Fault-related Rocks: A Photographic Atlas

by Arthur W. Snoke (Editor), Jan Tullis (Editor), Victoria R. Todd (Editor)

Overview

This is a richly illustrated reference book that provides a unique, comprehensive, and up-to-date survey of the rocks and structures of fault and shear zones. These zones are fundamental geologic structures in the Earth's crust. Their rigorous analysis is crucial to understanding the kinematics and dynamics of the continental and oceanic crust, the nature of earthquakes, and the formation of gold and hydrocarbon deposits. To document the variety of fault-related rocks, the book presents more than six hundred photographs of structures ranging in scale from outcrop to submicroscopic. These are accompanied by detailed explanations, often including geologic maps and cross sections, contributed by over 125 geoscientists from around the world.

The book opens with an extensive introduction by Arthur W. Snoke and Jan Tullis that is itself a major contribution to the field. Fault-related rocks and their origins have long been controversial and subject to inconsistent terminology. Snoke and Tullis address these problems by presenting the currently accepted ideas in the field, focusing on deformation mechanisms and conceptual models for fault and shear zones. They define common terminology and classifications and present a list of important questions for future research. In the main, photographic part of the book, the editors divide the contributions into three broad categories, covering brittle behavior, semi-brittle behavior, and ductile behavior. Under these headings, there are contributions on dozens of subtopics with photographs from localities around the world, including several "type" areas.

The book is an unrivaled source of information about fault-related rocks and will be important reading for a broad range of earth scientists, including structural geologists, petrologists, geophysicists, and environmental specialists.

Originally published in 1998.

The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Product Details

ISBN-13:	9780691600734
Publisher:	Princeton University Press
Publication date:	07/14/2014
Series:	Princeton Legacy Library , #410
Pages:	634
Product dimensions:	8.50(w) x 10.80(h) x 1.50(d)

Read an Excerpt

Fault-related Rocks

A Photographic Atlas

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By Arthur W. Snoke, Jan Tullis, Victoria R. Todd

PRINCETON UNIVERSITY PRESS

Copyright © 1998 Princeton University Press
All rights reserved.
ISBN: 978-0-691-01220-9

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CHAPTER 1

AN OVERVIEW OF FAULT ROCKS

Arthur W. Snoke and Jan Tullis

A brief discussion of nomenclature and classification 4
Deformation mechanisms 9
Cracking and frictional sliding 10
Pressure solution 11
Mechanical twinning 12
Dislocation creep 12
Diffusion creep and grain-boundary sliding 14
Comments on fault- and shear-zone models 15
Concluding remarks 17

The characteristics of faults and shear zones are strikingly variable. One goal of this Atlas is to provide a visual overview of the characteristics of the rocks associated with faults and shear zones. Such rocks are broadly called fault rocks (Sibson, 1977) and range from brittle, incohesive breccia or gouge at the highest structural levels in the Earth's crust to high-temperature, crystal-plastic mylonites in the deep crust or upper mantle.

In this overview, we include (1) a brief discussion of fault-rock nomenclature and classification. (2) a summary of the important grain-scale deformation mechanisms operative within fault and shear zones, (3) comments on crustal-scale fault- and shear-zone models, and (4) some important questions regarding fault rocks and fault and shear zones. Although this essay thus touches upon some of the major topics that have dominated research on fault rocks and fault and shear zones during the past twenty or more years, our treatment is not meant to be exhaustive.

At the outset of this overview, it is important to discuss the commonly used terms brittle and ductile. Some authors have used these terms genetically, but as Rutter (1986) points out, they are best used to describe the macroscopic nature of deformation. If the deformation involves a through-going discontinuity (a fault), it is brittle; if the deformation is continuous and homogeneously distributed, it is ductile. Brittle deformation involves cracking and frictional sliding, whereas ductile deformation can be accomplished by a variety of deformation mechanisms ranging from cataclastic flow to crystal-plastic processes (see the later section on deformation mechanisms). Thus, the use of the terms brittle and ductile depends on the scale that one is referring to: for example, cataclastic flow involves cracking on the grain scale, but because the cracks remain distributed the deformation is macroscopically ductile. However, cataclastic flow may occur within the gouge or cataclasite of a (brittle) fault zone.

