Geologic setting of hydrothermal activity at the northern Gorda Ridge

The northern end of the northern segment of the rift valley of the Gorda Ridge was investigated with Sea Beam bathymetry, SeaMARC IA and II side-scan sonar, surface-towed magnetics, and near-bottom camera-temperature tows to determine the geologic setting of large chemical anomalies in the water column indicative of intense hydrothermal discharge. The magnitude of the water column anomalies is comparable to that associated with the high-temperature Sea Cliff hydrothermal field situated on the east wall of the rift valley 25 km to the south. Water column andseafloor hydrothermal indicators and tectonic trends converge at a seamount at 42°57.3'N, 126°33.0'W situated 4.5 km east of the rift valley axis on a terrace as the most likely source area for the hydrothermal discharge. Comparison with the geologic setting of the Sea Cliff hydrothermal field confirms the presence of an anomalous axis-oblique (340°–357°) tectonic trend that permeates the structure of the northern Gorda Ridge. Additional sinistral offset along an axis-perpendicular structural trend (292°) at the Sea Cliff field indicates a southward increase in deformation of the northern Gorda Ridge along axis-transverse trends.