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Introduction to Structural Geology


Structural Geology In Geosciences

  1. Structural Geology aims to characterise deformation structures (geometry), to characterize flow paths followed by particles during deformation (kinematics), and to infer the direction and magnitude of the forces involved in driving deformation (dynamics). A field-based discipline, structural geology operates at scales ranging from 100 microns to 100 meters (i.e. grain to outcrop).
  2. Tectonics aims at unraveling the geological context in which deformation occurs. It involves the integration of structural geology data in maps, cross-sections and 3D block diagrams, as well as data from other Geoscience disciplines including sedimentology, petrology, geochronology, geochemistry and geophysics. Tectonics operates at scales ranging from 100 m to 1000 km, and focusses on processes such as continental rifting and basins formation, subduction, collisional processes and mountain building processes etc.
  3. Geodynamics focusses on the forces that drive mantle convection, plate motion and deformation of Earth's material. Geodynamics is concerned with deep mantle processes such as mantle convection, cold drips, hot plumes and their links to plate motion, including dynamic plate subsidence and uplift, and plate tectonic processes. Geodynamics involves working at scales > 100 km. Numerical modeling is at the core of modern geodynamics.


The Scientific Approach

As all scientists, structural geologists follow research strategies that call upon concepts such as: fact, hypothesis, model, theory, and law. A good understanding of these terms is essential to all scientists.

A fact is a bit of truth. For a structural geologist a “fact” could be the dip direction of a bedding plane. Having measured that north is to the left on the photo on the right, then it is a fact that the dip direction of the bedding is to the south.

An hypothesis is an assumed fact. It is a short statement one makes to go further into a reasoning. For instance, assuming that the bottom right photo contains the stretching lineation (hypothesis), then one can infer that the sense of shear deduced from the tilling of K-feldspars in this orthogneiss is top to the right.

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A model is a self-consistent framework providing a coherent explanation for the observed facts. A model combines as many facts as possible and as little hypotheses as possible. A good model allows to make verifiable predictions. A model is proven wrong if key predictions are not verified. It can be iteratively strengthened via minor modifications to account for both the facts and the predictions.

A theory is a very robust model which accounts for a large number of independent facts; and whose numerous predictions have been verified over a long period of time. Evolution and Plate Tectonics are two theories.

A law is a simple, fundamental concept that is always verified by experiments and that underpin our understanding of the world. For instance, the law of gravity and the laws of thermodynamics underpin our understanding of Physics and Chemistry.

Truth is not a scientific concept.

Workflow of Structural Geology & Tectonics

Structural Geology and Tectonics combines two aspects:

1/ Description and analysis of 3D structures and microstructures (Structural Geology sensu stricto).

Structural geologists are concerned with features resulting from deformation. These include fractures, faults, folds, boudins, shear zones, cleavages (also knows as schistosities), foliations and lineations.

From the analysis of these structures, they aim at understanding finite strain (i.e., the ultimate product of long, sometimes polyphased deformation histories), and incremental strain (i.e., the small increments of deformation, the accumulation of which leads to the finite strain).

They are interested to understand “strain fields” by mapping deformation features such as foliations and stretching lineations that tell us the orientation of the principal shortening direction and principal lengthening direction respectively.

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In the case of faults and shear zones, they are interested to understand their kinematics (i.e., the relative sense of motion of the blocks they separate), and the magnitude of the displacement involved.

They are interested to infer the direction of maximum and minimum stress directions from small deformation features such as centimeter-scale extensional fractures and associated stylolitic joints.

2/ Design of tectonic models (Tectonics).

The purpose of these models is to explain the deformation history that led to the observed 3D strain fields. Tectonic models incorporate a broad range of data from other disciplines. No matter how tectonicists design these models (following hours of pure rational thinking or via flashes of insight after a heavy night), tectonic models should always be:

  • Physically valid: They must be obey the law of physics, sounds trivial but not easy to meet this requirement without computational modelling.
  • Testable: They must provide testable predictions (structural, sedimentological, petrological, geochemical, geophysical ...) that can be verified.
  • Robust: They must explain a large number of unrelated facts,
  • Lean: Hypotheses should be kept at a minimum compared to the number of fact models explain.