bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear wave structure of the upper crust of the Albuquerque‐Belen basin from Rayleigh wave phase velocities
Authors: Schlue J. W. , Singer P. J. , Edwards C. L.
Published: February 2008

Fundamental mode Rayleigh wave phase velocities have been obtained for the Albuquerque‐Belen basin of the Rio Grande rift. Source and instrumental characteristics limited useful data to the period band 6–12 s. Inversion of the phase velocity data yielded a model for the shear velocity of the upper 20 km of the basin. The model contains two velocity inversions, one near a depth of 6 km, the other near 18 km. The velocity inversion near 18 km is attributed to the effects of a midcrustal magma body known from previous studies; the cause of the low‐velocity zone near 6 km is not known, but it may be caused by an upper crustal magma body. The average of the velocities for the upper crustal model presented here (βave ≃ 3.33 km/s) is comparable to those reported in previous studies.
Strain accumulation in southern California, 1973–1984
Authors: Savage J. C. , Prescott W. H. , Gu Guohua
Published: February 2008

Repeated surveys of selected lines from five trilateration networks along the San Andreas fault in southern California have been used to deduce the 1973–1984 strain accumulation records at five localities. The secular rate of engineering shear strain accumulation is about 0.3 μrad/yr with the plane of maximum shear parallel to the local strike of the San Andreas fault. The secular rate of accumulation of areal dilatation is negligible. The data were examined to detect evidence for fluctuations in the rate of strain accumulation. For this examination, 19 lines were removed from the data set: four because they exhibited an obvious coseismic offset and 15 others because they contained at least one very anomalous measurement that could reasonably be attributed to a survey blunder. (The incidence of such blunders appears to be one in every 75 measurements.) The remaining data consist of 104 lines with an average of 10 measurements each. Although the strain accumulation plots for the five networks may exhibit marginally significant temporal fluctuations, we are not convinced that those fluctuations are greater than could be attributed to survey error. In particular, we are unable to demonstrate that the 1973–1979 southern California strain anomaly reported by Savage and others is real. Given the uncertainty in the random and systematic errors in measurement, the strain measurements in southern California are marginally consistent with linear‐in‐time strain accumulation. The strain accumulation plots for the Salton network clearly established that, unlike the deformation reported after the 1940 Imperial Valley earthquake, no acceleration in the shear strain rate has yet been observed following the 1979 Imperial Valley earthquake.
Partitioning of K, U, and Th between sulfide and silicate liquids: Implications for radioactive heating of planetary cores
Authors: Murrell M. T. , Burnett D. S.
Published: February 2008

The possibility of heating of planetary cores by K radioactivity has been extensively discussed, as well as the possibility that K partitioning into the terrestrial core is the reason for the difference between the terrestrial and chondritic K/U. We had previously suggested that U and Th partitioning into FeFeS liquids was more important than K. Laboratory FeFeS liquid, silicate liquid partition coefficient measurements (D) for K, U, and Th were made to test this suggestion. For a basaltic liquid at 1450°C and 1.5 GPa, DU is 0.013 and DK is 0.0026; thus U partitioning into FeFeS liquids is 5 times greater than K partitioning under these conditions. There are problems with 1‐atm experiments in that they do not appear to equilibrate or reverse. However, measurable U and Th partitioning into sulfide was nearly always observed, but K partitioning was normally not observed (DK ≲ 10−4). A typical value for DU from a granitic silicate liquid at one atmosphere, 1150°C, and low f02 is about 0.02; DTh is similar. At low f02 and higher temperature, experiments with basaltic liquids produce strong Ca and U partitioning into the sulfide liquid with DU > 1. DTh is less strongly affected. Because of the consistently low DK/DU, pressure effects near the core‐mantle boundary would need to increase DU by factors of ∼103 with much smaller increases in DU in order to have the terrestrial K and U abundances at chondritic levels. In addition, if radioactive heating is important for planetary cores, U and Th will be more important than K unless the lower mantle has K/U greater than 10 times chondritic or large changes in partition coefficients with conditions reverse the relative importance of K versus U and Th from our measurements.
Formulation of ice shelf dynamic boundary conditions in terms of a Coulomb rheology
Authors: MacAyeal D. R. , Shabtaie S. , Bentley C. R. , King S. D.
Published: February 2008

Coastal boundaries where fast flowing ice shelves shear past stagnant, grounded ice are typically riven with surface crevasses, seawater‐filled basal crevasses, and tidal strand cracks. Here we formulate a boundary condition describing stress transmission through these fractured boundaries in terms of the Coulomb law. As a result of this formulation, agreement between finite element simulations of the Ross Ice Shelf flow and field observations is improved over agreement obtained with formulations which do not account for ice failure. Our results additionally suggest that shear stress transmitted through ice shelf boundaries is lower than previously thought.
A study of the source parameters of some large earthquakes of northeastern North America
Authors: Ebel John E. , Somerville Paul G. , McIver J. D.
Published: February 2008

