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 degreesC 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 degreesC 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 are-parallel (NE-SW) and are-normal (NW-SE) components of oblique convergence onto sinistral strike-slip and ductile thrust faults, respectively, at lower crustal levels. (C) 2001 Elsevier Science Ltd. All rights reserved.