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.     

A theory of concentric, kink and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers: IV. Development of sinusoidal and kink folds in multilayers confined by rigid boundaries

This part concerns folding of elastic multilayers subjected to principal initial stresses parallel or normal to layering and to confinement by stiff or rigid boundaries. Both sinusoidal and reverse-kink folds can be produced in multilayers subjected to these conditions, depending primarily upon the conditions of contacts between layers. The initial fold pattern is always sinusoidal under these ideal conditions, but subsequent growth of the initial folds can change the pattern. For example, if contacts between layers cannot resist shear stress or if soft elastic interbeds provide uniform resistance to shear between stiff layers, sinusoidal folds of the Biot wavelength grow most rapidly with increased shortening. Further, the Biot waves become unstable as the folds grow and are transformed into concentric-like folds and finally into chevron folds. Comparison of results of the elementary and the linearized theories of elastic folding indicates that the elementary theory can accurately predict the Biot wavelength if the multilayers contain at least ten layers and if either the soft interbeds are at most about one-fifth as stiff as the stiff layers, or there is zero contact shear strength between layers.Multilayers subjected to the same conditions of loading and confinement as discussed above, can develop kink folds also. The kink fold can be explained in terms of a theory based on three assumptions: each stiff layer folds into the same form; kinking is a buckling phenomenon, and shear stress is required to overcome contact shear strength between layers and to produce slippage locally. The theory indicates that kink forms will tend to develop in multilayers with low but finite contact shear strength relative to the average shear modulus of the multilayer. Also, the larger the initial slopes and number of layers with contact shear strength, the more is the tendency for kink folds rather than sinusoidal folds to develop. The theoretical displacement form of a layer in a kink band is the superposition of a full sine wave, with a wavelength equal to the width of the kink band, and of a linear displacement profile. The resultant form resembles a one-half sine curve but it is significantly different from this curve. The width of the kink band may be greater or less than the Biot wavelength of sinusoidal folding in the multilayer, depending upon the magnitude of the contact shear strength relative to the average shear modulus. For example, in multilayers of homogeneous layers with contact strength, the Biot wavelength is zero so that the width of the kink band in such materials is always greater than the Biot wavelength. In general, the higher the contact strength, the narrower the kink band; for simple frictional contacts, the widths of kink bands decrease with increasing confinement normal to layers. Widths of kink bands theoretically depend upon a host of parameters — initial amplitude of Biot waves, number of layers, shear strength of contacts between layers, and thickness and modulus ratios of stiff-to-soft layers — therefore, widths of kink bands probably cannot be used readily to estimate properties of rocks containing kink bands. All these theoretical predictions are consistent with observations of natural and experimental kink folds of the reverse variety.Chevron folding and kink folding can be distinctly different phenomena according to the theory. Chevron folds typically form at cores of concentric-like folds; they rarely form at intersections of kink bands. In either case, they are similar folds that develop at a late stage in the folding process. Kink folds are more nearly akin to concentric-like folds than to chevron folds because kink folds form early, commonly before the sinusoidal folds are visible. Whereas concentric-like folds develop in response to higher-order effects near boundaries of a multilayer, kink folds typically initiate in response to higher-order shear, as at inflection points near mid-depth in low-amplitude, sinusoidal fold patterns. Chevron folding and kink folding are similar in elastic multilayers in that elastic “yielding” at hinges can produce rather sharp, angular forms.     

A theory of concentric, kink, and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers: I. Introduction

