John Shervais - Academia.edu (original) (raw)

Papers by John Shervais

Ophiolites and oceanic crust: new insights from field studies and the Ocean Drilling Program, 2000

to the more common low-silica ONBs at the Apollo 15 site. Brown R. \V (19"7) A sample filsion tec... more to the more common low-silica ONBs at the Apollo 15 site. Brown R. \V (19"7) A sample filsion technique fi_r whole rock anak..sis with the electron microprobe. G_Jch#n. O_smocbim. Acta, 41. 43"5-438. _ _:= " _L Chappcll B. _ and Green D. H. (19"3) Chemical composition and pctrogenctic relationships in tlic AIx_llo i'3 marc ba.,_ths. Fx_rt't_

Geology of San Francisco and Vicinity: San Francisco Bay Region, California: July 1–7, 1989, 1989

Snake River Plain Play Fairway Analysis - Phase 1 CRS Raster Files. This dataset contains raster ... more Snake River Plain Play Fairway Analysis - Phase 1 CRS Raster Files. This dataset contains raster files created in ArcGIS. These raster images depict Common Risk Segment (CRS) maps for HEAT, PERMEABILITY, AND SEAL, as well as selected maps of Evidence Layers. These evidence layers consist of either Bayesian krige functions or kernel density functions, and include: (1) HEAT: Heat flow (Bayesian krige map), Heat flow standard error on the krige function (data confidence), volcanic vent distribution as function of age and size, groundwater temperature (equivalue interval and natural breaks bins), and groundwater T standard error. (2) PERMEABILTY: Fault and lineament maps, both as mapped and as kernel density functions, processed for both dilational tendency (TD) and slip tendency (ST), along with data confidence maps for each data type. Data types include mapped surface faults from USGS and Idaho Geological Survey data bases, as well as unpublished mapping; lineations derived from maxim...

The Western Snake River Plain provides us with a unique opportunity to study the evolution of bas... more The Western Snake River Plain provides us with a unique opportunity to study the evolution of basaltic volcanism. In addition to studying the horizontal variation at the surface of the plain, erosion by the Snake River allows us to study the vertical variation as well. With the aid of both a GSA graduate student research grant and a USGS EDMAP grant we mapped the Dorsey Butte quadrangle and southern half of the Little Joe Butte quadrangle at a scale of 1:24,000. The quadrangles are located within both the Ada and Elmore counties of southwestern Idaho, approximately 30 km west of Mountain Home and 2 km north of Grand View. The two maps will extend westward the eight 7.5 minute quadrangles previously mapped by Shervais et al. (2002). The result of this work will be an area encompassing 530 square miles centered around Mountain Home that have been geologically mapped at a scale of 1:24,000. The geology of this area consists predominantly of basaltic volcanism, lake sedimentation, and quaternary faulting. Lava flows, shield volcanoes, and cinder cones dominate the landscape with a concentration of phreatomagmatic deposits along the cliffs of the Snake River. An eight-meter stratigraphic column was generated from a phreatomagmatic outcrop within the Basaltic Tuff and Breccia of Hayland School. The base of the unit consists of subaerial scoria fall deposits. Above the scoria deposits sedimentary material is present within cross-bedded and massive layers that may be the product of channel flow, subaqueous reworking of material, and/or subaqueous debris flow. The final stages of the Hayland School eruption involved a de-watering of the system resulting in an increase in the proportion of basaltic clasts and finally a basaltic lava flow. The periods of hydration and de-watering of the volcanic system are likely the product of damming of the ancestral Snake River and/or growth of the volcanic edifice above water level

The costs and risks associated with the exploration for and assessment of geothermal reservoirs a... more The costs and risks associated with the exploration for and assessment of geothermal reservoirs are high and the results are not predictable. Geothermal resources have the advantage of base-load generation, but this does not command a premium price in the utility market. Internationally, there are a number of different business models that contribute to the viability of geothermal power generation. Often, the resource is government owned, and national companies undertake exploration and delineation drilling. Once the resource has been identified, private firms are brought in to build the power plants and deliver power to the grid. In the US, although the resource may be on Federal land, the government is not involved in exploration and assessment activities, and exploration and assessment risk is assumed by the developer. However, the government does participate by sponsoring research in exploration methodology, as illustrated by DOE’s Geothermal Play Fairway program. We would exten...

Geochim Cosmochim Acta, 1999

Geophysical Prospecting, 2015

The Snake River Geothermal Drilling Project was undertaken to better understand geothermal system... more The Snake River Geothermal Drilling Project was undertaken to better understand geothermal systems across the Snake River Plain volcanic province. A series of surface and borehole seismic profiles were obtained to provide insights into volcanic stratigraphy and test the capabilities of engineering-scale seismic imaging in such terranes. The Kimberly site drilled through 1.9 km of mostly rhyolite, with thin sedimentary interbeds in the upper part of the section. The Kimama site drilled through 1.9 km of mostly basalt with sedimentary interbeds at ß200 m depth and 1700 m depth. The Mountain Home site contained numerous sediment and volcanic rock layers. Downhole and surface vibroseis seismic results suggest sedimentary interbeds at depth correspond with low-velocity, high-temperature zones that relate to reflections on seismic profiles. Our results suggest that eruption flow volumes can be estimated and flow boundaries can be imaged with surface seismic methods using relatively high-fold and wide-angle coverage. High-frequency attenuation is observed at all sites, and this deficit may be countered by acquisition design and a focus on signal processing steps. Separation of surface and body waves was obtained by muting, and the potential for large static effects was identified and addressed in processing. An accurate velocity model and lithology contacts derived from borehole information improved the confidence of our seismic interpretations.

Special Paper 438: Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson, 2008

The Elder Creek ophiolite, which crops out along the South, Middle, and North Forks of Elder Cree... more The Elder Creek ophiolite, which crops out along the South, Middle, and North Forks of Elder Creek, is the largest exposure of mid-Jurassic Coast Range ophiolite in the northern Coast Ranges of California. The Elder Creek ophiolite contains almost all of the components of a classic ophiolite (mantle tectonites, cumulate ultramafi cs and gabbro, dike complex, volcanics), although most of the volcanic section has been removed by erosion and redeposited in the overlying Crowfoot Point breccia. It differs from classic ophiolite stratigraphy in that it has substantial volumes (25%-30% of the complex) of felsic plutonic rocks intimately associated with the other lithologies. The felsic lithologies include hornblende diorite, quartz-diorite, tonalite, and trondhjemite, which crop out in four distinct associations: (1) as rare, small pods within the sheeted dike complex, (2) as the felsic matrix of igneous breccias (agmatites), (3) 1-25-m-thick dikes that crosscut cumulate or isotropic gabbro, and (4) sill-like plutons up to 500 m thick and 3 km long that intrude the upper part of the plutonic section. Typical phase assemblages include quartz, plagioclase, hornblende, and pyroxene, in a hypidiomorphic texture. The Elder Creek tonalite-trondhjemite-diorite (TTD) suite spans a wide range in composition: 54%-75% SiO 2 , 3.3%-14.3% FeO*, and 2.7%-6.4% MgO; all are low in K 2 O (<0.7%). The large sill-like plutons are generally higher in silica (average 69% SiO 2) than the dikes, pods, and agmatite matrix (average 60% SiO 2). Mg#'s range from 65 to 17, with Cr up to 227 ppm at 58% silica. High-Mg diorites with 4%-7% MgO at 53%-58% SiO 2 are common in the dike suite, but relatively high MgO, Mg#, and Cr values are found in the large plutons as well. The major-and trace-element characteristics are consistent with partial melting of subducted, amphibolite-facies oceanic crust at relatively low pressures (5-10 kbar) outside the garnet stability fi eld. Melting of subducted oceanic crust at these pressures can only occur during the collision and subduction of an active spreading center. Subsequent reaction of these melts with the overlying mantle wedge has increased their refractory element concentrations. The occurrence of zircons with inherited Pb isotope characteristics implies the involvement of subducted sediments containing an ancient zircon component. Formation of the Elder Creek TTD suite was a transient event associated with ridge collision-subduction. This is consistent with previous models for the Coast Range ophiolite and other suprasubduction-zone ophiolites; it is not consistent with an ocean-ridge spreading-center origin.

