摘要:

A technique for determining relative ages of regions of the lunar surface using orbital photography was developed from a model of small‐impact erosion. The erosion model relates the shape of a crater to the integrated flux of debris that has impacted the surface since that crater was fresh. The shape of the most modified crater of a particular diameter is thereby related to the relative age of the surface. Application of this analysis to orbital photography reveals that the major mare units vary in the accumulation of impacts by more than a factor of 3. Comparison of these data with crystallization ages determined from samples collected during the Apollo 11 and 12 missions indicates that the impact fluxes were decreasing during the stages of mare formation. An exponentially decaying flux for the last 3.5 b.y. with a half‐life of 0.6 to 1.4 b.y. is compatible with the data. The data indicate (1) that the mare units sampled by the Apollo 11 and 12 missions represent major mare units of old and moderate age, respectively; (2) such major mare volcanism lasted on the order of 0.5 b.y. after the Apollo 12 basalts crystallized 3.3 b.y. ago; and (3) certain small localized volcanic features may be considerably younger (≳2 b.y.). The major mare unit at the Apollo 15 landing site (Hadley‐Apennine) is expected to have a crystallization age of 3.5 ±0.1 b.y., whereas the crystallization age of the youngest unit in the Marius Hills region (a small mare ridge) is expected to have an age of 1 to 2 b.y., depending on the details of the impact flux

ISSN:0148-0227    

DOI:10.1029/JB077i002p00279

年代:1972

数据来源: WILEY

摘要:

The delay‐time‐function method of analyzing seismic refraction data assumes that the configuration of the boundary between the upper layers and the basal refractor can be described by a simple mathematical function of position; e.g., a combination of polynomials and Fourier series. Lateral velocity variations and velocity anisotropy in the refractor can also be included in the analysis. Proper allowance for shot and receiver offset is made by using an iterative technique. Application of a two‐dimensional version of the method to the 1963 Lake Superior seismic data yields results comparable with those of previous investigators. Differences between previous structural interpretations are in some cases caused by the use of different data sets; in other cases these structural differences are caused by disparities in the offset allowance. The Gambler seismic profile, the southern part of which lies along the axis of the Midcontinent gravity high in Wisconsin, has delay times of 6.0 sec or greater beneath the gravity high and an abrupt 2.5‐sec change at the northern extremity of this gravity high. Structural complexities and the scant velocity‐depth information prevent precise conversion of these delay times to depths. However, estimates are that the mantle is possibly 60 km deep beneath the gravity high, decreases in depth, possibly by faulting, 15 to 25 km at the northern edge of the high, and is typically about 40 km deep beneath the western part of

ISSN:0148-0227    

DOI:10.1029/JB077i002p00297

年代:1972

数据来源: WILEY

摘要:

A compilation of the best available elasticity data on single crystals and polycrystalline aggregates shows that the ‘Voigt‐Reuss‐Hill average’ is neither proven nor disproven experimentally. Because the average of the bounds has no theoretical significance, the true moduli may lie anywhere between them. The ambiguity is removed by Kröner's recent theory for a perfectly disordered aggregate, which specifies a unique modulus for an idealized geometry. Theoretical calculations on aggregates of geophysically interesting crystals, and on mixtures of such crystals (i.e., rocks), shows that the ‘perfectly disordered’ theory differs numerically from the VRH average. In cases where this difference is small, the VRH average is thereby given some theoretical support; however, in certain situations of interest, it is large (e.g., in a highly porous rock, where data confirm Kröner's theory). The seismic parameter Φ of a multicomponent aggregate is not equal to the molar‐averaged Φ's of the components; the difference is significant for mixtures of the simple oxides: stishovite, MgO, wüstite. The bulk modulusKof an aggregate, like its shear modulus, depends on the time scale of the experiment that determines it. The short‐term (acoustic)Kis Kröner'sKeff; the long‐term (static)Kis Reuss'KR, which is ≤Keff. Hence the seismic parameter Φ and the density gradient within the earth are only indirectly related. Where arguments are given to show the near‐equality of short‐ and long‐term Φ's, the conclusion follows that the material cannot contain major fractions of widely different p

ISSN:0148-0227    

DOI:10.1029/JB077i002p00315

年代:1972

数据来源: WILEY

摘要:

The 375°C bump appearing in the induced thermoluminescence (TL) glow curve of natural quartz was examined by an isothermal annealing technique. The thermal stability of this bump was found to vary over different parts of the bump. The extrapolated half‐lives of the TL peaks at room temperature were found to be unsuitable for dating a Precambrian sample. The possible errors introduced in a previous report by Kual et al. are discuss

ISSN:0148-0227    

DOI:10.1029/JB077i002p00328

年代:1972

数据来源: WILEY

摘要:

An excited oxygen gas has been used to oxidize pure magnetite at temperatures between 50°C and 200°C. The oxidation products are either maghemite or hematite, depending on the water content of the initial material. In all cases the intensity of remanent magnetization, ARM or TRM, is reduced on oxidation with only small changes in the microscopic coercive force spectra. A chemical remanent magnetization, which is relatively weak with respect to the initial remanence, is always acquired during oxidation. These results are useful in the interpretation of intensity changes in some linear magnetic anomalies of ocean floors and in the interpretation of changes in direction and intensity of remanence in land outcrops that have undergone low‐temperature oxidat

ISSN:0148-0227    

DOI:10.1029/JB077i002p00334

年代:1972

数据来源: WILEY

摘要:

Study of the eruptions of three geysers located in two regions of the United States shows clearly that the interval between eruptions of a geyser is strongly influenced by variations in gravity due to earth tidal forces, variations in barometric pressure, and the buildup and release of tectonic strains usually associated with earthquake activity. Barometric pressure and tidal forces, both of which vary periodically, induce corresponding periodic fluctuations in the interval between eruptions, both increasing barometric pressure and increasing variation in gravity shortening the interval. The aperiodic tectonic strains associated with earthquakes produce consistent and repeatable patterns of changes in eruption intervals.

ISSN:0148-0227    

DOI:10.1029/JB077i002p00342

年代:1972

数据来源: WILEY

摘要:

Frequency and amplitude characteristics were studied for microearthquakes recorded on a local network of vertical‐component seismographs in the Fairview Peak area of central Nevada. Maximum trace amplitude of the events decreased with focal distance approximately asR−1.9. Site amplification, by a factor of 2, was observed for the only station not located on rock. Comparison of observed spectra for these events with theoretical spectra given by Brune indicates stress drops ranging from 0.04 to 0.60 bar. Calculated source dimensions and seismic moments for microearthquakes in this area are in agreement with results of earlier studies. When location of the events is considered, the variation in stress drop appears to be related to differences in the faulting mechani

ISSN:0148-0227    

DOI:10.1029/JB077i002p00351

年代:1972

数据来源: WILEY

摘要:

A calculation of the viscosity of the core at the boundary of the inner and outer core that supposes that boundary to be a melting transition and uses the Andrade hypothesis sets limits of 3.7<η<18.5 cp. The corresponding kinematic viscosities are such that the precessional system explored by Malkus would be unstable. Whether it would be sufficiently unstable to overcome a severely subadiabatic temperature gradient cannot be determined

ISSN:0148-0227    

DOI:10.1029/JB077i002p00360

年代:1972

数据来源: WILEY

摘要:

An attempt was made to detect cosmic‐ray‐produced14C in australites and far‐east tektites. No14C activity was found outside of statistical fluctuations. If a terrestrial age of 104years is assumed for australites, a maximum cosmic‐ray exposure time of 103years is obtained for these tektites on the basis of a 2‐σ statist

ISSN:0148-0227    

DOI:10.1029/JB077i002p00367

年代:1972

数据来源: WILEY

摘要:

Compressional‐ and shear‐wave velocities to 10 kb and the chemistry for naturally occurring polycrystalline aggregates of magnesite MgCO3, siderite FeCO3, and rhodochrosite MnCO3are reported. Velocity‐density relationships for the magnesite‐siderite series show a dependence on mean atomic weight similar to that observed for the olivine series. Iron substitution for magnesium in carbonates decreases the shear modulus and increases Poisson's ratio. The bulk modulus, however, changes relatively little with iron substitution. Manganese substitution, in carbonates affects the velocities and elastic parameters in much the same way

ISSN:0148-0227    

DOI:10.1029/JB077i002p00369

年代:1972

数据来源: WILEY