Crustal Deformation and Folds

Pamela J. W. Gore
Georgia Perimeter College

Objectives

  1. Define the basic type of stresses to which rocks may be subjected.
  2. Define the basic types of behavior that rocks may exhibit when subjected to stress.
  3. Contrast the various types of folds (anticlines, synclines, monoclines, basins, domes).
  4. Contrast the different types of faults (normal, reverse, thrust, strike-slip), and describe the type of stress that produces each.
  5. Understand the basics of strike and dip.
  6. Identify various folds and faults on geologic maps.

Deformation refers to the changes in volume or shape of a body of rock

Stresses

Stress is the FORCE acting on a body of rock.

Strain is the response of a rock to stress. It generally involves a change in shape or volume of the rock.

Types of stress:

  1. Compressional
  2. Tensional
  3. Shear
Types of strain:
  1. Elastic deformation - changes in shape of rock are reversible. Deform it, remove the stress, and it returns to its original shape (like a rubber band or a piece of elastic)
  2. Plastic deformation - changes in shape of rock are permanent and not reversible (like folding).

    Rock Behavior

    1. Brittle - the rock breaks
    2. Ductile - the rock flows or bends (folds are produced)
Once the elastic limit is surpassed, rocks will deform plastically if the rock is ductile or they will fracture (rupture) if the rock is brittle.

Describing the orientation of layers of rock

When we describe the orientation of sedimentary rock units, we must keep in mind that these rocks were originally deposited as sediment in horizontal (flat) layers. Tectonic forces cause the rock layers to be folded and uplifted, and sedimentary rocks can be in any orientation, including vertical.


Garden of the Gods, Colorado Springs, CO.

If we examine a small area of a layer of rock, we can describe its orientation in space using two directional components:

  1. The angle at which the rock "dips" (with respect to the horizontal) - called dip
  2. The compass direction along which the bed of rock trends (with respect to north) - called strike

For instance, in the image below, the rocks dip to the right at approximately a 45o angle.

Eldorado Canyon, south of Boulder, CO

To determine the strike direction, it is necessary to find the compass direction of an imaginary horizontal line on the surface of the bed of rock (also called a bedding plane).

The direction in which the imaginary horizontal line trends (with respect to north) is measured with a compass.

The direction is described as "so many degrees to the east (or west) of north", or in a more condensed version, "N degrees E" or "N degrees W" .

For example, N20E would refer to a horizontal line on a bedding plane that trends 20 degrees to the east of north, when plotted on a map.
As another example, N65W would refer to a horizontal line on a bedding plane that trends 20 degrees to the east of north, when plotted on a map.

In addition to the strike, the dip angle (and general direction) is also noted. Dip angle and direction are written such as "45oE" or "5oW".
Note that a 5o dip is nearly horizontal, and an 85o dip is nearly vertical.

When the orientation of a layer of rock is plotted on a map, the strike direction is perpendicular to the dip direction. If the rock dips due East, its strike will be due North (may also be described as north-south).

Dip and strike are recorded on maps using special symbols that resemble a "T" with an elongated top bar.
The top bar of the "T" is oriented on the map in the precise orientation of the strike of the rock unit.
The short vertical bar of the "T" points in the direction of dip.
There is almost always a number accompanying these strike and dip symbols, and that number refers to the angle of dip.

See examples in your textbook. You may also see these symbols on geologic maps in the laboratory or in the lab manual.

Types of folds

During mountain building or compressional stress, rocks may deform plastically to produce folds. Generally, a series of folds is produced, much as a carpet might wrinkle when you push on one end. The up-folds and the down-folds are adjacentt o one another, and grade into one another. In geology we give each a separate descriptive name.

Basic types of folds:

  1. anticlines - upfolds (on the left in the diagram above)
  2. synclines - downfolds (on the right in the diagram above)
Anticline and Syncline

Fold diagrams used with permission of Bruce E. Herbert, Texas A & M University, Big Bend Virtual Field Trip


Sideling Hill, a mountain in western Maryland,
cut by an interstate highway,
exposing a syncline in Late Paleozoic strata.

Folds
Chilhowee Group Quartzite
Mountain City Window

Chevron folds in shale
North Carolina or Tennessee

Folds in the Precambrian Baltimore Gneiss
Towson Dome, near Towson, Maryland
Hartley Mill Road, off Glenarm Road

Anticlines

Anticline
Wills Creek Formation (Silurian)
North of Hancock, Maryland, Rt. 70-522 exit ramp

Synclines

Syncline, eastern Tennessee

Syncline
Vandeveer Formation, Crab Orchard Mountain Group,
Pennsylvanian
near Ozone, Tennessee, Route 40/75 eastbound

Syncline
Cambro-Ordovician Limestone,
Charleton, MD (near Clear Spring) along Railroad Tracks

Overturned Folds


Overturned folds in the Baltimore Gneiss
Baltimore County, Maryland
Ensor Mill Road, along stream, off I-83.

Parts of a fold:

  1. fold axis
  2. axial plane
  3. limb

Fold shapes:

  1. symmetrical
  2. asymmetrical
  3. overturned - tipped in one direction so that one of the limbs is overturned
  4. recumbent - lying on its side
  5. dome
  6. basin

Orientation of the axis of the fold:

  1. plunging - the axis of the fold dips
  2. non-plunging - the axis of the fold is horizontal

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Page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston Campus, Clarkston GA
Page created November 6, 1998
Modified July 17, 1999
Links updated August 15, 2009

© PAMELA J. W. GORE, 1998-2009