bluuebunny/crossref_metadata_2025 huggingface dataset:
Geodetic strain measurements in Washington
Authors: Savage J. C. , Lisowski M. , Prescott W. H.
Published: February 2008

Two new geodetic measurements of strain accumulation in the state of Washington for the interval 1972–1979 are reported. Near Seattle the average principal strain rates are 0.07 ± 0.03 μstrain/yr N 19°W and −0.13 ± 0.02 μstrain/yr N71°E, and near Richland (south central Washington) the average principal strain rates are −0.02 ± 0.01 μstrain/yr N36°W and −0.04 ± 0.01 μstrain/yr N54°E. Extension is taken as positive, and the uncertainties quoted are standard deviations. A measurement of shear strain accumulation (dilatation not determined) in the epoch 1914–1966 along the north coast of Vancouver Island by the Geodetic Survey of Canada indicates a marginally significant accumulation of right‐lateral shear (0.06 ± 0.03 μrad/yr) across the plate boundary (N40°W strike). Although there are significant differences in detail, these strain measurements are roughly consistent with a crude dislocation model that represents subduction of the Juan de Fuca plate. The observed accumulation of strain implies that large, shallow, thrust earthquakes should be expected off the coast of Washington and British Columbia. However, this conclusion is not easily reconciled with either observations of elevation change along the Washington coast or the focal mechanism solutions for shallow earthquakes in Washington.
Geodetic analysis of reservoir depletion at the Geysers Steam Field in northern California
Authors: Denlinger Roger P. , Isherwood William F. , Kovach Robert L.
Published: February 2008

Reservoir depletion at the Geysers from 1974 to 1977 is evident in measured changes in gravity, surface strain, and pore pressure drainage. The drainage area increased about 20%, the maximum gravity decrease was about −120 μGal, and the maximum elevation change was about 6 cm during this period. Since the net mass withdrawal is known, it may be combined with the gravity change to estimate a drainage volume. The maximum drainage volume is 25 km3. Because the depth of the caprock and extent of the pore pressure drainage is known, this volume limit implies that no significant mass withdrawal occurs below a depth of 4 km. The ratios of surface elevation changes to horizontal contraction coinciding with the drainage area imply an equant drainage geometry, assuming that fluid production produces negative dilatation. Using the same cylindrical geometry used to model the gravity, negative dilatational strain rates of 4 to 5 × 10−5/yr were found to produce the observed surface displacements. The likelihood of boiling in the system to produce steam combined with apparent large bulk moduli (from seismic velocities) and small pore pressure declines suggest that most of the reservoir contraction is due to cooling.
Microscale chemical effects of low temperature alteration of DSDP basaltic glasses
Authors: Ailin‐Pyzik I. B. , Sommer S. E.
Published: February 2008

A novel application of X ray fluorescence, using selected area analysis, has been used to study the effect of low temperature alteration on trace elements in DSDP basaltic glasses. Major and minor element distributions were detailed through the use of electron microprobe analyses on the same sample areas. Samples include pillow basalt rinds and hyaloclastite sections from Deep Sea Drilling Project Site 396B. The glasses were found to release approximately 50% of the original Si and Al, 65% of the Mg and Na, and over 90% of the Ca originally present, during alteration to palagonite. Fe and Ti were found to be immobile, and K was increased 40‐fold by concentration from seawater. For the trace metals, over 25% of the Zn, Cu, and Ni were released, 40% of the Mn, and over 10% of the Cr. These changes apply only to the conversion of fresh glass (sideromelane) to palagonite (smectite) and do not include the effects of authigenic phillipsite and calcite reprecipitated locally. Differences between the effects of low temperature weathering on the crystalline basalts and the glasses appear to be primarily a function of the susceptibility of the primary mineral phases to attack, with the glass, as the least stable phase, most altered.
The Vredefort radioelement profile extended to supracrustal strata at Carletonville, with implications for continental heat flow
Authors: Nicolaysen L. O. , Hart R. J. , Gale N. H.
Published: February 2008

The dominant features of the geology in the Klerksdorp‐Carletonville area in South Africa are a thick sequence of Precambrian stratified rocks of the Witwatersrand basin and the underlying Archean basement granite. The total crustal thickness is ∼36 km, and the average measured heat flow is 46 mW m−2. At Vredefort, 50 km to the south of the Klerksdorp‐Carletonville area, Archean granite forms the core of an updomed and overturned sequence of strata. Recent geochemical, geophysical, and geological studies provide evidence that the Vredefort basement granite has also been overturned, exposing a ∼15 km thick section of the Archean crystalline crust. Profiles across the Vredefort basement, together with boreholes through the overlying stratified rocks, provide a unique opportunity of measuring the contribution of crustal radioactive heat production to surface heat flow. Adopting two extreme models for heat production in the lowermost crust, the total contribution to surface heat flow from radioactive heat generation in the crust is calculated at between 29 and 34 mW m−2. Heat flow from the mantle in the southwestern Transvaal sector of the Kaapvaal craton is estimated at between 17 and 12 mW m−2. This low uppermost mantle heat flux is about half of that estimated for an oceanic lithosphere at equilibrium. Accordingly, the low flux provides evidence for a lithosphere thickness about double that of an old oceanic region. The unusually high estimate of crustal heat production and the corresponding low mantle heat flow are due mainly to the occurrence of a heat production hump beneath an upper zone of exponential decline. Published statements that the reduced heat flow is an acceptable approximation to heat flow at the base of the stable continental crust are not supported by the present work.
A critical assessment of estimation methods for activation volume
Authors: Sammis Charles G. , Smith John C. , Schubert Gerald
Published: February 2008

We have compared estimates of the activation volume V* based on several theoretical models with measured values in metals, alkali halides, and olivine. The theoretical methods tested include one based upon the empirical correlation between activation energy and melting temperature and several which are based upon simple elastic models for the defect structure. For metals and olivine, the melting relation works well, but for alkali halides, the melting model predicts too large a V* by an approximate factor of 1.5. Of the elastic models, the dilatational strain energy model introduced by Zener (1942) provides reasonable estimates of V* for metals and olivine, but it also overestimates V* for alkali halides. Zener's assertion that the experimental value of V* should be bounded by theoretical values calculated from strain energy models which assume pure shear (Keyes, 1963) and pure dilatation is supported by the available data for metals, oxides, and alkali halides. These provide upper and lower estimates for the variation of viscosity with depth in the mantle.
Magnetization of the Austell Gneiss, northwest Georgia Piedmont
Authors: Ellwood Brooks B. , Abrams Charlotte
Published: February 2008

The Austell gneiss is located in the Piedmont 16 km west of Atlanta, Georgia, and 8 km northwest of the Brevard fault zone. This metamorphosed granite gneiss lies refolded in the nose of the Austell‐Frolona antiform, a doubly plunging structure of regional extent. Subsequent to intrusion, the unit has undergone four episodes of deformation. The first and second generation folds (F1 and F2) are coaxial but not coplanar. The two later fold events were less intense and have little effect on local outcrop patterns in the study area. The age of the unit has been estimated to be greater than 325 m.y. but less than 365 m.y. The induced and remanent magnetic properties for at least six cores drilled at each of 10 sites within the Austell gneiss have been determined. The magnetic foliation for the unit is approximately parallel with field estimates for S1 foliation planes exhibited by mineral orientations within the body, indicating post‐emplacement magnetic grain mobility. The stable remanent magnetization exhibited by the body was therefore probably acquired during or slightly after the F1 or F2 deformational events as a result of cooling from metamorphic temperatures (∼600°C), or, alternatively, from secondary magnetite grain nucleation and growth. A paleopole position calculated for these data without structural correction, ϕ = 126.2, θ = 34.4, dp = 6.6, dm = 13.2, can be explained by magnetic acquisition in at least three possible geologic settings. From simple to complex, these are the following: First, since a large oval of confidence is observed for the unit and minor tilting can not be resolved, the data may be explained by magnetization of the Austell at its present site during cooling from metamorphic temperatures. Second, a northwestward thrust event may have resulted in bringing the Austell to its present location. Third, the data can be explained by two major translational events. The Austell may have initially been magnetized and then moved to the northeast by transcurrent strike slip faulting >1000 km into juxtaposition with the southern Appalachians, followed by thrusting of the unit to the northwest to its present location. In support of this last interpretation is the similarity between the paleopole for the Austell and paleopoles determined for the Maritime Provinces of North America, which were suggested to have moved ∼1500 km to the northeast during the mid to late Paleozoic. Against such an interpretation is the lack of geologic evidence in support of large‐scale left‐lateral strike slip faulting in the southern Appalachians.
Structural fabric of the Paleozoic Golconda Allochthon, Antler Peak Quadrangle, Nevada: Progressive Deformation of an Oceanic Sedimentary Assemblage
Authors: Miller E. L. , Kanter L. R. , Larue D. K. , Turner R. J. , Murchey B. , Jones D. L.
Published: February 2008

The Golconda allochthon in Nevada consists of structurally complex mid to late Paleozoic basinal sedimentary and volcanic rocks deformed under low temperature and pressure conditions and emplaced onto the North American shelf in the Permo‐Triassic. Above the basal thrust in the Antler Peak area, the allochthon consists (in ascending structural order) of late Early to early Late Permian thick bedded chert (‘Pumpernickel subunit 1 ’); Pennsylvanian and Early Permian thin bedded chert, argillite and siltstone (‘Pumpernickel subunit 2’); Mississippian siliciclastic turbidites (‘Jory unit’) and undated chert and argillite of the ‘Trenton unit.’ Open to tight folds in subunit 1 trend north‐south and are east‐vergent. Folds in thinner bedded subunit 2 vary considerably in style. Fold axes in this unit are distributed within the mean axial plane (oriented N‐S, 60°W) but exhibit a west‐northwest trending data gap. Boudin axes measured in zones of highly disrupted layering (interpreted as zones of movement or shear) exhibit similar patterns without data gaps. These girdled distributions in part reflect rotation of these elements upon greater strain towards the east‐west trending direction of tectonic transport or towards parallelism with the X axis of the strain ellipsoid. Asymmetry of folds indicate movement was west to east. Locally developed late stage folds in subunit 2 are subhorizontal, trend north‐south, and are compatible with fold‐fault solutions that indicate east‐directed thrusting. Striations measured within fault zones yield bidirectional east‐west slip lines. Our fold and fault data indicate that east‐west shortening of these basinal sediments was accomplished by folding and east‐directed thrust faulting during a single progressive deformational event. This conclusion is supported by the presence of a major west‐dipping thrust fault in the map area that places Mississippian over Permian rocks. The allochthon formed as an east‐vergent submarine fold and thrust belt that was subsequently thrust on the continental shelf. Autochthonous rocks beneath the Golconda thrust are virtually undeformed, suggesting that most of the structure in the allochthon formed prior to its ultimate emplacement.
On the outer layers of the core and geomagnetic secular variation
Authors: Le Mouël Jean‐Louis , Courtillot Vincent
Published: February 2008

