Understanding Tectonic Stress, Deformation, and Geological Structures

Posted on Mar 9, 2025 in Geology

Tectonic Stress and Strain

Tectonic Stress

Rock may be subjected to three types of stress:

  1. Compression: These forces cause shortening and thickening of the rocks. These pressures originate from convergent plate margins and transform faults.
  2. Tension: These forces produce stretching and thinning of the rocks. They can also arise when rocks are released from compressive stress.
  3. Shear: These are pressures acting on a surface in opposite directions. Shear stress is developed at faults and along plate boundaries.

Tectonic Deformation

Tectonic forces produce two types of deformation in rocks:

  1. Plastic Deformation: This occurs when rocks behave in a ductile manner and fold due to their plasticity. Plastic deformations are continuous and result in folds.
  2. Fracture or Break: This occurs when rocks behave in a brittle manner and fracture when their strength is exceeded. These are called discontinuities. Examples include joints, faults, and thrust faults. Thrust sheets are a type of fault.

Folds

Folds are continuous plastic deformations produced by compressive stress that cause the curvature of structures, such as stratification surfaces or schistosity, that were initially flat.

Elements of a Fold

  1. Hinge: This is the area of maximum curvature of the fold. The line joining the hinge points is called the hinge line.
  2. Trend: This is the angle formed by the fold axis with the geographic north-south direction.
  3. Axial Plane: This is the plane containing all hinge lines. The axial plane divides the fold into two parts: flanks or limbs.
  4. Dip Slope: This is the angle formed by the surfaces of each of the limbs with a horizontal plane.
  5. Core: The innermost part of a tight fold.

Classification of Folds

By Profile

  1. Antiform: These have a convex profile upwards. When the oldest rocks are in the core, these folds are called anticlines.
  2. Synform: These have a concave upward profile. These folds are called synclines when the youngest rocks are in the core.

By Symmetry

  1. Symmetric Folds: These have axial symmetry about the axial plane.
  2. Asymmetric Folds: These do not have axial symmetry about the axial plane.

Faults

Faults are discontinuous deformations where movement occurs along a fracture.

Elements of a Fault

  1. Fault Plane: The fracture surface that divides the ground into two blocks or lips. The hanging wall moves up relative to the footwall, which moves down.
  2. Trend of the Fault: The angle formed by a horizontal line on the fault plane with the geographic north-south direction.
  3. Dip of the Fault: The angle formed by the fault plane with an imaginary horizontal plane.
  4. Fault Displacement: The distance, measured on the fault plane, separating two points that were once adjacent before the fracture occurred.

Types of Faults

  • Normal Fault: Normal faults are caused by tensional stress that produces extension and thinning of the crust.
  • Reverse Fault: These are produced by compressive or shear stress that shortens and thickens the crust.
  • Strike-Slip Fault: This is due to compressive or shear stress. The blocks move horizontally, so the fault displacement is parallel to its trend.