RESULTS FROM PRIOR SUPPORT Project Title: Collaborative Research: Crustal Evolution of the Bering Shelf-Chukchi Sea (Stanford P.I.'s Simon Klemperer and Elizabeth Miller) Dates: 03/01/94-8/31/97 Amount awarded: $903,000 Award Number: Continental Dynamics Division NSF EAR 9317087 The slightly submerged Bering Shelf-Chukchi Sea region comprises over 50% of the total U.S. continental shelf area and forms a broad isthmus of continental crust connecting the North American and Asian continents (Fig. 1). Although harsh climate and remoteness make this a difficult area to work in, there has been much interest in the stratigraphic framework and evolution of sedimentary basins on this shelf because of substantial oil and gas reserves in the region. However, the overall geologic and tectonic evolution of the continental crust beneath this region, the continuity of geologic structures between North America and Russia, and the plate-tectonic origin of the Arctic Ocean remain poorly known. This project was carried out as part of a multidisciplinary, multi-institutional, international collaborative effort with Russian scientists with the goal of better understanding the evolution of continental crust beneath this region and thus addressing the questions above. Our efforts involved the collection of two parallel seismic reflection profiles that imaged for the first time the deep crust and mantle beneath the region, forming a crustal transect across the entire width of the region of submerged continental crust connecting the North American and Asian continents. In addition, two full field seasons of work were carried out which took ten separate field parties to points in northeast Russia and westernmost Alaska. These field efforts were undertaken in order to place constraints on the geologic interpretation of the seismic data. The project was similar in scope and impact to the U.S.G.S. Trans-Alaska Crustal Transect (TACT) in eastern Alaska. Together, the two efforts have provided the scientific community with a wealth of new data and fresh insight into the make-up of the Alaskan part of the North American continent and its westward link to the Arctic Russian part of the Asian continent. Data collection went as planned, and the project was, on all counts, highly successful. Our results, together with timely contributions by other workers in the region are published in "Tectonic Evolution of the Bering Shelf-Chukchi Sea and Adjacent Landmasses", Geological Society of America Special Paper 360. This volume focuses on the integration of the new data sets, assessment of their relevance for the evolution of continental crust beneath the Bering-Chukchi region, the evidence for continuity of structures and stratigraphy between the Arctic portion of the Asian and North American continents and the implications of these data for the plate tectonic evolution of the Arctic. From the contents of this volume, it is clear that this project provided an excellent opportunity for quite a number of institutions and working groups to participate in the data collection and interpretation phases of the original project. It should be noted that P.I. Jaime Toro of West Virginia University was a senior level graduate student and post-doc at Stanford while The Bering Strait Project was conceived and carried out. Our most interesting and exciting discoveries are listed briefly below: 1. The northern part of the seismic line images what appears to be intact Precambrian crust and its overlying sedimentary cover, which possibly includes Late Precambrian as well as Paleozoic and Mesozoic stratified sequences. This segment of the Chukchi Shelf may be representative of the crustal fragment that rifted away from the Canadian Arctic margin during opening of the Canada Basin, one of the main ocean basins of the Arctic Ocean. It is the first time the deeper part of this crust has been imaged, and the data provide important constraints on plate tectonic reconstruction of the Arctic.

