摘要:

The single‐crystal elastic constants, the Raman and IR frequencies, the isotropic average of the elastic constants, and the first pressure derivatives of these quantities have been calculated for stishovite from a modified rigid ion model. In addition to Coulomb interaction, short‐range repulsive interaction between first and second nearest neighbors and angle‐bending forces for the oxygen‐silicon‐oxygen angles are included. The parameters of the model are determined from a least squares fit to the available zero‐pressure elastic and optical data in the isostructural sequence SiO2, GeO2, and SnO2. The results for the isotropic bulk modulus and for the shear modulus are K = 3.145 Mbar and μ = 2.03 Mbar and for their pressure derivatives, (∂K/∂P) = 7.0 an

ISSN:0148-0227    

DOI:10.1029/JB081i014p02453

年代:1976

数据来源: WILEY

摘要:

The equilibrium boundary between diamond and graphite has been determined over the temperature interval 1100°C–1625°C. A piston‐cylinder apparatus and a ‘zero‐friction’ cell were used. Pressures were computed by force/area measurements and are independent of any calibration points. Visual observations of diamond dissolution or diamond growth were used in fixing the position of the boundary. Our newly determined boundary has the equationP(kbar) = 19.4 +T(°C)/40 kbar. Corrections for the effect of pressure on the emf of thermocouples have been made. Our newly determined boundary is almost coincident with the 1961 boundary published by the General Electric Company (Bundy

ISSN:0148-0227    

DOI:10.1029/JB081i014p02467

年代:1976

数据来源: WILEY

摘要:

Twelve granite and gabbro cylinders have been stress‐cycled under confining pressures of 0.5, 1.5, and 5.0 kbar. No consistent trend appeared in the minimum stress required for dilatancy as a function of cycle number at the highest pressure, but at 1.5 kbar and below a decrease was indicated. Dilatant volumetric strain at differential stresses corresponding to the frictional strength of the rock persists at a value of a few parts in 104at all confining pressures and is essentially unaffected by cyclin

ISSN:0148-0227    

DOI:10.1029/JB081i014p02471

年代:1976

数据来源: WILEY

摘要:

The effect of pressure on the lattice parameter, and hence on the volume, of two samples of grossularite garnet (Ca3Al2Si3O12) has been measured at room temperature up to 250 kbar with the use of a diamond anvil highpressure X ray diffraction camera. The pressure to which the sample was subjected was measured by means of the NaCl internal pressure standard. A least squares fit of the pressure‐volume to the Birch‐Murnaghan equation yields a bulk modulus value of KO= 1.74 ± 0.04 Mbar when a (∂K/∂P)∣P=0value of 4.25 (Halleck, 1973) is used. This KOvalue is in agreement with the value of 1.704 Mbar obtained by Halleck [1973], using ultrasonic techniques. A comparison of the bulk moduli of grossularite, pyrope, and almandite shows that KO∝ Vo−1for Ca and Mg garnets as expected from static lattice models. On the other hand, KO∝ Vo−5for the Mg‐Fe garnet solid solutions. This behavior has not been explained b

ISSN:0148-0227    

DOI:10.1029/JB081i014p02475

年代:1976

数据来源: WILEY

摘要:

This paper reports the results of ultrasonic measurements of the second‐order elastic constants of a single‐crystal specimen of almandine‐spessartine garnet as a function of pressure and temperature. The density of the sample, ρ0= 4.2396 ± 0.0016 g/cm3, is consistent with the composition (Fe0.52, Mn0.46,Ca0.01)3Al2Si3O12as determined by microprobe analysis. The pertinent results,Ks= 1763.3 ± 2.3 kbar, (∂Ks/∂T)T= 4.59 ± 0.16, and (∂Ks/∂P)P= −0.172 ± 0.005 kbar/0C, whereKsis the adiabatic bulk modulus, are related systematically to other garnet results published by previous investigators. In addition to the foregoing conventional ultrasonic data we have obtained a measurement of the higher‐order property (∂2Ks/∂T∂P) of 0.9±0.6×10−30C−1. The magnitude of this parameter suggests that it could significantly effect seismic velocity and density equations of state which are used commonly to model the earth's mantle. Assuming that the second‐order elastic constants Cijof garnets are related linearly to the molar fraction of their constituent end‐members, the present data have been combined with the work of previous investigators in order to derive the elastic properties throughout the garnet solid solution series. The relevant' results forKsfrom the least squares analysis for the various end members are as follows (in kilobars); pyrope, 1658±13; almandine, 1801±7; spessartine, 1723±10; grossular, 1705±13. These data provide the basis for calculating the elastic wave velocitie

