bluuebunny/crossref_metadata_2025 huggingface dataset:
Travel time inversion: A geometrical approach
Authors: Garmany Jan , Orcutt John A. , Parker Robert L.
Published: February 2008

A geometric formulation of the seismic travel time problem is given based upon the use of slowness as an independent variable. Many of the difficulties in the conventional treatment (e.g., singular kernels) are thereby, avoided. Furthermore, it is shown that the inverse problem possesses an inherently linear formulation. In this formalism we are able to provide extremal solutions giving upper and lower depth bounds using linear programing. This approach has been compared with two well‐known nonlinear extremal inversions. We find our technique to be easier to implement and find that it often generates superior results.
An analysis of isostasy in the world's oceans: 3. Aseismic ridges
Authors: Detrick R. S. , Watts A. B.
Published: February 2008

Cross‐spectral techniques have been used to analyze the relationship between gravity and bathymetry on 26 profiles across the Walvis and Ninetyeast ridges. The resulting filters or transfer functions have been used to study the state of isostasy at these ridges. Transfer functions for the eastern Walvis Ridge and the Ninetyeast Ridge profiles can be best explained by an Airy‐type thickening of the crust beneath these ridges. The crustal thicknesses required are in the range 15–25 km, in good agreement with available seismic refraction data. The transfer function for the western Walvis Ridge can be best explained by a flexure model in which the oceanic lithosphere is treated as a thin elastic plate overlying a weak fluid. The effective elastic plate thickness required is in the range 5–8 km. These estimates for the elastic thickness are substantially less than those determined from flexural studies of loads on older crust but are similar to estimates determined for sea floor topography at midocean ridges. These observations are consistent with the formation of aseismic ridges near spreading centers on lithosphere that is young, thin, and relatively weak. The differences in isostasy between the eastern and western Walvis Ridge are attributed to an off‐axis shift relative to the South Atlantic spreading center of the ‘hot spot’ forming the Walvis Ridge about 80 m.y. B.P. These observations suggest that the isostatic parameters determined for these aseismic ridges were ‘frozen’ in at the time of their formation at or near a spreading center and have not significantly changed through time.
Geos 3 data processing for the recovery of geoid undulations and gravity anomalies
Authors: Rapp Richard H.
Published: February 2008

Approximately 2000 arcs of Geos 3 altimeter data have been analyzed to determine geoid undulations and mean and point gravity anomalies. Since the altimeter data are subject to orbit error and bias, an error model was developed to remove such errors by comparing the altimeter geoid to the geoid implied by the Gem 9 coefficients. Using a one‐ or two‐parameter error model and introducing crossover constraints, one primary adjustment involving 854 arcs distributed globally and six regional adjustments were performed. Typical a priori crossover discrepancies were ±8 m, while a posteriori discrepancies were of the order of ±0.6 m. The adjusted (bias removed) altimeter geoid undulations were used to construct 2‐m contour interval geoids in selected regions and to determine gravity anomalies using least squares collocation. Using the data available up to September 1977, 377 equal‐area anomalies of 5° and 12,144 mean anomalies of 1° × 1° had been determined by February 1978. The average predicted accuracy was ±3 mGal for the 5° blocks and ±7 mGal for the 1° blocks in areas where sufficient altimeter data were available. These accuracy estimates appear to be reasonable when comparisons are made with terrestrial ground truth. The altimeter undulations were also used to derive point anomaly variations over areas of geophysical interest such as trenches and seamounts.
Gravity model improvement using Geos 3 (GEM 9 and 10)
Authors: Lerch Fancis J. , Klosko Steven M. , Laubscher Roy E. , Wagner Carl A.
Published: February 2008

The spaceborne altimeter missions of Geos 3 (50‐cm accuracy) and the future Seasat (10‐cm accuracy) require precise knowledge of the radial position of the spacecraft to be most‐effective. Though errors in previous gravity models have produced large uncertainties in the orbital position of Geos 3, significant improvement has been obtained with new geopotential solutions, Goddard Earth Model (GEM) 9 and 10. The solution for GEM 9 was derived by combining laser data from Geos 3, Lageos, and Starlette; S band measurements on Landsat 1; and data from 26 other satellites used in previous solutions. GEM 10 is a combination solution containing a global set of surface gravity anomalies along with the data in GEM 9. Radial errors of Geos 3 for 5‐day arcs have been reduced from about 5 m to 1 m on the basis of orbital intercomparisons, station navigations, and analyses employing crossover points from passes of altimetry. The use of highly accurate laser data in a constrained least squares solution has permitted GEM 9 to be a larger field than previous derived satellite models, GEM 9 having harmonics complete to 20 × 20 with selected higher‐degree terms. The satellite data set has approximately 840,000 observations, of which 200,000 are laser ranges taken on nine satellites equipped with retroreflectors. GEM 10 is complete to 22 × 22 with selected higher‐degree terms out to degree and order 30 amounting to a total of 592 coefficients. Comparisons with surface gravity and altimeter data indicate a substantial improvement in GEM 9 over previous satellite solutions; GEM 9 is in even closer agreement with surface data than the previously published GEM 6 solution which contained surface gravity. In particular, the free air gravity anomalies calculated from GEM 9 and a surface gravity solution by Rapp (1977) are in excellent agreement for the high‐degree terms (13 ≤ l ≤ 22). From these terms an estimate is made of the gravity anomalies for the upper mantle. The mass constant of the earth, GM, has been estimated from the laser data as 398,600.64±0.02 km3/s2, a value which is principally determined from Lageos. The speed of light used was 299,792.5 km/s. Geocentric station positions were determined for approximately 150 stations in GEM 10. These station coordinates, their mean sea level heights, and altimetry data provide an estimate for the mean radius of the earth of ae = 6,378,139 ± 1 m. Accuracy estimates derived for the potential coefficients have been verified with independent data sets. These produce commission errors in geoid heights of 1.9 m and 1.5 m (global rms values), respectively, for GEM 9 and 10.
Tectonic evolution of the Coral Sea Basin
Authors: Weissel Jeffrey K. , Watts A. B.
Published: February 2008

