Anderson’s theory of faulting predicts that strike-slip faults should be characteristic of the wrench regime, and that those faults should be near-vertical. Most major strike-slip faults are found to dip steeply, confirming Anderson’s prediction. As a result, their traces tend to be very straight lines on geological maps.
The damage zones around strike-slip faults tend to contain a mixture of structures that we normally associate either with horizontal shortening (as in thrust belts) or with horizontal extension (as in rifts). Cross- sections through strike slip faults tend to be quite confusing, for this reason. The best way to show what’s going on in a strike-slip zone is in map view.
In map view, a strike-slip fault zone may be idealized as a zone of simple shear. For the sake of argument we will draw a dextral strike-slip zone. (For a sinistral strike-slip zone everything would be in mirror image.) In a dextral strike-slip a zone, it’s possible to show that the most rapid shortening takes place along a line 45° clockwise from the shear-zone boundary, and the most rapid extension takes place along a line 45° counter-clockwise from the shear- zone boundary. For a sinistral strike-slip zone, the opposite senses of rotation apply.
This idealized pattern of strain rates leads to some predictions of how structures will form. We predict that extensional structures like joints, veins, and normal faults, will form perpendicular to the extension direction, and shortening structures like folds, thrust faults, and
cleavage planes will develop perpendicular to the shortening direction.
It is also quite common to see subsidiary strike-slip faults, which may include Riedel shears. Dextral synthetic Riedel shears would be predicted at about 15° clockwise from our overall dextral strike-slip zone, while sinistral antithetic Riedel shears would be about 75° clockwise from the overall direction of dextral strike slip. (Reverse everything for a sinistral zone.)
All these orientations work for fault zones where the overall amount of strain is small. Strike-slip deformation includes a strong component of rotation, so as deformation continues, all the structures will be rotated. One of the challenges in interpreting strike-slip motion is that structures may have rotated out of the orientations in which they formed.
Transtension
The above predictions apply to ideal strike-slip motion. However, many of the most interesting structures in strike-slip zones are formed where there are departures from ideal strike-slip.
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For example, a combination of strike-slip motion with extension is called transtension. In transtension there is a component of crustal thinning, along with strike slip, so transtension zones tend to subside and form sedimentary basins.
One common environment of transtension is at a releasing bend (also known as a releasing stepover) on a strike-slip fault. If a dextral fault steps to the right, or a sinistral fault steps to the left, the resulting bend is said to be releasing, and the rocks adjacent to the fault are affected by transtension.
A releasing bend typically develops a localized, parallelogram-shaped subsiding area called a pull-apart basin. In the Los Angeles area, pull-apart basins associated with the San Andreas system host important natural resources of oil and natural gas. In Nova Scotia, the Stellarton Basin, also a pull-apart basin on a dextral strike-slip fault system, was a prolific producer of coal in the 20th century.
In cross-section, pull-apart basins tend to be bounded by families of faults that steepen downwards and merge into a single fault or shear zone at depth. Individual faults may have normal, strike slip, or oblique slip. This type of fault array is called a negative flower structure.
Figure 9. Transtension and transpression at releasing and restraining bends on a dextral strike-slip fault.
Transpression
A combination of strike-slip motion with shortening is called transpression. In transpression there is a component of crustal thickening, along with strike slip, so transpression zones tend to form narrow uplifts, ranges of hills or mountains.
One common environment of transpression is at a restraining bend (also known as a restraining stepover) on a strike- slip fault. If a dextral fault steps to the left, or a sinistral fault steps to the right, the resulting bend is said to be restraining, and the rocks adjacent to the fault are affected by transpression.
A restraining bend typically develops a localized uplift. Parts of the transverse ranges of California, and the Southern Alps of New Zealand, are associated with transpression at restraining bends along major transform faults.
In cross-section, transpressional ranges tend to be bounded by families of faults that steepen downwards and merge into a single fault or shear zone at depth. Individual faults may have reverse, strike slip, or oblique slip. This type of fault array is called a positive flower structure.
Anderson, E. W. 1905. The dynamics of faulting. Transactions of the Edinburgh Geological Society 8, 387-402.
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• Lab 9. Field Mapping
Introduction
Geologists in the field often need to be able to record the relationships between a set of outcrops so as to collect the most useful information for geological map construction in the shortest time. Mapping geologists always carry some kind of base map on which to record their observations. For mapping large areas this might be a topographic map at any scale between 1:10,000 and 1:50,000. For a small area it may be a sheet of gridded paper on which the geologist records both geologic and topographic information. Recording information directly on the map while in the field is essential; that way, cross-cutting relationships such as faults, intrusions, and unconformities can be swiftly and directly portrayed on the map. Also, you will find out whether you need to collect more evidence for a critical relationship before you leave the field. Modern methods of surveying, particularly the use of the Global Positioning System (GPS), have greatly assisted map-making at large scale. However, portable GPS units often give errors of 5 m or more in location. Detailed field relationships may still need to be surveyed using tape-and-compass or pace-and-compass methods.
This exercise will take place at a location where you can practise mapping techniques with a variety of rock types and structures. Your instructors or teaching assistants will show you the area to be covered. For this exercise you will need a notebook, a clipboard and a sheet of graph paper, coloured and lead pencils, a compass-clinometer, and your legs!
You will also need to be appropriately dressed and equipped for working outdoors. For all geological fieldwork, it is important to carry clothing and equipment appropriate to the range of possible conditions you may encounter.
At the University of Alberta, this lab will take place in the Geoscience Garden, a facility that is set up to enable you to practise mapping techniques with a variety of rock types and structures without leaving campus. Despite its nearby location, you will still need to be prepared for work outdoors. The weather in Edmonton can be unpredictable. You will probably need gloves and you may also need a waterproof coat and footwear. Alternatively, if it is sunny you may need sunscreen and a hat.
Make sure that your compass-clinometer is correctly set for magnetic declination at your location.
The Geological Survey of Canada has a useful declination calculator at:
Magnetic declination calculator: https://geomag.nrcan.gc.ca/calc/mdcal-en.php
For example, in Edmonton AB, Canada, at the beginning of 2020, the declination was 13.8° East
Assignment
The mapping area is large and contains a wide variety of rocks. Your teaching assistants will designate an area for the mapping exercise.
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