bluuebunny/crossref_metadata_2025 huggingface dataset:
Age and significance of Precambrian Basement Samples from northern Illinois and adjacent states
Authors: Hoppe W. J. , Montgomery C. W. , Van Schmus W. R.
Published: February 2008

Geochronologic results are reported for 12 localities where deep drilling has penetrated Precambrian basement in Iowa, Illinois, Indiana, Michigan, and Kentucky. For the northern Illinois deep drill holes UPH 1 and UPH 3, a zircon U‐Pb age of 1463±10 m.y. was obtained. Rb‐Sr data for these samples indicate mild disturbance of this system but are not interpreted in terms of any specific younger event. U‐Pb ages for other localities fall in the interval 1450–1510 m.y. and indicate that this was a major period of igneous activity throughout the southern Great Lakes region. Rb‐Sr results indicate that alteration was ubiquitous and that most ages in the range 1100–1400 m.y. probably represent disturbed systems. The results obtained so far tend to support interpretations in which Kentucky, Indiana, and Illinois are primarily underlain by a rhyolite and epizonal granite terrane that is 1450–1500 m.y. old; older Proterozoic rocks may be absent or more deeply buried.
Electrical conductivity structure of the San Andreas Fault in central California
Authors: Phillips Wayne J. , Kuckes Arthur F.
Published: February 2008

The electrical conductivity structure of a section of the San Andreas fault is investigated by means of an electromagnetic induction experiment that utilizes a grounded electric dipole source. Location of the study area is 18 km southeast of Hollister, where the fault runs adjacent to the Gabilan Mountains. The rerults indicate a sharp contrast in conductivity across the fault zone. The resistivity of the sediments on the northeast side of the fault is ≈ 20 Ω m compared to ≈ 1000 Ω m for the Gabilan granites on the southwest side. There is evidence for a zone of high conductivity beneath the Gabilans. The high conductivities beneath the Gabilans correlate with the results of earlier seismic and gravity studies. A gravity survey in the area indicates the presence of a pocket of low‐density rocks located ≈ 1–9 km beneath the Gabilans and ≈ 2 km southwest of the surface trace of the fault. The depth to the conducting layer there correlates reasonably well with the upper bound of these rocks. Farther away from the fault there is a discontinuity in seismic velocity at a depth of 12 km. The depth to the conducting layer in this area 13 km. The low‐density, highly conducting rocks adjacent to the fault zone are interpreted as sedimentary rocks like those on the northeast side of the fault, or fault gouge. Farther from the fault zone the seismic velocities and electrical conductivity at depth are consistent with the hypothesis that the Gabilans are underlain by a slab of serpentinized oceanic crust.
The American‐Antarctic Ridge
Authors: Lawver Lawrence A. , Dick Henry J. B.
Published: February 2008

The America‐Antarctic Ridge forms the boundary between the South America and Antarctic plates in the South Atlantic. A recent cruise on R/VMelville surveyed and dredged the America‐Antarctic Ridge between the Conrad Fracture Zone at 56°S, 4°W and a spreading center at 60°S, 18.5°W. This section of the plate boundary consists of five transform faults and six spreading center segments. Prior to our work the plate boundary was generalized on the basis of seismicity and was thought to consist of only two or three spreading centers offset by as many transform faults. Magnetic anomaly correlations indicate that the spreading rate on the America‐Antarctic Ridge near the Bouvet triple junction is about 9 mm/yr (half rate). The ridge has the rough topography and great relief characteristic of slowspreading ridges. The average relief of the median valleys (2.55±0.33 km) is even greater than that of the Mid‐Atlantic Ridge (1.36±0.47 km (Shih, 1980)), consistent with the slower spreading rate of the America‐Antarctic Ridge. Dredging of the fracture zones recovered diabase, greenstone, gabbro, and abundant peridotites (in addition to pillow basalt): rocks which are rare from fast‐spreading ridges. On the basis of the new bathymetrie and magnetic data we have revised the pole of rotation for the Antarctic and South American plates. Depending on the actual instantaneous plate velocities for the American‐Antarctic Ridge at the Bouvet triple junction, the newly determined Antarctic‐South America pole of rotation is either 85°S, 22°E, if the half spreading rate on the American‐Antarctic Ridge is 9.2 mm/yr, or between 84.5°S, 14°E and 85.50S, 19°E, if the rate is 9.0 mm/yr.
Autoregressive harmonic analysis of the Earth's polar motion using homogeneous international latitude service data
Authors: Chao B. Fong
Published: February 2008

