T13D-1568
Transition from rift basins to intra-cratonic basins in the Barents Sea as revealed in potential field signature and paleogeograph
A new compilation of potential field data and of paleogeographic maps provides a new fundament to constrain the basin architecture and crustal pattern of the Barents Sea region. Until now, most studies of the Barents Sea have been limited to either the Norwegian western part or the Russian eastern part of the Barents Sea. However, the Western and Eastern Barents Sea show a large set of different features, expressed in subsidence pattern, distribution of depocenters, basin geometry and evolution, crustal thickness as well as upper mantle structure. While the Western Barents Sea basins are typical rift basins affected by the opening of the North Atlantic, the Eastern Barents Sea basins show distinctive features such as large extension and wavelengths, thick sequence of sediments and a flat Moho which are normally associated with cratonic or intracratonic basins. Detailed study of the gravity field and the isostatic state of the Barents Sea Region indicate further that the Eastern Barents Sea basins are underlain by high lithospheric mantle densities, an observation often made for intra-cratonic basins. To define successful exploration models and hydrocarbon plays a good understanding of the basin evolution and the paleogeographic settings is needed. We compile a series of paleogeographic maps from selected time slices spanning the Early Silurian to the Eocene time interval. The paleogeographic maps help to predict the distribution and understand the formation of the main depocenters and intrinsic sedimentary facies, which also confirms the differences between the Eastern and Western Barents Sea. Our results show that the transition zone between the Eastern and Western Barents Sea is of special interest for understanding the Barents Sea geological evolution. In the gravity and magnetic fields the transition is coinciding with a change of anomaly pattern. Especially, large-scale anomalies in the magnetic field point to the existence of intrusions in intermediate crustal levels, which may be related to a suture zone of a possible Timanian origin. http://www.ngu.no
T13D-1569
Paleomagnetic Data From Alaska and Northeast Russia: Their Role in modeling the Opening of the Arctic Ocean.
The mode of origin of the Arctic Ocean has long been the focus of considerable debate. The conventional model involving a counterclockwise rotation of the Arctic Alaska terrane away from the Canadian Arctic Islands should have been an easy problem for a paleomagnetic answer. In practice almost all paleomagnetic studies in the region show a pervasive near-vertical magnetic overprint. Only two reliable studies exist for Arctic Alaska, one is from borehole core samples from the Early Cretaceous Kuparuk River Formation, and the other is from the late- Early Cretaceous Nanushuk Formation. The Kuparuk paleomagnetic pole is well constrained in time, has a good reversal stratigraphy indicating a primary magnetization and because it is part of a major oilfield is well understood in terms of its geological setting. The Nanushuk data pass a fold test, but are in the long Cretaceous normal chron, so no reversal test is possible. The Kuparuk data, when looked at in the light of recent apparent polar wander paths for North America, do not support the conventional rotation model. The Nanushuk data show that Arctic Alaska was in place with respect to North America at the time the rocks were magnetized. The Kuparuk paleomagnetic pole is permissive for many other models, including a rotation from an initial position nearer to Greenland or locations adjacent to Siberia. Attempts to constrain these possibilities are being made by comparing the Alaskan data with that from Siberia. Only two data points are available from the Chukotka block, and both of these are from the Okhotsk-Chukotka volcanic belt of latest Early Cretaceous age and give paleomagnetic poles close to those expected for the block being in-place with respect to present geography. Using data from the Kolyma-Omolon Superterrane, currently adjacent to the Chukotka block allows more constraints on the possible relative locations of the major blocks involved in the opening of the Arctic Ocean. These and other constraints result in an ever-evolving set of cartoons of possible paleogeographies.
T13D-1570
Evidence From Detrital Zircon U-Pb Analysis for Suturing of Pre-Mississippian Terranes in Arctic Alaska
Detrital zircon U-Pb ages of pre-Mississippian sandstones were determined using SHRIMP and LA-ICPMS techniques for four key geographic parts of the Arctic Alaska terrane, northern Alaska. In the northeastern Brooks Range, a sample of quartz-rich turbidites from the Proterozoic Neroukpuk Quartzite yielded zircon ages ranging from 980 Ma to 2.9 Ga with clusters at 980-1100 Ma, 1680-1850 Ma and 2220-2660 Ma. Quartz and chert-bearing sandstone in the Tulageak well from Ordovician-Silurian argillite in basement beneath the North Slope yielded a broad spectrum of ages between 1.0 to 2.1 Ga and 2.8 Ga, including peaks at 1.0-1.2 and 1.5-1.7 Ga. Paleozoic zircons cluster at 390 and 440 Ma in this sample, indicating it is Devonian. Lithic sandstone from the Silurian Iviagik Group at Cape Dyer on the Lisburne Peninsula yielded a variety of ages from 450 to 1600 Ma, with a large peak at 475-600 Ma and several grains between 1.9 and 2.5 Ga. In contrast to the broad distributions of the latter two samples, zircons in metamorphosed Proterozoic-Cambrian(?) lithic sandstone from the an unnamed metagraywacke unit near Mt. Snowden on the Dalton Highway in the southern Brooks Range are largely 600-650 Ma with lesser clusters at 1050-1200 Ma and 1600-1900 Ga. Samples of quartz-rich Mississippian sandstone at the base of the unconformably overlying Mississippian to Triassic Ellesmerian sequence near three of the pre-Mississippian sample locations were also analyzed. Mississippian sandstones from the West Dease well (near the Tulageak well) and at Cape Dyer on the Lisburne Peninsula display zircon distributions similar to those found in the underlying pre-Mississippian samples, indicating the Mississippian clastic strata are locally derived and that the observed zircon distributions are representative of a broad area. However, the Mississippian Kekiktuk Conglomerate, which rests on the Neroukpuk Quartzite in the northeastern Brooks Range, also contains a variety of ages between 560 and 900 Ma and 1200-1450 Ma that are not seen in the Neroukpuk sample, as well as a cluster of ages at 320-390 Ma. This sample indicates that the Neroukpuk is not the only source of zircons for Mississippian strata in the northeast Brooks Range. The samples from the northeastern Brooks Range contain zircon distributions similar to those reported from autochthonous North American strata in east-central Alaska and are strikingly different from those in western and southern parts of the Arctic Alaska terrane. Peaks at ~1.8 Ga are subdued or missing in the latter samples and 1.5-1.6 Ga grains, a magmatic gap in Laurentia, are present in the Lisburne Peninsula and North Slope samples. In view of these data, field and seismic evidence for significant Devonian deformation in northern Alaska, and likely plate configurations for that time, northern Alaska probably records early to mid-Paleozoic closing of the Iapatus Ocean and collisional suturing of non-Laurentian continental blocks with northwestern North America. This suggests that Devonian deformation in Arctic Alaska represents an important, northern element of the Caledonian deformational system that probably once linked up with Caledonian structures in the Canadian Arctic Islands and adjacent continental margin region.
T13D-1571
Constraints on a Palaeobathymetric Model of the Northern North Atlantic
The Fram Strait between Svalbard and Greenland is the only deep water connection between the Arctic Ocean and the northern North Atlantic. A detailed understanding of this region, however, is difficult due to the absence of clear marine magnetic and other geophysical data. Within the last years the Alfred Wegener Institute conducted several geophysical data sets in order to solve some of the remaining problems in the Fram Strait and the adjacent basins (Molloy and Boreas Basin). In this contribution we present a new aeromagnetic data grid, new seismic reflection data and existing gravimetric data that provide new insights into the geodynamic evolution of this area. These data are the basis for estimations of initial opening scenarios, and palaeobathymetric maps. Seismic reflection lines crossing the entire Molloy and Boreas basins show rough basement topography and a deep axial valley. Roughness calculations also indicate ultra-slow spreading rates in the whole area. Using the age information of the aeromagnetic data, the thermal subsidence for oceanic crust corrected for the sediment load fits the observed basement depth reasonably well in the Boreas and Molloy basins. A compilation of additional seismic lines in this area contributes the basement depth and sediment distribution. Combining all information of the seismic data, the aeromagnetic data and the gravity data, a model of the evolution of the northern North Atlantic is created. The latest results of this palaeobathymetric study for the Fram Strait and its subsequent basins are shown.
