bluuebunny/crossref_metadata_2025 huggingface dataset:
Sodium weakening of heavitree quartzite: Preliminary results
Authors: Jaoul Olivier
Published: February 2008

The electronic band structure of quartz is altered by the presence of small amounts of impurities such as aluminum, transition metals, alkalies or H2O. Silicon and oxygen selfdiffusion are enhanced by an increase in the concentration of ionized defects such as charged oxygen vacancies or silicon interstitials due to HOH acting as a shallow electronic acceptor. Sodium, by contrast, is a shallow electronic donor and retards the diffusion of these species. To evaluate the effect of the sodium dopant on strength we performed a simple creep experiment (two runs, P = 1.5 GPa, T = 1073, 1163°K, σ = 0.2 to 1 GPa) on, Heavitree quartzite (.2 mm grain size) containing 350 ppm at. Na. These doped samples deformed 10 times faster than equivalent undoped ones (at the same stress and temperature). Simple diffusion‐control of the creep rate is therefore unlikely but an alternative proposed by Hirsch seems consistent with the data. Using Hirsch's model, the weakening could be interpreted as due to an increase in the dislocation glide velocity through an increase of the charged kink population. This implies electronic transfers are strongly influenced by the nature and the abundance of the impurities (including hydroxyls). Sodium could favour the nucleation of negatively Charged kinks. Although this is preliminary and was performed using specimens containing grain boundaries and other impurities (Al), we decided to report it rapidly because of its importance to geology: rocks (like quartzites in this example) not only carry water, but also many different impurities which certainly affect their strengths.
The interaction of a subducting lithospheric slab with a chemical or phase boundary
Authors: Christensen Ulrich R. , Yuen David A.
Published: February 2008

We have used a finite element model of time‐dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle. A temperature‐dependent and non‐Newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection. The 650‐km discontinuity is taken as either a chemical or phase boundary or as a combination of both. It is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position. When the compositional density contrast is greater than about 5%, the descending slab is deflected sidewards at the boundary and two‐layer convection prevails. A resulting depression of the boundary in the range of 50–200 km should be detectable with seismic methods. Below 5% density difference the slab plunges several hundred kilometers into the lower mantle, and below 2% it will probably not stop before reaching the core‐mantle boundary and extensive mixing would be expected. With a pure phase change a negative Clapeyron slope of about −6 MPa/K (−60 bar/K) is required to establish a type of “leaky” double‐layer convection. A more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle. With the present uncertainties about the physical nature of the 650‐km discontinuity, a variety of convective styles appears possible on dynamical grounds.
Active faulting and tectonics of the Ningxia‐Hui Autonomous Region, China
Authors: Qidong Deng , Sung Fengmin , Zhu Shilong , Li Mengluan , Wang Tielin , Zhang Weiqi , Burchfiel B. C. , Molnar Peter , Zhang Peizhen
Published: February 2008

Strike‐slip, thrust, and normal faulting all seem to play an active role in the tectonics of Ningxia. In the southernmost part of the region a major left‐lateral strike‐slip fault enters the region from the neighboring Gansu province to the west and trends about S65°E. This fault is very clear on Landsat imagery and on aerial photos, and the portion in eastern Gansu and Ningxia broke in the Haiyuan earthquake of December 16, 1920. Displacements of 5–10 m caused by that earthquake are clear in numerous localities and accord with a revised value of the seismic moment of 1.2×1021N m. The eastern end of the Haiyuan fault terminates in a narrow south trending fold and thrust zone. Several other similar, north to northwest trending fold and thrust belts are present in the area about 50–200 km northeast of the Haiyuan fault and divide it into small, apparently relatively undeformed blocks 10–40 km in dimensions. The geometry of the structures in the fold and thrust zones and the apparently shallow depths at the time of deformation suggest that current deformation is similar to that that occurred in the fold and thrust belt of the Idaho‐Wyoming Rocky Mountains. North of this area, both the Helan Shan (a horst) and the Yinchuan graben are bounded by clear, active northerly trending normal faults, in some cases with right‐lateral strike‐slip components. The overall deformation, hence, seems to include dominant components of east‐west left‐lateral strike‐slip movement, northeast‐southwest crustal shortening, and northwest‐southeast extension. We interpret the extension as a response to a northeast directed force applied to the Ordos block and both this northeast directed force and the left‐lateral slip on the Haiyuan fault to the eastward displacement of material on the northeast edge of the Tibetan plateau with respect to Eurasia north of it.
Geophysical and isotopic constraints on mantle convection: An interim synthesis
Authors: Davies Geoffrey F.
Published: February 2008

Strontium and lead isotopic data from oceanic rocks seem to require not just two or three but many ancient mantle sources, and this would remove the geochemical rationale for mantle layering, although it would not preclude layering. Seamount data require that at least some small heterogeneities are present in the uppermost mantle. There is no compelling independent evidence that rocks with chrondritic neodymium isotopes are derived from a primitive mantle source. It follows that isotopic correlations do not necessarily constrain the proportion of depleted mantle or the mean silicate (or “bulk earth”) values of Sr and Pb isotopic compositions. Several recent geophysical arguments concerning the mantle viscosity profile, aseismic extensions of Benioff zones, and the angles and shapes of Benioff zones indicate that mantle flow associated with the plates penetrates well into the lower mantle. This would probably preclude chemical layering at or above the transition zone. These arguments suggest a mantle with heterogeneities of many sizes and compositions at all depths and with any stratification confined to roughly the bottom half. Such stratification might result from a pressure‐induced viscosity increase with depth or from gravitational settling and would help to explain the longevity of heterogeneities, including primordial He and Xe isotopic signatures, and the longevity and low horizontal velocities of some hot spots. Possible sources of mantle heterogeneities include recycled oceanic and continental crust, detached continental lithosphere, and surviving primitive mantle.
A comparison of velocity and attenuation between the Nicobar and Bengal Deep Sea Fans
Authors: Jacobson R. S. , Shor G. G. , Bée Michel
Published: February 2008

The compressional wave velocity and attenuation measurements from a deep source, deep receiver seismic refraction experiment from the Nicobar Fan, Indian Ocean, are presented and compared to a similar set of measurements in the Bengal Fan. The two deep sea fans consist of thick sections of turbidites derived from the same sediment source but have slightly different depositional histories. Travel time inversion of the data into a velocity depth model uses the tau‐zeta linear inversion scheme. We present a revised reparameterization of the travel time data and improved error analysis for the inversion process. Although the velocity structures with depth are morphologically similar, there exist discrete differences between the two stations. In the Nicobar Fan, the initial velocity (assumed) is 1.513 km/s, with an initial gradient of 2.32 s−1. The velocity increases smoothly with depth, while the velocity gradient decreases rapidly to a constant 0.81 s−1. The Bengal Fan station showed the same trend, although the velocity gradient at depth was only 0.67 s−1. The attenuation profiles are similar only in gross structure, with the Nicobar Fan values of Q−1 being a factor of 3 to 4 less than the Bengal station. The differences in velocity and attenuation structure can be attributed to a number of causes, including porosity and sedimentation rates (which are related to the distance from the sediment source) and the presence of a normal fault near the Nicobar Fan site. This fault may act as a conduit for water removal in the presence of overburden pressure. It is believed that porosity changes, due to whatever cause, dominate the lateral and vertical variations of velocity, velocity gradients, and (together with intrabed multiples) attenuation in thick sedimentary sections.
Scaling of ground motion parameters, state of stress, and focal depth
Authors: McGarr A.
Published: February 2008

The ground motion parametersRv and ρRa, where R is hypocentral distance, v is peak velocity, ρ is density, and a is peak acceleration, are found observationally to be strong functions of stress state and linearly related to focal depth z. If the stress state and focal depth are properly taken into account, then the ground motion parameters depend on the seismic moment M0, as expected from basic scaling principles. That is, Rv ∝ M01/3, and ρRa is independent of earthquake size. Regression lines fitted to observations, covering broad ranges in seismic moment and focal depth, indicate that for extensional and compressional tectonic regimes, corresponding to normal and thrust or reverse faulting earthquakes, respectively, peak acceleration is given by ρRa = ‐ 1.08 MPa + 3.06 (MPa/km)z (normal) and ρRa = 5.65 MPa + 8.76 (MPa/km)z (thrust), and for peak velocity, Rv/M01/3 = 10−4(m2/s) (N m)−1/3 [3.00+0.69 (km−1)z] (normal) and Rv/M01/3 = 10−4(m2/s) (N m)−1/3 [4.63+1.82 (km−1)z] (thrust). For strike‐slip earthquakes the data currently are insufficient to define regression fits, but such lines would definitely lie between those for the normal and thrust stress regimes. These equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space. It follows that for comparable hypocentral depth, peak acceleration in a compressional tectonic regime, for example, much of the eastern United States, is a factor of about 3 greater than in an extensional regime, such as Nevada, and for peak velocity a similar comparison yields a factor of 2 difference. The similarity in behavior of the peak acceleration parameter to crustal strength, estimated from Byerlee's law of friction, with respect to depth and stress state, indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high‐frequency peak ground motion. Analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter, as recorded at a typical surface site, that depend on stress state, specifically, a (normal) ≲0.5 g and a (thrust) ≲1.9 g. Thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the prediction of seismic ground motion.
A geophysical study of the Manila Trench, Luzon, Philippines: 2. Fore arc basin structural and stratigraphic evolution
Authors: Lewis Stephen D. , Hayes Dennis E.
Published: February 2008

