The electrical structure of the Slave craton

The Slave craton in northwestern Canada, a relatively small Archean craton (600×400 km), is ideal as a natural laboratory for investigating the formation and evolution of Mesoarchean and Neoarchean sub-continental lithospheric mantle (SCLM). Excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information becoming available.

Over the last 5 years deep-probing electromagnetic surveys were conducted on the Slave, using the natural-source magnetotelluric (MT) technique, as part of a variety of programs to study the craton and determine its regional-scale electrical structure. Two of the four types of surveys involved novel MT data acquisition; one through frozen lakes along ice roads during winter, and the second using ocean-bottom MT instrumentation deployed from float planes.

The primary initial objective of the MT surveys was to determine the geometry of the topography of the lithosphere–asthenosphere boundary (LAB) across the Slave craton. However, the MT responses revealed, completely serendipitously, a remarkable anomaly in electrical conductivity in the SCLM of the central Slave craton. This Central Slave Mantle Conductor (CSMC) anomaly is modelled as a localized region of low resistivity (10–15 Ω m) beginning at depths of 80–120 km and striking NE–SW. Where precisely located, it is spatially coincident with the Eocene-aged kimberlite field in the central part of the craton (the so-called “Corridor of Hope”), and also with a geochemically defined ultra-depleted harzburgitic layer interpreted as oceanic or arc-related lithosphere emplaced during early tectonism. The CSMC lies wholly within the NE–SW striking central zone defined by Grütter et al. [Grütter, H.S., Apter, D.B., Kong, J., 1999. Crust–mantle coupling; evidence from mantle-derived xenocrystic garnets. Contributed paper at: The 7th International Kimberlite Conference Proceeding, J.B. Dawson Volume, 1, 307–313] on the basis of garnet geochemistry (G10 vs. G9) populations.

Deep-probing MT data from the lake bottom instruments infer that the conductor has a total depth-integrated conductivity (conductance) of the order of 2000 Siemens, which, given an internal resistivity of 10–15 Ω m, implies a thickness of 20–30 km. Below the CSMC the electrical resistivity of the lithosphere increases by a factor of 3–5 to values of around 50 Ω m. This change occurs at depths consistent with the graphite–diamond transition, which is taken as consistent with a carbon interpretation for the CSMC.

Preliminary three-dimensional MT modelling supports the NE–SW striking geometry for the conductor, and also suggests a NW dip. This geometry is taken as implying that the tectonic processes that emplaced this geophysical–geochemical body are likely related to the subduction of a craton of unknown provenance from the SE (present-day coordinates) during 2630–2620 Ma. It suggests that the lithospheric stacking model of Helmstaedt and Schulze [Helmstaedt, H.H., Schulze, D.J., 1989. Southern African kimberlites and their mantle sample: implications for Archean tectonics and lithosphere evolution. In Ross, J. (Ed.), Kimberlites and Related Rocks, Vol. 1: Their Composition, Occurrence, Origin, and Emplacement. Geological Society of Australia Special Publication, vol. 14, 358–368] is likely correct for the formation of the Slave's current SCLM.     

The influence of subsurface moisture on rill system evolution

Rill development studies have focused almost exclusively on surface erosion processes and critical threshold hydraulic conditions. Characteristic rill features, such as arcuate headcuts and knickpoints, are morphologically similar to the ‘theatre-headed’ valleys which have been associated with ‘sapping’ processes at various scales. This paper reports on laboratory experiments designed to identify linkages between surface flow hydraulics, subsurface moisture conditions and rill development. Experiments were carried out in a 16·57 m² flume under simulated rainfall with soil samples up to 0·15 m depth in which moisture conditions were monitored by miniature time-domain reflectometer probes. Tests showed complex responses in which some rill incision reflected surface flow conditions, but major rill system development with markedly enhanced sediment yield was closely associated with high soil moisture contents. It was not possible to measure seepage forces directly, but calculation and observation indicate that these were less important than reduction in soil strength with saturation, which resulted in increased effective runoff erosivity. This caused concentrated undercutting along the water table at rill walls, while slightly stronger surface layers above the water table formed microscarps. These retreated along the water table into interrill surfaces, producing residual pediment transport slopes. The microscarps eventually disappeared when the water table reached the surface, eliminating differential soil strength. The experiments showed complex relationships between surface and subsurface erosional processes in evolving rill systems, strongly influenced by soil moisture dynamics. The very small topographic and hydraulic head amplitudes indicate that seepage forces and ‘sapping’ were minimal. The dominant effect of soil moisture was reduction of soil strength with saturation, and increased runoff entrainment. Experimental conditions were not unusual, either for agricultural fields or natural hillslopes, and the intricate interrelationship of surface and subsurface erosion processes observed is probably not uncommon. Attempts to link specific morphologic features at rill scale to dominance of surface or subsurface processes alone are therefore unlikely to be successful or reliable. © 1998 John Wiley & Sons, Ltd.     

