ESTIMATING MAGNITUDES OF SHEAR HEATING IN STRIKE-SLIP SHEAR ZONES

Shear heating in high strain zones may be an important driver of metamorphism and may contribute significantly to the thermal budget of orogenic systems. Paleostress estimates in plastically deformed rocks are high enough that deformation is expected to produce as much as 1-10 μW/m³ in ordinary crustal shear zones. Although shear heating is expected to be significant, few estimates have been made of how much shear heating actually occurs in natural shear zones. We have constructed a numerical model of shear heating that is adaptable to a wide range of parameters that can be measured in the field or can be estimated based on microstructural data, including shear zone width, duration of deformation, rheology, shear stress, deformation temperature and strain rate. A key feature of our model is the continual feedback between shear heating and rock strength. Most rocks have a temperature dependent rheology and are expected to become weaker with increasing temperature, producing less heat due to deformation. Accounting for all these factors, this model allows realistic estimates of shear heating to be made for specific natural shear zones. Generally, our modeling results show that greenschist facies shear zones can experience more than 100°C heating given a quartz-dominated rheology. Conversely, typical amphibolite facies shear zones may only produce on the order of 10-50°C of shear heating. More specifically, a one kilometer wide shear zone starting at 300°C and accommodating a displacement of 3 cm/yr for 10 million years could produce 125°C of shear heating if the deformation is dominated by quartz dislocation creep. Such a shear zone is predicted to experience a drop in shear stress from 65 MPa to 21 MPa during deformation. Exhumed strike-slip shear zones occur throughout the Appalachians and provide an excellent laboratory for studying the importance of shear heating in natural settings. We discuss estimates of shear heating based on data from the Norumbega shear zone in Maine, USA and potentially the Bowen’s Creek fault in Virginia.