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41.
The geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied, but such is not the
case for their internal fabric. The Tertiary Western Red Hills epigranites of the Isle of Skye are a classic example of such
intrusions. Using anisotropy of magnetic susceptibility measurements, we present the first structural data of their internal
magmatic fabric. The magnetic foliations, equated with the magmatic flow planes, have strikes which roughly follow the walls
of the different intrusions. They dip steeply toward the convex wall of each intrusion. The lineations, or maximal magnetic
susceptibility axes, generally have shallow plunges, except in the latest granite intrusion. These structures appear to be
related to the compressional deformation of each intrusion toward the end of its crystallization. This shortening would be
a consequence of a radial and compressive stress field acting after each injection of magma. This radial stress field is interpreted
as the effect of high magma pressures originating from the acid magma chamber underlying the ring-dyke complex at a shallow
depth.
Received: 10 October 1995 / Accepted: 4 June 1997 相似文献
42.
We demonstrate that the Miocene alkali granites of Yemen which emplaced along the Great Escarpment are contemporaneous with the stretching of the southern Red Sea continental crust. There is a gradient of Miocene extension towards the granites as well as a change of the geometry of tilted blocks around them. Geometrically, most of these granites seem to be emplaced passively in the necked areas of a stretched continental crust. These necked areas correspond to previous syn-plume Oligocene NNW–SSE to N–S trending areas of localized extension marked by basaltic dyke swarms. These early extensional areas were associated with the emplacement of the flood basalt volcanic pile and were probably localized over large basement discontinuities. 相似文献
43.
Natural speciation of Zn at the micrometer scale in a clayey soil using X-ray fluorescence, absorption, and diffraction 总被引：1，自引：0，他引：1
Alain Manceau Matthew A. Marcus Olivier Proux Nicolas Geoffroy 《Geochimica et cosmochimica acta》2004,68(11):2467-2483
Combined use of synchrotron-based X-ray fluorescence (SXRF), diffraction (XRD), and absorption (EXAFS) with an X-ray spot size as small as five micrometers allows us to examine noninvasively heterogeneous soils and sediments. Specifically, the speciation of trace metals at low bulk concentrations and the nature of host minerals can be probed with a level of detail unattainable by other techniques. The potential of this novel analytical approach is demonstrated by determining the Zn species in the solid phases of a pristine horizon of a clayey acidic soil (pH 4.5-5.0) having a Zn concentration of 128 mg/kg. The sample presents a differentiated fabric under the optical microscope with traces of localized manganiferous, ferriferous and argillaceous accumulations. The high chemical and textural heterogeneity of this soil offers an opportunity to identify new Zn species and to confirm the existence of others proposed from published least-squares fits of bulk averaged EXAFS spectra. As many as five to six Zn species were observed: sphalerite (ZnS), zincochromite (ZnCr2O4), Zn-containing phyllosilicate and lithiophorite, and Zn-sorbed ferrihydrite or Zn-phosphate, the results being less definitive for these two last species. Bulk EXAFS spectroscopy applied to the powdered soil indicated that Zn is predominantly associated with phyllosilicates, all other species amounting to < ∼10 to 20% of total zinc. The role of lithiophorite in the sequestration of zinc in soils had been inferred previously, but the firm identification of lithiophorite in this study serves as an excellent demonstration of the capabilities of combined micro-SXRF/XRD/EXAFS measurements. The micro-EXAFS spectrum collected in an area containing only phyllosilicates could not be simulated assuming a single Zn structural environment. Two distinct octahedrally-coordinated crystallographic sites (i.e., two EXAFS components) were considered: one site located within the phyllosilicate structure (isomorphic cationic substitution in the octahedral sheet) and another in the interlayer region in the form of a Zn-sorbed hydroxy-Al interlayered species. This second subspecies is less certain and further investigation of the individual EXAFS spectrum of this component is needed to precise its exact nature and the uptake mechanism of zinc in it. 相似文献
44.
Magnetic and plagioclase linear fabric discrepancy in dykes: a new way to define the flow vector using magnetic foliation 总被引：1，自引：0，他引：1
In basaltic dykes the magnetic lineation K1 (maximum magnetic susceptibility axis) is generally taken to indicate the flow direction during solidification of the magma. This assumption was tested in Tertiary basaltic dykes from Greenland displaying independent evidence of subhorizontal flow. The digital processing of microphotographs from thin sections cut in (K1, K2) planes yields the preferred linear orientation of plagioclase, which apparently marks the magma flow lineation. In up to 60% of cases, the angular separation between K1 and the assumed flow direction is greater than 45°. This suggests that the uncorroborated use of magnetic lineations in dykes is risky. A simple geometrical method is proposed to infer the flow vector from AMS in dykes based solely on magnetic foliations. 相似文献
45.
