GP41A-0812 0800h
Mesozoic oceanic terranes - key to our understanding of Meso-Pacific History
Along the eastern Pacific margin of Asia Mesozoic oceanic complexes are widely distributed. The structure and geometry of these accreted oceanic complexes vary from: (1) dismembered ophiolite nappes; (2) blocks in serpentinite melange and olistostromes; (3) offscraped fragments of the oceanic crust in a sheared argillite matrix which is interpreted to represent an accretionary prism sequence. The sedimentary strata associated with the oceanic complexes are mostly various types of siliceous sediments: radiolaria chert and jasper, grey cherts and siliceouc argillite and in lesser quantities, limestones. In many ophiolitic sections, sediments form the cover sequences of the various MORB-type lavas. The following sedimentary sequences have been mapped and described in the Koryak Mountains: The Pekul'ney Ridge, Mainitz, Alkatvaam, Ekonay, Kuyul (Penzhina) Ukelayat and Olutor terranes. Similar sequences are known from Kamchatka, Sakhalin, Taigonos and Elistratov Peninsulas, in Udskaya Guba, Priamurie, and in Sikhote-Alin. The composition and character of microfauna in these mostly pelagic sequences indicate accumulation of siliceous strata at lower latitudes than their present localities (Bragin, 1991; Vishnevskaya, 1990). Paleomagnetic data available on the oceanic complexes tend to support their allochthoneity, and indicate significant northward motion. As such, these oceanic complexes are particularly important for paleotectonic reconstructions of the Paleo Pacific ocean floor. Analysis of all data and paleomagnetic results from Late Jurassic - to Cretaceous accreted oceanic units allow one to conclude that the kinematics of the Koryak and Kamchatka oceanic terranes and their accretion is defined by the presence and interactions of four oceanic plates, the Izanagi, Farallon, Kula and Pacific plates with the continental margins of North American and Siberian continents. The older, Triassic - to Middle Jurassic complexes are the main source of information on the composition and structure of the more ancient Meso-Pacific Ocean. Correlation and age of these oceanic fragments together with a study of their structure and their time of accretion, provides a unique data set that is much more complete than that possible with deep-ocean drilling of Pacific Ocean floor.
http://pangea.stanford.edu/research/structure/
GP41A-0813 0800h
Deformation History of Central Chukotka (Alarmaut Uplift) Northeastern Arctic Russia
The Anyui-Chukotka fold-belt is a regionally extensive zone of folding and lesser thrust-faulting that extends from 162° W longitude to the eastern end of Chukotka along the Russian Arctic coast. The southern border of this fold-belt is the South-Anyui suture zone. Most investigators consider the South-Anyui suture as a result of Early Cretaceous collision of Eurasia with the Chukotka microcontinent during the closure of the Anyui Ocean. Despite its regional extent and significance in terms of regional tectonics, there is little published data on the style and geometry of deformation within the Anyui-Chukotka fold-belt and there are few age constraints on its development. Structural data was collected within the Anyui-Chukotka fold-belt in area located north of Bilibino, east of the Alarmaut Uplift, and south of Pevek. Strata involved in deformation are mostly thick sequences of Triassic slate and fine-grained quartzite, but locally Permian (?), Carboniferous, and Devonian (?) strata are also exposed locally. Three deformational episodes are distinguished: 1) The earliest deformation (D1) was regional folding and minor thrust-faulting associated with penetrative cleavage (S1) that is steeply dipping where the effects of younger deformation are absent. Folds range from outcrop-scale to map-scale and are oriented WNW-ESE. D1 structures are best preserved along a belt parallel to the South Anyui suture in the southern part of the study area. D1 is the result of the first compression phase, which is likely the main collisional deformation in the area. Abundant quartz veins formed during this phase of deformation. 2) The second deformation phase (D2), which affects a region of at least 3000 km2 along and east of the Alarmaut massif, dies out structurally upwards and to the south. D2 produced a strong high-strain foliation (S2) which is mostly gently dipping to flat. Quartz veins were deformed and folded, bedding and S1 are isoclinally folded, with the axial plane of these folds being parallel to S2. Boudinage and pinch-and-swell structures are also evident, and together with the isoclinal folds and transposition of previous fabrics argue for very high strains. Stretching lineations are locally developed within S2 but are not common. Near the granitic batholith of the Alarmaut massif, new-formed biotite grows within the S2 cleavage. Our interpretation is that D2 formed as a result of vertical shortening or flattening, but its regional significance is not clear, except that it post-dates upright folding in the Chukotka fold belt and may be coeval with intrusion of widespread granitoid plutons (undated). 3) Finally, there are small and outcrop-scale kink folds formed during D3 that fold S2, and there is local development of late-stage crenulation cleavage. Both sets of structures represent small strains.
GP41A-0814 INVITED 0800h
Tectonics of the South Anyui suture: present day version
The South Anyui Suture (SAS) is one of the most prominent tectonic elements of North-East Asia separating the Verkhoyansk-Kolyma and Anyui-Chukotka fold belts. Following the Seslavinsky (1979), Parfenov (1984), and Natal'in (1984) the SAS is recognized as result of late Mesozoic collision between Siberia and North America continents. Many aspects of the SAS geology and evolution remain poorly understood: (i) the timing of the oceanic basin in-ception and existence; (ii) the inner structure and deformational history; (iii) the age and genesis of the ophiolites; (iv) the island-arc units distribution and age. Main results of our research in the SAS are: (1) The north vergent thrust structure is recog-nized. The autochthone includes shelf and slope sequences of the Chukotka microcontinent. The allochthone thrust package consists of ophiolitic, metamorphic, terrigenous, and island-arc rocks; (2) Late Mesozoic subduction related chaotic and intra oceanic island arc units are recognized; (3) A Bathonian-Callovian age of basalt-chert oceanic assemblages, previously reported as Late Jurassic to Neocomian; (4) A new 40Ar/39Ar data for: (i) Vurguveem ophio-litic gabbro 312.2 ± 11.1 I, (ii) diabase dike from Aluchin ophiolite 226.6 ± 10.5 I, (iii) amphibolites in the metamorphic sole of Uyamkanda ophiolite 239.1 ± 3.8 I, (iv) green-schists at the base of tectonic sheet 156.5 ± 3.9, 120.6 ± 3.5, 115.2 ± 1.3, 108.4 ± 1.2, and 104.1 ± 2.8 I. According new data tectonic evolution of SAS can be suggested the following: (1) Anyui oceanic basin existed from Mississippian to Neocomian; (2) Northern (present-day coordi-nates) margin of ocean was passive, and southern margin was active; (3) Oceanic spreading was stopped in Oxfordian, probably simultaneously with start of Canada basin opening (Grantz et al. 1998); (4) The collision accompanied by subduction of Chukotka passive mar-gin beneath the Siberia active margin; (5) Pre-collisional deformations was related with sub-duction; (6) Collisional deformations during Hauterivian to Aptian include two phases: early - thrust fault related and late - strike-slip fault related; (7) Post-collisional Albian to Senoma-nian deformations was produced by extensional event. Supported by RFBR (grant 02-05-64217), FCP "Integratsiya" (grant 0382).
