The Blythe 30' x 60' quadrangle in southeastern California and southwestern Arizona displays complex geology that includes Mesozoic contractional deformation, metamorphism, and magmatism and Cenozoic extensional deformation and magmatism. The scope of the present map is limited to bedrock units of Miocene and older age because the younger deposits have not been mapped in sufficient detail across the quadrangle to support a systematic compilation. Mapping and topical studies by previous investigators (refer to accompanying pamphlet) resulted in recognition of the following regionally significant geologic features: (1) variably metamorphosed and deformed Paleozoic to early Mesozoic sedimentary rocks stratigraphically correlative with cratonal platform strata of the Colorado Plateau region; (2) Jurassic plutonic and volcanic rocks; (3) thick sequences of moderately to weakly metamorphosed sedimentary rocks of the Jurassic to Cretaceous McCoy Basin; (4) ductile folds and faults of the Late Cretaceous Maria Belt; and (5) Miocene detachment faults in the Big Maria and Plomosa Mountains. A major recent discovery is the recognition of the Late Cretaceous to Paleogene Orocopia Schist structurally below undated gneiss in the northern Plomosa Mountains. This northernmost outcrop area of Orocopia Schist yet found in western Arizona demonstrates that the entire Blythe quadrangle likely is underlain by this extensive, tectonically underplated subduction complex. In addition, post-middle Miocene transtensional deformation has been documented in the northern La Posa Plain, including recognition of left-lateral motion on two northeast-striking faults in the northern Plomosa Mountains and at Mesquite Mountain.