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161.
湘黔渝毗邻区大地构造位置位于扬子陆块东南缘,发育有完整的南华纪沉积记录,同时也是我国南华纪“大塘坡式”沉积型锰矿的重要富集区。通过对区内南华系剖面的详细调查与研究,结合室内综合分析,应用“优势相”成图方法编制了南华纪早世、中世、晚世岩相古地理图件。研究表明:受Rodinia超大陆全球性裂解作用影响,南华纪时期扬子陆块东南缘发生广泛的裂解作用,形成武陵次级裂谷盆地和雪峰次级裂谷盆地,构成“堑-垒”式古地理格局,南华系是在这一背景下形成的“楔状”沉积体;划分出大陆相组、过渡相组和海相组三大沉积相组,识别出河流相、湖泊相、三角洲相、海岸相以及浅海陆棚相等沉积相及若干沉积亚相等。系列图件的编制恢复了南华纪时期的沉积盆地演化与古地理变迁,建立了沉积盆地演化模式,为区域上沉积型锰矿的预测与寻找提供了一定的沉积学依据。 相似文献
162.
A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust. 相似文献
163.
The Bengal Basin originated during the collision of India with Eurasia and Burma. The provenance analysis of the Chittagong Tripura Fold Belt (CTFB), which is the folded eastern flank of the Bengal Basin as well as the Neogene belt of the Indo-Burman Ranges (IBR) is key to better understand the possible sources of sediment input from the complex interplay of the Indian, Eurasian and Burma plates. We report new whole rock geochemical and detrital zircon U–Pb data from the upper Neogene sandstones of Tipam-Dupi Tila formations (Pliocene to Plio-Plestocene succession) from the CTFB. Detrital zircon U–Pb age spectra show three predominant peaks at <200 Ma, 480–650, ∼800–1000 Ma. The geochemical discriminations and elemental ratios of Eu/Eu* (∼0.70), La/Sc (∼16.13), La/Co (∼15.76), Th/Sc (∼2.95), La/Th (∼5.67), Th/Co (∼2.87), Cr/Th (∼4.63) as well as Chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies for the Tipam and Dupi Tila formations are suggestive of a dominantly felsic source area experiencing moderate to intensive chemical weathering (Chemical index of alteration, CIA - 57 to 81) and have a recycled provenance orogen related to active continental or passive margin settings. Integrated geochemical and zircon U–Pb studies reveal that the main sediment input might have been from the Himalayan orogen with significant arc-derived detritus, possibly from the Gangdese arc as well as from the Burma magmatic arc. 相似文献
164.
《地学前缘(英文版)》2020,11(5):1821-1840
New,integrated petrographic,mineral chemistry,whole rock geochemical,zircon and titanite UPb geochronology,and zircon Hf isotopic data from the Montezuma granitoids,as well as new geochemical results for its host rocks represented by the Corrego Tingui Complex,provides new insights into the late-to post-collisional evolution of the northeastern Sao Francisco paleocontinent.U-Pb zircon dates from the Montezuma granitoids spread along the Concordia between ca.2.2 Ga to 1.8 Ga and comprise distinct groups.Group I have crystallization ages between ca.2.15 Ga and 2.05 Ga and are interpreted as inherited grains.Group II zircon dates vary from 2.04 Ga to1.9 Ga and corresponds to the crystallization of the Montezuma granitoids,which were constrained at ca.2.03 Ga by the titanite U-Pb age.Inverse age zoning is common within the ca.1.8 Ga Group III zircon ages,being related to fluid isotopic re-setting during the Espinhaco rifiting event.Zircon ε_(Hf)(t) analysis show dominantly positive values for both Group I(-4 to+9) and Ⅱ(-3 to+8) zircons and T_(DM2) model ages of 2.7-2.1 Ga and 2.5-1.95 Ga,respectively.Geochemically,the Montezuma granitoids are weakly peraluminous to metaluminous magnesian granitoids,enriched in LILES and LREE,with high to moderate Mg#and depleted in some of the HFSE.Their lithochemical signature,added to the juvenile signature of both inherited and crystallized zircons,allowed its classification as a shoshonitic high Ba-Sr granitoid related to a late-to post-collisional lithosphere delamination followed by asthenospheric upwelling.In this scenario,the partial melting of the lithospheric mantle interacted with the roots of an accreted juvenile intra-oceanic arc,being these hybrid magma interpreted as the source of the Montezuma granitoids.The Corrego Tingui Complex host rocks are akin to a syn-to late-collisional volcanic arc granitoids originated from the partial melting of ancient crustal rocks.The results presented in this study have revealed the occurrence of juvenile rocks,probably related to an island arc environment,that are exotic in relation to the Paleo-to Neoarchean crust from the Sao Francisco paleocontinent's core. 相似文献
165.
