bluuebunny/crossref_metadata_2025 huggingface dataset:
Tidal forcing of basal seismicity of ice stream C, West Antarctica, observed far inland
Authors: Anandakrishnan S. , Alley R. B.
Published: February 2004

The seismicity rate beneath the downglacier 85 km of ice stream C, West Antarctica, is modulated by the tide. The tide beneath the Ross Ice Shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude. This tidal forcing travels up the ice stream as an attenuating wave at approximately 1.6m s−1and is detectable 85km from the grounding line. We successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity η and thicknesshband calculate that the substrate has an apparent stiffness η/hbofO(108) Pa s m−1. This finding suggests that the conditions of the till layer at the bed of ice stream C are similar to those of ice stream B and that the reason for the recent stagnation of ice stream C is other than loss of till. We further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress.
Shear wave anisotropy in the Erzincan Basin and its relationship with crustal strain
Authors: Gamar Fatiha , Bernard Pascal
Published: February 2004

The analysis of S wave polarization from a large number of aftershocks of the 1992 Erzincan earthquake has revealed a clear S wave splitting, which we interpret in terms of seismic anisotropy in and around the Erzincan basin and which concerns at least the first few kilometers of the crust. In the sedimentary basin, our interpretation of the origin of the anisotropy is the alignment of cracks due to the stress field (extensive dilatancy anisotropy (EDA) model) within the deep sediments and possibly below, in the mean direction N335°. This is rotated by 20° from the compressive stress direction N355° deduced from other aftershock studies. This difference may either be explained by uncertainties in the stress direction or by a stress distortion related to the activity of some nearby active faults. In the outcropping basement, to the south, the fast S direction is N315°, significantly different from the present regional stress direction, which implies that the anisotropy is controlled either by a possible, though yet unreported, rock foliation or by cracks related to a local, strongly distorted stress field. Following the preferred hypothesis of stress rotations and of the EDA model, we use a two‐dimensional model to calculate the direction of the elastic stress resulting from the combined activity of the adjacent segments on the North Anatolian fault east to the basin (NAE) and within the basin (NAB) with a similar stress release. The resulting trajectories of the maximal compressive stress in the shallow crust are mainly controlled by the NAE segment, owing to its much greater length, and correctly fit the inferred crack directions in the bedrock sites as well as in the basin sites, provided that the stress release on the segments is of the order of the regional deviatoric stress amplitude stored in the shallow crust (1 km in depth). In this model, the stress and crack orientation inferred from shear wave anisotropy can be explained by some slip deficit on the NAB segment with respect to the NAE segment, which is independently suggested by the historical earthquakes in the area. Studies of the crack‐induced anisotropy in the shallow crust can thus bring important information on the spatial and possibly temporal variations of crustal strain and stress near active faults, in particular, near the limits of the fault segments marked by jogs, steps, or bends, and might constrain the relative potential of adjacent segments for future earthquake ruptures.
A continuum mixture model of ice stream thermomechanics in the Laurentide Ice Sheet 1. Theory
Authors: Marshall Shawn J. , Clarke Garry K. C.
Published: February 2004

We employ a continuum mixture framework to incorporate ice streams in a three‐dimensional thermomechanical model of the Laurentide Ice Sheet. The ice mass is composed of a binary mixture of sheet ice, which deforms by viscous creep, and stream ice, which flows by sliding and/or sediment deformation at the bed. Dynamic and thermal evolutions are solved for each component in the mixture, with coupling rules to govern transfer between flow regimes. We describe two different transfer mechanisms: (1) creep exchange, the nourishment of ice streams by viscous creep inflow from the surrounding ice sheet, and (2) bed exchange, the activation, growth, and deactivation of ice streams, perpetrated by transfers of bed area between flow constituents. This paper develops the underlying mixture theory. We express the governing equations for mass, momentum, and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics. A companion paper in this issue explores mixture and ice stream behavior in applications with the Laurentide Ice Sheet.
Quantifying Anderson's fault types
Authors: Simpson Robert W.
Published: February 2004

Anderson [1905] explained three basic types of faulting (normal, strike‐slip, and reverse) in terms of the shape of the causative stress tensor and its orientation relative to the Earth's surface. Quantitative parameters can be defined which contain information about both shape and orientation [Célérier, 1995], thereby offering a way to distinguish fault‐type domains on plots of regional stress fields and to quantify, for example, the degree of normal‐faulting tendencies within strike‐slip domains. This paper offers a geometrically motivated generalization of Angelier's [1979, 1984, 1990] shape parameters ϕ and ψ to new quantities named Aϕ and Aψ. In their simple forms, Aϕ varies from 0 to 1 for normal, 1 to 2 for strike‐slip, and 2 to 3 for reverse faulting, and Aψ/ranges from 0° to 60°, 60° to 120°, and 120° to 180°, respectively. After scaling, Aϕ and Aψ agree to within 2% (or 1°), a difference of little practical significance, although Aψ has smoother analytical properties. A formulation distinguishing horizontal axes as well as the vertical axis is also possible, yielding an Aϕ ranging from −3 to +3 and Aψ from −180° to +180°. The geometrically motivated derivation in three‐dimensional stress space presented here may aid intuition and offers a natural link with traditional ways of plotting yield and failure criteria. Examples are given, based on models of Bird [1996] and Bird and Kong [1994], of the use of Anderson fault parameters Aϕ and Aψ for visualizing tectonic regimes defined by regional stress fields.
Testing self‐organized criticality by induced seismicity
Authors: Grasso Jean‐Robert , Sornette Didier
Published: February 2004