The observed correlation between the decade variations in the earth's rotation and geomagnetic secular variation probably can be accounted for by electromagnetic core‐mantle coupling as suggested by Bullard long ago. The new observations, combined with a simplified earth model, provide some indications on the thickness of the outer core layers which are responsible for geomagnetic secular variation. This thickness appears to be much smaller than previously thought (of the order of 100 to 200 km). The radial velocity of the fluid at the base of the layer is found to be of the order of 10−6 m s−1, about 2 orders of magnitude smaller than the horizontal velocity in the layer.
An inverse approach to signal correlation
Authors: Martinson Douglas G. , Menke William , Stoffa Paul
Published: February 2008

A simple mathematical inverse method is used to correlate two time series D(x) and R(t), where these two signals are related to each other by the mapping function x(t). The mapping function describes stretching and squeezing of one signal with respect to the other. The method assumes that D(x) and R(t) are known, while x(t) is not. The mapping function is parameterized in terms of a sum of simple functions of unknown coefficients ai;. These coefficients are estimated from the time series with the assumption that the best coefficients are those which maximize the coherence between R(t) and D[x(t)]. The maximization is performed iteratively, beginning with some initial estimate of ai. The standard error in the estimates of ai and the uncertainty implied in x(t) is calculated. The effect of noise in the signals and sharp slope changes in the mapping function are assessed by empirical testing. Results suggest that mapping functions containing features with periods down to 6% of the signal length and as sharp as hiatuses can be recovered even if the signals contain significant noise. The method is applied to determining differential sedimentation rates from O18 profiles measured in two cores and to measuring differential spreading rate about a mid‐ocean ridge by using marine magnetic anomalies.
An investigation of ulvospinel composition and cation migration during maghemitization in Deep Sea Drilling Project leg 61 titanomagnetites
Authors: Steiner Maureen B.
Published: February 2008

Titanomagnetites of basalts and diorites from an off‐axis volcanic episode in the western Pacific Ocean (leg 61, Deep Sea Drilling Project) were studied for their ulvospinel composition and for changes in the iron‐titanium ratio with progressive low temperature oxidation. Thermomagnetic analysis and microscopy indicate that these Fe/Ti spinels are relatively little low temperature oxidized despite their Cretaceous age. Microprobe investigation shows that the Fe/Ti spinels have a wide range of ulvospinel contents and that the range directly reflects the range of parent rock chemistries. This is an important demonstration that ulvospinel contents are not constant but vary within a restricted range of compositions, namely, at least x = 0.50 to x = 0.75 (Fe3−xTixO4). Recognition of this fact is important in attempting to elucidate changes in cation ratio as a result of progressive low temperature oxidation. Examination of samples of different oxidation states for cation ratio changes associated with low temperature oxidation displayed lower titanium in the more oxidized samples and Ti enrichment in the less oxidized ones. This, however, appears to be a reflection of changing x values with progressive crystallization of individual sills, rather than a consequence of low‐temperature oxidation. In the more than 100 grains analyzed, no evidence was found for a more oxidized shell on the exteriors of grains, suggesting that the oxidation process did not proceed uniformly with respect to grain geometry.
Determination of in situ attenuation from full waveform acoustic logs
Authors: Cheng Chuen Hon , Toksöz M. Nafi , Willis Mark E.
Published: February 2008

Methods for the determination of in situ P and S wave attenuation from full waveform acoustic logs are developed. For P waves, the peak amplitude ratios of the refracted P waves from two different receivers can be used with geometrical spreading taken into account. For S waves, owing to the contamination by the guided waves, its attenuation cannot be determined directly. Instead, S wave attenuation is determined from the attenuation of the guided waves using the partition coefficients (normalized partial derivatives of the phase velocity with respect to the body wave velocities). Analytical forms of these partition coefficients are presented here, along with examples for a number of different rock formations (granite, limestone, sandstone and soft sediments). The results show that in high velocity rocks, the fluid attenuation controls the guided wave attenuation except near the cutoff frequency of the pseudo‐Rayleigh wave. For low velocity rock formations, especially in the case where the S wave velocity is lower than the fluid velocity, the S wave attenuation is the main contributor to the guided wave attenuation. Synthetic microseigmogram calculated with the measured body wave attenuation agrees well with the actual microseismograms.
Constraints on crustal structure in eastern Iceland based on extremal inversions of refraction data
Authors: MacKenzie Kevin , McClain Jim , Orcutt John
Published: February 2008

In the summer of 1978 the Iceland Research Drilling Project undertook the drilling of a deep crustal hole near Reydarfjörder in eastern Iceland. As a part of this project, the Scripps Institution of Oceanography and the University of Washington undertook a small‐scale seismic refraction experiment near the drill site in an attempt to compare surface geophysical measurements with observations of samples from and logging in the hole. Using recent advances in the methods of extremal inversion of seismic data, we have determined an approximate one‐dimensional velocity structure for the drill site. This structure indicates that the 1.9‐km hole failed to penetrate the layer 2 layer ‐3 transition which was at some 3.0–4.5 km beneath the drill site. The transition appears to be rather abrupt, unlike that beneath the ocean, with velocity increasing from 5.2–5.5 km/s in the upper crust to about 6.7 km/s in layer 3. We observe a steep eastward dip and a shallow westward dip in the lower crust away from the nearby Thingmúli and Reydarfjördur volcanic centers, respectively, in agreement with previous work associating shallow depths to layer 3 with Tertiary volcanic centers as a result of increasing metamorphic grade and increased dyke swarm intensity.
Zones of polarity reversal of stable remanent magnetization within some basaltic flows of the Iceland Research Drilling Project core
Authors: Schoenharting G. , Ghisler M.
Published: February 2008

Magnetic and ore microscopic investigations of flow units 326.2, 320.1, 263.2, and 298.2 reveal that extensive remagnetization of rocks has taken place in the lower portion of the drill hole leading to multiple polarity changes of NRM within single units. After optimum AF and thermal demagnetization a middle portion of stable normal polarity remanence remains with a top and bottom portion of reversed stable magnetization. The reversed direction is interpreted to be of primary origin, whereas the normal direction could arise from a viscous magnetization acquired at elevated temperatures which is stabilized by both grain subdivision during hydrothermally induced oxidation and by subsequent cooling. Microscopic study shows the presence of secondary magnetite in a few samples from flow bottoms of units 298.2, 320.1, and 326.2. For the latter two units, secondary magnetite appears to be of minor importance, whereas secondary magnetite apparently causes very high natural remanence and susceptibility values in samples from unit 298.2.
Millisecond accurate monitoring of seismic travel times over 13‐ and 18‐kilometer baselines
Authors: Leary Peter C. , Malin Peter E.
Published: February 2008

Using a repeatable airgun source we have monitored the travel times of several seismic phases along 13‐ and 18‐km baselines from Bouquet Reservoir to boreholes on the San Andreas fault near Palmdale, California. The first arriving phases refract at a depth of 2–3 km, while one later arrival is possibly a reflection from 10 km. The majority of some 200 travel time determinations for the most prominent phases were made between August 1979 and March 1980; isolated measurements were made in November 1978 and March 1981. There exists ample geodetic and other evidence for significant strain changes during this period. Allowing for source variation due to fluctuating reservoir water levels, we observed no travel time changes to the limit of our precision of ±1–2 ms for the first arrivals and ±3–5 ms for secondary arrivals, or about 0.1% of the total travel time. An earlier study, which monitored rays penetrating only 500 m into the crust, found that travel times varied by up to 0.3% over a period of several days. The variations showed some correlation with strain changes due to local seismic activity and were not permanent. To understand this different behavior of crustal rocks between 500 and 2000 m, we investigate the depth dependence of their velocity‐strain sensitivity. Using measured P and S velocity profiles to gauge the rate at which confining pressure increases velocity, we find that 1 bar of overpressure at 500 m can vary the P velocity by 0.1%, while 1 bar at 2000 m will vary the velocity by only 0.01%. Applying the O'Connell‐Budiansky physical model for cracked rock to the velocity depth profiles, we can further characterize the local crust as having a high crack density, a mobile fluid component, and an average crack aspect ratio of 2.5×10−3. The combination of a mobile fluid content and high crack density suggests that fluids play a role in the transient velocity anomalies observed at a depth of 500 m and argues against significant undersaturation phenomena at seismogenic depths on the San Andreas fault.
Refinements of the polar motion frequency modulation hypothesis
Authors: Carter William E.
Published: February 2008

In a previous paper (Carter, 1981) I suggested that the Chandler component of polar motion may be frequency modulated, as a function of the instantaneous polar motion magnitude. A composite of observational records compiled by S. C. Chandler and the International Latitude Service series suggests a variation in the magnitude at a frequency of approximately 0.0118 cycles per year (cpy). The spectrum produced by the resulting frequency modulation would depend on the modulation index. Comparison of spectra synthesized with various degrees of frequency modulation and those derived from observational data, suggest a frequency increase of 0.15 to 0.30 cpy/arc sec decrease in the polar motion magnitude. This degree of frequency modulation would cause ‘splitting’ of the Chandler component, producing dual spectral peaks at periods of 427 and 439 days, similar to that reported by several investigators beginning with Chandler (1901a, b).
Remagnetized cratonic Cambrian strata from southern Nevada
Authors: Gillett Stephen L.
Published: February 2008

Stratigraphic sections of Cambrian strata in southeastern Nevada are akin to thin, cratonic facies exposed in the Grand Canyon; their structural setting is much more complicated, however, from Mesozoic and Cenozoic tectonism. Paleomagnetic samples from two sections through these strata appear to have been completely remagnetized. Coarse‐grained, lightcolored sandstone from the Tapeats Sandstone yields scattered magnetizations, residing in hematite, that appear to reflect protracted diagenetic acquisition of remanence. A hematitic sandstone in the Bright Angel Shale yields relatively consistent ‘Paleozoic’ directions of magnetization, but petrographic study shows that the hematite results from diagenetic oxidation, and stratigraphic arguments suggest that the oxidation was not penecontemporaneous. Gray limestones of the Jangle and Muav Limestones yield a magnetization, residing in magnetite, that may reflect late Tertiary remagnetization, being imposed during uplift related to the onset of Basin and Range deformation. In any case, this magnetization differs greatly in direction from a hematite magnetization reported from slightly younger Muav Limestone in the Grand Canyon. Both the sampling sites also appear to have been tectonically rotated, but whether this rotation is true or a geometric artifact of the tilt correction cannot be determined from the present data. These results suggest that paleomagnetic data from rocks as old as Cambrian must be scrutinized very carefully before their magnetizations can be accepted as penecontemporaneous, and such scrutiny must include geologic data.
Amplitude constraints in linear inversions of seismic data
Authors: Garmany Jan
Published: February 2008

Linear programming inversions of seismic data may include constraints on the second derivative of the delay time, d2τ/dp2 = −dX/dp = −X′. Geometric ray theory predicts that the second derivative of the delay time is proportional to the inverse of the (complex) seismic amplitude squared, so that the use of this additional constraint constitutes the incorporation of amplitude information in linear inversions. The realizability constraint that velocity be a single valued function of depth must be included explicitly. For higher‐order derivatives of delay time than the second, the realizability constraint is nonlinear and cannot be included in a linear scheme. Given that the data are adequate to allow identification of the major prograde and retrograde branches and the intervening caustics, useful bounds on X′ can be established. The function X′ is calibrated with respect to measured amplitudes by requiring that the T(X) curve obtained by successively integrating the X′ curve satisfy the observed travel times. This calibration may be cast as a linear least squares problem. Applications to data from the ROSE (Rivera Ocean Seismic Experiment) area reveal a welldefined layer 2‐layer 3 transition, an apparently isotropic and nearly homogeneous layer 3, and a Moho transition that is quite gradual. This transition appears to be more rapid in the fast Pn direction.
Theory of the development of geothermal systems charged by vertical faults
Authors: Bodvarsson G. S. , Benson S. M. , Witherspoon P. A.
Published: February 2008

A two‐dimensional model of fault‐charged hydrothermal systems has been developed that considers the transient development of such systems including the effects of heat losses to the confining layers.The model can be used for theoretical studies of the development of fault‐charged reservoirs. It can also be used to estimate the rate of recharge from the fault source and the time of evolution, using temperature data from wells. The model has been applied to the hydrothermal system at Susanville, California. A reasonable match with the areal temperature distribution in the primary aquifer and the temperature profiles of individual wells was obtained. This allowed an estimate of the recharge rate from the fault into the hydrothermal system to be obtained. As the calculated recharge rate (9×10−6 m3/s m) into the Susanville hydrothermal system proved to be quite significant, a threefold increase in the potential of the Susan ville hydrothermal anomaly for space heating purposes is predicted.
The determination of the internal magnetic field in magnetic grains
Authors: Smith Guy , Merrill Ronald T.
Published: February 2008

For a number of years it has been supposed that for multidomain materials, the demagnetization factor N can be determined from either hysteresis loop or initial susceptibility measurements. Unfortunately, these methods involve a subtle circular argument which invalidates the entire analysis, ruling out the use of either of the aforementioned measurements in the determination of N. On examining possible alternatives to these methods we find the only viable one to be ferromagnetic resonance spectroscopy. Data are presented from a single crystal of magnetite which demonstrate that a reasonable value for N can be derived although only for the saturated state.
Subkilometer Martian volcanoes: Properties and possible terrestrial analogs
Authors: Frey Herbert , Jarosewich Martha
Published: February 2008

Kilometer to subkilometer cones with large central craters are the most numerous volcanic structures in the northern plains of Mars. Based on examination of more than 12,000 high resolution Viking orbiter images, most of these very small cones are located in parts of Eastern Acidalia and Utopia‐Isidis‐Elysium Planitia. Measurements of base and pit diameter for some 2000 well‐resolved features show small variations in the histogram of base diameters between and within regions but remarkable similarity in the distribution of crater/cone diameter ratios. This suggests that the small cones had a similar origin with only minor variations in local conditions. The Acidalia cones are generally larger, more widely distributed, and occur on a greater variety of terrains than those in Utopia‐Isidis‐Elysium. Based on morphology, distribution, and the crater/cone ratio histograms, the smallest Martian cones may be similar to Icelandic pseudocraters. Somewhat larger, steeper cones with smaller crater/cone ratios which also occur in Eastern Acidalia may be analogs of terrestrial cinder cones.
A model of Martian slope winds: Implications for eolian transport
Authors: Magalhães Julio , Gierasch Peter
Published: February 2008

Observations of dark Martian wind streaks not associated with visible topographic points of origin (‘coalesced dark streaks’) show these features to favor regions with slopes of 1°–10°. To investigate the circulation over such slopes, we adopt a Boussinesq fluid and assume a constant eddy diffusivity parameterization for small‐scale mixing. The equations of motion are scaled to determine the range of slopes over which a strong local circulation due to slope winds can exist. For nighttime conditions, our estimated range compares well with the observed range of slopes favored by coalesced dark streaks. For daytime conditions, the slope wind solution is not valid. A simple one‐dimensional analytic model is solved to determine friction velocities and velocity and temperature profiles. For reasonable choices of the input parameters, we find friction velocities and wind velocities suggestive of active downslope eolian transport on slopes of 1°–10°. Friction velocities are largest over surfaces with low thermal inertia and large roughness length.
Raman spectra of olivine up to 65 kilobars
Authors: Besson J. M. , Pinceaux J. P. , Anastopoulos C. , Velde B.
Published: February 2008

Raman spectra of natural forsteritic olivine crystals under pressure up to 65 kbar show that peaks due to three major SiO tetrahedral vibrations vary similarly under pressure, which indicates that the intra‐tetrahedral modes are not significantly affected. The SiO4 tetrahedra can be considered to be structurally independent of their immediate environment.
A captured island chain in the coast range of Oregon and Washington
Authors: Duncan Robert A.
Published: February 2008

K‐Ar and 40Ar−39Ar geochronologic data reveal the Paleocene to Eocene eruptive history of volcanic centers which produced the basaltic basement rocks of the Coast Range of Oregon and Washington. Volcanism was short lived at each center and migrated with time from the northern and southern ends toward the center, near the present mouth of the Columbia River. A plot of crystallization ages against geographic location produces a distinct V‐shaped pattern. An origin by hot spot volcanism centered on or near a spreading ridge in early Tertiary time best explains the observed age distribution. Limited geochemical data support this model, as the basalts exhibit attributes of both spreading ridge and hot spot‐related volcanism. Icelandic volcanism may be a presentday analog. Absolute motion modeling shows that the Yellowstone hot spot could have generated an island chain with approximately the correct paleoazimuth and volcanic propagation rate. Shortly after formation, this oceanic volcanic lineament collided with North America and rotated into its present marginal setting. The western edge of North America subsequently crossed over the hot spot, perhaps triggering late Eocene to Oligocene volcanism in the Coast Range, the Miocene Columbia River basalts in eastern Washington and Oregon and most recently, the basalts of the Snake River Plain and the Yellowstone area.
Adcumulus growth of anorthosite at the base of the lunar crust
Authors: Morse S. A.
Published: February 2008

Unzoned plagioclase crystals imply isothermal adcumulus solidification and cannot result from reactive equilibrium crystallization in dry magma. Extraction of latent heat upward through lunar crust results only in chilled or cotectic mesocumulate rocks, not anorthosite or adcumulates. Adcumulus growth of anorthosite requires exchange of refractory for incompatible components between the site of crystal growth and nearby fresh magma, and the extraction of calories from growing crystals to the flowing, supercooled magma. Supercooling can be acquired at remote sites like upwellings and then be carried by convective flow beneath crust which is accreting plagioclase by flotation or in situ nucleation and growth. If transport distances are larger than the scale of rockbergs, cotectic norites and troctolites may result. Impact‐induced upwellings tend to promote growth of buoyant rather than mafic crust, hence impacts are not solely destructive.
Lunar Regolith: Petrology of the <10 μm fraction
Authors: Devine James M. , McKay David S. , Papike James J.
Published: February 2008

It has been known for some time that the part of the lunar regolith smaller than 10 micrometers (<10 μm) is chemically distinct from the bulk soil. Specifically, it is enriched in Al, Ca, Na, K, light rare earth elements, and Th, and depleted in Mg, Fe, Mn, and Sc. Three models have been proposed to explain those systematics. The Exotic Component model holds that a fine‐grained, KREEPy exotic component was distributed on a global scale, perhaps by the Imbrian impact. In the Simple Comminution model, the <10 μm chemical systematics are due to differential comminution processes with fine‐grained melt‐rock mesostasis and feldspar concentrating in this fraction. The Soil Mixing model states that the chemical systematics are due to mixing of compositionally distinct soils that have different grain‐size distributions. To determine which model is correct, modal petrographic data were obtained by scanning transmission electron microscopy with energy dispersive X ray analysis for the <10 μm fractions of four Apollo soils: 10084, 14163, 15271, and 64501. Modal results are consistent with chemical data regarding feldspathic and KREEP enrichments in the finest fraction. Soil glass contents, crystalline silica, and plagioclase/mafic mineral ratios are greater in the <10 μm fraction than in the coarser fractions for all soils. Grain‐size distributions are inconsistent with a fine‐grained exotic component derived from distant sources. Comminution of local lithologies, vertical mixing, and local lateral transport control the <10 μm soil composition.
The dynamic tensile strength of ice and ice‐silicate mixtures
Authors: Lange Manfred A. , Ahrens Thomas J.
Published: February 2008

