Palaeomagnetic comparison of a new fit of East and West Gondwanaland with the Smith and Hallam fit

Palaeomagnetic data for five time intervals in the Phanerozoic are compared for a new fit of East (Antarctica, Australia, India) and West (Africa, Madagascar, South America) Gondwanaland and the hitherto durable fit of Smith and Hallam. The dispersion of the Palaeomagnetic poles is marginally less on the Smith and Hallam fit for four time intervals and marginally greater for the remaining one (Permo-Triassic). The Permo-Triassic poles of East Gondwanaland are evenly distributed between India and Australia and the decreased dispersion of the poles for this period on the new fit of East and West Gondwanaland is paralleled by the decreased dispersion of the poles for both India and Australia. Within the limitations of the analysis imposed by the data, the palaeomagnetic comparison shows that there is little to choose between the two fits.     

Revised tectonic implications for the magnetic anomalies of the western Weddell Sea

Ten years after the USAC (U.S.–Argentina–Chile) Project, which was the most comprehensive aeromagnetic effort in the Antarctic Peninsula and surrounding ocean basins, questions remain regarding the kinematics of the early opening history of the Weddell Sea. Key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the Weddell Sea and merging the USAC data set with the other magnetic data sets in the region. For this purpose we reprocessed the USAC data set using a continuation between arbitrary surfaces and equivalent magnetic sources. The equivalent sources are located at a smooth crustal surface derived from the existing bathymetry/topography and depths estimated by magnetic inversions. The most critical area processed was the transition between the high altitude survey over the Antarctic Peninsula and the low altitude survey over the Weddell Sea that required downward continuation to equalize the distance to the magnetic source. This procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion.The enhancement of the Mesozoic sequence permits refining the interpretation of the seafloor-spreading anomalies. In particular, the change in shape and wavelength of an elongated positive in the central Weddell Sea suggests that it was formed during the Cretaceous Normal Polarity Interval. The older lineations in the southwestern Weddell Sea are tentatively attributed to susceptibility contrasts modeled as fracture zones. Numerical experimentation to adjust synthetic isochrons to seafloor-spreading lineations and flow lines to fracture zones yields stage poles for the opening of the Weddell Sea since 160 Ma to anomaly 34 time. The corresponding reconstructions look reasonable within the known constraints for the motions of the Antarctic and South America plates. However, closure is not attained between 160 and 118 Ma if independent published East Antarctica–Africa–South America rotations are considered. The lack of closure may be overcome by considering relative motion between the Antarctic Peninsula and East Antarctica until 118 Ma time, an important component of convergence.     

Apparent polar wandering for the Atlantic-bordering continents: Late Carboniferous to Eocene

We present a compilation of reliable paleomagnetic pole positions from five continental plates (North America, Europe, the Iberian Peninsula, Africa, and South America) for ten time intervals ranging from Late Carboniferous to Eocene. Only well-dated results obtained by demagnetization techniques have been used. Paleomagnetic poles are plotted with respect to the paleo-positions of the continents, as reconstructed from correlations of marine magnetic anomalies in the Atlantic Ocean by Pitman and Talwani and from the fit by Bullard et al. The poles from North America, Europe and the younger poles from Africa show a very good grouping for most of the ten intervals considered, and a continuous apparent polar wandering path is obtained. These data have been used to construct paleolatitude maps for most intervals; thus the relative positions of the continents were established from sea-floor spreading data and their absolute positions on the globe were determined from paleomagnetic data. The older data from South America and the other Gondwana continents show a systematic deviation from those of the northern continents for Late Paleozoic and Early Triassic time periods. An explanation is offered in a different continental reconstruction between Laurasia and Gondwanaland before Middle Triassic times.     

