Fractals, chaos, self-organized criticality and tectonics

Some geological structures have simple geometrical forms and can be analysed using deterministic equations. Examples include alluvial fans and many sedimentary basins. But most geological structures are complex and appear to defy mathematical analyses. Yet in the complexity there is an order. Complex geological structures generally obey fractal statistics. Examples include topography, distributions of earthquakes and faults, and mineral deposits. An unresolved question is whether the fractal order is simply the result of scale invariance or the result of governing equations that yield deterministic chaos. In order to try to answer this question a variety of slider-block models have been considered. The stick-slip behaviour of slider-block models is a simple analogy to earthquakes. A pair of slider-blocks has been shown to behave chaotically. Models that use many slider-blocks exhibit self-organized criticality and generate fractal statistics similar to the statistics of regional seismicity.     

Modelling of segment structures: Boudins, bone-boudins, mullions and related single- and multiphase deformation features

Finite element modelling has been used to simulate the development of segment structures, deformed layer segments separated by veins, such as boudins, mullions, and bone-boudins. A parameter sensitivity analysis is used to compare the influence of the nature of the flow, the relative viscosities of veins in necks and the host rock, and the initial geometry of the layer segments. Parameter fields have been determined for the relative viscosity of veins and layers, and the kinematic vorticity number of flow. Reworked segment structures can have several shapes such as bone-, bulging, shortened bone-boudins and their asymmetric equivalents such as domino- and shearband-boudin geometry. The model for asymmetric reworked segment structures is applied to such features from the Lower Ugab Metaturbidites in NW Namibia. The model suggests that these structures form where the neck veins are stronger than the boudinaged layer, with a significant simple shear component of the bulk flow. The quartz filled necks in the Lower Ugab are therefore stronger than the quartz-rich wall rock in greenschist facies where the progressive deformation occurred. Bone-boudins are usually interpreted to form in transpressional flow, but simulations of the rotation of tension gashes show that they can also form in simple shear or slightly transtensional shear flow.     

大兴安岭北段伸展构造

本文对大兴安岭北段额尔古纳七卡地区发育的大型区域伸展滑脱构造开展了详细的研究,并系统地总结讨论了大兴安岭地区伸展构造及其与大兴安岭隆升的成因联系。查明七卡伸展构造带总体走向NE,滑脱面倾向NW,变形为具有减薄特征的简单剪切应变,发育SL构造岩,运动指向标志J显示向NW的伸展滑脱。结合区域上嫩江科洛伸展构造、嘎啦山伸展构造和磋岗伸展构造的对比研究表明,大兴安岭地区的伸展构造应该是区域伸展作用的产物,形成时间为早白垩世。随着大规模区域伸展作用,大兴安岭地区发生强烈的岩浆底侵,导致大兴安岭快速隆升,从而使其形成东侧向SE和西侧向NW方向滑脱的伸展构造。     

福建省德化邱村金矿床地质特征和找矿方向

福建省德化县肫村金矿床主要受构造断裂带控制,北东向断裂和近东西向断裂与成矿关系密切。主断裂既为导矿构造,也是容矿构造。单矿体定位于其主构造欠一级构造或次级构造与其它构造的交汇部位,形成富金矿包,闪长玢岩可作为该区主要的找矿标志。     

前陆褶皱冲断带冲起构造发育特征:基于砂箱构造物理模拟实验研究

冲起构造广泛发育于挤压逆冲构造体系和走滑构造体系,具有极其重要的油气勘探价值,因而备受关注。砂箱模拟模型已经被证明是强大的可视化工具,用来模拟不同岩层中复杂的构造现象。基于相似初始砂箱构造模型条件下不同挤压速率(0.3 mm/s、0.1 mm/s、0.005 mm/s)变形过程,揭示出褶皱冲断带发育过程中的典型两类冲起构造：叠加冲起构造和单一冲起构造,它们对于褶皱冲断带演化过程及其油气勘探具有明显不同的重要性。砂箱物理模拟实验揭示楔形体扩展变形序列对前陆褶皱冲断带冲起构造发育类别具有重要控制作用,即褶皱冲断带前缘以前展式扩展变形为主,主要发育单一冲起构造;后缘以后展式扩展变形为主,主要发育叠加冲起构造。     

东海西湖凹陷第三系反转构造及其对油气聚集的影响

东海西湖第三纪凹陷经历了早期裂陷和晚期挤压的构造作用,形成了现今规模宏大的反转构造。反转构造是先存的张性断裂后期受到挤压逆冲反转而形成的一种挤压构造与拉伸构造在垂向上叠加的复合构造。按反转强度的差异,本区的反转构造可划分为简单断展型和穿透断展型2种基本类型;按反转构造的几何样式,又可分为简单后冲反转、复合“Y”字型反转、“火”字型反转和花状反转等。凹陷内发育有东缘、中部、西斜坡边缘等3个反转构造带。沿中央凹陷带的反转作用最强,东缘次之,西斜坡边缘最弱。     

Influence of structural position on fracture networks in the Torridon Group,Achnashellach fold and thrust belt,NW Scotland

In fold-and-thrust belts rocks undergo deformation as fold geometries evolve. Deformation may be accommodated by brittle fracturing, which can vary depending on structural position. We use 2D forward modelling and 3D restorations to determine strain distributions throughout folds of the Achnashellach Culmination, Moine Thrust Belt, NW Scotland. Fracture data is taken from the Torridon Group; a thick, coarse grained fluviatile sandstone deposited during the Proterozoic. Modelling infers a correlation between strain and simple curvature; we use simple curvature to infer how structural position and strain control fracture attribute variations in a fold and thrust belt.In high curvature regions, such as forelimbs, fracture intensities are high and fractures are short and oriented parallel to fold hinges. In low curvature regions fractures have variable intensities and are longer. Fracture orientations in these regions are scattered and vary over short distances. These variations do not relate to strain; data suggests lithology may influence fracturing. The strain history of fold structures also influences fracturing; structures with longer deformation histories exhibit consistent fracture attributes due to moderate-high strain during folding, despite present day low curvature. This is in contrast to younger folds with similar curvatures but shorter deformation histories. We suggest in high strain regions fracturing is influenced by structural controls, whereas in low strain regions lithology becomes more important in influencing fracturing.     

A Raman spectroscopic study of glasses along the joins silica-calcium aluminate,silica-sodium aluminate,and silica-potassium aluminate

Aluminosilicate glasses with compositions along the joins silica-calcium aluminate, silica sodium aluminate and silica-potassium aluminate have been prepared by conventional and solar melting techniques and studied by Raman spectroscopy. The Raman spectra of crystalline calcium aluminate, anorthite and silica polymorphs are discussed in relation to their crystal structures, and compared with the spectra of the corresponding glasses. The glass and crystal spectra are generally comparable, suggesting similar vibrational structures. These crystals have structures based on tetrahedral aluminosilicate frameworks, and a similar molecular structure is suggested for the glasses, although it is noted that the Raman spectra do not directly characterize the aluminate polyhedra. Within the three glass series, our interpretation of the unresolved high-frequency bands shows the appearance of discrete bands near 1120, 1000, 930 and 890 cm^?1 as the silica content is decreased. This is compared with the behaviour of high-frequency bands in simple silicate systems, and used to suggest that the four bands in the aluminosilicate systems are due to stretching vibrations of silicate tetrahedra bound to one, two, three and four aluminium atoms. The spectra of calcium, sodium, potassium and lithium aluminosilicate glasses with similar silica contents are compared, and interpreted by the above model. This is used to construct a simple model for the effect of metal cation on aluminosilicate molecular groups in the glass structure, consistent with the results of calorimetric studies on similar systems.     

沁水盆地胡底井田地质特征及煤层气赋存规律

沁水盆地由于其良好的储气条件,多年来一直是国内外煤层气学者的研究对象。胡底井田位于樊庄区块的中西部,通过对其地质特征和煤储层的各项特征研究,探讨了区内煤层气的赋存规律及影响因素,得到以下认识：本区构造简单,煤层较厚且变质程度高,吸附能力强,含气量大,封存条件好,煤层气资源蕴藏丰富;受褶曲构造影响,在井田中部含气量较低,由中部向西含气量逐渐增高,向东含气量先增大后减小,南北方向也呈现起伏性变化;煤层气含量与煤层埋深基本呈正相关变化;煤层埋藏史、水文地质及煤层封盖等条件使本区形成了良好的煤层气富集区。     

甘肃辛曲金矿床地质特征及找矿标志

甘肃辛曲金矿位于甘肃省大水金矿田的西北部,属构造蚀变岩型金矿床。矿体受NNE向张扭性断裂控制,与印支期岩浆活动及脉岩有时空成因联系,具有矿体大,矿化均匀,物质组分简单的特征。文章在总结了矿床的地质背景、矿化类型、特征及富集规律的基础上提出了找矿标志。     

山门银矿近矿围岩蚀变特征

山门银矿床围岩蚀变呈面型及线型分布,受岩性及构造控制。蚀变作用具有多期、多阶段性,蚀变矿物组合在时间上表现由复杂到简单,由高温到低温的演化特点,在空间上具有明显的分带现象。矿化与硅化、黄铁矿化、伊利石—绢云母化极为密切。文中探讨了不同蚀变岩段蚀变岩带化学成分变化、元素的带入以及微量元素,成矿元素由红化带到硅化带逐渐富集规律。     

盘县马依东二井田构造特征及成因分析

利用井田勘探资料及野外地质资料,对井田构造基本形态、断层发育特征及其分布规律进行分析,总结出该井田的构造模式、构造成因、构造组合特征及其演化规律。研究认为井田的构造格局是多期构造运动综合作用、相互叠加的结果。井田构造形态为一略有起伏的平缓单斜,区内NE向的高角度正断层,多属张扭性,EW向构造早于NE向构造,构造复合部位较复杂,其余区段较简单;伸展构造是井田构造的主体样式,这一结论对下一步继续勘探及煤矿开采具有指导意义。     

京、津、唐地区中元古代沉积型海泡石粘土矿床

京、津、唐地区沉积型海泡石粘土矿分布广泛。含矿岩系为蓟县系雾迷山组富镁碳酸盐岩,区域构造简单,岩浆活动微弱。研究成果表明:本区海泡石粘土矿矿点多,储量丰富,矿层厚度大,埋藏浅,易开采。矿石矿物组合简单,化学成分富镁钙低硅,物理-化学性能优良。经选矿试验海泡石粘土矿可选性良好,回收率高,成本低廉,开发前景可观。      

Vectorial Formulation of Direction Cosines for Anisotropic Geologic Structures from Their Geologic Angle Measurements

On the basis of vectorial formulation, a series of relatively simple closed-form mathematical equations is presented to determine the direction cosines of the local coordinate axes associated with planar and linear structures in an anisotropic geologic material from their geologic angle measurements (i.e., strike and dip for planar structures and trend and plunge for linear structures). The equations are then applied to calculation of global anisotropic hydraulic conductivity and elastic stiffness tensors with respect to the geographical coordinate axes from their local counterparts with respect to the material symmetry axes.     

Stromatolites of the lower Missoula Group (Middle Proterozoic), Belt Supergroup,Glacier National Park,Montana

The mode of formation and environmental setting of stromatolites from the lower Missoula Group (ca. 1.1·10⁹ years old) in Glacier National Park, Montana, have been determined. The stromatolite-bearing interval in the lower Missoula Group was deposited in a shallow, intermittently exposed setting of very low relief, the stromatolites forming during periods of submergence. In situ carbonate precipitation was the dominant process involved in the formation of encrusting stromatolitic laminae. This precipitate was deposited within, and probably beneath, algal mats, most likely as a result of the photosynthetic removal of carbon dioxide by the mat-building microscopic algae. Calcite also was precipitated in several types of open-space structures occurring within these stromatolites. Other laminae were produced by the organic stabilization of detrital particles; by the solely physical accumulation of terrigenous material; and probably, by bacterially induced precipitation of iron sulfide which was later oxidized to form hematite layers.Three forms of filamentous microfossils, two of which appear to be oscillatoriacean cyanophytes and the third of which is probably either a cyanophyte or filamentous bacterium, have been detected in these structures. In addition, hematitic pillar-shaped microstructures, interpreted to have been produced by filamentous bacteria, are abundant locally.In gross morphology, the lower Missoula Group stromatolites are simple, unbranched, domal structures ranging from several millimeters to several decimeters in both height and diameter. Physical conditions played a major role in determining the macrostructure of these stromatolites. Of particular importance were the shape of the positive sediment-surface irregularities upon which the stromatolites initially formed, the rate of sedimentation between stromatolite colonies, and the deposition of layers of terrigenous material on stromatolite growth surfaces. The effect of biological factors on stromatolite structure is clearly seen in those portions of stromatolites relatively free of terrigenous material; biological activity was apparently greatest on positive irregularities of the growth surface, resulting in preferential enhancement of such irregularities and development of second- and higher-order hemispheroidal structures.     

Slump folds and gravity slide structures in a Lower Palaeozoic marginal basin sequence (the Skiddaw group), NW England

Major folds with associated thrust faults and a major olistostrome are described from the Skiddaw Group, a Lower Ordovician turbidite sequence in the Lake District of NW England. The style and geometry of the structures are shown to be compatible with their generation as submarine slumps or slide masses although they are much larger than any slump structures hitherto described from Britain. The predominant strain is shown to be simple shear. Spatial and temporal variations in strain permit a developmental model to be erected. The opposing vergence of the structures across the mapped area indicates a relatively narrow, probably fault controlled, depositional basin.     

