bluuebunny/crossref_metadata_2025 huggingface dataset:
Interpretation of core and well log physical property data from drill hole UPH‐3, Stephenson County, Illinois
Authors: Daniels Jeffrey J. , Olhoeft Gary R. , Scott James H.
Published: February 2008

Laboratory and well log physical property measurements show variations in the mineralogy with depth in UPH‐3. Gamma ray values generally decrease with depth in the drill hole, corresponding to a decrease in the felsic mineral components of the granite. Correspondingly, an increase with depth in mafic minerals in the granite is indicated by the magnetic susceptibility and gamma ray measurements. These mineralogic changes indicated by the geophysical well logs support the hypothesis of fractionation during continuous crystalization of the intrusive penetrated by UPH‐3. Two fracture zones and an altered zone within the granite penetrated by drill hole UPH‐3 are defined by the physical property measurements. An abnormally low magnetic susceptibility response in the upper portion of the drill hole can be attributed to alteration of the rock adjacent to the sediments overlying the granite. Fracture zones can be identified from the sonic velocity, neutron, and resistivity measurements. A fracture zone, characterized by low resistivity values and low neutron values, is present in the depth interval from 1150 to 1320 m. Low magnetic susceptibility and high gamma ray values indicate the presence of felsic‐micaceous pegmatites within this fracture zone. An unfractured region present from a depth of 1380 m to the bottom of the hole is characterized by an absence of physical property variations. The magnetic susceptibility and gamma ray measurements indicate a change in the amount of mafic minerals at the base of this otherwise homogenous region of the drilled interval. Abrupt changes and repeated patterns of physical properties within the drill hole may represent interruptions in the crystalization process of the melt or they may be indicative of critical temperatures for specific mineral assemblages within the intrusive.
Systematic change of focal mechanism with depth in the Western Great Basin
Authors: Vetter Ute R. , Ryall Alan S.
Published: February 2008

For most areas in the western Great Basin, focal mechanisms show a consistent pattern of primarily strike‐slip motion for shallow events and oblique or normal slip for deeper events. However, orientation of the axis of least principal stress (T axis) is different for different areas: NW–SE for western Nevada and the Mono Lake region, and NE–SW for the Mammoth Lakes area. Along the remainder of the Sierra Nevada frontal fault zone, T axes show both orientations. In general the change in mechanism with depth is interpreted as being caused by increasing overburden pressure, resulting in rotation of the maximum compressive stress (P axis) from horizontal at depths less than about 6 km to vertical at depths greater than about 9 km. The absence of normal‐slip events at depths greater than 10 km in one area (Mono‐Excelsior‐Luning zone) may be explained by a larger horizontal compressive stress than exists in areas that do have normal faulting at such depths. In some areas conjugate right‐ and left‐lateral shear on nearly vertical fractures may be associated with the formation of clusters of magma‐filled dikes at shallow depths. Assuming strike‐slip faulting to be characteristic of earthquakes with depth less than 6 km and normal faulting for events deeper than 10 km, extrapolation of measured shear stress in the upper few kilometers of the crust provides a rough estimate of the maximum and minimum principal stress as a function of depth. At about 3 km depth we find that S1 and S3 are both horizontal, with values of about 104 and 55 MPa, respectively. At depth of 20 km, S1 is vertical and equal to the overburden pressure, about 530 MPa; S3 is horizontal and about 260–300 MPa.
Paleomagnetism of the Late Cretaceous‐Early Tertiary north‐central Montana Alkalic Province
Authors: Diehl J. F. , Beck M. E. , Beske‐Diehl S. , Jacobson D. , Hearn B. C.
Published: February 2008

Paleomagnetic results have been obtained from the Late Cretaceous‐early Tertiary igneous complexes of the north‐central Montana alkalic province. Data from 94 sites in Eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82.0°N, 170.2°E (A95= 3.5°; k = 18.6), while 36 sites in Paleocene intrusions yield a paleomagnetic pole at 81.8°N, 181.4°E (A95= 5.4°; k = 20.2). These poles differ by only 1.6° and are not significantly different statistically. The 130‐site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander (APW) during the period of magnetization of the igneous centers. Postmagnetization structural complications in these rocks are minimal. The presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration. The northcentral Montana data together with other early Tertiary, Cretaceous, and mid‐Tertiary paleomagnetic results require modification of our earlier APW chronology [Diehl et al., 1980]. It now appears that APW relative to North America since the Early Cretaceous consists of a polar still‐stand during much of the Cretaceous (120–75 m.y. B.P.), a period of rapid movement in latest Cretaceous time (75–65 m.y. B.P.), and a period of slow polar movement thereafter. The onset of this period of rapid APW correlates well with a major change in plate motions at ∼80 m.y. and the beginning of the Laramide Orogeny.
Lunar and terrestrial crust formation
Authors: Walker David
Published: February 2008

