MOHO AND LOWER CRUSTAL REFLECTIVITY BENEATH A YOUNG RIFT BASIN - RESULTS FROM A 2-SHIP, WIDE-APERTURE SEISMIC-REFLECTION EXPERIMENT IN THE VALENCIA TROUGH (WESTERN MEDITERRANEAN)

We present new images of the lower crust and Moho beneath the Valencia Trough-a young rift basin in the western Mediterranean. These images were obtained from a two-ship, wide-aperture reflection experiment and show several features not distinguishable on previously available con ventional single-ship reflection profiles. The Moho, which was previou sly only seen intermittently, can now be confidently traced throughout the basin. We have constructed a present-day depth-to-Moho map and es timated the degree of crustal thinning for the whole basin. Crustal th inning is at a maximum in the centre of the basin, where beta values r each 3.15 +/- 0.25. At the margins of the basin the beta value decreas es to 1.5 +/- 0.1. The reflective character of the lower crust and Moh o is different beneath different parts of the basin. We have been able to correlate these differences with the amount of stretching. We ther efore interpret the variations of the observed lower crustal reflectiv ity as having been caused by the most recent (Neogene) stretching even t that opened the Valencia Trough. Along the Iberian margin there is w ell-developed lower crustal reflectivity consisting of 1-2 s two-way t ime (TWT) of 1-4 km long, near-horizontal reflectors underlain by a mo re continuous, although not significantly stronger, reflector interpre ted to be the reflection Moho. Offshore, this lower crustal reflective unit thins rapidly, such that it is undetectable 40-50 km from the co astline where the crust has been stretched by a factor of 1.7 +/- 0.1. As the lower crustal reflectivity becomes undetectable the reflection Moho becomes a robust, continuous event. Where beta exceeds 2.4 +/- 0 .2, however, the Moho is a weak event and difficult to trace. We infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centr e of the basin and enhanced the Moho where extension was moderate. The Balearic margin is somewhat anomalous in that there appears to have b een flexural loading of the crust due to thrusting and folding that oc curred at the same time as extension in the Valencia Trough. The lower crust shows evidence of weak, but locally variable lower crustal refl ectivity. It is possible that the lower crustal reflectivity was prese rved simply because the Moho was flexed downward and so decompression, and hence melting, of the upper mantle was restricted. This suggests that the melting itself rather than the extension is the primary mecha nism of lower crustal modification.