Abstract. Geophysical evidence for lower continental crustal earthquakes in almost all collisional orogens is in conflict with the widely accepted notion that rocks, under high grade conditions, should flow rather than fracture. Pseudotachylytes are remnants of frictional melts generated during seismic slip and can therefore be used as an indicator of former seismogenic fault zones. The Fregon Subdomain in Central Australia was deformed under dry sub-eclogitic conditions of 600-700 • C and 1.0-1.2 GPa during the intracontinental Petermann Orogeny (ca. 550 Ma) and contains abundant pseudotachylyte. These pseudotachylytes are commonly foliated, recrystallized, and cross-cut by other pseudotachylytes, reflecting repeated generation during ongoing ductile deformation. This interplay is interpreted as evidence for repeated seismic brittle failure and post-to inter-seismic creep under dry lower-crustal conditions. Thermodynamic modelling of the pseudotachylyte bulk composition gives the same PT conditions of shearing as in surrounding mylonites. We conclude that pseudotachylytes in the Fregon Subdomain are a direct analogue of current seismicity in dry lower continental crust.

The strike-slip Davenport Shear Zone in Central Australia developed during the Petermann Orogeny (~550 Ma) in an intracontinental lower crustal setting under dry subeclogite facies conditions (~650°C, 1.2 GPa). This approximately 5-km-wide mylonite zone encloses several large low-strain domains, allowing a detailed study of the initiation of shear zones and their progressive development. Quartzo-feldspathic gneisses and granitoids contain compositional layers, such as quartz-rich pegmatites, mafic bands, and dykes, which should preferentially localize viscous deformation if favorably orientated. This is not observed, except for long, continuous, and fine-grained dolerite dykes. Instead, many shear zones, typically a few millimeters to centimeters in width but extending for tens of meters, commonly exploited pseudotachylytes and are sometimes parallel to a network of little overprinted fractures. The recrystallized mineral assemblage in the sheared pseudotachylyte is similar to that in the host gneiss, without associated hydration due to fluid-rock interaction. Lack of localization in quartz-rich, coarser-grained (typically >50 μm) rocks compared to mafic dykes, precursor fractures, and pseudotachylytes implies that localization in the dry lower crust preferentially occurs along elongate, planar fine-grained layers. Transient high stress repeatedly initiated fractures, providing finer-grained, weaker, planar precursors that localized subsequent ductile shear zones. This intimate interplay between brittle and ductile deformation suggests a local source for lower crustal earthquakes, rather than downward migration of earthquakes from the shallower, usually more seismogenic part of the crust.Shear zone nucleation and strain localization in the lower crust have a major impact on the architecture of mountain belts, and the study of these processes yields insights into the rheology of the Earth's crust. Shear zone nucleation develops from an initial perturbation in rock viscosity, with strain localization being HAWEMANN ET AL.219

Abstract. Geophysical evidence for lower continental crustal earthquakes in almost all collisional orogens is in conflict with the widely accepted notion that rocks, under high grade conditions, should flow rather than fracture. Pseudotachylytes are remnants of frictional melts generated during seismic slip and can therefore be used as an indicator of former seismogenic fault zones. The Fregon Domain in Central Australia, was deformed under dry subeclogitic conditions during the intracontinental Petermann Orogeny (ca. 550 Ma) and contains abundant pseudotachylyte. These pseudotachylytes are commonly foliated, recrystallized, and crosscut by other pseudotachylytes, reflecting repeated generation during ongoing ductile deformation under generally dry conditions. This interplay is interpreted as a cycle of seismic brittle failure and post- to inter seismic creep under dry lower crustal conditions. Thermodynamic modelling of the pseudotachylyte bulk composition gives conditions of shearing of 600–700 °C and 1.0–1.2 GPa, the same as in surrounding mylonites. We conclude that pseudotachylytes in the Fregon Domain are a direct analogue of current seismicity in dry lower continental crust.

The Musgrave Block in central Australia exposes numerous large‐scale mylonitic shear zones developed during the intracontinental Petermann Orogeny around 560–520 Ma. The most prominent structure is the crustal‐scale, over 600 km long, E‐W trending Woodroffe Thrust, which is broadly undulate but generally dips shallowly to moderately to the south and shows an approximately top‐to‐north sense of movement. The estimated metamorphic conditions of mylonitization indicate a regional variation from predominantly midcrustal (circa 520–620°C and 0.8–1.1 GPa) to lower crustal (~650°C and 1.0–1.3 GPa) levels in the direction of thrusting, which is also reflected in the distribution of preserved deformation microstructures. This variation in metamorphic conditions is consistent with a south dipping thrust plane but is only small, implying that a ≥60 km long N‐S segment of the Woodroffe Thrust was originally shallowly dipping at an average estimated angle of ≤6°. The reconstructed geometry suggests that basement‐cored, thick‐skinned, midcrustal thrusts can be very shallowly dipping on a scale of many tens of kilometers in the direction of movement. Such a geometry would require the rocks along the thrust to be weak, but field observations (e.g., large volumes of syntectonic pseudotachylyte) argue for a strong behavior, at least transiently. Localization on a low‐angle, near‐planar structure that crosscuts lithological layers requires a weak precursor, such as a seismic rupture in the middle to lower crust. If this was a single event, the intracontinental earthquake must have been large, with the rupture extending laterally over hundreds of kilometers.

Abstract. Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.