![]() ![]() Symbols in plan view indicate the sense of displacement along the fault. Figure 10.3.5: Block model and plan view depictions of left-lateral (a) and right-lateral (b) strike-slip faulting resulting in the offset of a dyke in plan view. Symbols in plan view indicate the type of fault and are always drawn on the hanging wall side. Black arrows on the south-facing side of each block indicate the sense of displacement along the fault. Figure 10.3.4: Block model and corresponding plan view depictions of reverse (a) and normal (b) faulting. ![]() Figure 10.3.3: Depiction of reverse, normal, and strike-slip faults. Reverse faults happen during compression while normal faults happen during extension. Most strike-slip faults are related to transform boundaries. ![]() Map symbols for these strike-slip faults are illustrated in Figure 10.3.5. On strike-slip faults the motion is typically only horizontal, or with a very small vertical component, and as discussed above the sense of motion can be right lateral (the far side moves to the right), as in Figure 10.3.2, or it can be left lateral (the far side moves to the left). This is known as a strike-slip fault because the displacement is along the “strike” or the length of the fault. The third situation is where the bodies of rock are sliding sideways with respect to each other, as is the case along a transform fault (see Lab 1). The map symbols for these types of faults are illustrated in Figure 10.3.4. If the fault develops in a situation of compression, then it will be a reverse fault because the compression causes the hanging wall to be pushed up relative to the footwall. If the fault develops in a situation of extension, then it will be a normal fault, because the extension allows the hanging wall to slide down relative to the footwall in response to gravity. The body of rock above the fault is called the hanging wall, and the body of rock below it is called the footwall. The terms hanging wall and footwall in the diagrams apply to situations where the fault is not vertical. There are several kinds of faults, as illustrated on Figure 10.3.3, and they develop under different stress conditions. Figure 10.3.2: A fault (white dashed line) in intrusive rocks on Quadra Island, B.C. The pink dyke has been offset by the fault and the extent of the offset is shown by the white arrow (approximately 10 centimetres). Because the far side of the fault has moved to the right, this is a right-lateral fault. If the photo were taken from the other side, the fault would still appear to have a right-lateral offset. 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 10.3.2). ![]() through central Yukon and into Alaska, show evidence of hundreds of kilometres of motion, while others show less than a millimetre. Some large faults, like the San Andreas Fault in California or the Tintina Fault, which extends from northern B.C. Earthquakes don’t necessarily happen on existing faults, but once an earthquake takes place a fault will exist in the rock at that location. You may recall from lecture that an earthquake involves the sliding of one body of rock past another. FaultingĪ fault is a boundary between two bodies of rock along which there has been relative motion (Figure 10.1.3d). (right), both showing fracturing that has resulted from expansion due to removal of overlying rock. Figure 10.3.1: Granite in the Coquihalla Creek area, B.C. There is no movement on a fracture plane. Fractures, by definition, do not displace rock. Fracturingįracturing is common in rocks near the surface, either in volcanic rocks that have shrunk on cooling (Figure 10.1.3a), or in other rocks that have been exposed by erosion and have expanded (Figure 10.3.1). 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. ![]()
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