Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin

Late Cretaceous-Paleogene Exhumation History and Evolution of Paleotopography in the Gaize Basin, Central Tibet

A series of terrestrial sedimentary basins along the Bangong-Nujiang Suture Zone (BNSZ) record a wealth of information regarding the uplift and geomorphological evolution of the central Tibetan Plateau. The uplift, exhumation, and sedimentation processes in these basins and the surrounding orogenic belts are of significant scientific importance for understanding the tectonic evolution of the central Tibetan Plateau. Taking the Gaize Basin as the research focus, low-temperature thermochronology and thermal history modeling were conducted on sandstone samples to analyze their exhumation history and driving mechanisms during the Late Cretaceous to Paleogene. Based on stratigraphy and the youngest detrital zircon ages, the samples were collected from the Upper Jurassic to Lower Cretaceous Shamuluo Formation; thermal history modeling using apatite and zircon (U-Th)/He (AHe/ZHe) results reveals two significant exhumation events: 87-70 and 56-26 Ma. The rapid cooling-exhumation event during the Late Cretaceous is associated with crustal shortening and thickening caused by the collision of the Lhasa-Qiangtang terrane; the Paleogene exhumation event is linked to tectonic uplift driven by the India-Asia collision and the continued northward subduction of the Indian continent. Continuous thrusting and uplift on both sides of the basin strengthened river cutting and erosion and gradually converted the external drainage system to an internal drainage system, and the high-relief terrain was progressively leveled and filled. The height of the surface uplift brought on by sediment accumulation was approximately 0.4 km. After Oligocene, high-altitude, low-relief terrain had already been established.

The raising and westward expansion of central Tibet

Understanding the Cenozoic growth history of the Himalaya-Tibetan Plateau (HTP) is essential for elucidating the underlying geodynamic mechanism and interactions among topography, biosphere and atmosphere. However, the spatial-temporal evolution of the HTP, especially that of the Paleogene Central Tibetan Valley (CTV), remains hotly debated. In this study, through radiometric geochronology, plant assemblages, oxygen and clumped isotope paleoaltimetries, we reconstruct the uplift history of the east-west-oriented Luolong Basin in eastern Tibet. Results show that the Luolong Basin was at 0.6 (+0.2/-0.4) km at ca. 54-46 Ma, then rose to 2.9 ± 0.9 km at ca. 44 Ma. The newly discovered Luolong Flora indicates the Eocene CTV extending into eastern Tibet, and that the valley was higher in the east, sloping to the west, inferring a westward progressive rise of the valley floor. Integrated evidence from paleomagnetism, magmatism and seismic tomography suggests that the birth of the near modern plateau is attributed to the stepwise delamination (drip) of the subducted Lhasa lithosphere from east to west.

The last of their kind: Is the genus Scutiger (Anura: Megophryidae) a relict element of the paleo-Transhimalaya biota?

The orographic evolution of the Himalaya-Tibet Mountain system continues to be a subject of controversy, leading to considerable uncertainty regarding the environment and surface elevation of the Tibetan Plateau during the Cenozoic era. As many geoscientific (but not paleontological) studies suggest, elevations close to modern heights exist in vast areas of Tibet since at least the late Paleogene, implicating the presence of large-scale alpine environments for more than 30 million years. To explore a recently proposed alternative model that assumes a warm temperate environment across paleo-Tibet, we carried out a phylogeographic survey using genomic analyses of samples covering the range of endemic lazy toads (Scutiger) across the Himalaya-Tibet orogen. We identified two main clades, with several, geographically distinct subclades. The long temporal gap between the stem and crown age of Scutiger may suggest high extinction rates. Diversification within the crown group, depending on the calibration, occurred either from the Mid-Miocene or Late-Miocene and continued until the Holocene. The present-day Himalayan Scutiger fauna could have evolved from lineages that existed on the southern edges of the paleo-Tibetan area (the Transhimalaya = Gangdese Shan), while extant species living on the eastern edge of the Plateau originated probably from the eastern edges of northern parts of the ancestral Tibetan area (Hoh Xil, Tanggula Shan). Based on the Mid-Miocene divergence time estimation and ancestral area reconstruction, we propose that uplift-associated aridification of a warm temperate Miocene-Tibet, coupled with high extirpation rates of ancestral populations, and species range shifts along drainage systems and epigenetic transverse valleys of the rising mountains, is a plausible scenario explaining the phylogenetic structure of Scutiger. This hypothesis aligns with the fossil record but conflicts with geoscientific concepts of high elevated Tibetan Plateau since the late Paleogene. Considering a Late-Miocene/Pliocene divergence time, an alternative scenario of dispersal from SE Asia into the East, Central, and West Himalaya cannot be excluded, although essential evolutionary and biogeographic aspects remain unresolved within this model.

