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Calvert, A., Sandvol, E., Seber, D., Barazangi, M., Roecker, S., Mourabit, T., Vidal, F., Alguacil, G., and Jabour, N.

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

Journal of Geophysical Research, 105, 10871-10898, 2000.


A number of different geodynamic models have been proposed to explain the extension that occurred during the Miocene in the Alboran Sea region of the western Mediterranean despite the continued convergence and shortening of northern Africa and southern Iberia. In an effort to provide additional geophysical constraints on these models, we performed a local, regional, and teleseismic tomographic travel time inversion for the lithospheric and upper mantle velocity structure and earthquake locations beneath the Alboran region in an area of 800 x 800 km^2. We picked P and S arrival times from digital and analog seismograms recorded by 96 seismic stations in Morocco and Spain between 1989 and 1996 and combined them with arrivals carefully selected from local and global catalogs (1964-1998) to generate a starting data set containing over 100,000 arrival times. Our results indicate that a N-S line of intermediate depth earthquakes extending from crustal depths significantly inland from the southern Iberian coat to depths of over 100 km beneath the center of the Alboran Sea coincided with a W to E transition from high to low velocities imaged in the uppermost mantle. A high-velocity body, striking approximately NE-SW, is imaged to dip southeastwards from lithospheric depths beneath the low-velocity region to depths of ~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 near an isolated cluster of deep earthquakes. In addition to standard tomographic methods of error assessment, the effects of systematic and random errors were assessed using block shifting and bootstrap resampling techniques, respectively. We interpret the upper mantle high-velocity anomalies as regions of colder mantle that originate from lithospheric depths. These observations, when combined with results from other studies, suggest that delamination of a continental lithosphere played an important role in the Neogene and Quaternary evolution of the region.

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Copyright Statement

An edited version of this paper was published by AGU. Copyright 2000 American Geophysical Union.

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Key Figures and Captions

Figure 15. Evolutionary model showing map view and two cross sections modified from working model proposed by Platt and Vissers (1989) (compare with convective removal model shown in Figure 14, see reference) to explain results from this tomography study and Platt et al.'s (1998) more recent results. Thick dashed lines show suggested position for connection of delaminated lithosphere to surface lithosphere. GB, Guadalquivir Basin. Circles show locations of present subcrustal seismicity. Not the body shown in the upper mantle in the "Present" panel of BB' is attached to Iberia to the west of the cross section. Patterns are the same as those shown in Figures 2 and 14 (see reference). The crooked arrows shown in the Strait of Gibraltar mark the relative motion of Africa to Iberia remaining before present.

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