Planetary crusts may be accreted, produced in primordial differentiation, or built up piecemeal by serial magmatism. The existence of old, polygenetic, laterally heterogeneous, partial melt rocks in the lunar highlands suggests that the moon produced its early crust by serial magmatism. This view can be reconciled with lunar Eu anomalies, previously thought to support the magma ocean model of crust formation, if complications in the fractionation of mare basalts are recognized. Phase equilibrium and magmatic density information for mare basalts suggest a model in which plagioclase fractionation can occur even though plagioclase is not a near‐liquidus phase. The cryptic fractionation of clinopyroxene in MORB provides a precedent for this model. The necessity for a lunar magma ocean is questioned, but a role for a terrestrial magma ocean of sorts at depth is suggested.
Dissipative melting as a mechanism for core formation
Authors: Turcotte D. L. , Emerman Steven H.
Published: February 2008

Cosmochemical studies strongly favor a near‐homogeneous accretion of the earth. These studies also show that core segregation probably occurred within the first 105 years of earth history. Mechanisms of core formation have received relatively little attention. The principal purpose of this paper is to examine dissipative melting as a possible mechanism for core segregation. For a large iron body migrating through the mantle, the potential energy lost by the body is dissipated by frictional heating. If the body has a radius greater than about 30 km, the frictional heating is sufficient to melt a path through which the body can fall. If the iron body is liquid (as expected) with a low viscosity, it would penetrate the mantle as a diapir. The problem of an immiscible liquid body melting its way through a solid is solved, and a family of diapir shapes is obtained. We find that dissipative heating may be a viable mechanism for core segregation if sufficiently large bodies of liquid iron can form.
Apollo 14 very low titanium glasses: Melting experiments in iron‐platinum alloy capsules
Authors: Chen H.‐K. , Lindsley D. H.
Published: February 2008

This paper describes two techniques that have been developed to produce Fe‐Pt alloy capsules for high‐pressure experiments, and reports liquidus‐phase relations of the Apollo 14 very low titanium glasses determined using Fe‐rich capsules (aFe∼0.6). The liquid is multiply saturated with olivine and clinopyroxene at ≥22 kbar. The multiple saturation is at least 3 kbar higher than that determined using pure Fe capsules and corresponds to a source region at least 60 km deeper if olivine and clinopyroxene were the residual phases. However, we still need independent data on iron activity or oxygen fugacity of the glasses in order to choose a container of optimum composition. Preliminary experiments in Fe‐poor alloy capsules suggest that the valence state of iron and the crystallization sequence in the melt have changed, possibly as a result of oxidizing materials entrapped during the iron‐plating processes. The FeO content of the charge decreases linearly with increasing run duration in experiments using pure Fe capsules. The observation that iron‐rich globules grow with time suggests that the equilibrium Fe° (bleb) + Fe2O3 (liq) = 3 FeO (liq) might be established in the liquid at high pressure. If this explanation is correct, an appreciable amount of ‘FeO’ in the liquid could actually be Fe2O3, and some natural lunar volcanic glasses may contain ferric iron as well.
Morphology and chemistry of projectile residue in small experimental impact craters
Authors: Hörz Friedrich , Fechtig H. , Janicke J. , Schneider E.
Published: February 2008

Small‐scale impact craters (5‐7 mm in diameter) were produced with a light gas gun in high purity Au and Cu targets using soda lime glass (SL) and man‐made basalt glass (BG) as projectiles. Maximum impact velocity was 6.4 km/s resulting in peak pressures of approximately 120–150 GPa. Copious amounts of projectile melts are preserved as thin glass liners draping the entire crater cavity; some of this liner may be lost by spallation, however. SEM investigations reveal complex surface textures including multistage flow phenomena and distinct temporal deposition sequences of small droplets. Inasmuch as some of the melts were generated at peak pressures >120 GPa, these glasses represent the most severely shocked silicates recovered from laboratory experiments to date. Major element analyses reveal partial loss of alkalis; Na2O loss of 10–15% is observed, while K2O loss may be as high as 30–50%. Although the observed volatile loss in these projectile melts is significant, it still remains uncertain whether target melts produced on planetary surfaces are severely fractionated by selective volatilization processes.
Interpretation of integrated‐disk photometry of Callisto and Ganymede
Authors: Pang Kevin , Ajello Joseph M. , Lumme Kari , Bowell Edward
Published: February 2008

Integrated phase curves of Callisto and Ganymede have been interpreted using the Lumme‐Bowell theory. The brightness of these satellites up to about 80° solar phase angle can be explained by the theory, but at larger phase angles (∼120°), the observed brightness drops off more rapidly than predicted. We suggest a physical explanation for the discrepancy: single regolith particles must have phase functions which are much more elongated in the forward or backward scattering directions than allowed for by the Lumme‐Bowell theory. The model parameters derived for Callisto and Ganymede therefore can only approximately represent the surface‐scattering properties. For the dark hemisphere of Ganymede, we derive: ρ (roughness) = 1.0, g (asymmetry factor of the single particle scattering function) = −0.2,
(single‐scattering albedo of a particle) = 0.9. For the eastern hemisphere of Callisto, we obtain ρ = 1.0 and for the western hemisphere ρ = 0.4, with g = −0.2 and
= 0.5 for both hemispheres. The hemispheric asymmetry in Callisto's surface texture can be explained by invoking the formation of an ice film on the trailing side. Such an explanation is consistent with Voyager detailed photometry and thermometry of Callisto. Earth‐based polarimetric measurements of Callisto are reinterpreted so as not to conflict with the Voyager results.
Fluid inclusions in stony meteorites
Authors: Warner Jeffrey L. , Ashwal Lewis D. , Bergman Steven C. , Gibson Everett K. , Henry Darrell J. , Lee‐Berman Richard , Roedder Edwin , Belkin Harvey E.
Published: February 2008

