bluuebunny/crossref_metadata_2025 huggingface dataset:
Geoid anomalies in the vicinity of subduction zones
Authors: McAdoo David C.
Published: February 2008

The regional geoid of the southwest Pacific is matched reasonably well by results from a model of the upper mantle density structure (including slabs) associated with subduction zones of the region. Estimates of the geoid are obtained from Geos 3 and Seasat radar altimeter data. These data are very well suited to the task of detecting intermediate wavelength (600–4000 km) geopotential variations, which contain the dominant signature of subducting slabs. Gravimetric profiles across trench/island arc complexes resolve primarily short wavelengths. The model represents subducting slabs as thin surfaces of anomalous mass per unit area. These surfaces are positioned using published seismicity results which detail the configuration of the Benioff zones. Crustal effects are ignored. Effects due to the contrast between the young thermal lithosphere of the behind arc regions (marginal basins) and the older lithosphere seaward of the trench are modeled, Results indicate that the New Hebrides slab possesses an average areal density anomaly of about 3 × 105 g cm−2. This is about 3 times that which is estimated for the Tonga‐Kermadec slab. Both of these inferred density anomalies are somewhat less (∼10 and 65%) than anomalies predicted by a thermal model. Consistent with the results of other studies, additional modeling suggests that slabs worldwide may be an important source of large, long‐wavelength gravity highs; i.e., they may contribute substantially to geopotential power of harmonic degree as low as three or four (subduction, geoid anomalies, slab, lithosphere).
Melt segregation from partially molten source regions: The importance of melt density and source region size
Authors: Stolper Edward , Walker David , Hager Bradford H. , Hays James F.
Published: February 2008

The compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals. Consequently, the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth. The increasingly olivine‐normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth. Once vertical permeability is established by melt generated during partial melting, buoyancy‐driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale. Limits to this process are provided by cooling of the source region (freezing melt in) and rigidity of the crystalline matrix (mechanically trapping melt). Source region size influences these limits strongly: consequently, small, partially molten diapirs (∼km in diameter) may be able to trap large melt fractions (≳30%), but larger source regions would be unable to do so. The reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation. Diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent; this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma. Melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets—∼500 km on the moon, ∼250 km on Mars, ∼100 km on earth—correspond to similar pressures (25–35 kbar); at greater pressures, melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy. This may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time.
Tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts
Authors: Verosub Kenneth L. , Moores Eldridge M.
Published: February 2008

Paleomagnetic samples of many Deep‐Sea Drilling Project (DSDP) basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid‐ocean ridges. Rather than arising from peculiarities of the geomagnetic field, the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes. Several lines of evidence support this conclusion: observations of the Basin and Range Province, an extensional tectonic regime occurring in continental crust; observations of the Troodos complex, Cyprus, an oceanic extensional tectonic regime now exposed on land; and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid‐oceanic ridges. The tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30° to 50°, although rotations as large as 70° to 90° are not uncommon. The paleomagnetic consequences of these rotations can be calculated for DSDP basalts if the original strike and latitude of a ridge are known. At four DSDP sites for which adequate data are available, namely sites 319, 332, 410, and 417, rotations ranging from 50° to 70° are required to explain the paleomagnetic results. If the original strike and location of the ridge are not known, as is the case for most of the older parts of the Pacific Ocean, the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations. This information, combined with a similar but independent relationship derived from the marine magnetic anomalies, puts severe constraints on the original strike and location of the ridge.
Calibration validation for the GEOS 3 altimeter
Authors: Martin C. F. , Kolenkiewicz R.
Published: February 2008

The absolute bias calibration for the GEOS 3 intensive mode altimeter has been measured by using two satellite passes whose ground tracks were within 1 km of the Bermuda laser station. The Bermuda laser tracked on the two passes—rev 4553 on February 25, 1976, and rev 5471 on April 30, 1976—and was supported by two other NASA lasers on one pass and by the NASA Spacecraft Tracking and Data Network on the other pass. For each pass, the altimeter data around Bermuda was smoothed and extrapolated to the point closest to overhead at the laser site. This point was used for calibration, eliminating almost entirely the effects of geoid model error on the resulting altimeter bias estimate. After correcting for tide heights and sea state effects, the two passes give calibration biases which are in agreement to within 3 cm and have a weighted mean of −5.61 ± 0.16 m for correcting altimeter measurements to the center of mass of the spacecraft (i.e., including the antenna tracking point correction). For data correction, this bias must be subtracted from the measured values. Since a sea state bias correction has been used in the bias estimation, a different bias is more appropriate for data users not employing a sea state bias correction. For such users, a bias of −5.51 m, appropriate for moderate seas (H⅓ ≅ 2 m), is recommended. It was found impossible to reconcile the two calibration passes, as well as a set of altimeter crossovers in the middle of the GEOS 3 calibration area, without allowing for a data time tag error. On the basis of a selected set of four crossovers and an assessment of probable sources of timing error, it was concluded that one interpulse period (10.24 ms) should be added to the data time tags. This time tag correction should be used with the above bias value.
Simultaneous inversion for hypocenters and lateral velocity variations: An iterative solution with a layered model
Authors: Hawley B. W. , Zandt G. , Smith R. B.
Published: February 2008

