Structural geology and tectonics
The word shape is derived from the Latinword struere, to construct, and lets say:
"A geologic shape is a geometrical con?Guration of rocks, and structural geology deals with the geometry, distribution and formation of systems".
It should be added that structural geology only deals with structures created during rock deformation, not with primary structures formed by sedimentary or magmatic processes. However, deformation structures can form through the modiﬁcation of primary structures, such as folding of bedding in a sedimentary rock.
The closely related word tectonics comes from the Greek word tektos, and both structural geology and tectonics relate to the building and resulting structure of the Earth’s lithosphere, and to the motions that change and shape the outer parts of our planet. We could say that tectonics is more closely connected to the underlying processes that cause structures to form:
"Tectonics is attached with outside and frequently local methods that generate a feature set of structures in a place or a location".
By external we mean external to the rock volume that we study. External processes or causes are in many cases plate motions, but can also be such things as forceful intrusion of magma, gravity-driven salt or mud diapirs, ﬂowing glaciers and meteor impacts. Each of these “causes” can create characteristic structures that deﬁne a tectonic style, and the related tectonics can be given special names. Plate tectonics is the large-scale part of tectonics that directly involves the movement and interaction of lithospheric plates. Within the realm of plate tectonics, expressions such as subduction tectonics, collision tectonics and rift tectonics are applied for more speciﬁc purposes.
Glaciotectonics is the deformation of sediments and bedrock (generally sedimentary rocks) at the toe of an advancing ice sheet. In this case it is the pushing of the ice that creates the deformation, particularly where the base of the glacier is cold (frozen to the substrate).
Salt tectonics deals with the deformation caused by the (mostly) vertical movement of salt through its overburden. Both glaciotectonics and salt tectonics are primarily driven by gravity, although salt tectonics can also be closely related to plate tectonics. For example, tectonic strain can create fractures that enable salt to gravitationally penetrate its cover. The term gravity tectonics is generally restricted to the downward sliding of large portions of rocks and sediments, notably of continental margin deposits resting onweak salt or overpressured shale layers. Raft tectonics is a type of gravity tectonics occurring in such environments. Smaller landslides and their structures are also considered examples of gravity tectonics by some, while others regard such surﬁcial processes as non-tectonic. Typical nontectonic deformation is the simple compaction of sediments and sedimentary rocks due to loading by younger sedimentary strata.
Neotectonics is concerned with recent and ongoing crustal motions and the contemporaneous stress ﬁeld. Neotectonic structures are the surface expression of faults in the form of fault scarps, and important data sets stem from seismic information from earthquakes and changes in elevation of regions detected by repeated satellite measurements.
At smaller scales, microtectonics describes microscale deformation and deformation structures visible under the microscope.
Structural geology typically pertains to the observation, description and interpretation of structures that can be mapped in the ﬁeld. How do we recognize deformation or strain in a rock? “Strained” means that something primary or preexisting has been geometrically modiﬁed, be it cross stratiﬁcation, pebble shape, a primary magmatic 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.
Being able to apprehend tectonic deformation depends on our know-how of number one systems.
The ensuing deformation shape additionally depends at the initial fabric and its texture and shape. Deforming sandstone, clay, limestone or granite effects in signi?Cantly distinctive systems due to the fact they reply otherwise. Furthermore, there is often a close dating among tectonics and the formation of rocks and their number one structures. Sedimentologists enjoy this as they observe variations in thickness and grain length inside the striking wall (down-thrown aspect) of syndepositional faults. This is illustrated in Fig. 1, where the sluggish rotation and subsidence of the down-faulted block offers space for thicker strata near the fault than farther away, resulting in wedge formed strata and regularly steeper dips down phase. There is likewise a facies variation, with the coarsest-grained deposits forming near the fault, which may be attributed to the fault-induced topography visible in Fig. 1.
Another close relationship between tectonics and rock forming processes is shown in Figure 2, where forceful rising and perhaps inﬂating of magma deforms the outer and oldest part of the pluton and its country rock. Forceful intrusion of magma into the crust is characterized by deformation near the margin of the pluton, manifested by folding and shearing of the layers in Figure 2. 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, all through, and after deformation may offer important records about the pressure?Temperature situations all through deformation, and might incorporate textures and systems re?Ecting kinematics and deformation history. Hence, sedimentary, magmatic and metamorphic strategies may additionally 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. Nevertheless, there will be no strain without a stress ﬁeld that exceeds the rock’s resistance against deformation. 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.
