S.D. Sokolov, G.Ye. Bondarenko, O.L. Morozov, V.A. Shekhovtsov, S.P. Glotov, A.V. Ganelin, I.R. Kravchenko-Berezhnoy

South Anyui suture, northeast Arctic Russia: Facts and problems
Geological Society of America Special Paper 360, 2002, pp. 209-224

The South Anyui suture zone extends from the eastern part of the Laptev Sea into western Chukotka (northeastern Arctic Russia) and is everywhere marked by Mesozoic terrigenous turbidite and fragments of ophiolitic sequences. Ophiolitic sequences of both Paleozoic and Mesozoic age are believed to be present. The units flanking the suture are known to host suprasubduction-zone arc complexes of late Paleozoic-Late Jurassic age on the south and of Late Jurassic-Early Cretaceous age on the north. The Paleozoic Anyui oceanic basin is proposed to have been connected with the Taimyr and Polar Urals ocean basins. The relationship of the Anyui ocean basin with the paleo-Pacific remains debatable. In the Late Jurassic-Early Cretaceous (or, possibly, beginning in the Late Paleozoic), the South Anyui basin became separated from the Pacific by a convergent boundary. The accretionary melange and approximately coeval suprasubduction-zone complexes are regarded as indicators of convergent plate motion and boundaries between the oceanic plates of the South Anyui basin and the Siberian continent, and the Chukotka microcontinent. Four principal stages of deformation are distinguished in the South Anyui zone (Natal'in, 1984). The first two stages gave rise to fold and thrust-type structures. The two later stages involved strike-slip faulting. The most important interaction between the Chukotka microcontinent and Siberia during latest Jurassic and Early Cretaceous time was likely related to oblique collision between the two continental masses that involved lengthwise dextral strike-slip motion along the South Anyui suture.
In northeastern Russia, Mesozoic erogenic structures (Mesozoides) occupy a vast area extending from the Siberian craton on the west to the Okhotsk-Chukotka continental-margin volcanic belt to the east (Fig. 1). Farther east, the Koryak-Kamchatka fold belt occurs along the Pacific margin of northeastern Russia. Collisional processes played an important role in the formation of the Mesozoic orogenic belts of northeastern Russia (Parfenov, 1984; Natal'in, 1984; Parfenov et al., 1993; Zonenshain et al., 1990; Pushcharovsky et al., 1992; and others). One of these collisional belts is the South Anyui suture zone (Fig. 1). Since the works by Seslavinskiy (1970) and Til'man (1973), the South Anyui suture zone has customarily been viewed as a highly deformed zone that formed during and after the closure of a rift-related late Mesozoic oceanic basin. Later closing of the South Anyui ocean basin was linked to the collision between the Siberian continent and the Chukotka microcontinent, which belonged either to the hypothetical Arctida (or Hyperborea) continent (Seslavinskiy, 1979; Zonenshain and Natapov, 1988, 1990) or to the North American plate (Parfenov, 1984; Natal'in, 1984). At the same time, Parfenov (1984) indicated that accretionary processes may have also taken place along this boundary. Thus, it is more accurate to consider the South Anyui suture zone as one of the collisional-accretionary belts that are widespread around the edges of the Siberian craton (Berzin et al., 1994). The collisional nature of the South Anyui suture zone is not questioned, yet the age of formation of the ocean basin, its life span, and size prior to collision are open to discussion. These issues are critical to paleotectonic reconstructions of the Arctic and to understanding the interaction between the Eurasian and North American plates. Unfortunately the South Anyui suture zone remains poorly known, and the available data do not in all cases agree, giving rise to diverse, often contradictory views on the structure and history of the South Anyui suture zone. The most controversial issues regarding the geology of the South Anyui suture zone are (1) the timing of the inception and existence of the South Anyui oceanic basin and its original size; (2) the age of the sedimentary complexes involved in the formation of the South Anyui suture zone; (3) the age and genesis of the ophiolites; (4) the inner structure of the South Anyui suture zone and the age and sequence of deformations; (5) the spatial distribution and age of island-arc complexes in and adjacent to the South Anyui suture zone, and what they tell us about the convergent boundaries that flank the basin; and (6) the dynamics of the collision of the continental masses of Chukotka (North America) and Siberia (Asia) that developed during the evolution of this zone. This chapter is intended to summarize and analyze existing and new geological data in order to reach a more reasoned interpretation of the evolution of the South Anyui suture zone, and discusses the challenges and prospects that face future studies in the South Anyui suture zone.
