bluuebunny/crossref_metadata_2025 huggingface dataset:
The 1989 Loma Prieta earthquake imaged from inversion of geodetic data
Authors: Árnadóttir Thóra , Segall Paul
Published: February 2004

We invert geodetic measurements of coseismic deformation from the 1989 MS7.1 Loma Prieta earthquake to determine the geometry of the fault and the distribution of slip on the fault plane. The data include electronic distance measurements, Global Positioning System and very long baseline interferometry vectors, and elevation changes derived from spirit leveling. The fault is modeled as a rectangular dislocation surface in a homogeneous, elastic half‐space. First, we assume that the slip on the fault is uniform and estimate the position, orientation, and size of the fault plane using a nonlinear, quasi‐Newton algorithm. The best fitting dislocation strikes N48°±4°W and dips 76°±9°SW, consistent with the trend of the aftershock zone and moment tensor solutions. Bootstrap resampling of the data is used to graphically illustrate the uncertainty in the location of the rupture plane. The 95% confidence envelope overlaps the aftershock zone, arguing that there is not a significant discrepancy between the geodetic data and the aftershock locations. Second, we estimate the slip distribution using the best fitting uniform slip fault orientation but increase the fault length to 40 km and the downdip width to 18 km. The fault is divided into 162 subfaults, 18 along strike and 9 along dip. Each subfault is allowed to have constant right‐lateral and reverse components of slip. We then solve for the slip on each subfault that minimizes a linear combination of the norm of the weighted data residual and the roughness of the slip distribution. The smoothing parameter, which determines the relative weight put on fitting the data versus smoothing the slip distribution, is chosen by cross validation. Simulations indicate that cross‐validation estimates of the smoothing parameter are nearly optimal. The preferred slip distribution is very heterogeneous, with maximum strike slip and dip slip of about 5 and 8 m, respectively, located roughly 10 km north of the hypocenter. There is insignificant dip slip in the southeastern most part of the fault, causing the rake to vary from nearly pure right‐lateral in the southeast to oblique right‐reverse in the northwest. The change in rake is consistent with a uniform stress field if the fault dip increases by about 10° toward the southeast, as indicated by the aftershock locations. There was little slip above 4 km depth, consistent with the observation that there was little, if any, surface rupture.
Onset of thermal convection in fluids with temperature‐dependent viscosity: Application to the oceanic mantle
Authors: Davaille Anne , Jaupart Claude
Published: February 2004

Heat flow measurements through old seafloor demonstrate that the oceanic lithosphere is heated from below away from hot spot tracks. We reevaluate the hypothesis of small‐scale convection beneath the lithosphere with laboratory experiments in fluids whose viscosity depends strongly on temperature. Rayleigh numbers were between 106 and 108 and viscosity contrasts were up to 106. A layer of fluid was impulsively cooled from above, and a cold boundary layer grew at the top of the fluid layer. After a finite time, convective instabilities developed in the lowermost part of the boundary layer, while the upper part remained stagnant. The variation of surface heat flow as a function of time reflects the three‐dimensional nature of the flow and the presence of a thick lid. At viscosity contrasts greater than 103, this variation is very similar to what is observed on the oceanic lithosphere. For small times, heat flow follows the behavior of a half‐space cooled from above by conduction. Some time after the onset of convection, it deviates from the conductive evolution and settles to a value which seems almost constant over a length of time equal to a few multiples of the onset time. The occurrence of small‐scale convection is difficult to detect in global data sets of seafloor depths. The onset of convection is marked by a small “trough” in the local subsidence curve but does not occur at the same time everywhere because of the probabilistic nature of the instability process. Later instabilities occur independently of each other and, at any given age, involve a region of small horizontal extent below a thick lid. The characteristics of the instability depend on the function describing the variation of viscosity with temperature. Scaling laws are derived for the onset time and for the surface heat flow. The requirement that small‐scale convection supplies 45 mW m−2 to the oceanic lithosphere provides a relationship between the activation enthalpy for creep and the asthenosphere viscosity. For a range of activation enthalpy of 250 to 600 kJ mol−1, the asthenosphere viscosity must be between 3×1018 and 4×1017 Pa s. The thickness of the stagnant lid and the temperature difference driving small‐scale convection are predicted to be about 80 km and 200°C, respectively.
Mechanics of monoclinal systems in the Colorado Plateau during the Laramide orogeny
Authors: Yin An
Published: February 2004

