bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic modeling of the reflective lower crust: Petrophysical and geological evidence from the Ivera Zone (northern Italy)
Authors: Holliger Klaus , Levander Alan R. , Goff John A.
Published: February 2008

Crustal‐scale seismic wave equation modeling demonstrates that wavelength‐scale velocity fluctuations can dominate the seismic response over a wide range of offsets and that a wide range of velocity distributions qualitatively can explain deep seismic data. This range can be narrowed by studying the detailed petrophysical and structural properties of middle and lower crustal exposures. The Ivrea Zone, in the Southern Alps, is a well‐studied sliver of extended lower continental crust brought into its present upright position as a result of late Alpine lithospheric shortening. The high degree of structural and petrophysical complexity and in particular the limited exposure of the Ivrea Zone favor a statistical rather than deterministic approach to develop a two‐dimensional crustal‐scale seismic model. For this purpose we digitized two 1:25,000 geological maps from the central Ivrea Zone on a 60‐m grid. We find that the Ivrea Zone is characterized by a bimodal velocity distribution, and a band‐limited self‐affine structure having a fractal dimension of 2.7, a characteristic horizontal scale of 500 to 1000 m, and an aspect ratio around 4.0. Assuming statistical stationarity, a realization of this stochastic model was used to develop synthetic seismograms typical of “Ivrea‐type” lower continental crust. Our results indicate that (1) Ivrea‐type lower crust can explain qualitatively commonly observed lower crustal reflectivity, (2) relatively minor changes in bulk composition and/or metamorphic grade may lead to dramatic changes in reflectivity, which cannot be explained by current generic models on the origin of lower crustal reflections, and (3) travel time interpretation of the synthetic wide‐angle data may lead to erroneous large‐scale velocity structures.
A global analysis of heat flow from Precambrian terrains: Implications for the thermal structure of Archean and Proterozoic lithosphere
Authors: Nyblade Andrew A. , Pollack Henry N.
Published: February 2008

Previous studies of heat flow from Precambrian terrains have yielded two different relationships: a global temporal relationship between heat flow and tectonic age, and a regional spatial relationship between heat flow and the proximity of Archean cratons. We analyze heat flow from tectonically stable Precambrian terrains worldwide to address questions associated with these two heat flow patterns: (1) Is the spatial relationship a global pattern? (2) Do the two heat flow relationships have a common underpinning? In answer to the first question, our data analysis reveals a widespread spatial pattern between heat flow and the proximity of Archean cratons, which is characterized by low heat flow in Archean cratons and Proterozoic terrains adjacent to cratonic margins and higher heat flow in Proterozoic terrains that are more than a few hundred kilometers from a craton. To address the second question, we examine three previous interpretative models of Precambrian heat flow: (1) simple cooling of a thermal boundary layer, (2) thicker lithosphere in Archean terrains than in Proterozoic, and (3) greater heat production in Proterozoic crust than in Archean. Model 1 predicts essentially no change in heat flow in terrains older than ∼1.5 Ga and therefore does not likely provide a common underpinning for the Precambrian heat flow patterns. Models 2 and 3, when combined with the special structural configuration of sutures, can independently yield both the spatial and temporal heat flow distributions and thus alone or together may be considered candidates to explain the Precambrian heat flow patterns. However, thermal models of the lithosphere in which the heat flow patterns are explained entirely by variations in crustal heat production cannot satisfy constraints on upper mantle temperatures beneath Archean cratons derived from xenolith thermobarometry. The thermobarometry constraints can be satisfied by models in which differences in lithospheric thickness are the principal factor controlling the surface heat flow distributions. If the heat flow patterns result primarily from variations in lithospheric thickness, then a different temporal heat flow relationship for Precambrian terrains can be proposed, one in which heat flow varies inversely with the age of stabilization of the lithosphere.
Geometry of flow paths for fluid transport in rocks
Authors: David Christian
Published: February 2008

Results from numerical simulations on two‐dimensional networks used as an analog to pore space in porous rocks are presented to emphasize the existence of preferential paths for transport processes in heterogeneous media. The simulations show that hydraulic flow and electrical current are mostly driven in the so‐called “critical paths” when the pore size distribution has a decreasing exponential‐like shape in opposition to nearly homogeneous or uniform like distributions. The geometry of these flow paths is controlled by spatially correlated large conductances and can be described by a tortuosity factor, τ2. The amount of flow carried by the critical paths is higher than that predicted by statistical or effective medium models which do not take into account the role of spatial correlation. Another interesting result is that the geometry of the critical paths may change in response to alterations in the properties of the pore space, like, for example, the reduction in connectivity due to pressure‐induced pore closure. The simulations also show that the critical paths for hydraulic flow are different from those for electrical current. A statistical analysis shows that the hydraulic tortuosity is higher than the electrical tortuosity by a factor of 1.5 on the average which implies that one must be cautious in using the electrical tortuosity in the equivalent channel model for determining the permeability of rocks. As the numerical simulations have been done on distributions of pore or crack geometries typically encountered in rocks, the existence of critical paths highlighted in this paper is likely to play an important role in the phenomenology of transport processes in the Earth's crust.
Introduction to Atlantic Hydrothermal Activity
Authors: Rona Peter A. , Thompson Geoffrey
Published: February 2008

