bluuebunny/crossref_metadata_2025 huggingface dataset:
Episodic spreading and rift propagation: New paleomagnetic and geochronologic data from the Afar Nascent passive margin
Authors: Courtillot V. , Achache J. , Landre F. , Bonhommet N. , Montigny R. , Féraud G.
Published: February 2008

A recent model proposes that Afar is one of the few areas in the world where the formation of a passive margin by rift propagation can be observed directly. Forty sites spread over approximately 10,000 km2 in the Republic of Djibouti, between the Asal and Abhe rifts, have been sampled for paleomagnetic and geochronologic study. Most basalt samples have a single stable magnetic component, often already clear in the natural remanent magnetization. The remanent direction of sites younger than about 1 Ma is consistent with the expected axial dipole direction, whereas sites between 1.35 and 2.25 Ma old form a microblock that was rotated by 14.5°±7.5° clockwise, probably between 1.3 and 1 Ma. The rotation is attributed to rift propagation. This study shows that spreading and rift propagation in Afar are episodic and lead to alternate “effusive” and “tectonic” phases that may be of general significance for accreting plate boundaries.
A detailed magnetotelluric/audiomagnetotelluric study of the Jemez Volcanic Zone, New Mexico
Authors: Ander Mark E. , Goss Ron , Strangway David W.
Published: February 2008

Supplement is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, D.C. 20009. Document B84‐003; $10.00. Payment must accompany order.

The Jemez volcanic zone is an alignment of late Cenozoic volcanic centers located in northern New Mexico and Arizona. It trends N52°E and is >500 km in length. Previous investigators suggest the zone is associated with elevated heat flow and anomalously low P wave velocity from 25 to 140 km depth measured along a line from Mount Taylor to the Jemez Mountains. It may also be associated with a Precambrian crustal‐age boundary. A detailed study, consisting of 119 audiomagnetotelluric (AMT) and 25 magnetotelluric (MT) soundings, was performed in a 161 km2 area located ∼50 km NW of the zone center. The AMT soundings were spaced 1.6 km apart in a grid pattern and had a frequency range from 12 Hz to 10.2 kHz. The AMT soundings were used to detect any near‐surface electrical structures or lateral variations. None were found, although terrain effects near the edges of mesas were observed. Agreement between the AMT and MT results from 12 to 50 Hz was excellent. Electrical strike direction is undefined at less than 1‐s period. From 1 to 100 s the average tipper strike is N30°W, aligning with the dominant surface structures. Greater than 100 s, it rotates abruptly to ∼N60°. This direction represents the electrical strike within metamorphic Precambrian basement rocks and suggests control of the deep electrical structure beneath the study area by the Jemez zone. Two‐dimensional MT modeling indicates three relatively homogeneous layers. The resistivity of the deepest layer, ≥15 km depth, is 50 ohm‐m in the northwest but decreases to 5 ohm‐m southeastward, toward the zone. These results, plus preliminary results from a regional MT study that surrounds the detailed AMT/MT study, suggest that the Jemez zone is associated with anomalously high electrical conductivity at shallow depths relative to surrounding regions. Magma intrusion along the zone is the most probable explanation for this anomaly.
The effect of temperature and impurity content on indentation hardness of quartz
Authors: Evans Brian
Published: February 2008

The results of some new and previously published data from indentation hardness tests on natural and synthetic quartz as a function of temperature provide several interesting conclusions. Indentation hardness appears to be affected by the concentration of some cationic impurities but not by others. A previous study showed that quartz hardness could be affected by electrolytically injecting lithium, but for the six natural quartz samples used in this study, no correlation between structural hydroxyl content and hardness existed. Several natural crystals showed hardening caused by heat treatments at temperatures of 600°C or higher for times of the order of several hours. Two synthetic crystals were tested, and samples from one showed a correlation of hardness with total hydroxyl content. The hardness of the weakest synthetic crystal was never less than 50% of the average of the natural quartz values at all temperatures up to 850°C.
The effect of coordinate system rotations on spherical harmonic expansions: A numerical method
Authors: Goldstein Jacob D.
Published: February 2008

A numerical method is presented for effecting the transformation of a spherical harmonic expansion under an arbitrary rotation of the coordinate system. Included are results of numerical tests with expansions to degree and order 180. These results demonstrate the high accuracy of the algorithm.
Postseismic deformation due to subcrustal viscoelastic relaxation following dip‐slip earthquakes
Authors: Cohen Steven C.
Published: February 2008

