The Mule Mountains thrust system crops out discontinuously over a 100 km-strike length in the Blythe-Quartzsite region of southeast California and southwest Arizona. Along the thrust system, middle and upper crustal metamorphic and plutonic rocks of Proterozoic and Mesozoic age are thrust north-northeasterward (015¿ to 035¿) over a lower plate metamorphic terrane that formed part of the Proterozoic North American craton, its Paleozoic sedimentary rock cover, overlying Mesozoic volcanic and sedimentary rocks, and the intruding Jurassic and Cretaceous granitic rocks. Stratigraphic, petrologic, and Pb isotopic ties for Jurassic granitoids and for Jurassic(?) and Cretaceous sedimentary rocks across the various parts of the thrust system indicate that related crustal blocks are superposed and preclude it from having large displacments. The thick-skinned thrust system is structurally symmetrical along its length with a central domain of synmetamorphic thrust faults that are flanked by western and eastern domains where lower plate synclines underlie the thrusts. Deformation occurred under low greenschist facies metamorphic conditions in the upper crust. Movement along the thrust system was probably limited to no more than a few tens of kilometers and occurred between 79¿2 Ma and 70¿4 Ma. The superposition of related rocks and the geometry of the thrust system preculed it from being a major tectonic boundary of post-Middle Jurassic age, as has been previously proposed. Rather the thrust system forms the southern boundary of the narrow zone of Cretaceous intracratonic deformation, and it is one of the last tectonic events in the zone prior to regional cooling. ¿American Geophysical Union 1990