Transected folds with opposite patterns in Terena Formation (Ossa Morena Zone,Portugal): anomalous structures resulting from sedimentary basin anisotropies

The Terena Formation is located in the central part of the Ossa-Morena Zone (OMZ) and outcrops in the core of a latter (D3) first order syncline. This Formation is a Lower Devonian flysch and shows an unusual “Z” shape, with a central sector trending nearly N-S, and the tips trending NW-SE. This central sector is crossed by the cleavage (NW-SE) showing an apparent dextral (clockwise) transection pattern, anomalous and opposite to the regional widespread sinistral (anti-clockwise) transpression. The same sector with cartographic dextral transection, shows at outcrop scale, mesoscopic folds with a sinistral transection. During the Lower Devonian a N-S trending basin was developed as an effect of an early tectonic deformation phase. This trough was filled with turbidites and its elongated geometry determined the shape of the main syncline. We propose that the dextral transection pattern, at cartographic scale, result from the superposition of the NW-SE upright S3 cleavage on this major regional structure controlled by a sedimentary trough. The mesoscopic folds, observed on the upper levels of the sedimentary sequence were not influenced by the topographic anisotropy of the basin, and therefore they developed a left transection, according to the regional deformation mechanisms.

The “Z” shape of the syncline could be explained as a consequence of two major tectonic shear zones situated along the north and south boundaries of the OMZ, respectively the Tomar-Badajoz-Cordoba Shear Zone and the South Iberian Suture, lined by the Beja-Acebuches Ophiolitic Complex. Both shear zones have a sinistral transpressive character and were active during late Variscan tectonic events.     

http://www.sciencedirect.com/science/article/pii/S1674987113000030

The Dabashan orocline is situated in the northwestern margin of the central Yangtze block,central China.Previous studies have defined the orthogonal superposed folds growing in its central-western segment thereby confirming its two-stage tectonic evolution history.Geological mapping has revealed that more types of superposed folds have developed in the eastern segment of the orocline,which probably provides more clues for probing the structure and tectonic history of the Dabashan orocline.In this paper,based on geological mapping,structural measurements and analyses of deformation,we have identified three groups of folds with different trends （e.g.NW-,NE-and nearly E-trending folds） and three types of structural patterns of superposed folds in the eastern Dabashan foreland （e.g.syn-axial,oblique,and conjunctional superposed folds）.In combination with geochronological data,we propose that the synaxial superposed folds are due to two stages of ～N-S shortening in the west and north of the Shennongjia massif,and that oblique superposed folds have been resulted from the superposition of the NW-and NE-trending folds onto the early ～ E-W folds in the east of the Shennongjia massif in the late Jurassic to early Cretaceous.The conjunctional folds are composed of the NW-and NE-trending folds,corresponding to the regional-scale dual-orocline in the eastern Sichuan as a result of the southwestward expansion of the Dabashan foreland during late Jurassic to early Cretaceous,coeval with the northwestward propagation of the Xuefengshan foreland.Integration of the structure and geochronology of the belt shows that the Dabashan orocline is a combined deformation belt primarily experiencing a twostage tectonic evolution history in Mesozoic,initiation of the Dabashan orocline as a foreland basin along the front of the Qinling orogen in late Triassic to early Jurassic due to collisional orogeny,and the final formation of the Dabashan orocline owing to the southwestward propagation of the Qinling orogen during late Jurassic to early Cretaceous intra-continental orogeny.Our studies provide some evidences for understanding the structure and deformation of the Dabashan orocline.     

吉南太古宙岩群中的大型构造置换及其对BIF矿体的控制

以吉林省白山市板石沟铁矿区为典型实例,系统地论述了太古宙岩群中广泛发育的大型和区域性构造置换的几何特征和对BIF矿体的控制规律。本区构造置换主要表现为:在紧闭同斜褶皱发育的持续变形过程中形成S₁≈S₀,并伴有钩状褶皱、石香肠构造和各类型线理的生成。按照主构造面均匀性法则,划分了构造均匀区段,并对其中8个区段(Ⅰ～Ⅷ)进行了详细的SFLπ组构分析。基于构造置换规律研究,提出本区褶皱轴与包络线双向找矿的理论与方法,经工程验证,取得显著的经济效益。     

断层相关褶皱对砂岩构造裂缝发育的控制约束

在砂岩储层中进行构造裂缝发育规律的研究,对储层预测具有一定的现实意义。文中以库车坳陷东部地区砂岩中发 育的断层转折褶皱和断层传播褶皱为研究对象,统计分析构造裂缝密度大小与断层距离的关系,探讨断层相关褶皱对裂缝 发育程度的规律性认识。分析结果发现:在断层相关褶皱中,构造裂缝的密度随着距断层距离的增大而减小,二者存在明 显的指数关系;并且在靠近断层和转折端的附近,存在一个构造裂缝密度骤减的范围,定义这个范围为“断层 - 转折端共 控构造裂缝带”。断层相关褶皱带内“断层 - 转折端共控构造裂缝带”的发现与认识,对构造裂缝的定量化研究以及油藏数 值模拟等都具有一定的指导意义。     

反正切函数拟合下的地震数据体断裂刻画方法

利用界面的形状或者其不连续性是基于叠后地震数据刻画断裂的两种主要方法,根据实际地震资料的特点,两种方法各有其优劣性.对于同界面形状相关的断裂刻画方法,较常用的为地震曲率属性.受实际断裂模型的制约,在曲率属性的基础上,曲率变化率表现出对断裂更强的相关性及敏感性.本文发展了以反正切函数为准线的柱面拟合法来刻画断裂,主要包括两个步骤,分别为:一、柱面直母线的拟合;二、柱面准线拟合及曲率变化率的求取.实际地震资料计算结果表明,该方法对断裂刻画的精度高于传统方法,能够揭示更多的断裂细节,特别是对于地震剖面上不易识别的小断距断裂,亦能够清晰的刻画,同时具有较高的信噪比.     

The structural evolution of the Northern Hastings Block and southern Nambucca Block,southern New England Orogen,eastern Australia

The Hastings Block is a weakly cleaved and complexly folded and faulted terrain made up of Devonian, Carboniferous and Permian sedimentary and volcanic rocks. The map pattern of bedding suggests a major boundary exists that divides the Hastings Block into northern and southern parts. Bedding north of this boundary defines an upright box-like Parrabel Anticline that plunges gently northwest. Four cleavage/fold populations are recognised namely: E–W-striking, steeply dipping cleavage S₁ that is axial surface to gently to moderately E- or W-plunging; F₁ folds that were re-oriented during the formation of the Parrabel Anticline with less common N–S-trending, steeply dipping cleavage S₂, axial surface to gently to moderately N-plunging F₂ folds; poorly developed NW–SE-striking, steeply dipping cleavage S₃ axial surface to mesoscopic, mainly NW-plunging F₃ folds; and finally_, a weakly developed NE–SW-striking, steeply dipping S₄ cleavage formed axial surface to mainly NE-plunging F₄. The Parrabel Anticline is considered to have formed during the D₃ deformation. The more intense development of S₂ and S₃ on the western margin of the Northern Hastings Block reflects increasing strain related to major shortening of the sequences adjacent to the Tablelands Complex during the Hunter–Bowen Orogeny. The pattern of multiple deformation we have recorded is inconsistent with previous suggestions that the Hastings Block is part of an S-shaped orocline folded about near vertically plunging axes.     

