bluuebunny/crossref_metadata_2025 huggingface dataset:
Grazing impacts on Mars: A record of lost satellites
Authors: Schultz Peter H. , Lutz‐Garihan Anne B.
Published: February 2008

Grazing impacts can be identified on the basis of the elongate shape of the resulting crater and a distinctive pattern of ejecta deposits. Over 170 such impact craters larger than 3 km are recognized on Mars, and they represent more than 5% of the total crater population of the ridged plains. In contrast, the moon exhibits only one comparable example larger than 3 km on the maria, a frequency consistent with theoretical estimates for an isotropic influx of impactors. Many Mars grazers appear to occur along great circles. The most recent examples generally impacted in an east‐west direction, whereas older grazers impacted in more northerly directions. We interpret the excessive number of grazers and the common impact directions as the result of satellites whose orbits tidally decayed with time. If all orbits originally had small inclinations similar to the orbits of Phobos and Deimos as well as the most recent grazers, then the change in impact direction with time can be explained as the result of shifts in the crust due to changes in the martian moments of inertia. The locations of the projected orbital axes (orbit‐pole points) on the martian surface indicate that the geographic poles of Mars originally were situated at lower latitudes. More than 95% of the mass represented by these proposed satellites impacted prior to the emplacement of the volcanic plains of Lunae Planum. The estimated combined mass of grazing impactors would form a satellite at least 225 km in diameter. These results may provide new clues for the origin of Phobos and Deimos and perhaps the angular momentum of Mars.
Kreep glass and the exotic provenance and formation of polymict breccia 66055
Authors: Ryder Graham , Blair Elizabeth
Published: February 2008

Polymict breccia 66055 from the Apollo 16 site is unique in that it contains abundant brown KREEP glass (0.3–0.4% K2O) and related brown glassy breccias which have fluid forms. During assembly it consisted of hot and cold debris which flowed together. Most of the rock consists of clasts of poikiloblastic breccia (granulitic impactite) and mesostasis‐rich impact melt (VHA basalt), both of which predate the assembly of 66055. In contrast, the KREEP glass was created in the impact which produced the rock. KREEP compositions are sparse among glasses in soils and other breccias at the Apollo 16 site, and the rock would not have formed locally; it must be exotic. Terrains in which 66055 might have formed exist to the west of the landing site. Suitable igneous target materials for the KREEP glass have not positively been identified in 66055, so the petrogenesis of Apollo 16 KREEP remains enigmatic, although the presence of a ‘granitic’ glass fragment embedded in KREEP glass suggests that it is igneous. A local volcanic origin is more consistent with the spatial relationships, as deduced from orbital geochemical data, than is a basin ejecta deposit.
Chemical composition of the Howardite parent body deduced from Kapoeta primary ‘mafic’ magmas
Authors: Smith M. R. , Schmitt R. A.
Published: February 2008

Chemical data are presented for three ‘mafic’ clasts extracted from the Kapoeta howardite. Bulk compositions and petrologic observations suggest that two of these lithic clasts represent olivine‐plagioclase bearing orthopyroxenites. Chondrite‐relative refractory LIL abundances of two of the clasts are inferred to represent primary Mg‐rich magmas produced by extensive (≲ 70%) partial melting of a source composition indistinguishable from the silicate fraction of average CH‐CL ordinary chondrites, with the exception of the depletion of the alkalis Na and K by a factor of 13±1 in the source composition. A metal‐free and volatile depleted Kapoeta Parent Body (KPB) is subsequently deduced and is shown to compare very well with other similarly derived Achondrite Parent Body (APB) and Howardite Parent Body (HPB) estimates but not to parent body estimates derived from inferences based on eucrite phase equilibrium studies. Other implications suggest that the KPB is heterogeneous with respect to Fe/Mg ratios.
Primary igneous carbon in ureilites: Petrological implications
Authors: Berkley John L. , Jones John H.
Published: February 2008

The ureilite meteorites are carbonaceous olivine‐pyroxene achondrites. They typically contain up to 4 wt.% carbon (carbonaceous matrix) as graphite, diamond, and lonsdaleite. Shock degradation has effectively obliterated primary textures in the carbonaceous matrix of previously described ureilites, a factor that has hampered efforts to explain the origin of this material. In contrast, the Antarctic ureilite ALHA78019 displays perfectly preserved primary textures in the carbonaceous matrix characterized by euhedral graphite blades intergrown with Fe‐Ni metal and sulfide (diamonds are absent). This petrographic feature suggests that most graphite in ureilites originated by crystallization from a C‐rich metallic phase. Assuming that fO2; is controlled by C‐CO‐CO2 reactions, the compositions of silicates and metals in ureilites imply a two‐stage redox history. The noble gases and rare earths of ureilites are discussed in light of this model.
Heat transport in porous cometary nuclei
Authors: Smoluchowski R.
Published: February 2008

