bluuebunny/crossref_metadata_2025 huggingface dataset:
Extension in the Rio Grande Rift
Authors: Cordell Lindrith
Published: February 2008

A positive gravity anomaly along the axis of the Rio Grande rift reflects a volume of anomalous mass added at the base of the crust and intruded into the crust. Part of this volume can be associated with vertical uplift of the crust. The remainder of this anomalous volume, plus the volume of surficial graben fill, can be associated with horizontal crustal extension. The volume of crustal uplift in the Rio Grande rift is unknown, but this term can be eliminated by means of an independent equation provided by assumption of generalized isostatic equilibrium. The volume and mass equations combined provide a solution for extension of the crust in terms of the following parameters: total anomalous mass deficiency in the mantle lithosphere, total anomalous mass excess in the crust and its density contrast, total anomalous mass deficiency of surficial graben fill and its density contrast, and the volume of material eroded from the uplift. Using standard density estimates and masses determined by equivalent‐source modeling of gravity profiles, I obtained 1‐km extension at 37°N (Colorado‐New Mexico border), 13‐km extension at 35°N (Albuquerque, New Mexico), and 24‐km extension at 33°N in southern New Mexico. These estimates are of necessity very poorly constrained and may be as much as ±60% in error. Taking results at face value, the best fitting Euler pole occurs at about 41°N, in north central Colorado. The calculated opening angle is 1.32°. For an approximately 30‐m.y. duration of the extensional system (late Oligocene to the present), calculated average angular velocity is 7.8×10−4 rad/m.y.; calculated average spreading half‐rate at 33°N (for example) is a stately 0.04 cm/yr. The ratio of extension to uplift increases southward.
Permafrost, heat flow, and the geothermal regime at Prudhoe Bay, Alaska
Authors: Lachenbruch Arthur H. , Sass J. H. , Marshall B. V. , Moses T. H.
Published: February 2008

Temperature measurements through permafrost in the oil field at Prudhoe Bay, Alaska, combined with laboratory measurements of the thermal conductivity of drill cuttings permit an evaluation of in situ thermal properties and an understanding of the general factors that control the geothermal regime. A sharp contrast in temperature gradient at ∼600 m represents a contrast in thermal conductivity caused by the downward change from interstitial ice to interstitial water at the base of permafrost under near steady state conditions. Interpretation of the gradient contrast in terms of a simple model for the conductivity of an aggregate yields the mean ice content (∼39%), and thermal conductivities for the frozen and thawed sections (8.1 and 4.7 mcal/cm s °C, respectively). These results yield a heat flow of ∼1.3 HFU, which is similar to other values on the Alaskan Arctic Coast; the anomalously deep permafrost is a result of the anomalously high conductivity of the siliceous ice‐rich sediments. Curvature in the upper 160 m of the temperature profiles represents a warming of ∼1.8°C of the mean surface temperature and a net accumulation of 5–6 kcal/cm2 by the solid earth surface during the last 100 years or so. Rising sea level and thawing of ice‐rich sea cliffs probably caused the shoreline to retreat tens of kilometers in the last 20,000 years, inundating a portion of the continental shelf that is presently the target of intensive oil exploration. A simple conduction model suggests that this recently inundated region is underlain by near‐melting ice‐rich permafrost to depths of 300–500 m; its presence is important to seismic interpretations in oil exploration and to engineering considerations in oil production. With confirmation of the permafrost configuration by offshore drilling, heat conduction models can yield reliable new information on the chronology of arctic shorelines.
The role of lithospheric stress in the support of the Tharsis Rise
Authors: Willemann Raymond J. , Turcotte Donald L.
Published: February 2008

The Tharsis rise on Mars is a broad topographic high covered by volcanic plains and capped by large volcanoes. The geoid high associated with Tharsis shows that shallow isostatic compensation is not complete. We hypothesize that the Tharsis rise can be approximated as an axisymmetrical igneous construct. Linear theory for the deflection of planetary lithospheres is used to show that the lithospheric stresses required partially to support the construct are reasonable and consistent with the observed radial grabens around Tharsis. The computed thickness of the elastic lithosphere is between 110 and 260 km depending on the assumed values for crustal thickness and crustal density. The computed thickness of the Tharsis load is between 40 and 70 km. In this model the height of the geoid is not specified a priori. Therefore, the agreement between the observed and computed geoid is evidence for the validity of our model. The contribution of Tharsis to the gravitational flattening of Mars is computed as ΔJ2 = −8.8 × 10−5. After a correction is applied for the flattening, the geoid low around Tharsis is more nearly axisymmetrical. The tectonics of the Tharsis region are briefly reviewed, and it is argued that all observations are consistent with the loading model.
Martian volcanic materials: Preliminary thickness estimates in the Eastern Tharsis Region
Authors: De Hon René A.
Published: February 2008