T13D-1572
Seismic studies along the East Greenland margin between 72°N- 81°N
Due to previous offshore investigations along the East Greenland margin between 72°N-81°N a detailed seismic stratigraphy could not be developed. The history of the Northern Hemisphere is a subject of controversy. Especially speculations about the onset of the glaciation of East Greenland varies from Plio/Pleistocene to Eocene. The seismic dataset, gathered in 2002 and 2003 along the North East Greenland margin in combination with analyses on the ODP sites 908, 909 and 913 give for the first time the possibility to provide an age estimation of the sediments between 72°N-81°N. First results show the greatest glacial sediment deposits are located beneath the East Greenland continental shelf. A basement high in the Greenland Basin (77°N- 75.8°N), presumably a 230 km wide basement structure prevents continuous sediment transport from the shelf into the deep sea area in times before 14 Myrs. On the basis of our correlation we assume the ice-rafting in the Greenland Basin has probably occured since the middle Miocene. Additionally the compilation of sediment thickness provide an insight into the regional sediment distribution along the northern East Greenland margin. The sediment thickness ranges between 1 km in the Greenland Basin and up to 2.5 km in the Boreas and Molloy basins. A partially interpolation and extrapolation of ages in the region between 72 and 81°N provides a first age estimation along the northern East Greenland margin. The latest results along the northern East Greenland margin will be introduced.
T13D-1573
Horizontal Movements in the Eastern Barents Sea Constrained by Numerical Models and Plate Reconstructions
The eastern Barents Sea basins, west of Novaya Zemlya, were mildly inverted between Late Permian and Early Jurassic times, as indicated by mild folds in the basin sediments. Previous studies have suggested that the crustal part of Novaya Zemlya was thrust westward, but the magnitude of this compressive movement is not well known. Our aim is to provide an order-of-magnitude constraint on the amount of shortening associated with the displacement of Novaya Zemlya and the inversion of the eastern Barents Sea basins by combining numerical models and plate reconstructions in an iterative process. We use a 2D thermo-mechanical finite-element method to model inversion of a predefined basin. The total amount of shortening imposed on the models is first constrained by plate reconstructions for the Barents Sea region for the late Paleozoic to early Mesozoic. We assume that the shortening was caused by westward movement of the Siberian plate, but the magnitude of this westward displacement in plate reconstructions is highly uncertain due to the allochthonous nature of the rocks of Novaya Zemlaya and the scarcity of palaeomagnetic data in the region. Our models show that shortening localizes in the model basin and at Novaya Zemlya and that westward propagation of deformation is more efficient when the strength of the lower crust is reduced. By comparing the inversion obtained in the numerical models to the inferred inversion structures in the eastern Barents Sea basin we further constrain the amount of shortening that caused the inversion and therewith improve the plate reconstructions for the region. Our models indicate that the westward movement of Novaya Zemlya occurred in the Late Triassic-Early Jurassic (220-190 Ma) and was limited in magnitude to 100-200 km, which is considerably less than previous estimates (500-700 km).
