High- and Low-Angle Normal Fault Activity in a Collisional Orogen: The Northeastern Granada Basin (Betic Cordillera)

Abstract

Understanding active tectonics and seismicity in extensional settings requires the analysis of high-angle normal faults (HANFs) and the transfer of deformation at depth. The debate surrounds the role of low-angle normal faults (LANFs) in triggering high magnitude earthquakes. The central Betic Cordillera is an active seismic zone affected by the NNW-SSE Eurasia-Nubia convergence and orthogonal extension. The seismicity and present-day stress determined by earthquake focal mechanisms reveals the activity of a NE-SW extensional system in the shallowest 12 km of the Granada Basin. The structure of the sedimentary infill, as derived by geological field and gravimetric techniques, suggests the formation of a half-graben tilted to the N-NE. Seismologic data suggest the activity of HANFs above 6–7 km depth and a LANF zone around 6–12 km depth, with related earthquakes of up to Mw 4.0 and 20° to 30° fault dips. High-precision leveling lines highlight the importance of the Granada Fault in the system, with average vertical displacement rates of 0.35–1.1 mm/yr. These data suggest creep fault behavior at the surface and increased seismicity at depth. The upper crustal extension in the collisional Betic Cordillera is accommodated by a top-to-the-WSW extensional detachment related to westward motion and rollback in the Gibraltar Arc and the gravitational collapse of the cordillera, in a framework of NNW-SSE shortening. This comprehensive study draws a new scenario that advances understanding of relationships between HANFs and LANFs.