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Foliation and cleavage are terms for penetrative tectonic planar structures in rocks. Tectonic foliations go hand in hand with folds and lineations and form the most common type of structure encountered in metamorphic rocks, and their wide occurrence makes them particularly important for deciphering the deformation history of rocks. Primary foliations such as bedding are needed to initiate buckle folds and to observe folds in rocks in general. And where strain markers are absent, tectonic foliations provide us with useful and widespread strain information, since most foliations are associated with perpendicular shortening. Cleavage and foliations also create slates and schists that are of significant economic importance all over the world. In this chapter we will introduce basic terminology and discuss how and under what conditions different types of foliations initiate.
Turbidite layers, deformed by folding and thrusting in the foreland of the Variscan orogeny in the Almograve area, southwest Portugal. White surfaces are quartz veins that formed in the competent sandstone layers.
The San Rafael monocline in southern Utah. The steep limb of this monocline is easily visible, forming ridges of resistant Triassic-Jurassic rock layers (hogbacks). The fold is interpreted as a fault-propagation fold that formed above a reactivated basement fault during the Laramide orogeny.
Deformed rocks and their structures and fabrics can be studied and mapped, and we had a glimpse of some methods and techniques in Chapter 1. Each structure reflects a change in shape and perhaps transport within a given reference frame. We generally refer to these changes as deformation, and as we inspect deformed rocks we automatically start to imagine what the rock could have looked like before the deformation started and what it has gone through. If we want to understand the structures we need to understand the fundamentals of deformation, including some useful definitions and mathematical descriptions. That is the topic of this chapter.
Traditionally, extensional structures have received less attention than their contractional counterparts. However, the tide turned in the 1980s when it was realized that many faults and shear zones traditionally thought to represent thrusts carried evidence of being low-angle extensional structures. First recognized in the Basin and Range province in the western USA, it is now clear that extensional faults and shear zones are widespread in most orogenic belts. Most would agree that the study of extensional structures has significantly changed our understanding of orogens and orogenic cycles. The current interest in extensional faults is also related to the fact that many of the world’s offshore hydrocarbon resources are located in rift settings, and many hydrocarbon traps are controlled by normal faults. Also, the development of most hydrocarbon reservoirs requires a sound understanding of extensional faults and their properties and complexities.
Restoring a geologic cross-section or map to its original, pre-deformational state is an important part of making a structural interpretation. We want to be able to restore our deformed section to a geologically feasible undeformed section. For simplicity, we usually assume that either length or area (or volume in three-dimensional analyses) is preserved. If preserved, the section is balanced, meaning that length, area or volume of the restored section “balances” that of the strained section (the interpretation). Such exercises were first performed in areas of contraction, and are now routinely applied to extensional areas. Balancing puts important constraints on geologic interpretations, although there is no guarantee that a balanced section is correct. In this chapter we look at the basic premises and methods for balancing and restoration, mostly in sections and map view, and point out some of their usefulness and shortcomings.
As structural geologists we need to make objective observations and analyses based on our knowledge of structural geology. We then have to put our local observations together to come up with or evaluate a larger, regional model. We have to put our observations together in time, to construct a history of deformation, or perhaps come up with a model that incorporates sedimentary information, intrusive relations or metamorphic data. Combining structural observations with other information is always necessary, and in this chapter we will have a very brief look at a few relevant examples, particularly the separation of deformation into phases, metamorphic petrology, P–T–t paths and depositional patterns. The treatment will be brief, and is meant to point out some important principles and directions rather than discussing examples and methods in detail.
Brittle structures such as joints and faults are found almost everywhere at the surface of the solid Earth. In fact, brittle deformation is the trademark of deformation in the upper crust, forming in areas where stress builds up to levels that exceed the local rupture strength of the crust. Brittle structures can form rather gently in rocks undergoing exhumation and cooling, or more violently during earthquakes. In either case, brittle deformation by means of fracturing implies instantaneous breakage of crystal lattices at the atomic scale, and this type of deformation tends to be not only faster, but also more localized than its plastic counterpart. Brittle structures are relatively easily explored in the laboratory, and the coupling of experiments with field and thin-section observations forms the basis of our current understanding of brittle deformation. In this chapter we will look at the formation of various small-scale brittle structures and the conditions under which they form.