We examine the hypothesis proposed in recent years by several authors that the crust is in a self‐organized critical (SOC) state. This hypothesis has been suggested on the basis of the observation of power law distributions, such as the Gutenberg‐Richter law for earthquakes and the fault length distribution, and of the fractal geometry of sets of earthquake epicenters and of fault patterns. These self‐similar properties are shared by simplified models of the crust exhibiting a spontaneous organization toward a critical point characterized by similar scale‐invariant properties. The term “critical” is here used in the sense of phase transitions such as the Curie point in magnetism. The usefulness of a hypothesis is measured by its predictive and explanatory power outside the range of observations that have helped defined it. We thus explore how the SOC concept can help in understanding the observed earthquake clustering on relatively narrow fault domains and the phenomenon of induced seismicity. We review the major reported cases of induced seismicity in various parts of the world and find that both pore pressure changes (±Δp) and mass transfers (±Δm) leading to incremental deviatoric stresses of <1 MPa are sufficient to trigger seismic instabilities in the uppermost crust with magnitude ranging up to 7.0 in otherwise historically aseismic areas. Once triggered, stress variations of at least 1 order of magnitude less but still larger than the ∼0.01 MPa tidal stress are enough to sustain seismic activity. We argue that these observations are in accord with the SOC hypothesis as they show that a significant fraction of the crust is not far from instability and can thus be made unstable by minute perturbations. This property is shared by simplified models of SOC. Not all perturbations, however, trigger seismic activity; this is also compatible with the SOC hypothesis which embodies naturally the existence of large heterogeneities in the stress field. The induced seismicity is found to obey generally the Gutenberg‐Richter law up to a magnitude cutoff which correlates well with the width of the local seismogenic bed, ranging in size from that of mine pillars for mining‐induced seismicity to the thickness of brittle sedimentary beds in the vicinity of dams or depleted hydrocarbon reservoirs. In conclusion, the properties of induced seismicity and their rationalization in terms of the SOC concept provide further evidence that potential seismic hazards extend over a much larger area than that where earthquakes are frequent.
Study of plate deformation and stress in subduction processes using two‐dimensional numerical models
Authors: Hassani Riad , Jongmans Denis , Chéry Jean
Published: February 2004

Two‐dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere. The subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence. The lithosphere is assumed to be underlain by an in viscid asthenosphere. Different rheological laws have been considered for the lithosphere, including elasticity and elastoplasticity. The modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters: the density contrast between lithosphere and asthenosphere (Δρ = ρL ‐ ρA) and the coefficient of friction along the subduction plane. Varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc‐back arc system. Extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction, while negative Δρ values and/or high friction leads to a compressional regime. The coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0.1. The results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions.
Effects of deviatoric stress on undrained pore pressure response to fault slip
Authors: Wang Herbert F.
Published: February 2004

The influence of deviatoric stress on coseismic water level changes is examined using an empirical relation between induced pore pressure and stress. Skempton [1954] characterized undrained pore pressure response in clayey soils to triaxial loading using two coefficients A and B. The undrained pore pressure response under axial loading can be either larger (A>1/3) or smaller (A<1/3) than that predicted from the mean normal stress alone. For Indiana limestone the undrained pore pressure response under axial loading is characterized by A<1/3. Skempton's equation is generalized to a polyaxial state of stress by making the undrained pore pressure a linear combination of the mean normal stress and the octahedral shear stress, which is proportional to the square root of the second invariant of the deviatoric stress tensor. Dependence of the undrained pore pressure on the deviatoric stress components is consistent with coseismic water level changes observed on the trace of the fault plane near Parkfield California, for an A value less than 1/3. Regions of pore pressure decrease are enlarged relative to regions of pore pressure increase.
Southern California Permanent GPS Geodetic Array: Continuous measurements of regional crustal deformation between the 1992 Landers and 1994 Northridge earthquakes
Authors: Bock Y. , Wdowinski S. , Fang P. , Zhang J. , Williams S. , Johnson H. , Behr J. , Genrich J. , Dean J. , van Domselaar M. , Agnew D. , Wyatt F. , Stark K. , Oral B. , Hudnut K. , King R. , Herring T. , Dinardo S. , Young W. , Jackson D. , Gurtner W.
Published: February 2004