《地学前缘(英文版)》2020,11(3):807-820
The fungus Ophiocordyceps sinensis is endemic to the vast region of the Qinghai-Tibetan plateau(QTP).The unique and complex geographical environmental conditions have led to the "sky island" distribution structure of O.sinensis.Due to limited and unbalanced sample collections,the previous data on O.sinensis regarding its genetic diversity and spatial structure have been deemed insufficient.In this study,we analyzed the diversity and phylogeographic structures of O.sinensis using internally transcribed spacer region(ITS) and 5-locus datasets by a large-scale sampling.A total of 111 haplotypes of ITS sequences were identified from 948 samples data of the fungus O.sinensis,with representing high genetic diversity,and 8 phylogenetic clades were recognized in O.sinensis.Both the southeastern Tibet and the northwestern Yunnan were the centers of genetic diversity and genetic differentiation of the fungus,and they were inferred as the glacial refugia in the Quaternary.Three distribution patterns were identified to correspond to the 8 clades,including but not limited to the coexistence of widely and specific local distributive structures.It also revealed that the differentiation pattern of O.sinensis did not fit for the isolation-by-distance model.The differentiation into the 8 clades occurred between 1.56 Myr and6.62 Myr.The ancestor of O.sinensis most likely originated in the late Miocene(6.62 Myr) in the northwestern Yunnan,and the Scene A-C of the Qinghai-Tibetan movements may have played an important role in the differentiation of O.sinensis during the late Miocene-Pliocene periods.Our current results provide a much clearer and detailed understanding of the genetic diversity and geographical spatial distribution of the endemic alpine fungus O.sinensis.It also revealed that the geochronology resulting from paleogeology could be cross-examined with biomolecular clock at a finer scale. 相似文献
166.
《地学前缘(英文版)》2020,11(3):843-853
The combination of U-Pb and Lu-Hf compositions measured in zircon crystals is a remarkably powerful isotopic couplet that provides measures on both the timing of mineral growth and the radiogenic enrichment of the source from which the zircon grew.The U-Pb age documents the timing of zircon crystallization/recrystallization and Hf isotopes inform on the degree to which the host melt was derived from a radiogenic reservoir(e.g.depleted mantle) versus an unradiogenic reservoir(e.g.ancient continental crust),or some mixture of these sources.The ease of generating large quantities of zircon U-Pb and Lu-Hf data has been in large part facilitated by instrument advances.However,the dramatic increase in time constrained zircon Lu-Hf analyses in the Earth science community has brought to the fore the importance of careful data collection and reduction workflows,onto which robust geological interpretations may be based.In this work,we discuss the fundamentals of Lu-Hf isotopes in zircon,which then allows us to provide a robust,accessible,methodology for the assessment of data quality.Additionally,we discuss some novel techniques for:data visualization—that facilitates better transparency of data interpretation;integration of geographic information—that may reveal spatial trends where temporal trends were only apparent before;and some novel statistical evaluation tools—that may provide more rigorous interand intra-sample comparisons. 相似文献
167.
P. Seethalakshmi 《Geomechanics and Geoengineering》2020,15(2):123-139
ABSTRACTMicaceous soil is believed to be detrimental for civil engineering constructions due to the effect of high compressibility, low compacted density and low shear strength. Individual mica particle has numerous intact mica flakes foliated over each other making it flexible upon loading and rebound upon unloading due to its low hardness and resilient nature. Hence, micaceous soils with mica content more than 10% are considered undesirable for highway pavements, embankments and railway track constructions. When platy mica particles are sufficiently numerous to interact with spherical sand particles, bridging and ordering phenomena are augmented within the soil mass creating unique sand-mica particle orientation (MS microstructure) unlike sand-sand particle orientation (PS microstructure). The current experimental research was conducted to evaluate the variation in stress–strain, pore pressure and effective stress path response of Sabarmati sand under the influence of mica (sand with 30% mica and pure sand) with MS and PS microstructure respectively. Effect of particle crushing on stress–strain and pore pressure response was also studied on Sabarmati sand with MS and PS microstructure. Distinctive macroscopic response was observed in Sabarmati sand with MS microstructure under the influence of mica as well as mica particle crushing. 相似文献
168.
Multiple ridges across prograding coasts may display variable geometries, commonly expressed through varying elevations. Changes in ridge elevation have been traditionally related to the occurrence of fluctuating progradation rates, which might, in turn, be driven by shifting environmental conditions. Here, we explore the geometry and growth mechanisms of multiple ridges, generated at Barreta Island (Ria Formosa, southern Portugal), as a consequence of the rapid progradation of the island over the last 70 years, following the artificial fixation of the downdrift Faro-Olhão inlet with jetties in 1955. The variability in the morphology of these features was analysed in combination with available wind and wave data, in order to better distinguish growth mechanisms and understand the main parameters determining the final geometry of the observed ridges. The results suggest that (1) most of the identified ridges fall in the beach ridge classification, as they have been mostly built by marine processes, and (2) the parameters derived from, or closely related to wave climate variability (e.g. progradation rates, storm occurrence) can jointly explain most of the observed morphological changes, while aeolian processes played a secondary role. Indeed, ridge geometry appears mainly controlled by progradation rates, with higher ridges associated with lower progradation rates. Progradation rate, in turn, is mostly related to longshore wave power, storminess, and the occurrence storm groups. Yet, the final configuration of ridges can also be affected by runup levels and onshore winds. Therefore, establishing the relation between ridge geometry and wave climate is not a straightforward task, because of the complex processes and interactions that control coastal morphodynamics. © 2019 John Wiley & Sons, Ltd. 相似文献
169.
To investigate the seismic response of a pile group during liquefaction, shaking table tests on a 1/25 scale model of a 2 × 2 pile group were conducted, which were pilot tests of a test project of a scale-model offshore wind turbine with jacket foundation. A large laminar shear box was utilized as the soil container to prepare a liquefiable sandy ground specimen. The pile group model comprising four slender aluminum piles with their pile heads connected by a rigid frame was designed with similitude considerations focusing on soil–pile interaction. The input motions were 2-Hz sinusoids with various acceleration amplitudes. The excess pore water pressure generation indicated that the upper half of the ground specimen reached initial liquefaction under the 50-gal-amplitude excitation, whereas in the 75-gal-amplitude test, almost entire ground was liquefied. Accelerations in soil, on the movable frames composing the laminar boundary of the shear box, and along the pile showed limited difference at the same elevation before liquefaction. After liquefaction, the soil and the movable-frame accelerations that represented the ground response considerably reduced, whereas both the movable frames and the piles exhibited high-frequency jitters other than 2-Hz sinusoid, and meantime, remarkable phase difference between the responses of the pile group and the ground was observed, all probably due to the substantial degradation of liquefied soil. Axial strains along the pile implied its double-curvature bending behavior, and the accordingly calculated moment declined significantly after liquefaction. These observations demonstrated the interaction between soil and piles during liquefaction. 相似文献
170.
Understanding the hydrologic connectivity between kettle holes and shallow groundwater, particularly in reaction to the highly variable local meteorological conditions, is of paramount importance for tracing water in a hydro(geo)logically complex landscape and thus for integrated water resource management. This article is aimed at identifying the dominant hydrological processes affecting the kettle holes' water balance and their interactions with the shallow groundwater domain in the Uckermark region, located in the north-east of Germany. For this reason, based on the stable isotopes of oxygen (δ18O ) and hydrogen (δ2H ), an isotopic mass balance model was employed to compute the evaporative loss of water from the kettle holes from February to August 2017. Results demonstrated that shallow groundwater inflow may play the pivotal role in the processes taking part in the hydrology of the kettle holes in the Uckermark region. Based on the calculated evaporation/inflow (E/I) ratios, most of the kettle holes (86.7%) were ascertained to have a partially open, flow-through-dominated system. Moreover, we identified an inverse correlation between E/I ratios and the altitudes of the kettle holes. The same holds for electrical conductivity (EC) and the altitudes of the kettle holes. In accordance with the findings obtained from this study, a conceptual model explaining the interaction between the shallow groundwater and the kettle holes of Uckermark was developed. The model exhibited that across the highest altitudes, the recharge kettle holes are dominant, where a lower ratio of E/I and a lower EC was detected. By contrast, the lowest topographical depressions represent the discharge kettle holes, where a higher ratio of E/I and EC could be identified. The kettle holes existing in between were categorized as flow-through kettle holes through which the recharge takes place from one side and discharge from the other side. 相似文献