Monoclines developed in the Colorado Plateau region during the Laramide orogeny are divided into western and eastern groups by a broad NNW trending antiform through the central part of the plateau. In the western group the major monoclines verge to the east, whereas in the eastern group the major monoclines verge to the west. Paleogeographic reconstruction based on paleocurrent indicators and sedimentary facies distribution suggests that the broad antiform was developed during the Laramide orogeny and was coeval with the formation of the monoclines in the plateau. This relationship implies that the monoclines were drag folds verging towards the center of the plateau as a response to the antiformal warping of the plateau. To simulate the warping of the plateau region and the stress distribution that produced the variable trends of the monoclines, an elastic thin plate model considering in‐plane stress was developed. This model assumes that (1) sedimentation in the Laramide basins provided vertical loading along the edge of the plateau region, (2) frictional sliding was operating along the Laramide faults on the northern and eastern boundaries, and (3) the greatest regional compressive stress was oriented in the N60°E direction and was applied uniformly along the western and southwestern sides of the plateau. Buoyancy due to instantaneous isostatic adjustment of crustal thickening or magmatic addition was also considered. The result of the model suggests that the frictional strength of the Uinta thrust system on the northern side of the plateau is at least 2 times greater than that along the Park Range and Sangre de Cristo thrust systems on the eastern side of the plateau in order to explain the observed monoclinal trends and the warping pattern within the plateau during the Laramide orogeny.
Segmentation and a nontransform ridge offset on the Reykjanes Ridge near 58°N
Authors: Searle R. C. , Field P. R. , Owens R. B.
Published: February 2004

Multibeam bathymetry, deep‐towed side scan sonar, gravity, and magnetic data over a 50 km square area all suggest the presence of a second‐order, nontransform offset on the obliquely spreading Reykjanes Ridge (North Atlantic ocean) near 58°N latitude. It is the first such offset to be recognized on the Reykjanes Ridge. This region is characterized by a shallow median valley containing en echelon axial volcanic ridges (AVRs) similar to those found on more northerly parts of the Reykjanes Ridge. The side scan sonar shows that all the AVRs are constructional in nature. The one immediately south of the offset basin backscatters strongly, is unmarked by faulting, and so appears extremely young. Other AVRs appear older, having lower backscatter and, off axis, being cut by faults and fissures. At 57°55′N the progression of AVRs is interrupted by a 600‐m‐deep, 20 km × 10 km basin. Residual mantle Bouguer anomalies, corrected for two‐dimensional lithospheric cooling, display a high of at least 8 mGal over the basin. Two small, off axis basins occur roughly along the flow line from this basin and are also characterized by gravity highs. The basins are interpreted as regions of crustal thinning and are believed to represent the discontinuous trace of the ridge offset, which has thus been in existence for at least 2 m.y. and has been slowly propagating south. Inversion of the magnetic field shows a very low magnetization (∼3–4 A m−1) associated with the offset basin, which is interpreted as indicating a reduced magnetic layer thickness due to poor magma supply to the offset area. The Brunhes‐Matuyama reversal boundary and Anomaly 2 traces are offset about 3 km dextrally across the proposed offset trace. The AVR immediately north of the offset displays high amplitudes of magnetization which steadily increase southward toward its tip adjacent to the offset, suggesting the presence of increasingly highly fractionated basalts toward the AVR tip. Polarity transition widths across the Brunhes‐Matuyama boundary are comparable with those on other slow spreading ridges but are wider than individual AVRs, probably because AVRs overlap in the crustal accretion zone. Transition widths are also asymmetrical across the spreading center, being narrower (∼4.5 km) to the east than the west (∼8 km), suggesting a certain degree of asymmetry in the accretionary and rifting processes at the ridge axis.
Apatite fission track thermochronometry from central Alberta: Implications for the thermal history of the Western Canada Sedimentary Basin
Authors: Ravenhurst Casey E. , Willett Sean D. , Donelick Raymond A. , Beaumont Christopher
Published: February 2004

Apatite separated from 47 Cretaceous and Tertiary drill core and surface samples from the foreland basin strata of the Western Canada Sedimentary Basin (WCSB) have been analyzed for their apatite fission track (AFT) age and confined track length distribution. Thermal histories of the fission track data were then estimated using a constrained random search inversion technique based on the Durango apatite annealing model. Most importantly, this technique provides an estimate of the peak postdepositional temperature experienced by each sample with error bounds determined by the precision of the fission track data. Most apatite samples retain at least some tracks that formed prior to the time of maximum Cenozoic burial, assumed to be coincident with maximum temperature. Seven apatite samples were fully annealed during burial, during or following the Laramide Orogeny, and provide a minimum age of 42 Ma for heating. Lower Cretaceous samples from near the deformation front were fully annealed and attained temperatures of at least 119°–138°C during maximum burial. In contrast, stratigraphically equivalent samples at the northeastern end of the transect near the Cold Lake heavy oil and tar sand deposits did not exceed 80°C. Paleogeothermal gradients calculated using bounded estimates of the peak temperatures experienced by the samples, together with maximum burial estimates from coal moisture studies, range from ∼20°C/km near the deformation front to as high as 60°C/km near the cratonic edge of the basin. This variation has the same trend as present geothermal gradients, which range from ∼30°C/km to 45°C/km, but indicates a greater contrast at the end of the Laramide. The results are consistent with the concept of heat transport by basinal scale fluid flow.
An exact inversion for anisotropic moduli from phase slowness data
Authors: Miller Douglas E. , Spencer Carl
Published: February 2004

