摘要:

Convection driven by thermal buoyancy in the presence of the Coriolis force occurs in planetary atmospheres and interiors. In order to model convection subject to nearly spherically symmetric distributions of gravity and temperature, a hemisphere has been constructed which can be rotated about its axis of symmetry. The angular velocity is selected such that the paraboloidal surfaces of equal potential match closely surfaces of constant radius within the hemispherical fluid shell. While the baroclinicity of the fluid state is still noticeable, quantitative measurements can be obtained which can be compared with the theoretical calculations for the spherically symmetric case. The drift of the convection columns has been measured and a transition from the singly periodic state of convection to a modulated state has been visualized and determined quantitatively.

ISSN:0094-8276    

DOI:10.1029/92GL00574

年代:1992

数据来源: WILEY

摘要:

New classes of inertial oscillation mode in the form of equatorially trapped boundary waves are discovered. The structure and frequency of the waves are hardly affected by the presence of an inner sphere if the azimuthal wavenumberwhereη= ri/ro, the radius ratio of a spherical shell. Also discovered is that equatorially symmetric inertial waves belong to particular subclasses of thermal instabilities. The problem of the inertial waves can therefore be understood within the framework of instability theory. The findings provide valuable insights into many geophysical phenomena, not only for the Earth's fluid core but atmospheres and oceans as well

ISSN:0094-8276    

DOI:10.1029/92GL00357

年代:1992

数据来源: WILEY

摘要:

The value of the parameter ν from low pressure laboratory measurements disagrees with the value from seismic observations. The parameter ν relates the isobaric change in shear velocityVsto the change in compressional velocityVp. Seismic evidence indicates ν exceeds 2.0 in the lower mantle, whereas data on a variety of minerals at high temperatureTand ambient pressurePresult in lower values. We reconcile these differences. Ourab initiomodel calculations on MgO show that ν increases withPand is 2.0–2.5 at lower mantle pressures. There is no need to assume partial melting to explain the seismic data. These calculations also provide insight into thePandTdependence of the dimensionless parameter Γ ≡ −(1/αG)(∂G/∂T)p, whereGis the isotropic shear modulus and α is the volume thermal expansivity. Using measured values of thermoelastic parameters coupled with thermodynamic identities, we seek constraints on δs≡ −(1/αKs)(∂Ks/∂T)p, whereKsis the adiabatic bulk modulus, and confirm thatPcauses δsto decrease. We find Poisson's ratio increases withPandT. Altogether these results show that for MgO, ν increases from 1.3 at ambient conditions to over 2 at lower mantle conditions. We expect other mantle minerals to behave similarly. Therefore we find that reconciliation of the mineral physics approach with that of seismic tomography concerning ν does not require special assumptions about

ISSN:0094-8276    

DOI:10.1029/92GL00774

年代:1992

数据来源: WILEY

摘要:

We perform travel time analyses for the Kuril and the Mariana subduction zones to investigate whether a lower mantle high‐velocity zone exists immediately beneath the deepest earthquakes in these regions. We use P and PKIKP wave arrival time data read directly and accurately from WWSSN short‐period vertical component records and corrected for the effect of lateral heterogeneity far from the source regions using a differential residual sphere technique. The corrected residuals for the deep Kuril events show that a high velocity zone extends in the down‐dip direction of the Kuril slab in the lower mantle. On the other hand, the residuals for the deep Mariana events show no evidence for a high‐velocity zone in the down‐dip direction beneath the deepest earthquakes. A reasonable interpretation of these results is that the Kuril slab penetrates into the lower mantle (h≥670 km) but that the Mariana slab does not extend vertically into the l

ISSN:0094-8276    

DOI:10.1029/92GL00366

年代:1992

数据来源: WILEY

摘要:

The impact of lateral variations in the thickness of the lithosphere on surface topography, horizontal intraplate deformation and stress accumulation is studied for plates that drift with respect to the highly viscous lower mantle and the transition zone. The lithosphere and upper mantle are described by a viscoelastic Maxwell rheology within the framework of a finite element scheme which allows the modeling of heterogeneous lithospheric structures in 2D vertical cross sections. The geophysical signatures are found to be extremely sensitive to lateral viscosity contrasts which interact with the upper mantle flow. This mechanism can contribute to a certain extent and in concert with the other driving forces of plate tectonics to the evolution of back‐arc basins, to the explanation of the largest angle of subduction in west‐dipping slabs and to the initiation of subduction of an oceanic lithosphere underneath a stable continental

