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81.
Keith A.W Crook 《Journal of Structural Geology》1980,2(3):289-303
The extended evolution of fore-arc regions which leads to their eventual incorporation into stable kratonic continental crust is elucidated by a general model based upon observations from the modern circum-Pacific and the Palaeozoic Tasman Geosyncline.Fore-arc regions widen during subduction in the manner described by Karig & Sharman (1975). Their history, after subduction has ceased, depends upon the thickness of the accretionary prism formed during subduction. Where the prism is thick (ca. 20 km) kratonization is a single-step process. The fore-arc region remains above sea-level; post-arc silicic volcanics accumulate due to granitoid plutonism, the magmas being derived by melting of the subduction complex and from the oceanic lithosphere trapped beneath it. The volcanic arc subsides, becoming the site of a fore-deep.Intermediate-thickness accretionary prisms (ca. 16 km) are kratonized in a two-step process. They remain at shelf depths, while their associated volcanic arcs sink to comparable depths. Both acquire a post-arc shallow marine sequence of typical platform-cover facies. They are then deformed and intruded by granitoids when the crust attains critical thickness (ca. 20 km).Thin accretionary prisms (≤ 12 km) require a three-step process for kratonization. They and their associated arcs sink to bathyal depths. They are overwhelmed by prograding post-arc flysch deposits of continental origin. Deformation of the post-arc flysch and plutonism occur when critical crustal thickness (ca. 20 km) is attained. A transitional tectonic regime ensues, with molasse-like transitional basins preferentially sited over the extinct volcanic arcs and the thinner parts of buried accretionary prisms.The model satisfactorily explains the Late Proterozoic-Palaeozoic evolution of southeast Australia, where a 1000 km wide tract of continental crust was accreted to the Australian Kraton in 250–300 Ma, beginning as a S.W. Pacific-type oceanic terrain. It has been found useful for interpreting geosynclinal terrains in other continents.According to the model, the dynamic processes that contribute to kratonization are systematically causally connected. Kratonization is a unified, internally deterministic and self-sustaining phenomenon. The model has implications for the origin, ‘stratigraphy’ and composition of upper and lower continental crust; the origins and tectonic settings of ophiolites, granitoids, paired metamorphic belts and transitional basins; and for the nature and causes of orogenesis. 相似文献
82.
A Spectrum-Independent Procedure for Correcting Eddy Fluxes Measured with Separated Sensors 总被引:1,自引:0,他引:1
We investigate flux underestimates in eddy correlation measurements that are caused by horizontal separation of the sensors. A common eddy correlation setup consists of a sonic anemometer and a humidity sensor which, because of its bulk, must be placed some distance away from the sonic path, leading to a flux loss (of latent heat). Utilizing an additional fast temperature sensor placed near the humidity sensor, we develop a procedure for correcting for this loss. The procedure simultaneously corrects the sensible heat flux for the difference between true temperature and sonic temperature. Our correction procedure, which does not depend on the shape of the cospectrum, is then compared to the widely-used procedure following Moore (1986), which assumes a cospectral model ('Kansas Model). Both correction methods are applied to data collected within the internal boundary layer over a rice paddy, downwind of arid land. Under conditions of good fetch, they were found to agree well. Under poor fetch conditions, the model-based correction tended to be too small, while the spectrum-independent combined correction was robust. The latter is thus recommended for situations where the cospectral shape can be expected to deviate from the 'Kansas shape. 相似文献
83.
Neutron activation determination of La, Ce, Sm, Eu, Tb, Yb, Lu, Ta, Hf, Sc, Co and Th in potassic lavas from the Birunga and Toro-Ankole regions show that the rocks are characterized by high rare earth element (REE) contents (161–754 ppm) and form two groups based upon differing La/Yb ratios. One group is made up of katungite, ugandite and mafurite with La/Yb =146–312, and the other of rocks of the leucitite and phonolitic tephrite series, La/Yb =30–56. The trace element content of the ugandite group is similar to that of kimberlites. The data do not indicate any trends of differentiation or simple relationships between the two groups of rocks, although katungite is unlikely to be parental to rocks of lower La/Yb ratios. It is unlikely that in terms of La/Yb ratios that partial melting of mica-garnet-lherzolite mantle can form katungite because of the very small amounts of partial melting required (0.2%), although the La/Yb ratios of 150–200 (ugandites, mafurites) and 30–60 (leucitites, phonolitic tephrites) can be accounted for by 0.3–1.5% and 1–9% melting respectively, if the REE are then concentrated without further La and Yb fractionation. Partial melting of mantle which has been metasomatized by alkaline earths and REE bearing fluids or mixing of carbonatite and nephelenite are also compatable with the observed geochemistry of the lavas. It is considered that gas transfer processes which selectively enrich the light REE may have obscured REE evidence pertaining to early partial melting and/or differentiation processes and therefore that REE geochemistry is of little use in determining the petrogenetic processes involved in the formation of potassic lavas. 相似文献
84.
85.
Sedimentary and igneous rocks of the Swaziland Sequence of South Africa (>3,200 my. old) contain total carbon (0.09–8.28%), organic carbon (0.04–1.95%) and sulfur (9–53,000 ppm). Amounts of organic carbon are generally high in the lower part of the Sequence (based on two samples from the Theespruit Formation). Sulfur content varies widely with no obvious correlations or trends. These rocks may contain clues to the early carbon and sulfur cycles on earth. 相似文献
86.
Despite the long history of the continuum equation approach in hydrology, it is not a necessary approach to the formulation of a physically based representation of hillslope hydrology. The Multiple Interacting Pathways (MIPs) model is a discrete realization that allows hillslope response and transport to be simultaneously explored in a way that reflects the potential occurrence of preferential flows and lengths of pathways. The MIPs model uses random particle tracking methods to represent the flow of water within the subsurface alongside velocity distributions that acknowledge preferential flows and transition probability matrices, which control flow pathways. An initial realization of this model is presented here in application to a tracer experiment carried out in Gårdsjön, Sweden. The model is used as an exploratory tool, testing several hypotheses in relation to this experiment. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
87.
Susan TAYLOR Keith W. JONES Gregory F. HERZOG Claire E. HORNIG 《Meteoritics & planetary science》2011,46(10):1498-1509
Abstract– To determine the role played by sulfides in the formation of vesicles and FeNi metal beads, we mapped the locations and tabulated the numbers of sulfides, metal beads, and vesicles in 1583 sectioned micrometeorites (MMs) using conventional microscopy and in 190 whole MMs using synchrotron computed microtomography (SCMT). Both the section and the SCMT images show that sulfides melt, coalesce, and migrate to the MMs surface. The decomposition of sulfides may occur during all these stages. Given the sulfide morphologies and compositions that we see in section, we think the breakdown of Ni sulfides produces the FeNi beads. The SCMT images show that metal beads are common in melted MMs, >50% have them. Vesicles in porphyritic and scoriaceous MMs are also probably formed as sulfides decompose. Not only do sulfides abut the vesicles but also the temperatures at which sulfides decompose overlap those at which MM surfaces first melt and temporarily seal, suggesting that S gases could produce most of these vesicles. As the vesicle shapes and patterns of distribution differ among MM classes, tomography can be used to nondestructively screen for specific types of MMs. Tomography is a powerful tool for visualizing the three‐dimensional distribution of metal beads, sulfides, mean densities, and vesicles in MMs. 相似文献
88.
Keith D. Hamill 《新西兰海洋与淡水研究杂志》2013,47(5):1057-1059
Toxic cyanobacteria have been linked to dog deaths in the Waikanae River (1998) and Mataura River (1999 and 2000), New Zealand. These were Oscillatoria‐like species which formed benthic mats. This is the first time in New Zealand that animal deaths have been linked to toxins from benthic cyanobacteria. 相似文献
89.
Mapping block-and-ash flow hazards based on Titan 2D simulations: a case study from Mt. Taranaki,NZ 总被引:1,自引:0,他引:1
Jonathan N. Procter Shane J. Cronin Thomas Platz Abani Patra Keith Dalbey Michael Sheridan Vince Neall 《Natural Hazards》2010,53(3):483-501
Numerical models for simulation of mass flows are typically focussed upon accurately predicting the paths, travel times and
inundation from a single flow or collapse event. When considering catchment-based hazards from a volcano, this is complicated
by often being faced with several possible scenarios. Over the last 800 years at Mt. Taranaki/Egmont, a number of dome growth
and collapse events have resulted in the genesis and emplacement of block-and-ash flows (BAFs). Each BAF was directed northwestward
by a breach in the crater rim. The latest dome collapse events in the AD 1880s and AD 1755 inundated the northwestern flank
and had run-out lengths 10 km from source. Future activity of this type could have a devastating effect on the Taranaki region’s
communities, infrastructure and economy. Hazard planning has involved constructing volcanic hazard maps based upon the areas
inundated by past volcanic flows, with little consideration of present-day topography. Here, a numerical geophysical mass
flow modelling approach is used to forecast the hazards of future comparable BAF events on NW Mt. Taranaki. The Titan2D programme
encompasses a “shallow water”, continuum solution-based, granular flow model. Flow mechanical properties needed for this approach
include estimates of internal and basal friction as well as the physical dimensions of the initial collapse. Before this model
can be applied to Taranaki BAFs, the input parameters must be calibrated by simulating a range of past collapse events. By
using AD 1860 and AD 1755 scenarios, initial collapse volumes can be well constrained and internal and basal friction angles
can be evaluated through an iterative approach from previous run-out lengths. A range of possible input parameters was, therefore,
determined to produce a suite of potentially inundated areas under present-day terrain. A suite of 10 forecasts from a uniformly
distributed range were combined to create a map of relative probabilities of inundation by future BAF events. These results
were combined in a GIS package to produce hazard zones related to user-specified hazard thresholds. Using these input parameter
constraints, future hazard forecasts for this scale and type of event can also take into account changing summit and topographic
configurations following future eruptive or collapse events. 相似文献
90.
Susan H. BeVille Benjamin B. Mirus Brian A. Ebel George G. Mader Keith Loague 《Environmental Earth Sciences》2010,61(6):1249-1257
Hydrologically driven mass wasting in the form of landslides on steep slopes is a worldwide occurrence. High-profile events
in, for example, Brazil, Chile, the Philippines, Puerto Rico, and Venezuela during the last three decades all clearly illustrate,
based upon significant losses of life and property, that hydrologically driven slope instability in developed (urban) areas
can be a major geologic/environmental hazard. The focus of this study is the 1973 hydrologically driven Lerida Court landslide
in Portola Valley, CA, USA. Physics-based hydrologic-response simulation, with the comprehensive Integrated Hydrology Model,
was employed to forensically estimate the spatiotemporal pore pressure distributions for the Lerida Court site. Slope stability,
driven by the simulated pore pressure dynamics, was estimated for the Lerida Court site with the infinite slope/Factor of
Safety approach. The pore pressure dynamics for the Lerida Court site were reasonably captured by the hydrologic-response
simulation. The estimated time of slope failure for the Lerida Court site compares well with field observations. A recommendation
is made that hydrologically driven slope stability estimates including variably saturated subsurface flow be standard protocol
for development sites in steep urban settings. 相似文献