bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanisms and depths of Atlantic transform earthquakes
Authors: Engeln Joseph F. , Wiens Douglas A. , Stein Seth
Published: February 2008

We have studied 40 earthquakes on seven major transforms along the Mid‐Atlantic Ridge. The majority show the expected transform‐parallel strike‐slip motion on steeply dipping fault planes. Some “anomalous” mechanisms, suggesting either unusually shallow dip or a component of dip‐slip motion, occur near ridge‐transform intersections. The vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform. Centroid depths, determined by waveform inversion, are limited to a shallow zone corresponding approximately to the area above the 400°C isotherm of the thermal structure expected from lithospheric cooling. The depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600°C isotherm. Assuming that the areas above the 400°C or 600°C isotherms fail seismically, moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion, suggesting that some aseismic slip occurs if the time period sampled is representative. In contrast, on the Vema Transform the inferred seismic slip is greater than predicted unless faulting extends to the 650°C isotherm. Alternatively, the Vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history. In contrast to transform events, centroids for oceanic intraplate earthquakes extend to approximately the 750°C isotherm. This result is intriguing if depths reflect lithospheric strength as a function of depth. Transforms have much higher strain rates than intraplate areas, so if both had the same rheology the limiting isotherm for transforms should be higher. Alternatively, if a weaker rheology were appropriate, the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra‐plate regions. Transforms may thus be either weaker and/or have higher temperatures than expected. In the latter model the required high temperatures, perhaps due to intrusion at depth, predict measurable heat flow anomalies. The second possibility, unusual rheological weakness, is reminiscent of the unexplained weakness of the San Andreas fault, where heat flow data exclude anomalously high temperatures.
Seismic domains within the Gissar‐Kokshal Seismic Zone, soviet central Asia
Authors: Leith William , Simpson David W.
Published: February 2008

By evaluating seismological data from the belt of shallow intraplate seismicity known as the Gissar‐Kokshal seismic zone in Soviet Central Asia in conjunction with recent data on the shallow crustal structure and longer‐term geologic deformation, we show that certain size‐classes of earthquakes are characteristic of adjacent (and in some places overlapping) geologic/tectonic settings; we call these settings seismic domains. They are both geologically and seismologically distinct; i.e., they contain different rocks and structures and have different frequency‐magnitude relationships. For example, in terms of number of events, Gissar‐Kokshal seismicity is dominated by the microearthquakes that have occurred within the sedimentary rocks of the Vakhsh fold/thrust belt. These are all shallow and have occurred mostly within the frontal portions of the thrust sheets. More rarely, moderate‐size (M < 5.5) earthquakes have ruptured the thrust ramps. On the other hand, the major strain release in this region has accompanied the large (5.5 < M < 8.1) shallow earthquakes on basement faults in the southern Gissar range adjacent to the fold/thrust belt. Within these ranges there is relatively little microearthquake activity. That dichotomy indicates that a significantly different seismic hazard is associated with each of these areas, but the seismological distinction is obscured along the front of the fold/thrust belt, where the domain of small shocks in the sedimentary rocks overlaps the domain of large earthquakes on basement faults. To explain the localization of the large basement events along the southern flank of the fold/thrust belt adjacent to the fold/thrust belt, we hypothesize a mechanism of thrust sheet loading. Thrust loads are greatest in this region, since thrust sheets have overridden large basement block uplifts beneath the Illiac and Peter‐the‐First range faults.
Intracaldera volcanic activity, Toledo Caldera and Embayment, Jemez Mountains, New Mexico
Authors: Heiken Grant , Goff Fraser , Stix John , Tamanyu Shiro , Shafiqullah Muhammad , Garcia Sammy , Hagan Roland
Published: February 2008

The Toledo caldera was formed at 1.47±0.06 Ma during the catastrophic eruption of the lower member, Bandelier Tuff. The caldera was obscured at 1.12±0.03 Ma during eruption of the equally voluminous upper member of the Bandelier Tuff that led to formation of the Valles caldera. Earlier workers interpreted a 9‐km‐diameter embayment, located NE of the Valles caldera (Toledo embayment), to be a remnant of the Toledo caldera. Drill hole data and new K‐Ar dates of Toledo intracaldera domes redefine the position of Toledo caldera, nearly coincident with and of the same dimensions as the younger Valles caldera. The Toledo embayment may be of tectonic origin or a small Tschicoma volcanic center caldera. This interpretation is consistent with distribution of the lower member of the Bandelier Tuff and with several other field and drilling‐related observations. Explosive activity associated with Cerro Toledo Rhyolite domes is recorded in tuff deposits located between the lower and upper members of the Bandelier Tuff on the northeast flank of the Jemez Mountains. Recorded in the tuff deposits are seven cycles of explosive activity. Most cycles consist of phreatomagmatic tuffs that grade upward into Plinian pumice beds. A separate deposit, of the same age and consisting of pyroclastic surges and flows, is associated with Rabbit Mountain, located on the southeast rim of the Valles‐Toledo caldera complex. These are the surface expression of what may be a thicker, more voluminous intracaldera tuff sequence. The combined deposits of the lower and upper members of the Bandelier Tuff, Toledo and Valles intracaldera sediments, tuffs, and dome lavas form what we interpret to be a wedge‐shaped caldera fill. This sequence is confirmed by deep drill holes and gravity surveys. This fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse. We interpret the Toledo‐Valles caldera complex to be a pair of nearly coincident trapdoor calderas, with the hinge on the west side and thick caldera fill in the east.
Hydrothermal alteration in the Baca Geothermal System, Redondo Dome, Valles Caldera, New Mexico
Authors: Hulen Jeffrey B. , Nielson Dennis L.
Published: February 2008

