Figure G-28 is a generalized map of a portion of the southwestern United States showing the Basin-and- Range geologic province and bordering regions. The Basin-and-Range province derives its name from the north–south-trending basins and ranges that occur there. Two late Cenozoic deformational fields are recognizable in this area (Wright, 1976).

Fig. G-28

1. Examine Field I on the map. This field, which includes central and northern Nevada and western Utah, is characterized by listric normal faults. What are the two horizontal principal stresses in this field, and how are they oriented? Draw your answer in the space provided below the map.

2. Examine Field II, which includes southern and westernmost Nevada and eastern California. Notice that this field contains a combination of normal and strike-slip faults. These faults have all been active during the same general time interval, but not necessarily exactly synchronously. Focusing first on the normal faults, in the space provided below the map, draw the orientation of the horizontal principal stresses indicated by the normal faults in Field II.

3. Notice that the strike-slip faults in Field II include both dextral and sinistral faults. What horizontal principal stresses, and in what orientations, are indicated by these strike-slip faults? Draw your answer in the space provided below the map.

4. You should now recognize that your answers have revealed a stress-orientation paradox in Field II. In the space provided near the bottom of Fig. G-28, use one or two succinct sentences to state what this paradox is, including a description of the orientation of the stress ellipsoid responsible for each type of fault. Finally, think of at least one hypothesis to explain this paradox, and write your hypothesis in the space provided. If you can think of more than one hypothesis, all the better.

Transcribed Image Text:

0 0 California Normal fault (Direction of dip indicated) Strike-slip fault 100 100 200 300 San Andreas Fault 200 SIERRA NEVADA BLOCK 400 Nevada "// BASIN AND 500 km 300 miles Field I Field II -- [{} RANGE 2² 1 1 - 1 Utah Arizona COLORADO PLATEAU Hoover Dam