The homogeneous set of 80‐year‐long (1900–1979) International Latitude Service (ILS) polar motion data is analyzed using the autoregressive method (Chao and Gilbert, 1980), which resolves and produces estimates for the complex frequency (or frequency and Q) and complex amplitude (or amplitude and phase) of each harmonic component in the data. Principal conclusions of this analysis are that (1) the ILS data support the multiple‐component hypothesis of the Chandler wobble (it is found that the Chandler wobble can be adequately modeled as a linear combination of four (coherent) harmonic components, each of which represents a steady, nearly circular, prograde motion, a behavior that is inconsistent with the hypothesis of a single Chandler period excited in a temporally and/or spatially random fashion), (2) the four‐component Chandler wobble model “explains” the apparent phase reversal during 1920–1940 and the pre‐1950 empirical period‐amplitude relation, (3) the annual wobble is shown to be rather stationary over the years both in amplitude and in phase, and no evidence is found to support the large variations reported by earlier investigations, (4) the Markowitz wobble is found to be marginally retro‐grade and appears to have a complicated behavior which cannot be resolved because of the shortness of the data set.
Utilization of seismically recorded infrasonic‐acoustic signals to monitor volcanic explosions: The El Chichon Sequence 1982—A case study
Authors: Mauk Frederick J.
Published: February 2008

Infrasonic‐acoustic signals from five explosive eruptions of El Chichon volcano during March 29 through April 4, 1982, were recorded by a microbarograph array and Seismic Research Observatory (SRO) seismograph collocated near McKinney, Texas. Analyses of these signals for frequencies from 0.0033 to 0.025 H have demonstrated functional relationships of amplitude to frequency consistent with ω−1 for microbarometric data, ω−2 for vertical mode seismic data, and ω−3 for radial mode seismic data. These observed slopes are in agreement with those theoretically predicted by Sorrells (1971). Estimations of kinetic energy releases by the explosions of El Chichon are presented and range from 4.9×1022 ergs to 2.0×1023 for the largest eruption on April 4, 1982, to 4.2×1021 to 6.1×1021 ergs for the smallest eruption on April 3, 1982. The method of Posey and Pierce (1971) to estimate the explosivity from long‐period infrasonic signals is not likely to be applicable, strictly speaking, to the shorter‐period data. Thus it is believed that the energy release for the smallest eruption is overestimated by possibly an order of magnitude. Comparable estimations of volcanic explosivity from SRO radial seismic data and microbarometric array data strongly recommend using SRO and International Deployment Accelerometers network data for far‐field analyses of volcanic explosions and construction of a rigorous volcanic explosivity scale.
Late Cretaceous‐Early Tertiary tectonic history of southern Central America
Authors: Gose Wulf Achim
Published: February 2008

The paleomagnetic analysis of samples from three Late Cretaceous through Eocene sites in southwesternmost Nicaragua yields pole positions close to the present pole. The results from five equivalent sites in the Pacific coastal zone of Costa Rica, on the other hand, suggest a 14° northward translation in post‐Eocene time. This movement cannot have been initiated by the development of the Cocos‐Nazca spreading center and it seems likely that the extensive Late Miocene plutonism in Costa Rica defines the time of collision of the Nicoya Complex (sensu largo) and the Costa Rican island arc. The Nicaraguan as well as the Costa Rican data are very different from paleomagnetic results from Honduras, and the tectonic boundary between northern and southern Central America, therefore, must lie to the north and east of the Nicaraguan sites.
Seventh Foray: Whitlockite‐rich lithologies, a diopside‐bearing troctolitic anorthosite, Ferroan anorthosites, and KREEP
Authors: Warren Paul H. , Taylor G. Jeffrey , Keil Klaus , Kallemeyn Gregory W. , Shirley David N. , Wasson John T.
Published: February 2008

Seventeen nonmare samples, most of them pristine, are characterized, and implications of the new data are discussed. Five pristine samples are from Apollo 14, near the center of the KREEP‐rich zone in the moon's western hemisphere. Three of them are alkali anorthosites rich in Ca‐phosphate (whitlockite), which apparently crystallized from magmas with REE contents roughly 6× those of high‐K KREEP. The alkali anorthosites probably formed from Mg‐rich magmas that assimilated large amounts of urKREEP, but some might have formed by metasomatism of ferroan anorthosite by urKREEP. The gabbronorite/norite classification scheme is not well suited to western hemisphere lithologies, probably due mainly to the overriding effects of longitude‐petrochemistry correlations. A diopside‐bearing Mg‐rich lithology indicates that a low degree of melting was not a prerequisite for producing gabbroic (high‐Ca pyroxene‐rich) Mg‐rich magmas, and suggests that some source regions of Mg‐rich magmas were relatively Ca‐rich. Several pristine KREEP fragments from Apollo 15 station 2 are texturally and compositionally much like other pristine KREEP, and thus reinforce the evidence that KREEP is highly uniform. The pristine anorthosites, on the other hand, demonstrate further that lunar anorthosites are diverse.
Subdivision of the Mg‐suite noritic rocks into Mg‐gabbronorites and Mg‐norites
Authors: James Odette B. , Flohr Marta Kempa
Published: February 2008

Mg‐suite noritic rocks can be divided into two groups, the Mg‐gabbronorites and the Mg‐norites. The rocks of these groups differ in ratios of high‐Ca pyroxene to total pyroxene, compositions of pyroxene and plagioclase, assemblages of Ti‐, Nb‐, and Zr‐bearing minerals, compositions of chrome spinel, bulk‐rock Ti/Sm and Sc/Sm, and measured ages. The two groups probably crystallized from different types of parent magmas. Two hypotheses are offered for the differences in composition of the parent magmas. One hypothesis ascribes the differences to compositional heterogeneity of the mantle source areas. The other hypothesis ascribes the differences to variations in extent of partial melting of the mantle source regions and variations in extent of assimilation of the anorthosite and the highly differentiated residual liquid that were produced during the primordial lunar differentiation.
Petrology and comparative thermal and mechanical histories of clasts in breccia 62236
Authors: Nord Gordon L. , Wandless Mary‐Virginia
Published: February 2008

Lunar breccia 62236 contains large lithic fragments of troctolite, norite, and anorthosite. The mafic phases, olivine, inverted pigeonite, and augite, fill interstitial areas between larger plagioclases and appear to be cumulate phases with extensive adcumulus growth. Pyroxene compositional homogeneity indicates that cation exchange during cooling was limited to an area of about 1 mm. Primary augite and pigeonite both contain 30 μm‐wide lamellae of the other along ‘001’. Pigeonite inverted to orthopyroxene without retaining any crystallographic orientation and subsequently exsolved fine lamellae of augite on (100). Profiles across orthopyroxene‐augite interfaces obtained in the analytical transmission electron microscope show an increase of ∼5% wollastonite in augite within 0.5 μm of the interface, suggesting that short‐range cation exchange continued to temperatures below 500°C. The entire sample has undergone heterogeneous shock deformation. Shock melting of the troctolitic clast suggests pressures of 200–300 kb and well‐developed basal twinning in augite from the norite clasts suggests pressures of 50–300 kbars. The present evidence indicates that 62236 contains parts of a slowly cooled microlayered adcumulate that has been heterogeneously shocked several times and combined into the present breccia.
The effects of high pressure and high temperature on some physical properties of ocean sediments
Authors: Morin Roger , Silva Armand J.
Published: February 2008

A series of laboratory experiments was conducted with four ocean sediments, two biogenic oozes and two clays. Permeability and thermal conductivity were directly measured as a function of porosity, and the testing program was designed to identify any dependence of these physical properties upon hydrostatic pressure and temperature. The results show no discernible effects of pressure, within the range of 2–60 MPa, upon the permeability of any of the samples. Temperature effects, from 22° to 220°C, upon this property are accounted for by applying a viscosity correction to the permeating seawater. Previous investigations have suggested the existence of a pressure‐induced and/or a temperature‐induced breakdown of the absorbed water which surrounds clay particles, thereby promoting an increase in sediment permeability. Our experimental findings cannot confirm this phenomenon and fail to provide a satisfactory solution to the conflicting data which now exist between the pore water velocities inferred from nonlinear thermal profiles of ocean sediments and those fluid velocities derived from Darcy's law and laboratory permeability data. The effects of sizeable variations in pressure and temperature upon sediment thermal conductivity are found to reflect closely the behavior of the conductivity of the liquid phase alone under these same changes in environmental conditions. This is not surprising due to the relatively narrow range of high porosities encountered in this study. Empirical equations are developed which allow sediment thermal conductivity to be calculated as a function of temperature and void ratio. A hydrostatic pressure correction term is also presented.
The electrical conductivity of the upper mantle as estimated from satellite magnetic field data
Authors: Didwall Edna M.
Published: February 2008

The electrical conductivity of the upper mantle is estimated from low‐latitude magnetic field variations (magnetic storms) caused by large fluctuations in the equatorial ring current. The data base is derived from magnetic field magnitude data measured by satellites OGO 2, 4, and 6, which offer better global coverage than land‐based observatories. The procedures of analysis consist of 1) separation of the disturbance field into internal and external parts relative to the surface of the earth, 2) estimation of an electromagnetic response function Q(ω) which relates the internally generated magnetic field variations to the external variations due to the ring current, and 3) interpretation of the estimated response function using theoretical response functions for assumed conductivity profiles. Special consideration is given to possible oceanic and ionospheric effects. Best estimates of the geomagnetic response function Q(ω) for 0.2 to 2.0 cpd indicate an upper mantle conductivity of the order of 10−2 S/m. This conductivity is reasonable, especially when laboratory measurements of conductivity versus temperature for olivine are considered.
Topographic amplification of tectonic displacement: Implications for geodetic measurement of strain changes
Authors: Mctigue David F. , Stein Ross S.
Published: February 2008

