SEISMIC ANISOTROPY AND MANTLE CREEP IN YOUNG OROGENS

We discuss the origin of anisotropy in the crust and uppermost mantle in terms of processes that are controlled by temperature, rheology, and deformation. We disregard anisotropy within sedimentary basins and instead concentrate on anisotropy owing to the lattice preferred orientation (LPO) of anisotropic minerals. We focus on young mountain belts that have thick crustal roots and high geothermal gradients. We compare the orientation of this Pn anisotropy with the direction of compressive stress, which nearly always shows a mountain-perpendicular component in the upper crust. From the discrepancy of directions between the fast axis of mantle anisotropy and compressive crustal stresses, we therefore postulate the existence of a decoupling layer. This layer is generally found in the middle or lower crust in all areas with high geothermal gradients and especially in young and warm mountain belts. A decoupling layer corresponds to a low-viscosity layer capable of creep owing to tectonic stress. Tectonic escape or “lateral extrusion” was first suggested for southern China by Molnar & Tapponier (1975). It is today considered to be a lithospheric process and not limited to the crust.