La depresión submarina de Guaracayal, estado Sucre, Venezuela: Una barrera para la propagación de la ruptura cosísmica a lo largo de la falla de el pilar
Résumé
The underwater Guaracayal depression, in the Cariaco gulf, Sucre state, Venezuela, was first mapped from a bathymetric survey performed in the 80's. A high resolution reflection seismic survey, acquired with the Venezuelan R/V Guaiquerí II in the Cariaco gulf in early 2006, reveals that this depression is an active pull-apart basin on the submarine trace of the right-lateral strike-slip El Pilar fault, based on its geometry and the freshness of the sea-bottom fault scarps. The Guaracayal basin floor is ~15m deeper than the 80m deep flat bottom of the central sector of the gulf. This small pull-apart basin, which is about 8km long in east-west direction and 2km across, lies inside a dextral transtensive overlap of the El Pilar fault submarine trace, which cuts in two parts what was proposed before as a unique fault segment extending between Cumaná and Casanay-Guarapiche. This 2km-apart stepover may be a sufficient barrier to fault rupture propagation, as evidenced by contemporary and historical seismicity. Accordingly, this geometry may explain by itself the seismogenic segmentation exhibited by the 80km long fault section extending between Cumaná and Casanay during at least the last two seismic cycles in west-east direction (1797-1684 and 1929-1997). However, this behaviour does not preclude that the whole Cumaná-Casanay segment may break at once in the future.