摘要:

We consider the consequences of strong stable stratification at the top of the Earth's fluid outer core, concentrating on the generation of the geomagnetic secular variation. We assume that the core near the coremantle boundary is both strongly stably stratified and free of Lorentz forces: we find that this set of assumptions severely limits the class of possible motions, none of which is compatible with the geomagnetic secular variation. Relaxing either assumption is adequate: tangentially geostrophic flows are consistent with the secular variation if we relax the assumption that the core is strongly stably stratified (but retain the assumption that Lorentz forces are negligible); purely toroidal flows (consisting in this case of zonal flows together with horizontally polarised MAC‐waves) may explain the secular variation if we include Lorentz forces. We are unable to discriminate between these two case

ISSN:0094-8276    

DOI:10.1029/GL017i012p02081

年代:1990

数据来源: WILEY

摘要:

A new, more reliable technique has been used to determine the ancient intensity of the earth's magnetic field using samples of baked clay from eight sites in the Eastern Mediterranean. The resultant power spectrum reveals a single strong component with a period of 1430 years. The reproducibility of the results obtained with the new method also makes limited archaeomagnetic dating with a precision of between about one and two hundred years between 900 BC and AD 400 possible.

ISSN:0094-8276    

DOI:10.1029/GL017i012p02085

年代:1990

数据来源: WILEY

摘要:

High‐resolution x‐ray diffraction data were collected from 10 to 433K on a homogeneous polycrystalline specimen of Mg0.9Fe0.1SiO3‐perovskite. Rietveld structural refinement and x‐ray absorption fluorescence measurements demonstrate that Fe substitutes for Mg and not Si. The thermal expansion behavior is anisotropic, with the orthorhombic distortion becoming less with increasing temperature. The volumetric thermal expansion, which is dominated by the decrease in octahedral tilts, is 1.9×l0−5K−1between 150 and 373K. No evidence of conversion to enstatite could be found

ISSN:0094-8276    

DOI:10.1029/GL017i012p02089

年代:1990

数据来源: WILEY

摘要:

High‐pressure X‐ray diffraction and electron‐microscopy experiments show that fayalite (Fe2SiO4) transforms at around 350 kbar to an amorphous phase that can be quenched down to 1 bar. This room‐ternperature vitrification represents a thermodynamically driven fusion to a glass, as implied by the existence of a maximum above 100 kbar in the metastable extension of the melting curve of fayalite. The diffraction measurements, a thermodynamic analysis of the melting curve of fayalite, and the difference in bulk modulus between fayalite and Fe2SiO4liquid are consistent with a reversible switch of silicon in the amorphous phase from a high coordination number at pressure down to 4‐fold coordination

ISSN:0094-8276    

DOI:10.1029/GL017i012p02093

年代:1990

数据来源: WILEY

摘要:

The flexural creep responses of a Co‐Mg olivine‐plus‐basalt aggregate at temperatures near 1100°C have been transformed to give attenuation spectra (QE−1vs. ω) as functions of temperature. The experimental creep response of this mantle‐analog material is well‐modeled as a Burgers solid, displaying a steady‐state creep that is preceded by a transient deceleration. Calculated attenuation spectra are based on the Burgers solid viscosities and elastic constants obtained from the creep experiments. The attenuation becomes total and zero in the low‐ and high‐frequency limits, respectively, as is appropriate for polycrystalline olivine having a diffusional creep rheology. At intermediate frequencies (10−4Hz to 10−2Hz), QE−1experiences a local maximum. Both the magnitude (QE−1≈15) and central frequency of this loss maximum increase with temperature. The attenuation peak is an anelastic loss compatible with the transient creep response; it is inferred to represent a melt flow mechanism that adjusts the size of melt‐bearing triple‐grain junctions in respon

ISSN:0094-8276    

DOI:10.1029/GL017i012p02097

年代:1990

数据来源: WILEY

摘要:

To investigate the kinetics of porous flow in partially molten peridotite, a melt migration couple – formed from a disc of fine‐grained olivine plus ∼15% of a synthetic K‐Al silicate glass and a disc of poly crystalline olivine – was heated at 1255°C under a confining pressure of 300 MPa for 2 hr. Driven by capillary forces, silicate liquid in the source disc infiltrates along three‐grain junctions into the sink disc. To analyze the resulting melt migration profile in terms of compaction theory, the equations developed byMcKenzie[1984] to describe porous flow of a liquid in a deformable matrix were modified by replacing the buoyancy force term with a capillary force term. The governing equations were then solved numerically with the initial and boundary conditions specified by the experimental design. Comparison of the melt migration profile obtained from the experiment with those generated by numerical simulation demonstrates that compaction theory provides a good description of the experimental results provided that the permeability of these solid‐liquid materials increases linearly with increasing l

