bluuebunny/crossref_metadata_2025 huggingface dataset:
Deep‐tow and sea beam studies of dueling propagating ridges on the East Pacific Rise near 20°40′S
Authors: Macdonald Ken C. , Haymon R. M. , Miller S. P. , Sempere J.‐C. , Fox P. J.
Published: February 2008

Nontransform ridge axis discontinuities, such as propagating rifts, overlapping spreading centers (OSCs) and saddle points, occur frequently along the mid‐ocean ridge system. We used Sea Beam and deep‐tow to study an example of a relatively large nontransform offset near 20°40′S on the East Pacific Rise (EPR). The two spreading center tips are 15 km apart and separated by a diffuse shear zone. A broad V‐shaped wake of disturbed topography and magnetics occurs on the flanks of the EPR near the offset. On a fine scale, the offset is tectonically much more complex than either the typical OSC or propagating rift structures for two reasons. First, it is a “dueling propagator” in which structures produced by successive short‐term episodes of northward and southward propagation have overprinted each other, with migration toward the south prevailing over the long term. Second, its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an OSC system. The offset's rate of migration to the south has been ∼20 mm/yr over the last 2 m.y., far less than total opening rate of 162 mm/yr. Distinct propagation events may have been much more rapid, however, surging north and south at rates of 160–230 mm/yr. In the process, several fossil spreading ridges 30–50 km long have been abandoned and rafted to the side. We propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned, propagating rift tip. We call this process “self‐decapitation.” Dueling propagation at 20°40′S may be caused by competing magmatic sources at shallow points along the EPR: one to the north near 20°S and one to the south near 21°30′S. The 20°40′S dueling propagator occurs at a long‐wavelength low in the axial depth profile between these competing sources. In general, the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge.
Pore pressure changes induced by slip on permeable and impermeable faults
Authors: Rudnicki J. W. , Hsu Tze‐Chi
Published: February 2008

Pore pressure changes due to a ramp introduction of slip on permeable and impermeable faults in a fluid‐saturated rock mass are calculated for the purpose of evaluating water well level fluctuations. The calculations demonstrate the importance of coupling between deformation and fluid diffusion at observation points less than 5(4ct0)½, where c is the diffusivity and t0 is the rise time. The decay of pore pressure in the results here is due entirely to fluid mass diffusion. An approach that neglects diffusion and assumes that the pore pressure is proportional to the mean normal stress would predict a ramp pore pressure response. At distances greater than 5(4ct0)½ the pore pressure decays so slowly that the neglect of diffusion may be appropriate. For both permeable and impermeable faults, the pore pressure decays more rapidly for shorter slip zone lengths and longer rise times. However, the pore pressure change calculated for the impermeable fault is larger, particularly for observation points near the fault, and decays less rapidly than for the permeable fault. These differences suggest that fault permeability can be a significant factor in the response of water wells near faults and care should be used in inferring details of the slip distribution if hydrologic conditions are not known. These results are applied to a water well level change observed by Lippincott et al. A satisfactory fit to the data is obtained by uniform slip over a fault length of about 1.5 km and a rise time of 8 hours. Although the slip magnitude is not well constrained by the fit, the range of possible values includes the 0.5 to 1.0 cm inferred by Lippincott et al. using a different approach.
Volcanic hazards and public response
Authors: Peterson Donald W.
Published: February 2008

Although scientific understanding of volcanoes is advancing, eruptions continue to take a substantial toll of life and property. Some of these losses could be reduced by better advance preparation, more effective flow of information between scientists and public officials, and better understanding of volcanic behavior by all segments of the public. The greatest losses generally occur at volcanoes that erupt infrequently where people are not accustomed to dealing with them. Scientists sometimes tend to feel that the blame for poor decisions in emergency management lies chiefly with officials or journalists because of their failure to understand the threat. However, the underlying problem embraces a set of more complex issues comprising three pervasive factors. The first factor is the volcano: signals given by restless volcanoes are often ambiguous and difficult to interpret, especially at long‐quiescent volcanoes. The second factor is people: people confront hazardous volcanoes in widely divergent ways, and many have difficulty in dealing with the uncertainties inherent in volcanic unrest. The third factor is the scientists: volcanologists correctly place their highest priority on monitoring and hazard assessment, but they sometimes fail to explain clearly their conclusions to responsible officials and the public, which may lead to inadequate public response. Of all groups in society, volcanologists have the clearest understanding of the hazards and vagaries of volcanic activity; they thereby assume an ethical obligation to convey effectively their knowledge to benefit all of society. If society resists, their obligation nevertheless remains. They must use the same ingenuity and creativity in dealing with information for the public that they use in solving scientific problems. When this falls short, even excellent scientific results may be nullified.
Magma flow directions in dikes of the Koolau Complex, Oahu, determined from magnetic fabric studies
Authors: Knight Michael D. , Walker George P. L.
Published: February 2008

