Quantifying Strain across a Paleotransform Fault Using Incremental Deformation, Bogota Peninsula, New Caledonia

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault preserved within the mantle section of the Massif du Sud, a Cretaceous ophiolite in New Caledonia. Field measurements document systematic rotations of foliation, lineation, and pyroxenite dikes across a 50-km region. These rotations are centered on and symmetric about a 3-km wide high strain zone, suggesting this region records mantle deformation beneath a true transform fault (and not a fracture zone).

We used these changing fabric orientations to model the kinematics of deformation recorded by the ophiolite. The shear zone was subdivided into three regions (farfield, nearfield, and central high strain zone) with approximately homogeneous deformation. The main ophiolite was also included in our modeling, as it presumably records ridge-related deformation. Shear zone localization was modeled as the result of superimposed increments of deformation from initial fabric creation at the ridge to final transform motion within the high strain zone. We used a grid search to generate a variety of incremental deformation matrices testing potential shear plane orientations, shear sense, and transpressional versus transtensional kinematics. The best models were determined by comparing (1) the orientation of the strain ellipse with field foliation and lineation, and (2) the predicted and observed rotations of pyroxenite dikes across the shear zone.

Our modeling suggests that fabrics within the main Massif du Sud formed on the south flank of an obliquely spreading mid-ocean ridge trending ESE. Fabrics in the Bogota Peninsula shear zone, in contrast, record nearly perfect dextral simple shear along a transform fault, striking N to NNE, suggesting an acute angle between the transform fault and ridge segments. The model region between the central high strain zone and the main ophiolite has more complicated kinematics, which may reflect alternation between transcurrent motion and spreading-related magmatism.