Estructura del sector central de la Cuenca de Jaca (Pirineos meridionales)

Resum

The structure of the Jaca basin, a deformed proximal foredeep of the southern Pyrenees, is presented on the basis of two balanced sections across its central part. Tertiary sedimentary rocks which constitute the basin fill are deformed by south-vergent fold and thrust systems, which are younger towards the foreland. In the northern part of the basin, a first system of low-angle thrusts (Larra system) is deformed by a dominant, second-generation system of folds and thrusts. The southern part of the basin is deformed into upright detached folds, and it is bordered by the pyrenean frontal thrust complex of the Externa! Sierras. The Externa! Sierras display folded thrusts at the surface, which are thought to be underlain by a basal, non-emergent thrust that propagates into the foreland. Each thrust system in the cover can be related to a major basement thrust, existing to the north or deduced beneath the basin itself, on the basis of variations of structural elevation and stratigraphic level. The earliest Larra thrust system can be linked to the Lakora basement thrust to the north; the second, dominant system of folds and thrusts to the Gavarnie thrust, and the Externa! Sierras thrust front to the Guarga basement thrust. A thin and relatively homogeneous pre-Tertiary cover beneath the basin sediments leads the Alpine architecture of the Hercynian basement to exert a major control on the large-scale geometry and map pattern of the basin. The preserved structures account for some 30% shortening of the Jaca basin as it was incorporated in the orogen, accomplished from late Lutetian to late Oligocene-earliest Miocene times. As indicated by syntectonic sedimentation, the Larra system developed from the mid-late Lutetian to the Bartonian, the Gavarnie thrust from the Priabonian to the early Oligocene, whereas the Guarga thrust begun to emerge in the External Sierras from the late early Oligocene. Not until this time complete detachment of the basin occured, shortly before it was abandoned as a sedimentary locus and reworked into the foreland Ebro basin.