Topics

• FAULTS, FAULTS AND FAULTS

Structural Geology 1 Deformation

• Elastic deformation:All solid materials, including rocks, are elastic. They change their shape or volume under strain / stress but regains its original shape once removed. • Brittle deformation: Rocks can break when they can no longer sustain the forces acting on them. The plane along which a rock breaks is called a fault. • Ductile deformation: rocks can also flow when forces act on them. The rocks act similar to a thick fluid. Examples are folds

Structural Geology 2 Deformation

Distortion depends on: ! Temperature Stress and strain ! Stress (confining pressure) ! Strain rates ! Composition

• Stress: force/area (dimensionally

• normal stress: ⟂ to area

• shear stress: ∥ to area

• Strain: deformation, change in shape, resulting from stress (producing translation, rotation, volume/area changes)

Structural Geology 3 Stress components

Structural Geology 4 Brittle structures

• Joints: fractures with no relative movement of adjacent parts

• Faults: fractures with relative displacement (at multiple scale, from micro- to macro-) of the separate parts

Structural Geology 5 Joints & pressure/solution

• Joints are perpendicular to minimum stress (σ3)

• Same for filled cracks (where minerals can precipitate, e.g. calcite)

• Pressure-solution surface (stylolites, they seem like cranic bone sutures…) are perpendicular to maximum stress (σ1). Formed typically in carbonates

• Along stylolites material is dissolved and eventually re-precipitated in cracks.

Structural Geology 6 Structural Geology 7 2917-CH07.pdf 11/20/03 5:10 PM Page 143 Joints

σ1

1 2 Trace of twist hackle σ3 σ2 Origin Hackle fringe Mirror Twist hackle Mist Tip Plume line axis Arrest line Twist hackle Propagation direction (a)

Inclusion Structural Geology 8

FIGURE 7.3 (a) Block diagram showing the various components of an ideal plumose structure on a joint. The face of joint 1 is exposed; joint 2 is within the rock. (b) Simple cross- σ3 sectional sketch showing the dimple of a joint origin, controlled (b) by an inclusion.

(a)

(b)

FIGURE 7.4 Types of plumose structure. (a) Straight plume. (b) Curvy plume. (c) Plume with many arrest lines, Arrest line Cycle suggesting that it opened (c) repeatedly.

7.2 SURFACE MORPHOLOGY OF JOINTS 143 2917-CH07.pdf 11/20/03 5:10 PM Page 145

7.3.2 Joint Sets and Joint Systems “orthogonal” or “conjugate” to imply that the pair of joint sets formed at the same time. However, as you will Describing groups of joints efficiently requires a fair see later in this chapter, nonparallel joint sets typically bit of jargon. Matters are made even worse because not form at different times. So, we use the terms merely to all authors use joint terminology in the same way, so denote a geometry, not a mode or timing of origin. it’s good practice to define your terminology in con- As shown in Figure 7.6a, many different configura- text. We’ll describe joint patterns here and give the tions of joint systems occur, which are distinguished explanations of why various different groups of joints from one another by the nature of the intersections form later in the chapter. between sets and by the relative lengths of the joints in A joint set is a group of systematic joints. Two or the different sets. In joint systems where one set consists more joint sets that intersect at fairly constant angles of relatively long joints that cut across the outcrop comprise a joint system, and the angle between two whereas the other set consists of relatively short joints joint sets in a joint system is the dihedral angle. If the that terminate at the long joints, the throughgoing joints two sets in a system are mutually perpendicular (i.e., the are master joints, and the short joints that occur between dihedral angle is 90°), we call the pair an orthogonal ∼ the continuous joints are cross joints (Table 7.1). system (Figure 7.6a), and if the two sets intersect with a In the flat-lying sedimentary rocks that occur in dihedral angle significantly less than 90° (e.g., a dihe- continental interior basins and platforms (e.g., the dral angle of 30° to 60°), we call the pair a conjugate Midwest region of the United States), joint sets are Jointsystem (Figure 7.6a). patterns Many geologists use the terms

Orthogonal (+) joints Conjugate (X) joints J joints

Sigmoidal joints

Joints fanning hk 0 joints around fold

ac joints

c b Strike parallel joint a

Columnar joints Cross-strike joint

bc joints

(b)

(a) Structural Geology 9 FIGURE 7.6 (a) Traces of various types of joint arrays on a bedding surface. (b) Idealized arrangement of joint arrays with respect to fold symmetry axes. The “hk0” label for joints that cut diagonally across the fold-hinge is based on the Miller indices from mineralogy; they refer to the intersections of the joints with the symmetry axes of the fold.

