TECTONIC EVENTS RECORDED IN THE MT. HAY BLOCK, CENTRAL AUSTRALIA: IMPLICATIONS FOR INTERPRETATION OF HIGH-GRADE TECTONIC EVENTS IN THE NORTH CAROLINA BLUE RIDGE

The Mt. Hay block is located in the Arunta inlier, central Australia. The predominantly mafic, penetratively deformed, Mt. Hay granulites record peak metamorphic conditions of ~8-9 kb and ~800-900°C. Retrogression only occurs adjacent to minor mylonite zones associated with uplift. Km-scale structural domains record the Yambah (1790–1770 Ma) and Strangways (1740–1690 Ma) events on the Northern Australia Craton continental margin. The older Mt. Hay ridge structural domain is a 10-km scale sheath-like fold composed of lineation-dominated tectonites recording N-side up shear. Lineation and weak foliation are inclined moderately NE. The overprinting Capricorn ridge domain is a >6 km-thick shear zone (CRSZ) composed of foliation-dominated tectonites recording S-side up shear. Foliations dip steeply SSE and lineations pitch steeply E. Parallelism of grain shape fabrics and lithologic boundaries, transposition of lithologic boundaries into parallelism with the foliation, and locally sheath folds, all indicate high strain.

By removing rotation due to Paleozoic uplift, the CRSZ restores to a moderately dipping, N-side-down normal shear zone, the footwall of which is the Mt. Hay sheath-like fold. Shear in the Mt. Hay ridge remains N-side up. The restored CRSZ lineation orientation is oblique to the Proterozoic plate boundary normal, indicating oblique divergence. Regional studies and this work indicate that the Mt. Hay block records a switch from a continental magmatic arc setting during the Yambah event to a back-arc setting during the Strangways event.

Even though these domains are structurally distinct, prior work has shown that the granulites are geochemically indistinguishable, that zircon geochronology in both domains records both events, and that both lack prograde or retrograde metamorphism that could have occurred between the two events. Excellent exposure of the Mt. Hay block allows the detailed mapping required to document the overprinting relationships between the two domains. Over the 100m-scale transition, shear sense is ambiguous and fabric type grades from lineation- to foliation-dominated. This study holds implications for tectonic studies of high grade Blue Ridge terranes, where, for example, exposure is not as continuous.