Seafloor hydrothermal research has advanced rapidly from local to global scope through a sequence of discoveries. Hydrothermal research at seafloor spreading centers began in the mid‐1960s with the discovery of hot metalliferous brines and sediments ponded in deeps along the slow spreading (half rate 1 cm yr−1) axis of the Red Sea [Chamock, 1964; Miller, 1964; Swallow and Crease, 1965; Miller et al., 1966; Hunt et al., 1967; Bischoff, 1969]. At the same time a hydrothermal metalliferous component was identified in sediments of the East Pacific Rise [Skomyakova, 1965; Arrhenins and Bonatti, 1965; Boström and Peterson, 1966]. Geophysicists recognized that heat flow measurements at spreading centers could only be explained by convective cooling of the crust with circulating seawater [Elder, 1967; Lister, 1972].
A generalization of the demagnetizing tensor for nonuniform magnetization
Authors: Newell Andrew J. , Williams Wyn , Dunlop David J.
Published: February 2008

The demagnetizing tensor for ferromagnets is generalized to include interactions between uniformly magnetized bodies. This “mutual” demagnetizing tensor is symmetric, has a trace of zero, and has other simple geometric properties. The tensor is then used to develop an expression for the macroscopic magnetic field in non‐uniformly magnetized bodies of arbitrary shape. Finally, the theory is applied to a block model of magnetization and explicit formulae for the tensor components are given.
Brittle and semibrittle deformation of synthetic marbles composed of two phases
Authors: Dresen Georg , Evans Brian
Published: February 2008

To investigate the influence of rigid, second‐phase particles on the strength of marble in the semibrittle deformation regime, we performed conventional triaxial mechanical tests on synthetic samples formed by hot isostatic pressing two‐phase aggregates of fine‐grained calcite and 5–20 wt % of either SiC, Al2O3, or SiO2. The samples were tested at strain rates of 10−5 s−1 and at room temperature over a range of pressures from 5 to 300 MPa. Microstructures were studied using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Adding rigid inclusions with incoherent matrix/particle interfaces increased the range of pressure over which the transition between localized brittle failure and crystal plastic flow occurred. The strength of the pure marbles was inversely proportional to porosity between 0 and 8%. In the brittle field, two‐phase aggregates are generally 50 MPa weaker than pure marbles of similar porosity. Second phases and associated cracklike porosity tend to distribute brittle deformation in our samples. Aggregates with 20% inclusions show stable cataclastic flow at 5 MPa confining pressure. At the same pressure, pure samples fail catastrophically along a distinct fault. At higher pressure in the semibrittle field, the strengths of pure and two‐phase aggregates converge. The deformation resistance of the material containing particles is consistently higher than that of the pure samples. TEM observations also suggest an additional contribution to the deformation resistance and hardening rate by interactions between the dispersed particles and crystal defects.
Helium isotope and gas discharge variations associated with crustal unrest in Long Valley Caldera, California, 1989–1992
Authors: Sorey M. L. , Kennedy B. M. , Evans W. C. , Farrar C. D. , Suemnicht G. A.
Published: February 2008

The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within ∼4 months by a large increase in3He/4He in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 RA in July 1990 and subsequently declined to values near 5RA. Potential sources of the3He‐rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a3He‐rich gas chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7RA), the persistence of relatively high values (>5RA) over a period of 3 years, the increase in the flux of total He relative to gases in air‐saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased3He/4He ratios in the fumarolic gas, and that the intrusive process may have persisted for ∼1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome‐south moat region during the 1989–1991 period resulted in at most relatively small changes in3He/4He in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of3He‐rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with Mammoth Mountain.
Moist convection and the injection of volcanic ash into the atmosphere
Authors: Woods Andrew W.
Published: February 2008

If unsaturated water vapor is carried upward by a volcanic eruption column, it may eventually become saturated owing to the decrease in temperature of the column as it expands through decompression and transfers heat to entrained air. Heat released as a result of the subsequent condensation of water vapor causes the air within the column to expand. We show that this increases the buoyancy and therefore the total height of rise of the column. The increase in height is significant in relatively small sub‐Plinian and Strombolian eruptions in which mass eruption rates lie in the range 103 to 106 kg/s. In such eruptions, the latent heat released as the entrained water vapor condenses may provide the main source of heat which drives the ash and clasts upward. The height of rise then becomes relatively insensitive to the mass flux erupted at the vent and depends primarily upon the vapor loading of the atmosphere. In a moist atmosphere, ash may rise several kilomtres higher than in an eruption of comparable strength in a dry environment. Moist convection leads to much wider ash dispersal, particularly from very small eruptions. Subsequently, rain flushing of ash from umbrella clouds may result from the water which forms through the condensation of entrained vapor; the ash provides natural condensation nuclei for some of this entrained vapor, whose mass may be much greater than that of the ash. In small columns (< 15km), the dominant source of water for the formation of accretionary lapilli may be derived from the entrained vapor. In larger Plinian eruptions (> 107 kg/s), the latent heat released by condensation of vapor is relatively small in comparison with the thermal energy provided by the hot clasts and therefore moisture has no significant effect upon the eruption column dynamics; furthermore, the mass of water vapor originating from the erupted volatiles is usually comparable to, or greater than, that entrained from the ambient air. Our model also shows that, if the erupting mixture becomes buoyant, then the eruption columns associated with phreatomagmatic eruptions ascend nearly as high as Plinian columns with the same mass eruption rate. This is because the water which is vaporized by the hot ash at the source, condenses higher in the column and thereby restores its latent heat to the ascending ash.
Queen Charlotte Area Cenozoic tectonics and volcanism and their association with relative plate motions along the northeastern Pacific Margin
Authors: Hyndman R. D. , Hamilton T. S.
Published: February 2008

Cenozoic relative plate interactions along the Queen Charlotte margin, based on published models for the North America and northeast Pacific plates, are related to the tectonic and igneous history of the region. Plate motion models indicate convergence and subduction prior to the Eocene. Since that time, there has been transcurrent motion with varying small amounts of oblique extension or compression, assuming that the Pacific‐America‐Farallon triple junction was to the south of the region. A late Miocene or early Pliocene (about 4 Ma) to present period of oblique convergence is well resolved. The present Queen Charlotte fault zone along the west coast of the islands may have initiated more recently. Within the Cenozoic transcurrent regime, a period of oblique extension in the mid‐Tertiary (36–20 Ma) in one model is supported by a variety of volcanic and tectonic evidence. A small plate motion change at about 20 Ma may have resulted in the transfer of the Yakutat terrane from the North America plate to the Pacific plate and the resulting motion of the terrane northwestward to its present position along the margin of Alaska. The onset of Tertiary Masset volcanism that is extensive on the Queen Charlotte Islands corresponds within a few million years to the time of major plate reorganization at 43 Ma that has tectonic expression around the entire Pacific basin. The period of most extensive Masset volcanism and plutonism appears to correlate with the model oblique extension in the mid‐Tertiary for one model. The geochemistry and physical volcanology of the Masset volcanic s are indicative of an extension regime. The main syntectonic deposition in the Queen Charlotte Basin (Skonun Formation) as inferred from seismic reflection and well data also appears temporally correlated with this time interval of oblique extension. A variety of data, including plate models, dikes, normal faults, basin subsidence, crustal thickness from seismic refraction, and present and paleo‐heat flow, indicate mid‐Tertiary crustal extension of at least 20% in the Queen Charlotte region, with up to 150% (β up to 2.5) in the main Queen Charlotte Basin. Posttectonic basin subsidence and deposition may correlate with the model time interval of general transcurrent or oblique convergence motion from 20 to 4 Ma. More recent shortening deformation observed in outcrop and seismic sections of the northern parts of the basin may correlate with plate model oblique convergence from 4 Ma to the present. The latter convergence is associated with underthrusting that formed a trough or trench and an accretionary sedimentary prism off the west coast of the Queen Charlotte Islands and with inferred uplift and erosion of the western part of the islands.
Equation of state and high‐pressure phase transitions of stishovite (SiO2): Ab initio (periodic Hartree‐Fock) results
Authors: Sherman David M.
Published: February 2008

The possibility that free SiO2 may exist at the core mantle boundary motivates a first‐principles investigation of the equation of state and possible phase transitions of stishovite using periodic Hartree‐Fock theory. Using a moderately extended basis set, the calculated equation of state is shown to be in close agreement with experimental results taken under hydrostatic conditions. When fit to a third‐order Birch‐Murnaghan equation, the Hartree‐Fock total energies give V0 = 46.1 A3, K0 = 328 GPa, and K′ = 4.0. Moreover, the calculated equation of state is very close to that recently obtained using the linearized augmented plane wave method. The bonding in stishovite is unchanged by compression to 80% of its zero‐pressure volume (corresponding to a pressure near 110 GPa). The possibility that stishovite might transform to a denser phase at high pressure is investigated by calculating the free energy of SiO2 in the modified‐fluorite and α‐PbO2 structures. The enthalpy of SiO2 with the fluorite and modified‐fluorite structure is too high to allow those structures to be adopted. The 0 K enthalpy of silica in the α‐PbO2 structure is within 10 kJ/mol of that of stishovite, and a transition to such a structure cannot be ruled out in the temperature regime of the lower mantle. However, the density of α‐PbO2 structure SiO2 is only 2% greater than that of stishovite.
Magnetization of the La Palma Seamount Series: Implications for seamount paleopoles
Authors: Gee Jeff , Staudigel Hubert , Tauxe Lisa , Pick Thomas , Gallet Yves
Published: February 2008

Paleopoles determined from seamount magnetic anomalies constitute the major data source for the Pacific apparent polar wander path, but relatively little is known about the processes of remanence acquisition in seamounts. Since magnetic anomalies reflect both natural remanence (NRM) and the induced field, it is important first to assess whether the NRM is likely to represent an original field direction and second to constrain the magnitude of the induced component. To this end, we present paleomagnetic data from an uplifted, subaerially exposed section through a seamount on La Palma, Canary Islands. The Pliocene Seamount Series of La Palma comprises a >6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts. The structural tilt of the Seamount Series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting (e.g., viscous remanence). The NRM provides a poor indication of the original magnetization direction, although the characteristic magnetization of many La Palma samples is compatible with the original pretilt direction. Hydrothermal alteration has resulted in the production of Ti‐poor magnetite and an increasing contribution of hematite with increasing degree of alteration. More importantly, well‐defined magnetization directions which deviate from any reasonable geomagnetic direction at La Palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization. Based on a comparison of the magnitude of low‐stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization, we conclude that viscous and induced magnetization probably account for 15–25% of the total magnetization of seamounts. The resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4° to 16° for Cretaceous seamounts in the Pacific.
Estimate of the radial orbit error by complex demodulation
Authors: Francis O. , Bergé M.
Published: February 2008

Complex demodulation of the sea surface heights computed from the altimeter range measurements provides an original way to characterize and empirically remove the radial orbit error in the spectral domain. The complex demodulation method is described and applied to a simulated series of altimeter data and to Geosat altimeter data. The results are interpreted in terms of gravity field error contributions using Kaula's linear theory of satellite dynamics. The mean sea surface and the oceanic tides contributions to the observed modulations are also discussed. About 90% of the energy around 1 cycles/rev is attributable to the time‐varying component of the radial orbit error, whereas the energy at 2 and 3 cycles/rev is mainly due to the sea surface topography signal. The estimated full radial orbit error in Geosat altimeter data using GEM‐T1 orbits has a rms of 69 cm.
A utilitarian approach to modeling non‐Gaussian characteristics of a topographic field
Authors: Goff John A.
Published: February 2008

This paper develops a general framework for the analysis of the moments greater than 2 of a topographic field. This framework uses “iterated” expectation to reduce a statistical moment function to component parts involving the vertical (disjoint) moment of the same order, lower moments, and two‐point conditional expectations. In this way it is possible to isolate the unique informational contribution of each moment. Use of this framework necessitates a “bootstrap” or perturbation method, where lower moments are modeled first and then are used as constraints in the modeling of higher moments. Functional modeling of any moment is thus reducible to characterization of the disjoint moment (e.g., skewness, kurtosis) and the two‐point conditional expectation. In this paper, I demonstrate how it is possible to “design” a statistical model most sensitive to specific non‐Gaussian topographic characteristics by solving for the two‐point conditional expectation under an invertable mapping between Gaussian and non‐Gaussian fields of interest. Mappings of this sort are useful both for the fact that they can be intuitive descriptions of non‐Gaussian characteristics and for their utility in generating non‐Gaussian synthetic topography. The primary intent of this methodology is to forge a link between physical topographic characteristics, the information we want to know, and statistical moments, our tool for quantitatively measuring topographic fields. In addition, mapping models can be used to calculate the skewness and kurtosis (or higher moments) of topographic slopes directly. The applicability of these methods is demonstrated for mapping models which create vertical and lateral asymmetry and peakiness in a topographic field.
Subduction of the lithosphere and back arc dynamics: Insights from physical modeling
Authors: Shemenda Alexander I.
Published: February 2008

Physical modeling of the subduction of an elastico‐plastic lithosphere underlained by a low‐viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by Δρ (Δρ = ρ1 ‐ ρa where ρa and ρ1 are the asthenosphere and lithosphere densities). At Δρ = 0, a steady state process develops which requires a horizontal lithosphere compression of σh = (0.3–0.4)σs ≈ 108 Pa, where σs is the lithosphere yield limit. The central part of the leading wedge of the overriding plate is nonisostatically elevated in this case, especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200–400 mGal. A decrease in Δρ (Δρ<0) results in an increase of compression, growth of the frontal arc, and flattening of the Benioif zone, and, when compression is very intensive, a failure of the lithosphere and the formation of a new subduction zone either at the back arc zone abutting the volcanic arc, directly at the volcanic arc, or at the area just behind the outer rise, depending on the specific conditions. At Δρ>0, compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction. At some point, compression changes into tension. Tensional nonhydrostatic stresses grow during the process to a value near σh = (0.1–0.2)σs ≈ 0.4 × 108 Pa. Both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the Benioif dip angle accompany this process. The gravity anomaly maximum is displaced into the volcanic arc in this subduction regime, where it is normally less than 150 mGal. If some weaknesses are present in the overriding plate, tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment (the arc plate) starts to move counter to the subduction plate. The arc plate slides along the underthrusting plate's surface without loss of contact and causes back arc opening. Hydrostatic suction prevents the plates from being separated. This effect is caused by the peculiarity of a mechanical system whereby a low‐viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate. If the overriding lithosphere is not weakened enough to fail under the existing tension, then the hanging slab breaks down under its own weight when it reaches a specific length. Both interacting plates undergo isostatic recovery in this case, and the whole cycle is repeated again. The hydrostatic suction effect also provides back arc tension under other conditions, such as when asthenospheric flow forces the subducted plate to move away from the island arc.
Abyssal Hill Segmentation: Quantitative analysis of the East Pacific Rise flanks 7°S–9°S
Authors: Goff John A. , Malinverno Alberto , Fornari Daniel J. , Cochran James R.
Published: February 2008

The recent R/V Maurice Ewing EW9105 Hydrosweep survey of the East Pacific Rise (EPR) and adjacent flanks between 7°S and 9°S provides an excellent opportunity to explore the causal relationship between the ridge and the abyssal hills which form on its flanks. These data cover 100% of the flanking abyssal hills to 115 km on either side of the axis. We apply the methodology of Goff and Jordan (1988) for estimating statistical characteristics of abyssal hill morphology (rms height, characteristic lengths and widths, plan view aspect ratio, azimuthal orientation, and fractal dimension). Principal observations include the following: (1) the rms height of abyssal hill morphology is negatively correlated with the width of the 5‐ to 20‐km‐wide crestal high, consistent with the observations of Goff (1991) for northern EPR abyssal hill morphology; (2) the characteristic abyssal hill width displays no systematic variation with position relative to ridge segmentation within the EW9105 survey area, in contrast with observations of Goff (1991) for northern EPR abyssal hill morphology in which characteristic widths tend to be smallest at segment ends and largest toward the middle of segments; (3) abyssal hill rms heights and characteristic widths are very large just north of a counterclockwise rotating “nannoplate”, suggesting that the overlap region is being pushed northward in response to microplate‐style tectonics; and (4) within the 7°12′S–8°38′S segment, abyssal hill lineaments are generally parallel to the ridge axis, while south of this area, abyssal hill lineaments rotate with a larger “radius of curvature” than does the EPR axis approaching the EPR‐Wilkes ridge‐transform intersection.
Fault plane determinations for three small earthquakes along the San Jacinto Fault, California: Search for cross faults
Authors: Mori Jim
Published: February 2008

Using rupture directivity and aftershock locations, three earthquakes (ML 4.2 – 4.3) were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane. All three events occurred near the San Jacinto fault zone during December 1989 and February 1990 and had a large component of strike‐slip motion with nodal planes oriented in the northeast and northwest directions. Smaller earthquakes were used as empirical Green functions to deconvolve the path and site complications from the short‐period P waveforms. The resultant far‐field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data. The results of the waveform inversion showed that the earthquake near the Anza Gap (December 2, 1989) and the earthquake near San Bernardino (December 28, 1989) ruptured northwest trending planes. The earthquake near Buck Ridge (February 18, 1990) appeared to rupture a northeast trending plane, but the waveform results for this event were not considered as reliable. Available aftershock data for the three events were also examined using master event relocations. The aftershock sequence following the Buck Ridge event showed a clear lineation trending toward the northeast. The results of this study indicate that the Anza and San Bernardino earthquakes ruptured areas on or parallel to the San Jacinto fault, while the Buck Ridge event ruptured a northeast trending cross fault. In addition to determining the fault plane, the waveform analysis resolved details of the slip distribution for these relatively small earthquakes.
Observational constraints on models of the Hawaiian Hot Spot Swell
Authors: Wessel Pål
Published: February 2008

