We analyze gravity data over the Juan de Fuca and northern Gorda Ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate (28 mm/yr half rate). The Cleft Segment, at the south end of the Juan de Fuca Ridge, has an axial high morphology while the northern segment of the Gorda Ridge has a rift valley. Residual mantle Bouguer anomalies (RMBA) on the northern Gorda Ridge are elevated relative to the Cleft Segment by 10--20 mGal, indicating thinner crust and/or a colder mantle. The minimum value (-50 mGal) of the RMBA along the Juan de Fuca Ridge is over Axial Seamount and gradually increases south toward the Blanco Transform. The observed RMBA are interpreted to result from along axis variations in crustal thickness and mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. We estimate that mantle temperatures are elevated by 30¿--40 ¿C beneath Axial Seamount, resulting in an excess crustal thickness of ~1.7 km. The Cleft Segment is associated with crust that is estimated to be only 300--700 m thicker, and mantle temperatures are only 10¿--15 ¿C higher than beneath the northern Gorda Ridge.

However, even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology. These results are consistent with the predicted sensitivity of recent thermo-mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates. We attribute the systematic differences in axial morphology, crustal thickness, mantle temperature, and lithospheric strength along the Juan de Fuca/Gorda ridge system to the presence of the Cobb thermal anomaly at Axial Seamount. The hotter mantle beneath the Juan de Fuca Ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner, weaker lithosphere than along the Gorda Ridge. ¿ American Geophysical Union 1995