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Unit 36

SURFACE EXPRESSIONS OF SUBSURFACE STRUCTURES

Unit Overview

This unit examines geologic features resulting from interactions between lithospheric plates. The main sections are as follows:

  • Terminology of structure
  • Fault structures
  • Fold structures
  • Regional deformation

There is a strong relationship between geologic structure and surface landscape. Landscape features, often in the form of outcrops (a locality where exposed rock occurs), include faults and folds.

Plate tectonics are primarily responsible for faulting and folding. A fault is a fracture in crustal rock involving the displacement of rock on one side of a fracture with respect to rock on the other side. Faulting results when brittle rocks come under stress and break because they cannot bend or fold. Faults can be compressional, tensional, or transverse.

The vertical movement of blocks results from both converging and diverging plates, and the associated faults are compressional faults and tensional faults, respectively. Transverse faults are associated with horizontal motion along the fault plane. Common forms of compressional, tensional, and transverse faults are reverse, normal, and transcurrent faults, respectively.

Folding can occur in place of, or in addition to, faulting. All rocks have the ability to fold, and the resulting folds are primarily compressional features. Common topographic structures indicative of folding are anticlines and synclines. An anticline is an arch-like fold with the limbs dipping away from the axis, while a syncline is trough-like with limbs dipping toward the axis. Younger rocks constitute the cores of synclines in a sedimentary context. Anticlines and synclines are often plunging, thus their axes dip. Over large areas of stable lithosphere, faulting or folding is minimal; instead, slight deformation may exist.

Unit Objectives

  • To introduce basic terminology used in describing rock structure
  • To distinguish between types of fault movements and the landforms they produce
  • To discuss the folding of rocks and relate it to the landforms produced


Glossary of Key Terms

Anticline An arch-like upfold with the limbs dipping away from its axis.
Dip The angle at which a rock layer tilts from the horizontal.
Fault A fracture in crustal rock involving the displacement of rock on one side of the fracture with respect to rock on the other side.
Folding Bending or warping in layered rock.
Graben A crustal block that has sunk down between two fairly parallel normal faults in response to tensional forces.
Horst A crustal block that has been raised between two reverse faults by compressional forces.
Normal fault A tensional fault exhibiting a moderately inclined fault plane that separates a block that has remained fairly stationary from one that has been significantly down-thrown.
Overturned fold An extremely compressed fold that doubles back on itself with an axial plane that is oriented beyond the horizontal.
Primary landform A structure created by tectonic activity.
Recumbent fold A highly compressed fold that doubles back on itself with an axial plane that is near horizontal.
Reverse fault The result of one crustal block overriding another along a steep fault plane between them; caused by compression of the crust into a smaller horizontal space.
Secondary landform A landform that is the product of weathering and erosion.
Strike The compass direction of the line of intersection between a rock layer and a horizontal plane.
Strike-slip fault See transcurrent fault.
Syncline A trough-like downfold with its limbs dipping toward its axial plane.
Thrust fault A compressional fault in which the angle of the fault plane is very low; sometimes called an overthrust fault.
Transcurrent fault A transverse fault in which crustal blocks move horizontally in the direction of the fault; also known as a strike-slip fault because movement at a transcurrent fault occurs along the strike of a fault.
Transform fault A special case of transcurrent faulting in which the transverse fault marks the boundary between two lithospheric plates that are sliding past each other; California's San Andreas Fault is a classic example.


Unit Outline

  • Terminology of structure
    • Strike is the compass direction of the line of intersection between a rock and a horizontal plane
    • Dip is the angle at which a rock tilts from the horizontal
      • direction of dip
    • An outcrop is a prominently exposed area of rock
  • Fault structures
    • A fracture in the crust involving displacement of rock on one side as opposed to the other
    • A joint is a fracture with no displacement
    • Where plates converge, there are compressional stresses
    • Where plates diverge, there are tensional stresses
    • Where plates slide past one another horizontally, there are transverse stresses
    • Compressional faults
      • one block rides over the other where rock collides, forming a reverse fault
      • upthrown and downthrown blocks
      • parallel echelon faults
      • a thrust fault is formed when the angle of a fault plane is very low
      • horsts are raised blocks between reversed faults
    • Tensional faults
      • normal faults are produced when one block remains fairly stationary, and the other block is downthrown as the plates pull apart
        • rift-valley topography
      • grabens are sunken blocks raised between two reverse faults
    • Transverse faults
      • movement of blocks is lateral, no upthrown or downthrown blocks
      • fault is transcurrent, movement is in the direction of the fault
      • transform faults are unique because it marks a plate boundary, movement occurs at the strike of the fault; called strike-slip faults as well
    • Field evidence of faulting
      • examine erosion, weathering of faults
      • sides are smooth, glass-like areas on a scarp that suggest faulting
  • Fold structures
    • Rocks also respond to stress by folding
    • Characteristic of sedimentary layered rock
    • Folding and faulting usually occur together
    • Anticlines and synclines
      • anticlines are upfolds
      • synclines are downfolds
      • core of synclines has younger rock
    • Plunging folds and landscapes
      • a plunge is a dip in the axis of an anticline or syncline
      • many anticlines and synclines overturned or recumbent
      • primary landforms created by tectonic action
      • secondary landforms made by weathering and erosion
  • Regional deformation
    • Crust often deformed in a minor way
      • called diastrophism or crustal warping
      • epeirogeny refers to vertical movement of crust with little or no bending or breaking of crust


Review Questions

  1. What is a horst? A graben? How are they formed?
  2. Define the terms diastrophism, crustal warping, and epeirogeny.
  3. Draw a sketch of an anticline and a syncline.