摘要:

Horizon A, a prominent and extensive reflective zone in the North Atlantic deep‐sea sediments, has been sampled by the Joides drilling project. Cores show that the top of the horizon is generally a synchronous zone of Early‐Middle Eocene age, and that its reflective nature is the result of the layers of chert. The horizon appears to be synchronous with a similar horizon in the South Atlantic and with horizon A″ of the Caribbean, but synchronism with horizon A′ in the Pacific is not definitely established. The great extent of horizons A and A″ suggests that Eocene chert layers may be important stratigraphic markers in marine deposits in continen

ISSN:0148-0227    

DOI:10.1029/JB075i029p05645

年代:1970

数据来源: WILEY

摘要:

Eighty‐seven new heat‐flow values have been obtained in the Caribbean Sea and the Gulf of Mexico. Values in the Caribbean are, in general, uniform; only 5 of the 59 existing values lie outside the range 1–2 μcal cm−2sec−1. Above‐average heat flow is found in the area of the Cayman trough and the Yucatan basin. The excess flux may represent energy released during shear at the boundary between the Caribbean and Atlantic crustal plates. Resolution of the heat‐flow pattern in the area of the Antilles Island arc is inhibited by too few values, some of which are of low reliability; below‐average heat flow is found east of the arc and above‐average heat flow west of it. The average heat flow in the Venezuelan basin is nearly the same as the average of all ocean basins. In the Colombia basin, heat‐flow values trend from below average in the east to above average in the west. Through most of the Gulf of Mexico, the heat flow is uniformly low. A decrease of temperature gradient with depth in the sediment is characteristic of most of the Gulf measurements, and is not completely accounted for by known processes. The area of knolls, which includes the Sigsbee knolls, has uniformly high heat flow, 2.1 μcal cm−2sec−1, which supports the hypothesis that the area is und

ISSN:0148-0227    

DOI:10.1029/JB075i029p05655

年代:1970

数据来源: WILEY

摘要:

It is shown that the upper mantle is probably unstable. The instability expresses itself in diapirism under the ridges and sinking of the lithosphre under the island arcs. Both processes exert a force on the plate that can move it against the viscous drag from the flow of the asthenosphere. Order of magnitude estimates of the forces and of the required energy demonstrate that the process is plausible and that the drift rate can be determined from the resistance the plate encounters at its edges, particularly below the island arcs where it plunges into the mesosphere. The proposed mechanism can be tested by studying, e.g., the deep structure of the ridges, the stress field in the plates, and the gravity field over ridges and trenches.

ISSN:0148-0227    

DOI:10.1029/JB075i029p05671

年代:1970

数据来源: WILEY

摘要:

Electron paramagnetic resonance (EPR) spectra of samples of the meteorites Bruderheim, Homestead, and Pueblito de Allende have been recorded for spectrometer frequencies of ∼10 and 35 GHz at room temperature. Nuclear magnetic resonance (NMR) spectra of1H,23Na,27Al, and29Si were recorded over a range of frequencies from 8 and 16 MHz at room temperature. The principal component of the EPR spectrum of a powdered‐sample, nonmagnetic fraction of the three meteorites was attributed to Fe3+, ∼1% of the total Fe present, in a crystal field with an axial component 0.1 Cm−1<|D|<0.2 Cm−1and a rhombic to axial ratio, λ, 0.2<|λ|<0.3. From the data it was deduced that Fe3+was also present in crystal fields with different values of |D| and |λ|. Spectra of two chondrule fragments from Pueblito de Allende were attributed to Mn2+in one case and to Fe3+and Cr3+in the other. However, the Cr3+was also found in matrix material. Evidence for weak ferromagnetism was deduced from the field dependence of this resonance. Analysis of the NMR spectra showed that these internal fields did not extend to or were small at the hydrogen, sodium, aluminum, and s

ISSN:0148-0227    

DOI:10.1029/JB075i029p05681

年代:1970

数据来源: WILEY

摘要:

The isotopic composition of krypton and xenon in the Leoville carbonaceous chondrite is reported for both total melt and stepwise heating experiments. The enrichment of heavy xenon isotopes in the low‐temperature fractions is paralleled by an enrichment of heavy krypton isotopes. Evidence is presented in support of simple mass fractionation as the mechanism responsible for excess heavy xenon isotopes in carbonaceous chondrite

ISSN:0148-0227    

DOI:10.1029/JB075i029p05693

年代:1970

数据来源: WILEY

摘要:

Potential field data has been analyzed in the two‐dimensional wave number domain to obtain the ratio of intensity of magnetization to the density (J/ρ). A two‐dimensional fast Fourier transform is used to obtain convolution, upward continuation, vertical and horizontal derivatives, and reduction of the total field to the pole. A coherency test is used on the two sets of data to evaluate the validity of the calculatedJ/ρ ratio for each wavelength. A high‐coherency value is assumed to arise if the gravity and magnetic anomalies are caused by the same body. A theoretical prismatic model and a field example from northern Saskatchewan illustrate the tec

ISSN:0148-0227    

DOI:10.1029/JB075i029p05702

年代:1970

数据来源: WILEY

摘要:

A systematic study of the travel times and apparent velocities of precursors of the seismic core phasePKPPKPindicate that these phases are reflections from the mantle. The strongest reflection is from a depth of 630 km. In order of confidence, other reflectors were found at depths of 280, 520, 940, 410 (very weak), and 1250 km (tentative). The weakness of the 410‐km reflection was surprising in view of the large velocity increase at this depth indicated by refraction and Love‐wave studies. This transition region must be broader than the others or must involve a smaller density jump. Reflections were observed that were possibly from the top and bottom of the low‐velocity zone at depths of 50 and 130 km, respectively. The above reflections are interpreted in terms of the following solid‐solid phase changes, in order of increasing depth: pyroxene‐garnet solid solution, olivine → β spinel, β spinel → spinel and pyroxene → spinel + stishovite, spinel → post‐spinel, and garnet → ilmenite or oxides. A spin‐spin transition in Fe2+may be responsible for one of the deeper discon

ISSN:0148-0227    

DOI:10.1029/JB075i029p05713

年代:1970

数据来源: WILEY

摘要:

A method of using surface‐wave amplitude spectra from shallow earthquakes for focal depth determination is proposed. The theoretical basis of the method is established with the aid of the normal mode theory for surface‐wave excitations caused by dislocation sources in a multilayered medium. Various practical aspects, such as the crustal thickness, the spatial and temporal behavior of the source, and the attenuation, are considered and shown to have relatively small effects on amplitude spectral shapes of surface waves as compared to the focal depth. The method requires that the fault‐plane solution of an earthquake be known beforehand and also that the length of the fault be no longer than 10 km. An uncertainty of not more than 15° in the fault‐plane solution will not seriously impair its applicability. Several earthquakes with known focal depths are used to test the proposed method. The tests show that the focal depths determined from amplitude spectra of surface waves agree excellently with those obtained from body‐wave tr

ISSN:0148-0227    

DOI:10.1029/JB075i029p05729

年代:1970

数据来源: WILEY

摘要:

This study examines space‐time patterns in the distribution of major earthquakes of the Alaska‐Aleutian seismic zone. The evidence suggests that major earthquakes of this zone tend to progress in time from east to west. Extrapolation of past trends indicates that a major Alaska‐Aleutian earthquake may occur near 56°N, 158°W between about 1974 and 1980. Three kinds of evidence indicate that earthquakes of about magnitude 7.7 and larger should be used to identify space‐time earthquake patterns in the Alaska‐Aleutian seismic zones. (1) Space‐time graphs of earthquakes of about magnitude 7.7 and larger show strong linear trends. (2) Aftershock zones of successive large earthquakes (M≥ 7.7) are approximately adjacent. (3) The direction of fracture propagation is generally away from the focal zone of the previous adjacent large earthquake, which suggests that the concentration of stress before the event was greatest near the region of the adjacent earlier earthquake. Since this pattern is reasonably consistent, the linear trends of large earthquakes in this seismic zone are probably caused by some physical phenomena rather than some unusual chance distribution. The space‐time distribution of the U.S. Coast and Geodetic Survey (CGS) epicenters for 1961–1967 suggest that past trends will continue. The epicenters show a distinct seismicity gap in the region predicted for the next major Alaska‐Aleutian earthquake. In the past, such gaps have often occurred before major earthquakes. A suggestion of linear trends is also found in the space‐time distribution of large earthquakes in some other circum‐Pacific seismic zones. Although present information of these other areas is not sufficient for making predictions, there are at least two regions that should be monitored and studied further. The Commander Islands and the northern Kuriles both appear to be likely locations for

ISSN:0148-0227    

DOI:10.1029/JB075i029p05745

年代:1970

数据来源: WILEY

摘要:

The effect of pressure on the lattice parameter, hence the molar volume, of four ferromagnesian garnet specimens was determined up to 328 kb at room temperature by means of the X‐ray diffraction technique employing a diamond‐anvil high‐pressure cell. Using a Birch‐Murnaghan equation with a (∂KT/∂P)Tvalue of 5.45, the isothermal bulk modulus at zero pressureKohas been computed to be 1.90±0.06 Mb for the synthetic end member pyrope, 1.77±0.06 Mb for the natural pyrope (Pyrope60Almanditesi31), 1.73±0.06 Mb for the natural almandite (Pyrope22Almandite72), and 1.68±0.05 Mb for the synthetic end member almandite. An equation relating the bulk modulus to the composition of a binary solid solution was derived. It was found that the bulk modulus of the almandite‐pyrope solid solutions is inversely proportional to the mole fraction and is expressed by:whereXpyis the mole fraction of pyrope. The seismic parameter for these solid solutions can be expressed as a function of the mea

ISSN:0148-0227    

DOI:10.1029/JB075i029p05757

年代:1970

数据来源: WILEY