Showing papers in "Science China-earth Sciences in 2011"

Timing, scale and mechanism of the destruction of the North China Craton

Abstract: The North China Craton (NCC) is a classical example of ancient destroyed cratons. Since the initiation of the North China Craton Destruction Project by the National Natural Science Foundation of China, numerous studies have been conducted on the timing, scale, and mechanism of this destruction through combined interdisciplinary research. Available data suggest that the destruction occurred mainly in the eastern NCC, whereas the western NCC was only locally modified. The sedimentation, magmatic activities and structural deformation after cratonization at ~1.8 Ga indicate that the NCC destruction took place in the Mesozoic with a peak age of ca 125 Ma. A global comparison suggests that most cratons on Earth are not destroyed, although they have commonly experienced lithospheric thinning; destruction is likely to occur only when the craton has been disturbed by oceanic subduction. The destruction of the NCC was coincident with globally active plate tectonics and high mantle temperatures during the Cretaceous. The subducted Pacific slab destabilized mantle convection beneath the eastern NCC, which resulted in cratonic destruction in the eastern NCC. Delamination and/or thermal-mechanical-chemical erosion resulted from the destabilization of mantle convection.

Palaeozoic tectonic evolution of the Tianshan belt, NW China

Abstract: The Chinese Tianshan belt is a major part of the southern Central Asian Orogenic Belt, extending westward to Kyrgyzstan and Kazakhstan. Its Paleozoic tectonic evolution, crucial for understanding the amalgamation of Central Asia, comprises two stages of subduction-collision. The first collisional stage built the Eo-Tianshan Mountains, before a Visean unconformity, in which all structures are verging north. It implied a southward subduction of the Central Tianshan Ocean beneath the Tarim ac-tive margin, that induced the Ordovician-Early Devonian Central Tianshan arc, to the south of which the South Tianshan back-arc basin opened. During the Late Devonian, the closure of this ocean led to a collision between Central Tianshan arc and the Kazakhstan-Yili-North Tianshan Block, and subsequently closure of the South Tianhan back-arc basin, producing two su-ture zones, namely the Central Tianshan and South Tianshan suture zones where ophiolitic melanges and HP metamorphic rocks were emplaced northward. The second stage included the Late Devonian-Carboniferous southward subduction of North Tianshan Ocean beneath the Eo-Tianshan active margin, underlined by the Yili-North Tianshan arc, leading to the collision between the Kazakhstan-Yili-NTS plate and an inferred Junggar Block at Late Carboniferous-Early Permian time. The North Tianshan Suture Zone underlines likely the last oceanic closure of Central Asia Orogenic Belt; all the oceanic domains were consumed before the Middle Permian. The amalgamated units were affected by a Permian major wrenching, dextral in the Tianshan. The correlation with the Kazakh and Kyrgyz Tianshan is clarified. The Kyrgyz South Tianshan is equivalent to the whole part of Chinese Tianshan (CTS and STS) located to the south of Narat Fault and Main Tianshan Shear Zone; the so-called Middle Tianshan thins out toward the east. The South Tianshan Suture of Kyrgyzstan correlates with the Central Tianshan Suture of Chinese Tianshan. The evolution of this southern domain remains similar from east (Gangou area) to west until the Talas-Ferghana Fault, which reflects the convergence history between the Kazakhstan and Tarim blocks.

China's lakes at present: Number, area and spatial distribution

Abstract: Based on 11004 satellite images from CBERS CCD and Landsat TM/ETM, changes in the spatial characteristics of all lakes in China were determined following pre-established interpretation rules. This dataset was supported by 6843 digital raster images (1:100000 and 1:50000), a countrywide digital vector dataset (1:250000), and historical literature. Comparative data were corrected for seasonal variations using precipitation data. There are presently 2693 natural lakes in China with an area greater than 1.0 km2, excluding reservoirs. These lakes are distributed in 28 provinces, autonomous regions and municipalities and have a total area of 81414.6 km2, accounting for ∼0.9% of China’s total land area. In the past 30 years, the number of newly formed and newly discovered lakes with an area greater than 1.0 km2 is 60 and 131, respectively. Conversely, 243 lakes have disappeared in this time period.

New eolian red clay sequence on the western Chinese Loess Plateau linked to onset of Asian desertification about 25 Ma ago

Abstract: The expansion of inland Asia deserts has considerably influenced the environmental, social and economic activities in Asia. Aridification of inland Asia, especially timing of the initiation of Asian desertification, is a contentious topic in paleoclimatology. Late Cenozoic eolian loess-red clay sequences on the Chinese Loess Plateau, which possess abundant paleoclimatic and paleo-environmental information, can be regarded as an indicator of inland Asia desertification. Here we present a detailed magnetostratigraphic investigation of a new red clay sequence about 654 m in Zhuanglang located at the western Chinese Loess Plateau. Sedimentological, geochemical, mineralogical, and quartz morphological lines of evidence show that the red clay is of eolian origin. Magnetostratigraphic correlations indicate that this core sequence spans from 25.6 to 4.8 Ma, and typical eolian red clay appears as early as 25 Ma. This extends the lower limit of the red clay on the Chinese Loess Plateau from the previously thought early Miocene back into the late Oligocene. This new red clay record further implies that the inland Asia desertification was initiated at least by the late Oligocene. This sequence provides a unique high-resolution geological record for understanding the inland Asia desertification process since the late Oligocene.

