The central and southern sections of the Gorda Ridge are 

 spreading at slow rates (full-rate 2.2-2.4 cm/y; Atwater and 

 Mudie, 1973; Riddihough, 1980) and exhibit a well-defined rift 

 valley characteristic of slow-spreading oceanic ridges. The 

 rift valley is sediment-starved to latitude 41°15'N where the 

 southern 100 km is filled with terrigenous sediment up to 500 m 

 thick. A series of volcanic centers consisting of irregular 

 domes projecting upwards through the sediment fill are spaced at 

 15 to 20 km intervals along this southernmost section which is 

 part of the Escanaba Trough (Fig. 6). 



Hydrothermal Activity and Mineralization 



A survey of chemical and physical properties of the water 

 column over the rift valley of the Gorda Ridge to determine the 

 state of hydrothermal activity was carried out cooperatively by 

 investigators from NOAA, Oregon State University and the 

 University of California and coordinated by the Gorda Ridge 

 Technical Task Force of the Minerals Management Service in 1985 

 (Baker et al., 1987). The survey comprised ten widely spaced 

 stations along the length of the Gorda Ridge (Fig. 6) utilizing 

 CTD (conductivity, temperature, depth )-transmissometer (light 

 attenuation by suspended particulate matter) profiles and water 

 samples for analysis of chemical indicators of hydrothermal 

 activity (dissolved manganese, particulate aluminum/iron ratio, 

 methane, radon-222, helium-3). The survey revealed ongoing 

 hydrothermal venting at the two stations at the northern section 

 of the Gorda Ridge (station GR-14 at 42°44.7'N, 126°43.7'W; 

 station GR-15 at 42°56.7'N, 126°35.6'W), ambiguous values of the 

 various hydrothermal indicators at the stations in the central 

 section, and no hydrothermal signal at the single station over 

 the Escanaba Trough at the southern section of the Gorda Ridge. 

 More detailed water column work at the two northernmost stations 

 confirmed the hydrothermal activity (Collier et al., 1986; Kadko, 

 1986), and recovered suspended mineral particles (anhydride, 

 atacamite) indicative of high-temperature black smoker-type 

 venting (Nelsen, 1987). Recent dives with DSRV Alvin in the 

 NESCA area of the Escanaba Trough found and sampled active hot 

 springs associated with massive sulfide deposits at the flanks of 

 a volcanic center (Fig. 5; Campbell et al., 1988b; Zierenberg et 

 al., 1988). 



Northern Gorda Ridge 



The 10 by 10 km area centered on station GR-14 at the 

 northern section of the Gorda Ridge was investigated with 

 bathymetry, seafloor photography and video, and dredging by NOAA 

 and U.S. Geological Survey (USGS) cruises in 1986 building on a 

 prior seafloor data base of NOAA Sea Beam work (Malahoff, 1985) 

 and USGS-University of Hawaii SeaMARC II side-scan sonar imagery 

 (Clague et al., 1984; Clague and Holmes, 1987). Inactive and 

 active hydrothermal sites were identified within this area and 



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