The deformation of the earth following a dip‐slip earthquake is calculated by using a three‐layer rheological model and finite element techniques. The three layers are an elastic upper lithosphere, a standard linear solid lower lithosphere, and a Maxwell viscoelastic asthenosphere. Attention is focused on the magnitude of the postseismic subsidence and the width of the subsidence zone that develops due to the viscoelastic response to coseismic reverse slip. Detailed analysis for a fault extending from the surface to 15 km with a 45° dip reveals that postseismic subsidence is sensitive to the depth to the asthenosphere but is only weakly dependent on lower lithosphere depth. The greatest subsidence occurs when the elastic lithosphere is about 30 km thick, and the asthenosphere lies just below this layer (asthenosphere depth = 2 times the fault depth). The extremum in the subsidence pattern occurs at about 5 km from the surface break of the fault and lies over the slip plane. At this location the subsidence reaches 60% of the coseismic uplift after about 30 Maxwell times. Unlike the horizontal deformation following a strike‐slip earthquake, significant vertical deformation due to asthenosphere flow persists for many times the Maxwell time of the asthenosphere. As a consequence, the relaxation of the lower lithosphere has only a secondary effect on uplift and subsidence; it does, however, substantially increase horizontal straining. The width of the postseismic subsidence zone also depends strongly on the depth to the asthenosphere and shows little sensitivity to the presence of a slowly relaxing lower lithosphere. Parametric adjustment of the lithosphere thickness and fault dip angle over a considerable range of values in a simple model of an elastic lithosphere and viscoelastic asthenosphere reveals that the postseismic subsidence is greatest when the fault penetrates through about one half the lithosphere. Previous estimates of the asthenosphere viscosity (∼1019 Pa s) and lithosphere thickness (∼30–40 km) for the region of Japan affected by the 1896 Riku‐u earthquake are confirmed by model calculations.
A faulting model for the 1979 Imperial Valley earthquake
Authors: Archuleta Ralph J.
Published: February 2008

By comparing synthetic particle velocities with the near‐source strong motion data we have constructed, by trial and error, a faulting model for the 1979 Imperial Valley earthquake. The calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the Imperial Valley and the spatial variation of the slip rate parameters on the fault plane. The independent slip rate parameters are (1) the strike‐slip rate amplitude, (2) the dip‐slip rate amplitude, (3) the duration that slip rate is nonzero (the rise time of the slip function) and (4) the rupture time, which determines when the slip rate is initiated. Our faulting model has the following principal features: (1) Faulting occurred on the Imperial fault and on the Brawley fault, rupture on the Brawley fault being triggered by rupture on the Imperial fault. (2) The Imperial fault is a plane 35 km long and 13 km wide with a strike of 323°, measured clockwise from north, and a dip of 80° NE. The Brawley fault is a 10 km long and 8 km wide plane with a strike of 360° and a dip of 90°. (3) Faulting on the Imperial fault is primarily right‐lateral strike slip with a small component of normal dip slip in the sediments at its northern end. The larger strike‐slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1.0 m/s. The duration varies on the fault with a maximum of 1.9 s, which is considerably shorter than the total time for the rupture to take place. (4) The rupture velocity on the Imperial fault is highly variable. Locally, it exceeds the shear wave velocity and, in one instance, the compressional wave velocity. The average rupture velocity, though, is less than the shear wave velocity. (5) Although the slip on the Brawley fault contributes only about 4% of the total moment, it greatly affects the ground motion at nearby stations. (6) The total seismic moment is 6.7×1018N m where the Imperial fault contributes 6.4×1018N m and the Brawley fault contributes 2.7×1017N m. In the process of trying almost 300 faulting models, we found that given the elastic parameters of the medium, the synthetic seismograms were most sensitive to the specification of the rupture time. Although the slip rate amplitudes are linearly related to the data, the rupture time and the duration are not. The parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault.
Effects of physical fault properties on frictional instabilities produced on simulated faults
Authors: Okubo Paul G. , Dieterich James H.
Published: February 2008

Laboratory studies of large‐scale simulated faults show that physical properties of the fault, specifically normal stress and fault roughness, significantly influence the unstable shear failure behavior of the fault. In addition, the experiments provide insights into important length‐scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension. Stick‐slip shear failures have been generated along a 2‐m‐long simulated fault in a block of Sierra white granite. Experiments were conducted at normal stresses between 0.6 and 4 MPa with two different prepared roughnesses of the simulated fault: a smooth fault with a profilometer measured roughness of 0.2 μm and a rough fault with a measured roughness of ∼80 μm. High‐speed records of shear strains and slip velocities clearly show both the propagation of the stick‐slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved. Both the dynamic stress drops of the stick‐slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress. Although critical slip‐weakening displacements are insensitive to normal stress, they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault. Slip velocities in these experiments are influenced by roughness and are proportional to stress drop. Although the onset of slip is sharply defined, the termination of slip is more gradual without a clear stopping phase. Measured rupture velocities on the rough fault are lower than those on the smooth fault. When a length scale based on rupture nucleation dimensions, directly proportional to critical displacements, is introduced, the data suggest that at fault lengths close to the nucleation lengths, rupture velocity and slip velocity increase with increasing fault length.
Modern uparching of the Gulf Coastal Plain
Authors: Jurkowski G. , Ni J. , Brown L.
Published: February 2008

