Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy最新文献

Early on May 2, 2000, selective availability (SA) — the intentional dithering of clocks on GPS satellites — was terminated. The amplitude of variations in transmitter clocks, formerly tens of meters, was reduced by orders of magnitude. These clocks are now much more predictable, with the possibility that the International GPS Service (IGS) can produce, in addition to real-time orbits, real-time clocks that are of significantly higher quality than those in the broadcast ephemeris. Furthermore, the meaning of high in “high rate ground network” to support low-Earth-orbiter (LEO) missions needs to be re-examined.

Early Precambrian rocks are widespread distributed in northern China and constitute the metamorphic crystalline basement of the North China craton. An oblique successive rock sequence of the continental lower crust in the northern part of the craton has been recognized. Nearby this metamorphic terrane, some xenoliths of granulites in Cenozoic basalts are reported to represent the recent lower crust. Therefore, it is possible to establish a typical Precambrian continental lower crust cross-section in the North China craton to understand features of the lower crust.

This exposed cross-section of lower crust can be divided into five layers from south (lowermost crust) to north (upper-lower crust) by faults. Along the section northward, the metamorphic grade gradually decreases from high-pressure granulite facies, middle-pressure granulite facies, middle-low-pressure granulite facies to amphibolite facies. The metamorphic pressures of these five layers are, respectively, 12 to 14 kbar, 9 kbar, 7 to 8 kbar, 6 kbar and 5 kbar. The petrological compositions of the five layers show a change from gabbroic granulites, intermediate-felsic orthogneiss to metamorphosed supracrustal rocks. Geochemically, the lowermost crust (gabbro) and lower crust (intermediate-acid orthogneiss) are relatively poorer in Si and Al compared to the middle-lower crust and upper-lower crust. The cross-section demonstrates a depletion trend of heat-producing elements and some large ion lithophile elements, such as decreasing abundance of Th, U, K, Rb and Sr from the upper-lower crust to the lowermost crust. The rocks in lowermost crust and lower crust only contain CO fluid inclusions, whereas the rocks in the middle-lower crust and upper-lower crust usually contain H₂O fluid inclusions.

Isotopic data of garnet-bearing mafic granulites, which represent the lowermost crust, indicate that the lower crust in this area formed in Neoarchean, and uplifted and exposed to surface in late Paleoproterozoic. However, the study of granulite xenoliths in this area seems to demonstrate that the recent lower crust and Precambrian lower crust have some similarity and also some difference in geochemistry of trace elements and Sr, Nd and Pb isotopes. It is possible that the Precambrian crust is unstable in late geological processes and replaced by recent lower crust, and some old materials were added to the recent lower crust. The formation of the recent lower crust is mainly linked with magma underplating process related to Cenozoic basaltic volcanics.

Ground-based GPS meteorology has been developed during the last decade. Beside the GPS traditional application for precise geodesy, the GPS system has proved to be a powerful tool in atmospheric studies, such as climatology and meteorology. During 1999, at the Space Geodesy Center (CGS) of the Italian Space Agency (ASI) an operative and automatic system was developed in order to produce GPS tropospheric parameters on a daily basic for the Italian GPS permanent stations. In this work the operational processing of the GPS data is described. Moreover, the validation of the GPS tropospheric parameters through a comparison with other independent GPS solutions as well as independent techniques is presented. The GPS internal validation of zenith tropospheric delay (ZTD) shows a bias of 5 mm (absolute values) and a standard deviation ranging from 4 to 20 mm. For a few stations a comparison is performed between the GPS derived precipitable water vapor (PWV) and both the one computed using the MM5 and the one measured by the water vapor radiometer (WVR) and radiosondes (RAOB). The agreement between GPS and MM5 is within the range of 2–7 mm; for GPS, WVR and RAOB the standard deviation ranges from 0.85 to 1.62 mm.

A preliminary boron isotope study of tourmaline was carried out for a number of sediment-hosted massive sulfide deposits and granite-related hydrothermal Sn deposits in China. Tourmalines from the Yindongzi-Tongmugou (Devonian) and the Qingchengzi (Proterozoic) Pb-Zn deposits show δ¹¹B values of −7.6 to −13.7‰, which are within the typical δ¹¹B ranges of massive sulfide deposits worldwide. At Dachang Sn-polymetallic deposit, the hydrothermal fluids responsible for the formation of banded tourmalinites have remarkable different δ¹¹B values (−1 to −5‰) from those of quartz-tourmaline veins near the granite contacts (∼ −10‰), which indicate their different origins. Together with other geological and geochemical evidence, the banded tourmalinites at Dachang are of submarine hydrothermal exhalative origin. Overall, the boron isotopic compositions of tourmaline from massive sulfide deposits are mainly controlled by source rock compositions, and non-marine evaporites may have played an important role in the massive sulfide ore formation in same cases. Other controlling factors include temperatures, water/rock ratios, and seawater entrainments.

