bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermoelastic stress: How important as a cause of earthquakes in young oceanic lithosphere?
Authors: Bratt Steven R. , Bergman Eric A. , Solomon Sean C.
Published: April 2008

We present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere (less than 35 m.y. in age). In support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m.y. where cooling rates are relatively high. Most near‐ridge earthquakes have focal depths between the Moho and the depth of the 800°C isotherm. Thrust and strike‐slip events dominate the shallowest seismicity, while the focal mechanisms of the deeper events are generally characterized by normal faulting. To assess the importance of thermal stress in the generation of near‐ridge earthquakes, we model the lithosphere as an elastic half‐space in which the response to cooling is computed using the method of thermoelastic displacement potentials. A key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere. A further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature. Normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half‐space cooling. Models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust‐ and normal‐faulting earthquakes near mid‐ocean ridges. The addition of a “ridge push” stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m.y. but probably contributes little to the stress field at younger ages. Though these simple models are limited by several simplifying assumptions, particularly in the immediate vicinity of the ridge axis, they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere.
Signal detection techniques applied to the Chandler Wobble
Authors: Gross Richard S.
Published: April 2008

A sudden excitation event of the Chandler wobble should induce the earth's rotation pole to undergo damped harmonic motion. This type of motion has been searched for in the observations of the Chandler wobble using techniques based upon the concept of a matched filter. Although the signal detection techniques used here were not sensitive enough to detect any such isolated sudden excitation events; the result that was obtained is consistent with a randomly excited model of the Chandler wobble.
The 1979 Palmdale, California, strain event in retrospect
Authors: Savage J. C. , Gu Guohua
Published: April 2008

Although the long‐term (1971–1984) strain accumulation across the San Andreas fault near Palmdale, California, is simply right‐lateral shear at the rate of 0.20±0.01 μstrain/a (tensor shear), annual measurements indicate that an anomalous contraction perpendicular to the fault accumulated in the interval 1973–1978 and was abruptly released in mid‐1979. This anomaly is referred to as the 1979 Palmdale strain event. More frequent measurements of strain since 1979 suggest a more irregular accumulation of strain than prior to 1979. This irregularity may be due to measurement errors larger than previously estimated. As a consequence of this post‐1979 irregularity the 1979 strain event no longer appears as anomalous as it had in the shorter (1971–1980) record. To provide independent support for existence of the 1979 Palmdale strain event, we review gravity measurements at Palmdale by Jachens (1983) and water level measurements in wells near Palmdale by Merifield and Lamar (1981) that have previously been shown to provide a marginal correlation with the 1979 strain recovery. These data taken together with the original strain observations suggest a real strain anomaly did occur near Palmdale in 1979, but the evidence is not overwhelming.
A numerical study of mineral zoning formed during the cooling of finite volume grains
Authors: Wilson Clark R. , Smith Douglas
Published: April 2008

A numerical model is used to study compositional profiles formed by cation exchange and diffusion at constant cooling rates in spherical mineral grains. Fe‐Mg zoning in garnet‐olivine pairs is used as an example. Zoned concentration profiles can be described by an exponential function; changes in the parameters of the function can be used to evaluate the effects of changes in inclusion size, cooling history, diffusion parameters, and exchange enthalpy. The composition of an olivine inclusion within garnet depends strongly on inclusion size due to finite volume effects, as does zoning in the garnet surrounding the inclusion. Thus estimates of equilibration temperatures obtained from small inclusions of one phase within another require a size correction. However, for a spherical garnet within an olivine matrix, the effect is small for garnet radii greater than 1000 μm. The shapes of zoned concentration profiles are useful for bounding both the rate of cooling and the final temperature. In addition, the concentration ratios (Fe garnet)/(Fe olivine) may be used to estimate the cooling rate once the final temperature is bounded.
Polymetamorphism in the Panamint Mountains, California: A 39AR‐40AR study
Authors: Labotka Theodore C. , Warasila Robert L. , Spangler Robert R.
Published: April 2008

