Under similar confining pressures, halite (rock salt) is more susceptible to ductile deformation than is granite, which will more likely fracture. 1. CHAPTER 10: Folds, Faults and Rock Deformation. A dip-slip fault in which the block above the fault has moved downward relative to the block below. The definition of folds are when rocks bend due to force. Folds and faults . Figure 10.9: Folds are a result of ductile deformation of rocks in response to external forces. Unlike dipping beds, the plunge of a fold axis is in the same direction as the strike of the axial plane. 20o NW). Rock bands appearing on one side of the fold axis are duplicated on the other side. Folds generally occur at great depths in the earth’s crust where the rock layers are exposed to high temperatures and pressures. •Describe the differences between elastic, brittle, and ductile deformation. In the nineteenth century, the predominant theory was that since its hot, molten formation, the Earth has been slowly cooling and thus shrinking and collapsing, like the skin of a drying apple, which produces mountain-building structures such as faults and folds. Complex Folds • Formed by intense deformation in mountain ranges. Figure 10.10: A fold can be divided by an imaginary surface called the axial plane. Types of faults. Definition of a Fault: Faults are fractures along which movement of one block with respect to others has taken place. Figure 10.22c: Shear forces typically produce strike-slip faults where one block slips horizontally past the another. (a) The hanging wall is the block of rock above an inclined fault plane. 7. 3. Release of confining pressure causes the exhumed rock to expand and fracture, thereby producing joints. Here, sections of rock move past each other. Figure 10.5: In the field, a geologist can reconstruct the geometry of folds by: (a) measuring the strike and dip of various strata exposed in outcrops, (b) noting which direction the beds become younger. They occur as single isolated folds or in periodic sets. This is known as brittle deformation. Figure 10.6: Faults can form in response to any one of the three types of forces: compression, tension and shear: The type of fault produced, however, depends on the type of force exerted. The strike of a surface is the direction of a line formed by the intersection of a rock layer with a horizonal surface. 4. The dip is measured at right angles to the strike and is a measure of the angle at which the surface tilts relative to a horizontal surface. This movement may vary from a few centimetres to many kilometres depending on the nature and magnitude of the stresses and resistance offered by the rocks. FOLDS, FAULTS AND GEOLOGIC MAPS Objectives •Define three types of stress. Figure 10.25: Normal faults result from tensional forces and typically form rift valleys. • Usually the result of multiple episodes of folding. If you are at an office or shared network, you can ask the network administrator to run a scan across the network looking for misconfigured or infected devices. Start studying Chapter 9: Folds, Faults, and Geologic Maps. Sedimentary rocks are more flexible than the metamorphic, and when the thrust is not intense enough to move them fold as if they were a pie… 3. Folds constitute the twists and bends in rocks. 8. Expansion can occur if erosion strips away the overlying rocks to exhume once deeply buried rocks. Folds and reverse faults are the dominant structures in unmetamorphosed rocks exposed in a belt that may be tens or hundreds of kilometres wide. Rocks deep within the crust under high confining pressures deform by folding. also reveal differences in the joint systems at limb and hinge positions on large folds or different distances from large faults. The rock will contort and change shape without fracturing. 5. In brittle deformation, a continuous, force is applied to a rock. Figure 10.6: Rocks that were originally deposited in horizontal layers can subsequently deform by tectonic forces into folds and faults. They are easily visualized by the loss of horizontality of the strata. 5. Structural Geology 2. Folding is caused due to compressive stresses. This website contains information on faults and associated folds in the United States that are believed to be sources of M>6 earthquakes during the Quaternary (the past 1,600,000 years). 10.18: Synclines show the opposite trend. Figure 10.22b: Compressional forces typically push the hanging wall upward relative to the footwall, producing a reverse fault. Figure 10.6: Rocks that were originally deposited in horizontal layers can subsequently deform by tectonic forces into folds and faults.Folds constitute the twists and bends in rocks.Faults are planes of detachment resulting when rocks on either side of the displacement slip past one another. 2. Similarly, rocks with • which refers to a surface across which there is a discontinuity in displacement, strain, and/or fold style. The down-faulted block in a rift valley is called a graben while the uplifted block is referred to as a horst. Folds & Faults. Figure 10.7: Rocks are defined as brittle or ductile on the basis of the way they are deformed by forces. What Determines Whether a Rock Bends or Breaks? 1. The fracture itself is called a fault plane.When it is exposed at the Earth's surface, it may form a cliff or steep slope … Drillers experience great problems with confining pressure. For anticlines, the horseshoe or hairpin shape closes in the direction that the anticline plunges. Fold Mountains. Confining pressures within the earth are caused by the weight of the overlying rock pushing downward and from all sides. The type of rock also determines the type of deformation. Joints aid in weathering by providing channels where water and air can reach deep into the formation. The strike is described in terms of direction such as N 10o W. 3. Please enable Cookies and reload the page. In effect, the body is squeezed into itself. Folds form under … (a) Fig. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Holes drilled within the earths crust tend to remain open at shallow depths, but at greater depths holes tend to squeeze shut due to the increase in confining pressure. A fault plane divides a rock unit into two blocks. Definition of faults. Another way to prevent getting this page in the future is to use Privacy Pass. • (c) measuring any structural deformations within the rocks. Fig. 2. Tethys geosyncline. Introduction Structural geology : is the study of three dimensional distribution of rock units with respect to their deformational histories The study of geologic structures has been of prime importance in economic geology Folded and faulted rock strata commonly forms traps … The dip is indicated in terms of angle and direction (e.g. •Define and describe synclines, anticlines, and other types of folds. Folds are bends in rocks that are due to compressional forces. Although the angle of inclination of a specific fault plane tends to be relatively uniform, it may differ considerably along its length from place to place. 3. 2. Figure 10.22d: Oblique faults occur where there is both a strike-slip and dip-slip component to the fault. 4. The plunge of a fold can be described as the angle a fold axis makes with a horizontal surface. In thrust faults, the hanging wall moves almost horizontally over the footwall. 9. Source: USGS, 1970. 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