STRAIN LOCALIZATION PROCESSES IN THE UPPER MANTLE SECTION OF AN OCEANIC PALEOTRANSFORM FAULT, BOGOTA PENINSULA SHEAR ZONE, NEW CALEDONIA

The Bogota Peninsula Shear Zone (BPSZ), in east-central New Caledonia, is interpreted as an oceanic paleotransform fault exposed in upper mantle rocks (spinel harzburgites). The BPSZ has an estimated width of 25 km but lacks clearly defined boundaries. Deformation of the upper mantle rocks in the BPSZ involves reorientation of foliation and lineation, stretching and shearing of orthopyroxene grains, folding and boudinage of orthopyroxene bands, and grain size reduction. Spinel and orthopyroxene fabric anisotropy increases from the host rocks to the mylonites of the BPSZ, which we interpret to reflect the localization of strain within the shear zone, along a strain gradient. Olivine recrystallized grain size paleopiezometry shows a corresponding increase in differential stress ranging from 8–9 MPa in the host rocks to 43 MPa in the mylonites. Two types of mylonites are distinguished based on differences in microstructural configuration, recrystallized grain size distribution, and stress level. Higher stress (30–43 MPa) mylonites show a core-and-mantle like microstructure and have a bimodal olivine grain size distribution. Lower stress (10–20 MPa) mylonites are characterized by alternating bands of coarser-grained, monomineralic (olivine) domains and a fine-grained polymineralic matrix surrounding orthopyroxene porphyroclasts. Using constraints from geothermometry, we estimate the depth of formation of both types of mylonite. We further explore the processes leading to localization of strain at different time-scales and lithospheric levels in the oceanic transform fault.

Our observations in the BPSZ raise questions about the nature of deep roots of major fault zones. If strain localization is caused solely by strain weakening, then stress in the BPSZ should be lower relative to the host rocks. Rather, the opposite relation is observed. We suggest that stress and strain are likely higher in the BPSZ because the strain rates are higher, consistent with viscous deformation.