Evidence of early Cretaceous collisional-style orogenesis in Northern Fiordland, New Zealand and its effects on the evolution of the lower crust

Structural, metamorphic, and kinematic data from a well-exposed section of lower crustal rocks in northern Fiordland, New Zealand, reveal a history of intense contractional deformation and high-P metamorphism at the roots of a convergent orogen. High-P (>14 kbars) granulite facies garnet-clinopyroxene-bearing reaction zones occur adjacent to anorthositic veins within gabbroic and dioritic gneiss. These veins and reaction zones were variably deformed by two phases of high-P granulite facies deformation. Quantitative kinematic analyses, conducted using systems of rotated veins and reaction zones, indicate that the first phase produced steeply dipping shear zones within a sinistral pure-shear-dominated flow regime (W_k = 0.69). This deformation occurred at conditions of P = 14.0 ± 1.3 kbars and T = 676 ± 34°C and resulted in subhorizontal, arc-parallel (NE-SW) stretching and up to 60% subhorizontal shortening in high strain zones of the lower crust at depths >45 km. The second phase of deformation occurred at P = 14.1 ± 1.2 kbars and T = 674 ± 36°C and produced vertically stacked, gently dipping ductile thrust faults that accommodated are-normal (NW-directed) displacement. These features reflect major tectonic thickening of the crust, oblique convergence, and high-P metamorphism during the collision of the roots of a convergent orogen, represented by plutons of the Median Tectonic Zone in eastern Fiordland, with the paleo-Pacific margin of Gondwana, represented by western Fiordland. Distinctive kinematic styles suggest that this collision resulted in a partitioning of the arc-parallel (NE-SW) and arc-normal (NW-SE) components of oblique convergence onto sinistral strike-slip and ductile thrust faults, respectively, at lower crustal levels.