Plateau Margin Research Articles

Sources and spatio-temporal distribution of aerosol polycyclic aromatic hydrocarbons throughout the Tibetan Plateau

With the recent rapid development of urbanization, atmospheric pollutants such as polycyclic aromatic hydrocarbons (PAHs) have attracted wide attention, particularly in remote regions. The Tibetan Plateau (TP), known as the third pole is adjacent to areas with heavy atmospheric pollution, such as South and East Asia. However, the spatial distribution and sources of PAHs on the TP remain unclear. Thus, we investigated the sources and spatio-temporal distributions of PAHs on the TP by combining aerosol sample data from six sites, including Ngari (NG), Laohugou (LHG), Beiluhe (BLH), Nam Co (NMC), Everest (EV), and Yulong (YL), in 2014 and 2016. The average concentrations of 15 PAHs at the six sites ranged from 3.4 to 15.2 ng m−3, with a decreasing trend from the marginal to inner areas of the plateau. The highest concentration was that in YL in the southeastern part of the TP, with an average of 15.2 ng m−3. The PAH concentrations in NG, NMC, and YL were higher in autumn and winter and lower in summer. High molecular weight PAHs usually exists in the particulate phase whereas tricyclic PAHs can change from particulate to gaseous phase, therefore it can indicate long-range transport. Tricyclic PAHs were the dominant PAHs on the TP (44%–58%), indicating long-range atmospheric transport as the major source of PAHs. Principal component analysis (PCA) and diagnostic ratio analysis showed that biomass and coal combustion were the major sources of PAHs in inland areas of the TP; however, marginal plateau areas were affected by fossil fuel emissions. Compared with levels in Beijing and other urban sites, the toxic equivalent quantity (TEQ) was low (0.36–1.06 ng m−3), suggesting a low risk to human and ecosystem health.

Late Cenozoic topographic evolution of the Eastern Cordillera and Puna Plateau margin in the southern Central Andes (NW Argentina)

The Andes constitute an important orographic barrier in the southern hemisphere, impacting atmospheric circulation, the amount and distribution of rainfall, and Earth-surface processes in a highly asymmetric manner. In the Central Andes of NW Argentina, the Andean Plateau (Puna) and the intermontane basins of the adjacent Eastern Cordillera constitute geological archives that furnish spatiotemporal information on surface uplift and associated paleo-environmental change. Today, rainfall in NW Argentina is focused along the windward flanks of the Eastern Cordillera, while its intermontane basins and the Puna constitute high-elevation regions with strongly reduced rainfall. The present-day influence of these topographic characteristics on precipitation patterns is reflected by systematic changes in the hydrogen stable isotope ratios of meteoric water. Proxy archives from basin strata record past environmental conditions and provide insight into the nature of topographic growth through time. We present 34 new hydrogen isotopic compositions of volcanic glass (δDg), extracted from tuffs interbedded with the sedimentary deposits of three different regions across the eastern Andean margin between ca. 24°S and 25°S latitude. Combined with new zircon U-Pb geochronology, our data show clear spatiotemporal variations in δDg correlating with topographic growth and ensuing orographic effects during the Miocene and Pleistocene. In particular, variations in δDg indicate that the Pastos Grandes (Puna) and the Toro (Eastern Cordillera) basins had attained modern elevations by ca. 8 and 6 Ma, respectively. A strong positive shift in the δDg record of the Toro Basin after 840 ka coincides with deposition of thick fluvial/alluvial conglomerates suggesting orographically induced aridification due to progressive range uplift to the east. Other δDg records from the Eastern Cordillera show similar, but diachronous events emphasizing the importance of regional tectonics for environmental change with implications for the formation of orogenic plateaus.

Episodic Lithospheric Deformation in Eastern Tibet Inferred From Seismic Anisotropy

AbstractMechanisms for uplift and deformation of the Tibetan Plateau remain vigorously debated; hypotheses include stepwise growth, distributed thickening, and crustal channel flow, each with a distinct anisotropic signature. We have developed a new azimuthally anisotropic shear velocity model for the lithosphere beneath eastern Tibet, based on ambient noise tomography from 643 seismic stations. In our model, the Tibetan upper crust is characterized by strong anisotropy with fast axes that correlate with surface geology and mantle anisotropy, suggesting the occurrence of coherent deformation. However, a much different picture emerges in the middle and lower crust, where anisotropy is disordered and weaker beneath the plateau than along its margins, inconsistent with the prediction of large‐scale eastward crust flow in eastern Tibet. Our observations are best explained by heterogeneous crustal thickening beneath the plateau with complex flow in the middle and lower crust, accompanied by asthenospheric upwelling along the southeastern plateau margin.