GP41A-0815 0800h
Late Cretaceous - Eocene evolution of the Kronotsk arc
Eastern peninsulas of Kamchatka and probably Komandorskiy Islands form Kronotsk paleoarc. Main components uniting these blocks in a single structure are Paleocene-Eocene subduction-related volcanics. The lowest part of this formation on the Kronotsk peninsula was dated as the Late Senonian. Paleomagnetic data show that, 60-40 Myr ago, Kronotsk arc undergo large northern drift after a nearly equal period of southern drift. The southern part of the Kamchatskiy Mys peninsula, Africa block, is interpreted as a fragment of the accretionary prism of the Kronotsk arc, related to period of the southern drift. There are five main parts of this prism: Olenegorsk gabbro (50-70 Ma); Smaginsk Fm (Albian-Senomanian, 110-95 Ma): hot-spot basaltes and pelagic sediments; Pickezh Fm (Campanian - Maastrichtian, 85-65 Ma): tuffites in the lower part and subarcosic sandstones in the upper; and Soldatsk ultramafics. These parts of the prism are mostly separated by the large thrusts, but locally we saw the konglobrechia with gabbroic and diabasic clasts in the lowest parts of the Smaginsk and Pickezh sequences. The transition from the Pickezh Fm to Pickezh sanstones was always described as gradual. Six published paleomagnetic determinations (from Campanian to Bartonian, 80-40 Ma) of Kronotsk arc volcanics, kinematics of the large plates in the Northern Pacific, and some geological data allow us to reconstruct the drift of the Kronotsk arc at the end of Cretaceous and the first half of Paleogene. 80-60 Myr ago, Kronotsk arc marked a southern margin of the North American Plate (or a little plate with the very similar kinematics) when the Kula plate was consumed in the Kronotsk while the Kula-Pacific Ridge and Hawaiian hot spot were placed to the south. The apron of tuffs and tuffites overlapped the slopes of the newly arc and neighboring oceanic structures. One of the latter, Smaginsk oceanic plateau on the Kula plate was partly separated from this plate and attached to the Kronotsk accretionary prism. Simultaneously with these intra-oceanic events, there was a maximum of Laramie orogeny on the nearest North American continent. A huge sequence of turbidit fans was formed along western margin of Cordilleras. The Upper Pickezh sanstones are probably a small part of one of these fan included in the Kronotsk accretionary prism. At the Late Cretaceous and beginning of the Paleogene, the Kula-Pacific Ridge moved aside Obrutchev Rise and approached the Kronotsk. Therefore, the part of Kula plate south of the Kronotsk arc was constantly diminishing. As a result, 60-55 Myr ago, the subduction zone was blocked by the ridge and the western part of Kula plate was attached to the Pacific plate. This collision is the most probable cause of the strong deformation of the accretionary prism and of the emergence of the outer nonvolcanic arc that supplied the intra-arc basin (Stolbovsk Series of the Kamchatskiy Mys peninsula) with the ofiolitoclastics. The continuation of the Kronotsk volcanism after this collision is probably caused by a new subduction zone placed north of the Kronotsk arc.
GP41A-0816 0800h
Triassic terrigeneous deposits of Western Chukotka: sedimentation, mineral composition, deformations (NE Russia)
Chukotka's Triassic terrigeneous deposits form three different complexes: Lower-Middle Triassic complex, Upper Triassic Karnian complex and Upper Triassic Norian complex. The studied part of western Chukotka is composed of variably deformed, folded and cleaved rhythmic deposits. All the complexes are represented by rhythmic intercalation of sandstones, siltstones and mudstones. Unfortunately, macrofaunas are not numerous in the Triassic deposits, and in some cases deposits are dated by analogy or comparison to the units dated with macrofaunas. Triassic deposits are composed of hemipelagic sediments, low-density flows, high-density flows, and shelf facies associations. Relationships between the facies were reconstructed with structural researches and data. During the Triassic, sedimentation was represented by continental slope progradation. Petrographic study of mineral composition has established the sandstones as graywackes (classification diagram by Shutov, 1972). Although Triassic sandstones are similar in outcrops and classification, the enclosed rocks fragment grains are different. Rock fragment grains in sandstones are composed of diverse lithologies, and we chart the evolution of their composition from the Lower to the Upper Triassic. Sandstones with (clasts) rock fragments of lower metamorphic grade rocks dominate at the base of Triassic deposits, sandstones with fragments of higher grade metamorphic rocks dominate in the Later Triassic deposits. This different show us that the Triassic represents an unroofing sequence where(sours of) erosional processes that produced the clastic material eroded more deeply buried rocks through time. Supported by RFFI (grants 02-05-64217, 03-05-64915).
GP41A-0817 0800h
GIS Analysis of Spatial Distribution of Granitoid and Au-Quartz Deposits in the Metaturbidite Terranes of the Northeastern Asia
Au-quartz deposits of the Northeastern Asia metaturbidite terranes have close spatial and temporal relations with granitoid intrusions. This was the basis for many years discussion about the ore genesis. Multi-variant calculations by the GIS tools on the area bounded by the North-Asian Craton Margin (Verkhoyansk fold belt), the Kular-Nera turbidite terrane, the Polousno-Debin terrane of accretionary wedge, the Nagondzha turbidite terrane, and the Viliga turbidite terrane were carried out in order to examine the possible dependence of coefficient of the host rocks saturation by granitoid (relatively area of granitoids on the modern surface) from the intensity of the gold-quartz mineralization (the amount of lodes per unit of the area). The terrane boundaries were taken from the GIS Compilation of Geophysical, Geologic, and Tectonic Data for Circum-North Pacific (ed. W. Nokleberg and M. Digges, 1999); the granitoid boundaries were digitized from the geological map of the USSR North-East , the scale of 1:1500000 (ed. M. Gorodinsky, 1980). The terranes area was gradually filled with the regular net of similar clipping cells (circles with the overlapping). Thus, the crossing of the neighbouring circles has formed the hexagon. The circle radius varied from 20 to 40 km for the different series of calculations. Calculation of the ratio of granitoid and metaturbidite was done in every cell. The obtained values were given to the cell centers. Isolines were built from the regular point's distribution coverage. After that, statistic parameters of the gold-quartz deposits distribution within the fields bounded by isolines, were analyzed. The maximum amount of deposits is within the interval of 0 to 15% of granitoid, with the average value 7,7% with the circle of 20 km in radius, and the value of 8.8% with the circle of 40 km in radius. At the cell of 40 km in radius, the distribution is the more close to the normal one. Thus, the granitoid saturation zone from 5 to 10% is the most favorable to locate of the gold-quartz deposits. The obtained values are lower than those obtained during the analysis of the gold-bearing province in general which is 12-14%. An interpretation of gold-quartz deposits maximum location in the zone of 5-10% of saturation by granitoid is possible on the basis of the metamorphic model, that supposes the mobilization of fluids and metals at the boundary of greenschist facies and hornfels i.e., relatively high temperature metamorphism with the low pressure. The obtained in the isolines picture of the granitoid spatial distribution reflects the superposition of collision and subduction belts, and also the intensity of these tectonic events, the data may be used at the paleotectonic reconstruction.
