Recent and present-day stresses in the Granada Basin (Betic Cordilleras):: Example of a late Miocene-present-day extensional basin in a convergent plate boundary

The diffuse convergent boundary between the Eurasian and African plates in the western Mediterranean is associated with a seismicity zone more than 300 km wide. Although the two plates are converging NW-SE, the Betic and Rif Cordilleras contain extensional structures that have been active since the Miocene. The extensional tectonics in the region, which occurred simultaneously with the uplift of the cordillera, have been analyzed in the southeastern sector of the late Miocene to recent Granada Basin, using earthquake focal mechanisms, the determination of paleostresses from the study of the orientation and kinematics of microfaults, and the study of the major structures. Both the geological surface data and the focal mechanisms indicate present-day regional conditions of NE-SW extension, with triaxial to prolate stress ellipsoids. However, the stress field is heterogeneous, with local variations in stress over time, with different stresses sometimes even acting simultaneously in adjacent areas. The most frequent changes consist of pluridirectional or NE-SW extension, favored by the prolate character of the stress ellipsoids, and NW-SE subhorizontal compression, favored by the regional tectonic setting. Strike-slip faults are scarce even though they are the most likely structure to be expected in a region with SW-NE extension and NW-SE compression. Seismicity is concentrated in the upper crust and may correspond to the activity of low- to high-angle normal faults similar to the surface faults, although they can not be correlated with them. The lower cutoff of this seismicity probably coincides with the 300 degrees C isotherm and suggests a low thermal gradient for the region. Present-day regional stresses have sigma(1) vertical at the surface but in depth plunge toward the SW.