KINEMATICS AND GEOMETRY OF EARLY ALPINE, BASEMENT-INVOLVED FOLDS, SW PELVOUX MASSIF, SE FRANCE

The Pelvoux basement massif lies within the NW corner of the external are of the western Alps. In the Late Cretaceous-Early Eocene, this bas ement block was uplifted 1-3 km while the basement-cover interface and overlying thin Mesozoic cover were folded (on the south side) by SSE- vergent and (on the west side) by WSW-vergent fold systems under anchi zone metamorphic conditions with subordinate thrusts on overturned lim bs. These recumbent to semi-recumbent basement-involved folds face out ward around the SW corner of the massif and die out rapidly away from the massif. At the SW corner of the massif, fold interference in Mesoz oic strata shows that SSE-vergent folding commenced before WSW-vergent folding. A single cleavage is present in the Mesozoic carbonates, whi ch transects most folds and is most intense at the SW corner. No equiv alent cleavage has been detected in underlying basement lithologies. I t is proposed that uplift and associated folding of basement and cover was achieved in broad, inward dipping shear zones. Similar structures on the N and NE sides of the Pelvoux massif suggest that the whole ma ssif was uplifted as a basement block pop-up. Two possible origins for this three dimensional basement uplift are proposed. Either the regio nal contractional direction rotated from SSE-NNW to SW-NE, or the west ern and southern slopes of the Pelvoux paleogeographic high were obliq uely folded in a N-S contractional regime. The latter history is favou red here. This N-S contraction could have been generated by a regional sinistral transpression along the pre-existing NE-SW fault system of the Tethyan passive margin which may, in turn, have been related to th e sinistral migration of the Iberian plate south of the European plate . The uplifted Pelvoux massif then formed a positive feature that (a) formed a paleogeographic high in the Tertiary foreland basin and (b) m ay have acted as an obstruction in the path of the advancing late Alpi ne deformation front in late Oligocene times and hence influenced the evolution of the external Alpine arc.