Axial topographic relief associated with ridge-transform intersections

Abstract

The axial depths of mid-ocean ridges are consistently observed to increase along-strike from the central part of a ridge segment to its intersection with a transform fault. Variations in the magnitude of this axial relief probably reflect variations in the magmatic and tectonic processes that characterize plate accretion and seafloor spreading at each individual segment. Constrained by observations from 41 ridge-transform intersections, we investigate the relative effect on axial depths of transform offset, segment length, regional depth, and spreading rate. We first solve for a model of ridge segment relief as a function of the age offset of the transform and the segment length, both scaled by spreading rate. Through a series of regression analyses covering a range of spreading rate scalings, we determine a model which predicts segment relief at both fast and slow spreading centers to within 240 m standard deviation. We find that half-segment scale deepening is more than a direct thermal effect of the juxtaposition of a cold lithospheric edge and the end of a ridge at a transform fault. A second inversion problem is solved to determine the parameters that contribute to the absolute depth of the ridge axis. We can show that the absolute depth of the shallowest section of a ridge segment is strongly dependent on regional depth and that this dependence is more complex at slow spreading ridges reflecting the common, but not ubiquitous, existence of a median valley there. These results, in combination with the segment relief analysis, clearly indicate that the axial depth at a ridge-transform intersection is a function of both long wavelength processes, such as may control hot spot distribution, and local tectonic factors including, but not limited to, transform offset.