STRAIN LOCALIZATION WITHIN THE DUCTILE AND BRITTLE REGIMES: THE DEVELOPMENT OF THE ATLANTIS BANK OCEANIC DETACHMENT FAULT SYSTEM, SOUTHWEST INDIAN RIDGE

The mechanics of low-angle detachment fault formation remains controversial as some geologic field evidence for their formation is at odds with conventional Andersonian faulting theory. One key to understanding detachment faulting is the quantification of processes that allow strain localization within both the brittle and ductile regimes. We examine gabbroic fault rocks of the Atlantis Bank oceanic detachment fault system, a ~900 km² oceanic core complex formed along the ultra-slow-spreading Southwest Indian Ridge between ~13-10 Ma. We interpret the process of strain localization to determine how grain size, strain rate, and deformation mechanisms are related in order to describe the initiation and evolution of the low-angle normal detachment fault system of Atlantis Bank.

We use the flow law creep parameters for synthetic aggregates of both plagioclase and amphibole in order to construct deformation mechanism maps given the grain size, temperature and LPO signature of each sample. We use electron backscatter diffraction (EBSD) to measure the presence or absence of a lattice preferred orientation (LPO) in plagioclase and amphibole. Results suggest that high-T plagioclase deformation is accomplished by dislocation creep in the {010} <100> slip system; at low-T, grain size reduction through grain boundary migration recrystallization results in diffusion creep deformation. The amphibole fabric is dominated by the {100} <001> slip system, which develops in pressure solution. We use microprobe-determined mineral compositions for thermometry and for use in the flow laws. Microtextures indicate that high-T fabric is overprinted by lower-T fabric; thermometry confirms fabric development over the range 850º C - 300º C. From the deformation maps, we estimate the strain rates associated with detachment faulting range from 10e-13 to 10e-10 s^-1.

These results demonstrate that (1) the detachment fault system initiated as a distributed ductile shear zone that, with time and increased displacement, localized into a discrete brittle fault surface, (2) the fault system operated under very fast geologic strain rates, and (3) strain localization is dominated by plagioclase rheology and is achieved in part by dynamic recrystallization of plagioclase.