The force balance in plate tectonics is fundamentally important but poorly known. Here, we show that two prominent and seemingly unrelated observations—trench-parallel gravity anomalies along the Nazca–South America margin that coincide with the rupture zones of great earthquakes, and a rapid slowdown of Nazca–South America convergence over the past 10 m.y.—provide key insights. Both result from rapid Miocene-Pliocene uplift of the Andes and provide quantitative measures of the magnitude and distribution of plate coupling along the Nazca–South America margin. We compute the plate-tectonic force budget using global models of the faulted lithosphere coupled to high-resolution mantle circulation models and find that Andean-related plate-margin forces are comparable to plate-driving forces from the mantle, and they have sufficient magnitude to account for pronounced bathymetry variations along the trench. Our results suggest that plate coupling, gravity anomalies, and bathymetry variations along a given trench are all controlled by long-term stress variations in the upper portion of plate boundaries and that an explicit budget of driving and resisting forces in plate tectonics can be obtained. For the convergent margin considered here, spatial variations in the effective coefficient of friction associated with the distribution of lubricating sediments entering the trench are, by comparison, of minor importance.