Xiumian Hu

Direct stratigraphic dating of India-Asia collision onset at the Selandian (middle Paleocene, 59 ± 1 Ma)

The collision of India with Asia had a profound influence on Cenozoic topography, oceanography, climate, and faunal turnover. However, estimates of the time of the initial collision, when Indian continental crust arrived at the Transhimalayan trench, remain highly controversial. Here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when Asian-derived detritus was first deposited onto India in the classical Sangdanlin section of the central Himalaya, which preserves the best Paleocene stratigraphic record of the distal edge of the Indian continental rise. Deep-sea turbidites of quartzarenite composition and Indian provenance are replaced upsection by turbidites of volcano-plutoniclastic composition and Asian provenance. This sharp transition occurs above abyssal cherts yielding radiolaria of Paleogene radiolarian zones (RP) 4–6 and below abyssal cherts containing radiolaria of zone RP6 and calcareous shales with nannofossils of the Paleocene calcareous nannofossil zone (CNP) 7, constraining the age of collision onset to within the middle Paleocene (Selandian). The youngest U-Pb ages yielded by detrital zircons in the oldest Asia-derived turbidites indicate a maximum depositional age of 58.1 ± 0.9 Ma. Collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 ± 1 Ma. This age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern Himalaya, and suggests that, within the resolution power of current methods, the India-Asia initial collision took place quasi-synchronously in the western and central Himalaya.

Provenance of the Upper Cretaceous–Eocene Deep-Water Sandstones in Sangdanlin, Southern Tibet: Constraints on the Timing of Initial India-Asia Collision

The first arrival of sedimentary material from Asia onto the Indian continental margin provides a minimum constraint on the timing of initial India-Asia collision. A combination of petrology, detrital Cr-spinel geochemistry, and zircon U-Pb dating of the Upper Cretaceous to Eocene deep-water succession at the Sangdanlin section, southern Tibet, provides evidence for major provenance change within the strata. The Upper Cretaceous–Paleocene Denggang Formation quartzarenites contain zircons with dominant Proterozoic-Ordovician U-Pb ages, with an additional age peak of Early Cretaceous, which we interpret to be derived from the northern Indian margin. By contrast, the lithic sandstones of the Early to Middle Eocene Sangdanlin and Zheya formations are dominated by zircons younger than 200 Ma, showing one major peak at ∼80–125 Ma and two subdominant peaks at ∼54–70 and ∼180–196 Ma, comparable to those from the Gangdese magmatic arc. Cr-spinels in the Sangdanlin and Zheya formations are abundant and characterized by extremely low TiO2 wt%, indicating an ophiolitic source. We consider the Sangdanlin and Zheya formations syncollisional, deposited in a foredeep basin, with the provenance being the Gangdese arc and the Yarlung Zangbo suture zone. The abrupt sedimentary provenance change between the Denggang and Sangdanlin formations denotes the onset of India-Asia continental collision occurring before the late Ypresian (∼50 Ma). Comparison of our data with those from coeval strata along the Himalaya suggests limited diachroneity in the India-Asia continental collision process.

Xigaze forearc basin revisited (South Tibet): Provenance changes and origin of the Xigaze Ophiolite

