THE CAPRICORN RIDGE SHEAR ZONE (CRSZ): AN EXPOSED NATURAL LABORATORY IN CENTRAL AUSTRALIA AND ITS IMPLICATION ON STRAIN LOCALIZATION DURING CRUSTAL EXTENSION IN A NORMAL SENSE SHEAR ZONE

The Capricorn Ridge shear zone (CRSZ) in the Mt. Hay block of central Australia is a ~6 km-thick normal-sense shear zone comprised of gabbroic, noritic, and quartzofeldspathic granulite gneisses. The CRSZ records a combination of normal-sense shear and flattening in the deep crust ~1760-1690 Ma. Transposition of an earlier deformation fabric into the CRSZ foliation and development of sheath folds indicates high penetrative strain. Strain intensity increases towards the major lithologic contacts where the gneisses exhibit sharp contacts with fine-grained mylonites. The mylonites and gneisses have similar shear sense and overall subparallel foliation and lineation, suggesting that the mylonites record the second stage of a single deformational event.

An undergraduate research group used thin section analysis and geothermobarometry to constrain deformation conditions in samples from a traverse across a strain gradient from gneiss into mylonite. Chessboard sub-grains in quartz and subgrains in feldspar indicate deformation temperatures were >700°C in the gneisses except in those immediately adjacent to the mylonites. Biotite rims around hornblende and garnet in the mylonites and adjacent gneisses indicate minor retrogression due to cooling and/or hydration. Combined with previous geothermobarometry, new results (e.g., GASP, garnet-orthopyroxene) suggest that deformation occurred during cooling and uplift (from ~800°C to ~730°C and ~8kb to ~5kb).

The sharp grain-size transition between gneisses and mylonites (500 to <10 μm), combined with petrologic and microstructural observations, suggest that earler deformation spanned the ~6 km width of the CRSZ and later deformation rapidly localized in mylonite zones within the highest-strain gneisses. The localization was not related to a significant temperature drop, as recrystallized grains in a gneiss, and porphyroclasts in an adjacent mylonite, both record the same temperature (730 ± 30°C and 725 ± 30°C, respectively). Instead, strain localization may have been related to an influx of fluids, as suggested by the abundance of sillimanite (possibly a result of alkali leaching from feldspar-rich granitoid protoliths) and biotite (hydrous alteration product) in the mylonites.