Middle Valley

Middle Valley is an axial rift valley at the northern end of the Juan de Fuca Ridge near the intersection with the Sovanco fracture zone. The valley is completely filled with a few hundered meters up to 2 km of turbiditic sediments. Hydrothermal fluids are presently being discharged at two vent fields about 3 km apart, Bent Hill and the Dead Dog vent field. Hydrothermal discharge in these areas appears to be focused along extensional faults. At the Bent Hill clastic sulfide layers are interbedded with hemipelagic and turbiditic sediments along the flanks of the sulfide mound.

Seafloor spreading is thought to have shifted from Middle Valley to West Valley around 10-15 k.y. ago. The Dead Dog field supports hydrothermal vent faunal communities, that include symbiont-bearing bivalves (clams) and tubeworms. Areas of active venting in Middle Valley were tentatively identified using the heat flow data of Davis et al. (1987) and located more precisely using deep-towed video and still-camera mapping. Several submersible expeditions have since sampled and surveyed the hydrothermal fields. The Ocean Drilling program conducted  extensive scientific drilling operations in Middle Valley, beginning in 1991 with Leg 139, defining the areal extent and depth of the Bent Hill sulphide deposit, and installing subseafloor sensors in two CORKed boreholes (858G and 857D). These borehole sensors have been collecting data since the 1990's, using battery-powered data loggers that are visited by submersible for data downloads.

More Information

Not yet instrumented

Depth: 2400 m
Setting: Seismically active area on the northern end of the Juan de Fuca ridge
Studies: Plate tectonics, seismicity, hydrothermal vent systems and ecosystems, mid-ocean ridges