Online

Dianiska, Kathryn Elise

Geology

2015

MS

In this thesis, I present field, microstructural, and Electron Backscatter Diffraction (EBSD) analyses of rock fabrics from high strain zones in exposures of lower crustal Cretaceous plutons at Breaksea Entrance, Fiordland, New Zealand. The interplay between deformation and metamorphism occurs across multiple scales at the root of a continental arc. I show a series of steps in which retrogressive metamorphism is linked to the accommodation of deformation. I define three main phases of deformation and metamorphism at Breaksea Entrance. The first phase (D1) involved emplacement of dioritic to gabbroic plutons at depths up to 60 km. The second phase (D2) is characterized by deformation and metamorphism at the granulite and eclogite facies that produced high strain zones with linear fabrics, isoclinal folding of igneous layering, and asymmetric pressure shadows around mafic aggregates. New structural analyses from Hāwea Island in Breaksea Entrance reveal the development of doubly plunging folds that define subdomes within larger, kilometer-scale gneiss domes. The development and intensification of S2 foliations within the domes was facilitated by the recrystallization of plagioclase and clinopyroxene at the micro-scale (subgrain rotation and grain boundary migration recrystallization), consistent with metamorphism at the granulite and eclogite facies and climb-accommodated dislocation creep. EBSD data show a strong crystallographic preferred orientation in plagioclase during D2 deformation. The third phase (D3) is characterized by deformation and metamorphism at the upper amphibolite facies that produced sets of discrete, narrow shear zones that wrap and encase lozenges of older fabrics. Structural analyses reveal a truncation and/or transposition relationship between the older S2 and the younger S3 foliations developed during D3. Progressive localization of deformation during cooling, hydration, and retrogression, resulted in the breakdown of garnet and pyroxene to form hornblende, biotite, fine plagioclase and quartz. EBSD data show a strong crystallographic preferred orientation in hornblende. During D3, hornblende and biotite accommodated most of the strain through fluid-assisted diffusion creep. The last two events (D2 and D3) reflect a transition in deformation and metamorphism during exhumation, as well as a focusing of strain and evolving strain localization mechanisms at the root of a continental arc. An examination of structures at multiple scales of observation reveals that fabrics seen in the field are a composite of multiple generations of deformation and metamorphism.