The pattern of sediment distribution within the Manila Trench subduction zone west of Luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin. Multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic/hemipelagic sediment layer and oceanic basement in the northern part of the Manila Trench. These turbidites were transported from the uplifted collision zone of Taiwan, southward along the Manila Trench by gravity‐controlled processes. Similarly, thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the North Palawan block and the central Philippines. Sedimentation within the Manila Trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis. In contrast, the fore arc basin system of the Manila Trench receives sediment through submarine canyons incised into the West Luzon continental slope. The rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench. The size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench. A thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region. This generalization may apply to other convergent plate margins as well. The uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins. This variability results in the formation of several different types of fore arc morphology along the Manila Trench. The overall seismic stratigraphy of the West Luzon Trough fore arc basin is remarkably uniform, with seismic sequences continuous for up to 110 km along strike. The unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long‐term (approximately 10 Ma) uplift of the accretionary prism and relatively short‐term (approximately 1–2 Ma) variations in sediment supply to the fore arc basin, modulated by eustatic sea level changes.
Near‐surface in situ stress: Introduction
Authors: Engelder Terry , Sbar Marc L.
Published: February 2008

From plots of in situ stress versus depth it is apparent that horizontal stress is several times the vertical stress at depths of <100 m within the earth's crust [Hast, 1958; Herget, 1973; Haimson, 1978]. Brown and Hoek's [1978] compilation of earth stress shows that the average of principal horizontal stresses at zero depth varies between 2.7 and 40.5 MPa. The literature contains no consensus on why the horizontal stresses should be so high at shallow depths. Possible mechanisms include plastic yield and subsequent elastic deformation accompanying unloading on removal of overburden [Seagar, 1964], topography [Voight, 1966], and creep under tectonic forces [Hast, 1969].
Near‐surface in situ stress: 2. A comparison with stress directions inferred from earthquakes, joints, and topography near Blue Mountain Lake, New York
Authors: Plumb Richard , Engelder Terry , Sbar Marc
Published: February 2008

At 11 outcrops within 100 km of Blue Mountain Lake, New York, we measured strain relaxation during overcoring of “surface,” “doorstopper,” and Bureau of Mines borehole deformation gauges. The majority of measurements showed a maximum expansion ε1 parallel with the contemporary tectonic stress field. To confirm further the orientation of in situ stress, at two sites, vertical fractures were induced at borehole walls using a packer‐fracturing technique. Several cores from each site were then tested for mechanical anisotropy using ultrasonic, compressibility, and thin section analyses. The orientations of mechanical anisotropy had a poor correlation with the preferred orientation of microcracks observed in thin section. The various techniques for measuring in situ stress orientations gave internally consistent results where ε1 generally aligned with topographic contours and often the mechanically stiff direction of the core. Furthermore, et aligned with the known contemporary tectonic stress, local p axes of earthquakes, Precambrian structures, and local joints. We interpret the alignment of ε1 and other structures to be the result of a feedback between the contemporary tectonic stress (ENE in the northeastern United States) and the process of jointing during the development of local topography. Hence ε1 is controlled by local structures and is a reflection of the contemporary tectonic stress but not a direct measure of it.
Introduction and tribute to S. Thomas Crough 1947–1982
Authors: Thompson George A. , Morgan Jason
Published: February 2008

Tom Crough's friends remember him as warm, shy, inspiring. He was slow to discover his flair for geophysics. He graduated in economics in 1969 from the Woodrow Wilson School at Princeton. After graduation, he and Faith, a zoology graduate of the University of Ohio, were married; Tom described meeting Faith as the best thing that ever happened to him.
Origin and subsidence of Guyots in Mid‐Pacific Mountains
Authors: Winterer Edward L. , Metzler Christopher V.
Published: February 2008

Morphologic and magnetic data suggest that the Mid‐Pacific Mountains formed during Early Cretaceous time as a broad ENE trending double chain of midplate island seamounts over a mantle hot spot as the Pacific plate moved westward and slightly southward. Dredge, drill core, and reflection seismic data indicate that coral‐rudistid reefs grew on the subsiding seamounts and evolved to atolls and banks, largely burying the volcanic foundations. Magnetic data indicate that by late Aptian time, about 110 Ma, the seamounts were located at about 20°–25°S, which we suggest was near the fringes of the latitudinal zone of vigorous reef growth, where upward growth rates could just keep up with subsidence. A broad uplift probably related to the widespread regional emplacement of Aptian volcanics as oceanic plateaus, seamounts, and deep‐water flows and sills raised the Mid‐Pacific Mountain reefs out of the water, and both reflection seismic and isotopic data suggest that a karstic topography developed on many of the emergent reefs. As subsidence recommenced, the reefs could hot grow upward apace with subsidence. Renewed volcanism in Late Cretaceous time in the easternmost Mid‐Pacific Mountains maintained islands for a while, as at Horizon Guyot, but probably without large reefs. Elsewhere, pelagic conditions have prevailed as the guyots sank to their present‐day depths. These depths are systematically related to the inferred age of the volcanic foundations, being greatest on the youngest volcanoes.
A new isostatic model for the East Pacific Rise crest
Authors: Madsen John A. , Forsyth Donald W. , Detrick Robert S.
Published: February 2008