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2. The seismic data and our field data provide possible answers to why the Brooks Range, the northern terminus of the North American Cordillera, diminishes in elevation and disappears westward towards Russia. Tertiary-age extensional or transtensional basins provide evidence for thinning of the once-thickened crust of the Bering Shelf, and an older extensional event of Late Cretaceous age (described below) also appears to have helped reduce the region to sea-level again. 3. From the Seward Peninsula to Saint Lawrence Island, the crust appears to have been extensively modified by magmatism and crustal flow, possibly accompanied by extension. The details of this history of magmatism help constrain the evolution of this crust through time (e.g. Amato and Wright, 1997). Xenolith suites collected by K. Wirth as part of a supplemental NSF Research Opportunity Award provide important insight into the age and nature of this crust and suggest that the effects of this magmatic event are considerably more important with depth in the crust. Culminations of high-grade metamorphic rocks, or gneiss domes, appear to be the consequence of this magmatic event and may have resulted from flow of the crust while at elevated temperatures. 4. The seismic data indicate a rise in Moho depths beneath the basins of the Bering Shelf edge, indicating that faulting or stretching related to basin development involves the entire crust. The data have implications for why and how these basins formed, apparently shortly after the active margin jumped southward from the Bering Shelf edge to the Aleutians. 5. Our new data indicate that both older and younger Paleozoic and Mesozoic stratigraphy and structure, as well as superimposed Cretaceous structures and events, carry across the Bering Strait from Alaska to Russia. This makes it highly improbable that a plate boundary or suture exists between Alaska and Chukotka. Our evolving geological database is one of our more important contributions to understanding the plate tectonic reconstructions of the Arctic. Publications resulting from this award: The Stanford group produced 15 published papers, including GSA Special Paper 360. In addition, 3 Ph.D. and 1 M.S. theses were produced at Stanford. See the complete list of publications in the References section of this proposal. Women and minorities involved: 1 female professor, 2 female graduate students in geophysics, 1 male minority graduate student and 1 female Russian scientist; several undergraduate students at McCalester University, Minnesota, under the direction of Karl Wirth (RUI supplement).

Collaborative Research: Does the Brooks Range Fold and Thrust Belt

Continue Into Arctic Russia? Broader Scientific Context

We propose to improve geologic constraints on plate tectonic models of the Arctic region by studying the tectonic evolution of Wrangel Island and coastal Chukotka, Russia. These two areas serve as windows into the immense region of continental crust beneath the East Siberian Shelf which comprises a large part of the Arctic Alaska-Chukotka plate. Our goals are 1) to establish with greater certainty the kinematics and timing of Mesozoic deformation on Wrangel Island and northern Chukotka and 2) to compare both the structural history and the stratigraphic successions of Wrangel Island and Chukotka to those of the Brooks Range and North Slope Alaska. These studies will have far-reaching implications because the Arctic Alaska-Chukotka plate plays a pivotal role in the mode of formation of the Amerasian Basin of the Arctic Ocean. The Arctic Ocean Basin and surrounding

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continental shelves remain one of the least studied parts of the earth's crust. Two major sub-basins exist, the Eurasian and Amerasian basins (Fig. 1). Well-developed magnetic anomalies in the Eurasian Basin can be used to reconstruct the early Tertiary to recent sea floor spreading related to the Gakkel Ridge, which is the extension of the North Atlantic spreading ridge into the Arctic (e.g. Jackson and Gunnarson, 1990) (Fig. 1). In contrast, the origin of the Amerasian Basin, which includes the Amerasian side of the Lomonov Ridge, the Makarov Basin, the Alpha-Mendeleev Ridge, the Canada Basin and the Chukchi Borderland, is much more poorly understood (Fig. 2). Despite extensive satellite and airborne magnetic and gravity surveys, the history of the Amerasian Basin with its complex ridges, basins, and margins is still unknown. To compound the problem, this basin is fringed on its south side by the immense East Siberian continental shelf, which is also

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virtually unexplored. Current thinking on the origin of the Amerasia Basin hinges on the existence of the Arctic Alaska-Chukotka microplate. The Arctic Alaska-Chukotka plate includes all of northern Alaska and Chukotka (a portion of NE Russia) as well as the offshore shelfal regions including Wrangel Island and the East Siberia Shelf (Fig. 1).

The preferred current working model for the formation of the Amerasian Basin calls upon rotational opening by seafloor spreading about a pole near the McKenzie Delta which restores the Arctic Alaska-Chukotka plate to a position alongside the Canadian Arctic margin (Figs. 2 and 3) (Grantz et al., 1990; Lawver et al., 2002). This model best satisfies all stratigraphic and structural data from the once-adjacent parts of Alaska and Canada (Fig. 2b) and is compatible with a set of poorly developed and low amplitude magnetic anomalies within the Canada Basin itself (see discussion in Grantz et al., 1990, 1994). Although there is wide variation in the way the geometry of Arctic Alaska-Chukotka has been defined by different workers (as illustrated in figures 2 and 3), the rotational model dictates 1) that the margin of the Amerasian Basin along the Lomonosov Ridge is a right-lateral strike slip fault, and 2) that the Makarov Basin is the same age as the Canada Basin (Lawver et al., 2002). These ideas in turn imply that the other major intrabasinal highs, such as the Alpha-Mendeleev Ridge, are younger features, perhaps a hot-spot track (e.g. Forsyth and others, 1986). Discussions during this summer's NSF-sponsored workshop on the Amerasian Basin (http://www.geo-prose.com/amerasian/info.html) emphasized how little is actually known about the

