bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear wave velocities and crustal structure of the Eastern Snake River Plain, Idaho
Authors: Greensfelder Roger W. , Kovach Robert L.
Published: February 2008

The lithospheric velocity structure beneath the eastern Snake River Plain (ESRP) is inferred from data on Rayleigh wave dispersion (periods 6 s to 40 s) and teleseismic shear wave delays, as well as other data from refraction profiling, heat flow, volcanic history, and geology. The unusual velocity structure indicates a 12‐km thick high‐velocity layer (depth 8 to 20 km), with P and S velocities of 6.54 and 3.87 km/s, respectively; this overlies a 21‐km thick lower crust, with P and S velocities of 6.82 and 3.61 km/s, respectively. P velocities and layer thicknesses are based on our interpretation of refraction data (Braile et al., 1979) and are similar to the results of Sparlin et al. (1979). Upper‐mantle S velocity is 4.1–4.2 km/s, and a lid appears to be absent or weakly developed. Although the mild shear velocity inversion in the lower crust might result from viscous effects (possibly due to partial melt), an anomaly in mineral composition is the preferred explanation. This may be a major depletion of quartz, as a result of its fusion into the voluminous rhyolitic magmas which erupted onto the ESRP between 10 and 6 m.y. ago. The overlying high‐velocity layer is thought to comprise acidic and basic intrusions of late Cenozoic age, and Archean granitoid rocks whose velocity and density may have been increased by thermal metamorphism related to plutonism.
Permeability, underpressures, and convection in the oceanic crust near the Costa Rica Rift, eastern equatorial Pacific
Authors: Anderson Roger N. , Zoback Mark D.
Published: February 2008

In situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at DSDP site 504B. These measurements have relevance for the transition from convective to conductive heat flow on the south flank of the Costa Rica Rift. Conventional ‘slug’ and constant rate injection tests were made below a hydraulic packer set at various depths in the hole. The packer was first set in a massive flow unit 37 m below the sediment‐basement interface. The bulk permeability of the 172.5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys (4×10−10 cm2). Measurements over 3‐ and 15‐m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2–4 millidarcys. These values are thought to be accurate to ±30%. Formation pore pressures were found to be approximately 8 bars (∼2%) below hydrostatic. Interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment. Furthermore, the agreement between observed and modeled underpressures implies that hole 504B penetrated an active ocean crustal convection system. The thick sedimentary layer, layers of basal chert, and massive flow basalts above the layer 2A pillow flows apparently form an impermeable lid, effectively isolating the convection system from the ocean.
Post 12 m.y. rotation of southwest Washington
Authors: Magill James R. , Wells Ray E. , Simpson Robert W. , Cox Allan V.
Published: February 2008

Paleomagnetic field directions from the basalt of Pack Sack Lookout are compared to those from the Pomona Member of the Saddle Mountains Basalt of the Columbia River Basalt Group. The Pomona crops out over a wide region on the Columbia Plateau east of the Cascade Range, and the basalt of Pack Sack Lookout crops out well to the west of the Cascades about 30 to 60 km east of the Washington coast. Our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12‐m.y.‐old flow that erupted in western Idaho and flowed to the Washington coast. The magnetic data further indicate that southwest Washington has undergone clockwise rotation with respect to the Columbia Plateau and stable North America. The data suggest that both a large‐scale regional rotation of ∼16° and locally complex small‐scale rotations exist, the two being present to different degrees in the eastern and western regions of southwest Washington. The Cascade Range appears to coincide with the tectonic boundary, separating rotated and unrotated regions of Washington state. Rotation of southwest Washington appears to have been associated with the rotation of large pieces of western Oregon and southern Washington, possibly as a result of either a ridge‐push force from the Basin and Range province or shear along the Pacific‐North America plate boundary.
Geoid height‐age relation from SEASAT altimeter profiles across the Mendocino Fracture Zone
Authors: Sandwell David T. , Schubert Gerald
Published: February 2008

Twenty‐eight SEASAT altimeter profiles crossing the Mendocino Fracture Zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age, between ages of 15 and 135 m.y. An unbiased estimate of the overall geoid offset along each profile is determined from a least‐squares fit of the along‐track derivative of the geoid to the geoid slope predicted from a simple two‐layer gravitational edge effect model. Uncertainties based upon the statistical properties of each profile are also determined. A geoid slope‐age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets. The results are in agreement with geoid slope‐age relations determined from symmetrically spreading ridges (Sandwell and Schubert, 1980). However, the fracture zone estimates have smaller uncertainties and show less scatter. A comparison of these results with the geoid slope‐age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age (20–40 m.y.). A plate cooling model with a thickness of 125 km is most compatible with the geoid slope‐age estimates, although significant deviations occur; these may indicate that the lithospheric thermal structure is not entirely age dependent.
Crustal structure of the Mariana Trough
Authors: Ambos Elizabeth L. , Hussong Donald M.
Published: February 2008

