STRESS FIELDS ASSOCIATED WITH OCEANIC TRANSFORM FAULTS

Abstract

Although their formation is commonly attributed to shear stresses, oceanic transform faults contain many structural elements indicating extension across them. Examples include transform-parallel tension fractures, normal faults, grabens, dykes and small-scale spreading centres. At the junctions between transform faults and adjacent ridge segments there are commonly sets of oblique fractures. A detailed field study of oblique fractures in the vicinity of a junction between a mid-ocean ridge and an oceanic fracture zone in North Iceland shows that they are mostly tension fractures and normal faults. This paper presents the results of a boundary-element study of the stress field associated with the junction between an oceanic transform fault and its ridge segments. The results show that uniaxial tensile loading (plate pull) parallel with the transform fault tends to lock it but also gives rise to shear-stress concentration in a zone that coincides with the fault. This shear stress is primarily responsible for the strike-slip faulting and associated earthquakes in the transform fault. To generate the transform-parallel fracture-zone graben, normal faults, tensile fractures, dykes and spreading centres, however, requires tensile loading parallel with the ridge axis. The results suggest that biaxial (ridge-parallel and ridge-perpendicular) tensile loading generates a stress field that: (1) unlocks and makes slip easier on the transform faults; and (2) explains the oblique fractures at the ridge-transform junctions.