Constraints on the tectonic evolution of the westernmost Mediterranean and northwestern Africa from shear wave splitting analysis

- Infer seismic anisotropy from SK(K)S and direct S splitting measurements.
- Observe channeling of mantle flow beneath the High Atlas.
- Detect the edge of subducted slab from changes in shear-wave splitting.
- Observe the interaction of slab, craton, and thin lithosphere on mantle flow.

The westernmost Mediterranean mantle and lithosphere have evolved into their current configuration due to complex interactions between the African and Eurasian plates. To help unravel the regional tectonics, we use new broadband seismic data across the Gibraltar arc and into southern Morocco to infer azimuthal seismic anisotropy and flow patterns for the upper mantle based on shear wave splitting analysis. A deep (>600 km) earthquake in April 2010 was recorded by the array and allowed us to compare 31 direct S measurements with 235 teleseismic SK(K)S events from 3 years of deployment. The patterns of apparent fast polarization orientations and delay times suggest three major tectonic domains when interpreted jointly with recent tomographic images of the subducted slab: (1) a subducted slab related toroidal flow domain centered upon the Alboran Sea and southern Spain, leading to complex splits, (2), a region where the west African craton deflects mantle flow in the Anti-Atlas and High Plateaux, and, (3), an intermediate domain across the central High Atlas. Across the axis of the mountain belt a coherent, regional maximum of delay times is observed for both S and SKS splitting measurements, with polarizations predominantly parallel to the strike. We interpret this as possible SW-NE channeling of mantle flow beneath the region with a thinned lithosphere and slow seismic velocities beneath the central High Atlas Mountains.