T13D-1574
New U-Pb SHRIMP-RG Geochronology and Isotope Geochemistry of the Main and Northern Granitoid Belts of NE Russia
The Main and Northern granitoid belts of NE Russia extend for 1500 km along the boundary between the Kolyma- Omolon superterrane and the North Asian craton. On the basis of Early Cretaceous 40Ar/39Ar biotite ages (Layer et al., 2001) and of the per-aluminous composition of some of the granitoids, the Main Belt was originally interpreted to have formed by crustal melting during collision of the Kolyma-Omolon superterrane against the North Asia craton (Parfenov, 1991). This interpretation is problematic because there is no evidence that there ever was sufficient crustal thickening in the Verkhoyansk-Kolyma orogen to produce crustal melting. Also, the chemical signature of the granitoids is ambiguous. For example, in a Rb vs. Y+Nb tectonic discrimination diagram the Main Belt compositions overlap those of the Andean granitoids or the Sierra Nevada batholith, and the REE pattern for the Main Belt is similar to that of the Sierra Nevada. New U/Pb SHRIMP-RG zircon ages from 10 granitoids distributed along the length of the Main Belt are Late Jurassic, that is, older the onset of collisional deformation. Specifically, the Dogdo batholith has a 207Pb- corrected 206Pb/238U mean age of 148±2 Ma (n=10/10). The Pravo-Tuostakh batholith is 151±2 Ma (n=9/11). Two samples of the Chibagalakh batholith, which is about 200 km long, have mean ages of 148±2 Ma (n=10/10), and 147±2 Ma (n=9/13) with a second cluster at 154±2 Ma (n=4/13). The Porozhnotsepinsky batholith is155±2 Ma (n=9/11). The Khayargastakh batholith has ages that increase progressively from 142 to 158 Ma with an overall mean of 152±5 Ma (n=8/8). The Chuguluk batholith has a mean age of 151±1 Ma (n=8/9). The isotopic ratios are generally concordant and have very low or no common Pb. Several plutons yielded two populations of Late Jurassic ages with dispersion greater than expected from the analytical uncertainties which we interpret in terms of two zircon components encompasing the time span of magmatism in the area. We only found evidence of significant inheritance of 1.9 to 2.0 Ga crust in zircons of the Trud and Nelkan plutons which are located in the Tas-Kystabyt belt, west of the Main Belt in an area that is underlain by the North Asia craton. Four Northern belt granitoids have 206Pb/238U mean ages ranging from 134 ±0.9 to 126 ±1.2 Ma. Our isotopic studies (in progress) show a pattern of increasing initial 87Sr/86Sr from the Northern belt (0.7052), to the Main Belt (0.7074), to the Tas-Kystabyt Belt (0.714). This reflects progressively greater involvent of crustal material in the source region. The new isotopic data demonstrates that magmatism in the Main Belt started by ~154 Ma and most of the granitoids had been emplaced by ~147 Ma, that is, during the Late Jurassic (Kimmeridgian to Tithonian). Therefore the Main Belt is largely coeval with the Uyandina-Yasachnaya volcanic arc (Oxfordian- Tithonian) located on the western active margin of the Kolyma-Omolon superterrane. The Main belt granitoids were also produced by subduction beneath the Kolyma-Omolon. The Tas-Kystabyt belt requires a second subduction zone along the Verkhoyansk margin of North Asia in order to produce a belt of batholiths on either side of the boundary with the Kolyma-Omolon superterrane.
T13D-1575
Mid-Ocean Ridge Subduction Offshore Alaska During the Cretaceous
We present a framework for the tectonic development of the Arctic region through a set of regional plate and ocean floor reconstructions since the early Cretaceous. In order to understand the effect of time-dependent geometries of mid-ocean ridges, subduction zones and collisional plate boundaries on Arctic basin evolution and reactivation through time, we reconstruct now subducted ocean floor, including portions of tectonic plates which have now entirely vanished, and restore their plate boundary configurations and subduction history. We reconstruct paleo-oceans by creating "synthetic plates", the locations and geometry of which are established on the basis of magnetic lineations and fracture zones, geological data and the rules of plate tectonics. The absolute position of the Pacific Plate and its surrounding plates is restored using a Pacific hotspot reference frame, whereas all other plates are reconstructed based on an African-Indian hotspot reference system. This approach is required because the Pacific Plate was entirely surrounded by subduction zones in the Cretaceous, and therefore Pacific Ocean plates cannot be related to other tectonic plates via relative plate motions. Our reconstructions reveal that the Izanagi-Farallon spreading ridge was subducted underneath Alaska from about 120-100Ma. Prior to 120 Ma the northern portion of the Izagani-Farallon plate boundary was a convergent boundary according to our reconstructions, implying that between 140 and 120 Ma a subducting slab was overridden by the Alaskan North Slope and possibly other associated terranes. The Izanagi-Farallon subduction zone (before 120 Ma) and mid-ocean ridge (after 120 Ma) was oriented roughly orthogonal to the overriding plate. Trench subduction would have been associated with negative dynamic topography on the overriding plate, whereas an eastward migrating slab window underneath North Slope and its border terranes may have resulted in asthenospheric upwelling and extension. Mid-Cretaceous (Aptian to Santonian) rocks are missing over much of the Alaska Peninsula, presumably eroded, and the widespread absence of rocks of this age suggests uplift and erosion of the entire terrane during a portion of Aptian to Santonian time. These observations generally support our model, but the relative roles of trench and ridge subduction for causing the widespread regional erosion or for triggering the opening of the Canada basin remain open.
