ABSTRACT The upper Middle Eocene to Lower Miocene Sespe Formation is the youngest part of an ~7-km-thick Cretaceous–Paleogene forearc stratigraphic sequence in coastal southern California. Whereas Upper Cretaceous through Middle Eocene strata of southern California record a transition from local (i.e., continental-margin batholith) to extraregional (i.e., cratonal) provenance resulting from Laramide deformation (75–35 Ma), the Sespe Formation records the reversal of this process and the re-establishment of local sediment sources by Middle Miocene time. In contrast to underlying dominantly marine strata, the Sespe Formation primarily consists of alluvial/fluvial and deltaic sandstone and conglomerate, which represent terminal filling of the forearc basin. Prior to Middle Miocene dissection and clockwise rotation, the Sespe basin trended north-south adjacent to the west side of the Peninsular Ranges. The integration of paleocurrent, accessory-mineral, conglomerate, sandstone, and detrital zircon data tightly constrains provenance. Sespe sandstone deposited in the Late Eocene was supplied by two major rivers (one eroding the Sonoran Desert, to the east, and one eroding the Mojave Desert, Colorado River trough area, and Transition Zone, to the north), as well as smaller local drainages. As the Farallon slab rolled back toward the coast during the Oligocene, the drainage divide also migrated southwestward. During deposition of the upper Sespe Formation, extraregional sources diminished, while the Peninsular Ranges provided increasing detritus from the east and the Franciscan Complex provided increasing detritus from the west (prerotation). As the overall flux of detritus to the Sespe basin decreased and deposition slowed, nonmarine environments were replaced by marine environments, in which the Miocene Vaqueros Formation was deposited. The provenance and paleogeographic information presented herein provides new insights regarding the unique paleotectonic setting of the Sespe forearc from the Late Eocene through earliest Miocene. Nonmarine sedimentation of the Sespe Formation initiated soon after cessation of coastal flat-slab subduction of the Laramide orogeny and terminated as the drainage divide migrated coastward. Competing models for movement along the Nacimiento fault system during the Laramide orogeny (sinistral slip versus reverse slip to emplace the Salinian terrane against the Nacimiento terrane) share the fact that the Peninsular Ranges forearc basin was not disrupted, as it lay south and southwest of the Nacimiento fault system. The northern edge of the Peninsular Ranges batholith formed a natural conduit for the fluvial system that deposited the Sespe Formation.

Abstract Department of Defense military land use of the desert southwest includes a wide spectrum of military weapons testing, force-on-force training, and various types of flight training. The desert southwest provides a critical asset for the U.S. military— open space. Installations in the desert southwest tend to be much larger than installations in other regions of the nation, with several exceeding 400,000 ha. This open-space asset has allowed the military to historically establish large training areas and ranges on installations and to define expansive air maneuver regions above these ranges and above the vast public lands of other agencies. It also offers critical training and testing areas that are analogs to similar worldwide environments where the military operates. Training and testing activities are conducted in the three-dimensional land and air space that replicates the modern battle space. Land and air space use is highly variable among installations depending on mission requirements. Natural resource management challenges include the large spatial extent of lands and air space under Department of Defense management, highly variable military land-use requirements, significant endangered species regulatory and conservation requirements, encroachment and Base Realignment and Closure requirements, competition for water resources, and climate change. Department of Defense natural resource managers attempt to meet these challenges through interagency cooperative agreements, integrated natural resource management plans, and Department of Defense sustainable range programs.

