bluuebunny/crossref_metadata_2025 huggingface dataset:
Duration of S1, the most recent sapropel in the eastern Mediterranean Sea, as indicated by accelerator mass spectrometry radiocarbon and geochemical evidence
Authors: Mercone D. , Thomson J. , Croudace I. W. , Siani G. , Paterne M. , Troelstra S.
Published: September 2002

Slowly accumulated (<5 cm kyr−1) and rapidly accumulated (5–20 cm kyr−1) sediments have been compared to define the initiation and termination times of the most recent sapropel (S1) in the eastern Mediterranean Sea. The Ba/Al ratio has been employed as a more persistent index of productivity than Corg. Accelerator mass spectrometry radiocarbon dating of pelagic foraminifera indicates a maximum duration for increased Ba/Al levels in Sl from ∼9500–6000 (uncorrected radiocarbon convention years B.P.) in the rapidly accumulated sediments and ∼9500–5300 years B.P. in the slowly accumulated sediments. This difference is ascribed to bioturbation affecting the slower accumulated Sl sediments. In the two most rapidly accumulated Sl units, from the Adriatic and Aegean Seas, there is a “saddle” of lower values centered on 7500 years B.P. in the Corg and Ba/Al profiles, so that the visual Sl unit appears as a doublet. Geochemical evidence indicates that this intervening period is best interpreted as an episode of increased ventilation and bottom water oxygenation during the period of sapropel accumulation.
Control of organic carbon accumulation in the Late Quaternary equatorial Atlantic (Ocean Drilling Program Sites 664 and 663): Productivity versus terrigenous supply
Authors: Wagner Thomas
Published: September 2002

Organic geochemical records of the last 940 kyr are presented for equatorial Atlantic Ocean Drilling Program (ODP) sites 663 and 664 and discussed with regard to the development of ocean productivity and African paleoclimate. Proportions of marine and terrigenous organic matter (OM) are estimated from elemental, pyrolytic, isotopic, and petrologic data. Spectral analyses reveal a strong power at the eccentricity and obliquity band, indicating a close response of tropical organic sedimentation to the climatic evolution at high latitudes. The orbital covariance of organic carbon with biogenous opal and terrigenous records favor that glacially enhanced dust supply and surface water mixing were primary controls for deposition of organic carbon. Wind‐borne supply of terrigenous OM contributes 26 to 55% and 0 to 39% to the bulk OM based on microscopic and isotopic records, respectively. Admixture of C4 plant matter was approximated to contribute up to 16% to the bulk organic fraction during peak glacial conditions.
Younger Dryas Interval and outflow from the Laurentide Ice Sheet
Authors: Moore T. C. , Walker J. C. G. , Rea D. K. , Lewis C. F. M. , Shane L. C. K. , Smith A. J.
Published: September 2002

A boxmodel of the Great Lakes is used to estimate meltwater flow into the North Atlantic between 8000 and 14,000 calendar years B.P. Controls on the model include the oxygen isotopic composition of meltwaters and lake waters as measured in the shells of ostracodes. Outflow rates are highest when oxygen isotopic values of the lake waters are most negative, denoting a maximum glacial meltwater component. Flow rates reach maximum values before the onset of the Younger Dryas and after it ends. These maxima appear to be correlative with the major meltwater pulses MWP 1A and 1B. Although the resumption of North Atlantic Deep Water formation may be tied to the reduction in ice sheet melting, neither the onset nor the end of the Younger Dryas, as recorded in the Greenland Ice Sheet Project (GISP2) records, appear tied to maxima in meltwater outflow from the Laurentide ice sheet.
Significance of the sedimentary Al∶Ti ratio as an indicator for variations in the circulation patterns of the equatorial North Atlantic
Authors: Zabel Matthias , Bickert Torsten , Dittert Lars , Haese Ralf R.
Published: September 2002

This paper presents results of geochemical investigations conducted on three gravity cores recovered along an E‐W transect (3°–5°N) in the equatorial Atlantic. Considering the high terrigenous content of the sediments (approximately 12–74 wt%), Al and Ti concentrations are predominantly associated with the input of lithogenous phases. The terrigenous fraction of western equatorial Atlantic sediments is more influenced by material derived from the Amazon River than by deposition of African dust particles. As a possible route of transport, the North Equatorial Countercurrent is considered. Fluctuations in Al/Ti records generally reflect changes in the low‐latitude insolation cycle (19–23 kyr) and thus indicate a close coupling to variations of the trade‐wind systems and continental climate changes. However, characteristics of the terrigenous signals in the eastern and western equatorial Atlantic differ significantly. While the precessional signal in the Al/Ti record and low latitude portion in the δ18O record are in phase in the eastern equatorial Atlantic, variations in the terrigenous fraction westward of the Mid‐Atlantic Ridge precede the marine signal by 2–3 kyr, which we ascribe to a shift between trade‐wind systems of the Northern and Southern Hemispheres.
A comparative study of the northwest Africa and eastern equatorial Pacific upwelling zones as sources of CO2 during glacial periods based on boron isotope paleo‐pH estimation
Authors: Sanyal Abhijit , Bijma Jelle
Published: September 2002

Paleo‐pH reconstructions based on boron isotopic composition of foraminifera have been used to estimate glacial‐interglacial changes in surface ocean pCO2 of the northwest African upwelling zone. On comparison with a similar study for the eastern equatorial Pacific upwelling zone, it can be concluded that the two major upwelling zones acted quite differently during the glacial periods as compared to today. While the pH of the surface ocean off northwest Africa was 0.2±0.07 units higher during the glacial period compared to that during Holocene, there was no significant glacial‐interglacial change in the surface ocean pH in the eastern equatorial Pacific. Carbonate chemistry reconstructions based on the estimated pH changes suggest that the ocean‐atmosphere pCO2 gradient off northwest Africa was lower by at least 70±40 µatm during glacial periods compared to during the Holocene. In contrast, the ocean‐atmosphere pCO2 gradient in the eastern equatorial Pacific was higher by at least 80±40 µatm during glacial periods as compared to during the Holocene. Hence the eastern equatorial Pacific upwelling system was a significantly larger source of CO2 to the atmosphere, while the one off northwest Africa was a significantly smaller source of CO2 during the last glacial period. The pCO2 reconstructions further indicate that in spite of higher glacial productivity compared to during the Holocene, neither of the two areas became a sink of CO2.
Dansgaard‐Oeschger and Heinrich event imprints in Alboran Sea paleotemperatures
Authors: Cacho Isabel , Grimalt Joan O , Pelejero Carles , Canals Miquel , Sierro Francisco J. , Flores José Abel , Shackleton Nick
Published: September 2002

Past sea surface temperature (SST) evolution in the Alboran Sea (western Mediterranean) during the last 50,000 years has been inferred from the study of C37 alkenones in International Marine Global Change Studies MD952043 core. This record has a time resolution of ∼200 years allowing the study of millennial‐scale and even shorter climatic changes. The observed SST curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period. Comparison of this Alboran record with δ18O from Greenland ice (Greenland Ice Sheet Project 2 core) shows a strong parallelism between these SST oscillations and the Dansgaard‐Oeschger events. Five prominent cooling episodes standing out in the SST profile are accompanied by an anomalous high abundance of Neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals. These features and the isotopic record reflect drastic changes in the surface hydrography of the Alboran Sea in association with Heinrich events Hl–5.
Climatically sensitive eolian and hemipelagic deposition in the Cariaco Basin, Venezuela, over the past 578,000 years: Results from Al/Ti and K/Al
Authors: Yarincik K. M. , Murray R. W. , Peterson L. C.
Published: September 2002

Al/Ti and K/Al ratios in bulk sediment are used to interpret wind‐blown and hemipelagic sources of deposition to a 578 kyr record in the Cariaco Basin, Venezuela (Ocean Drilling Program site 1002). Graphical and cross‐spectral analyses indicate that these ratios vary extremely closely with planktonic foraminiferal δ18O, with both ratios being significantly higher during interglacials and lower during glacials. K/Al indicates that during glacials the lower sea level that results in relative basin isolation increases the relative proportion of kaolinite derived from local rivers draining the relatively humid Venezuelan margin. Al/Ti decreases during glacials, suggesting greater proportions of eolian rutile sourced from the northern Sahara (due to increased wind strength and/or aridity). This interpretation is consistent with previous studies of the chemistry and mineralogy of Saharan‐derived eolian matter in the Caribbean and with a mass balance determining the effect of changes in eolian rutile accumulation on the bulk sedimentary Al/Ti ratio.
Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit
Authors: Williams George E.
Published: September 2002

Over the past decade the analysis of sedimentary cyclic rhythmites of tidal origin, i.e., stacked thin beds or laminae usually of sandstone, siltstone, and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation, has provided information on Earth's paleorotation and the evolving lunar orbit for Precambrian time (before 540 Ma). Depositional environments of tidal rhythmites range from estuarine to tidal delta, with a wave‐protected, distal ebb tidal delta setting being particularly favorable for the deposition and preservation of long, detailed rhythmite records. The potential sediment load of nearshore tidal currents and the effectiveness of the tide as an agent of sediment entrainment and deposition are related directly to tidal range (or maximum tidal height) and consequent current speed. Hence the thickness of successive laminae deposited by tidal currents can be a proxy tidal record, with paleotidal and paleorotational values being determined by analysis of measured records of lamina and cycle thickness. The validity of the findings can be investigated by testing the primary, observed values for internal self‐consistency through application of the laws of celestial mechanics. Paleotidal and paleorotational values provided by late Neoproterozoic (∼620 Ma) tidal rhythmites in South Australia are validated by these tests and indicate 13.1±0.1 synodic (lunar) months/yr, 400±7 solar days/yr, a length of day of 21.9±0.4 h, and a relative Earth‐Moon distance a/a0 of 0.965±0.005. The mean rate of lunar recession since that time is 2.17±0.31 cm/yr, which is little more than half the present rate of lunar recession of 3.82±0.07 cm/yr obtained by lunar laser ranging. The late Neoproterozoic data militate against significant overall change in Earth's moment of inertia and radius at least since 620 Ma. Cyclicity displayed by Paleoproterozoic (2450 Ma) banded iron formation in Western Australia may record tidal influences on the discharge and/or dispersal of submarine hydrothermal plumes and suggests 14.5±0.5 synodic months/yr and a/a0 = 0.906±0.029. The combined rhythmite data give a mean rate of lunar recession of 1.24±0.71 cm/yr during most of the Proterozoic (2450–620 Ma), suggesting that a close approach of the Moon did not occur during earlier time. Concentrated study of Precambrian tidal rhythmites promises to illuminate the evolving dynamics of the early Earth‐Moon system and may permit the lunar orbit to be traced back to near the time of the Moon's origin.
Resolution and coverage analysis for ionospheric tomography
Authors: Biswas Chaitali , Na Helen
Published: September 2002

This paper presents results of investigations of the relationship between optimal image resolution and associated sky‐coverage for ionospheric tomography. Several measures of the variation of image information content as a function of resolution are presented for understanding the trade‐offs between information content and computational resources. A new quantitative description of imaging geometry that enables sky‐coverage analysis for this limited information tomographic system is introduced here. This paper also presents examples of commonly encountered nonideal system geometries, diagnosis of associated image artifacts, and improvements achieved by minor logistical modifications to the imaging system.
Modeling ELF radio atmospheric propagation and extracting lightning currents from ELF observations
Authors: Cummer Steven A. , Inan Umran S.
Published: September 2002

Observations of extremely low frequency (ELF) radio atmospherics (sferics), the transient electromagnetic fields radiated by lightning discharges, are used to determine the current moment waveforms of vertical lightning discharges. In order to extract this information the propagation of radio atmospherics from source to receiver must be modeled accurately, especially in view of the important role played by the D and E regions of the ionosphere at these long (> 200 km) wavelengths. We model broadband ELF sferic waveforms by adapting a single‐frequency ELF propagation code to calculate an ELF propagation impulse response under the assumption of horizontal ionospheric homogeneity, with which we extract the source lightning current waveform from an observed ELF sferic waveform using a deconvolution method based on linear regularization. Tests on modeled sferics indicate that the method is accurate and relatively insensitive to noise, and we demonstrate the application of the technique with a sprite‐associated sferic. Since ELF sferics can often be observed many thousands of kilometers from the source discharge, the technique developed here represents a powerful new method of remotely sensing lightning current waveforms.
Comparison of atmospheric parameters derived from meteor observations with CIRA
Authors: Cervera Manuel A. , Reid Iain M.
Published: September 2002

In this paper we compare monthly averages of the atmospheric parameter T/
(where T is temperature and p is pressure) derived from the decay of underdense meteor echoes with the CIRA (1986) atmospheric model. The meteor data were collected with the Buckland Park VHF radar situated ∼40km north of Adelaide, Australia. We examine the overall agreement between the meteor observations and CIRA as well as seasonal differences between the two. Comparison is made with the results of Hocking et al. [1997]. Our results are complimentary to those of Hocking et al.; our data were obtained in the Southern Hemisphere as opposed to the Northern Hemisphere. A discussion on the effect of the geomagnetic field on the diffusion of meteor trails and its effect on the measurement of atmospheric parameters is also included. We note that the geomagnetic field is a very important consideration when using meteors for the derivation of atmospheric temperatures and pressures above heights of around 92–93 km. This effect is required to be taken into account above these heights as failure to do so leads to errors in the interpretation of the data. Recent researchers have avoided this problem by restricting their data to below 90 km.
Second‐order difference equations in diffraction theory
Authors: Senior Thomas B. A. , Legault Stéphane R.
Published: September 2002

A technique is presented to solve a class of second‐order functional difference equations that arise in diffraction theory. Branch‐free solutions are obtained by linearly combining branched functions satisfying first‐order equations derived from the second‐order difference equation. The approach used is conceptually simple, and the underlying analysis is relatively straightforward. Analysis and computations both demonstrate that the resulting solutions have the desired analytical properties and recover the known expressions in the proper limit.
Two‐dimension visualization of rain cell structures
Authors: Goldhirsh Julius
Published: September 2002

A methodology is described for the simulation of two‐dimensional rain rate fields based on a formulation derived from radar measurements of other investigators and modeled point rain rate distributions. This simulation enables a determination of typical radar clutter maps of rain backscatter and estimates of terrestrial path attenuation as a function of distance and azimuth. Rain rate fields are derived for three diverse climatological regions, and a step‐by‐step methodology is presented for determining them. Parameters are tabulated, enabling the determination of rain rate fields for any of the 15 International Telecommunication Union‐Radio Communication rain rate zones. Examples giving typical path attenuation at 10 GHz versus distance profiles and 3‐GHz radar backscatter profiles are calculated. A modified formulation that predicts cumulative slant path fade distributions for Earth‐satellite scenarios using many of the elements of the simulation technique is reviewed. As a partial validation of the described rain rate field simulation technique, the cumulative fade distribution was calculated at 28.56 GHz and compared to the derived distribution from a previous 3‐year measurement program involving the COMSTAR satellite. The calculated and measured levels were observed to give close agreement over the attenuation range from 3 to 25 dB, resulting in a root‐mean‐square deviation smaller than l dB and a standard error of approximately 10%.
An objective evaluation of available LF/MF sky‐wave propagation models
Authors: Wang John C. H.
Published: September 2002

This paper presents results of an extensive comparative study in which all four major field‐strength prediction methods are compared with measured data. The most recent data bank has been used. These methods are analyzed in detail. Factors affecting sky‐wave propagation are discussed. Some historical background is also presented.
A hybrid UTD‐MM analysis of the radiation from a composite slot‐blade cavity‐backed antenna on realistic aircraft configurations
Authors: Tseng Huan‐Wan , Pathak Prabhakar H.
Published: September 2002

A hybrid method which combines the uniform geometrical theory of diffraction (UTD) with a numerical moment method (MM) technique is presented for the analysis of the radiation from a composite slot‐blade cavity‐backed antenna located on realistic aircraft configurations. The excitation of the antenna is due to a delta gap generator in a thin‐wire probe feed that is placed inside a rectangular cavity. The MM procedure is used to solve coupled integral equations that are formulated for the unknown equivalent electric current distribution on the blade and the unknown equivalent magnetic current distribution over the slot, when this slot‐blade antenna configuration resides on an infinite ground plane. The UTD is next used to efficiently and accurately calculate the radiation from these equivalent current distributions for the slot‐blade antenna combination when the latter are now located on realistic aircraft geometries instead of a fiat ground plane. Calculated radiation patterns of the composite slot‐blade cavity‐backed antennas placed on a Boeing 737 commercial airplane and an F‐16 fighter aircraft are presented to illustrate the utility of this hybrid approach.
Application of the Dopplionogram to Doppler‐sorted interferometry measurements of ionospheric drift velocity
Authors: Parkinson M. L. , Breed A. M. , Dyson P. L. , Morris R. J.
Published: September 2002

The Dopplionogram was developed as a method of displaying Doppler shifts along the frequency axis of ionograms recorded using B‐mode soundings of the Dynasonde, an early type of HF digital ionosonde. The basic idea of recording Doppler shifts in an ionogram format is applied and extended to the Doppler velocity mode of the Digisonde Portable Sounder‐4 (DPS‐4), a related and more recent type of digital ionosonde. In order to describe our mode of operation a Dopplionogram is redefined to mean a set of stepped‐frequency soundings that yields a set of ionospheric Doppler shifts particular to the chosen transmission frequencies. Extension of the technique to include Doppler‐sorted interferometry (DSI) analysis of the Doppler spectra facilitates a detailed analysis of ionospheric velocity variations in time and group height. This revitalized approach to DSI should prove useful for the study of ionospheric dynamics for which knowledge of the height profile of electric currents, drift velocity, and neutral winds is required. The technique is demonstrated using measurements of polar cap plasma winds obtained with a DPS‐4 located at Casey, Antarctica (66.3°S, 110.5°E).
A new data acquisition concept for digital ionosondes: Phase‐based echo recognition and real‐time parameter estimation
Authors: Wright J. W. , Pitteway M. L. V.
Published: September 2002

The problem of identifying echoes in digital ionosonde observations is discussed, starting with an off‐line simulation of the procedure long used in real time by the dynasonde. In that method, echoes are defined as amplitude‐modulus peaks from rapid (10‐μs) sampling of receiver quadrature outputs; coincidence of peak time among a small number (4–8) of repeated pulses has served well to reject impulsive noise while deciding the complex‐amplitude samples to be retained for off‐line analyses. This paper presents a new method of echo recognition dependent on all of the phase information contained in the same rapid sampling. No change is made in basic sounding function, but conversion to physical parameters is done first, for all 10‐μs samples; the consistency of these parameters in five dimensions over four or more samples defines an echo, or at least a partial “glint.” In an example recording, the new method recognizes about 60% more echoes and glints, giving better trace continuity than the earlier method. Measurement resolution among proximate echoing regions is improved. Weighted means of the physical parameters within their defining samples yields better accuracy. Almost incidentally, these results would be available immediately in a real‐time implementation of the method.
A comparison of low‐frequency radio noise amplitude probability distribution models
Authors: Chrissan D. A. , Fraser‐Smith A. C.
Published: September 2002

One of the most commonly modeled statistics in atmospheric radio noise studies is the noise envelope voltage amplitude probability distribution (APD). Although a number of models have been introduced to characterize atmospheric noise envelope APDs, the quantity of real data that exist to verify their accuracy is somewhat limited, especially in the ELF and VLF bands. This paper presents the results of a statistical analysis in which thousands of hours of ELF/VLF noise are processed to derive APDs, which are then compared with various APD models to determine which of the models is most accurate. The error criterion used to find the optimal parameters of each APD model, as well as to compare the models against each other, is the expected value of the log error squared (where the log error is the difference in decibels between the data histogram and the model histogram). This criterion provides a means by which the models may be evaluated and compared numerically. The most accurate model is found to depend on geographic location, time of year and day, bandwidth, and center frequency, but two of the simplest models (i.e., each with only two parameters) are found to give extremely good performance in general. These are the Hall and alpha‐stable (or α‐stable) models, both of which approximate the Rayleigh distribution for low‐amplitude values but decay with an inverse power law for high‐amplitude values. This paper concludes that the Hall model is the optimal choice in terms of accuracy and simplicity for locations exposed to heavy sferic activity (e.g., lower latitudes) and the α‐stable model is best for locations relatively distant from heavy sferic activity (e.g., the polar regions).
The updated ionospheric delay model to improve the performance of GPS single‐frequency receivers
Authors: Afraimovich E. L. , Chernukhov V. V. , Demyanov V. V.
Published: September 2002

A regional updating technique is suggested for the global ionospheric delay model (Klobuchar's [1987] model). The updating procedure uses series of values of the ionospheric delay (ID) which are reconstructed from distance measurements at two Global Positioning System working frequencies. We consider the situation where Klobuchar's model is updated from measurements from a single ground‐based reference station. We exemplify the use of a regional updating of the ID model under undisturbed ionospheric conditions, as well as under solar eclipse and magnetic storm conditions. In all cases presented, using the proposed technique the standard deviation between predicted and measured ID values was reduced by a factor of 1.2–2.8 as compared with Klobuchar's original model used under the same conditions. A tentative estimate is made of the variation in the ionospheric rms error in determining the user's location (UPL) through the use of the regional updating technique. For the examples considered in this study, on applying the regional updating of Klobuchar's model, the ionospheric rms error in determining the UPL was found to be lower by a factor of 1.3–2.6 than that obtained by using the same model without updating.
Range imaging using frequency diversity
Authors: Palmer Robert D. , Yu Tian‐You , Chilson Phillip B.
Published: September 2002

The need exists for measurements with high vertical resolution when observing the variety of atmospheric processes with extremely small vertical extent, such as microscale turbulence and scattering layers associated with inertia gravity waves. For example, recent in situ observations have shown that both humidity and temperature “sheets,” with thicknesses of the order of meters, exist throughout the lower atmosphere. Hampered by bandwidth constraints, however, standard pulsed radar systems have shown only limited usefulness in the detection of such phenomena. Frequency domain interferometry can be used to estimate the position and thickness of a single scattering layer within the resolution volume. Using two closely spaced frequencies, the method is derived under the restrictive assumption of a single, Gaussian‐shaped layer. We will now introduce range imaging (RIM), which fully exploits the general advantages of frequency diversity. Using a set of closely spaced transmitter frequencies, a generalized method based on constrained optimization will be used to reconstruct high‐resolution images of the average power density as a function of range. The technique will be studied using simulated radar data and will be shown to be capable of resolving complex structures similar to Kelvin‐Helmholtz billows, which can be much smaller in vertical extent than the resolution volume.
Cumulative distributions of rainfall rate and microwave attenuation in Singapore's tropical region
Authors: Zhou Z. X. , Li L. W. , Yeo T. S. , Leong M. S.
Published: September 2002

The microwave attenuation due to rainfall in tropical regions has not been very widely studied yet. In Singapore's tropical environment, line‐of‐sight microwave communication links were set up and have been operated for several years to study the microwave attenuation characteristics due to tropical rainfall. In this paper the experimental results are presented, including the cumulative distributions of rainfall rate, the cumulative distributions of microwave attenuation, the relationship between specific attenuation and rainfall rate, and frequency scaling formulae. The results show that the rainfall rate and the microwave propagation characteristics in Singapore over a 10‐month experiment period are out of accord with International Radio Consultative Committee predictions.
Studies on polarizabilities and scattering behavior of small spherical particles
Authors: Sharma Reena , Sihvola Ari
Published: September 2002

It is possible to relate the response of a sphere to an applied uniform static electric field with the scattering from a small spherical particle due to plane wave incidence. The limits up to which these relations between the polarizability and the extinction, scattering, and radar cross sections of a sphere are valid in the dynamic range are presented here. From the observations it can be concluded that radar cross section is a very good measure in predicting the polarizability. A related aspect studied here is the evaluation of the effective permittivity of a sparse mixture of spherical inclusions using a generalized Maxwell Garnett mixing rule. This is compared with extinction of a plane wave by a slab of n spherical inclusions sparsely located. The extinction by such a slab is calculated using the quasistatic approximation to Mie theory, and also using the full Mie theory, as the size of the inclusions is increased. The studies have been carried out for both lossless and lossy inclusions. The generalized mixing rule was found to be quite accurate in predicting the value of effective permittivity up to size parameters of 0.5 at least for small εr of the inclusion.
Calibrating the bulk rheology of active obliquely convergent thrust belts and forearc wedges from surface profiles and velocity distributions
Authors: Platt J. P.
Published: September 2002

