Online

Lincoln, Emily Sarah

Geology

2021

M.S.

Fiordland, New Zealand provides one of the best-known and deepest (to 65 km) exposures of an Early Cretaceous magmatic arc root known to geologists. These exposures allow for us to study tectonic deformational processes at varying crustal depths, including the role of pre-existing structures on later reactivation. The well-preserved Grebe shear zone (GSZ) marks the boundary between major basement terranes in southern Fiordland and has undergone multiple episodes of deformation during the Cretaceous and Cenozoic time periods. The primary focus of this study is to recognize and characterize the differing phases of deformation that occurred along this shear zone. To investigate these phases, we have conducted structural, finite strain, fault-slip, and kinematic analysis, on structural measurements and samples taken from Fiordland. We use these methods in concert to identify and differentiate the deformational styles. In southern Fiordland, the GSZ is characterized by a narrow zone of protomylonitic-mylonitic fabric within amphibolite retrogressed to greenschist facies rock. Finite strain analysis on feldspar aggregates from samples in and around the GSZ produced primarily oblate ellipsoids, indicative of shortening across the shear zone. Asymmetrical shear sense indicators present in thin sections oriented parallel, perpendicular, and oblique to lineations also suggest a component of sinistral obliquity in shear zone fabrics. This coupled with a deflection of foliations in surrounding rock towards parallelism with the shear zone boundary is consistent with transpressional deformation. This deformation is localized to a zone of ductile deformation where components of sinistral strike-slip and shortening are accommodated in close proximity to the shear zone (non-partitioned). This deformational event is associated with the formation of the shear zone and is overprinted by a separate transpressional event that took place during the Cenozoic. Fault-slip analysis showed that this reactivation event is accommodated in primarily brittle faults in where one set accommodates mostly or purely strike-slip motion, and another that accommodates mostly or purely reverse motion (partitioned). This contrasting style of transpression implies that the Cretaceous ductile shear zone influenced the behavior of strain during Cenozoic reactivation.