Introduction

This slide show contains a series of field photographs of the wide variety of features commonly referred to as "faults" (this is actually naming, not describing the feature). The field photographs are intermingled with some textbook sketches, maps and illustrations. There are several references to journal articles within the slide show.

Definitions

I'll start with a few definitions. When using this SlideShow viewer you can click on "View Introduction" at any time to check one of these definitions. When you click on "View Slides" you will return to the same point in the slideshow..

• Faults: Planes across which there has been lateral displacement. That is the in-plane displacements are non-zero. The term "fault" is often used to loosely describe a zone across which there has been shear displacements.
• Shear zone: Tabular bodies across which there has been non-zero shear displacements. The distinction is that one is a discrete plane and the other is a finite-width body. 
• Fault Zone: A shear zone that contains faults within the zone, and perhaps even bounding the zone on one side or another is referred to as a fault-zone. 
• Slip:
• Shift: Since we may be dealing with a shear zone rather than a discrete failure plane, I use the term shift to refer to offset (rather than slip).
• Deformation band: These appear to be one of the mechanisms of faulting (more below) that actually form as shear-fractures.

Mechanisms

What we refer to as a "fault" is a structure that is the result of one of several different  mechanisms (e.g., Johnson, 1995). Since what we commonly call "faults" can form in many mechanisms, there is not necessarily a direct relationship between fault orientation and the stress field. It is important to recognize the possible mechanism, and the stage of development of that mechanism when working with faults. An excellent example of stages in the development of a fault one can be found in earthquake rupture. See the Landers Earthquake slideshow for more information on the stages. 

Some mechanism for faulting (where a fault is a plane of discontinuity) are:

• Faulted-Joints. Here joints (which form with the in-plane displacements are zero, and the normal displacement is non-zero), slip to become faults (e.g. Cruikshank, Zhao and Johnson 1991).
• Zones of faulted joints (Martel, Pollard & Segall, 1988; Martel & Pollard 1989)
• Hinges of kink folds (e.g., Aydn, 1973)
• Along shear zones (e.g., Deformation bands; Aydin 1978, Aydin & Johnson 1978)

Faults form in "families" (as so-called conjugate sets). These faults all work in tandem to give the resulting deformation . These conjugate sets can also be found within a fault zone, and fault zones themselves will form conjugate sets. (see, for example, Oertel, 1965; Reches and Dieterich, 1983; Reches, 1983). It is essential that you correctly identify the mechanism of faulting, the scale of the fault, and how it related to larger structures and smaller structures.

A fault by any other name ...

When we talk about "normal", "reverse", or "strike-slip" faults, we are referring to their displacements relative to the current horizontal. We are also taught to think of normal faults representing extension, and reverse faults representing shortening. While these are true statements, I think a better way of thinking of all faults as the product of shortening. Normal faults and reverse fault sets have BOTH a direction of shortening and a direction of extension. One of the examples of faults within the slide show comes from the Owyhee area of eastern Oregon, in the Oregon-Idaho graben. The graben contains many normal faults, in sense that they bound a structural low (the graben). However, when you look at the major faults you see that their geometry requires shortening normal to the fault -- even though they are normal faults in a graben (Griffiths 2002)!  Faults are compressional structures. Depending on their orientation to the maximum compression direction we give them different names. I would try to forget the  names and focus on a description of the fault characteristics, orientation, and direction of displacement (remember that displacement has three components, so you should be able to determine all three components).

So, a couple of important things to always consider about geologic structures (here faults)

• The mechanism
• Scale of features
• Relation of features at different scales.

References

Aydin, A., 1973, Field study and theoretical analysis of some small faults in Montana, Wyoming, and Utah [Masters thesis]: Department of Geology, Stanford University, Stanford, California, 51 p.

Aydin, A., 1978a, Small faults formed as deformation bands in sandstone: Pure and Applied Geophysics, v. 116, p. 913-930.

Aydin, A., and Johnson, A. M., 1978b, Development of faults as zones of deformation bands and as slip surfaces in sandstones: Pure and Applied Geophysics, v. 116, p. 931-942.

Cruikshank, K. M., Zhao, G., and Johnson, A. M., 1991, Analysis of minor fractures associated with joints and faulted-joints: Journal of Structural Geology, v. 13, no. 8, p. 865-886.