The existence and break-up of the Antarctic land bridge as indicated by both amphi-Pacific distributions and tectonics

Amphi-Pacific disjunct distributions between South America and Australasia are correlated with the breakup and changing palaeo-climate of Gondwana. For a long period, with a temperate climate, Antarctica formed a land bridge between Australia and South America, allowing species to disperse/vicariate between both continents. Dated phylogenies in the literature, showing sister-clades with a distribution disjunction between South America and Australia, were used for the correlation. The initiation of the Antarctic Circumpolar Current, and a change to a colder Antarctic climate is associated with the opening of the Drake Passage between South America and Antarctica at c. 30 Ma, and the final separation of Australia and Antarctica along the South Tasman Rise at c. 45 Ma. The distribution data highlighted the existence of a “southern disjunct distribution” pattern, which may be the result of continental vicariance/dispersal. This is strongly indicative of a connection between Antarctica, South America and Australia; which later provided a dispersal pathway and facilitated vicariance after break up. The taxa that likely dispersed/vicariated via Antarctica included all species with a more (sub)tropical climate preference. Twelve distributions, younger than 30 Ma, are interpreted as the result of long distance dispersal between South America and Australia; these taxa are suited to a temperate climate. The climatic signal shown by all taxa is possibly a consequence of the Australian plate's asynchronous rifting over tens of millions of years in combination with climate changes. These events may have provided opportunities for tropical and sub-tropical species to disperse and speciate earlier than what we observe for the more temperate taxa.     

Hotspots in the Southern Oceans — an absolute frame of reference for motion of the Gondwana continents

The geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time; that is, motion of an order of magnitude less than the relative motion between plate pairs. In most cases it is concluded that inter-hotspot movement cannot be discerned for the period 100 m.y. to Present and that widely distributed hotspots in the Atlantic and Indian Oceans provide a frame of reference for plate motions following the disintegration of Gondwanaland, which is independent of paleomagnetism. This frame of reference is “absolute” in that it gives the motion of the lithosphere with respect to the mantle (= hotspots). The absolute motion model indicates that Africa and Antarctica are now moving only very slowly, that there has been significant relative movement between East and West Antarctica since the Cretaceous, and prescribes the relative motion between the Somali and African plates.     

New Somali Basin magnetic anomalies and a plate model for the early Indian Ocean

The oldest portions of the Indian Ocean formed via the breakup of Gondwana and the subsequent fragmentation of East Gondwana. We present a constrained plate model for this early Indian Ocean development for the time period from Gondwana Breakup until the start of the Cretaceous Normal Superchron. The motions of the East Gondwana terranes are determined using new geophysical observations in the Somali Basin and existing geophysical interpretations from other coeval Indian Ocean basins. Within the Somali Basin, recent satellite gravity data clearly resolve traces of an east–west trending extinct spreading ridge and north–south oriented fracture zones. A thorough compilation of Somali Basin ship track magnetic data allows us to interpret magnetic anomalies M24Bn through M0r about this extinct ridge. Our magnetic interpretations from the Somali Basin are similar in age, spreading rate, and spreading directions to magnetic anomalies previously interpreted in the neighboring Mozambique Basin and Riiser Larsen Sea. The similarity between the two magnetic anomaly datasets allows us to match both basin's older magnetic anomaly picks by defining a pole of rotation for a single and cohesive East Gondwana plate. However, following magnetic anomaly M15n, we find it is no longer possible to match magnetic picks from both basins and maintain plausible plate motions. In order to match the post-M15n geophysical data we are forced to model the motions of Madagascar/India and East Antarctica/Australia as independent plates. The requirement to utilize two independent plates after anomaly M15n provides strong circumstantial evidence that suggests East Gondwana breakup began around 135 Ma.     

