bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessing the accuracy of thermoluminescence for dating baked sediments beneath late Quaternary lava flows, Snake River Plain, Idaho
Authors: Forman Steven L. , Pierson James , Smith R. P. , Hackett W. R. , Valentine G.
Published: February 2004

Baked sediments beneath lava flows on the Snake River Plain, Idaho, with independent age control by either 14C or K/Ar dating were analyzed to evaluate the accuracy of the thermoluminescence (TL) technique. The age of flows ranges from ∼2 to 100 ka and multiple TL analyses by the total bleach method yielded ages that overlap at one sigma with independent chronologic control. The TL signal of one sample of baked sediment beneath a lava flow with an inferred age of at least 641±54 ka was near saturation, perhaps reflecting a relatively high environmental dose rate, and is not datable by TL. This study underscores several major limitations of luminescence geochronology, the natural spatial and temporal variability in environmental radioactivity and the susceptibility of silicate minerals to the growth and retention of a luminescence signal. Despite these limitations, the results demonstrate the utility of luminescence geochronology to date volcanic eruptive events during the Quaternary.
Effects of multiple phase transitions in a three‐dimensional spherical model of convection in Earth's mantle
Authors: Tackley Paul J. , Stevenson David J. , Glatzmaier Gary A. , Schubert Gerald
Published: February 2004

Numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three‐dimensional flow pattern, with cylindrical features and linear features that behave differently in their ability to penetrate the 670‐km discontinuity. The dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth, resulting in frequent “avalanches” of upper mantle material into the lower mantle. The effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670‐km phase change, resulting in smaller and more frequent avalanches, and a wider range of morphologies. Large quantities of avalanched cold material accumulate above the coremantle boundary (CMB), resulting in a region of strongly depressed mean temperature at the base of the mantle. The 670‐km phase change has a strong effect on the temperature field, with three‐distinct regions being visible: (1) the upper mantle, containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum; (2) the midmantle, containing quasi‐cylindrical avalanche conduits and characterized by a low amplitude, broad spectrum; and (3) the deep mantle, containing large pools of cold, avalanched material and characterized by a high amplitude, ultra‐red (i.e., long wavelength) spectrum. The effect on the velocity field is very different. Flow penetration across the 670‐km phase change is strongly wavelength‐dependent, with easy penetration at long wavelengths but strong inhibition at short wavelengths. Thus, when comparing numerical models with long wavelength seismic tomography, diagnostics based on the density field, such as the radial correlation function, are much more sensitive to the effects of phase transitions than those based on the velocity field. The amplitude of the geoid is not significantly affected by the partial layering, because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670‐km discontinuity. Avalanches are associated with geoid lows. However, a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in Earth.
Chemical evolution and periodic eruption of mafic lava flows in the west moat of Long Valley Caldera, California
Authors: Vogel T. A. , Woodburne T. B. , Eichelberger J. C. , Layer P. W.
Published: February 2004

Continuous core from research drill hole Inyo‐4 through a thick 300 m thick sequence of mafic lava flows in the west moat of the Long Valley Caldera has provided an unusual opportunity to investigate the chemical evolution of this exceptionally complete record of postcaldera mafic magmatism. Lavas are mainly basalts and trachyandesites ranging from 48 to 58% SiO2 having a nearly fourfold range in MgO contents. The lavas fall into five distinct chemical groups with little or no compositional overlap. These groups correlate remarkably well with stratigraphic position, and they define a trend toward more evolved compositions with time. The groups appear to represent periodic eruptions from a continuously evolving magma body. Preliminary 40Ar/39Ar dates indicate that these lavas erupted over a span of at least 0.264 m.y. between 0.415 Ma and 0.151 Ma. Except for the most evolved group, the chemical variation within a group was dominated by crystal fractionation. Except from the least evolved groups, the chemical contrasts between adjacent groups were dominated by assimilation. Warming of the crustal environment facilitated increasing assimilation. A zoned lava flow (more mafic upward) followed by a mafic flow in an otherwise progressively evolved sequence of flows provides evidence for eruption from a zoned magma reservoir. Deeper, more mafic portions of this zoned magma body were drawn up to shallower levels in the chamber during a period of high eruption rates. The heterogeneity of mafic clasts in the vent breccia dike beneath the 600‐year‐old South Inyo phreatic explosion crater indicates that these breccia clasts dropped >300 m down the vent from the overlying lava sequence during waning stages of the phreatic activity, rather than forming by brecciation of an older feeder dike, as previously proposed (Eichelberger et al., 1988).
Introduction to the Special Section Northeast Japan: A Case History of Subduction
Authors: Finn Carol , Kimura Gaku , Suyehiro Kiyoshi
Published: February 2004

Subduction‐related tectonic events such as strike‐slip faulting, uplift of high‐pressure metamorphic rocks in the forearc, back arc spreading, arc magmatism, and continental collisions have shaped northeast Japan, making it a case history for subduction. Much of the record of these events is preserved and can be used to reconstruct the tectonic history of the region.

