mechanism of folding

MECHANISM OF FOLDING Introduction Folds are the most commonly occurring structure in the earth's crust. This fold is the one of the best marker to know the deformation of particular rock mass or landform in geological past. They Occurs in micro, meso and macro scale. Their geometry and style of folding gives idea about type of deformation. They are also important by economically for oil traps, searching ore and mineral exploration. Why folds are generated and how they evolve in the rock mass such questions are arriving in mind. To know about the folds we need to do field observation and study with lab with experiments. From this study we can know the folding history which helpful to understand the mechanism.Mechanism is the process by which different types of folds are generated. Mechanism A force or stress leads to deformation of rock layers. These rock layers tend to balance the deformation by folding of layers with conservation of volume in the rock mass. The process or mechanism to form fold by applied force or stress depends on the how layers respond, actively or passively. This deformation occurs with several mechanism are follows, A) Buckling B) Passive folding C) Bending D) Flexural slip E) Flexural flow F) Kinking G) Chevron folding A) Buckling- It is a folding process in which shortening occurs parallel to layering. For buckling viscosity contrast is necessary between competent layers and surrounding incompetent host rock (matrix). This results in formation of rounded fold and parallel fold of more or less sinusoidal shape. Consider application of compression on two competent layers close to each other. These two layers behave like a single layer.In another case these two competent layers are far apart from each other. They behave as individual layers and deform individually into folds with a different wavelengths. Example – figure (A1) as compressional force is applied on either side of block, initially parallel shortening occurs which leads to increase in the thickness of layer. Then, folding without change in thickness starts. This results in parallel folds. Figure (A1) Strain distribution at hinge zone of buckled layer is such that upper part of layer is under extension due to stretching while inner part is under contraction due to shortening as shown in figure​ (A2) Figure (A2):strain distribution at hinge zone of buckle fold These two outer and inner parts of buckled fold are separated by a no strain surface known as neutral surface. Extensional and compressional strain increased from the neutral surface towards outer and inner part of buckled fold. There is no extension parallel to the fold axis Mostly buckling occurs in single competent layer but it can occur in multilayer rocks. We can study multilayer rocks buckling by following experiment, figure (A3) Figure (A3): Folding of multilayered rocks, experiments by Currie et al.(1962). As shown in below figure (A4) two competent layers but with different thickness.Application of compressional force leads to deformation of these layers such that thin layer started deforming into small folds prior to thick layer. At the same time thick layer started shortening and thickening. After some time thick layer started deforming into large fold, i.e. with large wavelength. (Thin layer Ld​ ​< thick layer Ld​ ​). Further development of folding is controlled by thick layer. Figure (A4): folding steps in thin and thick layer In case of a viscosity contrast where the host rock is competent and deforming layer is incompetent,after application of compressional force incompetent layer forms the cusp pointing towards the competent host rock, while competent layers deforms into bulbous fold towards incompetent layer. This results in mullion structure as shown in the figure (A5) below. Figure (A5) Viscosity contrast affects the rate of buckling and shortening. If the viscosity contrast is more, the rate of buckling is high and rate of layer shortening is low. If viscosity contrast is low, the rate of buckling is low and rate of layer shortening is high. This can be observed in the following figure (A6)- Layers of marl, limestone, dolomite and quartzite in a shale subject to homogeneous compression lead to folding. Least competent layer i.e. marl deforms in the way of shortening and thickening with less amount of folding while more competent layer i.e. quartzite deforms into buckling. Simultaneously moderately competent layers, i.e. limestone and dolomite, are affected by layer shortening and buckling according to their competency. Above experiment is carried out with the help of tools of fluid dynamics. Where, L= Limestone, M= Marl, Q = Quartzite, D = Dolomite. Figure (A6): folding occurs due to viscosity contrast in different layers B) PASSIVE FOLDING – Passive folding involves passive flow of rock mass.This folding occurs when passive layers are mechanically not affecting the folding. Viscosity contrast between deforming layers and surrounding host rock is not affecting the folding because these layers passively flow with the rest of rock. Layers maintain their constant thickness parallel to axial surface during the flow. This results in similar folds. Figure (B1): folding due to simple shear Passive folding is formed due to simple shearing of layers known as shear fold. These shear folds are also known as similar fold. Passive folds mostly occur in rocks of ductile regime. They are generated by simple shear and rarely by pure shear. Passive folding mechanism also produces harmonic folds as shown in figure (B2). In multilayer sequence this type of fold has a common axial trace and similar wavelength, amplitude and geometry along the axial surface. Due to passive nature of layers there is no effect on the shape of folding. C) BENDING – Figure (B2): Harmonic fold When a pair of force acts at high angle to a layer it bends the rock layer as shown in figure (C0). Bending occurs due to directed force upon the layer. Bending is generated in rock layers due to following mechanisms. Figure (C0): Bending due to force applied at high angle C1) Fault Block UpliftmentOverlying sedimentary layers are subject to forced upward movement due to upliftment of block along fault plane. This upliftment leads to bending of overlying sedimentary layers forming monoclinal fold. It will bend until the sedimentary layer reaches its rupture limit. C2) Ramp – Figure (C1): Bending above reactivated fault In thrust fault zone, thrust sheets are moving over a ramp structure due to force.Moved thrust sheets become bends which lie over the fault plane. Finally, this results in bending of layers and formation of folds. Figure (C2): Bending above thrust ramp C3) Forceful Intrusion – Intrusion of magma or salt leads to bending of the overlying sedimentary layers as shown in below figure, Figure (C3): Bending above salt diapir and magma intrusion at shallow depth C4) Boudins – Layer parallel extension of competent layer surrounded by ductile host rock tends to separate pieces of competent layer, while incompetent host rock occupies the separation gap between successive boudins because of the weight of the overlying layers. This mechanism leads to bending of incompetent host rock between the boudins. D) FLEXURAL SLIP – Figure (C4): Bending in between boudins This folding is generated when layers slip over other layers along a weak layer interface while no slip at the hinge of the fold. Mostly this slip occurs for maintaining the thickness of layers results parallel folding.Slip will occur until the fold limb reaches to maximum dip i.e. interlimb angle ​~ 60degree.​ ​As the dip of limb increases frictional resistance also increases due to tightness of fold. This friction results in slickenlines on the slipping surface. These slicken lines are perpendicular to hinge of a fold. Flexural slip folding commonly occurs in upper crustal brittle regime where sedimentary layers are folded. For occurrence of this mechanism folding deforming medium should be layered such that strong layers are separated by thin weak plane or layers. For example, thin shale planes between limestone or sandstone. Figure (D1): Flexural slip showing slip at limb, decreasing slip at hinge E) FLEXURAL FLOW – Flexural slip mechanism in a folding event is converted into flexural flow or flexural shear only when shear strain in the limbs under the plastic regime condition. Strain is zero at the hinge and increases towards the limbs because shear strain depends on the orientation of a layer. This orientation changes as layer rotates from original position which leads to increase in shear strain. In this fold strainis increases away from the hinge towards the limb. Layers maintain its thickness by shear strain results in parallel fold. Figure (E1): Flexural flow maintains thickness by shearing at limb F) KINKING – Kink bands are centimetre to meter wide with sharp boundaries. Wide kink zone layers become known as kink fold. These folds have sharp hinges, straight limbs and asymmetrical geometry. As compressive stress is applied on thin layers kink bands which are oblique to the direction of applied stress are generated. Along the kink band simple shear generates which results in kink folding. Ffigure (F1): Kninking due to simple shear G) CHEVRON FOLDING – An angular fold which has sharp hinge and straight limb is called a chevron fold. Experimental study proved that mechanism of formation of this fold is from merging of conjugate set of kink bands as shown in figure (G1). These conjugate sets of kink bands are merged together due to compressional stress resulting 50% shortening in kink band layers leads to form chevron fold. But 50% shortening in layers of kink band cannot occur naturally, since this mechanism of formation is not applicable to naturally occurring chevron folds. Figure (G1): Chevron folding by merging of conjugate set of kink bands Chevron fold is formed by flexural slip of multilayered rock at the time of layer parallel shortening as shown in figure (G2). Flexural slip occurs between competent layers due to application of stress. This results in formation of angular fold where strain occurs only at the hinge of the fold. Folded layers are stretched at the outer part of fold and shortened at the inner part. Figure (G2): Chevron folding by flexural slip of multilayered rock Abstract We should know that, in folding history which mechanisms are actively leads for deformations in different part of rocks result in different type of folds. To discover the main mechanism we can use fold geometry and strain distribution with knowledge of viscosity contrast, layer thickness, deformed medium composition and, carrying the experiments and interpreting their results. By knowing the mechanism of folding we can interpret what kind of deformation leads form present dominant structure in particular area. Mechanism of folding is studied with above mentioned factor and parameters. Deformation occurs with several mechanism are buckling, passive folding, bending, flexural slip, flexural flow, kinking, chevron folding. Buckled fold form due to layer parallel shortening with viscosity contrast between deforming layer and host rock. If buckling occurs in multilayer rocks multilayers behave like a single layer deforming into folding. In case of two competent layers with different thickness deforms such a way that thick layer controlled the further folding of layers.Thick layers produce longer-wavelength folds than thin layers.​ ​ Mullion structure formed due to opposite relation of buckling viscosity contrast ratio. Passive folds are involves passive flow of layers with whole rock. There is no effect of viscosity contrast in folding.This folds are formed due to s​imple shearing of passive layers leads to ​maintain their constant thickness parallel to axial surface. This results in formation of similar fold. Passive folding mechanism also produces harmonic folds. Bending occurs when forces act across the layering. Upliftment of fault block leads to bending of overlying sedimentary layers forming monoclinal fold. Thrust sheets become bends which lie over the fault plane results in bending and formation of folds. Intrusion of magma or salt leads to bending of the overlying sedimentary layers. In boudin structure layer parallel extension of competent layer surrounded by ductile host rock tends to separate pieces of competent layer leads to bending of incompetent host rock between boudins separation gap. Flexural slip folding is generated when layers slip over other layers along a weak layer interface while no slip at the hinge of the fold. This slip occurs for maintaining the thickness of layers results parallel folding. Commonly occurs in upper crustal brittle sedimentary regime. Flexural flow also called flexural shear. Flexural flow generates only when shear strain in the limbs under the plastic regime. Layers maintain its thickness by shear strain results in parallel fold. Wide kink zone layers become known as kink fold. Kink band are oblique to the direction of applied stress. Due to simple shear results in kink fold. Chevron fold is angular fold with sharp hinge and straight limbs. Experimental study proved that merging of conjugate set of kink bands with 50% shortening leads to form chevron fold. 50% shortening cannot occur naturally. This fold formed by flexural slip of multi-layered rock at the time of layer parallel shortening. This results in formation of angular fold with strain only at the hinge of the fold. Reference 1) 2) 3) 4) 5) Structural geology by Haakon Fossen Foundations of Structural Geology - R. G. Park Analysis of Geological Structures ​By Neville J. Price, John W. Cosgrove Earth Structure (2nd Edition), 2003 by W.W. Norton & Co, New York Development of Chevron Folds by John G. Ramsay ​Department of Earth Sciences, University of Leeds, Leeds LS2 9JT England 6) Selective buckling of composite layers with contrasted rheological properties, a theory for simultaneous formation of several orders of folds by Hans Ramberg