The hotspot-influenced western Galapagos Spreading Center (GSC) spreads at an intermediate rate (45-55 mm/yr) and has an axial high that is appreciably larger in amplitude than many sections of fast spreading ridges such as the East Pacific Rise. Moving westward from 91&Mac176;W away from the Galapagos hotspot, the amplitude of the axial high decreases as the depth of the axial magma lens reflector increases (observed in our multi-channel seismic data). We investigate the cause of the axial high using a model that determines the flexural response to loads resulting from the thermal and magmatic structure of the lithosphere (Shah and Buck, 2001). In this model, low-density material underlying the ridge axis was originally assumed to be hot and partially molten crust but we now extend it to include partial melt in the mantle. The low-density material rapidly cools and becomes denser away from the ridge axis imparting downward loads on the lithosphere. These loads, combined with thermal contraction stresses, depress the flanks of the axis downward such that the ridge axis stands on a topographic high. Using this model, we are able to predict the decrease in amplitude of the axial high with increasing magma lens depth by either decreasing the amount of low-density material beneath the ridge axis or by allowing the crust to cool more slowly as it moves off axis.

Previous applications of this model to other axial highs show that both the observed topography and gravity can be created by low-density material near the ridge axis and melt contained entirely within the crust. However, results of our calculations reveal that the unusually large axial high of the GSC requires that either the crust below the magma lens contains an extremely large amount of melt (up to ~35%), or alternatively, the melt extends well below the crust (up to ~70 km) in a narrow region below the ridge axis. It thus appears likely that the elevated mantle temperature and crustal production associated with the Galapagos hotspot maintains a significant amount of melt in the mantle. Using multi-channel seismic imaging of the axial magma lens, seismic refraction data, gravity and bathymetry measurements we constrain the amount of melt needed in the crust versus the mantle. Future mantle seismic studies could be used to further test our models.

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