Insights of Active Extension Within a Collisional Orogen from GNSS (central Betic Cordillera, S Spain)

The coexistence of shortening and extensional tectonic regimes is a common feature in orogenic belts. The westernmost end of the Western Mediterranean is an area undergoing shortening related to the 5 mm/yr NNW-SSE convergence of the Nubia and Eurasia Plates. In this region, the Central Betic Cordillera shows a regional ENE-WSW extension. Here, we present GNSS-derived geodetic data along a 170 km-long transect orthogonal to the main active normal faults of the Central Betic Cordillera. Our data indicate that the total extension rate along the Central Betic Cordillera is 2.0 +/- 0.3 mm/yr. Extension is accommodated in the eastern (0.8 +/- 0.3 mm/yr in the Guadix-Baza Basin) and western (1.3 +/- 0.3 mm/yr in the Granada Basin) parts of the Central Betic Cordillera, while no extension is recorded in the central part of the study area. Moreover, our data permit us to quantify, for the first time, short-term fault slip rates of the Granada Fault System, which is one of the main seismogenic sources of the Iberian Peninsula. We deduce a fault slip rate of similar to 1.3 +/- 0.3 mm/yr for the whole Granada Basin, with 0.9 +/- 0.3 mm/yr being accommodated in the Granada Fault System and 0.4 +/- 0.3 mm/yr being accommodated in the southwestern sector of the Granada Basin, where no active faults have been previously described at the surface. The heterogeneous extension in the Central Betic Cordillera could be accommodated by shallow high-angle normal faults that merge with a detachment at depth. Part of the active extension could be derived from gravitational instability because of underlying over-thickened crust.