bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of an impact‐generated gas cloud on the acceleration of solid ejecta
Authors: Vickery Ann
Published: February 2008

A large amount of petrologic, geochemical, isotopic, and rare gas evidence strongly indicates that the shergottite, nakhlite, and Chassigny (SNC) meteorites originated on a parent body large enough to have sustained recent geological activity, probably Mars. The most straightforward interpretation of the cosmic ray exposure evidence requires that the protoshergottites were at least 5 and, more likely, 15 m in diameter; this large size combined with the 5 km/s escape velocity for Mars presents a formidable dynamic problem. This study investigates the possibility that the entrainment of large blocks of solid ejecta within the rapidly expanding vapor cloud produced by an impact provides sufficient additional acceleration to overcome this problem. A large number of impact parameters, such as impactor and target material, impact velocity, times of ejection, and crater size, were varied to determine their relative effects. Ejection time was found to be the single most important variable: Rocks entrained at very early times are invariably crushed by high dynamic pressures, and rocks ejected at late times, after the gas density has become very low, are little affected by the gas. Only rocks ejected within a fairly narrow window of intermediate times have their velocities significantly changed. Rocks initially ejected at very high velocities, such as those predicted by the Melosh spallation model, are strongly decelerated by the gas. Rocks injected into the gas stream with velocities much different from the average gas velocity at times early enough that the gas density is still significant are often subjected to dynamic pressures great enough to crush them. Thus the net effect of a large, rapidly expanding, impact‐generated gas cloud on solid ejecta is to homogenize the velocity distribution and to reduce the average size. For 30‐km diameter craters the only rocks with final velocities greater than 5 km/s were those whose initial velocities were greater than or approximately equal to 5 km/s. Since the average final velocity increases with crater size, it may be necessary to invoke a crater much larger than 30 km to explain the Martian origin of SNC meteorites, if the proto‐SNCs had diameters of greater than 1 m. On the other hand, if the proto‐SNCs were only a few tens of centimeters or less, gas acceleration of ejecta from a 30‐km‐diameter crater is consistent with the Martian origin of these unusual meteorites.
Ray equation migration of wide‐angle reflections from southern Alaska
Authors: McMechan George A. , Fuis Gary S.
Published: February 2008

A Kirchhoff‐style prestack depth migration algorithm is implemented for wide‐angle reflections in two‐dimensional variable‐velocity (acoustic) media. Extrapolation of both source and recorder wavefields is performed by ray tracing. The imaging condition at each point in the image space is the total source to recorder travel time. Since the migration algorithm is based on ray tracing, migration can be performed for any medium through which rays can be traced. There is no inherent limitation on the size of the region to be imaged; regions up to 3.6 × 103 km2 are utilized in this study. The algorithm is illustrated by migration of two synthetic and three real data sets from the U.S. Geological Survey's Trans‐Alaska Crustal Transect program. Images are produced of inferred subduction assemblages in southern Alaska. Emphasis in this paper is on obtaining reflector positions and shapes rather than detailed structural interpretations.
Unusually high stress drops associated with shallow moonquakes
Authors: Oberst Jürgen
Published: February 2008

New estimates of stress drops and energy release associated with shallow moonquakes, believed to represent the only lunar seismic events of tectonic origin, have been made, taking into account (1) properties of source spectra, (2) radiation pattern of foci, (3) anelastic attenuation of seismic waves, (4) response of the seismometer, and (5) intense scattering of seismic signals. The results indicate that the three largest events show body wave magnitudes of mB > 5.5 and stress drops of Δσ > 100 MPa. The values for stress drops are high compared with those generally associated with most terrestrial events, but they are compatible with results predicted from thermoelastic models of the moon. Presence of high stresses in the lunar crust may indicate that at the beginning of its history the moon was originally molten not only in the outer surface region, but throughout its whole volume.
Spectral reflectance of carbonate minerals in the visible and near infrared (0.35–2.55 um): Anhydrous carbonate minerals
Authors: Gaffey Susan J.
Published: February 2008

Reflectance spectroscopy in the visible and near infrared (0.35–2.55 um) offers a rapid, inexpensive, nondestructive technique for determining the mineralogy and gaining information on the minor element chemistry of carbonate minerals and rocks. Spectra of all commonly occurring anhydrous end‐member carbonate minerals contain seven strong absorption bands at wavelengths >1.6 um due to vibrations of the carbonate radical. Positions, widths, and spacing between carbonate bands are diagnostic of mineralogy. Differences in positions of carbonate bands between spectra of different minerals are primarily due to differences in mass of the major cation, with cation electronegativity playing a secondary role. Spectra of calcite group minerals may contain absorption features due to transition metal cations such as Fe and Mn, which can also aid in mineral identification. Spectra confirm the occurrence of both cations in the divalent state. Fe2+ produces a broad feature near 1.1 um whose position and degree of doubling are related to the size and degree of distortion of the octahedral site, as predicted by crystal field theory. Mn2+ and Fe2+ produce very strong absorption bands in the minerals for which they are the major cation (rhodochrosite and siderite, respectively), making it easy to distinguish these from other carbonate minerals, even when presence of water bands may make it difficult to determine accurately carbonate band positions.
Discrimination of iron alteration minerals in visible and near‐infrared reflectance data
Authors: Townsend Timothy E.
Published: February 2008

