bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of maximum horizontal principal stress magnitude from stress‐induced well bore breakouts in the Cajon Pass Scientific Research borehole
Authors: Vernik Lev , Zoback Mark D.
Published: February 2008

In this paper we utilize the mechanical properties of core samples, detailed observations of stress‐induced well bore breakouts and estimates of the magnitude of Shmin from hydraulic fracturing experiments to construct a vertical profile of the maximum horizontal principal stress SHmax to 3.5 km depth in the Cajon Pass borehole. As in essentially all other boreholes, the hydraulic fracturing stress measurements in the Cajon Pass borehole yielded appreciably more (and more reliable) data on the magnitude of the least principal horizontal stress, Shmin, than the maximum principal horizontal stress, SHmax. To utilize the breakout observations to constrain the magnitude of SHmax, a brittle failure criterion was used that is based on the effective strain energy concept. This failure criterion also allows us to account for the polyaxial stress around the borehole and to incorporate the role of pore fluid pressure on rock strength in various ways. A comparison of the SHmax profile estimated from the breakouts with estimates of SHmax obtained from the hydraulic fracturing tests indicates that they compare fairly well for the case when the effect of pore pressure on hydraulic fracture initiation is negligible. The overall stress state to a depth of 3.5 km in the well corresponds to a normal/strike‐slip faulting regime. The SHmax profile indicates several marked decreases in stress magnitude associated with major fault zones, an observation consistent with the Shmin profile obtained from hydraulic fracturing tests.
Micromechanical modeling of healed crack orientations as a paleostress indicator: Application to Precambrian granite from Illinois and Wisconsin
Authors: Jang Bo‐An , Wang Herbert F.
Published: February 2008

We have used a micromechanical, four‐grain model to simulate intragranular cracks produced during primary cooling of a granitic pluton in a horizontally anisotropic stress field. The orientations of vertical, intragranular cracks were obtained using the criterion that they are perpendicular to the maximum principal tensile stress when it exceeds the local tensile strength. The preferred orientation was always parallel to the maximum regional compressive stress direction. The sharpness of the distribution peak increases with the ratio of the maximum to minimum horizontal stress. We applied the model results to new and previous data on Precambrian granites from Wisconsin and Illinois. Our new data are healed crack orientations and fluid inclusion properties in unoriented granite core from Illinois borehole UPH‐2. The fluid inclusion measurements and isotopic age dates indicate that most healed cracks were formed during the primary cooling of the pluton. The fluid inclusion characteristics and shape of the crack orientation distribution were almost identical to those previously determined for UPH‐3, 1 km away (Kowallis et al., 1987), suggesting that the preferred orientation of healed cracks in granite from UPH‐2 is the same as for UPH‐3 (N25°W ± 5°). The orientation distribution from the micromechanical model for a stress ratio of 4 is very similar to the healed crack orientation distribution measured in several Precambrian granites from the midcontinent region. We conclude that healed crack orientations are useful indicators of the paleostress field and that they may be used as a tool for orienting core in stable, structurally simple regions.
A low‐velocity zone in the basement beneath the Valles Caldera, New Mexico
Authors: Roberts Peter M. , Aki Keiiti , Fehler Michael C.
Published: February 2008

We present quantitative results of forward modeling applied to a suite of travel time delays observed for teleseismic p wave first arrivals recorded in the Valles Caldera, New Mexico, in 1987. We used a ray tracing method to construct a two‐dimensional model of the caldera's subsurface p wave velocity structure for the vertical cross section defined by the linear seismometer array geometry used to collect the data. Our initial trial models were based on observations of a systematic pattern in the recorded p wave delays, which indicated that an isolated, low‐velocity zone (LVZ) exists in the Precambrian basement beneath the caldera's surface layers. Thus the primary goal of the forward modeling was to determine the size, shape, location, and velocity contrast of the LVZ. Due to incomplete ray coverage we could not uniquely constrain all parameters defining the LVZ, and our final model thus represents a first look at the caldera's midcrustal structure. We placed upper and lower bounds on most model parameters by investigating the ranges of models that fit the data well. We determined that the LVZ must be lens shaped, pinching out laterally on each side, and is located beneath the center of the caldera's resurgent dome. Its width is approximately 17 km from tip to tip, and the depth from the surface to its center is 10–13 km. We could not determine an upper bound for the LVZ's thickness, but we found it could be no smaller than 8 km from top to bottom. For this thickness, the P wave velocity was constrained to lie between 3.5 and 4.2 km/s. Compared with a homogeneous half‐space model, the best LVZ model reduced the rms of the residual delays for 21 events by 70%, equivalent to a 91% decrease in the residual variance. The low p wave velocity suggests that the LVZ could be a region of partial melt, possibly related to the Bandelier ignimbrite or postcaldera rhyolite eruptions.
Sources of unsteady column dynamics in pyroclastic flow eruptions
Authors: Valentine Greg A. , Wohletz Kenneth H. , Kieffer Susan W.
Published: February 2008

