bluuebunny/crossref_metadata_2025 huggingface dataset:
Regional scale evaporation and the atmospheric boundary layer
Authors: Parlange Marc B. , Eichinger William E. , Albertson John D.
Published: February 2004

Evaporation into the atmosphere is fundamental to the fields of hydrology, meteorology, and climatology. With evolving interest in regional and global hydrologic processes there is an increasing recognition of the importance of the study of evaporation and land surface water balances for length scales of the order of 10 km. To obtain regional scale fluxes of water vapor, heat, and momentum, it is important to understand transport in the atmospheric boundary layer (ABL), which is defined to be that part of the atmosphere directly influenced by the land surface. In this review we briefly summarize some current models of evaporation and the ABL and discuss new experimental and computational opportunities that may aid our understanding of evaporation at these larger scales. In particular, consideration is given to remote sensing of the atmosphere, computational fluid dynamics and the role numerical models can play in understanding land‐atmosphere interaction. These powerful modeling and measurement tools are allowing us to visualize and study spatial and temporal scales previously untouched, thereby increasing the opportunities to improve our understanding of land‐atmosphere interaction.
Three‐dimensional electromagnetic modeling using finite difference equations: The magnetotelluric example
Authors: Mackie Randall L. , Smith J. Torquil , Madden Theodore R.
Published: February 2004

We have developed a robust and efficient finite difference algorithm for computing the magnetotelluric response of general three‐dimensional (3‐D) models using the minimum residual relaxation method. The difference equations that we solve are second order in H and are derived from the integral forms of Maxwell's equations on a staggered grid. The boundary H field values are obtained from two‐dimensional transverse magnetic mode calculations for the vertical planes in the 3‐D model. An incomplete Cholesky decomposition of the diagonal subblocks of the coefficient matrix is used as a preconditioner, and corrections are made to the H fields every few iterations to ensure there are no H divergences in the solution. For a plane wave source field, this algorithm reduces the errors in the H field for simple 3‐D models to around the 0.01% level compared to their fully converged values in a modest number of iterations, taking only a few minutes of computation time on our desktop workstation. The E fields can then be determined from discretized versions of the curl of H equations.
Static image principle for anisotropic layered medium using transmission line analogy
Authors: Ermutlu M. E.
Published: February 2004

Anisotropic medium in electrostatic problems has been a great interest for geophysicists because it allows the use of more realistic models in geophysical studies. Problems involving planar anisotropic medium have been studied by different authors and image principles of these kinds of problems, like an anisotropic half‐space bounded by a perfectly electrically or magnetically conducting plane, an impedance surface, or two similar transverse anisotropies, have been applied. Same examples are also considered by using transmission line analogy. This paper deals with a static problem involving layered anisotropic structure consisting of three different media with similar transverse anisotropies. A transmission line analogy is exploited to derive an electrostatic solution for reflection and transmission problems. A quasi‐physical interpretation of these solutions in terms of image sources is given.
Inversion of diffusive transient electromagnetic data by a conjugate‐gradient method
Authors: Wang Tsili , Oristaglio Michael , Tripp Alan , Hohmann Gerald
Published: February 2004

Inversion of three‐dimensional transient electromagnetic (TEM) data to obtain electrical conductivity and permeability can be done by a time‐domain algorithm that extends to diffusive electromagnetic (EM) fields the imaging methods originally developed for seismic wavefields (Claerbout, 1971; Tarantola, 1984). The algorithm uses a conjugate‐gradient search for the minimum of an error functional involving EM measurements governed by Maxwell's equations without displacement currents. The connection with wavefield imaging comes from showing that the gradient of the error functional can be computed by propagating the errors back into the model in reverse time and correlating the field generated by the backpropagation with the incident field at each point. These two steps (backpropagation and cross correlation) are the same ones used in seismic migration. The backpropagated TEM fields satisfy the adjoint Maxwell's equations, which are stable in reverse time. With magnetic field measurements the gradient of the error functional with respect to conductivity is the cross correlation of the backpropagated electric field with the incident electric field, whereas the gradient with respect to permeability is the cross correlation of the backpropagated magnetic field with the time derivative of the incident magnetic field. Tests on two‐dimensional models simulating crosswell TEM surveys produce good images of a conductive block scatterer, with both exact and noisy data, and of a dipping conductive layer. Convergence, however, is slow.
Solution of 2.5‐dimensional problems using the Lanczos decomposition
Authors: Allers A. , Sezginer A. , Druskin V. L.
Published: February 2004

We consider the problem of electrical conduction in the context of geophysical prospecting and assume that the conductivity of the Earth is constant in a direction perpendicular to the probing plane. The resulting boundary value problem is reduced to two dimensions via a Fourier transform with respect to this direction. To date, the typical method of solution involves solving several of these two‐dimensional problems and computing the approximate inverse Fourier transform numerically. We propose a more efficient approach in which the inverse Fourier integral is taken analytically. This method involves the computation of an analytic function of the matrix approximation to a differential operator using its Lanczos decomposition. After deriving the method we present numerical verification of its validity and a discussion of its computational cost, which approaches that of two‐dimensional problems.
An iterative method for inverse scattering problems based on an exact gradient search
Authors: Barkeshli S. , Lautzenheiser R. G.
Published: February 2004

A new iterative method based on an exact gradient search for inverse scattering problems is developed. In this scheme the two unknowns (the electric field and the electrical parameter within the object) are solved simultaneously. The bilinear property of the coupled integral equations is employed to drive the gradient of the residual ∇Φ. The exact gradient formulation allows one to perform an efficient line search for the solution. Some numerical results based on two‐dimensional inverse scattering problems are presented to demonstrate the efficiency of the algorithm.
Fade measurements on the wide bandwidth HF channel
Authors: Rifkin Robert , Perry B. Dan
Published: February 2004

This paper presents the results of HF channel fading measurements on 2000 and 3000 km midlatitude paths using a 1‐MHz instantaneous bandwidth direct‐sequence pseudonoise (DSPN) probing signal. The channel impulse response is estimated by correlating the received probe signal with a local DSPN reference. The channel is characterized in terms of the autocovariance and distribution functions of the received power. Bandwidth‐dependent characteristics are obtained by Fourier transforming the measured impulse response and then applying appropriate filtering. The measured long‐term wideband power statistics show a median standard deviation of 2–3 dB and median decorrelation times of 37 to 57 s. Both median fade depth and median incremental fade depth always decrease with increasing bandwidth.
Rapid 2.5‐dimensional forward modeling and inversion via a new nonlinear scattering approximation
Authors: Torres‐Verdín Carlos , Habashy Tarek M.
Published: February 2004

We introduce a novel approximation to numerically simulate the electromagnetic response of point or line sources in the presence of arbitrarily heterogeneous conductive media. The approximation is nonlinear with respect to the spatial variations of electrical conductivity and is implemented with a source‐independent scattering tensor. By projecting the background electric field(i.e., the electric field excited in the absence of conductivity variations) onto the scattering tensor, we obtain an approximation to the electric field internal to the region of anomalous conductivity. It is shown that the scattering tensor adjusts the background electric field by way of amplitude, phase, and cross‐polarization corrections that result from frequency‐dependent mutual coupling effects among scatterers. In general, these three corrections are not possible with the more popular first‐order Born approximation. Numerical simulations and comparisons with a 2.5‐dimensional finite difference code show that the new approximation accurately estimates the scattered fields over a wide range of conductivity contrasts and scatterer sizes and within the frequency band of a subsurface electromagnetic experiment. Furthermore, the approximation has the efficiency of a linear scheme such as the Born approximation. For inversion, we employ a Gauss‐Newton search technique to minimize a quadratic cost function with penalty on a spatial functional of the sought conductivity model. We derive an approximate form of the Jacobian matrix directly from the nonlinear scattering approximation. A conductivity model is rendered by repeated linear inversion steps within range constraints that help reduce nonuniqueness in the minimization of the cost function. Synthetic examples of inversion demonstrate that the nonlinear approximation reduces considerably the execution time required to invert a large number of unknowns using a large number of electromagnetic data.
A comparative study of an absorbing boundary condition and an artificial absorber for truncating finite element meshes
Authors: Özdemir T. , Volakis J. L.
Published: February 2004

The type of mesh termination used in the context of finite element formulations plays a major role on the efficiency and accuracy of the field solution. In this work we evaluate the performance of an absorbing boundary condition and an artificial absorber (a new concept) for terminating the finite element mesh. This analysis is done in connection with the problem of scattering by a finite slot array in a thick ground plane. The two approximate mesh truncation schemes are compared with the exact finite element ‐ boundary integral (FEM‐BI) method in terms of accuracy and efficiency. It is demonstrated that both approximate truncation schemes yield reasonably accurate results even when the mesh is extended only 0.3 wavelengths away from the array aperture. However, the artificial absorber termination method leads to a substantially more efficient solution. Moreover, it is shown that the FEM‐BI method remains quite competitive with the FEM‐artificial absorber method when the fast Fourier transform is used for computing the matrixvector products in the iterative solution algorithm. These conclusions are indeed surprising and of major importance in electromagnetic simulations based on the finite element method.
Study of measured and predicted reliability of the ionospheric HF communication channel at high latitudes
Authors: Thrane E. V. , Jodalen V. , Stewart F. , Saleem D. , Katan J.
Published: February 2004

This paper describes observations of the reliability of the ionospheric HF communication channel within and close to the northern auroral zone. Automatic equipment has been developed to measure reliability and virtual reflection height as a function of frequency in an hourly cycle. Measurements have been made over two propagation paths within Norway. The data show diurnal and seasonal variations as well as dependence of the measured parameters on geomagnetic conditions. The observations have been compared with predictions obtained from the Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC) prediction code. This code includes a model of the high‐latitude and polar ionospheres and their dependence upon geomagnetic disturbances. The results of the comparison show that ICEPAC represents an improvement over previous codes but that transmission losses are not properly included.
Three years of C band signal measurements for overwater, line‐of‐sight links in the mid‐Atlantic coast: 1. Fade statistics
Authors: Goldhirsh Julius , Dockery G. Daniel , Musiani Bert H.
Published: February 2004

Three years of fade statistics are described for two overwater, line‐of‐sight propagation linksalong the mid‐Atlantic coast of the United States operating at 4.7 GHz. Single‐terminal, joint fade, and fade duration statistics derived from time series of received signal losses due to refractive and diffractive effects are described for combined‐year, annual, and monthly cases. In particular, year‐to‐year and month‐to‐month variabilities in the statistics and also the efficacy of employing space diversity for the two links to mitigate fade margin requirements are examined. Sustained deep fade (SDF) events due to severe subrefraction during the 3‐year period tended to dominate the statistics. Statistics have been culled in terms of contiguous month groupings during which SDF events occurred (November–July) and did not occur (August–October) over the 3‐year period. Although the year‐to‐year variability in the annual fade statistics was relatively small, the year‐to‐year variability in the monthly statistics was large, especially during the April–July period, which separated the periods during which deep subrefractive fades were maximum and the periods during which they were nonexistent.
Model‐assisted ionospheric tomography: A new algorithm
Authors: Raymund T. D. , Bresler Y. , Anderson D. N. , Daniell R. E.
Published: February 2004

Ionospheric tomography uses total electron content (TEC) records collected by longitudinally aligned stations, which receive a beacon satellite orbiting overhead. The electron density distribution is reconstructed for the region bounded by the satellite orbit and the line of ground receivers. A new reconstruction algorithm is described which satisfies the TEC records, makes use of an ionospheric model, and allows the incorporation of complementary measurements. The new algorithm also accepts relative (rather than absolute) TEC as input data. A comparison of the new algorithm and one used recently shows significant improvement over early techniques, particularly when a scaled ionogram is included in the data.
Three‐dimensional ray paths in cylindrical ducts: Upper branch cold plasma O and X modes
Authors: James H. G.
Published: February 2004

The guiding or ducting of O and X mode waves along magnetic‐field‐aligned density irregularities has been studied by integrating the Haselgrove equations in cylindrical coordinates r, ϕ, z, where the z axis is parallel to the magnetic field (B). The density profile of the irregularity is a monotonie function of the radius r; linear and Gaussian functions of r are applied. The density rises from a minimum at r = 0 to a maximum at its outer limit, usually defined by the cutoff condition for the frequency and mode in question. Quasi‐helical paths are obtained whose shape projected onto the r‐ϕ plane perpendicular to B generally resembles a rotating ellipse. These shapes are similar to the orbital path of a particle in a central potential field. Indeed, for O mode propagation restricted to r and ϕ and a linear radial density profile, a closed‐form solution is obtained in the form of an elliptical integral. Peculiarities are found, such as rays that are perfectly circular and others that have a fixed quasi‐elliptical shape in the r‐ϕ plane. Other parameters varied in the study include the relative depletion of the axial density below ambient density and the ray starting position and direction. The results suggest that three‐dimensional effects may be observable in certain areas of high‐frequency research in the ionosphere‐magnetosphere.
A neural network to identify stray signals in ionograms
Authors: Koschmieder T. H.
Published: February 2004

A neural network will be used to identify stray signals in ionograms. The signals to be identified come from high‐frequency (HF, 2–30 MHz) transmitters that emit during the collection of the ionosonde data for the ionogram. Unfortunately, the ionosonde accepts the HF signals as valid and records them as data. The developed neural network correctly identifies 85% of the stray HF signals.
An approximate solution for scattering by an impedance wedge at skew incidence
Authors: Syed Hasnain H. , Volakis John L.
Published: February 2004

Diffraction coefficients are derived from an approximate solution of the governing functional difference equations to characterize the scattering by an impedance wedge illuminated at skew incidence. This solution recovers the exact known diffraction coefficients for an impedance half plane and a wedge at normal incidence. To validate the approximate solution for other wedge angles, a comparison is made with numerical data obtained from a moment method code applicable to skew incidence on a polygonal impedance cylinder.
Galileo entry probe recovery of fine structure in the zonal wind profile at Jupiter
Authors: Atkinson D. H. , Pollack J. B.
Published: February 2004

The algorithm developed to recover the zonal (east‐west) wind profile at Jupiter by Doppler tracking of the Galileo probe is well established in theory and verified by computer simulation. We extend the Doppler wind recovery algorithm to include detection and extraction of microscale wind structure. When the probe encounters regions of high wind shear, turbulence, atmospheric waves, or nonsteady aerodynamics (buffeting) a rapidly varying component is introduced into the probe telemetry Doppler data. These effects can be isolated by their respective Doppler signatures. Analysis of residuals allows fine structure in the wind profile to be located and retrieved. The overall wind recovery is a three‐step process: (1) The large‐scale zonal wind profile is recovered by converting the probe telemetry frequency residuals (as caused by the Doppler effect) to a velocity and back‐projecting it into the local horizontal east‐west direction at the probe location. The zonal winds are derived from the velocity residuals by a least squares algorithm. (2) Regions of high zonal wind shear are recovered numerically relative to the large‐scale wind structure and serve as a correction to the previously derived large‐scale profile. (3) The probe trajectory is updated to reflect both the large‐ and small‐scale wind structure, and the large‐scale wind profile is recalculated. The Galileo Doppler wind experiment described previously and the wind microstructure retrieval algorithm outlined here are the only reasonable means by which the vertical profile of the zonal winds at Jupiter can be directly sampled.
Series expansion of electrostatic potential radiated by a point source in isotropic Maxwellian plasma
Authors: Béghin C.
Published: February 2004

A new algebraic approach is proposed to calculate the electrostatic potential distributed around a point source in isotropic Maxwellian plasma. The method derives a power series expansion of the radial distance from the source with frequency‐dependent coefficients. Distance and frequency are normalized to the Debye length and to the plasma frequency, respectively, so that the expression keeps its entire generality whatever the experimental conditions might be. The proposed method is based upon the Mittag‐Lefler expansion of the inverse of the plasma dispersion function for the infinite series of Landau poles. After mathematical clarification of the validity of this expansion, a significant correction of the previous works leads to a self‐consistent interpretation of the true contribution of the higher‐order poles at large distance from the source. The power series expansion is compared to the classical so‐called “Landau wave approximation” which is proved to include in reality the contribution of higher‐order poles independently from the plasma temperature. For practical use the power expansion is needed to obtain a precise result at distances from the source shorter than about 15 Debye lengths, while the Landau wave approximation gives correct results at larger distances. This work provides all necessary baselines for precise three‐dimensional modeling of mutual impedance devices to be used in space plasma experiments where the Debye length is comparable to the spacecraft size.
The Ionospheric Communication Analysis and Prediction Program (IONCAP): Reliability and service probability calculations
Authors: Spaulding A. D. , Stewart F. G.
Published: February 2004

This paper presents an analysis and explanation of the system performance calculations performed by the reliability and service probability subroutines of the Ionospheric Communication Analysis and Prediction Program (IONCAP) and an IONCAP derivative, termed VOACAP. A review of the three components needed to properly statistically describe the performance of a communications system or link is presented. These components are needed to account for the short‐term and long‐term statistical variations of the desired signal and the interference and also the inherent prediction errors. A detailed numerical example is given for illustration and explanation. Suggestions for modification and improvement are also given.
Virtual heights for oblique path calculations
Authors: Reilly Michael H.
Published: February 2004

The propagation model of the Ionospheric Communications Analysis and Predictions Program (IONCAP) program, which is designed for fast throughput calculations in the prediction of high‐frequency sky wave system performance, makes use of virtual heights, as calculated from known height profiles of the electron density. Use of these for oblique path calculations relies on the well known flat Earth theorems of Breit‐Tuve and Martyn, with additional curved Earth correction terms. These approximate corrections are examined in the light of exact results from other models, and alternative strategies are discussed.
A long wave transverse electric‐transverse magnetic noise prediction model
Authors: Warber C. R. , Field E. C.
Published: February 2004

This paper describes a computerized physical model that predicts both horizontally and vertically polarized noise in the ELF to LF band (10 Hz to 60 kHz). Since naturally occurring radio noise in this band is produced by lightning and propagates to the receiver via the Earth‐ionosphere waveguide, the model starts with average lightning flash density data from which it calculates radiated power for horizontal and vertical noise. Adjustments are made to the radiated power to account for seasonal and latitudinal differences in the lightning processes. The noise power is then integrated over fairly large geographic areas to formulate horizontal and vertical equivalent noise transmitters. The power radiated from each of these transmitters is propagated to the receiver location using standard anisotropic long wave propagation algorithms and well‐known models of the Earth‐ionosphere waveguide. From the received power the model predicts RMS noise, standard deviation, voltage deviation VD, and the amplitude probability distribution of the noise for both polarizations. Since the model is based on theory, it can also predict these parameters under disturbed ionospheric conditions. The model's generally good agreement with RMS noise data is demonstrated.
RADARC model comparisons with Amchitka radar data
Authors: Headrick J. M. , Root B. T. , Thomason J. F.
Published: February 2004

Data collected with the AN/TPS‐71 relocatable over‐the‐horizon radar (ROTHR) has been used to validate the RADARC Model. For the first 10 days of January, April, and July, 1200 and 0000 UTC, the vertical ionograms, oblique backscatter soundings, radar ground backscatter amplitudes, and noise levels have been compared with model predictions. The differences between the model and median h′ƒ soundings are small enough to have negligble effect on predictions. The coverage predicted for oblique backscatter soundings agrees well with observed median data except for sporadic E effects which were underestimated. Medians of observed ground backscatter levels agree fairly well with predictions. The actual noise that controlled radar performance was the most significant deviation from the model; all nighttime and January day noise was higher than that predicted, and this was due to spread‐in‐Doppler clutter appearing as noise.
Characterizing potential offshore seismic sources using high‐resolution geophysical and seafloor sampling programs: An example from Cape Egmont fault zone, Taranaki shelf, New Zealand
Authors: Nodder Scott D.
Published: February 2004

Standard marine geological techniques (high‐resolution seismic reflection methods, piston cores) are used to investigate the neotectonics and paleoseismology of an active part of an offshore fault system, the Cape Egmont fault zone, Taranaki continental shelf, New Zealand. The main active structure within the zone, the Cape Egmont fault, is a reactivated Pliocene to Recent normal to oblique‐slip fault that is associated with shallow crustal seismicity and exhibits a 53‐km long, 1‐to 5‐m high, low‐angle drape fold or “scarp” at the seafloor. Four, possibly five, fault segments varying in length from 7–51 km are delineated using geometric, structural, and stratigraphic information. Empirical estimates of maximum credible earthquake magnitudes, based upon seismic moment calculations, surface rupture lengths, and vertical surface offsets, suggest that the prehistoric earthquake(s) that formed the present seafloor surface trace probably had a magnitude of at least MW 6.4(MW, moment magnitude) and could have been as large as MS 7.3–7.4(MS, surface wave magnitude). Tentative maximum recurrence intervals for such events, inferred for each segment, range from 8000–13,000 years, with lower magnitude events having correspondingly shorter recurrence intervals of 500–900 years. Without careful study of offshore late Quaternary faults and fault zones, particularly where they are contiguous with areas of active onshore deformation, existing regional seismic hazard predictions may be inaccurate.
Plumes in central Panthalassa? Deductions from accreted oceanic fragments in Japan
Authors: Kimura Gaku , Sakakibara Masayuki , Okamura Makoto
Published: February 2004

Accretion of a large number of seamounts or fragments of huge oceanic plateaus is recorded in the geology of Japan. Major accretion occurred twice; a late Early Carboniferous seamount chain accreted in latest Permian to Middle Jurassic time, and a Late Jurassic oceanic plateau accreted in the Early Cretaceous. The Late Jurassic plateau now in northern Japan and Sakhalin is here named the “Sorachi Plateau” and a Tithonian age for it is well documented on the basis of microfossils from pelagic cherts and limestones. Petrochemical characteristics of basalts suggest two kinds of sources; depleted mantle under oceanic lithosphere and enriched mantle related to plume upwelling. All the accreted fragments in Japan can be traced back to the central part of the Panthalassa by using paleomagnetic data and plate “trajectory analysis”. The timing of generation of the accreted seamount chain and the plateau is consistent with a time of global change. Their place of origin, in the mid‐Pan thalassa, retains a positive residual geoid anomaly and slow Vp in the lower mantle, which suggest a past upwelling of hot material from the core‐mantle boundary.
Vertical axis rotations in the Las Vegas Valley Shear Zone, southern Nevada: Paleomagnetic constraints on kinematics and dynamics of block rotations
Authors: Sonder Leslie J. , Jones Craig H. , Salyards Stephen L. , Murphy Kathleen M.
Published: February 2004

Paleomagnetic study of vertical axis rotations at 23 localities along the right‐lateral Las Vegas Valley Shear Zone (LVVSZ) in southern Nevada indicates that clockwise rotations generally increase with proximity to the LVVSZ, reaching 100° at the closest localities. Rates of rotation determined at four localities range from l°/m.y. to 12°/m.y. The maximum characteristic size of rotating blocks is 2–4 km, considerably smaller than the dimensions of the zone of deformation associated with the shear zone (∼100 km in length, ∼15 km in half width). Thus the deformation does not accord with kinematic models of block rotations that assume uniform rotations within domains having dimensions of the order of the width of the deforming zone. Instead, the deformation appears to be quasi‐continuous when considered on a scale of > ∼10 km. We suggest that it is consistent with the deformation of a highly plastic layer of upper crustal material, mechanically decoupled from deeper parts of the crust, and that the across‐strike distribution of rotations is controlled by the rheological properties of the upper crust, the length of the shear zone, and the total amount of offset across the shear zone.
Subduction, platform subsidence, and foreland thrust loading: The late Tertiary development of Taranaki Basin, New Zealand
Authors: Holt W. E. , Stern T. A.
Published: February 2004

Borehole, seismic, and gravity data are used to investigate deformation of continental lithosphere at a Miocene collisional zone. Deformation is manifested in the three following principal forms: a long wavelength (>500 km) platform subsidence ascribed to mantle convection; flexural deformation on a scale of 100–200 km due to crustal thrusting at the eastern boundary of the Taranaki Basin; and a ductile thickening, evident on the deep seismic section of Taranaki Basin, that occurs on a scale of ∼10 km. Evidence for flexural deformation principally comes from the deep seismic section that shows a 150‐km wavelength bending of the Moho down toward the major zone of thrusting within the Taranaki Fault Zone. Paleowater depths, however, provide evidence for an initial early Miocene regional subsidence that is too long in wavelength to be explained by flexure induced from thrust sheet loading. Instead, we propose that this broad "platform subsidence" was driven by loading from a deep source, probably subduction‐induced flow in the mantle. By ∼22–19 Ma, 1–2 km of water existed over most of the area now occupied by South Taranaki Basin. By ∼19–17 Ma the water depth in the zone east of the Taranaki Basin, the Taranaki Fault Zone, had been replaced by rock due to submarine thrusting and crustal thickening. This build up of submarine topography in the Taranaki Fault Zone constitutes part of the load (25±8 MPa) that created and maintains South Taranaki Basin. Gravity data place further constraints on loading at the thrust front and point to an additional intracrustal loading, equivalent to 15±7 MPa over a 50‐km‐wide zone. This intracrustal load is explained as being due to thick‐skinned thrusting bringing denser, lower‐crustal rocks nearer to the surface in the thrust zone. The complete load on the Taranaki foreland is therefore in three parts; the submarine‐topographic load, the intracrustal load, and the loading of infilling sediments.
Statistical examination of the existence and relative motion of the Jalisco and Southern Mexico Blocks
Authors: Bandy William , Pardo Mario
Published: February 2004

The continental lithosphere of Mexico south of the Trans‐Mexican volcanic belt may consist of two lithospheric blocks; the Jalisco and Southern Mexico Blocks. The existence of these two blocks can be examined employing the statistical F test (formulated to test for the presence of significant misclosure around a plate circuit) to plate motion data derived from marine magnetic anomaly lineations, earthquake slip vectors, and transform fault azimuths. The result of the F test applied to the Pacific‐Cocos‐North American plate circuit indicates that there is no significant (at the 99% confidence level) misclosure around this plate circuit. Therefore if the Southern Mexico Block exists, its motion relative to the surrounding lithospheric plates is too small to be resolved from these data. In contrast, the result of the F test applied to the Rivera‐Pacific‐North American plate circuit indicates the presence of a significant misclosure about the Rivera‐Pacific‐North American plate circuit, the cause of which is uncertain. One interpretation of the misclosure around the Rivera‐Pacific‐North American plate circuit is that it is due to the presence of an independent Jalisco Block. However, it is more likely that this misclosure is, instead, primarily due to the effects of recent changes in the relative motion between the Pacific and Rivera plates. Assuming the second explanation is correct, a new Rivera‐North American Euler pole location (21.8°N, 110.4°W) is determinable solely from earthquake slip vectors located along the Rivera‐North American boundary.
Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: The North Mernoo Fault Zone, offshore New Zealand
Authors: Barnes Philip M.
Published: February 2004

A complex transition from subduction at the Hikurangi margin to transpressive, continental collision across the Southern Alps occurs in the NE South Island region of New Zealand. Offshore, the southern limit of oceanic subduction is constrained by the Chatham Rise, a submerged continental plateau on the Pacific Plate, which is presumably too buoyant for normal subduction. In the NW corner of the rise is the North Mernoo Fault Zone (NMFZ), a 100 by 300 km region of basement‐involved, active normal faulting of late Miocene (circa 8–6 Ma) to Recent age. The NMFZ evolved contemporaneously with the rotation and development of the adjacent plate boundary zone. Growth faulting is characteristic within the sedimentary cover, which is generally less than 2 km thick, but the distribution of faulting has varied temporally. The amount of mid Pliocene‐Recent extension is estimated to be not more than a few percent. The NMFZ consists of an array of overlapping, south dipping normal faults, which are typically 2–5 km apart and which trend roughly E–W at a high angle to the plate boundary zone. Late Quaternary surface traces are widely distributed on the mid to upper continental slope, but many surface scarps are poorly preserved due to extensive erosion of the seafloor. The western end of the fault zone crosses submarine canyons at the southern end of the Hikurangi Trough and almost abuts two fold and thrust fault systems on the NE South Island continental margin. Although few data constrain the kinematics of extension, a single MW 6.1 earth quake at the western end of the NMFZ suggests planar rotating faults with a component of left‐lateral oblique slip. Two possible models of regional extension are considered; (1) lateral buckling of the upper crust, in which faulted slivers of crust are peeled off as the Chatham Rise slides past the transpressive Marlborough fault system toward the Southern Alps and (2) flexure of the edge of continental Pacific Plate as the Chatham Rise is bent downward into the southern end of the Hikurangi subduction zone. Improved seismological data and deep crustal seismic reflection profiles may help to constrain the kinematics of deformation.
Paleomagnetism of tilted dikes in fast spread oceanic crust exposed in the Hess Deep Rift: Implications for spreading and rift propagation
Authors: Hurst Stephen D. , Karson Jeffrey A. , Verosub Kenneth L.
Published: February 2004

Tilted dikes occur within the pillow lava, sheeted dike, and gabbroic sections of 1 million‐year‐old, fast spread oceanic crust exposed on the walls of the Hess Deep Rift. During recent Alvin dives, oriented samples from both tilted and vertical dikes were collected from several of the outcrops. These are the first oriented paleomagnetic samples ever collected directly from outcrops on the ocean floor and were obtained with the help of a specially constructed geological compass. Remanent magnetic inclinations measured on the dike samples are significantly different from the near‐horizontal inclinations expected. The present attitudes of the dikes and their remanent magnetizations indicate that the dikes have been technically rotated since their intrusion. Submersible observations from other dive programs that examined parts of nearby escarpments suggest that the structural complexity observed in the Alvin study areas may be only a local phenomenon, or at least, not a pervasive characteristic of oceanic crust. Although tectonic rotations such as those observed may have taken place as a result of the opening of the Hess Deep Rift, crosscutting vertical dikes and other geologic considerations suggest that substantial tectonic rotations occurred at or very near the East Pacific Rise axis, probably as a result of intermittent axial collapse. The observed structures suggest that tectonic processes beneath fast spreading ridges and the internal structure of crust produced at them may be significantly more complex than previously assumed.
On the gravitational potential of the Earth's lithosphere
Authors: Coblentz David D. , Richardson Randall M. , Sandiford Michael
Published: February 2004

The mean potential energy of the lithosphere
is useful for defining the tectonic reference state (TRS) of the Earth and can be used to constrain the ambient state of stress in the plates. In the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the TRS would remain undeformed. Thus the difference between the potential energy of a lithospheric column and the TRS determines whether the column is in an extensional, joeutral, or compressional state of stress. We evaluate
and intraplate variations about this mean, using a simple, first‐order lithospheric density model. This model assumed that the continental geotherm is linear, and density variations below a depth of 125 km have negligible influence on
, and is consistent with observed geoid anomalies across continental margins.
is estimated to be 2.379 × 1014 N m−1, which is equivalent to the potential energy of both near sea level continental lithosphere (−160 to +220 m for an assumed crustal density, ρc, in the range 2800–2700 kg m−3) and cooling oceanic lithosphere at a depth of 4.3 km. With the exception of Eurasia, which has anomalously high mean potential energy (
= 2.383 × 1014 N m−1), the mean potential energies of the continental plates are nearly identical to the global mean
. The mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere. Both the global and plate mean potential energies are relatively insensitive to a wide range in ρc. The potential of the mid‐ocean ridges (
), 2.391 × 1014 N m−1, is greater than the global mean, which is consistent with the divergent nature of the ridges. Elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to
, suggesting that in the absence of external forces, continental regions will be in a slightly extensional state of stress. The importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the South American, Nazca, Indo‐Australian, and Pacific plates.
Depositional history of the Eocene Chumstick Formation: Implications of tectonic partitioning for the history of the Leavenworth and Entiat‐Eagle Creek fault systems, Washington
Authors: Evans James E.
Published: February 2004

The Chumstick basin opened as an extensional half‐graben prior to 51 Ma, and was subsequently modified by two episodes of tectonic partitioning of drainage prior to basin deformation. Initially, westward flowing fluvial systems formed a unified depositional system with the Swauk basin. Tectonic partitioning of drainage at 51–49 Ma and at 44–42 Ma was controlled by localized uplift on the Leavenworth (LFZ), Eagle Creek (ECFZ), and Entiat (EFZ) fault zones and led in each instance to the truncation of regional depositional systems, modification and reversal of paleoflow, and internal drainage. Relief on the LFZ at 51–49 Ma may be the result of isostatic uplift of the extensional footwall, producing the Swauk and Chumstick basins as a pair of west facing half grabens. The earliest convincing evidence for the onset of oblique slip in the region is at about 48 Ma (folding in the Swauk basin) or about 44–42 Ma (probable transpressive uplift at left‐stepping bends of the LFZ, development of a transtensional step‐over basin between the ECFZ and EFZ, horsetail splays in the ECFZ, and possible flower structures in the LFZ and ECFZ in the Chumstick basin). Each episode of tectonic partitioning was followed by proximal onlap and overtopping of fault zones, to reestablish regional flow systems. The Chumstick Formation was deformed by dextral transpression between 37–34 Ma, and is unconformably overlain by the Oligocene Wenatchee Formation. The Chumstick basin is an example of an extensional basin modified by subsequent strike‐slip tectonics, thus caution should be used in applying idealized basin models.
Forearc tectonic evolution of the South Shetland Margin, Antarctic Peninsula
Authors: Maldonado A. , Larter R. D. , Aldaya F.
Published: February 2004

The main provinces of the South Shetland margin, Antarctic Peninsula characterized on the basis of multichannel seismic, long‐range side scan sonar and swath bathymetry data, include from northwest to southeast (1) the oceanic crust of the former Phoenix Plate, flexed down toward the margin and affected by normal faulting in the upper crust; (2) a narrow trench, with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge; (3) an accretionary prism with a complex internal structure, the toe of which is overthrust above the youngest trench deposits; (4) a midslope forearc basin, with an asymmetric synformal structure; and (5) the continental shelf, which includes two distinct tectonic provinces. Calculations of the late Cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6.7 Ma from about 60 mm/yr, resulting from the slowing and eventual cessation of spreading on the Antarctic‐Phoenix ridge. Once spreading had completely ceased (3.5–2.4 Ma), the convergence rate at the trench equalled the rate of extension in Bransfield Strait, which was probably less than 10 mm/yr at first but may have increased since 1.3 Ma, and this, in turn, would imply a corresponding increase in convergence rate. Above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges, laterally and vertically segmented by normal faults. Most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles. This suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope. The forearc basin records a long history of subsidence, during which the depocenter migrated landward, paralleling retrogradational erosion of the distal margin of the basin. Subduction of the Hero Fracture Zone at approximately the same location since 10 Ma may have enhanced tectonic erosion of the accretionary prism. In the forearc basin a synform is coincident with a bathymetric reentrant, but its axis is oblique to the trends of both the basin and the fracture zone. The origin of this synform and the final phase of uplift of the blueschist facies terrane on which Smith Island stands are probably related to their position above the subducting fracture zone ridge. The present elevation of Smith Island is thought to result from uplift which followed a ridge crest‐trench collision directly southwest of the Hero Fracture Zone at 5.5 Ma. The northwestern edge of the Mesozoic to early Cenozoic arc batholith, trending northeast–southwest on the shelf, is proposed to have acted as a backstop to the accretionary prism during Cenozoic subduction.
Structural evolution along the inner forearc of the obliquely convergent Hikurangi margin, New Zealand
Authors: Kelsey Harvey M. , Cashman Susan M. , Beanland Sarah , Berryman Kelvin R.
Published: February 2004

The accretionary margin of the Hikurangi forearc on the southeast coast of the North Island of New Zealand is part of the leading edge of the Australian plate, which is overriding the obliquely converging Pacific plate. We investigate the last 10 m.y. of deformation history of the innermost (western) quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the Eketahuna area on the east coast of the North Island. The Eketahuna area is ideal for such a study because emergence of the margin in the Quaternary has exposed a complete late Neogene rock record. This record has allowed us to chronicle the strain history. From 10 Ma to about 2.5 Ma this forearc region was the locus of subsidence and marine deposition. In the latest Pliocene this part of the margin began to shorten through folding and reverse faulting, bringing an end to basin filling. The period of shortening was brief, and by the late Pleistocene, reverse faulting had ceased and was immediately succeeded by dextral strike‐slip faulting, in some cases along the same faults. Presently, the dominant strain regime in the inner quarter of the forearc is strike‐slip faulting. This structural history illustrates that, over time, the pattern of strain partitioning has changed in the Hikurangi forearc. The switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m.y. may be a response to more than 10° of clockwise rotation in the southern Hikurangi forearc in Pliocene and Pleistocene time. This rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging Australian and Pacific plates in this area at the southernmost end of the Kermadec‐Hikurangi subduction system. The inboard portion of this young accretionary margin is exceptionally well exposed today, probably in part because of the late Neogene subduction of relatively thick, buoyant crust of the Hikurangi‐Chatham plateau.