IMPOSED SHEAR LOCALIZATION IN THE MANTLE SECTION OF AN OCEANIC PALEOTRANSFORM (BOGOTA PENINSULA SHEAR ZONE, NEW CALEDONIA) (Invited Presentation)

Strain localization, often in the form of shear zones, is inferred to occur in the deep sections of plate boundaries. This localization may result from either weakening processes or imposed stress/velocity gradients, the latter of which is poorly explored. We document the rock microstructure, deformation mechanisms, and spatiotemporal stress variations in the mantle section of a lithospheric-scale strike-slip fault system, to evaluate the role of imposed boundary conditions on strain localization.

The Bogota Peninsula Shear Zone (BPSZ), New Caledonia, is an ancient oceanic transform fault exhumed from upper mantle depths. Ductile structures in the BPSZ harzburgites formed at temperatures > 820 °C and indicate that differential stress varies spatially and temporally. Spatial variation is observed as an increase in differential stress with strain toward localized zones of high strain; stress increases from 6–14 MPa in coarse grained tectonites, to 11–22 MPa within 1–2 km wide mylonite zones. Temporal stress variation is inferred by the formation of micro-deformation zones (MDZs) that show brittle-followed-by-ductile deformation; they form at orientations consistent with Riedel shears in brittle fault zones, truncate and offset relict grains in the host harzburgites, and contain fine-grained recrystallized olivine grains. The MDZs record stresses of 22-81 MPa, which are 2–6 times higher than the background stresses in the surrounding mantle rocks.

We interpret the observed microstructures and stresses in the BPSZ as the result of earthquake-related deformation in the transform system. We attribute the increase in stress in the MDZs to imposed localization induced by downward propagation of the seismic rupture into the underlying mantle. Our results suggest that the fine-grained MDZs do not represent weak zones; they form by dislocation creep at transient high-stress deformation. This work shows that earthquake-imposed stress gradients can effectively localize shear in the mantle section of transform faults. Thus, for plate boundary deformation zones, strain localization may occur as a result of imposed high-stress boundary conditions rather than weakening mechanisms.