We have developed a new isostatic model for the East Pacific Rise (EPR) that explains the origin of both the axial topographic high and the rise crest gravity anomaly. Our model involves a thin elastic plate, with a constant thickness crust, that is broken at the ridge crest. We assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly. We have used this model, and two conventional isostatic models, to examine the isostasy of three segments of the EPR: at 9°–l4°N, 6°–11°S and 16°–21°S. Rather than modeling individual profiles we have stacked (summed) the gravity and bathymetry data in each area and computed a single, 200‐km‐long, mean, mirrored profile for each different ridge segment. These stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly. In all three areas the best fitting model parameters are similar and give average compensation depths of about 6–7 km below the seafloor (near the base of the oceanic crust) and an upper limit on the flexural rigidity of the plate that increases from about 1018 N m at the ridge axis to about 3×1019 N m 5 km from the axis and 1020 N m 25 km or more from the rise crest. These results indicate that the EPR crest is compensated at much shallower depths than suggested by previous investigators. Our results are consistent with the presence of a small, low‐density magma chamber in the lower crust, although at least part of the compensating root for the axial high must extend into the upper mantle. We interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished.
Viking Bistatic Radar Experiment: Summary of results in near‐equatorial regions
Authors: Simpson Richard A. , Tyler G. Leonard , Schaber Gerald G.
Published: February 2008

Viking bistatic radar data have been processed using Hagfors' scattering function to obtain estimates of rms surface roughness σB from ground tracks primarily in Mars' equatorial region. Roughness varies from as little as 0.75° rms east of Solis and Sinai Plana to at least 8° on the western slope of Arsia Mons; these values are appropriate to effective horizontal scales of about 25 m. Estimated roughness in cratered terrain is generally σB≲5°, implying the existence of smoothing processes (possibly aeolian) on scales less than 100 m, which are not apparent in orbital images. There is good agreement between bistatic roughness estimates and earth‐based results at ground track intersections near the equator in both cratered terrain and plains. Surface tilt probability density functions
(β) derived from these observations are usually more sharply peaked than the Hagfors function for similar values of σB. Both the observed forms and the Hagfors model are considerably sharper near β = 0° and fall off more slowly at large tilts than Gaussian forms. Earlier results obtained using the Gaussian model (Simpson and Tyler, 1981) appear to have overestimated surface roughness by 30–40%. We infer that the true surface has more flat segments and more “edges” than does a gently undulating surface described by Gaussian statistics.
The evolution of craters and calderas on young seamounts: Insights from SEA MARC I and Sea beam sonar surveys of a small seamount group near the axis of the East Pacific Rise at ∼10°N
Authors: Fornari Daniel J. , Ryan William B. F. , Fox Paul J.
Published: February 2008

A northwest trending group of seamounts located just west of the East Pacific Rise (EPR) axis has been mapped using high‐resolution Sea MARC I side‐scan sonar and Sea Beam multibeam bathymetry. These seamounts have 9–13 km basal diameters, and the shapes of the volcanos change from conical domes near the EPR axis to truncated cones and larger, elliptical truncated cones farther away from the rise axis. The detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions (craters to large complex calderas) and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow. Our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse. The creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera. The concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau. The steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera. The steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain. As a seamount evolves, the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits. The final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau.
Geometrical form of aseismic ridges, volcanoes, and seamounts
Authors: Angevine C. L. , Turcotte D. L. , Ockendon J. R.
Published: February 2008

Many volcanic structures have a regular geometrical form that suggests a simple and universal mechanism for the growth. We suggest that the surface of these volcanic structures is a surface of constant hydraulic potential. When an eruption begins, it follows a path to the point on the surface that has the minimum hydraulic resistance. This resistance includes both the frictional resistance to the flow and the gravimetric potential associated with elevation. We determine the shape of a two‐dimensional volcanic edifice formed from a line source and compare the shape with bathymetric profiles across several aseismic ocean ridges. We also compare the predicted shape of an axisymmetric volcanic edifice formed from a point source with several profiles from the Cascade Range in the western United States and with a number of volcanic islands and seamounts in the Pacific.
Inhomogeneous structure of the senegal lithosphere from deep magnetotelluric soundings
Authors: Ritz M.
Published: February 2008

