bluuebunny/crossref_metadata_2025 huggingface dataset:
A precursorlike change in coda excitation before the Western Nagano Earthquake (Ms = 6.8) of 1984 in central Japan
Authors: Sato Haruo
Published: February 2008

Temporal variations of coda duration were studied in relation to a crustal earthquake (September 14, 1984) which took place near the NW corner of the Kanto‐Tokai microearthquake observation network of the National Research Center for Disaster Prevention in Japan. Vertical seismograms recorded at station GER close to the main shock epicenter (Δ = 24 km) between February 1982 and December 1984 were analyzed. A precursorlike change was found in the relation between coda duration and average magnitude for 185 earthquakes which occurred in a small volume in the middle of the aftershock region. The average magnitude is the arithmetic mean of station magnitudes which are calculated from the maximum amplitudes of direct waves. Coda durations for earthquakes of the same average magnitude were significantly longer during the 16‐month period preceding the main shock than those before and after this period with a confidence level of 99.9%. The increase in coda durations was about 3 s on average in this period. The coda duration anomaly was significant for lapse time shorter than 30 s, which corresponds to a travel radius less than ∼50 km. Although no clear temporal change was found in the residuals of station magnitude measured at GER from average magnitude, an attenuation change in the preevent period can not be ruled out due to the large scatter of data. Crack formation, however, is the most plausible mechanism for an increase in scattering intensity around the focal region in the stress accumulation stage and is consistent with other precursory observations. Clusters of cracks in the heterogeneous earth medium might be big enough to scatter high‐frequency seismic waves, even though each individual crack may be very small.
Crustal genesis on the Oregon Plateau
Authors: Carlson Richard W. , Hart William K.
Published: February 2008

The areal and temporal distribution and the chemical and isotopic characteristics of Cenozoic volcanic rocks from the Oregon Plateau reflect the changing tectonic conditions affecting this area over the last 20 m.y. Following subduction zone related calc‐alkaline volcanism, flood basalt eruptions occurred in the time interval 18–14 Ma. This primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the Juan de Fuca ridge that resulted in changes in the geometry of subduction against the Pacific Northwest. The compositions of the mid‐Miocene flood basalts show signs of extensive crystal fractionation, in some cases accompanied by crustal contamination. Roughly 11 m.y. ago, basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions. This implies that as extension in the Oregon Plateau proceeded, the crust became less of an impediment to the passage of relatively unfractionated mantle melts. Initial Sr, Nd, and Pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava. Primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area. These observations suggest that an area of some 130,000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern North America over the last 20 m.y. The intensity of basalt volcanism as manifest in the Columbia River Group and in the Oregon Plateau does not appear to reflect the presence of a “hot spot” beneath the area. Both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by “back arc” spreading in a manner analogous to the formation of the marginal basins of the western Pacific. This spreading initiated along the western border of the Archean Wyoming craton with the location of mantle ascent, melt production, and extraction controlled by the changing geometry of subduction, the existence of a zone of lithospheric weakness, and the presence of a thick, low‐density, subcontinental mantle “keel” existing beneath the Archean craton in this area.
In situ observation of the olivine‐spinel phase transformation in Fe2SiO4 using synchrotron radiation
Authors: Yagi Takehiko , Akaogi Masaki , Shimomura Osamu , Suzuki Toshihiro , Akimoto Syun‐Iti
Published: February 2008

Detailed in situ observations of the olivine‐spinel phase transformation in Fe2SiO4 were performed using a cubic anvil type of high‐pressure and high‐temperature X ray diffraction apparatus combined with synchrotron radiation. An accurate equilibrium phase boundary, density increase at the transition, and thermal expansion of spinel phase were determined as follows: Transition pressure P (GPa) = 2.75 + 2.5 × 10−3 T (°C); Δρ at 5.3 GPa and 1000°C = 9.5%; mean volume expansion between 25°C and 1000°C at 5.3 GPa = 2.0 × 10−5 K−1. Comparisons of the observed and calculated equilibrium phase boundaries indicate that when the effect of cation disorder is taken into account, the observed phase boundary becomes consistent with thermochemical data. The thermal expansion of the spinel phase observed in its stable field differs significantly from that observed at atmospheric pressure. Nonuniform formation of the diffraction peaks of the spinel phase was not observed in the present study, which indicates that the mechanism of the olivine‐spinel transition may depend on the stress state of the specimen.
Stress rotation during the Coalinga Aftershock Sequence
Authors: Michael Andrew Jay
Published: February 2008

