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Research ArticleCenozoic history of the Bering Sea and its northwestern margin

VADIM D. CHEKHOVICH, DMITRY V. KOVALENKO AND GALINA V. LEDNEVA

Institute of the Lithosphere, Russian Academy of Sciences, Staromonetny per., 22, Moscow 109180, Russia;Email: <[email protected]>

Abstract Tectonic reconstructions based on the geodynamic analysis of geologic, paleomagnetic, structural and kinematic data of Cenozoic age from the western Bering Sea region are proposed in the present paper. The most active tectonic and magmaticprocesses took place in the Komandorsky segment of the Bering Sea, exemplified by theLate Cretaceous–Early Eocene Olutorsky Arc and Eocene–Oligocene Govena–KaraginskyArc, which was built on the structures of the Olutorsky Arc. A model of the complex collision of these two arcs with the paleocontinental margin, which considers rotations of the geological blocks from the various structural zones of the western margin of the BeringSea in the horizontal plane (paleomagnetic data), was proposed by the authors. Accordingto this model the collision of the flanks of the Olutorsky and Govena–Karaginsky arcs tookplace in the Eocene, before the collision of the central parts in the Miocene.

Key words: Cenozoic, Kamchatka, paleomagnetism, tectonic reconstructions.

MAIN STRUCTURES OF THE BERING SEA FLOOR

KOMANDORSKY BASIN

The structure of the crust in this basin is similarto that of oceanic crust. It is on average 12–14 kmthick, and the overlying sedimentary cover variesfrom 1 to 3 km in thickness. In the northern part of the basin, to the south of the shelves of Olutorsky Bay and Karaginsky Island, a narrow (~ 30–40 km) trough filled with more than 6 km ofsediments has been found (Fig. 1; Cooper et al.1976; Bogdanov 1988). Oceanic tholeiitic basaltsdated at 9.8 Ma have been recovered from underthe Late Cretaceous–Pliocene sedimentary coverfrom Ocean Drilling Program (ODP) Hole 191,drilled to the west of the submarine ShirshovRidge (Rubenstone 1984; Harbert et al. 1987;Valyashko et al. 1993).

ALEUTIAN BASIN

The thickness of the crust in the Aleutian basinattains 15–16 km. The sedimentary cover in itscentral part is 4–6 km thick. The thickness of sed-iments along the western slope of Shirshov Ridge,

INTRODUCTION

The Komandorsky and Aleutian basins arebounded by the fold-and-thrust structures of Kam-chatka and the Koryak Highlands (Fig. 1). The for-mational history of these structures is intimatelyrelated to the formation and evolution of thesebasins and the submarine Shirshov Ridge that separates them. Not all of the recently obtainednumerous geological, stratigraphic and paleomag-netic data on the folded framing of the Bering Sea have yet been used for tectonic reconstruc-tions of this region (Kovalenko 1996; Rumyantseva1996; Brandon et al. 1997; Kovalenko & Remizova1997; Levashova 1997; Palechek 1997). Thepresent paper focuses on the analysis and compi-lation of these new data, and makes an attempt toconnect geological and tectonic theories by meansof the actual geological data obtained for theKoriak Highlands and deep water basins withknown kinematics of the Pacific plates. Tectonicreconstructions for these regions are proposed inconclusion.

Accepted for publication October 1998.© 1999 Blackwell Science Asia Pty Ltd.

The Island Arc (1999) 8, 168–180

Cenozoic history of the Bering Sea 169

which bounds this basin in the west, graduallydecreases to 1 km. In the north, a large troughfilled with sediments up to 11 km thick extendsalong the continental slope of the Koryak High-lands for ~ 500 km. A similar trough is also foundalong the eastern slope of Bowers Ridge in theAleutian basin. This trough extends for ~ 800 kmand is filled with sediments, the thickness of whichexceeds 10 km in the center (Cooper et al. 1987;Bogdanov 1988).

Based on the interpretation of linear magneticanomalies, the Aleutian basin was previously con-sidered to be Late Cretaceous in age (Cooper et al.1976). Doubts about this conclusion, however, surfaced later (Bogdanov & Neprochnov 1984;Cooper et al. 1992). Cooper et al. (1992), based onthe analysis of all existing geophysical data, rec-ognized that the transverse Vitus arch, which hasa nearly northeast strike, was possibly an exten-sional axis in this basin in the Paleogene (Cooperet al. 1992). In this case, the extended troughs

filled with sediments in the north and south of thebasin might be interpreted as trenches related toPaleogene subduction zones of the Aleutian crust.

SHIRSHOV RIDGE

The crust of the straight, north–south-trendingShirshov Ridge, which separates the Koman-dorsky and Aleutian basins, is ~ 18 km thick (Bog-danov & Neprochnov 1984). Although its slopesare asymmetrical, this ridge is not accompanied bystructures along the basin margins that might beinterpreted as deep water trenches. From theridge, metamorphic rocks were dredged, which are similar to those of layer 3 of the oceanic crust(the age of metamorphism is 47 Ma; Sukhov et al.1987; Kepezhinskas 1990). The rock types found in this region are Triassic quartzites and micro-quartzites (Tsukanov et al. 1984), Late Cretaceous(Campanian–Maastrichtian) cherts similar to sediments of open ocean (Bogdanov et al. 1983),

Fig. 1 Main structural elements of the Bering Sea and its northwestern margin. 1, continents: (a) Siberian and (b) Chukotka and Arctic Alaska; 2,Okhotsk–Chukotsk volcanic belt (Albian to Senomanian); 3, Western Kamchatka–Koryak volcanic belt (R- 2–3); 4, Apuka–Vyvenka volcanic belt (N–Q); 5,rift-derived volcanic belt (Maastrichtian–Danian); 6, large Neogene–Quaternary basins; 7, Koryak accretionary prism (structural zones: I, Penzhina; II,Ust’-Bel’sk; III, Vega; IV, Pekul’ney; V, Mayna; VI, Ekonay); 8, flysch (K2–R- ); 9, Olutorsky accretionary prism; 10, Aleutian Island Arc (R- 2

1–Q2), (a) Aleut-ian segment, (b) Komandorsky segment; 11, (a) isobaths, (b) isopachs; 12, (a) large faults and (b) suture; 13, (a) strike–slips, (b) subduction zone, (c)spreading centers.

Paleogene (Oligocene) tripolite and silty muds,Miocene tuffs dated at 16.5 Ma (Cooper et al. 1976),and calc-alkaline andesitic basalts and daciticandesites. The basalts and andesites are dated bythe K/Ar method at 4 Ma (Sukhov et al. 1987).

According to gravimetric surveys from satel-lites, Shirshov Ridge is very different from islandarcs. The gravimetric character of Bowers Ridge,for example, is almost identical to that of modernisland arcs (Sandwell & Smith 1992), in contrast toShirshov Ridge.

BOWERS RIDGE

The underwater Bowers Ridge has an arcuateshape and is asymmetrical: the eastern slope of theouter part of the arc is steep. It is accompanied bya trough filled with sediments exceeding 10 km inthickness. The crustal thickness in the highestpart of Bowers Ridge reaches 27–30 km. There aregood grounds for interpreting this ridge as anancient island arc (Scholl et al. 1975; Bogdanov1988).

ALEUTIAN ISLAND ARC

The Aleutian island arc is divided morphologicallyinto the Aleutian and Komandorsky segments,whose boundary approximately coincides with thestrike of the Shirshov Ridge, but it is nonethelesscommonly considered to be one single ensimaticisland arc (Scholl et al. 1975; Tsvetkov 1990). Thereare, however, significant distinctions in the struc-ture and evolution of the Aleutian and Koman-dorsky segments. The basement of the volcanicsuccession (Eocene–Holocene) of the Aleutiansegment is composed of a tholeiitic rock series,which is built up by calc-alkaline volcanics locallyintercalated with arc tholeiites. On Attu Island, atthe western terminus of the Aleutian segment, thestructure of the volcanic succession is much morecomplicated. In this location, the Eocene oceanicseries structurally underlies the island arc series.In the Komandorsky segment, the basement of the succession is composed of a bimodal basaltic–rhyolitic series (Ivashchenko et al. 1984). TheOligocene deposits do not show any features of volcanic activity (Tsvetkov 1990). The Miocene volcanics are represented by thin layers ofshoshonites, which are not known at all in theAleutian segment. The Paleogene conglomeratesof the Komandorsky segment consist of debris ofexotic continental-derived rocks (Ivashchenko et al. 1984), which are not known in the Aleutian

Islands. Based on this rock type, it has been suggested that the Komandorsky segment has a continental basement. According to seismicsounding data, the thickness of the crust of the Komandorsky segment attains more then 30 km,which significantly exceeds that of the Aleutian arc proper. The features discussed in a previoussection led researchers to propose that the Komandorsky segment includes exotic blocks,which was later confirmed by paleomagnetic data(Bazhenov et al. 1992; Levashova 1997; Pecherskyet al. 1997).

