Structural geometry in the eastern Pyrenees and western Gulf of Lion (Western Mediterranean)

We present new seismic data from the Gulf of Lion located east of the Pyren ees on the continental shelf of the Mediterranean Sea. The deep penetration LISA (Ligurian-Sardinia Sea) seismic lines, the shots of the LISA cruise r ecorded on land, and the high definition ELF seismic sections allow us to p resent a complete picture of the tectonics in this area from the surface to the Motto level, and also to document late Miocene-early Pliocene extensio nal tectonics in the area. Previous studies show a prominent thinning of th e crust observed from the Pyrenees towards the Gulf of Lion. The Moho depth varies from 48 km beneath the Axial Range crust (thickened during the Pyre nean Eocene Orogeny) to 21 km below the Catalan Basin in the Gulf of Lion. This crustal thinning occurred mainly during the early Miocene extension of the Mediterranean Sea. Balanced reconstructed geological sections derived from reflection and refraction seismic data allow us to evaluate the stretc hing factors at the crustal level. A maximum extension of 25 km is computed for the Catalan Basin area. This extension is related to detachments that penetrate the crust as deep as I I km to the base of the brittle crust. The se intra basement detachments have been confused in the past with the Paleo zoic acoustic basement. The detachments show a clear listric shape and the geometry of horst and grabens can be explained by a hanging wail and footwa ll configuration with isostatic rebound of the footwall. The uplift in the Eastern Pyrenees (Alberes and Canigou Massifs), on the other hand, is relat ed to the late Miocene-early Pliocene extension we mapped in the area. Thes e elevated features, probably formed by isostatic rebound, are surrounded b y deep basins such as the Roussillon and El Emporda depressions. A 1.7 km u plift during the late Miocene-early Pliocene is computed in the offshore pa rt of the Alb res Massif. The cause of this Late Miocene-early Pliocene ext ension is not well explained although an uplift related to the Messinian de siccation or a thermal anomaly in the mantle have been proposed. The relati onship between the Eastern Pyrenees and the Gulf of Lion is governed by NE- SW transfer faults. These faults represent the southwestern limits of the G ulf of Lion basins. Although much seismicity is recorded in the Eastern Pyr enees, we do not see evidence of present tectonics in the Gulf of Lion as t he extensional faults were active only until the early Pliocene. Therefore, the present stress is probably compressional, as shown by the focal mechan isms of the earthquakes, although the trends of compressional vectors are d ivergent in Spain and France. (C) 2001 Elsevier Science Ltd. All rights res erved.