BEDROCK GEOLOGIC MAP COMPILATION OF THE BLYTHE 30X60’ QUADRANGLE, SE CALIFORNIA AND SW ARIZONA

The Blythe 30 x 60’ ( 1:100,000-scale) Quadrangle in SE California and SW Arizona portrays complex geology that includes Mesozoic thrusting, metamorphism, and magmatism; mid-Miocene crustal extension and volcanism; and late Miocene distributed normal and strike-slip faulting. A new bedrock geologic map of the Blythe quadrangle is being compiled through a collaborative effort between the U.S. Geological Survey (USGS) and the Arizona Geological Survey (AZGS). This interim map shows bedrock units only, but future versions will add detailed surficial mapping that is ongoing as part of the USGS Lower Colorado River Corridor Mapping Project. The map melds two 2006 USGS map publications: the west half of the Blythe quadrangle published as USGS SIM 2922, and the east half published as Digital Data Series DS-2006-225. In addition, new geologic mapping published since 2006 (AZGS DGM-107, AZGS DGM-116, AZGS DGM-121, and AZGS DGM-122) has been compiled into the database. AZGS also contributed an unpublished 1:100,000-scale compilation of the Plomosa Mountains in the eastern part of the quadrangle.

Major geologic features exposed in the Blythe quadrangle include: (1) thick sequences of moderately to weakly metamorphosed sedimentary rocks of the Jurassic to Cretaceous McCoy Basin; (2) ductile folds and faults of the Late Cretaceous Maria Fold and Thrust Belt; and (3) Miocene detachment faults in the Big Maria and Plomosa Mountains. Our understanding of these regionally important features is substantially improved as a result of recent mapping by AZGS in the Dome Rock and Plomosa Mountains. The most significant recent discovery, by John Singleton and Evan Strickland at Colorado State University, is recognition of the Late Cretaceous to Paleogene Orocopia Schist structurally below undated gneiss in the northern Plomosa Mountains (AZGS Contributed Map CM-17-A). This is the northernmost outcrop area of Orocopia Schist yet found in western Arizona.