Marginal continental and within-plate neoproterozoic granites and rhyolites of Wrangel Island,Arctic region

The paper presents new data on the U–Pb zircon age, as well as results of isotopic geochemical analysis, of granites and rhyolites from Wrangel Island. The U–Pb age estimates of granites and rhyolites are grouped into two clusters (~690–730 and 590–610 Ma), which imply that these rocks crystallized in the Late Neoproterozoic. Granitic rocks dated back to 690–730 Ma are characterized by negative εNd(t) values and Paleoproterozoic Sm–Nd model age. The older inherited zircons corroborate the ancient age of their crustal source. The granitic rocks pertain to involved peraluminous granites of type I, which form at a continental margin of the Andean type and can be compared with coeval granites and orthogneisses from the Seward Peninsula in Alaska. Rhyolites and granites ~590–610 Ma in age are distinguished by a moderately positive εNd(t) and Mesoproterozoic model age. It is suggested that they have a heterogeneous magma source comprising crustal and mantle components. The geochemical features of granites and rhyolites correspond to type A granites. Together with coeval OIB-type basalts, they make up a riftogenic bimodal association of igneous rocks, which are comparable with orthogneisses (565 Ma) and gabbroic rocks (540 Ma) of Seward Peninsula in Alaska.     

Neoproterozoic granitoids on Wrangel Island

Based on geochronological U–Pb studies, the age of Wrangel Island granitoids was estimated as Neoproterozoic (Cryogenian). Some granitoids contain zircons with inherited cores with an estimated age of 1010, 1170, 1200, and >2600 Ma, assuming the presence of ancient (Neoarchean–Mesoproterozoic) rocks in the Wrangel Island foundation and their involvement in partial melting under granitoid magma formation.     

The salt composition of rivers in Wrangel Island

Pioneer information about the chemical composition of river water in Wrangel Island has been obtained. It is shown that the water composition reflects the lithogeochemical specifics of primary rocks and ore mineralization. In contrast to many areas of the Russian Far North, river waters of the island are characterized by an elevated background value of total mineralization (i.e., total dissolved solids, TDS) (0.3–2 g/l) and specific chemical type (SO₄-Ca-Mg). This is related to abundance of Late Carboniferous gypsiferous and dolomitic sequences in the mountainous area of the island. It has also been established that the salt composition of some streams is appreciably governed by supergene alterations of the sulfide mineralization associated with the quartz-carbonate vein systems. They make up natural centers of surface water contamination. Waters in such streams are characterized by low pH values (2.4–5.5), high TDS (up to 6–23 g/l) and the SO₄-Mg composition. These waters are also marked by anomalously high concentrations of heavy and nonferrous metals, as well as REE, U, and Th.     

Tectonic elements of the Arctic region inferred from small-scale geophysical fields

Satellite altimetry data, Bouguer anomalies, anomalous magnetic field, bottom topography, and Love wave tomography for the deepwater part of the Arctic Ocean Basin and East Siberian Sea have made it possible to detect several new regional tectonic elements. The basin area, 700 km wide and 1800 km long, extending from the Laptev Sea to the Chukchi Borderland is a dextral strike-slip zone with structural elements typical of shearing. The destruction of the Eurasian margin surrounding the Amerasia Basin occurs within this zone. The opening of the Amerasia Basin is characterized by intense plume magmatism superimposed on normal slow spreading in several areas of the paleospreading axis. Magma was supplied through three conduits with minor offsets, the activity of which waned partly or completely by the end of basin formation. The main central conduit formed the structure of the Alpha Ridge. The dextral strike-slip system, which displaces the Gakkel Ridge and structural elements in the basement of the Makarov Basin, most likely extends to the northern termination of the Chukchi Borderland.     

Active Tectonics of the Eastern Arctic: New Data from Geological and Geophysical Studies at Cape Thomas (West of Wrangel Island)

Geotectonics - The article presents the results of the first comprehensive geological and geophysical studies of active tectonics in the western part of Wrangel Island, carried out during the...     

Evidence for an ice-free Wrangel Island, northeast Siberia during the Last Glacial Maximum

¹⁰Be and ²⁶Al surface exposure ages from 22 tors and bedrock samples from Wrangel Island, northeast Siberia, indicate that the East Siberian and Chukchi shelves were ice-free during the Last Glacial Maximum (LGM). The paucity of glacial landforms and deposits, the absence of erratics and the presence of radiocarbon dates on plant and mammal fossils that span the LGM suggest that Wrangel Island also remained free of extensive glacial ice during the LGM. The lack of moisture due to the continental climate on the emergent Bering Land Bridge is the most likely reason for limited ice in this part of the Arctic. Alternative interpretations regarding the age and origin of 'glaciogenic' bedforms on the Chukchi shelf should be considered.     

北极地区地质构造及主要构造事件

北极地区范围很广,北极圈面积达2 100×10⁴ km²,区域地质复杂。通过对北极地区区域地质编图,笔者认为前寒武纪主要由波罗的、劳伦和西伯利亚三大克拉通,以及其间的微板块或地块组成。主要造山带包括新元古代-早寒武世的贝加尔造山带、晚志留世-早石炭世的加里东造山带、晚古生代-早中生代的海西造山带、晚中生代的上扬斯克造山带、新西伯利亚造山带与楚科奇-布鲁克斯造山带。根据北极地区区域地质构造特征,显生宙以来经历的构造事件大致包括:新元古代-早寒武世的贝加尔运动,致使波罗的古陆与斯瓦尔巴-喀拉地块碰撞造山;晚泥盆世-早石炭世的加里东运动,在劳伦古陆周边形成规模巨大的加里东造山带;晚古生代的海西运动,波罗的古陆与西伯利亚古陆的碰撞造山形成海西造山带;北极阿拉斯加-楚科奇微板块裂离加拿大边缘,侏罗纪加拿大海盆开始张开;早白垩世,阿拉斯加-楚科奇微板块继续与西伯利亚碰撞,阿纽伊洋(Anyui Ocean)消亡,形成上扬斯克-布鲁克斯造山带。受北极调查程度影响,许多问题有待进一步研究。     

Late Saalian and Eemian palaeoenvironmental history of the Bol''shoy Lyakhovsky Island (Laptev Sea region, Arctic Siberia)

Palaeoenvironmental records from permafrost sequences complemented by infrared stimulated luminescence (IRSL) and [Formula: See Text]Th/U dates from Bol'shoy Lyakhovsky Island (73°20'N, 141°30'E) document the environmental history in the region for at least the past 200 ka. Pollen spectra and insect fauna indicate that relatively wet grass-sedge tundra habitats dominated during an interstadial c. 200-170 ka BP. Summers were rather warm and wet, while stable isotopes reflect severe winter conditions. The pollen spectra reflect sparser grass-sedge vegetation during a Taz (Late Saalian) stage, c. 170-130 ka BP, with environmental conditions much more severe compared with the previous interstadial. Open Poaceae and Artemisia plant associations dominated vegetation at the beginning of the Kazantsevo (Eemian) c. 130 ka BP. Some shrubs (Alnus fruticosa, Salix, Betula nana) grew in more protected and wetter places as well. The climate was relatively warm during this time, resulting in the melting of Saalian ice wedges. Later, during the interglacial optimum, shrub tundra with Alnus fruticosa and Betula nana s.l. dominated vegetation. Climate was relatively wet and warm. Quantitative pollen-based climate reconstruction suggests that mean July temperatures were 4-5°C higher than the present during the optimum of the Eemian, while late Eemian records indicate significant climate deterioration.     

The deglaciation of Clyde Inlet,northeastern Baffin Island,Arctic Canada

The behaviour of ice sheets as they retreated from their Last Glacial Maximum (LGM) positions provides insights into Lateglacial and early Holocene ice‐sheet dynamics and climate change. The pattern of deglaciation of the Laurentide Ice Sheet (LIS) in arctic fiord landscapes can now be well dated using cosmogenic exposure dating. We use cosmogenic exposure and radiocarbon ages to constrain the deglaciation history of Clyde Inlet, a 120 km long fiord on northeastern Baffin Island. The LIS reached the continental shelf during the LGM, retreated from the coastal lowlands by 12.5 ± 0.7 ka (n = 3), and from the fiord mouth by 11.7 ± 2.2 ka (n = 4). Rapid retreat from the outer fiord occurred 10.3 ± 1.3 ka (n = 6), with the terminus reaching the inner fiord shortly after 9.4 ka (n = 2), where several moraine systems were deposited between ca. 9.4 and ca. 8.4 ka. These moraines represent fluctuations of the LIS during the warmest summers since the last interglaciation, and this suggests that the ice sheet was responding to increased snowfall. Before retreating from the head of Clyde Inlet, the LIS margin fluctuated at least twice between ca. 7.9 and ca. 8.5 ka, possibly in response to the 8.2 ka cold event. Copyright © 2006 John Wiley & Sons, Ltd.     

