Seismic velocities at elevated pressures of igneous rocks at DSDP Holes 60-454 and 60-458

The Deep Sea Drilling Project, in addition to providing valuable information on the history and processes of development of the ocean, has significantly contributed to our knowledge of the chemical and physical nature of the upper oceanic crust. Among the important physical properties of the crust are its seismic velocity and structure, the interpretation of which requires laboratory studies of seismic velocities in oceanic rocks.

40Ar-39Ar dating of rocks drilled at sites 60-458 and 60-459

40Ar-39Ar dating of a high-MgO bronzite andesite from near the top of basement drilled at Site 458 shows the characteristic symptoms of artificially disturbed samples - i.e., an inverse staircase-type age spectrum, approximate linearity on an isochron plot, and concordance between total fusion age and isochron age. From conclusions based on other artificially disturbed samples (Ozima et al., 1979), we suggest that the reference isochron age (33.6 Ma) approximates the age of the sample. A basalt from deeper in Hole 458 gives an isochron age of 19.1 ± 0.2 Ma, which is slightly younger than the plateau age of 21.4 ± 1.0 Ma. Both ages are, however, considerably younger than the age of fossils in the overlying sediments (30 - 34 Ma). The age discrepancy may be explained if the 40Ar-39Ar age represents the age of secondary minerals, which formed later. No useful age data were obtained from a basalt sample recovered from Hole 459B.

Minerals, geochemistry and x-ray power characteristics at DSDP Sites 54-424, 60-458 and 60-459

Secondary minerals filling veins and vesicles in volcanic basement at Deep Sea Drilling Project Sites 458 and 459 indicate that there were two stages of alteration at each site: an early oxidative, probably hydrothermal, stage and a later, low-temperature, less oxidative stage, probably contemporaneous with faulting in the tectonically active Mariana forearc region. The initial stage is most evident in Hole 459B, where low-Al, high Fe smectites and iron hydroxides formed in vesicles in pillow basalts and low-Al palygorskite formed in fractures. Iron hydroxides and celadonite formed in massive basalts next to quartz-oligoclase micrographic intergrowths. Palygorskite was found in only one sample near the top of basement in Hole 458, but it too is associated with iron hydroxides. Palygorskite has previously been reported only in marine sediments in DSDP and other occurrences. It evidently formed here as a precipitate from fluids in which Si, Mg, Fe, and even some Al were concentrated. Experimental data suggest that the solutions probably had high pH and somewhat elevated temperatures. The compositions of associated smectites resemble those in hydrothermal sediments and in basalts at the Galapagos mounds geothermal field. The second stage of alteration was large-scale replacement of basalt by dioctahedral, trioctahedral, or mixed-layer clays and phillipsite along zones of intense fracturing, especially near the bottom of Holes 458 and 459B. The basalts are commonly slickensided, and there are recemented microfault offsets in overlying sediments. Native copper occurs in one core of Hole 458, but associated smectites are dominantly dioctahedral, unlike Hole 459B, where they are mainly trioctahedral, indicating nonoxidative alteration. The alteration in both holes is more intense than at most DSDP ocean crust sites and may have been augmented by water derived from subducting ocean crust beneath the fore-arc region.

(Table 1) Summary of acoustic properties at DSDP Holes 60-453 and 60-459B

Laboratory measurements of physical properties are important because the results may be applied to the interpretation of seismic and other types of geophysical data, and because they can be used to estimate the in situ physical properties of different lithologies present beneath the sea floor. In this chapter, wet-bulk densities and compressional-wave velocities, measured at elevated confining pressures, are reported for a suite of seven sediment samples recovered on DSDP Leg 60. Of the seven samples studied, two are mudstones, two are vitric tuffs, and three are chalks. All but one of the samples are from Hole 459B, near the eastern limit of the Mariana fore-arc region. In five cases, velocities were measured parallel and perpendicular to bedding to test for velocity anisotropy.

Petrografia, geoquímica e geocronologia das rochas metavulcânicas e metaplutônicas dos Greenstone Belts faina e Serra de Santa Rita : implicações para o ambiente tectônico

Dissertação (mestrado)—Universidade de Brasília, Instituto de Geociências, Pós-Graduação em Geologia, 2016.

Constraining the active tectonic deformation of Andaman-Nicobar Arc in the background of December 26,2004 Great Sumatra-Andaman Earthquake

The Andaman-Nicobar Islands in the Bay of Bengal lies in a zone where the Indian plate subducts beneath the Burmese microplate, and therefore forms a belt of frequent earthquakes. Few efforts, not withstanding the available historical and instrumental data were not effectively used before the Mw 9.3 Sumatra-Andaman earthquake to draw any inference on the spatial and temporal distribution of large subduction zone earthquakes in this region. An attempt to constrain the active crustal deformation of the Andaman-Nicobar arc in the background of the December 26, 2004 Great Sumatra-Andaman megathrust earthquake is made here, thereby presenting a unique data set representing the pre-seismic convergence and co-seismic displacement.Understanding the mechanisms of the subduction zone earthquakes is both challenging sCientifically and important for assessing the related earthquake hazards. In many subduction zones, thrust earthquakes may have characteristic patterns in space and time. However, the mechanism of mega events still remains largely unresolved.Large subduction zone earthquakes are usually associated with high amplitude co-seismic deformation above the plate boundary megathrust and the elastic relaxation of the fore-arc. These are expressed as vertical changes in land level with the up-dip part of the rupture surface uplifted and the areas above the down-dip edge subsided. One of the most characteristic pattern associated with the inter-seismic era is that the deformation is in an opposite sense that of co-seismic period.This work was started in 2002 to understand the tectonic deformation along the Andaman-Nicobar arc using seismological, geological and geodetic data. The occurrence of the 2004 megathrust earthquake gave a new dimension to this study, by providing an opportunity to examine the co-seismic deformation associated with the greatest earthquake to have occurred since the advent of Global Positioning System (GPS) and broadband seismometry. The major objectives of this study are to assess the pre-seismic stress regimes, to determine the pre-seismic convergence rate, to analyze and interpret the pattern of co-seismic displacement and slip on various segments and to look out for any possible recurrence interval for megathrust event occurrence for Andaman-Nicobar subduction zone. This thesis is arranged in six chapters with further subdivisions dealing all the above aspects.

Surface sediment samples from several fore-arc basins west and southwest of the Indonesian Archipelago, analyzed by planktonic foraminifera, stable oxygen and carbon isotopic signals and opal and CaCO3 contents in bulk sediment

A total of 69 surface sediment samples from several fore-arc basins located west and southwest of the Indonesian Archipelago was analyzed with respect to the faunal composition of planktonic foraminifera, the stable oxygen and carbon isotopic signal of a surface-dwelling (Globigerinoides ruber) and a thermocline-dwelling (Neogloboquadrina dutertrei) species, and the opal and CaCO3 contents in bulk sediment. Our results show that the distribution pattern of opal in surface sediments corresponds well to the upwelling-induced chlorophyll concentration in the upper water column and thus, represents a reliable proxy for marine productivity in the coastal upwelling area off S and SW Indonesia. Present-day oceanography and marine productivity are also reflected in the tropical to subtropical and upwelling assemblages of planktonic foraminifera in the surface sediments, which in part differ from previous studies in this region probably due to different coring methods and dissolution effects. The average stable oxygen isotopic values (d18O) of G. ruber in surface sediments vary between 2.9 per mill and 3.2 per mill from basin to basin and correspond to the oceanographic settings during the SE monsoon (July-October) off west Sumatra, whereas off southern Indonesia, they reflect the NW monsoon (December-March) or annual average conditions. The d18O values of N. dutertrei show a stronger interbasinal variation between 1.6 per mill and 2.2 per mill and correspond to the upper thermocline hydrology in July-October. In addition, the difference between the shell carbon isotopic values (d13C) of G. ruber and N. dutertrei (Delta d13C) appears to be an appropriate productivity recorder only in the non-upwelling areas off west Sumatra. Consequently, joint interpretation of the isotopic values of these species is distinctive for different fore-arc basins W and SW of Indonesia and should be considered in paleoceanographic studies.

Isothermal Section of the V-Si-B System at 1600 A degrees C in the V-VSi(2)-VB Region

In recent years, the Me-Si-B (Me-metal) ternary systems have received considerable attention aiming at the development of high-temperature structural materials. Assuming that any real application of these materials will rely on multicomponent alloys, as is the case of Ni-base superalloys, phase equilibria data of these systems become very important. In this work, results are reported on phase equilibria in the V-Si-B system, and are summarized in the form of an isothermal section at 1600 A degrees C for the V-VSi(2)-VB region. Several alloys of different compositions were prepared via arc melting and then heat-treated at 1600 A degrees C under high vacuum. All the materials in both as-cast and heat-treated conditions were characterized through x-ray diffraction, scanning electron microscopy, and selected alloys via wavelength dispersive spectroscopy. A negligible solubility of B in the V(3)Si, V(5)Si(3) (T(1)), and V(6)Si(5) phases as well as of Si in V(3)B(2) and VB phases was noted. Two ternary phases presenting the structures known as T(2) (Cr(5)B(3)-prototype) and D8(8) (Mn(5)Si(3)-prototype) were observed in both as-cast and heat-treated samples. It is proposed that at 1600 A degrees C the homogeneity range of T(2) extends approximately from 5 at.% to 12 at.% Si at constant vanadium content and the composition of D8(8) phase is close to V(59.5)Si(33)B(7.5) (at.%).

Biomass burning in Brazil`s Amazonian ""arc of deforestation"": Burning efficiency and charcoal formation in a fire after mechanized clearing at Feliz Natal, Mato Grosso

Estimates of greenhouse-gas emissions from deforestation are highly uncertain because of high variability in key parameters and because of the limited number of studies providing field measurements of these parameters. One such parameter is burning efficiency, which determines how much of the original forest`s aboveground carbon stock will be released in the burn, as well as how much will later be released by decay and how much will remain as charcoal. In this paper we examined the fate of biomass from a semideciduous tropical forest in the ""arc of deforestation,"" where clearing activity is concentrated along the southern edge of the Amazon forest. We estimated carbon content, charcoal formation and burning efficiency by direct measurements (cutting and weighing) and by line-intersect sampling (LIS) done along the axis of each plot before and after burning of felled vegetation. The total aboveground dry biomass found here (219.3 Mg ha(-1)) is lower than the values found in studies that have been done in other parts of the Amazon region. Values for burning efficiency (65%) and charcoal formation (6.0%, or 5.98 Mg C ha(-1)) were much higher than those found in past studies in tropical areas. The percentage of trunk biomass lost in burning (49%) was substantially higher than has been found in previous studies. This difference may be explained by the concentration of more stems in the smaller diameter classes and the low humidity of the fuel (the dry season was unusually long in 2007, the year of the burn). This study provides the first measurements of forest burning parameters for a group of forest types that is now undergoing rapid deforestation. The burning parameters estimated here indicate substantially higher burning efficiency than has been found in other Amazonian forest types. Quantification of burning efficiency is critical to estimates of trace-gas emissions from deforestation. (C) 2009 Elsevier B.V. All rights reserved.