Studies of marine magnetic anomaly data from the Coral Sea basin reveal magnetic lineations which strike N70°W almost parallel to the northern margin of the Queensland Plateau. The lineation pattern is best developed in the eastern part of the basin, and we can confidently identify a sequence of anomalies 26 (and perhaps 27) through 24 repeated about an extinct spreading center along the long axis of the basin. The extinct spreading center is associated with a short‐wavelength (λ
40 km) free air gravity anomaly low of 10–20 mGal and a region of rough acoustic basement. We infer that the Coral Sea basin began opening at about 62 m.y. B.P. at a half‐spreading rate of 2.4 cm/yr and that spreading ceased at about 56 m.y. B.P. Although the Coral Sea ceased opening at the same time as the Tasman Sea, finite rotations determined for anomaly 26 lineations from these two basins are distinctly different. We thus infer that at least one additional plate boundary was active in the Paleocene and that it met the Coral Sea and Tasman Sea plate boundaries at a triple junction near the eastern end of the Coral Sea basin.
Aleutian subduction zone seismicity, volcano‐trench separation, and their relation to great thrust‐type earthquakes
Authors: Davies John N. , House Leigh
Published: February 2008

A seismic network has been operated in the Shumagin Islands and adjacent Alaska Peninsula, Alaska, since July 1973. Hypocentral cross sections based on the first 4 years of data from this network show a well‐defined Benioff zone that is about 10 km thick and extends to about 180‐km depth. Focal mechanism and strong‐motion accelerograph data for an mb = 6.0 earthquake at 40‐km depth indicate a slip plane subparallel to the Benioff zone. This event is interpreted as being directly caused by the underthrusting of the Pacific lithosphere beneath the Shumagin Islands. Comparison of the Shumagin hypocentral cross section with cross sections from other regions of the Aleutian arc shows that at depths greater than 40 km (1) the Benioff zones are nearly congruent and (2) their positions relative to the volcano line are nearly the same along the entire length of the arc. This comparison also reveals that the eastward widening of the volcano‐trench separation occurs predominantly in the shallowly dipping extension of the Benioff zone from its position at 40‐km depth to where it outcrops near the trench; this extension is herein termed the main thrust zone. Rupture zones of the 1964, the 1965, and possibly the 1938 great earthquakes along the Aleutian arc are confined to the main thrust zone. This observation and the very low level of seismic activity at the present time in the main thrust zone compared to that in the Benioff zone below 40‐km depth suggest that seismic activity in the main thrust zone is dominated by great earthquakes and their aftershocks, followed by long intervals of quiescence. The Benioff zone below 40‐km depth shows more continual seismic activity. For the Aleutian arc there exists a weak correlation between the width of the main thrust zone and the rupture length and magnitude (Mw) of great earthquakes.
Intraplate seismicity on bathymetric features: The 1968 Emperor Trough Earthquake
Authors: Stein Seth
Published: February 2008

The April 28, 1968 (mb = 5.5, Ms = 5.2), earthquake in the Emperor Trough, a long linear deep east of the Emperor Seamounts in the North Pacific, is the only recorded seismic event in this region. The focal mechanism, determined using body and surface waves, shows faulting striking along the trough with the east side thrust upward. This mechanism is consistent with bathymetry showing the east flank 500 m higher than the west and suggests that the earthquake represents part of continuing deformation at the trough. This earthquake is one of a large number of intraoceanic plate events that occur on major bathymetric features. The focal mechanisms of earthquakes on such features probably represent the reactivation of already present faults or weak zones. The orientations of intraplate stresses inferred from the focal mechanisms may differ significantly from the true stress directions.
Self‐consistent imbedding and the ellipsoidal model for porous rocks
Authors: Korringa J. , Brown R. J. S. , Thompson D. D. , Runge R. J.
Published: February 2008