The problem of recovering density‐normalized elastic moduli of a transversely isotropic anisotropic medium from data consisting of qP or qSV phase velocities measured in multiple directions is addressed. Previous studies have used linear fitting methods with approximate forms of the dispersion relation. Here, it is shown that with algebraic manipulation, and a prior estimate of the squared shear velocity along the symmetry axis (A55), it is possible to use simple linear methods with an exact alternative form of the anisotropic dispersion relation. The method is demonstrated with an application to data from a walkaway vertical seismic profile (VSP) experiment and then used as a tool to address several questions raised by that experiment. It is shown that given data with realistically achievable accuracy, the prior estimate of A55 cannot be improved by optimizing the fit to qP data. It is shown that a near perfect fit by a transversely isotropic medium with a vertical symmetry axis (TIV) model to qP data collected in a single vertical plane does not rule out azimuthal anisotropy. Finally, it is shown that a variation of the method, combined with an algorithm suggested by Hood and Schoenberg, suggests a practical way to determine from walkaway VSP data, all the parameters of an orthorhombic medium formed by adding vertical fractures to a transversely isotropic medium.
Controls on hinge‐parallel extension fracturing in single‐layer tangential‐longitudinal strain folds
Authors: Lemiszki Peter J. , Landes John D. , Hatcher Robert D.
Published: February 2004

The stress history results from a published viscous layer folding solution are used as the basis for a fracture mechanics analysis of the factors that control hinge‐parallel extension fracturing in tangential‐longitudinal strain folds. The analysis incorporates published results for the change in sedimentary rock mode I fracture toughness at increasing confining stress to examine the relationship between regional strain rate, depth of burial, pore fluid pressure, initial crack size, layer viscosity, and the amount of fold shortening required for the propagation of a bed‐perpendicular, hinge‐parallel extension fracture. Tangential‐longitudinal strain folding of layers can occur at all scales in a foreland thrust system and is the result of the buckling and bending of stratigraphic units during the development of décollement, fault bend, and fault propagation folds. Hinge‐parallel extension fractures oriented perpendicular to bedding are a common fracture set observed in tangential‐longitudinal strain folds. The fractures propagate as a result of local tensile stresses that develop by the stretching of layers in the outer arc of fold hinges during bending. We considered a range of geologically reasonable boundary conditions to show that at one extreme, fracturing can occur as a result of only minor shortening by folding to the other extreme where a tight fold can form with no associated extension fracturing. For folds formed at shallow depths, where the confining stress on the system is less than the bending stresses in the layer and where the confining stress has not greatly increased the fracture toughness of the rock, hinge‐parallel extension fractures can grow under hydrostatic fluid pressure conditions. As depth increases, however, much higher pore fluid pressures are required to cause fracturing under similar strain rates. The observed controls are used to hypothesize how hinge‐parallel extension fracturing in fault bend folds can vary spatially and temporally across a thrust belt as a function of strain (thrusting) rate, the amount of bending at thrust ramps, and the depth of folding.
Thermal regime of the southern Basin and Range Province: 1. Heat flow data from Arizona and the Mojave Desert of California and Nevada
Authors: Sass J. H. , Lachenbruch Arthur H. , Galanis S. P. , Morgan Paul , Priest S. S. , Moses T. H. , Munroe R. J.
Published: February 2004

With about 150 new heat flow values, more than 200 values of heat flow are now available from the crystalline terranes of southern California, the Basin and Range Province of Arizona, and Paleozoic sedimentary rocks of the southwestern Colorado Plateau (CP). Heat flow ranges from about 5 mW m−2 on the CP near Flagstaff, Arizona, to more than 150 mW m−2 in the crystalline rocks bordering the Salton Trough in SE California. The heat flow pattern within this region is complex, although it correlates with regional physiographic and tectonic features. Unlike the adjacent Sierra Nevada Batholith where heat flow is a linear function of near‐surface radiogenic heat production, no statistically significant correlation exists between heat flow and heat production in the study area, possibly because of its complex tectonic history, involving lateral movement of basement terranes, and relatively young heat sources and sinks of different strengths, ages, and durations. Contemporary and Neogene extensional tectonism appears to be responsible for the very high heat flow (>100 mW m−2) associated with the Salton Trough and its neighboring ranges, the Death Valley fault zone and its southward extension along the eastern boundary of the Mojave block, and zones of shallow depth (<10 km) to the Curie isotherm (as inferred from aerornagnetic data) in west central Arizona. Low (<60 mW m−2) heat flow in the Peninsular Ranges and eastern Transverse Ranges of California may be caused by downward advection associated with subduction and compressional tectonics. Relatively low heat flow (67±4 mW m−2) is also associated with the main trend of metamorphic core complexes in Arizona, and the outcropping rocks in the core complexes have a low radioactive heat production (1.3±0.3 μW m−3) compared to the other crystalline rocks in the region (2.1±0.2 μW m−3).
Microgranitic enclave shapes and magmatic strain histories: Constraints from drop deformation theory
Authors: Williams Q. , Tobisch O. T.
Published: February 2004

