bluuebunny/crossref_metadata_2025 huggingface dataset:
Intrusion of horizontal dikes: Tectonic significance of Middle Proterozoic diabase sheets widespread in the upper crust of the southwestern United States
Authors: Howard Keith A.
Published: February 2008

Initially horizontal sheet intrusions of Middle Proterozoic diabase are abundant in a region 650 by 300 km across in Arizona and California. The diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences. Massive granite is the most common basement host for the sheets, probably because it fractured more easily than foliated hosts during sheet emplacement. Steep feeder dikes are rare compared to the sheets. The diabase in many places is exposed in fault blocks that were tilted during Tertiary tectonic extension. Structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust, to depths of at least 13 km. Sheet intrusion implies a vertical orientation of the least compressive stress, so I conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1.1 Ga. Magma overpressures, water encountered by rising magma, vertical changes in the crustal stress regime, and flotation of low‐density granite all may be important factors for sheet intrusion. The stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event. The diabase province contrasts structurally with similar‐age provinces of basaltic magmatism elsewhere in North America that show evidence of tectonic extension, such as the midcontinent rift. The consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks. Sheet intrusions in the geologic record may be seriously underreported. Their recognition is important for the interpretation of seismic reflection profiles of continental crust. Sheets in Arizona may be responsible for the Bagdad reflection sequence, which extends to depths of at least 15 km.
Differential rotation between lithosphere and mantle: A consequence of lateral mantle viscosity variations
Authors: Ricard Yanick , Doglioni Carlo , Sabadini Roberto
Published: February 2008

The description of plate motions in the so‐called hotspot reference frame introduces a global rotation of the lithosphere with respect to the mantle. This rotation, called toroidal field of degree 1, is roughly westward. It reaches an amplitude of about 2 cm/yr and has been consistently found in the different generation of plate tectonic models. Various authors have tried to relate this observation to the deceleration of the Earth's rotation, to polar wander, or to tidal drag. However, these different physical mechanisms cannot explain the requested amplitude. In this paper, we compare the values of this rotation vector using different relative plate motion models expressed in the hotspot reference frame. In a model Earth with lateral viscosity variations, a differential rotation is predicted. The observed net lithospheric rotation is consistent with the dynamics of a model Earth where the asthenospheric viscosity below the oceans is at least one order of magnitude lower than underneath the continents. This relative westward drift of the lithosphere may account for the significant structural differences between east or west dipping subduction zones.
Limited extension during peak Tertiary volcanism, Great Basin of Nevada and Utah
Authors: Best Myron G. , Christiansen Eric H.
Published: February 2008

The relative timing and magnitude of middle Tertiary extension and volcanism in the Great Basin (northern Basin and Range province) of the western United States remain controversial. To constrain the timing, we present 31 stratigraphic sections from the central part of the province, together with data from other studies in the Great Basin. Especially significant in this record of regional paleogeographic and associated tectonic conditions are thick sections of many well‐dated ash flow sheets emplaced during the period of the most voluminous, or peak, volcanic activity about 31–20 Ma. From these data we make the following conclusions: (1) Extension prior to the period of peak volcanism was apparently localized. (2) Extension during peak volcanism (the ignimbrite flareup) was minor and in places possibly related to magmatic processes in the shallow crust, rather than to regional tectonic processes. Angular unconformities and interbedded epiclastic deposits within sequences of volcanic rocks from 31 to about 22–20 Ma that would manifest synvolcanic faulting, tilting, and erosion are limited. (3) In the Great Basin as a whole, major extension and peak volcanism correlate poorly in space as well as time. (4) Essentially dip‐slip faults cutting the entire conformable volcanic sequence are common in the Great Basin and indicate a widespread episode of extension after peak volcanism. Southward sweeping Tertiary volcanism in the Great Basin reflects migration of the mantle magma supply that powered crustal magma systems. We suspect this migration was related to progressive southward foundering and steepening of dip of a subducting oceanic plate (after an earliest Tertiary near‐horizontal configuration beneath the continental lithosphere) and consequent backflow of asthenospheric mantle into the widening wedge between the plates. In the northern Great Basin, where the sweep was rapid, we postulate that relatively small volumes of mantle‐derived magma were inserted as dikes into the lower, locally extending crust which was unusually warm because of Mesozoic compressional thickening; crustal magma systems so powered were repeatedly tapped to feed modest volume eruptions of chiefly intermediate composition lava and minor silicic ash flow tuff. As the sweep stagnated in the central southern Great Basin, copious volumes of mafic magma were inserted into the crust, apparently mostly as extensive horizontal sheets, or sills, in a nonextending, uplifting crust in a state of nearly isotropic horizontal stress. These sills and the high mantle power input optimized crustal magma generation, creating huge volumes of silicic magma that vented as large volume ash flows, chiefly about 31–20 Ma. After about 22–20 Ma, the volcanic‐capped plateau collapsed in a widespread network of north striking extensional faults as plate boundary compressive forces were overcome by spreading forces within the uplift. Eruption of lava again became the dominant mode of volcanism.
Analysis of lateral coherency in wide‐angle seismic images of heterogeneous targets
Authors: Gibson Bruce S.
Published: February 2008

The character of deep reflections recorded in wide‐angle seismic experiments often suggests fine‐scale layering in the structure of the reflecting target. The lateral continuity of wide‐angle reflections is enhanced, however, because energy arriving at a long‐off set receiver is confined to a narrow range of apparent slowness. The distribution of energy with slowness was studied by Levander and Gibson (this issue), who show that the restricted range amounts to a dip filter. Their energy‐slowness distribution is used here to relate lateral correlation in a reflected wave field to the correlation properties of a randomly heterogeneous target. Tests with finite difference synthetic data from Levander and Gibson confirm a simple convolutional relation between the statistics of the wave field and those of a target with small‐magnitude velocity variations. Field data recorded in the Basin and Range Province, Nevada, show a progressive increase in reflection continuity with increasing source‐receiver offset, as expected. Interpretation of reflector heterogeneity for this data set, however, is complicated by noise contamination and scattering above the target.
Fluid‐mechanical models of crack propagation and their application to magma transport in dykes
Authors: Lister John R. , Kerr Ross C.
Published: February 2008

The ubiquity of dykes in the Earth's crust is evidence that the transport of magma by fluid‐induced fracture of the lithosphere is an important phenomenon. Magma fracture transports melt vertically from regions of production in the mantle to surface eruptions or near‐surface magma chambers and then laterally from the magma chambers in dykes and sills. In order to investigate the mechanics of magma fracture, the driving and resisting pressures in a propagating dyke are estimated and the dominant physical balances between these pressures are described. It is shown that the transport of magma in feeder dykes is characterized by a local balance between buoyancy forces and viscous pressure drop, that elastic forces play a secondary role except near the dyke tip and that the influence of the fracture resistance of crustal rocks on dyke propagation is negligible. The local nature of the force balance implies that the local density difference controls the height of magma ascent rather than the total hydrostatic head and hence that magma is emplaced at its level of neutral buoyancy (LNB) in the crust. There is a small overshoot beyond this level which is calculated to be typically a few kilometres. Magma accumulating at the LNB will be intruded in lateral dykes and sills which are directed along the LNB by buoyancy forces since the magma is in gravitational equilibrium at this level. Laboratory analogue experiments demonstrate the physical principle of buoyancy‐controlled propagation to and along the LNB. The equations governing the dynamics of magma fracture are solved for the cases of lithospheric ascent and of lateral intrusion. Volatiles are predicted to be exsolved from the melt at the tips of extending fractures due to the generation of low pressures by viscous flow into the tip. Chilling of magma at the edges of a dyke inhibits cross‐stream propagation and concentrates the downstream flow into a wider dyke. The family of theoretical solutions in different geometries provides simple models which describe the relation between the elastic and fluid‐mechanical phenomena and from which the lengths, widths and rates of propagation can be calculated. The predicted dimensions are in broad agreement with geological observations.
Evaluation of paleomagnetism as a correlation criterion for Mogollon‐Datil Ignimbrites, southwestern New Mexico
Authors: McIntosh W. C.
Published: February 2008

Paleomagnetic and rock magnetic data show that 36–24 Ma ignimbrite outflow sheets in the Mogollon‐Datil volcanic field carry thermoremanent magnetizations (TRMs) that provide useful, reliable correlation criteria. Over 90% of 404 ignimbrite sites exhibit well‐grouped (α95<10°) TRMs carried primarily by finely dispersed, high‐temperature‐oxidation assemblages of magnetite and hematite. Chemical remanence or randomly directed, lithic‐hosted magnetizations obscure the TRM in a few sites in altered or poorly welded, lithic‐rich tuff. TRM directions are generally laterally and vertically consistent within individual ignimbrite outflow sheets and show no evidence of syncooling secular variation or subblocking temperature flowage. The chief source of within‐unit variation in site‐mean directions is uncertainty in structural corrections, particularly in areas of strong tectonic extension. Nine regional ignimbrite outflow sheets show distinctive to unique unit‐mean TRM directions, which provide strong correlation criteria. Paleomagnetic correlation of these and other units is most powerful when combined with 40Ar/39Ar age determinations and lithologic, stratigraphic, and geochemical data. Thick (>500 m) intracaldera ignimbrite sequences agree in polarity with associated outflow sheets but show less consistent TRM directions, probably reflecting syncooling secular variation and discrepancies between eutaxitic foliations and the syncooling paleohorizontal. Mean virtual geomagnetic pole positions (VGPs) of normal and reverse polarity ignimbrites are nearly antipodal (175.6° apart), showing no evidence of long term differences in the zonal harmonics of the normal and reverse polarity states. Less antipodal mean VGPs (161.2° apart) from normal and reverse polarity lavas are interpreted as defects in the data set, probably including incompletely removed normal polarity viscous overprints. The mean paleomagnetic pole position for Mogollon‐Datil ignimbrites (n=68 independent unit‐mean VGPs) is located at (78.8°N, 166.6°E, K=19.8, A95=4.0, σ =18.2°).
Chemical remanent magnetization in synthetic magnetite
Authors: Pick Thomas , Tauxe Lisa
Published: February 2008

As a magnetic grain produced by a chemical process below its blocking temperature grows through a critical volume in the presence of a magnetic field, its moment becomes blocked and it acquires a chemical remanent magnetization, CRM. Despite its importance to paleomagnetism, the properties of CRM and the controls on its behavior are still poorly understood. We have modified techniques developed by Taylor et al. [1987] to synthesize magnetite at room temperature and atmospheric pressure, whereby magnetite is produced by alteration of “green rust” under reducing conditions. The period of synthesis is twenty‐four hours and a viscous component acquired over this amount of time can be demagnetized with alternating fields of 10 mT. The average direction of the CRM parallels the direction of the applied field and the intensity increases with increasing intensity of the applied field. The intensity is linearly related to the applied field up to approximately 1 mT, consistent with existing CRM theory developed by analogy to the acquisition of thermal remanent magnetization (TRM). Above about 1 mT, however, the CRM intensity approaches saturation at a rate substantially lower than that predicted by the TRM analog theory. Furthermore, the ratio of the CRM to an anhysteretic remanence acquired in the same field strongly depends on the intensity of the growth field. Analogy to TRM predicts this ratio to be independent of growth field. Both, the principal eigenvector of the anisotropy of isothermal remanence (AIR) matrix becomes more aligned with growth field and the percent anisotropy increases as the intensity of the growth field increases. The above observations suggest that the model of CRM acquisition based on analogy to TRM acquisition is incomplete. We therefore propose a revised model which incorporates the increased alignment of easy axes with growth field. Finally, the alignment of magnetic anisotropy ellipsoids of our synthetic samples with the growth field suggests a means to distinguish CRM from TRM in natural remanence.
Heat flow constraints on the South Pacific Superswell
Authors: Stein Carol A. , Abbott Dallas H.
Published: February 2008

The South Pacific superswell has been defined as a large region of anomalously shallow bathymetry, low Love wave velocities, and low effective elastic thicknesses relative to those predicted for its age. These phenomena have been interpreted as reflecting a combination of lithospheric reheating and thinning, and dynamic uplift due to mantle flow. We use heat flow data to better constrain the thermal structure of this region and examine the predictions of various possible models. The average heat flow for the superswell region does not differ significantly from that for lithosphere of similar ages elsewhere on the Pacific plate. Given their uncertainties, the heat flow data imply that thermal lithospheric thickness exceeds 60 km, but cannot discriminate between greater thicknesses. The lack of observed high heat flow appears not to be explained by biases due to water circulation in the thin sediment cover, since the superswell heat flow is not higher than for sites elsewhere with similar sedimentary environments. The Darwin Rise has been proposed as a fossil superswell in the Cretaceous, on the basis of the many similar characteristics to the South Pacific superswell. We find that the Darwin Rise heat flow values do not exceed those for similar ages elsewhere in the Pacific and Atlantic. This observation suggests that any thermal effects associated with the formation of the Darwin Rise are no longer present, and it is consistent with the idea of a fossil superswell. The surface heat flow data thus provide no evidence that the temperatures in the uppermost portion of the lithosphere; are significantly higher in the entire superswell region than in other areas of comparable age. This observation is intriguing given the suggestion that the thin effective elastic thicknesses inferred from seamount loading may reflect reheating of the lithosphere. Models in which the plate thicknesses and/or the basal temperatures are increased to yield temperatures high enough to explain the low effective elastic thicknesses predict surface heat flow much higher than observed. Reheating the lithosphere, as is proposed for hot spots, yields temperatures adequate to explain the effective elastic thicknesses only if reheating occurs at very shallow depths, and again implies a surface heat flow much greater than observed. Hence, unless shallow reheating is somehow localized beneath the seamounts, the thinner elastic thicknesses may reflect mechanical, rather than thermal, weakening of the lithosphere.
Introduction to Special Section on EDGE and Related Seismic Projects, Onshore‐Offshore California
Authors: Mooney Walter D.
Published: February 2008

The continental margins hold a special interest to Earth scientists because they mark the complex transition from oceanic to continental crust. This transition often involves a number of interacting processes, including subduction, accretion, and deformation of the oceanic lithosphere and growth and modification of the continental lithosphere, particularly the crust. The past decade has seen a great deal of progress in geophysical studies of continental margins, in large part due to the successful application of marine multi channel seismic (MCS) surveys that have provided unprecedented details on the deep structure.

