3-D Modeling of Differential Exhumation of Ultrahigh-Pressure Metamorphic Rocks Driven by Increasing Plate Divergence

Petrological and seismic constraints suggest differential sampling depth for ultrahigh pressure (UHP) metamorphic rocks exposed in different segments of the fossil subduction zone of the Western Alps. However, the mechanisms for the observed differential exhumation remain to be understood. Here, we account for the continental margin subduction within double subduction systems coupled with three-dimensional (3-D) geodynamic models to investigate the potential for this phenomenon. We tested two end-member scenarios of upper-plate motion, including Adria counterclockwise rotation and divergent motion away from the trench. Results show that both scenarios can produce a unidirectional increase in the metamorphic peak of exhumed UHP rocks along the orogen strike. Only in the simulations where the counterclockwise rotation of the upper plate acts as the primary driver of exhumation did the resulting characteristics of the deep structure fit with those observed in the Western Alps. They include the presence of an exhumed mantle wedge beneath the southern UHP domes and its absence beneath the northern UHP domes. Our results can be exported to other subduction zones where kinematic constraints suggest a component of upper-plate rotation, such as eastern Papua New Guinea, and to older subduction zones where the kinematics is poorly assessed.