereas the economic and environmentalconstraints are as yet prohibitive for exploration and develop­ment activities in more remote offshore targets, includingthose with proven unique reserves.

Igneous rocks in the cover sequences are the products ofpulses of extensional trap magmatism recorded in mid-lateDevonian, latest Permian to early Triassic, and late Jurassie toearly Cretaceous time intervals. The magmatic activity musthave been one of the important factors for the generation andpreservation of hydrocarbons, but only first approaches haveso far been made towards a solution ofthis problem.

The South Kam Basin

The distribution of seismic and aeromagnetic investigationswithin the South Kara Basin is very uneven (Figs. 1, 2): thesouth-western part of the basin (between Novaya Zemlya andthe Yamal Peninsula) is much better surveyed, especially nearthe Yamal Peninsula, whereas north and east of the YamalPeninsula, the seismic coverage is less systematic, and aero­magnetic data are virtually missing. About 60,000 km of MCSlines shot over the South Kara Basin using modern technolo­gies resulted in the discovery of 16 large prospects. Only twoofthem were drilled, and both turned out to be unique gas con­densate fields in the lower Cretaceous sequences (Leningrads­koye and Rusanovskoye).

5

20'E 40'E 60'E aOE 100E

70N

40'E 60'E 80E

Fig. 3: Depth to basement in the Barents - North Kara and the South Kara Basins (km); from GLEBOVSKY et al. (1997) and YNIIOkeangeologia archive data. LineA - B = loeation of geologie al profile shown in Figure 4.

I: 12

km

+ +

K-Kz

P?-T

SE

Late Paleozoic?(Hercynian?)

SOUTH KARA SEALÄl---:.c:---------------,. 0

rr

:-------:-----...-;:~ 4

____;--..,--~~ 8'-.---+ + +

NOYAYAZEMLYA

r-r

++

BARENTS SEA

X'

+Latest Precambrian (Baikalian)

++

++ +

km

NW

A 8Fig. 4: Sehematie geologieal profile along line A - B aeross the Barents Sea, Novaya Zemlya and the South Kara Sea (for loeation see Figs. land 3). Geologiealage symbols indieate the stratigraphie range of major sequenees established and/or presumed in the Barents - North Kara and the South Kara basins. Inseribedages refer in basin areas to the final eratonization event in the tectonic basement, and on Novaya Zemlya - to the main deformational event in the epieratoniesequenees broadly eoeval with the adjaeent basin fill. Generalized from MAGE data (e.g. IVANOVA & KAvUN 1997) and YNIIOkeangeologia numerous archivereports.

6

The striking discovery by the very first wells of these twogiants occurred in the same stratigraphic interval that yieldsthe bulk of production on the adjacent mainland. This positi­vely suggests that the South Kara Basin represents an offshorefrontier continuation of the extremely rich West Siberianhydrocarbon province and must therefore have much incommon with the latter. This is particularly true for the upperpart ofthe sedimentary cover which accommodates the majorreservoirs (GRAMBERG & SUPRUNENKO 1995), but evidence isnot yet sufficient to apply this similarity concept to the earlierbasin history.

Indeed, drilling in the South Kara Basin was stopped at thebase of the Aptian at approximately 2.0-2.5 km depth afterpenetrating a continuous upper early Cretaceous to Quaternarysuccession of predominantly marine terrigenous sedimentswith appreciable proportions of siliceous rocks. The drilledinterval is practically identical in composition and thicknesswith coeval sequences on the Yamal Peninsula where similarlithologies were cored for another 2-3 km further down thesection, through lowermost Cretaceous (pre-Aptian) andJurassie strata. Units of this age evidently continue offshorethe Yamal Peninsula and can reliably be traced by seismic datathroughout much of the South Kara Basin; however, correla­tion of older (pre-Jurassic) units which make up the lower 5-6km of the basin fill and the nature of the basement remainsambiguous.

The Laptev Basin

Seismic coverage in the Laptev Sea is much less dense than inthe western seas (Fig. 5a). It amounts to 22,000 - 23,000 km ofMCS lines with about half of their total length acquired byBGR. The data are mostly wide-spaced regional profilesmeasured with different acquisition parameters by severalorganizations, almost completely without refraction control.Together with the lack of borehole evidence and a high degreeof variability of the seismic-geological environments, thiscreates ground for a variety of interpretations of even thosedistinct reflectors which are identified in all data sets, andcertainly precludes the recognition of individual oil and gasprospects. The interpretation of aeromagnetic data fromwidely spaced flight-lines (Fig. 2) enabled the compilation of arough depth to basement sketch (Fig. 6), which appeared gene­rally consistent with seismic information. Repeated attemptsto use potential field evidence for better perception of thenature of the basement and the age of basin fill did not,however, appear particularly conclusive. Hydrocarbon expec­tations in the Laptev Basin are quite optimistic due to thepresence of a thick sedimentary cover the accumulation ofwhich was probably in large part associated with depositionalprocesses in the latest Mesozoic and Cenozoic paleodelta ofthe Lena River. Resource motivations are, however, as yet tooweak to stimulate offshore drilling in the harsh natural andlogistic environment of the Laptev Sea shelf, and the existingviews on its tectonic structure and history are still essentiallybased on speculative concepts rather then on direct geologicalevidence; the process of adaptation of these views to morerecent geophysical data and associated modern ideas is justbeginning.

