The relationship between plate curvature and elastic plate thickness : a study of the Peru-Chile Trench

Abstract

The age of the Nazca plate where it enters the Peru and northern Chile trenches varies from 30 Ma in the north to 45 Ma in the south as its dip beneath the South American continent steepens from 13° to 30°. If the elastic thickness Te of oceanic lithosphere depends only on its age, and therefore thermal state, we would expect that Te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth and geoid anomalies would increase from the Peru Trench in the north to the northern Chile Trench further south. We find that just the opposite is true: the lithosphere appears stiffer outboard of the Peru Trench than it does further south, and the isotherm controlling the elastic/ductile transition must be 600°C or greater if the thermal structure of the plate is that predicted by the standard thermal plate model. Because the decrease in plate stiffness to the south is correlated with a decrease in the minimum radius of curvature of the flexed plate over the outer rise and outer trench wall, we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains. The fact that the more highly bent segment of subducting lithosphere also dips at a steeper angle at greater depth beneath the continent might suggest that the amount of inelastic weakening of the lithosphere could be predicted from seismic images of the down going slab, but we find little support for this correlation worldwide. The forces and moments controlling the shallow deformation of the plate seaward of the trench do not appear to be linked to upper mantle processes which impose the dip at greater depth. Finally, we consider the possibility that the elastic thickness of the lithosphere would be reduced for trenches that are highly arcuate in map view, again due to inelastic yielding. If such a relationship exists, the effect for oceanic lithosphere is much smaller than what is documented for continental plates where they underthrust highly arcuate fold belts.