Mechanics and Evolution of Long-Offset Oceanic Transform Faults: Insights From Numerical Experiments

The majority of oceanic transform faults appear to fit a definition of a vertical quasi-planar strike-slip fault with a relatively narrow (few km) zone of deformation expressed in surface morphology. These faults generally lie on small circles centered on Eulerian poles of plate rotation. However, recent bathymetric and gravimetric surveys have shown that several long-offset transforms faults (for example, the Romanche on the Mid-Atlantic Ridge (MAR) and the Andrew Bain on the Southwest Indian Ridge (SWIR)) have very wide and complex deformation zones that do not lie on small circles. We study the mechanisms and evolution of broad deformation zones associated with long-offset transforms faults using numerical models. We developed finite element models of spreading segments of the oceanic lithosphere separated by a transform fault using the commercial software package Abaqus. The three-dimensional model domain is comprised of two layers: a brittle crust with yielding defined by the Mohr-Coulomb criterion, underlain by a viscous mantle with deformation governed by temperature-dependent power law creep. Creep in the mantle occurs in response to far-field velocity boundary conditions (imposed to simulate passive spreading), thermal buoyancy of mantle upwelling (active spreading), and thermoelastic stresses resulting from the lithospheric cooling. In the upper brittle layer we introduce strain weakening such that the yield stress decreases with increasing plastic strain; this serves to focus deformation and produce localized shear zones, which serve as a proxy for the development of new fault zones. We perform a series of simulations to assess two competing mechanisms by which a wide transform deformation zone might develop. The first mechanism involves a change in the regional spreading direction of a system of two equal-length transform faults linked by a single orthogonal ridge segment. This puts the system into transtension, which causes the initial configuration to transition to a single wide deformation zone. We simulate this mechanism by imposing a horizontal velocity boundary condition on the ridge-parallel sides of a model domain with an orthogonal transform-ridge-transform geometry. The second mechanism involves the relief of thermoelastic stresses by contraction of the lithosphere; in the transform-perpendicular direction, this contraction leads to the widening of the transform domain. The initial configuration for this mechanism is a single long-offset transform fault linking two orthogonal spreading ridge segments; we simulate thermal contraction by employing a fully coupled temperature-displacement analysis, in which changes in temperature produce changes in stress via a thermal expansion coefficient.