CRETACEOUS-TERTIARY BASIN EVOLUTION IN THE LHASA TERRANE OF SOUTHERN TIBET: RESPONSES TO TERRANE COLLISION, ARC-TRENCH TECTONICS, AND PROGRESSIVE UNDERTHRUSTING OF GREATER INDIA

The Lhasa terrane lies between the Indus-Yarlung suture (to the south) and the Banggong suture (to the north) in the southern part of the Tibetan Plateau. A thick (~6-8 km) succession of Cretaceous through Tertiary strata in the Nyima basin records the tectonic history of the terrane and possible linkages with Himalayan tectonic events. Collision of the Lhasa terrane with southern Asia during Late Jurassic–Early Cretaceous time produced a southward-directed thrust belt in the Banggong suture zone, which supplied voluminous quartzose sediment to a peripheral foreland basin system located on the Lhasa terrane. Mid-Cretaceous initiation of an arc-trench system along the southern fringe of the Lhasa terrane and an associated north-directed thrust belt was accompanied by development of a retroarc foreland basin system that filled with fluvial deposits (feldspathic litharenites) derived partly from the Gangdese magmatic arc to the south. An interlude of widespread marine carbonate deposition during Aptian-Albian time marked the transition from northerly to southerly provenance polarity. Upper Cretaceous–Tertiary depositional systems document a complex interplay among alluvial fan, lacustrine fan delta, eolian, fluvial, playa, and deep lacustrine environments, as well as continued growth of contractile structures and arc-related magmatism across much of the Lhasa terrane. The abundance of lacustrine and eolian deposits reflects internal drainage and desertification. Preliminary oxygen isotopic data from lacustrine marls suggest that paleoelevation during the early to middle Tertiary was on the order of 4 km. Crustal shortening alone is insufficient to account for this several thousand meter elevation gain, calling for the addition of crust from a source outside of the Nyima basin region. We propose that additional crustal thickening was the result of northward underthrusting of Greater Indian continental crust and lithosphere during the early stages of Himalayan orogenesis. The record of Himalayan shortening provides support for this hypothesis, and suggests that Tertiary growth of the Tibetan Plateau required the direct addition of Indian material.