bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleomagnetism of Cretaceous and Eocene strata, San Miguel Island, California, borderland and the northward translation of Baja California
Authors: Champion Duane E. , Howell David G. , Marshall Monte
Published: February 2008

The history of poleward motion of the Baja California peninsula is a subject of continuing geologic investigations, including paleomagnetic studies during the past decade. Geologic correlations seem not to necessitate northward movement, while paleomagnetic data suggest movement of 1000 km or more. We have made paleomagnetic measurements on Cretaceous and Eocene sedimentary rocks of San Miguel Island, California, which are western lithofacies of rocks cropping out on the Baja peninsula. A total of 97 drill cores taken from Upper Cretaceous turbidites, though weakly magnetized, were found to clean best in response to thermal demagnetization, revealing a characteristic remanence. The characteristic magnetization has an inclination 43.9°, declination of 24.0°, and an α95of 3.9°. This direction is 19° shallower and rotated 50° clockwise from the expected direction for the present site position and corresponds to a paleolatitude of 26°N. A total of 117 drill cores of middle Eocene rocks revealed characteristic magnetizations in response to either alternating field or thermal demagnetizations. Some samples suffer from a secondary thermal magnetization from early Miocene volcanic activity, requiring a thermal cleaning that incompletely removes the overprint. A mean direction of magnetization can be derived from the intersection of planes fitted to the paths of demagnetization. The characteristic magnetization for the Eocene rocks has an inclination of 31.9°, a declination of 71.0°, and an α95of 3.8°. This direction is 19° shallower and rotated 79° clockwise from the expected Eocene direction for this site and corresponds to a latitude of 17°N. These paleomagnetic data, in combination with recently published data from the Baja peninsula and Peninsular Ranges of southern California, indicate motion trajectories that mimic the North American craton for the period Cretaceous to late Eocene time. During the Oligocene the Baja peninsula and adjoining regions to the west moved northward relative to the craton, arriving at a pre‐Gulf of California position not later than earliest Miocene time. A comparison of the northward motion of the Baja peninsula with that of the Santa Lucia‐Orocopia allochthon shows that both areas underwent rapid poleward translations, but these motions occurred at different times.
The association between ground gas radon variations and geologic activity in Hawaii
Authors: Thomas Donald M. , Cuff Kevin E. , Cox Malcolm E.
Published: February 2008

Soil gas radon activities have been recorded at 10 stations on Kilauea volcano for approximately 5 years. Long‐term average radon activities were found to vary substantially among the sites monitored. Spatial variations appear to be controlled predominantly by (1) near‐surface hydrothermal activity, (2) thickness and permeability of soil cover, and (3) the presence of subsurface fracturing. Temporal changes in radon activity show minor correlation with meteorological changes but a strong correlation with seismic and volcanic events on Kilauea: magmatic intrusions were associated with contemporaneous changes in radon concentrations, whereas seismic events were associated with precursory radon anomalies. Such changes may be caused by release of trapped radon from rocks by microfracturing under broadly distributed stress changes associated with intrusion events and preceding seismic events.
Particle geochemistry of volcanic plumes of Etna and Mount St. Helens
Authors: Varekamp Johan C. , Thomas Ellen , Germani Mark , Buseck Peter R.
Published: February 2008

Particles in volcanic plumes include vapor condensates and reaction products of ash with vapors or condensed liquids, in addition to abundant silicate particles. In the 1980 Mount St. Helens plume we detected abundant (Na, K) Cl crystals as well as Ca sulfates, the latter commonly as overgrowths on ash or anthropogenic particles. Many of the chloride particles contained zinc and cadmium. High‐temperature fumarolic incrustations showed strong enrichments of arsenic, zinc, alkalis, and iron. At Etna we did not detect chloride crystals but found abundant Al, Fe, and Ca sulfates. Sulfuric acid droplets were ubiquitous in both plumes. Bulk analyses of fumarole incrustations at Etna showed an enrichment in the rare earth elements (REE). The chemical and textural data of the plume particles indicate that chloride particles form relatively early and react later with sulfuric acid droplets to form sulfates. At Etna, aluminum as well as some REE are probably transported as volatile fluorine compounds. The differences in plume chemistry between Etna and Mount St. Helens are most likely related to differences in F/Cl ratios of the vapors. Based on the abundance of particulate chlorine in plumes, we suggest that only a small fraction of the total chlorine released during an eruption might reach the stratosphere in the vapor phase.
Advances in soil gas geochemical exploration for natural resources: Some current examples and practices
Authors: McCarthy J. Howard , Reimer G. Michael
Published: February 2008

Field studies have demonstrated that gas anomalies are found over buried mineral deposits. Abnormally high concentrations of sulfur gases and carbon dioxide and abnormally low concentrations of oxygen are commonly found over sulfide ore deposits. Helium anomalies are commonly associated with uranium deposits and geothermal areas. Helium and hydrocarbon gas anomalies have been detected over oil and gas deposits. Gases are sampled by extracting them from the pore space of soil, by degassing soil or rock, or by adsorbing them on artificial collectors. The two most widely used techniques for gas analysis are gas chromatography and mass spectrometry. The detection of gas anomalies at or near the surface may be an effective method to locate buried mineral deposits.
Mechanical aspects of postglacial volcanism and tectonics of the Reykjanes Peninsula, southwest Iceland
Authors: Gudmundsson Agust
Published: February 2008