Thermal fluids circulating in the active hydrothermal system of the resurgent Redondo dome of the Valles caldera have interacted with their diverse host rocks to produce well‐zoned alteration assemblages, which not only help locate permeable fluid channels but also provide insight into the system's thermal history. The alteration shows that fluid flow has been confined principally to steeply dipping normal faults and subsidiary fractures as well as thin stratigraphic aquifers. Permeability along many of these channels has been reduced or locally eliminated by hydrothermal self‐sealing. Alteration from the surface through the base of the Miocene Paliza Canyon Formation is of three distinctive types: argillic, propylitic, and phyllic. Argillic alteration forms a blanket above the deep water table in formerly permeable nonwelded tuffs. Beneath the argillic zone, pervasive propylitic alteration is weakly developed in felsic host rocks but locally intense in deep intermediate composition volcanics. Strong phyllic alteration is commonly but not invariably associated with major active thermal fluid channels. Phyllic zones yielding no fluid were clearly once permeable but now are hydrothermally sealed. High‐temperature alteration phases at Baca are presently found at much lower temperatures. We suggest either that isotherms have collapsed due to gradual cooling of the system, that they have retreated without overall heat loss due to uplift of the Redondo dome, that the system has shifted laterally, or that it has contracted due to a drop in the water table. The deepest Well (B‐12, 3423 m) in the dome may have penetrated through the base of the active hydrothermal system. Below a depth of 2440 m in this well, hydrothermal veining largely disappears, and the rocks resemble those developed by isochemical thermal metamorphism. The transition is reflected by temperature logs, which show a conductive thermal gradient below 2440 m. This depth may mark the dome's neutral plane, which separates an upper permeable zone of extensional fracturing from a lower, less permeable compressional regime. The Baca hydrothermal system is similar to those which have formed ore deposits in other calderas: particularly, Creede (Colorado) type epithermal silver base metal veins and stockworks. Recent scientific drilling has also intersected a deep zone of strong phyllic alteration and molybdenum mineralization in the Valles caldera's ring fracture system, a setting which localized a large stockwork molybdenite orebody in the nearby Questa caldera.
Effect of heterogeneities in D″ on the decay rate of PDiff
Authors: Menke William
Published: February 2008

We use physical models to examine the effect of scatterers in the D″ region of the mantle on the decay rate of the Pdiff phase in the core shadow. Scatterers are in the size range 7.5–81 km, and measurements of the decay rate are made in the period range 3–60 s. We find that the presence of scatterers in D″ can change the decay rate of Pdiff by as much as 50%. The heterogeneity does not affect the decay rate for periods greater than about 10–20 s. At shorter periods, the decay rate can be either lower or higher than the laterally homogeneous case, depending on the scatterer geometry. We find that a layer of small (7.5 km) scatterers can decrease the decay rate, while layers of larger scatterers (41–81 km) mainly increase it. We did not identify any major effect of scatterer aspect ratio on the decay rate in the one relevant test that we performed (dikes and sills on the core‐mantle boundary).
An approach to modeling present‐day deformation in southern California
Authors: Rundle John B.
Published: February 2008

This paper presents an approach to modeling the complete, time‐dependent deformation in southern California. We make use of a variety of techniques to include the far‐field plate motion, stress relaxation in the asthenosphere, aseismic fault slip within the elastic lithosphere, and the complex, three‐dimensional nature of the faults within the southern California region. Among the conclusions of the work here are the following: (1) the data exhibit a marginal preference for a relatively thin lithosphere in southern California, (2) the most sensitive determinant for lithospheric thickness and viscosity of the asthenosphere is the rotation rate of the motion vectors toward or away from major faults, (3) the major faults in southern California are slipping at depth, at or near their average Holocene rates, (4) a shallow, active decollement beneath the Transverse Ranges is apparently needed to realistically model the strain changes observed there, and (5) this modeling technique offers the best approach for understanding the complex deformation field in southern California.
Global mean sea surface based upon the Seasat altimeter data
Authors: Marsh James G. , Brenner Anita C. , Beckley Brian D. , Martin Thomas V.
Published: February 2008

