Weak and Slow, Strong and Fast: How Shear Zones Evolve in a Dry Continental Crust (Musgrave Ranges, Central Australia)

The strike-slip Davenport Shear Zone in Central Australia developed during the Petermann Orogeny (~ 550 Ma) in an intracontinental lower crustal setting under dry sub-eclogite facies conditions (~ 650 °C, 1.2 GPa). This ca. 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 pegmatite dykes, 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.