Since 1990, acquisition of geoscientific data from ice breakers, submarines and geopotential field studies from satellites and aircrafts present a new level of opportunities to test our models for the first order geologic history of the Arctic Ocean Basin. The most significant results over the last decade are: 1. new insight into the seismic stratigraphic architecture of Lomonosov Ridge which support the origin of the ridge as a continental fragment, and 2. gravity evidence for thin crust at the Gakkel Ridge with its implication for a large role of tectonic extension at the spreading center, and 3. seismic reflection evidence for crustal scale detachments below the Laptev Sea Shelf.
Accumulating geological evidence and growing recognition of the importance of distributed deformation in the Canadian Arctic Archipelago to account for the Early Cenozoic relative motion between Greenland and North America, represent new insight into the behaviour of the lithosphere and resolution of the Nares Strait enigma.
During the Polish Geophysical Expedition in 1985 deep seismic sounding measurements were performed in transition zones between the western Spitsbergen and Knipovich Ridge, as well as between the northern Spitsbergen and Yermak Plateau. Five intersecting profiles of about 1200 km total length were performed. The interpretation was made using ray-tracing method.
Clear enough refracted waves were found up to distance of 200 km and reflected waves were recorded along of the whole profile length. In general, the crust in the investigated area can be divided in two parts. P-wave velocities in the uppermost crust are in the range of 6.3-6.5 km/s and in the lowermost crust of about 7.2 km/s. In the north-western part of the region we have found the P-wave velocity in the uppermost crust of about 5.5 km/s. The Moho discontinuity was determined at the depth of about 25 km beneath the land and of about 14-20 km beneath the sea. The rift in Knipovich Ridge was found rather as between continental crustal blocks. There is rather no the old oceanic crust in the study area. Seismic boundary at the depth of about 70 km was observed, too.
Arctic Ocean is unique among the oceans of the world in that half of its area is underlain by continental shelf, primarily the wide European and Siberian continental shelves. The Laptev, East Siberian and Chukchi seas overlie the extensive shelf of the northeastern Asia. To the north, this continental margin is bordered by Eurasia and Amerasia oceanic basins which contain large submarine ridges and plateaus: Gakkel, Lomonosov, Alpha-Mendeleev and Chukchi. Extending the terrestrial geology of the islands and mainland, one could assume that this margin occupies an area where Siberian Craton, Taimyr, New Siberian-Chukchi and Verkhoyansk-Kolyma fold belts abut and form the basement of the shelf sedimentary basins. However the structure and tectonic history of this vast area for a long time have remained poorly known.
The new offshore geophysical data, mainly the available seismic reflection profiles and altimeter derived marine gravity field, obtained in the last decade, provide a preliminary delineation of the tectonic fabric of the margin. These latter combined with the onshore bedrock studies allow making a suggestion that the continental margin was strongly affected by the rifting process since mid of Mesozoic, almost immediately after the series of terranes amalgamated to Siberian continent. The main rift zones extend from the shelf edge toward the mainland. These are the Laptev Rift System, New Siberian and Vil'kitskii rifts in the East Siberian Sea, and North Chukchi rift basin in the Chukchi Sea. These structures represent both active and aborted rifts related to extension episodes in the Arctic Ocean, namely: initial rifting in the Canada Basin (Middle Jurassic to Hauterivian), opening of the Makarov Basin (53-80 Ma?) and spreading in the Eurasia Basin (0-58 Ma). Thus it is suggested that the spreading migrated westward during the Cretaceous-Cenozoic time and led to separation and fragmentation of the continental blocks (Lomonosov and Mendeleev ridges, Chukchi Plateau) and subsidence of the rift basins on the northeastern continental margin of the Asia.
