the rift valley axis ( Rona and Speer, 1987, and in preparation). 

 A minimum estimate of the convective heat flux from the black 

 smokers in the central area of the mound is 10' W. This value is 

 intermediate between the values of 10° W and 10° W estimated for 

 small to large hydrothermal fields at the fast-spreading East 

 Pacific Rise (Macdonald et al., 1980; Converse et al., 1984; 

 Little et al . , 1987). The composition of the solutions 

 discharging from the black smokers exhibits both similarities and 

 differences from those discharging from black smokers at the East 

 Pacific Rise (Table 3). The concentration in the solutions of 

 major elements including iron, manganese and silicon are similar 

 to those at the East Pacific Rise site at 21°N ( Edmond et al., 

 1986). Minor elements including the rare earths and certain 

 isotopes including boron are different indicating differences in 

 the evolution of the solutions (Klinkhammer et al . , 1986; Edmond 

 et al., 1986; Campbell et al., 1988a). Reconnaissance of an 1800 

 km section of the rift valley of the Mid-Atlantic Ridge between 

 the TAG Hydrothermal Field at latitude 26°N and 12°N using 

 geological, geophysical, and geochemical indicators suggests that 

 other hydrothermal sites are present at a spacing of tens of 

 kilometers (Rona et al . , 1982; Rona, 1984; Klinkhammer et al., 

 1985). 



HYDROTHERMAL ACTIVITY AT THE GORDA RIDGE 



Characteristics 



The Gorda Ridge is a 300 km long volcanic mountain range 

 situated between 200 and 300 km west of northern California and 

 southern Oregon (Fig. 6). On the north it is separated from the 

 faster-spreading Juan de Fuca Ridge (full-rate 6 cm per year) by 

 the Blanco Fracture Zone and it terminates on the south at the 

 Mendocino Fracture Zone. The Gorda Ridge has been interpreted as 

 part of a deformation zone that accommodates differential motion 

 between the Juan de Fuca plate to the north and the Pacific plate 

 to the south and west (Fig. 6; Wilson, 1986; Stoddard, 1987). 

 The ridge is divided into three sections by right-lateral 

 structural discontinuities near 42°26'N and 41°26'N (Fig. 5). 

 The northern section is spreading at an intermediate full-rate of 

 5.5 cm per year (Riddihough, 1980), although its morphologic and 

 petrologic characteristics are more like those of the slow- 

 spreading Mid-Atlantic Ridge (full-rate < 4 cm/y) than any other 

 oceanic ridge in the Pacific (Fig. 4). Both the northern Gorda 

 Ridge and Mid-Atlantic Ridge have well-defined rift valleys, 

 although the width and relief of the rift valley of the northern 

 Gorda Ridge are about one-half that of the Mid-Atlantic Ridge and 

 the floor is 1 to 2 km shoaler. Spatially and temporally 

 closely associated lava types at the northern Gorda Ridge exhibit 

 a diversity in range of percentage of melting and degree of 

 compositional evolution similar to that of the Mid-Atlantic Ridge 

 (Davis and Clague, 1987). 



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