Our new stratigraphic, sedimentological, and micropaleontological analysis, integrated with basalt geochemistry, sandstone petrography, and detrital-zircon U-Pb and Hf isotope data, suggests the revision of current models for the geological evolution of the Asian active margin during the Cretaceous. The Xigaze forearc basin began to form in the late Early Cretaceous, south of the Gangdese arc, during the initial subduction of the Neotethyan oceanic lithosphere under the Lhasa terrane. Well-preserved stratigraphic successions document the classical upwardshallowing pattern of the forearc-basin strata and elucidate the origin of the associated oceanic magmatic rocks. The normal midocean-ridge basalt (N-MORB) geochemical signature and stratigraphic contact with the overlying abyssal cherts (Chongdui Formation) indicate that the Xigaze Ophiolite formed by forearc spreading and represents the basement of the forearc sedimentary sequence. Volcaniclastic sedimentation began with thick turbiditic sandstones and interbedded shales in the late Albian–Santonian (Ngamring Formation) followed by shelfal, deltaic, and fl uvial strata (Padana Formation), with fi nal fi lling of the basin by the Campanian age. Forearc sandstones do not show the classical trend from feldspatholithic volcaniclastic to quartzo-feldspathic plutoniclastic compositions, indicating limited unroofi ng of the Gangdese arc prior to collision. U-Pb age spectra of detrital zircons are unimodal with a 107 Ma peak in the lower Ngamring Formation (104–99 Ma), bimodal with a subordinate additional peak at 157 Ma in the middle Ngamring Formation (99– 88 Ma), and multimodal with more abundant pre-Mesozoic ages in the upper Ngamring and Padana Formations (88–76 Ma). These three petrofacies with distinct provenances document the progressive erosional evolution of the Gangdese arc, with uplift of the central Lhasa terrane and expanding river catchments to include the central Lhasa terrane during the Late Cretaceous.

Upper Cretaceous carbon- and oxygen-isotope stratigraphy of hemipelagic carbonate facies from southern Tibet, China

A high-resolution carbon-isotope curve derived from Upper Cretaceous hemipelagic sediments cropping out at Tingri, southern Tibet, shows similarities to patterns established on other continents, notably in the presence of a well-defined positive excursion across the Cenomanian–Turonian boundary where δ13C values exceed 3.5‰. From the upper Turonian to the lower Campanian, δ13C values generally decline, apart from a minor positive excursion in the middle Coniacian: a trend that departs from that recorded from Europe. Relatively low δ13C values (c. 1‰) at the Santonian–Campanian and Campanian–Maastrichtian boundaries in Tibet define a prominent broad positive excursion centred in the middle Campanian and terminated by an abrupt fall towards the close of the stage. When compared with data from Europe and North Africa, the δ13C values of the Tibetan section are generally lower by c. 1.5‰, except for the middle Campanian positive excursion where values (δ13C c. 2‰) are comparable with those documented from Europe and North Africa. These differences are interpreted as reflecting variable mixing of water masses carrying different carbon-isotope signatures, such that areas close to the major sinks of marine organic carbon recorded higher δ13C values than those located in more distal regions. Oxygen-isotope ratios, albeit affected by diagenesis, may record a palaeotemperature signal.

Upper Oligocene–Lower Miocene Gangrinboche Conglomerate in the Xigaze Area, Southern Tibet: Implications for Himalayan Uplift and Paleo-Yarlung-Zangbo Initiation

The Gangrinboche Conglomerate, exposed along the Yarlung-Zangbo suture zone, records a crucial stage of the Himalayan orogeny. The type section of these strata in the Xigaze area, southern Tibet, including the Qiuwu and Dazhuka Formations, was studied by integrated stratigraphic, sedimentologic, petrographic, and geochemical techniques. Palynological data and detrital zircon U-Pb ages indicate that the Qiuwu Formation was deposited during the latest Oligocene to the earliest Miocene (most probably ∼23 Ma), while the overlying Dazhuka Formation was deposited during 23–18 Ma. The Qiuwu Formation was deposited in a deltaic setting, and detritus was entirely derived from the Gangdese magmatic arc in the north. The Dazhuka Formation, in contrast, was deposited in mainly braided river environments and contains clasts derived from both the Gangdese arc in the north and the Himalayan orogen in the south. Clasts derived from the south first occur at the base of the Dazhuka Formation and increase in abundance upsection to become predominant at the top of the formation. This indicates active Early Miocene uplift and accelerated erosion of the Himalayan belt. Paleocurrent data from the Dazhuka Formation show westward axial sediment transport, which together with mixed provenance from both sides of the basin indicates that a paleo-Yarlung-Zangbo River running parallel to the suture zone initiated at the very start of the Miocene, although with flow direction opposite to that of the present. Flow reversal and establishment of the modern eastward-flowing course must have occurred later on in the Neogene, possibly initiating rapid uplift and focused erosion of the Namche-Barwa syntaxis. Basin subsidence at the close of the Paleogene and subsequent development of a major longitudinal paleo-Yarlung-Zangbo took place contemporaneously with initiation of the South Tibetan Detachment System and Main Central Thrust farther to the south, probably reflecting onset of the “hard collision” phase of the Himalayan orogeny.

