摘要:

Previous paleomagnetic studies have revealed that pre‐Quaternary pelagic clay with neither siliceous nor calcareous fossils has unstable remanent magnetization, and that alternating field (AF) demagnetization is not effective to remove the secondary magnetization. This study applied thermal demagnetization on pelagic clay from the South Pacific, and recovered characteristic directions. The observed paleolatitudes agree with those from plate tectonic analysis, which proves that the thermal demagnetization worked well. The magnetic overprint is estimated to be viscous remanent magnetization (VRM) from VRM acquisition experiment. A hypothesis is proposed that the magnitude of the VRM of pelagic clay has been controlled by the grain size of magnetic minerals of eolian origin, which has been subjected to the intensity of the global atmospheric circulation. This can explain the Late‐Pliocene unstable‐to‐stable transition of the remanence, which has been observed widely in the

ISSN:0094-8276    

DOI:10.1029/GL013i013p01438

年代:1986

数据来源: WILEY

摘要:

Ancient secular variation in Central Mexico was determined from paleomagnetic measurements on 45 independent lava flows ranging in age (14C and K‐Ar dates) from 2,500 to 580,000 years B.P. All the analyzed flows are characterized by a normal polarity placing them within the Brunhes Chron. The paleosecular variation analysis yielded an angular standard deviation from the field of an axial dipole of 15.3° with 95% confidence limits of 13.4° and 17.9°, and that of the corresponding Virtual Geomagnetic Poles (VGPs) of 13.9° with confidence limits of 12.1° and 16.1°. These values are in agreement with those predicted by most statistical models of secular variation, but divergent from those obtained from low latitudes (19°N) such as Hawaii and

ISSN:0094-8276    

DOI:10.1029/GL013i013p01442

年代:1986

数据来源: WILEY

摘要:

Paleomagnetic results from Neogene volcanic and sedimentary formations in central Aegea indicate that Evia and Skyros have undergone a 48° and 26° clockwise rotation respectively. The Volos region does not show any significant rotation in the last 3 Ma. The rotation of Evia is about twice as large as the one measured from coeval formations in the external Hellenic arc. The results are interpreted in terms of a published model of distributed deformation by faulting, which also provides an explanation for the significant differential rotation of Evia and Skyro

ISSN:0094-8276    

DOI:10.1029/GL013i013p01446

年代:1986

数据来源: WILEY

摘要:

A paleomagnetic study of the Late Precambrian Roxbury formation, consisting of extrusives and clastic sediments, has yielded a steeply downward directed and pre‐folding characteristic magnetization (D/I = 219°/71°, α95 = 5°, k = 311, paleopole at 13°N, 267°E, N = 4 sites), which indicates a significantly higher paleolatitude (55°) than would be expected if the Boston basin were part of the equatorial North American craton in the latest Precambrian and earliest Paleozoic. This characteristic magnetization reveals dual polarities and is further supported by a positive conglomerate test. A ubiquitous post‐folding late Paleozoic overprint is present in nine sites (D/I = 183°/14°, α95 = 8°, k = 42), with a paleopole at 41°S, 285°E. The pre‐folding magnetization resides in hematite, which is inferred to have formed during early oxidation of the rocks; the high stability of this hematite may have prevented its magnetization from being reset during the late Paleozoic chemical event responsible for the magnetic overprint. The Boston basin has a marked geological similarity to the Avalon basement terranes in Nova Scotia and Newfoundland, as well as the Armorican Massif in France, and the high paleolatitudes observed for all these terranes suggest a common paleogeographical affinity; a likely paleolocation is near the northwestern margin of Gondwana which was located at the southpole in the latest Precambrian

ISSN:0094-8276    

DOI:10.1029/GL013i013p01450

年代:1986

数据来源: WILEY

摘要:

We derive the long wave length equivalents of the constitutive relations for finely stratified fluids as suggested by Saito and Abe [1984]. The results show the transversely isotropic relations characterized by two effective viscosities. One is associated with shear stress applied on the plane parallel to the layering and the other is with normal stress applied perpendicular to the layering. If thin soft layers exist in the low velocity zone, there may be a significant difference in these two viscosities. A simple calculation simulating mantle flow with large apparent anisotropy shows the possible existence of convection having large aspect ratio.