An iterative inversion technique has been developed that uses the direct P and S wave arrival times from local earthquakes to compute simultaneously a three‐dimensional velocity structure and relocated hypocenters. Crustal structure is modeled by subdividing flat layers into rectangular blocks. An interpolation function is used to smoothly vary velocities between blocks, allowing ray trace calculations of travel times in a three‐dimensional medium. Tests using synthetic data from known models show that solutions are reasonably independent of block size and spatial distribution but are sensitive to the choice of layer thicknesses. Application of the technique to observed earthquake data from north‐central Utah shows the following: (1) lateral velocity variations in the crust as large as 7% occur over 30‐km distances, (2) earthquake epicenters computed with the three‐dimensional velocity structure were shifted an average of 3.0 km from locations determined assuming homogeneous flat layered models, and (3) the laterally varying velocity structure correlates with anomalous variations in the local gravity and aeromagnetic fields, suggesting that the new velocity information can be valuable in acquiring a better understanding of crustal structure.
Deep volcanic tremor and magma ascent mechanism under Kilauea, Hawaii
Authors: Aki Keiiti , Koyanagi Robert
Published: February 2008

Deep harmonic tremor originating at depths around 40 km under Kilauea was studied using records accumulated since 1962 at the Hawaii Volcano Observatory of the U.S. Geological Survey. The deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island‐wide network of seismometers. The period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode. The tremor appeared to be primarily composed of P waves. We interpret the observed period and amplitude in terms of the stationary crack model of Aki et al. (1977) and find that the seismic moment rates for deep tremors are considerably larger than those for shallow‐tremors suggesting more vigorous transport for the former. We propose a kinematic source model which may be more appropriate for deep tremor. According to this model, a measurable quantity called ‘reduced displacement’ is directly proportional to the rate of magma flow. A systematic search for deep tremor episodes was made for the period from 1962 through 1979, and the amplitude, period, and duration of the tremor were tabulated. We then constructed a cumulative reduced‐displacement plot over the 18‐year period. The result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface. The total magma flow estimated from the reduced displacement is however, one order of magnitude smaller than that estimated by Swanson (1972). It may be that most channels transport magma aseismically, and only those with strong barriers generate tremor.
Venus gravity: A high‐resolution map
Authors: Reasenberg R. D. , Goldberg Z. M. , MacNeil P. E. , Shapiro I. I.
Published: February 2008

The Doppler data from the radio tracking of the Pioneer Venus Orbiter (PV0) have been used in a two‐stage analysis to develop a high‐resolution map of the gravitational potential of Venus, represented by a central mass and a surface mass density. The two‐stage procedure invokes a Kalman filter‐smoother to determine the orbit of the spacecraft, and a stabilized linear inverter to estimate the surface mass density. The resultant gravity map is highly correlated with the topographic map derived from the PV0 radar altimeter data. However, the magnitudes of the gravity variations are smaller than would be expected if the topography were uncompensated, indicating that at least partial compensation has taken place.
Viscosities in the system albite‐anorthite
Authors: Cranmer D. , Uhlmann D. R.
Published: February 2008

Viscosities of several compositions in the albite‐anorthite system (Ab80An20, Ab50An50, and Ab25An75) were measured over a wide range of temperature (800°–1600°C). The log η versus 1/T relations for all compositions show continuous curvature. In the high‐temperature region (T>1200°C) the viscosity increases monotonically with increasing Ab content, but the change of viscosity with temperature increases with increasing An concentration. Hence the viscosity‐temperature relations show a region of crossover, and at sufficiently low temperatures (T≲750°C) the viscosity increases with increasing An content. The predictions of models based on free volume theory and the semiempirical models of Bottinga and Weill (1972) and Shaw (1972) are in close accord with the data (within ± 0.2 in log10η in nearly all cases).
Calderas in the Precambrian terrane of the St. Francois Mountains, southeastern Missouri
Authors: Sides J. Ronald , Bickford M. E. , Shuster R. D. , Nusbaum R. L.
Published: February 2008