Amerasian Basin and how conflicting the interpretations are for the origin of these various parts of the basin. Final discussions underscored the need, despite logistic difficulty and cost, for additional

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research to understand the origin and evolution of the various elements of the Amerasian Basin. This will require detailed geophysical surveys and the establishment of sites for ocean drilling in order to conclusively determine the age of basement and its overlying sedimentary cover. These conclusions echoed recommendations made more than ten years ago by the 1991 National Academy of Science's "Opportunities and Priorities in Arctic Geoscience" (//www.nap.edu/openbook/0309044855/html). Geologists at the Amerasian Basin Workshop pointed out that, although several studies have compared the stratigraphic and structural history of the northern Alaska to the Canadian Arctic (i.e. Grantz et., 1979; Embry and Dixon, 1990; Sherwood, 1994; Toro et al., 2004), very little data is available from the Russian portion of the Arctic Alaska-Chukotka plate to permit similar correlations. It is clear that a very important step towards placing constraints on models for the origin and evolution of the Amerasian Basin is to study key localities in Arctic Russia in order to establish whether the geological history of these areas is compatible with what is known about the North American portion of Arctic Alaska-Chukotka and whether there are stratigraphic matches between the conjugate margins as predicted by the plate models.

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Questions concerning the structural integrity of the Arctic Alaska-Chukotka plate

The Brooks Range fold-and-thrust belt and its foredeep to the north, the North Slope Basin, are the main structural elements of the Alaskan part of the Arctic Alaska-Chukotka plate (Fig. 4). Deformation in the Brooks Range began with emplacement of ophiolites onto the passive margin of Arctic Alaska in the Middle Jurassic and culminated with large-magnitude, thin-skinned thrust faulting in the Early Cretaceous (Moore et al.,1994). Contractile structures are overprinted by a strong extension-related metamorphic and deformational event along the southern flank of the Brooks Range dated as 120-90 Ma (Miller and Hudson, 1991; Little et al., 1994; and Toro et al., 2002). Although deformation in the Brooks Range began prior to the inferred time of opening of the Canada Basin, the most intense shortening and maximum subsidence rates in the foredeep coincide with the proposed timing of rotation of Arctic Alaska-Chukotka (135-120 Ma) (Grantz et al., 1990, 1994; Cole et al., 1997; Lawver et al., 2002). Episodic compressional deformation in the Brooks Range resumed in the Late Cretaceous (summary in Moore et al., 2002) but these younger contractile structures, related to far-field effects of Pacific margin convergence, die out westward and do not occur in the westernmost Brooks Range (e.g. O'Sullivan et al., 1997).

In Chukotka, a broad belt of contractile structures, known as the Chukotka fold-belt, involves rocks as young as Jurassic (Baranov, 1996). However, the exact timing of deformation is not known as reported stratigraphic and cross-cutting igneous relations are conflicting (discussion in Miller et al., 2002). Thus this fold belt may, or may not, be coeval with Brooks Range deformation. Establishing the time relationship between these two fold-and-thrust belts will place constraints on the timing, nature, and progression of internal deformation of the Arctic Alaska-Chukotka plate. Although the match of the Alaskan North Slope margin to Arctic Canada proposed by the rotational model is straightforward, the dimensions of the Chukotka portion of the plate require internal deformation in order to restore it to the Canadian Arctic margin by simple rotation. Current plate models use considerable freedom in reshaping Chukotka without taking into consideration the structural history of the region (since it is not well-known). For example, Grantz et al. (1990b) eliminated about 50% of the East Siberian Shelf in his restoration (Fig. 2). In the most updated model, Lawver et al. (2002) arbitrarily sever the western-most part of Chukotka even though both geological and geophysical data suggest that the plate continues to the west (Fig. 3).

