LATE CENOZOIC FOUNDERING OF MANTLE LITHOSPHERE IN THE WESTERN CORDILLERA OF SOUTHERN PERU

The relationship between surface uplift and crustal thickening in the building of extreme (>3 km) elevation in the central Andes is debated. End member hypotheses include (1) coupled pure shear crustal thickening and concomitant slow (i.e., km/10’s Myr) surface uplift; and (2) decoupled crustal thickening and rapid (i.e., km/Myr) surface uplift driven by foundering of mantle lithosphere. We tested these with new laser ablation ICP-MS Lu-Hf isotopic analysis of zircons sampled from ignimbrite deposits in Oligocene – Miocene hinterland basins in the Western Cordillera of southern Peru, interpreted in the context of igneous whole rock geochemical data. Pulsed juvenile magmatism is roughly coeval with a transition in hinterland basin deposition, from fluvial to evaporitic-lacustrine, and with recently reported rapid surface uplift between 22 and 10 Ma. Results support a decoupling between crustal thickening and surface uplift in the northern CAP during the late Cenozoic due to the formation and subsequent foundering of dense lower lithosphere via Rayleigh-Taylor instability, after protracted crustal thickening and shortening.