Fracture zones and offsets of the East Pacific Rise

Abstract

The East Pacific rise follows a gentle sinuous curve through the eastern Pacific basin on even the most recent published maps [Menard, 1964]. However, new soundings from Scripps Institution expeditions Amphitrite, CrissCross, RisePac, Step I, and Zephyrus, plus outside sources, permit extensive recontouring which shows quite a different pattern. Details of the topography will be published elsewhere; this note is intended to call attention to major features discovered and their possible significance to tectonics. The crest of the rise is offset in at least seventeen places, and in ten of these the displacement occurs on a typical linear fracture zone (Figure 1). Elsewhere the existence of fracture zones may be assumed but cannot be demonstrated with available soundings. Between offsets, the crest trends almost north-south in most places. In contrast, fracture zones trend roughly east-west, although one differs by as much as 45° from that trend. Thus, in a general way, the apparent sinuosity results from rectilinear steps. Similar offsets of the crest of other parts of the oceanic rise-ridge system are commonplace and appear characteristic [Menard, 1958, 1960, 1964; Heezen et al., 1961, 1964; Sykes, 1963].