Provenance Analysis in the Nima Basin during Paleogene and Its Implications for the Decline of the Tibetan Central Valley

It is unclear what caused the Bangong Nujiang suture zone in the central Tibetan plateau to rise from less than 2 km in early Cenozoic to more than 4 km at present. The zircon U-Pb ages and trace elements of samples from the Niubao Formation in the Paleogene of the Nima basin were analyzed and tested. Combined with the isostasy theory, the surface uplift height of the Nima Basin during the Cenozoic period was calculated. The zircon U-Pb age results of the Niubao formation are consistent with the ages of the Lhasa terrane on the south side of the basin, the Qiangtang terrane on the north side, and the uplift in central. The zircon Eu/Eu* results show that the crust in central part of Tibetan plateau thickened by ∼20 km in Paleogene, resulting in ∼3 km surface uplift. Sediments created a total of about 1 km of surface uplift throughout the Paleogene, and the deposition rate began to slow down significantly at ∼40 Ma. Therefore, it is inferred that in the early Cenozoic, the uplift of the valley was mainly caused by sedimentation. With the continuous downward subduction of the Indian plate, at about 40 Ma, factors such as crustal shortening dominated the uplift of the central valley, and the uplift caused by deposition only accounted for a very small part. In general, the uplift of the Central Valley in the Paleogene was mainly affected by crustal shortening, but a quarter of the surface uplift was caused by the accumulation of sediments.

Molecular phylogeny of mega-diverse Carabus attests late Miocene evolution of alpine environments in the Himalayan-Tibetan Orogen

The timing, sequence, and scale of uplift of the Himalayan-Tibetan Orogen (HTO) are controversially debated. Many geoscientific studies assume paleoelevations close to present-day elevations and the existence of alpine environments across the HTO already in the late Paleogene, contradicting fossil data. Using molecular genetic data of ground beetles, we aim to reconstruct the paleoenvironmental history of the HTO, focusing on its southern margin (Himalayas, South Tibet). Based on a comprehensive sampling of extratropical Carabus, and ~ 10,000 bp of mitochondrial and nuclear DNA we applied Bayesian and Maximum likelihood methods to infer the phylogenetic relationships. We show that Carabus arrived in the HTO at the Oligocene-Miocene boundary. During the early Miocene, five lineages diversified in different parts of the HTO, initially in its southern center and on its eastern margin. Evolution of alpine taxa occurred during the late Miocene. There were apparently no habitats for Carabus before the late Oligocene. Until the Late Oligocene elevations must have been low throughout the HTO. Temperate forests emerged in South Tibet in the late Oligocene at the earliest. Alpine environments developed in the HTO from the late Miocene and, in large scale, during the Pliocene-Quaternary. Findings are consistent with fossil records but contrast with uplift models recovered from stable isotope paleoaltimetry.

Pulsed rise and growth of the Tibetan Plateau to its northern margin since ca. 30 Ma

The onset of mountain building along margins of the Tibetan Plateau provides a key constraint on the processes by which the high topography in Eurasia formed. Although progressive expansion of thickened crust underpins most models, several studies suggest that the northern extent of the plateau was established early, soon after the collision between India and Eurasia at ca. 50 Ma. This inference relies heavily on the age and provenance of Cenozoic sediments preserved in the Qaidam basin. Here, we present evidence in the northern plateau for a considerably younger inception and evolution of the Qaidam basin, based on magnetostratigraphies combined with detrital apatite fission-track ages that date the basin fills to be from ca. 30 to 4.8 Ma. Detrital zircon-provenance analyses coupled with paleocurrents reveal that two-stage growth of the Qilian Shan in the northeastern margin of the Tibetan Plateau began at ca. 30 and at 10 Ma, respectively. Evidence for ca. 30 and 10 to 15 Ma widespread synchronous deformation throughout the Tibetan Plateau and its margins suggests that these two stages of outward growth may have resulted from the removal of mantle lithosphere beneath different portions of the Tibetan Plateau.

The rise and demise of the Paleogene Central Tibetan Valley

Reconstructing the Paleogene topography and climate of central Tibet informs understanding of collisional tectonic mechanisms and their links to climate and biodiversity. Radiometric dates of volcanic/sedimentary rocks and paleotemperatures based on clumped isotopes within ancient soil carbonate nodules from the Lunpola Basin, part of an east-west trending band of basins in central Tibet and now at 4.7 km, suggest that the basin rose from <2.0 km at 50 to 38 million years (Ma) to >4.0 km by 29 Ma. The height change is quantified using the rates at which wet-bulb temperatures (T_w) decline at land surfaces as those surface rise. In this case, T_w fell from ~8°C at ~38 Ma to ~1°C at 29 Ma, suggesting at least ~2.0 km of surface uplift in ~10 Ma under warm Eocene to Oligocene conditions. These results confirm that a Paleogene Central Tibetan Valley transformed to a plateau before the Neogene.

Relict groups of spiny frogs indicate Late Paleogene-Early Neogene trans-Tibet dispersal of thermophile faunal elements

BACKGROUND

The Himalaya-Tibet orogen (HTO) presents an outstanding geologically active formation that contributed to, and fostered, modern Asian biodiversity. However, our concepts of the historical biogeography of its biota are far from conclusive, as are uplift scenarios for the different parts of the HTO. Here, we revisited our previously published data set of the tribe Paini extending it with sequence data from the most western Himalayan spiny frogs Allopaa and Chrysopaa and using them as an indirect indicator for the potential paleoecological development of Tibet.

