bluuebunny/crossref_metadata_2025 huggingface dataset:
Source component mixing in the regions of arc magma generation
Authors: Arculus Richard J. , Powell Roger
Published: February 2008

Most recent workers attribute the main features of island arc basalt geochemistry to variable contributions of at least two source components. The major source appears to be the peridotitic wedge of upper mantle overlying the subducted slab, but the nature of the second component and the processes by which the sources become mixed during genesis of arc magmas are in dispute. A metasomatic addition to the wedge resulting from devolatilization in the slab is the simplest explanation of the marked enrichment of the alkali and alkaline earth elements with respect to the rare earths in island arc basalts, together with the variably developed trends in Pb, Sr, and Nd isotopic data toward sedimentary contaminants. However, lack of the correlations between relative degrees of trace element fractionation and radiogenic isotopic ratios expected of such processes requires a more complex explanation. Alternative models that suggest that all of the characteristics of island arc basalts can be accounted for by melting of an intraoceanic, hot spot type of mantle source also face specific difficulties, particularly with regard to the strong depletions of trace high‐field‐strength elements in arc compared with hot spot magmas. A possible resolution of these specific geochemical difficulties may lie in dynamic transport processes within the wedge linked with the slab through coupled drag, and the marked depression of mantle isotherms in subduction zones. Inefficient escape of melts and subsequent repeated freezing within the overturning wedge can lead to local mineralogic and geochemical heterogeneity of the peridotite overlying the slab. Fluids released from the slab may infiltrate the heterogeneous wedge and preferentially scavenge the alkalis and alkaline earths with respect to the rare earths and high field strength elements from locally enriched portions of the wedge. Incorporation of such metasomatic fluids in renewed melting at shallower but hotter levels within the wedge can give rise to the trace element and isotopic systematics generally observed in arc basalts. Furthermore, subsequent melting of wedge‐type peridotite in nonsubduction zone environments can result in complementary enrichment of the high field strength elements compared with arcs, and in the general isotopic similarity of hot spot and arc magmas. Although it is likely that the wedge‐type peridotite in any arc is heterogeneously veined by previous inefficient melt extraction episodes, it is possible that the subduction zone environment is most conducive to the generation of veining.
Two contrasting styles of interaction between basic magmas and continental crust in the British Tertiary Volcanic Province
Authors: Thompson R. N. , Morrison M. A. , Dickin A. P. , Gibson I. L. , Harmon R. S.
Published: February 2008

Some of the magmas that were extruded to form the 59 Ma, early, plateau‐forming, basalt‐dominated floods in the British Tertiary Volcanic Province mixed with small amounts of sialic melt during their uprise through the continental crust, while others underwent concomitant fractional crystallization and crustal assimilation. Magma batches of the first group originated in the uppermost asthenosphere and mostly ponded at the Moho, where they crystallized fractionally to form a basalt‐benmoreite suite. The major element compositions of members of this series show that they underwent negligible further fractional crystallization between the Moho and the surface. Nevertheless, they paused long enough during their uprise to dissolve their high‐pressure phenocrysts and, in many cases, to mix with small amounts of acid melt from lower crustal, granulite‐facies, Archaean Lewisian leucogneisses. Basalts and tholeiitic andesites of the second group outcrop locally at the base of the lava piles in Mull and Skye. Their major element compositions show that they equilibrated within the upper third of the crust, under conditions approximating to anhydrous 1‐atm cotectic equilibria. Most of these lavas are in SW Mull, where Proterozoic Moine metasediments form the uppermost crust. The major and trace element and isotopic compositions of the SW Mull basal lavas show that they assimilated substantial amounts of Moine metasediments progressively as they fractionated within the upper crust. Pb isotope data reveal that, before their final upper crust evolution, some of the SW Mull basal lavas had already mixed with small amounts of lower crust acid melts.
Effects of silicate liquid composition on mineral‐liquid element partitioning from Soret diffusion studies
Authors: Lesher C. E.
Published: February 2008

Thermal (Soret) diffusion of major, minor, and trace components of naturally occurring silicate melts ranging in composition from tholeiitic basalt to high‐silica rhyolite is examined at 1 GPa, mean temperatures of 1380°–1535°C, and temperature gradients of 50°–80°C mm−1. Soret diffusion in silicate melts is channeled along network former/network modifier compositional vectors indistinguishable from those observed in silicate liquid immiscibilty studies, but in the former case, gradients are continuous between the compositional (and thermal) extremes. Si, the chief network former, invariably segregates to the hot end of the temperature gradient as do Rb, K, and Na in melts of mafic to intermediate bulk composition. Ba, Sr, Ca, Mg, Mn, Fe, La, Ce, Nd, Y, and Ti always show separations antithetic to Si, while Rb, K, and Na fractionate from Si only in silicic melts. Li and Al display little separation in a temperature gradient. Soret separations of alkali and alkaline earth cations are consistent with their participation in the network as charge compensators for trivalent network formers (i.e., Al3+, etc.). These network formers as well as network modifiers show fractionations from Si that depend directly on cation field strength and size, and the polymerization state of the melt. Temperature and absolute concentration have no measurable effect on the fractionation behavior of these components relative to that of SiO2, the principal determinant of melt structure. The melt compositional dependence of element partitioning is characterized using these data. Twofold to fourfold changes in mineral‐liquid partition coefficients for minor and trace elements are expected over the melting range for typical basalts, and similar or greater variabilty is expected during differentiation of silicic magmas. These expectations corroborate reported variability of mineral‐liquid partition coefficients from studies of natural igneous rock samples and experimental phase equilibria.
A Rayleigh wave dispersion study between El Paso, Texas and Albuquerque, New Mexico
Authors: Sinno Y. A. , Keller G. R.
Published: February 2008