Four major NE trending postglacial volcanic and tectonic fissure swarms (volcanic systems) occur on the Reykjanes Peninsula, and the westernmost three are the main subject of this paper. Two main types of basaltic volcanoes are associated with these systems: shields of picrite and olivine tholeiite and tholeiite fissures. The average volume of 26 shields is 1.11 km3, and the total production is 29 km3, whereas the corresponding figures for lavas from 101 volcanic fissures are 0.11 km3 and 11 km3. The tectonic fractures are either tension fractures or normal faults of widths up to 20 m, throws up to 10 m, and lengths up to several kilometers. The volcanism and tectonics can be explained by magmatic pressure changes in ellipsoidal magma reservoirs located beneath the fissure swarms. A magmatic pressure increase of the order of 10 MPa is found to be sufficient for an excess uplift of the order of several meters, which is all that is needed to account for the fractures and measured dilation in the fissure swarms. It is concluded that most shield volcanoes, in particular the picrite shields and the large olivine tholeiite shields, formed during the early postglacial period and that their formation was facilitated by the stress field generated as a result of rapid uplift and bending of the crust above the reservoirs. Since that time the reservoirs have become independent systems, the volcanism has been confined to fissures, and the production rate has decreased significantly. During typical fissure eruptions (0.015 km3), only the uppermost several hundred meters of the source reservoir, depending on its magma content, supply magma to the eruption.
An iridium anomaly in the Middle‐Lower Jurassic of the Venetian Region, northern Italy
Authors: Rocchia Robert , Boclet Daniel , Bonté Philippe , Castellarin Alberto , Jéhanno Celestine
Published: February 2008

A significant iridium enrichment has been found in a Jurassic marine sequence, about 180 million years old, outcropping in the Alps of the Venetian region. The maximum iridium concentration of 3.2 ± 0.2 ng·g−1 occurs in a brown crust 2–3 mm thick, containing essentially iron hydroxide. This crust, characterized by a nearly total absence of detritic components, lies on top of upper Lias limestones, and is overlaid with Bajocian‐Bathonian limestones. The origin of the Ir anomaly is not clearly understood yet but could be explained by either a drop in the sedimentation rate resulting in the concentration within a thin sediment layer of the normally infalling cosmic dust and/or by an increase of cosmic material infall following an asteroid or comet impact(s).
A thermoluminescence study of experimentally shock‐loaded oligoclase and bytownite
Authors: Hartmetz Christopher P. , Ostertag Rolf , Sears Derek W. G.
Published: February 2008

The thermoluminescence (TL) of samples of terrestrial oligoclase and bytownite shock‐loaded to 10.5–45 GPa have been measured. The TL sensitivities of both feldspars decreased 10‐fold after shock‐loading to 25–32 GPa and, for the oligoclase, 25‐fold after shock‐loading to greater pressures. The shock‐induced changes for oligoclase are very similar to those of the Kernouve meteorite, in which the TL phosphor is oligoclase, after similar shock‐loading. The data indicate that ordinary chondrites of shock facies d‐f have suffered shock pressures over 25 GPa, consistent with petrographic estimates. Petrographic studies on the shocked samples suggest that the lowering of TL sensitivity is caused by maskelynization (and, at the highest pressures, melting). The oligoclase samples shocked above 34 GPa showed changes in their glow curve shapes similar to those produced by annealing Amelia albite and a type 3.4 ordinary chondrite above 700°C; the dominant peak moves from 132 ± 8°C to 230 ± 12°C. Changes in the glow carve shape for bytownite resembled those produced by annealing certain shergottites. In addition to a major peak at 138 ± 3°C, a peak appeared at 178 ± 8°C. For albite, the changes were shown to be associated with small degrees of disordering, and we suggest that shock‐induced disordering, or some process closely associated with the onset of disordering, is occurring in the trace amounts of feldspar that survive maskelynization after shock‐loading to pressures >34 GPa.
Roberts victor eclogites: Ancient oceanic crust
Authors: MacGregor Ian D. , Manton William I.
Published: February 2008

The two groups of Roberts Victor eclogites may be further distinguished by their 18O/16O ratios; one above (I) and one below (II) the mantle value (5.7). In the group II suite systematic variation of the major elements with oxygen isotopes matches that calculated for oceanic volcanic rocks altered by circulating seawater in ridge crest hydrothermal systems. A negative linear correlation between 18O/16O and 87Sr/86Sr(WR) and an associated positive correlation with K2O (and Rb) of the anhydrous eclogite assemblage with 87Sr/86Sr(WR) are also products of hydrothermal alteration. The chemical and isotopic similarity of the eclogites to hydrothermally altered oceanic crust supports the conclusion that they are subducted fragments that have been metamorphosed from metabasalts to eclogites. During metamorphism the elements are fractionated into two components. One, enriched in the radiogenic isotopes of U, Th, K, and Rb, and light rare earth elements, is concentrated in a hydrous, interstitial, mobile component, and the other, enriched in Nd relative to Sm, is concentrated in the anhydrous, immobile, garnet and clinopyroxene host. The mobile component is comparable in chemistry to rocks of the MARID suite and may well play an important role in mantle metasomatism. Nd/Sm and Pb/Pb ages indicate an Archean age (approximately 2.47 b.y.) for the metamorphism. Depth estimates indicate a skewed distribution with most eclogites coming from the depth range 165–190 km. The concentration of eclogites correlates with the inflection in the Lesotho geotherm and appears to mark a disrupted layer of subducted Archean oceanic crust separating a shallower region of undeformed, refractory, Mg‐rich peridotites from deformed, Fe‐rich, more “fertile” pyrolite. This boundary may be interpreted as the “petrologic” base to the continental lithosphere.
Underlying fracture zone nature of Astrid Ridge off Antarctica's Queen Maud Land
Authors: Bergh Hugh W.
Published: February 2008

