On the criticality of return flows in viscous accretionary wedges and its implications for deep-crustal exhumation in subduction zones

Abstract. In subduction zones, the accretionary wedges play a vital role in mediating the burial processes of incoming oceanic sediments and eventually their return pathways to the surface. A direction of the previous tectonic models invoked the standard corner flow theory, assuming a slab-parallel shear and a rigid, fixed overriding plate, to elucidate the crustal recycling processes in tectonic wedges. To deal with more complex subduction-collisional settings, where they have deformable overriding plates, and associate a horizontal slab migration (advance or rollback) component during subduction, we develop a generalized corner flow model to revisit the problem of return flow mechanics, providing a criticality analysis of the return flows as a function of the geometric, kinematic, and rheological conditions in accretionary wedges. A new set of analytical solutions is presented to evaluate the limiting conditions in which a wedge can set in significant return flows, leading to focused exhumation of the deep-crustal materials. The theoretical results suggest that, for moderate wedge-taper angles (~30^o), the viscosity ratios (µ_r) between the overriding plate and the wedge ≥ ~10³ provide favourable tectonic settings for the return flow kinematics in accretionary wedges. Decrease in µ_r, or addition of slab roll back weakens the return flows, whereas slab advance greatly strengthens the return flows. The analytical solutions are also utilized to demonstrate reversals in the shear-sense patterns across the wedge. We expand this study by reproducing some of the theoretical flow patterns in laboratory experiments. It is shown from the theoretical model that the total pressure in the accretionary wedge dynamics becomes close to the lithostatic value when the rheological setting has low-viscosity (10¹⁹ Pa s) wedge materials.