Interplay Between Brittle and Ductile Deformation in the Lower Crust (Musgrave Ranges, Central Australia)

Abstract

The localization of deformation in dry lower continental crust and the implied rheology have been investigated on the basis of field and laboratory observations from a well exposed and preserved section in Central Australia. The strength of the lower crust is the deciding factor determining whether deformation of crust and mantle are closely linked or decoupled, which is important for understanding the overall response to strain of continental plates and, in particular, the development of fault zones at depth. The usual assumption is that the upper part of the crust is relatively strong and deforms in a brittle manner, with deformation accommodated by stable frictional sliding and episodic seismic rupture. Consequently, fault rocks developed in this part of the crust are dominantly cataclasites and pseudotachylytes. As temperature rises with depth, the rocks begin to weaken and deform in a viscous manner, with an area of mutual overprinting cataclasites, pseudotachylytes and mylonites, termed the brittle-ductile (or frictional-viscous) transition zone. The depth of this zone is usually around 10-15 km, depending on the geothermal gradient, which is supported by the observation that most large continental earthquakes are located within this depth range. As this zone is marked by the onset of ductility in quartz, deeper parts of the crust are expected to flow rather than fracture. However, geophysical records show earthquakes even in the lower parts of the continental crust, at depths down to 40-50 km. Rocks from these depths are seldom exhumed to the surface and therefore rarely preserved. The central Musgrave Ranges in Central Australia provide a unique insight into the architecture of the continental lower crust, as several lithospheric scale shear zones that developed during the ca. 550 Ma Petermann Orogeny are preserved with outstanding exposure. The Petermann Orogeny localizes deformation in an intraplate position between the cratons of Australia, which amalgamated during the ca. 1200 Ma Musgravian Orogeny. This event reached granulite facies metamorphism in the core of the orogen, with partial melting and breakdown of water-bearing minerals producing effectively dry rocks. Metamorphic conditions during the later Petermann Orogeny reached sub-eclogitic facies (650-700 °C, 1.2 GPa) in the Fregon Subdomain, which was thrusted over the lower grade Mulga Park Subdomain in the north by the Woodroffe Thrust. In the hanging wall, the Davenport Shear Zone accommodated strike-slip movement in an overall transpressional setting. Strain is distributed heterogeneously in this 5 km wide mylonite zone and it encompasses several low strain domains where the initial stages of shear zone development are still preserved. The shear zone foliation varies from moderately to steeply dipping towards the SSW, with a sub-horizontal stretching lineation plunging towards the ESE and WNW. In low strain domains, fine grained dolerite dykes localize deformation, while quartz-rich pegmatites and mafic enclaves remain largely undeformed, even if these supposedly weaker layers are oriented in a favorable orientation for shearing. Instead, narrow shear zones (a few centimeters wide and several tens of meters long) crosscut lithological boundaries and many of those exhibit parallel fractures or features characteristic of pseudotachylytes, such as injection veins or breccias, demonstrating that most narrow shear zones nucleate along brittle precursors. Sheared pseudotachylyte is also found as clasts in a second generation of pseudotachylyte, illustrating a cyclical interplay of brittle fracturing and viscous shearing. The recrystallized paragenesis in the pseudotachylyte therefore gives information not only on the pressure-temperature conditions during shearing but also during pseudotachylyte emplacement. Thermodynamic modeling on the basis of local bulk compositions of sheared pseudotachylytes, derived from quantified X-ray maps using XMapTools, give P-T estimates similar to those derived from the mylonites (600-700 °C, 1.1-1.3 GPa). While differential stresses necessary to produce pseudotachylyte must be high (~ 1 GPa) in dry lower crustal rocks without significant pore fluid pressure, the grain size of quartz in adjacent mylonites is relatively coarse, in the range 50-100 microns, which indicates longterm flow stress on the order of 10 MPa or less. However, subgrains are evident in almost all quartz grains, with sizes below 5 and down to 1 micron. This can be interpreted as a non-steady state recrystallized grain size, resulting from transient high stresses. Another indicator for high stress events is the occurrence of fractured and plastically deformed garnet in a matrix of comparatively weak quartz and feldspar. Multiple generations of fractures show overprinting relationships and often induced lattice distortions in the garnet. Crystal plastic behavior is further manifested by subgrain rotation recrystallization. Dislocations are visible in TEM-images (transmission electron microscopy) and are mostly organized in dislocation walls, indicating recovery. Transient high stresses in the mid- to lower crust have been attributed to the downward propagation of the seismogenic zone during large earthquakes. However, the large amounts of pseudotachylyte throughout the Fregon Subdomain would require a tremendous amount of seismicity in the upper crust. In the case of the heterogeneously deformed Davenport Shear Zone, temporal and spatial variations in stress might instead be explained by the local interaction of stronger, relatively undeformed blocks within a network of narrow shear zones.