OBLIQUE RIFTING IN A SLOW-SPREADING RIDGE

IN oceanic rifts, the spreading direction is usually nearly perpendicular to the ridge axis, but at the Reykjanes1 and Mohns2-4 ridges this direction is highly oblique (30-degrees to 40-degrees to the axis). These ridges possess axial valleys containing oblique raised features (highs), corresponding to second- or third-order ridge axis discontinuities5. At the Reykjanes ridge, Searle and Laughton1 observed small highs which they interpreted as volcanic domes. Here we present a fault map of the Mohns ridge obtained from image processing of Seabeam data. We argue that the oblique highs in this ridge are of tectonic origin. The axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading. We describe laboratory experiments on brittle-ductile models which support this interpretation, in which stretching at 30-degrees to a deformable band reproduces the en echelon horst-and-graben pattern of the Mohns ridge.