Tourmalines from the granite-related Yidong Sn deposit display a gradual increase in δ¹¹B value from within the granite (−13.9‰), to the contact (−12.7 to −11.9‰) and the country rocks (−11.4 to −11.3‰). It is suggested that these variations reflect the boron isotope fractionation during magmatic degassing and magmatic-hydrothermal ore-forming processes with ¹¹B preferentially partitioned into hydrothermal solutions. The similar and limited δ¹¹B variations (−19.5 to −13.9‰) in granitic tourmalines from the granite-related Sn deposits in South China suggest that these Sn-rich granites may have similar source rocks from the basement of metasedimentary rocks and volcanic rocks in this region.

I explore the statistics of integrated water-vapor (IWV) and the ability of GPS to provide information that reflects large-scale weather systems. I demonstrate that the power spectral density (PSD) is a much more robust estimator of IWV statistics than the structure function is for long time scales. I then use the PSD to investigate the IWV statistics of simulated atmospheres. I use a random walk to approximate the Kolmogorov behavior of local atmospheric turbulence. The atmospheres I simulate contain the turbulent behavior sumperimposed on systematic changes to the IWV that might be associated with the passage of fronts. I demonstrate that the PSD associated with such frontal systems might be quite similar to those for pure turbulent behavior. This result may explain why many previous studies that examined PSDs of IWV over a site concluded that the spectra was Kolmogorov. I also derive the first cross-power spectrum of IWV using GPS sites separated by ∼500 km. At this separation, the local turbulence may be assumed to have become decorrelated. I find that even this long wavelength signal consists of components at a wide range of temporal frequencies. The IWV cross power spectrum contains significant energy even at temporal frequencies of 2–4 cycles per hour.

A subset of stations that are part of the International GPS Service (IGS) submit hourly RINEX observation files in addition to the daily files to be used for near real time applications. Ideally, concatenating the hourly files will result in the original 24 hour file. If this is indeed the case, there is no need for submitting every observation twice and the concatenated hourly files can replace the daily file. In order to compare such concatenated files with the original daily files, each data field has to be inspected according to the RINEX format definition. Differences have to be removed before any replacement could be performed. By using a RINEX inspection software tool on a subset of stations, the concatenated version of the daily file was in agreement with the original file in 94% of the conversions. This suggests that it may be possible for IGS to completely depend on the hourly files for the creation of the daily files in the future, thereby eliminating the need for transferring each observation twice.

The experimental characterisation of strain localisation in a porous sandstone is presented for the brittle and the transition between the brittle and semi-brittle regimes. Localisation occurs through bands and the observations show a continuous evolution from a dilative strain type inside the bands at low mean stress to a compactive strain type at higher mean stress. The orientation of the bands changes also with the stress level and the onset of localisation is systematically detected before the peak stress. Microscope observations suggest two steps during the localisation process. A theoretical analysis of the localised strain inside bands is presented using the framework of bifurcation theory. The notion of dilating and compacting shear bands is introduced and this allows to show that a continuous transition from pure extension bands to dilating and compacting shear bands and to pure compaction bands is expected. These theoretical predictions are in accordance with the experimental observations, providing an interesting framework to investigate the brittle to semi-brittle transition regime.

We present an overview of experimental results that we obtained in the last decade on the permeability evolution with stress and the analysis of stress-induced damage in sandstones. The goal of these studies was to get a fundamental knowledge on the interplay between deformation at the microscale level, fluid flow processes and the macroscopic rock properties. In a first part we present experimental data on the effect of stress on permeability, with emphasis on the influence of stress path and failure mode. In a second part, results from a microstructural study on the mechanics of deformation in Berea sandstone are shown. Although both studies have not been done on the same rock, some general conclusions are drawn on the influence of mechanical deformation on permeability with implications for field studies.

The aim of this study was to evaluate the role of microfossils and clay in the compaction of chalk facies sediments. To meet this aim, chalk sediments with varying micro texture were studied. The sediments have been tested uniaxially confined in a stainless-steel compaction cell. The sediments are: 1) Pure carbonate chalk with mudstone texture from Stevns Klint (Denmark), 2) Relatively pure chalk sediments with varying content of microfossils from the Ontong Java Plateau (Western Pacific), 3) Clay-rich chalk and mixed sediments from the Caribbean. The tested samples were characterised by porosity, carbonate content, and microfossil content. The texture was quantified by analysis of backscattered electron micrographs of polished sections. The samples do not compact along a common stress — porosity trend. This observation can be explained as a result of the differences in content of microfossils and fine-grained silica and clay. Samples with relatively pure chalk mud supported texture compact along a common stress — matrix porosity trend. Microfossils thus have a passive role, apparently because they are supported by the chalk mud. Samples with fine-grained silica and clay can be modelled to follow the same trend if we assume that a part of the fine-grained silica and clay are in the supporting frame and that the remaining silica and clay has a passive pore-filling role. The modelled part of the clay and silica in the frame varies from 0% to 100%. Porosity and sonic velocity variations for a given degree of consolidation may be a key to indirect interpretation of content of microfossils and porefilling fine-grained silica and clay in chalk.