P A R T

I FUNDAMENTALS

C H A P T E R I

NATURE OF STRUCTURAL

GEOLOGY

DEFORMATION OF THE EARTH'S CRUST

The start of any journey into unfamiliar territory is often spurred by dreaming, a kind of dreaming that spawns not lightheadedness but intense curiosity and the setting of goals. Our journey will explore the architecture of the crust of the planet on which we live. We will be concerned primarily with architectural forms that have developed through deformation as a response to forces and stresses.

Deformation is a word that is used in several ways. It refers to the structural changes that take place in the original location, orientation, shape, and volume of a body of rock. It refers to the physical and chemical processes that produce the structural changes. And it refers to the geologic structures that form to accommodate the changes. Any body of rock, no matter how hard, will deform if the conditions are right. This concept emerges in historic photographs of a fence line located at the site of the Hebgen Lake earthquake, a destructive quake that wracked southwestern-most Montana in 1959. As a result of shifts in the ground surface, the fence was forced to shorten. Where shifts were modest, shortening was accommodated by bending (Figure 1.1A). But where shortening exceeded the bending limit of the wooden slats, the fence fractured and splintered abruptly (Figure 1.1B).

Structural deformation results from stresses (for now, think of "pressures") that exceed rock strength. When strength is exceeded, the rock will fail by brittle (fracture) or ductile (flow) deformation, depending on how the physical environment has affected the ability of the rock to resist the stresses. For example, the ability of a rock to withstand stresses decreases with increasing temperature. Stresses are created in nature in countless ways: The weight of thousands of meters of sediments within a depositional basin creates a vertical stress that generally results in the thinning and compaction of the sediments as they are buried deeper and deeper. The forceful intrusion of magma can "shoulder aside" rocks and

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DEFORMATION OF THE EARTH'S CRUST 3

Figure I.I (A) Buckled fence and (B) broken fence, Hebgen Lake earthquake area, Montana. Fences, like rocks, respond in different ways to shortening. (Photograph by J. R. Stacy. Courtesy of United States Geological Survey.)

produce folding and faulting or stretching and thinning of the country rock that is invaded. The cooling of an igneous body, such as a basalt lava flow, causes shrinkage and contraction expressed in columnar jointing (Figure 1.2). The slow, steady "head-on" convergence of plates at plate boundaries produces major fault systems and fold belts, in some cases raising beds to vertical orientations (Figure 1.3). The spreading apart of plates along the oceanic ridges stretches the oceanic crust by faulting, rifting, and the injection of swarms of dikes. Where tectonic plates slide past one another, such as along the San Andreas fault in California, the buildup of stress results in sudden punctuated movement announced by earthquakes. The gravitational collapse of volcanoes above evacuated magma chambers can produce enormous craterlike calderas, like Crater Lake in Oregon (Figure 1.4).

Stresses that cause deformation generally build slowly but persistently, but in some situations incredibly high stresses "just show up." We have in mind Meteor Crater, located in northern Arizona, where asteroid impact created a bull's-eye of deformational destruction (Figure 1.5A). The now-

Figure 1.2 Columnar joints formed in basalt, exposed at San Miguel Regla, Hidalgo, Mexico. (Photograph by C. Fries. Courtesy of United States Geological Survey.)

4 CHAPTER I NATURE OF STRUCTURAL GEOLOGY

Figure 1.3 Steeply inclined limestone beds of Cretaceous age in the Central Andes east of Lima, Peru. The lake in the foreground is at an elevation of 14,000 ft (over 4 km); rocks in the background reach 18,000 ft (over 5 km). The limestone beds were originally deposited below sea level! (Photograph by G. H. Davis.)

upturned, pervasively fractured and distorted sedimentary rocks never knew what hit them (Figure 1.5B), as revealed by the telltale presence of a peculiar microscopic mineral texture aptly named "shocked" quartz (Figure 1.5C).

ARCHITECTURE AND STRUCTURES

Jacob Bronowski (1973), in his superb set of essays entitled The Ascent of Man, suggests that our conception of science today is a description and exploration of the underlying structures of nature, and he points out that words like "structure," "pattern," "plan," "arrangement," and "architecture" constantly occur in every description that we try to make. He believes:

The notion of discovering an underlying order in matter is man's basic concept for exploring nature. The architecture of things reveals a structure below the surface, a hidden grain, which when it is laid bare, makes it possible to take natural formations apart. . . . (From The Ascent of Man by J. Bronowski, p. 95. Published with permission of Little, Brown and Company, Boston, copyright 1973.)

Bronowski's remarks apply beautifully to structural geology, which can be most succinctly defined as the study of the architecture of the Earth's crust, insofar as it has resulted from deformation (Billings, 1972, p. 2). The expression "architecture of the Earth" is very appropriate because structural geology addresses the form, symmetry, geometry, and certainly the elegance and artistic rendering of the components of the Earth's crust on all scales (Figure 1.6). At the same time, structural geology focuses on the strength and mechanical properties of crustal materials, both at the time of their deformation and now.

Although architecture and structural geology have much in common, the challenges of the architect and the structural geologist are quite different. The architect designs a structure, perhaps a building or a bridge,

5 ARCHITECTURE AND STRUCTURES

A

Figure 1.4 (A) Eruptive formation of prehistoric volcano, Mount Mazama. (6) Caldera collapse of the volcanic edifice into the emptied magma chamber. (C) Lake forms within the caldera structure. At the center of the lake minor eruptions build a small cinder cone, part of which (not shown) is Wizard Island. [Drawing by Charles R. Bacon. Courtesy of United States Geological Survey (1988).]

B

C

6 CHAPTER I NATURE OF STRUCTURAL GEOLOGY

Figure 1.5 (A) Oblique aerial photo of Meteor Crater, Arizona. (Photograph and copyright by Peter Kresan). (6) Diagrammatic representation of the formation of Meteor Crater, and the accompanying upturning of formerly horizontal strata. [After Shoemaker (1979), fig. 4, p. I I.] (C) Shocked quartz collected from Meteor Crater. The rock, derived from the Coconino Sandstone, is 75% quartz, 20% coesite, and 5% glass. The light-colored quartz grains are deformed to fit into a mosaic. Individual quartz grains are close to 0.1 mm in diameter. The black opaque areas and the medium gray areas are the main regions of coesite and stishovite. (Photomicrograph by Susan Kieffer).

giving due attention to function, appearance, geometry, material, size, strength, cost, and other such factors. Then the architect supervises the process of construction daily, or perhaps weekly, making changes where necessary. In the end, the architect may be the only person who is aware of discrepancies between the original plan and the final product.

In contrast, the structural geologist is greeted in nature by what looks like a finished product, like the structural product shown in Figure 1.7, and is challenged to ask a number of questions. What is the structure? What starting materials were used? What is the geometry of the structure? How did the materials change shape during deformation? What was the source of the stresses that caused the deformation? And what was the sequence of steps in construction? Attempts to answer these questions generate even more questions. When was the job done? How long did it take? What were the temperature and pressure conditions? How strong were the materials? And, why "on earth" was it done?

The complexity of interpreting natural systems hit home to me in the reflections of a small pool I encountered amid dense underbrushwithin the bush of eastern Canada. Rock exposure is poor in this region, and thus clues regarding structural history are meager. Yet the surface waters of this pool were marked by foam patterns that conveyed geological insight. Delicately fashioned, these patterns resembled the layering of rocks deformed under hot, deep conditionsexactly the kind of rocks that are exposed in the immediately surrounding area. This pool became my one-day laboratory. Patterns of movement on the surface of the pool were both complex and ever-changing. Seeing the patterns come and go, my mind shifted to what would happen to these structures when winter set in. Some single pattern would be frozen; one of an infinite number of patterns would be preserved; and yet, that pattern might or might not be representative of the kinds of motion I had watched. I began to realize more fully that every geologic record we examine is but one out of millions of possible frozen records, stop-action points, tiny scenarios from a much longer and more complex drama that we never will know in full.

I was reminded of all this later in Utah when I looked down into a pool beneath a bridge and saw flow patterns of the same kind (Figure 1.8). But this time, the pattern carried more symbolism. The ordered patterns,

Figure 1.6 White House Ruin in Canyon de Chelly, Arizona, a sublime blend of the architecture of nature and that of the Ancient Ones. (Photograph by G. H. Davis.)

Figure 1.7 Geologist confronting the structure of nature, in this case an exfoliation jointing in granite near Shuteye Peak in the Sierra Nevada. (Photograph by N. K. Huber. Courtesy of United States Geological Survey.)