This study considers spatial and temporal changes of the stress regime during the 1983 Coalinga aftershock sequence. In both cases the observed changes manifest themselves as rotations of the most compressive stress axis. Inversion of 165 M≥3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24°E of north, while in the northern and central zones it is oriented 51°E of north. Statistical analysis by use of non‐parametric bootstrap resampling shows that this rotation is significant at above the 95% confidence level. Inverting the focal mechanisms of 122 M ≥3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock. The compressional axis rotates from an azimuth of 62°E of north to 47°E of north. Statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95% confidence level, subject to the assumption that 80% of the fault planes can be correctly picked from the two nodal planes. As the number of fault planes picked correctly varies from 50% (random choices) to 100% the significance of the rotation varies from 87% to 98%. Further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks. Active folds in the area suggest that the pre‐main shock stress regime had a compressional axis with a NE‐SW orientation. Thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock. This stress anomaly may be causally related to the occurrence of the aftershocks.
Heterogeneous kinetics and diffusion boundary layers: The example of reaction in a fracture
Authors: Casey William H.
Published: February 2008

At steady state both surface‐ and transport‐controlled reaction rates can be expressed in a single form which resembles a first‐order rate law. This rate law includes a dimensionless parameter analogous to the Biot number of heat transfer, and the Damköhler number of combustion physics. This parameter is useful for predicting the rate‐controlling step reaction for overall mineral dissolution or growth. For cases in which this parameter is much larger than 1, the reaction rate is controlled by solute transport through a boundary layer. When the parameter is much less than 1, the reaction rate is surface controlled. Because the activation energies for surface‐controlled reactions are generally much larger than those of diffusion, one anticipates that reaction mechanisms may vary from surface to transport control with increased temperature. As an illustration, this parameter is used to predict the rate‐controlling step reaction for quartz, calcite, and brucite dissolution as a function of temperature.
Determining planes along which earthquakes occur: Method and application to earthquakes accompanying hydraulic fracturing
Authors: Fehler Michael , House Leigh , Kaieda Hideshi
Published: February 2008

Although it is generally accepted that earthquakes occur along preexisting faults, the distribution of earthquake locations is often so smeared that the underlying fault or joint structures along which the earthquakes occur cannot be inferred from visual inspection of location plots. We present a statistical method for identifying fault or joint planes within what may otherwise appear to be an amorphous earthquake location set. The method takes all the hypocenters in an event set three at a time in order to determine the strikes and dips of all possible planes within the event set. A procedure for correcting for the shape of the region in which the earthquakes occur is applied. After correction, the orientation (one or a few) that is seen most often is taken as that of the zone of preexisting fault(s) or joint(s). We applied the method to a set of hypocenters determined for microearthquakes that accompanied a hydraulic injection into crystalline rock. The method was able to resolve successively five statistically significant orientations (planes) along which most of the microearthquakes occurred. The first two planes determined by the method are parallel to one nodal plane from each of the two most commonly found fault plane solutions. One of the two planes intersects the injection well bore at a location where water is known to have entered the rock during the injection. The planes identified thus coincide with the major fluid paths during the hydraulic injection.
Measurement of the melting curve of Mg0.9Fe0.1SiO3 at lower mantle conditions and its geophysical implications
Authors: Heinz Dion L. , Jeanloz Raymond
Published: February 2008

An experimental determination of the melting of Mg0.9Fe0.1SiO3 under lower mantle conditions is presented. Samples were compressed and laser heated (Nd‐YAG) in a Mao‐Bell‐type diamond cell. The temperature fields generated in the sample are measured using a simple tomographic technique and spectroradiometer. Mg0.9Fe0.1SiO3 in the perovskite structure melts at 3000 ± 300 K independently of pressure over the range of 22–65 GPa. The melting curve of Mg0.9Fe0.1SiO3 represents an experimental upper bound upon the geotherm from 700 to 1500 km. This melting curve corresponds to a homologous temperature of 0.70 or 0.85 for one‐ or two‐layer mantle convection, respectively. This is consistent with either one‐ or two‐layer mantle convection and a lower mantle of constant viscosity (based upon homologous temperature arguments).
Morphology and tectonics of the Kane Transform from Sea Beam bathymetry data
Authors: Pockalny Robert A. , Detrick Robert S. , Fox Paul J.
Published: February 2008

High‐resolution Sea Beam data have been used to construct an accurate, regional scale, bathymetric map of the Kane Transform, a large‐offset, slowly slipping (30 mm/yr), central North Atlantic transform. The Kane Transform trends about 100°, nearly orthogonal to the regional trend of the adjacent Mid‐Atlantic Ridge (MAR) rift valley. The transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the MAR. The ridges narrow and deepen toward the transform valley over distances of 10–20, km, and the intervening troughs widen into distinct closed‐contour depressions that appear to be relict nodal basins. The morphologic elements found on either side of the transform valley exhibit along‐strike continuity and represent undisrupted displacement markers. The swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins, and ridges that integrate to create a distinctive transform‐parallel grain. This interval, the transform tectonized zone (TTZ), marks the time‐averaged location of strike‐slip tectonism as parcels of crust move from each intersection along the transform interface. The TTZ is widest (∼6 km) near the eastern ridge‐transform intersection (RTI) and narrows to a strand less than 3 km wide along the central and western portions of the transform. Along the eastern portion of the transform valley a narrow (∼3 km) median ridge several hundred meters high can be traced for more than 40 km along the valley floor, and a similar, but shorter, feature is also present near the western RTI. At both RTIs the neovolcanic zone from the adjacent rift valley can be traced, as a series of constructional edifices, across the entire intersection area to the Opposing transform valley wall. At the eastern RTI the neovolcanic zone wraps around the eastern edge of the nodal basin, while at the western RTI it appears to bisect a paired nodal basin located in the intersection area. A line drawn connecting the ends of each neovolcanic zone lies within the narrow TTZ and has a strike of 098°. Using the bathymetric map of the Kane Transform as a record of the time‐integrated tectonic evolution of this large, slowly slipping ridge‐transform‐ridge plate boundary, we have constructed a simple model for the evolution of RTIs. Our model emphasizes the interplay between temporal variations in crustal production rates (i.e., variations in the volume of basaltic melt production) along oceanic spreading centers and extensional tectonism. This model can explain the existence of nodal basins at RTIs and relict nodal basins along transforms and fracture zones, as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks.
Distribution of Late Cenozoic volcanic vents in the Cascade range: Volcanic arc segmentation and regional tectonic considerations
Authors: Guffanti Marianne , Weaver Craig S.
Published: February 2008

Spatial, temporal, and compositional distributions of approximately 4000 volcanic vents formed since 16 Ma in Washington, Oregon, northern California, and northwestern Nevada illustrate the evolution of volcanism related to subduction of the Juan de Fuca plate system and extension of the Basin and Range province. Vent data were obtained from published map compilations and include monogenetic and small polygenetic volcanoes in addition to major composite centers. On the basis of the distribution of 2821 vents formed since 5 Ma, the Cascade Range is divided into five segments, with vents of the High Lava Plains along the northern margin of the Basin and Range province in Oregon forming a sixth segment. Some aspects of the Cascade Range segmentation can be related to gross structural features of the subducting Juan de Fuca plate. The orientation of the volcanic front of segments one and two changes from NW in northern Washington to NE in southern Washington, paralleling the strike of the subducting Juan de Fuca plate. Segments one and two are separated by a 90‐km volcanic gap between Mount Rainier and Glacier Peak that is landward of the portion of the subducting plate having the least average dip to a depth of 60 km. A narrow, N‐S trending belt of predominantly andesitic vents in Oregon constitutes a third segment, which is landward of the seismically quiet portion of the subduction zone. The narrowness of this segment may indicate steep dip of the subducting plate beneath the Cascade arc in Oregon. Vents are sparse between segment four (containing the Mount Shasta and Medicine Lake centers) and segment five (containing Lassen Peak), where the Juan de Fuca and Gorda North plates are characterized by differing age, amounts of subcrustal seismicity, and probably geometry. From the relation between seismicity at depth of 60 km and the position of the volcanic front of vents formed since 5 Ma, transitions between subducting‐plate segments of varying geometry likely occur near boundaries between independently defined volcanic segments in northern Oregon and northern California. In the Basin and Range province east of the Cascade arc, volcanism migrated into the region adjacent to the Cascade Range during the interval 5–10 Ma. Since 5 Ma, the impingement of the two provinces is characterized by cessation of basin‐range volcanism in southern Oregon, continuation of basaltic volcanism in northeastern California where the impingement process may not yet be complete, and contraction of the area of mafic volcanism around Mount Shasta, Medicine Lake, and Lassen Peak. In central Oregon where the northern margin of basin‐range volcanism (the High Lava Plains) intersects the Cascade arc, impingement of basin‐range extensional volcanism approximately coincides in time and space with the development of the High Cascade graben between Three Sisters and Mount Jefferson.
Global albedo variations on Mars: Implications for active aeolian transport, deposition, and erosion
Authors: Christensen Philip R.
Published: February 2008