Magnetochronology of Mid-Miocene mammalian fauna in the Lanzhou Basin, northeastern Tibetan Plateau: Implications for Asian mammal migration

The Quantougou (QTG) Fauna in central Lanzhou Basin is an important late Mid-Miocene fauna on the northeastern Tibetan Plateau margin, but its numerical age remains a matter of debate. Here, we present a new magnetostratigraphic record for a fluvio-lacustrine section to further constrain the age of the QTG Fauna. Results suggest that the studied section spans from polarity chrons C5Cn.2n to C5n.2n or C5An.1n, with ages of ca. 16.5 Ma to 10 ​Ma or 16.5 Ma to 12 ​Ma. The QTG Fauna is located at the top of polarity chron C5r.3r or C5Ar.2r, which corresponds to an age of 11.7 Ma or 12.8 ​Ma for the fauna. Accordingly, the associated Myocricetodontinae (a subfamily of Gerbillidae, Rodentia) is suggested to have appeared in the Lanzhou Basin at 11.7 ​Ma or 12.8 ​Ma, which is the oldest Myocricetodontinae in East Asia but is still much younger than the ~20 ​Ma appearance of this subfamily in West and South Asia. Our age data support the interpretation that East Asian Myocricetodontinae originated from South Asia. The QTG fauna further suggest a dry and open grassland environment, which is consistent with global cooling after the Mid-Miocene Climatic Optimum.

Tectonic evolution of the eastern margin of the Tibetan plateau: Insight from crustal structures using P wave receiver functions

The eastern margin of the Tibetan plateau is under ongoing expansion. Rise and growth of the Longmenshan are results of the interaction between the Songpan-Ganzi Terrane and the Sichuan Basin. In order to understand the block interactions in the eastern plateau margin, we studied the crustal structures using P wave receiver functions (including H-k staking, CCP stacking, and velocity inversion) with the waveforms recorded by 115 portable stations. The Moho, indicated by clear Ps phases, is generally deeper in the Songpan-Ganzi Terrane than in the Sichuan Basin, while a step is notable under the Longmenshan fault zone. We found a low velocity anomaly in the mid-lower crust below the Songpan-Ganzi Terrane and Longmenshan fault zone, which may imply the accumulated partial melting materials in the deep crust under the eastern plateau margin. Clear P-to-s converted phases are found within the crust of the Sichuan Basin. They look as apparent huge anticline structures, which may be caused by ancient crustal deformation under the stable craton.

СТРОЕНИЕ И УСЛОВИЯ ОБРАЗОВАНИЯ КРАЕВОГО ПЛАТО И ОСТРОВА СОКОТРА. ЧАСТЬ 2. ФИЗИЧЕСКОЕ МОДЕЛИРОВАНИЕ

The authors studied the formation conditions and the character of deformations of the margin plateau and Socotra Island using the physical modeling. In experiments, a plate with elastic-plastic properties lying on a liquid base was subjected stretching. Plate’s zones imitating the continental or oceanic lithosphere in the model had different thicknesses. In accordance with natural analogues, various heterogeneities were set in the zones: sections, linear reduced zones (rift heating zones) etc. The authors considered the tectonic structure of Socotra Island. The submerged plateau and Socotra Island were investigated using physical modeling, the conditions of formation and the nature of deformations. It is shown that the formation of the submerged Socotra plateau was caused by the interaction of two rift cracks, advancing towards each other. One crack transformed into a spreading ridge (the Sheba Ridge), and the other disappeared, forming an aulacogen-type structure (graben Guardafui). During the process of transition from rifting to spreading, Socotra Island rotated, both in horizontal and vertical planes, which led to jumps of the rift axis and the formation of an asymmetric structure of conjugated continental margins.

