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How late Archean tectonics could be seen to have operated in the Slave Province is illustrated. Lithospheric thinning and stretching, with the formation of rifted margins (to continental or island arc fragments), and lithospheric flexural loading of the kind familiar in arcs and mountain belts could be discerned.
Thin cherty sedimentary layers within the volcanic portions of the 3,500 to 3,300 Ma-old Onverwacht and Fig Tree Groups, Barberton Greenstone belt, South Africa, and Warrawoona Group, eastern Pilbara Block, Western Australia, contain an abundant record of early Archean life. Five principal types of organic and probably biogenic remains and or structures can be identifed: stromatolites, stromatolite detritus, carbonaceous laminite or flat stromalite, carbonaceous detrital particles, and microfossils. Early Archean stromatolites were reported from both the Barberton and eastern Pilbara greenstone belts. Systematic studies are lacking, but two main morphological types of stromatolites appear to be represented by these occurrences. Morphology of the stromalites is described. Preserved early Archean stromatolites and carbonaceous matter appear to reflect communities of photosynthetic cyanobacteria inhabiting shallow, probably marine environments developed over the surfaces of low-relief, rapidly subsiding, simatic volcanic platforms. The overall environmental and tectonic conditions were those that probably prevailed at Earth's surface since the simatic crust and oceans formed sometime before 3,800 Ma. Recent studies also suggest that these early Archean sequences contain layers of debris formed by large-body impacts on early Earth. If so, then these early bacterial communities had developed strategies for coping with the disruptive effects of possibly globe-encircling high-temperature impact vapor clouds, dust blankets, and impact-generated tsunamis. It is probable that these early Archean biogenic materials represent organic communities that evolved long before the beginning of the preserved geological record and were well adapted to the rigors of life on a young, volcanically active Earth during late bombardment. These conditions may have had parallels on Mars during its early evolution.
This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.
Observations of the late (t = 20-1500 days) bolometric light curve and the gamma-lines and X-rays from supernova 1987A are compared to theoretical models. It is found that 0.073 + or - 0.015 solar masses of freshly synthesized Ni-56 must be present to fit the bolometric light curve. The results place limits on the luminosity and presumed period of the newly formed pulsar/neutron star. In the second half of the paper, the problem of computing the luminosities in gamma-ray lines and in X-rays from supernova 1987A is addressed. High-energy observations suggest the development of large-scale clumping and bubbling of radioactive material in the ejecta. A model is proposed with a hydrogen envelope mass of about 7 solar masses, homologous scale expansion velocities of about 3000 km/s, and an approximately uniform mass distribution.
An X-ray-selected sample of 128 late-type (F-M) stars analyzed. These stars were identified as optical counterparts to serendipitous X-ray detections made by the Einstein Observatory Extended Medium Sensitivity Survey. Once identified as X-ray sources, the sample was followed up by an extensive program of optical observations, including high-resolution and low-resolution spectroscopy and photometry. Spectral types, luminosity classes, distances, X-ray luminosities, projected rotation rates (v sin i), radial velocities, and binary status have been determined for the sample.
Monte Carlo techniques are used to study the energy deposition of gamma rays from the radioactive decay of Co-56 in Type Ib and Type II SN. It is found that above an electron fraction of about 0.1, direct excitations are negligible, and the line emission arises from thermal processes. The results indicate that the emission is dominated by neutral and singly ionized lines, including forbiden O I, forbidden Ca II, forbidden C I, and forbidden Si I. The late spectrum is shown to be useful as a diagnostic of the nucleosynthetic structure of the ejecta. The present model has been applied to observations of the Type Ib SN 1985F, and it is suggested that Type Ib SN come from stars of greater than 25 solar masses.
Radiation cataractogenesis induced by small acute doses of particulate radiations and photons in the New Zealand white rabbit (Oryctolagus cuniculus), the beagle dog (Canis familiaris) and the rhesus monkey (Macaca mulatta) is discussed in the context of the use of animal models to assess the radiation hazards faced by humans during lengthy sojourns in deep space. Attention is paid to (1) the importance of lifespan studies with long-lived species - the above animals have median lifespans in captivity of 5-7, 13-14 and 25 years, respectively; and (2) the magnitudes of possible dose thresholds for cataractogenesis from sparsely ionizing radiations and the modifications of those thresholds by the late degenerative phase of the phenomenon.