海南屯昌为火山岩构造属性及其年代学研究

海南屯昌变火山岩与下古生界长英质片岩互层产出,其原岩为钙碱性粗安－安山岩和流纹岩组合,它们与海相碎屑岩一想构成岛弧环境的沉积－火山岩系。岩石化学特征表明,该变火山岩系属于岛弧区玄武质岩浆分异的产物,而玄武质岩浆产生则与富集型MORB地幔（FMM）经历中等程度部分熔融有关。变火山岩Ti相对于Zr和Y严重亏损可能与岛弧区俯冲深度有限相关,而其εNd（527Ma）（8．3－9．9）与P、Ti、Nb、K、Ba、Sr和Ce负异常一并指示交代富集型地幔源区（可能为EMII）的特点。锆石U－Pb年代学证实屯昌变火山岩形成于527Ma,并经历过印支期热－构造事件的叠置改造,通过研究变火山岩系将为约束海南岛及其邻区特别是特提斯域构造演化事件提供某些重要信息。     

Tectonic geomorphology in parts of Koyna region,Maharashtra

Geomorphic signatures in parts of Koyna region were analysed using satellite images and morphometric indices (stream length gradient index and valley width to height ratio) with limited field checks in key areas for understanding the role of tectonics in this seismically active zone. The Koyna River zone, particularly downstream of Koyna dam wall up to Kajali Nadi, is characterised by incised narrow valleys, steep escarpments and structurally controlled drainage. Presence of knick points at consistent elevations between 825 m to 675 m, steep gradients in an otherwise gradual longitudinal profile and valley width to height ratio less than 1.0 suggest readjustment of the streams along NW-SE and N-S directions. The escarpment slopes in general have en echelon arcuate ridge pattern ～ N45°-N 225° in the western part, and then becomes ～N 10°- N 190° near Donachiwadi, almost orthogonal to the E-W Koyna River. The Donachiwadi fault runs parallel to the escarpment slopes observed near Kodoli. The fluvial system in Koyna thus indicates rejuvenation controlled by basement propagated structures.     

Tectonic evolution of the Iceland region,North Atlantic

Major hypotheses on the formation of the Iceland region are considered. It is noted that plate- and plume-tectonic genesis is the most substantiated hypothesis for this region. Model estimations of the effect of hot plume on the formation of genetically different oceanic ridges are obtained. Computer calculations are performed for the thermal subsidence rate of aseismic ridges (Ninetyeast and Hawaiian-Emperor) in the asthenosphere of the Indian and Pacific oceans. Comparative analysis of the calculated subsidence rates of these ridges with those in the Iceland region (Reykjanes and Kolbeinsey ridges) is performed. The results suggest that the thermophysical processes of formation of the spreading Reykjanes and Kolbeinsey ridges were similar to those of the aseismic Ninetyeast and Hawaiian-Emperor ridges: the genesis of all these ridges is related to the functioning of a hotspot. Analysis of the heat flux distribution in the Iceland Island and Hawaiian Rise areas is carried out. Analysis and numerical calculations indicate that the genesis of Iceland was initially characterized by the plume-tectonic transformation of a continental rather than oceanic lithosphere. The level of geothermal regime near Iceland was two times higher (100 mW/m²) relative to the Hawaiian Rise area (50 mW/m²) because the average lithosphere thickness of the Reykjanes and Kolbeinsey ridges near the Iceland was approximately two times less (40 km) relative to the thickness of the Pacific Plate (80 km) in the Hawaiian area. The main stages of evolution of the Iceland region are based on geological and geothermal data and numerical thermophysical modeling. The Cenozoic tectonic evolution of the region is considered. Paleogeodynamic reconstructions of the North Atlantic in the hotspot system at 60, 50, and 20 Ma are obtained.     

