Strength profile across a continental-scale shear zone

Microstructural and textural analyses of quartz mylonites from the Main Central thrust allow identification of switches of deformation mechanisms caused by reductions in pressure and temperature during exhumation, and they provide quantitative constraints on stress history. At some localities there is an evolution of samples experiencing shifts to higher stress deformation that may be a result of interaction between end-member mechanisms where viscous flow cannot accommodate all the imposed displacement, which leads to “semi-brittle” deformation. Alternatively, the pulses of high stress associated with earthquakes might also be involved in generating bimodal populations of dynamically recrystallized grains. At other localities, however, there is an evolution of samples experiencing switches to lower stress deformation. The latter microstructures may be results coeval decrease of strain rate and temperature. These two contrasting deformation histories should be a focus of our future studies of megathrust systems.