SHEAR HEATING IN THE SANDHILL CORNER MYLONITE ZONE, MAINE

The Sandhill Corner mylonite zone (SCmz) is part of the Norumbega fault system, which stretches across the state of Maine and accommodated dextral strike-slip deformation at c. 300 Ma. Shear zones like the SCmz have the potential to generate significant amounts of heat during deformation and may contribute significantly to the thermal budget of deforming metamorphic systems. Although geodynamic models often predict significant shear heating during crustal deformation, very little is known about the amount of shear heating that actually occurs in natural shear zones. I use a combination of numerical modeling, microstructural data and field observations to constrain potential magnitudes of shear heating in the SCmz. Numerical models use a temperature dependent power law rheology to calculate shear stresses and simulate progressive softening during heating and thermal diffusion. These models can be compared with observed, shear stresses, deformation temperatures, durations of deformation and shear zone width to assess potential magnitudes of shear heating.

Recrystallized quartz grain size piezometry suggests that the SCmz experienced shear stresses of 30-40 MPa. A combination of grain boundary bulging and subgrain rotation recrystallization suggests that deformation occurred at temperatures of 350-400°C. A finite shear strain of γ = 30 has previously been estimated in the SCmz, which in a 0.5 km wide shear zone could be produced in 3-5 Ma at convergence velocities of 3-5 cm/yr. My numerical models suggest that these deformation conditions are consistent with 50-100°C of shear heating in the SCmz. Significant thermal gradients surrounding the shear zone are likely obscured by the amphibolite facies host rocks of the SCmz. Furthermore, our modeling suggests, that the thermal anomaly associated with shear heating is much wider than the shear zone. Little variation in deformation temperature is expected to occur within the SCmz even if shear heating is very significant.