If the far side moves to the right, as in Figures 13.23 and 13.26 (right), it is a right-handed, right-lateral,or dextral strike-slip fault. These happen where shear stress causes bodies of rock to slide sideways with respect to each other, as is the case along a transform boundary. Strike-Slip Faultsįaults where the motion is mostly horizontal and along the “strike” or the length of the fault are called strike-slip faults (Figure 13.26 bottom). Reverse faults shorten and thicken the crust (Figure 13.25, right). Compression produces reverse faults, pushing the hanging wall up relative to the footwall. This permits the hanging wall to slide down the footwall in response to gravity (Figure 13.25, left). Tension produces normal faults, in which the crust undergoes extension. Source: Photo- Gold Hill Mine, Yukon Territory, by Eric A. These terms were originally used by miners to describe the rocks above and below an ore body. Figure 13.24 The hanging wall (or headwall) of a fault is the rock above the fault. These terms were originally used by miners to describe the rocks above and below an ore body (Figure 13.24). In dip-slip faults we identify rock above the fault as the hanging wall, (or headwall) and the rock beneath as the footwall. Dip-Slip Faultsĭip-slip faults are so named because the dominant motion involves moving up or down the dipping (tilting) fault plane. We describe faults in terms of how the rocks on one side of the fault move relative to the other. Source: Steven Earle (2015) CC BY 4.0 view source Types of Faultsĭifferent kinds of faults develop under different stress conditions. Figure 13.23 View looking down on a fault (white dashed line) in intrusive rocks on Quadra Island, British Columbia. The pink dyke has been offset approximately 10 cm by the fault (length of the white arrow). This could be the edge of a bed or dike as in Figure 13.23, or it could be a landscape feature, such as a fence or a stream. Other faults show only centimetres of movement. In order to estimate the amount of motion on a fault, it is necessary to find a feature that shows up on both sides of the fault, and has been offset by the fault. Some large faults, like the San Andreas fault in California or the Tintina fault, extending from northern British Columbia through central Yukon and into Alaska, show evidence of hundreds of kilometres of motion. Source: Steven Earle CC BY 4.0 view sourceĪ fault is a boundary between two bodies of rock along which there has been relative motion (e.g., Figure 13.23). Figure 13.22 Joints developing to accommodate the larger horizontal component of compression (large red arrows). The joints accommodate the larger compression stress (larger red arrows) by allowing the rock to stretch in the up-down direction (along the green arrows). Joints can also develop in a rock a rock under compression as a way to accommodate the change in shape (Figure 13.22). Source: Steven Earle (2015) CC BY 4.0 view source Figure 13.21 Joints developed in the hinge zone of folded rocks. Joints can develop where rocks are being folded, because the hinge zone of the fold is under tension as it stretches to accommodate the bending (Figure 13.21). Nevertheless, it is possible for joints to develop where the overall regime is one of compression. Source: HylgeriaK (2010) CC BY-SA 3.0 view source Figure 13.20 Half Dome at Yosemite National Park is an exposed granite batholith that displays exfoliation joints, causing sheets of rock to break off. Exfoliation joints, which make the rock appear to be flaking off in sheets (Figure 13.20), occur when a body of rock expands in response to reduced pressure, such as when overlying rocks have been removed by erosion. It can also be from a body of rock expanding. The tension can be from a rock contracting, such as during the cooling of volcanic rock (Figure 13.9, upper left). Most joints form when the overall stress regime is one of tension (pulling apart) rather than compression. Rygel (2008) CC BY-SA 3.0 view source Jointing Figure 13.19 Joint sets have broken these siltstone and shale beds into long rectangular planks. Joints with a common orientation make up a joint set (Figure 13.19). If there is no movement of one side relative to the other, and if there are many other fractures with the same orientation, then the fractures are called joints. If rocks on one side of the break shift relative to rocks on the other side, then the fracture is a fault. When rocks break in response to stress, the resulting break is called a fracture.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |