Evaluating the role of climate and tectonics during non-steady incision of the Yellow River: evidence from a 1.24 Ma terrace record near Lanzhou,China

The competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain. Most of recent studies have attributed terrace formation to climatic changes and held that, even in tectonically active settings, climate variations control cycles of terrace planation and abandonment. Based on field investigations of loess-paleosol sequences, magnetostratigraphy and optically stimulated luminescence (OSL) dating, we develop a new chronology for a spectacular flight of terraces along the Yellow River near Lanzhou, China over past 1.24 Ma. All the terraces are strikingly similar in that they have several meters of paleosol developed directly above fluvial deposits on the terrace treads, suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates. However, the ages of terraces cluster in two relatively short time periods (1.24–0.86 Ma and 0.13 Ma – present). During the intervening time between 0.86 Ma and 0.13 Ma, terraces either did not form or were not preserved. We suggest that this record indicates that rock uplift rates varied through time and influenced terrace formation/preservation. Thus, our results demonstrate the utility of deep chronologic records from fluvial terraces for deconvolving the effects of tectonics and climate on fluvial incision.     

Late Cenozoic fluvial dynamics of the River Tana,Kenya, an uplift dominated record

The Late Cenozoic development of the River Tana in Kenya has been reconstructed for its central reach near its confluence with the River Mutonga, which drains the Mount Kenya region. Age control for this system has been provided by K–Ar and Ar–Ar dating. Between 3.21 and 2.65 Ma a major updoming occurred, in relation to the formation of the Kenyan rift valley. The tilting related to this doming has been reconstructed from lava flows that preserve former river gradients. Linear projection of these trends to the current rift valley rim suggests a net updoming of the eastern Gregory Rift valley by at least ∼1 km during 3.21–2.65 Ma. In contrast, since 2.65 Ma the Tana system has been mainly subject to relatively minor epeirogenic uplift. Changing climatic conditions combined with continuing uplift yielded a typical staircase of strath terraces with at least 10 distinct levels. A more detailed reconstruction of the incision rates since 215 ka has been made, by correlating mineralogically fingerprinted volcaniclastic Tana deposits with dated tephras in a lake record. These volcaniclastic sediments were deposited during glacial periods, contemporaneous with lahars. The reconstructed incision rates for the three youngest terraces are ∼0.1–0.2 mm a⁻¹, thus considerably faster than the overall average rate of valley incision since the Mid-Pliocene, of 0.06 mm a⁻¹. A plausible uplift history has been reconstructed using the estimated ages of the Tana terraces and marine terraces on the Indian Ocean coastline. The result suggests an increase in the rate of incision by the River Tana at ∼0.9 Ma, an observation typical in most European river terrace staircases. The reconstructed Late Quaternary development of Tana valley indicates that a similar Quaternary uplift mechanism has operated in both Europe and East Kenya, suggesting a globally applicable process.     

Terrace staircases of the River Euphrates in southeast Turkey,northern Syria and western Iraq: evidence for regional surface uplift

We present the first overall synthesis of the terrace deposits of the River Euphrates in SE Turkey, northern Syria, and western Iraq, combining new observations with summaries of data sets from different reaches that had previously been independently studied on a piecemeal basis. The largest number of terraces observed in any reach of the Euphrates is 11, in western Iraq, where this river leaves the uplands of the Arabian Platform. In many other localities not more than 5 or 6 terraces have previously been identified, although we infer that some of these are resolvable into multiple terraces. These terraces are typically formed of gravel, principally consisting of Neotethyan ophiolite and metamorphic lithologies transported from Anatolia. Although older gravels are also evident, most of the Euphrates terrace deposits appear, given the chronologies that have been established for different parts of the study region, to date from the late Early Pleistocene onwards, the cold stages most often represented being interpreted as marine Oxygen Isotope Stages 22, 16, 12, 8, 6 and/or 4, and 2. The formation of this terrace staircase reflects regional uplift of the Arabian Platform. Estimated amounts of uplift since the Middle Pliocene decrease southeastward from almost 300 m in SE Turkey to ∼150 m in western Iraq. Uplift rates increased in the late Early Pleistocene, the uplift estimated since then decreasing from ∼110 m in SE Turkey to a minimum of ∼50 m in the Syria–Iraq border region, then increasing further downstream across western Iraq to ∼70 m. Numerical modelling of this uplift indicates a relatively thin mobile lower-crustal layer, consistent with the low surface heat flow in the Arabian Platform.     