Waveforms of body and surface waves for six moderately large north eastern North American earthquakes are analyzed for source information. The earthquakes studied are the 1925 Charlevoix, Quebec, earthquake (MS ≃ 6.4), the 1935 Timiskaming, Quebec, earthquake (MS ≃ 6.0), the 1939 Charlevoix, Quebec, earthquake (mb ≃ 5.4), the 1940 Ossipee, New Hampshire, earthquakes (ML ≃ 5.3 and 5.1), and the 1944 Cornwall, Ontario‐Massena, New York, earthquake (M ≃ 5.1). The focal depths for all of the earthquakes for which determinations could be made (1925, 1935, 1939, and December 20, 1940) were about 8 to 10 km. The waveforms of the 1925, 1935, 1939, and 1940 events are consistent with thrust mechanisms, although there is some ambiguity in the strike direction for each of the events. The favored focal mechanism for the 1925 and 1939 events had northeast‐southwest nodal planes. For the 1935 event the available data suggest that the earthquake occurred on a northwest‐southeast oriented fault plane. The strike of the nodal planes for the December 20, 1940, event is quite uncertain, but a north‐south orientation is preferred. No body waves that could be modeled were collected for the December 24, 1940, or 1944 events. The stress drops for the events scatter widely but are comparable to those from other intraplate earthquakes. There is a difference in source complexity from event to event, with the December 20, 1940, event having had one simple source, while the 1935 shock had three separate energy releases. The source time functions found for the 1935, 1939, and December 20, 1940, earthquakes from the short‐period teleseismic body waves resemble those for some shocks from California. The focal mechanisms found here suggest that the minimum principal deviatoric stress is vertical and that the horizontal principal deviatoric stresses, both compressional, are controlled primarily by plate‐driving forces.
Magma ascent by porous flow
Authors: Scott David R. , Stevenson David J.
Published: February 2008

Porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle. Recent work has identified the importance of matrix deformation and compaction in this process. We present finite‐difference calculations based on a generalized form of Darcy's law that includes matrix deformation, in two‐dimensional Cartesian and cylindrical coordinates. We emphasize the existence of solitary wave solutions, called magmons. These waves are regions of locally high porosity that ascend through regions of low, uniform porosity. They differ from diapirs, where the liquid and matrix ascend together. The one‐dimensional waves that were previously reported are found to be unstable in two dimensions, breaking down to form two‐dimensional waves of circular cross section. Both the development of the instability and the form of the two‐dimensional waves are supported by theoretical analysis. Numerical experiments in which two‐dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves. The conservation can be very good; in other cases, larger waves appear to grow at the expense of smaller waves. Although the quantification of the relevant permeability and rheological parameters remains uncertain, geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year. In our preliminary assessment of the relevance of these results to igneous processes, we find that magmons are unlikely to be important in regions of broad upwelling, such as beneath spreading centers. Here the transport of liquid adjusts to match the supply by melting, and compaction processes are not important. If liquid is supplied from below to a stable partially molten region of the asthenosphere, it is expected to ascend in magmons. This may occur beneath oceanic volcanic centers or in subcontinental mantle. It is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity, if higher level processes do not obscure this influence. The waveform of a magmon ascends faster than the liquid within the magmon, so new liquid is taken in from above, while original liquid is lost from below. Consequently, magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface.
Paleomagnetic results from the TRiassic of the Yangtze Platform
Authors: Opdyke Neil D. , Huang K. , Xu G. , Zhang W. Y. , Kent D. V.
Published: February 2008

Two widely separated localities have been sampled from Triassic Formations of the Yangtze platforms. The first is from the border area between Sichuan and Guizhou provinces, where the Early Triassic Yelang Formation was sampled at 15 sites, located on both flanks of an anticline. Characteristic high‐temperature components were isolated from nine sites, the remainder were severely overprinted by a recent field. Two polarities are present, one directed toward the northeast with shallow positive inclinations, and the other to the southwest with almost horizontal inclinations. The northeast group passes the fold test at the 95% level. The second area of study was from the city of Nanjing, Jiangsu Province, where 19 sites were drilled from three formations ranging in age from the lower to upper Triassic. The samples are severely overprinted with a component that is almost vertical and whose origin is unclear. However, in 23 of the samples, a high‐temperature component was isolated directed to the northeast and positive and to the southwest and negative. The fold test is indeterminate. The pole positions from the two localities are Sichuan 46.3°N, 219.2°E, α95 = 10.9°; Nanjing 44.8°N, 223.6°E, α95 = 9.3°, which are not significantly different from each other and fall near two other recent studies of Triassic rocks from the Yangtze block. The pole positions given above are significantly different from the Triassic of Eurussia, Siberia, the North China block, and Thailand, indicating that these different components of eastern Asia were not sutured together in their present configuration until after the Triassic.
Paleomagnetic evidence for the timing of collapse and resurgence of the Lake City Caldera, San Juan Mountains, Colorado
Authors: Reynolds Richard L. , Hudson Mark R. , Hon Ken
Published: February 2008

Rocks of the 23.1‐m.y.‐old Lake City caldera consist of the compositionally zoned Sunshine Peak Tuff, postcollapse intracaldera lava flows, and resurgent quartz syenite intrusions. Declinations of reversely magnetized (I = −45° to −75°) Sunshine Peak Tuff change from easterly (D = 93°–130°) throughout most of the tuff to southerly (D = 195°–207°) within the late eruptive phases. The postcollapse lava flows are also reversely magnetized, and their emplacement appears to have spanned resurgence of the caldera. In contrast, the uppermost levels of the resurgent intrusion record two distinct components of magnetization: a reversed component isolated by thermal demagnetization typically at temperatures greater than 400°C and a normal component defined by thermal demagnetization at lower temperatures. A baked‐contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity. Assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data, the time for caldera development (from eruption of the ash flow tuffs to emplacement of the resurgent intrusion) was less than about 300,000 years on the basis of best estimates of durations of reversed polarities in the early Miocene. The estimated 300,000‐year time span for the development of the Lake City caldera is very close in duration to the radiometrically determined time spans of much younger (Pleistocene) calderas having similar dimensions and similar volumes of erupted material.
Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya
Authors: Hillhouse J. W. , Cerling T. E. , Brown F. H.
Published: February 2008

The Koobi Fora Formation, a Pliocene and Pleistocene sequence of sedimentary deposits northeast of Lake Turkana, has yielded numerous fossils and stone artifacts of early hominids. Stratigraphic correlation of the hominid‐bearing deposits throughout the Turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies. We have reinterpreted previously published magnetostratigraphy from the upper part of the Koobi Fora Formation because the original stratigraphy and dating of tuffs have been revised. In our reinterpretation we include previously unpublished data from the uppermost part of the formation. The upper magnetozones correlate with parts of the Brunhes Normal‐Polarity Chron and Matuyama Reversed‐Polarity Chron (about 0.6–0.85 Ma) and are separated from the magnetozones of the upper part of the Matuyama (2.0–1.25 Ma) by a disconformity. The Olduvai Normal‐Polarity Subchron is represented within the Matuyama, but the lower part of the Matuyama (2.4–2.0 Ma) is missing due to an erosional disconformity. We have also determined magnetozones in the lower part of the Koobi Fora Formation, which had not been sampled for paleomagnetism during the earlier studies. Our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of Koobi Fora tuffs with dated tuffs in the Shungura Formation of southern Ethiopia. The tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the Koobi Fora and Shungura formations. The lower part of the Koobi Fora spans the interval from about 4 Ma to 2.4 Ma, within the Gilbert Reversed‐Polarity and Gauss Normal‐Polarity chrons. Rock magnetic studies indicate that detrital magnetite carries most of the stable remanence, although hematite contributes to the remanence as indicated by thermal demagnetization. The hematite, which presumably formed by postdepositional oxidation of original iron oxide grains, carries chemical remanent magnetization (CRM) that diminishes the sharpness of polarity zone boundaries. The CRM accumulated continuously within the uppermost 10 m of sediment below the land surface, so that the CRM reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70,000 years in sections where the sedimentation rate was approximately 15 cm/1000 years.
Temporal variations in the electrical resistivity of the Earth's crust
Authors: Morrison H. F. , Fernandez R.
Published: February 2008

An electrical resistivity monitoring experiment was conducted on a portion of the San Andreas fault near Hollister, California, in 1979 and early 1980. Receiving dipoles up to 20 km away from a 140‐A 2‐km transmitting dipole were used to monitor apparent resistivities with accuracies as high as 0.1% for some of the sites. Several of the transmitter‐receivers clearly show long‐term changes in resistivity greater than the data errors; annual variations at a site outside the fault zone exceed 10%, and sites in the fault zone show month to month changes of a few percent. There are also significant variations in the vicinity of the transmitter which would be expected to produce correlated changes at other sites, but this was not observed in the data. There were no earthquakes of magnitudes greater than 3.0 during this period of high‐accuracy monitoring so that no conclusions can be drawn about changes in resistivity associated with major strain events. Future experiments to relate resistivity changes to earthquakes must not only have high sensitivity but must also be designed to distinguish such an event from normal background fluctuations that we have measured. Unless the resistivity variations associated with the event are of the order of several percent their detection will be a truly formidable task.
Geochemical characteristics of underground fluids in some active fault zones in China
Authors: Huixin Shi , Zuhuang Cai
Published: February 2008

