EXHUMATION OF MANTLE AT THE SOUTHWEST INDIAN RIDGE: CONSTRAINTS ON TEMPERATURE AND DEPTH OF WATER INGRESS DURING DETACHMENT FAULTING

The Southwest Indian Ridge (SWIR), one of the slowest spreading mid-ocean ridges (MORs) (~14-16 mm/yr), exposes broad regions of mantle peridotite, indicating that faulting dominates plate spreading, locally with little or no magmatism. Nineteen fresh, foliated peridotite mylonites dredged from the ~44–45 °E segment of the SWIR in 2018 were analyzed to estimate deformation conditions beneath the ridge axis. Recent microseismicity studies elsewhere along the SWIR predict a thick elastic lithosphere (~11–17 km) and, consequently, a deep brittle-ductile transition, suggesting that detachment faults transport peridotite mylonites from >11 km depth to the surface. Petrography, electron backscatter diffraction, and energy-dispersive x-ray spectrometry reveal a mylonitic matrix comprised of olivine, orthopyroxene, secondary amphibole, serpentine, and spinel-group minerals, with grain sizes of 2–5 µm. Porphyroclasts of olivine, orthopyroxene, and clinopyroxene imply the samples are deformed lherzolites. Crystallographic preferred orientations of matrix olivine show A, B, E, and AG slip systems; samples straddling the A, B and E slip system transition implies the olivine contained water (< ~700 ppm H/Si) during deformation, at differential stresses of ~300–400 MPa. Deformation mechanism maps (wet olivine flow laws) and the olivine piezometer suggest deformation initiated at ~1050 °C in the dislocation creep field, proceeded down temperature via diffusion creep in the matrix, and ended with low-temperature plasticity at ~500 °C. Undeformed pargasitic to tremolitic amphibole reflects the down-temperature (~700–500 °C) metamorphic history of these mylonites as they were unroofed to the surface, with hydration of most clinopyroxene to amphibole due to the ingress of seawater. The assumption of a linear temperature gradient indicates that the ingress of seawater occurs to depths of 11–15 km. Numerical models of slow spreading MORs suggest that reduced magmatism at the ridge axis facilitates large-offset detachment faulting, but the length of exposed footwalls measured from 60-m resolution bathymetry are less than or equal to the estimated ~11–17 km depth of the brittle-ductile transition, implying that multiple, cross-cutting faults are required to unroof peridotite mylonites to the surface.