During the first several minutes of an explosive volcanic eruption, the flow in the column above the vent is unsteady even if the discharge from the vent is steady. At a fixed location in the column, parameters such as temperature and ash content change with time until steady flow conditions are established. For conditions believed to be typical of fountain‐forming silicic eruptions of intermediate volume, numerical simulations show unsteadiness in temperature, plume diameter, mass flux, vertical velocity, and particle concentration. In addition to the steady mass flux from the vent, we have identified three sources of mass and heat flux into the column: (1) recirculation of pyroclastic flow material into the base of the column (low elevation inward flow); (2) ash entrained from the top of the pyroclastic flow by atmospheric inflow back toward the column (higher‐elevation inward flow); and (3) waves reflected within the pyroclastic flow if it encounters topographic obstacles. The thermal and mass flux pulses cause variations in column diameter and fountain height. Changing column diameter would be expected to be manifested in changes in fallout deposits (e.g., by shifts in clast isopleths). These temporal variations (if observed, for example, by satellite measurements of temperature pulses or pulses or inferred from field observations of deposit stratigraphy) could be misinterpreted as indicating source variations such as source mass flux, volatile content, or vent diameter, when, in fact, they result from fluid mechanical processes.
Pressure derivatives of the bulk modulus
Authors: Hofmeister A. M.
Published: February 2008

Simple relations between the first (or second) pressure derivative of the bulk modulus and first (or second) thermal Grüneisen parameter γTh (or qTh) are derived from equations that relate KT(P) to vibrational frequencies Vi as a function of pressure for cubic structures or those that have only one nearest neighbor distance, by assuming that the mode Grüneisen parameters, γi = −∂ ln vi/∂ ln V, are approximately equal. The free volume equation results, which holds within experimental uncertainties for about 70% for solids meeting the symmetry requirements. Agreement is poor for compounds with the rutile structure, which have the largest range of γi observed. Another result is a three‐parameter equation of state, KoK″o = (5/3 ‐ K′o)2 ‐ (5/3 ‐ K′o)qTh(0) ‐ (K′o ‐ 1)2. All available experimental determinations of K″o are bracketed by the spectroscopic limits for qTh. Use of the average value of zero for qTh (or qTh calculated from other thermodynamic parameters) gives the proper sign for K″o and nearly the same values as experiment, suggesting that this approximate equation can be used predictively. The equations involving K″o and K′o are probably valid for any structure, if the γi are roughly equal, although this most likely occurs for solids that compress nearly isotropically. The new formula, with qTh of zero, give values similar to that obtained from the Vinet‐Ferrante‐Rose‐Smith equation at low K′o gives the same values as the Birch‐Murnaghan (third‐order finite strain) equation near K′o of 4, and gives nearly equal values as the Grover‐Getting‐Kennedy equation of state at high K′o. Physically reasonable positive values of K″o as K′o approaches zero are obtained only from the new equation of state and that of Vinet et al. Comparison of the new equation of state with third‐order finite strain suggests that the latter is also likely to be incorrect at high K′o and thus should be cautiously applied to V(P) for liquids.
Mantle layering from ScS reverberations: 4. The lower mantle and core‐mantle boundary
Authors: Revenaugh Justin , Jordan Thomas H.
Published: February 2008

This is the final installment in a four‐part sequence that examines the nature of mantle layering required by the multiple‐ScS phases and internal reflections observed within the reverberative interval of SH‐polarized seismograms. In this paper, long‐period, SH‐polarized, multiple‐ScS phases reflected once from a mantle discontinuity (first‐order reverberations) are used to search for abrupt shear‐wave impedance contrasts in the lower mantle. Beneath the geographic regions sampled, the depth interval of 1000–2600 km (Bullen's region D′) appears free of any distinct, radial layering, in agreement with the majority of recent seismic models and the notion of near‐adiabatic compression of a compositionally homogeneous lower mantle. To pass undetected, discontinuities in D′ must be either small (SH‐polarized reflection coefficient R0<1.0%), highly intermittent or subject to lateral depth variations well in excess of 50 km. At greater depths, corresponding to the D″ region of the lowermost mantle, we find evidence for a reflector of long‐period seismic energy situated an average of 325 km above the core‐mantle boundary (CMB), similar to the discontinuity proposed by T. Lay and coworkers. Our results indicate a 4.4% increase in shear wave impedance and a 1.7% increase in density if the 2.75% increase in shear velocity proposed by Lay and Helmberger (1983) holds true for our study area. The discontinuity is observed beneath less than one‐third of the 18 seismic corridors examined and does not appear to be a ubiquitous feature of the lower mantle. This, plus an observed correlation of discontinuity depth and lower‐mantle velocity heterogeneity, and the lack of experimental evidence for deep‐mantle phase transformations strongly favor a compositional origin. Mantle‐side layering nearer the CMB, perhaps associated with a thin chemical boundary layer, would mimic the effects of the surface crust, producing a complex wave train of first‐ and higher‐order reverberations superposed upon all other reverberative interval arrivals. If present and unaccounted for, CMB structure would drive estimates of crustal structure and QScS obtained by multiple‐ScS waveform inversion away from their true values. Compared to independent estimates, our results admit little bias. Monte Carlo tests are used to assess the sensitivity of this technique to CMB layering, which decreases with decreasing boundary layer thickness. Nonetheless, the results limit CMB transition zones in excess of 20 km to impedance contrasts less than 12% (8% for a 40‐km transition) beneath much of the western Pacific, Australia, Melanesia, and Indonesia.
Magnetic properties of ilmenite70‐hematite30: Effect of transformation‐induced twin boundaries
Authors: Nord Gordon L. , Lawson Charles A.
Published: February 2008

