Transition from extrusion to flow tectonism around the Eastern Himalaya syntaxis

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.