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Middle-lower crustal flow in response to the India-Eurasia collision: Structural evidence from the southern Chong Shan belt within the Sundaland block, southeastern Tibetan Plateau

In this contribution, we present new structural, microstructural, fabrics, and geochronological data from the southern Chong Shan complex, one of the metamorphic complexes in the southeastern Tibetan Plateau that were sheared and exhumed during the India-Eurasia convergence. The NW-SE−striking complex is comprised of a central high-grade metamorphic zone (Unit I) flanked by two low-grade metamorphic zones (units II and III) on the northeastern and southwestern sides, respectively. High-grade metamorphic rocks (e.g., amphibolites, sillimanite-mica schists) of up to amphibolite facies, of the Proterozoic Chong Shan group and granitic intrusions of Permo-Triassic to Cenozoic in age in Unit I are characterized by high-temperature deformation. Units II (i.e., the Wuliangshan group) and III (i.e., the Lancang group) on both sides of the high-grade Unit I consist of metamorphic rocks of low greenschist facies (e.g., phyllites) with low-temperature deformation. The high- and low-grade units possess consistent kinematics, i.e., northwestward motion of the core rocks relative to the two limbs, and they are separated by large scale shear discontinuities. Thereby, the high- and low-grade units are kinematically linked but mechanically decoupled. Zircon laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating of syn-shearing granitic dikes reveals that ductile shearing occurred from 29 to 19 Ma.
 Structural analysis reveals that these units constitute an A-type dome that has long axis parallel to the stretching lineations and fold axes of outcrop-scale A-type folds. It is shown that three stages of deformation contributed to the formation of the southern Chong Shan dome, during which subhorizontal shearing were in connection with regional doming. The events occurred as the consequence of middle to lower flow that led to lateral crustal flow and vertical exhumation of crustal masses. Therefore, the lateral crustal flow was not only limited along the boundary high strain zones of the Sundaland block, but distributed within the southeastern Tibetan Plateau. We would argue that the tectonic extrusion of the Sundaland block occurred through ductile crustal flow of a viscous middle and lower crust in the plate interior combined with concurring channel flow along the block margins.