Plate Tectonics III.

Geologic Structures (Chapter 9 in textbook).

A. Stress can be either:

1. Compressional.
2. Tensional.
3. Shear.

B. Strain is the response and can be BRITTLE (breakage occurs) or DUCTILE (plastic).

1. IF BRITTLE:

   a. Faults:

i. Strike Slip (movement in strike direction), caused by SHEAR stress.

ii. Dip Slip (movement in vertical (dip) direction).

** Can be either:

• Normal dip slip (caused by Tensional stress)

• Reverse dip slip (caused by Compressional stress)

- Classified based on the type of movement which ultimately is controlled by the type of stress present.

The diagrams below show how fault geometries relate to different stress regimes:

	<-- This diagram shows a block cut by NORMAL faults. What type of stress causes this type of structure? Click on the diagram for a larger view.
The diagram on the right shows a REVERSE dip slip fault. What type of stress causes this structure? Click on the picture for a larger view -->

2. IF DUCTILE (plastic):

a. Folds (caused by slow application of COMPRESSIONAL stress):

i. Anticlines (either non-plunging or plunging).

ii. Synclines (either non-plunging or plunging).

The diagrams below illustrate how folding of the Earth's crust occurs:

	<-- The diagram on the left shows a pair of folds, a syncline and an anticline. Click on the image for a larger version.
Unequal stress over an area can produce plunging folds, and erosion planes off the folds exposing a pattern of lines and curves on the map view. The diagram on the right shows a pair of plunging anticlines separated by a plunging syncline. Click on the diagram for a larger version -->

Related Links:

Here is an online, zoomable geologic map of the Grand Canyon.

Here is a link to several very good Geology topic resources from the University of Tromso, Norway. About half way down the list is a nice Flash video that demonstrates the concepts of stress, strain, and deformation of rocks (scroll down to the one labeled "Deformation of rocks -- Structural geology", and check out some of the others on the page.