GRANULITES AND BEYOND: A PERSPECTIVE FROM THE AUSTRALIAN OUTBACK (Invited Presentation)

Mechanical models of the lithosphere assume a strong upper rheological layer and a weak middle to lower rheological layer separated by a steady-state “brittle-plastic transition” occurring at a depth of ~15 km. As the lower crust is thought to be too hot to deform by brittle fracture, frictional sliding at great depth is commonly considered to be a paradox. However, the seismic record shows that earthquakes occur at depths well below the brittle-plastic transition. Many granulite facies rocks are anhydrous “i.e. dry”, due to the removal of volatiles by dehydration reactions during prograde metamorphism. Consequently, a granulite middle-lower crust would be very strong, especially in the absence of free water, and would likely fracture before deforming plastically. In contrast, a “wet’ mid-crust containing hydrous minerals would be weak and flow.

The E-trending Musgrave Province (MP) is a Mesoproterozoic, granulite to amphibolite facies terrane in the centre of the Australian continent and preserves evidence of two contrasting styles of orogenesis; high-T igneous and metamorphic episodes at ~1600 and ~1200 Ma followed by heterogeneous, high-strain, lower-T events ascribed to the Petermann Orogeny (PO; ~630 - 530 Ma). The oldest rock is felsic orthogneiss, with ~1600-1540 Ma protolith ages, and is unconformably overlain by ~1.4 Ga metasedimentary rocks. The most widespread tectono-thermal event in the MP is the ~1220-1150 Ma Musgravian Orogeny, which produced high- to ultrahigh-T granulites (T ~900 °C, P ~0.7 GPa) and voluminous anhydrous syn- to post-tectonic granitoids in the mid-crust. Two major mafic dyke swarms post-date the Musgravian Orogeny and were not deformed or metamorphosed prior to the PO. The PO resulted in a series of major regional-scale shear zones which were active at middle to lower crustal levels. The Woodroffe Thrust (WT), one of the more prominent shear zones, is a crustal-scale structure that extends east-west over 600 km. Across the WT, we have an inverted section: a seismogenic hanging wall containing abundant pseudotachylyte that was tectono-stratigraphically below a ductile footwall represented by mylonites. Ordovician–Carboniferous zircon U-Th/He ages have been linked to reactivation, during the Alice Springs Orogeny, of the WT, and other major Petermann related structures.