We recognize the problems in terminology highlighted by Rutter (1986), the importance of specific deformation mechanisms as argued by Schmid and Handy (1991), and the evidence of both brittle and crystal-plastic deformation in some low-grade, polyphase mylonitic rocks. However, in an attempt to organize a diverse group of fault rocks, we have chosen a classification based upon macroscopic features rather than genetic factors. Thus, in the organization of this Atlas, we have grouped the plates into three categories: brittle behavior, semi-brittle behavior, and ductile behavior.

Major fault zones are commonly rooted in the middle to lower crust (e.g., see Ramsay, 1980a; Coward. 1984; Figure 0.1) and evolve over a considerable period of time. Consequently, a fault zone with a significant dip-slip component may consist of a wide variety of fault rocks representing a range in P, T, and fluid conditions (e.g., Sibson, 1977; Figure 0.2). Fault rocks from different depths in the crust will record different deformation mechanisms. Fault zones commonly exhibit a complex overprinting of fault-rock microstructures. recording changes in deformation mechanisms during progressive deformation or deformation during exhumation. It is common to observe crystal-plastic microstructures overprinted by brittle fracturing. The reverse of this situation. in which brittlely deformed rocks were overprinted by high-temperature metamorphic processes (e.g., recrystallization and grain growth), may also be common, but the early brittle history is difficult, if not impossible, to recognize after the superposition of a higher-temperature event.

In the case of large-displacement, normal-sense fault zones, crystal-plastic deformation microstructures in the footwall block are juxtaposed against brittle deformation microstructures of the hanging-wall block (e.g., Cordilleran-type metamorphic core complexes; also see Passchier, 1984). For large-displacement, reverse-sense fault zones, crystal-plastic deformation in the hanging-wall block (e.g., mylonitic rocks) is juxtaposed above brittle deformation of the footwall block. Schmid and Handy (1991) noted the common juxtaposition of fault rocks of different character across major dip-slip fault zones and concluded that such a distribution indicates that the rate of displacement and shearing far exceeded the rate of thermal equilibration across the fault zone. In other cases, fault rocks have developed over a protracted period of time, as when an old shear zone was reactivated by a significantly younger tectonic event. Many authors have inferred that long-established ductile shear zones influence the location of younger, brittle fault systems. Older brittle faults can also influence the location of younger brittle faults, e.g., in the Sevier fold-and-thrust belt of the U.S. Cordillera, late listric normal faults are developed near ramps of older thrust faults (Royse and others. 1975: Royse, 1993).

A brief discussion of nomenclature and classification

Nomenclature associated with fault rocks remains controversial (Tullis and others, 1982; Schmid and Handy, 1991) despite many attempts at systematic classifications (e.g., Spry, 1969; Higgins, 1971; Zeck, 1974; Sibson, 1977; White, 1982; Wise and others, 1984; Heitzmann, 1985). A problem that has plagued virtually all classification schemes is the use of genetic terms in describing or naming fault rocks when the deformation mechanisms that produced the texture and fabric of the rocks are poorly known or incorrectly inferred. On the other hand, devising a completely nongenetic classification has proven difficult (Schmid and Handy, 1991). We will return to this issue of fault-rock nomenclature in a later part of this overview, but first we present the historic usage of several important terms commonly associated with fault rocks. It should be noted that most of the terms and concepts were based on observations of quartzo-feldspathic rocks.

The term mylonite was introduced by Lapworth (1885) to describe rocks found within the Moine thrust zone (specifically at Arnaboll Hill, Eriboll) in northwest Scotland. Lapworth described the field setting and overall characteristics of mylonite as follows (1885, p. 559):

The most intense mechanical metamorphism occurs along the grand dislocation (thrust) planes, where the gneisses and pegmatites resting on those planes are crushed, dragged, and ground out into a finely-laminated schist (Mylonite, Gr. mylon, a mill) composed of shattered fragments of the original crystals of the rock set in a cement of secondary' quartz, the lamination being defined by minute inosculating lines (fluxion lines) of kaolin or chloritic material and secondary crystals of mica. Whatever rock rests immediately upon the thrust-plane, whether Archsean, igneous, or Palaeozoic, &c., is similarly treated, the resulting mylonite varying in colour and composition according to the material from which it is formed. The variegated schists which form the transitional zones between the Arnaboll gneiss and Sutherland mica-schists are all essentially mylonites in origin and structure, and appear to have been formed along many dislocation planes, some of which still show between them patches of recognisable Archaean and Palaeozoic rocks. These variegated schists (Phyllites or Mylonites) differ locally in composition according to the material from which they have been derived, and in petrological character according to the special physical accidents to which they have been subjected since their date of origin—forming frilled schists, veined schists, glazed schists, &c., &c. The more highly crystalline flaggy mica-schists, &c., which lie generally to the east of the zones of the variegated schists, appear to have been made out of similar materials to those of the variegated schists, but to have been formed under somewhat different conditions. They show the fluxion-structure of the mylonites; but the differential motion of the component particles seems to have been less, while the chemical change was much greater. In some of these crystalline schists (the augen-schists) the larger crystals of the original rock from which the schist was formed, are still individually recognisable, while the new matrix containing them is a secondary crystalline matrix of quartz and mica arranged in the fluxion-planes. While the mylonites may be described as microscopic pressure-breccias with fluxion structure, in which the interstitial dusty, siliceous, and kaolinitic paste has only crystallised in part; the augen-schists are pressure-breccias, with fluxion-structure, in which the whole of the interstitial paste has crystallised out. The mylonites were formed along the thrust-planes, where the two superposed rocksystems moved over each other as solid masses; the augen-schists were probably formed in the more central parts of the moving system, where the all-surrounding weight and pressure forced the rock to yield somewhat like a plastic body. Between these augenschists there appears to be every gradation, on the one hand to the mylonites, and on the other to the typical mica-schists composed of quartz and mica. Like the mylonites, the crystalline augenites and micalites present us with local differences in chemical composition (calcareous, homblendic, quartzose, &c.), suggestive of Archaean, igneous, or Palaeozoic origin. They also show similar structural varieties due to secondary physical changes (frilled, veined, glazed, &c.), as well as others due to the presence of special minerals (garnet, actinolite, &c., &c.).

Although Lapworth did not include illustrations with his initial description of mylonite, Teall provided an informative set of photomicrographs of mylonitic rocks from the Moine thrust zone, including an example of Lapworth's type mylonite (Teall's plate 1, figure 1). Teall wrote the following in regard to this sample (1918, p. 2):

During our first day's excursion in the Eribol district he [Lapworth] led us to Arnabol Hill, showed us Archaean gneiss resting almost horizontally on Cambrian quartzite, explained that this was due to an overthrust fault, and, pointing to the lower portion of the gneiss, used words to this effect'. If you want to study the microscopic structure of rolled out gneiss take a specimen from there. That was my first introduction to a special type of dynamic metamorphism, and here is the specimen I collected on that occasion.

It is a compact, cherty-looking rock, showing a streaky aspect on a joint surface. It possesses no marked cleavage, but has a tendency to break along certain planes which have a faint silky appearance suggestive of slickensides. Looked at simply as a hand specimen, and in the light of the knowledge previously available, it would have been difficult if not impossible to determine its mode of origin. But the field evidence settled this beyond a doubt, for it could be traced, in a few feet, into rocks which could be identified as portions of the Lewisian gneiss. This is the kind of rock for which Prof. Lapworth subsequently proposed the term mylonite. Under the microscope ... the streaky structure becomes still more marked. Colourless lenticles are seen to be separated by sinuous lines of darker material. Small grains of felspar may sometimes be recognised in these lenticles, which consist for the most part of irresolvable crypto-crystalline material. The dark streaks are also irresolvable.

At the type locality of mylonite, nonfoliated structureless fault rocks are scarce (White, 1982), and were not recognized by Lapworth (1885) as part of the fault-rock suite at Arnabol Hill, Eriboll. Subsequently, Grubenmann and Niggli (1924) introduced the term cataclasite which they described as an "aphanitic, structureless rock" (White, 1982). Although mylonite and cataclasite were originally coined as distinct micro-structural terms based on field and thin section examination, their distinctiveness became blurred when the term cataclastic rocks was introduced by Waters and Campbell (1935) as a collective name for all rocks of the gouge–breccia–cataclasite–mylonite kindred. This collective association led to the misleading implication that such rocks have developed chiefly by frictional processes or cataclasis. The pervasiveness of this misconception among geologists is perhaps best demonstrated in Higgins' (1971) extensive review paper entitled "Cataclastic Rocks." For example, Higgins defines mylonite as follows (1971, p. 7):

A coherent microscopic pressure-breccia with fluxion structure that may be megascopic or visible only in thin section and with porphyroclasts generally larger than 0.2 mm. These porphyroclasts make up from about 10 to about 50 percent of the rock. Mylonites generally show recrystallization and even new mineral formation (neomineralization) to a limited degree, but the dominant texture is cataclastic.

