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J.W. Shervais

Publications and source records attributed to J.W. Shervais.

6 recordsLinked to original sources
USGS2012

Hotspot: the Snake River Geothermal Drilling Project--initial report

The Snake River volcanic province (SRP) overlies a thermal anomaly that extends deep into the mantle; it represents one of the highest heat flow provinces in North America. The primary goal of this project is to evaluate geothermal potential in three distinct settings: (1) Kimama site: inferred high sub-aquifer geothermal gradient associated with the intrusion of mafic magmas, (2) Kimberly site: a valley-margin setting where surface heat flow may be driven by the up-flow of hot fluids along buried caldera ringfault complexes, and (3) Mountain Home site: a more traditional fault-bounded basin with thick sedimentary cover. The Kimama hole, on the axial volcanic zone, penetrated 1912 m of basalt with minor intercalated sediment; no rhyolite basement was encountered. Temperatures are isothermal through the aquifer (to 960 m), then rise steeply on a super-conductive gradient to an estimated bottom hole temperature of ~98°C. The Kimberly hole is on the inferred margin of a buried rhyolite eruptive center, penetrated rhyolite with intercalated basalt and sediment to a TD of 1958 m. Temperatures are isothermal at 55-60°C below 400 m, suggesting an immense passive geothermal resource. The Mountain Home hole is located above the margin of a buried gravity high in the western SRP. It penetrates a thick section of basalt and lacustrine sediment overlying altered basalt flows, hyaloclastites, and volcanic sediments, with a TD of 1821 m. Artesian flow of geothermal water from 1745 m depth documents a power-grade resource that is now being explored in more detail. In-depth studies continue at all three sites, complemented by high-resolution gravity, magnetic, and seismic surveys, and by downhole geophysical logging.

Snake River↗
USGS2010

Multi-stage origin of the Coast Range ophiolite, California: Implications for the life cycle of supra-subduction zone ophiolites

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the forearc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks vith arc tholeiitic or (roore rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift.

California↗
USGS1991

Intrinsic Nd, Pb, and Sr isotopic heterogeneities exhibited by the Lherz Alpine Peridotite Massif, French Pyrenees

Optically clear, acid-leached Cr-diopside, Al-augite, pargasitic amphibole, and phlogopite from peridotite and ultramafic dikes of the Lherz massif (French Pyrenees) exhibit chemical and isotopic heterogeneities indicative of a complex history. The data cover a wide range on the 143 Nd/ 144 Nd vs. 87 Sr/ 86 Sr covariation diagram, encompassing much of the mantle array and a large area below the array. Pb-isotopic signatures cover a wide range on Pb-Pb correlation diagrams, with several samples falling in the mid-ocean ridge basalt (MORB) field, but with others defining trends towards the enriched mantle II (EM II) and high-mu (HIMU) endmembers of Zindler & Hart (1986). Peridotites display the largest range in isotopic compositions, which formed in response to variable degrees of partial melting and subsequent enrichment during the intrusion of the ultramafic dikes. The ultra-mafic dikes represent at least four generations. Dike isotopic compositions range from ‘depleted’ to ‘enriched’, relative to Bulk Earth, reflecting the average composition of their source regions and subsequent melt-wall rock interactions. No simple isochronous relationships are observed on the 143 Nd/ 144 Nd vs. 147 Sm/ 144 Nd diagram for any lithologic group, demonstrating that the rock assemblages were not created by simple mixing of two homogeneous endmembers. Some ultramafic dikes, which represent low-temperature melt fractions, are enriched in the heavy isotopes of Sr, Nd, and Pb; any magmas derived from partial melting of these dikes are likely to have enriched isotopic signatures, particularly when the degrees of partial melting are only a few per cent.

Pyrenees↗
USGS1975

Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States

Ultramafic xenoliths in basalts from the western United States are divided into Al-augite and Cr-diopside groups. The Al-augite group is characterized by Al, Ti-rich augites, comparatively Fe-rich olivine and orthopyroxene, and Al-rich spinel, the Cr-diopside group by Cr-rich clinopyroxene and spinel and by Mg-rich olivine and pyroxenes. Both groups have a wide range of subtypes, but the Al-augite group is dominated by augite-rich varieties, and the Cr-diopside group by olivine-rich lherzolites.

Physics and Chemistry of the Earth↗