The Taimyr Peninsular is a key element in Arctic geodynamics because it records both contractional tectonics and subsidence in late Palaeozoic and Mesozoic times. The interactions of continental collisions and plume activity responsible for these processes are poorly understood, and yet they formed many of the features that were inherited in the Cenozoic development of the region. Existing models divide Taimyr into three major structural-stratigraphic provinces, separated by collisional sutures. In general the southern block is Siberia, the northern block part of an Arctic continental mass attached to Euramerica, and the central domain is a Proterozoic accretionary terrane that was welded to one or other continental margin. The timing of these suturing events, their geodynamic causes and associated deformation, are all controversial.
A traverse through 100 km of the southern and central domains of the fold belt reveals a history of stable continental margin sedimentation from Riphean to Permian times. No continental suture can be discerned between the southern and central domains, indeed this rather arbitrary boundary corresponds simply to the coincidence of a fault zone with a sedimentary facies contrast. Carbonates and more distal carbonate-rich to black shales dominate the lower Palaeozoic, with terrigenous shallow marine and deltaic sediments taking over in the late Carboniferous. These clastic sediments may reflect uplift and foredeep formation associated with final closure of the Uralian ocean, but the southern Taimyr depocentre continued filling through to the Triassic, when dolerites and basalts were generated on the side of the Siberian plume. The major structural features in the transect indicate a north-south compression and right-lateral strike slip, suggesting a transpressional regime which deformed all pre-Jurassic rocks in the same kinematic framework.
Most tectonic reconstructions of the Arctic rely on the presumption that a northern continent collided with Siberia in the Permo-Carboniferous, at the end of the Uralian orogeny. However, the age of the major deformation in central and southern Taimyr is Late Triassic because Permo-Triassic sills and volcanics are deformed in the same kinematic setting as the host sedimentary rocks, while Lower Jurassic sediments are unconsolidated and undeformed. The only evidence for possible pre-Triassic deformation is early contractional structures of unknown age but in the same kinematic orientation as the Triassic folding. Southern Taimyr may have formed a distal foreland to a late Palaeozoic collision, but juxtaposition of northern with central Taimyr was part of a more important transpressional phase in the Late Triassic. The true "Uralian suture" probably lies north of the central fold belt. Triassic dextral transpression may have resulted from plume-related extension in Siberia, or closure of the "South Anyui" oceanic embayment of Pangea. However, it is doubtful that oceanic crust extended as far west as Taimyr in the Mesozoic.
Reconstructions of the high Arctic basin for the Mesozoic, place Svalbard adjacent to northeastern Greenland, the N-trending Caledonian structures of this Barents Shelf archipelago lying discordantly to both the North and East Greenland fold belts. The Svalbard Caledonides are disrupted by several longitudinal faults across which correlation is difficult. Over thirty years ago, W. B. Harland proposed that these differences, taken together with similarities between eastern Svalbard and the East Greenland Caledonides, could be best explained by invoking large (in the order of 1000 km) strike-slip displacements on at least one of the main faults. Subsequently, this hypothesis has been variously elaborated and questioned. New investigations combining local structural/stratigraphical analysis and isotope-age/provenance studies have substantially changed the face of the Svalbard Caledonides; nevertheless the basic terrane assembly hypothesis remains intact.
Comparability of the classical Middle and Upper Hecla Hoek Neoproterozoic and Cambro-Ordovician successions of eastern Svalbard with the Eleonore Bay Group and overlying strata of similar age in East Greenland, formed an essential platform for the original terrane hypothesis. It is now known that these successions on Nordaustlandet were deposited on a Grenvillian-age basement of turbidites intruded by c 950 Ma granites; calc-alkaline volcanites of similar age separate the overlying Neoproterozoic successions from the folded older complex. The Nordaustlandet Terrane, forming the western part of the Barentsia micro-continent, is separated from the Caledonian complexes in western Ny Friesland by a c 5 km thick packet of mica schists (Planetfjella Group) of uncertain age (Neoproterozoic or Early Palaeozoic) and provenance. The West Ny Friesland Terrane, previously thought to be an essential part of Svalbard´s Eastern Tearrane together with the Nordaustlandet Terrane, is now known to be dominated by a high amphibolite facies, antiformal thrust stack (the Atomfjella Antiform), largely composed of c 1750 Ma basement granites and younger metasediments of probable Mesoproterozoic age.
