Tectonic Facies Concept Based upon An Alpine Model as Applied to the Geology of Tibet

The three classical tectonic units of the Swiss Alps are the Helvetic, Penninic and Australpine nappes, south of Alpine Molasse. The Australpine nappes are the overriding units of basement and cover nappes; their forward movement above a collapsed backarc basin is the motor of alpine deformation. The Penninic nappes are mainly mélanges, metamorphosed to different metamorphic facies and caught in a subduction zone. The Helvetic are the cover thrusts that were stripped off from their underlying continental basement, and thrust over the Molasse. The terms Australpine, Penninic and Helvetic, unfortunately, have also been used to designate the paleogeography of the rocks in those units, south, within and north of a south-facing backarc basin, traditionally known as the Alpine Geosyncline. In order to differentiate them by different sets of terms, Hsu proposed the names Rhaetic, Celtic, Alemannic for the tectonic facies; these were the names of the peoples living mainly in the Australpine, Penninic and Helvetic regions, respectively.

Recognizing the teconic facies in China, Hsu discerned that mountains were mainly collapsed backarc basins and proposed the archipelago model of orogenesis, in which ophiolites represent the remnants of oceanic crust of a former backarc basin. Thus the Zhanbo ophiolites are not the suture between Eurasia and India, but the remnants of one or more backarc basins behind a south-facing arc, which has been uplifted to become the Himalayas. In the west, two zones of Zhanbo mélanges mark the backarc basins separated by a relict arc, which is represented by its Paleozoic cover correlative to the Everest Sequence of the Himalayas. In the east, the flysch sediments associated with the mélanges indicate that those deepsea turbidites were deposited in the same backarc basins. The suture zone between Tibet and Gondwana is marked by the Boundary Thrust in the southern foothills of the Himalayas.