A composite section of eight Grande Ronde Basalt flows delineates the margin of the Columbia River Basalt Group on this portion of the eastern flank of the Cascade Range. The Grande Ronde Basalt flows belong to the following units (in descending stratigraphic order): Sentinel Bluffs Member (Basalt of Museum 2 and Museum 1; Basalt of Stember Creek; and upper and lower flows of the Basalt of McCoy Canyon), Ortley member (informal), Grouse Creek member (informal “Meeks Table” flow), and Wapshilla Ridge Member. All these Grande Ronde Basalt flows display similar intraflow structures (cooling joint patterns) and lithology, but they are separable by chemical compositions (i.e., TiO 2 , MgO, P 2 O 5 , Cr, Ba, and Zr). Individual Grande Ronde Basalt flows can range in thickness from 8 to 180 m, with the maximum total thickness of the Grande Ronde Basalt section being 555 m. As the Grande Ronde Basalt flows advanced into the map area, they covered plains, filled stream-cut valleys and canyons up to 160 m deep, and surrounded extinct volcanoes 750 m tall. In post–Grande Ronde Basalt time, the Grande Ronde Basalt flows were deformed into a series of ENE-striking anticlines, synclines, and associated faults that define this portion of the Yakima Fold Belt. During this same time, transpressional deformation activity increased folding and thrust faulting in the Cle Elum–Wallula Lineament, a structural segment of the Olympic-Wallowa Lineament. In addition, series of NNW-striking, dextral strike-slip and normal faults were developed with displacements up to 4.5 km on the strike-slip faults and 1 km on the normal faults. The N-striking Goat Creek and NW-striking Indian Flat and Devils Slide faults merge with the White River fault to the west. These faults, along with the E-NE–striking Bethel Ridge anticline and NNW-striking Cleman Mountain anticline, form the major structures in this area.

The Lovejoy basalt represents the largest eruptive unit identified in California, and its age, volume, and chemistry indicate a genetic affinity with the Columbia River Basalt Group and its associated mantle-plume activity. Recent field mapping, geochemical analyses, and radiometric dating suggest that the Lovejoy basalt erupted during the mid-Miocene from a fissure at Thompson Peak, south of Susanville, California. The Lovejoy flowed through a paleovalley across the northern end of the Sierra Nevada to the Sacramento Valley, a distance of 240 km. Approximately 150 km 3 of basalt were erupted over a span of only a few centuries. Our age dates for the Lovejoy basalt cluster are near 15.4 Ma and suggest that it is coeval with the 16.1–15.0 Ma Imnaha and Grande Ronde flows of the Columbia River Basalt Group. Our new mapping and age dating support the interpretation that the Lovejoy basalt erupted in a forearc position relative to the ancestral Cascades arc, in contrast with the Columbia River Basalt Group, which erupted in a backarc position. The arc front shifted trenchward into the Sierran block after 15.4 Ma. However, the Lovejoy basalt appears to be unrelated to volcanism of the predominantly calc-alkaline Cascade arc; instead, the Lovejoy is broadly tholeiitic, with trace-element characteristics similar to the Columbia River Basalt Group. Association of the Lovejoy basalt with mid-Miocene flood basalt volcanism has considerable implications for North American plume dynamics and strengthens the thermal “point source” explanation, as provided by the mantle-plume hypothesis. Alternatives to the plume hypothesis usually call upon lithosphere-scale cracks to control magmatic migrations in the Yellowstone–Columbia River basalt region. However, it is difficult to imagine a lithosphere-scale flaw that crosses Precambrian basement and accreted terranes to reach the Sierra microplate, where the Lovejoy is located. Therefore, we propose that the Lovejoy represents a rapid migration of plume-head material, at ~20 cm/yr to the southwest, a direction not previously recognized.

Recent heat-flow studies in the Western United States, especially the Cordillera, are discussed and summarized and a new heat-flow map is presented. The major features of the map have already been described: high heat flow in the Northern Rocky Mountains, Columbia Plateau, High Cascades, and Basin and Range provinces (the Cordilleran thermal anomaly zone); high heat flow along the San Andreas-Gulf of California transform system; high heat flow in the Southern Rocky Mountains; moderate heat flow in part of the Colorado Plateau; and low heat flow along the Sierra Nevada and the coastal provinces of Oregon and Washington. In addition, much detail is apparent in the Cordilleran thermal anomaly zone. Very high heat flow (greater than 2.5 HFU) is found in the Cascades, and part of the Brothers fault zone in Oregon, part of the Snake River Plain in Idaho, Yellowstone in Wyoming, the Battle Mountain “high” in Nevada, the Geysers area and the Imperial Valley in California, and the Rio Grande rift in New Mexico. Areas of low heat flow are associated with part of the Columbia Basin in Washington, the eastern part of the Snake River Plain in Idaho, and the Eureka “low” in Nevada. The heat-flow map is very complicated because it includes the effects of crust and mantle radioactivity and magmatic heat sources, regional hydrology, and thermal refraction due to structurally related thermal conductivity contrasts. In the active tectonic areas there are energy losses associated with volcanism, intrusion, and hydrothermal convection. Such losses may not be measured in a typical heat-flow survey. These losses are evaluated and shown to be a significant part of the total heat flow in many areas. Heat flow is compared to the geographical distribution of volcanism, plutonism, hydrothermal activity, and average topography. The regions of high heat flow correlate well with the areas of Cenozoic volcanism, plutonism, and thermal spring activity. The lack of a one-to-one correlation of areas of active plutonism to regional concentrations of thermal springs is shown. The relationship of topography to heat flow is complicated, and it appears that in general the average composition of the crust changes during a major continental thermal event so that the relationships between topography and heat flow may change during the evolution of the thermal event. Detailed heat-flow interpretation relies on the relationship between heat flow ( Q ) and radioactive heat generation ( A ). The Basin and Range plot of Q versus A applies only to areas where the most recent volcanic event is older than 17 m.y. In areas of younger thermal events, the reduced heat flow (that is, the heat flow measured at the surface less heat production from crustal radioactive sources) is generally higher than 1.4 HFU, and hydrothermal convection and volcanism are major mechanisms involved in the total energy transfer. Furthermore, transitions between thermal provinces are narrow (generally less than 20 km); therefore, the sources directly responsible for the surface-measured variations in heat flow must be in the crust. Thermal boundaries also usually appear in areas of contemporary seismicity. As a synthesis of the discussion, a map of energy release (as opposed to heat flow measured at the surface) and a simple model of a Cordilleran thermal event are presented. The generalized map of energy release shows total thermal energy transfer from the mantle, including nonconductive energy losses. This map shows highest heat flow along the eastern and western borders of the Cordilleran thermal anomaly zone and a smoother variation of heat flow within the zone than does the heat-flow map. During a continental thermal event typical of the Cordilleran thermal anomaly zone, which may be infinitely more varied in intensity and duration than an oceanic spreading event, the conductive heat flow will be only a part of the total energy loss, and the dominant heat-transfer mechanisms change with time over the life of the event. For the areas where a thermal event is young, volcanism and plutonism may be the prime energy-loss mechanisms, but in spite of the high overall energy loss, large areas of young volcanism may have very low conductive heat flow because of the dominant effect of hydrothermal convection as a mechanism for plutonic energy loss. As the thermal event decays, regional hydrothermal convection and heat conduction become the dominant heat-transfer mechanisms. For areas where thermal events are older than 17 m.y., heat conduction is the dominant heat-transfer mechanism although hydrothermal convection may be locally significant.

