bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of seismicity due to ruptures on noncoplanar interactive faults
Authors: Yamashita Teruo
Published: February 2004

We study the statistical property and temporal variation of activity due to ruptures on two noncoplanar interactive faults. Each fault is assumed to consist of cracks and intervening asperities. The ruptures are assumed to occur in the form of catastrophic coalescence of the coplanar cracks on each fault. We apply fracture mechanics to the analysis of the interactions among cracks and to the consideration of fracture condition. Our fault model gives rise to a complex frequency distribution of event sizes even if no externally imposed spatial inhomogeneity is assumed for the distributions of cracks and asperities and a homogeneous statistical distribution is assumed for fracture strengths. For the smaller events the frequency distribution of the event sizes obeys the Gutenberg‐Richter magnitude‐frequency law. It is also shown that the number of smaller events increases as the contribution of noncoplanar interactions increases. We generally observe the periodic recurrence of events after transient change at the beginning of each sequence of activity. The length of the transient part increases considerably as the contribution of noncoplanar interactions increases. The recurrence period at the steady periodic state also lengthens with increasing contribution of noncoplanar interactions. Hence if the temporal variation of activity is observed in a fixed length of time, then the activity appears to be much more complex when the contribution of noncoplanar interactions is larger. It is also observed that events with smaller recurrence intervals are more abundant when the influence of noncoplanar interactions is large.
Correlation between seafloor heat flow and basement relief: Observational and numerical examples and implications for upper crustal permeability
Authors: Fisher Andrew T. , Becker Keir
Published: February 2004

There is a strong positive correlation between bathymetric and well‐navigated heat flow data from the Galapagos Mounds hydrothermal area and the east flank of the Juan de Fuca Ridge. A similar correlation is apparent at Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) site 504, where it provides a constraint on the intensity and geometry of ridge flank hydrothermal circulation. We test the implications of this correlation with a hydrogeological model of the shallow crust that contains significant permeability mainly within a few thin layers (tens of meters thick) in the upper few hundred meters of basement, consistent with observations in DSDP/ODP holes into upper oceanic crust. These simulations are compared directly to others in which the same bulk permeability is represented more homogeneously within the upper crust. A model with thin permeable zones and subtle relief beneath a flat seafloor can explain the variations in seafloor heat flow observed on one part of the Juan de Fuca Ridge east flank, as can a model with a flat seafloor and basement top but relief within the most permeable zone. We explore the importance of layered heterogeneities versus explicit permeability anisotropy in these simulations through a series of parametric tests. Some form of permeability anisotropy appears to be required in order to achieve efficient lateral heat transport within the upper volcanic crust so as to produce the common seafloor heat flow‐basement relief correlation. This permeability anisotropy may result directly from the primary architecture of the volcanic crust. Numerical results suggest that within an anisotropic system such as the upper oceanic crust, the length scale of heat flow variations (and therefore the length scale of underlying hydrothermal convection cells) is not an indication of the depth extent of fluid flow. Rather, the length scale of heat flow anomalies may reflect the length scale of relief along aquifer boundaries, while the depth of vigorous flow is constrained by the depth extent of significant permeability, and may be unresolvable with surface heat flow measurements.
Black‐box modeling of the subglacial water system
Authors: Murray Tavi , Clarke Garry K. C.
Published: February 2004

Measurements of water pressure beneath Trapridge Glacier, Yukon Territory, Canada, yield the following generalizations about subglacial conditions in the studied region: (1) Even over short distances the subglacial water system is highly heterogeneous. (2) The subglacial water system consists of at least two distinct components which we refer to as the “connected” and “unconnected” water systems. (3) Regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system. (4) Large spatial pressure gradients can exist within the unconnected water system, and between the connected and unconnected systems. (5) Rapid pressure variations can occur in the unconnected water system. (6) Pressure variations in the unconnected water system do not match those in the connected system and can, in fact, be strongly anticorrelated with pressure variations in the connected system. If the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing, the input‐output relation between forcing and response can be efficiently represented as a low‐order nonlinear ordinary differential equation. The response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of ∼1.7 hours and ∼7.4 hours that we believe are associated with process rates for substrate compression and pore water diffusion, respectively.
Use of renormalization group theory to explain the large variation of domain states observed in titanomagnetites and implications for paleomagnetism
Authors: Ye Jun , Merrill Ronald T.
Published: February 2004

