摘要:

Icequakes have been recorded with an eight‐geophone array on an active glacier. All the events appear to originate from extensional faulting near the surface of the glacier. The absence of events occurring at depth suggests that the creep‐instability mechanisms proposed for shear faulting are not operative at depth in the glacier. Most icequakes occurred as single events within the zone of marginal crevasses. Swarms of icequakes were occasionally recorded. The individual events in a swarm were distributed along a straight line several hundred meters long. The activity seemed to progress from one end of this line to the other at a velocity of about 30 m/sec. The line often extended through a crevasse‐free section of the gl

ISSN:0148-0227    

DOI:10.1029/JB075i008p01351

年代:1970

数据来源: WILEY

摘要:

The stratigraphy, structural framework, geological provinces, and evolution of the southern Argentine territory and adjoining continental shelf are discussed. Four main basins (Salado, Colorado, San Jorge, and Magallanes) and several smaller ones occur in the region and are separated by positive areas. With the exception of several uplifts near the Andean region, most of these positive areas are cratonic. The southern hills of Buenos Aires, parts of the Tandil high, and scattered localities in the Deseado and northern Patagonia massifs are relics of Paleozoic basins. The basement, as considered here, is made up of Precambrian and Paleozoic rocks in the Salado and Colorado basins. In the San Jorge and Magallanes basins the basement includes younger rocks, as young as Late Jurassic in the latter area. The present‐day basins are essentially post‐Jurassic features. In Early Cretaceous time continental sediments, generally redbeds, were laid in the Salado and Colorado areas. In the San Jorge basin the Lower Cretaceous deposits are tuffaceous, and in the Magallanes basin they are marine. After a Subhercynian tectonic phase, the sedimentation continued during later Cretaceous time, being nonmarine except in the Magallanes basin. A reversal of the regional slope—from Pacific to Atlantic—occurred at the close of the Cretaceous, in many areas probably with no interruption of the deposition. As a consequence, the sea invaded the basins in the Paleocene, and during the Eocene the marine deposition continued in the Magallanes basin. There are no evidences of sediments of Eocene age in the areas of the Salado and Colorado basins; in the San Jorge basin the sediments are nonmarine. A withdrawal of the seas from the Argentine territory occurred during the Oligocene, according to recent data, and a general subsidence with marine deposition occurred in all the basins in the area in the Miocene. This subsidence was followed by a Pliocene emergence and, during the Quaternary, by several short‐lived advances of the sea near the present‐day Atlantic coast. The main tectonic features in the area are faults trending NW‐SE or NNE‐SSW, with subsidiary N‐S and, to a lesser extent, E‐W directions; thus the differential movements between the basement blocks formed by the faults gave origin to the basins. Successive tectonic phases, which, in part, were contemporaneous with the deposition, faulted and distorted the blocks. This block tectonics also has an important bearing on the igneous activity in Patagonia from Cretaceous time onward. Although the ‘modern’ fault pattern is essentially a post‐Middle Jurassic feature, some faults are pre‐Jurassic and others were formed along pre‐existing tectonic lines. Arcuate trends in the sedimentary cover in several basins (Colorado, offshore; San Jorge) are very likely a manifestation of the above‐mentioned differential movements, perhaps with some horizontal displacement. Close to the Andean area the tectonic lines show predominant N‐S directions, with evidences of compres

ISSN:0148-0227    

DOI:10.1029/JB075i008p01363

年代:1970

数据来源: WILEY

摘要:

The new global tectonics and sea‐floor spreading hypotheses imply downwarping and descent of the lithosphere in island‐arc regions. These downgoing slabs of crust and upper‐mantle material will affect the local temperature regime. Geophysically observable variables of surface‐heat flux and gravity and seismic travel times will be influenced by the temperature. Using a quasi‐dynamic computational scheme and a finite difference solution of the conservation of energy equation we have determined the effects of spreading rate, adiabatic compression, radioactivity, phase changes, and strain heating on the temperature regime of a downgoing slab. Effects of these parameters on surface heat flux, gravity anomalies, and local seismic travel time are also estimated. Predicted travel‐time anomalies show good agreement with observed anomalies in Japan and Tonga‐Kermadec. Several implications on the localized mechanical behavior of the upper mantle and the evolution of a downwarping slab are made on the basis of our computat

ISSN:0148-0227    

DOI:10.1029/JB075i008p01397

年代:1970

数据来源: WILEY

摘要:

The major east‐west fracture zones north of the Hawaiian ridge all undergo marked changes in topography, magnetic characteristics, and trend as they cross a narrow strip extending northwest from 156°W, 21°N to 174°W, 43.5°N. This strip, termed the bending line, marks the location of a major change in the direction of sea‐floor spreading in the north Pacific Ocean. Extrapolation of present spreading rates indicates an early Cretaceous age for the bending line. Minor structural features including shear zones and a small fracture zone found in the well‐mapped area east of the bending line between the Murray and Pioneer fracture zones may have resulted from the reorientation of spreading centers. Later formation of the Musicians seamounts may have been controlled by a zone of weakness along the ben

ISSN:0148-0227    

DOI:10.1029/JB075i008p01421

年代:1970

数据来源: WILEY

摘要:

Twenty‐eight now focal mechanisms were determined for intermediate and deep‐focus earthquakes in the Indonesia‐Philippine region. At intermediate depths beneath simple planar sections of the Sunda and Philippine arcs the axes of minimum compressive stress have a preferred orientation down the dip of the seismic zone, suggesting that the descending slab of lithosphere is under extension in this depth range. Deep‐focus mechanisms beneath the Sunda arc have nearly vertically dipping axes of maximum compressive stress, implying that the descending slab is under compression at great depth. These data are consistent with models in which gravitational body forces are important factors in the dynamics of the lithosphere. Focal mechanisms from events located near regions where the seismic zones deviate strongly from a planar configuration show a striking reorientation of the stress axes. In the Banda Sea and northern Celebes regions the seismicity indicates possible contortions in the underthrust slabs. The orientations of the stress axes in these regions are consistent with such contortion and give additional support to the idea that the lithosphere is a stres

ISSN:0148-0227    

DOI:10.1029/JB075i008p01431

年代:1970

数据来源: WILEY

摘要:

Incremental diameter‐frequency relations for twelve lunar regions are presented for four classes of craters (sharp to soft), 0.02 to 2 km diameter. Sharp craters have similar frequencies on all regions. Soft‐crater densities vary by factors up to 100, not only between highlands and maria, but between maria regions. Highlands lack the large excesses on the maria of soft craters; these excesses cluster about 0.8 km near Hyginus and at Ranger 8 and 9 impact sites, but about 0.2 km on Mare Imbrium, and at both or intermediate sizes for Bonpland and Sinus Medii. No simple large‐scale erosive process on impact craters yields such excesses or the observed regional variations in crater density and percentage mix of sharp to soft craters. Crater populations for individual areas also seem inconsistent with saturation impact gardening: observable craters are not geometrically saturated, and frequency relations are not strictly parallel with slopes of −3. Uniform or episodic erosive or covering processes also generally fail to produce the observed excesses of soft craters, and a covering episode should produce a bump in the frequency of sharp craters that is not observed. These conclusions hold independently of any regional or vertical variations in structure that might affect energy‐size scaling. Layering inferred from other studies apparently was not produced by impact gardening; the depth of the gardened layer (or effective depth of accretion or depletion) is probably less than 1 meter. Our data are consistent with this plausible (but not unique) interpretation: (1) Subsequent to extensive early blanketing, impact cratering has occurred onhighlands and old crater walls, with a probable admixture of endogenic and secondary cratering; (2) The maria underwent similar processes, but have many additional endogenic craters, probably collapse features formed in lava flows. Our counts of craters by class should also be useful for refining alternative models for cratering an

ISSN:0148-0227    

DOI:10.1029/JB075i008p01445

年代:1970

数据来源: WILEY

摘要:

High delay resolution (20 μsec) radar studies of the moon at 6‐, 7.5‐, and 12‐meter wavelengths show that the echo strength at the leading edge peak and the total radar cross section are very sensitive to the position of the subradar point. These phenomena are explained in terms of a variable thickness, attenuating regolith. Furthermore, when the subradar point is in a region of what is believed to be thick regolith, the over‐all backscattermg behavior of the moon is very nearly independent of wavelength in the 68‐cm to 7.5‐meter interval, and there is strong evidence that this is true over an even wider interval. For low angles of incidence, the angular power law is shown to be consistent with backscattering from a normally distributed surface with a Gaussian autocorrelation function and a unidirectional rms slope = tan 4.2°. At high‐incidence angles, the quasi‐specular scattering is thought to be due to reflections from bare rock surfaces. A crude estimate for the dielectric constant of the subregolithic material is made, resulting in a value of 13. The diffuse component of the echoes is thought to be primarily due to volume backscattering from wi

ISSN:0148-0227    

DOI:10.1029/JB075i008p01467

年代:1970

数据来源: WILEY

摘要:

Instead of the expansion in spherical harmonics currently used in satellite geodesy, the geopotential in this analysis is represented by the potential of a simple layer distributed over the surface of the earth. Density values of this layer for 48 surface elements have been determined from Baker‐Nunn camera observations of 4 satellites for 5 weeks. The solution closely approximates the one for the 1966 Smithsonian Institution Standard Earth and Anderle's solution (1967) based on Doppler data. The rms discrepancy in geoidal heights between those two solutions and the solution of this analysis amounts to ±15.5 m for both cases. Existing gravity anomalies are combined with the solution obtained from the satellite observations. The combination does not markedly differ from the satellite solution because the determination of the geopotential from satellite data is strong

ISSN:0148-0227    

DOI:10.1029/JB075i008p01483

年代:1970

数据来源: WILEY

摘要:

A wide‐band, tidal to seismic frequency, accelerometer has been developed that is of simple internal geometry and low drift (<10−9g/day). Initial tidal and earth‐mode frequency investigations are described along with detailed design features and internal noise measure

ISSN:0148-0227    

DOI:10.1029/JB075i008p01493

年代:1970

数据来源: WILEY

摘要:

Aeromagnetic survey data is in error due to diurnal variations, navigational inaccuracies, and instrumental drift. There is a corresponding discrepancy between the readings at the point of intersection between any two lines. The main problem in correction procedures is how to prorate this discrepancy between the two lines. We formulate this as a problem in statistical estimation and find all solutions under the assumption that the systematic errors are varying slowly along a line of data. From this class of solutions we select as “Optimal” the one that results in least adjustment of the data. The resulting algorithm leads to a simple and efficient computer prog

ISSN:0148-0227    

DOI:10.1029/JB075i008p01507

年代:1970

数据来源: WILEY