Thermomechanical evolution of a ductile duplex

The thermomechanical evolution of a midcrustal ductile duplex in central Australia has been reconstructed through space and time using ⁴⁰Ar/³⁹Ar thermochronology, flow stress estimates, cross-sectional restoration of dislocation creep microstructures, and microstructural and structural analysis. A critical aspect of this analysis is the identification of populations of white micas in quartzite mylonites that have neocrystallized below their closure temperature and which record the time when ductile deformation ceased. In dating these micas the mylonitic microstructures have effectively been dated. The time-temperature history of the duplex has been constrained through multidomain thermal modeling of K-feldspar ⁴⁰Ar/³⁹Ar data. The modeling demonstrates that a temperature gradient existed across the duplex during its formation. The concept of microstructural continuity during ductile deformation has great potential for elucidating the kinematic evolution of ductile duplexes. Mapping of the deformation mechanisms and recrystallized grain sizes of quartzites deformed under greenschist facies conditions has been used to evaluate tectonic offsets that occurred after microstructural freezing. This analysis shows that the duplex formed as a forward propagating thrust system accommodating ∼60 km of convergence between the upper and lower plates of the megathrust, with a significant fraction of the displacement occurring after microstructural freezing. Finally, using the data as input to published flow laws for quartz aggregates provides a strain rate history for the duplex. Although uncertainties are clearly large, the timing of highest-estimated strain rates during duplex evolution does, indeed, correlate with the highest rates of convergence between the upper and lower plates of the megathrust system (according to regional cooling history studies) and with coeval sedimentation in adjoining molasse basins.