Heat transport in cometary nuclei is important because it leads to the formation of larger tails and comae after perihelia than before, to early outbursts caused by phase transitions, and to fragmentation. The role of gases in pores in icy (H2O and H2o + Co2) nuclei in the heat transport has been investigated in the viscous and Knudsen diffusion regions. It appears that in pure H2o‐ices the heat flow is affected by the presence of pores but is affected only very close to the perihelion by water vapor contained in them. On the other hand, the presence of CO2 vapor in the pores significantly increases the heat flow in mixed nuclei at temperatures above 140°–150°K. Thus for heliocentric distances between 3 and 5 AU, the above mentioned phenomena should be considerably enhanced in proportion to the admixture of CO2 ices. The overlap and channel formation by pores in nuclei with high porosities is discussed.
Anelastic response of the Earth to a dip slip earthquake
Authors: Melosh H. J. , Raefsky A.
Published: February 2008

The deformation of the earth's surface immediately following an earthquake is well modeled by a dislocation in an elastic half space. Some time after the event, however, the elastic stresses begin to relax due to flow in the more fluid regions of the upper mantle (the asthenosphere). This relaxation causes further slow displacements observable at the earth's surface. This paper examines the pattern and timing of these postseismic motions for a vertical dip slip fault and a 30° dip thrust fault. A variety of rheological models are studied. These models encompass variations in asthenosphere thickness, mesosphere viscosity, fault depth, and, most important, power law flow. Both Newtonian (n = 1) and non‐Newtonian (n = 3) Maxwell rheologies are considered for the asthenosphere and mesosphere. The results for a vertical dip slip fault show that a peripheral upwarp develops within a distance of a few lithosphere thicknesses from the rupture zone on the downthrown side of the fault (a comparable downwarp develops on the upthrown side). These peripheral warps grow to a maximum height of about 5% of the total fault slip some time after the earthquake event and then collapse at much greater times. The vertical motions for a 30° dip thrust fault show that after the earthquake a general uplift occurs in the region near the rupture zone. Horizontal displacements propagate asymmetrically away from the rupture zone, the largest displacements occurring in the overthrust plate. The principal result of the comparison of n = 1 and n = 3 rheological laws is that the basic pattern of uplifts and horizontal motions is very similar: the major difference is the time dependence of the development of the pattern. In spite of the complex geometrical structure of the subduction zone the displacements ui(1)(t) for Newtonian rheology are nearly the same as ui(3)(t′) for the non‐Newtonian rheology if t′ ∼ t3. Thus although the qualitative pattern of the displacements is not diagnostic of power law flow, the great difference in time dependence allows a clear discrimination of the two flow laws.
On the thermal effects of groundwater flow: 1. Regional scale systems
Authors: Smith Leslie , Chapman David S.
Published: February 2008

Numerical solutions of the equations of fluid flow and heat transport are used to quantify the effects of groundwater flow on the subsurface thermal regime. Simulations are carried out for a vertical section through a basin with a distance of 40 km separating the regional topographic high and low. Emphasis is placed on understanding the conditions under which advective effects significantly perturb the thermal field. The transition from conduction‐dominated to advection‐dominated thermal regimes is sharp and depends primarily on the topographic configuration of the water table, the magnitude and spatial distribution of permeability, hydraulic anisotropy, and the depth of active flow. Deviations of surface heat flow from the background heat flux are a measurable effect of groundwater flow and depend on the same factors. Our results show that from 0% to almost 100% of the section may have surface heat flow significantly different from background heat flow, depending upon the nature of the hydrogeologic environment. A limited spatial variability in a distributed set of heat flow measurements and/or linear temperature‐depth profiles does not ensure that surface heat flow measurements are not disturbed. The results of our simulations suggest that knowledge of the complete environment of a site, including the water table configuration and subsurface flow system, combined with more closely spaced heat flow measurements may be necessary to unravel the true background heat flux in active flow regions.
Optical tracking of synchronous Earth's satellites for geophysical purposes
Authors: Catalano S. , McCrosky R. , Milani A. , Nobili A. M.
Published: February 2008

