bluuebunny/crossref_metadata_2025 huggingface dataset:
A regionalized upper mantle (RUM) seismic model
Authors: Gudmundsson Ólafur , Sambridge Malcolm
Published: February 2004

Seismic velocity heterogeneity in the Earth's mantle is strongly concentrated near its top. The shallow heterogeneity of the mantle correlates strongly with surface tectonics. We use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle (RUM) reference model. We use a select set of teleseismic travel times to minimize the mapping of mislocation into structure. The data selection emphasizes the robustness of individual picks. The form of the RUM model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of Earth. Together the profiles constitute a three‐dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom. This is achieved by irregularly sampling a detailed regionalization of the globe, by detailed mapping of subducted lithosphere in the mantle as defined by seismicity, and by combining these structures in an irregular grid in which bookkeeping is efficiently handled. The resulting RUM model includes subducting slabs as sharp fast features in the upper mantle. Old continents are fast; young oceans are slow. Models have been derived for both compressional and shear velocity. The RUM model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model. It can be useful as a reference model for individual tectonic regions. Travel times are efficiently generated for the RUM model. Mislocations of explosions of known location are significantly reduced when corrections for the RUM model are applied to travel time residuals for a spherically symmetrical Earth model.
Inversion of gravity data to determine the terrain density distribution in southwest Japan
Authors: Nawa Kazunari , Fukao Yoshio , Shichi Ryuichi , Murata Yasuaki
Published: February 2004

We propose a new method of inverting gravity data for the lateral density variation of the terrain above sea level. The density variation is modeled by a piecewise step function. We impose an a priori condition that the Bouguer anomaly distribution expressed by a third‐order spline function is smooth relative to the topographic distribution. No a priori condition is imposed on the density distribution. The Akaike's Bayesian Information Criterion is used to obtain optimum trade‐off parameters controlling the smoothness of the Bouguer anomaly surface against its fitness to the observed anomaly values. We apply the method to about 45,000 gravity observations in southwest Japan. The calculated density distribution, in general, correlates well with major geologic features: the density of sedimentary, volcanic, or granitic units increases as their age increases in a way consistent with laboratory measurements of rock density and age. In some regions, however, where the geologic features described in the literature are likely to be surficial and to be underlain by other geologic units, the estimated densities deviate from this general trend. Some active faults sharply offset topography and geology, resulting generally in a step in the Bouguer anomaly. In such cases the estimated densities tend to be unreasonably large or small. Synthetic tests indicate that a larger number of data points and spline knots are required near the offset to eliminate this problem.
Displacement field for an edge dislocation in a layered half‐space
Authors: Savage J. C.
Published: February 2004

The displacement field for an edge dislocation in an Earth model consisting of a layer welded to a half‐space of different material is found in the form of a Fourier integral following the method given by Weeks et al. [1968]. There are four elementary solutions to be considered: the dislocation is either in the half‐space or the layer and the Burgers vector is either parallel or perpendicular to the layer. A general two‐dimensional solution for a dip‐slip faulting or dike injection (arbitrary dip) can be constructed from a superposition of these elementary solutions. Surface deformations have been calculated for an edge dislocation located at the interface with Burgers vector inclined 0°, 30°, 60°, and 90° to the interface for the case where the rigidity of the layer is half of that of the half‐space and the Poisson ratios are the same. Those displacement fields have been compared to the displacement fields generated by similarly situated edge dislocations in a uniform half‐space. The surface displacement field produced by the edge dislocation in the layered half‐space is very similar to that produced by an edge dislocation at a different depth in a uniform half‐space. In general, a low‐modulus (high‐modulus) layer causes the half‐space equivalent dislocation to appear shallower (deeper) than the actual dislocation in the layered half‐space.
Detection of lower mantle scatterers northeast of the Marianna subduction zone using short‐period array data
Authors: Kaneshima Satoshi , Helffrich George
Published: February 2004

Short period University of Washington, northern California, and southern California seismic array records of a sequence of deep Mariana subduction zone earthquakes show unexpected clear arrivals nearly 90 and 105 s following P. We measure slowness, arrival time, and arrival azimuth of these phases relative to P using array techniques to find where they emanate in the mantle. By combining slowness and back azimuth deviations and the move‐out with depth of the arrivals from different earthquakes, we find their most probable origin is through S‐to‐P conversion at point scatterers in the mid‐ to lower mantle at 25.7°N 148.2°E 1590 km and 25.5°N 151.0°E 1850 km respectively, separated by ∼300 km. These features indicate elastically distinct material in the middle of the lower mantle, which, if fragments of subducted slabs, are not related to contemporary subduction. They may represent upwelling convective material from the deep mantle or D″, but they are not purely thermal in nature. As chemically distinct bodies, they may represent potential reservoirs of the isotopic and trace element components recognized geochemically.
Geochronology and geochemistry of pre‐Jurassic superterranes in Marie Byrd Land, Antarctica
Authors: Pankhurst R. J. , Weaver S. D. , Bradshaw J. D. , Storey B. C. , Ireland T. R.
Published: February 2004

