Metamorphic and Thermal History of a Fore-Arc Basin: the Fossil Bluff Group, Alexander Island, Antarctica

The Himalia Ridge Formation (Fossil Bluff Group), AlexanderIsland is a 2·2-km-thick sequence of Upper Jurassic–LowerCretaceous conglomerates, sandstones and mudstones, derivedfrom an andesitic volcanic arc and deposited in a fore-arc basin.The metamorphic and thermal history of the formation has beendetermined using authigenic mineral assemblages and vitrinitereflectance measurements. Metamorphic effects include compaction,pore-space reduction, cementation and dissolution and replacementof detrital grains by clay minerals (smectite, illite/smectite,corrensite and kaolinite), calcite, chlorite, laumontite, prehnite,pumpellyite, albite and mica, with less common quartz, haematite,pyrite and epidote. The authigenic mineral assemblages exhibita depth-dependence, and laumontite and calcite exhibit a strongantipathetic relationship. Detrital organic matter in the argillaceouslayers has vitrinite reflectance values (R_o) ranging from 2·3to 3·7%. This indicates considerable thermal maturation,with a systematic increase in reflectivity with increasing depth.There is good correlation of metamorphic mineral assemblageswith chlorite crystallinity and vitrinite reflectance values—allindicating temperatures in the range of 140 ± 20°Cat the top of the sequence to 250 ± 10°C at the baseof the sequence. The temperatures suggest a geothermal gradientof 36–64°C/km and a most likely gradient of 50°C/km.It is suggested that this higher-than-average gradient for afore-arc basin resulted either from rifting during basin formationor from a late-stage arc migration event. KEY WORDS: Antarctica; diagenesis; fore-arc basin; low-temperature metamorphism; vitrinite reflectance     

The First Upper Triassic Vertebrate Locality in Antarctica

In December 1999 we collected bones of two small reptiles from Upper Triassic mudstone on Mount Dearborn in southern Victoria Land, Antarctica. One specimen includes indeterminate limb fragments of a small reptile and the other is a tusk of a small dicynodont. We found both specimens in loose clasts on a slope of fine-grained strata above the second major channel-form sandstone above the base of member C in the Lashly Formation. The fossils occur in medium gray mudstone of the same lithology as the surrounding bedrock. Lashly member C at Mount Dearborn and in southern Victoria Land is composed of fining-upward cycles. At Mount Dearborn the cycles begin with channel-form, medium- to fine-grained sandstone. Quartz pebbles and cobbles occur at the base and in lenses with mudstone clasts. Lenses of coal occur along bedding planes and scours. The cycles fine upward gradually from sandstone to siltstone and carbonaceous mudstone with coal at the top. The Dicroidium flora is better preserved in the upper part of the cycles. A palynomorph assemblage from one of the bone-bearing samples suggests a Late Triassic (Carnian) age for the locality.This is the first report of Upper Triassic vertebrates in Antarctica and also of terrestrial vertebrates in Victoria Land. All previous vertebrate localities have been in non-carbonaceous beds in the central Transantarctic Mountains. The discovery expands the sequence of tetrapod occurrences in Lower, Middle and Upper Triassic and Lower Jurassic rocks in the Transantarctic Mountains. It appears that reptilian faunas managed to inhabit high latitudes (>60°), at least intermittently, for a period of 50 million years, suggesting a relatively warm climate for parts of Antarctica during those times.     

南极长城站区寒冻风化特征

通过对南极长城站区寒冻风化速率的观测和分析，结合气温等因素的变化，讨论了该区寒冻风化与影响因素之间的关系。研究表明，该区夏季风化速率为冬季的36－7．5倍；海拔较高的地方风化速率比海拔低的高1．28－3．35倍；壁朝向东、东南的风化强度比朝向北、北西的大2．6－16倍。低温年份的风化速度仅有高温年份的7．2％－16．6％，与世界其它冰缘区的风化速率对比，本区寒冻风化速率属中等水平。     

Late Cretaceous flora of the Hidden Lake Formation,James Ross Island (Antarctica), its biostratigraphy and palaeoecological implications

Plant fossils from the volcano-clastic marine deposits of the Coniacian Hidden Lake Formation of James Ross Island in Antarctica are described based on their macromorphology. Stratigraphic positions of fossiliferous horizons and details of the lithostratigraphic situation of the middle part of the Hidden Lake Formation are published for the first time. The flora consists primarily of leaf impressions and petrified wood. There are also small amounts of mesofossils, dispersed cuticles and charcoalified wood. The megafossils typically occur fragmented, underpinning their allochtonous origin. The plants are described in systematical order. This contribution in contrast to earlier observations reports a high diversity of pteridophytes (11 taxa) and conifers (6 taxa). Angiosperms representing families Nothofagaceae, Atherospermataceae, probably Lauraceae and Sterculiaceae are the most abundant and common plant groups of the flora (12 taxa). The presence of the tropical fern family Marattiaceae and rarity of the genus Nothofagus are of interest, arguing for the prevalence of a warm temperate to tropical humid climate during the Coniacian in this part of Antarctica.     

Geomorphology of the Ablation Point massif, Alexander Island, Antarctica

A coloured geomorphological map at the approximate scale of 1:50,000 is presented for the Ablation Point massif area. The main gcomorphological features have been described, such as ice and snow cover, glacial landforms and deposits (and chronology), ice marginal lakes, melt pools, gelifluction landforms and patterned ground and valley-slope landforms. The area is thought to be a good analogue for glacial-age maritime northwest Europe.     

New cyathealean tree fern,Yavanna chimaerica gen. et sp. nov., from the Early Cretaceous of Livingston Island,Antarctica

A new genus and species of cyathealean fern, Yavanna chimaerica gen. et sp. nov., is erected for several permineralized stems recovered from the Early Cretaceous (Aptian) Cerro Negro Formation, which crops out at the South Shetland Islands, Antarctica. The new species is characterized by solenostelic erect stems with medullary bundles traversing the pith, and surrounded by persistent petiole bases and adventitious roots. Proximal petiole bases present a one-parted modified omega-shaped trace, which becomes three-parted distally. The anatomy of the new fern show similarities with both Thyrsopteris elegans Kunze and with the Cyatheaceae s.s., suggesting that it is a representative of an extinct lineage among the Cyatheales.     

The deglacial history of NW Alexander Island,Antarctica, from surface exposure dating

Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic ¹⁰Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the ¹⁰Be ages are in the range 10.2–21.7 ka, showing a general trend of progressive ice-sheet thinning since at least 22 ka until 10 ka. The data also provide a minimum estimate (490 m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic ³He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46 ka, probably reflecting exhumation by periglacial processes.     

Stratigraphy and structure of Deception Island, South Shetland Islands, Antarctica

Deception Island is an active volcano with a large structural collapse caldera that has been interpreted by several investigators as having been generated by different processes. The general stratigraphy and structural study of the volcano is presented, with special emphasis on the distribution and structural disposition of the different stratigraphic units. The rocks of Deception Island have traditionally been divided into pre- and post-caldera products. A subdivision of the pre-caldera deposits, called ‘pre- and syn-caldera deposits,’ has been observed in two main units. Bulk magnetic susceptibility analyses are used to support the field stratigraphy presented. The localization and distribution of dikes is used to assign the central collapse of the volcano to rifting induced by tensional tectonics occurring in the Bransfield Strait and the presence of transcurrent faults around the island.     

A new ornithopod (Dinosauria,Ornithischia) from the Upper Cretaceous of Antarctica and its palaeobiogeographical implications

A new ornithopod dinosaur from the Upper Cretaceous (Maastrichtian) Snow Hill Island Formation, at James Ross Island, Antarctica is here described. This new taxon, named as Morrosaurus antarcticus gen. et sp. nov., is represented by a fragmentary right hind limb belonging to a medium-sized individual. Our phylogenetic analysis nests the new taxon in a monophyletic clade of Southern Hemisphere ornithopods that includes most Patagonian and Antarctic ornithopods. Several members of this group share a slender and bunched foot with narrow metatarsal IV, expanded chevrons, and bowed humerus without deltopectoral crest. Several features indicate that these ornithopods exhibit adaptations for a specialized cursorial mode of life. The recognition of Patagonian and Antarctic Ornithopoda belonging to a monophyletic clade reinforces palaeobiogeographical signals indicating that Patagonia, Antarctica and Australia shared a common Late Cretaceous terrestrial fauna.     

Lithostratigraphy of Upper Jurassic-Lower Cretaceous strata of eastern Alexander Island,Antarctica

As a result of recent field work, the 4000m thick Kimmeridgian-Albian Fossil Bluff Formation of eastern Alexander Island is elevated to Group status (Fossil Bluff Group). Four formations are recognised within the group (which in ascending order are the Ablation Point, Himalia Ridge, Spartan Glacier and Pluto Glacier Formations), characterised by slumped strata, conglomerates, mudstones and sandstones, respectively. Type sections are selected, and representative lithologies and faunas described for each of these stratigraphic units. This succession constitutes probably the best exposed sequence through the Jurassic-Cretaceous boundary at a southern high palaeolatitude.     

The Seismic Structure of Wilkes Land/Terre Adelie, East Antarctica and Comparison with Australia: First Steps in Reconstructing the Deep Lithosphere of Gondwana

A determination of the seismic structure of the crust and uppermost mantle of East Antarctica, in the region of Casey station, Wilkes Land and Dumont DUrville station, Terre Adelie is presented. High-fidelity waveforms from teleseismic earthquakes recorded at stations CASY and DRV (1996-2001) are used to calculate the seismic receiver function, the signal produced as energy passes through layers in the seismic velocity structure under the receiving station. The receiver functions are stacked to improve the signal-to-noise ratio and then modelled using an inverse algorithm to find the structure that best fits the observed waveform at each station. Inferences are made regarding the tectonic structure, in particular, the crustal thickness and character of the seismic Moho.The crustal thickness under Casey Station is found to be 30 km (+/- 2 km) with a fairly sharp Moho, considerably less than Dumont D'Urville Station, where the crustal thickness is 42 km, and there IS a significant low velocity region the deep crust. The structure of the Wilkes Land lithosphere is comparable to that of the Albany-Fraser Orogen, Western Australia, part of its conjugate margin. This places a new constraint on the relative position of East Antarctica and Australia in the reconstruction of Gondwana, and earlier, supercontinents. A recent reinterpretation of Antarctic geology proposes tectonic province boundaries trending perpendicular to the coast with counterparts in southern Australia. Seismic techniques, determining structure beneath regions with no surface exposure, are vital tools in testing such tectonic hypotheses, towards the reconstruction of Gondwana to full lithospheric depth.     

Palaeoecology and taxonomy of Pentoxylales from the Albian of Antarctica

Reproductive pentoxylalean material from the Albian Triton Point Formation, Fossil Bluff Group of Alexander Island, Antarctica is the youngest record of this group globally. Leaves are referred to Taeniopteris daintreei McCoy, ovulate reproductive organs to Carnoconites cranwelli Harris, and stem material to Pentoxylon sp. Occurring in leaf litter layers on palaeosol surfaces the sedimentology suggests that pentoxylalean plants grew upon areas of the floodplain distal to the river channel, where they covered the forest floor in a similar habit to modern brambles. The forest floor was shared with Equisetales and liverworts, an abundance of ferns [Cladophlebis Brongniart, Gleichenites Goeppert, Phyllopteroides Medwell, Sphenopteris (Brongniart) Sternberg], as well as angiosperms. Bennettitales, Ginkgoites Seward and Nilssonia Brongniart were rare within these communities, as were coniferous trees.     

