STRAIN MARKER SHAPE PREFERRED ORIENTATION ACROSS A STRAIN GRADIENT IN A GRANULITE FACIES NORMAL SHEAR ZONE, MT. HAY BLOCK, CENTRAL AUSTRALIA

The >5 km thick, granulite facies, Capricorn ridge shear zone (CRSZ) is exposed on the NE margin of the Mt. Hay block, part of the Arunta inlier in central Australia. Foliations and lineations in the CRSZ are inclined steeply SSE. Parallelism of grain shape fabrics and lithologic boundaries, and locally sheath folds, all indicate high strain. S>L (foliation stronger than lineation) fabrics, mesoscale structures recording extension perpendicular to the lineation, and S-side-up shear sense indicators suggest deformation involved flattening and shear. Foliation intensity increases across strike towards the edges of the three km-thick granulite units (anorthositic; intermediate; gabbroic + quartzofeldspathic). The fabric intensity pattern is interpreted as strain localization adjacent to the contacts (Waters-Tormey and Tikoff, 2007).

The CRSZ cross-cuts an older granulite facies structural domain exposed in Mt. Hay ridge to the south. Mineral rims record similar temperatures in both domains (776±38°C for CRSZ and 816±27°C for Mt. Hay, calculated for previously reported pressures of ~8 kb). On Mt. Hay ridge, L>S to L>>S (lineation much stronger than foliation) fabrics record N-side-up shear (Staffier, 2007). Across a ~1 km transition zone, these older fabrics are rotated and completely transposed into the CRSZ. By removing rotation due to uplift, the CRSZ restores to a moderately dipping (30-50°N), N-side-down normal shear zone.

Shape preferred orientation (SPO) intensities have been calculated using strain markers across one foliation intensity gradient in the CRSZ. In addition to compositional layer thickness, feldspar porphyroclasts and polycrystalline feldspar “blebs” in gabbroic granulite, and polycrystalline quartz ribbons in quartzofeldspathic granulite, were measured in the field; pyroxene, biotite and magnetite grains in gabbroic granulite were measured in thin section. All data are from the same outcrops for comparison and have been analyzed using Flinn diagrams, and SPO and ELLIPSOID programs (Launeau and Robin, 2005). Marker SPOs indicate flattening deformation except the quartz ribbons. The degree of SPO intensity increase across the foliation gradient depends on the marker. The relative magnitudes of change for the “blebs”, ribbons and compositional layer thickness are most similar.