The geometrical profiles and surface velocity distributions in active forearc wedges depend on the bulk rheology and the surface tractions. Analytical and numerical solutions of the force balance and corner flow circulation equations indicate that the mechanical parameters should, in principle, all be directly determinable from the observational variables. The analysis suggests the following preliminary conclusions. (1) The surface profiles of forearc wedges are consistent with either a bulk plastic rheology or a viscous rheology with a transition to plastic or Coulomb behavior in the upper few kilometers. (2) Corner flow circulation in the absence of accretion is diagnostic of wedges with a bulk viscous rheology. The surface strike‐normal velocity and the surface profile both depend on the bulk viscosity and the viscous coupling at the base, allowing these mechanical variables to be determined: current estimates of this velocity in some wedges suggest the bulk viscosity is not less than 3×1019 Pa s. (3) Obliquely converging nonaccreting plastic and Coulomb wedges show no distributed strike‐parallel shear. If the obliquity is above a critical angle, the wedge is separated from the upper plate by a strike‐slip fault, defining a forearc sliver. The critical angle of obliquity, the velocity of the forearc sliver, and the wedge geometry allow the boundary tractions to be determined. In obliquely convergent viscous wedges the strike‐parallel velocity is distributed throughout the wedge, decaying exponentially from the rear to the front. Its value at the rear and the length scale for the decay allow the bulk viscosity and the coupling at the rear to be determined. Slip vectors in some forearc wedges and thrust belts suggest complete partitioning of the strike‐parallel component of motion, characteristic of a bulk viscous rheology. Others show partitioning above a limiting angle of obliquity, suggesting bulk plastic behavior. There are no documented cases of active margins where there is an absence of partitioning for obliquity ∼45°, which appears to rule out Coulomb behavior as a general description for the bulk rheology of forearc wedges.
Quaternary convergent margin tectonics of Costa Rica, segmentation of the Cocos Plate, and Central American volcanism
Authors: von Huene R. , Ranero C. R. , Weinrebe W. , Hinz K.
Published: September 2002

Along Costa Rica, new geophysical data indicate considerable control of Quaternary convergent margin tectonics by the subducting lower plate. Three types of ocean crust enter the subduction zone: (1) Cocos Ridge with its underlying thick crust stands 2 km high, (2) on its north flank is normal crust covered 40% by seamounts, and (3) along the adjacent Nicoya margin the underthrust crust has a smooth sea floor. A 3‐ to 10‐km‐wide base of slope frontal prism varies little opposite different subducting crusts except where subducting seamounts eroded it. Once the breaching seamount has passed the prism it is quickly restored. The effect of oceanic crust on continental margin structure is most evident in the middle and upper slope. Where Cocos Ridge and its flanking seamounts subduct, erosion is pronounced relative to the stable slope where smooth lower plate subducts. Aligned upper plate features above lower plate segment boundaries extend more than 120 km landward of the trench axis and correspond in varying degrees with volcanic arc segmentation. The offset of volcanoes across the Costa Rica/Nicaragua border corresponds with a change in crustal structure and depth of the lava source. Subducted sediment shows little correlation with the slab signal in volcanic arc lavas but the magnitude of faulting associated with ocean plate flexure adjacent to the trench axis parallels it well. Thus fluids in ocean crust fractures and bound water in serpentinite may have a recognizable geochemical effect in arc lavas.
Extension in the southern Ryukyu arc (Japan): Link with oblique subduction and back arc rifting
Authors: Fabbri Olivier , Fournier Marc
Published: September 2002

Compilation of earthquake focal mechanisms, morphological analysis, and examination of mesofractures help to clarify the Cenozoic and recent stress history in the southern Ryukyu arc between the Ryukyu trench and the southern Okinawa Trough. (1) An older, pre‐Miocene, intermediate‐type stress field (vertical stress component σv = σ2) is attested by conjugate strike‐slip faults and shear joints in Eocene rocks of Ishigaki Island. The σ1 axis trends N90°E to N110°E in present‐day coordinates. (2) Normal faults and extensional joints in Mio‐Pliocene strata of Iriomote and Yonaguni Islands and fault scarps in Pleistocene strata at Yonaguni allow us to reconstruct a Miocene to Pleistocene stress field of tensional type (σv = σ1). The σ3 axis trends about NW–SE. The effects of this stress field can be traced back in Eocene rocks of Ishigaki Island. (3) Evidence for the youngest, Holocene to present‐day stress field comes from the analysis of the Harvard Moment Tensor Catalog (1976–1997) for shallow earthquakes, Quaternary fault scarps visible on the two islands of Miyako and Yonaguni, and extensional joints in Holocene rocks of Yonaguni. This stress field is tensional. Two directions of extension are inferred: a regional, arc‐perpendicular (N–S) one and a local, arc‐parallel (N60°E) one in the easternmost part of the area near Miyako. The record of Neogene to Quaternary tensional stress fields demonstrates that the extension in the Okinawa Trough can be traced until the arc region. Arc‐oblique to arc‐perpendicular stretching can be directly correlated with oceanward rifting in the back arc basin, while the arc‐parallel stretching reflects the increase of the arc curvature. The predominance of extensional features, the absence of shortening or transcurrent deformation structures, and a weak interplate coupling do not support the forearc sliver model for the present‐day southern Ryukyu arc.
Archean continental assembly in the southeastern Superior Province of Canada
Authors: Calvert A. J. , Ludden J. N.
Published: September 2002

Between 1988 and 1993, seismic reflection and refraction surveys were acquired across the medium‐ to high‐grade Opatica plutonic gneiss belt, the low‐grade Abitibi greenstone belt, and the Pontiac metasedimentary belt, all of which form part of the late Archean Superior Province. Shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 Ma of various exotic terranes against a backstop provided by the Opatica belt. This rapid southward growth of the Archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65‐km depth in the upper mantle below the Opatica belt. In contrast to the mainly orthogneissic Opatica and Pontiac belts, the midcrust of the Abitibi belt comprises metasedimentary and igneous rocks, plus imbricated units of unknown affinity. Relict midcrustal accretionary complexes of substantial size, whieh are indicative of primary suture zones, are interpreted near the northern and southern limits of the Abitibi belt. An interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous. Evidence of large‐scale extension appears to be confined to the Southern Volcanic Zone of the Abitibi, which developed into a half graben as the original suture zone was reactivated in extension. Unusually high seismic P wave velocities, 7.5–8.2 kms−1, are present in the lower 8 km of the Abitibi crust, and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust. Crustal xenolith studies suggest that this process may be linked to early Proterozoic magmatism.
Rheology predictions across the western Carpathians, Bohemian massif, and the Pannonian basin: Implications for tectonic scenarios
Authors: Lankreijer Anco , Bielik Miroslav , Cloetingh Sierd , Majcin Dušan
Published: September 2002

On the basis of extrapolation of failure criteria, lithology, and temperature models, we predict the rheology of the lithosphere for several sections through the Carpathians and surrounding regions. Our models show significant lateral variations in rheology for the different tectonic units, with important implications for the tectonic evolution. The Theologically strong lithosphere of the Polish Platform area contrasts with the weak lithosphere of the Pannonian basin, indicating that the arcuate shape of the Carpathian orogen is primarily caused by an inherited curvature of an ancient embayment in the foreland, with the Pannonian units passively filling the space. The Polish Platform and the Moesian Platform exhibit a similar rheological anisotropy caused by NW–SE trending weakness zones paralleling the Tornquist ‐ Teisseyre zone. This anisotropy was the main controlling factor on the behavior of the lithosphere in this area since Cadomian times, as documented by the geological evolution of the Sudety Mountains and the Mesozoic Polish Trough, including the Late Cretaceous Alpine inversion and the Neogene development of the Carpathian foreland. This rheological anisotropy appears to have a major controlling impact on the development of at least the eastern part of the European lithosphere. Rheology predictions for the Bohemian massif support the idea that the rigid lithosphere of the Bohemian massif governed the bending of the Alpine‐Carpathian transition zone, expressed in the large‐scale wrench movements opening the Vienna basin. In the foreland area, detachment levels are predicted for upper and lower crustal levels, leading to a decoupling of crustal and subcrustal flexure in most areas. Comparison with basin formation models indicates that our predictions for effective elastic thickness (EET) are similar to those derived from flexural models for the foreland area. Also, EET predictions from extensional basin models in the Pannonian region yield values close to our findings.
Kinematics of basin development during the transition from terrane accretion to strike‐slip tectonics. Late Cretaceous‐early Tertiary Cantwell Formation, south central Alaska
Authors: Cole Ronald B. , Ridgway Kenneth D. , Layer Paul W. , Drake Jeffery
Published: September 2002

The Cantwell basin was formed during Late Cretaceous time in the suture zone between the Wrangellia composite terrane and the former continental margin of southern Alaska. The Late Cretaceous (∼80–70 Ma) lower Cantwell Formation represents the initial fill of this basin and includes ∼4000 m of sedimentary rock. Between Maastrichtian and late Paleocene time (∼70–60 Ma), rocks of the lower Cantwell Formation were deformed by nearly eastwest trending folds and north vergent thrust faults. This deformation can be linked to the northward accretion and suturing of the Wrangellia composite terrane to southern Alaska. During late Paleocene to early Eocene time (∼60–55.5 Ma), at least 2750 m of volcanic rocks of the upper Cantwell Formation were deposited above the folded lower Cantwell Formation. New40Ar/39Ar ages for the upper Cantwell Formation range from 59.8+/−0.2 to 55.5+/−0.2 Ma and reveal that the unconformity between the lower and upper Cantwell Formations represents a 10–20 million year hiatus. Following volcanism the Cantwell Formation was crosscut by northeast trending folds, northwest trending normal faults, north trending left‐lateral slip faults, northeast trending reverse faults, and east trending right‐lateral slip faults. These structures are consistent with northwest‐southeast compression in a zone of right‐lateral simple shear along a fault with the same trend (east‐northeast) as that of the present McKinley strand of the Denali fault system. Cantwell basin kinematics are compatible with an escape tectonics model for south central Alaska whereby the Cantwell basin was formed, folded, and partially uplifted during northward terrane accretion and suturing and was deformed again during the westward transfer of terranes along right‐lateral strike‐slip fault systems.
Multistage accretion and exhumation of the continental crust (Ivrea crustal section, Italy and Switzerland)
Authors: Handy M. R. , Franz L. , Heller F. , Janott B. , Zurbriggen R.
Published: September 2002