Important experiments by Halgedahl (1991) demonstrate that a material can reach dramatically different domain states on cooling when the experiment is repeated many times. Renormalization group theory is applied to show that small thermal fluctuations close to the magnetic ordering temperature produce “predomain” structures. These predomain structures usually vary from one experiment to the next, even when the experimental conditions do not change. On cooling, these predomain structures are primarily responsible for determining the final domain states. Transdomain processes also occur during cooling, but these processes appear to be less important. Theory is presented to show that domain observations might be very useful to discriminate between primary thermal remanent magnetization and many forms of secondary magnetizations. In particular, a grain growth chemical remanent magnetization on the average will be characterized by a narrower distribution of magnetic domains in a given size grain than that associated with a thermal remanent magnetization.
Strength and rheology of the western U.S. Cordillera
Authors: Lowry Anthony R. , Smith Robert B.
Published: February 2004

Effective elastic thickness Te depends primarily on temperature, composition, and state of stress of the lithosphere. In this paper, we examine high‐resolution spectral estimates of Te and their relationships to regional heat flow, age of the lithosphere, seismic properties, stress orientations, and earthquake focal depths of the western U.S. Cordillera. The relationship of Te to heat flow indicates that ductile flow accommodates long‐term (∼106 to 108 years) isostatic response at different levels of the crust and upper mantle, depending principally on age (and, by implication, bulk composition) of the lithosphere. Isostatic response is primarily controlled by the upper mantle in Archean lithosphere of the middle Rocky Mountains, whereas Te depends on lower crustal flow in Early Proterozoic lithosphere of the Colorado Plateau. The Yellowstone‐Snake River Plain volcanic field and significantly extended regions in the Basin‐Range and northern Rocky Mountains are associated with latest Proterozoic aged lithosphere and indicate middle to upper crustal control of long‐term Te. We also show that azimuthal variations of Te reflect deviatoric stress in the lithosphere. Te is found empirically to approximate the 95th percentile focal depth of background seismicity. The latter relationship is inconsistent with brittle‐ductile control of focal depth, indicating that another rheological transition (e.g., from stick‐slip to stable sliding frictional behavior) is responsible. Tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere. Moreover, we find a spatial correlation of the Intermountain Seismic Belt to a marked transition in Te, implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current‐day plate boundary interactions.
Time‐varying fluid flow at the top of Earth's core derived from definitive geomagnetic reference field models
Authors: Voorhies Coerte V.
Published: February 2004

The effects of a time‐varying fluid velocity have been included in the estimation of frozen‐flux core surface flow. This has been done to improve the quality, performance, and accuracy of kinematic analyses of the geomagnetic secular variation. The derived flows consist of a reference velocity plus a steady acceleration and thus evolve linearly with time. The method features simultaneous solution for the initial radial geomagnetic field component at the core‐mantle boundary and the subadjacent, time‐varying fluid motion. The trade‐off between time‐averaged spatial complexity of the derived flows and misfit to the weighted Definitive Geomagnetic Reference Field models for 1945–1980 is explored. At fixed flow complexity, simultaneous solution for both initial geomagnetic field and steadily accelerating flow gives substantial reductions in the square‐weighted residuals per degree of freedom when compared with previous solutions for either initial field and steady flow or steady flow alone. The apparent timescale for flow change varies nonmonotonically with flow complexity between about 23 and 41 years; the single maximum value is associated with unit negative slope of the log‐log trade‐off curve and is used to help identify a preferred solution. This solution has an rms speed of but 7.50 km yr−1 and an rms acceleration of 0.183 km yr−2, or 1.84×10−13 m s−2. The latter indicates extremely weak net forces upon the liquid outer core.
Theoretical and numerical issues in the determination of reflector depths in seismic reflection tomography
Authors: Bube Kenneth P. , Langan Robert T. , Resnick Jeffrey R.
Published: February 2004

Seismic reflection tomography obtains an estimate of the subsurface slowness field and the location of strong reflectors by minimizing the difference between measured travel times from seismic reflection events and the corresponding travel times computed from a model of the subsurface. We present some theoretical results for the undiscretized problem regarding the possible ambiguity between slowness and depth. These results indicate that the depths of the reflectors are determined in theory except for edge effects, but a sufficiently large aperture at the reflector is necessary to resolve this ambiguity in practice. The slowness field, however, does have some undetermined features. These results have strong implications for how the tomography problem should be discretized and regularized to compute solutions which are accurate in the features of the model which are well determined from the travel time data. In particular, the slowness model should not be discretized much more coarsely than the reflectors as a way of regularizing the problem because that may force the computed reflector depths to try to match aspects of the travel time data which are caused by features in the slowness field.
Effects of geothermal development on deformation in the Long Valley Caldera, eastern California, 1985–1994
Authors: Sorey M. L. , Farrar C. D. , Marshall G. A. , Howie J. F.
Published: February 2004

