ANALYSIS OF MICROSTRUCTURES IN CORE FROM THE VICTORIA LAND BASIN, ANTARCTICA

This work analyzes microstructures in core recovered offshore from Cape Roberts, in the westernmost Ross Sea of West Antarctica, in order to understand the rifting evolution of the Victoria Land Basin and its relationship to the uplifting of the Transantarctic Mountain rift flank.

Our studies focus on textures, fabrics and grain-scale structures observed in microfaults, veins, and clastic dikes indicative of the deformation mechanisms that produced those structures and the mechanical state of the sediment during deformation. This data, as well as macrostructural features observed in the core are used to infer relative timing of faulting and sedimentation.

Microfaults are abundant and display two main types of textures. Some microfaults are characterized by grain-size reduction, poorly sorted angular grains and preferred orientation of clays and/or clast long axes parallel to fault zone walls; these are associated with brittle shear of dewatered and cohesive sediment. Others are ‘shear zones’ where bedding drag, sediment smearing, and no grain-size reduction indicate pre-lithification ductile flow of sediment by sliding of grains due to abundant pore fluid.

Veins commonly follow pre-existing fault planes and exhibit carbonate and pyrite growth in patches and less commonly sparry calcite. Abundant fibrous calcite perpendicular to vein walls suggests opening-mode origin.

Clastic dikes are present throughout the core and sometimes follow fault planes, indicating that injections of liquefied sediment used pre-existing faults as conduits for ‘dewatering bursts’.

Pre-lithification shearing and cataclasis of cohesive and lithified material appears to be the mode of faulting. Diagenetic clays, zeolites and precipitation of calcite and sulfides along fault planes post-date microfaulting. Preliminary data indicate that sediment-filled and cataclastic faults exhibit the same orientations throughout the core, implying tectonic stresses controlled both types. This suggests that faulting may have taken place at the same time as deposition in the rift basin.