Although Lapworth's (1885) original definition of mylonite may seem to focus on brittle deformational aspects because he used terms such as "crushed," "ground out," and "shattered," a complete reading of Lapworth's article as well as Teall's (1918) description, as pointed out by White (1982), indicates that crystal-plastic processes were also recognized as essential in mylonite formation. Teall clearly recognized the difference in behavior of quartz and feldspar (1918, p. 3): plagioclase "illustratejs] the cataclastic effects in a very beautiful manner," whereas quartz "yields more readily to the deforming stresses." Christie (1960, p. 86; 1963) recognized the importance of neomineralization in the development of mylonites in association with the Moine thrust zone in the Assynt region. He referred to such mylonitic rocks as blastomylonites and suggested that they graded into the regionally metamorphosed Moine schists. He referred to these mylonitic rocks as "primary mylonitic rocks" and argued that they were formed during the progressive metamorphism of the Moine schists in a discrete "movement-zone" associated with the Moine thrust. Christie (1960. 1963) also recognized another group of fault rocks, which he referred to as "secondary mylonitic rocks"; these fault rocks were clearly cataclastic and he reasoned that they were related to a separate, later phase of brittle deformation associated with the thrust zone.

In spite of the recognition of crystal-plastic deformation processes and neomineralization in mylonites of the Moine thrust zone, for about fifty years after Teall (1918), the brittle (cataclastic) aspects of the mylonite-generating process were generally emphasized (e.g., Hsu, 1955; Spry, 1969, table 8; Higgins, 1971, table 1). This was due in part to the lack of success in producing crystal-plastic deformation of silicates in the laboratory. However, Carter and others (1964) included a few pictures (e.g., their plate 9) of quartzites experimentally deformed at high pressures and temperatures and compared the textures in these samples to naturally deformed mylonitic quartzites from the Moine thrust zone. The authors concluded that analogous textures in both the experimentally and naturally deformed quartzites originated by recrystallization rather than by "crushing" or "granulation." Somewhat later, Tullis and others (1973) experimentally deformed quartzites over a wide range of conditions within the crystal-plastic field; their photomicrographs show flattened original grains progressively replaced with increasing strain and temperature by small, new syntectonically recrystallized grains. In that same year. Bell and Etheridge (1973) and White (1973) recognized that syntectonic recrystallization during relatively high-temperature ductile deformation was responsible for the grain-size reduction in many natural mylonites.

(Continues...)

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Excerpted from Fault-related Rocks by Arthur W. Snoke, Jan Tullis, Victoria R. Todd. Copyright © 1998 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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Table of Contents

Preface

Contributors' addresses

An overview of fault rocks 3

Brittle Behavior

Geometrical characteristics and microstructures 19

Cataclasis and gouge development 51

Pseudotachylyte 75

Fluid-related features 129

Semi-Brittle Behavior

Geometrical characteristics and microstructures 141

Fluid-related features 203

Ductile Behavior

Strain partitioning and other geometric characteristics 225

Microstructures 255

Foliation development 291

Composite foliations in mylonites 305

Porphyroclasts in mylonites 333

Folds in mylonitic zones 359

Evidence of coaxial deformational histories 369

Metamorphic grade variations 381

Fluid-related features 465

Microstructural development of mylonitic rocks of specific composition 477

References cited 595

Index 615

What People are Saying About This

From the Publisher

"This atlas is a significant contribution to the field in illustrating the broad range of structures and microstructures encountered in fault-related rocks. I know of no publication with such extensive illustrations of this important group of rocks."—John M. Christie, University of California, Los Angeles

Recipe

"This atlas is a significant contribution to the field in illustrating the broad range of structures and microstructures encountered in fault-related rocks. I know of no publication with such extensive illustrations of this important group of rocks."—John M. Christie, University of California, Los Angeles

Editorial Reviews

"Some stunning images make this library reference book a superb account of what happens to solid rocks when they flow or break apart on faults."

New Scientist

"Some stunning images make this library reference book a superb account of what happens to solid rocks when they flow or break apart on faults."

New Scientist

"This atlas is a significant contribution to the field in illustrating the broad range of structures and microstructures encountered in fault-related rocks. I know of no publication with such extensive illustrations of this important group of rocks."—John M. Christie, University of California, Los Angeles

From the Publisher