Sixteen new heat flow stations plus a seismic reflection, bathymetric, and magnetic survey of the north flank of the Galapagos Spreading Center have provided additional data to further delineate a low heat flow zone approximately 50 km wide running from 85.5¿W to 88¿W. This anomalous, low heat flow zone appears to be related to a graben which is found as a continuous, fault-bounded trough, offset by fracture zones and trending parallel to the present spreading center. Thin sediment, symmetric magnetic anomalies, shoaling topography, and a topographic step between the graben and the present spreading center suggest that it is the central rift valley of a fossil spreading center which jumped to the south ~4 m.y.B.P. The low heat flow measurements appear to be a manifestation of our inability to measure convective heat transfer in this area of thin sediment cover. The thin sediment cover appears to be caused (1) the youth of the fossil ridge crest relative to the surrounding sea floor and (2) burial by extrusive activity which might have continued after the major components of sea floor spreading jumped to the presently active ridge crest.