7.3 JOINT ARRAYS 145 Faults

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C A B D C

E B A

(a) (b)

0 20 No vertical exaggeration km NE SW

0

km

8

(c) FIGUREFaults 8.13 (a) Cross-sectional sketch showing various types of fault terminations. The fault terminates at the ground surface at point A; at point B, the fault has been cut by a pluton; at C and D, one fault cuts another; at E, the fault was eroded at an unconformity. (b) Termination of a fault by merging with another fault (at point A), or by horsetailing (at point B) and dying out into a zone of ductile deformation (at point C). (c) A series of ramps merging at depth with a basal detachment. • Fracture separating blocks (semi-spaces) with relative movement/offsetoccurred where the fault surface between already exists,the blocks but there fault trace, the greater the displacement. Indeed, recent is no slip beyond the tip of the fault (Figure 8.15a). A lit- work supports this idea, though the details remain con- tle later, after more fault-tip propagation, there is troversial. Faults that are meters long display offsets • Faultsincreased have slip in thea beginningcenter of the fault and (Figure an 8.15b).end (tip),typically where on theoffset order of centimeters or less, whereas approachesAs a consequence, zero the displacement changes along the faults with lengths on the order of tens of kilometers length of the fault and the magnitude of displacement have typical offsets on the order of several hundreds of must be less than the length of the fault. Considering meters. Figure 8.15c shows a plot of fault length ver- • Hangingthis relation, wall we might = above expect afault general plane relationship (as seensus offset,from based above) on examination of thousands of faults between fault length and displacement: the longer the that occur in a variety of lithologies and range in length • Foot wall = below fault plane (as seen from above)

Fault reaching surface Blind fault

Tip

Trace

Fault plane Tip line Tip line

Structural Geology 11

(a) (b) FIGURE 8.14 Tip lines for (a) an emergent fault and (b) a blind fault.

178 FAULTS AND FAULTING Faulting

• Normal/direct (A) fault ➔ accommodates extension (length increase)

• Reverse (C) fault ➔ accommodates compression (length reduction

• Transcurrent (B) fault ➔ neither extension nor compression (transform using only in plate tectonics context, large-scale)

• Intermediate cases exist (e.g. transtension, transpression)

Structural Geology 12 Normal faulting

Structural Geology 13 Structural Geology 14 Structural Geology 15 2917-CH08.pdf 11/20/03 5:11 PM Page 173

U D

Map view

(a) (c)

Cross section (b) FIGURE 8.6 Basic map symbols for (a) normal fault, (b) thrust fault, and (c) strike-slip fault.

Cutoff line a preexisting contact is called a cutoff, and in three dimensions (Figure 8.7), the intersection between a fault and a preexisting contact is a cutoff line. If the truncated contact lies in the hanging-wall block, the truncation is a hanging-wall cutoff, and if the trun- cated contact lies in the footwall, it is a footwall cutoff. When combining map and cross-sectional surfaces Footwall Hanging-wall with topography, we create a more realistic block dia- cutoff cutoff gram, giving us a three-dimensional representation of a (a) (b) region’s geology. Consider an area that is characterized FIGURECompr. 8.7 Block diagrams showing nomenclature the different by a low-angle reverse faulting (a thrust). Where ero- symbols for representing (a) dip-slip faults and (b) strike-slip sion cuts a hole through a thrust sheet, exposing rocks faults (here, left-lateral). In (a) we also mark footwall and of the footwall, the hole is a window and the teeth are hanging-wall cutoffs. drawn outwards from the hole (Figure 8.8). An

Klippe

Thrust-fault trace

Allochthon Thrust-fault trace

Autochthon Window

Thrust fault

FIGURE 8.8 Block diagram illustrating klippe, window (or fenster), allochthon (gray), and autochthon (stippled) in a thrust-faulted region. Note that the minimum fault displacement is Structural Geology 16 defined by the farthest distance between thrust outcrops in klippe and window.

8.2 FAULT GEOMETRY AND DISPLACEMENT 173 Stress & faulting

Structural Geology 17 Dip: normal vs. reverse

• Normal faults tend to be at high angle (~60° from horizontal)

• Reverse (thrust) faults tend to be at low angle (~30° from horizontal)

• Low-angle normal faults do exist and they can accommodate large extension: detachment faults

• Faults can be inverted (i.e. an original normal fault recycled later on as inverse or vice versa)

Structural Geology 18 Fault and displacement

Structural Geology 19 Kinematics: striae

• Striae (slickensides) are type of linear non penetrative fabric linked to local linear deformation along the direction of slip.