We describe fluid inclusions in five stony meteorites: diogenite ALHA 77256 and chondrites Bjurbole (H4), Faith (H5), Holbrook (L4), and Juin (H5). This brings to seven the number of stony meteorites in which fluid inclusions have been confirmed. The fluid inclusions in diogenite ALHA 77256 display a vapor bubble that decreases in volume from −180°C, the lowest temperature attainable in our microthermometric runs, to homogenization of liquid plus vapor to liquid. Homogenization temperatures are reproducible in each inclusion, and range from 25°C to over 225°C; some vapor plus liquid inclusions remain at 225°C, the highest temperature in our microthermetric experiments. On cooling, the fluid in some inclusions apparently freezes, as indicated by deformation and immobilization of the vapor bubble at low temperatures. However, temperatures of melting are difficult to observe and are not reproducible. Preliminary laser Raman spectroscopy shows symmetric and antisymmetric stretch bands characteristic of H2O. Microthermometric data suggest that the fluid in diogenite ALHA 77256 is aqueous with a high solute content. Fluid inclusions discovered in four chondrites have similar properties.
Mixing of passive heterogeneities by mantle convection
Authors: Olson Peter , Yuen David A. , Balsiger Derick
Published: February 2008

The kinematic processes by which thermal convection can mix together large‐scale mantle heterogeneities are described theoretically and with a series of numerical experiments. We have analyzed the time evolution of a large‐scale passive, diffusive scalar anomaly subject to two models of mantle flow: (1) turbulent flow, in which material elements are elongated exponentially with time, and (2) laminar flow, in which material elements are elongated according to a power law function of time. Mixing time scales are computed for both models. They are found to depend on the initial volume of the heterogeneity, the sample volume (the volume of the mantle which is averaged in process of measuring the anomaly), and the Peclet number, the ratio of strain rate to diffusive transport rate. Predictions from the two models have been tested against numerical simulations of mixing in a cellular convective eddy, at high Peclet numbers. The transport equation was integrated using the Lagrangian method of tracking particles, with the particle density being proportional to the anomaly magnitude. Diffusion was included by adding Gaussian‐random displacements at each time step. Simulations were made with up to 40,000 particles. Mixing times from the numerical experiments agree most closely with predictions from the laminar theory. The theory and the numerical experiments indicate the following: (1) Mixing by creeping flow in the mantle is a cascade phenomenon. Individual Fourier components of an anomaly are transferred up the wave number spectrum at a rate proportional to the mean Lagrangian strain rate. Because of the cascade effect, the time scale for mixing thermal anomalies is short, in the range 75–200 m.y., depending on the initial scale of the anomaly. The time scale for mixing large‐scale compositional anomalies is far longer, approximately the age of the earth. (2) Mixing of compositional anomalies tends to produce a laminated upper mantle, in which ancient heterogeneity is stretched into thin subhorizontal layers.
Implications of Jeffreys‐Lomnitz Transient Creep
Authors: Strick Ellis
Published: February 2008

In 1958 Jeffreys proposed a power law generalization of the logarithmic transient creep earlier attributed to Lomnitz. Although Jeffreys' power law form was admittedly defective in that it became unbounded at infinite time, he did apply it to the viscoelastic behavior of the earth‐moon system. Since then it has been successfully applied by many investigators to mantle rehology and Chandler wobble. Experimental seismic studies indicate that most rock types exhibit the almost constant Q behavior which Lomnitz showed to be associated with his logarithmic creep. In this paper, we study not only the Q behavior related to Jeffreys' power law creep but also other mechanical properties such as a precise spring‐dashpot ladder network realization are developed. In addition, a very simple physically realizable modification of this ladder network leads to a boundedness at long times of Jeffreys' creep in a manner which does not affect his successful application at finite times.
Study of the source processes of the 1965, 1968 and 1978 Oaxaca Earthquakes using short‐period records
Authors: Tajima Fumiko
Published: February 2008

The source processes of the 1965, 1968, and 1978 Oaxaca earthquakes are studied using teleseismic short‐period records which contain more detail of the high‐frequency component of the source‐rupture process than long‐period seismograms. The locations and the fault mechanism solutions of these earthquakes are similar and the recording stations are chosen common to the three earthquakes, so that the relative comparison of the source processes should be meaningful. The source process of each earthquake that was obtained is as follows: The 1965 event was a break of a single high‐stress zone. The 1968 and 1978 events were complex breaks of several high‐stress zones. The complexity of the source process has a good correlation with the foreshock activity (mb≥4.0) in the upcoming rupture zone, that is, the 1965 event had no foreshocks, the 1968 event had several foreshocks, and the 1978 event had one foreshock. For the 1965 event the moment value, 7×1025 dyn cm, which is estimated for the single source is much smaller than that obtained from a surface wave analysis, that is, 2.2×1027 dyn cm by Chael and Stewart.
A seismic refraction study of the Oregon Cascades
Authors: Leaver Donald S. , Mooney Walter D. , Kohler William M.
Published: February 2008

