FABRIC TRANSPOSITION IN THE LOWER CRUST DURING EXTENSIONAL COLLAPSE IN WESTERN FIORDLAND, NEW ZEALAND

We investigate how the lower crust accommodates extensional collapse after orogeny by performing microstructural and electron backscatter diffraction (EBSD) analyses on an amphibolite sample from the Resolution Island shear zone (RISZ), located in Fiordland, New Zealand. The sample was deformed at upper amphibolite facies conditions (T= 650-750 °C, P= 9-14 kbar) during the ~95-88 Ma interval as a granulite/eclogite facies orthogneiss dome was denuded by a system of amphibolite facies extensional shear zones. EBSD maps reveal irregular grain boundaries between plagioclase and hornblende, along with localized zones of cataclasis. Average intragrain misorientation maps reveal higher strain in plagioclase and biotite relative to hornblende. Pole figure data show hornblende lattice preferred orientation (LPO) with {100} parallel to foliation and <001> parallel to lineation. Biotite LPO patterns show {001} parallel to foliation and <100> forming a girdle within this foliation. Plagioclase LPO patterns show {001} and <110> forming weak maxima sub-parallel to foliation and parallel to lineation, respectively, and {011} and <100> forming stronger maxima parallel to foliation and lineation, respectively. The pole figures of all three constituent minerals show a spread in maxima consistent with progressive fabric development during rotation about the X kinematic direction.

We interpret the spread in maxima as evidence of granulite fabric rotation into parallelism with amphibolite facies shear zones, which is consistent with field relationships that show transposition of steep, granulite facies foliations into subhorizontal amphibolite facies foliations along the margins of orthogneiss domes. Transposition was achieved by crystal-plastic deformation in plagioclase and biotite and oriented growth of hornblende. We observe typical LPO patterns for hornblende and biotite, and the plagioclase LPO patterns are consistent with a transition from higher to lower temperature slip systems. Our results demonstrate a grain scale response to fabric transposition during retrogression from granulite to upper amphibolite facies metamorphism, consistent with a changing style of deformation and metamorphism at the root of a Mesozoic continental arc.