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

摘要

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.