Wrangel Island is significant because it offers the best place to test the continuity of structures from Arctic Alaska into Chukotka. It is also one of the few existing windows into the geology of the immense East Siberian Shelf. The structure of Wrangel Island has been interpreted as a north-vergent fold-and-thrust belt involving late Proterozoic though Triassic rocks (Kos'ko et al., 1993) (Fig. 5). Although the general geometry of structures (north-vergent, folds with axial plane cleavage, low metamorphic grade) has been described by Kos'ko et al. (1993), the exact age of folding and thrust faulting is constrained only as post-Triassic. The limited seismic data available from the Chukchi Sea together with satellite gravity data hints that the structures on the island connect to those in the Lisburne Hills of Alaska via a basement high known as the Herald Arch (Fig. 4) (Grantz et al. 1990b). In U.S. waters, where more complete seismic coverage exists (e.g. Grantz and May, 1987; Klemperer et al., 2002) the Herald Arch is underlain by a major thrust fault. This fault is inferred to extend to the north of Wrangel Island. This connection is significant because recent work (Moore et al., 2002) has shown that thrusting in the Lisburne Hills took place in the Early Cretaceous, simultaneously with the main episode of deformation in the Brooks Range orogen. We propose to test this link by dating the timing of deformation on Wrangel Island by apatite fission track and 40Ar/39Ar thermochronology, the same techniques employed successfully by Moore et al. (2002).

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There are important differences in the structural geology of Wrangel Island compared to that

of Alaska that would argue against connecting the two belts of deformation. First, Wrangel Island does not exhibit the thin-skinned style of deformation seen in the northern Brooks Range and the

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magnitude of shortening appears to be much lower (Fig. 5) (Kos'ko et al 1993). Secondly, no foredeep basin has been identified in Chukotka. This may be a function of the lack of knowledge of the geology of the northern part of the East Siberian Shelf, or it may point to fundamental differences in the tectonic history of Chukotka relative to Arctic Alaska. Our thermochronologic work will shed light on the thickness of stratigraphic section that has been stripped by erosion from Wrangel Island which may suggest an equivalent basin in the unexplored off-shore region. Third, in Chukotka there are no ophiolite nappes in a structural position comparable to those of the Brooks Range.

Late-Stage Structural Features Cutting the Arctic Alaska-Chukotka Plate

The Hope Basin, a major early Tertiary feature of extensional to transtensional origin occupies the continental shelf between Wrangel Island, the Chukotka mainland, and the western tip of Alaska (Fig. 4) (Tolson, 1987; Elswick et al., 2003). The extent of the basin is revealed by a prominent low in the satellite gravity (Fig. 4). Seismic data from the U.S. portion of the basin shows

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that basin formation was controlled by large NW-trending normal faults. The linear mountain front along the south side of Wrangel Island might be controlled by an east-west-trending normal fault associated with the northern margin of the western extent of the Hope Basin. A secondary objective of our research will be to examine evidence for these faults and to determine, via fission-track thermochronology, whether Tertiary tectonics play a significant role in the evolution of the island.

Stratigraphic Framework of Wrangel Island and Chukotka

Basement Depositional basement to Paleozoic sequences is known only with certainty from Wrangel Island where it consists of deformed Late Proterozoic clastic and volcaniclastic rocks with lesser volcanic and intrusive components (Wrangel Complex of Kos'ko et al., 1993) (Fig. 5). Intrusive rocks yielded U-Pb ages of 699± 2 Ma and felsic volcanic rocks yielded an age of 633 Ma (Kos'ko et al., 1993). Similar U-Pb ages are reported from orthogneisses in the Nome Group of the Seward Peninsula and from the Schist Belt of the southern Brooks Range, Alaska (e.g. Patrick and McCleland, 1995; Amato, 1996; Karl et al., 1989). K-Ar ages from the Wrangel Complex range from 575 to 115 Ma (Kos’ko et al., 1993). This wide scatter is probably the result of Late Proterozoic metamorphism overprinted by one or more younger thermal events. In Alaska, Late Proterozoic rocks are so highly deformed and metamorphosed that their stratigraphic relationship to overlying Paleozoic rocks is unclear. Wrangel Island is perhaps the only location in the entire Arctic Alaska-Chukotka plate where the history of these basement rocks can be studied in sufficient detail to make informed correlations with other Neoproterozoic successions of the circum-Arctic. It is also the only place where their structural relationship to the overlying Paleozoic sequence can be observed.

