ABSTRACT This guide presents an eight-hour, in-person tour of intersecting geologic and human history in western Nevada, USA. A 25 megaton phreatomagmatic blast created a mile-wide (1.6-km-wide) maar, now filled by Soda Lake. The magnitude 7 Dixie Valley earthquake ripped along more than 45 km of the Stillwater Range front in 1954. The 12 kiloton Shoal nuclear test in 1963 created a 50-m-wide cavity in solid granite.

ABSTRACT Shorelines formed by terminal lakes record past changes in regional moisture budgets. In the western Great Basin of North America, winter precipitation accounts for nearly half of the annual total and is well correlated with northeast Pacific storm track activity and moisture transport. We evaluated these relationships and found that historical precipitation between 1910 and 2012 was better correlated to moisture transport (0.78, p < 0.01) than to storm track activity (0.54, p < 0.01) because moisture transport better captures dynamics associated with the Sierra Nevada rain shadow. We derived modern analogs of enhanced and reduced storm track activity and moisture transport from reanalysis products and used associated winter precipitation anomalies with these analogs as inputs to a coupled water balance and lake evaporation model of the Walker Lake basin. Simulated lake-level responses were compared with a radiocarbon-dated lakeshore chronology spanning the past 3700 yr. Wet analogs developed from winters in the 90th and 75th percentiles for storminess and moisture transport produced lake levels that exceeded estimated late Holocene highstands by 50 m. Dry analogs (10th and 25th percentiles) produced lake levels corresponding to Medieval megadrought lowstands. The twentieth century is shown to be as wet as any century in the past 3700 yr. Our results demonstrate the sensitivity of terminal lakes to winter season circulations and highlight the value of using moisture transport as a predictor of cool season precipitation and to evaluate how past or future changes in regional circulations will influence the water balance of dryland regions.

ABSTRACT This paper summarizes the hydrological variability in eastern California (central Sierra Nevada) for the past 3000 yr based on three distinct paleoclimate proxies, δ 18 O, total inorganic carbon (TIC), and magnetic susceptibility (chi). These proxies, which are recorded in lake sediments of Pyramid Lake and Walker Lake, Nevada, and Mono Lake and Owens Lake, California, indicate lake-level changes that are mostly due to variations in Sierra Nevada snowpack and rainfall. We evaluated lake-level changes in the four Great Basin lake systems with regard to sediment-core locations and lake-basin morphologies, to the extent that these two factors influence the paleoclimate proxy records. We documented the strengths and weaknesses of each proxy and argue that a systematic study of all three proxies together significantly enhances our ability to characterize the regional pattern, chronology, and resolution of hydrological variability. We used paleomagnetic secular variation (PSV) to develop paleomagnetic chronostratigraphies for all four lakes. We previously published PSV records for three of the lakes (Mono, Owens, Pyramid) and developed a new PSV record herein for Walker Lake. We show that our PSV chronostratigraphies are almost identical to previously established radiocarbon-based chronologies, but that there are differences of 20–200 yr in individual age records. In addition, we used eight of the PSV inclination features to provide isochrons that permit exacting correlations between lake records. We also evaluated the temporal resolution of our proxies. Most can document decadal-scale variability over the past 1000 yr, multidecadal-scale variability for the past 2000 yr, and centennial-scale variability between 2000 and 3000 yr ago. Comparisons among our proxies show a strong coherence in the pattern of lake-level variability for all four lakes. Pyramid Lake and Walker Lake have the longest and highest-resolution records. The δ 18 O and TIC records yield the same pattern of lake-level variability; however, TIC may allow a somewhat higher-frequency resolution. It is not clear, however, which proxy best estimates the absolute amplitude of lake-level variability. Chi is the only available proxy that records lake-level variability in all four lakes prior to 2000 yr ago, and it shows consistent evidence of a large multicentennial period of drought. TIC, chi, and δ 18 O are integrative proxies in that they display the cumulative record of hydrologic variability in each lake basin. Tree-ring estimations of hydrological variability, by contrast, are incremental proxies that estimate annual variability. We compared our integrated proxies with tree-ring incremental proxies and found a strong correspondence among the two groups of proxies if the tree-ring proxies are smoothed to decadal or multidecadal averages. Together, these results indicate a common pattern of wet/dry variability in California (Sierra Nevada snowpack/rainfall) extending from a few years (notable only in the tree-ring data) to perhaps 1000 yr. Notable hydrologic variability has occurred at all time scales and should continue into the future.

