Distortions, rotations and crustal thinning at ridge–transform intersections

Abstract

WHEN an oceanic ridge intersects a transform fault, the newly formed oceanic crust may be severely distorted, frequently exhibiting major rotations of fault blocks^1-4. Such effects typically extend over a 10-km-wide strip parallel to the transform. By contrast, studies of contemporary oceanic transforms suggest that deformation is restricted to a single fault trace within the transform domain^5-9. This apparent inconsistency can be reconciled if the rotations occurred at the ridge-transform intersection during accretion of the crust. Here I develop a kinematic model that considers the effect of an ephemeral period of stretching on a region that broadens as the transform is approached along the ridge. The strains calculated from this model indicate extension at the ridge, simple shear at the transform and composite strains in the region of distributed deformation. The model predicts very rapid rotation rates (100° in 10⁴-10⁵ yr). The implications for the topography and structure of ridge-transform intersections are considered, and the alternating periods of extension by stretching and by intrusion are illustrated by reference to contemporary transforms on the Mid-Atlantic Ridge. The model may also explain the common occurrence of cross-cutting dykes near transform faults in ophiolites.