Extension and Lithosphere Dynamics in the Andes inferred from the 2016 Parina (Huarinchancara) Earthquake

The 2016 Mw 6.1 Parina earthquake led to extension of the south Peruvian Andes along a previously unknown normal fault with evidence of 10-20 m of Holocene slip. We used radar geodesy, seismology, geomorphology and field mapping to determine a source model for this event and show that extension at Parina is NE-SW, which is parallel to the direction of shortening in the adjacent sub-Andean lowlands. In addition, we use earthquake source models and GPS data to demonstrate that shortening in the foreland is parallel to gradients in the local topography. Together these observations imply that forces resulting from spatial variations in gravitational potential energy are important in controlling the pattern of deformation in the Andes. We present new calculations of these forces and infer that the average viscosity of the lithosphere in south Peru is >3x10²¹ Pa s, and the weakest faults within the South American foreland have effective coefficients of friction <0.2. Additionally, the onset of extension in the high Andes 5-9 Ma, and the slowing of shortening rates in the crust of the Bolivian Altiplano between 3 and 10 Ma, imply an orogen-wide change in the force balance perpendicular to the mountain belt since the late Miocene. We propose that the onset of shortening on weak, low-angle detachments in the Andean foreland reduced frictional shear tractions on the base of the overriding lithosphere, leading to extension in the highest parts of the range. A generalised version of this model, in which mountain belts weaken their forelands through sedimentation coeval with uplift and foreland basin subsidence, is likely to affect the evolution of the force balance in most mountain belts.