Mars displays a variety of volcanic materials from high standing constructs to smooth plains. These materials are evaluated in terms of their susceptibility to thickness determination, and a representative area is analyzed. The Martian ridged plains are much like lunar maria in both surface expression and apparent mode of emplacement. Flooded craters within the ridged plains materials provide an estimate of the thickness distribution. In the Tharsis region, ridged plains materials occur as part of the layered sequence on the east flank of the Tharsis dome. Partially buried craters indicate that the ridged plains materials range from zero meters thick at their eastern limit to over 1.5 km thick westward on the flank of the dome where they are overlain by younger materials. These materials are responsible for only a small portion of the total relief of the Tharsis dome.
Shock‐induced color changes in nontronite: Implications for the Martian fines
Authors: Weldon Ray J. , Thomas Warren M. , Boslough Mark B. , Ahrens Thomas J.
Published: February 2008

Riverside nontronite, a candidate for the major mineral in the Martian fines, becomes both redder and darker upon shock loading between 180 and 300 kbar. The change from olive‐yellow (2.5 Y 6/6) to strong brown (7.5 YR 4/6) in the 300‐kbar sample brackets the range of color observed at the Viking lander sites. Optical microscopy, X‐ray diffraction, optical, infrared, and 57Fe Mössbauer spectroscopy were applied to understand the physical basis of the color change. The Riverside nontronite experienced partial dehydroxylation, probably due to shock‐induced heating, that changed the coordination of the Fe3+ in the octahedral layer of the clay to a mixture of 4‐ and 6‐fold or a distorted 5‐fold coordination. These changes in the clay cause the O2−‐Fe3+ charge transfer absorption edge to shift from the near ultraviolet into the visible, producing a redder and darker phase. The absorption spectra of both impacted and nonimpacted Riverside nontronite contains the basic features of the reflectance spectra of the bright regions of Mars: a steep drop in absorption from the near UV into the visible and a featureless near IR region. Calculations indicate that significant impact induced color changes (and dehydration) can occur on Mars, though it seems likely that the mechanism would be more effective, volumetrically, at producing variations in color rather than affecting the absolute color.
The solar wind interaction with Mars revisited
Authors: Slavin James A. , Holzer Robert E.
Published: February 2008

Owing to a paucity of observational data, no clear consensus has been reached concerning the general nature of the solar wind interaction with Mars. In particular, the previous analyses are still at odds regarding the existence of a small intrinsic field magnetosphere at Mars as opposed to a Venus‐type ionospheric interaction (e.g., Russell, 1978a,b; Dolginov, 1978b,c). This study contributes to the resolution of this question in three ways. First, an improved determination of effective obstacle altitude and shape is obtained from the Mars 2,3, and 5 bow shock encounters through the use of a recently published catalog of gasdynamic flow solutions (Spreiter and Stahara, 1980a,b). Second, building upon the Pioneer Venus findings at a field‐free planet (Brace et al., 1980; Elphic et al., 1980), it is shown that the Martian ionosphere cannot support a Venus‐type ionopause at the obstacle altitudes inferred through our modeling of the bow wave observations even when maximal induced ionospheric magnetic fields and solar maximum EUV levels are assumed. These results allow an effective Mars magnetic dipole moment of 1.4 (±0.6) × 1022 G‐cm3 to be determined that stands off the solar wind over the dayside hemisphere at altitudes ranging from∼500 km at the subsolar point to ∼1000 km near the terminator with no direct aid from the ionosphere under average solar wind/magnetospheric conditions. Third, a search of published Mars and Mariner radio occultation measurements produced no evidence for the existence of an ionopause at Mars in agreement with the Viking study of Lindal et al. (1979). Rather, the electron density altitude profiles appear qualitatively consistent with the Martian ionosphere terminating in a chemopause associated with the effects of magnetospheric convection as first proposed by Bauer and Hartle (1973). After a review of the various arguments in the literature, as supplemented by the results of this study, we conclude that Mars most probably possesses a small intrinsic field magnetosphere.
Tracer diffusion in granitic melts: Experimental results for NA, K, RB, CS, CA, SR, BA, CE, EU to 1300°C and a model of calculation
Authors: Jambon Albert
Published: February 2008

Tracer diffusivity of Na, K, Rb, Cs, Ca, Sr, Ba, Eu, and Ce has been measured in natural and dehydrated obsidians in the range 300°–1300°C, 0–4 kbar, using radiotracers and an etching technique. The results complement and extend previous results on similar material. Diffusivity ranging from 10−16 to 10−7 cm2/s have been observed. An Arrhenius law is observed for all species, with activation energies E ranging from 20 to 120 kcal/mol. A strong correlation of E with ionic radius and squared formal charge (Z2) offers guidelines for model calculation of the activation energy. A mechanistic model of transport through an elastic medium is adapted to the present data. It permits a fair prediction of the activation energy as a function of r and Z according to E = 128(r−1.34)2 + 33 Z2/(r+1.34) + 8 kcal/mol, with a misfit comparable to the experimental uncertainty. The correlation between E and the preexponential factor Do (compensation law) is discussed within the framework of the proposed model. The major conclusion is that local configuration (size, charge, coordination, etc.) is the major control for cationic diffusivity and not the properties of the matrix, such as chemical composition, viscosity, etc.
SNC meteorites: Evidence against an asteroidal origin
Authors: Ashwal Lewis D. , Warner Jeffrey L. , Wood Charles A.
Published: February 2008

