bluuebunny/crossref_metadata_2025 huggingface dataset:
Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry
Authors: Smith Walter H. F. , Sandwell David T.
Published: February 2004

The southern oceans (south of 30°S) are densely covered with satellite‐derived gravity data (track spacing 2–4 km) and sparsely covered with shipboard depth soundings (hundreds of kilometers between tracks in some areas). Flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave‐lengths 15–160 km, if sediment cover is thin and seafloor relief is moderate. At shorter wave‐lengths, the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface; at longer wavelengths, isostatic compensation cancels out most of the gravity field due to the seafloor topography. We combine this theory with Wiener optimization theory and empirical evidence for gravity noise‐to‐signal ratios to design low‐pass and band‐pass filters to use in predicting bathymetry from gravity. The prediction combines long wavelengths (>160 km) from low‐pass‐filtered soundings with an intermediate‐wavelength solution obtained from multiplying downward continued, band‐pass‐filtered (15–160 km) gravity data by a scaling factor S. S is empirically determined from the correlation between gravity data and existing soundings in the 15–160 km band by robust regression and varies at long wave‐lengths. We find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99% level, and S may be related to the density of seafloor materials in these areas. The prediction has a horizontal resolution limit of 5–10 km in position and is within 100 m of actual soundings at 50% of grid points and within 240 m at 80% of these. In areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features. Images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts. Because the prediction relies on the gravity field at wavelengths <160 km, it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them. Therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation. The prediction is available from the National Geophysical Data Center in both digital and printed form.
Poroelastic stressing and induced seismicity near the Lacq gas field, southwestern France
Authors: Segall Paul , Grasso Jean‐Robert , Mossop Antony
Published: February 2004

Hundreds of shallow, small to moderate earthquakes have occurred near the Lacq deep gas field in southwestern France since 1969. These earthquakes are clearly separated from tectonic seismicity occurring in the Pyrenees, 25 km to the southwest. The induced seismicity began when the reservoir pressure had declined by ∼30 MPa. Repeated leveling over the field shows localized subsidence reaching a maximum of 60 mm in 1989. Segall (1989) suggested that poroelastic stressing, associated with volumetric contraction of the reservoir rocks, is responsible for induced seismicity associated with fluid extraction. To test this model, we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields. We find that the relationship between average reservoir pressure drop and subsidence is remarkably linear, lending support to the linear poroelastic model. Displacements and stresses are computed based on a priori knowledge of the reservoir geometry, material properties, and reservoir pressure changes. The computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling. Stress perturbations accompanying gas extraction, computed using the same parameters, are found to be ∼0.2 MPa or less. Changes in Coulomb failure stress are computed assuming that slip occurs on optimally oriented planes. The predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compressive stress axis is vertical. Accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state.
Introduction to the Special Section on the Transition Zone
Authors: King Scott D.
Published: February 2004

“The transitional layer appears to hold the key to a number of major geophysical problems.” This often quoted phrase, from the abstract of Birch's [1952, p. 227] work on the transition zone, is still true today, some 42 years later. In fact, the many studies related to the transition zone since Birch's work have often raised more challenging questions than they have answered. These questions include: Does the lower boundary of the transition zone mark a change in bulk composition between the upper and lower mantle? What is the mechanism of deep focus earthquakes and why does seismic activity cease near the bottom of the transition zone? How deeply are volatiles recycled into the mantle and does the transition zone serve as a volatile rich reservoir? Our view of the transition zone places important constraints on these and many other questions.
Gravity analysis over the Aral, Black, and Caspian Seas from a combined GEOS 3/Seasat/Geosat altimeter data set
Authors: Welker Jean E. , Au Andrew Y. , Brown Richard D.
Published: February 2004

GEOS 3, Seasat and Geosat satellite altimetry data over the Aral, Black and Caspian Seas are processed using least squares collocation to predict geoid undulations and free‐air gravity anomalies on a 0.25° × 0.25° grid. A 180 × 180 geopotential model provides a reference surface for the collocation procedure and the framework for a regional map. Using local empirical covariance functions computed by an iterative numerical convolution technique, prediction accuracies of 24, 8, and 10 mGal were achieved for the Aral, Black, and Caspian Seas, respectively. Over the Caspian Sea, a hybrid theoretical/empirical covariance function was used to obtain reliable short‐wavelength gravity anomalies. The resulting regional gravity anomaly map confirms tectonic structures postulated by other investigators, reveals gravitational evidence of river sediments, and generally improves our knowledge of the geophysics of inland seas.
Structure of the Kuril Trench from seismic reflection records
Authors: Klaeschen Dirk , Belykh Igor , Gnibidenko Helios , Patrikeyev Slava , von Huene Roland
Published: February 2004

