A KM-SCALE LOWER CRUSTAL SHEAR ZONE RECORDING OBLIQUE EXTENSION IN A BACK-ARC SETTING, MT HAY BLOCK, CENTRAL AUSTRALIA

A ~ 6 km thick section of a lower crustal shear zone is exposed in the Mt. Hay block (~10 km x 5 km) in central Australia. Mt. Hay granulite-grade tectonites (700-800°C, 7-8 kb) are primarily mafic to intermediate in composition, with crystallization ages of ~1810 Ma. Deformation is attributed to the Strangways orogenic event (~1800-1780 Ma).

A south-side up sense of shear and a strong, steeply E- to NE-plunging stretching lineation are penetrative throughout the Mt. Hay block except in discrete shear zones associated with younger uplift. Foliations dip steeply to the S or SE except where deflected around lower strain lenses. Lower strain lenses (0.5 – 10 km in longest exposed dimension) contain primarily mafic granulite (± quartzofeldspathic granulite) and felsic augen gneisses, characterized by L- and LS- (lineation > foliation) tectonite fabrics. Primary igneous features are observed at high angles to the lineation.

The highest strain zone is exposed on Capricorn ridge, on the NE part of the Mt. Hay block. This area is also the most lithologically diverse part of the Mt. Hay block. Gradients from lower strain lenses to higher strain zones are marked by fabric gradients from LS to SL (foliation > lineation), coincident with a progressive recrystallization of felsic augen gneisses to completely recrystallized blastomylonites. These gradients occur over 200-300 m across strike. Lineations and foliations across the fabric gradients are subparallel.

Previous geochemical and petrological analyses indicate that the Mt. Hay protoliths were generated in a back-arc setting on an active margin of the north Australian craton. We propose that the lower crustal fabrics are consistent with oblique transtension in a back-arc setting. Transtension generates and preserves L- and LS-tectonite fabrics such as those in the Mt. Hay block. Decameter-scale, high strain, SL-fabric zones record strain localization between the lower strain zones (lithons). This strain pattern is common in shear zones at the meso-scale and consistent with seismic images of the lower-crust in extensional terranes. Similarities in composition and fabric between the Mt. Hay block and correlative granulite blocks to the east suggest that the Neoproterozoic Strangways orogenic event records an extensional, rather than a contractional, tectonic setting.