GP41A-0818 0800h
Role of the Interference of Different Geodynamic Settings in Metallogenesis of Upper Kolyma Basin Area (Magadan Region)
The modern territory of the Magadan region is area of the two types of geodynamic environments (collisional and subductional), which defined the specifity of granitoid magmatism and metallogeny. Yana-Kolyma I- and S-type granitoids of the ilmenite series: diorite-granodiorite and granite-leucogranite magmatic suites (155-135 Ma) was related with plutonic-metamorphic collisional process (Goryachev, 2003). Gold-quartz veins, granitoid-related gold veins and stockworks, tin, and tungsten lode deposits in the Yana-Kolyma collisional zone existed during the period of 155-135 Ma and were related by this process. The Uda-Murgal magmatic arc (150-110 Ma) is located to the east (in present coordinates) of this collisional zone. The andesite - dacite volcanism early stage corresponds to the emplacement of gabbrodiorite-granodiorite-granite plutons I-type granitoids dated at 150-135 Ma. During this early stage of formation of the Uda-Murgal magmatic arc were formed of epithermal gold-silver, tin-silver, mesothermal granitoid related gold veins, skarn tungsten, and skarn silver-base metal lodes. The late stage (135-100 Ma) was marked by the formation in the back arc magmatism, local volcanic structures filled with magmatic rocks of dacite-rhyolite composition, plutonic I-type granites suites with withinplate geochemical signature, and A-type alkali granites. At that time epithermal gold-silver, shallow-depth gold-silver-quartz veins, granitoid-related gold stockworks, copper-molibden-porphyry and silver-cobalt-bismuth lode deposits were formed. According to tectonic reconstructions (Parfenov, 1995; Nokleberg et al, 1998; Tectonics, ., 2001), at the Late Jurassic - Early Cretaceous time (155-100 Ma) the Pacific margin of northeast Asia presented an area of coexistence of the Yana-Kolyma collisional zone (Goryachev, 1998) and the Uda-Murgal continental marginal magmatic arc (Parfenov, 1995; Sokolov et al., 1999; Goryachev, 2002). The new structure data about different folded plans of southeastern part of the Ayan-Uryakh antiklinorium (Goryachev I., 2003) and character of granitoid assemblages and complex metallogeny in the Upper Kolyma river basin and Northern Priokhot'ye part of Magadan region (Goryachev, 2002; Goryachev, 2004) demonstrate that this area is the zone of "interference" of different geodynamic environments where is characterized by a combination of magmatic suites and metalogeny characteristics of different origin but single age (Goryachev, 2002). The research was conducted with the support of Integration projects FEB-SB RAS No 65, 69, 71.
GP41A-0819 0800h
Tectonic History and Metallogeny of the Chukchi Terrane
The Chukchi Terrane consists of the Anyui, Wrangel, Chaun and Bering Sub-Terranes; at present, it is the northern part of the Pacific Folded Belt. Its basement is composed of folded rocks of Proterozoic and Paleozoic, which crop out on Wrangel Island, the Kuul and Alyarmaut Uplifts, Chukotka and Seward Peninsulas. Early Precambrian blocks may be also present in it. The above-mentioned sub-terranes have a similar development history and are featured by the same type of Mesozoic metamorphism, magmatism and metallogeny. Its tectonic history has been as follows: 1. Late Proterozoic and Early Paleozoic (pre-Visean) magmatism, thrusting, isoclinal folding. Scarce occurrences of gold and stibnite mineralization. 2. An unconformity at the base of Visean - Mid Carboniferous section with basal conglomerates contain granite pebbles and boulders. 3. Permian uplifting (continental), reduced sedimentation. 4. Maximum sedimentation in Triassic time accompanied by Early Triassic rifting and dominating turbidite rocks. The problem is the provenance areas for large amounts of terrigenous rocks. 5. Lack of sediments characterisitc of the greatest part of early Jurassic (post-Sinemurian), middle Jurassic and late Jurassic (pre-Volgian) time periods. Uplifting is correlative with the extension stage in the South-Anyui Ocean. 6. Late Jurassic - Neocomian. Intense uplifting processes occured at the end of late Jurassic and in early Cretaceous and associated with intrusion of subduction- and collision-related granitoids (147-140 Ma). The Nutesyn marginal continental arc was forming over the southern periphery of terrane and flysch processes occurred there through Neocomian. Fault-related depressions were developing in the north of the area under consideration. Since late Jurassic, as a result of an approach (with a right-side fault shifting) of the Novosibirsk-Chukchi Super-Terrane and the northeastern edge of the Asiatic Craton, the Yuzhno-Anyui Ocean began to close from west to east. Insignificant polymetallic and gold-tin-polymetallic mineralization types, and major tin lode deposits (Val'kumei) were developing. This was the first stage of tin mineralization. 7. Termination of the ocean closing process at the end of Neocomian (Barremian - early Albian); an outburst of granitoid magmatism (120 - 100 Ma) and formation of granite metamorphic domes. The main stage in the history of gold-quartz mineralization (Karalveem, Sovinoe, Ozyornoe, Ichuveem, Rock Creek, Big Hurrah, etc.) and may be silver-base metallic (Chechekuyum), and tin lode deposit Pyrkakai (cassiterite-quartz stockworks. 8. Cessation of an intense magmatic activity in late Albian - Campanian related to formation of the post-accretion Okhotsk-Chukchi Volcanic Belt (Chukotka). Termination of development of granite metamorphic domes; the third cycle of subduction-related granite magmatism (90 - 70 Ma). The main stage in the history of tin-tungsten-beryllium (Iultin, Lost River) and gold-silver mineralization types (Valunistoye). 9. Extension processes at the end of Neogene associated with formation of intraplate alkali basalt volcanoes.
GP41A-0820 0800h
Metallogenesis of Gold and Silver in Northeast Russia
Three genetic series of ore lode deposit types in Notheast Russia are distinguished: hydrothermal-metamorphogenic (early collision stage), hydrothermal plutonogenic granitoid (late collisional stage), and volcanogenic (post-collisional stage). Metallogenesis in the hydrothermal-metamorphogenic series is more or less exclusively gold mineralization (gold-quartz veins, and disseminated gold-sulfide mineralization). In the Yana-Kolyma metallogenic belt, gold mineralization of this genetic type occurs as lenticular quartz bodies. In the Allakh-Yun and West Verkhoyansk belts it is present as zones of stratified quartz veins. The hydrothermal-plutonogenic lode deposits related to granitoid suites were produced by ore-magmatic systems (OMS) with similar geochemical specialization for gold that most probably had a palingenetic crustal origin (Rb-Sr and Pb isotopic data). As the collision proceeded, large granitoid plutons were emplaced to form extensive belts (150-140 and 130-120 Ma), within which local ore-magmatic fields were formed. The intermediate-depth magmatic chambers (15-18 km depth) of the OMSs generated the low-sulfide gold-quartz lode deposits, while in hypabyssal magmatic chambers (1-2 km depth) granitoid-related gold lode deposits are produced. The volcanogenic series of shallow-depth ore lode types are silver-base metal, gold-silver-antimony, and silver-mercury. Subduction processes occurring along the Okhotsk active continental margin could have reactivated the earlier strike-slip fault zones, which served as the ore-controlling structures for the development of Late Cretaceous (95-70 Ma) subvolcanic magmatism and the formation of diversified mineralization (silver-base metal, gold-silver-antimony, and silver-mercury). The earliest is silver-base metal mineralization associated with subvolcanic granite porphyries and located in tin ore fields, thus confirming our supposition about the activation of deep horizons of staniferous OMSs. Gold-silver-antimony and silver-mercury mineralizations in narrow metallogenic belts are separate from silver-base metal ore. When the earlier metallogenic belts are cut by later ones, polychronic and polygenetic ore lode deposits are formed. An example is Adycha-Taryn fault zone where gold occurs together with antimony (Sarylakh and Sentachan lode deposits), and gold-mercury (Kyuchus) and tin mineralization are found in combination with silver-antimony mineralization (Kupol'noye and Alyaskitovoye). The combination of various OMSs in Verkhoyansk fold belt led to the formation of large polychronical and polygenetical tin-silver-base metal (Prognoz), or gold-silver (Nezhdaninskoye) lode deposits. The research was conducted with the support of RFBR projects (01-05-65485, 03-05-64980, 03-05-96010, Integration project 65, 69, 71 SB-FEB RAS.