Precambrian continent assembly and dispersal events of South Indian and East Antarctic Shields

An Archaean continent ‘SIWA’, an acronym for South India–Western Australia, comprising the Bastar–Dharwar craton, the Yilgarn craton, the Napier Complex, and the Vestfold Hills has been identified from palaeomagnetic and spatio-temporal data. This assembly was dispersed in three phases with the development of the proto-Indian ocean. The first and second events ~2350 and ~2000 Ma were related to the separation of the Yilgarn craton and the Napier Complex, respectively, to form a proto-Indo-Antarctic ocean and the Cuddapah basin. The proto-ocean was closed ~1650 Ma by the collision of the Lambert Terrane of East Antarctica and the Bastar–Dharwar craton. This collision, associated with ultra-high temperature (UHT) granulite facies metamorphism, is identified in the southern domain of the Eastern Ghats and the Oygardens domain of East Antarctica. The third extensional event between 1500 and 1200 Ma was associated with the separation of the Vestfold Hills block and a second phase of opening of the proto-Indian ocean, and the development of a series of basins on the western side of the Eastern Ghats (the Chhatisgarh, Khariar, Ampani, Indravati, and Sabari basins). The closing of this ocean basin during the Eastern Ghats–Rayner orogeny at ~950 Ma was related to the amalgamation of India and East Antarctica to form the supercontinent Rodinia. During the Neoproterozoic, this part of Rodinia was involved in orogenic collapse/extension and deposition of the Sodruzhesvo Group. The Pan-African Prydz Bay orogeny at ~550 Ma caused the closing of the basin to form East Gondwanaland.     

A new magnetic map of the Weddell Sea and the Antarctic Peninsula

As part of the Antarctic Digital Magnetic Mapping Project (ADMAP) workers from VNIIOkeangeologia (Russia), the British Antarctic Survey (UK) and the Naval Research Laboratory (USA) have brought together almost all of the available magnetic data in the area 0–120°W, 60–90°S. The final map covers the whole Weddell Sea and adjacent land areas, the Antarctic Peninsula and the seas to the west, an area comparable in size with that of the USA. This paper describes the methods used during the compilation of the map and reviews briefly some of the main features shown on it. Distinct magnetic provinces are associated with Precambrian rocks of the East Antarctic craton, highly extended continental crust in the Weddell Sea embayment, the arc batholith of the Antarctic Peninsula, and oceanic crust of the northern Weddell Sea, which was created as a direct consequence of South America–Antarctica plate motion and oceanic crust generated at the Pacific–Antarctic ridge. The magnetic anomaly map thus provides an overview of the fragmentation of south-western Gondwana and the tectonic development of the Weddell Sea sector of Antarctica.     

Intracontinental movements in Western Gondwanaland: a palaeomagnetic test

Early Cretaceous intracontinental movements within Africa and/or South America, to account for misfit problem in the pre-drift reconstruction of Western Gondwana, have been tested using palaeomagnetic poles from both continents. Each continent has been considered as comprising separate subplates according to the boundaries proposed by Pindell and Dewey (1982) and Curie (1983): i.e., northern Africa ( ), southern Africa ( ), northern South America (nsa) and southern South America ( ). Visual and statistical distribution of Late Permian to Jurassic poles from the African subplates indicate two distinct groups before rotating relative to , while after the rotation they become indistinguishable. The distribution and sparsity of data from South America limit the study in this continent as the data are statistically indistinguishable before and after rotation of with respect to . When the test is extended to the reconstruction of Western Gondwana, the tightest grouping of the poles occurs when an intracontinental movement is assumed within Africa. This suggests that the misfit between the two continents is probably due to intracontinental deformation within Africa and not in South America. If confirmed, this may have profound implications in the kinematics of the processes related to the opening of the South and Central Atlantic Oceans and also the origin of the West African Rift System.     

Cenozoic Exhumation of Larsemann Hills,East Antarctica: Evidence from Apatite Fission-track Thermochronology