Microgranitic (mafic) enclaves are common components of plutonic bodies and may represent the result of mingling of mafic and felsic magmas. These enclaves are often ellipsoidal or elongate in shape. In order to constrain the conditions under which such enclaves were generated and deformed by their host pluton, we apply drop deformation theory to model their distortion. Our model takes into account the change in viscosity of both the host and enclave with temperature, composition and the amount of suspended solids in enclave/host systems, and it assumes that simple shear flow predominates in the plutonic system. We focus on the deformational history of enclaves over the time period in which they remain liquid and are either more or less viscous than their granitic host. We find that the most stable enclaves with respect to being broken up by their entraining flow are generally the most mafic enclaves: those with silica contents near those of the host granite are highly unstable. This is in reasonable accord with field observations of the abundance of enclaves of different chemistry. Most deformation of microgranitic enclaves is likely to occur at relatively high temperatures, probably between 950°C and 1050°C, and therefore, the enclaves record magmatic strain of the host over only a limited temperature‐time range in the host's cooling history. Furthermore, the observation of apparent deformation of these enclaves in a liquid regime coupled with our calculated dependences of enclave deformation on flow velocity implies that magmatic flow velocities are likely to be below 10 m/yr in enclave‐bearing plutonic systems.
Negative‐polarity seismic reflections along faults of the Oregon accretionary prism: Indicators of overpressuring
Authors: Moore J. Casey , Moore Gregory F. , Cochrane Guy R. , Tobin Harold J.
Published: February 2004

Either thrusting of higher over lower impedance sediment or reduction of impedance locally in the fault zone can produce negative‐polarity reflections along the protothrusts and the frontal thrust of the Oregon accretionary prism. The vertical displacement of protothrusts averages only 27 m, is uncorrelated to the occurrence of polarity reversals, and is insufficient to generate the impedance contrasts associated with the negative‐polarity reflections. The negative‐polarity reflections from the protothrusts are probably due to localized impedance reduction in the fault zone. Waveform modeling of these negative‐polarity reflections indicates they develop due to a single interface with negative impedance contrast. This seismic signal could be produced by a fault zone with a sharp upper boundary with low impedance fault material and a transition down section to sediment of higher impedance. An impedance inversion due to the 1‐km throw on the frontal thrust could create the observed negative‐polarity reflections. Interval velocities do not decrease across the frontal thrust, but core‐scale velocity measurements indicate a velocity decrease in the frontal thrust. Therefore the negative‐polarity reflections are apparently produced by a localized decrease in impedance. Waveform modeling of frontal thrust reflections suggests the low impedance interval is about 20 m thick with discrete upper and lower boundaries. Moderately dipping faults are an efficient path for fluid expulsion across the turbidites with low equivalent vertical permeability. Fluid migration from the overpressured section up the faults can locally reduce the velocity sufficiently to cause the negative‐polarity reflections.
Inhomogeneous plane waves and cylindrical waves in anisotropic anelastic media
Authors: Krebes E. S. , Le Lawrence H. T.
Published: February 2004

In isotropic anelastic media, the phase velocity of an inhomogeneous plane body wave, which is a function of Q and the degree of inhomogeneity γ, is significantly less than the corresponding homogeneous wave phase velocity typically only if γ is very large (unless Q is unusually low). Here we investigate inhomogeneous waves in anisotropic anelastic media, where phase velocities are also functions of the direction of phase propagation θ and find that (1) the low phase velocities can occur at values of γ which are substantially less than the isotropic values and that they occur over a limited range of oblique directions θ, and (2) for large positive values of γ, there are ranges of oblique directions θ in which the inhomogeneous waves cannot propagate at all because there is no physically acceptable solution to the dispersion relation. We show examples of how the waves of case 1 can occur in practice and cause a number of anomalous wave propagation effects. The waves of case 2, though, do not arise in practice (they do not correspond to any points on the horizontal slowness plane). We also show that in the decomposition of a cylindrical wave into plane waves, inhomogeneous plane waves occur whose amplitudes grow in the direction of phase propagation and that this direction is away from the receiver to which they are contributing. The energy in these waves does, however, travel toward the receiver, and their amplitudes decay in the direction of energy propagation. We also show that if the commonly used definition for the quality factor in an isotropic medium, Q = −Re(μ)/Im(μ) where μ is a complex modulus, is applied to an anisotropic anelastic medium in order to study absorption anisotropy, a generally unreliable measure of the anelasticity of inhomogeneous wave propagation in a given arbitrary direction is obtained. The more fundamental definition based on energy loss (i.e., 2π/Q = ΔE/E) should be used in general, and we present some basic formulas for this quantity, as well as others, for plane waves in transversely isotropic anelastic media.
Paleomagnetism of the Permian Wallowa terrane: Implications for terrane migration and orogeny
Authors: Harbert William , Hillhouse John , Vallier Tracy
Published: February 2004

