GP44A-01 16:00h
Apatite Fission-Track Thermochronology of the Southern Verkhoyansk Fold-and-Thrust Belt, Russia
The Verkhoyansk fold-and-thrust belt of eastern Yakutia is one of the largest Mesozoic compressional belts. It involves an immense prism of strata deposited on the passive margin of the North Asian craton from Late Proterozoic through Jurassic time. Current models suggest that shortening in the Verkhoyansk was the result of collision and accretion of terranes against the Asian margin. The onset of deformation is constrained by the beginning of rapid subsidence in the Pri-Verkhoyansk foreland basin during Late Jurassic and by a poor quality $151\pm1$ Ma $^{40}$Ar/$^{39}$Ar age from the Sette-Daban zone of the Southern Verkhoyansk. The central part of the fold-and-thrust belt is underlain by low-grade Late Paleozoic basinal clastic rocks intruded by granodioritic plutons (119-123 Ma, $^{40}$Ar/$^{39}$Ar biotite). To the west there is a 25km-wide belt of intensely deformed Early to Middle Paleozoic basinal strata known as the Sette-Daban zone, which has a fan-shaped cleavage pattern. Still further west, lies the 40-80 km wide Kyllakh Zone composed of a series of west-vergent thrusts that involve Late Proterozoic-Jurassic platformal strata. Conodont alteration indices reveal that there were up to 14 km of tectonic and sedimentary burial in the Sette-Daban, while vitrinite reflectance data show that foreland basin sedimentation was minimal. We carried out apatite fission-track (AFT) thermochronology on 17 samples collected along a 120km long transect between the Allakh-Yun' and Yudama rivers. One apatite sample from Lower Jurassic sandstone of the foreland is not reset, yielding an age of $210\pm10$ Ma. Five samples from the frontal thrust sheets yielded central ages ranging from $78\pm6$ to $91\pm9$ Ma. However, all these samples have mean track-length distributions of less than 13.2 micrometers, indicative of relatively long residence within the PAZ. These ages reflect erosional denudation as a response to thrust uplift beginning in mid- to Late Cretaceous time followed by slow cooling during the Tertiary. A high-quality AFT age from the Tarbagganakh pluton is $90.5\pm3.2$ Ma. Track-length modeling shows that this sample underwent rapid cooling from 120 Ma (approximate intrusive age) to about 80 Ma, followed by slow cooling after the Late Cretaceous. Five other ages from the Allakh-Yun' zone are younger, ranging from $74\pm3$ Ma to $49\pm3$ Ma. The older ages are compatible with those from the frontal thrusts. Track-length modeling of the youngest age reveals two episodes of rapid cooling, one in the Early Cretaceous and a second one between 50 and 30 Ma. This second cooling episode may reflect late-stage deformation. Four samples from Sette Daban zone also yielded younger AFT central ages ranging from $45\pm6$ to $60\pm6$ Ma. Our preliminary interpretation is that these latest Cretaceous-early Tertiary ages reflect uplift associated with deformation along the N-S striking Burkhala left-lateral fault which cuts this portion of the thrust-belt.
GP44A-02 16:15h
Geochronology and Tectonic Significance of Mafic and Ultramafic Rocks From Yakutia, Russia
Mafic and ultramafic (ophilite) rocks from the western edge of the Kolyma-Omolon Superterrane (KOS) in central Yakutia record a Paleozoic to Mesozoic history of formation, accretion and post accretion deformation. Previous geochronologic and geochemical work focused on the Uyandina and Munilkan ophiolites, from the northern Chersky Range. Amphibole ages from these gabbros and amphibolites are 370 to 430 Ma reflecting the age of ocean crust, while micas record obduction and amalgamation of the KOS at 170 to 174 Ma. New data from the Ucha (Indigirka), Debin, Garbyn'ya ophiolite fragments and gabbros from the Tommot massif in the central and southern Chersky Range indicate that these units have different ages and thus record different aspects of the tectonic history of the region. For example, mafic rocks from the Ucha ophiolite have ages of 150 to 160 Ma, reflecting either younger oceanic crust or resetting at the time of arc volcanism. The Debin mafic-ultramafic fragment has an age of 144 Ma, reflecting the time of collision of the KOS. Ages from the highly sheared Garbyn'ya ophiolite reflect an even later resetting event at 90 Ma and may reflect the time of reactivation of strike-slip fault movement parallel to the Chersky Range. This age is also seen in samples from altered granitic rocks in the central Chersky range, again reflecting later faulting. Less well understood are similar ages from the Alazaya uplift in the central KOS and their relation to contemporaneous events in the South Anyui Suture zone or to Okhotsk Chukotka volcanism. The Tommot massif has late Devonian to Carboniferous ages, perhaps recording mid-paleozoic rifting. New 40Ar/39Ar geochronology from arc-related volcanic rocks from the Uyandina-Yasachnaya volcanic arc of the Chersky Range, confirms the late Jurassic age of subduction. These ages are consistent with previously published ages from subduction- and collision-related plutonic rocks recording the accretion of the KOS to Eurasia.
GP44A-03 16:30h
Timing and Kinematics of Eocene Olyutorsky Arc-Continent Collision
The collision of the Cretaceous Olyutorsky terrane with the northeast Asian margin was critical event in Cenozoic evolution of the northern Pacific. Remnants of the far-traveled Olyutorsky terrane can be traced for almost the entire length of the Kamchatka Peninsula, and likely extend eastward into the Bering Sea as the Shirshov and Bowers Ridge for an aggregate length of ~ 3000 km. The Vatyna Thrust representing the collisional suture between Ukelayat Flysch in the lower plate and Olyutorsky arc can be traced for 2000 km from southern Koryakia to Kamchatka. The Olyutorsky terrane comprises Late Cretaceous MORB-type basalt, chert, island-arc volcanic rocks, and associated zoned Alaskan-type intrusions. The Ukelayat flysch consists of a 10-to-15 km-thick zone of deformed Late Cretaceous to Eocene turbidites. In southern Koryakia and the Lesnovsk highlands brittle fault zone kinematic indicators suggest that the initial phase of collision was controlled by oblique arrival of the Olyutorsky terrane to southeast-facing NE Asian margin. Subsequently, orthogonal convergence prevailed as indicated by later NW-SE directed imbricate thrusting and NE-SW trending tight to isoclinal folds developed in the Ukelayat flysch. Stratigraphic and cross-cutting relationships in the Lesnovsk highlands provide tight control on the timing of thrusting. Here, detrital fission-track grain-ages and biostratigraphic data suggest that the deformed Lesnaya group rocks span the Upper Cretaceous to Middle Eocene. Further, the Vatyna Thrust is cross-cut by the Shamanka granitic massif and unconformably overlain by Kinkil Formation felsic volcanic rocks both of which yield 45 Ma U/Pb zircon ages, placing a Middle Eocene lower age limit on the timing of arc obduction. Low temperature thermochronologic data indicate that post-orogenic exhumation in this region proceeded slowly (~ 50 m/myr). Geochronologic data and regional structural observations from Sredinny Range (southern Kamchatka) indicate that collision began in the Late Paleocene. Syn-to-post collisional exhumation was initially rapid (~3 km/myr) in the Middle Eocene. In total, the high-grade core of the Sredinniy Range has been exhumed to depths of ~30 km. These new timing constraints indicate that arc obduction proceed diachronously. To explain this difference in the collisional and post-collisional tectonic histories, we argue that a transform fault likely separated the southern and northern portions of Olyutorsky terrane.