Data from 23 magnetotelluric sites were used to determine electrical conductivities within the crust and upper mantle in Senegal (West Africa) along a profile about 600 km long. Data have been obtained in a variety of tectonic zones: the West African craton (stable since 1850±250 m.y.), the West African mobile belt (with ages in the range of 350–650 m.y.), and the Senegal sedimentary basin. An earth model is derived from two‐dimensional modeling. The model shows several heterogeneities extending deep in the lithosphere and, perhaps more deeply, in the asthenosphere, to a depth of about 460 km. At the stations in the western part of the basin the average resistivity is anomalously low (20–30 ohm m) down to about 10 km. The basin anomaly in the upper crust is assumed to mark an initial rift zone; it may reveal a pattern of crustal weakness in this area. Beneath this zone, the resisitivity is in the range of 1000–3000 ohm m down to at least 300 km. In the eastern basin the resistivity is decreasing in the depth range of 20–30 km, and the top of the ultimate mantle conductor is at a depth of 300 km. At the sites in the marginal part of the West African craton, conductive layers can be recognized in the depth range of 30–40 and 80–100 km. The resistivity drops to 10 ohm m at a depth of about 460 km. The model in this region is compared with available geological and geophysical information, and an attempt is made to explain some of the observed discontinuities by plate tectonics. It is suggested that the major discontinuity which separates the basin and the mobile belt and extends to about 460 km depth could be interpreted as marking a Precambrian subduction slab dipping eastward. In the West African craton a low resistivity has been determined for a depth range of 130–150 km. The upper 130 km were found to have the rather high resistivities of 1000–3000 ohm m. A sharp rise in conductivity occurs at a depth of about 460 km beneath the craton. In contrast, a surprising feature in the Kedougou region (in the craton) is that the major portion of the lower crust is conductive. The existence of a Birrimian geosynclinal pair in eastern Senegal consisting of a eugeosynclinal marine andesitic volcanic trough, particularly volcanics with ophiolite affinity, and a miogeosynclinal sedimentary sequence could characterize a modern plate tectonic phase dominated by widespread ocean opening and continental collision. The conductive material in the lower crust might be explained by the buried ocean crust.
Upper mantle shear structure beneath the northwest Atlantic Ocean
Authors: Grand Stephen P. , Helmberger Donald V.
Published: February 2008

We have modeled the SH motion from earthquakes in the northwest Atlantic Ocean to derive an oceanic upper mantle shear velocity model. The signals were recorded on long‐period WWSSN and Canadian network stations on the east coast of North America. The travel times and waveforms of seismograms, in the distance range 11°–16°, were used to constrain the lid structure. These data indicate a fast (4.75 km/s) lid of about 100 km thickness in the older western Atlantic. From 16° to 28° the S waves pass through triplications due to velocity jumps near 400 and 660 km. The branches from both discontinuities are visible in the S wave data. These arrivals were modeled using synthetic seismograms to obtain accurate travel times. The two triplications are also apparent in SS data from 30° to 50°. The triplication curves derived from the S waves agree with the SS data. Given the lid structure, the waveforms and travel times from the more distant data put tight constraints on the shear velocities at greater depths. We find shear velocities, from 200 to 400 km, to be slow in relation to a shear model of the Canadian shield, derived by a similar technique, indicating continent‐ocean heterogeneity to about 400 km.
In situ stress determination from hydraulic injection test data
Authors: Cornet F. H. , Valette B.
Published: February 2008

Laboratory and field results have shown that the orientation of a hydraulically propagated fracture is not always perpendicular to that of the minimum principal stress but may be influenced by local weakness planes such as preexisting natural fissures: the lower the flow rate, the stronger the weakness planes influence. This observation has led to a new stress determination method based on the measurement of the normal stress supported by fracture planes, with various dips and strikes. This new method provides means to evaluate the precision of the stress determination and can be applied to situations in which none of the principal stresses is vertical. When all the fracture planes are parallel to the borehole axis, the method is still applicable if fast flow rate reopening pressures can be measured and if the principal stress directions are assumed to be independent of depth. Validity of the method is illustrated by analyzing results from two sets of data, one of which has been discussed previously in the literature.
Acoustic measures of partial gas saturation in tight sandstones
Authors: Murphy William F.
Published: February 2008

The acoustic properties of tight sandstones are very sensitive to partial gas/water saturation. Measurements at 5 kHz show that compressional velocity drops sharply with undersaturation. Extensional and shear losses at 5 kHz are large in both fully and partially saturated sands. The shear loss Qs−1 declines linearly with increasing gas content. The extensional loss Qe−1 peaks at 0.80 water saturation, Q−1 is frequency dependent. The attenuation is well explained by hydrodynamic relaxation in the grain contacts. The measurements indicate velocity dispersions consistent with this mechanism. Ultrasonic velocities at 500 kHz are roughly 10–25% higher than acoustic velocities at 5 kHz. The ultrasonic velocities exhibit a weak dependence on saturation. Frame moduli are unrelaxed at these frequencies.
Effects of thermal cycling on magnetic properties of lunar analogs
Authors: Barron Andrew M. , Shive Peter N.
Published: February 2008

