摘要:

Cores obtained to a depth of 2164 meters in the Antarctic ice sheet at Byrd station have undergone considerable relaxation since they were drilled. This relaxation, manifested as a density decrease of the ice with time, could still be detected more than 16 months after the cores had been pulled to the surface. The greatest measurable relaxation, of the order 0.6% expansion, occurred in cores from around 800 meters' depth. Ice from 400–900 meters proved to be much more brittle than deeper ice and was characterized by an abundance of highly compressed air bubbles. These bubbles had disappeared completely by 1100 meters, most probably as a result of the diffusion of gas molecules into the ice lattice, and this, together with the formation below 1200 meters of an oriented crystal fabric (characterized by a strong vertical orientation of crystallographiccaxes) is believed to be responsible for the greatly increased ductility of the deeper cores. Relaxation of ice from the brittle zone occurs primarily as a result of expansion of pressurized air bubbles. In deeper, bubble‐free ice, however, relaxation can be attributed to the creation of new space resulting from the formation of cleavage cracks, plate‐like voids and cavities, especially the latter, which become filled with gas derived from the air originally dissolved under pressure in the ice

ISSN:0148-0227    

DOI:10.1029/JB076i011p02533

年代:1971

数据来源: WILEY

摘要:

The semi‐isolated basins and series of basins of intermediate to normal oceanic depths that lie behind island arc systems are termed marginal basins, and are believed to be of extensional origin. Marginal basins, although differing in size and amount of sediment fill, are all underlain by oceanic crust, and can be divided into actively spreading inter‐arc basins and older inactive basins. The inactive basins are grouped on the basis of age as reflected by crustal heat flow; inactive basins with above‐normal heat flow are apparently younger than those with normal heat flow. Volcano‐tectonic rift zones with associated silicic tuffs are found in the tectonic position of inter‐arc basins in some continental trench‐arc systems and are thought to have been formed by similar extensional process. Large scale crustal extension and demonstratable trench migration imply displacement of the Benioff zone. However, the portion of the Benioff zone shallower than 150 km, and lying between the volcanic chain and the trench, remains relatively undeformed along the trend of arc systems in which the dip of the deeper segment of the Benioff zone changes markedly. Geometric relationships suggest that, as the inter‐arc basin opens, the trench is forced to migrate and the Benioff zone is flattened, except in areas where crust can be consumed in a second trench behind the extensional zone. Active extension is restricted to arc systems with earthquakes deeper than about 350 km. The available data can be explained by a thermal diapir of shear‐heated mantle material, bouyantly rising beneath the inter‐arc basin from the upper surface of the under‐thrust lithosphere. Hydrostatic forces associated with such a diapir are felt capable of over‐coming the compressional forces in the mantle produced by shearing along the upper surface of the lithosphere and by the bending of the lithosphere. The existence of low density, high temperature upper mantle is indicated by the lack of a large gravity anomaly over the shallow oceanic crust of the inter‐arc basin, by high heat flow there, and by anomalously high attenuation of shear waves passing through the upper mantle benea

ISSN:0148-0227    

DOI:10.1029/JB076i011p02542

年代:1971

数据来源: WILEY

摘要:

Seismic refraction studies in the Melanesian Borderland (the area between Australia on the west and New Zealand and Tonga on the east) show extreme diversity of crustal structure. The Lord Howe rise and the Norfolk ridge are topped by thick sediments and have deep crustal roots and thick layers of material with the same compressional wave velocity as the Australian continental crust. The Kermadec and Lau ridges, on the other hand, have structure and velocities typical of normal island arcs. The South Fiji basin structurally resembles an oceanic area with additional sedimentary fill. The western part of the Fiji plateau has thin sediments and has the structure of an area of normal deep‐ocean basin that has been uplifted two kilometers. All the features are compatible with the hypothesis that the area has been disrupted and fragments of continental material have been separated from the Australian mas

ISSN:0148-0227    

DOI:10.1029/JB076i011p02562

年代:1971

数据来源: WILEY

摘要:

‘Pure‐path’ phase and group velocities of mantle Rayleigh waves in combination with mantle Love wave velocities and worldwide averages of free oscillation data are used to derive upper mantle models with the smallest possible number of parameters. The shear‐velocity solutions between the crust‐mantle boundary and 400 km appear to be unique for an assumed set of averaging lengths, despite the fact that the density solutions are nonunique over the same depth interval. Oceanic models are characterized by a well‐developed low‐velocity zone. For the set of data used in this study, the thickness of the oceanic lithosphere proves to be a critical parameter that influences the density solutions. If the thickness of the lithosphere is 80 km or less, a reversal in density distribution is necessary to explain the data. If the thickness is 100 km or greater, the densities between 10 and 400 km can be represented by a single average value. The shield data do not require a low‐velocity channel when the velocity in the lid is 4.60 km/sec or less, but they do require a low‐velocity channel for higherSnvelocities. The characteristic feature of tectonic models is high shear velocity between 200 and 400 km (>4.80 km/sec). This feature could be explained by the thrust of oceanic lithosphere under

ISSN:0148-0227    

DOI:10.1029/JB076i011p02587

年代:1971

数据来源: WILEY

摘要:

Two mechanisms of material transport in dikes, viz., flow of a viscous magma and explosive entrainment of fragments in a gas, are examined quantitatively. For magma intrusion the asymptotic behavior of the system is examined by investigating the heat and fluid flow problems separately. For a given set of physical and geometrical conditions, limiting values of variables consistent with the mechanism may be computed. For some dikes too narrow to be satisfactorily explained by the magma flow representation, properties of an alternative model are developed. This model postulates that on the formation of a deep fissure a very sharp pressure gradient would develop at its base. The pressure gradient would lead to the formation of a shock wave and explosive evolution of volatiles that would be accelerated to near sonic velocity and entrain substantial quantities of fragmental material.

ISSN:0148-0227    

DOI:10.1029/JB076i011p02602

年代:1971

数据来源: WILEY

摘要:

The compressions of the cell parameters of a natural olivine were studied to pressures greater than 100 kb using a high‐pressure, in situ, X‐ray powder‐diffraction method. Thea(smallest) and theb(largest) cell parameters of orthorhombic olivine are found to have nearly the same compression values (xp/xo) to 100 kb and are more compressible than theccell parameter. The subsequent volume calculations indicate that olivine is less compressible than has been reported from static work and from equation‐of‐state calculations based on ultrasonic elastic constan

ISSN:0148-0227    

DOI:10.1029/JB076i011p02610

年代:1971

数据来源: WILEY

摘要:

Shock‐wave data for the high‐pressure phases of a number of rocks and minerals have been reanalyzed using a revised seismic equation of state to constrain the zero‐pressure properties of the high‐pressure phases. The anomalously low values ofdK/dPresulting from a previous analysis are thereby removed. The inferred zero‐pressure densities of the high‐pressure phases are reduced by an average of 4%, and the values of the zero‐pressure seismic parameter Φ0are reduced by up to 30%. bringing them into approximate agreement with the hypothesis of the molar additivity of Φ. For most of the materials considered, the derived pressure trajectories of density versus the seismic parameter Φ are consistent wth shock‐wave data on such materials as MgO, Al2O3, and SiO2(stishovite) where no zero‐pressure assumptions are required. Iron‐rich compounds may require further revision. It is demonstrated that in poorly constrained cases the Birch‐Murnaghan equation can produce a singularity indK/dPat high pressure. Possible crystal structures of the high‐pressure phases are considered using the revised zero‐pressure densities. It seems likely that olivines with less than about 10‐mole % FeO content can transform to a phase significantly denser than the isochemical mixture of oxides, in contrast to olivines with higher iron content. The possibility that electron spin transitions occur in iron‐rich compounds is considered, but no strong evidence has been obtained. The derived zero‐pressure densities of the high‐pressure phases are usually not of sufficient accuracy to distinguish between all alternative structures, but in some cases an alternative structure to that

ISSN:0148-0227    

DOI:10.1029/JB076i011p02617

年代:1971

数据来源: WILEY

摘要:

Vertical temperature gradients lead to density inversions in polar ice sheets, a situation that may lead to thermal convection in viscous fluids. The conditions necessary to initiate and maintain thermal convection in ice sheets are examined. The principal difficulty arises from the viscoplastic nature of ice and the importance of crystal fabric on ice flow. Available data indicate that critical Rayleigh numbers for convection can be attained by dislocation creep, but not by diffusion creep, the former being strongly dependent on grain fabric whereas the latter is strongly dependent on grain size. At the central ice divide no grain fabric should exist prior to convection, and once begun, convection flow should create a favorable fabric in this region. Once begun, therefore, convection flow should reinforce itself. Polar ice sheets can be considered as laboratories for studying the probability of convection in the earth's mantle. Having no phase changes and being heated from below, polar ice sheets resemble a simplified, miniature mantle, and can duplicate the boundary conditions imposed on the mantle by the various convection models. Convection appears possible for ice thicknesses, which may or may not require basal melting. This is analogous to mantle convection, which may or may not extend to the earth's fluid core. Conditions for convection are most favorable at the central ice divide of the Antarctic ice sheet, and a borehole should be drilled in this region to investigate the mode of heat transport.

ISSN:0148-0227    

DOI:10.1029/JB076i011p02628

年代:1971

数据来源: WILEY

摘要:

The probable arrangement of the continents of the Americas, Europe, and Africa before the opening of the Atlantic was determined byBullard et al.[1965]. This is the well‐known ‘Bullard fit,’ which has been reproduced in many articles. On this fit the most anomalous region is that which comprises Mexico, Central America, the Gulf of Mexico, and the Caribbean Sea. On the Bullard fit, the distance from the shores of Venezuela to the shores of Texas is about 1100 km. At present this distance is about 3300 km. It appears that North America moved northwestward with respect to South America by about 2200 km since the time shown by Bullard. However, this extension cannot be explained at present in terms of known sea‐floor sp

ISSN:0148-0227    

DOI:10.1029/JB076i011p02639

年代:1971

数据来源: WILEY

摘要:

Two‐dimensional conductivity models relating the seismic low‐velocity zone in the upper mantle to a zone of enhanced electrical conductivity satisfy the observed anomalous magnetic variation fields in the western United States. The upper mantle structure of the Colorado Plateau is intermediate between that of the Basin and Range and southern Rockies provinces, characterized by a well‐developed low‐velocity zone and high electrical conductivity, and the structure of the Great Plains, where mantle conductivities are low and the low‐velocity laye

ISSN:0148-0227    

DOI:10.1029/JB076i011p02643

年代:1971

数据来源: WILEY