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41.
Piergiorgio Scarlato Paola Tuccimei Silvio Mollo Michele Soligo Mauro Castelluccio 《Bulletin of Volcanology》2013,75(9):1-8
To better understand the mechanisms leading to different radon background levels in volcanic settings, we have performed two long-term deformation experiments of 16 days using a real-time setup that enables us to monitor any variation of radon activity concentration during rock compression. Our measurements demonstrate that, in the case of highly porous volcanic rocks, the emanating power of the substrate changes as a function of the volcanic stress conditions. Constant magmatic pressures, such as those observed during dike intrusions and hydrothermal fluid injections, can result in pervasive pore collapse that is mirrored by a significant radon decrease until a constant emanation is achieved. Conversely, repeated cycles of stress due to, for example, volcano inflation/deflation cycles, cause a progressive radon increase a few days (but even weeks and months) before rupture. After rock failure, however, the formation of new emanation surfaces leads to a substantial increase of the radon signal. Our results suggest that surface deformation in tectonic and volcanic settings, such as inflation/deflation or constant magmatic pressures, have important repercussions on the emanating power of volcanic substrates. 相似文献
42.
Francesca?CifelliEmail author Liliana?Minelli Federico?Rossetti Giorgia?Urru Massimo?Mattei 《International Journal of Earth Sciences》2012,101(3):787-802
Anisotropy of magnetic susceptibility (AMS) analysis has been carried out in the thermometamorphic aureole surrounding the
Late Miocene Monte Capanne pluton (Elba Island, Central Italy). The identification and separation of the main carriers of
the magnetic susceptibility by low-temperature and high-field AMS measurements demonstrate that a correct knowledge of the
magnetic fabric is needed in order to use AMS for tectonic interpretations. Magnetic fabric data, combined with structural
data from the aureole, and their comparison with data from the pluton itself, were used to constraint the mode of pluton emplacement.
Results document an intimate linkage between the magmatic flow pattern and the syn-metamorphic fabrics acquired during pluton
emplacement in the host rocks. The magnetic/structural fabric in the aureole rocks is dominated by flattening deformation
and no systematic relationship with any regional tectonic feature is observed. These results suggest that local processes
induced by magma ascent in the upper crust might have played a primary role in space generation for pluton emplacement in
the Tuscan Magmatic Province, suggesting a revaluation of the modes of pluton emplacement during the post-orogenic evolution
of the northern Apennine system as a whole. 相似文献
43.
An abandoned estuary within Marajó Island: Implications for late Quaternary paleogeography of northern Brazil 总被引：1，自引：0，他引：1
Several previous studies have recognized paleochannels as the most typical geomorphologic features imprinted in the modern
landscape of Marajó Island, northern Brazil. A characterization of these paleochannels by detailed mapping has not yet been
accomplished, despite their great potential for helping reconstructing the succession of events responsible for the origin
and evolution of the drainage system at the mouth of the Amazon River during the Quaternary. The present study aims to provide
a detailed mapping and characterization of these paleochannels using remote sensing (i.e., SRTM, Landsat 5-TM, and Landsat
7-ETM+), integrated with sedimentological and radiocarbon data obtained from outcrops and cores. The mapped paleochannels
allowed for the first time correlation of the channel-like morphologies with fining-upward sedimentary successions, typical
of channel deposits. The study focuses on the Lake Arari area located in the northeast of Marajó Island, where a complex fluvial
network dominated by a funnel-shaped structure attributed to a Late Pleistocene paleoestuarine system was recognized. The
inner estuary was connected to three fluvial channels that ran from the east-southeast throughout an area currently occupied
by Marajó Bay, implying that this bay did not exist when the estuarine system was active. Separation of the eastern side of
Marajó Island due to tectonic causes related to capture of Tocantins River by northeastward-orientated faults would have interrupted
the fluvial inflow, ultimately causing the abandonment of the estuarine system. Today, the central part of the funnel-shaped
structure related to the Late Pleistocene paleoestuary is occupied by Lake Arari, where sediments started to accumulate no
earlier than the mid-Holocene. It is proposed that this lake might have developed from the Late Pleistocene estuary as the
fluvial inflow was cut off during the detachment of Marajó Island. Marajó Island serves as an example of a very dynamic site
of changing landscapes throughout the Quaternary as a result of tectonic activity. 相似文献
44.
45.
Carmela Freda Mario Gaeta Valeria Misiti Silvio Mollo Daniela Dolfi Piergiorgio Scarlato 《Lithos》2008,101(3-4):397-415
The Alban Hills ultrapotassic volcanic district is one of the main districts emplaced during Quaternary time along the Tyrrhenian margin of Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition is mostly K-foiditic. Differentiated products (MgO < 3 wt.%) are characterised by low SiO2 concentration (< 50 wt.%) and geochemical features indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the Alban Hills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performed experiments on the Alban Hills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonated conditions. Experiments were carried out at 1 atm, 0.5 GPa and 1 GPa, temperatures ranging from 1050 to 1300 °C, and H2O and CaCO3 in the starting material up to 2 and 7 wt.%, respectively. The experiments performed at 0.5 GPa are the most representative of the Alban Hills plumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases. Nevertheless, our experimental results demonstrate that the occurrence of CaCO3 in the starting material strongly affects phase relations. Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This early crystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic (Alban Hills parental composition) to phonolitic compositions. This is in contrast with micro-textural evidence showing late crystallization of magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO3-bearing experiments (i.e., simulating magma–carbonate interaction) the magnetite and phlogopite stability fields are strongly reduced. As a consequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in a differentiation trend moving towards K-foiditic compositions. These experimental results are in agreement with micro-textural features and chemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data. Magma–carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems. However, the uniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled by different factors, the dynamics of the plumbing system being one of the most important. 相似文献
46.
Federico Lucci Federico Rossetti John Charles White Hadi Shafaii Moghadam Alireza Shirzadi Mohsen Nasrabady 《Arabian Journal of Geosciences》2016,9(10):573
Calc-alkaline arc magmatism at convergent plate margins is volumetrically dominated by metaluminous andesites. Many studies highlighted the importance of differentiation via fractionation processes of arc magmas, but only in the last decades, it has been demonstrated that not all rock-forming minerals may affect the evolution of calc-alkaline suites. In particular, a major role exerted by Al-rich hornblende amphibole as fractionating mineral phase has been documented in many volcanic arc settings. The aim of this work is to understand the role of the Tschermak molecule (CaAlAlSiO6) hosted in the hornblende and plagioclase fractionation assemblage in driving magma differentiation in calc-alkaline magmatic suites. We explore this issue by applying replenishment–fractional crystallization (RFC) and rare earth element–Rayleigh fractional crystallization (REE-FC) modeling to the Sabzevar Eocene (ca. 45–47 Ma) calc-alkaline volcanism of NE Central Iran, where hornblende-controlled fractionation has been demonstrated. Major element mass balance modeling indicates RFC dominated by a fractionating assemblage made of Hbl52.0–52.5 + Pl44.1–44.2 + Ttn3.3–3.9 (phases are expressed on total crystallized assemblage). REE-FC modeling shows, instead, a lower degree of fractionation with respect to RFC models that is interpreted as due to hornblende and plagioclase resorption by the residual melt. Calculations demonstrate that fractionation of the Tschermak molecule can readily produce dacite and rhyolite magmas starting from a calc-alkaline andesite source (FC = ca. 30 %). In particular, the Tschermak molecule controls both the heavy rare earth elements (HREE) and light rare earth element (LREE) budgets in calc-alkaline differentiation trends. 相似文献
47.
The Miocene Barreiras Formation in the Middle Rio Capim area records an incised valley system for which facies analysis and ichnology (Skolithos, Ophiomorpha, Planolites, Gyrolithes, Taenidium) suggest an estuarine character. Three stratigraphic units are recognized (from bottom to top): Unit 1 includes an inner estuarine tidal channel complex and tidal flat/salt marsh deposits; Unit 2 consists of estuarine bay/lagoon and flood tidal delta deposits related to the estuary mouth; and Unit 3 includes a tidal channel with a tidal point bar, as well as tidal flat/salt marsh deposits similar to those from Unit 1. These units and their bounding surfaces record the history of relative sea level changes in the estuary. After a sea level drop, the valley was inundated and formed an amalgamated sequence boundary and transgressive surface. Transgression (Unit 1) promoted the landward shift of flood tidal deltas and lagoon settings (Unit 2). The system then moved seaward, with the superposition of inner estuarine deposits (Unit 3) over Unit 2. Facies architecture seems to have been controlled by tectonics, as shown by: the paleovalley orientation according to the main tectonic structures of the basin; the presence of faults and fractures that displace the basal unconformity; and the abundance of soft sediment deformation. 相似文献
48.