FOLDED MARGINS OF THE DEEP WATER BASINS

The northwestern folded margin of the deep waterbasins is composed of both pre-Cenozoic and Ceno-zoic deposits. In general, continental, accretionaryand neo-autochthon structures were recognizedwithin the study area (Fig. 1).

The eastern boundary of the united Siberian andArctic Alaska continents (Til’man & Bogdanov1992) is marked by the Okhotsk–Chukotsk vol-canic belt (Filatova 1988). The wide expanse of the Koryak accretionary complex is located to thesouth. This complex is composed of several ter-ranes, which are different in age and origin (conti-nental, island arc and oceanic) and are separatedby fields of Late Jurassic and Early–middle Cre-taceous rocks. These rocks are mainly terri-geneous deposits that were presumably formedwithin marginal basins (Stavsky et al. 1990). In thenorth and west, the fold-and-thrust structures of this accretionary system are sealed up by theMaastrichtian molasse deposits, and are alsolocally sealed in the north by a belt of Maastricht-ian–Danian rift-derived volcanics (Filatova 1988).In the east, the thrusting took place at least up to late Eocene. The Western Kamchatka–Koryakvolcanic belt is the main neo-autochthon withrespect to the Koryak accretionary system. Thisbelt extends from Anadyr’ Bay in the north to Shelikhov Bay in the southwest. Further to the southwest, this belt is bounded by the hypo-thetical continuity of the Komandorsky strike–slip (Andieva & Grigorenko 1997). Most of theresearchers interpret this belt as a subduction-related continental structure (Filatova 1988;Stavsky et al. 1990). According to Filatova (1988),the ancient Benioff zone dipped to the north-northwest at an angle of 35–40° (K2O distributionin volcanics). It should be emphasized that the ages of volcanics in the southwestern andnortheastern areas of this belt are different. In the

170 V. D. Chekhovich et al.


southwest, in the area of the Kamchatka Isthmus,the middle Eocene volcanics are united to form the Kinkil Group (Gladenkov et al. 1990). In thenortheast, age variations are wider and range frommiddle Eocene to Oligocene (Filatova 1988).

The Olutorsky accretionary system (Olutorskyterrane) is located adjacent relative to the deepwater basins of the Bering Sea (Figs 1,2). The com-plexes composing this terrane are allochthonousand are overthrust onto the flysch deposits of theUkelayat Group in the north and the LesnovskGroup in the south. The minimal overlap is believedto be 15–20 km (Bogdanov et al. 1982), while themaximum is suggested to be more than 100 km. Thefrontal zone of this terrane is composed of island arcand oceanic deposits mainly of Late Cretaceous age(Bogdanov et al. 1982), accompanied by ultramafic–mafic massifs (Ledneva 1995). The geological struc-tures of this zone have a northern vergence. Thecentral zone of the Olutorsky Range has a similarstructure, but its structures are severely discor-dant with respect to that of the frontal zone, andhave a southeasterly vergence. In the matrix of theCampanian–Maastrichtian flyschoid–olistostromalsuccession, found along the eastern slope of theRange, there are chert olistoliths of Late Creta-ceous and Jurassic ages (Palechek 1997).

The Cenozoic deposits can be divided into threecomplexes, which form zones parallel to the coast. The relationships among these complexesare either tectonic or unknown. The Il’pinsky–Pakhachinsky zone, which is composed of Paleo-gene–early Miocene volcanomitic terrigeneousdeposits (Chekhovich 1993), is located to the southof the frontal zone. The significant feature of unitsthat compose this belt is their continuity of age fromthe Maastrichtian–Danian to the middle Miocene,forming a continuous succession (Tarasenko et al.1970; Gladenkov et al. 1988a,b; Chamov 1991, 1994).Sedimentary rocks of this complex usually con-cordantly overlie the Late Cretaceous island arcvolcanics.

The Govena–Karaginsky zone of volcanic–sedimentary deposits is located futher to thesoutheast. The maximum width of this belt asexposed in the northeastern Malinovsky Range is~ 40–50 km. The width of the zone decreases to thesouthwest and becomes only ~ 8–12 km wide onKaraginsky Island. The northeastern part of thiszone is composed of lavas and volcanic breccias andthe southwestern part mainly contains varioustuffs. According to paleontological and isotopicdata, the age of these deposits ranges fromEocene, possibly late Paleocene, to Oligocene

(Serova 1970; Chekhovich et al. 1990; Chamov1994).

