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Judge, Phoebe A.

Geology

2006

MS

In this thesis, I present structural, kinematic, and stress inversion data from the Darran Range in northern Fiordland, New Zealand. The ~800 kin² Darran Range has excellent exposure of faults within the upper crust, and provides an opportunity to study strain localization and fluid-induced weakening processes adjacent to the obliquely convergent Australia-Pacific plate boundary. Fault-slip data collected in the region outline several distinct zones of deformation, as well as the presence of elevated pore fluid pressure in the upper crust. Strain localization and weakening mechanisms, including elevated fluid pressure, may assist in explaining differences in deformation, such as the width of the deformation zone and the continued reactivation of major faults, near collisional plate boundaries. Kinematic solutions and stress inversions reveal spatial variations in the degree of strain localization and strike-slip partitioning adjacent to the Alpine Fault. Stress inversions of fault-slip data from major fault segments within ~10 krn of the plate boundary show evidence of elevated pore fluid pressure. Geometrical and frictional constrains on the analysis of stress tensors calculated from stress inversions indicate that the coefficient of friction is extremely low (mu = 0.10) near the Alpine Fault. Compression axes are oriented ~60° from the dominantly northeasterly strike of the plate boundary, and this orientation, combined with the low coefficient of friction, suggest a weakening of the crust around the Alpine Fault Zone. Deformation within 10 km of the plate boundary is characterized by reverse, oblique-reverse, and strike-slip fault populations. At distances greater than 10 km to the southeast of the plate boundary zone, deformation is characterized by oblique-reverse and strike-slip motion on reactivated steep, brittle faults; vertical motion is predominantly localized at lithologic boundaries. This deformation results in the extrusion of wedge-shaped blocks in the Darran Range. Cross-cutting relationships and kinematic analysis indicate the superposition of stress fields in northern Fiordland, including an older phase of normal faulting from the Late Cretaceous - Early Tertiary. Descriptions of strain localization mechanisms, such as fluid-induced weakening and the partitioning of strain in different regions, improves our understanding of deformation processes at collisional plate boundaries, and how the upper crust responds to tectonic stresses. The dominant strain localization mechanism in northern Fiordland is elevated pore fluid pressure in the near-boundary deformation zone. The inherited structure of the region assists in the localization of strain along rheological and lithologic boundaries, but it is subordinate to fluid-induced weakening.