摘要:

Theoretical calculations and some preliminary experimental observations show that the liquidus and solidus temperatures for each of a wide range of upper mantle peridotite and komatiite compositions become coherent at high pressures. This suggests that these materials may have been liquids in eutectic‐like equilibrium (rather than peritectic‐like) with mantle assemblages. It is shown that the major element geochemistry of 83 mantle peridotites and 61 komatiites define a trend which is not primarily due to an olivine control process; rather it is interpreted to represent the pressure‐induced compositional trace of eutectic liquids in equilibrium with mantle assemblages from 4 to 15 GPa. If the bulk Earth is chondritic in composition these phase equilibrium constraints would imply that the upper mantle had formed from the whole mantle as an ultrabasic partial melt, the transition zone and lower mantle being the complementary eclogite and pyroxenite residua. This is consistent with mineralogical interpretations of seismic data for the present‐day Earth which call for a peridotite upper mantle and pyroxene‐like transition zone and low

ISSN:0094-8276    

DOI:10.1029/GL012i009p00541

年代:1985

数据来源: WILEY

摘要:

This paper reports an analysis of electromagnetic data from a site occupied on the floor of the Tasman Sea during an Australia‐U.S. research program, the Tasman Project of Seafloor Magnetotelluric Exploration. This site (37°33′S, 155°58′E, depth 4450 m) is in the central portion of the Tasman Sea, and near both a fossil spreading ridge and the Tasmantid Seamount Chain. The extensive electric and magnetic data sets, covering 60 days, exhibit the signals of a most energetic oceanic environment. Conspicuous features of the electric field include an unusually large component induced by mesoscale fluid motions, as well as sporadically occurring high‐frequency signal probably induced by ocean turbulence.These motional fields contribute a significant background noise to the ionospheric storm and substorm magnetic activity from which the seafloor magnetotelluric impedance tensor is estimated, but permit a usable frequency band of 0.06 to 5.5 cph. Over this band the seafloor impedance is found to exhibit frequency‐independent skew and anisotropy. The skew suggests the presence of some nearby lateral conductivity heterogeneity. The anisotropy is characterised by a principal axes ratio of 3.5, the major axis being oriented 24° clockwise from true north.Tentative inversion of the data implies the presence of an unusually high conductance in the uppermost seafloor. Altogether, information on upper mantle conductivity may reach a 500 km depth. The principal magnetotelluric axes are approximately aligned with both the trend of the seamount chain and with the Australian coastline. The ensemble of data from eight other sites occupied in the project should permit an assessment of the relative contribution of these

ISSN:0094-8276    

DOI:10.1029/GL012i009p00545

年代:1985

数据来源: WILEY

摘要:

The heat flow over Precambrian shields is generally lower than over other continental provinces. Previous observations at 9 sites of the West African shield have shown that heat flow ranges from 20 mW m−2in Niger to 38‐42 mW m−2in Liberia, Ghana and Nigeria. Since some of these values are lower than expected for Precambrian shields, it is important to find out whether or not they are representative of the entire shield before trying to derive its thermal structure. In this paper, we present new heat flow determinations from seven sites of the West African shield. These indicate that the surface heat flow is comparable with that of other Precambrian shields in the

ISSN:0094-8276    

DOI:10.1029/GL012i009p00549

年代:1985

数据来源: WILEY

摘要:

Thrust faulting in the Yukon‐Tanana Upland of east‐central Alaska has long been hypothesized, but few thrust faults have been definitely identified. In the Boundary area near the international border between Alaska and Canada metamorphic rocks of the amphibolite facies have been thrust over rocks of the greenschist facies. Subordinate thrust faults and imbrication occur within both groups of rocks. Brecciation, slickensides, slickenfibers, rootless, isoclinally folded quartz bands, and abrupt lithologic changes from gneiss and amphibolite to quartz‐white mica‐chlorite schist characterize the thrust zone. Ultramafic rocks occur locally on or near the thrust plane. Thrusting is postulated to have occurred when a terrane composed of the amphibolite facies rocks converged with rocks along the continental margin of North America in Late Triassic and Early Jurass

ISSN:0094-8276    

DOI:10.1029/GL012i009p00553

年代:1985

数据来源: WILEY

摘要:

Five contiguous 12‐13 km fault segments form a sawtooth geometry on the southernmost San Andreas fault. The kinematic and morphologic properties of each segment depend on fault strike, despite differences of strike between segments of as little as 3 degrees. Oblique slip (transpression) of fault segments within the Indio Hills, Mecca Hills and Durmid Hill results from an inferred 8:1 ratio of dextral slip to convergence across the fault zone. Triggered slip and creep are confined almost entirely to transpressive segments of the fault. Durmid Hill has been formed in the last 28 ± 6 ka by uplift at an average rate of 3 ± 1 m