Three seismic refraction profiles were shot in the Mariana Trough in the vicinity of the proposed axial rift spreading center. One long east‐west trending line was shot across the axial high region, the elevated portion of the trough associated with the rift zone. The two other lines were shot in a rough north‐south orientation subparallel to the trend of the axial rift, on crusts 1 and 5–6 m.y. old. Comparison of the results of the ray‐tracing analysis for these two profiles indicated that thinning of layer 2A (velocity of 3.3 km/s) and development of a distinct layer 2B (velocity of 5.3 km/s) and layer 3A (velocity of 6.7 km/s) take place with increasing crustal age, implying a process of crustal evolution. The presence of large‐scale crustal faults was noted in several instances. In addition, a consistent but low upper mantle seismic velocity of 7.7 km/s was measured for all three lines. These characteristics correlate with those previously observed for other areas of young oceanic crust. When compared with major active spreading centers such as the Mid‐Atlantic Ridge and East Pacific Rise, the Mariana Trough appears similar to the former in most respects. Both the Mid‐Atlantic Ridge and Mariana Trough are slow spreading (1–2 cm/yr half rate) and are characterized by rough topography, an axial rift, and a thick layer 2. Like the Mid‐Atlantic Ridge, the Mariana Trough also has no low‐velocity zone in the crust that could be interpreted as a magma chamber at the spreading center.
Magnetic anomalies and tectonic fabric of marginal basins north of New Zealand
Authors: Malahoff Alexander , Feden Robert H. , Fleming Henry S.
Published: February 2008

Detailed airborne magnetic studies conducted over the region of the S.W. Pacific marginal basins extending from the Solomon Islands to New Zealand suggest that three major phases of basin formation and island arc development have occurred in this region. Development of the Tasman Sea took place during the Late Cretaceous‐Paleocene. Development of the basins to the east of the Tasman Sea occurred predominantly during the Oligocene as well as during the Upper Miocene to Recent. The South Fiji Basin, consisting of the Kupe and Minerva Abyssal Plains, is marked by the presence of possibly two RRR triple junction spreading centers that were active between the times of anomalies 13 to 7 (36–25.5 m.y.). The Kupe Abyssal Plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center. The western limb appears to have been subducted beneath the present site of the Three Kings Rise. This seafloor spreading phase (calculated half‐spreading rate of 35 mm/yr) was coincident with the overthrusting phase of the New Caledonia ultramafic rocks. During that period, active volcanism along the then continuous Solomons‐New Hebrides‐Fiji‐Lau Island arc was taking place. Magnetic anomalies from 1 to 4 (0–8 m.y. B.P.) are seen to extend along a clearly defined lineation pattern over the North Fiji Basin. Analysis of the magnetic anomalies suggests that during the Upper Miocene, at about the time of magnetic anomaly 4 (8 m.y.), the Solomons–Lau Island arc commenced breaking up through the development of an active spreading center (calculated half‐spreading rate of 35 mm/yr) located between the New Hebrides and Fiji portions of the island arc. Portions of the northeastern sector of the Minerva Abyssal Plain were subducted beneath the North Fiji Basin during this period between 0–8 m.y. B.P. At the time of about anomaly 3 to 2′ (4.5–3.5 m.y.) the development of the North Fiji Basin extended into the Lau Basin and Havre Trough, thus separating the Tonga and Lau Ridges. As a result, a distinct series of active spreading centers developed southward along the axis of the Havre Trough–Lau Basin. The Havre Trough is marked by present seismic and submarine volcanic activity. Residual magnetic anomalies 1–2 (0–1.8 m.y. B.P.) form a symmetrical pattern around the axis of the trough extending south from the Lau Basin to the active Taupo‐White Island Volcanic Zone of New Zealand. Phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of Pacific Plate motion.
Convective thinning of the lithosphere: A mechanism for the initiation of continental rifting
Authors: Spohn Tilman , Schubert Gerald
Published: February 2008

Thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting. The lithosphere‐asthenosphere boundary marks the transition from a quasi‐rigid behavior of the mantle on geological time scales to a viscous one. The boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure. Additional heat carried to the boundary will cause it to rise and the lithosphere to thin. A one‐dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle. The ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile. The rate of thinning is determined by the initial and final equilibrium thicknesses, the thermal diffusivity, the transition temperature profile and the plume temperature profile. It depends only slightly on any latent heat associated with the lithosphere‐asthenosphere transition. During a large fraction of the time between the initial and final equilibrium states, the lithosphere thins at a rate which is inversely proportional to the square root of time t. Even 300‐km‐thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes. This heat contributes to an enhanced surface heat flow. Because of the constant level of radioactive heat production in the crust, the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere. For large increases in asthenospheric heat flow, the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs. Uplift due to thermal expansion upon lithospheric thinning is of the order of 102–103 m. A Rayleigh‐Taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism.
Controlled electromagnetic sources for measuring electrical conductivity beneath the oceans: 1. Forward problem and model study
Authors: Chave Alan D. , Cox Charles S.
Published: February 2008

Exact closed‐form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one‐dimensional earth are derived from the Maxwell equations. Numerical methods for the evaluation of the solutions are given, including correction for the finite size of real sources. Simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic, petrologic, and laboratory data, and their electromagnetic response is modeled. Horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range, and the horizontal electric field offers superior received signal performance. Reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges. Estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0.01–10 Hz are presented. The ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle, especially low conductivity zones, is demonstrated. If the mantle conductivity is low enough, horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved.
Paleomagnetic evidence for the continuity and independent movement of a distinct major crustal block in the southern Appalachians
Authors: Ellwood Brooks B.
Published: February 2008

The magnetization of 22 granitic and gneissic southern Appalachian rock units, with estimated cooling ages of 415–250 m.y., has been determined. Included are 842 samples from 114 sites within 19 granites (100 sites) and 3 gneisses (14 sites) located in North Carolina, South Carolina, and Georgia. Samples from 57 of these sites in 14 granites and 3 gneissic bodies yield a remanent magnetism after af demagnetization which exhibits good within‐site and/or between‐site directional consistency. Data for units which cooled to temperatures <300°C between 350–240 m.y., a period of apparently only slight North American plate motion, can be divided into two groups. A mean paleopole calculated for the first of these groups, group A (derived from six granites and gneisses 365–325 m.y. in age), located in the vicinity of Atlanta, Ga., is coincident with well‐defined Lower Carboniferous North American paleopoles. Site paleolatitude is estimated to be ∼11°S. Group B granites (six units) range in age from 350–250 m.y., are located to the SE of an arc drawn from Columbia, S.C., through Athens, Ga., to Macon, Ga., are apparently anomalous, and lie in a crustal block >20,000 km2 in size. The southeastern extent of this block is unknown due to Coastal Plain onlap. A mean paleopole for this group, with corrections for maximum tilt estimates, exhibits good precision but has a paleosite latitude of ∼10°N. Without tilt correction the paleopole for group B still exhibits an anomalous paleosite latitude of ∼4°N. These data indicate that the group B block (Elberton‐Sparta Crustal Block) lying to the SE has an apparent paleosite latitude corresponding to magnetization at a location to the north of the zone containing group A units. The preferred explanations for these data are (1) a northwest postemplacement tilting of the Elberton‐Sparta Crustal Block during the Alleghenian orogenic event in the southeastern Appalachians, or (2) emplacement as a microplate caught between North America and Africa during the collision of these continental masses. Both interpretations appear to be consistent with the available geological, geochemical, and geophysical data for the area. Of those granites which comprise a ∼400‐m.y.‐old group, only one granite, Lowrys, in South Carolina, exhibits good within‐site and between‐site precision. A paleopole calculated for the sites sampled in this unit exhibits fair precision and is consistent with magnetization acquisition as part of North America in Middle Paleozoic time.
Seismic properties, density, and composition of the Icelandic crust near Reydarfjördur
Authors: Christensen Nikolas I. , Wilkens Roy H.
Published: February 2008

Compressional and shear wave velocities, densities, and porosities have been measured to confining pressures of 6 kbar for 135 samples from a deep borehole in Eastern Iceland. The measurements are from a random sampling of dikes and sills through the 2 km hole. Velocities within the flow units are relatively low at the tops, reach maximum values within central portions, and decrease near the bases. This systematic variation correlates well with variations in porosity and bulk density. The dike units, on the other hand, have relatively uniform velocities, which are approximately equivalent to maximum velocities measured within flow units. Below 700 m the dike and flow velocities show systematic increases with depth which are related to changes in mineralogy and independent of porosity. The velocity gradient (dVp/dZ)T within this region is well defined by the laboratory velocities and approximates 0.6 s−1, in good agreement with field refraction measurements. High velocities near the base of the hole are related to increasing alteration with depth and, in particular, an increase in epidote content. The abundance of epidote in the rocks recovered from the lower portion of the drillhole supports a metamorphic origin for the layer 2‐layer 3 boundary in this region.
Chemical stratigraphy, Iceland Research Drilling Project, Reydarfjordur, eastern Iceland
Authors: Flower M. F. J. , Pritchard R. G. , Brem G. , Cann J. R. , Delaney J. , Emmerman R. , Gibson I. L. , Oakley P. J. , Robinson P. T. , Schmincke H.‐U.
Published: February 2008

