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Abstract: Glaciovolcanic sequences are widespread in northern Victoria Land and their interpretation enables estimates of past ice thicknesses and other defining parameters of the coeval ice sheet to be made with considerable certainty. The terrestrial sequences are mainly Late Miocene in age (c. 12–5Ma) and conform to five main types: aa lava-fed deltas, volcanic sheet-like sequences, a glaciolacustrine sequence, scoria cones and tuff cones. Apart from the environmentally undiagnostic pyroclastic cones, the volcanic sequences were erupted in association with a glacial cover that was typically, and possibly never much more than, a few hundred metres thick (<300m). Despite gaps in the record, no evidence was observed for prolonged subaerial (i.e. fully ice-free) eruptive conditions, suggesting either that ice-free conditions did not occur or else were of such limited areal extent or short duration that they left no record. These observations are consistent with the presence of a thin persistent Late Miocene ice dome or icefield draping the pre-Miocene topography in northern Victoria Land for the period, although it may have been confluent with the greater East Antarctic Ice Sheet similar to conditions present today. The presence of glacially emplaced sequences right down to present datum suggests that the Transantarctic Mountains in the region were already uplifted to their current elevation prior to the volcanism. The high terrain would have helped the ice sheet to establish and persist. The glacial thermal regime varied from wet-based and dynamic, to cold-based (frozen to its bed) and presumably relatively stable. A major transition from wet-based ice to cold-based ice might have occurred between 9.7 and 7.5Ma consistent with published interpretations of the regional landscape evolution. Alternatively, the glacial thermal regime was dominantly polar since the earliest eruptions but with several temporary changes to a wet-based (sub-polar or temperate) more dynamic regime. [Copyright &y& Elsevier]

Abstract: New aeromagnetic data image the extent and spatial distribution of Cenozoic magmatism and older basement features over the Admiralty Block of the Transantarctic Mountains. Digital enhancement techniques image magmatic and tectonic features spanning in age from the Cambrian to the Neogene. Magnetic lineaments trace major fault zones, including NNW to NNE trending transtensional fault systems that appear to control the emplacement of Neogene age McMurdo volcanics. These faults represent splays from a major NW–SE oriented Cenozoic strike-slip fault belt, which reactivated the inherited early Paleozoic structural architecture. NE–SW oriented magnetic lineaments are also typical of the Admiralty Block and reflect post-Miocene age extensional faults. To re-investigate controversial relationships between strike-slip faulting, rifting, and Cenozoic magmatism, we combined the new aeromagnetic data with previous datasets over the Transantarctic Mountains and Ross Sea Rift. Two key observations can be made from our aeromagnetic compilation: 1) Cenozoic alkaline intrusions along the margin of the Ross Sea Rift lie oblique to the NW–SE strike-slip faults and are not significantly displaced by them; 2) the Southern Cross and the Admiralty Blocks are much more significantly affected by Cenozoic magmatism compared to the adjacent tectonic blocks, thereby indicating major tectono-magmatic segmentation of the Transantarctic Mountains rift flank. We put forward three alternative tectonic models to explain the puzzling observation that major Cenozoic alkaline intrusions emplaced along the Ross Sea Rift margin show no evidence for major strike-slip displacement. Our first model predicts that the alkaline intrusions were emplaced along left-lateral cross-faults, which accommodated distributed right-lateral shearing. In contrast, our second model does not require major distributed strike-slip shearing, and relates the emplacement of Cenozoic alkaline intrusions to sea-floor spreading in the Adare Basin, coupled with intracontinental transfer faulting. The third model is an attempt to reconcile the two opposing hypothesis and relies on a recent inference, which postulates that opening of the Adare Basin relates to fault splaying from the Balleny strike-slip fault zone. A low seismic velocity anomaly in the upper mantle appears to extend from the Ross Sea Rift under the Admiralty and Southern Cross Blocks of the Transantarctic Mountains. Lateral flow of hot upper mantle from the rifted region to the rift flank may explain the observed tectono-magmatic segmentation of the Transantarctic Mountains. We infer that this process caused a regional upwarp of the Curie isotherm under the Admiralty and Southern Cross Blocks of the Transantarctic Mountains, and facilitated extensional faulting, renewed uplift, and volcanism in the Neogene. [Copyright &y& Elsevier]