A broad doming of the Gulf coastal plain between Jackson, Mississippi, and New Orleans, Louisiana is indicated by leveling surveys carried out in 1934, 1959, 1964, and 1969. The progressive arching over a distance of 275 km attains a maximum average rate of 3.4±0.5 mm/yr relative to Jackson. Additional leveling in the area indicates that the observed pattern of movement could be elongate, with its major axis parallel to the coast. The observed uplift is consistent, at least in sign, with longer‐term trends indicated by deformed terraces in Mississippi and Louisiana. Oil, gas, and water withdrawal mechanisms as well as systematic leveling errors appear inadequate to explain the wavelength and magnitude of the observed elevation changes. The pattern of uplift resembles that which might be expected from flexure due to sedimentary loading at the mouth of the Mississippi delta. Simple models of flexure of a thin elastic plate overlying a viscous half space caused by sediment loading during the Holocene do predict rates similar to those evidenced by leveling, if relatively low mantle viscosities (3. × l019 Pa s; 1 Pa s = 10 poise) are used. These observations may represent direct measurements of ongoing dynamic flexure at a passive continental margin.
Paleomagnetism of abbott seamount and implications for the latitudinal drift of the Hawaiian Hot Spot
Authors: Sager William W.
Published: February 2008

The magnetic anomaly of Abbott Seamount, located near the Hawaiian‐Emperor bend, has been analyzed for its paleomagnetic information. Short‐wavelength “noise” components of the anomaly, assumed to result from small magnetization inhomogeneities or reversals in the upper layers of the volcano, were filtered out by upward continuation. The polarity of the bulk of the seamount is reversed, and its virtual geomagnetic pole is located at 75.5°N, 4.6°E. This pole lies close to Pacific paleomagnetic poles of late Eocene age and is consistent with the age of the edifice to be expected from its position in the Hawaiian chain. The paleolatitude of the seamount, 17.5°N±4.4°, is not significantly different from the present latitude of the Hawaiian hot spot, approximately 19.5°N. This finding implies that the Hawaiian hot spot (and by inference other hot spots) has remained relatively fixed with respect to the spin axis since the Eocene. However, other paleolatitudes from the Hawaiian‐Emperor chain suggest that systematic variation of the hot spot's latitude may have occurred. Comparison of the Hawaiian‐Emperor paleolatitude data with paleomagnetic data from central Pacific piston cores and with north Pacific paleoequator data from Deep Sea Drilling Project cores indicates that the apparent latitude drift since the Eocene has been largely the result of long‐term nondipole components in the geomagnetic field rather than relative motion between the hot spot and the spin axis. Conversely, the variation that occurred prior to the Eocene appears to contain a significant amount of such motion coupled with an indeterminate amount of nondipole effect.
Seismic estimates of stress release
Authors: Boatwright John
Published: February 2008

Five seismic estimates of stress release are compared for eight multiply recorded aftershocks of the 1975 Oroville, California, earthquake. The dynamic stress drop, the arms stress drop, and the apparent stress provide stable estimates which are strongly correlated across the data set. The uncertainties of these estimates (the standard error of the mean divided by the mean) are approximately 15%. Estimates of the Brune stress drop are uncorrelated with estimates of the average static stress drop calculated from the seismic moments and the rupture geometries of the aftershocks, determined from the signal durations of the recorded S waves. The Brune stress drops are most strongly correlated with the arms stress drops, implying that the Brune stress drop provides an estimate of the average dynamic stress drop, as Brune originally proposed.
A technique for analyzing the thermal and uplift histories of eroding orogenic belts: A Scandinavian example
Authors: Royden L. , Hodges K. V.
Published: February 2008

Geothermometric and geobarometric data from pelitic schists in the upper plate of the Forsa thrust in the northern Norwegian Caledonides are interpreted as recording part of the uplift trajectory of this area during erosion and slow cooling. A method developed in this paper for reconstructing the thermal history of orogenic belts from such data is based on a general mathematical description of thermal conduction in a lithospheric slab with an arbitrary initial temperature distribution and uplifted at a constant rate. With certain restrictions the uplift trajectory (Tz path) of any rock parcel can be calculated from only a few temperature‐depth points along that uplift trajectory. By applying this method to geothermometry and geobarometry data from near the Forsa thrust we can (1) extrapolate Tz paths measured over a limited depth range upward to the surface, (2) recover geotherms present at various times during the uplift and erosion history of the mountain belt, (3) obtain Tz paths for rock parcels above and below those for which PT data are available, and (4) use PT data in combination with geochronological data to obtain accurate estimates of uplift rates. Our analysis suggests that shortly after nappe emplacement, the nappe stack was hot (near 800°C in the lower part of the nappe stack), while the underlying autochthon was cold (near thermal equilibrium prior to nappe emplacement).The maximum burial depth of rocks near the Forså thrust was probably between 35 and 60 km.
A viscoelastic coupling model for the cyclic deformation due to periodically repeated Earthquakes at subduction zones
Authors: Thatcher Wayne , Rundle John B.
Published: February 2008

Vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic (Maxwell) half space are obtained and compared with the observed deformation cycle (coseismic strain release, postseismic transients, interseismic strain accumulation) from Japan. The viscoelastic effects, including the influence of buoyant restoring forces, are obtained using the method developed by Rundle, and plate convergence and strain accumulation are incorporated following the procedure suggested by Savage. The resulting deformation cycle is compared with that of an analogous elastic half‐space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault. Cyclic deformation is similar but not identical for the two models, and observations from southwest Japan suggest the superiority of the viscoelastic coupling model. In particular, inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half‐space model and results in agreement with the observed interseismic movement pattern. Several aspects of the postseismic deformation, its landward migration, and its transition to the interseismic phase of the cycle are explained as well, but the short duration of near‐trench transients relative to those observed farther inland is not matched. The success of a buried slip model in explaining early postseismic near‐trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle/elastic in the short term and ductile for longer‐term deformation, and such a modification may reconcile remaining discordant observations. However, reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of Japan, and these regional differences remain unexplained.
Focal mechanisms and aftershock locations of the Songpan earthquakes of August 1976 in Sichuan, China
Authors: Jones Lucile M. , Han Weibin , Hauksson Egill , Jin Anshu , Zhang Yaoguo , Luo Zhuoli
Published: February 2008

The precursory swarm, three mainshocks (M = 7.2,6.7, 7.2), and aftershocks of the Songpan earthquakes have been reanalyzed using both local and teleseismic data. The three mainshocks of this sequence occurred on the Huya fault over a 7‐day period. Relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence. Each mainshock occurred on a separate strand, each one south of the strand activated in the previous mainshock, and the aftershock zones of each mainshock appear to abut rather than overlap. Fault plane solutions determined by matching teleseismic P waveforms at World‐Wide Standard Seismograph Network stations with synthetic seismograms are consistent with the observed aftershock zones. The first and third mainshocks (M0 = 1.3 ×1 019 and 8.4 × 1018 N m, respectively) showed almost identical senses of motion, a combination of reverse and left‐lateral strike‐slip motion, on parallel strands, striking N15°W, that were separated by a large rightstepping en echelon offset. The second mainshock (M0 = 4.0 × 1018 N m), occurred in this offset on a fault at a steep angle (∼125°) to the other two strands and showed almost pure reverse motion. Differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock. These changes in normal stresses may have given rise to the longer time between the first and second events (5 days) as compared with the time between the second and third events (30 hours). A precursory swarm that preceded the Songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone. The time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in Soviet Central Asia.
Single‐crystal elastic properties of the spinel phase of Mg2SiO4
Authors: Weidner Donald J. , Sawamoto Hiroshi , Sasaki Satoshi , Kumazawa Mineo
Published: February 2008

The single‐crystal elastic moduli of the spinel (gamma) phase of Mg2SiO4 have been experimentally determined from Brillouin spectroscopy at ambient conditions. They are C11 = 327, C44 = 126, and C12 = 112 (GPa) yielding a bulk modulus of 184. and a shear modulus of 119 GPa. This value of bulk modulus is lower than expected from previous measurements. The elastic properties of the spinel and modified spinel (beta) phases of Mg2SiO4 are quite similar with the greatest differences related to the c axis of the beta which is relatively soft under compression. The polycrystal 1ine acoustic velocities differ by only 1% for these two phases. Thus there should be no seismic discontinuity associated with the beta to gamma phase transition in the mantle. The similarity of elastic properties of these two phases suggests that effects of pressure, temperature, and iron content on the elastic properties should be similar for both phases. Estimates of the effect of iron content are made from the extant data on fayalite spinel in conjunction with the data presented here. The derivatives of elastic moduli with respect to iron content are very similar to those observed for the olivine phase. Based on comparisons with aluminate analogue compositions in the spinel structure, there is no justification to predict the pressure or temperature derivatives of the elastic moduli of these high pressure phases at this time.
Residual plutonium migration in soil of Nagasaki
Authors: Mahara Yasunori , Miyahara Shojiro
Published: February 2008

The residual 239+240Pu concentration in the Nagasaki soil was observed 36 years after the plutonium atomic bombing of Nagasaki in 1945. The highest level of 239+240Pu contamination was discovered in the soil at the edge of the Nishiyama reservoir, a level about 10–30 times higher than in other areas of Nagasaki. In the Nishiyama district, 97% of the total plutonium in the soil was retained to a depth of 0.3 m, but the remaining 3% was dispersed over a 0.3‐ to 2.25‐m depth at approximately 0.037 Bq/kg of dry soil. The mean 239+240Pu migration rate in the Nishiyama soil for 36 years was estimated at 1.25 mm yr−1, compared to the infiltration rate of rainwater of 2500 mm yr−1.
The mineralogy and petrology of compositionally zoned ash flow tuffs, and related silicic volcanic rocks, from the McDermitt Caldera Complex, Nevada‐Oregon
Authors: Conrad Walter K.
Published: February 2008