METHODS

We obtained sequence data of two mitochondrial loci (16S rRNA, COI) and one nuclear marker (Rag1) from Allopaa samples from Kashmir Himalaya as well as Chrysopaa sequence data from the Hindu Kush available from GenBank to complement our previous data set. A Maximum likelihood and dated Bayesian gene tree were generated based on the concatenated data set. To resolve the inconsistent placement of Allopaa, we performed different topology tests.

RESULTS

Consistent with previous results, the Southeast Asian genus Quasipaa is sister to all other spiny frogs. The results further reveal a basal placement of Chrysopaa relative to Allopaa and Nanorana with an estimated age of ca. 26 Mya. Based on the topology tests, the phylogenetic position of Allopaa as a sister clade to Chaparana seems to be most likely, resulting in a paraphyletic genus Nanorana and a separation from the latter clade around 20 Mya, although a basal position of Allopaa to the genus Nanorana cannot be entirely excluded. Both, the placements of Chrysopaa and Allopaa support the presence of basal Paini lineages in the far northwestern part of the HTO, which is diametrically opposite end of the HTO with respect to the ancestral area of spiny frogs in Southeast Asia. These striking distributional patterns can be most parsimoniously explained by trans-Tibet dispersal during the late Oligocene (subtropical Chrysopaa) respectively early Miocene (warm temperate Allopaa). Within spiny frogs, only members of the monophyletic Nanorana+Paa clade are adapted to the colder temperate climates, indicating that high-altitude environments did not dominate in the HTO before ca. 15 Mya. Our results are consistent with fossil records suggesting that large parts of Tibet were characterized by subtropical to warm temperate climates at least until the early Miocene. They contradict prevalent geological models of a highly uplifted late Paleogene proto-Plateau.

A Middle Eocene lowland humid subtropical "Shangri-La" ecosystem in central Tibet

Tibet's ancient topography and its role in climatic and biotic evolution remain speculative due to a paucity of quantitative surface-height measurements through time and space, and sparse fossil records. However, newly discovered fossils from a present elevation of ∼4,850 m in central Tibet improve substantially our knowledge of the ancient Tibetan environment. The 70 plant fossil taxa so far recovered include the first occurrences of several modern Asian lineages and represent a Middle Eocene (∼47 Mya) humid subtropical ecosystem. The fossils not only record the diverse composition of the ancient Tibetan biota, but also allow us to constrain the Middle Eocene land surface height in central Tibet to ∼1,500 ± 900 m, and quantify the prevailing thermal and hydrological regime. This "Shangri-La"-like ecosystem experienced monsoon seasonality with a mean annual temperature of ∼19 °C, and frosts were rare. It contained few Gondwanan taxa, yet was compositionally similar to contemporaneous floras in both North America and Europe. Our discovery quantifies a key part of Tibetan Paleogene topography and climate, and highlights the importance of Tibet in regard to the origin of modern Asian plant species and the evolution of global biodiversity.

Revised chronology of central Tibet uplift (Lunpola Basin)

Knowledge of the topographic evolution of the Tibetan Plateau is essential for understanding its construction and its influences on climate, environment, and biodiversity. Previous elevations estimated from stable isotope records from the Lunpola Basin in central Tibet, which indicate a high plateau since at least 35 Ma, are challenged by recent discoveries of low-elevation tropical fossils apparently deposited at 25.5 Ma. Here, we use magnetostratigraphic and radiochronologic dating to revise the chronology of elevation estimates from the Lunpola Basin. The updated ages reconcile previous results and indicate that the elevations of central Tibet were generally low (<2.3 km) at 39.5 Ma and high (3.5 to 4.5 km) at ~26 Ma. This supports the existence in the Eocene of low-elevation longitudinally oriented narrow regions until their uplift in the early Miocene, with potential implications for the growth mechanisms of the Tibetan Plateau, Asian atmospheric circulation, surface processes, and biotic evolution.

Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene

Paleotopographic reconstructions of the Tibetan Plateau based on stable isotope paleoaltimetry methods conclude that most of the Plateau's current elevation was already reached by the Eocene, ~40 million years ago. However, changes in atmospheric and hydrological dynamics affect oxygen stable isotopes in precipitation and may thus bias such reconstructions. We used an isotope-equipped general circulation model to assess the influence of changing Eocene paleogeography and climate on paleoelevation estimates. Our simulations indicate that stable isotope paleoaltimetry methods are not applicable in Eocene Asia because of a combination of increased convective precipitation, mixture of air masses, and widespread aridity. Rather, a model-data comparison suggests that the Tibetan Plateau only reached low to moderate (less than 3000 meters) elevations during the Eocene, reconciling oxygen isotope data with other proxies.

Lithospheric foundering and underthrusting imaged beneath Tibet

Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of plateau uplift and volcanism. Here we show a T-shaped high wave speed structure in our new tomographic model beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.

The timing and mechanism of uplift of the Tibetan plateau continues to be a source of debate. Here, the authors present a new tomographic model revealing a T-shaped high wave speed structure beneath South-Central Tibet and interpret this an upper-mantle remnant from lithospheric foundering.