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Brittle Failure: Joints II / Faults I
Ch. 7: p. 134-‐137 / Ch. 8: p. 151-‐
1. Fault-‐Related Fractures: Both pinnate fractures and tailcracks (wing cracks) can form in different locations around the edge of a shear fracture.
[Fig. 7.3. Formation of secondary cracks around the edge of a shear fracture]
2. Fracture Shape in 3D: Although opening fractures look like lines on a map that terminate at a point (the tip), remember they actually planes in 3D.
The tip must completely encircle this plane in 3D, marking the boundary between fractured and unfractured rock. This boundary is called the tipline (also fracture periphery and fracture front).
The motion sense relative to this tipline defines the fracture mode.
[Figure: Fracture tipline]
3. Fracture Shape in 3D: If a joint is completely contained within a rock layer, its tipline shape may be approximately circular or elliptical, with the long axis horizontal (confined within a bed or beds, especially where beds have variable lithologies).
[Figure: Joint tiplines in outcrop]
4. Fracture Surface Markings: A joint is comprised of 2 planar surfaces bounded by a tipline. They used to be seamlessly joined but split apart due to stresses applied to the rock.
As the surfaces pull apart, small jogs along the plane on the granular scale create a distinct ornamentation, also called surface markings.
[Figure: Surface markings along a joint in a thin bed] [Fig. 7.24a. Ornamentation on the surface of a joint in graywacke, Norway]
5. Fracture Surface Markings: It is mostly defined by plumose structure or hackle. It is best viewed when the lighting is oblique to the surface. Plumose usually diverges from a single point (the nucleation point of fracture growth), creating a fanning effect.
[Fig. 7.24a. Ornamentation on the surface of a joint in graywacke, Norway]
6. Fracture Surface Markings: If joint growth is confined to a single bed, the hackle may feather away from a central line of symmetry, fanning outwards in the direction of growth. Periods of no growth are marked by arrest lines or rib marks.
[Fig. 7.24b. Examples of rib marks on joint surfaces] [Fig. 7.25. Within a single bed, hackle fans out away from a central symmetry line. Periods of hesitation in growth create rib marks]
7. Fracture Surface Markings: Towards the edge of a joint, the fracture surface may break down into a number of smaller joints oriented at an angle to the main joint face, forming an en echelon pattern. This is twist hackle (or a twist hackle fringe) and forms because the stresses change in the region of the tipline before growth ceases.
[Fig. 7.24c. Twist hackle along the top and bottom of a joint confined to a single bed] [Fig. 7.26. Formation of twist hackle]
8. Fracture Surface Markings: Fracture ornamentation is very useful for determining joint propagation directions, providing insight into the sequence of events. Other useful tools are crosscutting relationships and interactions between joints. 9. Timing of Fracture Formation: A joint system may have multiple joint sets that formed at different times and under different stress conditions. We need to unravel the age sequence to determine the order of fracturing events. We use crosscutting relationships to determine age, but with joints, older joints cut younger joints. This is because younger joints cannot grow through the open space of an existing joint, so must terminate against it.
[Figure: Fracture sequence]
10. Joint Intersections: A joint will either terminate orthogonal to an older joint (T-‐intersections) or curve parallel to it. Why does this happen?
Ambiguous crosscutting relationships can occur. If an old joint is closed by later stresses or becomes mineralized, a younger joint could crosscut it.
Joints can also propagate up and over an older joint in 3D or down and under.
[Fig. 7.29. A younger joint growing towards an older joint may curve to become either parallel or perpendicular to the older joint]
11. Ambiguous Joint Relationships: Younger joints can have ambiguous crosscutting relationships. (A) Growth under and around an older joint. (B) Growth in opposing directions away from an older joint. (C) Simultaneous growth where one goes up and over and the other goes down and under. (D) Simultaneous growth away from a common point. (E) Shear offsets create apparent terminations.
Faults I: Terminology and Architecture
12. Faults: Faults are types of shear fractures with observable offsets (typically > 1 m). Shear motions occur parallel to the fault plane, allowing one side of the fault to move oppositely to the other side.
Faults can be several meters to > 1000 km long. At the cm scale, shear offsets produce shear fractures. At the microscopic scale, they are microfaults.
Faults are generally thought of as planar features, but they can have width, so are technically tabular. However, the width of a fault zone is much smaller than the amount of motion as well as the trace length of a fault, so depending on the scale of observation, faults can safely be approximated as being planar.
20. Fault Geometry: Normal faults that dip towards each other form a down-‐dropped block called a graben. Normal faults dipping away from each other form an uplifted block called a horst.
A major fault may have smaller faults nearby that either dip in the same direction (synthetic faults) or in the opposite direction (antithetic faults).
[Fig. 8.5. Fault geometries in normal faulting environments]
21. Fault Evidence: Fault slip can be resolved into a number of components that tell us about the relative motions of the two sides of the fault. We rely on markers (e.g., beds, contacts, intrusions) that have been relatively displaced.
[Figure: Relatively displaced beds in the Triassic Moenkopi Formation, Arches National Park, UT]
22. Fault Kinematics: Become familiar with the following terminologies regarding fault kinematics:
[Fig. 8.6. Components of motion along a normal fault having a right-‐lateral component of slip]
23. Fault Kinematics: Slip: also called the net slip, true slip, or the displacement discontinuity. It is the sum of the two displacement vectors on each side of the fault (which are mutually opposed). Slip represents the cumulative result of many slip events.
[Fig. 8.6. Slip vectors connect two points across the fault plane that used to be together. The slip vectors of different slip events may be different. Also, the slip vectors can vary across the surface of the fault]
24. Fault Kinematics: Slip trend and plunge: the horizontal projection of the slip vector points in the compass direction of slip and is called the slip trend or azimuth. The angle it makes with the fault plane is called the plunge.
[Fig. 8.6. Slip trend and plunge are not shown on this figure] [Figure: Slip trend and plunge]
25. Fault Kinematics: Rake: the angle between the strike of the fault and the slip vector, measured in the plane of the fault. Also called the pitch. The slip vector is sometimes discernable from scratches imbedded in the fault plane, called slickenlines or grooves. The polished fault surface is a slickenside.
[Fig. 8.6. The rake is typically measured as an acute angle, but here angle f is obtuse. The benefit of always using the angle measured away from the true strike (obeying the right-‐hand rule) is that the exact direction of the slip vector in space is obtained] [Figure: Slickenlines]
26. Fault Kinematics: Dip separation: any cross section through the fault produces a component of motion parallel to the fault trace, also called the apparent slip or offset. Only in a cross section oriented parallel to the slip vector does this become the true slip. In nature, we typically observe apparent slip, not true slip.
[Fig. 8.6. Any cross section through the fault that shows a component of motion along the fault trace is showing offset, apparent slip, or dip separation. If the cross section is parallel to the slip vector, the offset is equal to the true slip]
27. Fault Kinematics: Strike separation: this is the offset measured along the fault trace in map view. The offset sense can be counter-‐intuitive depending on the relative dips of the beds and the fault and the true strike-‐slip component of motion.
[Fig. 8.6. Strike separation is observed in map view. Here, an apparent left-‐lateral offset is actually produced by right-‐lateral motion! Also, strike separation can occur when only dip-‐slip motions happen because of the dips of the beds]
28. Fault Kinematics: Throw: this is the vertical component of motion and is the same regardless of the orientation of the cross section relative to the slip vector.
[Fig. 8.6. Throw is caused by dip-‐slip motion components and is simply the height of the scarp or the vertical component of slip]
29. Fault Kinematics: Heave: this is the horizontal component of the dip-‐slip vector and so is only accurately measured in a cross section oriented perpendicular to fault strike. It is used to estimate the fault-‐perpendicular strain across a region.
[Fig. 8.6. Heave is the horizontal component of motion measured parallel to the dip direction. For any other cross section orientation, the horizontal component is greater than the heave]
30. Footwall and Hanging wall Cutoffs: The heave component of motion results in the top of fault scarp and the bottom of the fault scarp being separated in map view. The fault trace thus has a width to it defined by the footwall cutoff and the hanging wall cutoff.
[Fig. 8.8. Footwall and hanging wall cutoffs define a certain width to fault traces in map view. This is how faults are most accurately represented on a map]
31. Fault Kinematics: Separation: the relative offset of a marker from one side of the fault to the other, measured perpendicular to the strike of the marker. In cross section, if the beds are horizontal, this is also called the stratigraphic separation and is equal to the throw.
[Fig. 8.6. Separation is different to dip separation and strike separation. However, if beds strike perpendicular to the fault, the strike separation and the separation are identical]