A 275‐km‐long reversed refraction profile in the Oregon Cascades, two shallow earthquakes of magnitude 5 in southern Washington, a shallow earthquake of magnitude 4.6 in northern California, and a previously published analysis of the Bouguer gravity field are used to develop a crustal P wave velocity model for the Oregon Cascades. Travel time analysis of the refraction profile indicates a crustal structure characterized by surface layers with P wave velocities that vary from 2.9 to 5.2 km/s and thicknesses that vary from 2.5 to 5.0 km, upper crustal velocities of 6.1 to 6.5 km/s between the depths of 3 and 29 km, lower crustal velocities near 7.0 km/s between the depths of 29 and 44 km, and a mantle reflector at a depth of 44 km. Comparison with synthetic seismograms supports this structure and shows that the lower crustal and Mono transitions can be better modeled by continuous velocity gradients than by first‐order discontinuities. Arrival times from two shallow earthquakes in southern Washington in 1981 across 14 telemetry stations in the Oregon Cascades show an apparent velocity of 7.62 km/s. On the basis of a previously published gravity analysis, the mean north‐south component of the dip to the crust‐mantle boundary is estimated at 1° down dip to the south. The apparent velocity from the earthquakes and the dip estimate from gravity indicate that the true Pn velocity is 7.70 km/s. Arrival times from a shallow earthquake in northern California in 1978 across telemetry stations in the Washington Cascades are consistent with this model and indicate that the upper mantle velocity reaches 8.20 km/s at a depth of approximately 100 km.
The Yermak Plateau: Formed at a triple junction
Authors: Jackson H. Ruth , Johnson G. Leonard , Sundvor Eirik , Myhre Annik M.
Published: February 2008

The Yermak Plateau is an arcuate marginal plateau located seaward of northwestern Spitsbergen. The plateau is divided into two zones based on bathymetric trends and magnetic characteristics that have roughly coincident boundaries. North of latitude 82°N where the bathymetric contours trend NE, the magnetic amplitudes are large, whereas the southern portion strikes NNW and exhibits a subdued magnetic signature. Beneath the northern section of the plateau crustal refraction velocities of 5.0 km s−1 underlain by 7.2 km s−1 are recorded, while farther south velocities of 4.3, 6.0, and 8.0 km s−1 are measured. The 6.0 km s −1 velocity is similar to ones reported on the continental shelf south of Spitsbergen. Dredged material, gravity, heat flow, and seismic reflection and refraction data indicate the northern region of the plateau is oceanic and formed in conjunction with the Morris Jesup Rise. In contrast, the southern section is thinned continental crust formed by the initial rifting process at a triple junction north of Greenland. Plate reconstructions for anomalies 24 and 13 are consistent with this interpretation. Little space is required for the Yermak Plateau at anomaly 24 because it is composed of oceanic and thinned continental crust. The development of the Yermak Plateau and the Morris Jesup Rise, paired aseismic ridges, took place when the North American, Eurasian, and Greenland plates were moving independently.
Lower mantle convection and geomagnetism
Authors: McFadden P. L. , Merrill R. T.
Published: February 2008

The statistical properties of recent marine magnetic anomaly time scales, covering the period from 165 Ma to the present, are analyzed via the methods presented in the companion paper (McFadden, this issue). The data no longer support the hypothesis that during the last 86 Ma there were two periods, separated by a discontinuity at about 45 Ma, when the properties were stationary. Instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 Ma until it reached zero, resulting in the Cretaceous Normal Polarity Interval. At about 86 Ma ago the process of reversals began again, and the rate of reversals increased linearly with time. There is a slight indication that the rate of reversals reached a maximum about 10 Ma ago and is again decreasing. Analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states, nor need such a concept be invoked to explain long intervals without reversals, such as the Cretaceous Normal Polarity Interval. Quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years. Our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals.
The Fréchet Derivatives of electromagnetic induction
Authors: Chave Alan D.
Published: February 2008

The Fréchet derivatives of the fundamental toroidal and poloidal magnetic modes of electromagnetic induction are examined in detail. The response functions for both modes are shown to be Fréchet differentiable in an L2 norm for general conductivity structures and arbitrary source frequency‐wavenumber morphology. Perturbation forms of the modal Green functions are derived and used to examine the Fréchet kernels for a seafloor controlled source and a Kelvin wave model. In both cases, the TM mode possesses superior resolution ability, especially for low relative conductivity contrasts at depth. The results suggest that induction by the ocean tides can see details of the lithospheric structure at depths of at least 50 km.
Introduction and digest to the Special Issue on Chemical Effects of Water on the Deformation and Strengths of Rocks
Authors: Kirby Stephen H.
Published: February 2008

The important role of pore pressure in promoting such brittle processes as cataclasis, hydraulic fracturing, large‐scale faulting, and earthquakes within the crust is widely accepted in geology and geophysics [Hubbert and Willis, 1957; Hubbert and Rubey, 1959; Handin, 1958; Handin et al., 1963; Brace and Martin, 1968; Healy et al., 1968; Raleigh et al., 1976; Sibson, 1973, 1980; Raleigh and Evernden, 1981]. Provided that fluid pressure is fully communicated with rock pore space, the effective normal stresses that control crack growth, macroscopic fracture, and friction are reduced by the magnitude of the fluid pressui'e. Beyond this physical effect of pore fluids, there are chemical effects of water on the strength of rocks that are also important in governing differential stresseg and flow in the continental crust. Some of these chemical effects of water on rock deformation have long been recognized.
Experimental deformation of topaz crystals: Possible embrittlement by intracrystalline water
Authors: Lee Russell W. , Kirby Stephen H.
Published: February 2008

Crystallographically oriented single‐crystal prisms of gem quality topaz (composition Al2SiO4 (OHx− F1 ‐ x)2 where x = 0.04 ± 0.01) were deformed at a confining pressure of 1.50 GPa, a temperature of 800°C, and a strain rate of 2×10−5 s−1. Under nearly identical conditions, all crystals of anhydrous rock‐forming minerals that have been tested to date, such as olivine, quartz, feldspars, pyroxenes, and refractory oxides, deform plastically; in contrast, our topaz crystals failed by brittle fracture regardless of the orientation of the compression direction. No optical evidence for plastic deformation was detected. Another suite of experiments with compression perpendicular to the (001) cleavage at T = 100°–950°C and a strain rate of 2×10−5 s−1 displayed two regimes of behavior: (1) at T >400°C, fracture strength was independent of temperature, and fracture occurred on one or two surfaces parallel to {103}; (2) at T <400°C, the fracture strength increased rapidly with decreasing temperature, no macroscopic stress drop was observed, and many closely spaced conjugate fractures formed on (103) and (103). The anomalous brittleness of topaz compared to anhydrous silicate and oxide crystals indicates that intracrystalline “water” plays a role in the embrittlement. We suggest that water within the topaz crystals promotes fracture in ways similar to the mechanisms of slow crack growth aided by environmental moisture.
Flow strengths of quartz aggregates: Grain size and pressure effects due to hydrolytic weakening
Authors: Kronenberg Andreas K. , Tullis Jan
Published: February 2008

Novaculites and quartzite ranging in grain size from 1.2–211 μm have been experimentally deformed at confining pressures of 350–1620 MPa under conditions of constant displacement rate and constant deviatoric stress in order to determine the effects of both grain size and pressure on the rheology of quartzite. The amount of water available to the samples was varied so that flow strengths for the entire suite of samples could be compared at several, nominally equal water concentrations; samples were vacuum dried at 800°C for 12 hours, left as is, or sealed in Pt jackets with 0.03–0.4 wt % water added. Novaculites deformed at 800°C and 10−6 /s−1 in the presence of 0.4 wt % water show a continuous decrease in flow strength with increasing confining pressure over the range 350–1590 MPa. At high confining pressures 950–1600 MPa, constant displacement rate experiments show three distinct grain size effects, corresponding to the three levels of water concentration: (1) “grain boundary hardening” for vacuum‐dried samples, (2) grain size independent strength for as is samples, and (3) “grain boundary weakening” for samples deformed in the presence of water. Although grain boundary‐dislocation interactions may lead to grain boundary hardening and grain boundary sliding to weakening, the details of the mechanical data, in combination with microstructural observations, are inconsistent with existing models of intrinsic grain boundary effects. Instead, the strength‐grain size relations are interpreted as resulting from variations in structurally incorporated water, which, in turn, are controlled by diffusion of water to and from the grain boundaries. Finally, constant stress experiments show that the activation energy Q and stress exponent n for creep do not depend on grain size, so that the different strengths observed must be incorporated in the preexponential terms. However, both Q and n show a continuous decrease with increasing amounts of available water, from Q = 300 kJ/mol and n ≃ 4 for vacuum‐heated samples, to Q = 130 kJ/mol and n = 2.6 for water‐added samples.
A discussion of impulses and jerks in the geomagnetic field
Authors: Alldredge Leroy R.
Published: February 2008

French and United Kingdom workers have published reports describing a sudden change in the secular acceleration, called an impulse or a jerk, which took place in 1969. They claim that this change took place in a period of a year or two and that the sources for the alleged jerk are internal. This paper questions their method of analysis, pointing out that their method of piecemeal fitting of parabolas to the data will always create a discontinuity in the secular acceleration where the parabolas join and that the place where the parabolas join is an a priori assumption and not a result of the analysis. It is also shown that the jerk defined as an approximation to the third time derivative is mostly of external origin. A short review of the uses of the terms impulses and jerks is given in the introduction.
Tectonic study of Cascade Range and Columbia Plateau in Washington State based upon magnetotelluric soundings
Authors: Stanley William D.
Published: February 2008

