Retroarc deformation and exhumation near the end of the Andes, southern Patagonia

Abstract

The southern Patagonian Andes constitute the narrow, high-latitude end of the Andean orogen belt in South America, where inherited basin paleogeography, subduction processes, retroarc crustal thickening, and late Cenozoic glaciation have collectively influenced their unusual tectonic and physiographic evolution. New zircon and apatite (U–Th)/He thermochronology from the Patagonian Andes between 50°30′ and 51°30′S suggest concentrated exhumation across the retroarc (leeward) side of the orogen since early Miocene time. Zircon (U–Th)/He (ZHe) ages range from 44 to 10 Ma; oldest ages are recorded in the Patagonian batholith and along the far eastern frontal foreland monocline. Regionally-uniform ZHe ages between 22 and 18 Ma, located across a ∼75 km wide-zone of the Patagonian retroarc thrust-belt indicate widespread early Miocene cooling through the ZHe partial retention zone. Mesozoic sedimentary and volcanic rocks in this region have been exhumed from at least 5 to 6 km depths. Early Miocene denudation of the thrust-belt, deformation, and increased foreland sedimentation rates coincided with opening of the Scotia Sea, suggesting a causal response of the foreland to changes in plate dynamics. The apatite (U–Th)/He (AHe) ages from a similar region range from 11 to 3 Ma; the youngest ages (6–3 Ma) are spatially clustered within the more deeply-exhumed central thrust domain. We interpret these AHe ages to record >1–2 km of erosional denudation associated with late Cenozoic glaciation and fluvioglacial processes in Patagonia beginning ca. 7 Ma.

Zircon results indicate that since ca. 22 Ma, long-term exhumation rates have been highest across the western and central thrust domains (0.22–0.37 mm yr−1), and significantly lower along the eastern thrust front (0.10–0.17 mm yr−1). Since ∼7 Ma, apatite results from these same regions suggest comparable and slightly higher denudation rates (0.14–0.46 mm yr−1), particularly within the eastern thrust domain, consistent with efficient erosional processes acting in the retroarc region. These results from the Patagonian retroarc region, particularly the predominance of <7–4 Ma AHe ages, provide a new understanding for regional orogenic erosion models that are largely based on existing data from the windward regions. We suggest that retroarc denudation was enhanced by widespread Miocene structural uplift and unroofing of the fine-grained siliciclastic rocks of the marine Cretaceous Magallanes/Austral foreland basin. Furthermore, its location at the southern tip of the orogen, may have allowed additional moisture to reach the leeward side, leading to sustained late Cenozoic erosional denudation.