Metamorphosed upper Precambrian rocks in the Panamint Mountains show textural evidence for polymetamorphism. Previous isotopic studies have established the age of metamorphism as middle to late Mesozoic. This study was undertaken to try to determine the ages of the two metamorphic events using the 39Ar‐40Ar stepwise degassing technique of the K‐Ar method on biotite, muscovite, and hornblende separates. The resulting spectra indicate that metamorphism occurred during the intervals 170–150 Ma ago and 80–70 Ma ago. Many analyses yield ambiguous results because of slow cooling following metamorphism, contamination of the separates by refractory phases containing extraneous argon, and in some cases, the superposition of the two metamorphic events. The ages of metamorphism correspond to periods of emplacement of granitic rocks in the vicinity of the Panamint Mountains, suggesting that the high heat flux responsible for metamorphism was associated with the emplacement of the Sierra Nevada batholith. The ages of metamorphism and granite emplacement are diachronous with the development of major thrust faults in the western Great Basin, which suggests that metamorphism might have been triggered by the formation of nappes rather than causing the formation of nappes.
The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism
Authors: Mankinen Edward A. , Prévot Michel , Grommé C. Sherman , Coe Robert S.
Published: April 2008

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.
A Lower Ordovician paleomagnetic pole from the Oneota dolomite, Upper Mississippi River Valley
Authors: Jackson Mike , Van der Voo Rob
Published: April 2008

Of 149 oriented specimens of the lower Ordovician Oneota dolomite subjected to stepwise thermal or alternating‐field demagnetization, 50 contained a characteristic shallow east‐southeasterly or west‐northwesterly component of magnetization identifiable on orthogonal vector projections. For 48 other specimens the demagnetization trajectories defined greats‐circle paths which intersect close to the characteristic direction. The site‐mean directions for the characteristic component were determined by using both the characteristic directions and great‐circle data, and these yield a formation‐mean direction obtained from 10 sites with a declination of 105°, an inclination of +2°, and a precision parameter k of 17.6 (α95 = 11.9°). The mean of the site virtual poles falls at 10.4° N, 166.4° E (K = 48.0, A95 = 7.0°), which is close to previously published late Cambrian paleopoles. Both polarity groups have predominantly positive inclinations, suggesting the presence of an uncleaned downward component whose unblocking‐temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization. We interpret this downward component as a drilling‐induced remanent magnetization, and we hope to have removed its contribution by averaging the dual‐polarity results. Multicomponent magnetizations were also present in samples collected from blocks of Oneota dolomite incorporated into the overlying New Richmond sandstone (lower Ordovician), but only six out of eight samples yielded well‐defined greats‐circle paths. The best fit intersection of these defines a northerly and steep recent component, while the directions of the higher‐stability components are different for each block. Due to the small number of samples and imperfect resolution of magnetic directions, the conglomerate test is not conclusive; nevertheless it suggests that the stable remanence in the Oneota dolomite was acquired prior to the deposition of the New Richmond sandstone.
Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview
Authors: Hill David P. , Bailey Roy A. , Ryall Alan S.
Published: April 2008

Geological, chronological, and structural studies of the Long Valley‐Mono/Inyo Craters area document a long history of related volcanic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km3 prior to its climatic caldera‐forming eruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8–10 km deep with smaller cupolas as shallow as 4–5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1–0.2 km3 by additional magma since mid‐1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8–6.2 earthquakes that occurred in the region beginning in 1978 had non‐double‐couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low‐viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500‐year intervals over the past 2000–3000 years, whereas the Long Valley magma chamber has erupted at about 200,000‐year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity.
Precursors to the Bishop Tuff Eruption: Glass Mountain, Long Valley, California
Authors: Metz J. M. , Mahood G. A.
Published: April 2008

The Glass Mountain volcanic complex represents the oldest rhyolites of the Long Valley magma system. At least 30 high‐silica rhyolites erupted in two major episodes: a minimum of 4.8 km3 (DRE) of lava erupted sporadically between 2.1 and 1.2 Ma, and a minimum of 10.5 km3 erupted at a greater volumetric rate 1.1–0.8 Ma. The rhyolite complex was truncated as a result of caldera collapse on eruption of the Bishop Tuff at 0.73 Ma. Early Glass Mountain lavas are biotitebearing and were cooler and less fluid than later ones, some of which contain pyroxene. This correlates with a crude, long‐term trend with time toward less evolved rhyolite compositions; all Glass Mountain lavas are more evolved than the late‐erupted Bishop Tuff. A large, integrated, magma chamber may not have been present until ∼1.1 Ma, as evidenced by the greater chemical homogeneity of lavas of the younger eruptive episode. Glass Mountain is flanked by a pyroclastic apron containing ash‐flow tuffs and pumice fall deposits, and correlatives of some of the latter have been found as far away as Ventura County, California, and Beaver County, Utah. Explosive activity occurred throughout the eruptive history of Glass Mountain and is not restricted to more evolved compositions. Although apparent volumetric eruptive rates increased about six fold after 1.1 Ma, the eruptions apparently were not large enough to depressurize the magma chamber and initiate a caldera‐forming eruption. As Long Valley is a tectonically active area, release of pressure on the magma chamber due to rupture along the Hilton Creek‐Hartley Springs fault zone seems the most likely mechanism for triggering the catastrophic eruption of the Bishop Tuff.
Constraints on the upper crustal structure of the Long Valley‐Mono Craters Volcanic Complex, eastern California, from seismic refraction measurements
Authors: Hill David P. , Kissling E. , Luetgert J. H. , Kradolfer U.
Published: April 2008

During the summers of 1982 and 1983, field work was completed on 10 new seismic refraction profiles in the Long Valley‐Mono Craters region located within the technically active zone between the eastern Sierra Nevada and the Basin and Range province. The four profiles presented here sample the upper 7–10 km of the crust within the western half of Long Valley cladera, the Mono craters ring fracture system, and the “normal” crust north and northeast of Long Valley. The entire area shares a common crystalline basement consisting primarily of Mesozoic granitic rocks. At a depth of 2 km beneath the topographic surface, the P wave velocity is uniformly 5.6 km/s beneath which the velocity increases with a gradient of about 0.1 s−1. Clear secondary arrivals observed on a profile crossing the west part of the caldera support evidence from 1972 seismic refraction profiles for a reflecting boundary at a depth of 7–8 km beneath the west margin of the resurgent dome, which may represent the top of a magma chamber. Structural details resolved in the upper 2 km of the crust lead to the following conclusions: (1) P wave velocities in the crystalline basement at depths shallower than 2 km vary from 3.6 to 5.0 km/s. This variation reflects differences in the degree of fracturing of the upper portion of the basement. (2) An apparently homogeneous basement in the upper 7–10 km beneath the Mono Craters ring fracture system indicates that any extensive magma chamber must be at least 10 km deep there. (3) The downdropped crystalline basement beneath the western half of Long Valley caldera dips genty (5°–10°) to the northeast. Apparent offsets along steeply dipping caldera‐bounding faults are 1.0–1.7 km along the northern, western, and southern sides of the caldera with largest offsets in the north. (4) Rocks forming the caldera fill fall into essentially three P wave velocity groups. From the surface downward these velocity groups are 100–400 of 1.2–1.8 km/s material corresponding to unconsolidated fluvial or lacustrine deposits or highly fractured rhyolite flows, 200–400 m of 2.8–3.1 km/s material corresponding to sparsely jointed postcaldera rhyolite, rhyodacite and basalt flows, and approximately 1000 m of 3.9–4.4 km/s material corresponding to deposits of Bishop Tuff.
“Slow” physics of large continental ice sheets and underlying bedrock and its relation to the Pleistocene ice ages
Authors: Birchfield G. Edward , Grumbine Robert W.
Published: April 2008

Very simple two‐component models of the earth's cryosphere‐lithosphere system are analyzed. The models consist of a continental ice sheet coupled to a responding earth model. Because of relatively long response times of the bedrock, at least over shorter wavelengths, long‐period (of the order of 50,000 years) internal or “free” oscillations in the system occur over a wide range of parameters and parameterizations. The oscillatory mechanism operates primarily in the ablation zone and in particular at the firn line. These free oscillations could play a dynamic role in the response of the climate system to eccentricity forcing. A simple analysis presented strongly suggests that for the simplest rheological models of the earth response to Pleistocence ice sheets, elastic response is confined to a layer considerably less than the thermal thickness of the lithosphere. Furthermore, because of the striking lack of geological evidence for large depressions of bedrock at the leading edge of growing continental ice sheets, predicted by viscoelastic models of the earth, more sophisticated rheological models of the earth's uppermost layer appear necessary.
The role of water in the deformation of olivine single crystals
Authors: Mackwell S. J. , Kohlstedt D. L. , Paterson M. S.
Published: April 2008

Hydrothermal treatment experiments have been performed on single crystals of San Carlos olivine at 1100° to 1300°C and 300‐ and 1500‐MPa confining pressure, with the oxygen fugacity around the specimen constrained near the Fe/FeO buffer. The diffusion coefficients for the hydroxyl species giving rise to the sharp band and broadband features in the infrared spectrum of hydrothermally treated olivine have been determined to be greater than 10−10 m2 s−1 under all pressure and temperature conditions tested; the solubility of the hydroxyl species seems to vary between crystals but to show little variability for specimens from the same crystal over the temperature range investigated at 300‐MPa confining pressure. Crystals hydrothermally treated at 1300°C, 300‐MPa confining pressure, and 10−5 s−1 strain rate are a factor of 1.5 to 2.5 weaker than those treated in an anhydrous environment. Microstructural investigations suggest that this reduction in strength is due to enhancement of the rate of climb of dislocations in the “wet” experiments. First‐order calculations suggest a stress exponent of n ∼ 2.5 and a water fugacity exponent of m ∼ 1/5 for deformation under hydrous conditions, assuming that the deformation obeys a power law relation.
Statistical analysis of the mean heat flow/reduced heat flow relationship for continents and its tectonothermal implications
Authors: Pujol Jose , Fountain David M. , Anderson Donald A.
Published: April 2008

Previous analysis of the relationship between mean heat flow and reduced heat flow qr indicated that
, suggesting that the upper crustal radiogenic contribution to heat flow was coupled to changes in qr. Statistical reexamination of this relationship reveals that this empirical relationship results for several reasons and that other interpretations may apply in stable tectonic areas. Importantly, the slope of the
line changes significantly when certain points are excluded from the regression with the strongest effect exerted by the Basin and Range and eastern Australia. Because these areas were tectonically active during the Cenozoic, they may be outliers, exhibiting disproportionate influence on the regression results. Because
and qr exhibit large errors and are correlated, we used a computer simulation method to fit a straight line to correlated variables with errors. We find that the slope of the line is reduced and that the fit is sensitive to outliers. If we restrict the analysis to more stable tectonic regions, we determine a
relationship with a small slope and large intercept. Inclusion of data from regions which have been tectonically active in the Cenozoic gives results closer to the
relationship. We suggest that this previous result reflects the presence of data from thermally perturbed regions in the
data set. This, in conjunction with the nearly horizontal line found for stable regimes, indicates that coupling between the upper crust and mantle‐lower crust system is not implied. Furthermore,
for stable regions is about 27 mW/m2 with a small standard deviation (4 mW/m2). The average upper crustal radiogenic component is nearly equivalent but has a large standard deviation (14 mW/m2), suggesting that variations in
for heat flow provinces in stable regions is largely controlled by upper crustal radiogenic variations.
Planar high‐angle faulting in the basin and range: Geodetic analysis of the 1983 Borah Peak, Idaho, earthquake
Authors: Stein Ross S. , Barrientos Sergio E.
Published: April 2008

Geodetic elevation changes record the broad‐scale deformation associated with the M = 7.0 October 28, 1983, Borah Peak, Idaho, earthquake on the Lost River fault. The crest of the Lost River Range rose 0.2 m, and adjacent Thousand Springs Valley subsided 1.0 m, in relation to reference points 45 km from the main shock epicenter. The deformation was modeled by dislocations in an elastic half‐space. A planar fault with uniform dip slip of 2.05±0.10 m, dipping 47°±2°SW and extending to a vertical depth of 13.3±1.2 km, fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution. The geodetic moment is 2.6±0.5×1019 N m (2.6±0.5×1026 dyn cm), and the estimated static stress drop is 2.9±0.4 MPa (29±4 bars). Tests for coseismic slip on listric faults (which flatten with depth) and on detachments (horizontal faults or shear zones) showed fits to the geodetic data that are inferior to those for planar high‐angle faults. No detectable coseismic slip occurred on the Mesozoic White Knob thrust fault, although the low‐angle thrust sheet intersects the south end of Lost River fault near the 1983 mainshock epicenter. If the high‐angle Lost River fault abuts a flat‐lying detachment fault or shear zone, such a structure must lie at depths of >12 km, near the brittle‐ductile transition, where stick‐slip behavior gives way to creep. The depth and geometry of faulting at Borah Peak is similar to that inferred from seismic and geodetic evidence for the 1954 M = 7.2 Fairview Peak, Nevada, and the 1959 M = 7.3 Hebgen Lake, Montana, events, suggesting that if detachments are active at these localities, they are deep and most likely slip by creep.
Seismic anisotropy in the upper oceanic crust
Authors: Stephen R. A.
Published: April 2008

Seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at Deep Sea Drilling Project (DSDP) site 504B on DSDP leg 92. Particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis. There are four diagnostic patterns: (1) Along symmetry axes, where vertically polarized shear waves (SV) are generated but horizontally polarized shear waves (SH) are not generated, the particle motions are purely vertical, (2) for azimuths at which both SV and SH are generated and the SH velocity is significantly faster than SV, a cruciform pattern with horizontal first motion is observed, (3) for azimuths at which both are generated and the SV velocity is significantly faster than SH, a cruciform pattern with vertical first motion is observed, and (4) for azimuths at which both are generated and SV and SH velocities are similar elliptical particle motions are observed. The shear wave particle motions and compressional wave travel times (from a 2‐km radius circle) are consistent with an anisotropic model with hexagonal symmetry. The compressional wave velocity has a two theta azimuthal variation between 4.0 and 5.0 km/s. The symmetry axis is horizontal with an azimuth of N20°W±10°. The spreading direction at the site (6 m.y. age) is north‐south. The observed seismic anisotropy is most probably caused by the preferred orientation of large‐scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting.
The McKinley Sequence of granitic rocks: A key element in the accretionary history of southern Alaska
Authors: Lanphere Marvin A. , Reed Bruce L.
Published: April 2008

The McKinley sequence of granitic rocks consists of several discrete plutons in the central Alaska Range. Most of these plutons crop out south of the Denali fault system (DFS) in the Talkeetna quadrangle. Plutons of the McKinley sequence largely intrude deformed upper Meszoic flysch between the DFS and the northern edges of Wrangellia and the Peninsular terrane, which jointly make up the Talkeetna superterrane. The average K‐Ar age of biotite from nine granites of the McKinley sequence is 57.3 Ma; Rb‐Sr data for whole rock samples indicate that the McKinley sequence cannot be older than 60 Ma. A selected suite of 20 samples of granite and granodiorite range in SiO2 from 65.9 to 77.6%. All 20 samples are corundum normative, and 18 are moderately peraluminous. Initial 87Sr/86Sr ratios range from 0.7054 to 0.7085. The σ18O values range from +11.2 to +14.6‰. These high and variable Sr isotopic ratios, peraluminous nature, rare earth element patterns, and high σ18O values suggest that granitic rocks of the McKinley sequence crystallized from hybrid magmas produced by assimilation of sedimentary rocks by a mantle‐derived melt. Mesozoic flysch is the likely source of the crustal component of the hybrid magmas. Geologic evidence suggests that the Talkeetna superterrane collided with stable Alaska after Early Cretaceous time. The flysch basin, lying south of stable Alaska, was closed by northward movement of the Talkeetna superterrane; maximum age for basin closure and terrane accretion is middle Cretaceous (Cenomanian). Paleomagnetic evidence indicates that all terranes north of the DFS have been part of stable Alaska since the Paleocene and that northward movement of Wrangellia was completed by 50 Ma. Granitic rocks of the McKinley sequence may be products of terrane accretion; the granitic rocks crystallized from hybrid magmas produced during terrane collision and deformation of the flysch basin. Isotopic ages of the McKinley sequence establish the time of final accretion of the Talkeetna superterrane as Paleocene.
Paleomagnetic and petrographic investigation of the Taum Sauk Limestone, southeast Missouri
Authors: Dunn William J. , Elmore R. Douglas
Published: April 2008

Paleomagnetic results from 17 sites in one quarry of the Cambrian (Dresbachian Stage) Taum Sauk Limestone (member Bonneterre Formation) reveal two components of magnetization. On the basis of thermal demagnetization, coercivity spectrum analysis, and petrographic evidence, both components reside in hematite. Sites can be divided into three groups based on behavior during thermal demagnetization and petrographic criteria. The first group consists of massive pink limestones and contains a component with declination 270°, inclination −11°, corresponding to a Cambrian pole at 175.6°E, 3.6°S. The second group of sampling sites consists of specimens with numerous burrows filled with hematite intergrown with calcite. The sites are dominated by the second component of magnetization which has declination 147° and inclination 10° and corresponds to a reversed pole position at 133.1°E, 37.3°N. A significant fraction of this component, which is interpreted to be Carboniferous in age, resides in hematite intergrown with calcite in the burrows. Petrographic relationships indicate that the calcite and hematite were acquired before dolomite, which places a maximum age on dolomitization. The third group consists of massive pink limestones with variable amounts of burrowing and contains both directions of magnetization in roughly equal proportions, with closely overlapping blocking temperature spectra. The presence of two stable directions of magnetization from one location with neither being dominant or even present throughout the quarry indicates the significant effect that diagenetic and sedimentologic processes may have on the magnetization of sedimentary units.
Counting 81Kr atoms for analysis of groundwater
Authors: Lehmann B. E. , Oeschger H. , Loosli H. H. , Hurst G. S. , Allman S. L. , Chen C. H. , Kramer S. D. , Payne M. G. , Phillips R. C. , Willis R. D. , Thonnard N.
Published: April 2008