ISSN:0148-0227    

DOI:10.1029/JB081i014p02483

年代:1976

数据来源: WILEY

摘要:

Each Lamont‐Doherty sonobuoy located on well‐dated crust has been carefully analyzed to determine crustal structure down to oceanic layer 3. Results from the Atlantic and the Pacific are compiled separately in order to study crustal structure as a function of plate age in both oceans, since they have very different spreading rates.Layer 2A (refraction velocity about 3.6 km/s) in the North Atlantic is 1.5 km thick at the ridge crest and thins consistently to about 100 m as the crust ages to about 60 m.y. Layer 2A in the east Pacific is 0.7 km thick at the ridge and thins to about 100 m at about 30 m.y. This difference in thickness is probably attributable to the much faster spreading rate in the Pacific. A poorly refractive acoustic basement layer about 200 m thick with similarities to layer 2A but not necessarily composed of the same materials is measured sporadically in the Pacific M‐Series plates and even less consistently in the Atlantic. This layer is not recorded in the Cretaceous or the Jurassic quiet zones.Regressions of refraction velocities in layer 2A as a function of age show that its velocity increases from about 3.3 km/s at the ridge crests to that of layer 2B on crust about 40 m.y. old. There is no corresponding increase of velocity with age in any of the deeper layers.The high resolution of the air gun/sonobuoy records shows that the layer 2B refraction line breaks directly to layer 3 velocities (46 times in the present work) or to a line with a velocity of 6.1 km/s (114 times in the present work), which we call layer 2C. The variance of the velocities in 2C is one fourth that of 2A and 2B, which indicates relative lithological uniformity in 2C.It does not seem likely that layer 2A really thins; what appears to be a thinning of the layer may actually be the result of an increase of its refraction velocity with age. However, the 2A/2B interface is well‐defined by large amplitude refractions from 2B on crust that is younger than about 30 m.y., which seems to rule out a transitional zone at the base of layer 2A where it ‘converts’ to 2B. The seismic observations seem to require a diagenetic process or repeated basaltic intrusions; both processes raise serious

ISSN:0148-0227    

DOI:10.1029/JB081i014p02490

年代:1976

数据来源: WILEY

摘要:

The thermal, kinematic, and dynamic structures of mantle plumes are modelled by a two‐dimensional, natural convection boundary layer rising in a fluid with a temperature‐dependent, power law constitutive relationship between shear stress and strain rate. An analytic similarity solution is found for the case of upwelling adjacent to a vertical, isothermal, stress‐free plane. Temperature, vertical velocity, shear stress, and effective viscosity profiles are calculated for upwelling from a depth of 400 km in a mantle with either a Newtonian or a non‐Newtonian olivine rheology and for plume ambient mantle temperature differences between 100°C and 400°C. The temperature excess of dry olivine plumes is limited by the constraint of subsolidus conditions. If inhomogeneities with concentrated heat sources exist in the mantle, thin thermal boundary layers less than a few tens of kilometers can be maintained over vertical distances of several hundred kilometers, even for plume temperatures only 100°C above the temperature of the ambient mantle. The velocity boundary layers of upwelling plumes can extend to much larger lateral distances, e.g., several hundred kilometers, especially for excess plume temperatures of 100°C. These conclusions are valid for both olivine (non‐Newtonian) and Newtonian plumes. Olivine plumes generally have upward velocities in excess of 10 cm/yr, except for large values of the activation volume, say, 30 cm3moh−1, in which case the velocities are only a few centimeters per year. The temperature dependence of the viscosity plays a significant role in the structure of these boundary layer flows. Non‐Newtonian olivine plumes rise faster and are narrower than the Newtonian ones. This general conclusion must be taken with caution, since Newtonian rheological parameters are uncertain. An increase in activation volume‐broadens the olivine plumes and lowers their velocities. Viscous heating is unimportant in these flows. Stresses are generally found to lie in the range 1–10 bars. Lowering the plume temperature relative to that of the ambient mantle induces a thickening of the thermal and viscous boundary layers and an increase in the thickness of the viscous boundary layer relative to that of the thermal one, Olivine and Newtonian plumes thicken according to the ⅙ power, ¼ respectively, of the vertical distance fr