A sequence of volcaniclastic sedimentary and pyroclastic rocks of Permian age (late Leonardian or early Guadalupian; ≈260 Ma) in the Wallowa terrane of Oregon and Idaho has been sampled for paleomagnetic study. Forty‐six sites (222 cores) were collected from folded, laminated siltstone, mudstone, and tuffaceous sandstone beds exposed along the Snake River in Hells Canyon. Several stratigraphic sections, within the 1500‐ to 2500‐m‐thick Hunsaker Creek Formation, were sampled over a stratigraphic interval of about 700 m. Thermal demagnetization revealed three components of magnetization. The A component has the lowest unblocking temperatures (generally less than about 200°C), has normal polarity, and fails the fold test. The B component is removed between heating steps of about 200°C and 550°C, is of normal polarity, and also fails the fold test. The C component, observed between heating steps of approximately 550°C and 660°C, is always of negative inclination. The grouping of this component improves slightly upon unfolding, with the optimum grouping occurring at 95% unfolding, using sites withn≥ 3 andk≥ 10, (k95/k0= 1.18), or 100% using sitesn≥ 3 (k95/k0= 1.51). This component is interpreted as the characteristic remanent magnetization acquired during deposition of the Hunsaker Creek Formation based on the following observations: The better grouping of the high‐temperature magnetic component after unfolding and the consistent reversed polarity seen in samples from the different sections. The resulting paleolatitude, calculated using inclination‐only statistics, of 24°±12° N (n≥ 3,k≥ 10), or 26°±9°N (n≥ 3 sites), implies a northern hemisphere position of the Wallowa terrane during the Early Permian and possible southward transport of this terrane with respect to North America. If the Wallowa terrane was attached to the Wrangellia superterrane, our result indicates a northern hemisphere location for that superterrane during the Permian.
Seismic structure of the deep Kurile Subduction Zone
Authors: Ding Xiao‐Yang , Grand Stephen P.
Published: February 2004

Three‐dimensional modeling of P wave travel time residuals was performed to investigate the deep slab‐related velocity structure within the Kurile subduction zone. Travel times of P waves from earthquakes at all depths within the Kurile subduction zone to World‐Wide Standard Seismograph Network and Canadian stations were measured. Instead of analyzing the travel time residuals of a single event, relative travel time residuals of P waves to common stations have been analyzed and used in this study to isolate near source contributions from the total travel time residuals. The effect of uncertainties in long wavelength heterogeneity far from the source in the lower mantle and the effect of uncertainties in the reference Earth model have been significantly reduced by analyzing relative travel time residuals. The effect of uncertainties in station corrections are eliminated by relative travel time residuals. Our measured P wave travel time data clearly indicate that high‐velocity anomalies exist in the lower mantle beneath the Kurile subduction zone but they do not appear to be caused by simple slab continuation into the lower mantle. The travel time modeling was accomplished using a fully three‐dimensional finite difference technique for wave propagation within the source region. The modeling results show that high‐velocity anomalies extend several hundred kilometers into the lower mantle but they appear to be very complicated. A large number of models consisting of simple slab continuation into the lower mantle were tested and the results show that none ofthese models was able to produce synthetics which match the observed P wave travel time data. The slab‐related velocity anomalies in the upper mantle of the Kurile subduction zone are generally consistent with the seismicity. The high‐velocity anomalies in the lower mantle below the Kurile subduction zone are broad and geometrically change from the southern to the northern Kuriles. Below the southern Kuriles, the high‐velocity anomalies appear to be subhorizontal. However, beneath the central Kuriles, there are broad, about 600‐km‐wide, high‐velocity anomalies on the continental side of the slab which extend several hundred kilometers into the lower mantle. The high‐velocity anomalies found below the northern Kuriles are broad and on both the continental and the ocean side of the slab, and extend several hundred kilometers into the lower mantle.
Active faulting and block movement associated with large earthquakes in the Min Shan and Longmen Mountains, northeastern Tibetan Plateau
Authors: Chen She Fa , Wilson C. J. L. , Deng Qi Dong , Zhao Xiao Lin , Zhi Li Luo
Published: February 2004

The Min Shan and central and southern parts of the Longmen Mountains form the middle segment of the north trending earthquake zone at the eastern edge of the Tibetan Plateau. The Min Shan Mountains are a Cenozoic to Quaternary uplift zone that has undergone south directed block movements to produce the 1933 Diexi earthquake (M = 7.5) on the Min Jiang Fault and the 1976 Songpan earthquake swarm (M = 7.2, 6.7, 7.2) on the Huya Fault. The Min Jiang Fault on the western boundary of the uplift zone is dominated by reverse movement with a right‐lateral component. The Huya Fault on the eastern margin of the Min Shan Uplift Zone is also dominated by reverse movement but is associated with a left‐lateral component. The uplift and block movement of the Min Shan Mountains are probably related to the compression at the eastern end of the Kunlun Fault which is a major northwest trending strike‐slip fault within the Tibetan Plateau. Quaternary deformation in the Longmen Mountains is characterized by relatively continuous thrusting on major faults and Himalayan folding events in foreland basin deposits, and no earthquakes of M ≥ 7 have been recorded there.
The origin of Kenya rift plateau‐type flood phonolites: Evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism
Authors: Hay David E. , Wendlandt Richard F. , Wendlandt Eric D.
Published: February 2004