Major element data are reported for lavas and dikes from a 3.2‐km section through the eastern Iceland lava pile. The section includes a 1.2‐km subaerial exposure and a 1.9‐km sequence drilled by the Iceland Research Drilling Project (IRDP). Alteration increases down‐section, although primary magma chemistry may be interpreted from FeO* Al2O3, TiO2, P2O5 and (to a lesser extent) SiO2 in addition to immobile trace elements. Lava compositions range from MgO‐rich tholeiite to SiO2‐rich icelandite, while dikes range from tholeiite to tholeiitic andesite. Three stratigraphic divisions based on volcanologic and trace element criteria (e.g., Ce/Yb and Zr/Y) are confirmed by major elements. Upper and lower groups are predominantly basaltic while the middle group includes a high proportion of silicic lavas. Variation within groups conforms to simple models combining fractional crystallization and accumulation of phenocryst plagioclase. During build‐up of the pile, eruptive phases commenced with silica‐rich magma and frequently terminated with basaltic or intermediate plagioclase cumulates. Dike chemistry resembles that of upper group lavas and also the nearby Breiddalur volcano. Middle group lavas probably derive from a buried silicic complex, while the lower group may represent the flank of a more deeply buried complex. Chemical stratigrapy is consistent with iterative burial with spreading of lenticular basaltic‐silicic complexes. Magma supply, fractionation, and mixing are considered in the framework of such kinematic models, although it is premature to draw direct analogies to the active Neogene zone of Iceland.
The Iceland Research Drilling Project: Synthesis of results and implications for the nature of Icelandic and oceanic crust
Authors: Robinson Paul T. , Hall James M. , Christensen Nicolas I. , Gibson Ian L. , Fridleifsson Ingvar Birgir , Schmincke Hans‐Ulrich , Schonharting Gunther
Published: February 2008

Study of a 3‐km‐thick section of Tertiary basalts in eastern Iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the Mid‐Atlantic Ridge. Deep drilling at Reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3.5 km. Three major stratigraphic units composed largely of basalts are recognized, each characterized by distinctive major and trace element patterns. Each of these units is believed to be the product of a separate episode of mantle melting and fractionation and probably reflects accumulation on the flanks of a central volcano. Extensive fractionation in shallow magma chambers led to intermediate and silicic lavas, which mark the cores of the central volcanic complexes. Dike density does not increase downward below about 1.5‐km crustal depth, and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes. Secondary mineral assemblages are dominated by smectite and zeolites near the top of the section and by epidote and quartz near the base. In general, the temperature of alteration increases downward, but the distribution of individual minerals is irregular. Locally, earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins. The upper surface of greenschist facies metamorphism crops out at the surface in Thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the Reydarfjordur borehole, located only 3 km east of the edge of Thingmuli. Thus, the metamorphic zones clearly crosscut the lava stratigraphy. Physical property profiles correlate well with variations in secondary mineralogy, and the top of layer 3 velocities corresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth. Magnetic susceptibility decreases with depth and is extrapolated to zero at about 4.3‐km crustal depth close to the estimated upper limit of greenschist facies metabasalts. A much greater crustal thickness, leading to the emplacement of high level magma chambers within the extrusive part of the crust, is thought to account for many of the differences in crustal structure and evolution between Iceland and the rest of the Mid‐Atlantic Ridge.
Strain hardening and strength of clay‐rich fault gouges
Authors: Morrow C. A. , Shi L. Q. , Byerlee J. D.
Published: February 2008

Frictional sliding experiments at confining pressures from 50 to 400 MPa were performed on thin layers of clay‐rich fault gouges from locations along the San Andreas and Hayward faults in California as well as pure clays from other locations. Both dry and saturated, drained samples exhibited a strain hardening that increased systematically with increasing confining pressure for the each particular gouge. In addition, the amount of strain hardening was greater with progressively stronger gouges among the suite of different samples. Tests using various lubricating mediums at the sample ends and different jacketing materials all showed that these possible constraints on frictional sliding had no effect on the strain‐hardening process. Therefore, the observed strain hardening was a material property of the fault gouges. The presence of water lowered the strength, coefficient of friction, and amount of strain hardening of the samples. The coefficient of friction ranged from around 0.21 to 0.58 under saturated, drained conditions at 200‐MPa confining pressure with the exception of pure montmorillonite. This sample had anomalously low strength and friction (μ ∼ 0.13) in relation to the other specimens. The strength of the gouges did not correlate well with mineral composition, grain size distribution, or location along the San Andreas fault; in particular, gouge samples from creeping and ‘locked’ sections of the fault showed no systematic differences in strength.
The differential path phase comparison method for determining pressure derivatives of elastic constants of solids
Authors: Peselnick L.
Published: February 2008

An ultrasonic method is presented which combines features of the differential path and the phase comparison methods. The proposed differential path phase comparison method, referred to as the ‘hybrid’ method for brevity, eliminates errors resulting from phase changes in the bond between the sample and buffer rod. Define r(P) [and R(P)] as the square of the normalized frequency for cancellation of sample waves for shear [and for compressional] waves. Define N as the number of wavelengths in twice the sample length. The pressure derivatives r′(P) and R′ (P) for samples of Alcoa 2024‐T4 aluminum were obtained by using the phase comparison and the hybrid methods. The values of the pressure derivatives obtained by using the phase comparison method show variations by as much as 40% for small values of N (N < 50). The pressure derivatives as determined from the hybrid method are reproducible to within ±2% independent of N. The values of the pressure derivatives determined by the phase comparison method for large N are the same as those determined by the hybrid method. Advantages of the hybrid method are (1) no pressure dependent phase shift at the buffer‐sample interface, (2) elimination of deviatoric stress in the sample portion of the sample assembly with application of hydrostatic pressure, and (3) operation at lower ultrasonic frequencies (for comparable sample lengths), which eliminates detrimental high frequency ultrasonic problems. A reduction of the uncertainties of the pressure derivatives of single crystals and of low porosity polycrystals permits extrapolation of such experimental data to deeper mantle depths.
On elastic waves in prestressed solids
Authors: Tolstoy I.
Published: February 2008

Simple explicit solutions of the dynamical equations for a prestressed solid under horizontal compression in a gravity field are derived to establish the magnitudes of two factors influencing seismic wave propagation velocities: (1) The overburden pressure which, at depth z, leads to wave velocity changes of order ρgz/μ, where ρ is rock density and μ is the rigidity. Compressional velocity increases with depth, whereas shear velocity decreases: at depths of the order of 150 km this effect is of the order of 5%. (2) The deviatoric stress, or stress difference P, which leads to anisotropic velocity changes of order P/μ; these depend both on the direction of propagation and the polarization of the waves. This phenomenon is only likely to be important for seismic theory under conditions approaching internal instability and buckling.
Influence of particle aggregation on deposition of distal tephra from the MAy 18, 1980, eruption of Mount St. Helens volcano
Authors: Carey Steven N. , Sigurdsson Haraldur
Published: February 2008

The May 18, 1980, eruption of Mount St. Helens (MSH) produced an extensive ashfall deposit in Washington, Idaho, and Montana with a minumum volume of 0.55 km3 (tephra). An unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ENE of MSH near Ritzville, Washington. Grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained (mean size, 22 μm), poorly sorted, polymodal, and rich in glass shards and pumice fragments. A computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics, particularly the thickness versus distance relationship. The model was constrained by observations of the eruption column height, elevation of major ash transport, lateral spreading of the eruption plume, and atmospheric wind structure in the vicinity of MSH. Results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central Washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause. For the model to fit the observed characteristics of the deposit, significant particle aggregation of ash finer than 63 μm must be invoked. The best fit occurs when ash less than 63 μm is aggregated into particles several hundred microns in diameter with a settling velocity of 0.35 m/s. Support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at Pullman, Washington, during the May 18 eruption (Sorem, 1982). The premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics, variations in component abundances, and thickness versus distance relationship of the May 18 MSH ash fall deposit.
Magmatic model for the Mount St. Helens blast of May 18, 1980
Authors: Eichelberger J. C. , Hayes D. B.
Published: February 2008

Analytical and numerical solutions to the hydrodynamic equations of motion, constrained by physical properties of juvenile ejecta in the Mount St. Helens blast deposit, were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of May 18, 1980. Evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone, substantial vesicularity of this ejecta, and continued vesiculation of large juvenile clasts after eruption. Observed or inferred ejecta velocities of 100 to 250 m/s are shown to require 0.2 to 0.7 wt % water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick. These conditions imply total magmatic water contents of 0.7 to 1.7 wt %, respectively. Such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice.
Attenuation of S waves in the lithosphere due to scattering by its random velocity structure
Authors: Sato Haruo
Published: February 2008

Recent seismological studies suggest that the amplitude attenuation of S waves in the lithosphere may be primarily due to scattering by its random inhomogeneity. There is, however, a disturbing discrepancy between statistical scattering theory and observation with regard to the asymptotic frequency dependence of Q−1 in the high‐frequency limit. Although the observed Q−1 decreases with frequency above 1 Hz, the usual mean wave formalism predicts Q−1 of media with random velocity fluctuation increases with frequency; we take a mean over waves with large travel time fluctuations that are caused by long‐scale velocity fluctuations compared with the wavelength studied. We propose a new statistical averaging method; mean wave is defined after the correction of travel time fluctuations. The random media with a mean square fractional velocity fluctuation of 5.0×10−3 and a correlation distance of 4.0 km will explain the observed and partially conjectured Q−1 for frequencies from 0.05 to 30 Hz.
Strain behavior of a granite and a Graywacke sandstone in tension
Authors: Schock R. N. , Louis H.
Published: February 2008

Westerly granite and Lance graywacke sandstone were deformed with one of the principal stresses tensile at the same time that the principal strains were monitored. Both rocks exhibited inelastic behavior prior to failure. Pronounced dilatancy and softening in shear took place in the granite. When Young's modulus varied, it did so continuously between compression and tensile stress states with no apparent discontinuity at zero axial stress, implying that cracks played the same role in tension as in compression. Failure was observed whenever the tensile stress exceeded a critical value, in agreement with predictions from Griffiths' failure criterion. The sandstone exhibited approximately twice as much strain per unit stress change as the granite and showed evidence of grain‐to‐grain movement as well as crack closure, results consistent with comparisons determined earlier when all stresses are compressive.
Characteristics of microcracks in amphibolite and granulite facies grade rocks from southeastern Pennsylvania
Authors: Padovani Elaine R. , Shirey Steven B. , Simmons Gene
Published: February 2008

Our observations of the microcracks in a small suite of samples of amphibolite and granulite facies rocks of variable composition show that (1) most cracks are partly or completely sealed, (2) the minerals that seal microcracks are unrelated to phase equilibria associated with granulite facies metamorphic conditions, and (3) the present crack porosity is related to bulk composition rather than to metamorphic grade. All observed cracks (open, healed, sealed) appear to have formed in response to tectonic stresses associated with retrograde metamorphism.
An assessment of 40Ar‐39Ar dating of incompletely degassed xenoliths
Authors: Gillespie A. R. , Huneke J. C. , Wasserburg G. J.
Published: February 2008

The possibility of measuring the age of eruption of Pleistocene lavas by 40Ar‐39Ar analysis of entrapped ancient potassic xenoliths is demonstrated by a study of model systems. Upon inclusion in the hot magma such xenoliths are commonly only partially degassed of radiogenic 40Ar which has accumulated in them since their original crystallization. The residual 40Ar will increase the apparent K/Ar age of the xenolith. However, if a xenolith is of Cretaceous age or younger, then a plateau in its 40Ar‐39Ar age spectrum giving the age of eruption is expected to extend over 25–50% of the total 39Ar released if degassing of the xenolith in the magma exceeded 90% and if the phases in the xenolith are characterized by sufficiently different diffusion dimensions or activation energies. If diffusion was from a bimodal population of spheres, then the radii must differ by a factor of 10 or more (or the diffusion coefficients by a factor of 100 or more); or if the spheres were equal in size (and in diffusion coefficients), then the activation energies must differ by a factor of at least 1.5. That such requirements may be realized in real xenoliths containing K‐feldspars is expected from published activation energies for microcline and from data determined on a granitic xenolith which was degassed in an early Pleistocene basalt flow. The experimental results appear to establish that old xenoliths may contain Ar in distinctive phases which degas at sufficiently different temperatures as to permit determination of the age of degassing or eruption.
Rapid F‐K migration of zero offset marine reflection data
Authors: Carter Jerry A. , Frazer L. Neil
Published: February 2008

Recently developed frequency‐wave number migration algorithms have made it possible to migrate digitally recorded zero‐offset reflection data economically. The greatest contributions to the speed of these methods lie in the well‐known symmetries of the Fourier transform and the assumption of a constant velocity section. Several problems have been addressed by using a newly proposed algorithm. The problem of incorrect phase shifts caused by the assumption of a two‐dimensional medium is solved by utilizing a three‐dimensional derivation of the theory. Another problem results from the assumption of a constant migration velocity. The parts of the record section with root mean square velocities less than the migration velocity will be overmigrated and the parts where the rms velocity is greater than the migration velocity will be undermigrated. Given a velocity‐depth function, the rms velocity to a selected portion of a section is a good estimate of the velocity necessary to migrate that portion correctly. Errors in the rest of the migrated section can be reduced by applying the time‐stretching formula of Stolt (1978) prior to migration. Beyond the spatial Nyquist frequency, aliasing noise can become a problem. The amount of aliasing is dependent upon the shot spacing, the frequency content of the source, and the dip of the reflector. The shot spacings commonly in use today are sufficiently small to avoid spatial aliasing; however, shot spacings should be decreased when steeply dipping reflectors are expected and the source frequency is high. The use of automatic gain control before migration tends to reduce the signal‐to‐noise ratio of the migrated section and should be avoided if possible, especially for record sections with large areas of low reflection strength.
Absence of silicic volcanism on Mars: Implications for crustal composition and volatile abundance
Authors: Francis P. W. , Wood C. A.
Published: February 2008

On Earth, eruptions of silicic magmas with volumes of 1–100 km3 are generally associated with stratovolcanoes or composite cones, whereas larger (100–1000 km3) ignimbrite deposits are erupted from resurgent calderas and related structures. Small volumes of silicic rocks are generated by differentiation within basic magma chambers associated with stratovolcanoes, but large silicic deposits are largely confined to areas where the granitic crust is thicker than 25 km. As mantle‐derived magmas rise through the thick granitic crust, fractional crystallization and assimilation of hydrous crustal rocks concentrate volatiles and SiO2 in the magma, yielding explosive silicic eruptions. On Mars, under similar chemical conditions, ignimbrite eruptions could take place over a broader range of physical conditions than on earth, but no large volume ignimbrite deposits can be identified positively. Explosive basic to ultrabasic eruptions may be responsible for small, cinder conelike features on Mars, and perhaps for the ancient, flank‐scoured paterae. Such eruptions may have been driven by near‐surface water/ice‐magma interactions or volatile rich magmas. The paterae represent a unique style of volcanism that stopped∼2 b.y. ago. We suggest that the volatiles associated with these features were derived from mantle sources, and that the cessation of patera formation may coincide with the termination of the period of maximum planetary degassing. Volcanism since∼2 b.y. ago has been dominantly effusive, except for minor explosive ash events such as those which blanketed part of the summit of Hecates Tholus. We propose that such eruptions were driven by volatiles generated by differentiation of a magma chamber, similar to Icelandic explosive eruptions. The apparent lack of large scale silicic volcanism on Mars is circumstantial evidence that Mars is deficient in volatiles and lacks a granitic crust.
Mars: Stratigraphy and gravimetry of Olympus Mons and its aureole
Authors: Hiller Konrad H. , Janle Peter , Neukum Gerhard Ph. O. , Guest John E. , Lopes Rosaly M. C.
Published: February 2008

Olympus Mons and its surroundings have been investigated using impact crater statistics. Crater retention ages of all major geological units show that their emplacement occurred during three time intervals. The underlying fractured plains are part of the northern plains in general (N(1) > 4 × 10−3 (km−2)). This time is followed by the time interval represented by crater retention ages N(1) = 8 ×10−4 to 3 × 10−3 (km−2). Some lava flows in the fractured plains, the smooth plains, and the aureole materials were emplaced during this time. The aureole probably developed by some sort of collapse of the outer parts of an ancient volcano (‘Proto‐Olympus Mons’) underlying the present shield. Its remnants may show in the scarp and small parts of the shield not covered by younger lava flows. The third interval covers crater retention ages N(1) = 5 × 10−5 to 3 × 10−4 (km−2). This volcanic activity followed a period of dormancy. A series of lava flows forming caldera and present shield surface and flooding a depression around the shield are distinguishable. The regional stratigraphy is discussed as part of the global geological history. The question of the genesis of the aureole material is addressed by gravity models. The topographic mass of Olympus Mons contributes about 80% to the aureole gravity high northwest of the mountain. The gravity high of the entire aureole can be explained by a density model consisting of the topographic masses of Olympus Mons and the aureole materials. It does not require additional high‐density materials underneath the aureole. Thus aureole materials of an allochthonous origin by collapse are in agreement with the gravity data. However, the gravity data do not preclude an autochthonous and volcanic origin of the aureole materials. In the latter case, possible subsurface intrusions should have no density contrasts with the surroundings.
Mars: Definition and characterization of global surface units with emphasis on composition
Authors: McCord T. B. , Singer R. B. , Hawke B. R. , Adams J. B. , Evans D. L. , Head J. W. , Mouginis‐Mark P. J. , Pieters C. M. , Huguenin R. L. , Zisk S. H.
Published: February 2008