A mean sea surface map has been computed for the global ocean areas between 70°N latitude and 62°S latitude based upon the 70‐day set of Seasat altimeter data recorded in 1978. A combination of regional crossover adjustment techniques and a global reference surface, using the best available geodetic parameters for Seasat, has formed the basis for the computation. The overall rms value of the crossover residuals in the open ocean areas was 13 cm after the adjustment. A contour map of the crossover residuals is very similar in nature to earlier contour maps of the collinear pass results for mesoscale variability. The mean sea surface is presented in the form of a global contour map, a 0.5° × 0.5° grid, and a computerilluminated image.
Small Martian valleys: Pristine and degraded morphology
Authors: Baker Victor R. , Partridge John B.
Published: February 2008

The equatorial heavily cratered uplands of Mars are dissected by two classes of small valleys that are intimately associated in compound networks. Pristine valleys with steep valley walls preferentially occupy downstream portions of compound basins. Degraded valleys with eroded walls are laterally more extensive and have higher drainage densities than pristine valleys. Morphometric and crater‐counting studies indicate that relatively dense drainage networks were emplaced on Mars during the heavy bombardment about 4.0 b.y. ago. Over a period of approximately 108years, these networks were degraded and subsequently invaded by headwardly extending pristine valleys. The pristine valleys locally reactivated the compound networks, probably through sapping processes dependent upon high water tables. Fluvial activity in the heavily cratered uplands generally ceased approximately 3.8–3.9 b.y. ago, coincident with the rapid decline in cratering rates. The relict compound valleys on Mars are morphometrically distinct from most terrestrial drainage systems. The differences might be caused by a Martian valley formation episode characterized by hyperaridity, by inadequate time for network growth, by very permeable rock types, or by a combination of factors.
Uranium series dating of Allan Hills ice
Authors: Fireman E. L.
Published: February 2008

Uranium 238 decay series nuclides dissolved in Antarctic ice samples were measured. Ice from the Allan Hills, Cul de Sac site, that contains a large concentration of fine volcanic glass shards, has high 226Ra, 230Th, and 234U activities but similarly low 238U activities compared to Antarctic ice samples without volcanic shards. The 226Ra, 230Th, and 234U excesses are proportional to the shard content. The 238U decay series results are consistent with the assumption that alpha decay products recoiled into the ice from the fine shards. The age of the Cul de Sac ice is (325±75) × 103 yr from this method of uranium series dating.
Equation of state and stability of fluorite‐structured SiO2
Authors: Bukowinski M. S. T. , Wolf George H.
Published: February 2008

A number of workers have proposed the fluorite structure for a hypothetical postrutile phase of silica. However, this phase has never been observed, nor has the existence of any other postrutile phase been established unambiguously. Estimates of the density of this phase, based on empirical systematics of analogue compounds and theoretical arguments and calculations, differ widely. We have therefore investigated the equation of state and dynamic and elastic stability of SiO2 (fluor) using a first principles band structure calculation and pair potentials derived from the modified electron gas model. Our analysis indicates that it is unlikely that the density of the fluorite phase exceeds that of stishovite anywhere in the mantle. The theoretical phonon spectrum exhibits dynamic instability at pressures below 170 GPa, suggesting that even if the fluorite phase were produced at high‐pressure, it would be unquenchable and hence unobservable in unloaded experimental samples. However, it is also possible that the dynamic instability makes the fluorite phase a stable superionic conductor at lower mantle conditions. The relatively low density, and low‐pressure dynamic instability of the theoretical fluorite‐structured silica, is most likely due to strong oxygen‐oxygen repulsive interactions, coupled with an inherent instability of the simple cubic oxygen sublattice.
Introduction to Special Section on the Rio Grande Rift
Authors: Keller G. R.
Published: February 2008

With the aid of a Penrose Conference in 1974 and an international rift conference held in 1978 in Santa Fe, New Mexico, the Rio Grande rift has become widely recognized as a major Cenozoic continental rift zone. As a result of the 1978 Santa Fe meeting, the American Geophysical Union published a special volume of papers concerned with the Rio Grande rift [Riecker, 1979], and the New Mexico Geological Society recently published another volume focused on this rift [Baldridge et al., 1984]. These volumes are a manifestation of the research activity which lead to the formation of the Rio Grande rift consortium whose purpose is to foster rift‐related research and communication. This organization has sponsored several special sessions at geological and geophysical meetings and has generally increased the awareness of this important feature.
Discussion on displacement analysis: Detection of crustal deformation
Authors: Guohua Gu , Prescott William H.
Published: February 2008

