COLORADO RIVER STRATIGRAPHY, THE COLORADO RIVER EXTENSIONAL CORRIDOR, AND RECONSTRUCTED CRETACEOUS PLUTONS IN TOPOCK GORGE, ARIZONA AND CALIFORNIA

Geologic mapping of the Topock 7.5’ quadrangle, CA-AZ (http://pubs.usgs.gov/sim/3236) highlights the stratigraphic record of Colorado River-related landscape evolution superposed on the structurally complex older Colorado River extensional corridor in Topock Gorge. Post-Miocene landscape evolution is recorded by aggradational-degradational cycles of locally deformed sedimentary sequences, including the Pliocene Bouse Formation and four Colorado River-laid aggradational sequences, from oldest to youngest - the Pliocene Bullhead alluvium, the younger boulder conglomerate of Bat Cave Wash, the upper Pleistocene Chemehuevi Formation, and Holocene alluvium. Thick (1–3 km) deformed Miocene sections of volcanics, sedimentary breccias, and conglomerate record the structural and erosional evolution of the Colorado River extensional corridor. Four major Miocene low-angle normal faults and a steep block-bounding Miocene fault divide the deformed rocks into major structural plates and structurally thick tilt blocks along the eastern side of the Chemehuevi Mountains core complex. The low-angle faults attenuate >10 km of crustal section, superposing supracrustal and upper crustal rocks against originally deeper gneisses and granitoids. The block-bounding Gold Dome fault zone juxtaposes two large hanging-wall blocks, each tilted 90°, and splays at its tip into folds that deform layered Miocene rocks. A 15–Ma synfaulting intrusion occupies the triangular zone or gap where the folded strata detached from an inside corner along this fault between the tilt blocks. Late Cretaceous (72–76 Ma) and locally mylonitic Miocene (19 Ma) plutons intrude tilted sections of Proterozoic gneisses as thick as 8 km. Restoration of Miocene extension confirms dismembered parts of the Late Cretaceous Chemehuevi Mountains Plutonic Suite in Arizona as cupolas capping the sill-like Chemehuevi Mountains batholith exposed in California.