摘要:

Some aspects of the dynamical behavior of magma chambers, replenished from below with hotter but denser magma, have been modeled in a series of laboratory experiments. In previously reported work the fluids used were aqueous solutions of comparable viscosity, and thus the results should be applicable to basaltic magma chambers, in which the magmas do not vary greatly in viscosity. In that case, the lower layer cools by convective heat transfer to the fluid above, and crystallization causes the density of the residual liquid in the lower layer to decrease. When the density becomes equal to that in the upper layer, sudden overturning and intimate mixing take place. The present paper reports experimental results that allow us to extend the application to systems in which there is a large viscosity ratio between the resident and the injected fluid, for example, to calcalkaline magmas, where magma viscosity can vary by as much as 5 orders of magnitude. The largest viscosity ratio in our experiments (about 3000) was achieved using cold glycerine for the upper layer, above a hot denser KNO3solution. The most striking new feature with the very viscous upper layer is that now less dense fluid is released immediately and continuously from the interface and rises as plumes through the upper layer. Further crystallization occurs in the plumes, and the crystals fall out, but there is little mixing, and a layer of depleted KNO3solution is eventually deposited at the top. The transfer process between the layers is dominated by interfacial effects, with the high‐viscosity upper layer acting as a nearly rigid lid that allows buoyant fluid to accumulate just below the interface and then rise in localized plumes across the interface into the viscous layer. This physical picture is supported by a series of experiments in which the viscosity ratio is varied systematically; the mixing behavior changes gradually between that described above for a large viscosity ratio and the sudden over‐turning characteristic of layers with comparable viscosity. The importance of the viscosity ratio, rather than just an increase in viscosity, is confirmed by experiments in which both viscosities are increased by the same factor; the overturning process is then slower, but symmetrical. Other variations suggested by previous experiments are also described: the release of gas by a chemical reaction, to model the release of volatiles following an overturning event in a magma chamber; the effect of a cold, immiscible layer above the cooling crystallizing fluid; the influence of two viscous layers with a density step between them; and the constraining effects of a density (with corresponding viscosity) gradient in the upper region. The experiments indicate that whatever the stratification, whether it be in layers or continuous, the form of the initial motion in the upper fluid is determined by the viscosity ratio between the two fluids immediately adjacent to the interface. Geological applications are not examined in detail in this paper, but the experiments suggest that both sudden overturning (characteristic of magmas of nearly equal viscosity) and continuous release (when the upper layer is much more viscous) are viable mechanisms for magma mixing in the appropriate circumstan

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06857

年代:1984

数据来源: WILEY

摘要:

The systematics of tributary junction angles are an important link between the topology of a drainage network and its topography because the angle of junction depends on the relative slopes of the intersecting tributaries. I present here a model for stream junction angle as a function of position (link magnitude)within the network which quantitatively predicts the observed down‐network increase of junction angle for tributaries joining progressively larger recipient links. Good agreement between model and data exists for mature drainage networks mapped at varying scales. Additionally, the dependency of junction angle on relative tributary slopes suggests that as individual links within the network modify their slopes to evolve toward graded longitudinal profiles, there exists a central tendency within the network, modulated by structural and lithologie controls, for evolution to an equilibrium or “graded“ planimetric pattern. Extraction of process‐related (as opposed to purely topological) information from network planimetric pattern is important in terrestrial and planetary remote sensing appli

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06878

年代:1984

数据来源: WILEY

摘要:

Recent radar images of the mountains of Ishtar Terra, Venus, at approximately 3‐km radar resolution show a series of linear bands of alternatively greater and lesser backscatter; the bands are generally aligned with the topographic strike of the mountain ranges. We test the hypothesis that these band features are tectonic in origin, the product either of folding during lithospheric compression or of block faulting during lithospheric extension. Tested models for folding include uniform elastic and viscous plates overlying in viscid substrates, layered plates, and a viscous half space in which viscosity decreases exponentially with depth. The uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on Venus, about 15 to 20 km, if the elastic or high‐viscosity layer is, at most, a few kilometers in thickness, though the required compressive stresses are several kilobars in magnitude. For the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness, and similar results hold. Layered elastic or viscous plates, however, can fold at the required wavelength with subkilobar stresses and, therefore, are favored over uniform plate models. Tested extensional models include graben and horst formation in a surface elastic‐brittle layer, imbricate normal faulting, and necking of a surficial layer that is everywhere in a state of extensional failure. The spacing between adjacent extensional features can match the spacing between bands on Venus if the surficial brittle layer is no more than a few kilometers in thickness. Both compressional and extensional models for band formation are therefore consistent with radar images of Venus mountain ranges at the image resolution presently available. Additional evidence, however, including the linearity and continuity of individual bands, the relationship between band trends and topographic contours in some regions near the ends of mountain ranges, and the very different manifestations of lithospheric extension in other highland areas of Venus, leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of Ishtar Terra as a result of horizontal compression of the Venus lithosphere. Independent arguments on the thermal structure of the Venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of Venus is approximately 1–10 km thick. We therefore suggest that the surficial layer of elastic‐brittle or high‐viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the Venus highlands at the time of ban

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06885

年代:1984

数据来源: WILEY

摘要:

Remote sensing and photogeologic data were used to determine the composition and origin of lunar dark‐haloed craters. Near‐infrared reflectance spectra were obtained for numerous dark‐haloed impact craters located on either the ejecta blankets of large impact craters or Imbrian and Nectarian age light plains deposits. Spectral, thermal, radar, and photogeologic data conclusively demonstrate that Copernicus H and other dark‐haloed craters on the ejecta blanket of Copernicus excavated mare basalts from beneath lighter surface deposits rich in highlands material. Analyses of reflectance spectra of dark‐haloed craters on light plains indicate that in every instance these craters exposed mare basalt which had previously been covered by varying thicknesses of highlands debris. In the Schiller‐Schickard region a relatively thick highlands unit was emplaced as a result of the Orientale impact event. On the interior of Balmer basin, marelike basalt has been excavated from beneath much thinner highlands deposits emplaced by nearby craters. Mafic orbital geochemical anomalies are commonly associated with regions with abundant light plains deposits which exhibit dark‐haloed impact craters. Apparently, some mare material has been incorporated into the regolith either by local mixing during the emplacement of the highlands debris or by subsequent vertical mixing. The results of recent remote‐sensing, photogeologic, and lunar sample studies strongly indicate that mare volcanism was a significant process during much of the pre‐Imbrian and may have been initiated as early as 4.2–4.3 Ga. These very early volcanic episodes contributed materials to the lunar surface which were incorporated into the upper portion of the highlands crust by subsequent impact mixing. Models concerning the extent and duration of mare volcanism as well as those involving the composition and thermal evolution of the lunar interior will

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06899

年代:1984

数据来源: WILEY

摘要:

The average S wave travel time residuals from 11 earthquakes each in Tibet and the Himalaya imply that there are marked lateral variations in structure beneath Tibet and the Himalaya. Average S‐P wave residuals, measured with respect to the Jeffreys‐Bullen (J‐B) tables, for each of 11 earthquakes in the Himalaya are less than +1 s. Average J‐B S‐P residuals from 10 of 11 earthquakes in Tibet, however, are greater than +1 s, even when corrected for the large crustal thickness of Tibet. The largest values, 2.5, 3.2, 3.8, 4.5, and 4.9 s, are for five events in central and northern Tibet, and they imply that the average velocities in the crust and upper mantle of these portions of Tibet are 4–10% lower than those beneath the Himalaya. Therefore it is unlikely that a shield structure underlies these parts of Tibet, unless these S‐P residuals are due to variations in structure deeper than 250 km. S‐P residuals from two other events each in western and eastern Tibet are 1–2 s late and hence suggest that velocities beneath these areas are also somewhat lower than beneath shields. The mean S wave velocity seems not only to be different beneath Tibet and the Himalaya but also seems to increase from a minimum beneath northern Tibet to values close to those of the average earth toward the east, south, and west.Appendix A is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐010; $2.50. Payment m

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06911

年代:1984

数据来源: WILEY

摘要:

We have compared synthetic seismograms with long‐period body waves for nine earthquakes with epicenters in the Himalayan arc to determine depths of foci and to improve fault plane solutions. Focal depths are shallow (10–20 km). Inferred slip vectors are locally perpendicular to the mountain range; they plunge very gently (∼10°( in the eastern sections of the range and more steeply (∼25°) in western sections. Assuming India to be a rigid plate, the radially oriented slip vectors imply that southern Tibet extends at about half the rate of underthrusting in the Himalaya and therefore probably at about 5–10 mm/yr. The shallow depths and gentle dips of the fault planes, at least for the events in the eastern half of the range, are consistent with coherent underthrusting of the Indian plate beneath, at least, the Lesser Himalaya. The steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the Great

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06918

年代:1984

数据来源: WILEY

摘要:

Regionalized Rayleigh and Love wave attenuation coefficients have been measured across the Basin and Range province of western United States in the frequency range 0.02–0.2 Hz. The measurements were made by using the methods of Tsai and Aki (1969) and Yacoub and Mitchell (1977) adapted to work on any number of events simultaneously. Rayleigh waveQvalues at low frequency approach values near 40, which are significantly lower than previous measurements in the western United States. Love waveQvalues, on the other hand, are quite high at low frequencies. We suggest that interference between fundamental and higher modes may explain the Love wave observations. The Rayleigh and Love wave attenuation measurements have been inverted simultaneously for shear‐wave attenuation as a function of depth. A frequency‐independentQmodel is consistent with both Rayleigh wave and short‐period Love wave attenuation data. The shear‐waveQ(Qβ) model is characterized by lowQβin the lower crust (Qβ∼100) andQβdecreasing in the upper mantle with lowest values (Qβ∼30) beneath 60 km depth. Forward modeling shows that a high‐Qlower crust or upper mantle lid is inconsistent with the data. Our interpretation of these results is that the lithosphere is poorly developed beneath the Basin and Range and that the partially molten asthenosphere may reach very shallow depths, possibly to the base of the crust. This interpretation of theQmodel in conjunction with a number of geological and geophysical evidences suggests that attenuation mechanisms involving partial melt predominate in the lower crust and upper mantle of

