摘要:

A new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described. The magnetic fabric of a collection of Trenton Limestone specimens has been determined using this method, as well as by conventional anisotropy of magnetic susceptibility. The results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the Trenton in which the bulk magnetic susceptibilities are low. A model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses. The original fabric is inferred to have been isotropic; the anisotropy resides in secondary magnetite of Late Paleozoic age. It is argued that the observed magnetic fabric must therefore be Alleghenian or younger in age. Our method has the potential to determine paleostress directions in carbonates elsewhere, provided our assumptions are correct.

ISSN:0094-8276    

DOI:10.1029/GL012i006p00333

年代:1985

数据来源: WILEY

摘要:

Volcanic flows of the Late Ordovician Dunn Point Formation contain a generally univectorial magnetization which passes the fold test. Its steep direction (D = 335°, I = −61°, k = 79, α95= 4.2°, paleopole at 2°S, 136°E) is unlike any known for North America or for the Avalon terrane for post‐Ordovician time, so that a primary age of the magnetization appears to be very likely. The paleolatitude (42°S) for the Avalon terrane derived from this result is much higher than that predicted for the area on the basis of the cratonic North American apparent polar wander path, and a substantial post‐Ordovician displacement (>3500 km) of Avalon with respect to the craton can be deduced. In all likelihood the Avalon terrane did not collide with North America until Middle Devonian time. This collision produced the Acadian orogeny. High Ordovician paleolatitudes have also been obtained for northern Africa and for several localities in Hercynian Europe (the Armorica plate) and it is inferred that these areas may have drifted together until Late Ord

ISSN:0094-8276    

DOI:10.1029/GL012i006p00337

年代:1985

数据来源: WILEY

摘要:

A fundamental limiting factor in the precision of magnetostratigraphic correlation is stratigraphic completeness. Sadler [1981] has suggested a method by which the expected completeness of any given stratigraphic section can be calculated given the thickness, duration, and depositional environment. The technique is probabilistic and requires the investigator to specify a meaningful short‐term time scale at which completeness is to be estimated. For magnetostratigraphy, the short‐term time scale is defined by the duration of the polarity chron or other polarity feature of interest. Sadler's method allows the probability of observing such features to be quantified. It is a useful tool for evaluating the reliability of magnetostratigraphic correlations and for judging between alternative correlati

ISSN:0094-8276    

DOI:10.1029/GL012i006p00341

年代:1985

数据来源: WILEY

摘要:

We investigate global variations in the pattern of aftershock area expansion associated with large subduction zone earthquakes. The expansion of aftershock areas with time is relatively small in the subduction zones of Alaska, the Aleutians, Mexico, and parts of the Kuriles and South America. These subduction zones are interpreted to be characterized by moderate to strong interplate coupling. In contrast, the extent of aftershock expansion is greater for events within the subduction zones of northeast Japan and western and south Pacific where interplate coupling is inferred to be weak. These results are interpreted in terms of an asperity model where a fault zone is represented by a distribution of strong spots (asperities) and weaker zones. The immediate aftershock area is assumed to extend over an area occupied by asperities, and the subsequent expansion to occur into the surrounding weaker zones. If large asperities abut each other on a fault plane, interplate coupling is strong and little expansion of aftershock area occurs. Smaller and more sparsely distributed asperities cause weak interplate coupling and significant expansion of aftershock area. Small and densely distributed asperities result in moderate interplate coupling and little aftershock expansion.

ISSN:0094-8276    

DOI:10.1029/GL012i006p00345

年代:1985

数据来源: WILEY

摘要:

The 1979 Imperial Valley earthquake caused the Imperial fault to rupture 10 to 15 km southeast of the epicenter, in addition to the well documented rupture to the northwest. This is established from the properties of the initialS‐wave arrival at strong‐motion stations in Mexico at the southern end of the rupture zone. First, the amplitude of thisS—velocity pulse is larger at stations 10 to 15 km southeast of the epicenter than at those within 2 km of the epicenter. Second, the pulse shapes and polarizations at three stations south of the epicenter are consistent with southeastward propagation. Third, travel time considerations rule out contributions to these pulses from faulting originating more than 4 km northwest of the epic

ISSN:0094-8276    

DOI:10.1029/GL012i006p00349

年代:1985

数据来源: WILEY

摘要:

A comparison is presented between explosion‐generated teleseismic P signals from single events of comparable magnitude in the Tuamotu Islands, Nevada, and E. Kazakh recorded by the Wake Island Hydrophone Array. Spectral analysis of these signals indicates that attenuation (presumably under the source) is greatest for the Tuamotu travel path and least for the E. Kazakh travel path. The high frequencies (>3 Hz) present in E. Kazakh signals, which enhance the detectability of these signals on the ocean bottom near Wake (where noise levels are low in this range), are generally absent in signals from Nevada and the Tuamotu

ISSN:0094-8276    

DOI:10.1029/GL012i006p00353

年代:1985

数据来源: WILEY

摘要:

To examine the topography and gravity anomalies due to mantle convection, we have carried out finite difference calculations of thermal convection in a fluid layer with a viscosity exponentially decreasing with temperature. Both surface topography and gravity anomalies are shown to be positive over regions of ascending flow and negative over regions of descending flow. These results differ significantly from those of McKenzie (1977) which for similar conditions predict negative topography and gravity anomalies above rising plumes. At large Rayleigh number, the amplitude of surface topography is found to depend on Rayleigh number to the seven‐ninths power as predicted by boundary layer theory. These results are applied to test the hypothesis that the linear small‐scale gravity undulations in the Central Pacific Ocean (Haxby and Weissel, 1983) are caused by convective rolls in a layer at the base of the lithosphere. For a convecting layer thickness one‐half the observed gravity wavelength and with plausible values of flexural rigidity and heat flux, we show that convection can produce gravity anomalies of the observed magnitude with a layer viscosity comparable to that determined by post‐glacial rebound. However, a large increase of viscosity with depth is required to confine convection to a thin layer. If these anomalies are actively maintained by convective stresses, one possibility is that layered convection may result from compositional stratification of the

ISSN:0094-8276    

DOI:10.1029/GL012i006p00357

年代:1985

数据来源: WILEY

摘要:

We have reexamined the role played by transient rheology in the interpretation of mantle viscosity. This investigation has been carried out by comparing the amplitude responses with the data of secular variation of, the relative sea‐level histories at sites well within the ice margins and at the ice margin like the city of Boston. A linear Burgers’ body rheology has been assumed in the lower mantle. The data near the edge of the ice load proves most sensitive to the transient viscosity structure. The non‐monotonic behavior of sea‐level data near Boston can be explained both by a steady‐state lower mantle viscosity of 1022P with a thick lithosphere and also by a transient lower mantle rheology but with a thin lithosphere. The long‐term viscosity of the lower mantle in this second model has a steady‐state value of a

ISSN:0094-8276    

DOI:10.1029/GL012i006p00361

年代:1985

数据来源: WILEY

摘要:

The Northeast Japan arc is associated with westward subduction of the Pacific plate beneath the Eurasian plate. At the southernmost region of the arc, the Philippine Sea plate subducts northward into the mantle wedge between the Eurasian plate and the Pacific slab. Akagi volcano located at this region yields volcanic rocks with combined Sr‐O isotope relationships which are explained in terms of the mixing of the mantle material from the mantle wedge and deep ocean and/or continent derived sediments conveyed with the subducting Philippine Sea plate. This implys that the Philippine Sea plate continues to the location just under Akagi volcano, although the seismic Philippine Sea plate does not reach just under the volcan

ISSN:0094-8276    

DOI:10.1029/GL012i006p00365

年代:1985

数据来源: WILEY

摘要:

Initial measurements of the Venus bow shock obtained by Pioneer Venus in 1979 near solar maximum indicated that the bow shock was on average 2.44 RVfrom the center of the planet in the terminator plane. This is 0.35 RVfurther from Venus than observed by Venera 9/10 in 1976. In the past this discrepancy has been attributed to some effect of the solar cycle. Recent measurements by Pioneer Venus support this interpretation. In 1980 the distance to the bow shock reached a maximum of 2.45 RVand since then has been almost steadily declining toward the distance measured by Venera near solar minimum. The variation in bow shock position is well correlated with the sunspot number and the F 10.7 cm flux over this period. We attribute this behavior to the variation in the neutral atmosphere of Venus with the solar cycle and its subsequent effect on the mass‐loading of the solar win

ISSN:0094-8276    

DOI:10.1029/GL012i006p00369

年代:1985

数据来源: WILEY