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 i n 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 n ew swath bathymetry and magnetic data acquired aboard the B/O Hesperid es during the Galapagos'96 experiment. Within similar to 350 km of the Galapagos hotspot the ridge axis is associated with an East Pacific R ise (EPR)-like axial high. At increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional ax ial morphology (TAM). The axis in this transitional region is typicall y a broad zone (similar to 20 km wide) consisting of very rough volcan ic and fault-generated topography. West of 95 degrees W, this TAM evol ves 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 occu rs over a distance of similar to 200-300 km along-axis and is accompan ied 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 morpholog y are primarily caused by variations in magma supply along the GSC due to the entrainment and dispersal of plume mantle from the Galapagos h otspot. However, the changes in morphology are not symmetric about the Galapagos FZ at 91 degrees W. The axial high topography extends farth er east of the 91 degrees W FZ than to the west, and the rift valley w hich develops west of 94 degrees W is not found at comparable distance s along the GSC east of the hotspot. Axial depth variations are also a symmetric across the 91 degrees W FZ. This asymmetry in both morpholog y and axial depth variation is attributed to a full spreading rate inc rease along the GSC from 46 mm/yr at 97 degrees W to 64 mm/yr at 85 de grees W. Off-axis depth changes are symmetric about the 91 degrees W F Z and suggest that 15-40% of on-axis depth variation is dynamically su pported.