RHEOLOGICAL CONTRAST CONTROLLING THE DEVELOPMENT OF PAIRED SHEAR ZONES, FIORDLAND, NEW ZEALAND

Field and detailed microstructural analysis were used together with numerical modeling to examine the effect of reaction and annealing extent on the rheology of lower crustal rocks. We present data from a natural laboratory (Fiordland, New Zealand) where high-P granulite facies two-pyroxene-pargasite orthogneisses partially reacted to garnet granulite either side of felsic dykes. The metamorphic transition to garnet granulite not only changes the modal percentage of phases but also their shape and grain size. The reaction extent and annealing is most advanced close to the dykes while further away the reaction did not go to completion. As a consequence the modal percent of the rheologically hard phase garnet decreases away from the felsic dykes. Grain size decreases from maximum in the dykes to minimum in partially reacted host rock, while aspect ratios of garnet clusters increase and approach those of mafic clusters in the unreacted host rock. Post-reaction deformation localized in those areas that experienced minor to moderate reaction extent involving grain size reduction. This localization produces two spaced shear zones at either side of the felsic dykes, so-called paired shear zones. Our study shows that the effect of reaction and annealing extent on rheology is not only dependent on the change in modal percent of rheologically hard phases but also importantly the grain size. If grain size is sufficiently reduced, then deformation will occur by grain size sensitive deformation mechanisms such as grain boundary sliding, weakening the rock substantially and localizing deformation in partially reacted areas. However, if the mode of rheologically hard phases increases and grain size remains similar to the host rock or also increases, then the reacted rock is strengthened.