Black ellipses indicate strain, as discussed in Chapters 2 and 3. The strain deformation pattern within and around the pluton can be explained in terms of diapirism, where the intrusion ascends and squeezes and shears its outer part and the surrounding country rock to create space. Based on He et al. Ellipses in this figure illustrate the shape of enclaves inclusions , and it is clear that they become more and more elongated as we approach the margin of the pluton.
Hence, the outer part of the pluton has been flattened during a forceful intrusion history. Metamorphic growth of minerals before, during, and after deformation may also provide important information about the pressure—temperature conditions during deformation, and may contain textures and structures reflecting kinematics and deformation history.
Hence, sedimentary, magmatic and metamorphic processes may all be closely associated with the structural geology of a locality or region. These examples relate to strain, but structural geologists, especially those dealing with brittle structures of the upper crust, are also concerned with stress. Stress is a somewhat diffuse and abstract concept to most of us, since it is invisible. We can create a stress by applying a force on a surface, but at a point in the lithosphere stress is felt from all directions, and a full description of such a state of stress considers stress from all directions and is therefore three-dimensional.
There is always a relationship between stress and strain, and while it may be easy to establish from controlled laboratory experiments it may be difficult to extract from naturally formed deformation structures.
It embraces structures at the scale of hundreds of kilometers down to micro- or atomic-scale structures, structures that form almost instantaneously, and structures that form over tens of millions of years. A large number of subdisciplines, approaches and methods therefore exist within the field of structural geology.
The oil exploration geologist may be considering trap-forming structures formed during rifting or salt tectonics, while the production geologist worries about subseismic sealing faults faults that stop fluid flow in porous reservoirs; Section 8.
The engineering geologist may consider fracture orientations and densities in relation to a tunnel project, while the university professor uses structural mapping, physical modeling or computer modeling to understand mountain-building processes.
The methods and approaches are many, but they serve to understand the structural or tectonic development of a region or to predict the structural pattern in an area. In most cases structural geology is founded on data and observations that must be analyzed and interpreted. Structural analysis is therefore an important part of the field of structural geology. Structural data are analyzed in ways that lead to a tectonic model for an area.
By tectonic model we mean a model that explains the structural observations and puts them into context with respect to a larger-scale process, such as rifting or salt movements. For example, if we map out a series of normal faults indicating E—W extension in an orogenic belt, we have to look for a model that can explain this extension.
This could be a rift model, or it could be extensional collapse during the orogeny, or gravity-driven collapse after the orogeny. Age relations between structures and additional information radiometric dating, evidence for magmatism, relative age relations and more would be important to select a model that best fits the data.
It may be that several models can explain a given data set, and we should always look for and critically evaluate alternative models. In general, a simple model is more attractive than a complicated one.
There is thus a need to simplify and identify the one or few most important factors that describe or lead to the recognition of deformation structures that can be seen or mapped in naturally deformed rocks. Field observations of deformed rocks and their structures represent the most direct and important source of information on how rocks deform, and objective observations and careful descriptions of naturally deformed rocks are the key to understanding natural deformation.
Indirect observations of geologic structures by means of various remote sensing methods, including satellite data and seismic surveying, are becoming increasingly important in our mapping and description of structures and tectonic deformation.
Experiments performed in the laboratory give us valuable knowledge of how various physical conditions, including stress field, boundary condition, temperature or the physical properties of the deforming material, relate to deformation. Numerical models, where rock deformation is simulated on a computer, are also useful as they allow us to control the various parameters and properties that influence deformation.
In contrast, naturally deformed rocks represent end-results of natural deformation histories, and the history may be difficult to read out of the rocks themselves.
Numerical and experimental models allow one to control rock properties and boundary conditions and explore their effect on deformation and deformation history. Nevertheless, any deformed rock contains some information about the history of deformation. The challenge is to know what to look for and to interpret this information.
Numerical and experimental work aids in completing this task, together with objective and accurate field observations. Numerical, experimental and remotely acquired data sets are important, but should always be based on field observations. Rocks contain more information than we will ever be able to extract from them, and the success of any physical or numerical model relies on the accuracy of observation of rock structures in the field. Direct contact with rocks and structures that have not been filtered or interpreted by people or computers is invaluable.