The 1.3 b.y. crystallization ages and other features of the nine SNC meteorites (shergottites, nakhlites,and chassignites) distinguish them from all other meteorites which apparently formed more than 3 b.y. earlier. Proposed models for the origin of the young SNC meteorites include internal melting, either within a large planet such as Mars or within a large asteroid, and impact melting on one or more asteroids. Although an asteroidal source is dynamically more reasonable, we discuss petrologic, geochemical, and isotopic observations that strongly argue against an asteroidal origin for SNC meteorites, either as impact melts or as endogenically derived melts. Cumulate textures, as displayed by the SNC meteorites, have never been observed in terrestrial or lunar impact melts. Even the largest impact melt sheets cooled relatively quickly by thorough mixing with cool clastic debris, and the resulting rocks show obvious lithic and/or mineral clasts derived from the target materials. No clasts or remnants of lasts are present in SNC meteorites. The chemical and isotopic homogeneity of impact melt rocks is not displayed by SNC meteorites. Differences in initial 87Sr/86Sr among some of the shergottites, which are otherwise chemically similar, preclude an origin in the same impact melt sheet. These relationships indicate that the SNC meteorites did not form within an impact melt sheet, either on an asteroid or anywhere else. Instead, their parental magmas probably formed by internal melting within their parent body. Although thermal models of large asteroids ( > 800 km diameter) can be devised to allow endogenic melting as young as 1.5 b.y. ago, isotopic and geochemical data for SNC meteorites preclude them from having formed in this way. Specifically, the isotopic signature of the ancient (∼4.6 b.y.) large‐scale heating event which established the shergottite sources is inconsistent with such asteroidal thermal models which require cold accretion of materials with initially low thermal conductivities. The thermal models further require that the young melts were restricted to the asteroid's core; total fragmentation of such asteroids would be required to liberate these rocks into Earth‐crossing orbits. Destruction of an 800 km asteroid would have produced an immense swarm of asteroids with similar orbits. No Hirayama family with sufficient mass has been identified in the asteroidal belt. If retention of residual garnet in the shergottite sources is a requirement, as indicated by geochemical modeling, this would directly imply a parent body larger than the largest asteroid (Ceres, diameter = 1020 km). A planet‐sized parent body such as Mars still remains the most likely candidate for the SNC meteorites.
Impact induced dehydration of serpentine and the evolution of planetary atmospheres
Authors: Lange M. A. , Ahrens Thomas J.
Published: February 2008

Shock recovery experiments in the 25 to 45 GPa range on antigorite serpentine determine the amount of shock‐induced loss of structural water as a function of shock pressure. Infrared absorption spectra of shock recovered samples demonstrate systematic changes in the amount of structural water and molecular, surface adsorbed water. These yield qualitative estimates of shock‐induced water loss and demonstrate that a portion of the shock released structural water is readsorbed on interfacial grain surfaces. Determination of the post‐shock water content of the shocked samples relates shock‐induced water loss and shock pressure. Based on the present results and theoretical predictions, we conclude that shock pressures of from 20 to ∼6O GPa induce incipient to complete water loss, respectively. This result agrees closely with theoretical estimates for total dehydration. The dehydration interval and the activation energies for dehydration in shocked samples decrease systematically with increasing shock pressure as experienced by the sample. We believe the present experiments are applicable to describing dehydration processes of serpentine‐like minerals in the accretional environment of the terrestrial planets. We conclude that complete loss of structural water in serpentine could have occurred from accretional impacts of ∼3 km/sec when earth and Venus have grown to about 50% of their final size. Accreting planetesimals, impacting Mars, never reached velocities sufficient for complete dehydration of serpentine. Our results support a model in which an impact generated atmosphere/hydrosphere forms while the earth is accreting.
Thermal annealing of experimentally shocked feldspar crystals
Authors: Ostertag R. , Stöffler D.
Published: February 2008

