MESOZOIC-CENOZOIC GEOLOGICAL EVOLUTION OF THE HIMALAYAN-TIBETAN OROGEN AND WORKING TECTONIC HYPOTHESES

The Himalayan-Tibetan orogen culminated during the Cenozoic India - Asia collision, but its geological framework and initial growth were fundamentally the result of multiple, previous ocean closure and intercontinental suturing events. As such, the Himalayan-Tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general, and how the Earth's highest and largest orogenic feature formed in specific. This paper synthesizes the Triassic through Cenozoic geology of the central Himalayan-Tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere-scale cross-sections and paleogeographic maps. We suggest that north-dipping subducting slabs beneath Asian continental terranes associated with closure of the Paleo-, Meso-, and Neo-Tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing. These trench retreat events created ophiolites in forearc extensional settings and/or a backarc oceanic basins between rifted segments of upper-plate continental margin arcs. This process may have occurred at least three times along the southern Asian margin during northward subduction of Neo-Tethys oceanic lithosphere: from similar to 174 to 156 Ma; 132 to 120 Ma; and 90 to 70 Ma. At most other times, the Tibetan terranes underwent Cordilleran-style or collisional contractional deformation. Geological records indicate that most of northern and central Tibet (the Hoh-Xil and Qiangtang terranes, respectively) were uplifted above sea level by Jurassic time, and southern Tibet (the Lhasa terrane) north of its forearc region has been above sea level since similar to 100 Ma. Stratigraphic evidence indicates that the northern Himalayan margin of India collided with an Asian-affinity subduction complex - forearc - arc system beginning at similar to 60 Ma. Both the Himalaya (composed of Indian crust) and Tibet show continuous geological records of orogenesis since similar to 60 Ma. As no evidence exists in the rock record for a younger suture, the simplest interpretation of the geology is that India - Asia collision initiated at similar to 60 Ma. Plate circuit, paleomagnetic, and structural reconstructions, however, suggest that the southern margin of Asia was too far north of India to have collided with it at that time. Seismic tomographic images are also suggestive of a second, more southerly Neo-Tethyan oceanic slab in the lower mantle where the northernmost margin of India may have been located at similar to 60 Ma. The geology of Tibet and the India -Asia suture zone permits an alternative collision scenario in which the continental margin arc along southern Asia (the Gangdese arc) was split by extension beginning at similar to 90 Ma, and along with its forearc to the south (the Xigaze forearc), rifted southward and opened a backarc ocean basin. The rifted arc collided with India at similar to 60 Ma whereas the hypothetical backarc ocean basin may not have been consumed until similar to 45 Ma. A compilation of igneous age data from Tibet shows that the most recent phase of Gangdese arc magmatism in the southern Lhasa terrane initiated at similar to 70 Ma, peaked at similar to 51 Ma, and terminated at similar to 38 Ma. Cenozoic potassic-adakitic magmatism initiated at similar to 45 Ma within a similar to 200-km-wide elliptical area within the northern Qiangtang terrane, after which it swept westward and southward with time across central Tibet until similar to 26 Ma. At 26 to 23 Ma, potassic-adakitic magmatism swept southward across the Lhasa terrane, a narrow (similar to 20 km width), orogen-parallel basin developed at low elevation along the axis of the India - Asia suture zone (the Kailas basin), and Greater Himalayan Sequence rocks began extruding southward between the South Tibetan Detachment and Main Central Thrust. The Kailas basin was then uplifted to >4 km elevation by similar to 20 Ma, after which parts of the India - Asia suture zone and Gangdese arc experienced >6 km of exhumation (between similar to 20 and 16 Ma). Between similar to 16 and 12 Ma, slip along the South Tibetan Detachment terminated and east-west extension initiated in the northern Himalaya and Tibet. Potassic-adakitic magmatism in the Lhasa terrane shows a northward younging trend in the age of its termination, beginning at 20 to 18 Ma until volcanism ended at 8 Ma. We interpret the post-45 Ma geological evolution in the context of the subduction dynamics of Indian continental lithosphere and its interplay with delamination of Asian mantle lithosphere.