GP41A-0821 0800h
Structure and evolution of the inner parts of collisional orogens of Verkhoyansk-Chukotka area (North-Eastern Asia)
The Mesozoic Verkhoyansk-Chukotka orogenic area consists of Verkhoyansk-Kolyma and Chukotka-Anuyi orogenic belts. External part of Verkhoyansk-Kolyma orogen (Verkhoyansk fold-and-thrust belt) includes Paleozoic to Mesozoic shelf sediments and turbidite of the Asian continental margin. The inner part (Chersky belt) consists of several continental terranes. These complexes have been tectonically overlapped by allochthones of ophiolite and polymetamorphic rocks. Back part (Alazeya-Oloy belt) contains deformed Paleozoic to Mesozoic island arc volcano-terrigenous rocks. Early accretion deformations (Middle Jurassic) were connected with the Kolyma-Omolon microcontinent and Asia craton amalgamation. This event was fixed by thrusts, nappes, and recumbent and overturned folds of north-east vergence. Late oblique collisional (Late Jurassic-Neocomian) deformations were characterized by combine kinematics with sinistral transpression component, and counter clockwise rotation of convergent structures. Chukotka-Anuyi orogen is limited in the south by allochtones of island arc rocks of the Alazeya-Oloy belt. The northern vergent nappes of the inner part consist of ophiolite, Late Paleozoic-Mesozoic volcano-terigenous rocks and accretion melange. The autochthone includes high-deformed Triassic turbidite of the Chukotka-Arctic Alaska microcontinent passive margin. The nappes were overlapped Hauterivian-Barremian neoautochthone. The north vergent nappes were formed during early Valanginian-Hauterivian event of collisions. The structures of the Verkhoyansk-Kolyma and Chukotka-Anyui orogenic belts are formed as a result of accretion and collision of Kolyma-Omolon and Chukotka-Arctic Alaska microcontinents to the continental margins of Siberian and North-Asian cratons. Orogenic belts of the Verkhoyansk - Chukotka area appeared as a result of closing small oceanic basins, representing segments of Protoarctic ocean. Supported by RFFI (grants 02-05-64217, 03-05-64915, 03-05-96074).
GP41A-0822 0800h
New Geochemical and Geochronological Constrains on the Tectonic Evolution of the Okhotsk Terrane
The Okhotsk Terrane is a continental block with Proterozoic basement which lies east of the South Verkhoyansk fold-and-thrust belt of eastern Siberia. It has been proposed that thrusting in the South Verkhoyansk occurred as a result of Late Jurassic-Early Cretaceous collision of the Okhotsk Terrane against the North Asia craton followed by plate convergence that produced the Uda-Murgal volcanic arc. Ten zircon grains from a biotite plagiogneiss of the Upper Maya uplift, analyzed using the Stanford/USGS SHRIMP-RG, yielded a weighted mean $^{207}$Pb/$^{206}$Pb age of $2624\pm13$ Ma, corrected for $^{204}$Pb. TIMs analysis yielded a statistically undistinguishable upper intercept of $2595\pm26$ Ma. These ages are similar to those reported from the hornblende granulite complex of the Kukhtuy uplift of the central Okhotsk Terrane. 2.6 Ga ages are also common in the North Asia craton. The Neoarchean gneisses of the Upper Maya are intruded by meta-aluminous biotite-hornblende granodiorites and quartz diorites of the Mastakh pluton. Mineralogical composition, REE distribution, and negative Ta, Nb, Zr, and Ti anomalies of the Mastakh pluton are comparable to continental margin, subduction-related plutons in western North America and the Urals. Ten zircons separated from the Mastakh pluton yielded a $^{207}$Pb-corrected weighted mean $^{238}$U/$^{206}$Pb age of $376\pm4$ Ma (Late Devonian). This age suggests that the Mastakh pluton is related to Late Devonian calc-alkaline volcanic rocks of the Matiy formation that are widespread in the Okhotsk Terrane. $^{40}$Ar/$^{39}$Ar analysis of biotites from the Mastakh pluton yielded a weighted mean plateau age of $355\pm1$Ma (Early Carboniferous), therefore that the pluton underwent slow cooling after emplacement. During the Middle Devonian to Early Carboniferous the eastern margin of the North Asia craton underwent a regional episode of rifting. Several major rift grabens and aulacogens formed along the Verkhoyansk margin. The best known of these is the Vilyui basin, but the stratigraphic section of the Sette-Daban zone of the South Verkhoyansk also contains thick rift-related strata. Rift structures are also associated with major fields of basaltic dikes, which in the South Verkhoyansk strike north-south. Stratigraphic links suggest that the Okhotsk Terrane formed part of the North Asia craton until the Middle Paleozoic. We hypothesize that in the Late Devonian to Early Carboniferous the Okhotsk Terrane rifted and moved a relatively small distance away. Because of the existence of coeval subduction-related magmatism on the Okhotsk Terrane, we view this event as back-arc rifting. A poor quality $^{40}$Ar/$^{39}$Ar from phyllites of the Sette-Daban zone suggests that compressional deformation was on-going in the Verkhoyanks fold-and-thrust belt by Latest Jurassic or early Neocomian time. This compressional deformation likely resulted from closure of this back-arc basin and oblique collision of the Okhotsk Terrane against the Verkhoyansk margin along the Bilyakchan fault. Subduction-related magmatism of the Uda-Murgal arc system continued on the Okhotsk Terrane during Triassic to Early Cretaceous time. At about 120 Ma magmatism stepped inboard, perhaps due to flattening of the subduction zone, producing a belt of large calc-alkaline plutons in the core of the Verkhoyansk belt.