Paleozoic

The oldest fossiliferous units on Wrangel Island are Upper Silurian to Lower Devonian shallow marine shelf carbonates and siliciclastic strata (700 m). These are only locally exposed and their basal contact with basement has not been mapped (Fig. 5). Lower to Upper Devonian marginal marine to marine shelf sandstone, conglomerate and slate (up to 1200 m) rest directly on the Wrangel Complex in several locations. Conglomerates at the base of the section contain clasts of the underlying basement that display deformational fabrics that pre-date the enclosing sediments. The relations on Wrangel Island provide excellent documentation of a latest Proterozoic to earliest Paleozoic orogenic event in a continental or volcanic arc setting (Kos'ko et al., 1993). Lower Carboniferous strata rest disconformably on Devonian strata but are difficult to differentiate because they are similar in composition. Non-marine to shallow marine siliciclastic sandstone, slate, conglomerate, minor carbonate and gypsum are present (350 m). Exposures of mafic and felsic volcanic rocks, most likely part of the Carboniferous succession, were noted by Kos'ko et al. (1993). The lower to upper Carboniferous consists of 500-1000m of distinctive shallow marine fossiliferous limestone, slate and argillite. Lower to Upper Permian strata (750 m) are platformal to basinal limestone, siliciclastic rocks and slate. There is an upwards progression to basinal facies and by Late Permian, the carbonate platform was drowned.

The Paleozoic stratigraphic succession of mainland Chukotka (Figs. 6, 7) is similar to that of Wrangel Island, although the oldest rocks mapped are Devonian shallow marine clastics and carbonates. These are unconformably overlain by Carboniferous to Permian calcareous siltstones and shales (700m) (Fig. 7). Triassic

The Triassic successions of Chukotka are 3-5 km thick, and consist mostly of slate, siliceous argillite and fine- to medium-grained quartz-rich sandstone, deposited in a basinal environment (Fig. 7).

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Across many hundreds of kilometers in Chukotka, the basal contact of the Triassic is mapped as conformable with underlying Permian or disconformable on Carboniferous strata (Figs. 6, 7). The lower part of the Triassic is notable in that it contains abundant gabbro/diorite sills and dikes. These yielded conventional K-Ar whole rock ages ranging from 250-190 Ma, but have not been dated by more precise methods (Bychkov and Gorodinsky, 1992; Gelman, 1963; Ivanov and Milov, 1975; discussion in Miller et al., 2002). The Triassic of Wrangel Island is lithologically similar to that of the mainland but considerably thinner (800-1500m) and lacks the gabbro sills and dikes.

The Triassic of Chukotka and Wrangel contrasts with the thinner Triassic of Alaska's Brooks Range and North Slope. Beneath the North Slope, Triassic deposits are mainly shelfal to non-marine clastics with minor carbonates and are less than 600 m thick (Moore et al., 1994). These units are very well known as they include both the principal sandstone reservoir in the Prudhoe Bay oil field

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and an important petroleum source rock (the Shublik Fm.). In the Brooks Range, Triassic deposits are chert and black shale characteristic of a starved basin setting (Moore et al., 1994). These differences between the Triassic of Alaska and Chukotka have not been reconciled in terms of a coherent paleogeographic model. In particular, the exact time of formation and plate tectonic setting of Triassic basin development in Chukotka remain unexplained.

