Rapid Change in Upper Crustal Velocities for 0-7 Ma Slow-Spreading Mid-Atlantic Ridge Crust: Evidence for Continuing Low-Temperature Hydrothermal Circulation

Chemical and thermal exchange between the oceanic lithosphere and seawater has been found to influence ocean chemistry, igneous differentiation, and the alteration of upper oceanic crust. We present new data on young (0-7 Ma), slow spreading rate (~16 mm/yr) oceanic crust in the western South Atlantic to further characterize the seismic structure near the MAR at 30°S and to contribute to the global record of the rate of P-wave velocity change with crustal age. Three ridge-normal high-resolution tomographic 2-D velocity models of the upper oceanic crust were built on downward-continued streamer data acquired during the Crustal Reflectivity Experiment Southern Transect. A previous study by Kardell (2019) found that velocities on the Western flank increased from ~2.4 km/s at 0 Ma to ~4.2 km/s at 6 Ma. The observed velocity increase from 0 - 7 Ma is evidence of a decrease in porosity of upper crust, which is attributed to progressive hydrothermal mineralization within extrusive formations on the flanks of the ridge axis. Using two additional velocity profiles spaced 10 km to the North and South of line 1F, we explore the spatial variability of seismic velocities in this important age range.