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.

Responses to the India-Eurasia plate collision vary significantly in different regions. In Southeastern Tibetan Plateau, the tectonic extrusion of the Sundaland block accommodated the tectonic convergence between the two plates. However, there have been extensive controversies over the mechanism of extrusion of the block. In this study, we focus on macro- and micro-scopic structural analysis, kinematics, timing of shearing and thermal histories of several typical metamorphic complexes in order to understand the tectonic processes driving the deformation of the complexes and extrusion of the block. It is shown that dome structures cored by the metamorphic rocks are widely developed in Southeast Tibetan Plateau. The cores are composed of high-grade metamorphic and high temperature deformed rocks, while the mantle parts are characterized by low-grade metamorphic rocks and low temperature deformation. Thermochronological data reveal that most of the domes began to be exhumed since 30 Ma, while the initiation of doming was diachronous at different places and mostly through two-stage cooling histories. In most of the domes, shear discontinuities exist between the core and mantle parts. We show that the formation and exhumation of the dome structures are related to subhorizontal middle and lower crustal flow, during which shearing, folding and exhumation are simultaneous. The middle and lower crustal flow resulted in lateral crustal flow and vertical exhumation of crustal masses, which absorbed a large amount of deformation of the lateral escape of Sundaland block during India-Eurasia collision.

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.