Field and laboratory studies were conducted to relate the mineralogy of rock and soil samples containing hematite (α‐Fe2O3), goethite (α‐FeOOH), and jarosite (KFe3(SO4)2(OH)6) to their reflectance properties in visible and near‐infrared wavelengths. Field reflectance measurements of regolith containing one or more of these minerals were made in the Goldfield, Nevada, mining district with a four‐channel radiometer. The mineralogy and reflectance properties of regolith samples collected from these field measurement sites were then determined by laboratory studies. Mossbauer spectroscopy was used to identify the iron minerals present and their relative proportions. The reflectance spectra of samples containing hematite and goethite were characterized by a reflectance minimum near 900 nm. The location of this minimum could be accounted for by the relative proportions of hematite and goethite present in the samples. Under certain conditions it may be feasible to estimate the relative proportions of hematite and goethite on the earth's surface by determining the location of this near‐infrared reflectance minimum in high spectral resolution remote sensing data. In addition, the field measurements suggest that the ratio of band passes at 852 and 982 nm may be used to distinguish qualitatively among hematite, goethite, and jarosite. Thus maps of the distribution of these minerals may be derived from scanner images acquired in those band passes.
Orientational data on the state of stress in northeastern Mexico as inferred from stress‐induced borehole elongations
Authors: Suter Max
Published: February 2008

Preferential elongation directions are presented from 46 subvertical wells in northeastern Mexico from depth intervals ranging from 161 to 4,912 m. The measurements document a remarkably consistent west‐east to northwest‐southeast direction of the contemporary least horizontal stress in the upper crust of this intraplate region. In the regional neotectonic framework, the data suggest that Mexico north of the trans‐Mexican volcanic belt is being stretched in west‐east to northwest‐southeast direction. The measurements from the area of the Laramide Chihuahua and Coahuila fold‐thrust belts define the southern extend of the Basin and Range—Rio Grande rift stress province of the southwestern United States into northern Mexico. The least horizontal in situ stress directions recorded in the Gulf Coastal Plain (Burgos and Tampico‐Misantla Basins) are oblique to the continental margin of the Gulf of Mexico, but subparallel to the direction measured in the area of the Basin and Range—Rio Grande rift stress province. This suggests that the stress distribution in these basins is not mainly the result of gravitational loading as in the Gulf Coast stress province of the United States, but is controlled by the same lithospheric stress system that characterizes the Basin and Range—Rio Grande rift stress province. This is also supported by the north‐south trending zone of Oligocene‐Quaternary alkaline volcanism that crosses the Tampico‐Misantla Basin. Furthermore, the data indicate that the active deformation of the Mexican Ridges fold belt, east of the study area in the Gulf of Mexico cannot be caused by a compressional external tectonic load. The direction of the least horizontal in situ stress measured in the area of the Laramide Sierra Madre Oriental fold‐thrust belt in Hidalgo and San Luis Potosí States is consistent with that of the other zones. However, wellbore elongations were recorded less frequently and show a rather large circular variance, which suggests that the minimum and maximum horizontal far‐field stresses are about equal in this area.
Deformation near the Long Valley Caldera, eastern California, 1982–1986
Authors: Savage J. C. , Cockerham R. S. , Estrem J. E. , Moore L. R.
Published: February 2008

Annual surveys of trilateration and leveling networks in and around Long Valley caldera in the 1982–1985 interval indicate that the principal sources of deformation are inflation of a magma chamber beneath the resurgent dome and right‐lateral strike slip on a vertical fault in the south moat of the caldera. The rate of inflation of the magma chamber seems to have been roughly constant (0.02 km3/yr) in the 1982–1985 interval, but the slip rate on the south moat fault has decreased substantially. In addition, there is evidence for a shallow source of dilatation (possibly dike intrusion) beneath the south moat of the caldera in 1983 and less certain evidence for a deep source (possibly magma chamber inflation beneath Mammoth Mountain) in the western caldera in 1983–1985. Deformation in the 1985–1986 interval as inferred from trilateration alone seems to be similar to that observed in 1984–1985.
On the presolar origin of the “normal planetary” noble gas component in meteorites
Authors: Huss Gary R. , Alexander E. Calvin
Published: February 2008

