摘要:

Variations in the amplitude of radar echoes from the bottom of the grid western half of the Ross Ice Shelf have been analyzed. Contrary to the results of a similar analysis performed for the grid eastern sector of the ice shelf, bands of low signal strength downstream from both Crary Ice Rise and the Siple Coast do not correlate with modern flow lines. The difference in direction between the radar bands downstream of Crary Ice Rise and the present velocity vectors and the absence of a comparable trend farther east suggest to us that the grounding line around Crary Ice Rise retreated within the last 1000 years. This hypothesis is reinforced by the observation of several domes and hollows in ice thickness downstream of Crary Ice Rise which are similar to a hollow now located in the wake of the ice rise and a dome on its eastern flank. We interprete this as evidence for a rapid increase in flow around the ice rise which carried downstream the ice topography formed around the ice rise. Similar but less detailed evidence found downstream of the Siple Coast suggests that there was a regional retreat of the West Antarctic grounding line.

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00409

年代:1984

数据来源: WILEY

摘要:

Impact experiments in Newtonian fluids with a range of viscosities of 10−3to 60 Pa s demonstrate that transient crater volume and shape (depth‐to‐diameter ratio) depend on target viscosity as well as on gravity. Volume is reduced, and depth‐to‐diameter ratio is increased for cratering events in which viscosity plays a dominant role. In addition to being affected by target kinematic viscosity, viscous scaling is most strongly influenced by projectile diameter, less strongly by projectile velocity, and least strongly by gravity. In a planetary context, viscous effects can occur for craters formed by small or slow moving impacting bodies, low planetary surface densities, high surface viscosities, and low gravity values; conditions all likely for certain impacts into the icy satellites of Saturn and Jupiter, especially if liquid mantles were still present beneath solid crusts. Age dating based on crater counts and size‐frequency distributions for these icy bodies may have to be modified to account for the possibility that viscosity‐dominated craters were initially smaller and deeper than their gravity‐controll

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00417

年代:1984

数据来源: WILEY

摘要:

The kinematic processes by which thermal convection can mix together large‐scale mantle heterogeneities are described theoretically and with a series of numerical experiments. We have analyzed the time evolution of a large‐scale passive, diffusive scalar anomaly subject to two models of mantle flow: (1) turbulent flow, in which material elements are elongated exponentially with time, and (2) laminar flow, in which material elements are elongated according to a power law function of time. Mixing time scales are computed for both models. They are found to depend on the initial volume of the heterogeneity, the sample volume (the volume of the mantle which is averaged in process of measuring the anomaly), and the Peclet number, the ratio of strain rate to diffusive transport rate. Predictions from the two models have been tested against numerical simulations of mixing in a cellular convective eddy, at high Peclet numbers. The transport equation was integrated using the Lagrangian method of tracking particles, with the particle density being proportional to the anomaly magnitude. Diffusion was included by adding Gaussian‐random displacements at each time step. Simulations were made with up to 40,000 particles. Mixing times from the numerical experiments agree most closely with predictions from the laminar theory. The theory and the numerical experiments indicate the following: (1) Mixing by creeping flow in the mantle is a cascade phenomenon. Individual Fourier components of an anomaly are transferred up the wave number spectrum at a rate proportional to the mean Lagrangian strain rate. Because of the cascade effect, the time scale for mixing thermal anomalies is short, in the range 75–200 m.y., depending on the initial scale of the anomaly. The time scale for mixing large‐scale compositional anomalies is far longer, approximately the age of the earth. (2) Mixing of compositional anomalies tends to produce a laminated upper mantle, in which ancient heterogeneity is stretched into thin subhorizont

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00425

年代:1984

数据来源: WILEY

摘要:

In 1958 Jeffreys proposed a power law generalization of the logarithmic transient creep earlier attributed to Lomnitz. Although Jeffreys' power law form was admittedly defective in that it became unbounded at infinite time, he did apply it to the viscoelastic behavior of the earth‐moon system. Since then it has been successfully applied by many investigators to mantle rehology and Chandler wobble. Experimental seismic studies indicate that most rock types exhibit the almost constant Q behavior which Lomnitz showed to be associated with his logarithmic creep. In this paper, we study not only the Q behavior related to Jeffreys' power law creep but also other mechanical properties such as a precise spring‐dashpot ladder network realization are developed. In addition, a very simple physically realizable modification of this ladder network leads to a boundedness at long times of Jeffreys' creep in a manner which does not affect his successful application at finite ti

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00437

年代:1984

数据来源: WILEY

摘要:

This study of the active tectonics of Burma and surrounding regions is based mostly on an interpretation of Landsat imagery and on fault plane solutions for shallow and intermediate earthquakes. Fold axes in the Indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the Indoburman ranges. Fault plane solutions of shallow earthquakes, however, do not show underthrusting at the present time. Instead, P axes are oriented roughly north‐south, parallel, not perpendicular, to the fold axes in the ranges. P axes for fault plane solutions of intermediate shocks also trend north‐south, parallel to the strike of the subducted slab, and may indicate that the hanging slab is being dragged north by India through the surrounding asthenosphere. The Sagaing fault probably accommodates most of the right‐lateral slip of India past Indochina, but India's northward movement also causes internal deformation within Burma, Thailand, and Yunnan. Differences between the tectonic fabric seen on the Landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion, in the orientations of strain, and in the style of deformation in the late Cenozoic. Presumably, these changes reflect the transition from subduction to collision and strike‐slip motion as India penetrated into Eurasia past Burma. Large parts of Burma and Thailand may have rotated several tens of degrees clockwise in this

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00453

年代:1984

数据来源: WILEY

摘要:

Reversed refraction surveys were conducted along and across the Lomonosov Ridge as part of the 1979 Lomonosov Ridge Experiment. Interpretation of the strike profiles indicates a 5‐km‐thick upper crustal layer with a velocity of 4.7 km/s overlying a 15‐ to 20‐km‐thick layer of 6.6 km/s material. An upper mantle velocity of 8.3 km/s is indicated by a few pnarrivals. High‐amplitude reflection events recorded from this boundary can be successfully modeled by a transition zone of rapidly changing velocity over a depth interval of 5 km. Ray trace modeling of the dip profiles suggests a root structure extending to about 28‐km depth flanked by crust thinning to a depth near 13 km beneath the Makarov Basin and a more gradual thinning to near 16 km beneath the Fram Basin. The similarity between the crust of the Lomonosov Ridge and that beneath the Barents and Kara seas supports the suggestion that the ridge is a slice rifted from the

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00473

年代:1984

数据来源: WILEY

摘要:

The oceanic crust created during this first few million years of accretion in the Norwegian‐Greenland Sea lies at an unusually shallow depth for its age, has a smooth upper surface, and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate, seaward‐dipping reflectors. These have been attributed to lava flows generated during a brief period of subaerial seafloor spreading. We describe the results of inversions of digitally recorded sonobuoy measurements and two‐ship expanded spread profiles collected over the oceanic crust adjacent to the Norwegian passive margin. We find that the crust of the deep Lofoten Basin is indistinguishable from normal oceanic crust in thickness and structure. Closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2, while the layer 3 region retains approximately the same thickness. The area over which the seaward‐dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in “Layer 2” thickness. In the very oldest crust immediately adjacent to an escarpment that probably marks the continent‐ocean boundary, we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke‐lava interface in the thick crust. Comparing the structure of the thick crust to that of eastern Iceland, we find a strong resemblance, especially in the expansion in thickness of material with layer 2 velocities. These results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous, building a thick pile of lavas erupted from a subaerial s

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00483

年代:1984

数据来源: WILEY

摘要:

Several investigators have proposed that compressional wave anisotropy in indurated, deep‐sea carbonates is caused by a preferred orientation of calcite c axes normal to bedding. To test this hypothesis, we have used X ray pole figure goniometry to measure calcite fabrics in three anisotropic, calcareous sediment samples from Deep‐Sea Drilling Project hole 516F, on the Rio Grande Rise. Anisotropy in these samples ranges from 3.4 to 12.0%. The suggested concentration of c axes is present but very weak; concentrations of c axes normal to bedding are in the range 1.1–1.3% per 1% area. We used the Voigt‐Ruess‐Hill averaging scheme to calculate velocities from the X ray fabric data and found that even in nonporous, pure carbonate rocks, having the observed concentrations of c axes, the acoustic anisotropy would be less than 1%. We conclude that preferred orientation of calcite is not a significant contributor to compressional wave anisotropy in deep‐se

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00503

年代:1984

数据来源: WILEY

摘要:

A series of laboratory experiments was conducted with four ocean sediments, two biogenic oozes and two clays. Permeability and thermal conductivity were directly measured as a function of porosity, and the testing program was designed to identify any dependence of these physical properties upon hydrostatic pressure and temperature. The results show no discernible effects of pressure, within the range of 2–60 MPa, upon the permeability of any of the samples. Temperature effects, from 22° to 220°C, upon this property are accounted for by applying a viscosity correction to the permeating seawater. Previous investigations have suggested the existence of a pressure‐induced and/or a temperature‐induced breakdown of the absorbed water which surrounds clay particles, thereby promoting an increase in sediment permeability. Our experimental findings cannot confirm this phenomenon and fail to provide a satisfactory solution to the conflicting data which now exist between the pore water velocities inferred from nonlinear thermal profiles of ocean sediments and those fluid velocities derived from Darcy's law and laboratory permeability data. The effects of sizeable variations in pressure and temperature upon sediment thermal conductivity are found to reflect closely the behavior of the conductivity of the liquid phase alone under these same changes in environmental conditions. This is not surprising due to the relatively narrow range of high porosities encountered in this study. Empirical equations are developed which allow sediment thermal conductivity to be calculated as a function of temperature and void ratio. A hydrostatic pressure correction term is also pr

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00511

年代:1984

数据来源: WILEY

摘要:

A simple network model of pore space in rocks has been developed with which permeability and bulk modulus as a function of confining pressure can be calculated. Pores are modeled as straight conduits with circular, elliptic, or tapered cross sections. The interconnection of these conduits is modeled by emplacing them in regular two‐dimensional hexagonal, square, or triangular networks. Flow through each conduit is modeled using Poiseuille's law. Flow through the network is calculated based upon the analogy of fluid flow in Darcy's law to current flow in Ohm's law. An estimate of the effective bulk modulus is obtained by summing the contributions of the individual pores. The effect of confining pressure on permeability and bulk modulus is determined by the solid properties and the shape of the pores. For appropriate aspect ratio distributions, the permeability and bulk modulus characteristics of the network model are similar to those found for laboratory rock samples. For rocks of moderate porosity, like sandstone, the model predicts that the bulk modulus is most affected by small, low aspect ratio pores. In contrast, for rocks like granite and tight sandstones (permeability less than 0.5 mdarcy) in which there are relatively few round pores, the bulk modulus and the permeability are both controlled by easily deformed pores. These types of responses are observed in experimental data. The success of the model in predicting these general bulk modulus and permeability responses indicates that while the models presented are too simple to represent a rock completely, the network theory approach is a promising method for modeling porous medi

ISSN:0148-0227    

DOI:10.1029/JB089iB01p00527

年代:1984

数据来源: WILEY