Influence of Initial Slab Dip, Plate Interface Coupling, and Nonlinear Rheology on Dynamic Weakening at the Lithosphere-Asthenosphere Boundary

Abstract

The slab dip and long-term coupling along the plate interface can vary both between and within subduction zones. However, how the initial slab dip and resistance at the plate interface affect the dynamic viscous resistance of the asthenosphere at the lithosphere-asthenosphere boundary (LAB) is less understood. This paper presents two-dimensional (2D) visco-plastic models that examine the surface plate velocity and dynamic weakening of the asthenosphere as a function of three values of initial slab dip ($30^o$, $45^o$, $60^o$) and six upper bounds on the plate interface coupling ($3.1\times 10^{20}$, $1\times 10^{21}$, $3.1\times 10^{21}$, $1\times 10^{22}$, $3.1\times 10^{22}$, $1.0\times 10^{23}Pa\cdot s$). The models use a composite viscosity in the upper mantle and examine both the instantaneous and time-dependent flow. The instantaneous models show that decreasing the plate interface coupling bound results in an increase in the subducting plate speed and extent of dynamic weakening of the asthenosphere adjacent to the LAB (sub-LAB asthenosphere), peaking for models with a slab dip of $45^o$. The time-dependent results show the surface plate motion and thickness of the weakened asthenosphere evolve during several million years of subduction, with models with an initial slab dip of $30^o$ and weakest plate interface coupling bound producing the fastest subducting plate speeds and greatest dynamic weakening in the sub-LAB asthenosphere over time. The results show the surface motion is correlated with dynamic weakening in the sub-LAB asthenosphere due to the effects of the strain-rate-dependent rheology. This reduced viscous resistance to slab sinking facilitates subduction and mantle flow over time, thus facilitating plate tectonics.