Microstructures and seismic properties of amphibolites along the southern Ailao Shan–Red River belt: Implications for Oligo-Miocene crustal deformation in the southeastern Tibetan Plateau

The mechanism of Oligo–Miocene crustal deformation in the southeastern Tibetan Plateau has long been debated. The two widely cited models of rigid block extrusion and lower-crustal flow still show significant differences in explaining the rheological properties and deep geodynamic characteristics of the continental crust in the southeastern Tibetan Plateau. Here, we present new microstructural, crystallographic preferred orientation (CPO), and seismic property data from amphibolites along the southern Ailao Shan–Red River shear zone to constrain deformation mechanisms and their geodynamic implications. Amphibole CPOs exhibit variable strengths and fabric types and are primarily formed by syn-tectonic orientation-selective growth during amphibolite-facies metamorphism. In strongly deformed domains, however, dislocation creep becomes the dominant deformation mechanism and locally overprints earlier metamorphic fabrics. At the rock scale, seismic anisotropy in the studied amphibolites is primarily controlled by amphibole modal abundance and CPO strength. The amphibolites display strong seismic anisotropy (AVp = 6.90%–15.36% and max AVs = 5.14%–10.34%), both of which are positively correlated with amphibole modal abundance and CPO strength. At the regional crustal scale, however, amphibolites should not be regarded as the sole contributor to seismic anisotropy. Their modeled delay times are consistent with observed values, but crustal seismic anisotropy is jointly controlled by lithological heterogeneity, foliation geometry, and mineral-scale CPO development. This implies that interpretations of crustal anisotropy in orogenic belts should not rely on a single lithological or geodynamic model, but instead consider the integrated effects of multi-stage deformation and heterogeneous crustal composition. The observed anisotropy records both spatial variations and multi-stage deformation of the middle to lower crust, with anisotropy in high-strain belts reflecting subvertical foliations and horizontal lineations associated with crustal flow, and that in low-strain regions reflecting the folding of biotite-bearing metasedimentary rocks. Collectively, these observations support a geodynamic model involving early tangential shearing associated with lower-crustal channel flow followed by transpressional deformation and strike-slip shearing during the Oligo–Miocene.