Observations at 0.3–3.0 μm by the Viking infrared thermal mapper have been used to determine the temporal and spatial variations of the albedo of the Martian surface and atmosphere. These data were collected into 1° × 1° latitude, longitude bins, for local times of 6–10 H, 10–14 H, and 14–18 H (24 H equals 1 Martian day), and for 10° Ls bins throughout the Viking mission. The major variations in albedo were associated with the two global dust storms of 1977, The first storm began near Ls 205°, and the second storm began between Ls 273.5° and 274.0°, consistent with Viking imaging and thermal observations. For optically thick atmospheric dust, the observed albedo was 0.37–0.40. Over all regions the atmosphere cleared substantially between the two storms, with many southern hemisphere regions returning to their prestorm albedos. In general, the northern hemisphere atmosphere had a higher dust content throughout the storm phase, and retained dust longer during the decay phases, than the southern hemisphere. Southern hemisphere dark regions were not measurably brighter following the global storms, suggesting little net deposition of dust. In contrast, the northern hemisphere dark regions of Syrtis Major and Acidalia Planitia were measurably brighter following the storm, indicating the deposition of ∼7–45 μm of dust per year. These surfaces subsequently darkened over the following months, returning to prestorm albedo values prior to the next dust storm season. The material raised from these surfaces, and atmospheric dust observed locally over several other dark regions during the clear periods, may account for the general dust haze observed throughout the year. It appears that dust is also deposited in bright, low‐inertia regions, where it remains. The albedo data suggest active dust transport into the north, with net deposition in the bright, low‐inertia regions. This south‐to‐north transport of dust during global storms is consistent with atmospheric circulation models, which predict the presence of cross‐equatorial Hadley circulation during this season. Because of the close connection between dust transport and atmospheric circulation at present, it is possible that under reversed conditions of north‐to‐south circulation during epochs when the maximum solar heating occurs in the north, the current deposits would be eroded and transported back to the south. This continual reworking of an active, surface dust layer would therefore not require substantial erosion of the surface to account for the high level of aeolian erosion and transport observed at present.
Seismotectonics of northern Alaska
Authors: Estabrook Charles H. , Stone David B. , Davies John N.
Published: February 2008

Data from earthquakes occurring in northern Alaska collected by the Geophysical Institute of the University of Alaska, Fairbanks, have been reprocessed using a seismic velocity model developed for the Dall City and Fort Yukon areas of north central Alaska. A study of the relocated events shows that microearthquakes occur in a zone roughly parallel to, but south of, the crest of the Brooks Range. The events also show that the Kobuk, central Kaltag, Porcupine, Dall, Rampart, northwest Tintina, and Eskimo Lakes faults in northern Alaska and northwest Canada are seismically active, with the Eskimo Lakes fault being most active. Eight new focal mechanisms were determined for northern Alaska which show strike‐slip faulting south of the Brooks Range. This movement is consistent with the known movement on the associated faults. The orientation of the pressure axes derived from these and other fault plane solutions is consistent with Pacific‐North American plate convergence. Large‐scale shearing appears to extend northward from the Pacific‐North America transform boundary into arctic Alaska and allows the interpretation that the Seward Peninsula extensional zone is a large‐scale pull‐apart feature.
HENGILL TRIPLE JUNCTION, SW ICELAND 2. ANOMALOUS EARTHQUAKE FOCAL MECHANISMS AND IMPLICATIONS FOR PROCESS WITHIN THE GEOTHERMAL RESERVOIR AND AT ACCRETIONARY PLATE BOUNDARIES
Authors: Foulger G. R.
Published: February 2008

The Hengill ridge‐ridge‐transform triple junction is a complex tectonic unit containing two volcanic systems, one active and one extinct, and widespread geothermal resources. The area exhibits continuous, small‐magnitude earthquake activity. Good constraint was obtained for the radiation patterns of 178 small earthquakes within the area, of which 50% are non‐double couple and are interpreted as resulting from tensile crack formation. These events occurred in the depth range 2–7 km and indicate that at these depths in the area extensional stresses must be prevalent, that outweigh the compressive effect of the overburden. The tensile crack events were mingled with double couple events, and were confined to the high‐temperature geothermal area. Outside of this, seismic faulting was almost entirely shear. During a 3‐month recording period, tensile crack type events were of low magnitude and accounted for only 2% of the seismic stress release. The fault plane of the shear events was deduced from the orientation of the tensile crack strikes. The continuous small‐magnitude seismicity is attributed to the action of circulating groundwater fluids on hot rock at depth which remove heat and cause contraction of the rock, thereby reducing the effective confining stress. This process induces cooling contraction cracks that release thermal and regional stress in a mode that is consistent with the regional stress regime. In close proximity to the accretionary plate boundary, this stress regime is extensional, and some of the fractures formed are tensile cracks. The spatial distribution of the continuous earthquake activity represents a map of the heat source of the geothermal area. In the Hengill area the earthquake locations indicate that the high‐temperature geothermal area is fueled by two main distinct heat sources. These are associated with the two volcanoes, and separate geothermal fields that exhibit contrasting reservoir characteristics. Continuous small‐magnitude earthquake activity is commonly associated with geothermal areas worldwide, and the results from the Hengill area indicate that they may be used elsewhere as a prospecting tool to gain unique information about the heat source.
Variation in sulfur dioxide emissions related to earth tides, Halemaumau Crater, Kilauea Volcano, Hawaii
Authors: Connor Charles B. , Stoiber Richard E. , Malinconico Lawrence L.
Published: February 2008

Variation in SO2 emissions from Halemaumau crater, Kilauea volcano, Hawaii is analyzed using a set of techniques known as exploratory data analysis. SO2 flux was monitored using a correlation spectrometer. A total of 302 measurements were made on 73 days over a 90‐day period. The mean flux was 171 t/d with a standard deviation of 52 t/d. A significant increase in flux occurs during increased seismic activity beneath the caldera. SO2 flux prior to the this change varies in a systematic way and may be related to variation in the tidal modulation envelope.
Subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab/mantle interface
Authors: Helffrich George R. , Stein Seth , Wood Bernard J.
Published: February 2008

Seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested. These studies, employing waves reflected and converted at the slab/mantle interface, suggest that a velocity contrast of 5–10% occurs over a distance of 10–20 km to depths of at least 350 km. We have investigated whether such sharp contrasts can result from thermal structure, compositional differences, phase changes, or other effects such as preferred mineral orientation. Using a mantle and slab temperature model, a petrologic model of mantle and slab mineralogy, and a data base of elastic properties of mantle and slab phases, theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints. At 9.6 GPa and 1000°C (conditions modeled at the slab/mantle interface at ∼280‐km depth), cooler temperatures in the slab interior produce, at constant composition, only approximately a 1.75% Vp increase into the slab in any of the peridotitic and eclogitic mineralogies investigated. Variation in Vp among the different bulk compositions is only approximately 0.5%. These contrasts are substantially lower than those obtained from the seismological studies and indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts. We propose that phase transformations in mantle and slab mineralogies, notably elevation of the olivine‐spinel phase change normally occurring at 400‐km depth, are important factors in these sharp contrasts. Other contributing effects may include preferential mineral orientation near the slab surface and, at shallower depths, the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface.
The Irpinia (Italy) 1980 earthquake: Detailed analysis of a complex normal faulting
Authors: Bernard Pascal , Zollo Aldo
Published: February 2008

A detailed analysis of near‐source strong motion and leveling data, together with the results of teleseismic waveform modeling by Westaway and Jackson (1987) and aftershock studies by Deschamps and King (1984), allows a satisfactory kinematic description of the complex normal faulting associated with the magnitude Ms = 6.9, November 23,1980, Irpinia earthquake (southern Italy). The three main rupture episodes, starting at about 0, 20 and 40 s, are here associated with better constrained source parameters than in previous studies. We first evaluated the triggering time for the 11 closest accelerometers by using the well‐recorded 40‐s strong motion phases. This gave the absolute time and location of the dominant episodes of faulting. The first rupture propagated mainly toward the northwest on a NE dipping normal fault, at a mean velocity close to 3 km/s for about 20 km, and continued 15 km further on smaller subfaults. It was associated with surface breakage in the southeastern part. The second rupture started from the southeastern end of the first rupture, about 18 s after it, and propagated about 20 km toward the southeast on a low‐angle normal fault dipping 20°NE. It was associated with secondary faulting on steeper planes reaching the surface. The third and last episode at 39 s nucleated near the first hypocenter, at shallower depth, and the rupture possibly propagated on a 10‐ to 15‐km‐long normal fault, striking SE, antithetic to the first activated fault. A clear correlation appears between the strength of the geological formations and the existence of surface breakage and shallow aftershock activity.
Chemical and mineralogic trends within the Timber Mountain‐Oasis Valley Caldera Complex, Nevada: Evidence for multiple cycles of chemical evolution in a long‐lived silicic magma system
Authors: Broxton David E. , Warren Richard G. , Byers Frank M. , Scott Robert B.
Published: February 2008

Rocks of the Miocene and Pliocene Timber Mountain‐Oasis Valley (TM‐OV) caldera complex in the southwest Nevada volcanic field are dominantly rhyolites and quartz latites (trachytes). Minor basaltic to dacitic rocks were erupted peripherally to the caldera complex and in the moat of the youngest caldera. We divide the petrologic evolution of the caldera complex into a series of petrochemical cycles based on systematic changes in rock chemistry, modal petrography, and mineral chemistry as a function of time. Each petrochemical cycle is characterized by periods of systematic differentiation toward more siliceous or rhyolitic compositions. Breaks between cycles are generally abrupt and follow either times of major ash flow tuff eruption or episodes of relatively mafic (basaltic to dacitic) volcanism. New cycles begin with magma compositions that are less silicic than rhyolites erupted at the end of the preceding cycle. The systematic chemical changes toward more silicic compositions indicate that each cycle represents periods of progressive magmatic differentiation and suggest that members of a cycle successively evolved from a common parental magma. Some of the chemical trends within cycles are consistent with crystal fractionation of the observed phenocrysts and accessory minerals. However, other mechanisms must have operated in conjunction with crystal fractionation to produce the observed chemical variations in these rocks. Progressive buildup of volatiles and the depression of liquidus temperatures is suggested in some cycles by the decline in phenocryst abundances and the resorption of quartz. The occurrence of multiple petrochemical cycles is consistent with the interpretation that a series of magma bodies were successively emplaced and differentiated in the upper crust beneath TM‐OV. Members of petrochemical cycles were periodically erupted during the evolution of these magma bodies, providing a record of their differentiation. An alternative interpretation is that a large upper crustal silicic magma body was present beneath TM‐OV for most of its history and that new cycles of differentiation began after major ash flow eruptions, episodes of magma replenishment, and breakdown in compositional zonation.
Fractal mapping of digitized images: Application to the topography of Arizona and comparisons with synthetic images
Authors: Huang J. , Turcotte D. L.
Published: February 2008

The Earth's topography generally obeys fractal statistics; after either one‐ or two‐dimensional Fourier transforms the amplitudes have a power law dependence on wave number. The slope gives the fractal dimension, and the unit wave number amplitude is a measure of the roughness. In this study, digitized topography for the state of Arizona (7 points/km) has been used to obtain maps of fractal dimension and roughness amplitude. The roughness amplitude correlates well with variations in relief and is a promising parameter for the quantitative classification of geomorphology. Significant variations in fractal dimension are also found. For Arizona the mean fractal dimension for two‐dimensional Fourier spectral analyses is D = 2.09; for one‐dimensional Fourier spectral analyses the mean fractal dimension is D = 1.52, close to the Brown noise value D = 1.5. Synthetic two‐dimensional images have also been generated for a range of D values. For D = 2.1, the synthetic image has a mean one‐dimensional spectral fractal dimension D = 1.56, consistent with our results for Arizona. These results are also consistent with those of previous authors and show that it is not appropriate to subtract one from the two‐dimensional fractal dimension of topography in order to obtain the one‐dimensional fractal dimension.
Episodic venting of hydrothermal fluids from the Juan de Fuca Ridge
Authors: Baker E. T. , Lavelle J. W. , Feely R. A. , Massoth G. J. , Walker S. L. , Lupton J. E.
Published: February 2008

Evidence of large‐scale episodic venting of hydrothermal fluids was initially discovered in August 1986 in the form of a 130‐km3 radially symmetric “megaplume” over the southern Juan de Fuca Ridge. We report here on the discovery in September 1987 of a second, smaller megaplume about 45 km north of the location of the first megaplume. The 3He/heat, 3He/dissolved Mn, and 3He/dissolved silica ratios in both megaplumes were typical of high‐temperature vent fluids. Evidence from long‐term records of current flow over the southern Juan de Fuca Ridge, and from the mineralogy and Mn chemistry of megaplume particles, makes it unlikely that the second megaplume was a reencounter of the first. A plume model that relates the heat flux to the observed plume rise height of ∼1000 m finds that the total heat content of the fluids that formed the megaplumes was 1016–1017 J, or equivalently a fluid volume of 3–8 × 107 m3 at 350°C. The geometry and suspended particle population of the first megaplume imply that such features are formed within a few days time. The extraordinary heat and volume fluxes associated with megaplumes (102–103 greater than ordinary vent fields), as well as their typical hydrothermal chemistry, suggest that they resulted from tectonic or hydraulic fracturing that suddenly increased the permeability of the hydrothermal fluid reservoir in the axial crust. The flux of hydrothermal heat from continuous venting and episodic megaplumes on the southern Juan de Fuca Ridge is presently 4 − 10 × 109 W, a factor of 5–10 greater than various geophysical model calculations for this ridge segment. This imbalance may be symptomatic of a recent surge in the local cycle of magmatic activity.
Approximate analytical solutions for a cohesive fold‐and‐thrust wedge: Some results for lateral variation in wedge properties and for finite wedge angle
Authors: Fletcher Raymond C.
Published: February 2008

An existing solution for a perfectly plastic fold‐and‐thrust wedge with finite wedge angle is shown to be invalid. An approximate solution for a cohesive Coulomb wedge is obtained under the restrictions of small wedge angle and a weak basal surface. The effect of lateral variation in wedge properties is explicitly included. Using a similar method, the exact solution for a perfectly plastic wedge with horizontal upper surface and large wedge angle
is perturbed to give an approximate solution for a wedge with nonvanishing surface topography. The correction to the wedge profile obtained from the slender wedge approximation enters at
. The correction yields a 6% maximum reduction in slope for a wedge angle of 15°.
Caribbean arc tholeiites
Authors: Walker James A.
Published: February 2008

The occurrences of arc tholeiites in the two active subduction zones involving the Caribbean plate, Central America and the Lesser Antilles, are quite restricted. Volcanic rock suites exhibiting strong iron enrichment are limited to five volcanoes, all in Central America. Three of the five occur along the Nicaraguan portion of the volcanic front beneath which the arc crust is thinnest. On the other hand, volcanic rock suites with low absolute concentrations of large ion lithophile elements and light rare earth elements are largely confined to two northern islands in the Lesser Antilles, St. Kitts and St. Eustatius, and solely confined to the Nejapa‐Granada cinder cone alignments of Nicaragua in Central America. These tholeiitic characteristics, strong iron enrichment and low concentrations of certain incompatible elements, are produced by dissimilar processes. Strong iron enrichment results from extensive fractional crystallization at low pressure or with low water contents, perhaps in shallow magma bodies, whereas low absolute concentrations of specific incompatible elements are governed by relatively high degrees of source melting. The high degree of melting is made possible by a greater flux of material from the subducted plate. Crustal contamination exerts only a secondary and local control on incompatible element concentrations, at least in Central America.
Multivariate analysis of geomagnetic array data: 2. Random source models
Authors: Egbert Gary D.
Published: February 2008

In this paper I develop a spatial stochastic process model for randomly varying electromagnetic source fields observed over a one‐dimensional Earth. With this model the complex covariance between any two electromagnetic field components, measured at the same or different locations on the Earth's surface, can be computed. These calculations yield the expected values for the elements of the spectral density matrix (SDM; the matrix of averaged cross products of all components measured in the array) for synthetic arrays with arbitrary station locations. The model can thus be used to study the properties of the eigenvalues and eigenvectors of these synthetic array SDMs. For the case where both the array size and the skin depth of the electromagnetic fields in the conductive Earth are small compared to typical source length scales, the synthetic SDMs exhibit two larger eigenvalues, which correspond to nearly uniform source fields, followed by three smaller eigenvalues, which correspond to a set of canonical gradients. Qualitative consideration of more general models with three‐dimensional conductivity and arbitrary random sources shows that the eigenvalues of a geomagnetic array SDM represents a sort of discrete spatial power spectrum. The corresponding eigenvectors form a natural basis which any realizable field can be expanded in. These results provide strong justification for a generalized transfer function/response space model for the analysis of geomagnetic array data.
Estimation of vertical fracturing from measured elastic moduli
Authors: Hood Julie A. , Schoenberg Michael
Published: February 2008

A finely layered medium or a system of parallel fractures in an otherwise homogeneous elastic background renders the medium anisotropic for long wavelengths. The anisotropy increases in complexity as the number of different systems incorporated into the medium increases. Using the group calculus formulation for layered media developed by Schoenberg and Muir, the effects of the individual constituents can be separated arithmetically after the properties of each constituent are transformed. When orthorhombic behavior results from a set of parallel fractures perpendicular to closely spaced bedding planes, the contribution of the fractures to the elastic stiffness matrix can be removed. The average fracture compliances can be evaluated by requiring the background to be transversely isotropic. The remaining stiffness matrix gives the elastic properties of the transversely isotropic background as if the background had no vertical fractures.
Igneous history of the Koyukuk Terrane, western Alaska: Constraints on the origin, evolution, and ultimate collision of an accreted island arc terrane
Authors: Box Stephen E. , Patton William W.
Published: February 2008

The Koyukuk terrane of western Alaska consists of volcanic, volcaniclastic, and plutonic rocks which range from Late Paleozoic to Early Cretaceous in age. The terrane crops out in a U‐shaped belt which is roughly paralleled by outer belts of ultramafic rocks, oceanic plate basalts and cherts, and retrograded blueschist facies rocks of continental protolith. These rocks have been interpreted as components of a volcanic arc terrane that collided with the North American continental margin in Early Cretaceous time. The Koyukuk terrane consists of four time‐stratigraphic units: (1) pre‐Middle Jurassic basalts, (2) Middle and Late Jurassic granitic rocks, (3) lower Lower Cretaceous volcanic rocks, and (4) upper Lower Cretaceous volcanic rocks. Limited chemical data from the basalts of unit 1 indicate that they were erupted in a nonarc tectonic environment, possibly in an oceanic island or back arc setting. Units 2, 3, and 4 have the characteristics of subduction‐related volcanic rocks (i.e., depleted Nb and Ta and enriched alkaline elements, relative to the light rare earth elements). Unit 3 contains tholeiitic, calc‐alkaline, and alkaline rocks with chondrite‐normalized rare earth element patterns that range from flat (LaN/YbN = 1) to highly light rare earth element enriched (LaN/YbN > 15). The highly alkaline or shoshonitic lavas were erupted toward the end of unit 3 time (Valanginian) during the final stages of arc‐continent collision. These alkaline lavas could have been derived by very small degrees of partial melting of a similar source to that of the earlier arc lavas. Unit 4 lavas are also alkaline or shoshonitic, but their incompatible element composition indicates that they were derived from a different source than that of the earlier arc lavas. These late alkaline lavas are chemically similar to crosscutting mid‐Cretaceous plutons whose isotopic compositions (Arth et al., this issue (a)) suggest derivation by partial melting of distinctly older subcontinental lithosphere. We speculate that the parental magmas of unit 4 lavas may also have been derived by partial melting of this subcontinental mantle which was underthrust beneath the Koyukuk arc terrane during the final stage of arc‐continent collision.
The crustal structure of the Wrangellia Terrane along the East Glenn Highway, eastern‐southern Alaska
Authors: Goodwin Erik B. , Fuis Gary S. , Nokleberg Warren J. , Ambos Elizabeth L.
Published: February 2008

