Structural controls on orogenic gold mineralisation in the Klondike goldfield,Canada

The Klondike Schist that forms the basement rocks for the famous Klondike placer goldfield was emplaced as km-scale thrust slices in Early Jurassic time, along with some thin (10 to 30 m-scale) slices of greenstone and ultramafic rocks. Permian metamorphic fabrics in the schists were deformed during thrust emplacement by structures formed as the rocks passed through the brittle–ductile transition. Early-formed thrust-related structures were almost-pervasive recumbent folds that affected both the schist and greenstone/ultramafic slices and imposed a spaced cleavage with minor recrystallisation of micas. These structures gave way to shallow-dipping phacoidal cleavage near (within <100 m of) thrust structures. Thrust-related structures have been overprinted locally by well-defined steeply dipping reverse fault-fold zones, and associated upright folding on regional (km) to mesoscopic (m) scales. The fold-fault zones occur as two orthogonal sets of structures oriented NW to N and NE to E. Some of these steeply dipping fault zones have been reactivated by Late Cretaceous normal faulting. Orogenic (mesothermal) gold-bearing veins were emplaced in local sites of extension during or after formation of the compressional fault-fold zones and before normal fault reactivation. Over 400 veins (m to cm-scale) observed in this study imply a general NW strike for mineralised structures (W to N), but with a broad scatter of orientations. Vein emplacement was controlled principally by fold axial surfaces of kink folds of the fault-fold generation. However, some other local extension sites have opened along preexisting structures to host veins locally, including metamorphic foliation and spaced cleavage planes. In addition, irregular extensional fractures with no obvious structural control host some veins. The Klondike mineralised veins formed as swarms with broad regional structural control, but represent relatively diffuse mineralised zones, with numerous scattered small veins, compared to most orogenic vein systems. These diffuse vein swarms appear to be sufficient sources for the rich and geographically localised placer gold deposits that formed in overlying gravels during erosion of the Klondike Schist basement.     

Stick-slip model for kink band formation in shear zones and faults

Differential shear stresses acting along or adjacent to a non-planar fault surface or shear zone may cause uneven acceleration during slip. Alternatively, at the initiating and closing stages of motion of parts of a stick-slip fault, localised shear stresses may be variable. Stress variation of this nature causes zones of relative compression and tension, especially close to the “stick” zones on the fault. In fissile rocks adjacent to the fault, kink bands form in zones of local relative compression, while stratal extension features such as veins, fractures and extensional crenulations might be expected in the corresponding zones of relative tension. Repetitive motion on the fault should therefore cause the development of a suite of kink bands superimposed on each other and on any complementary extensional structures. Field evidence indicates that the extensional structures are not developed to the same extent as the kinks, perhaps due to ductile flow during layer-parallel extension of phyllosilicate rocks.

The advantages of this model are that it does not require bulk shortening of the shear zone relative to the enclosing less strained rocks, nor does it depend on complex stress orientation changes.     

渤海湾盆地隐性断裂带识别及其地质意义

隐性断裂带是区域或局部应力场、基底断裂体系活动、潜山块体扭动影响下在沉积盆地盖层中产生的断裂趋势带, 它是断裂带的一种类型, 与显性断裂伴生于沉积盆地, 但由于其不具有显性断裂的固有特征, 隐蔽性强而常常被忽略.可以通过以下7个方面有效识别隐性断裂带: ①小型显性构造(小断层、小褶皱、断块)呈雁列式、断续状、错位对称式反映的大型隐性断裂带; ②基底断裂体系活动在沉积盆地盖层中产生的隐性断裂带; ③潜山、凹陷分布形成的线状、调节型或者侧列式隐性断裂带; ④砂体分布反映的隐性断裂带; ⑤油藏排列、分布、走向反映的隐性断裂带; ⑥相干体切片显示的隐性断裂带; ⑦断层叠覆端、末端、深大断裂分段活动等局部应力场变化形成的隐性断裂带.应用上述方法开展渤海湾盆地隐性断裂带识别研究, 综合国内外的调研结果, 建立了隐性断裂带的类型划分方案体系并阐述了不同级别构造单元中隐性断裂带的类型特征.研究表明, 隐性断裂带具有多方面的地质意义, 可以作为调节构造带调节盆地不均匀伸展活动, 分隔凹陷、隆起等构造单元; 控制盆地沉积相带发育分布, 影响断陷湖盆砂体的展布范围; 控制圈闭组合排列、改造输导体系, 形成串珠状、带状油气富集区; 深断裂活动触发地震的过程中会在地表形成隐性断裂带, 因此, 隐性断裂带的识别和分析具有现实意义.      

