1. Faults and Faulting 1 Lecture 16 – Spring 2016
Structural Geology
Faults and Faulting 1
Lecture 16 – Spring 2016
Image: Photo by Martin Miller
Conjugate Normal faults, Canyonlands National Park, Utah
Photo by Dr. Martin Miller

2. Fault Definition
Faults are fractures on which slip develops primarily by brittle deformation processes
Figure 8.1a in text

3. Fault Zone Definition
Fault zones are brittle structures in which loss of cohesion and slip occur on several faults within a band of definable width
Figure 8.1b in text

4. Fault Splays
Displacement sometimes occurs along small, subparallel brittle faults, or slip along a larger fault off which many smaller faults, called fault splays, diverge
Figure 8.1c in text

5. Anastomosing Faults Slip may occur on anastomosing faults
Anastomosing refers to a group of wavy, subparallel surfaces which merge and diverge
Braided streams flowing across glacial outwash may show an anastomosing pattern
Figure 8.1d in text

6. Shear Zones
Shear zones involve deformation by cataclasis, or by crystal plastic deformation mechanisms
The rock body must not lose mesoscopic cohesion within the shear zone
Strain is distributed across a band of definable width
Figure 8.1e in text

7. Scale of Faults
Faults develop at scales from microscopic to megascopic in the lithosphere
Figure 8.2 in text
Fractured feldspar grain in
photomicrograph
Fault trace in aerial photo
They affect many rock properties, and are therefore heavily studied. Among the properties faults affect:
Control of spatial arrangement of rock - they may create puzzles that challenge those attempting to produce geologic maps
Topography - faults modify the landscape
Distribution of economic resources - they may act as petroleum traps, or suddenly terminate veins or oil-bearing strata
Fluid migration - faults may greatly increase permeability
Deformation - faults cause strain, and sometimes rotate or translate, or both, rock units
Mesoscopic faults in outcrop

8. Fault Terminology
Geologists have developed a jargon devoted to the description of faults and fault movements
Some terms have been borrowed from miners, since many ore deposits are found along faults
Wall - rock adjacent to the fault
Fault block - A volume of rock that has moved as a result of movement along the fault - for non-vertical faults, we can use two terms borrowed from miners, hanging wall and footwall

9. Steepness of Dip Angle
Faults are often described using strike and dip, or dip angle and dip direction
The steepness of the dip is then described as:
Horizontal - 0º
Subhorizontal º
Shallow or low-angle º
Moderate º
Steep or high-angle º
Subvertical º
Vertical - 90º
Listric - Faults that are steep near the earth’s earth, but curve to become shallow at depth
The terminology shown assumes the fault surface is planar, or nearly so.
Some faults have curving surfaces.
We have previously seen that listric faults are steep near the surface, but becoming increasing shallow at depth.
Fault movement is described by a vector, the net-slip vector. Vectors have both direction and magnitude, so both must be given.

10. Net-Slip Components
Diagram shows the dip-slip and strike-slip components of the net- slip
Note that the rake angle is measured from the horizontal to the direction of net-slip on the fault plane
This can be done by specifying the plunge and bearing, also called the rake, of the fault, and the sense of slip, or shear sense. (Figure 8.3)
Figure 8-3 in text

11. Types of Faults
Shear sense describes the movement of one wall relative to the other (up or down, left or right)
If net slip direction and dip slip direction are within 10º, the fault is said to be a dip-slip fault
It the net slip is within 10º of the strike, it is a strike-slip fault
If neither condition holds, the fault is an oblique-slip fault

12. Normal Dip-Slip Faults
Dip-slip faults may be described by giving the relative movement of the hanging wall
If the hanging wall moves down, the fault is normal
Figure 8.4a in text

13. Normal Dip-Slip Movement
Normal2.gif
Blocks move to illustrate movement

14. Detachment Fault
A regional, low-angle, listric normal fault formed during crustal extension
Commonly called detachment faults
Detachment surface in Whipple Mountains
Gneissic basement complex below
Young volcanic rocks and scraps of basement above

15. Reverse Dip-Slip Faults
If the hanging wall moves up, it is a reverse fault
The dip angle may be described as low (0-30º), intermediate (30-60º), or high (60-90º)
Figure 8.4b in text