Marie Byrd Land, Antarctica, is a major part of the proto‐Pacific supercontinental margin. On the basis of new geochronological and geochemical data relating to its pre‐Jurassic evolution, Marie Byrd Land is subdivided into western or interior (“Ross”) and eastern or exterior (“Amundsen”) provinces, equivalent to two superterranes in New Zealand. The Ross province is characterized by Cambrian? metagraywackes and I‐type orthogneiss dated at 505±5 Ma by U‐Pb SHRIMP (Sensitive High Resolution Ion Microprobe). Its magmatic record consists of Devonian‐Carboniferous (375±5 Ma and circa 339±6 Ma), predominantly I‐type granitoids, and further minor granitic magmatism in Permo‐Triassic times. This Paleozoic history is comparable to that of the Gondwana margin in northern Victoria Land, western New Zealand, and SE Australia. The Amundsen province has no observed Paleozoic graywacke succession; evidence from Rb‐Sr and U‐Pb SHRIMP dating supports calc‐alkaline granitoid events in Ordovician/Silurian (450–420 Ma) and Permian (276±2 Ma) times. The latter may be the previously unknown source of Permian volcanic detritus in the Ellsworth and Transantarctic mountains. The Amundsen province is considered to be the equivalent of the Median Tectonic Zone of New Zealand, and arc magmatism of comparable ages is found in the Antarctic Peninsula and Thurston Island. The underlying lithosphere of the two provinces may be distinguished by Nd isotope data; granitoids and metasedimentary rocks of the Ross province have Meso‐Proterozoic Nd model ages, generally 1300–1500 Ma, compared to 1000–1300 Ma for the Amundsen province. On the basis of published palaeomagnetic data, the two provinces amalgamated to form Marie Byrd Land in mid‐Cretaceous times, only shortly before rifting of the New Zealand continental block away from Antarctica.
High‐temperature deformation of dry diabase with application to tectonics on Venus
Authors: Mackwell S. J. , Zimmerman M. E. , Kohlstedt D. L.
Published: February 2004

We have performed an experimental study to quantify the high‐temperature creep behavior of natural diabase rocks under dry deformation conditions. Samples of both Maryland diabase and Columbia diabase were investigated to measure the effects of temperature, oxygen fugacity, and plagioclase‐to‐pyroxene ratio on creep strength. Flow laws determined for creep of these diabases were characterized by an activation energy of Q = 485±30 kJ/mol and a stress exponent of n = 4.7±0.6, indicative of deformation dominated by dislocation creep processes. Although n and Q are the same for the two rocks within experimental error, the Maryland diabase, which has the lower plagioclase content, is significantly stronger than the Columbia diabase. Thus the modal abundance of the various minerals plays an important role in defining rock strength. Within the sample‐to‐sample variation, no clear influence of oxygen fugacity on creep strength could be discerned for either rock. The dry creep strengths of both rocks are significantly greater than values previously measured on diabase under “as‐received” or wet conditions [Shelton and Tullis, 1981; Caristan, 1982]. Application of these results to the present conditions in the lithosphere on Venus predicts a high viscosity crust with strong dynamic coupling between mantle convection and crustal deformation, consistent with measurements of topography and gravity for that planet.
Extremely thin oceanic crust in the Proto‐Indian Ocean: Evidence from the Masirah Ophiolite, Sultanate of Oman
Authors: Peters Tjerk , Mercolli Ivan
Published: February 2004

The Masirah Ophiolite is a good example of thin oceanic crust. Below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1–1.5 km, the gabbroic lower crust barely exceeds 500 m in thickness. In spite of this reduced thickness, the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence. The crust was formed during the uppermost Jurassic (circa 150 Ma) when the Indian‐Madagascar plate separated from the African‐Arabian plate and is therefore related to the opening of the coeval Somali basin. Geological relationships indicate that this portion of oceanic crust was formed at a ridge‐transform intersect. The peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment, rather than the consequence of a tectonic thinning. The cooling effect due to the vicinity of two large continental lithospheric blocks (Indian‐Madagascar and African‐Arabian plates) during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness.
Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Ophiolite, Cyprus
Authors: Varga Robert J. , Gee Jeffrey S. , Staudigel Hubert , Tauxe Lisa
Published: February 2004

Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous‐age Troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge‐and‐groove structures termed hot slickenlines found throughout the complex. A unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin. Significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back‐flow of magma during the waning stages of intrusion. Surface lineations generally lie subparallel to the direction of flow inferred from AMS determinations on cores within 5 cm of dike margins. Surface lineations also lie subparallel to the long axis (ε1) of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from AMS cores. Correlation of surface lineations with interior indicators of flow (AMS, plagioclase trachytic texture) indicate that the surface features are good proxies for grain‐scale magma flow directions during dike propagation in Troodos dikes. Orientations of surface flow features in the dikes of the Troodos ophiolite indicate an approximately equal mix of subhorizontal to near‐vertical magma flow, contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers. Our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest.
Mineral abundance determination: Quantitative deconvolution of thermal emission spectra
Authors: Ramsey Michael S. , Christensen Philip R.
Published: February 2004