The West Ny Friesland Terrane is bounded to the west by an Old Red sandstone (ORS) graben, deposited on a basement of unknown character. The boundary (the Billefjorden Fault zone) to the Ny Friesland orogen has been previously thought to be the main terrane boundary on Svalbard; new evidence from northwestern Spitsbergen suggests that the Breibogen-Bockfjorden Fault may be a more important structure. West of this fault, Svalbard´s Northwestern Terrane is dominated by migmatites intruded by Grenvillian-age granites and influencing a Mesoproterozoic or older succession of schists and marbles. The Northwestern Terrane is probably composite with a small area on Biskayerhalvøya containing an eclogite-bearing complex, including Grenville-age (c 960 Ma) granites and gabbros, some evidence of late Neoproterozoic magmatism and Caledonian (c 455 Ma) high grade metamorphism.
Along the west coast of Spitsbergen, Tertiary folding and thrusting is superimposed on the Caledonian complexes and the Palaeozoic relationships are disrupted. Nevertheless, a Grenville-age metamorphic assemblage has been identified in southern areas, unconformably overlain by Neoproterozoic successions including tillites. In west central areas, this Southwestern Terrane contains an Early Ordovician blueschist-eclogite complex, apparently thrust onto the Neoproterozoic successions and unconformably overlain by Late Ordovician to Early Silurian limestones and turbidites. This Early Palaeozoic history has been compared with that of Pearya, in marked contrast to Svalbard´s eastern terranes, of East Greenland affinities.
The bedrock of Nordaustlandet and adjacent islands forms the eastern part of Svalbard´s Eastern Caledonian Terrane, and at the same time, the westernmost and only exposed part of the so-called Barentsia craton. The oldest exposed unit, the Mesoproterozoic Brennevinsfjorden Group (partly turbiditic shales and sandstones), is unconformably overlain by andesitic to dacitic volcanic and volcaniclastic rocks of the Kapp Hansteen Group, and intruded by quartz porphyries, granites and augen gneisses of Grenvillian age. These rocks form a basement to the overlying metasediments of the Neoproterozoic Murchisonfjorden Supergroup. The whole rock complex was folded into open, upright to W-vergent, N-S-trending antiforms and synforms in Caledonian time and intruded by Caledonian granites.
Conventional U-Pb zircon dating has given an age of c. 960 Ma for the syn-tectonic Laponiafjellet granite and 940 Ma for the post-tectonic Kontaktberget granite (Gee et al., 1995). The Fonndalen and Ringåsvatnet augen gneisses of central Nordaustlandet and the Kapp Hansteen Group volcanites and related quartz porphyries has also been dated to 940-970 Ma, using a combination of conventional U-Pb zircon and monazite dating, single zircon Pb-evaporation dating, and ion microprobe spot analyses of zircons. Abundant inherited zircons range in age from c. 1200 to 1800 Ma, with occasional late Archean grains. The geochemistry of these rocks suggests that this late Grenvillian magmatism occurred in a volcanic arc or syn-collisional setting, with large sedimentary input to the magmas.
Conventional U-Pb zircon and monazite dating and single zircon Pb-evaporation dating indicate Caledonian ages of 410-420 Ma for the high-magnetic Djupkilsodden pluton (Gee et al., in press) and the Rijpfjorden and Nordkapp granites. Inherited zircons again are abundant. The geochemistry of the Nordkapp and Rijpfjorden granites suggests a crustal anatectic origin and a syn-collisional setting. The quartz monzonites of the Djupkilsodden pluton may have a more deep-seated source. Initial single zircon Pb-evaporation data provide evidence for Caledonian migmatization and magmatism in the far northeast (Dammflya, Nordmarka, Kvitöya) at 430-450 Ma, slightly earlier than the late-tectonic plutons. This would support the idea of a Caledonian Iapetus suture running east of Svalbard, and disqualify ideas of Barentsia as a stable cratonic area during Caledonian orogeny.
