Influence of tectonic overpressure on P-T paths of HP-UHP rocks in continental collision zones: thermomechanical modelling

The principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial/exhumation depth from pressure given by geobarometry. However, pressure deviation from lithostatic, i.e. tectonic overpressure/underpressure due to deviatoric stress and deformation, is an intrinsic property of flow and fracture in all materials, including rocks under geological conditions. In order to investigate the influences of tectonic overpressure on metamorphic P-T paths, 2D numerical simulations of continental subduction/collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle. The experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab, in the subduction channel and within the overriding plate during continental collision. The main overpressure region that may influence the P-T paths of HP-UHP rocks is located in the bottom corner of the wedge-like confined channel with the characteristic magnitude of pressure deviation on the order of similar to 0.3 GPa and 10-20% from the lithostatic values. The degree of confinement of the subduction channel is the key factor controlling this magnitude. Our models also suggest that subducted crustal rocks, which may not necessarily be exhumed, can be classified into three different groups: (i) UHP-rocks subjected to significant (>= 0.3 GPa) overpressure at intermediate subduction depth (50-70 km, P = 1.5-2.5 GPa) then underpressured at depth >= 100 km (P >= 3 GPa); (ii) HP-rocks subjected to >= 0.3 GPa overpressure at peak P-T conditions reached at 50-70 km depth in the bottom corner of the wedge-like confined subduction channel (P = 1.5-2.5 GPa); (iii) lower-pressure rocks formed at shallower depths (< 40 km depth, P < 1 GPa), which are not subjected to significant overpressure and/or underpressure.