Rapid Eocene Exhumation in the Southern Patagonian Andes and Implications for Regional Tectonics

Abstract:

Thermal histories fossilized in ancient Cordilleran orogens preserve records of cyclic as well irregular convergent margin processes. Deconvolving the mechanisms behind these thermal signals is particularly important for understanding orogenesis at the ends of plate boundaries where changes in plate convergence and organization modify and overprint the thermal record of the upper crust. New thermal history modeling of nine samples from the southern end of the Patagonian Andes incorporates data from the zircon fission track (ZFT) and zircon (U-Th)/He (ZHe) systems to resolve an apparent synchronous phase of rapid cooling between 51°S and 53.5°S. Cooling initiation between ~ 50 Ma and ~45 Ma is observed in all nine samples collected in the retroarc region. Although the timing of rapid cooling is synchronous at these latitudes, the modelled thermal histories support an increase of the magnitude of exhumation progressively to the south. The absence of longitudinal/orogen-perpendicular cooling trends suggests that this phase of rock cooling is not driven by erosion of an eastward propagating thrust belt, as previously considered. Instead, cooling is coincident with the development of a regional unconformity and coeval plateau magmatism, and may suggest that rock uplift and erosion is linked to uplift above an asthenospheric window during subduction of the Aluk-Farallon spreading ridge. The thermal histories constrained by bedrock samples from the hinterland region have important implications that can be used to refine models of sediment routing, deformation, and regional paleogeography including orocline development and the proposed early opening of the Drake Passage.