Ordovician integrative stratigraphy and timescale of China

In this chapter, starting with a brief review of the research history and current status in the studies of the Ordovician chronostratigraphy in China, the subdivision of the Ordovician System, definition and recognition of its series and stage boundaries, and possible stratigraphic gaps are discussed in details in order to establish a multidisciplinary stratigraphic correlation through an integrated approach including lithostratigraphy, biostratigraphy, radiometric dating, chemostratigraphy and magnetostratigraphy. Being internationally accepted, the Ordovician System is now subdivided into three series and seven stages, in ascending order, Lower(Tremadocian, Floian), Middle(Dapingian, Darriwilian) and Upper series(Sandbian, Katian,Hirnantian). Three of the seven "Golden Spikes" defining the bases of the Ordovician stages, which were established in 1997–2007, are located in China. As a regionally applied chronostratigraphy, the Ordovician System was subdivided in China into Lower(Xinchangian, Yiyangian), Middle(Dapingian, Darriwilian) and Upper series(Neichiashanian, Chientangkiangian,Hirnantian). This scheme agrees largely with the standard international classification, which can actually be directly applied to China, except for some special circumstances where the Neichiashanian and Chientangkiangian stages of the Upper Ordovician are used. Based on the new studies in recent years and distinctions and differences recognized in the development of the Ordovician System in the constituent terranes of China, a new framework for correlation among the major Chinese palaeoplates or terranes, e.g. South China, North China(including Tarim and Qaidam) and Xizang(Tibet)-western Yunnan, has been established. However, it has been recognized herein that uncertainties still remain on defining the base of the Tremadocian,Dapingian and Katian, and on the correlation between different mega-facies. More specifically, for the Tremadocian, the precise correlation of its base will depend on the better-defined conodont taxonomy, while for the Dapingian and Katian, on the correlation between different mega-facies. It is worthwhile to note that the chemostratigraphic studies of the Ordovician System in China produced the carbonate δ13 C curves for the Darriwilian(Middle Ordovician) and Katian(Upper Ordovician), which show significant differences from the composite global curve. Record of the Ordovician isotopic dating is relatively rare in China, with only three reliable ages from zircons that are all from the upper Katian to Hirnantian of the Upper Ordovician.Abundant bentonite beds in the Upper Ordovician of South China will also provide unique opportunities to advance the isotopic dating and related researches. Studies on the Ordovician magnetostratigraphy need to be significantly enhanced in China, as currently all the available results are restricted to the Lower Ordovician of North China, although they can be correlated with those known from other parts of the world. The analysis of the durational unevenness of the seven stages in the Ordovician supports the possibility to further subdivide the long-durational Tremadocian, Darriwilian and Katian stages, each into two substages.     

Ediacaran integrative stratigraphy and timescale of China

Ediacaran successions occur widely in various depositional facies in South China and yield a series of fossil Lagerst?tten, providing a complete fossil record for the evolution of marine ecosystems after the terminal Cryogenian global glaciation. Carbonate-dominated Ediacaran successions in shallow water facies in South China record a nearly complete δ~(13)C profile that may reflect variations of marine carbon isotopic composition during the Ediacaran Period. The Ediacaran fossils andδ~(13)C profiles from South China permit stratigraphic correlation and subdivision of the Ediacaran strata. Based on biostratigraphic, chemostratigraphic, and geochronometric data from the Ediacaran successions in South China, we propose that the Ediacaran System in China can be subdivided into two series, with three stages in each series. The lower series is characterized by acanthomorphic acritarchs and the upper series by Ediacara-type macrofossils, and the two series are separated by the declining limb of a pronounced δ~(13)C negative excursion(EN3) in the upper Doushantuo Formation. The basal boundary of Stage1 is the same as the basal boundary of Ediacaran System, which has been defined at the base of the cap carbonate unit. Stage 2 represents the first radiation of Ediacaran microscopic organisms, with δ~(13)C feature representing by positive values(EP1). The base of the Stage 2 is placed at the first appearance level of a spiny acritarch species. Stage 3 is characterized by the occurrence of more diverse acritarchs and δ~(13)C feature EP2, with its basal boundary defined by a δ~(13)C negative excursion(EN2) occurring in the middle Doushantuo Formation. The basal boundary of Stage 4 is the same as the upper series. Stage 5 is marked by the occurrence of macrfossils of Miaohe biota, and its lower boundary can be placed at the level where δ~(13)C values transition from positive to negative in MNE, or the first appearance level of macrofossils of the Miaohe biota. Stage 6 is characterized by the occurrences of Ediacara-type Shibantan biota and Gaojiashan biota, with its lower boundary defined by the first appearance level of Conotubus hemiannulatus. The formal establishment of the aforementioned series and stages requires further and more detailed integrative stratigraphic study on the Ediacaran successions in China. Some of the Ediacaran successions in South China have great potential to become global standards in Ediacaran subdivision.     

