Duplex structures in the lewis thrust sheet, crowsnest pass, rocky mountains, Alberta, Canada

The Lewis thrust sheet of the southern Canadian Rocky Mountains contains many spectacular examples of small-scale duplex structures. This paper presents the results of a detailed analysis of such structures found in the Mississippian carbonates of the Banff Formation at Crowsnest Pass, southwestern Alberta.Foreland dipping, hinterland dipping and antiformal stacked duplexes are found in the hangingwall of the Lewis thrust. Out-of-sequence thrusts, back thrusts and folds that push out of the plane of the cross-section, termed lateral lobes, give rise to complex internal geometries. Dominant slip vectors are towards 080–090° but the complex fault geometries have generated significant variations in slip away from this direction. The duplex structures occur as discrete thrust fault-bounded packages with each package having different slip vectors. The panels above and below the duplex structures show consistent slip vectors towards 080–090° whereas the duplexes exhibit a wide scatter of slip vectors from 350–160°. The stacking of duplexes with many horses can be likened to the stacking of many inverted soup bowls, herein termed turtle back structures, and will involve a wide scatter of slip directions, particularly if the horses are of limited lateral extent. Such a stacking mechanism involving out-of-section movement invalidates the assumption of two-dimensional plane strain in the plane of the cross-section that contains the regional tectonic transport direction. Correctly balanced cross-sections cannot be constructed through such stacked duplex structures as described in this paper.     

Pressure Solution of Sulphides in Some Massive Sulphide Zinc—lead Deposits of Western Canada：Its Significance in Mobilization of Ore—forming Materials

Pressure solution is a common phenomenon in massive sulphide zinc-lead deposits of western Canada and may have been an important factor leading to the mobilization of ore-forming materials during diagenesis, deformation and metamorphism of sedimentary ores.The control of ductile shear zones over gold mineralization could be explained in view of pressure solution of gold-bearing miner-als under shearing stress and the tesultant mobilization of this metal.     

Pyrite deformation in stratiform lead-zinc deposits of the Canadian Cordillera

Pyrite textures in five stratiform lead-zinc deposits from lower to upper greenschist facies environment of the Canadian Cordillera are described and discussed in terms of deposition/early diagenesis, deformation, metamorphism and hydrothermal alteration processes. Overgrowth is an important process during both diagenesis and deformation. Diagenetic and deformational overgrowths can be distinguished. Diffusive mass transfer, involving pressure solution and oriented overgrowth of pyrite is the main deformation mechanism in pyrite deposits at low metamorphic grades. Although diffusive mass transfer favours fine-grained mineral aggregates, its effect on coarse pyrite grains has also been identified. Ore minerals dissolved by pressure solution may be transported, with the assistance of pore fluids within fractures and grain boundaries, over distances significantly greater than the scale of individual grains to give a range of pressure solution/overgrowth textures. The textural modification of pyritic ores from the early stages of diagnesis, through metamorphism and deformation, to post deformation thermal annealing, has important implications for the distribution of trace elements and isotopic compositions in pyritic ores.     

Single-domain magnetite in the Jimberlana Norite,Western Australia

The feeder series gabbros of the Jimberlana Norite possess a large, stable NRM. Koenigsberger ratios of 20 or greater suggest that single-domain grains carry most of the NRM. Coercivity spectra extend up to 2000 Oe. Volumes of magnetite satisfying single-domain requirements occur principally as intergrowths of host magnetite between exsolution phases. Small discrete magnetite grains do occur in the Ca-rich pyroxenes but are thought to contribute only a minor fraction of the NRM.     