The South Anyui suture zone is an important tectonic unit of Northeast Asia (Fig. 1). It terminates structures of the Kolyma loop, including the arcuate belt of allochthonous terranes (Zonenshain et al., 1990) as well as island-arc terranes of the Alazeya-Oloy fold zone (Til'man, 1973), and separates the Anyui-Chukotka fold belt from the Siberian craton (Fujita and Newberry, 1983; Parfenov, 1984; Pushcharovsky et al., 1992). Structures of the Kolyma loop and the Anyui-Chukotka fold belt are briefly described in the following. The Kolyma loop structures are thrust over the Verkhoyansk fold belt (Parfenov, 1984; Parfenov et al., 1993; Oxman et al., 1995), which is composed of thick shelfal and continental-slope sediments deposited along the margin of the Siberian craton and is referred to as the Verkhoyansk complex. The Verkhoyansk complex is Mississippian to Early Cretaceous in age. Important constituents of the Kolyma loop include microcontinental fragments with an early Paleozoic to Cretaceous sedimentary cover, such as the Okhotsk, Omolon, Prikolymsk, and Omuliovka blocks. These are separated by zones of imbricate thrust sheets composed of late Paleozoic and Mesozoic deposits similar to the Verkhoyansk complex (Fig. 1) (Gagiyev, 1992; Natapov and Shul'gina, 1991; Savostin et al., 1994). Over a significant portion of its length (-700 km), the South Anyui suture zone is bounded by the structures of the Alazeya-Oloy fold zone (Til'man, 1973; Parfenov, 1984) (Fig. 1). This zone consists of a number of terranes composed of Paleozoic metamorphic rocks, Paleozoic to Mesozoic clastic and sedimentary-volcanic complexes, and ophiolites (Parfenov et al., 1993; Nokleberg et al., 1994). Late Paleozoic to Mesozoic island-arc complexes are believed to be widespread within the Alazeya-Oloy zone (Seslavinskiy, 1979; Parfenov, 1984; Natal'in, 1984; Natapov and Surmilova, 1986; Natapov and Shul'gina, 1991). The Anyui-Chukotka fold belt of Til'man (1973), Parfenov et al. (1993), and Nokleberg et al. (1994) includes (1) the Western Chukotka terrane, (2) the Vel'may terrane, and (3) the Eastern Chukotka terrane (Fig. 1). The Western Chukotka terrane is composed of Paleozoic and Mesozoic, largely clastic strata deposited in a passive continental-margin setting. Triassic tur-bidites show a southerly direction of sediment transport (modern reference frame) (Til'man, 1973; Morozov, 1996). The Vel'may terrane is composed of volcanic, clastic, and chert-bearing sequences that have yielded Upper Triassic fauna, as well as minor ultramafic, gabbro, and plagiogranite bodies (Tynankergav and Bychkov, 1987). The Eastern Chukotka terrane is composed of Proterozoic metamorphic rocks and their Paleozoic sedimentary cover (Zhulanova, 1990); these are markedly similar to the Seward Peninsula complexes (Alaska) and are regarded as a single terrane (Nokleberg et al., 1994).
The South Anyui suture zone can be traced for more than 1600 km from the eastern Laptev Sea in a southeasterly direction to the north part of the Primorsk depression (Rusakov and Vinogradov, 1969; Spektoret al., 1981; Sekretov, 1993) into the upper reaches of the Bol'shoi Anyui River (Fig. 2). The South Anyui suture zone is not yet clearly defined. Some workers define the South Anyui suture zone to comprise only late Mesozoic clastic deposits and the associated ophiolitic sequences of the Polyarninsk block and the Bol'shoi Lyakhovsky Island (Radziwill, 1964; Til'man, 1973; Dovgal' et al., 1975; Seslavinskiy, 1970; Parfenov, 1984; Til'man and Bogdanov, 1992; Drachev and Savostin, 1993). Others believe the South Anyui suture zone (sensu lato) to be much broader and to include the Paleozoic to lower Mesozoic sedimentary and volcanic sequences and Paleozoic ophiolites found in the adjacent Yarak-vaam terrane (Ged'ko et al., 1991; Lychagin et al., 1991). Natal'in (1984) suggested that the South Anyui suture zone also includes the Late Jurassic-Early Cretaceous complexes of the Nutesyn and Oloy zones. To the southeast, rocks of the South Anyui suture zone are unconformably overlain by volcanic rocks of the Albian-Upper Cretaceous Okhotsk-Chukotka volcanic belt, and its southward continuation from there is debatable (Natal'in, 1984; Til'man and Bogdanov, 1992; Zonenshain et al., 1990). In general, its continuation is believed to be defined by sporadic outcrops of ultramafic rocks, gabbro, and clastic-chert-volcanic sequences in the vicinity of the Vel'may terrane in eastern Chukotka (Natal'in, 1984; Parfenov, 1984). The South Anyui suture zone consists chiefly of highly deformed clastic deposits (flysch) that were previously referred to as Upper Jurassic to Lower Cretaceous (Radziwill, 1964; Dovgal' et al., 1975; Seslavinskiy, 1970, 1979; Til'man, 1973; Natal'in, 1984; Parfenov, 1984). In some localities, however, litho-logically similar rocks have yielded Upper Triassic faunal assemblages (Bychkov and Solov'yev, 1992; Glotov, 1995). These fossil finds bring into question the assignment of all of these rocks to the Late Jurassic to Early Cretaceous. In the South Anyui suture zone, rocks mapped as terrigenous clastic deposits (flysch) locally contain exposures of ultramafic rocks, gabbro, and chert that have been previously interpreted as fragments of dismembered ophiolites (Pinus and Sterligova, 1973; Dovgal'etal., 1975; Natal'in, 1984; Parfenov, 1984; Lychagin, 1985; Lychagin et al., 1991). Most workers (Pinus and Sterligova, 1973; Til'man, 1973; Seslavinskiy, 1979; Natal'in, 1984; Lychagin et al., 1991; Parfenof, 1984) assign the mafic rocks