Cretaceous-Cenozoic Geological Evolution of Tibet: Tectonic Interpretations and Outstanding Questions (Invited)

The India-Asia collision, although profound, is only the most recent in a series of orogenic events that has modified the architecture of the Asian lithosphere. For instance, large parts of central Tibet (Lhasa and Qiangtang terranes) underwent >50% upper-crustal shortening, and likely substantial elevation gain, between Cretaceous and Eocene time in response to Lhasa - Qiangtang continental collision and Andean-style orogenesis along the southern margin of Asia. Findings by independent groups of Gangdese-arc-age detrital zircons in 52-50 Ma Tethyan Himalaya (TH) strata indicate that TH-Asia collision was ongoing by this time. This collision timing is consistent with multiple other, albeit less direct lines of evidence and suggests that a magmatic flare-up within the Gangdese arc (culminated at 52-51 Ma) occurred during subduction of TH lithosphere. Low-temperature thermochronologic data indicate that very low erosion rates, and likely plateau-like conditions considering the shortening history, were established in large parts of central Tibet at or by 50-45 Ma. The temporal-spatial distribution of subsequent shortening and exhumation is consistent with plateau growth northward and southward from central Tibet since the Eocene. The Cenozoic magmatic record of Tibet shows intriguing temporal-spatial patterns. Between 45 Ma and 30 Ma, volcanism swept >600 km northward from the Indus-Yarlung suture (IYS) and then back southward between 30 Ma and 25 Ma. These magmatic sweeps may have been produced by underthrusting and subsequent rollback of subducting TH lithosphere. Recent stratigraphic and structural studies suggest localized extension and elevation loss along the IYS at ~25 Ma, which is explainable in a slab rollback scenario, followed within a few million years by uplift back to near-modern elevations, perhaps in response to breakoff of TH lithosphere and northward underthrusting of Indian lithosphere. This hypothesis of TH - Indian lithosphere subduction can explain how ~2000 km of India-Asia convergence was accommodated south of the IYS since ~50 Ma (with the remaining ~1000 km accommodated by shortening of Asian lithosphere). Outstanding questions include: (1) What are the explanations for major, coeval geological changes in the Lhasa terrane, Gangdese forearc, IYS, and TH at 65-63 Ma, which have led some workers to argue for initiation of India-Asia collision at this time? (2) What was the nature of the subducted TH lithosphere and its former paleogeographic and tectonic relationships to Indian cratonic lithosphere? (3) Why has only <50% of the estimated 2000 km of post-50 Ma convergence south of the Indus-Yarlung suture been documented as shortening within the Tethyan-Himalayan thrust belts? (4) Why did Asian lithosphere in Pamir and Tibet behave so differently in response to collisional orogenesis?