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981.
Javier F. Pacheco Carlos Valds-Gonzlez Hugo Delgado Shri K. Singh F. Ramn Zuiga Carlos A. Mortera-Gutirrez Miguel A. Santoyo Jaime Domínguez Ricardo Barrn 《Journal of South American Earth Sciences》1999,12(6):73
An earthquake swarm occurred during February and March 1997 in the vicinity of the Tancitaro Volcano, in the southern part of the tectonically complex Michoacan Triangle. A study of these events provides an opportunity to map the active faults in the area and to learn if the orientation and the sense of motion on these faults are consistent with the mapped faults and the alignment of cinder cones in the region. The foci of 230 earthquakes, which could be located, are distributed between 10 and 18 km depth, and show an alignment in, roughly, a NE direction. The focal mechanisms and seismic moments of the 27 best-recorded events were determined by waveform modeling of P and S waves. These mechanisms show two distinct patterns. More than 50% of the solutions are left-lateral strike–slip mechanisms with a normal component. The preferred fault plane strikes NE. Another group of events, probably caused by triggered seismicity on the Chapala–Oaxaca fault zone, shows left-lateral strike–slip mechanisms with a large-thrust component on NW-trending faults. S wave splitting shows 1–2.5% crustal-anisotropy. The direction of the anisotropy coincides with the NE alignment of events, and the preferred nodal plane. This is also the alignment of cinder cones, suggesting that preexisting fractures and cracks are responsible for the seismicity and anisotropic behavior of the crust. The resulting stress orientation, NE compression, is the one expected for the fore-arc region. We conclude that although Michoacan Triangle lies in the Trans-Mexican Volcanic Belt, it does not form part of this stress province where the stress orientation is NS extension. 相似文献
982.
Prof. E. -R. Neumann Dr. J. Martí Dr. J. Mitjavila E. Wulff-Pedersen 《Mineralogy and Petrology》1999,65(1-2):113-139
Summary The Válencia Trough of northeastern Spain represents a poorly understood region of Cenozoic extension-related magmatism in western Europe. We present chemical data on mafic xenoliths and their host basalt from the Roca Negra cinder cone in the Catalán Volcanic Zone in the Válencia Trough. The xenoliths consist of augite+hornblende+ oxides±plagioclase±apatite±olivine::Lbiotitelorthopyroxene, and range from clinopyroxenites and hornblendites to gabbroic rocks. The major and trace element compositions of the xenoliths are compatible with formation as cumulates from a range of olivine tholeiitic to strongly alkaline magmas, and show a strong affinity to the Middle Miocene to Recent rift-related magmatism in the Cátalan Volcanic Zone. The xenoliths formed at temperatures of 1110–1180 °C, pressures of 2–7 kb (with 10 kb as the upper limit), and oxygen fugacities corresponding to 1–2 log units above the NNO oxygen buffer. The estimated pressure range implies formation in magma chambers within the crust, possibly also near the crust/mantle boundary (underplating). The presence of crustal cumulates indicates that the total volume of magmas emplaced into the crust in the Cátalan Volcanic Zone may be significantly larger than indicated by surface exposures. Isotopic compositions of the xenoliths are87Sr/86Sr: 0.703537–0.703647,143Nd/144Nd: 0.512732–0.512766,206Pb/204Pb: 18.097–19.106,207Pb/204Pb: 15.522–15.579, and208Pb/204Pb: 37.850–38.794. This range suggests the involvement of two mantle source components. One of these may be the low-velocity component recognized in Cenozoic magmatism in the eastern Atlantic Ocean to central Europe and western Mediterranean. The other component, which differs from those involved in other domains of Cenozoic volcanism in Europe and adjacent areas, may be located in the lithospheric mantle underlying NE Spain, which was metasomatized during late Variscan to early Alpine deformation.