Thermal cycling of lunar basalt analogs between liquid nitrogen temperature (−196°C) and room temperature has no apparent influence on their remanent coercivity spectra. Initially, samples were cycled up to 100 times in a refrigerator over a period of about 30 min. per cycle. In a second set of experiments, samples were dipped directly into liquid nitrogen up to 100 times at about 1 min. per cycle. Comparison of AF demagnetization curves of weak field anhysteretic remanent magnetization (ARM) before and after cycling revealed no systematic differences. Calculations based on a. model of spherical iron grains within olivine or troilite indicate that it is unlikely that the iron will crack under thermal stress. Thus, thermal cycling does not appear to provide an explanation for increasing the stability of remanence in samples from the lunar surface.
Apollo 16 site geology and impact melts: Implications for the geologic history of the lunar highlands
Authors: Spudis Paul D.
Published: February 2008

The Apollo 16 mission provided direct samples of two lunar highlands geological units: the Descartes and Cayley Formations. The Descartes Formation is interpreted as hummocky Nectaris basin ejecta, reworked by secondary impacts from the Imbrium basin. The Cayley Formation was emplaced as a consequence of the Imbrium impact and incorporated large quantities of the local substrate, which in this region consists mostly of Nectaris ejecta. The composition of the upper crust in this area is primarily anorthositic gabbro; local craters near the site would produce an impact melt of this composition. I estimate the total volume of pure melt within the Apollo 16 site deposits to be about 10% or less, significantly smaller than previous estimates.Two chemically defined melt groups found at Apollo 16 are of broadly low‐K Fra Mauro (LKFM) basalt composition and could not be produced by local craters in the region. The volumetically most important melt group at Apollo 16 is of aluminous LKFM composition and appears to have formed in one event at 3.92 aeons; those rocks probably represent Nectaris basin impact melt. Imbrium basin impact melt may also be present, in quantities of less than 1%. The model presented here suggests that the Descartes Formation (Nectaris ejecta) is a two‐component mixture of anorthositic clastic debris (>90% by volume) laden with aluminous LKFM Nectaris basin impact melt (>10% by volume). If the age of the Nectaris basin is 3.92 aeons as suggested, then there is probably no in situ primordial surface dating from the time of crustal formation, and the total thickness of the highlands “megaregolith” may be greater than a factor of ten higher than the currently quoted value(1–2 km).
Grain size evolution and fractionation trends in an experimental regolith
Authors: Hörz Friedrich , Cintala Mark J. , See Thomas H. , Cardenas Francisco , Thompson Thomas D.
Published: February 2008

The comminution of blocky planetary surfaces into fine‐grained regoliths was simulated by impacting a fragmental gabbro target 200 times with stainless steel projectiles (diameter = 6.35 mm; mass = 1.2g; Vi = 1.35 km/s). Initially the target fragments were 2–32 mm in size with a mean diameter of 15.2 mm, which was reduced to 0.4 mm after 200 shots. For the first 60 shots, comminution was linear with cumulative kinetic energy and, thus, essentially linear with time. The comminution efficiency, however, decreased at later stages. This could be due to a change in fracture mechanism from predominantly intergranular to predominantly intragranular and/or due to a change in partitioning of the projectile's kinetic energy from collisional fragmentation or large fragments at early times to cratering in a progressively finer‐grained medium at later stages. Mineral‐specific, preferential comminution is abundantly observed in the modal make‐up of various grain‐size fractions: plagioclase comminutes more readily than pyroxenes. Plagioclase—initially present at 54% (vol.)—comprises typically 65–70% of the 125–250 μm fraction; pyroxenes (initially at 33%) are depleted to 20–25%. This fractionation occurs in the earliest samples analyzed and is not a time‐dependent phenomenon. Bulk chemical analyses reveal a corresponding increase in Al2O3, CaO, and Na2O, and a decrease in FeO and MgO. The finest grain sizes are generally the most severely affected by differential comminution. Similar, if not exactly identical, trends are abundantly documented for lunar soil samples. These experimental findings illustrate that differential comminution is capable of producing fractionation in the finest grain sizes. They diminish the need for prominent lateral and vertical transport of components otherwise “foreign” to a lunar soil‐sampling site as postulated by some workers on the basis of multivariate chemical mixing models.
Micas in experimentally shocked gneiss
Authors: Lambert P. , MacKinnon I. D. R.
Published: February 2008

Biotites and muscovites from a gneiss have been experimentally shocked between 18 and 70 GPa using powder‐propellant guns at NASA Johnson Space Center and at the California Institute of Technology. This study shows that shock in biotite and muscovite can produce homogeneous and devolatilized glasses within microseconds. Shock‐deformed micas display fracturing, kinking, and complex extinction patterns over the entire pressure range investigated. However, these deformation features are not a sensitive pressure indicator. Localized melting of micas begins at 33 GPa and goes to completion at 70 GPa. Melted biotite and muscovite are optically opaque, but show extensive microvesiculation and flow when observed with the SEM. Electron diffraction confirms that biotite and muscovite have transformed to a glass. The distribution of vesicles in shock‐vitrified mica shows escape of volatiles within the short duration of the shock experiment. Experimentally shocked biotite and muscovite undergo congruent melting. Compositions of the glasses are similar to the unshocked micas except for volatiles (H2O loss and K loss). These unusual glasses derived from mica may be quenched by rapid cooling conditions during the shock experiment. Based on these results, the extremely low H2O content of tektites may be reconciled with a terrestrial origin by impact. Release of volatiles in shock‐melted micas affects the melting behavior of coexisting dry silicates during the short duration of the shock experiment. Transportation and escape of volatiles released from shock‐melted micas may provide plausible mechanisms for the origin of protoatmospheres on terrestrial planets, hydrothermal activity on phyllosilicate‐rich meteorite parent bodies, and fluid entrapment in meteorites.
Initial results from VC‐1, First Continental Scientific Drilling Program Core Hole in Valles Caldera, New Mexico
Authors: Goff Fraser , Rowley John , Gardner Jamie N. , Hawkins Ward , Goff Sue , Charles Robert , Wachs Daniel , Maassen Larry , Heiken Grant
Published: February 2008

Valles Caldera 1 (VC‐1) is the first Continental Scientific Drilling Program (CSDP) core hole drilled in the Valles caldera and the first continuously cored well in the caldera region. The objectives of VC‐1 were to penetrate a hydrothermal outflow plume near its source, to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera Jemez fault zone, arid to core the youngest volcanic unit inside the caldera (Banco Bonito obsidian). Coring of the 856‐m well took only 35 days to finish, during which all objectives were attained and core recovery exceeded 95%. VC‐1 penetrates 298 m of moat volcanics and caldera fill ignimbrites, 35 m of precaldera volcaniclastic breccia, and 523 m of Paleozoic carbonates, sandstones, and shales. A previously unknown obsidian flow was encountered at 160 m depth underlying the Battleship Rock Tuff in the caldera moat zone. Hydrothermal alteration is concentrated in sheared, brecciated, and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth. Alteration assemblages consist primarily of clays, calcite, pyrite, quartz, and chlorite, but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m. Carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the Madera Limestone above 550 m and in the Madera and Sandia formations below 700 m. This corresponds with zones of most intense calcite and quartz veining. Thermal aquifers were penetrated at the 480‐, 540‐, and 845‐m intervals. Although these intervals are associated with alteration, brecciation, and veining, they are also intervals where clastic layers occur in the Paleozoic sedimentary rocks.
Depth estimates of large earthquakes on the Island of Hawaii since 1940
Authors: Eissler Holly K. , Kanamori Hiroo
Published: February 2008

Although hypocenters of earthquakes on the island of Hawaii are now routinely assigned to within 5 km, depth was a poorly determined parameter until the early 1960's. However, the 1950–1960 period was very active both in volcanic eruptions and large earthquakes. Source depths for the 12 largest Hawaiian earthquakes (magnitude 6 or greater) since 1940 are estimated from the ratios of body and surface wave amplitudes recorded at Pasadena, California. Excitation functions for Rayleigh waves are calculated as a function of source depth for the two dominant periods in the Pasadena records, 8s and 20s. Theoretical body wave amplitudes are determined from synthetic seismograms. Calculated ratios are very sensitive to source depth; for example, amplitudes of 8‐s Rayleigh waves diminish by a factor of 300 between depths of 10 km and 50 km. This is a much larger effect than the fault geometry, which we estimate to be a factor of 4 between representative focal mechanisms. Estimated depths for post‐1960 earthquakes agree fairly well with the instrumental depths. In general, large earthquakes near the volcanic flanks and fault systems are shallow (≤20 km). Two earthquakes of magnitude 6 occurred under the volcanoes Mauna Loa (in 1950) and Kilauea (in 1951); they preceded major eruptions by 3 days and 14 months, respectively, and had the largest depth estimates at 40–55 km and 35–50 km. MS values assigned from global amplitudes are compared with those assigned from Pasadena amplitudes alone, for 70 events in 1973–1974 with 5.1≤ MS ≤ 6.0. The global values are only slightly larger (0.05 magnitude units) than the Pasadena values, indicating that Pasadena amplitudes are on the average representative of the event magnitude.
High‐temperature permeability and groundwater chemistry of some Nevada test site tuffs
Authors: Moore D. E. , Morrow C. A. , Byerlee J. D.
Published: February 2008

