How Plates Pull Transforms Apart: 3-D Numerical Models of Oceanic Transform Fault Response to Changes in Plate Motion Direction

Abstract:

Observations along oceanic fracture zones suggest that some mid-ocean ridge transform faults (TFs) previously split into multiple strike-slip segments separated by short (<~50 km) intra-transform spreading centers and then reunited to a single TF trace. This history of segmentation appears to correspond with changes in plate motion direction. Despite the clear evidence of TF segmentation, the processes governing its development and evolution are not well characterized. Here we use a 3-D, finite-difference / marker-in-cell technique to model the evolution of localized strain at a TF subjected to a sudden change in plate motion direction. We simulate the oceanic lithosphere and underlying asthenosphere at a ridge-transform-ridge setting using a visco-elastic-plastic rheology with a history-dependent plastic weakening law and a temperature- and stress-dependent mantle viscosity. To simulate the development of topography, a low density, low viscosity ‘sticky air’ layer is present above the oceanic lithosphere. The initial thermal gradient follows a half-space cooling solution with an offset across the TF. We impose an enhanced thermal diffusivity in the uppermost 6 km of lithosphere to simulate the effects of hydrothermal circulation. An initial weak seed in the lithosphere helps localize shear deformation between the two offset ridge axes to form a TF. For each model case, the simulation is run initially with TF-parallel plate motion until the thermal structure reaches a steady state. The direction of plate motion is then rotated either instantaneously or over a specified time period, placing the TF in a state of trans-tension. Model runs continue until the system reaches a new steady state. Parameters varied here include: initial TF length, spreading rate, and the rotation rate and magnitude of spreading obliquity. We compare our model predictions to structural observations at existing TFs and records of TF segmentation preserved in oceanic fracture zones.