Cretaceous magmatism An immense belt of Jura?-Cretaceous plutons and volcanic rocks extends across most of

Chukotka (Gorodinski, 1980). In northern Chukotka this belt is represented by older, syn- to post-folding, compositionally heterogeneous plutons and dike swarms. Unconformably overlying, gently-dipping silicic to mafic volcanic rocks and associated dikes of the Okhotsk-Chukotka belt are younger. The age span, geochemistry, and tectonic affinity of the older plutonic rocks are not well known. These data are of critical importance because they will help to date structural events in Chukotka and will help determine the plate tectonic setting of this magmatism (i.e. mantle-derived vs. crustal melts; subduction vs. hot-spot origin). The Okhotsk-Chukotka volcanic belt is generally regarded as related to subduction along the Pacific plate boundary (Belyi, 1977a, 1977b, 1978). It is important in the context of this proposal because it stitches the Arctic Alaska-Chukotka plate to the rest of Northeast Russia (Fig. 1). New 40Ar/39Ar ages from the southern portion of the belt indicate that the majority of the volcanic sequence was erupted between 85.5 ± 1.3 Ma and 73.6 ± 0.7 Ma (Hourigan and Akinin, 2004). This age-range is 15 Ma younger than the Albian to Early Cenomanian age range accepted in the Russian literature on the basis of fossil flora (i.e. Fig. 7). Thus, carrying out modern geochronology and geochemistry on the older cross-cutting plutons and the overlapping volcanic rocks in Chukotka will improve the constraints on both the timing of deformation in Chukotka and the arrival of Chukotka to its present position. Methodology

Dating regional folding and thrust-faulting A wide spectrum of stratigraphic, geochronologic and thermochronologic tools are available

to help date structures and fabrics formed during specific deformational events. Of these, stratigraphic and sedimentologic relations with fossil age control or cross-cutting relationships with igneous rocks are the most robust, but in the absence of straightforward relations such as these, a variety of radiometric dating techniques can date regional deformational events indirectly. These methods include U-Pb dating of zircon and monazite for high grade rocks; 40Ar/39Ar dating of hornblende, muscovite, biotite, and K-feldspar for intermediate to low-grade rocks; and fission-track dating of apatite for low-temperature events. These techniques date the time a particular mineral cooled below the temperature at which radioactive daughter isotopes are retained (closure temperature). These ages can be used to set limits on the age deformation particularly when growth of specific minerals can be linked to a particular event. Successful application of these thermochronologic techniques requires a sampling strategy that takes advantage of the exposure of different crustal levels across an orogen or map-scale structure and uses multiple mineral systems with different closure temperatures . Good examples of previous studies by the PI's and their collaborators that have used these different techniques to date deformation in western Alaska and the Bering Strait region are those of Moore et al. (2002), Toro et al. (2002), Amato et al. (2002 ), and Akinin and Calvert (2002). See the Facilities section for a description of the geochronological laboratories available for this project.

In the area of our proposed field work in coastal Chukotka, sedimentary, volcanic, and volcanoclastic units ranging from late Paleozoic to Late Cretaceous have been mapped. Their degree of deformation will provide a good stratigraphic bracket for regional folding in the Chukotka

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foldbelt. In addition, cross-cutting granitic to gabbroic plutons and dikes have been mapped but never dated. The cross-cutting relationships of these igneous rocks with structures is likely to provide reliable control on the time-span of regional deformation. Igneous rocks will be dated with a combination of U-Pb zircon and 40Ar/39Ar hornblende methods if appropriate. We will also examine reported higher grade rocks and migmatites of the Velitkinai massif (Fig. 6) to determine if the there is a link between the igneous-metamorphic fabrics and the regional deformation.

On Wrangel Island there are no stratigraphic relations that bracket deformation except that it post-dates deposition of Triassic sediments which are involved in folding and thrusting (Fig. 5). In addition, no cross-cutting intrusive rocks have been mapped, therefore we must rely entirely on thermochronologic techniques. Apatite fission-track thermochronology is a method for reconstructing the time-temperature history of rock samples within the temperature window of 60-125°C (e.g. Dumitru, 2000). Assuming a nominal geothermal gradient of ~25°C/km, this temperature window is equivalent to a depth window of ~2-5 km beneath the earth's surface. A complete thermal history can be determined because fission tracks, are generated continuously, but are erased rapidly near 125 degrees, and very slowly near 60°C. The length distribution of fission tracks in a sample can be modeled to determine the possible cooling histories experienced by the rock. On Wrangel Island, where 5-10 km of structural section is exposed due to the southward tilt of units, the fission-track method can provide information on the timing of thrust burial and subsequent erosional exhumation back towards the earth's surface. This approach, was successfully used by T. Dumitru in the Lisburne Hills of Alaska (Moore et al., 2002) and will be applied again to our samples from Chukotka and Wrangel Island. Dumitru's CV and the description of Stanford's fission-track analytical facilities are included under Supplementary Documents and Analytical Facilities.