Although many geological, geophysical, and geochemical data have been collected in Japan for the last 30 years, interpretation of these data into a plate tectonic context did not really begin until about 10 years ago. Much of the data and interpretation on northeast Japan have been published in Japanese or as individual local studies. The primary goal of this special section is to present new investigations covering a wide range of topics and scales and place them in a tectonic framework. The papers in this special section focus on three main subjects: Mesozoic aspects of northeast Japan; the relation of Japan Sea opening with the tectonics and geochemistry of northeast Japan in the Tertiary; and the geophysical setting. The purpose of this overview is to link these subjects and provide a tectonic framework for north‐ east Japan and eastern Asia since the Mesozoic.
Seismic constraints on shallow crustal emplacement processes at the fast spreading East Pacific Rise
Authors: Christeson G. L. , Purdy G. M. , Fryer G. J.
Published: February 2004

We present the results of nine on‐bottom seismic refraction experiments carried out over young East Pacific Rise crust. The experiments are unusual in that both the source and receiver are located within a few meters of the seafloor, allowing high‐resolution determinations of shallow crustal structure. Three experiments were located within the axial summit caldera (ASC) over “zero‐age” crust. The seismic structure at these three locations is fundamentally the same, with a thin (<60 m) surficial low‐velocity (<2.5 km/s) layer, a 100 to 150‐m‐thick transition zone with velocities increasing by ∼2.5 km/s, and a layer with velocities of ∼5 km/s at a depth beneath the seafloor of ∼130–190 m. The surficial low‐velocity layer and transition zone are defined as seismic layer 2A, and the ∼5 km/s layer is defined as the top of layer 2B. Both the surficial low‐velocity layer and the transition zone double in thickness within ∼1 km of the rise axis. We model layer 2A as the extrusive sequence and transition zone and the 2A/2B boundary as the top of the sheeted dikes. The primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from ∼150 m to ∼300 m within 1 km of the ASC. The thickening of the extrusive layer is interpreted to be due to lava that either overflows the ASC walls, is emplaced through eruptions outside of the ASC, or travels laterally from the ASC through conduits. The most probable cause for the thickening of the transition zone is sill emplacement outside of the ASC, either from magma that does not reach the surface in an off‐axis eruption or magma that is transported laterally during the drainage process creating the ASC. We suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2A.
Three‐dimensional numerical modeling of detachment of subducted lithosphere
Authors: Yoshioka S. , Wortel M. J. R.
Published: February 2004

Recent seismological studies suggest that slab detachment has occurred in the Mediterranean and the New Hebrides subduction zones. Subducted slabs in these regions are recognized to be torn at depths ranging from 100 to 300 km, presumably caused by the lateral migration of the tear along the strike of the slab. To investigate the physical mechanism of the slab detachment and in particular its migration, we constructed a viscoelastic three‐dimensional finite element model and introduced a small initial tear from one side of the slab. We investigated spatio‐temporal variations in the state of stress within the slab, as a function of tear length, rheology, and a variety of force distribution. Our results show that an area of high shear stress concentration of the order of several hundred megapascals forms near the tip of the tear inside the slab, which is probably sufficient to cause further lateral migration of the tear. The stress concentration increases with the length of the tear and lower viscosity values of the surrounding mantle and increases with downdip tension. From our modeling, we conclude that favorable conditions for slab detachment are characterized by a high interplate frictional force at a subduction zone and a low convergence rate, forming in‐plate tensional stress at intermediate depths. Such a condition is indeed observed in the Dinarides/Hellenic and the New Hebrides subduction zone.
Surface wave scattering from sharp lateral discontinuities
Authors: Pollitz Fred F.
Published: February 2004

The problem of surface wave scattering is re‐explored, with quasi‐degenerate normal mode coupling as the starting point. For coupling among specified spheroidal and toroidal mode dispersion branches, a set of coupled wave equations is derived in the frequency domain for first‐arriving Rayleigh and Love waves. The solutions to these coupled wave equations using linear perturbation theory are surface integrals over the unit sphere covering the lateral distribution of perturbations in Earth structure. For isotropic structural perturbations and surface topographic perturbations, these solutions agree with the Bom scattering theory previously obtained by Snieder and Romanowicz. By transforming these surface integrals into line integrals along the boundaries of the heterogeneous regions in the case of sharp discontinuities, and by using uniformly valid Green's functions, it is possible to extend the solution to the case of multiple scattering interactions. The proposed method allows the relatively rapid calculation of exact second order scattered wavefield potentials for scattering by sharp discontinuities, and it has many advantages not realized in earlier treatments. It employs a spherical Earth geometry, uses no far field approximation, and implicitly contains backward as well as forward scattering. Comparisons of asymptotic scattering and an exact solution with single scattering and multiple scattering integral formulations show that the phase perturbation predicted by geometrical optics breaks down for scatterers less than about six wavelengths in diameter, and second‐order scattering predicts well both the amplitude and phase pattern of the exact wavefield for sufficiently small scatterers, less than about three wavelengths in diameter for anomalies of a few percent.
Age and correlation of a paleomagnetic episode in the western United States by 40Ar/39Ar dating and tephrochronology: The Jamaica, Blake, or a new polarity episode?
Authors: Herrero‐Bervera Emilio , Helsley C. E. , Sarna‐Wojcicki A. M. , Lajoie K. R. , Meyer C. E. , McWilliams M. O. , Negrini R. M. , Turrin B. D. , Nolan J. M. Donnelly , Liddicoat J. C.
Published: February 2004

