bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of heat transfer processes and geothermal pattern in the Alberta Basin, Canada
Authors: Bachu Stefan
Published: February 2008

The transfer of heat from the crystalline basement of sedimentary basins to the atmosphere can be influenced to different degrees by the movement of formation waters within the complex structure of aquifers and aquitards in the basin. Past studies of the geothermal regime in the Western Canada Sedimentary Basin have shown the existence of a low geothermal gradient (low heat flux area) in the foothills region of southwestern Alberta, and of a high geothermal gradient (high heat flux area) in the lowlands in northeastern Alberta, close to the Precambrian Shield. These distributions of geothermal gradients and heat fluxes were attributed to the effects of basin wide groundwater flow. Hydrogeological studies in selected parts of the basin, and dimensional analysis applied to heat transfer processes show that the permeability of the sediments, and indeed the fluid velocities, are too low to play a significant role in the transport of terrestrial heat in the Alberta part of the Western Canada Sedimentary Basin. On a regional scale, the actual distributions of the heat flux and geothermal gradients are probably due to crustal thickening and/or increased radiogenic heat generation in the basement. Thermal anomalies, which may be due to granitic intrusions, are superimposed over this trend. At an intermediate scale, the geothermal field is controlled by topography, stratigraphy, and lithology of the sediments. Only on a local scale is the convection of heat important.
Fracture‐accommodated strain in the Diamond Joe Stock, Arizona
Authors: Rogers Ralph D. , Gerla Phil
Published: February 2008

Fractures filled with hydrothermal minerals form a dominantly radial pattern in the Diamond Joe stock. These fractures record a portion of the deformation experienced by the pluton from the time that it was emplaced until it cooled to ambient conditions. In individual outcrops, fracture orientations are very consistant, to within ± 10°. Where fractures cross k‐feldspar phenocrysts, no shear offset is observed, indicating that the fractures have opened by dilation. We have measured the apertures of these veins and define a two‐dimensional deformation gradient matrix to characterize the uniaxial strain accommodated by each fracture. We assume that each fracture represents one increment of strain and superimpose all increments in an individual outcrop to compute the fracture‐accommodated strain for that outcrop. We calculate the orientation and magnitude of the principal strains for each outcrop as well as the area change accommodated by the fractures. All of the outcrops have experienced a decrease in area that ranges from <1% to >5%. A portion of this area change is interpreted as thermal contraction. However, in the central part of the pluton, additional contraction is present. In this portion of the pluton the amount of strain, and area change, correlates well with fracture abundance. We interpret this additional contraction to result from volume changes associated with crystallization and/or vapor exsolution during crystallization of the pluton.
Absence of strain accumulation in the Shumagin Seismic Gap, Alaska, 1980–1987
Authors: Lisowski M. , Savage J. C. , Prescott W. H. , Gross W. K.
Published: February 2008

Measurements of the deformation of a trilateration network in the Shumagin seismic gap in the interval 1980–1987 failed to detect any significant strain accumulation (observed extension rate in the direction of plate convergence 0.00±0.03 μstrain/yr). Dislocation models of the subduction process and measurements at a comparable network at a known seismic subduction zone (Nankai Trough, Japan) suggest that a rate of the order of −0.2 μstrain/yr should have been observed if the main thrust zone beneath the Shumagin Islands were locked. The simplest explanation of the observed absence of strain accumulation in the Shumagin seismic gap is that the main thrust zone beneath the Shumagin Islands is not presently locked. Other possible explanations depend upon very particular circumstances.
Laboratory modeling of convection in magma chambers: Crystallization against sloping floors
Authors: Martin Daniel , Campbell Ian H.
Published: February 2008

Crystallization against inclined floors in magma chambers commonly releases a light depleted fluid which drives compositional convection. We describe the fundamental fluid mechanics of this process and derive equations describing the buoyant compositional boundary layer adjacent to the top of the crystal pile. The question of whether magma chambers become zoned as a result of compositional convection from sloping floors, as has been previously suggested, is addressed in a laboratory study. Because crystal sizes in magma chambers are usually smaller than the thickness of the compositional boundary layer, an appropriate laboratory model is thermal convection from a smooth, sloping heat exchanger. We investigate the circumstances under which this configuration leads to thermal stratification and find that only when the experimental tank has an extension with an unheated floor does a filling box process lead to significant stratification. This geometry, however, is geologically unrealistic, requiring that crystallization be confined to only part of the floor. We conclude that compositional convection from inclined floors will not lead to compositional stratification in magma chambers. Experiments investigating the same problem by cooling and crystallizing aqueous solutions are misleading partly because the size of crystals grown from aqueous solutions is extremely large compared with the thickness of the compositional boundary layer, whereas this is not usually true in magma chambers. This difference has a major influence on the nature of convection and we suggest that caution should be applied in extending the results of aqueous solution experiments involving crystallization to magma chambers. The petrological consequences of this work are (1) the fluid mechanics of crystallization at an inclined floor and horizontal floor are similar and suggestions, made elsewhere, concerning the geological implications in the latter case apply equally to the sloping floor case, (2) large volumes of homogeneous magma can exist in a magma chamber when crystallization is taking place against an inclined floor, and (3) models describing the crystallization of layered intrusions which involve the interaction of crystallization against an inclined floor with compositionally stratified magma require the stratification to be established by an external process.
An investigation of the complexity of the earthquake source time function using dynamic faulting models
Authors: Das Shamita , Kostrov B. V.
Published: February 2008

By constructing more than 30 numerical models of spontaneous earthquake rupture on heterogeneous faults, the complexity of the far‐field pulse shapes (the source time function), that is, the pulse consisting of several distinct subpulses, is investigated. The influence of multiple strength barriers and/or dry friction on the far‐field waveforms has been studied using the simple model of a square fault with uniformly spaced square barriers. A central asperity is allowed to break to initiate the dynamic process. It is found that the pulse duration is shorter the greater the density of barriers (ratio of barrier size to barrier spacing). The presence of friction has been found to have three different consequences. First, the slipping area is reduced giving rise to a shorter pulse duration. Second, the rate of slip propagation is reduced which increases the pulse duration. Third, the frictional stress drop, if present, increases the amplitude of the pulse. The interaction of these three effects together with the effect of the barriers results in different pulses depending on the relative role of these effects. Breaking of multiple asperities with relatively short time delay between their fracturing produced a single pulse with ripples in its rising portion, the pulse duration depending on the total time required for the asperities to break. Breaking of, say, two asperities with sufficient time delay between them produced a double pulse which is in accord with the asperity model of complex earthquakes. In one case of a fault with barriers and friction, an additional peak was observed in some directions from the source, the peak form and amplitude being essentially the same as those of the peaks due to breaking of asperities. One may conclude that each subpulse observed on a complex far‐field pulse is not necessarily interpretable as being due to the failure of an asperity on the fault. Furthermore, for the subpulses which can be related to asperity fracture, the duration is not determined by the size of the corresponding asperity but rather by the physical conditions on the fault surrounding the asperity.
The effect of the terrain on airborne gravity and gradiometry
Authors: Tziavos I. N. , Sideris M. G. , Forsberg R. , Schwarz K. P.
Published: February 2008

With the development of airborne gravimetry and gradiometry, efficient algorithms are needed to compute gravitational effects of the topography. In this paper, such algorithms based on the fast Fourier transform (FFT) are developed and used for studying effects of terrain representation (mass lines or prisms), height data resolution needed, and the number of expansion terms required to approximate the basically nonlinear terrain effect integrals. In addition to airborne gravity and gradiometry, improved formulas are also given for terrestrial gravity, i.e., the classical terrain correction. Results of the method are given for a very rough terrain in the Kananaskis area of the Canadian Rocky Mountains assuming a constant density value of 2.67 g/cm3. Comparisons have been done for height data grid spacings of 0.1, 0.2, 0.5, and 1 km at two different flight altitudes. Results indicate that to obtain gravity or gravity gradient effects at accuracies of 0.3–0.5 mGal and 1 Eotvos (E) unit, respectively, a 0.5 km×0.5 km grid is needed for a flying altitude of 1 km above the topography. Lowering the flight altitude to 600 m, a typical value for gradiometer surveys, requires height data given at a grid spacing of 0.25 km, as well as the inclusion of third‐order terms in the expansions of the terrain correction integrals. The advantage of the FFT method is computational speed: the typical CPU time for computing gravity and all gravity gradient tensor elements on a 114×74 point grid was about 15 s on a large mainframe (CDC Cyber 175).
Geologic evidence for a magma chamber beneath Newberry Volcano, Oregon
Authors: MacLeod Norman S. , Sherrod David R.
Published: February 2008

At Newberry Volcano, central Oregon, more than 0.5 m.y. of magmatic activity, including caldera collapse and renewed caldera‐filling volcanism, has created a structural and thermal chimney that channels magma ascent. Holocene rhyolitic eruptions (1) have been confined mainly within the caldera in an area 5 km in diameter, (2) have been very similar in chemical composition, phenocryst mineralogy, and eruptive style, and (3) have occurred as recently as 1300 years ago, with repose periods of 2000–3000 years between eruptions. Holocene basaltic andesite eruptions are widespread on the flanks but are excluded from the area of rhyolitic volcanism. Basaltic andesite in fissures at the edge of the rhyolite area has silicic inclusions and shows mixed basalt‐rhyolite magma relations. These geologic relations and the high geothermal gradient that characterizes the lower part of a drill hole in the caldera (U.S. Geological Survey Newberry 2) indicate that a rhyolitic magma chamber has existed beneath the caldera throughout the Holocene. Its longevity probably is a result of intermittent underplating by basaltic magma.
High‐resolution seismic tomography of compressional wave velocity structure at Newberry Volcano, Oregon Cascade Range
Authors: Achauer Ulrich , Evans John R. , Stauber Douglas A.
Published: February 2008

Compressional wave velocity structure is determined for the upper crust beneath Newberry Volcano, central Oregon, using a high‐resolution active‐source seismic‐tomography method. Newberry Volcano is a bimodal shield volcano east of the axis of the Cascade Range. It is associated both with the Cascade Range and with northwest migrating silicic volcanism in southeast Oregon. High‐frequency (∼7 Hz) crustal phases, nominally Pg and a midcrustal reflected phase, travel upward through a target volume beneath Newberry Volcano to a dense array of 120 seismographs. This arrangement is limited by station spacing to 1‐ to 2‐km resolution in the upper 5 to 6 km of the crust beneath the volcano's summit caldera. The experiment tests the hypothesis that Cascade Range volcanoes are underlain only by small magma chambers. A small low‐velocity anomaly delineated about 3 km below the summit caldera supports this hypothesis for Newberry Volcano and is interpreted as a possible magma chamber of a few to a few tens of km3 in volume. A ring shaped high‐velocity anomaly nearer the surface coincides with the inner mapped ring fractures of the caldera. It also coincides with a circular gravity high, and we interpret it as largely subsolidus silicic cone sheets. The presence of this anomaly and of silicic vents along the ring fractures suggests that the fractures are a likely eruption path between the small magma chamber and the surface.
Petrology and hydrothermal mineralogy of U.S. geological survey Newberry: 2. Drill core from Newberry Caldera, Oregon
Authors: Keith Terry E. C. , Bargar Keith E.
Published: February 2008

U.S. Geological Survey Newberry 2 was drilled to a depth of 932 m within Newberry caldera. The bottom‐hole temperature of 265°C is the highest reported temperature of any drill hole in the Cascades region of the United States. The upper part of the stratigraphic section penetrated by Newberry 2 consists of caldera fill below which are increasingly more mafic lavas ranging from rhyodacite at 501 m to basalt at 932 m. Measured temperatures shallower than 300 m are less than 35°C, and rock alteration consists of hydration of glass and local palagonitization of basaltic tuffs. Incipient zeolitization and partial smectite replacement of ash and pumice occurred throughout the pumiceous lithic tuffs from 300 to 500 m. Higher‐temperature alteration of the tuffs to chlorite and mordenite occurs adjacent to a rhyodacite sill at 460–470 m; alteration minerals within the sill consist of pyrrhotite, pyrite, quartz, calcite, and siderite. Below 697 m the rocks are progressively more altered with depth mainly because of increased temperature along a conductive gradient from 100°C at 697 m to 265°C at 930 m. Fluid inclusions in quartz and calcite indicate that temperatures in the past have been higher than at present, most likely due to local confining pressures between impermeable lava flows. Flow breccias are more altered than the adjacent dense massive lava flows, regardless of composition, because of their much higher permeability. Hydrothermal minerals in this zone are mainly mixed‐layer chlorite‐smectite, quartz, calcite, and pyrite. Chlorite becomes more abundant than mixed‐layer clays near the bottom of the hole. In the lowest two lava flows, epidote, anhydrite, and scarce hematite occur locally. Alteration and leaching in the basal 2 m are unique and have led to the postulation of a localized two‐phase fluid zone consisting mainly of steam and CO2. The hydrothermal system of Newberry 2 is a simple evolving system associated with the evolution of Newberry Volcano. Only a few localized highly altered intervals where fracturing controlled fluid access occur in the core. There are no crosscutting fractures to indicate multiple hydrothermal systems. Chemical analyses of altered rocks and equivalent fresh rocks indicate that little chemical migration has taken place.
Tapping of melt by veins and dikes
Authors: Sleep Norman H.
Published: February 2008

Abundant cross‐cutting dikes have been observed in mantle peridotites. These dikes are inferred to have formed while the material was a mostly crystalline mush beneath a ridge axis. The presence of these dikes indicates that the mush was able to fracture. The physics of the growth of vein and dikes and the extent to which they tap melt are investigated in this paper. Once small veins are initiated, they tend to open if the fluid pressure within them exceeds the least principal stress. Porous flow into the veins is necessary for them to open. This implies a melt pressure gradient and that the melt pressure in growing veins is less than the regional pressure. The opening rate and pore pressure decrease are obtained considering both porous flow and viscous deformation around the vein. The stress concentration factor driving vein lengthening is largest for veins near the compaction length. If the critical stress concentration factor is sufficiently low, vein growth and tapping of the veins by dikes reduces the melt pressure to near the least principal stress. Then the matrix of a material undergoing deviatoric strain is compacted because the melt pressure is less the “pressure” of the solid. Retention of significant melt directly beneath the axes of fast ridges is expected because the deviatoric strains are low there. However, computed contours of melt fraction are weakly dependent on the efficacy of tapping of melt by dikes. Numerical models indicate that about half of the melt is tapped by dikes if the shape of the conduit is thermally controlled. Tapping of melt by density‐driven porous flow is enhanced if the melt viscosity is low owing to high temperatures. This process, rather than dikes, was probably important in the Archean and is now important beneath hotspot ridges, such as Iceland. Shear strain rates in the asthenosphere are large enough that significant quantities of melt should be extracted if partial melt exists in intraconnected pores and the critical stress concentration factor for vein growth is exceeded.
Numerical studies of dissolution‐induced moving boundary problems from aqueous diffusion in porous media
Authors: Mundell John A. , Kirkner David J.
Published: February 2008