Attributes of Quaternary lakes and lake basins which are often important in the environmental prehistory of semideserts are discussed. Basin-floor and basin-closure morphometry have set limits on paleolake sizes; lake morphometry and basin drainage patterns have influenced lacustrine processes; and water and sediment loads have influenced basin neotectonics. Information regarding inundated, runoff-producing, and extra-basin spatial domains is acquired directly from the paleolake record, including the littoral morphostratigraphic record, and indirectly by reconstruction. Increasingly detailed hypotheses regarding Lake Bonneville, the largest late Pleistocene paleolake in the Great Basin, are subjects for further testing and refinement. Oscillating transgression of Lake Bonneville began about 28,000 yr B.P.; the highest stage occurred about 15,000 yr B.P., and termination occurred abruptly about 13,000 yr B.P. A final resurgence of perennial lakes probably occurred in many subbasins of the Great Basin between 11,000 and 10,000 yr B.P., when the highest stage of Great Salt Lake (successor to Lake Bonneville) developed the Gilbert shoreline. The highest post-Gilbert stage of Great Salt Lake, which has been one of the few permanent lakes in the Great Basin during Holocene time, probably occurred between 3,000 and 2,000 yr B.P.
A model of the tectonic evolution of the eastern Indian Ocean from the Late Cretaceous to the present is constructed. Reinterpretations of seafloor magnetic anomalies are used to place constraints on spreading rates and the timing of major reorganizations. Vertical deflection profiles from 22 repeat cycles of Geosat altimeter data are used to determine the tectonic fabric associated with fracture zones. The Geosat data place constraints on paleospreading directions. Consideration is given to the implications of the model for the relative motions between India, Australia, and Antarctica.
A spectroscopic survey of nine X-ray selected late type stars was conducted. These stars are serendipitously discovered EINSTEIN X-ray sources, selected from two large x-ray surveys: the Columbia Astrophysical Laboratory survey (five stars) and the CFA Medium Sensitivity survey (four stars). Four of the Columbia survey stars were found to be short period binaries. The fifth was found to be an active single G dwarf. None of the Medium Sensitivity survey stars were found to be either binaries or active stars. Activity was measured by comparing the H-alpha and the CaII infrared triplet (8498, 8542) lines in these stars to the lines in inactive stars of similar spectral type. A correlation was found between the excess H-alpha lime emission and V sin(i) and between the excess H-alpha line emission and X-ray luminosity. No correlation was found between the infrared line emission and any other measured quantity.
A dynamical model of the Pleistocene ice ages is presented, which incorporates many of the qualitative ideas advanced recently regarding the possible role of ocean circulation, chemistry, temperature, and productivity in regulating long-term atmospheric carbon dioxide variations. This model involves one additional term (and free parameter) beyond that included in a previous model (Saltzman and Sutera, 1987), providing the capacity for an asymmetric response. It is shown that many of the main features exhibited by the delta(O-18)-derived ice record and the Vostok core/delta(C-13)-derived carbon dioxide record in the late Pleistocene can be deduced as a free oscillatory solution of the model.
Study of several stars in the late B and early A spectral types shows that very high rotators are associated with shell characteristics (sometimes not detected at all in the visible spectra) and also with C IV and some Si IV spectral absorption features which can be explained by circumstellar phenomena superimposed over stellar metallic blends. These particularities are evidenced by comparison with other spectra of low and high rotators in the same spectral range. HD 119921, a star with similar characteristics to the other ones of the sample, is given special attention. A possible scenario is suggested to explain the observed superionization features.
