Regional heat flow variations across the sedimented Juan de Fuca Ridge eastern flank: Constraints on lithospheric cooling and lateral hydrothermal heat transport

Seafloor heat flow has been estimated continuously along a transect on the sedimented eastern flank of the Juan de Fuca Ridge to provide constraints on the scale and rate of lateral heat and fluid transport in the upper igneous crust, and on the total flux from the young lithosphere beneath. The profile extends from 20 km east of the ridge axis, where turbidite sediments lap onto the oceanic crust at an age of less than 1 Ma, and ends 110 km east of the axis over 3.6 Ma crust. Estimated heat flow is derived from the combination of the depth to igneous basement determined from seismic reflection profiles, and basement temperatures measured in nine Ocean Drilling Program boreholes along the transect. The strategy is based on the knowledge that in this young area upper basement temperatures are locally homogenized by vigorous hydrothermal circulation, and hence vary smoothly between drilling sites. At the young end of the transect near the region of extensive outcrop, heat flow is only 15% of that predicted for lithosphere of this age. Heat flow increases systematically to the east, reaching the full predicted level about 20 km from the point of sediment/basement onlap. The anomaly can be accounted for by mixing and/or eastward flow of cool water in the upper igneous crust beneath the sediment at a Darcian rate of the order of 1 m yr(-1). A geochemical signature of "fresh" seawater in basement is observed many tens of kilometers farther to the east, although the thermal signature is no longer present; along the remainder of the transect the heat flow conforms very well to that predicted by simple boundary layer cooling theory once the thermal effects of rapid sedimentation on this ridge flank are accounted for.