Deformation and fabric transposition in the lower crust during extensional collapse in western Fiordland, New Zealand

To investigate how the lower crust accommodates extensional collapse after orogeny I performed microstructural and electron backscatter diffraction analyses on three amphibolite samples (P1, P3 and P4) from the Resolution Island shear zone (RISZ), located in Breaksea Entrance, Western Fiordland, New Zealand. These samples were deformed at upper amphibolite facies conditions (T= 750 °C, P= 9-14 kbar) during extension around the ~95-88 Ma interval as a granulite/eclogite facies orthogneiss dome was denuded by a system of upper-amphibolite facies extensional shear zones. Retrograde metamorphism formed assemblages including hornblende and biotite after garnet and pyroxene, while plagioclase was ubiquitous. Misorientation analyses reveal a progressive shift of tilt-type misorientations within the sample coordinate systems of low and high strain amphibolite samples. I interpret this reorientation of misorientation axes as a grain scale representation of the transposition of steep retrogressed granulite fabrics into parallelism with the horizontal fabrics of the RISZ. I use this progression to study how plagioclase and hornblende accommodated extensional deformation in lower crustal shear zones. Microstructural and misorientation data suggest dislocation creep in plagioclase accommodated by grain boundary migration and subgrain rotation during late stage granulite-facies deformation and initial extensional upper-amphibolite facies deformation followed by dislocation glide. Hornblende microstructural and misorientation data suggest initial oriented growth, followed by dislocation creep accommodated by subgrain rotation, and lastly rigid body rotation during late extensional deformation. Average intragrain misorientation maps reveal progressive strain accommodation in hornblende and continuous accommodation in plagioclase. Hornblende lattice preferred orientation (LPO) patterns reveal progressive alignment of poles to (100) and [001] to the amphibolite foliation and lineation, respectively, from sample P4, to P1, and P3. Plagioclase LPO patterns reveal distinct fabric between samples with no noticeable progression of development. Plagioclase misorientation analyses results suggest initial slip on (010) along [100] and [001] followed by activation of multiple slip systems suggested by the LPO data. These results are significant because they demonstrate: (1) that the grain scale reorientation of steep retrogressed granulite fabric reflects a change from vertical to lateral flow in the lower crust during extension that preceded rifting and continental breakup and (2) that hornblende may plastically deform along with plagioclase to accommodate extensional strain in the deep crust.