Paleozoic polyphase strain and rheological evolution of the Shangdan shear zone, Qinling Orogen, central China

The Shangdan Shear Zone (SSZ) preserves a continuous record of deformation localization across the Paleozoic subduction-collision cycle of the Qinling Orogen. This study integrates field-based finite strain, kinematic vorticity (W_k), and crystallographic preferred orientation analyses alongside quartz rheometry and zircon U–Pb geochronology to quantify the thermomechanical evolution of the orogenic crust. Deformation manifests initially as diffuse, lower-crustal flow (>650 °C) during oceanic-continental subduction (D₁, ca. 540–500 Ma), where high-temperature conditions and anatexis suppressed strain concentration. During this phase, regional compression was accommodated by pure-shear-dominated flattening (W_k = 0.24–0.41). The transition to continental collision (D₂, ca. 432–395 Ma) triggered mid-crustal strain localization facilitated by fluid-assisted weakening. Syn-deformational fluid influx amplified strain rates to 10^−12.58–10^−15.46 s⁻¹—approximately two orders of magnitude higher than in subsequent stages—under differential stresses of 33.95–45.12 MPa, facilitating sinistral transpression (W_k = 0.27–0.48). During post-collisional deep subduction (D₃, ca. 357–330 Ma), strain localization intensified within narrowing felsic bands as the system exhumed and cooled to 300–400 °C. This stage is marked by a kinematic reversal to dextral simple shear (W_k = 0.53–0.74) under differential stresses of 24.82–44.68 MPa and pronounced mineral-scale strain partitioning, yielding a vorticity difference of ≈0.15–0.20 between quartz and feldspar. These results identify a succession of strain localization mechanisms within the orogenic cycle, transitioning from thermal anatexis during subduction to fluid-assisted weakening during collision and mineral-scale rheological partitioning during deep subduction. The findings demonstrate that syn-deformational fluid influx is the key driver of intense strain localization. Furthermore, mineral-scale partitioning ensures that plate interfaces remain active and accommodate kinematic reorganization even as the system cools and exhumes. This quantitative framework identifies the SSZ as a dynamic determinant of lithospheric strength that enables the crust to sustain localized deformation across diverse crustal levels throughout the orogenic lifecycle.