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8 CHAPTER I NATURE OF STRUCTURAL GEOLOGY

Figure 1.8 Deformed foam layers in a Rocky Mountain pool. Paper cup (upper right corner) is being blown by the wind from right to left, while the water is being pulled by gravity toward the lower right. (Photograph by G. H. Davis.)

guided ultimately by the flow of water under the compelling tug of gravity, were being modified simultaneously by the competing movement of a paper cup, blown by the wind, superimposing the imprint of its wake. In similar fashion, we see in matters of geologic record the complex, interfering effects of competition among the agents of gravity, heat, and tectonic stress in fashioning architectural form. And we see in the paper cup the influence we humans have on the natural environment.

PLATE TECTONICS AND STRUCTURAL GEOLOGY

Plate tectonics provides an essential backdrop for understanding the significance of structures, especially regional structures. It is the basis for understanding the dynamic circumstances that give rise to deformational movements. Plate interactions create rock-forming environments, which in turn give rise to the fundamental, original properties of regional rock assemblages. Furthermore, plate motions, both during and after the construction of regional rock assemblages, generate the stresses that impart to rocks their chief deformational characteristics.

Plate motions in the past have been responsible for shaping orogenic belts (or simply orogens), which are long, broad, and generally linear to arcuate belts in the Earth's crust where extreme mechanical deformation and/or thermal activity are concentrated. The Appalachians, Alps, Andes, and Himalayas are examples. Major regional structures abound in orogens, and these reflect systematic distortion (i.e., change in shape) of the crust in which the structures are found. Mountain systems are a physiographic expression of orogenic belts, but the presence of mountains is not integral to our view of an orogen. Ancient orogens, still recognizable as sites of regional distortion, are beveled to flatlands in the interior of continents. And of the presently forming orogens, the structurally interesting parts may not lie in the mountains, but instead may be 10, 50, or even 700 km below the Earth's surface. In this perspective, mountains, if they exist at all, are just the roofline of an orogen.

The generally accepted view among geologists today is that orogenic belts evolve through the interference of slowly moving rigid plates com-

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Front MatterPrefaceTable of ContentsPart I. Fundamentals1. Nature of Structural Geology1.1 Deformation of the Earth's Crust1.2 Architecture and Structures1.3 Plate Tectonics and Structural Geology1.4 The Fundamental Structures1.5 Concept of Detailed Structural Analysis1.6 Descriptive Analysis1.6.1 The Basis1.6.2 The Scale of Things1.6.3 Structural Elements

1.7 Kinematic Analysis1.7.1 General Approach1.7.2 Penetrative Deformation1.7.3 Slip, Flow, and Distortion

1.8 Dynamic Analysis1.8.1 General Approach1.8.2 Physical Models1.8.3 Mathematical Models

1.9 Two Examples of Detailed Structural Analysis1.9.1 Detailed Structural Analysis of a Pizza1.9.2 Detailed Structural Analysis of the San Manuel Fault

1.10 The Time Factor

2. Kinematic Analysis2.1 Strategy2.2 Translation2.2.1 General Concept2.2.2 Displacement Vectors2.2.3 Slip on Faults

2.3 Rotation2.3.1 General Concept2.3.1 Geological Examples

2.4 Strain2.4.1 General Concept2.4.2 The Ground Rules2.4.3 The Magic of Strain2.4.4 The Strain Ellipse2.4.5 Looking at Lines inside an Ellipse2.4.6 Describing Changes in Lengths of Lines2.4.7 Change in Length of a Deformed Belemnite Fossil2.4.8 Expressing Changes in Length Due to Folding and Faulting2.4.9 Line Length Changes When a Circle Becomes an Ellipse2.4.10 Angular Shear: Measure of Change in Angles between Lines2.4.11 Shear Strain2.4.12 The Finite Strain Ellipse2.4.13 Calibrating the Finite Strain Ellipse2.4.14 Evaluating the Strain of Lines in a Body2.4.15 The Fundamental Strain Equations2.4.16 Calculating the Variations in Strain2.4.17 The Mohr Strain Diagram2.4.18 Using Mohr Circle Strain Analysis at the Outcrop Scale2.4.19 Another Example: The Retrodeforming of Regional Strain2.4.20 The Finite Strain Ellipsoid and Plane Strain2.4.21 The Strain Ellipsoid and its Application2.4.22 Dilational Changes2.4.23 Coaxial and Noncoaxial Strain2.4.24 Pure Shear and Simple Shear2.4.25 Just a Word on Progressive Deformation

2.5 General Shear, and Changing the Shapes of Things, in Practice2.6 The Issue of Structural Compatibility2.7 On to Dynamics

