Oceanic Mantle Dynamics and Plate Reorganization at Transform Zones
Chad Hall, Mike Gurnis, and Luc Lavier
Seismological Laboratory, Cal Tech
Transform faults and fracture zones have often been used as sites for determining the strength of
oceanic lithosphere and viscosity of its asthenosphere. While previous studies focused primarily on flexure
induced by differential subsidence and thermal stresses, we are exploring the effects of viscous flow in the
lowermost portions of the thermal boundary layers on transform structure and dynamics. We are also
investigating whether transform topography and gravity profiles can help constrain the strength of the
contact zone between lithospheric age offsets. Determing the contact zone properties is critical in assessing
models of intra-oceanic plate boundary reorganization events such as subduction initiation.
While the processes and geological conditions which lead to the formation intra-oceanic
subduction zones are unknown, it has been hypothesized since the early 1970’s that some trenches were
initiated at a transform fault(s) or fracture zone(s). Two physical factors potentially make these preferred
sites for incipient subduction: they are zones of weakness compared to normal oceanic lithosphere, and the
offset in lithospheric age introduces a localized thermal buoyancy force which places tensional stress on the
offset. We are using relatively recent advances in computational mechanics to assess the likelihood of
intraoceanic subduction initiation at lithospheric age offsets.
Our two-dimensional numerical models have an elasto-plastic-viscous rheology, meaning they
account for elastic plate-bending, the self-consistent formation of plastic shear bands (faults), and viscous
flow in warmer regions. We constrain the elasto-visco-plastic rheology using laboratory measurements, and
explore how the mechanical stability of the offset varies as a function of the following parameters: the
strength of the faulted contact zone, age difference of the offset, and far-field convergence rate.
Topography across model fracture zones varies significantly with the rheology of the contact and
asthenosphere. The largest age offsets currently observed are > 80 My on the Southwest Indian Ridge, and
the existence of such a large offset fracture zones suggests that buoyancy forces from lateral thermal
gradients may not be sufficient to nucleate a subduction zone. Determining whether or not such selfnucleation can occur is important in ultimately knowing if the formation of new subduction zones typically
causes the onset of large changes in plate motions, or vice versa. Accordingly, we will test models with
different far-field stress conditions and convergence (or divergence) rates imposed across the fracture zone.