摘要:

Assuming that the Archean mantle was hotter and the oceanic crust thicker than at present, we estimate that the Archean oceanic lithosphere had a "sandwich" rheology, with the lower crust behaving as a soft, ductile layer. Accordingly, the Archean oceanic lithosphere had a rheological structure similar to present‐day continental lithosphere but unlike present‐day oceanic lithosphere. The thickness of the ductile layer is a decreasing function of age, while its strength is an increasing function of age. The buoyancy problem in Archean subduction could be overcome by delamination of oceanic upper crust, but mass balance calculation indicates that most Archean oceanic crust must have been recycled. Where the right dynamic conditions existed for delamination, oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in "greenstone belts" that are characteristic of Archean terra

ISSN:0094-8276    

DOI:10.1029/GL015i010p01077

年代:1988

数据来源: WILEY

摘要:

Intermediate and deep focus earthquakes in Wadati‐Benioff zones are thought to occur in the cold interiors of downgoing slabs which are significantly stronger than the warmer mantle. Given that earthquakes in oceanic lithosphere appear restricted by an isotherm, and hence a given value of lithospheric strength, we investigate whether a similar formulation is useful for subducting plates. Strength in downgoing slabs should be affected by both pressure and temperature, an effect previously treated using a depth‐dependent limiting temperature for seismicity [Wortel, 1982]. We find this limiting temperature implies that a possible limiting strength increases strongly with depth, unless either the temperatures were too low or the activation volume too large. Comparison of the analytic model used by Wortel with numerical thermal models appears to exclude the first possibility. We explore the second possibility by using the numerical thermal model to compute strength contours for flow law constants reported from laboratory experiments, and find that the expected pressure strengthening is large enough that the slab should have considerable strength well below the deepest seismicity. We conclude that if laboratory results are applicable to these conditions, either a strongly depth‐dependent limiting strength exists or factors in addition to strength control the distribution of subduction zone earthq

ISSN:0094-8276    

DOI:10.1029/GL015i010p01081

年代:1988

数据来源: WILEY

摘要:

A vertical seismic profile data set showing shear‐wave splitting from The Geysers, California, can be approximately fit with synthetic seismograms calculated with geometrical ray theory for anisotropic models of aligned cracks in the uppermost crust. Limited agreement between data and synthetics can be obtained if the cracks are constrained to be vertical, but a much better fit is possible if the cracks dip at an angle of 55 degree

ISSN:0094-8276    

DOI:10.1029/GL015i010p01085

年代:1988

数据来源: WILEY

摘要:

We calculate the demagnetizing temperature necessary to remove viscous magnetic moments from assemblies of single domain magnetite grains. It is shown that this cleaning temperature may be the same as that predicted by Pullaiahet al[1975] depending on the fraction of the viscous remanence removed. Experimental data by Williams ]1986] are shown to be in good agreement with the numerical calculations.

ISSN:0094-8276    

DOI:10.1029/GL015i010p01089

年代:1988

数据来源: WILEY

摘要:

Experimental studies have demonstrated that demagnetization temperatures for removing thermoviscous magnetizations (TVRMs) in rocks are often higher than predicted by Pullaiah et al.'s time‐temperature (t‐T) nomogram based on Néel's single domain (SD) theory. Better agreement has been found for t‐T relationships based on Walton's calculations invoking volume distributions. It has been pointed out, however, that Walton's formula applies for t‐T conditions for acquisition of magnetization only, not to demagnetization. We applied numerical methods for assessing the true influence of distributions in SD volume. As it turns out, for reasonable volume distributions, the differences between our results and Pullaiah et al.'s simpler approach are marginal; the differences in demagnetization temperature are significant only for moderate‐term low‐temperature acquisition. Discrepancies between experiments and theory appear because rocks contain predominantly pseudo‐single and multi‐domain grains to which SD theor

ISSN:0094-8276    

DOI:10.1029/GL015i010p01093

年代:1988

数据来源: WILEY

摘要:

Magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks, and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes. We have recently demonstrated however in laboratory produced samples that, as theoretically expected, susceptibility anisotropy is dependent on particle size. Multidomain particles of magnetite, or similar strongly magnetic particles with shape anisotropy, exhibit a maximum susceptibility parallel to their easy (remanent) magnetization axes whereas uniaxial single‐domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes. Hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state. By demonstrating uniaxial single‐domain anisotropy characteristics for the first time in a rock specimen, we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro‐ or ferrimagnetic particles in rocks. In particular, apparently foliated rocks can actually be lineated (and vice‐versa). Moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence. Remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of al

ISSN:0094-8276    

DOI:10.1029/GL015i010p01097

年代:1988

数据来源: WILEY

摘要:

A new paleomagnetic secular variation (PSV) record from the late Quaternary Wilson Creek beds of Mono Lake, California contains a distinctive periodic vector waveform that follows and is almost certainly related to the Mono Lake excursion. A comparison of all published PSV results from the Wilson Creek beds suggests that the magnetic field at Mono Lake went through an interval (36,000‐28,000 ybp) of very low amplitude PSV followed by the Mono Lake excursion (28,000‐27,000 ybp) and four subsequent recurrences (27,000‐12,500 ybp) of the excursion waveform with relatively diminished amplitudes (lower than the excursion amplitude but higher than PSV amplitudes at Mono Lake since 12,500 ybp). This pattern clearly suggests that the Mono Lake excursion is related to ‘typical’ PSV; it also suggests that the core dynamo process responsible for PSV is capable of near impulse onset, persistence over a time scale of 15,000 years, and quasi‐periodic behavior that is non‐wave‐length dis

ISSN:0094-8276    

DOI:10.1029/GL015i010p01101

年代:1988

数据来源: WILEY

摘要:

Application of the skewness method to 14 marine magnetic anomaly profiles across the Galapagos ridge provides a precise estimate of the equatorial contribution of the long‐term non‐dipole field. The average inclination anomaly (observed remanent inclination minus dipole inclination) so determined from the central anomaly over the Galapagos ridge is −3.3° ± 0.9°. This result agrees closely with estimates of the non‐dipole field contribution at equatorial latitudes as predicted from prior paleomagnet

ISSN:0094-8276    

DOI:10.1029/GL015i010p01105

年代:1988

数据来源: WILEY

摘要:

With the development of satellite altimetry, it is possible to infer the geostrophic velocity of the surface ocean currents, if the geoid and the position of the satellite are known accurately. Errors in current geoid models and orbit computations, both due primarily to errors in the Earth's gravity field model, have limited the use of altimeter data for this purpose. The objective of this investigation is to demonstrate that altimeter data can be used in a joint solution to simultaneously estimate the quasi‐stationary sea surface topography, ζ, and the model for the gravity field. Satellite tracking data from twelve satellites were used along with Seasat altimeter data for the solution. The estimated model of ζ compares well at long wavelengths with the hydrographic model of ζ. Covariance analysis indicates that the geoid is separable from ζ up to degree 9, at which point geoid error is comparable to the signal

ISSN:0094-8276    

DOI:10.1029/GL015i010p01109

年代:1988

数据来源: WILEY

摘要:

Heat flow measurements across the Washington‐Oregon convergent margin show several characteristic features: a well defined peak at the foot of the overthrust, a gradual decrease landward across the lower slope reaching a minimum at the ridge, and a gradual rise further landward. Constrained by geological and seismic evidence on the rate of stacking and the dimension of stacked thrust sheets and by drilling results on sediment properties, we use a two‐dimensional time‐dependent finite element procedure to model the stacking of overthrust slices of sediments and to calculate the thermal structure beneath the lower slope of the accretionary prism; the calculated heat flow profile is in good agreement with the measurements. Advective heat transfer due to stacking of overthrust sheets and topographic changes are shown to play a dominant role in controlling the surface heat flow and the thermal state of the accretionary

ISSN:0094-8276    

DOI:10.1029/GL015i010p01113

年代:1988

数据来源: WILEY