Equations are obtained for the effective elastic moduli for a model of an isotropic, heterogeneous, porous medium. The mathematical model used for computation is abstract in that it is not simply a rigorous computation for a composite medium of some idealized geometry, although the computation contains individual steps which are just that. Both the solid part and the pore space are represented by ellipsoidal or spherical ‘grains’ or ‘pores’ of various sizes and shapes. The strain of each grain, caused by external forces applied to the medium, is calculated in a self‐consistent imbedding (SCI) approximation, which replaces the true surrounding of any given grain or pore by an isotropic medium defined by the effective moduli to be computed. The ellipsoidal nature of the shapes allows us to use Eshelby's theoretical treatment of a single ellipsoidal inclusion in an infinite homogeneous medium. Results are compared with the literature, and discrepancies are found with all published accounts of this problem. Deviations from the work of Wu, of Walsh, and of O'Connell and Budiansky are attributed to a substitution made by these authors which, though an identity for the exact quantities involved, is only approximate in the SCI calculation. This reduces the validity of the equations to first‐order effects only. Differences with the results of Kuster and Toksöz are attributed to the fact that the computation of these authors is not self consistent in the sense used here. A result seems to be the stiffening of the medium as if the pores are held apart. For spherical grains and pores, their calculated moduli are those given by the Hashin‐Shtrikman upper bounds. Our calculation reproduces, in the case of spheres, an early result of Budiansky. An additional feature of our work is that the algebra is simpler than in earlier work. We also incorporate into the theory the possibility that fluid‐filled pores are interconnected. This is achieved with the use of Gassmann's equation.
The crustal structure of the Ontong Java and Manihiki oceanic plateaus
Authors: Hussong D. M. , Wipperman L. K. , Kroenke L. W.
Published: February 2008

Air gun‐sonobuoy precision echo recorder data acquired on the Ontong Java Plateau and the Manihiki High Plateau demonstrate that these features have upper crustal seismic velocity structures strikingly similar to those observed in deep ocean basins. Each of the crustal layers of the oceanic plateaus, however, tends to be proportionally thicker than its ocean basin counterparts. Our measurements show that the crustal layers of the Ontong Java Plateau and the Manihiki Plateau are 5.0 and 3.1 times thicker than ocean basin crust, respectively. The proportionality also holds for other oceanic plateaus, such as the Shatsky Rise, where the structure reported by Den et al. (1969) is 2.7 times thicker than ocean basin crustal layers. Layer 2A on the three plateaus is poorly predicted by this model. The oceanic plateaus considered in this paper have essentially the same mean crustal density as the much thinner ocean basin crust. These observations, in addition to the oceanic affinities and apparent age progression of the upper crustal rocks of the Ontong Java Plateau, lead us to conclude that this region, as well as most other oceanic plateaus, probably formed as an integral part of the lithosphere at an accreting plate margin.
Frictional metamorphism, argon depletion, and tectonic stress on the Alpine Fault, New Zealand
Authors: Scholz Christopher H. , Beavan John , Hanks Thomas C.
Published: February 2008

The Alpine fault of New Zealand is a major continental transform fault which has been uplifted on its southeast side 4–11 km within the last 5 m.y. This uplift has exposed the Alpine schists, which have been metamorphosed from the adjacent Torlesse graywackes. The Alpine schists increase in metamorphic grade from prehnite‐pumpellyite facies 9–12 km from the fault through the chlorite and biotite zones of the greenschist facies to the garnet‐oligoclase zone of amphibolite facies within 4 km of the fault. These metamorphic zone boundaries are subparallel to the fault for 350 km along the strike. The K‐Ar and Rb‐Sr ages of the schists increase with distance from the fault: from 4 m.y. within 3 km of the fault to approximately 120 m.y. 20 km from the fault. Field relations show that the source of heat that produced the argon depletion aureole was the fault itself. Adopting a friction model for the fault and applying the known geological history of motion of the Alpine fault, we show that the metamorphism resulted from frictional heating during the 360‐km right lateral slip on the Alpine fault during the Mesozoic (Rangitata) period of fault motion (140–80 m.y. ago). Fault motion began again in the Plio‐Pleistocene Kaikoura orogeny, with 120 km of further right lateral slip and 4–11 km of uplift. Frictional heating during this episode produced the argon depletion. Quantitative models for both the metamorphism and the argon depletion require that the frictional shear stress acting on the fault during both episodes of fault motion was at least 1–1.5 kbars. If, alternatively, it is assumed that all fault motion took place in the last 30 m.y., this does not change our conclusion with respect to the stress required for the argon depletion.
Heat flow in western Virginia and a model for the origin of thermal springs in the folded Appalachians
Authors: Perry L. D. , Costain J. K. , Geiser P. A.
Published: February 2008

