New geologic mapping and tephrochronologic assessment of strata in extensional basins surrounding Knoll Mountain (Nevada, USA) reveal a geologic history linked to tectonic development of the Yellowstone hotspot and Snake River Plain to the north, and to the Ruby–East Humboldt–Wood Hills metamorphic core complex to the south. Data from these areas are utilized to present a paleogeographic reconstruction of northeastern Nevada–southcentral Idaho depicting the architecture of extensional faulting and basin development during collapse of the Nevadaplano over the past 17 m.y. Knoll Mountain is a northeast-trending horst along the southern margin of the Snake River Plain and track of the Yellowstone hotspot. The horst is bounded on the east by the Thousand Springs fault system and basin, and on the west by the Knoll Mountain fault and basin, where streams currently drain north into the Snake River Plain. The Knoll and Thousand Springs basins form half-grabens that are filled with the ca. 16 Ma to ca. 8–5 Ma Humboldt Formation, which was deposited in alluvial, eolian, and lacustrine environments during slip along range-bounding faults and a series of late-stage synthetic intrabasin faults. Structural, chronologic, and sedimentologic assessment of the Humboldt Formation in the Knoll basin indicates that it records overall southward fluvial drainage with slip along the Knoll Mountain fault beginning ca. 16 Ma and continuing to at least 8 Ma, and that between 8 and ca. 5 Ma, a west-dipping intrabasin fault system had developed. Between ca. 8–5 Ma to ca. 3 Ma, several fundamental changes took place, beginning with the cessation of faulting followed by widespread erosion that in turn was followed by deposition of older alluvium. The reversal of drainage direction from south to north flowing in the Knoll basin also took place during this time period, but its age relative to the widespread erosion or older alluvium is unknown. An integration of our work with previous studies north of Knoll Mountain reveal that the Knoll Mountain and intrabasin faults terminate to the north in the vicinity of the Jurassic Contact pluton, and that this area forms an accommodation zone separating broadly coeval and colinear faults bounding the ca. 10–8 Ma north-trending Rogerson graben, the northern end of which merges with the Snake River Plain. Furthermore, an integration of our work with previous work south of Knoll Mountain reveals that the Knoll Mountain fault formed part of a >190-km long, west-dipping fault zone that included the Ruby–East Humboldt detachment. This fault zone, which we refer to as the Knoll-Ruby fault system, had an extensive hanging-wall basin, the KnollRuby basin. The Knoll-Ruby fault system was a prominent structure facilitating collapse of the Nevadaplano in northeastern Nevada between ca. 16 and ca. 8–5 Ma, and its central part produced partial exhumation of high-grade, mid-crustal metamorphic rocks in the Ruby–East Humboldt–Wood Hills metamorphic core complex. By 8–5 Ma, during the waning stages of extension along the Knoll-Ruby fault system, a series of intrabasin faults developed at about the same time as the integration of streams to form the incipient eastern reaches of the Humboldt River system. Profound changes in tectonics and paleogeography took place between ca. 8–5 Ma and ca. 3 Ma, that included the extinction of the Knoll-Ruby and intrabasin basin fault systems followed by southward migration of significant tectonism away from the Snake River Plain, resulting in development of a set of modern normal faults responsible for uplift of the southern Snake Mountains, Ruby Mountains, East Humboldt Range, and Pequop Mountains. These new faults cut and dismembered the central and southern part of the Knoll-Ruby fault system and basin, effectively ending any fluvial connection between the northern and southern parts of the Knoll-Ruby basin. Since ca. 8–5 Ma to the present, the Knoll Mountain region has remained relatively tectonically quiescent, and continued subsidence in the Snake River Plain to the north induced capture of the drainage system in the Knoll basin and reversed the drainage direction from south to north flowing. Our new findings indicate that (1) the Knoll-Ruby fault system and associated intrabasin faults were active until ca. 8–5 Ma, which is younger than the 12–10 Ma age generally recognized for cessation of major extension elsewhere in the northern Nevada region; (2) although this fault system was responsible for partial exhumation of core-complex metamorphic rocks, it extended well GEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01318.1 16 figures; 3 tables; 1 supplemental file CORRESPONDENCE: camillerip@apsu .edu CITATION: Camilleri, P., Deibert, J., and Perkins, M., 2017, Middle Miocene to Holocene tectonics, basin evolution, and paleogeography along the southern margin of the Snake River Plain in the Knoll Mountain– Ruby–East Humboldt Range region, northeastern Nevada and south-central Idaho: Geosphere, v. 13, no. 6, p. 1901–1948, doi:10.1130/GES01318.1. Received 24 January 2016 Revision received 3 April 2017 Accepted 2 August 2017 Published online 25 September 2017 For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. © 2017 Geological Society of America THEMED ISSUES: Cenozoic Tectonics, Magmatism, and Stratigraphy of the Snake River Plain–Yellowstone Region and Adjacent Areas and Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/6/1901/3994098/1901.pdf by guest on 02 November 2019

Thick late Miocene nonmarine evaporite (mainly halite and gypsum) and related lacustrine limestone deposits compose the upper basin fill in half grabens within the Lake Mead region of the Basin and Range Province directly west of the Colorado Plateau in southern Nevada and northwestern Arizona. Regional relations and geochronologic data indicate that these deposits are late synextensional to postextensional (ca. 12-5 Ma), with major extension bracketed between ca. 16 and 9 Ma and the abrupt western margin of the Colorado Plateau established by ca. 9 Ma. Significant accommodation space in the half grabens allowed for deposition of late Miocene lacustrine and evaporite sediments. Concurrently, waning extension promoted integration of initially isolated basins, progressive enlargement of drainage nets, and development of broad, low gradient plains and shallow water bodies with extensive clastic, carbonate, and/or evaporite sedimentation. The continued subsidence of basins under restricted conditions also allowed for the preservation of particularly thick, localized evaporite sequences prior to development of the through-going Colorado River.The spatial and temporal patterns of deposition indicate increasing amounts of freshwater input during the late Miocene (ca. 12-6 Ma) immediately preceding arrival of the Colorado River between ca. 5.6 and 4.9 Ma. In axial basins along and proximal to the present course of the Colorado River, evaporite deposition (mainly gypsum) transitioned to lacustrine limestone progressively from east to west, beginning ca. 12-11 Ma in the Grand Wash Trough in the east and shortly after ca. 5.6 Ma in the western Lake Mead region. In several satellite basins to both the north and south of the axial basins, evaporite deposition was more extensive, with thick halite (>200 m to 2.5 km thick) accumulating in the Hualapai, Overton Arm, and northern Detrital basins. Gravity and magnetic lows suggest that thick halite may also lie within the northern Grand Wash, Mesquite, southern Detrital, and northeastern Las Vegas basins. New tephrochronologic data indicate that the upper part of the halite in the Hualapai basin is ca. 5.6 Ma, with rates of deposition of similar to 190-450 m/m. y., assuming that deposition ceased approximately coincidental with the arrival of the Colorado River. A 2.5-km-thick halite sequence in the Hualapai basin may have accumulated in similar to 5-7 m. y. or ca. 12-5 Ma, which coincides with lacustrine limestone deposition near the present course of the Colorado River in the region.The distribution and similar age of the limestone and evaporite deposits in the region suggest a system of late Miocene axial lakes and extensive continental playas and salt pans. The playas and salt pans were probably fed by both groundwater discharge and evaporation from shallow lakes, as evidenced by sedimentary textures. The elevated terrain of the Colorado Plateau was likely a major source of water that fed the lakes and playas. The physical relationships in the Lake Mead region suggest that thick nonmarine evaporites are more likely to be late synextensional and accumulate in basins with relatively large catchments proximal to developing river systems or broad elevated terranes. Other basins adjacent to the lower Colorado River downstream of Lake Mead, such as the Dutch Flat, Blythe-McCoy, and Yuma basins, may also contain thick halite deposits.

We used tephrochronology for upper Neogene deposits in the Espanola 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 Espanola 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 Espanola 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.

Santa Cruz Province, Argentina, has some of the richest fossil mammal localities in the world. However, the absolute and relative ages of its fossil localities have long been a source of confusion and debate. In particular, there has been longstanding disagreement about the relative ages of the fossils from the western part of the province in deposits of the Pinturas Formation compared with those from the numerous localities of the Santa Cruz Formation along the Atlantic coast. Drawing on recent studies of the tuffaceous sediments in many classic fossil localities, and studies of fossil representatives of marsupials, rodents, and primates, we provide a synthesis of the temporal relationship among fossil localities throughout the province. There is broad agreement between the results of the tephrochronology and mammalian paleontology. Both tephra correlations and paleontological comparisons indicate that the lower units of the Pinturas Formation are older than the sections of the Santa Cruz Formation preserved at Monte Leon and Cerro Observatorio, supporting Ameghino's suggestion that part of the Pinturas Formation represents a distinct faunal zone. However, the upper unit of the Pinturas Formation seems to correspond in age with the lower part of the sections at Monte Leon and Cerro Observatorio.