Study of the brittle fracture process under uniaxial compression

Static friction along inclined cracks in photoelastic models increases with both the loading and frictional displacement before the larger crack-wall asperities are broken or completely over-ridden. Elastic shocks resulting from successive stick-slip during this stage can be repeated more or less indefinitely with repeated loading and unloading. Locked-in residual stresses, especially around crack tips, result when the model is unloaded, because of frictional coupling between crack walls. Favorable crack arrays for initial growth in the model include particular sets of en-echelon cracks inclined 45° to the stress axis, but the critical orientation may be smaller in brittle rock if crack-wall friction in rock is greater than in the model. Axial growth of en-echelon cleavage cracks, inclined at angles smaller than 45°, was observed in feldspar during deformation of a pegmatite having a mineralogy and texture similar to granite. Their growth follows predictions derived from photoelastic model studies.Crack growth in the pegmatite begins between half and two-thirds of the ultimate strength. The first flaws to grow, however, also include pre-existing axially oriented cleavage cracks in the feldspar. Crack growth occurs randomly throughout the specimen as stress is increased, without much evidence that grain boundaries are activated for crack growth. But when the applied stress approaches the ultimate strength, two new features are observed. There is an abrupt development of finite frictional slip along favorably inclined flaws and grain boundaries, beginning with displacements of the order of the dimensions of grain-boundary asperities. Crack growth still occurs at various locations throughout the specimen at this stage, but there is also a detectable concentration of growth along potential shear zones. Flaw-wall friction appears to be one of the critical factors that determine the pegmatite's ultimate strength and the instability of through-going fracture.     

Shear zone deformation determined from sigmoidal tension gashes

The potential of using sigmoidal tension gashes as strain markers for assessing strain localisation in shear zones is discussed. The appropriate analytical methods for this purpose depend on the assumed mechanism of tension gash formation. Two such models are considered. The first is one in which the curvature of the gash is produced by passive rotation of different segments of the gash in response shear strain gradients across the shear zone. The other model is one in which the curvature of the gashes is governed by the folding of the competent rock bridges between adjacent gashes. In the latter case, the tension gashes progressively grow within spaces created by the buckling bridges and therefore lead to a bulk dilatation of the shear zone. However, for the folded bridge mechanism to continue to operate beyond shear strains greater than unity requires a significant volume loss which in turn may signal the increase of the shear strength of the zone. The geometrical characteristics of gash arrays resulting from these two mechanisms are described and criteria given for the recognition of the two types of gash arrays. A new graphical method is proposed for the analysis of deformation in shear zones containing folded-bridge tension gashes. Tension gash arrays from Marloes, West Wales are used as examples of the procedures for shear zone analysis.     

