Comparison of cell- and vertex-centered discretization methods for flow in a two-dimensional discrete-fracture–matrix system

Simulations of flow for a discrete-fracture model in fractured porous rocks have gradually become more practical, as a consequence of increased computer power and improved simulation and characterization techniques. Discrete-fracture models can be formulated in a lower-dimensional framework, where the fractures are modeled in a lower dimension than the matrix, or in an equi-dimensional form, where the fractures and the matrix have the same dimension.     

Stacking of records of nuclear explosions registered by the world network of seismic stations

The methods and results of stacking of seismograms of nuclear explosions registered by the world network of seismic stations from 1980 until 1988 are presented. Stacking of seismograms obtained at one seismic station from the sources located in the same place allows us substantially to increase the useful signal/noise ratio. This, in turn, allows more confident correlation of various seismic waves coming to the seismic stations after the first arrivals, particularly, converted and reflected waves from different boundaries inside the Earth.     

Unusual spatial-temporal dynamics of geomagnetic disturbances during the main phase of the extremely strong magnetic storm of November 7–8, 2004

The spatial dynamics of geomagnetic variations and pulsations, auroras, and riometer absorption during the development of the main phase of the extremely strong magnetic storm of November 7–8, 2004, has been studied. It has been indicated that intense disturbances were observed in the early morning sector of auroral latitudes rather than in the nighttime sector, as usually takes place during magnetic storms. The unusual spatial dynamics was revealed at the beginning of the storm main phase. A rapid poleward expansion of disturbances from geomagnetic latitudes of 65°–66° to 74°–75° and the development of the so-called polar cap substorm with a negative bay amplitude of up to 2500 nT, accompanied by precipitation of energetic electrons (riometer absorption) and generation of Pi2–Pi3 pulsations, were observed when IMF B _z was about ?45 nT. The geomagnetic activity maximum subsequently sharply shifted equatorward to 60°–61°. The spatial dynamics of the westward electrojet, Pi2–Pi3 geomagnetic pulsations, and riometer absorption was similar, which can indicate that the source of these phenomena is common.     

Old, Low-Mass, Metal-Rich (SMR) Stars

The authors of this paper try to disentangle the many problems arisen from a new enlarged sample of nearby low-mass,metal-rich-stars. These stars have reliable absolute magnitudes, deduced from Hipparcos parallaxes, precise bolometric corrections, effective temperatures and metal abundances from high resolution detailed spectroscopic analyses. Their ages have been derived from a grid of isochrones calculated with up to date physics. The main goal of this paper is to determine the ages of the slightly evolved SMR stars. Among those with well determinated ages about 80% of them have intermediate ages of (2 to 5 Gyr), but only 20% have ages of 8 Gyr or more. Nevertheless, the existence of very old metal-rich stars is confirmed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The concentration of molecular H2 and CH4 in the stratosphere

Stratospheric profiles of CH₄ and H₂ over eastern Texas have been derived up to 31 km altitude from air samples collected aboard a balloon and analyzed by gas chromatography. For H₂, contamination during flight and measurement presented a problem which has been resolved only recently. The earlier profiles require corrections which are rather large for the highest altitudes.The three profiles indicate an increase of the H₂ concentration in the lower stratosphere from about 0.5 p.p.m. per volume at the tropopause to about 0.8 p.p.m. at around 27 km altitude. Above that altitude the H₂ concentration decreases again. An air sample collected between 44 and 62 km by a rocket-borne cryogenic sampler had an H₂ concentration of 0.4 p.p.m.The five CH₄ profiles showed a decrease in CH₄ concentration with altitude generally with a steeper gradient directly above the tropopause and a weaker gradient at higher altitudes reaching 0.9 p.p.m. at 30 km altitude. The CH₄ concentration in the rocket sample was 0.25 p.p.m., in good agreement with the gradient obtained from the balloon samples.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Proterozoic–Paleozoic development of the basement of the Central Andes (18–26°S) — a mobile belt of the South American craton

The Late Precambrian–Early Paleozoic metamorphic basement forms a volumetrically important part of the Andean crust. We investigated its evolution in order to subdivide the area between 18 and 26°S into crustal domains by means of petrological and age data (Sm–Nd isochrons, K–Ar). The metamorphic crystallization ages and t_DM ages are not consistent with growth of the Pacific margin north of the Argentine Precordillera by accretion of exotic terranes, but favor a model of a mobile belt of the Pampean Cycle. Peak metamorphic conditions in all scattered outcrop areas between 18 and 26°S are similar and reached the upper amphibolite facies conditions indicated by mineral paragensis and the occurrence of migmatite. Sm–Nd mineral isochrons yielded 525±10, 505±6 and 509±1 Ma for the Chilean Coast Range, the Chilean Precordillera and the Argentine Puna, and 442±9 and 412±18 Ma for the Sierras Pampeanas. Conventional K–Ar cooling age data of amphibole and mica cluster around 400 Ma, but are frequently reset by Late Paleozoic and Jurassic magmatism. Final exhumation of the Early Paleozoic orogen is confirmed by Devonian erosional unconformities. Sm–Nd depleted mantle model ages of felsic rocks from the metamorphic basement range from 1.4 to 2.2 Ga, in northern Chile the average is 1.65±0.16 Ga (1σ; n=12), average t_DM of both gneiss and metabasite in NW Argentina is 1.76±0.4 Ga (1σ; n=22), and the isotopic composition excludes major addition of juvenile mantle derived material during the Early Paleozoic metamorphic and magmatic cycle. These new data indicate a largely similar development of the metamorphic basement south of the Arequipa Massif at 18°S and north of the Argentine Precordillera at 28°S. Variations of metamorphic grade and of ages of peak metamorphism are of local importance. The protolith was derived from Early to Middle Proterozoic cratonic areas, similar to the Proterozoic rocks from the Arequipa Massif, which had undergone Grenvillian metamorphism at ca. 1.0 Ga.     

An 40Ar/39Ar study of a polymetamorphic complex in the Arunta Block,Central Australia

A polymetamorphic complex in the Arunta Block, Central Australia, which has undergone metasomatism involving introduction of Rb and loss of Sr has been investigated by ⁴⁰Ar/ ³⁹Ar step and total-degassing techniques in an attempt to date five metamorphic events (M1–M5) identified microstructurally. The complex isotopic data and apparent age spectra obtained have been interpreted in the light of constraints provided by an associated tectonometamorphic study, together with independent geochronological controls from the immediate study area and elsewhere in the Arunta Block. This has led to the delineation of ages of 1,780–1,820 M.y. for M1, 1,690–1,720 M.y. for M2, 1,450–1,500 M.y. for M3, 1,030–1,060 M.y. for M4 and 370–510 for M5. The polyphase M5 event is correlated with the Alice Springs Orogeny, previously accorded younger age limits of 330–370 M.y. The latter are interpreted to reflect closure of mica K/Ar systems as uplift continued subsequent to the close of the event, possibly culminating in minor high-level brittle deformation and the development of pseudotachylytes in the study area.Apparent age spectra recorded in this study do not all conform with generally accepted patterns of behaviour of mineral systems which have undergone polythermal histories. Two hornblendes have survived subsequent high temperature metasomatic reheating without loss of argon or resetting of their ages. Their resistance to argon loss may be attributable to low water contents, due to their growth under anhydrous granulite facies conditions, and may reflect a relationship between argon loss from hornblende and the water content of hornblende, which decreases with increasing metamorphic grade (Kostyuk and Sobolev 1969). Other samples display false plateaux in their apparent age spectra and in three cases the spectra are indistinguishable from those of undisturbed spectra.The age data recorded in this study suggest that during basement reactivation loss of argon is mainly confined to the vicinity of shear zones, implying that diffusion of argon from minerals in the deep crust may be dependent more on deformation than on reheating. However, thermal activity may also be associated with active shear zones, through the medium of superheated fluids streaming up the shear zones from below. The wide spread of K/Ar and ⁴⁰Ar/³⁹Ar ages (330–510 M.y.), recorded for the Alice Springs Orogeny in the Arunta basement, may reflect such controls.