We determined the dynamic tensile strength of ice and ice silicate mixtures at strain rates of ∼104 s−1. At these strain rates, ice has a tensile strength of ∼17 MPa, and ice‐silicate mixtures with 5 and 30 wt % sand content have strengths of ∼20 and 22 MPa, respectively. These values lie significantly above tensile strengths of ∼1.6 MPa for ice and of ∼5−6 MPa for frozen silt, measured at strain rates of ∼10−2 to 10° s−1, but markedly below values found for a variety of rocks at comparable strain rates. Results of the present experiments are used to derive parameters for continuum fracturing models in icy media, which are used to determine relations between tensile strength and strain rate, and to predict stress and damage histories as well as size frequency distributions for ice and ice‐silicate fragments. It is found that tensile strength σM is related to strain rate by σM ∝ ε[0.25–0.3], similar to results obtained for other geological materials. The increase of small fragments relative to larger fragments with increasing strain rate, as predicted by the continuum model, is a result which parallels findings in laboratory impact experiments.
Three‐dimensional crust and upper mantle structure of the Eurasian Continent
Authors: Feng Chi‐Chin , Teng Ta‐Liang
Published: February 2008

Group velocities of fundamental mode Rayleigh waves of 109 wave paths within Eurasia have been measured by an improved frequency‐time analysis technique. Stochastic inversion theory is applied to the mixed‐path measurements to extract pure path group velocities of 10°×10° grid elements that form the subdivisions of Eurasia. Then a three‐dimensional crust and upper mantle structure of the Eurasian continent to a depth of 300 km is constructed. The average crustal thickness of Eurasia is found to be about 40 km. Abnormally thick crust from 50 to 65 km is obtained in areas of Tien Shan, the Altai mountains, Afghanistan, Pamir, Tibet, and Burma. Among them, Tibet has the least dense crust and highest elevation.This implies that at least part of the elevation of Tibet is due to isostatic compensation. The upper mantle seismic velocity of the tectonically active regions of Asia is considerably lower than that of the European and Siberian cratons. If high velocity implies high rigidity, this explains that large‐scale tectonic deformations of Eurasia occur in regions with weak upper mantle that deforms more plastically. Regions with stronger upper mantle remain relatively undeformed. This is especially evident for the Tertiary tectonic deformation as a consequence of the collision of India with Eurasia.
Shock temperatures of SiO2 and their geophysical implications
Authors: Lyzenga Gregory A. , Ahrens Thomas J. , Mitchell Arthur C.
Published: February 2008

The temperature of SiO2 in high‐pressure shock states has been measured for samples of single‐crystal α‐quartz and fused quartz. Pressures between 60 and 140 GPa have been studied using projectile impact and optical pyrometry techniques at Lawrence Livermore National Laboratory. Both data sets indicate the occurrence of a shock‐induced phase transformation at ∼70 and ∼50 GPa along the α‐ and fused quartz Hugoniots, respectively. The suggested identification of this transformation is the melting of shock‐synthesized stishovite, with the onset of melting delayed by metastable superheating of the crystalline phase. Some evidence for this transition in conventional shock wave equation of state data is given, and when these data are combined with the shock temperature data, it is possible to construct the stishovite‐liquid phase boundaries. The melting temperature of stishovite near 70 GPa pressure is found to be 4500 K, and melting in this vicinity is accompanied by a relative volume change and latent heat of fusion of ∼2.7% and ∼2.4 MJ/kg, respectively. The solid stishovite Hugoniot centered on α‐quartz is well described by the linear shock velocity‐particle velocity relation, us = 1.822 up + 1.370 km/s, while at pressures above the melting transition, the Hugoniot centered on α‐quartz has been fit with us = 1.619 up + 2.049 km/s up to a pressure of ∼200 GPa. The melting temperature of stishovite near 100 GPa suggests an approximate limit of 3500 K for the melting temperature of SiO2 ‐bearing solid mantle mineral assemblages, all of which are believed to contain Si4+ in octahedral coordination with O2−. Thus 3500 K is proposed as an approximate upper limit to the melting point and the actual temperature in the earth's mantle. Moreover, the increase of the melting point of stishovite with pressure at 70 GPa is inferred to be ∼11 K/GPa. Using various adiabatic temperature gradients in the earth's mantle and assuming creep is diffusion controlled in the lower mantle, the current results could preclude an increase of viscosity by more than a factor of 103 with depth across the mantle.
Observations of volcanic tremor at Mount St. Helens Volcano
Authors: Fehler Michael
Published: February 2008