The southern margin of the East European Craton: new results from seismic sounding and potential fields between the North Sea and Poland 总被引：1，自引：0，他引：1
U. Bayer M. Grad T. C. Pharaoh H. Thybo A. Guterch D. Banka J. Lamarche A. Lassen B. Lewerenz M. Scheck A. -M. Marotta 《Tectonophysics》2002,360(1-4):301-314
The extension of eastern Avalonia from Britain through the NE German Basin into Poland is, in some sense, a virtual structure. It is covered almost everywhere by late Paleozoic and younger sediments. Evidence for this terrane is only gathered from geophysical data and age information derived from magmatic rocks. During the last two decades, much geophysical and geological information has been gathered since the European Geotraverse (EGT), which was followed by the BABEL, LT-7, MONA LISA, DEKORP-Basin'96, and POLONAISE'97 deep seismic experiments. Based on seismic lines, a remarkable feature has been observed between the North Sea and Poland: north of the Elbe Line (EL), the lower crust is characterised by high velocities (6.8–7.0 km/s), a feature which seems to be characteristic for at least a major part of eastern Avalonia (far eastern Avalonia). In addition, the seismic lines indicate that a wedge of the East European Craton (EEC) (or Baltica) continues to the south below the southern Permian Basin (SPB)—a structure which resembles a passive continental margin. The observed pattern may either indicate an extension of the Baltic crust much farther south than earlier expected or oceanic crust of the Tornquist Sea trapped during the Caledonian collision. In either case, the data require a reinterpretation of the docking mechanism of eastern Avalonia, and the Elbe–Odra Line (EOL), as well as the Elbe Fault system, together with the Intra-Sudedic Faults, appear to be related to major changes in the deeper crustal structures separating the East European crust from the Paleozoic agglomeration of Middle European terranes. 相似文献
46.
Jean Delteil Jean-Yves Collot Ray Wood Richard Herzer Stéphane Calmant David Christoffel Mike Coffin Jacky Ferrière Geoffroy Lamarche Jean-Frédéric Lebrun Alain Mauffret Bernard Pontoise Michel Popoff Etienne Ruellan Marc Sosson Ruppert Sutherland 《Marine Geophysical Researches》1996,18(2-4):383-399
The Geodynz-sud cruise on board the R/V l'Atalante collected bathymetric, side-scan sonar and seismic reflection data along the obliquely convergent boundary between the Australian and Pacific plates southwest of the South Island, New Zealand. The survey area extended from 44°05 S to 49°40 S, covering the transition zone between the offshore extension of the Alpine Fault and the Puysegur Trench and Puysegur Ridge. Based on variations in the nature and structure of the crust on either side of the margin, the plate boundary zone can be divided into three domains with distinctive structural and sedimentary characteristics. The northern domain involves subduction of probably thinned continental crust of the southern Challenger Plateau beneath the continental crust of Fiordland. It is characterized by thick sediments on the downgoing slab and a steep continental slope disrupted by fault scarps and canyons. The middle domain marks the transition between subduction of likely continental and oceanic crust defined by a series of en echelon ridges on the downgoing slab. This domain is characterized by a large collapse terrace on the continental slope which appears to be due to the collision of the en echelon ridges with the plate margin. The southern domain involves subduction of oceanic crust beneath continental and oceanic crust. This domain is characterized by exposed fabric of seafloor spreading on the downgoing slab, a steep inner trench wall and linear ridges and valleys on the Puysegur ridge crest. The data collected on this cruise provide insights into the nature and history of both plates, and factors influencing the distribution of strike-slip and compressive strain and the evolution of subduction processes along a highly oblique convergent margin. 相似文献
47.
Multibeam echosounders are becoming widespread for the purposes of seafloor bathymetry mapping, but the acquisition and the use of seafloor backscatter measurements, acquired simultaneously with the bathymetric data, are still insufficiently understood, controlled and standardized. This presents an obstacle to well-accepted, standardized analysis and application by end users. The Marine Geological and Biological Habitat Mapping group (Geohab.org) has long recognized the need for better coherence and common agreement on acquisition, processing and interpretation of seafloor backscatter data, and established the Backscatter Working Group (BSWG) in May 2013. This paper presents an overview of this initiative, the mandate, structure and program of the working group, and a synopsis of the BSWG Guidelines and Recommendations to date. The paper includes (1) an overview of the current status in sensors and techniques available in seafloor backscatter data from multibeam sonars; (2) the presentation of the BSWG structure and results; (3) recommendations to operators, end-users, sonar manufacturers, and software developers using sonar backscatter for seafloor-mapping applications, for best practice methods and approaches for data acquisition and processing; and (4) a discussion on the development needs for future systems and data processing. We propose for the first time a nomenclature of backscatter processing levels that affords a means to accurately and efficiently describe the data processing status, and to facilitate comparisons of final products from various origins. 相似文献
48.
Pressure variations in a magma reservoir may cause deformation at the surface and a redistribution of the stress in the surrounding rock. In this study, we use two‐dimensional numerical models and elaborate how magma chamber inflation and deflation affect the stress field around and surface displacement. We test how a pre‐existing normal fault near the magma reservoir may influence the pattern of stress. We demonstrate the possibility of initiating both normal and reverse slip on faults during the inflation of the magma reservoir. The Coulomb failure stress changes are calculated during the periods of pressure variation. An increase of Coulomb failure stress can be predicted above and below the magma chamber during increasing magma chamber pressure that may encourage earthquakes. This process can produce cracks and fault growth encouraging magma propagation along the cracked zone. A different distribution of the stress change is expected in the case of subsequent deflation of the overpressured magma reservoir. In this case, seismicity is expected on a plane at equal depth than the magma chamber, laterally offset from the extent of the magma chamber. Magma could propagate laterally from the magma reservoir into zones where cracks have been generated, but only if the resolved shear stress on the fault is small compared with the excess magma pressure. 相似文献
49.
Jess I. T. Hillman Geoffroy Lamarche Arne Pallentin Ingo A. Pecher Andrew R. Gorman Jens Schneider von Deimling 《Marine Geophysical Researches》2018,39(1-2):205-227
Using automated supervised segmentation of multibeam backscatter data to delineate seafloor substrates is a relatively novel technique. Low-frequency multibeam echosounders (MBES), such as the 12-kHz EM120, present particular difficulties since the signal can penetrate several metres into the seafloor, depending on substrate type. We present a case study illustrating how a non-targeted dataset may be used to derive information from multibeam backscatter data regarding distribution of substrate types. The results allow us to assess limitations associated with low frequency MBES where sub-bottom layering is present, and test the accuracy of automated supervised segmentation performed using SonarScope® software. This is done through comparison of predicted and observed substrate from backscatter facies-derived classes and substrate data, reinforced using quantitative statistical analysis based on a confusion matrix. We use sediment samples, video transects and sub-bottom profiles acquired on the Chatham Rise, east of New Zealand. Inferences on the substrate types are made using the Generic Seafloor Acoustic Backscatter (GSAB) model, and the extents of the backscatter classes are delineated by automated supervised segmentation. Correlating substrate data to backscatter classes revealed that backscatter amplitude may correspond to lithologies up to 4 m below the seafloor. Our results emphasise several issues related to substrate characterisation using backscatter classification, primarily because the GSAB model does not only relate to grain size and roughness properties of substrate, but also accounts for other parameters that influence backscatter. Better understanding these limitations allows us to derive first-order interpretations of sediment properties from automated supervised segmentation. 相似文献
50.
Dissolved organic matter fluorescence spectroscopy as a tool to estimate biological activity in a coastal zone submitted to anthropogenic inputs 总被引：16，自引：0，他引：16
Here we report on an investigation of the three-dimensional excitation-emission-matrix (EEM) fluorescence spectra of unconcentrated water samples collected in 1996, 1998 and 1999 at a site particularly propitious for macro-algae development. The degradation of these macro-algae was studied to determine the influence of their exudates on natural water EEM fluorescence spectra. This work demonstrates that biological activity is one of the major factors involved in the formation of the blue-shifted fluorescence band observed in marine waters (β component Ex/Em=310–320 nm/380–410 nm); our study also shows that fluorescence can be used to evaluate the biological activity both quantitatively and to determine its different phases. 相似文献