The recent installation of a WWSSN station at El Paso, Texas (EPT), makes it possible to analyze Rayleigh wave dispersion along the axis of the Rio Grande rift. Analysis of dispersion along the path extending from El Paso, Texas, to Albuquerque, New Mexico (EPT‐ALQ), indicates an average crustal thickness of approximately 32 km and an upper mantle shear wave velocity of approximately 4.25 km/s. This result is in agreement with recent seismic refraction studies in the area. Combined analysis of the dispersion and refraction data yields Poisson's ratios of 0.24, 0.27, and 0.29 in the upper crust, lower crust, and upper mantle, respectively. The observed phase velocities are very similar to those reported for the East African rift.
Electrical conductivity of water‐undersaturated crustal melting
Authors: Wannamaker Philip E.
Published: February 2008

Water‐undersaturated melting in the crust can occur at lithostatic pressure in the presence of an H2O‐CO2 fluid, of no CO2 or fluid but with all H2O bound structurally in hydrous minerals, or of an insufficient amount of H2O fluid to saturate a melt at liquidus temperatures. The composition of any fluid in equilibrium with possible source rocks depends on the metamorphic grade of the rocks; the fluid at lithostatic pressure in ductile granulite facies rocks would be CO2‐rich, while an increased fraction of H2O in a fluid is probable in lithologies of lower metamorphic grade or in rocks of the brittle regime. With limited amounts of pore fluid and hydrous minerals, melting at constant pressure is a highly nonlinear function of temperature and may extend over a broad temperature range. Electrical conductivity of the resultant feldspathic liquids can be estimated to first order from early conductivity measurements on granite under conditions of excess water and recent theories on dissolution of H2O in aluminosilicate magmas. Experimental data suggest especially for the deep crust that conductivity of melt in equilibrium with its source residuum falls as melt fraction increases due to dilution of water in spite of increased temperature. At the lesser pressures of high‐level magma chambers, pressure‐temperature‐water content relations of hydrous melt conductivity should be clarified by further laboratory examination. Melt fraction and temperature estimates derived from field electrical surveys are complicated seriously by melt phase tortuosity in the crystalline matrix, by H2O content of the source rock, and by the possibility of an H2O‐CO2 fluid.
Downdip tensional earthquakes beneath the Tonga Arc: A double seismic zone?
Authors: Kawakatsu Hitoshi
Published: February 2008

Characteristics of seismicity at intermediate depth beneath the Tonga arc are studied. Special attention is paid to newly identified downdip tensional events because the dominance of downdip compression has been known in this area. We reexamine the relatively shallow (<70 km) downdip tensional events, reported in the Harvard centroid moment tensor solutions, by modeling bodywave waveforms of World‐Wide Standard Seismograph Network seismograms and conclude that they are in fact intraplate downdip tensional events. Furthermore, these downdip tensional events are consistently located below and seaward of downdip compressional events and appear to constitute a double seismic zone. The result of a relative relocation technique applied to these earthquakes also supports this observation, which was originally based on routinely determined epicenters. The subducting slab beneath the Tonga arc has been considered as a prototype of a compressionally loaded slab. The existence of a double seismic zone in Tonga, therefore, would suggest that double seismic zones, located at intermediate depth in subducting slabs, are more common than previously thought.
Magnetic mineralogy of the lower continental crust
Authors: Frost B. Ronald , Shive Peter N.
Published: February 2008

Rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths, as parts of high‐grade metamorphic and plutonic terrains, and as obducted lower crustal cross sections. A review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks. Thermodynamic considerations indicate that Tipoor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust, regardless of the primary magmatic composition of the spinel phase. Furthermore, it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle. Thus magnetite in the deep crust is the most likely candidate for the source of the long‐wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys. If so, the entire crust should be magnetic in most continental areas, including shields. In areas of high heat flow the bottom of the magnetic crust may lie considerably above the Moho.
Geotomography using refraction fan shots
Authors: Pavlis Gary L.
Published: February 2008

This paper introduces a new method for imaging lateral variations in the seismic velocity structure of the earth. The discussion is centered around the geometry of a pilot experiment conducted in Salt Creek valley near Bloomington, Indiana, but the methodology is more general in scope. In the pilot experiment, 24 explosions were fired at equal intervals around a circular area 190 m in diameter and recorded by geophones positioned diametrically opposite the source. Travel time residuals for the fan shots are inverted to estimate lateral velocity variations in a two‐dimensional, bowl‐shaped image reconstruction region under the circular array. A simple damped least squares inversion worked poorly when delay times were included as additional free parameters in the solution. A parameter separation procedure was more successful. The value of these data in determining structure was analyzed using synthetic data and resolving power calculations. Structure could be determined to high accuracy with little distortion in the center of the circular region where rays crossed from all directions, but results were comparatively poor near the fringes of the region where angular coverage was more limited. Inversion of the Salt Creek data indicates the observed variations in the residuals can be almost completely accounted for by variations in the weathered layer. The refractor velocity is nearly constant to a precision of 0.005 s/km, but there is a suggestion of a slight velocity decrease in the refractor at higher elevations above the water table.
Characteristics of surge‐type glaciers
Authors: Clarke Garry K. C. , Schmok Jeffrey P. , Ommanney C. Simon L. , Collins Sam G.
Published: February 2008

Glaciers can be divided into two classes according to their flow behavior: normal (relatively steady annually averaged flow rate) and surge‐type (pronounced cyclic flow variations having a typical periodicity of 10–100 years). We have examined the population statistics of 2356 glaciers in the St. Elias Mountains, Yukon Territory, Canada, and estimate that 151 (6.4%) of these glaciers are surge‐type. To explore how various glacier attributes are associated with surging, we compare the probability of surging associated with various subsets of the complete population to appropriate reference values. In this way, potential influences on surge tendency can be examined. For the 55 drainage basins analyzed, there is a pronounced spatial variation in the concentration of surge‐type glaciers, but no obvious environmental control can be evinced. Within the study area the greatest concentration lies in the northern St. Elias Mountains, a region of high topographic elevation that is experiencing rapid tectonic uplift. Analysis of the influence of length on surge tendency reveals that long glaciers have a significantly greater probability of being surge‐type than short glaciers. The surge probability increases monotonically from 0.61% for very short glaciers (0–1 km) to 65.1% for long glaciers (10–75 km). This result suggests that ice sheets and ice caps, or at least portions of them, should have a high probability of surging. Tributary glaciers have a greater tendency to surge than trunk glaciers, presumably because they may themselves be surge‐type and may additionally participate in surges of the trunk glacier. The nonrandom geographical distribution of surge‐type glaciers is not simply a consequence of the variation from basin to basin of the glacier length distribution. Surge‐type glaciers tend to have a higher overall elevation than normal glaciers: the elevation of the highest point of the accumulation zone, the elevation of the snow line, and the elevation of the lowest point of the ablation zone, on average, exceed the corresponding elevations for normal glaciers. There is no significant difference between the overall slopes of surge‐type and normal glaciers, although there is a tendency for surge‐type glaciers to have greater slope in the accumulation zone and lesser slope in the ablation zone than normal glaciers. Although the prevalent flow direction for glaciers in the Yukon data set is to the north, surge‐type glaciers tend to flow to the east and southeast. This orientation influence is probably explained by the fact that many of the longest glaciers also flow to the east and southeast.
Mantle circulation and the lateral migration of subducted slabs
Authors: Garfunkel Z. , Anderson C. A. , Schubert G.
Published: February 2008

The irregular motions of plates and the irregular distribution of plate consumption and generation require that subduction zones and descending lithospheric slabs migrate laterally. Absolute plate motions indicate that slab migration is generally retrograde; that is, it is opposite to the direction of motion of the plates to which the slabs are attached, at rates of 10–25 mm yr−1. As a result, the downward motions of slabs are generally steeper than their dips, probably because of their negative buoyancy relative to the surrounding mantle. An important consequence of lateral slab migration is the displacement of material away from one side of the slab and inward flow of an equal volume toward the other side. This generates a mass flux in the mantle that is comparable in magnitude with the flux involved in the overturn of the oceanic lithosphere. The flow induced by slab migration is, therefore, an important part of the large‐scale mantle circulation associated with plate motions. Two‐dimensional numerical models with retrograde slab migration explicitly prescribed as a boundary condition show that migrating slabs, in contrast with nonmigrating ones, are at an angle to streamlines in the surrounding mantle. Accordingly, the parallelism of Benioff‐Wadati zones and streamlines cannot be used to discriminate among mantle flow models. Laterally migrating slabs do not separate streamlines that turn in different directions at depth, and these slabs do not turn backward beneath the plates to which they are attached. Instead, these slabs become parts of the circulations beneath the overriding plates, and even at depth they continue to move away from their oceanic ridge sources. Migrating slabs do not separate mantle convection cells. The numerical simulations also show that slab migration slows the flow under the attached plate by diverting part of it to the circulation beneath the overriding plate. The enhanced flow under the overriding plate pulls it more forcefully toward the subduction zone by increasing both the magnitude of the basal drag and the length of the overriding plate subject to a trench‐directed drag. This drag on the overriding plate is the source of the “trench suction” force that appears in models of plate‐driving mechanisms. Retrograde slab migration increases trench suction and may, thereby, play an important role in initiating and maintaining back arc spreading. Over long periods of time, migrating slabs sweep out large volumes of the mantle, altering flow patterns and dispersing material to great distances. By preventing the formation of regular closed cells, slab migration helps to mix the mantle and make it more chemically homogeneous. Slab migration is an important factor that causes mantle flow to be geometrically complex and time dependent.
Role of cracks in progressive permeability reduction during flow of heated aqueous fluids through granite
Authors: Vaughan Peter J. , Moore Diane E. , Morrow Carolyn A. , Byerlee James D.
Published: February 2008

Permeability was measured continuously during an experiment in which a heated, aqueous fluid was pumped through a cylindrical sample of Westerly Granite. Fluid flowed radially outward from an axial borehole down a temperature gradient to the outer edge of the sample. The fluid temperature in the borehole was maintained at 300°C, while a pore pressure difference of 0.5 MPa provided the driving force for fluid flow. Permeability decreased by a factor of 25 during the 2‐week experiment. Scanning electron microscope examination of this altered sample indicates that about half of the grain boundaries involving quartz are cracked or were cracked at some time during the experiment and now contain a filling material. Grain boundaries between the two feldspars are closed in the starting material and in all areas of the altered sample that were examined. Intragranular cracks in all three major minerals are open in the starting material. These observations suggest that initially, the quartz forms a more effective fluid pathway than do the feldspars. About half of the intragranular cracks near the borehole are open, whereas the other half contain a massive deposit which is either Si‐rich or Ca‐rich. In most cracks at the outer edge of the altered sample a Si‐rich filling is present. In crack intersections in this area the filling has a platy texture. At several times during the experiment the discharged fluids were sampled and chemically analyzed. The silica content of fluids passing through the sample was calculated as a function of radius, using rate equations for the dissolution and precipitation of silica. The calculated silica concentrations for the discharged fluids are approximately in agreement with measured values. A separate calculation of the reduction of crack porosity due to uniform dissolution and reprecipitation of quartz indicates that these processes caused a maximum of 10% crack porosity change for an experiment with a borehole temperature of 300°C. Assuming that reprecipitation of quartz was homogeneous, this porosity change corresponds to a permeability reduction of 27%, whereas the observed value was 96%. Therefore other processes besides the simple homogeneous precipitation of a layer of quartz must be operating to reduce permeability. Four possible processes are (1) nonhomogeneous precipitation, (2) precipitation in critical narrow places in cracks, (3) precipitation of other minerals in addition to quartz, and (4) crack healing.
Ocean signals in tide gauge records
Authors: Chelton Dudley B. , Enfield David B.
Published: February 2008

Tide gauges are designed to measure changes in water level relative to land. However, vertical motions of the earth's crust manifest themselves as apparent water level changes in tide gauge records. These crustally induced changes are often small in amplitude relative to the wide range of oceanic processes which affect water level in coastal regions. Vertical crustal motion can best be studied by first removing oceanic variability from the time series. In this paper we summarize the major oceanic signals in tide gauge records. We take the approach that the oceanic signals are unwanted “noise” in the data. Methods are described for removing or at least reducing the various oceanic signals. These oceanic signals span a broad range of time scales from tides to interannual variability associated with the El Niño phenomenon and secular sea level change from a number of oceanographic effects.
Melting of a dry peridotite KLB‐1 up to 14 GPa: Implications on the Origin of peridotitic upper mantle
Authors: Takahashi Eiichi
Published: February 2008

Melting phase relations of a fertile lherzolite KLB‐1 have been studied in the pressure range from 1 atm to 14 GPa (140 kbar). Olivine is the liquidus phase at all pressures studied. The second mineral to crystallize changes with increasing pressure; chromian spinel (1 atm), Ca‐poor orthopyroxene (up to 3 GPa), pigeonitic clinopyroxene (up to 7 GPa), pyrope‐rich garnet (above 7 GPa). The melting temperature interval of the peridotite is more than 600°C wide at 1 atm but narrows to about 150°C at 14 GPa. The partial melts along the peridotite solidus become increasingly more MgO‐rich as pressure increases throughout the pressure range studied. At 5–7 GPa, the partial melts formed within 50°C of the solidus contain more than 30 wt % MgO and are very similar to Al‐undepleted‐type peridotitic komatiite which is common in Archean volcanic terrains. Due to the increase of enstatite component in clinopyroxene solid solution at high pressure and temperature, the orthopyroxene liquidus field narrows as pressure increases and disappears at 3.5 GPa. Harzburgites which are common in the basal peridotite in ophiolite suites may have been produced as residues by partial melting at relatively shallower depths (< 100 km). Lherzolites showing protogranular textures might have been produced as single‐pyroxene peridotite residue by partial melting at relatively greater depths (> 100 km). A diapiric model is consistent with the genesis of komatiite magma by partial melting of mantle peridotite at 150–200 km depth. Based on the following observations, (1) convergence of the liquidus and solidus of the peridotite at pressures > 14 GPa, (2) the near solidus partial melt composition very close to the bulk rock at 14 GPa, and (3) change in liquidus mineral from olivine to majorite garnet at pressures between 16 and 20 GPa in preliminary experiments, it is proposed that the upper mantle peridotite was generated originally as a magma (or magmas) by partial melting of the primitive earth at 400–500 km depth.
Residual geoid anomalies of the North Atlantic Ocean and their tectonic implications
Authors: Jung Woo‐Yeol , Rabinowitz Philip D.
Published: February 2008

