Two contradictory plate-kinematic models, (1) collision extrusion and (2) mid- to lower-crustal-level flow, have been proposed for inducing crustal deformation surrounding the eastern Tibetan Plateau in response to the Cenozoic collision of India-Eurasia. A hybrid tectonic model indicating extrusion followed by crustal flow is established by new structural, petrological, and geochronological data obtained from the Gaoligong shear zone in western Yunnan, China. The 40Ar/39Ar ages for muscovite (34−31 Ma), biotite (23 Ma), and K-feldspar (33−21 Ma) mineral separates from the Gaoligong group, and a matrix crosscutting quartzofeldspathic dike produce a cooling path. By interlinking the synkinematic metamorphic conditions with the reconstructed cooling path, we find that the temperature dropped from 550 °C around ca. 34 Ma to 350 °C at ca. 32 Ma, and dropped again to 275 °C around ca. 22 Ma. The timing and kinematic mechanism of four ductile deformation events, followed by brittle deformation, is deciphered. The earliest D1 event deformed the Early Cretaceous granite (zircon U-Pb ages of 118−78 Ma), forming WNW-ESE−striking upright folds during the closure of the Neo-Tethyan ocean. The D2 event was associated with large-scale eastward thrusting and folding with top-to-the-N/NE sense of shear under upper-amphibolite-facies conditions between 50 and 35 Ma due to India-Eurasia continent-continent collision. D3 formed moderate NE-dipping, NW-SE−striking, left-lateral shear zones that accommodated the southeastward extrusion of Indochina around 35−28 Ma. The sinistral sense of shear S/C fabrics defined by muscovite folia with foliation-bounded myrmekite indicates that deformation occurred under middle- to lower-amphibolite-facies metamorphic conditions. The 28 Ma to 15 Ma steep N-S−trending, right-lateral Gaoligong shear belt (D4) is the dominant structural feature in this region. Chloritization of biotite and boudinaged sillimanite along S/C2 fabrics indicate that the crystalline rocks retrograded from amphibolite-facies into greenschist-facies conditions. This coincided with clockwise cataclastic flow around the Eastern Himalaya syntaxis due to gravitational collapse after delamination of the thickened Tibetan Plateau.
Transition from extrusion to flow tectonism around the Eastern Himalaya syntaxis
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Yu-Ping Chiu, Meng-Wan Yeh, Kuang-Hsuan Wu, Tung-Yi Lee, Ching-Hua Lo, Sun-Lin Chung, Yoshiyuki Iizuka; Transition from extrusion to flow tectonism around the Eastern Himalaya syntaxis. GSA Bulletin doi: https://doi.org/10.1130/B31811.1
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