Arctic plesiosaurs from the Lower Cretaceous of Melville Island,Nunavut, Canada

An expedition to Melville Island in Nunavut, Canada, recovered the fragmentary fossils of several plesiosaurs from non-marine deposits of the Hauterivian–Aptian Isachsen Formation. These plesiosaur fossils are some of the oldest Early Cretaceous records of the group in North America, and they likely predate the formation of a continuous Western Interior Seaway. The plesiosaurs from Melville Island appear to be primarily juveniles, and would have been living in a region that experienced at least seasonally cool temperatures. The presence of these fossils in a fluvial deposit support previous suggestions that juvenile plesiosaurs may have preferentially inhabited shallower waters rather than open marine environments. These fossils also show that polycotylid plesiosaurs were able to rapidly disperse and colonize high latitude coastal regions, as their occurrence in Arctic Canada only slightly postdates the first confirmed appearance of the group in Australia.     

Upper Silurian lithistid sponge reefs on Somerset Island, Arctic Canada

The Upper Ludlow Douro Formation contains the first reported Silurian sponge reefs. These relatively small (5–35 m diameter), mound-shaped structures contain, on average, 35% lithistid demosponges. Reefs are surrounded by irregular haloes of crinoid debris; abundance and diversity of all fossil groups decreases away from the reefs. Each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts; trepostomate bryozoans, solenoporacean algae and rhynchonellid brachiopods are locally common. The bulk of each reef consists of lime mudstone with abundant lithistid sponges. This is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges, calcareous algae, trepostomate bryozoans and stromatoporoids. This zonation, in which a sponge colonization community was replaced by a coral diversification community, is similar to that reported from some Middle Ordovician, Upper Jurassic and Holocene sponge reefs. The Douro sponge reefs were relatively low structures, with about 3 m maximum topographic relief. They grew on a broad carbonate platform, probably in warm, tranquil, turbid waters of normal or near-normal marine salinity. Periodic influxes of terrigenous mud adversely affected reef size, and caused biotic changes. Some of the reef lime mud was derived from non-reef sources, but significant quantities were also produced on the reefs. Reefs underwent synsedimentary lithification, bioerosion and minor storm erosion. Fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation. Originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite. Early dissolution of siliceous spicules is common in reef environments, and may have caused fossil sponges to be under-represented in ancient reefs.     

Pleistocene raised marine deposits on Wrangel Island, northeast Siberia and implications for the presence of an East Siberian ice sheet

Two previously undocumented Pleistocene marine transgressions on Wrangel Island, northeastern Siberia, question the presence of an East Siberian or Beringian ice sheet during the last glacial maximum (LGM). The Tundrovayan Transgression (459,000–780,000 yr B.P.) is represented by raised marine deposits and landforms 15–41 m asl located up to 18 km inland. The presence of high sea level 64,000–73,000 yr ago (the Krasny Flagian Transgression) is preserved in deposits and landforms 4–7 m asl in the Krasny Flag valley. These deposits and landforms were mapped, dated, and described using amino acid geochronology, radiocarbon, optically stimulated luminescence, electron spin resonance, oxygen isotopes, micropaleontology, paleomagnetism, and grain sizes. The marine deposits are eustatic and not isostatic in origin. All marine deposits on Wrangel Island predate the LGM, indicating that neither Wrangel Island nor the East Siberian or Chukchi Seas experienced extensive glaciation over the last 64,000 yr.     

Radiocarbon Geochemistry of Modern and Ancient Arctic Lake Systems, Baffin Island, Canada

The accuracy of Arctic lake chronologies has been assessed by measuring the¹⁴C activities of modern carbon sources and applying these isotopic mass balances to dating fossil lake materials. Small (<1 km²) shallow (<25 m) Arctic lakes with watersheds <12 km²have soil and peat stratigraphic sections with¹⁴C activities ranging from 98 to 51% Modern. The¹⁴C activity of particulate organic carbon, dissolved organic carbon, and dissolved inorganic carbon from lake and stream waters ranges from 121 to 95% Modern. The sediment–water interface of the studied lakes shows consistent¹⁴C ages of ∼1000¹⁴C yr, although the¹⁴C activity of living aquatic vegetation is 115% Modern. Radiocarbon measurements of components of the lacustrine carbon pool imply that the ∼1000¹⁴C yr age of the sediment–water interface results from deposition of¹⁴C-depleted organic matter derived from the watershed.     

