摘要:

The Apollo 14 lunar landing provided a greater amount of information on the mechanical properties of the lunar soil than previous missions because of the greater area around the landing site that was explored and because a simple penetrometer device, a special soil mechanics trench, and the modularized equipment transporter (Met) provided data of a type not previously available. The characteristics of the soil at shallow depths varied more than anticipated in both lateral and vertical directions. While blowing dust caused less visibility impairment during landing than on previous missions, analysis shows that eroded particles were distributed over a large area around the final touchdown point. Measurements on core‐tube samples and the results of transporter track analyses indicate that the average density of the soil in the Fra Mauro region is in the range of 1.45 to 1.60 g/cm3. The soil strength appears to be higher in the vicinity of the site of the Apollo 14 lunar surface experiments package, and trench data suggest that strength increases with depth. Lower‐bound estimates of soil cohesion give values of 0.03 to 0.10 kN/m2, which are lower than values of 0.35 to 0.70 kN/m2estimated for soils encountered in previous missions. The in situ modulus of elasticity, deduced from the measured seismic‐wave velocity, is compatible with that to be expected for a terrestrial silty fine sand in the lunar gravitational

ISSN:0148-0227    

DOI:10.1029/JB077i029p05641

年代:1972

数据来源: WILEY

摘要:

The electrical conductivity of a synthetic basalt has been determined at atmospheric pressure in air at temperatures between 700° and 1600°C. The conductivity increases by about 2 orders of magnitude in the melting interval between 1130° and 1263°C. It is suggested that partial fusion in the seismic low‐velocity zone of the upper mantle may account for the correlation of increased electrical conductivity with development of this

ISSN:0148-0227    

DOI:10.1029/JB077i029p05665

年代:1972

数据来源: WILEY

摘要:

Seventy‐eight reliable sites in Upper Triassic diabase from southeastern Pennsylvania yield a paleomagnetic pole at 62.0°N, 104.5°E, about 0.5° from an earlier pole calculation based on a preliminary study of 20 sites. Site poles have a Fisherian distribution, with an angular standard deviation of 7.4°, considerably less than the dispersion predicted by models based on the present geomagnetic field. This distribution of site poles suggests that geomagnetic secular variation during the Late Triassic consisted mainly of dipole wobble, with little or no contribution from nondipole sources. No reversals of the field are recorded in Upper Triassic igneous rocks from eastern North America, although reversals are known elsewhere in the Triassic section. Available evidence indicates that, although North America was actively drifting during the Late Triassic and Jurassic, polar wandering during that time interval was minimal. A model based on work by E. Irving, A. Cox, P. Goldreich, and A. Toomre is used to predict a correlation between general convective overturn in the upper mantle, polar wandering, high intensities of the nondipole component of the geomagnetic field, and high frequencies of geomagnetic rev

ISSN:0148-0227    

DOI:10.1029/JB077i029p05673

年代:1972

数据来源: WILEY

摘要:

Recent geophysical data from the Iceland plateau have revealed that asymmetrical sea‐floor spreading is apparently occurring on the mid‐ocean ridge. The western (Greenland) side is spreading at an average half rate of 0.77 cm/year out to anomaly 5, whereas the eastern side is spreading at an average half rate of 0.82 cm/year. Magnetic anomalies strongly suggest an extinct spreading axis on the Iceland plateau between 10 and 30 m.y. B.P. The quiet magnetic field and seismic reflection data support the hypothesis that the Jan Mayen ridge and 75 km of sea floor to its west are continental in ori

ISSN:0148-0227    

DOI:10.1029/JB077i029p05688

年代:1972

数据来源: WILEY

摘要:

One hundred and fifty‐eight new heat‐flow measurements in and around the marginal basins of the western Pacific are presented. Most of the measurements were collected in the Melanesian area and show that the region of high heat flow and shallow crustal elevation landward of the island arcs in the western Pacific includes the Fiji plateau and Lau basin. The high heat flow and shallow crustal depth of these two provinces can be accounted for by the recent intrusion of magma by a process similar to that at midocean ridges. The mean heat flow of the south Fiji basin, the Tasman abyssal plain, and the Coral Sea basin are below 1.5 μcal/cm2sec, and the average depths of these provinces are greater than 4.5 km. These data are compatible with a simple sea‐floor‐spreading origin for these marginal basins if the process occurred more than 30

ISSN:0148-0227    

DOI:10.1029/JB077i029p05697

年代:1972

数据来源: WILEY

摘要:

High heat flow and shallow crustal depths have been observed in many marginal basins landward of the trench‐island arc system in the western Pacific. All the basins in the north‐western Pacific where adequate heat‐flow data exist, except the deep west Philippine basin, show a high mean heat flow. This is in marked contrast to the southwestern Pacific, where only the Fiji plateau and Lau basin have high heat flow. All active midocean ridges show a uniform relationship between heat flow and elevation: high and shallow where the ridge is young, and uniform and deep where the oceanic crust is old. In the absence of magnetic data in the marginal basins, this relationship has been used to determine whether or not these basins have the same thermal and dynamic structure as the midocean ridges. The heat flow and elevation data from the southwestern Pacific basins are compatible with the ridge data, whereas heat flow and elevation data from the Japan abyssal plain, Parece Vela basin, and the Shikoku basin in the northwestern Pacific have too high a heat flow for their mean depth. Convincing evidence has been presented that the marginal basins of the northwestern Pacific owe their origin and development to large‐scale crustal extension landward of the island arc system. However, the intrusion of oceanic crust associated with this extension cannot alone adequately account for the heat flow and elevation data. Three other factors are discussed that, in addition to the intrusion process, may account for the observed distribution of heat flow and crustal depth: (1) continuous incipient intrusion, which raises the ‘effective’ thermal conductivity of the crust and hence the heat flow in these basins; (2) development of a thinner lithospheric plate landward of the island arc system of the northwestern Pacific; and (3) dynamic forces acting on the upper mantle landward of the downgoing slab, decreasing the observed elevation of the marginal basins. There are problems with all three of these factors, but a model using only the second is preferred, because it is simple to develop and is compatible with rigid plat

ISSN:0148-0227    

DOI:10.1029/JB077i029p05705

年代:1972

数据来源: WILEY

摘要:

Nine sites in two Cretaceous units, the La Teta lava and the Maruayan diorite (approximately 12°N, 72°W), from the Guajira peninsula of Colombia give a virtual geomagnetic pole in central equatorial Africa. The Cretaceous pole for eight sites in La Teta lava is located at 10°N, 21°E(δp= 10°, δm= 19°). This pole is about 90° from Cretaceous poles determined by other workers for North and South America. Our results, combined with the results of other workers from circum‐Caribbean sites, are interpreted in terms of large tectonic rotations of Caribbean sites during and after Cretaceous times. A paleotectonic reconstruction of the Americas was made by superimposing Cretaceous virtual paleomagnetic poles. It is concluded that the Guajira and Greater Antilles sites occupied latitudes about 10° south of their present latitudes, possibly off northwestern South America, in Cretaceous times. Eleven other sites from two diorite bodies in the Guajira peninsula gave inconclus

ISSN:0148-0227    

DOI:10.1029/JB077i029p05720

年代:1972

数据来源: WILEY

摘要:

Fault plane solutions of earthquakes in southern Africa indicate that the least compressive stress is oriented approximately E‐W nearly parallel to that in the northern part of the rift system. Seismic moments, source dimensions, and stress drops were determined for eight earthquakes from body‐ and surface‐wave spectra by using the theory of Brune (1970). Spectral estimates of these quantities for the 1966 earthquake in the Republic of Zaire agree well with those observed in the field. Relatively higher stress drops are found for events not associated with rift faulting. If higher stress drops indicate higher rock strength, these higher stress drops, with other geological and geophysical data, suggest that the northern part of the rift system is similar to ocean ridges and behaves as a plate boundary but that the southern part is different and is not a plate boundary. The tectonics associated with the northern part of the rift system appear to be extending sout

ISSN:0148-0227    

DOI:10.1029/JB077i029p05731

年代:1972

数据来源: WILEY

摘要:

The tectonic history within a continental rift is inferred from K‐Ar ages and paleomagnetic measurements from oriented samples of basaltic dikes. The stresses required to produce the strain distribution (dikes) are consistent with extension in the plane of the earth, which varies episodically as a function of geologic time; furthermore, an average paleomagnetic pole is calculated to be consistent with the present‐day pole position and indicates that large‐scale rotation of African continental blocks is untenable. An average rate of continental crust of ∼2 cm/104years is generated over a 70‐km traverse along the western margin of the Ethio

ISSN:0148-0227    

DOI:10.1029/JB077i029p05744

年代:1972

数据来源: WILEY

摘要:

The photographic records of shallow submarine eruptions show distinctive color differences in the expanding cloud indicative of contact surfaces between different subterranean fluids initially in the volcanic pipe. A fluid dynamic model is developed that incorporates the essential features of the blast wave expansion of a high‐temperature condensible fluid supporting a vaporization wave. On the basis of this model, several necessary conditions are proposed, and these are shown to lead to the different forms that a submarine volcanism can take. Slight modification of one of these forms yields an explanation of geyser eruption

ISSN:0148-0227    

DOI:10.1029/JB077i029p05755

年代:1972

数据来源: WILEY