Basin and Range

Uncertainty surrounds the fate of the orogenic plateau in what is now the Great Basin in western Utah and Nevada, which resulted from the Mesozoic and earliest Cenozoic contractile deformations and crustal thickening. Although there is some consensus regarding the gravitational collapse of the plateau by extensional faulting and consequent crustal thinning, whether or not the plateau existed during the middle Cenozoic Great Basin ignimbrite flareup – one of the grandest expressions of continental volcanism in the geologic record – had remained in doubt. We use compositions of contemporaneous calc-alkaline lava flows as well as configurations of the ignimbrite sheets to show that the Great Basin area during the middle Cenozoic was a relatively smooth plateau underlain by unusually thick crust. We compare analyses of 376 intermediate-composition lava flows in the Great Basin that were extruded at 42–17 Ma with compositions of >6000 analyses of the late Cenozoic lava flows in continental volcanic arcs that correlate roughly with known crustal thickness. This comparison indicates that the middle Cenozoic Great Basin crust was much thicker than the present ca. 30 km thickness, likely as much as 60–70 km. If isostatic equilibrium prevailed, this unusually thick continental crust must have supported high topography. This high terrain in SE Nevada and SW Utah was progressively smoothed as successive ignimbrite outflow sheets were emplaced over areas currently as much as tens of thousands of square kilometres to aggregate thicknesses of as much as hundreds of metres. The generally small between-site variations in the palaeomagnetic directions of individual sheets lend further support for a relatively smooth landscape over which the sheets were draped. We conclude that during the middle Cenozoic, especially towards the close of the ignimbrite flareup, this Great Basin area was a relatively flat plateau, and because it was also high in elevation, we refer to it as an Altiplano. It was not unlike the present-day Altiplano-Puna in the tectonically similar central Andes, where an ignimbrite flareup comparable to that in the Great Basin occurred at ca. 9–3 Ma. Outflow ignimbrite sheets that were deposited from 35 to 23 Ma on the progressively smoothed Altiplano in south-eastern Nevada were derived from source calderas to the west. Of the 12 major sheets from seven sources, nine are distributed unevenly east of their sources while the remaining three sheets are spread about as far east as west of their sources. This eccentricity of sources near the western margin of 75% of the sheets indicates the existence of a NS-trending topographic barrier in central Nevada that restricted westward dispersal of ash flows. In a symmetric manner, eastward dispersal of ash flows from sources farther west seemed to have been impeded by this same topographic barrier. The westward dispersal was controlled in part by westward-draining stream valleys incised in the sloping flank of the Great Basin Altiplano in western Nevada and adjacent California; at least one of these ash flows travelled as far west as the western foothills of the Sierra Nevada. The nature and origin of the implied topographic barrier are uncertain. It is possible that heavy orographic precipitation on the western slope of the Altiplano and consequent focused denudation and isostatic uplift created a NS-trending topographic high at the crest of the western slope and facing the smoothed Altiplano to the east. The barrier also lies near and essentially parallel to the buried western edge of the Precambrian basement and to a zone of thermal-diapiric domes that were spawned in thickened crust as the basement edge was overrun by late Palaeozoic–Mesozoic thrust sheets.

Apatite fission-track (FT) and (U–Th)/He analyses are used to constrain the low-temperature thermal history of the San Gabriel and San Bernardino Mountains (SGM and SBM), which are part of the southern California Transverse Ranges. FT ages from 33 SGM samples range from 3 to 64 Ma. Helium ages, ranging from 3 to 43 Ma, were obtained from 13 of these samples: all of the He ages are the same or younger than their respective FT ages. FT ages from 10 SBM samples were older, ranging from 45 to 90 Ma. The FT and ...

The Laramide orogeny is a classic yet controversial mountain-building event that resulted , in the southwest United States, in uplifts , sedimentation, and magmatism that can be used to constrain the onset of this event in the region and expand our knowledge of Late Cretaceous to Paleogene tectonism. The McRae Formation marks the onset of depo-sition in the Laramide Love Ranch Basin, which was located to the northeast of the west-northwest–trending coeval Rio Grande uplift in south-central New Mexico, but its age is not well constrained. A previously published late Maastrichtian age for the McRae Formation was based on the presence of dinosaur bones in the upper of two members of the formation. We obtained new U-Pb dates from one dacite clast and three ash-fall tuffs from the lower Jose Creek Member and from one ash-fall tuff from the lower part of the overlying Hall Lake Member of the McRae Formation. The clast yielded a date of 75.0 ± 1.1 Ma, whereas the ages of the tuffs, in ascending stratigraphic order, are 74.9 ± 0.7 Ma, 74.7 ± 0.6 Ma, 75.2 ± 1.3 Ma, and 73.2 ± 0.7 Ma. These dates indicate that the onset of Laramide deposition in the Love Ranch Basin must have occurred earlier, in late Campanian time, similar to deposition in the Laramide Ringbone Basin in southwestern New Mexico. In addition, U-Pb zircon dates of 75.7 ± 1.3 Ma and 75.0 ± 2.8 Ma were obtained on the Twin Peaks stock and on a da-cite sill, respectively, in the Burro Mountains of southwestern New Mexico. These dates are similar to those of other Laramide arc magmatic centers in southern New Mexico, which have a limited range of ages from 75 to 70 Ma, including the Hidalgo Formation in the Little Hatchet Mountains, the Silver City–Pinos Altos region, and the Copper Flat porphyry system. These new and previously published dates indicate that during the onset of Laramide deformation in southwestern and south-central New Mexico, the angle of subduction of the Farallon plate may have been steep enough to allow partial melting of an asthenospheric wedge, resulting in arc magmatism far inboard of the trench.