Digital recordings of ground motion during tremor episodes accompanying eruptions at Mount St. Helens Volcano in the state of Washington on August 7 and October 16–18, 1980, are studied. The spectra of the vertical component waveforms contain at least two dominant peaks at 1.0 and 1.3 Hz for all events recorded during both eruptions that were studied. Spectra of horizontal ground motion show peaks at 0.9 and 1.1 Hz. The relative amplitude of the two peaks changes between tremor episodes and during single tremor episodes and shows no consistent relation to amplitude of ground motion. Spectra of long‐period earthquakes are very similar to those of tremor events, suggesting that tremor is composed of many long‐period earthquakes that occur over a period of time. The unique waveform of tremor events observed at Mount St. Helens must be due to a source effect, since the relative amplitude of the two dominant peaks changes during tremor episodes. The path effect on tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of Mount St. Helens. The consistency of the location of the spectral peaks for the wide range of tremor amplitudes means that there must be a physical length at the source that is constant, independent of the amplitude of motion at the source. Variations in amplitude of motion generated during tremor events must be due to variations in the force driving the tremor. Amplitudes of ground motion varies between 0.11 and 4.7 μm. Seismic moment rates during the two eruptions are calculated using the model of Aki et al. (1977) and found to vary between 6×1018 and 1×1020 dynes cm s−1, which are larger than values found by Aki et al. (1977), who studied amplitudes of shallow tremor events recorded during the October 1963 eruption of Kilauea volcano in Hawaii. Study of tremor amplitudes recorded at Corvallis, Oregon, leads to the conclusion that tremor accompanying the cataclysmic May 18, 1980, eruption was at least one order of magnitude larger in amplitude than tremor during August and October.
Regional deformation of the Earth model for the San Diego Region, California
Authors: Snay Richard A. , Cline Michael W. , Timmerman Edward L.
Published: February 2008

A mathematical model for historical horizontal crustal deformation is presented for the southern California region that extends northward from Mexico for approximately 100 km to 33.5°N latitude. Model parameters are estimated from triangulation, trilateration, and astronomic azimuth data observed during the past century. Also geologic and seismic data indirectly participate in specifying the model. The model relates crustal deformation to the episodic movement experienced during large earthquakes (M ≥ 6), secular slip rates on geologic faults, and secular strain rates over large geographic areas. The episodic movement is modeled in accordance with the theory of dislocation in an elastic halfspace. For the secular motion the modeled region is partitioned into a mosaic of districts that are allowed to individually translate, rotate, and homogeneously deform as a linear function of time. The derived strain pattern supports the hypothesis that the modeled region spans the Pacific‐North American plate boundary. Secular shear strain is maximum in the NW direction parallel to the major faults, and shear strain rates decrease with distance from a NW–SE trending strip. To the region's north this strip is 40 km wide and is bounded on the west by the San Jacinto fault zone and on the east by the southern extension of the San Andreas fault. To the south this strip converges in width onto the Imperial fault. The secular velocity for a point near the city of San Diego relative to a point at the region's northeast corner is 4.3±3.1 cm/yr in the direction N 17°W±5°. This rate provides a rough estimate for the relative velocity between the Pacific and North American plates.
Oxygen isotope study of Precambrian granites from the Illinois Deep Hole Project
Authors: Shieh Yuch‐Ning
Published: February 2008

Seventeen whole rock samples and 27 mineral separates from the Precambrian granites in drill cores UPH‐1, ‐2, and ‐3 have been analyzed. The δ180 values for quartz remain exceedingly uniform (δ =7.6–8.6‰) throughout the entire length of the three cores and are indicative of their primary igneous values. The δ180 for the feldspars are also uniform for samples located 70 m or deeper below the unconformity (δ =6.0–6.9‰) However, in the upper 70 m of the granite body, a gradual upward increase of δ180 in the feldspars is observed (to 8.8‰). Similarly Δ18 quartz‐feldspar show typical plutonic values (1.4–2.1) in the main part of the Precambrian sections except in the upper 70 m, where ΔQ‐F become progressively smaller upward until a reversed fractionation (negative ΔQ‐F) is observed in samples several meters below the unconformity. The increase of δ180 in the feldspars is interpreted as a result of hydrothermal alteration of the granites at low temperatures (probably between 110°–260°C). The source of the hydrothermal fluid responsible for the alteration most likely derived from the formation waters in the upper Cambrian‐Ordovician sedimentary rocks overlying the unconformity. The alteration occurred at least 135–200 m.y, ago when there was also a lead mobilization event. The relative low primary 18O/16O ratios in the granites suggest that the granites may have derived from partial melting of low −180 metamorphic rocks in the lower crust.
Influence of seismicity and rainfall on episodic creep on the San Andreas Fault System in central California
Authors: Schulz Sandra , Burford Robert O. , Mavko Barbara
Published: February 2008

Simple wire‐type extensometers (creepmeters), placed across traces of the San Andreas fault system in central California, have produced nearly continuous records of aseismic fault slip (fault creep) for 12 years. The records indicate seasonal and other changes in creep activity that may be responses to variations in local seismicity and rainfall. One frequent type of response is a sudden, episodic movement, or creep event. Onset times of creep events on records from four creepmeters were compared with origin times of local earthquakes and onset times of rain. Seismicity appeared to affect long‐term creep rates but did not consistently trigger creep events. Rainfall produced varying responses on the instruments, but when compared annually to total creep, number of local magnitude 2+ earthquakes, number of creep events, and total amount of movement in the form of creep events, total rainfall showed no significant linear correlation with any of the variables. The dry part of the annual rainfall cycle appeared to delay onset of creep events. These ‘delayed’ events were consistently larger than events during the rainy season.