Geochemical investigations support the petrogenesis of Kenya rift plateau‐type flood phonolites (14–11 Ma) by partial melting of an alkali basaltic material at lower crustal pressures. High‐pressure/high‐temperature experiments on a natural plateau phonolite (Hay and Wendlandt, this issue) document multiple saturation of augite, andesine, titanomagnetite, and phlogopite at 0.7 GPa, 1000°C, XCO2 = 0.42, with amphibole appearing at 975°C. A least squares solution to major element modeling, involving subtraction of the compositions of near‐liquidus augite, andesine, titanomagnetite, and olivine (Fo67; hypothesized product of incongruent melting of hydrous phases) from a Kenya Miocene alkali basalt composition, indicates that plateau phonolites can be derived by 15 wt % fusion of this hypothetical parental material (∑R2 = 0.07). Alkali basaltic magmas may have injected and/or underplated the lower crust in southern Kenya during prior rift‐related basaltic volcanism (23–14 Ma). Bulk Earth values of (87Sr/86Sr)i and εNd near zero for four plateau phonolite samples are consistent with a mantle‐derived parental composition. Three of these four samples reflect little, if any, postmelting modification; one sample may have evolved by fractional crystallization (high Rb/Sr, low Ba, Sr and Mg #). A fifth sample may show evidence of assimilation and fractional crystallization processes (elevated radiogenic Sr and Pb, large negative Eu anomaly, and low Ba, Sr, and Mg #). Much of the geochemical variation among plateau phonolite lavas, however, can be ascribed to melting of a predominantly alkali basaltic source with contributions from a lower crustal protolith. A mantle‐derived source is also supported by Sr‐Nd‐Pb isotope data for the phonolites, which indicate that the alkali basaltic source can be described in terms of high U/Pb (HIMU) and enriched mantle (EM1 and EM2) components.
Estimation of the thermal conductivity anisotropy of rock with application to the determination of terrestrial heat flow
Authors: Deming David
Published: February 2004

A moderately large data set of mostly sedimentary and metamorphic rocks compiled from the literature shows an inverse correlation between thermal conductivity anisotropy and thermal conductivity perpendicular to bedding. The empirical relationship can be represented by a simple two‐component end‐member mixing model. One end‐member is anisotropic (λx/λz ∼ 2.5) and has a relatively low thermal conductivity perpendicular to bedding (λz ∼ 1 W/m‐K); the other end‐member is isotropic and has a relatively high thermal conductivity perpendicular to bedding (γ ∼ 4 W/m‐K). The proposed model may be used to derive a correction for anisotropic effects observed during thermal conductivity measurements on mixtures of randomly oriented aggregates. Although the accuracy and reliability of a universal correction procedure remain to be determined, the proposed correction scheme should enhance our ability to estimate accurately heat flow in sedimentary basins where generally only borehole cuttings are available for thermal conductivity measurements.
Paleomagnetic investigation of late Neogene vertical axis rotation and remagnetization in central coastal California
Authors: Horns Daniel M. , Verosub Kenneth L.
Published: February 2004

Outcrops of shallow marine sedimentary rocks of the Neogene Purisima Formation and Santa Cruz Mudstone occur throughout the Santa Cruz Mountains and along the coast of Santa Cruz and San Mateo counties in California. Within the Santa Cruz Mountains, the Purisima Formation has been extensively folded and offset by reverse faults. Along the coast, the Purisima Formation and Santa Cruz Mudstone are only gently folded although the Purisima Formation is cut by the San Gregorio Fault Zone. Previous paleomagnetic studies have shown that vertical axis rotation has occurred within some parts of the San Gregorio Fault Zone. As part of a kinematic study of the San Gregorio Fault Zone, this paleomagnetic investigation of the Purisima Formation and Santa Cruz Mudstone was conducted to determine the extent of vertical axis rotation. The characteristic paleomagnetic directions of the Purisima Formation and Santa Cruz Mudstone, which we interpret as the primary directions, indicate that a clockwise vertical axis rotation of 35° to 60° has occurred throughout the San Gregorio Fault Zone, whereas no rotation has occurred away from the fault zone. This result suggests that vertical axis rotation is fundamentally related to shear across the San Gregorio Fault Zone. Our work also indicates that while the Purisima Formation and Santa Cruz Mudstone along the coast have retained their primary magnetizations, the Purisima Formation within the Santa Cruz Mountains has been remagnetized after folding.
Ultradeep crustal metamorphism: A prospective view
Authors: Harley S. L. , Carswell D. A.
Published: February 2004