High‐resolution paleomagnetic records from two sites near Pringle Falls, Oregon, are compared with similar records from Summer Lake, Oregon, ∼170 km to the southeast: Paoha Island, in Mono Lake, ∼660 km to the southeast and Benton Crossing, in Long Valley, approximately 700 km to the southeast, in east‐central California. The sequences at Pringle Falls contain a distinctive coarse pumice‐lapilli tephra layer which we have dated as 218±10 ka by 40Ar/39Ar step‐heating of plagioclase feldspar. Stratigraphically, this tephra is closely associated with a suite of several other tephra layers that bracket the interval studied paleomagnetically. Each tephra layer is distinguished by the unique chemical composition of its volcanic glass shards. The pumice layer dated at Pringle Falls is correlated with layers at three of the other localities. Using all the tephra layers, we can correlate the lake stratigraphic sequences and associated paleomagnetic records among the four distant localities. Additional age control is obtained from a fifth locality at Tulelake in northern California, where the stratigraphic interval of interest is bracketed between ∼171±43 and approximately 140 ka. Characteristics of the paleomagnetic records indicate virtually identical paleofield variation, particularly the geometry of a normal to normal (N‐N) geomagnetic polarity episode. The observed paleofield behavior resembles the Blake geomagnetic polarity episode, but is significantly older than the generally accepted age of the Blake episode. Either the age of the Blake episode is significantly underestimated, or the polarity episode documented here is older, perhaps the Jamaica episode, or is an as yet unreported episode. A corollary of the latter option is that paleomagnetic polarity episodes of different ages may have similar transition polar paths, a conclusion implying that a common mechanism is involved.
Constraints on melt migration from mantle plumes: A trace element study of peridotite xenoliths from Savai'i, Western Samoa
Authors: Hauri Erik H. , Hart Stanley R.
Published: February 2004

Geochemical signals of high degrees of melting and migration of basaltic and carbonatitic melts are present in harzburgite xenoliths from the island of Savai'i, Western Samoa. These xenoliths are extremely depleted in clinopyroxene and have refractory mineral compositions. Ion microprobe trace element data delineate three groups of xenoliths. Peridotites depleted in the light rare‐earth elements (LREE) contain clinopyroxenes which have trace element characteristics of garnet but at lower absolute concentrations, and they have the lowest olivine Ca‐Al‐Na concentrations and the lowest equilibration temperatures (<960°C). This group of peridotites is significantly more depleted in trace elements than Hawaiian xenoliths and abyssal peridotites. These peridotites can be modeled as residues of 33–42% fractional melting from a pyrolite composition, with generally more than half of this melting taking place in the garnet Iherzolite stability field. Compared with abyssal peridotites, the LREE‐depleted Savai'i xenoliths originated in a melting regime characterized by higher rates of upwelling, probably within a mantle plume. Intermediate samples display U‐shaped REE patterns with variable, negative anomalies in Ti and Zr, intermediate olivine Ca‐Al‐Na concentrations, and intermediate temperatures (960–1050°C). These features can be modeled by percolation of basaltic melts through harzburgite at a transient stage of magma‐mantle disequilibrium. The existence of substantial intragranular trace element heterogeneity, even within single samples, argues against homogeneous porous flow, and clinopyroxene distribution suggests a diffuse channel network with channel spacing on a length scale of 1 cm. LREE‐enriched peridotites, with the highest olivine Ca‐Al‐Na concentrations and highest equilibration temperatures (1020–1170°C), are the end‐member of this melt migration process, in which the mantle has approached equilibrium with through‐going melts. The geochemistry of the Savai'i xenoliths testifies to complex processes of plume‐lithosphere interaction, fractional melting, and heterogeneous buoyancy driven flow of plume‐derived melt fractions with highly variable compositions.
Long‐lived mid‐ocean ridge segmentation: Constraints on models
Authors: Kane Kimberlee A. , Hayes Dennis E.
Published: February 2004