Late Quaternary river terrace sequences in the eastern Kunlun Range,northern Tibet: A combined record of climatic change and surface uplift

The Kunlun Range, a reactivated orogenic belt, constitutes the northern margin of the Tibetan Plateau. The extreme relief and major landforms of the Kunlun Range are a product of late Cenozoic tectonics and erosion. However, well-developed late Quaternary terraces that occur along the northern slope of the Kunlun Range probably resulted from climatic change rather than surface uplift. The terrace sequences formed in thick Quaternary valley fills and have total incision depths of 50–60 m. Optically stimulated luminescence dating was employed to place time controls on the valley fills and associated terraces. Dating results suggest that periods of significant aggradation were synchronous between different rivers and correspond to the last glacial stage. The abrupt change from aggradation to incision occurred between 21.9 ± 2.7 and 16 ± 2.2 ka, coincident with the last glacial–interglacial transition. Additional terraces developed during the late glacial period and early to middle Holocene. Based on a broader set of chronological data in northern Tibet, at least four regional incision periods can be recognized. Chronological data, terrace elevation profiles, and climate proxy records suggest that these terracing periods were triggered by cool and/or wet climatic conditions. A geometric survey of the riverbed longitudinal profile suggests that surface uplift serves as a potential dynamic forcing for long-term incision. A model is proposed for terrace formation as a response to climatic perturbation in an uplifted mountain range.     

Evolution of Ataúro Island: Temporal constraints on subduction processes beneath the Wetar zone, Banda Arc

Ataúro is a key to understanding the late stage volcanic and subduction history of the Banda Arc to the north of Timor. A volcanic history of bi-modal subaqueous volcanism has been established and new whole rock and trace element geochemical data show two compositional groups, basaltic andesite and dacite–rhyolite. ⁴⁰Ar/³⁹Ar geochronology of hornblende from rhyo-dacitic lavas confirms that volcanism continued until 3.3 Ma. Following the cessation of volcanism, coral reef marine terraces have been uplifted to elevations of 700 m above sea level. Continuity of the terraces at constant elevations around the island reflects regional-scale uplift most likely linked to sublithospheric processes such as slab detachment. Local scale landscape features of the eastern parts of Ataúro are strongly controlled by normal faults. The continuation of arc-related volcanism on Ataúro until at least 3.3 Ma suggests that subduction of Australian lithosphere continued until near this time. This data is consistent with findings from the earthquake record where the extent of the Wetar seismic gap to a depth of 350 km suggests slab breakoff, as a result of collision, commenced at ∼4 Ma, leading to subsequent regional uplift recorded in elevated terraces on Ataúro and neighbouring islands.     

黄河晋陕峡谷河流阶地和鄂尔多斯高原第四纪构造运动

本文通过黄河晋陕峡谷河段21个地点的阶地横剖面观察和阶地对比研究,确定出6级宽谷阶地序列,同时,依据17个放射性测年数据对阶地定年。6级宽谷阶地序列揭示了鄂尔多斯高原第四纪区域造陆隆起和局部构造变形,区域造陆隆起的发生时间比之青藏高原隆起要滞后0．113～0．25Ma,而且,平均区域造陆隆起量比之青藏高原的内部和边缘的隆起量小得多。晋陕峡谷河段发生过3期加积作用幕,指示着造陆隆起景观中的气候变化时间线。渭河盆地北缘的韩城断裂悬崖带记录着构造基准面下降,影响的河流长度仅为21～127km。     

Early Holocene fluvial activity from the sedimentology and palaeohydrology of gravel terrace in the semi arid Mahi River Basin,India

Palaeocompetence analysis and palaeodischarge estimation techniques are applied to a late Pleistocene–early Holocene gravel terrace in the Mahi River Basin, western India. Terrace sedimentology, comprising gravels overlain by sand lithofacies suggests a gradual change in palaeohydrological conditions marking a switch from braided to meandering fluvial styles. The discharge values for the gravel bedforms based on the clast size and the cross bed set thickness are estimated between ∼150–180 m³ s⁻¹ comparable with the present day observed values albeit with a much higher competence. Results indicate that fluvial aggradation occurred under low discharge conditions with intermittent high discharge events depositing longitudinal gravel bars. The incision of these gravel bars and the formation of terraces can be attributed to the higher discharge regime post 9.2 ka. The study further indicates that whereas the aggradation of the gravel terrace during the early Holocene was controlled by the large sediment influx, the incision that followed was in response to the increase in the discharge and competence of the river flow.     

Cenozoic thermo-tectonic evolution of the Gangdese batholith constrained by low-temperature thermochronology

Gangdese batholith in the southern Lhasa block is a key location for exploring the Tibetan Plateau uplift and exhumation history. We present the new low-temperature thermochronological data from two north–south traverses in the central Gangdese batholith to reveal their cooling histories and corresponding controls. Zircon fission track ages show prominent clusters ranging from 23.7 to 51.6 Ma, apatite fission track ages from 9.4 to 36.9 Ma, apatite (U–Th)/He ages between 9.5 and 12.3 Ma, and one zircon (U–Th)/He age around 77.8 Ma. These new data and thermal modeling, in combination with the regional geological data, suggest that the distinct parts of Gangdese batholith underwent different cooling histories resulted from various dynamic mechanisms. The Late Eocene–Early Oligocene exhumation of northern Gangdese batholith, coeval with the magmatic gap, might be triggered by crust thickening followed by the breakoff of Neotethyan slab, while this stage of exhumation in southern Gangdese batholith cannot be clearly elucidated probably because the most of plutonic rocks with the information of this cooling event were eroded away. Since then, the northern Gangdese batholith experienced a slow and stable exhumation, while the southern Gangdese batholith underwent two more stages of exhumation. The Late Oligocene–Early Miocene rapid cooling might be a response to denudation caused by the Gangdese Thrust or related to the regional uplift and exhumation in extensional background. By the early Miocene, the rapid exhumation was associated with localized river incision or intensification of Asian monsoon, or north–south normal fault.     

Pleistocene fluvial terraces from northern France (Seine,Yonne, Somme): synthesis,and new results from interglacial deposits

The Seine, Yonne and Somme are the main rivers draining northwestern France and flowing into the Channel. During Pleistocene cold stages they were tributaries of the “Fleuve Manche”. They are characterised by well-developed stepped terrace systems showing up to 10 incision steps for the last 1 Ma. After 15 years of research and the synthesis of stratigraphical, sedimentological, bioclimatical data and numerical dating, these terrace systems are interpreted as the response of the fluvial environments to climatic cyclicity, superimposed on a background of slow tectonic uplift. The comparison of these three terrace systems shows that the incision budget is similar within the studied area (about 55–65 m/1 Ma), implying relative homogeneity of uplift. In the main part of the area, beyond marine influence, the analysis of each stepped alluvial formation shows regular sediment balance generally composed of coarse periglacial gravels (pleniglacial), covered by fine-grained sediments locally overlain by calcareous tufa (interglacial). However, in the Lower Seine valley, close to the present coast, the low terrace includes two estuarine interglacial units interbedded within periglacial gravel units. Nevertheless, until recently evidence of full interglacial conditions remained elusive in fluvial contexts, especially in the Somme. To address these issues a new research programme has been launched on tufa deposits that are the best candidate to register the climatic optimum. Initial results from la Celle (Seine), Saint-Acheul and Caours (Somme) allow palaeoenvironmental reconstructions of MIS 11 and 5e, and thus confirm the importance of calcareous tufa to define a reference record of Pleistocene Interglacials for northern France. Moreover, it has been previously demonstrated that the main incision leading to terrace formation can be attributed to the transition between interglacial and glacial (during early glacial phases). The new results from interglacial records have important implications concerning the precise occurrence of the major incision phases within the climatic cycle as they provide the starting point immediately preceding downcutting.     

黄河中游响应气候变化和地表相对抬升发育阶地研究

揭示河流系统响应气候变化和地表抬升的机制是理解流域地貌演化以及水系发育过程的基础,其核心难题是如何充分认识它们在阶地发育中扮演的角色。以往的研究倾向于分开讨论气候变化和地表抬升在河流阶地发育中的作用,认为河流堆积/侧蚀和下切行为分别与冰期和间冰期气候对应,或者将阶地作为地表抬升的直接证据。首先,从上下游河段对比的视角初步解释了黄河中游响应气候变化和地表相对汾渭盆地抬升发育阶地的过程。1.2 Ma以来黄河下蚀鄂尔多斯地块和峨眉台地分别形成了7级阶梯状阶地和6级堆积阶地序列。黄土地层分析结合年代学研究揭示这些阶地面都直接上覆一层古土壤,指示它们形成于气候由冰期向间冰期的过渡阶段,即使在沉降的盆地依然如此。然而,黄河中游并没有在1.2 Ma以来的每一次冰期向间冰期转换都发育阶地,说明气候虽能通过控制河流堆积-侧蚀与下切行为的转换决定阶地的形成时代,但其本身并不是阶地形成的唯一控制因素。在峨眉台地沉降的背景下,黄河无法形成正常的阶梯状阶地序列,取而代之的是堆叠的阶地序列（阶地越年轻拔河高度越大）;而当鄂尔多斯地块相对汾渭盆地抬升缓慢时,黄河仅能在极为干旱的冰期向间冰期过渡阶段形成阶地;相比之下,它们相对汾渭盆地抬升速率都足够快速时,驱动黄河近乎对每一次的冰期向间冰期转换都能做出响应而发育阶地。以上黄河中游阶地与气候和地表抬升的对比模式揭示出,快速地表抬升也是阶梯状阶地序列发育不可或缺的因素,能驱使河流在冰期向间冰期过渡阶段显著下切,拉大相邻阶地面垂直距离从而利于后期保存。因此,研究认为黄河中游发育的系列阶地是响应气候变化和地表相对汾渭盆地抬升的结果。     

Estuarine–fluvial floodplain formation in the Holocene Lower Tagus valley (Central Portugal) and implications for Quaternary fluvial system evolution

We present a brief synthesis of the Quaternary fluvial record in the Lower Tagus Basin (central Portugal), concentrating on factors controlling infill and incision. The Holocene part of the record forms the focus of this paper and guides the questioning of the basic assumptions of the established Quaternary fluvial evolution model, in particular the link between sea-level change and fluvial incision-deposition. We suggest that several incision-aggradation phases may have occurred during glacial periods. Major aggradation events may overlap with cold episodes, while incision appears to concentrate on the warming limb of climate transitions. The complex stratigraphy of the Quaternary record in the Lower Tagus valley is influenced by repeated base-level and climate changes.This paper submits the first chronostratigraphic framework for valley fill deposits in the Lower Tagus area. Sea-level rise forced aggradation and controlled deposition of the fine-grained sedimentary wedge underlying the low-gradient Lower Tagus floodplain. Investigations have focused on the lower Muge tributary, where rapidly aggrading estuarine and fluvial environments were abruptly established (∼8150 cal BP) as sea level rose. Base level at the valley mouth controlled the upstream extent of the fine-grained backfill. Tidal environments disappeared abruptly (∼5800 cal BP) when the open estuary at the Muge confluence was infilled by the Tagus River. The decrease and final still stand of sea-level rise led to floodplain stabilisation with peat (∼6400–5200 cal BP) and soil formation (∼5200–2200 cal BP). Localised renewed sedimentation (∼2200–200 cal BP) is linked to human activity.     

Late Miocene–Quaternary rapid stepwise uplift of the NE Tibetan Plateau and its effects on climatic and environmental changes

The way in which the NE Tibetan Plateau uplifted and its impact on climatic change are crucial to understanding the evolution of the Tibetan Plateau and the development of the present geomorphology and climate of Central and East Asia. This paper is not a comprehensive review of current thinking but instead synthesises our past decades of work together with a number of new findings. The dating of Late Cenozoic basin sediments and the tectonic geomorphology of the NE Tibetan Plateau demonstrates that the rapid persistent rise of this plateau began ~ 8 ± 1 Ma followed by stepwise accelerated rise at ~ 3.6 Ma, 2.6 Ma, 1.8–1.7 Ma, 1.2–0.6 Ma and 0.15 Ma. The Yellow River basin developed at ~ 1.7 Ma and evolved to its present pattern through stepwise backward-expansion toward its source area in response to the stepwise uplift of the plateau. High-resolution multi-climatic proxy records from the basins and terrace sediments indicate a persistent stepwise accelerated enhancement of the East Asian winter monsoon and drying of the Asian interior coupled with the episodic tectonic uplift since ~ 8 Ma and later also with the global cooling since ~ 3.2 Ma, suggesting a major role for tectonic forcing of the cooling.     

