Oblique seafloor spreading across intermediate and superfast spreading centers

Abstract

We show that oblique seafloor spreading occurs in several regions where obliquity, α, was not recognized before. These include the slow spreading centers of the Red Sea (α≈20°), intermediate spreading centers of the Cocos–Nazca plate boundary between 91°W and 94°W (α≈9°), and superfast spreading centers of the East Pacific Rise at the Nazca–Pacific plate boundary between 29°S and 32°S (α≈10°) and perhaps between ≈16°S and ≈22°S (α≈4°). Thus, oblique spreading occurs across slow, intermediate, and superfast spreading centers, but not across fast spreading centers. Across intermediate and superfast spreading centers, oblique spreading does not occur in staircase-mode plate geometries, but does occur in oblique-mode plate geometries. Across slow and intermediate spreading centers, obliquity tends to decrease with increasing spreading rate, while across fast and superfast spreading centers it tends to increase with increasing spreading rate. Oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these. Using prior methods, we estimate the ratio of overpressure to remote tectonic stress for several spreading centers. In particular we show along a segment of the Cocos–Pacific plate boundary that magma overpressure is only one-fourth as large as remote tectonic stress, consistent with a prior inference from other observations. The highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume, but not all ridge segments near plumes spread obliquely. For one set of estimates of plume fluxes, the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate.