Modelling tectonic styles and ultra-high pressure (UHP) rock exhumation during the transition from oceanic subduction to continental collision

The tectonic styles of the transition from oceanic subduction to continental collision and their implications for exhumation of UHP material are tested in a series of numerical thermal-mechanical models that address subduction-collision dynamics at constant convergence rate. Reference models examine the sensitivity to continental mantle lithosphere density, pro-continental margin temperature and rheology, and retro-continental margin rheology. Three tectonic styles are observed: (1) continental subduction without significant deformation of the over-riding continent (favoured by relatively dense mantle lithosphere and a strong retro-continental margin), (2) deformation and subduction of the leading edge of the retro-continent (favoured by low-density mantle lithosphere and strong retro-continental crust), and (3) backthrusting of the retro-continental margin and subsequent double subduction (favoured by low-density mantle lithosphere and weak retro-continental crust). Continental (U)HP material is formed in all three tectonic styles, although it is only efficiently exhumed in the first case. Three mechanisms contribute to (U)HP exhumation, their importance depending on the evolving geometry of the subduction zone, the strength of the continental margin crust and the density of the subducting mantle lithosphere. Weaker/hotter continental margins decouple in the subduction channel and are subsequently exhumed by buoyancy, and/or driven circulation of material and/or forced expulsion in response to insertion of strong continent into the subduction channel (plunger effect). Stronger/colder continental material is subducted without detachment and is then transported laterally, underplating the retro-continental mantle lithosphere. Neither surface erosion nor slab break-off are required for UHP exhumation.