A synthesis of the sedimentary evolution of the Demerara Plateau (Central Atlantic Ocean) from the late Albian to the Holocene

Abstract The Demerara Plateau, off the French Guiana and Surinam margin, corresponds to a prominent sub-horizontal marginal plateau. The interpretation of a large set of seismic data acquired along the Demerara Plateau and integration with previous seismic stratigraphy and drill studies enables a better understanding of the sedimentary evolution of the outer Demerara marginal plateau. Since the end of the transform activity (late Albian), three sedimentary evolutionary stages have been identified. 1) The Pre-contourite Stage (late Albian to early Miocene) is strongly controlled by the structure of the margin that influences the distribution pattern of post-transform deposits through differential thermal subsidence between transform and divergent segments. 2) The Transitional Stage (middle Miocene to early Pliocene) is expressed as a current-controlled erosive surface resulting from the onset of bottom-current interactions due to the establishment and strengthening of the modern thermohaline circulation that is coeval with the progressive closure of the Isthmus of Panama. 3) The Contourite Stage (middle Pliocene to Holocene) is characterized by the development of a contourite depositional system that is linked to the activity of the North Atlantic Deep Water circulation and by a complex interaction between alongslope and downslope processes. Our study highlights that marginal plateaus might be particularly prone to develop contourite features since they regionally influence ocean current dynamics in guiding and accelerating bottom contour currents. Marginal plateaus should thus be considered as a key target to study bottom current dynamics and paleocirculation patterns.

Phylogeny of spiny frogs Nanorana (Anura: Dicroglossidae) supports a Tibetan origin of a Himalayan species group.

Recent advances in the understanding of the evolution of the Asian continent challenge the long‐held belief of a faunal immigration into the Himalaya. Spiny frogs of the genus Nanorana are a characteristic faunal group of the Himalaya–Tibet orogen (HTO). We examine the phylogeny of these frogs to explore alternative biogeographic scenarios for their origin in the Greater Himalaya, namely, immigration, South Tibetan origin, strict vicariance. We sequenced 150 Nanorana samples from 62 localities for three mitochondrial (1,524 bp) and three nuclear markers (2,043 bp) and complemented the data with sequence data available from GenBank. We reconstructed a gene tree, phylogenetic networks, and ancestral areas. Based on the nuDNA, we also generated a time‐calibrated species tree. The results revealed two major clades (Nanorana and Quasipaa), which originated in the Lower Miocene from eastern China and subsequently spread into the HTO (Nanorana). Five well‐supported subclades are found within Nanorana: from the East, Central, and Northwest Himalaya, the Tibetan Plateau, and the southeastern Plateau margin. The latter subclade represents the most basal group (subgenus Chaparana), the Plateau group (Nanorana) represents the sister clade to all species of the Greater Himalaya (Paa). We found no evidence for an east–west range expansion of Paa along the Himalaya, nor clear support for a strict vicariance model. Diversification in each of the three Himalayan subclades has probably occurred in distinct areas. Specimens from the NW Himalaya are placed basally relative to the highly diverse Central Himalayan group, while the lineage from the Tibetan Plateau is placed within a more terminal clade. Our data indicate a Tibetan origin of Himalayan Nanorana and support a previous hypothesis, which implies that a significant part of the Himalayan biodiversity results from primary diversification of the species groups in South Tibet before this part of the HTO was uplifted to its recent heights.

Evaluation and treatment of osteochondral injury of knee joint

To investigate evaluation and treatment of osteochondral injury of knee joint and its effectiveness. Between January 2010 and January 2016, 17 patients with osteochondral injury of knee joint were admitted. There were 2 males and 15 females, with an average age of 19.3 years (range, 15-33 years). The causes of injury included the sprain in 14 cases and knee hyper-extension and varus due to violence in 3 cases. The osteochondral injury located at patella in 8 cases, lateral femoral condyle in 4 cases, medial femoral condyle in 1 case, and tibial plateau in 4 cases. There were 15 cases of fresh fractures and 2 cases of old fractures. The Lysholm score of the knee joint was 31.6±2.3. After open reduction of osteochondral fractures of 14 cases, the absorbable rods (9 cases), absorbable cartilage nail (3 cases), or absorbable sutures (2 cases) were selected for fixation. The osteochondral fractures at the medial tibial plateau margin (non-weight-bearing area) in 3 cases were removed. The incision fat liquefaction occurred in 1 case after operation and healed after debridement. The other incisions had primary healing. All 17 patients were followed up 6 months to 2 years (mean, 13 months). Thirteen of 14 patients with internal fixation had good fractures healing without traumatic arthritis; 1 case of patella osteochondral fracture did not heal. Three patients with non-weight-bearing osteochondral removal had no narrowing of the medial joint space and traumatic arthritis during the follow-up. The Lysholm score of knee joint at 1 year after operation was 91.3±1.1, which significantly improved when compared with preoperative score ( t=7.136, P=0.001). For the osteochondral injury of the knee joint, the osteochondral block with full-layer cancellous bone can be treated with open reduction and internal fixation; while osteochondral block with punctate cancellous bone can be directly remove.

