Folds - Warping of Strata PDF

Compiled By: Prof. Fathy Hassan Folds – warping of strata produced by compressive deformation  Folds Folds form when layers are bent into curved or kinked shapes. Their forms can tell a lot about how, why and under what conditions they formed. They can form in many ways - buckling, d...

Compiled By: Prof. Fathy Hassan Folds – warping of strata produced by compressive deformation  Folds Folds form when layers are bent into curved or kinked shapes. Their forms can tell a lot about how, why and under what conditions they formed. They can form in many ways - buckling, due to motion on a fault or shear zone, flow at high temperatures, gravity sliding, etc. Folds – warping of strata produced by compressive deformation Range in scale from microscopic features to regional- scale domes and basins Indicators of compression and shortening Folds A fold is a wave- like structure in layered rock that is produced by ductile deformation. Folds may be linked together to form trains of many folds, or be an isolated single structure. Folds may be linked together to form trains of many folds Petroleum Traps ▪ Folds are important economically. They trap buoyant hydrocarbons, and may contain ores. Types of Folds  Youngest rocks in middle Oldest rocks in middle Think of an Egg Carton! Domes & Basins Symmetrical Asymmetrical Overturned Overturned Wegener’s matching of mountain Asymmetrical ranges on different continents Fold Terminology  Anticline Anticline Symmetrical Symmetrical Non-plunging plunging Plunge  Plunging Anticline Fossil Evidences  Plunging vs. non- plunging Syncline Anticline Fold Classification ▪ Folds are classified & described based on the following: ▪ 1- 3-D shape – cylindrical vs. non-cylindrical ▪ 2- Facing – whether fold is right-side-up or not. ▪ 3-Orientation of axial plane and hinge line. ▪ 4- Shape in a profile plane.  Shape and orientation In general, folds are made up of a hinge that connects two usually differently oriented limbs. The hinge may be sharp and abrupt, but more commonly the curvature of the hinge is gradual, and a hinge zone is defined. A spectrum of hinge shapes exists, from the pointed hinges of kink bands and chevron folds (sharp and angular folds) to the well-rounded hinges of concentric folds. Classification of folds relative to hinge curvature is referred to as bluntness. (a) Kink band ,where the bisecting surface, i.e. the surface dividing the interlimb angle in two, is different from the axial surface. (b) Chevron folds (harmonic). (c)Concentric folds, where the arcs are circular. (d) Box folds, showing two sets of axial surfaces.  1- 3-D Shape The point of maximum curvature of a folded layer is located in the centre of the hinge zone and is called the hinge point. Hinge points are connected in three dimensions by a hinge line. The hinge line is commonly found to be curved, but where it appears as a straight line it is called the fold axis. This takes us to an important element of fold geometry called cylindricity. Folds with straight hinge lines are cylindrical. A cylindrical fold is any surface that can be generated by moving a line parallel to itself. That direction is parallel to the hinge line. Features like domes and basins are not cylindrical folds. A cylindrical fold has an a long-strike shape that approximates a portion of a cylinder. Cylindrical Fold  Cylindrical Fold Only A has straight hinge line Move baton Poles to bedding planes will themselves be co-planar (follow a great parallel to paper circle ) if the fold has a cylindrical geometry. Cylindrical fold: surface wraps partway around cylinder; hinge line is straight. curvilinear fold: curved hinge line (noncylindrical). 2- Orientation of Axial Surface and Hinge Line The axial surface, or axial plane when approximately planar, connects the hinge lines of two or more folded surfaces. The axial trace of a fold is the line of intersectionof the axial surface with the surface of observation, typically the surface of an outcrop or a geologic section. The axial trace connects hinge points on this surface. Note that the axial surface does not necessarily bisect the limbs. It is also possible to have two sets of axial surfaces developed, which is the case with so-called box folds.  Axial Surface Axial surface: imaginary surface that joins hinge lines of different folded surfaces (layers) typically called axial Fold axis: line that joins plane, but not points of maximum curvature always a planar on folded surface surface 2- Orientation of Axial Surface and Hinge Line The orientation of a fold is described by the orientation of its axial surface and hinge line. These two parameters can be plotted against each other, as done in the Figure. Commonly used terms are upright folds (vertical axial plane and horizontal hinge line) and recumbent folds (horizontal axial plane and hinge line). 3- The orientation of the hinge line is described in terms of its plunge: Horizontal, shallow, steep, Vertical. Fold Classification ▪ Folds are described in terms of the orientation of their axial plane: ▪ An upright fold is one whose axial surface is vertical or nearly so. ▪ An inclined fold has an axial surface that dips significantly; one limb may be upside down or overturned. ▪ A recumbent fold has a nearly horizontal axial surface. One limb is upside down. Classification According to Orientation of Axial Surface and Hinge Line Recumbent Fold 3- Facing – Whether Fold is Right-side-up or not If the age of the rocks are not known, then those structures are termed antiform and synform. An antiform is a structure where the limbs dip down and away from the hinge zone, whereas a synform is the opposite, trough-like shape. Syncline is a trough-shaped fold where layers get younger toward the axial surface. A synformal anticline is an anticline because the strata get younger away from its axial surface. At the same time, it has the shape of a synform, i.e., it is synformal. Similarly, an antiformal syncline is a syncline because of the stratigraphic younging direction, but it has the shape of an antiform. Technically, a synformal anticline is the same as an anticline turned upside down, and an antiformal syncline looks like an inverted syncline. A monoclinal fold is a sub-cylindrical fold with only one inclined limb.  Fold Classification ▪Y = Younger ▪O = Older ▪Which folds are anticlines and which are synclines? ▪Which are downward facing? Monocline 4- Shape in a Profile Plane Folds are commonly described or classified according to tightness. Tightness is characterized by the opening or interlimb angle, which is the angle enclosed by its two limbs. Based on this angle, folds are separated into gentle, open, tight and isoclinal. Folds usually come in groups or systems, in these cases they can, akin to mathematical functions, be described in terms of wavelength, amplitude, inflection point and a reference surface called the enveloping surface. The enveloping surface is the surface tangent to individual hinges along a folded layer. Fold Tightness: measure of interlimb angle Tight fold Gentle fold Close fold Isoclinal Fold Fold Classification ▪In normal profile view, folded surfaces can be divided up into limbs and hinges. ▪ If the hinge is sharp, that point is called the hinge point otherwise it is called a hinge zone. ▪ Fold limbs commonly curve, and the location where segments of opposite convexity join is called the inflection point. It is the place where the fold is setting up for the next hinge. Fold Classification 4- Shape in a Profile Plane Wavelength: distance between hinges of successive folds Amplitude: half height of structure measured from crest to trough Interlimb angle gives qualitative estimate of intensity of folding; …smaller the angle, greater the intensity Arc length: The length measured along the layer from one hinge to another of the same type – e.g., anticline to anticline. Cylindrical Fold: A folded surface generated by moving a straight line parallel to itself. Profile Plane: the surface perpendicular to the hinge line. Fold Terminology Fold Classification ▪ The median trace connects the inflection points of the fold and parallels the enveloping surface ▪ The fold height is the distance between the hinge line and median trace measured along the hinge surface. ▪ The fold width is the distance between the inflection points. ▪ An arc length measures the distance along the layer from one inflection point to the other. ▪ Fold symmetry: ▪ A symmetric fold has a hinge surface perpendicular to the median surface. ▪ An asymmetric fold does not have a perpendicular hinge surface and also has limbs of different length.  Enveloping surface: Defines Limit of Fold Enveloping surface (antiform) (connect crests) Antiform: “hill” Enveloping surface (synform) (connect troughs) Synform: “hill” Fold Profile ▪ Fold shape in the profile plane is described by the interlimb angle (left) and the curvature of the hinge zone. ▪ Chevron or kink folds are characterized by sharp, highly curved hinge zones. Other folds have rounded hinge zones.  Fold Profile Class I Class I ▪Single layers of folds are classified on the curvature of the inner and outer layers and by the layer thickness changes that occur from the limbs to the hinge zone. Three fold classes Class II Class are recognized: III ▪ Class I: the inner curvature is greater than the outer ▪ Class II: the inner and outer curvatures are the same ▪ Class III: the outer curvature is greater than the inner ▪ Only class I folds can have no thickness change Dip isogons Fold Profile are a graphic representation of these curvature relationships. A dip isogon is a line that connects points of equal inclination on the inner and outer surface of a folded layer.  Fold Profile Class I: folds all have isogons that fan outward in anticlines because inner curvature > outer. Layer thickness does not change Class II: Fold Profile folds all have parallel isogons because inner curvature = outer They are also characterized by thinner limbs and thicker hinges  Fold Profile Class III: folds all have isogons that converge upward in anticlines because inner curvature < outer. They also have thinner limbs and thicker hinge zones ▪ Class I folds that have constant thickness are called parallel or concentric folds. ▪ Because concentric fold anticlines have increasing curvature downward, they Typically there will be must die out along a some sort of ‘room decollement or problem’ at the core of bedding fault. the fold. ▪ Class II folds are called similar, because each hinge curvature is the same. They do not have to die out and always have thicker hinges than limbs. ▪ Display some degree of limb thinning and hinge thickening. Similar Folds  Symmetry and order For symmetric folds the bisecting surface coincides with the axial plane. Hence, the kink band is not symmetric. In fact, this is how we distinguish between kink bands and chevron folds: chevron folds are symmetric while kink bands have one long and one short limb. Symmetric folds are sometimes called M-folds, while asymmetric folds are referred to as S-folds and Z-folds. Distinguishing between S- and Z-folds may be confusing to some of us, but Z-folds have short limbs that appear to have been rotated clockwise with respect to their long limbs. Chevron (a) Kink band ,where the bisecting surface, i.e. the surface dividing the interlimb angle in Fold two, is different from the axial surface. (b) Chevron folds (harmonic). ▪Z-folds have short Symmetry and order limbs that appear to have been rotated clockwise with respect to their long limbs. ▪ Z-folds thus mimic the letter Z when considering the short limb and its two adjacent long limbs. S- folds imply a counter- clockwise rotation, and resemble the letter S. Symmetry and order Fold systems consisting of folds with a consistent asymmetry are said to have a vergence. The vergence can be specified, and the vergence direction is given by the sense of displacement of the upper limb relative to the lower one. We can also relate it to the clockwise rotation of the inclined short limb, where a clockwise rotation implies a right-directed vergence. Large folds tend to have smaller folds occurring in their limbs and hinge zones. The largest folds are called the first-order folds, while smaller associated folds are second- and higher-order folds. The latter are also called parasitic folds. First-order folds can be of any size, but where they are map scale, we are likely to observe only second- or higher-order folds in outcrops. If a fold system represents parasitic (second-order) folds on first- order synformal or antiformal structure, then their asymmetry or vergence indicates their position on the large-scale structure. Parasitic folds have a vergence directed toward the hinge zone. Fold vergence is defined as the direction an axial plane is rotated away from a perpendicular to the median line. Clockwise rotation implies a right-directed vergence Counter-clockwise rotation implies a left-directed vergence Dextral Sinistral Z- S- fold fold  M M: symmetric Z: left limb hinge of antiform and right S limb of synform S: right limb of antiform and left limb Z M of synform Abu Roash – Parasitic folds- Cairo- Alexandria Desert Road Folding: mechanisms and processes We will start out by considering active folding (buckling). We will then look at passive folding, and then consider bending. Active folding or buckling (Class 1B folds) Active folding or buckling is a fold process that can initiate when a layer is shortened parallel to the layering. The folds in the competent layer approximate Class 1B folds (constant layer thickness). If there are two or more folded competent layers, then the incompetent layers in between are folded into Class 1A and Class 3.  Examples of bending in various settings and scales: (a) between boudins; (b) above thrust ramps; (c) above reactivated faults; and (d) above shallow intrusions or salt diapirs. Folding: mechanisms and processes The outer part of the competent layer is stretched while the inner part is shortened. The two parts are typically separated by a neutral surface. Passive folding (Class 2 folds) Passive folding is typical for rocks where passive flow occurs, i.e. where the layering exerts no mechanical influence on the folding. In these cases the layering only serves as a visual expression of strain with no mechanical or competence contrast to neighboring layers. Such layers are called passive layers. Perfectly passive folds produced by simple shear are Class 2 (similar) folds. Strain distribution in the hinge zone of a folded limestone layer in shale. Outer-arc stretching is separated from inner-arc shortening by a neutral surface. Folding: mechanisms and processes Passive folding produces harmonic folds where the layering plays no mechanical role and therefore no influence on the fold shape. Bending Bending occurs when forces act across layers at a high angle and may involve more than one mechanism, unlike buckle folds where the main force acts parallel to a layer. Classic geologic results of bending are the forced folds created in sedimentary layers blanketing faulted rigid basement blocks. Special Folds ▪ Fault Related Folds ▪ Fault Bend – generated by fault curvature ▪ Fault Propagation – generated by fault terminations on a ramp Fault Bend ▪ Detachment Folds – Generated by fault terminations on a flat. Fault Propagation Minor Folds and Foliation Are Clues to Much Larger Structures ▪ Pressure flattens and/or aligns minerals in a rock ▪ platy or sheet-like structure reflects the direction in which pressure was applied. ▪ Slate, schist, and gneiss foliated. ▪ In every case, the foliation is: ▪ In the direction of least resistance ▪ At right angles to the direction of greatest compression. How Geologists Use These Clues Here's an outcrop that might be seen in the field. Minor Folds and foliations can be used to determine the axe of the major fold. Thank You