Abstract. A total of 11 700 km of multichannel seismic reflection data were acquired during recent reconnaissance surveys of the wide, shallow shelves of the Laptev and western East Siberian Seas around the New Siberian Islands. To the north of the Laptev Sea, the Gakkel Ridge, an active mid-ocean ridge which separates the North American and Eurasian Plates, meets abruptly the steep slope of the continental shelf. Extension has affected the Laptev Shelf since at least the Early Tertiary and has resulted in the formation of three major, generally north-south trending rift basins: the Ust' Lena Rift, the Anisin Basin, and the New Siberian Basin. Our data indicate that the rift basins on the Laptev Shelf are not continuous with those on the East Siberian Shelf. The latter shelf can best be described as an epicontinental platform which has undergone continuous subsidence since the Late Cretaceous. The greatest subsidence occurred in the NE, manifested by a major depocentre filled with inferred (?)Late Cretaceous to Tertiary sediments up to 5 s (twt) thick. On the basis of deep reflection data we revise and adjust Mesozoic domain boundaries around the New Siberian Islands.
Abstract. The study area is part of the Anyui subterrane of the Chukotka microplate, a key element in the evolution of the Amerasia Basin, located in Western Chukotka, Northeast Russia. The subterrane contains variably deformed, folded and cleaved rhythmic Triassic terrigenous deposits which represent the youngest stage of widespread marine deposition which form three different complexes: Lower-Middle Triassic, Upper Triassic (Carnian) and Upper Triassic (Norian). All of the complexes are represented by rhythmic interbeds of sandstone, siltstone and mudstone. Macrofaunas are not numerous, and in some cases deposits are dated by analogy to, or by their relationship with, other units dated with macrofaunas. The deposits are composed of pelagic sediments, low-density flows, high-density flows, and shelf facies associations suggesting that sedimentation was controlled by deltaic progradation on a continental shelf and subsequent submarine fan sedimentation at the base of the continental slope. Petrographic study of the mineral composition indicates that the sandstones are lithic arenites. Although the Triassic sandstones appear similar in outcrop and by classification, the constituent rock fragments are of diverse lithologies, and change in composition from lower grade metamorphic rocks in the Lower-Middle Triassic to higher grade metamorphic rocks in the Upper Triassic. This change suggests that the Triassic deposits represent an unroofing sequence as the source of the clastic material came from more deeply buried rocks with time.
Abstract. The compilation, synthesis, description, and interpretation of regional geology and tectonics of major regions, such as Northeast Asia (Eastern Russia, Mongolia, Northern China, South Korea, and Japan) and the Circum-North Pacific (the Russian Far East, Alaska, and Canadian Cordillera), requires a complex methodology that includes: (1) definitions of key terms; (2) compilation of a regional geodynamics map that can be constructed according to modern tectonic concepts and definitions; and (3) formulation of a comprehensive tectonic model. This article presents a summary of the regional dynamics and tectonics of Northeast Asia as developed for a new summary geodynamics map of the region. This article also describes how a high-quality summary geodynamics map and companion tectonic analysis substantially aids in the understanding of the origin of major rock units, major structures, and contained mineral and fuel resources, and provides important guidelines for new research. * Prepared in memory of Leonid M. Parfenov, the leader of the geodynamics map team for the International collaborative project on NE Asia tectonics and metallogenesis.
Abstract. The Tommot pluton is located within the continental Omulevka terrane of the inner zone of the Verkhoyansk-Kolyma Mesozoic orogen. It is a small complex (~12 km2) composed of alkaline-ultramafic rocks, alkaline and subalkaline gabbroids, and alkaline and quartz syenites. The pluton is unique both in the composition and age of its constituent rocks. Mineralogical-petrographical and geochemical studies of the rocks indicate that the alkaline rocks resulted from the melting of depleted mantle horizons. K-Ar, Rb-Sr, and 40Ar/39Ar age determinations confirm a Paleozoic age of the rocks. Formation of the alkaline rocks is related to Middle Paleozoic rifting which occurred as two discrete events: a Late Devonian event, which affected the marginal part of the Siberian continent, and a Late Carboniferous event that reflects internal deformation of the Omulevka terrane or late-stage extension. A spatially associated alkali granite, the Somnitel'nyy pluton, is Late Jurassic–Early Cretaceous in age and is synchronous with accretion of the Kolyma-Omolon Superterrane to Siberia in the Mesozoic.
