ISSN:0148-0227    

DOI:10.1029/JB084iB01p00145

年代:1979

数据来源: WILEY

摘要:

In this paper we elaborate on suggestions in the recent literature that the mantle convects uniformly through its entire depth. The main novel feature introduced here, which leads to a satisfactory account of earth temperatures, it the assumption of a thermal boundary layer at the mantle. Such a layer is produced by the contrast of thermal conductivities of core and mantle if, as is now generally believed, the effects of radiative heat transfer in the mantle are small. On assuming in agreement with much current geochemical thinking that the core is mainly a solution of FeS in Fe, it is possible to estimate crudely the temperature of the core‐mantle boundary as 4000±500°K. The temperature curve in the mantle can be approximated by adding two boundary layer temperature drops that can be calculated to the adiabat in between; this is also calculable, and the sum of these three agrees roughly with the numerical value just given. It has long been known that the Rayleigh number of the mantle is so large as to make convection likely. Lately, Golitsyn has introduced a scaling analysis that allows one to express the depth of the convective zone as a function of known parameters; this yields a depth in rough agreement with the depth of the entire mantle. Finally, we discuss the likelihood that mantle convection has been going on through the entire life of the earth, beginning with the early formation of the core; this has obvious geological implicati

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00147

年代:1979

数据来源: WILEY

摘要:

We made 29 in situ doorstopper strain relaxation measurements distributed among eight sites spaced on a 35‐km transect running from the foothills of the San Gabriel Mountains, across the San Andreas fault, into the western Mojave desert southeast of Palmdale, California. This was a pilot study to test if such measurements can detect the regional stress field and any modification of it in a tectonically complex area. Strain was measured by overcoring strain gauge rosettes which had been bonded to the flattened bottom of a shallow borehole. Measurements of stress were repeatable at each site. We found NNE trending maximum compressive stress (σ1) at our sites farthest from the fault. This orientation is parallel to the σ1inferred from fault plane solutions of major southern California earthquakes and compares favorably with deep hydrofracture stress measurements made near our sites. Nearer the San Andreas fault, the orientation of σ1was approximately east‐west north of the fault, and northwest‐southeast to northsouth south of the fault. The repeatability of measurements at a site and the favorable comparison of our measurements with Tullis' [1977] near‐surface stress measurements suggest that we reliably determined the stress field orientation present at a site. It was not possible, using the available data, to distinguish the stress caused by residual or topographic effects from tectonically applied stress or to account for modification of the stress field by decoupling across

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00156

年代:1979

数据来源: WILEY

摘要:

The horizontal and vertical components of gravity change when tectonic stresses deform the earth because mass is redistributed relative to the gravity meter. We analyze the change in gravity resulting from deformation in a homogeneous elastic half‐space. We derive expressions in closed form which give the change in horizontal and vertical components of gravity measured at the surface for any specified distribution of dislocations at depth. For example, the change in the vertical component of gravity observed by a gravity meter fixed in space above an infinitely long thrust fault is found to be proportional to the local change in height, whereas the change due to a spherically symmetric source of dilatation is zero. Analysis of the change in the horizontal component shows that error in measurements of uplift resulting from changes in level is negligible for these source

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00165

年代:1979

数据来源: WILEY

摘要:

A review of geodimeter measurements made along the ‘big‐bend’ section of the San Andreas fault in southern California indicates no significant increment in strain during the period of major uplift (late 1959 to mid‐1963). Specifically, no evidence of an increment in compressional strain normal to the San Andreas fault at the time of the uplift was found. Geodolite measurements at four networks along the big bend independently indicate that the strain rate during the 1974–1977 episode of subsidence was essentially a uniaxial north‐south compression at the rate of about ⅓ μstrain/yr. Whether the 1974–1977 rate is significantly different from earlier rates determined by triangulation is not clear owing to a rather large variability in the earlie

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00171

年代:1979

数据来源: WILEY

摘要:

Downhole physical property measurements obtained using standard oil field logging tools in ocean crust hole 396B (leg 46) of the Deep Sea Drilling Project indicate that for the upper 200 m of oceanic layer 2 near the mid‐Atlantic ridge in situ sonic velocities, densities, and electrical resistivities are significantly lower and the porosities significantly higher than laboratory measurements on the recovered basalts. In situ sonic velocities vary from about 1.5 to about 4.8 km/s with a mean of about 3.1 km/s. Densities vary from about 1.55 to about 2.6 g cm−3. Porosities are unexpectedly high, ranging from about 13% to about 41%. Electrical resistivities vary from about 5 to about 90 ohm‐m. The mean sonic velocity is typical of values obtained for the upper part of oceanic crust of the age at this site (middle Miocene) using marine seismic refraction techniques. Laboratory values for small ocean ridge basalt samples are typically much higher, about 6 km/s. The agreement between refraction and logging velocities, combined with the lack of radial variation in electrical resistivity away from the hole, indicates that formation damage due to drilling is relatively minor and that the values obtained by logging more nearly reflect the true in situ values than laboratory measurement. The major lithologic boundaries can be detected using each of the tools, and combinations of tools can differentiate between rock types. The high in situ porosity and relatively low in situ electrical resistivity imply that this part of the crust contains extensive large scale porosity and is probably quite permeable. Most of the porosity probably occurs between uncemented or weakly cemented pieces of pillow basalt, which makes up most of the upper crust at this

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00178

年代:1979

数据来源: WILEY

摘要:

In this work, plausible ocean bottom structures have been represented by stacks of homogeneous, isotropic, nonabsorbing, plane horizontal layers. Generalized ray theory was used to calculate the seismic responses of these bottom models in a frequency band of 1–30 Hz for a receiver near the ocean's surface and located 2–20 km away from a sound source also near the ocean's surface. The models' responses depend most strongly upon the velocity structure of the basement rock and on the presence or absence of a zone of partial lithification between unconsolidated sediments and basement. The thickness and velocity gradients in the unconsolidated sediments have a smaller effect on the models' seismic responses. Joint consideration of both basement head wave and reflection amplitudes constrains basement velocity structure much more than separate consideration of either phase, thus emphasizing the importance of accurate recording and careful analysis of bottom reflections as well as refracted arrivals in marine refraction profiles. The remainder of the step responses and seismograms are available on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J79‐001; $1.00. Payment must accompany

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00189

年代:1979

数据来源: WILEY

摘要:

Compressional‐wave velocities have been measured for propagation directions parallel and perpendicular to bedding in 11 calcareous deep sea sediments at confining pressures to 1.0 kbar. The samples, recovered from subbottom depths of 0.39–0.75 km in the western South Atlantic on DSDP leg 39, range in wet bulk density from 1.83 to 2.30 g/cm−3and in porosity from 23 to 48%. The sediments exhibit significant velocity anisotropy at all pressures, with velocities in the bedding plane higher than those measured in the vertical direction. At 0.1 kbar, horizontal velocities (Vh) range from 1.80 to 2.99 km/s, while vertical velocities range from 1.65 to 2.54 km/s. Corresponding ranges of ΔV = (Vh‐Vv) and anisotropy (A = 2(Vh‐Vv)/(Vh+Vv)) are 0.10–0.45 km/s and 5.2–16.3%, respectively. Both ΔV and A increase markedly with depth of burial: ∂ΔV/∂z = 0.62 km/s/km, and ∂A/∂z = 19%/km. The failure of anisotropy to decrease with increasing confining pressure suggests that this phenomenon is not produced by the alignment of cracks. However, the elastic properties of calcite are such that an alignment ofCaxes perpendicular to bedding would produce the observed velosity distribution. Suggested mechanisms for producing the fabric are: (1) the alignment of certain microfossils such as Discoaster during compaction, (2) epitaxial growth of aligned forms during diagenesis, and (3) recr

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00205

年代:1979

数据来源: WILEY

摘要:

A combination of electrical sounding methods has been used to study the vertical resistivity structure of the crust on the southern extension of the Canadian Shield in northern Wisconsin. Direct current dipole‐dipole resistivity soundings were made at transmitterreceiver separations of 1 m to 40 km. Electromagnetic transient soundings were made at ranges of 5 to 40 km and with frequency bandwidths of about 0.5 to 10 Hz. The soundings were made in a region where the gross subsurface structure is laterally uniform, so horizontal, plane‐layered models were used to interpret the data. Layered earth models were randomly generated and tested against the observed data to determine the range of models that fit the data. A four‐layer model with the following range for the resistivities and thicknesses fits the data: (1) A surface layer, comprised mainly of glacial till, has a few hundred ohm meters resistivity down to depths of a few tens of meters. (2) A bedrock layer has a resistivity in the range of 3000 to 7000 ohm m down to depths of 4.5 to 11 km. (3) A deeper, high‐resistivity layer has resistivities of greater than 100,000 ohm m down to depths of 14 to 22 km. (4) A lower layer has resistivities of from 50 to 1500 ohm m. The interpreted resistivities down to depths of about 5 to 10 km are consistent with resistivities of wet rocks containing microfractures. Between 5 to 10 km and 15 to 20 km, we interpret much higher resistivities, which may indicate fewer original microfractures or healed microfractures at these depths. The low resistivities interpreted at depths below 15 to 20 km cannot be explained by usual models for the temperature and composition of the lower crust. The low resistivities can be explained by a higher temperature than is usually postulated for the lower crust, or by water of hydration in th

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00212

年代:1979

数据来源: WILEY

摘要:

We report geodetic and astrometric results from the analysis of fringe frequency observations from a series of three long base line interferometry (LBI) experiments carried out in 1973 between the 46‐m antenna of the Algonquin Radio Observatory, Lake Traverse, Canada, and the 2‐m antenna at Chilbolton Field Station, Chilbolton, England. The rms deviation from the mean of the estimates of the length and orientation of the 5251‐km equatorial component of the base line from all three experiments is 1.05‐m and 0.015″, respectively. The experiments also yielded positions of five extragalactic radio sources. The reported positions, each of which is from only a single experiment, have uncertainties of about 0.2″ in declination (except for low declination sources) and about 0.01 s in right ascension. The LBI determination of the length and orientation of the equatorial component of the base line is compared to the corresponding values derived from Naval Weapons Laboratory 9D (NWL‐9D) coordinates for the antennae. The two length measurements agree in scale within quoted experimental errors, however, the NWL‐9D coordinate frame is found to be rotated 0.867″±0.1″ to the east relative to the average terrestrial frame of the Bureau International de l'Heure (BIH), (LBI coordinate frame). This is in good agreement with the expected misalignment of 0.65″±0.2″. The differences in the rates of the clocks used at each end of the base line were also determined and compare

ISSN:0148-0227    

DOI:10.1029/JB084iB01p00229

年代:1979

数据来源: WILEY