A linear retrieval (spectral deconvolution) algorithm is developed and applied to high‐resolution laboratory infrared spectra of particulate mixtures and their end‐members. The purpose is to place constraints on, and test the viability of, linear spectral deconvolution of high‐resolution emission spectra. The effects of addition of noise, data reproducibility, particle size variation, an increasing number of minerals in the mixtures, and blind end‐member input are also examined. Thermal emission spectra of 70 mineral mixtures ranging from 2 to 15 end‐members and having particle diameters of 250–500 μm were obtained. Deconvolution results show that the assumption of linear mixing is valid and enables mineral percentage prediction to within 5% on average with residual errors of less than 0.1% total emissivity. One suite (21 distinct mixtures), varying from <10 μm to 500 μm, was also prepared to test the limits of the model at decreasing particle sizes. Incoherent volume scattering at grain diameters less than several times the wavelength (∼60 μm) produces significant changes in spectral band morphology and hence, an increase in the root‐mean‐squared (RMS) error of the model Because of this, it appears that spectral mixing remains essentially linear to ∼60 μm (using the 250–500 μm size fraction as end‐members). Below this threshold, the linear retrieval algorithm fails. However, with the appropriate particle diameter end‐member spectra for the corresponding mixtures, the errors are reduced significantly and linearity continues through to the 10–20 μm size fraction. Additions of increasing amounts of noise cause a deviation of an additional 2.4%, whereas variability due to spectrometer reproducibility produces an average error of 4.0%. The model is also able to detect accurately minerals in mixtures containing 15 end‐members, well beyond the number of geological significance. Extensive error analysis and model testing confirm the appropriateness of linear deconvolution as a useful and powerful tool to examine complexly mixed emission spectra in the laboratory and the field. The results of this study provide a foundation for remote sensing analyses of thermal infrared data from current airborne and future satellite instruments planned for Earth and Mars.
Paleomagnetism, paleogeographic origins, and uplift history of the Coast Range ophiolite at Mount Diablo, California
Authors: Hagstrum Jonathan T. , Jones David L.
Published: February 2004

Divergent paleogeographic origins have been proposed for the Coast Range ophiolite of western California which are testable using paleomagnetic methods. Paleomagnetic data for Middle Jurassic pillow lavas and diabase sills of the Coast Range ophiolite at Mount Diablo, northern California, indicate that they contain two components of remanent magnetization. The characteristic directions have normal and reversed polarities and apparently are carried by Ti‐poor magnetite. This magnetization is inferred to have been acquired during emplacement and seafloor alteration at an ancient spreading ridge. The paleolatitude calculated from its structurally corrected mean direction is 20°N ± 9° and agrees with the expected direction for stable North America; this result is also consistent with the concordant paleolatitude (32°N ± 8°) recently determined for Upper Jurassic Coast Range ophiolite at Stanley Mountain in southern California. In addition, clockwise vertical axis rotation of Mount Diablo (143° ± 11°) is indicated by the characteristic magnetization direction. An overprint component is inferred to have been acquired during uplift of Mount Diablo since the Miocene.
Gravitational coupling in a triaxial ellipsoidal Earth
Authors: Szeto A. M. K. , Xu S.
Published: February 2004

Gravitational coupling within an axially symmetric, ellipsoidal Earth gives rise to torques among the mantle, outer core, and inner core. Such torques act along “lines of nodes”, i.e., orthogonal to axes of symmetry. In a triaxial Earth model, small deviations from axial symmetry modify the torques in direction as well as magnitude. The perturbative torques depend on three Euler angles specifying the long axis of the inner core relative to that of the mantle shell. Their magnitudes, excluding angular dependences, are ∼400 times (i.e., of the order of the inner core boundary flattening) smaller than their biaxial analogs. While triaxiality has a negligible impact on coupling considerations along the line of nodes, it creates gravitational coupling along orthogonal directions that have not been previously investigated. The gravitational coupling torques on the inner core and the mantle that take these triaxial perturbations into account are summarized. To the extent that fluid pressure and electromagnetic effects have not been explicitly modeled here, the present calculations are intended to provide only an order of magnitude estimate of coupling within the Earth along directions not previously examined. It is found that the axial torque on the inner core produces a differential rotation of less than 0.02°/yr and thus fails by at least one order of magnitude in supplying a mechanism for inner core differential rotation suggested by seismological evidence [Song and Richards, 1996]. This conclusion breaks down only if one is prepared to accept an inner core obliquity surpassing 10°.
Synthetic rough fractures in rocks
Authors: Glover P. W. J. , Matsuki K. , Hikima R. , Hayashi K.
Published: February 2004