Three multichannel seismic reflection records across the Kuril convergent margin provide the first deep data in this area. The records are located across the southern tip of Kamchatka, the central Kuril arc, and the northern extension of Hokkaido Island platform which show three distinct tectonic regimes. The lower slope contains a wedge‐shaped buttress surrounded by low velocity sediments. Underplating sediment uplifts the buttress, as indicated by faults that displace its upper surface. The middle slope is a block of acoustic basement, which has a rough surface with significant arcward dipping faults. The middle slope is separated from the upper slope along a steep arcward dipping reflection, the “middle‐slope boundary.” The upper slope structure off Kamchatka is different from that off Hokkaido. Off Kamchatka a regular stratified sediment section has been uplifted, tilted, and dips seaward. Off Hokkaido, stronger uplift has tilted regional blocks seaward and arcward. Along the southern line north of Hokkaido, well‐studied nonthrust earthquakes with lateral motions occur beneath the middle slope boundary and on the boundary of the subducting plate. Thrust earthquakes occur under the middle and upper slope, whereas tsunami earthquakes occur under the lower slope. High‐amplitude reflections along the lower boundary of the wedge‐shaped buttress and along the active decollement, indicate high fluid concentrations which reduce the friction along the tectonic units so that through the weak coupling, slow rupture may extend up to the seafloor from a tsunami earthquake.
Propagation velocity of a natural hydraulic fracture in a poroelastic medium
Authors: Renshaw Carl E. , Harvey Charles F.
Published: February 2004

The propagation rate of a natural hydraulic fracture is limited by the rate that fluid flows from the saturated rock into the void space created by fracture growth. Unlike induced hydraulic fractures, natural hydraulic fractures can not be modeled by specifying rates of fracture growth or fluid flow into the fracture as boundary conditions. Numerical solutions of the governing equations for natural hydraulic fracture growth in a poroelastic medium indicate that growth rates are primarily controlled by the hydraulic conductivity κ, the storage Sp′ and the initial flaw length 2αo. Computationally more efficient models which partially and completely decouple material stresses from fluid pressures give similar results. Results for several rock types indicate that, although the rate of fracture propagation is limited by fluid flow, fracture growth still accelerates. Results are generalized in dimensionless plots of fracture length versus time for various values of the dimensionless parameter ϕ = (1‐ν)/(GSp′), where G is the shear modulus and ν is the drained Poisson's ratio.
Curie point depth in northeast Japan and its correlation with regional thermal structure and seismicity
Authors: Okubo Yasukuni , Matsunaga Tsuneo
Published: February 2004

Heat flow measurements are high on the back arc side and low on the forearc side of the Tohoku arc, but convective heat transfer in volcanic areas affects the heat flow measurements, complicating the determination of the thermal structure from heat flow measurements alone. Curie point depths deduced from previous magnetic analyses show a sudden change of the Curie isotherm between the volcanic front and the Japan Trench. A magnetic belt extends from west Hokkaido to the eastern limit of Tohoku. The location is west of and parallel to the Cretaceous subduction zone. To determine the geometry of the source of the magnetic belt, detailed magnetic analyses were carried out using forward modeling and recently developed spectral analysis techniques. The magnetic analyses indicate that sources of the magnetic belt extend to significant depths. The result reveals that the Curie isotherm varies from 10 km in the back arc to 20 km or deeper at the eastern limit of Tohoku. The boundary between seismic and aseismic zones in the overriding plate correlates with the inferred Curie isotherm, indicating that seismicity in the overriding plate is related to temperature.
Crustal structure and seismicity beneath the forearc off northeastern Japan
Authors: Suyehiro Kiyoshi , Nishizawa Azusa
Published: February 2004

Marine seismological and other data in the Japan Trench area (north of 38.5°N) were used to infer the relation of subducting plate structure, seismicity, and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates. Previous crustal models from ocean bottom seismographic refraction (OBS) surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data. A wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting Pacific plate crust. Its dip angle increases to about 7°, 110 km landward of the trench axis. A large increase beneath the deep sea terrace is required to reach the well‐defined angle of 25° beneath the Tohoku east coast. A large wave speed gradient within layer 2, commonly observed under normal oceans, seems to vanish beneath the inner trench slope at about 10‐km depth. Within the overriding plate, apparently brittle material with a P wave speed of ∼6 km/s can be found as near as 35 km from the trench axis. The upper limit of the seismogenic zone of interplate low‐angle thrust events is about 15‐km depth from OBS seismicity and large‐event analyses. Both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates.
Unified theory of electrical conduction in firn and ice: Site percolation and conduction in snow and firn
Authors: Shabtaie S. , Bentley C. R.
Published: February 2004

The conduction process in firn can be modeled by the theory of percolation and conduction in disordered systems. Effectively, firn is an infinite cluster of ice crystals with a statistically defined geometry. The properties of such clusters can be studied by random resistor network lattices, where the connectivity of the continuous conducting material can be controlled stochastically. These mixtures of conducting and insulating materials show a percolation threshold at which the conduction abruptly ceases. The structure of cold polar solid ice shows one ice crystal packed by 14 to 16 neighbors. As the density decreases, the number of nearest neighbors diminishes, which reduces the number of clusters that could contribute to the conduction process. From the volume fraction of ice at pore closeoff and the symmetry of the firn we obtain a critical volume fraction ≃0.08 (density equal to 0.07 Mg m−3) at which the conductivity should vanish. This agrees well with estimates of the critical volume fraction from percolation theory for a Bethe lattice with coordination number z = 14–16 and for a seven‐dimensional (7‐D) to eight‐dimensional (8‐D) hypercubic lattice (for which also z = 14–16). Conductivity in random networks obeys the power law σ(χ) ∝ (χ‐χc)t, where χ and χc are the volume fraction and critical fraction, respectively. The exponent t depends on the geometry and dimensionality of the lattice. We find that for firn, t = 2.7, in good agreement with both exponents calculated numerically for 7‐D to 8‐D hypercubic lattices and exponents obtained by extrapolation of theoretical values of t. These experimental observations and analyses explain why Looyenga's empirical mixture equation approximates the transport properties of firn. Furthermore, the analyses indicate that for the current to be transported from one crystal to up to 14 to 16 neighbors the ionic impurities must either be located in the ice lattice by substitution (i.e., with bulk conduction taking place) or form a coating of impurities that surround the crystal like a shell. These results do not support the concept of conduction along three‐grain boundaries, which implies z ≃ 4. In that case, according to site percolation theory, the conductivity would have to vanish at densities less than ∼0.4 Mg m−3, and the conductivity exponent t would be less than 1.8. These values are incompatible with measured conductivities in firn. Furthermore, it is physically impossible to form three‐grain boundaries at low densities.
Repeating waveform initiated by a 180–190 ka geomagnetic excursion in western North America: Implications for field behavior during polarity transitions and subsequent secular variation
Authors: Negrini Robert M. , Erbes Daniel. B. , Roberts Andrew P. , Verosub Kenneth L. , Sarna‐Wojcicki Andrei M. , Meyer Charles E.
Published: February 2004

New paleomagnetic, lithologic, and stratigraphic data are presented from the sediments of Lake Chewaucan in the Summer Lake Basin, Oregon. The new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from this locality. A complex, yet distinct, waveform is observed in all three components of the paleomagnetic vector. The waveform begins as the 180–190 ka Pringle Falls/Long Valley/Summer Lake II geomagnetic excursion and continues for two cycles after the excursion, until the record is interrupted by an unconformity that we correlate to the oxygen isotope stage 6/5e boundary. The waveform's directional morphology in virtual geomagnetic pole (VGP) space is defined by two clockwise loops followed by a distinctive counterclockwise, clockwise, counterclockwise looping sequence. The VGP paths of the two cycles after the excursion are rotated 180° about Earth's spin axis with respect to the VGP paths of the excursion cycle. The waveform also consists of a relative paleointensity variation which repeats during the two cycles after the excursion. The average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur during the excursion. This observation indicates that excursion‐initiated secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions. Because of the similarities noted previously between this excursion and full polarity transitions (Trie et al., 1991), our new observations constrain models for a wide range of field behavior including polarity transitions, excursions, and secular variation.
High‐pressure IR spectra of lattice modes and OH vibrations in Fe‐bearing wadsleyite
Authors: Cynn Hyunchae , Hofmeister Anne M.
Published: February 2004

High‐pressure infrared (IR) spectra of most lattice modes and of hydrous impurities of Fe‐bearing wadsleyite were measured from O to 23 GPa. At ambient conditions, 29 of the 35 expected IR bands were revealed through a combination of reflection and absorption measurements. Absorption spectra of a magnesium end‐member at ambient conditions were similar. The pressure dependence of eight OH vibrations were established to 12 GPa. The degree of hydrogen bonding is inferred from the values of the hydroxyl frequencies and their response to pressure: Five hydroxyls with low v have negative ∂v/∂P and thus moderate to strong hydrogen bonding, whereas three OH groups with high v have near‐zero or positive values of ∂v/∂P and thus weak or no hydrogen bonding. This information, the response of the OH bands to Fe/Mg exchange, and X ray data were used to assign the most intense OH bands to the structural sites, O(1) and O(2). These assignments of hydroxyl vibrations are consistent with previous theoretical models. Our study of high‐pressure OH vibrations in nominally anhydrous wadsleyite indicates that hydrogen impurities in wadsleyite are stable during compression. Our data are consistent with the previously observed minor structural change at around 9 GPa which is possibly due to a slight change in the compression mechanism.
The Gangotri granite (Garhwal Himalaya): Laccolithic emplacement in an extending collisional belt
Authors: Scaillet Bruno , Pêcher Arnaud , Rochette Pierre , Champenois Michel
Published: February 2004

The Gangotri Miocene leucogranite is composed of several laccoliths (6–7 km long, 1.5–2 km thick), which can be divided into two sets of lenses: the southern lenses, intruded in the lower part of the High Himalaya sedimentary cover, and the northern lenses, intruded in an older porphyritic biotite‐bearing granite. In both cases, the magmatic fabric is commonly weak with no dominant stretching direction, although a rough E‐W trend is present in the northern lenses. The magnetic fabric is characterized by the fact that drastic changes in the direction, in the magnitude of the magnetic parameters, or in the fabric type (planar or linear) may take place over very short distances. Measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses, in agreement with the large dispersion observed in both the field and magnetic lineations. The comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the High Himalaya range and is not related to a southward directed thrust event associated with the Main Central Thrust. This is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses. The presence of these dikes indicates in turn that magma ascent occurred by fracture propagation. The spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the Tibetan Slab or from the intrusion of independent laccoliths. The use of the elastic bending theory of Pollard and Johnson (1973) shows that in both hypotheses, the current laccolith sizes are compatible with a laccolithic mode of magma emplacement. However, neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the Tibetan Slab and the overlying Tibetan metasedimentary series controlled the level of magma emplacement. Rather, flat‐lying collapse structures, which intersected the upward propagating magma dikes, are the most likely causes of magma arrest. Such a mechanism was favored by the schist‐rich lithology of the metasedimentary host rocks. In addition, field relationships indicate that the melt supply through the dike system was a continuous, rather than pulsed, process. Existing numerical treatments on the rates of magma transport through fractures show that in such a case, the laccoliths could have been built in less than 100 years. This short time of emplacement, the small size of the laccoliths, and their peripheral disposition relative to the Badrinath granite suggest that the Gangotri lenses may represent the initial stage of pluton accretion in the High Himalaya which ultimately gave rise to a much larger massif such as the Manaslu granite.
A Mesozoic time scale
Authors: Gradstein Felix M. , Agterberg Frits P. , Ogg James G. , Hardenbol Jan , van Veen Paul , Thierry Jacques , Huang Zehui
Published: February 2004

We present an integrated geomagnetic polarity and stratigraphic time scale for the Triassic, Jurassic, and Cretaceous periods of the Mesozoic Era, with age estimates and uncertainty limits for stage boundaries. The time scale uses a suite of 324 radiometric dates, including high‐resolution 40Ar/39Ar age estimates. This framework involves the observed ties between (1) radiometric dates, biozones, and stage boundaries, and (2) between biozones and magnetic reversals on the seafloor and in sediments. Interpolation techniques include maximum likelihood estimation, smoothing cubic spline fitting, and magnetochronology. The age estimates for the 31 stage boundaries (in mega‐annum) with uncertainty (millions of years) to 2 standard deviations, and the duration of the preceding stages (in parentheses) are Maastrichtian/Danian (Cretaceous/‐Cenozoic) is 65.0±0.1 Ma (6.3 m.y.), Campanian/Maastrichtian is 71.3±0.5 Ma (12.2 m.y.), Santonian/Campanian is 83.5±0.5 Ma (2.3 m.y.), Coniacian/Santonian is 85.8±0.5 Ma (3.2 m.y.), Turonian/Coniacian is 89.0±0.5 Ma (4.5 m.y.), Cenomanian/Turonian is 93.5±0.2 Ma (5.4 m.y.), Albian/Cenomanian is 98.9±0.6 Ma (13.3 m.y.), Aptian/Albian is 112.2±1.1 Ma (8.8 m.y.), Barremian/Aptian is 121.0±1.4 Ma (6.0 m.y.), Hauterivian/Barremian is 127.0±1.6 Ma (5.0 m.y.), Valanginian/Hauterivian is 132.0±1.9 Ma (5.0 m.y.), Berriasian/Valanginian is 137.0±2.2 Ma (7.2 m.y.), Tithonian/Berriasian (Jurassic/Cretaceous) is 144.2±2.6 Ma (6.5 m.y.), Kimmeridgian/Tithonian is 150.7±3.0 Ma (3.4 m.y.), Oxfordian/Kimmeridgian is 154.1±3.2 Ma (5.3 m.y.), Callovian/Oxfordian is 159.4±3.6 Ma (5.0 m.y.), Bathonian/Callovian is 164.4±3.8 Ma (4.8 m.y.), Bajocian/Bathonian is 169.2±4.0 Ma (7.3 m.y.), Aalenian/Bajocian is 176.5±4.0 Ma (3.6 m.y.), Toarcian/Aalenian is 180.1±4.0 Ma (9.5 m.y.), Sinemurian/Pliensbachian is 195.3±3.9 Ma (6.6 m.y.), Hettangian/Sinemurian is 201.9±3.9 Ma (3.8 m.y.), Rhaetian/Hettangian (Triassic/Jurassic) is 205.7±4.0 Ma (3.9 m.y.), Norian/Rhaetian is 209.6±4.1 Ma (11.1 m.y.), Carnian/Norian is 220.7±4.4 Ma (6.7 m.y.), Ladinian/Carnian is 227.4±4.5 Ma (6.9 m.y.), Anisian/Ladinian is 234.3±4.6 Ma (7.4 m.y.), Olenekian/Anisian is 241.7±4.7 Ma (3.1 m.y.), Induan/Olenekian is 244.8±4.8 Ma (3.4 m.y.), Tatarian/Induan (Permian/Triassic) is 248.2±4.8 Ma. The uncertainty in the relative duration of each individual stage is much less than the uncertainties on the ages of the stage boundaries.
The core‐mantle boundary region
Authors: Loper David E. , Lay Thorne
Published: February 2004

The 75 year history of the American Geophysical Union has accompanied great advances in our understanding of the physics and chemistry of the transition zone between the Earth's core and mantle. The core‐mantle boundary (CMB) is the most significant internal boundary within our planet, buried at remote depths and probably forever hidden from direct observation; yet this region is very important to our understanding of the dynamic Earth system. The thermal and chemical processes operating near the CMB have intimate relationships to fundamental events in Earth history, such as core formation, and continue to play a major role in the planet's evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the mode of thermal convection of the Earth. The D″ region, comprising the lowermost 300 km of the mantle, is known to be highly heterogeneous in material properties on large and small scales, presumably due to thermal and chemical variations, while the outermost core is much more uniform. The substantial knowledge that we now have about this remote region is testimony to the remarkable progress made in geophysical remote sensing, prompted by prodigious increases in data, computational power, and experimental methodologies used to investigate deep earth structure and processes. In the past decade in particular there have been unprecedented multidisciplinary advances in understanding the CMB region, and there are excellent prospects for developing a comprehensive understanding of this region in the next few decades. Fundamental issues yet to be resolved include the causes of layering extensively observed in D″, whether or not downwelling slabs accumulate at the base of the mantle, whether plumes arise from the CMB region to feed hotspots at Earth's surface, the extent of core‐mantle chemical reactions, the relative importance of topographic versus electromagnetic coupling across the CMB, and the degree to which mantle structure influences the geomagnetic field and its reversals. This overview highlights the progress and future directions in geophysical investigations of the CMB region.
Frequency domain inversion of strong motions: Application to the 1992 Landers earthquake
Authors: Cotton F. , Campillo M.
Published: February 2004

We present a frequency domain inversion in which the observed earthquake strong ground motions are used to constrain the space‐time dependence of slip on a fault. Green's functions are numerically evaluated and the parameters describing the rupture are the local slip, rupture time and rise time. These parameters are simultaneously evaluated without additional constraints. This procedure allows for large variations in the local rupture velocity. The June 28, 1992 Landers earthquake (Mw = 7.3) offers an exceptional opportunity to apply this technique to a major strike‐slip event. We model the rupture evolution, including local differences in slip durations and variations in rupture velocity. Our final results are in good agreement with other inversion studies, geodetic and surface observations. The main discrepancies occurred at depth and at the end of the Johnson Valley fault. We show that a relatively low resolution could be an explanation for these differences. Rupture velocity and slip are extremely heterogeneous, both along strike and with depth. A moment of 0.90×1020 N m was found. The slip distribution shows that this event consists of a series of regions of high slip (subevents) separated by regions of relative low slip. Approximately 50% of the moment was released on the Homestead Valley fault; in this region of large slip, the rupture velocity inferred by our inversion is well constrained and is equal to 3.0 km/s at depth and 2.5 km/s near the surface. Our inversion favors the hypothesis that the duration of the slip at each point of the fault is of the order of the duration of rupture of each subevent.
Significance of seismic reflections beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California
Authors: Malin P. E. , Goodman E. D. , Henyey T. L. , Li Y. G. , Okaya D. A. , Saleeby J. B.
Published: February 2004

The focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface, with the Tehachapi Mountains in California being the case in point. As a prime example of the lower crust, these mountains expose Cretaceous gneisses that formed 25–30 km down in the Sierra Nevada batholith and appear to be underlain by the ensimatic Rand schist. Integrated geophysical and geological studies by the CALCRUST program have produced a cross section through this post‐Mid‐Cretaceous structure and suggest a general model for its development. Seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the San Joaquin Basin's Tejon embayment. Two south dipping reverse faults on the rim of the Tejon embayment were discovered in the reflection data and verified in the field. The faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the Rand schist. The upper and lower crusts are separated by a zone of laterally discontinuous reflectors. Reflections from the lower crust form a wedge, the base of which is a nearly flat Moho at 33 km. Regional geological relations and gravity models both suggest that the reflective zone corresponds to the Rand schist and the newly recognized faults account for its Neogene exposure. Alternatively, the reflective zone maybe part of the gneiss complex, suggesting that the schist either lies deeper or is not present under the gneisses. If the Rand schist underlies the Tehachapi Mountains and Mojave region to their south, a model for their evolution can be constructed from regional geological relations. It seems that during Late Cretaceous Laramide subduction the protolith of the schist was thrust eastward beneath the Mojave. Along this portion of the Cordilleran batholithic belt the subduction was evidently at very low angles. The bottom of the batholith was removed and replaced by a thick section of schist, fluids from which weakened the overlying batholith. This thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat‐lying slabs. When emplacement of the schist ended in latest Cretaceous/earliest Paleocene, the underlying mantle rose, compensating for the extension and providing material for magmatic underplating. In the Neogene, transpression and rotation of the upper crust along the San Andreas and Garlock faults resulted in the exposure of the schist.
Experimental constraints on the dynamics of the partially molten upper mantle: Deformation in the diffusion creep regime
Authors: Hirth Greg , Kohlstedt David L.
Published: February 2004

Deformation experiments have been conducted to investigate the effect of melt fraction and grain size on the creep behavior of olivine aggregates in the diffusion creep regime. Both nominally melt‐free and melt‐added samples display stress exponents (n = 1.0 ± 0.1) and grain size exponents (p = −3.0 ± 0.5 for nominally melt‐free, p = −3.2 ± 1.2 for melt‐added) indicative of grain boundary diffusion creep. The activation energy for creep of the nominally melt‐free aggregates is 315±35 kJ/mol. An abrupt change in the rheological behavior of the partially molten aggregates occurs at a melt fraction of 0.05. Below this melt fraction the influence of melt on strain rate is rather modest. For example, at a melt fraction of 0.04 the strain rate of melt‐added samples is enhanced by a factor of ∼3 relative to that of melt‐free aggregates. This result is consistent with previously published theoretical models for solution‐precipitation enhanced grain boundary diffusion creep in which the melt phase is present along three‐grain junction tubules and four‐grain junction corners. A comparison with published diffusion data indicates that deformation is limited by transport of Si along melt‐free grain boundaries under both melt‐free and melt‐present conditions. At melt fractions above ∼0.05, the strain rate enhancement is significantly greater than that predicted by the theoretical models. For example, at a melt fraction of 0.07 the strain rate of melt‐added samples is enhanced by a factor of ∼25 relative to that of melt‐free aggregates. Micro structural observations of both hot‐pressed and deformed aggregates with melt fractions greater than ∼0.05 demonstrate that a significant number of two‐grain boundaries are “wetted” by melt. These boundaries provide rapid transport paths not accounted for in the theoretical models. The presence of wetted grain boundaries at melt fractions less than ∼0.19 indicates that the melt topology in the olivine‐basalt system is affected by anisotropic interfacial energies. There is no difference in the strength of partially molten aggregates deformed with or without added water. This result is consistent with the observation that the solubility of water in basalt is ∼3 orders of magnitude greater than that in olivine and supports the conclusion that deformation is limited by transport along melt‐free grain boundaries at all conditions tested.
The origin of Kenya rift plateau‐type flood phonolites: Results of high‐pressure/high‐temperature experiments in the systems phonolite‐H2O and phonolite‐H2O‐CO2
Authors: Hay David E. , Wendlandt Richard F.
Published: February 2004

Near‐liquidus melting relations have been determined for a mafic, plateau‐type, flood phonolite from the Kenya rift at 0.5, 0.7, 0.9, and 1.2 GPa, with H2O added through saturation, and at 0.7 GPa with H2O and CO2 added. Mixed‐volatile experiments at 0.7 GPa delineate a near‐liquidus multiple saturation of augite, andesine, phlogopite, oxides, and apatite at 1000°C, XCo2 = 0.42, with calcic amphibole melting above 975°C. The multiple saturation and phase assemblage are interpreted to indicate that plateau phonolites were in equilibrium with the residuum of a parental alkali basaltic composition at 0.7 GPa consisting of augite, andesine, titanomagnetite, and olivine (a product of incongruent melting of phlogopite and, possibly, amphibole). This evidence for lower crustal equilibration refutes suggestions that plateau phonolites are low‐pressure differentiates. Their enormous volumes (about 50,000 km3), restricted eruptive period (14–11 Ma), uniform major element compositions, and the paucity of associated mafic‐intermediate rocks also argue against a deep origin by fractional crystallization. A two‐stage process for the origin of the phonolites is consistent with the thermal evolution of the rift. The lower crust was pervasively injected by alkali basaltic magmas during the period of voluminous eruption of early to middle Miocene basalts. Rising isotherms during rift evolution caused subsequent partial melting of this predominantly basaltic lower crust in the late Miocene, generating the plateau phonolites.
Dikes emplaced into fractured basement, Timna Igneous Complex, Israel
Authors: Baer Gidon , Beyth Michael , Reches Ze'ev
Published: February 2004

Dikes are usually envisioned as arrays of parallel segments dilated perpendicular to the direction of the least compressive stress. We describe here four dikes of highly irregular shape intruded in the fractured basement in the Timna Igneous Complex, southern Israel. The dikes include a doleritic dike, 2.3 km long and 1.6 m to 32 m thick, and three andesitic dikes, up to 1.5 km long and 8 m thick. The dikes each display significant variations of dip (up to 60°), strike (up to 160°) and thickness. The thickness variations correlate better with the segment attitude than with the position along the dikes. We show that the irregular shapes of the Timna dikes are the result of emplacement into fractured host rock under different paleostress states and driving pressures. Three dilation styles that differ by the geometry of the initial cracks are analyzed: an array of randomly oriented cracks (style A), a single linear crack (style B), and an array of interconnected, nonparallel cracks (style C). The analysis of style A provides the stress state during dike emplacement, including the orientations of the three principal stresses (σ1 ≥ σ2 ≥ σ3), the stress ratio ϕ = (σ2 ‐ σ3)/(σ1 ‐ σ3), and the normalized driving pressure R = (Pm ‐ σ3)/(σ1 ‐ σ3). The stress ratio ϕ indicates the shape of the stress ellipsoid and it ranges from ϕ = 0 for σ2 = σ3 (prolate ellipsoid) to ϕ = 1 for σ1 = σ2 (oblate ellipsoid). The normalized driving pressure R indicates the relative magnitude of the internal magma pressure Pm with respect to the tectonic stresses, and it ranges from R = 0 for Pm = σ3 to R = 1 for Pm = σ1. We found that for three dikes in Timna, ϕ ∼ 0.25, indicating small differences between the two horizontal principal stresses, and for one dike ∼ 0.9, indicating a large difference between the two horizontal principal stresses. The normalized driving pressure R is about 0.08 in two horizontally propagating dikes and about 0.25 in two vertically propagating dikes. Style B predicts an elliptical thickness profile along the dike due to dilation of a linear crack; this prediction agrees with the profile of one of the dikes. The predicted thicknesses due to dilation of the interconnected array of cracks (style C) are in good agreement with the thickness variations of the doleritic dike, and in fair agreement with two of the andesitic dikes. Deviations from the ideal geometry suggest separate stages of propagation and dilation in some of the dike segments.
Fractal geometry characterization of geothermal reservoir fracture networks
Authors: Watanabe K. , Takahashi H.
Published: February 2004

As a new procedure for modeling geothermal energy extraction systems, a two‐dimensional modeling technique for subsurface fracture networks on the basis of “fractal geometry” is presented. Models of fracture networks are generated by distributing fractures randomly in space and by using the fractal relation between fracture length r and the number of fractures N expressed with a fractal dimension D as N = Cr−D, where C is a constant that signifies the fracture density within the rock mass. This procedure makes it possible to characterize geothermal reservoirs by parameters measured from field data, such as from core sampling. In this characterization the fracture density parameter C of a geothermal reservoir is used as a parameter to model the subsurface fracture network. Using this fracture network model, the connectivities of the water flow paths between wells are calculated by means of a Monte Carlo simulation, and the result is then compared with that derived from a percolation model. We show that many fewer fractures are required to connect two wells for the fracture network model than for the percolation model. The transmissivities between wells for the fracture network model are also obtained as a function of the fracture density parameter C. The results show that the transmissivities in geothermal reservoirs are significantly dependent upon the fracture density of rock mass, and they can be predicted from the fracture density parameter C of the reservoirs.
Velocity structure of a gas hydrate reflector at Ocean Drilling Program site 889 from a global seismic waveform inversion
Authors: Singh Satish C. , Minshull Timothy A.
Published: February 2004

Strong subhorizontal reflectors in the upper few hundred meters of oceanic sediments have been imaged by seismic reflection profiling adjacent to many continental margins. These reflectors are known as “bottom simulating reflectors” (BSRs) as they run roughly parallel to the seabed. BSRs generally have been interpreted as representing the base of a methane gas hydrate stability field. A series of boreholes was drilled in October–November 1992 during leg 146 of the Ocean Drilling Program to examine fluid discharge and its relationship to the BSR, in the Cascadia accretionary wedge. Independently, we performed a global waveform inversion of seismic reflection data collected near site 889. We use a multistep strategy for global waveform inversion of seismic reflection data. The strategy is based on the Bayesian inference theory, in which the a posteriori probability distribution is determined by the a priori probability distribution and the likelihood function relating the data with the model. Three different likelihood functions have been defined relating different parts of the data with different wavelengths of model parameter variation. First, global nonlinear search algorithms are used to estimate the large‐scale features of one‐dimensional velocity variations; then a nonlinear iterative search is used to retrieve the velocity structure in fine detail. At each stage, bounds on the model parameters are estimated. The strategy has been implemented in the slowness and intercept‐time domain. We find that the BSR corresponds to a 15–20 m zone where the velocity drops from about 1.8 km/s to a minimum of 1.4 km/s, suggesting the presence of free methane gas in the low‐velocity zone, and a 15–20 m transition zone where the velocity gradually increases to a velocity of the surrounding sediments. Thus the total thickness of the low‐velocity zone is about 30–40 m. A small or negligible velocity increase just above the BSR, suggests that the hydrate fills less than 10% of the pore space in the sediments. Our result was confirmed by the drilling. The velocity derived using a vertical seismic profile and sonic log from the drill hole agree closely with our result. Waveform inversion can recover detailed and well‐constrained velocity variations from a high‐amplitude target such as a BSR; combined with the detailed interpretation of these velocity variations which the results from site 889 will provide, the inversion approach is a powerful tool for analysis of BSRs elsewhere.
Paleostress magnitudes determination by using fault slip and fluid inclusions planes data
Authors: Lespinasse M. , Cathelineau M.
Published: February 2004

Paleostress magnitudes can be estimated by using fault slip data, rupture and friction laws (Angelier, 1989) for dry conditions. However, their estimation is difficult if fluids are present during deformation, the lithostatic load and the fluid pressure being generally unknown. This paper shows that the quantitative estimation of the lithostatic load and the fluid pressure during a tectonic event can be derived from paleofluid analysis in fluid inclusion planes, (FIP). As FIP are healed mode I cracks oriented in a consistant manner relative to regional or local structures, stress and fluid features may be obtained for a given deformation event. This approach has been applied to a fault system affecting an Hercynian granite of the NW French Massif Central. A NW‐SE compression has been defined from a population of 51 faults characterized by orientations around N60°E to N110°E for dextral strike slips and N135°E to N175°E for sinistral movements. The stress ratio has been determined with fault slip data around 0.52±0.08 and by using Angelier's method, the ratio ψ (σ3/σ1) was estimated around 0.27±0.03, and the friction coefficient μ was estimated around 0.58±0.1. FIP orientation were measured in a sample collected at 1 meter from the fault. The dominant trend is NW‐SE, vertical or dipping toward the SW. Inclusion fluids from FIP are characterized by homogenization temperatures in the 260°–380°C range with a mode around 300°C and melting temperatures from 0°C to −1.5°C (0–5% equivalent NaCl) with a mode around −1.0°C (1.7% equivalent NaCl). These data yield, assuming a geothermal gradient in the 60°–80°C/km range, and hydrostatic conditions, a fluid pressure estimate of about 50±10 MPa and a σv around 132±10 MPa. Thus a major stress ratio σ1‐σ3 can be estimated in the 70–105 MPa range.
Delineation of intrabasin structure in a dilational jog of the San Jacinto Fault Zone, southern California
Authors: Park Stephen K. , Pendergraft Darin , Stephenson William J. , Shedlock Kaye M. , Lee Tien Chang
Published: February 2004

Three high‐resolution seismic reflection lines were acquired in the northern part of the San Jacinto graben. The graben, a pull‐apart basin formed by a dilational right step of the San Jacinto fault zone, has been previously interpreted as a simple rhombochasm. The reflection survey located at least one significant and previously unidentified intrabasin fault, referred to here as the Farm Road strand. This fault lies approximately halfway between the Claremont and Casa Loma strands of the San Jacinto fault zone. At the north end of the basin, the southwestern boundary of the graben is interpreted to be the newly identified Farm Road strand and not the Casa Loma strand as was previously thought. The identification of this intrabasin fault allows us to infer that the San Jacinto basin comprises coalescing subbasins and is not a simple pull‐apart basin with an unusually large length:width ratio. The distances between the en echelon Casa Loma, Farm Road, and Claremont strands are between 1 and 2 km. This close spacing would likely permit an earthquake rupture to jump between strands and thus propagate through the San Jacinto basin.
Geochemistry of granitic melts produced during the incongruent melting of muscovite: Implications for the extraction of Himalayan leucogranite magmas
Authors: Harris Nigel , Ayres Mike , Massey John
Published: February 2004

Granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium, both between restite and melt, and within individual phases during crystallization. Miocene leucogranites from the Himalayas exemplify melt formation from a pelitic source through muscovite breakdown. Trace elements Rb, Sr, and Ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling. Equilibrium between melt and restite was established for Sr under fluid phase‐absent conditions, but some disequilibrium is suggested by Rb abundances, consistent with known diffusion rates for these elements. Abundances of Zr and Ce in the granites, which provide essential structural constituents of the accessory phases zircon and monazite, indicate that the Himalayan granite melts were saturated in these elements if formed at temperatures of 670–700°C but slightly undersaturated if formed at somewhat higher temperatures. Ion microprobe traverses of Y and heavy rare earth element abundances across single garnets from Himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth, thus constraining the evolution of magmatic compositions during melt formation and crystallization. In some cases, progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant, indicating that minimum solidus temperatures were maintained throughout the melting event. This suggests that thermal buffering rather than melt extraction provided the rate‐determining step in magma evolution.