Even from currently available reconnaissance data the Laptev

Basin emerges as one of the world's most intriguing tectonicsites, first of all because of the specific nature of its junctionwith the Eurasian oceanic basin whose spreading lineamentsabut almost orthogonally against the base of the Laptev Seaslope. An evident manifestation of this structural coupling isthe configuration ofthe Laptev Basin with its general seawarddipping morphology, emphasized by the absence of island­capped basement highs in the outer shelf. In this respect, theLaptev Basin is markedly different from the western EurasianArctic margin. Another major distinction is a much morevariable depth of the acoustic basement which in the easternLaptev Basin forms a prominent high, whereas the westernLaptev Basin is dominated by a major basement depression.Within the latter, most Russian authors tend to recognize indi­vidual relative highs and lows believed to mark different struc­tural units (DRACHEV 1998a, 1998b, IVANOVA et al. 1989,VINOGRADOV 1984); however, recent seismic evidenceobtained by BGR (HINZ et al. 1997, 1998) suggests thepresence of a single, relatively flat-floored depression with arather constant depth to basement in the 4-5 s TWT interval.

On Figure 7, the main structural elements mentioned above arenamed the West Laptev Deep and the East Laptev High; theformer corresponds to the Ust' Lena rift, and the latter to theEast Laptev uplift of HINZ et al. (1997, 1998). The profileshown in Figure 7 traverses these major structures only in theirboundary zone, but it nevertheless illustrates the principalcrustal characteristics observed across their entire extent. Ex­tensional faulting is quite evident in the East Laptev Highwhich is dissected into the Laptev and the Kotel'nyi Horsts bythe Anisin Basin. A "crustal partition zone" recognized byHINZ et al. (1997) in the Laptev Horst is particularly specta­cular in this respect. A more subdued, relatively simpleseismic structure in the much deeper Ust' Lena Rift is traditio­nally explained in terms of longer subsidence history.However, it could also represent the result of greater degree ofextensional stretching and accompanying reworking of conti­nental crust within a relatively short length of geological time.A brief review of the main existing concepts and a stratigra­phic interpretation proposed for the observed seismic featuresare given in discussion section.

The Eastern Basins in the East Siberian and Chukchi Seas

Aeromagnetic data obtained along wide-spaced flight lines(Fig. 2) provided the source for depth to basement calculationsindicating the existence of very deep depressions of the mag­netic basement in the Eastern Basins, descending below 16 km(Fig. 6). Seismic data obtained by western companies are pro­prietary, whereas Russian seismic profiles in the EasternBasins are too few (Figs. 5a, b), and in most cases have insuf­ficient depth resolution to enable reliable verification of theaeromagnetic evidence and/or gravity modeling. Direct geolo­gical correlation is hampered by the scarcity of onshorebedrock outcrop and the total absence of deep stratigraphicwells in the area. The lack of ground truth for geophysical datapermits only hypothetical interpretations of the geologicalstructure of the Eastern Basins. An example of such a largelypresumptive model for the structure of the East Siberian shelfis given in Figure 8.

The principal structures recognized in this profile are a large

7

A 1200E

800N

1200E 1500E

---,, ',

AST SIBEroAN SEA _,- ---~,,

\..,

165°E

75°N

Fig. 5: Seismic data coverage in the Laptev Sea and East Siberian Sea (A) andChukchi Sea (B). Dotted lines = refraction lines; thin and bold lines = MCSlines acquired by Russian expeditions and by BGR; broken lines = MCS linesacquired by Western Geophysical; bold lines C - D and E - F show locations ofgeological profiles on Figures 7 and 8.

depocenter called in this paper the New Siberian Deep, aprominent basement high underlying the De Long Islands, andanother unnamed major sedimentary basin associated with theoutermost shelf and continental slope. A noteworthy feature ofthe New Siberian Deep is the marked difference between theacoustic and magnetic basement surfaces (see also Fig. 6)although their positions are more similar beneath the slopebasin. In the New Siberian Deep, the thickness of indisputablecover sequences overlying the acoustic basement varies from1-2 to 5-6 km; the greatest part of this section is believed toconsist of Cretaceous and Tertiary sediments, whereas a latestMiocene to Quaternary age is assigned to the uppermost thin,relatively persistent veneer. Ages given to older coversequences which are assumed to be present beneath the acou­stic basement in the New Siberian Deep, as weIl as to the evendeeper magnetic basement, are purely speculative.

It appears highly probable that the structural pattern inferredin the western part of the East Siberian Sea (Fig. 8) extendsinto its eastern and north-eastern parts which are almostcompletely unstudied by seismic methods but are also charac­terized by alternating large elevations and depressions of themagnetic basement. Preliminary analysis of limited seismicand potential field data in the Chukchi Sea suggests itsprobable structural continuity with the North Slope of Alaskawhich gives reasons for a positive assessment of the hydro­carbon prospects ofthe Eastern Basins.

nON

68°N

170 0W

170 0W

176°W

176°W

-J.- 1-- __,-_ ..... --.LI ""' .......... I -1- 1-.. I" ....., ..... ,1- __1

..,. - -'i-: _ ,',- L _ t ": - -J, ,'--,...',' '--...I-"'-_' 7,_(., ",- - '- " ,- .., - II , -I..... I ......