Late Cretaceous evolution of the Coqen Basin (Lhasa terrane) and implications for early topographic growth on the Tibetan Plateau

The tectonic evolution of the Lhasa terrane (southern Tibetan Plateau) played a fundamental role in the formation of the Tibetan Plateau. However, many uncertainties remain with regard to the tectonic and paleogeographic evolution of the Lhasa terrane prior to the India-Asia collision. To determine the early tectonic processes that controlled the topographic evolution of the Lhasa terrane, we analyze the Cretaceous strata exposed in the Coqen Basin (northern Lhasa subterrane), which comprises the Langshan and Daxiong Formations. The Langshan Formation unconformably overlies the volcanic rocks of the Lower Cretaceous Zelong Group and consists of similar to 80 m of Orbitolina-bearing limestones, which were deposited in a low-energy, shallow marine environment. Micropaleontological analysis indicates that the Langshan Formation in the Coqen Basin was deposited from late Aptian to early Cenomanian times (ca. 113-96 Ma). The overlying Daxiong Formation (similar to 1700 m thick) consists of conglomerate, coarse sandstone, and siltstone with interbedded mudstone, and represents deposits of alluvial fans and braided rivers. The Daxiong Formation was deposited after the early Cenomanian (ca. 96 Ma) and accumulated until at least ca. 91 Ma, indicating accumulation rates of greater than 0.3 km m.y.(-1). By combining paleocurrent data, sandstone petrology, detrital zircon U-Pb ages, and Hf isotope analysis, we demonstrate that the Daxiong Formation was derived from Lower Cretaceous volcanic rocks and pre-Cretaceous strata in the northern Lhasa subterrane. During Late Cretaceous time, two thrust systems with opposite vergence were responsible for transforming the northern Lhasa subterrane into an elevated mountain range. This process resulted in the evolution from a shallow marine environment (Langshan Formation) into a terrestrial depositional environment (Daxiong Formation) on the southern margin of the northern Lhasa subterrane. Given the regional paleogeographic context, we conclude that the Daxiong Formation in the Coqen Basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern Lhasa subterrane, which implies early topographic growth of the northern Lhasa subterrane in southern Tibet prior to the India-Asia collision.

Late Paleozoic magmatism in South China: Oceanic subduction or intracontinental orogeny?

A gneissic granite with an U-Pb age of 313±4 Ma was found in northeastern Fujian Province, South China. It is an S-type granite characterized by high K2O, Al2O3 and low SiO2, Na2O contents with high A/CNK ratio of 1.22 for the whole rock. Zircons with stubby morphology from the gneissic granite yield 206Pb/238U ages ranging from 326 to 301 Ma with a weighted average age of 313±4 Ma, and negative ɛHf(t) values from −8.35 to −1.74 with Hf model ages (TCDM) of 1.43 to 1.84 Ga. This S-type granite probably originated from late Paleoproterozoic crust in intracontinental orogeny. Integrated with previous results on paleogeographic reconstruction of South China, the nature of Paleozoic basins, Early Permian volcanism and U-Pb-Hf isotope of detrital zircons from the late Paleozoic to early Mesozoic sedimentary rocks, we suggest the occurrence of a late Paleozoic orogeny in the eastern Cathaysia Block, South China. This orogenic cycle includes Late Carboniferous (340–310 Ma) orogeny (compression) episode and Early Permian (287–270 Ma) post-orogenic or intraplate extension episode. Therefore, the late Paleozoic magmatism in the southeastern South China probably occurred during the intraplate orogeny rather than the arc-related process.

