摘要:

Although rarely reported, the principal magnetic carrier associated with rocks of basaltic composition can be titanomagnetite (Fe3−xTixO4) very rich in titanium (x>.75). If not oxidized during cooling, such a pristine magnetic carrier will possess a Curie temperature too low to insure the acquisition of a stable thermoremanent magnetization (TRM). A stable chemical remanent magnetization (CRM) ultimately may be acquired through room temperature oxidation of the titanomagnetite to titanomaghemite; however, such a process can occur during a time interval far removed from that of initial cooling. A case study is presented which dramatically illustrates the spurious nature of paleomagnetic results associated with such an occurrenc

ISSN:0094-8276    

DOI:10.1029/GL011i008p00681

年代:1984

数据来源: WILEY

摘要:

The 198 Ma Lower Jurassic Copper Mountain intrusions outcrop in the Quesnellia Subterrane of Terrane I in the Canadian Cordillera. Thermal demagnetization at 550 ± 10°C defines a single stable remanence component of 25.9°, 41.2° (α95=3.6°) in 11 reliable sites. The resulting pole position of 11.8°E, 57.3°N (δp= 2.7°, δm= 4.4°) is discordant for the North American craton and reflects 39 ± 6° of clockwise rotation and 9 ± 5° of northward translation. Combined with previous paleomagnetic and geologic data, a consistent model for the geotectonic evolution of Quesnellia and Te

ISSN:0094-8276    

DOI:10.1029/GL011i008p00685

年代:1984

数据来源: WILEY

摘要:

Body wave synthetic seismograms are used to study the source parameters and rupture time history of the doublet of large (MS= 6.9, 7.0) earthquakes which occurred within five hours of one another near the town of Ometepec, Mexico on June 7, 1982. The events are thrust events consistent with the subduction of the Cocos Plate beneath the North America plate. Our results indicate that these are not the simple, single source events previously thought to be typical of the Middle America Trench in this region. While the first event had a simple source time function, the second did not. This relatively complex behavior can not be explained by any tectonic complications that have been proposed to account for the complexity of the 1970 Chiapas earthquake and the 1973 Colima earthquake. Multiple earthquake sequences in 1962, 1950, and 1928 have occurred in the general area of the two 1982 Ometepec earthquakes suggesting that earthquake doublets are characteristic of this region.

ISSN:0094-8276    

DOI:10.1029/GL011i008p00689

年代:1984

数据来源: WILEY

摘要:

A comparison between parameterized and two dimensional finite element reservoir cooling models is described. The models feature strongly temperature dependent Newtonian rheology and thermal boundary conditions simulating sills cooled from above. Cooling histories predicted by the parameterized scheme are found to be in excellent agreement with the more exact finite element calculations for viscosity variations of up to 105occurring at a given time in the reservoir.

ISSN:0094-8276    

DOI:10.1029/GL011i008p00693

年代:1984

数据来源: WILEY

摘要:

Two dimensional acoustic images of sedimentary rocks were generated in order to characterize centimeter‐scale heterogeneities and microstratigraphy. The images are based on dense sampling of the compressional phase velocity at frequencies near 1 MHz. The acoustic observations are compared to visual and x‐ray images of the same samples. The results are intrepreted in terms of microscopic compositional variations. Sedimentary rocks are more variable on the scale of a centimeter than we expected. The homogeneous Berea sandstone exhibits a ±3% random variation in velocity. Heterogeneous rocks show 20 and 30% velocity variations. In the particular case where porosity is the sole compositional inhomogeneity, the acoustic and radiographic images agree. In general however, the acoustic images yield different information from the visual and radiographic im

ISSN:0094-8276    

DOI:10.1029/GL011i008p00697

年代:1984

数据来源: WILEY

摘要:

Thermal ion composition measurements from several successive dusk sector passes by the DE‐1 satellite show the formation of the new outer plasmasphere and double plasmapause following a sharp decrease in geomagnetic activity. In less than one day after the magnetic activity decrease, the outer plasmasphere formed and consisted of cold, essentially Maxwellian plasma with ion composition and thermal energy characteristics generally similar to those of the inner plasmasphere, albeit at significantly lower densities. There is also evidence that at times the thermal O+density is comparable to the H+density within the plasmaspher

ISSN:0094-8276    

DOI:10.1029/GL011i008p00701

年代:1984

数据来源: WILEY

摘要:

Bursts of energetic electron and proton precipitation from the magnetosphere and artificially stimulated by a powerful ground‐based Soviet VLF emitter (F = 19.1 kHz, L = 4.0), were detected during the ESSA experiment aboard the AUREOL‐3 satellite (ARCAD‐3 project) at altitudes of 1500 ‐ 2000 km in the MLT morning sector. Stimulated electron and proton precipitations were clearly recorded in 7 overpasses out of 17 during the period 1 ‐ 25 December 1981. The precipitation zone corresponded to the projection on the ionosphere of the ring current region (L = 2.8 to 3.6), and was displaced equatorward with respect to the location of the VLF transmitter. The intensities of the artificial 0.1 ‐ 2 keV electron fluxes ranged from 104to 105(cm² sec ster keV)−1and those of the 60 ‐ 220 keV protons from 100to 10² (cm² sec ster keV)−1. The 1.8 keV electron bursts were delayed by about 1 sec with respect to the VLF pulses, and the 110 keV proton fluxes were delayed by about 3 ‐ 5 sec. Results are discussed in terms of both Cerenkov and cyclotron resonance effects between VLF nonducted waves and ma

ISSN:0094-8276    

DOI:10.1029/GL011i008p00705

年代:1984

数据来源: WILEY

摘要:

Measurements taken by the solar wind experiment aboard the ISEE 2 satellite show evidence for H+ions, originating from neutral hydrogen moving with a speed much lower than that of the solar wind.Positive ion fluxes are measured as a function of energy and flow direction and accumulated over a time of few hours, in which the solar wind and magnetic field parameters remain approximately constant. A model given by Vasyliunas and Siscoe (1976) is used to predict the expected location of these ions in velocity space. A consistent correlation is found between predictions and measurements in three periods of time; order of magnitude estimates of the ion flux range between 104and 105cm−2s−1. In the region surrounding the Earth, the neutral hydrogen consists primarily of interstellar and geocoronal gas. The present preliminary results presented do not allow to evaluate the relative contributions of the two populations to the observed flu

ISSN:0094-8276    

DOI:10.1029/GL011i008p00709

年代:1984

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL011i008p00713

年代:1984

数据来源: WILEY

摘要:

Wind observations were obtained during the first nine months of the SEQUAL field program from a station maintained at St. Peter and St. Paul (SPP) rocks. Winds are very steady from May to November and very unsteady during April when the Intertropical Convergence Zone (ITCZ) crosses the site. Wind stress components are computed and comparison with 1979 data shows that the onset of the winds occurred one month earlier in 1983 but is of similar amplitude. The spectral analysis of the wind shows a significant increase in the variance for periods between 3‐5 days and 10‐15 d

ISSN:0094-8276    

DOI:10.1029/GL011i008p00715

年代:1984

数据来源: WILEY