Syntectonic Fluid Flux during Rift Deformation: Record from the MIS Core, Victoria Land Basin, Antarctica

The McMurdo Ice Shelf project recovered 1285 m of Neogene sedimentary core from the Victoria Land rift basin, Antarctica. The core contains 1475 faults, veins, and clastic dikes. Veins, mainly calcite, constitute at least 625 of this population, include at least 12 vein types, and commonly have multiple generations of calcite fill. Microstructural analysis of core structures is focused on the role of fluids in deformation, as well as the structural control on fluid pathways during lithification, compaction and diagenesis in this unique glaciated rift basin setting. Initial results show that calcite veins from 1-15 mm thick are associated with discrete faults; many have dip-slip slickenfibers, and calcite fills voids along pull-aparts, indicating veins formed during fault displacement. Intricately branching webs of hairline veins form planar arrays around thicker veins; the typical conjugate geometries suggest that these vein complexes represent fault zones. The diffuse, pervasive nature of the veining suggests that tensile strength of the rocks was low and fluid pressures were high. Opening-mode veins are common, are filled by calcite fibers and sparry calcite that grew normal to vein walls, and show multiple episodes of opening and fill. Tightly folded, vertical veins are ubiquitous in the core and indicate the host sediment was cohesive enough to support fracture, but was not fully lithified, and accommodated vein buckling during compaction. Vertical fibrous calcite veins occur as pressure shadows on the margins of pebbles, documenting formation during vertical loading and compaction. Generations of vein material track kinematic changes as indicated by fault-parallel slickenfibers overprinted by opening-mode calcite fibers. Calcite vein generations and vein cross-cutting relations document the relative chronology of deformation and fluid flux. High pore fluid pressures were clearly important throughout the progressive deformation history, and rift-related structures exerted a fundamental control on dewatering of the rift basin strata.