related to focusing of hydrothermal upwelling through fractured 

 oceanic crust with enhanced permeability at the intersection of 

 two regional tectonic trends ( Rona and Clague, 1986, 1989). A 

 predominant tectonic trend common to all oceanic ridges (Fig. 3) 

 is represented by fault scarps subparallel to the 022° 

 orientation of the rift valley axis. A subsidiary anomalous 

 tectonic trend related to the regional stress field of the 

 northern Gorda Ridge is represented by faults and fissures with a 

 350° orientation. The presently inactive hydrothermal site 

 occurs where the two tectonic trends intersect at the east side 

 of the rift valley floor at a depth of 3100 m near 42°44'N, 

 126°44.5'W. An assemblage of features is present within a small 

 area (c. 1 x 1 km) at this location comprising sheet flows, 

 fissures, partially altered talus at the base of fault scarps, 

 and material with the appearance in photographs of hydrothermal 

 precipitates. Facets of angular talus composed of fresh pillow 

 fragments recovered by dredging are coated white with the mineral 

 boehmite formed by high-temperature alteration of the basalt 

 (Howard and Fisk, 1988). The active hydrothermal site occurs 

 where the two tectonic trends intersect at mid-depth on the east 

 wall of the rift valley. A linear ridge surmounted by a row of 

 hills centered at 42°45.5'N, 126°42.2'W about 1 . 5 km long 

 parallel to the valley axis by 0.5 km wide rises up to 80 m above 

 the 2700 m isobath. This ridge directly underlies the chemical 

 and physical anomalies indicative of hydrothermal activity 

 measured in the overlying water column (Collier et al . , 1986; 

 Kadko et al., 1986; Baker et al . , 1987). The hills surmounting 

 the ridge are formed by fractured pillow lava flows. Relatively 

 dark sediment inferred to be metalliferous, biological debris, 

 and numerous fish recorded on video images at the southern end of 

 the hills suggests proximity to black smoker-type venting from 

 faults and fractures (Rona and Clague, 1986, 1989). Hydrothermal 

 phenomena in the GR-14 area of the northern Gorda Ridge are 

 targeted for a scheduled 1988 dive series with DSV Sea Cliff . 



Southern Gorda Ridge 



Polymetallic massive sulfide samples were recovered, their 

 surface distribution partially mapped and evidence of relatively 

 recent hydrothermal and volcanic activity was found at sites in 

 the southern section of the Gorda Ridge. The massive sulfides 

 were initially recovered by dredging at the flanks of one of the 

 volcanic centers at 40°45'N in the Escanaba Trough on a USGS 

 cruise in 1985 (Morton et al . , 1987). Seafloor photography, 

 dredging and heat flow measurements conducted on that cruise at 

 two of the volcanic centers revealed fresh pillow lava flows and 

 anomalously high heat flow values through adjacent sediment 

 (Abbott et al., 1986). Surface ship and submersible observations 

 with the U.S. Navy DSV Sea Cliff in 1986 provided additional 

 information. Multiple sulfide occurrences were observed in two 

 areas each about 5 by 10 km along the sediment-filled axis of the 

 northern ( NESCA site centered at latitude 41°00'N, longitude 



20 