This paper presents a study of tectonic activity in China by means of fluid‐geochemical methods. It is observed that deep fluids with characteristic chemical and isotopic compositions (high values of He and H2 and very low value of tritium) have migrated upward at some active faults. Many fluid‐geochemical anomalies were observed during periods of seismic activity, suggesting the usefulness of fluid‐geochemical methods for earthquake prediction. To properly select monitoring springs (or wells) and chemical components, local geological conditions have to be fully considered.
Cenozoic global plate motions
Authors: Gordon Richard G. , Jurdy Donna M.
Published: February 2008

Cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean‐lithosphere reference frame. Plate motions were analyzed over six time intervals divided by ages (10, 25, 43, 48, and 56 Ma) chosen, as much as possible, to coincide with key plate reorganizations. Alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the Pacific and North American plates. The circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the Atlantic region are assumed to be fixed relative to the hot spots in the Pacific region. Throughout the Cenozoic, the hot spot and mean‐lithosphere reference frames have been in continual, slow relative motion. The rate of motion is nonuniform, however, most of the motion having occurred during the middle Cenozoic. The net Cenozoic rotation of the lithosphere relative to the hot spots is described by a right‐handed rotation of 7° about a Euler pole at 46°S, 87°E, which yields a 5° displacement of the north poles of the two reference frames. This motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other. Analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the Cenozoic. Plate speeds correlate with latitude, plates moving faster near the equator than near the poles throughout the Cenozoic. As at present, continental plates (except for the Indian plate) moved slower than oceanic plates throughout the Cenozoic. Even the structure of the velocity fields as revealed in a contour of root‐mean‐square velocities in equatorial bands persists throughout the Cenozoic. The migration of the paleomagnetic axis over time is also compared to the hot spot and mean‐lithosphere reference frames. The paleomagnetic axis has shifted 5°–10° relative to the hot spot frame, and a lesser amount relative to the mean‐lithosphere frame.
Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post‐imbrium melt bombs
Authors: Borchardt R. , Stöffler D. , Spettel B. , Palme H. , Wänke H. , Wacker K. , Jessberger E. K.
Published: February 2008

Irregularly shaped, slag‐like impact‐melt bombs with variolitic crystallization texture, mostly 1 to 10 cm in size, occur throughout the regolith of the Apollo 16 landing site. We have analyzed 16 samples from Stations 11 and 13 near North Ray Crater using the microscope, electron microscope, microprobe, neutron activation, and 40Ar‐39Ar age‐dating techniques. We interpret the melt rocks as rapidly quenched, high‐velocity ejecta (“glass bombs”) from impact craters less than 1.5 km in diameter located in the vicinity of the Apollo 16 landing site. A comparison between glass bombs and typical regolith samples from various sampling stations reveals differences in their bulk chemical compositions. More than 80% of the glass bombs cannot be derived from the melting of the regolith, but can be produced by melting the subregolith basement. The differences in chemical composition of metal particles in glass bombs and regolith samples are consistent with these findings. Mixing calculations show that the glass bombs were formed by melting of two or more of the most abundant lithologies (granulites, feldspathic melt breccias, anorthosites, and various mafic melt breccias of VHA and LKFM affinity) that constitute the subregolith basement. From these results, we conclude that the subregolith basement represents a polymict megabreccia target. The glass bombs are post‐Cayley in age. They were produced by several different impact events more than 370 Ma ago by which they were emplaced in proximity to the North Ray Crater area. They were buried under a younger regolith layer until the North Ray impact event 50 Ma ago brought them up to the lunar surface again, and since then they have been exposed to cosmic radiation.
Mass spectrometric study of the Mercury isotopes in the Allende meteorite
Authors: Nier A. O. , Schlutter D. J.
Published: February 2008

Isotopic abundance ratios for mercury were determined by mass spectrometry in six samples of bulk material and in one sample of chondrules from the Allende meteorite. A primary purpose of the work was to attempt to verify the anomalous ratios reported for 196Hg/202Hg by neutron activation. Measurements were made on the mercury released at temperatures of 250°, 450°, 600°C, and in some cases, higher temperatures. The precision of the measurements was such that if an anomaly of the magnitude reported exists, it should have been seen. The isotopic abundance ratios for the other mercury isotopes were also measured. Within the errors of measurement these agreed with normal terrestrial values.
Identification of cosmogenic argon components in Allende by laser microprobe
Authors: Kirschbaum C.
Published: February 2008

New techniques are presented for using a laser microprobe to determine the spallation argon systematics of calcium‐aluminum inclusions. The 38Ars amounts determined for melilite and anorthite in a coarse‐grained inclusion from Allende are 2.9 × 10−8 and 1.3 × 10−8 cc/g, respectively. The ratio of the amounts is consistent with the calcium contents of these two minerals. The 38Ars amount determined for a fine‐grained inclusion from Allende is 1.1 × 10−8 cc/g. Calcium and potassium amounts were determined from irradiated samples of the same inclusions so that production of 38Ar from calcium during the cosmic ray exposure of Allende could be determined for these samples. The production observed was 12.4 ± 2.1 × 10−8 ccSTP 38Ar/g Ca for the coarse‐grained inclusion and 9.9 ± 2.4 ccSTP 38Ar/g Ca for the fine‐grained inclusion. No evidence of unusual exposure was observed in the two inclusions studied.
Physical properties of shocked and thermally altered nontronite: Implications for the Martian surface
Authors: Boslough M. B. , Venturini E. L. , Morosin B. , Graham R. A. , Williamson D. L.
Published: February 2008

Both shock recovery and thermal annealing experiments have been carried out on Riverside nontronite, an iron‐rich smectite clay that closely matches the composition, but neither the color nor magnetic properties, of the Martian fines. Nontronite was recovered from peak shock pressures between 7 and 32 GPa, with calculated peak mean bulk temperatures ranging from 125° to 1100°C. Other nontronite samples were heated in air for one hour at temperatures ranging from 400° to 1000°C at 100°C increments. Both shocked and heat‐treated samples were analyzed by magnetic measurements, X ray diffraction, and Mössbauer spectroscopy. Nontronite powders shock‐loaded to peak calculated temperatures of 650°C to 750°C show a dramatic increase in ferromagnetic saturation magnetization (Ms). A similar increase in magnetization in the same temperature range is observed in isothermally annealed powders at atmospheric pressure. The X ray diffraction and Mössbauer studies show, however, that for samples that experienced temperatures of 600°C or more, the shocked material has little structural similarity to the isothermally annealed nontronite. This suggests that the ferromagnetic properties may be due to a minor phase. There is evidence from Mössbauer spectra for metallic iron in the shocked samples and for superparamagnetic Fe2O3 in the annealed samples. In addition to magnetic and structural changes, the shock and heating experiments altered the color of the nontronite from olive‐yellow toward reddish‐brown, yielding material that matches the observed properties of the Martian fines much more closely than the starting material does. If nontronite is the major component of the Martian fines as some workers suggest, some mechanism for altering its properties is necessary, the most likely being repeated impacts onto the surface of Mars.
Models of the magnetic field at the core‐mantle boundary for 1715, 1777, and 1842
Authors: Bloxham Jeremy
Published: February 2008

Models of the magnetic field at the core‐mantle boundary for 1715.0, 1777.5, and 1842.5 are presented, derived from original observations rather than from charts or grid point values. The models, produced using the method of stochastic inversion, have surprisingly good resolution of the core field, resolving small‐scale features which have only recently been identified in modern data. The models show that secular variation is low over much of the core‐mantle boundary, with clearly identifiable static, slowly varying features in the core field. Westward drift is confined to a relatively small region, in which the secular variation is more vigorous. It is concluded that the effects of magnetic diffusion should not be ignored in studies of the secular variation.
Paleomagnetism of red cherts: A case study in the Inuyama Area, central Japan
Authors: Shibuya Hidetoshi , Sasajima Sadao
Published: February 2008

This study is an attempt to isolate and identify the primary magnetization in red cherts. Red chert samples from a middle Triassic sequence in the Inuyama area (35.4°N, 137.0°E), central Japan have a multicomponent magnetization, which is delineated by thermal demagnetization. Two of the components have high coercivity and high blocking temperatures; one is carried by hematite and the other by magnetite. The former was concluded to be the primary magnetization for two reasons: (1) it predates intraformational folding, and (2) it records geomagnetic reversals. The other component, which accounts for the dominant part of natural remanent magnetization, was probably acquired at a time long after the deposition. Two paleomagnetic results were drawn from the Middle Triassic red chert sequence: (1) The mean inclination of the samples in the sequence was 1.4° ± 6.8°; therefore the paleolatitude amounts to 0.7° ± 3.4°, in contrast to the paleolatitude expected from the paleopole of Eurasian continent, which is about 70°, and (2) the magnetostratigraphy of this sequence indicates that two geomagnetic epochs in the Middle Triassic are considerably longer (more than several million years) than the average of those in the Cenozoic.
Displacements of the Earth's surface due to atmospheric loading: Effects on gravity and baseline measurements
Authors: Van Dam T. M. , Wahr J. M.
Published: February 2008

Atmospheric mass loads and deforms the earth's crust. By performing a convolution sum between daily, global barometric pressure data and mass loading Green's functions, we estimate the time dependent effects of atmospheric loading, including those associated with short‐term synoptic storms, on surface point positioning measurements and surface gravity observations. We calculate the response for both an oceanless earth and an earth with an inverted barometer ocean. Load responses for near‐coastal stations are significantly affected by the inclusion of an inverted barometer ocean. Peak to peak vertical displacements are frequently 15–20 mm with accompanying gravity perturbations of 3–6 μ Gal. Baseline changes can be as large as 20 mm or more. The perturbations are largest at higher latitudes and during winter months. These amplitudes are consistent with the results of Rabbel and Zschau (1985), who modeled synoptic pressure disturbances as Gaussian functions of radius around a central point. Deformation can be adequately computed using real pressure data from points within about 1000 km of the station. Knowledge of local pressure, alone, is not sufficient. Rabbel and Zschau's hypothesized corrections for these displacements, which use local pressure and the regionally averaged pressure, prove accurate at points well inland but are, in general, inadequate within a few hundred kilometers of the coast.
A potpourri of regolith breccias: “New” samples from the Apollo 14, 16, and 17 landing sites
Authors: Jerde Eric A. , Warren Paul H. , Morris Richard V. , Heiken Grant H. , Vaniman David T.
Published: February 2008

Forty suspected regolith breccias from the Apollo 14, 16, and 17 landing sites were studied as part of a search for regolith samples exotic to the small traverse areas associated with these missions, as well as a general effort to constrain the nature and origins of regolith breccias. Of these 40 samples, 31 are indeed regolith breccias. Regolith breccias from Apollo 14 exhibit much greater compositional diversity than their soil (sensu stricto) counterparts: 14004,55 displays incompatible element concentrations about 1.4× those found in typical Apollo 14 regolith, while 14315 is radically different in many respects (higher Al, lower Mg and Fe, lower incompatible elements) from all other Apollo 14 regolith materials, and hence probably exotic to the Apollo 14 traverse area. Application of “mixing” models to Apollo 14 regolith materials suggests that whereas “normal” Apollo 14 regolith contains only slightly more ferroan anorthosite (FA) than alkali anorthosite (AA) and far more KREEP than FA and AA combined, 14315 contains more FA than KREEP and roughly 10 times more FA than AA. Three “new” regolith breccias from Apollo 16 bring to 20 the total number of regolith breccias that have been analyzed for Mg and Fe from the site. The molar Mg/(Mg + Fe) (or mg*) ratios of lunar regolith breccias are of great interest, because lunar‐meteorite ALHA81005 has a much higher mg* than two other lunar‐meteoritic regolith breccias (Y791197 and Y82192/3), even though these three samples are remarkably similar in all other compositional respects. A bimodality in mg* among Apollo 16 regolith breccias may result from: (1) statistical scattering among the small number of samples, (2) a tendency for older regolith breccias from this site to be compositionally distinct from their younger counterparts, or (3) differences between the Cayley and Descartes Formations. Among 10 samples from Apollo 17, all but one proved to be typical Apollo 17 regolith breccias. However, 72504,10 is >99% pure pyroclastic glass, compositionally identical to the Apollo 17 orange glass (74220), even though 72504,10 was obtained at a location over 4 km from Shorty Crater where 74220 was obtained. The nearly pure concentration of 74220‐identical glass spherules in 72504,10 attests to the widespread distribution of these distinctive deposits.
Ferric sulfates on Mars
Authors: Burns Roger G.
Published: February 2008

Evidence is presented for the possible formation and existence of ferric sulfato complexes and hydroxo ferric sulfate minerals in the permafrost on Mars. Acidic groundwater, derived from atmospheric oxidation of volcanogenic H2S to H2SO4 aerosols, promoted chemical weathering of fayalitic olivines, iron‐rich pyroxenes, plagioclase feldspar, and pyrrhotite‐pentlandite mineral assemblages in crustal ultramafic and basic igneous rocks. The acidic groundwater entered into electrochemical reactions with the iron sulfides, yielding dissolved FeSO4+, Fe(SO4)2−, and FeOH2+ complex ions, and the precipitation of basic ferric sulfate minerals such as those belonging to the roemerite, copiapite, botryogen, and jarosite‐alunite groups. These phases are stabilized at low temperatures and pH in Martian permafrost. The occurrence of jarosites in terrestrial arid regions suggests that they could also survive on the surface of Mars. Melting of the permafrost and raising of the pH may have initiated the hydrolysis of dissolved ferric sulfato complex ions and led to the precipitation of FeOOH, which reacted with precipitated silica to form phyllosilicates. Alternatively, degradation of the hydrolysate FeOOH to Fe2O3 during sublimation of permafrost exposed on Mars' surface may account for the presence of eolian maghemite suspected to be the magnetic mineral observed on the Viking Landers.
Nonlinear generation of elastic waves in crystalline rock
Authors: Johnson Paul A. , Shankland Thomas J. , O'Connell Richard J. , Albright James N.
Published: February 2008

The nonlinear interaction of two elastic waves at frequencies ƒ1 and ƒ2 in an elastically nonlinear material can give rise to a collimated wave at the difference frequency ƒ1 ‐ ƒ2. Because the amplitude of a difference frequency beam is proportional to the degree of elastic nonlinearity of the material through which it passes, amplitude should be higher in a material containing microcracks such as rock than it is in uncracked materials such as metals, single crystals, or water in which nonlinear elastic interactions have previously been observed. The “nonlinear signal” is important for investigating the nonlinear properties of rocks. Such a beam has already proved useful as a low‐frequency acoustic source in water and may ultimately be useful in geophysical exploration. In this paper, our observations of nonlinear signal generation in experiments with crystalline rocks are presented. Three criteria must be fulfilled in such experiments to establish that nonlinear interactions take place in the rock and not in the associated experimental apparatus: (1) The frequency of the observed nonlinear signal must precisely equal the difference frequency Δƒ = ƒ1 ‐ ƒ2, (2) the amplitude of the nonlinear signal must be proportional to the product of the amplitudes of the primary beams, and (3) the trajectory of the nonlinear signal, which is a function of the input trajectories, wave types, frequencies, and rock velocities, must match that predicted by theory. We observed signals that satisfy the above three criteria in the frequency range from 0.1 to 1.0 MHz.
A quantitative approach to bedrock velocity resolution and precision: The Lithoprobe Vancouver Island Experiment
Authors: Mayrand Louis J. , Green Alan G. , Milkereit Bernd
Published: February 2008

Bedrock velocities obtained from the inversion of first‐break picks of multichannel seismic reflection data have been used to aid the interpretation of shallow reflectors. The trade‐off between resolution and stability of inversion results has been examined. Models have been optimized by minimizing the variability of static corrections, and a Monte Carlo method has been used to estimate their precision. A velocity of 4.6 ± 0.4 km/s for the Leech River Complex (LRC) compares with velocities of 5.9 ± 0.6 and 5.8 ± 0.3 km/s for the neighboring geological units. Velocity contrasts associated with the change in lithologies may therefore be a contributing factor for generating reflections from the faults that bound the LRC. Velocities of 5.5 ± 0.4 and 4.6 ± 0.6 km/s on either side of the San Juan fault suggest that an event identified on several shot gathers may be a reflection from a steeply dipping fault.
Character and distribution of borehole breakouts and their relationship to in situ stresses in Deep Columbia River Basalts
Authors: Paillet Frederick L. , Kim Kunsoo
Published: February 2008

The character and distribution of borehole breakouts in deeply buried basalts at the Hanford Site in south central Washington State are examined in light of stress indicator data and hydraulic‐fracturing stress data by means of acoustic televiewer and acoustic waveform logging systems. A series of boreholes penetrating the Grande Ronde Basalt of the Columbia River Basalt Group were logged to examine the extent of breakouts at depths near 1000 m. Breakouts occur discontinuously throughout the interiors of most flows. In some boreholes the distribution of borehole wall breakouts closely correlates with the incidence of core disking. Differences in the distribution of breakouts and disking are attributed to differences in failure mechanisms. A thin interval of breakout‐free basalt occurs near the upper and lower limits of flow interiors, with many intervals of breakouts terminating at the intersection of oblique fractures with the borehole. Hydraulic‐fracturing stress measurement results obtained from four deep boreholes indicate anisotropic horizontal principal stresses, with maximum principal stress along an approximate north trending axis, consistent with the east trending orientation of breakouts. Acoustic waveform logs indicate that there is no measurable difference between the seismic properties of breakout‐free flow tops and flow interiors. The highly coherent waveforms obtained in almost all flow interiors indicate that damage to the borehole wall associated with breakout formation remains confined to the thin annulus of stress concentration.
On predicting changes in the geomagnetic field
Authors: Alldredge Leroy R.
Published: February 2008

The present method of using constant secular variation rates to forecast magnetic components at a given site or to forecast spherical harmonic coefficients is known to be inaccurate. A new predictive method using trend and trigonometric functions fitted to known past values is used to extrapolate for a few years into the future. This provides an improvement over the usual linear extrapolation method. This method applied to the spherical harmonic coefficients (SHC) in the series of International Geomagnetic Reference Fields (IGRF) provides an analytic function for each SHC so that charts can be compiled for a few years in the future without using the idea of a constant secular variation. In this case, the trend and trigonometric functions utilize the same number of Fourier and trend coefficients as are now present in the IGRF models on which they are based so that they can completely replace the IGRF information.
Role of geophysics in identifying and characterizing sites for high‐level nuclear waste repositories
Authors: Wynn Jeffrey C. , Roseboom Eugene H.
Published: February 2008

Evaluation of potential high‐level nuclear waste repository sites is an area where geophysical capabilities and limitations may significantly impact a major governmental program. Since there is concern that extensive exploratory drilling might degrade most potential disposal sites, geophysical methods become crucial as the only nondestructive means to examine large volumes of rock in three dimensions. Characterization of potential sites requires geophysicists to alter their usual mode of thinking: no longer are anomalies‐being sought, as in mineral exploration, but rather their absence. Thus the size of features that might go undetected by a particular method take on new significance. Legal and regulatory considerations that stem from this different outlook, most notably the requirements of quality assurance (necessary for any data used in support of a repository license application), are forcing changes in the manner in which geophysicists collect and document their data.
Antarctic ice streams: A review
Authors: Bentley Charles R.
Published: February 2008

An ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice. The distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice ‐‐ many Antarctic glaciers are intermediate in character. The “Ross ice streams,” which flow through the West Antarctic inland ice into the Ross Ice Shelf, are distinct in character, differing even from other ice streams in the marine ice sheet of West Antarctica. Their surface elevation profiles are low, their bed slopes are low and smooth, and their driving stresses diminish monotonically downglacier. In transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs, than other ice streams. Many models for the fast sliding of glaciers have been applied to the Ross ice streams; most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed. Most models are only semi physical at best, so need to be “tuned” by fitting to assumed parameters at some place or places in the real ice sheet. The recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models. The difficulty may be resolved if it is true, as recent experiments suggest, that ice stream B, and by extrapolation other Ross ice streams as well, slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock.
Glaciological studies on Rutford Ice Stream, Antarctica
Authors: Doake C. S. M. , Frolich R. M. , Mantripp D. R. , Smith A. M. , Vaughan D. G.
Published: February 2008

Rutford Ice Stream drains part of the West Antarctic ice sheet into Ronne Ice Shelf. Much of the drainage basin has a bed well below sea level and is therefore an area which could undergo substantial change if a climatic warming were to cause sustained thinning of Ronne Ice Shelf. Snow accumulation data suggest that an accumulation rate of 0.51 ± 0.12 Mg m−2 yr−1 is required for balance. An extensive ice movement survey network was established across the grounding line in 1978–1980 and was extended for more than 100 km upstream in 1984–1986. Velocities at five sites have been calculated using doppler satellite position measurements. When the velocities are combined with cross‐sectional areas of the ice stream measured by radio echo sounding, mass flux figures show that drag at the sidewalls and on the base must decrease downstream toward the grounding line. The increasing influence of buoyancy forces on ice stream motion is also shown by the steady rise in the ratio of bedrock depth to ice thickness going downstream along the network. Below the grounding line that was found to cross the middle part of the earlier network is a zone stretching for a further 100 km where it can now be shown that the ice stream is intermittently grounded. In these grounded areas the glacier bed is raised with respect to the surrounding seabed and forms eskerlike features. Surface elevations over the grounded areas are less than 30 m above those needed for hydrostatic equilibrium.
Deformation around the Creede Caldera: A consequence of isostatic adjustment following Caldera Formation
Authors: Gephart John W.
Published: February 2008

The pattern of deformation around the Creede caldera (26.5 Ma), southwest Colorado, may provide clues to the physical mechanisms of caldera evolution, particularly resurgent doming. This paper considers the hypothesis that the deformation is due to isostatic relaxation of surface load (topography) following caldera formation, involving uplift of the caldera floor and subsidence of a surrounding tumescent dome. This hypothesis is tested by constructing finite difference models of relaxing topography which have as parameters the mechanical properties of rocks surrounding the caldera. The goal is to find models that predict spatial and temporal patterns of deformation matching those observed around the caldera, as inferred from: (1) the present structural form of the region as indicated by the surface distribution of stratigraphic units and (2) past structural forms as indicated by the locations, orientations, and ages of faults, which are presumed to be due to a combination of regional tension and stresses caused by bending at the surface. Successful models of the deformation have been found using a variable viscosity half‐space overlain by a thin elastic plate. In general, the nature of deformation at the surface is strongly dependent on the radial variation of viscosity at shallow depths beneath the elastic plate and is much less sensitive to variation of viscosity with depth. Models with relatively low viscosities beneath the caldera and relatively high viscosities in the surrounding region accurately predict uplift of the caldera floor which is rapid compared to the rate of marginal subsidence. The results indicate that the rate of deformation is governed by the flow of material with viscosities characteristic of wallrock (roughly 1019–1024 Pa s) rather than magma. A gap in faulting around the caldera at about one caldera radius outside the wall suggests the existence of a relatively low radial viscosity gradient in that position; higher viscosity gradients inside and outside of this low may reflect the margins of two nested magma bodies at depth (a central stock within a larger batholith). In order to match the observed pattern of faults around the caldera the orientation of the regional tension that is superimposed on the bending stresses must rotate clockwise with time. This change in stress direction is consistent in sense but earlier than that proposed by others for the western United States.
Chemistry of hydrothermal solutions from the southern Juan de Fuca Ridge
Authors: Von Damm Karen L. , Bischoff James L.
Published: February 2008

Fluids from three vent fields on the southern Juan de Fuca Ridge were sampled in September 1984 using the DSRV Alvin. The fluids are uniquely high in both chloride, which ranges up to twice the seawater value, and in metal content. Simple vapor‐liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids. The cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes: (1) subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and (2) dissolution of a chloride‐rich phase combined with a possible small loss of gas during sampling procedures. Although measured temperatures were all less than 300°C, quartz geothermometry suggests that the fluids have equilibrated at greater than 340°C. Quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber.