Simulating the role of slab steepening and roll back in exhuming subducted continental lithosphere using 2D geodynamic numerical modelling (Saih Hatat Window, Oman Mountains)

Although continental lithosphere can enter the subduction zone, how buoyant continental crust sinks below 100 km of depth and is then exhumed remains hard to understand. Exhumation of continental lithosphere is testified by the occurrence at the surface of high pressure rocks in several orogenic belts. Despite the existence of different models describing the exhumation of high pressure rocks, they consider only tectonic settings where both upper and lower plates are either continental or oceanic.

In the Oman mountains, high pressure continental rocks crop out in the Saih Hatat Window surrounded by obducted oceanic lithosphere. Here, oceanic obduction occurred during subduction of continental lithosphere, setting a peculiar framework where a denser lithosphere overrides the subducting lighter continental crust.

In this study we used 2D thermo-mechanical geodynamic numerical modelling to investigate the mechanisms that drive exhumation of the continental lithosphere beneath obducted oceanic lithosphere. We validate the geodynamic numerical models with the pressure-temperature-time estimates from the Oman Mountains natural samples and we compare the final architecture reproduced by our models with the regional tomographic models available from the Oman region, which allow to infer the presence of a NE-dipping steep slab beneath the Oman mountains.

Our models reproduce the subduction of continental rocks up to ~150 km, with crustal material returning up to the surface guided by the steepening of the slab dip and by slab roll back. The result is a stack of continental material surrounded by obducted oceanic lithosphere. Concluding, exhumation of continental lithosphere is accompanied by a slight heating at the bottom of the exhumed continental crust, triggered by asthenosphere flow after the slab dip increased.