Five new heat flow values in the Blue Ridge (37°02′N, 80°12′W), Valley and Ridge (38°00′N, 79°50′W; 37°54′N, 79°54′W; 38°14′N, 79°49′W), and Alleghany Plateau (37°12′N, 82°06′W) provinces of Virginia are 36.3±1.3 mW/m2, 317±20 mW/m2, 48.3±0.3 mW/m2, 49.5±1.5 mW/m2, and 55.8±1.7 mW/m2, respectively. The value of 36.3 mW/m2 in the Blue Ridge agrees with values predicted by the contour map of Sass et al. (1976) for this part of the eastern United States; the values of 49.5 and 48.3 mW/m2 in the Valley and Ridge province do not. The high value of 317±20 mW/m2 at Hot Springs, Virginia, is the result of convection along a fault zone and is not regionally representative. The value of 48.3 mW/m2 determined at a site about 8 km southwest of Hot Springs, Virginia, is regionally representative of the heat flux. New data support a model of moderately deep circulation of meteoric water entering near‐vertical sedimentary beds at relatively high topographic elevations and circulating downward via bedding plane permeability and fractures within Silurian quartzites and/or adjacent carbonate units. The moderately deep circulation is then intersected by vertical transverse linear fracture zones along which the water rises. All of the warm springs are located adjacent to topographic gaps which have developed along zones of weakness associated with the transverse fracture zones.
Brief note for consideration of active seismic exploration on Mars
Authors: Tittmann B. R.
Published: February 2008

Considerable controversy has arisen regarding the existence of water and water ice below the surface of Mars. Results of laboratory internal friction measurements on strongly outgassed rocks, recently made as part of an effort to interpret the high seismicQvalues observed in the lunar crust, show that moisture absorbed in pores and cracks causes the lowQvalues. Unfortunately, the Viking seismology investigation found a surprisingly inactive planet. This note makes the case for active seismic exploration in order to ascertain the existence of liquid water in the crust fromQdeterminations.
Landslides in Valles Marineris, Mars
Authors: Lucchitta Baerbel K.
Published: February 2008

Large landslides in the Martian equatorial troughs have been investigated with respect to morphology, geologic structure of the troughs, time of emplacement, similarity to terrestrial landslides, and origin and mechanism of transport. Viking Orbiter images provided a base for morphologic analyses, mapping, crater counts, and volumetric considerations. Results showed that the morphologic variations of the landslides can be attributed mainly to their degree of confinement on trough floors. Many prominent landslides appear to be of similar age and were emplaced after a major episode of faulting that created scarps of great relief. Most sliding apparently occurred after these scarps were dissected into spurs, gullies, and tributary canyons. Much of the slide activity coincided with a major late eruptive phase of the Tharsis volcanoes, as deduced from the crater density, over the total deposit area, of 570±130 craters larger than 1 km in diameter per 106 km2. The huge size of many landslides is due to their occurrence on fault scarps that may have attained several kilometers in height in the absence of vigorous fluvial erosion on Mars. The mechanical efficiency of the Martian landslides is high but in accord with predictions from large landslides on earth. The slides may have originated as gigantic mudflows with slump blocks at their heads; water that may have existed in aquifers in the wallrock below and behind a lid of ice may have been liberated through minor disturbances from Marsquakes. Liquefaction and collapse of extensive wall sections may then have ensued, and the debris rushed out onto the trough floors at speeds exceeding 100 km/h.
Seasonal and secular variation of wind streaks on Mars: An analysis of Mariner 9 and Viking data
Authors: Thomas P. , Veverka J.
Published: February 2008

Viking orbiter observations extending over 1 Martian year have been used in conjunction with Mariner 9 data obtained in 1971–1972 to study the seasonal and secular behavior of several kinds of wind streaks. Most bright streaks, inferred to consist of dust storm fallout in the lees of obstacles, have changed very little in form or orientation over a period of 3 Martian years. Some are extremely stable and have experienced no effective eolian action over the 3 years. A few bright streaks changed rapidly during global dust storms; these streaks are located in areas subject to both global and topographic winds. Viking images have shown for the first time that dark, erosional streaks are stable from the time of their formation after major dust storms until the onset of the next episode of major storm activity. Available evidence shows that the large, dark streaks in Oxia Palus consist of material deflated from dune fields within the associated craters. These streaks lengthened secularly since 1972; changes appear to occur episodically during southern summer. The great majority of all streaks reflect winds during the period from late southern spring to early southern fall, although some changes occur throughout the year. The global pattern of wind streaks and the variability of the streaks thus depend strongly upon the current south‐north asymmetry of seasons on Mars.
Recession of Martian north polar cap: 1977–1978 Viking observations
Authors: James Philip B.
Published: February 2008

The regression curve for the 1977–1978 sublimation of the north polar cap of Mars is extracted from Viking orbiter imaging data. The recession observed during the spring season is quite consistent with previous observations and is in excellent agreement with telescopic data obtained concurrently. A receding late winter boundary is identified, but owing to hood obscuration this boundary cannot be definitely attributed to a surface cap.
Bursts of seismicity as long‐term precursors of strong earthquakes
Authors: Keilis‐Borok V. I. , Knopoff L. , Rotvain I. M. , Sidorenko T. M.
Published: February 2008