Abstract Porphyry Cu deposits, the major source of many metals currently utilized by modern civilization, form via the interplay between magmatism, tectonism, and hydrothermal circulation at depths ranging from about 2 to as much as 10 km. These crustal-scale features require the deep crustal formation of a hydrous and oxidized magma, magma ascent along extant permeability fabrics to create an upper crustal convecting magma chamber, volatile saturation of the magma chamber, and finally the episodic escape of an ore-forming hydrothermal fluid and a phenocryst-rich magma into the shallow crustal environment. Three general fluid regimes are involved in the formation of porphyry Cu deposits. These include the deep magma ± volatile zone at lithostatic pressure, an overlying zone of transiently ascending magmatic-hydrothermal fluids that breaches ductile rock at temperatures ~700° to 400°C, and an upper brittle zone at temperatures <400°C characterized by hydrostatically pressured nonmagmatic and magmatic fluids. Critical structural steps include the formation of the magma chamber, magmatic vapor exsolution and collection of a hydrothermal fluid in cupola(s), and episodic hydrofracturing of the chamber roof in order to create the permeability that allows a hydrothermal fluid to rise along with a phenocryst-bearing magma. The interplay between stress produced by far-field tectonics and stress produced by buoyant magma and magmatic hydrothermal fluid creates the fracture permeability that extends from the cupola through an overlying ductile zone where temperatures exceed ~400°C into an overlying brittle zone where temperatures are less than ~400°C. As a consequence, during each fluid escape and magma intrusion event, the rising hydrothermal fluid ascends, depressurizes, cools, reacts with wall rocks, and precipitates quartz plus sulfide minerals, which seal the permeability fabric. A consistent vein geometry present in porphyry Cu deposits worldwide is formed by steeply dipping veins that have mutually crosscutting orientations. Two general orientations are common. The principal vein orientation generally consists of closely spaced sheeted veins with orientations reflecting the far-field stress. Subsidiary veins may be orthogonal to the main vein orientation as radial or concentric veins that reflect magma expansion and extensional strain in the wall rocks as they are stretched by ascent of the buoyant magma and fluids. Episodic magmatic-hydrothermal fluid-driven hydrofracturing creates permeability that is commonly destroyed, as well as locally enhanced, by vein and wall-rock mineral precipitation or dissolution and by wall-rock hydrothermal alteration, depending upon fluid and host-rock compositions. The pulsing character of porphyry Cu magmatic-hydrothermal systems, in part produced by permeability creation and destruction, creates polyphase overprinted intrusive complexes, associated vein networks, and alteration mineralogy that reflects temporal temperature fluctuations beginning at magma temperatures but continuing to low temperatures. Temperature oscillations locally allow external nonmagmatic fluids to access principally the marginal areas but also in some cases the center of the porphyry Cu ore zone at ~<400°C between porphyry dike emplacement events. Over time, the upper part of the source magma chamber at depth cools and crystallizes downward and is accompanied by diminishing magmatic fluid input upward, leading to cooling and isothermal collapse of the porphyry system. Cooling permits the access of external circulating groundwater into the waning magmatic-hydrothermal plume. Magmatic-hydrothermal fluids dominate at temperatures >400°C at pressures transient between lithostatic and superhydrostatic. The external, nonmagmatic saline formation waters or meteoric waters dominate the surrounding and overlying brittle crust at temperatures <400°C at hydrostatic pressures, except where they may mix with buoyantly rising magmatic-derived fluids. Exhumation requires substantial topographic relief, precipitation, and time (typically >1 m.y.) and may enhance overprinted relationships and telescope low-temperature on high-temperature hydrothermal alteration assemblages. Synmineral propagation of faults into or out of a porphyry Cu hydrothermal system in the brittle regime at <400°C can provide an escape channel through which a metalliferous fluid may depart, potentially to form lateral quartz-pyrite veins, overprinted polymetallic Cordilleran lode veins, or an epithermal precious metal-bearing deposit at shallow crustal depths.