A crustal section is exposed across the Ailao Shan Tectonic Belt (ALTB) that is suggested to be the accommodation zone of southeastward extrusion of the Sundaland block during the Indian-Eurasian collision. A highly sheared high-grade metamorphic unit (HMU) is separated from the low-grade metamorphic unit (LMU) by an ultramylonite belt, i.e., the previously defined ‘Ailao Shan fault’. Rocks in the three units possess identical structural and kinematic characteristics. The ultramylonites exhibit brittle-ductile deformation characteristics in localized middle crustal high strain zones. Geothermometry analyses reveal contrasting deformation P-T conditions across the ultramylonite belt, i.e., 610 ~834 ℃, 0.4~0.6 GPa in the HMU and ca. 400 ℃ in the LMU, consistent with microstructural observations and quartz C-axis fabric analysis. The HMU and LMU are kinematically linked while mechanically decoupled, implying shearing of the two units at different crustal levels in the same strain field. Progressive stratified middle to lower crustal flow was responsible for the concurring high- and low-temperature fabrics at different crustal levels. They were juxtaposed during crustal flow in response to extrusion of the Sundaland block at ca. 30~21 Ma. Exhumation of lower crustal rocks and incision of a thick pile of middle crustal masses were attributed to doming during lower crustal flow. The previously defined ‘Ailao Shan fault’ occurred as a tectonic discontinuity (TDC) that may have inherited preexisting basement/cover contact along the ALTB. Ubiquitous occurrence of TDCs in middle crust provides a potential explanation for the middle crustal low-velocity and high-conductivity zone beneath the SE Tibet Plateau.

Understanding the mechanisms of strain localization is the key to our understanding of the transition from steady-state flow to unstable flow in the middle crust. In this paper, studies on deformation of gneisses sheared at mid-crustal level along the Jinzhou detachment fault zone, Liaodong peninsula, North China, reveal that biotite grains, as pre-existing weak-phase, have important influences on deformation of middle-crustal rocks. High phase strength contrasts between biotite grains and other mineral phases resulted in stress concentrations during shearing and occurrences of microcracks at the tips of biotite grains. Consequently, microcracks are formed either along contacts between high strength mineral grains or propagate into the mineral grains. The microcracks filled with biotite grains and fine-grained feldspar aggregates continue to nucleate, propagate, and coalesce in the rocks, while basal plane slip and grain boundary sliding (GBS) operate in biotite grains and fine-grained feldspar aggregates, respectively. These processes lead to a transition from load-bearing framework (i.e., coarse plagioclase grains) to interconnected weak phase (i.e., biotite grains and fine-grained feldspar aggregates), and the formation of incipient strain localization zones (SLZs). With the propagation and linkage of the SLZs, high stress concentrations at the tips of SLZs lead to nucleation of fractures. At the same time, there occurs an abrupt increase in strain rates that result in the transition from dislocation creep and GBS (velocity strengthening) to unstable slip (velocity weakening). The processes are accompanied by occurrence of mid-crustal earthquakes, and formation of pseudotachylite veins along with SLZs.

A crustal section is exposed across the Ailao Shan Tectonic Belt (ALTB) that is suggested to be the accommodation zone of southeastward extrusion of the Sundaland block during the Indian-Eurasian collision. A highly sheared high-grade metamorphic unit (HMU) is separated from the low-grade metamorphic unit (LMU) by an ultramylonite belt, i.e., the previously defined ‘Ailao Shan fault’. Rocks in the three units possess identical structural and kinematic characteristics. The ultramylonites exhibit brittle-ductile deformation characteristics in localized middle crustal high strain zone. Geothermometry analyses reveal contrasting deformation P-T conditions across the ultramylonite belt, i.e., 610~834 ℃, 0.4~0.6 GPa in the HMU and ca. 400 ℃ in the LMU, consistent with microstructural observations and quartz C-axis fabric analysis. The HMU and LMU are kinematically linked while mechanically decoupled, implying shearing of the two units at different crustal levels in the same strain field. Progressive stratified middle to lower crustal flow was responsible for the concurring high- and low-temperature fabrics at different crustal levels. They were juxtaposed during crustal flow in response to extrusion of the Sundaland block at ca. 30~21 Ma. Exhumation of lower crustal rocks and incision of a thick pile of middle crustal masses were attributed to doming during lower crustal flow. The previously defined ‘Ailao Shan fault’ occurred as a tectonic discontinuity (TDC) that may have inherited preexisting basement/cover contact along the ALTB. Ubiquitous occurrence of TDCs in middle crust provides a potential explanation for the middle crustal low-velocity and high-conductivity zone beneath the SE Tibet Plateau.