In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics.
The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons.
North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the 40Ar/39Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands.
The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the 40Ar/39Ar dates.
A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented. 相似文献
We describe a method for measuring the 40Ar/36Ar ratio and the 84Kr/36Ar ratio in air from bubbles trapped in ice cores. These ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured 15N/14N. Both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on Earth’s climate. The overall precision of the 40Ar/36Ar method (1 standard error of the mean) is 0.012‰ for a sample analyzed in duplicate, corresponding to ±0.6 m in reconstructed firn thickness. We use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation. About 100 g of ice is used for a duplicate pair of analyses. An example of the technique applied to the GISP2 ice core yields an estimate of 11 ± 3K of abrupt warming at the end of the last glacial period 15,000 years ago. The krypton/argon ratio can provide a diagnostic of argon leakage out of the bubbles, which may happen (naturally) during bubble close-off or (artifactually) if samples are warmed near the freezing point during core retrieval or storage. Argon leakage may fractionate the remaining 40Ar/36Ar ratio by +0.007‰ per ‰ change in 84Kr/36Ar, introducing a possible bias in reconstructed firn thickness of about +2 m if thermal diffusion is not accounted for or +6 m if thermal diffusion effects are quantified with measured 15N/14N. Reproducibility of 84Kr/36Ar measured in air is about ±0.2‰ (1 standard error of the mean) but is about ±1‰ for ice core samples. Ice core samples are systematically enriched in 84Kr/36Ar relative to atmosphere by ∼5‰, probably reflecting preferential size-dependent exclusion of the smaller argon atom during bubble entrapment. Recent results from the Siple Dome ice core reveal two climate events during the last deglaciation, including an 18-m reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr BP and a partial or total removal of the firn during an ablation event at 15.3 kyr BP. 相似文献
The COUPLEX1 Test case (Bourgeat et al., 2003) is devoted to the comparison of numerical schemes on a convection–diffusion–reaction problem. We first show that the results of the simulation can be mainly predicted by a simple analysis of the data. A finite volume scheme, with three different treatments of the convective term, is then shown to deliver accurate and stable results under a low computational cost. 相似文献
The soil solution sampling by ceramic cups allows pesticide transfer monitoring in the soil during long times. The ageing of material involves a bias in the sampling results. In laboratory, the comparison of two types of ceramic suction cups, new and installed in situ during four years, shows a modification of the hydrodynamic properties and a possible evolution of the adsorption capacity of the matrix. The passage rate, as well qualitative as quantitative, is better for the old material. Recommendations about site management are finally exposed. To cite this article: N. Domange et al., C. R. Geoscience 336 (2004).相似文献
Nickel speciation in a nickel hyperaccumulating plant (Sebertia acuminata) and its associated soil of southern New Caledonia was studied using various analytical methods. The soil is formed of iron oxides (goethite, hematite), which contain almost all the nickel. The available nickel is probably linked to the organic matter in the litter. Sebertia acuminata, acts as a nickel pump, and concentrates the metal in its leaves. It partitions nickel and silica; nickel is concentrated in the cells (probably in the vacuoles) as organometallic complexes, whereas silica forms the framework of the cells, and the phytolithes. A thorough study of these plants seems essential in order to define the soil–plant relations, and to propose appropriate ways for ecological restoration. To cite this article: N. Perrier et al., C. R. Geoscience 336 (2004).相似文献
The unexplored area of Azarhare in central Morocco is studied thanks to three sections composed of five lithological facies Main biostratigraphical and sedimentological results concern (1) the identification of Late Visean biozones, with important presence of problematic algae Ungdarella, (2) the regional extension of deposit sequences SD5, SD6 and SD7 previously defined, (3) and an analysis of the diagenetic kaolinite. To cite this article: A. Karim et al., C. R. Geoscience 337 (2005).相似文献
The potential of radar imagery in geological exploration was investigated at a study site in Mauritania (Akjoujt region). Compared with optical images, the results obtained show how radar imagery can help not only in detecting surface geological structures such as dykes and veins, but also mapping subsurface structures beneath a shallow layer of sand (palaeochannels). The mapping potential was found to be much better at long wavelengths than at short ones (L-band, compared with C- and X-band). As for optical images, their contribution is much more limited in the mapping of surface geological structures, and inappropriate for detecting subsurface structures. We conclude that spatial remote sensing enables the improvement of existing geological maps and the optimization of cartographic surveying. To cite this article: N. Baghdadi et al., C. R. Geoscience 337 (2005).相似文献
Correlations presented by Spycher et al. (2003) to compute the mutual solubilities of CO2 and H2O are extended to include the effect of chloride salts in the aqueous phase. This is accomplished by including, in the original formulation, activity coefficients for aqueous CO2 derived from several literature sources, primarily for NaCl solutions. Best results are obtained when combining the solubility correlations of Spycher et al. (2003) with the activity coefficient formulation of Rumpf et al. (1994) and Duan and Sun (2003), which can be extended to chloride solutions other than NaCl. This approach allows computing mutual solubilities in a noniterative manner with an accuracy typically within experimental uncertainty for solutions up to 6 molal NaCl and 4 molal CaCl2. 相似文献
