摘要:

Permeability of crystalline rocks results mainly of cracks. Using percolation theory and a statistical approach, Dienes has recently derived a model which appears to be very adequate to describe and calculate permeability in such rocks at a scale which is larger than the scale of the cracks. Using this model, we show that three microstructural parameters are of primary importance : the crack shape factor A, the mean crack length and the average crack spacingAlthough the third one, is frequently presented as the most important parameter, we suggest that A andare indeed as much important. Percolation transition depends on andbut high permeabilities are obtained if either A or are large. Using these results we discuss time constants for permeability evolution by taking into account slow crack growth processes. We suggest that slow crack growth could result in large increases of permeability with or without developing a macroscopic fracture. Such effects are of interest for seismogenesis, underground repositories and other problems concerned with fluid flow in the crust.

ISSN:0094-8276    

DOI:10.1029/GL013i005p00460

年代:1986

数据来源: WILEY

摘要:

Single crystal of the orthorhombic perovskite modification of MgSiO3have been grown from a melt at 27 GPa and 1830°C. The starting material of orthoenstatite was sealed in a platinum capsule with water. Crystals of good quality up to 200 microns in size were obtained, which are promising for the structural refinement, Brillouin spectroscopy, and other measurements. Some crystals, however, exihibit twinning on the (110) plane

ISSN:0094-8276    

DOI:10.1029/GL013i005p00464

年代:1986

数据来源: WILEY

摘要:

The interaction of basalt melt with mantle harzburgite at low pressure might result in silica enrichment of the melt by assimilation of orthopyroxene. Experimental tests of this hypothesis show that silica‐rich liquids (56 wt %) are produced by melt‐orthopyroxene reaction at 1200° to 1250°C. These silica‐rich liquids are enriched in Na2O and K2O over that found in the starting basalt melt which indicates that the alkalis diffused against a concentration gradient into the silica‐rich liquid. The liquids produced in this way are chemically similar to high‐magnesium andesites o

ISSN:0094-8276    

DOI:10.1029/GL013i005p00467

年代:1986

数据来源: WILEY

摘要:

Some studies of global paleomagnetic data have found an offset of the magnetic pole during the Plio‐Pleistocene which has been interpreted as indicating a period of rapid True Polar Wander, with a rate of movement comparable to the present‐day rate of polar motion deduced from astronomical observations. We show that much of the polar offset determined from the paleomagnetic data may be due to deviations in pole position caused by persistent non‐dipole zonal components of the geomagnetic field. A correction of paleomagnetic poles for the long‐term non‐dipole field reduces the polar offset and thus suggests a slower or shorter episode of True Polar Wander over the past 5 mill

ISSN:0094-8276    

DOI:10.1029/GL013i005p00471

年代:1986

数据来源: WILEY

摘要:

Geosat, a Navy satellite launched in March 1985, will provide the first long‐term, global altimeter data set. These observations will enable construction of monthly sea level maps of the equatorial Pacific, much like those presently derived from island tide gauges, but with greatly improved spatial resolution and coverage. During the first half of the Geosat mission, the satellite ground track will not repeat, requiring time series analyses to be based on altimetric height differences at crossover points. We have used the 3‐month Seasat data set in the equatorial Pacific to test the feasibility and accuracy of this procedure. Crossover differences were sorted into diamond‐shaped areas with zonal and meridional dimensions of 200 and 400 km, respectively. The sea level time series in each of these areas was then determined using objective analysis. These computations show agreement within 5 cm between altimeter and island tide gauge observations on time scales greater than 1 month. Using this technique, we propose to construct monthly maps of sea level variability in the equatorial Pacific for the duration of the Geosat mission (approximately 3 y

ISSN:0094-8276    

DOI:10.1029/GL013i005p00475

年代:1986

数据来源: WILEY

摘要:

Geophysical studies of the major boundary fault(s) in eastern Long Valley caldera have led to significantly different models being proposed for the subsurface structure of this area. Pakiser originally interpreted gravity data to propose a steep vertical offset across the eastern boundary fault(s) which may be as great as 5 km. On the other hand, Hill employing seismic refraction data, and Abers, who reinterpreted the available gravity data in the light of the "hard" seismic constraints of Hill, suggested that the offset across this fault may be much more gentle. However, new magnetotelluric data may call for a revision of these models. Based on these recent MT results, offsets along the eastern boundary fault appear to be somewhat steeper than recent models would suggest. Our current thinking would favor a model more in keeping with the one originally envisaged by Pakiser, but with less throw (approximately 2 km) across the faults.

ISSN:0094-8276    

DOI:10.1029/GL013i005p00479

年代:1986

数据来源: WILEY

摘要:

Simulations of natural smoothing of paleomagnetic reversal records suggest that illustrative features of actual field behavior may be made undiscernible even when relatively short integration times are associated with the acquisition of magnetic remanence. The results of this exercise further caution against the sole use of logs of component behavior (i.e. declination, inclination and intensity) for analysis, since time‐averaging of the complete vector field not only smooths, but also can significantly alter the component paleomagnetic waveform

ISSN:0094-8276    

DOI:10.1029/GL013i005p00483

年代:1986

数据来源: WILEY

摘要:

The influence of dislocations upon carrier mobility and diffusivity is studied through measurements of the electrical conductance of single crystals of olivine during creep deformation in the temperature range 1610‐1725 K, under controlled oxygen fugacity and under uniaxial stresses of 60 to 140 MPa. The results presented indicate that under the conditions examined here the generation and movement of dislocations and other defects associated with plastic deformation does not significantly affect the carrier concentration in olivine. One possible explanation of this conclusion may be that dislocation climb in our creep experiments is controlled by the diffusion of electrically neutral defects. In addition, the results indicate that the mobility of the defect responsible for conduction is not significantly perturbed by dislocation effects or that these effects do not exis

ISSN:0094-8276    

DOI:10.1029/GL013i005p00487

年代:1986

数据来源: WILEY

摘要:

A joint‐inversion of magnetic satellite (MAGSAT) and free air gravity data has been conducted to quantitatively investigate the cause for Bouguer gravity anomaly over Central Indian plateaus and possible fold consequences beside Himalayan zone in the Indian sub‐continent due to collision between Indian and Eurasian plates. The appropriate inversion with 40 km crustal depth model has delineated after discriminating high density and magnetisation models, for the first time, about 1500 km long hidden ridge structure trending NW‐SE. The structure is parallel to Himalayan fold axis and the Indian Ocean ridge in the Arabian Sea. A quantitative relief model across a representative anomaly profile confirms the ridge structure with its highest point nearly 6 km higher than the surrounding crustal level in peninsular India. The ridge structure finds visible support from the astro‐geoidal c

ISSN:0094-8276    

DOI:10.1029/GL013i005p00491

年代:1986

数据来源: WILEY