Structural Geology and Structural Analysis

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1 Structural geology and structural analysis

Structural geology is about folds, faults and other The e-modules for this chapter, Introduction deformation structures in the lithosphere – how they and Spherical projections, provide further appear and how and why they formed. Ranging support on the following topics: from features hundreds of kilometers long down to • Stress and strain microscopic details, structures occur in many differ- • Rheology ent settings and have experienced exciting changes • Structures in stress and strain – information that can be ours • Tectonic regimes if we learn how to read the code. The story told • Schmidt net by structures in rocks is beautiful, fascinating and • Projection interesting, and it can also be very useful to society. • Constructions Exploration, mapping and exploitation of resources • Software such as slate and schist (building stone), ores, • Convention 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.

2 Structural geology and structural analysis

1.1 Approaching structural geology not with primary structures formed by sedimentary or magmatic processes. However, deformation structures What can today be defined as modern structural geology can form through the modification of primary struc- was born out of field observations. Today indirect obser- tures, such as folding of bedding in a sedimentary rock. vations can also be made from remote data such as seis- The closely related wordtectonics comes from the mic data and satellite data, but the fact that observations Greek word tektos, and both structural geology and tec- trigger questions in the mind of the open-minded student tonics relate to the building and resulting structure of the remains the same. Answers can be sought through more Earth’s lithosphere, and to the motions that change and careful and systematic observations, which commonly in- shape the outer parts of our planet. We could say that tec- volve field measurements, thin section studies, plotting and tonics is more closely connected to the underlying pro- analyzing structural data, utilize more advanced methods cesses that cause structures to form: such as radiometric dating, and so on. In addition, we can set up physical experiments in the lab or use numerical Tectonics is connected with external and often regional modeling to further explore and test our hypotheses. processes that generate a characteristic set of structures Each of the different methods that structural geologists in an area or a region. use in their pursuit of a satisfactory answer has its advan- tages and limitations. Observations of structures, be it in By external we mean external to the rock volume that we the field or from remote sensing imagery, portray the final study. External processes or causes are in many cases plate results of deformation processes, while the actual defor- motions, but can also be such things as forceful intrusion mation history is, in most cases, unknown. The evolution of magma, gravity-driven salt or mud diapirs, flowing gla- of structures through progressive deformation can be ob- ciers and meteor impacts. Each of these “causes” can cre- served in laboratory experiments, but how representative ate characteristic structures that define atectonic style, are such minute-, hour- or perhaps week-long observa- and the related tectonics can be given special names. Plate tions of geologic histories that span thousands to millions tectonics is the large-scale part of tectonics that direct- of years in nature? And how well do meter-scale models ly involves the movement and interaction of lithospheric reproduce kilometer-scale natural examples? Numerical plates. Within the realm of plate tectonics, expressions modeling, where we use computers, physics and mathe- such as subduction tectonics, collision tectonics and rift matical equations to model deformation, is hampered by tectonics are applied for more specific purposes. simplifications required for the models to be runable with Glaciotectonics is the deformation of sediments and today’s codes and computers. Besides, input parameters bedrock (generally sedimentary rocks) at the toe of an ad- such as material properties or preexisting heterogeneities vancing ice sheet. In this case it is the pushing of the ice may be uncertain. Nevertheless, by combining different that creates the deformation, particularly where the base approaches we are able to obtain realistic models of how of the glacier is cold (frozen to the substrate). structures can form and what they mean. Field studies Salt tectonics deals with the deformation caused by will always be important, as any modeling, numerical or the (mostly) vertical movement of salt through its over- physical, must be based directly or indirectly on accurate burden (see Chapter 20). Both glaciotectonics and salt and objective field observations and descriptions. Objec- tectonics are primarily driven by gravity, although salt tivity during fieldwork is both important and challenging, tectonics can also be closely related to plate tectonics. For and field studies in one form or another are the main rea- example, tectonic strain can create fractures that enable son why many geologists choose to become geoscientists! salt to gravitationally penetrate its cover, as discussed in Chapter 20. The termgravity tectonics is generally 1.2 Structural geology and tectonics restricted to the downward sliding of large portions of rocks and sediments, notably of continental margin de- The word structure is derived from the Latin word stru- posits resting on weak salt or overpressured shale layers. ere, to build, and we could say: Raft tectonics is a type of gravity tectonics occurring in such environments, as mentioned in Chapter 20. Small- A geologic structure is a geometric configuration of er landslides and their structures are also considered ex rocks, and structural geology deals with the geometry, amples of gravity tectonics by some, while others regard distribution and formation of structures. such surficial processes asnon-tectonic . Typical non- tectonic deformation is the simple compaction of sedi- It should be added that structural geology only ments and sedimentary rocks due to loading by younger deals with structures created during rock deformation, sedimentary strata.