无背索斜拉桥主塔受力及抗震性能分析

为探讨主塔横梁对无背索斜拉桥结构的静动力特性影响,以某无背索斜拉桥为原型,采用三维静\,动力有限元计算方法,分析了实心横梁、大空心横梁、小空心横梁与无横梁四种主塔横梁方案的静、动力结构特性。静力计算结果表明:在最不利荷载作用下,四种对比方案中,实心横梁的拉索应力接近安全系数临界值;横梁重量越大,主梁挠度及主塔顺桥向变形值越小,且未设置横梁的方案在长期荷载作用下产生主跨侧的收缩徐变变形值。动力计算结果表明:大空心横梁方案结构横桥向刚度最大塔底轴力最大、横桥向弯矩值最大;无横梁时横桥向剪力最大,且无横梁方案使得主塔刚度较小,阵型易出现主塔横向失稳。综合考虑结构静、动力特性与施工便利性和后期维护,建议该结构采用实心主塔横梁。     

宽频带流动地震台站中地震计浅井式安装研究

在井下地震观测中,各种地表背景噪声具有显著衰减作用,针对中国地震科学台阵现有大量非专用浅井式设计的Guralp CMG系列地震计,以及目前多个新获批项目在高背景噪声区域的观测需求,设计一种野外实用浅井观测方式,在北京国家地球观象台进行不同深度、不同井壁材质的观测对比实验,分析研究观测数据的背景噪声功率谱密度,结果表明,随着浅井深度的增加,地震背景噪声在垂直、水平向得到改善,改善程度随频率变化有所不同。分析认为,在华北现有地质环境下,6 m浅井是较为经济的观测方式,性价比高、占地小、施工简便,可用于后续多个项目的宽频带流动地震台阵观测。     

青藏高原东南缘热流估算及与地震活动相关性分析

青藏高原东南缘地区内部构造运动强烈,是地热资源发育与地震事件频发的活动地区.大地热流记录了发生在地球深部各种作用过程的热学信息,可以作为地质构造活动和地震活动研究的有效约束,但是大范围的热流数据测量很难实现,因此,本文根据居里面深度结合放射性元素分布等计算了青藏高原东南缘的大地热流分布.首先,通过地表放射性元素的分布计算出地表产热量的分布,然后,利用相关地热参数之间的关系迭代计算出该地区地壳上下层的热导率分布,最终估算出地表热流及地下不同深度处热流值的分布.本文结果表明:(1)青藏高原东南缘的大地热流位于44～108mW·m~(-2)之间,平均75mW·m~(-2),符合研究地区西南高、东北低的背景趋势,地壳内部热流值随深度的增加而降低.大部分地区地表热流异常与实际地热带分布相吻合,如川西、藏东南与滇西地区等地为地热高值区,川东和楚雄等地为热流低值区.(2)结合其他地球物理探测结果,总结了地壳内部热流与地震事件的联系:在地热梯度带地区,当两侧地层在一定深度范围内存在明显物性差异时,地震事件高发.     

不同地壳速度结构对安徽地区地震定位深度的影响

地震深度定位对地壳速度结构模型有较大的依赖性。选取2017年安徽及周边M_L 1.5以上地震,使用PTD与单纯型定位方法,分别配置华南模型与AH2015模型进行重新定位,研究不同模型对安徽地震深度的定位影响。研究表明,使用PTD方法,配置AH2015模型时定位深度略大,且符合安徽区域地震实际深度的记录台站较多;使用单纯型定位法,配置两种模型所得定位深度差距不大,且深度分布均匀。说明PTD方法定位地震深度对地壳速度结构模型的依赖程度较大,AH2015模型比华南模型更加符合安徽区域实际地壳结构。     

河北区域DSQ型水管倾斜仪端点一致性分析

河北区域地震前兆台网6个地震台站配备DSQ型水管倾斜仪进行地倾斜观测,选取2009—2016年记录的观测数据,对水管倾斜仪两端点一致性进行相关性分析。结果表明:各台站水管倾斜仪同一分量2个端点的相关系数、方差及均值、潮汐参数无显著性差异,相关性较高,表征产出的数据稳定、可靠。     

互联网+社区地震应急桌面演练的设计与实施——以青海省海西州为例

地震灾害应对实践表明,社区作为社会治理的基本单元,在抢救生命、安置群众生活等方面发挥着重要作用。应急演练是有效提升民众防灾减灾意识和社区应对灾害能力的重要途径。近年来,社区开展了不同程度的地震应急演练,但在专业性和实效性方面仍存在很大提升空间。本文基于互联网+应急演练理念,提出基于互联网云服务的分布式、多社区、不限人数、同步或异步开展的应急桌面演练新模式,并在青海省海西蒙古族藏族自治州进行了试点应用。实践表明,互联网+社区地震应急桌面演练融合了传统桌面演练与实战演练的优势,可针对社区管理层开展地震灾害全过程桌面推演,能有效促进社区管理层应急组织协调能力的提升。     

Dynamic mechanisms controlling the topography of Longmenshan area

The Longmenshan fault, which defines the eastern edge of the Tibetan Plateau, is one of the steepest margins of the plateau with a sharp elevation drop of about 4 km over a distance less than 100 km across the Longmenshan fault. The mechanism which is responsible for controlling and maintaining the elevation difference is highly debated. Using multiple observations including seismic velocity model, Moho depth, effective elastic thickness of the lithosphere, we conducted a quantitative study for elucidating the contributions from crust and lithospheric mantle by an integrated analysis of lithospheric isostasy and flexure. It is shown that the topography of the Longmenshan fault is supported by both lithospheric isostasy and flexure statically, and lower crustal channel flow and mantle convection dynamically. Different mechanisms have different weights for contribution to the topography of the Songpan-Ganzi block and the Sichuan Basin. The static and dynamic support contribute roughly the same to the topographic difference of ~4 km between the two sides of the Longmenshan fault. The static topographic difference of ~2 km is mainly resulted from the lithospheric isostasy, while the dynamic one of ~2 km is contributed by the uprising of the accumulated material in the lower crust beneath the Songpan-Ganzi block and the downward drag force caused by the upper mantle convection under the Sichuan Basin. It is thus suggested that the lower crustal flow and upper mantle convection are dynamic forces which should be taken into account in the studies on the dynamics in the Longmenshan and surrounding regions.