Role of the overriding plate in the subduction process: Insights from numerical models

Active convergent margins are primarily shaped by the interplay among the subducting plate, overriding plate, and mantle. The effect of important forces, like far-field mantle flow, overriding plate motion, and inter-plate coupling, however, remains partially ambiguous. In a preliminary attempt to clarify their role, a self-consistent, viscoelastic, plane-strain, mechanical finite element model, in which subducting plate, overriding plate and mantle interact dynamically, is developed. In this quasi-static framework with a freely moving slab, trench, and inter-plate fault, the role of a compressive overriding plate on subduction zone kinematics, morphology and stress-state is characterized. A slab interacting solely with a semi-analytical three-dimensional mantle flow formulation shows that local non-induced mantle flow influences slab geometry and kinematics, adding an important dynamic term to the system. The impact of an overriding plate on this system is determined completely by overriding plate trench-ward motions and is only pertinent if the overriding plate actively advances the trench. A trench-ward moving overriding plate indents the slab and thereby enforces trench retreat and decreases slab dip. It also stimulates over-thrusting of the overriding plate onto the slab, and thereby permits mountain building within the overriding plate. Frictional resistance is observed to have a dominant local effect within the overriding plate as it is increasingly dragged down, thereby inhibiting the growth of overriding plate topography. A distinguishable effect on large-scale trench motions and deep slab dip is, however, absent for re-normalized friction coefficients ranging up to about 0.2. Minor additional effects include a decrease in plate motions of about 15% and slab bending stresses of about 10%.