Showing papers in "Journal of Geophysical Research in 1981"
TL;DR: In this article, an initial moment tensor is derived using one of the variations of the method described in detail by Gilbert and Dziewonski (1975), where perturbations to the elements of the moments are found simultaneously with changes in the hypocentral parameters.
Abstract: It is possible to use the waveform data not only to derive the source mechanism of an earthquake but also to establish the hypocentral coordinates of the ‘best point source’ (the centroid of the stress glut density) at a given frequency. Thus two classical problems of seismology are combined into a single procedure. Given an estimate of the origin time, epicentral coordinates and depth, an initial moment tensor is derived using one of the variations of the method described in detail by Gilbert and Dziewonski (1975). This set of parameters represents the starting values for an iterative procedure in which perturbations to the elements of the moment tensor are found simultaneously with changes in the hypocentral parameters. In general, the method is stable, and convergence rapid. Although the approach is a general one, we present it here in the context of the analysis of long-period body wave data recorded by the instruments of the SRO and ASRO digital network. It appears that the upper magnitude limit of earthquakes that can be processed using this particular approach is between 7.5 and 8.0; the lower limit is, at this time, approximately 5.5, but it could be extended by broadening the passband of the analysis to include energy with periods shorter that 45 s. As there are hundreds of earthquakes each year with magnitudes exceeding 5.5, the seismic source mechanism can now be studied in detail not only for major events but also, for example, for aftershock series. We have investigated the foreshock and several aftershocks of the Sumba earthquake of August 19, 1977; the results show temporal variation of the stress regime in the fault area of the main shock. An area some 150 km to the northwest of the epicenter of the main event became seismically active 49 days later. The sense of the strike-slip mechanism of these events is consistent with the relaxation of the compressive stress in the plate north of the Java trench. Another geophysically interesting result of our analysis is that for 5 out of 11 earthquakes of intermediate and great depth the intermediate principal value of the moment tensor is significant, while for the remaining 6 it is essentially zero, which means that their mechanisms are consistent with a simple double-couple representation. There is clear distinction between these two groups of earthquakes.
2,610 citations
TL;DR: In this article, the effects of mean winds and gravity waves on the mean momentum budget were investigated and it was shown that the existence of critical levels in the mesosphere significantly limits the ability of gravity waves to generate turbulence.
Abstract: It has been suggested (Lindzen, 1967, 1968a, b; Lindzen and Blake, 1971; Hodges, 1969) that turbulence in the upper mesosphere arises from the unstable breakdown of tides and gravity waves. Crudely speaking, it was expected that sufficient turbulence would be generated to prevent the growth of wave amplitude with height (roughly as (basic pressure)−1/2). This work has been extended to allow for the generation of turbulence by smaller amplitude waves, the effects of mean winds on the waves, and the effects of the waves on the mean momentum budget. The effects of mean winds, while of relatively small importance for tides, are crucial for internal gravity waves originating in the troposphere. Winds in the troposphere and stratosphere sharply limit the phase speeds of waves capable of reaching the upper mesosphere. In addition, the existence of critical levels in the mesosphere significantly limits the ability of gravity waves to generate turbulence, while the breakdown of gravity waves contributes to the development of critical levels. The results of the present study suggest that at middle latitudes in winter, eddy coefficients may peak at relatively low altitudes (50 km) and at higher altitudes in summer and during sudden warmings (70–80 km), and decrease with height rather sharply above these levels. Rocket observations are used to estimate momentum deposition by gravity waves. Accelerations of about 100 m/s/day are suggested. Such accelerations are entirely capable of producing the warm winter and cold summer mesopauses.
1,967 citations
TL;DR: In this article, it is suggested that the partial pressure of carbon dioxide in the atmosphere is buffered, over geological time scales, by a negative feedback mechanism, in which the rate of weathering of silicate minerals (followed by deposition of carbonate minerals) depends on surface temperature, which in turn depends on the carbon dioxide partial pressure through the greenhouse effect.
Abstract: It is suggested that the partial pressure of carbon dioxide in the atmosphere is buffered, over geological time scales, by a negative feedback mechanism, in which the rate of weathering of silicate minerals (followed by deposition of carbonate minerals) depends on surface temperature, which in turn depends on the carbon dioxide partial pressure through the greenhouse effect. Although the quantitative details of this mechanism are speculative, it appears able to partially stabilize the earth's surface temperature against the steady increase of solar luminosity, believed to have occurred since the origin of the solar system.
1,837 citations
TL;DR: In this article, an approximate analytic solution for the radiative transfer equation describing particulate surface light scattering, taking into account multiple scattering and mutual shadowing, was derived for the interpretation of reflectance spectroscopy of laboratory surfaces and the photometry of solar system objects.
Abstract: An approximate analytic solution is derived for the radiative transfer equation describing particulate surface light scattering, taking into account multiple scattering and mutual shadowing. Analytical expressions for the following quantities are found: bidirectional reflectance, radiance coefficient and factor, the normal, Bond, hemispherical, and physical albedos, integral phase function and phase integral, and limb-darkening profile. Scattering functions for mixtures can be calculated, as well as corrections for comparisons of experimental transmission or reflection spectra with observational planetary spectra. The theory should be useful for the interpretation of reflectance spectroscopy of laboratory surfaces and the photometry of solar system objects.