The above data add to the growing evidence for a branch of the Grenville Orogen reaching into the present-day high Arctic, as well as underline the importance of Caledonian magmatism in western Barentsia.
Gee DG, Johansson Å, Ohta Y, Tebenkov AM, Krasil'scikov AA, Balashov YuA, Larionov AN, Gannibal LF & Ryungenen GF, Precambrian Research, 70, 215-234, (1995).
Gee DG, Johansson Å, Larionov AN & Tebenkov AM, Polarforschung, (in press).
Research on northeastern Svalbard in recent years has demonstrated that the classical Neoproterozoic and Palaeozoic (Middle and Upper Hecla Hoek) successions were deposited on a basement complex of Grenvillian age. In the central part of Nordaustlandet, the Middle Hecla Hoek Murchisonfjorden Supergroup is underlain by granites of Grenvillian age (c. 950 Ma) with related augen gneisses, intruded syntectonically into late Neoproterozoic turbidites (Brennevinsfjorden and Helvetesflya groups). Volcanic units of calc-alkaline affinities, the Kapp Hansteen and Svartrabbane groups, of approximately the same Grenvillian age as the underlying basement, separate the Neoproterozoic strata from the underlying basement. The Caledonian deformation throughout western and central Nordaustlandet is dominated by up-right to W-vergent folding with a well developed cleavage in the Neoproterozoic strata and low greenschist facies metamorphism at the base of the 6 km thick succession. Within the cores of the major Caledonian, generally S-plunging anticlines migmatites occur that have been inferred by previous authors to be either Caledonian or Grenvillian in age. In central Nordaustlandet, they are cut by Late Caledonian (c. 415 Ma) granites.
Central Nordaustlandet, in the area between Rijpdalen-Duvefjorden in the north and Wahlenbergfjorden in the south, is dominated by a major fold, the Rijpdalen Anticline, cored by the Caledonian Rijpdalen granite. The major unconformity between the Grenvillian-age basement complex and the overlying Murchisonfjorden Supergroup strata is well exposed locally (eg in Galtendalen) in the western limb of the Rijpdalen Anticline; here, the lower units (Westmanbukta, Persberget and Meyerbukta Formations) appear to be readily correlatable with successions in the type areas of western Nordaustlandet. In the eastern limb of the Rijpdalen Anticline, the Persberget quartzite and overlying Westmanbukta multicoloured shales are similarly developed, but the Meyerbukta Formation, dominated by limestones and shales further west, is largely composed of sandstones (quartzites) and shales, with a notable paucity of limestone. It has here been called the Djevleflota Formation.
In the eastern limb of the Rijpdalen Anticline, subordinate folds plunge south and the Persberget and Djevleflota Formations are well exposed in the Kvartsithaugen and Innvikhøgden Synclines. In the area south of Innvika, the migmatite front cuts across the axis of the Innvikhøgden Syncline and the Djevleflota metasedimentary rocks are incorporated in the migmatites; quartzites and subordinate marbles occur as conspicuous mappable palaeosome in the latter. Thus in this critical area of central Nordaustlandet, there is field evidence supporting the interpretation that migmatization is of Caledonian age.
