LATERAL VARIATION OF THE CRUST IN THE IBERIAN PENINSULA - NEW EVIDENCE FROM THE BETIC CORDILLERA

New results from a seismic refraction/wide-angle reflection survey carried out in the Betic Cordillera in autumn 1989, contribute to a better picture of its deep structure. One NW-SE profile cuts across the Iberian Massif, the external and internal Betics. The structure of the crust in the Iberian Massif shows characteristics similar to those found in previous experiments. The lower crust is found as a distinct layer, 12 km thick, with an average velocity of 6.8 km . s-1; the Moho is found at about 35 km depth. This structure extends southeastward until a 3-4 km upwelling of the Moho, about 30 km north of the present-day surface boundary between the external and internal Betic units. Further southeast the Moho deepens to 38 km and the lower crust is no longer seismically detected. The absence of differentiated lower crust beneath part of the external Betics may be related with the Mesozoic rifting of the South-Iberian passive margin. Instead, this absence under the internal Betics may be caused by rifting in conjunction with the collisional evolution of the orogen. A WNW-ESE profile lying in the internal Betics shows the presence of a prominent reflector at 10-12 km depth. This seems to be a widespread feature in the internal Betics and may be interpreted as a detachment surface. The Moho is found at 38 km depth rising strongly in the easternmost Betics. Seismic data suggest a thin crust in the offshore area southwest of Malaga, probably containing a massive zone of high-velocity rocks which is also supported by available geophysical and geological data.