We compared the magnetic fabric with the macroscopic surface lineation direction (previously assumed to be parallel with magma flow direction) for 35 dikes of the Koolau complex, Oahu. Anisotropy of magnetic susceptibility (AMS) was determined for 530 samples from 59 dikes (including a sill and a pluglike intrusive body) and two filled lava tubes, and statistically significant AMS clusters were found in all but five dikes. For the 35 lineated dikes, the mean maximum AMS direction (χ1) coincides with the macroscopic lineations to within 25° in 18 and to within 15° in eight. The AMS ellipsoid shape in about half of the samples is prolate (cigar shaped) and in about half is oblate (pancake shaped). We interpret the AMS to be determined by shape anisotropy of ferromagnetic grains. The mean total anisotropy is 2.2%, and total anisotropy (H) ranges from 0.24% to as high as 14.12%. The principal AMS axes for samples from a dike are tightly grouped on equal‐area plots, and for most of the dikes the minimum AMS axes plot near to the pole of the dike. Our AMS measurements enable us to infer absolute magma flow directions for half the dikes studied; the maximum AMS axes of samples from one side of the dike form a tight cluster on an equal‐area plot, and those from the other side of the dike form another tight cluster. The two clusters are close together (separated by 10–30°) for narrow dikes (<100 cm wide), and are farther apart (30–60°) for wide dikes and are symmetrically disposed on either side of the plane of the dike. We infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient, and hence indicate the flow azimuth, i.e., the absolute magma flow direction. Nearly half of the dikes measured (25) gave paired AMS plots; in 12 the magma flowed up at low oblique angles 5–30° from the horizontal, in five it flowed down at 5–20° in four it flowed horizontally (<5°), in three it flowed up at a steep angle (50–75°), and in one it flowed down at a steep angle. The magma flow in general was rarely vertical or horizontal.
Geochemistry of the 1984 Mauna Loa Eruption: Implications for magma storage and supply
Authors: Rhodes J. M.
Published: February 2008

The 1984 eruption of Mauna Loa volcano, Hawaii, produced about 220×106 m3 of lava of remarkably homogeneous composition. No significant compositional changes occurred during the 22‐day eruption, and along the 20 km vent system, even though crystallinity increased substantially, accompanying a decline in the eruption rate. Such homogeneity in lava composition is typical of many historic Mauna Loa eruptions. The 1984 lava, along with most other homogeneous Mauna Loa lavas, is multiply saturated, crystallizing olivine, clinopyroxene, and plagioclase. Bulk compositions are “perched” at the intersection of an olivine‐control trend with a trend for more differentiated lavas. There is no evidence to indicate that the 1984 lava is a differentiate of the 1975 magma, or that it results from mixing a new batch of magma with differentiated melt remaining in the magmatic plumbing system after the 1975 eruption. It is proposed that these lavas are erupted directly from a small, continuously replenished magma reservoir located about 3–4 km beneath the summit caldera. Successive eruptions therefore provide a sampling of magma chamber processes, and small but discernible changes in lava composition reflect the competing processes of magma supply, differentiation, and eruption.
Valles Caldera region, New Mexico, and the emerging continental scientific drilling program
Authors: Goff Fraser , Gardner Jamie N.
Published: February 2008

Valles caldera is best known in recent years as an excellent example of a resurgent caldera [Smith and Bailey, 1968] and as the site of a high‐temperature geothermal system [Dondanville, 1978]. However, Valles caldera and the surrounding Jemez Mountains volcanic field possess a rich history of geologic research that dates back to the late 1800s. Through the years, the research focus has changed as different economic and scientific factors have exerted their influence. Early work emphasized mining activity, while modern work has stressed volcanology and, later, geothermal development. Only in the last 5 years has it been possible to view the region as a dynamic, integrated magma‐hydrothermal system having a complex evolution lasting more than 13 m.y. [Gardner et al., 1986; Goff and Nielson, 1986; Self et al., 1986].
A physical model for earthquakes: 2. Application to southern California
Authors: Rundle John B.
Published: February 2008

The purpose of this paper is to apply ideas developed in a previous paper to the construction of a detailed model for earthquake dynamics in southern California. The basis upon which the approach is formulated is that earthquakes are perturbations on, or more specifically fluctuations about, the long‐term motions of the plates. This concept is made mathematically precise by means of a “fluctuation hypothesis,” which states that all physical quantities associated with earthquakes can be expressed as integral expansions in a fluctuating quantity called the “offset phase.” While in general, the frictional stick‐slip properties of the complex, interacting faults should properly come out of the underlying physics, a simplification is made here, and a simple, spatially varying friction law is assumed. Together with the complex geometry of the major active faults, an assumed, spatially varying Earth rheology, the average rates of long‐term offsets on all the major faults, and the friction coefficients, one can generate synthetic earthquake histories for comparison to the real data. The result is a set of slip‐time histories for all of the major faults which are similar to data obtained by geologic trenching studies. However, the patterns of earthquake occurrence are evidently chaotic (in the technical sense), meaning that although there is an element of periodicity to the events, the patterns shift, change, and evolve with time. Time scales for pattern evolution seem to be of the order of a thousand years for average recurrance intervals of a hundred years or so. In the synthetic slip histories for the model faults a series of events on the Big Bend is identified which resembles the pattern of events leading up to the 1857 Fort Tejon earthquake. One of the events is similar enough to the 1857 event that it is taken as a model of that earthquake. The horizontal surface deformation 130 years after the model event is calculated for comparison to the data observed by various methods during the past decade in southern California. For example, horizontal surface rates of strain and velocities are calculated and compared to U.S. Geological Survey trilateration data, to very long baseline interferometry line length changes, and to laser ranging data on the San Andreas fault experiment baseline. With a few exceptions, agreement is good.
Geoid and topography for infinite Prandtl number convection in a spherical shell
Authors: Bercovici D. , Schubert G. , Zebib A.
Published: February 2008