In using geodetic networks to detect crustal motion there is always a question about what to use as a reference. If the stations in a subset of the network can be shown to remain fixed relative to one another, then this subset can be used as a reference for determining the motion of the remaining stations in the network. We derive such a solution for the displacement of a geodetic network, referred to as a stationary station solution. The advantage of this technique over other solutions to the same problem is that it neither depends on a single station nor makes assumptions about the movement of all the stations of the network. Application of the technique to a geodetic network near Hollister, California, indicates that stations between the faults are stationary, although for the block between the Calaveras and San Andreas faults the stations are only marginally stationary relative to one another. The technique is then extended to give the total block motion between the faults.
A comparison of the regional slope characteristics of Venus and Earth: Implications for geologic processes on Venus
Authors: Sharpton Virgil L. , Head James W.
Published: February 2008

The range of 3° by 3° regional slopes of the earth and Venus is similar (approximately 0.0°–2.4°); however, the surface distribution of these values differs significantly. On earth, cratonic and abyssal plains form extensive regions of 0.0° slope. Within these regions a variety of features (mid‐ocean ridges, volcanic island chains, subduction zones, and folded mountains) have regional slope characteristics influenced by seafloor spreading and plate recycling, as well as an active weathering regime. Continental margins form laterally continuous zones of relatively high slope (passive margins up to 1.9°; active margins up to 2.4°). The plains provinces of Venus are much more rugged than earth's plains and are marked by numerous closely spaced circular and linear features (0.1°–0.2° regional slope) concentrated into broad linear zones of global extent. Although Venus highlands are bounded by narrow zones of relatively steep slope, the margins of Aphrodite Terra and Beta Regio are not as steep as earth's continental margins and appear to be best developed parallel to the trends of major chasmata within these regions. Linear trends formed by some highland margin segments extend into the plains provinces as rugged belts densely populated with circular and linear features. Ishtar Terra's margins are significantly steeper and more continuous than other highland margins and are comparable to passive margins on earth. The Venus highlands do not contain appreciable smooth, flat interior regions, implying that highland topography is not significantly modified by erosion or deposition. Systematic variations in the density of plains features, elongate planitia, highland margin trends, and aligned highland topography form several major great‐circle‐like patterns oriented at generally less than 45° to the equator and differing in character from both the mosaiclike patterns of terrestrial lithospheric plates, and the subdued tectonic fracture grids of the smaller terrestrial one‐plate planets (the moon and Mercury).
Band‐limited global scalar magnetic anomaly map of the Earth derived from Magsat data
Authors: Arkani‐Hamed J. , Strangway D. W.
Published: February 2008

The magnetometer satellite (Magsat) data are separated into two distinct sets: the dawn and the dusk sets. The two sets are analyzed independently because of the dawn‐dusk asymmetry of the external magnetic field components of the data. Two scalar magnetic anomaly maps, the dawn and the dusk maps, are derived using the spherical harmonic expressions of the respective data set. The significant correlation of the two maps in the harmonic range of 18–41 indicates the repeatability of the Magsat data in these harmonics. The shortest repeatable wavelength is about 920 km. A band‐limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps. The map is then downward continued to about 10 km altitude. The downward continued magnetic anomalies are relatively more localized. The main features of the anomalies are as follows: (1) Continents have relatively stronger magnetic anomalies than oceans, especially the younger oceans, (2) many circum‐Pacific subduction zones have magnetic signatures, and (3) there is no overall correlation between anomalies and major shields.
Continental crustal underplating: Thermal considerations and seismic‐petrologic consequences
Authors: Furlong Kevin P. , Fountain David M.
Published: February 2008

The addition of crustal material to the base of the continental crust, crustal underplating, may play an important role in the evolution of the continental crust and lithosphere. We have evaluated the potential for this mechanism to increase the total thickness of the crust, and using petrologic and experimental data determined the likely seismic velocity structure which results. Modeling results indicate that mantle‐derived crustal material can add more than 10 km to the thickness of the crust. Depending on the composition of this underplated material and the local thermal structure, seismic velocities in this added layer will be in a range of values intermediate between velocities appropriate for crust and mantle materials (7.0–7.8 km/s), or values interpreted to be more typical of mantle materials (7.8–8.1 km/s). Comparison of the predicted seismic signature with velocity data from refraction experiments and laboratory determinations of inferred lower crustal samples indicates a strong correlation between our predictions and observation. If a significant volume of the continental crust has been added by this mechanism, the age and isotopic composition of near‐surface crustal rocks may be a relatively poor indicator of lower crustal and uppermost mantle evolution.
Extensional tectonics at The Geysers Geothermal Area, California
Authors: Oppenheimer David H.
Published: February 2008

Comparisons between the yearly volume of steam withdrawn from The Geysers, California, geothermal reservoir and the number of earthquakes indicate that most of the seismicity is induced. To establish the inducing mechanism, an estimate of the stress field orientation was calculated from inversion of 210 fault plane solutions. Below 1 km depth The Geysers is undergoing uniaxial extension with the least principal stress σ3 oriented near horizontal at approximately 105° azimuth. The consistency between the local and regional stress fields indicates that the regional tectonic stress is much larger than the stresses induced locally through geothermal activities. The stress data are equivocal as to whether earthquakes are induced due to conversion of aseismic to seismic slip or by increased shear stresses arising from reservoir contraction. The observed tendency for downward growth of seismicity may arise from a preferential fracture orientation that is near vertical in conjunction with a stress state where the gradient of σ1 exceeds that of σ3. Evidence of extension at The Geysers and in the surrounding Clear Lake region indicates a direct relationship to the presence of the Quaternary Clear Lake Volcanics.
Thermal activity and ground deformation at Phlegrean Fields, Italy: Precursors of eruptions or fluctuations of quiescent volcanism? A contribution of geochemical studies
Authors: Martini Marino
Published: February 2008

The ground uplift that occurred at Phlegrean Fields during the “crisis” of 1982–1984 is analyzed by taking into account the volcanological history of the area, the available geophysical information, and the data obtained through our systematic geochemical study. The chemical composition of the products of a unique historical eruption in A.D. 1538 displays a strong evolved character and fits the hypothesis of a progressive reduction in size of the magma body, which probably behaved as a closed system. The bicarbonate contents of the thermal waters in the area has increased with time, suggesting inputs of carbon dioxide into the shallow aquifers. Temperatures at the fumaroles have not changed for at least a hundred years, indicating that boiling aquifer at depth feeds the surface manifestations. Significant variations in the ratios H2S/CO2 and H2/N2 observed at the same time suggest that both the rate of ground uplifting and the increase of seismic energy release may be related to the fluctuation of a convective flow from a deep heat source. Tectonic activity can modify the permeability of deep structures and thereby trigger increased activity in surface manifestations.
Steady flows at the top of Earth's core derived from geomagnetic field models
Authors: Voorhies Coerte V.
Published: February 2008

Select models of the main geomagnetic field and its secular variation are extrapolated to the base of an insulating mantle and used to estimate the adjacent fluid motion of a perfectly conducting outer core. The assumption of steady motion provides formally unique solutions and is tested along with that of no upwelling. The hypothesis of no upwelling is found to be substantially worse than that of steady motion. Although the actual motion is not thought to be steady, the large‐scale secular variation at the top of the core can be adequately described by a large‐scale, combined toroidal‐poloidal circulation which is steady for intervals of at least a decade or two. The derived flows include a bulk westward drift but are complicated by superimposed jets, gyres, and surface divergence indicative of vigorous vertical motion at depth. The circulation pattern and key global properties including rms speed, upwelling, and westward drift are found to be fairly insensitive to variations in modeling parameters.
Analysis of borehole seismograms from Long Valley, California: Implications for caldera structure
Authors: Elbring Gregory J. , Rundle John B.
Published: February 2008

A three‐component seismometer was emplaced in a borehole in Long Valley caldera for a period of 2 months to record local earthquakes to the south, mostly in the Sierra Nevada. The seismic records were plotted in depth versus time sections to produce “vertical hypocentral profiles” for both the vertical and horizontal components. The data collected and displayed in this way provide good resolution of the crustal structure at depth and avoid the attenuation and complications introduced by the near‐surface caldera fill. The record sections were then modeled with two‐dimensional ray tracing to match the observed travel times. Although nonreversal of the profiles and uncertainties in hypocentral locations introduce ambiguities, the final interpretation reveals two separate low‐velocity bodies which we have tentatively identified as magma beneath the resurgent dome in Long Valley. The first of these is a small (2 km by 6 km) body with a depth to top of approximately 3.7 km beneath the surface at the southern end of the resurgent dome. The other body appears larger and lies at a depth of 5.5 km beneath the northern end of the resurgent dome.
Separated two‐phase flow and basaltic eruptions
Authors: Vergniolle Sylvie , Jaupart Claude
Published: February 2008

Fluid dynamical models of volcanic eruptions are usually made in the homogeneous approximation where gas and liquid are constrained to move at the same velocity. Basaltic eruptions exhibit the characteristics of separated flows, including transitions in their flow regime, from bubbly to slug flow in Strombolian eruptions and from bubbly to annular flow in Hawaiian ones. These regimes can be characterized by a parameter called the melt superficial velocity, or volume flux per unit cross section, which takes values between 10−3 and 10−2 m/s for bubbly and slug flow, and about 1 m/s for annular flow. We use two‐phase flow equations to determine under which conditions the homogeneous approximation is not valid. In the bubbly regime, in which many bubbles rise through the moving liquid, there are large differences between the two‐phase and homogeneous models, especially in the predictions of gas content and pressure. The homogeneous model is valid for viscous lavas such as dacites because viscosity impedes bubble motion. It is not valid for basaltic lavas if bubble sizes are greater than 1 cm, which is the case. Accordingly, basaltic eruptions should be characterized by lower gas contents and lower values of the exit pressure, and they rarely erupt in the mist and froth regimes, which are a feature of more viscous lavas. The two‐phase flow framework allows for the treatment of different bubble populations, including vesicles due to exsolution by pressure release in the volcanic conduit and bubbles from the magma chamber. This yields information on poorly constrained parameters including the effective friction coefficient for the conduit, gas content, and bubble size in the chamber. We suggest that the observed flow transitions record changes in the amount and size of gas bubbles in the magma chamber at the conduit entry.
Apollo 16 impact‐melt splashes: Petrography and major‐element composition
Authors: See Thomas H. , Hörz Friedrich , Morris Richard V.
Published: February 2008

We made petrographic and major‐element chemical analyses of 50 Impact‐Melt Splash (IMS) samples from the Apollo 16 landing site to assess their origin and the nature of their subregolith source. These melts possess volumes that are significantly larger than those of agglutinates but smaller than those represented by fragments of (generally holocrystalline) melt sheets. Most of these melts occur as splashes that drape various host rocks. The majority appear to have been deposited onto their hosts inside growing crater‐cavities or during ballistic flight and subsequently “piggy‐backed” to their collection site(s). Macroscopically, IMS's exhibit a glassy appearance but display varied crystallinity under the microscope. Schlieren‐rich glasses dominate the holohyaline areas. Most IMS's contain generally feldspathic monomineralic and lithic clasts, while purely regolithic materials (i.e., agglutinates) are absent. Crystalline portions are predominately spherulitic, but definitive criteria to distinguish between crystallization from a melt or solid state devitrification are lacking. More than half of the IMS's possess a distinct crystallization front that we attribute to devitrification. Chemically, most IMS's are fairly homogeneous on the scale of a thin section; variation among samples is much greater (e.g., Al2O3 varied from ∼24 to 32 wt%; FeO from ∼2 to 6 wt%). Such a wide range of melt compositions suggests that more than one impact generated these 50 IMS's. The chemistry of most IMS's is unlike the local regolith and appears to represent varied mixtures of VHA impact‐melt breccias and anorthosite (i.e., are dimict‐like). The host rocks are predominantly dimict breccias (70%) or one of their component lithologies (VHA impact‐melt breccias and anorthosite). Because of the short surface‐residence times for these samples (determined from microcrater populations on all samples, and absolute exposure ages of 10 specimens), many IMS's might be associated with South Ray and Baby Ray Craters. In the absence of absolute chronological ages for melt formation, we cannot distinguish between a “young” scenario, in which the melts were formed during excavation of South Ray and/or Baby Ray Craters, and an “old” scenario, in which previously formed melts were exposed as part of the discontinuous ejecta of either or both of these craters. We prefer the former, in part because of the tight compositional cluster of most IMS's. In either case, we conclude that most of these samples or their progenitors resided at depth within the South Ray and/or Baby Ray targets. Furthermore, the Cayley Formation in the southern part of the Apollo 16 landing site is a polymict deposit predominantly composed of VHA impact‐melt breccias and anorthosites. Both lithologies must be intimately mixed as they are in dimict breccias, and large, coherent units of individual lithologies appear to be limited to <50 m in dimensions.
A chemical and petrological model of the lunar crust and implications for lunar crustal origin
Authors: Spudis Paul D. , Davis Philip A.
Published: February 2008

We utilize a variety of lunar sample and orbital geochemical data in conjunction with current knowledge of impact‐cratering processes to develop a chemical and petrological model of the lunar crust. Orbital chemical data indicate that the upper highlands surface on the moon has the bulk composition of “anorthositic gabbro” (Al2O3 26–28 wt %); greater than 90% of the area covered is dominated by material having compositional affinities with the ferroan anorthosites, rather than with Apollo‐type Mg‐suite rocks. Considerations of the cumulative bombardment history of the moon indicate that the outer zone of impact brecciation extends tens of kilometers into the crust; given the resolution of the orbital gamma ray data, the ferroan‐anorthositic composition derived for the highlands surface may represent the bulk composition of the upper half of the lunar crust. The observed enrichment in “noritic” components in basin ejecta with increasing basin size, together with considerations of impact melt petrogenesis in lunar basins, suggest that the bulk composition of the lower lunar crust is “noritic” (Al2O3 ∼ 20 wt %); samples of lower crustal material are probably present in the Apollo collections in the form of low‐K Fra Mauro (LKFM) and very high alumina (VHA) basaltic impact melts. Our estimated value of total crustal Al2O3 content (24–25 wt %) suggests that the lunar crust contains too much aluminum to have originated by a purely “serial magmatism” mechanism. The large abundance of plagioclase within the lunar crust is probably a result of global‐scale fractionation of plagioclase in early lunar history. The “magma ocean” hypothesis of crustal origin more readily explains the bulk composition of the lunar crust.
West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance
Authors: Shabtaie Sion , Bentley Charles R.
Published: February 2008

Extensive airborne radar soundings, closely tied to accurately located ground stations made over a portion of the West Antarctic inland ice sheet and the Ross Ice Shelf, have enabled us to map the boundaries of the ice streams and their flow bands on the Ross Ice Shelf. The surfaces of the active ice streams, A and B, are heavily crevassed, but there are no visible crevasses on ice stream C. However, the existence of numerous crevasses at a depth of about 35 m implies that ice stream C ceased to be active about 250 years ago. There is a complex zone at the head of ice stream B that suggests that ice stream B is currently widening and advancing toward the interior of the ice sheet. The grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance, and the character of the radar echoes. The inland ice is more extensive than previously mapped, and includes a large, weakly grounded area downstream of ice streams A and B, where the surface elevations are barely above hydrostatic equilibrium. Crary Ice Rise extends 70 km upstream of the previously mapped position. To relate surface elevation data from satellite observations to sea level, three geoidal models were tested. We conclude that the GEM 10C model is the best of the three for this section of Antarctica. A new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams A and B, but not on ice stream C, whose surface contours show signs of stagnation. Ice stream C shows a strongly positive and ice stream B a significantly negative net balance rate. Ice streams A and D also show a significantly negative net flux, whereas ice streams E and F do not. The overall net balance of this part of the West Antarctic inland ice is suggestively negative (−23 ± 15 km3 yr−1). From the measured flux into the Ross Ice Shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0.12 ± 0.03 m yr−1.
Heat flow and seismic observations in the northwestern Banda Arc
Authors: Van Gool Mathijs , Huson Willem J. , Prawirasasra Rachmat , Owen T. R.
Published: February 2008

Results of heat flow measurements in the centers of three basins in the NW Banda Arc carried out during the Indonesian‐Dutch Snellius II expedition in 1985 all show high heat flow values. The average heat flow in the north Buru basin, on the outside of the arc, is 161 mW/m2. Two small, NS to NW‐SE elongated subbasins situated in the Lucipara basin show heat flow values of 175 and 134, mW/m2, respectively. The high heat flow measured in both the north Buru and Lucipara basins is interpreted to be the result of recent EW strike‐slip movement in the northwestern Banda Arc.
The 1886–1889 aftershocks of the Charleston, South Carolina, Earthquake: A Widespread burst of seismicity
Authors: Seeber L. , Armbruster J. G.
Published: February 2008

A systematic search of contemporary newspapers in South Carolina, North Carolina, Georgia and eastern Tennessee during the 1886–1889 (inclusive) aftershock sequence of the August 31, 1886 earthquake near Charleston, South Carolina has provided more than 3000 intensity reports for 522 earthquakes as compared to 144 previously known earthquakes for the same period. Of these 144 events, 138 were felt in Charleston/Summerville and had been assigned epicenters in that area. In contrast the new data provide 112 well‐constrained macroseismic epicenters. The 1886–1889 seismicity is characterized by a linear relation between log frequency and magnitude with a slope b≈1, a temporal decay of earthquake frequency proportional to time−1, and a low level of seismicity prior to the main shock. These are frequently observed characteristics of aftershock sequences. By 1889, the level of seismicity had decreased more than 2 orders of magnitude, reaching approximately the current level in the same area. The 1886–1889 epicenters delineate a large aftershock zone that extends northwest about 250 km across Appalachian strike from the coast into the Piedmont and at least 100 km along strike near the Fall Line of South Carolina and Georgia. An abrupt change in stress and/or effective strength is required over this zone. If this change can only occur in the near field of a single fault dislocation, this fault must be larger horizontally than the thickness of the seismogenic zone by an order of magnitude and must be shallow dipping. The correlation between the area of intensity VIII in the main shock with the area of large aftershocks is consistent with this hypothesis. The lack of a major fault affecting the post‐Upper Jurassic onlap sediments also favors a shallow dipping active fault, possibly a Paleozoic‐Mesozoic southeasterly dipping fault or detachment that may outcrop northwest of the aftershock zone. The 1886–1889 aftershocks occupy the same area as the South Carolina‐Georgia seismic zone as defined by recent seismicity. The portion of this zone in the Piedmont is the only area of the southeastern United States where reservoir‐induced seismicity is unambiguously recognized. Seismicity elsewhere in the Southeast, such as in the Virginia seismic zone, is deeper and apparently unrelated to reservoir loading. Thus, the 1886–1889 aftershock zone is currently manifested by a zone of seismicity with unique characteristics.
Effects of frictional heating on the thermal, hydrologic, and mechanical response of a fault
Authors: Mase Charles W. , Smith Leslie
Published: February 2008

The mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating. In this study we formulate a fault model which incorporates the effects of frictional heating on the thermal, hydrologic, and mechanical response of a small patch of the failure surface. This model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant. The problem has two time scales: a characteristic slip duration and a characteristic time for thermal pressurization. The slip duration is set by the fault geometry. The characteristic time for thermal pressurization is set by the slip rate, the friction coefficient, and the thermal and hydraulic characteristics of the medium. The response of the fault depends on the relative magnitude of these two times. Results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response. For earthquakes occurring across zones of low porous medium compressibility (< 10−9 Pa−1) and permeability (< 10−18 m2) the characteristic time for thermal pressurization is small. In this case, frictional heating can cause fluid pressures to approach lithostatic values, the shear strength to approach zero, and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium. Whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault. Moderate slip events where shear strains exceed two cause substantial strain weakening and, consequently, large stress drops, accelerations, and displacements. Thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones. Both the dynamic stress drop and total displacement decrease for zones with larger compressibility, permeability, or width. If the compressibility or permeability exceeds 10−8 Pa−1 or 10−14 m2 or the shear strain is less than one, then the effects of frictional heating may be negligible and the fault will exhibit no strain‐weakening characteristics. Consequently, the patch acts as barrier that halts or resists further fault motion. Extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface, explaining many of the features of active faulting (e.g., barriers, nonuniform slip, rupture stoppage, random ground accelerations, strong motions, and frequency‐magnitude relations).
Gravity anomaly at a Pleistocene lake bed in NW Alaska interpreted by analogy with Greenland's Lake Taserssauq and its floating ice tongue
Authors: Barnes David F.
Published: February 2008

A possible example of a very deep glacial excavation is provided by a distinctive gravity low located at the front of a valley glacier that once flowed into glacial Lake Aniuk (formerly Lake Noatak) in the western Brooks Range. Geologic and geophysical data suggest that sediments or ice filling a glacially excavated valley are the most probable cause of the 30–50 mGal anomaly. Reasonable choices of geometric models and density contrasts indicate that the former excavation is now filled with a buried‐ice thickness of 700 m or sediment thicknesses greater than 1 km; comparable anomalies are not known for other glaciated lacustrine valleys. However, many fiords do exceed 1 km in depth, and Crary found one nearly 2 km deep in Antarctica. In studying this fiord, he suggested the probable increased efficiency of excavation directly behind the point where an outlet glacier becomes afloat to form the Ross Ice Shelf and where it thus has a vertical component of motion and a mechanism for debris removal. Floating glacier ice tongues are now rare in the Arctic, but they exist in maritime parts of northern Ellesmere Island and Greenland. Studies of ice movement, environment, and morphology of another large floating glacier tongue in a perennially frozen lake in the Angiussaq Mountains of northern Greenland suggest that Pleistocene Lake Aniuk could have had a similar environment, water temperature, and near‐stable water level and that it could have maintained both a floating polar glacier tongue and a perennial ice cover. No direct evidence of efficient excavation was observed in Greenland, but efficient glacial erosion behind a floating polar ice tongue could explain the excavation that caused the Alaskan gravity anomaly.
Realistic, yet simple bottom boundary conditions for glaciers and ice sheets
Authors: Lliboutry Louis
Published: February 2008

Over a nondeformable and impermeable bed, the sliding law has to be a relationship between the bottom shear stress τb, the sliding velocity u, and the effective pressure N = pi — pw, where pi is the ice load and pw is the pressure in the interconnected water cavities. Three horizontal scales are recognized: 0.1 m, 10 m, and 10 km. Classical theories consider the smallest scale; they are of dubious value because temperate ice is neither dry nor totally impermeable. The intermediate scale yields sliding laws to be used in glacier modeling; melting‐refreezing processes may then be ignored. For modeling large, cold ice sheets, the sliding law at the largest scale is needed; it is suggested that sliding occurs only when a bottom temperate layer exists. How to tackle non‐Newtonian rheology is discussed. At the intermediate scale a microrelief model consisting of two superimposed “bumpy profiles” is favored. If the smaller one had bumps of equal height, τb would be double valued, but this model is unrealistic (the ice avalanche at the Allalingletscher in 1965 may be explained otherwise). Therefore a model with the smaller bumps of unequal height is adopted. It yields an asymptotic sliding law at large sliding velocities τb = ƒN + cuN1−n, with ƒ and c as adjustable parameters. A third one has to be introduced at low velocities. Although the correct interpolation function between both extreme cases remains unknown, some qualitative results about kinematic waves are obtained.