The western part of the Ny Friesland peninsula, located in northeastern Spitsbergen, includes a ca. 10 km thick packet (the Lower Hecla Hoek) of concordantly foliated metasediments and granitic gneisses. Modern mapping with detailed structural and isotopic studies, during the last eight years provide evidence that the Lower Hecla Hoek succession is tectonostratigraphic, including at least four thrust sheets, from the base the Finlandveggen, Rekvika, Nordbreen, and Dirksodden Nappes. The thrust sheets contain late Palaeoproterozoic (1730-1760 Ma) granitic gneisses overlain by metasedimentary rocks. Detrital zircons from four metasedimentary formations at the different tectonostratigraphic levels have been dated by the single zircon Pb-evaporation technique. In addition, U/Pb ionprobe (NORDSIM) geochronology on zircons from a meta-dolerite in the Rekvika Nappe has been carried out.The detrital zircon ages from the three upper nappes (Rekvika, Nordbreen and Dirksodden Nappes) show a consistent age-pattern, with three well defined age-groups, one at 1700 to 1750 Ma, a more prominent and diverse group ranging from 1850 to 2050 Ma and a significantly older group at 2500 to 2800 Ma. A few zircons in the quartzite of Rekvika Nappe yield Mesoproterozoic ages of 1320 and 1500 Ma. U/Pb-ionprobe data (NORDSIM) on zircons from a meta-dolerite cutting these sediments yield an age of 1302 ±25 Ma. This age is interpreted to be the intrusion age of the dyke but an inherited origin of the zircons is possible. The metasediments in the lowest thrust sheet show younger provenance ages of 1180 to 1700 Ma and older of 2500 to 2700 Ma; they lack the middle Paleoproterozoic group so dominant in the other tectonostratigraphic levels.A correlation of the sediments in the different thrusts (except those in the lowermost thrust sheet) is probable due to their similarity in composition and provenance ages. The detrital zircon ages show that the metasediments in the four thrust sheets are younger than the granitic gneisses of 1730-1760 Ma age. The age of the meta-dolerite suggest that they where deposited in earlier than 1300 Ma and later than 1320 Ma based on the youngest detrital zircon in the metasediments. It is likely that the granitic gneisses acted as a basement to the sediments and contributed with the 1730-1750 Ma old zircons found in most of the samples. The lowest thrust (Finnlandveggen Nappe) contain metasediments that has to younger than 1180 Ma, they can be as young as Silurian.
Crustal extension during and following the continental collision is well documented in the Arctic Caledonian fold belt. Models for the extensional collapse of the Caledonides are mainly based on geoscientific data from Scandinavia. However, for a more complete understanding of the evolutionof the Caledonides, knowledge of the crustal structure of East Greenland is vital.
We present results from a multidisciplinary geophysical study of North-East Greenland carried out by the Alfred Wegener Institute for Polar and Marine Research between 1993 and 1997. Seismic refraction studies revealed a pronounced topography of the Moho and a west-dipping lower crustal reflector beneath the fjord region of East Greenland. These deep crustal structures are related to Late Caledonian extensional structures at the surface. Simple-shear type extension along a west-dipping shear zone delineated by the lower crustal reflector could explain the observations. Thus, the East Greenland Caledonides probably thinned along a lower-crustal analogue of the west-dipping extensional detachments exposed in Norway. On a crustal scale, we find no evidence for a symmetric extension of the East Greenland and Scandinavian Caledonides as suggested from surface geology.
Exhumation of the Caledonian North-East Greenland Eclogite Province cannot be accomplished by our model. Instead, a synthesis of geoscientific data reveals marked differences in crustal structure of East Greenland north and south of about 76N indicating a contrasting crustal evolution of a northern part ofthe East Greenland Caledonides, hosting the eclogites, and a southern part, described by our model.
This subdivision of the East Greenland Caledonides into differently evolving parts has to be considered when developing models, e.g. for the exhumation of the Caledonian eclogite provinces or for the evolution of the Arctic Caledonides.