Most models for the origin and evolution of the Snake River Plain (SRP) volcanic province focus on the central role of the Yellowstone hot spot and its effects on the lithosphere of North America in response to plate mo- tions relative to this hot spot. Movement of the North American plate over the Yellowstone hot spot has resulted in a

Lichens form a conspicuous and diverse biota in the Caliente Field Office (CFO). Their varied colors can be seen on the cliffs along the highway through Rainbow Canyon and on boulders and junipers along mountain biking trails. Recently a trail in Cedar City, Utah was even named for its lichens (Miller 2015). Prior to surveys by the authors and colleagues in Great Basin National Park (Carter et al. 2019), no full-scale inventory of lichens had been conducted in the Great Basin Desert. Floristic lichen inventories have been conducted in the Mojave (Knight et al. 2002, Sweat et al. 2004, Jackson et al. 2005, Knudsen et al. 2013, Proulx and St. Clair 2013, Proulx et al. 2016) but this is the first study to survey the lichen flora of the Mojave and Great Basin along an ecotonal gradient. This report and anno- tated checklist are the product of three years of field work carried out in the CFO from 2016 to 2018. Specimens from these surveys are deposited at the Brigham Young University Herbarium of Non-Vas- cular Cryptogams (BRY-C).

This work establishes a robust baseline of lichen diversity for the CFO. A total of 361 taxa of lichens, 57 lichenicolous fungi and 3 allied fungi are reported from the field office. Twelve species are new reports for North America (Arthonia hertelii, Ascochyta candelariellicola, Caloplaca teicholyta, Cercidospora melanophthalmae, Endococcus karlstadtensis, Lawalreea lecanorae, Lichenochora epinashii, Lichenostigma gracilis, L. triseptatum, Megaspora rimisorediata, Polycoccum evae and Psorotichia numidella var. flageyna). An additional 56 species of lichens and 46 species of licheni- colous fungi, many probably new to science, are discussed and require further study. This report pro- vides an annotated checklist of the currently documented lichen flora of the Caliente Field Office and Basin and Range National Monument, including distribution, habitat, substrate preferences and conser- vation status for each species. Rare species and habitats are discussed, and detailed notes are provided for species ranked as critically imperiled (S1 rank). This report is accompanied by a field guide that includes photographs and field identification notes for over 180 common lichen species of the Great Basin Desert.

The highest diversity of lichens in the CFO is found on high-elevation mountain peaks, especially Highland Peak, which also hosts a diverse, closed canopy, mesic forest of White Fir and actively recruiting Bristlecone Pines. While high-elevation sites are per unit area the richest, Pinyon–Juniper woodlands host the most diverse assemblage of lichen species. The northeastern portion of the Mojave hosts an unusual species assemblage, potentially owing to its location in the ecotone of three floristic regions: Mojave, Great Basin and Colorado Plateau.
As evidence mounts demonstrating that the southern Great Basin biota is at high risk of climate change impacts (Bradley 2010, Still and Richardson 2015), of primary interest is whether lichens can be utilized as early-warning bioindicators of climate change (Root et al. 2014). Candidate bioindicator species are presented and preliminary species distribution and local conservation status is given for each species. Further research is required in order to build upon this robust baseline and create a suite of tools useful for land management, including field-testing monitoring methods, and implementing long-term monitoring plots for candidate bioindicator species.

Geologic mapping, supported by 40Ar/39Ar and U-Pb geochronology and geochemistry of sedimentary and volcanic rocks, reveals the details of the Cenozoic depositional and tectonic history of the eastern Piñon Range and central Huntington Valley in the north-central Basin and Range Province, Nevada (USA). Cretaceous to Miocene supracrustal successions were studied in detail in order to compare the geologic evolution of the upper crust near the Ruby Mountains–East Humboldt Range (RMEH) metamorphic core complex (MCC) with the magmatic, metamorphic, and deformational history of the deep crust in the developing MCC. During the well-documented Late Cretaceous– Oligocene history of partial melting and infrastructure development within the RMEH, surface deposits in Huntington Valley reflect general tectonic quiescence, with evidence for the development of the shallow Elko Basin, minor extension, and eruption of southward-younging ignimbrite flare-up volcanism. Thin, discontinuous successions ...