bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal core‐mantle interactions and time‐averaged paleomagnetic field
Authors: Gubbins David
Published: February 2008

Recent studies of historical geomagnetic secular variation and lower mantle seismic velocity suggest the Earth's magnetic field is controlled by lateral variations in mantle temperature. In this paper the average expected magnetic field pattern is compared with the time‐averaged field obtained from paleomagnetic pole positions from the last 5 m.y. The historical record suggests that the time‐averaged inclination anomalies are negative (“far‐sided”) in the Atlantic region north of 30°S and the west Pacific and positive in the South Atlantic, extreme North and South Pacific, and east Pacific. The pole positions give positive inclination anomalies in the South Atlantic and extreme North Pacific, and negative anomalies elsewhere, in agreement with the theory except for the east Pacific. This suggests that the historical record is not long enough to give an accurate picture of long term secular change in the eastern Pacific but that elsewhere the time‐averaged field of paleomagnetism is predicted satisfactorily. The preponderance of “far sidedness” in the paleomagnetic record is a consequence of the present distribution of temperature at the base of the mantle rather than a fundamental property of the geodynamo, and it may therefore change on the long time scale of mantle convection. In the historical record declinations average to negative values in the Atlantic hemisphere and positive values in the Pacific which, if typical of longer‐term behavior, could explain the preponderance of positive declinations for reversed polarity, which are mainly in the Atlantic hemisphere.
Tidal triggering of earthquake swarms at Kilauea Volcano, Hawaii
Authors: Rydelek Paul A. , Davis Paul M. , Koyanagi Robert Y.
Published: February 2008

Between 1967 and 1983, four earthquake swarms occurred on Kilauea Volcano, Hawaii, with durations ranging from 68 to 156 hours. Plots of the number of events per hour show a remarkable modulation having diurnal and semidiurnal periodicities. We test the hypothesis that these fluctuations are tidally triggered. A correlogram showing the correlation coefficient between the swarm series and tidal strain oriented at 20° intervals and phase shifted at 1‐hour intervals gives the orientation and phase of the tidal strain most closely resembling the swarm plots. Maximum correlation occurs at different orientations and phases for three of the four swarms, which occur in tectonically different parts of the volcano. Two of the swarms which occur in the same region can be fit with a common tidal forcing function even though they occurred more than a year apart. The discrete Fourier transform of the swarm data shows a dominance of diurnal energy over semidiurnal energy, compared with the opposite dominance for the transform of the theoretical tidal function at the times of the swarms. We checked whether the diurnal dominance might be generated by cultural noise by raising the threshold magnitude of the earthquakes used without noticeably affecting the results. Also the phase of the correlation varies from swarm to swarm relative to local Hawaii time. Diurnal dominance remains not completely understood, but tidal influences appear to be the best explanation for the modulation of the activity. These observations limit the rate of tectonic stress change in the rift zones of Kilauea to less than the tidal stress rate.
Relationship between gases and volcanic activity of Mount Etna in 1986
Authors: Le Cloarec M. F. , Pennisi M. , Ardouin B. , Le Roulley J. C. , Lambert G.
Published: February 2008

The long‐lived 222Rn decay products 210Pb, 210Bi and 210Po have been monitored in the plumes of several vents at Mount Etna (Sicily) from May to October 1986. The results show that the four main craters of this volcano emit gases whose compositions are different from each other. The 210Bi/210Pb ratios for the plumes have similar mean values, (close to 25), which correspond to a degassing time of 1.5 to 2.7 days, according to the model of Lambert et al. (1985/86). In contrast, 210Po/ 210Pb ratios have very different mean values in each plume: 35 at the Voragine crater, 20 at the Bocca Nuova crater, and 14 at the South East crater. These figures enable us to calculate proportions of deep magma of 50%, 29% and 19% in the degassing cells of these craters respectively. Moreover, the SE crater appears to be a secondary degassing vent, not directly related to the main magma reservoir. The evolution of these ratios has been related to variations in volcanic activity.
A time series of vent fluid compositions from 21°N, East Pacific Rise (1979, 1981, 1985), and the Guaymas Basin, Gulf of California (1982, 1985)
Authors: Campbell Andrew C. , Bowers Teresa Suter , Measures Chris I. , Falkner Kelly Kennison , Khadem Mitra , Edmond John M.
Published: February 2008

