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A strike slip fault11/18/2023 ![]() ![]() Ī fault is boundary between two bodies of rock along which there has been relative motion (Figure 12.4d). As we discussed in Chapter 11, an earthquake involves the sliding of one body of rock past another. Earthquakes don’t necessarily happen on existing faults, but once an earthquake takes place a fault will exist in the rock at that location. Some large faults, like the San Andreas Fault in California or the Tintina Fault, which extends from northern B.C. Figure 12.11 A depiction of joints developed in a rock that is under stress. įinally joints can also develop when rock is under compression as shown on Figure 12.11, where there is differential stress on the rock, and joint sets develop at angles to the compression directions. Figure 12.10 A depiction of joints developed in the hinge area of folded rocks. Note that in this situation some rock types are more likely to fracture than others. Ī fracture in a rock is also called a joint. There is no side-to-side movement of the rock on either side of a joint. Most joints form where a body of rock is expanding because of reduced pressure, as shown by the two examples in Figure 12.9, or where the rock itself is contracting but the body of rock remains the same size (the cooling volcanic rock in Figure 12.4a). In all of these cases, the pressure regime is one of tension as opposed to compression. Joints can also develop where rock is being folded because, while folding typically happens during compression, there may be some parts of the fold that are in tension (Figure 12.10). (right), both showing fracturing that has resulted from expansion due to removal of overlying rock. Figure 12.9 Granite in the Coquihalla Creek area, B.C. Fracturingįracturing is common in rocks near the surface, either in volcanic rocks that have shrunk on cooling (Figure 12.4a), or in other rocks that have been exposed by erosion and have expanded (Figure 12.9). īy: David Beasley, Earth.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 fracturing or faulting. ![]() The study was published in the Journal of Structural Geology. “What you’re left with at the end is a long fault with abandoned segments on either side, which is something we see in the field all the time,” Cooke said.”It’s a nice confirmation that our lab experiments replicate what is going on within the Earth.” The process starts simply, advances to a complex peak, and simplifies again before lengthening into a “through-going” or continuous single, surface crack, the research found. The study identifies four stages in fault evolution: pre-faulting, localization, linkage and slip. “Another especially surprising finding is that fault irregularities, which are inefficient, persist rather than the system forming a straight, efficient fault,” she said. This is the first experiment to document this. One surprising finding was that faults “shut off along the way,” Cooke said. “We have captured very different conditions for fault formation in our experiments that represent a range of conditions that might drive faulting in the crust,” Cooke said. The scientists record strain localization and fault evolution that represents millions of years at the scale of tens of kilometers during the maturation of strike-slip faults. One scenario begins with a pre-existing fault, another with localized displacement beneath the clay, and a third that is characterized by a displacement across a wider zone of shear beneath the clay. In two-hour experiments, the researchers load a tray with kaolin, also known as china clay, prepared so its viscosity and length scale to that of the earth’s crust. The researchers have developed methods that allow them to see faults grow in very, very fine detail, at a finer resolution than anyone has documented before,” Cooke said. We set up the conditions for faulting on a small scale and watch them unfold.” “When I give talks to other geologists I put up a picture of a fault and ask, wouldn’t you love to be able to see exactly how that formed?,” said structural geologist Michele Cooke. In addition to a video description, University of Massachusetts at Amherst researchers re-enacted the events in wet clay in the lab. A new study reveals how strike-slip faults form in the Earth’s crust, as two tectonic plates slide past one another, generating earthquakes. ![]()
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