With 10 Figures 相似文献
Herkunft und Bedeutung mafischer Xenolithe aus känozoiscben Extensions-bezogenen Vulkaniten aus dem Valencia Trog, Nordost-Spanien
Zusammenfassung Der Válencia Trog in Nordost-Spanien stellt eine noch wenig verstandene Region mit känozoischen Extensions-bezogenern Magmatismus in Westeuropa dar. Wir geben hier chemische Daten über mafische Xenolithe und ihre basaltischen Muttergesteine aus dem Roca Negra Kegei in der vulkanischen Zone von Catalan im Valencia Trog. Die Xenolithe bestehen aus Augit+Hornblende+Oxiden±Plagioklas±Apatit±Olivin±Biotit±Orthopyroxen und reichen in ihrer Zusammensetzung von Klinopyroxeniten und Homblenditen his zu gabbroischen Gesteinen. Die Haupt- und Spurenelementzusammensetzung der Xenolithe entspricht einer Kumulatbildung aus Olivin-tholéitischen bis stark alkalischen Magmen and zeigt eine deutliche Affinität mit dem mittelmiozanen bis rezenten Rift-bezogenen Magmatismus in der vulkanischen Zone von Catalan. Die Xenolithe bildeten sich bei Temperaturen von 1.110 bis 1.180 °C und bei Drucken von 2–7 kb (mit 10 kb als Obergrenze) und bei Sauerstoff-Fugazitäten, die 1 bis 2 log Einheiten über dem NNO Sauerstoff-Puffer liegen. Die abgeschätzten Drucke weisen auf Bildung in Magmakammern innerhalb der Kruste bin, womölich auch in der Nähe der Kruste-Mantelgrenze (Underplating). Das Vorkommen von krustalen Kumulaten zeigt, daß das gesamte Volumen von Magma, das in die Kruste der vulkanischen Zone von Catalan eingedrungen ist, wesentlich größer ist als das, das man aus Oberflächen-Ausbissen entnehmen k6nnte. Die Isotopen-Zusammensetzung der Xenolithe sind87Sr/86Sr: 0.703537–0.703647,143Nd/144Nd: 0.512732–0.512766,206Pb/204 Pb: 18.097–19.106,207Pb/204Pb: 15.522–15.579, and208Pb/204Pb: 37.850–38.794. Dies läßt erkennen, daß es sich bier um zwei verschiedene Mantel-Quellen handelt. Eine davon könnte die Niedrig-Geschwindigkeitskomponente sein, die man in känozoiscben Magmatiten des östlichen Atlantischen Ozeans, sowie in Zentraleuropa und im westlichen Mittelmeer erkannt hat. Die andere Komponete unterscheidet sich von jenen, die in anderen Bereichen von känozoischem Vulkanismus in Europa und benachbarten Gebieten vorkommen. Sie könnte im lithosphärischen Mantel unterhalb von Nordost-Spanien liegen, der während der variszischen bis zur frühalpidischen Deformation metasomatisch beeinflußt wurde.
With 10 Figures 相似文献
983.
For first time, during 1991, seismic activity was recorded during an eruption at Colima volcano. We analyze these data to
obtain a stress pattern using a composite focal mechanism technique. From the analysis of regional seismicity, the Tamazula
Fault and the Armeria River appear as active features and the dip of the slab east of the Jalisco Block is approximately 12°.
Southwest of Colima volcano a vertical alignment of seismic events was observed. We estimate five different composite focal
mechanism solutions from our data set, which indicate a change of the stress field at the volcano after the 1991 eruption.
These solutions suggest that the stress field in the volcanic edifice was controlled by stresses related to the emplacement
of magma superimposed on the regional stress field. No evidence of active local faults in the volcanic edifice was found.
We propose a model for the eruptive process that involves tilting of the volcanic edifice.
Received: 15 October 1995 / Accepted: 26 October 1998 相似文献
984.
We analyzed more than 1700 earthquakes related to the 1982 eruption of El Chichon volcano in southern Mexico. The data were
recorded at specific periods throughout the whole eruptive interval of March to April 1982, by three different networks. The
seismic activity began several months before the first eruption on 28 March. During this period the seismicity consisted of
hybrid and long-period shallow earthquakes most likely related to processes of faulting, fracturing, and fluid movement underneath
the volcano. The foci of events occurring before the eruption circumscribe an aseismic zone from approximately 7 to 13 km
below the volcano. After the eruption, the seismic activity consisted of tectonic-type earthquakes that peaked at 1200 events/h.
This later activity occurred over a wide range of depths, mostly between 5 and 20 km, that includes the former aseismic zone
and is roughly limited by the major tectonic faults in the area.
Received: 19 May 1998 / Accepted: 13 June 1999 相似文献
985.
The Citlaltépetl Ignimbrite records one of the largest explosive events during the Holocene activity of Citlaltépetl Volcano
(Pico de Orizaba). Multiple pyroclastic flow units, a fall deposit, and some lahar units were emplaced between 8500–9000 y
B.P. as a result of repetitive but discrete explosive events. The whole ignimbrite resulted from discrete fluctuations in
eruptive intensity that decreased with time. The initial pyroclastic flow pulse was by far the most violent and widespread
event, and its deposits show conspicuous variations in structure and texture that could be associated with different mechanisms
of transport and emplacement. Subpopulation Sequential Fragmentation Transport (SFT) analyses were carried out in order to
determine the physical mechanisms that selectively concentrate or remove particles in the moving flows. We suggest that lateral
and temporal changes in the flow rheology, in which fluidization, yield strength, entrainment of atmospheric air, and sedimentation
played a dominant role in flow propagation and emplacement, may imprint a unique signature in the grain-size spectra. The
lowermost unit of the Citlaltépetl Ignimbrite can be envisaged by a model in which progressive aggradation near the vent became
replaced by en masse emplacement farther outward.