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06929

年代:1984

数据来源: WILEY

摘要:

A marine seismic refraction study conducted between Cape Simpson and Prudhoe Bay, Alaska, provides data for subsurface structural and geological cross sections of the western Beaufort Shelf. The results suggest that the western Beaufort Shelf is underlain by a sedimentary prism with beds dipping and prograding to the northeast. Correlation of the refraction data with wells drilled on land and offshore reflection profiles permits tentative identification of geologic sequences on the basis of their seismic velocity. This study associates near‐surface velocities of 1.60–1.65 km/s with Quaternary sediments and 1.82–2.35 km/s velocities with Tertiary strata. Velocities of 1.60–3.40 km/s are correlated to Upper Cretaceous rocks, 2.10–4.14 km/s to Lower Cretaceous beds, 5.66–5.68 km/s to Mississippian‐Pennsylvanian carbonates, and 4.24–6.08 km/s to the Franklinian basement composed of argillites and phyllites. West of Cape Halkett, the sedimentary section is mainly Lower Cretaceous, whereas east of Cape Halkett, Upper Cretaceous and Tertiary strata dominate. Higher velocities (6.40–7.07 km/s) are thought to represent the crystalline basement, probably silicic in composition. Although no seismic velocities typical of the upper mantle are present on the record sections, a minimum depth calculation places the Mohorovicic discontinuity no shallower than 20 km. The interpretation indicates that the stratigraphic sequence present onshore at Prudhoe continues to the offshore and implies that the western Beaufort Shelf may be a good prospect for oil. The structures derived from seismic data indicate that the Beaufort continental margin is not different from Atlantic‐type margins and add further evidence to the concept of a rifted and rotated Beaufort

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06941

年代:1984

数据来源: WILEY

摘要:

Five seismic estimates of stress release are compared for eight multiply recorded aftershocks of the 1975 Oroville, California, earthquake. The dynamic stress drop, thearmsstress drop, and the apparent stress provide stable estimates which are strongly correlated across the data set. The uncertainties of these estimates (the standard error of the mean divided by the mean) are approximately 15%. Estimates of the Brune stress drop are uncorrelated with estimates of the average static stress drop calculated from the seismic moments and the rupture geometries of the aftershocks, determined from the signal durations of the recordedSwaves. The Brune stress drops are most strongly correlated with thearmsstress drops, implying that the Brune stress drop provides an estimate of the average dynamic stress drop, as Brune originally proposed.

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06961

年代:1984

数据来源: WILEY

摘要:

The ground motion parametersRv and ρRa, whereRis hypocentral distance, v is peak velocity, ρ is density, and a is peak acceleration, are found observationally to be strong functions of stress state and linearly related to focal depthz. If the stress state and focal depth are properly taken into account, then the ground motion parameters depend on the seismic momentM0, as expected from basic scaling principles. That is,Rv ∝M01/3, and ρRa is independent of earthquake size. Regression lines fitted to observations, covering broad ranges in seismic moment and focal depth, indicate that for extensional and compressional tectonic regimes, corresponding to normal and thrust or reverse faulting earthquakes, respectively, peak acceleration is given by ρRa = ‐ 1.08 MPa + 3.06 (MPa/km)z(normal) and ρRa = 5.65 MPa + 8.76 (MPa/km)z(thrust), and for peak velocity,Rv/M01/3= 10−4(m2/s) (N m)−1/3[3.00+0.69 (km−1)z] (normal) andRv/M01/3= 10−4(m2/s) (N m)−1/3[4.63+1.82 (km−1)z] (thrust). For strike‐slip earthquakes the data currently are insufficient to define regression fits, but such lines would definitely lie between those for the normal and thrust stress regimes. These equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space. It follows that for comparable hypocentral depth, peak acceleration in a compressional tectonic regime, for example, much of the eastern United States, is a factor of about 3 greater than in an extensional regime, such as Nevada, and for peak velocity a similar comparison yields a factor of 2 difference. The similarity in behavior of the peak acceleration parameter to crustal strength, estimated from Byerlee's law of friction, with respect to depth and stress state, indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high‐frequency peak ground motion. Analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter, as recorded at a typical surface site, that depend on stress state, specifically, a (normal) ≲0.5 g and a (thrust) ≲1.9 g. Thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the predict

ISSN:0148-0227    

DOI:10.1029/JB089iB08p06969

年代:1984

数据来源: WILEY