3. Dynamic Analysis3.1 Concept of Dynamic Analysis3.2 Force3.2.1 Definition of Force3.2.2 Mass and Weight3.2.3 Units of Force3.2.4 What Does a Newton Feel Like?3.2.5 Forces as Vectors3.2.6 Forces in the Subsurface World3.2.7 Types of Forces3.2.8 Definition of Load

3.3 Stress3.3.1 Definition of Stress3.3.2 Units of Stress3.3.3 Museum-Piece Stress Calculation3.3.4 Calculating Stress Underground3.3.5 A Fuller Definition of Stress3.3.6 Stress and Stress Tensor Analysis3.3.7 Setting Up an Example of Stress Analysis3.3.8 The Stress Calculations3.3.9 Resolving Normal Stress and Shear Stress3.3.10 Computing the Stress Tensor3.3.11 The Stress Ellipse and the Stress Ellipsoid3.3.12 The Special Case of Hydrostatic Stress3.3.13 The Stress Equations3.3.14 The Mohr Stress Diagram3.3.15 Images of Stress

3.4 Experimentally Observed Relationships between Stress and Strain3.4.1 Objectives and Hurdles3.4.2 The Value of Laboratory Deformational Experiments3.4.3 Sample Preparation3.4.4 Types of Tests3.4.5 Pressures, Stresses, and Loads3.4.6 Measuring Shortening3.4.7 Measuring Strain Rate3.4.8 A Standard Axial Compression Test3.4.9 Compression Test at Higher Confining Pressure3.4.10 Still Higher Confining Pressure3.4.11 Strength and Ductility3.4.11.1 Lithology3.4.11.2 Confining Pressure and Pore Fluid Pressure3.4.11.3 Temperature3.4.11.4. Strain Rate3.4.11.5 Time3.4.11.6 Preexisting Weaknesses3.4.11.7 Size

3.5 Elastic, Plastic, and Viscous Models of Rock Behavior3.5.1 The Need for Models of Behavior3.5.2 Elastic Behavior3.5.3 An Example of the Significance of Young's Modulus3.5.4 Plastic Behavior3.5.5 Viscous Behavior3.5.6 Summary

3.6 Conclusion

4. Deformation Mechanisms and Microstructures4.1 Crystalline Structure and the Strength of Solids4.1.1 Bonding and the Lattice of Crystals4.1.2 Elastic Deformation of a Lattice4.1.3 Exceeding the Elastic Limit4.1.4 Slip Systems and Crystallographic Control4.1.5 Slip Systems and Bonding4.1.6 Theoretical Strength of Crystals4.1.7 Crystals and Lattices in the Real World4.1.8 Defects4.1.8.1 Point Defects4.1.8.2 Line Defects4.1.8.3 Planar Defects

4.2 Deformation Mechanisms4.2.1 The Main Mechanisms4.2.2 Microfracturing, Cataclasis, and Frictional Sliding4.2.2.1 Formation of Microcracks4.2.2.2 Microcracks and Grain-Scale Fractures4.2.2.3 Cataclasis and Cataclastic Flow

4.2.3 Mechanical Twinning and Kinking4.2.3.1 Mechanical Twinning4.2.3.2 Conditions Favoring Mechanical Twinning4.2.3.3 Determining Strain and Stress from Mechanical Twins4.2.3.4 Kinking

4.2.4 Diffusion Creep4.2.4.1 Diffusion4.2.4.2 Volume-Diffusion Creep4.2.4.3 Grain-Boundary Diffusion Creep4.2.4.4 Superplastic Creep

4.2.5 Dissolution Creep4.2.5.1 Processes of Dissolution Creep4.2.5.2 Conditions Favoring Dissolution Creep

4.2.6 Dislocation Creep4.2.6.1 Types of Dislocations4.2.6.2 Interactions with the Lattice and Strain Hardening

4.2.7 Recovery and Recrystallization4.2.7.1 Recovery and Rotation Recrystallization4.2.7.2 Boundary-Migration Recrystallization4.2.7.3 Conditions Favoring Recrystallization

4.2.8 Summary

4.3 Deformation Experiments4.3.1 Creep Experiments4.3.2 Recovery-Recrystallization Experiments4.3.3 Analogue Experiments4.3.4 Using Computers to Model Deformation and the Development of Crystallographic Preferred Orientations

4.4 The Brittle-Ductile Transition4.5 A Few Final Thoughts

Part II. StructuresPart III. Descriptive AnalysisReferencesAuthor IndexSubject Index