Numerical modelling of mantle exhumation in inverted rift systems

The tectonic exhumation of mantle material is a well-known phenomenon and may occur during both rifting and subsequent (large-scale) basin inversion. However, the processes leading to the exhumation of dense and therefore negatively buoyant (sub-)lithospheric mantle material remain poorly understood. We therefore conducted a series of thermomechanical simulations using the geodynamics code ASPECT (coupled with FastScape for the inclusion of surface processes) testing the impact of various parameters on mantle exhumation in inverted rift systems.

We find that rift duration strongly impacts mantle exhumation, both during the rift phase, as well as during subsequent inversion. When only limited rifting is applied, the dense mantle material cannot reach the surface as the overlying crustal layers remain connected. Basin inversion then tends to create a symmetric pop-up structure by reactivating rift boundary faults, and the dense mantle material is forced down by the thickening of low-density crustal layers on top of it. Only after certain amount of extension, the crust is sufficiently thinned so that mantle material can be exhumed. This mantle material may then remain near the surface or be further exhumed during basin inversion. Such further mantle exhumation is favoured if asymmetric reactivation of the rift basin occurs, so that mantle material is thrust on top of the downgoing plate.

The establishment of such asymmetric orogenic systems allowing for efficient mantle exhumation is further promoted by having only short-lived tectonic quiescence between rifting and inversion, so that no thermal equilibration of the exhumed mantle domain can occur. As a result, the rift basin remains a weakness that is readily exploited during inversion. Longer periods of tectonic quiescence restore the strength of the lithosphere, so that delayed inversion generates more symmetric structures, with limited opportunities for mantle exhumation.

Within this tectonic context, erosion efficiency is another key factor. First, more efficient erosion during inversion removes crustal material so that the mantle can be exhumed (even in symmetric orogenic systems). Second, efficient erosion also leads to the development of asymmetric orogenic systems, thus doubly contributing to mantle exhumation. Somewhat similarly, high plate velocities during inversion introduce larger amounts of crustal material into the system, which erosion cannot remove in a timely manner, whereas slow plate velocities allow erosion more time to remove material. Hence, mantle exhumation is positively correlated to erosion efficiency, and is negatively correlated to plate velocities during inversion

Finally, serpentinization of mantle material can occur close to the Earth’s surface (i.e. in the uppermost kilometres) and strongly reduces the material’s density and brittle strength. Although our models so far only show a limited effect of serpentinization, the overall weakness of serpentinized mantle material at the rift basin floor seems to reduce localization of inversion-related deformation, thus generating more symmetric inversion systems with limited mantle exhumation.