Relationship of quartz LPO fabrics in mylonites near the Alpine Fault, New Zealand to the attitude of the shear zone boundary

Abstract:

The active Alpine fault self-exhumes its own ductile shear zone roots and has known kinematics. Within ~1 km of the fault, the foliation is subparallel to the shear zone boundary in which it formed at amphibolite-facies conditions. Using EBSD, we analysed quartz Lattice Preferred Orientations (LPOs) of mylonites along a central part of the fault. The samples were mostly taken from naturally outcropping rocks, complemented by a few sections from core from the DFDP-2B hole—rocks that accommodated a range of finite strains and that have diverse quartz contents. All the LPOs feature a single (or strongest) girdle of c-axes hat is inclined ~28 ±4° away from the pole to the shear zone boundary (SZB) in a sense that is synthetic to the bulk shear. A point maximum of a-axes is inclined at the same angle relative to the shearing direction. This girdle is perpendicular to C’ extensional shear bands in the rock, not to the bulk shear zone boundary, whereas the <a> maximum is parallel to the slip direction of the shear bands. These relationships prevail across large variations in quartz content and finite shear strain magnitude. We infer that quartz LPOs are not always reliable indicators of SZB attitude, and they do not necessarily undergo an obvious rotation relative to the SZB as a function of increasing finite strain. Both the C’ shears and the LPOs formed as late-incremental features at orientations controlled by the instantaneous geometry of a non-simple shear flow. We suggest that that the C’ planes were aligned to planes of maximum shear-strain-rate. The data can be explained by flow in a thinning and stretching shear zone that deforms in plane strain at a W_k of ~0.7 to ~0.85. In support of this, inversions of seismic focal mechanism data yield an orientation of σ₁ for the brittle crust of the central Southern Alps “natural laboratory” that approximately coincides with the predicted orientation of the contractional instantaneous stretching axis for the above-cited type of shear zone.