1.2 Structural geology and tectonics 3

Neotectonics is concerned with recent and ongoing steeper dips down section. There is also a facies variation, crustal motions and the contemporaneous stress field. with the coarsest-grained deposits forming near the fault, Neotectonic structures are the surface expression of faults which can be attributed to the fault-induced topography in the form of fault scarps, and important data sets stem seen in Figure 1.1. from seismic information from earthquakes (such as focal Another close relationship between tectonics and rock mechanisms, Box 10.1) and changes in elevation of re- forming processes is shown in Figure 1.2, where forceful gions detected by repeated satellite measurements. rising and perhaps inflating of magma deforms the outer At smaller scales, microtectonics describes microscale and oldest part of the pluton and its country rock. Force- deformation and deformation structures visible under the ful intrusion of magma into the crust is characterized by microscope. deformation near the margin of the pluton, manifested by Structural geology typically pertains to the observa- folding and shearing of the layers in Figure 1.2. Ellipses tion, description and interpretation of structures that can in this figure illustrate the shape of enclaves (inclusions), be mapped in the field. How do we recognize deforma- and it is clear that they become more and more elongated tion or strain in a rock? “Strained” means that something as we approach the margin of the pluton. Hence, the out- primary or preexisting has been geometrically modified, er part of the pluton has been flattened during a forceful be it cross stratification, pebble shape, a primary magmat- intrusion history. ic texture or a preexisting deformation structure. Hence strain can be defined as a change in length or shape, and recognizing strain and deformation structures actually requires solid knowledge of undeformed rocks and their primary structures. Rim syncline Rim syncline

Being able to recognize tectonic deformation depends on Neoproterozoic

our knowledge of primary structures. Granodiorite Mesoproterozoic Stratigraphic thinning The resulting deformation structure also depends on the initial material and its texture and structure. De-

forming sandstone, clay, limestone or granite results in Archaean ? 33kkmm significantly different structures because they respond differently. Furthermore, there is often a close relation- Figure 1.2 Structural geology can be linked to processes and ship between tectonics and the formation of rocks and mechanisms other than plate stresses. This illustration of a their primary structures. Sedimentologists experience granodioritic pluton southwest of Beijing, China, portrays the this as they study variations in thickness and grain size in close connection between forceful intrusion of magma, strain and folds in the country rock. Black ellipses indicate strain, the hanging wall (down-thrown side) of syndepositional as discussed in Chapters 2 and 3. The strain (deformation) faults. This is illustrated in Figure 1.1, where the gradual pattern within and around the pluton can be explained in terms rotation and subsidence of the down-faulted block gives of diapirism, where the intrusion ascends and squeezes and more space for thicker strata near the fault than farther shears its outer part and the surrounding country rock to create away, resulting in wedge-shaped strata and progressively space. Based on He et al. (2009).

Half-graben basin

1–5 km Mudstone Sandstone Basement Conglomerate

Figure 1.1 Illustration of the close relationship between sedimentary facies, layer thickness variations and syndepositional faulting (growth faults) in an area of active crustal extension.

