Structural geology

Structural geology is the study of the three-dimensional distribution of rock units to their deformational histories. The primary goal of structural geology is to use 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 geometries. This understanding of the dynamics of the stress field can be linked to important events in the regional geologic past; a common goal is to understand the structural evolution of a particular area to widespread rock deformation patterns (e.g., mountain building, rifting) due to plate tectonics.

Use and importance[]

The study of geologic structures has been of prime importance in economic geology, petroleum geology, and mining geology.[1] Folded and faulted rock strata commonly form traps to accumulate and concentrate fluids such as petroleum and natural gas. Faulted and structurally complex areas are notable as permeable zones for hydrothermal fluids and the resulting concentration areas for a base and precious metal ore deposits. Veins of minerals containing various metals commonly occupy faults and fractures in structurally complex areas. These structurally fractured and faulted zones often occur in association with intrusive igneous rocks. They often also occur around geologic reef complexes and collapse features such as ancient sinkholes. Deposits of gold, silver, copper, lead, zinc, and other metals, are commonly located in structurally complex areas.

Structural geology is a critical part of engineering geology, concerned with the physical and mechanical properties of natural rocks. Structural fabrics and defects such as faults, folds, foliations, and joints are internal weaknesses of rocks that may affect the stability of human-engineered structures such as dams, road cuts, open pit mines, and underground mines or road tunnels.

Geotechnical risk, including earthquake risk, can only be investigated by inspecting a combination of structural geology and geomorphology.[2] Also, areas of karst landscapes that are underlain by underground caverns and potential sinkholes or collapse features are essential for these scientists. Also, areas of steep slopes are potential collapse or landslide hazards.

Environmental geologists and hydrogeologists or hydrologists need to understand structural geology because structures are groundwater flow sites and penetration, which may affect, for instance, seepage of toxic substances from waste dumps or seepage of salty water into aquifers.

Plate tectonics is a theory developed during the 1960s which describes continents' movement by way of the separation and collision of crustal plates. In a sense, it is structural geology on a planet scale and is used throughout structural geology as a framework to analyze and understand global, regional, and local scale features.[3]

Methods[]

Structural geologists use a variety of methods to (first) measure rock geometries, (second) reconstruct their deformational histories, and (third) calculate the stress field that resulted in that deformation.

Geometries[]

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 ductile extended; fold axes; and intersection lineations, the trace of a planar feature on another planar surface).

Illustration of measurement conventions for planar and linear structures

Measurement conventions[]

The inclination of a planar structure in geology is measured by strike and dip. The strike is the line of intersection between the planar feature and a horizontal plane, taken according to the right-hand convention, and the dip is the magnitude of the inclination, below horizontal, at right angles to strike. For example, striking 25 degrees East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.

Alternatively, dip and dip direction may be used as this is absolute. Dip direction is measured in 360 degrees, generally clockwise from North. For example, a dip of 45 degrees towards 115 degrees azimuth, recorded as 45/115. Note that this is the same as above.

The term hade is occasionally used and is the deviation of a vertical plane, i.e. (90°-dip).

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 challenging to measure directly. In this case, the lineation may be measured from the horizontal as a rake or pitch upon the surface.

Rake is measured by placing a protractor flat on the planar surface, with the flat edge horizontal, and measuring the lineation anglewise from horizontal. The lineation orientation can then be calculated from the rake and strike-dip information of the plane it was measured from, using a stereographic projection.

If a fault has lineations formed by movement on the plane, e.g., slickensides, this is recorded as a lineation, with a rake, and annotated as to the indication of throw on the fault.

Generally, it is easier to record strike and dip information of planar structures in dip/dip direction format as this will match all the other structural information that may be recording about folds, lineations, etc. However, there is an advantage to using different formats that discriminate between planar and linear data.

Plane, fabric, fold, and deformation conventions[]

The convention for analyzing 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.

Planar structures are named according to their order of formation, with original sedimentary layering the lowest at S0. Often it is impossible to identify S0 in highly deformed rocks so that numbering may be started at an arbitrary number or given a letter (SA, for instance). In cases where there is a bedding-plane foliation caused by burial metamorphism or diagenesis, this may be enumerated as S0a.

If there are folds, these are numbered as F1, F2, etc. Generally, the axial plane foliation or cleavage of a fold is created during folding, and the number convention should match. For example, an F2 fold should have an S2 axial foliation.

Deformations are numbered according to their order of formation, with the letter D denoting a deformation event. For example, D1, D2, D3. Folds and foliations, because deformation events form them, should correlate with these events. For example, an F2 fold with an S2 axial plane foliation would result from a D2 deformation.

