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

It is a common practice in rheological modeling to take the extrapolated co mpressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental dusts, respectively. Such a p ractice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase. To test this assumption, we performed defo rmation experiments on synthetic layered and particulate quartz-anorthite ( 50:50) composites and aggregates of the end-members in a Paterson gas-mediu m apparatus at a confining pressure of 300 MPa, a constant strain rate of 1 0(-5) s(-1) and temperatures from 1273 to 1473 K. Under these conditions, a northite 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 defo rmed specimens, and the LPO pattern is interpreted as a result of dislocati on slip on the (010)[100] system. Our experimental results show that the bu lk flow strength of layered composites increases with decreasing thickness of the layers. Thin-layered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz-anorthite mixture (i.e., par ticulate composites) with the same modal composition but weaker than pure q uartz aggregates. Our experimental results together with the theoretical ov erview presented previously in the materials science literature demonstrate that (I) compressive flow strength of a polyphase rock cannot be represent ed by that of the weak phase and (2) thin-layered rocks compressed normal t o the layering are theologically stronger than homogeneous, isotropic mixtu res under the same deformation conditions. Thus weak-phase-based rheology w ill lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate.