Exhumation of the southern Tibetan plateau margin reflects interplay between surface and lithospheric dynamics within the Himalaya-Tibet orogen. We report thermochronometric data from a 1.2-km elevation transect within granitoids of the eastern Lhasa terrane, southern Tibet, which indicate rapid exhumation exceeding 1 km/Ma from 17-16 to 12-11 Ma followed by very slow exhumation to the present. We hypothesize that these changes in exhumation occurred in response to changes in the loci and rate of rock uplift and the resulting southward shift of the main topographic and drainage divides from within the Lhasa terrane to their current positions within the Himalaya. At ∼17 Ma, steep erosive drainage networks would have flowed across the Himalaya and greater amounts of moisture would have advected into the Lhasa terrane to drive large-scale erosional exhumation. As convergence thickened and widened the Himalaya, the orographic barrier to precipitation in southern Tibet terrane would have strengthened. Previously documented midcrustal duplexing around 10 Ma generated a zone of high rock uplift within the Himalaya. We use numerical simulations as a conceptual tool to highlight how a zone of high rock uplift could have defeated transverse drainage networks, resulting in substantial drainage reorganization. When combined with a strengthening orographic barrier to precipitation, this drainage reorganization would have driven the sharp reduction in exhumation rate we observe in southern Tibet.Tibet-Himalaya | thermochronometry | landscape evolution T he Himalaya-Tibet orogenic system, formed by collision between India and Asia beginning ca. 50 Ma, is the most salient topographic feature on Earth and is considered the archetype for understanding continental collision. Geophysical and geologic research has illuminated the modern structure and dynamics of the orogen (1). Nonetheless, how the relatively low relief and high elevation Tibetan plateau grew spatially and temporally and what underlying mechanism(s) drove the patterns of plateau growth remain outstanding questions.In the internally drained central Tibetan plateau, evidence from carbonate stable isotopes suggest that high elevations persisted since at least 25-35 Ma (2, 3). Sustained high elevations since shortly after collision commenced have also been used to explain low long-term erosion rates in the internally drained plateau interior (4-6). In contrast to the central plateau, the externally drained Tibetan plateau margins serve as the headwaters for many major river systems in Asia. Because externally drained rivers provide an erosive mechanism to destroy uplifted terrane, understanding why these rivers have not incised further and more deeply into the Tibetan plateau is essential to decipher how the plateau grew. Recent research in the eastern (7, 8) and northern (9) Tibetan plateau indicates that erosion rates have increased significantly since ∼10 Ma. These increases suggest that rock uplift rates have also increased and that the plateau has expanded to the e...
The Neoproterozoic Era was an interval characterized by profound environmental and biological transitions. Existing age models for Neoproterozoic nonglacial intervals largely have been based on correlation of carbonate carbon isotope values, but there are few tests of the assumed synchroneity of these records between basins. In contrast to the ash-poor successions typically targeted for Neoproterozoic chemostratigraphy, the Tonian to Cryogenian Tambien Group (Tigray region, Ethiopia) was deposited in an arc-proximal basin where volcanic tuffs suitable for U-Pb geochronology are preserved within the mixed carbonate-siliciclastic sedimentary succession. The Tambien Group culminates in a diamictite interpreted to correlate to the ca. 717-662 Ma Sturtian snowball Earth glaciation. New physical stratigraphic data and high-precision U-Pb dates from intercalated tuffs lead to a new stratigraphic framework for the Tambien Group that confirms identification of negative δ 13 C values from Assem Formation limestones with the ca. 800 Ma Bitter Springs carbon isotope stage. Integration with data from the Fifteenmile Group of northwestern Canada constitutes a positive test for the global synchroneity of the Bitter Spring Stage and constrains the stage to have started after 811.51 ± 0.25 Ma and to have ended before 788.72 ± 0.24 Ma. These new temporal constraints strengthen the case for interpreting Neoproterozoic carbon isotope variation as a record of large-scale changes to the carbon cycle and provide a framework for age models of paleogeographic change, geochemical cycling, and environmental evolution during the radiation of early eukaryotes.
