ABSTRACT Steens Mountain, a fault-block in the northern Basin and Range Province, rises 1.7 km above flanking basins and drives hydrologic systems that include hot springs, fresh-water streams, and cold artesian wells in the Alvord Valley. It also feeds freshwater streams, desert wetlands, and shallow fresh-water and alkali lakes in the Harney Basin. Steens Mountain melt water from the winter snow pack partitions to surface-water and groundwater systems. How the composition of these fluids evolve along the various flow paths as a result of differences in the geology, interaction with geother-mal aquifers, surface storage time, degree of evaporation, and biology will be examined. Deep-seated flow paths feed Alvord Valley hot springs, which discharge to the east, in the rain shadow of Steens Mountain. The largest of these hot spring systems— Borax Lake—along with features at Mickey Hot Springs, offer ample opportunity to investigate how biosignatures form and become preserved in hydrothermally precipitated sinter deposits. Surface water moving off the westward-dipping slope of Steens Mountain passes through wetland environments to Malheur Lake in Harney Basin. This key point along the Pacific flyway provides wonderful wildlife viewing and the chance to ponder the impacts of biology on lake chemistry. Finally, we will visit the saline-alkaline Harney Lake, the terminal sump for the water moving through Malheur Lake and all of the nearly 40,000 km 2 Harney Basin. At this locale, the focus will be on the influence of evaporative processes on water composition.

Field, petrographic, geochemical, and limited chronologic information allow for construction of a composite stratigraphic section for the approximately 1-km-thick exposure of basaltic lavas in the Pueblo Mountains region of the Oregon Plateau. Comparison of the Pueblo basalts with those of Steens Mountain and the Columbia Plateau suggests that these flood-basalts were derived from a common mid-Miocene magmatic event, possibly marking the initiation of back-arc spreading along the western margin of the Wyoming craton. Geochemical and Sr-isotopic variations within the Pueblo basalts and, more specifically, vertical variations within the Pueblo composite section, define two distinct groups of basalt and three phases of evolution. Within-group variations are modeled by low-pressure fractional crystallization, whereas the between-group variations are attributed primarily to different degrees of interaction between crustal materials and ascending Pueblo primary magmas during the three distinct phases of volcanism.

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.

Abstract An appraisal of Oregon's prospects as a possible future oil province requires consideration of two widely separated regions where thick sections of unmetamorphosed marine sediments occur: (1) the Coast Range Province of Tertiary rocks, and (2) the Central Oregon Province of Mesozoic and Paleozoic sediments. Between these two regions is the north-south-trending Cascade Range of late Tertiary volcanics. The remainder of the state, except for the metamorphic and plutonic masses of the Klamath Mountains in the southwest, and the Blue-Wallowa Mountains in the northeast, is covered by a thick section of volcanic rocks, with interbedded continental sediments, of Tertiary to Recent age, which conceal any pre-Tertiary marine sediments which may be present. Two parts of this vast concealed area, the Harney basin in southeastern Oregon, and the Vale-Ontario area in central easternmost Oregon, have attracted considerable attention from wildcatters and promoters, and are also discussed briefly for that reason.

Seismicity, fault-plane solutions, and Cenozoic geology are used to infer contemporary west to northwest extension and components of lateral slip between subplates of the western North American plate. Seismicity of the western interior of the Cordillera is characterized by earthquakes that occur in broad zones, up to 150 km wide, in the Nevada and the Intermountain seismic zones. Epicenters are scattered and when accurately located many do not coincide with mapped faults. Focal depths are shallow and seldom exceed 20 km. Secondary seismicity occurs around the margins of the Colorado Plateau, in central Idaho, and in eastern Oregon and Washington. The contemporary strain pattern of the Western United States, as interpreted from fault-plane solutions, suggests that three intraplate lithospheric blocks—the Sierra Nevada, the Great Basin-High Lava Plains, and the Northern Rocky Mountains-Columbia Plateau are moving west to northwest as “slivers” between the obliquely converging Pacific and North American plates. Intraplate extension within the Great Basin is primarily accommodated by north-south normal faulting in north-central Nevada and along the Wasatch Front, but significant components of strike-slip faulting occur along active seismic zones in southwestern Nevada and along the northern Intermountain seismic belt in Montana. A thin crust, ~25 km, characterizes the east margin of the Great Basin, with average P n -velocities of ~7.5 km /s. A crustal low-velocity layer, at 5 to 15 km depth, coincides with the eastern margin of the Great Basin. A thin crust, with an apparent P n -velocity of ~7.7 km /s, occurs on the northwest margin of the Great Basin and beneath parts of the Oregon-High Lava Plains. The central part of the Great Basin has a thicker crust, ~30 km, and higher P n -velocities, 7.7 to 7.9 km /s. The Colorado Plateau and the Rocky Mountains have thicker crusts, 40 to 50 km, and P n -velocities of ~7.8 km/s. An upper-mantle diapir or an upwelling thermal mechanism, facilitated by stress relaxation above the now-truncated Farallon subducting plate, is postulated to have uplifted, extended, and heated the crust of the Great Basin beginning at ~20 m.y. Late Cenozoic centers of volcanism appear to have progressed outward from a northern Great Basin thermal center in two divergent directions—northwesterly along a combined extensional-strike-slip zone of deformation in the High Lava Plains of southeastern Oregon and northeasterly along an extensional zone marked by the Snake River Plain. Lithospheric heating is thought to have produced a broad uplift of the Great Basin with concomitant crustal thinning and diminishing of upper-mantle P n -velocities. The thermal mechanisms are hypothesized to have produced laterally divergent mantle flow to form symmetric zones of crustal thinning and low P n -velocities that presently mark the east and west margins of the Great Basin.

Except for volcanoes in the Adel Mountains and near the Black Hills, the first 10 m.y. of the Cenozoic was a time of igneous quiescence. Basaltic volcanism in the eugeosyncline west of the present-day Cascade volcanic arc began in early Eocene time, was most intense between 54 and 44 m.y. ago, and tapered off slowly, with injection of basaltic and alkali trachyte dikes continuing until the Oligocene. In Oligocene time the eugeosynclinal rocks became welded to the continental margin, shorelines shifted westward, and volcanic activity west of the Cascade arc virtually ceased. Igneous activity began about 55 m.y. ago over a broad region in Washington, northern Idaho, and Montana, and during the early Eocene this activity swept southward across all the Northwestern United States. Volcanism, plutonism, regional high heat flow with associated geothermal circulation systems, block faulting, and ductile deformation at depth associated with tectonic denudation as a consequence of diapiric rise of plutonic rocks occurred synchronously during an intense culmination between 50 and 43 m.y. ago (Challis episode). Volcanism and deformation then died out abruptly as the locus of igneous eruption centers shifted south into Nevada-Utah and west into the Cascade arc, where volcanic activity persisted through the rest of the Cenozoic. Following widespread tectonic and igneous quiescence between about 38 and 18 m.y. ago, volcanic activity suddenly commenced over a large region with rapid eruption of Columbia River and related basalts. Volcanism, with bimodal chemistry, began during this episode (Columbia, 13 to 16 m.y. ago) in southwestern Idaho and over all of southeastern Oregon. Between 13 m.y. ago and today, bimodal igneous activity migrated eastward across Idaho to produce the Snake River Plain-Yellowstone volcanic field. In Oregon, at the same time, siliceous volcanic centers retreated westward. As a consequence rhyolitic volcanic centers today are most active only in Yellowstone and close to the Cascade Range. Striking synchroneity is shown by pulses of more intense igneous activity in the Cascade and Snake River Plain-Yellowstone regions.