Orogenic plateaus can exist in a delicate balance in which the buoyancy forces due to gravity acting on the high topography and thick crust of the plateau interior are balanced by the compressional forces acting across their forelands. Any shortening or extension within a plateau can indicate a perturbation to this force balance. In this study we present new observations of the kinematics, morphology and slip rates of active normal faults in the South Peruvian Altiplano obtained from field studies, high resolution DEMs, Quaternary dating and remote sensing. We then investigate the implications of this faulting for the forces acting on the Andes. We find that the mountains are extending ~NNE-SSW to ~NE-SW along a normal fault system that cuts obliquely across the Altiplano plateau, which in many places reactivates Miocene age reverse faults. Radiocarbon dating of o set late Quaternary mor...