Trench Outer Rise Flexure Models with Laterally Variable Plate Rigidity Derived from Oceanic Lithosphere Strength Profiles

Abstract:

We generate model bathymetry and free-air gravity grids for the seafloor seaward of subduction zones that capture the broad trends of deformation due to lithospheric flexure. By using a thin elastic plate formulation with rigidity variations along both horizontal dimensions and which accounts for the effects of plastic yielding, our models are able to reproduce the observed rapid change in curvature at the outer trench wall. Forward models for flexural deflection and plate rigidity are fitted to satellite altimetry-derived marine gravity anomalies jointly with shipboard sonar soundings and swath bathymetry data sets. The estimated parameters are the applied vertical shear and bending moment at the trench axis, and the data misfits are minimized with respect to the L1-norm subject to Tikhonov regularization for smooth variation of the load parameters along the strike of the trench. We perform pre-processing of the input data to enhance the recovery of the flexural signal. Short-wavelength features such as seamounts are isolated using a directional median filter and then excluded from the parameter estimation process. The advantage of adopting anisotropic filtering over similar methods that separate local scale topography from regional swells is that it provides consistent performance in feature detection with minimal user supervision. Our preliminary results from several examples show that the plate rigidity progressively decreases with increasing proximity to the trench axis. These zones of plate weakening correspond to the occurrence of trench-parallel seafloor fractures at the outer trench wall as seen in high-resolution bathymetry data. We seek to determine whether a correlation exists between the distribution or morphology of these fractures and the amount or trend in the reduction of plate rigidity for outer rise regions across the Pacific Rim.