4.3: Jointing and Faulting
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Brittle Deformation
A body of rock that is brittle—either because it is cold or because of its composition, or both— is likely to break rather than fold when subjected to stress, and the result is jointing or faulting.
Jointing
A joint is a fracture in a rock in which no displacement or side-to-side movement has occurred. Most joints form where a body of rock is expanding because of reduced pressure, which is the case of Half Dome in Yosemite National Park (Figure \(\PageIndex{1}\)). Joints can also form where the rock itself is contracting but the body of rock remains the same size (the cooling volcanic rock in (Figure \(\PageIndex{2}\)). In all of these cases, the pressure regime is one of tension as opposed to compression.
Joints can also develop when rock is under compression as shown on Figure \(\PageIndex{3}\), where there is differential stress on the rock, and joint sets develop at angles to the compression directions.
Faulting
A fault is a boundary between two bodies of rock along which there has been relative motion. An earthquake occurs when one body of rock slides past another. Earthquakes don’t always happen on existing faults, but once an earthquake occurs, a fault will form at that location. Some large faults, like the San Andreas Fault in California, show evidence of hundreds of kilometers of movement, while others show less than a millimeter. In order to estimate the amount of motion on a fault, we need to find some geological feature that shows up on both sides and has been offset (Figure \(\PageIndex{4}\)).
There are several kinds of faults and they develop under different stress conditions. The terms hanging wall and footwall apply to situations where the fault is not vertical. The body of rock above the fault is called the hanging wall, and the body of rock below it is called the footwall (Figure \(\PageIndex{5}\)). When a fault is vertical, there is no obvious hanging wall or footwall. Hanging wall and footwall are mining terminology. Many mines are in fault zones. When a miner is walking within an adit, or horizontal tunnel within the Earth, their feet are on one side of the fault, the footwall, and their head could bump on the other side of the fault, the hanging wall.
Dip-Slip Faults
A dip-slip fault is a fault which moves in the dip direction of the fault plane. In other words, the hanging wall is moving up or down relative to the footwall.
Normal faults are dip slip faults in which the hanging wall moves down relative to the footwall. Normal faults are created by extension of the crust and commonly occur at divergent plate boundaries, where the crust is being stretched.
The following 3D model is of a normal fault. Notice how the hanging wall has moved down relative to the footwall.
"Normal fault" by paulinkenbrandt via Sketchfab is licensed under CC BY 4.0.
The following video illustrates the formation of a normal fault. There is no audio.
In a normal fault, the block above the fault moves down relative to the block below the fault. This fault motion is caused by extensional forces and results in lengthening.
Normal faults are very common in both the Basin and Range and Mojave Desert provinces of California where the crust is undergoing extension. In these locations, the crust is being pulled apart, which can result in down dropped blocks known as grabens and raised blocks called horsts.
Reverse and thrust faults are created by compressional forces within the crust. In both reverse and thrust faults the hanging wall moves up relative to the footwall. Thrust faults are a particular type of reverse fault that is dipping at a very low angle (<30º). Often, thrust faults underlie other structural features like anticlines (Figure \(\PageIndex{7}\)).
Reverse faults, thrust faults, and compressional forces commonly occur at convergent plate boundaries, where the crust is either overlapping at subduction zones or colliding at continent-continent collisions. Reverse and thrust faults are common in the Transverse Ranges and parts of the Coast Range provinces of California where the crust is actively experiencing compression.
The following 3D model is of a reverse fault. Notice how the hanging wall has moved up relative to the footwall.
"Reverse Fault" by paulinkenbrandt via Sketchfab is licensed under CC BY.
The following 3D model is of a thrust fault. Like a reverse fault, the hanging wall has moved up relative to the footwall, however the fault is dipping at a much shallower angle.
"Thrust Fault" by paulinkenbrandt via Sketchfab is licensed under CC BY.
The following video illustrates the formation of a reverse fault. There is no audio.
In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small.
Strike-Slip Faults
Strike-slip faults (Figure \(\PageIndex{9}\)) are created by shear stresses within the crust. They commonly occur at transform plate boundaries: the San Andreas fault zone of California is a transform plate boundary between the Pacific and the North American plates that consists of a system of strike-slip faults.
Strike-slip faulting is common in the Peninsular Ranges province of southern California and the Coast Ranges province of central California, where many faults are parallel to the orientation of the San Andreas fault zone. Strike-slip faults move with mostly horizontal motion and typically are vertical or near vertical and thus do not have hanging walls or footwalls. These faults are named strike-slip because the direction of motion or slip is along the line of their strike.
The following 3D model is of a strike-slip fault where the two fault blocks are moving past one another. In this example, far side has moved to the right of the observer, indicating that it is a right-lateral strike-slip fault.
"Strike Slip" by paulinkenbrandt via Sketchfab is licensed under CC BY.
The following video illustrates the formation of a strike-slip fault. There is no audio.
In a strike-slip fault, the movement of blocks along a fault is horizontal. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral. If the block on the far side moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces.
Oblique Slip Faults
It is not uncommon for faults to have components of both strike-slip and dip slip. If that is the case, the fault is said to have oblique slip. The Corona Heights fault in San Francisco has experienced both strike-slip and dip-slip motion. We can tell because the fault is covered in slickenside surfaces, surfaces that indicate the direction of slip on the fault either with slickenlines, grooves in the surface of the fault, or slickenfibers, mineral fibers that have grown in the direction of fault slip. The slickenlines here indicate that the block to the left has moved relatively up and away from where the photographer is standing in Figure \(\PageIndex{10}\).
The following video further describes the Corona Heights fault, an oblique slip fault in downtown San Francisco.