A time series study of four hot spring fields at 21°N on the East Pacific Rise demonstrates that the solution compositions of mid‐ocean ridge hydrothermal vents are stable on a decadal time scale. The Ocean Bottom Seismometer (OBS), National Geographic Smoker (NGS), and South West (SW) vents show no significant changes in the major element composition of their end‐member fluids since 1979. Two of the vents, OBS and SW, have maintained exit temperatures near 350°C since 1979. The NGS vent has remained close to 270°C since 1981 after cooling from 350°C in 1979. Only one vent, Hanging Gardens, shows important changes in solution chemistry from 1981 to 1985. In 1985 this vent appears to have cooled ∼10°C relative to the other vents. The depth of the reaction zone, as determined by quartz geobarometry, has increased by about 1 km at this vent. The large fluxes of compositionally stable fluids from these vents cannot be explained by transient conditions and unique circumstances. Rather, the constancy of small interfield variations in solution composition is more consistent with control by rock buffering. A 4‐year time series study of hydrothermal vents in the Guaymas Basin also shows no significant changes in the major element composition of the end‐member fluids. Interfield differences in solution chemistry have been maintained from 1982 to 1985. This phenomenon is best explained in terms of an equilibrium model.
High‐temperature hydrothermal alteration of ultrmafic cumulates from the base of the sheeted dikes in the Josephine ophiolite, NW California
Authors: Kimball Karen L.
Published: February 2008

Ultramafic cumulate pods occur locally just below the base of the sheeted dike complex in the Josephine ophiolite in north‐western California. These ultramafic rocks occur with metagabbros, and both are extensively intruded by mafic dikes. The ultramafic cumulates contain evidence of high‐temperature hydrothermal alteration (early alteration possibly above 900°C), which continued to progressively lower temperatures (<350°C). This hydrothermal alteration has been partially obscured by later low‐grade regional metamorphism. Petrographic evidence for the hydrothermal alteration includes (1) hydration of primary clinopyroxene to hornblende, (2) supersolvus secondary hornblende, (3) subsolvus coexisting hornblende + actinolite and actinolite + chlorite, (4) alteration of hornblende to chlorite, (5) alteration of orthopyroxene and olivine to serpentine, and (6) serpentine veining. The composition of the altering fluid appears to change with temperature.
Effects of source depth on near‐source seismograms
Authors: Flynn Elizabeth C. , Stump Brian W.
Published: February 2008

Source depth effects are examined for five 115‐kg trinitrotoluene (TNT) explosions buried at depths ranging from the optimum cratering depth of 1.8 m to the fully contained depth of 11.5 m. Data were recovered at near source ranges from 17 to 228 m. The waveforms are dominated by P and SV‐Rayleigh energy. Depth effects are evident in the increase of P to SV‐Rayleigh amplitude ratios and in the twofold increase of high‐frequency energy for the deeper sources. Theoretical propagation depth effects are modeled by Green's functions calculated for a velocity gradient which approximates the velocity structure of the experimental site. The effects of depth on the explosion source function are predicted using the scaling laws of Mueller and Murphy (1971). These models did not reproduce the observed twofold increase of high‐frequency energy. The discrepancy between the model and observations is attributed to increased coupling of high frequency P wave energy for fully contained sources. Energy calculations confirm that the shallowest event coupled 40% and the fully contained event 80% of the total seismic energy into the P wave. Source coupling efficiencies ranged from 0.7∼1.0% for the near surface source to 1.5∼2.9% for the fully contained explosion.
Alteration in the Madera limestone and Sandia formation from core hole VC‐1 Valles Caldera, New Mexico
Authors: Keith Terry E. C.
Published: February 2008

Core hole VC‐1 penetrated the southwestern ring fracture zone of the 1.1 Ma Valles caldera and at a depth of 333 m intersected the top of the Paleozoic section including the Abo Formation, Madera Limestone, and Sandia Formation, reaching a total depth of 856 m. The Paleozoic rocks, which consist of thin‐bedded limestone, siltstone, mudstone, sandstone, and local conglomerate, are overlain by volcanic rocks of the caldera moat that are less than 0.6 Ma. Diagenetic and at least three hydrothermal alteration stages were identified in the Madera Limestone and Sandia Formation. Diagenetic clay alteration was pervasive throughout the sedimentary rocks. Volcanic activity at 16.5 Ma and continuing through the formation of the Valles caldera resulted in high thermal gradients, which caused recrystallization of diagenetic clay minerals. Interstratified smectite‐illite is the most diagnostic clay mineral throughout the section; structurally, the illite component in the ordered interstratified illite‐smectite changes gradationally from 70% at the top of the Madera Limestone to 95% at the base of the section in the Sandia Formation. Pyrite that occurs as small clots and lenses as well as finely disseminated is interpreted as being of diagenetic origin, especially in organic‐rich beds. Low permeability of much of the Paleozoic section precluded the deposition of hydrothermal minerals except in fractures and intergranular space in some of the more permeable sandstone and brecciated horizons. Three stages of hydrothermal mineral deposition are defined. Stage I is widespread and includes mainly chlorite, calcite, pyrite, and interstratified smectite‐illite that was formed prior to caldera development at temperatures approximating 200°C. Stage II is characterized by quartz, sericite, and scarce sulfides deposited locally by fluids at approximately 275°C shortly before or at the time of early caldera‐related volcanism. The Stage III hydrothermal event, associated with formation of the Valles caldera, caused complex brecciation accompanied by deposition of phengite and molybdenite at temperatures locally approaching 300°C mostly in rocks underlying the Madera Limestone. During the present drilling program, thermal aquifers were encountered at 483 m (113°C), 533 m (125°C), and 845 m (184°C), very close to the Stage II and III mineralized zones; however, measured temperatures of the present hydrothermal system are much lower than during the main hydrothermal events. Good permeability has been maintained in these three levels for a long time while the rest of the Madera Limestone and Sandia Formation was sealed by hydrothermal mineral precipitation.
Light lithophile element (boron, lithium) abundances in the Valles Caldera, New Mexico, VC‐1 core hole
Authors: Higgins Michael D.
Published: February 2008

