Multichannel seismic reflection data acquired between 8°50 and 9° 50 N and between 12° 30 and 13° 30 N along the East Pacific Rise provide a three-dimensional view of the young oceanic crust. Thicker crust does not always correlate with shallower ridge bathymetry, broader axial cross section, or more negative mantle Bouguer or subcrustal gravity anomaly. Variable thickness of the crust-mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result. It is hypothesized that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick Moho transition zone and a subcrustal gravity low.