Long‐lived, large scale segmentation of the mid‐ocean ridge, as defined by variations in subsidence rate of the ocean crust, zero‐age depth, and geoid rate, has been documented along three mid‐ocean ridge systems: the South Atlantic, the Pacific‐Antarctic, and the Southeast Indian. Many of the observations in these three areas are inconsistent with standard thermal models for the generation of oceanic crust and lithosphere. Three models for subsidence rate and geoid rate and two models for zero‐age depth are examined. Sensitivity tests are done to determine how much the thermal and physical “constants” in these models must vary in order to explain the observations and whether or not such values are plausible. Only the zero‐age depth variations can be adequately explained using existing models. An internally consistent, qualitative model is presented which can explain the data; it involves regional and local temperature changes in the mantle both along and across strike of the mid‐ocean ridge.
Stress modeling in the Andes: Constraints on the South American intraplate stress magnitudes
Authors: Richardson Randall M. , Coblentz David D.
Published: February 2004

Regions of high topography which exhibit both compressional and extensional deformation provide a natural laboratory to place constraints on horizontal intraplate stress magnitudes. An elastic finite element analysis of the lithospheric stress field in the Cordillera Bianca region of Peru has been used to evaluate the lithospheric stress state and constrain the South American intraplate stress magnitudes. The orthogonal orientation of the Andean high topography to the east–west regional stress field in conjunction with the well‐documented change in deformation style from compression to extension across the Cordillera provides a unique setting for the stress modeling. A two‐dimensional finite element grid consisting of an assembly of isotropic, elastic quadrilateral elements in a state of plane strain was used to represent the lithosphere. An isostatically balanced crustal density model was used to calculate the stresses associated with the lateral density variations between the Andean topography and the Brazilian shield. In some models horizontal stress was concentrated at midcrustal depths by including vertical variation in Young's modulus. The modeling results indicate bounds of 10 and 75 MPa for the magnitude of the average horizontal stress averaged over a 100‐km‐thick lithosphere. A preferred value of 25 MPa is obtained on the basis of fitting all of the data for the rheological models considered and predicting a transition from extension to compression near the 3000‐m elevation contour. Such a far‐field average stress is consistent with a ridge push origin. Because the modeling performed in this study assumes that the topographic load is entirely compensated by the buoyancy of the deflected Moho, these estimates represent upper bounds. The effect of incorporating thermal support for the topographic load of the Cordillera Bianca would be to further reduce our estimate of the intraplate stress magnitudes. Analysis of the distribution of calculated stress with depth indicates that previous assumptions about the vertical plane shear stress within the lithosphere being negligible are acceptable for the geometry of the Andes.
Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon
Authors: Kelemen Peter B. , Dick Henry J. B.
Published: February 2004

Results of a field study demonstrate that melt flow in the shallow mantle was focused along actively deforming ductile shear zones in the Josephine peridotite in SW Oregon. Intergranular flow of ascending liquids dissolved pyroxene and precipitated olivine, forming zones of dunite replacing harzburgite. Syntectonic formation of dunite was focused along near‐vertical shear zones, with both vertical and dextral displacement, in the shallow mantle at less than 30 km depth and temperatures between 950 and 1100°C. The migrating magmas which formed dunite and related pyroxenite included high‐Mg, calc‐alkaline andesites characteristic of subduction‐related magmatic arcs. Explanations for focusing of melt flow along shear zones include the presence of a pressure gradient between strong and weak material during active deformation (Stevenson, 1989) and rapid infiltration of liquid when grain boundaries are aligned in mantle tectonites (Waff and Faul, 1992). Focused flow of melt may, in turn, lead to localized deformation and the development of shear zones. It is likely that melt‐lubricated shear zones provide an important locus for both melt extraction and strain in the shallow mantle beneath spreading ridges and in subduction‐related magmatic arcs.
Nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks
Authors: Mancktelow Neil S.
Published: February 2004

A subduction shear zone can be modeled as a long narrow channel, with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces: the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential, which combines the effect of both pressure and buoyancy. If the channel walls are effectively rigid, very slight narrowing or broadening of the channel (convergence angles <1°) can result in very dramatic changes in the (nonlithostatic) pressure distribution along the channel. The geometry of the subducting plate, which is forced to bend under the overriding plate, suggests that the channel should initially narrow downward and then gradually broaden. A model assuming this geometry, with initial channel width 1500 m, minimum width ∼600 m and width at 100 km depth of again ∼1500 m, a maximum viscosity of 1019 Pa s, and a convergence rate of 8 cm/yr reaches pressures >2 GPa in the channel at only 40 km depth. The model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment (∼650 m). The practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls. At 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle, which should be strong enough to support a significant tectonic overpressure. Episodic failure of the upper plate to produce great earthquakes at 30–40 km focal depth could vent overpressured fluid from the channel, allowing a cyclical buildup and release of both rock and fluid pressure. Topography on the subducting plate (e.g., seamounts and thinned continental crust) may lead to an anvil‐like jamming of the channel and local high overpressures. Tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks. A decrease in the convergence rate or cessation of subduction, with a consequent general warming within the channel and associated viscosity decrease, promotes exhumation by buoyant reverse flow. The most rapid reverse flow occurs in the region of previously greatest overpressure. Since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation, preservation of high‐pressure assemblages at the surface in fossil subduction zones is promoted for such a model.
Oxygen isotope evidence for hydrothermal alteration within a Quaternary stratovolcano, Lassen Volcanic National Park, California
Authors: Rose Timothy P. , Criss Robert E. , Mughannam Andrew J. , Clynne Michael A.
Published: February 2004

