bluuebunny/crossref_metadata_2025 huggingface dataset:
Nature of the H chondrite parent body regolith: Evidence from the Dimmitt breccia
Authors: Rubin Alan E. , Scott Edward R. D. , Taylor G. Jeffrey , Keil Klaus , Allen Jaclyn S. B. , Mayeda T. K. , Clayton R. N. , Bogard D. D.
Published: February 2008

The Dimmitt H chondrite regolith breccia consists of (in vol.%) 40% H4 and H5 chondrite clasts, 3% impact melt rock clasts, 0.5% shocked H chondrite clasts, 1.5% exotic clasts (including carbonaceous and LL5 chondrites), and 55% gas‐rich matrix. The LL5 clast is the best documented example of an ordinary chondrite in a host of a different compositional group. The matrix contains unequilibrated material, which differs from typical H3 material in having little (0.2 vol.%) fine‐grained opaque silicate matrix, and having 20% of the olivines with compositions in the range Fa21–24. About 10–15% of this unequilibrated material is probably derived from graphite‐magnetite‐rich chondrites and 2% from H3.0–3.5 chondrites. The absence of H3 clasts suggests that most of the unequilibrated material was derived from unconsolidated type 3 components. Many exotic clasts may have been derived from planetesimals that accreted to the H chondrite parent body prior to regolith development. One slowly cooled melt rock clast formed beneath a 500‐m‐thick melt breccia pile on the floor of a large impact crater and was later excavated by additional impacts, incorporated into the regolith and consolidated with other components to form the Dimmitt breccia.
Chemical weathering and diagenesis of a cold desert soil from Wright Valley, Antarctica: An analog of Martian weathering processes
Authors: Gibson Everett K. , Wentworth Susan J. , McKay David S.
Published: February 2008

Weathering, diagenesis, and chemical alteration of a soil profile from the Dry Valleys of Antarctica have been studied as an analog to soil development within the Martian regolith. Soil samples from a one‐meter‐deep soil pit on Prospect Mesa, Wright Valley, have been examined for their major element concentrations, water soluble cations and anions, carbon, sulfur, and water concentrations, and related petrographic characteristics of weathering in a cold, dry environment. Petrographic study of the soil samples indicates that most silicate mineral and lithic fragments exhibit some degree of alteration. Chemical alteration occurs in samples both from above and from within the permanently frozen zone. Concentrations of water soluble cations (Na+, K+, Ca2+) and anions (Cl−, SO42−, NO3−) decrease significantly from the surface to the permanently frozen zone, suggesting major movement of water soluble species. Enrichments in secondary mineral abundances correlate with the water soluble ion concentrations. Formation of zeolites is observed throughout the soil column and may be potential reservoirs for volatile storage within the regolith. From the detailed study of the Dry Valley soils, an idealized soil profile for the regolith has been developed and is suggested as applicable to other planetary surfaces, especially the Martian nearsurface environment. The soil profile consists of four zones: aeolian, salt, active, and permanently frozen.
A detailed record of the Lower Jaramillo Polarity Transition from a southern hemisphere, deep‐sea sediment core
Authors: Clement Bradford M. , Kent Dennis V.
Published: February 2008

A detailed record of the lower Jaramillo (reversed to normal) polarity transition was obtained from a southern hemisphere, deep‐sea sediment core (latitude = 35.91°E, longitude = 59.97°E) The record consists of over 850 samples taken across 140 cm of section. The transition itself is recorded across approximately 70 cm and is represented by more than 475 specimens from about 160 levels giving intermediate directions. The transition is identified by a nearly 180° shift from directions in good agreement with a reversed, axial dipole field to those closely aligned with a normal, axial dipole field for the core site latitude. The inclinations shallow gradually early in the reversal and pass through very steep negative values (−80°) late in the transition. The declinations show little appreciable variation until the inclinations have moved through the near vertical, and then slowly approach values in agreement with a normal polarity field. An intensity low accompanies the directional change during which the intensity drops to less than 15% of the maximum values observed in this sample interval. The intensity fluctuation spans a wider interval than the directional change, decreasing prior to any systematic change in the directions and then increasing to pre‐transition levels by the same depth at which the directions have stabilized. The VGP path constructed for this reversal is longitudinally constrained to a certain extent, between 140° and 230° for intermediate VGP latitudes and is roughly centered 120° from the site longitude. This path is therefore a far‐sided VGP path in Hoffman's [1977] terminology. Assuming a constant sedimentation rate (67m/Ma) through the Jaramillo Subchron, the duration of the transition is estimated to be 11,200 years to 4,500 years (depending on the criteria) for the directional change, whereas the associated intensity variation occurred over 15,000 to 20,000 years. Considered together with records of the most recent reversal(Matuyama/Brunhes) in light of current transitional field models, this record strongly suggests that the lower Jaramillo transitional field was dominated by different harmonics than the Matuyam/Brunhes transitional field.
Geomagnetic paleointensities from excursion sequences in lavas on Oahu, Hawaii
Authors: Coe Robert S. , Gromme Sherman , Mankinen Edward A.
Published: February 2008

Paleomagnetic data demonstrating three late Tertiary excursions in the direction of the geomagnetic field recorded in sequences of basaltic lavas on the island of Oahu, Hawaii were published by R. R. Doell and G. B. Dalrymple in 1973. We have determined geomagnetic paleointensities by the Thelliers' method for 14 lavas from the three sites. During these experiments, considerable difficulty was encountered because of the presence of titanomaghemite in many lavas and the contamination of natural remanent magnetization by lightning in many others. Moreover, we often observed the production of spurious high‐temperature chemical remanent magnetization during the Thellier experiments. An analysis of this particularly troublesome problem is presented. Two of the sites showed low paleointensities associated with angular departures of the paleomagnetic field direction from that of a geocentric axial dipole, which suggests that these excursions represent aborted reversals or fragments of reversals. At the third site, however, the paleointensity did not become low as the field diverged. This excursion may reflect the variation of a large nondipole source near Hawaii.
Simultaneous inversion for lateral velocity variations and hypocenters in the Yellowstone Region using earthquake and refraction data
Authors: Benz Harley M. , Smith Robert B.
Published: February 2008

A three‐dimensional upper crustal P wave velocity structure of the Yellowstone region was determined by simultaneous inversion using Pg refracted arrival times and earthquake‐generated direct P wave arrival times. The velocity structure was modeled by a two‐layer block configuration with the velocity of the surface layer fixed. The inversion was performed on a 14.4‐km‐thick second layer (5×6 blocks) using an optimal block size of 21.2×18.3 km and incorporated the method of separation of parameters. Results show volumes of higher velocities, approximately 6.3 km/s, north of the Yellowstone caldera that correlate with gravity anomaly highs and are interpreted to be associated with near‐surface crystalline basement. The upper crust beneath the caldera showed an average velocity of 5.8 km/s with prominent low‐velocity zones of 4.9 and 5.2 km/s in the northeast and southwest portions of the caldera, respectively. The 4.9‐km/s low‐velocity layer in the northeast corner of the caldera coincides with a localized‐20‐mGal Bouguer gravity low and the largest area of hydrothermal activity in Yellowstone National Park. Relocating earthquakes with the new velocity model showed a systematic migration of epicenters radially toward the caldera by an average of 1.2 km relative to locations using a homogeneous velocity model.
Permeability of fault gouge under confining pressure and shear stress
Authors: Morrow C. A. , Shi L. Q. , Byerlee J. D.
Published: February 2008

The permeability of both clay‐rich and non‐clay gouges, as well as several pure clays, was studied as a function of confining pressures from 5 to 200 MPa and shear strain to 10. Permeability ranged over four orders of magnitude, from around 10−22 to 10−18 m2 (1 darcy = 0.987 × 10−12 m2). The lowest values were characteristic of the montmorillonite‐rich and finer grained non‐clay gouges. Illite, kaolinite, and chlorite had intermediate permeabilities, while the highest values were typical of the serpentine and coarser grained non‐clay gouges. Grain size was an important factor in determining permeability, particularly for the clay‐rich samples. The coarse grained gouges were the most permeable and decreased in permeability after shearing. Conversely, the fine grained gouges had characteristically lower permeabilities that did not vary significantly after various amounts of shearing. The permeabilities of the non‐clay samples were not significantly different than those of the clays. Therefore, comminuted rock flours can be equally as effective in reducing the flow of water as the characteristically low permeability clay gouges. The strengths of the samples were quite variable. The non‐clay gouges were consistently the strongest, with yield points (beginning of nonelastic behavior) around 850 MPa, while montmorillonite had an anomalously low strength in relation to all the other gouges at 250 MPa. Strength of the saturated samples under drained (low pore pressure) conditions did not correlate with high or low permeability. However, the low permeabilities of these gouges could be a factor in the measured low shear stresses along fault regions if excess pore pressures were created as a result of shearing or compaction, and this pressure was unable to dissipate through a thick section of the material.
Chemical kinetics of water‐rock interactions
Authors: Lasaga Antonio C.
Published: February 2008

The recent literature on the kinetics of water‐rock interactions is reviewed. The data are then extended to provide a quantitative framework for the description of weathering and alteration. The available experimental data on dissolution of silicates verifies quantitatively the usual mineral stability series in sedimentary petrology. The rate of hydration of carbonic acid is shown to be a possible limiting factor in water‐rock interactions. The framework is developed to enable use of laboratory dissolution experimental results and thermodynamics to arrive at a rate law applicable up to equilibrium and therefore applicable to natural systems. The kinetic justification for the significance of a water‐rock ratio is discussed. With a proper treatment of fluid flow, the equations are applied to the weathering profile leading to the development of bauxites from nepheline syenites.
Diffusional crack healing in quartz
Authors: Smith David L. , Evans Brian
Published: February 2008

Annealing fractured single crystal quartz in the presence of pore fluid at 200 MPa (2 kb) pressure at 400° and 600°C for various periods resulted in the reorganization of initially planar cracks into arrays of spherical and tubular fluid inclusions, a process termed crack healing. Samples heated to 600°C for several days with no added pore fluid showed no optically measurable crack healing. The amount of healing in samples with added pore fluids is a function of the temperature, time, initial concentration of silica in the pore fluid, and the initial crack dimensions. Several lines of evidence indicate that cracks heal as silica is locally transported by diffusion along the crack surface and/or through the pore fluid. Extensive crack healing occurs at temperatures as low as 400°C in times as short as 1–2 days. Estimation of the thermal activation parameters for crack healing suggest that microcracks in quartz will have geologically short lifetimes at temperatures of 200°C or greater.
Effects of zeta potential on microcrack growth in rock under relatively low uniaxial compression
Authors: Ishido Tsuneo , Nishizawa Osamu
Published: February 2008

The seismic velocity, attenuation, and acoustic emission were measured to detect and monitor microcrack growth in samples of Inada granite as they were held under constant uniaxial compression. The samples were saturated with two kinds of aqueous solutions, aluminum nitrate solutions and potassium nitrate solutions, and the ζ potentials at the rock (mineral)‐water interface were varied with different concentration of the salts in the solutions. The axial stress was held constant at 94 MPa, that is, the stress at the onset of dilatancy for the present sample. At this stress level, the development of microcracks, which was indicated mainly by the decrease in velocity (>10%) of elastic waves propagating in the direction perpendicular to the axis of stress, was found to occur after certain time has elapsed and to be strongly affected by the ζ potential. An “incubation period” for the microcrack growth (i.e., brittle creep) becomes shorter, and subcritical crack growth rate becomes higher as the ζ potential approaches zero. A model of tensile cracking in a rock loaded in compression (circular hole model) was proposed to explain the observed time‐dependent microcracking, and some chemomechanical mechanisms of the ζ potential effects are discussed.
Fluid participation in deep fault zones: Evidence from geological, geochemical, and 18O/16O relations
Authors: Kerrich R. , La Tour T. E. , Willmore L.
Published: February 2008

Fluid incursion into fault zones and their deeper level counterparts, brittle‐ductile shear zones, is examined in a number of different crustal environments. At the Grenville front, translation was accommodated along two mylonite zones and an associated boundary fault. The high‐ (MZ II) and low‐temperature (MZ I) mylonite zones formed at 580 to 640°C and 430 to 490°C, respectively, in the presence of fluids of metamorphic orgin indigenous to the immediate rocks. A population of posttectonic quartz veins occupying brittle fractures were precipitated from fluids with extremely negative δ18O at 200 to 300°C. The water may have been derived from downwards penetration into fault zones of low‐18O precipitation on a mountain range induced by continental collision with, uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults. At Lagoa Real, Brazil, Archaean gneisses overlie Proterozoic sediments along thrust surfaces and contain brittle‐ductile shear zones locally occupied by uranium deposits. Following deformation at 500 to 540°C, in the presence of metamorphic fluids and under conditions of low water to rock ratio, shear zones underwent local intense oxidation and desilicification. All minerals undergo a shift of −10‰, indicating discharge of meteoric water recharged formation brines in the underlying Proterozoic sediments up through the Archaean gneisses during overthrusting: about 1000 km3 of solutions passed through these structures. At Yellow‐knife, a series of large‐scale shear zones developed by brittle‐ductile mechanisms, involving volume dilation with migration of ∼5 wt % volatiles into the shear zone from surrounding metabasalts. This early deformation involved no departures in redox state or whole rock δ18O from background states of Fe2+/ΣFe = 0.72 and 7 to 7.5‰, respectively, attesting to conditions of low water/rock. Shear zones subsequently acted as high‐permeability conduits for pulsed discharge of >9 km3 of reduced metamorphic hydrothermal fluids at 360–450°C. The West Bay fault, a late major transcurrent structure, contains massive bull quartz that grew at 200–300°C from fluids of 2 to 6‰ (possibly formation brines). In general, flow regimes in these fault and shear zones follow a sequence from conditions of high temperature and pressure with locally derived fluids at low water to rock ratios (during initiation of the structures) to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution and at shallower crustal levels there was incursion of oxidizing fluids from near‐surface reservoirs into the faults.
High fluid pressures during regional metamorphism and deformation: Implications for mass transport and deformation mechanisms
Authors: Etheridge M. A. , Wall V. J. , Cox S. F. , Vernon R. H.
Published: February 2008

Evidence is presented to support the conclusion that pore fluid pressures tresses PF during regional metamorphism are generally greater than or equal to the minimum principal compressive stress S3. The resultant very low effective stresses σ lead to significantly increased porosity and permeability, even at moderate to high metamorphic temperatures. Permeabilities between 10−18 and 10−15 m2 and are considered to be common, resulting in rapid fluid migration and the dominance of advective (infiltrative) over diffusive mass transport, even over relatively small distances. In view of the importance of intergranular mass transport to rock deformation during metamorphism, a mobile, high‐pressure fluid will have substantial rheological effects, especially in polyphase rocks. The fluid is capable of influencing the rate of dislocation creep in a number of ways. More importantly, advective mass transport along fluid pressure gradients can give rise to a solution transfer deformation mechanism that competes with conventional pressure solution. The rate of deformation by advective mass transport could be controlled by a number of processes, including dissolution kinetics, advective transport rates, and the rate of crack growth. A specific deformation model, based on advective transport rate control,is developed, which can produce strain at competitive rates but with stress and temperature dependences of unusual form.
Martian north polar hazes and surface ice: Results from the Viking Survey/Completion Mission
Authors: Christensen Philip R. , Zurek Richard W.
Published: February 2008

Viking infrared observations of the Martian north polar regions taken during the 1979–1980 Viking Survey/Completion mission have been used to determine the location and nature of the retreating polar cap edge and of an accompanying atmospheric ice haze. The haze is composed of water ice, and its visible opacity is approximately unity, equivalent to 1–2 precipitable microns of water ice. During the polar cap retreat (aerocentric longitude Ls ∼340°–60°), the haze is nearly always present and extends from at most a few degrees south of the polar cap edge to an undetermined distance over the cap interior. The transition from ice‐free ground to a predominantly ice‐covered surface at the cap edge spans 5°–10° of latitude. The ability of the Viking infrared observations to distinguish haze from surface condensate yields a more accurate seasonal regression for the polar cap than can be determined from visual images. Generally, the infrared observations presented here yield a regression similar to those determined from earth‐based and/or spacecraft visual data for the same and for previous northern springs. However, there are differences concerning details of the cap retreat; the thermal data indicate that the Martian seasonal polar cap did extend south of 60°N during this particular late winter and that the midspring resumption of the cap retreat occurs at Ls∼50.
The atmospheric inventory of xenon and noble gases in shales: THE PLASTIC BAG EXPERIMENT
Authors: Bernatowicz T. J. , Podosek F. A. , Honda M. , Kramer F. E.
Published: February 2008

Noble gases in five shales have been examined in the traditional protocol for trapped gas analysis and also in a new protocol in which the shales are sealed in air prior to analysis. The second protocol eliminates the possibility of gas loss in the preanalysis laboratory vacuum exposure characteristic of the traditional protocol. This experiment is designed to further test the “shale hypothesis”, which stipulates that atmospheric noble gases occur in the same proportions as “planetary” gases in meteorites and that the factor of approximately 23 deficiency of air Xe relative to planetary Xe is made up by Xe stored in shales or other sedimentary rocks. Previously available data do not support this hypothesis but could be criticized as possibly seriously underestimating shale Xe contents because of experimental limitations. The procedure used in this study circumvents these limitations, and the results still do not support the shale hypothesis. From these results it appears quite unlikely that sedimentary rocks can account for the hypothesized missing Xe and, unless some other atmospheric reservoir can be found to contain it, it correspondingly appears that the best working hypothesis is that atmospheric noble gases are simply not planetary. If so, either the chemical history of the earth must account for preferential storage of Xe in the earth's interior or solar nebula process responsible for noble gas incorporation in solids were significantly different for meteorites and the earth.
Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones
Authors: Schwartz David P. , Coppersmith Kevin J.
Published: February 2008

Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp‐derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400–666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south‐central segment of the San Andreas fault suggest that the M 8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constant b value) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes. Only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well.
The role of an intracrustal asthenosphere on the behavior of major strike‐slip faults
Authors: Turcotte D. L. , Liu J. Y. , Kulhawy F. H.
Published: February 2008

Measurements of strain accumulation adjacent to the San Andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault. Such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake. The restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike‐slip faults has been attributed to the damping effect of a viscoelastic asthenosphere. However, to explain the narrowness of the zone on the San Andreas fault, it is necessary to conclude that the thickness of the lithosphere is 10–20 km. This does not appear to be consistent with the relatively low surface heat flow measurements. In this paper an alternative four layer model is proposed to explain the behavior of the San Andreas fault. An upper elastic plate extends to a depth of about 15 km, the depth of the deepest seismicity on and adjacent to the fault. Beneath this upper elastic plate is a soft, intracrustal asthenosphere that exhibits a viscoelastic behavior. The viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz, which is known to have a very soft rheology, or the presence of water that can lead to pressure solution creep or to both effects. Beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks. And beneath this second elastic layer is the asthenosphere. There is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in California and elsewhere. We show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the San Andreas fault.
Amplification of moment and strain energy release due to interaction between different size fault slip zones
Authors: Rudnicki J. W. , Hirashima K. , Achenbach J. D.
Published: February 2008

Previous work of Rudnicki and Kanamori, which used collinear crack solutions to examine quantitatively the effects of fault slip zone interaction on moment, stress drop, and strain energy release, is extended by considering the effects of interaction between different size slip zones. The calculations demonstrate that the presence of a large preexisting slip zone can substantially amplify the low frequency seismic signal due to slip on a nearby smaller slip zone. For example, if the length of the larger slip zone is l, that of the smaller is l/10, and the distance between the nearest ends of the zones is l/100, the seismic moment due to slip on the larger zone induced by slip on the smaller zone is about 5 times that due to slip on an isolated zone of length l/10. Furthermore, because of the interaction between slip zones, the moment due to slip on the smaller zone is about 2.5 times the value for an isolated zone of the same size and subjected to the same effective stress. Conversely, estimates of stress drop based on the measured value of the moment and the total length on which slip occurs substantially underestimate the actual stress drop. Although the stress drop on the larger slip zone is zero, the induced slip there does contribute to WO, the difference between the strain energy change and the frictional work. In fact, it is demonstrated that WO = τeM/2μ where τe is the effective stress, M is the actual value of the seismic moment, and μ is the shear modulus.
Tectonic analysis of fault slip data sets
Authors: Angelier Jacques
Published: February 2008

Using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations, it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio ϕ = (σ2−σ3)/(σ1−σ3) between principal stress values (σ1≥σ2≥σ3, compression being positive). No assumption is made concerning the orientation of fault planes relative to stress axes, so that reactived faults are taken into account as well as newly created ones. Qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years. The practical limitation of the methods and the necessity for critical field observations are emphasized. These methods can be applied to focal mechanisms of earthquakes. A more complex analysis of heterogeneous data sets, involving an iterative separation of different stress systems, is also discussed. This analysis enables one to distinguish successive faulting events. Careful qualitative study in the field is in all cases essential.
Spatial seismicity variations and asperities in the New Hebrides Seismic Zone
Authors: Habermann R. E.
Published: February 2008

The term “asperity” has been used in many recent studies to describe sections of faults with higher strength than the surrounding zones. The level of background seismicity expected in such zones is unclear. They may appear as quiet zones because of high strength or active zones because of high stress. In this work the problem of seismicity levels in asperities is addressed by identifying regions of anomalously high or low seismicity and comparing the locations of these regions with the locations of asperities as determined from other observations. The regions of anomalously high or low seismicity are identified using a quantitative technique which relies on spatial rate comparisons done with the z test for a difference between two means. This technique allows the determination of the significance of spatial rate differences and the uniqueness of zones of outstanding seismicity. Four active zones, which make up 10%, and eight quiet zones, which make up 16% of the length of the New Hebrides seismic zone, were recognized using this technique. Asperities in the New Hebrides seismic zone were identified using characteristics which include large event initiation and stopping, clusters of aftershocks and moderate events, foreshock activity, and earthquake stress drops. Eight regions which showed two or more of these characteristics were identified. They make up 34% of the length of the seismic zone. The overlap between the asperities and the active and quiet regions provides a test of possible relationships between these zones. Thirty four percent of the asperity area is active, as opposed to the 10% expected, and 6% of the asperity area is quiet, as opposed to the 16% expected. This suggests that in the asperities in the New Hebrides seismic zone, stress concentration overcomes increased strength and that phenomena other than increased strength can cause seismic quiescence. Nearly all of the asperities in the New Hebrides seismic zone show high background seismicity or high stress drops, which suggests that likely zones of mainshock initiation can be identified using background seismicity.
Subducted slabs and the geoid: Constraints on mantle rheology and flow
Authors: Hager Bradford H.
Published: February 2008

Geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion. The total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow‐induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself. These boundary deformations, and hence the total gravity field, depend on the radial distribution of effective viscosity. If the internal density contrasts can be estimated, as is the case for subducted slabs, useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system. The degree 4–9 components of the observed long‐wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs. The (positive) sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more. The amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs, however. The amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers. Mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long‐wavelength geoid anomalies over subduction zones.
The geometry of back arc thrusting along the Eastern Sunda Arc, Indonesia: Constraints from earthquake and gravity data
Authors: McCaffrey Robert , Nábělek John
Published: February 2008

The Flores earthquake of December 23, 1978, represents the first seismological evidence for active back arc thrusting behind the eastern Sunda Arc. This mb = 5.8 earthquake occurred north of Flores Island, where seismic reflection profiling has revealed back arc thrusting, interpreted to be a reaction to compression of the arc following collision with the Australian continent. We investigate the source mechanism and depth of the Flores earthquake by inversion of long‐period P waveforms and relocate the earthquake's epicenter incorporating arrival time data from local stations. We find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11.5 km (7 km below the seafloor). The best fitting fault plane solution is consistent with active southward thrusting of the floor of the Flores Basin beneath the volcanic arc along a 30° dipping fault plane. The morphology of the thrust zone and the free‐air gravity profile over the Flores Basin in the epicentral region resemble those of oceanic trenches, and the observed gravity field is best interpreted if the crust of the Flores Basin dips smoothly below the accreted wedge at an average angle of about 6°. The position of the earthquake hypocenter south of the Flores Thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates. The Flores Thrust is thus the surface expression of a deep‐seated thrust zone and may represent the initial stage of polarity reversal of the eastern Sunda Arc.
Lead 210 and polonium 210 in Mount St. Helens ash
Authors: Nevissi A. E.
Published: February 2008

During the Mount St. Helens eruptions, lead 210 and polonium 210 were measured in volcanic ash samples. Polonium 210 was consistently higher than lead 210 in all samples, with the 210Po/210Pb ratios ranging from 2 to 12. The overequilibrium of the ratios is due to the enrichment of polonium compounds in the volcanic gases from hot magma relative to less volatile lead compounds.
Junction angles in drainage networks
Authors: Pieri David C.
Published: February 2008

The systematics of tributary junction angles are an important link between the topology of a drainage network and its topography because the angle of junction depends on the relative slopes of the intersecting tributaries. I present here a model for stream junction angle as a function of position (link magnitude)within the network which quantitatively predicts the observed down‐network increase of junction angle for tributaries joining progressively larger recipient links. Good agreement between model and data exists for mature drainage networks mapped at varying scales. Additionally, the dependency of junction angle on relative tributary slopes suggests that as individual links within the network modify their slopes to evolve toward graded longitudinal profiles, there exists a central tendency within the network, modulated by structural and lithologie controls, for evolution to an equilibrium or “graded“ planimetric pattern. Extraction of process‐related (as opposed to purely topological) information from network planimetric pattern is important in terrestrial and planetary remote sensing applications.
Segregation vesicles of ocean floor basalts: 2. Their use as paleovertical indicators
Authors: Bideau D. , Prévot M. , Lecaille A.
Published: February 2008

Pillow lava samples dredged from the rift valley of the Mid‐Atlantic Ridge (FAMOUS area) are oriented according to their cooling position on the seafloor (paleovertical) by the segregation vesicle technique (Bideau et al., 1977). Assuming that the spatial distribution of the oriented vesicles corresponds to a Fisherian distribution, it is possible to compute the dispersion coefficient K from the characteristics of the planar distributions (Von Mises distributions). From a theoretical point of view a high probability of finding crescent‐shaped segregation vesicles in thin sections of ocean floor basalts is suggested, and the distribution of the oriented crescents should have higher precision parameter k in sections approaching the paleovertical. However, from a set of 100 rocks, only 27 samples exhibit enough segregation vesicles to provide meaningful directional averages. The paleovertical indicated by segregation vesicles is usually poorly defined in spite of the large number (commonly, ∼100) of angular determinations for each rock. The mean precision parameter K is low (∼3), the λ 95% confidence semi‐angles are large (typically, 15°). Moreover, the calculated magnetic inclinations are almost randomly distributed, which suggests that either the preferential orientation of the segregation vesicles does not correspond to the paleovertical or that the rocks rotate between the temperature of segregation (>1000°C) and the temperature of magnetization (<300°C, generally). Within one of our pillows the occurrence of rotations at high temperature is demonstrated by a progressive rotation in the average direction of the axes of segregation vesicles from the margin toward the interior of the pillow. The amount of rotation, as measured in successive parallel thin sections, amounts to 35° between two sections only 9 cm apart. In an effort to select the samples the most suitable for reorientation purposes a subset of samples was selected, based on petrological criteria, where the segregation vesicles probably developed under the quietest conditions. The preferential orientation of the segregation vesicles of these samples may have been controlled by gravity alone (no systematic effect of flow‐induced rotation) during the segregation process. A second subset of rocks was independently selected using statistical criteria in order to select the more precise directional data. Only seven samples compose the intercept of these two subsets. These rocks show less scatter in inclinations, but two of them have negative inclinations (−23°, −42°), although sampled within the limits of the Bruhnes magnetic anomaly. The most plausible interpretation of these negative inclinations seems to be the occurrence of rotations before the rocks were magnetized but after segregation vesicle solidification. This study shows that the use of segregation vesicles for reorienting ocean floor basalts for paleomagnetic studies has to be critically reconsidered.
Calderas: A planetary perspective
Authors: Wood C. A.
Published: February 2008

Caldera‐bearing volcanoes are the largest volcanic edifices on a planet and are expressions of long‐lived, near‐surface magma chambers. On earth only three morphological classes of calderas are common: shield calderas (e.g., Mauna Loa, Hawaii), stratocone calderas (e.g., Crater Lake, Oregon), and ash flow calderas (e.g., Valles Caldera, New Mexico). Each of these classes has a characteristic caldera size, magma type, and tectonic location. No calderas have been recognized on the moon or Mercury, but Mars has large shield calderas as well as the unique highland patera calderas. The latter may represent an explosive, ultramafic style of volcanism, unlike any volcanic activity in modern earth history. Radar images suggest that Venus has at least one large caldera‐bearing shield volcano and possible calderas of other types. Jupiter's satellite Io has abundant, large shieldlike calderas but no shield massifs. The number and variety of calderas on a planet may be proportional to planet mass, consistent with a previous suggestion that volcanic complexity increases with planet size. Io is an exception, but heating in Io is generated tidally rather than by the decay of radioactive elements, so that its calderas are not related to its size. The occurrence of shield volcanoes on four solar system bodies suggests that thermal and tectonic mechanisms (i.e. hot spots) giving rise to construction of shield mountains and associated collapsed summits are basic processes in planetary geology. Hot spot volcanism is expected to be most important on planets lacking plate tectonics. On earth, large shields have lifetimes of only a few million years, but on Mars, large shields may have been active for 1.5×109 years. Shield calderas on earth are smaller than 20 km in diameter, but known or suspected shield calderas on Mars, Venus, and Io have considerably larger diameters. Assuming that caldera diameters are related to magma chamber diameters and depths, magma chambers on other planets are significantly larger and/or deeper than their counterparts on earth. Terrestrial magma chambers are small because terrestrial shield volcanoes are small in comparison to shields on Mars and Venus, and terrestrial shields are small because the earth's mobile plates move growing shields away from their deep sources, thus distributing magma along a narrow chain rather than piling it up around a central vent. Finally, the long lava flows of Mars are also explained if the volume of erupted magma has any simple relation to magma chamber volume.
Downsag calderas, ring faults, caldera sizes, and incremental caldera growth
Authors: Walker George P. L.
Published: February 2008

Not all calderas conform to the currently favored model, in which a cylindrical block subsides as in cauldrons of deeply eroded volcanoes. Some calderas are downsagged structures, a fact deduced mainly from the backtilting of ignimbrite sheets assumed originally to have possessed an outwardly directed slope. Few calderas have postcaldera vent rings; postcaldera vents more commonly lie on a transverse line, occupy a central position, or are scattered, indicating that any possible caldera ring faults were not sufficiently significant lines of weakness to guide uprising magmas. About as many volcanoes have vent rings outside the caldera as inside, and vent rings are known also where caldera faults are lacking. These and other lines of evidence suggest that vent rings on intermediate or silicic volcanoes are more likely to be situated on cone sheet‐type fractures, caused by upwardly directed magmatic pressure and related to updoming, than on ring faults related to caldera subsidence. The prevailing view of caldera formation is a catastrophic one, but some calderas grow incrementally in response to a succession of moderatesized eruptions. On some volcanoes having a small caldera, the present caldera may be merely the youngest of a succession generated by relatively small‐magnitude events. Attention is drawn to the fact that cauldrons and ring‐dike complexes are a feature of intracontinental cratonic volcanoes. It is significant that the anomalously large Tertiary calderas of the Basin and Range Province, which conform more closely to the currently favored caldera model than do the calderas in epicontinental settings, also cut Precambrian crust.
Ignimbrites of the Eastern Snake River Plain: Evidence for major caldera‐forming eruptions
Authors: Morgan Lisa A. , Doherty David J. , Leeman William P.
Published: February 2008

The eastern Snake River Plain is a predominantly rhyolitic province, analogous to the present‐day Yellowstone Plateau volcanic field but older and further evolved. The widespread Quaternary basaltic lavas that now blanket the eastern Snake River Plain appear to have erupted after major rhyolitic activity ceased and represent less than 20% of the total volume of volcanic rock near the north‐central margin of the plain. The rhyolites show a general age progression that becomes younger northeastward toward Yellowstone. Three major ignimbrite sheets, comparable in magnitude to those of the Yellowstone Plateau field, have been correlated on both sides of the eastern Snake River Plain from the Arco and Pocatello areas to margins of the Yellowstone Plateau. These 4.3‐ to 6.5‐Ma‐old ignimbrites are referred to herein as the Heise volcanic field. Studies of facies changes and lateral variations in the three major ignimbrites of the Heise field permit identification of related Calderas that are now buried beneath the plain. Petrologic studies suggest that, although the ignimbrites overlap compositionalIy, each was derived from an isotopicalIy distinct magma that contained a significant component of crustal material.
Mid‐Tertiary ash flow tuff cauldrons, southwestern New Mexico
Authors: Elston Wolfgang E.
Published: February 2008

This summary characterizes 28 known or suspected mid‐Tertiary ash flow tuff (ignimbrite) cauldrons, up to 40 km in diameter, in southwestern New Mexico. A combination of postcauldron block faulting and erosion has resulted in widely ranging levels of three‐dimensional exposures, down to plutonic roots. The evolution of the largest resurgent cauldrons followed the course determined by other workers for the Valles caldera (New Mexico) and Lake City cauldron (Colorado), with local variations. Five principal stages can be recognized: (1) precursor, (2) caldera collapse, (3) early post‐collapse volcanism, (4) major ringfracture volcanism, and (5) hydrothermal activity. Development can be terminated at any stage; stages can also be repeated. Resurgent doming can occur during stage 3 or later; doming can also be enhanced by reactivation of cauldron structures by postvolcanic basin and range faulting. Even where the surface of caldera‐fill tuff is domed, the base may have quite a different configuration. Recognition of resurgent doming may be difficult in deeply eroded cauldrons. A proposed modification in the definition of “resurgence” places emphasis on one or more magma pulses after a caldera‐forming eruption; doming is one of several effects that may or may not occur when magma invades a previously subsided cauldron. In some cauldrons, postcollapse lavas can be divided into (1) “cauldron lavas,” i.e., relatively mafic crystal‐rich disequilibrium assemblages derived from shallow defluidized residues of caldera‐forming ash flow tuff eruptions, and (2) “framework lavas,” i.e., relatively siliceous crystal‐poor near‐equilibrium assemblages derived from an evolving siliceous pluton at deeper levels, below the cauldron complex. Special features of New Mexico cauldrons include zoned outflow sheets; zoning may be normal (most siliceous at the bottom) or reversed. Posteruption potassium metasomatism may mask some chemical trends. Several styles of eruption can be recognized among mid‐Tertiary cauldrons of southwestern New Mexico. The largest ones tend to occur in clusters, interpreted as surface expressions of buried composite plutons (e.g., Mogollon Plateau). In an early stage of development, or on the fringe of a cluster, cauldrons tend to be asymmetrical trapdoor structures, formed by repeated small eruptions separated by quiescent periods. During the height of activity, cauldrons formed by eviscerating eruptions, commonly followed by resurgence. The youngest known cauldron is shallow and resulted from asymmetrical subsidence and collapse of caldera walls.