Quantifying magnitudes of shear heating in metamorphic systems

Abstract

Estimated magnitudes of stress and strain rate in crustal rocks suggest that shear heating should contribute significantly to the thermal budget of deforming metamorphic systems. A simple one-dimensional thermal model is used to calculate magnitudes of shear heating in ductile shear zones based primarily on quartz flow laws, with consideration of additional models for wet feldspar and biotite. We calculate shear heating for likely ranges of key parameters, so that constraints can be placed on particular natural systems based on inferred stresses, durations of deformation, shear zone widths and metamorphic temperatures. By exploring large parameter spaces, our results should be appropriate for estimating shear heating experienced by a wide range of shear zones. Heating due to ductile deformation is highly dependent on these parameters and ranges from negligible to tens of degrees for most plausible deformation scenarios, and up to 200 °C in extreme cases. Results also predict that wet feldspar rheologies should produce significantly less shear heating than quartz and biotite schist rheologies. The width of a shear heating thermal anomaly can greatly exceed the width of the shear zone itself. For example, in a 0.5 km wide shear zone that experiences a maximum temperature increase of 100 °C in its core after approximately 1 Myr of deformation, a region ~1.0 km wide heats by at least 90 °C. Thermal weakening in our quartz rheology models is very rapid, so differential stresses on the order of 100 MPa should be short lived. These results can be used to better constrain magnitudes of shear heating in natural systems and can be extrapolated to infer the contribution of shear heating to evolving orogenic systems. We apply our results to the strike-slip Davenport and Norumbega shear zones (Australia and USA, respectively), also making tentative predictions for the thrust-sense Himalayan Main Central Thrust.