Beneath Gibraltar

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Science  03 Jan 2003:
Vol. 299, Issue 5603, pp. 19
DOI: 10.1126/science.299.5603.19c

In 1755, a magnitude 8.5 earthquake ripped through the area west of Gibraltar; the event, and its related tsunami and fires, killed tens of thousands in Lisbon, the Gulf of Cadiz area in southwest Spain, and Morocco, and provided key raw material for Voltaire's Candide. Pinning the quake on a particular source has been difficult, however, largely owing to the complex, diffuse tectonics of the African-Eurasian plate boundary in the Gibraltar region.

Gutscher et al. offer new insights in a study of Gibraltar tectonics. Using reflection seismology, they show that a large, chaotic submarine deposit in the Gulf of Cadiz, commonly interpreted as a gravity slide, is more likely to be an accretionary wedge associated with active compressional tectonics. And a tomographic cross section generated from seismic travel times shows a steeply east-dipping body of cold, dense material, stretching from beneath the Gulf of Cadiz, west of Gibraltar, to beneath the Alboran Sea, east of Gibraltar, that they interpret as subducting Mesozoic lithosphere. The Alboran Sea thus is likely to be a back-arc extensional basin related to rollback of the subducting slab, and active subduction in the Gulf of Cadiz to the west constitutes a possible source for the Great Lisbon Earthquake. — SW

Geology30, 1071 (2002).

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