and cherts to the Upper Jurassic to Lower Cretaceous sequence because of the presumed age of the enclosing clastic deposits. However, no direct evidence exists for the age of the ophiolitic rocks. The first direct ages for the ophiolitic sequences were determined from rocks that make up south-vergent thrust slices in the Polyarninsk block (Fig. 2). Here, ophiolitic sequences comprise serpentinized oceanic lherzolite and harzburgite, orthoamphibolite, cumulate gabbro, anorthosite, plagiogranite, sheeted diabase dikes, and oceanic pillow basalts that are associated with Middle to Late Jurassic radiolarian cherts (Ged'ko et al., 1991) (Fig. 2, cross section 3-4). Another ophiolite is associated spatially with upper Paleozoic sedimentary-volcanic rocks of probable island-arc affinity (Lychagin et al., 1991). Subsequent studies have provided evidence for a Paleozoic age for some of the ophiolitic rocks of the South Anyui suture zone (Lychagin, 1985; Lychagin et al., 1991; Ged'ko et al., 1991). In Bol'shoi Lyakhovsky Island, dismembered ophiolites are thrust to the northwest over Permian-Triassic shale and sandstone similar to those of the Chukotka terrane (Drachev and Savostin, 1993). The ophiolites comprise orthoamphibolite (473 Ma; K-Ar whole-rock dating, Drachev and Savostin, 1993), lherzolite, harzburgite, and pillow basalt (291 + 62 Ma; Sm-Nd mineral and whole-rock dating, Drachev and Savostin, 1993). The ophiolites are overthrust by Early Cretaceous island-arc an-desites and crosscut by granitoids (100-110 Ma; K-Ar whole-rock dating, Drachev and Savostin, 1993). The Aluchin and Vurguveem ophiolites were included by Ged'ko et al. (1991) and Lychagin et al. (1991) as parts of the South Anyui suture zone (sensu lato). These workers suggested that these ophiolites (Fig. 2) are late Paleozoic, based on the oldest K-Ar data from plagiogranite intrusions of the Vurguveem ophiolite (250 Ma, whole rock, Palymskaya and Palymsky, 1975) and Aluchin ophiolite (231 Ma, K-Ar data, whole rock, Dovgal' et al., 1975; and 374 Ma, K-Ar data, whole rock, Glotov, 1995). Upper Triassic conglomerates and sandstones were deposited unconformably on the Aluchin ophiolite and older complexes of the Yarakvaam terrane (Orlovka River), and contain spinel, chromite, and clasts of ultramafic rocks, gabbro, and plagiogranite (Dovgal' et al., 1975; Cecile et al., 1991). There are pebbles of Vurguveem gabbro and plagioganite in Upper Triassic and Middle Jurassic rocks of the Yarakvaam terrane (Palymsky and Palymskaya, 1975; Shekhovtsov, 1991). However, assignment of the Aluchin and Vurguveem ophiolites to the South Anyui suture zone remains debatable.
In 1997 and 1998, we carried out geological studies in the eastern part of the South Anyui suture zone. We focused our efforts in a transect (Fig. 2) that crosses the South Anyui suture zone from the Alazeya-Oloy folded zone to the south to the Anyui-Chukotka fold belt to the north. A preliminary summary of these studies, in which we describe the structural elements of the zone from south to north, is given in the following. These elements include the Yarakvaam terrane, the South Anyui suture-zone complex, and the Kulpolney island-arc complex. The Yarakvaam terrane, which is the most structurally intact terrane of the South Anyui suture zone, is bounded by shear zones on the south, north, and west and is overlapped by Okhotsk-Chukotka volcanic belt volcanic rocks on the east (Figs. 1 and 2). The Vurguveem (Gromadnensky) ophiolite massif occurs along the boundary between the Yarakvaam terrane and the South Anyui suture zone (Figs. 2 [segment 1'-2'] and 3). The ophiolitic rocks make up a thrust package that verges northward and consists of layered gabbro, ferro-gabbro, and gabbronorite with a minor amount of ultramafic rocks and orthoamphibolite (Lychagin, 1985; Lychagin et al., 1991). The mafic rocks are cut by plagiogranites and granodi-orites that yield two groups of K-Ar whole-rock dates (Palymskaya and Palymsky, 1975; Shekhovtsov, 1991): 252-207 Ma (latest Permian to Late Triassic) and 179-147 Ma (Middle to Late Jurassic). During field work in the south part of the Vurguveem massif in 1998, we mapped a suite of compositionally variable (diabase-andesite-dacite) sheeted dikes. The dikes are subvertical and trend chiefly 100°-140°SE. The lower part of the dike complex contains gabbro and gabbro-diabase screens, and the upper part contains screens of extrusive and pyroclastic rocks. Compositionally similar dikes and sills and their extrusive counterparts are widespread in a thick Carboniferous-Permian sedimentary-volcanic sequence of island-arc affinity belonging to the Yarakvaam terrane (Parfenov et al., 1993). This implies that the late Paleozoic sedimentary-volcanic sequence unconformably overlies the plutonic rocks of the Vurguveem massif. Basal conglomerates of Early Permian (?) age were described by Palymskaya and Palymsky (1975). Therefore, the plutonic rocks of the Vurguveem massif together with the overlying island-arc volcanic rocks might compose a single late Paleozoic arc and suprasubduction-zone ophiolitic complex. We interpret the gabbro to be a fragment of lower crust associated with an island arc, perhaps similar to the Tonsina assemblage in Alaska (De Bari and Coleman, 1989). The island-arc volcanic rocks are among the oldest stratified rocks of the Yarakvaam terrane. These stratified rocks consist of shallow-marine and continental sedimentary and volcanic rocks (mafic through felsic volcanic rocks, and pyroclastic and clastic rocks with