The elevation history of the Tibetan Plateau promises insight into the mechanisms and dynamics that develop and sustain high topography over tens of millions of years. We present the first nearly continuous Cenozoic elevation history from two sedimentary basins on the southern Tibetan Plateau within the latest Cretaceous to Eocene Gangdese arc. Oxygen isotope and Δ47 clumped isotope compositions of non-marine carbonates allow us to constrain carbonate formation temperature and reconstruct the paleo-precipitation record of the Eocene to Pliocene Manuscript Stable isotope paleoaltimetry in the India-Asia collision zone Numerous investigations have reconstructed the elevation history of the Himalayas and Tibetan Plateau using the oxygen isotopic composition of non-marine carbonates (Garzione et
A salient geomorphic feature of the Yarlung River is its abundance of large knickpoints, which in many cases coincide with north-south trending rifts. Across one of these rifts, near the town of Jiacha, the Yarlung falls nearly 500 m from an elevation of ~3500 m over 80 river kilometers, making this the second largest knickpoint on the river. We propose that the Jiacha knickpoint represents a wave of incision migrating upstream through the drainage network in response to a downstream base level fall, not a disturbance in the channel to due rift tectonics. Longitudinal profile slope-area and chi () analysis of Yarlung River tributaries and those of several major rivers in southeastern Tibet indicate severalknickpoints are present at ~3500 m elevation,all resulting from a single regional-scale base level fall. Retreat rates calculated from celerity modeling indicate that the Jiacha knickpoint was located at the upstream edge of the Namche Barwa massif at ~10 Ma, a history consistent with apatite 4 He/ 3 He thermochronometry data and thermokinematic modeling from that region. These data suggest the Yarlung River has flowed in its present course through this area since at least 10 Ma and imply that at least500 m of incision occurred within this canyon over this time period. The spatial scale of these observations suggests that these knickpoints resulted from surface uplift of southeastern Tibet of 500 to2500 m just prior to ~10 Ma. Additionally, our mapped knickpoint locations indicate that reorganization of the drainage network just east of the Namche Barwa massif occurred prior to this time.
While proxy records have been used to reconstruct late Quaternary climate parameters throughout the European Alps, our knowledge of deglacial climate conditions in the Maritime Alps is limited. Here, we report temperatures recorded by a new and independent geochemical technique—cosmogenic noble gas paleothermometry—in the Maritime Alps since the last glacial maximum. We measured cosmogenic 3He in quartz from boulders in nested moraines in the Gesso Valley, Italy. Paired with cosmogenic 10Be measurements and 3He diffusion experiments on quartz from the same boulders, the cosmogenic 3He abundances record the temperatures these boulders experienced during their exposure. We calculate effective diffusion temperatures (EDTs) over the last ∼22 ka ranging from 8°C to 25°C. These EDTs, which are functionally related to, but greater than, mean ambient temperatures, are consistent with temperatures inferred from other proxies in nearby Alpine regions and those predicted by a transient general circulation model. In detail, however, we also find different EDTs for boulders from the same moraines, thus limiting our ability to interpret these temperatures. We explore possible causes for these intra-moraine discrepancies, including variations in radiative heating, our treatment of complex helium diffusion, uncertainties in our grain size analyses, and unaccounted-for erosion or cosmogenic inheritance.
Orogenic topography results from the complex processes imposed by climate, tectonics, and their feedbacks mainly through river incision (e.g., Champagnac et al., 2012;Whipple, 2004). In this context, fluvial topography and exhumation rate histories can provide an archive of climatic (e.g., Bender et al., 2020) and tectonic (e.g., Schildgen et al., 2007) changes with time. As fluvial topography evolves over geological time, the control it exerts on erosion would also change with time (e.g., Champagnac et al., 2012), but how the control changes through time remains ambiguous. The Tibetan Plateau is the highest plateau on Earth and is characterized by high topographic relief at its margins (Montgomery & Brandon, 2002;Yin, 2006) formed through river incision (Figure 1) and these rivers are expected to be very sensitive to climate and tectonic changes (e.g., Nie et al., 2018). There are several large river drainages including the Yangtze, Mekong, Salween, and Yarlung across the southern and southeastern Tibetan Plateau and Himalayas (Figure 1). Their unusual geometries and topographies have been considered to be the result of tectonic deformation and drainage reorganization (e.g., Wang, Scherler, et al., 2014;Zhang et al., 2019), as well as climate change (e.g., Nie et al., 2018). Therefore, rivers in the Tibetan Plateau and the Himalayas are natural laboratories to investigate the complex links among tectonics, climate, surface process, and drainage reorganization.The west-to-east flowing Yarlung River, located within the Indian-Asian Collision Zone (IACZ, including the Gangdese arc, the Yarlung Zangbo Suture Zone, and the Tethyan Himalaya), is one of the largest rivers that flows from the Tibetan Plateau (Figures 1 and 2). Its source is high in southern Tibet and the Yarlung River cuts through the eastern syntaxis before flowing through Siang River into the Brahmaputra River.
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