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31.
Abstract Slab detachment is a geophysical instability whose manifestation can be revealed by seismic tomography. Evidence of this phenomenon is in the Dinarides/Hellenic and the New Hebrides subduction zones. Subducted slabs in these regions are torn horizontally at depths ranging from 100 to 300 km. We constructed a viscoelastic three-dimensional finite element model and investigated the state of stress. We found that an area with high stress concentration of the order of several hundred MPa is formed near the tip of the tear inside the slab, which can cause lateral migration of the tear. Favorable conditions for slab detachment are characterized by large interplate frictional force at a subduction zone and small slab resistance force deeper down. Stress concentration increases with the down-dip tension inside the slab. The phenomenon of slab weakening has also been studied from a thermal-mechanical standpoint, using a two-dimensional convection model with non-Newtonian, temperature-dependent rheology. The stress-dependent rheology plays an important role in causing local weakening of the descending slab. In strongly time-dependent situations the fast descending slab is not strong everywhere but has a weak region in the middle, making it vulnerable to slab detachment. The presence of viscous heating will enhance slab detachment tendency by further weakening the interior by the frictional heating. Besides these effects, there are other mechanisms which can also weaken the slab interior and help to make slabs more pliable and susceptible to detachment. 相似文献
32.
Alkaline rocks were intruded into the Precambrian basement in southern West Greenland during at least five separate episodes. The Tupertalik carbonatite intrusion was emplaced around or before 2650 m.y.; it is now metamorphosed in granulite facies but is recognisable by its trace element content. Lamprophyre dykes were intruded at ca. 1800 m.y. and again at ca. 1200 m.y. At ca. 600 m.y. the Sarfartôq carbonatite intrusion and extensive kimberlite dykes were emplaced in the Holsteiborg-Strømfjord region. At 225-115 m.y. the Qaqarssuk carbonatite complex was emplaced in the Sukkertoppen region, and numerous lamprophyre and kimberlite dykes were emplaced between Fishenasset and Ivigtut. All these episodes are correlatable with contemporaneous alkaline activity in Canada and Scandinavia and substantiate the periodicity of carbonatite emplacement. The Mesozoic suite of intrusions is related to rifting prior to continental break-up and the formation of the North Atlantic Ocean. The Eocambrian suite of 600 m.y. is thought to have formed under similar conditions during continental break-up and the formation of the Iapetus ocean. 相似文献
33.
Poul Klenz Larsen 《Environmental Geology》2007,52(2):375-383
This paper presents an investigation of the decay of bricks in the chancel vault of Brarup Church located close to the Baltic
Sea at the island of Falster in Denmark. The aim of the work was to study a peculiar phenomenon in order to prescribe appropriate
treatment. Although protected by a lime plaster, some bricks were pulverized up to 50 mm deep from the topside. The decay
occurred in a random pattern over the structure, with undamaged bricks positioned next to deteriorated bricks. The brick structure
was investigated by Mercury Intrusion Porosiometry. All bricks had a majority of pores in the range 1–10 μm, but the decayed
bricks also had a fraction of pores with a diameter less than 100 nm. The difference may be due to a lower firing temperature,
or it may be a consequence of the salt decay. Salt analysis proved that the deteriorated bricks were contaminated with sodium
chloride, which could have degraded the brick structure. However, the powdering of the bricks may be caused by the precipitation
of gypsum, which was found in the brick powder but not in the deteriorated brick itself. According to calculations with the
computer program RUNSALT, the precipitation of gypsum is much influenced by the presence of sodium chloride. The gypsum precipitates
due to changes in temperature between 0 and 30°C and variations in relative humidity between 75 and 95%. Seasonal climatic
changes in the attic were measured to cover this interval. Perhaps sodium chloride acted as a catalyst that facilitated the
accumulation and precipitation of gypsum at the topside of the bricks. A sacrificial plaster, which is the traditional treatment
for salt contaminated structures, would not prevent such decay. 相似文献
34.
The retrieval of cloud microphysical properties using satellite measurements and an in situ database
Christophe Poix Guy Febvre Anne Fouilloux Howard Larsen Jean-Francois Gayet 《Annales Geophysicae》1996,14(1):98-106
By combining AVHRR data from the NOAA satellites with information from a database of in situ measurements, large-scale maps can be generated of the microphysical parameters most immediately significant for the modelling of global circulation and climate. From the satellite data, the clouds can be classified into cumuliform, stratiform and cirrus classes and then into further sub-classes by cloud top temperature. At the same time a database of in situ measurements made by research aircraft is classified into the same sub-classes and a statistical analysis is used to derive relationships between the sub-classes and the cloud microphysical properties. These two analyses are then linked to give estimates of the microphysical properties of the satellite observed clouds. Examples are given of the application of this technique to derive maps of the probability of occurrence of precipitating clouds and of precipitating water content derived from a case study within the International Cirrus Experiment (ICE) held in 1989 over the North Sea. 相似文献
35.
Arthur Rodgers Hrvoje Tkalcic David McCallen Shawn Larsen Catherine Snelson 《Pure and Applied Geophysics》2006,163(1):55-80
We report site response in Las Vegas Valley (LVV) from historical recordings of Nevada Test Site (NTS) nuclear explosions
and earthquake recordings from permanent and temporary seismic stations. Our data set significantly improves the spatial coverage
of LVV over previous studies, especially in the northern, deeper parts of the basin. Site response at stations in LVV was
measured for frequencies in the range 0.2–5.0 Hz using Standard Spectral Ratios (SSR) and Horizontal-Vertical Spectral Ratios
(HVR). For the SSR measurements we used a reference site (approximately NEHRP B ``rock' classification) located on Frenchman
Mountain outside the basin. Site response at sedimentary sites is variable in LVV with average amplifications approaching
a factor of 10 at some frequencies. We observed peaks in the site response curves at frequencies clustered near 0.6, 1.2 and
2.0 Hz, with some sites showing additional lower amplitude peaks at higher frequencies. The spatial pattern of site response
is strongly correlated with the reported depth to basement for frequencies between 0.2 and 3.0 Hz, although the frequency
of peak amplification does not show a similar correlation. For a few sites where we have geotechnical shear velocities, the
amplification shows a correlation with the average upper 30-meter shear velocities, V30. We performed two-dimensional finite difference simulations and reproduced the observed peak site amplifications at 0.6 and
1.2 Hz with a low velocity near-surface layer with shear velocities 600–750 m/s and a thickness of 100–200 m. These modeling
results indicate that the amplitude and frequencies of site response peaks in LVV are strongly controlled by shallow velocity
structure. 相似文献
36.
Evolution of metamorphic volatiles during exhumation of microdiamond-bearing granulites in the Western Gneiss Region, Norway 总被引:1,自引:0,他引:1
Rune B. Larsen Elizabeth A. Eide Ernst A. J. Burke 《Contributions to Mineralogy and Petrology》1998,133(1-2):106-121
Fluid inclusions in garnet, kyanite and quartz from microdiamond-bearing granulites in the Western Gneiss Region, Norway,
document a conspicuous fluid evolution as the rocks were exhumed following Caledonian high- and ultrahigh-pressure (HP–UHP) metamorphism. The most important of the various fluid mixtures and daughter minerals in these rocks are: (N2 + CO2 + magnesian calcite), (N2 + CO2 + CH4 + graphite + magnesian calcite), (N2 + CH4), (N2 + CH4 + H2O), (CO2) and (H2O + NaCl + CaCl2 + nahcolite). Rutile also occurs in the N2 + CO2 inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions. Volatiles composed of N2 + CO2 + magnesian calcite characterise the ambient metamorphic environment between HP–UHP (peak) and early retrograde metamorphism. During progressive decompression, the mole fraction of N2 increased in the fluid mixtures; as amphibolite-facies conditions were reached, CH4 and later, H2O, appeared in the fluids, concomitant with the disappearance of CO2 and magnesian calcite. Graphite is ubiquitous in the host lithologies and fluid inclusions. Thermodynamic modelling of the
metamorphic volatiles in a graphite-buffered C-O-H system demonstrates that the observed metamorphic volatile evolution was
attainable only if the f
O2 increased from c. −3.5 (±0.3) to −0.8 (±0.3) log units relative to the FMQ oxygen buffer. External introduction of oxidising
aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the f
O2. The oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental
sediments previously subducted during an extended period of continental collision in conjunction with the Caledonian orogeny.
Received: 15 December 1997 / Accepted: 25 May 1998 相似文献
37.
Tectonics of sedimentary basin formation: models and constraints 总被引:3,自引:0,他引:3
S. Cloetingh L. O. Boldreel B. T. Larsen M. Heinesen L. Mortensen 《Tectonophysics》1998,300(1-4):1-11
38.
Jrgen Gutzon Larsen 《Lithos》1981,14(4):241-262
Megacrysts and polymineralic fragments of extraordinary diversity from a Tertiary monchiquitic dyke of Ubekendt Ejland comprise three groups: (1) Cr-diopside-fassaitic diopside + olivine, Fo90.5?81.5 + CrAl spinels. (II) Fassaitic salite-ferrisalite + KTi-pargasite-ferropargasite + apatite + AlTi-magnetite, (III) Scapolite + hyalophane + potassium feldspar + nepheline + analcime. By comparison with mineralogy and phase relations in the host rock and experimental data from alkaline rocks the megacrysts are related to a sequence of crystallization from primitive monchiquitic to potassic phonolitic magmas rich in H2O and CO2 at 5–11 kb. Group I megacrysts formed at temperatures of 1300-1150°C and group II between ? bar at the latter temperature. High Pco2 may have stabilized the scapolite in the more evolved liquid and K-feldspar and nepheline began to crystallize at ca. 800°C possibly together with the ferrisalite. 相似文献
39.
40.
The Sarfartôq carbonatite complex was emplaced in lower Palaeozoic time in a weakness zone within the Precambrian shield. Dolomitic magma intruded in two major stages of activity. In the first stage a steeply dipping conical body of concentric sheets of rauhaugite was formed, while in the second stage several batches of magma were emplaced into the surrounding marginal shock-zone as concentric and radial beforsite dykes and agglomerates. Hydrothermal activity gave rise to several phases of mineralisation in veins and shear zones. The accompanying fenitisation was of the Na-type. The whole complex covers about 90 km2. The main rock-forming minerals are dolomite-ankerite, apatite, orange reversely pleochroic phlogopite, richterite-arfvedsonite and magnetite. Important accessories are pyrochlore, zircon and niobian rutile. A complete mineral list is given, together with microprobe data on mineral chemistry. The dolomitic magmas were poor in SiO2, Al2O3 and K2O in relation to other carbonatites. Nb, U and LREEs are strongly enriched in pyrochloremineralised zones where the Nb content may be up to 40%. Some shear zones are strongly enriched in Th and HREEs (specifically Eu) and lesser Pb and Zn. Niobium, uranium, rare earth elements and phosphorus occur in economically interesting concentrations. 相似文献