The Govena–Karaginsky accretionary prism isin the southernmost position. It extends from thecoast of Olutorsky Bay to Karaginsky Island.Various complexes were mapped in this accre-tionary prism, including Late Cretaceous ophioliteand island arc complexes, and Eocene–Oligoceneflysch. The Eocene flysch deposits contain olis-toliths composed of rocks from the ophiolite andisland arc complexes as well as from flows ofoceanic basalts (Chekhovich et al. 1990). Litholog-ical features and textures of the sedimentaryrocks, as well as the presence of abyssal micro-fossils, suggest that these deposits accumulated ina deep water trough. The structure of the prismhas a southeasterly vergence (Chekhovich 1993).

Neogene to Quaternary volcanics are neo-autochthonous relative to the suprasubductionApuka–Vyvenka volcanic belt (Kepezhinskas 1990).

PALEOMAGNETIC DATA

Reliable paleomagnetic data are known for Cenozoic as well as Cretaceous deposits of the Olutorsky region (Kovalenko 1992a,b, 1996;Kovalenko & Remizova 1997) (Fig. 2).

According to paleomagnetic data, the Santonian–Campanian deposits of the Il’pinsky Peninsula,Karaginsky Island, Olutorsky Range (Kovalenko1992a,b) and Kamchatka Isthmus (Levashova 1997)were formed to the south of their present position (~ 40° north latitude). According to Kovalenko(1992a,b) and Kovalenko and Remizova (1997), paleolatitudes of the middle Eocene deposits of theIl’pinsky–Pakhachinsky and Govena–Karaginskyzones are similar to their present ones. Statisticalcomparison of paleolatitudes calculated for LateCretaceous and Paleogene complexes of the Olutorsky terrane with ‘expected’ paleolatitudescalculcated for the same regions based on paleo-magnetic poles of the North America and Eurasia inthe Late Cretaceous and Paleogene (Beck 1980;Demarest 1983), showed that collision of the Olu-torsky terrane with Eurasia is hardly possible untilthe Miocene (Fig. 3b). The collision of this terranewith North America is possible in the Miocene, buthardly possible since the Eocene (Fig. 3a).

Some blocks of Il’pinsky Peninsula and Kara-ginsky Island, composed of the Cretaceous andPaleogene deposits, underwent counterclockwiserotation about a vertical axis. The Paleogene islandarc volcanics exposed on the southern slopes of


Fig. 2 Geological scheme of the Olutorsky terrane (Chekhovich 1993). 1–3, oceanic complexes: 1, ophiolite (K2); 2, oceanic basalts (K1ab–K2t); 3,oceanic alkaline basalts; 4–5, trough and accretionary prism complexes: 4, flysch (R- 1–R- 3); 5, sedimentary melange (K2–R- 3); 6–10, arc complexes: 6,chert–volcanogenic (K2st–cp); 7, volcanogenic–fragmental (K2cp–d); 8, hyperbasite–basite and basite–felsic massifs (K2–R- ); 9, volcanogenic–sedimentary(R- 2–3); 10, volcanogenic–terrigeneous (K2–R- 3); 11–12, continental margin complexes: 11, Apuka–Vyvenka (N2–Q); 12, Western Kamchatka–Koryak belts;13, rift Apuka complex (R- –N); 14, Neogene–Quaternary deposits; 15, Ukelayat–Lesnovsk flysch deposits (K2–R- 1); 16, (a) thrusts and (b) steep faults. Ir,Iruney thrust; Vt, Vatyna thrust. Structural zones: A, frontal; B, Il’pi–Pakhacha; C, Govena–Karaginsky; D, Govena–Karaginsky accrecionary prism; E,central zone of the Olutorsky Range; F, Olutorsky Peninsula. Dashed arrows with angles of confidence indicate deviations of post-folding magnetizationvectors from the remagnetization field direction. Solid arrows with angles of confidence correspond to declinations of vectors of pre-folding magnetiza-tion. Roman numerals are identification numbers for magnetic value; arabic numerals indicate deviation angles of vectors of post-folding magnetizationfrom the direction of remagnetization and correspond to vector inclinations for pre-folding magnetization of non-remagnetized deposits. Directions XXII,XXIII are from Savostin and Kheyfets (1988); XXI is from Levashova (1997).

b

the Malinovsky Range (the Govena–Karaginskyvolcanic zone) were not rotated in the horizontalplane. The late Senonian deposits of the OlutorskyRange, which also originated at a latitude of40–50°N, are rotated clockwise about an angle of60–110°. According to Kovalenko (1996), this rota-tion took place in the Paleogene. It is important toremember that the southeasterly vergence of thefolded structures of the Olutorsky Range is notconcordant with the nearly northern vergence ofmost of the structures of the Olutorsky terrane(Fig. 2).

