ABSTRACT The Santa Catalina and Rincon Mountains north and east of Tucson, Arizona, form one of the largest core complexes on Earth. Both ranges consist primarily of Eocene leucogranites that intrude Proterozoic and late Cretaceous granitoids, and two Oligocene plutons. Mylonitic fabrics are well developed on the southern flank of the Santa Catalina Mountains and the southwestern flank of the Rincon Mountains. The corrugated form of the two ranges reflects the grooved form of the ca. 15–30 Ma Catalina–San Pedro detachment fault exposed primarily at the foot of the ranges. Normal displacement on two younger high-angle normal faults is responsible for much of the substantial relief of the ranges. This field guide is focused on fault rocks and mylonitic fabrics in the footwalls of the detachment fault and the high-angle Pirate normal fault, and includes description and analysis of shear-zone kinematics and processes, U-Pb geochronology of leucogranites, and core-complex geomorphology.

An evaluation of the poorly understood Cenozoic hydrologic history of the American Southwest using combined geological and biological data yields new insights with implications for tectonic evolution. The Mesozoic Cordilleran orogen next to the continental margin of southwestern North America probably formed the continental divide. Mountain building migrated eastward to cause uplift of the Rocky Mountains during the Late Cretaceous to early Tertiary Laramide orogeny. Closed drainage basins that developed between the two mountain belts trapped lake waters containing fish of Atlantic affinity. Oligocene-Miocene tectonic extension fragmented the western mountain belt and created abundant closed basins that gradually filled with sediments and became conduits for dispersal of fishes of both Pacific and Atlantic affinity. Abrupt arrival of the modern Colorado River to the Mojave-Sonora Desert region at ca. 5 Ma provided a new conduit for fish dispersal. Great dissimilarities in modern fish fauna, including differences in their mitochondrial deoxyribonucleic acid (DNA), indicate that late Miocene runoff from the Colorado Plateau did not flow down the Platte or Rio Grande, or through the Lake Bonneville Basin. Fossil fishes from the upper Miocene part of the Bidahochi Formation on the Colorado Plateau have characteristics that reflect a habitat of large, swift-moving waters, and they are closely related to fossil fishes associated with the Snake and Sacramento Rivers. This evidence suggests that influx of fishes from the ancestral Snake River involved a major drainage, not merely small headwater transfers.

The upper Miocene to lower Pliocene Bouse Formation in the lower Colorado River trough of the American Southwest was deposited in three basins—from north to south, the Mohave, Havasu, and Blythe Basins—that were formed by extensional faulting in the early to middle Miocene. Fossils of marine, brackish, and freshwater organisms in the Bouse Formation have been interpreted to indicate an estuarine environment associated with early opening of the nearby Gulf of California. Regional uplift since 5 Ma is required to position the estuarine Bouse Formation at present elevations as high as 555 m, where greater uplift is required in the north. We present a compilation of Bouse Formation elevations that is consistent with Bouse deposition in lakes, with an abrupt 225 m northward increase in maximum Bouse elevations at Topock gorge north of Lake Havasu. Within Blythe and Havasu Basins, maximum Bouse elevations are 330 m above sea level in three widely spaced areas and reveal no evidence of regional tilting. To the north in Mohave Basin, numerous Bouse outcrops above 480 m elevation include three widely spaced sites where the Bouse Formation is exposed at 536–555 m. Numerical simulations of initial Colorado River inflow to a sequence of closed basins along the lower Colorado River corridor model a history of lake filling, spilling, evaporation and salt concentration, and outflow-channel incision. The simulations support the plausibility of evaporative concentration of Colorado River water to seawater-level salinities in Blythe Basin and indicate that such salinities could have remained stable for as long as 20–30 k.y. We infer that fossil marine organisms in the Bouse Formation, restricted to the southern (Blythe) basin, reflect colonization of a salty lake by a small number of species that were transported by birds.

Abstract The Granite Wash Mountains are located in west-central Arizona and are contiguous with the Harcuvar Mountains to the northeast and the Little Harquahala Mountains to the south. They are part of the Maria fold and thrust belt, a belt of large folds and major thrust faults that trends east-west through west-central Arizona and southeastern California (Reynolds and others, 1986; Spencer and Reynolds, 1990). In the Granite Wash Mountains, late Mesozoic deformation related to the Maria belt affected a diverse suite of rock units, including Proterozoic crystalline rocks, Paleozoic carbonate and quartzose clastic rocks, and Mesozoic sedimentary, volcanic, plutonic, and hypabyssal rocks. This deformation was mostly deep seated and produced an assortment of folds, cleavages, and both ductile and brittle shear zones. Several discrete episodes of deformation occurred, resulting in refolded folds, folded and refolded thrust faults, and complex repetition, attenuation, and truncation of stratigraphic sequences. Greenschist-facies metamorphism accompanied deformation and was most intense along the major thrusts. Deformation and metamorphism were followed by emplacement of two Late Cretaceous intrusions and numerous Cretaceous to mid-Tertiary dikes.

The late Cenozoic geologic history of the Avawatz Mountains reflects two successive tectonic settings. Middle Miocene high-angle normal faulting along the northwest-striking Arrastre Spring fault was associated with deposition of a thick clastic sequence that forms the lower part of the Avawatz Formation. Conglomerate and sedimentary breccia in the lower part of the Avawatz Formation were shed southwestward across the fault from a presently unrecognized terrane to the east of the Avawatz Mountains. Faulting and conglomerate deposition occurred largely between 21 and 12 Ma. This period of faulting and sedimentation is interpreted as reflecting the regional Miocene extensional tectonic setting of the southern Basin and Range Province and Mojave Desert region. North-south shortening and associated folding and southward tilting of the largely Miocene Avawatz Formation occurred in late Miocene to Pliocene time. Interaction between the Garlock and Death Valley faults at their zone of intersection on the north flank of the Avawatz Mountains is the inferred cause of north-south compression and resultant folding and tilting of the Avawatz Formation. Continued movement on the two intersecting faults led to development of an increasingly arcuate reverse-fault system along the north and east flanks of the Avawatz Mountains. Quaternary deformation is characterized by northeast-directed reverse faulting, southwestward tilting, rapid uplift of the Avawatz Mountains, and probable counterclockwise rotation of faults and fault blocks in the northwestern Avawatz Mountains at the east end of the Garlock fault.