Natural fractures serve a very important function in the transport of fluid through rocks, as well as in the flow of electrical charge and heat. The creation of numerically synthesized fractures can aid the study of the physical processes connected with fractures. Synthetic fracture is the term used to describe fractures that are created numerically in such a way that they share the same mean geometrical characteristics as specific natural fractures measured by profiling, by a process known as tuning. We have modified methods for producing synthetic rough surfaces whose geometric properties are tuned to mimic natural fracture surfaces in rocks in order to create synthetic fractures that are statistically identical to those found in rocks. One important such modification has been the incorporation of a method that allows the surfaces to be matched at long wavelengths and unmatched at short wavelengths, with the degree of matching varying smoothly in between, as it does for real fractures. We have compared numerically synthetic fractures created using the new method with fractures created using an existing technique that uses a mismatch wavelength as a sudden discontinuity between matched and unmatched behavior, as well as data from fractures in real rocks. This comparison has shown that the new technique provides much more realistic numerically synthesized fractures than previous methods. Synthetic fractures created with the new method have been used in normal closure and fluid flow modeling, and the results are reported in a companion paper.
Morphology, eruption rates, and rheology of lava domes: Insights from laboratory models
Authors: Fink Jonathan H. , Griffiths Ross W.
Published: February 2004

The growth of lava domes can be either quiescent or violent, with transitions between styles of behavior commonly occurring with little warning. Here we propose that the behavior depends on the eruption rate, the magma rheology, and the thickness of the cooling surface. We present a model, based on laboratory simulations, field measurements, and photographic analysis, that relates the morphology and texture of a dome to the thickness of its cooled carapace, and thence to eruption conditions. A sequence of four main types of dome (spiny, lobate, platy, and axisymmetric) is identified in laboratory analog experiments with a Bingham plastic. These regimes are associated with progressively higher effusion rates, lower cooling rates, lower yield strengths, and (in real lava flows) decreasing tendency for explosive decompression during flow front collapse and are ordered according to the value of a single dimensionless number. The model allows an estimate of the yield strengths of the magma forming active domes based on data for the effusion rate and composition. It also permits the eruption rates of prehistoric or extraterrestrial lava domes and flows to be appraised from their morphology, if their compositions can be estimated. A comparison with the laboratory results suggests that the Venusian “pancake domes” are likely to have basaltic to basaltic andesitic composition.
Global magnetization models with a priori information
Authors: Purucker Michael E. , Langel Robert A. , Rajaram Mita , Raymond Carol
Published: February 2004

In an effort to explore the possible effects of change in integrated magnetization at the continent‐ocean boundary and to account for such effects in modeling, an inverse technique is developed which allows for the inclusion of a priori information in models of global crustal magnetization or susceptibility. This technique accounts for processing effects such as main and external field removal. An a priori model consisting of an ocean‐continent magnetic contrast, oceanic topography, and remanent magnetization in the Cretaceous quiet zones is constructed using equivalent source dipoles. Previous investigations using similar models utilize only forward modeling procedures. We show how this a priori model can be modified so that the resulting computed field, after removal of spherical harmonics below some specified degree (“main field” removal) and along track filtering (“external field” removal), matches that of the robust POGO‐Magsat anomaly map in a least squares sense. The dependence of the final model on the a priori information is also investigated. Between degrees 20 and 60 the final models are found to be almost identical for reasonable a priori conditions. An example from the Gulf of Mexico and surrounding Gulf Coast region serves to illustrate the utility of the technique. High heat flow,>40° C/km, is observed in much of the Gulf Coast region. The example suggests that the elevated heat flow persists at depth and has elevated the Curie point.
Utility of elastic models in predicting fault displacement fields
Authors: Gupta Anupma , Scholz Christopher H.
Published: February 2004

Although elastic models have long been used to model earthquake deformation, their application to fault problems is questionable, as accumulated fault strain is higher and the relevant timescales are longer. We test the utility of using elastic models to predict fault displacement fields by independently measuring the three‐dimensional slip (offset) distribution and displacement field of small normal faults. The displacement field is constrained from the topography of the deformed bedding planes; the slip distribution is constrained from stratal offsets in multiple sections of fault‐normal saw‐cuts. Using the observed slip distribution, we calculate both one‐ and three‐dimensional elastic displacement fields. We find that the large strain associated with fault growth can be accommodated with linear elastic models. Much of the remaining misfit between the data and the model may result from elastic interaction with other nearby faults, the inelastic zone around surrounding fault tips, and prefaulting irregularities in the measured bedding plane surface.
Rate and state of background stress estimated from the aftershocks of the 1989 Loma Prieta, California, earthquake
Authors: Gross Susanna , Bürgmann Roland
Published: February 2004