ISSN:0148-0227    

DOI:10.1029/JB081i014p02499

年代:1976

数据来源: WILEY

摘要:

Great thrust earthquakes should increase the potential to do work, and thus island arcs may be subject to large strain variations during the first few decades following an earthquake. As the stress returns to preearthquake levels, the strain differences across an island arc may become much more uniform. This suggests a method for qualitatively assessing long‐term earthquake risk and for earthquake prediction. A continuous extension of some island arcs during both the preseismic and the seismic stages of ground displacement may be responsible for the origin of the active interarc basins described by Kari

ISSN:0148-0227    

DOI:10.1029/JB081i014p02511

年代:1976

数据来源: WILEY

摘要:

With a generalized inverse technique, WWSSN (World‐Wide Standard Seismograph Network) long‐periodPandSHwave forms are analyzed from the Koyna earthquake. The effects of local plane‐layered earth structure near an imbedded point dislocation source are put in by using a modified plane‐wave ray theory which includes the standard reflection and transmission coefficients plus source corrections for radiation pattern and geometrical spreading. The generalized inverse compares synthetic seismograms to the observed ones in the time domain through the use of a correlation function. By using published crustal models of the Koyna region and primarily by modelling the crustal phasesP,pP, andsP, the first 25 s of the long‐period wave forms is synthesized for 17 stations, and a focal mechanism is obtained for the Koyna earthquake which is significantly different from previous mechanisms. The fault orientation is 67° dip to the east, −29° rake plunging to the northeast, and N16°E strike, all angles being ±6°. This is an eastward dipping, left lateral oblique slip fault which agrees favorably with the trend of fissures in the meizoseismal area. The source time duration is estimated to be 6.5±1.5 s from a triangular time pulse which has a rise time of 2.5 s, a tail‐off of 3.9 s, source depth of 4.5±1.5 km, and seismic moment of 3.2±1.4×1025dyn cm. Some short‐period complexity in the time function is indicated by modelling shortperiod WWSSN records but is complicated by crustal phases. The long‐periodPwave forms exhibit complicated behavior due to intense crustal phase interference caused by the shallow source depth and radiation pattern effects. These structure effects can explain much of the apparent multiplicity of the Koyna source. An interpretation of the Koyna dam accelerograms has yielded anS‐Ptime which can be used along with the IMD (Indian Meteorological Department) epicenter and present depth determination to place the epicenter direc

ISSN:0148-0227    

DOI:10.1029/JB081i014p02517

年代:1976

数据来源: WILEY

摘要:

Directions of natural remanent magnetization from Upper Devonian Catombal Group red beds retain after magnetic cleaning a large secondary component acquired after the rocks were folded. A computer program was developed to eliminate progressively this ‘hard’ secondary component, which was found to lie along the present dipole field direction with an average intensity 1.6 times that of the primary component. It is interpreted to be due to weathering. The best estimate of primary direction was taken as the one in which the site mean directions, after removal of some secondary component and unfolding, gave minimum dispersion. The Upper Devonian field direction at Wellington derived from this procedure is (171°, −14°), withA95= 8°. The southern hemisphere paleomagnetic pole (49°S, 312°E) obtained is not significantly different from the only published Australian pole position of Upper Devonian age, from the Lochiel Formation, at (58

ISSN:0148-0227    

DOI:10.1029/JB081i014p02531

年代:1976

数据来源: WILEY