Two magnetotelluric profiles have been recorded in the state of Washington in order to study crustal structures related to the Cascade Range volcanoes. A long magnetotelluric sounding profile was completed from near the Pacific Coast to the eastern part of Washington, covering some 400 km in length. This profile crossed the Cascade Range midway between Mount Rainier and Mount Adams. A shorter east‐west profile of data from southwest of Mount St. Helens to southeast of Mount Adams has also been interpreted. The west end of the long profile from near the Pacific Coast to the vicinity of Mount Rainier can be interpreted to indicate as much as 10 km of conductive Tertiary and possibly pre‐Tertiary sedimentary and volcanic rocks. These rocks have resistivities of 2–10 ohm m as demonstrated on the magnetotelluric soundings and electrical well logs. The interpretations show that these conductive materials are juxtaposed with a pluton in the Goat Rocks Wilderness, southeast of Mount Rainier. This feature and other characteristics on the shorter profile to the south are interpreted to be the cause of geomagnetic variation anomalies observed in the Washington Cascades in other research. The thick conductive units on the west end of the long profile and other assumed sedimentary units detected beneath basalts of the Columbia River region have significance in regional hydrocarbon exploration programs. Displacements in the thick conductive unit occur across the Mount St. Helens seismic zone defined by C. S. Weaver and the general pattern of seismicity in the Puget Lowland and south may be related to the interpreted sedimentary unit. Major differences in crustal electrical structures were observed in the Washington State volcanic Cascades compared to the High Cascades in Oregon and northern California. The main features of the magnetotelluric interpretations can be coupled to offshore seismic and gravity data to provide better constraints on the nature of paleosubduction of the Juan de Fuca plate.
Northward displacement and accretion of Wrangellia: New paleomagnetic evidence from Alaska
Authors: Hillhouse John W. , Gromme C. Sherman
Published: February 2008

Wrangellia is a vast terrane that accreted along the Pacific margin of North America from Oregon to central Alaska. Previous paleomagnetic studies of the Triassic volcanic rocks of Wrangellia in south‐central Alaska and British Columbia have given paleolatitudes of 10°–17°, which are anomalously low in comparison to results from similar‐aged rocks of nuclear North America. The discrepancy in paleolatitude implies substantial northward drift of the terrane relative to the craton. We have extended paleomagnetic sampling to the northern boundary of the terrane in the Mount Hayes and Healy quadrangles of the central Alaska Range. This collection of 46 Triassic lava flows yields a mean paleolatitude of 13.9° (α95 = 3.8°) the polarity of the data and hence the hemisphere of origin are not resolved with certainty. In contrast, Triassic paleomagnetic poles from the stable part of North America predict a paleolatitude of 42.1°N (α95 = 4.6°) for the region. The northernmost fragment of Wrangellia probably reached mainland Alaska in the middle Cretaceous, when Upper Jurassic and Lower Cretaceous flysch within the intervening basin underwent severe deformation. Tertiary transcurrent faulting has modified the positions of Wrangellian fragments in Alaska, but such displacements are probably minor, as indicated by paleomagnetic data from early Tertiary volcanic rocks that overlap the terrane.
Effects of attenuation on reflections: Experimental test
Authors: Bourbié Thierry , Nur Amos
Published: February 2008

A well‐known effect of seismic wave attenuation is the change in frequency content and amplitude of a pulse propagating through a lossy medium. A related aspect of wave propagation in lossy media is the frequency dependence of the reflection coefficients. This phenomenon is studied here experimentally and by computer simulation. It is found that attenuation contrast has a relatively large effect on reflections when the acoustic impedance (i.e., density times velocity) difference of the two media is small. While this is a common situation in the upper crust of the earth, very low acoustic impedance contrast is difficult to create in the laboratory. By using carefully chosen samples of polymer and silicon rubber, we show that with low acoustic impedance contrast and high attenuation contrast, an increase in the amplitude of reflections due to Q contrast is observable. We also show that there is no observable phase effect due to the attenuation contrast. By measuring and simulating the reflected signals at angles of incidence to 30°, we find that the Q contrast effect is important only at small angles and that it dies out at large ones.
Faulting associated with large earthquakes and the average rate of deformation in central and eastern Asia
Authors: Molnar Peter , Qidong Denq
Published: February 2008

We determine approximate average rates of deformation in portions of eastern and central Asia for the last 80 years. These rates are based on estimates of source dimensions, average displacements, seismic moments, and orientations of faults of all large earthquakes (M>7.5) and as many other major earthquakes (M≥7.0) as possible in Asia since 1900. A major part of this study is the compilation of these data. For separate parts of Asia we combine seismic moment tensors to estimate the average rotational strain (or deformation) tensors for these regions for the last 80 years. We estimate that shortening across the Tien Shan has occurred at 11 mm/yr (between 5 and 19 mm/yr). Left‐lateral shear on easterly trending planes in Mongolia, in northwestern Tibet, Gansu, and Ningxia, and on the Xianshuihe fault in western Sichuan has been very rapid, more than 10 mm/yr in each region, and apparently as rapid as 40 mm/yr in northwestern Tibet, Gansu, and Ningxia. Slower rates of a few to 10 mm/yr appear to characterize east‐west extension and conjugate north‐south shortening of Tibet. The earthquake history of the Himalaya is consistent with rates of underthrusting of more than 10 mm/yr but is probably less than the convergence rate of India with Eurasia of about 50 mm/yr. The overall strain field is consistent with a large part of India's penetration into Eurasia being absorbed by the extrusion of material out of India's way. As a result, southeast China moved at about 21 mm/yr east‐southeast with respect to Eurasia in the last 80 years with an uncertainty of about a factor of 2 in this estimated rate.
Magnetic properties of granitic rocks from the southern half of the Idaho Batholith: Influences of hydrothermal alteration and implications for aeromagnetic interpretation
Authors: Criss R. E. , Champion D. E.
Published: February 2008

The measured magnetic susceptibilities of rocks from more than 600 outcrops in the southern Idaho batholith have a good spatial correspondence with the observed intensities of magnetic anomalies. Koenigsberger ratios are low (≤0.3) in 75% of cored samples, indicating that remanent magnetizations are unimportant relative to induced magnetizations (which are related to susceptibilities) in aeromagnetic anomaly interpretations in this region. Extremely low susceptibilities (χ<5×10−5 cgs) occur in the leucocratic granite and granodiorite plutons that comprise the ∼7000‐km2 interior of the batholith, and there is a corresponding broad aeromagnetic low over the same region. Several zones of “high” susceptibility rock (χ>25×10−5) within the eastern and western parts of the batholith are the sources of positive aeromagnetic anomalies; the associated lithologies include Mesozoic tonalite and granodiorite, as well as a diverse Tertiary group of epizonal stocks and batholiths. Intense meteoric‐hydrothermal activity associated with the emplacement of these Tertiary plutons caused a reduction in χ in some rocks, although coarse igneous magnetite is surprisingly resistant to hydrothermal corrosion and oxidation. However, under conditions of lower water/rock ratios, a significant increase in χ commonly occurred in the leucocratic plutons, generally reflecting growth of hydrothermal magnetite. Because the measured susceptibilities do not necessarily represent more than the uppermost part of the 14‐ to 20‐km thick crust that gives rise to magnetic anomalies, an apparent discordance between susceptibility measurements and susceptibilities inferred from aeromagnetic data can occur. Such discordance patterns occur near the margins of the batholith, where granitic rocks are thin, and also in hydrothermally altered regions, where subsurface Tertiary plutons are present. Magnetic models suggest that the Idaho batholith thickens to ∼20 km in its east‐central part, which is underlain by enormous Tertiary intrusive bodies.
Lateral offsets and revised dates of large prehistoric earthquakes at Pallett Creek, southern California
Authors: Sieh Kerry E.
Published: February 2008

Recent excavation and new radiocarbon dates of sediments at Pallett Creek are the basis for new conclusions regarding the late Holocene history of the San Andreas fault. Systematic dissection of a 50‐m‐long, 15‐m‐wide, 5‐m‐deep volume of earth, centered on the fault, enables documentation in three dimensions of fault patterns, lateral offsets, and vertical deformation associated with large earthquakes of the past. The excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 A.D., with an average recurrence interval of about 145 years. Prehistoric slip events that occurred in 1720±50, 1550±70, 1350+50, 1080±65, and 845±75 A.D. have lateral offsets that are comparable to those of the most recent great earthquake of 1857. Thus all of these events represent earthquakes of large magnitude. The lateral offsets of two other events, in 935±85 and 1015±100 A.D., are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events. The new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years but suggest a monotonic decrease in individual intervals to below this range during the past 900 years. On the basis of these data, the probability of a large earthquake with surficial fault rupture at this site is between 0.2 and 5% during 1984 and 7 and 60% by the year 2000.
The pattern of anomalous geomagnetic variation fields over the midcontinent gravity high
Authors: Prugger Arnfinn F. , Woods Dennis V.
Published: February 2008

Magnetometer array data, collected by H. Porath and co‐workers from the University of Texas at Dallas in the midwest United States during the fall of 1969, have been reanalyzed to further define the nature of the reported geomagnetic variation anomaly in the region of the midcontinent gravity high. Variation events with a wide range of horizontal source field polarization and frequency have been digitized from the original film records. These data have been used in a vertical‐field response arrow (induction vector) analysis. The results indicate that there is no significant overall conductivity anomaly associated with the geologic structure causing the gravity feature. Rather, the pattern of anomalous variation fields is one of isolated anomalies at high frequency. These anomalies are coincident with postulated fault structures perpendicular to the gravity high lineament and with localized regions of high heat flow.
Seismic evidence for an ancient rift beneath the Cumberland Plateau, Tennessee: A detailed analysis of broadband teleseismic P waveforms
Authors: Owens Thomas J. , Zandt George , Taylor Steven R.
Published: February 2008

Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of Regional Seismic Test Network station RSCP are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source‐equalized horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat‐lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of RSCP. The results from events arriving from the northeast show a high‐velocity midcrustal layer not present in results from the southeast azimuth. This velocity structure can be shown to support the idea that this feature is part of a Keweenawan rift system. Another interesting feature of the derived velocity models is the indication that the crust‐mantle boundary beneath the Cumberland Plateau is a thick, probably laminated transition zone between the depths of 40 and 55 km, a result consistent with interpretations of early refraction work in the area.
Shock wave equations of state using mixed‐phase regime data
Authors: Watt J. Peter , Ahrens Thomas J.
Published: February 2008

A method is given that uses Hugoniot data in the mixed‐phase regime to constrain further equation of state (EOS) parameters of low‐ and high‐pressure phases of materials under‐going phase transformations on shock loading. We compute the relative proportion of low‐ and high‐pressure phases present in the mixed‐phase region and apply additional tests to the EOS parameters of the separate low‐ and high‐pressure phases by invoking two simple requirements: the fraction of high‐pressure phase (1) must increase with increasing shock pressure, and (2) must approach one at the high‐pressure end of the mixed‐phase regime. We apply our analysis to previously published data for potassium thioferrite, KFeS2, and pyrrhotite, Fe0.9S. We find that including the mixed‐phase regime data in the KFeS2 analysis requires no change in the published high‐pressure EOS parameters. For Fe0.9S we must modify the high‐pressure phase EOS parameters to account for both the mixed‐phase and high‐pressure phase Hugoniot data. Our values of zero‐pressure density, bulk modulus and first pressure derivative of the bulk modulus of the high‐pressure phase of Fe0.9S are 5.3 Mg/m3, 106 GPa, and 4.9, respectively.
Deformation of Long Valley Caldera, Mono County, California, from 1975 to 1982
Authors: Denlinger Roger P. , Riley Francis
Published: February 2008

Teleseismic and gravity data acquired in 1973 support the existence of a large body of partial melt (magma) beneath the resurgent dome of Long Valley caldera in eastern California. Whereas intermittent reoccupation of vertical and horizontal control networks in and near Long Valley caldera revealed only very small, probably tectonic movements between 1932 and 1979, a leveling survey in September 1980 showed a 250‐mm uplift of Highway 395 on the west side of the resurgent dome. This uplift was apparently related to expansion of the subjacent magma body beginning sometime between July 1979 and September 1980 and may have been coseismic with the Mammoth Lakes magnitude 6 earthquake sequence of May 25–27, 1980. These earthquakes ushered in a period of almost continuous low‐level seismicity, punctuated by relatively severe swarms in the southern moat. Vertical and horizontal displacements within Long Valley caldera between 1979 and 1982 define a striking radially symmetric pattern of uplift and horizontal distension that is consistent with expansion of the body of partial melt inferred from teleseismic and gravity data. Specifically, the observed deformation of the caldera floor approximates the theoretical deformation due to expansion of a spherical magma chamber at a depth of at least 7 km. The calculated maximum uplift is 430 mm and is located south of the center of the resurgent dome. Significant deviations from this simple model appear to be associated with either movement of mapped faults within and adjacent to the caldera or folding of weak surface deposits.
Caldera volcanoes of the Taupo Volcanic Zone, New Zealand
Authors: Wilson C. J. N. , Rogan A. M. , Smith I. E. M. , Northey D. J. , Nairn I. A. , Houghton B. F.
Published: February 2008

The Taupo volcanic zone (TVZ) has been active since 2 Ma and has erupted >104km3of dominantly rhyolitic magma during the last 1 m.y. Most of the volcanism is concentrated in a 125×60 km area forming the central TVZ and is expressed largely as six major caldera volcanoes, Rotorua, Okataina, Kapenga, Mangakino, Maroa, and Taupo, marked by localized collapse of the underlying basement and clustering of known or inferred vent sites. These centers have activity spans from 150 to 600 ka and have each erupted at least 300 to 1000 km3of magma. All centers except Rotorua are known or inferred to have had complex histories of multiple caldera collapse, which have occurred alongside general basement collapse within the TVZ accompanying regional extension. Deep‐seated NE trending basement lineations and/or faults have influenced vent sites at Okataina, Maroa, and Taupo. Welded ignimbrites are prominent in the pre‐140 ka record; their absence since then is attributed to the effects of surface water on eruption styles rather than to a change in eruptive behavior. Volcanism from the centers has been overwhelmingly rhyolitic (>97% SiO269–77 wt%) with minor high‐A1 basalt and dacite and traces of andesite, mostly as lithic fragments in ignimbrites from Okataina and Mangakino. Although insignificant in volume, the basalt is important as a low‐Si end‐member in mixing relationships with the rhyolite (at one extreme generating the dacites) and occasionally as a trigger for the rhyolitic eruptions. The current average rhyolite magma eruption rate from the central TVZ is ∼0.27 m3s−1, equally divided between Okataina and Taupo, a figure close to the long‐term average for the last 1.1 Ma. However, geothermal heat flow data imply that a further 1.4–1.8 m3s−1of magma may be intruded within the crust. The ratio of inferred intruded material to erupted material is higher at centers where lava extrusions are volumetrically significant (Okataina, Maroa), and this is correlated with lower phenocryst equilibration temperatures in the eruptives. Evidence for resurgent doming and long‐term (>105years) magma cycles documented at similarly sized rhyolitic calderas in the western United States is absent from the TVZ centers; this is attributed to the young faulted crust of the region, preventing the formation of sufficiently large high‐level magma chambers. In overall terms, the central TVZ is comparable in size and longevity to the Yellowstone system, but its individual eruptions have very much shorter recurrence intervals and smaller volumes.