We used tephrochronology for upper Neogene deposits in the Española Basin and the adjoining Jemez Mountains volcanic field in the Rio Grande rift, northern New Mexico, to correlate key tephra strata in the study area, identify the sources for many of these tephra, and refine the maximum age of an important stratigraphic unit. Electron-microprobe analyses on volcanic glass separated from 146 pumice-fall, ash-fall, and ash-flow tephra units and layers show that they are mainly rhyolites and dacites. Jemez Mountains tephra units range in age from Miocene to Quaternary. From oldest to youngest these are: (1) the Canovas Canyon Rhyolite and the Paliza Canyon Formation of the lower Keres Group (ca. <12.4–7.4 Ma); (2) the Peralta Tuff Member of the Bearhead Rhyolite of the upper Keres Group (ca. 6.96–6.76 Ma); (3) Puye Formation tephra layers (ca. 5.3–1.75 Ma); (4) the informal San Diego Canyon ignimbrites (ca. 1.87–1.84 Ma); (5) the Otowi Member of the Bandelier Tuff, including the basal Guaje Pumice Bed (both ca. 1.68–1.61 Ma); (6) the Cerro Toledo Rhyolite (ca. 1.59–1.22 Ma); (7) the Tshirege Member of the Bandelier Tuff, including the basal Tsankawi Pumice Bed (both ca. 1.25–1.21 Ma); and (8) the El Cajete Member of the Valles Rhyolite (ca. 60–50 ka). The Paliza Canyon volcaniclastic rocks are chemically variable; they range in composition from dacite to dacitic andesite and differ in chemical composition from the younger units. The Bearhead Rhyolite is highly evolved and can be readily distinguished from the younger units. Tuffs in the Puye Formation are dacitic rather than rhyolitic in composition, and their glasses contain significantly higher Fe, Ca, Mg, and Ti, and lower contents of Si, Na, and K. We conclude that the Puye is entirely younger than the Bearhead Rhyolite and that its minimum age is ca. 1.75 Ma. The San Diego Canyon ignimbrites can be distinguished from all members of the overlying Bandelier Tuff on the basis of Fe and Ca. The Cerro Toledo tephra layers are readily distinguishable from the overlying and underlying units of the Bandelier Tuff primarily by lower Fe and Ca contents. The Tshirege and Otowi Members of the Bandelier Tuff are difficult to distinguish from each other on the basis of electron-microprobe analysis of the volcanic glass; the Tshirege Member contains on average more Fe than the Otowi Member. Tephra layers in the Española Basin that correlate to the Lava Creek B ash bed (ca. 640 ka) and the Nomlaki Tuff (Member of the Tuscan and Tehama Formations, ca. 3.3 Ma) indicate how far tephra from these eruptions traveled (the Yellowstone caldera of northwestern Wyoming and the southern Cascade Range of northern California, respectively). Tephra layers of Miocene age (16–10 Ma) sampled from the Tesuque Formation of the Santa Fe Group in the Española Basin correlate to sources associated with the southern Nevada volcanic field (Timber Mountain, Black Mountain, and Oasis Valley calderas) and the Snake River Plain–Yellowstone hot spot track in Idaho and northwestern Wyoming. Correlations of these tephra layers across the Santa Clara fault provide timelines through various stratigraphic sections despite differences in stratigraphy and lithology. We use tephra correlations to constrain the age of the base of the Ojo Caliente Sandstone Member of the Tesuque Formation to 13.5–13.3 Ma.

Abstract Clarence Edward Dutton (1841–1912), a Yale University graduate, served in the Civil War and, as a captain in the United States Army, was detailed for duty with John Wesley Powell’s US Geographical and Geological Survey of the Rocky Mountain Region and, later, the US Geological Survey. He spent 10 years in geological research in the Colorado Plateau region, studying its geomorphological and stratigraphic evolution, and coined the term ‘isostasy’. His monograph Tertiary History of the Grand Cañon District is renowned for its particular charm, with sublime landscape descriptions and superb illustrations. Dutton later undertook studies in volcanic geology in Hawaii, the Cascades and on the Colorado Plateau. Dutton studied the effects of the catastrophic Charleston earthquake of 31 August 1886 and later engaged in irrigation research in the American West. Dutton critically reviewed many major geological concepts, including the causes of volcanism, earthquakes and, especially, orogeny. His pioneering work on stratigraphy, geomorphology, isostasy and seismology place him at the forefront of nineteenth century researchers and reflects his ability to generalize from field observations and formulate principles that constitute explanations of the observed facts.

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.