An opportunity arose in 1986 to augment, at a relatively modest cost, an MCS survey commissioned by the Pacific Gas and Electric Company (PG) across the central California margin. At the initiative of the Houston Area Research Center (HARC) and Rice University, funds were provided to the M CS contractor to record several additional MCS profries, and land seismographs were deployed to record the marine air gun shots and land shots prepared by the U.S. Geological Survey. Ocean bottom seismographs were deployed on two of the profiles by Oregon State University. This study constitutes the first of many profiles of opportunity completed or planned by EDGE, a National Science Foundation and industry funded consortium of institutions committed to geophysical studies of continental margins.
Thermal anomalies in the central Indian Ocean: Evidence for dewatering of the Bengal Fan
Authors: Corrigan Jeff D.
Published: February 2008

Seafloor heat flow measurements in the central Indian Ocean are locally higher than predicted by lithospheric cooling models. These anomalously high heat flow values occur in a region affected by folding and faulting of oceanic crust and overlying distal Bengal Fan sediments. Three closely spaced sites drilled during Ocean Drilling Program Leg 116 dated the onset of deformation in this area at about 7–8 Ma. Here apatite fission track thermochronometric data from sites 717 and 718 and numerical simulations are used to (1) quantify the effects of Bengal Fan sedimentation on seafloor heat flow, (2) evaluate the thermal history of sediments penetrated at these two sites and (3) constrain mechanisms responsible for anomalous seafloor heat flow in this area. At the rates sediments were deposited along the distal fan (≤ 350 m/m.y.). numerical simulations predict a 6% to 9% reduction in present‐day heat flow at the seafloor relative to heat flow through the base of the lithosphere. At site 717, thermal history simulations assuming heat transfer dominated by conduction are consistent with available down‐hole temperature measurements and apatite fission track data. Site 718 is located 8 km south of Site 717 and 2 km south of a high‐angle fault that offsets oceanic crust by about 300 m. A previously published detailed heat flow survey conducted around site 718 documented seafloor heat flow ranging from 44 to 166 mW/m2 over kilometer‐scale distances. In addition, a monotonic 10 to 20% reduction in the mean length of fission tracks in apatite from 560 to 960 m below the seafloor at site 718 implies bottom‐hole temperatures of the order of 50°–60°C for a time period greater than a few million years. Both the seafloor heat flow and apatite fission track data argue for localized, possibly intermittent, advection of heat by lateral, updip migration of pore fluids in the vicinity of site 718. Taken together, the heat flow data and temperature range inferred from the apatite fission track data at site 718 imply a fluid source in the upper crust. Upward movement of pore fluids at velocities of less than 1 cm/yr through a basement‐sediment boundary maintained at temperatures ranging from 65° to 80°C is capable of explaining both sets of observations. As a hypothesis to reconcile existing observations, I suggest that the anomalous thermal structure in the central Indian Ocean represents a joint response to compaction‐driven dewatering of the Bengal Fan and local enhancement of the vertical permeability in the upper crust and sediment pile due to intraplate deformation.
Seismic reflection/refraction mapping of faulting and regional dips in the Eastern Alaska Range
Authors: Brocher Thomas M. , Nokleberg Warren J. , Christensen Nikolas I. , Lutter William J. , Geist Eric L. , Fisher Michael A.
Published: February 2008

We present the results of a Trans‐Alaska Crustal Transect (TACT) investigation of the upper 2–5 km of the eastern Alaska Range in the vicinity of the Denali fault based on seismic reflection/refraction data, laboratory measurements of rock velocities, and structural mapping. The Denali fault is a major dextral slip structure mappable for more than 2000 km separating the Wrangellia and adjacent terranes to the south from the Yukon‐Tanana and adjacent terranes to the north. Geologic mapping suggests over 400 km of dextral slip has occurred on the fault, yet within the upper 1.5 km of the crust along the TACT corridor, basement rocks juxtaposed along the Denali fault reveal no significant seismic velocity differences, although the fault zone itself is associated with a minor lowering of velocity. The lack of seismic velocity contrast adjacent to the fault is in agreement with laboratory measurements of elastic wave velocities of samples from terranes bordering the fault. Laboratory measurements of elastic wave velocities of the metasedimentary mica‐quartz schists comprising the Yukon‐Tanana basement are highly anisotropic because of preferred orientation of mica and predict significant variations in velocity accompanying variations in foliation dip. Although other interpretations are possible, the northward shallowing of foliation dip of basement rocks in the Yukon‐Tanana terrane combined with the strong anisotropy associated with these highly foliated rocks can explain an observed northward increase in seismic velocity within this terrane. Seismic reflections from basement rocks within the Yukon‐Tanana terrane may originate from variations in anisotropy with depth and/or changes in composition reflecting different proportions of sandstone and shale in the protolith.
Surface deformation and energy release rates for constant stress drop slip zones in an elastic half‐space
Authors: Wu M. , Rudnicki J. W. , Kuo C. H. , Keer L. M.
Published: February 2008