T13D-1576
Siberian Origins of Neoproterozoic to Upper Triassic Rocks of Arctic Alaska
Evidence for a connection of the Arctic Alaska plate (including Chukotka) with Siberia from Cambrian until Late Triassic time can be made on the basis of paleobiogeography. Arctic Alaska contains a number of biogeographically distinctive megafossils for select time intervals, notably the Middle Cambrian, Early and Late Ordovician, Early and Middle Devonian, Mississippian, and Late Triassic. Middle Cambrian trilobites are strictly Siberian in affinity, but also show close affinities with coeval trilobites from the Farewell terrane of SW Alaska. Late Ordovician brachiopods, gastropods, trilobites, and ostracodes are known from the Shublik Mountains, NE Brooks Range and York Mountains of the Seward Peninsula. Affinities are likewise primarily with Siberia (sharing the primarily Siberian pentameroid brachiopod genera Tcherskidium and Eoconchidium and the strictly Siberian trilobite genus Monorakos), but also with the Farewell terrane. Late Early Devonian and Middle Devonian brachiopods and calcareous green algae from Arctic Alaska are similarly allied with Siberia and the Farewell and Alexander terranes of southern Alaska. Early Mississippian faunas from the lower part of the Lisburne Group and underlying Endicott Group contain relatively widespread fauna, including taxa recognized both in North America and Eurasia, consistent with the relatively cosmopolitan paleobiogeographic conditions of this interval. However, Late Mississippian brachiopod fauna from the upper part of the Lisburne Group contain many brachiopods of strictly Eurasian affinities, notably the gigantoproductids, which are unknown in cratonic North America, but widespread across Eurasia and even North Africa. Late Mississippian lycopods from this terrane have previously been noted as demonstrating strong Angaran affinities. Permian faunas of Arctic Alaska show strong affinities as well with the Siberian Arctic, virtually lacking any fusilinids and reefal buildups, which in contradistinction are commonly found in the Canadian Arctic Islands Richly diverse Upper Triassic fauna (halobiid and monotid bivalves, brachiopods) are present in the both the Shublik Formation and Otuk Group. These show closer affinities with NE Siberia rather than to western or northern North America, suggesting close spatial relationships between Siberia and Arctic Alaska at least until Late Triassic time. Sedimentary provenance studies in eastern Brooks Range Precambrian rocks indicate age ranges that are dissimilar to Proterozoic detrital-zircon ages from clastic rocks of the northern Canadian Cordillera and Canadian Arctic Islands where a detrital source within the Grenville orogen is indicated. Paleocurrent directions for the Neoproterozoic Katakturuk Dolomite in the northeast Brooks Range and similar-age units in the adjacent Victoria Island and Amundsen Basin are in approximately 100 degree opposition for a counterclockwise rotational- restored Arctic Alaska. Upper Devonian clastics of northern Alaska are in 180 degree opposition to coeval units in the Canadian Arctic Islands when the Arctic Alaska plate is restored in the rotational model. Therefore, based on paleobiogeography, sediment provenance, stratigraphy and sedimentology, tectonic models for the opening of the Canada Basin must take into account that Triassic and older rocks in Arctic Alaska have Siberian origins or were deposited proximal to Siberia.