Thermal history of the western Central Taurides fold-thrust belt: Implications for Cenozoic vertical motions of southern Central Anatolia

AbstractThe modern physiography of central Turkey is dominated by the 1-km-high Central Anatolian Plateau and the Central Tauride mountains that form the southern plateau margin. These correspond to a Cretaceous–Eocene backarc extensional province and forearc fold-thrust belt, respectively. The extent to which the morphology of the Miocene plateau was inherited from the physiography of the Cretaceous–Eocene subduction zone that assembled the Anatolian crust has not been tested but is important if we are to isolate the signal of Miocene and younger subduction dynamics in the formation of the modern plateau margin. There is no known stratigraphic record of the post-Eocene pre-Miocene evolution of the Taurides. We therefore collected rock samples across the Taurides and used zircon (U-Th)/He (ZHe), apatite (U-Th)/He (AHe), and apatite fission-track (AFT) low-temperature thermochronometers to constrain cooling; we interpret these thermochronometers to signal erosional exhumation. We use inverse thermal modeling to aid interpretation of our results and find that: (1) thermochronometers across the Taurides were reset as a result of heating by the emplacement of the Antalya and Bozkır nappes; (2) AFT and ZHe Eocene cooling ages are related to structurally driven uplift and erosional exhumation on major thrust culminations; (3) dispersed AHe ages record low rates of Oligocene–early Miocene cooling and hence low rates of erosional exhumation; and (4) fast rates of cooling were determined for samples along the margin of the Köprüçay Basin. We interpret that early Miocene cooling is a signal of active erosion of the western Central Taurides at a time of marine sedimentation in the Mut Basin on the southern Central Taurides, and these differing histories may reflect evolution above the Antalya and Cyprus slabs. Our thermochronological data, the enigmatic development of the Antalya Basin, and thrusting within the basin may be explained as the surface expression of stepwise delamination of the Antalya slab from the Tauride hinterland to its current position below the Gulf of Antalya since early Miocene time over a distance of ∼150 km.

Late Miocene Hinterland Crustal Shortening in the Longmen Shan Thrust Belt, the Eastern Margin of the Tibetan Plateau

AbstractLong‐term (million year time scale) fault‐slip history is crucial for understanding the processes and mechanisms of mountain building in active orogens. Such information remains elusive in the Longmen Shan, the eastern Tibetan Plateau margin affected by the devastating 2008 Wenchuan earthquake. While this event drew attention to fault deformation on the foreland side (the Yingxiu‐Beichuan fault), little is known about the deformation history of the hinterland Wenchuan‐Maoxian fault. To address this gap, thermochronological data were obtained from two vertical transects from the Xuelongbao massif, located in the hanging wall of the Wenchuan‐Maoxian fault. The data record late Miocene rapid cooling and rock exhumation at a rate of 0.9–1.2 km/m.y. from ~13 Ma to present. The exhumation rate is significantly higher than that in the footwall (~0.3–0.5 km/m.y.), indicating a differential exhumation of ~0.6 km/m.y. across the fault. This differential exhumation provides the first and minimum constraint on the long‐term throw rate (~0.6 km/m.y) of the Wenchuan‐Maoxian fault since the late Miocene. This new result implies continuous crustal shortening along the hinterland fault of Longmen Shan, even though it has not been ruptured by major historic earthquakes. Our study lends support to geodynamic models that highlight crustal shortening as dominating deformation along the eastern Tibetan Plateau.