Twin domains and boundaries arising from the order‐disorder transition (Tcr ∼ 1025°C) in ilmenite70‐hematite30 have little effect on intrinsic magnetic properties (TCurie, Jsaturation) but a large effect on extrinsic properties (isothermal remanent magnetization (IRM), thermoremanent magnetization (TRM), and alternating field (AF) and thermal demagnetization). At twin domain sizes between 800 and 1000 Å, JIRM reaches a maximum, and the orientation of acquired TRM changes from reverse to normal, suggesting that the twin domains and magnetic domains are coincident below this size and the twin boundaries are also magnetic walls. Above 800–1000 Å, free‐standing magnetic walls can form, reducing the IRM. Wall pinning by the twin boundaries is also demonstrated by AF demagnetization of both IRM and TRM. Thermal demagnetization of TRM reconfirms the Fe‐enriched nature of the twin boundaries (“x” phase), a necessary condition for self‐reversal. Samples annealed within the miscibility gap (below 600°C) show increased Curie temperatures caused by compositional fluctuations (observed by transmission electron microscopy) forming within the two‐phase field.
Effects of loading velocity, stiffness, and inertia on the dynamics of a single degree of freedom Spring‐Slider System
Authors: Gu Yaojun , Wong Teng‐fong
Published: February 2008

Numerical simulation of the nonlinear dynamics of a single degree of freedom spring‐slider system can provide useful insight into the effects of loading velocity, friction environment, elastic stiffness, and inertia on the stress drop and dynamic behavior of earthquakes. In our numerical simulations using a rate‐ and state‐dependent friction law, we varied the loading velocity by 7 orders of magnitude, the stiffness by 2 orders of magnitude, the mass by 5 orders of magnitude, and the velocity‐weakening parameter (B‐A) by a factor of 160. Four stability regimes were identified in the load point velocity‐stiffness space when inertia was taken into account. For cyclic stick‐slip instabilities, the stress drop amplitude increases with decreasing load point velocity, with decreasing stiffness, with increasing velocity weakening parameter B‐A, and with decreasing mass. Simple scaling relations for the static stress drop, dynamic stress drop, “static” friction, and shear fracture energy with the natural logarithm of load point velocity were observed. Using the scaling relation for static stress drop, we infer the normalized velocity weakening parameter b‐a (with respect to normal stress) to range from 0.0011 to 0.0039 for triaxial experiments on saw cut Westerly granite specimens sandwiched with a simulated gouge layer of ultrafine quartz at pressures from 10 MPa to 100 MPa. Unless we take b‐a throughout the seismogenic layer to be much higher than experimental measurements, the effect of loading velocity by itself is not sufficient to account for the increase of stress drop with increasing earthquake recurrence time inferred from seismological data. To the extent that our simple system can be treated as an analog for earthquakes, our study shows that the velocity weakening parameter (B‐A) controls the magnitude of the static stress drop far more than the loading velocity. If velocity weakening increases systematically with increasing healing time, then the coupled effect of loading velocity variation and change in friction environment can account for the seismological observations.
The role of seamount volcanism in crustal construction at the Mid‐Atlantic Ridge (24°–30°N)
Authors: Smith Deborah K. , Cann Johnson R.
Published: February 2008

