Evidence for a Paleo-Spreading Center in the Oman Ophiolite: Mantle Structures in the Maqsad Area

Abstract

Mantle diapirs about ten kilometers in size have been recognized in the Oman ophiolite. A detailed study of the best preserved of these diapirs, cropping out in the Maqsad area, has been undertaken in order to understand melt migration processes in the asthenosphere beneath oceanic spreading centers. New results include the accurate location of the paleo-spreading axis related to the Maqsad diapir. It is evidenced by a 1-2 km wide dunitic corridor striking parallel to the sheeted dyke complex, bearing witness, in its structure and petrology, to pervasive soaking of the mantle by a basaltic melt; most of the upwelling flow is channelled at shallow depth within and beneath this horizon; away from it, the horizontal asthenospheric flow displays a clear divergent pattern. The zone of diverging flow extend 10-15 km on both flanks of the presumed paleo-ridge axis. The relations between the asthenospheric flow trajectories and a large mylonitic shear zone suggest that the Maqsad diapir corresponds to a late spreading event largely contemporaneous with early emplacement tectonics. Dykes facies and intrusion pattern at the periphery of the zone of diverging flow suggest that it ascended through a previously accreted lithospheric segment and that mantle diapirism beneath spreading centers is unsteady.