OCEANIC CORE COMPLEX DEVELOPMENT AT ATLANTIS BANK, SOUTHWEST INDIAN RIDGE

In slow-spreading mid-ocean ridge environments, spreading is accommodated by both magmatic and tectonic processes, where the latter may be manifested in the form of an extensional low-angle detachment fault system. Due to the multiple manifestations of spreading, slow-spreading ridges have unique features that include ‘inside-corner highs’, anomalous bathymetric highs intimately linked to the tectonic partitioning of spreading. Atlantis Bank, a paleo-inside-corner high representing 4.6 m.y. of crustal accretion, is a topographically anomalous, faulted feature formed initially at the intersection of the ultra-slow-spreading Southwest Indian Ridge (SWIR) and the Atlantis II Transform. It is part of a long-lived (2.1 m.y.) detachment fault system associated with an oceanic core complex (~900 km² in area) originally developed within oceanic lithosphere at the inside-corner high of the SWIR-Atlantis II Transform ridge-transform intersection.

The Atlantis Bank core complex exposes gabbro, oxide gabbro and rare serpentinized peridotite. It exhibits a dome-shaped corrugated detachment surface that has been subsequently cut by moderately-dipping transform-parallel normal faults, enabling Shinkai 6500 manned submersible sampling of the footwall, hanging wall, and detachment fault surface, providing details of the extensional fault system exposed over 39 km normal to the ridge axis. Ocean Drilling Program Hole 735B samples preserve a continuous vertical section through the interior of the detachment fault system. Together, the sample collections enable a 3-D understanding of the development of the detachment fault system.

Thermometry and microstructures of the granulite-grade footwall rocks and those of the detachment fault surface breccias suggest that deformation initiated at high temperatures (850º C) in the ductile regime, and continued to lower temperatures (600º C) and finally through the semi-brittle and brittle regimes as fault rocks were denuded along the detachment fault system. Preliminary analysis suggests that strain localization in gabbroic rocks was achieved through a variety of processes including: grain size reduction, grain boundary migration-recrystallization, development of lattice-preferred orientation, and fluid infiltration.