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Structural geology

Belongs to subject Structural geology

The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rocks, and ultimately, to understand the stress field that resulted in the observed strain and geometries. The study of geologic structures has been of prime importance in economic geology, both petroleum geology and mining geology. Folded and faulted rock strata commonly form traps that accumulate and concentrate fluids such as petroleum and natural gas. These structurally fractured and faulted zones often occur in association with intrusive igneous rocks. Structural geology is a critical part of engineering geology, which is concerned with the physical and mechanical properties of natural rocks. Structural geologists use a variety of methods to (first) measure rock geometries, (second) reconstruct their deformational histories, and (third) estimate the stress field that resulted in that deformation.

Primary data sets for structural geology are collected in the field. Structural geologists measure a variety of planar features (bedding planes, foliation planes, fold axial planes, fault planes, and joints), and linear features (stretching lineations, in which minerals are ductily extended; fold axes; and intersection lineations, the trace of a planar feature on another planar surface).

The inclination of a planar structure in geology is measured by strike and dip. Dip direction is measured in 360 degrees, generally clockwise from North. Fold axis plunge is measured in dip and dip direction (strictly, plunge and azimuth of plunge). The orientation of a fold axial plane is measured in strike and dip or dip and dip direction. Lineations are measured in terms of dip and dip direction, if possible. Often lineations occur expressed on a planar surface and can be difficult to measure directly. The convention for analysing structural geology is to identify the planar structures, often called planar fabrics because this implies a textural formation, the linear structures and, from analysis of these, unravel deformations. Deformations are numbered according to their order of formation with the letter D denoting a deformation event. For example, an F fold, with an S axial plane foliation would be the result of a D deformation. Metamorphic events may span multiple deformations. Intersection lineations in rocks, as they are the product of the intersection of two planar structures, are named according to the two planar structures from which they are formed. Stretching lineations may be difficult to quantify, especially in highly stretched ductile rocks where minimal foliation information is preserved. Rock microstructure or texture of rocks is studied by structural geologists on a small scale to provide detailed information mainly about metamorphic rocks and some features of sedimentary rocks, most often if they have been folded. Geologists use rock geometry measurements to understand the history of strain in rocks. Brittle deformation takes place in the shallow crust, and ductile deformation takes place in the deeper crust, where temperatures and pressures are higher.

By understanding the constitutive relationships between stress and strain in rocks, geologists can translate the observed patterns of rock deformation into a stress field during the geologic past. The following list of features are typically used to determine stress fields from deformational structures.

In perfectly brittle rocks, faulting occurs at 30° to the greatest compressional stress. The greatest compressive stress is normal to fold axial planes.

The mechanical properties of rock play a vital role in the structures that form during deformation deep below the earth's crust. The conditions in which a rock is present will result in different structures that geologists observe above ground in the field. The field of structural geology tries to relate the formations that humans see to the changes the rock went through to get to that final structure. Knowing the conditions of deformation that lead to such structures can illuminate the history of the deformation of the rock. Temperature and pressure play a huge role in the deformation of rock. Other vital conditions that contribute to the formation of structure of rock under the earth are the stress and strain fields.

When a rock is subjected to stresses, it changes shape. Stress induces strain which ultimately results in a changed structure. Elastic deformation refers to a reversible deformation. In other words, when stress on the rock is released, the rock returns to its original shape. This type of deformation is modeled using a linear relationship between stress and strain, i.e. a Hookean relationship.

Where σ denotes stress,

Plastic deformation refers to non-reversible deformation. The relationship between stress and strain for permanent deformation is nonlinear. One mechanism of plastic deformation is the movement of dislocations by an applied stress. It is a measure of resistance to deformation, specifically permanent deformation. During plastic deformation, a material absorbs energy until fracture occurs. The area under the stress-strain curve is the work required to fracture the material. Brittle materials have low toughness because low plastic deformation decreases the strain (low ductility). Ways to measure toughness include:

Resilience is a measure of the elastic energy absorbed of a material under stress. In other words, the external work performed on a material during deformation. The area under the elastic portion of the stress-strain curve is the strain energy absorbed per unit volume.

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