16. Reverse Dip-Slip Movement
Reverse2.gif
Blocks move to illustrate movement

17. Dip Slip Photos
Left photo.
Right photo:
Faults in felsic volcanic rock, Death Valley National Park
Near Klamath Falls, OR.
Both photos by Dr. Marvin Miller

18. Thrust Faulting Thrust faults are low-angle reverse faults
They sometimes move large distances (tens of kilometers)
Photo: Dr. Marvin Miller
Ramp Anticline, southern British Columbia, Canada
Note the minor thrust fault immediately to the right of the hammer
Photo:

19. Fault Video USGS Fault Video Clip
The movie further illustrates some of these terms.
Running time 1 minute 2 seconds.
Source: m mov converted to m avi
Source:
USGS Fault Video Clip

20. Dextral Strike-Slip Fault
Strike-slip faults are described as right-slip, or dextral, if the fault block opposite the observer moves to the observer’s right
Figure 8.4c in text

21. Right-Lateral Movement
Rightlat2.gif
Blocks move to illustrate movement

22. Sinistral Strike-Slip Fault
Strike-slip faults are described as left-slip, or sinistral, if the fault block opposite the observer moves to the observer’s left
Figure 8.4d in text

23. Left-Lateral Movement
Leftlat2.gif
Blocks move to illustrate movement

24. Strike-Slip Faulting Near Coos Bay, Oregon
Oblique-slip faults occur when the motion is a combination of strike-slip and dip-slip. The terminology is a combination of strike-slip and dip-slip as well.
Near Lillooet, British Columbia
Near Coos Bay, Oregon

25. Left-Lateral Normal Fault
Movement of the far block is up and to the left, relative to the observer
Figure 8.4e in text

26. Left-Lateral Reverse Fault
Movement of the far block is down and to the left, relative to the observer
Figure 8.4f in text

27. Right-Lateral Normal Fault
Movement of the far block is up and to the right, relative to the observer
Figure 8.4g in text

28. Right-Lateral Reverse Fault
Movement of the far block is down and to the right, relative to the observer
Figure 8.4h in text

29. Oblique-Slip Movement
The animation show oblique-slip motion. What type is it? (Left-lateral reverse)
Oblique2.gif
Blocks move to illustrate movement

30. Scissors Faults
It is also possible for rotation to occur on the fault surface
Rotated faults are sometimes referred to as scissors faults
Figure 8.4i in text

31. Extensional Faults
Figure 8.5ab
Faults also increase or decrease the distance between points on the opposing fault blocks
Faults that cause an increase in distance are called extensional faults
Extensional faults result in loss of section when examined along a line that crosses the fault and is perpendicular to the strata

32. Contractual Faults
Figure 8.5, ac
Faults that result in lessening of the distance are called contractual faults
Contractual faults result in a duplication of section when examined along a line that crosses the fault and is perpendicular to the strata

33. Using Symbols on Maps
Geologists plot geologic information on maps and cross-sections using symbols
A fault is a type of contact, so it is plotted using a thick line
The line has symbols attached to tell what type of fault it is

34. USGS Dip-Slip Fault Symbols
Fault symbol on map or cross-section
Normal dip-slip fault – ball on downthrown block
Used when ball and bar is not used – u = upthrown, d – downthrown
Normal fault on small scale maps – tick on downthrown side
Thrust Fault (sawteeth on upper plate)
Reverse fault on small sacle maps – R on upthrown block
Images from:
An excellent source for all types of geologic map symbols is

35. USGS Strike-Slip Fault Symbols
Strike-slip faults are indicated with half-arrows showing the direction of movement – upper, right-lateral; lower, left-lateral
Used on cross-section – a = away from observer, t = toward observer
Images:

36. Faults on Cross-Sections
Figure 8.7 in text
Cross-sections also use symbols. If the fault slip direction is close to the plane of the cross-section, half-arrows on either side of the fault indicate the direction of movement.
Note that the half-arrows here will not be used for a strike-slip fault. Movement in and out of the section plane for a strike slip fault is indicated by the head of an arrow (circle with a dot in it) for movement out of the plane (toward you) and the tail of an arrow (circle with an X) for a block moving into the lane. For dip-slip faults moving in or out of the section plane, the map symbols are placed on the hanging wall block.
Dip-slip and strike-slip faults are shown in cross-section

