Tibetan tectonic evolution inferred from spatial and temporal variations in post-collisional magmatism

Cenozoic magmatism on the Tibetan plateau shows systematic variations in space and time that must be considered in models concerning Tibetan tectonic evolution. After the India–Asia collision, which started in the early Tertiary and terminated the Gangdese arc magmatism in the Lhasa terrane made of the southern Tibetan plateau, widespread potassium-rich lavas and subordinate sodium-rich basalts were generated from ∼50 to 30 Ma in the Qiangtang terrane of northern Tibet. Subsequent post-collisional magmatism migrated southwards, producing ultrapotassic and adakitic lavas coevally between ∼26 and 10 Ma in the Lhasa terrane. Then potassic volcanism was renewed to the north and has become extensive and semicontinuous since ∼13 Ma in the western Qiangtang and Songpan–Ganze terranes. Such spatial–temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the Indian continental lithospheric mantle started thrusting under Asia by involving rollback and breakoff of the subducted Neo-Tethyan slab followed by removal of the thickened Lhasa lithospheric root. We propose that only after the lithospheric removal, which occurred at ∼26 Ma, could the Indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the Himalayan–Tibetan orogenesis. Consequently, the Tibetan plateau is suggested to have risen diachronously from south to north. Whereas the southern part of the plateau may have been created and maintained since the late Oligocene, the northern plateau would have not attained its present-day elevation and size until the mid-Miocene when the lower part of the western Qiangtang and Songpan–Ganze lithospheres began to founder owing to the push of the underthrust Indian mantle lithosphere.