Abstract. The Eurasia (EU) – North America (NA) plate boundary zone across Northeast Asia still presents many open questions within the plate tectonic paradigm. Constraining the geometry and number of plates or microplates present in the plate boundary zone is especially difficult because of the location of the EU-NA euler pole close to or even upon the EU-NA boundary. One of the major challenges remains the geometry of the Okhotsk plate (OK). whose northwestern portion terminates on the EU-OK-NA triple junction and is thus caught and compressed between converging EU and NA. We suggest that this leads to a coherent and understandable large scale deformation pattern of mostly northwest-southeast trending strike-slip faults which split Northwest OK into several extruding slivers. When the fault geometry is analysed together with space geodetic and focal mechanism data it suggests a central block which is extruding faster bordered east and west by progressively slower extruding blocks until the OK plate boundary faults are encountered. Taking into account elastic loading from both the intra-OK faults and the OK-Pacific (PA) boundary reconciles geodetic motions with geologic slip rates on at least the OK-NA boundary which corresponds to the Ulakhan fault.
Abstract. The sequence of formation of structures is established in the zone of junction of the eastern margin of the North Asian craton and the northeastern flank of the Kolyma-Omolon microcontinent, in the area of bend of the Kolyma structural loop. Detailed structural studies revealed two phases in the formation of Mesozoic structures – an early thrust phase and a late strike-slip phase. Structures formed during each of the phases are described. Thrust structures are represented by the Setakchan nappe on which the minimum amount of horizontal displacement is estimated at 13–15 km. Later superposed left-lateral strike-slip faults have a north strike. Formation of these latter structures occurred during the second phase of collision between the Kolyma-Omolon microcontinent and the eastern margin of the North Asian craton.
Abstract. A long tectonic zone composed of Upper Jurassic to Lower Cretaceous volcanic and sedimentary rocks is recognized along the Asian continent margin from the Mongol-Okhotsk fold and thrust belt on the south to the Chukotka Peninsula on the north. This belt represents the Uda-Murgal arc, which was developed along the convergent margin between Northeast Asia and Northwest Meso-Pacific. Several segments are identified in this arc based upon the volcanic and sedimentary rock assemblages, their respective compositions and basement structures. The southern and central parts of the Uda-Murgal arc were a continental margin belt with heterogeneous basement represented by metamorphic rocks of the Siberian craton, the Verkhoyansk terrigenous complex of Siberian passive margin and the Koni-Taigonos Late Paleozoic to Early Mesozoic island arc with accreted oceanic terranes. At the present day latitude of the Pekulney and Chukotka segments there was an ensimatic island arc with relicts of the South Anyui oceanic basin in a backarc basin. Accretionary prisms of the Uda-Murgal arc and accreted terranes contain fragments of Permian, Triassic to Jurassic and Jurassic to Cretaceous (Tithonian–Valanginian) oceanic crust and Jurassic ensimatic island arcs. Paleomagnetic and faunal data show significant displacement of these oceanic complexes and the terranes of the Taigonos Peninsula were originally parts of the Izanagi oceanic plate.
Abstract. Paleomagnetic results from the upper two thirds of the whole section of the Okhotsk-Chukotka Volcanic Belt (OCVB) volcanics exposed in the area around Lake El'gygytgyn, Chukotka yield stable, consistent magnetic vectors and well-preserved reversed directions. The magnetostratigraphy and 40Ar/39Ar geochronologic data reported here indicate that the sampled OCVB volcanics were erupted between about 90 and 67 Ma, and show no significant change in the apparent pole position over that time. The OCVB extends from northeast China, across Northeast Russia to the Bering Straight. This belt is made up of both extrusive and intrusive rocks, with the extrusive rocks and their associated sediments being dominant. The whole belt important in interpreting the paleogeography of the region because it overlies many of the accreted terranes of Northeast Russia. Most importantly, it overlies parts of the Chukotka-Alaska block which is thought to have moved out of the Arctic Ocean region, as well as terranes accreted from the south. These latter terranes have been rafted northwards on the paleo-plates of the Pacific, implying that the present relative paleogeography of all of the terranes overlain by the OCVB were essentially in place by 67 Ma, and possibly as early as 90 Ma. However, comparing our paleomagnetic pole position for the OCVB with those for North America and Eurasia (a proxy for Siberia) shows a statistically significant displacement of the OCVB pole to the south west. This implies that not only the OCVB, but the underlying terranes of northeast Russia, experienced southerly displacement with respect to the Siberian and North American platforms since the Late Cretaceous.
