ALBERT

Description: New tomographic images of the upper mantle beneath the westernmost Mediterranean suggest that the evolution of the region experienced two subduction-related episodes. First subduction of oceanic and/or extended continental lithosphere, now located mainly beneath the Betics at depths greater than 400 km, took place on a NW–SE oriented subduction zone. This was followed by a slab-tear process that initiated in the east and propagated to the west, leading to westward slab rollback and possibly lower crustal delamination. The current position of the slab tear is located approximately at 4°W, and to the west of this location the subducted lithosphere is still attached to the surface along the Gibraltar Arc. Our new P-wave velocity model is able to image the attached subducted lithosphere as a narrow high-velocity body extending to shallow depths, coinciding with the region of maximum curvature of the Gibraltar Arc, the occurrence of intermediate-depth earthquakes, and anomalously thick crust. This thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained P-wave tomographic model. We show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structure.

Description: We have determined the three-dimensional P-wave velocity structure of the crust and uppermost mantle beneath the Iberia–Africa collision zone using local earthquake tomography. We have inverted arrival times of first-arriving P phases listed in the bulletins of the Instituto Geográfico Nacional and phases picked by us on permanent stations of other regional networks and on temporary broadband stations deployed in the frame of the TOPO-IBERIA project. In total we have used 38,624 P-wave arrival times from 2362 local events recorded at 120 seismic stations. The most remarkable result is the imaging of a large high velocity body following approximately the western Alboran coastline, with a horizontal dimension of at least 200 km and extending in depth from the surface down to 24 km. This body, not imaged previously with this extent using seismic tomography, coincides with surface exposures of subcontinental mantle (peridotites) in Iberia and Africa and with a belt of positive gravity anomalies. We have also found a marked contrast in the seismic velocities of the middle and lower crust of the Alboran basin, coinciding with the location of the Trans-Alboran Shear Zone. We attribute this contrast to widespread magmatic intrusions in the eastern part of the basin, resulting in higher P-wave velocities than in the west. This contrast would also explain the different orientation of the Trans-Alboran Shear zone with respect to the surface features and faults in the Alboran basin. We also image thick crust beneath the Betics and Rif, accompanied by downgoing lithosphere of the Iberian foreland and Gulf of Cadiz beneath the Betic–Rif–Alboran system.