Crustal structure under the central and eastern part of the Betic Cordillera

Abstract

In 1974 and 1975 a network of crustal seismic refraction profiles across the Betic Cordillera was established with shotpoints at sea off the Spanish coast. The longest profile with a length of 440 km runs parallel to the general strike through the centre of the Betic Cordillera. Two other profiles lie south of the main profile closer to the coast and perpendicular to the east–west strike, respectively. The interpretation of P waves indicates pronounced lateral variations of the structure and thickness of the crust and upper mantle. Two different crustal blocks seem to exist on either side of the north-east trending Palo-mares and Alhama de Murcia fault system in the eastern Betic Cordillera. East of this fault system the crust is only 23 km thick with a shallow high velocity layer of 6.9 km s−1. West of this line in the central and southern part of the Betic Cordillera the crust–mantle boundary lies at depths of 39 km and 24 km, respectively. In this area a velocity of 6 km s−1 is reached at about 3 km depth. A layer of reduced P-wave velocity follows between 7 and 12 km depth with a velocity of 5.4 km s−1. At its lower boundary the velocity increases to 6.2 km s−1 and reaches values between 6.6 and 6.7 km s−1 at a depth of 18 km under the south coast and 24 km under the centre of the Betic Codillera east of Granada. A thin lid of about 6 km thickness and a Pn velocity of 8.1 km s−1 lies on top of a rather thick layer with the reduced velocity of 7.8 km s−1. At a depth of 63 km the velocity increases again to 8.3 km s−1. Deep-reaching structural differences mark the east-west trending boundary between the internal and external zones of the central Betic Cordillera.