Apparent western termination of the North Pyrenean Fault and tectonostratigraphic units of the western north Pyrenees, France and Spain

Abstract

The surface geology of the North Pyrenees, southern France, is markedly different east and west of longitude 0°25′W (a line just west of Lourdes). East of this longitude the North Pyrenean fault (NPF), the major structure in the North Pyrenees, is clearly identifiable, and this fault and other faults in the North Pyrenean zone are subvertical. West of longitude 0°25′W the NPF is not identifiable at the earth's surface, suggesting to some that the NPF is not continuous and is not a regionally significant feature and that low‐angle as well as high‐angle faults are present in the fault zone. While east of Lourdes there is both a relatively continuous belt of Mesozoic metamorphic rock and a geophysically detected 15‐km step in depth to the Moho, neither is clearly recognized west of Lourdes, again bringing into question the continuity of the NPF. However, in order to accommodate opening of the Bay of Biscay, as much as 400 km of translation along a continuous NPF or North Pyrenean fault zone (NPFZ) is required. We have compiled a map covering a 100‐km‐long area in the western Pyrenees. The compilaation is based on our mapping and published maps by others. The compilation and our field work provide the data to delineate four new tectonostratigraphic units. These units are coherent stratigraphic sequences characterized by marked differences in stratigraphic thickness and depositional history. Each unit is bounded by regional faults that delineate four tectonic sheets. The principal stratigraphic differences between the four tectonostratigraphic units were produced during deposition in Cretaceous basins along the northern Pyrenees. The regional bounding faults are reverse and were formed during the main phase of the Pyrenean orogeny, that is, during Late Cretaceous and Tertiary times. Some are reactivated basin‐boundary faults and some of these are reactivated faults of Cretaceous or older age. In the western Pyrenees the NPF, which was an active left‐slip fault during the Late Jurassic and Early Cretaceous, is cut by these faults and is covered by tectonic sheets. These sheets, formed during the Paleogene, account for the apparent termination of the NPF near longitude 0°25′W and obscure the step in the Moho.