Structural analysis

Many structural tactics span thousands to hundreds of thousands of years, and maximum structural data describe the ?Nal made of a protracted deformation history. The history itself can simplest be found out via careful evaluation of the data. When searching at a fold, it could not be apparent whether it shaped by way of layer parallel shortening, shearing or passive bending. The identical factor applies to a fault. What part of the fault fashioned ?Rst? Did it form with the aid of linking of person segments, or did it grow from a unmarried factor outward, and in that case, become this factor inside the vital part of the prevailing fault floor? It won't usually be smooth to reply such questions, but the technique need to constantly be to analyze the ?Eld records and examine with experimental and/or numerical models.

The analysis of the geometry of structures is referred to as geometric analysis. This includes the shape, geographic orientation, size and geometric relation between the main (first-order) structure and related smaller-scale (second-order) structures. The last point emphasizes the fact that most structures are composite and appear in certain structural associations at various scales. Hence, various methods are needed to measure and describe structures and structural associations.

Geometric evaluation is the conventional descriptive approach to structural geology that most secondary structural geologic analytical strategies construct on.

Fig. 2. Listric normal fault showing very irregular curvature in the sections perpendicular to the slip direction. These irregularities can be thought of as large grooves or corrugations along which the hanging wall can slide.

Shape is the spatial description of open or closed surfaces such as folded layer interfaces or fault surfaces. The shape of folded layers may give information about the fold-forming process or the mechanical properties of the folded layer, while fault curvature may have implications for hanging-wall deformation (Figure 1) or could give information about the slip direction (Figure 2).

Orientations of linear and planar structures are perhaps the most common type of structural data. Shapes and geometric features may be described by mathematical functions, for instance by use of vector functions. In most cases, however, natural surfaces are too irregular to be described accurately by simple vector functions, or it may be impossible to map faults or folded layers to the extent required for mathematical description. Nevertheless, it may be necessary to make geometric interpretations of partly exposed structures. Our data will always be incomplete at some level, and our minds tend to search for geometric models when analyzing geologic information. For example, when the Alps were mapped in great detail early in the twentieth century, their major fold structures were generally considered to be cylindrical, which means that fold axes were considered to be straight lines. This model made it possible to project folds onto cross-sections, and impressive sections or geometric models were created. At a later stage it became clear that the folds were in fact non-cylindrical, with curved hinge lines, requiring modification of earlier models.

Fig 3, Synthetic structural data sets showing different degree of homogeneity. (a) Synthetic homogeneous set of strike and dip measurements. (b) Systematic variation in layer orientation measurements. (c) Homogeneous subareas due to kink or chevron folding. (d, e) Systematic fracture systems. Note how the systematics is reflected in the stereonets.
 Fig. 4. Lineation data from subareas defined in the previous figure. The plots show the variations within each subarea, portrayed by means of poles, rose diagrams, and an arrow indicating the average orientation. The number of data within each subarea is indicated by “n”.

In geometric analysis it is very useful to represent orientation data (e.g. Fig. 3 and 4) by means of stereographic projection. Stereographic projection is used to show or interpret both the orientation and geometry of structures. The method is quick and efficient, and the most widely used tool for presenting and interpreting spatial data. In general, geometry may be presented in the form of maps, profiles, stereographic projections, rose diagrams or threedimensional models based on observations made in the field, from geophysical data, satellite information or laser scanning equipment. Any serious structural geologist needs to be familiar with the stereographic projection method.

Strain and kinematic evaluation

Geometric description and analysis may form the basis for strain quantification or strain analysis. Such quantification is useful in many contexts, e.g. in the restoration of geologic sections through deformed regions. Strain analysis commonly involves finite strain analysis, which concerns changes in shape from the initial state to the very end result of the deformation. Structural geologists are also concerned with the deformation history, which can be explored by incremental strain analysis. In this case only a portion of the deformation history is considered, and a sequence of increments describes the deformation history.