1,816 citations
TL;DR: A model for the photochemistry of the global troposphere constrained by observed concentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented in this paper.
Abstract: A model for the photochemistry of the global troposphere constrained by observed concentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented. Data for NO and NO2 are insufficient to define the global distribution of these gases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO2, HO2NO2, CH3O2, CH3OOH, CH2O, and CH3CCl3 are calculated as functions of altitude, latitude, and season. Results imply that the source for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 107 tons N yr−1. Global sources for CO and CH4 are 1.5 × 109 tons C yr−1 and 4.5 × 108 tons C yr−1, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH4 (25%), with the balance from burning of vegetation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH4 provides sources of tropospheric O3 similar in magnitude to loss by in situ photochemistry. Observations of CH3CCl3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definitive test requires better definition of release rates for CH3CCl3 and improved measurements for its distribution in the atmosphere.
1,762 citations
TL;DR: The flow behind an interplanetary shock was analyzed through the use of magnetic field and plasma data from five spacecraft, with emphasis on the magnetic cloud identified by a characteristic variation of the latitude angle of the magnetic field.
Abstract: The flow behind an interplanetary shock was analyzed through the use of magnetic field and plasma data from five spacecraft, with emphasis on the magnetic cloud identified by a characteristic variation of the latitude angle of the magnetic field. The size of the cloud was found to be about 0.5 AU in radial extent and greater than 30 deg in azimuthal extent, with its front boundary almost normal to the radial direction. Because the field direction of the magnetic cloud as it moved past the spacecraft was observed to rotate nearly parallel to a plane, it is thought that the field configuration of the cloud was essentially two-dimensional. These results further suggest that the lines of force in the magnetic cloud formed loops, but it could not be determined whether these loops were open or closed.
1,575 citations
TL;DR: In this article, the authors studied the effect of pre-emptive and preemptive gradients in T and O 2 in a variety of compositionally zoned ash flow tuffs.
Abstract: Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.
1,448 citations
TL;DR: In this article, the authors studied the instability of a boundary layer for a range of physical parameters (Rayleigh number, amounts of thickening, and boundary conditions) and derived expressions that related the growth of the instability and the time needed to remove the boundary layer as a function of the amount of horizontal shortening (f), the Rayleigh number (R), and the ratio (a/d) of the thicknesses of the rigid and fluid layers.
Abstract: When crust thickens during crustal shortening, the underlying mantle lithosphere must shorten and thicken also, causing the submersion of cold, dense material into the surrounding asthenosphere. For a range of physical parameters the thickened boundary layer that forms the transition from the strong lithosphere to the convecting asthenosphere may become unstable, detach, and sink into the asthenosphere, to be replaced by hotter asthenospheric material. We have studied the instability of a thickened boundary layer for a range of physical parameters (Rayleigh number), amounts of thickening, and boundary conditions. In all cases the fluid was overlain by a rigid, conducting layer. Extensive numerical experiments were made for fluids with stress-free boundary conditions, heated either from below or from within. From a simple physical description of the observed pattern of flow we derived expressions that related the growth of the instability and the time needed to remove the thickened boundary layer as a function of the amount of horizontal shortening (f), the Rayleigh number (R), and the ratio (a/d) of the thicknesses of the rigid and fluid layers. In our opinion, observations and theory agree well (within 10% for R > 105) and show that the speed with which the thickened boundary layer is removed increases with increasing f, R, and a/d. A limited series of runs with no-slip boundary conditions suggests approximately the same functional relationships but with the process 0–30% slower than with stress-free boundaries. For Rayleigh numbers comparable to those appropriate for upper mantle convection (105–107) the removal of the boundary layer occurs rapidly, in times less than the thermal time constant of the overlying rigid plate. Using typical values for the physical parameters in the earth, the boundary layer is removed in times less than the duration of deformation in some collision zones (30–50 m.y.). Thus we suspect that often the lower lithosphere is removed during the process of crustal shortening, causing the overlying crust and uppermost mantle to warm rapidly. This process is likely to contribute to the development of regional metamorphism and to the generation of latetectonic or posttectonic granites. We suspect, in fact, that in some cases the entire mantle lithosphere may detach from the lower crust during crustal shortening, exposing the crust to asthenospheric temperatures.
971 citations
TL;DR: In this paper, isotopic analyses of 75 samples from the Samail ophiolite indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8 km-thick oceanic crustal section; locally, the H_2O even penetrated down into the tectonized peridotite.