The distribution of carbon monoxide in the lower atmosphere of Venus

We have obtained spatially resolved near-infrared spectroscopy of the Venus nightside on 15 nights over three observing seasons. We use the depth of the CO absorption band at 2.3 μm to map the two-dimensional distribution of CO across both hemispheres. Radiative transfer models are used to relate the measured CO band depth to the volume mixing ratio of CO. The results confirm previous investigations in showing a general trend of increased CO abundances at around 60° latitude north and south as compared with the equatorial regions. Observations taken over a few nights generally show very similar CO distributions, but significant changes are apparent over longer periods. In past studies it has been assumed that the CO latitudinal variation occurs near 35 km altitude, at which K-band sensitivity to CO is greatest. By modeling the detailed spectrum of the excess CO at high latitudes we show that it occurs at altitudes around 45 km, much higher than has previously been assumed, and that there cannot be significant contribution from levels of 36 km or lower. We suggest that this is most likely due to downwelling of CO-rich gas from the upper atmosphere at these latitudes, with the CO being removed by around 40 km through chemical processes such as the reaction with SO₃.     

Galactic Spiral Arms and Dynamo Control Parameters

We discuss the effects of galactic spiral arms on the -coefficient, turbulent diffusivity and turbulent energy density of the interstellar turbulence. We argue that the -coefficient and the dynamo number are larger in the interarm regions, whereas the kinetic energy density of turbulence is larger in the arms; the turbulent magnetic diffusivity can be only weakly affected by the spiral pattern.     

Rotation of a two-component model neutron star in the GTR

Equations are obtained for the dynamics of the rotation of a two-component model neutron star within the framework of the generai theory of relativity. It is shown that for steady rotation of the star’s normal component, Ω_c = const, the angular velocity Ω_s of the superfluid component depends on the coordinates and is Ω_c + ω, where ω is the nondiagonal component of the metric tensor. Translated from Astrofizika, Vol. 40, No. 3, pp. 403–412, August, 1997.     

A quantitative evaluation of Playfair's law and its use in testing long‐term stream erosion models

Playfair's law (J. Playfair, illustrations of the Huttonian Theory of the Earth, 1802) requires any two tributaries in a river network to lower at the same rate near their junction. Although this law holds exactly at the junction, it is unclear how well it holds in the vicinity of the junction. This issue has practical importance because Playfair's law has been used to estimate parameters for detachment‐limited models of erosion. If the incision rate of a stream is modelled as βA^mSⁿ, where β is an erodibility parameter, A is the area drained by the stream, and S is the local gradient of the channel, then the ratio of the parameters m/n can be estimated from junctions by assuming that Playfair's law holds over the distance used to determine S for each tributary. In this paper, Playfair's law and associated m/n estimates are evaluated for simulated basins with constant and temporally varying uplift rates (or baselevel lowering rates). The results demonstrate that estimates of m/n may be biased for basins with upward‐concave stream profiles because the local slope must be approximated with an average upstream slope. In addition, when uplift rate varies temporally, knickpoints are shown to travel through the basins with constant vertical velocity. Because incision rates vary within the basin, Playfair's law only holds exactly at the junctions. These effects are more important when slopes are measured over longer distances. Finally, measurement techniques are presented which address these potential biases. Copyright © 2001 John Wiley & Sons, Ltd.     

