bluuebunny/crossref_metadata_2025 huggingface dataset:
On the use of regional joint sets as trajectories of paleostress fields during the development of the Appalachian Plateau, New York
Authors: Engelder Terry , Geiser Peter
Published: February 2008

To compare the orientation and development of jointing with the orientation and magnitude of finite strain recorded in the Upper Devonian rocks of the Appalachian plateau, New York, we mapped systematic joint sets on an area of 20,000 km2. In this area, Wedel [1932] mapped folds with limb dips of less than a degree and axes that change strike by 30° from 090° in the east to 060° in the west. We observed two different cross‐strike joint sets that maintain their approximate cross‐strike position from east to west. Yet, in detail the angle between the sets is 18°±2° in the east and 30°±4° in the west. In many outcrops one joint set parallels the direction of maximum compressive strain (εƒ) as recorded by deformed fossils, whereas the other joint set never parallels εƒ. Rare calcite‐filled joints are oriented parallel to the direction of εƒ. In addition, the calcite‐filled joints both cut and are cut by solution cleavage. These observations suggest that the joint set paralleling εƒ formed during the deformation event represented by the deformed fossils. The joint set that does not parallel εƒ somehow reflects a deformational event other than that producing fossil distortion, as suggested by strain relaxation experiments. In an outcrop of the Machias formation, where the direction of εƒ is 15° from the trace of a cross‐strike joint set, the tensor average from 14 subsurface strain relaxation tests shows in situ maximum comprehensive strain (εoc = 10−4) parallel to cross‐strike joints but not the direction of εƒ. Strain relaxation is coaxial with a fabric anisotropy indicated by sonic velocity tests. Our idea is that the orientations of cross‐strike joints parallel to εoc were controlled by the same rock property that causes strain relaxation on overcoring, whereas the orientations of the cross‐strike joints parallel to εƒ were controlled by the stress field causing the fossil distortion. Placing these structural data in a regional context allowed us to construct a dynamic and kinematic model for the structural evolution of the New York plateau. The model indicates variation in both boundry conditions and material behavior through a series of four distinct deformational events, which begin prior to lithification and end in the Recent. Thus our analysis suggests that the structural features we have used represent a set of highly sensitive tools for investigating the deformational history of the Appalachian foreland.
Stress gradient in arc–back arc regions and plate subduction
Authors: Nakamura Kazuaki , Uyeda Seiya
Published: February 2008

Trajectories of the maximum horizontal tectonic stress σHmax have been compiled for five regions of plate convergence, i.e., the Alaska‐Aleutian, central Europe, Hellenic‐Aegean, and southwest and northeast Japan. For southwest and northeast Japan, stress trajectories have been considered for both the present and Miocene times. The results indicate that in most cases a systematic stress gradient exists in the crust of the overlying plate: the stress is, from the plate boundary landward, compressional, shear, and tensional even in the region where active back arc spreading is not taking place. In some regions, σHmax maintains its direction subparallel with that of convergence as it changes from σ1 to σ2 (σ1>σ2>σ3) in the back arc, whereas in other regions, σHmax direction is changed to become parallel to the arc. Such a change appears to require some mechanisms in addition to simple landward stress attenuation. Some discussion is given on such mechanisms. None of the models proposed so far, however, seems to explain the observed extensional tectonics in back arc regions adequately.
Scattering and attenuation of shear waves in the lithosphere
Authors: Aki Keiiti
Published: February 2008

Recent measurements of Qβ−1 of S waves in the lithosphere for frequency range from 0.02 to 25 Hz show a remarkable frequency dependence and systematic variation from place to place depending on the current tectonic activity. In order to test the hypothesis that the attenuation of S waves may be caused by the loss of energy by scattering due to heterogeneity, the amplitudes of S and coda waves were measured simultaneously for 900 local earthquakes in the central Japan for various frequency bands. The data were interpreted using the S to S single‐scattering theory for coda waves, which is now supported by the observed agreement between Q of coda and Qβ, and also by the observed identical local site effect on coda and S waves. The result shows that the loss of energy from S waves required to explain coda amplitudes agrees with the attenuation of S waves within the uncertainty of measurements. This conclusion is supported by several recent studies on Q of coda, Q of Lg, Qβ, and coda excitation by other workers.
Gravitational stability of water over steam in vapor‐dominated geothermal systems
Authors: Schubert Gerald , Straus Joe M.
Published: February 2008

Vapor‐dominated geothermal systems exist in a most extraordinary state: a condensate or water layer several hundred meters thick overlies a main steam zone of undetermined thickness. Why doesn't the water fall down? We show that a heavy fluid phase overlying a light phase of the same fluid in a porous medium can be stable provided the permeability in the vicinity of the interface is not too large. The special behavior of a phase change interface in response to disturbances of the boundary account for the stabilization. Heavy fluids overlying light fluids in a porous medium are unstable, irrespective of permeability, if the fluids are immiscible. Application of the stability analysis to vapor‐dominated geothermal systems shows that the condensate layer can be stabilized if the permeability near its base is no larger than about 40 nm2 (0.04 millidarcy).
Upper mantle structure of northwestern Eurasia
Authors: Given J. W. , Helmberger D. V.
Published: February 2008