Boron and lithium abundance distributions are very different in the sedimentary and volcanic sections of the VC‐1 core hole, Valles caldera, New Mexico. In the sedimentary section the wide variations in boron and lithium abundances probably reflect both diagenesis and introduction of these elements in early hydrothermal fluids. Late, higher‐temperature fluids appear to have leached boron and lithium from some rocks adjacent to fault zones and aquifers. The correlation of boron with lithium suggests that these elements behave in similar ways during diagenesis and hydrothermal alteration. The small variation of boron and lithium abundances in most of the vitreous volcanic rocks from the upper part of the core implies that their source was relatively homogeneous. Following primary devitrification, boron appears to have been stable, but lithium was mobile. Boron may have been stabilized by the development of boron‐bearing submicroscopic phenocrysts in the lavas, or boron may be less soluble than lithium in relatively cool groundwater.
Slip vectors and fault mechanics in the Makran Accretionary Wedge, southwest Pakistan
Authors: Platt John Paul , Leggett Jeremy K. , Alam S.
Published: February 2008

The Makran is a broadly east–west trending arcuate accretionary wedge currently forming where the oceanic floor of the Gulf of Oman is being subducted north‐northeastward beneath the south Asian margin. Two traverses across the Makran Coast Ranges, extending about 80 km inland from the coastline (which lies about 100 km north of the wedge front), show that the structure of this region is dominated by large back‐rotated south directed thrust faults and associated folds. Related structures include duplexes, fault gouge containing Reidel shears, and melange along fault zones. These structures are cut by later north directed backthrusts, out‐of‐sequence south directed thrusts, and apparently conjugate sets of NNE trending sinistral and NW trending dextral strike‐slip faults. Slip vectors were determined from slickenlines, fiber lineations, gouge fabrics, and Riedel shears. Vectors from early thrusts and bedding‐parallel slip surfaces, corrected for tectonic rotation, form a broad south directed concentration lying between the normal to the regional strike (varying from 160° to 180°) and the present plate convergence vector (200°). Locally, the concentration is bimodal. This pattern may represent the interaction between plate forces and body forces in the accretionary wedge. The late thrusts have widely variable slip vectors and stair‐step off the early thrusts. They overlap in orientation and slip vector with gently dipping sinistral strike‐slip faults, suggesting that they also stair‐step in plan off the latter. The steep strike‐slip faults represent the final tectonic stage, involving minor N‐S shortening and longitudinal extension. These three stages probably occurred in sequence in any one area but migrated diachronously southward as the wedge grew.
Application of pair analysis statistics to aftershocks of the 1984 Morgan Hill, California, Earthquake
Authors: Eneva Mariana , Pavlis Gary L.
Published: February 2008

We apply statistical analysis based on pairs of earthquakes to study spatial and temporal patterns of earthquake distribution in the aftershock zone of the 1984 Morgan Hill, California, earthquake (M = 6.2). We examine 6 years of preshock and 23 months of aftershock data, using frequency distributions of earthquake pairs. The nonrandomness in the observed spatial and temporal distribution of pairs is studied by comparison with simulated expected distributions. The significance of departures from the expected distributions is appraised by using tolerance limits that account for the scatter of the single trial expected frequencies. We find that the eathquake spatial distribution is nonuniform in two major ways. (1) Short‐distance clustering (distances less than 7 km) occurs in both the preshock and aftershock period. It is more pronounced and covers larger zones during the aftershock sequence. (2) A long‐distance nonuniformity is also observed (distances greater than 11 km) caused by concentration of events in two areas separated by a gap in earthquake activity. This feature is present in the preshock and early aftershock period but disappears in the late aftershock sequence. The distribution of time intervals between preshocks does not indicate any substantial nonrandom features. In contrast, the temporal distribution of aftershocks, although dominated by the classic power law decay in time, suggests the possible existence of characteristic time intervals that increase with magnitude and time after the mainshock.
Three‐dimensional P velocity structure of the crust below Newberry Volcano, Oregon
Authors: Stauber Douglas A. , Green Susan M. , Iyer H. M.
Published: February 2008

