摘要:

Seven new radar images of portions of Venus are presented, along with corresponding altitude contours. Each is of a circular region of 1500‐km diameter located near the Venusian equator. Areal resolution is, typically, 10×10 km; altitude resolution is 500 m. Much structure of geologic interest is reveal

ISSN:0148-0227    

DOI:10.1029/JB081i026p04807

年代:1976

数据来源: WILEY

摘要:

We have reexamined the structure of the lunar mantle by using data on the decay of shear wave amplitude with distance and the relative arrival times ofPandSwaves. The new analysis confirms our previous lunar model based primarily upon travel times and more closely defines certain properties of the lunar mantle. A negative shear wave velocity gradient of 0.0013 (km/s)/km, shear waveQof 4000, and Poisson's ratio of 0.250±0.025 are found for the upper mantle, which lies between the depths of 60 and 300 km. A rapid decrease of shear wave velocity with increasing depth is observed starting at around a 300‐km depth, associated with a lowerQfor shear waves. Poisson's ratio in the lower part of the middle mantle, which extends to a depth of about 1000 km, is estimated to be 0.36±0

ISSN:0148-0227    

DOI:10.1029/JB081i026p04818

年代:1976

数据来源: WILEY

摘要:

The ages of large martian channels have been studied by determining the relative abundances of craters superimposed on channels and adjacent terrains and by examining superposition relationships between channels and plains and mantle materials. The channels are extremely old, are spatially confined and temporally related to the ancient cratered terrain, and in many cases are related to the as yet poorly understood genetic processes of fretting and chaos formation. No evidence is found for recent channel activity.

ISSN:0148-0227    

DOI:10.1029/JB081i026p04825

年代:1976

数据来源: WILEY

摘要:

Theoretical mass flow rate and the particle trajectory equations of motion of granular material in saltation are used to correlate experimental data for the rate of erosion in the wake regions of wind‐tunnel‐model Martian craters. Vertical geometric distortion is inherent in the simulation because of the equivalent roughness height characteristic of a turbulent boundary layer, which is affected by material in saltation. It is thus necessary to distort topographic model geometry in the vertical direction. A systematic similitude which is based on erosion rate and equivalent roughness in saltation is shown to correlate time‐dependent model data as long as the model Reynolds number is higher than a critical

ISSN:0148-0227    

DOI:10.1029/JB081i026p04846

年代:1976

数据来源: WILEY

摘要:

The predictions of a simple model based on the concept that the lithosphere is a thermal boundary layer, somewhat analogous to a layer of ice on a pond, are in agreement with the relevant data from North America. The mean surface heat flow of tectonic provinces is approximately inversely proportional to the thickness of the lithosphere. If crustal thickness remains constant, a 40‐km increase in lithospheric thickness results in a 1‐km decrease in surface elevation, because unlike the ice analogy the lithosphere is denser than its underlying asthenosphere. It is suggested that part of the observed decrease of continental heat flow in the time following a thermal event is caused by cooling and thickening of the lithosph

ISSN:0148-0227    

DOI:10.1029/JB081i026p04857

年代:1976

数据来源: WILEY

摘要:

Two holes were drilled to depths of about 180 m in the Lower Klamath Lake basin south of Klamath Falls, Oregon, to obtain heat flow data and to provide estimates of the thermal conductivity of the valley fill. Twenty‐nine thermal conductivity determinations on eight cores give a mean conductivity of 1.82 mcal/cm s °C (0.75 W/m °K). Curvature in the upper 50 m of both terriperature profiles indicates a decrease in surface temperature of about 1.8°C, presumably resulting frorn reclamation of what was marshland in the early part of this century. A surprisingly low heat flow of 0.3 HFU (1 HFU = 10−6cal/cm2s = 41.8 mW/m2) was measured at site LS near the center of the basin. At site OC‐1, 7 km east of LS and 2 km from the Klamath Hills geothermal zone, the heat flow was 1.44 HFU, also a low value in this setting. Temperature profiles in 15 unused water wells in the area had linear gradients ranging from 47° to 170°C/km. The corresponding lower limits of heat flow (conductivities measured at the two heat flow sites being used) range from 0.8 to 3.1 HFU. These variations in heat flow evidently are caused by temperature variations in a convecting system within the near‐surface volcanic rocks and do not provide firm constraints on the nature of heat sour

ISSN:0148-0227    

DOI:10.1029/JB081i026p04863

年代:1976

数据来源: WILEY

摘要:

We solve the two‐dimensional time‐dependent heat flow equation across an idealized fracture zone boundary, which includes the process of lateral heat conduction across an initially abrupt temperature contrast. From this solution we can calculate the temperature field for any initial offset and at any sequential time. We can use this to calculate a theoretical free air gravity anomaly across the fracture zone given an assumed density variation with temperature. Results for a 30 m.y. initial offset show that both the amplitude and the shape of the anomaly are significantly altered by lateral heat conduction across the fracture zone even during the first few million years. Assumptions of a sharp density contrast across fracture zones are therefore only valid in very restricted regions close to the ridges. We also find that the theoretical anomaly is small in comparison to observations of free air gravity anomalies. These results add significant complications to attempts at finding unique models for the structure of the lithosphere from gravity anomalies across fracture zo

ISSN:0148-0227    

DOI:10.1029/JB081i026p04869

年代:1976

数据来源: WILEY

摘要:

The magnetic basement topography and associated magnetic anomaly lineation pattern in an area north of the New England seamounts lying between anomaly 31 and the continental margin were examined in light of the Mesozoic sea floor spreading history of the northwest Atlantic. Within the magnetic quiet zone the crust is typically oceanic as far landward as the slope anomaly and exhibits a normally polarized magnetism in the inner quiet zone. Magnetic lineations have been correlated with the Keathley sequence near Bermuda, and, based upon best estimates of the reversal time scale, cross‐strike spreading rates are found to be about 1.7 cm yr−1from −172 m.y. to −136 m.y. and between 0.9 and 1.0 cm yr−1from −136 m.y. to −72 m.y. Landward of the Keathley sequence, magnetic anomalies decrease in amplitude, although the intensity of magnetization of the normally polarized crust exhibits average values. Magnetic observations within a large area of the quiet zone surveyed in detail can be fitted to the basement topography if a Jurassic pole position of normal polarity is assumed. However, three zones trending subparallel to the Keathley lineations require a weak reversely polarized magnetism. These low values of remanent magnetization are attributed to either viscous remanence or contamination of the original crust by widespread volcanic flows or intrusives during periods of predominantly normal magnetic polarity. The normal and reversed polarity sequence resulting from this study is correlated with the sequence obtained from paleomagnetic measurements on land. Trends of isochrons within this relatively limited oceanic area necessitate an alteration in the shape of the ridge axis. A difference in spreading rate and direction is obvious on either side of the New England seamount chain and is discussed with respect to the history of early plate motions in the north

ISSN:0148-0227    

DOI:10.1029/JB081i026p04875

年代:1976

数据来源: WILEY

摘要:

The distribution of large shallow earthquakes along subduction boundaries does not agree with the distribution pattern that might be predicted from a simple model of plate tectonics. That is, along extensive sections of some island arcs, large shocks occurred infrequently or not at all during recorded history. Most of these zones of long‐term quiescence are nearly coterminous with segments of the margin where zones of seamounts, aseismic ridges, or other bathymetric highs of the underthrust slab appear to be interacting with the subduction process. This spatial correlation suggests that at least some of the long‐term absences of great shocks may result from a tectonic origin and not from temporary intervals of strain accumulation. The zones where rises interact with active trenches are also characterized in many instances by a near absence of low‐angle thrust type mechanisms, by gaps in intermediate depth hypocenters, and by gaps and offsets in the line of active volcanoes. Thus major departures from classic subduction activity may develop where significant bathymetric features interact with a convergent margin. To explain these observations, we favor the hypothesis that aseismic ridges or other uplifted regions may delineate zones of oceanic lithosphere which are relatively buoyant and resist subduction upon collision with an active trench. In place of ‘typical’ oceanic lithosphere therefore there may exist a broad spectrum of average densities for oceanic lithosphere, and the relative buoyancy of both the underthrusting and the overthrusting slabs near the subducting margin may be a dominant influence in the development of subduction tectonics and in the locations and frequency of great ea

ISSN:0148-0227    

DOI:10.1029/JB081i026p04885

年代:1976

数据来源: WILEY

摘要:

The interaction of fault slip and lithospheric creep is studied on the basis of an idealized viscoelastic model of the lithosphere. Under a constant average driving stress the model displays a periodic slip‐creep‐slip limit cycle. A simple formula is found that relates effective lithospheric viscosity to fault depth, long‐time average slip rate, and dynamic driving stress τAD= τA‐ τD, where τAis the average shear stress in the lithosphere and τDis the dynamic shear resistance to fault slip. An estimate is made of lithospheric viscosity i

ISSN:0148-0227    

DOI:10.1029/JB081i026p04897

年代:1976

数据来源: WILEY