Kinematic vorticity and tectonic significance of superposed mylonites in a major lower crustal shear zone, northern Fiordland, New Zealand

Abstract

New structural, metamorphic, finite strain and kinematic vorticity data from a 4 km-wide, subvertical shear zone in Fiordland, New Zealand reveal a history of deformation reflecting different tectonic regimes. An analysis of ductile fabrics within the shear zone and its wall rocks shows two distinctive stages of amphibolite facies mylonitic deformation ( D ASZ 2 and D ASZ 3) that are superimposed on older Paleozoic or early Mesozoic structures. Variations in strain intensity and well defined shear zone boundaries have allowed us to examine the progressive development of L ASZ 2– S ASZ 2 and L ASZ 3– S ASZ 3 fabrics and compare the types, kinematics and conditions of deformation that produced them. Mineral assemblages defining L ASZ 2– S ASZ 2 provided calculated peak conditions of 11.9±1.1 kbar and 581±34°C indicative of lower crustal depths. Mineral assemblages defining L ASZ 3– S ASZ 3 provided calculated peak conditions of 8.7±1.2 kbar and 587±42°C. Finite strain and kinematic vorticity studies show that D ASZ 2 involved ductile normal faulting and crustal thinning leading to decompression and exhumation of lower crustal rocks at some time during the Cretaceous rifting of ancestral New Zealand from Gondwana. D ASZ 3 represents an episode of mid-crustal dextral transpression that may have resulted from latest Mesozoic or Cenozoic oblique convergence. Reactivation of the subvertical S ASZ 3 foliation by cataclastic shear zones and brittle faults ( D ASZ 4) was accompanied by limited recrystallization at greenschist facies conditions. D ASZ 4 shear zones record upper crustal dextral strike-slip faulting that resembles late Tertiary deformation patterns associated with the Australian–Pacific transform plate boundary.