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.

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.

To better understand the mechanisms of crustal exhumation related to tectonic extension, we report on the progressive doming and detachment faulting of the Cretaceous Liaonan metamorphic core complex (MCC). The detachment fault zone of Liaonan MCC is comprised of two branches, i.e., the Jinzhou detachment fault zone (JDFZ) and the poorly-researched Dongjiagou shear zone (DSZ). Thus, integrated structural, microstructural, quartz c-axis fabrics, and fluid inclusion analysis, and U-Pb on zircon dating were performed on mylonites along the DSZ. In contrast with the JDFZ that possesses characteristics of detachment fault zone, the DSZ encompasses Archean gneisses and Neoproterozoic meta-sedimentary rocks, between which exists an obvious metamorphic contrast forming a tectonic discontinuity contact (TDC). However, rocks from both sides of the TDC possess structures and fabrics for identical geometries and kinematics that are consistent with those along the JDFZ. Thermometric analysis of fluid inclusions from syn-tectonic quartz veins (630 °C, 470 °C, 350 °C) and quartz c-axis fabric from mylonites along the DSZ show that the shearing penetrates throughout the Archean to Neoproterozoic rocks. Dating of zircons from syn-kinematic granitic dikes from DSZ yields an age ca. 134 Ma, which is similar to the ages of early shearing along the JDFZ (ca. 133~134 Ma). The results imply that the shearing initiated in both JDFZ and DSZ at an early stage, then progressive shearing continued, and finally developed the detachment faulting along the JDFZ. Based on the timing and processes of the regional extension, a geodynamic model of MCC’s is proposed.