Most folds in stratified rock are similar in form to ideal kink, concentric or chevron folds, in which there are discontinuities in slope or curvature of bedding planes. In this respect most folds appear to be closely related to faults, traces of which can be considered to be lines across which there are discontinuities of displacement of layers. Further, the close association of reverse faults and folds or monoclinal flexures seems to indicate that theories of faulting and folding should be closely related.The theory of characteristics is a mathematical tool with which we can obtain insights into processes involving discontinuities. Theoretical characteristic lines are directions across which certain variables might be discontinuous and they are directions along which discontinuities propagate. The theory has been widely applied in plasticity theory and in fluid mechanics and theoretical studies of faulting have suggested that faults are analogous to the lines of discontinuity predicted by plasticity theory. Elasticity and viscosity theories, on which theories of folding have been founded, exclude the existence of characteristic lines in the materials unless the equilibrium equations, rheological properties or strains are nonlinear. However, all folding theories are nonlinear to some extent and the theories can be modified so that they predict lines of discontinuity for some conditions of loading and deformation.Theories of folding will be developed in subsequent papers of this series in order to predict conditions under which characteristic lines can exist in multilayered materials and in order to determine the conditions that must be satisfied across and along the characteristic lines. The theory should help us to recognize lines of apparent discontinuity in natural and experimental folds and study of these lines should provide further understanding of mechanisms of folding.Experimental studies of folding of a wide variety of materials, including alternating layers of rubber and gelatin, modeling clay and grease or graphite, and potter's clay and rubber or cardboard, suggest that the patterns of folding in these materials begin with sinusoidal forms, transform into concentric or kink forms and then into chevron forms as the multilayers are shortened axially. A suitable theory of folding of multilayers should account for these observations.     

A theory of concentric, kink and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers: VII. development of folds within Huasna Syncline, San Luis Obispo County, California

Folds in the Huasna area of the southern Coast Ranges of California provide an opportunity to study different fold forms and to estimate dimensional and relative rheological properties of rocks at the time of folding. Plunging, concentric-like and chevron-like folds with wavelengths ranging from about 0.1 to 1 km are clearly visible in natural exposures at the south end of the Huasna syncline, which has a wavelength of 12–16 km. Examination of two fresh roadcut exposures in the Miocene Monterey Formation suggests that folding within part of the Monterey was accommodated primarily by layer-parallel slip between structural layers with thicknesses ranging from 30 to 43 m, even though lithologic layers range from a few mm to a few dm in thickness. This part of the Monterey is folded into a series of concentric-like folds, with chevron-like folds at their cores and with a ratio of wavelength to total thickness of layers of about . Theoretical analysis of multilayers, comprised of identical, elastic or elastic—plastic layers with frictionless contacts, indicates that the effective, or weighted-average thickness of structural layers corresponding with an ratio of 0.42 is about 41 m. Thus, the theoretical predictions are roughly in agreement with available data concerning these folds.Thicknesses of structural units in other folds of this area are inadequately known to closely check theoretical predictions, but most of the data are consistent with predictions. An exception is the Huasna syncline which has a larger wavelength than we would predict. There are several likely explanations for this discrepancy. Layers in the underlying Franciscan complex may have taken part in the folding, making our estimates of total thickness too small. The basement rocks may have been much softer, relative to the overlying sedimentary rocks, than we assumed. The Huasna syncline could be partly a result of gravitational instability of relatively low density, Miocene siliceous and porcelaneous shales, overlain by relatively high density, Pliocene sandstones.The Huasna syncline and some of the smaller folds in the Miocene rocks are doubly in the northwest—southeast direction. Further, the maximum compression was approximately normal to the traces of the large faults in this part of California.     

A theory of concentric, kink and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers: V. asymmetric folding in interbedded chert and shale of the franciscan complex, san francisco bay area, california

The origin of tight, asymmetric, kink-like or chevron-like folds in interbedded shales and radiolarian cherts of the Franciscan Complex in the San Francisco Bay area has been somewhat of a mystery for many years. Stephenson Ellen provided many clues as to the origin and indicated that the folds became asymmetric as a result of layer-parallel shear. He believed that the original folds were conjugate kink folds.As a result of reexamination of most of the folds studied by Ellen, of experimentation with elastic multilayers and of the theories developed in Parts III and IV of this series of papers, we believe that the original folds were mostly chevron rather than kink folds. Thus, we suggest that the folds formed by a combination of layer-parallel shortening and layer-parallel shear when the rocks were soft and pore pressures were high.Several lines of evidence suggest that typical folds in the Franciscan are asymmetric chevron folds. A combination of theory of finite simple shear and of experimentation with elastic multilayers indicates that the tight folds of the Franciscan could have been produced by smaller angles of simple shear if the original folds were typical chevron folds rather than typical kink folds. Several field observations, including thickening of shales but not of cherts in hinges of folds and lack of deformation of radiolaria in the cherts, indicate that the cherts were soft and the shales very soft at the time of folding. The pore-water pressures in the shales probably were high. Such conditions theoretically favor concentric-like and chevron folding, not kink folding. Finally, most of the asymmetric folds in a quarry exposure can be reconstructed geometrically as typical chevron folds but not as typical kink folds subjected to simple shear.     

Variscan stress regime rotation: Insights from the analysis of kink folds in the northern margin of the Bohemian Massif,South Poland

We aimed to determine variations in stress regimes during the youngest Variscan deformations in the northern part of the Bohemian Massif. For this purpose, we calculated the orientation of the principal stress and strain axes for kink folds observed in the metamorphic envelope of the Karkonosze Granite, using two methods: 1) the traditional method, incorporating structural diagrams (for conjugate kink folds only), and 2) butterfly diagram analysis. The use of both methods enabled us to determine the stress regime, based not only on conjugate but also on monoclinal kink bands. The obtained results prove that butterfly diagram analysis, when applied to monoclinal kink folds, yields reliable results, especially when calibrated using the internal friction angle (Ф) calculated for the conjugate structures.We identified two generations of kink folds: 1) an older one, developed under sublatitudinal shortening and most probably related to the Early Carboniferous terminal stages of the northwest-directed thrusting of the metamorphic units, and 2) a younger one; produced by north-south Variscan Carboniferous compression, and the emplacement and subsequent doming of the Karkonosze Granite. This is the first study on brittle-ductile structures observed commonly in the metamorphic units of the Bohemian Massif, showing their relation to the granitoid intrusion and complementing the tectonic models that usually omit kink folds.     

Upright folding of varying intensity on isoclinal folds of diverse orientation: A study from the early Precambrians of Western India

In the Early Precambrian Banded Gneissic Complex between Amet and Lawa Sardargarh in central Rajasthan, India, open to isoclinal upright folds with NNE-striking axial planes have affected westerly plunging isoclinal folds of reclined, inclined and rare upright attitudes. Interference patterns of diverse shapes ranging in scale from inches to miles have been caused by variable angle between the axes and axial planes of the early folds, angular variation in the attitudes of the two fold systems, and spatial variation in the tightness of the later folding.

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Zusammenfassung In dem früh-präkambrischen gebänderten Gneißkomplex zwischen Amet und Lawa Sardargarh in Mittel-Rajasthan, Indien, haben offene bis isoklinale, senkrechte Falten mit NNE-streichenden Axialflächen nach Westen abtauchende isoklinale Falten überprägt, die selbst eine geneigte, überkippte oder seltener senkrechte Lage aufweisen. Überlagerungsmuster von unterschiedlicher Form und Größe (von Zoll bis zu Meilen) wurden verursacht durch 1. verschiedene Winkel zwischen den Achsen und den Axialflächen früherer Falten, 2. Winkelveränderungen in der Anordnung der zwei Faltensysteme und 3. räumliche Veränderungen in der Dichte der späteren Faltung.

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Résumé Dans l'ensemble de gneiss zoné du Précambrien inférieur entre Amet et Lawa Sardargarh, à Rajasthan central aux Indes, des plissements ouverts à isoclinaux, de direction NNE, ont affecté des plissements isoclinaux reclinés, inclinés ou rarement verticaux, plongeant vers l'ouest. Des patrons d'interférence de formes diverses, à l'extension d'un centimètre au kilomètre, ont été causés par des angles variables entre les axes et les surfaces axiales des premiers plissements, des variations angulaires dans la disposition des deux systèmes de plissement, et des variations d'espace des plissements postérieurs.

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Cleavage-fold relationships and their implications for transected folds: an example from southwest Virginia,U.S.A.

