TECTONIC EVOLUTION OF THE LHASA TERRANE, SOUTH TIBET SINCE MIDDLE TRIASSIC: TRANSITION FROM OCEAN-CONTINENT MULTIPLE SUBDUCTION TO CONTINENT-CONTINENT COLLISION (Invited Presentation)

The Lhasa terrane, located between the Qiangtang and Himalayan terranes in southern Tibet, is regarded as an Andean-type continental margin in the Mesozoic. It was accreted to Eurasian continent during early Paleogene, marking the Indo-Asian collision.

(1) The tectonic framework of the Lhasa terrane showing three units

a) The Triassic-Miocene Gangdese accretionary magmatic belt in South Lhasa terrane, including four periods of magmatism: 240–130, 100–80, 65–40, 30–13 Ma, as well as Cretaceous backarc and forearc basins.

b) The Triassic central uplift belt in Central Lhasa terrane was intruded by syn-collisional plutons and overprinted by 65–40 Ma volcanic rocks.

c) The northern Jurassic-Cretaceous passive continental margin belt in Northern Lhasa terrane.

(2) Cretaceous proto-plateau in the Lhasa terrane

Cretaceous backarc basin in the south Lhasa terrane underwent crustal shortening and orogeny during 90–65 Ma and is overprinted by 65–40 Ma Linzhizong volcanics. The Lhasa terrane experienced a plateau process during 90–65 Ma due to the subduction of Neotethyan oceanic slab, as suggested by an unconformable contact between underlying Cretaceous sequences and overlying early Paleogene Linzhizong volcanics.

(3) Intracontinental faulting in the Lhasa terrane

The 19–10 Ma S-dipping Great Counter thrust and 38–21 Ma N-dipping South Gangdese thrust were triggered by Indo-Asian collision. The ~100 Ma N-dipping thrusts in North Lhasa are related to the collision between Qiangtang and Lhasa terranes. The 220–240 Ma S-dipping Sumdo thrust was related to Triassic orogeny. These large-scale intracontinental faulting system has caused crustal thickenning.

(4) The Lhasa terrane underwent tectonic transition of multiple ocean-continental subduction to continent-continent collision

a) Multi-stage intra-oceanic arcs and protracted active continental margin along southern margin of the Lhasa terrane during 240–130 Ma.

b) 100–80 Ma magmatic ‘flare-up’ event, probably attributable to slab rollback of the northward subducted Neotethyan oceanic lithosphere beneath the Lhasa terrane.

c) 65–40 Ma voluminous magmatic rocks, as product of tectonic transition from ocean-continent subduction to continent-continent collision.

d) Voluminous ~30–13 adakites, ultrapotassic volcanic rocks and Cu-hosting porphyry.