On the Chukotka mainland, appropriate samples collected from various parts of the stratigraphic succession across regional anticlinal structures and dated with both fission track (at shallow structural levels) and with 40Ar/39 Ar methods (at deeper levels) will provide good controls on the age of deformation. On Wrangel Island, Paleozoic and Triassic rocks are described mostly as slates. Rocks of this grade are difficult to date, but metamorphic basement rocks do have metamorphic white micas (Kos'ko et al., 1993). The available K-Ar ages from the Wrangel Complex range from Neoproterozoic to Early Cretaceous. We will use modern 40Ar/39Ar thermochronology on metamorphic white micas (closure temperature 350�50�C, McDougall and Harrison, 1988) and igneous K-feldspar in order to resolve the timing of the different thermal events. A carefully designed step-heating experiment of a K-feldspar sample can reveal a complete thermal history for the sample from above 400� to less than 200�C because of the existence of multiple diffusion domains in the mineral (Lovera et al., 1997).

Geochemistry and Geochrology of Igneous Rocks

A wide variety of dikes, sills, and intrusive bodies ranging from granite to gabbro have been mapped in our target field areas of the Chukotka mainland (Fig. 6). There has been little or no modern geochemistry or geochronology on any of these igneous rocks. Are the plutons of the Chukotka fold belt related to mantle-derived magmatism? Are they related to subduction along the South Anyui margin of the Arctic Alaska-Chukotka plate as implied by the plate reconstructions shown in Figs. 2 and 3? Or are they related to the onshore continuation of the Alpha-Mendeleev Ridge, a postulated Cretaceous hot-spot track (e.g. Lawver et al., 2002)? In order to answer these questions we will carry out U-Pb zircon dating of the major intrusive bodies accessible from our transects using the USGS/Stanford SHRIMP-RG. This work will be lead by our Russian collaborator Dr. V. V. Akinin, who is an experienced petrologist (see Supplementary Documents). We will also characterize the major and trace element composition of the Velitkinai granite massif and other selected intrusive bodies, and investigate their Sm, Nd, Sr, and Pb isotopic composition by thermal

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ionization mass spectrometry to help determine source region characteristics for the magmas in order to help constrain their tectonic setting.

Circum-Arctic Paleogeographic Correlations with the help of U-Pb geochronology

A more in depth geochronologic study of Neoproterozoic, Paleozoic, and Triassic strata of Wrangel Island and Chukotka will provide a better basis for comparisons of the Russian part of the Arctic Alaska-Chukotka plate to other circum-Arctic regions and will help to test whether Chukotka was once connected to the Canadian margin. Work on this aspect of the project will be carried out by Dr. Victoria Pease at no cost to NSF. She has experience working in the Russian Taymir, Nova Zemlaya, and the Siberian Islands (see letter of support and interest in Supplementary Documents). Field observations, together with U-Pb geochronology provides the best means of comparing the history of basement rocks from these different places. In addition, dating of detrital zircon suites from sedimentary rocks can provide information-rich data sets on the age and nature of source regions for clastic rocks, thus allowing linkage of basins to a particular continental mass (e.g. North America, Siberia, Eurasia).