Application of kinematic vorticity and gold mineralization for the wall rock alterations of shear zone at Dungash gold mining,Central Eastern Desert,Egypt

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in wall rocks alterations of shear zone at Dungash gold mine. Furthermore, it shows the relationship between the gold mineralization and deformation and also detects the orientation of rigid objects during progressive deformation. The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Neoproterozoic basement rocks. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. The kinematic vorticity number for the metavolcanic and metasedimentary samples in the Dungash area range from 0.80 to 0.92, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. Furthermore, we conclude that disseminated mineralization, chloritization, carbonatization and silicification of the wall rocks are associated with fluids migrating along shearing, fracturing and foliation of the metamorphosed wall rocks.     

Progressive deformation across a ductile shear zone: an example from the Singhbhum Shear Zone,eastern India

Structural investigations in the Precambrian Singhbhum Shear Zone of eastern India document an intimate relationship between micro- to meso-scale structures and the deformation history. Shear zone rocks are characterized by composite foliation, a well-developed stretching lineation, folds, shear planes, and quartz veins. These structures reflect thrusting of the Proterozoic north Singhbhum hanging wall block over the Archaean south Singhbhum footwall block. Microstructural analysis of multiple foliation and mylonitic rocks within the shear zone helps to define its progressive evolution. During progressive deformation, overprinting of microstructures resulted in incomplete transposition or complete erasing of previously formed structures and mineral assemblages, allowing room for new dynamic equilibrium structures to form. The dominant deformation mechanism was dissolution–recrystallization, with locally important fluid circulation responsible for transformation of the quartzo-feldspathic mass into phyllonite, and quartzites and schists into mylonite. Textural features suggest that the bulk deformation was non-coaxial, evolving from dominant pure shear in the early stage followed by simple shear in a single progressive strain history of the Singhbhum Shear Zone.     

澜沧南角河银多金属矿围岩蚀变及成矿机制

南角河银多金属矿为破碎带蚀变岩型,赋存在花岗岩与澜沧群接触带,以断裂、裂隙等构造低压带中有利容矿空间以填沉淀方式,形成形态简单的多金属矿床。     

Late Proterozoic periglacial aeolian deposits on the West African Platform, Taoudeni Basin, western Mali

The Late Proterozoic Bakoye 3 Formation is a predominantly aeolian unit deposited in the glacially influenced cratonic Taoudeni Basin of western Africa. The Bakoye 3 can be divided into five distal units, two proximal units, and a local upper massive sandstone. The basal Unit 1 shows a complex interfingering of aeolian and subaqueous structures, and is interpreted as the precursor of the overlying erg sequences. Unit 2 consists of compound, trough cosets of aeolian cross-strata dominated by grain-flow strata. The unit is interpreted to represent draas with superimposed, small, crescentic dunes. A super bounding surface marks the termination and planation of the erg. Unit 3 is distinguished from the underlying Unit 2 by its larger, overall simple sets of trough cross-strata, interpreted to represent simple, large, crescentic dunes. Unit 4 occurs only locally in laterally discontinuous, large troughs. In one case the trough is filled by small sets of tabular cross-strata dominated by grain-flow deposits. At another section, wedges of coarse-grained wind-ripple strata fill the trough. Unit 4 may represent remnants of ergs or, more likely, local deposition in depressions. The depressions, in the latter scenario, formed with the development of a second super surface that truncates Unit 3. Unit 5 consists of very large sets of wind-ripple cross-strata with less common sets of grain-flow deposits. These deposits are believed to represent enormous dunes with large plinths and subordinate slip face development. A third super surface separates Unit 5 from overlying marine deposits. Together, Units 1–5 represent the core of the ergs in a distal position relative to adjacent upland source areas. Proximally, aeolian deposits are simple, smaller, trough sets interpreted as moderate sized crescentic dunes. Coarse-grained braided stream deposits are prominent. Locally, the top of the Bakoye 3 is marked by channelized mass-flow deposits containing aeolian blocks, and is believed to have resulted from iceberg grounding. An overall environment for the Bakoye 3 is one of uplands marked by ice sheets, with outwash plains extending distally to aeolian ergs. Super surfaces, all marked by polygonal fractures and coarsegrained sediment, represent periods of erg termination that may be linked to glacial-fluvial-aeolian cycles.