Several components of planetary noble gases, including CCFXe (Xe‐X), s‐process Kr and Xe, and Ne‐E, are now thought to have had a presolar origin. We propose that the “normal planetary” noble gas component, which contains most of the planetary Ar, Kr, and Xe, is also presolar. This component was trapped in carbonaceous mantles as the mantles formed on grain surfaces in the molecular cloud. The presolar dust grains and their carbonaceous mantles were then incorporated into primitive meteorites. This model naturally explains: (1) the affinity of “normal planetary” noble gases for carbonaceous material in primitive chondrites; (2) the near absence of He and Ne in the “normal planetary” component; (3) the elemental fractionation patterns of the “exotic” and “normal planetary” components; (4) the similarity of the isotopic compositions of “normal planetary” and trapped solar Ar, Kr, and Xe; (5) the high noble gas content of the “normal planetary” gas carrier; (6) several aspects of the chemistry of the “normal planetary” gas carrier; (7) the correlation of planetary noble gases with the fine‐grained material in primitive chondrites; and (8) the widespread occurrence of the planetary fractionation pattern throughout the solar system.
Finite element models of non‐Newtonian crater relaxation
Authors: Thomas Paul J. , Schubert Gerald
Published: February 2008

Models of viscous crater relaxation proposed to date for the icy satellites of the outer solar system assume the behavior of a Newtonian medium, with viscosity η independent of effective stress τ. While this is reasonable if both effective stresses and temperatures are very low, laboratory data on ice indicate that, at effective shear stresses (τ ∼0.1 MPa) and temperatures (∼173 K) likely to occur in regions underlying craters, non‐Newtonian behavior is probable. Such material would have a viscosity‐effective stress relation given by η (τ) ∝ τ1−n, where n≈4. It is likely, due to the low activation energy of ice at these temperatures, that stress differences may affect the profile of relaxing craters to a greater extent than temperature differences. To investigate the effects of a non‐Newtonian rheology on the profiles of relaxing craters, two‐dimensional finite element simulations have been performed. Initially, effective stresses are highest in the region below the crater center, producing a zone of relatively low viscosity. Relaxation flow forces this region into a convex bulge as the crater relaxes. At later stages, the low viscosity region moves to beneath the crater rim, leading to enhanced relaxation of crater rims, compared to a Newtonian rheology. The reduction of effective stress as the crater relaxes increases viscosity in the region beneath the crater. As a result, relaxation rates are quite rapid compared to those in later stages. This is in marked contrast to the exponential behavior of depth with time associated with Newtonian relaxation. The short relaxation times observed for the craters modeled indicate that a silicate component may be required in the crusts of icy satellites to account for the observed crater populations.
Rheologic properties and kinematics of emplacement of the chaos jumbles rockfall avalanche, Lassen Volcanic National Park, California
Authors: Eppler Dean B. , Fink Jonathan , Fletcher Raymond
Published: February 2008

The Chaos Jumbles is a rockfall avalanche deposit that was emplaced by three separate events ∼ 300 years ago. Deposits from each event are distinguishable on the basis of morphology, size variation of large dacitic surface clasts, and by the color of both the matrix and entrained dacitic blocks. Steep lateral and distal deposit margins and surface features such as folds and apparent strike‐slip faults indicate that each rockfall avalanche had a finite yield strength and was being actively deformed and sheared throughout the body of the moving deposit, rather than strictly along a basal surface. Kinematic analysis of the three deposits indicates that each had a very low apparent coefficient of friction and was emplaced at velocities of up to ∼ 100 m/s. These data suggest that each rockfall avalanche can be modeled as a pseudoplastic material undergoing flow parallel compression above a frictionless base. This model allows calculation of deposit volumes ranging from ∼1.2 to 1.7 × 108 m3 and also suggests that a future rockfall avalanche from the same location would have a more restricted runout than the previous events.
Thermodynamic state of SO2 on Io's surface
Authors: Zent Aaron P. , Fanale Fraser P.
Published: February 2008

It has been suggested that surface conditions on Io might be conducive to significant SO2 adsorption on sulphur or alkali sulphides. A number of spectroscopic arguments for and against the high abundance of a SO2 adsorbate (versus frost) on Io have been made. We have measured SO2 adsorption isotherms on particulate sulphur and approached the question of SO2 adsorbate/ice from a thermodynamic perspective. Because of formidable experimental difficulties, data were not obtained at the very low temperatures and PSO2 characteristic of Io. However, data were obtained over a wide range of pressures and temperatures somewhat higher that Io's and extrapolated to Io surface conditions. Errors in estimating adsorptive capacity accrue through extrapolation of the adsorption isotherms; however, more significant is the addition of the solid SO2 equilibrium vapor pressure curve to the phase diagram. The formation of ice places a strict upper limit on adsorptive coverage at any temperature. We calculate that limit for Io based upon our data and show that no more than 1.4 × 10−2 monolayers of SO2 can adsorb on sulphur at Io temperatures. Given the assumption that sulphur forms the primary adsorbent on Io's surface, or the assumption that the adsorptive capacity of the other adsorbents is not substantially greater than that of sulphur, this explains the fact that the ν1 + ν3 band center position is in better agreement with that of frost than adsorbate.
Heat flow and heat production in the Namaqua Mobile Belt, South Africa
Authors: Jones M. Q. W.
Published: February 2008

