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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 55 records · Page 3Linked to original sources
USGS1981

Late Cenzoic rhyolites from the Kern Plateau, southern Sierra Nevada, California.

Four late Cenozoic rhyolite domes lie atop the Kern Plateau, 30 to 40km S-SE of Mount Whitney. K-Ar and 40Ar/39Ar dating shows that Monache, Templeton, and Little Templeton Mountains are all about 2.4Ma old; a small dome nearby is approx 0.2Ma old. The three older rhyolites have SiO2 = 73-74% and have steep, fractionated rare earth element (REE) patterns; the youngest has SiO2 = 76% and a concave-upward REE pattern with a large negative Eu anomaly, Monache rhyolite contains the unusual phenocryst assemblage almandine + fayalite + biotite + oligoclase. The 2.4Ma old rocks may be nearly unmodified partial melts of crustal sources, whereas the 0.2Ma old rhyolite may be a product of relatively shallow differentiation. The rhyolites and nearby basalts are coeval with mafic and silicic volcanic rocks in the Coso Range about 40km to the SE. Their generation and eruption may reflect intense tectonic extension at the margin of the Basin and Range province and concomitant relaxation of compressive stress in a W- NW direction, allowing melts to reach the surface in the adjacent Sierra Nevada.

California↗
USGS1983

Discovery of a Late Triassic basin north of Boston and some implications as to post-Paleozoic tectonics in northeastern Massachusetts (USA)

As deduced from various data the Triassic-Jurassic Middleton basin-trends NE and is probably approx 5.7km in length and no >0.5km in width. Two sets of dikes which have been K/Ar dated as Late Triassic-Early Jurassic have the same NNE strikes as strike-slip faults. There are indications that a tectonic event of some importance may have occurred in Middle Mesozoic time. -from Author

Massachusetts↗
USGS1984

Chromite from the Blue Ridge province of North Carolina

Accessory chromite in dunite shows a variety of textures that indicate alteration. One group, type A, consists of four types of chromite: clean chromite; lattice chromite, in which the invading chlorite occurs along three directions in the (100) plane; optically zoned chromite; and poikiloblastic chromite. Most of type A chromites are surrounded by chromian clinochlore. The other group, type B, consists of euhedral to subhedral chromite grains which are included in olivine or pyroxene. The accessory chromites define a trend exhibited by chromite from other areas that have undergone metamorphism. Olivine-spinel geothermometry indicates equilibration temperatures near 700 degrees C, roughly consistent with mineral assemblages in the host gneisses. Thus, the Blue Ridge dunites are metamorphic rocks and not primary mantle peridotites.

North Carolina↗
USGS1984

Evidence for tectonic emplacement of ultramafic and associated rocks in the pre-Silurian eugeoclinal belt of western New England: Vestiges of an ancient accretionary wedge

In northern Vermont, detailed 1:10,000 mapping of the Hazens Notch, Ottauquechee, Stowe, and Moretown formations in the 60 km 2 Jay area has shown that metasedimentary rocks and serpentinites are highly faulted to produce a tectonic stratigraphy in which serpentinites and talc-carbonate rocks occur as slivers along faults that separate contrasting lithic assemblages. Remnants of the ophiolite sequences of Quebec. In western Massachusetts, the ultramafic-bearing section of the Rowe Schist and the western part of the Moretown Formation are largely equivalent to the rocks in northern Vermont. Mapping at 1:24,000 scale and compilation work for the new bedrock map of Massachusetts have shown that the Rowe Schist consists of numerous lenses of greenschist, carbonaceous schist, gray schist, amphibolite, and serpentinite. Although the geometric arrangement of the lenses and serpentinites is strikingly similar to that seen in the Jay area, higher grade Taconian and Acadian metamorphism has partially obscured the fault fabrics seen in Vermont.

Massachusetts, Vermont↗
USGS1984

Lithotectonic assemblages as portrayed on the new bedrock geologic map of Massachusetts.

Scale of 1:250,000. The map units are grouped into eight lithotectonic packages. Five "zones" of older rocks that cover the whole state are, from west to east, the Taconic-Berkshire, Rowe-Hawley, Bronson Hill, Nashoba, and Milford-Dedham. In central and western Massachusetts, these zones are overlain by the Connecticut Valley and Merrimack "belts" of Silurian-Devonian, and local Carboniferous, strata; the two belts are distinguished from each other on the basis of the lithofacies of the Silurian strata. In eastern Massachusetts, the Milford-Dedham Zone also includes Silurian-Devonian, Carboniferous, Mesozoic, and Tertiary strata. A Mesozoic "basin" is separately identified in the Connecticut Valley region

Massachusetts↗
USGS1985

Geochemical mass-balance relationships for selected ions in precipitation and stream water, Catoctin Mountains, Maryland

Results of a study of input/output mass balances for major ions based on the chemical composition of precipitation and stream-water, geochemical reactions with different loading rates of hydrogen ion, and watershed processes influencing the chemical character of stream-waters in two small watershed areas are reported with a view to predicting the effect of additions of acidic rain to the watershed systems. Geochemical weathering processes account for the observed changes in the chemistry of stream flow. Although present in bedrock in extremely small quantities, calcite plays an important role in neutralization of the total hydrogen-ion input.

Maryland↗