Dynamics of Arc-continent Collision: The Role of Crustal-mantle Dynamics on Controlling the Evolution of Stress Regime

Abstract:

Arc-continent collision is the process by which intra-oceanic arc crust is accreted to continental margins and the most important mechanism that enables the growth of the continental crust since Phanerozoic times. We use numerical visco-plastic mechanical models to explore how crustal-mantle dynamics control the evolution of the stress regime in continental margins during arc-continent collision. We performed a series of simulations only varying the thickness of the arc as it has been suggested to control the density profile and rheology of intra-oceanic arcs and therefore the dynamics of collision. Modelling results show that arc-continent collision can evolve into two contrasting mechanisms: i) arc transference in thin arcs (15-31km in thickness); and ii) slab break-off in thick arcs (32-35 km in thickness). In turn, these two contrasting mechanisms trigger the partition of stress into extension in the continental margin and compression towards the subducting plate. We interpret that the partitioning in stress into compression and extension in all simulations is caused by a gravity-driven flow, which equilibrates the contrasts in gravitational potential energy (GPE) stored in the lithosphere during collision and episodes of lithospheric thickening. We argue that this gravity-driven flow applies a horizontal gravitational force directed from the collided arc towards the subducting plate (compressional) and the continental margin (extensional). Finally, we conclude that the large-scale mantle return flow emerged from slab-anchoring facilitates the stress partitioning by enhancing: i) compression and lithospheric thickening; and ii) the contrast in GPE between the accreted arc and the continental margin during collision. In the particular case of thin arc simulations, slab-anchoring also promotes further compression-thickening during post-collision and further storage of GPE.