Abstract

Fission track (FT) analysis is applied to assess the Mesozoic and Cenozoic thermal and denudational history of the Patagonian Andes between 44° and 51°S and the geologic and geomorphic response of late Cenozoic subduction of the active Chile rise mid-oceanic spreading center on the overriding plate. Seventy-two FT ages from 43 samples are presented. Zircon FT ages indicate fast post intrusion cooling of Cretaceous parts of the Patagonian batholith and previously unreported Miocene magmatic rocks south of 48°S. Metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early Carboniferous and Late Permian. Apatite FT data reveal initiation of accelerated cooling and denudation at ca. 30 Ma at the western margin of southern continental South America followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca. 12-8 Ma at the position of the present-day main topographic divide. This migration is proposed to be related to either coeval eastward migration of the retroarc deformation, the effects of subduction erosion in the overriding plate at the Peru-Chile trench or less likely, shallowing of the angle of subduction. East of the divide, <3 km of denudation has occurred since the Late Cretaceous. Enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca. 28-26 Ma that triggered orographically enhanced precipitation on the west side of the Patagonian Andes allowing increased erosion by fluvial incision and mass transport processes. The actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation.