Recently active contractile deformation in the forearc of southern Peru

Geomorphic and structural features of southern Peru (14–18°S) provide strong evidence for distributed crustal deformation along range-sub-parallel contractile structures. We use in situ produced cosmogenic radionuclides, in conjunction with field and remote mapping, to determine the ages of geomorphic features and find (1) ancient surfaces (>1 Ma) preserved as a result of very low surface erosion rates, (2) young (∼30 ka) low-relief pediment surfaces developed during recent landscape modifications, (3) active tectonic structures accommodating compressional stresses, and (4) Pleistocene river incision rates of ∼0.3 mm/yr consistent with longer-term rates. In this region of southern Peru, the steep western wedge (trench to arc area) of the Andean margin presently maintains the high topography of the Altiplano through a combination of uplift and contractile deformation along steep east-dipping faults and isostatic responses to the focused removal of large amounts of crustal material through canyon incision.