Foreland migration of orogenic exhumation during nappe stacking: Inferences from a high-resolution thermochronological profile over the Southeast Carpathians

Abstract

The exhumation mechanism of orogens showing one structural vergence, or single-sided orogens, is still not fully understood due to their common transition from periods of high convergence to slab retreat. The overprinting of inherited nappe stacks by subsequent back-arc extension makes it difficult to assess the low amounts of contractional exhumation observed, which are furthermore difficult to interpret due to long residence times in partial retention zones of typical low-temperature thermochronology markers. This is the case in many orogens worldwide and is often observed in the Mediterranean area as well. One notable exception is the Romanian segment of the Carpathian Mountains, where the back-arc extension took place far from the location where the nappe stack formed. To understand the mechanics of contractional exhumation in this single-sided orogen, we generated a new high-resolution low-temperature (apatite fission-track and U-Th/He) thermochronological transect across the Southeast Carpathian nappe stack and its foreland. This new dataset and its correlation with previous exhumation and kinematic studies demonstrate that the gradual accretion of sediments and crustal material was associated with an exhumation that migrated progressively towards the foreland throughout the entire contractional history. The exhumation style changed when the thin-skinned deformation was interrupted by periods of crustal accretion, associated either with the late Eocene onset of continental collision or with the late Miocene locking of the subduction system. The generally accepted moments of onset and termination of slab retreat are not directly visible in available exhumation data of the Southeast Carpathians. Our results are in agreement with modelling studies, which inferred that the formation of single-sided collisional wedges is primarily controlled by the pre-existing rheology of continental plates and is associated with large amounts of continental subduction.