Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton,Australia

The Archean Pilbara Craton contains five geologically distinct terranes – the East Pilbara, Karratha, Sholl, Regal and Kurrana Terranes – all of which are unconformably overlain by the 3.02‐ to 2.93‐Ga De Grey Superbasin. The 3.53–3.17 Ga East Pilbara Terrane (EP) represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3.53–3.43, 3.35–3.29 and 3.27–3.24 Ga. Each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3.72 Ga, and melting of crust to generate first tonalite‐trondhjemite‐granodiorite (TTG), and then progressively more evolved granitic magmas. In each case, plume magmatism was accompanied by uplift and crustal extension. The combination of conductive heating from below, thermal blanketing from above, and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust. These mantle melting events caused severe depletion of the subcontinental lithospheric mantle, making the EP a stable, buoyant, unsubductable continent by c. 3.2 Ga. Extension accompanying the latest event led to rifting of the protocontinent margins at between 3.2 and 3.17 Ga. After 3.2 Ga, horizontal tectonic forces dominated over vertical forces, as revealed by the geology of the three terranes (Karratha, Sholl and Regal) of the West Pilbara Superterrane. The c. 3.12‐Ga Whundo Group of the Sholl Terrane is a fault bounded, 10‐km‐thick volcanic succession with geochemical characteristics of modern oceanic arcs (including boninites and evidence for flux melting) that indicate steep Archean subduction. At 3.07 Ga, the 3.12‐Ga Sholl Terrane, 3.27‐Ga Karratha Terrane and c. 3.2‐Ga Regal Terrane accreted together and onto the EP during the Prinsep Orogeny. This was followed by development of the De Grey Superbasin – an intracontinental sag basin and widespread plutonism (2.99–2.93 Ga) as a result of orogenic relaxation and slab break off. Craton‐wide compressional deformation at 2.95–2.93 Ga culminated with 2.91‐Ga accretion of the 3.18 Ga Kurrana Terrane with the EP. This compression caused amplification of the dome‐and‐keel structure in the EP. Final cratonization was effected by emplacement of 2.89–2.83 Ga post‐tectonic granites.     

Monitored Natural Attenuation of Contaminants in the Subsurface: Applications

In recent years there has been increasing interest in the application of passive technologies to reduce or remove contaminants from the subsurface environment including soil and ground water. In most cases, the impetus for this interest lies in a perceived savings compared with more traditional remedial alternatives. In a few cases, the infrastructure at contaminated sites, such as buildings, paved areas, and utilities, makes the use of conventional remedial measures difficult and expensive.
To demonstrate that natural processes are effective in reaching established goals, it is necessary to determine that transformation processes are taking place at a rate that is protective of human health and the environment and that these processes will continue for an acceptable period of time. The basic conditions that must be present to confirm natural attenuation processes arc taking place are discussed along with the behavior of contaminated plumes, monitoring requirements, data analysis, rates of degradation, and mathematical modeling.     

A neodymium,strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust

In the Samail ophiolite,¹⁴⁷Sm-¹⁴³Nd,⁸⁷Rb-⁸⁷Sr, and¹⁸O/¹⁶O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and¹⁸O/¹⁶O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the¹⁴⁷Sm-¹⁴³Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 ± 12m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65–85 m.y. The initial¹⁴³Nd/¹⁴⁴Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in ε_Nd of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The⁸⁷Sr/⁸⁶Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (ε_Sr = ?19.7 to +30.5) and the δ¹⁸O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the¹⁸O/¹⁶O water-rock fractionation.     

An incentive mechanism for reducing emissions from conversion of intact and non-intact forests

This paper presents a new accounting mechanism in the context of the UNFCCC issue on reducing emissions from deforestation in developing countries, including technical options for determining baselines of forest conversions. This proposal builds on the recent scientific achievements related to the estimation of tropical deforestation rates and to the assessment of ‘intact’ forest areas. The distinction between ‘intact’ and ‘non intact’ forests used here arises from experience with satellite-based deforestation measurements and allows accounting for carbon losses from forest degradation. The proposed accounting system would use forest area conversion rates as input data. An optimal technical solution to set baselines would be to use historical average figures during the time period from 1990 to 2005. The system introduces two different schemes to account for preserved carbon: one for countries with high forest conversion rates where the desired outcome would be a reduction in their rates, and another for countries with low rates. A ‘global’ baseline rate would be used to discriminate between these two country categories (high and low rates). For the hypothetical accounting period 2013–2017 and considering 72% of the total tropical forest domain for which data are available, the scenario of a 10% reduction of the high rates and of the preservation of low rates would result in approximately 1.6 billion tCO₂ of avoided emissions. The resulting benefits of this reduction would be shared between those high-rate countries which reduced deforestation and those low-rate countries which did not increase their deforestation over an agreed threshold (e.g., half of “global” baseline rate).     

Thermal inertia mapping of Mars from 60°S to 60°N

Twenty-micrometer brightness temperatures are used to derive the thermal inertia for 81% of the Martian surface between latitudes ±60°. These data were acquired by the two Viking Infrared Thermal Mappers in 1977 and 1978 following the two global dust storms of 1977. The spatial resolution used is 2° in latitude by 2° in longitude and the total range in derived inertia is $\text{1}\text{to}\text{15 × 10}{}^{\mathrm{?3}}\text{cal cm}{}^{\mathrm{?2}}\text{sec}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{°}\text{K}{}^{\mathrm{?1}}$. The distribution of thermal inertia is strongly bimodal with all values of thermal inertia less than $\text{4 × 10}{}^{\mathrm{?3}}\text{cal cm}{}^{\mathrm{?2}}\text{sec}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{°}\text{K}{}^{\mathrm{?1}}$ being associated with three disjoint bright regions mostly in the northern hemisphere. Sufficient dust is raised in global storms to provide fine material adequate to produce these low-inertia areas but the specific deposition mechanism has not been defined. At the low resolution used, no complete exposures of clean rock were found. There is some tendency for darker material to be associated with higher thermal inertia, although the trend is far from one to one. The distribution of high- and low-inertia areas is sufficiently nonrandom to produce a variation in whole-disk brightness temperature with central meridian longitude. This variation and the change in surface kinetic temperature associated with dust storms are factors in establishing the whole-disk brightness temperature at radio and infrared wavelengths and will be important for those who use Mars as a calibration source.     

The Cosmic Foreground Explorer (COFE): A balloon-borne microwave polarimeter to characterize polarized foregrounds

The COsmic Foreground Explorer (COFE) is a balloon-borne microwave polarimeter designed to measure the low-frequency and low-ℓ characteristics of dominant diffuse polarized foregrounds. Short duration balloon flights from the Northern and Southern Hemispheres will allow the telescope to cover up to 80% of the sky with an expected sensitivity per pixel better than 100 μK/deg² from 10 GHz to 20 GHz. This is an important effort toward characterizing the polarized foregrounds for future CMB experiments, in particular the ones that aim to detect primordial gravity wave signatures in the CMB polarization angular power spectrum.     

Circular Polarization Variability in Extragalactic Sources on Time Scales of Months to Decades

We present previously unpublished circular polarization (cp) measurements at 4.8 and 8.0 GHz made with the University of Michigan 26-meter prime focus telescope during 1978–1984 and results of a new observational program during the past two years. Based on the preliminary analysis of our recent data, eight sources were detected at 4.8 GHz with average degrees of cp ranging from ≤ 0.1% to ≥ 1%.The results are compared with observations at other frequencies and with linear polarization and total flux density variability in the sources. The behavior of the cp variability observed to date is consistent with stochastic variations produced by mode conversion in transient, opaque emitting regions in the sources. The observed sign changes observed between different epochs and different frequencies are not consistent with the hypothesis that sources maintain a fixed handedness of cp.     

A linear mathematical model for the seismic inplane behaviour of brick masonry walls part 2: Determination of model parameters through optimization using experimental data

The parameters appearing in the mixture and effective modulus models proposed in Part 1 are determined through optimization by matching theoretical and experimental responses. The optimization analysis is performed in frequency space. The response quantities chosen to be matched are the complex frequency response functions (experimental and theoretical) relating the Fourier transforms of top and base accelerations of the wall. Computations in optimization analysis are carried out by introducing an object (error) function and minimizing it using the Gauss-Newton method. The results show that the mixture model is capable of predicting accurately the dynamic response of masonry walls up to a frequency which is well above the second modal frequency, whereas the effective modulus model describes the wall behaviour only up to the first modal frequency. Furthermore, it is shown that the mixture model is still valid when micro cracks, which may exist between the mortar and brick constituents, are present.     

The effect of Cr on the solubility of Al in orthopyroxene: experiments and thermodynamic modelling

The partitioning of chromium and aluminium between coexisting orthopyroxene and spinel in equilibrium with forsterite in the system MgO–Al₂O₃–SiO₂–Cr₂O₃ (MAS–Cr) has been experimentally determined as a function of temperature, pressure and Cr/(Cr + Al) ratio. Experiments were conducted at temperatures between 1300 and 1500 °C and at pressures from 5 to 54 kbar. Previous experimental results on the (Al, Cr)₂O₃ and Mg(Al, Cr)₂O₄ solid solutions have been combined with the present results plus relevant data from the CMAS system to derive a thermodynamic model for Al–Cr-bearing orthopyroxenes, spinels and corundum–eskolaite solid solutions. The orthopyroxene solid solution can be modelled within the accuracy of all experimental constraints as a ternary solid solution involving the components Mg₂Si₂O₆ ( E), MgAl₂SiO₆ (M) and MgCr₂SiO₆ (C), in which the activities are related to composition through the equations: The mole fractions are defined as where n _Al and n _Cr are the number of Al and Cr cations per orthopyroxene formula unit of six oxygens. These expressions reduce to one-site mixing for Mg₂Si₂O₆–MgAl₂SiO₆ orthopyroxenes in the Cr-free system, but are equivalent to two-site mixing for the exchange of Al and Cr between orthopyroxene and spinel, as required by the experimental data. We find W ^opx _EM =W ^opx _EC  = 20 kJ mol⁻¹ and W ^opx _MC =0. Received: 9 August 1999 / Accepted: 18 February 2000