Free surface displacements, stress intensity factors, and energy release rates are calculated for planar slip zones in an elastic half‐space subjected to a prescribed shear stress drop. Although the method can treat arbitrarily shaped planar zones and distributed stress drops, for simplicity, results are presented only for circular and elliptic zones and uniform stress drops. Calculations of the stress intensity factors and energy release rates for various geometries indicate that solutions for the half‐space differ by less than 10% from those in the full space if the distance from the slip zone center to the free surface is greater than the downdip width of the slip zone. In addition, the influence of the free surface is greater for decreasing dip angle. For slip zones that are near the free surface and, especially, those that break the surface there is a coupling between slip and normal relative displacement. That is, for a prescribed shear stress drop and zero normal stress change, slip induces relative normal displacement. As example applications, these solutions are used to reexamine the coseismic geodetic data from three earthquakes: 1966 Parkfield, 1983 Borah Peak, and 1987 Whittier Narrows. The geometries, moments, and stress drops are similar to those inferred in previous studies using dislocation methods. However, the stress drop inferred here may be more reliable because stress drop is one of the parameters adjusted to fit the observed surface deformations. In addition, the method makes it possible to estimate the critical energy release rate at the termination of rupture. Values for the Parkfield, Borah Peak, and Whittier Narrows earthquakes are 1.5.×106 J/m2, 1.2×106 J/m2, and 2×108 J/m2, respectively.
Mechanisms for the formation of Domal and Basinal Detachment Faults: A three‐dimensional analysis
Authors: Yin An
Published: February 2008

Detachment faults (i.e., regional low‐angle normal faults) in metamorphic core complexes of the North American Cordillera commonly exhibit doubly plunging antiformal (domal) and synformal (basinal) geometries. These geometries have been previously attributed to the superposition of two processes: (1) the antiforms and synforms with axes parallel to the extension direction originated as primary fault undulations or corrugations developed coevally with fault slip, and (2) the antiforms and synforms with axes perpendicular to the extension direction were produced by isostatic uplift due to tectonic denudation. However, the coaxial relationship between undulations of detachment faults and the sedimentary beds and metamorphic foliations in both the upper and lower plates of some detachment fault systems in the U.S. Cordillera suggests that the synforms and antiforms with axes parallel to the extension direction may have formed as folds, and that the domal and basinal geometry of detachment faults may have formed synchronously by a single process. A three‐dimensional, elastic thin plate model is developed to investigate possible mechanisms for the formation of domal and basinal detachment faults. This model explores the interactions among the vertical, horizontal, and basal shearing forces during the formation of warped detachment faults. The model considers the role of compression perpendicular to the extension direction and stress reduction parallel to the extension direction. The results of the model suggest that the stress reduction itself in the extension direction is insufficient to cause buckling of a thin elastic crust. Two mechanisms are most likely to explain the formation of domal and basinal geometries of detachment faults: (1) an upward pushing by undulatory crustal roots or buoyant synextensional plutons beneath the extensional belt, and (2) buckling caused by a compression perpendicular to the extension direction. Both mechanisms are consistent with the geologic constraints during the development of Cordilleran core complexes. In addition to isostatic uplift due to tectonic denudation, upward warping of detachment faults can also be caused by buckling, basal shearing, and buoyant forces.
A tilt and seismicity episode in the New Hebrides (Vanuatu) Island Arc
Authors: Mellors R. , Chatelain J.‐L. , Isacks B. L. , Hade G. , Bevis M. , Prevot R.
Published: February 2008

Tilt and seismicity have been monitored in the central New Hebrides island arc since 1978 using bench mark arrays, long tube water tiltmeters, borehole tiltmeters, and a local seismometer network. Releveling of the bench mark array on Efate island in late November 1986 revealed a 10 μrad tilt up to the NNW since the previous leveling in April 1986. The tilt event was preceded by a magnitude 5.9 thrust event that occurred on October 25, 1986, at a depth of 48 km and about 11 km NW of the tiltmeter instruments. Six days later, a shallow (<20 km) swarm of earthquakes occurred 12 km NNW of the tiltmeter instruments and 5 km north of the epicenter of the magnitude 5.9 earthquake. Closely coincident in time with the swarm, a 5 μrad tilt up to the NNW that occurred over a period of 5 days was recorded on both the 100 m baseline water tube tiltmeter and the borehole bubble level tiltmeter. A composite focal mechanism of 191 earthquakes selected from the swarm indicates a thrust mechanism with some component of strike slip. Calculations show that the seismic slip associated with a swarm of this magnitude is apparently inadequate to cause the observed surface deformation. Two similar shallow swarms in November 1987 and July 1988 have occurred within 15 km of the 1986 swarm but with no apparent surface deformation. The most likely explanation, supported by simple modeling, is that the swarm and tilt are the result of a magmatic intrusion from island arc volcanism. An alternate hypothesis is that both the seismicity and the tilt are due to an episode of largely aseismic creep in the upper crust.
High‐temperature deformation of diopside single crystal: 1. Mechanical data
Authors: Raterron Paul , Jaoul Olivier
Published: February 2008

Laboratory deformation experiments were carried out on diopside single crystals. Creep tests were made in a dead load apparatus at temperatures T = 1020 to 1320°C, axial compressive stresses σ = 50 to 170 MPa with strain rates (
) ranging from 2×10−9 s−1 to 4×10−7 s−1. The specimens were oriented such that mechanical twinning was not possible. The experiments were designed to activate {110}1/2〈a±b〉 slip systems. Our results demonstrate that they are the major operative systems for T > 1000°C and appear to be predominant over the (100)[c] slip system. The mechanical data are fitted to a power law ε˙ = A σn exp(−E*/RT). An inversion method was used to determine the parameter In (A), the activation energy E*, and the stress exponent n. Below a critical temperature Tc ≃ 1130–1140°C, E* is found to be 440 ± 30 kJ/mol associated with the {110}1/2〈a±b〉 slip systems, activated symmetrically without any contribution from the (100)[c] system (orientation [2]); E* is found equal to 740±30 kJ/mol if all three systems have the same resolved shear stresses (orientation [1]). For T > Tc, values of E* drop to 50±15 and 85±30 kJ/mol for orientations [2] and [1], respectively, so that the creep law of diopside becomes nearly temperature independent. The law derived from the orientation [2] crystals could be consistent with natural deformations. For this orientation and within the range of temperatures covered, n is 6.5±0.4. The strong decrease in the activation energy at Tc may be ascribed unambiguously to the occurrence of partial melting leading to microdroplets which pin the mobile dislocations (Ingrin et al., this issue). The high value found for the stress exponent (n) in the creep law suggests that clinopyroxene is softer than olivine at high stresses (at T = 900°C, stress σ ≥ 20 MPa and
), while olivine is softer at lower stresses. This competence inversion is predicted to occur within the ranges of stresses and strain rates expected in the lower crust and upper mantle.
Proper boundary conditions in reflection seismograms
Authors: Loewenthal Dan
Published: February 2008

During the last few years it became feasible to record in the marine environment the reflection seismograms of the acoustic pressure field along two horizontal streamers emersed one below the other. This enables a more exact treatment of the wave equation as a second‐order partial differential equation requiring two boundary conditions such as the pressure field and its gradient. According to Green's representation theorem, if the actual source is located above the two streamers, the wave field can be reconstructed for any point below them provided the medium parameters are known. Alternatively, if the dual acoustic field, namely, the pressure and particle velocity, is measured along a single horizontal streamer placed below the source, both dual fields may be reconstructed. As a first step to demonstrate the power of processing simultaneously the dual fields, we show how the source signature estimation, deghosting and deconvolution can be deterministically derived. As far as we know these processes cannot be done with a single set of boundary conditions.
Frequency dependent electrical properties of polycrystalline olivine compacts
Authors: Roberts Jeffery J. , Tyburczy James A.
Published: February 2008

The complex electrical properties of poly crystalline San Carlos olivine compacts were measured over the range of frequency l0−4–104Hz from 800° to 1400°C under controlled oxygen fugacity. The impedance data display a strong frequency dependence that is evidenced most clearly when the results are displayed in the complex impedance plane. A parameterized model of the frequency dependent electrical response using equivalent electrical circuits is presented. Two distinct conduction mechanisms of the sample are observed: grain interior and grain boundary conduction. Each occurs over a different range of frequency. The resistance of each mechanism adds in series resulting in a lower total DC conductivity for polycrystalline olivine than for either mechanism separately. The total DC conductivity is dominated by the grain interior conductivity above 1200°C, whereas the grain boundary conductivity has the strongest influence below 1000° C. Impedance spectra of natural dunite samples exhibit a similar type of frequency dependence. The grain interior conductivity displays a change in slope at 1344°C and has activation energies of 1.45 eV (800°–1344°C) and 4.87 eV (>1344°C). The grain boundary conductivity has an activation energy of 2.47 eV. In these cases, the ƒO2 for each experimental run was controlled at that of the wustite‐magnetite oxygen buffer. Experiments on samples with different grain sizes reveal no dependence of DC conductivity on grain size for either mechanism, although the relaxation time and real relative permittivity of the grain boundary mechanism are dependent on grain size. Because of the electrical response observed at low frequencies, care must be taken in the inversion of electromagnetic field observations using laboratory measurements made in the kilohertz range since they may not be the same as DC measurements. Impedance measurements must be performed over a range of relatively low frequencies to assess the role of grain boundaries on the overall electrical response of polycrystalline materials.
Shear wave constraints on a deep crustal reflective zone beneath Vancouver Island
Authors: Cassidy John F. , Ellis Robert M.
Published: February 2008

