Decoupled cooling and exhumation signals in subduction orogens

Abstract

Reconstructing thermal histories in thrust belts using thermochronometry is a widely used method to infer the age and rates of thrusting, a precondition to understanding the driving mechanisms of orogenesis. Along a thrust sheet, the time and temperature conditions at the switch between heating and cooling retrieved from thermal modeling are commonly interpreted as the onset of thrust-induced exhumation associated with thrustbelt development. In subduction orogens such as the Andes, this interpretation can be challenged by the intrinsic changes in basal heat flow imposed by changes in subduction regimes. We document a case in the northwestern Sierras Pampeanas in the Argentinean Central Andes in which independent constraints on the onset late Cenozoic thrusting derived from structural cross-cutting relationships allow us to explore alternative causes for Cenozoic cooling signals. Located at ~31°S Lat, the Villa Unión-Ischigualasto basin hosts a composite stratigraphic record associated with Triassic rifting developed onto the Paleozoic substratum of western Gondwana and the overlying Meso-Cenozoic foreland basin record. A multi-method approach including apatite fission-track, apatite and zircon (U-Th)/He analyses, vitrinite reflectance and clay mineralogy carried out along three stratigraphic profiles, and inverse thermal modeling reveals the thermal history patterns and allows inferring its triggering mechanisms. Despite an up to 5 km-thick Cenozoic overburden and unlike previously thought, the thermal peak in the basin is not due to Cenozoic burial but occurred in the Triassic, associated with an abnormally high heat flow of up to 90 mWm-2 and less than 2 km of burial, which heated the base of the Triassic strata to ~160°C. Following exhumation, attested by the development of an unconformity between the Triassic and Late-Cretaceous-Cenozoic sequences, Cenozoic re-burial increased the temperature to ~110°C at the base of the Triassic section and only ~50°C 4 km upsection, suggesting a dramatic decrease in the thermal gradient. The onset of Cenozoic cooling from those conditions occurred between ~10 and 8 Ma, approximately 5 My before the onset of thrusting that has been independently documented by exceptionally well preserved stratigraphic-cross-cutting relationships. We argue that the onset of cooling is associated with lithospheric refrigeration following a decrease in the angle of subduction of the Nazca slab, leading to the eastward displacement of the asthenospheric wedge beneath the South American plate. Our study places time and temperature constraints on an idea that has been previously discussed in the region and calls for a careful interpretation of exhumation signals in thrustbelts inferred from thermochronology only.