Long Valley caldera in east central California has been the site of crustal unrest in the form of seismicity and ground deformation in response to magmatic inflation since 1980. Uplift of the resurgent dome has totaled ∼0.6 m over the 1975–1992 period. Within this region of uplift, and near the southwestern edge of the resurgent dome, is the Casa Diablo area, which experienced relative subsidence of 0.17 m between 1985 and 1992. Geothermal fluid production began at Casa Diablo in 1985 to supply a binary electric power plant; currently, three such plants generate a total of about 40 MW of electricity. The plants are supplied by 170°C water pumped from depths near 150m; all the produced water is reinjected at cooler temperatures at depths near 600 m. Analyses of data from (1) regional leveling lines and a network of bench marks in the geothermal well field area, (2) tilt observations from an L‐shaped array of bench marks, and (3) calculations relating pressure and temperature changes to subsidence indicate that relative subsidence at Casa Diablo results mainly from reductions in pressure in the shallow production reservoir and overlying formations and reductions in temperature in the underlying injection reservoir. Secondary effects on deformation are caused by offsets along the Taylor‐Bryant fault bounding the west side of the well field.
Swave attenuation structure in Long Valley Caldera, California, from three‐componentS‐to‐Pamplitude ratio data
Authors: Sanders Christopher O. , Nixon Lesley D.
Published: February 2004

UsingS‐to‐Pamplitude ratios measured from three‐component seismograms we tomographically imaged spatial variations inSwave attenuation in parts of Long Valley caldera. Data from seismograms of local earthquakes in the Long Valley caldera region that occurred during 1984 were employed with theS‐to‐Pamplitude ratio technique with corrections for source radiation pattern to determineQS. Resolution is limited throughout most of the model; however, significant resolution and acceptable error in parts of the model allow meaningful interpretation of the results. The primary result is the image of a low‐QSanomaly at 7–8 km depth beneath the resurgent dome. This is independent evidence for high temperatures and possible magma accumulation at this location. Other regions of anomalously lowQSare marginally significant or poorly resolved; of these lowQSat 4–5 km depth beneath the Inyo Domes has the most potential significance. The similarity of the results from the three‐component data and from the vertical‐component data implies that vertical‐component data, which are generally more abundant, can be used with confidence.
The interaction of a plume with a rheological boundary: A comparison between two‐ and three‐dimensional models
Authors: van Keken Peter E. , Gable Carl W.
Published: February 2004

The discrete nature of volcanic hot spot chains, whose origin is believed to be related to mantle plumes, is suggestive that the plumes feeding them may be pulsating. A possible origin of the pulsations is the interaction of mantle plumes with a rheological interface in the transition zone. We have studied the time‐dependent, three‐dimensional interaction of an upwelling mantle plume with a rheological interface that separates a Newtonian lower mantle from a Newtonian upper mantle with a lower viscosity. Previous two‐dimensional (2‐D) work demonstrates that when a hot plume enters a non‐Newtonian layer with lower viscosity, the hot material is carried from the interface in pulsating, diapiric events. A purely Newtonian analog to this model mimics this behavior in a qualitative manner if the viscosity contrast across the interface is larger than 2 orders of magnitude. In this work, we have used this Newtonian model to make three‐dimensional (3‐D) modeling feasible. A combined spectral/finite difference code has been employed to solve the 2‐D and 3‐D convection equations in Cartesian coordinates. We have varied the Rayleigh number (scaled to the constant viscosity of the lower mantle) between 104 and 105 and the viscosity contrast of the layers between 10 and 1000. In 2‐D situations with low viscosity contrasts, a plume thins in a steady state fashion as it enters the upper layer. At moderate and higher viscosity contrasts (above 50), the plume shows a pulsating behavior. This transition from “thinning” to “pulsating” is found to be strongly dependent on viscosity contrast and only moderately dependent on Rayleigh number. The frequency of the pulsations is linearly dependent on Rayleigh number and viscosity contrast. In three dimensions, the plume thins less and is therefore less susceptible to disruption by shear and the plume moves through the upper layer in a steady state fashion. This disappearance of pulsating behavior in three dimensions indicates a case where conclusions about plume dynamics in two dimensions do not apply to 3‐D situations.
Interseismic deformation along the San Andreas Fault in southern California
Authors: Savage J. C. , Lisowski M.
Published: February 2004

