MODELING DEFORMATION ACROSS A PALEOTRANSFORM FAULT IN THE NEW CALEDONIA OPHIOLITE USING SHAPE PREFERRED ORIENTATION OF ORTHOPYROXENE GRAINS

The Massif du Sud ophiolite in New Caledonia exposes Cretaceous-age mantle rocks. Most of the ophiolite has subhorizontal foliations and lineations with weak fabric development. However, foliations rotate and steepen and the shape preferred orientation (SPO) of orthopyroxene grains in harzburgites rotate and increase in strength on the Bogota Peninsula. Pyroxenite dikes also rotate and deform more in this region than elsewhere in the Massif du Sud. The greatest deformation occurs within a 3-km wide area inside a broader 10-15 km zone, and the rotation of fabrics is roughly symmetric across this large shear zone. Previous workers have attributed these patterns of field fabrics to the preservation of a transform fault in the ophiolite.

We use the orthopyroxene SPO to model deformation in this region. Working on one side of the shear zone, which we assume to be an oceanic inside corner, we model its three-dimensional flow as a heterogeneous simple shear. Parameters include the strike of the transform, displacement along the transform, and decay of displacement away from the transform. SPO ellipsoids at 16 stations are treated as viscous clasts according to Eshelby's deformable ellipsoid theory. Among all possible parameter values, we find those that undeform the SPO ellipsoids to a minimally aligned initial state (or other prescribed initial conditions). We then use the resulting flow field to undeform foliation, lineation, and pyroxenite dikes, for comparison to results by other authors and for other ridge-transform systems.

This type of model represents an improvement on our previous models of some of the same field data, which assumed that all field fabrics began with the same initial orientation. This assumption may not be applicable in a ridge-transform system, where deformation near the inside corner are influenced by both spreading at the ridge and shearing along the transform fault. Because deformation is recorded in mantle rocks, our approach may provide insight into deeper lithospheric deformation beneath actively deforming ridge-transform systems.