Abstract: Isotopic analyses of 75 samples from the Samail ophiolite indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8-km-thick oceanic crustal section; locally, the H_2O even penetrated down into the tectonized peridotite. Pillow lavas (δ^(18)O = 10.7 to 12.7) and sheeted dikes (4.9 to 11.3) are typically enriched in ^(18)O, and the gabbros (3.7 to 5.9) are depleted in ^(18)O. In the latter rocks, water/rock ≤ 0.3, and δ^(18)O_(cpx) ≈ 2.9 + 0.44 δ^(18)O_(feld), indicating pronounced isotopic disequilibrium. The mineral δ^(18)O values approximately follow an exchange (mixing) trajectory which requires that plagioclase must exchange with H_2O about 3 to 5 times faster than clinopyroxene. The minimum δ^(18)O_(feld) value (3.6) occurs about 2.5 km below the diabase-gabbro contact. Although the gabbro plagioclase appears to be generally petrographically unaltered, its oxygen has been thoroughly exchanged; the absence of hydrous alteration minerals, except for minor talc and/or amphibole, suggests that this exchange occurred at T > 400°–500°C. Plagioclase δ^(18)O values increase up section from their minimum values, becoming coincident with primary magmatic values near the gabbro-sheeted diabase contact and reaching 11.8 in the diabase dikes. These ^(18)O enrichments in greenschist facies diabases are in part due to exchange with strongly ^(18)O-shifted fluids, in addition to retrograde exchange at much lower temperatures. The δ^(18)O data and the geometry of the mid-ocean ridge (MOR) magma chamber require that two decoupled hydrothermal systems must be present during much of the early spreading history of the oceanic crust (approximately the first 10^6 years); one system is centered over the ridge axis and probably involves several convective cells that circulate downward to the roof of the magma chamber, while the other system operates underneath the wings of the chamber, in the layered gabbros. Upward discharge of ^(18)O-shifted water into the altered dikes from the lower system, just beyond the distal edge of the magma chamber, combined with the effects of continued low-T hydrothermal activity, produces the ^(18)O enrichments in the dike complex. Integrating δ^(18)O as a function of depth for the entire ophiolite establishes (within geologic and analytical error) that the average δ^(18)O (5.7 ± 0.2) of the oceanic crust did not change as a result of all these hydrothermal interactions with seawater. Therefore the net change in δ^(18)O of seawater was also zero, indicating that seawater is buffered by MOR hydrothermal circulation. Under steady state conditions the overall bulk ^(18)O fractionation (Δ) between the oceans and primary mid-ocean ridge basalt magmas is calculated to be +6.1 ± 0.3, implying that seawater has had a constant δ^(18)O≈−0.4 (in the absence of transient effects such as continental glaciation). Utilizing these new data on the depth of interaction of seawater with the oceanic crust, numerical modeling of the hydrothermal exchange shows that as long as worldwide spreading rates are greater than 1 km^2/yr, ^(18)O buffering of seawater will occur. These conclusions can be extended as far back in time as the Archean (> 2.6 eons) with the proviso that Δ may have been slightly smaller (about 5?) because of the overall higher temperatures that could have prevailed then. Thus ocean water has probably had a constant δ^(18)O value of about −1.0 to +1.0 during almost all of earth's history.
895 citations
TL;DR: In this paper, the authors show that the concentration of Nd correlates well with eNd in the batholith rocks and support the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements.
Abstract: Plutonic igneous rocks of the Sierra Nevada batholith exhibit a range of Nd isotopic composition described by eNd = +6.5 to −7.6. Similar rock types from the Peninsular Ranges have eNd = +8.0 to −6.4. In both batholiths, eNd correlates strongly with initial 87Sr/86Sr. Decreasing eNd values are accompanied by increasing 87Sr/86Sr and increasing δ18O; the correlation with δ18O being more pronounced for the Peninsular Ranges. The eNd values show regular geographic variations, as was found previously for initial 87Sr/86Sr. Three metasedimentary country rock samples from the Sierra Nevada region have low eNd values (−11 to −16) and Precambrian model Sm-Nd ages (1.5 to 1.9 AE). The country rock eNd values, and those of primitive oceanic island arcs (eNd = +8), bracket the data for the batholith rocks. The Nd, Sr, and O isotopic data can be explained if the batholiths are mixtures of island arc and metasedimentary components, the latter being of both Paleozoic and early Proterozoic age. This model appears to be consistent with existing Pb isotopic data. Consideration of O-Sr isotopic relations and the variation of 147Sm/144Nd with eNd suggests that assimilation of crustal rocks by magmas rising from the mantle and undergoing fractional crystallization could have been the major process responsible for the mixing of crustal- and mantle-derived components. The isotopic data, when combined with assumptions about the structure of the crust beneath the batholiths, suggest that about 50% of the crustal material presently within the geographic boundaries of the batholiths and above the Moho represents juvenile crust derived from the mantle in the Mesozoic. The remaining material appears to be mostly derived from 1.8-AE crust, yielding an average crust formation age of nearly 1 AE for this section of the crust. This result, which may apply to large portions of the Cordillera, suggests that the average age of the North American continent may be greater than previously estimated. The concentration of Nd correlates well with eNd in the batholith rocks and supports the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements. A 1.5-AE Sm-Nd model age for sedimentary rocks of the Mesozoic(?) Calaveras Formation indicates that the Nd in this “oceanic” terrain is dominated by continental detritus and demonstrates the potential of Sm-Nd isotopic studies for aiding in construction of tectonic models.
873 citations
TL;DR: In this article, 11 passes of the ISEE satellites through the frontside terrestrial magnetopause were identified, where the plasma velocity in the magnetic field and boundary layer was substantially larger than in the magnetosheath, with a view to determining whether the velocity enhancements can be explained by magnetic field reconnection.
Abstract: Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause were identified, where the plasma velocity in the magnetopause and boundary layer was substantially larger than in the magnetosheath. The nature of the plasma flow, magnetic field, and energetic particle fluxes in these regions were examined, with a view to determining whether the velocity enhancements can be explained by magnetic field reconnection.
TL;DR: In this paper, Bagnold's energy-based total load sediment transport model is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed.
Abstract: Bagnold's energetics-based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed. In both the bedload and suspended load, the transport rate vectors are found to be composed of a velocity-induced component directed parallel to the instantaneous velocity vector and a gravity-induced component directed down slope. The model is applied to idealized surfzone conditions, leading to estimates of the local longshore and onshore-offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions. The model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions. The results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics.
TL;DR: In this paper, the ascent and emplacement of basaltic magma on the earth and moon is modeled by the application of geological and physical observations and constraints, provided that allowance is made for the coalescence of gas bubbles.
Abstract: The ascent and emplacement of basaltic magma on the earth and moon is modeled by the application of geological and physical observations and constraints. Relatively simple mathematical models of the motion of gas/liquid mixtures are shown to be adequate in the treatment of basaltic eruptions, provided that allowance is made for the coalescence of gas bubbles and that realistic geological and petrochemical constraints are applied to the numerical values of variables. Because gas exsolution from magmas on the earth and moon commonly occur at depths of less than 2 km, it is generally convenient to consider separately the rise of bubble-free magmatic liquid at depth in a planetary crust and the more complex motions occurring near the surface with gas exsolution.
TL;DR: In this article, it is shown that for a relative small range of ocean wave parameters, the likelihood that the transfer function is linear increases as the direction of wave propagation approaches the range direction, as the wavelength increases, and as the wave height decreases.
Abstract: Real and synthetic aperture radars have been used in recent years to image ocean surface waves. Though wavelike patterns are often discernible on radar images, it is still not fully understood how they relate to the actual wave field. The present paper reviews and extends current models on the imaging mechanism. Linear transfer functions that relate the two-dimensional wave field to the real aperture radar (SLAR) image are calculated by using the two-scale wave model. It is noted that a description of the imaging process by these transfer functions can only be adequate for low to moderate sea states. Possible other mechanisms that contribute to the visibility of waves by real aperture radar at higher sea states, such as Bragg scattering from spontaneously generated short waves at peaked crests or in wave breaking regions, and Rayleigh scattering from air bubbles entrained in the water and from water droplets thrown into the air by breaking waves, are discussed in a qualitative way. The imaging mechanism for synthetic aperture radars (SAR's) is strongly influenced by wave motions (i.e., by the orbital velocity and acceleration associated with the long waves). The phase velocity of the long waves does not enter into the imaging process. Focusing of ocean wave imagery is attributed to orbital acceleration effects. The orbital motions lead to a degradation in resolution which causes image smear as well as a SAR inherent imaging mechanism called velocity bunching. The parameter range for which velocity bunching is a linear mapping process is calculated. It is shown that linearity holds only for a relative small range of ocean wave parameters: The likelihood that the transfer function is linear increases as the direction of wave propagation approaches the range direction, as the wavelength increases, and as the wave height decreases. Linearity is required for applying simple linear system theory for calculating the ocean wave spectrum from the gray level intensity spectrum of the image. Although, in general, the full ocean wave spectrum cannot be recovered from the SAR image by applying simple linear inversion techniques, it is concluded that for many cases in which the ocean wave spectrum is relatively narrow the dominant wavelength and direction can still be retrieved from the image even when the mapping transfer function is nonlinear. Finally, we compare our theoretical models for the imaging mechanisms with existing SLAR and SAR imagery of ocean waves and conclude that our theoretical models are in agreement with experimental data. In particular, our theory predicts that swell traveling in flight (azimuthal) direction is not detectable by SLAR but is detectable by SAR.
TL;DR: In this article, the authors quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions and mantle flow, and the model is extended to the actual three-dimensional (spherical) plate geometry.
Abstract: Cooling and thickening of lithospheric plates with age and subduction result in large-scale horizontal density contrasts tending to drive plate motions and mantle flow. We quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions. First, two-dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions. We are unable to obtain platelike behavior in viscous models with traction-free boundary conditions. The piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes. If the lithosphere has a Newtonian viscous rheology, the net force on any plate is a strong function of the effective grid spacing used, leading to ambiguities in interpretation. Incorporating a rigid-plastic lithosphere, which fails at a critical yield stress, into the otherwise viscous model removes these ambiguities. The model is extended to the actual three-dimensional (spherical) plate geometry. The observed velocities of rigid-plastic plates are matched to the solution of the viscous Stokes equation at the lithosphere-asthenosphere boundary. Body forces from the seismically observed slabs, from the thickening of the lithosphere obtained from the actual lithospheric ages, and from the differences in structure between continents and oceans are included. Interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included. The driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three-dimensional global return flow and resistance to deformation at converging boundaries; the rms residual torque is ∼30% of the driving torque. The density contrasts within the plates themselves can reasonably account for plate motions. Body forces from convection in the interior may provide only a small net force on the plates. At converging boundaries the lithosphere has a yield stress of ∼100 bars; drag at the base of the plates is ∼5 bars and resists plate motion. The net driving forces from subducting slabs and collisional resistance are localized and approximately balance. Driving forces from lithospheric thickening are distributed over the areas of the plates, as is viscous drag. The approximate balance of these two forces predicts plate velocities uncorrelated with plate area, as observed. The model represents a specific case of boundary layer convection; the dynamical results are consistent with either upper mantle or mantle-wide convection.
TL;DR: In this article, the authors measured the ζ potentials and the streaming potential coefficients of silicate mineral-water and rock-water systems for purposes of estimating the magnitude of electrokinetic effects induced by water diffusion in the earth's interior.
Abstract: The ζ potentials and the streaming potential coefficients of silicate mineral-water and rock-water systems were measured for purposes of estimating the magnitude of electrokinetic effects induced by water diffusion in the earth's interior. In rock-water systems the adsorption reaction of H+ at a solid surface is proved to be a main factor determining the ζ potential. For typical crustal rocks the ζ potential in water of pH higher than 2 is commonly negative and increases in absolute magnitude as supporting electrolyte concentration decreases and as temperature increases. A physicochemical theory was developed on the above features of the ζ potential on the basis of the adsorption reaction at the solid-liquid interface. The experimental and theoretical results obtained in the present study of the ζ potential and the streaming potential coefficient in rock-water systems are applied to some geophysical problems: electromagnetic variations induced by water diffusion prior to earthquakes and the self-potential anomaly associated with hydrothermal convection.
TL;DR: In this article, an intact plutonic sequence within the Samail ophiolite in the Ibra area, southeastern Oman Mountains, reveals stratigraphic, structural, and petrologic details of oceanic layer 3.
Abstract: Geologic mapping of an intact plutonic sequence within the Samail ophiolite in the Ibra area, southeastern Oman Mountains, reveals stratigraphic, structural, and petrologic details of oceanic layer 3. Four measured stratigraphic sections, each spaced about 5 km apart across the southern flank of Jabal Dimh, define a time-transgressive progression within the ophiolite and reveal geometric and petrologic features of a spreading-ridge magma chamber. The sections show the following vertical sequence: (1) dunite (chr-ol cumulates ± harzburgite xenoliths) 0–200 m thick, grading up from a transition zone with harzburgite tectonite, (2) interlayered wehrlite-melagabbro-gabbro (cpx-ol and ol-cpx-pl cumulates) 0–100 m thick, (3) layered gabbro (chiefly ol-cpx-pl cumulates but including recurrent intervals of cumulus wehrlite and melagabbro) 2.6–5.5 km thick, (4) planar laminated nonlayered gabbro (chiefly ol-cpx-pl cumulates) 100–400 m thick, (5) hypidiomorphic (ol)-hb-cpx gabbro (high-level gabbro) 200–800 m thick, (6) small, discontinous diorite to plagiogranite bodies at or near the top of the gabbro. Cumulus textures (adcumulus > mesocumulus), planar lamination, and cumulus layering (phase, ratio, and grain size layers at mm to 10-m scale, commonly graded) within this sequence show that crystals accumulated from the base of the magma chamber upward to within a few hundred meters of the top; downward solidification from the roof was minor. Cyclicity within the cumulus sequence is represented by the recurrence of olivine-rich intervals (melagabbo and wehrlite) up to high stratigraphic levels and by hundreds of phase-graded layers (ol-rich at the bases to pl-rich at the tops), individually up to 5 m thick. Limited cryptic variation relative to closed-system layered intrusions and the limited range in solid-solution components of olivine (Fo69–90), plagioclase (An62–95) and clinopyroxene (En40–54, Fs4–16, Wo37–49) from the cumulus suite require replenishment of the magma during its crystallization history. Zig-zag normal and reverse cryptic variation differentiation trends (both in major solid-solution components and in minor element concentration) indicate that the replenishment took place in pulses followed by periods of magma mixing and crystal fractionation. Recurrent olivine-rich intervals commonly (but not always) coincide with reverse cryptic variation trends. They are considered to be the products of fresh draughts of primitive magma, with olivine and chromite as the only liquidus phases prior to extensive mixing with the more fractionated resident magma within the chamber. The sequence of crystallization is explained using the simplified tholeiitic basalt tetrahedron of Presnall et al. (1979). The plutonic sequence is roofed by sheeted dikes that are overlain by submarine basalt, indicating that the magma crystallized beneath a spreading ocean ridge. Major and trace element geochemistry of the dike complex is similar in many respects to that of mid-ocean ridge basalt (MORB) and yield Mg/Mg + Fe+2 ratios that overlap with ratios predicted for parent liquids of the cumulus suite based on crystal/liquid equilibria. Field evidence, including continuity of layering over large areas, distinctive layer sets that are mapped for several km along strike, and lack of chamber edge contacts indicates that crystallization of Jabal Dimh plutonics took place in a single, large, long-lived magma chamber rather than in small transitory chambers. Similarity of mineral compositions and tentative correlation of cryptic variation trends between stratigraphic sections favors a large, single-chamber model. The chamber shape, controlled by floor growth upward greatly exceeding roof growth downward as the chamber halves diverged from the paleo-spreading axis, must have been funnel-shaped in cross section; i.e., the floor sloped inward to the center from a sandwich horizon that developed just beneath the roof at both sides. This conclusion is based on the occurrence of cumulates up to a very high level in the stratigraphic sections and on geologic mapping which shows that cumulus layering locally approaches the base of the cumulus section at an angle. This vergence angle is <20°, well below inferred angles of repose for cumulates. Tentative correlation of cryptic variation between sections yields similar vergence angles. The scarcity of slump structures and of layer disruption within the main layered sequence verifies that the floor sloped rather gently, surely much less than 45°. Thus, the half-width of the chamber must have exceeded its vertical dimension, which averages ∼5 km. A half-width of ∼18 km is calculated from a preferred vergence angle of 15°. The cumulates crystallized within this spreading chamber from a magma body that was repeatedly replenished by primitive magma from a central feeder zone at the spreading axis. High-level gabbro and diorite through plagiogranite crystallized from stagnant residual magma at the outer edges of the chamber, beyond reach of replenishment from the center.
TL;DR: In this article, measurements from the Lepedea plasma instruments and the flux gate magnetometers on ISEE 1 and 2 are used to examine the nature of the hydromagnetic waves associated with the various classes of ions backstreaming from the earth's bow shock.
Abstract: Measurements from the Lepedea plasma instruments and the flux gate magnetometers on ISEE 1 and 2 are used to examine the nature of the hydromagnetic waves associated with the various classes of ions backstreaming from the earth's bow shock. The reflected ions, which are confined to a narrow energy and angular range, are accompanied by small amplitude (less than approximately 1/2 gamma peak to peak) left-handed waves at frequencies close to 1 Hz in the spacecraft frame. Diffuse backstreaming particles with a broad energy spectrum are associated with low frequency (approximately 30-s period), large amplitude (approximately 5 gamma peak to peak) waves. Intermediate particles are associated with a mixture of these two wave types. Often the waves associated with the diffuse beams steepen as if they were minishocks. The leading edge (trailing edge in the spacecraft frame) frequently appears to break up into a whistler mode wave packet. These discrete wave packets are right-hand polarized and have frequencies from below the proton gyrofrequency to well above it in the plasma frame and are blown back towards the earth by the solar wind.
TL;DR: In this article, an algorithm for estimating moisture content of a bare soil from the observed brightness temperature at 1.4 GHz is discussed and applied to a limited data base, based on a radiative transfer model calculation, with some modifications to take into account the effect of surface roughness.
Abstract: An algorithm for estimating moisture content of a bare soil from the observed brightness temperature at 1.4 GHz is discussed and applied to a limited data base. The method is based on a radiative transfer model calculation, which has been successfully used in the past to account for many observational results, with some modifications to take into account the effect of surface roughness. Besides the measured brightness temperatures, the three additional inputs required by the method are the effective soil thermodynamic temperature, the precise relation between moisture content and the smooth field brightness temperatures and a pair of parameters related to surface roughness. The procedures of estimating surface roughness parameters and of obtaining moisture content from observed brightness temperature are discussed. The algorithm is applied to observations from truck mounted and airborne radiometers. The estimated moisture contents compare favorably with the observations in the top 2 cm layer.
TL;DR: In this paper, the authors show that the principal means of methane escape is through the plants themselves as opposed to transport across the water-air interface via bubbles or molecular diffusion, and that existing data and flux-measurement methods are insufficient for reliable global extrapolations.
Abstract: Field measurements of methane fluxes from rice paddies, fresh water lakes, and saltwater marshes have been made to infer estimates of the size of these sources of atmospheric methane. The rice-paddy measurements, the first of their kind, show that the principal means of methane escape is through the plants themselves as opposed to transport across the water-air interface via bubbles or molecular diffusion. Nitrogen-fertilized plants release much more methane than unfertilized plants but even these measured rates are only one fourth as large as those inferred earlier by Koyama (1963, 1964) and on which all global extrapolations have been based to date. Measured methane fluxes from lakes and marshes are also compared to similar earlier data and it is found that extant data and flux-measurement methods are insufficient for reliable global extrapolations.
TL;DR: In this article, the convection potential drop across the polar cap is computed from data obtained on high-inclination low-altitude satellites and the potential measurements are correlated with various combinations of parameters measured simultaneously in the upstream solar wind.
Abstract: The convection potential drop across the polar cap is computed from data obtained on high-inclination low-altitude satellites. Potential measurements are correlated with various combinations of parameters measured simultaneously in the upstream solar wind. Most of the potential drop is successfully predicted by merging theory, although a significant background potential drop of 35 kV does not depend on IMF parameters and is attributed to a process other than merging. Results indicate that small values of the IMF are amplified by a factor of 5-10 at the dayside magnetopause, which, when taken into account, improves correlations between IMF parameters and polar cap potential drop. Potential drop is better correlated with IMF parameters than with geomagnetic indices, due to nonlinear response of the magnetosphere affecting geomagnetic activity indices.
TL;DR: In this paper, the stretching model of McKenzie is applied to the formation of passive continental margins, assuming local isostatic equilibrium, and its fit to the IPOD data on the Armorican and Galicia continental margins of the northeast Atlantic is discussed.
Abstract: The stretching model of McKenzie is applied to the formation of passive continental margins, assuming local isostatic equilibrium. We present the quantitative implications of the model; we then discuss its fit to the IPOD data on the Armorican and Galicia continental margins of the northeast Atlantic. The amount of brittle stretching observed in the upper 8 km of the prestretched continental crust reaches a maximum value of about 3. This large amount of thinning is comparable to the thinning of the whole continental crust observed by seismic refraction measurements and required by the model for the whole lithosphere. This agreement suggests that the simple stretching model is a good first approximation to the actual physical process of formation of the margin. It is thus possible to compute simply the thermal evolution of the margin and to discuss its petrological consequences. It is also possible to obtain a quantitative reconstruction of the edge of the continent prior to breakup. Finally, the large slope of the base of the lithosphere during the formation of the margin results in a force similar but opposite to the ‘ridge-push’ force acting on accreting plate boundaries.
TL;DR: In this article, a comprehensive computer simulation of the behavior of the earth's inner magnetosphere during a substorm-type event is reported, where the basic equations of the model are derived, and the inputs are described.
Abstract: Results of a comprehensive computer simulation of the behavior of the earth's inner magnetosphere during a substorm-type event are reported. It is pointed out that the computer model self-consistently computes electric fields, currents, and plasma distributions and velocities in the inner-magnetosphere/ionosphere system; parallel electric fields and ionospheric neutral winds, however, are not included. The basic equations of the model are derived, and the inputs are described. An overview of the results is also given. The first appendix contains derivations of general, useful laws of bounce-averaged gradient, curvature, and E x B drifts in a plasma with isotropic pitch angle distribution. The second appendix gives a description of the numerical method used in the simulation.
TL;DR: In this article, the fluxes of marine cyclic salts through that part of the Amazon River system draining past Obidos (80% of the basin) at the time of peak discharge in June.
Abstract: Analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the Amazon River system draining past Obidos (80% of the basin) at the time of peak discharge in June. Amazon precipitation chemistry can be devided into two principal components: marine and terrestrial. The marine component (determined from analyses of marine rain) consists of Na, K, Mg, Ca, and Cl in approximately sea-salt proportions, with S doubly enriched. The excess sulfur is probably derived from gas phase inputs. The terrestrial component makes an important contribution of K, Ca, S, and N, much of which can be related to biological emissions. The emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural ‘acid rain’ in the Amazon region with a pH from 4.7 to 5.7. This is about one tenth the acidity of polluted urban rain. The chloride content of lowland rivers, which drain regions lacking significant geologic sources of chloride, shows a systematic decrease in chloride with increasing distance from the ocean. This trend is used to define the cyclic salt background for Amazonian surface waters. Cyclic salts, in general, make only a minor contribution, relative to terrestrial inputs, to the chemistry of Amazon Basin rivers, even those draining intensely weathered terrains. An estimated 17.6%-Cl, 6.9%-Na, 1.3%-Mg, 3.6%-S, 0.4%-K, and 0.1%-Ca of the dissolved load at Obidos during peak discharge is cyclic.
TL;DR: In this paper, the subducted portions of the Panama basin and old Farallon plate have become segmented into three pieces recognized in this study, from north to south, they are (1) a "Bucaramanga" segment continuous with the Caribbean seafloor northwest of Colombia, (2) a ‘Cauca’ segment continued with oceanic crust (Nazca plate), and (3) an ‘Ecuador” segment at the northern end of the Subducted lithospheric plate which is dipping at a small angle to
Abstract: Spreading along the Cocos-Nazca plate boundary since the breakup of the Farallon plate in the Miocene has resulted in the formation of the Panama basin and a complex interaction of plates in and near northwestern South America. Current plate boundaries have been defined, and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes, considering only well-located events, and through focal mechanism determinations. The existence of relict plate boundaries, bathymetric features, and the Panamanian isthmus has affected the subduction process of the Nazca plate beneath South America and determined the present-day configuration of the subducting lithospheric plate. There is no single triple junction separating the Caribbean, South American, and Nazca plates. Instead, the Panamanian isthmus and surrounding areas are accommodating east-west compression (and a lesser degree of north-south compression) along a series of thrust faults striking NW to NE, and the Andean ranges of Ecuador, Colombia, and Venezeula are moving as a block NNE relative to the rest of the South American plates, along a system of faults following the front of the Eastern Cordillera. The subducted portions of the Panama basin and old Farallon plate have become segmented into three pieces recognized in this study. From north to south, they are (1) a ‘Bucaramanga’ segment continuous with the Caribbean seafloor northwest of Colombia, (2) a ‘Cauca’ segment continuous with oceanic crust (Nazca plate) currently being subducted beneath South America at the Colombia-Ecuador trench, and (3) an ‘Ecuador’ segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern Peru. The segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted.
TL;DR: The GEOS 1 and 2 spacecraft contain a set of particle and wave detectors which allow for a very comprehensive study of wave-particle interactions occurring within the equatorial region of the magnetosphere.
Abstract: The GEOS 1 and 2 spacecraft contain a set of particle and wave detectors which allow for a very comprehensive study of wave-particle interactions occurring within the equatorial region of the magnetosphere. This paper is devoted to interactions involving protons in the energy range 20 keV to 300 keV and ULF waves with frequencies below the proton gyrofrequency. It is shown that mose of the ion cyclotron waves (ICW's) detected in this frequency range have spectra whose charcteristic frequencies are organized in the vicinity of the He/sup +/ gyrofrequency. Simultaneous measurements of the ion composition in the thermal energy range (E< or approx. =110 eV) show these waves to be clearly associated with the abundance of cold He/sup +/ as well as the anisotropy of ions above 20 keV. The general characteristics of these helium-associated ULF events are presented in case studies of four events. The interpretation of this phenomenon is given in the present paper in terms of the propagation of ICW's in a He/sup +/ -rich plasma. It is shown that the shape of the cold plasma dispersion curve (for both parallel and non-parallel propgation) can adequately explain the main characteristics of the observed waves (frequency spectrum, polarization)more » as well as the differences between observations made onboard GEOS 1 and GEOS 2. The generation conditions of ion cylotron waves in such a multi-component plasma, as well as their quasi-linear effects on both the cold He/sup +/ ions and the hot protons, are discussed in a companion paper.« less
TL;DR: In this paper, a parameterization for bare soil is developed, which is to be incorporated in a mesoscale numerical prediction model, and generalized to accomodate 11 types of soil in addition to peat, using mean soil characteristics.
Abstract: A parameterization for bare soil is developed, which is to be incorporated in a mesoscale numerical prediction model. This parameterization is generalized to accomodate 11 types of soil in addition to peat, using mean soil characteristics. The sensitivity of the scheme to several soil parameters is evaluated by a series of one-dimensional simulations. It is shown that the most important soil characteristic is the soil moisture, which regulates the strength of the heat fluxes between the atmosphere and the ground.
TL;DR: The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the northern boundary of the Arabian-African plate as mentioned in this paper.
Abstract: The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the northern boundary of the Arabian-African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the Tethyan Sea, Gondwana Land continued its dispersal, and the Arabian-African plate drifted northward about 10°. These events combined with the opposite rotation of Eurasia and Africa initiated the closing of the Tethyan during the Late Cretaceous. At the early stages of closure, downwarping of the Arabian continental margin combined with the compressional forces of closure from the Eurasian plate initiated obduction of the Tethyan oceanic crust along preexisting transform faults, and still hot oceanic crust was detached along oblique northeast dipping thrust faults. Amphibolites developed at the base of the detached hot peridotite as it was thrust southward over oceanic volcanic and sedimentary rocks. Plate configurations combined with palinspastic reconstructions show that subduction and attendant large-scale island arc volcanism did not commence until after the Tethyan sea began to close and after the Samail ophiolite was emplaced southward across the Arabian continental margin. The Samail ophiolite nappe now rests upon a melange consisting mainly of pelagic sediments, volcanics, and detached fragments of the basal amphibolites which in turn rest on autochthonous shelf carbonates of the Arabian platform. Laterites and conglomerates with reworked laterites on the eroded upper surface of the ophiolite indicate a period of emergence prior to the deposition of shallow water Maestrichtian carbonates. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just north of the present Jaz Murian depression in Iran.
TL;DR: The global neutral gas temperature distribution and circulation of the thermosphere are calculated for equinox and solstice conditions by using NCAR's thermospheric general circulation model (TGCM) as discussed by the authors.
Abstract: The global neutral gas temperature distribution and circulation of the thermosphere are calculated for equinox and solstice conditions by using NCAR's thermospheric general circulation model (TGCM) The variables are determined on a 5° grid in latitude and longitude at 24 constant pressure surface layers in the vertical from about 90 to 500 km Global empirical models of electron density and neutral composition are used to specify ion drag and the neutral gas background properties The energy sources that drive the thermosphere include heating caused by the absorption of solar EUV and UV radiation and a high-latitude heat source associated with auroral processes Starting from a global average state, the TGCM reached a diurnal reproducible pattern in approximately 5 days and showed no apparent instabilities in the basic flow The calculated motion patterns indicate that the observed day-to-day variability in the thermosphere is likely because of variations in the heat and momentum sources such as the result of neutral plasma interactions and waves coming from the lower atmosphere The calculated horizontal structure of wind and temperature is qualitatively different between the upper and lower thermosphere This difference occurs because the diurnal and semidiurnal forcing are in phase in the optically thick lower thermosphere but out of phase in the optically thin upper thermosphere Using the latest calculated heating efficiencies and measured fluxes of EUV, the calculated diurnal temperature amplitude is 250 K, compared to 320 K predicted by the MSIS empirical model, indicating that solar heating is primarily responsible for maintaining the structure of the thermosphere The zonally averaged properties of the TGCM essentially agree with previous results obtained from a two-dimensional model of the zonally symmetric thermosphere However, for reasons discussed, the latitudinal variations of temperatures is somewhat larger than previously calculated Our previously estimated auroral heating terms need to be reduced by 30%–40% to get best agreement with observations
TL;DR: In this article, a model of the azimuthal current sheet by a finite thickness annulus of inner radius 5 Jovian radii, 5-Jovians thickness, and extending to about 50 Jovians radii provides detailed fits of the vector magnetic field perturbations observed in relation to the planetary field.
Abstract: Voyager 1 and 2 magnetic field observations confirm and extend the earlier Pioneer 10 detection of the Jovian magnetodisc, a region of enhanced charged particles and plasma and reduced magnetic field intensity located near the magnetic equatorial plane. Modeling of the azimuthal current sheet by a finite thickness annulus of inner radius 5 Jovian radii, 5-Jovian radii thickness, and extending to about 50 Jovian radii provides detailed fits of the vector magnetic field perturbations observed in relation to the planetary field for distances less than 30 Jovian radii. Field line geometry is also investigated, and better insight into the phenomena of charged particle absorption by the Galilean satellites is obtained which provides improved explanations of observed effects due to Ganymede.