ABSTRACT The Basin and Range hosted large pluvial lakes during the Pleistocene, which generally reached highstands following the Last Glacial Maximum and then regressed rapidly to near-modern levels. These lakes were large and deep enough to profoundly affect the crust through flexure; they filled basins formed by faults, and they locally modified pore pressure and groundwater conditions. A compilation of geochronologic constraints on paleoshorelines and paleoseismicity suggests temporal correlations between lake level and earthquake recurrence, with changes in earthquake rates as lakes regressed. In the northwestern Basin and Range, climatic and tectonic conditions differ from the rest of the province: The modern and glacial climate is/was cooler and wetter, glacial lakes were proportionally larger, and the crustal strain rate is lower. Numerous valleys host late Pleistocene and Holocene fault scarps and evidence of >M w 7 earthquakes in the last 15,000 yr. We compiled detailed lake hydrographs, timing of earthquakes and slip on faults, and other climatic and crustal data from Surprise Valley, Summer Lake, and the Fort Rock basin, along with additional data from other basins in the northwestern Basin and Range. We also present new mapping and topographic analysis of fault scarps that provides relative age constraints on the timing of slip events. Our results confirm temporal correlations, but the limited length of the paleoseismic record prevents definitive causation on the scale of the individual fault or lake basin. Taken together, however, data from all basins suggest that the faults in the northwestern Basin and Range could be acting as a system, with pluvial lake cycles affecting elastic strain accumulation and release across the region.

ABSTRACT The field trip guide describes nine stops that examine the mechanisms and timing of some of the abundant and often gigantic landslides that occur along the Winter Ridge–Slide Mountain escarpment in south-central Oregon. Subsidence of Summer Lake basin, situated in the northwestern Basin and Range province, has exposed a kilometer-thick Neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks in the bounding escarpment. Subsidence is accommodated on the 58-km-long Winter Rim fault system, a normal fault which is capable of producing M w ≈ 7 earthquakes with near-field, maximum horizontal acceleration approaching 1 g on the bedrock footwall. Gigantic rock slides cubic kilometers in volume scallop the southwestern portion of the escarpment, and their deposits run out as rock avalanches several kilometers onto the basin floor. Limit-equilibrium slope stability analyses support observations that these gigantic bedrock landslides initiate within the weak tuffaceous sedimentary rocks along shallow, east-dipping, planar failure surfaces one to two kilometers in length; are insensitive to groundwater fluctuations; and, are stable under static conditions. Strong ground motions appear requisite to trigger landsliding and are necessary to replicate the long, shallow failure surfaces. Landslide, colluvial, and lacustrine deposits on the hanging wall have undergone widespread post-emplacement deformation, which may involve large-scale seismogenic lateral spreading and flow sliding controlled by the saturated, fine-grained basin fill.

ABSTRACT The Searles Lake Formation in Searles Valley, southeastern California, represents deposition of the paleo–Owens River into a Pleistocene and Holocene pluvial terminal lake. A prior 32–10 ka estimated age for the upper part of the Searles Lake Formation relied on uncalibrated, conventional radiocarbon dates. We present accelerator mass spectrometer radiocarbon dates that indicate the base of the Searles Lake Formation at the Poison Canyon type section is 46 ka. That age is consistent with paleomagnetic data at Poison Canyon and the Tire Farm locality, which record high-latitude Southern Hemisphere virtual geomagnetic poles that we assign to the 41 ka Laschamp excursion. The presence of Searles Lake at 46–43 ka also is consistent with a Pacific storm track that extended south of 37.5°N at that time. At the head of Salt Wells Valley–Poison Canyon, sediments that we interpret as a Searles Lake highstand were radiocarbon dated at 14.1 ka.

Abstract Scientific investigations of Pleistocene pluvial lakes in the American West occurred in five phases. The pioneer phase prior to 1870 saw former lakes identified by missionary priests, fur trappers, military expeditions and railroad surveyors. The classic phase, between 1870 and 1920, linked initially with independent surveys and, after 1879, with the United States Geological Survey and with irrigation and mining ventures, saw most lakes identified and described by such worthies as Gilbert, Russell, Gale, Waring and Thompson. A consolidation phase from 1920 to 1955 provided synthesis and new data but, in the absence of age controls, saw much speculation about temporal links between pluvial lakes, glacial stages, and climate forcing. The initial dating phase between 1955 and 1980 saw radiocarbon dating applied to late Pleistocene lakes and their Holocene relics and successors. The integrative phase since 1980, supported by enhanced field, remote sensing, laboratory and dating techniques, has seen an array of issues involving pluvial lakes linked to changes in regional ecology and global climate. In the above sequence, progress from one phase to the next reflected changes in the intellectual climate and advances in scientific methods. Today, we reflect on the episodic but cumulative increase in knowledge about late Pleistocene pluvial lakes, especially for Lake Bonneville, Lake Lahontan and the eastern California lake cascade. The record of earlier Pleistocene lakes, in some cases successors to Miocene and Pliocene lakes, is less certain because of deformation and erosion or burial. Continuing challenges involve evaluation of the Pleistocene lake record as a whole in the context of late Cenozoic tectonic and climate change, and of contemporary environmental and water-resource issues.

ABSTRACT Sedimentary records were analyzed from three lakes in the Ruby Mountains and East Humboldt Range of northeastern Nevada. Lakes are rare in the arid Great Basin, and these represent the highest-elevation lacustrine records from this region. The three cores cover overlapping time intervals: One, from a lake located just beyond a moraine, is interpreted to represent the Last Glacial Maximum, extending back to 26 cal ka; another extends to deglaciation ca. 14 cal ka; and the third extends to deposition of the Mazama ash, ca. 7.7 cal ka. Multiproxy analysis focused on measurements of bulk density, organic matter content, C:N ratio, biogenic silica abundance, and grain-size distribution. Depth-age models were developed using optically stimulated luminescence (OSL) dating, along with accelerator mass spectrometry (AMS) 14 C dating of terrestrial macrofossils (wood and conifer needles), charcoal, and pollen concentrates (for deep sediment in one lake). Collectively, the three lakes record a series of discrete intervals spanning an unusually long stretch of time. These include the local Last Glacial Maximum (26.0–18.5 cal ka), local deglaciation (18.5–13.8 cal ka), the onset of biologic productivity (13.8–11.3 cal ka), early Holocene aridity (11.3–7.8 cal ka), deposition and reworking of the Mazama ash (7.8–5.5 cal ka), a neopluvial interval (5.5–3.8 cal ka), a variable late Holocene climate (3.8–0.25 cal ka), and a latest Holocene productivity spike (250 yr B.P. to the present) that may be anthropogenic. Data from all three lakes are presented, and the collective record of climate and environmental change for the Ruby Mountains and East Humboldt Range is compared with other paleorecords from the Great Basin.

ABSTRACT A large part of the northwestern United States has undergone extensive late Cenozoic magmatic activity yielding one of the great continental volcanic provinces on Earth. Within this broader area lies the High Lava Plains province, the focus of this field guide. For our purposes, the High Lava Plains is a middle and late Cenozoic volcanic upland, contiguous with and gradational into the Basin and Range province to the south. The High Lava Plains province of southeastern Oregon is characterized by thin, widespread Miocene-Pleistocene lava flows of primitive basalt and a belt of silicic eruptive centers. The rhyolitic rocks generally are successively younger to the northwest, describing a mirror image to the basalt plateau and rhyolite age progression of the Snake River Plain. The High Lava Plains is associated with a zone of numerous, small northwest-striking faults and lies at the northern limit of major Basin and Range normal faults. The abundant late Cenozoic bimodal volcanism occupies an enigmatic intracontinental tectonic setting affected by Cascadia subduction, Basin and Range extension, the Yellowstone plume, and lithospheric topography at the edge of the North American craton. The purpose of this field trip is to focus on the late Cenozoic lithospheric evolution of this region, through the lens of the High Lava Plains, by considering structural, geophysical, petrologic, and temporal perspectives. A grand tour southeast from Bend to Valley Falls, north to Burns, and then east to Venator, Oregon, takes participants from the eastern edge of the Cascade volcanic arc, across several basins and ranges in eastern Oregon, and onto the volcanic plateau of the High Lava Plains. Day 1 provides an overview of Newberry Volcano and the western edge of Basin and Range, including the Ana River and Summer Lake fault zones. On Day 2, the early magmatic and extensional history of the region is explored along the Abert Rim range-front fault. Participants are introduced to the bimodal volcanism within the High Lava Plains, with focus on the Harney Basin and Rattlesnake ignimbrite event. An evening session will highlight geophysical results from the High Lava Plains, including new data from one of the largest active-source seismic experiments to be conducted in North America. Day 3 activities examine early bimodal volcanic history of the eastern High Lava Plains and the late Miocene and Pliocene subsidence history on the east edge of the Harney Basin east of Burns, Oregon.