Abstract. Geochronologic and structural data from the terranes of the South Anyui suture zone record a protracted deformational history before, during and after an Early Cretaceous collision of the passive margin of the Chukotka-Arctic Alaska continental block with the active continental margin of the North Asian continent. Preceding this collision, the island arc complexes of the Yarakvaam terrane on the northern margin of the North Asian craton record Early Carboniferous to Neocomian ages in ophiolite, sedimentary, and volcanic rocks. Triassic to Jurassic amphibolites constrain the timing of subduction and intraoceanic deformation along this margin. The protracted (Neocomian to Aptian) collision of the Chukotka passive margin with the North Asian continent is preserved in a range of structural styles including first north verging folding, then south verging folding, and finally late collisional dextral strike slip motions which likely record a change from orthogonal collision to oblique collision. Due to this collision, the southern passive margin of Chukotka was overthrust by tectonic nappes composed of tectono-stratigraphic complexes of the South Anyui terrane. Greenschists with ages of 115–119 Ma are related to the last stages of this collision. The postcollisional orogenic stage (Albian to Cenomanian) is characterized by sinistral strike slip faults and an extensional environment.
Abstract. A seismic swarm lasting over two years occurred near the village of Neshkan, Chukotka, far northeastern Russia, beginning with a ML, 4.2 (4.1 mb) earthquake on 9 December 2002. The swarm generated considerable anxiety among the local populace and authorities. Two temporary seismic stations were deployed during the latter part of September 2003, and recorded over 150 events with magnitudes up to 3.0. Eighteen locatable events appear to form a northeast striking linear trend, parallel to other seismicity trends in Chukotka, extending 20 km to the southwest from the village. We interpret this trend as a previously unknown fault. A small pond located ~1 km west of the village drained and some apparent surface deformation was observed over the course of the earthquake sequence. Relocation of historic seismicity in the region shows that a magnitude 6.0 in 1996 may have ruptured an adjacent fault segment. Other, less well located but larger, teleseismic events earlier in the 20th century may also have occurred on or near this fault. The seismicity is consistent a proposed region of transtension along the northern boundary of a Bering plate.
Abstract. Strikingly similar Late Mesoproterozoic stratigraphic sequences and correlative U-Pb detrital-zircon ages may indicate that the Sette Daban region of southeastern Siberia and the Death Valley region of southwestern North America were formerly contiguous parts of a Grenville foreland basin. The Siberian section contains large numbers of detrital zircons that correlate with Grenville, Granite-Rhyolite, and Yavapai basement provinces of North America. The sections in both Siberia and Death Valley exhibit west-directed thrust faults that may represent remnants of a Grenville foreland thrust belt. North American detrital-zircon components do not occur in Siberian samples above a ~600 Ma breakup unconformity, suggesting that rifting and continental separation blocked transfer of clastic sediment between the cratons by 600 Ma. Faunal similarities suggest, however, that the two cratons remained within the breeding ranges of Early Cambrian trilobites and archeocyathans.
Abstract. U-Pb isotopic dating of seven granitoid plutons and associated intrusions from the Bilibino region (Arctic Chukotka, Russia) was carried out using the SHRIMP-RG. The crystallization ages of these granitoids, which range from approximately 116.9±2.5 to 108.5±2.7 Ma, bracket two regionally significant deformational events. The plutons cut folds, steep foliations and thrust-related structures related to sub-horizontal shortening at lower greenschist facies conditions (D1), believed to be the result of the collision of the Arctic Alaska-Chukotka microplate with Eurasia along the South Anyui Zone (SAZ). Deformation began in the Late Jurassic, based on fossil ages of syn-orogenic clastic strata, and involves strata as young as early Cretaceous (Valanginian) north of Bilibino and as young as Hauterivian-Barremian, in the SAZ. The second phase of deformation (D2) is developed across a broad region around and to the east of the Lupveem batholith of the Alarmaut massif and is interpreted to be coeval with magmatism. D2 formed gently-dipping, high-strain foliations (S2). Growth of biotite, muscovite and actinolite define S2 adjacent to the batholith, while chlorite and white mica define S2 away from the batholith. Sillimanite (± andalusite) at the southeastern edge the Lupveem batholith represent the highest grade metamorphic minerals associated with D2. D2 is interpreted to have developed during regional extension and crustal thinning. Extension directions as measured by stretching lineations, quartz veins, boudinaged quartz veins is NE-SW to NW-SE. Mapped dikes associated with the plutons trend mostly NW-SE and indicate NE-SW directed extension. 40Ar/39Ar ages from S2 micas range from 109.3±1.2 to 103.0±1.8 Ma and are interpreted as post-crystallization cooling ages following a protracted period of magmatism and high heat flow. Regional uplift and erosion of many kilometers of cover produced a subdued erosional surface prior to the eruption of volcanic rocks of the Okhotsk-Chukotka volcanic belt (OCVB) whose basal units (~87 Ma) overlie this profound regional unconformity. A single fission track age on apatite from granite in the Alarmaut massif yielded an age of 90±11 Ma, in good agreement with this inference.
