Dynamical evolution of cometary asteroids

We present results from long-term numerical integrations of hypothetical Jupiter-family comets (JFCs) over time-scales in excess of the estimated cometary active lifetime. During inactive periods these bodies could be considered as 'cometary' near-Earth objects (NEOs) or 'cometary asteroids'. The contribution of cometary asteroids to the NEO population has important implications not only for understanding the origin of inner Solar system bodies but also for a correct assessment of the impact hazard presented to the Earth by small bodies throughout the Solar system. We investigate the transfer probabilities on to 'decoupled' subJovian orbits by both gravitational and non-gravitational mechanisms, and estimate the overall inactive cometary contribution to the NEO population. Considering gravitational mechanisms alone, more than 90 per cent of decoupled NEOs are likely to have their origin in the main asteroid belt. When non-gravitational forces are included, in a simple model, the rate of production of decoupled NEOs from JFC orbits becomes comparable to the estimated injection rate of fragments from the main belt. The Jupiter-family (non-decoupled) cometary asteroid population is estimated to be of the order of a few hundred to a few thousand bodies, depending on the assumed cometary active lifetime and the adopted source region.     

Putting the census on the web: lessons from two case studies

Summary This paper addresses the digital dissemination of geographically referenced census information in the UK. A number of important weaknesses in the 1991 model of data access are identified, and the possibility of future access to census information via the World Wide Web is then addressed in detail. Two case studies demonstrate the potential to overcome some fundamental weaknesses in earlier access models, including the provision of integrated data and metadata, graphical interfaces to geographical datasets, and an integrated interface and analysis environment.     

Delineation of lithostructural domains in northern Orissa (India) from Landsat Thematic Mapper imagery

Image-based reconnaissance geological mapping at 1:100,000 scale using Landsat TM data has delineated a college of Precambrian lithostructural domains within a 50,000-km² region which encompasses the northern portion of the Archaean to Proterozoic granulite-grade Eastern Ghats Tectonic Province and the adjacent Archaean-Mesoproterozoic Singhbhum Craton. The domains identified in the present study display distinctive internal structures on satellite imagery. Most are bounded by clearly recognisable major shear zones and faults on imagery; displacement directions may frequently be ascertained through local reorientation of planar structures adjacent to large strike-slip shear zones and through juxtaposition of domains with different structuring.

The macroscopic/megascopic structural overview provided by the Landsat interpretation, supported by preliminary field investigation, suggests that the N-S shortening (E-W fold/thrust packages with associated NE and NW strike-slip faulting), is the dominant structural style in the northernmost part of the Eastern Ghats Tectonic Province, followed by regional dextral transpression, expressed in the form of major strike-slip faults. The largest of these structures (Kerajang Fault), which may be traced for over 250 km, has demonstrable Paleozoic-Mesozoic dextral motion associated with coal basin formation. A precursor Kerajang shear zone with dextral movement in excess of 100 km of indeterminate age may also have been instrumental in juxtaposing the Singhbhum Nucleus into its present position to the north of the Eastern Ghats Tectonic Province.     

Geochemical constraints on crustal anatexis: a case study from the Pan-African Damara granitoids of Namibia

 Major and trace element models of recently published vapour-absent mica dehydration melting experiments are used to identify granitoids generated by muscovite and biotite dehydration melting, and to distinguish between plagioclase-limited and biotite-limited, biotite dehydration melting. In the case of granitoids from the Pan-African Damara mobile belt (Namibia), many of the leucogranites and Salem-type granitoids may be modelled by biotite dehydration melting. The low Rb/Sr granitoids (e.g. Donkerhuk Onanis, Salem Onanis, Donkerhuk Nomatsaus, Salem Goas) probably reflect feldspar-limited, biotite dehydration melting (a pelitic source) whereas the high Rb/Sr suites (e.g. Bloedkoppie leucogranite, Stinkbank leucogranite, Salem Swakopmund, Leucocratic Stink bank granite) reflect biotite-limited, biotite dehydration melting (a greywacke source). Alaskites from the Damara belt have major element compositions which are consistent with muscovite dehydration melting, and their positive Eu anomalies are linked to high K₂O reflecting K-feldspar entrainment. Combined Zr and LREE (light rare earth element) solubility models indicate that insufficient time (probably less than 10⁴ years) had elapsed between melt generation and melt extraction to ensure that the alaskite melts attained their equilibrium concentrations of Zr and the LREEs. In contrast, the leucogranites and Salem-type granites have attained their equilibrium inventories of these trace elements. Combined Fe₂O₃ and MgO contents in some samples from two granitoids (the Salem Goas and Donkerhuk Onanis intrusions) are higher than those readily attainable by biotite dehydration melting indicating either: (1) that they contain a contribution from melts generated by incipient garnet breakdown or; (2) that they contain small amounts of an entrained ferromagnesian phase. Received: 24 April 1995/Accepted: 11 December 1995     

Air quality and view degradations due to copper mining and milling: Preliminary analysis and cost estimates for Green Valley,Arizona

Green Valley, Arizona is a retirement community located near major open-pit copper mines. Mining and milling activities create dust, which degrades air quality, and mine dumps and tailings banks, which degrade the viewscape. Although the tailings banks are highly visible, this study found that consumer surplus lost from dust-polluted air is more than twice that from degradation of viewscape. Consumer surplus lost from both air quality and viewscape degradation is estimated to be between $116,000,000 and $169,000,000. These preliminary estimates are based upon two studies: hedonic property values, and a contingent valuation survey of willingness to pay.     

Vietnamese activity in the Adelaide real estate market

Significant real estate activity by Vietnamese households in Adelaide did not occur until 1979, and since then the groups buying activity has been heavily concentrated in the northwest region of the metropolitan area. Therefore, the median housing price paid by Vietnamese buyers has not kept pace with the median price paid by all buyers. Concentration also causes Vietnamese buyers to be disproportionately active in the middle‐to‐lower end of the market, within which there has been a strong preference for rural properties and for houses rather than home units and maisonettes. Amongst Vietnamese buyers who subsequently sell their properties, the majority have not re‐entered the market and their capital gain from ownership has been marginal.     

Frictional characteristics of Teflon–steel interfaces under dynamic conditions

An experimental study to assess the frictional characteristics of Teflon–steel interfaces under dynamic conditions is described. Tests were conducted on unfilled Teflon sliding on stainless steel polished to a surface roughness of less than 0.05 μm CLA. The bearing pressure was 1.38, 3.45 and 6.9 N/mm², the frequency of sliding was 0.5 and 1 Hz and the acceleration of excitation, imposed by a small shaking table, was between 0.05 and 0.5g. It was found that friction drops with decreases in the acceleration of excitation and drops with increases in the bearing pressure. It was observed that Teflon–steel interfaces have the tendency to slide continuously regardless of the value of acceleration. This is very important in base isolation applications because it prevents high frequency input to the structure.     

Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites: variation with crystal fractionation and interaction with sea water

Lavas and pyroclastics on Ascension Island contain plutonic blocks that include fluid-inclusion-bearing peralkaline-granite. ¹⁸O/¹⁶O ratios, F and Cl have been analysed on whole rocks and/or minerals for lavas and granites, and D/H ratios and H₂O⁺ for comenditic obsidians and granites. Whole rock ¹⁸O/¹⁶O ratios of fresh alkali-basalt, hawaiite, trachyandesite, trachyte and comendite range from 6.0 to 6.9 with ¹⁸O tending to increase with increase in SiO₂. The ¹⁸O values of the granites are from 0.0 to 0.3 depleted in ¹⁸O relative to the comendites. Comenditic obsidians have D= –80±4 and H₂O⁺ 0.3 wt.% while amphiboles from the granites have D= –56±2 The O-isotope trend of the lavas is consistent with a crystal fractionation model. Fresh igneous rocks with ¹⁸O values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma. The D/H ratios and Cl contents ( 3,000 ppm) of the H₂O-poor comenditic obsidians represent undegassed primary magmatic values. The H-isotope compositions and low H₂O and Cl (167 ppm) contents of the granites are consistent with the major degassing (loss of >90% of initial H₂O) of an H₂Osaturated magma derived from the interaction of sea (or possibly meteoric) water with the H₂O-undersaturated comenditic melt. It is proposed that, associated with caldera subsidence and stoping, water was sucked in around the residual magma before the system had time to be sealed up. The H₂O-undersaturated magma consumed this H₂O with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks, at a pressure of about 1.5 kb. The granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites. Granites from oceanic islands may, in general, be a result of generating an H₂O-saturated acid melt by such direct or indirect crustal water-magma interaction processes.Contribution No. 603 Centre de Recherches Pétrographiques et Géochimiques     

Development of conjugate shear bands during bulk simple shearing

In rocks possessing a strong planar fabric, shear bands of constant shear sense and oriented at an oblique angle to the foliation are considered by many authors to be characteristic of a non-coaxial bulk deformation history, whereas conjugate shear bands are considered to indicate coaxial shortening. However, in two areas where bulk deformation history appears to be non-coaxial (Cap Corse, Corsica and Ile de Groix, Brittany), conjugate shear bands are observed. In order to investigate this problem, experiments were performed by bulk simple shearing using Plasticine as a rock analogue. When slip between layers of the model is permitted, shear bands of normal-fault geometry form with both the same and opposite shear sense as the bulk simple shearing at approximately the same angle with the layering (40°) irrespective of layer orientation in the undeformed state (for initial orientations of 50, 30 and 15°). Shear bands are initially formed within individual layers and may propagate across layer interfaces when further movement along these is inhibited. The existence of conjugate shear bands in Corsica and Ile de Groix is therefore not incompatible with a model of bulk simple shearing for these two regions. In field studies, one should perhaps exercise care in using shear bands to determine the kind of motion or the sense of bulk shearing.