In contrast to convergent or divergent boundaries, along a transform plate boundary plates slide horizontally past one another without the production or destruction of lithosphere. Transform plate boundaries, also called transform faults, are considered conservative plate margins because crust remains essentially intact even as the plates slide. The nature of transform faults was discovered in 1965 by Canadian geologist J. Tuzo Wilson, who proposed that large faults connect divergent and convergent boundaries and “transform” the motion between them. Most transform faults are found on the ocean floor, where they offset segments of divergent oceanic ridge systems, producing a steplike plate margin (Figure 19A). Notice that the zigzag shape of the Mid-Atlantic Ridge in Figure 19A  roughly reflects the shape of the original rifting that caused the breakup of the supercontinent Pangaea. (Compare the shapes of the continental margins of the landmasses on both sides of the Atlantic with the shape of the Mid-Atlantic Ridge in Figure 10.)

Transform faults in oceanic crust are part of prominent linear breaks in the seafloor known as fracture zones, which include both active transform faults and their inactive extensions into the plate interior (Figure 19B). In a fracture zone, the active transform fault lies only between the two offset ridge segments. On each side of the fault, the seafloor moves away from the corresponding ridge segment. Thus, between the diverging ridge segments, these adjacent slabs of oceanic crust are grinding past each other along a transform fault. Beyond the ridge crests, these faults are inactive because the rock on either side moves in the same direction. However, these inactive faults are preserved as linear topographic depressions, which are evidence of past transform fault activity. The trend (orientation) of these fracture zones roughly parallels the direction of plate motion at the time of their formation. Thus, these structures help geologists map the direction of plate motion in the geologic past.

Transform faults also provide the means by which the oceanic crust that is created at ridge crests can be transported to a site of destruction—the deep-ocean trenches. Figure 20 illustrates this situation. Notice that the Juan de Fuca plate moves in a southeasterly direction, eventually being subducted under the west coast of the United States and Canada (at a convergent boundary). The southern end of this plate is bounded by a transform fault called the Mendocino Fault. This transform boundary connects the Juan de Fuca Ridge to the Cascadia subduction zone. Therefore, it facilitates the movement of the crustal material created at the Juan de Fuca Ridge to its destination beneath the North American continent.

Like the Mendocino Fault, most transform fault boundaries are located within the ocean basins; however, some cut through continental crust. Examples include the earthquake-prone San Andreas Fault of California, New Zealand’s Alpine Fault, and the North and East Anatolian Faults of Turkey. Notice in Figure 20 that the San Andreas Fault connects a spreading center (divergent boundary) located in the Gulf of California to the Cascadia subduction zone (a convergent boundary) and the Mendocino Fault. Along the San Andreas Fault, the Pacific plate is moving toward the northwest, past the North American plate (Figure 21). If this movement continues, the part of California west of the fault zone, including Mexico’s Baja Peninsula, will become an island off the west coast of the United States and Canada in the distant geologic future. However, a more immediate concern is the earthquake activity triggered by movements along this fault system.

• At a transform boundary, lithospheric plates slide horizontally past one another. No new lithosphere is generated, and no old lithosphere is consumed. Shallow earthquakes signal the movement of these slabs of rock as they grind past their neighbors.

• The San Andreas Fault in California is an example of a transform boundary in continental crust, while the fracture zones between segments of the Mid-Atlantic Ridge are transform faults in oceanic crust.

• fracture zones: Linear seafloor features extending hundreds of kilometers from mid-ocean ridges that represent both the active and inactive portions of transform faults that offset ridge segments.

• transform plate boundary: A conservative plate boundary in which two lithospheric plates slide past one another without creating or destroying lithosphere. Equivalent to a transform fault.