Correlations of level changes with topography demand an assessment of the effect of an irregular free boundary on surface deformation. This is examined through a plane strain elastic model with topography of small slope, subjected to a change in the far‐field horizontal stress. To leading order, vertical surface displacements due to the topographic perturbation are proportional to the local relief. Elevation‐dependent uplift results from a compressional change, and downdrop results from a tensional change. The model further predicts that the ratio of elevation change to elevation is proportional to and of the same order of magnitude as the regional strain. Horizontal strains are locally perturbed by topography as well, with the magnitude scaling with the local slope. The predicted localization of level changes is very small in aseismic regions and cannot contribute significantly to measured correlations. A test case in southern California bears this out, with strains of order 10−6 accompanied by elevation change to elevation ratios of order 10−4. Releveling following the Nankaido‐Tonankai earthquakes, which induced large coseismic and postseismic strains, reveals scattered examples of elevation‐dependent level changes. However, when compared to modeled strains, the correlations are again at least an order of magnitude larger than the localization effect predicted by the elastic model. Although the topographic perturbation of vertical displacements appears to be unmeasurably small, local variations in horizontal strain or borehole dilatation across steep relief may be discernible with current technology.
Inversion of ocean bottom magnetotelluric data revisited
Authors: Oldenburg D. W. , Whittall K. P. , Parker R. L.
Published: February 2008

Three ocean bottom magnetotelluric data sets from sites on the Pacific plate are reinterpreted. The initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site. That work also suggested that the resistivity increased below the conductor. This analysis, which includes new methods for constructing one‐dimensional conductivity models, shows that the postulated increase in resistivity is not demanded by the data. It also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data. The previously reported trends with lithospheric age still exist, but they are not as strong as initially believed. Finally, it is shown rigorously that the different age sites are distinct in that no one‐dimensional model can account for all three data sets.
Seasat observations of lithospheric flexure seaward of trenches
Authors: McAdoo David C. , Martin Chreston F.
Published: February 2008

Lithospheric flexure seaward of deep ocean trenches is evident in Seasat altimeter observations of the marine geoid. In fact, mechanical models of lithospheric flexure can be tested directly on the Seasat altimeter data. We have used a simple elastic model for the oceanic lithosphere and, after least squares adjustments, have recovered estimates of model parameters including outer rise (OR) amplitude, OR wavelength, and effective lithospheric thickness. Effective lithospheric thicknesses have been recovered for six regions: the Mariana, the Kuril, the Philippine, the Aleutian, the Izu‐Bonin, and the Middle America OR's. These results support the proposition that effective thickness Te increases with age of lithosphere in approximate accord with the relation Te ≅ C · age1/2 where C ≅ 4 km m.y.−1/2. In fact, our altimetric results agree more closely with this relation than do published results based on bathymetric data. The close agreement with the thickness‐age relation suggests that there is no longer any need to assume that significant horizontal compression acts across the Kuril, Marianas, and Izu‐Bonin trenches. This thickness‐age relation implies that flexural strength of the oceanic lithosphere is temperature controlled.
Oxygen self‐diffusion in quartz under hydrothermal conditions
Authors: Dennis Paul F.
Published: February 2008

Oxygen self‐diffusion, Dox, has been monitored in single‐crystal quartz in the temperature range 515°–850°C under hydrothermal conditions. In the beta field, between 700° and 850°C, the data are represented by two linear Arrhenius relations for transport parallel and perpendicular to c. Values for Do (m2 s−1) are 2.09×10−11 parallel to c and 3.16×10−10 perpendicular to (
). Values for ΔH (kJ mol−1) are 138.54 parallel to c and 203.72 perpendicular to (
). At 700°C, in the total pressure range 11.5–100 MPa, Dox is independent of water (f(H2O)) and oxygen (f(O2)) fugacities between the Ni‐NiO and Fe3O4‐Fe2O3 buffers. The results are consistent with diffusion via a simple charged vacancy mechanism under an extrinsic point defect regime. Further experiments are required to confirm the nature of the mobile oxygen defect. A key aspect of the results is the observation that at the low water fugacities of the present experiments a hydrogen‐containing defect appears to play no role in the oxygen transport mechanism. This is in contrast to other published sets of data and leads directly to the requirement for detailed interlaboratory comparisons.
Effect of humidity on time‐ and velocity‐dependent friction in rocks
Authors: Dieterich James H. , Conrad Gerald
Published: February 2008

Friction experiments were conducted utilizing samples tested in a humid atmosphere (ambient laboratory humidity or argon bubbled through water) and samples that were carefully dried and tested in an atmosphere of very dry argon (H2O<1 ppm). Dry surfaces consistently yield a friction coefficient of 0.85 to 1.00 compared with 0.55 to 0.65 for the tests in a humid environment. Time‐dependent increases in the friction coefficient and large‐scale stick‐slip instability are characteristic of surfaces exposed to atmospheric water, but they are eliminated with careful drying. Direct dependence of frictional strength on slip speed persists under both dry and humid conditions.
The role of pore water circulation during the deformation of foreland fold and thrust belts
Authors: Engelder Terry
Published: February 2008

Volume‐loss strain accompanying pressure solution of calcite occurred within both the Umbria‐Marches Apennines of Italy and the Appalachian Mountains of western New York. Data from strain markers show that volume‐loss strain was greater within the shallow portions of the Apennines than within the Appalachians. Within the deeper portions of both fold and thrust belts, strain was nearly volume‐constant. Calcite solubility data suggest that downward circulation of meteoric water is necessary for the 35% volume‐loss strain of the limestones within the Apennines. Strain at a depth of about 1 km was volume‐constant and is interpretated as indicative of restricted pore fluid circulation. In the Appalachians, calcite comprises less than 1% of the clastic rocks, and a 10% volume‐loss of this calcite may occur during circulation of connate or dehydration water derived from dewatering of the shales but in an environment that restricts the circulation of meteoric water. Here, the volume of calcite removed (0.1% of the total rock) is so small that circulation of meteoric water is not necessary for strain by pressure solution.
Coherent tilt signals observed in the Shumagin Seismic Gap: Detection of time‐dependent subduction at depth?
Authors: Beavan J. , Bilham R. , Hurst K.
Published: February 2008

Repeated surveys of short level lines in the Shumagin Islands, Alaska, reveal coherent tilt signals associated with subduction of the Pacific plate beneath the North American plate in the Shumagin seismic gap. Ten years of steady tilt down toward the trench is interrupted during 1978–1980 by a rapid episode of reverse tilt. The “normal” tilt represents surface deformation as subduction occurs with the plate boundary locked to at least 60 km depth. Using all available tilt, sea level, and seismic data, we interpret the tilt reversal as due to an episodic reverse slip of ≈ 80 cm magnitude on the plate boundary between ≈ 70 km and ≈ 20 km depth, downdip from the main seismogenic zone, which remains locked. This event causes an increase of stress on the locked main thrust zone. We speculate that such events may be a regular process at subduction zones, that great plate boundary earthquakes may be more common during their occurrence, and that their onset may be detectable early enough to give warning of an increase in probability for the occurrence of a great earthquake.
Geologic and seismic velocity structure of the crust/mantle transition in the Bay of Islands Ophiolite Complex
Authors: Karson J. A. , Collins J. A. , Casey J. F.
Published: February 2008

Geological investigations of the Bay of Islands Ophiolite Complex show that while a typical layered ophiolite suite is present, the thickness of major lithologic units is extremely variable from place to place. The composition and internal structure of map‐scale (kilometers across) lithologic units as well as the contacts that bound them are laterally variable. Inferred velocity‐depth functions of the crust/mantle (“Moho”) transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure. The crust/mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick. The mafic/ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long. These units (megalenses) may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units. Solid‐state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units. Substantial relief and slopes of up to at least 12° over lateral distances of about 10 km occur on the top of the crust/mantle transition zone as well as other geologic unit contacts. Although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes, many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere.
Paleomagnetic and geologic data indicating 2500 km of northward displacement for the Salinian and related terranes, California
Authors: Champion D. E. , Howell D. G. , Gromme C. S.
Published: February 2008

Results of a pair of paleomagnetic studies of sedimentary rocks in the Salinian terrane of westernmost California indicate a northward displacement of about 2500 km since Cretaceous time. Stratigraphic relations suggest that the Salinian terrane became amalgamated with several others by the Late Cretaceous and all underwent the same northward translation. This composite terrane (allochthon) became sutured to cratonal North America by early Tertiary time. One paleomagnetic study involved Upper Cretaceous turbidites of the Pigeon Point Formation. Fine‐grained, thin‐bedded turbidites yielded the most reliable results, whereas medium‐ to coarse‐grained sandstones are not always stably magnetized. Contorted layers resulting from soft sediment deformation usually gave directional results parallel to those of undeformed sedimentary layers, thereby implying a postdepositional remanence process. There is no indication of inclination error or other processes affecting the fidelity of the remanent magnetization. Paleomagnetic results from four separate locations in the Pigeon Point Formation gave positive fold tests and mean inclination values near 36°. A mean geomagnetic latitude calculated from these results is 21.2°±5.3°, much more southerly than the value of 46.5°±2.3° calculated for Pigeon Point using paleomagnetic pole data from Late Cretaceous rocks from cratonic North America. Declination values are not antipodal for normal and reversed polarity sites. Analysis of depositional current directions from sandstone beds of the same outcrops shows that the difference is the result of Neogene rotations of small crustal blocks adjacent to the San Gregorio fault. In a sequence of Paleocene turbidites that crop out at Point San Pedro, four sample localities yielded positive fold tests with mean inclination values near −40°. Declination values, however, were spread over a range between 230° and 320°. Comparison with depositional current directions from the same outcrops indicates clockwise rotations related to Neogene right slip on the Pilarcitos fault. The mean geomagnetic latitude calculated for the Point San Pedro sediments is 24.5°±4.0° and is more southerly than the 43.3°±2.5° latitude expected for that location. For the Salinian terrane and the west coast of cratonic North America, absolute rates of latitudinal motion were 8.5 cm/yr northward and 3 cm/yr southward, respectively.
Dual radiotracer measurement of zoobenthos‐mediated solute and particle transport in freshwater sediments
Authors: Krezoski John R. , Robbins John A. , White David S.
Published: February 2008

Gamma spectroscopy methods have been applied to determine the effects of Stylodrilus heringianus and Pontoporeia hoyi, two freshwater benthic macroinvertebrates, on the reworking of sediments and the transfer of solutes across the sediment‐water interface. Natural lake sediments (sieved to remove organisms) and overlying water were contained in temperature‐regulated rectangular plastic cells. A submillimeter layer of sediment solids labeled with 137Cs was deposited on the sediment interface while overlying water was spiked with 22Na. After addition of Stylodrilus (oligochaete worms) and Pontoporeia (crustacean amphipods) to these microcosms, the vertical distributions of 137Cs (a tracer of particle transport) and 22Na (a tracer of solute transport) were determined at daily to weekly intervals for 3 months by scanning the length of the cells with a well‐collimated NaI detector. In cells with Stylodrilus, the 137Cs layer moved downward at a rate that decreased exponentially with time. The displacement of the layer is the result of the conveyor‐belt feeding mode of this organism. The rate of marked layer burial is consistent with that of other freshwater annelids (0.18×10−5 cm d−1 individual−1 m−2; 11.6°C). The exponential decrease in burial rate is ascribed to uniformly distributed feeding of Stylodrilus within the feeding zone of 4.4 cm. In cells with Pontoporeia, 137Cs activity was smeared downward in time owing to eddy diffusive mixing of sediments over a small range (1–2 cm). In cells without worms, the veneer of Cs active material remained at the interface while the penetration of 22Na into sediments was consistent with diffusion in free solution with small corrections for sediment porosity and sorption (KD = 0.17). The effective diffusion coefficient De for 22Na in this cell (8.2×10−6 cm2 s−1) was essentially the same as that for a cell that had been inhabited by worms for 3 weeks and then poisoned with formalin just before addition of 22Na. Thus the presence of biogenically reworked sediments (with pelletized materials and remnant burrow structures) did not affect solute transport. In cells with live Stylodrilus, penetration of 22Na within the feeding zone was considerably more rapid, implying an apparent De twice as high as in cells without worms. Inferences based on the particle reworking results were used to develop an illustrative transport model that includes advective as well as diffusive terms. Advective transport arises from the incorporation of 22Na into pore fluids moved downward as a result of conveyor‐belt feeding. The model indicates that within the feeding zone, solute transport is dominated by advection and that the apparent enhancement of De in pure diffusion models is really the result of solute flow induced by particle reworking. In cells with Pontoporeia, De is approximately twice that in control cells. In these cells, 22Na profiles may be treated theoretically without advection.
Location and configuration of magma bodies beneath Long Valley, California, determined from anomalous earthquake signals
Authors: Sanders Chris O.
Published: February 2008

Recordings of 281 small earthquakes which occurred recently near the southern edge of Long Valley caldera at depths from 1 to 15 km are used to map the subsurface geometry of shear wave attenuating bodies in the caldera. Signatures of these events recorded northwest, north, northeast, and east of Long Valley with ray paths through the caldera are often anomalous in that S wave arrivals have very low amplitudes, and high frequency P and S wave energy is missing for the same station‐event combinations. The volcanic and geothermal history of the region suggests that ray paths for these anomalous signals have passed through magma which absorbed much of the S wave energy and also attenuated the higher frequencies. With over 1200 normal and anomalous ray paths through the caldera we have located two apparently massive magma bodies in the central and northwest caldera as well as two more dispersed magma areas in the southern caldera and beneath Crowley Lake. The central magma body extends from 4.5 to at least 13 km beneath the surface, and the northwest body is as shallow as 5.5 km. These two magma bodies are probably connected below about 8 km. Secondary arrivals seen on select seismograms are tentatively interpreted as reverberations inside the central magma chamber and are consistent with the interpreted lateral extent of the magma. The northwest and central magma bodies lie beneath the northwest and southeast corners of the caldera's resurgent dome and probably connect beneath the dome's center. Also the shallower parts of these magma bodies and the southern anomalous area lie beneath the surface expression of the medial graben faults suggesting that structural features seen at the surface of the valley may be related to magma location at depth.
Earthquake swarm in Long Valley Caldera, California, January 1983: Evidence for dike inflation
Authors: Savage J. C. , Cockerham R. S.
Published: February 2008

The 1982–1983 deformation observed by trilateration and leveling surveys across the Long Valley caldera is apparently related to the 8.5‐km‐long by 8‐km‐deep vertical rupture surface defined by the January 1983 earthquake swarm that occurred in the south moat of the caldera. The observed deformation can be explained as follows. In late 1982, 0.03 km3 of magma was injected into a dike that dips 30° northward from the bottom of the rupture surface. The downdip dimension of this dike is 8 km. The dike inflation accounts for the uplift observed across the caldera as well as some of the horizontal deformation. Inflation of the dike generated a tension of about 3 bars across the vertical plane that was to become the rupture surface of the January swarm. This reduced the frictional stress on the rupture plane and perhaps triggered the slip that caused the January swarm. Right‐lateral slip (0.22 m) on the uppermost 2 km of the rupture plane during and after the January swarm accounts for the additional horizontal deformation observed. The model can be improved marginally if strike slip is admitted over the entire rupture surface and 0.006 km3 of magma is injected along that surface in the depth interval 3–8 km. The improvement in the model fit, however, is not sufficient to require shallow injection of magma. Thus we conclude that inflation of a dike at depth (8–12 km) dipping northward beneath the resurgent dome plus shallow right‐lateral slip on the rupture surface is a simple, but not unique, explanation of the observed deformation and seismicity.
Pyroclastic rocks and calderas associated with strongly peralkaline magmatism
Authors: Mahood Gail A.
Published: February 2008

Strongly peralkaline volcanic complexes tend to be shieldlike due to coalescence of low‐viscosity lavas from numerous vent areas and blanketing by pyroclastic flows and falls. Dense welding and rheomorphism commonly mask the nature of pyroclastic units; welded pumice falls are common. Relatively low eruptive columns produce poorly sorted pumice falls that are difficult to distinguish from topography mantling pyroclastic flows. Welded fall deposits are characterized by multiple, fine‐scale, welding reversals, welding zones that correspond with stratification, and by flattening of fiamme parallel to underlying slopes, whereas welded topography‐mantling ignimbrites may have fine‐grained basal layers, lithic lenses, imbricate fiamme, and variations in crystal content and fiamme size with topographic position. Calderas are relatively small, typically 3–9 km in diameter, with little evidence for single‐stage subsidence in excess of a few hundred meters. Many strongly peralkaline volcanic centers show evidence of two or more nested calderas, and partial reactivation of older ring fractures is common. Replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures, resulting in multicyclic collapses that reuse these structures. In several centers, the caldera‐forming unit is zoned from pantellerite to more crystal‐rich trachyte. Following collapse, trachyte lavas commonly erupt from a central vent on the caldera floor, building a cone that nearly fills the caldera. Reestablishment of isostatic equilibrium may be accomplished both by eruption of these lavas and by flow of trachytic and more‐mafic magma into the root zone beneath the volcano. Resurgent doming sensu strictu has not been documented in strongly peralkaline systems; at Pantelleria and Mount Suswa, however, the central portions of the cauldron blocks were uplifted and tilted. Magma reservoirs that feed explosive eruptions generally are shallow, as indicated by pyroclastic units that contain lithic fragments of the volcanic edifice (commonly including cognate syenite) but lack subjacent crustal lithologies. In addition, the overwhelming predominance of alkali feldspar as a phenocryst phase and the scarcity of quartz are consistent with pantellerite liquidus relations only at pressures <100 MPa. Pantellerite and trachyte shields may represent the mature stage of continental rift volcanism; once the flux of mantle‐derived magmas has been sufficient to generate a large mass of felsic liquid that can act as a density filter, mafic magmas rarely reach the surface.
Seismic quiescence and asperities in the Tonga‐Kermadec Arc
Authors: Wyss M. , Habermann R. E. , Griesser J.‐C.
Published: February 2008

Highly significant temporal and spatial seismicity rate changes were found along the TongasKermadec plate boundary. The standard deviate (z) test was used to compare systematically the rates in different periods and different volumes, and to test for uniqueness of anomalies found. The NOAA hypocenter data file showed a pronounced decrease in the number of small‐magnitude earthquakes reported after 1969 at the same time as such a reporting decline exists in the worldwide data. Therefore only earthquakes with mb≥4.9 were studied. A search for quiescence before recent mainshocks yielded two uniquely significant precursory anomalies, one missed mainshock and one false alarm. Quiescence started 63 months before the January 1976, M = 8.0, Kermadec; and 25 months before the June 1977, Tonga, M = 7.2, earthquakes. The December 1975, M = 7.5, shock was not preceded by a seismicity rate change. The seismic gaps in the Tonga‐Kermadec arc with sufficient seismicity for analysis (17°–22° and 31°–34°) show constant rates up to the present. Therefore, on the basis of the quiescence hypothesis, earthquakes are not forecast for these segments. The stress drops, Δσ, of 380 interface events were estimated using the mb/Ms method. The average Δσ was uniform along the island arc with the exception of two segments with average Δσ of less than half the normal values. These segments coincided with the two largest recent interface ruptures; however, in the northern Tonga the Δσ was low before the 1975 rupture, whereas in the Kermadec the 1976 aftershocks showed low Δσ. Asperities could not be identified on the basis of high stress drops. A systematic study of the seismicity rate as a function of space defined large variations along the arc. The areas of highest and lowest seismicity rate correlate with the northern end of the trench and with the Louisville ridge intersection, respectively. However, other large rate contrasts exists between arc segments without obvious tectonic differences. This implies that seismicity doughnut patterns do not reliably define precursors. One segment of outstandingly high seismicity rate (30°–31°S) is identified as a major asperity because it contains a large number of m>6 events and a large number of recent mainshock‐aftershock sequences and because it stopped the great 1976 rupture. We propose that the only other segment of high seismicity rate (near 20.5 °S) also represents a major asperity.
Near‐surface in situ stress: 3. Correlation with microcrack fabric within the New Hampshire Granites
Authors: Plumb Richard , Engelder Terry , Yale David
Published: February 2008

We studied the correlation between near‐surface in situ stress and the preferred orientation of microcracks at two quarries in the Milford granite and one quarry in the Conway granite, New Hampshire. The orientation of in situ stress was determined by overcoring doorstoppers and from the strike of induced borehole fractures produced by a pressurized packer. The preferred orientation of vertical microcracks was determined using ultrasonic measurements to determine P wave velocity Vp on core and in situ and from thin section analyses. In situ Vp anisotropy was determined from interborehole travel time data. At all three sites, directions of maximum compressive stress σ1 and induced borehole fractures are aligned with the preferred orientation of open microcracks determined from core Vp and thin section data. An analysis of the microcracking sequence within each pluton suggests (1) that the quarry grain resulted from cooling‐induced thermal stresses and (2) a method of distinguishing the paleostress axes at the time of cooling from contemporary stress.
Darwin reprise
Authors: Menard H. W.
Published: February 2008

The relief of Cretaceous guyots indicates that an enormous region of the Pacific plate was 3300–;4800 m deep about 100 Ma ago. The plate, then 13–64 Ma old, had depth anomalies of −170 to −1600 m. It was drifting over a group of overlapping midplate swells that comprised a bumpy Darwin Rise of smaller extent than originally believed. In 100 Ma much of the region has subsided only 1000–1200 m instead of the 2000–3000 m predicted by thermal models. A post‐Cretaceous sea level lowering of 200–500 m accounts for some of the difference. The remainder appears to require repeated thermal rejuvenation such as occurred in Tertiary time in the central South Pacific. The Jurassic and Early Cretaceous crust is not anomalously shallow just because it is old. It was shallow when it was young.
Digital image processing of Seabeam bathymetric data for structural studies of seamounts near the East Pacific Rise
Authors: Edwards M. H. , Arvidson R. E. , Guinness E. A.
Published: February 2008

Digital marine bathymetric data are rapidly increasing in both volume and variety. These data provide new perspectives on the morphology of the seafloor. They also present a number of problems with regard to developing display methods that convey the large amount of information contained in these data. We have used digital image processing techniques to reduce and display Seabeam multibeam data for three young (less than 5 Ma) seamounts on the eastern flank of the East Pacific Rise. Navigational errors in location were corrected by identifying offsets between features observed in crossing tracks and then translating the tracks relative to a control track. Images of the seamounts were then generated using spatial filtering techniques. The images reveal more information than contour maps because (1) more depth intervals can be displayed, and (2) the continuous tone nature of the images shown in shaded relief or color‐coded form more readily conveys morphologic information. The images suggest a significant degree of structural control on seamount shape, more so than can be delineated by examination of contour maps alone. In one case, a chain of seamounts strikes at an angle perpendicular to the abyssal hill direction, although each seamount within the chain is elongate in a direction parallel to the hills. In another case, a polygonally shaped, flat‐topped seamount overlies a region of the crust where the abyssal hills display two strike directions. Images of the third seamount demonstrate a complicated structural control on morphology, including a control on the distribution of satellite constructs. Finally, the image presentations clearly depict regions where cross tracks could not be registered because of (1) residual navigation errors and (2) the presence of steep slopes, where the depth and location estimates can vary depending on the angular relationship between the ship track azimuth and the strike of the slope.
On geoid heights and flexure of the lithosphere at seamounts
Authors: Watts A. B. , Ribe N. M.
Published: February 2008

The sea surface height has now been mapped to an accuracy of better than ±1 m by using radar altimeters on board orbiting satellites. The major influence on the mean sea surface height is the marine geoid which is an equipotential surface. We have carried out preliminary studies of how oceanic volcanoes, which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges, contribute to the marine geoid. Simple one‐ and two‐dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum. Previous studies based on gravity and bathymetry data and uplift/subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness Te increase with the age of the lithosphere at the time of loading. The models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid‐ocean ridge crest will be associated with relatively low amplitude geoid anomalies (about 0.4–0.5 m/km of height), while seamounts formed on relatively old lithosphere, on ridge flanks, will be associated with much higher amplitude anomalies (1.4–1.5 m/km). Studies of the Seasat altimetric geoid prepared by NASA's Jet Propulsion Laboratory support these model predictions; geoid amplitudes are relatively low over the Mid‐Pacific Mountains and Line Islands, which formed on or near a mid‐ocean ridge crest, and relatively high over the Magellan Seamounts and Wake Guyots, which formed off ridge. Direct modeling of the altimetric geoid over these features is complicated, however, by the wide spacing of the satellite tracks (which can exceed 100 km) and poor bathymetric control beneath individual satellite tracks. In regions where multibeam bathymetric surveys are available, models can be constructed that fit the altimetric geoid to better than ±1 m. Studies of geoid anomalies over the Emperor seamount chain, for example, suggest that these seamounts formed on 20–30 Ma old oceanic lithosphere, while anomalies over the Louisville ridge and the Hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 Ma. This indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting. We examine the implications of these results to studies which attempt (1) to separate temporal variations of the sea surface due to oceanographic effects, (2) to predict seamounts directly from satellite‐derived gravity and geoid data, and (3) to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due to mantle convection.