摘要:

Lateral slip on initially rift‐parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts. It occurs in southern Afar, where clockwise block rotations result from distributed dextral shear between the overlapping Ghoubbet Asal‐Manda Inakir and Manda Hararo‐Abhe Bad rifts. Faulting observed during the 1969, Serdo earthquakes and on SPOT images is consistent with the shear being taken up by left‐lateral slip on steep NW‐SE striking faults, which formed as normal faults before extensional strain became localized in the two rifts. This bookshelf faulting accounts quantitatively for the 14.5°± 7.5° rotation documented by paleomagnetism in the 1.8 ± 0.4 Ma old Afar stratoid basalts, given the 17.5 ± 5 mm/yr rate of separation between Arab

ISSN:0094-8276    

DOI:10.1029/GL017i001p00001

年代:1990

数据来源: WILEY

摘要:

Global sea‐level changes have been occurring since the last Pleistocene deglaciation. Recently the contribution of current postglacial rebound to local sea‐level variations has been removed by means of a radially stratified earth model in order to assess the amount of sea‐level rise due to recent climatic changes. We have studied the effects of lateral variations in the viscosity structure on the postglacial uplift rates along the continental margins. Finite‐element calculations in cylindrical geometry show that the spread in the cumulative vertical displacement may be affected by 20% in the presence of lateral rheological contrasts. But the vertical rates of deformation can attain differences of up to 50% for the different models. Horizontal deformation rates along continental margins are much more affected by the lateral variations in viscosity and can be used in the future as constraints on mantle r

ISSN:0094-8276    

DOI:10.1029/GL017i001p00005

年代:1990

数据来源: WILEY

摘要:

In order to throw some light on the rheological behavior of the upper mantle of the Earth, a detailed series of high‐temperature deformation experiments has been performed on olivine single crystals at 1 atm pressure under highly controlled thermodynamic conditions. The effects of stress, temperature, oxygen fugacity and orthopyroxene activity on the deformation rate of olivine have been carefully measured for the major high‐temperature slip systems. These experimental results have been extrapolated to the pressures and strain rates of the upper mantle to provide new insight into the mechanical behavior of the mantle and a better constrained approach to extrapolation of laboratory deformation data to the conditions present in the upper mantle. We suggest that extrapolations in stress provide the most reliable means for extending laboratory creep data to upper mantle strain rates. We also predict that, in the dislocation creep field, the bulk of the strain in olivine in the upper mantle is accommodated by the (010)[100] slip system. In addition, true mantle strengths are probably most accurately modelled using the experimental data for samples oriented favoring the (010)[001]slip system. However, the reliable prediction of mantle strengths requires highly accurate temperature versus depth relationships and good indications of the local oxygen fugacity in the man

ISSN:0094-8276    

DOI:10.1029/GL017i001p00009

年代:1990

数据来源: WILEY

摘要:

The Plasticity of single crystals of MgSiO3perovskite was studied (at room temperature) by the microhardness tests using a Vickers and a Knoop indenter. The Vickers hardness, Hv, of MgSiO3perovskite was determined to be 18 +/− 2 GPa, about a factor of two larger than the hardness of either enstatite or olivine, indicating a significantly larger resistance to plastic flow of this mineral than that of the upper mantle minerals. The normalized hardness, Hv/μ (μ: shear modulus), of MgSiO3perovskite is in reasonable agreement with those of titanate perovskites (SrTiO3, BaTiO3, CaTiO3), although a cubic perovskite SrTiO3has a significantly lower hardness than others. The Knoop tests showed anisotropic hardness consistent with the slip system

ISSN:0094-8276    

DOI:10.1029/GL017i001p00013

年代:1990

数据来源: WILEY

摘要:

A station bias in the travel‐times of P‐waves may exist in the International Seismological Centre (ISC) data base. It appears that low gain seismographs report systematically late arrival times. Average station delays confirm this with low gain stations generally slower than high gain stati

