Paper No. 1
Presentation Time: 9:00 AM


JOHANESEN, Katharine, Department of Geociences, SUNY Fredonia, Fredonia, NY 14063 and PLATT, John P., Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089-0740,

The upper margin of the Ronda peridotite massif, southern Spain, exhibits intense deformation associated with its exhumation from ~100 km along a mantle shear zone. We analyze the dynamically recrystallized grain size and lattice preferred orientations (LPO) of grain populations with increasing distance from the upper margin to determine the nature of this shear zone. The top of the peridotite massif is characterized as ~1 km of mylonite transitioning into a tectonite of variable thickness (~1-4 km). Beneath the tectonite is a granular peridotite, characterized by larger, more equant grains and textures that indicate melt percolation. Recrystallized grain sizes in the upper two zones vary between 85-130 μm and average ~110 μm with no statistically significant trend with regard to distance from the top of the mylonites or the transition to the tectonites. While the recrystallized grain size does not increase with distance from the margin, the proportion of recrystallized grains decreases with distance from the margin. Retort–style stretched orthopyroxene porphyroclasts in the marginal mylonite generally indicate a shear sense of top to the west. The recrystallized grains in the mylonite and tectonite zone share a weak, A-type LPO showing slip on (010) in the [100] direction. The coarser grains in the tectonite zone show a stronger version of the A-type LPO, indicating that dynamic recrystallization randomized grain orientation, weakening the LPO of the fine grains in these zones. Together, these observations support the conclusion that mylonite and tectonite recrystallized fabrics are related and represent progressive localization into the mylonite zone during exhumation.