Volcanic and sedimentary rocks and structures record the Tertiary structural evolution of the lower Colorado River region in southeastern Arizona and southwestern California. A late Eocene or early Oligocene (prior to ~33 Ma) episode of faulting is indicated by medium- to coarse-grained arkosic rocks in the Chocolate and southern Trigo Mountains. Much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from ~33 to 22 Ma, but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks. A major episode of extensional deformation and tilting occured after deposition of welded tuff about 22 Ma and affected the entire area from the Palo Verde Mountains on the west to the Kofa Mountains on the east. Extension-related faulting quickly waned and had largely ceased by about 20 Ma in the Kofa Mountains; elsewhere the timing is poorly constrained. By ~13 Ma, thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area. Volcanism in the lower Colorado River region may have been coincident with a broad structural depression now oriented east-west. The northern limit of the volcanic terrane defines a tilt-domain boundary. The northern boundary, reaching from the New Water Mountains in Arizona to the little Chuckwalla Mountains in California, ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south. The southern margin is less well defined but is subparallel to the northern boundary and to the Chocolate Mountains anticlinorium. ¿American Geophysical Union 1991