bluuebunny/crossref_metadata_2025 huggingface dataset:
Variations in SKS splitting across western Canada
Authors: Bostock M. G. , Cassidy J. F.
Published: February 2004

Measurements of SKS splitting parameters have been made at stations of the Canadian National Seismograph Network for events over the period 1993–mid1994. Here we report on results obtained from western stations. In the northern Cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upper mantle are being deformed coherently. Splitting parameters in the southern Cordillera, however, are oriented approximately perpendicular to the strike of major tectonic features, notably the Cascadia subduction zone. This contrast in the character of fast polarization directions between the southern and northern Cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from convergent along the southern B.C. coast to transform north of the triple junction. One plausible explanation is that upper mantle anisotropy in the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the Juan de Fuca ridge, subduction along the coast and associated back‐arc asthenospheric flow beneath the B.C. interior.
Total atmospheric ozone determined from spectral measurements of direct solar UV irradiance
Authors: Huber Martin , Blumthaler Mario , Ambach Walter , Staehelin Johannes
Published: February 2004

With a double monochromator, high resolution spectral measurements of direct solar UV‐irradiance were performed in Arosa during February and March, 1993. Total atmospheric ozone amount is determined by fitting model calculations to the measured spectra. The results are compared with the operationally performed measurements of a Dobson and a Brewer spectrometer. The total ozone amount determined from spectral measurements differs from the results of the Dobson instrument by −1.1±0.9% and from those of the Brewer instrument by −0.4±0.7%.
Is the Cauca‐Patia and Romeral Fault System left or rightlateral?
Authors: Ego F. , Sébrier M. , Yepes H.
Published: February 2004

Extrusion driven collision is controlled by strike‐slip faults of opposite senses whereas subduction related oblique convergence is accommodated by margin parallel faults of identical senses. Northern Andes geodynamics has been interpreted either as resulting from the obliquity of convergence between Nazca and South American plates or from the collision of the Carnegie Ridge with Ecuadorian margin. The sense of the Cauca‐Patia and Romeral Fault System (CPRFS) has been interpreted in two opposite manners: right or left‐lateral agreeing with oblique or collisional patterns, respectively. We analyze arguments which favor, South to 4°N, a right‐lateral motion along the CPRFS that is consistent with obliquity driven geodynamics. However, North of 5°N, the CPRFS is actually left‐lateral. These opposite motions between the northern and southern part of the CPRFS have to be accommodated around 5°N by N‐S stretching and/or shortening. Existing data support N‐S shortening around 5°N, this buttressing could be due to the collision between the Panama and Andean Blocks.
Biomass burning record and black carbon in the GISP2 Ice Core
Authors: Chýlek Petr , Johnson Bruce , Damiano P. A. , Taylor K. C. , Clement P.
Published: February 2004

We have determined the black carbon concentration in three sets of ice core samples from the GISP2 (Greenland Ice Sheet Project 2) ice core. The major peaks in black carbon concentration between 320 and 330 A.D. correlate well with peaks in ammonium concentration and with the dips in electrical conductivity measurements (ECM), which allows us to identify extensive forest fires in this time period. The average black carbon concentration during the 320 to 330 A.D. decade is found to be 2.1 µg of black carbon per 1 kg of snow melt water. The current snow (1989 and 1990) from the GISP2 site shows an average black carbon concentration of about 2.0 µ/kg suggesting that the rate of black carbon deposition at the GISP2 Greenland site during 1989–1990 was about the same as 1670 years ago.
Interseismic strain accumulation on the Himalayan crustal ramp (Nepal)
Authors: Pandey M. R. , Tandukar R. P. , Avouac J. P. , Lavé J. , Massot J. P.
Published: February 2004

The Departement of Mines and Geology has been monitoring the seismicity of the Central Himalayas of Nepal since 1985. Intense microseismicity and frequent medium‐size earthquakes (mL<4) tend to cluster beneath the topographic front of the Higher Himalaya. This 10–20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data. Both microseismic and geodetic data indicate strain accumulation on a mid‐crustal ramp that had been previously inferred from geological and geophysical evidence. This ramp connects a flat decollement under the Lesser and Sub‐Himalaya with a deeper decollement under the Higher Himalaya, and probably acts as a geometric asperity where strain and stress build up during the interseismic period. The large Himalayan earthquakes could nucleate there and probably activate the whole flat‐and‐ramp system up to the blind thrusts of the Sub‐Himalaya.
The relationship between kappa and temperature in energetic ion spectra at Jupiter
Authors: Collier Michael R. , Hamilton D. C.
Published: February 2004

A universal energy per charge kappa function fit is simultaneously applied to the spectra of Voyager 2 LECP proton, helium, oxygen, sulfur, and carbon ions during 33 Jovian plasma sheet crossings from 26 to 160 RJ. The fits yield an approximately linear relationship between high energy spectral index, κ, and core proton temperature of the form κ(TH) ≈ η · TH + κ0with η = 0.080 keV−1, κ0 = 2.86, and TH measured in keV. Core proton temperatures range from 5 to 35 keV with spectral indices ranging from 3 to 6.
Use of a hybrid code to model the Earth's magnetosphere
Authors: Swift Daniel W.
Published: February 2004

This paper presents a demonstration of the use of a hybrid code to model the Earth's magnetosphere on a global scale. A hybrid code calculates the interaction of particle ions and a massless electron fluid with the magnetic field. The demonstration is a simulation of the noon meridian plane of the magnetosphere. Innovative features of the code include a numerically generated curvilinear coordinate system, approximation of the cold ionospheric plasma as an MHD fluid that coexists with the kinetic plasma and subcycling of the magnetic field update to the particle push. These innovations allow the code to accommodate disparate time and distance scales. New results include particle acceleration in the cusp and nearly field aligned currents linking the cusp and polar ionosphere. It is pointed out that such a code is needed help resolve many of the outstanding questions regarding substorm dynamics and perhaps form the basis of a numerical space weather prediction scheme.
Variation of fractional electron density fluctuations inside 40 Ro observed by Ulysses ranging measurements
Authors: Woo Richard , Armstrong J. W. , Bird M. K. , Pätzold M.
Published: February 2004

The first measurements of fractional electron density fluctuations δne/ne, where δne is rms electron density fluctuation and ne is the mean electron density, have been carried out inside 40 Ro using 1991 Ulysses dual‐frequency S‐ and X‐band (13 and 3.6 cm) ranging (time delay) measurements. In the frequency band of ∼ 6 × 10−5 ‐ 8 × 10−4 Hz (periods of 20 min to 5 hr), δne/ne varies from a high near 20% in the slow wind close to the neutral line to a low of 1% in the fast wind far from the neutral line. For spatial wavenumber K ∼ 1.4 × 10−6 km−1 (period of 5 hr at 250 km/s), δne/ne is essentially independent of heliocentric distance over 0.03–1.0 AU in the slow wind; it is a factor of 30 lower in the fast wind than in the slow wind inside 0.1 AU, but exhibits dramatic growth with heliocentric distance inside 0.3 AU. This latter result reinforces current views of the evolution of MHD turbulence and the association of Alfvén waves with high speed streams based on in situ fields and particles measurements beyond 0.3 AU. That regions of enhanced density fluctuations near or above the neutral line coincide with regions of enhanced density confirms previous conclusions that they are the interplanetary manifestation of the heliospheric current sheet and extensions of coronal streamers. While the regions of enhanced density fluctuations lie within those of enhanced density, they have boundaries that are distinctly more abrupt, suggesting the separation of plasma of different nature and origin.
Large‐scale tectonic plate motions measured with the DORIS Space Geodesy System
Authors: Soudarin Laurent , Cazenave Anny
Published: February 2004

We have analysed approximately 3 years of Doppler tracking measurements of the DORIS system on the SPOT2 and Topex‐Poseidon satellites. Absolute velocities of 19 stations of the DORIS network have been computed through a dynamic orbit computation method. These stations are located on 9 major tectonic plates. For 14 stations, relative velocities across 28 plate boundaries involving 8 different plates, agree quite well with the NUVEL‐1 geological estimates, with a correlation coefficient of 0.982 and a slope of 1.00±0.02. This study provides the first space geodesy‐derived motion of the African plate which is based on data taken at 4 stations. Computed rotation vectors for the relative motions between the Eurasian, Pacific and African plates show a good agreement with NUVEL‐1 vectors. For the remaining sites which are typically located close to plate boundaries, measured motions depart significantly from the ones predicted by NUVEL‐1. The site of Fairbanks (Alaska) appears to move anomalously compared to earlier VLBI results.
Topography and tectonics of the central New Madrid seismic zone: Results of numerical experiments using a three‐dimensional boundary element program
Authors: Gomberg Joan , Ellis Michael
Published: February 2004

We present results of a series of numerical experiments designed to test hypothetical mechanisms that drive deformation in the New Madrid seismic zone. Experiments are constrained by subtle topography and the distribution of seismicity in the region. We use a new boundary element algorithm that permits calculation of the three‐dimensional deformation field. Surface displacement fields are calculated for the New Madrid zone under both far‐field (plate tectonic scale) and locally derived driving strains. Results demonstrate that surface displacement fields cannot distinguish between either a far‐field simple or pure shear strain field or one that involves a deep shear zone beneath the upper crustal faults. Thus, neither geomorphic nor geodetic studies alone are expected to reveal the ultimate driving mechanism behind the present‐day deformation. We have also tested hypotheses about strain accommodation within the New Madrid contractional step‐over by including linking faults, two southwest dipping and one vertical, recently inferred from microearthquake data. Only those models with step‐over faults are able to predict the observed topography. Surface displacement fields for long‐term, relaxed deformation predict the distribution of uplift and subsidence in the contractional step‐over remarkably well. Generation of these displacement fields appear to require slip on both the two northeast trending vertical faults and the two dipping faults in the step‐over region, with very minor displacements occurring during the interseismic period when the northeast trending vertical faults are locked. These models suggest that the gently dipping central step‐over fault is a reverse fault and that the steeper fault, extending to the southeast of the step‐over, acts as a normal fault over the long term.
An earthquake cycle along the south flank of Kilauea Volcano, Hawaii
Authors: Dvorak John
Published: February 2004

The intrusion of magma that led to the beginning of the January 3, 1983, eruption along the middle east rift zone of Kilauea Volcano, Hawaii, resulted in widening of the rift zone and compression of the adjacent portion of the south flank of the volcanic edifice. An earlier rift zone event, and subsequent eruption, on September 12, 1977, probably had a similar effect on the south flank. The compression built up since 1977 was relieved by a M = 6.1 earthquake on June 26, 1989. Horizontal displacements associated with the earthquake, determined by using the Global Positioning System (GPS), indicated seaward movement of the south flank of as much as 300 mm. Vertical displacements, measured by a conventional leveling technique and by GPS, indicated subsidence of as much as 250 mm near the east edge of the aftershock zone. These ground displacements can be modeled by horizontal slip of 1.2 m at an average depth of 6 km. The geodetic moment was 1×1026 dyn cm, a factor of 2 greater than the seismic moment, indicating a significant amount of ground movement was aseismic. The geodetic moment associated with the 1977 and 1983 rift zone events was about 0.8×1026 dyn cm, nearly equal to the geodetic moment of the 1989 south flank earthquake. The 6‐km depth of average slip during the earthquake was well above the 9‐km depth of the base of the volcano, which indicates the average depth where strain was released by the 1989 earthquake was well within the volcanic edifice. The strain released by the earthquake, and the initial compression caused by rift zone intrusions, occurred over a range of depths, indicating the south flank behaves more as a large elastic spring coupled to a sliding mass rather than a rigid block sliding along the base of the volcano.
Emplacement dynamics of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy
Authors: Quick James E. , Sinigoi Silvano , Mayer Adriano
Published: February 2004

The Ivrea‐Verbano Zone of northern Italy provides an opportunity to study directly the effects of intrusion of large volumes of mantle‐derived melts into the lower continental crust. Alpine uplift has exposed a complex of mafic to intermediate plutonic rocks, which had intruded the lower continental crust during the Permian. Field mapping reveals a gross arcuate structure within the complex that has survived tilting and uplift. In terms of the orientation during intrusion, banding and foliation are subhorizontal near the base of the complex and steepen gradually up section. Outcrop relations indicate that the cumulates were profoundly affected by extensional deformation while they still contained interstitial melts. The intensity of this hypersolidus deformation increases downward in the complex. The above characteristics are explained in terms of a model in which a huge volume of cumulates (8 km thick and >20 km long) crystallized from a small (≤1 km thick and ≤4 km wide), periodically refilled magma chamber which remained at a relatively fixed position through time. Large‐scale ductile deformation of the complex and transport of cumulates downward and outward from the magma chamber occurred as a consequence of extensional tectonics. This process is capable of generating large sheets of gabbro in the lower crust. Gabbro complexes formed in this manner would be characterized by (1) widespread evidence of synmagmatic deformation dominated by stretching, (2) downward increasing intensity of deformation, (3) upward steepening layering and foliation that is concave toward the center of spreading, and (4) a scarcity of crosscutting intrusive relationships.
Time dependent seismicity in the Indonesian region
Authors: Papadimitriou Eleftheria E. , Papazachos Basil C.
Published: February 2004

The Indonesian region was divided in 23 seismogenic sources and the interevent times of strong shallow mainshocks were determined and used for each source in an attempt at long‐term forecasting. The following relations have been obtained: log Tt = 0.15Mmin + 0.34Mp − 0.34 log mo + 6.54 and Mƒ = 0.92Mmin − 0.34Mp, + 0.53 log mo − 10.60, where Tt is the interevent time, measured in years, Mmin the surface wave magnitude of the smallest mainshock considered, Mp the magnitude of the preceding mainshock, Mƒ the magnitude of the following mainshock, and mo the moment rate in each source per year. On the basis of these relations and taking into account the time of occurrence and the magnitude of the last mainshock in each seismogenic source, time dependent conditional probabilities for the occurrence of the next large (Ms ≥ 7.0) shallow mainshocks during the next 10 years as well as the magnitudes of the expected mainshocks are determined.
Coulomb theory applied to accretionary and nonaccretionary wedges: Possible causes for tectonic erosion and/or frontal accretion
Authors: Lallemand Serge E. , Schnürle Philippe , Malavieille Jacques
Published: February 2004

