On the asymmetry of moustaches

A careful photometry of moustaches is carried out to reveal asymmetry of their emission in the far wings ( 1 Å), if it exists. The presence of background continuous emission in spectra of moustaches demands special care with the photometry, and makes the current method of comparison of rest-intensities inadequate. The other sources of errors are also discussed. The blue asymmetry, as a systematic difference of intensities between the blue and the red wing shows itself in some cases, being 2–3 times larger than the probable errors. In most moustaches investigated, the accompanying background continuous emission shows a rapid increase to the violet.     

Strain-related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland

Abstract TEM and XRD techniques were used to study crystal growth characteristics of the fabric-forming phyllosilicates which developed in response to low-grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding-parallel. TEM-measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD-measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of <25° C km^-1. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain-induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low-grade metamorphism is closely associated with tectonic fabric development.     

Infrared imagery of a turbulent intrusion in a stratified environment

A time sequence of airborne infrared imagery provides a unique view of phenomena associated with a turbulent tidal intrusion into a stratified bay. During flood tide, cooler water from the Strait of Juan de Fuca is observed to penetrate Sequim Bay (Washington, U.S.V.) as a turbulent jet. After separating from the shoreline, the jet collapses into the stratified middle part of the bay, forming a mushroom-shaped head consisting of a semicircular plunge front and areas of recirculating flow. As the plunge front advances into the estuary, a set of nonlinear internal waves emerges and propagates toward the relatively stagnant southern part of the bay, where they are a potential source of vertical mixing. This range of phenomena is expected based on laboratory studies, but has not been seen previously in a natural setting.     

The Apoteri volcanic formation — tholeiitic flows in the North Savannas Graben of Guyana and Brazil

Amygdaloidal tholeiitic flows, locally pillowed, have been mapped at intervals over a distance of 280 km along the margins of the North Savannas Rift Valley in southern Guyana and Brazil. The combined chemical and petrological evidence suggests that the formation represents an originally uniform continental tholeiitic sequence that has undergone variable secondary alteration involving increasing oxidation and ingress of soda, potash and water. The lavas are both spatially and temporally related to the rift faulting; field observations and K∶Ar age determinations on eleven samples from different localities indicate that the flows were erupted during the Early to Middle Jurassic (180-150 m. y.). It is shown that this tectono-magmatic event is synchronous with the initiation of the southern North Atlantic rift and the pan-Antarctic rift which resulted in fragmentation of the continents according to the scheme ofDietz &Holden (1970).     

Stable carbon isotopes (δ<Superscript>13</Superscript>C) of total organic carbon and long-chain <Emphasis Type="Italic">n</Emphasis>-alkanes as proxies for climate and environmental change in a sediment core from Lake Petén-Itzá, Guatemala

Sediment core PI-6 from Lake Petén Itzá, Guatemala, possesses an ~85-ka record of climate and environmental change from lowland Central America. Variations in sediment lithology suggest large and abrupt changes in precipitation during the last glacial and deglacial periods, and into the early Holocene. We measured stable carbon isotope ratios of total organic carbon and long-chain n-alkanes from the core, the latter representing a largely allochthonous (terrestrial) source of organic matter, to reveal past shifts in the relative proportion of C₃–C₄ terrestrial biomass. We sought to test whether stable carbon isotope results were consistent with other paleoclimate proxies measured in the PI-6 core, and if extraction and isotope analysis of n-alkanes is warranted. The largest δ¹³C variations are associated with Heinrich Events. Carbon isotope values in sediments deposited during the last glacial maximum indicate moderate precipitation with little fluctuation. The deglacial was a period of pronounced climate variability, e.g. a relatively warm and moist Bølling–Allerød, but a cool and dry Younger Dryas. Arid periods of the deglacial were inferred from samples with high δ¹³C values in total organic carbon, which reflect times of greater proportions of C₄ plants. These inferences are supported by stable isotope measurements on ostracod shells and relative abundance of grass pollen from the same depths in core PI-6. Similar trends in carbon stable isotopes measured on bulk organic carbon and n-alkanes suggest that carbon isotope measures on bulk organic carbon in sediments from this lake are sufficient to infer past climate-driven shifts in local vegetation.     

Digital modeling studies for evaluation of hydrogeological characteristics in Shadnagar granite basin,Andhra Pradesh,India

The use of digital models has increased significantly in recent years with the accessibility of fast computing machines. A variable dimensioned digital model was constructed for the Shadnagar granite basin using SPECTRUM-7 micro computer to integrate various hydrogeological characteristics and for their quantitative evaluation. The basin has an areal extent of 437 sq km and is demarcated with clear water divides in all directions. Transmissivity, recharge and discharge at each cell of the basin area were estimated by trial and error simulation of the hydrogeological phenomenon under steady state condition. Dynamic simulation at representative nodes facilitated the estimation of storage coefficient. The capability of the constructed model was established by the conformance of the simulated hydrographs with the actual behaviour of the ground water system. The entire studies ammended the earlier arrived estimates of various input/output hydrogeological parameters and evolved a methodology for efficient processing of aquifer simulation data     

Thermal structure of a fossil subduction wedge in the Western Alps

New peak metamorphic temperatures are obtained by Raman spectroscopy of Carbonaceous Material to document the thermal structure of the central Western Alps with high sampling resolution. We show that peak metamorphic T gradually increases eastward from <330 to 350 °C (ultra-Dauphinois to subbriançonnais units), ∼350 to more than 400 °C (Briançonnais domain including the Zone Houillère where metamorphic index minerals are rare) and from 350 to more than 500 °C (Liguro–Piemontese domain). Combined with other constraints on the metamorphic evolution, this dataset reveals a good preservation of the overall thermal structure of the fossil subduction wedge, with no particular thermal overprint during collision. However, local confrontation with P – T estimates and radiometric ages reveals more subtle variations within tectonic units and across the main contacts that are linked to the past activity of the major thrusts and extensional shear zones during subduction and exhumation.