Long‐and short‐period WWSSN seismograms from nuclear explosions in the Union of Soviet Socialist Republics are incorporated with apparent velocity observations to derive an upper mantle model for northwest Eurasia. The compressional waves from these explosions have several distinctive features that provide important new information about the character of the upper mantle in the region. The seismograms from 9° to 13° exhibit impulsive first arrivals Pn, implying a smooth, positive velocity gradient between depths of 60 and 150 km. There is a consistent pulse arriving about 2 s after Pn at the ranges from 13° to 17°, and at larger ranges there are distinct reflections from the two major discontinuities of the mantle. Synthetic seismograms displaying these features indicate a model that correlates with other velocity models from around the world, with a distinctive lid and low‐velocity zone. The arrival following Pn is modeled by positioning a low‐velocity zone between 150 and 200 km. The model is relatively smooth from a depth of 200 km down to 420 km, where a 5% jump in velocity produces a triplication from 15° to 23°. The observations from 21° to 26° clearly show another discontinuity at a depth of 675 km with a 4% change in velocity. These results suggest that stable continental regions may have a shadow zone that extends to beyond 17°. Below 250 km depth there is no distinguishable difference between the model proposed for northwest Eurasia and models derived for the United States.
The silica heat flow interpretation technique: Assumptions and applications
Authors: Swanberg Chandler A. , Morgan Paul
Published: February 2008

We have previously established a linear relation between temperatures based on the silica content of groundwater and regional heat flow and used the relation to prepare a new heat flow map of the continental United States. We now examine the assumptions upon which the relation is based, the accuracy to which groundwater silica data can be used to estimate regional heat flow, and the limitation of the technique. By averaging silica geotemperatures and traditional heat flow values over 1°×1° blocks, the linear regression is TSiO2 = mq + b, where m and b are constants determined to be 680±67°C m2 W−1and 12. ±5.1°C. The physical significance of b is mean annual surface temperature, and the product of m times thermal conductivity reflects the minimum mean depth to which groundwaters may circulate. These values are not sufficiently different from our earlier values (m = 670, b = 13.2) to justify using the newer values. To illustrate the application of the linear regression in predicting regional heat flow, data sets are presented from upstate New York, south central New Mexico, and Egypt. In each case, the predicted heat flow is tectonically reasonable and consistent with whatever traditional data are available.
Geoid height versus age for symmetric spreading ridges
Authors: Sandwell David , Schubert Gerald
Published: February 2008

Geoid height‐age relations have been extracted from Geos 3 altimeter data for large areas in the North Atlantic, South Atlantic, southeast Indian, and southeast Pacific oceans. Except for the southeast Pacific area, geoid height decreases approximately linearly with the age of the ocean floor for ages less than about 80 m.y. in agreement with the prediction of an isostatically compensated thermal boundary layer model (Haxby and Turcotte, 1978). The geoid‐age data for 0 to 80 m.y. are consistent with constant slopes of −0.094±0.025, −0.131±0.041, and −0.149±0.028 m/m.y. for the South Atlantic, southeast Indian, and North Atlantic regions, respectively. For ages greater than 80 m.y. the geoid‐age relation for the North Atlantic is nearly flat, indicating a reduction in the rate of boundary layer thickening with age. The uncertainties in the geoid slope‐age estimates are positively correlated with spreading velocity.
The active part of Charlie‐Gibbs Fracture Zone: A study using sonar and other geophysical techniques
Authors: Searle Roger
Published: February 2008

A short survey with Gloria side‐scan sonar and other geophysical instruments has revealed new information about Charlie‐Gibbs fracture zone between 29° and 36°W. The traces of two transform faults have been clearly delineated. They fit small circles about the pole of rotation with an rms error of only about 1 km, but they do not always follow the deepest parts of the transform valleys. The transforms are joined by a short spreading center at 31°45′W. The median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable Vine‐Matthews magnetic anomalies. A transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature. Considerable thicknesses of sediment appear to have been deposited in the northern transform valley from Norwegian Sea overflow water passing through the fracture zone, but transverse ridges have prevented the sediment reaching the southern valley.
Electrical properties of granite with implications for the lower crust
Authors: Olhoeft Gary R.
Published: February 2008

The electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature. Minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure, structurally bound water, oxygen fugacity, and other parameters. The effect of sulfur fugacity may be important but is experimentally unconfirmed. In addition to changing the magnitude of electrical properties, the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties. With increasing temperature, changes in water content retain large, but lessened, effects on electrical properties. Near room temperature, a monolayer of water will decrease the electrical resistivity by an order of magnitude. Several weight‐percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5. At elevated temperatures just below granitic melting, a few weight‐percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2. Above the melting temperature (650° to 1100°C depending upon water pressure), a few weight‐percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy. Remarkably, the few weight‐percent water must be present as free water. Experiments with hydrated hornblende schist (with structural water) indicate an electrical resistivity very similar to that for dry granite. The implications of these results, together with the findings of deep magnetic sounding and magnetotelluric surveys, suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle.
Toward a better theory of thermal remanent magnetization
Authors: Merrill R. T.
Published: February 2008

The major results of this paper are the pointing out of deficiencies in existing thermal remanent magnetization theories for nonuniformly magnetized grains and the suggestion of ways in which some of these deficiencies might be removed. Most models of pseudo single domain behavior fail to predict certain critical experimental observations of TRM, particularly those dealing with magnetic stability. At least part of the problem with understanding TRM in small nonuniformly magnetized grains is that the size and role of the demagnetization field has been grossly overestimated. TRM in small multidomain grains is probably acquired by domain wall movement, except for the smallest (submicron) grains in which pseudo single domain behavior associated with wall moments probably dominates. A collective response of domain walls seems to be required to explain magnetic stability observations, although the degree and the precise nature of this collective response has not yet been determined.
Geomagnetic variation sounding of the asthenosphere beneath the Juan de Fuca Ridge
Authors: Law L. K. , Greenhouse J. P.
Published: February 2008

Two ocean bottom magnetometers have been deployed at sites on either side of the Juan de Fuca Ridge, 20 and 40 km from the crest, near latitude 47°N. Twelve days of data were recorded simultaneously at the two sites, and these data have been used to determine the electrical conductivity structure beneath the region. The vertical field variations at both stations are of very low amplitude in the period range 0.25 to 8 hours, with no detectable phase reversal across the ridge. Electric fields in the ocean layer have been estimated from the attenuation of sea floor horizontal fields with respect to Victoria, and the resulting impedance spectra inverted to find resistivity as a function of depth using the technique of Oldenburg (1979). Resistivity models which fit the data within the specified bounds are characterized by a comparatively resistive crust and upper lithosphere overlying a low resistivity zone between depths of 55 and 75 km which in turn appears to be underlain by a higher resistivity section. The average resistivities are used to estimate temperature and melt fraction on the basis of the effective medium theory of Shankland and Waff (1976) for a basalt melt fraction within an olivine matrix. For the low resistivity zone, and depending on the water content of the melt (0 to 10%), the estimated temperature range is 1400°C to 1250°C and the corresponding melt fraction 0.05 to 0.09. The data suggest that the melt fraction decreases below 80 km. The conductivity profile from magnetotelluric data on 72 Ma ocean floor in the North Central Pacific (Filloux, 1977) has similar characteristics to the 1 Ma old conductivity structure near the Juan de Fuca Ridge, but with the low resistivity layer at depths of 140 to 220 km. In both cases, the resistivity minimum should be equated with the region of maximum partial melt within the asthenosphere, rather than with the base of the lithosphere. The increase in depth to this zone with increasing lithosphere age is in general agreement with plate tectonic concepts.
Curie temperature and lattice constant reference contours for synthetic titanomaghemites
Authors: Keefer Charles M. , Shive Peter N.
Published: February 2008

Curie point and lattice parameter measurements were made on 57 titanomaghemite samples produced from sintered titanomagnetites with x (mole fraction ulvospinel) values of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0. Oxidation was carried out in various atmospheres at temperatures as low as 113°C for durations of about 24 hours. The oxidation process was successful in producing homogeneous titanomaghemites with z (oxidation parameter) values between 0.0 and 1.0. Spinels with z < 0.6 are generally single phase, but the necessity of using temperatures greater than 300°C to produce strongly oxidized phases resulted in spinel oxidation in the presence of a rhombohedral phase. Curie temperature and lattice constant data are significantly different from results presented by Ozima and Sakamoto (1971), Readman and O'Reilly (1972), and Nishitani (1979). Although some of these differences may be due to unmixing (inversion), most of them apparently result from differences in techniques of titanomaghemite preparation and oxidation.
Parameterizations for the horizontally averaged temperature of infinite Prandtl number convection
Authors: Richter Frank M. , McKenzie Dan P.
Published: February 2008

Parameterizations for the horizontally averaged temperature of infinite Prandtl number numerical convection experiments are determined. The types of convection considered result from uniform heating from below, uniform heating from within, and isothermal boundaries with a higher temperature applied to the bottom. The method used is based on fitting simple analytical expressions to suitably non‐dimensionalized and rescaled numerical solutions, thus providing a concise and in most cases accurate description of the horizontally averaged temperature.
Experimental and theoretical basis of electrokinetic phenomena in rock‐water systems and its applications to geophysics
Authors: Ishido Tsuneo , Mizutani Hitoshi
Published: February 2008

The ζ potentials and the streaming potential coefficients of silicate mineral‐water and rock‐water systems were measured for purposes of estimating the magnitude of electrokinetic effects induced by water diffusion in the earth's interior. In rock‐water systems the adsorption reaction of H+ at a solid surface is proved to be a main factor determining the ζ potential. For typical crustal rocks the ζ potential in water of pH higher than 2 is commonly negative and increases in absolute magnitude as supporting electrolyte concentration decreases and as temperature increases. A physicochemical theory was developed on the above features of the ζ potential on the basis of the adsorption reaction at the solid‐liquid interface. The experimental and theoretical results obtained in the present study of the ζ potential and the streaming potential coefficient in rock‐water systems are applied to some geophysical problems: electromagnetic variations induced by water diffusion prior to earthquakes and the self‐potential anomaly associated with hydrothermal convection.
Stress relaxations at low frequencies in fluid‐saturated rocks: Attenuation and modulus dispersion
Authors: Spencer James W.
Published: February 2008

A new laboratory apparatus measures the complex Young's modulus of rock samples at frequencies between 4 and 400 Hz and at strain amplitudes near 10−7. There is negligible attenuation and modulus dispersion in vacuum‐dry rocks. In water‐saturated samples of sandstone, limestone, and granite there are large attenuation peaks which are caused by stress relaxations. The attenuation and modulus data can be described by Cole‐Cole distributions of relaxation times. The rock anelasticity therefore satisfies the Kramers‐Kronig integral relations. The stress relaxations occur at low frequencies, have narrow distributions of characteristic times, are thermally activated, and have activation energies (16–22 kJ/mol) which are characteristic of hydrogen bonds. The modulus defect and peak attenuation are shown to vary with different pore fluids (water, ethanol, and n‐decane). The results are explained in terms of the pore fluids lowering the surface free energy of the rock‐forming minerals. The relaxation process involves the movement of fluid molecules. The molecules, by bonding to the surface, reduce the surface energy and thus generate the frequency‐dependent softening in the rock.
Sr isotopic tracer study of the Samail Ophiolite, Oman
Authors: Lanphere Marvin A. , Coleman Robert G. , Hopson Clifford A.
Published: February 2008

We have measured Rb and Sr concentrations and Sr isotopic compositions in 41 whole‐rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite, diabase dikes, and gabbro and websterite dikes within the metamorphic peridotite. Ten samples of cumulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have mean 87Sr/86Sr ratios and standard deviations of 0.70314 ± 0.00030 and 0.70306 ± 0.00034, respectively. The dispersion in Sr isotopic composition may reflect real heterogeneities in the magma source region. The average Sr isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern midocean ridge basalt. The 87Sr/86Sr ratios of noncumulate gabbro, plagiogranite, and diabase dikes range from 0.7034 to 0.7047, 0.7038 to 0.7046, and 0.7037 to 0.7061, respectively. These higher 87Sr/86Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with seawater. Mineral separates from dikes that cut harzburgite tectonite have Sr isotopic compositions which agree with that of cumulate gabbro. These data indicate that the cumulate gabbro and the different dikes were derived from partial melting of source regions that had similar long‐term histories and chemical compositions.
Determination of earthquake source parameters from waveform data for studies of global and regional seismicity
Authors: Dziewonski A. M. , Chou T.‐A. , Woodhouse J. H.
Published: February 2008

It is possible to use the waveform data not only to derive the source mechanism of an earthquake but also to establish the hypocentral coordinates of the ‘best point source’ (the centroid of the stress glut density) at a given frequency. Thus two classical problems of seismology are combined into a single procedure. Given an estimate of the origin time, epicentral coordinates and depth, an initial moment tensor is derived using one of the variations of the method described in detail by Gilbert and Dziewonski (1975). This set of parameters represents the starting values for an iterative procedure in which perturbations to the elements of the moment tensor are found simultaneously with changes in the hypocentral parameters. In general, the method is stable, and convergence rapid. Although the approach is a general one, we present it here in the context of the analysis of long‐period body wave data recorded by the instruments of the SRO and ASRO digital network. It appears that the upper magnitude limit of earthquakes that can be processed using this particular approach is between 7.5 and 8.0; the lower limit is, at this time, approximately 5.5, but it could be extended by broadening the passband of the analysis to include energy with periods shorter that 45 s. As there are hundreds of earthquakes each year with magnitudes exceeding 5.5, the seismic source mechanism can now be studied in detail not only for major events but also, for example, for aftershock series. We have investigated the foreshock and several aftershocks of the Sumba earthquake of August 19, 1977; the results show temporal variation of the stress regime in the fault area of the main shock. An area some 150 km to the northwest of the epicenter of the main event became seismically active 49 days later. The sense of the strike‐slip mechanism of these events is consistent with the relaxation of the compressive stress in the plate north of the Java trench. Another geophysically interesting result of our analysis is that for 5 out of 11 earthquakes of intermediate and great depth the intermediate principal value of the moment tensor is significant, while for the remaining 6 it is essentially zero, which means that their mechanisms are consistent with a simple double‐couple representation. There is clear distinction between these two groups of earthquakes.
Aeromagnetic and radio echo ice‐sounding measurements over the Dufek Intrusion, Antarctica
Authors: Behrendt John C. , Drewry David J. , Jankowski Edward , Grim Muriel S.
Published: February 2008