Geoid anomalies and surface and lower‐boundary topographies are calculated for numerically generated thermal convection in an infinite Prandtl number, Boussinesq, axisymmetric spherical fluid shell with constant gravity and viscosity. Calculations were done for heating entirely from below (HFB) and entirely from within (HFW). Convective solutions were obtained for Rayleigh numbers Ra up to 20 times the critical Ra in the HFB case and 27 times critical in the HFW case. Bifurcations that occur with varying Ra are associated with different convective patterns. Boundary topography is positive at upwellings and negative at downwellings. Absolute topography is greater at the poles than at the equator due to poleward compression of the flow field; lower‐boundary topography is greater than at the surface due to radial compression of the flow field. The geoid is primarily dominated by the gravitational potential of surface masses due to surface boundary deformations; hence the geoid parallels surface undulations. In general, boundary deformation increases with increasing cell wavelength. This property may elucidate whether core‐mantle boundary (CMB) topography is due to mantle convection or mantle dregs (chemically dense material) which are only indirectly of dynamic origin, and hence are unlikely to have the same wavelength dependence in its associated topography. The ratios of boundary deformation and geoid to αΔTd (α, coefficient of thermal expansion; ΔT, characteristic temperature drop across the fluid shell; and d, shell thickness) maximize with increasing Ra at approximately 10 times Ra critical and then steadily decrease out to the maximum Ra calculated. Dimensionless geoid and topography in the HFB case are approximately 5 times greater than in the HFW case. At the highest Rayleigh numbers investigated, dimensionless peak‐to‐trough amplitudes of geoid and topography are weak functions of Ra, which lends some justification to estimating the dimensional values of these quantities with Earthlike parameters. For parameter values characteristic of the Earth's mantle, the geoid and upper and lower boundary topograpies are approximately 1 km, 10 km, and 15 km for the HFB case, and approximately 200 m, 2 km, and 3 km for the HFW case, respectively. The HFW values correlate with geophysical data more closely than the HFB values, suggesting a predominance of internal heating in the mantle. Although this result is qualfied by the low Rayleigh numbers and axisymmetry of the model calculations, our study emphasizes that dynamically induced topography and geoid are sensitive to the mode of heating in the Earth's mantle.
"Offsets of Late Quaternary morphology, rate of slip, and recurrence of large earthquakes on the Chang Ma Fault (Gansu, China)""
Authors: Peltzer Gilles , Tapponnier Paul , Gaudemer Yves , Meyer Bertrand , Guo Shunmin , Yin Kelun , Chen Zhitai , Dai Huagung
Published: February 2008

The Chang Ma fault, in the western Qilian Shan, Gansu province, shows evidence of consistent left‐lateral displacements despite a complex en échelon geometry. We have measured short‐ and long‐term offsets on two N70°–80°E striking segments of the fault, along the southern edge of the Ta Quen Kou basin, where it cuts late Quaternary terraces, transverse ridges, and drainage channels. To analyze the offsets of such structures at one site in the eastern part of the basin, we ran topographic profiles parallel and perpendicular to the fault over a distance of about 5 km. Spectral analysis and comparison of longitudinal profiles north and south of the fault allowed us to separate horizontal offsets of structures of different wavelengths, apparent on air photos and satellite images. Transverse profiles were used to determine the vertical throw, whose variations appear to result from changes in the fault strike. Small horizontal offsets of ridge crests and valley slopes are about 55 m. Along the N70°E striking segment of the fault, this displacement corresponds to an uplift of the northern block of about 27 m implying that the fault dips about 73° to the South. The measurements also constrain horizontal projection of the slip vector to about N78°E. A larger horizontal offset (≈500 m) of morphological swells of longer wavelength (≈1.6 km) is visible on the longitudinal profiles. Assuming that periglacial erosion has reshaped the surface of the lateral slopes and tops of ridges during the Würm glacial period, the smallest offsets of adjacent ridge crests may postdate the begining of the Holocene, yielding an average rate of left‐lateral slip on the fault of 5.5±2.2 mm/yr in the last 10±2 kyr. In the western part of the Ta Quen Kou basin, a somewhat slower rate of Holocene slip (4.6±.4 mm/yr) is obtained. Measurement of the horizontal displacement due to the 1932 earthquake there (6.2±0.6 m) would imply that the recurrence interval for similar earthquakes at this second site is 1350±540 years.
Possible breakup of dark terrain on Ganymede by large‐scale shear faulting
Authors: Murchie Scott L. , Head James W.
Published: February 2008