Utilizing teleseismic P residuals, we have detected a column of high P velocity material extending from within 10 km of the surface below the summit of Newberry Volcano, Oregon, to midcrustal depths near 25 km. We interpret this column to be the expression of a swarm of predominantly subsolidus gabbroic sills and dikes which were intruded as the volcano was built. The high P velocities observed below the volcano severely limit the size of magma chambers which could presently exist in the crust below Newberry Volcano. Those possible include a few percent of partial melt distributed through large volumes of a mafic intrusion zone in the midcrust; a few, smaller, higher melt fraction zones in the midcrust with dimensions less than 6 km and whose aggregate volume is only a few percent of enclosing volumes of 200 km3; small magma bodies with dimensions of a few kilometers located within the upper 10 km of the crust; or a mafic, crystal‐rich magma of arbitrary dimensions located in the upper few km. The low P velocities detected in the upper 4 km beneath the center of the summit caldera may be partially caused by a magma chamber in the second of these catagories.
Gravity model studies of Newberry Volcano, Oregon
Authors: Gettings M. E. , Griscom Andrew
Published: February 2008

Newberry Volcano, a large Quaternary volcano located about 60 km east of the axis of the High Cascades volcanoes in central Oregon, has a coincident positive residual gravity anomaly of about 12 mGals. Model calculations of the gravity anomaly field suggest that the volcano is underlain by an intrusive complex of mafic composition of about 20‐km diameter and 2‐km thickness, at depths above 4 km below sea level. However, uplifted basement in a northwest trending ridge may form part of the underlying excess mass, thus reducing the volume of the subvolcanic intrusive. A ring dike of mafic composition is inferred to intrude to near‐surface levels along the caldera ring fractures, and low‐density fill of the caldera floor probably has a thickness of 0.7–0.9 km. The gravity anomaly attributable to the volcano is reduced to the east across a north‐northwest trending gravity anomaly gradient through Newberry caldera and suggests that normal, perhaps extensional, faulting has occurred subsequent to caldera formation and may have controlled the location of some late‐stage basaltic and rhyolitic eruptions. Significant amounts of felsic intrusive material may exist above the mafic intrusive zone but cannot be resolved by the gravity data.
Global distribution of seamounts from Seasat profiles
Authors: Craig Claire H. , Sandwell David T.
Published: February 2008

Bathymetrie profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins, but only a small fraction of the seafloor has been sampled by ships. At the present exploration rate it will take several centuries to map significant portions of the seafloor topography. Satellite altimetry, which maps the topography of the equipotential sea surface, is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years. Using a model of a Gaussian‐shaped seamount loading a thin elastic lithosphere, we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile. The model predicts that the seamount diameter is equal to the peak‐to‐trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed. Moreover, the model suggests that these two measurements are relatively insensitive to the cross‐track location of the seamount. We confirm these model predictions using Seasat altimeter profiles crossing 14 well surveyed seamounts in the Pacific. We then apply the measurement technique to 26 × 106 million kilometers of Seasat profiles resulting in a new global set of seamount locations. Approximately one quarter of the seamounts identified in Seasat profiles were previously uncharted. Modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid; therefore the set of locations is not a straightforward sampling of the total seamount population, but is weighted toward seamounts which are poorly compensated. A preliminary analysis indicates considerable variations in population density and type across the oceans; most notable among them are the absence of seamounts in the Atlantic, variations in population density across large age‐offset fracture zones in the Pacific, the prevalence of small signatures in the Indian Ocean, and the existence of linear trends in the large seamounts of the west Pacific.
Role of the lithosphere in mantle convection
Authors: Davies Geoffrey F.
Published: February 2008

Plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath, and plate formation and subduction account for most of the heat transport from the Earth's interior. Correspondingly, mantle convection models must incorporate a stiff but mobile boundary layer, like the lithosphere, before they can reproduce the main features of mantle convection. A relatively easy way to accomplish this in numerical models is to combine a temperature‐dependent viscosity with an imposed, piecewise constant surface velocity boundary condition. It is shown how surface heat flux, topography, gravity, and geoid (but not plate velocities or stresses) can then be derived. Numerical models confirm that a lithosphere has a first‐order effect on the underlying flow structure. For internally heated models, approx‐imating the case of whole mantle convection, the calculated surface signatures are in first‐order agreement with observations, a level of empirical success which hitherto has not been approached by models of mantle convection. Companion papers exploit the observations more fully to constrain the main features of mantle convection.
Applications of attenuation tomography to Imperial Valley and Coso‐Indian Wells Region, southern California
Authors: Ho‐Liu Phyllis , Kanamori Hiroo , Clayton Robert W.
Published: February 2008

Spatial variations in seismic wave attenuation were tomographically imaged from observed S to P wave amplitude ratios in the Imperial Valley and Coso‐Indian Wells regions of southern California. In the Coso‐Indian Wells region a highly attenuating body (S wave quality factor Q β ≈ 30) coincides with a slow P wave anomaly mapped by Walck and Clayton (1987). This coincidence suggests the presence of a 3‐ to 5‐km depth magmatic or hydrothermal body in the Indian Wells region. In the Imperial Valley, slow P wave travel time anomalies and highly attenuating S wave anomalies were found in the Brawley seismic zone at a depth of 8–12 km. The effective S wave quality factor is very low (Q β ≈ 20), and the P wave velocity is 10% slower than the surrounding areas. These results suggest either magmatic or hydrothermal intrusions, or fractures at depth, possibly related to active shear in the Brawley seismic zone. This attenuation tomographic technique is shown to be useful in delineating the spatial variations in seismic wave attenuation and in estimating the effective seismic quality factor of attenuation anomalies.
Early Cretaceous paleolatitude of the Yukon‐Koyukuk province, Alaska
Authors: Hillhouse John W. , Gromme C. Sherman
Published: February 2008

We report results from a paleomagnetic study of the Yukon‐Koyukuk province, a key region for reconstructing the Cretaceous paleogeography of northern Alaska and northeastern Asia. The province lies between the displaced continental fragment of Arctic Alaska and the accreted terranes of southern Alaska. Although Lower Cretaceous volcanogenic rocks of the Yukon‐Koyukuk province preserve primary remanent magnetizations, the primary component is masked by a pervasive overprint. The over‐printing magnetization, which has dual polarity, was acquired after a Late Cretaceous orogeny left the region highly deformed. By fitting planes to tilt‐corrected thermal demagnetization data and solving for the best intersection of the corresponding remagnetization circles, we separated the primary component, presumably a vestige of the original thermoremanent magnetization, from the overprinting component of the natural remanence. The primary magnetization corresponds to a mean paleopole of Early Cretaceous age at 60.2°N, 159°E. The mean paleolatitude is 69°N with the 95% confidence ellipse ranging from 60° to 75°N. This analysis was successful because the tilt corrections of the sampled strata had sufficient variation to ensure a strong intersection of the remagnetization circles. We infer that Lower Cretaceous rocks of the Brooks Range and the Yukon‐Koyukuk province were once separated by a wide ocean basin that was closed by subduction, probably during the Cretaceous. The northern limb of the Angayucham terrane, which lies between the Brooks Range and the Yukon‐Koyukuk province, may contain remnants of the now‐closed ocean basin.
Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada
Authors: Blakely Richard J.
Published: February 2008

Estimates of the depth to the Curie temperature isotherm in Nevada are in accordance with other regional geologic and geophysical information and together can be explained in the context of present‐day tectonism. A method to estimate the depth extent of magnetic sources from the statistical properties of magnetic anomalies was applied to a statewide compilation of aeromagnetic data from Nevada. Basal depths of magnetic sources show no apparent correlation with the so‐called magnetic quiet zone, which trends northerly through the eastern part of the state, or with basin‐and‐range topography. However, certain correlations with published heat flow measurements are apparent and suggest that undulations in basal depth of magnetic sources are related in part to undulations in the Curie temperature isotherm. For example, an area of shallow basal depth (<10 km) near Battle Mountain corresponds to an area of exceptionally high conductive heat flow and indicates a shallow depth to the Curie temperature isotherm in this region. A narrow zone of shallow basal depth extends south from the Battle Mountain area along the 118°W meridian to at least latitude 38°N, which also is a zone of historic surface offsets and high‐magnitude earthquakes. The correspondence along the 118° meridian of shallow basal depth, high heat flow, high lower crustal seismic velocities, attenuated P and S wave arrivals, historic faulting, and large earthquakes suggests that they each are related to an active north trending spreading zone in this part of the Basin and Range province.
The May 18, 1980, eruption of Mount St. Helens: 3. Stability and chemistry of amphibole in the magma chamber
Authors: Rutherford Malcolm J. , Devine Joseph D.
Published: February 2008

Additional experiments have been done with the May 18, 1980, Mount St. Helens dacite and a more mafic, October 1980 sample to resolve questions concerning amphibole stability and dissolved volatiles in the magma chamber prior to the May 18 eruption. The experiments were done at 920°C, at fluid pressures of 220 or 320 MPa, and, in contrast to previous work, at an ƒO2 between the NNO and MnO‐Mn3O4 oxygen buffers. Fe‐Ti oxides are present in the melt under these conditions, and amphibole is stable when XH2O in the fluid is greater than 0.67. The An content of plagioclase in equilibrium with melt decreases with decreasing XH2O in the fluid and, in the amphibole‐bearing experiments, reaches the natural plagioclase rim compositions (An49) at an XH2O of 0.67. Under these H2O‐undersaturated conditions the experimentally produced amphibole, low‐Ca pyrpxene, and Ca‐rich pyroxene are compositionally equivalent to phenocrysts in the May 18 white pumice. The melts (glasses) in amphibole‐bearing experiments range from the average plagioclase melt inclusion composition [Rutherford et al., 1985] to slightly less evolved compositions as XH2O approaches 1.0.

Melt inclusions in natural amphiboles were analyzed, and the compositions were plotted on SiO2 variation diagrams along with experimental glass analyses. The amphibole melt inclusions define a liquid line of descent for the magina which extends from relatively primitive compositions (68 wt % SiO2, anhydrous basis) to the more evolved average plagioclase melt inclusion composition (73 wt % SiO2). The volatile content of the amphibole melt inclusions (difference method) reaches 5.0±0.5 wt %, which compares favorably with the volatile content of the amphibole‐bearing experimental melts produced at XH2O = 0.67.
Somali Basin, Chain Ridge, and origin of the Northern Somali Basin gravity and geoid low
Authors: Cochran James R.
Published: February 2008

The Northern Somali Basin, located between Chain Ridge and the Horn of Africa north of 4°N, is characterized by a distinct 5‐m geoid low and by large negative gravity anomalies. The boundaries of the basin are marked by steep gradients in both gravity and geoid. Basement in Northern Somali Basin is 1–2 km deeper than on the Carlsberg Ridge flank to the southeast or on the Sheba Ridge flank to the north with a sharp discontinuity across the boundary of the basin. The Western Somali Basin, to the south, was created in the Late Jurassic and Early Cretaceous by the movement of Madagascar away from Africa. Reinterpretation of the magnetic anomalies in the Western Somali Basin shows that they record both limbs of a mid‐ocean ridge that was active by M22 time (Kimmeridgian) and died soon after MO (Aptian). Magnetic and gravity data allow the relict ridge crest to be traced from Davie Ridge near the African coast to the Dhow‐VLCC‐ARS fracture zone complex at 50°E. Davie Ridge is a transform fault connecting the Western Somali Basin spreading center with a similar age spreading center in the Mozambique Basin. The Dhow‐VLCC‐ARS complex can be shown to continue north of the 4°N bathymetric high separating the Northern and Western Somali Basins and to intersect the African margin near 7°N, The Northern Somali Basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between East and West Gondwanaland. The original Northern Somali Basin was split apart by the northward motion of India in the Late Cretaceous and Chain Ridge formed along the new boundary. Thermal and gravity modeling shows that the flexure resulting from differential subsidence across Chain Ridge combined with the difference in lithospheric thermal structure (Late Jurassic vs. Early Tertiary) on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across Chain Ridge. The geoid step up from the basin up to the African coast can be modeled as an edge effect between the juxtaposed Jurassic oceanic and African continental crust. Thus, the geoid and gravity low over the Northern Somali Basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly.
A two‐dimensional seismic investigation of crustal structure under the Hawaiian Islands near Oahu and Kauai
Authors: Lindwall Dennis A.
Published: February 2008

Two expanding spread profiles (ESPs) were analyzed in detail to investigate the deep structure of the volcanoes and the underlying oceanic crust near Oahu. One ESP was located on the southeastern submarine flank of Kauai (ESP 10), and the other was located directly over the ridge axis between Oahu and Molokai (ESP 5). Two‐dimensional models of the entire crust were obtained by iterative comparison of both travel times and relative amplitudes of arrivals on the ray‐theoretical synthetic seismograms with those in the data. A 2.4‐km vertical offset in the preexisting crust, inferred to be a large fault zone, was found near the midpoint of ESP 10. This fault may form the southern wall of a graben or half graben in which Kauai sits. The layering and seismic velocities of the preexisting oceanic crust near Kauai have remained unmodified by the load of the volcanic pile. ESP 5 is located over a shallow 4‐km‐thick basin filled with volcaniclastics and possibly volcanic flows bordered by the islands of Oahu and Molokai, Penguin Bank, and two buried rift zones. Under this basin the main volcanic edifice is 4 to 5 km thick. The top of the preexisting oceanic crust is 8 to 9 km below sea level and is 3 to 4 km lower than the crust away from the islands. The underlying oceanic crust is 6 to 7 km thick excluding the Moho transition zone, which is of normal thickness and at a depth from 15 to 16 km. The existence of a large subcrustal plutonic body is refuted.
Chemographic and Thermodynamic Analysis of the Paragenesis of the Major Phases in the Vicinity of the 6120‐Foot (1866 m) Flow Zone, California State Well 2–14
Authors: Charles R. W. , Janecky D. R. , Goff Fraser , McKibben M. A.
Published: February 2008

The major phases found in the vicinity of the 1866m flow zone are K‐feldspar, epidote, pyrite, hematite, and chlorite. These are found to exist in primarily four, two‐phase assemblages, the dominant being K‐feldspar + pyrite, epidote + pyrite, epidote + hematite, and chlorite + pyrite. Complete reaction relations among these isobaric isothermal divariant phase assemblages oriented in log fO2 ‐ pH space are compatible with stable growth at 300°C. Sensitivity of reaction relations to total sulfur, temperature, pressure, and thermodynamic values is investigated. The aqueous fluid sampled at 1866 m is out of equilibrium with the more reducing phase assemblages lying stratigraphically above and below the flow zone as well as pyrite‐bearing assemblages in the flow zone and shows aspects of mixing with other, hotter, flow zones. High concentrations of CO2 may indicate contribution to the fluid of CO2 from hotter portions of the system. Low total sulfur and high total iron are due to possible gas loss during down hole sampling and reaction of the fluid with well casing and lost circulation materials.
Principal Component Analysis of Geodetically Measured Deformation in Long Valley Caldera, Eastern California, 1983–1987
Authors: Savage J. C.
Published: February 2008

Typical geodetic measurements of deformation consist of repeated surveys of a particular geodetic network. Such deformation data can be interpreted as a consequence of one or more self‐coherent sources by means of principal component analysis. A self‐coherent source is defined as any source that produces deformation that is time and space separable. Principal component analysis then gives the time and space factors that characterize the deformation attributed to each self‐coherent source. Geodetic measurements of deformation at Long Valley caldera provide two examples of the application of principal component analysis. A 40‐line trilateration network surrounding the caldera was surveyed in midsummer 1983, 1984, 1985, 1986, and 1987. Principal component analysis indicates that the observed deformation can be represented by a single coherent source. The time dependence for that source displays a rapid rate of deformation in 1983–1984 followed by less rapid but uniform rate in the 1984–1987 interval. The spatial factor seems consistent with expansion of a magma chamber beneath the caldera plus some shallow right‐lateral slip on a vertical fault in the south moat of the caldera. An independent principal component analysis of the 1982, 1983, 1984, 1985, 1986, and 1987 leveling across the caldera requires two self‐coherent sources to explain the deformation. The deformation pattern produced by the larger of these two sources appears to be roughly consistent with that found from the trilateration data. The deformation due to the second source is a nearly uniform tilt in the uplift profile. Presumably, that tilt is simply an artifact of systematic error in the leveling.
Oxygen fugacity of gases and rocks from Momotombo Volcano, Nicaragua: Application to volcanological monitoring
Authors: Benhamou G. , Allard P. , Sabroux J. C. , Vitter G. , Dajlevic D. , Creusot A.
Published: February 2008

The oxygen fugacity (fO2) and the fO2 versus T°C relationship of high‐temperature (600°–860°C) gas emissions from Momotombo volcano, Nicaragua, was determined from both field electrochemical measurements (electrolytic cell assembly) and thermodynamic computations on gas samples collected between 1978 and 1985. It was then compared with the intrinsic fO2 of fresh and altered lavas from the last eruption (1905), as measured between 500° and 1100°C in laboratory. The electrochemical results show that the oxygen fugacity of Momotombo fumaroles, at equivalent temperature, is much higher than that of the fresh 1905 lava (∼FMQ buffer) and closer to that of their altered wall rocks (∼FMQ buffer). The equilibrium O2 fugacities calculated from the chemistry of gas samples confirm this pattern. However, they suggest that the gas mixtures preserve the (variable) memory of a higher thermal equilibrium achieved at depth, under temperature and fO2 conditions of up to 1050°C and 10−9.0 atm, respectively, which correspond to the cross over between the fO2‐T gas and lava trends. These data thus support the idea that Momotombo volcanic gases, released in a period of increasing activity, escape from a shallow magma body before suffering a variable oxidation during their ascent through both unbuffered cooling and reactions with environmental fluids and rocks. This late oxidation is weaker at central fumaroles than at peripherical ones. While between 1978 and 1985 the temperature of the hottest fumarole increased from 750° to 865°–900°C, the equilibrium fO2 of the gas decreased by nearly one order of magnitude (at comparable equilibrium temperature). Such an evolution presumably reflects an increasing connection between the surface exhalations and the magma degassing at depth along with time. This work underlines the possibility of monitoring the processes of magma ascent and gas‐magma separation within a volcano before an eruption by continuously recording the changes of both oxygen fugacity and temperature in hot fumaroles.
Venus: Surface electromagnetic properties
Authors: Pettengill Gordon H. , Ford Peter G. , Chapman Bruce D.
Published: February 2008

Using Pioneer Venus high‐angle radar imaging data, we have attempted to correct values of the specular power reflection coefficient, previously reported by the Pioneer Venus radar altimetry experiment, for the effects of diffusely scattering small‐scale surface roughness. We have compared these results with values of surface radio emissivity obtained both from the Pioneer Venus radar altimetry experiment and from thermal radioemission observations of Venus made at the Very Large Array. The combined measurements provide credible estimates of the relative dielectric permittivity (ε) at 17‐cm wavelength for the majority of the Venus surface. The lowland regions are characterized by ε = 5.0 ±0.9, a value commonly associated with many types of compact dry basaltic or granitic rocks. Most highland areas, however, display very much larger values, rising to as high as ε = 39 ± 15 over Maxwell Montes. If the high values of permittivity derive from the loading of a host rock matrix with small embedded fragments of an electrically conducting mineral, as we believe, the required volume fraction of this material for Maxwell is about 14%, with many other highland regions containing as much as 10%. Suitable candidates for the conducting mineral are pyrite, other iron sulfides, or, depending on the oxidation state of the surface, possibly magnetite, hematite, or ilmenite.
Reflectivity method for dipping layers
Authors: Wenzel Friedemann , Sandmeier Karl‐Josef
Published: February 2008