A combined aeromagnetic and radio echo ice‐sounding survey (4200 km of traverse) made in 1978 in Antarctica over the Dufek layered mafic intrusion of Jurassic age suggests a minimum area of about 50,000 km2, making it comparable in size with the Bushveld Complex of Africa. Comparisons of the magnetic and subglacial topographic profiles illustrate the usefulness of this combination of methods in studying bedrock geology beneath ice‐covered areas. Rocks are exposed in only 3% of the inferred area of the intrusion. Magnetic anomalies measured a few hundred meters above outcrops of the intrusion range in peak‐to‐trough amplitude from ∼50 nT over the lowermost exposed portion of the section in the Dufek Massif to ∼3600 nT over the uppermost part of the section in the Forrestal Range. Theoretical magnetic anomalies, computed from models based on the subice topography fitted to the highest‐amplitude observed magnetic anomalies, required normal and reversed magnetizations ranging from 10−4 to 10−2 emu/cm3 having directions and magnetizations consistent with measurements previously made on oriented samples. This result is interpreted as indicating that the Dufek intrusion cooled through the Curie isotherm during one or more reversals of the earth's magnetic field.
Magnetotelluric response on vertically inhomogeneous Earth
Authors: Kao Dominic
Published: February 2008

Magnetotelluric response in an inhomogeneous earth having resistivity varying linearly with depth is investigated. Horizontal electric and magnetic fields in such a medium are given in terms of modified Bessel functions. Impedance and apparent resistivity are computed for (1) an inhomogeneous half‐space, (2) an inhomogeneous half‐space overlain by a homogeneous layer, and (3) an inhomogeneous layer as a transition zone sandwiched between two homogeneous layers. Results are presented graphically and are compared with those of Cagniard models and with homogeneous multilayer models. In the case of resistivity decreasing with depth, the results of the above inhomogeneous models are similar to that of Cagniard two‐layer models, with the equivalence of h1c = h1, + ρ1/α. In the case of resistivity increasing with depth the results of the above inhomogeneous models can not usually be fitted with Cagniard two‐layer models but can be fitted with homogeneous multilayer models if the parameters of the layers are favorably chosen. The reciprocal impedance as a function of frequency is more useful than the apparent resistivity for determining the rate of increase α and the thickness h1 of an overlying homogeneous layer.
Water frost and ice: The near‐infrared spectral reflectance 0.65–2.5 μm
Authors: Clark Roger N.
Published: February 2008

The spectral reflectance of water frost and frost on ice as a function of temperature and grain size is presented with 1½% spectral resolution in the 0.65‐ to 2.5‐μm wavelength region. The well‐known 2.0‐, 1.65‐, and 1.5‐μm solid water absorption bands are precisely defined along with the little studied 1.25‐μm band and the previously unidentified (in reflectance) 1.04‐, 0.90‐, and 0.81‐μm absorption bands. The 1.5‐μm band complex is quantitatively analyzed using a nonlinear least squares algorithm to resolve the band into four Gaussian components as a function of grain size and temperature. It is found that the 1.65 μm component, which was thought to be a good temperature sensor, is highly grain size dependent and poorly suited to temperature sensing. Another Gaussian component appears to show a dependence of width on grain size while being independent of temperature. The relative apparent band depths are different for frost layers on ice than for thick layers of frost and may explain the apparent band depths seen in many planetary reflectance spectra.
Explicit correction of ultrasonically determined elastic wave velocities for transducer‐bond phase shifts
Authors: Jackson Ian , Niesler Herbert , Weidner Donald J.
Published: February 2008

Comparative experiments, in which either sample length or transducer configuration is varied, provide a reliable, accurate, and model‐independent means of explicitly correcting ultrasonically determined elastic wave velocities, for transducer‐bond phase shifts. The results of a number of such comparative experiments, employing both lithium niobate and quartz transducers on fused silica and aluminium samples, indicate that an absolute accuracy of order ±1 ns in travel time is routinely attainable. The use of these correction procedures in future ultrasonic studies should greatly improve the reliability of measured pressure and temperature derivatives of mineral elastic moduli and thereby facilitate more meaningful interpretation of seismic profiles for the earth's interior.
Investigation of controversial polar motion features using homogeneous international latitude service data
Authors: Dickman S. R.
Published: February 2008

Standard time series analysis techniques have been applied to the homogeneous polar motion data recently published by the ILS‐IPMS [Yumi and Yokoyama, 1980] in order to study some of the more controversial features apparently possessed by the older ILS data. The magnitude and direction of the secular trend unbiased by the presence of harmonics in the data were determined, yielding a rate of polar wander ∼3.52×10−3 arc sec/yr (which extrapolates to ∼0.98°/m.y.) in direction 80.1°W longitude. The long‐period Markowitz wobble, which dominates the retrograde power spectrum of the data, has a signal to noise ratio in that spectrum of 21:1; its period is well‐determined as 31 years. Variations with time of the annual wobble and Chandler wobble were investigated using complex demodulation; the annual wobble was found to undergo relatively insignificant variations in amplitude and phase, in contrast to some analyses of the older ILS data, while the amplitude modulation and 1925–1940 phase change of the Chandler wobble were reconfirmed.
Subduction of the Nazca Plate beneath Peru as determined from seismic observations
Authors: Hasegawa Akira , Sacks I. Selwyn
Published: February 2008

A detailed reinvestigation of seismicity in the southern half of Peru using data from a local seismic network and a focal mechanism study using P wave first motions reveal that for the first 100 km of descent the slab (as defined by this seismicity) enters at a normal dip angle near 30° and that below this depth it is bent to a nearly horizontal angle. This horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere. ScSp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity. Beneath the region from southern Peru to northern Chile, the Nazca plate descends with an almost constant dip angle (∼30°) down to at least 300‐km depth. Between the 30° dipping plate in southern Peru and the horizontal profile beneath central Peru, there is a contortion over a 100‐km‐wide continuous lateral section of the subducting plate. We infer that, at least in the upper 150 km depth range, the descending Nazca plate is contorted rather than torn.
Volcanics and structure of the famous Narrowgate Rift: evidence for cyclic evolution: AMAR 1
Authors: Crane Kathleen , Ballard Robert D.
Published: February 2008

A near‐bottom photographic survey on the Mid‐Atlantic Ridge from the FAMOUS region south to Transform B reveals structural and volcanologic evidence for an alternating widening and narrowing rift valley. Extension wedges appear to be propagating south from Transform A and north from Transform B. These merge at Mt. Mars, where the rift valley is narrowest and shallowest (Narrowgate), giving the valley a symmetrical hourglass shape. Three major volcanic en echelon lineaments trend N10°E, 10° oblique to the strike of the inner walls. Faulting and fissuring are antisymmetrical across the rift valley in contrast to the major extension. This contrasting antisymmetry reflects the surficial distribution of stress across crust of varying strengths, whereas the large‐scale wedge phenomenon determines the periodicity of transition from a narrow valley to a wide valley. Using vector analysis, the extension wedges are propagating at about 9 cm/yr toward each other. At this rate a narrow rift valley will be fully transformed into a wide rift in 0.64 m.y.
Convective instability of a thickened boundary layer and its relevance for the thermal evolution of continental convergent belts
Authors: Houseman G. A. , McKenzie D. P. , Molnar Peter
Published: February 2008

When crust thickens during crustal shortening, the underlying mantle lithosphere must shorten and thicken also, causing the submersion of cold, dense material into the surrounding asthenosphere. For a range of physical parameters the thickened boundary layer that forms the transition from the strong lithosphere to the convecting asthenosphere may become unstable, detach, and sink into the asthenosphere, to be replaced by hotter asthenospheric material. We have studied the instability of a thickened boundary layer for a range of physical parameters (Rayleigh number), amounts of thickening, and boundary conditions. In all cases the fluid was overlain by a rigid, conducting layer. Extensive numerical experiments were made for fluids with stress‐free boundary conditions, heated either from below or from within. From a simple physical description of the observed pattern of flow we derived expressions that related the growth of the instability and the time needed to remove the thickened boundary layer as a function of the amount of horizontal shortening (f), the Rayleigh number (R), and the ratio (a/d) of the thicknesses of the rigid and fluid layers. In our opinion, observations and theory agree well (within 10% for R > 105) and show that the speed with which the thickened boundary layer is removed increases with increasing f, R, and a/d. A limited series of runs with no‐slip boundary conditions suggests approximately the same functional relationships but with the process 0–30% slower than with stress‐free boundaries. For Rayleigh numbers comparable to those appropriate for upper mantle convection (105–107) the removal of the boundary layer occurs rapidly, in times less than the thermal time constant of the overlying rigid plate. Using typical values for the physical parameters in the earth, the boundary layer is removed in times less than the duration of deformation in some collision zones (30–50 m.y.). Thus we suspect that often the lower lithosphere is removed during the process of crustal shortening, causing the overlying crust and uppermost mantle to warm rapidly. This process is likely to contribute to the development of regional metamorphism and to the generation of latetectonic or posttectonic granites. We suspect, in fact, that in some cases the entire mantle lithosphere may detach from the lower crust during crustal shortening, exposing the crust to asthenospheric temperatures.
Laboratory convection experiments: Effect of lateral cooling and generation of instabilities in the horizontal boundary layers
Authors: Nataf H. C. , Froidevaux C. , Levrat J. L. , Rabinowicz M.
Published: February 2008

Convection experiments are carried out in a tank with two isothermal heat sinks: The top plate and one of the sidewalls. Heating is supplied either by an isothermal bottom plate or by generation within the fluid. This situation is similar to that of the earth's subcontinental mantle in the presence of a neighboring subducting oceanic lithosphere. There, the mantle material loses heat not only to the base of the continental lithosphere but also to the cold dipping oceanic slab. The thermal structure of the convective fluid is observed by a variety of techniques, including differential interferometry and strioscopy. Two parameters characterize the observations: the usual ‘vertical’ Rayleigh number and a newly defined ‘lateral’ Rayleigh number. The lateral cooling induces a large roll with axis parallel to the cold wall. The variation of its width relative to the values of the two Rayleigh numbers has been determined. An application to the earth's upper mantle would predict rolls 5 times wider than high. The circulation in the large roll remains two‐dimensional for Rayleigh numbers up to about 3 × 104. Beyond this value, boundary layer instabilities are observed within the persisting large roll. Their period has been determined. The interferometric method is shown to be very useful for visualizing other time‐dependent processes such as the growth of the induced large rolls. This growth is rapid enough to allow us to propose the existence of such large rolls in the earth and argue that their action could have led to continental break‐up.
Terrestrial heat flow in Arizona
Authors: Shearer Charles , Reiter Marshall
Published: February 2008

New heat flow data in Arizona suggest that the southern Basin and Range has recently undergone less extension than other areas of the Basin and Range province. Heat flow–heat production data imply that magmatism and hydrothermal circulation in the crust and upper mantle beneath southern Arizona and northwestern Mexico may be less extensive than along the Rio Grande rift. A heat flow transition between the Basin and Range and the Colorado Plateau occurs within a lateral distance of 60 km in central Arizona. Predicted steady state temperatures suggest that the M discontinuity may be nearly coincident with an isotherm along this transition. The new heat flow measurements from depths greater than 650 m suggest that the mean value for the Arizona Basin and Range is 1.94 ± 0.12 HFU, indicating as little as 0.3 to 0.4 HFU difference between the Colorado Plateau and the Arizona Basin and Range. This difference could be attributed primarily to widespread magmatic intrusion in the Basin and Range.
An investigation of the remanence characteristics of three sedimentary units of the Silurian Mascarene Group of New Brunswick, Canada
Authors: Roy Jean L. , Anderson P.
Published: February 2008

Thermal, chemical, alternating field (AF) and two‐stage (AF followed by thermal) experiments were performed on 18 sites (217 specimens) of three sedimentary units (groups A, B and C) of the Mascarene Group of southwestern New Brunswick. Groups A and B of Pembroke age (late Late Silurian) apparently carry a dual polarity initial remanence yielding a pole at 087°E, 05°S. Group C of probably Waweig age (older than Pembroke) carries a magnetite remanence and a hematite remanence with the former probably being the initial and yielding a pole at 085°E, 28°N. This would indicate a pole shift of some 30° southwards relative to the sampling area during the Upper Silurian, in agreement with the hypothesis that considerable polar shift took place during the Siluro‐Devonian period. The extensive experimental work (over 4000 treatments) demonstrates the need to use several techniques in the pursuit of the magnetic history of the rock. For example, if the work had been limited to thermal treatment up to 650°C, an overprinted or more likely an apparent remanence could conceivably have been mistaken for the initial remanence. Conversely, the considerable information gathered through the use of different techniques allows one to advance and model the most probable scenario of remanence acquisition while contributing additional knowledge about the magnetic characteristics of sedimentary units. Results from this investigation, including those of a Mascarene Group intrusion also reported here, are discussed in the context of Siluro‐Devonian poles of North America. Different options are open to the interpreter attesting to the difficulties in establishing both the rock and remanence ages. It is concluded that more data from detailed investigations are needed before the evolution of the eastern seaboard and the Appalachians can be retraced by paleomagnetic means.
Isostasy and the ambient state of stress in the oceanic lithosphere
Authors: Dahlen F. A.
Published: February 2008

Two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium. The first is that the stress is hydrostatic below some depth, the level of compensation, and the second is that above that depth there are no vertical shear stresses. In the case of a planar two‐dimensional mid‐ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge, but to determine that component an additional assumption is required. It is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid‐ocean ridges, the deviatoric stress in the oceanic lithosphere is a minimum. The unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid‐ocean ridges with a magnitude in old ocean basins of a few hundred bars, in agreement with oceanic intraplate earthquake mechanisms.At any given distance from the ridge the stress is maximum at the seafloor, and it decreases with depth at about the same rate as the cooling of the lithosphere, so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans.
Radon content of groundwater as an earthquake precursor: Evaluation of worldwide data and physical basis
Authors: Hauksson Egill
Published: February 2008

The properties of a worldwide data set of 91 radon (222Rn) anomalies (the frequency of occurrence, the precursor time interval, and the distribution of peak amplitudes) are correlated with earthquake data such as the respective magnitude and epicentral distance. These anomalies were reported as precursors to earthquakes in the United States, USSR, China, Japan, and Iceland. Although the data set is incomplete and limited by experimental deficiencies, several consistent patterns emerge. Radon anomalies from different tectonic regions show similar patterns. The radon anomalies occur at greater epicentral distances for earthquakes of the larger magnitude. Anomalies preceding large earthquakes (M ≥ 6) are frequently observed at a distance of 100 to 500 km. These distances are larger than several times the rupture dimensions of the future earthquakes. The time from the onset of an anomaly to the time of the earthquake (the precursor time) increases with magnitude but decreases with distance between epicenter and radon station. In addition, radon anomalies are observed more frequently prior to large earthquakes than prior to small ones, indicating that the preparation zone increases in size as magnitude increases. The peak amplitude does not scale with magnitude but forms a consistent pattern with epicentral distance in that the larger the earthquake magnitude, the farther away the largest amplitudes tend to occur. The preparation zone of the earthquake where the anomalies occur forms an almost continuous annulus that expands with time away from the future rupture zone. The outer radius of this annulus scales with the earthquake magnitude. Model calculations indicate that strain fields of at most 10−6 to 10−8 strain caused the radon anomalies. If these strains are divided by the appropriate precursor time, minimum strain rates from 10−7 day−l to 10−10 day−1 are obtained. Such small strains and strain rates suggest that in most cases neither mechanical crack growth induced by dilatancy nor mechanical coupling between pore pressure and the rock matrix caused the anomalies. Large changes in the orientation of the local strain field, however, could occur and affect the local stress intensity factor. Since changes in the stress intensity factor can result in stress corrosion, the occurrence of radon anomalies is attributed to slow crack growth controlled by stress corrosion in a rock matrix saturated by groundwater.
Volcanic ash beds: Recorders of Upper Cenozoic silicic pyroclastic volcanism in the western United States
Authors: Izett Glen A.
Published: February 2008

Stratigraphic relationships and tephrochronologic information for volcanic ash beds (4.0–0.1 m.y.) of the Western Interior allow the recognition of at least 68 different ash falls. The average reoccurrence rate, based on the succession of the isotopically dated ash beds, is near one eruption every 57,000 years. The ash beds were grouped into two chemical types, rhyolitic and dacitic; the main property that distinguishes dacitic from rhyolitic ashes is the larger Ca and Mg and smaller K content in dacitic ashes. Dacitic volcanic ashes were defined to have more than 0.55 wt % Ca. Rhyolitic ashes were subdivided further into two types, W‐ and G‐type, based on the presence of biotite in the W‐type and its absence in G‐type. W‐type rhyolitic ashes are chalky white, have less than 0.55 wt % Fe, have colorless glass shards of pumiceous habit. G‐type rhyolitic ashes are light to medium gray, have from 0.55 to 2.0 wt % Fe in the glass shards, lack biotite, have colorless, platy, bubble wall, and bubble junction glass shards. Dacitic ashes are light gray to grayish brown, have more than 0.55 wt % Fe and Ca, and have a complex mixture of colorless to light brown shards, including pumiceous, fibrous, chunky, and bubble wall and bubble junction types. W‐type rhyolitic ashes possibly formed from magmas that were cooler (695° to 830°C) and thus more viscous than the hotter (870° to 1000°C) magma progenitors of G‐type ashes based on Fe‐Ti oxide mineral compositions. Primary microphenocrysts in the ash beds comprise assemblages of the following: quartz, sanidine, plagioclase, biotite, amphibole, clinopyroxene, orthopyroxene, fayalite, zircon, apatite, allanite, chevkinite, sphene, magnetite, and ilmentite. Fayalite and chevkinite only have been found in G‐type; biotite and sphene only have been found in W‐type. Rhyolitic ashes are highly evolved and contain 74–79 wt % SiO2; dacitic ashes vary from 67–77 wt % SiO2. Al ranges from 10.5 to 14.5 wt % Al2O3, Mg ranges from 0.06 to 1.0 wt % MgO, and Fe (as Fe3O4) ranges from 0.50 to 4.1 wt %. Dacitic ash beds have the most (0.80–3.8 wt % as Fe3O4) and rhyolitic ashes have the least (0.50–1.9 wt % as Fe3O4). Ca varies from an unusually low amount (0.25 wt % CaO) in a few rhyolitic ashes to as much as 2.2% in some dacitic ashes. Rhyolitic glasses show larger Eu deficiencies compared to dacitic glasses on rare‐earth element diagrams. W‐type rhyolitic glasses have moderately large La to Sm ratios and have La contents from about 30 to 100 times chondrite values; G‐type rhyolitic glasses also have moderately large La to Sm ratios and contain somewhat more La than do the W‐type rhyolitic ashes, generally from 100 to 300 times chondrites amounts.