Spatiotemporal precipitation variations in the arid Central Asia in the context of global warming

Abstract: This study analyzed the temporal precipitation variations in the arid Central Asia (ACA) and their regional differences during 1930–2009 using monthly gridded precipitation from the Climatic Research Unit (CRU). Our results showed that the annual precipitation in this westerly circulation dominated arid region is generally increasing during the past 80 years, with an apparent increasing trend (0.7 mm/10 a) in winter. The precipitation variations in ACA also differ regionally, which can be divided into five distinct subregions (I West Kazakhstan region, II East Kazakhstan region, III Central Asia Plains region, IV Kyrgyzstan region, and V Iran Plateau region). The annual precipitation falls fairly even on all seasons in the two northern subregions (regions I and II, approximately north of 45°N), whereas the annual precipitation is falling mainly on winter and spring (accounting for up to 80% of the annual total precipitation) in the three southern subregions. The annual precipitation is increasing on all subregions except the southwestern ACA (subregion V) during the past 80 years. A significant increase in precipitation appeared in subregions I and III. The long-term trends in annual precipitation in all subregions are determined mainly by trends in winter precipitation. Additionally, the precipitation in ACA has significant interannual variations. The 2–3-year cycle is identified in all subregions, while the 5–6-year cycle is also found in the three southern subregions. Besides the inter-annual variations, there were 3–4 episodic precipitation variations in all subregions, with the latest episodic change that started in the mid- to late 1970s. The precipitations in most of the study regions are fast increasing since the late 1970s. Overall, the responses of ACA precipitation to global warming are complicated. The variations of westerly circulation are likely the major factors that influence the precipitation variations in the study region.

Cratonization and the Ancient North China Continent: A summary and review

Abstract: Cratonization is a key geological process to form stable continental masses with a considerable scale. The Precambrian global cratonization and formation of supercratons in the world is an unrepeated event in the history of the Earth’s formation and evolution. Mainly based on study of early Precambrian geology in Eastern Hebei Region and combining other Archean regions in the North China Craton (NCC), the author proposes a two-stage cratonization model of the NCC. The first stage took place at the end of Neoarchean of ∼2.5 Ga (boundary time between Archean and Proterozoic), when several micro-blocks were amalgamated together with amphibolite-granulite facies metamorphism and intrusion of crustal-melting granites to form the present-scale NCC. The second cratonization event is cratonic reworking, corresponding to rifting-subduction-collision at 2.3–1.97 Ga and subsequent extension-uplifting related to upwelling mantle at 1.97–1.82 Ga, which could be linked to, respectively, assembly and breaking up of the Columbia Supercontinent. Three main Paleoproterozoic mobile belts in the NCC record that small remnant Neoarchean ocean basins and continental rift basins within the craton were opened and finally closed, and metamorphosed to greenschist-amphibolite facies at ∼2.0–1.97 Ga. After that, high-grade granulite facies (HT-HP and HT-UHT) metamorphism with abnormally high heat occurred at 1.97–1.82 Ga. A metamorphism-migmatization event that includes lower crust of the NCC uplifting as a whole, intrusion of mafic dyke swarms, continental rifting and anorogenic magmatic action took place in 1.82–1.65 Ga, marking that the second cratonization of the NCC was finally accomplished and started to evolve to a period of stable continent (platform).

Geochemical studies on the source region of Asian dust

Abstract: The North China and the neighbouring Mongolia in Asian Interior is characterized by extremely dry climate, resulted in one of the world’s major dust emission centres. Deciphering the source region of Asian dust is critical for revealing the mechanism of the dust production, interpreting the paleo-environmental records of eolian deposits, predicting the overall environmental effects of dust, and setting the strategies for the control of contemporary dust storms. This paper summarizes the geochemical methods applied to the source tracing of Asian dust. Nd-Sr isotopes were the most extensively studied source tracer of Asian dust and have been successfully applied in many cases. Geochemistry of detrital monomineral shows great theoretical advantages in source tracing and deserves further studies. The short-range transportation of Chinese loess with direction similar to that of the prevailing near surface wind is revealed. Source tracing also shows that the Asian dust has two ultimate material sources from the northern margin of the Tibetan Plateau and the Central Asian Orogen, which confirms the importance of mountain processes in the production of silt eolian particles. Based on the recent progresses on the source tracing of Asian dust, discussions are expanded on the natural background of Asian dust storms and potential anthropogenic influence, the materials evolution of the source regions of Asian dust and its relationships with climate changes and Tibetan uplift, and the role of Tibetan uplift in the Asian dust system.

China land soil moisture EnKF data assimilation based on satellite remote sensing data

Abstract: Soil moisture plays an important role in land-atmosphere interactions. It is an important geophysical parameter in research on climate, hydrology, agriculture, and forestry. Soil moisture has important climatic effects by influencing ground evapotranspiration, runoff, surface reflectivity, surface emissivity, surface sensible heat and latent heat flux. At the global scale, the extent of its influence on the atmosphere is second only to that of sea surface temperature. At the terrestrial scale, its influence is even greater than that of sea surface temperatures. This paper presents a China Land Soil Moisture Data Assimilation System (CLSMDAS) based on EnKF and land process models, and results of the application of this system in the China Land Soil Moisture Data Assimilation tests. CLSMDAS is comprised of the following components: 1) A land process model—Community Land Model Version 3.0 (CLM3.0)—developed by the US National Center for Atmospheric Research (NCAR); 2) Precipitation of atmospheric forcing data and surface-incident solar radiation data come from hourly outputs of the FY2 geostationary meteorological satellite; 3) EnKF (Ensemble Kalman Filter) land data assimilation method; and 4) Observation data including satellite-inverted soil moisture outputs of the AMSR-E satellite and soil moisture observation data. Results of soil moisture assimilation tests from June to September 2006 were analyzed with CLSMDAS. Both simulation and assimilation results of the land model reflected reasonably the temporal-spatial distribution of soil moisture. The assimilated soil moisture distribution matches very well with severe summer droughts in Chongqing and Sichuan Province in August 2006, the worst since the foundation of the People’s Republic of China in 1949. It also matches drought regions that occurred in eastern Hubei and southern Guangxi in September.

A high resolution simulation of climate change over China

Abstract: Multi-decadal high resolution climate change simulations over East Asia were performed by using The Abdus Salam International Centre for Theoretical Physics (ICTP) Regional Climate Model (RegCM3), nested within the NASA/NCAR global model FvGCM/CCM3. Two sets of simulations were conducted at 20-km grid spacings, one for present day (1961–1990) and one for the future climate (2071–2100, IPCC A2 scenario). Simulations of present climate conditions over China by RegCM3 and FvGCM were compared against observations to assess the model performance. Results showed that both models reproduced the observed spatial structure of 500 hPa height, surface air temperature and precipitation. Compared with FvGCM, RegCM3 provided increasing spatial detail of surface variables. Furthermore, RegCM3 improved the simulation of monsoon precipitation over the region. Changes in the mean temperature and precipitation were analyzed and compared between the two models. Significant warming in the end of the 21st century was simulated by both models in December–January–February (DJF), June–July–August (JJA), and the annual mean. In DJF, greater warming was simulated by FvGCM over Northeast and Northwest China, as well as the Tibetan Plateau, compared with RegCM. In JJA, RegCM3 simulated greater warming over northern China, Inner Mongolia, Northwest China, and the Tibetan Plateau. Simulated changes in DJF precipitation showed similar spatial patterns between the two models. In JJA, while FvGCM projected a prevailing increase of monsoon precipitation over China, which is in agreement with other global models, RegCM3 projected extended areas of decreased precipitation. Changes in the variability for annual mean temperature and precipitation also are presented.

Detrital zircon age model of Ordovician Wenquan quartzite south of Lungmuco-Shuanghu Suture in the Qiangtang area, Tibet: Constraint on tectonic affinity and source regions

Abstract: Early to Middle Ordovician strata, including Wenquan quartzite, occur widely in the Himalaya, Lhasa, and south Qiangtang blocks. The Wenquan quartzite occurs on the south side of the Lungmuco-Shuanghu Suture in the Qiangtang area, Tibet. A total of 145 analyses on detrital zircons from the quartzite show five age ranges of 520–700, ca. 800, 900–1100, 1800–1900, and 2400–2500 Ma, with particularly distinct age peaks of 625 and 950 Ma. The reliable youngest detrital zircon age is 525 Ma, and the oldest, 3180 Ma. Detrital zircons show large variations in Hf isotope composition, with depleted mantle model ages tDM(Hf) ranging from 750 to 3786 Ma. Based on data obtained in this study and by others, the main conclusions are as follows: 1) Low-grade metamorphic sedimentary rocks are distributed extensively in the south of the Lungmuco-Shuanghu Suture and are Phanerozoic in age; 2) Pan-African and Grenville-Jinning tectono-thermal events were well developed in the source region of the Wenquan quartzite; 3) the source region shows crustal addition and recycling of different periods; 4) Wenquan quartzite was derived from the Gondwana metamorphic basement, suggesting that the Qiangtang block is a Gondwanan fragment.

Mesozoic contraction deformation in the Yanshan and northern Taihang mountains and its implications to the destruction of the North China Craton

Abstract: Mesozoic contraction deformation in the Yanshan and Taihang mountains is characterized by basement-involved thrust tectonics, basement-cored buckling anticlines and ductile thrust and nappe tectonics. Most of these deformations are orientated west-east, west-northwest and northeast to north-northeast. The contraction deformations began in the Permian, continued through the Triassic and Jurassic and terminated in the Early Cretaceous, and constitute an important part of the destruction of the North China Craton. It is estimated, from balanced cross-section reconstructions, that the north-south shortening of the central part of the Yanshan belt before 135 Ma was around 38%. The initial crust thickness, pre-dating the major contraction deformation in late Paleozoic and early Mesozoic, was estimated to be around 35 km based on paleogeographic characteristics. Assuming that the inferred depth of ductile thrusting deformation, 20-25 km, was the crust thickness involved in the contraction deformation, and also assuming that the N-S contraction deformation was accommodated by vertical crust thickening, the thickness of the crust after the contraction deformation was expected to be around 47-50 km. This was the approximate crust thickness required for the eclogitization of the lower crust for delamination. The gravity potential accumulated by the isostatic uplift of the thickened crust, together with the decrease in crustal strength caused by the coeval magmatisms associated with the contraction deformation, led to the subsequent extensional collapse of the middle and upper crust although the regional stress regime associated with the plate interactions remained constant. It is inferred that the Mesozoic contraction deformations in the Yanshan and Taihang mountains were not only a significant tectonic process contributing to the destruction of the craton in middle and upper crust but also stimulated delamination at a deep level and the extension of the shallow crust. In other words, both the suspected delamination of the lower crust and upper mantle and the well constrained extension deformations of the shallow crust in the eastern North China Craton during the late Mesozoic are a consequence of crust thickening due to previous contractions. Extensional deformations could be expected to occur independently in the shallow crust, and are not necessarily associated with or responding to de-lamination at a deep level.

The discovery of the oldest rocks in the Kuluketage area and its geological implications

Abstract: The basement rocks in the Kuluketage area are composed predominately of tonalite-trondhjemite-granodiorite rocks, and occured mainly in Xinger and Korla. U-Pb dating of TTG gneiss near Korla yielded a late Neoarchean weighted mean 207Pb/206Pb age of 2.65 Ga, which is the oldest published age for the TTG rocks in the Kuluketage area and thus suggests that Archean terrane in the area was formed in the late Neoarchean. The Korla gneiss is much younger than the TTG rocks in the northern Altyn Tagh, eastern Tarim Craton, indicating that the oldest terrane of the Tarim Craton was exposed probably in the northern Altyn Tagh. Until late Neoarchean, the Tarim continent extends to the Kuluketage area and finally had generated a relatively large uniform Archean basement within the craton. Zircon Hf isotopic analyses of the TTG gneiss give low e Hf( t ) values (?5 to 1) with Paleoarchean to Mesoarchean two-stage model ages ( T DM 2) between 3.0 and 3.3 Ga, suggesting that the basement rocks in the northern Tarim Craton were derived dominately from partial melting of Paleoarchean to Mesoarchean juvenile crustal material. The Hf model ages, therefore, indicate that no continent crust older than 3.3 Ga existed in the Kuluketage area.

Prediction of permafrost changes in Northeastern China under a changing climate

Abstract: combined with the unsteady-state heat conduction model to simulate permafrost temperatures at present, and to predict those after 50 and 100 a. The results indicate that at present, sporadic permafrost occurs in the zones with MAGSTs of 1.5°C or colder, and there would still be a significant presence of permafrost in the zones with the present MAGSTs of 0.5°C or colder after 50 a, and in those of 0.5°C or colder after 100 a. Furthermore, the total areal extent of permafrost would decrease from 2.57×10 5 km 2 at present to 1.84×10 5 km 2 after 50 a and to 1.29×10 5 km 2 after 100 a, i.e., a reduction of 28.4% and 49.8% in the permafrost area, respectively. Also the permafrost would degrade more substantially in the east than in the west. Regional warming and thinning of permafrost would also occur. The area of stable permafrost (mean annual ground temperature, or MAGT≤1.0°C) would decrease from present 1.07×10 5 to 8.8×10 4 km 2 after 50 a, and further decrease to 5.6×10 4 km 2 after 100 a. As a result, the unstable permafrost and seasonally frozen ground would expand, and the southern limit of permafrost would shift significantly northwards. The changes in the permafrost environment may adversely affect on ecological environments and engineering infrastructures in cold regions. Avoidance of unnecessary anthropogenic changes in permafrost conditions is a practical approach to protect the permafrost environment. permafrost, Northeastern China, climate change, equivalent latitude model (ELM), prediction

Spatio-temporal framework of tectonic uplift stages of the Tibetan Plateau in Cenozoic

Abstract: Four intensive uplift periods, i.e., 60-35, 25-17 and 12-8 Ma (but 18-13 Ma in the Himalayas of the southern Tibet), and since about 5 Ma, can be determined on the Tibetan Plateau by synthetical analysis of low-temperature thermo-chronology data, sedimentary deposit records, and structural deformation records of different areas. The strong tectonic uplift periods in different areas on the Tibetan Plateau are penecontemporaneous, except for the Himalayan area of the southern Tibet, where a rapid uplift and exhumation period, controlled by the activity of the South Tibetan Detachment System faults, occurred during 18-13 Ma. These strong uplift and exhumation periods correspond well to intensive deformation activity periods, suggesting tectonically-controlled uplift and exhumation. The deposit records, such as the distribution of coarse clastic sediments, the distribution of tectonically-controlled basins, stratigraphic discontinuousness or unconformity, and fault-controlled geomorphologic evolution, also match well with the strong uplift and exhumation periods. Expanding processes of the plateau are also discussed.

Biostratigraphy and geography of the Ordovician-Silurian Lungmachi black shales in South China

Abstract: Based on the new material of seven Ordovician-Silurian boundary sections investigated recently, together with previously published data, we analyze the temporal and spatial distributions of the Lungmachi black shales, a key petroleum source bed widely distributed in South China. The Lungmachi black shales range in age from the Normalograptus persculptus Biozone of the uppermost Ordovician to the Spirograptus guerichi Biozone of the lower Telychian, and ten graptolite biozones can be recognized within this unit. The basal and upper contacts of the Lungmachi black shales are diachronous. The basal contact ranges from the N. persculptus to the C. cyphus biozones, a span of five graptolite biozones over two stages. The upper contact ranges from the D. pectinatus-M. argenteus Biozone to the Spirograptus guerichi Biozone, which spans four graptolite biozones over two stages. The Yichang Uplift resulted in the formation of the Hunan-Hubei Submarine High in the border area of Hubei, Hunan, and Chongqing. This is supported by a break in sedimentation in this area spanning all or part of the Hirnantian, and in many areas extending into the underlying Katian and overlying Rhuddanian. Comparison of the distribution of the Katian to Rhuddanian strata in this area indicates a growth and subsequent reduction in area of the Hunan-Hubei Submarine High particularly in the Hirnantian to early Rhuddanian. This may partly represent the influence of the process of formation and melting of ice sheet in Ordovician South Pole and consequent sea level change.

Circulation anomalies associated with interannual variation of early- and late-summer precipitation in Northeast China

Abstract: Summer rainfall is vital for crops in Northeast China. In this study, we investigated large-scale circulation anomalies related to monthly summer rainfall in Northeast China using European Center for Medium-Range Weather Forecast ERA-40 reanalysis data and monthly rainfall data from 79 stations in Northeast China. The results show that the interannual variation in rainfall over Northeast China is mainly dominated by a cold vortex in early summer (May–June) and by the East Asian summer monsoon in late summer (July–August). In early summer, corresponding to increased rainfall in Northeast China, an anomalous cyclonic anomaly tilted westward with height appears to the northwest and cold vortices occur frequently. In late summer, the rainfall anomaly is mainly controlled by a northward shift of the local East Asian jet stream in the upper troposphere and the northwest extension of the western Pacific subtropical high (WPSH) in the lower troposphere. The enhanced southwesterly anomaly in the west of the WPSH transports more moisture into Northeast China and results in more rainfall. In addition, compared with that in July, the rainfall in Northeast China in August is also influenced by a mid- and high-latitude blocking high over Northeast Asia.

A unified theory for elastic wave propagation through porous media containing cracks—An extension of Biot’s poroelastic wave theory

Abstract: Rocks in earth’s crust usually contain both pores and cracks. This phenomenon significantly affects the propagation of elastic waves in earth. This study describes a unified elastic wave theory for porous rock media containing cracks. The new theory extends the classic Biot’s poroelastic wave theory to include the effects of cracks. The effect of cracks on rock’s elastic property is introduced using a crack-dependent dry bulk modulus. Another important frequency-dependent effect is the “squirt flow” phenomenon in the cracked porous rock. The analytical results of the new theory demonstrate not only reduction of elastic moduli due to cracks but also significant elastic wave attenuation and dispersion due to squirt flow. The theory shows that the effects of cracks are controlled by two most important parameters of a cracked solid: crack density and aspect ratio. An appealing feature of the new theory is its maintenance of the main characteristics of Biot’s theory, predicting the characteristics of Biot’s slow wave and the effects of permeability on elastic wave propagation. As an application example, the theory correctly simulates the change of elastic wave velocity with gas saturation in a field data set. Compared to Biot theory, the new theory has a broader application scope in the measurement of rock properties of earth’s shallow crust using seismic/acoustic waves.

Effect of permafrost degradation on hydrological processes in typical basins with various permafrost coverage in Western China

Abstract: Monthly discharge of four rivers with various permafrost coverage and little anthropogenic influence was used to identify effects of permafrost degradation during the last 50 years, which has occurred because of significant increases in air temperature in the river regions. The basins of the Shule, Heihe, Shiyang and upper Yellow Rivers in northwestern China have 73%, 58%, 33% and 43% permafrost coverage, respectively. There is snow cover in the basins and no rain to supply rivers during winter. The monthly recession coefficient (RC) in winter reflects groundwater conditions. The RC has increased obviously for the Shule and Heihe rivers with 73% and 58% permafrost coverage, respectively, but did not increase for the Shiyang River, and decreased insignificantly for the upper Yellow River, which had less permafrost coverage. There is a distinct positive relationship between RC and annual negative degree-day temperature (NDDT) at the meteorological stations in the basins with high permafrost coverage. These results imply that permafrost degradation due to climate warming affects hydrological processes in winter. The effect is obvious in the basins with high permafrost coverage but negligible in those with low permafrost coverage. Permafrost degradation increases infiltration, enlarges the groundwater reservoir, and leads to slow discharge recession. The result means that hydrological processes are affected strongly by permafrost degradation in river basins with high permafrost coverage, but less in river basins with less permafrost coverage.

Three Gorges Dam stability monitoring with time-series InSAR image analysis

Abstract: In this paper, we carried out a combination of permanent scatterer and quasi permanent scatterer time-series InSAR image analyses to extract geometric information over the area of the Three Gorges Dam. For the first time, we measured and analyzed the deformation of the Three Gorges Dam and its surrounding area using 40 SAR images acquired from 2003 to 2008. Our results indicate that the temporal deformation of the left part of the dam has ceased and that the deformation of the dam was influenced by the changing level of the Yangtze River. Seasonal deformation due to varying temperature is also observed. The obtained results agree well with the published results of the Three Gorges Dam deformation obtained by employing conventional survey methods. We also found that there is an area of abnormal subsidence near Zigui County. This paper demonstrates the potential of time-series InSAR image analysis in the monitoring of dam stability and measurement of subsidence.

Modeling tree root diameter and biomass by ground-penetrating radar

Abstract: Roots play a key role in ecosystem functioning as they transfer water and nutrients from soil to plants. Traditional methods for measuring roots are labor-intensive and destructive in nature, which limits quantitative and repeatable assessments in long-term research. Ground-penetrating radar (GPR) provides a non-destructive method to measure plant roots. Based on the superiority of GPR with 2 GHz frequency, we developed a new, practical method to estimate root biomass. First, average root matter density was measured by collecting a small number of root samples. Second, under controlled, experimental conditions in a sandy area, a root diameter estimation model base on GPR was developed from which root diameter was estimated. Third, root volume was calculated using the estimated root diameter and assuming the shape of roots to be cylindrical. Finally, root biomass was estimated by averaging root matter density and root volume. Results of this study suggest the following: (1) the density of coarse roots with diameters greater than 0.5 cm is relatively uniform; (2) a new wave shape parameter, ΔT, extracted from profile data of 2 GHz frequency antenna is independent of root depth, thus enabling the construction of a root diameter estimation model with high accuracy; and (3) results of a field experiment demonstrated the GPR-based method to be feasible and effective in estimating biomass of coarse roots. These findings are helpful for improving GPR-based root diameter and biomass estimation models and suggest the potential of GPR data in studying root systems.

Geographic analysis-oriented Virtual Geographic Environment: Framework, structure and functions

Abstract: Previous research on the Virtual Geographic Environment (VGE) has focused mainly on representation rather than geographic analysis. However, geographic analysis plays a significant role in modern geography. To address this issue, this paper systematically examines theories and implementing VGE techniques that support geographical analysis and simulation. Based on its framework, VGE can be divided into four subtypes. These are the data environment, modeling environment, expression environment, and collaborative environment. The functions and key techniques of each are examined, and some case studies are discussed. This study provides direction for necessary new developments of advanced VGE platforms.

Recent trends in surface sensible heat flux on the Tibetan Plateau

Abstract: Over the last three decades, the Tibetan Plateau has exhibited a significant increase in air temperature and a significant decrease in wind speed. How the surface heat source has changed is an important issue in monsoon research. Based on routine meteorological data, this study investigates the differences between methods for estimating trends in surface sensible heat flux on the Tibetan Plateau for the period 1984–2006. One is a physical method based on micro-meteorological theory and experiments, and takes into account both atmospheric stability and thermal roughness length. The other approach includes conventional empirical methods that assume the heat transfer coefficient is a constant value or a simple function of wind speed. The latter method is used widely in climatologic studies. Results from the physical method show that annual mean sensible heat flux has weakened by 2% per decade, and flux seasonal mean has weakened by ∼2%–4% except in winter. The two commonly used empirical methods showed high uncertainties in heat flux trend estimates, although they produced similar climatologies. Annual mean heat flux has weakened by 7% per decade when a fixed transfer coefficient is used, whereas the trend is negligible when the transfer coefficient is assumed a function of wind speed. Conventional empirical methods may therefore misrepresent the trend in sensible heat flux.

Early Cretaceous extensional structures in the Liaodong Peninsula: Structural associations, geochronological constraints and regional tectonic implications

Abstract: Various extensional structures, e.g., half grabens, detachment faults, and metamorphic core complexes, were formed in the Liaodong Peninsula. There are two metamorphic core complexes (mcc’s) in the western part of the Peninsula, i.e. the Liaonan mcc and the Wanfu mcc. They share the same lower plate and constitute a conjugate mcc pair. The Dayingzi detachment fault system and the three half grabens are exposed in the central and eastern parts, respectively. U-Pb dating of zircons from syntectonic plutons in the lower plates of the detachment faults and volcanic rocks from half graben basins indicates that their formation spans from 135 to 106 Ma, although the individual structure may be formed at a particular stage. Despite the differences in age of formation, in the regional attitudes, and in rooting depths, the extensional structures have great similarities in their kinematics, geometrical asymmetry, and coeval tectono-magmatic activities etc. Macroscopically, the extensional structures constitute conjugate associations, but a particular one generally has asymmetric patterns. Early Cretaceous extensional structures extend from the Liaodong Peninsula to North China, Northeast China, South China, and eastern Mongolia and Transbaikal area in Russia. The extensional structures from different areas share many common features. The Liaodong Peninsula is the miniature of the East Asia with respect to the formation of extensional structures in Early Cretaceous. It is suggested that the interaction of the Izanagi Plate with Eurasia Plate is responsible for the extension of crust. The structural mobility of the lithosphere, partly attributed to the fluid flow at the depth, and detachment faulting in both the crustal and mantle lithosphere provide important constraints on the development of Early Cretaceous extensional structures in the East Asia.

Geochronology and significance of volcanic rocks in the Ning-Wu Basin of China

Abstract: The Ning-Wu (Nanjing-Wuhu) Basin is one of the most important volcanic basins in the Middle-Lower Yangtze River Valley, eastern China. It consists of four volcanic units, i.e., the Longwangshan, Dawangshan, Gushan, and Niangniangshan Formations. Their LA-ICP MS U-Pb zircon ages are 134.8±1.8, 132.2±1.6, 129.5±0.8, and 126.8±0.6 Ma, respectively. Results of this study indicate that all volcanic rocks in the Ning-Wu Basin were formed in the Early Cretaceous from 135 to 127 Ma, lasting 8–10 m.y. No Jurassic volcanic activities occurred in any of the volcanic basins of the Middle-Lower Yangtze River Valley, including the Ning-Wu Basin. These new chronological results provide significant evidence for further study of this region to improve our understanding of Mesozoic tectonic, magmatic, and metallogenic processes of eastern China.

Adakitic rocks and destruction of the North China Craton: Evidence from experimental petrology and geochemistry

Abstract: The general temporal-spatial consistency for the formation of adakitic rocks and lithospheric thinning in northern China provide a window to examine the processes and mechanism for the destruction of the North China Craton. With experimental petrology data, this paper demonstrates that the adakitic rocks in northern China are the products of partial melting of middle- to high-potassic metabasalts at the base of the lower continent crust. Based on the TiO2 solubility model, many adakitic rocks in Dabie, Jiaodong and the northern part of the craton appear to have been saturated with TiO2. This indicates the presence of a Ti-rich accessory phase in their source regions. This phase must be rutile based on the decreasing Nb/La with increasing La/Yb in the adakitic rocks. The adakitic magmas were, thus, derived from a depth of more than 50 km, based on pressures (higher than 1.5 GPa) for the stability of rutile in a metabasalt system. Because present-day crustal thickness is generally only 35 km, we suggest that the destruction of the North China Craton may have led to at least 15 km of thinning or delamination of the crust.

Application of the modern ophiolite concept with special reference to Precambrian ophiolites

Abstract: Much has been learned in the past 40 years about the great diversity of the internal structure and geochemical compositions of Phanerozoic ophiolites, indicating that these on-land fragments of ancient oceanic lithosphere formed in distinctly different tectonic settings during their igneous evolution. Recent studies in Archean and Proterozoic greenstone belts have shown that the Precambrian rock record may also include exposures of a diverse suite of ophiolite complexes as part of craton development in the early history of the Earth. We review the salient features of the Precambrian ophiolite record to highlight what has been learned about Precambrian oceanic spreading systems since the original Penrose definition of ophiolites in 1972. Some of the diagnostic, characteristic, typical, and rare aspects of ophiolites of all ages are presented in a table in order to help determine if tectonically deformed and metamophosed sequences in Precambrian shield areas may be considered as ophiolites. The results of this comparative study are important in that they enable researchers to more realistically characterize allochthonous mafic/ultramafic rock sequences as ophiolitic or non-ophiolitic. This approach is more deterministic in contrast to some other arbitrary classification schemes requiring three or four of the Penrose-style ophiolitic units to be present in the Precambrian record for a specific rock sequence to be considered ophiolitic. Once these tectonic fragments are recognized as remnants of ancient oceanic lithosphere, great progress shall be made in understanding early Earth history. We discuss the significance and implications of the Precambrian ophiolite record to constrain the mode and nature of the plate tectonics that operated in deep time.

Relationship of atmospheric boundary layer depth with thermodynamic processes at the land surface in arid regions of China

Abstract: The atmospheric boundary layer (ABL) is an important physical characteristic of the Earth’s atmosphere. Compared with the typical ABL, the ABL in arid regions has distinct features and is formed by particular mechanisms. In this paper, the depth of the diurnal and nocturnal ABLs and their related thermodynamic features of land surface processes, including net radiation, the ground-air temperature difference and sensible heat flux, under typical summer and winter conditions are discussed on the basis of comprehensive observations of the ABL and thermodynamic processes at the land surface carried out in the extreme arid zone of Dunhuang. The relationships of the ABL depth in the development and maintenance stages with these thermodynamic features are also investigated. The results show that the depth of the ABL is closely correlated with the thermodynamic features in both development and maintenance stages and more energy is consumed in the development stage. Further analysis indicates that wind velocity also affects ABL development, especially the development of a stable boundary layer in winter. Taken together, the analysis results indicate that extremely strong thermodynamic processes at the land surface are the main driving factor for the formation of a deep ABL in an arid region.

Vortex genesis over the Bay of Bengal in spring and its role in the onset of the Asian Summer Monsoon

Abstract: Physical processes associated with onset of the 1998 Asian summer monsoon were examined in detail using multi-source datasets. We demonstrated that strong ocean-atmosphere-land interaction in the northern Indian Ocean and tropical Asian area during spring is a fundamental factor that induces the genesis and development of a monsoon onset vortex over the Bay of Bengal (BOB), with the vortex in turn triggering onset of the Asian summer monsoon. In spring, strong surface sensible heating over India and the Indochina Peninsula is transferred to the atmosphere, forming prominent in situ cyclonic circulation, with anticyclonic circulations over the Arabian Sea and northern BOB where the ocean receives abundant solar radiation. The corresponding surface winds along the North Indian Ocean coastal areas cause the ocean to produce the in situ offshore currents and upwelling, resulting in sea surface temperature (SST) cooling. With precipitation on the Indochina Peninsula increasing from late April to early May, the offshore current disappears in the eastern BOB or develops into an onshore current, leading to SST increasing. A southwest-northeast oriented spring BOB warm pool with SST >31°C forms in a band from the southeastern Arabian Sea to the eastern BOB. In early May, the Somali cross-equatorial flow forms due to the meridional SST gradient between the two hemispheres, and surface sensible heat over the African land surface. The Somali flow overlaps in phase with the anticyclone over the northern Arabian Sea in the course of its inertial fluctuation along the equator. The convergent cold northerlies on the eastern side of the anticyclone cause the westerly in the inertial trough to increase rapidly, so that enhanced sensible heat is released from the sea surface into the atmosphere. The cyclonic vorticity forced by such sensible heating is superimposed on the inertial trough, leading to its further increase in vorticity strength. Since atmospheric inertial motion is destroyed, the flow deviates from the inertial track in an intensified cyclonic curvature, and then turns northward toward the warm pool in the northern BOB. It therefore converges with the easterly flow on the south side of the anticyclone over the northern BOB, forming a cyclonic circulation center east of Sri Lanka. Co-located with the cyclonic circulation is a generation of atmospheric potential energy, due to lower tropospheric heating by the warm ocean. Eventually the BOB monsoon onset vortex (MOV) is generated east of Sri Lanka. As the MOV migrates northward to the warm pool it develops quickly such that the zonal oriented subtropical high is split over the eastern BOB. Thus, the tropical southwesterly on the southern and eastern sides of the MOV merges into the subtropical westerly in the north, leading to active convection over the eastern BOB and western Indochina Peninsula and onset of the Asian summer monsoon.

Distribution, occurrence and enrichment causes of gallium in coals from the Jungar Coalfield, Inner Mongolia

Abstract: We collected eleven bench samples of No. 6 coal from the Heidaigou Surface Mine, Jungar Coalfield, Inner Mongolia, China, and four samples from the affiliated coal preparation plant. Based on these samples, we used inductively coupled-plasma mass spectroscopy, X-ray diffraction, scanning electron microscope with an energy-dispersive X-ray spectrometer techniques, and borehole exploration data, to investigate the distribution, occurrence and enrichment causes of gallium (Ga) in the coals. Our results show: (1) Gallium is significantly enriched in the coal seams from the study area, with an average content of 18.8–26.0 ppm. Gallium is distributed heterogeneously in the coals, and reaches ore-forming scales only in No. 6 coal of Heidaigou Surface Mine, not in the other mining districts of Jungar Coalfield. (2) On the horizontal plane, Ga is enriched in the main minable coals from the northern and middle part of the coalfield. In the vertical profile, Ga content in the coal seams is higher at the base of Taiyuan Formation (Nos. 8 and 9) and Shanxi Formation (Nos. 3 and 4) than at the top of the Taiyuan Formation. Within the identical coal seam, Ga content is higher in the benches near the roof and floor than in the middle section. (3) Gallium in the coals is associated mainly with kaolinite and boehmite. Additionally, Ga may be adsorbed to some extent by humic acid, resulting in a high level in weathering coal. (4) Geological factors affect Ga enrichment in coal, such as the property of parent rocks in the source area, the sedimentary environment, organic matter, structure, and past magmatic hydrothermal activity. Especially, Ga content in parent rocks plays a leading role. (5) The mobility and precipitation of trace elements like Ga are controlled principally by the geochemical behavior of the major element Al. Terrestrial and transgressive environments can cause the precipitation of bauxite, whereas marine-continental depositional environments may cause the separation of Ga from Al. In addition, Ga may migrate in the form of gas, and may be affected by the ground temperature. Thus, it is relatively enriched in high-volatile coal.

Rupture segmentation and slip partitioning of the mid-eastern part of the Kunlun Fault, north Tibetan Plateau

Abstract: The Kunlun Fault, an active fault on the border between the Bayan Har and Kunlun-Qaidam blocks, is one of the major left lateral strike-slip faults in the Tibetan Plateau. Previous research has not reached a consensus on agreeable slip rates along much of its length and the slip rate gradient along the eastern part, both of which play critical roles in a range of models for the eastward extrusion and thickened crust of the Tibetan Plateau. New slip rates have been determined at sites along the eastern part of the Kunlun Fault by dating deposits and measuring atop displaced fluvial terrace risers. Field investigations and interpretation of satellite images reveal geometrical features of the fault and the late Quaternary offset, new earthquake ruptures and surface-rupturing segmentation, from which long-term slip rates and earthquake recurrence intervals on the fault are estimated. The tectonic geomorphology method has determined that the long-term horizontal slip rates on the Tuosuohu, Maqin and Maqu segments from west to east are 11.2±1, 9.3±2, and 4.9±1.3 mm/a while their vertical slip rates are 1.2±0.2, 0.7±0.1, and 0.3 mm/a in the late Quaternary. Results indicate that the slip rates regularly decrease along the eastern ∼300 km of the fault from >10 to <5 mm/a. This is consistent with the decrease in the gradient such that at the slip rate break point is at the triple point intersection with the transverse fault, which in turn is transformed to the Awancang Fault. The vector decomposition for this tectonic transformation shows that the western and eastern branches of the Awancang Fault fit the slip-partitioning mode. The slip rate of the southwestern wall is 4.6 mm/a relative to the northeastern wall and the slip direction is 112.1°. The mid-eastern part of the Kunlun Fault can be divided into three independent segments by the A’nyemaqen double restraining bend and the Xigongzhou intersection zone, which compose the surface rupture segmentation indicators for themselves as well as the ending point of the 1937 M7.5 Tuosuohu earthquake. The average recurrence interval of the characteristic earthquakes are estimated to be 500–1000 a, respectively. The latest earthquake ruptures occurred in AD 1937 on the western Tuosuohu segment, as compared to ∼514–534 a BP on the Maqin segment, and ∼1055 to 1524 a BP on the Maqu segment. This may indicate a unidirectional migration for surface rupturing earthquakes along the mid-eastern Kunlun Fault related to stress triggered between these segments. Meanwhile, the long-term slip rate is obtained through the single event offset and the recurrence interval, which turn out to be the same results as those determined by the offset tectonic geomorphology method, i.e., the decreasing gradient corresponds to the geometrical bending and the fault’s intersection with the transverse fault. Therefore, the falling slip rate gradient of the mid-eastern Kunlun Fault is mainly caused by eastward extension of the fault and its intersection with the transverse fault.