Boundary Element Modeling of Two-Plate Interaction at Subduction Zones: Scaling Laws and Application to the Aleutian Subduction Zone

Overview & Synopsis

This work uses the boundary element method (BEM) to explore the dynamics of subduction of a dense lithospheric plate (subducting plate, SP) beneath an overriding plate (OP). For simplicity, the model is two dimensional, the plates are purely viscous, and the ambient fluid is infinitely deep. The negative buoyancy of the slab is the only driving force of the system. First, we study the SP kinematics focusing on two characteristic instantaneous velocities: the convergence speed (VConv) of the descending slab and the horizontal plate speed (USP) of the flat portion of the SP. We find that VConv is entirely controlled by the slab's geometry, by the width of the lubrication layer d2 separating the SP and the OP, and by the SP's flexural stiffness St. Turning to USP, we find that this parameter depends not only on d2 and St but also on the lengths LSP and LOP of the two plates. The dependence of USP on LSP is exactly logarithmic, both with and without an OP. Next, we explore the deformation of the OP, which occurs by a combination of extension/compression and bending. The OP deformation is compression dominated close to the trench and bending dominated along the remaining portion of the OP that undergoes significant deformation. For a positively buoyant OP, backarc extension is also observed. Finally, we estimate the subduction interface viscosity ηSI of the central Aleutian subduction zone, running our BEM model with the appropriate geometry according to Lallemand et al. (2015, https://doi.org/10.1029/2005GC000917). We find ηSI = (0.96–1.72) ×1020 Pa s.