Brokeoff volcano, a Quaternary stratocone located in the Lassen volcanic center in northern California, has been deeply eroded, exposing a 10‐km2 meteoric hydrothermal alteration zone at the core of the volcano. Portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone. The δ18O analyses of more than 100 whole rock samples, consisting primarily of andesitic lavas, vary from +9.8 to +0.6 per mil. The highest δ18O values occur in bleached, solfatarically altered rocks that have interacted with low‐pH, fumarolic hot springs associated with the present‐day hydrothermal system. Low δ18O values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures, mostly during the stratovolcanic stage (650–400ka) of the hydrothermal system, but probably continuing today at depth. Electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples (δ18O < +5.0) revealed the presence of discrete chlorite, suggesting that temperatures up to 200 to 250°C were attained in the shallow levels of the system. Two zones of pervasive meteoric hydrothermal alteration, defined by concentric 18O contours that are probably interconnected at depth, are located within the original topographic edifice of the volcano. The most intensely altered rocks within these equant zones of alteration define NNW trends that coincide with stream valleys and with regional structural patterns. A comparison of the characteristics of the 18O‐depleted zone at Brokeoff with those of more deeply eroded volcanic centers, such as the Comstock Lode mining district (Criss and Champion, 1991), permits the construction of composite 18O cross sections through a hypothetical intact stratovolcano. At both Brokeoff and Comstock, hydrothermal fluids were strongly focused into plumelike zones of intense 18O depletion. At Comstock, these low‐18O plumes are associated with faults. Although major fault displacements are not observed at Brokeoff, the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano.
Seafloor hydrothermal systems
Authors: Lowell Robert P. , Rona Peter A. , Von Herzen Richard P.
Published: February 2004

The discovery of seafloor hydrothermal systems approximately two decades ago has led to a major reassessment of the Earth's thermal and geochemical budgets and has revolutionized our understanding of biological processes. This review traces the development of the study of seafloor hydrothermal systems from the indirect evidence provided by conductive heat flow anomalies to the discovery of ≈ 350°C black smoker vents on the East Pacific Rise at 21°N. Although the review focuses on physical characteristics and processes, it outlines some key characteristics of vent fluid chemistry that provide constraints on physical models. Ridge crest systems have thermal power outputs ranging from 10 to 104 MW. They are transient systems, driven by magmatic heat sources, but episodic events such as megaplumes, the interplay between focused and diffuse venting, and other aspects related to their thermal, chemical, and biological evolution remain poorly understood. Advances will be made by continuing exploration and discovery to determine the full range of possible phenomena both on and off axis and in different tectonic settings. In order to understand the complete, integrated ridge system, however, future studies must include long‐term monitoring of an active system, deep drilling into the reaction zone, and mathematical modeling that incorporates both physical and chemical constraints.
Metamorphism and near‐trench plutonism during initial accretion of the Cretaceous Alaskan forearc
Authors: Barnett Daniel E. , Bowman John R. , Pavlis Terry L. , Rubenstone James R. , Snee Lawrence W. , Onstott Tullis C.
Published: February 2004

Cenozoic uplift of the eastern Aleutian forearc has exposed the tectonic join between the Jurassic crystalline hanging wall of the arc and the oceanic assemblages of the footwall accreted during Cretaceous and Tertiary subduction, the Border Ranges fault system (BRFS). The early stages of accretion along the BRFS in the Knik River area were characterized by high‐temperature ductile thrusting and metamorphism at low to moderate P/T (P from 300–700 MPa, Tmax about 600°C). Peak metamorphic conditions at the currently exposed levels can be explained by heat carried to the area by tonalite and trondhjemite plutons. Generation of tonalite and trondhjemite melts within the Cretaceous forearc, however, requires a heat source within or near the BRFS. Ridge subduction cannot be ruled out as the heat source; however, simple thermal models presented herein indicate that the temperatures required for anatexis within the forearc can be explained by transient heating on the fault by shear heating of dry lithologies during the earliest stages of thrust motion. Water released from subducted oceanic crust would result in generation of partial melts in the hanging wall if shear heating is significant or if the initial thermal gradient within the hanging wall is high. Further accretion of oceanic sediment would quench the system and temperatures rapidly evolve to the high P/T conditions of normal subduction zones. Thermochronologic data from this study show that the duration of high‐T‐low P/T conditions was short, of the order of about 15 m.y. or less. Exhumed forearcs here and elsewhere commonly record a period of low to moderate P/T conditions during subduction initiation in which shear heating likely played an important role.
Fractal analysis of three‐dimensional spatial distributions of earthquakes with a percolation interpretation
Authors: Robertson Michelle C. , Sammis Charles G. , Sahimi Muhammad , Martin Aaron J.
Published: February 2004