Precambrian igneous rocks in the St. Francois Mountains of southeastern Missouri consist of silicic volcanic rocks, mostly ash flow tuff, which crop out mostly in the western and southwestern part of the region, and epizonal granitic plutons which are mostly exposed in the northeastern part of the area. These relations suggest that the exposures in the northeast are of a somewhat deeper part of the crust and that rocks there are now exposed because of west‐southwesterly tilting and beveling by erosion. In the eastern St. Francois Mountains the occurrence of a thick rhyolitic ash flow tuff, Grassy Mountain ignimbrite, and its distribution relative to a large, high‐silica pluton, Butler Hill granite, suggest that the ash flow is the principal eruptive material of a caldera, the Butler Hill caldera, and that Butler Hill granite is the frozen magma chamber that was originally beneath it. Structural relations indicate that Butler Hill granite was emplaced into the volcanic rocks of the caldera during resurgent doming. Four small low‐silica plutons, Silvermine, Stono, Knoblick, and Slabtown granites, are distributed peripherally to the Butler Hill granite and are interpreted as ring plutons emplaced along the ring fracture of the caldera or along arcuate intracaldera faults, after resurgent doming and emplacement of Butler Hill granite. More than 3 km of silicic volcanic rocks occupy a basinal structure in the western St. Francois Mountains. These rocks lie with angular unconformity upon up‐domed older rocks of the Butler Hill caldera along an arcuate zone of megabreccia and disturbed structure on the western margin of the older caldera. This feature is interpreted as the topographic wall of a second caldera which lies to the west. The occurrence of the thick Taum Sauk rhyolite stratigraphically about 3 km higher than the rocks involved in the megabreccias may indicate that there was yet another caldera formed in this western part of the St. Francois Mountains.
Phase relationships of S‐type granite with H2O to 35 kbar: Muscovite granite from Harney Peak, South Dakota
Authors: Huang W. L. , Wyllie P. J.
Published: February 2008

Muscovite granite (13.8% muscovite, 4.8% normative corundum) was reacted, with varying percentages of H2O, in cold‐seal vessels at 2 kbar and in piston‐cylinder apparatus between 10 and 35 kbar. The diagrams illustrating melting/crystallization relationships are: P‐ T sections with both excess H2O and with no H2O added (0.66% H2O in rock); T‐XH2O sections at 15 kbar and 25 kbar showing H2O‐undersaturated conditions; the H2O‐undersaturated surface for the crystallization of quartz/coesite (small amounts of aluminosilicate minerals persist to higher temperatures). Glass compositions measured by electron microprobe from samples with 5% H2O at 15 kbar confirm that liquids are syenitic through at least 100°C above the solidus, as predicted from the effect of pressure on the Residua System. Results are explained successfully by phase relationships involving muscovite, quartz, and orthoclase in K2O‐Al2O3‐SiO2‐H2O, with reactions depicted in a Ptotal‐PeH2O‐T model, with special reference to the divariant surfaces in the region PeH2O < Ptotal. With reduced PeH2O (or aH2O), produced either by small amounts of H2O (and H2O‐undersaturation) or by CO2 + H2O mixtures, subsolidus dehydration reaction temperatures decrease, vapor‐present solidus temperatures increase, and muscovite stability in presence of liquid increases. In general, muscovite, biotite, and amphibole can be precipitated from magmas containing only a few tenths per cent H2O (although the H2O‐undersaturated liquids coexisting with crystals may contain 3% or more dissolved H2O). This particular granite cannot be a primary magma from mantle or subducted oceanic crust. It is a possible product of partial fusion of pelitic rocks between about 20 km and 40 km depth given sufficient H2O, and xenocrystal muscovite or sillimanite from the source rocks. The phase relationships are consistent with the idea of S‐type granites, but not sufficient to prove the origin of this rock. Additional tests require phase relationships of other associated granitic rocks, and details of geochemistry, geophysics, and field relationships.
Manaslu leucogranite: A collision signature of the Himalaya: A model for its genesis and emplacement
Authors: Le Fort Patrick
Published: February 2008

The Manaslu pluton is one of 10 leucogranites that formed in the overthrusted Higher Himalaya after the Indo‐Eurasian collision. Field and analytical data indicate that the underlying migmatites of the Tibetan Slab may be where the leucogranitic melts were generated. The Himalayan crustal thrusting of a hot slab over a rather cold volcano‐sedimentary pile [Le Fort, 1975a] provides the necessary release of fluids. These fluids cross the Main Central Thrust (MCT), induce the partial anatexis of the overheated Tibetan Slab, and produce a leucogranitic magma. The emplacement of the magma at first is located along the main disharmonic plane above the MCT, between the infrastructure and the superstructure. There it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities. Progressive emplacement of the granite proceeds as the convected fluids, including the fluids released by the saturated magma, dissolve the mainly calcareous host rocks of Tibetan sedimentaries. A frozen image of this ‘caving out’ progression is given by the extensive network of granitic dikes outside the pluton. The two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision. This two‐fold model, dominated by fluid activity, may be of importance for other leucogranites and granites.
Seismic reflections from the basal contacts of batholiths
Authors: Lynn Heloise B. , Hale Laura D. , Thompson George A.
Published: February 2008

