Outer trench slope flexure and faulting at Pacific basin subduction zones

Abstract

Flexure and fracturing of the seafloor on the outer trench wall of subduction zones reflect bending of the lithosphere beyond its elastic limit. To investigate these inelastic processes, we have developed a full nonlinear inversion approach for estimating the bending moment, curvature and outer trench wall fracturing using shipboard bathymetry and satellite altimetryderived gravity data as constraints. Bending moments and downward forces are imposed along curved trench axes and an iterative method is used to calculate the nonlinear response for 26 sites in the circum-Pacific region having seafloor age ranging from 15 to 148 Ma. We use standard thermal and yield strength envelope models to develop the nonlinear moment versus curvature relationship. Two coefficients of friction of 0.6 and 0.3 are considered andwe find that the lower value provides a better overall fit to the data. The main result is that the lithosphere is nearly moment saturated at the trench axis. The effective elastic thickness of the plate on the outer trench slope is at least three times smaller than the elastic thickness of the plate before bending at the outer rise in agreement with previous studies. The average seafloor depth of the unbent plate in these 26 sites matches the Parsons & Sclater depth versus age model beyond 120 Ma.We also use the model to predict the offsets of normal faults on the outer trench walls and compare this with the horst and graben structures observed by multibeam surveys. The model with the lower coefficient of friction fits the fault offset data close to the trench axis. However, the model predicts significant fracturing of the lithosphere between 75 and 150 km away from the trench axis where no fracturing is observed. To reconcile these observations, we impose a thermoelastic pre-stress in the lithosphere prior to subduction. This pre-stress delays the onset of fracturing in better agreement with the data.