Eight trilateration networks located along the San Andreas fault in southern California have been surveyed 8 to 19 times within a 14‐ to 17‐year interval between 1971 and 1992. The data, measurements of distances between the same 10 to 32 pairs of geodetic monuments within a network in each of the surveys, have been corrected for coseismic offsets from nearby earthquakes calculated from dislocation models. The corrected data, a total of 2027 measurements, are displayed in the form of plots of measured distance versus time for each of the 167 lines measured. The hypothesis that the interseismic deformation is steady is tested by examining whether deviations from linear fits to the data in the plots are within the range expected for observational error. A significant deviation from steady deformation is found only for the network located neax Palmdale, California. In that network, many of the measurements made in the survey of early 1982 deviate from the trend defined by the measurements in other surveys. The deviations are not of the form (fixed proportional error) one would generally expect from systematic survey error. If the survey of early 1982 is excluded, the remaining data at Palmdale are consistent with steady deformation. Thus the apparent strain event observed near Palmdale in 1982 was transient.
Variable viscosity convection and partial melting in the continental asthenosphere
Authors: Schmeling Harro , Bussod Gilles Y.
Published: February 2004

Convection beneath continents may be associated with the generation of partially molten zones at depth. Experimentally determined non‐Newtonian, temperature‐ and pressure‐dependent creep laws for olivine were used to model upper mantle convection in the presence and absence of a melt phase. In our models, convection is chaotic and exhibits a strong time dependence. Using model “dry” and “water‐undersaturated” solidus relations, partially molten regions develop at asthenospheric depths forming low‐viscosity regions. The existence of these regions is linked to convection and consequently varies in space and time with an episodicity of 5 to 10 m.y., in good agreement with observations based on seismic tomography and the episodicity and volumetric abundance of surface volcanics. For an average continental heat flux of about 53.3 mW/m2, maximum melt fractions range between 0 and 2% (dry solidus) and 2 and 4% (water‐undersaturated solidus). On the basis of experimental deformation studies, we employed an empirical relationship between stress and grain size to assess the grain size variations possible in the upper mantle. The grain sizes obtained (millimeters to centimeters) are in good agreement with grain sizes observed in natural examples. The smallest grain sizes occur along narrow zones of high stress within the deepest part of the lithosphere. The largest grain sizes are found in the overlying lithosphere and in partially molten asthenospheric regions. As our models scale well to natural observations, these results appear to preclude diffusion creep (n = 1) as a viable upper mantle steady state deformation mechanism.
Principal component analysis of interseismic deformation in southern California
Authors: Savage J. C.
Published: February 2004

Eight trilateration networks along the San Andreas fault in southern California have been surveyed repeatedly (8 to 19 times) within an approximately 15‐year interval between 1971 and 1992. The data for each network were analyzed by principal component analysis to represent the observed changes in line length as a superposition of individual modes: Lij ‐ Li = ∑k Aik Ck (tj), where Lij is the length of the ith line measured at the time tj, Li is the average length of the ith line, and AikCk(tj) is the kth mode. Each mode is described by a common time function Ck(tj) that is evaluated only at the times tj of the surveys, and the participation of each line in that mode is described by a constant amplitude Aik for that line. For all networks the deformation is largely described by the first mode, and the first‐mode time function is essentially linear in time. That is, the deformation in each of the networks appears to be steady. Two anomalies in the accumulation of deformation were observed: In the network near Palmdale a transient strain event occurred in the second mode in early 1982, and in the network within Cajon Pass, fluctuations in the accumulation of deformation somewhat greater than would be attributed to measurement error were found. Although both anomalies are formally significant given the estimated measurement error, I am not confident that either anomaly is real. The principal conclusion is that the accumulation of interseismic deformation in the southern California trilateration networks is steady within the precision of measurement.
Changing techniques and paradigms for the evaluation of magmatic processes
Authors: Bergantz George W.
Published: February 2004

The study of magmatic processes is at a juncture. New continuum models for the complex physics of crystallization and melting with convection and crystal‐melt segregation are now available, and the models are amenable to exploration by numerical methods. These models, which were largely developed in the fields of material science and mechanical engineering, will allow petrologists to consider a wide range of transport phenomena: crystal settling with reaction as fully multiphase flow, combined thermal and compositional buoyancy with realistic material property variations, open system and eruptive behavior and the appropriate system geometry and boundary conditions. The traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal‐melt mush, subject to reintrusion or mobilization. Percolative flow and reaction in this mush, including contaminants, may be as important as near‐liquidus processes in driving petrological diversity. Thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes. Melting of the crust following intrusion by basalt can be considered in light of two end‐members: the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes. Although more commonly cartooned, the large sill type may not be nearly as efficient as repeated diking in driving crustal melting, magma mixing, and mingling. The greatest challenge for those studying magma dynamics will be to key the models to well‐constrained geological examples and to raise the sophistication of the content of these models in light of well‐defined tests that have geological significance.
Crustal shear wave anisotropy in southern Hawaii: Spatial and temporal analysis
Authors: Munson Clifford G. , Thurber Clifford H. , Li Yingping , Okubo Paul G.
Published: February 2004

