bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data
Authors: Dunlop David J.
Published: October 2002

Although most paleomagnetic and environmental magnetic papers incorporate a Day plot of the hysteresis parameters Mrs/Ms versus Hcr/Hc, a comprehensive theory covering superparamagnetic (SP), single‐domain (SD), pseudo‐single‐domain (PSD), and multidomain (MD) (titano)magnetites is lacking. There is no consensus on how to quantify grain‐size trends within the Day plot, how to distinguish MD from SP trends/mixtures, or whether magnetite, titanomagnetites, and other minerals have distinctive trends by which they might be identified. This paper develops the theory of the Day plot parameters for MD, MD + SD, PSD, and SP + SD grains of titanomagnetite (Fe3 – xTixO4) with compositions x = 0 (TM0 or magnetite) and x = 0.6 (TM60). MD grains have a separate trend that intersects the curve for SD + MD mixtures. SP + SD mixtures generate a variety of trends, depending on the SP grain size. All SP + SD curves lie much above those for MD or SD + MD trends, as has been proposed, but not demonstrated, previously. Data for PSD‐size magnetites of many different origins fall along a single trend, but different levels of internal stress shift points for similar grain sizes along the “master curve.” In order to use the Day plot to determine grain size, one must have independent information about the state of internal stress. Theoretical model curves for SD + MD mixtures match the PSD magnetite and TM60 data quite well, although the SD→MD transition region in grain size is much narrower for TM60 than for magnetite. The agreement between PSD data and SD + MD mixing curves implies that PSD behavior is due to superimposed independent SD and MD moments, either in individual or separate grains, and not to exotic micromagnetic structures such as vortices. The theory also matches Mrs and Hc values in mechanical mixtures of very fine and very coarse grains, although nonlinear mixing theory is required to explain some Hcr and Hcr/Hc data.
Theory and application of the Day plot (Mrs/MsversusHcr/Hc) 2. Application to data for rocks, sediments, and soils
Authors: Dunlop David J.
Published: October 2002

New theoretical curves relating the hysteresis parametersMrs/MsandHcr/Hcfor single‐domain (SD), superparamagnetic (SP), pseudo‐single‐domain (PSD), and multidomain (MD) grains and their mixtures are applied to published data for natural materials. The Day plot ofMrs/MsversusHcr/Hchas been used to crudely classify samples into box‐like SD, PSD, and MD (or sometimes incorrectly, MD + SP) regions with arbitrary boundaries. New type curves for MD, PSD/SD + MD, and SD + SP grains and mixtures permit more subtle and precise modeling. The predicted MD trend and its junction with the PSD trend are observed in two data sets: for magnetite spherules from carbonate rocks and for temperature‐varying hysteresis results spanning the Verwey transition. The latter data are the basis of a suggested new method for pinpointing the PSD‐MD threshold size. Selected data for pottery clays, soils, and paleosols generally follow SD + MD type curves and indicate intermediate‐size PSD magnetite with narrow to broad size distributions. A lake sediment section with known grain‐size progression tracks in the predicted sense along the SD + MD trend. Selected data for glaciomarine and pelagic sediments are also generally compatible with SD + MD trends. Examples of remagnetized carbonate rocks, submarine basaltic glasses, and glassy rims of pillow basalts all follow predicted SP + SD or SP + PSD mixing curves, with a large range in volume fraction of SP grains (0–75%) but a narrow range of SP particle sizes: 10 ± 2 nm. Larger SP grains spanning the range to SD size (25–30 nm) are absent for unknown reasons. Oceanic dolerites, gabbros, and serpentinized peridotites in some cases fall in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.
Structure and elasticity of single‐crystal (Mg,Fe)O and a new method of generating shear waves for gigahertz ultrasonic interferometry
Authors: Jacobsen Steven D. , Reichmann Hans‐J. , Spetzler Hartmut A. , Mackwell Stephen J. , Smyth Joseph R. , Angel Ross J. , McCammon Catherine A.
Published: October 2002