Although seismic reflection data in batholithic terranes are scarce, datasets available to us from three different regions show surprising similarities. All three profiles, one across Precambrian rocks in Texas, and two across Mesozoic batholiths in California and Nevada, show strong subhorizontal reflections at depths of 6–10 km. These reflections are interpreted to originate from the base of the batholiths, and indicate that these batholiths are tabular in shape. The large amplitudes of the reflections require a marked contrast in acoustic impedance (product of velocity and density) between the batholith and the underlying rocks. Two of the profiles show reflective zones having appreciable thicknesses (0.5–2.5 km), while the third profile shows a single continuous reflection. The four most favored candidates for sub‐batholithic reflections are (1) underplated gabbro, (2) cumulate layers, (3) migmatites, and (4) thrust faults. Cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance.
Marine geology of the Hess Rise: 1. Bathymetry, surface sediment distribution, and environment of deposition
Authors: Nemoto Kenji , Kroenke Loren W.
Published: February 2008

New charts of bathymetry, acoustic character, and sediment distribution describe the Hess Rise, a large oceanic plateau in the central north Pacific. Discrete physiographic provinces on the Hess Rise are the High Plateau, shallower than 3900 m, trending N30°W; the Northeastern Flank, a smooth, gentle slope gradually increasing in depth to the northeast; the Woollard Abyssal Plain, extending farther to the northeast; the Volcanic Province with its high peaks and ridges along the southern margin of the Hess Rise; the Mendocino Fracture Zone to the south, expressed by broad, planar seafloor regions bordered by ridges and scarps; the Western Steps, formed by structural benches on the western side of the Rise; and the Emperor Deep, between the rise and the Emperor Seamounts. Five types of acoustic units have been mapped and interpreted: a transparent layer, predominantly of biosiliceous pelagic clay; a stratified layer, predominantly of nannofossil ooze; a diffuse layer of debris flows that seem to have originated mostly in the Volcanic Province; an opaque horizon commonly formed of volcaniclastic sediments that are usually found on the seafloor of the Mendocino Fracture Zone; and a hyperbolic horizon, indicating outcrops of igneous rock. The pronounced effect of bottom currents on the present‐day environment of deposition in the Hess Rise is evidenced by the presence of the opaque horizon, which is interpreted as an erosion surface, and by current moating, abrupt thinning of surface layers and truncation of subbottom reflectors. The widespread erosion on the seafloor of the Mendocino Fracture Zone is attributed to the flow of Antarctic bottom water.
On measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method
Authors: Edwards R. N. , Law L. K. , DeLaurier J. M.
Published: February 2008

Magnetotelluric sounding reveals a conductive zone beneath the Pacific Ocean at depths in excess of 60 km but does not resolve the resistivity of the lithosphere above this zone. Further resolution can be obtained by controlled source electrical methods. The simplest of these are the galvanic techniques. Dipole‐dipole resistivity sounding is not suitable because dipole separations of thousands of kilometers would be required to obtain values of the resistivity. A viable alternative is to measure on the ocean floor the magnetic field of a vertical bipolar source extending from the sea surface to the seafloor. Magnetometer transmitter separations of only a few kilometers are sufficient to determine the resistivity of a half space beneath the ocean. Sounding curves similar to those of the resistivity method may be constructed to resolve the resistivity of a layered lithosphere. The curves constructed are valid at alternating frequencies which are small compared with a skin frequency not in the ocean but in the lithosphere. The depth of penetration is of the order of half the transmitter‐receiver separation. Magnetic field amplitudes are in the range of picotesla for reasonable lithospheric resistivities and separations up to 10 times the length of the bipole. Modern instrumentation, modified for the ocean floor, can detect such signals at a range of 20 km at a frequency of about 0.02 Hz averaged over several hours.
An experimental study of magnetic viscosity in synthetic monodomain titanomaghemites: Implications for the magnetization of the ocean crust
Authors: Özdemir Özden , Banerjee Subir K.
Published: February 2008

A study of magnetic viscosity in well characterized synthetic monodomain titanomaghemites of composition Fe2.4Ti0.6O4 is described. It is found that the samples are capable of acquiring a viscous remanent magnetization (VRM) only when they are highly oxidized. The same assembly does not show significant magnetic viscosity for 0.1 < z < 0.6 (where z is the oxidation degree). This is a result of their being in the thermally stable monodomain state with high anisotropies. The observed increase in VRM for the highly oxidized material may be entirely due to the decrease in magnetic anisotropy. A decrease in the total number of Fe2+ as a functin of oxidation results in a decrease in coercive force, i.e., magnetic anisotropy of the initial monodomain titanomaghemites. It is thought that the dramatic decrease in magnetic anisotropy at room temperature may be the main cause of the observed increase in VRM for the highly oxidized material. Finally, we conclude that the ability of the magnetic layer of the ocean crust to acquire a VRM increases as the maghemitization degree increases.
A comparison of the magnetic properties of synthetic titanomaghemites and some oceanic basalts
Authors: Moskowitz Bruce M. , Banerjee Subir K.
Published: February 2008

The variation of some magnetic properties of (1) synthetic titanomaghemites and (2) oceanic basalts from DSDP site 417 D as a function of grain size and oxidation is presented. The synthesis of the initial titanomagnetite (x = 0.6) was carried out, using controlled fugacity and self‐buffering techniques. Single‐phase titanomaghemites were produced by heating the titanomagnetite in air at low temperatures (<350°C). The magnetic properties studied on the synthetic titanomagnetite (grain size ∼ 1 μm) show the following variations with increasing degrees of oxidation (0 ≤ z ≤ 0.6): (1) Curie temperature (Tc) increases, (2) saturation magnetization (Js) at room temperature increases, (3) Js measured at 77°K decreases for z ≲ 0.3 and then increases for 0.3 < z ≲ 0.6, (4) bulk and remanent coercivities (Hc and Hr) increase slightly initially (z < 0.2) and then decrease at both room temperature and 77°K, (5) susceptibility (χ0) decreases for z ≲ 0.3 and then increases, (6) the magnetic viscosity acquisition and decay coefficients (Sa and Sd) increase and (7) the median destructive fields for an anhystretic remanent magnetization and a viscous remanent magnetization increase for z ≲ 0.3 and then decrease. These magnetic data also suggest that the synthetic titanomaghemites are pseudo‐single domained (PSD) and with increasing degrees of oxidation these PSD grains become more SD‐like. The basalt samples from site 417D are divided into fine‐grained pillow basalts and coarse‐grained massive flows. Most of the magnetic properties measured are distinctly different between these two groups. The magnetic data indicate that the domain states of these basalts are probably PSD. Although it is difficult to separate the effects of grain size and oxidation on the magnetic properties of these basalts, it appears that grain size effects predominate. A preliminary investigation of the magnetic viscosity of the synthetic and oceanic titanomaghemites suggests that a combination of thermally activated and diffusive magnetic viscosity is responsible for their viscous behavior. Finally, we will discuss the roles of grain size and oxidation variation in oceanic basalts and their separate and combined effects on their magnetic behavior.
Paleomagnetic correlation of Columbia River basalt flows using secular variation
Authors: Bogue Scott W. , Coe Robert S.
Published: February 2008

A statistical method is developed to evaluate stratigraphic correlations that are based on the similarity of paleomagnetic directions. It involves comparison of the probabilities that such directions could have arisen by either simultaneous or random samplings of the ancient magnetic field. To calculate these probabilities one must estimate the effects of paleomagnetic errors and paleosecular variation, respectively. The method is tested successfully on sections of 8–10 Grande Ronde Basalt (Columbia River Basalt Group) flows at Sentinel Gap and Umtanum Ridge, which are separated by only 25 km and contain two flows thought to be the same on geological grounds. More significantly, for widely separated sections at Sentinel Gap and Butler Canyon the analysis again shows that simultaneous magnetization is a much more likely explanation of the general similarity of paleomagnetic directions than chance agreement of ancient field directions. The simplest interpretation of this result is that many of the flows at the two sites are the same, implying that series of Grande Ronde flows extended over distances of several hundred kilometers.
The inverse problem of constructing a gravimetric geoid
Authors: Zlotnicki Victor , Parsons Barry , Wunsch Carl
Published: February 2008

Computation of a single geoidal height from gravity acceleration data formally requires that the latter be known everywhere on the earth. We present a computational procedure based on linear inverse theory for estimating geoidal heights from incomplete sets of data. The same scheme can be used to estimate gravity accelerations from altimetry‐derived geoids. The systematic error owing to lack of data and the choice of a particular inverse operator is described by using resolution functions and their spherical harmonic expansions. An rms value of this error is also estimated by assuming a spectrum for the unknown geoid. The influence of the size of the data region, the spacing between data, the filtering applied to the data, and the model weighting function chosen are all quantified in a spherical geometry. The examples presented show that when low degree spherical harmonic coefficients are available—from satellite orbit analysis—a band‐passed version of the geoid can be constructed from local gravity data, even with a relatively restricted data set.
The Snake River Plain, Idaho: Representative of a new category of volcanism
Authors: Greeley Ronald
Published: February 2008

Studies of the volcanic geology of the Snake River Plain, Idaho, and comparison with other basaltic regions suggest a new category of volcanic activity, termed basaltic plains volcanism. Typified by the Snake River Plain, this style of volcanism is intermediate between basaltic flood (or plateau) eruptions and Hawaiian Volcanism. Characteristics that are common to both Hawaiian and plains volcanism are: multiple lava flow units which erupt primarily from point sources, formation of low shields, and frequent emplacement through lava tubes or channels. Characteristics that are common to both flood basalts and plains volcanism are: high volume flows, vents aligned along rift zones, and planar surfaces. The recognition of plains volcanism in other areas provides a means to interpret the style of eruption and volcanic history.
Hydrothermal changes related to earthquake activity at Mud Volcano, Yellowstone National Park, Wyoming
Authors: Pitt A. M. , Hutchinson R. A.
Published: February 2008

The Mud Volcano hydrothermal area in Yellowstone National Park is near the intersection of a 20‐km‐long zone of northeast trending normal faults with the eastern resurgent dome within the 600,000‐year‐old Yellowstone caldera. Recent crustal uplift along the northeast trending axis of the caldera is at a maximum (700 mm since 1923) near the Mud Volcano area. From 1973 through April 1978, less than 10 earthquakes (largest M 2.4) were located within 3 km of the Mud Volcano area. In May 1978, earthquakes began occurring beneath the hydrothermal area at depths of 1 to 5 km. The seismic activity continued until the end of November with intense swarms (100 events per hour) occurring on October 23 and November 7. The largest event (M 3.1) occurred on November 14 and at least 8 events were M 2.5 or larger. In December 1978, heat flux in the Mud Volcano hydrothermal features began increasing along a 2‐km‐long northeast trending zone. Existing mud cauldrons became more active, new mud cauldrons and fumeroles were formed, and vegetation (primarily lodgepole pine) was killed by increased soil temperature. The increase in heat flux continued through July 1979 then gradually declined, reaching the early 1978 level by June 1980. The spatial and temporal association of earthquakes and increased hydrothermal activity at Mud Volcano suggests that the seismic activity expanded preexisting fracture systems, premitting increased fluid flow from depths of several kilometers.
Interpretation of shallow electrical features from electromagnetic and magnetotelluric surveys at Mount Hood, Oregon
Authors: Goldstein N. E. , Mozley E. , Wilt M.
Published: February 2008

A magnetotelluric survey, with a reference magnetometer for noise cancellation, was conducted at accessible locations around Mount Hood, Oregon. Thirty‐eight tensor magnetotelluric (MT) and remote telluric stations were set up in clusters around the volcano except for the northwest quadrant, a wilderness area. Because of limited access, station locations were restricted to elevations below 1829 m, or no closer than 5 km from the 3424‐m summit. On the basis of the MT results, three areas were later investigated in more detail using a large‐moment, controlled‐source electromagnetic (EM) system developed at Lawrence Berkeley Laboratory and the University of California at Berkeley. One‐dimensional interpretations of EM and MT data on the northeast flank of the mountain near the Cloud Cap eruptive center and on the south flank near Timberline Lodge show a similar subsurface resistivity pattern: a resistive surface layer 400–700 m thick, underlain by a conductive layer with variable thickness and resistivity of <20 ohm m. It is speculated that the surface layer consists of volcanics partially saturated with cold meteoric water. The underlying conductive zone is presumed to be volcanics saturated with water heated within the region of the central conduit and, possibly, at the Cloud Cap side vent. This hypothesis is supported by the existence of warm springs at the base of the mountain, most notably Swim Warm Springs on the south flank, and by several geothermal test wells, one of which penetrates the conductor south of Timberline Lodge. The MT data typically gave a shallower depth to the conductive zone than did the EM data. This is attributed, in part, to the error inherent in one‐dimensional MT interpretations of geologically or topographically complex areas. On the other hand, MT was better for resolving the thickness of the conductive layer and deeper structure. The MT data show evidence for a moderately conductive north‐south structure on the south flank below the Timberline Lodge and for a broad zone of late Tertiary intrusives concealed on the southeast flank.
Oceanic plateaus, the fragmentation of continents, and mountain building
Authors: Nur Amos , Ben‐Avraham Zvi
Published: February 2008

Many anomalous rises in today's oceans may be submerged continental fragments detached from previous continents, ancient island arcs, or basaltic piles formed by hot spots and spreading centers. These rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins. Where such rises are being consumed at present, e.g., the Nazca Ridge, they cause cessation of volcanism, disruption of the downgoing slab, and possible shifts in plate boundary configuration. Many past rises, including numerous continental fragments, have been recognized within mountain belts as allochthonous terranes. They constitute a large portion of the orogenic belts in the North Pacific from Mexico through western North America, Alaska, east Siberia, Japan and in New Zealand. The orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes. Many terranes have been accreted with substantial deformation also in the Alpine chain, well before major continent‐continent collisions. It is suggested, therefore, that the accretion of fragments may be the common process of the deformation phase of mountain building. Subduction of normal oceanic crust may be insufficient for deformation, whereas full continent‐continent collision may not be necessary. The general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the Andes or not. Most of the accreted fragments with continental affinities in the Mesozoic‐Cenozoic orogenic belts of the world can be traced back to the breakup of Gondwana, beginning with a Pacifica domain in the Permian through a larger India domain in the early Mesozoic and continuing through the separation of the Somalia plate in the near future. The reasons for this 250 million year breakup process are not known, but some kind of thermal process, possible of mantle‐wide scale, is implied.
Attenuation of explosion‐generated pulse in rock masses
Authors: Blair D. P. , Spathis A. T.
Published: February 2008

Recordings were made of seismic pulses produced by explosive sources in rock underground at Mount Isa Mine, Queensland, Australia. Analysis of the experimental results in light of the theory of Kjartansson (1979) indicates that the simple rise time law τ = τ0 + CT/Q is inadequate for describing the attenuation of seismic pulses generated by a realistic source. The rise time law makes no allowance for the spectral characteristics of the source except by assigning a value of τ0 to each source and is an oversimplification of the process of source propagation through rocks; consequently, the constant C is source dependent. An alternative method of estimating pulse attenuation is presented, which overcomes the difficulties inherent in the rise time law.
Isostatic geoid anomalies on a sphere
Authors: Dahlen F. A.
Published: February 2008

Long‐wavelength geoid anomalies due to lateral variations in the density or thickness of a thin layer in local isostatic equilibrium on the surface of a sphere depend sensitively on the assumed state of stress within the layer. A number of common intuitive definitions of local isostasy generally believed to be essentially identical are associated with quite different states of isostatic stress, and the corresponding theoretical geoid anomalies can vary by more than a factor of 2. This sensitivity of the theoretical anomaly to the exact definition of local isostasy constitutes an obstacle to any proposed program of interpreting or exploiting observed global isostatic geoid anomalies such as those associated with the aging of the oceanic lithosphere.
Deformation of single‐crystal clinopyroxenes: 1. Mechanical twinning in diopside and hedenbergite
Authors: Kollé J. J. , Blacic J. D.
Published: February 2008

Laboratory deformation experiments were carried out on two single‐crystal clinopyroxenes: chrome diopside and hedenbergite. Tests were made in a Griggs solid medium, triaxial, hot creep tester. A confining pressure of 1000 MPa was applied in all experiments. Crystals were deformed at strain rates from 10−4 to 10−8 s−1 and at temperatures from 400°C to 1200°C. Two orientations of the crystals with respect to the maximum principal compressive stress were tested. The first orientation subjects the (100), [001] and (001), [100] mechanical twinning systems in clinopyroxene to a high resolved shear stress in the sense appropriate for mechanical twinning to occur. The second orientation subjects these systems to an equal resolved shear stress but in the opposite sense so that mechanical twinning is not possible. Effects of temperature and strain rate on the flow stress are observed for the two clinopyroxene compositions. Mechanical twinning on the system (100), [001] is observed to be the primary deformation mechanism in crystals oriented favorably for twinning. The resolved shear stress required for mechanical twinning on this system is 140 MPa for hedenbergite and 100 MPa for chrome diopside. The smaller twinning stresses for diopside are considered to be due to abundant inclusions in the crystals. The mechanical twinning stress is nearly independent of temperature from 400°C to 850°C and strain rate from 10−4 to 10−8 s−1. Above 850°–1000°C for diopside and 1000°C for hedenbergite, deformation occurs by dislocation glide and is strongly temperature and strain rate dependent. Crystals tested in the orientation for which mechanical twinning is not possible deform by two different dislocation glide mechanisms depending on temperature. Details of these experiments are reported in part 2. Results for mechanical twinning predict that this is not an important deformation mechanism in the earth unless resolved shear stress on the twinning system exceeds 140 MPa. Lack of dependence of twinning stress on strain rate implies that large, rapid strains will occur in this mineral if the twinning stress is reached. Independence of twinning stress in clinopyroxenes from temperature, strain rate, or composition allows use of this mineral as a geopiezometer. At plate margins where stresses are high, presence of mechanically twinned clinopyroxene grains allows determination of the orientation and magnitude of stresses responsible for the deformation.
Tectonic controls on tholeiitic and calc‐alkaline magmatism in the Aleutian Arc
Authors: Kay S. Mahlburg , Kay R. W. , Citron G. P.
Published: February 2008