Results of heat flow and heat production surveys at 13 localities in the Namaqua mobile belt, southern Africa, are presented. Twelve stations traverse the 1200–1100 m.y. old gneissic and metamorphic Bushmanland and Gordonia subprovinces. One station is situated in the Richtersveld subprovince, a 2000–1730 m.y. old volcano‐sedimentary domain mildly affected by the 1200–1100 m.y. old orogeny. Surface heat flow in the mobile belt ranges from 39 to 81 mW m−2 and averages at 61 ± 11 mW m−2. Surface heat production ranges from 0.8 μW m−3 in mafic rocks to extreme values of 6.9–12.3 μW m−3 in the Springbok district, western Namaqualand. High heat production in the Springbok district is associated with a thin, tabular granitic intrusive and is not representative of the basement. The mean heat production of the remaining stations in the mobile belt is 2.3 ± 1.1 μW m−3. Linear regression analysis of the heat flow‐heat production data yields a poor fit (correlation coefficient = 0.62) and imprecise estimates for the intercept (34 ± 6 mW m−2) and slope (11 ± 3 km). The eastern margin of the mobile belt has relatively low heat flow and heat production; geothermal, geological, and gravity data suggest a relative depletion of crustal radiogenic heat production in the region. However, the average Namaqua heat flow is higher than worldwide averages for Proterozoic terrains. There is no geological or geophysical evidence for recent mantle heating below the Namaqua mobile belt, suggesting that an elevated crustal radioactive component is largely responsible for the elevated heat flow. In this respect, the mobile belt is similar to several mid‐late Proterozoic heat flow provinces that are characterized by high average surface heat flow and probably have elevated crustal radiogenic heat input. These observations, and seismic data, suggest that the Namaqua crust consists of an upper layer that contributes 40–50%, on average, to the heat flow and a depleted lower layer; the inferred heat flow at midcrustal levels is 50–60% of the average surface heat flow. The mobile belt's average surface heat flow is 40–85% higher than that from the basement of the adjacent Archaean Kaapvaal craton, and its inferred midcrustal heat flow is similar to the lowest surface values in the craton. Heat production data from the Kaapvaal craton indicate that its upper crust is not depleted of heat sources. While enhanced radiogenic heat input from the Namaqua upper crust may account for part of the higher‐than‐craton heat flow, it cannot account for the full excess. Consequently, a larger equilibrium mantle heat flow, related to a reduced lithospheric thickness below Namaqualand, seems to be required by the new data.
Three‐dimensional velocity variations and their relation to the structure and tectonic evolution of the New Madrid Seismic Zone
Authors: Al‐Shukri Haydar J. , Mitchell B. J.
Published: February 2008

Ninety‐five teleseisms and 270 local earthquakes are used to determine three‐dimensional velocity models of the crust and upper mantle beneath the New Madrid seismic zone. P wave travel time residuals from teleseisms yield a three‐dimensional model of velocity perturbations in the crust and upper mantle to a depth of 156 km. Travel time residuals of local earthquakes lead to an upper crustal model which is consistent with the model for that region obtained from teleseismic data and provide details which can be related to geological structure. The local earthquake data also permit the resolution of tradeoffs between the upper and lower crust which may result from the analysis of teleseismic data only. Lateral variations in velocity occur both as broad continuous or nearly continuous bands and as small‐scale localized features. A band of anomalous velocities correlates in position and orientation with a basement rift system which has been delineated from gravity and magnetic data. This band occurs as a low‐velocity zone through the upper crust and uppermost mantle and may be offset beneath the branch of the New Madrid seismic zone having the most intense earthquake activity. After trade‐offs with upper crustal velocities are removed, the lower crust exhibits somewhat higher‐than‐average velocities in the rift region. Several localized regions of high velocity in the crust inferred from teleseismic data correlate with the sites of intrusions found from gravity and magnetic data. A number of localized regions of both high and low velocities occur throughout the upper mantle with no clear‐cut relation to near‐surface features.
Fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at Mirror Lake, New Hampshire
Authors: Hardin E. L. , Cheng C. H. , Paillet F. L. , Mendelson J. D.
Published: February 2008

Results are presented from experiments carried out in conjunction with the U. S. Geological Survey at the Hubbard Brook Experimental Forest near Mirror Lake, New Hampshire. The study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures. The collected data set includes a four‐offset hydrophone vertical seismic profile, full waveform acoustic logs at 5, 15, and 34 kHz, borehole televiewer, temperature, resistivity, and self‐potential logs, and borehole‐to‐borehole pump test data. Borehole televiewer and other geophysical logs indicate that permeable fractures intersect the Mirror Lake boreholes at numerous depths, but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis. The vertical seismic profiling (VSP) data indicate a single major permeable fracture near a depth of 44 m, corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis. VSP data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m; all correspond to major permeable fractures in the acoustic waveform data set. Pump test data confirm the presence of a hydraulic connection between the Mirror Lake boreholes through a shallow dipping zone of permeability at 44 m in depth. Effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the VSP data set. This discrepancy is attributed to the effect of fracture stiffness. A new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of VSPs.
Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness
Authors: Klein Emily M. , Langmuir Charles H.
Published: February 2008

Regional averages of the major element chemistry of ocean ridge basalts, corrected for low‐pressure fractionation, correlate with regional averages of axial depth for the global system of ocean ridges, including hot spots, cold spots, and back arc basins, as well as “normal” ocean ridges. Quantitative consideration of the variations of each major element during melting of the mantle suggests that the global major element variations can be accounted for by ∼8–20% melting of the mantle at associated mean pressures of 5–16 kbar. The lowest extents of melting occur at shallowest depths in the mantle and are associated with the deepest ocean ridges. Calculated mean primary magmas show a range in composition from 10 to 15 wt % MgO, and the primary magma compositions correlate with depth. Data for Sm, Yb, Sc, and Ni are consistent with the major elements, but highly incompatible elements show more complicated behavior. In addition, some hot spots have anomalous chemistry, suggesting major element heterogeneity. Thermal modeling of mantle ascending adiabatically beneath the ridge is consistent with the chemical data and melting calculations, provided the melt is tapped from throughout the ascending mantle column. The thermal modeling independently predicts the observed relationships among basalt chemistry, ridge depth, and crustal thickness resulting from temperature variations in the mantle. Beneath the shallowest and deepest ridge axes, temperature differences of approximately 250°C in the subsolidus mantle are required to account for the global systematics.
Till beneath ice stream B: 4. A coupled ice‐till flow model
Authors: Alley R. B. , Blankenship D. D. , Rooney S. T. , Bentley C. R.
Published: February 2008

A nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations. For the model we require one‐dimensional flow and continuity of ice and till, and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed. This allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity. An analytic, steady state solution for ice stream B, West Antarctica, shows that till viscosity decreases slowly downstream, probably in response to decreasing effective pressure downstream. Nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years, with a new steady state in about 200 years. Changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system.
Lava flows as kinematic waves
Authors: Baloga Stephen
Published: February 2008

A recently proposed model for the emplacement of lava flows is reinterpreted as a kinematic wave theory for the depth of the flow. With this interpretation the longitudinal profile of an advancing flow is considered to be a propagating waveform with its length limited continuously by the amount of lava extruded. The application of kinematic wave formalism provides new insights into the effects of time‐dependent source behavior on the shape and extent of the wave. Eruption behavior with a period of increasing flow depth at the source is shown to produce so‐called “breaking points” in the depth profile where the kinematic wave model itself could be invalid. Two models of such eruption behavior are investigated, and this is shown not to be the case. Specifically, when the conservation of lava volume is imposed, the breaking points always reside beyond the front of the wave. In general, it is found that source behavior propagates downstream faster than the flow advances. Thus only a fraction of the source time dependence is reflected in the shape of the kinematic wave. The remainder of the time‐dependent source effects must be deposited in the flow front, lateral margins, or solidifying crust or be dissipated irretrievably by processes not included in the model. The fraction of eruption history contained in the advancing waveform depends crucially on the nature of the flow rate/flow depth relation for the lava. Distal surface waves, budding at the nose of the flow, distal levee collapse, and other volcanological observations may be manifestations of time‐dependent source behavior that advanced beyond the kinematic wave front.
Investigations of relative plate motions in the South Atlantic using Seasat altimeter data
Authors: Shaw Peter R.
Published: February 2008

