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The thermal state within plate boundary fault zones is likely to influence the dimensions of large co-seismic fault ruptures, and strain localisation in underlying mylonitic shear zones. Vein microstructures, biotite chemistry and fluid inclusion data offer new insights into the thermal structure of the mylonitic shear zone down dip of the Alpine Fault, the main Pacific–Australian plate boundary structure through southern New Zealand. Quartz veins within the fault rocks were formed and deformed at temperatures ranging from > 500 °C to immediately above that of the brittle–viscous transition (∼ 325 °C), illustrating that fluids are present throughout the fault zone. Microthermometric data from primary, mixed CO 2 –H 2 O fluid inclusions in boudin-neck quartz–carbonate veins indicate that these inclusions were trapped at temperatures of 325 ± 15 °C and pressures of c. 40 MPa. Vein microstructures indicate that fluid temperatures were equal to rock temperatures. Assuming hydrostatic fluid pressure, the veins were trapped at depths of 2 O–CO 2 fluids in the shallower crust of > 500 kg m − 3 . Biotite grains in the mylonites are a mixture of porphyroclastic grains with high R = Ti/Mg# inherited from the parent Alpine Schist, and smaller, low R grains that recrystallised during mylonitisation, particularly those that are found within syn-mylonitic shear bands. Assuming a simple step-function model approximating diffusive equilibration, the changes in biotite chemistry indicate Δ T during mylonitic deformation of ∼−340 °C according to the Ti-in-biotite thermometer of Henry et al. (2005). A best-fit uplift P – T path to existing data and these new results indicate that the uplift path P – T gradient of fault zone rocks above the brittle–viscous transition is of the order of 40 °C km − 1 . Below the brittle–viscous transition, the uplift path P – T gradient is 10 °C km − 1 . The P – T profile within the fault rocks is comparable to that in the immediate hanging wall Alpine Schist, indicating there is no peak in the crustal isotherms in the fault hanging wall, as was indicated by previous numerical models, and analyses that did not consider data from the fault rocks themselves.

The prehnite-pumpellyite to greenschist facies rocks of the classic low-grade Otago schist terrane are subdivided into a number of textural zones based on the development of slaty cleavage and the appearance of segregation lamination in metagreywackes. In this study, conglomerates from fifteen localities have been studied in order to investigate the relationship between ductile strain and the sequence of textural zones. Strain measurements on lithic pebbles show a clear correlation between strain and textural zone, ranging from around 40% shortening in TZ IIA with the development of slaty cleavage to more than 70% in TZ IIIA corresponding to the appearence of segregation lamination. The much lower and more prolate strains of the quartzose pebbles compared to the corresponding lithic clasts may be explained by differential strain partitioning between clast and matrix. Microstructural evidence, including well-developed strain shadows and solution seams, indicates that solution-transfer was the dominant deformation mechanism. The development of quartz segregation laminae in TZ III is due to three related processes: the development of mica-rich solution seams demarcating quartzo-feldspathic microlithons; the development of amalgamated and recrystallised strain shadow overgrowths on clastic grains; and the formation, rotation and disruption of early formed veins. The large increase in strain from TZ II to TZ III is responsible both for increased segregation and for the rotation and flattening of segregation features into the foliation, thus producing a segregation lamination. Large strains developed by solution-transfer during the low-grade metamorphism of greywackes will lead inevitably to segregated schists.

Continental transform boundaries in detail consist of zones of fault-bounded blocks adjacent to the principal active transform fault(s). Relative movement of these blocks in response to plate boundary motions gives rise to differential vertical displacements of the earth's crust and hence exercises a degree of control on the tectonic evolution of sedimentary basins adjacent to the transform boundary. The Cenozoic sedimentary basins of southwest New Zealand have several features in common that may be attributed directly to the movement on the adjacent plate boundary. However, their location, detailed sedimentary evolution and tectonic development may best be understood in the context of the relative displacements of a number of fault-bounded blocks, in response to the plate motions. In particular, movements of the Fiordland block, bounded by the Alpine and Moonlight fault zones, exercised a large measure of influence on the development of the basins. The tectonic model presented is consistent with plate boundary motions inferred from marine magnetic anomalies while at the same time providing an explanation for many of the more detailed features of the basins.