Formation of orogen-perpendicular thrusts due to mechanical contrasts in the basal décollement in the Central External Sierras (Southern Pyrenees, Spain)

Two series of analogue models are used to explore the effect of ductile-frictional contrasts of the basal décollement on the development of oblique and transverse structures during thin-skinned shortening. These models simulate the evolution of the Central External Sierras (CES; Southern Pyrenees, Spain), which constitute the frontal emerging part of the southernmost Pyrenean thrust. The CES are characterised by the presence of N–S to NW–SE anticlines, perpendicular to the Pyrenean structural trend and developed in the hanging-wall of the thrust system. They detach on unevenly distributed Triassic materials (evaporitic–dolomitic interfingering). The models simulated the effect of adjacent ductile versus frictional décollements during shortening. Model Series A tests the thickness ratio between overburden and the ductile layer, whereas model Series B tests the width (perpendicular to the shortening direction) of frictional décollement. Model results confirms that deformation reaches further in areas detached on a ductile layer whereas above frictional décollement areas, shortening is accommodated by additional uplift and penetrative strain. This replicates the structural style of the CES: higher structural relief of N–S anticlines with regard to orogen-parallel structures, absence of a representative ductile décollement in the core, plunging towards the hinterland and foreland-side closure not thrusted by the South Pyrenean thrust.