MICROSTRUCTURAL ANALYSIS OF HYPERSOLIDUS FOLIATIONS AT THE ATLANTIS BANK OCEANIC CORE COMPLEX, SOUTHWEST INDIAN RIDGE

To investigate the initiation of detachment faulting at “hot detachment” oceanic core complexes, we focus on the Atlantis Bank oceanic core complex, Southwest Indian Ridge, where abundant high-temperature hypersolidus fabrics (i.e., those developed at high temperatures in the presence of melt) in the footwall rocks are assumed to have recorded the onset of detachment faulting and strain localization during magmatism. We examine hypersolidus fabrics in lower crust gabbro from the footwall of the Atlantis Bank core complex using drilling core samples from Ocean Drilling Program (ODP) Hole 735B. We use petrographic and microstructural analyses to describe the variation in hypersolidus fabric development, and we observe two primary fabric types that we designate as Fabric Type I and Fabric Type II.

In Fabric Type I, subhedral and elongate plagioclase laths define a strong foliation where plagioclase grains display microstructures such as undulose extinction and tapered deformation twins, which are indicative of very minor strain. In contrast, clinopyroxene grains display microstructures indicative of slightly more strain relative to plagioclase, as subgrains are common and systematically oriented perpendicular to foliation. In Fabric Type II, a moderate foliation is defined by subhedral and elongate plagioclase laths that exhibit undulose extinction and deformation twins, and smaller grains that show curved or lobate grain boundaries indicative of recovery through grain boundary mobility and minor recrystallization. Both clinopyroxene and olivine display evidence of minor strain such as undulose extinction, but olivine exhibits rare subgrains that are systematically oriented perpendicular to foliation. Late-stage igneous brown hornblende mantles clinopyroxene grains and displays straight extinction. Fabric Types I and II have strong to moderate foliations, yet the constituent minerals show relatively minor evidence of internal crystal plasticity, suggesting both fabrics formed in the presence of melt. However, the fabrics are differentiated by relative abundances of microstructures indicative of strain and recovery, where Type II fabrics may indicate more intense deformation associated with the onset of strain localization, possibly due to decreasing melt fraction approaching the solidus.