On the buckling analyses of embedded layers

The published buckling analyses of axially compressed layers embedded in a softer medium, or of multilayers, are based on the notion that the buckling load is P_cr; the so-called critical load which is obtained from a linear eigenvalue problem. The purpose of the present paper is to show that this assumption is generally not justified. For these cases a linearized buckling analysis is not sufficient and buckling of a layer, or of multilayers, has to be determined from a non-linear formulation.     

Layer parallel shortening in salt-detached folds: constraint on cross-section restoration

Cleavage-fissility perpendicular to bedding is a common feature in the external part of fold-and-thrust belts. Three techniques were used to determine the internal distortion in the frontal Southern Pyrenees: the analysis of strain markers such as burrows and rain drops, the measurement of fissility, and the measurement of anisotropy of magnetic susceptibility (AMS). The comparison of the three techniques showed a good fit although they differ in sensitivity to penetrative strain variations in the range of deformation values explored in the study case. On the regional scale, the values of layer parallel shortening (LPS) derived from the markers analysis are very constant and account for 16–23% of shortening. These values are two to three times larger than the shortening values calculated from the restoration of the macroscopic scale structures and indicate a good decoupling above the Cardona salt Formation. This study permitted an accurate restoration of the low-amplitude el Guix detached anticline.     

Fold propagation in single embedded layers

This paper investigates the folding of single competent layers embedded in a less competent matrix, where the competence contrast is about 10: 1. Folds result from buckling during layer-parallel compression. A geometrical study of natural examples shows that individual folds tend to be grouped into fold complexes.The amplitude varies from a maximum at the centre of a complex to a minimum at each end. Each complex is often centred about a sedimentary lens or nodule which may have triggered the folding and localized the complex. The formation of folds of this kind has been simulated experimentally by deformation of models made from paraffin waxes of known rheological properties. Early in the deformation of a model, buckling starts at a localized site of disturbance, producing only one fold. With further deformation, new folds appear at either side of the initial one. The buckling then propagates along the layering, further folds appearing serially in time and distance. The end result is a complex with many individual folds and a regularly periodic shape.With a competence contrast of 10: 1, the rate of fold propagation is slow, and formation of a periodic complex requires an overall shortening of at least 15%. The shapes of folds formed experimentally are similar to those formed naturally.     

Tectonic overpressure in competent mafic layers and the development of isolated eclogites

Abstract Variation in the state of stress during heterogeneous deformation should be reflected in variation in the effective pressure of metamorphic reactions, whether this is mean stress or the normal stress acting across the reacting interface. The magnitude of this pressure variation will determine whether it is discernible in the preserved metamorphic mineral assemblages of heterogeneously deformed rocks. The magnitude of the mean stress difference across a non-slipping interface between two materials with viscosity ratio >c. 20:1 is effectively equal to the maximum shear stress for flow in the more viscous material. Progressive shortening of the interface results in a higher mean stress in the more competent material, whereas extension results in a lower mean stress. For high-P/low-T eclogite facies conditions, current experimental data indicate that clinopyroxene- and garnet-rich layers of eclogite should be very strong and that pressure differences of up to 800 MPa (8 kbar) between competent layer and weaker matrix may be possible. Such high values can be obtained in widely separated competent layers for values of bulk stress in the overall multilayer that are much lower (by a factor approaching the viscosity ratio). Extrusion of material between more rigid plates, which has been proposed as a regional mechanism of lateral ‘continental escape’for both the Alps and the Himalayas, should also be accompanied by a lateral gradient in effective pressure; otherwise extrusion could not occur. Maximum mean stresses with magnitudes that are many times the maximum shear stress required for plastic flow should develop for deformation zones that are long relative to their width (e.g. around 20 times for a width-to-thickness ratio of 10). Tectonic overpressure in progressively shortened competent layers, particularly in regions of extrusion between more rigid plates, might help explain the occurrence of isolated layers and pods of low-T eclogite (<550°C) with estimated peak pressures markedly in excess of those in the surrounding matrix. It cannot explain the occurrence of isolated high-T eclogites, because at temperatures >c. 550°C, the dramatic weakening of clinopyroxene in the power-law creep field precludes the development of significant overpressures in eclogite layers.     

Localization conditions of lenticular layers of polymetallic ores in Irtysh zone of buckling,as in Belousovskoye ore deposit

Morphology of ore bodies is relatively simple along the strike but characteristically intricate across the strike (table 4) in the regional type of pyrite-polymetallic ore deposit (C₃-P₁). Positions of the ore bodies are structurally-lithologically controlled. Historically, every aspect of the deposit's modern structure may be related to geological evolution of the region, from the Devonian sedimentation to terminations of the Upper Paleozoic tectonic movements.     

Initiation of crustal shortening in the Himalaya

New monazite U/Th‐Pb petrochronological data from the Annapurna region of central Nepal outline a protracted thermal history spanning ~ 30 Ma from the early Eocene (c. 48 Ma) to the early Miocene (c. 18 Ma). Rare earth element data collected concomitant with the isotopic analyses are consistent with prograde metamorphism and crustal thickening between ~ 48 and 30 Ma and anatexis between ~ 28 and 18 Ma. The timing of metamorphism recorded in these rocks is consistent with records of crustal shortening derived from ultrahigh‐pressure rocks in the western Himalaya and exhumed metamorphic rocks in southern Tibet. Although previous records of early shortening/metamorphism related to the initial collision of India with Asia are spatially associated with the northern margin of the Indian plate, the ages presented in this study extend that early record south into the main Himalayan range. These new data provide important geological constraints on this early, poorly documented history.     

Gravity anomalies and crustal shortening in the eastern Mediterranean

Crustal shortening of the ocean floor in the eastern Mediterranean is recognized by a marked thickening of the sedimentary layer seaward of the Hellenic and Calabrian island arcs. Steep gradients and large negative free-air anomalies in the gravity field along with a highly uniform, low regional heat flow are manifestations of the thickened crust. Bodies of recently deformed sediment in and seaward of the Hellenic Trough reveal the style, polarity, and dynamics of the thickening mechanism.

A linear buried anticlinal structure, inferred from analysis of surface ship gravity profiles, may mark the site of contemporary intrabasinal underthrusting. The distribution of earthquakes beneath the Mediterranean Ridge supports the interpretation that the Anaximander, Ptolomy, and Strabo Mountains are features comparable to large basement nappes. Cyprus is one such structure, offset to the south, where the oceanic crust and part of the upper mantle have been involved in the décollement.     

水泥胶结钙质砂地层中单桩竖向承载特性试验研究

钙质砂属于岩土工程中一种特殊的岩土材料,除具有颗粒形状不规则、易破碎等特征,还具有胶结性。针对钙质砂具有胶结性的地质现状,通过室内模型试验研究了胶结钙质砂地层中钢管桩的承载能力、沉降情况及其影响因素,同时与未胶结钙质砂中的桩基承载特性进行了对比。研究结果表明：与未胶结钙质砂中的钢管桩相比,胶结钙质砂的相对密实度对桩基承载力影响程度明显减弱,桩的承载形式依然表现为端承桩,随着钙质砂胶结程度的提升,桩端阻力承载占比越来越高;胶结程度较高的钙质砂地层中桩身侧摩阻力发挥存在异步过程,这是因为桩基沉降时桩身下部破坏砂层形成了更为紧密的新接触面,该接触面对桩身的径向膨胀更为敏感;胶结钙质砂中桩基 q_s-S_u 线没有出现明显的硬化阶段,与未胶结钙质砂地层中桩基的 q_s-S_u多段折线变化规律不同,胶结钙质砂地层中桩基的 q_s-S_u曲线更为接近双曲线线型。     

西藏措勤盆地构造特征与地壳缩短

措勤盆地为青藏高原仅次于羌塘盆地的第二大海相盆地,笔者通过对盆地基底和盖层变形特征分析,将措勤盆地基底划分为北部拗陷、北部隆起、中部拗陷和南部隆起4个一级构造单元;盖层划分为北部拗褶带、北部冲断带、中部拗褶带、南部冲断带和南部拗褶带5个一级构造单元,并利用平衡剖面计算得到措勤盆地晚白垩世缩短约24%。     

Flaw propagation and buckling in clay-bearing sandstones

Many historically and culturally significant buildings have sandstones that contain swelling clay inclusions in the binding phase. Differential strains that evolve during wetting and drying cycles can generate stresses that are on the order of the strength of the stone, leading to degradation. Most damage observed in the field is surface delamination and buckling of the stone over a flaw, indicating that the damage is occurring during wetting. Classical buckling theory predicts buckling to occur at a particular aspect ratio, or flaw size. The results of this study confirm buckling theory experimentally. Through finite-element simulation and experiment, the study then explores a potential flaw propagation mechanism whereby nonuniform wetting patterns generate stress intensities capable of flaw propagation. As a result, small natural flaws can grow to the critical size necessary for buckling.     

粘性土-砂井地基单井固结模型的改进

单井固结模型的计算中通常将砂井周围土体简单划分为涂抹区和非涂抹区,不符合实际砂井周围土体的渗透系数分布复杂的事实。本文在Terzaghi固结理论的基础上提出了改进的单井固结模型,以一个待定参数流量系数Cq取代涂抹区和非涂抹区渗透系数来刻画砂井周围土体的横向渗透性特征,使单井固结问题得到高度简化又不失严密性。本文将改进模型用于非完整砂井单井固结的最终沉降量的数值计算,并将计算结果与谢康和改进法以及Hart法的解析解进行了比较,证明了改进模型数值解的可靠性。     

北京市地面沉降区含水岩组及压缩层划分

文章通过北京地面沉降区综合基础地质及地面沉降专项调查，查明了沉降区水文地质、工程地质条件及地面沉降分布现状，并在典型地面沉降区开展了钻探和各种水文地质及土工试验工作。根捃上述成果资料，首次对北京市地面沉降区的含水岩组及压缩层组进行了划分。为首都地面沉降网站建设及地面沉降预警预报系统建立奠定了基础。     

Estimation of shortening between the Siberian and Indian Plates since the Early Cretaceous

A comparative study of the amount of latitudinal shift of the Siberian Plate, the Lhasa Block, the Himalayan Block and the Indian Plate using palaeomagnetic data shows that the Himalayan Block belongs to the Indian Plate. The Neo-Tethys Ocean, which separated the Himalayan and Lhasa Blocks, opened to its greatest width of 31.9° in latitude in the Early Cretaceous. The extent of shortening of the latitudinal distance between the Siberian and Indian Plates amounts to 58.2° since the Early Cretaceous. It was found that after the determination of the greatest width of Neo-Tethys Ocean, the latitudinal shortening of the crust between the Indian and Siberian Plates reached 26.3°, or about 2760 km. On the basis of the palaeomagnetic data obtained from the Qaidam Block, the latitudinal shortening between the Himalayan and Qaidam Blocks reached 50.2° since the Early Cretaceous. It was concluded that over the past 50 Ma, since the Early Cretaceous, the latitudinal shift of the southern blocks or plates was larger than that of the northern ones, and that shortening between the Qaidam and Himalayan blocks was greater than the shortening between the Qaidam Block and the Siberian Plate. This differential latitudinal movement is the main cause of convergence, resulting in the compression and uplift of the Qinghai–Tibetan Plateau.     

Strain distribution in superposed buckling folds and the problem of reorientation of early lineations

Two series of experiments were carried out with soft model-materials in order to assess the relative importance of initial homogeneous strain, external rotation and late-stage strain in reorienting early lineations during superposed buckle-folding. In the first series cylindrical buckling folds were produced in embedded planar sheets containing a “lineation”. In the second series noncylindrical folds were produced by compression of a set of cylindrical folds. The experiments indicate that the ratio of buckle shortening to layer-parallel strain is much smaller when the principal extension is parallel to the fold-axis than in the case when the principal extension is perpendicular to the fold-axis. In very competent rocks, the reorientation of old lineations is mainly by external rotation and by the associated concentric longitudinal strain. In moderately competent rocks, the orientation of early lineations always changes by initial homogeneous strain before buckling becomes significant. Because of the unlike amounts of initial strain in layers of different competences, orientations of unrolled lineations may not be parallel in disharmonically folded layers of unlike competences. Under certain conditions the early lineation may become virtually parallel to the later fold-axis. The experiments indicate that the effects of late-stage strain in buckle-folding are largely restricted to the incompetent layers of a multilayer. Hence, if orientation data of early lineations in both competent and incompetent rocks are lumped together, the pattern of orientation may become quite complex. Even for a single competent layer, the pattern of early lineations can locally become complex because of the complex nature of concentric longitudinal strain (and strain resulting from stretching of middle surface of the layer), development of conical folds, development of shear strain along hinge zones of deformed early folds and also because of the development of different orders of folds in both the first and the second deformations.     

Petrological evidence for the development of refolded folds during a single deformational event

The Pelona Schist, which forms the lower plate of the Vincent thrust in the San Gabriel Mountains of southern California, has undergone a complex history of folding. The youngest folds in the schist (style 2 folds) range in shape from open to tight and fold both compositional layering and schistosity. These are superposed upon isoclinal folds with axial-plane schistosity (style 1 folds) that, in turn, overprint older isoclinal folds (also called style 1 folds). Samples from the hinges of style 2 folds contain two generations of muscovite. Muscovites of the older generation are parallel to the folded (style 1) schistosity. The newer muscovites recrystallized during and/or after style 2 folding. Microprobe analysis indicates that the two generations of muscovite are very similar in composition, although the new muscovites tend to have slightly higher paragonite and celadonite contents than the old muscovites. From the gross similarity of the two groups of muscovite, it is concluded that the style 1 and style 2 folds were produced during a single progressive deformation. The slightly higher paragonite and celadonite contents of the new muscovites are thought to indicate that both pressure and temperature were increasing during the deformation. This is consistent with the deformation being due to underthrusting of the Pelona Schist beneath the upper plate of the Vincent thrust.     

Two anisotropic layers in the Slave craton

Four years of recording global earthquakes using a broadband seismometer located at the Ekati diamond mine revealed variations with earthquake azimuth in the arrival of SKS phases. These variations can be modeled assuming two distinct layers of anisotropy in the lithosphere. The lower layer probably lies in the mantle, and the anisotropy aligns with both North American plate motion and the strike of mantle structures identified by previous conductivity and geochemical analyses, at ˜N50°E. The upper layer is hypothesized to result from regional structures in the uppermost mantle and the crust; these trends are distinct from the mantle trends.     

Static and Dynamic Moduli of the Seismogenic Layer in Italy

Summary Static and dynamic elastic moduli of Calcare Massiccio (mudstone-limestone) have been measured as a function of frequency over nine decades, using three different standard methods: uniaxial static compression, dual cantilever forced oscillations, and measurement of ultrasonic velocities. An accurate critical comparison using the same techniques on a poly-methyl-methacrylate (PMMA) material shows that an unresolvable 10 to 20% bias exists in the two low frequency standard techniques, whereas the ultrasonic measurements are more accurate and reproducible within 5%. No significant frequency dependence is found for Calcare Massiccio, which gives a Youngs modulus of (75±7) GPa and a Poisson#x2019;s ratio of (0.28#x2009;#x00B1;#x2009;0.02).     

Migration and shortening rates in the northern Apennines,Italy: implications for seismic hazard

Is compression across the northern Apennine fold‐and‐thrust system (Italy) still active? To address this question, we quantified the long‐term rates of migration and shortening of the system along with the measurement errors. Our approach integrates structural geology, seismicity patterns, and statistical treatment of tectonic activity. On the basis of recently published surface and subsurface data, we found a migration rate of 8.85 ± 0.61 mm yr⁻¹. The inception age of individual fold structures follow closely this average rate, indicating that the system has been migrating at a constant rate for the past 17 Myr. Cumulative shortening of the system also increases linearly through time at 2.93 ± 0.31 mm yr⁻¹. The location of the youngest structures in the easternmost portion of the system coincides with a significant peak of seismic moment released by historical earthquakes. We conclude that not only these easternmost thrusts are still active, but also that they generate earthquakes.