The role of outcrop-to-outcrop fluid flow in off-axis oceanic hydrothermal systems under abyssal sedimentation conditions

It has been proposed that ridge flank hydrothermal circulation by outcrop-to-outcrop (lateral) flow may be the dominant mode of oceanic hydrothermal circulation globally. In this model, the upper igneous crust is an aquifer overlain by low permeability sediments, and aquifer-ocean fluid exchange occurs through basement outcrops. Thermally induced pressure gradients drive fluid laterally from recharge outcrops to discharge outcrops. To test the global applicability of outcrop-to-outcrop flow, models of synthetic basement bathymetry representative of crust formed at different spreading rates, pelagic sediment supply and post depositional transport, and sediment hydraulic impedance are used to quantify the time-varying distribution of sediment and basement outcrops globally. Results suggest that basement outcrops may be 40-50% closer together than previously estimated. The modeled sediment and outcrop results are coupled with a two-dimensional model of outcrop-to-outcrop fluid flow and heat exchange in a vertically isothermal crustal aquifer to predict the spatial distributions of seafloor heat flow within the simulation region under this mode of hydrothermal circulation. It is found that both the time-varying average and standard deviation of modeled seafloor heat flow required by outcrop-to-outcrop flow simultaneously fit the global heat flow data if the average aquifer permeability decreases from similar to 10(-9) m(2) to similar to 10(-11) m(2) over the duration of the global heat flow deficit (to similar to 65 Myr). This permeability range is consistent with other estimates of upper crustal permeability on comparable spatial scales, and supports the proposition that outcrop-to-outcrop fluid flow may be the dominant mode of off-axis hydrothermal circulation globally.