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Structural Geology, prepared by Prof. Dr. Mundher A. Taha
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University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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Structural Geology: Deals with the origin , geometry and
Fig.(1-1)
University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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Fig.(1-4)
1.1 ROCK DEFORMATION
College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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Strain is the deformation produced by stress. A rock responds to tectonic
stress by elastic deformation , plastic deformation , or brittle fracture. An
elastically deformed rock springs back to its original size and shape when
the stress is removed. During plastic deformation, a rock deforms like
putty and retains its new shape. In some cases a rock will deform
plastically and then fracture (Fig. 1-6).
Figure 1-6 This rock (in the Nahanni River, Northwest Territories, Canada) folded plastically and then fractured.
Several factors control whether a rock responds to stress by elastic or plastic deformation or fails by brittle fracture:
nugget, and a rubber ball. If you strike quartz with a hammer, it shatters. That is, it fails by brittle fracture. In contrast, if you strike the gold nugget, it deforms in a plastic manner; it flattens and stays flat. If you hit the rubber ball, it deforms elastically and rebounds immediately, sending the hammer flying back at you. Initially, all rocks react to stress by deforming elastically. Near the Earth’s surface, where temperature and pressure are low , different types of rocks behave differently with continuing stress. Granite and quartzite tend to behave in a brittle manner. Other rocks, such as shale, limestone, and marble, have greater tendencies to deform plastically.
of a rock to behave in a plastic manner. It is difficult to bend an iron bar
University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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at room temperature, but if the bar is heated in a forge, it becomes plastic and bends easily.
During burial, both temperature and pressure increase. Both factors promote plastic deformation, so deeply buried rocks have a greater tendency to bend and flow than shallow rocks.
favors plastic behavior. Marble park benches in New York City have sagged plastically under their own weight within 100 years. In contrast, rapidly applied stress, such as the blow of a hammer, to a marble bench causes brittle fracture.
Enormous compressive forces can develop at a convergent plate boundary, bending and fracturing rocks in the tectonically active region. In some cases the forces deform rocks tens or even hundreds of kilometers from the plate boundary. Because the same tectonic processes create great mountain chains, rocks in mountainous regions are commonly broken and bent. Tectonic forces also deform rocks at divergent and transform plate boundaries.
Tectonic forces create three types of geologic structures: folds , faults , and joints.
A fold is a bend in rock (Fig. 1-7). Some folded rocks display little or no fracturing, indicating that the rocks deformed in a plastic manner.
Figure 1-7 A fold is a bend in rock. These are in quartzite in the Maria Mountains, California. University of Diyala
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Simple folds are divided into two types, that is, anticlines and synclines in the former, the beds are convex upwards, whereas in the latter, they are concave upwards. In the anticline when we move toward the core we can show the oldest rocks in contrary to the syncline we show the youngest rocks. The crestal line of an anticline is the line that joins the highest parts of the fold, whereas the trough line runs through the lowest parts of a syncline (Fig. 1.10a, b).
Figure 1-10 (a) symmetrical anticline and syncline, and the parts of a fold. (b) Asymmetrical anticline and syncline. (c) Axial plane of anticline and axial plane of syncline.
The hinge line of a fold is the line along which the greatest curvature exists and can be either straight or curved. However, the axial line is another term that has been used to describe the hinge line. The limb of a fold occurs between the hinges, all folds having two limbs. The axial plane of a fold is commonly regarded as the plane that bisects the fold and passes through the hinge line (Fig. 1.10c). A fold arching upward is called an anticline and one arching downward
is a syncline. The sides of a fold are called the limbs. Notice that a single
limb is shared by an anticline–syncline pair. A line dividing the two limbs
of a fold and running along the crest of an anticline or the trough of a
syncline is the fold axis. The axial plane is an imaginary plane that runs
through the axis and divides a fold as symmetrically as possible into two
halves. In many folds, the axis is horizontal, as shown in Figure 1–10a, b.
If you were to walk along the axis of a horizontal anticline, you would be
walking on a level ridge.
In other folds, the axis is inclined or tipped at an angle called the plunge , as shown in Figure 1–11. A fold with a plunging axis is called a plunging fold. If you were to walk along the axis of a plunging fold, you would be traveling uphill or downhill along the axis. Even though an anticline is structurallyUniversity of Diyala a high point in a fold, anticlines do not always form College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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topographic ridges. Conversely, synclines do not always form valleys Figure 1–11B.
Figure 1-11 Plunging anticline and plunging syncline, figures and photos. Landforms are created by combinations of tectonic and surface processes. The amplitude of a fold is defined as the vertical difference between the crest or the trough and the median line, whereas the wave length of a fold is the horizontal distance from crest to crest or trough to trough.