Abstract. The Chersky seismic belt represents a zone of deformation between the North American and Eurasian plates in northeast Russia. The belt extends from the Laptev Sea into the Chersky Range where it splits into two branches. One branch extends to Kamchatka and the Aleutian-Kurile Junction, while the other branch extends south towards Sakhalin Island. Focal mechanisms indicate a change from extension to transpression in the northern Verkhoyansk Range and generally left-lateral transpression in the Chersky Range extending to northern Kamchatka. The few focal mechanisms on the second branch suggest right-lateral transpression. A large number of faults, sub-parallel to the seismicity and presumed to be strike-slip, are visible in satellite imagery and topographic maps and are also associated with seismically generated landslides. These data support a model in which the Sea of Okhotsk forms the core of a separate Okhotsk microplate surrounded by diffuse boundaries on the north and west. Microseismicity in continental northeast Russia is most heavily concentrated within and between the fault systems along the northern boundary of the proposed Okhotsk plate and indicates a high level of deformation. The sense of slip on the faults (both from focal mechanisms and geology) are also generally consistent with the extrusion of the Okhotsk plate to the southeast as it is compressed between its larger neighbors. The northernmost part of the Okhotsk plate may be decoupled to some degree from the more stable central Sea of Okhotsk.
Abstract. The Pevek region of Arctic Russia provides excellent beach cliff exposure of sedimentary and igneous rocks that yield detailed information on the nature, progression and timing of structural events in this region. Regional folding and thrust faulting, with the development of a south-dipping axial plane cleavage/foliation developed during N-S to NE-SW directed shortening and formation of the Chukotka-Anyui fold belt. This deformation involves strata as young as Valanginian (136–140 Ma, Gradstein et al., 2004). Fold-related structures are cut by intermediate to silicic batholiths, plutons and dikes of Cretaceous age. Reported K-Ar whole rock and mineral ages on the granitoids range from 144 to 85 Ma, but to the south, more reliable U-Pb zircon ages on compositionally similar plutons yield a much narrower age range of ~120–105 Ma (Miller et al., this volume) and a pluton in Pevek yields a U-Pb age on zircon of 108.1±1.1 Ma with evidence for inheritance of slightly older 115 Ma zircons. Magmas were intruded during an episode of E-W to ENE-WSW directed regional extension based on the consistent N-S to NNW-SSE orientation of over 800 mapped dikes and quartz veins. Analysis of small-offset faults and slickensides yield results compatible with those inferred from the dikes. Younger tectonic activity across this region is minor and the locus of magmatic activity moved southward towards the Pacific margin as represented by the <90 Ma Okhotsk-Chukotsk volcanic belt (OCVB). A lengthy period of uplift and erosion occurred after emplacement of Cretaceous plutons and produced the peneplain beneath the younger OCVB. Based on our studies, we speculate that ~120–105 Ma magmatism, which heralds a change in tectonic regime from compression to extension, could represent one of the consequences of the inception of rifting in the Amerasian Basin of the Arctic, forming the Makarov Basin north of the Siberian shelf at this longitude. A synthesis of available seismic reflection, gravity and magnetic data for the offshore Siberian Shelf reveals a widespread, seismically mappable basement-sedimentary cover contact that deepens northward towards the edge of the shelf with few other significant basins. Various ages have been assigned to the oldest strata above the unconformity, ranging from Cretaceous (Albian – 112–100 Ma) to Tertiary (Paleocene–Eocene – ~60–50 Ma). The period of uplift and erosion documented along the Arctic coast of Russia at this longitude could represent the landward equivalent of the (yet undrilled) offshore basement-sedimentary cover contact, thus overlying sedimentary sequences could be as old as early Late Cretaceous. Although quite speculative, these conclusions suggest that land-based geologic, structural, petrologic and geochronologic studies could provide useful constraints to help resolve the plate tectonic history of the Arctic Ocean.