International Geology Review, 2015

Journal of Petrology, 1979

... The Al-augite suite ('ariegite' of Lensch, 1971; 1976) is characterized by a grey, ... more ... The Al-augite suite ('ariegite' of Lensch, 1971; 1976) is characterized by a grey, chrome-poor (<0-2 per cent Cr2O3), alumina-rich (6-5-10 per cent A12O3) clinopyroxene, and green, Cr-poor hercynite. Rocks of this suite cross-cut Cr-Di series pyroxenites and are less deformed. ...

Journal of Geophysical Research: Solid Earth, 1984

The 1978 AMAR expedition extended the investigation of the Mid-Atlantic Ridge that was begun by t... more The 1978 AMAR expedition extended the investigation of the Mid-Atlantic Ridge that was begun by the 1973-1974 FAMOUS expedition from the northern end of the FAMOUS rift (the original FAMOUS area) to the narrow central and southern end of the FAMOUS rift (Narrowgate region) and into the broad AMAR valley south of Fracture Zone B. Available field and geochemical data allow us to characterize in detail the volcanic-tectonic cycle for these two segments of the mid-ocean ridge system. A dynamic model of the crust-forming processes within these two rift valley segments is deduced from (1) variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins, (2) composition and petrographic relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution, and (3) detailed stratigraphic and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor. Each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition. The decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits. Intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes, the common evidence of crystal resorption, and the apparent disequilibrium between megacrysts, phenocrysts, and their enclosing glass. This indicates control by a crustal magma chamber. Liquid and crystal compositions and textural relations can best be explained by a three-stage crystallization history: (1) crystallization of olivine, plagioclase, and clinopyroxene onto the floor of a magma chamber, (2) partial resorption of mineral phases by a superheated, undersaturated liquid above the floor, (3) intratelluric crystallization of olivine and plagioclase (+ clinopyroxene) during rise of melt to the surface through the conduit system above the magma chamber. A magma chamber model is suggested for the AMAR-FAMOUS rift valleys that satisfies both seismic and geological constraints. We propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment (AMAR and Narrowgate regions) but that these thin and terminate near the intersecting fracture zones. Expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization. The FAMOUS-Narrowgate rift is currently in a contraction period so that in the northern FAMOUS region, primitive liquid has erupted at Mount Venus and Mount Pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the Narrowgate region. TECTONICS AND MORPHOLOGY Detailed descriptions of the structure of the FAMOUS-Narrowgate rift and AMAR rift valleys are provided elsewhere [Ballard and van Andel, 1977; Crane and Ballard, 1981; Macdonald, 1977, 1984]. Included here is a short summary of the morphological characteristics that are relevant to characterizing the volcanic-tectonic cycle. Absolute ages of the terrain within the rift valley segments are extremely difficult to determine, and standard age-dating techniques are difficult to apply for such young rocks. Using bottom photographs and individual diver's observations, an empirical relative time scale was derived based upon sediment cover, preservation of the fragile glassy margins, extent of tectonic fissuring [Crane and Ballard, 1981], and thickness of palagonite or Mn crusts on the rocks [Bryan and Moore, 1977]. These inferred age relationships are those used throughout this discussion. Relative ages of the different ridge segments have also been deduced by Van Wagoner and Johnson [1983] based on magnetization of the basaltic rocks. FAMO US-Narrowgate Rift The FAMOUS valley has a narrow V-shaped inner floor that is up to 3 km across. The width of the inner floor is broadest at the junction with transforms A (to the north) and B (to the south) and narrowest in the valley midsection called Narrowgate (Figure 2). This inner rift valley is bounded by steep, normal-faulted walls. The valley floor is bisected by a constructional volcanic lineament composed of central volcanoes that vary in volume, length/width ratio, and relative age and that are separated by sediment-filled lows [Crane and Ballard, 1981]. All of our samples are from the Narrowgate region whose central volcanoes are Uranus, Mars, and N Saturn (Figure 2). Geochemical data from the FAMOUS expedition are from the area north of Narrowgate, including the central volcanic highs, Venus and Pluto. These constructional highs were built along the then axial zone of most recent volcanism and tend to be progressively longer, narrower, and taller toward the narrow portion of the rift valley near Mount Mars. All of these volcanic edifices are built of pillowed lava flows that have been erupted onto a much older, sediment-covered, and tectonically disrupted floor. Crane and Ballard [1981] see no evidence of significant age difference among these central axial highs. The older floor in the north FAMOUS valley is exposed on the flanks of the broad central volcanoes as faulted rocks that were not covered by later flows. The oldestappearing section in the FAMOUS-Narrowgate rift is exposed north of Mount Mars, juxtaposed against younger-appearing flows. Mount Mars, within Narrowgate, has faulted flanks and is bisected by a small graben that postdates the volcanic pile. Mount Mars also lacks the abundant fresh glass described by the FAMOUS divers for the more northern central highs, Venus and Pluto (J. Moore, personal communication, 1982). Van Wagoner and Johnson [1983] contend that Mars is the most youthful volcanic high in the northern valley based upon the high magnetic intensity of samples collected there. The STAKES ET AL.' VOLCANIC-TECTONIC CYCLE OF THE FAMOUS

