A transform plate boundary is a margin between two lithospheric plates that constitutes a regional-scale transform fault. The best-known transform plate boundary is the San Andreas fault system, which accommodates the right-lateral displacement between the North American and Pacific plates. The northwards-moving Pacific plate is subducted at the Aleutian trench and at western Pacific island arcs. Western California, part of the Pacific Plate, comprises exotic terranes translated northwards and rotated through angles up to 90 degrees along the margin of the North American plate. In the north, the San Andreas fault system terminates at the Mendocino triple junction where it intersects the Mendocino Fracture Zone and Cascadia subduction. Here the North American and Pacific plates intersect the Gorda–Juan de Fuca plate. The easterly moving Gorda–Juan de Fuca plate is subducted beneath the American plate north of the San Andreas fault system termination forming the Cascade Range. The San Andreas fault system steps through a series of oblique spreading ridges and transform faults in the Gulf of California. It terminates at the Rivera triple junction in the southern Gulf of California (junction between the Rivera, Pacific and North American plates). South of this triple junction, the Rivera Plate is subducted beneath North America. The San Andreas Fault is the main transcurrent or strike-slip fault within a broad deformation zone that comprises hundreds of minor faults along western California. Changes in their orientation and relay stepping of faults result in localized dilatation or contraction. Some segments of the San Andreas and other major faults in the San Andreas fault system are locked. In such segments, built-up strain may finally be rapidly released, producing an earthquake. Other segments are undergoing slow, continuous deformation or creep. The lithosphere is thinner beneath the San Andreas fault system than for normal continental lithosphere.
The Alpine Fault Zone along the western South Island of New Zealand is an example of an obliquely convergent transform plate boundary separating two zones of subduction with opposite polarity. North of New Zealand, the Pacific Plate is obliquely subducted beneath the Indo-Australian plate at the Tonga-Kermadec trench. South of New Zealand, the Indo-Australian Plate is obliquely subducted beneath the Pacific Plate at the Puysegur Trench. Right-lateral displacement and horizontal shortening occur across a zone 93–125 mi (150–200 km) wide. Most displacement has occurred along the Alpine Fault, which comprises oblique thrusts linked by sub-vertical dextral transcurrent faults.
