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71.
High‐P tectono‐metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines,Italy
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Deborah Lo Pò Roberto Braga Hans‐Joachim Massonne Giancarlo Molli Alessandra Montanini Giuseppe M. Bargossi 《Journal of Metamorphic Geology》2018,36(1):23-39
Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly‐deformed basements, the loss of information on pre‐kinematic phases of mylonites hinders large‐scale correlations based on tectono‐metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P)–temperature (T)–deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine‐grained K‐white mica and chlorite enveloping mica‐fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg‐rich cores and Mg‐poor rims. The amphibolite contains green amphibole+plagioclase+garnet+quartz+ilmenite defining S1 with a superposed mylonitic fabric localized in decimetre‐ to centimetre‐scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P–T constraints that are linked to the reconstructed metamorphic‐deformational stages. For the first time an early high‐P stage at >11 kbar and 510°C was constrained, followed by a temperature peak at 550–590°C and 9–10 kbar and a retrograde stage (<475°C, <7 kbar), during which ductile shear zones developed. The inferred clockwise P–T–D path was most likely related to crustal thickening by continent‐continent collision during the Variscan orogeny. A comparison of this P–T–D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono‐metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established. 相似文献
72.
Piazza R Giuliani S Bellucci LG Mugnai C Cu NH Nhon DH Vecchiato M Romano S Frignani M 《Marine pollution bulletin》2010,60(12):2303-2310
Samples from nine Central Vietnam coastal lagoons, together with three soils and sediments collected in two freshwater reservoirs of the Thua Thien-Hué province, were analysed for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Total concentrations are low, from 192 to 2912 pg g−1 and depth profiles in Tam Giang-Cau Hai (TG-CH) sediment cores show only minor changes over time in PCDD/F input and composition. Octachloro dibenzo-p-dioxin (OCDD) is the prevailing congener (approximately 90%), indicating combustion as the main PCDD/F source to these coastal systems, whereas natural formation might be partly responsible for the presence at depth. 2,3,7,8-Tetrachloro dibenzo-p-dioxin (TCDD), largely sprayed together with Agent Orange over the study areas during the war (1961–1971), is absent or very low. This result supports the hypothesis of strong degradation soon after spraying. Multivariate statistical analyses account for the presence of local, short-range sources as observed in the northern part of the TG-CH lagoon. 相似文献
73.
We present a method to derive the 2D maps of the O2 (a1Δg) airglow emission at 1.27 μm from the OMEGA/MEx nadir observations. The OMEGA imaging capabilities allow monitoring the 2D distribution, daily and seasonal variation of the O2 emission intensities with a detection limit of 4 MR. The highest values, of the order of ∼31 MR, are found on the south pole for 11 h < LT < 13 h, during the early spring (186° < Ls < 192°) of martian year (MY) 27, according to the Mars Year numbering scheme of Clancy et al. [Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. Mars aerosol studies with the MGS TES emission phase function observations: Optical depths, particle sizes, and ice cloud types versus latitude and solar longitude. J. Geophys. Res. 108. doi: 10.1029/2003JE002058]. In the polar regions the day-by-day variability, associated with polar vortex turbulences, is obtained of the order of 30-50% as predicted by the model [Lefévre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. Three-dimensional modeling of ozone on Mars. J. Geophys. Res. 109, E07004. doi: 10.1029/2004JE002268] and found by SPICAM [Perrier, S., Bertaux, J.-L., Lebonnois, S., Korablev, O., Fedorova, A., 2006. Global distribution of total ozone on Mars from SPICAM/MEX UV measurements. J. Geophys. Res. 111, E09S06. doi: 10.1029/2006JE002681]. In the considered set of data a maximum of the O2 emission is observed between 11 h and 15 h LT in the latitude range 70-85° during early spring on both hemispheres, while for the southern autumn-winter season a maximum is found between 50° and 60° in the southern hemisphere for MY28. Increase of intensity of the O2 emission observed from Ls 130° to 160° at southern high latitudes may be explained by increase of solar illumination conditions in the maps acquired during the considered period.Atmospheric waves crossing the terminator on the southern polar regions are observed for the first time during the MY28 early spring. The spatial scale of the waves ranges from 100 to 130 km, and the intensity fluctuations are of the order of 4MR.This study confirms the high potentiality of O2 (a1Δg) day glow as a passive tracer of the martian atmosphere dynamics at high latitudes. 相似文献
74.
Francesco Soldovieri Giancarlo Prisco Svein-Erik Hamran 《Planetary and Space Science》2009,57(8-9):1076-1084
The exploitation of ground penetrating radar in Mars subsurface exploration is becoming assessed in remote sensing observations and is of timely interest for high resolution in situ prospecting of the first meters of the underground.In this framework, we deal with a novel processing approach based on microwave tomography. Aiming to achieve accurate and reliable “images” of the investigated subsurface region in order to detect, localize and possibly determine the extent and the geometrical features of the embedded layers while reducing at the minimum possible the “interpretation” of the diagnostics result.The feasibility of the microwave tomographic approach has been tested in realistic cases dealing with conditions analogue to the Mars subsurface ones. In particular, we will present the tomographic reconstruction results achieved by experimental data collected in a field survey at Svalbard Islands (Norway) with a time-domain GPR. 相似文献
75.
Giancarlo Della Ventura G. Diego Gatta Gunter J. Redhammer Fabio Bellatreccia Anja Loose Gian Carlo Parodi 《Physics and Chemistry of Minerals》2009,36(4):193-206
We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H2O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow···O donor–acceptor distances >2.7 Å. The FTIR spectra show two main absorptions. The first at 3,602 cm?1 is polarized for E ⊥ c and is assigned to the ν3 mode. The second, at 3,531 cm?1, is also polarized for E ⊥ c and is assigned to ν1 mode. A weak component at 4,108 cm?1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300–1,500 cm?1 range are strongly polarized for E ⊥ c, and are assigned to the vibrations of the CO3 group. 相似文献
76.
