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 car ried out in the Betic Cordillera in autumn 1989, contribute to a bette r picture of its deep structure. One NW-SE profile cuts across the Ibe rian Massif, the external and internal Betics. The structure of the cr ust 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 f ound 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 s urface 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 beneat h 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 colli sional evolution of the orogen. A WNW-ESE profile lying in the interna l 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 de pth rising strongly in the easternmost Betics. Seismic data suggest a thin crust in the offshore area southwest of Malaga, probably containi ng a massive zone of high-velocity rocks which is also supported by av ailable geophysical and geological data.