Constraining the timing of the India-Asia continental collision by the sedimentary record

Placing precise constraints on the timing of the India-Asia continental collision is essential to understand the successive geological and geomorphological evolution of the orogenic belt as well as the uplift mechanism of the Tibetan Plateau and their effects on climate, environment and life. Based on the extensive study of the sedimentary record on both sides of the Yarlung-Zangbo suture zone in Tibet, we review here the present state of knowledge on the timing of collision onset, discuss its possible diachroneity along strike, and reconstruct the early structural and topographic evolution of the Himalayan collided range. We define continent-continent collision as the moment when the oceanic crust is completely consumed at one point where the two continental margins come into contact. We use two methods to constrain the timing of collision onset: (1) dating the provenance change from Indian to Asian recorded by deep-water turbidites near the suture zone, and (2) dating the age of unconformities on both sides of the suture zone. The first method allowed us to constrain precisely collision onset as middle Palaeocene (59±1 Ma). Marine sedimentation persisted in the collisional zone for another 20–25 Ma locally in southern Tibet, and molassic-type deposition in the Indian foreland basin did not begin until another 10–15 Ma later. Available sedimentary evidence failed to firmly document any significant diachroneity of collision onset from the central Himalaya to the western Himalaya and Pakistan so far. Based on the Cenozoic stratigraphic record of the Tibetan Himalaya, four distinct stages can be identified in the early evolution of the Himalayan orogen: (1) middle Palaeocene-early Eocene earliest Eohimalayan stage (from 59 to 52 Ma): collision onset and filling of the deep-water trough along the suture zone while carbonate platform sedimentation persisted on the inner Indian margin; (2) early-middle Eocene early Eohimalayan stage (from 52 to 41 or 35 Ma): filling of intervening seaways and cessation of marine sedimentation; (3) late Eocene-Oligocene late Eohimalayan stage (from 41 to 25 Ma): huge gap in the sedimentary record both in the collision zone and in the Indian foreland; and (4) late Oligocene-early Miocene early Neohimalayan stage (from 26 to 17 Ma): rapid Himalayan growth and onset of molasse-type sedimentation in the Indian foreland basin.

Remagnetization of carbonate rocks in southern Tibet: Perspectives from rock magnetic and petrographic investigations

Netherlands Organization for Scientific Research (NWO) with a Rubicon grant [825.15.016]; Institute for Rock Magnetism (IRM) at the University of Minnesota; Instruments and Facilities program of NSF; ERC Starting Grant [306810]; NWO VIDI [864.11.004]

The Cenomanian-Turonian anoxic event in southern Tibet: A study of organic geochemistry

The Cenomanian-Turonian oceanic anoxic event (C/T OAE) is developed in southern Tibet. Organic geochemical study of the Cenomanian-Turonian sediments from the Gamba and Tingri areas shows that the mid-Cretaceous black shales in southern Tibet are enriched in organic carbon. The molecular analyses of organic matter indicate marine organic matter was derived from algae and bacteria. In the Gamba area, the organic matter is characterized by abundant tricyclic terpanes and pregane, which are predominant in 191 and 217 mass chromatograms, respectively. Pristane/phytane (Pr/Ph) ratios in the C/T OAE sediments are less than 1, demonstrating the domination of phytane. The presence of carotane can be regarded as a special biomarker indicating oxygen depletion in the C/T OAE sediments in the Tethyan Himalayas. In anoxic sediments, β-carotane and γ-carotane are very abundant. The β- and γ-carotane ratios relative to nC17 in the Cenomanian-Turonian anoxic sediments vary from 32.28 ∼42.87 and 5.10∼11.01.