The Santa Cruz and Pinturas Formations (SCF and PF) are two partially coeval formations in the southern part of Santa Cruz Province, Argentina, that were deposited during the Early to Middle Miocene. The SCF underlies the coastal plain between 47.0 degrees and 51.6 degrees S and extends from the Atlantic Coast into the Andean foothills. The PF has a more restricted distribution centered on eastern tributaries of the Rio Pinturas along the northern perimeter of the SCF. Both formations have abundant tephra and tuffaceous sediments with likely sources in volcanoes associated with emplacement of the late Cenozoic South Patagonian batholith. This study re-evaluates the age of the SCF and the relationship of the SCF to the PF, adding some radiometric dates to those previously published and using the methods of tephrochronology. Tephra samples were collected from 26 localities in the SCF and PF. Glass shards were analyzed by electron microscopy. Ten tephra samples were analyzed by the Ar-40/Ar-39 method: nine from the SCF and one from the PF. Results of these analyses, in conjunction with previous studies, indicate that there are at least 38 individual tephra layers in the SCF, while there are likely many more tephra than the six analyzed from the PF. Of the 38 tephra layers in the SCF, 16 are shared by two or more sections, with one key tephra, the Toba Blanca, present in eight and possibly nine localities from 51.6 degrees S northward to 47.0 degrees S, over a distance of similar to 525 km. Integrating results of the tephra correlations and radiometric ages indicates that the SCF spans the interval similar to 18 Ma to 16 Ma in the Atlantic coastal plain and similar to 19 to 14Ma in the Andean foothills, with a chronologic overlap between the PF and lower part of the SCF. With this tephrochronology in place, studies of space-time variations such as rates of sediment accumulation, composition of mammalian faunas, facies changes, and other aspects of the SCF and PF can be fruitfully pursued.

Sedimentary sequences in the Columbia Plateau region of the Pacific Northwest ranging in age from 16-4 Ma contain fallout tuffs whose origins lie in volcanic centers of the Yellowstone hotspot in northwestern Nevada, eastern Oregon and the Snake River Plain in Idaho. Silicic volcanism began in the region contemporaneously with early eruptions of the Columbia River Basalt Group (CRBG), and the abundance of widespread fallout tuffs provides the opportunity to establish a tephrostratigrahic framework for the region. Sedimentary basins with volcaniclastic deposits also contain diverse assemblages of fauna and flora that were preserved during the Mid-Miocene Climatic Optimum, including Sucker Creek, Mascall, Latah, Virgin Valley and Trout Creek. Correlation of ashfall units establish that the lower Bully Creek Formation in eastern Oregon is contemporaneous with the Virgin Valley Formation, the Sucker Creek Formation, Oregon and Idaho, Trout Creek Formation, Oregon, and the Latah Formation in the Clearwater Embayment in Washington and Idaho. In addition, it can be established that the Trout Creek flora are younger than the Mascall and Latah flora. A tentative correlation of a fallout tuff from the Clarkia fossil beds, Idaho, with a pumice bed in the Bully Creek Formation places the remarkably well preserved Clarkia flora assemblage between the Mascall and Trout Creek flora. Large-volume supereruptions that originated between 11.8 and 10.1 Ma from the Bruneau-Jarbidge and Twin Falls volcanic centers of the Yellowstone hotspot in the central Snake River Plain deposited voluminous fallout tuffs in the Ellensberg Formation which forms sedimentary interbeds in the CRBG. These occurrences extend the known distribution of these fallout tuffs 500 km to the northwest of their source in the Snake River Plain. Heretofore, the distal products of these large eruptions had only been recognized to the east of their sources in the High Plains of Nebraska and Kansas.

