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171.
Rebecca M. Flowers Samuel A. Bowring Kevin H. Mahan Michael L. Williams Ian S. Williams 《Contributions to Mineralogy and Petrology》2008,156(4):529-549
New U–Pb geochronology for an extensive exposure of high-pressure granulites in the East Lake Athabasca region of the western
Canadian shield is consistent with a history characterized by 2.55 Ga stabilization of cratonic lithosphere, 650 million years
of lower crustal residence and cratonic stability, and 1.9 Ga reactivation of the craton during lithospheric attenuation and
asthenospheric upwelling. High precision single-grain and fragment zircon data define distinctive discordia arrays between
2.55 and 1.9 Ga. U–Pb ion microprobe spot analyses yield a similar range of U–Pb dates with no obvious correlation between
date and cathodoluminescence zonation. We attribute the complex U–Pb zircon systematics to growth of the primary populations
during a 2.55 Ga high-pressure granulite facies event (~1.3 GPa, 850°C) recorded by the dominant mineral assemblage of the
mafic granulite gneisses, with subsequent zircon recrystallization and minor secondary zircon growth during a second high-pressure
granulite facies event (1.0 GPa, ~800°C) at 1.9 Ga. The occurrence of two discrete granulite facies metamorphic events in
the lower crust, separated by an interval of 650 million years that included isobaric cooling for at least some of this time,
suggests that the rocks resided at lower crustal depths until 1.9 Ga. We infer that this phase of lower crustal residence
and little tectonic activity is coincident with an extended period of cratonic stability. Detailed structural and thermochronological
datasets indicate that multistage unroofing of the lower crustal rocks occurred in the following 200 million years. Extended
lower crustal residence would logically be the history inferred for lower crust in most cratonic regions, but the unusual
aspect of the history in the East Lake Athabasca region is the subsequent lithospheric reactivation that initiated transport
of the lower crust to the surface. We suggest that a weakened strength profile related to the 1.9 Ga heating left the lithosphere
susceptible to far-field tectonic stresses from bounding orogens that drove the lower crustal exhumation. An ultimate return
to cratonic stability is responsible for the preservation of this extensive lower crustal exposure since 1.7 Ga. 相似文献
172.
B. R. Bowring 《Journal of Geodesy》1983,57(1-4):374-381
A simple equation to determine the distance on a meridian is developed using complex numbers. Very great accuracy is obtainable. Inverse and other forms are also presented. 相似文献
173.
Robert S. Hildebrand Paul F. Hoffman Samuel A. Bowring 《Journal of Volcanology and Geothermal Research》1987,32(1-3)
The 1875-1840-Ma Great Bear magmatic zone is a 100-km wide by at least 900-km-long belt of predominantly subgreenschist facies volcanic and plutonic rocks that unconformably overlie and intrude an older sialic basement complex. The basement complex comprises older arc and back-arc rocks metamorphosed and deformed during the Calderian orogeny, 5–15 Ma before the onset of Great Bear magmatism. The Great Bear magmatic zone contains the products of two magmatic episodes, separated temporally by an oblique folding event caused by dextral transpression of the zone: (1) a 1875-1860-Ma pre-folding suite of mainly calc-alkaline rocks ranging continuously in composition from basalt to rhyolite, cut by allied biotite-hornblende-bearing epizonal plutons; and (2) a 1.85-1.84-Ga post-folding suite of discordant, epizonal, biotite syenogranitic plutons, associated dikes, and hornblende-diorites, quartz diorites, and monzodiorites. The pre-folding suite of volcanic and plutonic rocks is interpreted as a continental magmatic arc generated by eastward subduction of oceanic lithosphere. Cessation of arc magmatism and subsequent dextral transpression may have resulted from ridge subduction and resultant change in relative plate motion. Increased heat flux due to ridge subduction coupled with crustal thickening during transpression may have caused crustal melting as evidenced by the late syenogranite suite. Final closure of the western ocean by collision with a substantial continental fragment, now forming the neoautochthonous basement of the northern Canadian Cordillera, is manifested by a major swarm of transcurrent faults found throughout the Great Bear zone and the Wopmay orogen.Although there is probably no single evolutionary template for magmatism at convergent plate margins, the main Andean phase of magmatism, exemplified by the pre-folding Great Bear magmatic suite, evolves as larger quantities of subduction-related mafic magma rise into and heat the crust. This results in magmas that are more homogeneous, siliceous, and explosive with time, ultimately leading to overturn and fractionation of the continental crust. 相似文献
174.