Ultradeep, or ultrahigh‐pressure (UHP), metamorphism of crustal rocks at depths greater than those of the quartz‐coesite and even graphite‐diamond transitions has been firmly established from petrological studies in at least three collisional belts. The typically disrupted structural settings of the UHP rocks raise important questions about the scale and extents of UHP metamorphism and whether any present example can be interpreted as a regional terrane across which metamorphic P‐T gradients can be defined. Improvements in the precision with which UHP metamorphism can be characterized will rely on extending experimental studies to a greater range of chemical systems and solid solutions and on developments in the application of microbeam techniques to experimental products. The role of fluids in continental crust undergoing subduction, important to any understanding of melt generation in convergent zones, needs to be considered further using the available UHP areas as natural laboratories and through high‐pressure experimental studies of the solubilities of minerals in aqueous and saline fluids. Pressure‐temperature paths provide an essential constraint on the choice of possible tectonic models to explain the exhumation of UHP metamorphic rocks and therefore must be determined in detail using all the petrological tools available and integrated with precise geochronology focused on the timing of mineral growth in relation to structural markers and P‐T evolution. Extension of thickened crust or a crustal wedge experiencing continued underplating is a favored mechanism for the uplift and exhumation of UHP areas which will continue to be evaluated as new P‐T‐t data constraining the rates of exhumation and cooling of these remarkable rocks become available.
Reflection tomography: How to handle multiple arrivals?
Authors: Delprat‐Jannaud F. , Lailly P.
Published: February 2004

Applications of reflection tomography for the determination of complex geologic structures calls for the generalization of this method so that it can take triplications and other multiple arrivals into account. In this way, we propose a new formulation of travel time inversion. It relies on the choice of an adequate parametric representation of travel time information: the parameters we have chosen for this representation are the receiver location and the ray parameter at the receiver, some quantities directly measured from seismic data. The forward problem involved in the solution of this new inverse problem consists in shooting rays from a receiver according to the measured values of the ray parameter at the receiver. We can thus predict for a given model the emergence point of the reflected ray (i.e., the shot location) and the associated reflection arrival time. The least squares formulation of the inverse problem consists in minimizing an objective function that measures the mismatch between predicted and actual shot locations on one side and predicted and actual reflection arrival times on the other side, for the considered receiver locations and the associated measured ray parameters. However, inversion of noise corrupted kinematic data calls for a realistic definition of the uncertainties associated with the data. In particular, those uncertainties should take into account the sensitivity of reflection arrival times and shot locations to an error in the measurement of the ray parameter at the receiver. The objective function to minimize being chosen, the solution of the inverse problem is performed by a Gauss‐Newton method, the Jacobian of the forward modeling operator being computed by the adjoint state technique. It is interesting to note that no two‐point ray tracing is required in our method which is therefore cheaper than classical reflection tomography. The effectiveness of this approach is illustrated on a difficult synthetic example with large lateral velocity variations and strongly noise corrupted data.
Calc‐alkaline magmatism, lithospheric thinning and extension in the Basin and Range
Authors: Hawkesworth Chris , Turner Simon , Gallagher Kerry , Hunter Arlëne , Bradshaw Tim , Rogers Nick
Published: February 2004

Most of the volcanic rocks in the Colorado River Trough (CRT) and the Mogollon‐Datil Volcanic Field (MDVF) in the Basin and Range exhibit calc‐alkaline major element trends and relatively low high field strength element abundances, similar to those erupted from the volcanoes of Aso and Towada in Japan. Such features are widely regarded as characteristic of subduction‐related magmatism, and yet the rocks in the Basin and Range were generated in response to lithospheric extension. The preextensional to synextensional rocks of the CRT and the MDVF have higher Na2O, K2O, and TiO2, in the range 47–55% SiO2, and relatively low Al2O3, and overall, they tend to have higher Sr contents and Zr/Y and La/Nb ratios than those from Aso and Towada. In addition, the basalts in the Basin and Range tend to be more aphyric than those in Japan, consistent with more rapid movement of magma through the crust during extension in the Basin and Range, and the rate of melt generation appears to have been significantly less in the Basin and Range than along recent destructive plate margins. The geochemical differences are attributed to smaller degrees of partial melting in the Basin and Range and to source regions that had been enriched in incompatible elements since the Proterozoic, resulting in parental magmas with higher alkali contents than those commonly observed in subduction‐related calc‐alkaline suites. Within the CRT the subsequent calc‐alkaline trend was due at least in part to mixing with crustal derived melts, whereas in the MDVF such trends reflect both crustal contamination and fractional crystallization involving magnetite and amphibole. The small volumes of magma with minor and trace element features similar to oceanic basalts indicate that relatively little melt was generated in underlying asthenosphere. Thus it is inferred that magmatism in the Basin and Range was not associated with a significant increase in temperature, such as might be attributed to a mantle plume, but rather it was in response to lithospheric extension. Calculations are presented which demonstrate that the magma volumes and inferred source regions, extension, present‐day heat flow, and topography are consistent with a model of convective lithospheric thinning after thickening in the Laramide and Sevier orogenies.
Comprehensive analysis of marine magnetic vector anomalies
Authors: Korenaga J.
Published: February 2004