- - <t-«; \I'" 'I- I , r-..... - .......... .,. --J..-' I I 7--L.. ..... I---0/. ---t- ~'"f_' , "7-, - - ..,. L... 1"""1_L- - .[_ I I

I I ..... r: -+ -I ..... ....J...... -1- ' .. I:l-Vrangel, I" , ,.. 0/. .J, - - 1. _ -'-.,. J-

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C J1 U I< CI rr t - _ S E A ~'-I_:'......1.:: ....., 1""_..... .I I ,-..... I -(,

, -'- -' "..-'" - ,- -, ',-!. I, .[, '-"'-...... l: ' -,... ,, --' ,t > .,.

-' ,,... -,............ I

B

nON

68°N

8

130'E 160'E 170'W

130'E 160'E 170'W

Fig. 6: Depth to basement in the Laptev and Eastern Basins (km) from VNIIOkeangeologia archive data; !ines C - D and E - F show locations of geologicalprofiles on Figures 7 and 8.

DISCUSSION

The Barents-North Kara Basin

The shelf edge of the Barents and Kara Seas is almost strictlyparallel to the spreading axis of the Eurasian Basin whichagrees with the definition ofthe Barents - North Kara Basin asa "classic" passive continental margin. Like many otherpassive margin basins associated with the evolution of modernoceans, the Barents - North Kara Basin displays a prolonged

pre-breakup depositional history apparently unrelated to theEurasian opening. Indeecl, formation of the sedimentary coverin the Barents - North Kara Basin began, perhaps, close to thePrecambrian/Paleozoic boundary and therefore preceded theonset of spreading in the Eurasian Basin by almost 500 Ma.On the contrary, Cenozoic sequences which would beexpected to accumulate on the Barents-North Kara marginsynchronously with the deepening of the Eurasian Basin, arevirtually absent. These evolutionary peculiarities of theBarents-Kara sector of the Eurasian Arctic margin are impor-

W

LOWER CRUST

4

s

S(TWT)

E

L A P T E V H G H ( E L U )

H 0 r s .~.~ Kotel'nyi Horst.- 0:

<a:l N-Q0

e v

Basement (Neoproterozoic?)

E AST

50 kmL- -.--!Io

L a p

KZ

Ust'Lena Rift

WEST LAPTEV DEEP

4

sJ!rI MANTLE')

12 J

S(TWT)

c DFig. 7: Schematic geologieal profile along !ine C - D in the Laptev Sea (for location see Figs. 5 and 6). Seismic boundaries are synthesized from interpreted BGR!ines 97-01 (eastern end) and 97-20 (HINZ et al. 1997). Geological age symbols indicate stratigraphie range assigned to basin fill sequences and/or to older coverunits be!ieved to occur in the acoustic basement. For further explanation see text in discussion section. ELU = East Laptev Uplift.

9

SWE AST S I B E R A N S E A

NE

SouthAnuiZOlle

NEW SIBERIAN DEEP DE LONG BASEMENT HIGHContinentul

dope

• /N-Q0- ,~:J.~~+---"-"--"-----"--"--",) ~t +1-/-- K-N /+Y+

4-i~.~ + +:,; +:,; ... ','"." --', '\'+ +~fj~ +++ ')Z .),'.8~ Si mid-Paleozoic? + .P 2_3-

MZ . .+:~Q (Ellesrnerian") + + ...+.J~

+ + + + +

early-mid Paleozoic?(Caledonian?)

km

EoI

100 kmI

km

F

Fig. 8: Sehematie geologieal profile aIong line E - F in the Bast Siberian Sea (for loeation see Figs. 5 and 6). Note differenee in position of magnetie basementsurfaee (dotted boundary) and of aeoustie basement (dashed boundary) in the New Siberian Deep. Geologieal age symbols indieate stratigraphie range provisio­nally assigned to basin fill sequenees and/or to older cover units believed to oeeur in the aeoustie basemeent. Inseeibed ages refer to tectonic events thought to beresponsible for basement eratonization in different zones. Adapted from DRACHEV et al. (1995, 1998), KOS'KO (1984), and VNIIOkeangeologia archive data.

tant both in a fundamental context and for resource evalua­tions.

A very prolonged sedimentation history spanning the greatestpart of the Phanerozoic is commonly interpreted in terms ofthe Barents - North Kara Basin evolution on the ancient EastEuropean platform. This concept is most readily applied to thewestern part of the Barents Sea shelf where a moderatethickness of Paleozoic and Mesozoic sequences is consistentwith low sedimentation rates characteristic for stable tectonicregimes. In the eastern Barents Sea Shelf, however, a verythick latest Permian-Triassie succession indicates vigorousaccumulation in a graben-like trough most likely associatedwith intense intracratonic extension and thinning ofthe Earth'scrust at about the Paleozoic/Mesozoic boundary. Such activa­tion of a hitherto quiet geodynamic environment coulel,perhaps, indicate the appearance of initial extensional stressesthe effects of which were restricted at this early stage to theformation of the East Barents failed rift. It was not until about200 Ma later when these stresses culminated in the separationof the Lomonosov Ridge and the opening of the EurasianBasin.

