The
structural geology, timing
of shearing, and tectonic implications
of the ASRR shear zone, one
of the most striking lineaments in Southeast Asia, have been the topics
of extensive studies over the past few decades. The Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS) and Day Nui Con Voi (DNCV) metamorphic massifs along the shear zone have preserved important information on its
structural and tectonic evolution. Our field
structural analysis, detailed micro
structural and fabric analysis, as well as the quartz, sillimanite and garnet fabric studies
of the sheared rocks from the massifs demonstrate the dominant roles
of three deformation episodes during Cenozoic tectonic evolution in the shear zone. Among the contrasting
structural and micro
structural associations in the shear zone, D
2 structures, which were formed at the brittle to ductile transition during large-scale left-lateral shearing in the second deformation episode, predominate over the
structural styles
of the other two deformation episodes. Discrete micro-shear zones with intensive grain size reduction compose the characteristic
structural style
of D
2 deformation. In addition, several types
of folds (early shearing folds, , and late-shearing folds, ) were formed in the sheared rocks, including discrete to distributed mylonitic foliation, stretching lineation and shear fabrics (e.g., mica fish, domino structures, as well as sigma and delta fabrics). A sequence
of microstructures from syn-kinematic magmatic flow, high-temperature solid-state deformation, to brittle-ductile shearing is well-preserved in the syn-kinematic leucocratic intrusions. Deformation structures from the first episode (D
1) are characterized by F
1 folds and distributed foliations (S
1) in rocks due to pure shearing at high temperatures. They are preserved in weakly sheared (D
2) rocks along the eastern margin
of the ALS belt or in certain low-strain tectonic enclaves within the shear zone. Furthermore, semi-brittle deformation structures, such as hot striae and discrete retrogression zones, are attributed to normal-slip shearing in the third deformation episode (D
3), which was probably locally active, along the eastern flank
of the DCS range, for example.
There are four quartz c-axis fabric patterns in the mylonitic rocks, including type A point maxima, type B Y point maxima with crossed girdles superimposition, type C quadrant maxima, as well as type D point and quadrant maxima combination. They are consistent with microscopic observations of microstructures of high-temperature pure shearing, low-temperature simple shearing and their superimposition. Integrated microstructural analysis and fabric thermometer studies provide information on both high temperature (up to 750 掳C) and dominant low-temperature (300-600 掳C) deformations of quartz grains in different rock types. Sillimanite and garnet fabrics, especially the latter, were primarily formed at the peak metamorphism during high-temperature pure shearing.
The above structural, microstructural and fabric associations were generated in the tectonic framework of the Indian-Eurasian collision. The low-temperature microstructures and fabrics are attributed to left-lateral shearing along the ASRR shear zone from 27 to 21 Ma during the southeastward extrusion of the Indochina block, which postdated high-temperature deformation at the peak metamorphism during the collision.