Single crystals of sanidine, orthoclase, microcline, oligoclase, and labradorite, as well as polycrystalline bytownite, were shocked experimentally to peak pressures as high as 45 GPa and subsequently annealed under dry conditions for various times at 800°C, 900°C, and 1000°C. These experiments reveal that alkali feldspars do not change their Al, Si‐distribution due to shock alone, but when subjected to shock pressures > 22 GPa and then annealed, microcline and orthoclase rapidly transform to sanidine, displaying a highly disordered Al, Si‐distribution. High diffusivity of K, Na, and Ca was observed in a labradorite sample shocked to 26 GPa. Subsequent annealing may thus disturb chemical, structural, and isotopic characteristics of shocked minerals and rocks more easily than of unshocked specimens. This is of considerable importance for the interpretation of isotopic age data of impact breccias. Recrystallization textures of annealed diaplectic plagioclase glasses are very distinctive and may be used as a relative measure of the shock pressure experienced by the samples.
Adsorption and excess fission XE: Adsorption of XE on vacuum crushed minerals
Authors: Bernatowicz T. J. , Kramer F. E. , Podosek F. A. , Honda M.
Published: February 2008

We report the results of Xe adsorption studies at low partial pressures of Xe (∼2×10−13 atm) for a terrestrial labradorite and lunar anorthosite 15415 which were crushed under high vacuum. The creation of fresh surfaces resulted in a distribution of surface adsorption potentials, with the most energetic, but least abundant, fractions of the surface dominating adsorption behavior at lower temperatures. Atmospheric exposure tends to eliminate the most energetic surface fractions. For sample 15415, surfaces with an adsorption potential as great as 13.7 kcal/mole were created, but these constituted only about 10−6 of the surface area. Observation of the adsorption behavior of this sample over a period of days revealed the gradual disappearance of the most energetic fractions of the surface, probably because of site competition effects caused by sorption of active gases in our high vacuum manifold. Our results consist of values of the Henry constants of Xe adsorption at various temperatures. We have used extrapolations based upon these data to evaluate the possible role of adsorption in the excess fission Xe effect observed in lunar highland gas‐rich breccias, for a model in which excess fission Xe is concentrated by adsorption while diffusing through the regolith. Our experimental results, and estimations of the sorptive properties of the in situ regolith based on the time scale of lunar surface Rn emanations, indicate that adsorption falls about five orders of magnitude short of accounting for the effect. We also infer that the adsorption characteristics of our vacuum‐crushed lunar sample are reasonable analogs to those of in situ lunar soil, so that the failure of adsorption to account for excess fission Xe cannot be attributed to significant differences in sorptive behavior of these materials.
Numerical models for the hydrothermal field at the Galapagos Spreading Center
Authors: Fehn Udo , Green K. E. , Von Herzen R. P. , Cathles L. M.
Published: February 2008

The heat flow distribution at the Galapagos Spreading Center is compared to results of two‐dimensional numerical models for the hydrothermal convection through oceanic crust. The model calculations are based on the equations for fluid flow through porous media adapted for the situation at spreading oceanic ridges. The temperature‐ and pressure‐dependent thermodynamic characteristics of water were used in the fluid flow equations. Models with average permeabilities of approximately 5×10−15 m2 and penetration depths between 2 and 5 km produce heat flow distributions compatible with the observations at the Galapagos Spreading Center. Because of the convective heat loss, temperatures within the hydrothermal layer are significantly lower than for conductively cooling crust. Two different types of convection cells develop. The one or two cells closest to the ridge axis are fixed in location with respect to the ridge axis. Convection there is characterized by high temperatures (>300°C), rapid flow rates, and low water to rock ratios (∼1). These cells remove most of the heat associated with the intrusion process at the ridge axis. Cells farther away from the ridge axis move with the moving plate and serve to prevent the oceanic crust from reheating. Temperatures there typically are moderate to low (<200°C), and flow velocities are lower than those in the axial cell, but water to rock ratios can be very high in these cells.
A detailed study of the Cobb Offset of the Juan de Fuca Ridge: Evolution of a propagating rift
Authors: Johnson H. Paul , Karsten Jill L. , Delaney John R. , Davis Earl E. , Currie Ralph G. , Chase Richard L.
Published: February 2008

The Cobb Offset on the northern portion of the Juan de Fuca Ridge has been identified as the tip of a northward propagating rift [Hey and Wilson, 1982]. Map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the Offset and permit detailed modeling of the recent evolution within this transform zone. The axis of recent spreading on the southern ridge segment bends from the normal ridge trend (N20°E) to a N‐S trend, north of 47°15′N. The spreading axis on the northern ridge segment generally defines a N20°E trend, except at the southern terminus, where the spreading center is offset slightly to the east. The two spreading centers overlap by about 33 km in the Offset vicinity, and there is evidence of recent volcanism on both segments. Present ridge axis morphology exhibits a transitional sequence from a symmetrical, axial high along the more ‘normal’ portions of each ridge segment to a grabenlike depression as the tip is approached. The magnetic anomaly patterns observed in the Cobb Offset vicinity are not consistent with the patterns predicted by models of continuous, northward propagation. The magnetic anomaly patterns of the Brunhes Epoch require an event of rapid northward propagation about 0.7 m.y. B.P., followed by a more gradual southward propagation in the middle Brunhes Epoch; most recently, the spreading center on the southern ridge has extended northward to its present configuration. Prior to the Brunhes Epoch, modeling of the magnetic anomaly patterns does not indicate a unique solution; however, net propagation has been northward. We present alternative models for the period beginning 1.7 m.y. B.P. In the first model, the Cobb Offset has evolved by a series of northward and southward events of propagation, with net advance to the north. In the second model, stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge. Both models indicate that propagation has been discontinuous and relatively rapid (compared to spreading rate) and that realignment of ridge segments near the propagator tip has been an important component.
The East Pacific Rise and the thermal model
Authors: Lewis Brian T. R.
Published: February 2008

