bluuebunny/crossref_metadata_2025 huggingface dataset:
Relative array analysis of upper mantle lateral velocity variations in southern California
Authors: Walck Marianne C. , Minster J. Bernard
Published: February 2008

Spatial averaging of short‐period teleseismic P wave arrivals across SCARLET constrains upper mantle lateral velocity variations beneath southern California. We compare dT/dΔ, back azimuth, and averaged arrival time estimates determined from the entire network for 154 events to the same parameters derived from small subsets of SCARLET. The resulting relative array diagrams and plots of ‘net subarray delays’ computed for 171 overlapping subarrays exhibit large anomalies that vary smoothly as a function of subarray position, implying substantial lateral heterogeneity under the network. Patterns of mislocation vectors for more than 100 subarrays permit identification of an east‐west striking high‐velocity antiform beneath the Transverse Ranges. Further constraints on both the lateral configuration and depth of the anomaly are obtained through thin lens analysis of the net delay data, where all the time perturbations are assumed to originate at a single depth. Projection of 2798 net delays requires a lens deeper than 100 km. A representative lens at 150‐km depth features low velocities beneath the Salton Trough as well as a concentrated high‐velocity zone between 116.3°–118.3°W and 33.7°–34.8°N, consistent with the relative array diagrams and the PKP‐delay contours of Hadley and Kanamori [1977]. Therefore the bulk of the high‐velocity material observed by Hadley and Kanamori may in fact lie at or deeper than 150 km rather than 40–100 km. This possibility prompts renewed speculation regarding the relationship of this body to the plate tectonic history of the region.
On the scaling of crater dimensions: 2. Impact processes
Authors: Holsapple K. A. , Schmidt R. M.
Published: February 2008

Existing theoretical and empirical scaling rules for cratering phenomena are commonly based upon implicit assumptions regarding the roles of the impactor conditions and the target properties. It is common, for example, to assume that only the kinetic energy of the impactor and the work against gravity in the target govern large impact craters. A more general case is considered here. A spherical impactor is characterized by its kinetic energy, momentum, and various material properties of both the impactor and the target. Under these general assumptions, scaling among both similar and nonsimilar events is studied. Similar events always satisfy the cube‐root mass scaling rule. The dependence on energy varies according to how the similarity is achieved. For constant mass or constant velocity tests, variations in gravity are required, and cube‐root energy scaling is obtained. For a strengthless material only, quarter‐root energy scaling is possible, but only for restricted variations in both impactor mass and velocity, which imply that the mass is proportional to the three‐quarters power of the energy. Scaling rules for the more common and more useful application to nonsimilar events (e.g., variable energy and/or velocity at constant gravity) are shown to be bounded by certain well defined limits. Dimensional analysis is used with plausible assumptions on the sign of the partial derivative of the crater volume with regard to each of the independent variables. Distinctions are made among different forms of cube‐root scaling obtained from strength‐dominated phenomena. Quarter‐root scaling is shown to be a distinct limit to gravity domination, occurring only when there is no dependence upon impactor velocity and target strength. Special cases of the general scaling obtained correspond to degenerate forms common in the literature. Of these, energy scaling, momentum scaling, strength scaling, and gravity scaling are shown to be the bounds for all permissible scaling rules. This result is shown to be in agreement with a comprehensive collection of data obtained from a survey of the literature. Included are experimental results for both geological materials and metals, analytical models, and finite difference code calculations.
The collision complex of the North Scotia Ridge
Authors: Ludwig William J. , Rabinowitz Philip D.
Published: February 2008

Single‐channel and multichannel seismic reflection profiles of the North Scotia ridge trending west‐east off southernmost South America show the north side to consist of a wedge of deformed sediments resting on a planar layered sequence. Relative movement of the ridge northward toward the Falkland plateau has resulted in deformation and uplift of the near‐surface sediment layers of the Falkland trough to form an accretionary wedge; these sediments are ≃ 1 to 3 s of two‐way travel time in thickness and lie above a well‐defined reflector whose velocity at the northern part of the trough is 3.6 km/s. The deeper layers extend to depths, in places, greater than 9.0 s of two‐way travel time beneath the north slope without any significant deformation. The deepest observed reflector is associated with a 5.9‐km/s basement. The deformed pile of sediments, or collision zone, of the North Scotia ridge is similar to that observed at other active, compressive plate boundaries.
The Notch Peak Contact Metamorphic Aureole, Utah: Paleomagnetism of the metasedimentary rocks and the quartz monzonite stock
Authors: Gillett Stephen L. , Hover Victoria C. , Papike James J.
Published: February 2008

The Notch Peak pluton, a Late Jurassic granitic stock, intrudes early Paleozoic miogeoclinal strata in the House Range, western Utah. As part of a detailed study to determine textural and chemical changes related to the intrusion, a paleomagnetic reconnaissance has been made of the pluton and its contact aureole. Steep, scattered magnetizations with both polarities present are found in the pluton upon alternating field (af) demagnetization, and they may reflect an original thermoremanence in magnetite that was acquired over at least one polarity reversal. Limestones and argillites in the Big Horse Canyon Member of the Orr Formation (Late Cambrian) were sampled at four stratigraphically controlled sites ranging from unaltered country rock to marble. The site farthest from the pluton is well behaved upon progressive thermal demagnetization and yields a low‐inclination, reversed Paleozoic direction by 400°C. The sites nearer the stock generally have a diffusely defined, two‐polarity, high‐inclination magnetization that reflects remagnetization by the pluton. This magnetization is removed above 300°C, and af demagnetization suggests that it resides in fine‐grained hematite. Of these sites, one appeared nearly unaltered in thin section, whereas the others range from hornfels to skarn. The remagnetization resulted from a late oxidation event related to the intrusion, perhaps from mobilization of groundwater; thus, the country rock has been remagnetized chemically rather than thermally. These results suggest that paleomagnetism may be a useful tool for detecting altered zones around plutons. In addition, no consistent, high‐blocking‐temperature magnetization was found in the aureole rocks; the magnetization of the hornfels was easily measurable at higher steps, but directions changed erratically, and this behavior in part results from the presence of pyrrhotite.
Contemporary block tectonics: California and Nevada
Authors: Hill David P.
Published: February 2008

Well‐determined fault plane solution and the gross pattern of late‐Cenozoic faulting in California and Nevada show a systematic relation between the orientation of fault planes and slip directions. In general, normal faults have northerly strikes, reverse faults have easterly strikes, and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes, respectively. Kinematically, this relation is consistent with the response of clusters of fault‐bounded crustal blocks to a regional stress field generated by the relative motion between the Pacific and North American plates. In this stress field, the greatest and least principal (compressive) stresses are restricted to northerly and easterly striking vertical planes, respectively. Clusters of crustal blocks bound by faults having the above attitudes form three basic regimes: (1) a spreading regime with a combination of normal and conjugate, strike slip faults, (2) a converging regime with a combination of reverse and conjugate, strike slip faults, and (3) a wrench regime with sets of subparallel, throughgoing, northwest striking (dextral) faults or northeast striking (sinstral) faults. These three regimes are typified by faulting patterns in the Basin and Range province (spreading), the Transverse Ranges (converging), and the San Andreas‐Garlock fault systems (wrench), respectively. The gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north‐south shortening and east‐west extension with the ratio of extensional to shortening strains (and the areal dilitation) decreasing systematically from spreading to wrench to compressional regimes. The wrench regime involves a component of net rotational deformation (clockwise for dextral slip and counter‐clockwise for sinstral slip), while deformation of the spreading and converging regimes is irrotational. Local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes. Maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength (Byerlee's law) along preexisting fractures and that pore pressure in the brittle crust is laterally uniform. A minimum strength difference between active, block‐bounding faults and block interiors is 15–30%. Simple arrangements of such block clusters mimic the gross kinematic pattern of Quaternary faulting in California and Nevada. Some implications for contemporary tectonics emphasized by this model involve the westward displacement of the Sierra Nevada block.with respect to the stable interior of the North American plates, oblique thrusting of the Salinian block over the Pacific plate, and a progressive increase in the offset of the San Andreas fault represented by the ‘big bend’ through the Transverse Ranges.
Northern East Pacific Rise: Evolution from 25 m.y. B.P. to the present
Authors: Mammerickx Jacqueline , Klitgord Kim D.
Published: February 2008

The northeast Pacific topography and magnetic lineations (25 m.y. B.P. to the Present) record the traces of three major spreading reorganizations. Only one spreading center is observed today, but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive Pacific‐Guadalupe plate boundary to a much shortened Pacific‐Cocos and Pacific‐Rivera plate boundary. The 25 m.y. B.P. plate reorganization culminated with the formation of the Guadalupe plate, bound by the Murray fracture zone to the north and the Cocos‐Nazca spreading ridge to the south. Between 25 and 12.5 m.y. B.P. spreading continued while the plate retained the same general outline. The 12.5‐11 m.y. B.P. reorganization resulted in the creation of a much shortened Pacific‐Cocos plate boundary located in its early stages over the Mathematician seamounts and a much reduced Cocos‐plate. The last reorganizaton (6.5–3.5 m.y. B.P.) resulted in the abandonment of the Mathematician spreading ridge as a Pacific‐Cocos plate boundary in favor of the East Pacific Rise.
On transformation plasticity
Authors: Poirier J. P.
Published: February 2008

After a short review of transformation plasticity and of the extant models, we propose an expression derived from microscopic first principles for the strain rate increase at constant stress or the stress drop at constant strain rate of solids driven through a phase transition. We also suggest the possibility of a premonitory contribution to transformation plasticity due to a weakening of elastic constants before the transition.
The sea slope problem revisited
Authors: Castle Robert O. , Elliott Michael R.
Published: February 2008