Abstract. The Cretaceous granitoid complexes of the Eastern Taigonos and the Prybrezhny Taigonos belts (southern part of the Taigonos Peninsula), Tanyurer pluton of the Okhotsk-Chukotka volcanic belt, and the Peekiney, Moltykan, and Telekay plutons of the Chaun tectonic zone are discussed in relation to their structural position, petrography, rock and mineral chemistry and physicochemical conditions of melt crystallization. These granitoid plutons were generated through melting of a compositionally heterogeneous crustal source, with direct contribution from mafic melts produced in the mantle wedge above active or extinct Benioff zones. Variations of the trace-element composition of granitoids are controlled to a greater extent by local compositional peculiarities of the source regions than by the geodynamic regime as such. The final crystallization of these plutons occurred at comparatively shallow depths, between 1–2 and 6–7 km, in a temperature interval of 700–770°C. The depth of emplacement of the bodies decreases with increasing distance from the areas with oceanic and transitional type crust, as does the degree of incompatible element enrichment of the mantle and crustal sources of melts. Variations in fo2 values at the late stages of crystallization of the plutons reach 3–4 orders of magnitude, exceeding the limits of the quartz-fayalite-magnetite (QFM) and nickel-nickel oxide (NNO) buffer equilibria, which likely results from local variations of the source composition.
Abstract. A paleomagnetic study was carried out on Paleogene sedimentary rocks from Bering Island, Komandorsky islands, located at the far western end of the Aleutian Island Arc. The age of these sediments has been debated at length, but the combination of magnetostratigraphy with the fossil record indicates that the base of the section is of early Eocene (approximately 55 Ma) and the top latest Eocene age. Paleomagnetic data were obtained from 260 samples from 60 individual bedding units. The combined data show a clockwise rotation R=26.3°±8.5°, F=8.1°±2.5° with respect to the North American Plate and R=38°±8.8°, F=8.7°±2.7° with respect to the Eurasian Plate. They also show a shallowing of the inclination which yields a paleolatitude of 53°, 12° south of its expected latitude. The shallowing may have a component due to compaction, but the wide variation in sampled lithologies, combined with internal consistency of the data set, would argue against the shallowing being significant. To compare these data with other Aleutian Arc data we compiled a comprehensive survey of all available data sets. Out of these we selected four islands for which the data passed basic reliability criteria, namely Umnak, Amlia, Amchitka and Medny islands. All four showed significant clockwise rotation with respect to both North American and Eurasian polar wander paths. Several mechanisms can generate the observed rotation, ranging from block rotation driven by oblique relative motion of the Pacific plate, through lateral transport along the curve of the arc, to whole-arc rotation about its eastern end. The distribution and age spread of the rotation data are insufficient to discriminate between mechanisms, but it seems likely that different mechanism may have operated at different times and in different locations.
Abstract. The Okhotsk terrane, located east of the South Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt, has Archean crystalline basement and Riphean to Early Paleozoic sedimentary cover similar to that of the adjacent the North Asian craton. However, 2.6 Ga biotite orthogneisses of the Upper Maya uplift of the Okhotsk terrane yielded Early Devonian 40Ar/39Ar cooling ages, evidence of a Mid-Paleozoic metamorphic event not previously known. These gneisses are also intruded by 375±2 Ma (Late Devonian) calc-alkaline granodiorite plutons that we interpret as part of a continental margin volcanic arc. Therefore, Late Devonian rifting, which affected the entire eastern margin of North Asia separating the Okhotsk terrane from the North Asian craton, was probably a back-arc event. Our limited 40Ar/39Ar data from the South Verkhoyansk metamorphic belt suggests that low grade metamorphism and deformation started in the Late Jurassic due to accretion of the Okhotsk terrane to the North Asia margin along the Bilyakchan fault. Shortening and ductile strain continued in the core of the South Verkhoyansk metamorphic belt until about 120 Ma due to paleo-Pacific subduction along the Uda-Murgal continental margin arc.
Abstract. Outcrops along the Inach River in the Uyandina basin and those along the Myatis' River in the Indigirka-Zyryanka basin were studied in detail and sampled for coal rank determinations. The Uyandina basin is an intramontane pull-apart basin characterized by extensional structures within the Moma rift system. The coal rank is below 0.3% vitrinite reflectance (Rr), which indicates shallow, immature conditions of basin formation and very low subsidence. The Myatis' River coal-bearing outcrops in the Indigirka-Zyryanka basin reveal compression induced by continent collision. The compressive deformation includes also lowermost Pliocene strata. Due to the position in the Verkhoyansk-Chersky fold belt adjacent to the Kolyma-Omolon microcontinent the Indigirka-Zyryanka basin has much in common with a foredeep, i.e. the asymmetry in thickness and tectonic structure. The vitrinite reflectance data (Rr) which range from 0.25% to more than 5% reinforce the accepted models that describe basin subsidence and geothermal history and the tectonic deformation.