Analysis of teleseismic receiver functions recorded in the northern Cascadia subduction zone show that large P‐to‐S converted phases are generated at the boundaries of a dipping layer of deep crustal reflectors (the “E” zone) beneath central Vancouver Island. These data provide the first shear wave constraints on this layer which is clearly observed in seismic reflection sections and is coincident with a region of low electrical resistivity interpreted from a magnetotelluric survey. Modeling of the amplitudes, polarities, and arrival times of the P‐to‐S converted phases demonstrates that the E zone is a prominent low‐velocity layer, with an S velocity 1.0±0.2 km/s lower than the regions above and below. The E zone dips 7°±5° in the direction N10°E±20° and extends from depths of 36.5 to 41 km at ALB‐B on central Vancouver Island. The estimated S velocity contrast, when considered with refraction P velocities, yields a Poisson's ratio of 0.34 for this zone. Given the uncertainty in ΔVp and ΔVs, this value may range from 0.27 to 0.38. The low P and S velocities and high Poisson's ratio combined with electrical resistivity measurements suggest a porosity of 0.1–1.9% and crack aspect ratios of 0.001–0.01, values consistent with a region containing thin, fluid‐saturated cracks and thus supportive of the interpretation of this feature as a major shear zone.
Neutrino tomography of the Earth
Authors: Nicolaidis Argyris , Jannane Mohammed , Tarantola Albert
Published: February 2008

Neutrino oscillations inside matter are very sensitive to the density of the medium traversed by the neutrinos. If the neutrino mixing angle (so far unknown) is not very small, then the neutrino oscillation length is of the size of the Earth, and we can use the phenomenon of neutrino matter oscillations to study the Earth's density distribution. Although neutrino attenuation data have already been proposed as a means to infer the density structure of the Earth, we show that neutrino oscillation data could also be used to make a density tomograph of the Earth. A numerical experiment in which neutrino beams from an accelerator are sent across the Earth, and are detected at six different locations on the Earth's surface, shows the feasibility of using neutrino oscillation data, with the advantage of using only currently available neutrino energies.
The recent crustal stress field in central Europe: Trajectories and finite element modeling
Authors: Grünthal G. , Stromeyer D.
Published: February 2008

The recent crustal stress field of Central Europe and especially of the adjoining areas to the east is presented in terms of the directions of maximum horizontal stress (SHmax). The analysis is based on fault plane solutions, in situ stress measurements, geologic fault slip determinations, and repeated precise geodetic triangulations. A bending of the direction of SHmax from the well‐known NW‐SE direction in the western part of the study area to directions of NE‐SW to E‐W in the eastern part is shown. First results on the recent crustal stress field of the study area were presented by Grünthal and Stromeyer (1986), who substantiated this tendency of bending in the central and eastern parts of the study area by few observations only. Therefore one aim of this paper was to compile observations on the areas with few data points. Generally, these additional data confirm the previously established pattern; in some areas, especially in the Pannonian basin, the stress features became more complicated compared with those solely based on a few data points. The second part of the paper presents steady state elastic finite element model calculations to provide some possible explanations of the observed stress orientations as a result of plate driving forces. Simulation of the North Atlantic seafloor spreading and the northward directed motion of the African plate by appropriate boundary loads produces a pattern of SHmax directions for the western part of the Eurasian plate which is compatible with the broad‐scale observed stress directions. Subregional anomalies such as the fanlike stress pattern perpendicular to the arc of the Western Alps or the radial directions around the Pannonian basin can be explained only when additional stress producing features are introduced overmodulating the regional field. Rigorous introduction of physically constrained model parameters for all these features was not feasible to date. Therefore the preliminary empirical model calculations presented in this paper are attempts to discuss which features have the most influence on the stress field. They are, in a regional scale, the North Atlantic seafloor spreading, the northward motion of the African and the Arabian plate, and obviously, subregionally, increased stiffness of the Apulian promontary as well as of the Bohemian massif.
The effect of low‐temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites: A case for acquisition of chemical remanent magnetization
Authors: Kelso Paul R. , Banerjee Subir K. , Worm Horst‐Ulrich
Published: February 2008

The effect of hydrothermal alteration on the thermoremanent magnetization (TRM) of synthetic titanomagnetite (TM40: Fe2.6Ti0.4O4) has been studied, to simulate the alteration that occurs in the oceanic crust. Pseudo‐single‐domain titanomagnetite grains, similar in size to those often found in oceanic basalts, were dispersed in a permeable but rigid glass matrix. This resulted in a TRM in the sample which was subsequently oxidized in acidic solutions while a magnetic field (0.1 mT) was applied perpendicular to the TRM direction. The experiments were conducted in a non‐magnetic stainless steel pressure vessel at 150°C in solutions of acidity varying from pH=2 to pH=7. In addition to being time and temperature dependent, the acquired chemical remanent magnetization (CRM) was also found to be very pH dependent. The degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions. Long‐term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating. It was found that during the alteration of TM40 to titanomaghemite the CRM is along the ambient field direction from the onset, and not partly or wholly along the TRM direction as has been found in previous air oxidation experiments. This has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples.
In situ stress measurements to 3.5 km depth in the Cajon Pass Scientific Research Borehole: Implications for the mechanics of crustal faulting
Authors: Zoback Mark D. , Healy John H.
Published: February 2008