Structural geology covers deformation structures shaped at or near the Earth?S floor, within the cool, upper part of the crustwhere rocks have a propensity to fracture, in the warmer, decrease crust wherein the deformation tends to be ductile, and intheunderlying mantle.It embraces systems at the size of loads of kilometers down to micro- or atomic-scale systems, structures that shape nearly instantly, and systems that form over tens of thousands and thousands of years.
A big number of subdisciplines, procedures and strategies consequently exist in the ?Eld of structural geology. The oil exploration geologist may be thinking about entice-forming structures fashioned during rifting or salt tectonics, at the same time as the manufacturing geologist worries about subseismic sealing faults (faults that prevent ?Uid ?Ow in porous reservoirs). The engineering geologist may don't forget fracture orientations and densities in relation to a tunnel project, at the same time as the college professor makes use of structural mapping, bodily modeling or pc modeling to apprehend mountain-building tactics. The techniques and processes are many, however they serve to recognize the structural or tectonic improvement of a vicinity or to expect the structural sample in an area. In most instances structural geology is founded on statistics and observations that ought to be analyzed and interpreted. Structural analysis is therefore an important a part of the ?Eld 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.
Structural information units
Planet Earth represents an incredibly complex physical system, and the structures that result from natural deformation reﬂect this fact through their multitude of expressions and histories. 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.
Experiments and numerical models no longer best assist us understand how external and internal physical situations control or are expecting the deformation structures that form, however additionally deliver information on how deformation structures evolve, i.E. They provide insights into the deformation records. In assessment, certainly deformed rocks constitute cease-effects of herbal deformation histories, and the history may be dif?Cult to study out of the rocks themselves. Numerical and experimental models permit one to control rock homes and boundary conditions and explore their effect on deformation and deformation history. Nevertheless, any deformed rock includes a few information about the history of deformation. The challenge is to understand what to look for and to interpret this facts. Numerical and experimental work aids in completing this project, collectively with goal and accurate ?Eld observations.
Numerical, experimental and remotely acquired information units are crucial, however should continually be based on ?Eld observations.
It is tough to overemphasize the importance of conventional ?Eld observations of deformed rocks and their structures. Rocks contain greater records than we can ever be capable of extract from them, and the achievement of any physical or numerical version is predicated on the accuracy of remark of rock structures within the ?Eld. Direct contact with rocks and structures which have no longer been ?Ltered or interpreted via humans or computers is beneficial.
Unfortunately, our capacity to make goal observations is limited. What we've got found out and visible within the past strongly in?Uences our visible impressions of deformed rocks. Any student of deformed rocks need to therefore teach himself or herself to be objective. Only then can we count on to discover the surprising and make new interpretations that can make contributions to our information of the structural development of a area and to the ?Eld of structural geology in trendy. Many structures are ignored until the day that a person factors out their lifestyles and meaning, upon which they all of a sudden appear ?Everywhere?. Shear bands in strongly deformed ductile rocks (mylonites). They were either neglected or taken into consideration as cleavage till the late 1970s, once they were well described and interpreted. Since then, they have been defined from almost each important shear region or mylonite zone inside the world.
Traditional ?Eldwork involves the use of simple gear consisting of a hammer, measuring tool, topomaps, a hand lens and a compass, and the facts amassed are especially structural orientations and samples for skinny segment research. This type of information series is still crucial, and is aided by using modern-day international positioning machine (GPS) gadgets and excessive-decision aerial and satellite tv for pc pictures. More superior and distinct work can also contain the usage of a portable laser-scanning unit, where pulses of laser mild strike the surface of the Earth and the time of return is recorded. This information can be used to build an in depth topographic or geometrical model of the outcrop, onto which one or extra high-resolution ?Eld photographs may be draped. An instance of the sort of version is shown in Figure 3, even though the gain of in reality shifting around in the model can not be validated by way of a ?At photograph. Geologic observations such as the orientation of layering or fold axes can then be made on a pc.
In many cases, the most vital manner of recording ?Eld statistics is via use of cautious ?Eld sketches, aided by way of photos, orientation measurements and different measurements that can be associated with the cartoon. Sketching additionally forces the ?Eld geologist to look at features and info which can in any other case be left out. At the equal time, sketches can be made to be able to emphasize relevant statistics and forget about beside the point details. Field sketching is, in large part, a be counted of exercise.
Satellite images, such as those shown in Figure 4a, c, are now available at increasingly high resolutions and are a valuable tool for the mapping of map-scale structures. 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 4b), with resolutions down to a few tens of centimetres in some cases. Both ductile structures, such as folds and foliations, and brittle faults and fractures are mappable from satellite images and aerial photos.
In the ﬁeld of neotectonics, InSAR (Interferometric Synthetic Aperture Radar) is a useful remote sensing technique that uses radar satellite images. 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 millimetre-scale changes in elevation and geometry of the surface, which may reﬂect active tectonic movements related to earthquakes. In addition, accurate digital elevation models (see next section) and topographic maps can be constructed from this type of data.
GPS records in popular are an crucial source of records that may be retrieved from GPS satellites to measure plate actions (Figure 5). Such information also can be amassed on the floor by using stationary GPS devices with right down to millimetre-scale accuracy.
DEM, GIS and Google Earth
Conventional paper maps are still useful for many ﬁeld mapping purposes, but rugged laptops, tablets and handheld devices now allow for direct digitizing of structural features on digital maps and images and are becoming more and more important. Field data in digital form can be combined with elevation data and other data by means of a Geographical Information System (GIS). 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 unfastened get admission to to geographic information exists, and this kind of statistics was revolutionized with the aid of the improvement of Google Earth within the ?Rst decade of this century. The particular information to be had from Google Earth and related sources of virtual facts have taken the mapping of faults, lithologic contacts, foliations and more to a new stage, both in terms of ef?Ciency and accuracy.
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 exploration for petroleum and gas. Most seismic data are thus from rift basins and continental margins.
|Fig. 6. Seismic 2-D line from the Santos Basin offshore Brazil, illustrating how important structural aspects of the subsurface geology can be imaged by means of seismic exploration. Note that the vertical scale is in seconds.|
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 6) are collected by boat, where a sound source (air gun) generates sound waves that penetrate the crustal layers under the sea bottom. 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 information 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 re?Ected from layer limitations in which there may be an growth in acoustic impedance, i.E. Wherein there's an abrupt exchange in density and/or the velocity with which sound waves journey within the rock. A lengthy line of microphones, onshore referred to as geophones and offshore referred to as hydrophones, file the re?Ected sound signals and the time they seem at the surface. These records are amassed in digital shape and processed by computers to generate a seismic photograph of the underground.
Seismic information 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 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 accountlateralchangesinrockvelocitythatmaycausevisual 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 accrued wherein the power emitted is suf?Ciently high to penetrate deep parts of the crust and even the upper mantle. Such traces are useful for exploring the massive-scale shape of the lithosphere. While broadly spaced deep seismic traces and local 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. 25m) that the data can be processed in three dimensions, and where sections through the cube can be made in any direction. 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 layers that are a certain distance apart (typically around 5–10m), and only faults with a certain minimum offset can be imaged and interpreted. 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 15–20m for identiﬁcation of fault throw.
Sophisticated methods of data analysis and visualization at the moment are to be had for 3-D seismic information units, beneficial for identifying faults and different systems which are underground. Petroleum exploration and exploitation typically rely on seismic three-D statistics units interpreted on computers by geophysicists and structural geologists. The interpretation makes it feasible to generate structural contour maps and geologic pass-sections that can be analyzed structurally in diverse methods, e.G. Through structural healing.
Three-D seismic statistics shape the inspiration of our structural knowledge of hydrocarbon ?Elds.
Other styles of seismic records also are of interest to structural geologists, particularly seismic data from earthquakes. This data offers us important facts approximately present day fault motions and tectonic regime, which in simple terms manner whether or not a place is undergoing shortening, extension or strike-slip deformation.
Offshore collection of seismic information is achieved by using a vessel that travels at about 5 knots whilst towing arrays of air guns and streamers containing hydrophones some meters beneath the floor of the water. The tail buoy enables the crew find the stop of the streamers. The air guns are activated periodically, which include each 25m (approximately each 10 seconds), and the ensuing sound wave that travels into the Earth is re?Ected back via the underlying rock layers to hydrophones at the streamer and then relayed to the recording vessel for in addition processing.
The few sound traces shown at the ?Gure suggest how the sound waves are each refracted across and re?Ected from the interfaces between the water and Layer 1, between Layer 1 and a pair of, and between Layer 2 and three. Re?Ection happens if there is an increase within the product among speed and density from one layer to the next. Such interfaces are referred to as re?Ectors. Re?Ectors from a seismic line image the top stratigraphy of the North Sea Basin (right). Note the upper, horizontal sea mattress re?Ector, horizontal Quaternary re?Ectors and dipping Tertiary layers. Unconformities like this one usually suggest a tectonic occasion. Note that maximum seismic sections have seconds (two-manner time) as vertical scale.
Credits: Haakon Fossen (Structural Geology)