A suite of 101 well-navigated heat-flow stations was used to investigate lithospheric cooling on the Mid-Atlantic Ridge near 37°N. Measurements in two topographic depressions marking the intersection of transform faults with axes of sea-floor spreading show evidence of hydrothermal circulation in basement rocks. One of these depressions appears to have been subjected to a recent bottom-water temperature change which severely biases the heat-flow data. Some of the evidence suggests that transform faults are more permeable than other regions of the ocean crust and therefore may cool much faster.

We also measured the heat flow in rift mountains extending from 10 to 35 km west of the spreading axis. The data here also suggest that a hydrothermal system is presently active in the crustal rocks. Although hydrothermal vents have not yet been unambiguously identified on the sea floor, hydrothermal circulation seems likely to be the dominant mode of cooling oceanic crust near spreading axes where sediment cover is thin (that is, few tens of metres) or nonexistent.

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