Adjacent segments of the Pacific-Antarctic ridge display significantly different morphologies and depth-age relationships over seafloor younger than 36 Ma. The spreading corridor southwest of Fracture Zone XII is characterized by a rift valley and an usually small subsidence constant of 226¿13 m/m.y.1/2, while the two spreading corridors immediately northeast of Fracture Zone XII have an axial high and a subsidence constant consistent with the global average. This abrupt variation in ridge morphology is not usually characteristic of medium-rate spreading centers, nor is such an abrupt variation expected of adjacent ridge segments that are spreading at the same rate. We suggest that a thermal anomaly beneath the ridge may influence the first-order effects of spreading rate and lithospheric cooling enough to produce the observed rift valley and axial high and the different subsidence constants. Although we are not certain what would produce the thermal anomaly here, we speculate that when the spreading rate on the Pacific-Antarctic ridge increased from slow to intermediate rates since 20 Ma, so did the need for materials for accretion, which may be supplied in part by along-axis asthenospheric flow from hotspots or a hot region to the northeast. A sufficient supply of hot asthenosphere may still be lacking in the ridge segment with the axial valley to the southwest, leaving it cooler and starved for accretionary materials. ¿ American Geophysical Union 1994