High‐temperature plastic deformation of quartz‐plagioclase multilayers by layer‐normal compression

Abstract

It is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts, respectively. Such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase. To test this assumption, we performed deformation experiments on synthetic layered and particulate quartz‐anorthite (50∶50) composites and aggregates of the end‐members in a Paterson gas‐medium apparatus at a confining pressure of 300 MPa, a constant strain rate of 10−5 s−1 and temperatures from 1273 to 1473 K. Under these conditions, anorthite deforms by recrystallization‐accommodated dislocation creep, while quartz is semibrittle. Electron backscatter diffraction measurements show a strong lattice‐preferred orientation (LPO) of anorthite developed in deformed specimens, and the LPO pattern is interpreted as a result of dislocation slip on the (010)[100] system. Our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers. Thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz‐anorthite mixture (i.e., particulate composites) with the same modal composition but weaker than pure quartz aggregates. Our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that (1) compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and (2) thin‐layered rocks compressed normal to the layering are rheologically stronger than homogeneous, isotropic mixtures under the same deformation conditions. Thus weak‐phase‐based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate.