Analyses of gravity data collected at a large nontransform offset (NTO) at 36.5 ¿S on the East Pacific Rise indicate that the southward ridge propagation there does not occur concurrently both on the seafloor and at depth. A large negative axial mantle Bouguer anomaly commonly observed at the East Pacific Rise does not extend from the north into the overlapping zone following the ridge axis observed in seafloor bathymetry data. We speculate that the southward ridge propagation at this offset is probably an episodic process: it starts with rapid rise-crest (dikes) propagation on the seafloor and is followed by slow propagation of the melt supply sources associated with the mantle upwelling. The gravity data collected at the 36.5 ¿S NTO provide new insight on the tectonic evolution of a large propagating nontransform offset and also provide important constraints on ridge-NTO models. ¿ American Geophysical Union 1996