bluuebunny/crossref_metadata_2025 huggingface dataset:
Series expansions for the incomplete Lipschitz‐Hankel integral Je0(a, z)
Authors: Mechaik Mehdi M. , Dvorak Steven L.
Published: February 2004

Bessel series expansions are derived for the incomplete Lipschitz‐Hankel integralJe0(a, z). These expansions are obtained by using contour integration techniques to evaluate the inverse Laplace transform representation for Je0(a, z). It is shown that one of the expansions can be used as a convergent series expansion for one definition of the branch cut and as an asymptotic expansion if the branch cut is chosen differently. The effects of the branch cuts are demonstrated by plotting the terms in the series for interesting special cases. The Laplace transform technique used in this paper simplifies the derivation of the series expansions, provides information about the resulting branch cuts, yields integral representations for Je0(a, z), and allows the series expansions to be extended to complex values of z. These series expansions can be used together with the expansions for Ye0(a, z), which are obtained in a separate paper, to compute numerous other special functions, encountered in electromagnetic applications. These include: incomplete Lipschitz‐Hankel integrals of the Hankel and modified Bessel form, incomplete cylindrical functions of Poisson form (incomplete Bessel, Struve, Hankel, and Macdonald functions), and incomplete Weber integrals (Lommel functions of two variables).
Phenomenology of transionospheric pulse pairs
Authors: Massey R. S. , Holden D. N.
Published: February 2004

Recent observations of transient radio impulses by an Earth‐orbiting satellite appear to be quite unlike any previously reported. They appear as pairs of brief (a few microseconds), noiselike bursts, separated by a few tens of microseconds, and are dispersed in a way that implies subionospheric origin. Over 300 of these events have now been observed. These “transionospheric pulse pairs” (TIPPs) have not yet been associated with any known source, although thunderstorms are suspected. The observations, made by the Blackbeard instrument on the ALEXIS satellite, are digitized records of the electric field in a passband from about 25 to 100 MHz. Ground‐based observations of lightning in this band appear quite different, even accounting for ionospheric dispersion: bursts of short pulses last hundreds of microseconds and have much lower power (when propagated to the satellite) than TIPP events. Signals that resemble the ground‐based data have been observed by Blackbeard but, being much weaker, are much less likely to trigger the instrument than are the strong pulse pair events. In this paper we analyze 97 of the early TIPP observations. We compute several parameters that describe the events: the location of the satellite at the time of reception, the energy in each pulse, the separation between pulses, the duration of each pulse, and the dispersion of each pulse. The statistical distributions of these parameters provide clues to and constraints on possible source mechanisms. The possibility that the pulses might be the direct and reflected signals from a high‐altitude source is considered and cannot be rejected by the data.
Distortion of gravity wave spectra of horizontal winds measured in atmospheric radar experiments
Authors: Rastogi Prabhat K. , Kudeki Erhan , Sürücü Fahri
Published: February 2004

Large disparities in upper mesospheric winds measured with different radar techniques have been attributed earlier to the possibility that MF radars are sensitive to the phase speed of waves, rather than to true winds. An alternative explanation attributes these disparities to biases due to horizontal gradients in the wind field. These biases are common to the Doppler beam swinging (DBS), spaced antenna (SA), and imaging Doppler interferometry (IDI) methods and arise because of spatial filtering of the wind field by the beam configuration. We develop a theoretical model for intrinsic frequency and vertical wave number spectra of DBS wind component estimates that includes the effect of spatial filtering on an ensemble of gravity waves. The model shows a strong enhancement of frequency components above a knee, at ∼1 hour intrinsic period. A similar model for the SA experiment is inherently intractable. We suggest, instead, that the DBS model should also hold for the spectra of SA wind component estimates. The knee is clearly evident in recently observed SA wind component spectra, but is less pronounced and occurs at a frequency lower than the DBS model predicts. Better agreement is expected with a model refined to include the effect of Doppler shifts due to background winds. Occurrence of the knee in observed SA wind spectra implies that MF radars are sensitive to winds rather than to phase speed of atmospheric waves.
VLF signatures of ionospheric heating by HIPAS
Authors: Bell T. F. , Inan U. S. , Danielson M. T. , Cummer S. A.
Published: February 2004

The amplitude and phase of subionospherically propagating VLF signals are known to be sensitive indicators of the electron density and temperature at D region altitudes. In this paper we present new observations at a station in Alaska positioned to provide measurements of VLF signals that have propagated beneath the ionospheric region heated by the high‐power auroral stimulation (HIPAS) HF heating facility near Fairbanks, Alaska. Analysis of data from HIPAS campaigns conducted in fall 1992 and spring 1993 has shown that in roughly 60% of the cases analyzed, the amplitude of the 23.4‐kHz signal from the NPM transmitter in Hawaii as observed in Fort Yukon, Alaska, exhibited a measurable change in amplitude with the same on/off modulation pattern as that of the HIPAS HF transmissions at 2.85 MHz. In almost 70% of the cases analyzed, the same signal exhibited similar measurable changes in phase. The amplitude changes ranged from −0.2 dB to +0.5 dB, and the sensitivity of the measurement was approximately ± 0.02 dB. The phase changes ranged from −4.5° to −0.3°, and the sensitivity of the measurement was typically ± 0.4°. It is demonstrated that the phase and amplitude changes can be used as diagnostic tools to determine characteristics of the ambient electron density profile above the HIPAS facility.
Transient excitation of a straight thin wire segment over an interface between two dielectric half spaces
Authors: Bretones Amelia Rubio , Tijhuis Anton G.
Published: February 2004

The transient excitation of a straight thin wire segment parallel to a plane interface between two homogeneous dielectric half spaces is analyzed by the continuous‐time, discretized‐space approach. The analysis is carried out in three steps. First, the subproblem of a single wire embedded in a homogeneous dielectric medium, excited by a voltage source and an incident electric field, is studied with the aid of a time domain integral equation. This equation is discretized in space and transformed to the frequency domain. This procedure results in a system of linear equations of a fixed dimension which is solved by marching on in frequency. Second, the subproblem of a horizontal, pulsed dipole over the interface between two homogeneous dielectric half spaces is considered. The reflected field in the upper medium is computed by spectral techniques. With the aid of a time domain Weyl representation, a fixed, composite Gaussian quadrature rule is derived for the semi‐infinite integral involved. The angular integral is evaluated in closed form. Third, for the complete problem, the reflected field in the upper medium is expressed as a superposition of contributions from dipole sources on the wire axis, and subsequently treated as a secondary incident field in the integral equation for the current on the wire. This integral equation is then solved by combining the techniques developed for the two subproblems. Representative numerical results are presented and discussed.
Passive detection of sporadic E using GPS phase measurements
Authors: Coco David S. , Gaussiran Thomas L. , Coker Clayton
Published: February 2004

A passive sporadic E detection technique based on a Global Positioning System (GPS) receiving system has been developed and tested in a midlatitude environment. This system detects the small‐scale total electron content (TEC) variations believed to be produced by electron density structures associated with sporadic E. The current GPS detection technique was able to detect ionosonde‐detected sporadic E conditions for 73% of the cases at high‐elevation look angles in a set of midlatitude summer observations. Several approaches have been identified that may significantly improve this detection ratio. These approaches include reducing GPS phase multipath, implementing time and space averaging, and investigating the use of high‐speed GPS TEC measurements. This technique provides a basic sporadic E detection functionality for applications where an ionosonde is not available. It also provides complementary ionospheric information in regions outside the ionosonde viewing area for applications where an ionosonde is available.
Analysis of electromagnetic wave direction finding performed by spaceborne antennas using singular‐value decomposition techniques
Authors: Ladreiter H. P. , Zarka P. , Lecacheux A. , Macher W. , Rucker H. O. , Manning R. , Gurnett D. A. , Kurth W. S.
Published: February 2004

By using two rotating noncollinear antennas or three spatially fixed noncoplanar antennas on a spacecraft, full information on the polarization and the direction of arrival of an electromagnetic wave can be obtained by measuring the voltages created by the electric field of the incident wave. The physical parameters (polarization and direction of arrival) of the incoming wave are related to the received voltages on the antenna system by the so‐called direction‐finding equations. Since the used antennas are generally of small directivity (electrically short monopoles or dipoles), the resulting system of equations is numerically close to singular, and generally no unique solution can be obtained for the physical parameters of the wave throughout the inversion process. However, there exists a very powerful tool for dealing with sets of equations that are singular or close to singular, known as singular‐value decomposition (SVD), which precisely focuses the problem. For illustration, this paper analyzes the direction‐finding equations for the Radio and Plasma Wave Science (RPWS) experiment on the Cassini spacecraft by using SVD techniques. It also compares the expected performances of RPWS with those of the Ulysses Unified Radio and Plasma Wave (URAP) experiment achieved at Jupiter for the kilometer and hectometer emissions. The RPWS experiment on Cassini, which will be launched in 1997, is supposed to observe wave phenomena between a few hundred Hertz and 16 MHz in the Saturnian magnetosphere.
High‐resolution wind profiling using combined spatial and frequency domain interferometry
Authors: Palmer R. D. , Huang X. , Fukao S. , Yamamoto M. , Nakamura T.
Published: February 2004

A novel approach to wind profiling is presented which is based on the hybrid use of spatial interferometry (SI) and frequency domain interferometry (FDI). Many algorithms exist that can be used to determine the wind field using SI. However, the imaging Doppler interferometry (IDI) technique is somewhat unique in that the wind field within the radar beam is angularly “imaged” using the Doppler sorting effect. The spatial locations of scatterers are determined by assuming a wind field across the beam and Fourier analyzing signals to sort Doppler velocities. Pulsed radar systems are limited in range resolution by the length of the transmitted pulse, and wind estimates are obtained for a discrete set of altitudes determined by sampling the continuous stream of signals. Frequency domain interferometry (FDI) can be used to determine the radial location of scattering layers within the resolution volume. Thus the combined use of FDI and IDI can provide the radial and angular location of particular scattering points. Using the Doppler sorting idea, a new wind profiling technique is presented which uses FDI to increase the altitude resolution of wind estimates obtained from IDI. Experimental data that illustrate the implementation of the algorithm are presented from the Middle and Upper (MU) Atmosphere radar.
A 64‐bit strong alternating code discovered
Authors: Markkanen Markku , Nygrén Tuomo
Published: February 2004

An up‐to‐date table of strong alternating codes is presented, containing four new 32‐bit codes and one 64‐bit code. The new codes were found using a novel search technique discovered by studying hidden regularities in older codes. The search method will be explained in a subsequent paper. It is expected that even longer alternating codes can now be found in the near future.
Relationship between scintillation and rain attenuation at 19.77 GHz
Authors: Matricciani Emilio , Mauri Mario , Riva Carlo
Published: February 2004

This paper reports a statistical relationship between 1‐s averaged rain attenuation (A, in decibels) and standard deviation (σ, in decibels) of simultaneous tropospheric scintillation in 1‐s intervals, derived from high‐resolution (50 samples/s) experimental 19.77 GHz attenuation time series recorded at Spino d'Adda (45.4°N) in a 30.6° slant path to satellite Olympus during an observation time of approximately 1 year. During rain the relationship between scintillation and rain attenuation, suitably separated, can be fit by the power law σ = 0.0391A5/12 formula derivable from a turbulent‐thin layer model.
Geophysical evidence for crustal thickness variations between the Denali and Tintina Fault Systems in west‐central Yukon
Authors: Lowe C. , Cassidy J. F.
Published: February 2004

Analyses of receiver functions recorded at two broadband seismic stations in the northern Cordillera indicate that the crust is thinner beneath Dawson (∼ 35 km) than Whitehorse (∼ 39 km). A simple two‐dimensional gravity model, constrained by the seismic results, shows that this change in crustal thickness occurs at about 63°N, in a zone approximately 35 km wide, where the Moho dips at ∼8° to the south. The Bouguer anomaly high associated with the thinner crust can be traced westward into east‐central Alaska where crustal thinning and extension are well documented; therefore we propose that west‐central Yukon (north of 63°N) has been extended also. On the basis of geological and other geophysical data we examine three likely time windows, in the Cretaceous, early Tertiary, and Present. We discuss data which indicate that early Tertiary and younger extension may be related to the transfer of motion from the Denali Fault System inboard to the Tintina Fault System. Earlier Cretaceous extension has been variously attributed to collision related back arc extension, syncollisional processes, or gravitational collapse of an overthickened crustal section. The relative importance of extension in these periods remains nebulus; however, the correspondence between the transition in crustal thickness determined in this study and a mapped boundary between “lower” plate and "upper" plate rocks suggests mid‐Cretaceous extension exerted a significant influence on current Moho depths.
Thermal and structural evolution of the intracratonic Arltunga Nappe Complex, central Australia
Authors: Dunlap W. James , Teyssier Christian , McDougall Ian , Baldwin Suzanne
Published: February 2004

The thermal and structural evolution of a Paleozoic thrust system in central Australia, the Arltunga Nappe Complex, has been examined through field mapping, petrographic, microstructural and chemical analyses, K/Ar and 40Ar/39Ar thermochronology, and multi‐diffusion‐domain (time‐temperature) modeling of K‐feldspar 40Ar/39Ar data. The Arltunga Nappe Complex contains three distinct tectonic units, bounded by regional faults and shear zones, that have undergone contrasting deformational and metamorphic histories. The cooling history of the three tectonic units has been constrained by K/Ar and 40Ar/39Ar isotopic age data from K‐feldspars, micas, and amphiboles. Results are consistent with a piggyback history of crustal imbrication spanning over 100 m.y., with the hottest rocks uplifted and cooled first and with imbrication acting progressively toward the foreland. Implications for the intracratonic Paleozoic tectonics of central Australia are (1) uplift within the structurally highest tectonic unit was initiated before 400 Ma; (2) continued piggyback imbrication, footwall failure, and uplift and exhumation of the crust resulted in duplexing in the lowest structural unit at approximately 325–300 Ma; (3) the contrast in thermal history between the tectonic units indicates that once widely separated crustal segments were juxtaposed during thrusting above a hinterland dipping megathrust, resulting in ∼85 km of crustal shortening.
Ar/Ar geochronology of ultrahigh‐pressure metamorphism in central China
Authors: Hacker Bradley R. , Wang Qingchen
Published: February 2004

The Dabie Shan of China contain the largest areal exposure of ultrahigh‐pressure regional metamorphic rock known on Earth. The thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation, preservation, and exhumation. Published ages of ultrahigh‐pressure metamorphism and subsequent cooling range from Archean to Jurassic. By analyzing 21 Dabie hornblende, phengite, and biotite samples by the 40Ar/39Ar method, we find that (1) cooling from peak metamorphic temperatures to ∼300°C occurred between about 206 and 178 Ma; (2) widespread Cretaceous ages reflect reheating by a postultrahigh‐pressure magmatic/extensional episode; (3) 40Ar/39Ar and K/Ar ages older than 230 Ma, and perhaps 210 Ma, are geologically meaningless due to the incorporation of excess 40Ar; and (4) Dabie ultrahigh‐pressure metamorphic rocks are temporally related to blueschist‐facies rocks farther west in the suture zone.
Paleomagnetism of the Dockum Group (Upper Triassic), northwest Texas: Further evidence for the J‐1 cusp in the North America apparent polar wander path and implications for rate of Triassic apparent polar wander and Colorado plateau rotation
Authors: Molina‐Garza Roberto S. , Geissman John W. , Van der Voo Rob
Published: February 2004

We report paleomagnetic data for 26 accepted sites collected in two sections of flat‐lying strata of the upper Carnian‐lower Norian (∼225 Ma) Dockum Group, northwest Texas. Six additional sites in coarse‐grained conglomeratic sandstones gave no usable results. The total assemblage of 26 VGPs is streaked along the Late Triassic ‐ earliest Jurassic track of the North America apparent polar wander path and their mean is inconsistent with the accepted upper Carnian‐lower Norian reference pole. In detail, 12 sites in grayish white (nonhematitic) sandstones have weak magnetizations (less than about 1 mA/m) carried by magnetite or maghemite that give a paleopole at 56.4°N–96.3°E (N=12 dual‐polarity VGPs; K=44.2; A95=6.6°) in close agreement with other results for upper Carnian‐lower Norian rocks in North America. The 14 remaining sites in tan and redcolored (hematitic) sandstones, siltstones, and claystones give high unblocking temperature characteristic magnetizations carried by hematite, with paleopoles at 59.0°N–53.8°E (normal; N=7 VGPs, K=62.4, A95=7.6°) and 59.3°N–77.8°E (reverse; N=7 VGPs, K=204.2, A95=4.2°). These poles fall along the younger track of poles and near the J‐I cusp of the North American APWP as defined by (unrotated) poles derived from Colorado plateau rocks. We suggest that the characteristic magnetization of the non‐hematitic sandstones is an “early” magnetization, acquired during or soon after deposition. However, the characteristic magnetization of the red bed sites is interpreted as a secondary magnetization, for which we infer an earliest Jurassic age. The secondary origin for this magnetization is supported by the observation of conflicting magnetostratigraphies. The extreme westward position of the poles derived from red bed sites, particularly those with normal polarity, confirms the general position of the J‐I cusp indicated by poles in the Piedmont province, the Newark basin, and the Colorado plateau; it also suggests that the magnitude of rotation of the Colorado plateau is no greater than about 5°. We compile an apparent polar wander path for North America, including Colorado plateau data, which suggests a fast rate of apparent polar wander throughout the Triassic period (about 0.8°/m.y.) with a gradual increase that preceded the opening of the Atlantic.
Kinematics around a large‐scale oblique ramp, southern Pyrenees, Spain
Authors: Holl James E. , Anastasio David J.
Published: February 2004

The Gavarnie thrust sheet of the southern Pyrenees is bound to the east and west by ∼40‐km‐long transverse zones producing a rectilinear thrust sheet. Within the Boltana region the northwest corner of the Gavarnie thrust sheet is well exposed. The locale is used to characterize the strain and kinematic patterns and detailed geometry of an oblique ramp over a variety of scales. The hanging wall of the Gavarnie thrust fault in the area was transported toward ∼200° and contains two regional‐scale, north‐south trending anticlines. In the vicinity of the oblique ramp the strike of the Gavarnie thrust fault and the trend of hanging wall fold axes are ∼0°–180°. The magnitude of shortening accommodated by regional‐scale fold and fault development is constant at ∼25% along the length of the ramp. The orientation of instantaneous and finite shortening axes on meso and grain scales was inferred from the attitudes of cleavage, mesoscale folds and faults, carbonate mechanical twins, and the geometry of finite strain. Finite and instantaneous shortening axes are subhorizontal and remain perpendicular to the Gavarnie thrust ramp within a ∼10‐km‐wide zone in the footwall and a ∼20‐km‐wide zone in the hanging wall. Outside of the halo of footwall and hanging wall ramp deformation, the principal shortening directions consistently trend ∼200°, parallel to thrust transport directions throughout the south Pyrenean thrust belt. The parallelism of instantaneous and finite principal strain axes suggests that grain‐scale deformation was rionrotational. Total shortening measured within the footwall of the Gavarnie thrust fault decreases from ∼60% adjacent to the thrust ramp to <10% approximately 10 km away from the ramp. While principal shortening directions determined over scales ranging from millimeters to kilometers show a consistent pattern in the Boltana region, they differ by as much as 90° from tectonic convergence directions between Iberia and Eurasia during the Alpine orogeny (∼80–18 Ma) determined from magnetic anomaly patterns in the central Atlantic and Bay of Biscay. Divergence between plate convergence directions and Alpine south Pyrenean shortening directions suggests that ∼200 km of NW–SE convergence was apparently partitioned into the north and south Pyrenean thrust zones which were shortened ∼167 km, the Iberian Range which accomodated ∼30 km of shortening, <10 km of shortening accomodated in the Catalan Coastal Range, and ∼100 km of right‐lateral strike slip accomodated within the internides.
Gravity fields in eastern Halmahera and the Bonin Arc: Implications for ophiolite origin and emplacement
Authors: Milsom John , Hall Robert , Padmawidjaja Tatang
Published: February 2004

Classic ophiolites, as exemplified by the Troodos Massif in Cyprus and the Papuan Ultramafic Belt in eastern New Guinea, are large overthrust masses which are generally associated with large positive gravity anomalies. However, similar rocks occurring in extensive fragmented terranes which have also been described as ophiolitic do not produce large gravity effects. The eastern part of the island of Halmahera, in northeastern Indonesia, is an ophiolite of this latter type. On the two eastern arms of the island, a Mesozoic ophiolitic basement is overlain by, and imbricated with, Upper Cretaceous and Paleogene arc volcanic and sedimentary rocks. Bouguer gravity values are generally in the range +50 to +150 mGal and are characterised by steep local gradients indicative of shallow sources. The Bouguer gravity average suggests that the crust is at least 20 km thick, and it must be even thicker if a significant part of the anomalous gravity field is due to the presence of a cold and therefore dense, lithospheric slab within the asthenosphere, associated with the present‐day subduction beneath Halmahera. The absence of any exposures of continental basement rocks or of quartzose sediments in eastern Halmahera suggests that these ophiolites have not been overthrust onto continental crust and that the thickening occurred in an intraoceanic island arc. The Paleogene arc was evidently characterised by volcanism occurring over an unusually wide area. In this it resembles the Izu‐Bonin volcanic arc, which, like Halmahera, has been situated at the margin of the Philippine Sea Plate throughout its history. The gravity field of the Halmahera ophiolite is comparable with that of the Bonin volcanic arc, but there is no Halmahera parallel to the very high gravity fields recorded over the Bonin Islands forearc ridge. The equivalents of this part of the Paleogene arc may be represented by the ophiolitic complexes now distributed along the northern margin of the orogenic belt in New Guinea.
A structural transect across the Mongolian Western Altai: Active transpressional mountain building in central Asia
Authors: Cunningham W. Dickson , Windley Brian F. , Dorjnamjaa D. , Badamgarov G. , Saandar M.
Published: February 2004

We present results from the first detailed geological transect across the Mongolian Western Altai using modern methods of structural geology and fault kinematic analysis. Our purpose was to document the structures responsible for Cenozoic uplift of the range in order to better understand processes of intracontinental mountain building. Historical right‐lateral strike‐slip and oblique‐slip earthquakes have previously been documented from the Western Altai, and many mountain fronts are marked by active fault scarps indicating current tectonic activity and uplift. The dominant structures in the range are long (>200 km) NNW trending right‐lateral strike‐slip faults. Our transect can be divided into three separate domains that contain active, right‐lateral strike‐slip master faults and thrust faults with opposing vergence. The current deformation regime is thus transpressional. Each domain has an asymmetric flower structure cross‐sectional geometry, and the transect as a whole is interpreted as three separate large flower structures. The mechanism of uplift along the transect appears to be horizontal and vertical growth of flower structures rooted into the dominant right‐lateral strike‐slip faults. The major Bulgan Fault forms the southern structural boundary to the range and is a 3.5‐km‐wide brittle‐ductile zone that has accommodated reverse and left‐lateral strike‐slip displacements. It appears to be linked to the North Gobi Fault Zone to the east and Irtysh Fault zone to the west and thus may be over 900 km in length. Two major ductile left‐lateral extensional shear zones were identified in the interior of the range that appear to be preserved structures related to a regional Paleozoic or Mesozoic extensional event. Basement rocks along the transect are dominantly metavolcanic, metasedimentary, or intrusive units probably representing a Paleozoic accretionary prism and arc complex. The extent to which Cenozoic uplift has been accommodated by reactivation of older structures and inversion of older basins is unknown and will require further study. As previously suggested by others, Cenozoic uplift of the Altai is interpreted to be due to NE‐SW directed compressional stress resulting from the Indo‐Eurasian collision 2500 km to the south.
Tectonic structure of the convergent Pacific margin offshore Costa Rica from multichannel seismic reflection data
Authors: Hinz K. , von Huene R. , Ranero C. R. ,   PACOMAR Working Group
Published: February 2004

The Middle America Trench between the Cocos Ridge and a well‐studied corridor off the Nicoya Peninsula has a more varied morphology and structure than previously reported. The morphological positive features on the lower plate significantly affect the upper plate structure. The Cocos Ridge has uplifted the margin opposite the Osa Peninsula. Northwest of Cocos Ridge, numerous seamounts on the oceanic crust sculptured the margin as they subducted. A seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted. The greater portion of the trench sediment is subducted beneath a lower slope accretionary mass. Beneath the middle and upper slope is a margin wedge consisting of a high‐velocity rock with few internal reflections. Its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area. Overlying this surface is a low‐velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred Eocene section beneath the shelf. The shelf basement is composed of Nicoya complex (ophiolite) with the same acoustic velocity, similar structure, and no apparent dividing geologic boundary with the margin wedge. We favor a seaward continuation of the Nicoya complex to the middle slope and emphasize the evidence for a non‐steady state Tertiary tectonic history.
Computer simulations of Alaskan neotectonics
Authors: Bird Peter
Published: February 2004

Ongoing deformation and fault slip (neotectonics) in the Alaskan/Bering Sea region were modeled with thin‐plate finite‐element methods by computing anelastic strain rate, stress, and velocity fields which represent time averages over several seismic cycles. Models included variable crust and lithosphere thickness, heat flow, and elevation and incorporated all faults known or thought to be active. Three sets of experiments, totaling 46 models, were computed with systematic variations of shear traction on the megathrust, fault friction within North America, internal friction of lithospheric blocks, and mantle creep strength. Model quality was scored by comparing predictions to geologic data on fault slip rates, the most compressive horizontal principal stress direction
, and geodimeter and very long baseline interferometry (VLBI) baseline rates. Best results were obtained with a driving traction of only 10∼15 MPa on the Aleutian megathrust and fault friction of only 0.17 within North America; this is clearly a “low‐stress” model. The fanning of
away from the syntaxis was reproduced but does not imply high deviatoric stress magnitudes within the North American plate. The Aleutian forearc west of Kodiak Island moves westward along strike at rates which increase to 65 mm yr−1 in the Komandorsky region. However, no crust is presently escaping westward from the Alaskan syntaxis. Instead, it is shortened and stacked by thrusting, principally in the vicinity of the Chugach and Wrangell Mountains. Because stresses in this model are low and are delicately balanced against topography, it is likely that the present regime represents a transient response to Plio‐Pleistocene glacial mass redistribution and differs from typical tectonic flow in the Tertiary.
Structural evolution of the northern Los Angeles basin, California, based on growth strata
Authors: Schneider Craig L. , Hummon Cheryl , Yeats Robert S. , Huftile Gary L.
Published: February 2004

Subsurface structure contour maps and cross sections show that the northern Los Angeles basin is underlain by a south facing monocline complicated by secondary faults and folds. The monocline and associated structures form a structural high, the northern Los Angeles shelf, which marks the northern margin of the Los Angeles central trough. Analysis of “growth strata,” sediments deposited during structural growth, shows that during the Miocene, the predominant structural style was extension. Growth strata show that at approximately the beginning of the Pliocene (5 Ma), contraction reactivated the Miocene normal faults as reverse faults and formed the monocline, its secondary structures, and the Los Angeles central trough. Lensing‐out of earliest Pliocene (Delmontian Stage) growth sandstones onto the flanks of the monocline and secondary structures (the South Salt Lake, the East Beverly Hills, and the Las Cienegas anticlines) indicates that these contractional structures all began to form at this time. All of the secondary structures became largely inactive prior to the deposition of upper Pico member of early Pleistocene age (1.2 Ma). However, thick accumulations of growth strata of the upper Pico member of the Fernando Formation within the Los Angeles central trough attest to continued monoclinal folding after the secondary structures became largely inactive. The growth strata record both vertical and horizontal components of structural growth of the monocline so that the dip of the blind fault zone causing the monocline (the Los Angeles fault) can be calculated. In the East Beverly Hills area, the dip of the Los Angeles fault is 61° and at Las Cienegas, the dip is 62°. These are maximum values based on the assumption that the growth strata bed lengths record all shortening. The Pliocene‐Pleistocene average fault slip rates for the Los Angeles fault are 1.1 – 1.3 mm/yr in the East Beverly Hills and 1.3–1.5 mm/yr at Las Cienegas. The resulting Pliocene‐Pleistocene horizontal convergence rates are 0.5 – 0.6 mm/yr and 0.6 – 0.7 mm/yr respectively. The Pliocene‐Pleistocene growth strata show increased dip with age, indicating that the monocline grew by progressive limb rotation rather than by kink‐band migration. Therefore, fault‐bend and fault‐propagation fold models based on kink‐band migration are inadmissible solutions to explain the growth of the monocline. We suggest a basement‐involved shear zone to explain the geometry of the northern Los Angeles shelf.
Variscan exhumation of a subducted Paleozoic continental margin: The basal units of the Ordenes Complex, Galicia, NW Spain
Authors: Martínez Catalán José R. , Arenas Ricardo , Díaz García Florentino , Rubio Pascual Francisco J. , Abati Jacobo , Marquínez Jorge
Published: February 2004

A structural and metamorphic study was carried out in the basal units of the Ordenes Complex in Spain, thought to represent a subducted part of the Paleozoic margin of Gondwana. According to their metamorphic evolution, this part of the margin was subducted at the onset of the Variscan Orogeny, becoming part of an accretionary complex developed below a colliding element built previously. Variations in the PT conditions of the first high‐pressure metamorphic event along the units indicate a polarity of the subduction to the west. Subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units, whereas the convergence continued. Recumbent folds and thrusts developed along with successive normal detachments. Compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading. The unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units.
Development of the accretionary prism along Peru and material flux after subduction of Nazca Ridge
Authors: von Huene R. , Pecher I. A. , Gutscher M.‐A.
Published: February 2004

Results from Ocean Drilling Program Leg 112 indicated tectonic erosion of the Peruvian convergent margin during subduction of the Nazca Ridge followed by renewed accretion against the erosional scar. Seismic images and swath mapping morphology show a regional geology shaped as subduction of Nazca Ridge migrated 800 km along the Peru Trench. The accretionary prism is imaged at two stages of development since upper Miocene time with depth‐migrated seismic records in which the “back stop” is well defined. From these data, material flux can be quantified. After the ridge crest subducted and accretion dominated over erosion, the prism grew rapidly to 10‐ to 15‐km width. During rapid growth, the margin taper was large and about 60 percent of the sediment supply accreted. As the prism growth slowed, only 30 percent accreted despite an increased trench sediment supply. Since the convergence rate changed little, the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted. The decreased prism growth may be partially controlled by varying the structure of the back stop. The force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary. Thus the shear stress transmitted across the plate boundary is insufficient to drive “out‐of‐sequence” thrust faults that would thicken the back stop or raise it beyond a limited height. A constant back stop height limits accretion, and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop. Nazca Ridge subduction affected the convergent margin mass flux for about 8 Ma. Mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest. When growth of the accretionary prism slowed, a greater terrigenous input through the trench was accommodated by sediment subduction. The resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling.
Far‐travelled Permian chert of the North Fork terrane, Klamath Mountains, California
Authors: Mankinen Edward A. , Irwin William P. , Blome Charles D.
Published: February 2004

Permian chert in the North Fork terrane and correlative rocks of the Klamath Mountains province has a remanent magnetization that is prefolding and presumably primary. Paleomagnetic results indicate that the chert formed at a paleolatitude of 8.6° ± 2.5° but in which hemisphere remains uncertain. This finding requires that these rocks have undergone at least 8.6° ± 4.4° of northward transport relative to Permian North America since their deposition. Paleontological evidence suggests that the Permian limestone of the Eastern Klamath terrane originated thousands of kilometers distant from North America. The limestone of the North Fork terrane may have formed at a similar or even greater distance as suggested by its faunal affinity to the Eastern Klamath terrane and more westerly position. Available evidence indicates that convergence of the North Fork and composite Central Metamorphic‐Eastern Klamath terranes occurred during Triassic or Early Jurassic time and that their joining together was a Middle Jurassic event. Primary and secondary magnetizations indicate that the new composite terrane containing these and other rocks of the Western Paleozoic and Triassic belt behaved as a single rigid block that has been latitudinally concordant with the North American craton since Middle Jurassic time.
New evidence of magmatic diapirs in the intermediate crust under the Dead Sea, Israel
Authors: Rabinowitz Nitzan , Steinberg Jean , Mart Yossi
Published: February 2004

Simultaneous inversion of P wave arrivals of 262 local earthquakes, recorded in the region of the Dead Sea, was carried out to model the velocity stratification in the underlying crust and uppermost mantle. The resulting tomographic model delineates several large magmatic domes, rising from the lower crust, at depth of approximately 20 km, and ascending more than 12 km into the overlying intermediate crust. Commonly, the domal ascent took place along the boundary faults of the Dead Sea rift. The domes are elliptical in shape, measuring nearly 5 × 15 km in diameter, and are spaced about 20 km apart. Earthquake foci are abundant around the diapirs but rare within them. The occurrence of these magmatic diapirs under the Dead Sea implies anomalously high temperatures with metastable stratification in the lower crust and suggests magmatic upwelling.
Stress pattern in Portugal mainland and the adjacent Atlantic region, West Iberia
Authors: Ribeiro A. , Cabral J. , Baptista R. , Matias L.
Published: February 2004

The Portuguese mainland territory is located close to the Azores‐Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [Cabral, 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (SHmax). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean SHmax trend. These differences in stress trend suggest a regional progressive rotation of the SHmax direction from NNW‐SSE to WNW‐ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW‐SE SHmax trend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continental crust.
Early Jurassic deformation in the Pine Forest Range, northwest Nevada, and implications for Cordilleran tectonics
Authors: Wyld Sandra J.
Published: February 2004

Triassic and Jurassic rocks in the Black Rock Desert of northwest Nevada comprise part of the Mesozoic volcanic arc successions found throughout the westernmost U.S. Cordillera. Detailed structural studies in the Pine Forest Range, located in the western Black Rock Desert, provide new insight into the Jurassic structural history of this part of the Mesozoic arc system. Regional Jurassic deformation and metamorphism in the Pine Forest Range affected a thick (∼10.5 km) section of lower(?) Paleozoic through Triassic strata that were tilted after deformation and are now exposed in approximately cross‐sectional view. At high stratigraphic levels, deformation produced a slaty cleavage and relatively open folds, both of which are concentrated in rheologically incompetent rock types, and was accompanied by metamorphism at low grade. At deeper stratigraphic levels, the regional foliation intensifies and becomes more pervasive, folds tighten, and metamorphic grade increases through greenschist into amphibolite grade. At the deepest stratigraphic levels, the rocks are amphibolite grade L‐S tectonites with a pronounced mylonitic fabric. The ∼2 km‐thick‐zone of mylonites is interpreted as a ductile shear zone that formed when the stratigraphic section was still upright, whereas the less deformed, younger rocks are interpreted to occupy the upper plate of the shear zone and to record a strain gradient away from the shear zone. A variety of structural and regional relations indicate that the shear zone is a ductile thrust fault along which younger rocks moved to the northwest (in present‐day coordinates) with respect to older rocks. The timing of regional Jurassic deformation and metamorphism is constrained by relations in and around two plutons dated by the U‐Pb zircon method. The 201±1 Ma Big Creek pluton was intruded as a sill‐like body along the upper levels of the shear zone and is associated with a variety of features indicating syntectonic intrusion, including internal textural features in plutonic rocks and metamorphic changes in wall rocks with proximity to the pluton. The 185±1 Ma Theodore pluton which intrudes across part of the shear zone has a thermal aureole which statically overprints the shear zone fabric. Deformation was thus ongoing at 201 Ma and over by 185 Ma. The Early Jurassic age of regional shortening deformation in the Pine Forest Range contrasts with the Middle to Late Jurassic (∼170–150 Ma) age of regional shortening documented in many other parts of the Mesozoic arc system, including the Klamath Mountains, Sierra Nevada, and Mojave Desert regions of California. It also contrasts with the record of Early Jurassic extensional tectonism documented in the Sierra Nevada, Mojave Desert, and western Arizona segments of the Mesozoic arc system. These conflicting relations indicate a complex tectonic evolution for the Jurassic U.S. Cordillera plate margin and lend support to the idea that disparate parts of this plate margin were juxtaposed along younger strike‐slip faults.
Late Quaternary reorientation of stress field and extension direction in the southern Gulf of Suez, Egypt: Evidence from uplifted coral terraces, mesoscopic fault arrays, and borehole breakouts
Authors: Bosworth William , Taviani Marco
Published: February 2004

Uplifted Pleistocene coral terraces and modern earthquakes show that several large normal faults are presently active in the southern Gulf of Suez rift basin. The footwall of one of these faults is exposed at Gebel el Zeit, where terraces at elevations of +10–18 and +42 m have been radiometrically dated as 125 ka and 426 ka, respectively. After correcting for eustatic sea‐level changes, this results in net tectonic uplift rates of about 0.1 m/kyr. Published interpretations and our own observations indicate that the average extension direction during the Miocene to Pliocene synrift history was 055°. Analysis of borehole breakouts and published earthquake fault plane solutions, however, suggests that the present‐day stress field in the southern Gulf has a 010°–020° Shmin orientation. Detailed structural observations show that a change in extension direction occurred in the late Pleistocene, with rotation of the stress field beginning prior to formation of the 125 ka terraces but after formation of older Pleistocene terraces whose ages are less tightly constrained. Using a horizontal slip direction of 015° and our observed net footwall uplift rate, we calculate a separation velocity between Sinai and Africa of 0.8–1.2 m/kyr. The proposed Pleistocene change in extension direction in the Gulf of Suez corresponds closely with the post‐500 ka change in extension direction documented in the Kenyan rift system and a similar change in extension direction recorded in the central Red Sea. These regional similarities in tectonic history suggest that the underlying causes of these events may be a plate‐scale phenomenon affecting the entire Afro‐Arabian rift system, rather than local changes in the Quaternary stress field.