BEDROCK GEOLOGIC MAP OF THE COLORADO RIVER EXTENSIONAL CORRIDOR IN THE PARKER 30’ BY 60’ QUADRANGLE, CALIFORNIA AND ARIZONA

The Parker 30' by 60’ quadrangle straddles the California-Arizona border along the lower Colorado River and contains a geologic record nearly two billion years long. Desert exposures record this history in geologic structures, Paleoproterozoic gneisses, Paleoproterozoic and Mesoproterozoic intrusions, metamorphosed Paleozoic and Mesozoic strata, Jurassic and Cretaceous intrusions, and varied Neogene rocks. Crust-building Paleoproterozoic gneisses >1.7 Ga formed before and during the Ivanpah orogeny were intruded by late kinematic ca. 1.64-Ga granodioritic batholiths. Mesoproterozoic ca. 1.4-Ga granite and quartz monzodiorite intrusion followed as part of a transcontinental anorogenic igneous belt. Diabase sheets of the 1.1-Ga southwestern Laurentia large igneous province widely intruded the older rocks.

The southern part of the quadrangle exposes metamorphosed Cambrian to Jurassic sedimentary formations and Jurassic volcanic rocks. Jurassic (ca. 170–160 Ma), middle Cretaceous (ca. 100 Ma), and Late Cretaceous (ca. 75 Ma) groups of plutons differ distinctively although each group ranges compositionally from diorite to granite. Late Cretaceous contractional tectonism in the Maria fold and thrust belt deformed and metamorphosed the Phanerozoic stratigraphic sections and an overthrust nappe of mostly Proterozoic gneiss. Similarities in structural style and orientation tie this part of the Maria fold and thrust belt to coeval structures northwest of the quadrangle in and near the Old Woman Mountains.

The Neogene Colorado River extensional corridor and its Whipple Mountains metamorphic core complex expose a record of tectonic extension, during which the crust doubled in width as magmatism inflated the crust, an imbricate series of down-to-NE faults sliced and tilted blocks of the upper crust, and volcanic and sedimentary materials including landslide breccias filled structural basins. Scattered late Neogene dextral and oblique faults overprinted those extensional structures as an early phase of the eastern California shear zone. The Colorado River became integrated through the area in the early Pliocene, depositing distinctive sediments and linking previously isolated basins.