The notion that the East Barents depression was formed byextensional stretching of the continental lithosphere is prima­rily based on geophysical evidence suggesting that up to 20km and more of sediments rest here directly on a high-velocitylower crust. This type of structure is encountered in manymajor continental rift grabens and usually attributed tocompensation processes beneath rapidly subsiding sedimen­tary basins. Alternative interpretations deny extensional riftingand propose that the lower crustal layer beneath the EastBarents basin fill represents a deeply buried oceanic base­ment, a relic of an ancient ocean that existed in the place of theEast Barents and North Kara Seas in late Precambrian andearly Paleozoic times. In the absence of direct ground truth,both models remain largely speculative, though in our view thesecond concept is deliberately conjectural and more difficultto reconcile with the existing geological data and isostaticconsiderations.

Regardless of the difference in opinions on the tectonic nature

10

ofparts ofthe Barents - North Kara Basin basement, it is clearthat a vast sedimentary basin existed here long before theEurasian opening and the formation ofthe modern continentalmargin. There is no reason to believe that this pre-Cenozoicdepositional area was limited to the present-day Barents ­North Kara shelf: more likely it encompassed the LomonosovRidge in its pre-drift position anel, perhaps, even partlyextended over the region which is now occupied by the Alpha­Mendeleev bathymetric highs. It is not clear whether thispresumably continuous basin marked a continental margin ofan already existing hypothetical Arctic paleo-ocean, or repre­sented an intracontinental depression completely surroundedby source areas. In either case, it appears that spreading of theEurasian Basin seafloor must have begun across a maturedepositional depression and continued for a lengthy periodbeneath a substantial thickness of pre-breakup sediments,including not only a pre-rift graben fill but also oldersequences of wide stratigraphie range (POSELOV et a1. 1998).Since the latter are known to include Paleozoic and Mesozoicplatform strata in the Barents - North Kara Basin, the presenceof deposits of similar age on the conjugate Lomonosov Ridgemargin is a logical assumption. However, their tectonic posi­tion on the ridge may differ from the structurally undisturbedplatform units in the Barents - North Kara Basin and maydisplay more similarities with deformed sequences ofNovayaZemlya and/or fold belts ofNortheast Russia.

The prolonged depositional history resulting in the accumula­tion of a great thickness of sediments on both passive marginsof the Eurasia Basin and also in its oceanic core suggests thatthis entire part of the Arctic region may possess a high oil andgas potential as already positively proven in the Barents ­North Kara Basin. Further assessments and related prospec­ting and exploration activities may profit from the considera­tion of some additional factors that were likely to influence theformation and distribution of hydrocarbons.

Indeeel, the absence of early Cenozoic cover sequences in theBarents - North Kara Basin and the very limited occurrence oflate Cenozoic strata mark a pronounced Tertiary erosional epi­sode associated with an elevated position of the entire Barents- North Kara lithosphere. The cause and mechanism ofthis up-

heaval, which occurred synchronously with the deepening ofthe adjacent oceanic basin, are poorly understood. It is concei­vable that this major tectonic event could have caused a consi­derable redistribution of initial hydrocarbon accumulations.Only few of them probably retained their original stratigraphicposition, whereas many others were, perhaps, partly orcompletely destroyed or, on the contrary, remobilized toproduce major secondary fields. The Prirazlomnoye oil fieldin the Pechora Sea is an example of original accumulation, butthe Shtockman and Ledovoye giant gas condensate fieldscould have been formed by an upward migration of hydrocar­bons into the Jurassie sequence (GRAMBERG & SUPRUNENKO1998).

One of the crucial scientific concerns in the Barents-NorthKara Basin is a better understanding of the transition from un­deformed Paleozoic and early Mesozoic cover units on theeastern Barents shelf to coeval folded sequences on NovayaZemlya. A possible solution of the problem is schematicallypresented in Figure 4, mainly on the basis of a common under­standing that the Mesozoic deformation on Novaya Zemlyawas caused by SE-NW lateral tectonic transport. Fieldevidence, however, suggests that at least in some parts of thearchipelago the structure is simpler and forms an almostsymmetrical anticlinorium (TRUFANOV et al. unpubl. data).

Among other controversial issues is the nature and the scopeofAlpine tectonism in West Spitsbergen. Some recent publica­tions (e.g. DARAGAN-SUSCHEV & EVDOKIMOV 1998) claim thattranspressional stresses related to the Cenozoic plate motionscaused the first major structural distortion in the entire,hitherto undisturbed area, but conventional concepts associa­ting the main deformational event with the Caledonianorogeny remain by far more commonly accepted.

The South Kam Basin

The ambiguity in identification of age and composition of thebase of the cover gives rise to different tectonic concepts of theearly basin history and the nature of the basement. A morecommonly accepted interpretation is based on analogy withnorthern West Siberia and assurnes that pre-Jurassic units arerepresented by latest Paleozoic(?)-Triassie volcanic-sedimen­tary molasse deposits in graben-like depressions of the base­ment. The latter is believed to represent an offshorecontinuation of the Hercynian continental crust which wasstretched and submerged to 10-12 km depth (Figs. 3,4) in thefloor ofmajor rift-related grabens underlying the deepest partsof South Kara Basin. As the result of these extensional events,originally mid-upper crustal layer acquired the position andvelocity parameters usually encountered in lower crust. Thisconcept is essentially based on the similarity with the EastBarents depression in the mechanism proposed for the modifi­cation of continental crust by extensional stresses, as well as inthe latest Paleozoic-Triassie age assigned to the formation ofthe main graben-controlled sequences (SHIPlLOV et al. 1998).