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Parallel or concentric folds are those where the strata have been bent into more or less parallel curves in which the thickness of the individual beds remains the same. From Figure 1.13a, it can be observed that, because the thickness of the beds remains the same on folding, the shape of the folds changes with depth and, in fact, they fade out. Parallel folding occurs in competent (relatively strong) beds that may be interbedded with incompetent (relatively weak, plastic) strata, Fig. 1.13a and 1.13A. Similar folds are those that retain their shape with depth. This is accomplished by flowage of material from the limbs into the crest and trough regions (Fig. 1.13b). Similar folds are developed in incompetent strata. However, true similar folds are rare in nature, for most change their shape to some degree along the axial plane. Most folds exhibit both the characteristics of parallel and similar folding Fig. 1.13 B.
Figure 1.13 Shows (a) Parallel folding. (b) Similar folding.
University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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Fig. 1.13 A_._ These are folds in Cretaceous strata exposed at Ernst Tinaja in Big Bend National Park. They show the disharmonic geometry and maintenance of bedding- perpendicular thickness that characterize a parallel fold style.
Fig. 1.13B. Shows photo of similar fold.
Several other common terms specify relative orientations of the limbs of the folds. A homocline comprises a surface, such as bedding, that has a uniform nonhorizontal attitude over a regional scale with no major fold hinge (Fig.1.14A). A monocline is a special type of fold with only one limb or a fold pair that has two long horizontal limbs connected by a relatively short limb (Fig.1.14B). A monocline may develop where sedimentary rocks sag over an underlying fault (Fig.1.15). A structural terrace is a fold pair with two long planar inclined limbs connected by relatively short horizontal limb (Fig.1.14C). A recumbent fold in which one limb is overturned i.e. rotated more than 90º (Fig.1.14D).
University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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a b Fig.1.1 7 (a) Chevron fold in limestone of Miocene age, Kaikuora, South Island, New Zealand, b , Chevron folds with flat-lying axial planes, Millook Haven, North Cornwall, UK
Fan fold is one in which both limbs are overturned ( Fig.1.1 8 A ). In the anticlinal fan fold, the two limbs dip toward each other; in the synclinal fan fold, the two limb dip away from each other. Kink bands are narrow bands, usually only a few inches or few feet wide, in which the beds assume a dip that is steeper or gentler than that in the adjacent beds (Fig.1 8 B).
Fig.1 8. Shows some varieties of folds. AP, axial plane. (A) Fan fold (B) Kink bands. A fracture may separate the kink band from the rest of beds.
C D Fig.18.C, modeling of kink band formation, D , photo of kink band figure.
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(Fig.1.19).
a b Fig.1.19. a, Mechanism of drag folds resulting from shearing, b, photo of drag folds in incompetent bed.
Dome
Figure 1.20 (a) Sedimentary layering dips away from a dome in all directions, and the outcrop pattern is circular or elliptical. (b) Layers dip toward the center of a basin.University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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Fig.1.20. A. Old tightness classification. B. Modern tightness classification where P=A/M ; is the ratio of the amplitude A of a fold measured along the axial surface, to the distance M measured between the adjacent inflection points that bound the fold. University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
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Fold classification using Isogons
This method is based on the construction of dip isogons: line joining points of equal dip on either side of the folded layer. Using three geometric parameters are as follow (1) The dip isogons; (2) the orthogonal thickness tα, which is the perpendicular distance between the two parallel tangents; (3) the axial trace thickness Tα, which is the distance between the two tangents measured parallel to the axial surface trace(Fig.1.21). The two measures of layer thickness tα and Tα are related by
tα= Tα cosα
Fig.1.2 1 Definition of the layer inclination α, the dip isogons, the orthogonal thickness tα, the axial trace thickness Tα used to define the style of folded layer.
If the lines of dip isogons converge toward the inner side of the fold, that is convergent isogons; if they diverge toward the inner surface, that is the divergent isogons; and when they are parallel, that is parallel isogons.
Three classes of folds have been recognized (Fig.1.22):
Class 1 , Convergent isogons imply that the inner arc curvature exceed that of the outer arc , which are subdivided in to three subclasses.
Sub-class 1A : strongly convergent
Sub-class 1B : parallel fold with isogons perpendicular to layering.
Sub-class 1C : weakly convergent.
Class 2 , parallel isogons, and similar fold, the lines of isogons are parallel to the axial surface.
Class 3, Divergent isogons, imply that the outer arc curvature exceed that of the inner arc.
University of Diyala College of Science Department of Petroleum Geology and Minerals Lectures in Structural Geology
