Cyclical one-way continental rupture-drift in the Tethyan evolution: Subduction-driven plate tectonics

Numerous continents have rifted and drifted away from Gondwana to repeatedly open ocean basins over the past-500 millionyears.These Gondwana-derived continents drifted towards and collided with components of the Eurasian continent to successively close the preexisting oceans between the two.Plate tectonics satisfactorily describes the continental drift from Gondwana to Eurasia but does not define the geodynamic mechanism of continuously rifting to collisions of continents in the Tethy an Realm.After reappraisal of geological records of the rift,collision and subduction initiation from the surface and various geophysical observations from depth,we propose that Eurasia-directed subducting oceanic slabs would have driven Tethyan system in the Phanerozoic.The Eurasia-directed subduction would have dragged the passive Gondwana margin to rift and drift northwards,giving birth to new oceans since the Paleozoic.The closure of preexisting oceans between the Gondwana-derived continents and Eurasia led to continental collisions,which would have induced the initiation of oceanic subduction in the Tethyan Realm.Multiple episodic switches between collision-subduction-rift repeatedly led to the separation of continental fragments from Gondwana and dragged them to drift towards Eurasia.The final disappearance of Neo-Tethy s would have induced collision of the Gondwana-derived continents with the Eurasian continent,giving rise to the Cenozoic Alpine-Zagros-Himalayan collisional system.Therefore,the Eurasia-directed oceanic subduction would have acted as a 'one-way train' that successively transferred the ruptured Gondwana continental fragments in the south,into the terminal in the north.In this regard,the engine of this "Tethyan one-way train" is the negative buoyancy of subducting oceanic slabs.     

Seismic tomography and continental drift

Based on data of seismic tomography, the structure of the mantle flows of the contemporary Earth and the continental drift are calculated. Results of calculation of the contemporary motion of continents and their future drift for 150 Myr are presented. The present-day positions of six continents and the nine largest islands are taken as an initial state. The contemporary temperature distribution in the mantle is calculated according to the data of seismic tomography. The 3-D distribution of seismic wave velocities is converted into the density distribution and then into the temperature distribution. The Stokes equation is numerically solved for flows in a viscous mantle with floating continents for the given initial temperature distribution. In this way, the velocities of convective flows are determined in the entire present-day mantle and the surface distribution for the Earth’s heat flux is obtained. The reliability of the calculated flows in the mantle is estimated by the comparison of the calculated velocities of the contemporary continents and oceanic lithosphere with data of satellite measurements. Further, evolutionary equations of convection with floating continents were numerically solved. The calculated structure of mantle flows, temperature distribution, and position of continents are presented for a time moment 150 Myr in the future. The resulting successive changes in the position of continents in time show how islands (in particular, Japan and Indonesia) will be attached to continents and how continents will converge, exhibiting a tendency toward the formation of a new supercontinent in the southern hemisphere of the Earth.     

Three-dimensional numerical model for the evolution of convection in a layer of a viscous fluid with six floating continents

A three-dimensional model is developed for describing the long-term evolution of the mantle convection with six continents, which includes the formation and breakup of a supercontinent. In contrast to the theory of floating continents [Trubitsyn, 2000; 2008], in which the drift was calculated from a system of convection equations for the mantle and the Euler equations for the continents, in the present work the continental motion is described in terms of a set of floating markers, which have anomalously high viscosity and are driven by the currents.     

Some remarks on continental drift

Summary The continental drift may be explained by an expanding Earth only. In fact, there is a differences in the rate of heat flow between continents and oceans. Principially, there is a possibility of deriving the value of ancient radii by palaeomagnetic and age measurements.     

Acquisition of long wavelength magnetic anomalies pre-dates continental drift

Using the low altitude MAGSAT satellite data, NASA recently published a new global magnetic map of anomalies. Using this new accurate map and paleomagnetic reconstructions of Gondwanaland, we point out that long wavelength anomalies were continuous in continents that are presently separated. Thus, continental drift would not significantly affect the geometry of these deep-source anomalies. Accordingly, the magnetization of the rocks responsible for these anomalies should mainly be induced.     

Theory of the evolution of the earth and the earth's crust

Summary The paleomagnetic field (declination and inclination) in different geological epochs is represented through the field of the so-called magnetic dipole. The optimal dipole coordinates so obtained show that during older geological times the optimal dipole lay outside the earth's centre and moved towards and around the earth centre. It is supposed that the optimal dipole trajectory in the earth coincides with earth core trajectory. This core motion brought about a motion of the masses of the mantle in a direction opposite to the motion of the core. This in turn brought about the break-down of Pangea and the separation of the continents. The drift of the continents, the relative changing of the earth's rotational axis and many other geological and geophysical phenomena may be explained through such a motion, that is through the motion of an eccentrical core towards the earth centre.     

扁率应力与地震

地球扁率的实测值要比理论值大,这证明地球内部还存在着微小的应力差。经计算,扁率变化所产生的地球形变与冰期中由地壳均衡所产生的地球形变在数值上大体相等。这表明冰期是地球扁率变化的一个原因。地壳均衡不仅能产生地壳的垂直运动,而且能产生地壳的水平运动,因而也是海底扩张、大陆漂移的力源之一。     

Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories

A method developed recently for constructing tectonic subsidence curves in early Paleozoic miogeoclines has produced new evidence for the breakup of a late Proterozoic supercontinent. Tectonic subsidence analyses in miogeoclines of eastern and western North America, northwestern Argentina, the Middle East and northwestern Australia limit the timing of the continental breakup to between 625 and 555 Ma. These results refine the implications of a much broader range of radiometric ages of rift-related igneous rocks and biostratigraphic ages of the transition from active extension to passive subsidence in miogeoclines.

The recognition of the timing and extent of rifting has led to testable hypotheses for latest Proterozoic and early Paleozoic continental histories. Breakup and onset of drift along an extensive system of continental fractures within a relatively short period of time would generate a large amount of young ocean floor, thereby reducing the volume of the global ocean basin and causing a sea level rise. Maximum reduction of ocean basin volume would postdate the time of breakup, probably by about 70 m.y., placing the transgressive peak at a time not older then about 510–520 Ma. That age agrees well with the time of maximum flooding on the continents close to the end of the Cambrian. Restriction of the breakup to between 625 and 555 Ma reduces the time gap between an essentially intact late Proterozoic supercontinent and the oldest reliable paleomagnetic reconstruction of the dispersed continents at about 560 Ma. A continental reconstruction produced by rotating Laurentia and Baltica into Gondwana a minimum distance from the 560 Ma position is consistent with limited geologic data. However, that reconstruction places Laurentia and Baltica in low latitudes which is difficult to reconcile with the absence of evaporites in syn-rift complexes in both continents.     

Continental drift,plateau uplift,and the evolutions of monsoon and arid regions in Asia,Africa, and Australia during the Cenozoic

Monsoon and arid regions in the Asia-Africa-Australia(A-A-A) realm occupy more than 60% of the total area of these continents. Geological evidence showed that significant changes occurred to the A-A-A environments of the monsoon and arid regions, the land-ocean configuration in the Eastern Hemisphere, and the topography of the Tibetan Plateau(TP) in the Cenozoic. Motivated by this background, numerical experiments for 5 typical geological periods during the Cenozoic were conducted using a coupled ocean-atmosphere general circulation model to systemically explore the formations and evolutionary histories of the Cenozoic A-A-A monsoon and arid regions under the influences of continental drift and plateau uplift. Results of the numerical experiments indicate that the timings and causes of the formations of monsoon and arid regions in the A-A-A realm were very different. The northern and southern African monsoons existed during the mid-Paleocene, while the South Asian monsoon appeared in the Eocene after the Indian Subcontinent moved into the tropical Northern Hemisphere. In contrast, the East Asian monsoon and northern Australian monsoon were established much later in the Miocene. The establishment of the tropical monsoons in northern and southern Africa, South Asia, and Australia were determined by both the continental drift and seasonal migration of the Inter-Tropical Convergence Zone(ITCZ), while the position and height of the TP were the key factor for the establishment of the East Asian monsoon. The presence of the subtropical arid regions in northern and southern Africa,Asia, and Australia depended on the positions of the continents and the control of the planetary scale subtropical high pressure zones, while the arid regions in the Arabian Peninsula and West Asia were closely related to the retreat of the Paratethys Sea. The formation of the mid-latitude arid region in the Asian interior, on the other hand, was the consequence of the uplift of the TP.These results from this study provide insight to the important roles played by the earth's tectonic boundary conditions in the formations and evolutions of regional climates during geological times.     

