EVIDENCE FOR MECHANICAL HETEROGENEITY IN A HIGH TEMPERATURE, HIGH STRAIN ZONE: MT HAY BLOCK, CENTRAL AUSTRALIA

A >5 km-thick high strain zone (Capricorn ridge) occurs within the Mt. Hay granulites (~800°, ~8 kb) in central Australia. Penetrative ductile deformation involved flattening and S-side-up shear (Strangways event, ~1780-1720 Ma). Foliations dip steeply SSW. Lineations plunge steeply SE. Evidence for high strain includes sheath folds and transposition of lithologic boundaries into parallelism with foliation.

Foliation intensity increases across strike towards the edges of the three km-thick granulite units that comprise Capricorn ridge (anorthositic; intermediate; pyroxenitic + quartzofeldspathic, abbreviated px + qtzfs). The lithologic contacts are gradational over several m and no fault-like discontinuity is observed, suggesting that fabric development was continuous across them. The fabric intensity pattern is interpreted to record strain localization across the contacts. The strain localization zones are interpreted as “attachment” zones between lithologic units that deformed with different competencies.

Studies across the px + qtzfs / anorthositic contact indicate that compositional domains have distinct microstructural characteristics that vary little with increasing strain. In the px granulite, grains are granoblastic to interlobate; grain aspect ratios are ≤3:1. In the qtzfs granulite, grains are interlobate and grain boundaries are aligned over distances that are several times the lengths of individual grains. Grain aspect ratios are ≤3:1 (multigrain quartz ribbons are ≤10:1). In the anorthositic granulite, grains are granoblastic and plagioclase grains are nearly equant. Grain size reduction occurs only in the qtzfs granulite (from ~0.5 to ~0.2 mm in longest dimensions). Preliminary data indicate no LPO in quartz in qtzfs granulite and in plagioclase in the anorthositic granulite. Evidence of intracrystalline strain includes tapered deformation twins in plagioclase and undulose extinction in quartz and feldspar (rarely in pyroxene).

Capricorn ridge contains evidence that compositional domains had different mechanical behaviors even at this relatively high homologous temperature of deformation. Further, deformation localized at lithologic contacts instead of within one rock type, and appears to have been accommodated by a combination of deformation mechanisms.