T13D-1577
Is There Evidence for Recent Compression Along the Northwind Ridge and Chucki Borderlands?
In the summer of 2005, USCG Icebreaker Healy crossed the Arctic Basin from Dutch Harbor, Alaska, to Tromsø, Norway, to collect geophysical data and take shallow cores in an effort to gain greater insight into the paleo- oceanographic, depositional, and tectonic histories of the Arctic Basin. Integrated geophysical data, including seismic reflection, seismic refraction, multi-bean swath mapping, and chirp data, were collected over the Northwind ridge and Chukchi Plateau from the Canada Basin out to the Mendeleev Ridge. These data sets provide insight into the tectonic evolution of the region and shed new light on several controversial aspects of tectonic models for the evolution of the Arctic basins. Significant extensional normal faulting is visible across the profile and there is clear evidence of growth faulting affecting older sediments below a major unconformity. Many of these normal faults appear to have been reactivated quite recently, with some displacing the seafloor. No evidence of tectonic inversion is found in the seismic images, which would be predicted by models for region that propose a thrust fault on the eastern side of the North-wind Ridge.
T13D-1578
U-Pb Zircon Provenance of Metasedimentary Basement of the Northwestern Terrane, Svalbard: A central East Greenland correlation
Svalbard's Caledonian and older bedrock consists of three main exotic terranes separated by north-south trending strike-slip faults. Early in the 19-century Kulling (1930, 1934) noted the striking similarity between the Neoproterozoic and Early Paleozoic sedimentary rocks from East Greenland and Svalbard's Eastern Terrane. After this pioneering work Harland et al. (1969) documented their remarkable similarities through detailed stratigraphic correlation of Neoproterozoic and Early Paleozoic sedimentary rocks. Recent work in the Northwestern and the eastern part of the Eastern terrane shows that this metasedimentary basement has a strong resemblance to the Krummedal and Smallefjord sequences of central East Greenland, with Grenville age (sensu lato) granitoid intrusion followed by Caledonian migmatization and granite genesis. We present the first LA-ICP-MS U-Pb zircon provenance study on quartzites and mica schists from the Kongsfjorden Group of the Northwestern Terrane of Svalbard. These results indicate a strong correlation with the Krummedal of central East Greenland and help to constrain paleogeographic reconstructions of the Northwestern Terrane. References: Kulling, O. 1930. Stratigraphic studies of the geology of Northeast Greenland. Meddelelser on Gronland, 74, 317- 346. Kulling, O. 1934. Scientific results of the Swedish-Norwegian Arctic Expedition in the summer of 1931. Part XI, The "Hecla Hoek Formation" round Hinlopenstredet. Geografiska Annaler, 16, 161-254. Gee, D. G. and Tebenkov, A. M., 2004. Svalbard: a fragment of the Laurentian margin.? In: Gee, D. G. and Pease, V. (eds) The Neoproterozoic Timanide Orogen of eastern Baltica. Geological Society, London, Memoirs, 30, 191- 206. Harland, W. B. 1969. Contribution of Spitsbergen to understanding of tectonic evolution of North Atlantic region. North Atlantic Geology and Continental Drift. Memoirs of the American Association of Petroleum Geologists, 12, 817-851.