Units of similar optical properties have been defined for a large part of the surface of Mars using primarily Viking 2 approach maps of red (0.59 μm) and violet (0.45 μm) normal albedo. Of available global data sets, spectral albedo most closely relates to surface mineralogy. The resultant unit map represents Mars at the time the Viking images were acquired (Ls = 103°–105°, morning, northern hemisphere summer in 1976). Computer processing of these data reveals clusters of albedo values which relate to specific, contiguous regions on the planet. Final unit definitions were determined by an interactive, iterative process which included scientific judgment on the part of the researchers; the results therefore are not the product of an unsupervised statistical classification. Some unit boundaries are of a gradational nature, while others are more discrete. Certain of the units are temporal and relate to the presence of condensates at the time of observation. Other units, by virtue of their redness and low albedo, are almost certainly not affected by condensates. A study of the albedo properties of analog materials measured in the laboratory and of telescopic spectrophotometry of Mars indicates that both hydroxylated (low temperature or secondary) and hematitic (high temperature or primary) oxidized materials exist on the surface. The most likely surface expression of global dust and/or high‐albedo soils is in the classical bright region Arabia. This material is probably a low‐temperature hydrated oxidation product of mafic volcanic glass, such as a palagonite or amorphous iron‐silica gel. Most low‐albedo features in the region studied have red and violet albedos consistent with a dark substrate (such as basalt) coated by, or mixed with, the fine‐grained alteration product mentioned above. There may be no genetic relationship between these two components in this majority of low‐albedo surface materials. Certain of the other low‐albedo regions are also the most red (highest R‐V) areas observed on the planet. These areas are consistent with a nonhydroxylated, more hematitic material such as that produced by high‐temperature hydrothermal alteration of basalt. Some of these dark, very red regions correlate with exposures of very ancient terrain. At this spatial resolution (10–20 km/pixel) there is no evidence of surface exposures of uncoated or unweathered basaltic rock or glass. Comparisons of the unit map with other data sets available to us revealed no striking detailed correlations. Larger‐scale correlations certainly do exist, however, among the various albedo data sets and with thermal inertia data.
Water vapor in Mars' Arctic: Seasonal and spatial variability
Authors: Davies Donald W.
Published: February 2008

The seasonal and spatial behavior of water vapor in Mars' arctic is described. These data were obtained by the Mars atmosphere water detector (MAWD) instruments on the Viking Orbiters. Maps of the spatial distribution of water vapor throughout the spring and summer, high resolution one‐dimensional scans, and seasonal histories of selected areas are presented. Peak water vapor amounts in excess of 100 precipitable microns occur shortly after summer solstice in the region immediately surrounding the permanent cap, at which time there is a great deal of fine‐scale structure in the vapor distribution. It is likely that the large amounts of vapor are derived from a surface or near‐surface frost deposit produced the previous year.
The seasonal CO2 cycle on Mars: An application of an energy balance climate model
Authors: James Philip B. , North Gerald R.
Published: February 2008

Energy balance climate models of the Budyko‐Sellers variety are applied to the carbon‐dioxide cycle on Mars. Recent data available from the Viking mission, in particular the seasonal pressure variations measured by Viking landers, are used to constrain the models. No set of parameters was found for which a one‐dimensional model parameterized in terms of ground temperature gave an adequate fit to the observed pressure variations. A modified, two‐dimensional model including the effects of dust storms and the polar hood reasonably reproduces the pressure curve, however. The implications of these results for martian climate changes are discussed.
Experimental determination of the spinel‐garnet boundary in a Martian mantle composition
Authors: Patera E. S. , Holloway John R.
Published: February 2008

The high pressure reaction orthopyroxene + clinopyroxene + spinel to garnet + olivine is applicable to the mantle of Mars. Experimental reversals of this reaction in the system CaO‐MgO‐FeO‐Al2O3‐SiO2 have been made in the range 1000°–1200°C using the bulk composition proposed by Morgan and Anders [1980]. At 1000°C, 1100°C, and 1200°C the garnet‐out equilibrium is bracketed at 10.8±0.5, 12.6±0.5, and 15.2±0.5 kbars, respectively. A dP/dT slope of 14.0 bar/K at 1000°C increasing to 26.0 bar/K at 1200°C has been inferred. Depending on bulk composition, the mantle of Mars is inferred to have a phase assemblage of either olivine + clinopyroxene + orthopyroxene + garnet or olivine + clinopyroxene + garnet + spinel. In the second, the partial melts will not be buffered by olivine and orthopyroxene and will have markedly lower silica activities than those of the terrestrial mantle.