This study seeks to determine the role of external ownership and control of branch manufacturing plants in a traditionally peripheral region of the United States, the states of Georgia and South Carolina. The data are derived from a questionnaire survey sent to branch plants in the two states. The analysis is based on whether the branch plants are locally, regionally, nationally or internationally headquartered. The study identified the degree to which headquarter location affects the spatial pattern of material inputs, market location, and communication flows and personnel movements. The authority for making various decisions is also analyzed for branch plants based on the location of the headquarters. 相似文献
175.
B. R. Bowring 《Journal of Geodesy》1977,51(1):17-32
To deal specifically with the case of a nearly antipodal geodesic of given terminal positions, an equation in azimuth is produced
for solution by the Newton-Raphson Method of successive approximation. When the azimuth is in the region of 90° it is found
that an approach is best made by a development in series when the aim is precision calculation. Degeneracy is examined theoretically
and graphically. 相似文献
176.
David M. Boore John D. Sims Hiroo Kanamori Sam Harding 《Physics of the Earth and Planetary Interiors》1981,27(2):133-142
Long-period teleseismic P, S and Rayleigh waves and geologic considerations indicate that the Montenegro earthquake involved thrust faulting on a plane striking nearly parallel to the Adriatic coast and dipping ca. 15° toward the Yugoslav mainland. There is some support from modeling of body waves recorded on long-period WWSSN instruments for a focal depth of 22 km, but the possibility of a multiple source and the difficulty of matching some of the detailed characteristics of the P- and S-wave forms reduce our confidence in the determination of the depth. Fortunately, the source orientation and moment of the event are not sensitive functions of the depth. The long-period (256 s) moment was 4.6 × 1019 Nm (4.6 × 1026 dyne-cm). The moment obtained by fitting the first cycle of P and S waves recorded on WWSSN long-period instruments is about four times smaller. This increase of moment with period is consistent with spectral estimates of the moment from SH waves recorded at SRO and ASRO stations. 相似文献
177.
A two-dimensional signal processing algorithm is developed to obtain smoothed estimates of the gravity disturbance vector
from vector measurements obtained by an inertial surveying system. The method differs from a conventional least squares regional
adjustment of such measurements in that it accommodates a signal model in the smoothing process. Using principles from the
physical theory of geodesy, it is shown that for a local region on the surface of the earth, an appropriate signal model is
obtained by applying the two-dimensional Laplacian operator to a function representing the surface disturbance potential and
equating the result to spatial white noise. The model of the vector measurement is the three orthogonal spatial derivatives
of a three dimensional disturbance potential evaluated at the surface contaminated by additive white noise. The problem of
simultaneous smoothing of all the gravity disturbance measurements from all survey traverses in the region is solved by representing
the surface disturbance potential by a two-dimensional Karhunen-Loeve expansion that makes no specific reference to either
the geometry or the ordering of the parameter space, thereby making no assumptions of causality, stationarity or isotropy.
The problem of estimating the gravity anomaly and the two vertical deflection components reduces to estimating the Karhunen-Loeve
coefficients which are uncorrelated and rapidly converging. Simulation results as well as smoothing of actual gravity disturbance
vector measurements obtained by the U.S. Army Engineer Topographic Laboratories (USAETL) with the Rapid Geodetic Survey System
(RGSS) at the White Sands Missile Range (WSMR) are presented in the paper. An analysis of these results shows that the optimal
two-dimensional smoother obtains a performance benefit relative to conventional regional least squares by a factor of 2 and
a benefit relative to single-traverse smoothed results by a factor of 4.
