Gravity anomaly map of the Pyrenean region and a comparison of the deep geological structure of the western and eastern Pyrenees

A gravity anomaly map, based on over 13,000 observations, accurately delineates the major features of the Pyrenean region. An extensive gravity anomaly low in the south of the region appears to result from crustal thickening associated with the Pyrenean mountain building episode. Several large positive gravity anomalies are apparent in the North Pyrenean Zone in the centre and west of the mountain chain, but are absent in the east. The interpretation of a series of gravity anomaly profiles along the Pyrenees has been controlled by existing geophysical data. The interpretations confirm the cause of the southern gravity anomaly low as thickened Iberian crust and demonstrate, as shown by previous seismic investigations, that the transition to much thinner European crust is quite abrupt. The sources of the positive anomalies in the North Pyrenean Zone are bodies in the upper crust with densities too high to be explained in terms of crustal materials. These are interpreted as segments of upper mantle that originated in transtensional N-S separation across the North Pyrenean Fault and were emplaced at shallow levels by thrusting during Pyrenean orogenesis. This is the first time that evidence has been presented for the presence of such mantle bodies in the Pyrenees. It is suggested that their absence in the east is caused by less extension between Iberia and Europe in this region during the transform motion between them prior to orogenesis, allowing only limited ascent of deep material. The greater extension in the west would have allowed the ascent of upper mantle into the space between Iberia and Europe, which would have been further elevated during the thrusting associated with Pyrenean orogenesis.