块状硫化物矿石中硫化物的压溶和增生及成矿意义——以加拿大西部矿床为例

加拿大西部块状硫化物矿石普遍地发生过硫化物的压溶和增生。增生作用根据增生体的成分可以分为同质增生和异质增生，根据动力环境可以分为静态增生和动态增生。三晶嵌接结构可以是静态增生的产物。压溶和增生是块状硫化物矿床成岩和变质过程中的重要作用。脉石矿物的压溶可使原生矿石就地加富，硫化物的压溶可使成矿物质发生再活化。增生可促进矿质沉淀。富含硫化物的地层之所以能成为地球化学障而有利于后期热液叠加和层控矿床的形成，硫化物晶芽的增生是一种重要机制。     

Listric extensional fault systems - results of analogue model experiments

Abstract Analogue models are a powerful tool for investigating progressive deformation in extensional fault systems. This paper presents exciting new insights into the progressive evolution of hanging wall structures in listric extensional terranes. Analogue models, scaled to simulate deformation in a sedimentary sequence, were constructed for simple listric and ramp/flat listric extensional detachments. For each detachment geometry homogeneous sand, sand/mica and sand/clay models were used to simulate respectively, deformation of isotropic sediments, of anisotropic sediments and of sedimentary sequences with competency contrasts. Roll-over anticlines with geometrically necessary crestal collapse graben structures are characteristic of the steepening-upwards segments of listric extensional fault systems in all of our models. With progressive deformation, crestal collapse grabens show hanging wall nucleation of new faults. Variations in graben size, amount of fault rotation and throw, are dependent on detachment curvature and amount of extension. Individual faults and associated fault blocks may significantly change shape during extension. Complex and apparently conjugate fault arrays are the result of superposition of successive crestal collapse grabens. Ramp/flat listric extensional fault systems are characterized by a roll-over anticline and a crestal collapse graben system associated with each steepening-upwards segment of the detachment and a ramp zone consisting of a hanging wall syncline and a complex deformation zone with local reverse faults. The roll-over anticlines and crestal collapse graben are similar in geometry to those formed in simple listric extensional systems. The models demonstrate that the geometry of the detachments exerts a fundamental control on the evolution of hanging wall structures. Analysis of particle displacement paths for these experiments provides new insights into the mechanical development of roll-over anticlines. Two general models for deformation above simple listric and ramp/flat listric extensional detachments have been erected.     

Influence of shear angle on hangingwall deformation during tectonic inversion

Tectonic inversion is a common phenomenon in island arc settings, especially in back‐arc basins. The reactivation of normal faults as thrusts, triggered by tectonic inversion, produces typical inversion fault‐related folds and thrusts in the hangingwall. These hangingwall inversion geometries are affected by two factors: the geometry of the underlying master fault and the angle of inclined simple shear relative to the regional dip of strata, in the case that the deformation is approximated by simple shear. This study employed numerical simulations to analyse the influence of the antithetic shear angle on the geometry of the hangingwall and displacement along the master fault. The simulation results reveal that a steeply inclined shear vector during extension produces a narrow, steep‐sided half‐graben, whereas a gently inclined shear produces a wide, open basin. After tectonic inversion, a tight anticline is formed under steeply inclined shear, whereas an open anticline is formed under gently inclined shear. Antithetic shear results in reduced total displacement along the master fault, and the greater the angle between the shear direction and the regional dip, the greater the displacement along the master fault. Because the deformation geometry of syn‐extension layers is affected by extension followed by contraction, a change in the shear angle during tectonic inversion produces a wide variety of deformation geometries. Comparison of the simulation results with the results of analogue modelling suggests that the shear angle decreases by 5° during the transition from extension to tectonic inversion and that such a change may be commonly observed in natural geological structures. These results highlight the benefits of numerical simulations, which can be used to readily examine a variety of constraining parameters and thereby lead to a better understanding of the mechanism of hangingwall deformation, avoiding erroneous estimates of the amount of fault displacement.     

Structural evolution of the Orange Basin gravity-driven system,offshore Namibia

The Orange Basin records the development of the Late Jurassic to present day volcanic-rifted passive margin of Namibia. Regional extension is recorded by a Late Jurassic to Lower Cretaceous Syn-rift Megasequence, which is separated from a Cretaceous to present day post-rift Megasequence by the Late Hauterivian (ca. 130 Ma) break-up unconformity. The Late Cretaceous Post-rift evolution of the basin is characterized by episodic gravitational collapse of the margin. Gravitational collapse is recorded as a series of shale-detached gravity slide systems, consisting of an up-dip extensional domain that is linked to a down-dip zone of contraction domain along a thin basal detachment of Turonian age. The extensional domain is characterized by basinward-dipping listric faults that sole into the basal detachment. The contractional domain consists of landward-dipping listric faults and strongly asymmetric basinward-verging thrust-related folds. Growth stratal patterns suggest that the gravitational collapse of the margin was short-lived, spanning from the Coniacian (ca. 90 Ma) to the Santonian (ca. 83 Ma). Structural restorations of the main gravity-driven system show a lack of balance between up-dip extension (24 km) and down-dip shortening (16 km). Gravity sliding in the Namibian margin is interpreted to have occurred as a series of episodic short-lived gravity sliding between the Cenomanian (ca. 100 Ma) and the Campanian (ca. 80 Ma). Gravity sliding and spreading are interpreted to be the result of episodic cratonic uplift combined with differential thermal subsidence. Sliding may have also been favoured by the presence of an efficient detachment layer in Turonian source rocks.     

On the resonant modes of a cavity and the dynamical properties of micropulsations

Certain classes of micropulsations are customarily explained in terms of guided (toroidal) and isotropic (poloidal) hydromagnetic waves m the magnetosphere. The physical properties of these waves are not well understood and their utility in explaining observed polarization patterns is questionable. In an effort to understand and explain the physics underlying these modes, a study is made of a cylindrical cavity (the hydromagnetic wedge), filled with a plasma having a large but finite conductivity and magnetized by an azimuthal magnetic field. Coupling between the toroidal and poloidal modes is effected by the inclusion of the Hall current in the generalized Ohm's law. Physically meaningful solutions to the wave equation are obtained and the toroidal eigenfunctions are demonstrated to be non-degenerate and well-behaved throughout the configuration, and exhibit for each mode a unique spatial resonance whose location, given by a line of force, is specified by the corresponding eigenvalue. The non-degenerate, discrete and spatially independent eigenvalues for the modes are shown to obey a selection rule that limits the spectrum. For a given mode, the states of polarization of the transverse field are determined and it is shown (as has been observed) that, depending on the line of force singled out, the magnetic polarization may be linear, elliptical or circular, right or left-handed, and whatever the state, it is immutable along the line of force. More complicated polarization patterns are derived and explained by superposing different modes vectorially. Classical concepts such as guided and isotropic modes and vibrating field lines are reinterpreted and evaluated in terms of the model. To examine the dependence of modal amplitude on source, the amplitude is expressed in terms of a sinusoidal driving pressure for a simple steady-state case. Symmetries of the model and the magnetosphere are specified and the detailed numerical results are ‘scaled’ for plasmaspheric application. The resonant spectrum, encompassing pc 2–4, is described and the variation of period spectrum with magnetic latitude and activity is presented. The agreement between the semi-quantitative analysis and the observational results is sufficiently close to indicate that the basic physics of the model encompasses the fundamental dynamics of pc activity.     

Structural control on syn-rift sedimentation,northwestern Red Sea margin,Egypt

Detailed field mapping of a 120-km-long segment of the northwestern Red Sea margin reveals the existence of a number of sediment-input sites where Lower Miocene sediments form coarse-grained fan delta systems. Localizations of the fan deltas at the sediment-input sites (Wadis Gassus, Guesis, Quwyh and Sharm El Bahari) are controlled by structural and topographic elements, including relay ramps between interacting normal fault segments, cross-trend transfer faults, reactivated Precambrian basement fabrics and the plunge directions of tilted fault blocks. Sedimentary facies and geometrical characteristics of the fan deltas indicate that they are progradational and “Gilbert-type” fan delta systems. Variations in size and composition of the conglomerate clasts reflect the heterogeneous lithologies of the Precambrian basement and pre-rift strata in the source areas along the rift margin. Palaeocurrent data show marked changes in paleoflow directions, from northeast in the northernmost fans to east and southeast directions in the southern fans. This change occurs across the Duwi accommodation zone which formed a topographic high separating the oppositely dipping half-grabens in the study area. The data presented in this paper clearly demonstrates that the structural architecture of the northwestern Red Sea margin together with its related topographic expressions played a fundamental role in controlling the drainage network systems, sediment dispersal and localization of the fan deltas in this part of the rift system.