摘要:

Two seismic profiles across the 1912 pyroclastic flow in the south and central branches of the Valley of Ten Thousand Smokes giveP‐wave velocities within the flow of from 0.38 to 0.92 km/sec, indicating considerable lateral variation in compaction and/or welding along the traverse lines. AverageP‐wave velocities of 2.8 km/sec were obtained for the underlying strata, providing a sharp discontinuity at the base of the ash flow. The bedrock underlying the south branch of the valley is displaced by a normal fault at the east end of the profile. A fissure fumarole line overlies this fault on the surface of the ash flow, indicating a deep rather than ‘rootless’ origin for these particular fumaroles. Maximum thicknesses of 50 and 25 meters were determined for the pyroclastic flow along traverses across the south and central branches of the valley, respectively. These depths are in approximate agreement with those proposed by Sbar, Matumoto, and Kienle on the basis of gravity and refraction surveys along similar profiles, but in strong disagreement with the estimated thicknesses up to 300 meters suggested by other

ISSN:0148-0227    

DOI:10.1029/JB075i014p02619

年代:1970

数据来源: WILEY

摘要:

Analysis of the sedimentary, volcanic, structural, and metamorphic chronology in mountain belts, and consideration of the implications of the new global tectonics (plate tectonics), strongly indicate that mountain belts are a consequence of plate evolution. It is proposed that mountain belts develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic‐type continental margins. These assemblages result from the events associated with the rupture of continents and the expansion of oceans by lithosphere plate generation at oceanic ridges. The earliest assemblages thus developed are volcanic rocks and coarse clastic sediments deposited in fault‐bounded troughs on a distending and segmenting continental crust, subsequently split apart and carried away from the ridge on essentially aseismic continental margins. As the continental margins move away from the ridge, nonvolcanic continental shelf and rise assemblages of orthoquartzite‐carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites (rise) accumulate. This kind of continental margin is transformed into an orogenic belt in one of two ways. If a trench develops near, or at, the continenal margin to consume lithosphere from the oceanic side, a mountain belt (cordilleran type) grows by dominantly thermal mechanisms related to the rise of calc‐alkaline and basaltic magmas. Cordilleran‐type mountain belts are characterized by paired metamorphic belts (blueschist on the oceanic side and high temperature on the continental side) and divergent thrusting and synorogenic sediment transport from the high‐temperature volcanic axis. If the continental margin collides with an island arc, or with another continent, a collision‐type mountain belt develops by dominantly mechanical processes. Where a continent/island arc collision occurs, the resulting mountains will be small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop on the oceanic side of the arc. Where a continent/continent collision occurs, the mountains will be large (e.g., the Himalayas), and the single trench zone of plate consumption is replaced by a wide zone of deformation. Collision‐type mountain belts do not have paired metamorphic belts; they are characterized by a single dominant direction of thrusting and synorogenic sediment transport, away from the site of the trench over the underthrust plate. Stratigraphic sequences of mountain belts (geosynclinal sequences) match those asciated with present‐day oceans, island arcs, and co

ISSN:0148-0227    

DOI:10.1029/JB075i014p02625

年代:1970

数据来源: WILEY

摘要:

Fifteen new heat‐flow measurements have been made as part of a general geophysical study west of the Juan de Fuca ridge. All but one are within 16 km of latitude 47°N, and they form a profile from the crest of the ridge to a point out on the Tufts abyssal plain. Values decrease from 4 μcal/cm2sec at the crest of the ridge to 0.2 on the abyssal plain to the west, with the exception of one station 60 km west of the ridge where the heat flow is 10.9. Study of the possible causes of a high local value suggests that the real heat flow near the station is unlikely to be less than 7 μcal/cm2sec, and therefore that the heat‐flow distribution on the western flank of the ridge is ab

ISSN:0148-0227    

DOI:10.1029/JB075i014p02648

年代:1970

数据来源: WILEY

摘要:

A model for erosion of the lunar surface by impact of small projectiles is developed that provides an analytic representation of the change of crater shape as a function of time. The model is applied to the erosion of craters approximately 1 meter to 1 km in diameter. The lifetime of a crater in this size range, which is steadily eroded by impact, is approximately proportional to its radius. The model predicts the observed steady‐state size frequency distribution of small lunar craters and the dependence of this distribution on the crater‐production cu

ISSN:0148-0227    

DOI:10.1029/JB075i014p02655

年代:1970

数据来源: WILEY

摘要:

The thermal emission spectrums (7.0 to 13.5 μ) of six selected areas on the lunar surface were measured from an altitude of 32 km and their spectral emissivities calculated. All spectrums show departures from black‐ or gray‐body emission. Emissivities in the 8.0‐ to 9.0‐μ region of the spectrum are significantly higher than those for wavelengths greater than 10 μ. Differences are noted in the wavelengths of peak emissivity, particularly between the highland areas and the maria. The interpretation of the differences in terms of lunar composition is discussed