Knowledge of the elastic properties and crystal chemistry of dense oxide structures plays an important role in interpreting the composition and mineralogy of planetary interiors. We report the effects of (Mg,Fe)2+ substitution and nonstoichiometry due to Fe3+ on the crystal structure, elastic constants (cij) and moduli (K0T, K0S, G0) of (Mg,Fe)O utilizing single‐crystal X‐ray diffraction and gigahertz ultrasonic interferometry with a new method of generating high‐frequency shear waves. The new acoustic technique features a P‐to‐S conversion by internal reflection on the oriented facet of a single‐crystal MgO buffer rod. In addition to periclase (MgO) and wüstite (Fe0.95O), we investigated Fe3+‐bearing (Mg,Fe)O single crystals prepared by interdiffusion having ΣFe/(ΣFe + Mg) = 0.06, 015, 0.24, 0.27, 0.37, 0.53, 0.56, 0.75, and 0.79, with ferric iron contents ranging from ∼1 to 12% of the total Fe. The elastic constants (c11, c12, c44) are determined from compressional and shear wave velocities in the [100] and [111] propagation directions in the range of 0.5–1.2 GHz. The c11 and c44 elastic constants soften from periclase to wüstite, whereas the c12 elastic constant increases. The rate of change in the elastic constants with composition (∂cij/∂x) is greatest between MgO and (Mg,Fe)O with ∼25 mol % FeO implying that substitution of Fe into periclase has a greater effect on the elastic properties than adding Mg to wüstite. The elastic anisotropy of (Mg,Fe)O has rather unusual behavior, being essentially constant for the range 0–25 mol % FeO but then decreases linearly with Fe content such that wüstite is elastically isotropic. The elastic properties of (Mg,Fe)O having similar total Fe but varying Fe3+ contents are identical within uncertainty. The isothermal compressibility of samples with ΣFe/(ΣFe + Mg) = 0.27, 0.56, and 0.75 is determined by single‐crystal X‐ray diffraction in a diamond anvil cell to ∼9 GPa. For these samples, K0T = 158.4(4), 155.8(9), and 151.3(6) GPa with ∂KT/∂P = 5.5(1), 5.5(1), and 5.6(2), respectively (where values in parentheses indicate standard deviations). The deviation of ∂KT/∂P from 4.0 corresponds to a difference in calculated density of about one percent for ferropericlase (Mg0.8Fe0.2)O at 30 Gpa from the value predicted by second‐order truncation of the Birch‐Murnaghan equation of state.
Geomagnetic excursions and paleointensities in the Matuyama Chron at Ocean Drilling Program Sites 983 and 984 (Iceland Basin)
Authors: Channell J. E. T. , Mazaud A. , Sullivan P. , Turner S. , Raymo M. E.
Published: October 2002

We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9–2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm kyr−1, respectively. For the older part of the record (>1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference δ18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic δ18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30–60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Réunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190–1215 (Cobb Mountain Subchron), 1255, 1472–1480, 1567–1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.
Structure and structural development of the Havre Trough (SW Pacific)
Authors: Delteil J. , Ruellan E. , Wright I. , Matsumoto T.
Published: October 2002

Northeast of New Zealand the Pacific plate is obliquely subducted beneath the overriding Australian plate. Major differences appear in the oblique‐convergence‐driven structures that occur along the plate boundary. While strike‐slip component of convergence is accommodated in the forearc domain to the south at the eastern North Island margin, we here substantiate that at least part of the strike‐slip component is accommodated within the back arc Havre Trough to the north offshore New Zealand. New swath bathymetry and structural data collected in the Havre back arc domain show that the basin as a whole displays oblique to basin axis structures that allow considering the basin to be the loci of extensive tectonics associated with dextral displacement of the Kermadec Arc Block relative to the Australian plate. Therefore, where the two plates are oceanic, the oblique convergence between Pacific and Australian plates is partitioned at the subduction trench for its normal‐to‐trench component and at the rear of the Kermadec Arc Block for part or all of its along‐strike component where it is intimately associated with extensional rifting within the back arc domain. The kinematic pattern of Australian‐Pacific plate boundary northeast of New Zealand that straddles a limit between continental and oceanic lithosphere allows addressing the specificity of the structural development, the deformation distribution, and the assessment of movement components that occur in a back arc basin associated with oblique convergence.
Aftershocks of microearthquakes as probes of the mechanics of rupture
Authors: Rubin Allan M.
Published: October 2002

Using a waveform cross‐correlation technique, Rubin and Gillard [2000] obtained precise relative locations for 4300 0.5 < M < 3.5 earthquakes occurring along 50 km of the San Andreas fault. This study adds to that another 5000 earthquakes distributed along 10 km of the San Andreas fault and 20 km of the Calaveras fault. Errors in relative location are typically tens of meters for earthquakes separated by hundreds of meters and, after correcting for time‐dependent station delays, meters for earthquakes separated by tens of meters. Along both faults, the minimum separation between consecutive earthquakes scales with magnitude in a manner consistent with a magnitude‐independent stress drop. By treating each earthquake on the San Andreas as if it were a main shock, scaling the distances to all subsequent earthquakes by main shock size, and stacking the results, a “composite” aftershock sequence is produced that has many of the characteristics predicted by rate‐and‐state friction models. Projected onto the fault surface, these aftershocks outline a quasi‐elliptical, roughly 4‐MPa stress drop main shock elongate in the slip‐parallel direction by ∼40%. At the ends of the major axes of this ellipse over twice as many aftershocks occur to the NW than to the SE, an asymmetry attributed to the contrast in material properties across the fault. Unlike the San Andreas, the Calaveras fault exhibits little P wave velocity contrast and no discernible aftershock asymmetry; however, the earliest part of the aftershock sequence on the Calaveras might be truncated by the ∼30‐s “blind” time of the network following a triggering event.
Constraints on the S wave velocity structure in a continental shield from surface wave data: Comparing linearized least squares inversion and the direct search Neighbourhood Algorithm
Authors: Snoke J. Arthur , Sambridge Malcolm
Published: October 2002

In their study of upper mantle structure beneath the Paraná Basin of SE Brazil, Snoke and James [1997] concluded, on the basis of a linearized least squares inversion (LLSI) of surface wave dispersion data, that a strong (5% contrast) low‐velocity zone (LVZ) beginning at a depth less than ∼150 km was not required to fit the data. They were unable to establish a quantitative estimate, however, on the maximum depth at which such a LVZ could be resolved by their data. Sambridge [1999a, 1999b] has introduced the Neighbourhood Algorithm (NA), a direct search method for nonlinear inversion which can be tuned to extract information from an ensemble of models in addition to finding a single best fit model. Applying NA to the Brazilian dispersion data quantifies the statistics of the ensemble of models classified as “acceptable” based on a data misfit criterion and a smoothness constraint. The NA best fit model is not significantly different from the LLSI best fit model, but the analysis of the ensemble of models provides new insights regarding how well constrained the model is. Synthetics runs show that for this data set, our modeling procedures could resolve a strong LVZ that began at a depth of 120 km but could not rule out such an LVZ beginning at a depth of 180 km.
An SH hybrid method and shear velocity structures in the lowermost mantle beneath the central Pacific and South Atlantic Oceans
Authors: Wen Lianxing
Published: October 2002

An SH hybrid method is developed for calculating synthetic seismograms involving two‐dimensional localized heterogeneous structures. The hybrid method is a combination of analytic and numerical methods, with the numerical method (finite difference) applied in the heterogeneous region only and analytical methods applied outside the region. Generalized ray theory solutions from a seismic source are used to initiate the finite difference calculation, and seismic responses at the Earth's surface are obtained from the finite difference output by applying the Kirchhoff theory. We apply the hybrid method and study SH wave propagation near the bottom of the mantle. The shear velocity structures and the interaction of SH waves with these velocity structures at the base of the mantle beneath the central Pacific and South Atlantic Oceans are of particular importance. The observed SH waves sampling these two regions of the core‐mantle boundary show very different characteristics across the epicentral distance range of 83°–108°. The SH waves sampling the core‐mantle boundary beneath the central Pacific show a linearly increasing delay of 4 s from 98° to 108° and discernible ScS phases up to an epicentral distance of 102°. The SH waves propagating through the base of the mantle beneath the South Atlantic Ocean, on the other hand, exhibit a linearly increasing delay of 10 s from 98° to 108°, discernible multiple ScS phases with a same slowness as the direct SH waves up to 108°, and rapid variations of waveforms across small epicentral distances. Synthetic tests indicate that while the observations sampling the central Pacific can be explained by a negative shear velocity gradient of 3% (relative to the preliminary reference Earth model) at the bottom 300 km of the mantle, those sampling the South Atlantic Ocean require a 300‐km‐thick bottom boundary layer with a larger negative velocity gradient (up to 10%) and steeply dipping edges. The negative velocity gradient at the base of mantle beneath the South Atlantic Ocean can be best explained by partial melt driven by a compositional change produced in the early Earth's history and a vertical thermal gradient within the layer, while that beneath the central Pacific may reasonably be attributed to pure thermal effects within a thermal boundary layer.
Effect of normal stress during rupture propagation along nonplanar faults
Authors: Aochi Hideo , Madariaga Raúl , Fukuyama Eiichi
Published: October 2002

We study a symmetrical three‐forked shear fault under simple triaxial stress. Rupture initiates in the central primary plane and propagates toward the branching point. One of the two branches is in the compressional quadrant, while the other is in the tensional domain. We study the following question: Does rupture always prefer propagating along the tensional branch because of the effect of normal stress? When the primary plane is located in the most favorable direction as determined by Mohr‐Coulomb criterion, one finds that the compressional branch is almost always preferred for various dynamic coefficients and confining pressures. Simultaneous rupture propagation on both branches appears only under a very special condition found by Aochi et al. [2000b]. These results indicate that shear stress changes produced by the rupture front dominate during dynamic rupture propagation, compared to normal stress changes. This result depends completely on the orientation of the whole fault system. In all cases, we can explain the branch selection by analyzing the applied initial shear stress and the fault properties. Change of normal stress may play a role similar to shear stress change for disjoint faults since the effect of normal stress change sometimes dominates.
Mechanisms of normal fault development at mid‐ocean ridges
Authors: Behn Mark D. , Lin Jian , Zuber Maria T.
Published: October 2002

