SEISMIC REFRACTION CONSTRAINTS ON THE GEOMETRY OF THE RONDA PERIDOTITIC MASSIF (BETIC CORDILLERA, SPAIN)

Large ultramafic masses known as the Ronda peridotites outcrop in the westernmost part of the Internal Zone of the Betic Cordillera in southern Spain. The Bouguer anomaly gravity map shows a pronounced positive anomaly (up to +110 mGal), although shifted eastwards with respect to the outcropping peridotites. Three seismic refraction profiles, one of them partly reversed, crossing the peridotites and an area displaying a gravity high were surveyed in 1977 and 1981. The length of the profiles ranges from 50 to 70 km. The "normal" crustal structure of the Internal Betics was derived from another 150 km long Almeria-Málaga profile along the south coast of Spain where an average crustal thickness of 25 km has been found. A P-wave interpretation of the seismic data indicates pronounced lateral variations in the upper crust based on first arrivals and one later seismic phase. The delay time of Pg arrivals corroborates the large offshore Cenozoic sediment thickness (1-3 km) close to the coast. Ray-tracing modelling of high apparent velocity first arrivals and secondary seismic phases, interpreted as reflections, shows the presence of a high-velocity (7.0-7.2 km/s) body beneath the gravity high. Its top is well defined whereas the bottom remains loosely constrained. It is suggested that the peridotites outcropping further north are slices of this ultramafic body. The Moho has been located at 15-20 km in the Malaga-Estepona area, the precise depth depending on the alternatives chosen when modelling the seismic data. © 1990.