A system of arcuate furrows in Galileo Regio and Marius Regio, two large blocks of ancient dark terrain, has previously been proposed to be the remnant of an originally more concentric furrow set that was disrupted by shear. Estimates of the two regions' poles of furrow concentricity indicate a considerable westward offset of the Galileo Regio pole. These measurements suggest the possibility of about 500 km of left‐lateral offset of the two areas; four additional, independent structural indicators support this hypothesis. There is evidence that shear deformation was concentrated along a major structural lineament which closely follows a small circle about 45° of arc of radius which encloses Galileo Regio, and that a smaller amount of distributed shear occurred across an adjacent 500‐ to 1500‐km‐wide band. There is also morphologic evidence for zones of minor right‐lateral shear between central and southern Marius Regio and between Barnard and Nicholson Regio, and for a zone of minor left‐lateral shear in Nun Sulci. Stratigraphic relations indicate that any major shear offsets occurred before and during the earliest stages of grooved terrain formation, before most grooves had formed and before virtually all light material had been emplaced. However, regionally dominant groove orientations are generally consistent with orientations expected for transtensional features: Either shear strain or the shear's driving mechanism may have had a long‐term effect on patterns of deformation of Ganymede's lithosphere.
Fault plane solutions for the 1984 Morgan Hill, California, Earthquake Sequence: Evidence for the state of stress on the Calaveras Fault
Authors: Oppenheimer David H. , Reasenberg Paul A. , Simpson Robert W.
Published: February 2008

Fault plane solutions for 946 aftershocks of the April 24, 1984, Morgan Hill, California, M6.2 earthquake reveal a pattern of complex faulting within a 10‐km‐wide zone surrounding the Calaveras fault. The fault plane solutions fall into three groups: strike‐slip mechanisms located along the Calaveras fault with north‐northwest striking dextral slip planes nearly parallel to the fault, strike‐slip mechanisms located northeast of the Calaveras fault with north striking dextral slip planes, and reverse mechanisms located southwest of the Calaveras fault with northeast or southwest dipping slip planes. The average azimuth of P axes for aftershocks located on the Calaveras fault is N10°±1°E. In contrast, the average azimuths of P axes for aftershocks northeast and southwest of the Calaveras fault are N49°±7°E and N37°±3°E, respectively. By assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field, we are able to infer from the observed average slip directions an orientation for σ1, the axis of maximum horizontal compression, 63°–80° from the local strike of the Calaveras fault. Such a high‐angle orientation of σ1 is incompatible with the classical Andersonian model of strike‐slip faulting but is consistent with an alternate model in which a weak Calaveras fault is driven by plate motion slightly convergent with respect to the San Andreas fault system in central California. If this inferred high‐angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 Morgan Hill main shock, the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions. The orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the Coulomb failure criterion. Areas with predicted increases in the Coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity. The invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 MPa. In addition to explaining a preponderance of the seismic observations of the aftershock sequence, the presence of a predominantly fault‐normal compressive regional stress field accounts for the contemporary development of folds, reverse faults, and topographic relief observed throughout the central California Coast Ranges.
Short‐period body wave constraints on properties of the Earth's inner core boundary
Authors: Cummins Phil , Johnson Lane R.
Published: February 2008

In this study we model short‐period waveform data in an attempt to constrain the P and S velocity structure at the Earth's inner core boundary (ICB). The data set consists of recordings from 10 events in the South Pacific, and the data selection criteria as well as methods of analysis were designed to avoid problems with receiver response often associated with short‐period waveform data. These data are modeled using a calculation technique that facilitates the consideration of a wide variety of models for the ICB. Results suggest that Qα in the inner core has the frequency dependence proposed by Doornbos (1983). The data determine the P velocities above and below the ICB to within a trade‐off that is well constrained by the data. For example, with a P velocity structure above the ICB given by PREM, the P velocity below the ICB is 11.03 ± 0.03 km/s. Similarly, for the data analyzed here the estimate of the S velocity at the top of the inner core trades off with the estimate of Qα, but we use the Qα model of Doornbos (1983) to estimate the jump in S velocity to be 3 ± 1 km/s.
Volcanomagnetic effects observed on Piton de la Fournaise Volcano (Réunion Island): 1985–1987
Authors: Zlotnicki J. , Le Mouel J. L.
Published: February 2008