New geophysical data are used to refine the extent and setting of Astrid Ridge, a volcanic edifice lying off the Princess Astrid Coast section (5°E to 20°E) of Queen Maud Land in Antarctica. New findings presented here include a well‐defined fracture zone, occurring as a narrow basement ridge which extends northeast from Astrid Ridge, and an adjacent set of Mesozoic magnetic anomalies M0 to M9, which are offset by 120 km from an identical, previously described set. These two sets of anomalies are combined with the conjugate Mozambique Basin anomalies to define an Africa‐Antarctica rotation pole at 10°S, 29°W for anomaly M2 time. The northern edge of Astrid Ridge and the sharp southern margin of the Mozambique Ridge on the African plate are probably related to the same cessation of excess volcanic activity near the end of the Cretaceous normal polarity interval. Both features had their eastern and western limits confined by a contiguous pair of fracture zones. A possible major swing in fracture zone trend prior to anomaly M9 is also suggested.
New constraints on Aegean subduction from Seasat Altimetry
Authors: Genthon P. , Souriau M.
Published: February 2008

Crustal structure profiles relative to the Aegean fore arc area are modeled according to geoid undulations and free air gravity anomalies, taking into account the subducted slab. The models involve three layers with fixed density but a variable geometry for their boundaries. The first profile, drawn across the western (convergent) branch of the arc, exhibits a large amount of subducted crustal material, and a local trough in the Moho under the so‐called Mediterranean Ridge, superimposed on a long‐wavelength flexure increasing toward the trench. The Moho topography suggests a lithosphere with no rigidity below the Mediterranean Ridge, while the flexure associated with the subduction implies an elastic thickness of 80 km. This discrepancy leads us to assume the presence of an additional load acting downward below the Mediterranean Ridge. On the second profile, drawn across the eastern (strike‐slip) branch, there is a deep trough in the Moho, far from the topographic trench. This Moho deepening represents a large isostatic unbalance and requires an active compensation mechanism. Thus both profiles, and especially the eastern one where the strike‐slip dominant motion does not allow a proper subduction, require a downward force acting seaward of the arc. On the latter branch, we propose that gravitational instability induces the growth of a cold bulb at the base of the old and dense African lithosphere. It is noted that, through viscous coupling with the top of the lithosphere, a 115‐km bulb can produce an important collapse of the Moho and that the resulting gravitational anomalies are comparable to the observations. The growth of such a cold lithospheric bulb may be the first stage of a southward jump of the Aegean arc.
Evaluation of the 1984 Alaska Global Positioning System campaign with the Bernese GPS Software
Authors: Beutler G. , Bauersima I. , Gurtner W. , Rothacher M. , Schildknecht T. , Mader G. L. , Abell M. D.
Published: February 2008

During the summer of 1984 five TI‐4100 global positioning system (GPS) receivers were used to measure an eight‐station network in Alaska and Canada, previously measured with mobile very long baseline interferometry (VLBI) systems, with baselines ranging from 300 km to almost 2700 km. The observations have been processed in the network mode with the Bernese GPS software using orbit improvement techniques. The comparisons of the GPS and VLBI solutions, established through a seven‐parameter Helmert transformation, show an agreement of the order of 0.1 ppm for all solutions. No significant scale factor between GPS and VLBI coordinates could be detected. These results demonstrate for the first time the power of orbit improvement in conjunction with network processing of GPS phase observations.
High‐frequency seismic attenuation of oceanic P and S waves in the western Pacific
Authors: Butler Rhett , McCreery Charles S. , Frazer L. Neil , Walker Daniel A.
Published: February 2008

Analyses of oceanic P and S data from an earthquake in the Kuril Islands which occurred along the trend of a 1500‐km‐long ocean bottom hydrophone array deployment near Wake Island have yielded constraints on high‐frequency seismic attenuation in the western Pacific. Spatial attenuation and Q were determined for group velocities between 8.2 and 3.0 km/s and for frequencies between 2.5 and 22.5 Hz. There is greater attenuation of oceanic P than oceanic S at all frequencies suggesting either the presence of significant bulk attenuation in the lithosphere or that oceanic P propagates as a leaky mode. The apparent attenuation of oceanic P and S is distance‐dependent, with lower attenuation beyond 30° Δ. The apparent Q of oceanic P increases with frequency as Q ∼ ƒ0.7, from about 300 at 2.5 Hz to 1500 at 17.5 Hz. The apparent Q of oceanic S increases with frequency as Q ∼ ƒ1.1, from about 400 at 2.5 Hz to 3000 at 22.5 Hz.
Heat flow and continental breakup: The Gulf of Elat (Aqaba)
Authors: Ben‐Avraham Zvi , Von Herzen Richard P.
Published: February 2008

