Transtensional basins embedded in the San Andreas fault system of Southern California (United States) and northwestern Mexico are filled with sediment derived from the Colorado River, which drains a large area of the western U.S. interior. The sediment is rapidly buried, heated, and mingled with intrusions in the deep basins to form a new generation of recycled crust along the active plate boundary. Using a range of values for total basin depth, relative volume of mantle-derived intrusions, and composition of early rift deposits, the volume of Colorado River–derived sediment in the basins is bracketed between 2.2 and 3.4 × 105 km3, similar to the volume of rock that likely was eroded from the Colorado River catchment over the past 5–6 m.y. The volumetric rate of crustal growth by sedimentation is ∼80–130 km3/m.y./km, comparable to growth rates in subduction-related island arcs and slow seafloor spreading centers. Sedimentary and basinal processes thus play a major role in crustal evolution and recycling in this setting, and may be important at other rifted margins where a large river system is captured following tectonic collapse of a prerift orogenic highland.