A transform boundary, sometimes called a strike-slip or conservative boundary, is where the lithospheric plates slide past each other in the horizontal plane. This movement is described based on the perspective of an observer standing on one of the plates, looking across the boundary at the opposing plate. Dextral, also known as right-lateral, movement describes the opposing plate moving to the right. Sinistral, also known as left lateral, movement describes the opposing plate moving to the left.
Most transform boundaries are found on the ocean floor, around mid-ocean ridges. These boundaries form aseismic fracture zones, filled with earthquake-free transform faults, to accommodate different rates of spreading occurring at the ridge.
Some transform boundaries produce significant seismic activity, primarily as earthquakes, with very little mountain-building or volcanism. This type of transform boundary may contain a single fault or series of faults, which develop in places where plate tectonic stresses are transferred to the surface. As with other types of active boundaries, if the plates are unable to shear past each other the tectonic forces will continue to build up. If the built-up energy between the plates is suddenly released, the result is an earthquake.
In the eyes of humanity, the most significant transform faults occur within continental plates and have a shearing motion that frequently produces moderate-to-large magnitude earthquakes. Notable examples include the San Andreas Fault in California, Northern and Eastern Anatolian Faults in Turkey, Altyn Tagh Fault in Central Asia, and Alpine Fault in New Zealand.
Transpression and Transtension
Bends along transform faults may create compressional or extensional forces that cause secondary faulting zones. Transpression occurs where there is a component of compression in addition to the shearing motion. These forces build up around the area of the bend, where the opposing plates are restricted from sliding past each other. As the forces continue to build up, they create mountains in the restraining bend around the fault. The Big Bend area, located in the southern part of the San Andreas Fault includes a large area of transpression where many mountains have been built, moved, and even rotated .
Transtension zones require a fault that includes a releasing bend, where the plates are being pulled apart by extensional forces. Depressions and sometimes volcanism develop in the releasing bend, along the fault. The Dead Sea found between Israel and Jordan, and the Salton Sea of California are examples of basins formed by transtensional forces.
When a geological feature is cut by a fault, it is called a piercing point. Piercing points are very useful for recreating past fault movement, especially along transform boundaries. Transform faults are unique because their horizontal motion keeps a geological feature relatively intact, preserving the record of what happened. Other types of faults—normal and reverse —tend to be more destructive, obscuring or destroying these features. The best type of piercing point includes unique patterns that are used to match the parts of a geological feature separated by fault movement. Detailed studies of piercing points show the San Andreas Fault has experienced over 225 km of movement in the last 20 million years, and this movement occurred at three different fault traces.