[1] A range of ages have been proposed for the timing of India-Asia collision; the range to some extent reflects different definitions of collision and methods used to date it. In this paper we discuss three approaches that have been used to constrain the time of collision: the time of cessation of marine facies, the time of the first arrival of Asian detritus on the Indian plate, and the determination of the relative positions of India and Asia through time. In the Qumiba sedimentary section located south of the Yarlung Tsangpo suture in Tibet, a previous work has dated marine facies at middle to late Eocene, by far the youngest marine sediments recorded in the region. By contrast, our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50.6–52.8 Ma, in broad agreement with the timing of cessation of marine facies elsewhere throughout the region. Double dating of detrital zircons from this formation, by U-Pb and fission track methods, indicates an Asian contribution to the rocks thus documenting the time of arrival of Asian material onto the Indian plate at this time and hence constraining the time of India-Asia collision. Our reconstruction of the positions of India and Asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early Eocene. We conclude the paper with a discussion on the viability of a recent assertion that collision between India and Asia could not have occurred prior to ∼35 Ma.

/pdf/timing-of-india-asia-collision-geological-biostratigraphic-4vizd3piru.pdf

Timing of India‐Asia collision: Geological, biostratigraphic, and palaeomagnetic constraints

The timing of India-Asia collision onset – Facts, theories, controversies

Siliciclastic sedimentary rocks derived from the southern Lhasa terrane, sitting depositionally upon rocks of the northern Indian passive continental margin, provide an estimate of the age of initial contact between the continental parts of the Indian and Asian plates. We report sedimentological, sedimentary petrological, and geochronological data from Upper Cretaceous-Paleocene strata in the Sangdanlin section, located along the southern flank of the Indus-Yarlung suture zone in southern Tibet. This is probably the most proximal, and therefore the oldest, record of the India-Asia collision. These strata were deposited by high-density turbidity currents (or concentrated density flows) and suspension settling of pelagic biogenic debris in a deep-marine setting. An abrupt change from quartz-arenitic to feldspatholithic sandstone compositions marks the transition from Indian to Asian sediment provenance. The abrupt compositional change is accompanied by changes in U-Pb ages of detrital zircons diagnostic of a sediment provenance reversal, from Indian to Asian sources. The timing of the transition is bracketed between ~60 Ma and 58.5 ± 0.6 Ma by detrital zircon U-Pb ages and zircon U-Pb ages from a tuffaceous bed in the upper part of the section. In the context of a palinspastically restored regional paleogeographic framework, data from the Sangdanlin section combined with previously published data from the northern Tethyan Himalaya and the frontal Nepalese Lesser Himalaya and Subhimalaya suggest that a flexural wave migrated ~1300 km southward across what is now the Himalayan thrust belt from Paleocene time to the present.

/pdf/paleocene-eocene-foreland-basin-evolution-in-the-himalaya-of-4l7j2r8wuy.pdf

Paleocene-Eocene foreland basin evolution in the Himalaya of southern Tibet and Nepal: Implications for the age of initial India-Asia collision

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 Earth9s 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 ∼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 ∼100 Ma. Stratigraphic evidence indicates that the northern Himalayan margin of India collided with an Asian-affinity subduction complex – forearc – arc system beginning at ∼60 Ma. Both the Himalaya (composed of Indian crust) and Tibet show continuous geological records of orogenesis since ∼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 ∼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. Seismic tomographic images are also suggestive of a second, more southerly Neo-Tethyan oceanic slab in the lower mantle where the northernmost margin of India may have been located at ∼60 Ma. The geology of Tibet and the India – Asia suture zone permits an alternative collision scenario in which the continental margin arc along southern Asia (the Gangdese arc) was split by extension beginning at ∼90 Ma, and along with its forearc to the south (the Xigaze forearc), rifted southward and opened a backarc ocean basin. The rifted arc collided with India at ∼60 Ma whereas the hypothetical backarc ocean basin may not have been consumed until ∼45 Ma. A compilation of igneous age data from Tibet shows that the most recent phase of Gangdese arc magmatism in the southern Lhasa terrane initiated at ∼70 Ma, peaked at ∼51 Ma, and terminated at ∼38 Ma. Cenozoic potassic-adakitic magmatism initiated at ∼45 Ma within a ∼200-km-wide elliptical area within the northern Qiangtang terrane, after which it swept westward and southward with time across central Tibet until ∼26 Ma. At 26 to 23 Ma, potassic-adakitic magmatism swept southward across the Lhasa terrane, a narrow (∼20 km width), orogen-parallel basin developed at low elevation along the axis of the India – Asia suture zone (the Kailas basin), and Greater Himalayan Sequence rocks began extruding southward between the South Tibetan Detachment and Main Central Thrust. The Kailas basin was then uplifted to >4 km elevation by ∼20 Ma, after which parts of the India – Asia suture zone and Gangdese arc experienced >6 km of exhumation (between ∼20 and 16 Ma). Between ∼16 and 12 Ma, slip along the South Tibetan Detachment terminated and east-west extension initiated in the northern Himalaya and Tibet. Potassic-adakitic magmatism in the Lhasa terrane shows a northward younging trend in the age of its termination, beginning at 20 to 18 Ma until volcanism ended at 8 Ma. We interpret the post-45 Ma geological evolution in the context of the subduction dynamics of Indian continental lithosphere and its interplay with delamination of Asian mantle lithosphere.

Mesozoic─Cenozoic geological evolution of the Himalayan-Tibetan orogen and working tectonic hypotheses

The timing of initial collision between India and Asia has remained controversial for half a century. This paper attempts to review this crucial and hotly debated argument, describing first the different methods used to constrain the age of collision and discussing next the rationale, results, inferences and problems associated with each. We conclude that stratigraphy represents the best direct way to unravel collision chronology. Other methods focusing on the magmatic, metamorphic or paleomagnetic record provide additional fundamental constraints, but cannot provide a robust direct estimate of collision onset. Initial collision in the central-eastern Himalaya is dated directly at the middle Paleocene (59 ± 1 Ma) by the abrupt change in sediment provenance recorded in trench settings. The quasi-synchronous unconformities documented along both Tethyan passive margin of India and active margin of Asia from Tibet to Zanskar-Ladakh confirm that orogeny was underway at the close of the Paleocene (56 Ma), well before the disappearance of marine seaways in the Himalaya during the early-middle Eocene (50–45 Ma). Sedimentary evolution and provenance changes in marine to fluvio-deltaic successions are recorded synchronously within error from the western to the central-eastern Himalaya, failing to provide conclusive evidence for diachronous collision. These coherent observations are hard to reconcile with three widely cited hypotheses invoking either Paleogene arc-continent collision or Late Cretaceous ophiolite obduction, or the protracted existence of a Greater India Basin, which are all not favored after discussing the geological evidence critically point by point. A scenario no more complex than the one involving solely the passive continental margin of India and the active continental margin of Asia is needed to explain the geological evolution of the nascent Himalaya. The collision between the Tethys Himalaya and the Transhimalayan arc-trench system does represent the collision between India and Asia. Because the Yarlung Zangbo Ophiolite is the forearc basement of the Asian active margin, its obduction onto India could not have preceded the initial closure of Neo-Tethys. Ophiolite obduction began when collision began, in the middle Paleocene.

The timing of India-Asia collision onset - facts, theories, controversies

https://www.lancaster.ac.uk/staff/najman/Najman%20et%20al%20EPSL%202008.pdf

The Paleogene record of Himalayan erosion: Bengal Basin, Bangladesh

The Cenozoic sedimentary succession of Bangladesh provides an archive of Himalayan erosion. However, its potential as an archive is currently hampered by a poor lithostratigaphic framework with limited age control. We focus on the Hatia Trough of the Bengal Basin and the adjacent fold belt of the Chittagong Hill Tracts which forms the outermost part of the west-propagating IndoBurmese wedge. We present a basin-wide seismic stratigraphic framework for the Neogene rocks, calibrated by biostratigraphy, which divides the succession into three seismically distinct and regionally correlatable Megasequences (MS). MS1 extends to NN15-NN16 (ca. 2.5–3.9 Ma), MS2 to NN19-NN20 (ca. 0.4–1.9 Ma) and MS3 to present day. Our seismic mapping, thermochronological analyses of detrital mineral grains, isotopic analyses of bulk rock, heavy mineral and petrographic data, show that the Neogene rocks of the Hatia Trough and Chittagong Hill Tracts are predominantly Himalayan-derived, with a subordinate arc-derived input possibly from the Paleogene IndoBurman Ranges as well as the Trans-Himalaya. Our seismic data allow us to concur with previous work that suggests folding of the outer part of the west-propagating wedge only commenced recently, within the last few million years. We suggest that it could have been the westward encroachment and final abutment of the Chittagong Hill Tracts fold belt onto the already-uplifted Shillong Plateau that caused diversion of the palaeo-Brahmaputra to the west of the plateau as the north-east drainage route closed.

/pdf/therecordof-himalayanerosionpreservedinthe-53bv2qqup1.pdf

Therecordof Himalayanerosionpreservedinthe sedimentaryrocksof the HatiaTroughof the Bengal BasinandtheChittagongHillTracts,Bangladesh

Abundances of s-capture process elements in stars with exquisite asteroseismic, spectroscopic, and astrometric constraints offer a novel opportunity to study stellar evolution, nucleosynthesis, and Galactic chemical evolution. We aim to investigate one of the least studied s-process elements in the literature, Ce, using stars with asteroseismic constraints from the Kepler, K2 and TESS missions. We combine the global asteroseismic parameters derived from precise light curves obtained by the Kepler, K2 and TESS missions with chemical abundances from the APOGEE DR17 survey and astrometric data from the Gaia mission. Finally, we compute stellar ages using the code PARAM. We investigate the different trends of [Ce/Fe] as a function of [Fe/H], [alpha/Fe] and age considering the dependence on the radial position, specially in the case of K2 targets which cover a large Galactocentric range. We, finally, explore the [Ce/alpha] ratios as a function of age in different Galactocentric intervals. The studied trends display a strong dependence of the Ce abundances on [Fe/H] and star formation history. Indeed, the [Ce/Fe] ratio shows a non-monotonic dependence on [Fe/H] with a peak around -0.2 dex. Moreover, younger stars have higher [Ce/Fe] and [Ce/alpha] ratios than older stars, confirming the latest contribution of low- and intermediate-mass asymptotic giant branch stars to the Galactic chemical enrichment. In addition, the trends of [Ce/Fe] and [Ce/alpha] with age become steeper moving towards the outer regions of the Galactic disc, demonstrating a more intense star formation in the inner regions than in the outer regions. Ce is thus a potentially interesting element to help constraining stellar yields and the inside-out formation of the Milky Way disc. However, the large scatter in all the relations studied here, suggests that spectroscopic uncertainties for this element are still too large.

/pdf/time-evolution-of-ce-as-traced-by-apogee-using-giant-stars-3rjc9hrd.pdf

E. Willett

Papers

The Paleogene record of Himalayan erosion: Bengal Basin, Bangladesh

Therecordof Himalayanerosionpreservedinthe sedimentaryrocksof the HatiaTroughof the Bengal BasinandtheChittagongHillTracts,Bangladesh

Time evolution of Ce as traced by APOGEE using giant stars observed with the Kepler, TESS, and K2 missions