One or more earthquakes with abnormally large numbers of aftershocks at the beginning of their aftershock sequences are possible long‐term precursors of a stronger earthquake. This precursor, named pattern B (bursts of aftershocks), was tested together with two other possible premonitory patterns, S and Σ, described previously. Pattern S (swarm) consists of the spatial clustering of earthquakes at a time when the seismicity of the region is above average. Pattern Σ consists of an increase in the sum of earthquake energies to the 2/3 (roughly) power, in a sliding time window. These three patterns were tested by retrospective long‐term prediction of earthquakes of Southern California, 1932–1977, with magnitudes ≥6.5. The total duration of identified periods of elevated probability of occurrence is about 14 years. Five out of six earthquakes occurred during these intervals. However, the patterns do not indicate the exact location of future strong earthquakes within the region. All three patterns seem to represent different projections of the same general pattern, of ‘bursts of seismicity,’ i.e., an abnormal clustering of earthquakes in the time‐space‐energy domain. Pattern B also precedes strong earthquakes in New Zealand and Italy; Pattern Σ precedes strong earthquakes in New Zealand.
Magnetic fabric of the Elberton Granite, northeast Georgia
Authors: Ellwood Brooks B. , Whitney James A.
Published: February 2008

The anisotropy of magnetic susceptibility of samples from 21 sites within the granitic Elberton pluton, located in northeast Georgia, has been used to determine a possible tectonic rotation, source, and flow direction for granite emplacement. The data indicate that the granitic magma may have been initially emplaced in the northeastern and south central portions of the body and then flowed as a result of plastic deformation to the northeast and southwest. Estimates of tectonic rotation are based on field evidence and on the assumption that the magnetic fabric within the body was initially horizontal. Present orientation of this fabric, then, indicates a rotation about a N‐NE strike, down to the southeast by possibly as much as 35°. A zone of high bulk susceptibility within the body, indicative of an excess in iron content, is closely associated with areas of pink discoloration of the granite. We suggest that the pink discoloration is the result of low‐temperature oxidation of magnetite to hematite in the feldspars.
Application of the Bingham distribution function in paleomagnetic studies
Authors: Onstott Tullis C.
Published: February 2008

Fisherian statistical parameters are frequently published for paleomagnetic data that form elongate directional distributions, despite the fact that they are strictly applicable to circularly symmetric distributions. Thus the Bingham statistical parameters provide better approximation to elongate paleomagnetic data sets. Because the Bingham parameters also pertain to directions dispersed along a great circle, they supply a statistical basis for describing the distribution of axes perpendicular to great circles intersecting at a common point, a problem that arises in the analysis of multicomponent magnetization, in the application of the Hargraves' correction technique, and in intersecting lunes. Application of the Bingham density function to paleomagnetic poles from Tertiary lava flows in Iceland reveals temporal fluctuations in the eccentricity of the data. Use of the Bingham density function in the analysis of intersecting great circles is illustrated by application to data from a lightning strike remagnetized basalt in northern Arizona.
Heat flow in the Coso Geothermal Area, Inyo County, California
Authors: Combs Jim
Published: February 2008

Obvious surface manifestations of an anomalous concentration of geothermal resources at the Coso geothermal area, Inyo County, California, include fumarolic activity and associated hydrothermally altered rocks. Pleistocene volcanic rocks associated with the geothermal activity include 38 rhyolite domes occupying a north trending structural and topographic ridge and numerous basaltic cinder cones and lava flows partly surrounding its southern half. In an investigation of the thermal regime of this Basin and Range geothermal area, temperature measurements were made in 25 shallow and 1 intermediate depth borehole. Geothermal gradients ranged from 25.3°C/km to 906.°C/km. The high gradients arise from convecting hot water and former convective transport of heat by dikes that fed the domes and flows. Thermal conductivity measurements were made on 312 samples from cores and drill cuttings. The resultant terrain‐corrected heat flow values range from 1.6 to 23. HFU. The actual process by which heat is transferred is rather complex; however, the heat flow determinations can be divided into two groups. The first group, less than 4.0 HFU, are indicative of regions with primarily conductive regimes, although deep‐seated mass transfer is implied. The second group, greater than 4.0 HFU, are characteristic of regions with considerable convective heat transfer in the shallow subsurface. The general shapes of the 3‐and 5‐HFU contours in the upper 35 m of the subsurface are essentially aligned with the major regional fault zones, suggesting that the high heat flows are the result of convective heat transfer caused by circulation of hot groundwater in these vertically permeable subsurface channels. The high heat flow values (>10 HFU) are essentially restricted to the rhyolite dome field and the associated surface thermal manifestations. Heat transferred by convection of water would be rapidly exhausted if it were not intermittently supplied with heat energy from depth; therefore the heat flow data substantiate the concept that the surface hydrothermal activity and associated volcanic rocks are products of a long‐lived magmatic system that has periodically erupted lava during the past 0.3–1.0 m.y. This magmatic system is the crustal heat source for the present‐day Coso geothermal system (1 HFU = 41.87 mW/m2).
Spectral reflectance and discrimination of plutonic rocks in the 0.45‐ to 2.45‐μm region
Authors: Blom Ronald G. , Abrams Michael J. , Adams Herbert G.
Published: February 2008

Visible and near‐infrared field spectral reflectance measurements of plutonic rocks were acquired in the 0.45‐ to 2.45‐μm region with a portable field reflectance spectrometer. These spectra were used to determine spectral signatures for the various rock types and to evaluate the separability of these rocks based on their spectral characteristics. A total of 135 samples were divided into 11 groups based on their mineralogy. These 11 groups approximately correspond to traditional rock classifications and include five granitic groups, three gabbroic groups, and three ultramafic groups. The positions, intensity, and presence of iron, CO3−2, and Al‐OH and Mg‐OH absorption bands varied among the 11 groups. Each rock group also had a range of albedos that is characteristic of the group. Stepwise linear discriminant analysis was performed on the spectral data to determine the separability of the 11 groups. Classification accuracy for 30 equally spaced wavelength bands between 0.45 and 2.45 μm was 78% with 10% serious misclassifications. The same analysis was repeated, limiting the spectral data to the wavelength regions corresponding to the proposed Landsat D thematic mapper scanner. Classification accuracy was 71% with 13% serious misclassifications. To assess the effects of weathering on spectral separability of plutonic rocks, a set of 47 spectra of weathered plutonic rocks was analyzed by using the classification functions developed in the preceding analyses. Classification accuracy of these spectra could not be as critically assessed because precise mineralogy was not known; however, serious classification errors totaled 17% with the 30‐channel data and 30% with the simulated Landsat D data. These results indicate that plutonic rock types can be successfully discriminated from one another with good results on the basis of their spectral characteristics in the 0.45‐ to 2.45‐μm region.
Episodic radon changes in subsurface soil gas along active faults and possible relation to earthquakes
Authors: King Chi‐Yu
Published: February 2008

Subsurface soil gas along active faults in central California has been continuously monitored by the Track Etch method to test whether its radon‐isotope content may show any premonitory changes useful for earthquake prediction. The monitoring network was installed in May 1975 and has since been gradually expanded to consist of more than 60 stations along a 380‐km section of the San Andreas fault system between Santa Rosa and Cholame. This network has recorded several episodes, each lasting several weeks to several months, during which the radon concentration increased by a factor of approximately 2 above average along some long, but limited, fault segments (∼100 km). These episodes occurred in different seasons and do not appear to be systematically related to changes in meteorological conditions. However, they coincided reasonably well in time and space with larger local earthquakes above a threshold magnitude of about 4.0. These episodic radon changes may be caused by a changing outgassing rate in the fault zones in response to some episodic strain changes, which incidentally caused the earthquakes.
Long‐term nondipole components in the geomagnetic field during the last 130 m.y.
Authors: Coupland David H. , Van der Voo Rob
Published: February 2008

All available paleomagnetic data for the last 130 m.y. meeting certain selection criteria are drawn from standard compilations and returned to precontinental drift site locations and orientations using spreading poles based on sea floor magnetic anomalies. The resulting data set is divided into six time periods, and global patterns of inclination and declination anomalies are used to identify important nondipole components. Spherical harmonic analysis is then performed to evaluate the corresponding coefficients. An axially symmetric quadrupole component (g20) is found to be important for at least the last 100 m.y., and an axially symmetric octupole component (g30) for the last 50 m.y. Possible correlations are noted between the variation with time of the reversal frequency of the geomagnetic field, compiled by A. Cox, the polarity bias published by Irving and Pullaiah, and our nondiple field magnitude. Both current explanations for the nondipole components seen in paleomagnetic data, the offset dipole model and Cox's zonal nondipole model, are incompatible with the large values of g30 found in this study. However, as a possible alternative, Cox's model can be generalized to accomodate the new information.
Propagating rifts on midocean ridges
Authors: Hey Richard , Duennebier Frederick K. , Morgan W. Jason
Published: February 2008

Spreading center jumps identified west of the Galapagos Islands near 95°W occur in a pattern consistent with the propagating rift hypothesis. A new rift is gradually breaking through the Cocos plate. Each successive jump is slightly longer than the preceding jump. The new spreading center grows at a new azimuth toward the west as the old one dies. The jumps are a manifestation of rift propagation. We extend the analysis of propagating rifts to the case of continuous propagation and predict patterns of magnetic anomalies and bathymetry consistent with the observed patterns. In particular, we correctly predict the trends of fossil spreading centers and V patterns of magnetic anomaly offsets required by the propagating rift hypothesis. Similar V patterns have been observed on many other spreading centers and have been interpreted in various ways. The propagating rift hypothesis appears to offer a simple explanation, consistent with rigid plate tectonics, for each of these patterns. This hypothesis may also have important implications for continental rifting.
Tectonic significance of paleomagnetic data from northern Venezuela
Authors: Skerlec G. M. , Hargraves R. B.
Published: February 2008

A paleomagnetic study, mostly of basic intrusive rocks of Cretaceous and Tertiary age from northern Venezuela, includes 153 samples from 31 sites, of which only 6 sites failed to give consistent results. Fifteen sites within the Caribbean Mountains have easterly declinations with shallow, mixed positive and negative inclinations. These anomalous vectors are interpreted to be the result of tectonic rotation about vertical axes rather than to reflect anomalous magnetic field orientations associated with a polarity transition. Although these sample sites are within igneous intrusive bodies with no control on the original attitude during acquisition of the natural remanent magnetization, the relative conformity of data from numerous separate intrusions requires an average 90° megatectonic rotation of both the Villa de Cura and Tinaco tectonic belts of the Caribbean Mountains. On the other hand, seven sites from the Paraguana Peninsula (and two sites from the younger Tertiary Falcon basin) have southerly declination and positive inclination. These vectors can be explained by rotation about horizontal east‐west axes during Cenozoic folding. A compilation of available paleomagnetic data from rocks of Cretaceous age in the Caribbean indicates that anomalous declinations are widespread in both the Greater Antilles and Caribbean Mountains. We interpret these data to support previous suggestions of megatectonic rotations of a more linear, north‐south trending Mesozoic island arc system that included portions of the Greater Antilles and Caribbean Mountains of Venezuela.
The conference on magnitude of deviatoric stresses in the Earth's crust and uppermost mantle
Authors: Hanks Thomas C. , Raleigh C. Barry
Published: February 2008

The magnitude of deviatoric stresses in the earth's crust and uppermost mantle, and especially the magnitude of shear stresses that resist plate motions across the major plate boundaries, is a matter of central importance to solid earth geophysics. Since currently available estimates vary by at least a factor of 10, even when the same features or set of observations are involved, such fundamental issues as the driving mechanism(s) of plate tectonics and the energetics of crustal faulting are entirely unresolved. More generally, there is very little in the study of active geologic processes of geophysical interest that does not involve material deformation of some kind or another; in understated terms the analysis of material deformation to reveal causative processes is poorly constrained when the magnitude of the deviatoric stresses involved is so uncertain.

Apart from the obvious problem of our inability to measure directly deviatoric stresses in the earth's crust and uppermost mantle through all but a small fraction of its depth—or sample directly this region in any other way at the present time—a confounding difficulty exists in the great range of time scales involved in geophysical processes of interest, from fractions of seconds for the spontaneous failure of small crustal earthquakes to billions of years for the relaxation of certain topographic and gravimetric features of the continental crust. It is probably naive then to expect that the same magnitude of deviatoric stresses should exist for all these geologic processes, given the great range of time scales over which they operate. Moreover, the earth's crust and uppermost mantle are heterogeneous enough, especially in continental regions, that significant spatial variations probably exist as well. Certainly, an important problem is whether the actual variations in space and time of deviatoric stresses in the earth's crust and uppermost mantle can be large enough to accommodate the order of magnitude differences as presently perceived or whether our present perceptions are only imagined, in one sense or another. Even so, we can be assured that at least some of our perceptions must be imagined, when deviatoric stress estimates differing by an order of magnitude or more are offered to explain the same basic processes or the same observations of them.
Mechanics of subducted lithosphere
Authors: Davies Geoffrey F.
Published: February 2008

Some simple mechanical models of subducted lithosphere, combined with observations of Benioff seismic zones, challenge some commonly accepted views of descending lithospheric slabs. The great thermal inertia of subducted slabs results in very large negative buoyancy forces, and phase changes may provide significant additional downward forces, but it is not clear what opposing forces are large enough to balance these. For slabs which do not penetrate beyond a depth of 650 km, the only reasonable possibilities seem to be frictional stresses of about 0.2 GPa (2 kbar) in shallow fault zones or viscous resistance from the surrounding mantle, which requires effective viscosities of at least 5×1021 Pa s (5×1022 P), significantly greater than viscosities inferred from post‐glacial rebound. Although more deeply penetrating slabs might encounter additional resistance below 650 km depth, this resistance seems unlikely to be able to provide significant support for the slab since the slab would probably buckle, whether or not compressional stresses are limited by brittle or ductile yielding; in any case, the slab would probably only be supported episodically, if at all, and the results of buckling or fracture should be more evident in Benioff zones. Again the only reasonable alternatives seem to be large shallow frictional stresses or a high‐viscosity upper mantle, which would slow the growth of buckling instabilities.
Secular, annual, and semiannual changes in the baseline level of the Earth's magnetic field at North American locations
Authors: Campbell Wallace H.
Published: February 2008

Secular, annual, and semiannual changes were obtained from the midnight values of field on specially determined days of quiet magnetospheric conditions. From a selection of North American observatories the latitude variation of these changes was evaluated from 20° to 65°. Results for the active and quiet sun years (1958 and 1965) were compared, and changes at Tucson were traced from 1958 through 1974. The year‐to‐year variance in the determination of a secular change seemed to be decreased by the present analysis method in comparison to a method which uses the annual mean of hourly values. The annual horizontal field variations were largely northward in early summer and southward in early winter; quiet year amplitudes of 2 to 3 γ about doubled in the solar active year. The amplitudes of the semiannual field vectors were about one half that of the annual vectors; the directions were apparently inconsistent from year to year.
A12O3 solubility in orthopyroxene in the system MgO‐Al2O3‐SiO2: A reevaluation, and mantle geotherm
Authors: Lane Douglas L. , Ganguly Jibamitra
Published: February 2008