T13D-1579
Active Faulting Along the Ulakhan Fault, Seimchan-Buyunda Basin, Northeast Russia
The Seimchan-Buyunda basin is a topographically flat, 30 x 100 km region that is located along the eastern margin of the Ulakhan fault in northeast Russia. The Ulakhan fault roughly parallels the accretionary boundary between the Kolyma-Omolon superterrane and Siberian craton and delineates the modern plate boundary between the Okhotsk block and the North American plate proper. The subsidence history of the Seimchan basin and scale of seismic activity of the Ulakhan fault remain widely speculative due largely to a lack of detail study in this region. However, preliminary evidence suggests that active subsidence in the basin is being driven by modern offsets along the proto-Ulakhan fault. Our recent field investigations along the Ulakhan fault near the southern edge of the Seimchan basin reveal sag features, pressure ridges, troughs, and a scarp sections that delineates the trace of the fault over ~10 km, across an alluvial fan. These features can be traced within the basin on satellite images to the northwest and southeast of our survey and show left-lateral strike-slip motion associated with the fault. Previous studies support recent offset of up to 24 km as evidenced by river offsets observed northwest of the basin in the Chersky Range. Recent uplift adjacent to the Ulakhan fault and subsequent subsidence within the Seimchan basin may have been recorded by the down cutting of the Kolyma and Indigirka Rivers that have left a series of preserved meander terrace plateaus along the southern and northern margins of the basin. If one accounts for the present location of abandoned incised meanders on the north side of the basin, a 24 km offset is also apparent. A tectonic reconstruction of the area for a time prior to this offset using presumed fault motions shows that the Ulakhan fault system may be responsible for the development of the Seimchan-Buyunda basin as a large pull-apart structure. Seismicity along the Ulakhan fault has been poorly documented due to a lack of local seismic stations. However, preliminary field deployment with short-period instruments indicates a considerably higher level of seismicity than previously expected. Given the location of the Ulakhan fault along the suture boundary, new evidence for left- lateral offset, and recent seismicity, the Ulakhan clearly represented a major locus of displacement in the Pliocene that continues to take up considerable strain at the edge of the North American plate.
T13D-1580
Neoproterozoic Mafic Magmatism in Central Novaya Zemlya, Additional Evidence From Zircon and Titanite U-Pb Ages
Novaya Zemlya is a banana-shaped set of islands stretching northward from the Polar Urals and separating the Barents and Kara Seas. The southernmost part of the islands is an integral component of the Timanides, a Neoproterozoic orogenic belt, which broadly follows the Urals along the eastern and northeastern margin of Baltica. The Timanian event at ca. 600-550 Ma created a folded and imbricated Late-Neoproterozoic basement, which was subsequently unconformably covered by a succession of Early Ordovician to Permian sediments. In more northerly parts of Novaya Zemlya the evidence for such an Early Paleozoic unconformity seems to dissipate, the geological picture being dominated by an apparently continuous sedimentary succession spanning most of the Paleozoic. There is, however, local evidence for the presence of a Precambrian basement comprising Late Neoproterozoic magmatic rocks and perhaps also Mesoproterozoic basement (Korago et al. 2004). In our study we have now obtained further evidence for Neoproterozoic plutonic activity in the region. Zircons were analyzed by ID-TIMS in four gabbroic and dioritic dikes from Matotchkin Strait and Mashigin Fjord in central parts of Novaya Zemlya. They yield U-Pb ages ranging from 716 to 704 Ma, which based on the uniform and characteristic morphology of the zircon populations and the similar titanite age from one sample can be interpreted as dating intrusion of the mafic magmas. These ages indicate a period of magmatism that predated Timanian convergence and collision, perhaps supporting the inference that central parts of Novaya Zemlya were not affected by the Timanian orogeny. The timing of these plutonic events at 716-704 Ma corresponds instead to that of arc magmatism on parts of the Siberian margin (e.g. Vernikovsky et al. 2003) pointing to a possible paleogeographic affiliation of these domains. References Korago, E.A., Kovaleva, G.N., Lopation, B.G. & Orgo,V.V. 2004. The Precambrian rocks of Novaya Zemlya. In Gee, D,G, & Pease, V.L. (eds.) The Neoproterozoic Timanide Orogen of Eastern Baltica. Geol. Soc. London Memoirs, 30, 135-143. Vernikovsky, V.A., Vernikovskayaa, A.E., Kotov, A.B., Sal'nikova, E.B., & Kovach, V.P. 2003. Neoproterozoic accretionary and collisional events on western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge. Tectonophysics 375, 147– 168.
