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, geoid, and gravity anomalies would increase from the Peru Trench in the north to the northern Cline Trench further south. We find that the opposite is true: the lithosphere appears stiffer outboard of the Peru Trench than it does offshore Chile. To explain deflections of the lithosphere seaward of Peru, the isotherm controlling the elastic/ductile transition must be near 800¿C, providing 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 an increase of plate curvature 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 magnitude of the reduction in strength, however, suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern Chile is characterized by 100-MPa-level tension. The more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent, but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise. ¿American Geophysical Union 1991