Heat flow measurements were made at five sites in the major basins of the Gulf of Elat (Aqaba), northern Red Sea. The gulf is located at the southern portion of the Dead Sea rift, which is a transform plate boundary. Vertical temperature gradients were measured with a probe which allows multiple penetration of the bottom during a single deployment, and thermal conductivities were determined by needle probe measurements on sediment cores. A mean heat flux of about 80 mWm−2 was found, and the values tend to increase from north to south. This latitudinal gradient corresponds to the general trend of gradual thinning of the continental crust of the gulf toward the Red Sea, where oceanic crust exists. The heat flow data, together with other geophysical data, are consistent with a propagation of rifting from the Red Sea northward along the Dead Sea rift, where the fractures are primarily shear. The gulf forms a transition between these two tectonic regimes, extensional Red Sea to the south and transform Dead Sea to the north.
Petrology and geochemistry of alkali gabbronorites from Lunar Breccia 67975
Authors: James Odette B. , Lindstrom Marilyn M. , Flohr Marta K.
Published: February 2008

Clasts of an unusual type of lunar highlands igneous rock, alkali gabbronorite, have been found in Apollo 16 breccia 67975. The alkali gabbronorites form two distinct subgroups, magnesian and ferroan. Modes and bulk compositions are highly varied. The magnesian alkali gabbronorites are composed of bytownitic plagioclase (Or2–5An82–89), hypersthene (Wo3–5En49–62), augite (Wo39–42En36–44), a silica mineral, and trace Ba‐rich K‐feldspar. The ferroan alkali gabbronorites are composed of ternary plagioclase (Or11–22An65–74), pigeonite (Wo6–9En35–47), augite (Wo38–40En29–35), Ba‐rich K‐feldspar, and a silica mineral. Trace minerals in both subgroups are apatite, REE‐rich whitlockite, and zircon. The magnesian and ferroan alkali gabbronorites appear to have formed by progressive differentiation of the same, or closely related, parent magmas; the compositional data indicate that these magmas were REE‐rich. The ternary plagioclase is probably a high‐temperature metastable phase formed during crystallization. In composition and mineralogy, the 67975 alkali gabbronorites show many similarities to Apollo 12 and 14 alkali norites, alkali gabbronorites, and alkali anorthosites, and all these rocks together constitute a distinctive alkali suite. In addition, the alkali gabbronorites show some similarities to KREEP basalts, Mg‐norites, and some felsites. These data suggest genetic links between some or all of these types of pristine rocks. Two types of relationships are possible. The first is that alkali‐suite rocks crystallized in plutons of KREEP basalt magma, and KREEP basalts are their extrusive equivalents. The second is that the alkali‐suite rocks and some felsites all crystallized in plutons of Mg‐norite parent magmas, and KREEP basalt magmas formed by remelting of these plutons. Additional studies are needed to resolve which of these hypotheses is correct.
Petrographic evidence for nonlinear cooling rates and a volcanic origin for Apollo 15 KREEP basalts
Authors: Ryder Graham
Published: February 2008

Most Apollo 15 KREEP basalts have textures indicative of linear cooling rates. Some samples have textures indicative of two‐stage cooling: 15434,8 contains orthopyroxene phenocrysts up to 2 mm long in a much finer grained, intersertal groundmass, and probably has accumulated orthopyroxene; 15404,5 has glomerocrysts of orthopyroxene and plagioclase, also in a much finer grained, intersertal groundmass; clasts in glassy breccia 15358 have abundant (20% to 40%), clear, undevitrified orange‐yellow intersertal glass (glassy intersertal textures) indicative of late quenching. Phenocrysts, by comparison with experiments on dynamic crystallization, do not grow from multiply saturated magmas (as are the Apollo 15 KREEP basalts) at linear cooling rates. Therefore the phenocryst‐bearing samples provide evidence for their initially slow cooling and later rapid cooling. Two‐stage cooling is common in volcanic, dynamic environments but cannot be expected in the static environment of crystallization of impact melt sheets, even basin‐scale ones. Therefore these Apollo 15 KREEP basalts, and by extrapolation all the other clast‐free, homogeneous, meteoritic‐siderophile‐absent Apollo 15 KREEP should be reexamined with the constraint that the Apollo 15 KREEP basalts were produced by igneous processes at 3.9 Ga, but managed to retain the trace element characteristics established at about 4.4 Ga. The common acceptance of total remelting by impact at 3.9 Ga, such as the Imbrium basin event, in the interpretation of the isotopic and chemical data is not permissible.
Mixing levels, the apennine front soil component, and compositional trends in the Apollo 15 soils
Authors: Korotev Randy L.
Published: February 2008

Variations in concentrations of lithophile elements in samples of Apollo 15 soil can be explained in terms of only four chemical components. Three components are represented by extrusive rock types found at the Apollo 15 site: mare basalt, KREEP basalt, and the pyroclastic green glass associated with breccia 15426. The fourth component, the highlands component, can be represented by soil from the Apennine Front, particularly the soil obtained from the 55–57 cm depth interval of station 2 core 15007/8. Among samples of Apennine Front soil, this one appears to be the least contaminated by mare materials and may be our most typical sample of the Apennine Front highlands. Although soil fron the Apennine Front is itself a complex mixture of many types of highlands rocks, it behaves as a single component with respect to mixing with the other three (rock type) components. The various subcomponents of the Apennine Front soil (e.g., melt rock and anorthositic norite) that have been used in some mixing models are not required as discrete components in order to explain the compositional variation among samples of Apollo 15 soil. Most of this compositional variation results from mare‐highlands mixing, i.e., Apennine Front soil with mare basalt. KREEP basalt, which probably derives from local (possibly unexposed) sources, is also a variable component of the soil and is most prevalent in the soil from stations LM, 6, and 9.
Remagnetization of the coast range ophiolite and lower part of the Great Valley Sequence in northern California and southwest Oregon
Authors: Frei Leah S. , Blake M. C.
Published: February 2008

Overprinted magnetizations have been found at four localities in the Middle Jurassic Coast Range ophiolite and the overlying Upper Jurassic and Lower Cretaceous Great Valley sequence in northern California and at one locality in the partially correlative Lower Cretaceous Days Creek Formation in southwest Oregon. At Del Puerto Canyon, on the east side of the Diablo Range, a pilot study of the upper Jurassic Lotta Creek Formation gives in situ paleomagnetic directions grouped around the present geomagnetic field, suggesting magnetic overprinting long after deposition and folding. We suggest that the loss of original magnetization could be the result of long burial at about 7 km depth, followed by Late Tertiary uplift; the possibility of chemical remagnetization, however, can not be excluded. Paleomagnetic data from sandstones from the Lower Cretaceous Great Valley sequence in the Wilbur Springs area suggest Cenozoic remagnetization that could be related to Pliocene and Pleistocene volcanic and hydrothermal activity in the area. The results from calcareous concretions in the Wilbur Springs area, from 21 sites from basalts and overlying sedimentary rocks of the Great Valley sequence at Stonyford, and from several sites in various rock types in the Coast Range ophiolite and Great Valley sequence near Paskenta suggest unblocking of magnetization during the long period of burial at depth of 7 km or more and remagnetization during tectonic uplift in the Late Cretaceous or Tertiary. Chemical remagnetization, however, can not be excluded. The paleomagnetic data from six sites in Lower Cretaceous sedimentary rocks of the Days Creek Formation in southwest Oregon, which overlies a dismembered ophiolite, fail the fold test. In situ paleomagnetic directions group near the expected Tertiary field directions. These sedimentary rocks may have been remagnetized during a major Eocene tectonic event known to have occurred in this region. The paleomagnetic results confirm that the Coast Range ophiolite and the seemingly little‐deformed Great Valley sequence in northern California, as well as correlative rocks in southwest Oregon have had a complex tectonic history. Once the nature and timing of these events are better understood, the timing of remagnetization may be further constrained, which, in turn, could give further insight into the nature of the tectonic events.
Vector constraints on western U.S. deformation from space geodesy, neotectonics, and plate motions
Authors: Minster J. Bernard , Jordan Thomas H.
Published: February 2008

The rate‐of‐slip vector on the San Andreas fault in central California estimated from geodetic and Holocene geological data (34 ± 3 mm/yr, N41°W ± 2°) is inconsistent with the prediction of rigid plate models such as RM2 (56 ± 3 mm/yr, N35°W ± 2°). This well‐known “San Andreas discrepancy” is diagnostic of plate deformation distributed both east of the fault in the Basin and Range and west of the fault along the California continental margin. We construct constraints on the integrated deformation rates across these two regions consistent with (1) the kinematical boundary conditions imposed by the rigid plate model, (2) neotectonic and paleoseismic estimates of deformation rates, (3) ground‐based geodetic measurements, and (4) rates of change observed by very long baseline interferometry along seven baselines to western U.S. sites. The space‐geodetic data on Basin and Range extension taken over a 4‐year interval are compatible with geological observations averaged over the Holocene; the best estimate of its integrated deformation rate, provided by the joint inversion of both data types, is 9.7 ± 2.1 mm/yr, N56°W ±10°, too small and in the wrong direction to account entirely for the San Andreas discrepancy. We therefore attribute much of it to deformation west of the Sierra Nevada‐Great Valley block accommodated, for example, on faults such as the San Gregorio‐Hosgri system of coastal California. The integral of this deformation, estimated by subtracting the Basin and Range contribution from the discrepancy vector, requires significant right‐lateral shear parallel to the San Andreas (13 ± 5 mm/yr) and some compression perpendicular to it (9 ± 3 mm/yr).
Terrestrial heat flow in Botswana and Namibia
Authors: Ballard Sanford , Pollack Henry N. , Skinner Neville J.
Published: February 2008