Chester, F. M., Evans, J. P., and Biegel, R. L., 1993, Internal structure and weakening mechanisms of the San Andreas fault: Journal of Geophysical Research, v. 98, no. B1, p. 771-786.

Davies, R. K., and Pollard, D. D., 1986, Relations between left-lateral strike-slip faults and right-lateral monoclinal kink bands in Granodiorite, Mt. Abbot Quadrangle, Sierra Nevada, California: Pure and Applied Geophysics, v. 124, no. 1/2, p. 177-201.

Griffiths, J., 2002. XXXXXX MS Thesis, Department of Geology, Portland State University. (PDF version available).

Johnson, A. M., 1995, Orientations of fault determined by premonitory shear zones: Tectonophysics, v. 247, p. 161-238.

Martel, S. J., 1990, Formation of compound strike-slip fault zones, Mount Abbot quadrangle, California: Journal of Structural Geology, v. 12, no. 7, p. 869-882.

Martel, S. J., and Pollard, D. D., 1989, Mechanics of slip and fracture along small faults and simple strike-slip fault zones in granitic rock: Journal of Geophysical Research, v. 94, no. B7, p. 9417-9428.

Martel, S. J., Pollard, D. D., and Segall, P., 1988, Development of simple strike-slip fault zones, Mount Abbot quadrangle, Sierra Nevada, California: Geological Society of America Bulletin, v. 100, no. 9, p. 1451-1465.

Oertel, G., 1965, The mechanism of faulting in clay experiments: Tectonophysics, v. 2, no. 5, p. 343-393.

Reches, Z., and Dieterich, J. H., 1983, Faulting of rocks in three-dimensional strain fields I. Failure of rocks in polyaxial, servo-controlled experiments: Tectonophysics, v. 95, p. 111-132.

Reches, Z., 1983, Faulting of rocks in three-dimensional strain fields II. Theoretical analysis: Tectonophysics, v. 95, p. 133-156.

Zhao, G., and Johnson, A. M., 1991, Sequential and incremental formation of conjugate sets of faults: Journal of Structural Geology, v. 13, no. 8, p. 887-895.

Slide Descriptions

1992-01-35

1992.04.05 - This is a zone of deformation bands in Navajo Sandstone in the Klodnike Bulffs area of Arches National Park Utah. Although it cannot be seen in this view, a slip surface has formed on one slide of the zone forming a fault plane. Deformation bands and zones of deformation bands are described in Aydin (1978) and Aydin & Johnson (1978). More detail of this zone is shown in the next slide (ID: 1992-01-36)

A couple of things to note about the zone:

• If you try to follow some of the purple layers you will see that they are smeared through shear zones up to about a cm wide. The coin used for scale is a US 5¢ piece (nickel), which is about 2.2 cm in diameter. As a general rule, the amount of shift across a deformation band increases with it's thickness. 
• In this view there are apparently two orientations of shear zones. One of the two orientations (dipping to the right) have formed a more continuous through-going set (their shift reflects the sense of slip on the fault plane at the edge of the zone).

1992-01-36

This is a repeat of the previous caption ...

1992.04.05 -

A couple of things to note about the zone:

• If you try to follow some of the purple layers you will see that they are smeared through shear zones up to about a cm wide. The coin used for scale is a US 5¢ piece (nickel), which is about 2.2 cm in diameter. As a general rule, the amount of shift across a deformation band increases with it's thickness. 
• In this view there are apparently two orientations of shear zones. One of the two orientations (dipping to the right) have formed a more continuous through-going set (their shift reflects the sense of slip on the fault plane at the edge of the zone).

1992-02-08

1992.04.07 - Pair of conjugate deformation bands in Slickrock member of Entrada Sandstone, in the Delicate Arch area of Arches National Park, Utah. Some sets of laminae have been marked. One of the two sets is more highly developed than the other. Although the right-dipping set appears to offset the left-dipping set, this is not the case throughout the area. Zhao & Johnson (1991) document how segments of conjugate deformation bands in the are form a very complex sequence of cross-cutting relations. Deformation bands and zones of deformation bands are described in Aydin (1978) and Aydin & Johnson (1978). The deformation bands are exposed on a joint surface.

