ALBERT

Description: The Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and the Federal Institute for Geosciences and Natural Resources (BGR) collected around 150 hours of new gravity, magnetic and ice-penetrating radar data from east and south of Princess Elisabeth station in Dronning Maud Land between 2013 and 2015. Survey lines were spaced 10 km apart. The 2013/2014 and 2014/2015 used different gravimeters; a LaCoste and Romberg AirSea gravimeter (LCR) at constant barometric altitude and a Gravimetric Technologies GT2A gravimeter at constant ground separation. Both surveys used a Scintrex Cs-3 caesium vapour magnetometer mounted in a tail boom with compensation for the airframe calculated using a fuselage-mounted three-component fluxgate magnetometer. The GT2A gravity data reflect the effects of short-wavelength density contrasts between basement rocks and the ice sheet more reliably than the LCR data. Cross-over analysis suggests the repeatability of data collection with the GT2A lies at the sub-milliGal level. A broad subglacial channel that separates eastern Sør Rondane from the Yamato Belgica Mountains is evident in the gravity data. In the south of the survey region, the data reveal a dendritic pattern of subglacial valleys that converge towards the SW. Strong NS-trending magnetic anomalies coincide with the Yamato-Belgica Mountains. Further west, subtler ESE-trending anomalies confirm proposals that the SE Dronning Maud Land province continues into the region south of eastern Sør Rondane. An unexpected feature of both data sets is the apparent termination of the anomaly patterns associated with the province at a NNW-trending anomaly running south of Princess Elisabeth.

Description: The geology of Sør Rondane has been the focus of intense research and occupies a key position for reconstructing the late Neoproterozoic to early Paleozoic geodynamic evolution in eastern Dronning Maud Land (DML). Sør Rondane appears to be located close to the supposed intersection of the East African-Antarctic Orogen (EAAO) and the Kuunga Orogen. The western part of Sør Rondane is subdivided in two distinct terranes. The amphibolite to granulite-facies NE terrane is mainly composed of metasupracrustal rocks, with detrital zircon ages in part younger than 750 Ma, deposited on older basement of unknown, possibly Rayner-type, crust (Shiraishi et al., 2008). Metamorphism has been dated by U-Pb on zircon at ca. 640-600 Ma and amphibolite-facies retrogression dated at ca. 590-530 Ma. The SW terrane is subdivided by the Main Shear Zone (MSZ) into two lithothectonic units, i.e. Pan- African greenschist- to granulite-facies metamorphic rocks with “East African” affinities in the N and a Rayner-age early Neoproterozoic gabbro-tonalite-trondhjemite-granodiorite (GTTG) complex with “Indo-Antarctic” affinities in the S. The GTTG complex has suffered Pan-African greenschist- to lower amphibolite-facies thermal overprint, but also contains large domains with only weak deformation except for its northern margin close to the MSZ. The deformation there is related to high shear strain along this structure. New zircon crystallisation ages of the GTTG cluster around 1000-930 Ma. It is interpreted to have formed along a juvenile oceanic arc, in which the wide age range might indicate a long-lasting accretionary orogen. The MSZ is characterized by a right-lateral sense of movement and high-strain ductile deformation under peak amphibolite-facies conditions. The structure can be traced over a distance of ca. 120 km between Lågkollane in the W and Lunckeryggen in the E and reaches several hundred meters in width. The MSZ cannot be traced further to the W where it seems to terminate at the north-eastern border of the NW-SE oriented prominent magnetically defined SE DML Province. The north-eastern border zone may coincide with a significant dextral shear zone that runs from the Schirmacher Oasis into the region S of Sør Rondane (Schirmacher- Rondane Lineament). The SE DML Province most likely consists of Rayner-age (1000-900 Ma) crust with evidence of intense Pan-African reworking indicated by new geochronological data and was part of a large Tonian Oceanic Arc Super Terrane (TOAST). The continuation of the MSZ into eastern Sør Rondane and beyond is not clear either, since it appears to terminate at a N-S oriented region with low magnetic signatures (central Sør Rondane corridor) that is possibly related to extensional tectonics. Crosscutting relationships with dated magmatic rocks bracket the activity of the MSZ between Latest Ediacaran to Cambrian times (c. 560- 530 Ma). Based on new combined aeromagnetic and structural results from a four-seasons survey of the greater Sør Rondane region, we propose that the crustal structural architecture of eastern DML and is strongly influenced by N-directed (with Africa/Antarctica restored to its original position in Gondwana) lateral extrusion of the EAAO. This process was likely driven by the combination of (i) indentation of the SE DML block towards the conjugate stable Kalahari- Grunehogna cratonic foreland, (ii) extensional collapse of the previously (c. 580-550 Ma) thickened and gravitational instable crust of central DML, and (iii) large-scale tectonic escape of crustal blocks in eastern DML along major shear zones such as the Schirmacher Rondane Lineament and MSZ towards an unconstrained yet unknown region at a lateral position of the EAAO.

