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 (D-1, D-2 and D-3) of Cenozoic deformation structures are recognized in the complex. The D-1 and D-2 structures constitute the dominant structural style of the Units I and II. The D-1 structures consist of penetrative S-1 foliations, mineral lineations (L-1), symmetrical intrafolial folds (F-1) and tectonic lenses of granitic and quartz veins in rocks from the Units I and II. Mylonitic foliations (S-2//S-1), NNW-SSE plunging subhorizontal stretching lineations (L-2), widespread A-type folds (F-2), asymmetric porphyroclasts and tectonic lenses form the dominant structural styles in the D-2 deformation. D-3 structures are new foliations 53 and hot striae L-3 in the normal-slipping shear zone (Unit III). The S-2//S-1 foliations define the domal framework of the DCS complex, which is truncated by the D-3 shear zone. The dome is NW-SE elongated and has a hinge line paralleling to the L-2//L-1 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 D-1 and D-2 deformations formed a progressive process from early pure shear-dominated (D-1) to simple shear-dominated (D-2) shearing. Shear sense indicators from different parts (Units I and II) of the dome are compatible with top-to-the southeast shearing during D-2 deformation. It is shown that the D-1-D-2 deformation was characterized by a successive process from penetrative flow (D-1) and ductile shearing at middle to lower crustal levels to doming (D-2) 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.

Automated summary

The study of the Diancang Shan complex in the Southeastern Tibetan Plateau reveals a complex structural evolution involving multiple deformation phases, with D-1 and D-2 structures dominating the Proterozoic core and Mesozoic limbs of the complex. The presence of D-3 structures in the shear zone suggests a more recent deformation event, providing insights into the progressive flow, ductile shearing, doming, and exhumation processes of the complex associated with the Indian and Eurasian plate collision.