intercalated limestone) and contain Carboniferous and Permian faunas and floras (Radziwill, 1964; Palymskaya and Palymsky, 1975; Yegorov, 1990; Shekhovtsov, 1991; Glotov, 1995) (Fig. 2, column B ). The Carboniferous-Permian sequence is riddled with mafic, intermediate, and felsic dikes and sills and is cut by minor plagiogranite, diorite, and gabbro-diabase intrusions. This late Paleozoic island-arc volcanic sequence is overlain unconformably by transgressive Upper Triassic clastic rocks. From the center to the northern boundary of the South Anyui suture zone, shallow-marine facies give way to deeper water turbidites and submarine-slump deposits (Shekhovtsov, 1991; Yegorov, 1990; Bychkov and Solov'yev, 1992). Faunal assemblages in these deposits include Tethyan species (Afizkiy, 1970). In the west, Upper Triassic conglomerate unconformably overlaps the Aluchin ophiolite (Dovgal' et al., 1975; Cecile et al., 1991). These relationships imply a pre-Late Triassic accretion or amalgamation of the Aluchin ophiolite to the Yarakvaam terrane (possibly between the end of the Permian and the beginning of the Late Triassic). Higher in the section, Lower to Middle Jurassic clastic rocks and Oxfordian to Valanginian pyroclastic-terrigenous rocks occur (Afizkiy, 1970; Shekhovtsov, 1991). The southwest part of the Yarakvaam terrane (Vukvaam zone, after Gulevich, 1975) displays a widespread sequence of compositionally variable island-arc volcanic rocks of Oxfordian to Volgian age (Natal'in, 1984). Along the boundary with the South Anyui suture zone west of the Yarakvaam terrane, the Volgian to Valanginian part of the succession comprises a volcanic sequence with layers of intercalated tuff, conglomerate, and sandstone. Basalts and andesites within the volcanic sequence are alkalic in composition. The island-arc kinship of these volcanic rocks is thus not readily apparent (Gulevich, 1975; Parfenov, 1984), and further studies are required to elucidate their geodynamic setting. It has long been proposed that the eastern part of the Alazeya-Oloy zone is underlain by older continental basement (Yablonsky massif) that is in turn part of the Omolon massif (Til'man, 1973; Grinberg and Rudich, 1981; Natal'in, 1984). However, the relationships just described between the Vurgu-veem gabbro and the intruding dikes and sills with the uncon-formably overlying Paleozoic island-arc suites are at variance with this hypothesis; hence, the interpretation that the Yarakvaam terrane is of island-arc affinity (Parfenov et al., 1993; Nokleberg et al., 1994) is more reasonable.
A system of high-angle fault-bounded tectonic slices that verge chiefly north make up the central part of the South Anyui suture zone (Figs. 2 and 3). Their constituents, from south to north, are as follows.
Basalt-chert complex
The basalt-chert complex makes up the southern, tectoni-cally bounded slices in the South Anyui suture zone and is overridden by the gabbro of the Vurguveem massif to the south (Figs. 2 and 3). The sequences within individual tectonic slices are fragmentary and differ in the number and thickness of basalt flows, presence or absence of chert (including red radiolarian chert), and the presence or absence of interpillow carbonate lenses and layers. The basalt-chert sequences in the tectonic slices are separated by melanges of tuff and terrigenous material. The matrix of olistostrome includes some clasts of felsic tuff. Geochemically, the basalts are similar to oceanic or backarc basin basalts (Natal'in, 1984; Lychagin et al., 1991). The cherts yield the radiolaria Haliodictya cf. Hojnosi Riedel and Sanfilippo, Stichocapsa convexa Yao, Williriedellum sp., and Zhamoidellum sp., which is a typical Bajocian-Kimmeridgian fauna (according to N.Yu. Bragin, Geological Institute, Russian Academy of Sciences, Moscow).
Sedimentary melange
The sedimentary melange consists of tuffaceous clastic rock exhibiting various degrees of tectonization. The melange contains olistoliths and blocks of various sizes composed of basalt, andesite, chert, gabbroic rocks, and plagiogranite that resemble parts of the Vurguveem ophiolite. The extrusive rocks comprise both mid-ocean ridge basaltlike rocks and medium-Ti, high-Al tholeiitic basalts, andesites, and andesitic dacites of island-arc affinity. The blocks of the melange are arranged in lenses and boudinaged layers within which the rocks are lithologically uniform. The matrix of the melange displays sedimentary structures, such as rocks within the layers showing submarine-slump structures, evidence of redeposition by bottom currents, and horizons of grain-flow deposits and pebbly mudstone or diamictite and turbidites. Some tectonic slices within the clastic terrigenous melange that are composed of turbidites yield Oxfordian to Volgian and Berriasian to Valanginian faunas (Radziwill, 1964; Radziwill and Radziwill, 1975; Glotov, 1995). Similar lithologies are found in the matrix of the melange, suggesting that the fragments in the matrix are of the same age as the rocks that compose the tectonic slices. Direct evidence for the age of the matrix is absent, however, and it may contain fragments of various ages. The sedimentary melange shows various degrees of tectonization, and in places it is metamorphosed to greenschist facies. Textures common to accre-tionary melanges, such as C-S tectonites, broken formations, and block-in-matrix structures, are widespread in the sedimentary melange unit and are comparable to those found in accretionary melanges elsewhere around the world (Cowan, 1985).
Terrigenous complex
The terrigenous complex comprises alternating terrigenous turbidite and shale with sulfide-carbonate concretions and calcareous sandstone intercalations. Sandstone grains consist of quartz (80%-90%), feldspar, and less abundant sedimentary rocks and pyroxene. Measurements of the persistent system of bottom current marks suggest a north-to-south clastic supply (present reference frame). These deposits were previously assigned to the Upper Jurassic to Lower Cretaceous (Radziwill, 1964; Seslavinskiy, 1979; Natal'in, 1984). Geologists from the Bilibino Geological Survey, however, established that the rocks that yield the Late Jurassic to Early Cretaceous faunas are distinguished by a more polymictic sandstone composition. These turbidites are lithologically similar to the Triassic deposits of the Anyui-Chukotka fold belt, which are tentatively dated as Triassic (Glotov, 1995). One turbidite sample, collected in 1997, yielded sporadic, poorly preserved conodont remains (as determined by V.A. Aristov, Geological Institute, Russian Academy of Sciences). Farther west along the strike of the transect, similar terrigenous rocks within the South Anyui suture zone have yielded Upper Triassic faunas (Glotov, 1995). The terrigenous clastic complex is thus more likely to be Triassic than Upper Jurassic to Lower Cretaceous in age.
Turbidite complex
This complex of subvertical to steeply northward-dipping tectonic slices consists of distal and proximal turbidites with interbeds of high-Ti subalkalic pillow basalt and picritic basalt. The clasts in the sandstones show evidence of explosive vol-canism of felsic composition. Faunal remains in the terrigenous rocks yield Volgian ages (Glotov, 1995). Preliminary geochem ical analysis suggests that the picritic basalts formed in a supra-subduction-zone setting that was undergoing extension (Sharaskin et al., 1983). This rock assemblage may have formed in the frontal (distal) part of an island arc. Tectonic slices composed of volcaniclastic turbidites with an admixture of tuffa-ceous material occur farther north. The amount of volcanic material increases toward the north, and grain-flow and clastic gravity-flow deposits such as olistostromes with olistoliths and pebbles of tuffs and volcanic rocks of mafic and intermediate to felsic composition appear. The turbidites yield Volgian to Valanginian faunas (Glotov, 1995). The coarsening of the turbidite elastics and the abundant volcanic clasts suggest that these deposits were derived from an arc and were deposited in a proximal forearc setting. The relationships between the turbidite and terrigenous complexes described here are tectonic. Fault-bounded wedges of shallow-water pebble conglomerate are present within the tectonic slices of Upper Jurassic to Lower Cretaceous polymict turbidites. Clast composition is diverse, but is dominated by lithologies derived from the erosion of the South Anyui suture zone units. These deposits can be regarded as deformed neoautochthonous strata. The age of the shallow-water conglomerate is Valanginian to Hauterivian according to Yegorov (1990), and Hauterivian to Barremian according to Dovgal' et al. (1975). The new ages (Yegorov, 1990) pose further limitations on the final time of closing of the South Anyui basin.
Kulpolney island-arc complex
This complex in the northern part of the South Anyui suture zone (Fig. 2) is dominated by various types of volcanic rocks that include pillow lavas and massive amygdular basalts, andesites, dacites, and their subvolcanic and pyroclastic counterparts. Volcanic flows are interbedded with thin lenses of tuff, ash beds, siliceous shale, and interpillow jasperoid chert. Field evidence indicates that, in places, the volcanic complex is in stratigraphic continuity with the underlying turbidite sequence. The upper part of the turbidite sequence includes tuff and lava horizons, and the lower part of the volcanic sequence contains tuffa-ceous layers. Lateral transitions and facies changes between the upper part of the turbidite section and the lower part of the volcanic section might also be present. These relationships imply a Late Jurassic to Early Cretaceous (?) age for the volcanic complex. The volcanic rocks display calc-alkaline and subalkaline trends and may have formed in an island-arc setting. Structurally upsection in the Kulpolney island arc, interme-diate-felsic extrusive rocks and associated tuffaceous and terrigenous deposits have stratigraphic positions with respect to underlying rocks that remain unclear. They either cap the Upper Jurassic to Lower Cretaceous island-arc sequence (Glotov, 1995) or make up the volcanic fill of the younger, and mostly overlying, Nutesyn depression of Early Cretaceous age (Shekhovtsov, 1991). The issue of what constitutes the basement of the Kulpolney island arc remains open to discussion. Most workers (Natal'in, 1984; Parfenov, 1984; Til'man and Bogdanov, 1992; Morozov, 1996) believe that this island-arc complex formed along the margin of the Chukotka microcontinent. Our investigations, however, have not found any direct relationships between the Kulpolney complex and the sedimentary cover of the Chukotka terrane. Furthermore, the thrust-bounded wedges of Lower Jurassic sandstone and siltstone occur along the boundary between the Chukotka terrane and the Kulpolney island-arc sequence. These clastic rocks have polymict compositions and differ from that of the cover of the Chukotka terrane (Glotov, 1995).
The first detailed structural studies in the South Anyui suture zone were carried out by Natal'in (1984), who identified thrust- and strike-slip-related deformation of various ages. Through our field work, we collected additional structural data and recognized four main stages of tectonic deformation of Mesozoic age, which we describe in the following. From oldest to youngest, these include thrusting of probable pre-Late Triassic age, Late Jurassic to Early Cretaceous thrusting and dextral strike-slip faulting, and post-Albian sinistral strike-slip faulting.