• Striae form on fault planes

• For dip-slip fault s (i.e. purely normal striae - if present - are // to dip direction

• For oblique faults striae direction is different from dip direction, i.e. there is

• They are one of several kinematic indicators

Structural Geology 20 Kinematics: striae

Source: U. SidneyStructural Geology 21 Extensional nomenclature

• Horst & graben geometry related to symmetrical extension

• Half-graben geometry related to asymmetrical extension

• Synthetic (fault) = same dip direction of master fault

• Antithetic (fault) opposite dip direction of master fault

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oblique-slip fault. As you can see in Figure 8.3, Dip-slip Hanging-wall Hanging-wall oblique-slip faults have both a strike-slip and a dip- faults block block slip component of movement. We describe the shear sense on a dip-slip fault with reference to a horizontal line on the fault, by saying that the movement is hanging-wall up or hanging- wall down relative to the footwall. Hanging-wall down Footwall faults are called normal faults, and hanging-wall up block faults are called reverse faults (Figure 8.4a and b). To Footwall block define sense of slip on a strike-slip fault, imagine that you are standing on one side of the fault and are look- (a) Normal (b) Reverse ing across the fault to the other side. If the opposite Strike-slip wall of the fault moves to your right, the fault is right- faults lateral (or dextral), and if the opposite wall of the fault moves to your left, the fault is left-lateral (or sinistral; Figure 8.4c and d). Note that this displace- ment does not depend on which side of the fault you are standing on. Finally, we define shear sense on an oblique-slip fault by specifying whether the dip-slip component of movement is hanging-wall up or down, (c) Right-lateral (or dextral) (d) Left-lateral (or sinistral) and whether the strike-slip component is right-lateral Oblique faulting or left-lateral (Figure 8.4e–h). Commonly, an addi- Oblique-slip tional distinction among fault types is made by adding faults reference to the dip angle of the fault surface; we recognize high-angle (>60° dip), intermediate-angle (30° to 60° dip), and low-angle faults (<30° dip). We provide descriptions of the basic fault types in Table 8.2, along with descriptions of other commonly used names (such as thrust and detachment). (e) Left-lateral/normal (f) Left-lateral/reverse You may be wondering where the terms “normal” and “reverse” come from. Perhaps normal faults were thought to be “normal” because the hanging-wall block appeared to have slipped down the fault plane, just like a person slips down a slide. It is a safe guess that geologists came up with the name “reverse fault” to describe faults that are the opposite of normal. Now you know! Structural Geology 23 We also distinguish among faults on the basis of (g) Right-lateral/normal (h) Right-lateral/reverse whether they cause shortening or lengthening of the layers that are cut. Imagine that a fault cuts and dis- places a horizontal bed marked with points X and Y (Figure 8.5a). Before movement, X and Y project to points A and B on an imaginary plane above the bed- ding plane. If the hanging wall moves down, then points X and Y project to A and B′. The length AB′ is greater than the length AB (Figure 8.5b). In other (i) Scissors fault words, movement on this fault effectively lengthens FIGURE 8.4 Block diagram sketches showing the different the layer. We call a fault which results in lengthening types of faults. of a layer an extensional fault. By contrast, the fault- ing shown in Figure 8.5c resulted in a decrease in the distance between points X and Y (AB > AB″). We call a fault which results in shortening of a body of rock a contractional fault. Contractional faults result in

170 FAULTS AND FAULTING Syn-sedimentary faults

• Faults can be active before, during or after sedimentation occurs.

• If faults are syn-sedimentary, they have an effect on sedimentation

• Accommodation space changes in relation (spatial AND temporal) with tectonic activity

• E.g. ➔ wedge-shaped sedimentary fills & growth faults

Structural Geology 24 Pre- / syn- / post-rift

Structural Geology 25 Example Exercise

• Find the dip and dip direction of the beds “B” • Locate the fault surface on the map; determine its dip and dip direction.

• Extend the structural contours of the bed “B” across the fault. Compare and name the fault type (normal, reverse, oblique, strike slip).

• Create a cross section perpendicular to the strike of the fault. What is the fault throw (displacement) ? Is this consistent with your response to the fault type in the previous answer ?

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answers.

fault strike, to confirm the above two

Draw a profile, perpendicular to the

displacement in m) ?

What is the fault throw (vertical

sandstone layer ?

What is the thickness of the

Is it a normal or a reverse fault ?

dip angle of the fault surface ?

fault. What is the dip direction and

Draw the structural contours of the

sandstone (A).

upper and lower surfaces of the

Draw the structural contours for the

!

!

!

!

!

! Exercise VI: Faults End Class

• Quick Summary • FAULTS • Exercises / Homework

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