Unfortunately, our ability to make objective observations is limited. What we have learned and seen in the past strongly influences our visual impressions of deformed rocks. Any student of deformed rocks should therefore train himself or herself to be objective.
Only then can we expect to discover the unexpected and make new interpretations that may contribute to our understanding of the structural development of a region and to the field of structural geology in general.
Shear bands in strongly deformed ductile rocks mylonites are one such example Figure They were either overlooked or considered as cleavage until the late s, when they were properly described and interpreted.
Since then, they have been described from almost every major shear zone or mylonite zone in the world. Traditional fieldwork involves the use of simple tools such as a hammer, measuring device, topomaps, a hand lens and a compass, and the data collected are mainly structural orientations and samples for thin section studies.
This type of data collection is still important, and is aided by modern global positioning system GPS units and high-resolution aerial and satellite photos. More advanced and detailed work may involve the use of a portable laser-scanning unit, where pulses of laser light strike the surface of the Earth and the time of return is recorded.
This information can be used to build a detailed topographic or geometrical model of the outcrop, onto which one or more high-resolution field photographs can be draped. An example of such a model is shown in Figure 1. Geologic observations such as the orientation of layering or fold axes can then be made on a computer. In many cases, the most important way of recording field data is by use of careful field sketches, aided by photographs, orientation measurements and other measurements that can be related to the sketch.
Sketching also forces the field geologist to observe features and details that may otherwise be overlooked. At the same time, sketches can be made so as to emphasize relevant information and neglect irrelevant details. Field sketching is, largely, a matter of practice.
An increasing amount of such data is available on the World Wide Web, and may be combined with digital elevation data to create three-dimensional models. Orthorectified aerial photos orthophotos may give more or other details Figure 1.
Both ductile structures, such as folds and foliations, and brittle faults and fractures are mappable from satellite images and aerial photos. Beams of radar waves are constantly sent toward the Earth, and an image is generated based on the returned information. The intensity of the reflected information reflects the composition of the ground, but the phase of the wave as it hits and becomes reflected is also recorded. Comparing phases enables us to monitor millimeter-scale changes in elevation and geometry of the surface, which may reflect active tectonic 5 6 Structural geology and structural analysis m Figure 1.
This type of model, which actually is three dimensional, allows for geometric analysis on a computer and provides access to otherwise unreachable exposures. The lower figures are more detailed views. Modeling by Simon Buckley. In addition, accurate digital elevation models see next section and topographic maps can be constructed from this type of data. GPS data in general are an important source of data that can be retrieved from GPS satellites to measure plate movements Figure 1.
Such data can also be collected on the ground by means of stationary GPS units with down to millimeter-scale accuracy.
Field data in digital form can be combined with elevation data and other data by 1. The image reveals graben systems on the east side of the Colorado River. An orthophoto b reveals that the grabens run parallel to fractures, and a high-resolution satellite image c shows an example of a graben stepover structure.
Left White arrows velocity vectors indicating motions relative to Europe. The vectors clearly show how India is moving into Eurasia, causing deformation in the Himalaya—Tibetan Plateau region. Right Strain rate map based on GPS data. Warm colors indicate high strain rates.
Similar use of GPS data can be applied to much smaller areas where differential movements occur, for example across fault zones.
See Kreemer et al. By means of GIS we can combine field observations, various geologic maps, aerial photos, satellite images, gravity data, magnetic data, typically together with a digital elevation model, and perform a variety of mathematical and statistical calculations.
A digital elevation model DEM is a digital representation of the topography or shape of a surface, typically the surface of the Earth, but a DEM can be made for any geologic surface or interface that can be mapped in three dimensions.
Surfaces mapped from cubes of seismic data are now routinely presented as DEMs and can easily be analyzed in terms of geometry and orientations. Inexpensive or free access to geographic information exists, and this type of data was revolutionized by the development of Google Earth in the first decade of this century. The detailed data available from Google Earth and related sources of digital data have taken the mapping of faults, lithologic contacts, foliations and more to a new level, both in terms of efficiency and accuracy.
Because of the rapid evolution of this field, further information and resources will be posted at the webpage of this book. Some seismic data are collected for purely academic purposes, but the vast majority of seismic data acquisition is motivated by exploration for petroleum and gas. Most seismic data are thus from rift basins and continental margins. Acquisition of seismic data is, by its nature, a special type of remote sensing acoustic , although always treated separately in the geo-community.
Marine seismic reflection data Figure 1. Microphones can also be put on the sea floor. This method is more cumbersome, but enables both seismic S- and P-waves to be recorded S-waves do not travel through water. Seismic data can also be collected onshore, putting the sound source and microphones geophones on the ground.
The onshore sound source would usually be an explosive device or a vibrating truck, but even a sledgehammer or specially designed gun can be used for very shallow and local targets.
The sound waves are reflected from layer boundaries where there is an increase in acoustic impedance, i. A long line of microphones, onshore called geophones and offshore referred to as hydrophones, record the reflected sound signals and the time they appear at the surface.
These data are collected in digital form and processed by computers to generate a seismic image of the underground. Seismic data can be processed in a number of ways, depending on the focus of the study. Standard reflection seismic lines are displayed with two-way travel time as the vertical axis. Depth conversion is therefore necessary to create an ordinary geologic profile from those data.
Depth conversion is done using a velocity model that depends on the lithology sound moves faster in sandstone than in shale, and yet faster in limestone and burial depth lithification 1. The tail buoy helps the crew locate the end of the streamers. The air guns are activated periodically, such as every 25 m about every 10 seconds , and the resulting sound wave that travels into the Earth is reflected back by the underlying rock layers to hydrophones on the streamer and then relayed to the recording vessel for further processing.
The few sound traces shown on the figure indicate how the sound waves are both refracted across and reflected from the interfaces between the water and Layer 1, between Layer 1 and 2, and between Layer 2 and 3.
Reflection occurs if there is an increase in the product between velocity and density from one layer to the next. Such interfaces are called reflectors. Reflectors from a seismic line image the upper stratigraphy of the North Sea Basin right. Note the upper, horizontal sea bed reflector, horizontal Quaternary reflectors and dipping Tertiary layers. Unconformities like this one typically indicate a tectonic event. Note that most seismic sections have seconds two-way time as vertical scale.
Seismic signals Tail buoy 0s Streamer hydrofones Air gun array Layer 1 Layer 2 Layer 3 1s leads to increased velocity. In general it is the interpretation that is depth converted. However, the seismic data themselves can also be depth migrated, in which case the vertical axis of the seismic sections is depth, not time.
This provides more realistic displays of faults and layers, and takes into account lateral changes in rock velocity that may cause visual or geometrical challenges to the interpreter when dealing with a time-migrated section. The accuracy of the depthmigrated data does however rely on the velocity model.
Deep seismic lines can be collected where the energy emitted is sufficiently high to penetrate deep parts of the crust and even the upper mantle. Such lines are useful for exploring the large-scale structure of the lithosphere.
While widely spaced deep seismic lines and regional seismic lines are called two-dimensional 2-D seismic data, more and more commercial petroleum company data are collected as a three-dimensional 3-D cube where line spacing is close enough c. The lines parallel to the direction of collection are sometimes called inlines, those orthogonal to inlines are referred to as crosslines, while other vertical lines are random lines.
Horizontal sections are called time slices, and can be useful during fault interpretation. Three-dimensional seismic data provide unique opportunities for 3-D mapping of faults and folds in the subsurface. However, seismic data are restricted by seismic resolution, which means that one can only distinguish 9 Structural geology and structural analysis Seawater 3 sec. Anticline Unconformity nc li n e 10 Salt Normal faults 5 sec. Normal faults 5 km Figure 1. Note that the vertical scale is in seconds.
Some basic structures returned to in later chapters are indicated. Seismic data courtesy of CGGVeritas. The quality and resolution of 3-D data are generally better than those of 2-D lines because the reflected energy is restored more precisely through 3-D migration. The seismic resolution of high-quality 3-D data depends on depth, acoustic impedance of the layer interfaces, data collection method and noise, but would typically be at around 15—20 m for identification of fault throw.