The southern California Permanent GPS Geodetic Array (PGGA) was established in 1990 across the Pacific‐North America plate boundary to continuously monitor crustal deformation. We describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19‐month period between the June 28, 1992 (Mw=7.3), Landers and January 17, 1994 (Mw=6.7), Northridge earthquakes. We compare displacement rates at four site locations with those reported by Feigl et al. [1993], which were derived from an independent set of Global Positioning System (GPS) and very long baseline interferometry (VLBI) measurements collected over nearly a decade prior to the Landers earthquake. The velocity differences for three sites 65–100 km from the earthquake's epicenter are of order of 3–5 mm/yr and are systematically coupled with the corresponding directions of coseismic displacement. The fourth site, 300 km from the epicenter, shows no significant velocity difference. These observations suggest large‐scale postseismic deformation with a relaxation time of at least 800 days. The statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets; two possible noise models fit the PGGA data equally well as described in the companion paper by Zhang et al. [this issue]; the pre‐Landers data are too sparse and heterogeneous to derive a reliable noise model. Under a fractal white noise model for the PGGA data we find that the velocity differences for all three sites are statistically different at the 99% significance level. A white noise plus flicker noise model results in significance levels of only 94%, 43%, and 88%. Additional investigations of the pre‐Landers data, and analysis of longer spans of PGGA data, could have an important effect on the significance of these results and will be addressed in future work.
Structural control on fault geometry: Example of the Grevena MS 6.6, normal faulting earthquake
Authors: Chiarabba C. , Selvaggi G.
Published: February 2004

In this paper we present a three‐dimensional velocity structure of the Kozani‐Grevena region, a relatively low‐seismicity area of the western margin of the Internal Hellenides, which was struck by a MS=6.6 earthquake on May 13, 1995. P wave arrival times from 656 aftershocks are inverted to resolve the velocity structure of the upper 12 km of the crust. The 1995, ruptured fault (Dheskati fault) lies at the southeastern end of a 35‐km‐long, normal fault (the Servia fault). The crustal heterogeneities identified by tomography help to constrain the geometry and structure of the 1995 fault and the rupture evolution during the mainshock. The aftershocks and the velocity structure define a fault dipping ∼35° toward NW, which from 5 km depth splays upward into two nearly parallel shallow faults. The slip during the mainshock and the aftershocks all were concentrated along the deeper portion of the fault, while the limited slip observed at the surface occurred on the northern of the two shallower splays (the Paleochori fault). It appears that the lithological heterogeneities of the shallow crust are responsible for the splaying of the main fault upward. Our work stresses the importance of crustal structure in determining fault geometry and segmentation and rupture evolution.
Seismic anisotropy beneath the southern Himalayas‐Tibet collision zone
Authors: Sandvol Eric , Ni James , Kind Rainer , Zhao Wenjin
Published: February 2004

Shear wave phases ScS, SKS, and SKKS, collected from the Project International Deep Seismic Sounding of Tibet and Himalayas II (INDEPTH‐II) array in the eastern Himalayas and Tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern Himalayas‐Tibet collision zone. No evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the Indus‐Tsangpo Suture. The lack of shear wave splitting beneath the stations in the vicinity of suture and the Tethyan Himalayas indicates that there is no appreciable large‐scale mantle polarization seismic anisotropy beneath this part of the collision zone. One explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened Indian lower continental lithosphere. North of the Gandese belt, the fast directions of the split shear waves are oriented in a northeast‐southwest direction, approximately parallel to the trend of a major left‐lateral strike‐slip fault zone that bounds the Gulu rift. The coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric‐wide coherent deformation.
Subduction‐related structures in the North Iberian Margin
Authors: Alvarez‐Marron J. , Rubio E. , Torne M.
Published: February 2004

The oblique convergence of Eurasia and Iberia since the Early Cretaceous, caused the formation of the Pyrenean intracontinental collisional orogen in the east, and progressed to continent‐ocean collision with subduction of the Bay of Biscay oceanic crust beneath the North Iberian Margin in the west. Two deep multichannel seismic profiles (IAM‐12 and ESCIN‐4), integrated with gravity modeling and other geological and geophysical data, provide the crustal‐scale architecture of this margin and its tectonic evolution during the convergence. The North Iberian Margin is modeled with a South or south‐southeast dipping oceanic crust beneath the outer part of the continental shelf. Mesozoic basins on the shelf were inverted during the Tertiary, and compressional activity continued until recent times in the ESCIN‐4 section, while a shallower, probably Neogene age basin is subjected to active recent erosion in the IAM‐12 section. In the oceanic areas, a marginal trough deepens and widens toward the east as a result of the regional east dip of the oceanic basement. The accretionary prism increases in size from west to east (18–56 km), and its internal structure and morphology varies along strike. The prism is buried by postcorivergence sediments in both sections and in the IAM‐12 section appears to have been active at least during Lutetian to Burdigalian times. The crustal‐scale structure of the North Iberian Margin is that of an arrested subduction zone in which a remnant oceanic basin was being consumed near two continental plates that collided obliquely.
A strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the Pacific
Authors: Ritsema Jeroen , Garnero Edward , Lay Thorne
Published: February 2004