Important results were achieved from the analy-sis of the post-folding magnetization of the LateCretaceous–Paleogene para-autochthon flysch andallochthonous island-arc and oceanic successionsof the Olutorsky terrane (Kovalenko & Remizova1997). These successions experienced two episodesof unidirectional deformation. The first episode ofdeformation caused the formation of tight andeven isoclinal folds of a nearly northern vergencein the western area of the Olutorsky terrane, andalso folds of southeastern vergence in the Olu-

torsky Range to the east. The second episode ofdeformation, recognized by the discordance of thepost-folding magnetization of the studied succes-sions and the magnetic field of the Earth, led to thetilting of these structures to various angles (from40 to 10°) from the south to the north (Kovalenko1996). The second episode of deformation might berelated to the final collision of the Olutorsky Arcas it moved northward.

GEODYNAMIC ANALYSIS OF THE GEOLOGICAL ANDSTRUCTURAL ELEMENTS OF THE BERING SEA ANDITS FOLDED FRAMING

(1) The identification of magmatic island arccomplexes, which indicate either active continentalmargins or island arcs, is most important for geo-dynamic analysis. Volcanic sequences of the Cre-taceous Okhotsk–Chukotsk, Eocene–OligoceneWestern–Kamchatka–Koryak, and Pliocene–Quaternary Apuka–Vyvenka belts can be inter-preted as continental margin arcs. The Cretaceousand Eocene (island-arc zone) allochthonous com-plexes of the Olutorsky terrane (Fig. 2) anddeposits of the present day Aleutian island arc,which began to form in the Eocene (Fig. 1), aretypical of island arcs. It is also important toremember that the complexes of the Western–Kamchatka–Koryak belt and Govena–KaraginskyArc are coeval. In the present-day configurationthe distance between these zone is ~ 100–150 km.It is possible that in the Paleogene, the Western–Kamchatka–Koryak belt and Govena–KaraginskyArc were separated by the oceanic crust being con-sumed under the Eurasian margin (Chekhovich1993).

(2) Cretaceous allochthonous and Paleogeneisland arc successions, which are mapped in theOlutorsky terrane, are not known in the areas tothe east of the Olutorsky Range (in particular in Alaska). This suggests that the Cretaceous–

Fig. 3 Paleolatitudes of formation of the Olutorsky terrane complexes,which are different in age. Roman numerals correspond to the paleo-magnetic data shown in Fig. 2.

Paleogene arc, remnants of which are found in the Olutorsky terrane, may have been limited by atransform fault on the northeast. This interpreta-tion is supported by other data. The existence ofthis fault might explain the large difference in thegeological evolution of the western and easternareas of the Koryak accretionary system. In thewest, accretionary processes ceased in the Maastrichtian (molasse age; Sokolov 1992). Sincethe cessation of magmatic activity in the Okhotsk–Chukotka volcanic belt, island arc magmatism hasoccurred only in the Eocene Western–Kam-chatka–Koryak belt. In the east (Ekonay zone),the formation of tectonic nappes occurred up untilthe middle Eocene (Ruzhentsev et al. 1992;Sokolov 1992) or Oligocene (Filatova 1988). Islandarc volcanics are also known in Maastrichtian–Paleocene (Kakanaut Formation) and Eocene–Oligocene successions (Sokolov 1992). The occur-rence of Jurassic and Cretaceous deposits in theShirshov Ridge, as well as the presence of Triassicand Cretaceous olistoliths in the Campanian–Maastrichtian flysch of the Olutorsky Range(aforementioned), indicate interaction of crustalblocks of different age in these regions. ShirshovRidge can thus be interpreted as a relic of thetransform fault. As will be described in the following, truncation of the arc by a transformfault can explain the clockwise rotation of the largeblocks of the Olutorsky Range in the horizontalplane.

(3) Another important element of the geo-dynamic analysis is the timing of the deformationof the Bering Sea and its margin. The timing of thedeformations of the Koryak accretionary systemwas discussed in the previous section.

