Seismic anisotropy from the Variscan core of Iberia to the Western African Craton: New constrains on upper mantle flow at regional scales

- Anisotropy beneath Iberia and N Morocco related to mantle flow in the asthenosphere.
- FPDs consistent with global mantle flow models outside the Gibraltar Arc.
- The fast velocity slab beneath the Gibraltar Arc deflects mantle flow around it.
- Possible presence of edge-driven convective cells beneath the High Atlas and SW Portugal.

The regional mantle flow beneath the westernmost Mediterranean basin and its transition to the Atlantic domain is addressed by inspecting the anisotropic properties of the mantle. More than 100 new sites, from the Variscan core of Iberia to the northern rim of the Western African Craton, are now investigated using the data provided by different temporary and permanent broad-band seismic arrays. Our main objective is to provide a larger regional framework to the results recently presented along the Gibraltar Arc in order to check the validity of the different geodynamic interpretations proposed so far.The significant variations in the retrieved anisotropic parameters suggest that different processes must be invoked to explain the origin of the observed anisotropy. Beneath the Variscan units of the Central Iberian Massif the new results show a moderate amount of anisotropy with fast polarization directions (FPD) oriented close to E-W. Those results can only be explained in terms of global mantle flow if models accounting for contributions from surface plate motion, net lithosphere rotation and density variations are taken into consideration. One of the major results presented is the significant number of good quality data without evidence of anisotropy (“nulls”) observed beneath permanent stations in southern Portugal. Those “nulls” can be explained by the presence of a predominantly vertical mantle flow associated to large variations in the lithospheric thickness. Beneath the Gibraltar Arc the FPD show a spectacular rotation, evidenced by the results presented by Díaz et al. (2010) and Miller et al. (2013). Those results are reviewed here taking also into consideration the geodynamic modeling presented recently by Alpert et al. (2013) and other geophysical and geodetic results. Further South, the analysis of new broad-band stations installed in the Moroccan Meseta and the High Atlas show a small degree on anisotropy and a large number of “null” events, pointing again to the presence of vertical flow in the mantle.The results favor an asthenospheric origin related to present-day mantle flow for the anisotropy observed from the Variscan core of Iberia to the northern rim of the West African Craton. This flow is deflected around the high velocity slab beneath the Gibraltar Arc and seems affected locally by vertical flow associated to edge-driven convective cells. The presence of significant backazimuthal variations in the anisotropic parameters retrieved from single events suggests that a second order contribution from an anisotropic layer within the lithosphere may also exist.