Split shear wave arrivals are analyzed in seismograms from local earthquakes in southern Hawaii recorded at five temporary arrays and one permanent network station. We identify split shear wave arrivals by their orthogonally polarized pulses, linear particle motions, and similar waveforms and estimate the delay time for the slow shear wave arrival (S2) using a waveform cross‐correlation method. Consistent leading shear wave polarizations were measured at the majority of our stations. Comparison of observed and predicted shear wave polarizations confirms that the former are due to anisotropy rather than earthquake source mechanism. Agreement between fast shear wave (S1) polarizations and independently estimated directions of the maximum horizontal compressive stress (σH) for the Ainapo and Punaluu Gulch arrays leads us to conclude that the predominant source of the observed anisotropy for these two areas is stress‐aligned cracks consistent with the extensive dilatancy anisotropy (EDA) hypothesis. Two distinct S1 polarization directions were observed over distances less than 1 km for the Bird Park and South Flank arrays. S1 polarizations parallel to the NE striking Kaoiki Pali fault system for the Bird Park array combined with a nonhorizontal maximum principal stress (σ1) for the South Flank region suggest stress‐induced cracks aligned by nearby faulting as a source for the observed anisotropy. Large station‐to‐station variations in S1 polarization and the relationship between delay time and event depth for arrays in the Kaoiki and South Flank regions provide evidence for anisotropy that is predominantly shallow rather than pervasive. Average delay times for the five arrays vary from about 100 to 230 ms, with standard deviations of the order of 30 ms. Estimated anisotropic velocity variations and crack densities exceeding 10% indicate that the upper crust of southern Hawaii is highly fractured. A search for possible temporal changes in delay time associated with the 1983 Kaoiki main shock (ML = 6.6), at a station near the epicenter, finds no evidence for change.
Seismotectonic constraints on the convergence rate between the Rivera and North American plates
Authors: Kostoglodov Vladimir , Bandy William
Published: February 2004

There are two significantly different types of models for the convergence rate between the Rivera and North American plates. The first type, the high‐rate model (Bandy, 1992), predicts convergence rates of approximately 5.0 cm/yr near the southern end of the Rivera‐North America subduction zone and between 2.0 and 3.0 cm/yr at its northern end. In contrast, the second type, the low‐rate model (e.g., DeMets and Stein, 1990), predicts convergence rates of between 2.0 and 3.3 cm/yr near the southern end of the Rivera‐North America subduction zone and between 0.6 and 1.7 cm/yr at its northern end. Seismotectonic relationships, which relate seismic characteristics of subduction zones (maximum magnitudes, maximum seismic depths, etc.) to plate tectonic parameters (convergence rates, age of the oceanic lithosphere, etc.) provide a means of distinguishing between the two different models. Three such relationships suggest that the Rivera‐North American and Cocos‐North American convergence rates should be roughly equal across the Rivera‐Cocos plate boundary, favoring the high‐rate model. Employing the high‐rate model, one can evaluate the magnitude and distribution of the strike‐slip component of forearc motion,Vss, produced by oblique convergence between the Rivera and North American plates. The analysis indicates both a progressive increase and clockwise reorientation ofVssnorthwestward along the plate contact zone of the Rivera‐North America subduction zone. Such a distribution inVssshould produce a northwestward movement of and NW‐SE oriented extension within the interior of the Jalisco Block, consistent with previous proposals of Jalisco Block motions. Also, such a distribution in Vss should produce a slight clockwise rotation of the Jalisco Block in the vicinity of Bahia de Banderas, consistent with paleomagnetic data.
H2O transport and release in subduction zones: Experimental constraints on basaltic and andesitic systems
Authors: Poli Stefano , Schmidt Max W.
Published: February 2004

Phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2.2 to 7.7 GPa, and 550°C to 950°C, in the presence of an aqueous fluid, in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge. Water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field (>70–80 km) are lawsonite (11 wt % H2O), Mg‐chloritoid (8 wt %), talc (5 wt %), and zoisite‐clinozoisite (2 wt %) in basaltic rocks; and lawsonite, zoisite‐clinozoisite, phengite (4 wt %) and staurolite (2 wt %) in andesitic compositions. The thermal stability of lawsonite at 6.0 GPa extends to ≈800°C and 870°C in basaltic and andesitic compositions, respectively. At pressures above amphibole‐out (2.3–2.5 GPa) lawsonite reacts through continuous reactions with steep positive dP/dT slopes to zoisite‐clinozoisite (until 3.0–3.2 GPa), and at higher pressures (to more than 7.7 GPa) to assemblages containing garnet + clinopyroxene and garnet + clinopyroxene + kyanite in basaltic and andesitic compositions, respectively. On the contrary, the breakdown of zoisite‐clinozoisite is mainly pressure‐sensitive. Phengite represents the hydrous phase with the largest stability field encountered in this study. In andesite, phengite is stable to more than 7.7 GPa and more than 920°C. Talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions. A model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships. Up to 1 wt % and 2 wt % H2O in basaltic and andesitic rocks, respectively, can be stored to depths beyond 200 km in cold subduction zones, mainly by lawsonite and phengite. Dehydration rates are high until amphibole‐out, and relatively low at greater depths. The amphibole‐out reactions are found to release a significant amount of water in a depth interval of several kilometers, however, they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab. Fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle, causing the generation of amphibole or phlogopite peridotite. At higher geothermal gradients, epidote/zoisite contributes to fluid flux to the mantle wedge at 100–120 km depth. The extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth.
Fracture characteristics of the Sheeted Dike Complex, Troodos ophiolite, Cyprus: Implications for permeability of oceanic crust
Authors: van Everdingen David A.
Published: February 2004

Fracture characteristics measured in the Sheeted Dike Complex of the Troodos ophiolite, Cyprus, indicate a decrease in fracture aperture and trace length with depth. Fractures, generally of a contraction‐on‐cooling origin, tend to be oriented either parallel or perpendicular to the dike margins. High‐temperature hydrothermal epidote and quartz fracture fillings are overprinted by late, low‐temperature calcite fracture fillings. Paleopermeabilities estimated on the basis of these data using three methods of permeability determination (parallel plate, matrix addition, and a stochastic approach) indicate higher on‐axis and slightly lower, late, possibly off‐axis, permeabilities than those measured in present‐day oceanic crust.
Postintrusive thermal evolution of continental crust: A moving boundary approach
Authors: Manglik A. , Singh R. N.
Published: February 2004

Intrusion of magmatic bodies into continental crust has been extensively used to explain high heat flow, anomalous seismic velocity structure, and gravity anomalies. These intrusions significantly control the crustal deformation by modifying the heat and mass budget of the crust. However, thermal modeling results based on simple cooling of intrusives ignore the effect of moving boundaries during magma solidification. In the present work we discuss a mathematical formulation of the thermal evolution of continental crust in presence of such magma solidification process. The solution is obtained by using the Fourier spectral approach in the space domain and a modified finite difference scheme in time domain. This solution provides the cooling history and thermal evolution within an intrusion unlike the solutions proposed earlier to adjust the effect of latent heat of intrusion in the form of apparent specific heat or apparent temperature of intrusion and is a general one in the sense that this includes the radiogenic heat sources distribution and an initial depth dependent temperature distribution in lithosphere. We apply this solution to study the thermo‐rheological evolution of continental lithosphere after magmatic underplating. We conclude that this effect keeps the intrusive body and the surrounding rocks at a higher temperature for a longer duration than when the effect is ignored or is adjusted in the form of increased specific heat. For an initially ductile lower crust and brittle upper mantle, magmatic underplating causes a change in the mode of deformation from brittle to ductile in the mantle layer and the latent heat effect included in the form of moving boundaries prolongs the duration of ductile deformation in this layer in comparison to the latent heat effect adjusted by increasing the specific heat of intrusion. The prolonged duration of ductility in lower crust/upper mantle due to the slow cooling of intrusion results in a weak lithosphere and is important in addressing the issues concerning crustal dynamics.
An investigation of a correlation/covariance method of signal extraction
Authors: Langel Robert A.
Published: February 2004

A harmonic correlation technique has been used effectively to isolate a primary signal from two independent data sets each of which is contaminated by the presence of interfering signals. Each of the two data sets is decomposed by a harmonic analysis, and then the correlation coefficients and amplitude ratios between the two decompositions are computed for each harmonic. Only harmonics which satisfy predetermined selection criteria are retained. Those criteria are in the form of requiring the correlation coefficient to exceed some magnitude, say, ρ0, and the amplitude ratio not to exceed some value, say, R0. The resulting pair of retained signals are then typically highly correlated. It is suggested that appropriate values of ρ0 lie between 0.25 and 0.6 and of R0 between 3.0 and 6.0, depending on the relative amplitudes of the primary and interfering signals. Critical examination of the technique shows that under many conditions it can be an effective tool for eliminating signals which are not a good approximation to the primary signal. This conclusion is confirmed by simulated examples of satellite magnetic anomaly data with interfering fields from ionospheric currents.
Long‐term earthquake clustering: A 50,000‐year paleoseismic record in the Dead Sea Graben
Authors: Marco Shmuel , Stein Mordechai , Agnon Amotz , Ron Hagai
Published: February 2004

