HEAT-FLOW AND REGIONAL UPLIFT AT THE NORTH-EASTERN BORDER OF THE EBRO BASIN, NE SPAIN

The north-eastern border of the Ebro foreland basin is characterized by relatively high elevation, the presence of Neogene to Present volcanism, and a high thermal gradient. These facts suggest that the area is affected not only by the topographic load of the Pyrenees but also by the opening of the Valencia trough. To examine this hypothesis, we first present the results of a surface heat-flow survey carried out in the Osona depression. After selecting those wells which are essentially affected by conductive heat transport and applying topographic and palaeoclimatic corrections, we obtain a corrected surface heat-flow density of 88 +/- 16 mW m(-2). Assuming this value as representative of the regional conductive heat flow, we relate the resulting lithospheric thermal structure to the topographic elevation assuming local isostatic equilibrium and considering a plausible range of crustal thickness and crustal density. The results obtained together with geothermometric and geobarometric data from xenoliths sampled in the nearby Olot volcanic zone indicate a regionally thinned lithosphere about 60-70 km thick. This thinning is related to the Neogene opening of the Valencia trough whose effects extend beyond the rift shoulder (Catalan Coastal Ranges) affecting a broad area of the Ebro basin. Two different uplift mechanisms are proposed as having taken place in the study area: a mechanical uplift related to Eocene compression, followed by a thermal uplift phase associated with asynchronous differential stretching produced during Neogene extension. The combination of these two mechanisms accounts for the observed vertical movements and the migration of the Middle Miocene and Plio-Quaternary volcanic activity.