Although many studies have shown that faults and fractures are self‐similar over a large range of scales, none have tested the fault structure for self‐similarity in three dimensions. In this study, earthquake hypocentral locations in central and southern California were used to illuminate three‐dimensional (3‐D) fault structures, for which we measured the fractal capacity dimension, D0(3‐D). Hypocentral distributions from the Joshua Tree, Big Bear, and Upland aftershock sequences, as well as background seismicity at Parkfield were found to be fractal, where D0(3‐D) increased with increasing event density, asymptotically approaching a stable value. The Joshua Tree data set stabilized at D0(3‐D) = 1.92 ± 0.02, the Parkfield data set asymptotically approached D0(3‐D) = 1.82, and the Big Bear data set approached D0(3‐D) = 2.01. As a test of the effects of location errors upon the measured value of D0(3‐D), the Upland aftershock data were located with both the southern California Hadley and Kanamori (1977) (H‐K) velocity model, and the more accurate Hauksson and Jones (1991) (H‐J) velocity model. Events located with the H‐K model asymptotically approached D0(3‐D) = 2.07, and events located with the H‐J model approached D0(3‐D) = 1.79, suggesting that improved hypocentral locations may decrease the measured fractal dimension. One interpretation of our results of D0(3‐D) ≤ 2.0 for all of the hypocentral data is that earthquakes only occur on the “percolation backbone” of a fault network, i.e., the active part of the network that accommodates finite strain deformation (Sahimi et al., 1993). We show that a percolation model that allows for healing of previously broken bonds is consistent with this interpretation.
Separation of intrinsic and scattering Q based on frequency‐dependent amplitude ratios of transmitted waves
Authors: Kang Ik Bum , McMechan George A.
Published: February 2004

Q estimates are made from in situ measurements of energy loss per cycle from transmitted P and S waves, as a function of frequency, for data from active‐source experiments. Assuming that intrinsic Q is frequency‐independent and scattering Q is frequency‐dependent over the frequencies of interest, the relative contributions of each, to a total observed Q, are estimated. Test examples are produced by computing viscoelastic synthetic seismograms using a pseudospectral solution with inclusion of relaxation mechanisms (for intrinsic Q) and a fractal distribution of scatterers (for scattering Q). The composite theory implies that when the total Q for S waves is smaller than that for P waves, intrinsic Q is dominant; when it is larger, scattering Q is dominant. In the inverse problem, performed by a global least squares search, intrinsic Q estimates are reliable as long as their values are sufficiently low so that their effects are measurable in the data. Estimates of scattering Q parameters are most reliable if the frequency bandwidth of the data corresponds to wavelengths both larger and smaller than the dominant scatterer size, and the estimates exhibit nonuniqueness if all the data are on one side or the other. In situ measurements are made from small‐scale linear array data from the North Sea, the New Madrid seismic zone, Turkey, and north Texas. All provide internally consistent and geologically reasonable results. For the P waves in all four data sets, intrinsic and scattering contributions are of approximately the same importance; for S waves the intrinsic contributions dominate.
A baseline for upper crustal velocity variations along the East Pacific Rise at 13°N
Authors: Kappus Mary E. , Harding Alistair J. , Orcutt John A.
Published: February 2004

A wide aperture profile of the East Pacific Rise at 13°N provides data necessary to make a high‐resolution seismic velocity profile of the uppermost crust along a 52‐km segment of ridge crest. Automated and objective processing steps, including τ − p analysis and waveform inversion, allow the construction of models in a consistent way so that comparisons are meaningful. A continuous profile is synthesized from 70 independent one‐dimensional models spaced at 750‐m intervals along the ridge. The resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability. The main features are a thin low‐velocity layer 2 A at the top with a steep gradient to layer 2B. The seafloor velocity is nearly constant at 2.45 km/s ±3% along the entire ridge. The velocity at the top of layer 2B is 5.0 km/s ±10%. The depth to the 4 km/s isovelocity contour within layer 2A is 130±20 m from 13°to 13°20′N, north of which it increases to 180 m. The increase in thickness is coincident with a deviation from axial linearity (DEVAL) noted by both a slight change in axis depth and orientation and in geochemistry. The waveform inversion, providing more details plus velocity gradient information, shows a layer 2A with about 80 m of constant‐velocity material underlain by 150 m of high velocity gradient material, putting the base of layer 2A at approximately 230 m depth south of 13°20′N and about 50 m thicker north of the DEVAL. The overall lack of variability, combined with other recent measurements of layer 2A thickness along and near the axis, indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity, but the dynamics of emplacement. The homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10% against which other observed variations in similar regimes can be compared.
Active thrust front of the Greater Caucasus: The April 29, 1991, Racha earthquake sequence and its tectonic implications
Authors: Triep E. G. , Abers G. A. , Lerner‐Lam A. L. , Mishatkin V. , Zakharchenko N. , Starovoit O.
Published: February 2004

