Description of faults

Lecture Index: Notebook terms./ Description of idealized fault components. / Fault zone rocks and structures. / Fault recognition at map scale. / A traditional fault classification. /


This figure is from a core drilled into basement rocks of South Carolina and shows a pseudotachylite injection vein (red arrow), a pseudotachylite slip surface with secondary associated, high angle oblique microfaults (violet arrow), and a chloritized fault breccia (yellow arrow). The slip here is parallel to the gneissic layering seen in the adjacent rocks. The rocks here are more mafic in composition, which is thought by some to promote pseudotachylite formation.

Notebook terms for week 2

Description of idealized fault components

Faults or a smaller portion of a fault can sometimes be idealized as a planar surface with a vector of slip in it and offset and truncated layers, as depicted in the simplified diagram to the right. The diagram to the right visits the basic geometry and terms you should become familiar with. This diagram depicts a simplified case of horizontal layers that are offset. Dipping layers will usually have plunging cutoff lines associated with them. Note that there is an infinite number of lines that potentially connect the two cutoff lines. This means that cutoff lines by themselves are not adequate to allow the direction and amount of motion on the fault to be determined.

In this case, the following components are used to describe the fault geometry:

Total (net) slip vector = strike slip vector plus dip-slip component vector. Since the triangle formed by these three vectors is by definition a right triangle, this is just one place that trig functions come in useful (as you will see in lab).

Here is an unlabeled diagram that you can use to practice and develop your understanding of basic fault geometry. If you click on here you should have a page size version that you can print. The purple arrow shows the net/total slip. Given this diagram and information try to label the diagram with the components labeled in the similar version above (cutoff lines, normal and strike-slip components, etc.). If you click here than you can find an answer. Realize that it will not help you nearly as much to just click to the answer - better learning comes from trying to work through it first.

Fault separations versus slip directions:

In reality and at larger scales faults are typically non-planar! This has some very important implications as you will see.

Turtleback detachment fault in Death Valley (dashed line) with rift fill fanglomerates and sandstones in hanging wall and mylonitized, brecciated and chloritized (retrograde metamorphism) basement rocks in the footwall. Note the small normal faults that displace the sandstone in the hanging wall and bend (sole) into the flatlying detachment.The sandstone layers inbetween these small normal faults have rotated clockwise (in this view) as this faulting occurred.

fault terminations:

Block diagram showing an interior fault surface with slip vectors shown in brown. The maximum slip is in the interior and the slip decreases towards the tip-line fault margin. The slip gradient depicted here, as expressed in unit decrease per unit length, is higher here than is typical for rocks.

This is a map of the slip amounts on the subduction fault during the devastating earthquake that caused the tsunamis that created so much havoc. This is the picture for one slip event. The net slip on a fault from many events can also show a similar distribution of a maximum toward the center of an ellipse that grades out to a fault tip with zero displacement. Image source from:

Fault zone rocks and structures

Some faults are pretty much a single and simple slip surface, but most faults are manifest as a zone with distinctive rocks and features related to the fault movement within them. Below are some examples. Fault images to feed your eyes.

View of a small normal fault in the Brule Formation strata of Slim Buttes, South Dakota, with the fault core and damage zones labeled. The tan line outlines an offset marker horizon.The damage zone in this case consists of joints parallel to the fault plane, some of which have small displacements. Joint clusters are common in this rock, and one interpretation is that this was originally a joint cluster that was reactivated as a slip plane as strain increased, a common type of history. Note how the fault core forms a fin and is erosionally resistent. This is in part because of differential cementation in the fault core, suggesting it may have served as a conduit for fluid flow at some point in time.

Fault recognition at map scale

How do you recognize faults in the field?

Map to right is part of the USGS Tapestry of Time geologic map and shows the truncation of the Appalachian structural features by the younger Cretaceous Coastal Plain sedimentary units (red arrows). Many traits clearly indicate that this contact is an angular unconformity - including the ages of the rocks involved, the interdigitate character of the contact, and the scale involved. However, subhorizontal faults can have a similar interdigitate map expression. Image source: .

A traditional fault classification

Andersonian classification: This classification is based both on observation of what types of faults are common, and on theory guided by the idea that the earth's surface tends to shape fault orientations. Real faults are more complicated, as we will see later in the course, but this is a useful starting classification.

The image to the right shows simplified schematic diagrams of the offset of layers with the arrow representing the direction of slip along the fault plan, which in three of the four examples are dip slip.

 fault type  fault dip  dip or strike slip  hanging wall motion  horizontal kinematics
 normal  60 degrees  dip slip  down  extension
 thrust  30 degrees  dip slip  up  contraction
 reverse  60 degrees  dip slip  up  contraction
 wrench  subvertical  strike slip  NA  both

Fault images to feed your eyes.

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