摘要:

The petrology, bulk chemistry, and Sr‐isotopic chemistry of ultramafic rocks from the equatorial mid‐Atlantic ridge indicate that: (1) the ultramafics are derived from the upper mantle; (2) they are not genetically related to oceanic gabbros and basalts; (3) they are residual and were depleted of lithophile elements at some early stage in the history of the earth; (4) they are similar to alpine‐type peridotites from the continents. These results suggest that a zone of alpine‐type residual peridotitic material exists in the upper mantle beneath the mid‐Atlantic ridge. This zone is tentatively identified with the anomalous, low‐density mantle body observed beneath the ridge in seismic and gravity profiles. The alpine‐type residual peridotitic upper mantle beneath the ridge was originally part of a layer of ‘continental’ mantle located below a pre‐Atlantic rift super‐continent (Pangea); it constituted the material left over from the differentiation of the sialic body from the primitive mantle. Upon rifting of the sialic body, the upper mantle ‘continental’ layer is carried along by convection currents except for a central block caught in a stagnant zone at the divergence of the two rising limbs; such block is left behind and is presently beneath the mid‐Atlantic ridge; fragments of it become exposed by upward intrusion. The stagnant ‘continental’ mantle should be present below the Atlantic and Indian ridges, which originated by rifting of continental blocks, but not below the East Pacific rise, because the latter probably initiated in a ‘protooceanic’ area. This hypothesis may help explain contrasting features of the two types of ridges; for instance, why a larger heat‐flow anomaly is associated with the East Pacific rise than wi

ISSN:0148-0227    

DOI:10.1029/JB076i017p03825

年代:1971

数据来源: WILEY

摘要:

A marine geophysical survey in the southwestern Barents Sea reveals an over‐all sequence of layered sediments. In the main part of this area relatively high seismic velocities (2.5–3.0 km/sec) occur near the sea floor, but a low‐velocity (1.85–2.2 km/sec) sedimentary wedge exists near the shelf edge. The low‐velocity section is believed to consist of material deposited during the Cenozoic when the Barents shelf was uplifted. However, the main portion of the sediments seem to be consolidated, of Mesozoic and Paleozoic age. Magnetic and gravity data suggest an irregular basement relief in the west and a smoother and probably deeper basement toward the east. The latter is associated with the Riphean structural complex, whereas the western area may reflect a continuation of the Ca

ISSN:0148-0227    

DOI:10.1029/JB076i017p03832

年代:1971

数据来源: WILEY

摘要:

The linear relation between heat flow and heat production suggests that in a gross sense the vertical distribution of crustal heat production beneath granitic rocks has a simple generalized form. Knowledge of the vertical gradient of heat production near the surface would permit selection between alternative simple models. However, attempts to determine a generalized gradient from heat‐production measurements from boreholes are complicated by the occurrence of inhomogeneities on all observable scales. Variations in heat production typically observed on the hand‐sample scale preclude meaningful estimates (even of the sign) of the generalized gradient in holes a few hundred meters deep, the depth typically drilled for heat‐flow‐heat‐production measurements. In holes 3 km deep, the uncertainty in the gradient due to small‐scale perturbations is generally reduced to acceptable levels with 100 or so samples. However, perturbations with wavelengths greater than 1 km and amplitudes sufficiently small to permit the linear heat‐flow relation, can still preclude meaningful estimates of gradient if the phase is unfavorable. Confident determinations of the trend of heat production with depth in granitic rock will require observations in several holes to depths of a few kilometers or in very large numbers of holes drilled to

ISSN:0148-0227    

DOI:10.1029/JB076i017p03842

年代:1971

数据来源: WILEY

摘要:

Vertical gradients of heat production were estimated from measurements on samples from eight boreholes in granitic rocks. Samples contained from 10 to 100 specimens, and the bore‐hole depths range from 350 meters to 3 km. For holes less than 1 km deep the scatter of results is consistent with theoretical expectations based on known small‐scale inhomogeneities. Gradient estimates from the deeper holes are not strongly influenced by such inhomogeneities, but their uncertainty cannot be evaluated statistically because of the possible effects of small perturbations of longer wavelength. The data suggest a general decrease in heat production with depth in the granitic rocks of a magnitude consistent with the exponential source model, although alternate models are not precluded. A 3‐km hole in schist did not exhibit this trend, consistent with the view that upward concentration of sources occurred in the presence of a melted phase. As uncertainties in the numerical results are large and difficult to evaluate, many more analyses will be needed to establish vertical heat‐production trends with con

ISSN:0148-0227    

DOI:10.1029/JB076i017p03852

年代:1971

数据来源: WILEY

摘要:

Body and surface waves for a shallow shock that occurred on November 25, 1965, in the middle of the Nazca plate indicate a double‐couple dip‐slip source with a horizontal pressure axis in the east‐west direction. This suggests that the Nazca plate is being compressed in the direction of plate motion. A focal depth of 9 km below the ocean bottom was precisely determined from a combination of body‐ and surface‐wave data. For a dip‐slip source, the azimuthal radiation pattern for Rayleigh waves is strongly frequency dependent, and this characteristic can be used for an accurate focal depth determination. The maximum effect of the continental margin on surface‐wave amplitude was estimated assuming conservation of energy without reflections or changes in mode. Love waves are potentially more affected than Rayleigh waves by the continental margin. The effects on Rayleigh‐ and Love‐wave amplitudes due to the continental margin were measured between Ga

ISSN:0148-0227    

DOI:10.1029/JB076i017p03861

年代:1971

数据来源: WILEY

摘要:

Samples from 24 different spots in the soil surrounding the group of meteorite craters at Henbury, Australia, were collected for a study of the nature and distribution of meteoritic debris dispersed by the impact of the Henbury iron meteorites. The magnetic fraction of the soil samples was extracted, and representative magnetic particles were examined, sectioned, and analyzed chemically by electron‐microprobe techniques. The particles that contained meteoritic material were almost exclusively in the form of microscopic spherules. All analyzed spherules were found to be made up of a mixture of elements from the soil and elements from the meteorite. From the known iron‐to‐nickel ratio of the main meteorite, it is possible to estimate the total fraction of meteoritic material in each of the particles analyzed. This fraction varies and is found to be dependent on the position with respect to the meteorite craters. The minimum mass of meteoritic material in these spherules in the soil surrounding the craters is estimated to be at least

ISSN:0148-0227    

DOI:10.1029/JB076i017p03880

年代:1971

数据来源: WILEY

摘要:

A structural study was made of meteorite crater no. 9 at Campo del Cielo, Argentina, by excavating it completely. Five tons of meteorite and iron‐shale fragments were found with the sediments within the partly refilled crater. The reconstructed true crater was elongated parallel to N 81°E and had an asymmetric extension of the crater floor southeastward from the low point. Greatest dimensions of the true crater were 25.7×20.0 meters, with a depth of 5.5 meters. Its excavated volume was 928 m3. The meteorite that formed the crater impacted at an angle of 22° with the horizontal. Energy of formation of crater 9 was 3.2×1017ergs, related to projectile energy by the sine of 22°. Part of the crater‐forming meteorite is almost certainly missing from the crater. The missing mass may be as great as ten to fifteen tons, and until this is found, the impact velocity cannot be established unequivocally. From the mass that was recovered, however, it is known that the impact velocity could not have been greater than 5.

ISSN:0148-0227    

DOI:10.1029/JB076i017p03896

年代:1971

数据来源: WILEY

摘要:

Two paleomagnetic studies of the Lisbon basalts by N. D. Watkins and A. Richardson and by R. Van der Voo bearing on the problem of the rotation of the Iberian Peninsula have lately been criticized with tectonic and paleomagnetic arguments. It was therefore concluded that a renewed study based on a new collection of samples might settle some points of discussion. As was reported previously, the remanent magnetizations of these Eocene lavas are of normal polarity only. An unfolding test proves the characteristic magnetizations to be pre‐folding. From 176 samples, a mean direction of magnetization has been found,D:352°,I:+40°, and α95:3°, giving unit weight to sites (N=33) in the statistical analysis. A comparison shows that there is such a slight difference between the paleomagnetic Late Cretaceous and Eocene data from Portugal and those from stable Europe, that any post‐Eocene rotation of the Iberian Peninsula cannot have been more than a few degrees. This conclusion does not conflict with marine geophysical work reported

ISSN:0148-0227    

DOI:10.1029/JB076i017p03913

年代:1971

数据来源: WILEY

摘要:

Several fundamental mechanical and optical principles involved in the design of automatic leveling instruments are reviewed. Optical‐mechanical relationships in the compensators of various automatic leveling instruments are used in the solution of a differential equation involving changes in gravity intensity and angles of tilt of the compensator support. A table is developed from the derived formulas, showing the errors to be expected from, a combination of various angles of tilt with several types of compensators, as influenced by variation in gravity intensity. Suggestions are presented for minimizing this source of error in the manufacture and utilization of automatic leveling instrument

ISSN:0148-0227    

DOI:10.1029/JB076i017p03922

年代:1971

数据来源: WILEY

摘要:

Formal solutions are given for the integral equations governing the mass or unidirectional magnetic dipole densities on equivalent strata. The densities are given in terms of infinite series in the derivatives of the potential fields observed on planes parallel to the strata. In the magnetic case, the formal solution applies to dipoles with moments normal to the strata. A truncation of the series leads to an approximation method that is applicable to the longwave components of the densities. An iterative approximation method given for the case of oblique field intensities and moments is applicable when the field and source vectors deviate little from the vertical.

ISSN:0148-0227    

DOI:10.1029/JB076i017p03932

年代:1971

数据来源: WILEY