Subduction initiation and subsequent burial-exhumation of (ultra)high-pressure rock

Subduction zones are one of the main features of plate tectonics, they are essential for geochemical cycling and are often a key player during mountain building. However, several processes related to subduction zones remain elusive, such as the initiation of subduction or the exhumation of (ultra)high-pressure rocks.

Collision orogens, such as the European Alps or Himalayas, provide valuable insight into long-term subduction zone processes and the larger geodynamic cycles of plate extension and subsequent convergence. For the Alps, geological reconstructions suggest a horizontally forced subduction initiation caused by the onset of convergence between the Adriatic and European plates. During Alpine orogeny, the Piemont-Liguria basin and the European passive magma-poor margin (including the Briançonnais domain) were subducted below Adria. The petrological rock record indicates burial and subsequent exhumation of both continental and oceanic crustal rocks that were exposed to (ultra)high-pressure metamorphic conditions during their Alpine burial-exhumation cycle. Moreover, estimates of exhumation velocities yield magnitudes in the range of several mm/yr to several cm/yr. However, published estimates of exhumation velocities, ages of peak metamorphic conditions and estimates for peak pressure and peak temperature vary partly significantly, even for the same tectonic unit. Consequently, many, partly significantly, contrasting conceptual models for subduction initiation (convergence versus buoyancy driven) or rock exhumation (channel-flow, diapirism, episodic regional extension, erosion etc.) have been proposed for the Alps. 

Complementary to the data-driven approach, mathematical models of the lithosphere and upper mantle system are useful tools to investigate geodynamic processes. These mathematical models integrate observational and experimental data with the fundamental laws of physics (e.g. conservation of mass, momentum and energy) and are useful to test conceptual models of subduction initiation and rock exhumation. Here, we present numerical solutions of two-dimensional petrological-thermo-mechanical models. The initial model configuration consists of an isostatically and thermally equilibrated lithosphere, which includes mechanical heterogeneities in the form of elliptical regions with different effective viscosity. We model a continuous geodynamic cycle of subsequent extension, no far-field deformation and convergence. During extension, the continental crust is necked, separated and mantle is exhumed, forming a marine basin bounded by passive margins. During the subsequent stage with no far-field deformation, the thermal field of the lithosphere is re-equilibrated above a convecting mantle. During convergence, subduction is initiated at one passive margin and the mantle lithosphere below the marine basin as well as the other passive margin are subducted. During progressive subduction, parts of the subducted continental upper crust are sheared-off the subducting plate and are exhumed to the surface, ultimately forming an orogenic wedge. For the convergence, we test the impact of serpentinites at the top of the exhumed mantle on orogenic wedge formation. We compare the model results with observational and experimental constraints, discuss the involved processes and driving forces and propose a model for subduction initiation and (ultra)high-pressure rock exhumation for the Alps.