We present a comprehensive description of the analysis of marine magnetic vector anomalies. The marine magnetic vector field can be measured with a shipboard three‐component magnetometer (STCM). The STCM has the advantage of measuring three components, but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton‐precession magnetometer. We have estimated the quality of STCM data, constructed an optimal filter, and evaluated the reliability of the filtered data. Then, we have organized an analytical procedure composed of four steps: (1) the discrimination between two‐ and three‐ dimensional magnetic sources; (2) the determination of magnetic boundaries; (3) the calculation of the boundary strikes; and (4) the estimation of the boundary magnetization contrasts. We show examples of these procedures using the data near the East Pacific Rise 28°S–31°S collected during leg 5 of the GLORIA expedition. All the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the surveyed area.
Fluid‐rock interaction in faults of the San Andreas system: Inferences from San Gabriel fault rock geochemistry and microstructures
Authors: Evans James P. , Chester Frederick M.
Published: February 2004

Optical and scanning electron microscopy and whole rock geochemical analyses are used to investigate variations in deformation mechanisms and fluid‐rock interactions in rocks at three sites on the San Gabriel fault, southern California: Pacoima Canyon, Bear Creek, and North Fork. At Bear Creek, unaltered and undeformed granite, granodiorite, and diorite protolith bound a fault core several meters thick that consists of foliated cataclasite on either side of 2–20 cm thick ultracataclasite layer. The foliated cataclasite contains clays and zeolite veins which developed by alteration of protolith during slip. The ultracataclasite consists of 20–100 μm diameter feldspar and quartz fragments embedded in a clay‐zeolite matrix. The matrix consists of grains <10 μm and is enriched in Fe, Mg, Mn, and Ti relative to the average composition of the protolith. In contrast, ultracataclasite at Pacoima Canyon contains little clay and zeolite and apparently evolved with little fluid‐rock interaction. Whole rock geochemical analyses of the fault rock compositions at both sites are best explained as a result of mechanical mixing with local redistribution of some elements in a closed system relative to fluids. At both sites the ultracataclasite compositions can be modeled as the result of mixing of the bounding foliated cataclasites. Similarly, the foliated cataclasites were derived by mixing the protoliths on the same side of the ultracataclasite layer. Whole rock analyses for rocks from the North Fork site, which lies on a major splay of the San Gabriel fault, suggest an open system relative to fluids. The concentration of immobile elements in the fault core relative to all protoliths is best explained by fluid‐assisted volume loss of 37%±10%. Overall, the results imply local and regional‐scale variations in the hydrologic setting along the San Gabriel fault that produced contrasting styles of deformation and fluid‐rock interactions.
Regional differences of random inhomogeneities around the volcanic front in the Kanto‐Tokai area, Japan, revealed from the broadening of S wave seismogram envelopes
Authors: Obara Kazushige , Sato Haruo
Published: February 2004

Broadening of seismogram envelopes around direct S waves of deep earthquakes that occurred in a lithospheric slab reveals random inhomogeneities in the upper mantle and crust. Regional differences in the character of envelope broadening were studied in relation to the volcanic front in the central part of Japan. A total of 58 earthquakes that occurred along the subducting Pacific plate ranging from 80 to 500 km in depth were observed at 73 stations of the Kanto‐Tokai seismic observation network and used in this analysis. Time lags of the maximum peak arrival and the half‐maximum arrival were measured from the onset of the direct S wave on root‐mean‐squared traces of horizontal component seismograms in frequency bands of 1, 2, 4, and 8 Hz. The envelope broadening is weak at stations located along the coastline of the Pacific Ocean, however, the dependence of time lags on the hypocentral distance and frequency becomes strong at stations near the volcanic front. Moreover, the time lags become very long with increasing hypocentral distances and frequencies west of the volcanic front. This broadening of the seismogram envelope can be interpreted as the effect of the diffraction and multiple forward scattering on the seismic wave propagation through random inhomogeneities. The parabolic approximation is applied to simulate the envelope broadening when the correlation distance of velocity inhomogeneities is longer than the seismic wavelength. The numerical simulation indicates that the dependence of the envelope broadening on the travel distance and frequency depends greatly on the type of the autocorrelation function for the randomness. By comparing the observed frequency dependence of the envelope broadening with the result of the numerical simulation, the type of the autocorrelation function was estimated to be close to Gaussian at stations east of the volcanic front and close to exponential west of the volcanic front. The regional difference in the randomness on both sides of the volcanic front might reflect the difference in the short‐wavelength component inhomogeneities.