Large-scale solar magnetic field: Latitudinal dependence

Large-scale solar magnetic fields in the latitude range 50° S–50° N are analyzed in detail for a long time interval (1915–1990). We are primarily concerned with the two types of large-scale fields forming the two-and four-sector patterns on the Sun. The rotation parameters of these structures are obtained for all latitudes considered. The contribution of the two-sector structure grows and that of the four-sector structure decreases toward high latitudes. The magnetic field is activated simultaneously over a wide latitude range. Since both magnetic-field systems exhibit quasi-rigid rotation, their current systems must either be concentrated in a narrow latitude range or be situated beneath the convection zone, where rotation is only weakly differential. A period of about three years is manifest in the difference between the rotation periods for the two types of magnetic field. Physically, this may imply that these oscillations are external with respect to any level, and there is some phase delay due to their propagation from one level to another. We can conclude with a fair degree of certainty that as the activity level rises, the rotation speed decreases, and vice versa.     

A fuzzy-Markov-chain-based analysis method for reservoir operation

In this study, a fuzzy-Markov-chain-based stochastic dynamic programming (FM-SDP) method is developed for tackling uncertainties expressed as fuzzy sets and distributions with fuzzy probability (DFPs) in reservoir operation. The concept of DFPs used in Markov chain is presented as an extended form for expressing uncertainties including both stochastic and fuzzy characteristics. A fuzzy dominance index analysis approach is proposed for solving multiple fuzzy sets and DPFs in the proposed FM-SDP model. Solutions under a set of α-cut levels and fuzzy dominance indices can be generated by solving a series of deterministic submodels. The developed method is applied to a case study of a reservoir operation system. Solutions from FM-SDP provide a range of desired water-release policies under various system conditions for reservoir operation decision makers, reflecting dynamic and dual uncertain features of water availability simultaneously. The results indicate that the FM-SDP method could be applicable to practical problems for decision makers to obtain insight regarding the tradeoffs between economic and system reliability criteria. Willingness to obtain a lower benefit may guarantee meeting system-constraint demands; conversely, a desire to acquire a higher benefit could run into a higher risk of violating system constraints.     

Paleomagnetic and palynologic analyses of Albian to Santonian strata at Bayn Shireh, Burkhant, and Khuren Dukh, eastern Gobi Desert, Mongolia

Cretaceous terrestrial sediments deposited in a series of intracratonic basins across the Gobi Desert region of southern Mongolia and northern China contain a unique and diverse vertebrate fauna. In 1996 an expedition jointly sponsored by the Mongolian Paleontological Center and the Hayashibara Museum of Natural Sciences revisited a number of famous vertebrate fossil localities in the eastern Gobi region of Mongolia and, as part of a broad geological and paleontological study, collected a series of paleomagnetic samples from measured sections at Bayn Shireh, Burkhant and Khuren Dukh, as well as from an unmeasured locality adjacent to Khuren Dukh. Expedition members also collected palynologic samples from Khuren Dukh and the adjacent locality. Paleomagnetic analysis shows that all the sites from which samples were collected display detrital remnant magnetization that is consistently normal in polarity. The measured Cretaceous magnetic directions are oriented to the east or northeast of the present day expected direction (declination 356.2°, inclination 65.2°), and they are wholly concordant with that expected for a mid-latitude Northern Hemisphere sampling locality, and with the directions for this period reported by other workers. These results, when considered in tandem with the known biostratigraphy, strongly suggest that the sedimentary deposits at all four localities in the eastern Gobi correlate to the normal polarity chron 34 (the Cretaceous Long Normal), which ranges in age from approximately 121 to 83.5 million years. Previous vertebrate, invertebrate and palynological data from Khuren Dukh suggest that the lower and middle parts of the stratigraphic interval exposed there (which have been assigned to the Shinekhudag Formation) are ‘Khukhtekian’ in age and correspond to the Aptian–Albian interval that can be broadly correlated to the older, Early Cretaceous part of the Cretaceous Long Normal, C34n. New palynologic data presented here indicate that these strata are no older than middle to late Albian. The rocks at Bayn Shireh (the Bayn Shireh Formation) have been assigned a ‘Baynshirenian’ biostratigraphic age that may range from Cenomanian to early Campanian. The magnetostratigraphy results presented here indicate that the strata at both the Bayn Shireh and Burkhant localities do not cross the Santonian/Campanian Stage boundary, however, as this is believed to lie at, or very near, the C34n/C33r reversal boundary. Thus, the Bayn Shireh Formation was most likely deposited near the end of the Cretaceous Long Normal Interval, no later than the latest Santonian.