The effect of mantle composition on density in the extending lithosphere   总被引：1，自引：0，他引：1  

Nina S.C. Simon  Yuri Yu. Podladchikov 《Earth and Planetary Science Letters》2008,272(1-2):148-157

The density distribution of the lithosphere is non-linear and discontinuous due to complex mineralogy and, most importantly, phase transitions. We evaluate the influence of changes in mantle composition on lithospheric density and its evolution during horizontal stretching, using thermodynamic calculations of the density as a function of pressure, temperature and composition. We also develop a simple parameterization based on end-member mineral reactions and geometric relationships between the geotherm and the phase boundary for comparison. The garnet–spinel peridotite transition leads to a moderate decrease in density of the mantle part of the lithospheric column at the initial stages of stretching. When the crust is sufficiently thinned and temperature is relatively high, plagioclase peridotite becomes stable in the upper part of the mantle. The density reduction due to the plagioclase-in reaction is controlled by bulk Al₂O₃ in the mantle and by the depth of the plagioclase-in reaction, which is mainly governed by the Na₂O/Al₂O₃ ratio. Since Na₂O and Al₂O₃ increase with the fertility of the mantle the phase transition effect is most pronounced for relatively fertile mantle (and strong extension) and can lead to 2.3% density reduction. This is equivalent to heating the entire lithosphere by 700 °C if only the effect of thermal expansion on density is taken into account. The formation of plagioclase peridotite can explain syn-rift uplift in sedimentary basins that experienced large mantle stretching without invoking an unrealistically strong increase in temperature. It might also be responsible for the break-up unconformity observed at continental margins.  相似文献   

A model of inter-annual variability in beach levels     

Simon P. Neill  Alan J. Elliott  Mohammad R. Hashemi 《Continental Shelf Research》2008

Beach profile data, collected twice per year at 19 stations over a 25 km length of coastline in Tremadoc Bay, have been analysed to quantify the inter-annual variability in beach levels over a 7 year period and the results compared against the output of a numerical model. Using hourly wind data as forcing, the morphological development of northern Tremadoc Bay was simulated by wave, tidal, longshore transport, total transport and bed level change models. The modelling methodology was efficient and innovative, allowing realistic simulations of long duration with a time step of 1 h, hence capturing the high frequency nature of wind events. The model was run for each of the 7 autumn/winter periods (generally November–April) and the modelled net change in beach levels compared with the data from all 19 stations. The model results had reasonable agreement with the beach profile surveys. However, the observed magnitude of bed level change in the bay lagged the model output by 1 year, indicating that sediment processes acting over a larger area are important in a relatively localised study of inter-annual variability.  相似文献   

Thermal–mechanical modeling of cooling history and fracture development in inflationary basalt lava flows     

Simon A. Kattenhorn  Conrad J. Schaefer 《Journal of Volcanology and Geothermal Research》2008,170(3-4):181-197

Thermal–mechanical analyses of isotherms in low-volume basalt flows having a range of aspect ratios agree with inferred isotherm patterns deduced from cooling fracture patterns in field examples on the eastern Snake River Plain, Idaho, and highlight the caveats of analytical models of sheet flow cooling when considering low-volume flows. Our field observations show that low-volume lava flows have low aspect ratios (width divided by thickness), typically < 5. Four fracture types typically develop: column-bounding, column-normal, entablature (all of which are cooling fractures), and inflation fractures. Cooling fractures provide a proxy for isotherms during cooling and produce patterns that are strongly influenced by flow aspect ratio. Inflation fractures are induced by lava pressure-driven inflationary events and introduce a thermal perturbation to the flow interior that is clearly evidenced by fracture patterns around them. Inflation fracture growth occurs incrementally due to blunting of the lower tip within viscoelastic basalt, allowing the inflation fracture to pivot open. The final stage of growth involves propagation beyond the blunted tip towards the stress concentration at the tapered tip of a lava core, resulting in penetration of the core that causes quenching of the lava and the formation of a densely fractured entablature. We present numerical models that include the effects of inflation fractures on lava cooling and which support field-based inferences that inflation fractures depress the isotherms in the vicinity of the fracture, cause a subdivision of the lava core, control the location of the final portion of the lava flow to solidify, and cause significant changes in the local cooling fracture orientations. In addition to perturbing isotherms, inflation fractures cause a lava flow to completely solidify in a shorter amount of time than an identically shaped flow that does not contain an inflation fracture.  相似文献   

Wave Energy from the North Sea: Experiences from the Lysekil Research Site     

Mats Leijon  Cecilia Boström  Oskar Danielsson  Stefan Gustafsson  Kalle Haikonen  Olivia Langhamer  Erland Strömstedt  Magnus Stålberg  Jan Sundberg  Olle Svensson  Simon Tyrberg  Rafael Waters 《Surveys in Geophysics》2008,29(3):221-240

This paper provides a status update on the development of the Swedish wave energy research area located close to Lysekil on the Swedish West coast. The Lysekil project is run by the Centre for Renewable Electric Energy Conversion at Uppsala University. The project was started in 2004 and currently has permission to run until the end of 2013. During this time period 10 grid-connected wave energy converters, 30 buoys for studies on environmental impact, and a surveillance tower for monitoring the interaction between waves and converters will be installed and studied. To date the research area holds one complete wave energy converter connected to a measuring station on shore via a sea cable, a Wave Rider? buoy for wave measurements, 25 buoys for studies on environmental impact, and a surveillance tower. The wave energy converter is based on a linear synchronous generator which is placed on the sea bed and driven by a heaving point absorber at the ocean surface. The converter is directly driven, i.e. it has no gearbox or other mechanical or hydraulic conversion system. This results in a simple and robust mechanical system, but also in a somewhat more complicated electrical system.  相似文献   