A new analytical technique with sufficient sensitivity to determine 81Kr concentrations in groundwater is described. The method utilizes single‐atom counting by laser resonance ionization spectroscopy (RIS) in combination with a small mass spectrometer to achieve the required selectivity and sensitivity. A first demonstration of the technique for groundwater analyses is presented in which about 1000 atoms of 81Kr were counted after the processing of gas extracted from only a few liters of water. The potential of the new method for groundwater dating in the range 50,000 to 1,000,000 years is discussed.
Temperatures within comet nuclei
Authors: Squyres Steven W. , McKay Christopher P. , Reynolds Ray T.
Published: April 2008

We have performed a theoretical study of temperatures beneath the surface of a comet's nucleus. We solve the one‐dimensional heat conduction equation for the outer portion of the comet. The upper boundary condition of the model is given by energy balance at the surface of the nucleus, including conduction of heat inward, radiation, insolation as modified by the coma, and sublimation. Our coma model assumes single scattering and includes attenuation of direct sunlight by dust grains, scattering of light onto the nucleus, and infrared radiation by dust grains. The lower boundary condition is zero net heat flux around an orbit. The thermal conductivity expression for the nucleus includes direct conduction at grain boundaries, radiative conduction, and Knudsen flow vapor diffusion. The thermal diffusivity of the nucleus and the resultant temperature profiles are shown to be strongly dependent on the physical properties of the material, including porosity, pore size, and compaction. The temperature profiles and the equilibrium temperature deep within the comet also depend on the functional relationship between thermal conductivity and temperature; the highest deep equilibrium temperatures are found for models where the thermal conductivity increases strongly with increasing temperature. The dependence of temperatures on the albedo and thermal emissivity of the nucleus is also calculated, as well as the variation of temperature with latitude for a variety of pole orientations. The effect of a dust mantle on subsurface temperatures is also investigated. All calculations are presented for short‐period comets with orbits that make them accessible for exploration by spacecraft rendezvous. In situ measurements of the thermal profile in the upper meter of a comet nucleus can substantially constrain the thermal diffusivity of the material, which in turn can provide significant information about the physical properties of the nucleus.
Fine fragments in high‐velocity impact experiments
Authors: Asada Noriaki
Published: April 2008

Ejection angle dependences of size and kinetic energy of high‐velocity fine fragments ejected by high‐velocity impact into basalt targets were studied by exposing secondary targets to the ejecta. According to the properties of the fine ejecta the ejection angles were divided into four regions: outer region (region I, 10° < θ < 40°), outer belt region (region II, 40° < θ < 45°), inner belt region (region III, 45° < θ < 55°) and inner region (region IV, θ > 55°), where θ was measured from the target surface. The size distributions of the fine ejecta were expressed by power law functions with power indices of about −3.5, except for region IV (about −3) in the size range of 1–70 μm. The secondary microcraters smaller than 100 μm in diameter were most abundant in region II, and their diameter distributions were also expressed by power law functions with power indices of about −3. Comminution energy, which was estimated from surface energy of the fine ejecta, was about 10% of the impact energy, and kinetic energy of the high‐velocity fine ejecta, which was estimated from formation energy of the secondary microcraters, was about 40% of the impact energy.
Simple shear of southern California during Neogene time suggested by paleomagnetic declinations
Authors: Luyendyk Bruce P. , Kamerling Marc J. , Terres Richard R. , Hornafius J. Scott
Published: April 2008

We have studied the paleomagnetism of various Neogene age rocks in southern California as a means of determining the amounts of Oligocene and younger tectonic rotation and translation which has occurred in this region. Our results suggest that fully 25% of this area, in particular the Transverse Ranges, has undergone extreme clockwise rotation. Work in southeastern California implies that 40° of clockwise rotation has occurred here, although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200°. The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°. Paleomagnetic results from the offshore islands suggests that San Clemente, Santa Barbara, and San Nicolas islands have not rotated but Santa Catalina has undergone about 90° of clockwise rotation. All of the northern Channel Islands, including Anacapa, Santa Cruz, Santa Rosa, and San Miguel, are implied to have rotated 70° or 80° clockwise. This result is also found for the Santa Monica Mountains east of Anacapa Island. In the Santa Ynez Range north of the Channel Islands, paleomagnetic study of the Monterey Formation also indicates large clockwise rotations of near 90°. These data also suggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphic control on the paleomagnetic data from the Monterey Formation implies that the rotation began about 16 m.y. ago and may be continuing today in the western region. Paleomagnetic inclination data from our study show that the northern Channel Islands, in particular, may have translated 15° northward since middle Miocene time. However, equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections, non dipole magnetic field behavior and right offset on the San Andreas fault system. Palinspastic reconstruction of southern California regions for the early Miocene implies that parts of the Transverse Ranges structures were once aligned with north trending extensional structures in the southwestern United States. We propose that Pacific‐American plate interactions both rifted the continental crust to create this pattern and rotated the western‐most structures within a dextral simple shear zone which had a half width of about 400 km.
Paleomagnetism and neogene tectonics of the Northern Channel Islands, California
Authors: Kamerling Marc J. , Luyendyk Bruce P.
Published: April 2008

Geologic observations and previous paleomagnetic studies have suggested that the western Transverse Ranges arrived at their anomalous east‐west orientation by clockwise tectonic rotation. A paleomagnetic study of the northern Channel Islands was undertaken, in order to test the extent of rotated areas and to develop constraints on tectonic models concerning the formation of the southern California Borderland. The islands of Anacapa, Santa Cruz, Santa Rosa, and San Miguel form an east‐west trending chain and are considered part of the Transverse Range physiographic province. Oligocene through Miocene volcanic and intrusive rocks occur on these islands, and were sampled for this paleomagnetic study. Eocene sandstones were sampled on San Miguel Island, and Eocene sandstones and Miocene siltstones were sampled on Santa Cruz Island. Paleomagnetic results from igneous units are characterized by declinations deflected clockwise by 69° to 81° from expected directions and inclinations which are too shallow by 10° to 25°. Normal and reversely magnetized units yield antipodal mean directions. The mean result is concordant to the directions obtained from individual units which passed fold and baked contact tests. These results suggest that the northern Channel Islands have been tectonically rotated into place since early(?) Miocene time as the outer borderland area translated northwestward within a large shear zone between the Pacific and North American Plates. The data from the northern Channel Islands when averaged with the data from the Santa Monica Mountains yield a result of I = 36.1° ± 5.1°, D = 72.6° ± 6.3° for the southern part of the western Transverse Ranges (52 units, 405 samples). The shallow paleomagnetic inclinations suggest a northward latitude translation of 14.0°+3.7° or −3.9° for the western Transverse Ranges. However, most plate tectonic reconstructions suggest only 4°±3° of northward translation. The discrepancy may be due to initial dips of the volcanic flows which would cause an artificial shallowing of inclination when the structural dip of the units was corrected to an assumed original horizontal. Other causes for the shallow inclinations could be the effects of an offset dipole field or nondipole field components, or motions of small tectonic plates between the North American and Pacific plates as the Farallan plate broke up and was subducted.
Tectonic implications of paleomagnetic poles from Lower Tertiary Volcanic Rocks, south central Alaska
Authors: Hillhouse John W. , Grommé C. Sherman , Csejtey Bela
Published: April 2008

We have determined the paleolatitude of lower Tertiary volcanic rocks in southern Alaska to measure possible poleward translation of the Wrangellia and the Peninsular terranes after 50 m.y. ago. Previous paleomagnetic studies have shown that in Triassic and Jurassic time these terranes were located near the equator and have moved at least 3000 km poleward relative to the North American craton. Our sample localities are in the northern Talkeetna Mountains in mildly deformed andesite and dacite flows (50.4, 51.3, 53.9, and 56.3 m.y. by K‐Ar) that overlap Lower Cretaceous flysch, Lower Permian volcanic rocks of Wrangellia, and Upper Triassic pillow basalt of the Susitna terrane. Results from 26 cooling units (23 of reversed polarity and 3 of normal polarity) give a mean paleomagnetic pole at 69.5°N, 179.6°E, α95 = 12.2°. Stratigraphic sections from opposite limbs of a syncline yield directional paths that pass the fold test, satisfying a necessary condition for primary origin of the magnetization. The corresponding mean paleolatitude (76°N) of the northern Talkeetna Mountains is 8°±10° higher than the latitude predicted from the Eocene reference pole for North America. Therefore, northward drift of the Talkeetna superterrane, which is the amalgamation of the Wrangellia and Peninsular terranes during and after Middle Jurassic time, was probably complete by 50 m.y. ago. Our results are consistent with paleomagnetic poles from uppermost Cretaceous and Paleocene volcanic sequences in Denali National Park, the Lake Clark region, northern Bristol Bay region, and near McGrath. These poles generally lie south of the cratonic poles, suggesting that the region between the Kaltag, Bruin Bay, and Castle Mountain faults has rotated counterclockwise relative to North America since the early Eocene.
Spherical cap harmonic analysis of geomagnetic secular variation over Canada 1960–1983
Authors: Haines G. V.
Published: April 2008

The new technique of spherical cap harmonic anaylsis has enabled a model of secular variation to be constructed that applies to an area of over 30 million square kilometers and a time interval of over 20 years. This technique is a regional analog of the ordinary method of spherical harmonic analysis and ensures that the curl and divergence of the secular variation vector are zero. By making each harmonic coefficient a polynomial in time, the model provides both spatial and temporal continuity. The secular variation data, from 1960 to 1983, comprise 1227 first central differences of annual mean values from 24 observatories in Canada, United States, Greenland, and Iceland, and 939 first central differences from 138 Canadian and American repeat stations, divided by their respective time intervals. The secular variation model includes terms up to spatial index 4 and temporal degree 2. Although such a model allows for 75 coefficients, only 39 were found to be statistically significant. The temporal behavior of the model at observatories and repeat stations is compared on time plots to that of the secular variation data itself. The scatter about the regression model, or standard error of estimate, was 11.8 nT/yr. The model is required both to update aeromagnetic survey data for producing Canadian geomagnetic main field charts at 1985.0 and to provide annual change information for the extrapolation of those charts up to 1990. The average secular variations derived from the Definitive (DGRF) and Provisional (PGRF) International Geomagnetic Reference Fields agree well with the model, but the average secular variation for 1980–1985 from the predictive International Geomagnetic Reference Field (IGRF) does not.
An integrated chemical and stable‐isotope model of the origin of Midocean Ridge Hot Spring Systems
Authors: Bowers Teresa Suter , Taylor Hugh P.
Published: April 2008

Chemical and isotopic changes accompanying seawater‐basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs, incorporating provision for addition and subtraction of a wide‐range of reactant and product minerals, as well as cation and oxygen and hydrogen isotopic exchange equilibria. The models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100° to 350°C, with an overall water‐rock ratio of about 0.5 being constrained by an assumed δ18OH2O at 350°C of +2.0 per mil (H. Craig, personal communication, 1984). This is a realistic model because: (1) the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust; (2) the water‐rock ratio is high (>50) at low temperatures and low (<0.5) at high temperatures; and (3) it allows for back‐reaction of earlier‐formed minerals during the course of reaction progress. The results closely match the major‐element chemistry (Von Damm et al., 1985) and isotopic compositions (Craig et al., 1980) of the hydrothermal solutions presently emanating from vents at 21°N on the East Pacific Rise. The calculated solution chemistry, for example, correctly predicts complete loss of Mg and SO4 and substantial increases in Si and Fe; however, discrepancies exist in the predicted pH (5.5 versus 3.5 measured) and state of saturation of the solution with respect to greenschist‐facies minerals. The calculated δDH2O is +2.6 per mil, in excellent agreement with analytical determinations. The calculated chemical, mineralogic, and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges (Humphris and Thompson, 1978; Stakes and O'Neil, 1982), as well as with data from ophiolites (Gregory and Taylor, 1981). Predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite‐epidote‐chlorite‐tremolite (greenschist) assemblage at 350°C. The models demand that the major portion of the water‐rock interaction occur at temperatures of 300°–350°C. Interaction at temperatures below approximately 250°C results in negative δ18OH2O shifts, contrary to the observed positive δ18O values of the fluids exiting at midocean ridge vents. Hydrogen isotope fractionation curves by Suzuoki and Epstein (1976), Lambert and Epstein (1980), and Liu and Epstein (1984), among others, are compatible with the model, and require δDH2O to increase at all temperatures as a result of seawater‐basalt interaction.