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R.C. Jachens

Publications and source records attributed to R.C. Jachens.

82 records · Page 5Linked to original sources
USGS1983

Aeromagnetic map and interpretation of geophysical data from the Condrey Mountain Roadless Area, Siskiyou County, California

The Condrey Mountain Roadless Area lies within the Klamath Mountains geologic province, a province composed of four eastward-dipping imbricate thrust slices or belts consisting predominantly of marine arc-related volcanic and sedimentary rocks, but also including significant amounts of ultramafic and other ophiolitic rocks (Irwin, 1981). From west to east the four Klamath Mountains thrust slices are called the western Jurassic belt, the western Paleozoic and Triassic belt, the central metamorphic belt, and the eastern Klamath belt. The Condrey Mountain Road less Area is located within a structural window in the western Paleozoic and Triassic belt, a window in which the structural dome comprised of the Condrey Mountain Schist is exposed (Coleman and others, 1983). North, east and south of the roadless area, the Condrey Mountain Schist is separated from the overlying western Paleozoic and Triassic belt by a low-angle regional thrust, whereas to the west the two units are in high-angle fault contact (Coleman and others, 1983). The Condrey Mountain schist consists mainly of sedimentary and volcanic rocks metamorphosed to greenschist facies. Metasedimentary rocks are exposed over most of the eastern and central parts of the structural window. Metavolcanic rocks occupy the western part of the window but also occur in small exposures within the metasedimentary rocks and along the eastern and southern margins of the window. Tabular bodies of metaserpentinite, the largest of which crops out near White Mountain, are contained within the metasedimentary rocks. Densities of hand samples from 11 sites scattered throughout the Condrey Mountain window average 2.66±0.05 g/cm 3 . The four samples of metavolcanic rocks yielded a higher average density than the seven samples of metasedimentary rocks (2.71±0.04 g/cm 3 versus 2.63±0.03 g/cm 3 ). Along the western edge of the Condrey Mountain Road less Area, numerous narrow north-trending zones of mineralized schist extend from near Copper Butte on the south, northward to Elliot Creek (Coleman and others, 1983). These zones contain abundant pyrite mineralization associated with pyrrhotite, chalcopyrite, qalena, and sphalerite. The Blue Ledge mine area, 2 mi north-northeast of Copper Butte, is located in a zone of mineralized schist. The western Paleozoic and Triassic belt that nearly surrounds the Condrey Mountain Schist is a melange of sedimentary, volcanic, and ultramafic rocks metamorphosed to amphibolite facies (Coleman and others, 1983). Only two samples of the metamorphic melange were collected near the Condrcy Mountain Road less Area, but extensive sampling of this unit southwest of the roadless area yielded an average sample density of 2.86±0.15 g/cm 3 (112 samples) (Jachens and others, 1983).

California↗
USGS1983

AIRYROOT: A Fortran program for calculating the gravitational attraction of an Airy isostatic root out to 166.7 km

This report describes a FORTRAN-77 program called AIRYROOT which calculates the gravitational attraction at sea level of an Airy isostatic root extending out to a radius of 166.7 km. The program incorporates an efficient algorithm devised by Parker (1912) which uses the fast Fourier transform to calculate potential field anomalies. Input is a rectangular grid in a projected coordinate system which contains values describing topography (elevations and depths in meters) or surface load (mass in g/cm2). Output is a rectangular grid of values of the gravitational attraction of the root in milligals (mGal). A strategy is outlined for combining this output with published results describing the attraction of the root beyond 166.7 km, for upward continuing the total attraction to appropriate observation elevations, and for subtracting the result from Bouguer gravity data to produce an isostatic residual gravity map. This program can also be used to make complete Bouguer corrections to marine gravity data given appropriate bathymetric data sets.

Open-File Report↗
USGS1982

Differential compaction mechanism for earth fissures near Casa Grande, Arizona

Precise gravity measurements indicate that earth fissures or tension cracks caused by ground-water withdrawal within a 10-km 2 area southeast of Casa Grande, Arizona, are associated with relief on the buried interface between the alluvial aquifer and underlying bedrock. All of the fissure zones; which have a cumulative length of >8.7 km, occur above either ridges or steps in the bedrock surface. Intersecting fissure zones overlie intersecting bedrock features, and the angle of intersection of the zones accurately reflects the angle between the bedrock features. These relations suggest that the fissures are forming in response to localized differential compaction caused by localized variations of aquifer-system thickness. Topographic profiles across fissures on undisturbed desert floor confirm differential compaction proportional to the variations in aquifer thickness. The occurrence of the fissures at points of maximum convex-upward curvature in profiles of both the topographic and buried bedrock surfaces indicates that the fissures result from tensile strains caused by bending of the strata above the buried bedrock features in response to the differential compaction. Tensile strains at failure are estimated to range from ∼ 0.02% to 0.2% on the basis of modeling of the bending process.

Geological Society of America Bulletin↗
USGS1980

Geophysical observations of Kilauea volcano, Hawaii, 1. temporal gravity variations related to the 29 November, 1975, M = 7.2 earthquake and associated summit collapse

Repeated high-precision gravity measurements made near the summit of Kilauea volcano, Hawaii, have revealed systematic temporal variations in the gravity field associated with a major deflation of the volcano that followed the 29 November, 1975, earthquake and eruption. Changes in the gravity field with respect to a stable reference station on the south flank of neighboring Mauna Loa volcano were measured at 18 sites in the summit region of Kilauea and at 4 sites far removed from its summit. The original survey, conducted 10-23 November, 1975, was repeated during a two-week period after the earthquake. The results indicate that sometime between the first survey and the latter part of the second survey the gravity field at sites near the summit increased with respect to that at sites far removed from the summit. The pattern of gravity increase is essentially radially symmetrical, with a half-width slightly less than 3 km, about the point of maximum change 1.5 km southeast of Halemaumau pit crater. Gravity changes at sites near the summit correlate closely with elevation decreases that occurred sometime between leveling surveys conducted in late September 1975 and early January 1976. The systematic relation between gravity and elevation change (-1.71 ?? 0.05 (s.e.) ??gal/cm) shows that deflation was accompanied by a loss of mass from beneath the summit region. Mass balance calculations indicate that for all reasonable magma densities, the volume of magma withdrawn from beneath the summit region exceeded the volume of summit collapse. Analysis suggests that magma drained from at least two distinct areas south of Kilauea caldera that coincide roughly with two reservoir areas active during inflation before the 1967-1968 Kilauea eruption. ?? 1980.

Journal of Volcanology and Geothermal Research↗