Fluid-Rock Interaction in the Granulites of Madagascar and Lithospheric-Scale Transfer of Fluids     

Eric Pili  Simon M.F. Sheppard  Jean-Marc Lardeaux 《Gondwana Research》1999,2(3):341

The nature of synmetamorphic fluids and their flow is examined in the granulitic lower crust of Madagascar, part of a Precambrian crustal-scale network of vertical ductile shear zones. Based on three independent data sets - field and satellite mapping, C-, O- and H-isotope geochemistry and gravimetry - this crust is divided into three zones: outside of shear zones, minor shear zones (<140 km long and 7 km wide), and major shear zones (>350 km long and 20–35 km wide). The major shear zones are rooted in and are controlled by the mantle. They tapped mantle-derived CO₂ with carbon fluxes of the same order of magnitude as oceanic ridge degassing. One major shear zone shows abundant phlogopite-diopside-apatite-calcite mineralizations (a well known paragenesis in mantle metasomatism) due to mantle-fluid infiltration and their interaction with the crust. Carbonatitic magmas possibly collected in the major shear zones at the base of the crust and may be the source for CO₂ upwellings as well as other metasomatic agents. Small-scale minor shear zones were controlled by crustal deformation processes and focused crustally-derived H₂O-rich fluids. Pervasive fluid circulation was restricted to the vicinity (< 100 m) of synmetamorphic plutons. Fluid absent conditions dominate everywhere else. Mantle-CO₂ flushing is not required for granulite genesis but is a consequence of the high associated heat flux. Fluid transfer at the mantle/crust interface is controlled by the tectonic setting and the associated geothermal gradient. The C- and O- isotope systematics of metamorphosed carbonates sampled on a regional scale within a known petrological and structural framework are shown to be of great help to identify the distribution of major fluid-rock interaction processes associated with plate tectonics.  相似文献   

Response to A. Henderson-Sellers and K. McGuffie     

Simon Shackley  Peter Young  Stuart Parkinson 《Climatic change》1999,42(3):611-617

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3-D Prestack-migration of Wide-angle Data from a Variscan Transition Zone     

F. Bleibinhaus  M. Bopp  M. Simon  H. Gebrande 《Pure and Applied Geophysics》1999,156(1-2):173-186

—In addition to the near normal-incidence observations within the German DEKORP 2 project in 1984, wide-angle observations have been carried out on a parallel profile across the boundary between the Saxothuringian and Moldanubian crust, approximately 50 km NE of the main transect to control three-dimensional variations. Explosion sources have been used for the entire survey, providing excellent conditions for wide-angle registrations. A velocity model has been derived on the basis of in- and off-line refraction measurements using a kinematic raytracer which was extended to three dimensions by interpolation of 2-D velocity fields between parallel sections. Although prestack-migration of the data led to aliasing effects due to large shot and geophone spacing, stable results were obtained by forming envelopes after single-shot migration. The migrated sections reveal a strongly reflective Moho at about 31 km depth and a steeply (50°) dipping intracrustal reflector, which seems to be related to the border between the two Variscan units.  相似文献   

Escape from a Crisis in Fokker-Planck Models     

Simon F. Portegies Zwart  Koji Takahashi 《Celestial Mechanics and Dynamical Astronomy》1999,73(1-4):179-186

Recent N-body simulations have shown that there is a serious discrepancy between the results of N-body simulations and the results of Fokker-Planck simulations for the evolution of globular and rich open clusters under the influence of the galactic tidal field. In some cases, the lifetime obtained from Fokker-Planck calculations is more than an order of magnitude smaller than those from N-body simulations. In this paper we show that the principal cause for this discrepancy is an over-simplified treatment of the tidal field used in previous Fokker-Planck simulations. We performed new Fokker-Planck calculations using a more appropriate implementation for the boundary condition of the tidal field. The implementation is only possible with anisotropic Fokker-Planck models, while all previous Fokker-Planck calculations rely on the assumption of isotropy. Our new Fokker-Planck results agree well with N-body results. Comparison of the two types of simulations gives a better understanding of the evolution of such clusters. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Diffusion metasomatism in silica-undersaturated sapphirine-bearing granulite from Rumdoodle Peak, Framnes Mountains, east Antarctica     

Daniel J. Dunkley  Geoffrey L. Clarke  Simon L. Harley 《Contributions to Mineralogy and Petrology》1999,134(2-3):264-276

At Rumdoodle Peak, near Mawson Base, east Antarctica, an enclave of metasedimentary granulite is enclosed in the Mawson Charnockite, an extensive c. 960 Ma intrusion. The enclave contains a disrupted layer of black spinel-orthopyroxene-phlogopite gneiss, which was truncated by a quartz-rich vein. A reaction band that developed between these units is composed of a sequence of mineral zones that contain spinel, orthopyroxene, sapphirine, cordierite and plagioclase. The sequence of mineral zones approximately matches that predicted by a model of closed system diffusion metasomatism, involving the exchange of Si for Fe and Mg. The reaction bands differ from the model in the presence of micro-scale disequilibrium textures, that include “double coronas” composed of cordierite surrounding sapphirine and sapphirine surrounding spinel. The growth of the reaction band was controlled by diffusion along intergranular pathways, with local equilibrium maintained adjacent to grain boundaries. The presence of corona textures is a result of slow reaction rates, due to limited diffusive exchange of Si and Al across mineral grains. Received: 15 January 1998 / Accepted: 7 September 1998  相似文献   

"That Very Garden of South America": European Surveyors in Paraguay   总被引：1，自引：1，他引：0  

Simon Naylor 《Singapore journal of tropical geography》2000,21(1):48-62

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