Brittle Structures - Teknoiot

4 Jun 2020

Brittle Structures

If you look at the photographs of rock outcrops, you’ll notice thin dark lines that cross the rock faces. These lines represent traces of natural cracks along which the rock broke and separated into two pieces during brittle deformation.  Geologists refer to such natural cracks as joints (figure above a, b). Rock bodies do not slide past each other on joints. Since joints are roughly planar structures, we define their orientation by their strike and dip, as described in (Describing the Orientation of Geologic Structures).

Joints develop in response to tensile stress in brittle rock: a rock splits open because it has been pulled slightly apart. Joints may form for a variety of geologic reasons. For example, some joints form when a rock cools and shrinks because the process makes one part of a rock pull away from the adjacent part. Others develop when rock formerly at depth undergoes a decrease in pressure as overlying rock erodes away, and thus changes shape slightly. Still others form when rock layers bend.

Geotechnical engineers, people who study the geologic setting of construction sites, pay close attention to jointing when recommending where to put roads, dams, and buildings. Water flows much more easily through joints than it does through solid rock, so it would be a bad investment to situate a water reservoir over rock containing lots of joints the water would leak down into the joints. Also, building a road on a steep cliff composed of jointed rock could be risky, for joint-bounded blocks separate easily from bedrock, and the cliff might collapse.

If groundwater or hydrothermal fluids seep through cracks in rocks, minerals such as quartz or calcite may precipitate out of the groundwater and fill the crack. A mineral-filled crack is called a vein. Veins may look like white stripes cutting across a body of rock (figure above c). Some veins contain small quantities of valuable metals, such as gold.

After the San Francisco earthquake of 1906, geologists found a rupture that had torn through the land surface near the city. Where this rupture crossed orchards, it offset rows of trees, and where it crossed a fence, it broke the fence in two; the western side of the fence moved northward by about 2 m. The rupture represents the trace of the San Andreas Fault. As we have seen, a fault is a fracture on which sliding occurs, and slip events, or faulting, can generate earthquakes. Faults, like joints, are planar structures, so we can represent their orientation by strike and dip.

Faults have formed throughout Earth history. Some are currently active in that sliding has been occurring on them in recent geologic time, but most are inactive, meaning that sliding on them ceased long ago. Some faults, such as the San Andreas, intersect the ground surface and thus displace the ground when they move. Others accommodate the sliding of rocks in the crust at depth and remain invisible at the surface unless they are later exposed by erosion.

Fault Classification

Not all faults result in the same kind of crustal deformation some accommodate horizontal shear, some accommodate shortening, and some accommodate stretching. It’s important for geologists to distinguish among different kinds of faults in order to interpret their tectonic significance. Fault classification focuses on two characteristics of faults: (1) the dip of the fault surface (seeDescribing the Orientation of Geologic Structures) the dip can be vertical, horizontal, or any angle in between; and (2) the shear sense across the fault by shear sense, we mean the direction that material on one side of the fault moved relative to the material on the other side. With this concept in mind, let’s consider the principal kinds of faults.

The different categories of faults.
  • Strike-Slip Faults: A strike-slip fault is a fault on which the slip direction is parallel to a strike line, a horizontal line on the fault surface (seeDescribing the Orientation of Geologic Structures). This means that the block on one side of the fault slips sideways, relative to the block on the other side, and there is no up-or-down motion. Most strike-slip faults have a steep to vertical dip. Geologists distinguish between two types of strike-slip faults based on the shear sense as viewed when you are facing the fault and looking across it. If the block on the far side slipped to your left, the fault is a left-lateral strike-slip fault, and if the block slipped to the right, the fault is a right-lateral strike-slip fault (figure above a). The faults that occur at transform plate boundaries are strike-slip faults.
  • Dip-Slip Faults: On a dip-slip fault, movement is parallel to the dip line, a line parallel to the slope of the fault surface  (seeDescribing the Orientation of Geologic Structures), so the hanging-wall block, meaning the material above the fault surface, slides up or down relative to the footwall block, the material below the fault surface (figure above b). If the hanging-wall block slides up, the fault is a reverse fault; a reverse fault with a gentle dip is also known as thrust fault. Reverse faults accommodate shortening of the crust, as happens during continental collision. If the hanging-wall block slides down, the fault is a normal fault. Normal faults accommodate stretching of the Earth’s crust, as happens during rifting.
  • Oblique-Slip Faults: On an oblique-slip fault, sliding occurs diagonally on the fault plane. In effect, an oblique-slip fault is a combination of a strike-slip and a dip-slip fault (figure above c).

Recognizing Faults

Recognizing fault displacement in the field.

How do you recognize a fault when you see one? The most obvious criterion is the occurrence of displacement, meaning the movement across a fault plane. Displacement offsets the layers in rocks, so that layers on one side of a fault are not continuous with layers on the other side (figure above a, b).

Faults may also leave their mark on the landscape. Those that intersect the ground surface while they are active can displace natural landscape features such as stream valleys or glacial moraines (figure above c), and human-made features such as highways, fences, or rows of trees in orchards. Displacement on a dip-slip or oblique-slip fault will make a step on the ground surface; this step is called a fault scarp (figure above a). And because faults tend to break up and weaken rock, the “fault trace” (the line of intersection between the fault and the ground surface) preferentially erodes to become a linear valley.

Faulting under brittle conditions may crush or break rock. If this shattered rock consists of visible angular fragments, then it is called fault breccia (figure above b), but if it consists of a fine powder, then it is called fault gouge. Some fault surfaces are polished and grooved by the movement on the fault. Polished fault surfaces are called slickensides, and linear grooves on fault surfaces are slip lineations or fault striations (figure above c). We specify the orientation of a slip lineation by giving its plunge and bearing (seeDescribing the Orientation of Geologic Structures).

Features of exposed fault surfaces.

Describing the Orientation of Geologic Structures

Specifying the orientation of planar and linear structures.

When discussing geologic structures, it’s important to be able to communicate information about their orientation. For example, does a fault exposed in an outcrop at the edge of town continue beneath the nuclear power plant 3 km to the north, or does it go beneath the hospital 2 km to the east? If we knew the fault’s orientation, we might be able to answer this question. To describe the orientation of a geologic structure, geologists picture the structure as a simple geometric shape, then specify the angles that the shape makes with respect to a horizontal plane (a flat surface parallel to sea level), a vertical plane (a flat surface perpendicular to sea level), and the north direction (a line of longitude).

Let’s start by considering planar structures such as faults, beds, and joints. We call these structures planar because they resemble a geometric plane. A planar structure’s orientation can be specified by its strike and dip. The strike is the angle between an imaginary horizontal line  (the strike line) on the plane and the direction to true north (figure above a, b). We measure the strike with a special type of compass (figure above c). The dip is the angle of the plane's slope more precisely, it is the angle between a horizontal plane

and the dip line (an imaginary line parallel to the steepest slope on the structure), as measured in a vertical plane perpendicular to the strike. We measure the dip angle with a clinometer, a type of protractor. A horizontal plane has a dip of 0°, and a vertical plane has a dip of 90°. We represent strike and dip on a geologic map using the symbol shown in figure above b.

A linear structure resembles a geometric line rather than a plane; examples of linear structures include scratches or grooves on a rock surface. Geologists specify the orientation of a line by giving its plunge and bearing (figure above d). The plunge is the angle between a line and horizontal in the vertical plane that contains the line. A horizontal line has a plunge of 0°, and a vertical line has a plunge of 90°. The bearing is the compass heading of the line, meaning the angle between the projection of the line on the horizontal plane and the direction to true north.

Credits: Stephen Marshak (Essentials of Geology)

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