The Al2O3 solubility in orthopyroxene (OPx) coexisting with pyrope has been determined at several P‐T conditions in the system MgO‐Al2O3‐SiO2 (MAS), using synthetic crystalline starting mixtures and approaching from both high‐ and low‐alumina sides. These results and the reversed experimental data of Danckwerth and Newton (1978) on Al‐OPx + spinel + forsterite equilibrium have been treated thermodynamically, using the solution model of Al‐OPx proposed by Ganguly and Ghose (1979), to express Al2O3 solubility in OPx as a function of P and T in both assemblages. The calculated alumina isopleths are in very good agreement with all available reversed experimental data. In addition, the univariant boundary between spinel peridotite and garnet peridotite in the MAS system, which is defined by the intersection of complementary alumina isopleths in these two fields, is in excellent agreement with experimental reversals of the boundary by Danckwerth and Newton. The spinel field isopleths have somewhat steeper dT/dP slopes than those calculated by previous workers but are still not sufficiently sensitive to pressure to be useful as a practical geobarometer. The garnet field isopleths are nearly equally sensitive to both P and T but are somewhat different from those of MacGregor (1974) and show mild curvature, concave toward the pressure axis. The univariant boundary shows moderate curvature, convex toward the temperature axis, with a pressure minimum of 19 kbar at 840°C. The ‘pyroxene geotherm’of Boyd (1973) has been reexamined in the light of the results of this study and the garnet‐clinopyroxene geothermometer of Ganguly (1979). Boyd's conclusion that the sheared garnet lherzolite nodules in kimberlite pipes have formed at higher P‐T conditions than the granular ones is reinforced; however, the inferred P‐T conditions do not seem to suggest an inflection of the geotherm between the two groups of nodules. The pyroxene or nodule geotherm overlaps the shield geotherm of Clark and Ringwood but progressively diverges from the latter toward higher temperature with increasing depth.
Vein mineral ages of old oceanic crust
Authors: Richardson Stephen H. , Hart Stanley R. , Staudigel Hubert
Published: February 2008

Vein smectites with large Rb/Sr enrichments from extensively altered basaltic oceanic crust in Deep Sea Drilling Project hole 417A in the western Atlantic define a highly constrained Rb/Sr isochron age of 108±3 m.y. This age is identical to a less well constrained age of 108±17 m.y. for vein smectites with lower Rb/Sr enrichments from adjacent hole 418A and to the 108 m.y. age of crust formation derived by paleontological and magnetic anomaly correlation. Reasonable agreement exists between the 87Sr/86Sr ratio of vein calcites from both sites and the seawater 87Sr/86Sr ratio at the time. Pervasive low‐temperature alteration in the contrasting environments of sites 417 and 418 appears to be coeval and essentially coincident with basement formation. Alteration may be used to advantage in determining ages of old oceanic crust.
Winona Basin, West Coast Canada: Crustal structure from marine seismic studies
Authors: Clowes R. M. , Thorleifson A. J. , Lynch S.
Published: February 2008

Winona basin is a deep water sedimentary basin forming part of Explorer plate and located off the northern end of Vancouver Island, British Columbia. During 1975 a deep seismic sounding survey, using explosive charges and multiple hydrophones as receivers, was carried out over the basin. Three reversed refraction profiles were shot, one along and two across the basin. A preliminary look at data along the basin indicated reasonable lateral homogeneity so interpretation was based on both travel time and amplitude characteristics through the use of synthetic seismograms. However, the two profiles across the basin indicated significant lateral variations in crustal structure, so a ray tracing approach was used. The sediments of Winona basin have a maximum thickness of approximately 5 km, consisting of ∼2 km of low‐velocity (average ∼2.5 km/s) layers and ∼3 km of higher‐velocity (∼4.3 km/s) material. Beneath the sediments there is a rapid increase in velocity to ∼5.4 km/s, the top of the oceanic crust. Below this and along the basin, the interpretation indicates three zones with thicknesses of 1 to 3 km and having low‐velocity gradients; these being separated by transition zones with depth intervals less than 1 km and having high‐velocity gradients. The three zones with low‐velocity gradients are represented approximately by 1 km at 5.4 km/s, 2.7 km at 6.2 km/s, and 2.5 km at 6.7 km/s. With the inclusion of the zones of high‐velocity gradient, an average subsedimentary crustal velocity of ∼6.3 km/s and thickness of ∼8 km are obtained. The upper mantle velocity is ∼7.8 km/s. Across the basin, the crustal model shows layers dipping eastward from the westward side and dipping westward from the eastward side, forming a bowllike structure. Two‐dimensional gravity calculations based on the seismic model are compatible with a gravity profile derived from a free air anomaly map. The thick crust and structural variations are consistent with, but do not provide direct evidence for, the complex plate interactions embodied in the tectonic reconstructions proposed by other scientists.