The Interdependence of Plate Coupling Processes, Subduction Rate, and Asthenospheric Pressure Drop across Subducting Slabs

Abstract:

One advantage of analytical models, in which analytic expressions are used for the various components of the subduction system, is the efficient exploration of parameter space and identification of the physical mechanisms controlling a wide breadth of slab kinematics. We show that, despite subtle differences in how plate interfaces and boundary conditions are implemented, results for single subduction from a 3-D semi-analytical model for subduction FAST (Royden & Husson, 2006; Jagoutz et al., 2015) and from the numerical finite-element model CitcomCU (Moresi & Gurnis, 1996, Zhong et al., 2006) are in excellent agreement when plate coupling (via shear stress on the plate interface) takes place in the FAST without the development of topographic relief at the plate boundary. Results from the two models are consistent across a variety of geometries, with fixed upper plate, fixed lower plate, and stress-free plate ends. When the analytical model is modified to include the development of topography above the subduction boundary, subduction rates are greatly increased, indicating a strong sensitivity of subduction to the mode of plate coupling. Rates of subduction also correlate strongly with the asthenospheric pressure drop across the subducting slab, which drives toroidal flow of the asthenosphere around the slab. When the lower plate is fixed, subduction is relatively slow and the pressure drop from below to above the slab is large, inhibiting subduction and slab roll-back. When the upper plate is fixed and when the plate ends are stress-free, subduction rates are approximately 50% faster and the corresponding asthenospheric pressure drop from below to above the slab is small, facilitating rapid subduction. This qualitative correlation between plate coupling processes, asthenospheric pressure drop, and rates of subduction can be extended to systems with more than one subduction zone (Holt et al., 2015 AGU Fall Abstract).

Jagoutz, O., Royden, L., Holt, A. & Becker, T. W., 2015, Nature Geo., 8, 10.1038/NGEO2418.

Moresi, L. N. & Gurnis, M., 1996, Earth Planet. Sci. Lett., 138, 15–28.

Royden, L. H. & Husson, L., 2006, Geophys. J. Int. 167, 881–905.

Zhong, S., 2006, J. Geophys. Res., 111, doi: 10.1029/2005JB003972.