MAGMATISM AND THE EVOLUTION OF HIGH STRAIN ZONES IN THE LOWER CRUST DURING LITHOSPHERIC EXTENSION AND OROGENIC COLLAPSE, FIORDLAND, NEW ZEALAND

Over 3000 km² of Mesozoic lower crust exposed in Fiordland, New Zealand record a history of magmatism, lower crustal melting, high-grade metamorphism, and the formation of a symmetric style of extensional detachments that border eclogite- and granulite-cored gneiss domes. We integrate the results of field mapping with structural analyses of superposed mineral fabrics, metamorphic petrology, and U-Pb (zircon, titanite) and Sm-Nd (garnet) geochronology to determine the time scales and dimensions of extensional deformation as the thermal structure and rheology of the lower crust changed over a 30 My period. Pluton chemistry and metamorphic minerals record crustal thickening to 50 km by ~130 Ma. Extension began by or after ~117 Ma following the emplacement of >1000 km² of mafic-intermediate magma into the lower crust. Magma emplacement occurred in ≤5 My, with cycles of lower crustal melting, and granulite–eclogite facies metamorphism outlasting each magmatic pulse by <5 My. In as little as 3-6 My, the lower crust had cooled to below titanite U-Pb closure temperatures (600-650°C). Superposed mineral fabrics record a progression from magmatic flow to high-T extensional deformation at the garnet granulite and eclogite facies (T=700-800°C, P=12-18 kbar) to lower-T extension at the upper amphibolite facies (T=550-650°C, P=7-9 kbar). During the high-T stage (117-112 Ma), high strain zones were delocalized across >1000 km² of lower crust. During the lower-T stage (111-90 Ma), narrow (<1 km thick) shear zones localized at pluton margins. The spatial distribution of these latter shear zones suggests that strain partitioning was controlled by the size, shape and distribution of plutons that gradually cooled, strengthened and eventually resisted flow. Vertical, dipping, and nearly concentric foliations that record both constriction and flattening surround these areas of strong, cooling lower crust. Petrologic and geochronologic data suggest that the change from delocalized to localized strain evolved equally quickly as the thermal structure of the lower crust changed from 117-111 Ma. This evolution appears to have inhibited the formation of one or more dominant extensional detachments and favored the formation of a network of symmetric, focused style of low-slip (≤15 km) shear zones that border the gneiss domes.