Fracture zones in the world's oceans appear clearly in the Seasat altimeter (geoid) data at wavelengths of about 100 km and less. In the North and South Atlantic these signatures are stable in form and amplitude from the ridge crest to crust tens of millions of years old and can be mapped across nearly the width of the basin. The signatures diminish in amplitude markedly on crust older than 70 to 80 m.y. B.P., apparently when thickly covered by sediments. Because fracture zone trajectories closely approximate plate motion flow lines, they may be used to infer the plate kinematic history of the ocean basin. In this paper I describe a new method for estimating the relative poles of opening, describing the history of motion between two plates by using fracture zone trajectories. The method is an iterative inversion whereby the trajectories of synthetic flow lines are linearized with respect to the pole parameters. When applied to the Seasat altimeter track data, a waveform inversion based upon this method is possible in which the altimeter data are modeled using a synthetic geoid fracture zone signal. This waveform inversion is applied to 17 prominent fracture zones in the central and South Atlantic to estimate the poles of opening between the South American and African plates. Altogether, 1022 Seasat fracture zone crossings are used; for each of these a waveform comparison is made. The new method is found to be robust and converges to a stable set of pole solutions even if the initial flow lines are small circles about a single pole that poorly approximates the observed trajectories. The solution poles document a Cenozoic change in plate motion near anomaly 13 time (36 m.y. B.P.) similar to that in the North Atlantic at anomaly 1.3–21 time (36–50 m.y. B.P.).
Reconstruction of Madagascar and Africa: Evidence from the Davie Fracture Zone and Western Somali Basin
Authors: Coffin Millard F. , Rabinowitz Philip D.
Published: February 2008

New seismic reflection, gravity, and magnetic data from offshore East Africa allow the Davie Fracture Zone to be traced from ∼11°S to its intersection with the Kenyan coast at ∼2°S, constraining the relative motion of Madagascar and Africa. Seasat‐derived free air gravity anomalies and slope/rise positive magnetic anomalies observed in shipboard data help to locate the continent‐ocean boundaries (COB) off the shore of East Africa and Madagascar. Seismic reflection data further document a diapirprovince off Madagascar, presumably conjugate to that observed off Kenya and Somalia. The Dhow and Very Large Crude Carrier (VLCC) basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of Madagascar and Africa. From these data we determine a predrift fit of Madagascar and Africa involving a 14.2° rotation of Madagascar to Africa about a pole at 10°N, 150°E. The geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the Comoros Basin and of the Somali Basin between Kenya and the Seychelles, and it does not conflict with onshore or offshore stratigraphy. Timing of the opening of the Western Somali Basin is constrained by Mesozoic marine magnetic anomalies and extrapolation to the interpreted COB and occurred between approximately 165 and 130 Ma.
Local crustal extension at Mount St. Helens, Washington
Authors: Weaver Craig S. , Grant Wendy C. , Shemeta Julie E.
Published: February 2008

Seismicity and the orientation of fault planes from focal mechanisms indicate that Mount St. Helens is located at a dextral offset along the St. Helens seismic zone (SHZ): earthquake swarms occurring in this offset are related to volcanic eruptions. Because motion on the SHZ is in a right‐lateral strike‐slip sense, this dextral offset creates extension within a volume of the crust between the offset fault segments. This offset geometry is similar to that of geothermal areas along the San Andreas fault system. We apply a model derived from these geothermal areas to Mount St. Helens and find that the major differences between Mount St. Helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments (l), to the seismogenic depth (h). At Mount St. Helens this ratio is < 1, whereas in the geothermal areas the ratio is ≈ 1. We propose that when l/h < 1 as at Mount St. Helens, the regional minimum principal stress does not completely dominate the small volume under extension, and as a consequence, the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where l/h ≈ 1. Late Quaternary volcanic vents near Mount St. Helens strike northeast, similar to the strike of a set of pre‐Quaternary faults and intrusive rocks that are mapped north of the volcano; in addition, the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault. Since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress, we infer that the spatial position of Mount St. Helens is controlled by the junction of the right‐stepping offset of the SHZ with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust. The sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of right‐lateral shear measured from geodetic lines around Mount St. Helens during June and July 1980.
A model for generation of ash clouds by pyroclastic flows, with application to the 1980 eruptions at Mount St. Helens, Washington
Authors: Denlinger Roger P.
Published: February 2008

Pyroclastic flows are defined as consisting of a dense basal avalanche underlying a turbulent ash cloud. The generation of ash clouds by the basal avalanche is commonly assumed to be closely related either to fluidization mechanisms within the avalanche or to airflow/flow front interactions. These mechanisms carry ash upward from the basal avalanche into the ash cloud in currents of gas or air, but so far no mechanisms have been proposed to keep the ash in turbulent suspension in the ash cloud. Here I consider the ash cloud turbulence that is generated in two ways: by temperature differences between the basal avalanche and overriding air, and by the motions of the blocks, lapilli, and ash on the outer surface of the basal avalanche. This turbulence would carry ash from saltation into turbulent suspension at the base of the ash cloud, and this transition would simultaneously occur from the upper surface and front of the basal avalanche. This approach quantifies one type of airflow/flow front interaction. The analysis incorporates studies of heat and mass transfer from rough surfaces during high Reynolds number flow. Equations for conservation of mass, momentum, heat energy, and ash particle concentration are formulated and presented in integral form. Model results show that for typical avalanche velocities and temperatues, temperature differences between the basal avalanche and the surrounding air are insignificant in generating the initial turbulence within the ash cloud. Instead, the initial distribution of particle sizes in the ash cloud is controlled by the turbulence generated by the saltating/cascading particles in the basal avalanche. Study of the morphology of observed ash clouds suggests that thermally driven convection often becomes significant hundreds of meters or more behind the flow front, and this may result from deposition of significant quantities of ash from the ash cloud onto the basal avalanche. The variations in ash cloud heights, morphologies, and particle sizes that are predicted by this model are consistent with published observations of small pyroclastic flows at Mount St. Helens in 1980.
A numerical model for simulating deformation of Mount St. Helens Volcano
Authors: Paul A. , Gratier J. P. , Boudon J.
Published: February 2008