Continental drift during the Palaeozoic

Rapid polar shifts relative to Gondwanaland are identified in the Late Ordovician and Carboniferous. These shifts form part of the “Common polar wander path” interpretation of the palaeomagnetic poles for the Gondwana continents during the Palaeozoic. For western Europe a transition occurs between the Ordovician and Silurian poles, but is of smaller magnitude than the Late Ordovician Gondwana shift. Similarly Carboniferous shifts with respect to Europe and North America are smaller than the Gondwana shifts. A third shift in Europe is dated as mid-Devonian, but could be as old as Late Silurian, and has no counterpart in Gondwanaland. The differences in timing and magnitudes of these shifts provide evidence of the predominant role of continental drift rather than polar wandering. Attempts to explain the data exclusively in terms of polar wandering lead to geologically and geometrically untenable conclusions. Whilst there were one or perhaps two supercontinents in most of the upper Palaeozoic, it seems Laurasia was itself a set of separate fragments.     

Wavelet analysis of the annual discharge records of the world’s largest rivers

The determination of the temporal variability of water resources is of high importance with respect to long-term water policy. This contribution provides a wavelet-based global analysis of 55 large river discharge fluctuations located on the five continents.     

Estimation of drift limits for different seismic damage states of RC frame staging in elevated water tanks using Park and Ang damage index

Damage to elevated water tanks in past earthquakes can be attributed to the poor performance of their supporting frame staging. In order to ascertain the performance of these elevated water tanks, it is crucial to categorize the damage in quantifiable damage states. Among various parameters to quantify the damage states, the top drift of frame staging can be conveniently correlated to the different damage levels. In literature, drift limits corresponding to different damage states of the frame staging of the elevated water tank are not available. In the present study, drift limits for RC frame staging in elevated water tanks corresponding to different seismic damage states have been proposed. Various damage states of the elevated water tank have been determined using the Park and Ang damage index. The Park and Ang damage index utilizes results of both pushover analysis and incremental dynamic analysis. Twelve models of elevated water tanks have been developed considering variation in staging height and tank capacity. Incremental dynamic analysis has been performed using the suite of twelve actual earthquake ground motions. Based on the regression analysis between damage indexes and drift, limiting drift values for each damage state are proposed.     

Identification of yield drift deformations and evaluation of the degree of damage through the direct sensing of drift displacements

Inter‐story drift displacement data can provide useful information for story damage assessment. The authors' research group has developed photonic‐based sensors for the direct measurement of inter‐story drift displacements. This paper proposes a scheme for evaluating the degree of damage in a building structure based on drift displacement sensing. The scheme requires only measured inter‐story drift displacements without any additional finite element analysis. A method for estimating yield drift deformation is proposed, and then, the degree of beam end damage is evaluated based on the plastic deformation ratios derived with the yield drift deformation values estimated by the proposed method. The validity and effectiveness of the presented scheme are demonstrated via experimental data from a large‐scale shaking table test of a one‐third‐scale model of an 18‐story steel building structure conducted at E‐Defense. Copyright © 2016 John Wiley & Sons, Ltd.     

非偶极子磁场西向漂移的频散特征

根据第8代国际参考地磁场的资料,本文采用Briggs方法对1900～2000年期间m=1～10次谐波的磁位的全球平均西向漂移速度进行了计算和分析,得出了与Hide的理论预测完全不同的频散特征,即地磁场西向漂移呈现出负频散特征.本文还就Briggs方法得到的结果与采用Malin纬度剖面法的结果进行了比较和讨论,结果表明,正是由于非偶极子磁场的某些谐波分量及其漂移速度的全球分布存在着纬度依赖性,导致了两种方法计算出的个别谐波的漂移速度存在着较大的差异.     

Fragmentation and assembly of the continents,Mid-carboniferous to present

Eighteen maps showing the motions of the major continents following the break-up of Wegener's Pangaea in the Early to Mid-Jurassic are presented. Palaeolatitudes are determined palaeomagnetically, palaeolongitudes mainly from sea-floor spreading evidence. The break-up commences with the opening of the southern North Atlantic in the mid-Jurassic and its extension north and south in the Cretaceous and Cenozoic. Concurrently the northern continents move northwards away from the southern continents (Gondwana) and a continuous east-west seaway is formed between them in the Cretaceous. Successive fragmentation of continents then created the Arctic, Indian and Antarctic Oceans. Five maps showing the disposition of land in the Permian are also given. These are based on the palaeomagnetic evidence and the idea of minimizing the motions required to bring the continents into their known Early Jurassic configuration. The Permian maps show Gondwana situated further east than in the Pangaea configuration of Wegener. There are severe problems in constructing palaeogeographical maps for the Triassic and none are presented. Palaeomagnetic results from smaller crustal fragments are also reviewed and the evidence for the former dismemberment of Eurasia and the western part of the North American cordillera are set out. The results indicate that most orogenies are to some degree collisional in nature.     

双重约束条件下的连续重力观测非线性漂移改正方法分析

连续重力观测去除固体潮后所得重力残差结果,除了受仪器故障、自然因素、人为因素以及供电系统干扰外,弹簧重力仪的长期漂移也成为重力变化的重要影响因素,故漂移改正不可忽略。本文以高台地震台近5年的连续重力观测资料为数据基础,采用多阶多项式拟合法,并以驰豫分析结果和流动重力场结果为约束,研究弹簧重力仪的非线性漂移特征及其漂移改正的合理性。研究结果表明：高台重力仪的线性和非线性漂移特征明显,严重影响了其他微弱重力信号的提取,而驰豫分析处理结果和流动重力场成为有效地辅助约束手段,在其双重制约下进行非线性漂移改正,使所得重力非潮汐结果更为准确可靠,为地震预测预报提供较为准确的参考依据。     

钢筋混凝土框架结构层间位移角与构件变形关系研究

层间位移角已作为检验建筑结构抗震性能的主要指标之一而被广泛应用。为实现该指标在钢筋混凝土框架结构基于位移的抗震设计中的应用,研究了层间位移角与构件变形之间的关系。首先,采用对部分子结构的弹性理论分析得到了弹性阶段梁变形对层间位移角贡献比例的计算公式。接着,采用对15层钢筋混凝土框架整体结构的非线性地震反应计算结果的统计分析得到了在塑性阶段梁变形对层间位移角的贡献比例回归计算公式。所建立的计算公式反映了梁与柱的相对刚度和强度比例、层闽塑性变形程度的影响。最后,应用该方法进行了一算例分析,计算结果和试验结果比较一致。利用本文提出的计算方法可以方便地把对框架结构的层间位移需求转变为对梁、柱构件的变形要求。     

地球磁场的西向漂移

西向漂移是地球主磁场长期变化最重要的特点之一，任何一个成功的地磁场起源理论都反能否合理地解释西漂作为检验标准之一。本文简要回顾地磁场西漂硬件历史，对计算西漂的各种方法和结果作了对比分析，对这些方法的物理依据，特点及局限性进行了讨论。     

施工缝对框架结构抗震性能影响的数值分析

为研究施工缝对框架结构抗震性能的影响,利用提出的施工缝模型,基于OPENSEES平台建模进行静力非线性分析和非线性动力时程分析。通过对比整浇框架与带缝框架的顶点最大位移、层间位移角、塑性铰出现和分布规律等明确施工缝对框架结构的抗震性能的影响程度。结果表明,施工缝使框架结构的变形和层间位移角显著增大,并且使8、9度区框架结构的层间位移角分布发生改变;施工缝使柱端更易出现塑性铰,更易发生"强梁弱柱"的破坏模式;在高烈度区,施工缝的影响比较显著,如果忽略其影响,将会高估框架结构的抗震性能。