Quaternary hinterland evolution of the active Banda Arc: Surface uplift and neotectonic deformation recorded by coral terraces at Kisar,Indonesia

Coral terrace surveys and U-series ages of coral yield a surface uplift rate of ∼0.5 m/ka for Kisar Island, which is an emergent island in the hinterland of the active Banda arc–continent collision. Based on this rate, Kisar first emerged from the ocean as recently as ∼450 ka. These uplifted terraces are gently warped in a pattern of east–west striking folds. These folds are strike parallel to more developed thrust-related folds of similar wavelength imaged by a seismic reflection profile just offshore. This deformation shows that the emergence of Kisar is influenced by forearc closure along the south-dipping Kisar Thrust. However, the pinnacle shape of Kisar and the protrusion of its metamorphic rocks through the forearc basin sediments also suggest a component of extrusion along shear zones or active doming.Coral encrusts the island coast in many locations over 100 m above sea level. Terrace morphology and coral ages are best explained by recognizing major surfaces as mostly growth terraces and minor terraces as mostly erosional into older terraces. All reliable and referable coral U-series ages determined by MC-ICP-MS correlate with marine isotope stage (MIS) 5e (118–128 ka). The only unaltered coral samples are found below 6 m elevation; however an unaltered Tridacna (giant clam) shell in growth position at 95 m elevation yields a U-series age of 195 ± 31 ka, which corresponds to MIS 7. This age agrees with the best-fit uplift model for the island. Loose deposits of unaltered coral fragments found at elevations between 8 and 20 m yield U-series ages of <100 years and may represent paleotsunami deposits from previously undocumented tectonic activity in the region.     

Spatial–temporal relationships of Mesozoic volcanic rocks in NE China: Constraints on tectonic overprinting and transformations between multiple tectonic regimes

LA-ICP-MS zircon U–Pb ages and geochemical data are presented for the Mesozoic volcanic rocks in northeast China, with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the Paleo-Asian Ocean, Mongol–Okhotsk, and circum-Pacific tectonic regimes. The new ages, together with other available age data from the literature, indicate that Mesozoic volcanism in NE China can be subdivided into six episodes: Late Triassic (228–201 Ma), Early–Middle Jurassic (190–173 Ma), Middle–Late Jurassic (166–155 Ma), early Early Cretaceous (145–138 Ma), late Early Cretaceous (133–106 Ma), and Late Cretaceous (97–88 Ma). The Late Triassic volcanic rocks occur in the Lesser Xing’an–Zhangguangcai Ranges, where the volcanic rocks are bimodal, and in the eastern Heilongjiang–Jilin provinces where the volcanics are A-type rhyolites, implying that they formed in an extensional environment after the final closure of the Paleo-Asian Ocean. The Early–Middle Jurassic (190–173 Ma) volcanic rocks, both in the Erguna Massif and the eastern Heilongjiang–Jilin provinces, belong chemically to the calc-alkaline series, implying an active continental margin setting. The volcanics in the Erguna Massif are related to the subduction of the Mongol–Okhotsk oceanic plate beneath the Massif, and those in the eastern Jilin–Heilongjiang provinces are related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The coeval bimodal volcanic rocks in the Lesser Xing’an–Zhangguangcai Ranges were probably formed under an extensional environment similar to a backarc setting of double-direction subduction. Volcanic rocks of Middle–Late Jurassic (155–166 Ma) and early Early Cretaceous (145–138 Ma) age only occur in the Great Xing’an Range and the northern Hebei and western Liaoning provinces (limited to the west of the Songliao Basin), and they belong chemically to high-K calc-alkaline series and A-type rhyolites, respectively. Combined with the regional unconformity and thrust structures in the northern Hebei and western Liaoning provinces, we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the Mongol–Okhotsk suture belt. The late Early Cretaceous volcanic rocks, widely distributed in NE China, belong chemically to a low- to medium-K calc-alkaline series in the eastern Heilongjiang–Jilin provinces (i.e., the Eurasian continental margin), and to a bimodal volcanic rock association within both the Songliao Basin and the Great Xing’an Range. The volcanics in the eastern Heilongjiang–Jilin provinces formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust, or both. Late Cretaceous volcanics, limited to the eastern Heilongjiang–Jilin provinces and the eastern North China Craton (NCC), consist of calc-alkaline rocks in the eastern Heilongjiang–Jilin provinces and alkaline basalts in the eastern NCC, suggesting that the former originated during subduction of the Paleo-Pacific Plate beneath the Eurasian continent, whereas the latter formed in an extensional environment similar to a backarc setting. Taking all this into account, we conclude that (1) the transformation from the Paleo-Asian Ocean regime to the circum-Pacific tectonic regime happened during the Late Triassic to Early Jurassic; (2) the effect of the Mongol–Okhotsk suture belt on NE China was mainly in the Early Jurassic, Middle–Late Jurassic, and early Early Cretaceous; and (3) the late Early Cretaceous and Late Cretaceous volcanics can be attributed to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.     

Late Cenozoic surface uplift,basaltic volcanism,and incision by the River Tigris around Diyarbakır,SE Turkey

We document the staircase of terraces of the River Tigris in the Diyarbakır area of SE Turkey, in the northern Arabian Platform, and improve control on the ages of these terrace deposits by dating of overlying basalt flows using the unspiked K–Ar technique. These fluvial terraces are formed of polymict gravel, including clasts derived from the Anatolian metamorphic terrane farther north as well as of local basalt. At least 9 Tigris terraces have been recognised so far, the highest of which, ∼200 m above present river level, marks the local transition from stacked deposition to fluvial incision, the timing of which is bounded between the mid Late Miocene and the Middle Pliocene. Our K–Ar dating indicates a hiatus in fluvial incision in the late Early Pleistocene, as basalts dated to 1.22 ± 0.02 and 1.07 ± 0.03 Ma overlie Tigris gravels at very similar levels, ∼60–70 m above the present river. The lower terraces record the subsequent entrenchment of the modern Tigris valley following an increase in incision rates in the early Middle Pleistocene, evident from the disposition of younger basalt, dated to 0.43 ± 0.02 Ma, capping fluvial gravel only ∼21–22 m above the present river level. Numerical modelling can account for the observed uplift history, as the response to coupling between surface processes and induced flow in the lower crust, with the mobile lower-crust thin (∼5–7 km thick), consistent with the known presence of a thick layer of mafic underplating at the base of the crust beneath the Arabian Platform. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cosmogenic 3He age estimates of Plio-Pleistocene alluvial-fan surfaces in the Lower Colorado River Corridor,Arizona, USA

Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of the LCR and its adjacent alluvial fans. Cosmogenic ³He (³He_c) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2 Ma, 2.2–1.1 Ma, 1.1 Ma to 110 ka, < 350 ka, < 150 ka, and < 63 ka. T1 and Q1 fans are especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since its inception ca. 4.2 Ma. ³He_c data suggest that the LCR began downcutting into the Bullhead Alluvium as early as 3.3 Ma and as late as 2.2 Ma. Younger Q2a to Q4 fans very broadly correlate in number and age with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in ³He_c ages preclude a temporal link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphology and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont and valley-floor aggradation in the LCR valley.     

Provenance of Lower Cretaceous Wölong Volcaniclastics in the Tibetan Tethyan Himalaya: Implications for the final breakup of Eastern Gondwana

Lower Cretaceous volcanic lithic arenites, widely distributed in the Tethyan Himalaya, provide insights into the continental breakup of Eastern Gondwana. In southern Tibet they are represented by the Wölong Volcaniclastics. The volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts, whereas rhyolitic/dacitic volcanism becomes the predominant source of the upper strata. Geochemical analyses of basaltic grains and of detrital Cr-spinels from the Wölong Volcaniclastics demonstrate the alkaline character of the volcanism and suggest “within-plate” tectonic setting for the volcanism. Zircon U–Pb ages confirm that this volcanism continued from ～ 140 Ma to ～ 119 Ma. Hf-isotope data on these Early Cretaceous zircons indicate that their parental magmas were mantle-derived, but in the later stage of magmatic activity mantle-derived magmas were mixed with partial melts derived from the continental crust.The Lower Cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of Greater India. The onset of volcaniclastic deposition in the Himalayas appears to become progressively younger toward the west, but it ended synchronously during the Late Albian (～ 102 Ma). The low volume of volcanic rocks and their intra-plate tectonic setting suggest that they are the result of decompressional melting along extensional deep-seated fractures cross-cutting the continental crust, and reflect changes in regional intra-plate tectonic stresses when Greater India began to separate from the Australia–Antarctica supercontinent.     

Combined U–Pb and Lu–Hf isotope data on turbidites of the Paleozoic basement of NW Argentina and petrology of associated igneous rocks: Implications for the tectonic evolution of western Gondwana between 560 and 460 Ma

New combined U–Pb and Lu–Hf isotope analyses on zircon from three turbidite deposits, and petrologic data for associated igneous rocks were used to study the evolution of the Paleozoic basement of Eastern Cordillera, NW Argentina. Maximum and minimum ages for turbidite deposits, considered to be part of the Puncoviscana Fm., are reported. In the Tastil area, turbidites were deposited in a fore-arc setting after 560 Ma and intruded at 534 Ma by the Tastil batholith. In the El Niño Muerto Hill area turbidites with maximum depositional age of 496 ± 11 Ma were intruded by high-K dacites at 483 ± 3 Ma. In the Río Blanco Valley, the turbiditic/hemipelagitic sediments, with maximum depositional age of 463 ± 11 Ma were contemporaneous with E-MORB/OIB volcanism. The U–Pb and Lu–Hf data permitted to distinguish two major periods of magmatic activity during Late Mesoproterozoic–Early Neoproterozoic (0.95 to 1.2 Ga) and Late Neoproterozoic–Early Paleozoic (0.75 to 0.46 Ga) times, the former dominated by the input of juvenile crust and the latter by arc magmatism and recycling of Meso- to Paleoproterozoic crust. On the basis of new data we suggest that western margin of Gondwana was controlled by subduction processes and accretion of small terrains during Neoproterozoic–Early Paleozoic times.     

Moraine dam related to late Quaternary glaciation in the Yulong Mountains,southwest China,and impacts on the Jinsha River

The Yulong Mountain massif is tectonically active during Quaternary and contains the southernmost glacierized mountains in China, and all of Eurasia. Past glacial remnants remain preserved on the east and west sides of the Yulong Mountains. A ridge of moraine protruded into the Jinsha River at the Daju Basin, damming the river, and forming a lake at the head of the Jinsha River. Cosmogenic ¹⁰Be and ²⁶Al provide exposure age dates for the moraine-based fluvial terraces left behind after the dam breached, and for moraine boulders on both the eastern and western sides of the Yulong Mountains. Our results yield exposure ages for the terraces that range from 29 ka to 8 ka, and a downcutting rate of 7.6 m/ka. The preservation of the remaining dam for over 10,000 years suggests stability of the moraine dam and gradual erosion of the dam during drainage of the dammed lake. From the relationship between exposure ages and elevations of the fluvial terraces located in different walls of the Daju fault, we obtain a late Quaternary dip-slip rate of about 5.6 m/ka for the Daju fault. The exposure ages of 10.2 ka and 47 ka for moraine boulders located in the east and west sides of the Yulong Mountains, respectively, coincide with warm periods in the late Quaternary. This implies that precipitation provides the major control for glaciations on the Yulong Mountains, a domain of the southwest Asian monsoon.     

High-resolution magnetostratigraphic study of the Paleogene-Neogene strata in the Northern Qaidam Basin: Implications for the growth of the Northeastern Tibetan Plateau

The Cenozoic terrestrial, intermontane Qaidam Basin on the northeastern edge of the Tibetan Plateau contains > 12 km of sedimentary rocks that potentially document the accommodation of India-Asia convergence and the growth of the plateau. The chronology remains incomplete, hindering cross-basin correlation between lithostratigraphic units and their further interpretation. Here we present a high-resolution magnetostratigraphy spanning > 5 km of Paleogene-Neogene sequence at Dahonggou in the Northern Qaidam Basin. Based on correlation with the geomagnetic polarity time scale (GPTS), we have dated the section to being between ~ 52 and ~ 7 Ma. The bottom conglomeratic unit, ranging from > 52 Ma to ~ 44 Ma, was deposited in high-energy environments (e.g., alluvial fan or braided river), reflecting the earliest deformation and uplift of the basin-bounding Qilian Shan fold-thrust belt in response to India-Asia collision. In addition, we identified two major increases in sedimentation rate at 25–16 Ma and after ~ 9.5 Ma and three phases of lesser increases at 52–44 Ma, 38–33 Ma, and 14.6–12.0 Ma. These increases in sedimentation rate are consistent with regional thermochronology and basin analysis studies, which revealed enhanced motion on basin-bounding thrust faults. We argue that these accelerated sedimentation rates indicate pulsed tectonism in the northeastern Tibetan Plateau. The pulse at 25–16 Ma may further relate to phases of strong rainfall linked to an intense monsoon at that time.