Stage 1
Deformation associated with stage 1 is best developed in the gabbroic rocks of the Vurguveem ophiolite (Fig. 4, A and B). It is represented by isoclinal Fl folds confined to dynamother-mally metamorphosed zones and amphibolite schists (metamorphosed gabbro (Fig. 4B). Fold axes trend roughly east-west. Tectonic transport was directed from south to north. Figure 4A shows plagiogranite veins affected by late-stage deformation supporting this transport direction. The age of plagiogranite is late Paleozoic (Palymskaya and Palymsky, 1975). This deformation took place later. We suppose that the deformation accompanied the late Paleozoic ophiolite accretion. This accre-tionary event is marked by Upper Triassic stratigraphic unconformity at the edge of the Yarakvaam terrane. In terms of geometry, later deformational structures in the Vurguveem ophiolite are similar to fabrics of stages 2, 3, and 4 (described in the following). Among these structures are the thrust faults of north and south vergence and the strike-slip faults that were documented by Natal'in (1984). According to Natal'in (1984), the tectonic history of the Vurguveem ophiolite plutonic rocks was increasingly complicated. We have observed very complicated fold structures of magmatic layering and the presence of high-temperature shear zones, but we believe that these are related to the magmatic and metamorphic history of the Paleozoic plutonic rocks of the Vurguveem ophiolite. We do not address these deformations, but focus instead on the identification and interpretation of the main tectonic fabrics of Mesozoic age.
Stage 2
This stage of deformation is characterized by two substages of deformation (a and b) of similar age (Figs. 3C and 4, C and D). Both substages are manifest in the accretionary melange, the Upper Triassic terrigenous complex, and the Upper Jurassic to Lower Cretaceous turbidites, and are characterized by isoclin-ally folded bedding (Fig. 4, C and D) and an axial-plane Sj penetrative cleavage. The fold axes trend north-northwest-south-southeast (Fig. 3, stereogram B), and the cleavage is strongly deformed by subsequent events.
Substage 2a
The characteristic feature of substage 2a is the southward vergence of folds and an axial-plane cleavage (Fig. 4D). We have identified substage 2a deformation only in some outcrops of the Late Triassic terrigenous complex and the Upper Jurassic-Lower Cretaceous clastic turbidites. The south-vergent folds of this substage are located in the northern part of the South Anyui suture zone. We propose that substage 2a deformation was synchronous with subduction beneath the Kulpolney island arc, in the forearc region or accretionary prism. The deformation of substage 2a is the result of the formation of an accretionary wedge in Late Jurassic-Early Cretaceous time.
Substage 2b
The deformation of substage 2b is characterized by northward vergence of the folds and development of an axial-plane cleavage (Figs. 3 [stereograms B and C] and 4C). This deformation is similar to the F3, F2, and F4 folds described by Natal'in (1984). These features are more widespread than substage 2a deformational features, especially in the southern part of the South Anyui suture zone, and the zones of dy-namothermal metamorphism are characterized by more intensive development and a greater strain. These zones locally also develop the ductile C-S tectonite fabrics. Substage 2b deformation may be due to compression directed roughly north-south. The lower age limit of this deformation is delimited by the fact that the folds affect the Upper Jurassic to Valanginian rocks of the terrigenous melange; hence deformation must be Valanginian or younger in age. We propose that deformation occurring in substage 2b is associated with the earliest phase of collision of the Siberian and Chukotka microcontinents. As a result, the deformation of substage 2b is superposed on the synaccretion deformation of substage 2a. This deformation began in the southern part of the South Anyui oceanic paleobasin and then migrated to the north.
Stage 3
Structures associated with stage 3 deformation are widespread in the South Anyui suture zone and its immediate vicinity. We believe that these structures are related to motion along the east-west- and west-northwest-east-southeast-trending subvertical dextral strike-slip faults. Conical folds of bedding with a new axial-plane cleavage are developed chiefly in this event and characterized by subvertical axes (Fig. 3, stereograms A and B). Lense-shaped zones of tectonic melange are also present and are thought to be related to this event. Boudin axes in these tectonic melange zones are subvertical (Fig. 3, stereogram C). C-S structures indicating dextral strike-slip fabrics are ubiquitous at all scales of observation (Fig. 5A). Z-shaped folds of bedding, foliation including the boundaries of tectonic slices, and boudins with subvertical long axes in the tectonic melange zones are common at all scales of observation. This is illustrated by the Z-shaped form of the Vurguveem ophiolite massif (Fig. 5B). The upper age limit for the dextral strike-slip faults is delimited by the fact that this deformation affects rocks as young as those of the Upper Jurassic-Lower Cretaceous turbidite complex.
Stage 4 (postcollisional)
Structures related to stage 4 deformation are developed in conjunction with roughly east-west-trending brittle sinistral strike-slip faults, the locations of which may be inherited from preexisting fault planes. Sinistral strike-slip faulting is demonstrated by the local development of pressed conical folds with subvertical axes within zones of high deformation only as thick as a few tens of meters. The domains between these zones show only open folds of bedding and cleavage and widely spaced tension fractures. The sinistral strike-slip faults offset the Albian-Cenomanian rocks of the Okhotsk-Chukotka volcanic belt, and are conjugate with north-northeast-south-southwest-trending strike-slip faults, to which belts of Upper Cretaceous dikes are confined.