Received: 24 March 1998 / Accepted: 9 October 1998 相似文献
986.
F. Díaz García J. R. Martínez Catalán R. Arenas P. González Cuadra 《International Journal of Earth Sciences》1999,88(2):337-351
An important detachment is described in the allochthonous Ordenes Complex, in the NW Iberian Massif, and its meaning is related
to the kinematics of contemporaneous convergent structures. The Corredoiras Detachment (CD) separates a hangingwall unit,
characterised by a medium-pressure metamorphic gradient, from a footwall high-pressure and high-temperature unit and an underlying
ophiolitic unit. An associated ductile shear zone, nearly 2000 m thick, developed in the lower part of the hangingwall unit,
where the Corredoiras Orthogneiss, a Lower Ordovician metagranite, was progressively transformed into augengneisses, mylonitic
and ultramylonitic gneisses. The attitude of the stretching and mineral lineation in the mylonites varies due to late refolding
at map scale, but the sense of movement can be estimated, being roughly top to the SE. According to crosscutting relationships,
the CD developed subsequent to the thrusting of the high-pressure/high-temperature unit onto the ophiolitic unit, and prior
to younger extensional detachments, upright folding and strike-slip tectonics. The geometric relationships of the CD with
the previous structures in the footwall unit, the subtractive character of the metamorphic gap between its hangingwall and
footwall, and the available isotopic data suggest that the CD is an early Variscan, ductile extensional detachment, the movement
of which was roughly simultaneous with the onset of thrusting of the allochthonous complexes over their relative autochthon.
Received: 17 November 1998 / Accepted: 4 April 1999 相似文献
987.
Origin of peraluminous granites and granodiorites, Iberian massif, Spain: an experimental test of granite petrogenesis 总被引:27,自引:0,他引:27
Antonio Castro Alberto E. Patiño Douce L. Guillermo Corretgé Jesús D. de la Rosa Mohammed El-Biad Hassan El-Hmidi 《Contributions to Mineralogy and Petrology》1999,135(2-3):255-276
The discrimination between potential source materials involved in the genesis of Iberian granites and granodiorites, as well
as the role of mantle-crust interactions, are examined using constraints imposed by melting experiments, melting-assimilation
experiments and Sr-Nd isotope systematics. The Sr-Nd isotope relationships indicate the existence of different genetic trends
in which juvenile mantle materials are involved by different mechanisms: (1) a source trend, traced by a particular evolution
of the pre-Hercynian basement and indicating mantle participation at the time of sedimentation; (2) a set of magmatic trends
traced by gabbro-tonalite-enclave-granodiorite associations, implying the incorporation of new mantle material at the time
of granite generation. These relationships strongly support a pure crustal origin for the peraluminous leucogranites, derived
from partial melting of crustal protoliths, and a hybrid origin for the peraluminous granodiorites. These granodiorites are
the most abundant granitic rocks of the Central Iberian zone (CIZ) of the Iberian massif, implying that processes of hybridisation
by assimilation and/or magma mixing played an important role in granitoid production during the Hercynian orogeny. These hypotheses
have been tested by means of melting and assimilation experiments. Melting experiments in the range 800–900 °C and at pressures
of 3, 6, 10 and 15 kbar indicate that: (1) several potential source materials such as Bt-Ms gneisses and metagreywackes are
suitable for the production of peraluminous leucogranite melts; (2) the melt compositions are always leucogranitic, regardless
of pressure; (3) pressure exerts a strong influence on the fertility of the source: experiments at 3 kbar produce more than
20 vol% of melt, compared with less than 5 vol% of melt produced at 10 and 15 kbar and at the same temperature. The melting-assimilation
experiments carried out at 1000 °C and 4, 7 and 10 kbar and using a proportion of 50% gabbro and 50% gneiss give high melt
proportions (more than 50 vol.%) and noritic residues. These melts have the composition of leucogranodiorites, and overlap
with part of the compositional range of peraluminous granodiorites of the Iberian massif. The generation of more mafic granodiorites
may be explained by the incorporation of some residual orthopyroxene to the granodiorite magmas. The low solubility of Fe + Mg
prevents the generation of granodiorite melts with more than 3 wt% of MgO + FeO at all crustal pressures. The large volumes
of peraluminous, hybrid granodiorites, produced by assimilation of crustal rocks by mantle magmas, imply that an important
episode of crustal growth took place during the Late-Palaeozoic Hercynian orogeny in the Iberian massif.
Received: 30 June 1998 / Accepted: 27 November 1998 相似文献
988.
The scope for generating bio-oils with relatively low oxygen contents via hydropyrolysis 总被引:3,自引:0,他引:3
The primary oils obtained in high yields from fast (fluidised-bed) pyrolysis of biomass generally have high oxygen contents (ca. 40% w/w). The scope for using pyrolysis under hydrogen pressure (hydropyrolysis), to give oils with much lower oxygen contents compared to normal pyrolysis has been investigated. Fixed-bed hydropyrolysis tests have been conducted on cellulose, sugar cane bagasse and eucalyptus wood using hydrogen pressures up to 10 MPa, with heating rates of 5 and 300°C min−1. A colloidal FeS catalyst was used in some tests (Fe loading of 5%, w/w) to increase overall conversions. Further, the attractive option of using a two-stage reactor in which the primary oil vapors are passed though a bed of hydrotreating catalyst is also described. Raising the hydrogen pressure from atmospheric to 10 MPa reduced the oxygen content of the primary oil by over 10% to below 20% w/w. The addition of a dispersed iron sulphide catalyst gave conversions close to 100% for all three biomass samples investigated at 10 MPa under conditions in the fixed-bed reactor where significant diffusional resistances existed and reduced the oxygen content of the bio-oil by a further 10%. Although NMR indicated that the oils became increasingly aromatic as more oxygen was removed, the increase in hydrogen pressure decreased the extent of overall aromatisation that occurs primarily due to the lower char yields obtained. In two-stage tests for cellulose, using a commercial sulphided Ni/Mo γ-Al2O3 catalyst at 400°C, increasing the hydrogen pressure from 2.5 to 10 MPa decreased the oxygen content of the oil by over 20% to 10% w/w. The H/C ratios were higher and O/C ratios smaller for the two-stage bio-oils compared to their single stage counterparts. However, the differences in the O/C ratios between the single and two-stage bio-oils increase with pressure. 相似文献
989.
J. C. Balanyá J. Galindo-Zaldívar A. Jabaloy G. Leitchenkov A. Maldonado J. Rodríguez-Fernández O. Vinnikovskaya 《Geo-Marine Letters》1999,18(3):215-224
The structure of the South Powell Ridge (SPR), separating the Late Cenozoic ocean-floored Powell Basin and the Mesozoic Weddell
Sea domain, is revealed by multichannel seismic data. The SPR appears as a basement high, bounded northward by transtensional
faults and by normal and major reverse faults to the south. These margin features seem to be linked to the Powell Basin southern
strike-slip margin and to the Jane Arc paleotrench, respectively. We suggest the ridge evolved from the Antarctic Peninsula
passive margin to become the deformational front of the Scotia/Antarctica Plate boundary, later being welded to the Antarctic
Plate.
Received: 18 August 1997 / Revision received: 4 May 1998 相似文献
990.
Santiago Hernández-León Lutz Postel Javier Arístegui May Gómez María Fernanda Montero Santiago Torres Carlos Almeida Eugen Kühner Ullrich Brenning Eberhard Hagen 《Journal of Oceanography》1999,55(4):471-482
Plankton biomass and indices of metabolism and growth [electron transport system (ETS), glutamate dehydrogenase (GDH) and aspartate transcarbamylase (ATC) activities] were studied over a 2,800 km east-west section of the tropical North Atlantic Ocean (21°N) in <200, 200–500 and >;500 µm size classes. On the large-scale, zooplankton (>;200 µm) enzymatic activities increased westward in the study section, where large cyanobacteria chains (Trichodesmium spp.) were observed. Parallel to it, an increase in medium calanoids (1–2 mm length) was observed towards the western part of the transect, whereas small calanoids (<1 mm) were dominant throughout the boundary area of the subtropical gyre. Microplankton ETS and mesoplankton ETS and ATC activities seemed to match the wave length of low frequency waves. Our results suggest that such waves are related to the observed enhancement of metabolic activity of micro- and mesoplankton. The large-scale and mesoscale variability observed give evidence of the inadequacy of assuming a steady-state picture of the euphotic zone of tropical and subtropical waters. 相似文献