GP41A-0823 0800h
Geodynamic setting, geochemistry, U-Pb datings and mafic enclave composition of granitoids from Eastern- and Prybrezhny Taigonos belts (southern part of Taigonos Peninsula, NE Russia)
The Taigonos Peninsula is divided into the following terranes: Avekov, Central Taigonos, and Beregovoi. The Avekov terrane consists of the Precambrian and Pz1 metamorphic complexes and the Central Taigonos terrane - Pz2-K1 island-arc complexes. The Central and Beregovoi terranes are separated by the Eastern Taigonos granitoid belt (ETb). The Beregovoi terrane represents a fragment of accretionary prism, which consists of T-K1 volcanogenic, siliceous and terrigenous sheets separated by serpentinite melange. Small plutons of the Pribrezhnyi Taigonos belt (PTb) intrude accretionary prism of the Beregovoi terrane. The 40Ar/39Ar datings from granites of the: (1) ETb are 103.3-A0.3 and 103.1-A0.5 Ma; (2) PTb are 100.9-A0.6, 101.3-A0.5, 103.5-A1.9 Ma. U-Pb datings on zircons indicate that granitoids of the ETb were emplaced and crystallized between 104.6-A1.1 and 97.0-A1.1 Ma, those of the PTb, between 106.5-A0.9 and 105.5-A0.9 Ma. ETb is mainly composed of tonalites and granodiorites. PTb is composed of small gabbro-diorite-tonalite plutons. Granitoids of ETb and PTb are metaluminous rocks, (Nb/Zr)n vs Zr and Rb vs Y+Nb show that they are subduction related rocks, they are characterized by LILE enrichment and HFSE depletion in relation to ORG, with distinct Ta and Nb depletion. Granites of ETb are more enriched in LREE, with more fractionated REE patterns. They also belong to high-K calc-alkaline and calc-alkaline series in terms of K2O and SiO2 concentrations, whereas granitoids of PTb, only to calc-alkaline series. Granitoids of both belts contain mafic enclaves (MME). Based on mineralogy and geochemistry MME from granitoids of PTb (gabbros, gabbro-diorites) are ascribed as plutonic derivatives of subalkaline magmatism that produces calc-alkaline island-arc series. P"CT estimates for rocks of gabbro-diorite-tonalite massif in the of PTb and enclaves in them show several stages of their evolution: 1) magmatic T= 800"C1000 3 and P= 5"C10 b; 2) recrystallization of enclaves, T=610-800 3, P= 5"C10 b; 3) metamorphism '=500"C590 3, =3"C4 b and ' 450"C500 3, 2 kb. Diversity of petrographycal types of ET pluton, large volume of granitoid magmatism in ETb, prevailing of granodiorite, tonalites and geochemistry of rocks allow to refer these granitoids to Cordilleran I-type granites; granitoids of PTb are similar to M-type subduction related granites. Supported by RFBR, grant 04-05-65132
GP41A-0824 0800h
Comparative Analysis of Biogeographic, Sedimentologic and Paleomagnetic Data and the Geodynamics of Terranes of Northeast Asia in Late Permian
We present the first consistent model of the relative locations of the most important tectonic structures in Northeast of Asia for Late Paleozoic time. This model is based on comparative analysis of paleomagnetic, sedimentologic and biogeographic data. Results of research by the authors and critically reviewed data of the other researchers are used. The current paleomagnetic data for Permian rocks from the Northeast region still remain scanty and are practically non-existent for some tectonic structures such as the Okhotsk microcontinent. Nevertheless we believe that it can be shown that there was no major (thousands of kilometers) horizontal motion between the separate tectonic blocks of Yana-Kolyma fold-and-thrust area, at least starting Middle Paleozoic. In paleogeographic terms Northeast Asia in the Permian represented a system of marine basins of various types. Okhotsk microcontinent was outboard from the Siberian craton to the southeast (present day coordinates). A system of deepwater marginal type marine basins lay to the east of the Siberian craton. The Koni-Taigonos volcanic arc was along the south edge, and. its erosion products formed deepwater fore-arc basins. Significant differences between the Permian bivalve communities on the Omolon microcontinent and contemporary communities of Verkhoyansk indicate the existence of the deepwater Ayan-Yuryakh trough basin. The strata of the latter are characterized as thick (up to 7 km) flysch deposits plus thick diamictites. Paleobiogeographic studies show that major biogeographic units can be clearly distinguished in the Verkhoyansk-Okhotsk on one side and Kolyma-Omolon biochores on the other, which can be currently ranked as sub regions. Verkhoyansk-Okhotsk sub region includes Verkhoyansk epicontinental sea shelf and the Okhotsk microcontinent shelf. These can be further subdivided into a number of provinces. The Kolyma-Omolon sub region includes continental shelves of the Omolon, Omulevka, Prykolyma microcontinents and the Koni-Taigonos arc. The degree of diversity of these two biochores is so great that it requires separate development and indicates the existence of a major biogeographical barrier during the Permian. The distinctions between the Verkhoyansk-Okhotsk and Kolyma-Omolon sub regions are found througout the whole Permian and over other faunal groups such as brachiopods and ammonites as well as over the rest of bivalve taxons. Multiple use of biogeographic, sedimentologic and paleomagnetic materials including new original data on sedimentology and paleomagnetism allowed a model of the relative positions of the basic tectonic structures of Verkhoyansk-Kolyma fold-and-thrust area in the second half of the Permian. . These studies have been supported by the Russian Foundation for Basic Research, grant N 03-05-96012-Arctic and Far East Branch Russian Academy of Sciences, Grant N 04-3-A-08-014.
GP41A-0825 0800h
Cenozoic Polarity Time Scale (CPTS) as the Tool of Dating and Correlation of the Cenozoic Strata in North-Eastern Russia
About 70 sections of continental and marine sediments covering he age through Paleocene to Pleistocene have been studied in North-Eastern Russia. Zones of polarity were used to determine the age and regional correlation of the strata with different biostratigraphic characteristic. On the basis of relation of magnetic zones to the CPTS, the dating of the major paleoclimatic and stratigraphic boundaries of the region fixed in the local stratons, has been obtained. There are the following events: chrones 25-24r - warming of the climate (boundary layers between Paleocene and Eocene ); chrones 13r - the beginning of the Eocene-Oligocene climatic pessimum; the boundary of the chrones 6Cn-6Cr - the climatic pessimum at the boundary of the Paleogene and Neogene; chrones 5Br - the middle of the chrone 5Adn - the first climatic optimum of the Miocene (the boundary of the Lower and Middle Miocene); chrones 5r - the lower part of the chrone 5n - the second climatic optimum of the Miocene ( the boundary interval between Middle and Lower Miocene). According to the paleomagnetic data, the supposed boundaries of the Miocene and Pliocene, Pliocene and Pleistocene are considered to be problematic and are fixed in the unique
GP41A-0826 0800h
Phanerozoic Paleomagnetic Reconstruction of Selected Parts of the Kolyma-Omolon Superterrane (Northeast Russia)
There are now sufficient paleomagnetic data for certain terranes of Northeast Russia (Omulevsky, Prykolymsky and Omolonsky terranes) to be able to determine temporal variations of paleolatitude and trace the dynamics of the displacement of these terranes over the whole Phanerozoic. The following paleotectonic development of the terranes studied is suggested: In Early Paleozoic (Cambrian - Early Ordovician Period) Prykolymski was located between 10° and 20° S. near the present day Northeast margin of the North Asian craton. In Early Silurian the Omulevsky terrane was located between the equator and 10° N, but by the end of Silurian it approached the craton and was located between 10° and 20° N near the current east margin of the latter. The Prykolymsky terrane has the same paleolatitude and location as was determined for the Omulevsky terrane for Late Silurian time. In the Middle Devonian their locations remained almost identical (between 30° and 40° N.). For Late Devonian time there are also data for Omolonsky. This terrane together with the Omulevsky and Prykolymsky terranes was located (at least from the Late Devonian) near the northern or northeast margin of North Asian craton in the west hemisphere between 30° and 40° N. and all three were rifting in a northerly direction. The data indicate the Prykolymsky terrane to be close to the craton in the Cambrian, maybe even in Riphean. For the Omulevsky terrane close correspondence with the craton is seen Ordovician time, data for earlier times being unavailable. Prykolymsky in the Middle-Late Devonian was rotated counterclockwise relative to the craton by 132±17.7°, for the Omulevsky terrane, from the Silurian to the Permian a counterclockwise rotation of 85-100° is observed. The Omolonsky terrane in Permo-Triassic turned 150° counterclockwise relative to the craton. During the Late Carboniferous and Permian times the scenario of the relative locations of the craton and blocks under study did not undergo significant changes. The Prykolymsky and Omulevsky terranes were located near the craton between 40° - 50° N. The Prykolymsky terrane in the Late Paleozoic was turned counterclockwise by 67,7±28.5° relative to the craton. There are conflicting data for Triassic time for the Omolonsky terrane. During the Jurassic period Omolonsky terrane was located at 60o N paleolatitude and by the Late Cretaceous it moved to 75° N. Omulevsky and Prykolymsky terranes were located tentatively on the same latitudes and this time. Paleotectonic evolution of the terranes described above is proposed based on the analysis of only paleomagnetic data. For complete justification (or refutation) of these locations it is necessary to involve additional geological information. The studies were supported by the Russian Foundation for Basic Research (project 03-05-96012 Arctica) and Far East Branch Russian Academy of Science (grant 04-3A-08-014).
GP41A-0827 0800h
Paleomagnetic results from accretional complexes of Taigonos peninsula and their paleotectonic application.
Mesozoic island-arc and oceanic rocks were sampled from accretional complexes of Uda-Murgal volcanic belt (Taigonos peninsula). Overprint components were obtained for: 1) Late Jurassic paleoarc basalts (D=10.6, I=72.7, k=44.6, a95=9.1); 2) Late Triassic oceanic sediments (D=70.5, I=73.7, k=61.1, a95=3.2); 3) Hauterivian forarc sediments (D=18.6, I=79.4, k=33.8, a95=3.3). Postfolding components, obtained for Late Jurassic basalts and Late Triassic oceanic sediments, have a good agreement with regional direction of remagnetization. Component, obtained in Hauterivian sediments indicate that these rocks were remagnetizated before folding. Prefolding component, isolated in Late Jurassic basalts (D=196.4, I=49.8, k=54.4, a95=5.7) implies paleolatitude 28.6 ±7.8°N. For Late Triassic sediments, prefolding components (D=217.3, I=4.5, k=79.6, a95=6.2) indicate a paleolatitude 2.3 ±3.2 °N. Assuming the prefolding direction, isolated in Late Jurassic paleoarc basalts, represent a primary magnetization, only one scenario is possible - studied island-arc basalts were formed on Izanagi plate. This result put very strict geometry constrains on configuration and relative positions of Paleopacific plates in Mesozoic.
GP41A-0828 0800h
Collision and Extension at Continental Margins: Example of the Sea of Okhotsk
The crustal structure of the northern Sea of Okhotsk region was reworked after the relict Okhotsk Sea plate collided with Eurasia about 55 Ma. Post-collisional extension and magmatic processes were likely initiated within the Okhotsk Sea plate and Asian margin along the remnant convergent plate boundary. The crustal structural pattern differs from the north and from the south by the remnant plate boundary. To the north, a series of northeast trending normal faults started to form during the Eocene extension within the Asian margin after collision with the Okhotsk Sea plate. To the south, a series of southeast trending normal faults was initiated due to extension within the Okhotsk Sea plate during the latest Oligocene through Early Miocene. Formation of Eocene volcanic rocks likely marks the magmatic processes which occurred at the beginning of Tertiary extension in the Sea of Okhotsk. Further studies are relevant to evaluate of earthquake potential and crustal movement along the remnant plate boundary and to correlate the deformation events within the Okhotsk Sea plate with the arc-continent collision event occurred at Kamchatka about 55 Ma. This work is funded by Russian Science Support Foundation.
GP41A-0829 0800h
Geophysical Fields and Geodynamics of Eastern Chukotka
The geology of Chukotka peninsula is one of the most important problems in Beringia development. The absolute age of some lithological assemblages, with the preservation of their composition, was changed by modern studies. This has resulted alternat explanation of geological development of some structures. For examples, for metamorphic assemblages, it is supposed that they have occurred as a result of tectonic activity and elevation to the surface of rocks warmed at the depth. This processes was synchronous with the formation of the Okhotsk-Chukotka volcanogenic belt (OCVB) The study of the deep composition by the geophysical methods should stimulate the knowledge of geological development of the Eastern Chukotka. The anomalous magnetic field of Chukotka peninsula is correlated by geological occurrences. Outcrops of sedimentary and metamorphic rocks, granitoid intrusions correspond to the calm, close to normal magnetic field. Within intrusion and at their boundaries, small in area, high gradient anomalies, associated with dikes of basic composition and zones of contact metamorphism, are observed. Zons of intensive linear anomalies are traced above the Kolyuchin-Mechigmen riftogenic depression. They are result of presence of high magnetic subvertical bodies of the ultrabasic composition of Triassic age. These anomalies are also traced in the Bering Sea. The rocks of OCVB, mosaic magnetic field with smoll isometric or ellipsoidal anomalies occur. The ultrabasic rocks of Triassic age occur in the gravity field by the local positive Bouguer anomalies up to +40 mGl. To the south from Kolyuchin Bay, their thickness reaches 10 km. In the region of the Mechigmen Inlet their thickness does not exceed 2.5-3 km. Probably ultrabasic rocks of the same thickness are located in the region to the north-east of the Kolyuchin Bay coast. The outcrops of granitoid intrusions are marked by negative anomalies of up to -20 to -25 mGl. The field character makes it possible to suppose that at depth, most of them are combined, and form the line of mass, large in area. Sedimentary deposits of Paleozoic correspond to the small ( up to +10 - +15 mGl) anomalies. Metamorphic domes are marked by small negative anomalies, that are composed of great minimum, associated with granitoid. Poorly negative gravity field, complicated by anomalies that are associated with granitoid, are observed above the rocks of OCVB. The regional gravity field of the Chukotka peninsula along the coastal is positive ( up to 10 mGl), but within the land , it is negative (up to -15 mGl). It is explained by the fact of post-glacial rebound after the melting of glacier falling into the Bering sea. This fact is testified by the rise of the Chukotka Sea coast. It is possible, that the line of earthquakes, having the spreading mechanism, is related to these processes. Minimum zones correspond to the regions of the largest seismic activity in the field of velocity of longitudinal and cross seismic waves. The analogous geodynamic environments are observed on the Scandinavia peninsula.