4 Structural geology and structural analysis

Metamorphic growth of minerals before, during, and model that explains the structural observations and puts after deformation may also provide important informa- them into context with respect to a larger-scale process, tion about the pressure–temperature conditions during such as rifting or salt movements. For example, if we map deformation, and may contain textures and structures out a series of normal faults indicating E–W extension in reflecting kinematics and deformation history. Hence, an orogenic belt, we have to look for a model that can ex- sedimentary, magmatic and metamorphic processes may plain this extension. This could be a rift model, or it could all be closely associated with the structural geology of a be extensional collapse during the orogeny, or gravity- locality or region. driven collapse after the orogeny. Any kind of relevant These examples relate to strain, but structural geol- data, such as relative age relations, radiometric dates, evi ogists, especially those dealing with brittle structures of dence for magmatism, and stratigraphic thickness and the upper crust, are also concerned with stress. Stress is facies variations, are used as we search for the model that a somewhat diffuse and abstract concept to most of us, best fits the data. It may be that several models can ex- since it is invisible. Nevertheless, there will be no strain plain a given data set, and we should always look for and without a stress field that exceeds the rock’s resistance critically evaluate alternative models. In general, a simple against deformation. We can create a stress by applying a model is more attractive than a complicated one. force on a surface, but at a point in the subsurface stress is felt from all directions, and a full description of such 1.3 Structural data sets a state of stress considers stress from all directions and is therefore three-dimensional. There is always a relation- Planet Earth represents an incredibly complex physi- ship between stress and strain, and while this relationship cal system, and the structures that result from natural may be easy to establish from controlled laboratory ex- deformation reflect this fact through their multitude of periments, it may be difficult to extract from naturally expressions and histories. There is thus a need to simplify formed deformation structures. and identify the one or few most important factors that Structural geology covers deformation structures describe or lead to the recognition of deformation struc- formed at or near the Earth’s surface, in the cool, upper tures that can be seen or mapped in naturally deformed part of the crust where rocks have a tendency to fracture, rocks. Field observations of deformed rocks and their in the hotter, lower crust where the deformation tends structures represent the most direct and important source to be ductile, and in the underlying mantle. It embraces of information on how rocks deform, and objective ob- structures at the scale of hundreds of kilometers down to servations and careful descriptions of naturally deformed micro- or atomic-scale structures, structures that form al- rocks are the key to understanding natural deformation. most instantaneously, and structures that form over tens Indirect observations of geologic structures by means of of millions of years. various remote sensing methods, including satellite data A large number of subdisciplines, approaches and and seismic surveying, are becoming increasingly impor- methods therefore exist within the field of structural tant in our mapping and description of structures and geology. The oil exploration geologist may be considering tectonic deformation. Experiments performed in the lab- trap-forming structures formed during rifting or salt tec- oratory give us valuable knowledge of how various physi- tonics, while the production geologist worries about sub- cal conditions, including stress field, boundary condition, seismic sealing faults (faults that stop fluid flow in porous temperature or the physical properties of the deform- reservoirs; Section 8.7). The engineering geologist may ing material, relate to deformation. Numerical models, consider fracture orientations and densities in relation where rock deformation is simulated on a computer, are to a tunnel project, while the university professor may also useful as they allow us to control the various parame- use structural mapping, physical modeling or computer ters and properties that influence deformation. modeling to understand mountain-building processes. Experiments and numerical models not only help us The methods and approaches are many, but they serve to understand how external and internal physical conditions understand the structural or tectonic development of a control or predict the deformation structures that form, region or to predict the structural pattern in an area. In but also give information on how deformation structures most cases structural geology is founded on data and ob- evolve, i.e. they provide insights into the deformation his- servations that must be analyzed and interpreted. Struc- tory. In contrast, naturally deformed rocks represent end tural analysis is therefore an important part of the field of results of natural deformation histories, and the history structural geology. may be difficult to read out of the rocks themselves. -Nu Structural data are analyzed in ways that lead to a tec- merical and experimental models allow one to control tonic model for an area. By tectonic model we mean a rock properties and boundary conditions and explore

1.4 Field data 5

their effect on deformation and deformation history. that have not been filtered or interpreted by other people’s Nevertheless, any deformed rock contains some infor- minds or computers is invaluable (Figure 1.3). mation about the history of deformation. The challenge Unfortunately, our ability to make objective observa- is to know what to look for and to interpret this infor- tions is limited. What we have learned and seen in the mation. Numerical and experimental work aids in com- past strongly influences our visual impressions of de- pleting this task, together with objective and accurate field formed rocks. Any student of deformed rocks should observations. therefore train himself or herself to be objective. Only then can we expect to discover the unexpected and make Numerical, experimental and remotely acquired data new interpretations that may contribute to our under- sets are important, but should always be based on field standing of the structural development of a region and to observations. the field of structural geology in general. Structures can be overlooked until the day that someone points out their 1.4 Field data existence and meaning, upon which they all of a sudden start to appear “everywhere”. Shear bands in strongly de- It is hard to overemphasize the importance of traditional formed ductile rocks (mylonites) are one such example field observations of deformed rocks and their structures. (Figure 16.25). They were either overlooked or considered Rocks contain more information than we will ever be able as cleavage until the late 1970s, when they were properly to extract from them, and the success of any physical or described and interpreted. Since then, they have been de- numerical model relies on the quality of observation of real scribed from almost every major shear zone or mylonite rock structures. Direct contact with rocks and structures zone in the world.

Figure 1.3 Direct contact with rocks is important. Make sure you enjoy many days in the field every year where you allow for open discussions and new ideas. Here Donna Whitney and Christian Teyssier are discussing gneissic structures in the Scandinavian Caledonides.

6 Structural geology and structural analysis

Fieldwork starts with visual observations and sketching modern field geologist is more likely to take advantage of (Figures 1.3 and 1.4). It then involves the use of basic tools digital maps and high-resolution digital satellite images such as a hammer, measuring device, topomaps, a hand lens and aerial photos that let you map structures and collect, and a compass, a camera, and the data collected are mainly plot and evaluate data more effectively by means of smart- structural orientations and samples for thin section studies. phones, tablets and more specialized digital devices with Global positioning system (GPS) units and high-resolution GPS capabilities. This makes for accurate positioning and aerial and satellite photos are important tools, and more ad- more efficient data collection. However, the basic con- vanced and detailed work may involve the use of a portable cepts remain the same. laser-scanning unit, where pulses of laser light strike the surface of the Earth and the time of return is recorded. Sketching and photographing In many cases, the most important way of recording field Mapping data is by making careful field sketches aided by photo- A topographic map onto which geologic contacts, local- graphs, orientation measurements and other measure- ity numbers and orientation measurements can be add- ments that can be related to the sketch. Sketching also ed has always been essential to the field geologist. The forces the field geologist to observe features and details

Figure 1.4 Sketching is an important field activity that serves several purposes. It makes you a better observer, gives you ideas, and helps you remember the outcrop. A sketch can be a reproduction of an outcrop where geologic structures are emphasized over uninteresting features (such as vegetation), or more of a principle sketch that illustrates concepts and models. This student sketch (by Gijs Henstra) is an example from a field trip to southern Utah.

1.4 Field data 7

that may otherwise be overlooked. Even making a draw- by either of two conjugate angles, which in the example ing from a good field photograph can reveal features that shown in Figure 1.5 are 060 and 240, respectively. I use the would otherwise have gone unnoticed. Another use of (American) right-hand rule, which says that when you sketches is to emphasize relevant information and tone look in the “correct” strike direction, the plane should be down or neglect irrelevant details. Many of us find it use- dipping to the right. Or, with respect to your right hand; if ful to make an overview field sketch in order to get the your right-hand thumb is pointing in the strike direction, overview of an outcrop. A more detailed drawing of par- your fingers should be pointing in the direction of dip. ticularly interesting structures or parts of the outcrop can In our example, the plane dips to the right when looking then be made. It is always a good idea to back up sketch- toward 060, not toward 240. Hence, the orientation of the es with pictures, most conveniently with a camera with a plane can be denoted 060/30. We always use three digits built-in or external GPS unit. Field sketching is, largely, a for strike to avoid confusion with the dip value. It may also matter of practice. be a good idea to denote the direction of dip, in our case to the SE. Now our plane is denoted 060/30 SE. Adding Taking measurements direction (SE) also distinguishes it from a lineation meas- There are a variety of manual compasses available to urement. You may also see this orientation written as N measure planar and linear structures, and we will not go 060 E/30 SW, meaning that the strike direction is 60 ° E of through the details of how to use them here. In addition, N. Planar structures are plotted as great circles or as poles there are several apps for smartphones that can be very to planes in spherical projection diagrams (Appendix B). useful (see online resources for more information). These Strike directions are sometimes represented by means of apps allow you to easily measure, plot and directly or rose diagrams, particularly for fractures (see Appendix B). indirectly present measurements on maps or imag- A plane can also be described by its dip direction. The es, but they have their own shortcomings with respect dip value is the same as above, but the strike value is now to accuracy, battery power, etc. that must be taken into exchanged for dip azimuth, or the horizontal direction account. Unfortunately, different notations are being used in which our plane is dipping. So if our plane is dipping for orientation measurements. In particular, planes can toward 150 (to the SE), its orientation is denoted 150/45 be represented by their strike/dip values, or by dip and or 45/150. Basically this adds 90° to our strike angle, so dip direction (dip azimuth), and although conversion is it is easy to go between the two, but also easy to confuse simple, unambiguous field notes are important to avoid if you get data from other geologists. Dip directions are confusion. plotted as poles in spherical projections, since they are Many geologists denote the orientation of a plane by its effectively lines. strike and dip. Strike is the intersection between our plane Linear structures are represented by their trend and and the horizontal plane, and the strike value is the angle plunge, where the trend is the projection of the linear struc- between the strike and north. This can be represented ture onto the horizontal plane, and the plunge is the angle

Bedding, inclined Figure 1.5 Description of Projection of lineation onto horizontal Trend (190) Bedding, inverted foliation and lineation in plane Plunge (24°) an outcrop, together with Bedding, vertical some commonly used map Dip azimuth: Bedding, horizontal symbols. The box shows how horizontal line Strike the foliation and lineation perpend. (060) Foliation, inclined to strike Dip Dip (30°) in the figure may appear (150) direction Foliation, vertical (150/30) on a map and as equal-area Cleavage, inclined projections.

N Lineation Cleavage, vertical Lineation

N Lineation, horizontal Thrust N 060 fault Fol. Foliation and lineation, same plane of bedding 30° Lin. 24° Foliation and lineation, same plane of foliation Normal fault (alternative Fold axis, inclined notations) Anticline Syncline Fold axis, horizontal

8 Structural geology and structural analysis

between this line and the lineation. In some cases pitch is digitally on handheld devices in the field. Also, geologic measured, as explained in Appendix B. Linear structures field data are commonly presented on three-dimensional are represented by poles in spherical projections. (3-D) models (such as Google Earth), where geologic Magnetic declination is the difference between true contacts and orientation data are projected onto elevation and magnetic north, which can be quite significant in models aided with remote sensing images. some places and must be accounted for. It can be done by adjusting your manual compass (but remember to Cross-sections readjust when moving to a different area) or the meas- A geologic map is not complete without one or more urements can be adjusted later (in a spreadsheet or by cross-sections (Figure 1.6), which is constructed by mak- rotating the data in a spherical projection program). If ing a topographic profile with surface information from you are using a smartphone app, it may or may not auto- field observations. Usually the section is chosen in the matically correct for declination. dip direction of important lithologic boundaries or the foliation, or perpendicular to major faults or fold hinges, Spherical projections which in many cases captures the tectonic transport dir Orientation data are plotted on spherical projections ection. The topographic profile is usually constructed (equal angle or area) and rose diagrams, and smartphone from digital elevation data. Then, with the aid of geophys- compass apps automatically plot data on the screen so that ical data and well information, the surface information we can immediately evaluate the results and, if necessary, is extended downward into the subsurface. The section add measurements. Convenient plotting applications are should be restorable to a reasonable pre-deformational available into which data can be entered or imported from situation, as discussed in Chapter 21. handheld devices. However, it is absolutely necessary to understand spherical projections in order to make full use Collecting oriented samples of the plots that they produce. Hence, review the basics It may be important to know the orientation of a sam- of plotting orientation data on equal angle and area ple, for example for kinematic analysis or for oriented diagrams, as presented in Appendix B and the e-module thin sections. To collect an oriented sample, put the Spherical projections. sample back in place, draw a strike and dip symbol on a dipping surface of the sample while it is in place, and Scanlines write the strike and dip values on the sample, as shown Scanlines are lines across structures such as fractures or in Figure 1.7. If there is a lineation it may be useful to deformation bands, where the location of each structure also mark it with an arrow and its trend/plunge, but or number of structures per meter is recorded. Scanline note that a lineation by itself does not contain sufficient data can be collected by stretching out a measuring tape information to reorient the sample. Also add sample across the structures of interest, or from cores or borehole number and position information, and take a picture of image data. Such data can be plotted in frequency graphs the sample in place before putting it into a well-marked such as that shown in Figure 9.13. Note that it is com- sample bag. monly desirable to measure perpendicular to structures, and where this is not possible, to make a simple geometric 1.5 Remote sensing and geodesy correction so that true spacing, for example of a joint set, can be found. Satellite images, such as those shown in Figure 1.8a and c, are now available at increasingly high resolutions and are Geologic maps a valuable tool for the mapping of map-scale structures. Geologic maps are important presentations of structures An increasing amount of such data is available on the as they are supplied with symbols representing various World Wide Web, and may be combined with digital el- planar and linear structures. The use of symbols differs, evation data to create 3-D models. Ortho-rectifiedaerial and some common ones are shown in Figure 1.6. Each photos (orthophotos) may give more or other details geological survey seems to have their own set of symbols, (Figure 1.8b), with resolutions down to a few tens of cen- in addition to the great variation between individual au- timeters in some cases. Both ductile structures, such as thors. Hence, the symbols used must be explained in the folds and foliations, and brittle faults and fractures are legend of the map. mappable from satellite images and aerial photos. While mapping used to involve paper and pencils, many In the field of neotectonics,InSAR (Interferometric geologists now draw contacts and add measurements Synthetic Aperture Radar) is a useful remote sensing

1.5 Remote sensing and geodesy 9

36 11 24 49 10 Srh 36 86 22 Bedding 18 60 26 15 22Horizontal 67 15 14 75 40 18 35 51 58 38 17 Vertical 12 63 50 75 12 17 8 14 80 48 54 25 Inclined 21 81 65 29 21 33 39 17 47 Sto 35 10 45 29 85 57 74 79 71 23 51 18 Overturned 65 c 35 25 23 15 Sw 58 45 20 18 15 45 Joint 12 27 28 48 60 48 53 65 Sb 30 23 47 Inclined 35 16 46 45 37 78 25 Vertical Swc 22 38 25 15 14 15 69 48 43 60 18 16 45 7 64 25 35 53 25 67 7 47 0 88 24 3 Anticline axial 25 51 53 31 17 43 trace 33 30 9 70 10 21 50 31 57 12 40 8 Dmh 12 4 52 35 Syncline axial 67 30 31 31 75 Dm 16 trace 26 19 25 32 9 73 25 30 Dm 8 13 Don 25 5 24 73 20 73 Thrust fault 5 53 5 45 (approximate) 22 15 11 17 2 Dop 37 23 76 59 4 8 DSk 76 9 25 40°25'00" 20 82 40°25'00" 74 78 Sto 75 SYNCLINE 11 25 75 12 15 23 35 38 85 Dbh 30 30 27 ANTICLINE 40 ANTICLINE 43 Sto St 28 35 35 45 RIDGE45 Swc 78 32 Sm 32 25 Sb 28 14 2 Sk MOUNTAIN 2 23 54 TUSCARORA 30 64 10 76 32 LIMESTONE 54 Srh 60 53 55 14 5 85 20 80 38 44 57 70 25 12 52 44 47 45 33 29 42 40 50 38 6 23 22 TUSCARORA 55 60 75 Dmh 36 6 62 52 25 67 34 6 48 46 Dciv 28 54 44 45 77 45 9 10 5 3 81 42 41 29 47 44 7 65 59 37 55 Srh 75 45 18 82 78 77 49 39 28 45 20 37 45 TN 41 49 2 60 MN 3 Dm 31 54 4 66 Sm 45 76 14 Sk Srh 67 72 46 10.5º 59 45 5 36 83 St 19 48 24 9 15 55 18 18 36 58 55 76 42 Dm 40 65 60 32 44 1St km 60 56 88 A' 40°22'30" 50 -77°37'30" -77°35'00" -77°32'30" CONTOUR INTERVAL 20 FEET -77°30'00"

A A' Mete rs Don Tuscarora Mountain 500 Don Tuscarora syncline Limestone Ridge Dop anticline Dop Dm Dm DSk anticline 250 Dmh Srh St 0 Oj Oj Swc Sb Obe -250 Obe Or Sm Srh 1 km -500 Sto Sk Figure 1.6 Example of a bedrock geologic map from the Ridge and Valley province (Appalachians) of Pennsylvania, with structural data and a cross-section (A–A'). The stratigraphic column of the Ordovician–Devonian sedimentary succession exposed on this map sheet is not shown here. Modified from McElroy and Hoskins (2011).

technique that combines two or more radar satellite images may reflect active tectonic movements related to earth- to map variations in ground surface deformation. Beams quakes or sliding. In addition, accurate digital elevation of radar waves are constantly sent toward the Earth from models (see next section) and topographic maps can be satellites, and an image is generated based on the returned constructed from this type of data. information. The intensity of the reflected information GPS data in general are an important source of data reflects the composition of the ground, but the phase of that can be retrieved from GPS satellites to measure plate the wave as it hits and becomes reflected is also recorded. movements (Figure 1.9). Such data can also be collect- Comparing phases enables us to monitor millimeter-scale ed on the ground by means of stationary GPS units with changes in elevation and geometry of the surface, which down to millimeter-scale accuracy.

10 Structural geology and structural analysis

1.7 Seismic data In the mapping of subsurface structures, seismic data are invaluable and since the 1960s have revolutionized our understanding of fault and fold geometry. Some seismic data are collected for purely academic purposes, but the vast majority of seismic data acquisition is motivated by 355 exploration for petroleum and gas. Most seismic data are 76E 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 355 separately in the geo-community. Marine seismic reflection data (Figure 1.10 and Box 1.1) are collected by boat, 76E N where a sound source (air gun) generates sound waves that penetrate the crustal layers under the sea bottom. Microphones (hydrophones) can also be put on the sea Figure 1.7 Collection of an oriented sample requires that the floor (OBS, or ocean bottom seismic). This method is orientation (strike/dip) of one of the faces is measured and more cumbersome, but enables both seismic S- and marked on the sample before it is permanently removed from P-waves to be recorded (S-waves do not travel through the outcrop. 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 1.6 DEM, GIS and Google Earth an explosive device or a vibrating truck, but even a sledge- hammer or specially designed gun can be used for very Conventional paper maps are still useful for many field shallow and local targets. mapping purposes, but rugged laptops, tablets and hand- The sound waves are reflected from layer boundaries held devices now allow for direct digitizing of structural where there is an increase in acoustic impedance, i.e. where features on digital maps and images and are becoming there is an abrupt change in density and/or the velocity more and more important. Field data in digital form can with which sound waves travel in the rock. A long line be combined with elevation data and other data by means of microphones, onshore called geophones and offshore of a Geographical Information System (GIS). By means of referred to as hydrophones, record the reflected sound GIS we can combine field observations, various geologic signals and the time they appear at the surface. These data maps, aerial photos, satellite images, gravity data, magnet- are collected in digital form and processed by computers ic data, typically together with a digital elevation model, to generate a seismic image of the underground. and perform a variety of mathematical and statistical cal- Seismic data can be processed in a number of ways, culations. A digital elevation model (DEM) is a digital depending on the focus of the study. Standard reflection representation of the topography or shape of a surface, seismic lines are displayed with two-way travel time as the typically the surface of the Earth, but a DEM can be made vertical axis. Depth conversion is therefore necessary to for any geologic surface or interface that can be mapped create an ordinary geologic profile from those data. Depth in three dimensions. Surfaces mapped from cubes of seis- conversion is done using a velocity model that depends mic data are now routinely presented as colored and shad- on the lithology (sound moves faster in sandstone than in ed DEMs and can easily be analyzed in terms of geometry shale, and yet faster in limestone) and burial depth (lith- and orientations. ification leads to increased velocity). In general it is the Inexpensive or free access to geographic informa- interpretation that is depth converted. However, the seis- tion exists, and this type of data was revolutionized by mic data themselves can also be depth migrated, in which the development of Google Earth in the first decade of case the vertical axis of the seismic sections is depth, not this century. The detailed data available from Google time. This provides more realistic displays of faults and lay- Earth and related sources of digital data have taken the ers, and takes into account lateral changes in rock velocity mapping of faults, lithologic contacts, foliations and that may create geometrical challenges when interpret- more to a new level, both in terms of efficiency and ing a time-migrated section. The accuracy of the depth- accuracy. migrated data does however rely on the velocity model.