The Lyman-alpha line of neutral hydrogen is probably the most important cooling channel for chromospheric plasma in late-type stars, yet it is also the least studied major line in the far ultraviolet. The scattering of much of the stellar Lyman-alpha flux by interstellar hydrogen, coupled with the geocoronal emission foreground, seriously complicates the analysis of the Lyman-alpha spectra. The influence of the local interstellar medium on the observed profiles was circumvented by observing stars with radial velocities sufficiently high to Doppler shift the center of the stellar emission line out of the interstellar absorption core. There are several stars that have high radial velocities by virtue of their presence in close binary systems. High resolution IUE (International Ultraviolet Explorer) spectra of Ly alpha line of two such eclipsing binary stars, AR Lac and TY Pyx, are obtained, at each orbital quadrature phase, when the projected orbital velocity is a maximum. By combining the spectra from opposite quadratures it is possible to piece together the entire stellar emission profiles. The third star in this study, delta Lep, is a single star with a high space velocity.
A model is developed for the young and ongoing kinematic deformation of the southern California crust and upper mantle. The kinematic model qualitatively explains both the overall seismic structure of the upper mantle and much of the known geological history of the late Cenozoic as consequences of ongoing convection beneath southern California. In this model, the high-velocity upper-mantle anomaly of the Transverse ranges is created through the convergence and sinking of the entire thickness of subcrustal lihtosphere, and the low-velocity upper-mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1-4 percent partial melt related to adiabatic decompression during mantle upwelling.
The results are presented of a survey of the coronal temperatures of late-type stars using the Einstein Observatory IPC. The spectral analysis shows that the frequently found one- and two-temperature descriptions are mainly influenced by the SNR of the data and that models using continuous emission measure distributions can provide equally adequate and physically more meaningful and more plausible descriptions. Intrinsic differences in differential emission measure distributions are found for four groups of stars. M dwarfs generally show evidence for high-temperature gas in conjunction with lower-temperature material, while main-sequence stars of types F and G have the high-temperature component either absent or very weak. Very hot coronae without the lower-temperature component appearing in dwarf stars are evident in most of the giant stars studied. RS CVn systems show evidence for extremely hot coronae, sometimes with no accompanying lower-temperature material.
The procedure of Ayres and Wiedemann (1989) was applied to a range of stellar-atmosphere models to study the CO Delta-v = 1 spectrum and to establish its use as a remote sensor of thermal conditions in late-type stars. Spectra were computed to examine the sensitivity of the CO Delta-v = 1 to fundamental stellar parameters and to assess the errors introduced into the spectrum intepretation by uncertain input parameters and non-LTE effects. Results of the sensitivity study demonstrate that CO fundamental spectra are useful probes for the temperature structure of the outer layers of cool stellar atmospheres, but that their value is limited by the uncertainties introduced by non-LTE effects. However, in stars with surface gravities of log g of about 1.5 and greater, the values of these uncertainties are reasonably small.
Recent tectonic models have resulted in conflicting descriptions of how the late Precambrian sutures of the Arabian-Nubian shield extend into northeast Africa. The Hamisana shear zone in northeastern Sudan is critical to this discussion because it truncates and disrupts two sutures, the Allaqi-Heiani and the Onib-Sol Hamed. Analysis of field structural data, Thematic Mapper imagery, and Rb-Sr and U-Pb geochronology suggests that the Allaqi-Heiani suture is the western extension of the Onib-Sol Hamed suture and that both make up the exposed parts of a far-traveled, polydeformed ophiolitic nappe complex. Subsequent deformation localized in the Hamisana shear zone disrupted this nappe and displaced the suture between 660 and 550 Ma during regional deformation associated with the Najd fault system. These results indicate that at least one suture extends westward into the interior of northern Africa.
Hydrodynamic constraints or driving forces that determine steady disk gas radial distributions are discussed, and observed gas distributions in late-type disks and correlations with disk rotation curves are analyzed. Evidence for quasi-steady states with radial gas flow is examined, and gas consumption rates and stellar density profiles are discussed, as are models of steady states with radial flows and more extreme 'driven' disks. The results indicate that the radial structure of a gas-rich disk can be simply and consistently accounted for as an azimuthally averaged hydrodynamic steady state in a fixed gravitational potential.