The temporal distribution of earthquakes in the Dead Sea Graben is studied through a 50,000‐year paleoseismic record recovered in laminated sediments of the Late Pleistocene Lake Lisan (paleo‐Dead Sea). The Lisan represents more than 10 times the 4000 years of historical earthquake records. It is the longest and most complete paleoseismic record along the Dead Sea Transform and possibly the longest continuous record on Earth. It includes unique exposures of seismite beds (earthquake‐induced structures) associated with slip events on syndepositional faults. The seismites are layers consisting of mixtures of fragmented and pulverized laminae. The places where the seismites abut syndepositional faults are interpreted as evidence for their formation at the sediment‐water interface during slip events on these faults. Thicker sediment accumulation above the seismites in the downthrown blocks indicates that a seismite formed at the water‐sediment interface on both sides of the fault scarps. Modern analogs and the association with surface ruptures suggest that each seismite formed during anML≥ 5.5 earthquake. The230Th‐234U ages of a columnar section, obtained by thermal ionization mass spectrometry, give a mean recurrence time of ∼1600 years ofML≥ 5.5 earthquakes in the Dead Sea Graben. The earthquakes cluster in ∼10,000‐year periods separated by quiet periods of similar length. This distribution implies that a long‐term behavior of the Dead Sea Transform should be represented by a mean recurrence of at least 20,000 year record. This observation has ramifications for seismic hazard assessment based on shorter records.
An Early Cretaceous paleomagnetic pole from Ponta Grossa dikes (Brazil): Implications for the South American Mesozoic apparent polar wander path
Authors: Raposo Maria Irene Bartolomeu , Ernesto Marcia
Published: February 2004

Paleomagnetic results from the Early Cretaceous Ponta Grossa dike swarm are reported. These dikes crosscut sediments and basement rocks associated with the Ponta Grossa Arch, a tectonic feature in the eastern border of the Paleozpic‐Mesozoic Parana Basin. These dikes are vertical or subvertical and trend mainly NW and NE. Samples from 127 dikes were submitted to both alternating field and thermal demagnetizations; 121 of them yielded characteristic directions interpreted as original thermal remanent magnetizations. Both normal and reversed polarities as well as intermediate directions (6 dikes) are recorded. The paleomagnetic pole is located at 30.3°E, 82.4°S (N = 115; α95 = 2.0°; k = 43.8). This pole along with the available paleomagnetic poles for the Paraná Basin volcanics (Serra Geral Formation), recalculated in this paper in the light of reported 40Ar/39Ar ages describe an apparent polar wander path segment corresponding to a clockwise rotation (∼6.9°) of the South American plate. This movement is compatible with the initial phase of the South Atlantic opening with rifting progressing from south tp north. However, the calculated plate velocity is higher than the predicted velocities reported so far based on oceanic floor magnetic anomalies. The new data reported in this paper allowed the calculation of a mean Early Cretaceous pole (60.3°E, 83.8°S; N = 7, α95 = 2.9°) which can be discriminated from the Middle‐Late Jurassic (191.3°E, 86.2°S; N = 3, α95 = 7.8°) and Late Cretaceous (346.5°E, 84.1°S; N = 5, α95 = 4.9°) mean poles for South America, indicating a more complex movement of the continent than the simple east‐west drift generally proposed.
Deep penetration seismic soundings across the northern margin of the South China Sea
Authors: Nissen Susan Spangler , Hayes Dennis E. , Buhl Peter , Diebold John , Bochu Yao , Zeng Weijun , Chen Yongqin
Published: February 2004

Twenty reversed, two‐ship expanding spread profiles (ESPs) with maximum source‐receiver offsets of ∼100 km were collected in three transects across the rifted northern margin of the South China Sea. Source‐receiver offset versus two‐way travel time (X‐T) data were mapped into the intercept time versus ray parameter (τ‐p) domain, and velocity‐depth solutions were obtained by a combination of τ‐sum inversion in the τ‐p domain and ray tracing in both the τ‐p and X‐T domains. Arrivals from the Moho were detected on 17 of the ESPs. The depths to Moho determined for individual ESP interpretations have reproducibilities of ±0.1 km to ±3 km; in most cases the Moho depth has been determined to within ±1.5 km. Moho depths determined in this investigation represent a significant improvement over previous estimates of Moho along the margin from gravity data. Variations in present‐day crustal thickness (measured from top of prerift basement to Moho) are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent South China Sea Basin. The ESP interpretations reveal that across the eastern portion of the south China margin, the crust appears to thin more or less continuously toward the continent‐ocean boundary. In the west, ESP interpretations also show a general trend of seaward crustal thinning but, in addition, indicate at least two instances of focused, localized crustal thinning. Crustal velocities and the relative proportion of upper crust (VP< 6.4 km/s) and lower crust (VP> 6.4 km/s) are used to identify areas of the south China margin with similar and contrasting crustal structures. Variations in these properties are believed to result primarily from contrasting, prerift crustal structure across the margin. However, magmatic underplating during rifting, depth dependent extension, and Pleistocene igneous intrusions may also have contributed to the variations in present crustal structure. Reliable information about variations in crustal thickness and velocity structure across and along the south China margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin.
Second term of the ray series and validity of the ray theory
Authors: Popov Mikhail M. , Camerlynck Christian
Published: February 2004