ISSN:0094-8276    

DOI:10.1029/GL017i001p00017

年代:1990

数据来源: WILEY

摘要:

Observations of shear‐wave splitting were obtained from temporary and permanent stations in western Nevada and southern California. Measurements of fast polarization azimuth ϕ and delay time δtwere made on both SKS and direct S phases. For stations in the northern Basin and Range the results were consistent, yielding an average of (ϕ, δt) = (+75 ± 8°, 0.9 ± 0.3 s). One station in the southern Basin and Range gave (+40 ± 11°, 0.4 ± 0.1 s) and one in the Mojave Desert gave (−54 ± 3°, 1.2 ± 0.1 s). δt= 0.9sis consistent with a 100km‐thick mantle layer characterized by 4% anisotropy. For the northern stations, the anisotropy appears unrelated to the present‐day extension or absolute plate motion. Rather, we suggest that it is ‘fossil’ anisotropy associated with pre‐Miocene extension, whose direction is about +68°. For the California station ϕ is nearly parallel to the strike of the San Andreas Fault system, and is attributed to shear strain associat

ISSN:0094-8276    

DOI:10.1029/GL017i001p00021

年代:1990

数据来源: WILEY

摘要:

The 1986 fissure eruption of Izu‐Oshima volcano was preceded and followed by strong volcanic tremor activities. The post‐eruption tremor activity consists of an alternating sequence of a tremor episode lasting about 10 min and the subsequent quiescence of about 40–50 min. We observed this post‐eruption activity by a 24‐channel seismic array deployed on the caldera floor. The semblance analysis of the recorded data revealed that high frequency (>5Hz) components of the tremor were radiated from two separated sources beneath the caldera. The tremor first occurred at one source and then migrated to the other source in a transition time of about a few minutes during each tremor episode. This transition suggests that the two sources are dynamically linked with each other in a magma‐filled crack

ISSN:0094-8276    

DOI:10.1029/GL017i001p00025

年代:1990

数据来源: WILEY

摘要:

High‐pressure phase relations have been examined for phlogopite + diopside with and without enstatite under vapor absent conditions in the pressure range of 5 to 13 GPa and in the temperature range of 1000 to 1300°C. Phlogopite in these systems can be stable up to 6–7 GPa and decomposes through pressure‐dependent reactions to crystallize phases including potassic amphibole. The experimental results suggest that phlogopite, which is one of main hydrous phases in the downdragged hydrated peridotite at the base of mantle wedge, plays an important role in the formation of magmas at the backarc side of a volcanic arc. The existence of potassic amphibole at higher pressure regions may imply the involvement of subduction component in magma generation in the region far away from the trenc

ISSN:0094-8276    

DOI:10.1029/GL017i001p00029

年代:1990

数据来源: WILEY

摘要:

The interplanetary structures of the magnetic field and the current near the sun are analyzed by MHD computer simulation for the case of the magnetic dipole configuration on the photosphere. It is shown that the solar wind in the closed field region corotates with the sun. The azimuth magnetic field is formed in the open region but remains very weak in the closed region. The poloidal current, which associates with the azimuth magnetic field, is found to come into and go out of the sun in the open and closed regions, respectively.

ISSN:0094-8276    

DOI:10.1029/GL017i001p00033

年代:1990

数据来源: WILEY

摘要:

One year periodic variations in the near Earth solar wind ion density and bulk flow velocity are reported. The variations show an inverse relationship between the ion velocity and density. The peak strength of the observed density variation ranges from 50–100% over the background. These variations imply either large scale mass loading inside the Earth's orbit or intrinsic solar modulations. Analyses of both near Earth and Pioneer Venus Orbiter spacecraft data provide a comparison at two different heliocentric distances. Several explanations for these variations are discusse

ISSN:0094-8276    

DOI:10.1029/GL017i001p00037

年代:1990

数据来源: WILEY