Journal of Asian Earth Sciences, 1999

The Dir-Utror volcanic series forms a NE±SW trending belt within the northwestern portion of the ... more The Dir-Utror volcanic series forms a NE±SW trending belt within the northwestern portion of the Kohistan island arc terrane in the western Himalayas of northern Pakistan. The Kohistan arc terrane comprises a diverse suite of volcanic, plutonic, and subordinate sedimentary rocks of late Mesozoic to Tertiary age, developed prior to and after suturing of the Indo-Pakistan and Asiatic continental blocks. The Dir-Utror volcanic series near Dir is dominated by basaltic-andesite and andesite, with subordinate basalt, high-MgO basalt, dacite, and rhyolite. Porphyritic textures are dominant, with less common aphyric and seriate textures. Plagioclase is the dominant phenocryst in ma®c to intermediate rocks, K-feldspar and quartz phenocrysts predominate in the dacites and rhyolites. Chlorite, epidote, albite, and actinolite are the most common metamorphic phases; blue-green amphibole, andesine, muscovite, biotite, kaolinite, sericite, carbonate, and opaques are widespread but less abundant. Phase assemblages and chemistry suggest predominant greenschist facies metamorphism with epidote-amphibolite facies conditions attained locally. Whole rock major element compositions de®ne a calc-alkaline trend: CaO, FeO, MgO, TiO 2 , Al 2 O 3 , V, Cr, Ni, and Sc all decrease with increasing silica, whereas alkalis, Rb, Ba, and Y increase. MORB-normalized trace element concentrations show enrichment of the low-®eld strength incompatible elements (Ce, La, Ba, Rb, K) and deep negative Nb, P, and Ti anomaliesÐ patterns typical of subduction related magmas. Ma®c volcanic rocks plot in ®elds for calc-alkaline volcanics on trace element discrimination diagrams, showing that pre-existing oceanic crust is not preserved here. All rocks are LREE-enriched, with La=16±112 Â chondrite, La/Lu=2.6±9.8 Â chondrite, and Eu/Eu Ã =0.5±0.9. Dacites and rhyolites have the lowest La/Lu and Eu/Eu Ã ratios, re¯ecting the dominant role of plagioclase fractionation in their formation. Some andesites have La/Lu ratios which are too high to result from fractionation of the more ma®c lavas; chondrite-normalized REE patterns for these andesites cross those of the basaltic andesites, indicating that these lavas cannot be related to a common parent. The high proportion of ma®c lavas rules out older continental crust as the main source of the volcanic rocks. The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is ma®c relative to normal continental margins. Andesites with high La, La/ Lu, K 2 O, and Rb may be crustal melts; we suggest that garnet-rich high-pressure granulites similar to those exposed in the Jijal complex may be restites formed during partial melting of the crust.