ISSN:0094-8276    

DOI:10.1029/GL012i009p00557

年代:1985

数据来源: WILEY

摘要:

Repeated first‐order leveling surveys conducted by the National Geodetic Survey (NGS) in 1972, 1974, 1976, 1978, and 1981 provide evidence of contemporary relative uplift near the junction of the San Andreas fault and the Brawley Seismic Zone. Uplift, which extends over a distance of about 5 km where crossed by the leveling line, apparently developed progressively between 1972 (possibly before) and 1978. Maximum relative uplift during this period reached 58 ± 4 mm. This spatially and temporally coherent pattern of uplift was interrupted between 1978 and 1981 possibly as a result of the 1979, M6.6 Imperial Valley earthquake. While the cause of the observed uplift is unknown, given the location, one interpretation is that it represents a zone of concentrated strain possibly associated with fault activi

ISSN:0094-8276    

DOI:10.1029/GL012i009p00561

年代:1985

数据来源: WILEY

摘要:

Teleseismic SV waves couple to P waves in the lithosphere in several ways. P waves converted from discontinuities in the lithosphere precede the main SV arrival as small amplitude Sp precursors. Large amplitude P waves follow the main SV arrival after conversion by reflection at the free surface, and post‐critical reflection from a discontinuity in the lithosphere. SPdiff denotes a converted P wave diffracted along the underside of the crust when a teleseismic SV wave strikes it from below near the critical angle of incidence.We observed several of these shear‐coupled P waves on seismograms of deep earthquakes recorded at broadband seismic stations (RSTN) in North America. For these paths, large amplitudes relative to SV characterize the lithospheric multiple near 50°, with amplitudes decreasing rapidly beyond 60°. Calculation of synthetic seismograms indicate that a post‐critical reflection from a discontinuity at 70‐80 km depth where compressional velocity increases abruptly to about 8.5 km/s produce the largest amplitude P arrival. The observations of these shear‐coupled P waves presents a previously untapped source of information about the fine structure of the u

ISSN:0094-8276    

DOI:10.1029/GL012i009p00565

年代:1985

数据来源: WILEY

摘要:

By appealing to the correspondence principle, it is possible to compute free oscillations of an anelastic earth model directly. This formulation reveals a source phase due to anelasticity which is not predicted by first order perturbation theory. For toroidal modes, this source phase is found to be largest for source components which are proportional to the radial strain scalar rather than to the radial displacement scalar. The source phase is found to increase with overtone number. Also, large relative differences in both excitation modulus and phase with respect to an elastic model are found when the elastic excitation becomes small. This effect may be large enough to bias estimates of source properties and elastic structure.

ISSN:0094-8276    

DOI:10.1029/GL012i009p00569

年代:1985

数据来源: WILEY

摘要:

Comparison of teleseismic P‐wave delays in the vicinity of the Central Nevada Test Site in Hot Creek Valley, Nevada, with P‐delay data over a wider region in the Great Basin, shows that upper mantle speeds under Hot Creek Valley stations are higher than the average for the Great Basin as a whole, but lower than those beneath Pahute Mesa. These observations indicate that the caldera complex in Hot Creek Valley may have a high speed root similar to that proposed to exist beneath the Silent Canyon caldera at Pahute Mesa, however the Hot Creek Valley anomaly is not as strong as the Pahute Mesa anomaly. These high‐speed upper mantle structures cause defocusing for outgoing rays resulting in a shadow zone at the distance range corresponding to many European stations. The shadow zone caused by the Pahute Mesa structure is much more pronounced and consequently magnitudes of Pahute Mesa explosions can be underestimated relative to the magnitude of Hot Creek Valley explosions of similar yield. When magnitude versus yield relationships developed using Pahute Mesa data are applied to other areas, such as Hot Creek Valley, they can result in a biased estimate of

ISSN:0094-8276    

DOI:10.1029/GL012i009p00573

年代:1985

数据来源: WILEY

摘要:

Large amplitude long period vertical oscillations of the thermosphere were detected over Chatanika, Alaska, just after an earthquake. The oscillations continued for at least nine hours. The earthquake registered 5.8 on the Richter scale, and its epicenter was about 1000 km from the observing site. The vertical velocity of the atmosphere was determined via incoherent scatter measurements of the plasma velocity parallel to the earth's magnetic field, and at times exceeded 100 m/s in the 300 to 450 km height range. Analysis of the oscillation frequency and its relation to the Brunt‐Vaisala frequency yields reasonable agreement with the theory for such a process [Row, 1967], although the indicated exospheric neutral gas temperature is on the high side of expected values. There is some evidence that the upper thermosphere may have been viscously heated by the oscillations, which may explain the high temperatur

ISSN:0094-8276    

DOI:10.1029/GL012i009p00577

年代:1985

数据来源: WILEY