Slow spreading ridge segments are characterized not only by small, closely spaced faults that develop near the segment center but also by large, widely spaced faults that develop near the segment ends, typically at the inside corner of a ridge‐offset intersection. In this study we investigate the competing effects of stress accumulation in the lithosphere and the yield strength of the lithosphere in controlling the location of normal fault formation and direction of propagation. Seismic velocity models from the Mid‐Atlantic Ridge in the Oceanographer‐Hayes region and 29°N and the East Pacific Rise at 9°N were used to estimate the along‐axis change in dynamic Young's modulus. Corresponding thermal and rheologic models were calculated to estimate the along‐axis variation in yield strength. We then develop a thin‐plate model to calculate the predicted location of fault initiation or reactivation and the subsequent propagation direction for different combinations of linear along‐axis gradients in Young's modulus and yield strength. On the basis of this model we define two modes of normal fault development at slow spreading segments: mode C (center) faults, which develop at the segment center and propagate outward, and mode E (end) faults, which develop at the segment ends and propagate inward. Mode C faults are predicted to form at ridges where the along‐axis variation in yield strength dominates the along‐axis accumulation of stress. Conversely, mode E faults are predicted to develop at ridges where stress accumulation toward segment ends overcomes the high yield strength in these locations. In addition to the accumulation of stress caused by along‐axis gradients in Young's modulus, we illustrate that shear stresses resisting relative plate motion along a transform fault will generate higher effective stress at inside corners, possibly concentrating mode E faulting in these locations. At fast spreading ridges, where along‐axis gradients in stress and lithospheric strength are relatively small, more uniform patterns of faulting are predicted. The results of this study quantify how the interplay between the along‐axis variations in stress state and the mechanical properties of the lithosphere controls the style of fault development at mid‐ocean ridge segments.
Source scaling of Pn and Lg spectra and their ratios from explosions in central Asia: Implications for the identification of small seismic events at regional distances
Authors: Xie Jiakang
Published: October 2002

Multiple station recordings of Pn and Lg spectra from an mb = 6.6 nuclear explosion and several mb ≤ 3.7 chemical explosions in central Asia are used to invert for source seismic moments (M0), corner frequencies (fc), and path‐variable Q models. A modified Mueller‐Murphy source model fits the nuclear explosion well and fits the chemical explosions reasonably well, although Pn from chemical explosions are complicated by event‐variable spectral fluctuations. New source spectral scalings are derived for underground explosions in a wide mb range between ∼3.7 and 6.6; they preserve features of the previous scalings, such as (1) larger PnM0 than LgM0, (2) quarter‐root scaling between fc and M0, and (3) higher Pnfc than Lgfc by factors of ∼4. A necessary and important consequence of the scalings is that the Pn/Lg spectral ratio grows rapidly in an intermediate frequency range between the fc of Pn and Lg. Since fc scales with event size, this frequency range shifts higher for smaller events. This magnitude dependence is directly confirmed using observed Pn/Lg ratios from the above explosions. It is also confirmed using Pn/Lg ratios observed at station WMQ using many mb ∼ 6 and mb ∼ 5 Soviet explosions. A procedure is proposed to account for then magnitude dependence of Pn/Lg ratios in the explosion identification.
Half graben versus large‐offset low‐angle normal fault: Importance of keeping cool during normal faulting
Authors: Lavier Luc L. , Buck W. Roger
Published: October 2002

We study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere. Two distinct fault patterns are seen depending on whether the brittle layer thickness HL is greater or less than a critical value HC. For a thin layer (HL < HC) a single high‐angle normal fault develops large offset with strong rotation of the abandoned footwall. For thick layers (HL > HC) a rift develops with multiple faults bounding an evolving set of grabens. Numerical results show that the value of HC depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension. In the region where the displacement occurs along the fault the brittle lithosphere thins, and we show that this promotes development of multiple faults. In models with heat transfer only by rock motion and conduction, the advective thinning effect can greatly reduce HC. With rift velocities greater than 1 cm yr−1, HC is reduced by a factor of 5 (to unreasonably small values). However, inclusion of simulated hydrothermal cooling increases HC. This allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick. Extension of layers thicker than 10 km tends to produce a rift with multiple graben structures. Finally, we compare the result of a simplified analytic model of extension to data. We find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions.
Influence of subglacial drainage system evolution on glacier surface motion: Haut Glacier d'Arolla, Switzerland
Authors: Mair Douglas , Nienow Peter , Sharp Martin , Wohlleben Trudy , Willis Ian
Published: October 2002

The relationship between the evolution of subglacial drainage system morphology and spatial patterns of glacier surface velocity was investigated using dye tracing experiments and ground surveying throughout the 1995 melt season at Haut Glacier d'Arolla, Switzerland. With the onset of high and variable melt season discharges, subglacial drainage changed from a predominantly distributed system to a predominantly channelized system. The change occurred later farther up glacier. During the period of drainage evolution the glacier was subjected to three periods of rapidly rising meltwater discharge. The magnitude and spatial pattern of the glacier's velocity response differed between these periods and can be explained in terms of the impact of the evolving drainage system morphology on the amplitude and spatial distribution of basal hydrological forcing. Increasing discharge through a distributed drainage system caused widespread basal forcing and high glacier velocity. Increasing discharge through incipient channels below moulins, not yet connected to the main channel, caused more localized basal forcing and slightly increased glacier velocity. Increasing discharge through a fully channelized drainage system caused no significant basal forcing and glacier velocity was not significantly different from the annual deformation flow pattern. Empirical orthogonal function analysis of flow patterns defined two distinct spatial modes of surface velocity which corresponded closely with the drainage system morphologies inferred to be present during each event. The relative importance of these modes changed through the melt season, suggesting a temporal change in the spatial pattern of hydrologically induced basal forcing.
Cenozoic magmatism in the western Ross Embayment: Role of mantle plume versus plate dynamics in the development of the West Antarctic Rift System
Authors: Rocchi Sergio , Armienti Pietro , D'Orazio Massimo , Tonarini Sonia , Wijbrans Jan R. , Di Vincenzo Gianfranco
Published: October 2002