Sophisticated methods of data analysis and visualization are now available for 3-D seismic data sets, helpful for identifying faults and other structures that are underground. Petroleum exploration and exploitation usually rely on seismic 3-D data sets interpreted on computers by geophysicists and structural geologists. The interpretation makes it possible to generate structural contour maps and geologic cross-sections that can be analyzed structurally in various ways, e.
Other types of seismic data are also of interest to structural geologists, particularly seismic information from earthquakes. This information gives us important information about current fault motions and tectonic regime, which in simple terms means whether an area is undergoing shortening, extension or strike-slip deformation. Buckle folding, shear folding, reverse, normal and strike-slip faulting, fault populations, fault reactivation, porphyroclast rotation, diapirism and boudinage are only some of the processes and structures that have been modeled in the laboratory.
The traditional way of modeling geologic structures is by filling a box with clay, sand, plaster, silicone putty, honey and other media and applying extension, contraction, simple shear or some other deformation. A ring shear apparatus is used when large amounts of shear are required. In this setup, the outer part of the disk-shaped volume is rotated relative to the inner part. Many models can be filmed and photographed during the deformation history or scanned using computer tomography.
Another tool is the centrifuge, where material is deformed under the influence of the centrifugal force. Here the centrifugal force plays the same role in the models as the force of gravity does in geologic processes. Ideally we wish to construct a scale model, where not only the size and geometry of the natural object or structure that it refers to are shrunk, but where also physical properties are scaled proportionally.
Hence we 1. This and similar models were made by H. Cadell to illustrate the structures of the northwest Scottish Highlands. With permission of the Geological Survey of Britain. We also need kinematic similarity, with comparability of changes in shape and position and proportionality of time. Great book, a must have for any geologist. Students are engaged through examples and parallels drawn from practical everyday situations, enabling them to connect theory with practice. This website uses cookies to improve your experience while you navigate through the website.
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Skip to content. This website uses cookies to improve your experience. Summary Folds and folding Boudinage Kinematics and paleostress Balancing and restoration Extensional regimes Summary Strike-slip, transpression and transtension Appendix A: More about the deformation matrix Each chapter starts with a general introduction, which presents a context for the topic within structural geology as a whole.
These introductions provide a roadmap for the chapter and will help you to navigate through the book. The main text contains highlighted terms and key expressions that you will need to understand and become familiar with. Many of these terms are listed in the Glossary at the back of the book. The Glossary allows you to easily look up terms whenever needed and can also be used to review important topics and key facts.
Each chapter also contains a series of highlighted statements to encourage you to pause and review your understanding of what you have read. Most chapters have one or more boxes containing in-depth information about a particular subject, helpful examples or relevant background information. Other important points are brought together in the chapter summaries.
Review questions should be used to test your understanding of the chapter before moving on to the next topic. Further reading sections provide references to selected papers and books for those interested in more detailed or advanced information.
In addition, there are links to web-based e-learning modules at the end of the chapters. Using these modules is highly recommended after reading the chapter as part of review and exam preparation. The modules provide supplementary information that complements the main text. These and instructors as a community additional geologic structures and present key aspects of structural resource.
It is mainly concerned with the structural and 5 and via rheology Chapter 6 to brittle deformation geology of the crust, although the processes and struc- Chapters 7 and 8. Of these, Chapter 2 contains material tures described are relevant also for deformation that that would be too detailed and advanced for some students occurs at deeper levels within our planet.
Further, remote and classes, but selective reading is possible. Then, after data from Mars and other planets indicate that many a short introduction to the microscale structures and aspects of terrestrial structural geology are relevant also processes that distinguish crystal-plastic from brittle beyond our own planet.
Three consecutive chapters subjects within this field. Making the selection has not then follow that are founded on the three principal been easy, knowing that lecturers tend to prefer their tectonic regimes Chapters 16—18 before salt tectonics own favorite aspects of, and approaches to, structural and restoration principles are presented Chapters 19 geology, or make selections according to their local and A final chapter, where links to metamorphic departmental course curriculum.
Existing textbooks petrology as well as stratigraphy are drawn, rounds off in structural geology tend to emphasize the ductile or the book, and suggests that structural geology and plastic deformation that occurs in the middle and lower tectonics largely rely on other disciplines.
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