THREE DIMENSIONAL EVOLUTION OF A LOWER CRUSTAL SHEAR ZONE DURING LITHOSPHERIC EXTENSION AND THE COLLAPSE OF ARC CRUST, FIORDLAND, NEW ZEALAND (Invited Presentation)

An exhumed section of lower crust in Fiordland records the localization of strain into <1 km thick shear zones during the extensional collapse of a Cretaceous orogen. We determined how this localization was accomplished by examining the Doubtful Sound shear zone (DSSZ), which separates Paleozoic paragneiss (hanging wall) from Early Cretaceous gabbroic-dioritic orthogneiss (foot wall). Pyroxene-bearing igneous assemblages in the foot wall were recrystallized at the garnet granulite facies (~800°C, 1.2 GPa), including during an early period of distributed high-T extension. DSSZ formation progressively rotated and transposed old fabrics into new orientations, and formed a new down-dip mineral stretching lineation orthogonal to lineations in the granulite during retrogression to upper amphibolite facies. Mineral shape fabrics, obtained in 3D from >10, 000 grains in 30 samples using the R_f/ϕ method, reveal variations in fabric ellipsoid geometries inside and outside the DSSZ. Ellipsoid axes match local measurements of foliations and lineations, suggesting a robust data set. Analyses of plagioclase, hornblende, clinozoisite, and clinopyroxene allowed us to examine the different roles of minerals in accommodating deformation. Variations in shape and orientation exhibited by different minerals indicate a sensitivity to changing deformation mechanisms as the DSSZ evolved. As expected, hornblende and pyroxene record lower strains than matrix plagioclase in the granulites. The hot, dry interior of the foot wall records the highest stretches and is dominated by L and L>S tectonites. In contrast, DSSZ amphibolites show mostly S=L and S>L tectonites and flattening. Surprisingly, matrix plagioclase, which typically is one of the weakest lower crustal phases, shows an apparent decrease in grain distortion and alignment, and increased grain size into the DSSZ from its foot wall. These patterns reflect the resetting of old fabrics in new directions, the increased importance of biotite and hornblende in accommodating strains, and increased plagioclase grain boundary mobility during recrystallization. The results illustrate the importance of hydration reactions and the breakdown of garnet and pyroxene in localizing strain and influencing deformation style, including increased flattening, in the shear zone.