10Be and OSL dating of Pleistocene fluvial terraces along the Hongshuiba River: Constraints on tectonic and climatic drivers for fluvial downcutting across the NE Tibetan Plateau margin, China

Accurate geochronologic constraints of fluvial surfaces along tectonically active mountain fronts can help clarify river responses to tectonic uplift and rapid climate change. In this study, we re-constrained abandonment ages of four fluvial terraces along the Hongshuiba River on the northern margin of the Qilian Shan, NE Tibet using 10Be depth profiles to date fluvial deposits, and OSL dating of capping loess. Comparison of chronological ages of the same surface derived from both boulders and depth profiles provides an opportunity to evaluate the suitability of different sampling methods for determining landform ages in arid climate. Correlating terrace ages with regional climate proxies suggests that terraces formed during an interglacial stage or during glacial to interglacial transitions, or during a period of strong summer monsoon or monsoonal weakening. The initial period of fluvial incision may primarily result from climate changes (e.g, monsoon evolution), and other factors that influence the ratio of sediment flux to water discharge. Similarities between fluvial incision rates and incremental tectonic uplift rates suggest a tectonic control on continuous river downcutting before ∼26 ka. In summary, climate forcing drives the initial abandonment of fluvial terraces in this region, while the amplitude of fluvial incision was mainly controlled by progressive tectonic uplift. However, comparison of regional fluvial incision rates (e.g. ∼6mm/a of the Hongshuiba River), exhumation rates from the Qilian Shan (∼0.5mm/a), denudation rates from the river catchments (0.3-0.8mm/a), and vertical slip rates of the northern Qilian Thrust (∼1mm/a) all suggest climate change driven forcing of incision since the Late Pleistocene to Holocene.

P and S Wave Tomography Beneath the SE Tibetan Plateau: Evidence for Lithospheric Delamination

AbstractThe SE Tibetan Plateau is located between the eastern Himalayan syntaxis and the stable Yangtze craton; its tectonic evolution is important for understanding the plateau expansion. However, interactions between the plateau and its adjacent subduction zone and craton are still unclear. In this study, we determine updated high‐resolution P and S wave tomographic models of the crust and upper mantle beneath the SE Tibetan Plateau. We confirmed the existence of a high‐velocity layer in the upper mantle down to 200‐km depth beneath the Sichuan basin, the core of the Yangtze craton, representing the stable cratonic root. Another high‐velocity body is located beneath Burma, reflecting the subducting Indian slab in the upper mantle and possibly the remnant Burman slab in the mantle transition zone. A strong low‐velocity column exists above the high‐velocity body in the transition zone and reaches the Moho under the Tengchong volcano. In addition, we find high‐velocity fragments in the upper mantle beneath the Yangtze craton and northern Indochina, which may reflect delaminated lithosphere beneath the SE Tibetan Plateau. The lithospheric delamination may be closely related to mantle flow extruded from the high plateau and possible thermal upwelling from the lower mantle. Our results provide new insight into the deep interactions between the Tibetan plateau and its surrounding blocks near the SE plateau margin.

A Numerical Study of Lithospheric Deformation and Strain Partitioning Across the Longmen Shan Orogenic Belt, Eastern Tibetan Plateau

AbstractThe Longmen Shan orogenic belt is the landform boundary between the eastern Tibetan Plateau and the Sichuan Basin. However, there are significant differences in lithospheric deformation across its southern and northern segments. We established numerical models to investigate the effect of rheological heterogeneity on lithospheric deformation across the eastern plateau margin. The results show that the rheological heterogeneity of the Longmen Shan controls the type of lithospheric deformation and the strain partitioning. When the Longmen Shan is weak or narrow, high strain is predicted between the plateau and the Sichuan Basin along the Longmen Shan, which helps to explain the lack of a Cenozoic foreland basin along the southwestern part of the Sichuan Basin. However, when the upper crust of the Sichuan Basin is weak, the deformation should extend into the interior of the basin, such as the Longmen Shan‐Longquan Shan thrust system, which provides sedimentary space for the Cenozoic sediments. In contrast, when the Longmen Shan is strong or wide, the deformation is diffuse over a broad transition zone. The strain is mainly localized along the boundary between the Songpan‐Ganzi terrane and the Longmen Shan. This result can be used to understand the uplift of the Min Shan. Compared to the observations, the high strength of the Longmen Shan corresponds to the distribution of the Precambrian crystalline basement. The rheological heterogeneity of the Longmen Shan is from the result of the amalgamation of ancient continents, which illustrates the control of the ancient continental margin on recent tectonics.

Phylogenetic and population structural inference from genomic ancestry maintained in present-day common wheat Chinese landraces.