Discrepancies in sea surface topography based on comparisons between the results of steric leveling and repeated geodetic levelings have identified what is known as the ‘sea slope problem.’ This problem is actually twofold: (1) the sea surface relief based on steric leveling differs significantly from that based on geodetic leveling along several generally north‐south coasts, and (2) successively propagated levelings between several widely separated tide stations indicate that the stationary sea slope seemingly has been changing with time, whereas differenced sea level means between these stations indicate that the sea surface relief has remained virtually invariant during the same intervals. Reexamination of the three reported discrepancies between geodetically and sterically determined sea slopes indicates that the Australian example is based on leveling of a quality inappropriate to the comparison. The discrepancy developed along the Atlantic coast of the United States is limited to the reach between Portsmouth–Hampton Roads, Virginia, and Key West, Florida, where the accuracy of steric leveling may be especially vulnerable owing to the dynamic effects of the Gulf Stream. Reconsideration of the example along the Pacific coast of the United States indicates that the various discrepancies are due chiefly to intrasurvey movement and resultant distortion of geodetically defined height differences between tide stations. Agreement between the results of steric and geodetic leveling along tectonically inactive north‐south coasts devoid of strong boundary currents is generally good. This observation supports the conclusions (1) that any directionally dependent systematic error in geodetic leveling is measurably insignificant and (2) that where allowance is made for the possible effects of major boundary currents or intrasurvey movement during levelings between tide stations, the sea slope problem tends to vanish.
Effects of regional uplift, deformation, and meteoric‐hydrothermal metamorphism on K‐Ar ages of biotites in the southern half of the Idaho Batholith
Authors: Criss R. E. , Lanphere M. A. , Taylor H. P.
Published: February 2008

K‐Ar ages of biotites from the southern half (Atlanta Lobe) of the Idaho batholith are younger than the true emplacement ages of the rocks. For Mesozoic plutons with true ages probably greater than 100 m.y., the K‐Ar ages range from 95 m.y. along the east and west margins of the batholith to 37 m.y. in the east‐central zone; there is a conspicuous age gap between 46 and 53 m.y. However, a north trending group of epizonal Eocene plutons in the east‐central zone has K‐Ar ages of 44±6 m.y., values which closely approximate the crystallization ages. These Eocene plutons, which were emplaced at shallow levels and underwent rapid cooling, are responsible for widespread (>10,000 km2) meteoric‐hydrothermal alteration of the Mesozoic country rocks. Partial chloritization and lowering of the K2O, deuterium, and 18O contents of the biotites accompanied the hydrothermal activity. Although the intensity of the hydrothermal effects is geographically correlated with the degree of lowering of the K‐Ar ages, the production of anomalously young K‐Ar ages is attributed in part to regional uplift and doming in addition to the hydrothermal metamorphism. In any given part of the Atlanta Lobe, the K‐Ar ages of the higher elevation samples become systematically older with increasing elevation. This effect is observed in almost all of the biotites, both those that have undergone intense hydrothermal alteration and those that show no evidence of any kind of hydrothermal effects. The only exceptions are biotites that underwent complete Ar loss; such samples occur only at low elevations and within the contact metamorphic aureoles of the Eocene plutons. Age versus elevation trend lines are linear, with slopes that indicate uniform late Cretaceous and early Tertiary uplift (∼0.10–0.14 mm/yr) and cooling of the terrane. The trend lines have almost identical slopes throughout the region, but the positions of the trend lines shift systematically upward to higher elevations from west to east, indicating that subsequent doming and faulting of the terrane modified the relative elevations of different subregions. The zone where this more recent uplift is most prominent almost coincides with the axis of the Eocene intrusions. Furthermore, the K‐Ar age versus elevation trend lines are systematically truncated both at low elevations and close to contacts with Eocene plutons, apparently because of a temperature increase due to widespread hydrothermal heating of the Mesozoic rocks by the Eocene batholiths. In rocks located at present‐day elevations below about 1000–1500 m in the central part of the Atlanta Lobe, biotites lost practically all their radiogenic Ar during the Eocene intrusive event.
Detrital remanent magnetization: Viscosity theory of the lock‐in zone
Authors: Denham Charles R. , Chave Alan D.
Published: February 2008

Post‐depositional remanent magnetization (PDRM) becomes permanent within a lock‐in zone, whose apparent time delay (depth) and time span (thickness) are related to the characteristic time constants of compaction and magnetic grain rotation. Two simple models of the PDRM lock‐in zone are described, using Yaskawa's idea of effective sedimentary viscosity, and Mooney's empirical relation for the viscosity of a concentrated suspension of spheres. The impulse response of the lock‐in zone determines the fidelity of the paleomagnetic recording.
Earthquakes at Loihi Submarine volcano and the Hawaiian Hot Spot
Authors: Klein Fred W.
Published: February 2008

Loihi is an active submarine volcano located 35 km south of the island of Hawaii and may eventually grow to be the next and southernmost island in the Hawaiian chain. The Hawaiian Volcano Observatory recorded two major earthquake swarms located there in 1971–1972 and 1975 which were probably associated with submarine eruptions or intrusions. The swarms were located very close to Loihi's bathymetric summit, except for earthquakes during the second stage of the 1971–1972 swarm, which occurred well onto Loihi's southwest flank. The flank earthquakes appear to have been triggered by the preceding activity and possible rifting along Loihi's long axis, similar to the rift‐flank relationship at Kilauea volcano. Other changes accompanied the shift in locations from Loihi's summit to its flank, including a shift from burst to continuous seismicity, a rise in maximum magnitude, a change from small earthquake clusters to a larger elongated zone, a drop in b value, and a presumed shift from concentrated volcanic stresses to a more diffuse tectonic stress on Loihi's flank. The 1971–1972 swarm began at depths of 20–50 km about 1 month before the shallow swarm started and suggests an upward migration of seismicity as the first stage of the 13‐month swarm. The seismic ‘root’ of Kilauea volcano is well defined by earthquakes which plunge to the south and southwest to a depth of 50–60 km, terminating in a region in which earthquakes are associated with deep harmonic tremor and magma. The zone of these deeper earthquakes and tremor lies between Kilauea, Loihi, and Mauna Loa volcanoes, may feed magma to all three volcanoes, and probably locates the Hawaiian hot spot.
Rapid intrusion of magma into wet rock: Groundwater flow due to pore pressure increases
Authors: Delaney Paul T.
Published: February 2008

Analytical and numerical solutions are developed to simulate the pressurization, expansion, and flow of groundwater contained within saturated, intact host rocks subject to sudden heating from the planar surface of an igneous intrusion. For most rocks, water diffuses more rapidly than heat, assuring that groundwater is not heated along a constant‐volume pressure path and that thermal expansion and pressurization adjacent to the intrusion drives a flow that extends well beyond the heated region. The forcing parameter for pressurization and flow is α ΔT, where α is a thermal expansion coefficient reflecting the overall expansion of water heated through the temperature difference ΔTbetween the initial ambient and intrusive values. Pore pressure increases due to heating are greatest when the intrusion is emplaced rapidly and where the intrusive contact is impervious to groundwater contained in stiff, impermeable rocks with high thermal diffusivities and porosities. The maximum velocity of water flowing in pores decays with the inverse square root of time and is insensitive to hydraulic properties of the host rocks. Pressures are lessened and flow directions are reversed with the onset of hydrothermal convection. This occurs at times ranging from hours to weeks after onset of intrusion. As magma rises into near‐surface rocks, steam can be generated. Solutions indicate that pressure increases and velocities are sensitive to the overall amount of expansion rather than the behavior of the water‐steam transition. Both the overall thermal expansion coefficient α and the temperature difference ΔTare greater in shallow (<1 km) environments than in deep (∼5 km) ones. Thus, for rocks with similar transport properties, pressure increases due to heating are greatest in shallow environments. Although solutions can be applied to rocks with a wide variety of properties, pressure increases are calculated for compliant quartz‐rich sedimentary rocks with a porosities between 1 and 20% and permeabilities between 1 darcy and 1 μdarcy, subject to temperature increases of 500 and 1000 K at depths ranging from 0.1 to 5 km in a region of hydrostatic pressures and normal geothermal gradient. Such rocks, with porosities greater than 5%, permeabilities less than a 0.1 mdarcy, and drained hydrostatic compressibilities of 10−4/MPa, undergo pressure increases greater than 10 MPa (100 bars)for conditions typical of water table depths of 2.5 km and heating to 500 K above ambient. Similar rocks, but with permeabilities less than 1 mdarcy, undergo pressure increases of 10 MPa for conditions typical of 1 km water table depth. Rocks commonly considered to be good aquifers undergo pressure increases of less than 1 MPa, primarily because of their high permeability. Although these estimates neglect the effects of fracturing and brecciation that may accompany such pressure increases, calculations indicate that pressure increases due to heating of cool groundwater can lead to failure of host rocks by a phreatic mechanism.
Upper crustal velocity structure in the ROSE area of the East Pacific Rise
Authors: Ewing J. I. , Purdy G. M.
Published: February 2008

We describe a method to constrain the seismic velocity structure of the upper 500–800 m of young oceanic crust. Conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust. The method described here requires the assumption of a linear velocity gradient and utilizes measured values of range, travel time, and slowness of the observed refracted waves. Results are reported from several refraction profiles that sample crust between 0 and 4 m.y. in age on the flanks of the East Pacific Rise at 12°N. These data give an average gradient in the uppermost 800 m of the crust of 3.5 s−1 and a velocity at the seafloor of 2.5 km/s. Ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor.
Ophiolites, synthetic seismograms, and ocean crustal structure: 1. Comparison of ocean bottom seismometer data and synthetic seismograms for the Bay of Islands ophiolite
Authors: Kempner W. C. , Gettrust J. F.
Published: February 2008

A series of synthetic Seismograms based on the Bay of Islands ophiolite data are compared with ocean bottom seismometer seismic refraction observations which sample 60‐m.y. crust in the northeastern Pacific. The general agreement between the synthetics for the ophiolite and the observations support the idea that some ophiolites are representative of normal ocean crust. This support extends to similarities in complex second arrivals in the synthetics and observations which often are not used in the interpretation of marine seismic data. The use of the ophiolite model also allows us to show how the interpretation of seismic data is affected by experimental design and interpretation procedures; as the sampling density increases and as more phases in the observations are accounted for, the velocity‐depth model more closely approaches that found for the ophiolite. The results of this modeling support sharper P velocity transitions within the crust than are found in young (<12‐m.y. age) oceanic crust. The need for a thin (approximately 1 km thick) P low‐velocity zone at the base of the crust is interpreted as support for serpentinization at the crust‐mantle boundary; serpentinization of the ultramafic component in the Bay of Islands ophiolite, therefore, may include a small preemplacement component.
Coda wave excitation due to nonisotropic scattering and nonspherical source radiation
Authors: Sato Haruo
Published: February 2008

Under the assumption that seismic coda waves may be modeled as a superposition of single scattered body waves due to scatterers randomly distributed in the three‐dimensional earth, we study how nonisotropic scattering and nonspherical source radiation patterns affect the coda wave excitation. The energy density of coda waves is represented by an integral over scattering angle or radiation angle. Properties of each integral kernel are analytically studied in detail. The energy density of coda waves is factored into 2 parts: a geometrical spreading factor and a function of the ratio of the lapse time measured from the earthquake origin time to the travel time of direct wave. The equations predict that the influence of nonisotropic scattering and that of nonspherical source radiation will be significant, particularly on the early part of coda waves.
Thick shell tectonics on one‐plate planets: Applications to Mars
Authors: Banerdt W. B. , Phillips R. J. , Sleep N. H. , Saunders R. S.
Published: February 2008

A theory for stress distributions in thick lithospheric shells on one‐plate planets is developed based on the zero frequency equations of a self‐gravitating elastic spherical shell overlying a strengthless fluid. Stress distributions in lithospheres are reviewed for both the compensated and flexural modes. In the former case, surface stresses only depend on surface topography, while for the latter case, it is shown for long wavelengths that stress trajectories are mainly dependent on the lithospheric lateral density distribution and not on elastic properties. Computational analyses are carried out for Mars, and it is found that isostatically compensated models correctly predict the graben structure in the immediate Tharsis region and a flexural loading model is satisfactory in explaining the graben in the regions surrounding Tharsis. A three‐stage model is hypothesized for the evolution of Tharsis: isostasy with north‐south graben formation on Tharsis, followed by flexural loading and radial graben formation on the perimeter of Tharsis, followed by a last stage of loading with little or no regional deformation. This model is consistent with the Martian lithosphere monotonically thickening over geologic time.
Volcanoes and volcanic provinces: Martian western hemisphere
Authors: Scott David H.
Published: February 2008

The recognition of some Martian landforms as volcanoes is based on their morphology and geologic setting. Other structures, however, may exhibit classic identifying features to a varying or a less degree; these may be only considered provisionally as having a volcanic origin. Regional geologic mapping of the western hemisphere of Mars from Viking images has revealed many more probable volcanoes and volcanotectonic features than were recognized on Mariner 9 pictures. These abundant volcanoes have been assigned to several distinct provinces on the basis of their areal distribution. Although the Olympus‐Tharsis region remains as the principal center of volcanism on Mars, four other important provinces are now also recognized: the lowland plains, Tempe Terra plateau, southern highlands (in the Phaethontis and Thaumasia quadrangles), and a probable ignimbrite province, situated along the highland‐lowland boundary in Amazonis Planitia. Volcanoes in any one province vary in morphology, size, and age, but volcanoes in each province tend to have common characteristics that distinguish that particular group.
Seismic crustal structure of the Proterozoic North Australian Craton between Tennant Creek and Mount Isa
Authors: Finlayson D. M.
Published: February 2008

Seismic recording along a 600‐km traverse across the southern part of the North Australian craton indicates that lateral velocity variations exist in the middle/lower crust. Taken together with other data from the Craton, there is evidence for midcrustal velocities increasing from east to west and from south to north. These trends could result from a greater proportion of high‐velocity granulites being emplaced at higher levels in the crust. Between Tennant Creek and Mount Isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites. Near‐surface (0–5 km) P wave velocities on the Tennant Creek Inlier increase from 5.47 km/s at the surface to about 6.2 km/s at 6‐km depth and correspond with low‐grade metamorphics of the Warramunga Group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains. In the Mount Isa Geosyncline, velocities of 6.03–6.15 km/s correspond to the Leichhart Metamorphics interspersed with granites. In the middle crust, P wave velocities of 6.85 km/s occur at a depth of 26 km near Tennant Creek, whereas such velocities are not evident until depths of about 37 km near Mount Isa. There is therefore a trend for midcrust velocities to increase from east to west. The lower crust is characterized by velocities of 7.3–7.5 km/s, and the upper mantle velocities of 8.16–8.20 km/s are reached at depths of 51–54 km. Exposed granulite facies rocks in central Australia are low in heat‐producing elements and are compatible with the low surface heat flow generally measured in Precambrian Australia. These data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust, through hornblende granulite, to garnet granulite in the lower crust.
Stratigraphy and structural evolution of Southern Mare Serenitatis: A reinterpretation based on Apollo Lunar Sounder Experiment data
Authors: Sharpton Virgil L. , Head James W.
Published: February 2008

The Apollo Lunar Sounder Experiment (ALSE) detected two subsurface reflecting horizons in southern Mare Serenitatis. These horizons appear to be regolith layers, >2 m thick, which correlate with major stratigraphic boundaries in southeastern Mare Serenitatis. Our analysis differs from previous interpretations and implies that the lower horizon represents the interface between the earliest mare unit (unit I; ∼3.8 b.y.) and the modified Serenitatis basin material below. The upper horizon represents the regolith developed by sustained impact bombardment of the unit I surface prior to the emplacement of the intermediate basalts (unit II; ∼3.5 b.y.). The latest volcanic episode (unit III; ∼3.2 b.y.) resulted in a very thin infilling (<400 m deep) in the southern portion of the basin and was undetected by ALSE. Profiles of the stratigraphic surfaces represented by the ALSE reflecting horizons are reconstructed for three stages in the basin filling history. These reconstructions permit the timing and magnitude of various volcanic and tectonic events within the Serenitatis basin to be assessed. On the basis of comparison with Orientale ring topography, the large subsurface arch is interpreted to be the peak‐ring‐related topography of the Serenitatis basin. The thickness of mare basalts across the ALSE ground track is highly variable: basalts are <400 m thick above the peak ring, while they reach a maximum thickness of ∼2.5 km in the depression between the first and second basin rings. Comparison with Orientale topography suggests that a major increase in basalt thickness of ∼6 km may occur ∼50 km inside the peak ring. This region was unsampled by the ALSE traverse. Extensive load‐induced deformation occurred after the emplacement of unit I (∼1.2 km thick, maximum). This loading of the lithosphere resulted in graben formation along the margins of the basin, major downwarping and tilting of the unit I surface, and production of discrete zones of faulting, buckling, and subsidence. Pre‐mare topography exerted a strong control over the tectonic patterns in the basin: Topographic highs such as ring structures underwent <400 m of subsidence, whereas intraring depressions underwent as much as 1.4 km of downwarping. Although the lithosphere was tectonically most active between the emplacement of units I and III, it continued to respond to the load of units I and II after the emplacement of unit III, as evidenced by ∼600 m of postunit III structural relief. The association of all mare ridges transected by the ALSE ground track with distinct offsets or depth changes in the subsurface layering indicates that these surface features are tectonic in origin.
The crust of Venus: Theoretical models of chemical and mineral composition
Authors: Barsukov V. L. , Volkov V. P. , Khodakovsky I. L.
Published: February 2008

Physico‐chemical modeling of the atmospheric‐lithospheric interaction on Venus is presented. The thermodynamic assessment is carried out in terms of Venera 11 and 12 and Pioneer Venus measurements and the dynamic atmospheric structure. The subcloud atmosphere is interpreted to be a zone of nonequilibrium chemical conditions while the near‐surface layer is presumed to be an equilibrium zone due to catalytic effects and low wind velocities. The mineral assemblages of the ‘weathering crust’ are calculated. The pyrite‐anhydrite‐magnetite assemblage is suggested as the buffering system resulting in reducing conditions (P02≈ 10−21 atm). The existence of hydration is problematic and is thermodynamically conceivable only for the sulfur enrichment of primary rocks (about 2 wt. %). The carbonate minerals are suggested to be unstable.
Petrography and mineralogy of two basalts and olivine‐pyroxene‐spinel fragments in achondrite EETA79001
Authors: Steele Ian M. , Smith Joseph V.
Published: February 2008

Three lithologies occur in shocked achondrite EETA79001: A1, exotic irregular crystals and clusters of coarse‐grained olivine (Fo81–55:CaO 0.2, Cr2O3 0.06, NiO 0.5 wt.%), low‐Ca pyroxene (En82Wo2 ‐ En58Wo12) and low‐Ti chromite (Cr2O3 59, FeO 27, TiO2 0.7 wt.%) A2 basaltic host to A1 containing pyroxene (mainly pigeonite En67Wo12 ‐ En52Wo13), maskelynite (An61–55Or0.7,), high‐Ti chromite (some rimming low‐Ti chromite), whitlockite, minor Cl‐apatite, ilmenite, pyrrhotite, and mesostasis; and B, a homogeneous basalt containing augite laths (∼En40–50Wo25–35) in a matrix of pigeohite‐augite, maskelynite (An62–56 Or1.2)ulvöspinelilmenite intergrowth, whitlockite, Cl‐apatite, and mesostasis. Mineral compositions indicate an oxidation state similar to that of shergottites. Lithology A1 represents an unmetamorphosed cumulate that was disrupted and partly resorbed by a basaltic magma. Incorporation of low‐Ca magnesian pyroxenes and olivine increased the Mg/Fe ratio of the basaltic magma responsible for A2, and resulted in abundant crystallization of pigeonite. Lithology B was generated from a distinct magma lacking fragments of A1. The parental spinel harzburgite of A1 was not annealed, and is probably a near‐surface igneous cumulate from a shergottite‐type basalt. Ringwood (?) and majorite (?) were found in shock veins.
The survival of solar flare tracks in interplanetary dust silicates on deceleration in the Earth's atmosphere
Authors: Fraundorf P. , Lyons T. , Schubert P.
Published: February 2008

Pulse heating experiments on magnesium‐rich olivine and pyroxene, two silicates often found in micrometeorites collected in the stratosphere, show that iron ion tracks remain detectable in the transmission electron microscope for temperature maxima up to ∼ 600°C. Assuming thermal emissivities near unity, this implies that micrometeorites with surface densities above 1 mg/cm2 are likely to have their track record erased, and that tracks in reentrant or low‐density micrometeorites smaller than 7 μm in size are likely to survive atmospheric entry.
Classification of the Johnson Space Center stratospheric dust collection
Authors: Mackinnon Ian D. R. , McKay David S. , Nace GeorgeAnn , Isaacs Andrew M.
Published: February 2008

The NASA Johnson Space Center stratospheric dust collection program has made available to the scientific community a rare set of materials for studies of (1) extraterrestrial materials germane to the early history of the solar system and (2) natural and man‐made terrestrial materials in the stratosphere. A review of the first 291 cataloged particles provides a basis for a reliable taxonomy of all stratospheric particles. Data used in this review include particle shape, size, bulk chemistry, and texture. Extraterrestrial materials occur in all three defined categories: spheres, aggregates, and fragments. Approximately 76% of aggregates are of probable extraterrestrial origin. Spheres contain the least amount of extraterrestrial material (∼43%). A number of new groups of stratospheric particles have been identified. These include aluminum and silicate aggregates, aluminum fragments, and aluminum prime spheres and fragments. Limitations to the use of this classification scheme are based primarily upon the lack of additional data to unambiguously define particle origin.
Heat flow on the western flank of the East Pacific Rise at 21°N
Authors: Becker Keir , Von Herzen Richard P.
Published: February 2008

We report 48 new heat flow measurements on the western flank of the East Pacific Rise at 21°N. The stations were taken in 1 to 30 m of sediment on 0.4 to 1.4 m.y. old crust. The low average measured value of ≥173 mW/2 (including 12 tilted minimum values) is about 1/3 the mean heat flow predicted for crust of this age range by cooling plate theory. This may be partly due to a bias of measurement locations in sediment ponds. Nevertheless, 4‐km running averages of measured heat flows, projected normally to the axis, display an oscillatory trend suggestive of cellular hydrothermal circulation in the basement. We sampled no more than one cycle of the apparent heat flow modulation, which is of ≈ 15‐km wavelength and ≈ 200‐mW/m2 amplitude. This variation appears to be uncorrelated with the topographic variation of ≈ 10‐km wavelength, Assuming that the porosity sensitive electromagnetic results of Young and Cox (1981) reflect a permeability distribution that restricts the depth of circulation to ≈ 1.4 km, average temperatures of circulation may be ≈ 50–60°C. Deeper, higher‐temperature circulation is allowable if a more uniform permeability‐depth distribution holds.
Some statistical properties of a sequence of historical Calabro‐Peloritan Earthquakes
Authors: Bottari A. , Neri G.
Published: February 2008

A statistical analysis is applied to the sequence of earthquakes, with epicentral intensity I0 ≥ VIII MSK, which occurred in the Calabro‐Peloritan Arc region (southern Italy) during the period 1600–1974. The study reveals, by the application of the Poisson test, an evident level of interdependence of the shocks which appear grouped in temporal clusters satisfying the simple Poisson process. In fact, the equations which characterize the generalized Poisson process give a good fit of the observed data, the distribution of the events in the clusters being of the Pareto type (discrete). The values 0.9 and 2.8 have been found for k′ and E parameters, which are the average number of clusters occurring in unit time and the parameter of the Pareto distribution, respectively. A subsequent elaboration of these results makes the calculation of some quantities, like the clustering length, possible. Finally, a statistical interpretation of the sequence, with the characterization of the clusters, is proposed.
Investigation of internal friction in fused quartz, steel, plexiglass, and westerly granite from 0.01 to 1.00 Hertz at 10−8 to 10−7 strain amplitude
Authors: Liu Hsi‐Ping , Peselnick Louis
Published: February 2008

A detailed evaluation on the method of internal friction measurement by the stress‐strain hysteresis loop method from 0.01 to 1 Hz at 10−8 to 10−7 strain amplitude and 23.9°C is presented. Significant systematic errors in relative phase measurement can result from convex end surfaces of the sample and stress sensor and from end surface irregularities such as nicks and asperities. Preparation of concave end surfaces polished to optical smoothness having a radius of curvature >3.6×104 cm reduces the systematic error in relative phase measurements to <(5.5±2.2)×10−4 radians. The values of QE−1 (internal friction under uniaxial compression) determined from the relative phase measurements are |QE−1–Qs−1|< 2.8×10−3 for the tool steel sample and |QE−1–Qs−1|< 2.2×10−3 for the Westerly granite sample, where Qs−1 is the internal friction of the fused quartz stress sensor under uniaxial compression. These values are consistent with those inferred from the relative modulus dispersion data also presented in this paper. The polymethyl methacrylate (PMM, trade name Plexiglass) sample shows high values of internal friction (QE−1 ≅5×10−2) with strong frequency dependence and with a maximum in QE−1 at ≅0.4 Hz.
Electrical conductivity of molten basalt and andesite to 25 kilobars pressure: Geophysical significance and implications for charge transport and melt structure
Authors: Tyburczy James A. , Waff Harve S.
Published: February 2008

Electrical conductivities of molten Hawaiian tholeiite and Crater Lake andesite were measured between 1200°C and 1400°C at atmospheric pressure and at pressures up to 17and 25 kbar, respectively. Isobaric plots of log σ versus 1/T (σ is electrical conductivity) are linear, with the exception of the zero pressure tholeiite melt data. Conductivities decrease with increasing pressure in both melts, with the andesitic melt exhibiting a greater pressure dependence. Between 5 and 10 kbar, abrupt decreases in the slopes of isothermal log σ versus P plots (i.e., decreases in activation volume) are observed for both rock melts. This discontinuity probably reflects changes in melt structure, as opposed to changes in conduction mechanism. In each pressure range, the data for each rock melt can be described reasonably well by an equation of the form σ = σ′0 exp[−(Ea + PΔV′σ)/kT], where σ′0 is a preexponential constant, Ea is the activation energy, and ΔV′σ is the activation volume. A qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density. Conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers. Minimum melt fraction estimates of 5–10% are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis.
Anomalous bulk viscosity of two‐phase fluids and implications for planetary interiors
Authors: Stevenson D. J.
Published: February 2008

A calculation is presented for the irreversible entropy production that accompanies the imposition of a pressure perturbation on a two‐phase medium consisting of a dilute suspension of one phase (as droplets or snowflakes) in another (liquid) phase of significantly different composition. No metastability is allowed, and the relaxation process is then dominated by the finite diffusivity of solute. The fluid medium behaves as though it has a very large bulk viscosity (typical value ∼1012 P in the low‐frequency limit). The minimum quality factor Q for acoustic or tidal pressure oscillations is found to be typically ∼102–103 and occurs at a frequency ω0 ∼ 4πDη
where D is the solute diffusivity and η is the number density of suspended inclusions of average radius
. For plausible parameter values, ω0 is in the range of planetary interest (e.g. 10−4 Hz). At ω ≲ ω0, Q ∝ ω−1;; at ω0 ≲ ω ≲ Ds−2, Q ∝ ω; and at ω ≳ Ds−2, Q ∝ ω½. The model is applied to helium rain clouds in the deep interiors of giant planets and is found to be capable in principle of providing a tidal Q ∼ 105, needed to explain the volcanism of Io and resurfacing of Enceladus. The model is also applied to the earth's outer core and found to be marginally capable of explaining the attenuation of radial modes (notably 0S0) and potentially capable of providing significant attenuation of earth tides. However, quantification and application of the model are difficult because of large uncertainties in the nature of the required two‐phase suspensions.
Crater ejecta scaling laws: Fundamental forms based on dimensional analysis
Authors: Housen K. R. , Schmidt R. M. , Holsapple K. A.
Published: February 2008

A model of crater ejecta is constructed using dimensional analysis and a recently developed theory of energy and momentum coupling in cratering events. General relations are derived that provide a rationale for scaling laboratory measurements of ejecta to larger events. Specific expressions are presented for ejection velocities and ejecta blanket profiles in two limiting regimes of crater formation : the so‐called gravity and strength regimes. In the gravity regime, ejecta velocities at geometrically similar launch points within craters vary as the square root of the product of crater radius and gravity. This relation implies geometric similarity of ejecta blankets. That is, the thickness of an ejecta blanket as a function of distance from the crater center is the same for all sizes of craters if the thickness and range are expressed in terms of crater radii. In the strength regime, ejecta velocities are independent of crater size. Consequently, ejecta blankets are not geometrically similar in this regime. For points away from the crater rim the expressions for ejecta velocities and thickness take the form of power laws. The exponents in these power laws are functions of an exponent, α, that appears in crater radius scaling relations. Thus experimental studies of the dependence of crater radius on impact conditions determine scaling relations for ejecta. Predicted ejection velocities and ejecta‐blanket profiles, based on measured values of α, are compared to existing measurements of velocities and debris profiles.
Travel times and station corrections for P waves at teleseismic distances
Authors: Dziewonski Adam M. , Anderson Don L.
Published: February 2008

Approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of P wave travel times and station residuals, including azimuthal effects. The events were selected from a catalog containing 160,000 earthquakes, and those having uniform distance and azimuthal coverage were systematically relocated and used to refine P wave travel times and station corrections. Station corrections are provided for 994 seismic stations. The station corrections involve three terms: the static effect and two cosine terms with appropriate phase shifts. They exhibit general consistency over broad geographic areas and, where coverage is dense, often show abrupt changes from one geological province to another. The cos 2θ terms appear to be due to upper mantle anisotropy, and they correlate with the stress direction in the crust.