Estimates of the tectonic stress state including loading rate and magnitude of background stress are derived from the spatial and temporal distribution of Loma Prieta aftershocks. This technique was previously applied to the Landers aftershock sequence [Gross and Kisslinger, 1997] and is based upon the seismicity model of Dieterich [1994]. Dieterich's theory suggests that background seismicity should be proportional to stress rate and the number of aftershocks in an area should be proportional to the stress step experienced in that area. We used two independently derived source models to compute the stress step from the mainshock and to determine how effective that stress step was in triggering aftershocks. A background stress state is then chosen which makes the stress steps at the aftershock locations most distinct from the stress steps at hypocenters of background seismicity. The best fitting background stress state has its greatest compressive stress plunging ≈ 17° to N13°E, and an intermediate stress very close in magnitude to the least principal stress. The small shear stresses at depth and low coefficient of friction suggest that high‐pressure pore fluids may be present inside active faults. The estimated stress rate of ∼ 70 Pa/d (0.25 bar/yr) is comparable to the stress rate found for the southern San Andreas Fault system. We found a best fitting “effective” coefficient of friction μ′ ≈ 0.2 for the Loma Prieta area, significantly less than μ′ ≈ 0.6 estimated for Landers in previous work. Variations in aftershock decay rate within the Loma Prieta aftershock zone are correlated with static stresses caused by postseismic slip as modeled by Bürgmann et al. [1997]. Some of the postseismic slip occurred on structures that did not slip during the mainshock, so the postseismic and coseismic stress step fields have different spatial distributions. The effectiveness of slowly accumulated postseismic static stresses in triggering aftershocks is especially interesting because dynamic stresses are insignificant in this case.
Variations of Cl, F, and S in Mount Etna's plume, Italy, between 1992 and 1995
Authors: Pennisi Maddalena , Le Cloarec Marie‐Françoise
Published: February 2004

This study investigates data on the Cl, F, and S concentrations of plume‐filter samples collected between 1992 and 1995 from Mount Etna. The samples were collected during eruptive and noneruptive activity and show fairly constant Cl/F ratios but wide variation in the Cl/S ratios. The determination of the Cl/F ratio (between 5 and 14) allows an estimate of F degassing to be derived that ranges from 10 to 30% of the initial F content of the magma. The variation in the Cl/S ratio fits an empirical two‐stage degassing model where gas with high Cl/S ratio is discharged during quiescient degassing and gas with low Cl/S ratios is discharged during eruptive degassing. The typical permanent degassing occurs from a depth at which chlorine exsolution exceeds sulfur exsolution (Cl/S > 1). The opposite of this behavior is observed during eruptive periods when significant sulfur degassing occurs because magma degasses at near atmospheric pressure (Cl/S ∼ 0.1). The Cl/S ratio in the plume, characterizing the type of degassing, has the potential to be a useful geochemical tool for monitoring the volcano.
Evaluation of thermal conductivity temperature corrections applied in terrestrial heat flow studies
Authors: Lee Youngmin , Deming David
Published: February 2004

We collated 117 sets of temperature‐dependent thermal conductivity measurements on minerals and rocks for the purpose of evaluating the comparative accuracy of different types of temperature corrections commonly applied in terrestrial heat flow studies. Data used consisted of measurements on 10 minerals, 59 igneous rocks, 19 sedimentary rocks, and 29 metamorphic rocks with a thermal conductivity range from 1.4 to 9.0 W/m‐K at room temperature (20–25°C). The data set included thermal conductivites measured at temperatures from 0 to 500°C. When accuracy was bench‐marked against the entire data set, corrections proposed by Sekiguchi [1984], Somerton [1992], Sass et al. [1992], Funnell et al. [1996], and Chapman et al. [1984] were found to have mean absolute relative errors of 6.9%, 7.7%, 8.5%, 14.2%, and 17.6%, respectively. The correction proposed by Somerton [1992] was found to have the lowest mean absolute relative error for minerals (11.3%) and sedimentary rocks (6.5%), while the correction proposed by Sekiguchi [1984] was found to have the lowest mean absolute relative error for igneous (5.4%) and metamorphic rocks (4.9%). The correction schemes were also found to differ in their systematic errors. Corrections proposed by Sekiguchi [1984], Somerton [1992], Sass et al. [1992], Funnell et al. [1996], and Chapman et al. [1984] were found to have mean relative errors of −3.3%, +2.6%, −5.8%, −11.0%, and +15.5%, respectively. Most of the temperature‐dependent thermal conductivity data used to evaluate correction schemes were measurements on air‐saturated rocks; however, the invariance of the relative rankings with respect to rock porosity suggests the rankings may be valid with respect to in situ conditions.
Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone
Authors: Baldwin Suzanne L. , Lister Gordon S.
Published: February 2004

Micas and potassium feldspars from the South Cyclades Shear Zone, Ios, Cyclades, Greece, yield varied and complex 40Ar/39Ar apparent age spectra. A correlation exists between 40Ar/39Ar apparent ages and the relative timing of different episodes of recrystallization and grain growth, as indicated by fabric and microstructural analysis. The 40Ar/39Ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth, and partial to complete resetting of argon systematics in potassium‐bearing minerals during Hercynian (M0), Alpine (M1) and late Oligocene ‐ early Miocene (M2) metamorphism. Deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals. Modeling suggests the Oligo‐Miocène thermal events were of insufficient magnitude and/or duration to completely reset the isotopic systematics in these samples. Our data lead to the concept of the argon partial retention and resetting zone (PRZ), defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium‐bearing minerals. Within the PRZ, some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained. Tectonic exhumation of the PRZ involves movement on crustal‐scale ductile shear zones, accompanied by strongly partitioned deformation and localized recrystallization. Recrystallization leads to resetting of argon systematics, and thus will result in heterogeneous 40Ar/39Ar age distributions within these ductile shear zones (e.g., in the South Cyclades Shear Zone).
Fluid permeability of deformable fracture networks
Authors: Brown Stephen R. , Bruhn Ronald L.
Published: February 2004

We consider the problem of defining the fracture permeability tensor for each grid block in a rock mass from maps of natural fractures. For this purpose we implement a statistical model of cracked rock developed by M. Oda, where the permeability tensor is related to the crack geometry via a volume average of the contribution from each crack in the population. In this model, tectonic stress is implicitly coupled to fluid flow through an assumed relationship between crack aperture and normal stress across the crack. We have included three enhancements to the basic model. (1) A realistic model of crack closure under stress has been added along with the provision to apply tectonic stresses to the fracture system in any orientation. The application of compressive stress results in fracture closure, and consequently, a reduction in permeability. (2) The fracture permeability can be linearly superimposed onto an arbitrary anisotropic matrix permeability. (3) The fracture surfaces are allowed to slide under the application of shear stress, causing fractures to dilate and result in a permeability increase. Through two examples we demonstrate that significant changes in permeability magnitudes and orientations are possible when tectonic stress is applied to fracture systems.
Geochemistry of basalts from the Hayes Transform region of the Mid‐Atlantic Ridge
Authors: Smith Susan E. , Casey John R. , Bryan Wilfred B. , Dmitriev Leonid , Silantyev Sergei , Magakyan Ruben
Published: February 2004

Fresh glassy aphyric and phyric basalts and dolerites have been recovered from the Mid‐Atlantic Ridge spreading segments adjacent to the Hayes Transform (33°40′N) and within the transform valley. Glass and whole rock compositions exhibit Mg numbers that range from 44.3 to 70.6, CaO/Al2O3 ratios of 0.57–0.87, Na2O contents of 1.84–3.49 wt %, (La/Yb)cn ratios of 0.40–1.39, 87Sr/ 86Sr ratios of 0.70270–0.70403, and 206Pb/204Pb ratios of 17.95–18.79. These basalts are some of the most primitive and most fractionated basalts from the Mid‐Atlantic Ridge. The Hayes Transform appears to mark a geochemical boundary between two major mantle provinces based on major, trace, rare earth element, and Sr‐Nd‐Pb isotopic compositions of these basaltic samples. Modeling of partial melting and fractional crystallization from minor, trace, and rare earth elements indicates that the southern Hayes basalts could have been generated by ∼20% melting of a fertile normal mid‐ocean ridge basalt (NMORB) source region, efficient pooling of melts, and low‐pressure fractionation along a magmatically robust spreading segment (HA‐1). The northern Hayes basalts are estimated to have been generated by ∼13 to ∼20% partial melting of a heterogeneous source region (enriched MORB (EMORB) and infertile NMORB source domains), incomplete pooling of melts, and moderate pressure fractionation along a magmatically starved spreading segment (OH‐3). Transform valley basalts show estimated extents of melting from ∼11 to ∼22% of a heterogeneous source region, efficient and inefficient pooling of melts, and moderate pressures of fractionation.
Minimum‐work mountain building
Authors: Masek Jeffrey G. , Duncan Christopher C.
Published: February 2004

Minimum‐work methods may provide a useful way to examine mountain‐building processes. Here we present forward models of crustal deformation in which faults form along paths that minimize the total (frictional and gravitational) work associated with slip. Our numerical model produces thrust wedges analogous to the wedges produced by viscous models but with explicit fault paths rather than diffuse internal deformation. The model reproduces the general geometry of real compressional belts, highlighting the basic competition between friction and topography (gravitational work) which may drive the evolution of orogenic zones. In particular, we replicate observed geologic features such as dendritic faulting patterns, thin‐skinned thrust systems, and back thrusting. While some modeled features differ from observations, the results suggest that for many applications minimum work may be a good approximation for macroscale crustal deformation.
The equivalent strength of geometrical barriers to earthquakes
Authors: Nielsen Stefan B. , Knopoff Leon
Published: February 2004

We present a quantitative framework for evaluating the influence of non‐planar fault geometry on repeated seismic ruptures. We model quasistatic ruptures on a non‐planar fault trace imbedded in a two‐dimensional elastic medium under in‐plane strain. Because of the presence of fault segments that are not parallel to the regional shear stress (i.e. bends), the apparent strength at a given point on the fault is not fixed, but fluctuates with normal stress. Compressional features behave as increasingly strong barriers to fracture unless the stored normal stress is released in order to unlock the fault. Since slip on the fault itself cannot get rid of the normal stress, this is achieved through the action of off‐fault morphological features such as secondary faulting, folding and vertical motions, that we introduce parametrically in the form of an aseismic relaxation. The apparent strength of a fault bend will stabilize in a narrow interval of values after repeated ruptures, characterized by a non‐dimensional “hardness” parameter, whereby the relaxation rate is scaled by the tectonic loading rate.

On a fault structure having several small, widely separated bends, three families of events can be identified whose frequency and magnitude depend on the hardness (relaxation) parameter and the geometry: small events that cluster in the tension zones of the bends, intermediate size ruptures involving a single interbend segment, and large ruptures that break through bends and link on or more interbend segments. Large multi‐segment events are most likely to occur for low values of the hardness, i.e., fast relaxation and slow loading rate.

Regions with compressional features act as barriers that stop most ruptures; stress is stored at these sites until they themselves break and initiate motion on the smoother, long reaches of the fault.
Lower crustal extension across the Northern Carnarvon basin, Australia: Evidence for an eastward dipping detachment
Authors: Driscoll Neal W. , Karner Garry D.
Published: February 2004

Asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults. Nevertheless, cogent evidence for their existence remains limited. Furthermore, even when inferred detachment faults are imaged seismically, it is difficult to demonstrate that differential displacement has occurred across these surfaces. We present tectonic and stratigraphic evidence from the Northern Carnarvon basin, northwest Australia, that documents the existence of an intracrustal eastward dipping detachment. By integrating sequence stratigraphy with kinematic and isostatic models of basin development, we conclude that the Northern Carnarvon basin was formed as a consequence of four extension events: (1) a broadly distributed late Permian event, (2) a predominantly localized Rhaetian event responsible for the inception of the Barrow and Dampier subbasins, (3) a localized Callovian fault reactivation within the Barrow‐Dampier subbasins, and (4) a Tithonian‐Valanginian event that generated large post‐Valanginian regional subsidence across the Northern Carnarvon basin with only minor accompanying brittle deformation and erosional truncation. The regional distribution and amplitude of the post‐Valanginian subsidence are not consistent with the minor amounts of Tithonian‐Valanginian brittle upper crustal extension observed on the margin. Large portions of the platform were emergent or at very shallow water depths prior to the Tithonian‐Valanginian extension. To match the distribution and magnitude of the post‐Valanginian “thermal‐type” subsidence requires significant lower crustal and mantle extension across the Northern Carnarvon basin. Such a distribution of extension implies the existence of an eastward dipping, intracrustal detachment having a ramp‐flat‐ramp geometry that effectively thinned the lower crust and lithospheric mantle. The detachment breached the surface close to the position of the continent‐ocean boundary, west of the Exmouth Plateau. The flat component of the detachment occurred at midcrustal depths (∼15 km) across the plateau and ramped beneath the Australian continent. Lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation, which may explain the paradox that both sides of many conjugate margins appear to be the “upper plate”.
The anatomy of a buried complex impact structure: The Mjølnir Structure, Barents Sea
Authors: Tsikalas Filippos , Gudlaugsson Steinar Thor , Faleide Jan Inge
Published: February 2004

Analysis of the prominent seismic disturbance at the 40‐km‐diameter Mjølnir impact structure, based on an extensive seismic reflection database, shows that the observed broad‐brimmed bowl‐shaped disturbance was formed as a result of the impact of an asteroid or comet during Volgian‐Berriasian time (149–141 M). Seismic mapping exhibits and visualizes a 850–1400 km3 disturbed volume and analysis of several structural features within the disturbance provides insight into major cratering processes, such as brecciation and excavation, melting, gravitational collapse of the transient crater, and structural uplift. A transient crater of 16 km in diameter and 4.5 km in depth is determined. From transient and final crater dimensions we obtain an estimate of the degree of gravitational collapse of the order of 2.5, considerably larger than the average expected values for typical terrestrial craters. The extensive collapse took place by low‐angle décollement surfaces at the periphery and by chaotic debris mass flows toward the center. Furthermore, we estimate the Mjølnir projectile to have been 0.9–3 km in diameter and that the physical impact released energy in the range of 2.4–53×1020 J corresponding to an earthquake of magnitude 8.3. The Mjølnir impact is not associated with a significant mass extinction. However, dissipation of the energy released during the Mjølnir impact was sufficient to have caused several short‐term, near‐field perturbations, such as large‐amplitude tsunami waves, affecting the Barents Sea region and possibly adjacent areas in the Arctic.
Hydrothermal fluid circulation through the sediment of Crater Lake, Oregon: Pore water and heat flow constraints
Authors: Wheat C. Geoffrey , McManus James , Dymond Jack , Collier Robert , Whiticar Michael
Published: February 2004

We present evidence for pore water flow through the sediment of Crater Lake, Oregon based on systematic variations in pore water chemical compositions and thermal gradients. Pore water was extracted from sediment by centrifugation and diffusive exchange using a gravity corer deployed from a surface vessel and a box corer and peepers deployed from the submersible Deep Rover in a known geologic context. Depth profiles of sediment temperature were measured using two probes deployed from the submersible. One probe was connected to the submersible whereas the other was self‐contained and deployed for 7 days. On the basis of measured and calculated depth profiles of pore water Na, Ca, Mg, K, Li, and temperature, we show that pore water upwells in zones of focused upflow at speeds of meters to hundreds of meters per year. These zones of focused flow are patchy and usually cover several square meters to hundreds of square meters and are marked by iron‐manganese‐rich crusts, bacterial mats, and saline pools. In contrast, most of the lake floor consists of sediment derived from the caldera walls and has a low heat flow with pore water velocities of millimeters per year. The chemical composition of the pore water that upwells through the sampled section of the sediment column differs from core to core. This difference results from mixing a hydrothermal fluid in igneous basement below the lake with lake water before the final ascent through the sediment column. Elemental ratios of this thermally and chemically altered fluid in basement match those calculated based on mass balance considerations. Calculation of mass balance and geothermometry constrain the temperature in basement and ultimately the power output, which is about 30 MW. This power output is in agreement with two other estimates that were calculated using temperature data from the water column and measurements of sediment heat flow.
A revision of the 1693 eastern Sicily earthquake and tsunami
Authors: Piatanesi Alessio , Tinti Stefano
Published: February 2004

The 1693 earthquake was a disastrous event affecting eastern Sicily, southern Italy, where it caused over 60,000 victims and total destruction of several villages and towns in the districts of Siracusa, Ragusa, and Catania. The earthquake was followed by a tsunami that struck the Ionian coasts of Sicily and the Messina Strait and was probably observed even in the Aeolian Islands. Historical documents on sea waves and flooding, though not abundant, allow us to form a picture of the tsunami first attack and inundation. The first water movement all along the Sicilian coastline was a strong sea withdrawal, followed by a violent sea return and coastal flooding. The main purpose of this research is to put constraints on the focal mechanism of this earthquake on the basis of the available documents on the tsunami (1) by simulating tsunami from different possible sources via numerical modeling based on finite element technique and shallow water approximation and (2) by choosing the source best fitting the tsunami data as the most plausible cause for this tsunami. The relevance of this study should also be evaluated in light of the circumstance that no certain indication on the earthquake fault can be deduced from the available macroseismic data alone. Solving the source problem for this event, one of the largest occurring in southeastern Sicily in historical times, is a significant contribution to understanding the seismotectonic regime of the region and to assessing the related earthquake and tsunami hazard/risk with implications on mitigation policies.
Geomagnetic polarity reversal model of deep‐tow profiles from the Pacific Jurassic Quiet Zone
Authors: Sager William W. , Weiss Chester J. , Tivey Maurice A. , Johnson H. Paul
Published: February 2004

The Jurassic magnetic “Quiet Zone” (JQZ) contains magnetic lineations, but their low amplitudes make correlation and interpretation difficult. Part of the problem is the separation of source and sensor for old, deep ocean crust. We increased anomaly amplitudes by collecting magnetic data along two deep‐tow profiles over western Pacific JQZ lineations. A magnetic polarity reversal timescale was constructed by matching deep‐tow anomalies with a simple, rectangular block magnetization model for oceanic crust. The polarity sequence covers ∼11 m.y. (156–167.5 Ma) and contains 88 pre‐M29 polarity chrons extending to Chron M41. A limitation of this model is its poor representation of the oldest anomalies upward continued to sea level. On deep‐tow profiles these anomalies have both long‐ and short‐wavelength components, but only the latter are easily modeled on a datum close to the source. An alternative polarity model was constructed to match the anomalies upward continued to sea level. This model retains only 44% of the deep‐tow model polarity chrons because of short‐wavelength attenuation by upward continuation. Because of the inferred periods and magnetization contrasts, we think many of the short‐wavelength anomalies represent paleofield intensity fluctuations. In contrast, polarity reversals have been documented by prior magnetostratigraphic work for the younger part of the timescale covered by our model. Thus our data may show a transition from a geomagnetic field behavior dominated by intensity fluctuations to one dominated by reversals.