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In order for plates to be able to move relative to one another, a third type of plate boundary is required. Transform plate boundaries are characterized by horizontal relative motion along fault systems that is parallel to the plate boundary segment that separates two plates (Figure 1.4). Because the rocks on either side slide horizontally past each other, transform fault systems are a type of strike‐slip fault system.

Figure 1.14 (a) Diagram depicting the convergence of India and Asia which closed the Tethys Sea.

Source: Courtesy of NASA;

(b) Satellite image of southern Asia showing indentation of Eurasia by India, the uplift of Himalayas and Tibetan Plateau and the mountains that “wrap around” India.

Source: From UNAVCO.

Transform faults were first envisioned by J.T. Wilson (1965) to explain the seismic activity along fracture zones in the ocean floor. Fracture zones are curvilinear zones of intensely faulted, fractured oceanic crust that are generally oriented nearly perpendicular to the ridge axis (Figure 1.15). Despite having been fractured by faulting along their entire length, earthquake activity is largely restricted to the transform portion of fracture zones that lies between offset ridge segments. Wilson (1965) reasoned that if sea floor was spreading away from two adjacent ridge segments in opposite directions, the portion of the fracture zone between the two ridge segments would be characterized by parallel relative motion in opposite directions. This would produce shear stresses that result in strike‐slip faulting of the lithosphere, frequent earthquakes and the development of a transform fault plate boundary. The exterior portion of fracture zones outside the ridge segments represents oceanic crust that was faulted and fractured when it was between ridge segments, then carried beyond the adjacent ridge segment by additional sea floor spreading. These exterior portions of fracture zones are appropriately called healed transforms or transform scars. They are no longer plate boundaries; instead, they are aseismic (lacking significant earthquakes) intraplate features because the seafloor on either side of them is spreading in the same direction (Figure 1.15). However, they do record the relative motion between two plates at the time they were actively forming.

Continental transform plate boundaries occur in continental lithosphere. The best known modern examples of continental transforms include the San Andreas Fault system in California (Figure 1.16), the Alpine Fault system in New Zealand, and the Anatolian Fault systems in Turkey and Iran. All of these are characterized by active strike‐slip fault systems of the type that characterize transform plate boundaries. In places where such faults bend or where their tips overlap, deep pull‐apart basins may develop in which thick accumulations of sedimentary rocks accumulate rapidly.

Figure 1.15 Transform faults offsetting ridge segments on the eastern Pacific Ocean floor off Central America. Arrows show directions of sea floor spreading away from the ridge. Oppositely directed arrows (black) indicate transform plate boundaries. Similarly directed arrows (red) indicate intraplate transform scars.

Source: William Haxby, with the permission of Columbia University Earth Institute; Copyright Marine Geoscience Data System.

Plates cannot simply diverge and converge; they must be able to slide past each other in opposite directions in order to move at all. Transform plate boundaries serve to accommodate this required sense of motion. Small amounts of igneous rocks form along transform plate boundaries, especially hybrid boundaries that have a component of divergence or convergence as well. They produce much smaller volumes of igneous and metamorphic rocks than are formed along divergent and convergent plate boundaries. Because they neither create nor destroy large volumes of crust/lithosphere, these boundaries are sometimes referred to as conservative plate boundaries.

Figure 1.16 Fracture zones, transform faults and ridge segments in the eastern Pacific Ocean and western North America. The San Andreas Fault system is a continental transform fault plate boundary.

Source: Courtesy of USGS.