Eco-environmental degradation in the northeastern margin of the Qinghai–Tibetan Plateau and comprehensive ecological protection planning

The regional hydrology and ecosystems of the northeastern margin of the Qinghai–Tibetan Plateau have changed over the past 40 years driven by intense human activity and regional climate changes. Annual mean air temperature has increased in the region. Streamflow from the northeastern margin of the Qinghai–Tibetan Plateau has decreased significantly. Overall, a number of Alpine step meadows and Alpine frigid meadows have seriously degraded. Degeneration of vegetation and grassland led to desertification and frequently induced dust storms. With the continuous increase in cultivated land area, grassland area in the region has dropped significantly since the 1960s. At present, degraded grassland occupies about 83% of total usable grassland area. As the number of livestock increased, range condition deteriorated and the carrying capacity was reduced. The forest area in the northeastern margin of the Qinghai–Tibetan Plateau has decreased by 20%, and the local ecosystem has become very fragile. Given the relatively stable weather conditions, the northeastern margin of the Qinghai–Tibetan Plateau can be characterized by its three major ecosystems: grassland ecosystem, forest ecosystem and wetland ecosystem, which are crucial in maintaining the ecological stability. Changes in these ecosystems could influence sustainable development in the region. To avoid further deterioration of the environment and ecosystems, it is important to establish and implement ecosystem protection planning. Some effective measures are essential in this respect, including technical and political considerations.     

Late cretaceous tectonic change of the eastern margin of the Tibetan Plateau — results from multisystem thermochronology

Partially due to lack of structural and sedimentary records to constrain the Jurassic-to-Cretaceous evolution, there was a missing process here in the eastern margin of Tibetan Plateau as it changed from the Paleo-Tethyan to Neo-Tethyan regime. Based on the analysis of 125 thermochronology ages (U/Pb, ⁴⁰Ar/³⁹A, ⁸⁷Rb/⁸⁶Sr, FT, U-Th/He) of igneous rocks from the eastern margin of Tibet, we propose a multisystem thermochronology approach to restore the cooling and emplacement of granites and decipher the missing process. Our integrated study suggests that a key Late Cretaceous (about 100Ma) tectonic change from the Paleo-Tethyan to Neo-Tethyan regime took place there. In the Late Triassic period, the initial emplacement of granite in the Songpan-Ganzi Fold Belt (SGFB) was characterized by a decrease in emplacement age and depth from west to east, and from north to south. Subsequently, all were followed by a very long period of slow cooling, which was followed by a rapid emplacement of about 100Ma. The intensive granite emplacement took place all over except northeastern SGFB, with a decrease in emplacement depth from west to east, which was linked with the far-field effect of Lhasa-Qiangtang collision. After this episode, the cooling history of granite in SGFB had a rapid emplacement on the subsurface under the control of the Neo-Tethyan regime. This process has control of the post Late Cretaceous regional magmatic activity and tectonics, as well as the sedimentary response in Sichuan and Xichang basin.     

Holocene climate change inferred from stratigraphy and OSL chronology of aeolian sediments in the Qaidam Basin,northeastern Qinghai–Tibetan Plateau

Paleoclimatic reconstruction based on aeolian sediments in the eastern Qaidam Basin (QB) has been hindered by the limited chronological data. Here we present 61 Optically Stimulated Luminescence (OSL) ages. On the basis of these OSL ages and the lithologic stratigraphy, we propose the ‘effective moisture index (EMI)’ for aeolian sediments to reconstruct the effective moisture change. Based on the EMI from twelve sections, the effective moisture change, moisture sources and relevant mechanisms for paleoclimatic change in the eastern QB are discussed. The results indicate that (1) aeolian deposition started at least before 12.4 ± 0.7 ka during the deglaciation, the paleosols developed at the early and mid-Holocene, and aeolian sand and loess accumulated at mid- and late Holocene; (2) effective moisture history was: hyper-arid at 12.8–11.6 ka, humid and variable at 11.6–8.3 ka, moderately humid and stable at 8.3–3.5 ka, and increasingly arid at 3.5–0 ka; (3) the effective moisture change was mainly controlled by the Asian summer monsoon (ASM), which mainly followed the change of Northern Hemispheric summer insolation, and the westerlies strengthened and increased the aridity in the QB when the ASM shrank.     

Palaeomagnetic results from Palaeogene red beds of the Chuan-Dian Fragment,southeastern margin of the Tibetan Plateau: implications for the displacement on the Xianshuihe–Xiaojiang fault systems

ABSTRACT

The mechanism of deformation associated with the Cenozoic collision of India with Asia along the eastern boundary remains a poorly understood aspect of the tectonic evolution of the southwestern South China Block (SCB). Consequently, we carried out a palaeomagnetic investigation of Palaeogene red beds of the Dayao area of Yunnan Province in order to contribute to understanding the Palaeogene evolution of the SCB. A characteristic higher temperature magnetic component (HTC), with an unblocking temperature from 660°C to 680°C, was determined by principal component analysis (PCA), and positive fold tests indicated that the remanence was a primary magnetization. The mean direction of the HTC from the Dayao area is Ds = 27.8° Is = 33.1° κ = 64.8, α₉₅ = 4.3° after tilt correction. Compared with other palaeomagnetic results from the SCB, our data suggest that the central part of the Chuan–Dian Fragment (CDF) experienced approximately 16.3 ± 4.7° clockwise rotation with respect to East Asia. Rotation of the CDF occurred along the left-lateral Xianshuihe–Xiaojiang Fault Systems (XSF-XJF), which exhibit an arc-shaped curve centred on the Eastern Himalayan Syntaxis. The XSF-XJF was approximated by a circle centred on a Euler pole at Lat. = 26.5° N, Lon. = 97.2° E (α₉₅ = 0.2°), based on 11 reference points selected from the fault system. The clockwise rotation of the CDF resulted in left-lateral shearing along the XSF-XJF system, with a left-lateral displacement of ~200 km. The nature of diverse intense local deformation along the Xianshuihe-Xiaojiang left-lateral strike-slip fault systems is also discussed.     

Chronology and tectonic significance of Cenozoic faults in the Liupanshan Arcuate Tectonic Belt at the northeastern margin of the Qinghai–Tibet Plateau

The Liupanshan Arcuate Tectonic Belt (LATB) is located at the northeastern margin of the Qinghai–Tibet Plateau. Major strike-slip and thrust faults in the Liupanshan area are prominent Cenozoic structures, which are critical in understanding and reconstructing the tectonic deformation history. This paper not only provides detailed investigations on geometric and kinematic characteristics of these faults in the LATB, but also dates the faults’ movements by electron spin resonance (ESR). The LATB underwent a succession of compression, extension and again compression tectonic deformation processes since the Cenozoic. The Liupanshan Curved Faults first experienced sinistral strike-slip shear during 57–61 Ma. The Liupanshan Curved Faults responded to the deformation caused by the eastward escape of the Qinghai–Tibet Plateau and acted as the northeastern boundary of the deformation. Timing for the formation of the Liupanshan Curved Faults shows that the collision of the Indian and Eurasian plates must have occurred earlier than these faults’ activity because the latter is reflected the far-field effect of the collision.     

Late Pleistocene lake and glaciation evolution on the northeastern Qinghai–Tibetan Plateau: a review

Late Pleistocene paleoclimatic history on northeastern Qinghai–Tibetan Plateau (QTP) has been reconstructed mainly from lake sediments; however, data regarding dry–wet climate changes reported in this region are still not clear and controversial. Based on shoreline features and highstand lacustrine sediments around lakes on the QTP, high lake level histories in this paper were summarized and compared with paleoclimatic records from lake sediments, ice core and glaciation evolution surrounding mountains on the NE QTP during late Pleistocene. The results indicate that periods of high lake level occurred at MIS 5, MIS 3 and early-middle Holocene and most likely corresponding to warm and wet climate periods, while periods of low lake level existed in intervening intervals, corresponding to cold and dry climate periods, which most likely coincide with glacial advances surrounding high mountains. With an exception, no wide glacial advance in study area was found during MIS 3, possibly suggesting that effective moisture is lower than that in the other region of NE QTP in this period.     

Further paleomagnetic results from the ~ 155 Ma Tiaojishan Formation,Yanshan Belt,North China,and their implications for the tectonic evolution of the Mongol–Okhotsk suture

A new paleomagnetic study on well-dated (~ 155 Ma) volcanic rocks of the Tiaojishan Formation (Fm) in the northern margin of the North China Block (NCB) has been carried out. A total of 194 samples were collected from 26 sites in the Yanshan Belt areas of Luanping, Beipiao, and Shouwangfen. All samples were subjected to stepwise thermal demagnetization. After removal of a recent geomagnetic field viscous component, a stable high temperature component (HTC) was isolated. The inclinations of our new data are significantly steeper than those previously published from the Tiaojishan Fm in the Chengde area (Pei et al., 2011, Tectonophysics, 510, 370–380). Our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation (PSV), but the PSV can be averaged out by combining all the virtual geomagnetic poles (VGPs) from the Tiaojishan Fm in the region. The mean pole at 69.6°N/203.0°E (A₉₅ = 5.6°) passes a reversal test and regional tilting test at 95% confidence and is thus considered as a primary paleomagnetic record. This newly determined pole of the Tiaojishan Fm is consistent with available Late Jurassic poles from red-beds in the southern part of the NCB, but they are incompatible with coeval poles of Siberia and the reference pole of Eurasia, indicating that convergence between Siberia and the NCB had not yet ended by ~ 155 Ma. Our calculation shows a ~ 1600-km latitudinal plate movement and crustal shortening between the Siberia and NCB after ~ 155 Ma. In addition, no significant vertical axis rotation was found either between our sampled areas or between the Yanshan Belt and the major part of the NCB after ~ 155 Ma.     

Unraveling the early–middle Paleozoic paleogeography of Kazakhstan on the basis of Ordovician and Devonian paleomagnetic results

It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the ~ 2000 km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north–central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle–Late Ordovician paleolatitudes and came to the conclusion that a nearly E–W trending active margin of the Kazakhstania landmass had existed at low (~ 10°S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet–Akbastau, and with the Aktau–Junggar microcontinent by the Ordovician–Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by ~ 45°. This changed the orientation of the Kazakhstania to NW–SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt.     

Geodynamics of the Barents–Kara margin in the Mesozoic inferred from paleomagnetic data on rocks from the Franz Josef Land Archipelago

New data on paleomagnetism and isotope geochronology of Jurassic and Early Cretaceous basic igneous rocks on Franz Josef Land Archipelago (FJL) represented by flows and dikes are discussed. The first paleomagnetic data obtained for these rocks offer the opportunity to suggest a model of spatial changes in the FJL block position during the Jurassic?Cretaceous. In the Early Jurassic, the block occupied a different position relative to Europe from the modern one. It was displaced in the northeasterly direction by a distance of approximately 500 km and rotated clockwise by about 40° relative to its modern position. By the Early Cretaceous, the FJL block occupied a position close to the present-day one avoiding subsequent substantial relative displacements. The data obtained are of principal significance for reconstructing the geodynamic evolution of Arctic structures in the Mesozoic and contribute greatly to the base of paleomagnetic data for the Arctic region, development of which is now in progress.     

First data on Early Carboniferous intrusive magmatism of the eastern margin of the Middle Urals: Geodynamic conditions and U–Pb isotope constraints

U–Pb LA ICP–MS dating of zircon from rocks of the Nekrasov gabbro–granitoid complex within the eastern margin of the Middle Urals was performed. The average U–Pb age calculated from three concordant measurements (326 ± 8 Ma) shows that their intrusion occurred at the Serpukhov Stage of the Early Carboniferous. According to the ideas on periodization of magmatic processes within the eastern sector of the Middle Urals, the formation of this complex corresponds to the final episodes of the continental marginal (supersubduction) magmatism.     

Assessing the ratio of archaeol to caldarchaeol as a salinity proxy in highland lakes on the northeastern Qinghai–Tibetan Plateau

The ratio of archaeol to caldarchaeol (the ACE index) has been proposed recently as an index for paleosalinity reconstruction and is based principally on archaeal core lipids (CLs) from coastal salt pans (Turich, C., Freeman, K.H., 2011. Archaeal lipids record paleosalinity in hypersaline systems. Organic Geochemistry 42, 1147–1157). We have examined possible relationships between salinity and ACE in both CLs and intact polar lipids (IPLs) from suspended particulate matter (SPM) and surface sediments of lakes and surrounding soils on the northeastern Qinghai–Tibetan Plateau. Our results showed that ACE values were positively correlated with salinity in all samples; however, CL ACE values were systematically higher than IPL ACE values, probably due to different degradation kinetics of intact polar (IP) archaeol and IP caldarchaeol. On the other hand, surface sediment ACE values from both CLs and IPLs were lower than SPM ACE values, probably due to enhanced production of caldarchaeol relative to archaeol in the sediment. Our results demonstrate that the ACE proxy reflects changes in salinity in diverse environments on the Qinghai–Tibetan Plateau, which is promising for paleosalinity reconstruction; however, caution should be used when applying the salinity proxy before we have a better understanding of degradation kinetics of archaeal IPLs and in situ production of caldarchaeol and archaeol in sediments.     

Structural control on the topography of the Laji–Jishi and Riyue Shan belts in the NE margin of the Tibetan plateau: Facilitation of the headward propagation of the Yellow River system

The Yellow River system, the largest river system in northern China, generally flows northeasterly through a series of linear mountain belts in the northeastern margin of the Tibetan plateau, the youngest of which are the Laji–Jishi Shan and Riyue Shan ranges, formed during late Cenozoic time due to NE–SW oblique shortening. As the product of the interaction between the tectonic process and the climate, the incision of the Yellow River system is a crucial parameter in models of the scale and timing of the crustal uplift and erosion in northeastern Tibetan plateau. Thus, whether the along-strike topographic feature of the Laji-Jishi Shan that is cut through by the Yellow River system and related streams is controlled by structural deformation or by erosion needs to be constrained. Our mapping shows that the variation in deformation along this mountain belt formed two structural saddles with relative low elevation in late Cenozoic time, through which the Yellow and Yaoshui Rivers cut into the plateau and drained a series of the Tertiary basins. The Yaoshui River is the tributary of the Huangshui River which itself flows into the Yellow River in the Lanzhou area. One saddle is present along the Yaoshui River valley, formed by NW–SE extension along the Riyue Shan Pass (RSP) normal fault, along which the Miocene and Mesozoic rocks were subsided against Proterozoic metamorphic rocks. These deformed rocks in the hanging wall are truncated by a sub-horizontal erosion surface at an elevation of 3200 m, on which terrace deposits are locally present, presumably middle Pleistocene in age. This terrace is incised by the Yaoshui River to an elevation of 3000 m, which yields 300 m of incision. Another saddle is along the Yellow River valley (the Xunhua-Linxia gouge) between the southern tip of the Laji Shan and the northern tip of the Jishi Shan, generated by en echelon folding. This structural saddle is underlain by the lower Cretaceous and Pliocene clastic rocks, which are truncated on the top by a rugged erosion surface at an average elevation of 3000 m. The Yellow River incised into this surface to an elevation of 1900 m, which yields 1100 m of incision. These two saddles, featured by topographic and structural low, were formed in the middle or late Miocene, and facilitated the headward propagation of the Yellow and Yaoshui Rivers, which initiated in early and middle Pleistocene time, respectively.     

Early Carboniferous paleogeography of the northern Verkhoyansk passive margin as derived from U–Pb dating of detrital zircons: role of erosion products of the Central Asian and Taimyr–Severnaya Zemlya fold belts

The first U–Pb dating of detrital zircons from the Lower Carboniferous sandstones in the frontal part of the northern Verkhoyansk fold-and-thrust belt showed that detrital zircon age spectra for the Lower Visean (Krestyakh Formation) and the Upper Visean–Serpukhovian (Tiksi Formation) rocks are quite different. The Early Visean sandstones contain up to 95% detrital zircons of Precambrian age, while those of Late Visean–Serpukhovian age, only 55%. The shape of age distribution plots of Precambrian zircons for both samples is similar, indicating that reworking of terrigenous sediments of the Krestyakh Formation or the same sources dominated in Early Visean time (crystalline basement of the craton, eroded Meso- and Neoproterozoic sedimentary complexes, and igneous rocks of Central Taimyr) contributed significantly to the accumulation of the Late Visean–Serpukhovian deposits. In the rocks of the Tiksi Formation, 45% of detrital zircons are of Paleozoic age, while 24% are Early Paleozoic, with prevailing Cambrian and Ordovician ages. Possible provenance areas with abundant igneous rocks of this age could be the Taimyr–Severnaya Zemlya and Central Asian fold belts extending along the northern, western or southwestern margins of the Siberia. The presence of Middle–Late Devonian zircons is thought to be related to the erosion of granitoids of the Yenisei Ridge and the Altai–Sayan region. Early Carboniferous detrital zircons probably had a provenance in igneous rocks of the Taimyr–Severnaya Zemlya fold belt, on the assumption that collision between the Kara block and the northern margin of the Siberian continent had already occurred by that time. In Early Visean time, sedimentation occurred in small deltaic fans, likely along steep fault scarps that formed as a result of Middle Paleozoic (Devonian–Carboniferous) rifting. The clastic material came from small rivers that eroded the nearby area. Late Visean–Serpukhovian time was marked by a sharp increase in the amount of clastic material and by the appearance of detrital zircons coming from new provenance regions, such as fold belts extending along the northern and southwestern margins of the Siberian continent. A large river system, which was able to transport clastic material over large distances to deposit it in submarine fans on the northern Verkhoyansk passive continental margin, had already existed by that time.     

Early Carboniferous Radiolarian Fauna from Heiyingshan South of the Tianshan Mountains and Its Geotectonic Significance

Abundant and well-preserved fossil radiolarians found from the Artencasher Formation, Heiyingshan of Baicheng County, Xinjiang Uygur Autonomous Region, are identified, including 15 species and 2 unnamed species in 9 genera. The fauna is dominated by the Family Entactiniidae of Spumellaria. According to the faunal characteristics, the radiolarians may be divided into five assemblages, namely, the Triaenosphaera sicarius, Entatinosphaera palimbola, Entactinia vulgaris, Belowea cf. variabilis and Archocyrtium sp assemblages. The fauna may be correlated with that from the Early Carboniferous of Frankenwald and Rein in Germany. Thus, ophiolite was formed in the Carboniferous, while the age of collision between the Ili plate and the Tarim plate is Early Carboniferous.     

Glauconites from the Late Palaeocene — Early Eocene Naredi Formation,western Kutch and their genetic implications

Glauconitic minerals are considered as one of the valuable input parameters in sequence stratigraphic analysis of a basin. In the present study glauconitic minerals are reported from subtidal green shale facies in the lower part of the Late Paleocene-Early Eocene Naredi Formation of western Kutch. On the basis of the foraminiferal assemblage the glauconite bearing beds are interpreted to have formed in a mid shelf depositional settings of an unstable marine conditions. XRD studies confirm the glauconite mineralogy of the green pellets and provide an estimation of glauconite maturity. Textural attributes of the glauconites confirm their derivation by different degrees of alteration of precursor feldspar grains. Because of the authigenic origin and autochthonous nature, these glauconites hold promise for understanding sequence stratigraphy of the Palaeogene succession of the western Kutch.     

Tectonic evolution of the southwestern margin of Pangea and its global implications: Evidence from the mid Permian–Triassic magmatism along the Chilean-Argentine border

The tectonic evolution of the southwestern margin of Pangea supercontinent is represented by the extensive late Paleozoic–Triassic magmatism along the southwestern margin of South America, including the Chilean Frontal Andes batholiths as part of the Choiyoi province. Several models have proposed cessation of subduction as the reason behind the vast amounts of felsic magmatism and apparent lack of typical arc magmas. Here, new U-Pb in zircon ages, and geochemical and isotope analyses (Rb-Sr, Sm-Nd, Re-Os) indicate that mid Permian–Triassic granitic magmatism originated in a subduction-related extensional setting (slab rollback). Subduction and anatexis of lower continental crust were the main magma-generation mechanisms, the latter caused by asthenospheric upwelling, decompression and subsequent accumulation of underplated basalts. A comparison with coeval igneous units along the Chilean-Argentine border allows extension of this model from at least 21° to 40°S. The key elements triggering slab rollback are low subduction plate velocities and convergence rates, which can be attributed to the assembly of Pangea supercontinent (mid Permian–Triassic). Therefore, subduction of the oceanic plate beneath South America has been a continuous process from early Paleozoic times onwards—rather than having a period without subduction before the onset of the Andean cycle as previous models have invoked. New geochronological constraints indicate that the peak of the voluminous crustal-derived magmatism and related explosive volcanism (Choiyoi province) was contemporaneous with the emplacement of the Emeishan and Siberian Traps LIPs, potentially conditioning the Earth system for the environmental collapse and biotic crises related to those LIPs. The observed tectonic changes, magmatism and related environmental implications could potentially be linked to the assembly of Pangea supercontinent.     

Late Triassic thickening of the Songpan–Ganzi Triassic flysch at the edge of the northeastern Tibetan Plateau

Growing geologic evidence documents incremental Mesozoic and early Cenozoic shortening and thickening of the Tibetan crust prior to the onset of the main Cenozoic orogenic event. The Tibetan crust shows spatial and temporal variability in thickness, style, and timing of thickening, and in plateau-forming processes. The Songpan–Ganzi area of northeastern Tibet provides evidence for shortening and thickening of the crust in Late Triassic time. An oil exploratory well (HC-1) of 7012.4 m located in the area shows at least six tectonic repetitions, resulting in more than ～46% thickening of the Triassic sequence. It indicates that the true thickness of the Songpan–Ganzi Triassic flysch is not 10–15 km as previously assumed, but not more than 3–5 km. Based on this evidence, combined with prior tectonostratigraphic studies, we propose that substantial crustal shortening and thickening, leading to initial plateau formation in the northeastern Tibetan Plateau, had already occurred during the Late Triassic.     

New basin modelling results from the Polish part of the Central European Basin system: implications for the Late Cretaceous–Early Paleogene structural inversion

Combined subsidence and thermal 1D modelling was performed on six well-sections located in the north-western Mid-Polish Trough/Swell in the eastern part of the Central European Basin system. The modelling allowed constraining quantitatively both the Mesozoic subsidence and the magnitude of the Late Cretaceous–Paleocene inversion and erosion. The latter most probably reached 2,400 m in the Mid-Polish Swell area. The modelled Upper Cretaceous thickness did not exceed 500 m, and probably corresponded to 200–300 m in the swell area as compared with more than 2,000 m in the adjacent non-inverted part of the basin. Such Upper Cretaceous thickness pattern implies early onset of inversion processes, probably in the Late Turonian or Coniacian. Our modelling, coupled with previous results of stratigraphic and seismic studies, demonstrates that the relatively low sedimentation rates in the inverted part of the basin during the Late Cretaceous were the net result of several discrete pulses of non-deposition and/or erosion that were progressively more pronounced towards the trough axis. The last phase of inversion started in the Late Maastrichtian and was responsible for the total amount of erosion, which removed also the reduced Upper Cretaceous deposits. According to our modelling results, a Late Cretaceous heat-flow regime which is similar to the present-day conditions (about 50 mW/m²) was responsible for the observed organic maturity of the Permian-Mesozoic rocks. This conclusion does not affect the possibility of Late Carboniferous–Permian and Late Permian–Early Triassic thermal events.