USTRITSKY (1985, 1998) stresses the similarity between theSouth Kara Basin and East Barents depression from a differentangle. He attributes the apparent absence of a "granite layer"in the South Kara Basin to an initial lack of continental crust

and assurnes that the high-velocity crustallayer at the base ofthe sedimentary fill represents a "basalt window" inheritedfrom the Riphean to early Paleozoic Uralian Ocean. Accordingto this concept, a hypothetical ocean was not closed in thisarea during the Hercynian Orogeny, and the oceanic floor waspreserved after the cratonization of the West Siberian base­ment. During much of Paleozoic time this oceanic depressionaccumulated mostly thin deep-water sediments. After theemergence of Hercynian denudation sources farther to thesouth, it was rapidly filled by thick clastic sequences andconverted into a shallow water basin. The latter continued itsgradual subsidence in late Mesozoic and Cenozoic time as theSouth Kara part of the Eurasian Arctic margin continentalshelf.

The presence ofPaleozoic, relatively deep-water lithologies onthe east coast of Novaya Zemlya is the main geological argu­ment used by USTRITSKY (1985) who interpretes these rocks aslower slope facies, transitional from a continental margin to anoceanic environment. However, other authors (e.g. KORAGO etal. 1998) believe that these rocks could equally likely indicatean uncompensated sedimentation in a deep intracratonictrough which was formed east ofNovaya Zemlya in the courseof mid-Paleozoic crustal extension and associated initialrifting of an ancient continentallithosphere.

To gain insight into the whole South Kara Basin history, it isobviously crucial to explain the nature of folding in the Paleo­zoic and lower Triassie sequences on Novaya Zemlya and todocument their transition to the South Kara basement andbasin units. These goals may, perhaps, be achieved by dedi­cated geological and geophysical studies on the archipelagoand in the nearby Kara Sea shelf, without waiting for furtherstratigraphic drilling which in the present-day economic situa­tion appears a very remote possibility.

The Laptev Basin

Unlike the Barents - Kara Seas sector, the Laptev Sea conti­nental margin cannot, strictly speaking, be regarded as "pas­sive" with respect to the Eurasian oceanic basin. Thespreading lineaments of this basin abut orthogonally againstthe base of the Laptev Sea slope and form one of the world'smost enigmatic T-junctions of that type. The abrupt termina­tion of the oceanic features is explained by the existence of aprominent linear divide recognized from morphostructural andpotential field evidence. This divide is described in the litera­ture as Khatanga-Lomonosov zone, or Severnyi Transfer, orCharlie Transform Fault. Most authors agree, however, thatthis major boundary did not prevent the further propagationonto the Laptev Sea upper slope and shelf of the extensionaltectonic regime resulting in the formation of rift-related horstand graben assemblages.

The opinions about the preceding basin history and the natureof the substratum affected by rifting are less unanimous. Amore traditional view implies that the West Laptev Deep andthe East Laptev High are the offshore portions of the ancientEast Siberian craton and the adjacent Late Mesozoic fold belt,respectively. In late Mesozoic and Cenozoic time, both pro­vinces underwent extensional downfaulting and were buried

11

under young basin sequences. The accumulation of these se­quences in the West Laptev Deep was merely a continuation ofa preceding lengthy platform history but in the east it markedthe change from a compressional tectonic regime in the foldbelt to an extensional one. Consequently, a much greater totalsediment thickness in the West Laptev Deep is attributedmainly to the presence of thick undeformed pre-Iate Mesozoicplatform strata (probably as old as Riphean), whereas in theeastern Laptev Basin the base of the sedimentary cover is cor­related with a peneplained surface of the late Mesozoicorogen. The extremely long depositional history and the muchwider stratigraphic range of the cover sequences assumed bythis concept for the western Laptev Basin provide the basis fora much more optimistic hydrocarbon evaluation than for theless mature eastern Laptev Basin (KIM 1998).

More recent interpretations postulate that throughout thegreatest part of the Laptev Sea shelf the base of the basinsequences is represented by a distinct, relatively young,seismic boundary. This boundary is assumed to be associatedeither with a structural unconformity post-dating the lateMesozoic deformations, or with a prominent erosional discon­tinuity believed to accompany the major re-orientation ofplatemotions at about the Mesozoic/Cenozoic time boundary, orwith a superposition of these stratigraphically close surfaces(DRACHEV 1998a, 1998b, DRACHEV et a!. 1998, HINZ et a!.1997, 1998). Consequently, the main distinction between thewestern and eastern Laptev Basin is seen in the intensity ofrift-related crustal extension and the subsidence reflected inthe thickness of the latest Mesozoic and Cenozoic basin fillsequences. This approach provides less optimism for hydro­carbon evaluations because it reduces the probability ofgeneration and preservation of hydrocarbons from organicsources in pre-rift sequences. At the same time, very activegeodynamic environments and vigorous Cenozoic clastic sedi­mentation could facilitate the appearance of relatively youngprimary accumulations, and/or the remobilization and redistri­bution of older organic matter leading formation of majorsecondary fields.

Elements of both approaches were used for the geologicalsolution proposed in Figure 7. Already "partitioned" but notyet appreciably thinned crust underlying the East Laptev Highis interpreted as consisting of three layers: a supracrustal sedi­mentary layer, a mid-crustal infrastructural layer, and a lowercrustallayer. Only the upper part ofthe supracrustal sedirnent­ary layer is represented by high-reflectivity units forming the1atestTertiary-Quaternary post-rift veneer and the main Ceno­zoic syn-rift basin fil!. The lower part of the supraerustal sedi­mentary layer is believed to be concealed beneath the acousticbasement where a pattern of scattered reflectors suggests thepresence of pre-rift sedimentary sequences probably correla­tive with the thick, moderately folded Paleozoic-Mesozoicsuccession on Kotel 'nyi Island. The assignment of the under­lying mid-crustal layer to a Neoproterozoic(?) crystallineinfrastructure follows a commonly accepted interpretation ofthe Kotel'nyi crustal block as an epi-continental "medianmassif". Extensional faulting is most evident in the supracru­stal sedimentary layer, except in its uppermost part whichmust have accumulated after the main phase of stretching. Theconfiguration of faults in the upper crust and their apparentabsence at deeper levels suggest that the top and the base of

12

the mid-crustallayer may delineate major detachment surfacesaccommodating the extensional stress.

Beneath the Western Laptev Deep these two mid-crustalreflectors and the descending base of the basin fill seem tomerge into one main interface separating the sedimentarylayer from its "solid" base. The latter appears to have beenreduced by stretching to a thickness of only 4-5 s TWT (about10 km?). The composition and properties, as well as theprimary nature of this mid-Iower crustal layer are open forassumptions and may, perhaps, be regarded as a tectoniccollage of heavily reworked infrastructural crustal assemb­lages and suprastructural sequences. Deepening of the basinfrom the East Laptev High westwards is shown in Figure 7(after BOR seismic data, HINZ et a!. 1997, 1998) as causedmainly by Cenozoic extension al faulting. However, a slightlybroader stratigraphic range is assigned to the Ust' Lena Riftfill to indicate that in this part of the Laptev Basin subsidenceand sedimentation could have already begun in Cretaceoustime. The possibility that older cover sequences are alsopresent in this basin fill is certainly not ruled out by theexisting data, although some geological evidence seems toindicate that the entire area presently occupied by the LaptevBasin may have been affected by late Mesozoic folding.

The leading role of Cenozoic rifting in the formation of theLaptev Basin is challenged by some authors who question theclose connection between the evolution of the Eurasian andLaptev basins. According to USTRITSKY (1998), the LaptevBasin basement was largely formed during the late Mesozoicorogeny as the result of accretion of allochthonous continentalterranes in the process of closure of a hypothetical pre-existingocean whose relics may still be present in the deepest parts ofthe Laptev Basin.

Apart from such ultramobilistic views, the main scientificquestions related to the Laptev Basin formation can besummarized as folIows:(i) the stratigraphic range ofthe basin fill in the Ust' Lena Rift

and the nature of the crustal substratum affected by Ceno­zoic rifting, or in other words, the credibility of the tradi­tional concept of an essentially epi-platform evolution ofthe western Laptev Basin versus its more recent interpreta­tions as a deeper and more strongly stretched portion of asingle Laptev shelf basin that evolved on a more or lessuniform late Mesozoic fold basement, and

(ii) the mechanism of interaction between the Eurasianoceanic basin and the Laptev shelf basins within theKhatanga-Lornonosov ("Severnyi Transfer") zone, or thevalidity of the speculation by HINZ et a!. (1997, 1998) thatsouth of the Charlie Transform Fault the Eurasian openingwas compensated by an equivalent amount of lateral stret­ching of the continental crust. According to this model, theintensity of extensional deformation in the Laptev Basingradually shifted from the Ust' Lena Rift to youngergrabens between the Anjou and De Long Islands, follo­wing an eastward migration ofthe Gakkel Ridge spreadingaxis.

The Eastern Basins

The lack of accessible seismic data precludes a substantialdebate of the geological structure and history of the EasternBasins. At this stage, they can only be discussed in a verypreliminary manner, mainly on the basis of indirect evidenceincluding the consideration of general analogies, remotecorrelations or geodynamic reconstructions. The southernmostperiphery of the New Siberian Deep is believed to be under­lain by the late Mesozoic South Anui Fold Zone and the west­north-west continuation of the Chukchi Fold Belt exposed inonshore outcrops on the mainland and the Lyakhovsky Islandsand traced by potential field data within a near-coastal offs­hore strip. The Lower Cretaceous unconformity (LCU) whichtruncates the top of this basement and manifests the maincompressional event throughout much of the Russian NorthEast is believed to form the acoustic basement in the entireEastern Basins. This fundamental assumption underlies themajority of commonly accepted seismic stratigraphie interpre­tations.

Northward from the South Anui Zone agentie descent of theacoustic basement is accompanied by a much steeper down­ward offset of the magnetic basement. Based predominantlyon an assumed structural continuity of the latter with Paleo­zoie complexes on the mainland and on WrangelIsland, wepropose a late mid-Paleozoic to early lower Cretaceous age forthe units between the magnetic and acoustic basements in theNew Siberian Deep. Whether these units are structurally morecomparable with moderately deformed Paleozoic-Mesozoicsuccession on Anjou Islands or with much stronger folded pre­late Mesozoic units on Lyakhovsky and WrangelIslands isopen for assumptions.

An older (Caledonian?) basement beneath the De Long Highis indicated by the presence of flyschoid sediments in islandoutcrops intruded by mafic sills and dikes; the latter yieldedlate Ordovician isotopic ages probably corresponding to thetime of the main tectonic event.

In our view, the lack of direct evidence for a late Mesozoieplate convergence anywhere within the vast East Siberian andChukchi Sea shelves may indicate that over the greatest part ofthe Eastern Basins the mid-Cretaceous compressional eventwas limited to structural reworking of the pre-existing crustand probably mainly consisted of refolding of early-midPaleozoic basement complexes and/or initial deformation ofthe pre-Late Mesozoic cover. Nevertheless, mid-Cretaceousorogenie assemblages are widespread on the mainland and theLyakhovsky Islands, and compressional features of presu­mably the same age have been documented on WrangelIsland(KOS'KO et al. 1990, 1993). This makes some authors (e.g.USTRlTSKY 1998) believe that geodynamic models postulatedfor the Russian North East and the Alaska North Slope mayalso apply to the entire area occupied by the Eastern Basins.According to this concept, the latter are underlain by a relati­vely young crust which was accreted in the form of exoticterranes and cratonized as late as in late Mesozoic time.

Whichever scenario of the Eastern Basins pre-Cenozoiccrustal history finds better support from future investigations,the post-mid Cretaceous geodynamic evolution of the eastern

Eurasian Arctic margin and its present-day structural relati­onship with the Amerasian Basin present a separate problem,since they seem to exemplify a peculiar setting not readilymatched elsewhere in the world, namely, a modern extensionalmargin evolving on the periphery of a "non-spreading"oceanic basin. A plausible explanation of this unusual combi­nation may not be achievable without better understanding ofthe nature and origin of the Amerasian Basin deep seabed.

CONCLUSIONS

Vast sedimentary basins underlying the Russian Arctic shelfand the basement highs which delineate and separate thesebasins are the principal components of the crustal structure ofthe Eurasian Arctic margin. Because of an outstanding oil andgas potential of these basins, the fundamental and applieddimensions of earth science exploration on the Eurasian Arcticmargin have always been closely interrelated. Research inte­rest in this field ranged from understanding the broad regulari­ties of crustal evolution of the entire Eurasian Arctic margin tomore specific objectives, such as

the recognition of syn-oceanic structural elements andtectonic events as opposed to features inherited from pre­oceanic evolution;the correlation of such syn-oceanic structures/events withthe peculiarities of geological history of the Eurasian andAmerasian oceanic basins, and of pre-oceanic featureswith the evolution ofthe adjacent mainland;the determination of the lithostratigraphic composition ofthe sedimentary cover and development of tectono-strati­graphie concepts for basin modeling;the identification of the factors influencing the generationand preservation of hydrocarbons and of the criteria todiscriminate between predominantly oil- and gas-bearingbasins.

Despite the apparent progress achieved during the pastdecades in the exploration of the Eurasian Arctic margin, espe­cially in its western sector, most ofthese problems continue tostand out as priority research challenges requiring furtherdedicated investigations. Of particular interest in a funda­mental context is the examination of the structural and evolu­tionary continuity between the Eurasian Arctic margin and theadjacent crustal assemblages on both the mainland and theoceanic sides with a view to unravel the formation of differentparts of the Eurasian Arctic margin in relation to Cenozoicgeodynamic processes in the Arctic Ocean.

From this angle, the Barents - North Kara Basin fully corres­ponds to the definition of a typical passive margin whose outeredge is almost strictly matched by the shape of the activeGakkel Ridge oceanic rift and the conjugate slice of conti­nental lithosphere underlying the Lomonosov Ridge. The pre­breakup depositional record ofthe Barents - North Kara Basinsuggests a prolonged extensional subsidence within a muchlarger basin which probably extended from the East Europeanplatform to far beyond the Lomonosov Ridge in its pre-driftlocation. The geodynamic positon of this vast area of sedimen­tation must have been wholly intracontinental at least untillateMesozoic time when its most distal part was probablyconverted into a continental margin bordering the inferred

13

opening of the Canada Basin. An unexplained feature of thehistory ofthe Barents - North Kara Basin is the Tertiary upliftwhich seems inconsistent with either the preceding subsidencehistory or with the Cenozoic deepening of the rest of theArctic geodepression.