Tracking data from synchronous earth's satellites can be analyzed to provide the resonant geopotential coefficients, mainly C22 and S22 which represent the ellipticity of the earth's equator. Very accurate tracking techniques (LASER, Doppler) could in principle be used to improve the present knowledge of these coefficients but nongravitational perturbations on the satellite orbit, in particular solar radiation pressure, cannot be modeled to the same high accuracy unless the satellite is an ad hoc geophysical satellite. On the other hand, geosynchronous satellites currently used for telecommunication purposes can be easily tracked with moderate size large field telescopes measuring the satellite angular coordinates with respect to some catalogue field stars. The accuracy is at least 2 arc sec (≃ 400 m), i.e., much worse than the accuracy of the LASER data but surely comparable to longitude errors due to the radiation pressure model. Moreover, optical tracking does not need any onboard device and is easily accomplished. We conclude that the organization of a worldwide optical tracking campaign would allow an improvement in the knowledge of the long‐wavelength geoid by at least one order of magnitude, and this is very interesting for many geophysical purposes.
Geomagnetic spherical harmonic analyses: 1. Techniques
Authors: Schmitz Dave R. , Cain Joseph C.
Published: February 2008

Improved techniques for direct least squares analysis of the main geomagnetic field are introduced and tested with simulated data. One improvement involves adding secular change data to the least squares analysis (as well as the usual spatial data). As a result, the secular variation (SV), as observed at fixed magnetic observatories and repeat stations, directly adjusts the SV terms of the spherical harmonic coefficients, while spatial data adjust both the spatial and SV terms. Simulations show that SV errors can be reduced by factors of 2 or 3 by introducing SV as data. The second refinement involved obtaining uncorrelated spherical harmonic coefficient sets through employment of a uniform distribution of data by area or by a compromise of an area‐weighting scheme. The latter was shown to reduce high cross correlations between spherical harmonic coefficients by up to a factor of 3 in simulated tests. This technique is especially useful in analyses of data sets which are unevenly distributed. Justification is given for the area‐weighting scheme and also for the employment of evenly distributed data sets, as may readily be obtained in the case of satellite data. A third refinement, though used previously, that of incorporating declination data weighted by the horizontal intensity and inclination data weighted by total intensity, was shown to have merit on the basis of observed error distributions.
The dependence of fracture mechanical and fluid flow properties on fracture roughness and sample size
Authors: Tsang Y. W. , Witherspoon P. A.
Published: February 2008

A parameter study has been carried out to investigate the interdependence of mechanical and fluid flow properties of fractures with fracture roughness and sample size. A rough fracture can be defined mathematically in terms of its aperture density distribution. Correlations were found between the shapes of the aperture density distribution function and the specific fractures of the stress‐strain behavior and fluid flow characteristics. Well‐matched fractures had peaked aperture distributions that resulted in very nonlinear stress‐strain behavior. With an increasing degree of mismatching between the top and bottom of a fracture, the aperture density distribution broadened and the nonlinearity of the stress‐strain behavior became less accentuated. The different aperture density distributions also gave rise to qualitatively different fluid flow behavior. Findings from this investigation make it possible to estimate the stress‐strain and fluid flow behavior when the roughness characteristics of the fracture are known and, conversely, to estimate the fracture roughness from an examination of the hydraulic and mechanical data. Results from this study showed that both the mechanical and hydraulic properties of the fracture are controlled by the large‐scale roughness of the joint surface. This suggests that when the stress‐flow behavior of a fracture is being investigated, the size of the rock sample should be larger than the typical wave length of the roughness undulations.
The stability of ground ice in the equatorial region of Mars
Authors: Clifford Stephen M. , Hillel Daniel
Published: February 2008

Consideration of the partial pressure of H2O in the Martian atmosphere and the range of mean annual temperatures at the Martian surface suggests that the occurrence of ground ice in equilibrium with the atmosphere is restricted to latitudes poleward of ±40°. However, there is a growing body of morphologic evidence that indicates that substantial quantities of ground ice may have been present in the equatorial regolith throughout Martian geologic time. The accepted explanation for this apparent contradiction has been that the H2O found near the equator is a relic, emplaced very early in Martian geologic history (>3.5 b.y. ago) and under substantially different climatic conditions. It is generally believed that this fossil ground ice layer has been preserved to the present day by the diffusion‐limiting properties of a relatively shallow layer ( ≲10 m) of fine‐grained regolith. To evaluate this hypothesis, the lifetime of a 200‐m layer of ground ice, buried below 100 m of ice‐free regolith, has been examined for the latitudes between ±30° on Mars. Twelve model pore size distributions, representative of silt‐ and clay‐type soils found on Earth, were selected to simulate the pore structure of the Martian regolith. The parallel pore model of gaseous diffusion was then used to calculate the flux of escaping H2O molecules from the buried ground ice layer. The potential effects of such factors as depth of burial, the Martian geothermal gradient, regolith porosity, adsorption, surface diffusion, and climatic change were also considered. Based on our analysis, the most important factors affecting the stability of equatorial ground ice appear to be (1) soil structure, (2) the magnitude of the geothermal gradient, and (3) the climatic desorption of CO2 from the regolith. We conclude that while the conditions necessary for the long‐term survival of ground ice may exist in isolated regions within the equatorial regolith, on the global scale it appears unlikely that a fossil ground ice layer has been preserved throughout Martian geologic history. If massive ground ice does presently exist in the equatorial regolith, then its presence may imply the existence of some continuous or periodic process of replenishment.
Multimode Rayleigh wave attenuation and Qβ in the crust of the Barents Shelf
Authors: Kijkol A. , Mitchell B. J.
Published: February 2008

