Paleotectonic Evolution of Tibet:
Basically, the plateau is a collage of continental fragments (terranes) that were added successively to the Eurasian plate during the Paleozoic and Mesozoic eras.The sutures between these micro plates are marked by scattered occurrences of ophiolitic material caught up between the crustal blocks during accretion. From north to south, the main Tibetan crustal blocks are the Kunlun, Songpan-Ganzi, Qiangtang, and Lhasa terranes (figure 3).
Figure 3. A simplified map showing major terranes in Tibetan plateau.
Lhasa Terrane which is bounded by the Yarlung-Zangbo Suture and Bangong-Nujiang sutures, collided with Qiangtang to the north in late Jurassic (figure 4) (Dewey et al. 1988). Subduction of the Indian oceanic lithosphere created forearc basin deposits and batholith emplacement. Active collision lasted until the early Late Cretaceous within the Lhasa terrane, resulting in at least 180 km shortening (Murphy et al. 1997).
Qiangtang terrane which lies north of Lhasa Terrane, is about 500 km wide in central Tibet. This terrane is characterized by a north-facing Triassic volcanic arc. The widespread Triassic volcanic deposits may have resulted from southward subduction of the Songpan-Ganze ocean basin (figure 4) (Kapp et al 1999b). Although the age and nature of basement rocks beneath Qiangtang is still not well known, recent studies resulted in the discovery of extensively distributed metamorphosed melange (blueshist) complexes which are controlled by detachment faults (Kapp et al 1997). In order to model how northern Tibet get thickened, understanding these deep crustal assemblages is essential.
Songpan-Ganze Terrane which is a long, narrow belt between Kunlun and Qiangtang terrane, formed by collision during Jurassic timeand completed by latest Jurassic (figure 4) (Dewey et al 1988). This terrane is characterized by intensely deformed sequence of Triassic-Jurassic strata of deep marine deposits. This deformation was contemporaneous with collision and subsequent continued convergence between North and South China and resulted in ~250 km shortening (Burchfiel et al 1995).
The Kunlun terrane is known by a broad Early Paleozoic arc that was later superposed by a Triassic arc. Together, they are referred to as the Kunlun batholith. Origin of the Triassic arc is still under much debate. Some authors interpret it as northward subduction of the Songpan-Ganze ocean beneath Paleozoic Kunlun basement (figure 4) (Zhang & Zheng 1994) and some others relate this arc to south dipping subduction on the north (figure 4) (Burchfield et al 1995). In the region, the Kunlun thrust system that resulted in at least 250 km of shortening, is still active.
Figure 4. Simplified evolution of Tibetan terranes during lower Mesozoic.
All these terranes are characterized by fold and thrust belts that are spatially associated with Tertiary foreland basin development (Yin & Harrison 2000). In general they commonly developed over wide region with regional decollements since the Indo-Asian collision at about 70 Ma. Estimates of total shortening implies at least 1400 km of north-south shortening within the Tibetan plateau.
Most recently, tomographic study accomplished by Van der Voo et al. (1999) brought light to the subduction history of Indo-Asian orogen after the Jurassic. They interpreted each of the high P-wave velocity anomalies in the mantle under India and adjacent areas as fossil slabs due to subduction of Tethyan oceanic lithosphere, or - as delaminated sub-continental Indian lithosphere. These results agrees with northward subduction of Lhasa terrane and supports the idea of an intra-Neo-Tethys subduction zone to the north of India during the Cretaceous (figure 5).
Figure 5. Evolutionary cartoons of Mesozoic Tethyan subduction zones and the Tertiary lithospheric delamination during the India-Asia collision
with a summary diagram of the tomographic results on the right [Van der Voo et al. 1999].