We report terrestrial heat flow measurements from 25 new sites in Botswana and Namibia. Twelve sites from the Archean Kaapvaal‐Zimbabwe Craton and the Limpopo Belt have a mean of 47 ± 4 mW m−2 (standard error of the mean) but show a trend of increasing heat flow from less than 40 mW m−2 within the cratonic nucleus to about 60 mW m−2 at its edge. Thirteen sites in the Proterozoic and Pan African mobile belts that surround the craton have a mean of 66 ± 3 mW m−2, increasing from about 60 mW m−2 at the cratonic margin to more than 70 mW m−2 several hundred kilometers away. Radiogenic heat production in near‐surface basement rocks was measured for four sites on the craton and seven sites in the mobile belts. The limited heat flow‐heat production data from the craton are insufficient to establish a correlation. Heat flow and heat production data from the mobile belts, when combined with published data from six sites in Zambia, exhibit a linear trend with an intercept of 50 ± 3 mW m−2 and a slope of 5.5 ± 1.1 km. Some but not all of the observed difference in surface heat flow between the mobile belts and the cratonic nucleus may be due to differences in crustal heat production. That part of the regional variation in surface heat flow which originates at greater depths implies a fundamental difference in thermal structure of the lithosphere between the craton and the mobile belts in southern Africa.
Preliminary bounds on the expected postclosure performance of the Yucca Mountain Repository Site, southern Nevada
Authors: Sinnock S. , Lin Y. T. , Brannen J. P.
Published: February 2008

On the basis of current data and understanding of site characteristics at Yucca Mountain, the likely performance range of a mined repository for spent nuclear fuel can be calculated. Low flux through the unsaturated zone results in groundwater travel times to the water table that probably exceed 10,000 years and may exceed 100,000 years, far longer than required by the Nuclear Regulatory Commission (NRC). The low flux will also limit releases of waste from the waste packages, probably to annual amounts less than one millionth of the mass of the waste inventory remaining 1000 years after repository closure; the corresponding releases of curies would be well within the allowable releases set by the NRC. Geochemical retardation by sorption and diffusion will slow radionuclide movement relative to groundwater flow by factors of hundreds to thousands for many waste species. In combination, these site conditions provide a high degree of confidence that no releases to the accessible environment will occur during the first 10,000 years after repository closure, the time period for which the Environmental Protection Agency (EPA) has set release limits. Carbon 14, technetium 99, iodine 129, and various nuclides of uranium sorb poorly on the tuffs along the flow paths and, together with uranium daughter products, will be the first radionuclides to arrive at the water table. The total radioactivity produced by these and later arriving contaminants will remain far below the allowable releases, even for periods of millions of years, if expected flux conditions prevail. If the flux is currently greater than the values inferred from the measured in situ moisture contents of the volcanic rocks or if it were to increase in the future, fracture flow and attendant short flow times to the water table could occur. Even if rapid fracture flow were to occur, release of wastes to the accessible environment would probably remain low with respect to the EPA's limits because diffusion of radionuclides from the fractures into the rock matrix would ensure slow migration of most of the wastes through the sorbing matrix.
Numerical experiments on permeability tensor and its application to jointed granite at Stripa Mine, Sweden
Authors: Oda Masanobu , Hatsuyama Yoshihiro , Ohnishi Yuzo
Published: February 2008

Rock masses containing a large number of geological discontinuities (called joints) are treated as homogeneous, anisotropic porous media. A permeability tensor, hydraulically equivalent to a flow network formed by joints, is formulated in terms of two tensors and a nondimensional scalar, both depending only on the geometrical aspects of joints such as the spacing, size, orientation, and aperture. Two‐dimensional numerical analyses on seepage flow through the networks support the validity of the present theory. The information about the geometry of joints, which is obtained from the ventilation drift at Stripa mine, Sweden, is interpreted to give the corresponding permeability tensor. The permeability tensor is compared with the result of the large‐scale hydraulic conductivity test, with the conclusion that it provides a reasonable prediction for the hydraulic properties of Stripa jointed granite.
Pressure‐temperature‐volume relationship for hexagonal close packed iron determined by synchrotron radiation
Authors: Huang Eugene , Bassett William A. , Tao Penguong
Published: February 2008

The equations of state of both body‐centered cubic (bcc) and hexagonal close packed (hcp) iron have been determined in a high‐temperature diamond anvil cell using synchrotron radiation as the X ray source. Four isothermal runs, 25°, 150°, 300°, and 450°C were carried out using gold as the pressure calibrant. The compressibility of bcc iron was determined up to 12 GPa at each temperature. The equation of state data for iron were found to be in reasonable agreement with measurements made by earlier diamond cell work and by the ultrasonic methods. The compressibility of hep iron was determined in the pressure range 14–24 GPa at various temperatures. By fitting the compression data at room temperature to the Birch‐Murnaghan equation, we determined the bulk modulus to be 212 ± 14 GPa assuming Ko′ = 4. Volume thermal expansion of hep iron is determined to be 1.001, 1.011, and 1.021 (±0.005) at 150°, 300°, and 450°C, respectively, in this pressure range. The da ratios of the hep phase remained constant at 1.60 ± 0.02 up to 300°C in this experiment.
Crack geometry in igneous rocks: A maximum entropy inversion of elastic and transport properties
Authors: Doyen Philippe M.
Published: February 2008

The maximum entropy method is used to infer the dimensions of cracks in igneous rocks from measurements made under confining pressure of their compressibility, d.c. electrical conductivity, and hydraulic permeability. The fluctuations of crack dimensions are characterized by a crack spectrum which gives the frequency of occurrence of cracks as a function of their cross‐sectional length and aspect ratio in the absence of stress. The cracks deform elastically and close under pressure. The resulting variations of the bulk properties are obtained by averaging the pressure‐dependent crack contributions over the unknown crack spectrum. The compressibility depends on the average of the square of the crack cross‐sectional length. The transport coefficients are proportional to some characteristic crack conductances which are expressed self‐consistently at each pressure in terms of the spectrum. The maximum entropy crack spectrum in Westerly granite is determined from laboratory measurements of the elastic and transport coefficients as a function of pressure. The predicted spectrum is comparable with the distribution of dimensions estimated from direct observations of cracks with a scanning electron microscope. The wide range of crack lengths contrasts with the existence of a constant crack width. The main features of the calculated spectrum are consistent with a model of growth of the microfissures during cooling of the granite.
Rapid soft bed sliding of the Puget Glacial Lobe
Authors: Brown N. E. , Hallet B. , Booth D. B.
Published: February 2008

The Puget lobe of the late Pleistocene Cordilleran ice sheet advanced into western Washington over a thick sequence of unlithified sediments. The basal drag due to sliding over this surface was calculated by (1) idealizing the sediment bed as a rigid planar surface scattered with roughness elements corresponding to individual sedimentary particles and (2) taking the minimum reconstructed sliding velocity to be 500 m yr−1. The calculated basal drag, minimized by choices of parameters, is 400 kPa, much higher than the reconstructed gravitational driving stress of 40 kPa, indicating that a rigid bed and low water pressure are not consistent with the glacier's rapid motion. High subglacial water pressure, averaging 90% of the ice overburden (10 MPa) is inferred from overconsolidation of subglacial clays. The occurrence and deformation of water‐deposited sediments within the till and the requirement to drain large quantities of water from the interface suggest that water pressure reached 99% of the ice overburden. Ploughing of till by ice‐entrained clasts, pervasive shearing, and, finally, ice‐bed separation become possible as water pressure increases. At the low inferred effective normal stress the basal drag is reduced from the rigid bed case by water layers which decouple the smallest particles from the glacier sole and ploughing which reduces the resistance offered by the largest particles. The limited shear strain observed in much of the till implies pervasive shearing did not contribute significantly to ice motion; basal motion was confined to the ice‐bed interface or to distinct faults within the substrate.
Propagation of a glacier surge into stagnant ice
Authors: Raymond Charles , Johannesson Tomas , Pfeffer Tad , Sharp Martin
Published: February 2008

The propagation of the surge of Variegated Glacier into its terminal lobe was observed by daily surveying of a longitudinal line of closely spaced markers extending up glacier from a “stagnant zone” of thin (≈40 m), nearly motionless ice near the terminus, across a “front zone,” to a “surge zone” of thicker (≈100 m), rapidly moving (>20 m d−1) ice above. Within the front zone there were large angles of upward motion (>30°), compressive longitudinal strain rates (>0.1 d−1), and upward vertical velocities (>5 m d−1). Within the surge zone the ice motion was nearly parallel to the ice surface, and longitudinal gradients were small. The deformation pattern produced microcracking, exfoliation, and buckling of surface ice, longitudinal cracking, longitudinal chasms, and an increase in glacier volume. The front zone propagated at an average speed of about 40 m d−1 or roughly twice the surge speed of the ice above the front. This factor is determined by the thickness jump across the front. Finite element modeling constrained by measured geometry and surface velocity was used to deduce the velocity and stress at depth. The velocity in the surging zone occurs almost entirely by sliding. The sliding rate drops abruptly beneath the center of the front zone to zero in the stagnant zone. A comparison of basal velocity and shear stress distributions indicates a highly “lubricated” area beneath the upper part of the front zone and a “locked” area beneath the lower part of the front zone. Normal stress at the bed is not significantly different from overburden, and there is no high compressive normal stress on the bed to act as a pressure dam to obstruct water flow through the front zone.
Deep Sea Drilling Project heat flow and models of the rifted West Rockall Margin
Authors: Hyndman R. D. , Roberts D. G.
Published: February 2008

Downhole thermal measurements for the southwest Rockall Bank rifted margin made on Deep Sea Drilling Project leg 81, plus those previously made on leg 48, give six reliable heat flow values in the narrow range 62–71 mW m−2, with an average of 66.5 mW m−2. The temperatures were obtained using a self‐contained downhole temperature‐recording device. A long probe is inserted through the drill bit into the undisturbed sediments below the bottom of the hole at intervals during drilling. The thermal disturbance from the penetration was extrapolated to equilibrium in situ temperatures using the cylindrical decay function. The thermal conductivities have been measured on core samples using the needle probe method supplemented with porosity‐conductivity relations. Heat production in the sedimentary‐volcanic sequence above basement probably makes a negligible contribution to the heat flow. There is no systematic heat flow trend with water depth, which ranges from 1650 to 1900 m, or with position, and the mean value is not significantly different from that expected from pure oceanic crust of age equal to the approximately 50‐Ma time of rifting. However, the unloaded basement depth is over 1 km shallower than expected for normal oceanic crust of the 50‐Ma rifting age. The heat flow combined with subsidence data fits a simple rifting model with crustal stretching and thinning by a factor of 3 or 4 over a zone at least 100 km wide, occurring about 50 Ma. The heat production in the stretched crust is taken to contribute 4 mW m−2, based on a prerift structure comparable to once adjacent North America. A major uncertainty in thermal and subsidence models for the margin is the effect of the earlier “Iceland hot spot” which produced extensive margin volcanism on the northern British Isles and Greenland and shoaling of the oceanic crust at about the time of west Rockall rifting.
Analysis of two‐color geodimeter measurements of deformation within the Long Valley Caldera: June 1983 to October 1985
Authors: Langbein John , Linker Mark , Tupper David
Published: February 2008

Line length changes from several baselines in a trilateration network within the Long Valley caldera clearly define a decrease in strain rate from June 1983 through October 1985. The data consist of more than 1600 length measurements on 23 baselines using a two‐color geodimeter, which has a precision of 0.2 ppm of the line length. Initial measurements made during the summer of 1983 show extension rates were as high as 6 ppm/yr on some baselines. The rates subsequently decreased steadily, to less than 1 ppm/yr during the summer of 1985. Ih contrast, the strain rates on a few other baselines were less than 0.2 ppm/yr averaged over the 2.3‐year interval. A model is constructed using these observations as well as yearly observations of a trilateration network within and near the caldera. The model contains two points of inflation located at 5 and 10 km beneath the resurgent dome of the caldera plus dextral slip on a fault plane within the south moat within an elastic half‐space. To mimic the observed time dependence in the two‐color geodimeter data, we conclude that the rates of both slip and inflation have steadily decreased over the survey interval. Because the two‐color geodimeter network is closer to all three sources than the single‐color geodimeter network, these data are important for defining the factor of 2–3 rate decrease of slip and inflation. Comparing our estimates of slip and inflation with similar estimates for the year prior to July 1983, we conclude that the rate decrease is probably a result of postseismic and postinflation relaxation following the earthquake swarm in the south moat in January 1983.
An aeromagnetic study of Mount St. Helens
Authors: Finn Carol , Williams David L.
Published: February 2008

Aeromagnetic data from surveys flown by the U.S. Geological Survey over Mount St. Helens, Washington, before and after the climactic May 18, 1980, eruption were used to determine the bulk magnetic properties of the volcano and to delineate a buried source. We assumed that most of the edifice of preeruption and posteruption Mount St. Helens has a magnetization direction near the present earth's field of 69° inclination and 20° declination and calculated its intensity as 4.1 A/m. After subtraction of magnetic anomalies due to topogaphy magnetized with this direction and intensity, the preeruption and posteruption surveys revealed nearly identical residual magnetic highs and lows, indicative that their sources were not altered or removed by the May 1980 eruption. The residual highs were explained by a 200‐m‐deep source lying mostly within the edifice of Mount St. Helens. The source could be terrain that predates Mount St. Helens, such as a buried ridge or a cone or a valley filled with lava. We calculated the magnetization of the material removed by the May 1980 eruption and found its intensity to be 4.2 A/m in a direction near the present earth's field, similar to that assumed for the volcano as a whole. This similar result confirmed the validity of the magnetization vector assumed for the entire edifice.
Studies of the seismic coda using an earthquake cluster as a deeply buried seismograph array
Authors: Spudich Paul , Bostwick Todd
Published: February 2008

Loosely speaking, the principle of Green's function reciprocity means that the source and receiver positions in a seismic experiment can be exchanged without affecting the observed seismograms. Consequently, the seismograms observed at a single observation location o and caused by a cluster of microearthquakes at locations {ei} are identical to the time series that would be measured by an array of stress meters emplaced at positions {ei}, recording waves generated by a source acting at o. By applying array analysis techniques like slant stacking and frequency‐wave number analysis to these seismograms, we can determine the directions and velocities of the component waves as they travel in the earthquake focal region rather than at the surface. We have developed a computationally rapid plane‐wave decomposition which we have applied to single‐station recordings of aftershocks of the 1984 Morgan Hill, California, earthquake. The analysis is applied to data from three seismic stations having considerably different site geologies. One is a relatively hard rock station situated on Franciscan metamorphics, one is within the Calaveras fault zone, and one is on semiconsolidated sand and gravels. We define the early coda to be the part of the coda initiating immediately after the direct S wave and ending at twice the S wave lapse time. The character of the S wave and early coda varies from being impulsive at the first station to highly reverberative at the last. We examine waves in sequential time windows starting at the S wave and continuing through the early part of the coda. At all seismic stations the early coda is dominated by a persistent signal that must be caused by multiple scattering, probably within 2 km of each seismic station. Despite clear station‐to‐station differences in the character of the early coda, coda Q values measured in the late coda (greater than twice the S lapse time) agree well among stations, implying that the mechanisms causing the varying behavior of the early coda do not control the coda decay rate at the stations we have considered. Coda Q values measured on horizontal components of motion agree within a factor of 2 with those measured on vertical components. We have not been able to determine the composition of the late coda because of a low signal‐to‐noise ratio. Our analysis technique, however, is quite appropriate for such a task.