Measurements of in situ stress orientation and magnitude at the site of the Cajon Pass research borehole have been made from depths of 0.9–3.5 km using the hydraulic fracturing technique and analysis of stress‐induced well bore breakouts. The results of these measurements support two important conclusions about the mechanics of crustal faulting. First, the magnitudes of measured in situ stresses indicate ratios of shear to normal stress on favorably oriented fault planes that are consistent with predictions based on Mohr‐Coulomb theory and laboratory‐determined coefficients of friction in the range of 0.6–1.0 assuming hydrostatic pore pressure (this is commonly known as Byerlee's law). Thus the stress measurements indicate that the frictional strength of the crust adjacent to the San Andreas fault is high (i.e., consistent with laboratory‐derived friction values) and that the level of shear stress in the crust adjacent to the San Andreas is principally controlled by its frictional strength. However, data on the orientation of maximum horizontal compression in the borehole from 1.75 to 3.5 km (N57°E ± 19°) indicate that the San Andreas must be quite weak as a complete absence of right‐lateral shear stress resolved on planes parallel to the ∼N60°W striking San Andreas fault is observed. The lack of right‐lateral shear stress on planes parallel to the San Andreas fault at this site is especially surprising as Cajon Pass is located along a section of the San Andreas which has not had a major earthquake since 1812 and is thus presumably quite “late” in the earthquake cycle. Nevertheless, both the orientation and magnitudes of stresses measured in the well are consistent with the style of active faulting in the area surrounding the drill site, most notably normal faulting and Quaternary age left‐lateral slip on the Cleghorn fault that parallels the San Andreas in the vicinity of the drill site (Meisling and Weldon, 1982; Weldon, 1986; R. J. Weldon et al., unpublished report, 1981). We argue that the stress state (and Quaternary fault offsets) observed in the Cajon Pass area could exist only if the San Andreas moved at low shear stresses comparable to seismic stress drops rather than the much higher values predicted by Byerlee's law, a conclusion consistent with the lack of frictionally generated heat flow along the San Andreas system (e.g., Brune et al., 1969; Henyey and Wasserburg, 1971; Lachenbruch and Sass, 1973, 1980). Taken together, the Cajon Pass in situ stress and heat flow measurements (Lachenbruch and Sass, this issue) support a conceptual model of the San Andreas system in which the San Andreas is extremely weak with respect to the surrounding crust.
Boundary conditions for rock fracture analysis using the boundary element method
Authors: Zeller Scott S. , Pollard David D.
Published: February 2008

Successful numerical models of faults, joints, and other fractures in a rock mass require specification of appropriate boundary conditions, but the choice may not be obvious. To illustrate potential pitfalls, a single two‐dimensional fracture is modeled by the displacement discontinuity method of boundary elements using both displacement boundary conditions and traction boundary conditions. The results are compared with the analytical solution for a mode I (opening) crack. It is shown that the traction boundary conditions can provide a satisfactory solution, while displacement boundary conditions, applied to a problem with both continuous and discontinuous displacement components, can introduce significant errors.
An approximate analytical model of shock waves from underground nuclear explosions
Authors: Lamb F. K. , Callen B. W. , Sullivan J. D.
Published: February 2008

We discuss an approximate analytical model for the hydrodynamic evolution of the shock front produced by a spherically symmetric explosion in a homogeneous medium. The model assumes a particular relation between the energy of the explosion, the density of the medium into which the shock wave is expanding, and the particle speed immediately behind the shock front. The assumed relation is exact for shock waves that are strong and self‐similar. Comparison with numerical simulations indicates that the relation is also approximately valid for shock waves that are neither strong nor self‐similar. Using the assumed relation and the Hugoniot of the ambient medium expressed as a relation between the shock speed and the postshock particle speed, one can calculate the radius and other properties of the shock front as a function of time. The model also allows one to investigate how the evolution of the shock wave is influenced by the properties of the ambient medium and how these properties affect the characteristic radius at which the shock wave becomes a low‐pressure plastic wave. The shock front radius versus time curves predicted by the model agree well with numerical simulations of explosions in quartz and wet tuff and with data from four underground nuclear tests conducted in granite, basalt, and wet tuff when the official yields are assumed. When the model is used instead to fit radius versus time data from the hydrodynamic phases of these tests, it gives yields that are within 8% of the official yields when piecewise‐linear approximations to the Hugoniots are used. This accuracy is comparable to the accuracy of other models.