Contrasting influence of sediments vs surface processes on retreating subduction zones dynamics

Abstract

Surface processes and sediments have been suggested to play a crucial role in subduction initiation and to speed up subduction and plate tectonics. The exact roles of sediments and surface processes and the way they influence slab retreat dynamics has however not been constrained yet. Here, we analyse these roles by using 3D numerical thermo-mechanical modelling code coupled to surface processes model in order to investigate large scale dynamics of retreating subduction zones. We focus on two regional-scale geodynamic scenarios: a slab retreating in a homogeneous oceanic domain and a slab retreating along a continental margin before entering an oceanic domain. We study the influence of the thickness of the sediment cover on the oceanic crust and the intensity of surface processes. We show that the sediment thickness and the surface processes both affect the retreat dynamics in a distinct manner, and, when the slab retreats along a continental margin, also influence its trajectory. A thick sediment cover leads to the building of a large accretionary prism which acts as a lock on the system and slows down the slab retreat. When the slab retreats along a continental margin, the thicker the sediment cover, the stronger the slab deviation from that margin. Surface processes have a more complex role which depends on the amount of sediments available. In systems where little sediments are available, a higher efficiency of surface processes leads to a slow down of slab retreat by accumulating sediments in the accretionary wedge. In systems where a lot of sediments are available, a higher efficiency of surface processes slightly speeds up the retreat, by eroding the overriding plate and the subducting plate respectively at the front and the back of the accretionary wedge, as well as the prism itself when reaching a high enough elevation, thereby decreasing the mechanical locking caused by the accumulation of sediments.