A detailed analysis of the topography and geoid associated with the Hawaii‐Emperor seamount chain reveals that while geoid/topography ratios are high over the southeastern part of the chain, they fall off with distance away from the current hot spot location. The topographic expression of the swell has two maxima near the intersections between the seamount chain and the Murray and Molokai fracture zones, respectively. Both geoid and topography rapidly decrease westward of the intersection with the Murray fracture zone. It appears that the region of the Pacific plate between the two fracture zones has been more susceptible to reheating and hot spot penetration than elsewhere along the chain. Alternatively, the observed pattern may be indicative of time variation in the strength of the Hawaiian hot spot.
The seismic source parameters of the 1991 Costa Rica Aftershock Sequence: Evidence for a transcurrent plate boundary
Authors: Fan Guangwei , Beck Susan L. , Wallace Terry C.
Published: February 2008

The April 22, 1991, Valle de la Estrella, Costa Rica earthquake (Ms = 7.6) was a back‐arc thrusting event associated with the underthrusting of the Caribbean plate beneath Central America. A network of three PASSCAL‐type, portable instruments was deployed to monitor the aftershock activity in southern Costa Rica 2 to 6 weeks after the main shock. The waveforms recorded on three‐component midperiod seismometers were used to recover source information for 15 small aftershocks (magnitudes between 3.2 and 4.4) with a linear moment tensor inversion method. We conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery. The longer‐period waveforms, in general, are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data. The earlier part of the seismic wave‐forms has proven to be the most important carrier of the source information. A gross crustal model can be used to describe the structure for the source study. The small changes in the waveform character resulting from the mislocation of the events, or inexact Green's functions generated from the oversimplified crustal model, do not prohibit us from the recovery of the source orientation at local distances. In contrast, the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information. Our focal mechanisms are generally in good agreement with P wave first‐motion fault plane solutions determined from a local short‐period network. The aftershocks show a clear spatial segmentation based on focal mechanism type. Most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock. In contrast, a cluster of aftershocks northwest of the main shock showed dominantly left‐lateral, strike‐slip motion on a northeasterly striking nodal plane. This suggests that a diffuse deformation zone exists in central Costa Rica and is characterized by left‐lateral strike‐slip motion. This diffuse, transcurrent deformation zone coincides with several geologic and geophysical features, and perhaps is a result of the slower subduction rate of the buoyant Cocos Ridge, than its adjacent segments along the Middle America Trench (MAT). The diffuse transcurrent boundary may intersect with the North Panama Deformed Belt (NPDB) near limon, Costa Rica, and is very likely a plate boundary for the proposed Panama block.
The initiation of rifting at constant tectonic force: Role of diffusion creep
Authors: Hopper John R. , Buck W. Roger
Published: February 2008

We have investigated the effect of diffusion creep on lithospheric extension using a one‐dimensional numerical model that assumes a constant force is available to drive extension. The model is motivated by the fact that continental areas with average heat flow should be too strong to rift using standard estimates of lithospheric strength. However, such areas do rift and diffusion creep is a mechanism by which lithosphere may deform at a lower stress level than is required for the usually assumed dislocation creep. We consider the evolution of strain rate and temperature at the center of an idealized pure shear rift. The strain rate will either increase or decrease with time depending on whether lithospheric weakening or strengthening dominates as extension progresses. Given an initial thermal condition and an assumed lithospheric rheology, the applied force must be such that the initial strain rate is greater than some critical value for weakening to dominate. The force at which this condition is met we will term the critical force. Diffusion creep is more efficient at smaller grain sizes and the model results indicate that mantle grain sizes would have to be less than 1 mm for diffusion creep to significantly reduce the critical force. We speculate that during the initial stages of continental rifting, grain size reduction and diffusion creep deformation mechanisms may have an important effect on lithospheric strength.
Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in Lac du Bonnet granite
Authors: Jansen D. P. , Carlson S. R. , Young R. P. , Hutchins D. A.
Published: February 2008

Concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined, 15‐cm cube of Lac du Bonnet granite from Atomic Energy of Canada Limited's Underground Research Laboratory. An electrical resistance cartridge heater, placed in a central vertical borehole, was used to cycle the sample to progressively higher peak temperatures between 75°C and 175°C. Tomography data were collected, at room temperature, before and after each thermal cycle. Acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycle. Microfractures opened above 80°C and eventually coalesced into a macroscopic fracture plane. The macroscopic fracture originated at the outer edges of the sample and then extended inward, parallel to the fast velocity direction, and eventually intersected the borehole. Both acoustic emission locations and slowness difference tomography clearly delineated the fracture plane. We attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acting upon a brittle, anisotropic medium.
The Tuzo Wilson Volcanic Field, NE Pacific: Alkaline volcanism at a complex, diffuse, transform‐trench‐ridge triple junction
Authors: Allan James F. , Chase Richard L. , Cousens Brian , Michael Peter J. , Gorton Micheal P. , Scott Steven D.
Published: February 2008

The Tuzo Wilson Volcanic Field (TWVF) consists of Quaternary alkaline volcanics erupted at the complex, diffuse triple junction between the Explorer, Pacific, and North American plates. It occurs in a region of distributed strain that lies between right‐lateral, strike‐slip faulting at the Queen Charlotte fault, seafloor spreading at the Explorer Ridge, and subduction at the Cascadia subduction zone. The TWVF is contained within a poorly defined graben structure and consists of two 500‐ to 700‐m‐high composite seamount volcanoes surrounded by numerous smaller vents, with a total edifice volume of about 12 km3. The erupted volcanics are large ion lithophile elements (LILE) and light rare earth element (LREE) enriched alkali basalts, hawaiites, mugearites, and benmoreiites. These volcanics also differ from mid‐ocean ridge basalts by lacking Fe enrichment and Al depletion trends and by having low relative Fe, Sc, and Cr and high relative Al for a given MgO content, reflecting significant high‐pressure pyroxene fractionation. They are petrologically similar to other alkaline volcanics found capping near‐ridge seamounts in the east Pacific. Petrogenetic modeling combined with analysis of Sr, Nd, and Pb isotopic data shows that TWVF volcanism is ephemeral in nature and that the TWVF lavas were derived by small amounts of melting (≤3%) of an amphibole‐bearing, LREE‐ and LILE‐enriched heterogenous mantle similar to that underlying the adjacent Explorer spreading ridge. The new volcanological, petrological, and geochemical data presented constrain both the petrogenetic origin of TWVF lavas and the tectonic processes occurring in the triple junction region. Current wisdom states that the TWVF represents either a site of seafloor spreading or a hotspot/mantle plume. Our new data are incompatible with these models and instead are consistent with other geophysical data in suggesting that the TWVF represents minor volcanism associated with pull‐apart structures developing between parallel strike‐slip faults in a region of distributed strain. In short, the TWVF represents “leaky transform” volcanism in an oceanic setting.
Motion of Kilauea Volcano during sustained eruption from the Puu Oo and Kupaianaha Vents, 1983–1991
Authors: Delaney Paul T. , Miklius Asta , Arnadottir Thora , Okamura Arnold T. , Sako Maurice K.
Published: February 2008

Kilauea erupted almost continuously from January 1983 through 1991. Although the summit began subsiding during the rift zone dike intrusion that initiated this eruption, remarkably steady ground surface motions began in late 1983 after a magnitude 6.6 earthquake beneath the slopes of nearby Mauna Loa volcano and continued until the onset of brief upper east rift zone earthquake swarms in late 1990. During these 7 years the summit and upper rift zones subsided up to 10–11 and 4–8 cm yr−1, respectively, and summit baselines contracted up to 6 cm yr−1. Baselines directed northward from the summit to stations on Mauna Loa extended at rates up to 7 cm yr−1, and a baseline from south of the summit to Mauna Loa extended 4 cm yr−1. Much of this extension is inconsistent with deformation caused solely by summit magma reservoir collapse and more likely reflects rifting as the south flank of the volcano moved seaward from the summit and rift zones. Farther from the summit, baselines crossing the south flank extended up to 2 cm yr−1, and a south flank tide gauge rose 2 cm yr−1; the lower east rift zone, 40–50 km from the summit, subsided about 2 cm yr−1. Motion on Kilauea, then, is broadly consistent with slip along low‐angle south flank faults, generating subsidence that is focused at the summit and along the rift system behind the faulting and uplift along the coastal south flank ahead of it. Dislocation models that combine these elements show that much of Kilauea's edifice migrated seaward, producing ground surface motions along the south flank of up to about 6 cm yr−1. The magnitude 6.1 earthquake of 1989 punctuated these motions along the eastern south flank, producing more than 25 cm of seaward displacement and, 15 km east of the epicenter, up to 24 cm of subsidence south of the lower east rift zone. Unlike the magnitude 7.2 south flank earthquake of 1975, the 1989 event was preceded neither by summit magma reservoir inflation nor by rift zone dike intrusions and accompanying compression of the south flank. Deformation was probably caused by the weight of the volcanic overburden and by ongoing dilation and slip within the rift system.
Aerosol dynamics and light‐scattering properties of a volcanic plume
Authors: Ammann M. , Burtscher H.
Published: February 2008

Field data from recent studies on volcanic aerosols are used to propose a model for the dynamic evolution of a volcanic plume from aerosol generation to stabilization of the plume cloud after the first minute of plume travel time. The aerosol has a large number of ultrafine particles with radii of 10–20 nm and a large mass of acid droplets with radii of several hundred nanometers. We investigate the size and concentration profile of these particles along the plume travel path, incorporating coagulation, dilution by turbulent mixing, and growth as function of relative humidity. The light‐scattering properties of the plume cloud are expressed in terms of the extinction coefficient, which is clearly dominated by the presence of acid droplets. The hygroscopic behavior of the droplets determines the rapidly changing visual appearance of the plume from a transparent haze to a bright, dense cloud with increasing humidity. When the chemistry of the magmatic gas does not favor the formation of acid droplets, Rayleigh scattering by high number concentrations of ultrafine particles explains the bluish color of the plume; the observability is restricted to special meteorologic and volcanologic conditions.
Active tectonics in southern Xinjiang, China: Analysis of terrace riser and normal fault scarp degradation along the Hotan‐Qira Fault System
Authors: Avouac Jean‐Philippe , Peltzer Gilles
Published: February 2008

The northern piedmont of the western Kunlun mountains (Xinjiang, China) is marked at its easternmost extremity, south of the Hotan‐Qira oases, by a set of normal faults trending N50E for nearly 70 km. Conspicuous on Landsat and SPOT images, these faults follow the southeastern border of a deep flexural basin and may be related to the subsidence of the Tarim platform loaded by the western Kunlun northward overthrust. The Hotan‐Qira normal fault system vertically offsets the piedmont slope by 70 m. Highest fault scarps reach 20 m and often display evidence for recent reactivations about 2 m high. Successive stream entrenchments in uplifted footwalls have formed inset terraces. We have leveled topographic profiles across fault scarps and transverse abandoned terrace risers. The state of degradation of each terrace edge has been characterized by a degradation coefficient τ, derived by comparison with analytical erosion models. Edges of highest abandoned terraces yield a degradation coefficient of 33 ± 4 m2. Profiles of cumulative fault scarps have been analyzed in a similar way using synthetic profiles generated with a simple incremental fault scarp model. The analysis shows that (1) rate of fault slip remained essentially constant since the aggradation of the piedmont surface and (2) the occurrence of inset terraces was synchronous at all studied sites, suggesting a climate‐driven terrace formation. Observation of glacial and periglacial geomorphic features along the northern front of the western Kunlun range indicates that the Qira glaciofluvial fan emplaced after the last glacial maximum, during the retreat of the Kunlun glaciers (12–22 ka). The age of the most developed inset terrace in uplifted valleys is inferred to be 10 ± 3 ka, coeval with humid climate pulses of the last deglaciation. The mass diffusivity constant (k=τ/T, being time B.P.) in the Hotan region is determined to be 3.3 ± 1.4 m2/103 years, consistent with other estimates in similar climatic and geologic environments of western China. These results imply a minimum rate for the Tarim subsidence of 3.5 ± 2 mm/yr. If Western Kunlun overthrusts the Tarim platform on a crustal ramp dipping 40°–45° to the south, it would absorb at least 4.5 ± 3 mm/yr of convergence between western Tibet and Tarim.
Pigeonite at solidus temperatures: Implications for partial melting
Authors: Bertka Constance M. , Holloway John R.
Published: February 2008

The results of partial melting experiments with iron‐rich and iron‐poor mantle compositions have been used to determine the pressure at which pigeonite is stable at mantle solidus temperatures. Pigeonite is stable at the solidus at lower pressure in an iron‐rich mantle (Mg# 75, ∼1.0 GPa, Mg# = atomic (Mg/(Mg+Fe))*100), than in an iron‐poor mantle (Mg# 90, >2.0 GPa). The stabilization of pigeonite in a peridotite mantle will result in the absence or low modal abundance of orthopyroxene in the subsolidus assemblage. The melting reaction which takes place at the solidus of a pigeonite‐bearing mantle assemblage produces orthopyroxene. Partial melting increases the mode of orthopyroxene in the residue. Orthopyroxene may be absent from igneous eclogites because they are the crystallization product of melts from a pigeonite‐bearing source. The high modal abundance of orthopyroxene in South African cratonic peridotites may be evidence of their origin as the residue of a high pressure, >2.0 GPa, partial melting event.