摘要:

The40Ar/36Ar ratio of the mantle bears important information about the extent of early degassing of the earth. Many researchers have agreed that40Ar/36Ar ratio in the depleted mantle as sampled by Mid‐Ocean Ridge Basalts (MORB) is as high as 28000. However, the40Ar/36Ar ratio in the undepleted mantle has been strongly debated. We have reexamined the noble gas data of He and Ar in MORB and hot spot volcanics using4He/40Ar vs.36Ar/40Ar and4He/40Ar vs. ³He/40Ar diagrams. In these diagrams, mixing lines between two components result in straight lines, which enable us to interpret data more easily than using diagrams with different denominators like a40Ar/36Ar vs. ³He/4He plot where all mixing lines appear as curves. We conclude that the40Ar/36Ar ratio in the undepleted mantle is much higher than the atmospheric value and should be higher than the observed highest values in the hot spot volcanics. We also suggest that high4He/40Ar ratios observed in MORB result from kinetic disequilibrium during partial melt

ISSN:0094-8276    

DOI:10.1029/95GL01893

年代:1995

数据来源: WILEY

摘要:

Seismic tomography (Tanimoto 1990, Su&Dziewonski 1991 and Masterset al.1992) has provided detailed three‐dimensional map of the shear wave velocity in the Earth's mantle. These tomographic results when characterized by the radial correlation function (Jordanet al.1993) shows no signature of the mantle transition zone. The radial correlation function and associated radial correlation length are subsets of the entire two‐point correlation and therefore here we propose an optimal characterization of the two‐point correlation in terms of its eigenfunctions. It is shown that the eigenrepresentation of the tomographic data converges rapidly and that the most energetic eigenfunctions reveal the signature of the mantle transition

ISSN:0094-8276    

DOI:10.1029/95GL01787

年代:1995

数据来源: WILEY

摘要:

The influences of two exothermic phase transitions, the olivine toβ‐spinel and theβ‐ to γ‐spinel transition, on Martian mantle convection have been studied with an axisymmetric spherical‐shell model. There are depth dependences in the thermal expansivity and gravity, which, taken together, decrease by a factor of between 2 to 3 across the mantle. We have discovered an unique mechanism for the amplification and superheating of megaplumes in Mars, which works particularly well because of the optimal heights of the two destabilizing exothermic phase transitions above the Martian core‐mantle boundary (CMB): The cold descending mantle flow is accelerated by its interactions with the phase transitions and the nearby CMB forces the flow to bend over sharply. Together these effects result in strong localized viscous heating, which will generate and accelerate a rising plume and may even cause melting in its deep interior and produc

ISSN:0094-8276    

DOI:10.1029/95GL02008

年代:1995

数据来源: WILEY

摘要:

Large lateral variations in the thickness of the lithosphere and viscosity of the mantle may be associated with continental roots. The effects of continental roots, or the tectosphere, on postglacial rebound are calculated using an axisymmetric flow model in which both the mantle and lithosphere are described as Newtonian fluids. Continental roots are shown to have a measurable effect on postglacial rebound, in particular, on the evolution of the peripheral bulge. Effects of continental roots are concentrated near the edge of the tectosphere, and thus often near ocean‐continent boundaries where many relative sea‐level histories are measured. The pattern and distribution of tectonic and seismic activity associated with deglaciation are affected by a tectosph

ISSN:0094-8276    

DOI:10.1029/95GL02012

年代:1995

数据来源: WILEY

摘要:

Rocks at depth in the crust acquire a viscous (i.e., time‐dependent) magnetization under the pressure‐temperature conditions at which they reside. There have been numerous studies on the effect of temperature on viscous magnetization but little work has been performed on the effect of hydrostatic pressure. We have studied viscous remanent magnetization at 22°C in a 0.1 mT field at 0.1 and 100 MPa for natural and synthetic multidomain magnetite. The viscous remanent magnetization was found to increase nearly linearly with log(time) at both pressures. There was little change in the rate of viscous acquisition between 0.1 and 100 MPa over the multidomain grain size range studied. Thus for rocks buried at depth in the earth the enhancement of magnetic viscosity by thermal fluctuations will dominate over effects due to hydrostatic pres

ISSN:0094-8276    

DOI:10.1029/95GL01791

年代:1995

数据来源: WILEY

摘要:

The precision of AMS determination is enhanced by measuring susceptibility in directions with a uniform orientation distribution that include the four body diagonals. Some standard 10.5 cm³ samples with mean susceptibility<100µSI possess too few “magnetic” grains for reliable petrofabric interpretation whatever the measurement strategy. We should only interpret their AMS if they pass fabric homogeneity

ISSN:0094-8276    

DOI:10.1029/95GL01910

年代:1995

数据来源: WILEY

摘要:

We describe the results of rock‐magnetic measurements of a sediment core from the Madeira Abyssal Plain which contains multiple turbidites. Previous studies of trace element (e.g. U) concentration and observations of sediment color changes have shown that turbidite emplacement has resulted in reductive diagenesis and the formation of paleoxidation fronts. We show that variations in sediment lithology and postdepositional sedimentary processes can be characterized very sensitively using magnetic measurements. Low concentrations of magnetic minerals occur in turbidites with a high organic content, reflecting the destruction of magnetic material via the process of reductive diagenesis. In addition, fossil redox fronts are readily identified by rock‐magnetic propert

ISSN:0094-8276    

DOI:10.1029/95GL01798

年代:1995

数据来源: WILEY

摘要:

We have studied the spatial configuration of earthquakes in the New Britain subduction zone by inverting regional and teleseismicP, PKP,andpParrival times using a hypocentroidal decomposition method. A previously unreported intermediate depth double seismic zone exists along a small, 70 km long segment of the slab. In this region, centroid moment tensor mechanisms suggest that the upper zone of seismicity shows horizontal, along‐strike compression and the lower zone shows downdip tension. This contrasts with most previously studied double seismic zones, which generally show downdip compression in the upper zone. We propose that along‐strike, rather than downdip compressional stresses exist in the upper seismicity plane due to membrane and lateral bending stresses resulting from the shape of the New Britain trench, which undergoes a 40° change in strike between 148° and 152° longitude. The double seismic zone is bounded by a tear in the slab which accommodates the majority of the change in strike that the slab must undergo. These results suggest that double seismic zones can show very rapid lateral variations controlled by the local tectonics, and that along‐strike membrane and bending stresses resulting from slab morphology can be significant in producing double seism

ISSN:0094-8276    

DOI:10.1029/95GL01806

年代:1995

数据来源: WILEY

摘要:

In oblique convergent subduction areas, an obliquity increase should produce a slip rate increase on trench parallel strike‐slip faults and thus the fore‐arc platelet should be stretched. Along the Great Sumatran Fault (GSF), that is associated with the oblique convergent Sunda Arc, a northward increase of the GSF slip rate occurs parallel with the convergence obliquity. Transpressional back‐arc deformation accommodates part of the GSF slip rate variation while no significant fore‐arc stretching is observed. The Sumatran case shows that oblique convergence may be accommodated by deformation of a 500‐km‐wide zone, from the fore‐arc to the bac

ISSN:0094-8276    

DOI:10.1029/95GL01793

年代:1995

数据来源: WILEY

摘要:

We use geodetic data taken over four years with the Global Positioning System (GPS) to estimate (1) motion between six major plates and (2) motion relative to these plates of ten sites in plate boundary zones. The degree of consistency between geodetic velocities and rigid plates requires the (one‐dimensional) standard errors in horizontal velocities to be ∼2 mm/yr. Each of the 15 angular velocities describing motion between plate pairs that we estimate with GPS differs insignificantly from the corresponding angular velocity in global plate motion model NUVEL‐1A, which averages motion over the past 3 m.y. The motion of the Pacific plate relative to both the Eurasian and North American plates is observed to be faster than predicted by NUVEL‐1A, supporting the inference from Very Long Baseline Interferometry (VLBI) that motion of the Pacific plate has sped up over the past few m.y. The Eurasia‐North America pole of rotation is estimated to be north of NUVEL‐1A, consistent with the independent hypothesis that the pole has recently migrated northward across northeast Asia to near the Lena River delta. Victoria, which lies above the main thrust at the Cascadia subduction zone, moves relative to the interior of the overriding plate at 30% of the velocity of the subducting plate, reinforcing the conclusion that the thrust there is locked beneath the continental shel

ISSN:0094-8276    

DOI:10.1029/95GL02006

年代:1995

数据来源: WILEY