A new approach for constraining the seismic shear velocity structure above the core‐mantle boundary is introduced, whereby SH‐SKS differential travel times, amplitude ratios of SV/SKS, and Sdiff waveshapes are simultaneously modeled. This procedure is applied to the lower mantle beneath the central Pacific using da.ta from numerous deep‐focus southwest Pacific earthquakes recorded in North America. We analyze 90 broadband and 248 digitized analog recordings for this source‐receiver geometry. SH‐SKS times are highly variable and up to 10 s larger than standard reference model predictions, indicating the presence of laterally varying low shear velocities in the study area. The travel times, however, do not constrain the depth extent or velocity gradient of the low‐velocity region. SV/SKS amplitude ratios and SH waveforms are sensitive to the radial shear velocity profile, and when analyzed simultaneously with SH‐SKS times, rnveal up to 3% shear velocity reductions restricted to the lowermost 190±50 km of the mantle. Our preferred model for the central‐eastern Pacific region (Ml) has a strong negative gradient (with 0.5% reduction in velocity relative to the preliminary reference Earth model (PREM) at 2700 km depth and 3% reduction at 2891 km depth) and slight velocity reductions from 2000 to 2700 km depth (0–0.5% lower than PREM). Significant small‐scale (100–500 km) shear velocity heterogeneity (0.5%–1%) is required to explain scatter in the differential times and amplitude ratios.
Tomographic imaging of deep velocity structure beneath the Eastern and Southern Carpathians, Romania: Implications for continental collision
Authors: Fan Guangwei , Wallace Terry C. , Zhao Dapeng
Published: February 2004

The Vrancea seismic zone, located beneath the “elbow” of the Eastern and Southern Carpathians, is characterized by intermediate‐depth seismicity associated with subduction and plate collision. Among many strong earthquakes that have occurred in the region, the 1977 Vrancea earthquake (March 4, 1977, Ms = 7.1, depth of 86 km) caused extensive damage and fatalities. The Vrancea seismic zone is defined by a tabular region of seismicity that extends from the surface to a depth of 200 km. The small size (about 80 km in length and 40 km in width) and geometry of the seismic zone have made it difficult to interpret the kinematics of subduction and continental collision in the Romanian Carpathians. To better understand the collision/subduction environment, body wave seismic tomography was used to image the deep structure beneath the Eastern and Southern Carpathians in Romania. A total of 7837 P and 2292 S arrival times from 433 shallow and intermediate‐depth earthquakes recorded by local and regional seismic stations were inverted for the three‐dimensional (3‐D) seismic velocity structure. The travel times and ray paths were calculated by an efficient 3‐D ray‐tracing technique. By utilizing the LSQR algorithm, P and S wave velocity perturbations were determined from the surface down to a depth of 200 km. The high‐resolution P wave tomographic images reveal a pattern of broad heterogeneity in the velocity structure of the region. One of the most important features is a high‐velocity body existing beneath the Carpathian arc at depths between 100 and 170 km. This high‐velocity body outlines a slab dipping near vertically where most of the intermediate‐depth earthquakes occur. Beneath the Transylvanian Basin, a high‐velocity zone is dipping toward the SE and may represent the base of lithosphere, which is in convergence with the subducted slab. Low‐velocity bodies are found along the forearc of the Carpathian foredeep down to depths of 70 to 80 km. These low‐velocity bodies may be correlated with the shallow sedimentary layer in the foreland and may represent crustal materials wedged and underthrust along with the subducted lithosphere.
Stress‐field rotation and rooted detachment faults: A Coulomb failure analysis
Authors: Wills Stewart , Buck W. Roger
Published: February 2004

Several well‐known mechanical models have shown that unusual boundary or loading conditions can alter principal‐stress orientations into configurations consistent with low‐angle normal faulting. Such models, however, have not demonstrated whether magnitudes of reoriented stresses are sufficient to initiate and promote slip on low‐angle surfaces. We present the results of simple Coulomb failure analyses to determine whether, and where, such models predict frictional slip, assuming geologically plausible boundary stresses, pore pressures, and rock strengths. Models that invoke a sizable shear traction at the base of the upper crust or spatially varying loads on the upper crust reorient principal stresses and failure planes but do not produce frictional failure on crustal‐scale detachments either in the absence of pore fluids or at hydrostatic pore fluid pressures. Models that reorient stresses by midcrustal dike intrusion produce slip on low‐angle surfaces at relatively deep crustal levels but only in the area of the dike tip; the low‐angle failure surfaces curve into a high‐angle orientation a short distance from the dike. All of these models also imply unsustainably high absolute tensile stresses in the upper 5 km of the crust and suggest that, in any system in which stresses are allowed to evolve over time, failure and stress release will occur on high‐angle faults before low‐angle ones have developed. These assertions are true even when near‐lithostatic pore pressures are assumed, unless there is an inhomogeneous, extraordinarily fortuitous distribution of pore pressures and rock strengths at the time of initiation of a new detachment fault. One model we tested, for example, required pore pressures exceeding 0.96 times lithostatic in the area of the hypothesized low‐angle normal fault, with lower pore pressures both above and below the detachment to prevent slip and stress release on high‐angle normal faults in the upper part of the modeled region and on low‐angle thrust faults in the lower part.
Detailed structure of the top of the melt body beneath the East Pacific Rise at 9°40′N from waveform inversion of seismic reflection data
Authors: Collier J. S. , Singh S. C.
Published: February 2004

We have applied waveform inversion to multichannel seismic reflection data collected at the East Pacific Rise at 9°40′N in order to determine the precise velocity structure of the magma body causing the axial magma chamber reflection. Our analysis supports the idea of a molten sill as previously suggested from forward modeling of seismic data from this location. Our inverted solution has a 30‐m‐thick sill with a P wave seismic velocity of 2.6 km s−1. Although not well constrained by the data we believe that the S wave velocity in the sill is not significantly different from 0.0 km s−1. The low P‐ and S wave velocities in the sill imply that it contains less than 30% crystals. The molten sill is underlain by a velocity gradient in which the P wave velocity increases from 2.6 to 3.5 km s−1 over a vertical distance of 50‐m. The shape of our velocity‐depth profile implies that accretion of material to the roof of the sill is minor compared to accretion to the floor. The underlying velocity gradient zone may represent crystal settling under gravity. We suggest that only material from the 30‐m‐thick layer can erupt.
Tidal station displacements
Authors: Mathews P. M. , Dehant V. , Gipson John M.
Published: February 2004

Theoretical modeling of the station displacements produced by tidal deformations of the Earth due to lunisolar gravitational forces is a necessary part of the analysis of space geodetic data. To attain the accuracies demanded by the precision of the data, a generalized version of the Love number formalism has to be used wherein the classical Love and Shida numbers are replaced by sets of Love number parameters with values that depend on the frequency of the tidal excitation. This paper presents theoretical expressions for the contributions from the various Love number parameters to the station displacement vector and a scheme for efficient computation of these contributions, taking into account the frequency dependence of these parameters as well as the imaginary parts that are present when the effects of mantle anelasticity are taken into account. A brief discussion of the permanent tide which arises from the zero frequency component of the tide‐generating potential is included.
Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution
Authors: Blakely Richard J. , Christiansen Robert L. , Guffanti Marianne , Wells Ray E. , Donnelly‐Nolan Julie M. , Muffler L. J. Patrick , Clynne Michael A. , Smith James G.
Published: February 2004

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north‐south within the magmatic regions and northwest‐southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest‐southeast dextral shear of the Walker Lane belt to the east‐west extension characteristic of the Cascade arc in central Oregon. Faulting along north‐south strikes in the volcanically active areas indicates the east‐west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.
Magnetic petrology of arc xenoliths from Japan and Aleutian Islands
Authors: Warner Richard D. , Wasilewski Peter J.
Published: February 2004

Xenoliths from Japan and the Aleutian Islands were studied for the purpose of identifying source rocks with sufficient magnetization to account for positive subduction zone Magsat anomalies. Two types of magnetic lithologies are identified: (1) amphibolites and hornblende gabbros containing up to 8.6% titanomagnetite, mostly as large, unoxidized, multidomain crystals, and (2) pyroxene gabbros and metagabbros with up to 20.3% titanomagnetite, usually in association with primary ilmenite. The first type is represented by xenoliths from Ichinomegata, Japan and Adak Island in the Aleutians. The second, restricted to Iki Island, Japan and Kanaga Island, Aleutians, has abundant coarse‐grained titanomagnetite which in most samples has been oxidized, producing secondary, trellis‐type ilmenite lamellae; a few Iki Island xenoliths show more advanced oxidation. Arc xenoliths show 3 orders of magnitude range in natural remanent magnetization and saturation remanence and about 2 orders of magnitude range in magnetic susceptibility. Nearly all Iki Island rocks and many from Ichinomegata and the Aleutians possess induced magnetization 2 A m−1 and greater, with the expectation of additional magnetization due to viscous remanence at elevated temperature. The TiO2 content of titanomagnetite, together with the presence of appreciable Al2O3 and MgO, ensure that most xenoliths have Curie points between 400° and 550°C. Remanence acquisition curves exhibit a range reflecting varying degrees of magnetic hardening due to oxidation. Correspondingly, each xenolith site groups distinctly on a plot of magnetic hysteresis ratios. This paper indicates that lithologies with total magnetizations of many A m−1 exist in lower crust arc settings. Their volumetric distribution is uncertain based on the limited xenolith population reported herein.
Paleointensity of the Earth's magnetic field in the Jurassic: New results from a Thellier study of the Lesotho Basalt, southern Africa
Authors: Kosterov Andrei A. , Prévot Michel , Perrin Mireille , Shashkanov Vladimir A.
Published: February 2004

Thellier paleointensity experiments carried out on the Jurassic (180 m.y. old) Karoo basalt, Lesotho, southern Africa, provided paleointensity estimates for 40 samples from 10 distinct lava flows with reversed magnetization. Mean paleointensity was found to be equal to 24.2 ± 11.3 μT (standard deviation), with a minimum of 12 μT and a maximum of 41.3 μT. This corresponds to a mean virtual dipole moment (VDM) of 4.6±2.0×1022 Am2. The between‐flow dispersion is ascribed to paleosecular variation, which thus would approach up to 40% of the mean paleointensity, as is observed from the published paleointensity data set covering last 20 m.y. This result yields further evidence for the period of low geomagnetic field in Jurassic to Early Cretaceous time (Mesozoic Dipole Low). A symmetrical intensity versus time pattern is suggested for this time interval, with a minimum VDM of 3×1022 Am2 in Late Jurassic and substantially higher (∼ 4.5×1022 Am2) VDMs in Early to Middle Jurassic and Early Cretaceous, respectively.
Distribution of large Pacific seamounts from Geosat/ERS‐1: Implications for the history of intraplate volcanism
Authors: Wessel Paul , Lyons Suzanne
Published: February 2004

We characterize the seamount distribution on the Pacific Plate using the gridded vertical gravity gradient (VGG, or geoid curvature) derived from Geosat and ERS‐1 satellite altimetry. The VGG amplifies short‐wavelength information and suppresses longer wavelength components, making it suitable for seamount detection purposes. Furthermore, the VGG over seamounts has a much more pronounced zero crossing than that of the free‐air anomaly (FAA); the distance to the zero crossing can be used as a proxy for seamount radius. After removing a regional field obtained by robust median filtering we identify seamount amplitudes and locations from local maxima in the VGG grid. The radius of a seamount is more difficult to estimate since seamounts tend to cluster and overprint each other's signals. Individual seamounts are modeled as Gaussian, axisymmetric objects loading an elastic lithosphere; the VGG over such features can be approximated by a simple analytical expression which we use to determine the zero‐crossing distances for overlapping seamounts. By using the VGG the maximum amplitude and distance to the zero crossing become largely independent of the elastic plate thickness and infill density. We do forward modeling of Gaussian seamounts and their gravimetric response and create a look up table that relates seamount FAA (in milligals), VGG (in Eötvös), and zero‐crossing distance (in kilometers) to actual height and radius (in kilometers). The frequency‐size distribution of these predicted seamount heights follows a power law for heights between 2 and 8 km. The seamount density (number of seamounts per area) is greatest in the central Pacific. We confirm earlier results suggesting that the majority of large seamounts are located in the western region of the Pacific Plate, on older crust. As crustal age increases, so does seamount density, peaking on 100–130 m.y. crust, supporting suggestions of high magmatism in the Cretaceous. We demonstrate that there may be an empirical relationship between the seamount VGG amplitude and the age of the lithosphere at the time of seamount formation and invert this relationship to predict seamount ages from VGG amplitudes. These pseudo ages have large uncertainties but, nevertheless, may be used to investigate temporal fluctuations in Pacific intraplate volcanism. Our results indicate that seamount intraplate volcanism attained a maximum level in the mid‐Cretaceous to Late Cretaceous, about 70–120 Ma, apparently contemporaneous with the formation of large oceanic plateaus in the Pacific.
Preeruption conditions and timing of dacite‐andesite magma mixing in the 2.2 ka eruption at Mount Rainier
Authors: Venezky D. Y. , Rutherford M. J.
Published: February 2004

Analytical, field, and experimental evidence demonstrate that the Mount Rainier tephra layer C (2.2 ka) preserves a magma mixing event between an andesitic magma (whole rock SiO2 content of 57–60 wt %) and a dacitic magma (whole rock SiO2 content of 65±1 wt %). The end‐member andesite (a mix of an injected and chamber andesite) and dacite can be characterized on the basis of the homogeneity of the matrix glass and phenocryst rim compositions. Many pumices, however, contain mixtures of the end‐members. The end‐member dacite contains a microlite‐free matrix glass with 74–77 wt % SiO2, orthopyroxene rims of Mg57–64, clinopyroxene rims of Mg66–74, and plagioclase rim anorthite contents of An45–65. The temperature and oxygen fugacity, from Fe‐Ti oxide compositions, are 930±10°C and 0.5–0.75 log units above NNO. The mixed andesite contains Mg73–84 orthopyroxene rims, Mg73–78 clinopyroxene rims, An78–84 plagioclase rims, and Mg67–74 amphibole rims. The temperature from Fe‐Ti oxides, hornblendeplagioclase, and two‐pyroxene geothermometry is 1060±15°C, and the oxygen fugacity is approximately one log unit above NNO for the injected andesite. The chamber andesite is estimated to be a magma with a ∼64–65 wt % SiO2 melt at 980°C and a NNO oxygen fugacity. We conclude that the andesitic and dacitic magmas are from separate magma storage regions (at >7 km and ∼2.4 km) due to differences in the bimodal whole rock, matrix glass, and phenocryst compositions and the presence or absence of stable hornblende. The time involved from the mixing event through the eruption is limited to a period of 4–5 days based on Fe‐Ti oxide reequilibration, phenocryst growth rates, and hornblende breakdown. The eruption sequence is interpreted as having been initiated by an injection of the 1060±15°C andesitic magma into the ∼980°C (>7 km) andesite storage region. The mixed andesitic magma then intersected a shallow, ∼2.4 km, dacitic storage system on its way toward the surface. The eruption became more dacitic over time, and the final products some show evidence of partial reequilibration between the andesite and dacite.
Powder X ray diffraction study of synthetic pyrope‐grossular garnets between 20 and 295 K
Authors: Bosenick Anne , Geiger Charles A.
Published: February 2004

The unit cell parameters of synthetic pyrope (Mg3Al2Si3O12) ‐ grossular (Ca3Al2Si3O12) and five pyrope‐grossular solid solutions have been measured between 20 and 295 K using X ray powder diffraction methods and their linear thermal expansions have been determined. The low‐temperature thermal expansion data, together with existing thermal expansion data between 295 K and 1000 K on synthetic end‐member pyrope and grossular, were compared to the heat capacities of garnets along this join. The thermal expansion of pyrope is greater than that of grossular over the whole temperature range from 20 to 1000 K, whereas the Cp of pyrope is only greater than that of grossular up to about 200 K. It was observed that the change in the thermal expansion of pyrope with respect to temperature is greater than that of grossular below 295 K. The thermal expansions of the solid solution compositions show a temperature dependence intermediate between the two end‐members. The volumes of mixing along the join show positive asymmetric deviations from ideality. A two‐parameter Margules fit to the data, incorporating recently published data on this join, gives WVCa‐Mg = 0.12±0.26 cm3/mol and WVMg‐Ca = 1.84±0.28 cm3/mol at 295 K. No measurable changes are observed in the mixing behavior below 298 K. These new volume of mixing data are in good agreement with another recent determination and remove much of the uncertainty on the mixing properties of pyrope‐grossular solid solutions. They will enable refinements of important petrologic geobarometers, which are widely used in pressure determinations of rocks of the lower crust and upper mantle. The change in molar volume of garnet can be interpreted using a model of rigid tetrahedral rotation. It is proposed that phonons related to rigid SiO4‐ tetrahedral and X site cation translations play an important role in the low‐temperature thermodynamic behavior of garnet.
Variations in axial morphology along the Galápagos spreading center and the influence of the Galápagos hotspot
Authors: Canales J. Pablo , Dañobeitia Juan José , Detrick Robert S. , Hooft Emilie E. E. , Bartolomé Rafael , Naar David F.
Published: February 2004

The Galápagos Spreading Center (GSC) is marked by systematic changes in axial morphology between the Inca Fracture Zone (FZ) at 85.5°W and the 95.5°W propagator. We analyze these changes using new swath bathymetry and magnetic data acquired aboard the B/O Hespérides during the Galápagos'96 experiment. Within ∼350 km of the Galápagos hotspot the ridge axis is associated with an East Pacific Rise (EPR)‐like axial high. At increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology (TAM). The axis in this transitional region is typically a broad zone (∼20 km wide) consisting of very rough volcanic and fault‐generated topography. West of 95°W, this TAM evolves into a 20–40 km wide, 400–1500 m deep axial valley typical of the slow spreading Mid‐Atlantic Ridge (MAR). There is not an abrupt change from axial high to rift valley along the GSC, but a distinct TAM occurs over a distance of ∼200–300 km along‐axis and is accompanied by a gravity‐estimated crustal thickening of >1–2 km. The boundary between an axial high and this TAM is quite abrupt and occurs along a segment that is less than 9 km long. These changes in axial morphology are primarily caused by variations in magma supply along the GSC due to the entrainment and dispersal of plume mantle from the Galápagos hotspot. However, the changes in morphology are not symmetric about the Galápagos FZ at 91°W. The axial high topography extends farther east of the 91°W FZ than to the west, and the rift valley which develops west of 94°W is not found at comparable distances along the GSC east of the hotspot. Axial depth variations are also asymmetric across the 91°W FZ. This asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the GSC from 46 mm/yr at 97°W to 64 mm/yr at 85°W. Off‐axis depth changes are symmetric about the 91°W FZ and suggest that 15–40% of on‐axis depth variation is dynamically supported.
Morphology and distribution of seamounts surrounding Easter Island
Authors: Rappaport Yoav , Naar D. F. , Barton C. C. , Liu Z. J. , Hey R. N.
Published: February 2004

We investigate the morphology and distribution of a seamount population on a section of seafloor influenced by both superfast seafloor spreading and hotspot volcanism. The population under investigation is part of a broad chain of seamounts extending eastward from the East Pacific Rise, near Easter Island. In order to define the morphological variability of the seamounts, basal shape, cross‐sectional area, volume, flatness, and flank slope are plotted against height for 383 seamounts with heights greater than 200 m, based on bathymetry data collected by GLORI‐B and SeaBeam 2000, during three cruises onboard the R/V Melville in the spring of 1993. Nearly complete swath mapping coverage of the seamounts is available for the analysis of size and shape distribution. We quantitatively describe the seamount population of this active region, in which seamounts cover ∼27% of the seafloor, and account for ∼4.2% of the total crustal volume. Over 50% of the total volume (61,000 km3) of seamounts used in this study is made up by the 14 largest seamounts, and the remaining volume is made up by the 369 smaller seamounts (>200 m in height). Our analysis indicates there are at least two seamount populations in the Easter Island‐Salas y Gomez Island (25°–29°S, 113°–104°W) study area. One population of seamounts is composed of short seamounts (<1200 min height) with variable flatness from pointy cones to flattened domes (flatness from 0.01 to 0.57) and predominantly steep flanks (slopes from 5° to 32°). A second population is of massive (>1200 m), shield‐like, pointy cones (flatness <0.2) and gentle slopes (from 5° to 15°). An exponential maximum likelihood distribution is fit to the binned raw frequency of height and gives a characteristic height of the seamount population of 308±12 m and an expected number of seamounts per 1000 km2 of 2.7±0.15. Many seamounts that have different slope and flatness relationships with height are formed next to each other. We speculate that the larger volcanoes (>∼1200 m) originate exclusively from a hotspot source, but only a portion of the smaller volcanoes (<∼1200 m) are formed from a hotspot source. The remainder would be presumably formed by a normal mantle or mixed source.
Determination of the preexponential frequency factor for superparamagnetic maghemite particles in magnetoferritin
Authors: Moskowitz Bruce M. , Frankel Richard B. , Walton Sarah A. , Dickson Dominic P. E. , Wong K. K. W. , Douglas T. , Mann Stephen
Published: February 2004

Magnetization and Mössbauer measurements on maghemite particles with an average particle diameter of 10 nm have been made in the temperature range from 5 K to 353 K spanning the superparamagnetic (SPM) and stable single domain (SD) regimes. The maghemite particles were produced within the iron‐storage protein ferritin, resulting in a narrowly‐sized, weakly interacting nanocomposite material called magnetoferritin. Experiments combining hysteresis measurements, low temperature remanence, and Mössbauer spectroscopy were used to characterize magnetoferritin and to provide experimental estimates of (1) the pre‐exponential frequency factor ƒ0 in the Néel‐Arhennius relaxation equation; (2) the SPM threshold size at room temperature for maghemite; and (3) the SD value of Hr/Hc at 0 K. The frequency factor was determined from the difference in blocking temperatures measured by dc magnetization and Mössbauer spectroscopy, yielding a value of f0≈109 Hz. This agrees well with the standard value and justifies the usually assumed superparamagnetic blocking condition of KV = 25 kT for remanence measurements. The SPM threshold size at room temperature for remanence measurements was estimated to be 20–27 nm and the extrapolated SD value at 0 K for Hr/Hc is 1.32. The latter value is slightly larger than the theoretical value of 1.09 but may be more appropriate for weakly interacting SD particles commonly found in sediments and soils. However, ƒ0 for ferrimagnetic magnetoferritin is a factor of 103 lower than was determined previously for native ferritin, which contains antiferromagnetic ferrihydrite cores. The difference in ƒ0 values between the two varieties of ferritin is probably related to the two different types of magnetic spin ordering of the core minerals and suggests that the higher value of ƒ0 is more appropriate for antiferromagnetic minerals like hematite and goethite, whereas the lower value is more appropriate for ferrimagnetic minerals like maghemite, magnetite, or greigite.