Abstract
AbstractThe 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.
Highlights
In simplified one-dimensional, constant strain-rate models of lithospheric strength (e.g. Goetze and Evans, 1979), depth variation in flow stress due to crystal-plastic flow is, for a specific rock composition, assumed to be controlled by temperature
The Davenport Shear Zone in Central Australia is a strike-slip fault zone developed during the Petermann Orogeny (~ 550 Ma) in an intracontinental lower crustal setting, with conditions of shearing estimated at upper amphibolite to eclogite facies (~ 650 °C, 1.2 GPa)
We present a model for a heterogeneously sheared dry lower crust involving cycles of brittle and ductile deformation leading to the repeated nucleation of shear zones and strain localization
Summary
In simplified one-dimensional, constant strain-rate models of lithospheric strength (e.g. Goetze and Evans, 1979), depth variation in flow stress due to crystal-plastic flow is, for a specific rock composition, assumed to be controlled by temperature. Laboratory determined flow laws for wet quartz are used to model the strength of uniform crust The simplest model of a quartz-only crust predicts the transition from dominantly brittle failure to dominantly ductile flow at about 300 °C, equivalent to a depth of 10-15 km, which is taken to be the base of the seismogenic zone Nazareth and Hauksson, 2004; Sibson, 1982) This zone, rocks are expected to flow homogenously with decreasing strength at increasing temperature. Field observations demonstrate that strain can be strongly partitioned and localized in such dry lower crust (Austrheim, 1987; Menegon et al, 2014, White, 2004)
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