Previous work on surface deformation associated with Volcanic activity has revealed significant departures from ideal elastic and homogeneous behavior. To account for the highly heterogeneous and discontinuous mechanical behavior of volcanic edifices, a numerical model named Bloc was used to model surface deformation data. The basic assumption of this model is that the volcanic edifice behaves mechanically as an assemblage of rock blocks that can move in relation to one another. The well‐documented eruption of Mount St. Helens in 1980 was used to test this model and to compare the fit between the observed and computed surface deformation. From the north‐south cross section of the volcano, several two‐dimensional models were developed to study the effect of various parameters (geometry, boundary conditions, coefficient of friction, etc.). The models were loaded with a step‐by‐step increasing internal pressure simulating the intrusion of magma within the volcano and with a horizontal acceleration simulating an earthquake. The deformation and failure patterns are highly dependent on the coefficient of friction on block boundaries. For the lower value of this coefficient (0.5), a good fit is found between the observed and computed surface displacements. The failure mode by gravitational sliding of the north flank is obtained with internal pressure loading alone as well as with internal pressure and an earthquake trigger. However, the second kind of loading gives a better fit with the particular deformation pattern observed.
Lake bottom magnetotellurics
Authors: Schultz Adam , Booker John , Larsen Jim
Published: February 2008

The study of the electrical conductivity structure of the lower crust and the upper mantle and midmantle promises to place important constraints on the thermobaric and compositional state of the earth's interior. Such work, particularly for depths above approximately 400 km, is best attempted by magnetotelluric sounding, an effort frustrated by the almost complete absence of stable long‐period electric field measurements. In this paper, we propose that proven techniques developed for seafloor magnetotellurics be applied also to deep freshwater lakes. The establishment of such long‐period electric and magnetic observatories in the thermally stable, low‐noise lake bottom environment will yield new insights on the deep electrical structure beneath the continents. A lake bottom long‐span electric field array was deployed in a deep freshwater lake adjacent to Seattle, Washington. A vector flux gate magnetometer was buried on the shore of the lake, and magnetotelluric data of high quality were collected. The analysis of the data described in this work clearly demonstrates the feasibility of establishing such lake bottom continental magnetotelluric observatories. Inversion of the data provides a view of structure beneath the site consistent with regional seismic and tectonic models.
Theoretical temperature and grain‐size dependence of domain state in X = 0.6 titanomagnetite
Authors: Moskowitz Bruce M. , Halgedahl Susan L.
Published: February 2008

Domain state calculations have been made for x = 0.6 titanomagnetite (TM60) as a function of grain size (a), temperature (T), stress (σ), and exchange constant (A), based on the equilibrium domain models of Amar and Kittel. Grains were assumed to be rectangular parallelepipeds, containing a simple array of uniformly spaced domains separated by planar, 180° Bloch walls, in zero magnetic field. To investigate the effects of residual stress upon domain number N, the domain wall energy was given in terms of either magnetocrystalline or uniaxial stress anisotropy. The effects of temperature upon N were modeled through the thermal variation of the material constants of TM60 which described magnetostatic (saturation magnetization) and domain wall (magnetocrystalline, magnetostriction, and exchange) energies. Calculations confirmed that both the Amar and Kittel models yielded very similar results at room temperature, regardless of whether stress or magnetocrystalline anisotropy was dominant. Rapid divergence between the two models occurred only close to the Curie temperature. Thus, significant discrepancies which have been noted between the predicted number of domains and the observed number of domains are not due to a lack of refinement in previous models, but must reflect uncertainties of a more fundamental nature. Systematic failure of particles to achieve absolute energy minimum states may not be sufficient by itself to explain this discrepancy. Higher levels of residual stress or lower values of the exchange constant, or both, may be necessary in order to reconcile theory with observation. The thermal models predicted that N will either increase or decrease, with heating, according to whether the wall energy falls more or less rapidly than the magnetostatic energy with temperature. Furthermore, the thermal dependence of N should be accounted for in models of thermoremanent magnetization. A simple model for calculating domain blocking temperatures (Tdb) was developed and predicted that (1) particles governed by magnetocrystalline anisotropy had Tdb values which were truncated well below the Curie temperature, while those particles governed by stress anisotropy had much higher Tdb values and (2) in all cases, very large multidomain grains had distributed Tdb values, while particles in the two‐ and three‐domain range, had higher and more discrete Tdb values. Domain calculations also confirmed that TM60 particles should exhibit less thermal sensitivity in their domain structure when controlled by stress anisotropy than when controlled by magnetocrystalline anisotropy. This may help to explain recent domain observations on natural TM60, which suggested that few domain state changes occurred with heating. The model results also predicted that the exchange constant in TM60 should vary with temperature approximately as Ms1−3.
Introduction to Juan de Fuca Ridge Special Section
Authors: Brett Robin
Published: February 2008

