SUBSIDENCE HISTORY, CRUSTAL STRUCTURE, AND THERMAL EVOLUTION OF THE VALENCIA TROUGH - A YOUNG EXTENSIONAL BASIN IN THE WESTERN MEDITERRANEAN

The Valencia Trough is an extensional basin that began rifting during the late Oligocene-early Miocene, yet it is located within a region of compression between the European and African plates. Seismic reflection, seismic refraction, well, gravity, and geoid data have been used to determine the relative importance of extensional and compressional processes in controlling the structure and evolution of the trough. Although the trough appears symmetric on bathymetric maps, seismic and gravity and geoid data indicate a fundamental asymmetry in its deep crustal and upper mantle structure. Two dimensional flexural backstripping techniques have been used to estimate the total amount of crustal stretching along a ''transect'' of the trough between the Catalan Coastal Ranges and Mallorca. There is good agreement between the predicted depth to the Moho based on a stretching model and the seismically constrained depth beneath the Catalan margin, but poor agreement beneath Mallorca. The seismic Moho beneath Mallorca can be explained by a model in which the crust was modified by the flexural effects of thrust/fold loading. Geoid data suggest that the differences in the crustal structure between the two margins extend into the upper mantle. The Catalan margin is characterized by a thermal and mechanical boundary, which marks the transition between cold, unstretched lithosphere and hot, thin lithosphere, while Mallorca is located above a broad region of lithospheric thinning. The subsidence history and amount of crustal thinning in the Catalan margin resemble that of typical Atlantic-type nonvolcanic margin. However, a narrow transition zone (< 60 km), the lack of any evidence for a ''conjugate'' margin, and the absence of oceanic crust suggest fundamental differences in the way that extensional processes may occur in convergent plate settings.