Juvenile Crust, Mantle Avalanches, and Supercontinents in the Last 1.6 Ga     

Kent C. Condie 《Gondwana Research》1999,2(4):543

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Sediment distribution and transport across the continental shelf and slope under idealized wind forcing     

Scott A. Condie  Christopher R. Sherwood   《Progress in Oceanography》2006,70(2-4):255

Resuspension, transport, and deposition of sediments over the continental shelf and slope are complex processes and there is still a need to understand the underlying spatial and temporal dynamical scales. As a step towards this goal, a two-dimensional slice model (zero gradients in the alongshore direction) based on the primitive flow equations and a range of sediment classes has been developed. The circulation is forced from rest by upwelling or downwelling winds, which are spatially uniform. Results are presented for a range of wind speeds and sediment settling speeds. Upwelling flows carry fine sediments (low settling speeds) far offshore within the surface Ekman layer, and significant deposition eventually occurs beyond the shelf break. However, coarser sediments quickly settle out of the deeper onshore component of the circulation, which can lead to accumulation of bottom sediments within the coastal zone. Downwelling flows are more effective at transporting coarse sediments off the shelf. However, strong vertical mixing at the shelf break ensures that some material is also carried into the surface Ekman layer and returned onshore. The concentrations and settling fluxes of coarse sediments decrease offshore and increase with depth under both upwelling and downwelling conditions, consistent with trends observed in sediment trap data. However, finer sediments decrease with depth (upwelling) or reach a maximum around the depth of the shelf break (downwelling). It is shown that under uniform wind conditions, suspended sediment concentrations and settling fluxes decay offshore over a length scale of order τ_s/ρf|w_s|, where τ_s is the wind stress, ρ the water density, f the Coriolis parameter, and w_s is the sediment settling velocity. This scaling applies to both upwelling and downwelling conditions, provided offshore transport is dominated by wind-driven advection, rather than horizontal diffusion.  相似文献   

Geochemistry of the Archean Bulawayan Group,Midlands greenstone belt,Rhodesia     

Kent C. Condie  N.M. Harrison 《Precambrian Research》1976,3(3):253-271

The Bulawayan Group in the Midlands greenstone belt can be divided into three formations. The Mafic Formation is composed principally of pillowed, low-K tholeiites and minor bedded chert. The Maliyami Formation and conformably overlying Felsic Formation are composed of calc-alkaline tholeiites, andesites, and dacites with andesites dominating in the Felsic Formation. Minor rhyolite quartz porphyries and ultramafic bodies also occur in the section. The Bulawayan Group near Que Que is perhaps the least altered and metamorphosed Archean greenstone succession known. The absence of andesite and related rocks, the association of bedded chert, and the consistently low K₂O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they represent Archean oceanic rise tholeiites. The compositions of tholeiites and andesites of the Maliyami Formation, however, suggest that they represent an emerging arc system. The Felsic Formation is interpreted as a more advanced stage in the evolution of this arc system.Trace-element model calculations favor an origin for Mafic Formation tholeiites involving about 30% partial melting of a lherzolite source. Similar calculations are consistent with an origin for Maliyami Formation tholeiites, Maliyami and Felsic Formation andesites, and Midlands rhyolites involving, respectively, 50, 20–30, and 10% equilibrium melting of eclogite or garnet amphibolite (of Mafic Formation tholeiite composition). The low K₂O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they were derived from an upper mantle source as depleted in these elements as the oceanic upper mantle is today.A plate tectonic model is proposed for the Bulawayan Group in which the Mafic Formation is derived from a depleted lherzolite source beneath a spreading center in a marginalsea basin and the Maliyami and Felsic Formations and associated rhyolites are produced by partial melting of eclogite in a descending slab located west of the basin.  相似文献   

Origin of Archean migmatites from the Gwenoro Dam area,Zimbabwe-Rhodesia     

Kent C. Condie  Philip Allen 《Contributions to Mineralogy and Petrology》1980,74(1):35-43

Archean migmatites in the vicinity of Gwenoro Dam in Zimbabwe-Rhodesia are composed chiefly of trondhjemite gneiss (TR), mafic tonalite (MT), amphibolite (AM), leuco-trondhjemite veins (LTR), and pegmatites. The gneiss is intruded in nearby areas with small tonalite plutons (TN). Geochemical model studies together with field relationships are consistent with the following model for migmatite production: AM is produced by partial melting of a partly depleted ultramafic parent in which neither garnet nor amphibole remain in the residue; TR and TN are produced by partial melting of undepleted to variably depleted amphibolite in which garnet does not remain in the residue; MT is produced by mixing of plagioclase-rich TR with AM; and LTR represents the solid residue after fractional crystallization of TR.  相似文献   

TTGs and adakites: are they both slab melts?   总被引：21，自引：0，他引：21  

Kent C. Condie   《Lithos》2005,80(1-4):33-44

Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings.

Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite, whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 °C.

Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni. These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth.  相似文献   

Precambrian clastic sedimentation systems   总被引：1，自引：0，他引：1  

P. G. Eriksson  K. C. Condie  H. Tirsgaard  W. U. Mueller  W. Altermann  A. D. Miall  L. B. Aspler  O. Catuneanu  J. R. Chiarenzelli 《Sedimentary Geology》1998,120(1-4):5-53

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A laboratory model of a convectively driven ocean     

Scott A. Condie 《Dynamics of Atmospheres and Oceans》1989,13(1-2)

Many large-scale flows in the ocean are driven by an imposed horizontal density gradient and the resulting circulation is strongly influenced by the Earth's rotation. Some of the essential features of such flows have been incorporated into a laboratory model by differentially heating and cooling the vertical end walls of a shallow rectangular cavity rotating about a vertical axis. Buoyancy driven boundary currents produced by the heating and cooling were unstable and the resulting eddy structures eventually dominated the system. A broad mean flow, perpendicular to the side walls, developed in the central region of the cavity. The resultant steady-state flow had a barotropic component consisting of two large-scale gyres of cyclonic and anticyclonic senses. In this paper, the evolution of the flow towards its final steady state is described. Measurements of the time-scales for the establishment of stratification in the cavity, point to the existence of two dynamical density adjustment modes.  相似文献   

Archean geotherms and supracrustal assemblages     

Kent C. Condie 《Tectonophysics》1984,105(1-4)

Metamorphic mineral assemblages suggest the existence of variable geotherms and lithospheric thicknesses beneath late Archean continental crust. Archean granite-greenstone terranes reflect steep geotherms (50–70°C/km) while high-grade terranes reflect moderate geotherms similar to present continental crust with high heat flow (25–40°C/km). Corresponding lithosphere thicknesses for each terrane during the late Archean are 35–50 km and 50–75 km, respectively.Early Archean ( 3.0 b.y.) greenstones differ from late Archean ( 2.7 b.y.) greenstones by the rarity or absence of andesite and graywacke and the relative abundance of pelite, quartzite, and komatiite. Mature clastic sediments in early greenstones reflect shallow-water, stable-basin deposition. Such rocks, together with granite-bearing conglomerate and felsic volcanics imply the existence of still older granitic source terranes. The absence or rarity of andesite in early greenstones reflects the absence of tectonic conditions in which basaltic and tonalitic magmas are modified to produce andesite.A model is presented in which early Archean greenstones form at the interface between tonalite islands and oceanic lithosphere, over convective downcurrents; high-grade supracrustals form on stable continental edges or interiors; and late Archean greenstones form in intracontinental rifts over mantle plumes.  相似文献   

Estimating time parameters of overland flow on impervious surfaces by the particle tracking method     

Manoj KC 《水文科学杂志》2013,58(2):294-310

Abstract

Travel time and time of concentration T_c are important time parameters in hydrological designs. Although T_c is the time for the runoff to travel to the outlet from the most remote part of the catchment, most researchers have used an indirect method such as hydrograph analysis to estimate T_c. A quasi two-dimensional diffusion wave model with particle tracking for overland flow was developed to determine the travel time, and validated for runoff discharges, velocities, and depths. Travel times for 85%, 95% and 100% of particles arrival at the outlet of impervious surfaces (i.e. T_t85, T_t95, and T_t100) were determined for 530 model runs. The correlations between these travel times and T_c estimated from hydrograph analysis showed a significant agreement between T_c and T_t85. All the travel times showed nonlinear relationships with the input variables (plot length, slope, roughness coefficient, and effective rainfall intensity) but showed linear relationships with each other.