The reflectivity method used for the calculation of synthetic seismograms for a layered elastic Earth is modified for the case where the stack of layers is inclined with respect to the free surface. The application of appropriate coordinate transforms and a stationary phase approximation results in a solution that holds if shot point and receivers are not in the vicinity of the area of outcrop and if multiple reflections between free surface and layers are excluded. The method is applied to a simple synthetic example which demonstrates that downdipping reflectors can drastically reduce the critical amplitudes in comparison to horizontal layering. A further application of the method concerns refraction data from the area of the German deep continental drilling location (Oberpfalz) where dipping layers revealed in the reflection section require their incorporation in the wide‐angle modelling.
Inhomogeneities near the core‐mantle boundary inferred from short‐period scattered PKP waves recorded at the Global Digital Seismograph Network
Authors: Bataille Klaus , Flatté Stanley M.
Published: February 2008

We analyze short‐period PKP precursor wave trains recorded at Global Digital Seismograph Network stations in the distance range 120° < Δ < 140° to infer the nature of inhomogeneities near the core‐mantle boundary. Travel times and particle motions are consistent with predictions of single‐scattered waves near the CMB. The dominant frequencies are around 1 Hz. The regions best sampled are beneath Indonesia, North America, Central Africa, and South America. Based on first‐order scattering theory, the characteristics of the wave number spectrum of the structural inhomogeneities are obtained for two hypotheses: volume inhomogeneities and topographic irregularities. For the range of wavelengths sampled by these data (between 10 and 70 km), the spectrum of inhomogeneities is best represented by a power law of index 5.3 for volume inhomogeneities and 6.8 for topographic irregularities. If the scattering is due only to volumetric inhomogeneities, we estimate the strength of the structural variations to be about 0.5% in P wave velocity for a 200‐km‐thick layer. If the scattering is due only to topographic irregularities, the topography has an rms height of 280 m. At present we can not distinguish between these two types of inhomogeneities.
A stochastic model for the creation of abyssal hill topography at a slow spreading center
Authors: Malinverno Alberto , Gilbert Lewis E.
Published: February 2008

The present study is an attempt to model abyssal hill topography, i.e., the randomly varying topography that is superimposed on the subsidence of the ocean floor due to thermal contraction. We assume that abyssal hill topography is created by faulting, and that it can be represented as the convolution of a series of stochastic fault offsets with a fault impulse response. As a constraint, the power spectrum of the model‐generated topography will have to resemble the power spectrum of a profile of abyssal hill topography across a typical slow spreading ridge such as found in the South Atlantic. Using a series of fault offsets positively correlated in space and fault impulse responses limited to a finite width w, profiles that statistically resemble abyssal hill topography have been generated. If the fault offsets are constrained to be positive, i.e., to cause uplift, the synthetic profiles contain a steady state depression at the axis. The fit of the synthetically derived topography to the observed topography is very sensitive to the width w of the fault impulse response. Our results suggest that the effects of a faulting event must be limited to distances of about 3.5 km and that tectonic activity must take place in the immediate vicinity of the inner wall of the axial valley. If faulting is indeed the process that creates abyssal hill topography, the best fit flexural impulse response to faulting implies a flexural rigidity of 5 × 1017 N m. If instead constructional volcanism contributes significantly to the topography, our study suggests that the emplacement of volcanic constructions must also take place within a narrow width w about the axis.
Slab diffraction of S waves
Authors: Cormier Vernon F.
Published: February 2008

Seismograms are synthesized by superposition of Gaussian beams for teleseismic S waves radiated by deep focus earthquakes located within the zone of high seismic velocities that define a descending lithospheric slab. The S wave synthetics demonstrate a phenomenon of slab diffraction that acts to lengthen the apparent pulse width of the S wave. Low‐frequency energy leaks out of the slab and travels to the receivers along paths in the lower‐velocity region surrounding the slab. Seismograms synthesized in fully three‐dimensional models of slab structure show that slab diffraction strongly depends on the azimuth of the receiver with respect to strike of the slab. Slab diffraction lengthens the pulse width of S waves in the entire azimuthal range on the dipping side of the slab. As the azimuth approaches the strike of the slab, the diffracted phase may evolve into a higher‐frequency, impulsive phase, which arrives after the minimum time arrival. Some slab models predict the slab phase to become a superposition of multipathed, ray theoretical arrivals as the azimuth approaches the strike of the slab. As the azimuth varies through the strike of the slab toward the side opposite the direction of dip, the slab phase is rapidly extinguished. Rapid variations in the complexity of S waveforms recorded along azimuths parallel to the strike of the slab are consistent with the predictions of synthetic seismograms. Evidence of slab diffraction in the S waveforms of the deepest focus earthquakes supports the hypothesis of slab penetration beyond 650 km depth. Broadband S waveforms from deep focus earthquakes in the Kuril‐Kamchatka slab suggest that this slab penetrates below 650 km and that it may suffer some distortion in shape and advective thickening below this depth.