Gravity profiles across the Cheyenne Belt, a precambrian crustal suture in southeastern Wyoming     

R.A. Johnson  K.E. Karlstrom  S.B. Smithson  R.S. Houston 《Journal of Geodynamics》1984,1(3-5)

Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm³. Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes.  相似文献   

Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites     

D.D. Bogard  L.E. Nyquist  P. Johnson 《Geochimica et cosmochimica acta》1984,48(9):1723-1739

Isotopic concentrations of the noble gases have been measured in several different phases of Elephant Moraine A79001 and in whole rock samples of Zagami and Allan Hills A77005, three meteorites which belong to the rare group of SNC achondrites that may have originated from the planet Mars. Shocked phases of EETA79001 contain a trapped Ar, Kr, and Xe component characterized by ${}^{84}\text{Kr}{}^{132}\text{Xe}$ ～15, ${}^{40}\text{Ar}{}^{36}\text{Ar}\text{> 2000}$, ${}^{129}\text{Xe}{}^{132}\text{Xe ≥ 2}$, and ${}^4\text{He}{}^{40}\text{Ar}\text{≤ 0.1}$. These elemental and isotopic ratios are unlike those for any other noble gas component except analyses of the Martian atmosphere made by Viking spacecraft. The isotopic composition of the trapped Kr shows an approximate 1% per mass unit enrichment of lighter isotopes compared to terrestrial Kr, and the traped Xe may show either a fission component or a fractionated enrichment of heavier isotopes compared to terrestrial Xe. It is hypothesized that these gases represent a portion of the Martian atmosphere which was shock-implanted into EETA79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. Cosmic ray-produced gases in the eight known SNC meteorites form three distinct groups with exposure ages of ～11 MY (Chassigny and the nakhlites), ～2.6 MY (Shergotty, Zagami, and ALHA77005), and ～0.5 MY (EETA79001). These ages suggest three distinct events and cannot have been produced by irradiation for a common time under greatly different shielding. Comparison of cosmogenic ${}^3\text{He}{}^{21}\text{Ne}$ measured in EETA79001 with two independent models for the production of this ratio as a function of shielding indicates that this meteorite was irradiated in space as a relatively small object. If the SNC meteorites were ejected from Mars ～ 180 My ago, the shock age of the shergottites, they must have been relatively large objects (>6 meters diameter) which experienced at least three space collisions to initiate cosmic ray exposure. Ejection from Mars by three events at the times of initiation of cosmic ray exposure would permit the ejected objects to have been much smaller (<1 meter diameter), but would require three such events on 1.3 Gy Martian terraine in the past ～10 MY and would not explain the common 180 MY shock age seen in all four shergottites.  相似文献   

Quasi-Lagrangian diagnostics of a Mediterranean cyclone: Isentropic results     

Prof. Dr. D. R. Johnson  D. K. Hill 《Meteorology and Atmospheric Physics》1987,36(1-4):118-140

  相似文献   

Location Analysis in Ground-Water Remediation Using Neural Networks   总被引：3，自引：0，他引：3  

Virginia M. Johnson  Leah L. Rogers 《Ground water》1995,33(5):749-758

  相似文献   

Large-scale volcanic cone collapse: The 1888 slope failure of Ritter volcano,and other examples from Papua New Guinea     

R. W. Johnson 《Bulletin of Volcanology》1987,49(5):669-679

A largely submarine avalanche amphitheatre that formed catastrophically in 1888 on Ritter volcano has been identified from a bathymetric survey. Collapse of the volcano in 1888 therefore is considered to have been caused by rapid, large-scale slope failure, rather than by cauldron subsidence, as previously supposed. Escarpments of pre-historic slope failures are common on other Papua New Guinea volcanoes. Directions of avalanching on some volcanoes in the Bismarck volcanic arc appear to be controlled by a regional stress pattern, and those for some volcanoes in the Fly-Highlands province on mainland Papua New Guinea point away from the regional centre of Pliocene uplift. Large amphitheatres such as at Doma Peaks in the Fly-High-lands province probably originated by multiple collapses.  相似文献   

Comments on P.K. Theobald's Presidential Address, Hannover, 1980     

S. M. Johnson 《Journal of Geochemical Exploration》1982,17(2)

  相似文献   

Solubility of rhodochrosite (MnCO₃) in water and seawater     

Kenneth S. Johnson 《Geochimica et cosmochimica acta》1982,46(10):1805-1809

The solubility product of rhodochrosite (MnCO₃) was measured in seawater, deionized water and dilute NaCl solutions. The solubility product extrapolated to infinite dilution at 25.0 C was (2.60 ± 0.07)× 10^?11. The stoichiometric solubility product measured in seawater of 34.27%. salinity was (3.24 ± 0.23) × 10^?9 at 25.0 C and (2.28 ± 0.24) × 10^?9 at 3.3 C. The stoichiometric solubility product is in good agreement with the value calculated from an ion association model. The enthalpy of the reaction is in fair agreement with the estimated value.  相似文献   

Folding of passive layers and forms of minor structures near terminations of blind thrust faults—application to the central Appalachian blind thrust     

Philip Berger  Arvid M. Johnson 《Journal of Structural Geology》1982,4(3):343-353

The existence of detachment surfaces or décollement zones beneath folded rocks of the Valley and Ridge and Plateau provinces of the Appalachians has been recognized as an important condition of folding. Large folds at the border between the two provinces resulted primarily from repetition of strata by thrusting of blocks over ramp faults that connect detachement surfaces at different horizons. Some investigators have suggested that folds in the Plateau province of Pennsylvania were produced by splay faults arising from detachment surfaces, but field observations and theoretical analyses by Sherwin and by Wiltschko & Chapple suggest that the folds are a result of buckling of multilayered rocks above a décollement. An exception may be the Burning Springs anticline in West Virginia, which appears to have formed at the termination of a detachment surface.Investigation of the translation of an homogenous, viscous material above a flat detachment surface that terminates laterally indicates that the termination produces a broad, low-amplitude anticline in passive layering as a result of thickening induced by a gradient of shear stresses in the vertical direction. This thickening above the termination of a detachment is a mechanism of folding. If the viscous fluid contained mechanical layering, the fold would become amplified by buckling. Computations of stresses in the material indicate that minor faults should be generated first near the termination of the flat detachment surface. The Burning Springs anticline probably was initiated by termination of a detachment surface and subsequently amplified by buckling.  相似文献   

Pleistocene volcanic rocks from the Fly-Highlands province of western Papua New Guinea: A note on new Sr and Nd isotopic data and their petrogenetic implications     

P.J. Hamilton  R.W. Johnson  D.E. Mackenzie  R.K. O'Nions 《Journal of Volcanology and Geothermal Research》1983,18(1-4)

RB-Sr and Sm-Nd isotopic and trace-element-abundance values have been determined for 15 mafic and intermediate rocks from six Pleistocene volcanic centres of the Fly-Highlands province. ⁸⁷Sr/⁸⁶Sr and _{N d} values range from 0.70362 to 0.70540, and +1.9 to +5.9, respectively. These new data can be accounted for by contamination of mantle-derived magmas by the continental crust through which the magmas have risen. They do not, however, preclude derivation of some of the Sr and Nd from subducted crust, nor are they inconsistent with Sr and Nd enrichments having taken place by means of mantle metasomatic events. Nevertheless, there is no Benioff zone beneath the Fly-Highlands province (although there is geological evidence for Cretaceous subduction). A preferred interpretation is that uncontaminated, mantle-derived magmas are related to the Pliocene crustal uplift that caused the development of the highlands and which formed in response to a mid-Tertiary continent/island-arc collision.  相似文献   

The interpretation of the isotopic ratio235U/238U in fission track dating: Comment     

V. E. McGee  N. M. Johnson 《Mathematical Geosciences》1983,15(3):497-498

  相似文献