摘要:

Geophysical profiles across portions of the Chagos‐Laccadive Ridge, one of the most prominent features in the Indian Ocean, have been examined. Several north‐south fracture zones were found along the margins of the ridge, which is composed of several segments of different origins. Some portions of the ridge are volcanic features formed as either ‘leaky’ transform faults due to the northeastward movement of India and the spreading activity of the Central Indian Ridge or from drift over a ‘hot‐spot’ area, while others, especially the Maldive Islands segment, may be microcontinents rifted away from India. No obvious magnetic lineations were found in t

ISSN:0148-0227    

DOI:10.1029/JB082i008p01295

年代:1977

数据来源: WILEY

摘要:

A series of models of the lunar interior are derived from topographic, gravitational, librational, and seismic data. The librational parameters and low‐degree gravity harmonics result primarily from surface height variations and only secondarily from lateral density variations. The moon departs from isostasy, even for the low‐degree harmonics, with a maximum superisostatic stress of 200 bars under the major mascon basins. The mean crustal thicknesses under different physiographic regions are: mascons, 30–35 km; irregular maria, 50–60 km; and highlands, 90–110 km. A possible composition consistent with our model is an anorthositic crust, underlain by a predominantly forsterite upper mantle which grades into a refractory rich lower mantle surrounding a pyrrho

ISSN:0148-0227    

DOI:10.1029/JB082i008p01306

年代:1977

数据来源: WILEY

摘要:

A paleomagnetic study of the Belchertown pluton, a syntectonic Devonian intrusion in the Bronson Hill anticlinorium, Massachusetts, indicates the presence in 23 samples from eight sites in primary quartz monzodiorites of a strong, stable, and consistently oriented remanent magnetization vector (D = 154.9°, I = −13.4°, K = 20.8° and α95= 12.4°) with a high Q value (7.0). The remanence is inferred to be of thermal and/or thermochemical origin acquired during the original crystallization and cooling of the pluton. Recrystallized equivalents of the primary rocks, which occur as a surrounding border zone, yield weak and inconsistent results (four samples from two sites) due to the absence of opaque minerals. The mean Belchertown vector is discordant with respect to the mean North American Devonian vector (D = 170° and I = +32°) in such a direction as to suggest that the entire pluton has undergone some 30°–60° of rotation to the northwest about a NE‐SW axis. The high Q value of the primary rocks and their isolation from magnetic country rocks by the surrounding non‐magnetic border zone allow computer modeling of the three‐dimensional shape of the pluton using the measured remanent vector and the total field aeromagnetic anomaly. The resulting model is consistent with the proposed amount and direction of tectonic rotation of a once roughly vertically symmetrical funne

ISSN:0148-0227    

DOI:10.1029/JB082i008p01315

年代:1977

数据来源: WILEY

摘要:

Plate impact techniques and laser interferometry instrumentation were used to obtain continuous one dimensional compression wave data through states of loading and unloading on Blair dolomite in the stress range of 0–6.5 GPa. These data are used to infer the dynamic stress‐strain response. Both the character of the wave propagation and a comparison with quasi‐static uniaxial strain data obtained by other workers suggest substantial strain rate dependence in this rock. The dynamic yield stress of 2.5 GPa determined from the wave propagation data was approximately a factor of 10 higher than that observed in the quasi‐static data. Wave propagation below 2.5 GPa is suggestive of viscoelastic response, and a nonlinear Maxwell constitutive equation fitted to these data provides a characteristic relaxation time of 0.01 μs. Comparison of dynamic and quasi‐static loading data suggests that the large difference in observed yield stress could be the result of a transition from failure by brittle fracture to failure by intracrystalline pl

ISSN:0148-0227    

DOI:10.1029/JB082i008p01325

年代:1977

数据来源: WILEY

摘要:

The single‐crystal elastic constants of SiO2, coesite (monoclinic, C2/c), have been determined from Brillouin scattering measurements on a 100‐μm crystal. The elastic constants inferred from acoustic velocities for 80 different propagation directions are (in units of Mbar) c11= 1.608, c22= 2.304, c33= 2.316, c44= 0.678, c55= 0.733, c66= 0.588, c12= 0.821, c13= 1.029, c15= −0.362, c23= 0.356, c25= 0.026, c35= −0.393, and c46= 0.099. This experimental technique provides a method of determining single‐crystal elastic properties for materials such as high‐pressure phases that are not available in sufficiently large sizes for ultrasonic

ISSN:0148-0227    

DOI:10.1029/JB082i008p01334

年代:1977

数据来源: WILEY

摘要:

A model for earthquake swarms in volcanic regions consists of the following concepts: (1) clusters of magma‐filled dikes exist within brittle volumes of the crust, (2) dikes within a cluster are systematically oriented with their long dimension in the direction of the regional greatest principal stress, and (3) a sequence of shear failures (an earthquake swarm) occurs along a system of conjugate fault planes joining en echelon offset dike tips at oblique angles. This model accounts for commonly observed geometric relations between surface faulting patterns, the hypocentral distribution of swarm earthquakes, and fault plane solutions in a variety of situations. Swarm areas dominated by strike‐slip faulting, however, provide the most compelling examples of the utility of the model. Specific examples considered here include a swarm on the east rift zone of Kilauea volcano, Hawaii, and swarms in the Imperial Valley, California, and the Reykjanes Peninsula, Iceland, which represent transitional zones between spreading centers and transform fau

ISSN:0148-0227    

DOI:10.1029/JB082i008p01347

年代:1977

数据来源: WILEY

摘要:

A practical method of continuous measurement of radon in groundwater for earthquake prediction, especially suitable for field observation, is described. The detector consists of a ZnS scintillation chamber for alpha counting and a separation chamber in which radon is emanated from groundwater to the gaseous phase. The radon concentration in the gaseous phase equilibrates with that in water according to the distribution coefficient. Alpha activities of radon and its daughters (218Po and214Po) in the scintillation chamber are counted and recorded continuously. The counting rate of about 1000 cpm/10−10Ci Rn/1 is obtained by the detector, which has an effective volume of 1.6

ISSN:0148-0227    

DOI:10.1029/JB082i008p01353

年代:1977

数据来源: WILEY