The tectonic position of Aleutian arc volcanic centers and their magmatic differentiation trends (calc‐alkaline or tholeiitic) appear to correlate. From 160°W to 175°E, the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates. The tholeiitic volcanoes are large, primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow, closed system differentiation. The calc‐alkaline volcanoes are smaller, more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult. Differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers. The chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma, probably derived from mantle peridotite. Conditions for the derivation of the two trends through crystal fractionation support the proposed model. The tholeiitic magmas show characteristics (i.e., no hydrous phenocrysts, Fe enrichment trend, parallel REE patterns, vitrophyric and esites and dacites) consistent with low‐pressure, high‐temperature crystallization in large shallow magma chambers. The calc‐alkaline magmas show characteristics (i.e., some hydrous phenocrysts, no Fe enrichment trend, nonparallel rare earth patterns, porphyritic lavas) consistent with higher pressure and lower temperature of crystallization than the tholeiitic series. Tertiary plutons also show both calc‐alkaline and tholeiitic trends and appear to be chemically similar to the Quaternary volcanoes. The larger plutons are calc‐alkaline and probably represent the extensive (1002 km ) magma chambers of the small calc‐alkaline volcanoes. The small, shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series.
Monitoring changes of stress along active faults in southern California
Authors: Clark Bruce R.
Published: February 2008

Measurements of stress changes using IRAD vibrating‐wire Stressmeters suggest that significant stress changes have occurred in a large area of Southern California near active faults in the past several years. Long‐term changes in stress follow a pattern of horizontal compression in approximately a NS direction relative to stress levels in the EW direction, in good agreement with the tectonic pattern established by youthful geologic structures, independent stress and strain measurements, and the recent seismic history of the area. The sensors are installed at a depth of approximately 20 m by wedging them against the walls of small‐diameter boreholes, producing an initial ‘prestress’ of 0.5–2MPa −20 bar). Consequently, they only measure changes from an initial prestress level and are not capable of determining absolute magnitudes of the stress field. By revising the standard procedures for installing and reading the sensors, we have improved the resolution level of the sensors when installed from approximately 15 kPa (150 mbar) to better than 3 kPa (30 mbar). Three sites are now located along the San Andreas fault between Elizabeth Lake and Wrightwood, and four sites are being monitored along the Sierra Madre fault system at the southern margin of the San Gabriel Range. A new net along the San Jacinto fault between Hemet and Anza is now established but will require a year or more of monitoring before providing a useful long‐term data sequence. Short‐term anomalies are superposed on the long‐term trends and may be useful for site‐specific earthquake prediction purposes. In one case, a short‐term anomaly was associated with a moderate earthquake 15 km from the measurement array. The stress‐change measurements may also provide useful data on near‐surface thermoelastic effects and on the symmetry of the earthquake source mechanism.
A gravity survey in the Reydarfjördur area, eastern Iceland, with interpretation
Authors: Schoenharting Guenther , Pálmason Gudmundur
Published: February 2008

A total of 63 new gravity stations was established in the Reydarfjördur area and around the Iceland Research Drilling Project (IRDP) drill site. Both previous gravity stations and the new ones have been incorporated into a new Bouguer anomaly map. The main gravity features of the area are a strong W‐E gradient in the vicinity of the IRDP drill site, an extended gravity minimum a few kilometers east of Reydarfjördur village, and a gravity maximum at the Reydarfjördur volcanic center. These features correlate with dike density distribution. Dikes, however, can explain only 10% of the observed gravity amplitudes. Relief on a zone of density gradient observed in flows from the IRDP core and extrapolated to about 1 km below the bottom of the hole has been modeled by assuming constant density sublayers. Relief amplitudes of the order of 1 km were necessary to explain the anomalies. We conclude that the main source of the gravity anomaly, at least in the vicinity of the drill site, lies above the depth of the layer 2/3 boundary or transition zone indicated for the general area by refraction seismics.
Detailed susceptibility log of Iceland Research Drilling Project drill core, Reydarfjordur, eastern Iceland
Authors: Schonharting G. , Hall J. M.
Published: February 2008

A detailed log of initial susceptibility, based on 6135 measurements made on drill core with a hand held meter, is presented for the 1919‐m‐long lower part of the IRDP Reydarfjordur crustal section. The log shows typical strong internal variation, with a central maximum, in flow susceptibility while dikes show little internal variation. Differences between dikes and flows and internal variations are explained in terms of variation in degree of deuteric and hydrothermal alteration, grain size, and abundance of iron‐titanium oxides. General decrease of flow susceptibility with depth is related to depth variation in hydrothermal alteration. This has the effect of removing iron from oxides probably for incorporation in nonmagnetic silicates. This effect is intense toward the bottom of the drill core. Linear extrapolation of the decrease of flow susceptibility with depth indicates a zero value at about 4 km beneath the original surface of the crustal section. Whether or not magnetization of the crust as a whole reaches zero level will depend on the dike density at this depth.