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 Jurassic Humboldt igneous complex in west-central Nevada consists of a comagmatic suite of intrusive and extrusive rocks and is tectonically intercalated with Triassic to Lower Jurassic shelf sequence and basinal successions. Its plutonic rocks include, from bottom to top, olivine-gabbro, melatroctolite, hornblende gabbro, microgabbro, and diorite transitional upward into quartz diorite, tonalite-granodiorite, and monzonite. Contacts between these plutonic subunits are commonly gradational, but mutual intrusive relations, characterized by the existence of brecciated and altered zones and xenoliths, are also common. Mafic to felsic plutonic rocks are cut by generally N–S to NW–SE striking dikes that form local dike swarms with one- and two-sided chilled margins. Dikes are composed of fine- to medium-grained rocks ranging in composition from basalts to andesites and feed into and/or are overlain by extrusive rocks consisting of lava flows intercalated with volcanic tuff and breccia. Lava flows at stratigraphically lower levels are more mafic and locally display pillow shapes reminiscent of submarine lava flows, whereas lava flows at higher levels are more felsic and are commonly interleaved with a fine-grained tuffaceous material. Volcanic rocks range in composition from basalts, basaltic andesites, andesites, to latites and dacites and mineralogically and texturally are similar to the dikes. The major element compositions of the analyzed rocks suggest relatively evolved basaltic magmas, whereas strongly incompatible trace element ratios (e.g., Ce/Ta) have high values typical of subduction related magmas. Lavas, dikes, and gabbros commonly display similar rare earth element (REE) patterns, although more felsic rocks are light rare earth (LREE) enriched, suggesting a cogenetic suite of rocks. These REE patterns are characteristic of basaltic andesites from volcanic arcs and suggest, coupled with field relations, that the rocks of the Humboldt complex might have evolved from subduction originated magmas in a volcanoplutonic arc setting. The tectonic nature of the contact between the Humboldt complex and the underlying Triassic-Jurassic sedimentary strata indicates that it was displaced eastward from its original arc environment following its igneous evolution. Both the Humboldt complex and the sedimentary strata are intruded at all structural levels by numerous northeast-striking dikes and dike swarms, which strongly altered and metasomatized their country rocks. These dike rocks are Miocene in age and have geochemical characteristics distinctly different from those of the Humboldt rocks. Based on regional correlations and paleogeographic reconstructions, we interpret the Humboldt igneous complex as the northern continuation of a Jurassic continental margin arc that extended from the Sonora Desert region in the south to northwestern Nevada and northern California in the north. This continental margin arc was a site of regional subsidence and crustal extension that accompanied magmatic activity and penecontemporaneous deposition of both arc and craton-derived detritus in submarine to subaerial conditions during much of the Jurassic. We emphasize that the coeval ensimatic arc terrane west and outboard of this extending continental margin arc might have represented a fringing oceanic realm that was subsequently collapsed into the outer continental margin of North America in Middle to Late Jurassic time.

We recognize two important paleogeographic links between the Klamath-Sierran segment and Mojave-Sonoran segment of the early Mesozoic arc in the southwest Cordillera: (1) both segments of the arc occupied an extensional graben-depression in Late Triassic to early Middle Jurassic time, and (2) the graben-depression acted as a long-lived (>40 m.y.) trap for Early and Middle Jurassic craton-derived quartz arenites deposited within both segments of the arc. The Early Mesozoic arc formed at a high angle to the truncated continental margin across cratonal, miogeoclinal, and eugeoclinal settings inherited from Paleozoic time. Intra-arc extension in the cratonal setting (Mojave-Sonoran Deserts) resulted in accumulation and preservation of thick terrestrial volcanic and sedimentary sequences. Intra-arc extension in the miogeoclinal setting (southern and central Sierra Nevada) resulted in deposition of very thick shallow marine and lesser deep marine successions. Intra-arc subsidence is also documented from deep marine to shallow marine successions in the eugeoclinal setting (northern Sierra and adjacent northwest Nevada). The Klamath-Sierran segment and Mojave-Sonoran segment of the arc evolved dissimilarly in late Middle Jurassic to Late Jurassic time. Contractional deformation documented in the Klamath-Sierran segment was not nearly as widespread or intense in the central Mojave Desert, and probably did not affect the southern Mojave to Sonoran Desert areas at all. Extensional tectonics, instead, continued to dominate the Mojave-Sonoran segment of the arc, probably due to continental rifting associated with the opening of the Gulf of Mexico.