Permeability measurements of samples of the Topopah Spring Member of the Paintbrush Tuff and the Bullfrog Member of the Crater Flat Tuff collected from the Nevada Test Site have been made at room temperature and in a temperature gradient. The room temperature permeabilities of the samples ranged from 0.8×10−18 to 64×10−18 m2 for the Topopah Spring Member and 0.5×10−18 to 8.4×10−18 m2 for the Bullfrog Member. These variable permeabilities probably reflect differences in crack and void configurations among the samples. The permeability of a given sample was only slightly affected by heating to maximum temperatures between 90°C and 250°C during experiments of up to 5 weeks' duration. The observed lack of permeability change is consistent with the hypothesis that the disposal of nuclear waste in tuff units in the unsaturated zone at the Nevada Test Site would not have a large effect on the permeability of the host rock. The fluids discharged from the tuff samples during the experiments were mostly dilute solutions of nearly neutral pH, whose compositions differed only slightly from the starting Jl3 ground water composition. Waters of this type should be relatively noncorrosive to stainless steel waste canisters.
Global duricrust on Mars: Analysis of remote‐sensing data
Authors: Jakosky Bruce M. , Christensen Philip R.
Published: February 2008

Global remote‐sensing data for Mars are analyzed to obtain a simple, self‐consistent model for the surface layer. The data sets discussed include radar cross‐section measurements at several wavelengths, radio whole‐disk thermal emission observations at two wavelengths, the global distribution of thermal inertia, deviations of diurnal temperatures from those of a homogeneous model, and thermal spectral estimates of surface rock abundance and of the thermal inertia of the nonrocky component of the surface. The data sets which most constrain the interpretation are the rock abundance map and the correlation of thermal inertia with radar cross section; these require the rock abundance to not vary significantly from place to place and simultaneously require the density and thermal inertia of the fines to vary in a consistent manner. The simplest model which can explain all of the data involves a global case‐hardened crust (“duricrust”) which varies spatially in its degree of formation. In general, low‐thermal‐inertia regions have a poorly‐developed crust and highinertia regions have a well‐developed crust there are, however, regions that consist of coarse particles and which do not fit this model (e.g., Chryse). This model is consistent with the ages of low‐inertia regions and with aeolian mechanisms for their development. The duricrust is thought to form via the mobilization of salt ions within a layer of water adsorbed within the regolith, and its formation may be associated with the exchange of water between the regolith and atmosphere which occurs on the 105‐ and 106‐year time scale.
Effect of clay on dielectric properties of oil‐sand media
Authors: Singh R. P. , Rankin D.
Published: February 2008

While the dielectric properties of oil itself are well known, neither the dielectric constant nor the resistivity of a multiphase system consisting of a matrix with clay, containing oil and more or less water, is at all well known. Presented herein are some measured values of the dielectric constant based on a preliminary study of multiphase systems consisting of (1) an unconsolidated clay‐sand mixture with varying percentages of oil over the frequency range 20 MHz to 1 GHz, and (2) a sandstone core with clay inclusions containing oil, over the same frequencies. The behavior of dielectric constant shown is significant in the interpretation of microwave logging and electromagnetic geotomography data and further improvement of these techniques.
Apollo 16 regolith breccias: Characterization and evidence for early formation in the mega‐regolith
Authors: McKay D. S. , Bogard D. D. , Morris R. V. , Korotev R. L. , Johnson P. , Wentworth S. J.
Published: February 2008

All of the Apollo 16 regolith breccias (18 specimens) have been characterized in terms of their petrography, grain‐size distribution, porosity, major and trace element composition, noble gas contents, and ferromagnetic resonance properties. These breccias vary significantly with respect to their density and porosity, with the more dense breccias showing significant shock damage. The regolith breccias resemble the soils in grain‐size distribution and in the relative proportions of major petrological components, except agglutinates. Many of the breccias are compositionally different from the Apollo 16 soils in that they lack an important mafic component present in the soils. Although some groupings occur, the petrologic and chemical compositions of the regolith breccias do not correlate with the station location of the samples. All but one of the breccias show some evidence of irradiation at the lunar surface (solar gases, measurable FMR, agglutinates), and analyses made on grain‐size separates from two disaggregated breccias indicate that this irradiation occurred before compaction when the breccia material was finely disseminated on the surface. However, the concentrations of surface irradiation parameters (solar gases, FMR, agglutinates) for most breccias are far less than seen in any lunar soils or in regolith breccias from other Apollo missions. Several breccias also contain unusually high trapped 40Ar/36Ar ratios of ∼8–12 and a significant fission Xe component in excess of that expected from in situ production. These observations suggest that the surface irradiation of these breccias occurred as early as 4×109 years ago. We conclude that most of the Apollo 16 regolith breccias were not formed from any known Apollo 16 soil. They appear to be well‐comminuted material that contains ancient regolith developed during the late stage heavy bombardment of the moon when large impacts were much more common relative to small impacts so that regoliths did not have time to significantly mature before being diluted by fresh ejecta and buried. This ancient megaregolith is significantly different from more recent lunar regolith but may be similar to asteroidal regoliths from which some brecciated meteorites have formed.