The Santa Cruz and Pinturas Formations (SCF and PF) are two partially coeval formations in the southern part of Santa Cruz Province, Argentina, that were deposited during the Early to Middle Miocene. The SCF underlies the coastal plain between 47.0 degrees and 51.6 degrees S and extends from the Atlantic Coast into the Andean foothills. The PF has a more restricted distribution centered on eastern tributaries of the Rio Pinturas along the northern perimeter of the SCF. Both formations have abundant tephra and tuffaceous sediments with likely sources in volcanoes associated with emplacement of the late Cenozoic South Patagonian batholith. This study re-evaluates the age of the SCF and the relationship of the SCF to the PF, adding some radiometric dates to those previously published and using the methods of tephrochronology. Tephra samples were collected from 26 localities in the SCF and PF. Glass shards were analyzed by electron microscopy. Ten tephra samples were analyzed by the Ar-40/Ar-39 method: nine from the SCF and one from the PF. Results of these analyses, in conjunction with previous studies, indicate that there are at least 38 individual tephra layers in the SCF, while there are likely many more tephra than the six analyzed from the PF. Of the 38 tephra layers in the SCF, 16 are shared by two or more sections, with one key tephra, the Toba Blanca, present in eight and possibly nine localities from 51.6 degrees S northward to 47.0 degrees S, over a distance of similar to 525 km. Integrating results of the tephra correlations and radiometric ages indicates that the SCF spans the interval similar to 18 Ma to 16 Ma in the Atlantic coastal plain and similar to 19 to 14Ma in the Andean foothills, with a chronologic overlap between the PF and lower part of the SCF. With this tephrochronology in place, studies of space-time variations such as rates of sediment accumulation, composition of mammalian faunas, facies changes, and other aspects of the SCF and PF can be fruitfully pursued.

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating Ar-40/Ar-39 isochron and plateau ages of 4.834 +/- 0.011, 4.76 +/- 0.03, <= 4.70 +/- 0.03, and 4.50 +/- 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies.Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek-Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past similar to 4.83 Ma.We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

The provenance and stratigraphic architecture of basin-filling Miocene sediments around the Gold Butte area, southern Nevada, and adjacent highlands record the erosion of fault blocks that progressively tilted during extension. This study focuses especially on upper Miocene correlatives of the red sandstone unit and the Muddy Creek Formation that were deposited during waning stages of extension. Upper parts of the underlying middle Miocene Horse Spring Formation are also addressed. The large east-tilted South Virgin-White Hills block, including the Gold Butte block, was the primary source of coarse detritus into the adjacent half-graben basins on both sides. Voluminous, very coarse-grained sediments were shed eastward down the back slope of this tilt block into the Grand Wash Trough. This suggests that there were large middle and late Miocene catchments on that side of the block, possibly inherited from a gentler dip slope early in the tilting history. The block uplifted and tilted during slip on the west-dipping South Virgin-White Hills normal fault that bounds the west side of the block. Its exposed footwall shed coarse-grained debris to the west. While the fault was active, this debris included rock-avalanche megabreccias. Longitudinal transport of coarse-grained sediment also occurred along the axes of basins on both sides of the block.In the late Miocene, fault death at ca. 10 Ma followed rotation of the South Virgin-White Hills fault, and the along-strike Quail Spring fault, from initial dips >55 degrees to dips <30 degrees. This cessation of faulting coincided with and likely caused an eastward shift in locus of faulting to the steeper Wheeler fault system. Coarse sediment shed from the South Virgin-White Hills tilt block gradually declined as deformation waned and limestone-rich sedimentation expanded onto the basin margins against the block. Where the rising sedimentary fills eventually bridged across the block and connected basins on either side, these bridge sites served to focus later integrated regional drainage-the Pliocene Colorado River.Progressive Miocene tilting of the highland block would have broadened its structural footwall on the west and narrowed its east-dipping back slope. Migration of the drainage divide by erosion and piracy, influenced by changing tilt slopes, can explain the modern position of the divide in the Gold Butte block as one that separates drainage roughly equally down the two sides.

Orographic precipitation of Pacific-sourced moisture creates a rain shadow across the central part of the Sierra Nevada (California) that contrasts with the southern part of the range, where seasonal monsoonal precipitation sourced to the south obscures this rain shadow effect. Orographic rainout systematically lowers the hydrogen isotope composition of precipitation (δ D ppt ) and therefore δ D ppt reflects a measure of the magnitude of the rain shadow. Hydrogen isotope compositions of volcanic glass (δ D glass ) hydrated at the earth's surface provide a unique opportunity to track the elevation and precipitation history of the Sierra Nevada and adjacent Basin and Range Province. Analysis of 67 well dated volcanic glass samples from widespread volcanic ash-fall deposits located from the Pacific coast to the Basin and Range Province demonstrates that between 0.6 and 12.1 Ma the hydrogen isotope compositions of meteoric water displayed a large (>40‰) decrease from the windward to the leeward side of the central Sierra Nevada, consistent with the existence of a rain shadow of modern magnitude over that time. Evidence for a Miocene-to-recent rain shadow of constant magnitude and systematic changes in the longitudinal climate and precipitation patterns strongly suggest that the modern first-order topographic elements of the Sierra Nevada characterized the landscape over at least the last 12 million years.

Late Miocene and early Pliocene sediments exposed along the lower Colorado River near Laughlin, Nevada, contain evidence that establishment of this reach of the river after 5.6 Ma involved flooding from lake spillover through a bedrock divide between Cottonwood Valley to the north and Mohave Valley to the south. Lacustrine marls interfingered with and conformably overlying a sequence of post–5.6 Ma fine-grained valley-fill deposits record an early phase of intermittent lacustrine inundation restricted to Cottonwood Valley. Limestone, mud, sand, and minor gravel of the Bouse Formation were subsequently deposited above an unconformity. At the north end of Mohave Valley, a coarse-grained, lithologically distinct fluvial conglomerate separates subaerial, locally derived fan deposits from subaqueous deposits of the Bouse Formation. We interpret this key unit as evidence for overtopping and catastrophic breaching of the paleodivide immediately before deep lacustrine inundation of both valleys. Exposures in both valleys reveal a substantial erosional unconformity that records drainage of the lake and predates the arrival of sediment of the through-going Colorado River. Subsequent river aggradation culminated in the Pliocene between 4.1 and 3.3 Ma. The stratigraphic associations and timing of this drainage transition are consistent with geochemical evidence linking lacustrine conditions to the early Colorado River, the timings of drainage integration and canyon incision on the Colorado Plateau, the arrival of Colorado River sand at its terminus in the Salton Trough, and a downstream-directed mode of river integration common in areas of crustal extension.

Field and geochronologic studies of Neogene sedimentary basins in northeastern Nevada document the paleogeographic and geologic evolution of this region and the effects on major mineral deposits. The broad area that includes the four middle Miocene basins studied—Chimney, Ivanhoe, Carlin, and Elko, from west to east—was an upland that underwent prolonged middle Tertiary exposure and moderate erosion. All four basins began to retain sediments at ca. 16 Ma. Eruption of volcanic flows in the Chimney and Ivanhoe basins produced short-lived (ca. 2 Ma), lacustrine-dominated basins before the dams failed and the streams drained to the southwest. In contrast, early, high-angle, normal faulting induced fluvial to lacustrine sedimentation in the Carlin and Elko basins, and volcanic flows further blocked drainage in the Carlin basin until the basin drained at ca. 14.5 Ma. The Elko basin, with continued synsedimentary faulting, retained sediments until ca. 9.8 Ma and then drained west into the Carlin basin. Sediment buildup in all basins progressively buried existing highlands and created a subdued landscape.

Over the course of its 16 m.y. history, the Yellowstone hotspot has produced silicic magmas exhibiting systematic, and often sympathetic, variations in isotopic and chemical composition, temperature and frequency of eruption. Nd and Hf isotopic ratios vary systematically from initial eruptions at similar to 16 Ma, contemporaneous with basaltic volcanism in eastern Oregon and Washington, to the present day Yellowstone Volcanic Plateau. Nd and Hf isotopic ratios co-vary and span the range of most terrestrial samples, reflecting mixing of mantle and crustal sources. Earliest erupted silicic magmas were hot (in excess of 1050 degrees C), relatively less evolved and have isotopic ratios within the range of contemporaneous Columbia River flood basalts. The transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the Precambrian craton at 15 Ma is marked by shifts in epsilon(Nd) from +4 to -11 and in epsilon(Hf) from +10 to -10. The duration of the transit yields a crustal magma source diameter of similar to 70 km. In the interval from 14 to 9 Ma, epsilon(Nd) systematically increases from -11 to -7, recording a minimum increase in the mantle component from 5% to 30%. The mantle component could be twice as great, depending upon the isotopic composition of crust and mantle reservoirs. In this same interval, peak temperatures of similar to 1000 degrees C occurred at 9 Ma. The last 9 m.y. are characterized by less frequent eruption of lower temperature (830-900 degrees C) and more compositionally evolved magmas. (c) 2006 Elsevier B.V. All rights reserved.