Gloria survey of the east pacific rise near 3.5°S: Tectonic and volcanic characteristics of a fast spreading mid-ocean rise

Abstract

A long-range, sidescan sonar (GLORIA) survey of an approximately 100-km square area of the East Pacific Rise crest between 3°S and 4°S extends results obtained in the same area by Lonsdale (1977) using Deep-tow. The axial, linear volcano was found to be continuous over a distance of 75 km. The presence of major inward and outward facing fault scarps was confirmed, but the GLORIA data show several distinct differences between the two fault sets. The inward dipping faults are more numerous, more closely spaced, and longer than the outward dipping ones, and their dip-slopes backscatter sound more extensively than those of the outward dipping faults; moreover most of them appear to be formed within 2 km of the axis, whereas the majority of the outward dipping faults begin to develop between 5 and 8 km from the axis. These differences suggest that the two sets of faults have different origins. The horizontal pattern of inward dipping faults is similar to those observed on other mid-ocean rises at all spreading rates, though the lengths and separations of the scarps are slightly, and their throws considerably, less than on slower spreading rises. This common horizontal pattern suggests that inward dipping faults on all rises have a common mode of formation regardless of spreading rate. Horizontal tension is probably the dominant factor, but an additional mechanism is needed to explain the polarization of fault dips that occurs in the region 2–8 km from the axis. The similarity of major fault spacings on the East Pacific Rise to those on slower spreading rises suggests that faulting is invariant in space, rather than time, and that the lithosphere where these faults are formed (about 2 km from the spreading axis) has a similar, small thickness for all spreading rates. This is attributed to the regulating effect of hydrothermal circulation and plate cooling.