Although fault‐bounded thrust sheets are common in the geological record, seismic evidence for their motion is sparse. The April 29, 1991, Racha earthquake (Ms = 7.0), the largest instrumentally recorded earthquake in the Greater Caucasus, is one of the largest recent earthquakes in continental thrust belts and provides evidence on mechanisms of thrust sheet motion. Using data from a deployment of Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) digital seismographs and various other instruments, we locate 1952 aftershocks occurring between May 7 and June 30, 1991. The aftershocks form a zone ∼70 km long and 10–25 km wide striking E‐W, following the Racha ridge at the southern boundary of the Greater Caucasus thrust system. Teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks. The solutions show thrust faulting with centroid depths of 3–10 km, comparable to depths of locally recorded aftershocks (∼2–12 km). The shallow‐dipping nodal plane, the aftershock distribution, and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping NNE at 20°–31° bounding the southern slope of the Greater Caucasus. This fault system thrusts the Greater Caucasus structures south over the Dzhirula basement massif. The inferred fault geometry suggests that the active fault is either a detachment between sediments and Dzhirula basement or cuts through the basement at shallow depths. The 1500‐m‐high Racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes. Hence the 1991 earthquake sequence shows that the western Greater Caucasus is accommodating plate convergence at a rate possibly comparable to the eastern Greater Caucasus (a few millimeters per year). Along‐strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area. No strong evidence for transfer structures could be found along strike, suggesting that differences in collisional style between the western and eastern Greater Caucasus may reflect differences in mechanical properties rather than differences in convergence rate. A June 15, 1991, event and its aftershocks, southeast of the primary aftershock zone along strike, show fault planes and slip vectors rotated ∼41° clockwise from the mainshock. This rotation is consistent with an along‐strike change in direction of the thrust front, near 44°E longitude, and demonstrates strong local structural or topographic control on slip direction. The rotation requires along‐strike shortening within the Greater Caucasus thrust system at a rate comparable to the rate of thrusting.
Magnetism in rocks
Authors: Dunlop David J.
Published: February 2004

Rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks, sediments, soils, and organisms. Its applications include environmental magnetism, magnetic anisotropy, sources of continental and oceanic magnetic anomalies, records of geomagnetic field variations and polarity reversals, and the paleomagnetic record of plate motions and the Wilson cycle. This paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas: pseudo‐single‐domain behavior; magnetic domains and micromagnetic structures; diagnostic tests of the type and stability of remanent magnetization; magnetic microanalysis; thermoviscous remagnetization; and chemical remanent magnetization. Other areas, including sediment and soil magnetism, are covered in a companion paper by Verosub and Roberts on environmental magnetism.
Reactions between molten iron and silicate melts at high pressure: Implications for the chemical evolution of Earth's core
Authors: Ito E. , Morooka K. , Ujike O. , Katsura T.
Published: February 2004

Reactions between molten iron and silicate melt were investigated with mixtures of pure iron and silicates (1Fe + 3MgSiO3 enstatite and 1Fe + 3(Mg0.9Fe0.1)2SiO4 olivine in volumetric ratio) as starting materials at pressures of 10–26 GPa and temperatures of about 2500°C. The results show that a certain amount of Si (up to about 2%) dissolves in molten iron from silicate melt and that the dissolution is enhanced with increasing pressure. Many small spherical blobs composed of SiO2 and FeO present in coalesced iron grains were interpreted as quenched immiscible liquid formed during cooling. Therefore O also dissolves in molten iron under the experimental conditions. No evidence for dissolution of Mg was obtained. The present study also indicates that Si and O are important light elements of Earth's core if core segregation occurred in the deep magma ocean. The chemical evolution of Earth's core is discussed on the bases of the current core formation model and the present experimental results.
Variations on the NeHT high‐resolution tomography method: A test of technique and results for Medicine Lake Volcano, northern California
Authors: Chiarabba C. , Amato A. , Evans John R.
Published: February 2004

To test alternate analysis techniques and verify principal structural results of earlier work, we computed the three‐dimensional compressional‐wave velocity structure of the upper crust beneath Medicine Lake volcano and surrounding parts of northern California, using damped least squares “local earthquake” inversions. Our data are travel times from high‐resolution (“NeHT”, after Nercessian, Hirn, and Tarantola (Nercessian, et al., 1984)) tomography experiments plus seismic refraction studies. This data set is inhomogeneous in ray density and ray orientation, though it is of nearly optimal homogeneity beneath the volcano itself. We used the damped least squares methods of Thurber and of Prothero, which differ in the way they trace rays. Both methods allow us to loosen the a priori constraints on raypaths implicit to a previous study done with the “ACH” (after Aki, Christofferson, and Husebye (Aki et al., 1977)) “teleseismic” inversion, and only the NeHT data set. We attempt to calculate the absolute value of the velocity, which is lost in the ACH approach, and to obtain more realistic raypaths. These gains are at the cost of solving a nonlinear problem with increased risk of missing the most realistic or the global‐minimum solution. We first performed inversions of the densely sampled region beneath the volcano, using a heterogeneous starting model taken from refraction and gravity data. These starting models include the primary velocity anomaly beneath the volcano, a large shallow high‐velocity lens. We also calculated a “graded” series of inversion models, progressing from a coarse grid of velocity nodes to a fine grid. The result at each “graded” inversion step becomes the starting model for the next finer step. This graded technique is sometimes recommended to assist discovery of the global‐minimum solution, but it may not have succeeded in finding the most realistic model for our inhomogeneous data set. The results of inversions computed with the heterogeneous starting model are very similar to the published ACH results, but results of the graded inversion differ significantly. Though the graded model did reproduce first‐order results of the ACH inversion, it differed in many important details and was sensitive to the method of raytracing. We are gratified by the first‐order similarities between all models and the excellent agreement between the ACH and the results with the heterogeneous starting model, but doubt that the graded inversion method reliably recovers the best solution from inhomogeneous data sets.
Measurement and interpretation of tidal tilts in a small array
Authors: Kohl M. L. , Levine J.
Published: February 2004

Strain‐induced tilts caused by variations in material properties can produce large perturbations in the tilt field near a material discontinuity. This is evidenced in data from a closely spaced array of borehole tiltmeters that has been operating at Piñon Flat Geophysical Observatory in southern California since 1987. The array is composed of three borehole tiltmeters located at depths of 24, 36, and 120 m. The tiltmeters have a sensitivity of a few nanoradians. Tidal measurements show differences between the boreholes of up to 50%. The largest percent difference occurs in the semidiurnal band where the tilt field measured in the 36‐m borehole differs by ∼10 nrad at the M2 frequency. This difference has been modeled as a strain‐induced tilt caused by the weathering layer.
Cyclic fluid flow through a regionally extensive fracture network within the Kodiak accretionary prism
Authors: Fisher Donald M. , Brantley Susan L. , Everett Mark , Dzvonik Joseph
Published: February 2004

Two types of periodic textures observed in veins from the Kodiak accretionary prism attest to cyclic fluid flow through a regionally extensive fracture network buried at 8–12 km depth: (1) crack‐seal microstructures with bands of mica inclusions and (2) collapse microstructures with jagged bands of residue embedded within euhedral crystals of quartz. The difference in texture reflects the closure of cracks: crack‐seal microstructures record the complete chemical sealing of the crack after each fracture event, whereas the collapse features record longer fluid‐filled periods followed by more rapid draining of fractures. Collapse features consist of pressure solution selvages trapped within veins and in the wall rock adjacent to euhedral growth terminations; the high concentrations of immobile elements in these selvages indicate that these fractures closed by collapse and penetration of quartz crystals into wall rock. Analysis of chemical composition on either side of four large euhedral growth veins and whole rock analysis of slates across the Kodiak Formation reveal local depletion of silica adjacent to veins but no evidence for long‐distance silica transport within the system. Both crack‐seal and collapse textures are observed in a regionally extensive vein system that displays a regular geometry, with thin, closely spaced (0.5–3 cm), near‐vertical crack‐seal veins that connect vertically and laterally with thicker euhedral growth veins arranged in widely spaced (∼500 mm) southeast dipping en echelon sets. The mesoscopic distribution and textural variability of the vein network suggests that the development of the vein system involved early nucleation and growth of vertical hydrofractures. As the fracture density increased, arrays of fractures locally provided zones of weakness and southeast dipping brittle‐ductile shear zones nucleated. These en echelon cracks remained open and provided small reservoirs of fluid. Textures show that en echelon fractures remain open but periodically grow by upward and downward propagation. Crack tips are then sealed with locally derived silica, and fluid drains back into en echelon fracture arrays. This local fluid movement is punctuated by less frequent events where the system links up over a greater distance, fractured reservoirs become interconnected, and the fluid within reservoirs is drained upward or laterally. Periodic inflation and deflation of en echelon arrays may reflect periodic slip on crosscutting faults and rupture of the seals that separate reservoirs.