<正>Does Cenozoic exhumation occur in the Larsemann Hills,East Antarctica? In the present paper,we conducted an apatite fission-track thermochronologic study across the Larsemann Hills of East Antarctica.Our work reveals a Cenozoic exhumation event at 49.8±12 Ma,which we interpret to be a result of exhumation caused by crustal extension.Within the uncertainty of our age determination, the timing of extension in East Antarctica determined by our study is coeval with the onset time of rifting in West Antarctica at c.55 Ma.The apatite fission-track cooling ages vary systematically in space, indicating a coherent block rotation of the Larsemann Hills region from c.50 Ma to c.10 Ma.This pattern of block tilting was locally disrupted by normal faulting along the Larsemann Hills detachment fault at c.5.4 Ma.The regional extension in the Larsemann Hills,East Antarctica was the result of tectonic evolution in this area,and may be related to the global extension.Through the discussion of Pan-Gondwanaland movement,and Mesozoic and Cenozoic extensions in West and East Antarctica and adjacent areas,we suggest that the protracted Cenozoic cooling over the Larsemann Hills area was caused by extensional tectonics related to separation and formation of the India Ocean at the time of Gondwanaland breakup.     

Evolution of the Pacific margin of the northern Antarctic Peninsula: an overview

The sector of the northern Antarctic Peninsula between the Tula and Shackleton Fracture Zones provides evidence for the subduction of south-east Pacific oceanic crust under Antarctic continental crust during Late Mesozoic through Miocene times. The pre-subduction depositional history of this sector includes the formation of a marine siliciclastic turbidite wedge (?Permian-Triassic) deposited in a marginal basin setting. It was folded and thrust retroarc before the Middle Jurassic to form the Trinity accretion foldbelt, which extended for several hundred kilometres along the Pacific margin of Gondwanaland. The foldbelt was deeply eroded and levelled under subaerial conditions, then unconformably covered either by Middle-Upper Jurassic alluvial to lacustrine deposits (in the north) or by Early Cretaceous basic lavas (in the south). The subduction-related magmatism, in the form of acidic effusions and intrusions, began in the northern Antarctic Peninsula during Middle Jurassic times and continued as predominantly basic lavas and agglomerates intruded by basic, intermediate and acidic plutons, and by a succession of dykes, during the Early to Late Cretaceous. Thus the inner magmatic are of the northern Antarctic Peninsula (northern Graham Land-Trinity Peninsula) was formed. An outward (north-westerly) migration of centres of magmatic activity with time (Cretaceous-Tertiary) towards the subduction trench, coupled with a northeastward shift of these centres along the Arc's length due to the counterclockwise rotation of Antarctica, produced the outer magmatic arc of the South Shetland Islands. Slight folding of Late Mesozoic and Tertiary magmatic suites occurred at several stages of subduction. Stronger folding and retroarc thrusting appeared locally as a result of the collision of the Aluk Ridge-Antarctic Peninsula during the Mid-Miocene. The latest plate tectonic event was the opening of the Bransfield Rift (Oligocene-Recent) as a spreading back-arc basin, associated with terrestrial and submarine volcanic activity.     

Evolution of the Kerguelen plume and its impact upon the continental and oceanic magmatism of East Antarctica

Petrological–geochemical study showed that the alkaline-ultramafics of the Jetty Oasis (rift zone of the Lambert glacier, East Antarctica) are similar in the age (117–110 Ma) and geochemistry to the ultrapotassic alkali basalts of eastern India (Jharia and Raniganj intrusions). Alkaline magmatism in India and Antarctica is related to the activity of the Kerguelen plume, which significantly affected the evolution of the entire eastern Indian Ocean, in particular, determined geodynamic peculiarities of the ocean opening (existence of non-spreading blocks, fragments of the Gondwana lithosphere in oceanic areas) and geochemical characteristics of erupted tholeiitic magmas. Enriched magma sources related to the Kerguelen plume were formed by melting of ancient Gondwana-derived continental fragments, which experienced multiple transformations during its evolution up to the formation of metasomatized mantle under the impact of the Kerguelen plume on the Antarctic and India margins.     

Structural geology of the Mesozoic Miers Bluff Formation and crosscutting Paleogene dikes (Livingston Island, South Shetland Islands, Antarctica) – Insights into the geodynamic history of the northern Antarctic Peninsula

The Antarctic Peninsula has been part of a magmatic arc since at least Jurassic times. The South Shetland Islands archipelago forms part of this arc, but it was separated from the Peninsula following the Pliocene opening of the Bransfield Strait. Dikes are widespread throughout the archipelago and are particularly accessible on the Hurd Peninsula of Livingston Island. The host rocks for the dikes are represented by the Miers Bluff Formation, which forms the overturned limb of a large-scale fold oriented 63/23 NW. The orientation of minor structures indicates a fold axis oriented NNE–SSW (24/0). Structural analysis of the dikes and their host rocks shows that the tectonic regime was similar to other parts of the archipelago and that only minor changes of the stress field occurred during dike emplacement.Based on crosscutting field relationships and geochemical data, six early Paleocene to late Eocene intrusive events can be distinguished on Hurd Peninsula. In contrast to calc-alkaline dikes from other parts of the South Shetland Islands, the majority of the Hurd Peninsula dikes are of tholeiitic affinity. Nd and Pb isotope data indicate a significant crustal component, particularly during initial magmatic activity.Plagioclase ⁴⁰Ar/³⁹Ar and whole rock K–Ar ages show that dike emplacement peaked during the Lutetian (48.3 ± 1.5, 47.4 ± 2.1, 44.5 ± 1.8 and 43.3 ± 1.7 Ma) on Hurd Peninsula and also further northeast on King George Island. Dike intrusion continued on Livingston Island at least until the Priabonian (37.2 ± 0.9 Ma). The type of magma sources (mantle, slab, crust and sediment) did not change, though their relative magmatic contributions varied with time.During Cretaceous and Early Paleogene times, the Antarctic Peninsula including the South Shetland Islands was situated southwest of Patagonia; final separation from South America occurred not before the Eocene. Thus, the geological evolution of Livingston Island is related as much to the development of Patagonia as of Antarctica, and needs to be considered within the history of southernmost South America.     

南极新生代海陆格局变迁对全球气候变化的影响

南极大陆记录了新生代以来地质演化中多次重大地质事件,包括大陆生长、裂解和离散、全球冷却和大陆尺度南极冰盖的发展等。尽管非常重要,但至今关于南极大陆新生代地质演化仍有诸多争论。文章主要针对塔斯曼通道和德雷克海峡贯通过程,系统总结并分析了南极洲、南美洲和澳大利亚的构造、岩浆和沉积演化历史。始新世晚期至渐新世早期开始发育的南极环极洋流（ACC）受德雷克海峡和塔斯曼通道扩张程度的控制。综合分析和对比研究表明,~34 Ma全球气候从"暖室"到"冷室"的转变与ACC开始的时间一致,表明构造通道的打开控制了ACC的发育,进而对全球气候产生了重要影响。最后,简要总结了南极作为一个完整的地球系统,其新生代地质演化如何控制海陆格局的变迁,并提出未来研究需要解决的关键问题。     