Geochemical and chronological data for Cenozoic plutons and dikes from northern Victoria Land (Antarctica), were used to propose a tectonic‐magmatic model for this portion of the West Antarctic Rift System (WARS). The seven major plutons are compositionally bimodal, with gabbroic and syenitic portions. Among the 180 studied dikes, most are 1 m thick and have alkali basalt‐basanite‐tephrite compositions, along with minor intermediate rocks. Trachytic‐rhyolitic dikes (up to 50 m thick) are by far less common. The 40Ar‐39Ar data for dikes indicate middle Eocene to early Oligocene ages, the oldest found to date for igneous activity throughout the WARS. The geochronological‐structural framework provides evidence for coeval emplacement of dikes on two main NW‐SE and N‐S striking trends, whereas plutonic‐subvolcanic activity occurred in adjacent crustal sectors at different times. Mafic dikes display major and trace element distribution typical of basalts from oceanic islands, such as high ratios of Nb and Ta to large ion lithophile and heavy rare earth elements, coupled with prominent negative K and Pb anomalies in the primitive mantle‐normalized multielement diagrams. Initial isotopic compositions are within the ranges of 0.70299–0.70372 for 87Sr/86Sr and 4.2–6.3 for εNd. These features are shared by younger Neogene lavas. Geochemical modeling for both mafic dikes and lavas indicates an enriched magma source characterized by a residual potassic hydrous phase, thus pointing out a mantle source nearly uniform throughout 50 m.y. Comparable geochemical signature are reported for the magma source of the other Cenozoic volcanic provinces of the WARS and the whole Antarctic Plate. The new data for the early igneous rocks of the rift put in evidence a chronologic‐structural link between magmatic evolution, regional tectonics and plate dynamics which suggests a model for WARS magmatism that is alternative to current plume hypotheses. We propose that magma genesis and emplacement are related to reactivation of preexisting translithospheric faults, which promoted local decompression melting of an enriched mantle that was previously veined during the decompression episode associated with the amagmatic late Cretaceous extensional rift phase.
Paleomagnetic overprints in ocean sediment cores and their relationship to shear deformation caused by piston coring
Authors: Acton Gary D. , Okada Makoto , Clement Bradford M. , Lund Steve P. , Williams Trevor
Published: October 2002

We use paleomagnetic data from cores collected at Ocean Drilling Program Site 1062 on the Bahama Outer Ridge, as well as observations from prior cruises, to quantify the nature and to assess the causes of drilling overprints. The largest overprint arises from an isothermal remanent magnetization that is imparted to the core during coring or core retrieval. A second type of overprint is apparent only in soft sediments collected with a piston corer and has coercivities and unblocking temperatures that completely overlap those of the primary remanent magnetization, which means it cannot be removed by standard demagnetization methods. This demagnetization‐resistant overprint can produce significant biases in paleomagnetic observations, particularly those from split core sections. We hypothesize that shearing, which occurs as a result of friction on the inside wall of the piston corer as it rips through sediments and cuts a core, deflects the ancient magnetization. We develop and test a model that predicts how shear deformation can rotate remanence‐carrying grains and deflect the paleomagnetic remanence of a split core section. Using the shear model predictions and directions measured on split core sections and U channel samples from the undeformed core center, we estimate the average deformation sustained and then correct the split core data for biases caused by shear deformation. The reduction in the deviations between corrected split core data and U channel data is statistically significant, indicating that the shear model is capable of accounting for a significant portion of the systematic biases observed in paleomagnetic observations obtained from piston cores.
Paleomagnetism of the early Triassic Semeitau igneous series, eastern Kazakstan
Authors: Lyons John J. , Coe Robert S. , Zhao Xixi , Renne Paul R. , Kazansky Alexey Y. , Izokh Andrey E. , Kungurtsev Leonid V. , Mitrokhin Dimitry V.
Published: October 2002

