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.

With a basic understanding of the nature of stress, we will now look at how we get information about stress in the crust and how to understand it. A large number of stress measurements have been performed globally over the last few decades. These measurements indicate that the stress conditions in the crust are complex, partly because of geologic heterogeneities (faults, fracture zones and compositional contrasts), and partly because many areas have been exposed to multiple phases of deformation, each associated with different stress fields. The latter is of importance because the crust has the ability to “freeze in” a state of stress and preserve remnants of it over geologic time. Knowledge of the local and regional stress fields has a number of practical applications, including evaluation of tunneling operations, drilling and stimulation of petroleum and water wells. Besides, knowledge of the present and past states of stress provides important information about tectonic processes, then and now.

Strain can be retrieved from rocks through a range of different methods. Much attention has been paid to one-, two- and three-dimensional strain analyses in ductilely deformed rocks, particularly during the last half of the twentieth century, when a large portion of the structural geology community had their focus on ductile deformation. Strain data were collected or calculated in order to understand such things as thrusting in orogenic belts and the mechanisms involved during folding of rock layers. The focus of structural geology has changed and the field has broadened during the last couple of decades. Today strain analysis is at least as common in faulted areas and rift basins as in orogenic belts.

Salt as a rock has properties and behavior that are very different from most other rocks. This implies that when sedimentary sequences containing salt layers are deformed, they develop their own characteristic and often very fascinating structural styles. Salt ridges, pillows, diapirs and even salt glaciers are special structures that are of importance in many settings. Even where the salt is restricted to a thin layer, it can control the structural expression and increase the areal extent of the deforming area because of its tendency to act as a décollement. Salt-related structures are of great importance to geologists working in regions of extensional as well as contractional tectonics, and are also important because many petroleum provinces contain salt layers or are deformed by salt tectonics. This chapter deals specifically with salt structures and salt tectonics, and provides an overview of salt structure geometries, processes and tectonic settings with examples from several places around the world.

Folds are eye-catching and visually attractive structures that can form in practically any rock type, tectonic setting and depth. For these reasons they have been recognized, admired and explored since long before geology became a science (Leonardo da Vinci discussed them some 500 years ago, and Nicholas Steno in 1669). Our understanding of folds and folding has changed over time, and the fundament of what is today called modern fold theory was more or less consolidated in the 1950s and 1960s. Folds, whether observed on the micro-, meso- or macroscale, are clearly some of our most important windows into local and regional deformation histories of the past. Their geometry and expression carry important information about the type of deformation, kinematics and tectonics of an area. Besides, they can be of great economic importance, both as oil traps and in the search for and exploitation of ores and other mineral resources. In this chapter we will first look at the geometric aspects of folds and then pay attention to the processes and mechanisms at work during folding of rock layers.

Faults disturb layered rock sequences, introducing “faults” or “defects” to the primary lithologic framework. While representing challenges to stratigraphers and geologists mapping rocks in the field or interpreting seismic data, faults are extremely intriguing structures that have fascinated structural geologists as much as they have frustrated stratigraphers, petroleum geologists and miners. We know much more about faults today than just a few decades ago, largely because of their importance to the petroleum industry. Faults also represent challenges associated with waste repositories and tunnel operations, and active faults are closely associated with earthquakes and seismic hazards. In this chapter we will focus on fault geometry, fault anatomy and the evolution of faults and fault populations, with examples and applications relevant to the petroleum industry.

Strain, and shear strain in particular, tends to localize into zones or bands. We have already looked at some types of strain localization structures, such as shear fractures and faults that form in the brittle regime. Localization also occurs in the plastic regime, where foliations and sheared markers tend to show continuity across the zone. Such classic shear zones form an important end-member in a spectrum of shear zones in which both microscale deformation mechanism and ductility vary. On the other end of this spectrum are faults with a measurable thickness. Shear zones can be many kilometers wide, but they also occur on the scale of a hand sample. We will look at shear zones and their internal structure and strain pattern in this chapter, going from a discussion of definitions via the ideal shear zone to different and more complex types of high-strain zones. The last part is devoted to kinematic structures, structures that can reveal the sense of movement in a shear zone, and shear zone growth.

In the plastic regime, layers tend to fold when shortened, particularly if there is a viscosity contrast between individual layers. In this chapter we will look at how layers that are being stretched can part into pieces known as boudins. Classic boudins represent the counterpart to buckle folds and provide solid evidence of layer-parallel extension that is preserved even if the layers experience later shortening and folding. Just like folds, boudins form in different ways and provide us with different types of information that are well worth our attention.