Significant improvements, both in understanding the evolution of zircons and in understanding the geotectonic and metallogenetic evolution of the complex terrain of southern Brazil, are obtained from a SHRIMP geochronology study and reviewed in this paper. The use of backscattered electron and cathodoluminescence images, prior to SHRIMP isotopic determinations, proved of enormous fundamental value for this technique. Zircon is a domainal open‐system mineral in many geological conditions; very old domains may be preserved, but the same crystal may show ages of younger tectonic events. Zircons may recrystallise inwards from the rims or outwards from the cores, and also along euhedral high‐U or metamict thin zones. Zircons also may be recrystallised during gold‐related hydrothermalism, phyllic alteration of granitic rocks. The precise dating of amphibolite dykes can be achieved by the identification and dating of magmatic zircons. Precambrian orogenies are identified along with the intervening intracratonic tectonic cycles of supercontinents in southern Brazil from 3300 to 470 Ma. Granulite protoliths were formed during the Jequié Orogeny (ca 2600 Ma), but extensive arc accretion occurred in the Palaeoproterozoic (ca 2250 Ma) Encantadas Orogeny. Late in the Transamazonian Cycle, granites were formed by crustal melting at about 2000 Ma in the Camboriú Orogeny. Both accretionary and collisional orogenies are also identified in the Neoproterozoic Brasiliano Cycle. These are the accretionary Passinho Orogeny (ca 900 Ma) and São Gabriel Orogeny (ca 700 Ma), that were succeeded by the collisional Dom Feliciano Orogeny (ca 600 Ma). Base‐metal and gold deposition occurred in juvenile island arcs and in late orogenic porphyry‐copper‐type magmatic‐hydrothermal settings during the Neoproterozoic. 相似文献
We present results of solar-wind parameters generated by 3D MHD models. The ENLIL inner-heliosphere solar-wind model together
with the MAS or Wang – Sheeley – Arge (WSA) coronal models, describe the steady solar-wind stream structure and its origins
in the solar corona. The MAS/ENLIL and WSA/ENLIL models have been tuned to provide a simulation of plasma moments as well
as interplanetary magnetic-field magnitude and polarity in the absence of disturbances from coronal transients. To investigate
how well the models describe the ambient solar wind structure from the Sun out to 1 AU, the model results are compared to
solar-wind measurements from the ACE spacecraft. We find that there is an overall agreement between the observations and the
model results for the general large-scale solar-wind structures and trends, such as the timing of the high-density structures
and the low- and high-speed winds, as well as the magnetic sector structures. The time period of our study is the declining
phase of Solar Cycle 23 when the solar activity involves well-defined stream structure, which is ideal for testing a quasi-steady-state
solar-wind model. 相似文献
The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane, a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian.
The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (-359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis. 相似文献
The importance of a national or regional network of meteorological stations for improving weather predictions has been recognized for many years. Ground-based automatic weather stations typically observe weather at a height of 2-10 m above ground level(AGL); however, these observations may have two major shortcomings. Large portions of data cannot be used if the station height is significantly lower than the model surface level; and such observations may contain large representativity errors as near-surface observations are often affected by the local environment, such as nearby buildings and tall trees. With the recent introduction of a significant number of mobile communication towers that are typically over40 m AGL in China, a campaign has been proposed to use such towers to build a future observing system with an observing height of 40 m. A series of observing system simulation experiments has been conducted to assess the potential utility of such a future observing system as part of a feasibility study. The experiments were conducted using the Weather Research and Forecasting model and its Rapid Update Cycle data assimilation system. The results revealed the possibility of improving weather forecasting by raising present weather stations to a height of 40 m; this would not only enable more observations to pass the terrain check, but should also reduce interpolation errors. Additionally, improvements for temperature, humidity and wind forecasting could be achieved as the accuracy of the initial conditions increases. 相似文献
In this contribution closed-form expressions are given for the minimal detectable biases of single- and dual-frequency pseudo-range
and carrier-phase data. They are given for three different single-baseline models. These are the geometry-free model and two
variants of the geometry-based model, namely the roving and stationary variants. The baselines are considered to be sufficiently
short such that orbital uncertainties in the fixed orbits and residual ionospheric and tropospheric delays can be assumed
absent. The stochastic model used is one that permits cross-correlation and the use of different variances for individual
GPS observables, including the possibility to weigh the observables in dependence on which satellite is tracked.
Received: 23 April 1997 / Accepted: 27 October 1997 相似文献
Phase folding algorithms are conventionally used in periodicity analyses using X-ray astronomy pulsar. These allow for accurate identification of the cycle and phase characteristics of the physical parameters of the periodic variation. Although periodic variations in earthquake activity have long been studied, this paper is the first to apply the phase folding algorithm to the analysis of shallow (<70 km) seismic data for the period 1973–2010. The goal is to study the phase distribution characteristics of earthquake frequencies and we see a connection between earthquake occurrence and solar and lunar cycles. First, the rotation of the Sun may play a significant role in impacting on the occurrence time of earthquakes with magnitudes of less than 6.0. This may be especially pertinent for earthquakes with magnitudes between 5.0 and 6.0, when the modulation ratio reaches 12 %. The Moon’s gravity, which is generally thought to have the greatest influence on the global environment, may actually play less of a role on earthquake timing than the rotation of the Sun. Second, when we consider the world to be divided into 72 local regions based on latitude and longitude, we can see that there are more than a dozen regions with significant non-uniform distributions of earthquake occurrence time. In these regions, the ratio of χ2 to the number of degrees of freedom far exceeds five. As a result, we posit that some factors associated with the Sun–Earth–Moon relationship may trigger earthquake activity under certain temporal and spatial conditions. 相似文献