In this contribution the palaeogeothermal pattern in the Tertiary and Cretaceous rocks of northern Greenland and Spitsbergen will be described on the basis of recently obtained vitrinite reflectance data. In northeastern Greenland, flat-lying Tertiary sediments rest with sharp unconformity on folded Upper Cretaceous and older strata. In contrast, in northernmost Greenland the Kap Washington Volcanics, partly extruded in Tertiary times, were involved in Alpidic deformation. In Spitsbergen, Tertiary rocks are found in small grabens in the west and in the large Central Basin in the east. The palaeotectonic history of the Central Basin is characterized by a hiatus lasting some 30 Ma between the Early Cretaceous and the Early Tertiary. However, no evidence of compressive deformation during this hiatus has been found. Subsequently, the Tertiary sediments and the underlying strata, together with the adjacent West Spitsbergen Fault-and-Thrust Belt, were affected by Alpidic compression. In general, the coalification pattern of the Greenland margin is documented by vitrinite reflectance (Rr) as follows: (1) Cretaceous: the mean Rr of the Upper Cretaceous in Depotbugt is 1.9-2.2%, the Upper Cretaceous along the Harderfjord Fault Zone 2.6-3.6% (mainly 3.2%), Upper Cretaceous of Herlufholm Strand Formation in central Herluf Trolle Land 2.5-3.0% (mainly 2.8%), Lower Cretaceous of southern Herluf Trolle Land 0.53-0.57%, Upper Cretaceous on Kilen 1.6-2.0%, and the Cretaceous beneath the Kap Washington Volcanics is 2.6-5.4%; and (2) Tertiary: Rr for the Lower Tertiary of Thyra Ø ranges from 0.49-0.61% (mainly 0.56%).The coalification pattern on Spitsbergen is completely different. Palaeocene and Eocene Tertiary rocks of the Central Basin show Rr ranging from 0.60% to more than 1% depending on the previous amount of overburden. The highest values are associated with the deepest zone of the Central Basin in southeastern Spitsbergen. They decrease eastwards and also westwards towards the West Spitsbergen Fault-and-Thrust Belt, which was formed mainly in Tertiary times. The underlying Cretaceous shows a similar pattern with somewhat higher values (0.62-1.4% Rr). The coalification pattern of the Tertiary in grabens in the west is asymmetric. On the western flank of the Forlandsundet graben Rr is generally over 2%, max 5.1%, and on the eastern flank Rr varies between 0.56 and 0.69% Rr.With respect to the isotopic composition of the organic matter, the 13C values of Tertiary samples from Spitsbergen vary from -22.2 to -28%, the heaviest carbon occurs in samples that have undergone tectonic deformation (cleavage). The Tertiary of Greenland yielded 13C values of -22 to -23.5%, similar to those of the underlying Cretaceous.
The tectonic map, scale of 1:2 500 000, covers 820 to 680 N lat. and includes the Yamal Peninsula to the West and the mouth of Lena River on the East. Occupying more than 2/3 of the map area are the southern marginal basins (Nansen and Amundsen basins and the Gakkel Oceanic Ridge) within the Polar Ocean which consists of the Kara Sea and Laptev Sea. The land part includes the Archipelagoes of Franz Josef Land, Novaya Zemlya, and Severnaya Zemlya; the Taymyr Peninsula, lower reaches of the Ob, Yenisey, Khatanga, and Lena Rivers; and on the continent, the West Siberian Lowland and the Anabar Highland. A large amount of geophysical data, rare drill boreholes studies and 1:200 000 scale geological maps of this territory were used to establish the tectonic architecture and history of the area. Two separate surfaces of cover structure are shown by isopleths covering the whole map: (1) depths to the assumed cover bottom (the acoustic basement, depths range up to 14 km) and (2) depths down to the lowest boundaries of the Jurassic, or sometimes the Cretaceous sediments within the cover. The continental crust includes the Early Proterozoic Siberian craton, Siberian Triassic trap complex; the Late Orecambrian Kara plate and the Svalbard plate; the Late Proterozoic Taymyr accretionary belt; the Paleozoic West Siberian plate, and the Late Mesozoic Kotelny fold belt. Continental rifts, 10-14 km deep, are localized within the western Siberian platform and in the northern Siberian craton. The suboceanic crust is widespread in specific areas, such as the Santa Anna Trough and Voronin Trough, localized near the Oceanic basin, and also in the Pur-Gydan massif between the Siberian craton platform. The oceanic crust areas are formed by spreading processes in the Nansen and Amundsen basins.
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