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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 811 records · Page 45Linked to original sources
USGS1979

Evidence for tectonic movement of the Las Positas Fault, Alameda County, California

On October 29, 1979, a new exposure of the Las Positas fault zone near Livermore, California, was created by excavation of a creek bank along Arroyo Seco. The face of the bank was cleaned by geologists of Lawrence Livermore Laboratory (Livermore, California) to obtain a better and more complete view of the fault. The excavation was undertaken by Lawrence Livermore Laboratory as part of a larger review of the earthquake hazards of the laboratory site. The Las Positas fault zone extends along the south side of Livermore Valley, passing just south of Lawrence Livermore Laboratory. The exposure revealed new evidence concerning not only the width of faulting in the Las Positas fault zone, but also its recency, style, and origin.

California↗
USGS1980

Metallogenic and tectonic significance of oxygen isotope data and whole-rock potassium-argon ages of the Nikolai Greenstone, McCarthy Quadrangle, Alaska

The Middle and (or) Late Triassic Nikolai Greenstone, part of the allochthonous terrane of Wrangellia, is typically altered and locally metamorphosed to prehnite-pumpellyite facies with chlorite and epidote as the most common secondary minerals. Intrinsic copper content averages 155 ppm, and two types of concentrations of copper in the Nikolai are common: (1) native copper fillings of amygdules and rubble zones typically near flow tops, and (2) veins and thin replacement zones that contain native copper and copper-iron sulfides in quartz-epidote or calcite gangue in faults and fractures. Oxygen isotope data from quartz and epidote from three copperbearing veins yield calculated ore fluid temperatures of approximately 200°C and 6180 of approximately +1 per mil in agreement with a metamorphicsegregation origin of these deposits, as suggested by Sinclair (1977). Seven K-Ar ages of chloritized greenstone, including those adjacent to veins fall on an initial argon diagram with a zero intercept and a slope which yields an isochron age of 112 + 11 m.y. The ages define a Cretaceous thermalmetamorphic episode which is responsible for alteration and mineralization. The episode is younger than a major Jurassic progeny, accompanied by granitic intrusion, in the area, and appears to be unaffected by minor granitic intrusion in the middle to late Tertiary. We believe the Cretaceous event is related to accretion of Wrangellia to its present relative position in North America. This age of accretion agrees with stratigraphic and structural evidence cited by other workers.

Alaska↗
USGS1980

Notes on the geology of Tibet and adjacent areas; report of the American plate tectonics delegation to the People's Republic of China

Mean water levels in wells across Georgia were from 3.03 feet higher to 11.67 feet lower in 1979 than in 1978, and in some areas were the lowest on record. Water levels in the principal artesian aquifer underwent a long-term decline during the period 1970-79. In some areas water levels dropped more than 10 feet. Wells tapping the Clayton Limestone in the Albany area showed a long-term decline during the period 1970-79, and in some wells water levels dropped more than 20 feet. Water levels in the Cretaceous aquifer system showed little fluctuation during 1979. In the Piedmont area mean water levels remained the same to 4 feet higher in 1979 than in 1978 and showed no long-term trend. (USGS)

Open-File Report↗