Recently acquired seismic refraction data from eastern‐southern Alaska provide new information on the structure and composition of the Wrangellia and adjacent terranes. The data comprise a 160‐km‐long refraction profile along the East Glenn (Tok‐Cutoff) Highway that was collected as part of the U.S. Geological Survey's multidisciplinary Trans‐Alaska Crustal Transect program. The upper 3 km of the Wrangellia terrane and associated rocks is characterized by low compressional wave velocities (Vp = 1.9, 3.3, 4.6, 5.6 km s−1) and high‐velocity gradients common to most onshore seismic refraction profiles. There is also clear seismic expression of the West Fork fault system as a steep, down‐to‐the‐southwest fault that separates the Peninsular terrane to the southwest and the metamorphic complex of Gulkana River to the northeast. In contrast, no seismic expression occurs for the Paxson Lake fault, which separates the Wrangellia terrane from the metamorphic complex of Gulkana River. Adjacent to the Denali fault, within the Wrangellia terrane, two high‐velocity bodies (Vp = 6.6 km s−1) occur in the upper crust. One of these extends to ∼10‐km depth and correlates with a late Paleozoic dioritic complex, suggesting that the Wrangellia terrane is at least 10 km thick in this part of Alaska. From 5 to 23 km depth, the crust appears seismically homogeneous, with velocity increasing from Vp = 6.2 to Vp = 6.6 km s−1. Beneath this level, the crust is less well resolved, although evidence exists for a low‐velocity zone between 23 and 26 km and a possible southwest dipping interface at 35 km. No identifiable mantle refraction or reflection is observed, possibly indicating a crust as thick as 55 km. The relatively low seismic velocities in the upper 23 km of the crust compare favorably with laboratory‐measured velocities on pelitic schists and intermediate‐composition plutonic rocks (granites and granodiorites), both of which are recognized in Wrangellia. We interpret the seismic velocities to indicate that silicic‐to‐intermediate‐composition rocks are important constituents of the basement of this part of Wrangellia. Geologic evidence indicates that the Alaskan part of the Wrangellia terrane is a Paleozoic and Mesozoic island arc: our seismic evidence indicates it may have been built mostly on continental crust as opposed to the fragment of Wrangellia from Vancouver Island which was probably built on oceanic crust.
Identification of lunar rock types and search for polar ice by gamma ray spectroscopy
Authors: Metzger Albert E. , Drake Darrell M.
Published: February 2008

The ability to map the surface composition of the Moon from orbit by means of gamma ray spectroscopy has been studied with attention to the possible use of small spinstabilized spacecraft. Two types of detectors were considered, a sodium iodide scintillator and a cooled germanium solid‐state device. The latter has superior sensitivity for all determinable elements, the difference ranging from factors of 3 to 11. A design for accommodating the preferred germanium detector gamma ray spectrometer (GeGRS) on either a small or large spacecraft has been devised. Detection sensitivity has been applied to typical rock compositions to determine measurement times and the ability to discriminate among representative lunar rock types. For sets comprising nine highland and 16 mare types, the most useful elements are found to be Mg, Al, K, Ti, Fe, U, and Th. Compositional distinctions combined with the sensitivity of the GeGRS instrument should allow each major rock type to be recognized in accumulation periods of 2 hours or less. The presence of two or more rock types in comparable abundance will require longer periods. Permanently shadowed polar areas of the Moon may be repositories of water ice. The expected instrument response to the resulting gamma ray and neutron fluxes has been analyzed. A neutron mode added to the spectrometer will be more sensitive to the possible presence of ice than the gamma ray mode, although without the application of compositional corrections based on analysis of the gamma ray data, most of the neutron mode sensitivity will be lost. With a pair of selected neutron absorbers, and a model which provides that 2.5% of the area above 75° latitude is occupied by trapping sites, the most sensitive of the configurations examined will provide a one year mission detection limit of 0.056% H2O by weight for each polar region. Given the postulated abundance of trapping sites, a determination of whether water is present in adequate abundance for resource utilization can be made within a week. These results are applicable to alternative spacecraft by simply adjusting the duty cycle factor.
Internally consistent thermodynamic data and equilibrium phase relations for compounds in the system MgO‐SiO2 at high pressure and high temperature
Authors: Fei Yingwei , Saxena Surendra K. , Navrotsky Alexandra
Published: February 2008

Thermochemical and thermophysical data for phases in the system MgO‐SiO2 are critically evaluated, refined, and supplemented by calculating phase equilibrium relations from the available calorimetric data. Lattice vibrational models are used to constrain further the entropies and heat capacities. Phases in the data base are MgO, SiO2 (coesite, stishovite), Mg2SiO4 (olivine, ß‐phase, spinel), and MgSiO3 (pyroxene, garnet, ilmenite, and perovskite). The internally consistent data set reproduces the experimentally determined phase relations in the system at pressures to 30 GPa and to melting temperatures.