Evidence for shear heating, Musgrave Block, central Australia

The phenomenon of shear-heating is generally difficult to recognise from pe trologic evidence alone. Establishing that shear zones attain higher temper atures than the surrounding country rocks requires independent evidence for temperature gradients. In the Musgrave Block, central Australia. thew is a clear spatial association between shear zones and interpreted elevated tem peratures. Eclogite facies shear zones that formed at similar to 550 Ma rec ord temperatures of similar to 650-700 degreesC. Outside the high-pressure shear zones, minerals with low closure temperatures such as biotite (simila r to 450 degreesC in the Ar-40-Ar-39 and Rb-Sr systems), preserve ages >800 Ma, suggesting that these rocks did not experience temperatures greater th an about 450 degreesC at similar to 550 Ma for any extended period. Thus, t he shear zones record temperatures that are similar to 200 degreesC higher than the surrounding country rocks. Simple calculations show that the combi nation of relatively high shear stresses (similar to 100 MPa) and high stra in rates (similar to 10(-11) s(-1)) for short durations (<1 Ma) can account for the observed apparent temperature variations. The evidence indicates t hat shear heating is the dominant mechanism for localised temperature incre ases in the shear zones, while the country rock remained at relatively lowe r temperatures. (C) 2001 Elsevier Science Ltd. All rights reserved.