Raimundo Almeida-Filho Dilce F. Rossetti Francisco J. Ferreira Carlos Beisl 《Quaternary Research》2009,72(1):103-73
The Pirapemas Lineament is a remarkable 200-km-long, NE-SW trending structure in the Barreirinhas Basin, one of the several Brazilian coastal basins. This lineament splits the study area in two sectors of distinctive morphology, drainage patterns, and sedimentary covers. Terrain northward of the lineament presents a smooth topography with sub-parallel to sub-dendritic drainage patterns, whereas a dissected plateau characterized by incised valleys and rectangular drainage pattern occurs southward, suggesting a structural control by joints and faults. Geological field data, crossed with thermal luminescence (TL) and optically stimulated luminescence (OSL) dating, revealed that the surface southward of the lineament consists mostly of Miocene and late Pleistocene sedimentary deposits, represented by the Barreiras Formation and the Post-Barreiras sediments, respectively. In contrast, relatively younger sands mantle most of the northward terrain, as indicated by well-preserved paleodune deposits that grade into active aeolian dunes of the Lençóis Maranhenses National Park. Geomorphological and geological data analysis suggests that the northern sector is under the effect of subsidence, proving that the Pirapemas Lineament is an active agent modeling the landscape in the region. Geophysical data (gravity and seismic) confirm that such a structural feature is the surface expression of an active deep-seated basement fault. 相似文献
49.
Davide Labagnara Mario Patrucco Piergiorgio Rossetti Valeria Pellegrino 《Environmental Earth Sciences》2013,70(2):857-868
The modern approach to the design and management of tunnel excavation, and muck reuse, can be influenced to a great extent by the possible presence of rock formations containing asbestos minerals. This situation creates problems concerning the protection of the workers’ health and the expectable environmental criticalities, while a drastic re-consideration of the muck destination is necessary. Since, in the case of carcinogens, corrective action following exposure or dispersion is not acceptable, detailed knowledge of the characteristics of the material to be excavated is all the more essential. Only on this basis it is possible to design the tunneling operations in a prevention through design approach, so that the tunnel driving techniques, fittings and technologies, and special equipment and modus operandi, can grant, where necessary, minimized risk conditions, and make a correct decision on whether, how and when these measures must be fully activated, since such an approach involves a remarkable impact on costs and work organization. In the Western Alps a detailed identification and quantification of the asbestos content in rocks is a difficult task, due to the fact that the asbestos in the host rocks, mainly serpentinite, shows a highly variable distribution, typical of ophiolitic belts throughout the world, as it is mostly associated with joints and shear zones. The possibilities of a predictive assessment of the asbestos content in the formations to be excavated are here discussed, with special reference to the reliability of the achievable results. 相似文献
50.
The interaction modalities of transfer zones connecting rift segments may be influenced by several factors. Amongst these, the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies. Sand-box models were made to investigate how the orientation, geometry and kinematics of transfer zones depend upon pre-existing basement anisotropies. Analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults. Strike-slip faults are present inside the transfer zone, provided that angle α (between the extension direction and the axis of the transfer zone) is less than 50°. Narrower transfer zones, striking oblique to the extension direction, occur for angle φ (between the direction perpendicular to the extension direction and the direction of the basement anisotropy) less than 90°; wider transfer zones, subparallel to the extensional direction, occur for φ > 90°. Increasing the overstep induces narrower transfer zones striking subparallel to the extension direction. Similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts. The comparison between experimental and natural data shows how the geometry, kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies: transfer zones along inherited basement structures set at lower angles to the extension direction, and display lower aspect ratios, than transfer zones where inherited anisotropies are absent. 相似文献