Tectonics of the eastern Arctic region

The Vendian (Baikalian), Late Devonian (Ellesmerian), and Mid-Cretaceous (Brookian) orogenies were three cardinal events in the history of formation and transformation of the continental crust in the eastern Arctic region. The epi-Baikalian Hyperborean Craton was formed by the end of the Vendian (660–550 Ma), when the Archean-Proterozoic Hyperborean continental block was built up by the Baikalian orogenic belt and concomitant collision granitoids. As judged from the localization of deepwater facies, the Early Paleozoic ocean occupied the western part of the Canadian Arctic Archipelago, western Alaska, and the southern framework of the Canada and Podvodnikov basins and was connected with the Iapetus ocean. The closure of the Early Paleozoic Arctic basins is recorded in two surfaces of structural unconformities corresponding to the pre-Middle Devonian Scandian orogenic phase and the Late Devonian Ellesmerian Orogeny; each tectonic phase was accompanied by dislocations and metamorphism. The Ellesmerian collision was crucial in the Caledonian tectogenesis. The widespread Late Devonian-Mississippian rifting probably was a reflection of postorogenic relaxation. As a result, the vast epi-Caledonian continental plate named Euramerica, or Laurussia, was formed at the Devonian-Carboniferous boundary. The East Arctic segment of this plate is considered in this paper. In the Devonian, the Angayucham ocean, which was connected with the Paleoasian and Uralian oceans [62], separated this plate from the Siberian continent. The South Anyui Basin most likely was a part of this Paleozoic oceanic space. The shelf sedimentation on the epi-Caledonian plate in the Carboniferous and Permian was followed by subsidence and initial rifting in the Triassic and Jurassic, which further gave way to the late Neocomian-early Albian spreading in the Canada Basin that detached the Chukchi Peninsula-Alaska microplate from the continental plate [25]. The collision of this microplate with the Siberian continent led to the closure of the South Anyui-Angayucham ocean and the development of the Mid-Cretaceous New Siberian-Chukchi-Brooks Orogenic System that comprised the back Chukchi Zone as a hinterland and the frontal New Siberian-Wrangel-Herald-Lisburne-Brooks Thrust Zone as a foreland; the basins coeval with thrusting adjoined the foreland. Collision started in the Late Jurassic; however, the peak of the orogenic stage fell on the interval 125–112 Ma, when ophiolites had been obducted on the margin of the Chukchi Peninsula-Alaska microplate along with folding and thrusting accompanied by an increase in the crust’s thickness, amphibolite-facies metamorphism, and growth of granite-gneiss domes. The magmatic diapir of the De Long Arch that grew within the continental plate in the Mid-Cretaceous reflected a global pulse of the lower mantle upwelling that coincided with the maximum opening of the Canada Basin. The present-day appearance of the eastern Arctic region arose in the Late Mesozoic and Cenozoic owing to the opening of the Amerasia and Eurasia oceans. Sedimentary basins of various ages and origins—including the Late Devonian-Early Carboniferous grabens, the spatially coinciding Late Jurassic-Early Cretaceous rifts related to the opening of the Canada Basin, the syncollision basins in front of the growing orogen, and the Cretaceous-Cenozoic basins coeval with strike-slip faulting and rifting at the final stages of orogenic compression and during the opening of the Eurasia ocean were telescoped on sea shelves.     

Collection of radiocarbon dates on the mammoths (Mammuthus primigenius) and other genera of Wrangel Island, northeast Siberia, Russia

We present and discuss a full list of radiocarbon dates for woolly mammoth and other species of the Mammoth fauna available from Wrangel Island, northeast Siberia, Russia. Most of the radiocarbon dates are published here for the first time. Of the124 radiocarbon dates on mammoth bone, 106 fall between 3700 and 9000 yr ago. We believe these dates bracket the period of mammoth isolation on Wrangel Island and their ultimate extinction, which we attribute to natural causes. The absence of dates between 9–12 ka probably indicates a period when mammoths were absent from Wrangel Island. Long bone dimensions of Holocene mammoths from Wrangel Island indicate that these animals were comparable in size to those on the mainland; although they were not large animals, neither can they be classified as dwarfs. Occurrence of mammoth Holocene refugia on the mainland is suggested. Based on other species of the Mammoth fauna that have also been radiocarbon on Wrangel Island, including horse, bison, musk ox and woolly rhinoceros, it appears that the mammoth was the only species of that fauna that inhabited Wrangel Island in the mid-Holocene.