Paleomagnetic results obtained from the early Triassic Semeitau igneous series (SIS) yield a high‐quality pole for eastern Kazakstan that is concordant with reference poles for Siberia and Europe, strongly supporting the hypothesis that Kazakstan was already joined with Eurasia at this time. Of the 25 sites that were measured, data were straightforward for 20 sites, 2 of which recorded normal polarity and 18 of which recorded reverse; 5 sites were discarded because of severe overprinting, unstable demagnetizations, or extremely scattered within‐site directions. Combining sites judged to be the same cooling unit and taking the average of 15 cooling units of equal weight, we calculated a pole located at 56°N, 139°E (N = 15, K = 24.6, α95 = 7.9) for the SIS. We consider this pole to be reliable. The sampling of temporally spaced extrusives and more slowly cooled intrusives and the presence of both polarities strongly suggest that this pole averages secular variation. The 40Ar/39Ar determinations on sanidine crystals from two extrusive units yielded highly consistent ages of 248.2 ± 0.5 and 248.8 ± 0.5 Ma. Comparison of our pole for the SIS with the Siberian reference pole shows relative translation (−8° ± 10°) and rotation (1°±16°) that are indistinguishable from zero at 95% confidence. This result fails to confirm hypothesized post‐Permian rotation of the Siberian craton relative to its margins and provides no evidence for left‐lateral motion on the east‐west shear zones in which the Semeitau is located. The similarity of ages between the SIS and the Siberian Traps supports speculation of a genetic relationship between the two.
Effect of pore geometry on VP/VS: From equilibrium geometry to crack
Authors: Takei Yasuko
Published: October 2002

Seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes, including oblate spheroids, tubes, cracks, and an equilibrium geometry controlled by a dihedral angle. The relative role of liquid compressibility and pore geometry on the “VP/VS” velocity ratio is clarified. The result clearly indicates that P and S velocity structures determined by seismic tomography can be used to verify whether interfacial energy‐controlled melt or fluid geometry (equilibrium geometry) is achieved. Relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio. As a function of the aspect ratio, a significant effect of pore geometry on “d ln VS/d ln VP”, the ratio of the fractional changes in VS and VP, is shown. Equilibrium geometry of the partially molten rocks, characterized by a dihedral angle of 20°–40°, corresponds to an aspect ratio of 0.1–0.15. The value of d ln VS/d ln VP expected for the texturally equilibrated partially molten rocks is shown to be 1–1.5, which is much smaller than that expected for cracks and dikes with an aspect ratio of <10−2–10−3. In the upper mantle low‐velocity regions the seismologically obtained value of d ln VS/d ln VP is within this range beneath the Bolivian Andes (1.1–1.4) but is as high as 2 beneath Iceland (1.7–2.3) and beneath northeastern Japan (2.0). The former region can be regarded as a region where equilibrium geometry is achieved, and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate.
A Holocene paleosecular variation record from 14C‐dated volcanic rocks in western North America
Authors: Hagstrum Jonathan T. , Champion Duane E.
Published: October 2002

A paleosecular variation (PSV) curve for western North America is presented on the basis of 94 virtual geomagnetic poles (VGPs) from dated volcanic rocks sampled at 446 sites. Approximately 60% of the paleomagnetic database has been previously published. A curve defined by “spherical smoothed splines” is fitted to the VGPs, ranked by the quality of the age determinations, where the data density is highest between 3690 and −30 years before present (B.P.) (A.D. 1950), between 7800 and 7050 years B.P., and between 14,060 and 12,700 years B.P. The younger segments of the curve derived from volcanic rocks are similar but less complex than other high‐resolution PSV curves derived from lacustrine sediments, particularly the record at Fish Lake, Oregon. The PSV record from lava flows (PSVL), however, is perhaps more reliable in its general shape and chronology because of the higher fidelity of volcanic rocks as magnetic field recorders and because of the greater density of 14C dates. The new PSVL record provides a partial Holocene master curve for western North America and will be of particular value in dating geological and archeological materials using paleomagnetic directions.
Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada
Authors: Prevec Stephen A. , Cawthorn R. Grant
Published: October 2002

The Sudbury Igneous Complex (SIC) and associated Ni‐Cu‐PGE mineralization has been interpreted in terms of a large meteorite impact event. In this study, the thermal relationship between the large cooling melt sheet and the surrounding country rock is examined in terms of its role in an evolving thermal gradient rather than as a passive receptacle for the melt sheet above. Thermal modeling of this environment is undertaken using physical and thermal constraints appropriate to the SIC and assuming heat dissipation from the 2.5‐km‐thick superheated melt sheet (≥1800°C) by either diffusion with zero convection or by rapid convection within the melt sheet. With zero convection, basal cooling produces a solid base, which lowers conductivity such that the immediate footwall rocks reach ≤1000°C, producing partial melting that extends 200 m into the footwall. In a rapidly convecting melt sheet the initial footwall chill is remelted and high temperatures maintained within the sheet close to the contact. This results in higher temperatures being attained in the immediate footwall (1100–1200°C), inducing complete melting of proximal footwall and partial melting to depths of 500 m below the melt sheet. Proximal footwall consists of Paleoproterozoic Huronian basalts, granitoids and sediments, exposed in the south range, overlying Archaean gneisses and granitoids. Total and partial melting of this material early in the cooling history of the melt sheet and the subsequent gravitational accommodation of these melts according to density would produce a basalt‐dominated basal liquid corresponding to the so‐called contact sublayer. The thermal aureole predicted by our models is consistent with that preserved around the north range of the SIC assuming ∼800 m of thermally induced erosion at the contact.
Modeling the radio echo reflections inside the ice sheet at Summit, Greenland
Authors: Miners W. D. , Wolff E. W. , Moore J. C. , Jacobel R. , Hempel L.
Published: October 2002

Radio echo surveys to determine the thickness of ice sheets often record reflections from inside the ice. To increase our understanding of these internal reflections, we have used synthetic seismogram techniques from early seismic modeling to construct two models. Both models were one‐dimensional; the first considered only primary reflections, while the second included both primary and multiple reflections. The inputs to both models were a radio pulse and data from the Greenland Ice Core Project (GRIP) core of length 3028 m. The ice core data consisted of a profile of the high‐frequency conductivity, calculated from dielectric profile (DEP) measurements, and a smooth profile of the real permittivity. The models produced synthetic radargrams which are the energy reflected from conductivity variations as a function of the two‐way travel time. Both models gave similar results, indicating that multiples do not alter the travel time of the reflections, i.e., no O'Doherty‐Anstey effect at our time resolution. One of the results was then processed to simulate the reflected energy passing through the receiver circuit of a radio echo system and then compared with a recorded trace. The processed result contained many of the larger reflections recorded below about 500 m, including nearly all the features from depths greater than 1500 m, in particular, several interstadial events in the Wisconsin age ice. Since high‐frequency conductivity variations are dominated by chemical changes which are caused by deposition on the surface of the ice sheet, it is possible to conclude that the reflections deep inside the Greenland ice sheet can be treated as isochrons.
Evolution mechanisms of an earthquake swarm under the Hida Mountains, central Japan, in 1998
Authors: Aoyama H. , Takeo M. , Ide S.
Published: October 2002

An earthquake swarm occurred under the Hida Mountains in central Japan in 1998. The focal area, which expanded from south to north, was distributed above the low‐velocity and high‐attenuation zone, which lies under the main range. During the swarm, there was a migration of focal areas with time delays. In this paper, we investigate the spatiotemporal distribution of earthquakes in the swarm and discuss the phenomenon that must have controlled the seismicity. Understanding the complicated spatial pattern of seismicity, we investigate the dependency of the occurrence of earthquakes on the static stress changes caused by preceding earthquakes. Our findings show that, for each of 18 major earthquakes (magnitudes 4.0 or higher), the estimated distribution of the resulting Coulomb failure stress corresponds to the expansion of focal areas and the distribution of the seismically quiescent region. However, the detailed structure of seismicity cannot be explained only by the static stress changes. We discuss two additional factors which may control delayed fractures: flow of underground water for the seismicity around the southernmost focal cluster and stress corrosion process for the leaps of seismicities. The results of this study suggest that the 1998 Hida swarm was strongly affected by the disturbances of a static stress field that was created by the swarm itself, although the magnitudes of static stress change are small. The seismic activity was initiated by swarm‐type seismicity around the southernmost cluster and then expanded northward gradually due to the increase in static stress.
Mechanical fault interaction within the Los Angeles Basin: A two‐dimensional analysis using mechanical efficiency
Authors: Cooke Michele L. , Kameda Ayako
Published: October 2002

Mechanical models examine deformation within eight different structural cross sections proposed by Davis et al. [1989] and Shaw and Suppe [1996] along a northeast‐southwest transect across the Los Angeles Basin, California. Horizontal contraction of the models, constrained by geodetic measurements, yields varying dip‐slip rates along frictionally sliding faults within the different cross sections. Mechanical efficiency analysis using effective stiffness and strain energy density assesses the overall fault system deformation as well as the partitioning of work between fault slip and host rock strain. The cross section interpreted by Shaw and Suppe [1996] has the best fit to paleoseismically determined slip rates and the greatest mechanical efficiency (greatest proportion of work toward fault slip); however, this model produces excessive reverse slip along the Newport‐Inglewood fault. A modified fault configuration with a wedge or blind Puente Hills thrust fault rather than a ramp‐detachment configuration better matches the paleoseismic data with slightly lower mechanical efficiency. Slip rates in the mechanical models based on interpretations of Shaw and Suppe [1996] have much closer match to the geologically determined rates than those estimated from kinematic models. This difference is due to (1) differing time spans of slip rate estimates and (2) deformable rather than rigid host rock in the mechanical models. The mechanical efficiency analysis provides quantitative indicators of overall fault system deformation, including the cumulative effect of interaction between individual faults. Assessment of effective stiffness and strain energy density furthers our understanding of two‐dimensional fault interactions in the Los Angeles Basin and offers great potential for future applications.
Crustal fissuring on the crest of the southern East Pacific Rise at 17°15′–40′S
Authors: Wright Dawn J. , Haymon Rachel M. , White Scott M. , Macdonald Ken C.
Published: October 2002