Antarctica — Before and after Gondwana

The origin of the Antarctic continent can be traced to a relatively small late Archaean cratonic nucleus centred on the Terre Adélie regions of East Antarctica and the Gawler Craton region of South Australia. From the late Archaean to the present, the evolution of the proto-Antarctic continent was remarkably dynamic with quasi-continuous growth driven by accretionary or collisional events, episodically punctuated by periods of crustal extension and rifting. The evolution of the continent can be broken into seven main steps: (1) late Palaeoproterozoic to middle Mesoproterozoic accretion and collision added crust first to the Antarctic nucleus's eastern margin, then to its western margin. These events resulted in the incorporation of the Antarctic nucleus within a single large continent that included all of Proterozoic Australia, a more cryptic Curnamona–Beardsmore Craton and most probably Laurentia. (2) Rifting in the middle to late Mesoproterozoic separated a block of continental crust of unknown dimensions to form an ocean-facing margin, the western edge of which was defined by the ancestral Darling Fault in Western Australia and its unnamed continuation in Antarctica. (3) Inversion of this margin followed shortly and led to the Grenville aged collision and juxtaposition of proto-Antarctica with the Crohn Craton, a continental block of inferred Archaean and Palaeoproterozoic age that now underlies much of central East Antarctica. The Pinjarra Orogen, exposed along the coast of Western Australia, defines the orogenic belt marking this collision. In Antarctica the continuation of this belt has been imaged in sub-ice geophysical datasets and can be inferred from sparse outcrop data and via the widespread dispersal of syn-tectonic zircons. (4) Tectonic quiescence from the latest Mesoproterozoic to the Cryogenian was the forerunner to Ediacaran rifting that separated Laurentia and the majority of the Curnamona–Beardsmore craton from the amalgam of East Antarctica and Australia. The result was the formation of the ancestral Pacific Ocean. (5) The rifting of Laurentia was mirrored by convergence along the opposing margin of the continent. Convergence ultimately sutured material with Indian and African affinities during a series of Ediacaran and Cambrian events related to the formation of Gondwana. These events added much of the crust that today defines the East Antarctic coastline between longitudes 30°W and 100°E. (6) The amalgamation of Gondwana marked a shift in the locus of subduction from between the pre-Gondwana cratons to Gondwana's previously passive Pacific margin. The result was the establishment of the accretionary Terra Australis and Gondwanide orogenies. These were to last from the late Cambrian to the Cretaceous, and together accreted vast sequences of Gondwana derived sediment as well as fragments of older and allochthonous or para-allochthonous continental crust to Gondwana's Pacific margin. (7) The final phases of accretion overlapped with the initiation of extension and somewhat later rifting within Gondwana. Extension started in the late Carboniferous, although continental separation did not begin until the middle Jurassic. Gondwana then fragmented sequentially with Africa–South America, India, Australia and the finally the blocks of New Zealand separating between the middle Jurassic and the late Cretaceous. The late Cretaceous separation of Antarctica and Australia split the original Antarctic nucleus, terminating more than 2.4 billion years of shared evolution. The slightly younger separation of New Zealand formed the modern Antarctic continent.     

西南极中—新生代古地磁与板块重建研究进展

根据西南极已发表的中—新生代古地磁数据,对西南极不同地块进行了古大陆重建。识别出古太平洋板块对西南极构造演化影响广泛的两次构造事件：一是120~100 Ma古太平洋板块内翁通爪哇-马尼希基-希库朗基大火成岩省的喷发与全球板块洋壳扩张速率高峰期引起的西南极瑟斯顿岛-埃茨海岸与东玛丽·伯德地快速南向移动;二是古太平洋-凤凰板块洋中脊在~100 Ma俯冲至西南极之下导致的以罗斯海区域为主的岩石圈伸展、瑟斯顿岛-埃茨海岸与玛丽·伯德地远离东南极以及南极半岛发生南向运动与顺时针旋转。证明太平洋板块俯冲与西南极板块运动的耦合关系。未来需要在西南极获得更多具有准确年代学限制的可靠的古地磁数据,这将对西南极的构造演化模式提供更多的制约,并有助于深入理解南极大陆的构造演化过程、板块生长与裂解的地球动力学机制。     

Juxtaposition of India and Antarctica During the Precambrian: Inferences from Geochemistry of Mafic Dykes

There are several geological, geochemical and geophysical evidences, which corroborate reconstruction of Gondwanaland and juxtaposition of India and Antarctica. Petrology of the Precambrian mafic dykes of East Antarctica and Central-East India also support juxtaposition of India and Antarctica. Mafic dykes of different generations are emplaced in the Archaean granite gneisses of these regions. These dykes appear to be an important tool to support juxtaposition of India and Antarctica. Geological and petrological data of the Central-East India Precambrian mafic dykes suggest four episodes of mafic magmatism in the region - three tholeiitic and one noritic (?). Similarly, East Antarctica also comprises four dyke suites, emplaced during three distinct periods. These suites are 2.4 Ga meta-tholeiites, 2.4 Ga high-Mg tholeiites, 1.8 Ga dolerites and 1.2–1.4 Ga dolerites. Geochemical compositions of these mafic dykes are compared and they show good relationships with each other. Similarities in petrological and geochemical characteristics of Precambrian mafic dykes of East Antarctica and Central-East India strongly support juxtaposition of these two continents.     

