Chapter 15 Geologic Structures

Chapter 15 Geologic - 2009 Allan Ludman and Stephen Marshak W.W Norton Company CHAPTER 15 INTERPRETING GEOLOGIC STRUCTURES PURPOSE To become

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1 CHAPTER 15 INTERPRETING GEOLOGIC STRUCTURES PURPOSE • To become familiar with the most common geologic structures (folds and faults). • To learn to recognize the presence of folded and faulted rock from landscape features. • To learn to interpret the deformation history of an area from geologic maps. MATERIALS NEEDED Colored pencils A fine-tipped black pen • Tracing paper 15.1 INTRODUCTION Rocks deform when plates collide in subduction zones, rift during seafloor spreading, or grind past one another in transform boundaries. Deforming forces stress rocks and when stress overcomes their cohesiveness, they experience strain – they change position, shape, and/or volume by folding or faulting. There are three kinds of stress, each exemplified by a type of plate boundary: compression is a head-on collision like the jaws of a vise (convergent margins); tension is a pulling apart (divergent margin); and shear is what happens between a pair of scissor blades (transform margin). Folds and faults (Figure 15.1d, e) are called geologic structures . Folds form when rocks bend as the result of compression between converging plates. Faults can develop at all plate boundaries and within plates when rocks break under more stress than they can accommodate by bending. Erosion of deformed rock produces distinctive landscapes so you can determine what kind of deformation took place from an airplane, a topographic map, or DEM. 15.2 GEOLOGIC MAPS, CROSS-SECTIONS, AND BLOCK DIAGRAMS A geologic map shows the outcrop pattern of different rocks on Earth’s surface and that pattern reveals their deformation history. Geologic maps are often coupled with cross-sections in a three- dimensional block diagram that interprets how the structure would appear at depth if a Grand Canyon © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company
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2 was cut into it (Figure 15.1a). Figure 15.1b is a simplified block diagram showing horizontal strata; Figures 15.1c-e are block diagrams of tilted, folded, and faulted rocks. These diagrams are simplified in that the map view is flat rather than showing the true topography. Figure 15.1: Block diagrams showing different types of deformation a. Construction of a block diagram b. Block diagram showing horizontal strata c. Block diagram showing tilted strata d. Block diagram showing folded strata e. Block diagram showing faulted strata Front cross-section Map view Fault Colors indicate different ages, patterns show different rock types Sandstone Limestone Shale Conglomerate
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3 15.2.1 Structures revealed by landscapes How would a real landscape differ from these simplified block diagrams? Rock types that are resistant to erosion form topographic highs that stand above valleys underlain by less resistant rocks (Figure 15.2). The pattern of the ridges and valleys makes it possible to recognize folds and faults from maps – and when flying overhead. Exercises 15.1 and 15.2 explore the relationships between topography and structural features.
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This note was uploaded on 03/03/2009 for the course GEOL 101 taught by Professor Jackel during the Spring '09 term at CUNY Queens.

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Chapter 15 Geologic - 2009 Allan Ludman and Stephen Marshak W.W Norton Company CHAPTER 15 INTERPRETING GEOLOGIC STRUCTURES PURPOSE To become

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