Residual geoid undulations of the North Atlantic Ocean are obtained and interpreted in terms of upper mantle processes. Starting from the 1° × 1° averaged Seasat altimeter‐derived geoid, the effects of long‐wavelength features were first removed by subtracting the degree and order 10 geoid of GEM 10C standard earth model. The empirical geoid‐age relationship of the North Atlantic after removing the long‐wavelength features shows closer affinity to the geoid‐age relationship of the plate model than to that of the half‐space model for the older part (greater than 100 m.y.) of the ocean floor. The geoid effects associated with the thermal evolution of the oceanic lithosphere mask the geoid signal associated with deeper processes. We thus removed these effects and compared the resulting geoid with residual depth in order to investigate upper mantle processes. A principal conclusion here is that the axis of the Azores geoid high associated with the Azores triple junction shifts east to Azores Islands after we account for the thermal effects. This may indicate that the main axis of smaller‐scale (upper 600 km) convective upwellings does not lie beneath the present‐day spreading center but beneath the Azores Islands. Further, areas of extensive volcanism such as the Bermuda Rise and Cape Verde Rise in the older part of the ocean floor (ages greater than 100 m.y.) exhibit relative residual geoid highs, which correlate very well with the residual depth anomalies.
Research on groundwater radon as a fluid phase precursor to earthquakes
Authors: Teng Ta‐Liang , Sun Liang‐Fang
Published: February 2008

Groundwater radon monitoring work carried out in southern California by the University of Southern California since 1974 is summarized here. This effort began with a sampling network over a locked segment of the San Andreas fault from Tejon to Cajon and was later expanded to cover part of the southern Transverse Mountain ranges. Groundwater samples were brought back weekly to the laboratory for high precision scintillation counting. Needs for more frequent sampling and less labor prompted the development of an economical and field worthy instrument known as the continuous radon monitor. About 10 have been installed in the network since early 1980. The groundwater radon content was found to show anomalous increases (mostly at a single station) before a number of moderate and nearby earthquakes. Our work is hampered by a lack of large earthquakes that may have a regional impact on radon anomalies and by the complexity of the underground hydrological regime. To circumvent this difficulty, we have chosen to monitor only deep artesian wells or hot spring wells.
The role of strain heating in the evolution of megathrusts
Authors: Pavlis Terry L.
Published: February 2008

On the basis of elementary energy considerations and the nonlinear temperature dependence of experimental flow laws, a model is developed for the interplay of thermal and mechanical processes along megathrust systems. The model is based on the observation that thrust systems involving large portions of the lithosphere have a two‐part history. During the early history of movement, dry lower crustal/upper mantle rocks are deformed in a shear zone where, because of the nonlinear effect of temperature‐dependent diffusion, strain‐heating rates are directly controlled by the initial thermal state; that is, heating is insignificant if initial temperatures are high but where temperatures are low, large temperature increases are possible. As a result, strain heating acts as a “thermal buffer” such that upon arrival of upper crustal rocks at depths of 20 km or more, the temperatures along the thrust would be controlled largely by the rheology of hanging‐wall rocks rather than the initial thermal state. Moreover, the relatively high melting temperatures for dry, lower crustal/upper mantle rocks implies that the temperatures achieved along the thrust should be relatively high. As thrusting progresses, this early history is successively eliminated as wet, upper crustal rocks are emplaced beneath the “preheated” hanging wall. The granitic melts that characterize many natural megathrust systems are probably generated when these wet metasedimentary rocks move beneath the hanging wall, either by hydrous fluxing of the hot hanging wall or by melting in the highest‐grade footwall rocks. Rapid uplift implied by both metamorphic studies and preservation of the inverted metamorphic sequence is interpreted as a late phase event created by “tectonic pileup” beneath the thrust. Thorough testing of the model will require thorough assessment of the time‐dependent problem. However, one‐dimensional, steady state models together with energy considerations indicate that the general model is allowable. The concept of “thermal buffering” by strain heating is important because it implies that megathrust systems are a natural system where metamorphic temperatures are controlled by rock rheology; hence they represent real‐world systems where a thermal and mechanical system are intimately linked. This rationale is used to extend the model by indicating a method in which metamorphic conditions along exhumed megathrusts might be used as a natural constraint on experimentally determined flow laws.
Micromechanics of thermally induced cracking in three crustal rocks
Authors: Fredrich Joanne T. , Wong Teng‐fong
Published: February 2008

