THE EFFECTS OF PARTIAL MELTING AND MAGMATISM IN THE LOWER CRUST ON THE EVOLUTION OF CONTINENTAL LITHOSPHERE: RESULTS FROM FIORDLAND, NEW ZEALAND

Studies of convergent orogens have shown that the strength and rheology of the lower crust strongly influence the behavior of deforming continental lithosphere. However, determining the effects of periodic magmatism and rapidly changing thermal and structural regimes in the lower crust on lithospheric evolution remains problematic. We present data from Fiordland, New Zealand that show how interactions among magmatism, metamorphism, crustal melting, and deformation in Early Cretaceous lower crust influenced the mechanical evolution of an entire orogen from lower to upper crustal levels (10-50 km paleodepths). At the deepest levels of the orogen a major batholith dominated by diorite was emplaced into the lower crust from 126 Ma to 116 Ma. Within and below this batholith arrays of tensile fractures formed repeatedly in granulite facies metagabbros and metadiorites during a ~20 Ma interval. The pressures (14-16 kbars) and temperatures (750-800°C) of fracturing are well constrained by garnet-clinopyroxene-bearing reactions zones that formed adjacent to the fractures. Fracturing coincided with the partial melting of metadiorites and alternated in time with focused ductile flow at similar conditions. Rapid cooling by ~200°C at lower crustal depths following batholith emplacement is indicated by intergrowths of kyanite, plagioclase and quartz in fabrics that envelope the granulites. These data and geochronology indicate that the Fiordland orogen possessed an unusually strong, mafic lower crustal root from ~116 Ma to 105 Ma. Six years of accumulated structural data provide direct evidence that a strong, mafic lower crust resulted in strong vertical coupling between upper and lower crust during orogenesis. We show how low volumes of partial melt, rapid crystallization of mantle-derived magmas, and a cold, dry, mafic lower crust controlled the degree of vertical coupling and the partitioning of deformation in this well exposed section of deforming continental lithosphere, and produced a narrow, focused structural style.