Presented at the Second International Symposium on Inertial Technology for Surveying and Geodesy, Banff, Canada, June 1–5, 1981. 相似文献
Sommaire Un algorithme de traitement du signal en deux dimensions est développé pour obtenir une estimation lissée du vecteur de la perturbation de la pesanteur à partir des mesures de vecteur obtenues avec un système d’arpentage inertiel. La méthode diffère d’un compensation régionale conventionnelle par moindres carrés de telles mesures, par le fait qu’elle contient un modèle du signal dans le processus de compensation. En s’appuyant sur les principes de la géodésie physique, il est montré que pour une région locale de la surface de la terre, un modèle approprié du signal est obtenu en appliquant l’opérateur à deux dimensions de Laplace à une fonction représentant le champ perturbateur à la surface de la terre et égalisant le résultat à un bruit blanc spatial. Le modèle du vecteur de mesures est défini par les trois dérivées spatiales de la fonction tridimensionnelle du potentiel perturbateur évaluées à la surface et contaminées par un bruit blanc. Le problème du lissage de toutes les mesures de gravité perturbatrice obtenues à partir des polygonales effectuées est résolu en représentant le potentiel perturbateur à la surface à l’aide d’un développement Karhunen-Loeve à deux dimensions qui ne fait aucunement référence à la géométrie ou à l’ordre des paramètres; ceci prévient toute dépendance spatiale des points adjacents. Le problème de l’estimation de l’anomalie de la gravité et des deux composantes de la déviation de la verticale se réduit à celle des coefficients Karhunen-Loeve qui sont non-corrélés et convergent rapidement. Les résultats de simulation aussi bien que le lissage des données du vecteur de perturbation de la pesanteur foumi par l’U.S. Army Engineer Topographics Labs (USAETL) sont présentés. L’analyse de ces résultats montre que le lissage optimal à deux dimensions améliore les résultats par un facteur 2 comparés aux résultats d’une compensation régionale par moindres carrés, et par un facteur 4 comparés aux résultats lissés d’une simple traverse.
Presented at the Second International Symposium on Inertial Technology for Surveying and Geodesy, Banff, Canada, June 1–5, 1981. 相似文献
178.
179.
Evidence of large submarine slides, presumably seismically triggered, is evident on the Israeli continental slope. This article
presents a study of the earthquake characteristics which were likely to have caused these slides. Four seismic records with
different characteristics were used as input, and propagation and response analyses were carried out. The results of these
analyses suggest that the slides were caused by one or more large magnitude events resulting in low frequency tremors propagating
through the slope profile. It is likely that these earthquakes were located on the Jordan Rift. 相似文献
180.
A combined geochronological, geochemical, and Nd isotopic study of felsic high-pressure granulites from the Snowbird Tectonic Zone, northern Saskatchewan, Canada, has been carried out through the application of integrated electron microprobe and isotope dilution thermal ionization mass spectrometry (ID-TIMS) techniques. The terrane investigated is a 400 km2 domain of garnet–kyanite–K–feldspar-bearing quartzofeldspathic gneisses. Monazite in these granulites preserves a complex growth history from 2.6 to 1.9 Ga, with well-armored, high Y and Th grains included in garnet yielding the oldest U–Pb dates at 2.62 to 2.59 Ga. In contrast, matrix grains and inclusions in garnet rims that are not well-armored are depleted in Y and Th, and display more complicated U–Pb systematics with multiple age domains ranging from 2.5 to 2.0 Ga. 1.9 Ga monazite occurs exclusively as matrix grains. Zircon is typically younger (2.58 to 2.55 Ga) than the oldest monazite. Sm–Nd isotope analysis of single monazite grains and whole rock samples indicate that inclusions of Archean monazite in garnet are similar in isotopic composition to the whole rock signature with a limited range of slightly negative initial Nd. In contrast, grains that contain a Paleoproterozoic component show more positive initial Nd, most simply interpreted as reflecting derivation from a source involving consumption of garnet and general depletion of HREE's. Our preferred interpretation is that the oldest monazite dates record igneous crystallization of the protolith. The ca. 2.55 Ga dates in zircon and monazite record an extensive melting event during which garnet and ternary feldspar formed. Very high-pressure (> 1.5 GPa) metamorphism during the Paleoproterozoic at 1.9 Ga produced kyanite from garnet breakdown, and resulted in limited growth of new monazite and zircon. In the case of monazite, this is likely due to the armoring and sequestration of early-formed monazite such that it could not participate in metamorphic reactions during the high-pressure event, as well as the depletion of the REE's due to melt loss following the early melting event. 相似文献