Development of Amphibole Crystal Preferred Orientations (CPOs) and Their Effects on Seismic Anisotropy in Deformed Amphibolites

Crystal preferred orientations and seismic anisotropy of amphibole play a crucial role in fingerprinting the rheological and physical properties of the deep crust. This study presents typical naturally deformed (banded, mylonitic, and ultramylonitic) amphibolites from the Ailao Shan-Red River shear zone (ASRR-SZ), southwestern China. In the banded amphibolite, coarse amphibole grains are highly elongated by dislocation creep and develop type I fabric of amphibole. In the mylonitic amphibolite, amphibole grains are characterized by porphyroclasts with core-mantle structure and fine-grained amphiboles in the matrix. Subgrain rotation recrystallization dominantly contribute to the grain size reduction process and display type III fabric of amphibole. In the ultramylonitic amphibolite, the fine-grained amphiboles are mainly deformed by mechanical rotation of prismatic grains and develop type IV amphibole fabric. The seismic anisotropies are calculated from crystal preferred orientation (CPO) of naturally deformed amphiboles and model fabric. They both display that the transition of amphibole fabric from type III through type I to type IV also comes along with the transition of AVp patterns from Vp(X) similar to Vp(Y) > Vp(Z) through Vp(X) > Vp(Y) > Vp(Z) to Vp(X) > Vp(Y) similar to Vp(Z). Under the same texture strength, the strongest seismic anisotropy (both AVp and Max. AVs) occurs in the type III fabric of amphibole, the weakest seismic anisotropy is related to type IV fabric and a medium seismic anisotropy in type I fabric. This work indicates that the CPO types of amphiboles need be carefully considered in interpreting observed seismic anisotropy at the deep crustal level.