Lateral subhorizontal middle to lower crustal flow in response to continental collision: Evidence from the Diancang Shan complex along the Ailao Shan-Red River belt, Southeastern Tibetan Plateau

How middle to lower crust in Southeastern Tibetan Plateau flowed in response to the Indian-Eurasian continental collision remains hotly debated in the last decades. Metamorphic complexes along crustal mobile zones, e.g., the Diancang Shan (DCS) complex along the Ailao Shan-Red River (ASRR) metamorphic belt may provide key knowledge on our understanding of the middle-lower crustal flow during the collision. In this contribution, integrated structural, microstructural, kinematic, and thermo-chronological studies of the DCS complex reveal that the complex constitutes a destructed A-type dome. High-grade metamorphic rocks of Proterozoic protoliths form the core (Unit I) that is covered by low-grade metamorphic rocks of Mesozoic age in the limbs (Unit II). The eastern limb of the DCS dome is cut by a normal shear zone (Unit III). Three phases (D1, D2 and D3) of Cenozoic deformation structures are recognized in the complex. The D1 and D2 structures constitute the dominant structural style of the Units I and II. The D1 structures consist of penetrative S1 foliations, mineral lineations (L1), symmetrical intrafolial folds (F1) and tectonic lenses of granitic and quartz veins in rocks from the Units I and II. Mylonitic foliations (S2//S1), NNW-SSE plunging subhorizontal stretching lineations (L2), widespread A-type folds (F2), asymmetric porphyroclasts and tectonic lenses form the dominant structural styles in the D2 deformation. D3 structures are new foliations S3 and hot striae L3 in the normal-slipping shear zone (Unit III). The S2//S1 foliations define the domal framework of the DCS complex, which is truncated by the D3 shear zone. The dome is NW-SE elongated and has a hinge line paralleling to the L2//L1 stretching lineations. Synthetic analysis of structural and microstructural styles, and kinematic vorticity data estimated by rotated rigid porphyroclast method (0.38–0.69) and oblique grain-shape/quartz c-axis-fabric method (0.72–0.98) implies that the D1 and D2 deformations formed a progressive process from early pure shear-dominated (D1) to simple shear-dominated (D2) shearing. Shear sense indicators from different parts (Units I and II) of the dome are compatible with top-to-the southeast shearing during D2 deformation. It is shown that the D1-D2 deformation was characterized by a successive process from penetrative flow (D1) and ductile shearing at middle to lower crustal levels to doming (D2) that was coeval with exhumation of the DCS complex since 27 Ma or earlier. We conclude that these processes, i.e., the penetrative flow, ductile shearing, doming and exhumation of the DCS complex, were attributed to lateral subhorizontal middle to lower crustal flow along the ASRR metamorphic belt in the Southeastern Tibetan Plateau during the Indian and Eurasian plate collision.