Online

Lindquist, Peter Carl

Geology

2020

M.S.

Rocks exposed in Fiordland, New Zealand provide a record of magmatic and tectonic processes that were active in the middle to lower crust of a magmatic arc during the Early Cretaceous. The George Sound shear zone (GSSZ) is one expression of those processes, and is a steep, lower-crustal shear zone that accommodated oblique sinistral motion within the continental margin of Gondwana. I have compiled structural and petrologic observations from five field areas that span the 50 km length of the exposed GSSZ. Directional and orientation statistics allow me to compare the orientation of fabrics at each field area to characterize the geometry of the GSSZ. Petrographic and microstructural analyses provide insight into the metamorphic history of rocks the GSSZ and the temperatures at which they deformed. Synthesizing these data, I construct a model of the large-scale architecture of the GSSZ and explore the tectonic, magmatic, and metamorphic processes that may have driven its evolution. The George Sound shear zone is defined by a zone of rocks that exhibit evidence of deformation at upper amphibolite facies conditions and dominantly sinistral kinematic indicators in N- to NE-striking fabrics. This zone varies from meters to kilometers in width, and contains segments that split into up to four separate branches. Apparent variations in the amount of strain accommodated by different fabrics within the shear zone suggest that, over time, strain localized into narrower zones. These strands of high-strain fabrics experienced extensive hydration metamorphism or are found in lithologically heterogeneous areas containing weaker lithologies, indicating that strain localization processes in the lower crust can vary at the kilometer scale within a single shear zone. The geometry of fabrics also varies along strike within the GSSZ, expressed, from north to south, as a shallowing of mineral lineation directions from 70° plunges to 0--30° plunges. Such variation may be the result of strain partitioning within the lower crust, with contractional structures adjacent to the GSSZ along its southern extent enabling more strain partitioning than in the north where the GSSZ appears to accommodate non-partitioned transpression. The architecture and along-strike variations in deformation processes in the GSSZ highlight the possible complexities of lower-crustal shear zones and the numerous factors that may control the rheology of the lower crust.