Almost all rock outcrops exhibit joints – thin extension fractures that penetrate rocks without any appreciable shear displacement – and many well-exposed regions show joint systems defined by sets of parallel and planar joints. Because there are so many of them, and because they weaken rocks and conduct fluids, they can be extremely important structures in the uppermost crust. Tunnel makers, reservoir engineers, solid rock hydrogeologists and magmatic geologists trying to understand intrusion mechanisms all have to deal with joints in one way or another, simply because they are weak and laterally extensive structures that easily affect geologic processes. For example, petroleum geologists and engineers are concerned with joints. Geologists do not want them in the cap rock of hydrocarbon structures, but production engineers deliberately create them in reservoir formations to enhance the flow of fluids into production wells.
Structural geology is about folds, faults and other deformation structures in the lithosphere – how they appear and how and why they formed. Ranging from features hundreds of kilometers long down to microscopic details, structures occur in many different settings and have experienced exciting changes in stress and strain – information that can be ours if we learn how to read the code. The story told by structures in rocks is beautiful, fascinating and interesting, and it can also be very useful to society. Exploration, mapping and exploitation of resources such as slate and schist (building stone), ores, groundwater, and oil and gas depend on structural geologists who understand what they observe so that they can present well-founded interpretations and predictions. In this first chapter we will set the stage for the following chapters by defining and discussing fundamental concepts and some of the different data sets and methods that structural geology and structural analysis rely on. Depending on your background in structural geology, it may be useful to return to this chapter after going through other chapters in this book.
In Chapter 3 we looked at how strain can be observed and measured in deformed rocks. The closely related concept of stress is a much more abstract concept, as it can never be observed directly. We have to use observations of strain (preferentially very small strains) to say something about stress. In other words, the deformation structures that we can observe tell us something about the stress field that the rock experienced. The relation is not straightforward, and not even the most precise knowledge of the state of stress can predict the resulting deformation structures unless additional information, such as mechanical or physical properties of the rock, temperature, pressure and physical boundary conditions, is added. The most basic concepts of stress are presented here, before looking at stress in the lithosphere and the relations between stress, strain and physical properties in the following two chapters.
In most of this book we study structures observable in thin sections, outcrops, maps and satellite photos. However, it is very useful and interesting to also take a closer look at the processes and mechanisms that take place from the grain scale down to that of atoms. This is a range that is more difficult to approach, especially the atomic scale, but a basic understanding is important and forms a foundation for a good understanding of mesoscale structures. The most important distinction is between brittle and plastic deformation mechanisms. Brittle deformation is sudden and violent: atomic lattices are forcefully torn apart and the lattice structure is forever damaged and weakened. Mechanisms in the plastic regime are more complicated and sluggish.
Linear structures go hand in hand with planar structures in deformed rocks, where they are mesoscopic structures pointing in a specific direction. We have already looked at the role of lineations that are found on slip surfaces and how they can reveal paleostress and kinematics. Lineations are even more common in metamorphic rocks, where they tend to be closely associated with strain and transport or shear directions. In this chapter we will sort out the different types of lineations that are commonly encountered in deformed rocks and discuss their origins and implications.
Strike-slip faults constitute an important class of faults that have been studied for more than 100 years. They first received attention in California, Japan and New Zealand, where very long strike-slip faults with considerable displacement intersect the surface of the Earth. They are known for their close association with devastating earthquakes, particularly in places such as California and Turkey. Understanding such faults and the tectonic regimes in which they occur is therefore of public as well as academic interest. In this chapter we will address the basic types of strike-slip faults, their formation and tectonic settings, and also look at transpression and transtension – three-dimensional deformations that link strike-slip, extensional and contractional regimes.
Stress and strain are related, but the relationship depends on the properties of the deforming rock, which themselves depend on physical conditions such as state of stress, temperature and strain rate. A rock that fractures at low temperatures may flow like syrup at higher temperatures, and a rock that fractures when hit by a hammer may flow nicely at low strain rates. When discussing rock behavior it is useful to look to material science, where ideal behaviors or materials (elastic, Newtonian and perfectly plastic) are defined. These reference materials are commonly used when modeling natural deformation. This is what we will do in this chapter, and we will focus on a very useful arena for exploring related rock deformation, which is the rock deformation laboratory. Experimenting with different media has greatly increased our knowledge about rock deformation and rheology.
In the previous chapters we have indicated that a close relationship exists between stress and faulting, for example according to Anderson’s tectonic stress regimes. It should therefore be possible to say something about the stress field at the time of faulting and fracturing, based on the orientation and nature of the faults and fractures. This is referred to as paleostress analysis, a field that is hampered by several assumptions. However, many paleostress analyses yield reasonable results, as can be verified by independent information. The fundamental input to paleostress analysis is kinematic observations of fault structures made in the field. Relevant structures and the fundamentals of paleostress analysis in the brittle regime are briefly presented in this chapter.