Description: East Antarctica consists of a number of cratonic fragments that amalgamated along distinct orogenic belts in late Neoproterozoic to early Palaeozoic times. These mobile belts include the c. 640 to 500 Ma old East African-Antarctic Orogen (EAAO) and the Kuunga Orogen, which seem to converge in Dronning Maud Land in the Atlantic sector of Antarctica. The polymetamorphic basement of Dronning Maud Land is characterized by rocks with Grenville-age protolith ages of c. 1130 to 1000 Ma in the west and rocks with early Neoproterozoic protolith ages of c. 1000 to 900 Ma in the east. These two provinces are separated by the prominent Forster Magnetic Anomaly, which is therefore interpreted to represent a suture zone. Four joint AWI-BGR international expeditions within the WEGAS (West-East Gondwana Amalgamation and Separation) and GEA (Geodynamic Evolution of East Antarctica) programmes between 2010 and 2015 have provided new combined geological and geophysical data that reveal a complex crustal architecture between central Dronning Maud Land and Lützow-Holm-Bay. The magnetic anomaly pattern changes significantly east of the Forster Magnetic Anomaly with apparently no indication of Maud-type crust. Particularly, the GEA II campaign (2011-12) targeted a series of previously unvisited nunataks in the largely ice- covered Borchgrevink-Isen between central Dronning Maud Land and Sør Rondane from Urna and Sørsteinen in the west to Blåklettane and Bergekongen in the east. This region is characterized by NW-SE trending distinct linear magnetic anomalies. This pattern is referred to as the SE Dronning Maud Land Province and was previously interpreted as a fragment of potentially older cratonic crust south of an Ediacaran to Cambrian mobile belt that crops out in Sør Rondane. New SHRIMP/SIMS U-Pb zircon ages and geochemical analyses, however, indicate that this region consists of Rayner-age (c. 1000 to 900 Ma) juvenile arc and metasedimentary cover rocks, which were intensely reworked by medium- to high-grade metamorphism and felsic melt injections between c. 630 and 520 Ma. The juvenile rocks are very similar to a gabbro-tonalite-trondhjemite-granodiorite (GTTG) suite in the southern SW Terrane of Sør Rondane, which yield crystallization ages of c. 1000 to 920 Ma based on U-Pb zircon geochronology. The juvenile character of this suite suggests a long-lived accretionary setting in early Neoproterozoic times. While the rocks in the Borchgrevink-Isen further west were intensely reworked in Pan-African times, the GTTG complex in Sør Rondane shows evidence of Pan-African up to lower amphibolite-facies thermal overprint, but still contains large domains with apparently only weak deformation. An exception is the northern margin of the GTTG complex where high-strain dextral shear is related to the prominent Main Shear Zone that is estimated to be of latest Ediacaran to early Cambrian age (c. 560 to 530 Ma). This structure, which we interpret as part of a fault system related to NE-directed lateral extrusion of the EAAO, separates the Rayner-age GTTG complex from a series of greenschist- to granulite-facies metasupracrustal rocks of mainly volcano-sedimentary origin. They in turn are separated from the amphibolite- to granulite-facies NE Terrane in the north and north-east by the Main Tectonic Boundary that is postulated by researches of the Japanese National Antarctic Programme. Available literature and our own new geochronological data indicate that peak and retrograde metamorphism in the NE and SW terranes was at c. 640 to 530 Ma. Both terranes were intruded by several granitoid magmatic pulses between c. 650 and 500 Ma. In contrast to “Indo-Antarctic” affinities of the GTTG complex south of the Main Shear Zone and the similar rocks of the SE Dronning Maud Land Province to the west, these units thus appear to have rather “East African” affinities. Furthermore, grey heterogeneous gneisses and augen-gneisses of the aforementioned meta-volcanosedimentary supracrustal rocks of the SW Terrane close to the Main Shear Zone gave zircon crystallization ages of c. 750 Ma. Such ages are unknown from the EAAO in central and western Dronning Maud Land west of the Forster Magnetic Anomaly. Taking all evidence together, we propose that the Forster Magnetic Anomaly separates distinctly different parts of the EAAO. These are (i) a reworked, mainly Grenville-age crust of the Maud Belt to the west representing the overprinted margin of the Kalahari Craton, and (ii) a part of the orogen dominated by early Neoproterozoic accretionary tectonics to the east, which we refer to as Tonian Ocean Arc Super Terrane (TOAST). The contrast between these two crustal units is also reflected in the geochemistry of voluminous late-tectonic granitoids across the whole belt. Based on our new geological and aerogeophysical evidence, the regional crustal structure of eastern Dronning Maud Land as a whole may tentatively be interpreted as reflecting large-scale lateral extrusion of the EAAO post-dating continental collision in the late Neoproterozoic and early Cambrian.

Description: Carbonate platforms form informative archives for paleoclimates and their internal structures can also hold crucial information about the tectonic history and carbonate evolution of the ocean basins. The Zhongsha atoll (Macclesfield Bank) forms the largest atoll system in the South China Sea with a surface area of 23500 km2. However, the internal structure and evolution of this atoll system is completely unknown. 2D multichannel seismic reflection data were acquired in 2017 over the Zhongsha atoll in the South China Sea to unravel the stratigraphy, geomorphology, depositional processes, and seismic facies of one of the world's largest atoll for the first time. This Neogene carbonate platform comprises more than 1 km thick carbonate sequence and overlies a metamorphic basement. The southeastern part of the atoll comprises a fault-controlled graben system, which was formed during the Cenozoic rifting stage of the South China Sea. Most of the faults trend NE-SW and E-W and terminate at or slightly above the top of Middle Miocene strata. Atolls and abundant organic reefs initiated on the positive relief and closely mimicked the underlying topography during the Early Miocene. Shallow-water carbonates continued growing through Middle Miocene to present times. Regional uplift led to subaerial exposure, termination of platform growth and karstification during the Miocene. We also reveal a number of fluid-flow features such as vertical sub-bottom venting features (chimneys and pipes), chaotic reflection zones, which provide the first evidence of active fluid venting in the area of Zhongsha atoll. The Neogene sedimentation history of Zhongsha atoll further provide an important paleoenvironmental context for future scientific drilling to better constrain the evolution of Asia Monsoon.

Description: 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 the 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 surveys and gridded datasets accessible under the Findable, Accessible, Interoperable, and Reusable (FAIR) data principles. With the goals of making the gridding process reproducible and allowing scientists to re-use the data freely for their own analysis, we introduce the new SCAR Bedmap Data Portal (https://bedmap.scar.org, last access: 1 March 2023) created to provide unprecedented open access to these important datasets through a web-map 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 (last access: 5 May 2023​​​​​​​). See the Data availability section for the complete list of datasets.

Description: Published

Description: 2695–2710

Description: OSA2: Evoluzione climatica: effetti e loro mitigazione

Description: JCR Journal