The Alps 2: Controls on crustal subduction and (ultra)high-pressure rock exhumation in Alpine-type orogens

Building on our previous results, we use 2-D upper mantle-scale thermomechanical numerical models to explore key controls on the evolution of Alpine-type orogens and the Alps per se, focusing on (ultra) high-pressure ((U) HP) metamorphic rocks. The models show that UHP rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit. Here we test the sensitivity of the models to surface erosion rate, crustal heat production, plate convergence/divergence rates, geometry of the subducting continental margin, and strength of the retrocontinent. Surface erosion affects crustal exhumation but not early buoyant exhumation. Metamorphic temperatures increase with crustal radioactive heat production. Maximum burial depth prior to exhumation increases with plate convergence rates, but exhumation rates are only weakly dependent on subduction rates. Onset of absolute plate divergence does not trigger exhumation in these models. We conclude that contrasting peak pressures, exhumation rates, and volumes of (U) HP crust exhumed in the Alps orogen primarily reflect along-strike contrasts in the geometry, thermal structure, and/or strength of the subducting microcontinent (Brianconnais) and continental (European margin) crust. The experiments also support the interpretation that the Western Alps (U) HP Internal Crystalline Massifs exhumed as composite, stacked plumes and that these plumes drove local crustal extension during orogen-scale shortening. For weak upper plate retrocrusts, postexhumation retrothrusting forms a retrowedge. Overall, these results are consistent with predictions using the exhumation number (ratio of buoyancy to side traction forces in the conduit), which expresses the combined parameter control of the depth/volume of crustal subduction and the transition to buoyant exhumation.