We suggest a criterion to examine the range of validity of the ray method, based on the theory of asymptotic series. This criterion examines the ratio of the second term of the ray series over the first one. For that purpose, we develop an algorithm for the computation of the second term of the ray series, together with the resolution of a system of differential equations for auxiliary functions, the determination of initial data near the source, and boundary conditions on interfaces. We apply this criterion to two‐dimensional gradient velocity models without an interface and with interfaces. Serious limitations for the use of ray methods arise from our results in at least three situations: (1) when the distribution of energy along the initial wave front is not uniform; (2) when the incident angle is relatively large (near grazing incidence); and (3) after a large number of reflections in a waveguide, with increasing distance from the source.
Effect of slab rheology on mass transport across a phase transition boundary
Authors: King Scott D. , Ita Joel
Published: February 2004

In this study we explore the impact of a strongly temperature‐dependent viscosity on convective flow in the presense of an endothermic phase transition. Temperature‐dependent viscosity strengthens the upper thermal boundary layer and its associated downwellings (slabs). Three temperature‐dependent rheologies ranging from weakly to strongly temperature‐dependent, as well as a constant viscosity are considered in a two‐dimensional Cartesian model. All other properties of the flow and phase transition are held constant. The slab is located in the center of the computational grid to avoid the direct influence of a sidewall on the downgoing flow. We find that when holding the volume averaged Rayleigh number constant, the strength of the downwelling does not noticeably affect its ability to penetrate the phase transition as observed by the isotherms. Stronger downwellings, however, do maintain a more coherent, narrow, slablike shape and significantly increase the vertical mass flux compared to weaker downwellings which tend to build up in a large pool of cold material above the phase transition. Compared to the strong slabs, there is widespread deformation of the cold downwelling material above the phase transition. This effect is most noticeable at large negative values of the Clapeyron slope, when the constant viscosity models are strongly layered. In contrast, we find that whether the slab descends along the side of the box or in the middle of the box has a significant influence on the ability of the slab to penetrate the phase boundary as observed by the isotherms, but the variation in the mass flux diagnostic is minor. We conclude that neither slab penetration nor mass flux diagnostics is a sufficient indicator of long‐term mixing of material between the upper and lower mantle.
Open‐system melting in the upper mantle: Constraints from the Hayachine‐Miyamori ophiolite, northeastern Japan
Authors: Ozawa Kazuhito , Shimizu Nobumichi
Published: February 2004

The Hayachine and Miyamori hydrous ultramafic complexes, northeastern Japan, exhibit not only diverse lithological variations from very primordial spinel lherzolites (olivine forsterite mol % = 89 (Fo89), spinel 100Cr/(Cr+Al) (Cr #) =10) to refractory harzburgites (Fo93, spinel Cr #=70) but also show diverse trace element characteristics in amphiboles and clinopyroxenes correlated with the major element behavior. Some of the most primitive lherzolites are from the Hayachine complex and contain a clinopyroxene with strongly light rare earth element (REE) depleted abundances. The slightly refractory aluminous spinel peridotites of the Miyamori complex forming kilometric blocks in chromian spinel peridotites, exhibit flat to weakly V‐shaped REE patterns with lower heavy REE (HREE) abundances. The chromite‐bearing peridotites of the Miyamori complex exhibit light REE (LREE) enriched patterns with further lower HREE abundances than the aluminous spinel peridotites. The most refractory clinopyroxene‐free harzburgite (Fo93) contains amphibole with the highest LREE/HREE ratio among the Miyamori‐Hayachine peridotites. The observed covariance between major and trace elements together with geological and petrological data for the complex is reproduced with an open‐system melting model, in which continuous supply of LREE‐enriched melt, melting, and melt segregation are assumed to be coupled. Nonmodal melting with specified amounts of trapped melt is also taken into consideration. The result shows that REE and trace element data for less refractory peridotites can be reproduced fairly well by the model. Accounting for the REE patterns of more refractory peridotites, however, proved less tractable, indicative of the importance of a melting and melt separation process involving melt migration and reactions during the decompression of the mantle material.