Evidence for shear heating, Musgrave Block, central Australia

The phenomenon of shear-heating is generally difficult to recognise from petrologic evidence alone. Establishing that shear zones attain higher temperatures than the surrounding country rocks requires independent evidence for temperature gradients. In the Musgrave Block, central Australia, there is a clear spatial association between shear zones and interpreted elevated temperatures. Eclogite facies shear zones that formed at ~ 550 Ma record temperatures of ~650-700°C. Outside the high-pressure shear zones, minerals with low closure temperatures such as biotite (~450°C in the ⁴⁰Ar-³⁹Ar and Rb-Sr systems), preserve ages >800 Ma, suggesting that these rocks did not experience temperatures greater than about 450°C at ~550 Ma for any extended period. Thus, the shear zones record temperatures that are ~200°C higher than the surrounding country rocks. Simple calculations show that the combination of relatively high shear stresses (~100 MPa) and high strain rates (~10^-11 s^-1) for short durations (<1 Ma) can account for the observed apparent temperature variations. The evidence indicates that shear heating is the dominant mechanism for localised temperature increases in the shear zones, while the country rock remained at relatively lower temperatures.