Tectonic Plate Dynamics: Deformations & Earthquakes

Tectonic Plate Dynamics

Efforts and Deformations

The dynamics of tectonic plates create high pressure, known as tectonic forces, which produce significant changes in rocks called tectonic deformations.

Tectonic Forces

Rocks can be subjected to three types of forces:

  • Compressional
  • Extensional
  • Shear

Tectonic Deformations

Tectonic forces produce two types of permanent rock deformation:

  • Plastic Deformation: Occurs when rocks exhibit ductile behavior and fold due to their plasticity. These are continuous deformations, with folds being the most common.
  • Fracture or Break: Occurs when rocks exhibit brittle behavior and fracture upon exceeding their resistance limit, called strength. These are major faults.

Types of Efforts

  • Compressional: Cause shortening and thickening of rocks. They originate from convergent plate margins and transform faults.
  • Extensional: Produce stretching and thinning of rocks. They are generated at diverging plate edges and may be caused by decompression of rocks when they are no longer subject to compressional forces.
  • Shear: Are pressures acting in opposite directions on a given surface. They develop in transform faults and tapering edges.

Folds

Folds are plastic and continuous deformations produced by compressional forces, causing the curvature of structures such as stratification or foliation, which are initially flat. The following elements are distinguished:

  • Hinge: The zone of maximum curvature of the fold. The line connecting hinge points is called the axis of the fold or hinge line.
  • Strike: The angle between the axis of the fold and the geographic north-south direction.
  • Axial Plane: The plane containing all hinge lines. It divides the fold into two parts called flanks or limbs.
  • Dip: The angle between the surfaces of each flank and a horizontal plane.
  • Nucleus: The innermost and tightest part of a fold.
Classification of Folds
  • By the shape of the profile:
  • Antiform: Convex upward. When the oldest rocks are at the core, these folds are called anticlines.
  • Synform: Concave upward. When younger materials are at the core, these folds are called synclines.
  • By symmetry with the axial plane:
    • Symmetrical: They have axial symmetry about the plane.
    • Asymmetrical: They lack axial symmetry about the plane.

Faults

Faults are discontinuous deformations where movement has occurred between blocks separated by the fracture. The main elements of faults are:

  • Fault Plane: The fracture plane that divides the land into two blocks or lips.
  • Fault Strike: The angle between a horizontal line on the fault plane and the geographic north-south direction.
  • Dip: The angle between the fault plane and an imaginary horizontal plane.
  • Fault Displacement: The distance measured along the fault plane that separates two points that were joined before the fracture.
Types of Faults
  • Normal Fault: Caused by extensional forces that produce the extension and thinning of the crust.
  • Reverse Fault: Produced by compressional or shear forces that shorten and thicken the ground.
  • Strike-Slip Fault: Due to compressional or shear forces, the blocks move horizontally, so the fault break is parallel to its strike.

Other Discontinuous Deformations

  • Joints: Fractures with no relative motion between the blocks separated by the rupture plane. They can be caused by any tectonic force. Their elements are:
    • Strike: The angle between a horizontal line on the fracture plane and the geographic north-south direction.
    • Dip: The angle between the fracture plane and an imaginary horizontal plane.
  • Fault-Fold: Discontinuous fractures formed when compressional forces exceed the mechanical strength of the rocks, breaking the continuity of the fold.
  • Thrust Nappes: Thrust faults with a very stretched fracture plane and a very large displacement. The allochthonous nappe is the block of rock that slides over the autochthonous. Erosion of a nappe exposes the autochthonous structures, known as tectonic windows. Klippen are also the result of erosion, sometimes appearing as small, isolated blocks of the nappe.
  • Earthquakes: Also known as seismic events or tremors, are ground vibrations of varying intensity that occur when energy is released suddenly and quickly. They can be produced by both artificial and natural causes. Seismic zones are located on the edges of tectonic plates.
  • Elastic Rebound: The mechanism that causes most earthquakes is elastic rebound. As rocks deform, they store elastic energy. If the rocks are deformed beyond their breaking point, they suddenly rupture and release the stored energy, producing vibrations (seismic waves).