Thermo-mechanical coupling response of a layered isotropic medium around a cylindrical heat source

Based on the governing equations of the thermo-elastic problem, the analytical layer-elements of a finite layer and an underlying half-space are obtained using the Laplace-Hankel transform and the characteristic value method. The cylindrical heat source is divided into several micro cylindrical heat sources, which can be approximately simulated by plane heat sources. Then, the global stiffness matrix for the problem is assembled and solved in the transformed domain, and a Laplace-Hankel transform inversion is taken to obtain the real solution. Finally, the influence of heat source types, division numbers, embedded depths and layered properties on the thermo-mechanical coupling response is investigated.     

A pseudo-coupled analytic fluid-structure interaction method for underwater implosion of cylindrical shells

Underwater implosion, the rapid collapse of a structure caused by hydrostatic pressure, is a fully coupled, highly dynamic and nonlinear fluid-structure interaction (FSI) problem. The primary motivation behind studying implosion is the short-duration, high-pressure pulse generated in the surrounding water. This paper presents a simplified analytic method to estimate the energy in the pressure pulse, based on potential flow theory. The method accounts for the varying fluid pressure and accompanying FSI. The focus is on long, thin, unstiffened metallic cylindrical shells that collapse in mode 2. The implosion pulse energy is shown to be equal to the maximum system kinetic energy developed during collapse. The kinetic energy is calculated using an energy balance approach and analytic solutions for plastic energy dissipation and energy required to compress the internal air. The time-varying fluid pressure, and subsequently the work done by the fluid on the cylinder, is found using a novel explicit time-stepping methodology. The result is a pseudo-coupled analytic solution for the fluid pressure time history and implosion pulse energy. Solutions for pulse energy agree with RANS numerical simulations within 5%.     

Geometry and evolution of fold-thrust structures at the boundaries between frictional and ductile detachments

Fold-thrust belts formed above a ductile detachment typically contain detachment folds, whereas those formed above frictional detachments contain fault-related fold complexes, such as imbricate thrust systems. Analog models, using silica sand to represent sediments and silicone gel to represent salt were conducted to study the fold geometry, fold-fault relations, and sequential development of structures formed in each setting and at the boundaries between the two settings. The results showed a relatively thinner wedge above a ductile detachment, so that the deformation front propagated farther forward than that above a frictional detachment. The thrust front connects across the two settings with a significant change in position and a resulting change in orientation. The geometry of the deformation front is strongly controlled by that of the detachment boundary, so that an oblique detachment boundary results in an oblique thrust front in the transition zone. Modifications in the taper geometry also result from the presence of a frictional belt behind a ductile belt, the width of the ductile detachment which limits the location of the deformation front, and the lateral propagation of thrust faults between the two regimes. The experimental models can be used to explain observed geometries in natural examples of fold-thrust belts marked by transitions between frictional and ductile detachments.     

A simplified 3 D.O.F. model of A FEM model for seismic analysis of a silo containing elastic material accounting for soil–structure interaction

The purpose of this study is the evaluation of dynamic behavior induced by seismic activity on a silo system, containing bulk material, with a soil foundation. The interaction effects between the silo and bulk material, as well as the effects produced between the foundation of the silo and the soil, were taken into account. Proposed simplified approximation, as well as the finite model, were used for analysis. The results, from the presented approximation, were compared with a more rigorous obtainment method. Initially, the produced simplified approximation, with elastic material assumption for the grain, could determine the pressures on the dynamic material along with displacements along the height of the silo wall and base shear force, etc., with remarkable precision. Some comparisons, via a change of soil and/or foundation conditions, were also made regarding the seismic pressure of the dynamic material pressure, displacement and base shear forces for both squat and slender silos. Comparing the analytical predictions to results from the numerical simulations produced good results. It can be concluded that the model can be used effectively to perform a broad suite of parametric studies, not only at the design stage but also as a reliable tool for predicting system behavior under the limit state of the system. The results and comprehensive analysis show that displacement effects and base shear forces generally decreased when soil was softer; however, soil structure interaction (SSI) did not have any considerable effects on squat silos and therefore need not be taken into practice.