Fissure densities and widths have been mapped along the axial zone of the superfast spreading southern East Pacific Rise (EPR) at 17°15′–40′S with the near‐bottom DSL‐120 and Argo II imaging systems. We observe that the youngest lava flows (on a relative age scale) are sparsely fissured and that there is a cumulative increase in fissure abundance with time that produces a strong positive correlation between fissure density and relative age of lava flows. Average fissure widths were used to estimate fissure depths. In the 17°15′–40′S area, calculated fissure depths are estimated to extend below the seismic layer 2A/2B boundary, and fissures are widest/deepest where lava flows are youngest. We interpret these wide fissures in relatively young flows to be eruptive fissures. Relatively young lava flows combined with high average fissure widths south of 17°25′S suggest that there may have been recent dike propagation along the ridge crest in this area. In comparison to the northern EPR at 9°–10°N the density of fissuring on the southern EPR is significantly higher, due in part to the higher occurrence of relatively older, more areally restricted pillow lava flows.
Rotating thermal convection experiments in a hemispherical shell with heterogeneous boundary heat flux: Implications for the Earth's core
Authors: Sumita Ikuro , Olson Peter
Published: October 2002

We report the results of thermal convection experiments in a rapidly rotating hemispherical shell with a heterogeneous heat flux at the outer boundary to model the effect of a thermally heterogeneous core‐mantle boundary on the convection in the Earth's outer core. A parameter study is made by varying the heat flux, size, and location of an anomalously heated patch for a range of Rayleigh numbers at ∼108 and a fixed Ekman number 4.7 × 10−6. Experiments show that fully developed, boundary‐driven convection occurs when the radial convective heat transfer from the heater exceeds that of the surrounding boundary region. The flow consists of a large‐scale cyclonic circulation originating from the heater and includes a radially extending spiralling front with a jet. The front is stationary when the sectorial high heat flux region is imposed at low latitudes but becomes unstable when it is imposed at high latitudes. The ratio of applied heat which is partitioned to radial and lateral heat transfer becomes fixed in this regime. Measurements also indicate that there is a close correlation between the flow direction and the statistics of temperature fluctuations. Applied to the Earth, the experiments suggest that there are two scales of flows in the core: fine‐scaled jets and slower, large‐scale flows. The large‐scale hemispherical structure of the core can be interpreted in terms of the boundary‐driven flow driven by the high heat flow region beneath east Asia.
Sonic waveform attenuation in gas hydrate‐bearing sediments from the Mallik 2L‐38 research well, Mackenzie Delta, Canada
Authors: Guerin Gilles , Goldberg David
Published: October 2002

The Mallik 2L‐38 research well was drilled to 1150 m under the Mackenzie Delta, Canada, and penetrated a subpermafrost interval where methane hydrate occupies up to 80% of the pore space. A suite of high‐quality downhole logs was acquired to measure in situ the physical properties of these hydrate‐bearing sediments. Similar to other hydrate deposits, resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework. In addition, sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed. We use monopole and dipole waveforms to estimate compressional and shear attenuation. Comparing with hydrate saturation values derived from the resistivity log, we observe a linear increase in both attenuation measurements with increasing hydrate saturation, which is not intuitive for stiffening sediments. Numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results. We also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate‐bearing sediments. We suggest that this model can be improved and extended, allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact.
Magnetostratigraphy, paleomagnetic correlation, and deformation of Pleistocene deposits in the south central Puget Lowland, Washington
Authors: Hagstrum Jonathan T. , Booth Derek B. , Troost Kathy G. , Blakely Richard J.
Published: October 2002

Paleomagnetic results from Pleistocene sedimentary deposits in the central Puget Lowland indicate that the region has experienced widespread deformation within the last 780 kyr. Three oriented samples were collected from unaltered fine‐grained sediments mostly at sea level to determine the magnetostratigraphy at 83 sites. Of these, 47 have normal, 18 have reversed, and 18 have transitional (8 localities) polarities. Records of reversed‐ to normal‐polarity transitions of the geomagnetic field were found in thick sections of silt near the eastern end of the Tacoma Narrows Bridge, and again at Wingehaven Park near the northern tip of Vashon Island. The transitional horizons, probably related to the Bruhnes‐Matuyama reversal, apparently fall between previously dated Pleistocene sediments at the Puyallup Valley type section (all reversed‐polarity) to the south and the Whidbey Island type section (all normal‐polarity) to the north. The samples, in general, are of sufficient quality to record paleosecular variation (PSV) of the geomagnetic field, and a statistical technique is used to correlate horizons with significant agreement in their paleomagnetic directions. Our data are consistent with the broad structures of the Seattle uplift inferred at depth from seismic reflection, gravity, and aeromagnetic profiles, but the magnitude of vertical adjustments is greatly subdued in the Pleistocene deposits.