Two fixed‐mesh, finite element formulations are presented for numerically solving the diffusional mass transport equations involving reversible heterogeneous reactions leading to moving boundaries in porous media. Both schemes assume the existence of local equilibrium between the solid and fluid phases and are based on particular forms of the weak variational statement of the governing differential equations. A front‐tracking‐type (FT) method is derived from the selection of the aqueous concentration as the primary variable, whereas a formulation similar to enthalpy‐type (ET) schemes used to simulate heat conduction with phase change results from the selection of the total component concentration as the primary unknown. The results are compared to analytical solutions for one‐ and two‐component systems involving the dissolution of quartz at 550°C and 1000 bars and calcium carbonate at 35°C, respectively. It was found that for problems with significant mass contributions by the reactive solid phase, the FT scheme yields more accurate results than the ET formulation for an equivalent grid and time discretization. While this numerical accuracy is a result of the ability of the FT scheme to capture the discontinuous behavior of the field variables at the moving interface, it is achieved through the use of a more complex numerical algorithm to obtain the converged solution. Realistic, multicomponent systems involving multiple moving boundaries and other chemical reactions increase the complexity of FT formulations, encouraging further development and modification of the ET method to improve its performance.
Ocean bathymetry and mantle convection: 1. Large‐scale flow and hotspots
Authors: Davies Geoffrey F.
Published: February 2008

Numerical models of plate‐scale convection confined to the upper mantle predict large deviations from observed ocean bathymetry, gravity, and geoid, while whole mantle models yield first‐order agreement with these observations. The upper mantle models fail because there is insufficient radioactivity in the upper mantle to explain the surface heat flux, the upper mantle must therefore be heated mainly from below, the resulting hot boundary layer generates buoyant material, and when this buoyant material rises to the base of the lithosphère, it generates large positive anomalies in topography (2 km), the geoid (30 m), and gravity (50 mGal). The concept of the magnitude of a hotspot swell is introduced: it is the rate at which new topography is created, expressed as a volume per unit time, and is a measure of the buoyancy and heat fluxes in the underlying plume. By this measure the major Pacific hots pots dominate the Earth's total plume flux, but plumes account for less than 10% of the Earth's total heat flux. This is comparable to the amount of heat likely to be coming from the core and supports the idea that plumes originate at the core mantle boundary and not at the 670‐km transition. These results indicate that mantle convection is dominated by a plate‐scale flow which penetrates throughout the mantle, with a secondary mode involving plumes rising from a weak thermal boundary layer at the bottom of the mantle.
Static sources of elastic deformation in a homogeneous half‐space
Authors: Kagan Y. Y.
Published: February 2008

After determining the relationship between cylindrical and spherical solid vector harmonics, we arrive at the complete and effective solution for the static deformation due to a multipolar source of arbitrary order in a homogeneous isotropic half‐space. Using the correspondence between spherical harmonics and seismic moment tensors obtained earlier, we construct a catalog of mirror image sources which yield zero traction on the plane. For each angular order number l the total number of these images is 15 for a general case, of which the first spheroidal source has seven images, the toroidal source has four images, and the second spheroidal source also has four images. We derive the conditions under which image point sources are converted into semi‐infinite line nuclei. We analyze several special cases of deformation in a half‐space, namely, axisymmetric, generalized Boussinesq‐Cerruti, thermoelastic deformations. We present several general and specific models of finite sources of elastic displacement in a half‐space and discuss possibilities for inverting complex extended sources of elastic deformation. The nonuniqueness of the inversion can be estimated objectively.
Paleomagnetic investigation of some volcanic rocks from the McMurdo volcanic province, Antarctica
Authors: Mankinen Edward A. , Cox Allan
Published: February 2008

Paleomagnetic data for lava flows from sporadic but long‐lived eruptions in the McMurdo Sound region are combined with previously published geologic and geochronologic data to determine the general eruptive sequence of the area. Lava flows in the Walcott Bay area were erupted during the Gauss Normal, Matuyama Reversed, and Brunhes Normal Polarity Chrons. The youngest flows on Black Island probably erupted near the boundary between the Gilbert and Gauss chrons. The most recent activity was concentrated on the volcanic edifices of Mounts Morning and Discovery and on Ross Island where Mount Erebus is currently active. Combining virtual geomagnetic poles calculated for 39 units sampled during this study with those of eight flows that were published previously yields a mean paleomagnetic pole at 87.3°N, 317.3°E (α95 = 6.3°). The ancient geomagnetic field dispersion about this mean pole is 23.5°, with upper and lower limits of 95% confidence equal to 27.4° and 20.5°, respectively. This value probably is a reasonable estimate of secular variation for the Antarctic continent during Pliocene and Pleistocene time.
Paleomagnetic constraints on rotation within Mount Abbot quadrangle, central Sierra Nevada, California
Authors: Ross Stephanie L.
Published: February 2008

The regional photolineament pattern in Mount Abbot quadrangle in the central Sierra Nevada is sigmoidal in plan view. The fractures in the southwest section of the quadrangle, near Florence Lake, strike N10°–20°E, while those in the central section, along Bear Creek, strike N50°–70°E and those in the northeast strike N25°–40°E. A paleomagnetic investigation was undertaken to determine if this sigmoidal pattern was formed by kinking of originally straight photolineaments after the rock mass cooled below the blocking temperature. If the inner limb of the kink rotated 40°–60° clockwise with respect to the outer limbs, then there should be a similar difference in the paleomagnetic declinations between the Florence Lake and Bear Creek areas. The paleomagnetic results indicate a clockwise difference in declination of 13.4°±7° of the Bear Creek area with respect to Florence Lake. Thus the sigmoidal pattern could not have formed entirely by rotation after cooling below the blocking temperature. It is also possible that the observed declination discrepancy is due to tilting, apparent polar wander, or paleomagnetic dispersion instead of rotation. Sigmoidal patterns, formed by kink folding, occur on the outcrop scale with inner limb rotations of 15°–30°; three of these kinks were sampled to test the paleomagnetic method in an area of demonstrable rotation. The data from these sites indicate that although no difference in declination is evident, a reheating event may have occurred which could have removed any rotation from the paleomagnetic record. Reheating, possibly due to the circulation of hydrothermal fluids, may have been causally related to the outcrop scale kinking, since reheating is not observed away from the outcrop scale kinks. The data further indicate that there may have been continuous deformation of the rock mass during the formation of the kinks.
Paleogeographic maps of the continents bordering the Indian Ocean since the Early Jurassic
Authors: Besse Jean , Courtillot Vincent
Published: February 2008

Paleomagnetic data from the African plate from the Early Jurassic onward are critically re‐evaluated and updated, resulting in a set of 30 paleopoles, from which an apparent polar wander path is derived using 20 Ma intervals. This path is then used, in conjunction with kinematic models of the Indian and Atlantic oceans, to place other continents bordering the Indian Ocean (Eurasia, India), islands and plateaus (Madagascar, Seychelles‐Mascaréne), and blocks involved in the collision of India with Eurasia (south Tibet, Indochina) in an “absolute” paleogeographic frame fixed with respect to Africa. Paleogeographic maps are presented for the Early Jurassic (pre‐breakup), the Jurassic‐Cretaceous boundary (anomaly Ml5), the Early Cretaceous (M2), the Late Cretaceous (A 34), the Cretaceous‐Tertiary boundary (A 29), the Early Eocene (A 24), the middle Eocene (A 21) and the Oligocene‐Miocene boundary (A 6). Observed and predicted paleomagnetic directions for landmasses other than Africa (i.e., for western Europe, China, Korea, India, and Australia, which are not used in deriving the maps) are found to be in very satisfactory agreement. The quality of reconstructions, as measured by rms discrepancies between observed and predicted paleolatitudes, is of the order of 5°. The main features of the paleogeographic maps are discussed, with the aim of relating oceanic and on‐land geodynamic events. The constraints provided by Tibetan data on the India‐Asia collision, and the 20 Ma reorganization of geology and plate boundaries in a band extending from the south China Sea, through Tibet, to the Red Sea system are discussed in more detail. We emphasize the episodic nature of the breakup of Gondwanaland and formation of the Indian Ocean, and subsequent interaction with Eurasia, in which six major discrete events are identified, separated by ∼30 Ma phases of rather uniform plate motion.
Analysis of P and S Wave Vertical Seismic Profile Data From the Salton Sea Scientific Drilling Project
Authors: Daley Thomas M. , McEvilly Thomas V. , Majer Ernest L.
Published: February 2008

As part of the Salton Sea Scientific Drilling Project at California State well 2–14, vertical seismic profile (VSP) data were collected from P and S wave sources at two distances from the well. Use of a three‐component geophone, along with rotation of the recorded data traces into a wave front‐based coordinate system, allows analysis of many aspects of seismic wave propagation properties around the well. Standard VSP analysis techniques were used to measure interval P and S wave velocities and to identify reflecting horizons both within and below the survey interval (from 455 to 1735 m). A reflection from below the survey interval, seen with both P and S sources, seems to be associated with a fractured reservoir near 2100 m. Indications of fracturing were observed, including vertical scattering of P waves from a zone near 915 m. Orthogonally polarized shear waves were generated at each offset to study anisotropy by travel time measurement and particle motion analysis of the shear wave arrivals. Three‐component particle motion analysis of shear wave arrivals was found to be effective for characterizing the subtleties in the S wave splitting throughout the various zones in the well. The SHt source (horizontal and transverse to the well) produced complicated, elliptical particle motion while the SV source (in‐line with the well)produced linear particle motion. The difference in linearity of particle motion from orthogonally polarized shear wave sources was unexpected and may be related to regional tectonics. Anomalous zones may be related to transition depths in the Salton Sea geothermal field. Travel time difference between SV and SH waves, while clearly observable, indicates only about 1% average anisotropy.
Mineral Reactions in Altered Sediments From the California State 2–14 Well: Variations in the Modal Mineralogy, Mineral Chemistry and Bulk Composition of the Salton Sea Scientific Drilling Project Core
Authors: Shearer C. K. , Papike J. J. , Simon S. B. , Davis B. L. , Laul J. C.
Published: February 2008

X ray diffraction‐reference intensity method (XRD‐RIM) modes, whole rock chemistry, and mineral chemistry were determined on 36 samples from the Salton Sea Scientific Drilling Project (SSSDP) core. These samples display a wide variation in grain size (shale to sandstone), bulk composition (SiO2 = 36.2–81.2 wt %; Al2O3 = 5.1–17.1 wt %; CaO = 1.2–21.1 wt % and (total Fe as Fe2O3) Fe2O3/(Fe2O3 + MgO) wt ratio = 0.48–0.98) and mineral assemblage. Seven mineral assemblages have been identified in this sample suite: (1) chlorite + calcite + quartz + plagioclase ± K‐feldspar ± illite (476–2559 m), (2) chlorite + quartz +plagioclase + K‐feldspar (918.7–1984.3 m), (3) chlorite + epidote + quartz ± plagioclase ± calcite ± Kfeldspar ± illite ± magnetite (905.6–2954.8 m), (4) chlorite + biotite ± calcite + quartz + plagioclase + K‐feldspar (2700.0–2819.2 m), (5) epidote + quartz ± plagioclase (1424.2–2954.8 m), (6) epidote + actinolite + quartz ± biotite ± chlorite ± plagioclase ± K‐feldspar ± anhydrite (2880.2–3021.2 m), and (7) epidote + clinopyroxene + actinolite + quartz (2484.7 m). These assemblages define three metamorphic zones in the SSSDP well: calcite‐chlorite zone, biotite zone, and actinolite zone. Accompanying these transitions in mineral assemblages are changes in the Si‐Al ordering in K‐feldspar and an increase in the anorthite component in plagioclase. Fe/(Fe+Al) in epidote shows very little systematic variation downhole (in the sediments) but can be highly zoned and shows considerable systematic variation between vein systems and sedimentary pore space. Quantitative modal mineralogy and whole rock chemistry can be utilized in determining mineral reactions and their importance and in evaluating the evolution of mineral assemblages. For example, epidote‐forming reactions precede actinolite‐epidote‐ and biotite‐epidote‐forming reactions in numerous bulk compositions and result in the formation of high modal abundances of epidote. The production of fluids with high XCO2 from epidote‐, actinolite‐, biotite‐, and diopside‐forming reactions in rocks in equilibrium with fluids with CO2 < 0.1 indicates extensive interaction between fluid and the shales. This is also indicated by the leaching of Cu, Zn, and Mn and the enrichment of Sr. Numerous elements, however, do not show evidence of exchange between sediments and fluid.
Structure and Stratigraphy Beneath a Young Phreatic Vent: South Inyo Crater, Long Valley Caldera, California
Authors: Eichelberger John C. , Vogel Thomas A. , Younker Leland W. , Miller C. Dan , Heiken Grant H. , Wohletz Kenneth H.
Published: February 2008

An 861‐m‐long hole has been cored on a slanted trajectory that passed directly beneath South Inyo Crater in the west moat of Long Valley Caldera, California. The purpose of the hole was to investigate the magmatic behavior that led to surface deformation and phreatic activity during the 600‐year‐old eruption of the Inyo vent chain. The hole was sited 216 m southwest of the crater, passed beneath its center at a depth of 566 m, and terminated 79 m northeast of the crater center at a depth of 810 m. Metamorphic basement was encountered at a depth of 779 m. The volcanic and sedimentary sequence consists solely of post‐Bishop Tuff caldera fill, including 319 m of moat basalt and 342 m of early rhyolite, and is nearly 900 m thinner than in a Unocal Corporation well 900 m to the southeast. Apparently, a major fault lies between the two holes and forms part of the western structural boundary of the caldera, 3–4 km inboard of its topographic boundary. Breccia zones that intrude the caldera fill were intersected at 12.0–9.3 m and 1.2–0.8 m SW and 8.5–25.1 m NE of the crater center. The largest breccia unit is symmetrically zoned from margins rich in vesicular rhyolite and locally derived rhyolite wall rock, with most clasts ≤ 0.1 m, to a center of up to 50 vol % basalt, with individual basalt clasts to 1 m in intersected length. The basalt appears to be an early feeder for the moat basalt sequence. The vesicular rhyolite is chemically distinct from both the rhyolite wall rock and previously recognized 600‐year‐old Inyo eruptives but matches fresh‐appearing pyroclasts in the crater ejecta. This component of the breccia is therefore interpreted as representing juvenile magma that vesiculated and fragmented as it rose along the margins of an older basalt intrusion. During this process, wall rock fragments were ejected from depths extending to at least 800 m. Progression to a magmatic eruption was probably prevented by rapid influx of groundwater into the feeder. The chemical data do not support the hypothesis that a single, simple dike exists under the entire segment of the Inyo chain that was active 600 years ago. If such a dike exists, it apparently tapped different magma chambers along its ∼ 10‐km length. The excess of surface extension over that needed to accommodate the intruded juvenile material may reflect the presence of a larger intrusion at depth, below the level of fragmentation, or the contribution of tectonic activity to the surface deformation.
Slab Decoupling in the Tonga Arc: The June 22, 1977, Earthquake
Authors: Lundgren Paul R. , Okal Emile A.
Published: February 2008

The June 22, 1977, earthquake in the southern Tonga arc is a large normal faulting event with downdip rupture propagation on a steeply dipping fault plane. We use a standard least squares inversion of P and SH waves for the source time function to determine a source consisting of two subevents located at depths of 40–55 and 105–125 km and a total time duration of 50 s. The ratios of the spectral amplitudes of the radial modes 0S0, 1S0 and 2S0 independently require a centroid depth of at least 100 km at their lower frequencies. The body wave analysis suggests a moment of 1.04 × 1028, and the 0S0 spectral amplitude suggests a value of 1.8–2.0 × 1028 dyn cm. The emerging picture is that of a complete rupture of the sinking lithospheric slab, starting from 40 km depth and propagating down to 125 km. This indicates that the decoupling of the overriding lithosphere from the slab in this area of the Tonga arc takes place by gravitational pull.
Structural Pattern of the Galapagos Microplate and Evolution of the Galapagos Triple Junctions
Authors: Lonsdale Peter
Published: February 2008

A 13,000‐km2 microplate intervenes to prevent the junction of the Nazca, Cocos, and Pacific plates in the eastern equatorial Pacific Ocean west of the Galapagos Islands. The microplate is rotating clockwise at 6°/m.y. about an “absolute” pole 400 km southeast of its center, so that although its northern margin has a similar motion to adjacent parts of the Cocos plate, its southern margin is 50% slower. The perimeter of the Galapagos microplate is best defined by bathymetry, and has been surveyed with Sea Beam and examined by Deep Tow. Two of the three triple junctions on its margins are ridge‐ridge‐ridge (RRR) junctions 160 km apart on the crest of the East Pacific Rise (EPR); the most like a classic RRR junction is the Pacific‐Nazca‐Galapagos intersection at 1°10′N, where the EPR axis is joined by a narrow volcanic ridge 70 km long and 1 km high. The Cocos‐Nazca‐Galapagos junction at the west end of the Cocos‐Nazca spreading center is inferred to be of the RFF type, but the “transform fault” branches are represented by large nontransform offsets. Lack of stable transform faults is related to rapid westward migration of the junction, caused by lengthening of the Cocos‐Nazca axis at 60–70 mm/yr. This rate, which was once required to maintain a stable Nazca‐Cocos‐Pacific RRR junction, has persisted even after birth of the microplate about 1 Ma. Continuing propagation of the Cocos‐Nazca axis has opened up the Hess Deep rift basin in the eastern EPR flank in advance of the eruptive propagator tip. This rift valley has a relief of more than 3 km from its uplifted rims to the floor, where severe attenuation exposes basal layers of the EPR crust. The similar Dietz Deep rift basin, at the southeastern margin of the microplate 70 km from Hess Deep, was formed when a lengthening Galapagos‐Nazca spreading axis rifted through another part of the EPR flank.
INTERMEDIATE PERIOD RESPONSE OF WATER LEVELS IN WELLS TO CRUSTAL STRAIN: SENSITIVITY AND NOISE LEVEL
Authors: Rojstaczer Stuart
Published: February 2008

The response of water levels in wells to earth tides indicates that wells can be used to detect small crustal strain. Vertical groundwater flow between the well intake and the water table can significantly attenuate this sensitivity. The attenuation of strain sensitivity as a function of frequency can be inferred from the response of water wells to atmospheric loading. For the wells examined in this study, significant attenuation due to water table drainage can occur when strain accumulates gradually over periods of days to weeks. Despite the presence of attenuation the wells are still sensitive strain meters over this range in period. At a frequency of 2.5 cycles/day, the noise level of the water level records examined is −130 dB relative to 1 strain2/Hz. This noise level is about the same as that reported for dilatometers and wire strain meters, but is at least 10 dB higher than that reported for laser strain meters. In the frequency band of 0.025 to 2.5 cycles/day, the noise level of the water level records examined increases roughly 25 dB‐per‐decade decrease in frequency. Some of this noise is due to the influence of atmospheric loading. When the effects of atmospheric loading are removed from the record, the noise level is reduced to roughly 20 dB‐per‐decade for frequencies above 0.08 cycles/day, a rate typical of high‐quality strain meters. For periods slightly less than a month, the wells have a lower noise level in terms of areal strain than that of the best geodetic distance measurements.
An Experimental Study of Permeability and Fluid Chemistry in an Artificially Jointed Marble
Authors: Marone Chris , Rubenstone James , Engelder Terry
Published: February 2008

To examine the amount of rock dissolution accompanying changes in joint permeability, deionized water was forced through axially split cylindrical samples of Vermont marble, subjected to a confining pressure of 60 MPa. Surface preparation and loading conditions varied among experiments during which permeability and fluid chemical composition were monitored with time. Time dependent permeability and fluid composition for freshly polished joints differed from joints which had been “etched” by the fluid during previous runs. For freshly polished surfaces, permeability decreased and fluid chemical concentrations increased during the first 50 to 100 hours of fluid flow. For the water‐etched surfaces, permeability was not time dependent, and a steady state chemical composition was reached after 20 to 40 hours. A calculation of the steady state chemical concentrations for coexisting calcite and dolomite in deionized water, at a fluid pressure of 10 MPa and a confining pressure of 60 MPa, shows that the steady state values reached in our experiments were supersaturated. Supersaturation was probably due to enhanced solubility at contact points together with precipitation inhibition. Our data indicate that fluid flow through fresh fractures causes dissolution of contact points and permeability reduction, depending on the morphology and state of the fracture surfaces.
Effect of far‐field slope on morphologic dating of scarplike landforms
Authors: Hanks Thomas C. , Andrews D. J.
Published: February 2008

The principal finding of this paper is that the far‐field slope has a first‐order effect on model age determinations of scarplike landforms in weakly consolidated terrains. Observationally, this can be demonstrated in two ways using the Lake Bonneville and Lahontan shoreline scarps as separate and combined data sets. Use of the reduced scarp slope, tan θs ‐ b (where θs is the maximum scarp angle and b is the far‐field or fan slope), instead of tan θs alone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope‐offset plot for both the separate and combined data sets. Theoretically, the argument can be put even more strongly, at least within the range of linear and nonlinear diffusion models that we consider here together with a mathematical transformation of the empirical approach of R. C. Bucknam and R. E. Anderson: When one correctly takes into account the far‐field slope, one will basically get the same age determination no matter which of these models one uses; conversely, without accounting properly for the effect of far‐field slope, one is virtually guaranteed to get an erroneous age determination, no matter which model is used.
Time‐dependent convection with non‐Newtonian viscosity
Authors: Christensen Ulrich R. , Yuen David A.
Published: February 2008

We have used a numerical model of bottom‐heated, two‐dimensional convection in boxes of aspect ratios 2.5 and 4.0 to study the differences in time‐dependent convection between Newtonian and stress‐dependent viscosity. The onset of time dependence due to boundary layer instability is found at approximately the same effective Rayleigh number for both rheologies. However, with increasing Rayleigh number the temporal and spatial fluctuations in the flow field become much more pronounced with non‐Newtonian rheology; also, the tendency for breakup of long cells into smaller ones is stronger. The presence of stress‐dependent rheology can cause long‐wavelength lateral viscosity variations up to an order of magnitude; this result could have strong implications for the interpretation of mantle viscosity from postglacial rebound.
An evaluation of thermoluminescence dating of paleoearthquakes on the American Fork segment, Wasatch fault zone, Utah
Authors: Forman Steven L. , Machette Michael N. , Jackson Michael E. , Maat Paula
Published: February 2008

The accuracy of the thermoluminescence (TL) method for dating Holocene paleoearthquakes was tested on fine‐grained sediments from three trenches excavated across the American Fork segment of the Wasatch fault zone, Utah. Sediments exposed in the trenches targeted for TL dating include A horizons of buried soils, loessial silts, and sag pond muds. This study indicates that these sediments were fully light‐bleached prior to deposition and are suitable for TL dating. Equivalent dose estimates by the regeneration, total‐ and partial‐bleach methods are in general agreement, and mean TL age estimates are concordant with radiocarbon ages. Combined radiocarbon and TL age estimates indicate three major episodes of faulting at 5.3±0.3, 2.6±0.3 and 0.5±0.2 kyr ago.
A self‐consistent model of melting, magma migration and buoyancy‐driven circulation beneath mid‐ocean ridges
Authors: Scott David R. , Stevenson David J.
Published: February 2008

Numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle‐derived magma at mid‐ocean ridges. Our model includes the following effects: melting due to decompression, magma migration by percolation, and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge. The distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material. The calculation of densities is based on a simple petrological model in which garnet‐spinel lherzolite melts to form a basaltic liquid and a harzburgite residue. We find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis, faster than the plate velocity, is confined laterally by stably stratified residual material beneath the newly formed plates. This effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included. Our results provide a simple explanation for the narrowness of the zone of crustal formation at mid‐ocean ridges. The model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced, perhaps giving rise to along‐axis variations in the character of seafloor spreading. The narrow, rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus. This may allow the liquid at depth to segregate into macroscopic channels, which would account for the consensus from experimental petrology that the liquids parental to MORB are derived from well below the base of the crust.
The 1759 Earthquake in the Bekaa Valley: Implications for earthquake hazard assessment in the Eastern Mediterranean Region
Authors: Ambraseys N. N. , Barazangi Muawia
Published: February 2008

Analysis of macroseismic data based on primary sources for large, though infrequent, historical earthquakes (Ms>6.5) that occurred along an approximately 350‐km‐long segment of the northern part of the Dead Sea fault system primarily in Lebanon and Syria for the period 1100–1988 reveals the following: (1) Ten events occurred in three relatively short periods (tens of years) with repeat times of 200–350 years; (2) the events most probably broke this north segment of the Dead Sea fault system, possibly including the westernmost segment of the East Anatolian fault system near the border between Syria and Turkey; (3) the lack of such large events during the past 100 years should not be interpreted to minimize potential earthquake hazard in this region; and (4) the Ms ≈ 7 plus earthquake on November 25, 1759, almost certainly produced surface faulting probably along the Yammouneh fault in the Bekaa valley and caused heavy destruction with great loss of life in numerous villages and towns, including Safad, Damascus, Beirut, and Baalbek. This main event was preceded by a Ms ≈ 6 plus foreshock on October 30, 1759, in the southern part of the epicentral area of the main shock near the towns of Safad and Qunaitra, which were almost totally destroyed with considerable loss of life.