By definition, strain applies to ductile deformation, i.e. deformation where originally continuous structures such as bedding or dikes remain continuous after the deformation. Ductile deformation occurs when rocks flow (without fracture) under the influence of stress. The opposite, brittle deformation, occurs when rocks break or fracture. However, modern geologists do not restrict the use of strain to ductile deformation. In cases where fractures occur in a high number and on a scale that is significantly smaller than the discontinuity each of them causes, the discontinuities are overlooked and the term brittle strain is used. It is a simplification that allows us to perform strain analysis on brittle structures such as fault populations.

Fig. 5. Abrasive marks (slickenlines) on fault slip surfaces give local kinematic information. Seismically active fault in the Gulf of Corinth.
Fig. 6. An example of how geometric analysis can lead to a kinematic model, in this case of sense of movement on a fault. (a) A fault where stratigraphy cannot be correlated across the fault. (b, c) Relative movement can be determined if layer rotation can be observed close to the fault. The geometry shown in (b) supports a normal fault movement, while (c) illustrates the geometry expected along a reverse fault.

Geometric description also forms the foundation of kinematic analysis, which concern show rock particles have moved during deformation (the Greek word kinema means movement). Striations on fault surfaces (Figure 5) and deflection of layering along faults and in shear zones are among the structures that are useful in kinematic analysis.

To illustrate the relationship between kinematic evaluation and geometric evaluation, keep in mind the fault depicted in Figure 6a. We can not correlate the layers from one aspect to the other, and we do not realize whether or not this is a normal or opposite fault. However, if we ?Nd a de?Ection of the layering along the fault, we will use that geometry to interpret the experience of motion at the fault. Figure 6 b, c indicates the one-of-a-kind geometries that we would count on for everyday and reverse movements. In other words, a ?Eld based totally kinematic evaluation is based on geometric analysis.

Dynamics is the study of forces that cause motion of particles (kinematics). Forces acting on a body generate stress, and if the level of stress becomes high enough, rocks start to move. Hence dynamics in the context of structural geology is about the interplay between stress and kinematics. When some particles start to move relative to other particles we get deformation, and we may be able to see changes in shape and the formation of new structures.

Dynamic analysis explores the stresses or forces that purpose systems to form and stress to accumulate.

In maximum instances dynamic analysis seeks to reconstruct the orientation and magnitude of the pressure ?Eld via studying a hard and fast of structures, typically faults and fractures. Returning to the example proven in Fig. 6, it maybe assumed that a sturdy force or pressure acted in the vertical path in case (b), and within the horizontal course in case (c). In exercise, the exact orientations of forces and pressure axes are dif?Cult or impossible to estimate from a unmarried fault structure, but may be predicted for populations of faults forming in a uniform strain ?Eld.

Applying stress to syrup gives a different result than stressing a cold chocolate bar: the syrup will flow, while the chocolate bar will break. We are still dealing with dynamic analyses, but the part of dynamics related to the flow of rocks is referred to as rheologic analysis. Similarly, the study of how rocks (or sugar) break or fracture is the field of mechanical analysis. In general, rocks flow when they are warm enough, which usually means when they are buried deep enough.“Deep enough” means little more than surface temperatures for salt, around 300 C for a quartz-rich rock, perhaps closer to 550 C for a feldspathic rock, and even more for olivine-rich rocks. Pressure also plays an important role, as does water content and strain rate. It is important to realize that different rocks behave differently under any given conditions, but also that the same rock reacts differently to stress under different physical conditions. Rheological testing is done in the laboratory in order to understand how different rocks flow in the lithosphere.

Fig 7. Scanning electron microphotograph of a millimeter-skinny region of grain deformation (deformation band) in the Entrada Sandstone close to Goblin Valley State Park, Utah.

Tectonic evaluation includes dynamic, kinematic and geometric evaluation at the dimensions of a basin or orogenic belt. This kind of evaluation may also consequently contain extra elements of sedimentology, paleontology, petrology, geophysics and different subdisciplines of geoscience. Structural geologists worried in tectonic analysis are once in a while known as tectonicists. On the opposite quit of the size range, some structural geologists analyze the systems and textures that may most effective be studied thru the microscope. This is the study of the way deformation takes place among and within person mineral grains and is referred to as microstructural evaluation or microtectonics. Both the optical microscope and the scanning electron microscope (SEM) (Fig. 7.) are beneficial equipment in microstructural analysis.

Credits: Haakon Fossen (Structural Geology)

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