Testing models of orogen exhumation using zircon (U–Th)/He thermochronology: Insight from the Ligurian Alps, Northern Italy

Abstract

Testing models of orogen exhumation requires precise constraint of the time–temperature paths of the exhumed rocks. The zircon (U–Th)/He (ZHe) thermochronometer has a closure temperature of ~140–210°C, and can provide temporal constraints on the exhumation history of rocks through a thermal range which crucially corresponds the onset of brittle behaviour of an exhuming orogen. We performed ZHe analyses to test the existing contradictory models for the exhumation of the Ligurian Alps. The ZHe ages indicate a very rapid (1.3–6.8mm/yr) and southward migrating exhumation of the orogen between ~32 and 25Ma. These high exhumation rates are unique within the Alpine belt and cannot be reconciled with existing geodynamic models of the evolution of the Ligurian Alps. We propose a model of tectonic denudation via detachment accomplished in the shallowest crust (<5km) as a result of the last orogenic phase of extension.