The Ivrea crustal section exposes in map view all levels of the southern Alpine continental crust, from ultramafic, mafic, and felsic granulite facies rocks of the deep crust (Ivrea‐Verbano Zone), through medium‐grade basement rocks (Strona‐Ceneri Zone and Val Colla Zone), to unmetamorphosed Permo‐Mesozoic sediments. The oldest part of the crustal section is preserved in the medium‐grade basement units, which are interpreted to be the overprinted remains of an Ordovician (440–480 Ma) magmatic arc or forearc complex. During Variscan subduction this arc was technically underplated by a Carboniferous accretion‐subduction complex (320–355 Ma) containing metasediments and slivers of Rheic oceanic crust presently found in the Ivrea‐Verbano Zone. During the late stages of Variscan convergence (290–320 Ma), lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions. This was broadly coeval with extensional exhumation and erosion of the Variscan‐overprinted Ordovician crust presently exposed in the Strona‐Ceneri and Val Colla Zones. Post‐Variscan transtensional tectonics (270–290 Ma) were associated with renewed magmatic underplating, mylonitic shearing, and incipient exhumation of the lower crust in the Ivrea‐Verbano Zone. This coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust. Early Mesozoic, Tethyan rifting of the southern Alpine crust (180–230 Ma) reduced crustal thickness to 10 km or less. In the lower crust, most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing. The lower crust was exhumed along a large, noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust. The composite structure resulting from this complex evolution is probably typical of thinned, late Variscan continental crust on the passive margins of western Europe. Alpine faulting and folding (20–50 Ma) fragmented the crustal section. The originally deepest levels of the crustal section in the Ivrea‐Verbano Zone as well as some segments of the basement‐cover contact were steepened, whereas other parts of the crustal section, particularly the Strona‐Ceneri Zone, underwent only minor to moderate Alpine rotation.
Geometry and Quaternary kinematics of fold‐and‐thrust units of southwestern Taiwan
Authors: Lacombe O. , Mouthereau F. , Deffontaines B. , Angelier J. , Chu H. T. , Lee C. T.
Published: September 2002

Structural and paleostress analyses provide new insights into the Quaternary kinematics of the outermost fold‐and‐thrust units of southwestern Taiwan Foothills. The frontal folds are interpreted as fault‐related folds, and their tectonic evolution through space and time is tightly constrained. Fold development is correlated with reef building on top of the anticlines. Moreover, we provide field evidence that NW–SE fault zones oblique to the structural grain of the belt probably acted as transfer fault zones during the the Quaternary fold‐thrust emplacement. Two successive Quaternary stress regimes are evidenced in southwestern Taiwan: A NW–SE compression, followed by a recent nearly E–W compression. The latter shows an along‐strike change from pure E–W contraction to the north to perpendicular N–S extension in the south. This southward decrease in N–S confinement probably represents the on‐land signature of the incipient Quaternary tectonic escape predicted by analogue and numerical modelling and evidenced at present‐day by Global Positioning System data.
Architecture and tectonic evolution of nonvolcanic margins: Present‐day Galicia and ancient Adria
Authors: Manatschal Gianreto , Bernoulli Daniel
Published: September 2002

A comparison of the reconstructed southeastern margin of the Tethys ocean with the present‐day Galicia margin shows that although both margins are of different age and had a different fate, their architectures and tectonic evolutions are very similar. Along both non‐volcanic margins the site of rifting shifted from a broad area in the future proximal margins to a localized area in its distal parts, accompanied by a change in the mode of extension. During the initial phase of rifting, extension was accommodated by symmetrically arranged listric faults which soled at midcrustal levels, indicating that deformation in the upper crust was decoupled from deformation in the upper mantle along a hot and weak lower crust. During advanced rifting, extension was dominated by simple shear along low‐angle detachment faults with a top‐to‐the‐ocean sense of movement. These shallow crustal structures formed a series of breakaways in the continental crust and cut into mantle rocks, indicating that now deformation in the upper crust and in the upper mantle was no longer decoupled. Cooling and strengthening of the lower crust during an initial stage of rifting apparently led to localization of deformation and a different style of deformation, documenting that the tectonic evolution of nonvolcanic margins is largely controlled by the thermal state of the lithosphere. Seafloor spreading initiated only after exhumation and exposure of the subcontinental mantle on the ocean floor and may have been accompanied by a loss of the yield strength of the upper mantle, due to a combination of simple shear extension, asthenospheric uplift, and increased melt production.