U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
How to Construct Seven Paper Models that Describe Faulting of the Earth
By Tau Rho Alpha* and John C. Lahr*
Open-file Report 90-257-A
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Although this program has been used by the U.S. Geological Survey, no warranty, expressed or implied, is made by the USGS as to the accuracy and functioning of the program and related program material, nor shall the fact of distribution constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith.
*U.S. Geological, Survey, Menlo Park, CA 94025
Description of Report
This report contains instructions and patterns for preparing seven three-dimensional paper models that schematically illustrate common earth faults and associated landforms. The faults described are: normal, reverse, right- and left-lateral strike-slip, and oblique-slip. There are also models and discussions of two fault-produced landforms, a graben and a horst.
These models are intended to help students and others visualize the principal classes of faults and learn some of the terminology used by geologists to describe faults. By constructing and examining these models, students will obtain a greater appreciation of the relationship between fault displacements and the landforms that result.
The date of this Open-File Report is 4/12/90 (version 1) . OF 90-257-A, paper copy; OF 90-257-B, 3.5-in. Macintosh diskette.
The date of version 2 of this Open-File Report is 2/7/92. OF 90-257-A, paper copy; OF 90-257-B, 3.5-in. Macintosh diskette.
Purchasers of the diskette version 2 of this report, which includes all of the text and graphics, can use HyperCard 2.0™ software (not supplied) to change the model (by adding geologic patterns, symbols, colors, etc.) or to transfer the model to other graphics software packages. Requirements for the diskette version 2 are: Apple Computer, Inc., HyperCard 2.0™ software, and an Apple Macintosh™ computer. If you are using System 7, we recommend using at least 3 MB of RAM with 1.5 MB of system memory available for HyperCard.
To see the entire page (card size: MacPaint), select "Scroll" from "Go" menu and move the hand pointer in the scroll window. If you are experiencing trouble with user-level buttons, select message from the "Go" menu. Type "magic" in the message box and press return. Three more user-level buttons should appear.
To order this report contact: USGS Information Services, Box 25286, Denver, CO 80225 OR CALL 1-800-USA-MAPS
A fault is a fracture surface within the earth on which slip or displacement has taken place. The total displacement on a fault may be less than a few centimeters or may be measured in hundreds of kilometers. Large displacements are commonly achieved by a series of sudden slips associated with earthquakes, but under some conditions involving slow slip, called creep. Many possible fault configurations are possible; the fracture surface may be planar or curved, and the slip may be uniform everywhere or may change from place to place, as in a rotational displacement or a displacement that becomes smaller and smaller and finally dies out. In this report we will focus on those portions of faults with uniform displacement on planar fracture surfaces (figure 1) and will not discuss complex faults or the details associated with the edges or intersections of faults (figure 2).
The three fundamental fault types are normal, reverse, and strike-slip (figure 1). Normal faults involve a dipping fracture surface on which the block above the fault plane, the hanging-wall block, is downthrown with respect to the block below, called the footwall block. Normal faults are common in regions of crustal extension. In contrast, reverse fault displacements, which are common in regions of compression, are such that the block above the fracture surface is uplifted with respect to the block below. Strike-slip faults generally involve no vertical motion, but instead are produced by two blocks that are sliding laterally past one another. The sense of lateral motion can be right lateral (dextral) or left lateral (sinistral). Imagine that you are standing on one side of the fault. If the other side has moved to the right, as may be indicated by offset streams, ridges, roads, fences, or other features that cross the fault, it is a right-lateral fault. If the other side has been offset to the left, the fault is left lateral. Few faults are, in fact, purely normal, reverse, or transverse, but instead combine transverse motion with either normal or reverse motion. This combined motion is termed oblique slip.
Figure 1. Simple fault types.
Figure 2. Complex fault types.
When faults extend to the Earths surface, displacing parts of the landscape, landforms are developed or modified. The portion of the fracture surface that is exposed by faulting is called the fault scarp (figure 3). Fault scarps may initially be angular and well defined, but over time they are modified by weathering and erosion on the upper portions while the lower portions become buried by eroded debris (talus). If a region is sliced by a series of subparallel normal faults with sufficient displacement, horst-and-graben topography may develop. A horst is a block that has remained high relative to those on either side, whereas a graben is depressed relative to the adjacent blocks (figure 4).
Figure 3. Elements of a fault.
Figure 4. A collection of faults.
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