西南极乔治王岛新生代古生物特征及古环境探讨

西南极乔治王岛发育有一套高钾低铝的拉斑玄武岩,夹火山碎屑岩,属于岛弧火山岩系列。该岛还保存了南极最长的冰川沉积记录,是研究南极冰盖演化历史的重要证据。乔治王岛出露的新生代陆相地层中含有丰富的植物叶、孢粉、茎干、无脊椎动物化石及鸟类足印的痕迹化石等,从始新世到早中新世,化石逐渐减少,表明植物多样性呈明显下降趋势,幸存的稀疏植被被严格限制在冰川周缘的苔原物种上。冰海相地层与古生物研究表明,晚渐新世海相地层主要对应高能环境,早中新世海相地层对应低能环境。对乔治王岛新生代古生物特征及古环境的探讨,不仅理解了古生物及多样性的变化趋势,也为重建南极古环境提供证据。      

Brittle faulting in the Prince Charles Mountains, East Antarctica: Cretaceous transtensional tectonics related to the break-up of Gondwana

The Lambert Glacier–Amery Ice Shelf occupies a narrow NNE–SSW-orientated fault-bound depression referred to as the Lambert Graben. Deep faults associated with this structure are recognised geophysically, and are interpreted to extend at least 700 km inland from the Antarctic coast. Kinematic and palaeostress data from quartz- and calcite-bearing faults, inferred to represent the surface expression of these deeper structures, suggest that a single faulting event occurred in response to NW–SE-directed extension, oblique to the axis of the graben. The bulk of the movement along these faults was dextral strike slip, accommodating components of both normal and reverse offset. In the northern Prince Charles Mountains, these faults disrupt the Permo-Triassic Amery Group and juxtapose it against Proterozoic basement. Equivalent strike-slip faults in the southern Prince Charles Mountains produce dextrally offset tectonic boundaries and metamorphic isogrades across the Lambert Glacier. The similarity in orientation between the palaeostress field calculated for these faults and the Cretaceous divergence vector between India and Antarctica strongly supports the inference that faulting was of Cretaceous age, and related to the break-up of Gondwana.     

Two-stage decompression in garnet-bearing mafic granulites from Sostrene Island, Prydz Bay, East Antarctica

Mafic garnet-bearing granulites from Sostrene Island, 150 km southwest of Davis Station on the coast of Prydz Bay, East Antarctica, exhibit two-stage symplectic coronas on garnet, formed after peak metamorphic conditions (M1). An outer corona of Opx (Mg₆₆) + Pl (An_94–97) + minor Hbl mantles a finer-grained inner corona of Opx (Mg₆₇) + Pl (An_95–96) + Spl (Mg₃₆). Both symplectites contain minor ilmenite–magnetite intergrowths. The finer-grained symplectite also occurs along a fracture cleavage in the garnet. The outer corona originated during a second metamorphic event (M2) via the reaction Grt + Cpx (Hbl) + SiO₂= Opx + Pl (1), whereas the inner corona formed later in response to decompression and minor deformation, resulting in the fracture cleavage in the garnet, according to the reaction Grt = Opx + Pl + Spl (2). The grossular content of the garent (X_Grs= 0.168) is almost exactly that which is required for the stoichiometric breakdown by reaction (2) (calculated X_Grs= 0.167). The mafic rocks are silica undersaturated, and the SiO₂ for reaction (1) was most probably derived externally from the surrounding felsic gneisses. Preferred P–T estimates for M1 based on garnet core (Prp₄₀Alm₄₂Grs₁₇Sps₁)–matrix Opx–Cpx–Hbl pairs are c. 10 kbar at 980°C. The fine-grained symplectite formed post-peak M2 at c. 7 kbar and 850°C. The enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar, which compare with conditions of c. 6 kbar and 775°C in the nearby Bolingen Islands. These lower P–T estimates are considered to be representative of the widespread 1100-Ma metamorphic event recognized in outcrops along the Prydz Bay coast. The high-P, high-T estimates derived from the garnet relics provide evidence for an earlier, possibly Archaean, high-grade metamorphic event.     

The Polonez Cove Formation of King George Island, Antarctica: stratigraphy, facies and implications for mid-Cenozoic cryosphere development

The middle to late Oligocene Polonez Cove Formation, exposed on south‐eastern King George Island, South Shetland Islands, provides rare evidence of mid‐Cenozoic West Antarctic cryosphere evolution. A revised lithostratigraphy and facies analysis and a review of the palaeoenvironmental significance of the formation are presented here. The diamictite‐dominated basal member of the formation (Krakowiak Glacier Member) records the presence and retreat of marine‐based ice on a shallow continental shelf. Five overlying members are recognized. These consist of basaltic‐sourced sedimentary rocks and lavas and represent a variety of shoreface and shallow continental shelf environments in an active volcanic setting. These units contain diverse reworked and ice‐rafted exotic clasts that become sparse towards the top of the formation, suggesting a continuing but waning glacial influence. New ⁴⁰Ar/³⁹Ar dates from interbedded lava flows indicate a late Oligocene age (25·6–27·2 Ma) for the Polonez Cove Formation, but are slightly younger than skeletal carbonate Sr‐isotope ages obtained previously (28·5–29·8 Ma). There is evidence for wet‐based subice conditions at the base of the Polonez Cove Formation, but no sedimentary facies to suggest substantial meltwater. This may reflect a subpolar setting or may result from lack of preservation or a high‐energy depositional environment. A northern Antarctic Peninsula/South Shetland Islands provenance is probable for most non‐basaltic clasts, but certain lithologies with possible origins in the Transantarctic and Ellsworth Mountains also occur sparsely throughout the formation. There is evidence to suggest that the presence of such far‐travelled clasts within subglacially deposited facies at the base of the formation reflects the advance of a local ice cap across marine sediments containing the clasts as ice‐rafted material. The presence of these clasts suggests that extensive marine‐based ice drained into the southern Weddell Sea region and that a strong Weddell Sea surface current operated both before and during deposition of the Polonez Cove Formation.     

Early Cambrian crustal shortening and a clockwise P–T–t path from the southern Prince Charles Mountains, East Antarctica: implications for the formation of Gondwana

Cambrian orogenesis (550–490 Ma) in the Lambert Province of the southern Prince Charles Mountains resulted in three successive stages of deformation. The earliest of these deformations resulted in the development of a layer‐parallel foliation (S1) that was folded into macro‐scale recumbent folds (F2). Subsequent deformation buckled the rocks into long‐wavelength (c. 20 km), SW‐ to NW‐trending antiformal closures (F3) mostly separated from each other by west to SW trending, steeply dipping, high‐strain zones. Metapelitic rocks from the region are divisible into two compositional types: a high‐Al, ‐Fe and ‐K type and a high‐Mg, ‐Ca and ‐Na type. In rocks of both composition, relic staurolite preceded the formation of upper amphibolite facies garnet + biotite + sillimanite ± muscovite mineral assemblages that record peak pressures and temperatures of c. 650–700 °C and 6–7 kbar. Subsequent decompression of c. 3 kbar is implied from texturally late plagioclase and a reduction in the modal abundance of garnet in the high‐Al, ‐Fe and ‐K metapelites, and from texturally late cordierite in the more magnesium rocks. This clockwise P–T–t path, with prograde heating followed by rapid decompression, is: (i) equivalent to that recorded in the same‐aged rocks at Prydz Bay located 600 km to the north, and (ii) similar to the modelled response of the crust to thickening following continent–continent collision. These results indicate that large areas of East Antarctica were thickened and rapidly exhumed, probably in response to collisional orogenesis during the Early Cambrian. This supports the inference that Early Cambrian orogenesis in the Prydz Bay–Prince Charles Mountains region of East Antarctica marks one of the fundamental lithospheric boundaries within Gondwana.     

基于Sentinel-1SAR数据的南极松岛冰川流速监测

南极入海冰川的流速对了解物质平衡和海平面上升具有重要意义。基于Sentinel-1 SAR数据,利用特征跟踪法提取了2015-2017年松岛冰川的冬夏季流速,分析了冰川流速的年际和季节变化特征。结果表明松岛冰川处于高速流动状态,流速最高达4 400 m·a^-1左右。沿主流线方向,2015-2017年2月（夏季）的平均流速分别是3 502 m·a^-1、3 529 m·a^-1、3 498 m·a^-1。2015-2017年8月（冬季）的平均流速分别是3 473 m·a^-1、3 498 m·a^-1、3 478 m·a^-1。2015-2017年松岛冰川主流线上平均流速为3 450~3 550 m·a^-1,流速总体保持稳定,没有明显变化。2015-2017年松岛冰川流速没有明显季节差异,冬夏季流速大致持平。基于裸露岩石作为控制点的分析表明误差为22~39 m·a^-1,冰川流速监测结果是可信的。     

Isotopic and geochemical constraints on the age and origin of granitoids from the central Mawson Escarpment, southern Prince Charles Mountains, East Antarctica

Granitoids from the central Mawson Escarpment (southern Prince Charles Mountains, East Antarctica) range in age from Archaean to Early Ordovician. U–Pb dating of zircon from these rocks indicates that they were emplaced in three distinct pulses: at 3,519 ± 20, 2,123 ± 12 Ma and between 530 and 490 Ma. The Archaean rocks form a layer-parallel sheet of limited extent observed in the vicinity of Harbour Bluff. This granitoid is of tonalitic-trondhjemitic composition and has a Sr-undepleted, Y-depleted character typical of Archaean TTG suites. ε_Nd and T_DM values for these rocks are −2.1 and 3.8 Ga, respectively. Subsequent Palaeoproterozoic intrusions are of granitic composition (senso stricto) with pronounced negative Sr anomalies. These rocks have ε_Nd and T_DM values of −4.8 and 2.87 Ga, indicating that these rocks were probably melted from an appreciably younger source than that tapped by the Early Archaean orthogneiss. The remaining intrusions are of Early Cambrian to Ordovician age and were emplaced coincident with the major orogenic event observed in this region. Cambro–Ordovician intrusive activity included the emplacement of layer-parallel pre-deformational granite sheets at approximately 530 Ma, and the intrusion of cross cutting post-tectonic granitic and pegmatitic dykes at ca. 490 Ma. These intrusive events bracket middle- to upper-amphibolite facies deformation and metamorphism, the age of which is constrained to ca. 510 Ma—the age obtained from a syn-tectonic leucogneiss. Nd–Sr isotope data from the more felsic Cambro–Ordovican intrusions (SiO₂ > 70 wt%), represented by the post-tectonic granite and pegmatite dykes, suggest these rocks were derived from Late Archaean or Palaeoproterozoic continental crust (T_DM ∼ 3.5–2.3 Ga, ε_Nd ∼ −21.8 to −25.9) not dissimilar to that tapped by the Early Proterozoic intrusions. In contrast, the compositionally more intermediate rocks (SiO₂ < 65 wt%), represented by the metaluminous pre-tectonic Turk orthogneiss, appear to have melted from a notably younger lithospheric or depleted mantle source (T_DM = 1.91 Ga, ε_Nd ∼ −14.5). The Turk orthogneiss additionally shows isotopic (low ¹⁴³Nd/¹⁴⁴Nd and low ⁸⁷Sr/⁸⁶Sr) and geochemical (high Sr/Y) similarities to magmas generated at modern plate boundaries—the first time such a signature has been identified for Cambrian intrusive rocks in this sector of East Antarctica. These data demonstrate that: (1) the intrusive history of the Lambert Complex differs from that observed in the adjacent tectonic provinces exposed to the north and the south and (2) the geochemical characteristics of the most mafic of the known Cambrian intrusions are supportive of the notion that Cambrian orogenesis occurred at a plate boundary. This leads to the conclusion that the discrete tectonic provinces observed in the southern Prince Charles Mountains were likely juxtaposed as a result of Early Cambrian tectonism.