Paleotectonic interpretation of the South Anyui suture zone
Previous studies together with new data establish the presence in the southern part of the South Anyui suture zone of chert-basalt complexes of Bajocian to Kimmeridgian age, which implies the former presence in the region of a South Anyui oceanic basin (sensu stricto). The sedimentary melange complex can be interpreted as the relic of an accretionary prism. Furthermore, the ophiolite elastics in the melange, derived from the erosion of the Yarakvaam terrane and Vurguveem ophiolite, together with the northerly vergence of the tectonic slices, suggest a spatial association of the accretionary prism with the Vukvaam segment of the island arc. This segment developed along the northern margin of the amalgamated terranes of the Alazeya-Oloy fold belt (Fig. 6B). Our studies also document the existence of clastic and volcanic deposits of the Kulpolney island arc developed in the northern part of the South Anyui suture zone (Fig. 6C). The Late Jurassic Epoch is characterized by the north-dipping subduction of the South Anyui oceanic crust beneath the Chukotka micro-continent (Natal'in, 1984) (Fig. 6C). The spatial relationships of the Upper Jurassic island-arc complexes with the turbidite deposits of the South Anyui suture zone further support this view. The main axis of volcanism related to the Kulpolney island arc was to the north. To the south, these volcanic rocks are inferred to interfinger with proximal and distal turbidites deposited in a forearc (Fig. 6C). Picritic basalts probably were erupted in local zones of extension in the frontal part of the forearc, similar to picrite-basalt volcanism in the Tonga arc (Sharaskin et al., 1983). The location of this arc along the margin of the Chukotka microcontinent (Fig. 6C) is only an inference. We cannot exclude the possibility that the Kulpolney island arc developed in an intraoceanic setting similar to the Koyukuk island arc of Alaska (Moore et al., 1994). Note that our data support the notion (Parfenov, 1984; Natal'in, 1984; Nokleberg et al., 1998) of a degree of similarity between the South Anyui suture zone complexes and the Angayucham terrane (Moore et al., 1994).
The new data on the age of the ophiolites, pelagic cherts, and basalts in the South Anyui suture zone and Alazeya-Oloy zones testify to the existence of an Anyui (South Anyui) oceanic basin between the Siberian and North American plates in Paleozoic and early Mesozoic time (e.g., Fujita and Newberry, 1983; Zonenshain and Natapov, 1988; Zonenshain et al., 1990; Golonka et al., 1994; Nokleberg et al., 1998). The age of formation and early history of this basin remain relatively poorly known. Its youngest deposits are pelagic chert and shale of Ba-jocian to Kimmeridgian age. In Paleozoic time the Anyui oceanic basin may have been connected, via Taimyr, with the paleo-Urals Ocean. The presence of late Paleozoic ophiolites in the Polar Urals region (Ruzhentsev and Aristov, 1998) (Fig. 1) suggests that the Polar Urals Ocean closed in late Paleozoic time. Simultaneously, through the collision of Siberia and the Kara microcontinent, the Taimyr basin ceased to exist (Vernikovsky, 1996). Various viewpoints exist concerning the eastward continuation of the Anyui oceanic basin. Most workers (Zonenshain et al., 1990; Parfenov, 1984; Til'man and Bogdanov, 1992) suggest that the Anyui basin was a reentrant of the paleo-Pacific. We believe that, starting at least in the late Paleozoic, a convergent boundary separated the Anyui basin from the northwest Pacific (Morozov, 1996; Sokolov et al., 1997) (Fig. 6A). At the end of the Paleozoic and in the early Mesozoic, after the folding in the western (present reference frame) branch of the Anyui basin within the Mesozoides of northeastern Russia, the Anyui oceanic basin was still in part preserved, but had a rather complex configuration (Fig 6B). The widespread record of late Paleozoic to early Mesozoic island-arc magmatism in the Alazeya-Oloy zone together with evidence for tectonic deformation of the ophiolite sequences of the Chersky Range (Oxman et al., 1995) suggest a shrinking of the Anyui basin. In pre-Late Triassic time, an amalgamation of island-arc terranes (e.g., Yarakvaam, Alazeya) and oceanic terranes (e.g., Aluchin) took place. The newly formed heterogeneous basement provided the site for the development of the younger Alazeya-Oloy island arc (Late Triassic to Lower Jurassic), which rimmed the southern margin of the South Anyui oceanic basin (present reference frame) (Seslavin-skiy, 1979; Parfenov, 1984; Natal'in, 1984). Paleomagnetic data analyzed by A.N. Didenko (Sokolov et al., 1997) suggest that the Kolyma loop terranes once formed a single megablock together with the Siberian plate, beginning at least in Middle Devonian time. Before the Middle Jurassic these terranes had not been rigidly connected with Siberia, yet their motions were characterized by similar paths. In post-Middle Jurassic time these blocks finally became permanently incorporated into the Eurasian plate. Geologic data (Parfenov et al., 1993) suggest that during this time, in the Middle Jurassic, the Kolyma loop terranes became a single Kolyma-Omolon super-terrane, and ophiolites were obducted in the Chersky Range (Fig. 6B) (Oxman et al., 1995). In Late Jurassic time new convergent boundaries developed along the modified margin of the northeastern Asian continent. At the Pacific boundary, the Uda-Murgal convergent margin developed upon a heterogeneous basement (Sokolov et al., 1997, 1998) (Fig. 6C). Geologic data suggest that this margin continued into the southern part of the Chukotka Peninsula (Morozov, 1996) and into the southern part of Alaska, probably into the To-giak terrane (Nokleberg et al., 1994), or the Chitina arc (Parfenov and Spector, 1997). This convergent boundary separated the South Anyui basin from the Pacific (Fig. 6C). In Late