Strain Accommodation, Metamorphic Evolution, And 3d Kinematics Of Transpressional Flow Within The Lower Crust Of A Cretaceous Magmatic Arc In Fiordland, New Zealand

The George Sound Shear Zone (GSSZ) exposed in Bligh Sound within Fiordland, New Zealand allowed us to reconstruct the kinematics of transpressive flow in >100 km2 of exhumed Cretaceous lower crust. We compare the three-dimensional characteristics of the deformation to theoretical models of transpression that assume steady-state flow in a homogeneous medium. This assumption is rarely the case for shear zones that experience metamorphism during deformation. We determined the three-dimensional kinematics of the GSSZ and evaluated the effects of metamorphism on strain accommodation and structural fabric evolution in the GSSZ to determine if metamorphism is an important parameter that transpressional models should account for. We found that metamorphism aided strain localization within the GSSZ and resulted in a style of structural fabric development that deviates from predictions made by theoretical models. We used foliation and lineation orientation data and field observations to determine GSSZ kinematics. Asymmetric pyroxene σ-porphyroclasts and hornblende fish show topdown-to-the-SW apparent normal shear sense with a sinistral component. The Z-axes of oblate SPO ellipsoids define the vorticity normal section and the moderately WNWplunging vorticity vector. Foliation deflections relative to the shear zone boundaries yielded a vorticity magnitude (Wk) of ≥0.8. Our kinematic results suggest that the GSSZ records inclined, triclinic transpression with sinistral, top-down-to-the-SW simple sheardominated flow. We used finite strain analysis and petrographic analysis to determine that metamorphism influences strain accommodation. Finite strain analyses were performed in 3D on 16 samples using the Rf/ɸ, Fry, and Intercept methods to determine the SPO fabric ellipsoids at different stages of deformation. Petrographic analysis was performed to identify metamorphic reactions using syn-kinematic minerals and constrain deformational temperatures using deformation mechanisms of plagioclase. Early deformation formed a ~13 km wide prolate fabric at granulite facies. Deformation later localized into a ~2-4.6 km wide oblate, mylonitic fabric at upper amphibolite facies. This fabric cross-cuts the prolate fabric and is characterized by metamorphic hornblende and biotite produced from retrogressive hydration reactions. Samples with syn-kinematic biotite contain more shear bands and display more grain size reduction of plagioclase than samples without this phase, suggesting these samples may have accommodated more strain. Changes in syn-kinematic metamorphic minerals were accompanied by steepening of stretching lineations and by changes in foliation orientation. Our analyses show that retrogressive hydration metamorphism aided strain localization within a cross-cutting oblate fabric, and the uneven distribution of biotite within this domain potentially influenced along strike variation in strain magnitude and fabric ellipsoid symmetry. Our results highlight the influence of fluid-induced metamorphism on shear zone evolution and call for new transpressional models to incorporate changes in rheology due to syn-kinematic metamorphism.