Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust

Abstract

Creep strength of the crust depends upon the rheology of the most common mineral, usually quartz. Recrystallized quartz grains in many high-grade shear zones from the middle to lower crust are typically large (millimeter sized), implying active grain boundary migration, but equivalents from old polymetamorphic and water-deficient basement sheared at similar crustal depths can be very small. For the latter, strong crystallographic preferred orientation (CPO) of quartz, with c axes aligned close to Y, progressive mis-orientation of crystals, subgrain and dislocation development, and core-mantle structures with recrystallized grains of 2-8 μm size, all point to dislocation glide dominantly on the prism <a> system with recrystallization by subgrain rotation. Recrystallized grain size piezometry of such quartz indicates high flow stress in fine-grained shear zones, while the synkinematic metamorphic mineral assemblage and the CPO are typical of amphibolite facies conditions. This is evidence that middle to lower crust is not inevitably weak due to its high temperature: water content also has an important influence. Show more