Strain Localized Deformation Variation of a Small-Scale Ductile Shear Zone

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 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 zones exhibit a typical transition from protomylonite, mylonite to extremely deformed ultramylonite, and decreasing mineral size from coarse-grained aggregates to extremely fine-grained mixed phases. Shearing sense indicators 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 <a> 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 µ m) mixed-phases in the 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 middle-deep crust seated crustal condition dominated by the temperature-controlled formation and rheological weakening.

Keywords

strain localization / ductile deformation / ultramylonite / microstructure / EBSD texture / Gaoligong shear zone

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