The geodynamic setting of the South Kara Basin is obscuredby its evident continuity with the West Siberian province. Thisis emphasized by the separation from the Barents-North KaraBasin by a prominent suture extending from the Polar Uralsthrough Vaigach Island, the Novaya ZemlyaArchipelago to theNorth Siberian Ramp. This suture probably started to developin the Polar Urals - Vaigach Island segment during Hercynianevents, culminated in the emergence of the early MesozoicNovaya Zemlya fold system, and completed its evolution inthe course of the Tertiary uplift of the North Kara shelf.However, this growing isolation of the South Kara Basin fromthe Barents - North Kara Basin did not seem to radically influ­ence the obvious similarities in their Mesozoic pre-breakupevolution which could also be characteristic of a considerablepart ofthe West Siberian province. Consequently, the apparentlack of a direct present-day connection with the Arctic Oceandeep seabed does not preclude an assignment of the SouthKara Basin to the passive margin of the Eurasian Basin.

The "classic" parallelism between the spreading lineaments ofthe Eurasian Basin and the western part of the Eurasian Arcticmargin changes in the Laptev Sea to a peculiar T-junctioncharacterized by the propagation of extensional stress from theEurasian Basin almost orthogonally across the continent­ocean boundary. This geodynamic setting caused noticeablelateral expansion of thinned continental crust beneath thewestern Laptev Basin but not to the extent of breakup andseparation by newly-forrned oceanic lithosphere. In contrast tothe Barents-Kara margin, the evidence of a preceding long­term extensional history in the Laptev Basin is by far lessconvincing. Here, the rapid transformation of normal conti­nental crust into an "overstretched" margin seems to haveoccurred shortly after the late Mesozoic compressional eventthat probably affected, at least to some degree, much of thearea presently occupied by the Laptev shelf. Such structuralrelationships are certainly not usual in typical passive marginsand not really covered by the commonly accepted terminology.

The conventional classification of the Eastern Basins as theeastern sector of the Eurasian passive margin, transitionalfrom north-eastern continental Asia to the central Arcticoceanic core, is consistent with the present-day morphostruc­tural evidence, but not, perhaps, as yet sufficiently well under­stood in a historical context, namely, how the latest Mesozoic ­Cenozoic subsidence of the Bastern Basins was related to theformation of the adjacent Ämerasian deep seabed. Within thelatter neither the presence of continuous oceanic crust, nor themode and the age of its formation, have been reliably esta­blished. An old view that relative bathymetric highs in theAmerasian Basin might reresent a submerged continuation ofthe eastern Eurasian.Arctic margin has never been decidedlyinvalidated. The history of the .Eastern Basins is also notadequately known to allow a confident interpretation of alter­nating compressional and extensional environments caused bythe interaction of lithospheric plates anc:l/or intraplate tectonicevents.

14

A common need in all Eurasian Arctic margin basins is the im­provement of information on the basement assemblages,particularly on age and geodynamic mechanism of theirformation. In the western Eurasian Arctic margin the finalcratonization of the basement is believed to have been asso­ciated with its last recorded deformation (Baikalian in theBarents-North Kara Basin, Hercynian in the South KaraBasin) succeeded by the beginning of accumulation of unde­formed platform sequences. In the eastern Eurasian Arcticmargin the initial, latest Precambrian or early-mid Paleozoicfolding failed to cause basement cratonization. Instead, after alengthy period of cover sedimentation, the late Cimmerianreactivation lead to various degrees of superposed deformationin the earlier basement complexes and/or to initial disturbancein the cover rocks. In both the western and eastern EurasianArctic margin, there are deep basement depressions characte­rized by the apparent absence of the "granite" crustal layer(the so called "basalt windows"). Extensional stretching andthinning of continental crust is seen as the main mechanismforming such a low in the Laptev Sea basement. It is not clear,however, to what extent this model, proposed for a specificCenozoic geodynamic environment, can be applied to othersimilar hollows, especially in the western Eurasian Arcticmargin where their formation is referred to a much earlierperiod of basin history.

A prolonged epi-continental depositional history which is re­liably documented in the western Eurasian Arctic margin andcan be assumed in the eastern Eurasian Arctic margin and,perhaps, also in much of the adjacent Arctic Ocean, underliesthe concept of an Arctic geodepression advanced by POGRE­BITSKY (1984). It also agrees with the observation by GRAM­

BERG (1998) that the youngest ocean of the planet is framed bythe oldest passive margin environments. A further develop­ment of the theoretical basis for studying the evolution of theArctic Basin in general and the Eurasian Arctic margin inparticular will require much closer focusing on issues relatedto expansion of continental crust in the course of its exten­sional stretching. At present, this problem is obviouslyneglected, and most palinspastic reconstructions seem to disre­gard the increase in lateral dimensions of continental massesin the course of their pre- and post-breakup history, mostnotably within extensional continental margins formingaround late Mesozoic and Cenozoic oceans. Another majorshortcoming of current fundamental thinking is its over­concentration on modeling the changes on the Earth 's face asthe main instrument of discerning global evolution, whereasthe studies of deep interior processes causing reorganizationsat the upper level of lithosphere are not sufficiently encou­raged.

It is believed that answers to these and other unsolvedquestions can best be achieved through an unbiased balancebetween rational ideas contained in both modern plate tectonichypotheses and alternative approaches to interpretation ofcrustal history based on more traditional concepts.

ACKNOWLEDGMENTS

Our thanks are due to Dr. Franz Tessensohn for his thoroughreading the manuscript and making valuable editorialimprovements.

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