Strain Localization and Seismic Properties in the Heterogeneous Ailaoshan-Red River Shear Zone, Southeast Asia

Strain localization is crucial in developing crustal-scale shear zones; however, a systematic analysis of deformation characteristics and their impact on seismic properties is still lacking. This study provides a comprehensive analysis of migmatites and granitic mylonites in the Ailaoshan-Red River shear zone (ASRR-SZ), incorporating detailed field observations, microstructure analysis, mineral crystallographic preferred orientations, rheological parameters, and seismic properties. Pre-existing compositional and mechanical anisotropies significantly influence strain localization in the ASRR-SZ. The southern part of the ASRR-SZ is primarily characterized by crustal anatexis, suggesting these regions as potential initiation sites for strain localization. Strain characteristics in the ASRR-SZ manifest in deformation temperatures (three ranges, ∼400–440°C, ∼470–500°C, and ∼730°C), differential stress (concentrated at 15.9–65.1 MPa), and strain rate (concentrated at 10⁻¹³–10⁻¹¹ s⁻¹). Notably, strain localization significantly alters the rock fabric and further affects the seismic properties of rocks. Significant differences in Vp values and orientations are noted between melanosomes (anisotropy of P-waves: AVp = 6.8%–17.9%, Max. Vp along the X-axis) and leucosomes (AVp = 3.4%–3.7%, Max. Vp along the Y-axis). The seismic velocities and AVp in granitic mylonites exhibit a linear correlation with quartz content, and deformation conditions strongly influence their orientation. For a middle to lower crust thickness of ∼25 km, the delay times between fast and slow polarized shear waves are 0.3–0.66 s for granitic mylonites, 0.37–0.7 s for melanosomes, 0.22–0.31 s for leucosomes, 0.27–0.58 s for amphibolites, and 0.57–2.7 s for schists. The average delay time (dt) of these rocks along the ASRR-SZ accounts for the observed delay time (dt = 0.58 s).