Indo-Antarctica, Gondwanaland, and the distortion of a granulite belt

A detailed examination of a Gondwanaland re-assembly of Peninsular India and part of East Antarctica permits several conclusions to be drawn largely from data from the Indian coastal belt, which includes Ceylon. These are that original major granulite metamorphism probably occurred at about 2000 m.y., and that the sinuosity of the granulite belt, now split into two coastal belts, is a later distortion, outlined by anorthosite occurrences in India, which probably occurred between 720 and 450 m.y. Brief mention is made of the continuation of this belt into Madagascar, and of the problem of what lay between Western Australia and India in Gondwanaland.     

Eocene to Miocene geometry of the West Antarctic Rift System

Tectonic models for the Late Cretaceous/Tertiary evolution of the West Antarctic Rift System range from hundreds of kilometres of extension to negligible strike-slip displacement and are based on a variety of observations, as well as kinematic and geodynamic models. Most data constraining these models originate from the Ross Sea/Adare Trough area and the Transantarctic Mountains. We use a new Antarctic continental crustal-thinning grid, combined with a revised plate-kinematic model based on East Antarctic – Australia – Pacific – West Antarctic plate circuit closure, to trace the geometry and extensional style of the Eocene – Oligocene West Antarctic Rift from the Ross Sea to the South Shetland Trench. The combined data suggest that from chron 21 (48 Ma) to chron 8 (26 Ma), the West Antarctic Rift System was characterised by extension in the west to dextral strike-slip in the east, where it was connected to the Pacific – Phoenix – East Antarctic triple junction via the Byrd Subglacial Basin and the Bentley Subglacial Trench, interpreted as pullapart basins. Seismic-reflection profiles crossing the De Gerlache Gravity Anomaly, a tectonic scar from a former spreading ridge jump in the Bellingshausen Sea, suggest Late Tertiary reactivation in a dextral strike-slip mode. This is supported by seismic-reflection profiles crossing the De Gerlache Gravity Anomaly in the Bellingshausen Sea, which show incised narrow sediment troughs and vertical faults indicating strike-slip movement along a north – south direction. Using pre-48 Ma plate circuit closure, we test the hypothesis that the Lord Howe Rise was attached to the Pacific Plate during the opening of the Tasman Sea. We show that this plate geometry may be plausible at least between 74 and 48 Ma, but further work especially on Australian – Antarctic relative plate motions is required to test this hypothesis.     

A window on West Antarctic crustal boundaries: the junction between the Antarctic Peninsula, the Filchner Block, and Weddell Sea oceanic lithosphere

A new airborne magnetic survey of the southeastern Antarctic Peninsula and adjacent Weddell Sea embayment (WSE) region suggests a continuity of geological structure between the eastern Antarctic Peninsula and the attenuated continental crust of the Filchner Block. This has implications for the reconstructed position of the Ellsworth–Whitmore Mountains block in Gondwana, which is currently uncertain. Palaeomagnetic data indicate that it has migrated from a Palaeozoic position between South Africa and Coats Land to its current position as a microplate embedded in central West Antarctica. The most obvious route for migration is between the Antarctic Peninsula and the Weddell Sea embayment. Evidence that geological structures are continuous across the boundary places constraints on the timing and pathway of migration. Magnetic textures suggest the presence of shallow features extending from the Beaumont Glacier Zone (BGZ) in the west for at least 200 km into the Weddell Sea embayment. These data suggest that the Eastern Domain of the Antarctic Peninsula and the stretched continental crust of the Filchner Block share a common recent, probably post-Early Jurassic, history. However, examination of deep anomalies indicates differences in the magnetic characteristics of the two blocks. The boundary may mark either the edge of extended continental crust, or a discontinuity between two, once separated, blocks. This discontinuity, or pre-Late Jurassic Antarctic Peninsula terrane boundaries to the west, may have allowed the passage of the Ellsworth–Whitmore Mountains block to its present location.