Linking Cenozoic (U)HP exhumation to orogen-scale deformation in the western Alps

We present a series of high-resolution 2D numerical geodynamic model experiments designed to address the problem of Cenozoic (ultra)high-pressure ((U)HP) rock exhumation in the western Alps. The model geometry is based on reconstructions of pre-Alpine paleogeography and includes an upper plate, ‘Adria’, with a pre-existing, accreted Austroalpine domain, a Piemont-Liguria Ocean, and a lower plate consisting of a Brianconnais microcontinent and a rifted margin (Valais domain and European margin). We begin by describing a model that accounts for the first-order stages of western Alpine evolution, from deep subduction of oceanic and continental rocks beneath the Adriatic upper plate and subsequent rapid exhumation by coeval thrusting and shallow synorogenic extension, to backthrusting of the (U)HP units during orogen-scale bivergent thrusting. We then explore how variations in certain model parameters (e.g., crustal rheology) give rise to different tectonic styles of the model system that may explain some of the along-strike variability evident in cross-sections of the western Alps. The models illustrate the important interplay between buoyancy-driven flow of material from within the subduction channel and shallow-level shortening and normal-sense shear. Synorogenic extension within the overall contractional orogen, recognized as playing a critical role in exhuming western Alpine (U)HP rocks, may have actually been driven by the buoyant return of (U)HP crust from below, rather than changing boundary conditions (e.g., slab-rollback). This mechanism requires that the buoyancy of the exhuming crust is sufficient to disrupt the overlying orogenic wedge, implying that in order to exhume (U)HP terranes must achieve a minimum critical size. This minimum requirement suggests that small (U)HP slices or fragments may be part of a larger composite body aggregated from different levels of the subduction channel, and/or they were buoyed from below by still buried low density (U)HP crust. Finally, although slab-breakoff likely played a role in the geodynamics of the western Alps, it is not required for (U)HP exhumation.