Reliable data on the age of deformation alsoexists for the central part of the Olutorsky terrane(to the west of the Olutorsky Range), the Kam-chatka Isthmus area and for the Karaginsky accre-tionary prism. It is possible that the central partof the Olutorsky terrane was deformed in theMiocene. This theory is supported by the absenceof any significant angular unconformities and dis-continuities in all the studied stratigraphic succes-sions (Mayny–Kakyny Range, Korf Bay, Il’pinskyPeninsula, Karaginsky Island, Malinovsky Range)from the Paleocene to middle Miocene (Tarasenkoet al. 1970; Gladenkov et al. 1988a,b; Chamov 1991,1994). Most of the Cenozoic deposits seem to havebeen formed by a single large tectonic event.Movement along the Vatyna thrust, along whichallochthonous deposits of the Olutorsky terranepresently overlap the Ukelayat flysch, probably

took place about the same time or a bit earlier (lateOligocene). This is supported by the presence ofthe late Eocene rocks among the deposits of theUkelayat flysch (37–48 Ma, fission-track (FT)dating of zircons from sandstones), as well as bythe cooling time of sandstones (20–25 Ma, apatitefrom the same sandstones collected below thethrust; Brandon et al. 1997). Studies of the meta-morphism of flysch deposits indicate that theserocks were not heated above 200–230°C.

In the Kamchatka Isthmus area, thrusting alongthe Iruney thrust took place earlier, in theearly–middle Eocene. Here, the allochthonousisland arc and oceanic deposits of the IruneyGroup are overthrust onto Cretaceous and earlyEocene flysch deposits, forming the LesnovskGroup. Both of these units are intruded by Eocenegranites and are overlain discordantly by themiddle Eocene Kinkil volcanics of the WesternKamchatka–Koryak belt (Fedorchuk & Izvekov1992).

The time of formation of the Karaginsky accre-tionary prism is determined by the presence of ophiolite and island arc complex rock debrisamong the middle Eocene flysch deposits, occur-ring in the accretionary prism at present(Kravchenko-Berezhnoy & Nazimova 1991; Chek-hovich 1993). The southeastern vergence of thisstructure indicates that the subduction zone wasdipping northwestward. The presence of oceanicbasalts in the flysch deposits of Karaginsky Island(Kravchenko-Berezhnoy et al. 1990) and KumrochRange (Tsukanov & Fedorchuk 1989) suggests thatspreading occurred in the fore-arc area.

KINEMATICS OF THE WESTERN PACIFIC IN THECENOZOIC

Most researchers agree that, at the boundary ofthe Mesozoic and Cenozoic, the kinematics of thePacific were determined by the motion of the Kulaand Pacific plates, and that the spreading ridgethat separated these two plates moved northward(Engebretson et al. 1985). The Kula plate movednorth-northwest towards Eurasia until 55 Ma, andthan to the northwest after this time (Lonsdale1988). Volcanic activity on the spreading ridge thatseparates the Kula and Pacific plates ceased at 43 Ma, which led to the amalgamation of the plates.About the same time, the direction of movement ofthe Pacific plate changed to western-northwest.The rate of convergence decreased between thePacific and Eurasian plates (3–4 cm/year) between



50 and 35 Ma, then increased in the late Oligoceneto the beginning of the Miocene.

Although the southward movement of Eurasia(and the North America Plate) was slow (1cm/year)in the Paleogene, it should not be ignored as well(Engebretson et al. 1985).

CONSTRAINTS OF TECTONIC RECONSTRUCTIONS

In the Late Cretaceous, an extensive continentalmargin, marked by the Okhotsk–Chukotsk vol-canic belt, existed in the Russian Northeast. Thelarge Khatyrka terrane (Maynitsky and Ekonayblocks) might have approached the subductionzone (in pre-Maastrichtian time), collided with thecontinent and caused the cessation of the subduc-tion (Stavsky et al. 1990).

At this time, the Olutorsky Arc (terrane) waslocated at 40–50°N. From this time, the subductionzone beneath the arc probably dipped to the southor southeast. In the east, the Olutorsky Arc wasbounded by a transform fault. To the east of thisfault, subduction of the lithospheric plates wasprobably going on beneath the continental margin,which is marked by the continuation thrusting inthese regions (Ekonay system of nappes) and bythe presence of Maastrichtian–Paleocene arc-typevolcanics (Fig. 3a). It is possible that the easternsegment of the Aleutian Arc initiated only at thePaleocene–Eocene boundary after the subductionzone had died, due to collision of the Umnak terranes with the continent (Stavsky et al. 1990).

According to paleomagnetic data (Fig. 4), duringthe Paleocene the Olutorsky Arc was transportedfrom 40 to 60°N, close to its present position. Sincethe Paleocene it experienced very little or no trans-port (Fig. 4). This might have been caused by achange of the spreading direction in the Kula Ridgefrom a nearly northern to a northwestern one(Lonsdale 1988) at the boundary of the Paleoceneand Eocene, and also by initiation of the AleutianArc at the same time (Rubenstone 1984). The subduction of lithosphere situated between the Olutorsky Arc and the continent resulted from the southward movement of the continent over thesubduction zone, the existence of which is docu-mented by volcanics of the Western Kamchatka–Koryak belt. In the middle Eocene, the western endof the Olutorsky Arc probably collided with the continental margin. In this case, non-simultaneouscollision of the western (Eocene) and more eastern(Miocene) segments can be explained by thearcuate shape of the continental margin (Figs 5–7).