Role of shear along horizontal plane in the formation of helicoidal structures

An unusual structural paragenesis, complicated by brachyanticlines, is revealed for the first time in the sedimentary cover of the West Siberian Plate by 3D seismic surveying. These are linear (in plan view) systems of en-echelon arranged low-amplitude normal faults related to wrench faults in the basement. On different sides off a wrench fault, the planes of normal faults dip in opposite directions, forming a helicoidal structure that resembles the blades of a propeller. In the section parallel to the wrench fault, the boundaries of the beds and normal fault planes dip in opposite directions as well. In the section across the strike of the normal faults converging toward the basement, the beds take the shape of an antiform with a crest sagged along the normal faults (flower structure). This structural assembly was formed as a result of interference of stress fields of horizontal shear in the vertical plane (induced by faulting in the basement) and in the horizontal plane (caused by gravity resistance of the cover). In this case, the displacements along the normal faults develop in both the vertical and, to a greater extent, horizontal directions, so that the faults in cover are actually characterized by normal-strike-slip kinematics. The regional N-S-trending compression of the West Siberian Plate is the main cause of shearing along the NW- and NE-trending faults in the basement, which make up a rhomb-shaped system in plan view. Petroliferous brachyanticlines, whose axes, notwithstanding tectonophysical laws, are oriented in the direction close to the maximum compression axis, are known in the large wrench fault zones of Western Siberia. Our experiments with equivalent materials showed that a local stress field arising at the ends of echeloned Riedel shears within a wrench fault zone may be a cause of the formation of such brachyanticlines. The progressive elongation of Riedel shears leads to the corresponding elongation of the brachyanticlines located between their ends. The performed study has shown that the known types of interference of elementary geodynamic settings such as horizontal shear along the vertical plane + horizontal compression (transpression) and horizontal shear along the vertical plane + horizontal extension (transtension) may be supplemented by combination of horizontal shears along the vertical and horizontal planes, resulting in tectonic lamination. By analogy, we propose to name this type of interference of elementary shear settings translamination. Petroliferous helicoidal structures arise in the given geodynamic setting of translamination.     

The geometry and kinematics of a suite of conjugate kink bands, southeastern Australia

A conjugate set of subvertical kink bands is exposed in coastal outcrops of well-foliated Ordovician turbidites near Mystery Bay, Australia. All kink bands with widths exceeding 3 cm have complex internal structures including compound and parasitic kinks, stepped kink boundaries, internal crenulations, variable kink angles and prismatic voids. The kink bands are interpreted to result from rotation of short foliation segments between fixed kink planes with subsequent widening and modification by layer-parallel shear external to the kink band. Layer-parallel shear of both sinistral and dextral sense accompanied kinking and indicates a variable stress system during kink band development.Conjugate kink bands are abundant and are used to estimate bulk strain orientations. In general, the dominant kink set of a conjugate pair is inclined at a lower angle to the external foliation than the weaker set and this angular disparity increases with increasing dominance of one set. These observations are at variance with relationships described from experimental bulk pure shear deformation of anisotropic materials. It is suggested that orthogonal constraints in these experiments restrict layer-parallel shear to within a developing kink band and are, therefore, unlike many natural kink systems. Simple shear experiments can produce structures geometrically similar to natural kink bands.     

The development of orientated symplectites during deformation

Abstract Orientated symplectites have been observed in deformed granulite facies metabasic rocks from the Ivrea-Verbano zone in northern Italy. The area underwent lower crustal extension, accommodated by movement on localized high- T shear zones. In areas of relatively low strain, such as at the margins of shear zones, symplectites of orthopyroxene, plagioclase and spinel have formed. The symplectites are vermiform and orientated parallel to the main foliation and in the regional stretching direction. The reaction was synkinematic with the deformation, and only developed in potentially dilatant grain boundaries in the rock. It was presumably inhibited in grain boundaries subjected to higher normal stress due to the relatively large volume increase involved in the reaction.
The observations support the interpretation that the deformation was related to regional extension under high- T granulite facies conditions, the symplectites forming as a result of decreasing pressure.     

张扭性盆地隐性断裂带识别、演化及控藏作用——以苏北盆地金湖凹陷为例

隐性断裂带是由区域或局部应力场或基底断裂活动影响下, 在凹陷沉积盖层中产生的断裂趋势带。重磁、三维地震资料处理解释成果证实苏北盆地金湖凹陷存在北东、北西两组基底断裂。部分北东向的基底断裂活动强烈, 控制了凹陷形成和演化; 北西向和部分北东向的基底断裂活动性较弱, 沉积盖层中形成了隐性断裂带。它们表现为成带分布的雁列式小断层、断断续续沿固定方向分布的小断层、一系列沉积扇体或油气圈闭成带成串分布等线状构造。随机测线的剖面上表现为地震同相轴没有明显错断或呈现杂乱反射。除上述两组隐性断裂带, 区域右行应力场在凹陷内部产生的东西方向的挤压应力分量也形成了断续状、串状的南北向隐性断裂带。研究表明, 隐性断裂带由"隐性"逐渐向"显性"过渡。经历了早期弱雁列式隐性期、早中期强雁列式隐性期、中期断续状隐-显期、中后期串状显-隐期4个隐性阶段, 最终演化为"显性"张扭性走滑断裂。沉积盖层中形成的这些隐性断裂带控制了储集砂体分布、改善了储层物性、使隐性圈闭成带成串分布, 是油气聚集成藏的有利区带。     

Kinematic analysis of an en échelon—continuous vein complex

An array of sigmoidal tension gashes from the Idaho—Wyoming thrust belt changes laterally into a continuous vein. Detailed mechanical twin analysis was used to determine the strain variation in the optically and chemically homogeneous blocky calcite filling. In the continuous portion of the vein complex, the shortening axes are parallel to the vein boundary. However, the orientation of the shortening axes in the tip areas of the sigmoidal gashes are at an angle of approximately 35° to the vein boundary, and are parallel to the trend of the tips. Twinning patterns in the central portions of the gashes record two principal strain axes of shortening of nearly equal magnitude with the maximum perpendicular to the vein trend. Everywhere in the vein complex the orientation of the maximum extension axis is parallel to the twist axis of the gashes. The petrofabric strain results show that the vein filling has largely recorded local strains. The pattern of variation in orientation of the principal strains in the vein complex is in close agreement with the theoretically determined stress distribution in similar structures. Our results show that the sigmoidal gashes were formed at the leading edge of a propagating vein and that the sigmoidal shape reflects changes in the local strain field rather than a remote shear. The orientation of these local strains closely corresponds to the orientation of the local stresses.     

Centrifuge modelling of plutons intruding shear zones: application to the Fürstenstein Intrusive Complex (Bavarian Forest,Germany)

Models consisting of a thick overburden resting on a buoyant layer were sheared and centrifruged in order to study the relationship between strike-slip shear zones and intrusions of buoyant material. Three experiments were carried out: In model 1, where the overburden consisted of a viscous material, no diapirs formed even after shearing for 40 mm (? = -1.07) and 27 min centrifuging. In models 2 and 3, where the overburden was semi-brittle, prescribed cuts at two different orientations (model 2: parallel to s1; model 3: perpendicular to s1) were initiated in the overburden in order to see whether such cuts acted as pathways for intrusion. In model 2 the prescribed cuts were used by the buoyant material as pathways when the cuts opened during shearing. Continued shearing widened the cuts and allowed the buoyant material to extrude on the surface of the model forming a coalesced elliptical sheet. In model 3. the cuts were closed during shearing and prevented the intrusion of the buoyant material. During further shearing, the cuts rotated and activated as strike-slip faults bounding pull-apart basins. Such pull-apart basins were not deep enough to tap the buoyant material. Nevertheless, the results of the experiments suggest that magma ascends in shear zones not as diapirs, but rises along preexisting pathways as dykes. Model results were used to evaluate emplacement of the Fürstenstein Intrusive Complex (FIC) in the Bavarian Forest, whose magnetic and structural inventory have been investigated in detail. The pluton consists of 5 magma batches, each with distinct magnetic fabrics. which are interpreted as the result of magma intrusion along opening and rotating tension gashes within the BPSZ stress field. Shear failure of the crust in the FIC area due to thermomechanical weakening provided the space for the emplacement of the last and biggest granite magma batch. Overall, the emplacement history of the FIC fits perfectly with the observations made during experiment 2 and indicates that magma ascent in shear zones is bound to tension gashes.     

The contemporary state of stress and strain at the western pericline of the Greater Caucasus

We collected materials on geological indicators of paleostresses at the western pericline of the Greater Caucasus mega-anticlinorium and within the large transverse flexure-fault zone (Anapa and Dzhiginka zones) limiting this mega-anticlinorium. Based on the data, we reconstructed local stress states in different tectonic zones. The reconstructed local stresses showed a considerable variation of the orientations axes of principal stress near the two zones. In a site adjacent to the flexure-fault zone and located near the western pericline of the Greater Caucasus mega-anticlinorium, the detachment systems of northeastern (NE–SW) strike are determined. Additionally, field structural studies proved elongation in the northwestern (NW–SE) direction. This was also verified by the reconstruction of orientations of minimum compression stress axes (maximum deviatory tension) implemented by cataclastic analysis of structural–kinematic information on the movements of the fault planes (tectonic cracks and minor ruptures). We found a well-expressed multistage regime of the northwestern (NW–SE) tension within the limits of the Semisam anticline. Tension deformations (along the axis of the main folded structure) are manifested in structures of different scales; the values of relative elongation are defined for some of them. At the western pericline of the Greater Caucasus mega-anticlinorium, in the Miocene deposits, a north–south (NNW) compression regime with steep inclinations of axes of maximum compression stresses was identified. In the boundary zone between the Northwestern Caucasus and transverse Kerch–Taman trough, an alteration of the orientations of main axes of normal stresses was found. These changes led to the replacement of horizontal-compression and horizontalshear (with a NE-oriented compression) settings, which are predominant in the Caucasus, with settings of horizontal tension (with steep NNW-oriented compression axes).     

Jumps in deformation gradients and particle velocities across propagating coherent boundaries

Grain boundaries and axial surfaces of folds are geological examples of thin transition zones, which can be modelled mathematically as surfaces of discontinuity. Here we consider coherent boundaries, across which no material lines are ever discontinuous, but deformation gradients and particle velocities can be so. A discontinuity in particle velocity requires that a coherent boundary propagate (migrate) through the material.The geometry and kinematics of coherent boundaries were studied by Hadamard (1903) and reviewed by Truesdell and Toupin (1960). Here, we rederive their results in a geological context, using modern notation and simple coordinates. We conclude that velocity gradient, stretching and spin can all be discontinuous. Applied to ideal kink bands in non-dilatant foliated materials, the theory predicts that kink boundaries rotate through the material at exactly the rate needed to maintain fold symmetry. In a polycrystalline aggregate, easily migrating coherent grain boundaries are mechanically analogous to slippery incoherent boundaries: both kinds support little shear stress.     

Mesozoic extension in the Basque–Cantabrian basin (N Spain): Contributions from AMS and brittle mesostructures

In this work we analyse and check the results of anisotropy of magnetic susceptibility (AMS) by means of a comparison with palaeostress orientations obtained from the analysis of brittle mesostructures in the Cabuérniga Cretaceous basin, located in the western end of the Basque–Cantabrian basin, North Spain. The AMS data refer to 23 sites including Triassic red beds, Jurassic and Lower Cretaceous limestones, sandstones and shales. These deposits are weakly deformed, and represent the syn-rift sequence linked to basins formed during the Mesozoic and later inverted during the Pyrenean compression. The observed magnetic fabrics are typical of early stages of deformation, and show oblate, triaxial and prolate magnetic ellipsoids. The magnetic fabric seems to be related to a tectonic overprint of an original, compaction, sedimentary fabric. Most sites display a NE–SW magnetic lineation that is interpreted to represent the stretching direction of the Early Cretaceous extensional stage of the basin, without recording of the Tertiary compressional events, except for sites with compression-related cleavage.Brittle mesostructures include normal faults, calcite and quartz tension gashes and joints, related to the extensional stage. The results obtained from joints and tension gashes show a dominant N–S to NE–SW, and secondary NW–SE, extension direction. Paleostresses obtained from fault analysis (Right Dihedra and stress inversion methods) indicate NW–SE to E–W, and N–S extension direction. The results obtained from brittle mesostructures show a complex pattern resulting from the superposition of several tectonic processes during the Mesozoic, linked to the tectonic activity related to the opening of the Bay of Biscay during the Early Cretaceous. This work shows the potential in using AMS analysis in inverted basins to unravel its previous extensional history when the magnetic fabric is not expected to be modified by subsequent deformational events. Brittle mesostructure analysis seems to be more sensitive to far-field stress conditions and record longer time spans, whereas AMS records deformation on the near distance, during shorter intervals of time.     