Metamorphic events may span multiple deformations. Sometimes it is useful to identify them similarly to the structural features for which they are responsible, e.g., M2. This may be possible by observing porphyroblast formation in cleavages of known deformation age, by identifying metamorphic mineral assemblages created by different events or geochronology.

Intersection lineations in rocks, as they are the intersection of two planar structures, are named according to the two planar structures from which they are formed. For instance, an S1 cleavage and bedding intersection lineation is the L1-0 intersection lineation (also known as the cleavage-bedding lineation).

Stretching lineations may be challenging to quantify, especially in highly stretched ductile rocks where minimal foliation information is preserved. Where possible, when correlated with deformations (as few are formed in folds, and many are not strictly associated with planar foliations), they may be identified similar to planar surfaces and folds, e.g., L1, L2. For convenience, some geologists prefer to annotate them with a subscript S, for example, Ls1, to differentiate them from intersection lineations, though this is generally redundant.

Stereographic projections[]

Stereographic projection of structural strike and dip measurements is a powerful method for analyzing the nature and orientation of deformation stresses, lithological units, and penetrative fabrics.

Rock macro-structures[]

On a large scale, structural geology studies the three-dimensional relationships of stratigraphic units within terranes of rock or geological regions.

This branch of structural geology deals mainly with the orientation, deformation, and stratigraphy relationships (bedding), which may have been faulted, folded, or given a foliation by some tectonic event. This is mainly a geometric science, from which cross-sections and three-dimensional block models of rocks, regions, terranes, and parts of the Earth's crust can be generated.

The regional structure study is vital in understanding orogeny, plate tectonics, and more specifically in the oil, gas, and mineral exploration industries as structures such as faults, folds, and unconformities are primary controls mineralization and oil traps.

Modern regional structure is being investigated using seismic tomography and seismic reflection in three dimensions, providing unrivaled images of the Earth's interior, faults, and the deep crust. Further information from geophysics such as gravity and airborne magnetics can provide information on the nature of rocks imaged in the deep crust.

Rock microstructures[]

Structural geologists study rock microstructure or texture of rocks 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.

The textural study involves measuring and characterizing foliations, crenulations, metamorphic minerals, and timing relationships between these structural features and mineralogical features.

Usually, this involves collecting hand specimens, which may be cut to provide petrographic thin sections analyzed under a petrographic microscope.

The microstructural analysis also finds application in multi-scale statistical analysis to analyze some rock features showing scale invariance (see, e.g., Guerriero et al., 2009, 2011).

Kinematics[]

Geologists use their measurements of rock geometries to understand histories of strain in the rocks. Strain can take the form of brittle faulting and ductile folding and shearing. Brittle deformation takes place in the shallow crust, and ductile deformation takes place in the deeper crust, where temperatures and pressures are high.

Stress Fields[]

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 is typically used to determine stress fields from deformational structures.

In perfectly brittle rocks, faulting occurs at 30° to the most significant compressional stress. (Byerlee's Law)
The most significant compressive stress is normal to fold axial planes.

References[]

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G.H. Davis and S.J. Reynolds (1996). The structural geology of rocks and regions (2nd ed.). Wiley. ISBN 0-471-52621-5

      ISBN 0-471-52621-5.

C.W. Passchier and R.A.J. Trouw (1998). Microtectonics. Berlin: Springer. ISBN 3-540-58713-6

      ISBN 3-540-58713-6.

B.A. van der Pluijm and S. Marshak (2004). Earth Structure - An Introduction to Structural Geology and Tectonics (2nd ed.). New York: W. W. Norton. pp. 656. ISBN 0-393-92467-X

      ISBN 0-393-92467-X. http://globalchange.umich.edu/ben/ES/earthstructure.htm.

V. Guerriero et al. (2011). "Improved statistical multi-scale analysis of fractures in carbonate reservoir analogues". Tectonophysics (Elsevier) 504: 14–24. doi:10.1016/j.tecto.2011.01.003

     doi:10.1016/j.tecto.2011.01.003. Bibcode: 2011Tectp.504...14G
         Bibcode: 2011Tectp.504...14G.

V. Guerriero et al. (2009). "Quantifying uncertainties in multi-scale studies of fractured reservoir analogues: Implemented statistical analysis of scan line data from carbonate rocks". Journal of Structural Geology (Elsevier) 32 (9): 1271–1278. doi:10.1016/j.jsg.2009.04.016

     doi:10.1016/j.jsg.2009.04.016. Bibcode: 2010JSG....32.1271G
         Bibcode: 2010JSG....32.1271G.

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