Are spreading centers perpendicular to their transform faults?

Abstract

A COMMON assumption in seafloor spreading is that mid-ocean ridge crests are aligned perpendicular to their transform faults and, hence, to their spreading directions. There are some well known exceptions to this rule, for example, the Reykjanes Ridge. Vogt et al.¹ suggested that spreading systems may take one of two configurations: either a transform faultless, oblique configuration or a perpendicular one. He then assigned the Reykjanes and certain older anomaly sets to the first category and the rest, the segmented, faulted ridges to the latter. We agree with this bimodal separation of ridge types, and here we discuss only the latter, the transform-faulted ‘perpendicular’ group. We examined all available detailed data from this group, and wherever we could find a fine scale map which included both transform fault and spreading centre we measured their trends. Of eight segmented, slow-spreading centres (half-rate less than 3 cm yr⁻¹) we did not find a case which was, in fact, perpendicular. All were 6–38° oblique, and all were oblique in the sense which shortens the connecting transform faults, that is, the configurations in Fig. 1a as opposed to those shown in Fig. 1b . Fast- and some intermediate-rate spreading centres, on the other hand, seem to be perpendicular within the errors of measurements. These results are particularly interesting for the constraints that they place upon models of spreading centres in which the ridge crest-transform fault angle is used as a measure of the relative amounts of energy dissipated by these two features as motion occurs across them.