Cosmogenic 3He exposure dating of the Quaternary basalts from Fogo,Cape Verdes: Implications for rift zone and magmatic reorganisation     

Jurgen P.T. Foeken  Simon Day  Finlay M. Stuart 《Quaternary Geochronology》2009,4(1):37-49

We have used cosmogenic ³He to date pre- and post-collapse lava flows from southwestern Fogo, Cape Verdes, in order to date rift zone magmatic reorganisation following the lateral collapse of the flank of the Monte Amarelo volcano. The post-collapse flows have exposure ages ranging from 62 to 11 ka. The analysis of multiple flow tops on each lava flows, often at different elevations, provides an internal check for age consistency and the exposures ages conform with stratigraphic level. The exposure ages suggest that volcanic activity along the western branch of the triple-armed rift zone was more or less continuous from before 62 ka to approximately 11 ka. The absence of magmatic activity for the last 11 kyr reflects a structural reconfiguration of the volcano and may be related to renewed flank instability. This volcanic hiatus is similar in duration to that observed in the Canary Islands. Replicate ³He exposure ages of a pre-collapse flow (123.0 ± 5.2 ka) brackets the time of the Monte Amarelo collapse between 62 ka and 123 ka. Reproducible cosmogenic ³He exposure ages of less than 123 ka from flows away from major erosion features demonstrates that the technique is a viable alternative to the radiocarbon, K/Ar and ⁴⁰Ar/³⁹Ar chronometers for dating recent volcanism in arid climate zones.  相似文献   

Large-eddy simulation of katabatic winds. Part 2: Sensitivity study and comparison with analytical models     

Simon L. Axelsen  Han van Dop 《Acta Geophysica》2009,57(4):837-856

Steady-state quasi-one-dimensional large-eddy simulations of slope winds over simple terrain are presented. The model results of up-slope flow are compared to previous simulations by Schumann (1990), and good agreement is found. Modelled downslope winds are compared to meteorological observations from two glaciers. The vertical profiles of velocity and buoyancy agree with the observations, whereas larger discrepancies are found between the modelled and the observed momentum and buoyancy flux profiles. Despite some discrepancies, the model captures the main characteristics of the observed downslope winds fairly well. The numerical model is used in a companion paper (Part II) to study how some external input parameters affect katabatic winds.  相似文献   

Response to Wilson, P. (2008): “Comment on Carr, S., and Coleman, C. (2007): An improved technique for the reconstruction of former glacier mass-balance and dynamics: Geomorphology 92, 76–90,” Geomorphology 99, 443–444     

Simon Carr  Chris Coleman 《Geomorphology》2009,106(3-4):383-384

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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