摘要:

Three global‐scale zones of possible tectonic origin are described as occurring along broad, low rises within the Equatorial Highlands on Venus (lat 50° N. to 50° S., long 60° to 310°). The two longest of these tectonic zones, the Aphrodite‐Beta and Themis‐Atla zones, extend for 21,000 and 14,000 km, respectively. Several lines of evidence indicate that Beta and Atla Regiones, located at the only two intersections of the three major tectonic zones, are dynamically supported volcanic terranes associated with currently active volcanism. Rift valleys south of Aphrodite Terra and between Beta and Phoebe Regiones are characterized by 75‐ to 100‐km widths, raised rims, and extensions of only a few tens of kilometers, about the same magnitudes as in continental rifts on the Earth. Horizontal extension on Venus was probably restricted by an early choking‐off of plate motion by high crustal and upper‐mantle temperatures, and the subsequent loss of water and

ISSN:0094-8276    

DOI:10.1029/GL009i005p00499

年代:1982

数据来源: WILEY

摘要:

Correlations have been observed between groundwater radon and thoron concentrations and carbon dioxide discharges at the Lake Hughes station of the Caltech radon monitoring network. The Lake Hughes site is one of three radon monitoring stations located near the "big bend" segment of the San Andreas fault which began to record anomalous radon levels in August 1981. Two stations, Lake Hughes and Lytle Creek, recorded anomalous increases in radon while the third, Sky Forest, recorded an anomalous decrease. Several weeks after the onset of the anomaly, strongly correlated radon fluctuations began at Lake Hughes and Lytle Creek. These radon spikes also were found to be phase anti‐correlated with barometric pressure fluctuations. Analyses of gas grab samples showed relatively high levels of CO2and ethylene in borehole air at Lake Hughes and Lytle Creek, while analyses of water samples showed relatively large increases in HCO3−at both sites. Isotopic analysis of one gas sample from Lake Hughes yielded a13C δ value of −22 ‰, which suggests that the CO2originates from the oxidation of organic material. The correlation in radon fluctuations at Lake Hughes and Lytle Creek and their common dependence on barometric pressure changes began shortly after the onset of the radon anomaly in August, and probably resulted from the simultaneous saturation of the water in these boreholes with carbon

ISSN:0094-8276    

DOI:10.1029/GL009i005p00503

年代:1982

数据来源: WILEY

摘要:

We have developed a technique for measuring from boreholes the radiation pattern of a high frequency electromagnetic antenna system. Three coplanar holes are required with a single transmitter in one and two identical receiving antennas in the other holes. Signal attenuation is measured along several ray paths at different angles from the source. The method does not depend on the subsurface to be electrically homogeneous but does require reflections and refractions to be negligible. Measurements in fractured granite demonstrate the utility of the technique to obtain antenna radiation patterns. A modification of this same method could be used for characterization of downhole acoustic sources.

ISSN:0094-8276    

DOI:10.1029/GL009i005p00507

年代:1982

数据来源: WILEY

摘要:

Seismograms from a local seismic network in the Virgin Islands portion of the northeastern Caribbean are used to determine a composite focal mechanism for 26 microearthquakes along the North America ‐ Caribbean plate boundary. Only one nodal plane of the focal mechanism could be constrained from P‐wave first motion data alone. P/SV amplitude ratios observed for these events were compared to theoretical amplitude ratios calculated for different focal mechanisms. This procedure constrained the dip of the second nodal plane to be shallower than about 50°, ruling out the possibility of a transform fault in this portion of the plate boundary. The resulting focal mechanism indicates that oblique underthrusting of the North American plate beneath the Caribbean plate occurs in the area. This oblique motion is accommodated along a thrust plane that dips at a relatively shallow angle beneath the Virgin Islands plat

ISSN:0094-8276    

DOI:10.1029/GL009i005p00511

年代:1982

数据来源: WILEY

摘要:

We have obtained paleomagnetic data for the southern tip of the middle Tertiary Chilliwack Composite Batholith, located on the Canada‐United States border about 125 km E of Vancouver, B.C. Thirty‐four separately oriented samples were collected along a road traverse 1.5 km long located along State Highway 22, about 20 km NE of Marblemount, Washington. The mean direction after magnetic cleaning is: D, 182.8°; I, −65.0°; α95, 1.5°. This corresponds to a paleomagnetic pole at 87.5°N, 267.5°E, close to other poles for Tertiary plutons from the North Cascades and only slightly displaced from Tertiary reference poles from the craton. We conclude that the North Cascades block has not been rotated or suffered major internal disruption since perhaps late Eocene time, in marked contrast to all the terranes that

ISSN:0094-8276    

DOI:10.1029/GL009i005p00515

年代:1982

数据来源: WILEY

摘要:

Convection in a fluid layer heated from below with two immiscible components A and B of the same average density exhibits different states of motion. Besides a solution describing convection in sublayers other solutions in which fluid B is surrounded by streamlines of fluid A or vice versa are possible. Potential applications to the problem of mantle convection are discussed.

ISSN:0094-8276    

DOI:10.1029/GL009i005p00519

年代:1982

数据来源: WILEY

摘要:

ISEE satellite measurements of magnetic fields in the boundary layer of the earth's magnetosphere have revealed an unexpected orientation of the field lines there that may be indicative of a ‘reverse draping.’ In many crossings of the north dawn boundary of the magnetosphere the direction of the boundary layer field lines was such as to suggest that northern high latitude segments of those field lines were being pulled tailward by the boundary layer plasma faster than were their equatorial segments. The observations are compatible with the view that plasma entry occurs at high latitudes (around the cusps) and that the field aligned current source is at high, rather than near‐equatorial, latitudes. Three‐dimensional plasma flow measurements made with the ISEE satellites showed the tailward flow in the boundary layer to have an equatorward component and this also suggests high latitude entry of plasma. Simultaneous magnetic field measurements with ISEE‐1 and 2 during a sequence of boundary crossings provided determinations of the electric currents in the boundary layer and at its interfaces with the plasma sheet and the magnetosheath. these currents were found to be largely field‐aligned and to have intensities in the range 3‐

ISSN:0094-8276    

DOI:10.1029/GL009i005p00523

年代:1982

数据来源: WILEY

摘要:

A particle code is used to simulate the magnetopause region in the high latitude geomagnetic tail in which the magnetic field undergoes a significant increase in going from the magnetosheath to the magnetotail lobe. The simulation indicates that plasma can flow from the magnetosheath to the lobe, which is accompanied by a drop in pressure and density. In the earth's inertial frame, the particles do work against the convection electric field. Hence the magnetopause region serves as a dynamo. The simulation also shows that the width of the transition region increases with time. In the earth's inertial frame this is seen as an expansion of the magnetopause thickness in the antisunward direction.

ISSN:0094-8276    

DOI:10.1029/GL009i005p00527

年代:1982

数据来源: WILEY

摘要:

We investigate the free escape of ions from the post shock plasma into the region upstream of a fast collisionless shock. We find that the accessibility of downstream ions to the upstream region depends strongly on the upstream magnetic field shock normal angle as well as the upstream plasma parameters. Post shock ions escape upstream in significant numbers for shock normal angles below about 45°, in rough accord with the observed transition between quasi‐perpendicular and quasi‐parallel shock structure. We also calculate upstream ion distribution functions that resemble those of the observed “intermediate” ions and i

ISSN:0094-8276    

DOI:10.1029/GL009i005p00531

年代:1982

数据来源: WILEY

摘要:

The equivalent current of an SC main impulse in the polar region is found to be a single vortex current system which encircles the cusp region when the interplanetary magnetic field is continuously northward in the vicinity of an interplanetary shock wave. The twin vortex current pattern hitherto believed as the current system of the main impulse tends to appear when the IMF is southward. We present evidence suggesting that this IMF dependence of the current pattern comes from the superposition of the twin vortex structure of the DP2 current or the substorm current system on the single vortex system of the main impulse.

ISSN:0094-8276    

DOI:10.1029/GL009i005p00535

年代:1982

数据来源: WILEY