A non-coaxial deformation involving pre-folding initiation of cleavage perpendicular to bedding is proposed to explain non-axial planar cleavage associated with mesoscopic folds in part of the Appalachian foreland thrust-belt of southwest Virginia. Folds are gently plunging, asymmetric, upright to slightly inclined, sinusoidal forms with non-axial fanning cleavage. They show extreme local variations in type and degree of transection and the consistency of transection direction. These relations are further complicated by hinge migration.Cleavage-fan angles, bedding-cleavage angles and δ transection values appear influenced by fold tightness, and in part by fold flattening strain. Fold flattening increments are considered simultaneous with folding. Axial surface traces, and not cleavage traces, coincide with the principal extension direction in fold profiles. Geometric modelling of cleavage fanning and bedding-cleavage angle variations for various theoretical folding modes suggest that folding in limestone and sandstone layers was by tangential longitudinal strain. Significant shape modification and change in bedding-cleavage relations occurred after limb dips of 40 and 50° were attained in limestone and sandstone respectively. Mud-rock class 1C folds with convergent cleavage fans show features transitional between buckling and flexural flow. Initiation of ‘cleavage’ fabrics during layer-parallel shortening prior to significant folding may be important for cleavage evolution in some deformed rocks.     

Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels,slumps, faults,folds and glacitectonics

Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.     

Can flat-ramp-flat fault geometry be inferred from fold shape?: A comparison of kinematic and mechanical folds

The inference of fault geometry from suprajacent fold shape relies on consistent and verified forward models of fault-cored folds, e.g. suites of models with differing fault boundary conditions demonstrate the range of possible folding. Results of kinematic (fault-parallel flow) and mechanical (boundary element method) models are compared to ascertain differences in the way the two methods simulate flexure associated with slip along flat-ramp-flat geometry. These differences are assessed by systematically altering fault parameters in each model and observing subsequent changes in the suprajacent fold shapes. Differences between the kinematic and mechanical fault-fold relationships highlight the differences between the methods. Additionally, a laboratory fold is simulated to determine which method might best predict fault parameters from fold shape. Although kinematic folds do not fully capture the three-dimensional nature of geologic folds, mechanical models have non-unique fold-fault relationships. Predicting fault geometry from fold shape is best accomplished by a combination of the two methods.     

断层相关褶皱的三维构造几何学分析:以川西三维地震工区为例

现有的断层相关褶皱理论均基于平衡剖面原理而建立的二维几何学模型。所谓的假三维模型则是通过一系列的二维剖面复合而成的,真正意义上的三维断层相关褶皱理论尚未确立,仍然处于探索阶段。本文采用Trishear 4.0和GoCad软件,分别构建一系列不同位移量的断层转折褶皱和三剪断层传播褶皱的二维正演剖面,将这些正演剖面按照一定的线性位移梯度平行排列,从而建立起断层转折褶皱和三剪断层传播褶皱的假三维理论模型并加以分析和讨论。进而选取川西南盆地中三维地震勘探所覆盖的邛西断层转折褶皱和盐井沟断层传播褶皱,作为真三维实际解释模型的两个实例。研究表明,邛西背斜是一个典型的剪切断层转折褶皱,盐井沟背斜是一个典型的三剪断层传播褶皱;断层相关褶皱的假三维理论模型与实际解释模型的对比分析,可以看出沿背斜走向的位移梯度是控制三维构造几何学特征的基本因素,然而自然界断层面几何学的复杂多变以及岩层力学性质的各向异性,是造成断层相关褶皱真三维理论模型难以建立的主要原因。     

The Ordovician chondrite from Brunflo, central Sweden: III. Geochemistry of terrestrial alteration

The fossil H chondrite Brunflo, found in a slab of Ordovician limestone from central Sweden, is pervasively altered to an assemblage dominated by calcite and barite. The meteorite is surrounded by a 15–20 cm wide zone of lighter colors than the unaffected limestone due to dissolution of hematite. Here we present detailed geochemical analyses of two meteorite samples, 14 limestone samples at distances from 0 to 29 cm along two profiles from the meteorite, and a reference sample of Brunflo limestone. Element concentrations in Brunflo and surrounding bleached limestone have been strongly disturbed during two stages of alteration (early oxygenated and deep burial). In the meteorite, the Ni/Co ratio has changed from an initial value of 20 to 0.8 and redox sensitive elements like V, As, Mo, Re and U are strongly enriched. The sulfur isotope composition of barite from Brunflo (δ³⁴S=+35‰) indicates initial loss of meteoritic sulfide, followed by later accumulation of sea water sulfate as barite. During deep burial under more reducing conditions, reduction processes supported by an externally derived reductant possibly derived from alum shale underlying the limestone, were largely responsible for the observed redox phenomena. In spite of massive redistribution of many elements, concentrations of Pt, Ir and Au remain at chondritic levels. The geochemistry and mineralogy of alteration determined for Brunflo are similar to those in “reduction spots” in red beds, where accumulation of a similar suite of elements (except Mo, Re) occurred as a result of isolated reduction activity.     

Transition from subduction- to exhumation-related fabrics in glaucophane-bearing eclogites, Oman: evidence from relative fabric chronology and Ar/Ar ages

Controversy over the age of peak metamorphism and therefore the tectonic evolution of the Arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane-bearing eclogites from the Saih Hatat window beneath the allochthonous Samail ophiolite in NE Oman. These eclogites contain relicts of earlier fabrics, structures and metamorphic assemblages and provide a record of change from subduction to exhumation. The eclogites are part of a mafic layer that was disrupted into boudins up to 0.5 km in length within a lower plate shear zone (As Sifah shear zone). The megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ENE-trending lineation and sheathlike isoclines enveloped by the flat-lying schistosity. The boudin-bearing layer is isoclinally folded with calc-schist, mafic schist and quartz–mica schist, where the regional folds have axes parallel to the NE-trending stretching lineation (a-type folds). Textural evidence suggests multiple growth events for garnet and clinopyroxene, requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins. The surrounding calc-schist and quartz–mica schist are both intensely deformed with transposition foliation containing an NE-trending lineation in phengite and asymmetric shear indicators such as C′-type shear bands and asymmetric pressure shadows around garnets, that give top-to-the-NE sense of shear. Although consistent ENE-trending lineations in all the boudins suggest that they have largely acted as passive, nonrotating rigid bodies, the presence of NE-vergent asymmetric mesofolds, extensive dynamic recrystallisation, multiple generations of phengites and a range of ⁴⁰Ar–³⁹Ar apparent ages within the megaboudins suggest, however, that they have not acted entirely passively during the later deformation. Phengites isolated from the high-P/low-T fabrics show groupings in ⁴⁰Ar–³⁹Ar apparent ages interpreted as distinct metamorphic/cooling intervals at 140–135, 120–98 and 92–80 Ma. Microstructural relations suggest that age groupings younger than 100 Ma reflect phengite growth during exhumation with the top-to-the-NE shearing. The older ages (120–110 Ma) from fabrics that give top-to-the-S shear sense may reflect growth during the subduction phase. The combination of groupings of apparent argon ages older than the crystallisation age of the Samail Ophiolite, the suggestion of different geothermal gradients, and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite. The glaucophane–eclogites are interpreted as having formed at different times under varying pressure–temperature conditions during underthrusting with variations in the rate of underthrusting, allowing thermal equilibration and/or rapid cooling at different crustal levels.     

3-D geomechanical restoration and paleomagnetic analysis of fault-related folds: An example from the Yanjinggou anticline,southern Sichuan Basin

We examine the development of the Yanjinggou anticline, a fault-propagation-fold in the southern Longmen Shan, through an integrated study of structural geometry, strain, and paleomagnetism. The 3-D structural and strain restoration models generated in our analysis reveal that the NE-trending Yanjinggou fold has a curved map trace that is convex to the southeast. The fold has three distinct regions characterized by different strain patterns: contraction in the core of the fold, extension in the outer arc, and a forelimb with distributed shear. To further understand the kinematics of the Yanjinggou anticline, we performed paleomagnetic analysis on 184 oriented samples collected across the structure. Anisotropy of magnetic susceptibility (AMS) measurements and stepwise thermal demagnetization were conducted. A strike test was applied to the high temperature component (HTC) in order to identify rotation around the arc. The result indicates that the Yanjinggou anticline is a progressive arc, with a minor initial curvature and a dominant secondary curvature related to vertical-axis rotation synchronous with thrusting. The primary curvature and initial development of the structure correlates with the growth of the southern Longmen Shan in Late Miocene. The secondary curvature correlates with displacement extending since Late Pleistocene toward the southeast into the central basin along the detachments that underlie the structure. Lateral gradients in displacement along this underlying detachment provide a mechanism for producing the vertical rotation of the anticline. AMS results and historical earthquake analysis imply that the fault-propagation fold, along with other NE trending structures in the southern Sichuan basin, are tectonically active and accommodate east-west crustal shortening in the basin. By integrating 3-D structural and strain restoration modeling with systematic AMS and paleomagnetic methods using statistical analysis, we closely constrain how the Yanjinggou anticline developed, and provide insights into the formation of fault-related folds with curved shapes in map view, which are common in other fold-and-thrust belts around the world.     

Fault geometry and mechanics of marly carbonate multilayers: An integrated field and laboratory study from the Northern Apennines,Italy

Sealing layers are often represented by sedimentary sequences characterized by alternating strong and weak lithologies. When involved in faulting processes, these mechanically heterogeneous multilayers develop complex fault geometries. Here we investigate fault initiation and evolution within a mechanical multilayer by integrating field observations and rock deformation experiments. Faults initiate with a staircase trajectory that partially reflects the mechanical properties of the involved lithologies, as suggested by our deformation experiments. However, some faults initiating at low angles in calcite-rich layers (θ_i = 5°–20°) and at high angles in clay-rich layers (θ_i = 45°–86°) indicate the important role of structural inheritance at the onset of faulting. With increasing displacement, faults develop well-organized fault cores characterized by a marly, foliated matrix embedding fragments of limestone. The angles of fault reactivation, which concentrate between 30° and 60°, are consistent with the low friction coefficient measured during our experiments on marls (μ_s = 0.39), indicating that clay minerals exert a main control on fault mechanics. Moreover, our integrated analysis suggests that fracturing and faulting are the main mechanisms allowing fluid circulation within the low-permeability multilayer, and that its sealing integrity can be compromised only by the activity of larger faults cutting across its entire thickness.     

Episodic Mesozoic constructional events of central South China: constraints from lines of evidence of superimposed folds,fault kinematic analysis,and magma geochronology

ABSTRACT

The South China block (SCB), located in the convergence zones of the western Pacific subduction tectonic domain and Tethyan tectonic domain, has experienced complex tectonic processes, including the continent–continent collision caused by the closure of the Palaeo-Tethys Ocean, and the subduction of the Palaeo-Pacific plate. However, due to complex intracontinental deformation and abundant magmatism, there are serious divergences in the corresponding records of the tectonic transformation from Palaeo-Tethys to Palaeo-Pacific. We have analysed the map-scale superimposed fold system developed in central SCB and also inverted the palaeo-stress field based on the fault–slip vectors. On this basis, the deformation styles and superposition mechanism of two-stage folds were recovered to establish the tectonic stress field in early Mesozoic and tectono-magmatic events by combining the chronology of the accompanying syntectonic magma. The early E–W/WNW-trending folds and Triassic magmatic system were identified; these were controlled by the NE–SW-trending compressive stress field, to coordinate with the collisions between the SCB and the Indochina block in the southwest, and the North China block in the north. The late NE/NNE-trending folds superimposed on the early folds in an orthogonal way to form a large-scale dome-basin superimposed fold system, which were controlled by the WNW–ESE-trending compressive stress field. According to the strata relations involved in deformation and the chronology data of magmatic rocks, it could be determined that NE/ENE-trending folds would be formed during the Mid- to Late Jurassic, corresponding to the westward subduction events of the Palaeo-Pacific plate. The establishment of the large-scale superimposed fold styles and the identification of fold deformation in the Triassic and Jurassic are important for understanding the early Mesozoic tectonics of South China, and even for all East Asia continent. In particular, it can provide important temporal and spatial constraints to explain the complex deformation process and geodynamic settings of South China in the early Mesozoic.     

Transition of metamorphic series from the Kyanite- to andalusite-types in the Altai orogen, Xinjiang, China: Evidence from petrography and calculated KMnFMASH and KFMASH phase relations

Metamorphic zones in the Chinese Altai orogen have previously been separated into the kyanite- and andalusite-types, the andalusite-type being spatially more extensive. The kyanite-type involves a zonal sequence of biotite, garnet, staurolite, kyanite, sillimanite and, locally, garnet–cordierite zones. The andalusite-type zonal sequence is similar: it includes biotite, garnet and staurolite zones at lower-T conditions and sillimanite and garnet–cordierite zones at higher-T conditions, but additionally contains staurolite–andalusite and andalusite–sillimanite zones at intermediate-T conditions. As relic kyanite-bearing assemblages commonly persist in the staurolite–andalusite, andalusite–sillimanite and sillimanite zones, it is not clear that the distinction is valid. On the basis of a reevaluation of phase relations modelled in KMnFMASH and KFMASH pseudosections, kyanite and andalusite-bearing rocks of the Chinese Altai orogen record, respectively, the typical burial and exhumation history of the terrane. Mineral assemblages distributed through the various zones reflect a mix of portions of the ambient P–T array and the effects of evolving P–T conditions. The comparatively low-T biotite, garnet and staurolite zones mostly preserve kyanite-type peak assemblages that only experienced minor changes during exhumation. Rocks in the comparatively high-T sillimanite and garnet–cordierite zones are dominated by mineral assemblages of a transitional sillimanite type, having formed by the extensive modification of earlier higher pressure assemblages during exhumation. Only rocks in the intermediate-T kyanite and probably some lower sillimanite zones were clearly recrystallized by late stage andalusite metamorphism, producing the staurolite–andalusite and andalusite–sillimanite zones. This andalusite metamorphism could not reach an equilibrium state because of limited fluid availability.     

Ultramafic Xenoliths from Bullenmerri and Gnotuk Maars, Victoria, Australia: Petrology of a Sub-Continental Crust-Mantle Transition

The basanite tuffs of Bullenmerri and Gnotuk maars, Victoria,enclose abundant xenoliths of spinel lherzolites, many of whichcontain amphibole ± apatite ± phlogopite. Thexenolith suite also includes cumulate wehrlites, spinel metapyroxenitesand garnet metapyroxenites. All xenolith types contain abundantlarge CO₂-rich fluid inclusions. Microstructural evidence forthe exsolution of spinel, orthopyroxene, garnet and rare plagioclasefrom complex clinopyroxenes suggests that all of the metapyroxeniteshave formed from clinopyroxene (± spinel ± orthopyroxene)cumulates by exsolution and recrystallization during coolingto the ambient geotherm. Pyroxene chemistry implies that a rangeof parental magma types was involved. Garnet pyroxenites showa series of reactions to successively finer-grained, lower-Pmineral assemblages, which imply a relatively slow initial upwardtransport of the xenoliths in the magma, prior to explosiveeruption. The same process has allowed crystallization of phenocrystsfrom small patches of interstitial melt within xenoliths oflherzolite, wehrlite and metapyroxenite. Critically selected P-T estimates for 16 garnet websteritesare consistent with published experimental studies of the spinel/garnetpyroxenite transition, and define a geotherm from 900 °C,11 kb to 1100 °C, 16 kb. Other published data extend thecurve down to c. 7 kb and up to 25 kb. This elevated geothermsuggests that the high regional heat flow is related to convectiveheat transfer by dike injection accompanying the vulcanism.T estimates for the lherzolites range from 850–1050 °C;comparison with the derived geotherm implies that the spinellherzolites are derived from depths of 30–55 km. Thiszone has low seismic velocities (V_p = 6.8–7.8 km/sec)and has thus previously been regarded as a thick, largely maficlower crust. The xenolith data show that this Mower crust' isdominantly ultramafic, with layers, dikes and some large bodiesof pyroxenites and mafic granulites. The anomalously low V_pmay be due to the high T, the high proportion of fluid-filledpore volume, and the magnesian composition of the lherzolites.The seismically defined Moho (V_p >8.0 km/sec) coincides withthe experimentally determined position of the spinel lherzolite-garnetlherzolite transition.