T13D-1581
RECENT RUSSIAN GEOPHYSICAL AND GEOLOGICAL INVESTIGATIONS ON SIBERIAN CONTINENTAL MARGIN
In July-August, 2005 new geophysical and geological data were acquired in the Mendeleev Rise (MR) region during "Arctic-2005" cruise aboard M/V "Akademik Fedorov". The study was concentrated in the southern part of MR in the area of its junction with East Siberian shelf. On-ice deep seismic sounding investigations (with offsets up to 250 km) and helicopter-supported seismic reflection soundings were performed along 600 km-long sub- longitudinal profile. Seismic survey was accompanied by on-ice gravity observations and geological sampling. Air-borne magnetic and air gravity measurements at scale 1:1,000,000 were also performed within a 100 km- wide corridor along the central seismic profile. Processing and analysis of new evidence included the compilation of deep seismic section, 2D seismic-gravity modeling of the Earth crust, 3D modeling of basement and Moho relief, and estimation of sediment and earth crust thickness. The results were integrated with earlier data and used for advanced structural and tectonic interpretations. The following main conclusions were obtained: Thickness of sediment cover along seismic line varies from 12 km in the south (in the North-Chukchi Trough) to 3-4 km in the northern MR. Crust thickness beneath MR is on the order of 30-35 km with a maximum value of 38 km in its southern part. The thinnest crust (28 km) is observed in the North-Chukchi Trough. Potential fields indicate existence of several blocks differing in gravity and magnetic anomalies. In the southern MR these blocks appear separated by grabens and display distinct continental characteristics accentuated by thickness of the crust, its seismic velocities and potential field pattern. At some of the shallowest (possibly eroded?) bathymetric highs the results of bottom sampling seem to point to the possibility of local derivation of coarse bottom debris. The proposed tectonic model implies structural continuity between MR and the adjacent East Siberian shelf. Brief information about the latest Russian geophysical and geological cruise "Arctic-2007" to the Lomonosov Ridge and its transition to the Siberian shelf will also be presented.
T13D-1582
SCANLIPS – A Study of Epirogenic Uplift of Scandinavia
Thermochronology data and geomorphological interpretation indicate that parts of the Scandinavian mountains have risen by over 1 km since the Miocene. This permanent uplift, the cause of which is still disputed, varies across Norway, being greatest in southern and northern areas and least in the central region. To investigate this the SCANLIPS project employs passive seismology, coupled with modelling of potential field data to determine variations in crustal properties and structure across Norway and Sweden. Initially we intend to test whether lateral variations in crustal structure and properties are correlated with the uplift pattern. This would suggest that the cause of the differential uplift lies in a modification of the crust. If the test of this hypothesis is null we will use the data to investigate the present day upper mantle structure for the cause. Between April and October 2006 28 seismometers were deployed at sites along a c. 600 km long profile between Trondheim in Norway and Harnosand in Sweden to record teleseismic arrivals. Receiver Functions have been calculated for teleseismic events recorded at these stations and then modelled to determine crustal velocity structure, estimate Vp/Vs and depth to Moho. Preliminary results suggest that crustal thickness increases eastward beneath Norway and then remains deep beneath the lower topography of central Sweden. Along the profile a gradual eastward increase in seismic velocity, including a very high velocity lower crust beneath Sweden explains the compensation of shallow topography by thick crust. Forward density and isostatic modelling shows that the introduction of the high-density lower crust adjusts both the gravity field and the isostatic compensation. Beneath Norway the crust thins rapidly toward the continental margin at a rate that is faster than the topography decreases. This suggests that at least part of the topography is supported by the flexural strength of the crust in the footwall of the More-Trondelag fault zone. Recently published results of Svenningsen et al. (2007) show a similar thickening below the high topography of southern Norway, indicating Airy type compensation. Further work is required before a direct comparison can be made of the crustal properties between the two regions and a possible cause for the differential uplift of Scandinavia determined. http://www.le.ac.uk/gl/rwe5/Neogene_uplift.html