Dynamic Asthenospheric Weakening Facilitating Plate Tectonic Motion

Subduction zones accommodate plate convergence, playing a critical role in plate tectonics. Two-dimensional, time-dependent, numerical models are presented that examine the role of initial slab dip and a strain-rate dependent viscosity on the early phase of subduction. The parameter sweep tests three initial slab dips (30°, 45°, and 60°) and four bounds on the lower limit of viscosity (1019 Pa·s, 1018 Pa·s, 1017 Pa·s, and no limit), simulating up to 6 Myr of subduction. Using a strain-rate dependent viscosity leads to a dynamic weakening of the asthenosphere (~ 1018 Pa·s) enveloping the slab and beneath the surface plates. As the slabs undergo rotation about the hinge, the models using initial slab dips of 30° and 45° display the most dynamic weakening of the asthenosphere and faster flow velocities. For all models, the dimensions and intensity of the weakened asthenosphere and subduction induced flow velocities change over time, with asthenospheric flow velocities peaking when the slab dip is at 45° and at a minimum when the slab dip is subvertical. This is because, when using the composite viscosity, the viscous support of the slab is dynamically modified by the bending torque. Shallower slabs have greater torque due to slab pull and larger velocity gradients along the length of the slab in the direction away from the pivot point at the slab hinge. The associated strain gradients cause more dynamic weakening of the asthenosphere, resulting in less viscous support of slab. The dynamic weakening in turn provides less overall impedance to subduction, and, over time, the models that started out with the shallower slab dip have faster motion. This implies that the dynamic weakening is important for sustaining the early stages subduction by reducing viscous resistance to subducting plate motion, both around the slab and beneath the base of the subducting plate.