Dale P. Cruikshank Eric Wegryn R.H. Brown B.J. Buratti T.B. McCord Y.J. Pendleton G. Filacchione P. Cerroni R. Jaumann K.H. Baines G. Bellucci Y. Langevin D.L. Matson P. Drossart 《Icarus》2008,193(2):334-343
Material of low geometric albedo (pV?0.1) is found on many objects in the outer Solar System, but its distribution in the saturnian satellite system is of special interest because of its juxtaposition with high-albedo ice. In the absence of clear, diagnostic spectral features, the composition of this low-albedo (or “dark”) material is generally inferred to be carbon-rich, but the form(s) of the carbon is unknown. Near-infrared spectra of the low-albedo hemisphere of Saturn's satellite Iapetus were obtained with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft at the fly-by of that satellite of 31 December 2004, yielding a maximum spatial resolution on the satellite's surface of ∼65 km. The spectral region 3-3.6 μm reveals a broad absorption band, centered at 3.29 μm, and concentrated in a region comprising about 15% of the low-albedo surface area. This is identified as the CH stretching mode vibration in polycyclic aromatic hydrocarbon (PAH) molecules. Two weaker bands attributed to CH2 stretching modes in aliphatic hydrocarbons are found in association with the aromatic band. The bands most likely arise from aromatic and aliphatic units in complex macromolecular carbonaceous material with a kerogen- or coal-like structure, similar to that in carbonaceous meteorites. VIMS spectra of Phoebe, encountered by Cassini on 11 June 2004, also show the aromatic hydrocarbon band, although somewhat weaker than on Iapetus. The origin of the PAH molecular material on these two satellites is unknown, but PAHs are found in carbonaceous meteorites, cometary dust particles, circumstellar dust, and interstellar dust. 相似文献
77.
Raúl R. Castro Giancarlo Monachesi Luca Trojani Marco Mucciarelli Massimo Frapiccini 《Journal of Seismology》2002,6(1):43-59
We studied spatial and temporal characteristics of seismic attenuation inCentral Italy using S- and coda- waves recorded by the MarchesanSeismograph Network from earthquakes located in the epicentral area ofthe 1997 Umbria-Marche sequence. The amplitude decay of the S waveswith distance was defined calculating empirical attenuation functions at 15frequencies between 1 and 25 Hz. We analyzed separately foreshocks andaftershocks and we found the same attenuation functions, suggesting thatthe possible temporal variations could be confined in a small area. Thefrequency dependence of Q
S was approximated by the equation Q
S=18 · f
2.0between 1 and 10 Hz. At higher frequencies (10–25 Hz), the frequencydependence of Q
s weakens, having an average value of Q
S=990. We also estimated Q from coda waves (Q
C) using the single-scattering models of Aki andChouet (1975) and Sato (1977). We found that Q
C=77 · f
0.6, (between 2 and 20Hz) at the western side of the mountain chain, using either foreshocks oraftershocks. This relation is consistent with previous estimates of Q
Creported for the Central Apennines. For a volume sampling the Colfioritobasin, the Apennines and the Marche region we found that Q
C=55 · f
0.8,indicating highattenuation below the mountain belt. To detect small temporal changes ofQ, we calculated spectral ratios of 5 temporal doublets located in theepicentral area and recorded at the closest station. We found temporalchanges of Q that vary from 27% to 56%, depending on the locationof the doublets. This variability suggests that the temporal change ofattenuation may depend on the spatial variation of Q and perhaps on thespatial distribution of tectonic stress in the epicentral area. 相似文献
78.
Giancarlo Serri 《Earth and Planetary Science Letters》1981,52(1):203-212
Ophiolites have been divided into two groups: high-Ti and low-Ti types. These can be discriminated by studying the fractionation trends of both gabbroic complexes (this work) and lavas and dykes [16], particularly in the TiO2/M.I. diagram. The first type typically shows MORB-like magmas whereas in the second the magma types have a spectrum of composition from mid-ocean ridge basalts to island arc tholeiites and boninite-like magmas often occur.High-Ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back-arc basin crusts as well as oceanic crust generated during the intermediate and late-stage opening of intraoceanic back-arc basins.Parental magmas and fractionation processes of low-Ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back-arc basins. 相似文献
79.
80.
Lateral flow of African mantle below the nearby Tyrrhenian plate: geochemical evidence 总被引:1,自引:0,他引:1
The Quaternary magmatism of the Southern Tyrrhenian basin represents a rare example of an active volcanic arc system where ocean island basalt (OIB) and island arc basalt (IAB) magmas coexist. Although there is general agreement on the importance of the Ionian oceanic lithosphere subduction in the genesis of the IAB magmatism, the tectono‐magmatic processes producing the coexisting OIB magmas are still poorly understood. Here we show that the geochemistry of the Quaternary Southern Tyrrhenian OIB‐type magmatism (i.e. Ustica island and Prometeo, a previously unknown submarine lava field) is very similar to that of OIB‐type volcanoes situated on the nearby African plate (i.e. Etna and Hyblean Plateau). Among the possible geodynamic scenarios proposed to explain the coexistence of OIB and IAB magmas in arc settings, we consider the development of a tear at the edge of the Ionian plate as the more likely mechanism to favour the flow of African asthenospheric mantle below the Tyrrhenian plate. 相似文献