GP41A-0830 0800h
Seismicity and Tectonic Stress Field of the Laptev Sea Shelf
The seismicity of the Laptev Sea shelf, Arctic Ocean, northeast Russia, is primarily concentrated in three north-south to northwest-southeast striking bands that link the mid-ocean Gakkel (Arctic Mid-Ocean) Ridge to the active deformation belts of the Verkhoyansk and Chersky Ranges. The main band extends from Gakkel Ridge to Yana Bay with events of magnitude 5.5-7.0, and is presumed to represent the main boundary between the North American and Eurasian plates. Two zones of weaker seismicity parallel the main band. The West Lena - Taimyr zone follows the western edge of the Laptev Sea shelf and extends from the Lena River delta to Taimyr Peninsula and on to Severnaya Zemlya. Over 300 weak earthquakes have been recorded from this zone with three significant clusterings of events. The first (1980) is located on the east side of Olenek Bay and the second on the west side (1987-1988) faulting). The third cluster occurs on Taimyr Peninsula and had a series of M 4-5 events over the past 50 years. All of these events have a normal faulting to transtensional focal mechanism. The mechanism for the 1990 event (strike 160, dip 63, slip -175) was obtained using Russian regional network data. Geological surveys show that active faults in Taimyr are expressed as linear canyons of northwest to northerly strike with highly fractured rocks. For the 1990 earthquake, this corresponds to normal faulting with a right-lateral strike-slip fault component. The other seismic band is located in the area of the New Siberian Islands and the East Siberian Sea is less considerably less active than the Lena-Taimyr band with only a few dozen weak earthquakes in 60 years. The largest of them (M = 4.9) was a thrust event that occurred in the East Siberian Sea. The two extensional bands are presumed to bound the region of active extension in the Laptev Sea rift system and delineate a Laptev Sea block. The thrusting in the East Siberian Sea may be a result of the reactivation of an old suture zone resulting from ridge induced compression in the adjoining continent.
GP41A-0831 0800h
Reconstruction of Cretaceous-Cenozoic Tectonic Stress Fields in the Vicinity of Diffuse Plate Boundary Between North American and Eurasian Plates (Laptev Sea Region)
We present reconstructions of the Cretaceous-Cenozoic tectonic stress fields for the Laptev Sea Region. This study is based on analysis of the mesostructures observed mainly within the Middle-Upper Paleozoic sedimentary rocks along the western part of the New Siberian Archipelago (Bel'kov Island) and the Northern Verkhoyansk Range. The earliest stage of deformation is expressed in the syn-sedimentary (slump) folds identified in the Upper Paleozoic clastic rocks in the both regions. It seems that they mark the earliest stages of the South Anyui oceanic basin development. The main compressional stage took place in the Early Cretaceous over the vast Siberian Arctic continental margin and is known as Late Mesozoic Orogeny. According to our results, studied areas reveal NE-SW and NW-SE compression stresses for Bel'kov Island and Northern Verkhoyansk Region respectively. Then, most likely in the Late Cretaceous, both regions were subjected to E-W compression followed by N-S compressional stage. The latter is well-determined in the Northern Verkhoyansk Range and much less expressed at the Bel'kov Island. During subsequent (latest Late Cretaceous-Cenozoic) rift-related stage the tension stress was oriented E-W to NE-SW, roughly orthogonally to the Laptev Sea rift system; though a secondary NW-SE extension stress is also recognized. The work was supported by INTAS grant 01-0762 (NEMLOR), MK-2487.2003.05, NS-1980.2003.5 and RSSF grants.
GP41A-0832 0800h
Russian East Arctic Shelf: Structural Outline
The Laptev Sea Basin is open towards the Eurasia Basin. A set of grabens have been discovered within the basin. The sedimentary cover thickness exceeds 10 km. The initial Precambrian basement was rejuvenated in the MZ. The sedimentary cover is dividable into late Precambrian to early Cretaceous and early Cretaceous to CZ structural stages. The Sviatoi Nos-Kotel'ny High and The De Long High mark the boundary between the Eurasia and Amerasia continental margins separating the Laptev Sea Basin from the basins underlying the East Siberian Sea (ESS) and the Chukchi Sea (CHS) shelf. Late Kimmerian basement and PZ and MZ moderately deformed strata were mapped within the Sviatoi Nos-Kotel'ny High. The De Long High is located on the NW ESS. The Caledonian tectonic basement is exposed on the New Siberian islands. PZ and MZ strata of the intermediate structural stage may be present offshore. The upper structural stage of the sedimentary consists of Cretaceous to CZ volcanics and siliciclastics. The ESS and CHS shelf is a huge basin filled with Pliocene and Quaternary sediments deposited on a complex structural ensemble. The Novaya Sibir' Megabasin is comprised of a series of structures. The most prominent one is the New Siberian Rift Basin with sediments up to 12 km thick. The New Siberian Basin parallels the different aged basement blocks which mark the boundary of the late Kimmerian deformation front. The Vil'kitsky Rift Basin is located on the NE ESS and N CHS. It projects as a continental slope basin to the NW and crosses the US/Russian boundary to the East. The depth of the basin exceeds 17 km. And contains an early Cretaceous molasse at the base of the sedimentary fill. Late Cretaceous sedimentation was fault controlled. Tertiary strata form a platform type syncline. The lowermost reflections on the south slope of the Basin are related to the intermediate structural stage. The Wrangel-Herald Ridge separates the Vil'kitsky Basin from the South Chukchi Basin. The late Kimmerian tectonic basement capping the ridge is exposed on Wrangel island. The South Chukchi Basin extends from the SE ESS to the SE CHS. The basin fill is late MZ to CZ strata up to 8 km thick, overlying the late Kimmerian basement. The basal portion of the sedimentary fill is believed to be the early Cretaceous molasse.
GP41A-0833 0800h
GIS Compilation of Geophysical Data for Northeast Russia and Adjacent Areas as a Tool for Constructing Tectonic Elements
A large part of Northeast Russia consists of extensive offshore continental shelf. To construct a comprehensive plate tectonic model for the circum-Arctic region we need to understand the geological links between the Northeast Russia mainland, its adjacent shelf and the ridges in the Arctic Ocean , the oceanic Gakkel Ridge, the continental Lomonosov Ridge and the enigmatic Mendeleev Ridge. Geological studies on thefew islands do not provide sufficient data to fully understand the tectonic evolution of the surrounding shelf. Modern GIS compilation of available geophysical data is important and useful for constructing the main geophysical and tectonic elements of the region. Data include public domain compilations such as Magnetic Anomalies of the Arctic and North Atlantic Oceans and Adjacent Land Areas (J. Verhoef et al., 1998), International Bathymetry, International Bathymetric Chart of the Arctic Ocean (R. Macnab et al., 2000) and the Arctic Gravity Project (R. Forsberg et al., 2001). Poor geophysical coverage of the Northeast Russian shelf and adjacent deep ocean areas is additionally complicated by the diversity of technologies applied during data acquisition. For example, gravity data includes land stations, marine gravity, aerogravity, satellite altimetry and submarine measurements. As a consequence, numerous artifacts appear at the junctions of neighboring survey areas resulting from different quality and level of detail of data. To eliminate this problem, original GIS for magnetic, bathymetry, gravity and seismic datasets available in Russia was compiled and this dataset enabled better definition of the true geophysical or tectonic elements and their discrimination from technologic artifacts.