37. Fenster
Thrust faults are often thin sheets, and erosion may open holes in them
A hole through a thrust sheet is called a fenster, or window
Fenster: An eroded area of a thrust sheet that displays the rocks beneath the thrust sheet
Triangular teeth point outward fenster are used on a map

38. Klippe
If erosion leaves an isolated remnant of thrust sheet, surrounded by exposed footwall, the remnant is called a klippe (German for cliff)
Klippe are indicated on a map by inward pointing teeth

39. Definitions
Autochthon: A body of rocks that remains at its site of origin, where it is rooted to its basement. Although not moved from their original site, autochthonous rocks may be mildly to considerably deformed
Allochthon - A mass of rock that has been moved from its place of origin by tectonic processes, as in a thrust sheet - Many allochthonous rocks have been moved so far from their original sites that they differ greatly in facies and structure from those on which they now lie

40. Thrust Sheet Diagram
Figure in text
Note inward pointing teeth indicating a klippe, and outward pointing teeth indicating a fenster (“window” on diagram)
Window (fenster) shows of the autochthon through the eroded allochthon
Klippe is a piece of allochthon surrounded by autochthon

41. Cutoffs
Figure 8.7 in text
Faults which cross geologic contacts will displace the contact, unless the net-slip vector is exactly parallel to the fault-contact intersection
The point of intersection on either a map or cross-section is called a cutoff

42. Marker Horizon
Marker horizons (or beds) are very useful when examining movement on a fault
A marker horizon is a distinctive layer, easily recognized on both sides of the fault
Red arrows indicate a marker horizon
Fault on Moores Gap Road
(Tennessee – JFC ’03)
Photo: Dr. Anton Oleinik

43. Separation
The distance between the separated parts of the marker horizon is the separation, which is not the same as the net slip unless the line along which separation is measured happens to parallel the net-slip vector

44. Components of Separation
Separation can be divided into seven components:
Stratigraphic separation
Heave
Throw
Strike separation
Vertical Separation
Horizontal separation
Dip separation

45. Stratigraphic Separation
Offset measured perpendicular to bedding

46. Horizontal Separation
Horizontal separation (H) - Offset measured in a horizontal direction along a line perpendicular to the offset surface
Figure 8.10c in text

47. Vertical Separation
Vertical separation (V) - Distance between two points on the offset bed as measured in a vertical direction
If borehole data is used, it is vertical separation that is measured between two parts of an offset marker horizon
Figure 8.10c in text

48. Dip Separation
Dip separation (D) - The distance between the offset horizons measured in the dip direction
Strike separation (S) - Distance between the offset horizons measured along the strike direction
Figure 8.10c in text

49. Components of Separation, 3
Heave - Horizontal component of the dip separation
Throw - Vertical component of the dip separation
Strike separation - Distance between the offset horizons measured along the strike direction
Figure 8.10a in text

50. Net-Slip
The net-slip vector is defined by its magnitude, direction (bearing and plunge), and the sense of slip
If two points that were adjacent prior to slip are visible on the fault surface, the net-slip can be directly measured
The points are called piercing points
Straight objects, such as underground pipes, telephone poles, or the edge of a road, because their intersection with the ground defines a line, and the intersection of the line with the fault surface provides two piercing points, can be used to measure net-slip

51. Calculating Net-Slip
More often, it is necessary to calculate the net-slip, rather than measure it directly
Calculation can be done using various forms of information, such as:
Separation between a marker horizon and the fault, along a specified line, plus the direction of slip.
Separation along two non-parallel lines, of a single plane and the fault.
Separation, along a specified line, of two non-parallel marker horizons.
Sometimes the net-slip cannot be determined. But some useful information may still be available.
Geologists look for slip lineations or shear-sense indicators.
Slip lineations are visible structures on the fault that paralleled the net slip, at least during the last step of progressive deformation, and possibly for an accumulation of incremental steps.
Slip lineations define the plunge and bearing of the net-slip vector. This can help to interpret movement along the fault in terms of overall tectonics, even without knowing the magnitude of movement. Shear-sense indicators are structures on the fault surface that define the direction one block moved with respect to the other.

52. Net-Slip Dimensions
The magnitude of net-slip on faults can range from a millimeter or so to thousands of kilometers
The San Andreas fault in California, probably the best known in the United States, has an accumulated movement of about 600 kilometers
Faults with large net-slips are major faults, those with small slips are minor or mesoscopic faults
It is sometimes possible to measure the movement of a single earthquake by looking at disrupted ground surfaces, if a major fault intersects the surface.
For many faults, the distance measured is a cumulative displacement from many earthquakes.

53. Non-Planar Faults Fault surfaces are not necessarily planar
Many large faults change direction either down-dip or along-strike
The change may be quite gradual
If it is sharp, it is called a fault bend
Fault bends are common in dip-slip faults which traverse many layers of differing mechanical strength
The trace of the fault begins to resemble a staircase

54. Bedding and Fault Plane Orientation
Figure 8.11a in text
Shows the situation prior to displacement
If a fault segment is flat, parallel to bedding, it is a flat
If it is inclined, cutting across bedding, it is a ramp
Provided no folding was present prior to faulting, the ramps will have inclinations of 30-45̊, and the flats will be subhorizontal

55. Bedding and Fault Plane Orientation, cont.
Figure 8.11b in text
After displacement
AB is a hanging wall flat on a footwall flat
BC is a hanging wall flat on a footwall ramp
CD is a hanging wall ramp on a footwall flat
DE is a hanging wall flat on a footwall flat
Fault bends in strike-slip faults cause the strike to change suddenly. Looking along the fault, we describe the direction of the fault bend by saying the fault steps to the right or left.
Fault segments may parallel bedding in either the footwall of the hanging wall, but cut across bedding in the opposite block

56. Restraining Bends
Bends along a strike-slip fault can cause the fold to compress together
These are called restraining bends
Compression caused by movement along the fault is called transpression
Figure 8-12a in text

57. Releasing Bends
Bends along a strike-slip fault can cause the fold to pull apart
The are called releasing bends
Extension caused by movement along the fault is called is called transtension
Figure 8-12b in text

58. Restraining vs. Releasing Bends
Left steps on right-lateral faults yield restraining bends
Right-steps on a right-lateral fault yield releasing bends
So the general rule is that when the step and movement are both dextral, or both sinistral, the bends is releasing
If they have opposite senses, the bend is restraining

59. Fault Termination Against Another Structure
A fault may terminate where it has been cut by a younger structure, such as another fault (C & D), an unconformity (E), or an intrusion (B), or at the ground surface (A)
Like joints, fault must terminate, and can do so in several different ways.
The Principle of Cross-Cutting Relationships can be used to determine the relative ages
Figure 8.13a in text

60. Emergent Fault
Faults can also terminate at the ground surface, or appear to
The San Andreas fault does terminate at the ground surface, and is called an emergent fault
Image: San Andreas Carrizo dis01bigb.jpg

61. Exhumed Faults
Other faults, blind when they formed, may be exposed by erosion to become exhumed faults
Figure 8.14a in text

62. Blind Faults
Blind faults are faults that terminated before reaching the surface
Figure 8.14b in text

63. Animation of Blind Fault
Blindani2.gif’
The resulting fold is called a monoclinal fold

64. Genoa Fault, near Carson City, NV
A fault plane has been exhumed by quarrying
Striations visible on the fault plane surface help constrain the sense of motion on the fault plane
The sedimentary wedge visible is stratified and probably formed by alluvial/colluvial deposition
Source:
Photo: Andrew Sorby

65. Death of a Fault
Figure 8.13b in text
Faults can also split, to form an anastamosing array, which may merge and diverge several times along its length
A fault splay may develop, with the fault splitting and dying out – these are called horsetails (B)
A fault dies when its displacement becomes less and less, finally reaching zero near the tip, in a zone of plastic deformation (C)

66. Termination against Detachment
Figure 8-13c in text
Ramps merging and terminating at depth with a basal detachment

67. Prediction of Fault Length or Displacement
In “a” the offset of XX’ is small
As the fault grows, the offset of XX’ increases
Figure 8.15ab in text

68. Fault Length and Displacement
Figure shows a log-log plot of the relationship between displacement and fault length
Despite a wide diversity of lengths and lithologies in the host rocks, all faults plotted lie between two parallel lines at a 45º angle
This suggests that we can predict either fault length or displacement, if we know the other
Figure 8.15c in text

69. Relationship Between Fault Length and Displacement
The longer the fault, the greater the displacement
This is a general relationship, supported by research within the last three decades

70. Fit to Data The best fit to the data is
D = C • Ln, with C =0.03, and n = 1.06
Where C is a constant, and n is called the fractal dimension