Samples of Westerly granite, Frederick diabase, and Oak Hall limestone were thermally cracked at room pressure to various peak temperatures. Scanning electon microscopy (SEM) was performed on ion‐thinned samples. The thermally induced crack density is dependent upon the temperature, thermal expansion mismatch, thermal expansion anisotropy, initial crack porosity, and grain size. The mode of propagation is different for grain boundary and intragranular cracks. Crack densities in the granite were quantified using stereological techniques. The thermally induced crack surface area per unit volume apparently has a quadratic dependence on the temperature increase, a physical interpretation for which can be formulated on the basis of energetic balance. Fracture mechanics models are developed to interpret thermal cracking. The predictions concerning thermal crack initiation temperature and crack propagation and arrest behavior agree well with observations for the granite and the diabase. The model predicts significant thermal cracking for the Oak Hall limestone, which contradicts the SEM observation. A possible explanation for this discrepancy is that the internal stresses due to thermal expansion anisotropy are relaxed by plastic flow in this relatively fine grained limestone.
The stratigraphy of Mars
Authors: Tanaka Kenneth L.
Published: February 2008

A detailed planetwide stratigraphy for Mars has been developed from global mapping based on Viking images and crater counting of geologic units. The original Noachian, Hesperian, and Amazonian Systems are divided into eight series corresponding to stratigraphic referents. Characteristic crater densities and material referents of each series are (1) Lower Noachian [N(16)] (number of craters > 16 km in diameter per 106 km2) > 200] basement material; (2) Middle Noachian [N(16) = 100–200] cratered terrain material; (3) Upper Noachian [N(16) = 25–100; N(5) = 200–400] intercrater plains material; (4) Lower Hesperian [N(5) = 125–200] ridged plains material; (5) Upper Hesperian [N(5) = 67–125; N(2) = 400–750] complex plains material; (6) Lower Amazonian [N(2) = 150–400] smooth plains material in southern Acidalia Planitia; (7) Middle Amazonian [N(2) = 40–150] lava flows in Amazonis Planitia; and (8) Upper Amazonian [N(2) < 40] flood‐plain material in southern Elysium Planitia. Correlations between various crater size‐frequency distributions of highland materials on the moon and Mars suggest that rocks of the Middle Noachian Series are about 3.92–3.85 b.y. old. Stratigraphic ages compiled for units and features of various origins show that volcanism, tectonism, and meteorite bombardment have generally decreased through Mars' geologic history. In recent time, surficial processes have dominated the formation and modification of rock units. The overall stratigraphy of Mars is complex, however, because of temporal and spatial variations in geologic activity.
Observations of body wave multipathing from broadband seismograms: Evidence for lower mantle slab penetration beneath the Sea of Okhotsk
Authors: Silver Paul G. , Chan W. Winston
Published: February 2008

We present observations of anomalously broad and complex SH waveforms from two deep‐focus earthquakes in the southern Kurils to Digital World‐Wide Standard Seismograph Network and Regional Seismic Test Network Stations in North America. By making use of broadband, three‐component seismograms and relying on a variety of phases, one can limit the class of possible explanations for these phases. In particular, it is shown that the anomalous arrivals are not due to source complexity, near‐receiver structure, a triplication from a lower mantle velocity gradient, a low‐Q zone in the mid‐lower mantle, or shear wave splitting. We show that these arrivals correspond to relatively fast paths and have enhanced amplitudes, indicating that they are most probably caused by multipathing due to interaction with a high‐velocity lower mantle feature below the source region. We examine the hypothesis that this feature may be a lower mantle extension of the Kuril slab. Ray‐tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the North American stations used in this study for a realistic slab model. Observations of reversed‐polarity arrivals are consistent with such an explanation, where it appears that two rays comprising the arrival emerge from different points on the focal sphere. One test of this explanation is that slab‐induced multipathing should not be seen along back‐of‐slab (oceanic) azimuths. Examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the SH waveforms. Identification of slab‐induced multipathing associated with deep‐focus events has two important consequences. First, it appears to require slab penetration beyond the 650‐km discontinuity, at least 300–400 km into the lower mantle under the Sea of Okhotsk. Second, these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities, low‐Q zones, near‐receiver structure, or source complexity.
Quaternary extension in southern Tibet: Field observations and tectonic implications
Authors: Armijo Rolando , Tapponnier Paul , Mercier J. L. , Han Tong‐Lin
Published: February 2008

We summarize evidence for Quaternary and active faulting collected in the field during three Sino‐French expeditions to southeastern Tibet (1980–1982). Detailed mapping of Quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the Himalayas in the last 2 ± 0.5 m.y. The maximum horizontal principal stress in south Tibet appears to be only the intermediate principal stress σ2, σ1 being vertical. South of the “chord” joining the eastern and western syntaxes of the Himalayan arc, extensional strains are principally localized within seven regularly spaced rift zones, three of which have been studied in some detail. The extension direction is determined to be N96° E ± 7° mainly from statistical averaging of strikes of newly formed normal faults. Throw rates on normal faults are evaluated for different time spans (2 ± 0.5 m.y., 60 ± 40 kyr, and 10 ± 2 kyr B.P.), using structural and topographic reliefs, as well as synglacial and postglacial vertical offsets. The rate of Quaternary extension is about 1% m.y.−1 along an 1100‐km‐long ESE traverse across south Tibet. This corresponds to a “spreading” rate of 1 ± 0.6 cm/yr. This rate and the divergent horizontal projections of slip vectors of earthquakes along the Himalayan front constrain the rate at which rigid India underthrusts southern Tibet to be 2 ± 1 cm/yr. Although most of the normal faults appear to be independent of, and nearly orthogonal to, the ≃E‐W Mesozoic‐Tertiary tectonic fabric, the Yadong‐Gulu rift appears to be guided for over 130 km by the older, oblique (≃NE‐SW) Nyainqentanglha range and fault zones along it. This reactivated zone is the most prominent left‐lateral strike‐slip fault system in SE Tibet. Excepting this zone, and the vicinity of the SE extremity of the Karakorum fault, Quaternary strike‐slip faulting is rare in south Tibet, i.e., south of the chord between the syntaxes of the Himalayan arc. North of the chord, the tectonic style is different. There minor conjugate strike‐slip faulting is widespread and appears to control Quaternary normal faulting, which is more diffuse and subdued than in south Tibet. Along the chord, the presence of a major zone of active right‐lateral, en échelon strike‐slip faults (Karakorum‐Jiali fault zone) probably reflects the greater facility of eastward extrusion in north central Tibet, in response to the northward push of India. The eastern Himalayan syntaxis may be an obstacle to such extrusion movements south of the chord.
Time‐domain apparent‐attenuation operators for compressional and shear waves: Experiment versus single‐scattering theory
Authors: Dubendorff Bruce , Menke William
Published: February 2008

We derive time‐domain apparent‐attenuation operators from both laboratory data and a single‐scattering theory. The scattering medium consists of a homogeneous elastic block containing parallel cylindrical voids, with volume fractions in the range from 0.66% to 3.75%. The attenuation operators were computed by first measuring (or theoretically predicting) the attenuation of spectral amplitude and then constructing a causal seismogram using the Kramers‐Kronig (KK) relation. Theory and experiment broadly agree, with the following exceptions: theory underestimated the overall level of attenuation, compressional wave attenuation is more poorly predicted than shear wave attenuation, and the theoretical attenuation operators tend to be larger in amplitude and less oscillatory than is observed. The KK attenuation operators are compared with attenuation operators derived from a least squares deconvolution (LSD) of the attenuated and unattenuated time series. The KK and LSD attenuation operators are superficially very different, reflecting different limitations in the methods. The LSD attenuation operators tend to be narrower than the KK attenuation operators and should not be used to measure pulse width.
Topographic effect on tilt, strain, and displacement measurements
Authors: Meertens Charles M. , Wahr John M.
Published: February 2008

Deformation and stress fields are locally modified by the presense of topography. An approximate analytical method is used to give simple expressions for the effect of arbitrarily shaped topography on vertical, horizontal, and surface tilt measurements. This provides a means of making corrections to observations without the use of finite element techniques. Corrections to tilt fields are found to be directly proportional to the local slope of the topography. Comparison is made between expressions for tilt and those for strain and displacement. Although the topographic effects on secular tilt, strain, and vertical displacement are not yet measurable, we emphasize that the topographic effect on precision tidal tilt and strain measurements can be significant.
P wave velocity variations in the Coso Region, California, derived from local earthquake travel times
Authors: Walck Marianne C. , Clayton Robert W.
Published: February 2008

Inversion of 4036 P wave travel time residuals from 429 local earthquakes using a tomographic scheme provides information about three‐dimensional upper crustal velocity variations in the Indian Wells Valley‐Coso region of southeastern California. The residuals are calculated relative to a Coso‐specific velocity model, corrected for station elevation, weighted, and back‐projected along their ray paths through models defined with layers of blocks. Slowness variations in the surface layer reflect local geology, including slow velocities for the sedimentary basins of Indian Wells and Rose valleys and relatively fast velocities for the Sierra Nevada and Argus Mountains. In the depth range of 3–5 km the inversion images an area of reduced compressional velocity in western and northern Indian Wells Valley but finds no major velocity variations beneath the Coso volcanic field to the north. These results are consistent with a recent study of anomalous shear wave attenuation in the Coso region. Between 5 and 10 km depth, low‐velocity areas (7% slow) appear at the southern end of the Coso volcanics, reaching east to the Coso Basin. Numerical tests of the inversion's resolution and sensitivity to noise indicate that these major anomalies are significant and well‐resolved, while other apparent velocity variations in poorly sampled areas are probably artifacts. The seismic data alone are not sufficient to uniquely characterize the physical state of these low‐velocity regions. Because of the Coso region's history of Pleistocene bimodal volcanism, high heat flow, geothermal activity, geodetic deformation, and seismic activity, one possibility is to link the zones of decreased P velocity to contemporary magmatic activity.
Folding in the Greenland Ice Sheet
Authors: Whillans I. M. , Jezek K. C.
Published: February 2008

The deformation of layering into folds is modeled for a linear viscous medium moving over a décollement. Folds are generated by flow variations caused by relief on the décollement, variations in friction, or both. The model is applied to folds forming now in the Greenland Ice Sheet near Dye 3, for which more complete data are available than for analogous solid earth situations and for which the décollement is at or near the bed. The folds (wavelength 4–8 km) are detected by radio reflection sounding. Measured surface deformation and deformation rate are used with the radar results to test the theory. Calculated fold amplitude is only 20% less than that measured, which indicates that the theory is substantially correct. Inversion of the data to calculate basal drag and velocity variations is not helpful for near Dye 3 because many different basal boundary conditions can lead to the observed deformations.
Prediction of rock electrical conductivity from mercury injection measurements
Authors: Katz A. J. , Thompson A. H.
Published: February 2008

We show that mercury injection can be used to characterize the portions of rock pore space that dominate both the electrical conductivity and the absolute permeability. The resulting new expression for the conductivity formation factor is fundamentally different from the classical Archie's law but will appear similar to it in some circumstances. The predictions of our theory for both conductivity and permeability agree with the experimental measurements within expected errors. The results indicate that the absolute permeability and the conductivity formation factor can be predicted from mercury injection measurements with no adjustable parameters.
evidence of Suppression of the Unequal Refraction Error in Geodetic Leveling
Authors: Mark Robert K. , Gilmore Thomas D. , Castle Robert O.
Published: February 2008

Analyses of historic data indicate that procedures in force since the nineteenth century have effectively suppressed the unequal refraction error (URE) in geodetic leveling. Although theoretical considerations argue that large refraction errors should accumulate where levelings traverse gentle slopes permitting long sight lengths, extensive examination of routinely developed results lends little support to this argument. Our analyses of the historic record consider (1) height differences based on otherwise similar levelings over the same route but characterized by substantially different sight length, (2) modeled refraction error differences and actually measured elevation changes on both a section‐by‐section and a line‐by‐line basis, (3) misclosures produced from levelings identified with significantly different procedural constraints in technically stable areas of comparable variation in terrain and climate, and (4) alternatively and sequentially developed heights in areas of tectonic stability and areas of persisting instability. All of these analyses indicate that the URE is generally small and unpredictable and hence inconsistent with both theoretical expectations and the results of two conceptually different experimental studies. This inconsistency probably is attributable to the inability of the theoretical models and the failure of the experiments to incorporate in their design those procedures, especially the rejection criteria at the setup, section, and circuit level, that normally minimize refraction error. Traditionally invoked procedural specifications may have suppressed the URE through biasing of observations toward less refractive conditions, whereas all of the theoretical models assume an averaged sampling of the atmosphere. Alternatively, a variety of real world complexities that are disregarded in the models may take on added importance in normally constrained leveling. Thus refraction corrections based on simplified models and mean atmospheric and terrain conditions tend to overcompensate substantially for refraction errors associated with routinely constrained surveys.
On the connection between mare basalts and picritic volcanic glasses
Authors: Longhi John
Published: February 2008

The evolution of the major element compositions of mare basaltic liquids and coexisting crystals in low‐pressure fractionation processes has been modeled in order to examine the postulated link between picritic volcanic glasses and mare basalts. Existing models of equilibrium and fractional crystallization [Longhi, 1977, 1982] have been modified to include expressions for the Cr‐spinel liquidus surface and the boundary between the armalcolite and ilmenite surfaces; average molar partition coefficients for armalcolite/liquid pairs are also included in the modifications. Results of the calculations show that most mare basalts are not derived by simple low‐pressure fractionation from parents with the compositions of picritic glasses as tabulated by Delano [1986a]. Also, there is no indirect evidence as yet for posteruption fractionation of picritic magmas in the form of sufficiently magnesian, mare‐type olivines at the Apollo 12 and 15 landing sites. However, the calculations do suggest that Luna 24 ferrobasalt and Apollo 11 high‐K basalts could have been derived by simple, near‐surface fractionation of known picritic compositions. Also, xenocrystic olivines with mare‐like concentrations of TiO2 and CaO in high‐Ti basalt 74275 provide evidence for the existence of a picritic magma at the Apollo 17 site. More importantly, the calculations indicate that the picritic magmas would have fractionated to produce basalts with bulk and mineral compositions generally similar to those of mare basalts. These latter results favor the hypothesis that mare basalts have fractionated, “nonprimary” compositions and that the small number of observed linkages between basalts and picritic parents is a consequence of limited sampling.