An analysis of approximately 6000 km2 of Sea Beam swaths indicates that the floor of the median valley of the Mid‐Atlantic Ridge (MAR) between 24°–30°N is strewn with near‐circular volcanoes. We have identified 481 seamounts in the height range 50–650 m. Because many of the features which appear to be seamounts do not meet our criteria fully, they have not been counted, and our numbers are likely a minimum estimate of the population. The large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the MAR. The summit height distribution of the MAR seamount population is consistent with the exponential frequency‐size distribution model for off‐axis eastern Pacific seamounts (Jordan et al., 1983; Smith and Jordan, 1987). The model parameters for the MAR population yield an average of about 195 seamounts per one thousand square kilometers, and a characteristic height of about 60 m. Shape statistics compiled from the 481 seamounts give parameters that are exactly the same, or cover the same range in values as those obtained from a study of Pacific seamounts in the height range 140–3800 m (Smith, 1988) implying a universal control on seamount construction. Based on the volcanic morphology, we identify 18 spreading segments along the ridge. All except two contain a prominent axial volcanic ridge. Many of the seamounts identified are associated with the axial ridges. From the Sea Beam swaths, we infer that the ridges are composed of piled up seamounts and hummocky flows, and interpret them to be the primary sites of crustal construction. This style of volcanism contrasts strongly with that observed at the East Pacific Rise where seamounts are virtually absent at the spreading axis. The construction of seamounts and hummocky flows may be directly related to the fact that the MAR is fed by magma chambers that are limited in size and frequency. We suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice; small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows. Using buoyancy arguments, the heights of the MAR seamounts are related to the depth of the magma bodies. We hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped, and then erupt. If this hypothesis is correct, the thickness of the brittle lid controls seamount height, and the exponential distribution of seamount summit heights implies that the thickness of the brittle Hd follows an exponential distribution in time and space with characteristic thickness 1.4–1.9 km. If seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle/ductile transition, then the lower crust of the MAR will be made up of the products of a multitude of small plutons.
Deepwater high‐resolution expanding spread and split spread seismic profiles in the Nankai Trough
Authors: Stoffa P. L. , Wood W. T. , Shipley T. H. , Moore G. F. , Nishiyama E. , Botelho M. A. B. , Taira A. , Tokuyama H. , Suyehiro K.
Published: February 2008

In deep water the source‐receiver offsets that are required for accurate velocity determinations cannot be achieved with single‐ship multichannel seismic methods. Two ships, one equipped with a multichannel receiving array and the other with a seismic source, have previously been employed to acquire common midpoint, expanding spread profiles, principally to determine deep crustal velocity structures. We extend this method to higher resolution in space and time to determine the velocities of sedimentary layers in deep water offshore Japan in the Nankai Trough. This high‐resolution two‐ship data acquisition method used a 13.1‐L water gun source array; a 1.6‐km, 96‐channel receiving array with 0.0166‐km active group; and shore‐based navigation. Analysis of the data was performed in the τ‐pdomain by successive downward continuation of the plane wave data. Interactive velocity analysis methods for both one‐dimensional and two‐dimensional Earth models are described for both common source/receiver and common midpoint profiles. Results in one of two areas surveyed show a low‐velocity zone below the subduction decollement which is consistent with models of low wedge taper, high pore fluid pressure, and reflection polarity reversal described previously by other researchers. The velocity profiles show the expected landward increase in velocity assumed to be due to lateral strain and porosity decrease, but the effect is small, only slightly greater than would be expected in an area of no lateral strain.
Sound velocities at high pressure and temperature and their geophysical implications
Authors: Duffy Thomas S. , Ahrens Thomas J.
Published: February 2008

Temperature coefficients of compressional and bulk sound velocities at pressures on the order of 100 GPa are obtained from Hugoniot sound velocity measurements for solid Al, W, Cu, Ta, and Mg2SiO4. The Hugoniot velocities are compared to third‐order finite strain extrapolations of velocities along the principal isentrope using ultrasonically determined coefficients. At low pressure, where thermal effects are minor, good agreement is found between the Hugoniot velocities and finite strain extrapolations. At high pressures, differences in velocities and temperatures are used to constrain temperature coefficients of velocity. For all materials studied except W, the temperature coefficients of velocity at pressures above 1 Mbar are a factor of 2 to 8 smaller in magnitude than zero‐pressure values. In shock‐melted materials, the Hugoniot sound velocities are close to finite strain velocities calculated from low‐pressure properties of the solid phase for Mo, Ta, Pb, Fe, and alkali halides. The temperature coefficient determined for the high‐pressure phases of forsterite above 100 GPa (| (∂VP/∂T)P| = 0.1 ± 0.1 m/s/K) is in agreement with estimates based on elastic and thermodynamic properties for the Earth. Our results indicate that |(∂VP/∂T)P| is a decreasing function of pressure in contrast to residual sphere studies which suggest |(∂VP/∂T)P| is nearly constant with depth in the Earth. In combination with mineral physics estimates of thermal expansivity at High pressure, it is estimated that (∂VP/∂ρ)P = 2 (km/s)/(g/cm3) for P > 100 GPa, with acceptable values ranging from 0 to 8. This overlaps the range of estimated lower mantle values based on seismic and geodetic data. Tomographic and free oscillation data require large increases in the parameter ν = (∂ ln VS/∂ ln VP)P under lower mantle conditions, relative to laboratory values. Available data for tungsten and aluminum yield ν values along the Hugoniot that are consistent with zero‐pressure values for these materials, although uncertainties are ± 50%. Temperature coefficients of velocity at high pressure are used to make improved estimates of the magnitude of thermal heterogeneities sampled by seismic tomography. Long‐wavelength compressional velocity anomalies at pressures in the 100–127 GPa range (2271–2891 km depth) in the lower mantle correspond to temperature variations of 120 ± 100 K, whereas those in the D″ region are likely to be a factor of 3 to 4 larger.
Periodic instabilities during compression or extension of the lithosphere 1. Deformation modes from an analytical perturbation method
Authors: Martinod J. , Davy P.
Published: February 2008

