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

Abstract

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) ∼ Vp(Y) > Vp(Z) through Vp(X) > Vp(Y) > Vp(Z) to Vp(X) > Vp(Y) ∼ 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.

Seismic Anisotropy of Mafic Blueschists: EBSD‐Based Constraints From the Exhumed Rock Record

“…This transition in the Vp pattern is accompanied by increased density of the [100] and [010] crystallographic axes in the Z‐ and Y‐directions respectively that implies a transition between the type‐IV and type‐I amphibole CPOs in these samples. The magnitude of AVp percentages of the blueschists in our compilation can be broadly ordered as AVp(Vp‐pattern I) > AVp(Vp‐pattern III) > AVp(Vp‐pattern IV) in agreement with the trend observed for the hornblende‐group amphiboles in amphibolites (Figure 8a; Ji et al., 2013; Kim & Jung, 2020; Liu & Cao, 2023). We infer from these results that the glaucophane CPO strength and type exerts a strong control on the magnitude and symmetry of the P wave seismic anisotropy in blueschists.…”

“…The upper-hemisphere spherical projections of the P wave velocities display three different seismic anisotropy patterns that were previously observed in amphibolite facies deep crustal rocks (Ji et al, 2013;Liu & Cao, 2023). We describe the patterns using the terminology of Liu and Cao (2023) where Vp(X) is the velocity in the X-direction (parallel to the lineation), Vp(Y) is the velocity in the Y-direction (normal to the lineation in the foliation plane), and Vp(Z) is the velocity in the Z-direction (normal to the foliation plane). In the blueschists with the highest AVp % magnitudes, the Vp pattern is Vp(X) > Vp(Y) > Vp(Z) and is correlated to a type-I CPO in glaucophane (Figure 5a).…”

“…Photos of hand samples illustrating the macroscopic fabrics are shown in Figure 3. Thin sections were prepared following the common methodology employed in petrofabric based anisotropy studies (e.g., Brownlee et al, 2017;Cao et al, 2013;Kang & Jung, 2019;Kim et al, 2013;Liu & Cao, 2023;Ward et al, 2012). One structurally oriented thin section, cut parallel to the lineation and perpendicular to the foliation, where the X-Z plane is contained in the thin section, was cut for each sample.…”

“…Amphibole with a type‐IV CPO displays a strong alignment of the [001] axes subparallel to the shear direction with the [100] and [010] axes describing a girdle subnormal to the shear direction (Figure 2c). Laboratory deformation experiments on amphibolites yielded correlations between amphibole CPO type and the magnitude of seismic anisotropy predicted in the samples (Kim & Jung, 2019, 2020), and studies of naturally deformed amphibolites reported further correlations between the amphibole CPO type and the symmetry of the Vp anisotropy patterns (Ji et al., 2013; Kim & Jung, 2020; Liu & Cao, 2023). In contrast, the contribution of glaucophane to the seismic properties of mafic blueschists is not as well understood due to the limited number of petrofabric based seismic anisotropy studies on mafic blueschists, and the potential contribution of other anisotropic phases to the bulk anisotropy (e.g., epidote, lawsonite, and phengite).…”

Deformation microstructures and seismic anisotropy of Yeoncheon amphibolites in the Imjingang Belt, South Korea

“…Amphibole is one of the representative minerals in the middle to lower crust that has a marked influence on the rheological characteristics of the continental crust (Aspiroz et al, 2007;Christensen and Mooney, 1995;Gao et al, 1999;Tatham et al, 2008). As amphibole is also an elastically anisotropic mineral (Brown and Abramson, 2016;Ko and Jung, 2015), LPO of amphibole may contribute notably to the seismic anisotropy of the middle to lower crust (Cao et al, 2010;Kim and Jung, 2019;Ko and Jung, 2015;Liu and Cao, 2023;Tatham et al, 2008;Wang et al, 2023). Recent experimental studies have classified LPOs of amphibole into four types (type-I, II, III, and IV) and reported that temperature, differential stress, and shear strain during deformation may govern the LPO of amphibole (Kim and Jung, 2019;Ko and Jung, 2015).…”