Geologic controls of hydrothermal discharge on the northern Gorda Ridge

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Abstract
An investigation of a 10 x 10 km area near an axial high of the rift valley of the northern Gorda Ridge by using bathymetry, side-scan sonar, and bottom photography where prior water-column surveys indicated ongoing high-temperature hydrothermal discharge reveals that the discharge is localized by specific tectonic and volcanic conditions. The discharge occurs where faults having a predominant ridge-parallel and an anomalous subsidiary ridge-oblique tectonic trend intersect at the southern end of a linear ridge on the east wall and at another site at the base of the east wall of the rift valley. Hydrothermal alteration of basalt talus and stratiform hydrothermal precipitates produced by past activity occur where faults and fissures with the same two tectonic trends intersect near a fault having a third ridge-transverse trend at a 1 x 1 km zone of sheet flows on the floor of the rift valley. Present and past hydrothermal discharge in this area is controlled by the intersection of an axis-parallel tectonic trend common to all spreading centers having axis-oblique trends related to the regional stress field near the shallowest and volcanically most productive part of the ridge segment.

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