Chapter 3 - Topographic and Morphologic Evidences of Deformation at Oceanic Transform Faults: Far-Field and Local-Field Stresses

Abstract

Oceanic transform faults are fundamental features of the spreading processes and record changes in both the local-field and the far-field stresses, due to either local factors or plate motion changes. The study of the morphologic evidences of deformation they undergo due to these changes is key to understand the thermal and the mechanical structure of the oceanic lithosphere. Different features result from transform deformation. The large transverse ridges observed at several transforms form as a flexural response to extension at the transform or the fracture zone. The thermal heating of the lithosphere as well as the serpentinization of mantle rocks play a minor role in this uplift. Median ridges are also tectonic features formed due to compressive or extensional stresses inside the transform domain and serpentinization does not appear to play a major role in their uplift. At fast spreading ridges, extension inside the transform domain often results in the formation of “leaky transforms.” These morphologies are rare at slow spreading ridges, because the colder lithosphere does not favor the inception of intra-transform segments. However, large offset transforms, under severe changes in plate motions, may develop this geometry. Several factors will control the response of transform faults to changes in plate motion. A major one is the length of the offset, but the magnitude of the change in plate motion is also important. Minor changes will be easily accommodated by both small and large offset transforms, often through small deformations inside the transform valley. Conversely, larger changes may have small impact on small offset transforms but result in strong deformation at larger ones. Large offset transforms are slower to respond to kinematic changes than small offset ones. Mega-transforms may readjust even more slowly, thus forming a multifault complex boundary. At even larger offsets, the transform is no longer able to adjust to severe changes and will be abandoned. Local stresses such as mantle flow and thermal contraction of the lithosphere may induce significant topography at transform faults through small bends of the principal transform displacement zone (PTDZ) inside the transform domain, thus resulting in localized transpression and transtension.