Abstract

I investigate the formation and deformation of oceanic lithosphere at the ultra-slow spreading southwest Indian Ridge (SWIR), with particular focus on the Atlantis Bank oceanic core-complex and the surrounding region. I use magnetic and bathymetric data for 450 km along axis to constrain the regional evolution of the plate boundary at the SWIR over the past 30 Ma. The shape of the plate boundary has changed significantly. These changes were accommodated by asymmetric plate-spreading rates and are driven by the rotation of ∼450-km-wide oblique first-order spreading segments to become more orthogonal to the plate spreading direction. Atlantis Bank, itself, rises to within 720 m of sea-level east of the Atlantis II Transform fault, it is an oceanic core-complex where detachment faulting exposed lower crustal gabbros and peridotites at the seafloor 10-13 Ma. I use magnetic data and Pb/U ages of igneous zircon to calculate the rate of slip on this detachment fault (14.1 km/Myr). This rate requires highly asymmetric spreading and that the exposed gabbro accreted during detachment faulting; it also suggests that more gabbroic crust accreted at depth below the detachment fault than was accreted above it onto the conjugate plate. I investigate how a vertical section of this gabbroic oceanic crust accreted by dating the crystallization of samples from the 1508 m deep ODP Hole 735B drilled into the summit of Atlantis Bank. The ages reveal that the samples accreted in <120 kyr, an observation consistent with rapid crustal accretion within 1.7 km of the ridge-axis. Atlantis Bank's shallow bathymetry made it an ideal locale to drill Hole 735B, but only ∼1 km of the >3 km of anomalous elevation can be explained by plate flexure due to detachment faulting. The uplift due to large transform-parallel normal faults observed at Atlantis Bank is quantified using flexural fault models and can account for the remaining uplift of Atlantis Bank. These faults formed during a period of transtension on the Atlantis II transform fault that initiated following a 10° counter-clockwise rotation in spreading direction ca. 19.2 Ma.