Jurassic to Early Cretaceous time the Svyatoi Nos-Oloy island arc developed along the boundary between the Asian continent and the South Anyui basin (Til'man et al., 1977; Natapov and Surmilova, 1986). The Vukvaam island arc is a representative part of this larger arc (Natal'in, 1984). The Kulpol-ney arc developed along the north side of the South Anyui basin. It can therefore be inferred that oceanic crust of the South Anyui basin was consumed by subduction along both its northern and southern boundaries, bringing about its rapid closure and demise. The Chukotka-Eurasia collision that resulted from this closure culminated at the end of the Early Cretaceous, approximately synchronous with the onset of seafloor spreading in the Canada basin (Embry and Dixon, 1990; Grantz and May, 1983; Grantz et al., 1990). After the collision the Nutesyn erogenic basin developed north of the South Anyui suture zone, and the Aynakhkurgen orogenic basin formed south of it. Both of these basins filled with Hauterivian to Albian sedimentary deposits and volcanic rocks. Our paleotectonic interpretation, which is presented in Figure 6, is a very general one and is offered as a working hypothesis that requires further refinement and verification. Future studies must focus on the ophiolites in order to (1) elucidate the time of inception of the South Anyui basin and (2) compare these ophiolites with coeval ophiolites in the Chersky Range, Polar Urals, Taimyr, and the northwest Pacific margin. The Paleozoic ophiolites (e.g., the Aluchin ophiolite) lack demonstrable volcanic members, whereas the South Anyui suture zone Mesozoic (Bajocian to Kimmeridgian) oceanic assemblages are not known to contain ultramafic-gabbroic complexes. This might be due to a poor knowledge of the geologic details of this region or to the poor degree of preservation of these ophiolites. Otherwise, a satisfactory explanation must be found for this fact, which appears to support the idea of the rift-related origin of the South Anyui suture zone (Seslavinskiy, 1970, 1979; DovgaT et al., 1975; Til'man et al., 1977). It remains unclear whether the Anyui basin was a vast ocean or a minor basin. Tackling this issue requires a paleomagnetic study of the terranes north and south of the South Anyui suture zone. The Late Jurassic to Early Cretaceous syncollisional defor-mational events that affected rocks of the South Anyui suture zone were closely related to thrust structures, which likely reflect the same event. The thrusting must have marked the initial stage of a continent-to-continent collision: during this stage collision developed orthogonally. The final stage of continental collision developed by oblique slip and was accompanied by transpression and dextral strike-slip faults along the South Anyui suture zone. Some thrusts may have formed through a symmetric or asymmetric flower structure-type arrangement at right angles to the suture. Such phenomena are widespread in zones of transpression (Sylvester, 1988; Woodcock and Fischer, 1986; Bondarenko, 1996). The map distribution of Upper Jurassic to Early Cretaceous island-arc complexes along the South Anyui suture zone (Fig. 1) shows gaps. These gaps might be due to segments where the continental and oceanic plates interacted along strike-slip faults. Some of the volcanic complexes currently categorized as arc related might have formed in pull-apart structures adjacent to strike-slip faults. That dextral displacements played an important role suggests that the collision involved a counterclockwise rotation of the Chukotka microcontinent relative to Siberia. This rotation may have been due to differential rifting in the Canada basin (Embry and Dixon, 1990). Specifying the dynamics of collision between the continental masses will require the determination of reliable timing of the deformation stages identified herein.
The tectonic interpretation proposed for the South Anyui suture zone remains a working hypothesis because many structural details and fundamental issues about the evolutionary history of this basin are not yet understood. At the same time, this geological summary highlights the key role of the South Anyui suture zone to our understanding of the tectonic history and palinspastic reconstructions of northeast Asia and the Arctic. Understanding this history would further refine the scenario for the interaction between the Eurasian, North America, and Pacific plates. In discussing the existing viewpoints and in criticizing the available geological data, our purpose has been to suggest approaches that will be lucrative for future South Anyui suture zone studies. We believe that the first priorities for study are (1) a more detailed study of the ophiolites, (2) a geochemical study of the Late Jurassic to Early Cretaceous volcanic rocks, (3) study of the stratigraphy and sedimentology of the terrigenous sequences, and (4) a paleomagnetic study of the terranes that flank the South Anyui zone.
We thank V.T. Burchenkov, director of the Anyui Governmental Mining and Geological Enterprise (Bilibino) for his help in our field work. A. Ishiwatari (Kanazawa University, Japan) assisted in the field study of the Aluchin and Vurguveem ophiolites. This work was supported by the Russian Foundation for Basic Research (projects 97-05-65711, 98-07-90015, 99-05-65649, and 00-07-90000), the International Association for the Promotion of Cooperation with Scientists from the New Independent States of the Former Soviet Union (INTAS) (grant 96-1880), the North Atlantic Treaty Organization (NATO) (grant 97-5739), and Young Scientists Russian Academy of Science grant 1999 to O. Morozov. We are grateful to the reviewers R.G. Coleman, M. Cecile, and B. Natal'in for well-disposed criticism. We also thank Elizabeth Miller and Arthur Grantz for their fruitful suggestions, which helped to improve this text.


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