1992-03-25

1992.04.17 - A faulted joint in Moab member of Entrada Sandstone, Garden Area, Arches National Park, Utah (Cruikshank et al., 1991). Here an earlier formed deformation band was first cut by a joint and then at some yet later time there was lateral displacement on the joint, making it a faulted joint. It is important to understand the history of this structure, since at the the time it formed the principle compressive stress was parallel to the joint. At some time later it was oblique to the joint, so there was shear displacement on that surface. If one assumed it formed as a shear fracture, then you would come up with an incorrect stress history. The orientation of the structure (the faulted-joint) is not related to any preferred orientation of fault formation, rather the faulting process took advantage of a pre-exiting plane. 

The next slide, 1992-03-27, shows the termination of the faulted-joint shown in this picture. The orientation of the faulted-joint is consistent between slides.

1992-03-27

1992.04.17 - Termination of faulted-joint ("kink") developed at end of faulted joint seen in the previous slide. Moab member of Entrada Sandstone, Garden Area, Arches National Park, Utah. The kink at the end of a faulted-joint propagates into the area of tension developed when the joint becomes faulted. The kink indicated the block on the left moved "down" relative to the right side of the photograph (see Cruikshank et al., 1991) (ID: 1992-03-25)

1992-03-33

1992.04.17 - Fault slip surface bringing Dewey Bridge Member of Entrada Sandstone (left) in contact with the underlying Navajo Sandstone (right). The slip surface is developed on the outside of a zone of deformation bands. There is about 20 m of throw across the fault. Delicate Arch area, Arches National Park, Utah.

1992-04-14

1992.04.20 - Zone of deformation bands. Buckskin spring area, Goblin Valley State Park, Utah (Aydin, 1978). This zone is similar to that shown earlier in 1992-01-35 & 1992-01-26. Here the individual deformation bands are more resistant to weathering than the rest of the zone, so they stand up in relief. There is about 1.5 m of throw across the zone of deformation bands.

1992-04-14

1992.04.20 - Zone of deformation bands. Buckskin spring area, Goblin Valley State Park, Utah (Aydin, 1978). Marco Antonellini for scale. The individual deformation bands are standing in relief. Within the zone there are three orientations of deformation bands. 

1992-04-37

Slide miss-numbered? 1992.04.20. Closeup of fault zone in 1992-04-14. 1992.04.20 - Zone of deformation bands. Buckskin spring area, Goblin Valley State Park, Utah (Aydin, 1978). This zone is similar to that shown earlier in 1992-01-35 & 1992-01-26. Here the individual deformation bands are more resistant to weathering than the rest of the zone, so they stand up in relief. There is about 1.5 m of throw across the zone of deformation bands.

1992-08-06

1992.05.11 - Small deformation bands in Dakota Sandstone, Salt Valley Anticline, at north end of Arches National Park, Utah. This is just a nice slide of a fault zone in a dm thick white sandstone bed.

1992-09-07 

1992.05.19 - Molly's Castle area Goblin Valley State Park, Utah (Aydin 1978). Slip surface on the edge of a zone of deformation bands. ??? for scale.

1992-09-09

1992.05.19 - Looking at zone of deformation bands. Note other deformation bands in footwall. Marco Antonellini for scale.

1992-09-13

1992.05.19 - Close up of zone of deformation bands. Slip surface is on left.

1992-09-15

1992.05.20 - Marco Antonolleni showing the width of a Zone of deformation bands in Navajo Sandstone. The red unit overlying the Navajo in this area is the Dewy Bridge Member of Entrada Sandstone.. This zone does not stand up in relief, and has no slip surface developed in this area. This zone of deformation bands can be traced to the slip surface shown in 1992-03-33. Delicate Arch area, Arches National Park, Utah.

1992-10-03

1992.05.21 - A single deformation band offsetting some cross-bedding in Slickrock Member of Entrada Sandstone. Willow Flats area, Arches National Park, Utah.

1988-N042-19

A step between tow deformation band segments. The steps resemble "duplex structures" in that there are a series of "ramps" connecting the two segments. In three-dimensions the outer boundaries of the stopovers form large elliptical pods, which contain the ramps. Southern Garden Area, Arches National Park, Utah. Arvid Johnson for scale.

1988-N042-21

Close-up of the structure shown in the previous slide.

1993-01-21