Thermochronometry and microstructures of quartz—a comparison with experimental flow laws and predictions on the temperature of the brittle–plastic transition

A gradient in quartz microfabrics across a major strike-slip shear zone (with a minor vertical component), active during the Oligocene in the Eastern Alps (Alto Adige, Italy), is correlated with new zircon fission track thermochronometric data and available Rb–Sr biotite ages to constrain the depth/temperature range of the recorded rheologic regimes. Distributed deformation in the semibrittle regime (i.e. beneath the brittle–ductile transition) is effective near the closure temperature for fission tracks in zircon (which we estimate as 280±30°C), with high-stress dislocation creep of quartz, microcracking, and pressure solution being active simultaneously. Steady state dislocation creep of quartz at moderate stress in the fully plastic regime is effective at temperatures above the closure temperature for the Rb–Sr and K–Ar systems of biotite (ca. 310±30°C) and below that for the K–Ar system of white mica (ca. 350±50°C). For the inferred temperatures and correlated flow stresses derived by paleopiezometry, the majority of available experimental flow laws for wet quartzite predict strain rates on the order of 10−13–10−14 s−1, consistent with the geological constraints. This finding supports the validity of the extrapolation of experimental flow laws to natural strain rates.