Cenozoic basin evolution beneath the Southern McMurdo ice shelf, Antarctica

Johnston, Lisa.

Fifty-two kilometres of of multi-channel seismic reflection data were acquired from the southern McMurdo Ice Shelf in preparation for future drilling as part of the Antarctic Drilling (ANDRILL) programme. These data provide the first opportunity to test existing hypotheses for basin evolution in this region. The seismic survey was acquired atop rvllO to rv220 m of floating ice and extended across ablation and accumulation zones of the ice shelf. Seismic processing included: datum static corrections to account for changes in the nature of the near-surface firn layer, and changes in the surface elevation across the survey area; residual static corrections to account for near surface ice shelf irregularities; and predictive deconvolution to suppress ice and firn layer multiples. A model for the ice shelf thickness was incorporated into the interval velocity model during depth conversion to ensure that the ice shelf structure did not impose non-static shifts on the seismic section. Within the depth converted seismic sections, several N to NE trending normal faults offset acoustic horizons by up to rv 150 m within the lower part of the section, and form a broad east-dipping, half-graben structure. These structures are interpreted as the southern extension of the ( early-Mesozoic to present) Terror Rift. The rift is offset slightly to the west beneath the SMIS, which may have been facilitated by an accommodation zone. This latest episode of rifting is associated with up to rv500 m of strata beneath the SMIS, which contrasts with the rvlOOO m thick succession recorded to the north of Ross Island (Fielding et al., 2005). This difference suggests that the field area is located at the tip of the rift basin, within a basin-linking accommodation zone where subsidence is less significant and erosion is more pronounced. The rift succession can be seperated into two main packages: a dislocated (?)earlyMiocene syn-rift package; and a relatively undeformed, post-rift package, seperated by a high amplitude syn-rift unconformity. The post-rift package infills the rift topography, and drapes over the graben system. Throughout this stage, basin evolution was also influenced by minor Neogene volcanic flexure, indicated by tilted strata and an ang~ lar unconformity. Neogene volcanism has also exerted a more direct effect, indicated by the presence of three volcanic features within the profiles, and thick successions of associated igneous material. Glacial erosion has also played an important role and is indicated by erosional horizons and glacio-geomorphic features within the sedimentary package and on the seafloor. The orientation of glacio-geomorphic features on the seafloor indicates an approximately NW direction for ice flow during recent ice sheet grounding events. These grounding events have probably removed the plio-pleistocene iii - •:". 'I succession from this area. Underlying the early-Miocene rift succession, reflectivity is low and acoustic horizons are dislocated. This unit may represent rift strata from an earlier Paleogene rift episode or may be acoustic basement. Support for the earlier rift episode is provided by the occurance of Eocene fossiliferous erratics around the margin of the SMIS, which has lead other workers to suggest that strata of this age may underly the SMIS and be uplifted within the footwall of a normal fault (Stilwell and Feldmann, 2000; Wilson, 2001). The structural setting revealed by the SMIS seismic reflection profiles is consistent with this hypothesis.

xvi, 230 p. : ill. (some col.), 5 diag., maps ; 30 cm. + 1 CD-ROM (4 3/4 in.); 5 sheets (folded)

2006Johnston

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