Although the thermal mode of lithosphere cooling can be used to predict the East Pacific Rise topography reasonably well, it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor. The misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography, generating a theoretical gravity anomaly smaller than that observed. It is shown, using data at 12°N as an example, that improved agreement with the thermal model is obtained if, to first order, the crust is treated as an isothermal, constant thickness, constant density layer with variable depth. This result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation. It implies that convective cooling of the whole crust is very rapid, even at the axis of fast spreading ridges. The oceanic Moho may represent the depth to which cracking and convective cooling extends.
Electromagnetic geophysics and global tectonics
Authors: Gough D. Ian
Published: February 2008

Lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics. The methods of investigation include classical four‐electrode resistivity depth probing, which is of special value in continental shields; magnetotellurics, long used on land and recently in the Pacific to show that in electrical terms the lithosphere is thicker, near an oceanic ridge, than had been expected; and magnetometer arrays, which are useful primarily in mapping highly conductive structures. Examples of the use of all three methods will be reviewed, with emphasis on the last. In conjunction with other geophysical, geological, and geochemical data, magnetometer arrays have been useful in mapping partial melt under the western United States, in discovering an extension of the southern end of the African rift system to the Atlantic coast, in detecting a major fracture zone in central North America which may be a Proterozoic interplate collision feature, and in discovering a conductive belt under southernmost Africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under Proterozoic Gondwanaland. Inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two‐ and three‐dimensional conductive structures. This remains valuable, however, particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures. Recent array studies now in progress in western Canada have defined two new conductivity anomalies. One of these may represent a local partial melt concentration under the Rocky Mountains Trench, the other a Precambrian rift valley in the lower crust previously suggested by others on the basis of seismological, gravity, and static magnetic data. Both are being investigated by more concentrated arrays.
The inverse problem of a spherically symmetric fluid Earth
Authors: Coen Shimon
Published: February 2008

It is shown that the density and bulk modulus profiles of a spherically symmetric fluid earth model are uniquely determined from the surface particle acceleration and pressure at two arbitrary frequencies. The excitation is a harmonically oscillating point source which is located on the surface of the earth.
The use of Magsat data to determine secular variation
Authors: Cain Joseph C. , Frayser Jeffory , Muth Lorant , Schmitz Dave
Published: February 2008

The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B83‐004; $2.50. Payment must accompany order.

A combined spatial and secular variation model of the geomagnetic field, labeled M061581, is derived from a selection of Magsat data. Secular variation (SV) data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model. The SV patterns from this new model are compared with those from the 1980 IGRF and with those for 1970 computed by the DGRF and with the 1960 patterns computed using the GSFC(12/66) model. Most of the features of the M061581 are identical in location and level with those of the 1980 IGRF. Together they confirm that the reversals in sign of field change seen over Asia and North America between 1965 and 1975 are reverting to the pre‐1965 states. The M061581 model gives −32 nT/yr for the dipole decay rate, larger than the 70% increase already reported since 1965. This abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger (−100 nT/yr) drop in field over the polar regions not indicated by the 1980 IGRF. This north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric (counterclockwise) afternoon Sqp cell increasing in intensity from the beginning (November 1979) to the end (June 1980) of the Magsat data collection period, coupled with an enhancement of its effect as the orbit lowered from the 350– 550 km initial altitudes to near 200 km just prior to burnup. This experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that ‘annual snapshots’ of the field by a satellite would allow easy and accurate models of secular change without the use of any surface data.
Earthquake locations and three‐dimensional crustal structure in the Coyote Lake Area, central California
Authors: Thurber Clifford H.
Published: February 2008

Previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure. Approximate ray tracing and parameter separation are important elements of the improved method. Application of the method to P wave arrival time data recorded by stations of the U.S. Geological Survey Central California Network yields a three‐dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the Coyote Lake earthquake of August 1979. Very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed. Improved estimates of the locations of earthquakes in the study area are also determined. The relocation of explosions indicates epicentral accuracies of the order of a kilometer or better. Based on the revised hypocentral locations, it is concluded that the San Andreas fault is vertical in this area, with no actual offset between the epicenters and the fault trace. In contrast, the Calaveras has two (or more) active fault surfaces, one nearly vertical and another dipping 75° to the northeast.
Opening of a fault and resulting slip due to injection of fluid for the extraction of geothermal heat: 2. Further details
Authors: Hayashi K. , Abé H.
Published: February 2008

Further details of the behavior of a fault with a fluid‐filled region are presented; an oblique fault is modeled by an interface across which two elastic bodies are in frictional contact and the tectonic stresses varying linearly with respect to depth are taken into account. It is shown that a fault can keep fluid if there exists a spatially fluctuating tectonic stress field superimposed on the linear depth‐dependent stress field, although the fluctuation amplitude must be larger than a lower limit when the fault is oblique. It is also revealed that except for the case of a horizontal fault, there is an upper limit for the size of the fluid‐filled region even if the fluctuating stress field is spatially periodic. As the fluid‐filled region is created, slip spreads simultaneously along the fault. This resulting slip is also analyzed.
Investigation of the mechanism of the olivine‐spinel transition in fayalite by synchrotron radiation
Authors: Furnish M. D. , Bassett W. A.
Published: February 2008

Evidence for a martensitic type mechanism for the olivine‐spinel (α − γ) transition in Fe2SiO4 has resulted from an investigation by rapid acquisition of diffraction data collected on fayalite subjected to high temperature and pressure in a diamond anvil cell using an intense X ray beam from the Cornell High Energy Synchrotron Source (CHESS). An energy‐dispersive technique employing a white X ray beam and a solid‐state detector made it possible to collect a complete diffraction pattern every 2 min as pressure was increased across the phase transition at 520°C. The series of patterns clearly showed that the γ phase peaks which appeared first were those also compatible with a structure whose unit cell dimensions are half those of the spinel unit cell. After the time of three to four spectrum acquisitions, all of the peaks of the spinel structure were present. This same effect was observed in two other series. An additional series was run at 400°C. As soon as the γ phase peaks appeared, the run was halted, and the sample was quenched and examined by the Debye‐Scherrer method. The only γ phase peaks observed were those also allowed for a structure with the smaller unit cell. These results are consistent with a restacking of the anions from the approximately hexagonal close‐packed (hcp) arrangement in the olivine structure to the nearly cubic close‐packed (ccp) arrangement in the spinel structure, followed later by a reordering of the cations.
A reevaluation of the isostatic rebound of Lake Bonneville
Authors: Nakiboglu S. Mete , Lambeck Kurt
Published: February 2008

Recent data pertaining to the uplift of the Provo and Bonneville shorelines of the late Pleistocene Lake Bonneville are evaluated in a reanalysis using a rigorous model of deformation of a plane earth comprising an elastic layer overlying a viscoelastic channel. The new observations constrain the wavelength and amplitude of the uplift of two shorelines formed at two different epochs, and they partly resolve the ambiguities in the plausible mechanical parameters found in previous solutions. The results indicate that the effective thickness of the elastic lithosphere is 28 to 30 km, equal to the seismically determined crustal thickness in the region. The estimate of the upper mantle viscosity in the region varies from (1.5–3.4) ×lO19 Pa s to (2.1–5.8)×l018 Pa s, depending on whether the upper mantle is assumed to be of uniform viscosity or the flow is confined to a narrow channel in the asthenosphere. The predicted rate of present uplift at the center of the desiccated lake is 0.06 mm/yr and 0.03 mm/yr for the shallow channel and uniform mantle models, respectively. These values are beyond the first‐order leveling accuracies, and the question of the existence of a narrow channel in the region cannot be resolved with the available observations.
Geochemical studies of abyssal lavas recovered by DSRV Alvin from Eastern Galapagos Rift, Inca Transform, and Ecuador Rift: 1. Major element variations in natural glasses and spacial distribution of lavas
Authors: Fornari Daniel J. , Perfit Michael R. , Malahoff Alexander , Embley Robert
Published: February 2008

Fifty three major element chemical analyses of volcanic glasses, collected by Alvin, from the eastern Gapalagos rift‐Inca transform‐Ecuador rift plate boundary are used to assess tectonic controls on petrogenesis and subrift magma chamber evolution. These lavas span a compositional range that includes mid‐ocean ridge basalts (MORB), FeTi basalts, basaltic andesites, and andesites. This entire chemical spectrum of rocks was recovered on each of several dives within the eastern Galapagos rift along short ( ∼2 km) traverses of the seafloor at the axis of accretion. This juxtapositon of chemically diverse lavas supports the idea that very small, shallow‐level isolated magma bodies are the eruptive sources for fractionated accretionary lavas in this rift segment. A distinct difference in chemical diversity occurs between two of the dives (1002 and 998) in the eastern rift. Lavas recovered east of the break exhibit high degrees of fractionation, while rocks from west of this divide have only MORB compo‐sitions. This break occurs approximately 60 km west of the Galapagos rift‐Inca transform intersection, and it is believed to mark an important east‐to‐west transition in subrift accretionary dynamics. East of this break, accretion is perturbed by transform (“cold edge”) plus attempted rift propagation effects, while west of it a more stable accretionary environment (large, buffered magma chambers which erupt only MORB spectrum lavas) prevails. Only extremely Fe‐ and Ti‐enriched lavas were recovered from the southern intersection of the Inca transform with the Galapagos rift. The abundance of FeTi basalts at this locale is considered to be an effect of attempted rift propagation through thick, cold Nazca plate lithosphere. Rocks from the Ecuador rift northern Inca transform intersection have similar MORB chemistries and are among the least fractionated basalts recovered from the Cocos‐Nazca plate boundary.
Shock experiments on feldspar crystals
Authors: Ostertag Rolf
Published: February 2008

Single crystals of sanidine, orthoclase, microcline, oligoclase, and labradorite as well as polycrystalline bytownite were shocked from 10.5 to 45 GPa and recovered. The direction of shock wave propagation in single crystals was always parallel to the crystallographic a‐axis. Optical observations reveal a continuous sequence of shock effects: fracturing starts to develop at <10.5 GPa, planar elements develop at 10.5–14 GPa, mosaicism at 18–26 GPa, transformation of the crystal into diaplectic glass begins at 26–34 GPa, and formation of melt glass is observed at ∼42 GPa. Microcline, however, remains weakly birefringent even at 45 GPa. Refractive indices of diaplectic glass drop sharply at shock pressures between 30 and 40 GPa. Xray investigations reveal only a minor expansion of the unit cell of sanidine, whereas the other feldspars display no systematic change of lattice constants. The structural state (Al,Si‐distribution) of feldspars is not affected by shock. EPR analyses reveal the disordering of the crystal structure and the formation of a short‐range order phase in feldpars at pressures as low as 10.5–14 GPa. The beginning of the formation of diaplectic glass in the range from 18–22 GPa is evident from IR‐spectra of shocked samples. It is assumed that diaplectic glass is an arrangement of grossly unchanged crystalline material and a glassy phase that is structurally indistinguishable from fusion‐formed glass. The influence of the chemical composition and initial structural state of feldspars on the development of shock effects is believed to be less than the influence of exsolution lamellae and alteration products. Based on the results of these investigations, pressure ranges are given for the lower and upper boundary of the mixed‐phase regime of the pV Hugoniot (p = shock pressure; V = specific volume) for all samples. Two independent methods for the determination of shock pressures from residual shock effects are proposed based on optical and IR‐spectroscopic analyses.
A model for the formation of the Earth's core
Authors: McCammon C. A. , Ringwood A. E. , Jackson I.
Published: February 2008

The recent discovery of a phase transformation in Fe0.94O by Jeanloz and Ahrens has allowed a more detailed development of a model for core formation involving oxygen as the principal light alloying element in the core. It is predicted, based on calculations, that an increasing pressure in the system FeO‐MgO will result in a gradual exsolution of an almost pure high‐pressure phase FeO(hpp), leaving an iron‐depleted (Fe, Mg)O rocksalt (B1) phase. We also predict that FeO(hpp) will form a low‐melting point alloy with Fe at high temperature and high pressure. On the basis of our interpretations, we have constructed a model for core segregation. Assuming the earth to have accreted from the primordial solar nebula as a relatively homogeneous mixture of metallic iron and silicate‐oxide phases, core segregation involving oxygen would commence at a depth where pressure is sufficiently high to cause exsolution of FeO(hpp) from the rocksalt phase, and temperature is sufficiently high to allow formation of an Fe‐FeO(hpp) melt. A gravitational instability arises, leading to vertical differentiation of the earth as molten blobs of the metal sink downwards to form the core and the residual depleted silicate material coalesces to form large bodies which rise diapirically upwards to form the mantle.
The effect of viscosity on impact cratering and possible application to the icy satellites of Saturn and Jupiter
Authors: Fink Jonathan , Gault Donald , Greeley Ronald
Published: February 2008

Impact experiments in Newtonian fluids with a range of viscosities of 10−3 to 60 Pa s demonstrate that transient crater volume and shape (depth‐to‐diameter ratio) depend on target viscosity as well as on gravity. Volume is reduced, and depth‐to‐diameter ratio is increased for cratering events in which viscosity plays a dominant role. In addition to being affected by target kinematic viscosity, viscous scaling is most strongly influenced by projectile diameter, less strongly by projectile velocity, and least strongly by gravity. In a planetary context, viscous effects can occur for craters formed by small or slow moving impacting bodies, low planetary surface densities, high surface viscosities, and low gravity values; conditions all likely for certain impacts into the icy satellites of Saturn and Jupiter, especially if liquid mantles were still present beneath solid crusts. Age dating based on crater counts and size‐frequency distributions for these icy bodies may have to be modified to account for the possibility that viscosity‐dominated craters were initially smaller and deeper than their gravity‐controlled counterparts.
Seismicity and tectonics of the Stiegler's Gorge Area, Tanzania
Authors: Bungum Hilmar , Nnko Abdueli A.
Published: February 2008

A microearthquake network with six stations and a diameter of about 50 km has been in operation in the Stiegler's Gorge area (Rufiji Basin), Tanzania, between 1978 and 1981. The network, which used radiotelemetry and digital recording at 30 Hz based on a triggering system, has recorded a large number of teleseismic and regional earthquakes in addition to several thousand local ones within the network. Most of the locally recorded earthquakes are in the magnitude range 0–4 measured on a duration magnitude scale which has been developed for the area, a scale which also satisfactorily covers regional earthquakes with distance up to 25°. An active and well‐defined seismicity zone has been delineated in the Stiegler's Gorge area, striking N140°E and dipping 45°NE, with events distributed between 0 and 40 km in depth. The time and space development of the local seismicity shows a fairly stable pattern, and a detailed focal mechanism study within the most active zone shows a predominance for normal faulting, with east‐west extensional movements. This is consistent with results from local geological studies, even though there is no clear correlation between the seismicity and locally mapped faults, and it is also consistent with our knowledge of the regional tectonics of the East African Rift System.
Focal mechanisms and depths of earthquakes in central Pakistan: A tectonic interpretation
Authors: Quittmeyer Richard C. , Kafka Alan L. , Armbruster John G.
Published: February 2008

Focal mechanisms and depths for seven earthquakes in central Pakistan were determined from an analysis of Rayleigh waves of 20‐to 50‐s periods. In east‐central Pakistan, the nodal planes for some solutions strike obliquely to the grain of surface structures. This observation supports the contention of other workers that a thin surficial unit, decoupled from the basement along a surface of decollement, characterizes this region. In west‐central Pakistan, relative movement between the Indian and Eurasian plates is at least partially accommodated by seismic slip along the Chaman fault. Other faults, which are situated to the east of and lie subparallel to the Chaman fault, may also take up some of the relative plate motion. Observed activity within the zone of convergent‐type structure in central Pakistan may be a result of the greater component of convergence across the Indian‐Eurasian plate boundary north of Quetta, Pakistan.

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Origin of Hawaiian tholeiite: A metasomatic model
Authors: Wright Thomas L.
Published: February 2008

Two voluminous magma types generated in the mantle underlying the Pacific plate are mid‐ocean ridge tholeiite (MORB) erupted at the East Pacific Rise spreading center and Hawaiian tholeiite (HT) erupted above the Hawaiian hot spot or melting anomaly. MORB has low initial 87Sr/86Sr ratios and low amounts of all incompatible trace elements including rare earths; chondrite‐normalized patterns are depleted in light rare earths. HT, by contrast, has higher initial 87Sr/86Sr and higher amounts of incompatible trace elements; chondrite‐nor‐malized patterns are enriched in the middle and light rare earths. HT is generally poorer in CaO and Al2O3 and much richer in total iron and TiO2 compared with MORB having the same MgO content. Primary magma compositions for the two volcanic systems are calculated in Fe‐Mg equilibrium with residual olivine (Fo92). MORB is generated by partial melting of a trace element depleted Iherzolite source leaving a residual assemblage dominated by olivine and orthopyroxene. The percentage of partial melting for a primary magma containing 15% MgO is calculated to be 35–42% in a source mantle having a heavy rare earth content of 3×chondrite and 33–35% MgO. HT, represented by Kilauea tholeiite, is generated by partial melting of a mixture of unmelted and residual mantle for MORB which has been modified by metasomatic addition of a nephelinitic fluid, amphibole, and minor amounts of apatite and Fe‐bearing phases such as sulfide and magnetite/ilmenite. This model yields a picritic magma in equilbrium with magnesian dunite at high (>40%) degrees of partial melting. The source also has 35% MgO before partial melting. Melting in both systems in polyvariant and not controlled by lower‐temperature invariant equilibria. The low‐velocity zone is considered to be the source of metasomatic fluids that are driven upward into the lowermost lithosphere in response to a thermal plume. Picritic primary magmas are produced by shear melting, localized in the zone of thinned and metasomatized lithosphere beneath Hawaii. Melt extraction is rapid and episodic at intervals of months to decades; magma is not stored in the mantle but passes upward to a plexus of storage reservoirs located 2–6 km beneath the surface of Kilauea. Kilauea primary magmas fractionate olivine during upward transport to reach bulk compositions of 13–14% MgO in storage. Different magma batches erupted to the surface, distinguished by different major and minor element compositons compared at similar MgO content, represent combinations of differing degrees of metasomatic enrichment, differing degrees of partial melting, and some effects of premelting mantle heterogeneity.