Abstract
Emplacement of the Median Batholith along the Gondwana margin has been argued to have resulted through magmatic and tectonic processes. Analysis of the Grebe Mylonite Zone (GMZ) an amphibolite facies Early Cretaceous ductile shear zone, provides new insight into the kinematics, conditions and processes that played a role in its deformation and exhumation. The GMZ forms the boundary between the Outboard Median Batholith and Eastern Province and the Inboard Median Batholith and the Western Province in Fiordland, New Zealand. Three units derived from Darran Suite magmas, (1) meta-diorite, (2) meta-granodiorite and (3) meta-granite dominate the GMZ and exhibit a north striking steeply west dipping foliation and moderately south plunging lineation. The grain boundary migration (GBM) mechanism dominates quartz recrystallisation and is inferred to represent deformation temperatures of ~ 600˚C and stresses of 6 to 22 MPa derived from paleopiezometry of recrystallised quartz size.
Crystallographic preferred orientation (CPO) data of quartz, plagioclase and hornblende were obtained using electron backscatter diffraction (EBSD). Hornblende [100] a-axes align preferentially perpendicular to foliation with [001] c-axes aligned parallel to lineation, producing an orthorhombic symmetry that provides no information on shear sense. Plagioclase fabrics are sub divided into three groups: [a](c), hybrid-A and hybrid-B, from these fabrics it is interpreted that crystal plastic slip was accommodated on the (001), (010) and the previously unrecognised (011) planes. Weakening of the plagioclase CPO from east to west is thought to indicate a shift towards grain boundary sliding (GBS) diffusion creep deformation mechanisms. Quartz CPOs show predominantly Y-maxima fabrics, [0001] c-axis directions plot within the plane of foliation indicating < a > prism slip at ~ 600˚C. Progressive rotation of quartz CPO fabrics towards the west are thought to represent an increasing temperature/strain rate gradient towards the western contact between the GMZ and the Puteketeke Pluton. Interpretation of quartz fabrics indicate an overall regional oblique sinistral west side down sense of motion relative to the Western Province. However, conflicting kinematics are observed between the units of the GMZ, strain partitioning in a dominantly coaxial strain geometry is proposed as the mechanism for the difference in kinematics.