ISSN:0094-8276    

DOI:10.1029/GL017i012p02101

年代:1990

数据来源: WILEY

摘要:

We tested the feasibility of using finite difference methods to model seismic propagation at ∼10 Hz through a two‐dimensional representation of an axial magma chamber with a thin, liquid lid. Our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of P and S energy propagation. The results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries. We observe a high‐energy, downward‐traveling shear phase, observable only with borehole receivers, that would be useful in studying the nature and shape of magma chambers. The ability of finite difference methods to model high‐order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near‐axis drilling of the East Pacific Rise in order to optimize the benefits from shear wave imaging of sub‐a

ISSN:0094-8276    

DOI:10.1029/GL017i012p02105

年代:1990

数据来源: WILEY

摘要:

More than twenty‐five years ago Miller and Low reported the existence of a threshold pore pressure gradient below which water would not flow through clay. Recent experimental observations of the shear strength of structured water on biotite surfaces have provided a physical basis for understanding this threshold gradient. The existence of this phenomenon has profound implications for the rheological properties of mature fault zones, such as the San Andreas, that contain large thicknesses of fault gouge. For example, a clay–filled fault zone about 1 km wide at the base of the seismogenic zone decreasing to zero width at the surface could support core fluid pressure equal to the maximum principal stress over the entire seismogenic zone. As a result, the fault would have near‐zero strength and the maximum principal stress measured on the flanks of the fault, would be oriented normal to the fault surface. Another consequence of the threshold gradient is that normal hydrostatic fluid pressures outside the fault zone could coexist with near‐lithostatic fluid pressures in the interior of the fault zone without the need for continual replenishment of the overpressured fluid. In addition, the pore pressure at any point should never exceed the local minimum principal stress so that hydrofracture will no

ISSN:0094-8276    

DOI:10.1029/GL017i012p02109

年代:1990

数据来源: WILEY

摘要:

Strain rates measured at four geodetic networks in eastern California situated between northern Owens Valley and the Transverse Ranges along a small circle drawn about the Pacific‐North America pole of rotation are remarkably consistent. Each exhibits 0.14 μrad/yr simple right‐lateral engineering‐shear‐strain accumulation across the local vertical plane tangent to the small circle. Local faults (e.g., Owens Valley, Garlock, Helendale) traversing these networks are not as closely aligned with the vertical planes of maximum shear‐strain accumulation as is the local tangent to the small circle. A fifth network slightly east of the small circle shows no significant strain accumulation. Thus, a shear zone trending N35° W from near the eastern end of the big bend of the San Andreas fault to northern Owens Valley is indicated by these data. This corresponds to the Eastern California shear zone proposed on geological evidence by Dokka and Travis. The shear zone carries ∼8 mm/yr of the Pacific‐North America relative plate motion from the San Andreas fault north‐northwest across the Mojave Desert into Owens Valley and the northern Basin and Range province. The shear zone observed at the surface may be a manifestation of a through‐goi

ISSN:0094-8276    

DOI:10.1029/GL017i012p02113

年代:1990

数据来源: WILEY

摘要:

TeleseismicsSandsScSwaveforms provide constraints on the attenuation structure above deep seismic rones. We estimate the average QβalongsSandsScSpaths beneath the Lau back arc by comparison withSand ScS waveforms. Q is determined using both time and frequency domain analyses of transverse long period records assuming Q is frequency independent. In the frequency domain technique, spectra from reflected (sSorsScS) and direct (SorsScS) waveforms are analyzed using least squares. The time domain method uses a grid search to determine the attenuation operator that, when convolved with the direct phase, produces the best fit to the reflected phase. In both methods the direct phase is convolved with a crustal response function to simulate the crustal interaction of the reflected phase. Large events subject to source directivity effects and source‐station combinations involving ray propagation down the subducting slab are not used. The resulting attenuation estimates show low Q values which increase with focal depth, suggesting high attenuation in the upper mantle and a rapid decrease in attenuation below 200–300 km. Lateral variations are also indicated with very high attenuation (Q ∼ 20–35) in the upper mantle near the active spreading centers and lower attenuation west of the Lau Ridge. Q‐temperature relationships extrapolated from laboratory experiments suggest that the low Q regions represent zones of asthenospheric parti

ISSN:0094-8276    

DOI:10.1029/GL017i012p02117

年代:1990

数据来源: WILEY