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Over 500 km2 of rock exposure in Fiordland, New Zealand records strain localization processes accompanying the formation of a steep, transpressional shear zone within the root of a Cretaceous continental magmatic arc. Here, we pair field observations with microstructural and petrographic analyses of the George Sound shear zone (GSSZ) to investigate how metamorphism and compositional variability influenced shear zone evolution in the lower continental crust. The northern portion of the 50 km-long GSSZ deforms a monzodioritic pluton where superposed mineral fabrics record a narrowing of the shear zone width over time. Early stage deformation was accommodated mostly by dynamic recrystallization of pyroxene and plagioclase, forming a steep zone of coarse, gneissic foliations over 10 km wide. Subsequent deformation created a 2 km-thick zone of mylonite containing fine-grained plagioclase, hornblende, biotite, and quartz. The latter three minerals formed during the hydration of older minerals, including igneous pyroxene. The change in mineralogy and grain size also produced thin (< 1 mm), weak layers that localized deformation in shear bands in the highest strain zones. The southern ~35 km of the GSSZ deforms a heterogeneous section of granite, diorite, and metasedimentary rock. In this area, the hydration of igneous assemblages also is pervasive but is not restricted to high-strain zones. Instead, the shear zone branches into four ≤1 km-wide strands that closely follow lithologic contacts. The thinnest branch occurs at the contact of a coarse-grained, dioritic pluton and a fine-grained granitic pluton. These patterns suggest that the factors that controlled strain localization in the GSSZ vary along its length. In the north, where its host rock is homogeneous, retrograde metamorphism helped localized strain into shear bands at the micro scale, mirroring a narrowing at the km scale. In the south, lithologic contacts created weak zones that appear to have superseded the effects of metamorphism, creating a series of thin, branching high-strain zones. These results suggest that the rheology of lower-crustal shear zones also varies significantly along their length and over time. Both of these factors can be used to generate improved models of continental deformation. 

Garnet ages for eclogite and granulite from the Western Fiordland Orthogneiss (WFO) provide a precise age for high-grade metamorphism and partial melting of the lower crust in a Cretaceous magmatic arc currently exposed in Fiordland, New Zealand. U/Pb zircon ages and pluton areas indicate that a high magmatic flux event between 118 and 115 Ma added >3,000 km2 of mid- to lower-crustal plutons. The high flux event was followed by high temperature metamorphism and partial melting which resulted in pervasive leucosomes, and trondhjemite layers and veins. At least 1,800 km2 of the newly added crust was metamorphosed to garnet granulite facies orthogneiss. Thermobarometry and phase diagram models indicate that garnet grew at 850 to 1,000°C and 12 to 14 kbar in this monzodiorite and diorite gneiss of the Misty, Malaspina, and Breaksea plutons. Sm-Nd garnet-rock isochrons for these three plutons of the WFO (>700 km2of lower crust) indicate that peak temperatures were reached at 111.7±1.0 Ma (N=16). The isotopic and chemical composition of zircon indicate that the Cretaceous arc flare-up was most likely triggered by partial melting and hybridization of subducted oceanic crust and enriched subcontinental lithospheric mantle directly prior to cessation of arc magmatism. The driving mechanism for the terminal magmatic surge is inferred to be propagation of a discontinuous slab tear beneath the arc, or a ridge-trench collision event between 136 and 128 Ma. The lack of ca. 112 Ma plutons in the western part of Fiordland negates a magmatic heat source for garnet granulite metamorphism. Therefore, we infer that high heat flow associated with mantle advection at the base of the arc after the magmatic surge continued for several m.y., heating the lower crust to granulite facies temperatures