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7.4.2. Faults

Geological faults

The faults are brittle deformation that occur when rocks can not absorb the stresses to which they are subjected. The stresses can be compressive, strain or shear. Materials break apart and the resulting fragments move relative to each other. If there were no displacement, we would speak of cleats.

Geometric elements of the faults

In order to classify the faults and understand how they have been formed, it is necessary to define the main elements of the faults:

  • Fault blocks or lips. They are each of the two pieces of rock that are separated by the fault plane.
    • If the fault plane is not vertical, a distinction is made:
      • Upper block: Block that is above the fault plane.
      • Lower block: Block is below the fault plane.
    • According to the relative movement of the blocks:
      • Raised block.
      • Sunken block.
  • Fault plane. It is the failure plane along which the blocks that separate in the fault move. It can be vertical, horizontal or inclined. The plane can be determined, as we saw when talking about the axial plane of the fold, with the direction and the dip. Due to the friction between the blocks, the fault planes can be polished giving rise to the fault mirrors. Fault striations , rectilinear marks that indicate the direction of movement of the blocks, may also appear on the fault plane.
  • Skip jump. It is the distance that one block has moved with respect to the other. It can be measured both laterally, horizontally or vertically. The net jump would indicate the sum of the three previous measurements, and would be indicated by the fault striations.

Interactive activity: Place each element of the fault in its corresponding place.

Types of failures

  • Normal or direct failure. The top block (the one above the fault plane) is the sunken block. It is produced by distensive efforts, since there is an increase in the surface.
Arturo González [CC BY-SA 4.0], via Wikimedia Commons

Video: Normal failure.

Reverse fault. The upper block coincides with the raised block. It is produced as a consequence of compressive efforts, since the surface area decreases.

By Arturo González (Own work) [CC BY-SA 4.0], via Wikimedia Commons

Video: Reverse Failure.

  • Vertical fault. They are not common, the displacement of the blocks occurs only vertically, nothing horizontally.
  • Tear or shear failure or direction. The displacement of the blocks occurs only horizontally. They originate from shear stresses.

Falla de desgarre

By The original uploader was Cferrero at English Wikipedia (Transferred from en.wikipedia to Commons.) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

Video: Tear failure.

Images: Fallas galleries.

Interactive activity: Types of failures.

Failure Associations

Faults, such as folds, usually appear associated with others, causing associations or fault systems. In regions affected by normal faults, the following are distinguished:

  • Horst or tectonic massif: A raised block that lies between two normal faults.
  • Graben: Block that remains lower as a result of distensive movements, between two normal faults.

Horst y graben

By Horst_graben.jpg: U.S. Geological Surveyderivative work: Gregors (talk) 11:17, 7 June 2011 (UTC) (Horst_graben.jpg) [Public domain], via Wikimedia Commons