Wrangel Island Field Work

Fieldwork on Wrangel Island will be carried out by members of the geologic team aboard the Oden at no cost to NSF. Our first objective is to study the structural style of deformation in the Triassic and Paleozoic part of the section which will involve measuring structural fabrics in the field and determining single or multiple phases of deformation. Our second objective is to collect a suite of samples for fission track apatite dating so that we can place good constraints on the age of folding and thrusting on Wrangel Island. The rationale for this sampling transect is described under methodology and several possible sample transect locations are shown on Fig. 5. The south dip of units along the southern side of Wrangel Island affords the greatest structural relief: 5-10 km of structural section are exposed over 10-20 km. About 15-20 samples will be collected across one of these transects. The base of the Paleozoic stratigraphic section and the underlying Wrangel Complex are described as containing various amounts of metamorphic white mica, which are appropriate targets to sample for 40Ar/39Ar dating to complement the fission track dating. Our final objectives are to collect Paleozoic sandstone and conglomerate for single-grain zircon provenance studies and determine more certainly the deformational, metamorphic and intrusive history of the Wrangel Complex, collecting samples that will provide data to match these rocks to other sequences along the margins of the Amerasian Basin. This work will be carried out mainly by our Swedish colleagues.

Chukotka Field Work

During July and August of 2005, one of the P.I.’s (Toro) together with chief Russian collaborator Dr. V. V. Akinin, and two graduate students will study two areas of the Arctic coast of Chukotka making use of a Russian all-terrain track vehicle (Fig.7). They will first conduct a structural and thermochronologic transect of about 100 km across the Sheiagskiy Range starting from the town of Pevek to the Arctic coast (Fig. 7). Here, folded Devonian to Triassic clastic sedimentary rocks are exposed and their deformational history can be studied. They will collect samples across this transect for apatite fission track and 40Ar/39Ar dating. The apatite fission track analyses will help constrain the history of cooling and erosion to help place limits on the timing of folding in the Chukotka foldbelt. 40Ar/39Ar thermochronology, together with structural measurements and petrographic studies, will help establish the age of metamorphism, its relationship to the plutons exposed in the area, and whether it is syn-or post-folding or both. In the process of achieving these

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objectives, the basic stratigraphic units of this region will be described and sampled for fossils, sedimentologic studies, and detrital zircon geochronology in order to better understand their correlation to other sequences of this age across Chukotka, on Wrangel Island and in Alaska After completing this work, Toro and Akinin will continue east to the large Velitkinai granitic massif in order to study the extensive contact metamorphism described in the Russian literature, the higher grade rocks and migmatites in the core of the massif, and key stratigraphic relationships between the Late Cretaceous volcanic and volcaniclastic rocks of the Okhotsk-Chukotka belt and the underlying deformed Devonian to Triassic strata. The Pevek Transect is designed to complement and augment simultaneous helicopter-based work from the Swedish icebreaker Oden on Wrangel Island and to a few points on the Chuktoka coastline.

Work Plan and Time Table Approximate Dates/Location

Personnel Task

Year 1 March1, 2005-February 28, 2006 Jan 2005 Toro, Akinin, Sokolov Begin logistic preparation and permitting for 2005 field work. Spring 2005 /Magadan

Akinin GIS Russian maps and photos in preparation for 2005 field work.

Spring 2005 Miller, Sokolov Preparation for Oden Icebreaker Beringia 2005 Expedition July-August 2005/Arctic

Miller, Sokolov Participation in Oden Icebreaker Beringia 2005 Expedition to Wrangel Island and north coast of Chukotka (no cost to NSF)

July-August 2005/Chukotka

Toro, grad. students, Akinin

American participants travel to Magadan to meet Akinin. All travel to Pevek for 2005 transects.

August 2005 /Stanford

Miller, Sokolov Oden Icebreaker Beringia 2005 Expedition ends in Nome Alaska, Miller and Sokolov return to Stanford for wrap-up

Sept.-Dec. 2005

All participants Preparation of 2005 samples for geochemistry, geochronology, and thermochronology and compilation of field data.

Dec. 2005 /Stanford

Miller, Toro, 2 Russian collaborators

Project Meeting at Stanford, presentation of preliminary results at AGU, analytical work at Stanford

Year 2 February 1, 2006-January 31, 2007 Jan- July 2006 All participants Geochronology/thermochronology, petrology, geochemistry and

sedimentology of selected samples. Sept.-Dec. 2007

All participants Final study of samples and data analysis /thermochronology, petrology/geochemistry and sedimentology. Outline final papers

Dec. 2007 /Stanford

Miller, Toro, 2 Russian collaborators

Project Meeting at Stanford, presentation of final results at AGU, any remaining analytical work at Stanford, manuscript work.

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