Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: new modelling and field data from Oman
The physical model presented reveals two principal regimes of continental subduction: a highly compressional (HC) regime and a low compressional (LC) regime characterised by high and low pressure between the overriding and subducting plates, respectively. The pressure is inversely proportional to the pull force, which depends on the difference between average density of the subducting lithosphere and density of the mantle. The subducting continental crust reaches a maximum depth which is proportional to the strength of the crust, inversely proportional to the interplate pressure, and is ca. 200 km on average. The crust then fails, forming a major crustal thrust. The location of failure depends on the regime of subduction (on the interplate pressure): in the HC regime, failure occurs near the front of the subduction (collision) zone, whereas the crust fails at greater depth under the base of the overriding plate in the LC regime. The failure is followed by buoyancy-driven uplift of the subducted crustal slice, while the lithospheric mantle keeps subducting. The uplift causes a normal sense displacement (formation of a normal fault) along the upper surface of the crustal slice. For the LC regime (failure of the crust near the base of the overriding plate), a rapid spontaneous crustal uplift (intrusion into the interplate zone) brings the deeply subducted crust to shallow depths. Under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails, forming a tectonic window. The rising material (the high-pressure rocks) is exhumed within this window. In the HC regime, uplift of the subducted crust, after its failure in front of the overriding plate, is possible only with erosion of the relief, which provides an unloading effect, allowing the subducted crustal slice to rise up. The exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger. The HC subduction regime has been shown earlier to match the Himalayan situation. The LC regime fits the situation in the Oman Mountains considered in this paper.