Variations in axial morphology along the Galapagos Spreading Center and the influence of the Galapagos hotspot

The Galapagos Spreading Center (GSC) is marked by systematic changes in axial morphology between the Inca Fracture Zone (FZ) at 85.5 degrees W and the 95.5 degrees W propagator. We analyze these changes using new swath bathymetry and magnetic data acquired aboard the B/O Hesperides during the Galapagos'96 experiment. Within similar to 350 km of the Galapagos hotspot the ridge axis is associated with an East Pacific Rise (EPR)-like axial high. At increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology (TAM). The axis in this transitional region is typically a broad zone (similar to 20 km wide) consisting of very rough volcanic and fault-generated topography. West of 95 degrees W, this TAM evolves into a 20-40 km wide, 400-1500 m deep axial valley typical of the slow spreading Mid-Atlantic Ridge (MAR). There is not an abrupt change from axial high to rift valley along the GSC, but a distinct TAM occurs over a distance of similar to 200-300 km along-axis and is accompanied by a gravity-estimated crustal thickening of >1-2 km. The boundary between an axial high and this TAM is quite abrupt and occurs along a segment that is less than 9 km long. These changes in axial morphology are primarily caused by variations in magma supply along the GSC due to the entrainment and dispersal of plume mantle from the Galapagos hotspot. However, the changes in morphology are not symmetric about the Galapagos FZ at 91 degrees W. The axial high topography extends farther east of the 91 degrees W FZ than to the west, and the rift valley which develops west of 94 degrees W is not found at comparable distances along the GSC east of the hotspot. Axial depth variations are also asymmetric across the 91 degrees W FZ. This asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the GSC from 46 mm/yr at 97 degrees W to 64 mm/yr at 85 degrees W. Off-axis depth changes are symmetric about the 91 degrees W FZ and suggest that 15-40% of on-axis depth variation is dynamically supported.