DUCTILE FLOW IN A THERMALLY-EVOLVING LOWER CRUST AND ITS RELATIONSHIP TO DEFORMATION IN THE MIDDLE AND UPPER CRUST OF A CORDILLERAN-STYLE MAGMATIC ARC IN WESTERN NEW ZEALAND

Studies of continental arcs have shown that ductile flow in a thermally evolving lower crust influences the style of deformation observed at shallow crustal levels during periods of extension. However, few field sites allow us to test directly models that describe how deformation at different crustal levels is related. A crustal section exposed in western New Zealand reveals how deformation in ancient lower crust was related spatially and temporally to extension in the middle and upper crust (10-50 km paleodepths) of an Early Mesozoic arc. In the deepest part of the section, at Doubtful Sound in Fiordland, a major batholith dominated by diorite was emplaced during the interval 126-116 Ma. By ~116 Ma igneous assemblages had recrystallized at granulite facies conditions involving temperatures of T>750°C and pressures of P≥12 kbars. The granulite facies assemblages formed part of a broadly distributed fabric that records subhorizontal stretching and top-down-to-the-NE and –SW displacements. Cutting this fabric are thin (<500 m thick) upper amphibolite facies shear zones that are locally mylonitic and record kinematics identical to the granulite facies fabric. U-Pb dates on zircon from a syntectonic dike within a thin shear zone yielded an age of 102 ± 1.8 Ma. The deformation ended before 88.4 ± 1.2 Ma. Published U-Pb dates on titanite suggest that the region had cooled to T<650°C by 113.5-111 Ma. These relationships show that deformation in the lower crust evolved toward a highly focused style of flow as it cooled. A comparison of deformation styles at different crustal depths after ~111 Ma implies that the extension affected the entire lithosphere within a narrow <100 km wide zone inside the arc. The data are consistent with models suggesting that this narrow, focused style of extension, including the development of metamorphic core complexes, was controlled by a cooling, viscous lower crust rather than hot, weak zones caused by magmatism. We contrast this style of extension with that observed in other arc systems.