Based on observations from both modem convergent margins and sandbox modeling, we examine the possible conditions favoring frontal accretion and/or frontal and basal tectonic erosion. Mean characteristic parameters (μ, μ*b and λ) are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the Coulomb theory is applicable. Natural observations reveal that “typical accretionary wedges” are characterized by low tapers with smooth surface slope and subducting plate, low convergence rates and thick trench sediment, while “nonaccretionary wedges” display large tapers with irregular surface slopes and rough subducting plate, high convergence rates and almost no trench fill. Sandbox experiments were performed to illustrate the effects of seamounts/ridges in the subduction zone on the deformation of an accretionary wedge. These experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer. This is followed by a tunnelling effect of the subducting seamount through the frontal wedge material, which results in considerable sand reworking. At an advanced subduction stage, the décollement jumps back from a high level in the wedge to its former basal position. We conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion. In contrast, several conditions may favor tectonic erosion of the upper plate. First, oceanic features, such as grabens, seamounts or ridges, may trap upper plate material during their subduction process. Second, destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism.
Paleoseismic evidence of clustered earthquakes on the San Andreas Fault in the Carrizo Plain, California
Authors: Grant Lisa B. , Sieh Kerry
Published: February 2004

Exposures we have excavated across the San Andreas fault contradict the hypothesis that part of the fault in the Carrizo Plain is unusually strong and experiences relatively infrequent rupture. The exposures record evidence of at least seven surface‐rupturing earthquakes which have been approximately dated by accelerated mass spectrometry radiocarbon analysis of detrital charcoal and buried in situ plants. Five large earthquakes have occurred since 1218 A.D. The most recent earthquake, event A, was the 1857 Fort Tejon earthquake, which we have associated with 6.6–10 m of dextral slip along the main fault trace. The penultimate earthquake, event B, most likely occurred within the period A.D. 1405–1510. Slip from either events B and C combined or from event B alone, totals 7–11 m. Three earthquakes, events C, D, and E, occurred in a temporal cluster prior to event B and after approximately A.D. 1218. The average recurrence interval within this cluster is 73–116 years, depending on assumptions. Events F and G occurred after 200 years B.C. A depositional hiatus between events E and F may hide evidence of additional earthquakes. Events B and D within the Carrizo cluster of A.D. 1218–1510 may correlate with events T (A.D. 1329–1363) and V (A.D. 1465–1495) at Pallett Creek on the Mojave “segment” of the fault. This suggests two fault ruptures similar in length to that of 1857. Events C and E apparently did not rupture the Mojave section, which suggests that the Carrizo segment has ruptured independently or in combination with segments to the north. Irregular repeat times of large earthquakes suggest a pattern of clustered events at the end of seismic “supercycles.”
The length‐scaling properties of topography
Authors: Weissel Jeffrey K. , Pratson Lincoln F. , Malinverno Alberto
Published: February 2004

The scaling properties of synthetic topographic surfaces and digital elevation models (DEMs) of topography are examined by analyzing their “structure functions,” i.e., the qth order powers of the absolute elevation differences: Δhq(l) = E{|h(x + l) ‐ h(x)|q}. We find that the relation Δh1(l) ≈clH describes well the scaling behavior of natural topographic surfaces, as represented by DEMs gridded at 3 arc sec. Average values of the scaling exponent H between ∼0.5 and 0.7 characterize DEMs from Ethiopia, Saudi Arabia, and Somalia over 3 orders of magnitude range in length scale l (∼0.1–150 km). Differences in apparent topographic roughness among the three areas most likely reflect differences in the amplitude factor c. Separate determination of scaling properties in the x and y coordinate directions allows us to assess whether scaling exponents are azimuthally dependent (anisotropic) or whether they are isotropic while the surface itself is anisotropic over a restricted range of length scale. We explore ways to determine whether topographic surfaces are characterized by simple or multiscaling properties. The difference between scaling exponents of Δh1(l) and
for the DEMs is small, but positive, and such divergence in the scaling exponents of the structure functions is consistent with multiscaling behavior. Exceedance and perimeter sets of fractional Brownian surfaces fail to yield the trivial fractal dimensions expected of sets of known monofractals, suggesting a practical limitation arising from the use of finite resolution data sets in the analysis. By comparing the hypsometry of “real” topography (represented as DEMs) with that of fractional Brownian surfaces, we show that synthetic surfaces based on Gaussian statistics are limited as models for natural topography. Hypsometric curves, which probably reflect the relative importance of tectonic and erosional processes in shaping topography, clearly show that statistical moments higher than the second are important in describing topographic surfaces. Scaling analysis is a valuable tool for assessing the quality and accuracy of DEM representations of the Earth's topography.
Influence of crystallization and entrainment of cooler material on the emplacement of basaltic aa lava flows
Authors: Crisp Joy , Baloga Stephen
Published: February 2004

A theoretical model is used to describe and investigate the effects of simultaneous crystallization, radiation loss, and entrainment of cooler material on the temperature of a well‐mixed core of an active aa lava flow. Entrainment of crust, levee debris, and base material into the interior of active flows has been observed, but the degree of assimilation and the thermal consequences are difficult to quantify. The rate of entrainment can be constrained by supplementing the theoretical model with information on the crystallization along the path of the flow and estimation of the radiative loss from the flow interior. Application of the model is demonstrated with the 1984 Mauna Loa flow, which was erupted about 30°C undercooled. Without any entrainment of cooler material, the high crystallization rates would have driven temperatures in the core well above temperatures measured by thermocouple and estimated from glass geothermometry. One plausible scenario for this flow, which agrees with available temperature and crystallinity measurements, has a high initial rate of entrainment during the first 8 hours of travel (a mass ratio of entrained material to fluid core of about 15% if the average temperature of the entrained material was 600°C), which counterbalances the latent heat from approximately 40% crystallization. In this scenario, the model suggests an additional 5% crystallization and a 5% entrainment mass ratio over the subsequent 16‐hour period. Measurements of crystallization, radiative losses, and entrainment factors are necessary for understanding the detailed thermal histories of active lava flows.
Micromagnetic modeling of two‐dimensional domain structures in magnetite
Authors: Xu Song , Dunlop David J. , Newell Andrew J.
Published: February 2004

Micromagnetic studies of two‐dimensional domain structures in various local energy minimum states were carried out for 1‐ and 5‐μm cubes of magnetite. Domain structures obtained for the cubes are relatively simple compared to ones from previous two‐and three‐dimensional calculations for smaller magnetite grains (Newell et al., 1993a; Williams and Dunlop, 1989, 1990). With an initially assumed one‐dimensional lamellar structure, the final two‐dimensional structure reveals the formation of closure domains at the grain surfaces. The magnetization directions in closure domains are found to be determined largely by the magnetostatic energy, rather than the magnetocrystalline anisotropy as normally expected. The number of body domains that gives the lowest‐energy state for the 1‐μm magnetite cube is 2, while the number for the 5‐μm cube is 4. These are much smaller equilibrium numbers of domains than the 6 and 13 domains obtained from a one‐dimensional model by Moon and Merrill (1985) and the 4 and 10 domains predicted from a quasi‐two‐dimensional analysis by Ye and Merrill (1991) for 1‐and 5‐μm cubes, respectively. The numbers of domains predicted are comparable to the numbers observed in magnetite grains of similar sizes.
A revisit to P wave travel time statics at teleseismic stations
Authors: Zhou Hua‐wei , Wang Hongwei
Published: February 2004

To account for local deviations from the one‐dimensional reference velocity model, the station correction formulation of Dziewonski and Anderson (1983) is extended to include the travel time variation with respect to the ray takeoff angle. The station statics are obtained through an iterative retrieval of the lower‐degree spherical harmonics normalized over the data takeoff angle range on station residual spheres, and similar event statics are introduced to decouple the effects of near‐station and near‐event heterogeneities. Except the zero degree constant term, all higher‐degree static terms are obtained on a regional rather than an individual station/event location. The P wave static corrections up to degree and order 2 are derived for 1343 stations using nearly 3 million International Seismological Center travel time residuals with respect to the iasp91 model and with event relocation and summary ray sorting. The resulting station statics account for nearly 27% of the variance of the P wave travel time residuals. Patterns of the zero‐degree statics correlate well with surface tectonic features and with previous results. At most stations over subduction zones the slowest direction of the secondorder statics, or the second‐degree in azimuth, is perpendicular to the trench.
Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier: 1. Observations
Authors: Meier Mark , Lundstrom Scott , Stone Dan , Kamb Barclay , Engelhardt Hermann , Humphrey Neil , Dunlap William W. , Fahnestock Mark , Krimmel Robert M. , Walters Roy
Published: February 2004

Measurements of glacier flow velocity and basal water pressure at two sites on Columbia Glacier, Alaska, are combined with meteorological and hydrologic data to provide an observational basis for assessing the role of water storage and basal water pressure in the rapid movement of this large glacier. During the period from July 5 to August 31, 1987, coordinated observations were made of glacier surface motion and of water level in five boreholes drilled to (or in one case near to) the glacier bed at two sites, 5 and 12 km from the terminus. Glacier velocities increased downglacier in this reach from about 4 m d−1 to about 7 m d−1. Three types of time variation in velocity and other variables were revealed: (1) Diurnal fluctuation in water input/output, borehole water level, and ice velocity (fluctuation amplitude 5 to 8%); (2) Speed‐up events in glacier motion (15–30% speed up), lasting about 3 days, and occurring at times of enhanced input of water, in some cases from rain and in others from ice ablation enhanced by strong, warm winds; (3) “Extra‐slowdown” events, in which, after a speed‐up event, the ice velocity decreased in about 3 days to a level consistently lower than that prior to the speed‐up event. All of the time variations in velocity were due, directly or indirectly, to variations in water input to the glacier. The role of basal water in causing the observed glacier motions is interpreted by Kamb et al. (this issue).
Lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths
Authors: Sleep Norman H.
Published: February 2004

Mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots. The fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material. It has been proposed either that (1) secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or (2) that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly. A parameterized method of modeling transient secondary convection is developed to examine this question. Calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity. In addition, vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the Hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like Cape Verde. This conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the Hawaiian swell.
Present‐day kinematics of the Middle East and eastern Mediterranean
Authors: Westaway Rob
Published: February 2004

The Middle East and eastern Mediterranean regions contain three major strike‐slip fault zones: the Dead Sea fault zone (DSFZ) and the North and East Anatolian fault zones (NAFZ and EAFZ). These accommodate northward motion of the African and Arabian plates relative to Eurasia and westward motion of the small Turkish plate. This study deduces an internally consistent solution for the present‐day kinematics of these plates, which is accurate to better than 20% and applicable throughout time since ∼5 Ma, by constraining the slip senses and rates on these strike‐slip fault zones. The 7 (±0.5) mm yr−1 left‐lateral slip rate on the DSFZ is associated with counterclockwise rotation of Arabia relative to Africa at 0.33° m.y−1 around an Eulcr pole near 33°N 23°E. The NAFZ takes up counterclockwise rotation of the Turkish plate relative to Eurasia at 0.83° m.y.−1 (±13%) around a pole at 31°N 35.5°E, with right‐lateral slip rate ∼15 mm yr−1. The resulting prediction for the EAFZ involves ∼14 mm yr−1 left‐lateral motion and ∼2 mm yr−1 of shortening between Arabia and Turkey. The triple junction of Africa, Arabia and Turkey is near the city of Kahramanmaras. This locality contains a narrow promontory, bounded by SSW‐trending left‐lateral faults at the edges of the Arabian and Turkish plates, which is loosely attached to the African plate, moving westward relative to its interior at ∼4 mm yr−1.
Phase transition Clapeyron slopes and transition zone seismic discontinuity topography
Authors: Bina Craig R. , Helffrich George
Published: February 2004

The depths, widths, and magnitudes of the 410‐km and 660‐km seismic discontinuities are largely consistent with an isochemical phase change origin, as is the observation that the topography on these discontinuities is negatively correlated and significantly smaller than predicted for chemical changes. While most thermodynamic studies of the relevant phase changes predict greater topography on the 410 than the 660, recent seismic studies suggest greater topography on the 660. The seismic results are consistent with some recent thermochemical studies which suggest that the Clapeyron slopes of the perovskite‐forming reactions exceed in magnitude those of the spinel‐forming reactions; however, we have reexamined the relevant Clapeyron slopes in light of other, more recent, experimental studies as well as the requirements of internal thermodynamic consistency. We conclude that the bulk of the evidence indicates a greater Clapeyron slope magnitude for the 410 than for the 660. Thus the recent seismic results are unexpected. One explanation might be that lateral temperature variations near 660 km depth exceed those near 410, consistent with a model of the 660 as a thermal boundary layer. An alternate interpretation, which requires neither a thermal boundary nor metastable olivine, is that the 410 does possess greater topography but is simply less visible seismically than the 660. This latter idea, and recent short‐period observations of P’410P’ seismic phases in conjunction with an elevated 660, is consistent with thermodynamic modeling of subduction zones illustrating the extreme broadening of the olivine α → β transition in cold slab interiors and, conversely, its sharpening in regions of high temperature.
Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier: 2. Interpretation
Authors: Kamb Barclay , Engelhardt Hermann , Fahnestock Mark A. , Humphrey Neil , Meier Mark , Stone Dan
Published: February 2004

The data presented in part 1 of this paper (Meier et al., this issue) are here used to assess the role of water input/output, water storage, and basal water pressure in the rapid movement of Columbia Glacier, Alaska. Consistently high basal water pressures, mostly in the range from 300 kPa below to 100 kPa above the ice overburden pressure, are responsible in an overall way for the high glacier flow velocities (3.5–9 m d−1), which are due mainly to rapid basal sliding caused by the high water pressure. Diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site. The water pressure fluctuation tracks the time‐integrated water input (less a steady withdrawal), as expected for the diurnally fluctuating storage of water in the glacier far from the terminus. At the downglacier site, the situation is more complex. Diurnal peaks in water level, which are directly related to intraglacial water storage as well as to basal water pressure, are shifted forward in time by 4 hours, probably as a result of the effect of diurnal fluctuation in water output from the glacier, which affects the local water storage fluctuations near the terminus. Because of the forward shift in the basal water pressure peaks, which at the downglacier site lead the velocity peaks by 6 hours, a mechanical connection between water pressure and sliding there would have to involve a 6‐hour (quarter period) delay. However, the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single, consistent control mechanism at both sites. The velocity variations in nondiurnal “speed‐up events” caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier. It appears that small variations in water pressure (≤100 kPa) sufficient to produce the observed velocity variations (15–30%) are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity. At the achieved level of observation the clearest (though not complication free) control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed.
Seismic response and construction of seaward dipping wedges of flood basalts: Vøring volcanic margin
Authors: Planke Sverre , Eldholm Olav
Published: February 2004

Geological and geophysical data from the >900 m of volcanic basement drilled at Ocean Drilling Program Site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins. The main part of the drilled section, corresponding to a prominent seaward dipping reflector sequence, consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments. The basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity, pore aspect ratio distribution and alteration. Stacks of laterally continuous basalt flows appear to have thin‐layer transverse isotropic properties for typical wavelengths in multichannel seismic data. Vertical seismic profiling and average sonic log velocities are similar, 3.77 km/s and 3.88 km/s, respectively, while comparable refraction velocities are 10–20% higher. Synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces, though the most continuous reflectors are related to thick flows. Seismic models based on the characteristic velocity and density basalt flow distributions and Site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows. The base of the dipping reflector sequence, reflector K, correlates with flows in the lower part of upper series basalts, while the transition from basaltic to underlying dacitic/andesitic lavas correlates with a locally defined reflector. A model for the breakup related volcanism includes (1) prebreakup dacitic/andesitic volcanism, (2) early breakup basaltic volcanism infilling the prebreakup relief, (3) main breakup stage with intense, focused volcanism, large subsidence and lava pounding, and (4) late breakup volcanism during a period of decreased subsidence and local off‐axis activity.
A structural fabric defined by topographic lineaments: Correlation with Tertiary deformation of Ellesmere and Axel Heiberg Islands, Canadian Arctic
Authors: Oakey Gordon
Published: February 2004

Digital topographic contours from four 1:250000 scale maps have been gridded to produce a digital elevation model for part of Ellesmere and Axel Heiberg islands in the Canadian Arctic Islands. Gradient calculations were used to define both east and west dipping slopes defining a pattern of lineaments that have been compared with mapped geological structures. In ice‐covered areas, where geological mapping was not possible, well‐defined topographic lineaments have been identified and are correlated to extensions of major structural features. The northeast‐southwest patterns of both topographic lineaments and mapped structures are strongly unimodal and support a single compressive event oriented at 67° west of north. This orientation is coincidental with the convergence direction calculated from the kinematic poles of rotation for Greenland relative to North America between 56 and 35 Ma. A minor secondary peak at 70° east of north is observed for thrust and normal fault solutions and is not directly related to the predicted convergence direction. Whether this represents a unique phase of deformation or is a subcomponent of a single event is not known. The agreement of structural components, lineament orientations, and convergence direction suggests an overwhelming overprint of Eurekan deformation on any preexisting structural fabric. This study confirms, for the first time, an excellent compatibility between geological and geophysical constraints for the timing and geometry of the Eurekan orogeny.
Contributions of Cretaceous Quiet Zone natural remanent magnetization to Magsat anomalies in the Southwest Indian Ocean
Authors: Fullerton Lawrence G. , Frey Herbert V. , Roark James H. , Thomas Herman H.
Published: February 2004

The Magsat magnetic anomalies over the Southwest Indian Ocean are modeled using a combination of induced plus viscous remanent magnetization (IM/VRM) and natural remanent magnetization (NRM). Two broad, roughly parallel, SW to NE trending triple‐peaked positive anomalies dominate the region, one lying south of Africa and the other north of Antarctica. Although these anomaly peaks generally correspond with the Agulhas Plateau/Maud Rise, Mozambique Plateau/Astrid Ridge, and Madagascar Ridge/Conrad Rise conjugate pairs, the IM/VRM contribution from structural characteristics (i.e., crustal thickness) accounts for only about 20% of the anomaly amplitudes. A spatially variable but observationally constrained NRM contribution in Cretaceous Quiet Zone (KQZ) crust is required to account for the location, shape, and amplitude contrast of these anomalies. Many crustal features in the Southwest Indian Ocean near Antarctica have little geophysical data to constrain their structure but do have tectonic conjugates near Africa for which much more geophysical data are generally available. Using geophysical and geological constraints from one member to model the magnetization structure of its conjugate reproduces the observed Magsat reduced‐to‐pole anomalies over both structures very well. This suggests that no significant alteration in their magnetization structure has occurred since the features split. Models of these conjugate structures show that IM/VRM reproduces the Magsat anomalies associated with non‐KQZ crust but that both IM/VRM and a dominant NRM component are required to explain the anomalies associated with KQZ crust.
Inversion for source parameters from sparse data sets: Test of the method and application to the 1951 (M=6.9) Kona, Hawaii, earthquake
Authors: Beisser Martin , Gillard Dominique , Wyss Max
Published: February 2004

We have tested a method originally developed by Beisser et al. (1990) to retrieve the source parameters (strike, dip, rake, and depth) of sparsely recorded earthquakes from the inversion of teleseismic waveform data. The complete wave train of all body waves is modeled using the reflectivity method. The parameter space of strike, dip, and rake is searched to find the source orientation which leads to the minimum misfit between the observed and the synthetic seismogram. The 1983 Kaoiki, Hawaii, earthquake (MS = 6.6) was chosen as a test case. The inversion of the full data set (16 stations) gave a fault plane solution similar to the best double couple moment tensor solution of Harvard University and National Earthquake Information Centre. These three solutions were averaged to create a standard solution. Sparse data sets were simulated by decimating the full data set, and the resulting fault plane solutions were compared with the standard. We found that as few as one to three stations were sufficient to retrieve the focal mechanism of the 1983 Kaoiki event. We applied this technique to the 1951 Kona, Hawaii, earthquake (MS = 6.9). A total of four stations and nine components were used to model the source parameters of this earthquake. The depth was estimated at 13±3 km. The fault plane solution was a decollement type with a near‐horizontal plane dipping at about 15° to the southwest and a near‐vertical plane striking NW‐SE. This observation supports a tectonic model for the Kona coast similar to that of Kilauea's south flank; the upper crust is pushed away from the center of Hawaii, slipping westward along a near‐horizontal plane of weakness.
A theoretical model for thrust‐induced deep groundwater expulsion with application to the Canadian Rocky Mountains
Authors: Ge Shemin , Garven Grant
Published: February 2004

This paper presents a numerical model for simulating deformation and induced fluid flow in fold‐and‐thrust belts. Unfractured rock strata are modeled as poroelastic media while fault zones are treated as plastic‐elastic media. We introduce the slip element technique into a finite element code to accommodate large deformations along faults. Thrust displacements, stress field changes, and the effects of thrust faulting on groundwater flow are investigated by solving the coupled stress and flow equations numerically. The calculation shows that when a thrust sheet is displaced along its fault surface, the displacement‐induced stress generates high pore pressure zones near the tectonic stress boundary and beneath low permeability ramps. These overpressures cause transient fluid flow across the thrust belt. Sensitivity studies on the hydrologic properties of the fault zone suggest that hydraulic conductivities within a fault play important roles in initiating slip deformation and in determining the extent of transient disturbances to the flow field. Low permeability can result in rapid pore pressure buildup in the fault, thereby reducing the effective strength of the fault which leads to earlier failures. A low‐permeability fault can also impede the movement of flow into the footwall, thereby limiting the tectonic impact on the flow system within the hanging‐wall. Application of the model to the McConnell Thrust in the Canadian Rockies indicates that the total volume of fluid flow induced by tectonic compression could have been of the order of 105 to 106 m3 over a time period of tens to hundreds of years accompanied by an average 100 m of thrust movement.
Natural controls of fluvial denudation rates in major world drainage basins
Authors: Summerfield M. A. , Hulton N. J.
Published: February 2004

We present a new compilation of estimates of modern rates of mechanical and chemical denudation for externally drained basins exceeding 5×105 km2 in area. These estimates are based on sediment and solute load data selected in order to represent natural rates as far as possible. Chemical denudation rates have been calculated by deducting the nondenudational component of solute load. Mechanical denudation rates range from 1 mm kyr−1 for the St. Lawrence and Dnepr basins to 670 mm kyr−1 for the Brahmaputra basin. Chemical denudation rates vary from 1 mm kyr−1 (Kolyma, Niger, Nile and Rio Grande basins) to 27 mm kyr−1 (Chiang Jiang basin). The Kolyma basin has the lowest (4 mm kyr−1), and the Brahmaputra basin the highest, overall rate of denudation (688 mm kyr−1). Relationships between denudation rates and a range of morphometric, hydrologic, and climatic variables are investigated through correlation and regression analysis. Morphometric variables, such as mean local relief, are accurately calculated for large basins for the first time by using the National Geophysical Data Center 10‐minute topographic database. Variables expressing basin relief characteristics and runoff are found to be most strongly associated with both mechanical and chemical denudation rates, with more than 60% of the variance in total denudation being accounted for by basin relief ratio and runoff. Basin area, runoff variability, and mean temperature, however, are only weakly associated with rates of denudation. Although direct comparisons cannot be made, it appears that rates of basin denudation derived from present‐day mass flux estimates are not, overall, significantly different from estimates of long‐term rates based on sediment volume and thermochronologic data. It therefore appears that the key factors identified as controlling denudation rates here are also applicable to the geological time spans relevant to the interaction between tectonic and denudational processes.