The multimode method of Cheng and Mitchell [1981] is used to study the attenuation properties of the crust throughout the Barents shelf. The sensitivity of the spectra to various source factors and to propagation factors which might characterize a shelf region are thoroughly studied. For plane‐layered models, the method is very sensitive to Qβ in the upper crust and to Qβ of sedimentary layers if those layers are thick and highly attenuating. It is far less sensitive to Qβ in the lower crust and to Qα. The spectra, for shallow strike slip earthquakes, are very insensitive to changes in strike, slip, and dip angles of the fault throughout the entire period range of interest but are very sensitive to variations in focal depth except in the long‐period range of the fundamental mode and the short‐period range of the higher modes. The fundamental mode, but not the higher mode, spectra for one path across the Barents shelf are, apparently, distorted at short periods by lateral heterogeneities along the travel path. Simple two‐layer, trial‐and‐error inversions yield a model of the Barents shelf with an upper crustal Qβ value of about 80. Three‐layer inversions with a sedimentary layer, however, yield an upper crustal Qβ of about 250, if the sediments are 1 km thick and are characterized by Qβ values of 40. Shear velocity models for the Barents shelf show variations between about 3.0 and 3.6 km/s in the upper crust along different paths. The lower crust shows smaller regional differences, but velocities vary between 3.5 and 4.0 km/s through the depth range 25–35 km. The variation of shear velocities in the upper mantle may correlate with those of the upper crust and vary regionally between about 4.3 and 4.8 km/s.
Cocorp and the continental crust
Authors: Oliver Jack , Cook Frederick , Brown Larry
Published: February 2008

This is a speculative paper on the continental crust, a major frontier of modern earth science. Seismic reflection profiling data from the COCORP project, coupled with other kinds of geophysical and geological observations, suggest some fresh perspectives on the crust. For example, there is evidence for great differences within the basement at a single site and from one COCORP site to another. A simple layered model is surely not an adequate representation of the crust in most areas. There is, however, the puzzling and rather common occurrence of limited zones of parallel, though not always flat, layered features at various depths in certain parts of the crust. Are the corresponding rocks metasedimentary? Or igneous? Or both? There are transparent zones from which little or no coherent reflected seismic energy is detected. Are the corresponding rocks intrusions? Or highly deformed metasediments? Can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust? There is evidence for large‐scale thrusting and related features that emphasizes the great importance of the Wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences. In some places the modern Moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting. The widely held concept of the Moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust.
The subduction of young lithosphere
Authors: Sacks I. S.
Published: February 2008

Studies, using a variety of techniques, of the subduction beneath western South America and the southwest Honshu‐Kyushu region of Japan indicate that volcano‐free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere. The subduction is initiated at normal dip angles, but the plate deforms at some depth (100 km under central Peru) to travel horizontally immediately beneath continental lithosphere. The most plausible reason for this geometry is that the subducted plate is buoyant. A model is developed constrained by age of the plate, bathymetry, and heat flow. Estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts. If the high‐density tranformation of crustal basalt to eclogite is retarded by low temperatures, the plate can remain buoyant to considerable depth for long periods of time. Heat flow data from western South America are consistent with this model.
A dislocation model of strain accumulation and release at a subduction zone
Authors: Savage J. C.
Published: February 2008

Strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface. This process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone, consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip. Because steady state subduction does not contribute to the deformation at the free surface, deformation observed there is completely equivalent to that produced by the slip cycle alone. The response to that slip is simply the response of a particular earth model to embedded dislocations. For a purely elastic earth model, the deformation cycle consists of a coseismic offset followed by a linear‐in‐time recovery to the initial value during the interval between earthquakes. For an elastic‐viscoelastic earth model (elastic lithosphere over a viscoelastic asthenosphere), the postearthquake recovery is not linear in time. Records of local uplift as a function of time indicate that the long‐term postseismic recovery is approximately linear, suggesting that elastic earth models are adequate to describe the deformation cycle. However, the deformation predicted for a simple elastic half‐space earth model does not reproduce the deformation observed along the subduction zones in Japan at all well if stick slip is restricted to the main thrust zone. As recognized earlier by Shimazaki, Seno, and Kato, the uplift profiles could be explained if stick slip were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km, but independent evidence suggests that stick slip at such depths is unlikely.
Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan
Authors: Ando Masataka , Ishikawa Yuzo , Yamazaki Fumihito
Published: February 2008

Shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth's surface is investigated using three‐component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in Honshu, Japan. Eighty nine high‐quality seismograms were selected from a period of 3 years. The data used in this study are restricted such that incidence angles are smaller than the critical angle of 30° to the earth's surface in order to avoid phase shifts in the shear wave train. To find directions of the maximum and minimum velocities in split shear waves, where shear waves are resolved into two phases with the maximum time separation, each set of the two horizontal component seismograms is rotated in the horizontal plane. The split shear waves thus obtained are again recombined after the correction of anisotropy, and the anisotropy‐corrected particle motion is compared with the focal mechanism for a cross‐check of the observed anisotropy. Directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station. The stereograms and the cross sections of seismic ray paths show that (1) the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations, and (2) the anisotropic region is separated into two parts: one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0° to 30° from the north (north anisotropy) and the other in the south with it trending 90° to 120° (south anisotropy). The maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions. The horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km. perhaps prevalent in west Honshu. However, the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles. If we assume the vertical extent of the anisotropic material to be 100 km, the maximum velocity contrast would be 4%. If we adopt a crack alignment model, for the observed anisotropy, cracks are inferred to be distributed vertically trending 0° to 30° in the north and 90° to 120° in the south. If we assume an olivine alignment model, olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0° to 30° and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90° to 120° and the slip plane being horizontal.
The possible bearing of the granite of the UPH Deep Drill Holes, northern Illinois, on the origin of Mississippi Valley ore deposits
Authors: Doe B. R. , Stuckless J. S. , Delevaux M. H.
Published: February 2008

The Pb‐Pb, Th‐Pb, and U‐Pb systems of whole‐rock drill‐core samples of the granite of UPH record at least one postcrystallization event. Treatment of the data on a concordia plot indicate the timing of this disturbance was probably 260 ±35 m.y. ago and probably no older than 400 m.y. ago. The upper intercept age on concordia is 1416±20 m.y and the Pb‐Pb isochron age for the four least disturbed rocks plus the four potassium feldspars is 1451±19 m.y. These ages are in close agreement with Rb‐Sr whole rock and U‐Pb zircon ages determined by others and should be the age of granite formation. Considerable Pb loss is indicated for many of the shallower samples that have a more red rather than pink color, probably to a factor of two to three or more as a result of the disturbance, although the granite remains Pb‐rich today (40–80 ppm). In some other studies of disturbed U‐Th‐Pb systems, the radiogenic lead has been taken up by potassium feldspars; however, the Pb for most samples seems to have been lost from the system for the granite of UPH. One sample with an unusually high Pb content of about 195 ppm has the most radiogenic Pb of any sample but ‘normal’ values of Th‐U. This sample has obviously gained radiogenic Pb (by more than a factor of four) and confirms that Pb was indeed mobilized in parts of the system. Within the limits of the uncertainties, equal amounts of Pb were lost or gained in both the U‐Pb and Th‐Pb systems, which is compatible with mobilization being restricted to lead. There is some tendency, however, for the more affected samples to have lower Th‐U values and higher U contents. A complex model in which U is gained, Pb lost, and 208Pb lost preferentially to 207Pb and 206Pb cannot be ruled out. U and Th have somewhat different crystallographic locations and decay energies so that 208Pb might be in more accessible sites than the other Pb isotopes. Fission track mapping of the samples indicates most of the U today is located in submicroscopic sites along edges and cleavages of biotite and iron oxides so U is readily available for remobilization. Lead extraction with only minor U or Th removal may have been accomplished by chloride‐rich brines. Lead isotope data on the granite of UPH indicates that some of the Pb in the ore in the nearby Upper Mississippi Valley lead‐zinc district may have been derived from these granites. An even better match is made with the ores of southeastern Missouri, although a granite like that of UPH would only be one end member (most likely the radiogenic end member) of a mixed source. Curiously, similar aged and heavy metal rich granites from southeastern Missouri are not a good match for theckn nearby ores. Basement rocks such as the granite of UPH must be reconsidered as an important contributor to the ores of the Mississippi Valley, but it is unlikely that they constitute the sole source.
Distribution and origin of igneous rocks from the landward slopes of the Mariana Trench: Implications for its structure and evolution
Authors: Bloomer Sherman H.
Published: February 2008

The landward slope of the Mariana Trench is composed largely of igneous rocks. Serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m. Seamountlike features on the trench slope break are the surface expression of serpentinite diapirs. Cumulate and massive gabbros are found; several varieties of volcanic rocks are common including boninites, altered and metamorphosed basalts, andesites, and dacites. The chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism. Together with the gabbros, these volcanic rocks represent what is probably a late Eocene arc complex. These rocks were probably the first volcanic products to result from the subduction of the Pacific plate beneath the Philippine Sea plate; their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since Eocene time. Most of this removal of material appears to have occurred during the early stages of subduction. There are isolated occurrences on the landward slope of rock assemblages including alkalic basalts, chert, hyaloclastites, upper Cretaceous siliceous sediments, and shallow water limestones. These assemblages are very similar to rocks dredged from seamounts on the offshore flank of the trench, and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion, there has been episodic accretion of seamount fragments to the landward slope.
Earthquakes in the Orozco Transform Zone: Seismicity, source mechanisms, and tectonics
Authors: Tréhu Anne M. , Solomon Sean C.
Published: February 2008

As part of the Rivera Ocean Seismic Experiment, a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the Orozco transform fault in the central eastern Pacific. We present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network. All epicenters are within the transform region of the Orozco Fracture Zone and clearly delineate the active plate boundary. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone (region 1). Most focal depths for these events are very shallow, within 4 km of the seafloor; several well‐determined focal depths, however, are as great as 7 km. No shallowing of seismic activity is observed as the rise‐transform intersection is approached; to the contrary, the deepest events are within 10 km of the intersection. First motion polarities for most of the earthquakes in region 1 are compatible with right‐lateral strike slip faulting along a nearly vertical plane, striking parallel to the spreading direction. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity in this region is more scattered than in the first, and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. Pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions. The effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three‐dimensional velocity grid. While findings for events in region 1 are not significantly affected, in region 2, epicentral mislocations of up to 10 km and azimuthal deflections of up to 45° may result from assuming a laterally homogeneous velocity structure. When corrected for the effects of lateral heterogeneity, the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending NW–SE; the focal depths, however, are poorly constrained. These results suggest an en echelon spreading center or leaky transform regime in the central transform region.
Inference of variations in the gravity field from satellite‐to‐satellite range rate
Authors: Kaula William M.
Published: February 2008

An analytic scheme for inferring variations of the gravity field from satellite‐to‐satellite range rate (low‐low) is developed. As a test, it is applied to a pair of satellites in polar orbit, at altitude 160 km and spacing 100 km, with 72 data points per revolution. An assumed gravity field of tesseral spherical harmonics up to the eighth degree is completely recovered in three iterations over 64 revolutions. It is apparent that data points at regular intervals enable the use of data analysis techniques that avoid massive matrix inversions.
Collision, rotation, and the initiation of subduction in the evolution of Sulawesi, Indonesia
Authors: Silver Eli A. , McCaffrey Robert , Smith Randall B.
Published: February 2008

The island of Sulawesi, Indonesia, has been shaped and deformed as a result of collision with the Sula platform, a sliver of continental material from the northern margin of Australia‐New Guinea. The collision has resulted in rotation of the north volcanic arm of Sulawesi and the development of the accretionary wedge of the North Sulawesi trench. The North Sulawesi trench changes laterally from a zone of no active deformation in the eastern part to a wide accretionary wedge in the west. Early stages of thrusting produce a steep frontal slope (8°–16°), indicative of relatively high basal shear stress, whereas the more advanced (western) zone of thrusting produces a gentle (2°) slope, consistent with low basal shear stress. Reported paleomagnetic data suggest post late Eocene counter‐clockwise rotation of the North Arm, and the offshore geophysics are explained by a pivot of the North Arm with respect to the Celebes basin about the eastern end of the arc. Convergence between the north Banda basin and Southeast Sulawesi is documented by the presence of the Tolo thrust. Its outcrop is strongly arcuate and its accretionary wedge varies in width from a minimum of a few kilometers at each end to a maximum of 30–40 km in the central part. The northern end transforms to the leftlateral Matano fault, with a reported offset of 20 km. The southern end of the thrust projects toward the deformed rocks of Buton, but the structural relations there are not clear. The Matano fault zone appears to connect westward with the Palu fault, which forms the western transform of the North Sulawesi trench. The Palu‐Matano fault system acts as a trench‐trench transform between the North Sulawesi trench and the Tolo thrust, and this system is described by the same rotation pole as that for the Sulawesi North Arm.
Electrical conductivity, temperatures, and fluids in the lower crust
Authors: Shankland Thomas J. , Ander Mark E.
Published: February 2008

