摘要:

Multispectral mosaics of the three icy Galilean satellites have been produced from low‐ to moderate‐resolution Voyager images. The digital image data have been calibrated using preflight and in‐flight calibration data and reduced to normal albedos for four‐color band passes, centered at 0.35, 0.41, 0.48, and 0.59 μm. The individual images have been geometrically corrected, mosaiked, and presented in a normal cylindrical map projection at a uniform scale of 8 km per pixel. Preliminary analysis of the normal albedo data shows that (1) the darkest terrains on the satellites are the dark regions on Callisto, (2) the brightest regions are bright plains regions on Europa and bright, fresh craters on Ganymede, (3), bright craters on Ganymede and its dark, cratered regions are both significantly brighter than comparable terrains on Callisto, and (4) Europa exhibits several distinct spectral units, with regions on the trailing hemisphere having lower relative ultraviolet spectral reflectance than areas on the leading hemisphere. This is consistent with proposals that Io‐related plasma impact on Europa's trailing side has altered its optical

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05789

年代:1983

数据来源: WILEY

摘要:

The results of cratering and fragmentation experiments on pure ice are reported. The projectiles used are cylindrical aluminum, poly‐carbonate, teflon, and pyrophyllite fired at velocities between 110 m/sec and 680 m/sec, with kinetic energies at impact between 2 and 500 joules. Crater diameters (pit diameters) in the ice were about two times larger than craters in the same energy range in basalts. The ratios of (pit diameter)/(spall diameter) are about three and the ratios of (depth of crater)/(spall diameter) are between 0.1 and 0.3 which are close to the depth/diameter ratios observed in basalts. The crater diameter in ice is also well expressed as a single function of the ‘late‐stage effective energy’ defined recently by Mizutani et al. [1983]. The specific energy for complete destruction of ice target is about 50 J/kg which is two orders of magnitude smaller than that of basalt. The present experimental data on cratering and fragmentation of ice show that the impacts associated with the largest craters on Callisto and Mimas must have severely fractured the whole satellites, and that those giant impacts with the kinetic energy of 1023to 1027joules probably affected significantly the evolution of the sat

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05806

年代:1983

数据来源: WILEY

摘要:

The chemical compositions of primary magmas of olivine tholeiite (OTB), high‐alumina basalt (HAB), and alkali olivine basalt (AOB) are obtained by the olivine maximum fractionation model for Quaternary magnesian basalts from the Northeastern Japan arc. These basalts are assumed to have fractionated only olivine crystals before eruption. The melting phase relations for three primary basalt compositions have been determined under both anhydrous and water‐undersaturated conditions. The AOB melt coexists with olivine, orthopyroxene, and clinopyroxene at 17 kbar and 1360°C under anhydrous conditions and at 23kbar and 1320°C in the presence of 3wt % water. The HAB melt also coexists with the above three phases at 15 kbar and 1340°C under anhydrous conditions and at 17 kbar and 1325°C in the presence of 1.5 wt % water. The OTB melt, on the other hand, coexists with olivine and orthopyroxene at 11 kbar and 1320°C under anhydrous conditions. The water contents in arc basalt magmas are estimated to be about 3, 1.5, and nearly O wt % for the AOB, HAB, and OTB, respectively, on the basis of the solubility limit of water in silicate melts. Based on these estimates and the experimental results, the AOB, HAB, and OTB magmas are suggested to segregate from the mantle at about 1320°C and at 23, 17, and 11 kbar, respectively. As the temperatures at the segregation of the magmas given above appear to be too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for the magma production in a subduction zone. Considering the heat of formation of melt in the diapir, the region with temperatures higher than 1400°C has to be present in the ma

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05815

年代:1983

数据来源: WILEY

摘要:

Crustal deformation associated with hydrofracture is modeled by a dipping rectangular dislocation beneath the surface of an elastic half space. The Burger's vector is taken normal to the rectangular surface. Analytical expressions for vertical displacements are obtained from integration of Volterra's equation by using Mindlin's point force solutions for the elastic halfspace. In the limits of 0° and 90° dip, the results agree with those obtained by Maruyama (1964) for Poisson's ratio 0.25. Expressions for arbitrary Poisson's ratio are given. For elongate dislocations the displacement field agrees with curves given by Pollard and Holzhausen (1979) for two‐dimensional, near surface fractures at various dips. At dip = 0°, a deep square dislocation gives displacements indistinguishable from those of Sun's (1969) model of a horizontal, penny shaped crack beneath the surface of an elastic half space. Modifying his result for shallow depths shows that they correspond to a dislocation of semi‐elliptical cross‐section with the surface displacement field asymptotically approaching a semi‐elliptical shape as depth decreases to zero. Similarly, the surface displacement field of the square dislocation approaches that of its rectangular cross‐section. Nonlinear inversion of surface tilt fields, associated with hydrofracture of deep boreholes, gives estimates of fracture geometry, position, and orientation along with their uncertainties. Two examples are presented, one of a near horizontal and one of a near verti

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05826

年代:1983

数据来源: WILEY

摘要:

Multichannel seismic reflection data collected near the southern Kenai Peninsula, Alaska, show deep events that could stem either from a linear diffractor or from a curved reflector. Whichever is the cause, it must be at a minimum depth of 12 km. If a linear diffractor causes the most obvious events, it strikes at a 60° angle across the surface trend of Mesozoic rocks. Thus the diffractor could be associated with a transverse tectonic boundary that, on the basis of other evidence, is thought to strike through the area of this study. Alternatively, these deep events could be reflections from rocks associated either with a detached piece of oceanic crust or with the downgoing Pacific plate. These associated rocks could be porous and overpressured sedimentary rocks that are squeezed between indurated or metamorphosed sedimentary rocks in the overthrust plate and igneous rocks in the downgoing oceanic crust

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05835

年代:1983

数据来源: WILEY

摘要:

Shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth's surface is investigated using three‐component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in Honshu, Japan. Eighty nine high‐quality seismograms were selected from a period of 3 years. The data used in this study are restricted such that incidence angles are smaller than the critical angle of 30° to the earth's surface in order to avoid phase shifts in the shear wave train. To find directions of the maximum and minimum velocities in split shear waves, where shear waves are resolved into two phases with the maximum time separation, each set of the two horizontal component seismograms is rotated in the horizontal plane. The split shear waves thus obtained are again recombined after the correction of anisotropy, and the anisotropy‐corrected particle motion is compared with the focal mechanism for a cross‐check of the observed anisotropy. Directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station. The stereograms and the cross sections of seismic ray paths show that (1) the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations, and (2) the anisotropic region is separated into two parts: one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0° to 30° from the north (north anisotropy) and the other in the south with it trending 90° to 120° (south anisotropy). The maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions. The horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km. perhaps prevalent in west Honshu. However, the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles. If we assume the vertical extent of the anisotropic material to be 100 km, the maximum velocity contrast would be 4%. If we adopt a crack alignment model, for the observed anisotropy, cracks are inferred to be distributed vertically trending 0° to 30° in the north and 90° to 120° in the south. If we assume an olivine alignment model, olivine crystals are inferred to be aligned in the north with theaaxis of the slip direction trending 0° to 30° and the slip plane normal to thebaxis being vertical and in the south anisotropy with the axis trending 90° to 120° and the slip pl

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05850

年代:1983

数据来源: WILEY

摘要:

Seismic imaging of the Elberton granite in the Georgia Inner Piedmont was undertaken to define its shape and to determine its relation to the southern Appalachian thrust. The recent Consortium for Continental Reflection Profiling (COCORP) lines in the southern Appalachians suggest a major thrusting of a crystalline block over sediments, although the extent of this thrust is subject to interpretation. The fine‐grained Elberton, of Late Devonian age, is the only major pluton crossed by the seismic traverse and is not well defined on the unmigrated common depth point (CDP) seismic section. Migration by beam‐forming reveals considerable variation in the direction of returning signal and provides a clearer image of the laminated reflections, ubiquitous in the southern Appalachians, which have been interpreted as the equivalents of the Valley and Ridge sediments beneath an allochthonous sheet. The upper 5 km portions of the migrated and CDP sections lack events, probably because of the necessary stripping (mute) of a large‐amplitude, early arrival from the seismic records before further processing. Filtering, and transformation of the data to the intercept‐slowness domain, to eliminate this interfering arrival are attempted to enhance the upper portion of the seismic sections Although these approaches do not give a consistent picture of the shallow section, a pattern of scatterers located at depths of 2–3.5 km emerges; these may correlate with the base of the intrusive body. This identification of the granite base, along with the presence of the thrust‐related reflectors beneath the pluton, suggests a prethrust or synthrust emplacement of the granite. However, tectonic interpretations of the Inner Piedmont based on the Elberton granite are severely limited by assumptions: that the pluton age corresponds to time of cooling, that the pluton is not fed by a narrow (seismically elusive) feeder, and that the thrust has not been reactivated after emplacement of the intrusive body. If all these assumptions hold, then the Late Devonian age of the granite is the earliest possible time o

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05865

年代:1983

数据来源: WILEY

摘要:

Supplement available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N. W., Washington, D.C., 2009. Document B83‐002; $2.50. Payment must accompany order.Teleseismic data were recorded within the Sierra Nevada to look for lateral variations in the upper mantle. The data were collected at both temporary and permanent stations, andPwave residuals were computed. After correcting thePresidual data for crustal and topographic effects, there is still a variation of as much as 0.5‐0.6 s from the north end of the Sierra Nevada to Mono Lake, located east of the central part of the range. In addition, there are significant variations in travel time patterns, depending on the azimuth of wave arrivals. Two simple modeling approaches have been used to infer the upper mantle velocity structure from the observed variations in travel time. In the first, it is assumed that the velocity variations are distributed throughout a depth range of about 60–160 km. One model that fits the data indicates a north‐to‐south trend in upper mantlePvelocities from about 7.9 km/s at the north end of the Sierra to 7.6 km/s near Mono Lake. Superimposed is a west‐to‐east trend from 7.85 km/s under the Sierra Nevada crest to 7.7 km/s east of Lake Tahoe. In the second modeling approach, a fixed velocity contrast is assumed between the lithosphere and asthenosphere with variations in travel time resulting from variations in lithospheric thickness. One model, consistent with the observations, indicates a thinning of the lithosphere from 110 km at the north end of the Sierra to 60 km n

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05874

年代:1983

数据来源: WILEY

摘要:

Two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault, accompanied by a progressive broadening of the zone of deformation adjacent to it. In the thin lithosphere model, transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere. For an earth model with a thick elastic lithosphere (plate thickness » depth of seismic slip), transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane. Data from the San Andreas fault indicate a long‐term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle. Observations support a cycle‐long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar. If so, the usually lower strain rates and broader deformation zone currently observed on the southern San Andreas represent a later evolutionary stage of the northern locked section, where a great earthquake is a more recent occurrence. Although the data allow some extreme models to be discarded, no sufficiently strong constraints exist to decide between the thin and thick lithosphere models. Regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time‐predictable

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05893

年代:1983

数据来源: WILEY

摘要:

The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B83‐004; $2.50. Payment must accompany order.A combined spatial and secular variation model of the geomagnetic field, labeled M061581, is derived from a selection of Magsat data. Secular variation (SV) data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model. The SV patterns from this new model are compared with those from the 1980 IGRF and with those for 1970 computed by the DGRF and with the 1960 patterns computed using the GSFC(12/66) model. Most of the features of the M061581 are identical in location and level with those of the 1980 IGRF. Together they confirm that the reversals in sign of field change seen over Asia and North America between 1965 and 1975 are reverting to the pre‐1965 states. The M061581 model gives −32 nT/yr for the dipole decay rate, larger than the 70% increase already reported since 1965. This abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger (−100 nT/yr) drop in field over the polar regions not indicated by the 1980 IGRF. This north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric (counterclockwise) afternoonSqpcell increasing in intensity from the beginning (November 1979) to the end (June 1980) of the Magsat data collection period, coupled with an enhancement of its effect as the orbit lowered from the 350– 550 km initial altitudes to near 200 km just prior to burnup. This experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that ‘annual snapshots’ of the field by a satellite would allow easy and accurate models of secular change without the use of any

ISSN:0148-0227    

DOI:10.1029/JB088iB07p05903

年代:1983

数据来源: WILEY