We argue that the plastic rheology of the lithosphere, rather than its “recoverable” elastic properties, is responsible for the systematic development of periodic instabilities during compression or extension. We use a linear perturbation model with analytical solutions to calculate the instability modes for various rheologies. The growth of such periodic instabilities is enhanced by the highly nonlinear stress‐strain rheologies encountered in the brittle layers of the lithosphere. On the contrary, ductile layers, deforming according to high temperature creep flow laws, tend to inhibit these instabilities. For oceanic domains, we assume that the only brittle layer is the upper part of the lithosphere. In compression, the only valid instability is a buckling whose wavelength is around 4 times the thickness of the brittle layer. Calculated wavelengths and growth rates are consistent with observations available for the Indian Ocean. For continental domains, a reasonable assumption is the existence of two plastic layers, one in the upper crust, the other in the upper mantle, for Moho temperatures between 450°C and 600°C. In compression and extension, two instabilities develop a long wavelength instability involving the whole lithosphere (coupling mode) and a short wavelength instability involving the crust and controlled by the upper brittle layer (intrinsic crustal mode). In compression, the coupling mode is a whole‐lithosphere buckling, with a wavelength about 4 times the thickness of the active lithosphere (the two plastic layers plus the intermediate ductile layer). In extension, the coupling mode is a boudinage of opposite phase in the two plastic layers and a folding of the intermediate ductile layer. The intrinsic crustal mode is a crustal buckling in compression, crustal boudinage in extension. Neither deflects the Moho. The intrinsic crustal mode is favored by an increase in thermal gradient and by a decrease in strength of the ductile lower crust.
A similarity solution for two‐phase water, air, and heat flow near a linear heat source in a porous medium
Authors: Doughty Christine , Pruess Karsten
Published: February 2008

Placement of a heat source in a partially saturated geologic medium causes strongly coupled thermal and hydrologic behavior. To study this problem, a recently developed semianalytical solution for two‐phase flow of water and heat in a porous medium has been extended to include an air component and to incorporate several physical effects that broaden its range of applicability. The problem considered is the placement of a constant‐strength linear heat source in an infinite homogeneous medium with uniform initial conditions. Under these conditions the governing partial differential equations in radial distance r and time t reduce to ordinary differential equations through the introduction of a similarity variable η = r/t1/2. The resulting equations are coupled and nonlinear, necessitating a numerical integration. The similarity solution developed here is used to investigate various physical phenomena related to partially saturated flow in low‐permeability rock, such as vapor pressure lowering, pore level phase change effects, and an effective continuum representation of fractured/porous media. Application to several illustrative problems arising in the context of high‐level nuclear waste disposal at Yucca Mountain, Nevada, indicates that fluid flow, phase changes, and latent heat transfer may have a significant impact on conditions at the repository.
Upper mantle heterogeneity below the Mid‐Atlantic Ridge, 0°–15°N
Authors: Bonatti E. , Peyve A. , Kepezhinskas P. , Kurentsova N. , Seyler M. , Skolotnev S. , Udintsev G.
Published: February 2008

Small‐scale variations in composition of mantle‐derived peridotites have been investigated in the 0°–15°N portion of the Mid‐Atlantic Ridge (MAR), thanks to a relatively close‐spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle‐equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a “cold” upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter‐Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (∼8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20′ FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°–3°N and 14°–15°N regions is associated spatially with light rare earth element enriched mid‐ocean ridge basalt showing a “hot spot”‐type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero‐age topographic highs in the 2°–3°N and 14°–15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O‐rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.
Primary magmas of mid‐ocean ridge basalts 1. Experiments and methods
Authors: Kinzler Rosamond J. , Grove Timothy L.
Published: February 2008

This paper reports experiments carried out between 9 and 16 kbar (0.9–1.6 GPa) using natural, primitive mid‐ocean ridge basalt compositions and synthetic analogs of mid‐ocean ridge basalts to investigate the effects of pressure, temperature, and variable bulk composition on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and plagioclase or spinel. For this low‐variance, five‐phase assemblage, equations involving pressure, melt NaK # ([Na2O+K2O]/[Na2O+K2O+CaO]; weight ratio), melt Mg # (Mg/[Mg+Fe2+]; total iron as Fe2+), and weight percent TiO2 in the melt predict temperature and major element compositions of magmas produced by melting spinel and plagioclase lherzolites at upper mantle pressures. The equations are estimated using a selected set of data from this experimental study and published experimental studies that report compositions of glasses coexisting with olivine, low‐Ca pyroxene, augite, and plagioclase and/or spinel. An experimental test of a liquid compositionally similar to melts produced in a subset of peridotite‐basalt sandwich experiments is presented. The composition tested was reported as multiply saturated (with orthopyroxene + augite + spinel + olivine) in the sandwich experiment, but it does not crystallize these phases at the conditions of the experiment. We exclude this liquid and the subset it represents (data from Fujii and Scarfe [1985] and Falloon and Green [1987]) from the data set used to constrain the melting equilibria. With estimates of the melt NaK #, melt Mg #, and weight percent TiO2 of the melt the quantitative descriptions of the melting equilibria can be used to predict the temperatures and major element compositions of melts from lherzolite. Methods are described for estimating these compositional parameters with the nonmodal batch melting equation (for Na2O, CaO, K2O, and TiO2) and a mass balance calculation (for MgO and FeO) from the initial composition and phase proportions of the mantle source, the amount of melt produced and the nature of the melting process, and the stoichiometric coefficients of the mantle melting reaction.
Propagation of a set of opening‐mode fractures in layered brittle materials under uniaxial strain cycling
Authors: Wu Haiqing , Pollard David D.
Published: February 2008

Surface textures commonly found on natural opening‐mode fractures (joints) in rock are indicative of cyclic loading. The effect of uniaxial strain cycling on the development of a set of fractures in layered brittle materials was studied using a series of model experiments on polymethyl methacrylate (PMMA) sheets covered with a thin brittle coating. Changes in five geometric parameters, including number of fractures, total length, average length, average spacing, and spatial density, were recorded during each experiment. Two major factors, strain rate and strain magnitude, are also considered under the cycling condition. Three groups of experiments are (1) for different strain magnitudes, (2) for different strain rates with the same total strain, and (3) for increasing strain cycling. It is found that few new fractures appear after a certain number of cycles as old fractures keep getting longer; and the geometric parameters, as functions of cycles, decrease or increase rapidly for the first several cycles and then tend not to change much. A greater strain or strain rate increases propagation velocity and fracture length. The critical value of strain for fracturing can be expressed as a function of cycles, and it decreases with increasing cycles. The fractures with different lengths have different propagation velocities, but all decrease very quickly after the first several cycles.
Gravitational stresses in anisotropic ridges and valleys with small slopes
Authors: Liao Jyh Jong , Savage William Z. , Amadei Bernard
Published: February 2008

We present an approximate analytical solution for the effects of topography on near‐surface gravity‐induced stresses in symmetric anisotropic ridges and valleys. The solution can be used for laterally constrained ridges and valleys consisting of isotropic, transversely isotropic, or orthotropic rock with horizontal or vertical layers. A parametric study on the effect of anisotropy type and degree of rock anisotropy on gravity‐induced stresses is presented. The magnitude of the predicted stresses is of the order of the characteristic stress ρgH, where H is the height of the ridge or depth of the valley. The approximate solution is limited to ridges and valleys with small slopes not exceeding 10%.
Disequilibrium fluid pressures and groundwater flow in the western Canada sedimentary basin
Authors: Corbet Thomas F. , Bethke Craig M.
Published: February 2008

Fluid pressures too low to be in equilibrium with the topography of the land surface occur in Cretaceous sediments over much of the western Canada sedimentary basin. We used a numerical model to study the origin of low pressure in southern Alberta and its effects on patterns of regional groundwater flow over the past 5 million years. The model accounts for changes in basin topography, conduction and advection of heat, cooling of pore fluid, and rebound of pore volume during erosion. Results show that the ∼3 MPa of underpressuring observed in this region could have formed because the pore volume of sediments expanded slightly as Pliocene‐Pleistocene erosion removed some of the confining load. Our calculations provide an estimate for the upper limit for the permeability of Cretaceous shales on a regional scale because results match observed pressures only if we assign vertical permeabilities less than 3 × 10−20 m2 to these sediments. Pore fluid cooled and contracted as erosion reduced the burial depth of the sediments, but this effect could not have played a significant role in generating low pressure unless Cretaceous shales in southern Alberta are extremely stiff (pore compressibilities of the order of 6 × 10−10 Pa−1) and have regional permeabilities of about 10−22 m2. In our simulations, erosion generates potential gradients that drive groundwater along deep aquifers toward regions of lowest pressure in adjacent aquitards. Groundwater, however, moves too slowly to transport a significant amount of heat. The observed west‐to‐east increase in the geothermal gradient across the study area most likely occurs because sediments that were deeply buried in the west are more compacted and hence more thermally conductive than those in the east.
Stress, grain size, and magnetic stability of magnetite
Authors: Xu Song , Merrill Ronald T.
Published: February 2008

Existing data of thermal variations of bulk coercivity Hc for various magnetite samples are shown to be linearly proportional to λ/Ms in the temperature range from the Verwey transition to the Neel temperature of magnetite, where λ is some appropriate magnetostriction constant and Ms is the saturation magnetization. This clearly indicates that the temperature stability of bulk coercivity in these magnetite samples is stress controlled, as concluded early by Hodych (1982, 1986) based on Hc data measured at low temperatures. However, no existing theories of bulk coercivity for multidomain grains are satisfactory at explaining this seemly simple λ/Ms dependency of Hc. Although other complexities may exist, we suggest a mechanism in which internal stress associated with dislocations in these samples not only causes domain wall pinning but also determines the anisotropy of domain walls. The theory thus developed is consistent with the observed temperature dependency of Hc. We further suggest that the temperature dependency of bulk coercivity of all magnetite grains is predominantly controlled by stress associated with dislocations, with a few exceptions such as for single‐domain (or nearly single‐domain) grains for which the stability is primarily controlled by grain shape.
Paleomagnetism of the Middle Jurassic Summerville Formation, east central Utah
Authors: Bazard David R. , Butler Robert F.
Published: February 2008

The paleomagnetism of the late Callovian(?) Summerville Formation was analyzed to obtain a late Middle Jurassic paleomagnetic pole for North America. A total of 281 samples were collected from 35 sedimentary horizons (sites) in a single locality in the San Rafael Swell area of east central Utah. Fifteen site‐mean characteristic remanent magnetization (ChRM) directions pass the reversals test and define at least five polarity zones within 52 m of stratigraphic section, suggesting that the ChRM was acquired upon, or soon after, deposition. Magnetizations of some specimens are complex, and several horizons yield anomalous site‐mean directions. Data analysis included filtering to provide different combinations of virtual geomagnetic poles for calculation of the paleomagnetic pole. However, editing the data did not change the pole position by more than 5°. The preferred paleomagnetic pole position is 56.3°N, 133.4°E (A95 = 7.2°; N = 11 sites). The Summerville Formation paleomagnetic pole is located near the ∼172 Ma Corral Canyon pole and is statistically indistinguishable from the ∼151 Ma Glance Conglomerate and ∼149 Ma Lower Morrison poles. The paleomagnetic pole from the Summerville Formation is located at a much lower latitude and more easterly longitude than the paleomagnetic pole obtained from the ∼165 Ma Moat Volcanics of New England. We propose that the Jurassic North American apparent polar wander path is an age‐progressive band at 55°N to 65°N latitude extending from ∼11°E longitude at ∼172 Ma to ∼150°E longitude at ∼149 Ma.
The hydraulics of density currents over estuarine sills
Authors: Chao Shenn‐Yu , Paluszkiewicz Terri
Published: February 2008

It is demonstrated here that density currents, being sensitive to the underlying topographies, are subject to topographic control in estuaries indented with mild sills. As a density current flows over a ridge, that current shoals downstream but deepens upstream, rendering the upstream flow more subcritical. The Earth's rotation weakens the topographic influence upstream and downstream, and the vertical mixing of momentum and density also weakens the influence. Conventionally, it was thought that the two‐layer estuarine circulation characterized by the upward entrainment of a landward undercurrent will be set up following the seaward passage of the density front. In the presence of bottom topographies, however, there should be a final phase of adjustment. During this phase there is a downward entrainment of the flow upstream of sills and a consequent modulation of the estuarine circulation.
Spatial variability of sea‐ice properties in the northwestern Weddell Sea
Authors: Eicken Hajo , Lange Manfred A. , Dieckmann Gerhard S.
Published: February 2008

Arrays of sea‐ice cores were taken at 0.25‐, 2‐, and 20‐m spacing from three floes of presumably 2 years' age in the northwestern Weddell Sea. Texture, salinity, chlorophyll a, and nutrient concentrations were determined in order to study the small‐scale variability of these properties. Their vertical distribution patterns are similar between core samples from a specific floe. Mean salinity exhibits generally low standard deviations between cores (mostly <0.3 ppt; in one case as high as 0.95 ppt). At specific depth levels, very high differences may occur, such as maximum salinity differences of more than 10 ppt at the top of floes. Mean chlorophyll concentrations vary up to one order of magnitude on small scales, which is as high as the regional variability exhibited by cores from the entire study area. Excepting ammonium, concentrations of nutrients vary much less than chlorophyll. Individual profiles and variability of nutrients are tied to chlorophyll concentrations. The variability of salinity and chlorophyll seem linked to the distribution of large‐scale, secondary pores within the ice.
Evidence of subduction within cold filaments of the northern California Coastal Transition Zone
Authors: Kadko David C. , Washburn Libe , Jones Burton
Published: February 2008

Profiles of 222Rn, chlorophyll, nutrients, oxygen, temperature, and salinity taken within a cold filament observed in satellite images of the northern California coastal transition zone provide evidence that there is strong vertical transport out of the surface layer by subduction processes occuring within these features. Layers of chlorophyll found below the euphotic zone have associated with them other characteristics of water that at one time had been in contact with the sea surface, including deficiencies of 222Rn with respect to 226Ra, elevated oxygen concentrations, and warm water on plots of temperature and salinity. The data indicate that the subsurface maxima in chlorophyll are derived from subducted surface layers and are not produced by in situ phytoplankton growth nor derived by particle settling. The presence of a 222Rn deficiency at depth further suggests that the subducted water had been in contact with the surface within the past week and indicates that the rate of vertical transport is approximately 25 m/d. This has important implications for the fate of the filaments and for the high primary productivity within them.
Remote sensing of the roughness of a fractal sea surface
Authors: Agnon Yehuda , Stiassnie Michael
Published: February 2008

One use of radar altimeters is to measure surface wind speeds through their effect on the roughness of the sea surface. The specular point reflection model is only appropriate for surfaces with roughness on scales which are large in relation to the radar wavelength. This may not be the case for ocean surfaces. Here we model the sea surface as a fractal on the relevant scales. This is based on Hasselmann's model for nonlinear wave action transfer. The radar cross section for nadir backscatter is derived and its dependence on the radar frequency is determined. The results are compared with the cross section obtained by the specular point model for a smoothed surface, yielding the appropriate cutoff wavenumber. Some existing measurements of the sea surface by altimeter and stereophotogrammetry are discussed, suggesting a smaller fractal dimension for short gravity waves. Dual‐frequency altimeters may help determine the synoptic spectral shape.
Internal waves and velocity fine structure in the Arctic Ocean
Authors: D'Asaro Eric A. , Morehead Mark D.
Published: February 2008

A series of closely spaced velocity profiles made in the ice‐covered Canada Basin of the Arctic Ocean during April 1985 are used to investigate the spatial scales and dynamics of small‐scale motions. The velocity fluctuations are very different from those found in mid‐latitudes, containing about 40–50 times less WKB normalized energy at low wave numbers and about 10 times less WKB normalized shear variance. The vertical wave number spectrum is much flatter, with the deficit in energy occurring primarily for wavelengths greater than 30 m. The horizontal wave number magnitude spectrum of horizontal kinetic energy can be estimated only for velocity fluctuations with vertical wavelengths near 40 m. Combined with estimates of kinetic and potential energy, this determines the distribution of energy between motions with various types of dynamics in this band. Near‐inertial frequency internal waves account for about two‐thirds of the energy. The data suggest such waves are generated during storms. Velocity fine structure accounts for about a quarter of the kinetic energy and has an absolute energy level comparable to that found at mid‐latitudes. It is dynamically distinct from internal waves and has a Rossby number near or greater than unity. High frequency internal waves account for a small fraction of the energy but carry most of the superinertial potential energy.
Equatorial and off‐equatorial fine‐scale and large‐scale shear variability at 140°W
Authors: Peters Hartmut , Gregg Michael C. , Sanford Thomas B.
Published: February 2008

In April 1987, the mean vertical shear was unusually low in the upper 350 m at 0°N, 140°W. The surface winds, the equatorial undercurrent, and the 20–30 day instability waves were at seasonal minima, and the equatorial zone was recovering from an El Niño event which had suppressed the undercurrent earlier that year. Unexpectedly, fine‐scale fluctuations with a vertical wavelength of 40 m and a period of about 12 days produced shears up to 0.03s−1, in contrast to the 0.01 s−1 related to the undercurrent. This “shear wave” accounted for most of the shear between 40 and 120 m. Its space and time scales are consistent with the dispersion characteristics of Doppler‐shifted equatorial inertia‐gravity waves. Turbulence measurements indicate that the shear wave was strongly damped on a time scale of one wave period or less. These equatorial observations differ markedly from those made at the same place in November 1984, when surface winds, the undercurrent, and instability waves were strong and when there was no shear wave. Measurements taken in transit to and from the equator show an increase in internal wave energy and shear toward the equator that can be described by a Garrett and Munk spectrum modified to have constant spectral density and increasing bandwidth as the Coriolis parameter approaches zero.
On the estimation of parameters of hydraulic models by assimilation of periodic tidal data
Authors: Das S. K. , Lardner R. W.
Published: February 2008

The estimation of both constant and position‐dependent parameters in a sectionally integrated hydrodynamical model of tidal flow is considered. It is assumed that the parameters of concern are bottom friction coefficients and water depth and that the available data consists of measured periodic values of water surface height at certain stations. The adjoint equation formulation is used to obtain the parameter equations, and several optimization algorithms are examined and compared for the iterative refinement of the parameter values. An empirical connection is established between the number of parameter values estimated and the minimum number of data stations required to estimate them.