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 sheer zone (wi th a minor vertical component), active during the Oligocene in the Eastern Alps (Alto Adige, Italy), is correlated with new zircon fission track therm ochronometric data and available Rb-Sr biotite ages to constrain the depth/ temperature range of the recorded theologic regimes. Distributed deformatio n in the semibrittle regime (i.e. beneath the brittle-ductile transition) i s effective near the closure temperature for fission tracks in zircon (whic h we estimate as 280 +/- 30 degrees 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 f or the Rb-Sr and K-Ar systems of biotite (ca. 310 +/- 30 degrees C) and bel ow that for the K-Ar system of white mica (ca. 350 +/- 50 degrees C). For t he inferred temperatures and correlated Bow stresses derived by paleopiezom etry, the majority of available experimental How laws for wet quartzite pre dict strain rates on the order of 10(-13)-10(-14) s(-1), consistent with th e geological constraints. This finding supports the validity of the extrapo lation of experimental How laws to natural strain rates. (C) 1999 Elsevier Science Ltd. All rights reserved.