Hexaploid common wheat is one of the most important food crops worldwide. Common wheat domestication began in the Fertile Crescent of the Near East approximately 10000years ago and then spread west into Europe and eastward into East Asia and China. However, the possible spreading route into and within China is still unclear. In this study, we successfully extracted DNA from single ancient wheat seeds and sequenced the whole genome of seven ancient samples from Xiaohe and Gumugou cemeteries in Xinjiang, China. Genomic inference and morphological observation confirmed their identity as hexaploid common wheat grown in prehistoric China at least 3200years before present (BP). Phylogenetic and admixture analyses with RNA-seq data of modern hexaploid wheat cultivars from both China and Western countries demonstrated a close kinship of the ancient wheat to extant common wheat landraces in southwestern China. The highly similar allelic frequencies in modern landraces of the Qinghai-Tibetan plateau with the ancient wheat support the previously suggested southwestern spreading route into highland China. A subsequent dispersal route from the Qinghai-Tibetan plateau margins to the Yangtze valley was proposed in this study. Furthermore, the common wheat populations grown in the Middle and Lower Yangtze valley wheat zones were also proposed to be established by population admixture with the wheat grown in the Upper Yangtze valley. Our study reports ancient common wheat sequences at a genome-wide scale, providing important information on the origin, dispersal, and genetic improvement under cultivation of present-day wheat landraces grown in China.

Growing the Tibet plateau: insights from crustal and Moho structure at the plateau margins

Growing the Tibet plateau: insights from crustal and Moho structure at the plateau margins

The thermochronologic record of erosion and magmatism in the Canyonlands region of the Colorado Plateau

The Cenozoic history of crustal deformation, surface uplift, and erosion of the central Colorado Plateau (southwestern USA) is distinct from and relatively poorly understood compared with surrounding regions in the North American Cordillera, including the Grand Canyon region and the southwestern Plateau margin. Here, we present new apatite thermochronologic results from Paleozoic-Mesozoic sandstones sampled in the Canyonlands region—the interior of the Colorado Plateau in southeastern Utah. The apatite (U-Th-Sm)/He (He) ages are highly dispersed, with both positive-slope and negative-slope age-effective U (eU) trends. Samples with apatite He results suitable for thermal history modeling are from the Abajo and La Sal mountains intrusive complexes and the Permian, Triassic, and Jurassic rocks sampled near the Colorado River at Lees Ferry, Arizona, and Hite, Utah. Samples with the richest thermal history information have positive-slope apatite He age-[eU] trends, with ages ca. 10 to 5 Ma at [eU] 60 ppm. Modeled thermal histories that best predict this pattern require two periods of rock cooling: one during the middle Cenozoic ca. 30 to 20 Ma and the other since the latest Miocene ca. 6 Ma. The most recent cooling documents the transition from a slowly eroding Miocene landscape to recent rapid erosion that likely postdates 6 Ma and the integration of the modern Colorado River. Middle Cenozoic rock cooling can be attributed to either ∼1 km of erosion between ca. 25 and 15 Ma, as previous workers have suggested in other parts of the Colorado Plateau region, or relaxation of a geothermal gradient transiently doubled by magmatism associated with the vigorous magmatic flare-up that swept through the region ca. 34 to 20 Ma. The ambiguity of using middle Cenozoic rock cooling as a proxy for erosion in this region means that this event should be described using a nongenetic term: the Middle Cenozoic Cooling Event.

Three‐Dimensional Model of the Lithospheric Structure Under the Eastern Tibetan Plateau: Implications for the Active Tectonics and Seismic Hazards

AbstractDetails of lithospheric structures in three‐dimensions (3‐D) are key to understanding the dynamics of crustal deformation and earthquakes in active orogenic systems. In this study, we develop a 3‐D model of the eastern Tibetan Plateau using the Skua‐Gocad software based on the latest Rayleigh wave tomography. We perform a quantitative modeling workflow to map the details of the Moho discontinuities and the high‐velocity anomalies. Then, we integrate the topography, major active faults, large earthquakes, and crustal Poisson's ratios to analyze the relationship between the shallow and deep structures of this region. Our study shows that the Moho is generally coupled with the topography. A steep Moho ramp exists under the plateau margin and its surface projection intersects the Longmen Shan (LMS) at a low oblique angle (~22°). Active deformation and large earthquakes are related to the steep Moho ramp under the plateau margin. Moreover, based on a 3‐D model of the Poisson's ratio perturbations, we find that deformation across the central LMS is characterized as a crocodile‐type wedge in the crust and lithospheric mantle, due to the resistance of the Yangtze craton and isostatic rebound. We suggest that a tectonic wedging model that contains both the upper‐crust thrusting and lower‐crustal thickening contribute to the LMS orogeny. Three‐dimensional visualization of the lithospheric structure is well suited to reveal the different levels of deformation and their relationships. Quantitative modeling methods provide effective constraints on the active blocks in the eastern Tibetan Plateau.

