Heat flow, sediment and pore fluid chemistry, and hydrothermal circulation on the east flank of Alarcon Ridge, Gulf of California

New seismic, heat flow, sediment and pore fluid geochemistry data from the east flank of Alarcon Ridge, at the mouth of the Gulf of California, provide evidence for vigorous hydrothermal circulation within young oceanic crust formed at a moderate-rate spreading center. Data and samples were collected 9-20 km from the ridge axis to assess the hydrologic state of 0.30-0.65 Ma seafloor. Conductive heat flow values are 15-55% of that input at the base of the lithosphere. Heat flow is highest near the center of a sediment-covered trough, and lowest along the trough margins, suggesting that trough-bounding faults and basement exposures may help to focus hydrothermal recharge. Sediment and pore fluid geochemistry data, in combination with reactive transport modeling, indicate that conditions within the shallow sediments are dominantly diffusive and reactive in two locations, but that bottom seawater recharges through the thin sediment layer with velocities on the order of 2-10 mm/yr at other sites. Seafloor heat flow appears to be entirely conductive, which is consistent with the slow rate of seepage inferred from pore fluid observations and modeling. Fluid recharge through sediments requires that basement is underpressured relative to hydrostatic conditions. We interpret these observations and inferences to indicate vigorous fluid flow in basement, and secondary seepage through overlying sediments. The heat flow deficit along the 11-km Alarcon Basin transect averages 440 mW/ m², equivalent to heat output of 5 MW per kilometer of spreading axis. This heat output is similar to the combined focused and diffusive heat output of a single basement outcrop on the east flank of Juan de Fuca Ridge, and suggests that sites of concentrated heat and fluid outflow may exist on the east flank of Alarcon Ridge.