GP44A-04 16:45h
Detrital Zircon Ages from Late Jurassic-Early Cretaceous Myrgovaam Basin Sandstones (Rauchua Trough), Western Chukotka, NE Russia
Late Jurassic to Early Cretaceous Myrgovaam Basin sediments (previously Rauchua Trough) are regionally significant because of the stratigraphic constraints they provide on the age and progression of deformation in the Chukotka fold belt, a possible along-strike continuation of the Alaskan Brooks Range fold-and-thrust belt. Existing descriptions of the structural and stratigraphic relations of Myrgovaam Basin sediments to underlying strata are contradictory. Some maps portray the basin fill as deposited unconformably over deformed Triassic and Jurassic strata of the Chukotka fold-belt. In other publications, the deposits are described as structurally detached and imbricated by N-verging thrust sheets (Baranov, 1996). Field studies reveal that underlying strata are tightly folded compared to overlying strata and that the contact is a structural discordance not an unconformity. More locally, we observed arkosic sandstones typical of the Myrgovaam Basin interbedded with underlying Late Jurassic strata or present as submarine channel deposits cut into older rocks, suggesting an original stratigraphic relationship. To reconcile these observations we suggest regional deformation post-dates deposition of Myrgovaam Basin deposits, and that the disharmony in deformational style between underlying thin-bedded Triassic sandstones and shales and (stratigraphically) overlying massive quartzites, is due to their different mechanical properties. Petrographic studies indicate that fine-grained Triassic-early Jurassic sandstones represent a distal recycled orogen source, while Myrgovaam Basin sandstones originated from a proximal orogenic source containing granitoid and crystalline basement rocks (microcline, biotite, muscovite and fragments of multiply deformed schist) and intermediate to felsic volcanic rocks. Laser Ablation ICPMS was used to date zircons (100 grains) from sandstones of the Myrgovaam Basin and compare them to those in Triassic sandstones (300 grains) and verify that Myrgovaam Basin deposits represent a major change in clastic source regions. Zircon populations from Triassic sandstones have age peaks in cumulative probability plots at 247, 298, 380, 453, 504 and 566 Ma (63 percent of zircon population). Only 12 percent of the grains are older than 1.8 Ga. In contrast, zircons from Rauchua Formation sandstones have age peaks at 180, 270, 322, 390-420 (43 percent of the grains). Over 40 percent of the zircons are 1.8-2.2 Ga. The immaturity of sandstones of the Myrgovaam Basin and their abundance of Precambrian zircons, suggest basement-involved faulting during deposition. Since Myrgovaam Basin deposits likely pre-date folding in the Chukotka fold-belt, faulting could be related to either the onset of rifting of the Arctic Alaska-Chukotka plate away from its parental continent or to the beginning of collision-related thrust faulting, but there are no known exposures of 1.8-2.2 Ga rocks in Chukotka. Jurassic zircons, representing a very small part of the population, suggest a proximal magmatic source and provide a maximum age for these strata.
GP44A-05 17:00h
Paleomagnetism of the Kamchatka region, northeastern Russia: Implications for the evolution of the northwest Pacific Basin
The Kamchatka Peninsula of northeastern Russia is located along the northwestern margin of the Bering Sea and consists of zones of complexly deformed accreted terranes. Along the northern portion of the peninsula, progressing from the northwestern Bering Sea inland the Olyutorsky, Ukelayat, and Koryak superterranes are accreted to the Okhotsk-Chukotsk volcanic-plutonic belt in northern-most Kamchatka. Together paleomagnetic results and their close agreement with the calculated terrane trajectories strongly suggest that an island arc system moving with the Kula plate collided with this region of North American plate approximately 50 Ma (Middle Eocene). Previously published plate motion models for oceanic plate systems in this region have suggested such a subduction boundary between the Izanagi and Farallon plates. In the model of Engebretson et al. [1985] this region becomes part of the Kula plate at 85 Ma, after initiation of Kula-Farallon motion. Paleomagnetic data show that fragments of an island arc are present in the Olyutorsky superterrane and East Kamchatka superterrane as predicted by this model; perhaps the tectonostratigraphic terranes of the northern Kamchatka represent an accreted fragment of an oceanic island arc system between the Izanagi and Kula plates. The coincidence of this age of accretion (approximately 50 Ma) and the age of initiation of subduction along the Aleutian arc (estimated as Late Paleocene to Early Eocene) suggests the possibility of a causal relationship. The small circle geometry of the present day Aleutian arc and the right angle intersection of magnetic anomalies interpreted, over the oceanic plate presently beneath the eastern Bering Sea, with the arc both strongly suggest that the region now occupied by the Aleutian arc subduction zone was originally a transform fault within an oceanic plate. Given this pre-existing zone of weakness within the subducting oceanic plate and its apparent close proximity to the region undergoing obduction of the Olyutorsky superterrane during the Early Eocene, we strongly favor this model for formation of the present-day Aleutian arc subduction zone. A plate tectonic animation of this scenario will be shown.
http://harbert.geology.pitt.edu
GP44A-06 17:15h
Deeply Exhumed Roots of an Eocene Arc-Continent Collision Zone, Sredinniy Range, Kamchatka
The Olyutorsky arc-continent collision is marked by the Vatyna Thrust, a regionally extensive suture zone, across which Late Cretaceous-Paleocene Olyutorsky island arc and retro-arc basinal rocks are juxtaposed against the continentally derived Late Cretaceous to Eocene Ukelayat flysch. Geologic relationships in northern Kamchatka and Southern Koryakia are relatively straightforward, and here, the timing of arc obduction is tightly constrained to the Middle Eocene. In southern Kamchatka, within the Sredinniy Range, the major elements of this collision zone are recognized flanking the N-S trending metamorphic core; however, internal structural complexity, old protolith and metamorphic age estimates (Archean to Early Cretaceous) and the high-grade of metamorphism (up to granulite facies) have precluded a straightforward interpretation for the occurrence high P-T metamorphic rocks in the context of this arc-continent collision. Most tectonic models invoke basement-involved thrusting of a previously accreted microcontinental block. We outline a new tectonic model based on recent field, microstructural, thermobarometric and thermochronologic work which demonstrates that the high-grade core of the Sredinniy Range was developed in the Early Eocene during arc-continent collision affecting stratigraphic equivalents of the Ukelayat Flysch and Olyutorsky arc in the absence of microcontinental basement. The Sredinniy Range comprises granulite to amphibolite facies gneiss and schist of continental affinity (Sredinniy Complex) structurally overlain by amphibolite to greenschist facies mafic schists (Malka Complex). The protolith age of the Andrianovka mafic schist within the Malka Complex is problematic with estimates ranging from Precambrian to Cretaceous. Metamorphosed cherts therein contain morphologically well-preserved recrystallized radiolaria. A nearly identical suite of moderately recrystallized radiolaria is found in the low-grade Iruney Group rocks, suggesting that Andrianovka Schist is likely an upgraded analog of the Iruney Group. Similarly, the origin and protolith ages for the Kamchatka Schist and Kolpakova Gneiss are highly debated with estimates ranging from Archean to Early Cretaceous. Our study utilizes minimum detrital zircon U/Pb grain-ages for these metasedimentary units to constrain maximum protolith depositional ages. The Kolpakova Gneiss and Kamchatka Schist give Late Cretaceous and Paleocene stratigraphic age estimates, respectively. Further, a favorable comparison between detrital zircon grain-age probability densities for the Kamchatka Schist and a sample of sub-greenschist facies Ukelayat Flysch suggests that the former is likely a metamorphosed analog of Cretaceous-Paleocene NE Russian marginal strata. U/Pb ages for zircon overgrowths observed in migmatites of the Kolpakova Gneiss and Th/Pb monazite ages indicate that peak metamorphic conditions, including partial melting, occurred at ~52Ma - broadly synchronous with the Olyutorsky arc-continent collision. Taken together, these new correlations suggest that high-grade metamorphic rocks of the Sredinniy Range are best viewed as the metamorphosed, deeply exhumed roots of an Eocene arc continent collision zone.