We have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles. Regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models; we avoided those data where purely conductive heat flow suggested temperatures more than about 1000°C, substantially higher than solidus temperatures and outside the range of validity of heat flow models. The resulting plots of log conductivity σ versus 1/T demonstrate that even low‐conductivity layers (LCL) have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits. In addition, technically active regions show systematically higher conductivities than do shield areas. Because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions, these materials probably account for the relatively higher conductivities of rocks in situ in the crust; free water in amounts of 0.01–0.1% in fracture porosity could explain crustal conductivities. Other possibilities are graphite, hydrated minerals in rare instances, and sulfur in combination with other volatiles. As most of the temperatures are less than 700°C, partial melting seems likely only in regions of highest heat flow where the conductive temperature profiles are inappropriate. Another result is that at a given temperature, crustal high‐conductivity layers (HCL) are more conductive by another order of magnitude and show more scatter than do LCL's. Because the differences between HCL's and LCL's are independent of temperature, we must invoke more than temperature increases as a cause for large conductivity increases; increased fluid concentration in situ seems a probable cause for enhanced conductivities in HCL's. From the point of view of these observations, it does not matter whether the fluids are in communication with the surface or trapped at lithostatic pressures.
Slip instability and state variable friction laws
Authors: Ruina Andy
Published: February 2008

The dependence of the friction force on slip history is described by an experimentally motivated constitutive law where the friction force is dependent on slip rate and state variables. The state variables are defined macroscopically by evolution equations for their rates of change in terms of their present values and slip rate. Experiments may strongly suggest that one state variable is adequate or prove that one is inadequate. Analysis of steady slip governed by a single state variable in a spring and (massless) slider predict oscillations at a critical spring stiffness k = kcrit. The critical stiffness kcrit is given by a simple formula and steady slip is stable for k > kcrit and unstable for k < kcrit. State variable friction laws may superficially appear as a simple slip rate dependence, slip distance dependence, or time dependent static friction, depending on experiment and testing machinery. Truly complicated motion is possible in a spring‐slider model if more than one state variable is used. Further consequences of state variable friction laws can include creep waves and apparent rate independence for some phenomena.
Spectral properties of mixtures of montmorillonite and dark carbon grains: Implications for remote sensing minerals containing chemically and physically adsorbed water
Authors: Clark Roger N.
Published: February 2008

The spectral properties from 0.4 to 3μm of montmorillonite plus dark carbon grains (called opaques) of various sizes are studied as a function of the weight fraction of opaques present. The reflectance level and band depths of the 1.4‐, 1.9‐, 2.2‐, and 2.8‐μm water and/or OH absorption features are analyzed using derived empirical relationships and scattering theory. It is found that the absorption band depths and reflectance level are a very nonlinear function of the weight fraction of opaques present but can be predicted in many cases by simple scattering theory. The 2.8‐μm bound water fundamental band is the most difficult absorption feature to suppress. The overtone absorptions are suppressed a greater amount than the fundamental but are still apparent even when 10–20 wt% opaques are present. Thus the band depth ratio of one overtone by the fundamental or other lower overtone varies as a function of the weight fraction of opaques present. The relationships observed and the simple scattering theory presented show that quantitative compositional remote sensing studies are feasible for surfaces containing complex mineral mixtures. The question of the uniqueness of quantitative remote sensing is discussed.
Chemical composition of the mantle
Authors: Anderson Don L.
Published: February 2008

The composition of primitive mantle (54 elements) is estimated by a mass balance approach that does not make a priori assignments of basalt:peridotite ratios or LIL contents of these components. It is also not necessary to assume that such ratios as Rb/Sr and K/U are the same as in the crust. Primitive upper mantle is treated as a four‐component system: crust, peridotite, LIL‐depleted basalt (MORB), and an LIL‐enriched component. These are combined to give chondritic ratios of the oxyphile refractory trace elements. The composition of the whole mantle is estimated by requiring chondritic ratios of the major elements as well. In this way one can estimate the volatile and siderophile content of the mantle. The primitive mantle has K = 152 ppm, U = 0.020 ppm, Th = 0.078 ppm, K/U = 7724, and Rb/Sr = 0.025. The ratios are significantly less than previous estimates. The inferred steady state heat flow, 0.9 μ cal/cm2 s, implies a substantial contribution of cooling to the observed heat flow. The crust and upper mantle may contain most of the terrestrial inventory of the incompatible elements, including K, U, and Th. There is no evidence that the chalcophiles are strongly partitioned into the core.
Brachina: A new type of meteorite, not a chassignite
Authors: Nehru C. E. , Prinz M. , Delaney J. S. , Dreibus G. , Palme H. , Spettel B. , Wänke H.
Published: February 2008

The Brachina meteorite, previously classified as a chassignite, is found to be a nearly primitive and unique meteorite. It is a fine‐grained, unshocked equigranular igneous rock containing small melt inclusions. It contains 80% olivine (Fo70), 5.5% clinopyroxene (Wo38En49Fs13), 10% plagioclase (An22), 0.5% chromite, 0.5% chlorapatite, 3% Fe‐sulfide, 0.3% pentlandite, and a trace of Ni‐rich metal; orthopyroxene (En70) and anorthoclase‐rich glass are found only in melt inclusions. Siderophiles are high in the sulfides, and the oxidation state is similar to that of LL‐chondrites. Major element and mineralogic data of Brachina are similar to Chassigny, but trace element data are dramatically different. The major differences are higher FeO/MnO, Ga/Al, and K/La ratios, W depletion, high siderophile content, near‐horizontal REE pattern at about 1× chondrite level, high refractory/nonrefractory metal ratios (e.g., Ir/Au), and chalcophile element depletion. Compositionally, Brachina is close to C2 chemistry, but has undergone some segregation of (Fe,Ni)S melt. These data, in addition to its near 4.5 b.y. age, and oxygen isotopic signature within the eucritic field, suggest that Brachina is not related to Chassigny or SNC meteorites. It may be related to primitive meteorites, such as silicate inclusions in IAB irons and winonaites, but is more oxidized; reduction would make it very similar to winonaites. The assemblage may have experienced the beginning of melting in its parent body. Since it has characteristics that differ from any other meteorite, we suggest it be called a ‘brachinite’.
Experimental deformation of polycrystalline H2O ice at high pressure and low temperature: Preliminary results
Authors: Durham W. B. , Heard H. C. , Kirby S. H.
Published: February 2008

Interest in the mechanical properties of water ice under the conditions in which it exists in the outer solar system has motivated the development and use of a new high‐pressure, low‐temperature triaxial deformation apparatus. Constant displacement rate tests on 70 samples of pure polycrystalline water ice have been performed at temperatures 77≤T≤258 K, confining pressures 0.1≤P≤350 MPa, and strain rates 3.5×10−6≤
≤3.5×10−4 s−1. In most cases, the ice polymorph tested was ice Ih. Both brittle and ductile behavior have been observed. Brittle behavior of ice, promoted by lower pressure, lower temperature, and higher strain rate, is analogous to that in rocks, with the important exception that brittle fracture strength becomes independent of confining pressure above 50 MPa pressure and the fracture angle is approximately 45° to the loading direction (i.e., the coefficient of internal friction is approximately zero). Ductile flow, the predominant behavior in our tests at T≥195 K, follows a law of form
= Aσn exp (−H*/RT) (σ is stress; R is the gas constant; A, n, H* are material constants). Three sets of material constants are required to fit the data, with changes in sets (or mechanisms) occurring near 243 K and 195 K. The value of n remains near 4 throughout the measured ductile field, but H* drops from 91 to 61 to 31 kJ/mole as temperature decreases. The maximum brittle strength measured was 171 MPa; the maximum ductile strength measured was 91 MPa. At confining pressures near the phase transition pressure of ice Ih → ice II, the ductile strength is observed to drop dramatically. Some overlap with previous work occurs at higher temperatures and lower pressures. Agreement with present work is generally good, both quantitatively in the values of n and H*, and qualitatively in the mechanism of deformation. Although the ductile strengths measured here are somewhat higher than expected on the basis of extrapolations of previous work, the low value of H* at T<195 K indicates that the ice Ih layer on icy bodies in the solar system is much weaker than has generally been predicted.
The magma ocean from the Fra Mauro shoreline: An overview of the Apollo 14 crust
Authors: Hunter Robert H. , Taylor Lawrence A.
Published: February 2008

Numerous coarse‐grained, polymineralic, texturally‐monomict, igneous clasts from the Apollo 14 breccias have been studied petrographically and mineralogically. The aims of these clast studies were to evaluate the range of crustal lithologies present at the Apollo 14 site, to bracket their constituent mineralogical compositions, and to assess their relative abundances. Troctolite and troctolitic anorthosite are the most abundant rock types. These are related to the Mg‐rich trend. Three distinct plutons may be represented. There is a distinct paucity of noritic lithologies. The second most abundant group consists of plagioclase cumulates (alkali‐rich anorthosites) with more evolved mineral compositions than the Mg‐rich trend cumulates; their provenance is indeterminate. Coarse‐grained ilmenite gabbros and mineralogically‐evolved monzonoritic and granitic clasts are widespread in occurrence but not abundant. An olivine‐gabbronorite clast (14305,92) is pristine. Petrographically and mineralogically, it shows a striking resemblance to Apollo 12 plivine mare basalts. The paucity of ferroan anorthosites at this and the Apollo 17 site is attributed to a higher density of Mg‐rich and related intrusives in the primitive anorthositic crust in comparison with other sites. Our data provide further support for widespread regional heterogeneities within the early lunar crust.