THERMOMECHANICAL EVOLUTION OF A DUCTILE DUPLEX

The thermomechanical evolution of a midcrustal ductile duplex in centr al Australia has been reconstructed through space and time using Ar-40 /Ar-39 thermochronology, flow stress estimates, cross-sectional restor ation of dislocation creep microstructures, and microstructural and st ructural analysis. A critical aspect of this analysis is the identific ation of populations of white micas in quartzite mylonites that have n eocrystallized below their closure temperature and which record the ti me when ductile deformation ceased. In dating these micas the myloniti c microstructures have effectively been dated. The time-temperature hi story of the duplex has been constrained through multidomain thermal m odeling of K-feldspar Ar-40/Ar-39 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 ductil e duplexes. Mapping of the deformation mechanisms and recrystallized g rain sizes of quartzites deformed under greenschist facies conditions has been used to evaluate tectonic offsets that occurred after microst ructural freezing. This analysis shows that the duplex formed as a for ward propagating thrust system accommodating similar to 60 km of conve rgence between the upper and lower plates of the megathrust, with a si gnificant 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. Alth ough 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.