ISSN:0094-8276    

DOI:10.1029/92GL00640

年代:1992

数据来源: WILEY

摘要:

A compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness, away from plateaus, is 6 km; no systematic increase of crustal thickness with spreading rate is observed. Instead, the data show large variations in crustal thickness at slow spreading ridges (3 – 8 km for half rates30 mm/yr). The large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid‐ocean ridges. Both data sets support the speculation of a transition from a 3‐D structure of crustal accretion at slow ridges to a 2‐D accretion pattern at fa

ISSN:0094-8276    

DOI:10.1029/92GL00161

年代:1992

数据来源: WILEY

摘要:

An extensive suite of downhole geophysical logs was acquired through a 70 m thick section of basalt drilled by the Ocean Drilling Program (ODP) into 110 Ma oceanic crust 225 km SW of Hawaii. These data include standard sonic, electrical, and nuclear logs, as well as electrical and acoustic images of the wellbore, and enable a vertically continuous characterization of the physical properties of shallow, old oceanic crust. Fracture porosity and alteration increase with depth in this hole, and considered together with results obtained elsewhere in both young and old (5 to 110 Ma) crust, suggest that physical properties gradients in the uppermost few hundred meters of oceanic basement are controlled by crustal morphology and alteration and are ubiquitously variable. As this hole was drilled primarily for pilot experiments at the first site of the Ocean Seismic Network, evaluation of the log data demonstrates the importance of carefulin‐situdata collection in order to emplace a broadband monitoring device below the seafloor for long‐term seismic measureme

ISSN:0094-8276    

DOI:10.1029/92GL00833

年代:1992

数据来源: WILEY

摘要:

The isotopic composition of Nd is uniform in metalliferous sediments formed at distances varying from>1000 km to within 10 km of the East Pacific Rise (EPR) palaeoridge. These data indicate that hydrothermal vent fluids, despite having concentrations more than 500 times greater, have no effect on the Nd isotopic composition of seawater. This implicates efficient scavenging of hydrothermal Nd by particulates, resulting in extremely short residence times (<1 year) close to the hydrothermal vents. Therefore Nd isotopic studies of ancient seawater precipitates, particularly metalliferous sediments, cannot be used to delimit the magnitude of past hydrothermal circulation without independent constraints on local scavenging rates (or residence times) relative to those of modern oceans.

ISSN:0094-8276    

DOI:10.1029/92GL00393

年代:1992

数据来源: WILEY

摘要:

We have detected a new powerful source of low‐frequency seismic energy which is associated with some violent volcanic eruptions. For the two eruptions for which we have identified this new source the spectrum is bichromatic, consisting of two narrow, phase coherent spectral lines. These bands were centered on 3.76 and 5.14 mHz for the April 4, 1982 El Chichón eruption, and on 3.68 and 4.44 mHz for the June 15, 1991 Mt. Pinatubo eruption. During the latter, 8 hour long eruption the resonance frequencies varied by less than 20μHz. The strength of the source is sufficient to excite globe circling surface waves. We conclude from these observations that volcanoes can excite “harmonic tremors” at much longer periods than previously observed. Our preferred source model involves feedback between local atmospheric oscillations and the eruption process. The physics responsible for this feedback remains subject to specu

ISSN:0094-8276    

DOI:10.1029/92GL00685

年代:1992

数据来源: WILEY

摘要:

One of the most controversial issues in geology today is the way the lithosphere extends. The Galicia margin is a particularly good place to adress this problem as it is fairly sediment starved, has clear upper crustal structure well imaged by a variety of seismic reflection profiles and has well control to calibrate seismic data. Observed subsidence is compared with extension of the uppermost crust of the entire margin as computed from the geometry of tilted fault blocks imaged on a series of parallel seismic lines. The pure shear (or stretching) model clearly accounts for the extension of the upper continental margin but not for the lower margin where the S reflector zone, a prominent zone of intra‐crustal reflectors, is present. I suggest that the S reflector zone is a detachment zone accomodating decreasing displacement towards the continent and that this detachment zone, active since the beginning of rifting, outcrops on the Canadian side and corresponds to the steep southeastern Flemish Cap continental slope. Pure shear is the mechanism which affects the whole lithosphere, but in the upper part of the crust, the rifting mechanism is locally simple shear where the detachment zone is activ

ISSN:0094-8276    

DOI:10.1029/91GL02984

年代:1992

数据来源: WILEY