Miocene to Quaternary basin evolution at the southeastern Andean Plateau (Puna) margin (ca. 24°S lat, Northwestern Argentina)

AbstractThe Andean Plateau of NW Argentina is a prominent example of a high‐elevation orogenic plateau characterized by internal drainage, arid to hyper‐arid climatic conditions and a compressional basin‐and‐range morphology comprising thick sedimentary basins. However, the development of the plateau as a geomorphic entity is not well understood. Enhanced orographic rainout along the eastern, windward plateau flank causes reduced fluvial run‐off and thus subdued surface‐process rates in the arid hinterland. Despite this, many Puna basins document a complex history of fluvial processes that have transformed the landscape from aggrading basins with coalescing alluvial fans to the formation of multiple fluvial terraces that are now abandoned. Here, we present data from the San Antonio de los Cobres (SAC) area, a sub‐catchment of the Salinas Grandes Basin located on the eastern Puna Plateau bordering the externally drained Eastern Cordillera. Our data include: (a) new radiometric U‐Pb zircon data from intercalated volcanic ash layers and detrital zircons from sedimentary key horizons; (b) sedimentary and geochemical provenance indicators; (c) river profile analysis; and (d) palaeo‐landscape reconstruction to assess aggradation, incision and basin connectivity. Our results suggest that the eastern Puna margin evolved from a structurally controlled intermontane basin during the Middle Miocene, similar to intermontane basins in the Mio‐Pliocene Eastern Cordillera and the broken Andean foreland. Our refined basin stratigraphy implies that sedimentation continued during the Late Mio‐Pliocene and the Quaternary, after which the SAC area was subjected to basin incision and excavation of the sedimentary fill. Because this incision is unrelated to baselevel changes and tectonic processes, and is similar in timing to the onset of basin fill and excavation cycles of intermontane basins in the adjacent Eastern Cordillera, we suspect a regional climatic driver, triggered by the Mid‐Pleistocene Climate Transition, caused the present‐day morphology. Our observations suggest that lateral orogenic growth, aridification of orogenic interiors, and protracted plateau sedimentation are all part of a complex process chain necessary to establish and maintain geomorphic characteristics of orogenic plateaus in tectonically active mountain belts.

Monoclinal flexure of an orogenic plateau margin during subduction, south Turkey

AbstractGeologic evidence across orogenic plateau margins enables the discrimination of the relative contributions of orogenic, epeirogenic and/or climatic processes that lead to growth and maintenance of those plateaus and their margins. Here, we discuss the mode of formation of the southern margin of the Central Anatolian Plateau (SCAP) and evaluate its time of formation using fieldwork in the onshore and seismic reflection data in the offshore. In the onshore, uplifted Miocene rocks in a dip‐slope topography show monocline flexure over >100 km, km‐scale asymmetric folds verging south, and outcrop‐scale syn‐sedimentary reverse faults. On the Turkish shelf, vertical faults transect the basal latest Messinian of a 10 km fold where on‐structure syntectonic wedges and synsedimentary unconformities indicate pre‐Pliocene uplift and erosion, followed by Pliocene and younger deformation. Collectively, Miocene rocks delineate a flexural monocline at plateau margin scale that is expressed along our on‐offshore sections as a kink‐band fold with a steep flank 20–25 km long. In these reconstructed sections, we estimate a relative vertical displacement of 3.8 km at rates of ca. 0.5 mm/y, and horizontal shortening values <1 %. We use this evidence together with our observations of shortening at outcrop, basin, plateau‐margin and forearc‐system scales to infer that the SCAP forms as a monoclinal flexure to accommodate deep‐seated thickening and shortening since >5 Ma, and to contextualize the plateau margin as the forearc high of the Cyprus subduction system.