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Over the past 60 years, scientists have strived to understand the past, present and future of the Antarctic Ice Sheet. One of the key components of this research has been the mapping of Antarctic bed topography and ice thickness parameters that are crucial for modelling ice flow and hence for predicting future ice loss and ensuing sea level rise. Supported by the Scientific Committee on Antarctic Research (SCAR), the Bedmap3 Action Group aims not only to produce new gridded maps of ice thickness and bed topography for the international scientific community, but also to standardize and make available all the geophysical survey data points used in producing the Bedmap gridded products. Here, we document the survey data used in the latest iteration, Bedmap3, incorporating and adding to all of the datasets previously used for Bedmap1 and Bedmap2, including ice-bed, surface and thickness point data from all Antarctic geophysical campaigns since the 1950s. More specifically, we describe the processes used to standardize and make these and future survey and gridded datasets accessible under the ‘Findable, Accessible, Interoperable and Reusable’ (FAIR) data principles. With the goals to make the gridding process reproducible and to allow scientists to re-use the data freely for their own analysis, we introduce the new SCAR Bedmap Data Portal (bedmap.scar.org, last access: 18 October 2022) created to provide unprecedented open access to these important datasets, through a user-friendly webmap interface. We believe that this data release will be a valuable asset to Antarctic research and will greatly extend the life cycle of the data held within it. Data are available from the UK Polar Data Centre: https://data.bas.ac.uk.

The Cape Darnley region in East Antarctica has been an area of scientific interest for a variety of disciplines over the last three decades. The recent acquisition of several high-resolution bathymetry datasets enabled the compilation of a detailed regional bathymetry grid. We present a high-resolution bathymetric compilation of the Cape Darnley region in East Antarctica, including areas of the Mac.Robertson Land shelf, slope and adjacent deep sea. A variety of data, single-beam and multibeam swath bathymetry and digitized depths from nautical charts were sourced from numerous institutions. The 100 m-resolution gridded bathymetric dataset improves previous bathymetric representations of the region and enables visualization of the seafloor morphology in unprecedented detail. The bathymetry grid has been constructed using a layered hierarchy approach based on the source of each dataset. This data compilation forms important baseline information for a range of scientific applications and end users including oceanographers, glacial modellers, biologists and geologists. The compilation will aid numerical modelling of ocean circulation, reconstruction of palaeo-ice streams and refinement of ice-sheet models.

The back-arc spreading at the southern Mariana is categorized as slow, but surface morphological and geophysical features of the spreading centers suggest that the spreading process is associated with enhanced melting in the upper mantle, due to water being derived from the subducted Pacific slab. A marine magnetotelluric experiment was performed along a transect across a segment at 13°N to reveal the key processes of melting, dehydration, and dynamics in the upper mantle, and their relationships to the surface characteristics. Our inversion model of electrical resistivity shows (a) a conductive body at 10–20 km depth beneath the spreading axis, and (b) another conductive area expanding asymmetrically under and around the conductive body. Away from the spreading center, there is (c) a resistive area thickening up to ∼40 km on the remnant arc side, and (d) another resistive area with a constant thickness of ∼150 km on the trench side. Implications of these model features are (a) a melt body beneath the spreading axis; (b) a hydrous mantle above the subducted slab and asymmetric passive decompression melting in the mantle wedge; (c) a residual mantle off from the spreading axis; and (d) a cold mantle wedge tip and the subducted Pacific mantle. The structure markedly contrasts with that in the central Mariana Trough at 18°N, suggesting that the horizontal distance between the location of the spreading center and the root of the buoyant upwelling above the subducted slab is a key parameter that controls the mantle dynamics beneath the back-arc spreading.

The southern Indian Ocean has several prominent aseismic ridges recognized as oceanic large igneous provinces (i.e., the Madagascar Ridge, Del Cano Rise, Crozet Plateau, and Conrad Rise) in the off-axis areas of the Southwest Indian Ridge (SWIR). However, previously obtained magnetic survey lines are sparse and not correctly aligned with the seafloor spreading direction; thus, the detailed spreading history, including the formation of these aseismic ridges, remains an open question. We reconstructed the tectonic history of two segments between the Discovery II and Gallieni fracture zones in the SWIR using newly obtained magnetic data (total and vector magnetic field) and an open-source magnetic dataset. We revealed that the southern Madagascar Ridge and the Del Cano Rise once formed a single bathymetric high and separated by at least Chron 30y, which is quite different from the global age model. In addition, the rises may have formed before Chron 34y, assuming an extinct ridge south of the Del Cano Rise. The two rises have been recognized as having formed by Marion hotspot plume-induced excess volcanism around the active spreading ridge of the SWIR, which can explain locally isostatically compensated thicker-than-normal crust. However, linear magnetic anomalies have not been observed across the main part of these rises, suggesting that magmatic activity controlled by seafloor spreading is unlikely. Like other aseismic ridges in the southern Indian Ocean, these two rises may possibly have been formed partly by continental fragments rather than plume-induced excess volcanism. Plain Language Summary The southern Indian Ocean contains several bathymetric high regions previously interpreted as oceanic large igneous provinces. Hotspot-ridge interaction is considered the key factor in the formation of such large volcanic structures. However, the detailed seafloor spreading history and origin of these large volcanic structures are not understood as well as those in other oceans because of the vast and remote Indian Ocean. We conducted magnetic surveys and reconstructed a new tectonic history of two segments along the Southwest Indian Ridge (SWIR). We found that the southern Madagascar Ridge and the Del Cano Rise once formed a single bathymetric high and that they separated by at least 66 million years ago. Moreover, the rises may have formed before 84 million years ago, assuming an extinct ridge south of the Del Cano Rise. The two rises might have formed by interaction between the Marion hotspot and SWIR; however, linear magnetic anomalies implying seafloor spreading are not observed in the main parts of these rises. This absence suggests that hotspot-ridge interaction cannot fully explain the origin of these rises. We cannot exclude the possibility that the two rises are partly continental fragments rather than products of hotspot-induced excess volcanism.

The 11th Symposium on Polar Science/Interdisciplinary sessions: [IN] New insights of ship-based interdisciplinary study in Soutnern Ocean, Thu. 3 Dec.

The 11th Symposium on Polar Science/Interdisciplinary sessions: [IN] New insights of ship-based interdisciplinary study in Soutnern Ocean, Thu. 3 Dec.

The 11th Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Poster presentations, Mon. 16 Nov. - Fri. 18 Dec.

The 11th Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Poster presentations, Mon. 16 Nov. - Fri. 18 Dec.

The 11th Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Poster presentations, Mon. 16 Nov. - Fri. 18 Dec.

The 11th Symposium on Polar Science/Special session: [S] Accelerating Arctic research: Recent progress and future prospect of Arctic research, Poster presentations, Mon. 16 Nov. - Fri. 18 Dec.

The Tenth Symposium on Polar Science/Special session: [S] Future plan of Antarctic research: Towards phase X of the Japanese Antarctic Research Project (2022-2028) and beyond, Tue. 3 Dec. / Entrance Hall (1st floor) at National Institute of Polar Research (NIPR)

The formation of Dense Shelf Water (DSW) and Antarctic Bottom Water (AABW) in the Southern Ocean is an essential part of the thermohaline circulation, and understanding this phenomenon is crucial for studying the global climate. AABW is formed as DSW flows down the continental slope and mixes with the surrounding waters. However, DSW formation and its descent remains a poorly resolved issue in many ocean models. We, therefore, simulated the formation and descent of DSW and investigated the model sensitivities to horizontal and vertical grid spacings. The Massachusetts Institute of Technology general circulation model (MITgcm) was used for the region off Cape Darnley in East Antarctica, one of the main AABW production areas, where historical and mooring data are available for comparison. Simulations with coarse horizontal grid resolutions of order (10 km) yielded high volumes of DSW on the shelf. However, the largest part of this DSW was transformed into intermediate water and advected westward. Horizontal model resolutions equal to or higher than 2 km were required to simulate the descent of DSW and a realistic AABW production. Simulated time series at a mooring located at a depth of 2,600 m showed periodic fluctuations in velocity and temperature of 0.3 m ⋅ s−1 and 0.5 °C, respectively, consistent with observations. We also found that high-resolution bathymetry datasets are crucial because the newly formed AABW volume was reduced by 20% when a smoother bathymetry was used on a 2-km resolution grid. Vertical resolution had little influence on model performance because the plume was much thicker (> 170 m) than the grids width. Therefore, reproducing the downslope flow of DSW and AABW formation in the Cape Darnley region can be achieved with a high horizontal resolution ( ≤ 2 km) and a relatively coarse vertical resolution ( ∼ 100 m on the continental slope).

The evolution of seafloor spreading of Africa, South America, and Antarctica is key to understanding the initial break-up of Gondwana. Vector geomagnetic surveys were conducted in the Natal Valley and Mozambique Ridge, off South Africa. We summarize the nature of the crust using the results of dense vector geomagnetic anomaly data, as well as satellite gravity data. Based on both inversion and forward analytical results, we identified areas of stretched continental crust, with basaltic magma intrusion in parts, as the northern Natal Valley, north part of the Mozambique Ridge, and north part of the southern Natal Valley. Oceanic crust was identified in the south part of the southern Natal Valley and south part of the Mozambique Ridge. Magnetic isochrons M0–M10 were identified in the south part of the southern Natal Valley. Clear magnetic lineations were observed in the south part of the Mozambique Ridge, where some areas were distorted by hotspot volcanism. The location of the continental ocean boundary in the Natal Valley, along with a four-stage model of tectonic evolution of the study area since about 183 Ma, are newly proposed.

Summary The Palaeognathae comprise the flightless ratites and the volant tinamous, and together with the Neognathae constitute the extant members of class Aves. It is commonly believed that Palaeognathae originated in Gondwana since most of the living species are found in the Southern Hemisphere [1–3]. However, this hypothesis has been questioned because the fossil paleognaths are mostly from the Northern Hemisphere in their earliest time (Paleocene) and possessed many putative ancestral characters [4]. Uncertainties regarding the origin and evolution of Palaeognathae stem from the difficulty in estimating their divergence times [1, 2] and their remarkable morphological convergence. Here, we recovered nuclear genome fragments from extinct elephant birds, which enabled us to reconstruct a reliable phylogenomic time tree for the Palaeognathae. Based on the tree, we identified homoplasies in morphological traits of paleognaths and reconstructed their morphology-based phylogeny including fossil species without molecular data. In contrast to the prevailing theories, the fossil paleognaths from the Northern Hemisphere were placed as the basal lineages. Combined with our stable divergence time estimates that enabled a valid argument regarding the correlation with geological events, we propose a new evolutionary scenario that contradicts the traditional view. The ancestral Palaeognathae were volant, as estimated from their molecular evolutionary rates, and originated during the Late Cretaceous in the Northern Hemisphere. They migrated to the Southern Hemisphere and speciated explosively around the Cretaceous-Paleogene boundary. They then extended their distribution to the Gondwana-derived landmasses, such as New Zealand and Madagascar, by overseas dispersal. Gigantism subsequently occurred independently on each landmass.

Autonomous Underwater Vehicles (AUVs) have a great potential for under ice explorations of both the Arctic and Antarctic Oceans. Most of the current under-ice AUVs are large and are used for long-distance survey. Survey results of these AUVs are extremely precise, but there is a trade-off between precision and running cost and/or risk of losing AUVs. In this paper, the authors propose a variable and compact AUV MONACA (Mobility Oriented Nadir AntarctiC Adventurer), which can offer the necessary and sufficient compositions for various survey needs by modular design. MONACA's range is smaller than conventional AUVs. However, the simple and compact AUV can reduce operation risk. Therefore, we are expecting MONACA to lower the hurdle of conducting challenging under-ice operations.

The Ninth Symposium on Polar Science/Interdisciplinary sessions: [IF] Toward Future Plan of the Arctic and Antarctic Science, Thu. 6 Dec. / 2F Auditorium, National Institute of Polar Research

The Ninth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 5 Dec. / Entrance Hall (1st floor), National Institute of Polar Research

The Ninth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 5 Dec. / 3F Seminar room, National Institute of Polar Research

We summarize U–Pb age data for the Lutzow–Holm Complex (LHC) in East Antarctica and propose the following geological subdivisions based on protolith ages, along the coast of Dronning Maud Land from southwest to northeast: the Innhovde Suite (INH, 1070–1040 Ma) composed mainly of felsic orthogneiss; the Rundvagshetta Suite (RVG, 2520–2470 Ma), mostly felsic orthogneiss with minor mafic and metasedimentary gneisses; the Skallevikshalsen Suite (SKV, 1830–1790 Ma), felsic to mafic orthogneiss with abundant dolomitic marbles, calc-silicates and other metasediments; the Langhovde Suite (LHV, 1100–1050 Ma), mostly felsic orthogneiss with minor mafic and calc-silicate gneisses; the East Ongul Suite (EOG, 630 Ma), with various orthogneisses and metasediments; and the Akarui Suite (AKR, 970–800 Ma) with diverse orthogneisses and paragneisses. The oldest crustal components of the LHC lie in the southern part of Lutzow–Holm Bay, and consist of late Neoarchean and Paleoproterozoic protoliths to charnockites and enderbites that dominate the Rundvagshetta and Skallevikshalsen Suites. This older domain is surrounded by gneisses and granulites with late Mesoproterozoic to early Neoproterozoic protolith ages, including the Innhovde Suite and the Langhovde Suite. The Akarui Suite contains diverse orthogneisses with Neoproterozoic protoloiths, and the youngest unit is the East Ongul Suite with protolith ages of ~630 Ma. Cape Hinode, located geographically within the Akarui Suite, underwent high-grade metamorphism at ~960 Ma that is much older than that which produced the gneisses and granulites of the surrounding LHC (~600–520 Ma). Cape Hinode is therefore exotic, independent of the surrounding LHC, and defined as the “Hinode Block”. The boundaries proposed in this paper are largely consistent with those inferred from magnetic anomalies, gravity anomalies, and bedrock topographical data.

By combining a Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) Earth Gravity Model (EGM) and in situ gravity data obtained from the Japanese Antarctic Research Expedition (JARE) surveys, we estimated the regional gravity field in the area of Syowa Station, a Japanese research station located in Lutzow-Holm Bay, East Antarctica. In situ data sets that were used consisted of land gravity data collected since 1967, shipborne data collected since 1985 and airborne gravity data collected in 2006. The GOCE direct (DIR) solution release 5 (R5) model was used as the long-wavelength reference of the gravity field. Using these data sets, we calculated gravity anomalies and geoid heights at 1-by-1′ grid by means of least-squares collocation. The resulting geoid height at Syowa Station was compared with a local height based on GPS, spirit leveling and tide gauge data. The result suggests that the sea surface height at Syowa Station is −1.57 m, which is consistent with a dynamic ocean topography model. During this investigation, we also evaluated GOCE EGMs and other recent EGMs by comparing them with the airborne gravity data. The results indicate that the GOCE DIR R5 produced the smallest RMS (Root Mean Square) differences and that the newer models performed nearly as well. These comparisons demonstrate the importance of using reliable in situ data when evaluating satellite-only EGMs.

Recently, a source of Antarctic Bottom Water (AABW) was identified off Cape Darnley at the eastern end of the Weddell-Enderby Basin. However, the behavior and long-term variability of Cape Darnley Bottom Water (CDBW) are not clearly understood. Hydrographic observations from 1974 to 2016 were compared, and a decade-long bottom temperature record was analyzed to clarify multidecadal changes in the CDBW in this region and its downstream influences. In the Cooperation Sea, CDBW spread northwestward with its deepest part reaching to approximately 4,900 dbar. CDBW freshening of 0.001-0.003 decade(-1) was revealed. In the Cosmonaut Sea, long-term AABW warming of approximately 0.01-0.03 degrees C decade(-1) was prominent in the deep basin, while freshening was detected on the upper continental slope. Spatial patterns suggest that an interbasin deep transport of excess freshwater is carried by CDBW and fed into the Weddell Gyre, which might act as an abyssal freshwater buffer. Plain Language Summary Global oceans' abyss is filled with the cold, dense water fed from the Antarctic coastal margin. The Weddell Sea is the most voluminous supplier of this bottom water. In addition to the well-known source of the bottom water, new source regions are discovered recently: Continental shelf off Cape Darnley, East Antarctica, is one of such regions. Vast parts of the ocean around Antarctica is experiencing freshening for these decades, possibly related to an accelerating ice mass discharge from the Antarctic continent. In contrast, the abyssal Weddell Sea has been known to be warming significantly, acting like a huge heat buffer. Our study shows the newly discovered bottom water off Cape Darnley is carrying an increasing amount of freshwater and feeding the excess freshwater into the abyssal Weddell Sea. This suggests that the Weddell Sea experiences changes that originate from a distant, continental source.

The second generation Antarctic magnetic anomaly compilation for the region south of 60 degrees S includes some 3.5 million line-km of aeromagnetic and marine magnetic data that more than doubles the initial map's near-surface database. For the new compilation, the magnetic data sets were corrected for the International Geomagnetic Reference Field, diurnal effects, and high-frequency errors and leveled, gridded, and stitched together. The new magnetic data further constrain the crustal architecture and geological evolution of the Antarctic Peninsula and the West Antarctic Rift System in West Antarctica, as well as Dronning Maud Land, the Gamburtsev Subglacial Mountains, the Prince Charles Mountains, Princess Elizabeth Land, and Wilkes Land in East Antarctica and the circumjacent oceanic margins. Overall, the magnetic anomaly compilation helps unify disparate regional geologic and geophysical studies by providing new constraints on major tectonic and magmatic processes that affected the Antarctic from Precambrian to Cenozoic times., Korea Polar Research Institute (KOPRI) programs, PM15040 and PE17050, Germany's AWI/Helmholtz Center for Polar and Marine Research, Federal Institute for Geosciences and Natural Resources, British Antarctic Survey/Natural Environmental Research Council, Italian Antarctic Research Programme, Russian Ministry of Natural Resources, U.S. National Science Foundation and National Space and Aeronautics Administration, Australian Antarctic Division and Antarctic Climate & Ecosystem Cooperative Research Centre, French Polar Institute, Global geomagnetic observatories network (INTERMAGNET)

The western Cosmonauts Sea, off the coast of East Antarctica, was a site of rifting of the Gondwana supercontinent and subsequent early seafloor spreading. To improve our understanding of the breakup of Gondwana, we conducted a marine magnetotelluric experiment to determine the electrical resistivity structure within the uppermost several hundred kilometers beneath the western Cosmonauts Sea. Magnetotelluric response functions at two sites, obtained after considering possible influences of non-plane magnetic field sources, suggest that these responses include distortions by topographic variations and conductive anomalies around the observation sites. Three-dimensional forward modeling confirmed that these distortions due to topographic variations and a thin (∼2-km thick) conductive layer immediately under the sites (mostly sediments) are severe. Furthermore, three-dimensional forward modeling to investigate the resistivity structure at deeper depths revealed an upper resistive layer (≥300 Ω-m), with a thickness of 90 Ma), and may suggest a conductive anomaly in the upper mantle produced by mantle convection and/or upwelling.

日本南極地域観測隊では「しらせ」船上での重力測定を実施しているが，重力計のドリフトやオフセットにばらつきがあることから，小西ほか（2006）では，第46次隊までのデータについて，衛星高度計データによる海域重力モデルを基準とした補正が行われている．しかし，それ以降の取得データは未補正であり，また，衛星高度計による海域重力モデルも新しくなっていることから，今回，第52次隊までのすべての船上重力データについて再処理を行った．具体的には，未処理だった第47次隊から第52次隊のデータに含まれる異常値を除去した後，第27次隊から第52次隊の全データについて，最近のモデルgrav.img.20.1を基準としてドリフトおよびオフセットの補正を行った．これらの処理の概要について報告するとともに，補正済みデータセットを利用したリュツォ・ホルム湾からアムンゼン湾沖にかけての地殻構造推定結果について述べる．, To compile consistent data sets of Japanese Antarctic Research Expedition (JARE) shipborne gravity data, we applied drift and offset corrections to data sets obtained from JARE-27 to -52 by using a new satellite altimetric gravity model as the reference. Konishi et al. (2006) had previously conducted drift and offset corrections for data obtained up to and including JARE-46 so that the shipborne data fitted the satellite altimetric gravity data of grav.img.11.1 (Sandwell, 2004). Following the method of Konishi et al. (2006), we extracted gravity values from the altimetric gravity model of grav.img.20.1 (Sandwell, 2012) along the ship tracks, and then compared these values with those of the shipborne gravity data. These comparisons revealed large discrepancies near the turning points of the ship tracks, and large drifts and offsets in some of the data sets. We removed data that contained large discrepancies, and applied drift and offset corrections to the remaining data by assuming polynomial functions of time as the drift model. These reprocessed data sets were used for estimating crustal structure near Syowa Station.

The Sor Rondane Mountains of eastern Dronning Maud Land, East Antarctica can be subdivided into two different crustal terranes: the NE-terrane and the SW-terrane. The former is underlain by basement rocks of amphibolite-facies (unit A) and granulite-facies (unit B), and the latter by granulite-facies (unit C) to greenschist-facies (units D and D′) rocks. The metamorphic evolution of the NE-terrane exhibits a clockwise pressure–temperature–time (P–T–t) path and the SW-terrane exhibits a counter-clockwise P–T–t path, and detrital zircon U–Pb ages are also different between the two terranes. The differences in the metamorphic evolution processes and detrital zircon provenance in these two regions can be explained by the collision of the NE-terrane and the SW-terrane which is constrained to have occurred at 600–650 Ma. The collision is interpreted to reflect convergence between the East Maud-East African orogen terrane and the Maud-Nampula terrane of the Kalahari Craton, which are bounded by the Main Tectonic Boundary and passes through the Sor Rondane Mountains. The RTP magnetic anomalies suggest this suture is cut by the subsequent collision boundary between the Kalahari Craton and the Rayner Belt, which is exposed in the Lutzow–Holm region. This collisional belt is considered to have formed at approximately 520–580 Ma.

In this paper we clarify the history of deformation in the Sor Rondane Mountains (SRMs), eastern Dronning Maud Land (DML), East Antarctica, and construct a form-line contour map of the metamorphic and plutonic rocks in order to comprehend their structural features and provide constraints on the collisional tectonics of East and West Gondwana. We divide the deformational history in the SRMs into 13 stages (D1–D13). The tectonic regime varied frequently from extension (D3–D4) to layer-normal compression and layer-parallel extension (D5), to compression (D6), extension (D7), sinistral transtension and sinistral strike-slip (D8), compression (D9–D11), and finally extension related to dextral shearing (D12–D13). D7 and D8 indicate major extensional tectonic activity in the southern part of the East African and Antarctic Orogen (EAAO) before the Pan-African compressional event, and after the c. 600 Ma peak of metamorphism. The Pan-African compressional event resulted in the formation of upright folds with horizontal axes that curve along the coastline in central to eastern DML during the D9 deformation that took place between 600 and 560 Ma. The coastline-parallel fold axes and subvertical axial-planes correspond to the X-axes and the XY-planes, respectively, of strain ellipsoids that were progressively rotated counterclockwise toward the central parts of a sinistral shear zone. Therefore, the curved fold axes and axial-planes suggest the EAAO acted as a zone of sinistral transpression during the collision of parts of East and West Gondwana. Around 560–550 Ma, during D12, parallel dyke swarms of granitic pegmatite were intruded along normal faults under a regime of NNE–SSW horizontal extensional stress. The extensional paleo-stress and its related structures suggest dextral rather than sinistral shearing took place along the north-trending EAAO during this late Pan-African event. There is the possibility of a reversal of trans-orogen asymmetry from sinistral to dextral in the southern part of the EAAO. The dyke swarms are considered to have been the heralds of the voluminous 530–500 Ma A-type granite intrusions in DML. At the same time, the Lutzow–Holm Complex was under a non-extensional tectonic regime, and may have been situated in a different orogen from the EAAO.

The area around Syowa Station, the Japanese Antarctic wintering station in Lutzow-Holm Bay, is widely considered as a junction of the continents of Africa, India, Madagascar, and Antarctica, according to a reconstruction model of Gondwana that considers the suture between East and West Gondwana. This area is therefore key for investigating the formation of Gondwana. To reveal the tectonic evolution that contributed to Gondwana's formation in this area, joint Japanese-German airborne geophysical surveys were conducted around Syowa Station in January 2006 during the 47th Japanese Antarctic Research Expedition, from 67° S to 73° S latitude and from 35° E to 45° E longitude. Ice radar, magnetic, and gravity data were obtained from onshore areas. Several characteristic features that are possibly related to the tectonic evolution of Gondwana were inferred, primarily from magnetic anomalies, as well as from gravity anomalies and bedrock topography. The boundaries of the Lutzow-Holm Complex, the Yamato-Belgica Complex, and the Western Rayner Complex are defined, but the inland extension of the boundary between the Lutzow-Holm and the Yamato-Belgica Complexes is unknown south of 71° S. The main geological structural trends of the Lutzow-Holm Complex derived from magnetic anomalies are NW–SE and are concordant with the geological results in the coastal region. However, nearly NE–SW-trending magnetic anomalies cut across the NW–SW magnetic anomaly trends, and NE–SW right lateral strike-slip faults were deduced from the magnetic and the gravity anomaly data of the Lutzow-Holm Complex. The Lutzow-Holm Complex was sub-divided into four blocks based on the estimated strike-slip faults. These strike-slip faults may have been generated during a younger stage of Pan-African orogeny, after the formation of NW–SE-striking geological structures. Cape Hinode, which is considered an allochthonous unit in the Lutzow-Holm Complex according to its surface geology, may have originated from the Rayner Complex and been displaced by right lateral strike-slip motions. These results provide new constraints on the tectonic evolution of Gondwana during the Pan-African orogeny.

A new hydrothermal vent site in the Southern Mariana Trough has been discovered using acoustic and magnetic surveys conducted by the Japan Agency for Marine-Earth Science and Technology's (JAMSTEC) autonomous underwater vehicle (AUV), Urashima . The high-resolution magnetic survey, part of a near-bottom geophysical mapping around a previously known hydrothermal vent site, the Pika site, during the YK09-08 cruise in June–July 2009, found that a clear magnetization low extends ∼500 m north from the Pika site. Acoustic signals, suggesting hydrothermal plumes, and 10 m-scale chimney-like topographic highs were detected within this low magnetization zone by a 120 kHz side-scan sonar and a 400 kHz multibeam echo sounder. In order to confirm the seafloor sources of the geophysical signals, seafloor observations were carried out using the deep-sea manned submersible Shinkai 6500 during the YK 10-10 cruise in August 2010. This discovered a new hydrothermal vent site (12°55.30′N, 143°38.89′E; at a depth of 2922 m), which we have named the Urashima site. This hydrothermal vent site covers an area of approximately 300 m×300 m and consists of black and clear smoker chimneys, brownish-colored shimmering chimneys, and inactive chimneys. All of the fluids sampled from the Urashima and Pika sites have chlorinity greater than local ambient seawater, suggesting subseafloor phase separation or leaching from rocks in the hydrothermal reaction zone. End-member compositions of the Urashima and Pika fluids suggest that fluids from two different sources feed the two sites, even though they are located on the same knoll and separated by only ∼500 m. We demonstrate that investigations on hydrothermal vent sites located in close proximity to one another can provide important insights into subseafloor hydrothermal fluid flow, and also that, while such hydrothermal sites are difficult to detect by conventional plume survey methods, high-resolution underwater geophysical surveys provide an effective means.