Mantle structure beneath the Alboran Sea from shear wave splitting

Abstract

Earthquake patterns and seismic tomography in the western Mediterranean are insufficient to precisely resolve the density structure beneath the Alboran Sea reflected in the complex surface tectonics. Here, we use seismic anisotropy to test a range of suggested structures. We show that SKS splitting observations in the region are best reproduced by mantle flow models that include a continuous, deeply extending, slab structure beneath the Alboran Sea, elongate along the Iberian margin from Granada to Gibraltar, where it curves southward toward the High Atlas. Detached slabs, slabs with spatial gaps, and drip-like features produced results inconsistent with the splitting observations, suggesting delamination processes are unlikely. We also find that the absolute reference frame choice when prescribing surface velocities is crucial due to the opposing surface flow orientation in this region between two end-member absolute plate motion models, Nuvel-1A in the no-net-rotation and HS3 reference frames. Allowing for high net rotations and forcing the associated shear into the upper mantle with a no-slip core-mantle boundary generates a south-directed shear within the Alboran domain that predicts splitting orientations most similar to the patterns observed along Gibraltar,without invoking slab rollback. Slab viscosities of ~250 times that of the upper mantle produce the best match to the observations by providing a balance between radial flow induced by buoyant downwelling and toroidal flow induced by stiff slabs. Splitting orientations and delay times in the Atlas Mountains are best matched with the addition of a stiff continental keel in NW Africa, which deflects flow northward, generating EW fast axes. Our results show that predictions of LPO and SKS splitting orientations from mantle flow models, when compared with observations, can be useful in distinguishing the spatial and depth extent of regional density structure that is otherwise ambiguous.

AAPG Datapages/Search and Discovery Article #90339 ©2019 AAPG Pacific Section Convention 2019, Long Beach, California, April 1-3, 2019