The McDermitt caldera complex, Nevada‐Oregon, is a composite collapse structure formed following eruption of three ash flow tuffs from a single compositionalIy zoned magma chamber. The major and early erupted portions of the tuffs are mildly peralkaline high‐silica comendite. Later erupted portions of two ash flow sheets are metaluminous low‐silica rhyolite. Associated with the caldera complex are precaldera dacite to rhyolite lava flows and postcaldera rhyolite domes and intrusives. Systematic variations in mineral compositions and whole‐rock chemistry throughout the entire silicic volcanic suite record the petrogenesis of highly silicic comendite through fractional crystallization of dacitic or rhyodacitic parental magma. Fractionation from daciterhyodacite to metaluminous rhyolite is characterized by initially decreasing light and middle rare earth elements; little change in Zr and Hf; and increasing U, Th, K, Rb, and Ba; controlled by removal of plagioclase, low and high‐Ca pyroxenes and significant (5%) amounts of apatite, magnetite, and ilmenite. Fractionation from rhyodacite‐low‐silica rhyolite to high‐silica comendite is characterized by increasing rare earth elements, U, Th, K, Rb, Zr, and Hf, with drastically decreasing Eu and Ba, controlled by removal of ternary‐alkali feldspar, quartz, ferroaugite‐ferrohedenbergite, fayalite and lesser ilmenite, magnetite, and apatite. Heavy rare earth element enrichment (HREE) from dacite to rhyolite is pronounced in ash flow tuffs and postcaldera volcanic rocks but not in precaldera lavas; this is possibly due to the variable presence of F as a HREE complexing agent during crystal fractionation in the caldera magma chamber. Fe‐Ti oxide compositions and phase equilibria indicate progressively decreasing magmatic temperature and oxygen fugacity: 865–940°C in dacite‐low‐silica rhyolite, with fo2 below the fayalite‐magnetite‐quartz buffer, and approximately 800°C in high‐silica comendite, with fo2 slightly above the wustite‐magnetite buffer. In the ash flow tuffs, observed patterns of vertical compositional zonation and phenocryst distributions reveal preeruptive processes important for the genesis of mildly peralkaline silicic magmas. Influx of heat is required to maintain crystallization of relatively mafic, high‐temperature mineral assemblages, notably with ternary feldspar, and prohibits further crystallization of derivative melts. Heat transfer occurred throughout a density‐stratified magma chamber via convection within compositionally discreet zones of low‐ and high‐silica rhyolite.
Propagation as a mechanism of reorientation of the Juan de Fuca Ridge
Authors: Wilson Douglas S. , Hey Richard N. , Nishimura Clyde
Published: February 2008

We present a revised model of the tectonic evolution of the Juan de Fuca ridge by propagating rifting. The new model has three different relative rotation poles, covering the time intervals 17.0–8.5 Ma, 8.5–5.0 Ma, and 5.0 Ma to the present. The rotation pole shifts at 8.5 and 5.0 Ma imply clockwise shifts in the direction of relative motion of 10° to 15°. At each of these shifts, the pattern of propagation reorganizes, and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges. The model, containing a total of seven propagation sequences, shows excellent agreement with the isochrons inferred from the magnetic anomaly data, except in areas complicated by the separate Explorer and Gorda plates. The agreement between model and data near the Explorer plate breaks down abruptly at an age of about 5 Ma, indicating that the probable cause of the rotation pole shift at that time was the separation of the Explorer plate from the Juan de Fuca plate.
Steady state, secondary convection beneath lithospheric plates with temperature‐ and pressure‐dependent viscosity
Authors: Fleitout Luce , Yuen David A.
Published: February 2008

We have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self‐consistently by the heat transported by convection. In this study of the interaction between the lithosphere and upper mantle, strongly temperature‐ and pressure‐dependent rheologies for both Newtonian and non‐Newtonian creep mechanisms are employed. For a strictly temperature‐dependent rheology an insignificant amount of heat, less than 12.5 mW/m2, can be transported convectively for an interior viscosity, 0(1021 Pas), compatible with postglacial rebound. On the other hand, for similar values of the interior viscosity, steady heat fluxes between 20 and 40 mW/m2 are produced by introducing pressure dependence into the rheology. For the temperature‐ and pressure‐dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated, once all of the rheological parameters are fixed. This finding may have important ramifications for parameterized convection. We employ both the single‐mode, mean field approximation and the complete two‐dimensional equations, using finite elements, to obtain solutions for the various types of rheologies. From evaluating the geophysically relevant observables, such as topography, free air gravity anomalies, surface heat flow, and stress fields in the lithosphere, we find that the lateral variations of these quantities predicted by a non‐Newtonian rheology are much smaller than those derived from a linear rheology. These results suggest that surface variations of geophysical observables are more compatible with a non‐Newtonian rheology in the upper mantle.
A two‐phase model of electrical conduction in polar ice sheets
Authors: Wolff E. W. , Paren J. G.
Published: February 2008

It has been suggested that the dc conductivity of polar ice sheets could be due to the presence of liquid layers at the grain boundaries. These layers would consist of a mixture of acids with water. We show that it is plausible that acids will be at three‐grain boundaries in polar ice. Using reliable data for H2SO4, HNO3, and HCl concentrations in ice at South Pole, we derive the correct magnitude and temperature dependence for its conductivity. The model explains the narrow range of ice conductivities found in polar regions.
Moho orientation beneath central California from regional earthquake travel times
Authors: Oppenheimer David H. , Eaton Jerry P.
Published: February 2008

Table Al is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐011; $2.50. Payment must accompany order.

