Mapping the Moho in the Iberian Mediterranean Margin by Multicoverage Processing and Merging of Wide-Angle and Near-Vertical Reflection Data

Abstract

Nonnal incidence seismic reflection data have been collected recently in the Iberian Mediterranean margin within the Spanish ESCI programme. The experiments include in-line recordings onshore of marine profiles, and provide coincident small and large-offset multicoverage of complex areas such as the onshore/offshore transition, where classic stacked images of the vertical reflectivity lack resolution and hamper a thorough comprehension of the structure and its lateral evolution. We obtained coherent and enhanced crustal images by developing a wide-angle multichannel processing analogous to the near-vertical conventional one and merging the final stacked and migrated sections.

All the wide-angle sections generated show clear images of the Moho reflectivity. In the Valencia trough, a combined transect documents the lateral evolution of the deep crustal reflectivity across strike of structures. A steady thinning of the crust is revealed in the Iberian margin, from a depth of 32 km beneath the Catalap ranges up to 19 km depth at 60 km seawards. A similar thinning rate is observed on the Balearic flank. In the central part of the trough the Moho is imaged at constant depths around 16-17 km. The lower crustal reflectivity as well as the velocity-depth and gravity results indicate that the thinning is accomodated mainly by the lower crust. Underplating features reported in continental passive Atlantic margins are not supported here by the low velocities of 6.4-6.5 km/s found in the lower crust. In the central part of the BeticsAlboran domain the wide-angle stacked section suggest, in agreement with previous gravity interpretations, that the thin Alboran Sea crust extends beyond the shoreline, up to 10-15 km inland where a Moho jump down of about 3 s TWT marks the southernmost limit of the internal Betics thick crust. The strong stretching rates inferred from the present crustal images in the Iberian Mediterranean margin are to be related with the interaction of extensional processes within a convergent regime between the African and Iberian plates, or even with shear tectonics in the Betics-Alboran domain.