ISSN:0148-0227    

DOI:10.1029/JB080i023p03285

年代:1975

数据来源: WILEY

摘要:

Synthesis of polar topographic data derived from the Mariner 9 radio occultation, ultraviolet spectrometer, and television imaging experiments provides new information on the behavior of polar volatiles and the topographic configuration of the martian polar layered deposits. Gentle slopes in the vicinity of the south pole may serve to shift the point of minimum annual solar insolation from the pole to a site within the perimeter of the offset residual frost cap. Localized defrosting which gives rise to the dark‐banded appearance of both residual caps correlates with a series of outward‐facing slopes descending from central topographic highs. Stability of the volatile involved apparently is largely insolation controlled. The south polar residual cap lies entirely higher (at lower pressure) than the northern cap, implying that the south residual cap is an unlikely site for any permanent surface deposit of solid carbon dioxide. Photogrammetric models of both residual caps reveal a series of regularly spaced topographic undulations descending from central topographic highs within the underlying layered deposits. Scarplike to troughlike in cross section, these features slope l°–5° and are 100–1000 m in local relief. The south polar layered deposits lie almost entirely at higher elevations than those in the north. Total thickness of the deposits is inferred to be 1–2 km in the south and 4–6 km

ISSN:0148-0227    

DOI:10.1029/JB080i023p03286

年代:1975

数据来源: WILEY

摘要:

A relatively detailed investigation of the Mid‐Atlantic Ridge crest at 26°N was conducted by using narrow‐beam bathymetric data, total earth's magnetic field measurements, and underwater photographs. The Mid‐Atlantic Ridge crest at 26°N appears to be hydrothermally active. The structural setting of this area is conducive to the occurrence of hydrothermal deposits. The walls of the rift valley are extensively faulted with blocks and steps ranging in size from kilometers to meters in width and relief. Underwater photographs show hydrothermal manganese associated with interpreted fault steps at depths between 3100 and 2500 m on the east wall, suggesting that the faults provide avenues for hydrothermal fluids. Small topographic highs in the floor of the rift valley are the sites of relatively recent volcanism and are believed to represent the top of an active dike emplacement zone. Bathymetric trend directions for this portion of the ridge crest are complex in comparison with plate rotation predicted trends for the Mid‐Atlantic Ridge. The bathymetric grain is a function of processes active in the ri

ISSN:0148-0227    

DOI:10.1029/JB080i023p03307

年代:1975

数据来源: WILEY

摘要:

Measurements of the K‐Ar ages of eight trachybasaltic dikes from the Rio de Los Molinos zone, Argentina, suggest that the dikes were intruded during at least two magmatic episodes at about 140 and 65 m.y. B.P. Normal and reversed magnetic remanences are observed in the older dikes; these suggest that during the older magmatic episode at least two events of intrusion occurred; a minimum period of 5000 years must have elapsed between these intrusions. The pole computed for the Upper Jurassic–Lower Cretaceous dikes (79°S, 8°E, α95= 18°) is reasonably close to the paleomagnetic poles of the Vulcanitas Cerro Colorado formation (118 m.y.) and the Serra Geral Formation (115–125 m.y.). This supports the interpretation that the South Atlantic Ocean was opened in Lower Cretace

ISSN:0148-0227    

DOI:10.1029/JB080i023p03315

年代:1975

数据来源: WILEY

摘要:

Coda waves from small local earthquakes are interpreted as backscattering waves from numerous heterogeneities distributed uniformly in the earth's crust. Two extreme models of the wave medium that account for the observations on the coda are proposed. In the single backscattering model the scattering is considered to be a weak process, and the loss of seismic energy by scattering is neglected. In the diffusion model the seismic energy transfer is considered as a diffusion process. Both models lead to similar formulas that allow an accurate separation of the effect of earthquake source from the effects of scattering and attenuation on the coda spectra. A unique difference was found in the scaling law of earthquake source spectra between central California and western Japan, which may be attributed to the difference in inhomogeneity length of the earth's crust. TheQof coda waves in the two regions is strongly frequency dependent with values increasing from 50–200 at 1 Hz to about 1000–2000 at 20 Hz. This observation is interpreted as a combined effect of variation ofQwith depth and frequency‐dependent composition of coda waves described below. The turbidity coefficient of the lithosphere required at 1 Hz to explain the observed coda as body wave scattering is orders of magnitude greater than previously known values such as those obtained by Aki (1973) and Capon (1974) under the Montana Lasa from the amplitude and phase fluctuations of teleseismicPwaves. From the high attenuation and turbidity obtained at this frequency we conclude that at around 1 Hz the coda is made of backscattering surface waves from heterogeneities in the shallow, low‐Qlithosphere. The highQobserved for the coda at frequencies higher than 10 Hz, on the other hand, eliminates the possibility that these waves are backscattering surface waves. We conclude that at these high frequencies the coda must be made of backscattering body waves from heterogeneities in the deep lithosphere. The low turbidities found for deep earthquake sources under western Japan are consistent with this model of coda wave gen

ISSN:0148-0227    

DOI:10.1029/JB080i023p03322

年代:1975

数据来源: WILEY

摘要:

A shallow earthquake focus mechanism is modeled by a plane strain shear crack extending at a nonuniform rate under the action of general loading. An equation of motion of the crack tip is determined in terms of a specific fracture energy, i.e., a surface energy, by applying an overall energy balance fracture criterion. If the specific fracture energy is specified, the motion can be determined. On the other hand, if any particular motion of the fault boundary is specified, the requisite specific fracture energy can be calculated from the equation of motion. A process of destruction of unbroken material is included in which the relative sliding of the faces of the fracture surface leads to complete removal of stress in the fractured area. The strength of the dynamic stress singularity at the crack tip is written as a product of an equivalent static stress singularity multiplied by a universal function of crack velocity, the universal function decreasing with increasing crack tip velocity.

ISSN:0148-0227    

DOI:10.1029/JB080i023p03343

年代:1975

数据来源: WILEY

摘要:

Pyroxene geotherms under continents (F. R. Boyd, 1973, 1975) and ocean basins (I. D. MacGregor, 1974) are characterized by inflections to steeper thermal gradients at temperatures above about 1050°C. The inflections are interpreted as marking the lid of the low‐velocity zone, and the steeper gradients are attributed to shear heating. Mechanical considerations suggest, however, that shear heating is likely to be negligible for viscosities and strain rates expected at the physical conditions of the inflections, and this suggestion seems to be borne out by reequilibration paths deduced for partially recrystallized specimens. With appropriate corrections applied to new and existing data for solid solution effects on temperature and pressure determinations the inflections disappear and are interpreted here as artifacts reflecting variations in the degree of depletion with depth. The oceanic and continental pyroxene geotherms derived are in accord with those estimated on the basis of other geophysical and geochemical observatio

ISSN:0148-0227    

DOI:10.1029/JB080i023p03349

年代:1975

数据来源: WILEY

摘要:

ScAlO3has been synthesized at 1000°C and 120 kbar as an orthorhombic perovskite with unit cell dimensionsa= 4.933 ± 0.003 Å,b= 5.226 ± 0.003 Å, andc= 7.193 ± 0.005 Å. Ions as small as 0.73 Å in radius can thus occupy the A site of this ABO3structure type. Refinement of the structure from X ray powder diffraction data shows that the oxygen atom coordination is fourfold and fivefold, the average Al‐O distance is 1.93 Å, and Sc3+has six near‐neighbor oxygen atoms at an average distance of 2.12 Å and two more at 2.64 Å to give an average eight‐coordinate distance of 2.25 Å. It is inferred that both Mg2+and Fe2+, with radii of 0.72 and 0.77 Å, respectively, as well as Mn2+(0.82 Å) and Ca2+, could occupy the A sites of silicate perovskites to give mantle assemblages denser than those available to any other known structural type. These densities are deduced from the systematics of perovskite unit cell volumes, and by comparison with Hugoniot data for shocked high‐pressure assemblages they are consistent with enstatite, bronzitite, fayalite, and dunite compositions forming perovskite or perovskite assemblages a

ISSN:0148-0227    

DOI:10.1029/JB080i023p03363

年代:1975

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB080i023p03371

年代:1975

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB080i023p03373

年代:1975

数据来源: WILEY