The relationship between regional stress field,fracture orientation and depth of weathering and implications for groundwater prospecting in crystalline rocks

In a uniform granite gneiss study area in central Zimbabwe, lineaments oriented parallel to the maximum regional compressive stress orientation exhibit the thickest regolith development, while lineaments oriented perpendicular to the maximum compressive stress show the shallowest development of weathered regolith. The principal fracture set orientations were mapped using aerial imagery. The regional stress field, estimated from global stress maps, was used to determine the stresses acting on each principal lineament orientation. Multi-electrode resistivity profiling was carried out across fractures with different orientations to determine their subsurface regolith conditions. The results indicate that the 360 and 060° lineaments, which are sub-parallel to the principal compressive stress orientation (σ₁) exhibit maximum development of the regolith, while 130° lineaments perpendicular to σ₁ do not exhibit significant regolith development. Since regolith thickness has been positively correlated with groundwater resources, it is suggested that fractures with orientations sub-parallel to the principal compressive stress direction constitute favourable groundwater targets. Knowledge of the regional stress field and fracture set orientations can be used as an effective low cost tool for locating potentially higher yielding boreholes in crystalline rock terrains.     

Passive kimberlite intrusion into actively dilating dyke–fracture arrays: evidence from fibrous calcite veins and extensional fracture cleavage

Calcite veins are invariably associated with en-echelon kimberlite dyke–fracture arrays. A detailed microstructural study of veining indicates four vein types. Type I stretched or ataxial veins are defined by high aspect ratio calcite fibers that are crystallographically continuous with calcite of the kimberlite matrix wall rock, by elongated phenocrystic phlogopite with sharp crystal terminations centered on contacts between adjacent calcite fibers and by phenocrystic phlogopite that grows or extends across these veins. Type I vein mineralogy indicates syn-dilational crystallization of vein minerals in local tensional areas within the kimberlite. Vein Types II (stretched to syntaxial elongate-blocky) and III (antitaxial) indicate late crystallization vein mineral growth during subsequent or repeated dilation. Calcite fibers in Type I to Type III veins are orthogonal to the contacts of their host dykes regardless of the orientation of vein margins. Type IV calcite veins, with blocky or mosaic/polycrystalline textures, are attributed to minor post-intrusion extension, which was potentially accompanied by repeated kimberlite intrusion within a given dyke array. Syn-crystallization/syn-intrusion Type I veins and an ubiquitous dyke-parallel fracture cleavage, in a zone up to 4 m on either side of dyke contacts, suggest that en-echelon kimberlite dyke–fracture arrays occupied the approximate center of zones of active dilation within the brittle carapace of the upper crust. Type II and III veins indicate that extension or dilation continued, independently of an occupying kimberlite fluid phase, after initial intrusion. Arrested mobile hydrofracturing, under low differential stress within the upper brittle or seismic carapace of the continental crust, followed by repeated dilation of the dyke–fracture system, is proposed as a mechanism for producing the features observed in this study. The conditions constrained in this study indicate passive dyke intrusion into dilating fracture arrays during crustal extension.     

Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)

The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones ₁) to steep inclination (shear zones ₂). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.     

A theory of concentric, kink and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers: VI. Asymmetric folding and monoclinal kinking

One of the rules of thumb of structural geology is that drag folds, or minor asymmetric folds, reflect the sense of layer-parallel shear during folding of an area. According to this rule, right-lateral, layer-parallel shear is accompanied by clockwise rotation of marker surfaces and left-lateral by counterclockwise rotation. By using this rule of thumb, one is supposed to be able to examine small asymmetric folds in an outcrop and to infer the direction of axes of major folds relative to the position of the outcrop. Such inferences, however, can be misleading. Theoretical and experimental analyses of elastic multilayers show that symmetric sinusoidal folds first develop in the multilayers, if the rheological and dimensional properties favor the development of sinusoidal folds rather than kink folds, and that the folded layers will then behave much as passive markers during layerparallel shear and thus will follow the rule of thumb of drag folding. The analyses indicate, however, that multilayers whose properties favor the development of kink folds can produce monoclinal kink folds with a sense of asymmetry opposite to that predicted by the rule of thumb. Therefore, the asymmetry of folds can be an ambiguous indicator of the sense of shear.The reason for the ambiguity is that asymmetry is a result of two processes that can produce diametrically opposed results. The deformation of foliation surfaces and axial planes in a passive manner is the pure or end-member form of one process. The result of the passive deformation of fold forms is the drag fold in which the steepness of limbs and the tilt of axial planes relative to nonfolded layering are in accord with the rule of thumb.The end-member form of a second process, however, produces the opposite geometric relationships. This process involves yielding and buckling instabilities of layers with contact strength and can result in monoclinal kink bands. Right-lateral, layer-parallel shear stress produces left-lateral monoclinal kink bands and left-lateral shear stress produces right-lateral monoclinal kink bands. Actual folds do not behave as either of these ideal end members, and it is for this reason that the interpretation of the sense of layer-parallel shear stress relative to the asymmetry of folds can be ambiguous.Kink folding of a multilayer with contact strength theoretically is a result of both buckling and yielding instabilities. The theory indicates that inclination of the direction of maximum compression to layering favors either left-lateral or right-lateral kinking, and that one can predict conditions under which monoclinal kink bands will develop in elastic or elastic—plastic layers. Further, the first criterion of kink and sinusoidal folding developed in Part IV remains valid if we replace the contact shear strength with the difference between the shear strength and the initial layer-parallel shear stress.Kink folds theoretically can initiate only in layers inclined at angles less than to the direction of maximum compression. Here φ is the angle of internal friction of contacts. For higher angles of layering, slippage is stable so that the result is layer-parallel slippage rather than kink folding.The theory also provides estimates of locking angles of kink bands relative to the direction of maximum compression. The maximum locking angle between layering in a nondilating kink band and the direction of maximum compression is . The theory indicates that the inclination of the boundaries of kink bands is determined by many factors, including the contact strength between layers, the ratio of principal stresses, the thickening or thinning of layers, that is, the dilitation, within the kink band, and the orientation of the principal stresses relative to layering. If there is no dilitation within the kink band, the minimum inclination of the boundaries of the band is to the direction of maximum compression, or to the direction of nonfolded layers. Here α is the angle between the direction of maximum compression and the nonfolded layers. It is positive if clockwise.Analysis of processes in terminal regions of propagating kink bands in multilayers with frictional contact strength indicates that an essential process is dilitation, which decreases the normal stress, thereby allowing slippage and buckling even though slopes of layers are low there.     

Linking stress field deflection to basement structures in southern Ontario: Results from numerical modelling

Analysis of stress measurement data from the near-surface to crustal depths in southern Ontario show a misalignment between the direction of tectonic loading and the orientation of the major horizontal principal stress. The compressive stress field instead appears to be oriented sub-parallel to the major terrane boundaries such as the Grenville Front, the Central Metasedimentary Belt boundary zone and the Elzevir Frontenac boundary zone. This suggests that the stress field has been modified by these deep crustal scale deformation zones. In order to test this hypothesis, a geomechanical model was constructed using the three-dimensional discontinuum stress analysis code 3DEC. The model consists of a 45 km thick crust of southern Ontario in which the major crustal scale deformation zones are represented as discrete faults. Lateral velocity boundary conditions were applied to the sides of the model in the direction of tectonic loading in order to generate the horizontal compressive stress field. Modelling results show that for low strength (low friction angle and cohesion), fault slip causes the stress field to rotate toward the strike of the faults, consistent with the observed direction of misalignment with the tectonic loading direction. Observed distortions to the regional stress field may be explained by this relatively simple mechanism of slip on deep first-order structures in response to the neotectonic driving forces.     

华北陆块北缘东段夹皮沟金矿带的剪切脉系

吉林夹皮沟金矿带中所发育的含金脉系,无一例外地赋存于规模不等、方向不同的剪切带之中。研究表明,由于区内NW向主剪切带的长时期右行走滑,诱发两侧岩块(尤其是东侧上盘的夹皮沟花岗-绿岩地体)中形成大量的次一级剪切褶皱群和剪切脉系,并产生强烈的渐进应变。区域右行走滑控制脉系的基本形态和构造方位,渐进应变则制约脉系内部的构造细节;据此可以对夹皮沟金矿带内次一级剪切脉系的几何方位和发育规律进行鉴别。属于R和D的含矿剪切裂隙在两维的延展上较为稳定;而T属于水平单剪体制下形成的张性裂隙,如果它生成后没有进一步叠加剪切应变的话,理论上其三维延展要比R和D差一些。研究不同力学性质的含矿裂隙的发育规律,在找矿勘探和成矿预测中具有重要的实际应用价值。