Part IV: five categories of geologic structures
Part IV includes Chapter 7 (Fractures), Chapter 8 (Faults), Chapter 9 (Folds), Chapter 10 (Fabrics), and Chapter 11 (Intrusions). For each category of structure, we use outcrop photographs, detailed maps, and thin sections to document the geometry of these structures and constrain the mechanics of their formation. For example, among many other applications in these chapters:
- Chapter 7 uses a map of the transition zone between two fracture domains in the Entrada Sandstone at Arches National Park, Utah, to reveal their relative age;
- Chapter 8 uses a map of the Frog fault and the Lone Mountain monocline in the Western Grand Canyon, Arizona, to document how faulting and folding are related;
- Chapter 9 uses Airborne Laser Swath Mapping (ALSM) at Sheep Mountain anticline, Wyoming, to prepare a structure contour map and quantify the 3D form of this fold;
- Chapter 10 uses thin sections from a right step in a fault from the Sierra Nevada, California, to study ductile fabrics that are characteristic of a shear zone; and
- Chapter 11 uses the geologic map of Mt Ellsworth, Henry Mountains, Utah, to investigate three stages in the growth of laccolithic intrusions.
The images below highlight the applications mentioned above for Chapters 7 and 8.
This Google Earth image shows two joint domains in the Entrada Sandstone at Arches National Park, UT. Figure 7.12 is a detailed structure map of the transition zone between these two joint domains. Some NW striking joints initiated as wing cracks near tips of NE striking joints. The development of the wing cracks striking to the northwest on the NE-striking joints is consistent with left-lateral shearing of these joints. This shear sense is associated with a counter-clockwise rotation of the regional horizontal principal stresses. Modified from Cruikshank and Aydin, 1995, Figure 9. Outcrop location at UTM: 12 S 613963.61 m E 4292665.01 m N.
Faulting and folding can be intimately related to one another. Here we use an exposure near the Grand Canyon (Figure 8.25a) to explore an example of these relationships. (a) Oblique Google Earth image looking southeast along strike of the Frog fault between Parashant Canyon and the Western Grand Canyon, AZ. Approximate UTM: 12 S 291315.00 m E 4003819.00 m N. (b) Schematic cross section based on GPS mapping of the Permian Esplanade formation showing the Frog fault and a synthetic fault. Fault geometry at depth is interpreted as either planar or listric. Modified from Resor, 2008, Figure 3B.