Fig. 4 Displacement of the Olutorsky terrane complexes relative to (a)North America and (b) Eurasia calculated by the methods of Beck (1980)and Demarest (1983) using paleomagnetic poles (Westphal et al. 1986).Roman numerals correspond to the paleomagnetic data shown in Fig. 2.F-difference between expected and calculated inclinations, according to Beck (1980).

Another possibility is that the Iruney arc was not asegment of the Olutorsky Arc and belonged to theKamchatka Arc, which collided with the continentin the Paleocene–Eocene. This collision may explainthe short duration of the continental arc-type volcanic activity in the Kamchatka Isthmus area(the age of the Kinkil volcanics of the Western Kamchatka–Koryak belt is middle–late Miocene).

In the Early Eocene the direction of subductionbeneath the Olutorsky Arc changed from the southeast to the northwest. The Eocene–Oligocene Govena–Karaginsky volcanics began toform and the Karaginsky accretionary prism wasinitiated; its southwesterly vergence indicates thesubduction direction. The establishment of this newsubduction zone probably caused extension andsubsidence of the lithosphere, which resulted in theformation of the Il’pinsky–Pakhachinsky zone and

accumulation of thick sediments there. In the Paleogene–Neogene the Il’pinsky–Pakhachinskyzone appears to be wedge-like in shape, wide in itssouthwestern part in the Litke Strait and narrow inits northeastern part, where the Cenozoic sedimen-tary deposits wedge out (Rumyantseva 1996). Theformation of the Govena–Karaginsky arc may haveresulted either from the subduction of the Pacific

plates, if the Komandorsky segment of the AleutianArc was established no earlier than 42 Ma, or wasrelated to spreading in the paleo-Komandorskybasin, if the Komandorsky segment was initiatedsimultaneously with the Aleutian Arc. The presenceof oceanic basalts in the Eocene flysch of Karagin-sky Island and the Kumroch Range suggests thatspreading processes did occur in these regions.


Fig. 5 Paleotectonic reconstructions for northern Pacific: (a) Late Cre-taceous–Paleocene (75–57 Ma); (b,c) Eocene (57–42 Ma). (b) Koman-dorsky segment established simultaneously with the Aleutian Arc; (c)Komandorsky segment established after the Aleutian Arc. 1, plate bound-aries: (i) subduction; (ii) strike–slip; (iii) spreading; 2, continent (EU,Eurasia; NA, North America); 3, terranes (Ka, Kamchatka; Ol, Olutorsky;Bo, Bowers; Um, Umnak); 4, back-arc spreading centers; 5, extinct plateboundaries; 6, fold-and-thrust belts. Plates: PAS, Pacific; KU, Kula; FA,Farallon. Arrows show plate movement directions. Kinematics of theoceanic plates of the Pacific Basin are taken from Lonsdale (1988) andEngebretson et al. (1985). Taken from Fig. 10 of Kovalenko andKravchenko-Berezhnoy (unpubl. data).

Fig. 6 Tectonic processes in the Bering Sea region. (a,b) Eocene(57–42 Ma). (a) Komandorsky segment established simultaneously withthe Aleutian Arc; (b) Komandorsky segment established after the Aleut-ian Arc; (c) Quaternary (25–20 Ma). Symbols as in Fig. 5.


The main (northeastern) part of the Olutorskyterrane, including the Govena–Karaginsky Arc,was situated quite far from the continental marginand was characterized by the strongest volcaniceruptions in the middle Eocene (Figs 5,6). In thelate Eocene the volcanic activity in this island arcdecreased and later ceased because of the changein the direction of the Pacific plate motion andestablishment of the Komandorsky strike–slipfault. The approach of the Olutorsky Arc to the con-tinent was caused by the southward motion of thecontinent, which resulted in the development of thecentral part of the Western Kamchatka–Koryakvolcanic belt in the late Eocene–Oligocene.

