Strain localized deformation variation of a small-scale ductile shear zone

Abstract

Abstract. A continental-scale strike-slip shear zone frequently presents a long-lasting deformation and physical expression of strain localization in a middle to lower crustal level. However, the deformation evolution of strain localization at a small-scale shear zone remains unclear. This study investigated < 10 cm wide shear zones developing in undeformed granodiorites exposed at the boundary of the continental-scale Gaoligong strike-slip shear zone. The small-scale ductile shear zone demonstrated a typical transition from protomylonite, mylonite to extremely deformed ultramylonite, and decreased mineral size from coarse-grained aggregates to extremely fine-grained mixed phase. Shearing senses such as hornblende and feldspar porphyroclasts in the shear zone are the more significantly low-strain zone of mylonite. The microstructure and EBSD results revealed that the small-scale shear zone experienced ductile deformation under medium-high temperature conditions. Quartz aggregates suggested a consistent temperature with an irregular feature, exhibiting a dominated high-temperature prism slip system. Additionally, coarse-grained aggregates in the mylonite of the shear zone were deformed predominantly by dislocation creep, while ultra-plastic flow by viscous grain boundary sliding was an essential deformation process in the extremely fine-grained (~50 μm) mixed-phase of ultramylonite. Microstructural-derived strain rates calculated from quartz paleopiezometry were on the order of 10−15 to 10−13 s−1 from low-strain mylonite to high strained ultramylonite. The localization and strain rate-limited process was fluid-assisted precipitation presenting transitions of compositions as hydrous retrogression of hornblende to mica during increasing deformation and exhumation. Furthermore, the potential occurrence of the small-scale shear zone was initiated at a deep-seated crustal dominated by the temperature-controlled formation and rheological weakening.