ISSN:0094-8276    

DOI:10.1029/GL013i013p01454

年代:1986

数据来源: WILEY

摘要:

Following a recent big deep event (mb=6.8) in the Fiji islands region, a seismic array in Hokkaido region, Japan recorded clear impulsive precursors to the P′P′ phases (P′ is another symbol for PKP and P′P′ is the surface reflection of P′) in the epicentral distance range of 70°‐72.4°. Reflection points of the phases are located 1.5°‐3.5° west of the Mid‐Atlantic ridge (13°S, 15°W). The precursors were also recorded at two WWSSN stations in Japan. Travel times and slowness measured by the array are best explained as the precursors being underside reflections from a depth of 660 km. The amplitude ratios of P′660P′/P′P′ are about 0.1, a value similar to previous observations. The observation suggests that there exists a sharp discontinuity at a depth of 660 km beneath the area

ISSN:0094-8276    

DOI:10.1029/GL013i013p01458

年代:1986

数据来源: WILEY

摘要:

Melt segregation can be driven by suction applied to the free surface of a partially molten region (dynamic forcing). Dynamic forcing may play a role in drawing melt towards eruption centers or in maintaining melt channels within the mantle. I examine the importance of dynamic forcing by means of a model in which melt is extracted by suction from a deformable porous medium with initially uniform porosity. The suction rapidly reduces the porosity near the free surface, and the melt extraction rate drops to zero after about 10³ yr. The total volume of melt extracted during this time is only about 0.05 km³ per km² of free surface area. These results suggest that dynamic forcing probably plays a negligible role in melt segregation, and that melt channels are unlikely to exist in the mant

ISSN:0094-8276    

DOI:10.1029/GL013i013p01462

年代:1986

数据来源: WILEY

摘要:

A successful correlation between floor and roof rocks is needed to study the effects of cooling rates on the nucleation, solidification, and trace element content of cumulate igneous rocks. In felsic intrusions, the most obvious assumption to test is that the core compositions of plagioclase feldspar correlate, because the mineral is abundant and resistant to subsolidus exchange. A test of that assumption is provided by the modal abundance of apatite, which peaks near a plagioclase composition of An 38 in both floor and roof rocks of the Kiglapait Intrusion. The test is made adequately sensitive by dividing the measured mode of apatite by an independently estimated residual porosity, a procedure which helps correct for large amounts of trapped liquid that produced apatite in the roof rocks. The procedure is applicable to other intrusions and its success here suggests that failures of correlation found elsewhere deserve another look.

ISSN:0094-8276    

DOI:10.1029/GL013i013p01466

年代:1986

数据来源: WILEY

摘要:

This work presents a detailed numerical study of the dynamical behavior of convection in a spherical shell, as applied to mantle convection. From both 2‐dimensional (120 radial and 360 tangential points) and 3‐dimensional (60 radial levels and spherical harmonics up to order and degreel=33, m=33), we show that for a spherical shell (with inner to outer radii ratio η=.62) convection becomes time‐dependent, withl=2 dominating, at a Rayleigh number of about 31 times supercritical for a constant viscosity, base‐heated configuration. This secondary instability is characterized by oscillatory time‐dependence, with higher frequencies involved, at slightly higher Rayleigh numbers. In the process of illustrating the onset of time‐dependence, we extend our analysis to show that the onset of weak turbulence in spherical‐shell convection takes place at about 60 times the critical Rayleigh number via a quasi

ISSN:0094-8276    

DOI:10.1029/GL013i013p01470

年代:1986

数据来源: WILEY

摘要:

The stirring of a small, passive heterogeneity in an unsteady circulation is investigated by following the boundary of the heterogeneity; the circulation mimics features of plate‐scale flow. After initial release, the heterogeneity is stirred and subsequently consists of at least one large "blob" connected to long, but thin tendrils. Tendrils exponentially lengthen and exponentially thin with time, as predicted by a turbulent mixing law, but the width of blobs decays about 100 times more slowly. The blobs mix slowly because they are smaller than the scale of flow and are sheared and occasionally unmixed by the laminar flow within the interiors of convection cells. The decay time of 1 to 100 km sized heterogeneities is on the order of billions of years for whole‐mantle convection and thus is in accord with isotopic observations which indicate the mantle has chemical heterogeneities persisting for billions of ye

ISSN:0094-8276    

DOI:10.1029/GL013i013p01474

年代:1986

数据来源: WILEY