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

Abstract

AbstractUnderstanding 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.