Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

A number of different geodynamic models have been proposed to explain the e xtension that occurred during the Miocene in the Alboran Sea region of the western Mediterranean despite the continued convergence and shortening of n orthern Africa and southern Iberia. In an effort to provide additional geop hysical constraints on these models, we performed a local, regional, and te leseismic tomographic travel time inversion for the lithospheric and upper mantle velocity structure and earthquake locations beneath the Alboran regi on in an area of 800 x 800 km(2). We picked P and S arrival times from digi tal and analog seismograms recorded by 96 seismic stations in Morocco and S pain between 1989 and 1996 and combined them with arrivals carefully select ed from local and global catalogs (1964-1998) to generate a starting data s et containing over 100,000 arrival times. Our results indicate that a N-S l ine of intermediate-depth earthquakes extending from crustal depths signifi cantly inland from the southern Iberian coast to depths of over 100 km bene ath the center of the Alboran Sea coincides with a W to E transition from h igh to low velocities imaged in the uppermost mantle. A high-velocity body, striking approximately NE-SW, is imaged to dip southeastwards from lithosp heric depths beneath the low-velocity region to depths of similar to 350 km . Between 350 and 500 km the imaged velocity anomalies become more diffuse. However, pronounced high-velocity anomalies are again imaged at 600 km nea r an isolated cluster of deep earthquakes. In addition to standard tomograp hic methods of error assessment, the effects of systematic and random error s were assessed using block shifting and bootstrap resampling techniques, r espectively. We interpret the upper mantle high-velocity anomalies as regio ns of colder mantle that originate from lithospheric depths. These observat ions, when combined with results from other studies, suggest that delaminat ion of a continental lithosphere played an important role in the Neogene an d Quaternary evolution of the region.