Reconstructing the ancestral Yellowstone plume from accreted seamounts and its relationship to flat-slab subduction     

J.Brendan Murphy  Andrew J. Hynes  Stephen T. Johnston  J.Duncan Keppie 《Tectonophysics》2003,365(1-4):185

Recent geodynamic analyses have emphasized the relationship between modern flat-slab subduction zones and the overriding of buoyant oceanic crust. Although most models for the evolution of the Late Mesozoic–Cenozoic Laramide orogeny in the southwestern United States involve flat-slab subduction, the mechanisms proposed are controversial. An examination of the geological evolution of the 60–50-Ma Crescent terrane of the Coast Ranges indicates that it was formed in a shallowing-upward Loihi-type oceanic setting culminating in the eruption of subaerial lavas. Plate reconstructions indicate that the Crescent terrane was emplaced into ca. 20-Ma crust, and the presence of subaerial lavas implies an uplift due to the plume of ca. 4.2 km, which we use to calculate a minimum buoyancy flux of 1.1 Mg s⁻¹, similar to that of the modern Yellowstone plume.Published paleomagnetic data indicate that the Crescent terrane was formed at a paleolatitude similar to that of the Yellowstone plume. The Crescent seamount was accreted within 5 My of the cessation of plume magmatism. Plate reconstructions indicate that it would have originated about 750 km to the west of the North American plate margin if it developed above a fixed Yellowstone plume, and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement.We interpret the Crescent terrane as due to the ancestral Yellowstone plume. Such a plume would have generated an elongate swell and related plateau that would have been overridden by the North American margin. Taken together, the relationship between flat-slab and overriding of oceanic plateau in Laramide times would have been analogous to the relationship between modern Andean flat-slab subduction zones and the Juan Fernandez and Nazca oceanic plateaus.  相似文献   

Planetary rings,by Larry W. Esposito     

Guy Consolmagno SJ 《Meteoritics & planetary science》2007,42(2):301-302

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Geomorphic and sedimentologic evidence for the separation of Lake Superior from Lake Michigan and Huron     

John W. Johnston  Todd A. Thompson  Douglas A. Wilcox  Steve J. Baedke 《Journal of Paleolimnology》2007,37(3):349-364

A common break was recognized in four Lake Superior strandplain sequences using geomorphic and sedimentologic characteristics. Strandplains were divided into lakeward and landward sets of beach ridges using aerial photographs and topographic surveys to identify similar surficial features and core data to identify similar subsurface features. Cross-strandplain, elevation-trend changes from a lowering towards the lake in the landward set of beach ridges to a rise or reduction of slope towards the lake in the lakeward set of beach ridges indicates that the break is associated with an outlet change for Lake Superior. Correlation of this break between study sites and age model results for the strandplain sequences suggest that the outlet change occurred sometime after about 2,400 calendar years ago (after the Algoma phase). Age model results from one site (Grand Traverse Bay) suggest an alternate age closer to about 1,200 calendar years ago but age models need to be investigated further. The landward part of the strandplain was deposited when water levels were common in all three upper Great Lakes basins (Superior, Huron, and Michigan) and drained through the Port Huron/Sarnia outlet. The lakeward part was deposited after the Sault outlet started to help regulate water levels in the Lake Superior basin. The landward beach ridges are commonly better defined and continuous across the embayments, more numerous, larger in relief, wider, have greater vegetation density, and intervening swales contain more standing water and peat than the lakeward set. Changes in drainage patterns, foreshore sediment thickness and grain size help in identifying the break between sets in the strandplain sequences. Investigation of these breaks may help identify possible gaps in the record or missing ridges in strandplain sequences that may not be apparent when viewing age distributions and may justify the need for multiple age and glacial isostatic adjustment models. This is the third in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.  相似文献   

Geography and Revolution – Edited by David N. Livingstone and Charles W. J. Withers     

Ron Johnston 《Singapore journal of tropical geography》2007,28(2):246-248

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New data on the Baraba Formation age (the Sredinnyi Range of Kamchatka)     

A. V. Solov’ev  T. N. Palechek  M. N. Shapiro  S. A. Johnston  J. I. Garver  D. M. Ol’shanetskii 《Stratigraphy and Geological Correlation》2007,15(1):112-119

In the Sredinnyi Range of Kamchatka, the Baraba Formation of continental conglomerates is assumed to be of the late Campanian age based on found flora remains, but data of isotopic geochronology suggest the Eocene age of these deposits. New data on radiolarians from cherty pebbles are considered in this work along with results of fission-track dating of zircons from pebbles and matrix of the Baraba conglomerates. Fission-track dates obtained for zircons from matrix approve the Eocene age of the Baraba Formation, and new dates characterizing pebbles are not contradicting this conclusion. The Baraba Formation structural position can hardly be lower, therefore, than that of the Irunei Formation.  相似文献   

The 1997 periastron passage of the binary pulsar PSR B1259−63     

S. Johnston  N. Wex  L. Nicastro  R.N. Manchester  A.G. Lyne 《Monthly notices of the Royal Astronomical Society》2001,326(2):643-648

We report here on multifrequency radio observations of the pulsed emission from PSR B1259−63 around the time of the closest approach (periastron) to its B2e companion star. There was a general increase in the dispersion measure (DM) and scatter-broadening of the pulsar, and a decrease in the flux density towards periastron although fluctuation in these parameters were seen on time-scales as short as minutes. The pulsed emission disappeared 16 d prior to periastron and remained undetectable until 16 d after periastron.
The observations are used to determine the parameters of the wind from the Be star. We show that a simple model, in which the wind density varies with radius as r ⁻², provides a good fit to the data. The wind is highly turbulent with an outer scale of ≤10¹⁰ cm and an inner scale perhaps as small as 10⁴ cm, a mean density of ∼10⁶ cm⁻³ and a velocity of ∼2000 km s⁻¹ at a distance of ∼50 stellar radii. We find a correlation between DM variations and the pulse scattering times, suggesting that the same electrons are responsible for both effects.  相似文献   

A Conceptual Framework for Enhancing the Utility of Rainfall Hazard Forecasts for Agriculture in Marginal Environments   总被引：1，自引：0，他引：1  

Muhammad?T.?UsmanEmail author  Emma?Archer  Peter?Johnston  Mark?Tadross 《Natural Hazards》2005,34(1):111-129

Semi-arid and dry sub-humid areas (especially in the tropics) are characterized by high inter-annual and intra-seasonal rainfall variability. Agriculture, which employs the bulk of the rapidly increasing populations, is largely rain-fed, low-input based and highly resource dependent. Recent spates of drought have, therefore, exacerbated the now-too-familiar specter of famine and starvation in these areas with glaring examples being the recurring episodes in sub-Saharan Africa since the great Sahel drought of 1969–1973. A great need for accurate and timely hazard forecast products in aid of agriculture thus exists.Several schemes are currently employed by various agencies around the globe in this direction. There does remain, however, a gap between product provision and user expectations. This paper examines this gap suggesting a five-point framework within which it can be addressed as an action agenda for the climate science community. The paper posits that changes are possible to existing methodologies (related to three of these points), which, within the context of current science, can greatly enhance the utility of forecast products for agriculture in marginal areas. The remaining two points have, however, been identified as requiring additional applied research and necessary pointers for addressing these issues are provided. First is the need for appropriate impact-related indicators for intra-seasonal and interannual rainfall variability that are easy to compute, amenable to forecasting and follow closely the experiences of farmers in marginal areas. The second is a consideration of appropriate forecast information formatting and communication medium that guarantee effective feedback between forecast producers and users. Specific examples of the status quo and of work currently underway are cited from southern Africa – a region currently attracting international attention as a result of recent droughts and the threat of famine.  相似文献   

XMM–Newton observations of PSR B1259−63 near the 2004 periastron passage     

M. Chernyakova †  A. Neronov  A. Lutovinov  J. Rodriguez  S. Johnston 《Monthly notices of the Royal Astronomical Society》2006,367(3):1201-1208

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Center and periphery: spatial variations in politics: Gottman, J.(ed), Beverly Hills and London: Sage, 1980.226pp.     

R.J. Johnston 《Applied geography (Sevenoaks, England)》1981,1(4):312-313

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A Mössbauer spectroscopic and X-ray diffraction study of the iron mineralogy of some sediments from the Southwestern Pacific Basin     

J.H Johnston  G.P Glasby 《Marine Chemistry》1982,11(5):437-448

The results of a ⁵⁷Fe Mössbauer spectroscopic and X-ray diffraction study of four pelagic clay sediments from the Southwestern Pacific Basin collected at increasing distances from New Zealand, on a New Zealand—Rarotonga transect, are presented. These show that, with increasing distance from New Zealand, the Fe³⁺/Fe²⁺ ratio of the sediment increases as a result of the increasing contribution of the X-ray amorphous secondary hydrous iron oxides, notably ferrihydrite, due to the longer sedimentation periods and consequent greater degree of oxidation of the sediments. This increase in the Fe³⁺/Fe²⁺ ratio is correlated with the change in colour of the sediments from pale yellowish to dusky brown, as well as a number of other sediment parameters. Apart from a slight increase in the relative abundance of montmorillonite with increasing distance from New Zealand, the abundances of the other clay minerals, illite and kaolinite, and of chlorite in the sediments are approximately equal. Quartz and feldspar with minor augite are the dominant detrital minerals. Calcite is present in minor amounts in some of the sediments.  相似文献