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.
Highlights
This study presents a detailed description of small-scale shear zones developing in unfoliated large intrusive granodiorite bodies at the boundary of the Gaoligong continental-scale shear zone (GLG-SZ) on the southeastern margin of the Tibetan Plateau
The new detail microstructural, electron backscatter diffraction (EBSD) texture, and geothermal data reveal that (1) strain localization in small-scale shear zones is characterized by the development of mylonite and ultramylonite with the increasing strain from rim to the center, (2) formation conditions and processes of the micro-shear zone are associated with the continentalscale GLG-SZ ductile shearing and exhumation
(2) The progressive development of microstructures suggests that the ductile deformation of small-scale shear zone is at least amphibolite facies conditions
Summary
Nucleation and initiation of a continental-scale shear zone occur within the deep crust or even mantle lithosphere in a specific thermal-structural architecture, where temperature-controlled rheological weakening plays a critical role in localizing future strike-slip shear zone (Cao and Neubauer, 2016 and references therein). Experiments and models on the deformation of rocks have been proposed to explain the formation of shear zones in varied scales, including the lithosphere's strength, the external conditions such as temperature, pressure, and fluid content, and the fact that rocks' rheology depends on their composition and grain size (e.g., Evans, 2000; Faulkner and Rutter, 2001; Collettini et al, 2009; Bense et al, 2013; Cao and Neubauer, 2016; Fossen and Cavalcante, 2017; Liu, 2017). The new detail microstructural, EBSD texture, and geothermal data reveal that (1) strain localization in small-scale shear zones is characterized by the development of mylonite and ultramylonite with the increasing strain from rim to the center, (2) formation conditions and processes of the micro-shear zone are associated with the continentalscale GLG-SZ ductile shearing and exhumation
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