摘要:

We present three‐layer interior models of Uranus and Neptune with central rocky cores, mantles of water, methane, and ammonia (the ‘ices’), and outer envelopes primarily composed of hydrogen and helium. The models incorporate a new H2O equation of state based on experimental data which is considerably ‘softer’ than previous H2O equations of state. Corrections for interior temperatures ∼5000°K are included in the models, and the thermal evolution of both planets is investigated using recent heat flow measurements. We find that evolutionary considerations are consistent with gravitational field data in supporting models with approximately solar abundances of ‘ice’ and ‘rock.’ Models with very low abundances of water, methane, and ammonia can be excluded. Evolutionary considerations indicate that initial temperatures and luminosities for Uranus and Neptune were not substantially higher than the present value. Both planets apparently have relatively small (∼1–2 earth masses) hydrogen‐helium envelopes, with Neptune's envelope smaller than Uranus'. A monotonic trend is evident among the Jovian planets: all have central rock‐ice cores of ∼15 earth masses, but with hydrogen‐helium envelopes which decrease in mass from Jupite

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00225

年代:1980

数据来源: WILEY

摘要:

Centrifuge experimental techniques provide possibilities for laboratory simulation of ground motion and cratering effects due to explosive loadings. The results of a similarity analysis for the thermomechanical response of a continuum show that increased gravity is a necessary condition for subscale testing when identical materials for both model and prototype are being used. The general similarity requirements for this type of subscale testing are examined both theoretically and experimentally. The similarity analysis is used to derive the necessary and sufficient requirements due to the general balance and jump equations and gives relations among all the scale factors for size, density, stress, body forces, internal energy, heat supply, heat conduction, heat of detonation, and time. Additional constraints due to specific choices of material constitutive equations are evaluated separately. The class of constitutive equations that add no further requirements is identified. For this class of materials, direct simulation of large‐scale cratering events at small scale on the centrifuge is possible and independent of the actual constitutive equations. For a rate‐independent soil it is shown that a small experiment at gravitygand energyEis similar to a large event at 1Gbut with energy equal tog3E. Consequently, experiments at 500Gwith 8 grams of explosives can be used to simulate a kiloton in the field. A series of centrifuge experiments was performed to validate the derived similarity requirements and to determine the practicality of applying the technique to dry granular soils having little or no cohesion. Ten shots using Ottawa sand at various gravities confirmed reproducibility of results in the centrifuge environment, provided information on particle size effects, and demonstrated the applicability of the derived similitude requirements. These experiments used 0.5–4 grams of pentaerythritol‐tetranitrate (PETN) and 1.7 grams of lead‐azide explosives. They were placed at zero depth of burial and were detonated at gravities up to 450G. These results provide rules for scaling crater dimensions in Ottawa sand over a range of more than 10 orders of magnitude in total energ

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00235

年代:1980

数据来源: WILEY

摘要:

Results from refraction experiments have been combined with local earthquake data to produce a preliminary model for the structure of the New Hebrides arc‐trench system. Maximum crustal thickness of about 28 km occurs beneath the island ridge. The crust is unusually thick (about 10 km) in the zone seaward (westward) of the trench, where the velocity of the deepest layer is 7.0 km/s. The crustal thickness of the Fiji plateau, east of the New Hebrides ridge, varies between 5 and 7 km. The velocities of the materials that form the inner trench wall are similar to those found in the island ridge. A zone of relatively high velocity (4.1 km/s) is encountered at a depth of about 1 km beneath the arc‐trench slope. This zone thickens toward the trench and is overlain by material with velocity typical of surficial sediments (≈ 2 km/s). This may be an accretionary deposit, or it may be of volcanic o

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00253

年代:1980

数据来源: WILEY

摘要:

Depths and reflection times to mantle have been computed in the west Pacific from 60 sonobuoy refraction solutions, many of which could be compared with observed mantle reflection depths from multi‐channel data obtained at the same time. After repicking some of these sonobuoy records, all were eventually adjusted to agree within 0.05 s with the observed mantle reflection times. This added constraint produces solutions that are clearly more reliable. Crustal velocities (exclusive of water and sediment) from the study area are rather tightly distributed about a mean value of 6.53 km/s with a standard deviation of only 0.23 km/s (n= 47). Results show that the crust thickens in a westerly direction from the west Pacific basin, where mantle depths are 11–11.5 km to a belt 200 km east of the Japan trench, coinciding with the outer gravity high, where mantle is at an average depth of 14 km. Several sonobuoys in the zone of maximum crustal thickness just east of the outer slope of the Japan trench record two deep reflectors about 0.6 s apart in the vicinity of the upper mantle. Two values of interval velocity obtained from a reducedT2/X2analysis of the layer bounded by these reflectors are 7.5 and 7.2 km/s. These sonobuoys and a few others with weaker double reflections are all located within the outer gravity high. To the south a well‐observed mantle reflection and its strong 8.2‐km/s refraction disappear from our records just as the crust begins its descent into the Bonin trench. Within the outer trench slope a 7.3‐km/s refractor, which is a weak arrival elsewhere, becomes the dominant refractor. The peculiar double reflector near mantle and the marked change in velocity structure and upper mantle reflectivity at the edge of the outer slopes seem to confirm the changes in upper mantle refraction velocity reported by Talwani et al. (1977) in the Curacao trench, Caribbean Sea, but our evidence is not conclusive. In spite of the ambiguity it is clear that velocity structures near active trenches cannot be confidently defined by correlating sections with similar v

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00267

年代:1980

数据来源: WILEY

摘要:

NNE normal fault trends characterize much of the northern Basin and Range province. These faults make sharp bends to NNW and ENE trends in north‐central Nevada in the vicinity of a mid‐Miocene rift characterized by a zone of diabase dike swarms, graben‐filling flows, and a coinciding aeromagnetic anomaly. Despite a roughly 45° change in the least principal stress direction since mid‐Miocene time, pre‐existing NNW‐ and ENE‐trending faults in the vicinity of the rift accommodated the extension whereas regionally, major crustal blocks were faulted along a NNE trend, approximately perpendicular to the modern least principal stress direction. An assumed uniform regional stress field (derived from geologic and geophysical indicators of the modern principal stress field) and the observed oblique slip on the pre‐existing faults were combined in an analysis utilizing an empirically derived frictional sliding law and the Coulomb failure criterion. This analysis constrained the ratio of the least principal stress to the greatest principal stress (S3/S1) as well as the inherent shear strength of intact crustal rocks, τc. While both parameters,S3/S1and τC, are functions of unknowns including pore pressure and the cohesion (frictional strength) of the pre‐existing faults, reasonable assumptions about these parameters lead to τcestimates that agree well with values obtained from laboratory experiments simulating crustal conditions. At a depth of 10 km, the analysis indicates that the minimum inherent shear strength of intact crustal rocks must range between 150–450 bars for zero pore pressure and 150–350 bars for hydrostatic pore pressure, whereas the corresponding maximum shear stresses at 10‐km depth are 970–1200 bars for zero pore pressure and 640–770 bar

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00275

年代:1980

数据来源: WILEY

摘要:

The electron band structure and charge density of MgO are investigated theoretically atV0/V= 1.0, 1.25, 1.50. It is found that compression increases the band gap to a value of the order of 10 eV at lower mantle conditions. The prediction of increasing band gap is consistent with the experimentally observed decrease of the index of refraction of MgO. The valence charge density shows a significant amount of delocalization which further increases with compression. Ionic radii and ionicities cannot be uniquely defined, and in particular, the distribution of the valence charge, and therefore the properties of the oxygen ‘ion,’ are strongly influenced by the nature of the cation. Some geophysically important consequences are a likely increase in the transparency of MgO and its silicates to thermal radiation and a very large electrical resistivity at lower mantle conditi

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00285

年代:1980

数据来源: WILEY

摘要:

Two types of shock wave experiments have been conducted on samples of Blair dolomite. The first employed a light‐gas gun to produce uniaxial strain, while the second used a spherical high‐explosive charge to cause radial divergent flow. Comparisons of data taken from these two types of experiments (at strain rates of 106/s and 104/s, respectively) with quasi‐static data (at 10−4/s) show that Blair dolomite is strongly time dependent in its behavior. This time dependence appears over the entire stress interval from 0.24 to 5

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00293

年代:1980

数据来源: WILEY

摘要:

Eleven detailed profiles of heat flow measurements have been completed over young oceanic crust of the Juan de Fuca ridge system. Individual measurements were spaced typically 0.4–1 km apart along multiple‐penetration lines from 3 to 20 km in total length, and each measurement was located with respect to structural and sedimentary features by simultaneous seismic reflection profiling. In all cases, the average heat flow is well below that predicted by simple conductively cooled spreading models, even when the sediment cover is thick and the nearest basement outcrop is 15 km away. This disparity is attributed to ventilated convective circulation of water in the crust. Large heat flow variability is common along all profiles. Variations are present at two scales. Small‐scale variations (from 1 km to a few kilometers between significant heat flow maxima), present in the younger profiles, probably reflect the influence of local venting and permeability variations on permeable layer cellular convection. Large‐scale variations (10–20 km between significant heat flow maxima), present in all profiles, may reflect the influence of regional circulation driven by cold water recharge at isolated basement outcrops. Laboratory experimental data indicate that normal cellular convection can coexist with larger‐scale bilateral flows, so that there is no simple way to extract the permeable layer thickness from the surface heat flow data. There is a considerable reduction in the amplitude of small‐scale heat flow variability over the range of crustal age studied (0.1–12.5 m.y.). This is probably caused by the thermal filtering effects of the sediment cover which increases from about 50 m near the ridge crests to over 700 m on the flanks.The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, D.C. 20009. Document J79‐007; $1.00. Payment mus

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00299

年代:1980

数据来源: WILEY

摘要:

Twelve new heat flow values in Wyoming are in the range 0.6–2.1 μcal/cm2s (25–88 mW/m2). Radioactive heat productions at eight localities range from ∼0 to ∼1.3 μμcal/cm3s (∼0−5.4 μW/m3). These data are consistent with the following interpretations: (1) The Laramie Mountains‐eastern Wyoming Basin area is a zone of normal heat flow (0.6–1.6 μcal/cm2s (25–67 mW/m2)) that is characterized by low flux (∼0.6 μcal/cm2s (∼25 mW/m2)) from the lower crust and upper mantle. (2) The eastern boundary of the Yellowstone caldera heat flow high (≥2.5 μcal/cm2s (≥105 mW/m2)) is narrow. (3) The heat flow is high (1.9–2.1 μcal/cm2s (79–88 mW/m2)) in parts of the Black Hills in northeastern Wyoming and western South Dakota. From the data presented, a major heat flow transition occurs between the Medicine Bow and Laramie mountains in Wyoming (0.6–1.3 μcal/cm2s (25–54 mW/m2)) and the Rocky Mountains in northern Colorado (2.2–3.0 μcal/cm2s (92–125 mW/m2)). The high flux in this part of the Southern Rockies may mean that the zone of high heat flow associated with the Rio Grande rift extends to the Colorado‐Wyoming Border. The normal heat flow in the Laramie Mountains‐eastern Wyoming Basin area implies submelting temperatures in the upper mantle. In contrast, the very high flux in northern Colorado may be related to high‐temperature, nonradiogenic heat sources in the lower crust and upper mantle because the width of the transition to normal flux in the Lar

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00311

年代:1980

数据来源: WILEY

摘要:

Oxygen self‐diffusion coefficients for forsterite single crystals were determined in different orientations by the single‐spectrum proton activation technique. In the temperature range 1275°–1625°C the diffusion coefficients in b direction can be represented by Db= 3.5 × 10−3exp [(−89 ± 3 kcal/m

ISSN:0148-0227    

DOI:10.1029/JB085iB01p00322

年代:1980

数据来源: WILEY