Analysis of relative Pn travel times recorded by 238 stations of the U.S. Geological Survey central California seismic network (CALNET) from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the Coast Ranges and Sierra Nevada foothills. By dividing the CALNET into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space, the orientation of a planar Moho beneath each subarray is calculated by inversion of relative Pn arrival times. The results show that the Moho dips to the northeast between 3° and 6° beneath the Coast Ranges and 8°–17° beneath the Great Valley and is horizontal beneath the foothills. Pn residuals west of the San Andreas fault are relatively early by 0.35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness. Unexplained Pn travel time variance improves by approximately 60% after correction for the travel time due to dip and mean Pn residual at each station. The Pn data corrected for the travel time due to a dipping Moho indicate an isotropic upper mantle velocity of 7.97±0.05 km/s beneath the Coast Ranges, Great Valley, and foothills. Evidence of upper mantle velocity anisotropy in the azimuthal variation in Pn apparent velocity corrected for dip is only marginally significant at the 5% level. However, large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust, unmodeled higher‐order mantle structure, and gradients in upper mantle velocity. Arrival times of Cape Mendocino earthquakes recorded in the Coast Ranges south of San Francisco Bay indicate lateral variations in upper mantle velocity. The 7.4 km/s apparent velocity estimate from travel times of the Oroville earthquake recorded in the Coast Ranges out to distances greater than 400 km is anomalous in that Pn is normally observed at distances greater than 200 km. These arrivals may originate from updip propagation along a 6.9 km/s layer in the lower crust of the Coast Ranges subparallel to the Moho.
Velocity‐porosity‐clay content systematics of poorly consolidated sandstones
Authors: Kowallis B. J. , Jones L. E. A. , Wang H. F.
Published: February 2008

Compressional velocities have been measured as a function of confining pressure for 14 West Delta Block (Louisiana) sandstone cores. The dry velocity data for these poorly consolidated sandstones at 10 MPa confining pressure plot on a linear velocity‐density trend with well‐consolidated sandstones but not loose sands. The dry velocity data also show linear trends of decreasing velocity with increasing porosity along contours of constant clay content similar to those obtained at 40 MPa by Tosaya and Nur (1982) for saturated sandstones. The spacing between clay contours indicates that a large portion of the velocity variation with clay content is due to microporosity, which is not included in point counting, within the clays. The slope of the velocity‐porosity trends indicates that cementation increases with decreasing porosity.
Petrographic studies of refractory inclusions from the Murchison meteorite
Authors: MacPherson Glenn J. , Grossman Lawrence , Hashimoto Akihiko , Bar‐Matthews Miryam , Tanaka Tsuyoshi
Published: February 2008

Mineralogical and petrographic data are presented for ten refractory inclusions recovered by freeze‐thaw disaggregation and heavy liquid separation from the Murchison C2 chondrite. One hibonite‐, corundum‐rich and two hibonite‐spinel inclusions are completely unlike any previously described objects. The corundum‐bearing inclusion has a hibonite core that is partially replaced by corundum, which is itself partially replaced by hibonite. It formed either by condensation during varying physico‐chemical conditions or by partial incongruent melting of hibonite at ∼210O°K to form corundum and liquid, followed by distillation of calcium from the melt and reaction of the residual liquid with some of the corundum upon cooling. The melting temperature required in the latter model is far above astrophysical estimates of nebular temperatures, except perhaps at the very center of the nebula. The hibonite‐spinel inclusions are beautiful lacy aggregates of euhedral hibonite plates, some of which were pseudomorphically replaced by spinel. Textures provide compelling evidence of a vapor‐solid condensation origin and clearly show, that spinel formed by direct reaction of hibonite with the solar nebular gas, without the intervention of CaAl4O7 and prior to condensation of melilite, in conflict with equilibrium thermodynamic calculations. Four blue hibonitespinel spherules are described, three of which differ from all previously studied members of this class in being intensely altered to phyllosilicate and calcite. One shows evidence of two stages of alteration to phyllosilicates, possibly one in the solar nebula and the other in the parent body. Two of three pyroxene spinel‐, forsterite‐rich inclusions described herein differ from previously studied members of this group, one in containing a peculiar amoeba‐shaped spinel‐pyroxene core suggestive of a vapor‐solid condensation origin and the other in containing a spheroidal core suggestive of crystallization from a melt. The Murchison parent body sampled materials that formed under a wide range of nebular conditions and contains many objects composed of more refractory phase assemblages than are found in most Allende inclusions.
Quantification of waste morphology in Martian fretted terrain
Authors: Kochel R. Craig , Peake R. Thomas
Published: February 2008