The U.S. Geological Survey and several other institutions have been studying the southernmost segment of the Juan de Fuca Ridge for several years by almost every means available to marine geologists and geophysicists [e.g., Normark et al., this issue]. Hydrothermal vent areas along the ridge were located by camera tows, by the presence of sulfides in dredge samples, and by locating hydrothermal plumes by geophysical and geochemical tows. Clearly, the only opportunity remaining for closer study of the vents and for in situ sampling of the sulfide minerals and associated hydrothermal fluids lay in the use of a deep manned submersible vessel. Accordingly, the U.S. Geological Survey (USGS) sponsored a series of dives in the area in the fall of 1984 using DSRV Alvin.
Mineralogical studies of sulfide samples and volatile concentrations of basalt glasses from the southern Juan de Fuca Ridge
Authors: Brett Robin , Evans Howard T. , Gibson E. K. , Hedenquist Jeffrey W. , Wandless M.‐V. , Sommer M. A.
Published: February 2008

Sulfide samples obtained from the U.S. Geological Survey's DSRV Alvin dives on the southern Juan de Fuca Ridge closely resemble those from the same area described by Koski et al. (1984). Major minerals include sphalerite, wurtzite, pyrite, marcasite, isocubanite, anhydrite, and chalcopyrite. Equilibrium, if attained at all, during deposition of most sulfides was a transient event over a few tens of micrometers at most and was perturbed by rapid temperature and compositional changes of the circulating fluid. Two new minerals were found: one, a hydrated Zn, Fe hydroxy‐chlorosulfate, and the other, a (Mn, Mg, Fe) hydroxide or hydroxy‐hydrate. Both were formed at relatively low temperatures. Lizardite, starkeyite, and anatase were found for the first time in such an environment. Sulfide geothermometry involving the system Cu‐Fe‐S indicates a vent temperature of <328°C for one sample. Fluid inclusion studies on crystals from the same vicinity of the same sample give pressure‐corrected homogenization temperatures of 268° and 285°C. Ice‐melting temperatures on inclusions from the same sample are about −2.8°C, indicating that the equivalent salinity of the trapped fluid is about 50% greater than that of seawater. Volatile concentrations from vesicle‐free basalt glass from the vent field are about 0.013 wt % CO2 and 0.16 wt % H2O. CO2 contents in these samples yield an entrapment depth of 2200 m of seawater, which is the depth from which the samples were collected.
Early Cenozoic crust at the Norwegian continental margin and the conjugate Jan Mayen Ridge
Authors: Skogseid Jakob , Eldholm Olav
Published: February 2008

Multichannel seismic profiles at the Vøring Plateau Margin off Norway and the northern Jan Mayen Ridge have provided a framework for the early Tertiary evolutionary history of these areas which were adjacent prior to the opening of the Norwegian Sea. We propose that their Paleogene evolution is quite similar and that the two acoustic basement reflectors, EE and JO, were formed in the early Eocene by extrusion of flow basalts. From the time of opening to about anomaly 23 time, oceanic seafloor was generated by Icelandic‐type spreading. During that time, the Central Jan Mayen Fracture was the main transform. The seaward dipping reflector sequences are flow basalts generated by the Icelandic spreading. The existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation. During the early Eocene subaerial spreading episode, which is related to the North Atlantic Volcanic Province, basalt flows also covered the adjacent thinned and heavily intruded continental crust. It is proposed that the continent‐ocean boundary is located just landward of anomaly 24B, a short distance seaward of the Vøring Plateau Escarpment. Reflector K is only recognized landward of this boundary in a region of “transitional crust.” The escarpment experienced syndepositional faulting, and the most elevated parts of the Vøring Marginal High did not subside below shallow water depths before Oligocene/Miocene time. We suggest that the Vøring Margin and the Jan Mayen Ridge represent “volcanic” passive margins. The evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust.