摘要:

The results of a reconnaissance bathymetric and magnetic survey of the mid‐Atlantic ridge between 42° and 45°N latitude show a symmetrical north‐south magnetic pattern parallel to the axis over the two southernmost of six profiles across the ridge. The southern profiles are also characterized by a well developed central rift valley. The four northern profiles do not contain obvious magnetic symmetry elements and are over a portion of the ridge characterized by rugged topography without a central rift. A detailed survey made over the ridge crest between 42°30′ and 43°30′N spanned the region of apparent change. Here a narrow zone transverse to the ridge axis shows abrupt termination of the symmetrical north‐south magnetic trends and central rift valley of the southern area. Dredged rocks and heat‐flow measurements in the two distinct areas also differ markedly. Basalts and relatively low heat flow (<0.9 μcal/cm2sec, two observations) characterize the southern area, even in the median valley, whereas serpentinized peridotites and very high heat flow (3–7 μcal/cm2sec, four observations) were observed in the northern area. The contrasting nature of the magnetic, topographic, petrologic, and heat‐flow character in the northern and southern areas suggests that the ridge crest at 43°N latitude is interrupted by a complex fracture zone. Model studies suggest that the configuration of magnetic bodies that could account for the symmetrical magnetic pattern of the southern area may be interpreted in terms of the Vine‐Matthews hypothesis of seafloor spreading. A change in spreading rate is suggested to have occurred 4 m.y. ago. However, it is difficult to identify specific magnetic features, and several models can be fitted to the observed profile. The determination of spreading history here may not be possible from t

ISSN:0148-0227    

DOI:10.1029/JB074i012p03069

年代:1969

数据来源: WILEY

摘要:

Several refraction profiles on the outer ridge north of Puerto Rico, combined with earlier studies, support the presence of an east‐west trending structural high or welt on the surface of the mantle. The determinations of shallow mantle depths lie along an east‐west line north of a closed free air (20 mgal) gravity high. The trend of the postulated ridge parallels the axis of the Puerto Rico trench and intersects the northwest‐trending magnetic lineations reported by other investigators at an angle o

ISSN:0148-0227    

DOI:10.1029/JB074i012p03082

年代:1969

数据来源: WILEY

摘要:

Seismic refraction measurements of the compressional wave velocity of the upper mantle, obtained in the northeast Pacific Ocean between California and Hawaii, have shown a dependence on direction indicating anisotropy of velocity with a maximum of 8.6 km/sec in a roughly easterly direction and a minimum of 8.0 km/sec. In order to test whether or not these observations represent true anisotropy, rather than geographical variation of velocity, special anisotropy experiments were conducted at two locations centered roughly at 31°N, 121°W and 35°N, 126°W, respectively. At each of these locations significant anisotropy was observed, agreeing in direction with the apparent anisotropy indicated for the entire region. The magnitude of the velocity difference, 0.3 km/sec, on any single station is half the over‐all variation of 0.6 km/sec. These observations do not prove the hypothesis of anisotropy throughout the entire region from California to Hawaii. However, they are consistent with this hypothesis and give encouragement to similar observations else

ISSN:0148-0227    

DOI:10.1029/JB074i012p03095

年代:1969

数据来源: WILEY

摘要:

Theoretical modeling shows that within the uncertainty of present measurements, Mars appears to have an iron core that has a radius in the range of 790 to 950 km and a mass in the range of 2.7 to 4.9% of the total mass of the planet. The composition of the Martian mantle probably does not differ greatly from the composition of the terrestrial mantle. The temperature of the interior of Mars may be between 800° and 1500°C, considerably lower than the temperature of the interior of the earth. The modeling was based on recent findings on the composition and structure of the terrestrial mantle and the very accurate determinations of the radius and mass of Mars by the Mariner 4 Mars probe. The implications of the results are discussed in relation to the physical condition and history of Mar

ISSN:0148-0227    

DOI:10.1029/JB074i012p03110

年代:1969

数据来源: WILEY

摘要:

During October–November 1966 900,000 m3of olivine basalt flowed from the flank of Izalco volcano, El Salvador. The total heat energy was approximately 1015calories. No measurable changes in gravity occurred at stations on the active cone between August 1964 and August 1967. In the summit crater fumaroles have surface temperatures as high as 540°C. The cooling rate of these fumaroles was 18°C/yr before the eruption and 45°C/yr after. Yearly temperature cycles due to wet and dry seasons are superimposed on the general cooling trend. The rate of gas emission at four fumaroles in November 1967 was 86 g/sec. The data from fumaroles and the volume of the flank eruption indicate that the volume of the high‐level magma storage beneath the crater was 3.8×106metric tons before the eruption and 1.4×106metric tons after. Four of the larger hot fumaroles contribute at least 10% of the heat loss from the high‐level magma storage, whereas heat conduction accounts for more than half the

ISSN:0148-0227    

DOI:10.1029/JB074i012p03119

年代:1969

数据来源: WILEY

摘要:

An observational study was made to determine the spectral reflectivity differences (color differences) on the lunar surface in the extended visible wavelength region. A 21‐filter (0.4–0.8 μ), double‐beam photoelectric photometer was designed, constructed, and used to observe differentially eighty‐three areas (some areas being observed many times) to an accuracy of 0.1 to 0.3%. Many color variations were observed up to 10% with some to about 60%. Relative color was dependent on phase angle, but there was no temporally varying luminescence. The broad‐band shape on spectral reflectivity curves suggested absorption features and possibly some less broad, low‐amplitude (0.2–0.5%) humps. Spectral curve shape appeared dependent on lunar morphology, but there was no universal dependence of color on brightness. These results are interpreted as indicating that color differences are caused mainly be compositional differences and that the shape of spectral reflectivity curves may give some indication of the rock and mineral composition of the

ISSN:0148-0227    

DOI:10.1029/JB074i012p03131

年代:1969

数据来源: WILEY

摘要:

A paleomagnetic investigation of the middle Oligocene (29.6 m.y.) Marys Peak sill (44°30′N, 123°35′W) demonstrates a relationship between the intrusion's cooling history and its remanent magnetization. The 390‐meter‐thick sill is vertically differentiated, varying in compositon from a gabbro to a diorite with associated pegmatite and aplite facies. From 7 to 10 cores per sampling site were taken at 26 sites separated vertically by 7 to 10 meters along a traverse through the sill. Directions and intensities of remanent magnetization were measured for all specimens at the original magnetization level and after alternating‐field demagnetization at 100−, 200−, and 400‐oe peak demagnetizing fields. Demagnetization behavior. Curie temperatures, and polished section examinations indicate that the remanence is stable. The directions of natural remanent magnetization (NRM) at individual sites are well grouped and significantly different from the present field. The site mean direction pattern changes systematically fromD≅ 20°,I≅ 50°, in the lower portion toD≅ 10°,I≅ 50°, in the central region toD≅ 25°,I≅ 50°, at the top. A mathematical approximation of the cooling history permits the transformation from the geometric record of the sill to a time record of the earth's magnetic field that existed when the sill cooled. Several models were evaluated and the results indicate that the range of the paleomagnetic record is approximately 2000 to 6000 years. It is concluded that the change in remanent magnetization direction could have been caused by a large nondipole field component or an

ISSN:0148-0227    

DOI:10.1029/JB074i012p03143

年代:1969

数据来源: WILEY

摘要:

Seismic events recorded by the individual subarrays at LASA were Fourier‐analyzed. For any particular subarray the amplitude spectrum was divided by the average spectrum for all twenty‐one subarrays. This process produced similar spectra for events with the same epicenter, suggesting that the average amplitude spectrum is that of a ‘primary’ signal and that each subarray has a distance and azimuth‐dependent transfer function that operates on this primary signal to produce the amplitude spectrum of the subarray. The assumption is made that the phased summation of the whole array is representative of the primary signal. The time‐domain transfer function associated with a subarray is then shown to consist of a series of time‐delayed delta functions, each associated with an amplitude coefficient. In other words, theP‐wave signal recorded at a particular subarray consists of a series of closely spaced individual arrivals. The interference of these multiple arrivals with one another causes the recorded signal variations between subarrays. It is suggested that deep crustal relief causes multipathing of a P wave inci

ISSN:0148-0227    

DOI:10.1029/JB074i012p03161

年代:1969

数据来源: WILEY

摘要:

An efficient Monte Carlo scheme has been implemented to generate families of velocity models, all of which are consistent with observations ofdT/dΔ versus Δ andTversus Δ. Uncertainties in the observations are translated into uncertainties in the velocity structure when we examine the envelope of the family of models passed. As an example of the application of the Monte Carlo technique, we have inverted a set ofdT/dΔ observations ofP‐wave arrivals measured at Tonto Forest Seismological Observatory in Arizona. These observations require that the resulting compressional velocity models exhibit two major discontinuities. The uncertainty of the depth to these discontinuities is 30–40 km. Contrary to other interpretations of these data, however, we have found models either with or without a low‐velocity layer below the M discontinuity. In addition, some models contained low‐velocity zones immediately above the two major discontinuities. The width of the envelope of models varies from 0.46 km/sec at 100 km to 0.06 km/sec at 10

ISSN:0148-0227    

DOI:10.1029/JB074i012p03171

年代:1969

数据来源: WILEY

摘要:

An investigation was made to determine the sources and properties of the seismic noise in the 20− to 40‐second period range at the large aperture seismic array (LASA) in Montana. The conventional and high‐resolution frequency‐wavenumber spectra are presented for the noise, as well as coherence results. These data show that the noise consists of two components. One component propagates across the array as fundamental‐mode Rayleigh waves and is believed to be caused by the action of surf on coastlines. The other component is nonpropagating, or incoherent, noise; some of its properties will be

ISSN:0148-0227    

DOI:10.1029/JB074i012p03182

年代:1969

数据来源: WILEY