GP41A-0834 0800h
A Possible Differentially Shortened Strike-slip Plate Boundary: the Okhotsk Plate Example.
The Okhotsk plate has been postulated based on a combination of GPS geodetic inversions (REVEL1), seimsicity, geologic and lineament data. Lying between the North American and Eurasian plates, its northwestern corner would appear to be undergoing compression in a scissors motion between the two bounding plates. Extrusion tectonics along multiple, large strike-slip faults within the Okhotsk plate itself have been suggested to allow the escape of material away from the apex of Eurasia-North America. The plate boundary between Okhotsk and North America has been suggested to be diffuse, based on widely scattered minor seismicity. However, the large, left lateral, Ulakhan fault has also been suggested as a candidate plate boundary. We present field geological and geomorphological evidence of the partitioning of deformation between the Ulakhan fault, and several parallel and oblique, linked faults. The Ulakhan fault strand appears to have a maximum displacement of 24 km based on river valley offsets and closing large pull apart basins. Some of the displacement from the Ulakhan fault appears relayed into the plate margin along oblique trending, thrust/oblique slip faults. Estimated shortening over these faults is equivalent to the amount of shortening relayed into the plate margin from the plate boundary. There may be several thrust/oblique slip faults along the Ulakhan fault, which leads to the interesting situation of a segmented, strike-slip plate boundary being actively shortened in a margin parallel direction. This may be the result of postulated extrusion of the Okhotsk plate due to North America/Eurasia convergence. Such a situation would have important consequences for the interpretation of GPS data in a plate tectonic context.
GP41A-0835 0800h
A Seismic Swarm Near Neshkan, Chukotka, Far Northeastern Russia
Felt earthquakes have been occurring almost daily near the village of Neshkan, Chukotka (67.05N, 173.02W), in far northeastern Russia, since December, 2002, shortly after the Denali earthquake in Alaska. The events occur in a region that has experienced several large events in the past, including four magnitude 7 events in the 1920s and magnitude 5-6 events in 1962, 1971, and 1996. As a result of this activity, a permanent seismic station was deployed at Neshkan in September, 2003, and two field stations were operated during the second half of that month. During the two week field deployment, over 150 events were recorded, the largest having a magnitude of 3. About twenty-five events were recorded well enough to locate; these form a linear trend from 1 km west of the village south-southwest over a distance of 20 km. In addition, a small pond drained near the village suggesting there has been some near surface deformation. We suggest these events are associated with a previously unknown fault which defines the eastern edge of the Kolyuchin basin. Analysis of the 1996 sequence suggests that that series may have ruptured the same fault as well. These events, in addition to relocations of small events in the central part of the peninsula, suggest that much of the recent seismic activity occurs on northeast striking faults and that their significance has previously been underrated. Earthquakes continue (through June, 2004) to be recorded at the Neshkan station, some with S - P times which indicate that many of the events are occurring within a few kilometers of the village; 600 events were recorded in December, 2003, alone. Hot springs, signs of active faulting, and quaternary volcanics are found throughout eastern Chukotka, some in close proximity to Neshkan. Combined with similar evidence on Seward Peninsula, it is suggested that an extensional regime exists near Neshkan and this raises the possibility that the events are related to magma migration.
GP41A-0836 0800h
Relationships Between Seismicity and GPS Determined Velocities in Northeast Asia
Published GPS data are equivocal about the existence of the Okhotsk and Amur plates in northeast Asia. Seismicity data, however, clearly delineate these plates, as well as possible additional blocks. The northern boundary of the Okhotsk plate is separated from both North America and Eurasia by diffuse zones of seismicity in northeast Russia. Seismicity in the Magadan region coincides with faults clearly visible in satellite imagery that are left-lateral based on offset rivers and focal mechanisms. The largest earthquakes (~M 5) are associated with specific faults. Although GPS velocities are generally consistent with the motion of North America, their residual velocity (with respect to North America) shows a gradient across this zone of seismicity with relative displacements increasing towards the Okhotsk plate. The eastern boundary of the Amur plate is generally accepted as lying along Sakhalin Island and bordering the Okhotsk plate. However, several bands of north-south seismicity exist west of this boundary along possible extensions of the Tanlu fault of northeastern China. This seismicity extends the Tanlu to intersect with an east-west striking zone of seismicity separating Amur and Eurasia which connects Lake Baikal and Sakhalin. This seismic extension of the Tanlu implies that a small block may exist between Okhotsk and Amur that extends south to include Korea. Although the events are small, the linear and arcuate trends in seismicity clearly imply active motion along specific faults. However, there is no clear signature of active faulting in the geomorphology or geology. GPS sites in eastern Amur are too close to active to constrain block motions, however, additional blocks may be needed in interpreting GPS results from Amur, Sakhalin, Korea, and northeast China, especially given the low velocities with respect to Eurasia.
GP41A-0837 0800h
The Setting of the Okhotsk-Sea Micro-Plate Northern Boundary and its Seismicity
The Sea of Okhotsk is surrounded by seismic zones. That is why we believe that its water area corresponds spatially to hard platform. The Okhotsk-sea micro-plate in triangular form is located at the joint of the Pacific, North-Amerikan, and Eurasian plates. It is bounded from the west by sub-meridional zone of deep sesmogenic faults spreading through the Sakhalin island. From the east, micro-plate is bounded by subduction zones, extending along the Kuril Islans and the eastern coast of Kamchatka. The northern boundary of the Okhotsk-sea plate is considered differently by different authors. The analysis of data of the marine seismic exploration by the method of common deep point, deep seismic sounding, gravitation field and the anomalous magnetic field, and also the seismological observation by the equipment make possible to conclude that the northern boundary of the Okhotsk-sea micro-plate has the sublatitudinal extending and spatially coincides with wide zone of deep faults confided to the coast. On land and offshore, this zone is marked by the line of riftogenic basins filled with Cenozoic continental sediments. On the continent, their thickness is not more than 1500-2000m, although on the offshore it reaches 8-10 km. The basins are mostly spread in meridian direction. The epicenters of the crust earthquakes are spatially related with these basins. This fact testifies of the right-lateral faults. Cenozoic sediments in basins of land are broken by numerous fault-shifts with the summary amplitude of horizontal displacements up to 1.5 km and by the vertical ones up to the first hundreds of meters. The northern boundary of the Okhotsk-sea micro-plate is marked by the change of the Earth's crust type: the continental type on the north, and transitional type on the offshore. Geological and geophisical data make possible to suppose that the northern boundary of micro-plate in primagadanskaya part coincides with the Benioff paleozone that closed its active existence in Paleogene.