Styles of Continental Deformation

If we look closely enough, we can find all the basic deformation structures—faults, folds, domes, basins, joints—in any zone of continental deformation. But when we view continental deformation at a regional scale, we see distinctive patterns of faulting and folding that relate directly to the tectonic forces causing the deformation. Figure 7.19 depicts the deformation styles typical of the three main types of tectonic forces.

Figure 7.19: The orientation of tectonic forces—(a) tensional, (b) compressive, and (c) shearing—determines the style of continental deformation. On a regional scale, the basic types of faulting shown in the inset figures can lead to distinctive, complex patterns of deformation.

Tensional Tectonics

In brittle crust, the tensional forces that produce normal faulting may split a plate apart, resulting in a rift valley—a long, narrow trough formed when a block of rock drops downward relative to its two flanking blocks along nearly parallel, steeply dipping normal faults (Figure 7.19a). Well-known examples include the Rhine Valley, the rift valleys of East Africa (Figure 7.20), and the Red Sea, as well as the rift valleys of mid-ocean ridges. As we saw in Chapter 5, these structures form basins that fill with sediments eroded from the rift walls as well as with volcanic rocks extruded from tensional cracks in the crust.

Figure 7.20: In East Africa, tensional forces are pulling the Somali subplate away from the African Plate, creating rift valleys bounded by normal faults (see Figure 2.8b). The rift valley shown here is filled by sediments and Lake Tanganyika, on the boundary between Tanzania and the Democratic Republic of Congo. The cross section has been vertically exaggerated by a factor of 2.5:1, which exaggerates the fault dips; the actual dips of the normal faults are about 60°.

Tension on the shallow continental crust usually produces normal faults with high dip angles, typically 60° or more. Below a depth of about 20 km, however, crustal rocks are hot enough to behave as ductile materials, and deformation occurs by stretching rather than by fracturing. This change in rock behavior causes the dip of the faults to flatten with increasing depth, which results in normal faults with curved fault surfaces (called listric faults), as shown in Figure 7.19a. The crustal blocks moving along these curved faults are tilted backward as the stretching continues.

The Basin and Range province, which is centered on the Great Basin of Nevada and Utah, is a good example of a region defined by many adjacent rift valleys. This region, which is now more than 800 km wide, has been stretched and extended in a northwest-southeast direction by a factor of 2 during the last 15 million years. Here, normal faulting has created an immense landscape of eroded, rugged fault-block mountains and smooth, sediment-filled valleys, some covered with recent volcanic rocks (see Figure 10.5). This tensional deformation, which appears to be caused by upwelling convection currents beneath the Basin and Range province, continues today.

Compressive Tectonics

In subduction zones, oceanic lithosphere slips beneath an overriding plate along a huge thrust fault, or megathrust. The world’s largest earthquakes, such as the great Tohoku, Japan, earthquake of March 11, 2011, which generated a disastrous tsunami that killed more than 19,000 people, are caused by sudden slips on megathrusts. Thrust faulting is also the most common type of faulting within continents undergoing tectonic compression. Sheets of crust may glide over one another for tens of kilometers along nearly horizontal thrust faults, forming overthrust structures (Figure 7.21).

Figure 7.21: The Keystone thrust fault of southern Nevada is a large-scale overthrust structure of a kind formed during episodes of continental compression. Compressive forces have detached a sheet of rock layers (D, C, B) and thrust it a great distance horizontally over a section of the same rock layers (D, C, B, A).

When two continents collide, the crust can be compressed across a wide zone, resulting in spectacular episodes of mountain building. During such collisions, the brittle basement rocks ride over one another by thrust faulting while the more ductile overlying sedimentary rocks are compressed into a series of great folds, forming a fold and thrust belt (Figure 7.19b). Large earthquakes are common in fold and thrust belts; a recent example is the great Wenchuan earthquake that hit Sichuan, China, on May 12, 2008, killing more than 80,000 people.

The ongoing collisions of Africa, Arabia, and India with the southern margin of the Eurasian continent have created fold and thrust belts from the Alps to the Himalaya, many of which are still active. The great oil reservoirs of the Middle East are trapped in anticlines formed by this deformation. Compression across western North America, caused by that continent’s westward movement during the opening of the Atlantic Ocean, created the fold and thrust belt of the Canadian Rockies. The Valley and Ridge province of the Appalachian Mountains is an ancient fold and thrust belt that dates back to the collisions that created the supercontinent Pangaea.

Shearing Tectonics

A transform fault is a strike-slip fault that forms a plate boundary. Transform faults such as the San Andreas can offset geologic formations by long distances (see Figure 7.7), but as long as they stay aligned with the direction of relative plate movement, the blocks on either side can slide past each other without much internal deformation. Long transform faults are rarely straight, however, so deformation patterns along these faults can be much more complicated. The faults may have bends and jogs that change the tectonic forces acting across portions of the plate boundary from shearing forces to compressive or tensional forces. Those forces, in turn, cause secondary faulting and folding (Figure 7.19c).

A good example of these complications can be found in Southern California, where the right-lateral San Andreas fault bends first to the left and then to the right as one moves along its trace from south to north (Figure 7.22). The segments of the fault on both sides of this “Big Bend” are aligned with the direction of relative plate movement, so the blocks slip past each other there by simple strike-slip faulting. Within the Big Bend, however, the change in the fault orientation causes the plates to push against each other, which produces thrust faulting to the south of the fault. This thrusting has raised the San Gabriel and San Bernardino mountains to elevations exceeding 3000 m and, during the last half century, has produced a series of destructive earthquakes, including the 1994 Northridge quake, which caused more than $40 billion in damage to Los Angeles (see Chapter 13).

Figure 7.22: Space Shuttle photograph showing an oblique view of the San Andreas fault system. The annotations illustrate how the deviations of a transform fault’s strike from the direction of plate motion can cause local extension and compression. Between the Gulf of California and the Salton Sea (near bottom of figure), the fault system jogs to the right in two major steps; the right-lateral fault segments (black lines), which are parallel to the Pacific-North America plate motion, are separated by rift valleys (in red) that are volcanically active, subsiding, and filling with sediments. As we look northward, the fault trace first bends to the left, away from the direction of plate motion, and then to the right, realigning with the plate motion in central California (near top of figure). This “Big Bend” in the San Andreas fault causes compression, which is taken up by reverse faulting in the Los Angeles region (middle of figure).

At the southern end of the San Andreas, between the Gulf of California and the Salton Sea, the boundary between the Pacific and North American plates jogs to the right in a series of steps. Within these jogs, the plate boundary is subjected to tensional forces, and normal faulting has formed small rift valleys that are volcanically active, rapidly subsiding, and filling with sediments. This extension occurs within 200 km of the Big Bend compression, demonstrating how variable the tectonics along continental transform faults can be!