Extensive geomorphic mapping in the northern Martian fretted terrain of Deuteronilus and Protonilus Mensae shows details of the wastedominated landform association in this region. Wasting appears to have occurred along the cratered terrain boundary (CTB) at the expense of old cratered terrain. North of the boundary remnants of old cratered terrain occur as island mesas surrounded by extensive debris aprons. Quantitative study using principal components analysis (PCA) reveals distinct spatial variations in the morphology of waste forms in the egion and supports the qualitative interpretations made during geomorphic mapping. These organized spatial patterns occur with respect to longitude and latitude, and therefore with reference to distance from the CTB, and provide clues to the degradational history of the region. Analyses of the individual landforms irrespective of their location to the CTB indicate strong structural control of their form. Mesas appear to evolve from equant forms to narrow ridges with large debris aprons after they become detached from the old cratered terrain. The success of PCA in explaining patterns in degradational landforms suggests that PCA may be an important technique in quantifying the geomorphic evolution of the Martian surface.
An improved thermal model for cometary nuclei
Authors: Weissman Paul R. , Kieffer Hugh H.
Published: February 2008

We have modified our earlier thermal model of cometary nuclei by incorporating a more accurate calculation of the opacity of the cometary dust coma. The new model uses a modified Sekanina‐Miller dust particle size distribution and a Delsemme‐Miller gas outflow dynamics model. The improved model has been applied to the case of the 1986 apparition of Halley's Comet, also incorporating a revised radius and albedo for the Halley nucleus. The estimated opacity of the Halley coma is reduced from that of the earlier model. The revised opacity is 0.12 at 1.0 AU and 0.43 at perihelion, 0.587 AU. This results in a decrease in the energy supplied to the nucleus from the coma by multiply scattered radiation and thermal emission, and brings the model into better agreement with the results of others. The coma feedback mechanism enhances nucleus gas production by a factor of 1.4 at 1.0 AU over that for a bare ice nucleus with no dust coma, and by a factor of 2.3 at perihelion. Predicted gas production at 0.9 AU postperihelion, the expected heliocentric encounter distance for the Giotto mission, is 4.4×1029 molecules per second. This agrees well with a recent estimate of 3.9×1029 molecules s−1 for the Halley gas production rate at 0.9 AU during the 1910 apparition.
39AR‐40Ar ages for the Apollo 15 green and yellow volcanic glasses
Authors: Spangler R. R. , Warasila R. , Delano J. W.
Published: February 2008

A technique using the laser microprobe to extract noble gases from individual small (100–750 μm in diameter), low potassium (≤0.09 wt % K2O), glassy samples was developed. This technique has an advantage in that an individual sample can be chemically characterized by an electron microprobe and then analyzed for its argon age, thus eliminating any ambiguities in the classification of the sample and the need for large quantities of samples. This technique was then used to study the argon isotopes and apparent 39Ar‐40Ar ages of three varieties of volcanic glass from Apollo 15: yellow volcanic, green volcanic group A, and green volcanic group D. The apparent solidification age for the glasses are 3.62±0.07, 3.41±0.12, and 3.35±0.18 aeons, respectively. The ages for group A and D green glasses agree well with previously reported ages for aliquots of ‘bulk’ green glass. A value of 40Ar/36Arsw = 0.5 ± 0.5 was used to correct the data since a 40Ar/36Arsw versus 39ArK/36Arsw plot failed to identify a well‐defined trapped component. No significant difference in the solidification age (>0.2 aeons) was found between the A and D groups of the volcanic green glasses. These ages for volcanic glasses are consistent with the prediction of Delano and Livi [1981] that Array II volcanic glasses are generally older (≥3.6 aeons) than Array I glasses. The average exposure ages for the Apollo 15 yellow volcanic and green volcanic (groups A and D) glasses from 15426 and 15427 were indistinguishable, with ages between 300 and 275 m.y.
Destruction of rocks by low velocity impact and its implications for accretion and fragmentation processes of planetesimals
Authors: Matsui Takafumi , Waza Toshihiko , Kani Kouki
Published: February 2008

Recent experimental results on impact destruction of rocks (basalt and tuff) are summarized. The mode of destruction and some empirical relations between the mass of the largest fragment Ml, the released kinetic energy divided by target mass Ei, and the work done during impact W are presented. The mass Ml can be related to both Ei and W:log (Ml/Mt) = a ‐ β log Ei and log (Ml/Mt) = b ‐ γ log W. An effect due to the difference in impact velocity on the Ml versus Ei relation can be observed, but there seems to be no effect due to impact velocity on the Ml versus W relation. Thus it may be suggested that W is a useful means of describing phenomena associated with the impact destruction of rocks. Target shape also affects the Ml versusEi relation but no large differences have been observed between various rocks. The size distribution of fragments can be adequately expressed in terms of an inverse power law relation, in which slope of the size distribution α (i.e., exponent) increases with increasing Ei:α = 0.90 + 0.36 log Ei for basalt and α = 1.94 + 0.24 log Ei for tuff. Based on the empirical α ‐ Ei relations, a fragmentation model of the parent bodies of Eos and Koronis families is presented. It is suggested that the parent bodies of Eos and Koronis families had been differentiated prior to fragmentation and were completely fragmented at Ei = ∼2 × 104 joule/kg and ∼2 × 103 joule/kg.
Temporal and spatial variability of seafloor spreading processes in the northern south Atlantic
Authors: Brozena John M.
Published: February 2008

The paper reports the results of an aeromagnetic study of the Mid‐Atlantic Ridge in the northern South Atlantic. The study consists of 38,000 km of low altitude magnetics tracks centered on the ridge covering nearly 700,000 km2 south of Ascension Island. Most tracks extend out to crust of anomaly 5B age. Large areal coverage coupled with high resolution allows comparison of crustal generation processes over space (along ridge strike) and time (across strike). Temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion. The total spreading rate increased sharply at 10 Ma and then decreased between 6 Ma and the present. The time variation of spreading rate is similar to that seen in other areas such as the North Atlantic and the southern and eastern Pacific. The plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones. In several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones. The periods of plate boundary modification correlate with the increase and decrease in spreading rate. Spatial heterogeneities are caused by the influence of the Ascension hotspot on the Mid‐Atlantic Ridge in the northern portion of the study area, and include two apparent seamount chains formed by the hotspot. Ascension Island and several of the seamounts appear to have been produced some distance away from the hotspot location (presently about 225 km southeast of Ascension Island) by channeled subaxial magma flow beneath the spreading center. The seamount chains, therefore, do not mark hotspot tracks. Large local variations in spreading rate and degree of asymmetry along strike are also found.
Tectonics of the Jemez Lineament in the Jemez Mountains and Rio Grande Rift
Authors: Aldrich M. J.
Published: February 2008

The Jemez lineament is a NE trending crustal flaw that controlled volcanism and tectonism in the Jemez Mountains and the Rio Grande rift zone. The fault system associated with the lineament in the rift zone includes, from west to east, the Jemez fault zone southwest of the Valles‐Toledo caldera complex, a series of NE trending faults on the resurgent dome in the Valles caldera, a structural discontinuity with a high fracture intensity in the NE Jemez Mountains, and the Embudo fault zone in the Española Basin. The active western boundary faulting of the Española Basin may have been restricted to the south side of the lineament since the mid‐Miocene. The faulting apparently began on the Sierrita fault on the east side of the Nacimiento Mountains in the late Oligocene and stepped eastward in the early Miocene to the Canada de Cochiti fault zone. At the end of the Miocene (about 5 Ma) the active boundary faulting again stepped eastward to the Pajarito fault zone on the east side of the Jemez Mountains. The north end of the Pajarito fault terminates against the Jemez lineament at a point where it changes from a structural discontinuity (zone of high fracture intensity) on the west to the Embudo fault zone on the east. Major transcurrent movement occurred on the Embudo fault zone during the Pliocene and has continued at a much slower rate since then. The relative sense of displacement changes from right slip on the western part of the fault zone to left slip on the east. The kinematics of this faulting probably reflect the combined effects of faster spreading in the Española Basin than the area north of the lineament (Abiquiu embayment and San Luis Basin), the right step in the rift that juxtaposes the San Luis Basin against the Picuris Mountains, and counterclockwise rotation of various crustal blocks within the rift zone. No strike‐slip displacements have occurred on the lineament in the central and eastern Jemez Mountains since at least the mid‐Miocene, although movements on the still active Jemez fault zone, in the western Jemez Mountains, may have a significant strike‐slip component. Basaltic volcanism was occurring in the Jemez Mountains at four discrete vent areas on the lineament between about 15 Ma and 10 Ma and possibly as late as 7 Ma, indicating that it was being extended during that time.
Chemical and isotopic characteristics of fluids within the Baca Geothermal Reservoir, Valles Caldera, New Mexico
Authors: White Art F.
Published: February 2008

Geochemical and isotopic fluid data from 10 wells completed in the Baca geothermal reservoir indicate the presence of both a normal enthalpy, lower‐chloride fluid and an excess enthalpy, higher‐chloride fluid. This chemical difference, along with enrichment of deuterium and oxygen 18 in the excess enthalpy fluid, can be explained by adiabatic cooling or boiling at 170°C during upward convection along the central fault system in the reservoir followed by conductive reheating during downward movement. Chemical modeling using the EQ3NR computer code indicates chemical stability with the mineral assemblage quartz, albite, K‐mica, epidote, chlorite, calcite, anhydrite, and pyrite, which is in agreement with observed minerals in well cuttings. This correlation, along with chemical geothermometers, implies a constant temperature range of 260°–320°C during evolution of the reservoir.