GP44A-07 17:30h
Mesozoic-Cenozoic reworking of the deep crust beneath the Bering Sea plate: Data from lower to middle crust xenoliths
Petrographic and geochemical investigation of lower to middle crustal xenoliths within Late Neogene alkaline basalts of the Bering Sea province (Enmelen volcanoes, Chukotka; Imuruk volcanic field, Alaska; Kookooligit volcanic field, Saint Lawrence island) were carried out to determine the composition, conditions of equilibration and age of the deep crust beneath the Bering Shelf. Mineral thermobarometry shows that xenoliths were derived from the middle to lower crust (~ characterized by pressures of 3 to 10 kb and elevated temperatures of 700 to 1050 C). Three groups of xenoliths can be distinguished: 1) charnockites and quartz- two pyroxene granulites enriched in REE and having Sr and Nd isotopic ratios close to BSE; 2) gabbroic restites and pyroxene-plagioclase cumulates which have relatively depleted and flat REE patterns, and positive Eu anomalies; 3) kelephytic (garnet) gabbro enriched in HREE and depleted in LREE. Radiogenic isotope in pyroxene and plagioclase separates from studied xenoliths have ratios fall into the major mantle array attributed to OIB-like mantle source (87Sr/86Sr =0.7040-0.70463; 143Nd/144Nd= 0.51252-0.51289; 206Pb/204Pb= 18.32-18.69). Isotopic data rule out a close genetic link between the host lava and xenoliths but shows possible relations of some xenoliths with Cretaceous calc-alkaline magmatic rocks associated with the Pacific Margin related Okhotsk-Chukotsk volcanic belt. Nunivak and Navarin xenoliths from the southern edge of the Bering shelf are different in their geochemistry and isotopic ratios (MORB-like signatures) from the rest of studied xenoliths and this might suggests diverse composition of the lower crust beneath the area. Zircons in charnockite and granulite xenoliths were dated by the U-Pb method using the SHRIMP RG and range from 60 to 107 Ma. The zonation patterns of the zircons, their geochemistry, U-Pb ages and how these characteristics vary with rock types and mineral association were used to reconstruct a general model for the petrologic evolution of the deep crust with added constraints provided by geophysical and geologic data. It can be concluded that the lower crust beneath the Bering Sea province has been modified significantly during Cretaceous (107 - 80 Ma) magmatic underplating and was later subjected to a thermal/metamorphic event at 75 - 60 Ma. These two events coincide with two major pulses of calc-alkaline magmatism represented by plutons and volcanic rocks at the surface. These two events occurred together with, and post-dated inferred extensional collapse of previously thickened crust beneath the Bering Shelf . Similar data from xenoliths across a broad area of the U.S., Russia and the Bering Shelf support the regional nature of these conclusions concerning the magmatic and tectonic evolution of the deep crust. Geotherms and metamorphic gradients derived from xenoliths study are too hot to be explained by relatively cold terrains involved in collision-type deformation and require the imput of heat into the crust by mantle derived magmas.
GP44A-08 17:45h
Detrital Zircon U-Pb Ages From Northern Alaska and Northern Canada: Implications for Opening of the Canada Basin
Detrital zircons from five Paleozoic and Mesozoic sandstones from the Brooks Range and North Slope of Alaska, and two Triassic sandstones from the Sverdrup Basin in northern Canada were dated using SHRIMP and laser ablation ICPMS geochronology. In Alaska, dated samples from allochthonous strata of the Brooks Range include the northern and southern lithofacies of the Devonian and(or) Mississippian Kanayut Conglomerate near the Trans-Alaska Pipeline and the Valanginian Tingmerkpuk Sandstone in the western Brooks Range. Also dated are zircons from autochthonous strata of the North Slope foreland, including the Lower Triassic Ivishak Sandstone in the Sadlerochit Mountains and a Hauterivian sandstone from the Jurassic and Lower Cretaceous Kingak Shale in the Tunalik well. In Canada, dated samples are from the Lower Triassic Bjorne Formation and the Upper Triassic Pat Bay Formation on the southeastern and northern margins of the Sverdrup Basin, respectively. The two Valanginian samples from Alaska and the Bjorne Formation in Canada yielded generally similar results dominated by 1.0 to 2.0 Ga zircons, with lower Paleozoic (especially 420 to 450 Ma) and less abundant Archean zircons. Samples from the Triassic Ivishak and Pat Bay Formations also are similar, but are dominated by 530 to 570 Ma zircons with less abundant older zircons. The two samples of Kanayut Conglomerate, in contrast, are dominated by 430 to 400 Ma zircons and yielded only minor amounts of Proterozoic zircons. The similarities between the Sverdrup Basin and northern Alaska samples suggest they were derived from similar source areas and lend support to models that call for opening of the Canada Basin by counterclockwise rotation of northern Alaska and Chukotka in the Early Cretaceous. Moreover, the Alaskan samples, all derived from generally northerly source areas, may indicate that unroofing in the source area progressed from strata rich in 450 to 400 Ma zircons in the Devonian to rocks yielding abundant Proterozoic and Archean zircons in the Early Cretaceous deposits. This pattern is consistent with the unroofing sequence interpreted from Nd isotopic data from the Sverdrup Basin by Patchett and others (2004, J. Geology, p. 39 to 57), who concluded that detritus from 450 to 350 Ma tectonism in the Caledonian and Ellesmerian orogens covered Arctic Canada until it was progressively removed in the Mesozoic.