ISSN:0148-0227    

DOI:10.1029/JB075i014p02662

年代:1970

数据来源: WILEY

摘要:

Thermal emission spectra of six different areas on the lunar surface are discussed in the light of laboratory studies of the spectral emissivity of rocks and minerals. It is shown that the emission maxima characteristics of lunar spectra are similar to emission maxima found in the spectra of particulate rock samples. Such emission maxima are shown to be diagnostic of general rock type, within limits that are defined. Differences in emission maxima of their respective spectra indicate a significant difference in bulk composition between the material ejected by the craters Copernicus and Theophilus and the surface material of the two circular maria, Imbrium and Serenitatis. Barring some unknown measurement error in the spectra, or some unanticipated effect of the lunar environment on surface spectral behavior, we can conclude that the crater materials display an emissivity maximum characteristic of silica‐poor basic rocks, while the circular maria display a maximum characteristic of silica‐rich ultrabasic rocks. If this preliminary result is confirmed by future research, it has profound implications for the origin and evolution of the lunar cr

ISSN:0148-0227    

DOI:10.1029/JB075i014p02671

年代:1970

数据来源: WILEY

摘要:

Magnetotelluric observations, consisting of measurements in the period range from1000 sec were made at five stations across the Wisconsin arch. Scalar and tensor apparent resistivity values were computed from the power density estimates of the horizontal field components. Electrical resistivity models of the earth at each of the five sites were obtained by fitting the apparent resistivity values of one‐dimensional models to the measured values. The preferred conductivity models show a layered crustal structure which agrees well with the crustal seismic profile at the four best sites. The most striking result is an order‐of‐magnitude increase in conductivity at about 12 km depth, which corresponds to an intracrustal velocity interface. Near the crust‐mantle boundary an increase in resistivity, comparable to that of the upper crust, is observed. Indications of a conductivity increase at 70 km depth were obtained at two of the stations; one of the models tenuously suggests another steeper conductivity increase at a depth of

ISSN:0148-0227    

DOI:10.1029/JB075i014p02683

年代:1970

数据来源: WILEY

摘要:

Focal mechanisms are presented for thirteen earthquakes between Hindu Kush and Sumatra, including four shallow‐focus events along the Himalayan mountain front and two events at intermediate depth beneath the Burmese mountains. All mechanisms are based on first‐motionP‐ andS‐wave data recorded by long‐period instruments. The mechanisms along the Himalayan front confirm the existence of thrust faulting for which there is post‐Mesozoic geologic evidence. Additional evidence for thrust faulting in central Asia comes from focal mechanisms based on other seismic data published recently by Russian investigators. The axis of minimum compression (theTaxis) at intermediate depths beneath the Burmese mountains is oriented approximately down the dip of the seismic zone, as areTaxes at intermediate depths in several other seismic zones. A mechanism solution consistent with strikeslip faulting and another consistent with normal faulting were derived from two shallow earthquakes in western China. One shallow earthquake within the Himalayas yielded a normal faulting mechanism, as did one event from the western margin and two events near the northern border of the Andaman Sea. Slip vectors consistent with underthrusting beneath the Himalayas have a nearly uniform north to northeasterly strike. This evidence, as well as the frequency of occurrence of large‐magnitude earthquakes, suggests that seismic slip at shallow depths may account for the convergence between the Indian Ocean and the Eurasian plates along the Himalayan m

ISSN:0148-0227    

DOI:10.1029/JB075i014p02699

年代:1970

数据来源: WILEY

摘要:

Earth dilatations due to earth tides and seismic waves cause periodic fluctuations of the fluid pressure within saturated porous formations. Pressures and water levels in wells that are connected with sufficiently confined subsurface fluids are also affected and provide a means of obtaining quantitative data on the earth strain. A theory is given of strain‐induced pressure fluctuations in a well‐aquifer system, and the results are discussed on the basis of the physical properties of common permeable rocks. The theory is based on a generalized form of Darcy's law involving an inertia term. It is concluded that observations on wells can provide quantitative data on dilatations and that well‐aquifer systems can under favorable conditions act as very sensitive strain m

ISSN:0148-0227    

DOI:10.1029/JB075i014p02711

年代:1970

数据来源: WILEY

摘要:

The case of the generalized repulsive potential in the central force approximation is used in Born's lattice theory to derive the elastic constants versus pressure for the NaCl, CsCl, and ZnS structures. The equations are rearranged so thatcijis a function ofKandP, the bulk modulus and pressure. The isothermal equation of state relatesK,P, and the density ρ, so that the elastic constants are specific functions of the relation betweenKandP, and ρ andP. As a special case of the theory, the parameter in the repulsive potentialnis evaluated by the measured valueKo′. The theory shows that the simple repulsionv(r) =b/rnaccounts for the experimental results, except for a small term due to noncentral bind

ISSN:0148-0227    

DOI:10.1029/JB075i014p02719

年代:1970

数据来源: WILEY