The earlier collision of the western segment ofthe Olutorsky Arc with the continent, and exist-ence of the transform fault to the east of it, makeit easy to explain the rotation of the blocks of theOlutorsky terrane (Fig. 2). Possible episodes of theOlutorsky terrane collision are shown in Fig. 7.Before the collision of the Iruney block, rotation ofsmall blocks was possible to the east along thetransform fault bounding the Olutorsky Arc. Aneastern component of motion of the arc developedduring the Eocene collision of the western end ofthe arc, which may have caused thrusting in the

western part of the arc as well as in its eastern end,and clockwise rotations. After this time, theeastern segment of the arc may have collided with a prominent part of the continental slope. Theexistence of this prominent part of a continentalslope is supported by the Late Cretaceous andEocene flysch deposits in coastal cliffs of theeastern part of the Olutorsky Range. This collisioncaused the formation of larger tectonic nappes andtheir clockwise rotation. The modern shape of theOlutorsky terrane, concave toward the continent,was formed due to fan-like clockwise or counter-clockwise rotations of terrane blocks in the finalepisode of collision, at the end of the Oligocene orMiocene. The degree of rotation increased withdecreasing distance from the eastern and westernends of the arc.

The subduction of the oceanic crust, fragmentsof which remained between the Olutorsky terraneand continent, can only explain the formation ofthe western and eastern parts of the WesternKamchatka–Koryak volcanic belt. To the east ofthe transform fault, volcanic activity cannot resultfrom this process. Therefore, attention should bepaid to the Aleutian basin, which has been sepa-rated from the Pacific plates by the Aleutian Arcsince the Eocene. The Koryak margin can haveinteracted only with the oceanic crust of the Aleutian basin, which may have enlarged in thePaleogene (Cooper et al. 1992). This extensioncould have been compensated for in two oppositedirections: the subduction under the Koryakmargin, or under the Bowers Arc (Fig. 6b,c). Thelatter is supported by the existence of troughsfilled with sediments. Extension in the Aleutianbasin might be reflected in the formation of theNavarin pull-apart basin in the western outer partof the Bering Sea shelf during the Paleogene(Worral 1991). Note that all tectonic processes,which took place in the Bering Sea region in theLate Cretaceous, Paleogene and Eocene, wereprobably much more complicated due to possibleinteraction between the Eurasian and NorthAmerica continents in these regions (Coe et al.1988).

Since the late Miocene, the evolution of marginsof the the Komandorsky basin has been different.This stage is characterized by the formation of volcanogenic deposits of the northern part of the Sredinny Range and Apuka–Vyvenka belt.These volcanics can be spatially divided into twoareas. One area includes numerous isolated nappesof exotic rocks situated in the Kamchatka Isthmusand the western part of the Olutorsky zone. These

Fig. 7 Possible consecutive episodes of collision of the Olutorskyterrane with concave continental margin. 1, proximal arc deposits (lavas,tuffs, lava breccias, tuffaceous sandstones and tuffaceous conglomer-ates); 2, distal arc deposits (tuffaceous siltstones; tuffaceous silicites andcherts); 3, continental slope; 4, (a) thrusts and (b) strike–slips; 5, decli-nation of magnetization vectors; 6, plate movement directions.

rocks are compositionally similar to the supra-subductional volcanics. Another area relates to the Apuka rift zone, which is located along the continuation of the inferred spreading axis of theKomandorsky basin. The petrological and geo-chemical features of these volcanics undoubtedlyindicate their rift-derived origin (Kepezhinskas1990).

After the subduction stopped and, as a result,the Pliocene–Quaternary volcanic activity in thefolded margin of the Komandorsky segment of theBering Sea ceased, a new interplate boundary,which is marked seismically at present (Worral1991), was initiated, and the Komandorsky basinbecame a part of the independent Beringia plate.

CONCLUSIONS

The Cenozoic as well as Mesozoic geological evo-lution of the western Bering Sea region was con-trolled by the interaction of the continental andoceanic plates. However, the interactions in theAleutian and Komandorsky segments, which wereseparated by a transform fault, were different.The oceanic crust of the Aleutian and Koman-dorsky basins was separated from the Pacific bythe Aleutian Arc and by the Olutorsky andGovena–Karaginsky arcs, respectively. The mostactive tectonic and igneous processes took place inthe Komandorsky segment because of the LateCretaceous–Early Eocene Olutorsky Arc and theEocene–Oligocene Govena–Karaginsky Arc, whichdeveloped on the structures of the Olutorsky Arc,and the complicated non-synchronous collision ofthese arcs with the continental margin. In theAleutian segment, tectonic processes were not soactive. This segment was also characterized by subduction processes, which formed the vol-canic deposits of the Eocene–Oligocene Western Kamchatka–Koryak belt of the northern boundaryof the Aleutian basin.

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