A permanent network of four to six telemetered magnetic stations has been installed on Piton de la Fournaise Volcano in October 1985 and 1986. Eight eruptive sequences have occurred since then, some of which affected only the summit whereas others affected the whole massif. Volcanomagnetic variations are observed with each of these volcanic events. The geomagnetic field intensity is measured simultaneously every minute at each station, with a precision of 0.25 nT (nanotesla). The main results are the following. Volcanomagnetic variations observed with eruptions have amplitudes of several nanoteslas to more than 10 nT with characteristic time durations ranging from l min or less to several months. A good correlation between the magnetic variations recorded at the different stations is observed. Spatial and temporal properties of the Volcanomagnetic field appear to be controlled by the features of the eruptions. Several days before the beginning of the December 1985; March, July, November–December 1986; and January 1987 eruptions, changes in the intensity differences are observed with amplitudes of a few nanoteslas (up to 5 nT) at stations located close to the active part of the rift zone. Variations are also observed in the most remote stations for larger eruptions in March 1986, November–December 1986, and January 1987. Correlations between magnetic variations, ground deformations, and seismic activity have been observed for the December 1985, March 1986, November–December 1986, and January 1987 events. Three main mechanisms could account for the observed Volcanomagnetic variations: piezomagnetic effects, electrokinetic effects, and electrical resistivity changes since variations in the stress field in the surroundings can modify the induced and natural magnetizations of magnetized rocks, the circulation of water through the interconnected pore network (microcracks, faults, aquifers), and also the electrical resistivity of geological structures through the modification of the properties of the pore network.
A new constraint on Saturn's zonal gravity harmonics from Voyager observations of an eccentric ringlet
Authors: Nicholson Philip D. , Porco Carolyn C.
Published: February 2008

The first experimental determination of Saturn's J6 is derived from the simultaneous solution of three linear relations involving Saturn's zonal gravity harmonics. Two of these relations arise from the combined analysis of Pioneer 11 Doppler tracking and satellite secular rate data by Null et al. (1981). The third is the result of the analysis of Voyager 1 and 2 observations of the narrow eccentric ringlet at 1.2908Rs by Porco et al. (1984). On the assumption that the eccentricity of this ringlet is forced by the satellite Titan, we redetermine the location of the Titan apsidal resonance to an accuracy of ±13 km, which in turn provides a strong constraint on Saturn's zonal gravity harmonics. Combining this constraint with the results of Null et al. and adopting a priori values for J8, J10, and J12 based on interior models, we obtain new estimates for the first three zonal gravity coefficients: J2 = (16,297±18) × 10−6, J4 = (−910±6l) × 10−6, and J6 = (107 ± 50) × 10−6, all normalized to an equatorial reference radius, Rs = 60,330 km. Linearized about these nominal values, the Titan ringlet constraint alone may be written in the form J2 − 1.507 (J4 + 910 × 10−6) + 1.586 (J6 − 107 × 10−6) − 1.430 (J8 + 10 × 10−6) + 1.181 (J10 − 2 × 10−6) − 0.922 (J12 + 0.5 × 10−6) + 0.692J14 − 0.505 J16 = (16,297 ±12) × 10−6.
Marine gravity of the southern ocean and Antarctic margin from Geosat
Authors: Sandwell David T. , McAdoo David C.
Published: February 2008

In November of 1986 the U.S. Navy satellite Geosat began collecting unclassified (gravity) altimeter data as part of its exact repeat mission (ERM). For national security reasons the Geosat orbit was arranged so that it closely follows the Seasat satellite altimeter ground track. However, there are two advantages of the Geosat data over the Seasat data. First, because of improvements in altimeter design, Geosat profiles are about 3 times more precise than Seasat profiles. This corresponds to an accuracy of 2–3 μrad (i.e., 2–3 mGal) for wavelengths greater than 20 km. Second, the Geosat altimeter data were collected when the Antarctic ice coverage was minimal (February 1987 to March 1987), while Seasat was only active during an Antarctic winter (June 1978 to September 1978). These new data reveal many previously uncharted seamounts and fracture zones in the extreme southern ocean areas adjacent to Antarctica. Seven large age‐offset fracture zones, apparent in the Geosat data, record the early breakup of Gondwana. Finally, the new data reveal the detailed gravity signatures of the passive and active continental margins of Antarctica. These data are an important reconnaissance tool for future studies of these remote ocean areas.
Dipole/quadrupole family modeling of paleosecular variation
Authors: McFadden P. L. , Merrill R. T. , McElhinny M. W.
Published: February 2008

Contrary to previous claims that the VGP scatter and its latitude variation deduced from the present field are atypical of those for the past 5 m.y., it is shown that it is in fact typical when correctly calculated. A simple and effective model for the paleosecular variation of lavas is developed by using this important observation together with the dipole and quadrupole families from dynamo theory. The model predicts a latitudinal variation of VGP scatter, S, given by
, where λ is the latitude and A and B are parameters that depend on the contribution to the scatter from the dipole family and the quadrupole family, respectively. For the present field these parameters are estimated (95% confidence limits) as A = 0.24±0.02 and B = 13.5°±0.6°, and for the past 5 m.y. as A = 0.23±0.04 and B = 12.8°±0.6°.
Estimation of lava extrusion and magma production rates for two flood basalt provinces
Authors: Baksi Ajoy K.
Published: February 2008

