4.6: Transform Faults and Fracture Zones

Transform faults are formed where two plates slide past each other. Numerous earthquakes occur along such faults as the edges of the two plates periodically lock and then break loose and slide past each other. The best-known transform fault is the San Andreas Fault in California, where a long section of the Pacific Plate is sliding northward past the North American Plate (Fig. 4-14). Most transform faults are much shorter than the San Andreas Fault.

Motions of plates at transform faults do not create the dramatic trenches, volcanoes, and mountain ranges of other types of plate boundaries. However, the movement of the two adjacent plates creates complicated stresses where the two plates meet, produces faults along which earthquakes occur, and forms low hills or mountains. Mountain building may be enhanced at bends on the plate boundary. One such bend on the San Andreas Fault is at the Santa Cruz Mountains south of San Francisco and was the site of the October 1989 Loma Prieta earthquake.

Numerous short transform faults are formed at divergent and convergent plate boundaries. As two plates move apart or together, they also rotate in relation to each other because the plates are moving on a spherical Earth (Fig. 4-22). The rate of spreading and creation of new oceanic crust will vary along the plate boundary when such rotation occurs. To accommodate the varying rates of spreading or subduction between two rigid plates, new plate material must be added or destroyed at different rates in adjacent segments of the plate boundary. This is accomplished by the creation of transform faults between short sections of the plate boundary (Fig. 4-23). In the transform fault region, the edges of the two plates slide past each other, one side moving slightly faster in one direction than the other, which is moving in the opposite direction.

Figure 4-22. (a) Because lithospheric plates have rigid shapes and move on a spherical surface, they must rotate with respect to each other, and the rate of formation or destruction of crust at any plate boundary must vary along the plate boundary’s length as shown by the spreading from A to B in this figure. The principle is similar to the movement of your eyelids as you open and close your eyes. The eye is exposed or covered more rapidly at the center than at the two corners. The rate of spreading cannot be equal along the plate boundary because the plates would need to bend and deform. This is shown by the vertical lines through the sequence C–D–E , which represent where the plate divergent boundary would be if spreading were uniform along the plate boundary. (b) Each of the points labeled with the same letter on either side of the Atlantic Ocean were joined at one time. Since the pairs are now separated by different distances, crust must have been formed at different rates along the oceanic ridge. Because the lithosphere is rigid and cannot stretch along its edge, variation in the rate of formation or destruction of crust is accommodated by the formation of transform faults between segments of the plate boundary.

You can demonstrate why this is necessary with a simple test using a book. With the book open on a flat surface, place one hand on the lower right corner of the right-hand page and press down so that it cannot move. With your other hand, lift all the pages (without the hard cover) at the top right corner slightly and grasp them between your finger and thumb. Hold these pages tightly together so that they cannot slide across each other, and try to lift and rotate these pages toward the lower left corner. Now loosen your grip on the top corner of the pages and try the same movement, allowing the pages to slide across each other. The resistance you felt and the distortion of the page edges is no longer there. Although the motions in this test are different from the motions of lithospheric plates on a sphere, the principle is the same. The book pages, when allowed to, slide across each other—the equivalent of transform faults between the pages.

Figure 4-23. Adjacent plates move past each other at a transform fault. (a) Most transform faults connect two segments of an oceanic ridge. The plates slide past each other in the region between the two ridge crests. This is the transform fault. Beyond this region, the two ridge segments are locked together and form a fracture zone. A transform fault can also connect (b) two adjacent segments of a trench or two trenches, or (c) an oceanic ridge and a trench.

Transform faults not only accommodate the changing rates of plate creation or destruction but also affect seafloor topography. To examine how transform faults work and how they influence topography consider the oceanic ridge transform fault depicted in Figure 4-23a. The transform fault is the segment of the plate boundary where the line of the oceanic ridge is offset. Along this segment, the sections of crust newly formed on each of the two plates slide past each other. However, once the spreading motion has transported the new oceanic crust past the central rift valley of the adjacent segment, it is no longer adjacent to the other plate. Instead, it is adjacent to another piece of its own plate. Because the two sides of this joint are now parts of the same plate and moving in the same direction, the two sides do not slide past each other. These areas are called fracture zones. The topographic roughness formed at the transform fault remains after the fracture zone has moved away from the plate boundary. Figure 3-4 shows numerous transform faults and associated fracture zones that cut perpendicularly through the oceanic ridges. At each transform fault, the line of the oceanic ridge crest is offset.

Some transform faults connect divergent and convergent plate boundaries, and others occur in subduction zones. The seafloor at subduction zones is covered by deep sediments that flow into any depressions and blanket the topography. Therefore, transform faults in subduction zones are less well defined by topography than their oceanic ridge counterparts are.