Utilizing the statistical approach of Cox and Dairymple (1967), it is shown that the geomagnetic polarity time scale of Harland et al. (1982) may contain errors of 0.85–1.0 m.y. for middle‐late Miocene times. A revised time scale, based on the work of McDougall et al. (1984), is correlated with the magnetostratigarphic units of the Columbia River Basalts (United States). Lava extrusion rates reached a peak of ∼275 km3/kyr for a period of ∼0.5 m.y. around 15.8 Ma during the Columbia volcanic episode. It is suggested that even higher rates of lava extrusion may have occurred in the Deccan Province (central India) ∼65 Ma. A simple energy budget calculation indicates that the Reunion hotspot could provide enough heat to melt the total volume of rock needed to produce the Deccan Traps during the ∼5 m.y. it would have taken the subcontinent to traverse the hotspot.
Capture of an olivine micrometeoroid by spacecraft in low‐Earth orbit
Authors: Rietmeijer Frans J. M. , Blanford George E.
Published: February 2008

Particulates associated with an impact feature in the solar maximum satellite MEB thermal control blanket are identified as magnesium‐rich, Mg/(Mg + Fe2+) = 0.66 ‐ 0.83, olivine and confirm that the impact feature was caused by an olivine micrometeoroid. At a relative impact velocity of ∼7 km s a considerable portion of the micrometeoroid was lost due to back splattering. Analytical electron microscope analysis of recovered particulates show several degrees of mineralogical and chemical degradation such as mechanical fragmentation, melting, loss of sulfur and iron, and formation of Fe, Ni grains. The results indicate that recovery of information is sufficient to establish the character of the impacted micrometeoroid.
Arc volcanic rocks characterize the landward slope of the Philippine Trench off northeastern Mindanao
Authors: Bloomer Sherman H. , Fisher Robert L.
Published: February 2008

Five dredge hauls from the landward flanks (9300–3600 m) of the Philippine Trench at 10°30′N, northeast of Mindanao, recovered densely plagioclase‐clinopyroxene phyric basalt, basaltic andesite, and andesite, with small amounts of diabase. These rocks are all transitional from an arc‐tholeiitic to arc‐calcalkaline volcanic series and have low Ni and Cr (10–30 and 30–70 ppm, respectively), low TiO2 (0.7–1.2%), and high incompatible element concentrations (Sr, 260–530 ppm; Ba, 170–410 ppm). A single dredge on the offshore slope of the trench included aphyric tholeiitic basalt similar to N‐type ocean ridge basalts sampled from other locations in the West Philippine Basin (TiO2, 1.0–1.2%; Ni, 70–80 ppm; Cr, 130 ppm; Ba, 2–10 ppm; Sr, 70–85 ppm). The landward samples are distinct from the offshore samples and are identical in texture and composition to arc volcanic rocks from southeast and northeast Mindanao. The Mindanao rocks are interpreted to be part of the Samar‐East Mindanao arc, which collided with the Central Mindanao‐Sangihe island arc in the late Oligocene/early Miocene. The volcanic rocks exposed on the landward slope of the Philippine Trench are, apparently, part of the eastern margin of the Samar‐East Mindanao arc. After the collision the Philippine Trench was initiated immediately adjacent to the arc massif; there is no evidence of any accretion of material from the subducted Philippine Sea crust. The polarity of the Samar‐East Mindanao arc is unknown, but there is no evidence of either a deformed volcanoclastic apron or an older forearc terrane along the trench. Such materials must have been limited in extent prior to collision, have foundered and been subducted as the trench was initiated, or have been faulted or rotated so that they are not locally exposed.
Three‐dimensional infinite Prandtl number convection in one and two layers with implications for the Earth's gravity field
Authors: Cserepes L. , Rabinowicz M. , Rosemberg‐Borot C.
Published: February 2008

The three‐dimensional planform of infinite Prandtl number convection is studied in one and two layers, with a numerical code which combines spectral and finite difference methods. The equations are solved in rectangular cavities supposing mirror symmetry on the sides. The one‐layer experiments serve mainly for testing the code. These experiments reveal the outstanding importance of the initial and boundary conditions in the evolution of the flow pattern. The main emphasis of this paper is on two‐layer convection models, with implications for the Earth's mantle and its gravity anomaly field. In this case, the interface between the two layers is fixed at a given depth, and coupling of the two circulations is ensured by the continuity of velocities and horizontal stress across the interface. The two‐layer cases have been run with Rayleigh numbers up to 130 times the critical value, the thickness of the lower layer being twice that of the upper one. The ratio of the lower layer viscosity to the upper one is gradually increased. When this ratio is small (between 1 and 10), viscous coupling prevails, and bimodal or square cells formed in the lower layer induce similar, but counterrotating cells in the thin upper layer. When the viscosity ratio is high (several hundred), thermal coupling becomes dominant: again, the convective structure of the lower layer is more or less duplicated in the upper layer, but now the flows of both layers rotate in the same sense. In these situations the gravity anomalies are positively correlated with the topography of the upper surface. When the viscosity ratio is moderate (e.g., 25), the flow direction of the rolls in the thin upper layer becomes perpendicular to that observed in the lower layer, leading to a balance between the conflicting effects of the viscous and thermal couplings. This allows the upper rolls to have an aspect ratio close to 1. In this case, gravity is positevely correlated with deflection of the interface, but no more correlation can be seen with surface topography, which is determined by small‐scale flow of the upper layer. These results suggest that the gravity and bathymetry anomalies of the Earth should be either positively correlated or not correlated at all. This has to be contrasted with earlier findings of two‐dimensional modeling [Cserepes and Rabinowicz, 1985] which allow the possibility of having anticorrelation between gravity and topography.
Magnetic mineralogy in an Archean crustal cross section: Implications for crustal magnetization
Authors: Shive Peter N. , Fountain David M.
Published: February 2008

A traverse across Precambrian rocks from the Kapuskasing uplift in Ontario provided 31 samples from a crustal cross section that is now exposed at the surface. About half the samples are dominantly paramagnetic. Magnetite, accompanied in a few samples by minor amounts of pyrrhotite, is the only significant magnetic mineral in the other half; magnetic susceptibilities of these samples are constant between room temperature and about 540°C, above which temperature they drop rapidly to zero. Magnetic susceptibility does not depend on rock type, apparent original depth in the crust, or degree of metamorphism. The average induced magnetization of the most magnetic units in this section is less than 1 A/m, which is much lower than the source requirements for the deep crust deduced from aeromagnetic and satellite magnetometer surveys.
Three‐Dimensional Gravity Modeling of the Geologic Structure of Long Valley Caldera
Authors: Carle Steven F.
Published: February 2008

A 48‐mGal gravity low coincides with Long Valley caldera and is mainly attributed to low‐density caldera fill. Gravity measurements by Unocal Geothermal have been integrated with U.S. Geological Survey data, vastly improving gravity station coverage throughout the caldera. A strong regional gravity trend is mainly attributed to isostasy. A “best fitting” (based on regional control of basement densities) Airy‐Heiskanen isostatic model was used for the regional correction. A three‐dimensional, multiple‐unit gravity modeling program with iterative capabilities was developed to model the residual gravity. The density structure of Long Valley caldera and vicinity was modeled with 22 discrete density units, most of which were based on geologic units. Information from drill hole lithologies, surface geology, and structural geology interpretations constrain the model. Some important points revealed by the three‐dimensional gravity modeling are that (1) the volume of ejected magma associated with the Bishop Tuff eruption is greater than previously thought, (2) the caldera structure is strongly influenced by precaldera topography and the extensions of major, active faults, (3) the main west ring fracture is coincident with the Inyo Domes‐Mono Craters fracture system, (4) a relatively low‐density region probably underlies the caldera, and (5) a silicic magma chamber may underlie Devils Postpile.
Vapor‐Dominated Zones Within Hydrothermal Systems: Evolution and Natural State
Authors: Ingebritsen S. E. , Sorey M. L.
Published: February 2008

Three conceptual models illustrate the range of hydrothermal systems in which vapor‐dominated conditions are found. The first model (model I) represents a system with an extensive near‐vaporstatic vapor‐dominated zone and limited liquid throughflow and is analogous to systems such as The Geysers, California. Such systems can evolve within low‐permeability barriers without changes in boundary conditions or rock properties, given an adequate supply of heat. Their scarcity in nature may be due to the need for a long‐lived, potent heat source and for a low‐permeability aureole that remains intact for significant lengths of time. Models II and III represent systems with significant liquid throughflow and include steam‐heated discharge features at higher elevations and high‐chloride springs at lower elevations, connected to and fed by a single circulation system at depth. In model II, as in model I, the vapor‐dominated zone has a near‐vaporstatic vertical pressure gradient and is generally underpressured with respect to local hydrostatic pressure. The vapor‐dominated zone in model III is quite different, in that phase separation takes place at pressures close to local hydrostatic and the overall pressure gradient is near hydrostatic. A relatively large number of high‐temperature systems in regions of moderate to great topographic relief are similar to either model II or model III; however, in most cases there are insufficient data to establish a single preferred model.
Erosional furrows formed during the lateral blast at Mount St. Helens, May 18, 1980
Authors: Kieffer Susan Werner , Sturtevant Bradford
Published: February 2008

Nearly horizontal, quasi‐periodic erosional features of 7‐m average transverse wavelength and of order 100‐m length occur in scattered locations from 3.5 to 9 km from the crater at Mount St. Helens under deposits of the lateral blast of May 18, 1980. We attribute the erosional features to scouring by longitudinal vortices resulting from flow instabilities induced by complex topography, namely, by streamline curvature in regions of reattachment downstream of sheltered regions, and by the cross‐flow component of flow subparallel to ridge crests. The diameter of the vortices and their transverse spacing, inferred from the distance between furrows, are taken to be of the order of the boundary layer thickness. The inferred boundary layer thickness (≈14 m at 9 km from the source of the blast) is consistent with the running length from the mountain to the furrow locations. By using knowledge of ablation patterns on bodies and lofting of dust in high‐speed flow, we are able to infer some features of the flow field within the blast. Within the furrows the erosion rate was of the order of 9 kg m−2 s−1, about 4 times greater than that expected from laboratory data obtained in flow free of longitudinal vortices. The orientation of furrows induced by the cross‐flow instability can be used to measure the upwash angle and estimate the flow Mach number: at the central ridge of Spirit Lake the Mach number is inferred to have been about 2.5, and the flow velocity approximately 235 m/s. The similarities and differences between the furrows reported here and channels observed at other volcanoes are discussed.
Displacement calculations from geodetic data and the testing of geophysical deformation models
Authors: Segall P. , Matthews M. V.
Published: February 2008

The inverse problem of estimating displacements on the Earth's surface from repeated geodetic measurements is usually mixed‐determined. Absolute displacements are underdetermined if the network is not tied to an external reference frame, but relative displacements may be overdetermined if the network is geometrically redundant. Least squares analysis of underdetermined linear systems may be carried out by optimizing some criterion of solution desirability over the class of solutions that fits the data. We review solution by constrained optimization, its connection with generalized matrix inverses, and currently used solution methods, such as the “inner coordinate” and “outer coordinate” solutions. In the context of fitting deformation models to geodetic data, we introduce a “model coordinate” solution that fixes the indeterminate components of the displacement field by minimizing the difference between the computed displacements and those predicted by a geophysical model. This gives an appropriate solution for model assessment. We also propose a means to exploit the overdetermination of some geodetic data by decomposing the model residuals from a geometrically redundant network into two orthogonal components; the “pure error,” which contains only observation errors, and the “lack of fit,” which may contain both observation errors and model misfit. Like the model coordinate displacements, this decomposition is useful in model assessment. Under certain statistical assumptions, the relative sizes of the pure error and lack of fit may be compared to test for significant model misfit.
Static compression and equation of state of CaO to 1.35 Mbar
Authors: Richet Pascal , Mao Ho‐Kwang , Bell Peter M.
Published: February 2008

The room temperature static compression of CaO has been measured in a diamond anvil cell to 1.35 Mbar. The compression of the Bl phase and the volume change at the Bl to B2 transition agree with previous diamond cell results. The B2 phase has been observed down to 527 kbar and seems slightly more compressible at room temperature than deduced from shock wave experiments. Eulerian as well as semiempirical equations of state account well for the data and yield similar bulk moduli for both phases as a function of pressure. Equation‐of‐state parameters are Ko = 1.11 ± 0.01 and 1.3 ± 0.2 Mbar and Ko′ = 4.2 ± 0.2 and 3.5 ± 0.5 for the Bl and B2 phases, respectively, and Vo = 14.8 ± 0.2 cm3/mol for the B2 phase. For a pressure of 550 kbar, at which no data extrapolations are required, one finds K = 3.04 and 2.96 Mbar with K′ values of about 3.0 and 2.5 for the Bl and B2 phases, respectively. Recent theoretical structure calculations predict room temperature compressions in fair agreement with the static results.
Paleomagnetism of mesozoic kimberlites in southern Africa and the cretaceous apparent polar wander curve for Africa
Authors: Hargraves R. B.
Published: February 2008

Including those previously published, good paleomagnetic data have been obtained from 48 of 51 sites in 28 separate kimberlites. Radiometric dates (by one or more methods) are available for 17 of these bodies and range from 81 to 145 Ma, with maxima around 90 and 120 Ma. These ages correlate with geochemical contrasts distinguishing what have been designated Group I (younger) and II (older) kimberlites [Smith et al., 1985b]. Stable magnetization appears to be associated with magnetite developed deuterically, after emplacement, but in most bodies, secular variation has not been completely averaged in the remanence acquired. The distribution of “VGPs” calculated is distinctly, but not sequentially, streaked yet correlates broadly with the older and younger petrochemical types. Mean North poles calculated on this basis yield KP1 (N = 14), 81–100 Ma, 64.1°N, 226.1°E, A95 = 5.2 and KP2 (N = 10), 113–145 Ma, 47.6°N, 269.9°E, A95 = 9.7. With the exception of one undated pipe (Meltonwold), which was not included in the averages, and the melilititic Male fissure at Swartruggens (in Group II average), all the kimberlites sampled are normally magnetized. Including these new data, 10 of 17 published African Cretaceous poles meet acceptability criteria specified, and define an apparent polar wander (APW) curve from ∼130 to 75 Ma. The curve appears to consist of two segments, trending latitudinally before ∼100 Ma, and meridionally thereafter. The latter trend is evident even in the distribution of the individual KP1 poles, and may be related to the true polar wander speculated by Gordon and Livermore (1987) to have occurred in that interval.