bluuebunny/crossref_metadata_2025 huggingface dataset:
Similar glacial and interglacial export bioproductivity in the Atlantic Sector of the Southern Ocean: Multiproxy evidence and implications for glacial atmospheric CO2
Authors: Frank Martin , Gersonde Rainer , van der Loeff Michiel Rutgers , Bohrmann Gerhard , Nürnberg Christine C. , Kubik Peter W. , Suter Martin , Mangini Augusto
Published: February 2004

We present time series of export productivity proxy data including 230Thex‐normalized deposition rates (rain rates) of 10Be, dissolution‐corrected biogenic Ba, and biogenic opal as well as authigenic U concentrations which are complemented by rain rates of total (detrital) Fe and sea ice indicating diatom abundances from five sediment cores across the Atlantic sector of the Southern Ocean covering the past 150,000 years. The results suggest that 10Be rain rates and authigenic U concentration cannot serve as quantitative paleoproductivity proxies because they have also been influenced by detrital particle fluxes in the case of 10Be and bulk sedimentation rates (sediment focussing) and deep water oxygenation in the case of U. The combined results of the remaining productivity proxies of this study (rain rates of biogenic opal and biogenic Ba in those sections without authigenic U) and other previously published proxy data from the Southern Ocean (231Pa/230Th and nitrogen isotopes) suggest that a combination of sea ice cover, shallow remineralization depth, and stratification of the glacial water column south of the present position of the Antarctic Polar Front and possibly Fe fertilization north of it have been the main controlling factors of export paleoproductivity in the Southern Ocean over the last 150,000 years. An overall glacial increase of export paleoproductivity is not supported by the data, implying that bioproductivity variations in the Southern Ocean are unlikely to have contributed to the major glacial atmospheric CO2 drawdown observed in ice cores.
Intercomparison of coral oxygen isotope data and historical sea surface temperature (SST): Potential for coral‐based SST field reconstructions
Authors: Evans Michael N. , Kaplan Alexey , Cane Mark A.
Published: February 2004

We examine the extent to which the large‐scale features of the sea surface temperature (SST) anomaly field are represented by a sparse observational network of coral oxygen isotope (δ18O) time series. Regression of annually averaged δ18O data against gridded estimates of local SST anomaly within the period 1856–1990 confirm the literature regression of δ18O anomaly on SST anomaly for Indo‐Pacific corals. However, while interannual SST variability is generally well represented by individual coral time series, observed decadal and secular variability does not always display a linear relationship to local SST anomaly. Instead, many records appear to better recover nonlocal, large‐scale phenomena, which in turn are related to either the coral local SST or SST covariant changes in seawater δ18O. We employ empirical orthogonal function (EOF) analysis to identify common patterns of variability in the coral data. We find two significant patterns which are interpretable as the oceanographic signature of the El Niño‐Southern Oscillation (ENSO) and as a near‐global warming in which the eastern equatorial Pacific cools. A third pattern weakly resembles the Pacific Decadal Oscillation. These modes are seen more clearly in a singular vector decomposition (SVD) of the covariance between the coral data and the dominant patterns of large‐scale historical SST variability. The results are consistent with those found in EOF and SVD analyses of SST data from the coral locations. As additional coral‐based proxy estimates become available, they will improve the resolution of the patterns recovered. These results suggest that a sparse network of coral data may be used to reconstruct interannual, secular, and decadal SST variability for preinstrumental periods, albeit with large uncertainty.
Phase relationships between millennial‐scale events 64,000–24,000 years ago
Authors: Shackleton Nicholas J. , Hall Michael A. , Vincent Edith
Published: February 2004

A core recovered on the Iberian margin off southern Portugal can be correlated with Greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over Greenland. Our age model identifies the base of every interstadial between 64,000 and 24,000 years ago and uses the Greenland Ice Core Project (GRIP) timescale. The oxygen isotope signal in benthic foraminifera (on this GRIP‐based timescale) is quite different from the planktonic record and resembles the temperature record over Antarctica when this is synchronized with Greenland using the record of methane in the atmospheric air in the polar ice cores. We interpret the benthic record as indicating significant fluctuations in ice volume during millennial events, and we suggest that Antarctic temperature changed as a function of ice volume.
Maastrichtian foraminiferal and paleoceanographic changes on Milankovitch timescales
Authors: MacLeod Kenneth G. , Huber Brain T. , Pletsch Thomas , Röhl Ursula , Kucera Michal
Published: February 2004

Milankovitch‐scale alternations in Maastrichtian hemipelagic strata from Ocean Drilling Program Hole 1050C (Blake Nose) provide a natural experiment of paleoceanography and foraminiferal paleoecology in a tropical/subtropical greenhouse ocean. Cycles are 30–50 cm thick and thought to reflect the ∼21 kyr precessional cycle. High planktic δ18O values are correlated with high planktic but low benthic δ13C values, indicating that cooler and/or more saline surface waters were associated with higher productivity. High‐productivity intervals are also characterized by high Ca concentrations; enrichment in feldspar and kaolinite; and high relative abundance of Heterohelix spp., Globigerinelloides spp., and Laeviheterohelix glabrans. Conversely, low‐productivity intervals have low planktic δ18O values; high Fe and Ti concentrations; enrichment in quartz, illite, and chlorite; and relative increases in Globotruncana spp. and Pseudoguembelina spp. Potential forcing mechanisms for observed covariation include cyclic variation in water column stratification, variation in continental nutrient fluxes, and changes in the intensity of upwelling.
Attenuation of millennial‐scale events by bioturbation in marine sediments
Authors: Anderson David M.
Published: February 2004

The discovery of large, millennial‐scale oscillations (1–10 kyr period) in climate records from ice cores and marine sediments in the North Atlantic has motivated the search to understand their cause and evaluate the geographic extent of this variability. One uncertainty in comparing marine geologic records of millennial‐scale variability is the extent to which mixing of sediment by organisms at the seafloor (bioturbation) may attenuate millennial‐scale events in the stratigraphic record. Evidence of attenuation of millennial variability can be found in cores with sedimentation rates in the range of 5–15 cm/kyr. Modeling the attenuation of a hypothetical input time series similar to GISP2 using a moderate level of mixing provides estimates of attenuation as a function of sedimentation rate. The amplitude of a 4 kyr duration event is reduced by 50% for sedimentation rates ∼10 cm/kyr and reduced by 20% for sedimentation rates of 20 cm/kyr. Minimal attenuation (<5%) is achieved only when the sedimentation rate exceeds 50 cm/kyr for a 4 kyr duration event and 70 cm/kyr for a 2 kyr duration event. The intensity of mixing plays a role in determining how much attenuation occurs. The amplitude of a 4 kyr duration event in a 10 cm/kyr core is reduced by 75% under strong mixing or reduced by 25% under weak mixing. This study indicates that the original amplitude of Dansgaard‐Oeschger (DO) millennial‐scale events observed during the last glacial may be twice the observed amplitude for cores with sedimentation rates 10–20 cm/kyr that have experienced moderate bioturbation. Regional comparisons of the amplitude of millennial‐scale variability will require cores with weak mixing or sedimentation rates exceeding 50 cm/kyr in order to avoid biased estimates of the amplitude of millennial‐scale (4 kyr duration) variability.
Paleoceanographic implications of the difference in deep‐sea sediment mixing between large and fine particles
Authors: Bard Edouard
Published: February 2004

More and more paleoceanographic studies are performed on different size fractions of deep‐sea sediments. Although it has been a controversial issue for some time, the bioturbation process depends on the particle size and can, in principle, decouple paleoceanographic records obtained in the same core. This problem is studied by using a simple mixing model which allows the derivation of analytical equations for both the expected phase shift and relative attenuation of signals carried by different size fractions. Depending on several parameters, the time shift and relative attenuation can be on the order of a few millenia and 50%, respectively. However, high sedimentation rates (> 10–20 cm/kyr) minimize these potential complications allowing us to study millenium‐scale events in well‐chosen sedimentary environments.
A latitudinal productivity band in the central North Atlantic over the last 270 kyr: An alkenone perspective
Authors: Villanueva J. , Calvo E. , Pelejero C. , Grimalt J. O. , Boelaert A. , Labeyrie L.
Published: February 2004

Productivity changes in the central North Atlantic Ocean have been traced by means of the total C37 alkenone contents along two sediment cores located at 43°N and 37°N. Both alkenone signals revealed the occurrence of discrete productivity events every 23 kyr. Spectral analyses highlight the presence of a dominant 23‐kyr periodicity in the alkenone signal, which is highly coherent to the precession index. However, a close comparison revealed small but relevant differences in the timing of several of the productivity events recorded at both locations. These asynchronies suggest that the alkenone maxima do not necessarily reflect a general increase of productivity over the North Atlantic. We propose that the events are related to a latitudinal band of productivity that moves northward and southward over time. Satellite‐derived productivity estimates show that the present location of this band is 45°–55°N. To illustrate this hypothesis we have constructed a conceptual model that reconstructs the temporal changes of productivity at one given location by assuming a productivity band that evolutes latitudinally over time. The model is able to reconstruct the main features of the alkenone records, namely, (1) the occurrence of discrete and abrupt productivity events, (2) the asynchrony at different latitudes, and (3) the bimodal pattern of many of the productivity peaks.
Response of the Mid‐Cretaceous global oceanic circulation to tectonic and CO2 forcings
Authors: Poulsen Christopher J. , Barron Eric J. , Arthur Michael A. , Peterson William H.
Published: February 2004

The mid‐Cretaceous was a period of unusually active tectonism that drove enhanced volcanic outgassing and high seafloor spreading rates. This intense tectonic activity is coincident with dramatic events in the marine environment, including oceanic anoxic events 1 (Aptian‐Early Albian) and 2 (Cenomanian/Turonian boundary), high biological turnover rates, and a thermal maximum. In this study, a series of mid‐Cretaceous ocean general circulation model experiments were completed using the Parallel Ocean Climate Model. These experiments demonstrate the effect of enhanced atmospheric CO2 concentrations and paleogeographic change on the global oceanic circulation. The experiments reveal that paleogeography, specifically the presence/absence of a marine connection between the North Atlantic and South Atlantic basins, may have governed the nature of the mid‐Cretaceous global oceanic circulation. In the absence of this connection, an Albian simulation is characterized by extremely warm, saline conditions throughout the North Atlantic and northern South Atlantic Oceans. With a gateway in a Turonian simulation, Antarctic Bottom Water ventilates the Atlantic basins. In both Albian and Turonian simulations the Pacific‐Indian basins are dominated by thermohaline circulation with deep water sources in the Southern Ocean. While atmospheric CO2 concentrations influence the global temperature and salinity, an increase from present‐day to 4 times present‐day levels alters the global circulation very little. Differences between the Albian and Turonian numerical simulations agree well with aspects of the marine record, supporting speculation that the climatic and oceanographic changes surrounding the Cenomanian‐Turonian boundary were driven by the initiation of a connection between the Atlantic Oceans.
An analytical, numerical, and experimental study of backscattering from multiscale soil surfaces
Authors: Mattia Francesco , Le Toan Thuy , Davidson Malcolm
Published: February 2004

The objective of this paper is to investigate the backscattering from multiscale soil surfaces through a comparison of analytical model predictions with numerical computations and experimental data. A numerical simulation of the backscatter from multiscale surfaces is performed in order to assess the sensitivity of the radar backscatter to the multiscale effect as a function of the roughness range of the surfaces, the polarization, and the incidence angle of the impinging electromagnetic wave. The numerical backscattering coefficients are compared with those obtained using the traditional integral equation method (IEM) as well as those obtained using a novel version of the IEM which has been adapted to model the backscattering from multiscale surfaces (MLS IEM). As a result, the surface conditions under which the traditional IEM can still predict well the backscatter from multiscale surfaces as well as the conditions under which traditional IEM fails are derived. Subsequently, 25 m long roughness profiles collected over the Marestaing test site located near Toulouse, France, are analyzed in order to estimate input parameters for the two versions of the IEM model. Then, a comparison is made between the single‐scale IEM and MLS IEM predictions and ERS synthetic aperture radar (SAR) data acquired over the site. The results show that over medium‐rough soils (i.e., harrowed soils) the SLS IEM is in good agreement (within 1 dB) with ERS SAR data, whereas MLS IEM underestimates the measurements of 2–3 dB. In contrast, over very smooth soils (i.e., rolled soils) the MLS IEM is in overall better agreement with ERS SAR data than SLS IEM. The latter overestimates the measurements of 3 dB.
GPS global detection of the ionospheric response to solar flares
Authors: Afraimovich E. L.
Published: February 2004

This author suggests the concept of a new technology for global detection (GLOBDET) of atmospheric disturbances of natural and technogenic origin, on the basis of phase measurements of the total electron content (TEC) in the ionosphere using an international GPS network. Temporal dependencies of TEC are obtained for a set of spaced receivers of the GPS network simultaneously for the entire set of “visible” (over a given time interval) GPS satellites (up to 5–10 satellites). These series are subjected to filtering in the selected range of oscillation periods using algorithms for spatiotemporal analysis of signals of nonequidistant GPS phased antenna arrays which are adequate to the detected disturbance. An analysis is made of the possibilities of using the GLOBDET when detecting the ionospheric response of solar flares. In this case it is best to make the coherent summation of the filtered series of TEC. Powerful impulsive flares of July 29, 1999, and December 28, 1999, were chosen to illustrate the practical implementation of the proposed method.
Errors of ionospheric parameter diagnostics by vertical sounding method
Authors: Denisenko P. F. , Beloff N. , Noranovich D. A. , Gough M. P.
Published: February 2004

The problem of RMS error determination in ionospheric parameters measured by the method of vertical HF soundings is considered here. This problem is solved for the case of topside ionospheric soundings when the height distribution of electron density is monotonic. An analytical expression is obtained which relates the variance of deduced parameters to the correlation function of errors in measured characteristics of sounding signals. The computing simulation provides for errors in real height determination, in reconstruction of vertical velocities of plasma motion, and in effective electron collision frequencies for the measurements of virtual heights, Doppler frequency shift, and radio wave absorption, respectively. Their dependencies on signal polarization, working frequency range, and geomagnetic latitude of observation point are determined. The obtained results are useful for planning plasma diagnostics experiments using HF sounding technique.
Nonturbulent layers in polar summer mesosphere: 2. Application of wavelet analysis to VHF scattering
Authors: Alcala C. M. , Kelley M. C.
Published: February 2004

In a companion paper [Alcala et al., this issue] we presented a wavelet analysis of rocket data obtained simultaneously with a VHF radar during a polar mesosphere summer echo event. Here we apply scattering theory to each of the wavelets in the reconstructed function and add up the individual electric fields scattered from the volume illuminated by the radar. The resulting scattering profile is quite similar to that observed if we take into account a small tilt of the layer, consistent with the present rocket data and with other rocket and radar measurements during similar conditions. The scattering power is overestimated by about an order of magnitude. This can be explained by either a scattering structure a factor of 5 smaller than the Fresnel scale, a tilt in the scattering layer, or a combination of the two effects.
Fourier transform analysis of a ridge waveguide and a rectangular coaxial line
Authors: Cho Yong H. , Eom Hyo J.
Published: February 2004

A novel technique based on the Fourier transform is applied to analyze a ridge waveguide and rectangular coaxial line. On the basis of the image theorem the ridge waveguide and rectangular coaxial line are transformed into a guide with an infinite number of grooves. The dispersion relations for the ridge waveguide and rectangular coaxial line are obtained in rigorous, yet simple series forms. Numerical computations show that our series solutions converge fast and agree with others.
International Reference Ionosphere 2000
Authors: Bilitza Dieter
Published: February 2004

The International Reference Ionosphere (IRI) is the international standard for the specification of ionospheric densities and temperatures. It was developed and is being improved‐updated by a joint working group of the International Union of Radio Science (URSI) and the Committee on Space Research (COSPAR). A new version of IRI is scheduled for release in the year 2000. This paper describes the most important changes compared to the current version of IRI: (1) an improved representation of the electron density in the region from the F peak down to the E peak including a better description of the F1 layer occurrence statistics and a more realistic description of the low‐latitude bottomside thickness, (2) inclusion of a model for storm‐time conditions, (3) inclusion of an ion drift model, (4) two new options for the electron density in the D region, and (5) an improved model for the topside electron temperatures. The outcome of the most recent IRI Workshops (Kühlungsborn, 1997, and Nagoya, 1998) will be reviewed, and the status of several ongoing task force activities (e.g., efforts to improve the representation of electron and ion densities in the topside ionosphere and the inclusion of a plasmaspheric extension) will be discussed. A few typical IRI applications will be highlighted in section 6.
A neural network approach to the microwave inverse scattering problem with edge‐preserving regularization
Authors: Gong Xing , Wang Yuanmei
Published: February 2004

A neural network approach to the inverse scattering problem for microwave tomographic reconstruction is presented. Although the technique of microwave tomography has been developed for more than two decades, it is still in its infancy in that it is necessary to solve the inverse scattering problem, which is well known as an ill‐posed and nonlinear problem and therefore difficult to deal with. To improve the inherent ill‐posedness of the problem, good regularization procedures are required. In this paper, an edge‐preserving regularization is proposed with a set of line processes to preserve the edge of the reconstructed image. Since the unknown dielectric permittivities are continuous complex variables and the line processes are binary variables, an augmented Hopfield network is applied to the mixed‐variable optimization problem. With this method, a priori knowledge can be conveniently incorporated into the optimization process, and inversions of large matrices are avoided. A numerical example of a simple model illuminated by the transverse magnetic incident waves is reported, and the advantages and limitations of the method are discussed.
An experimental contribution to the interpretation of mesosphere‐stratosphere‐troposphere radar echoes in frequency domain interferometric mode
Authors: Luce H. , Kubo K. , Fukao S.
Published: February 2004

Studies of atmospheric structures and dynamics at small scales require high resolution observations in space and time. Stratosphere‐troposphere radars offer high potentialities for such studies, in particular, with the dual‐frequency domain interferometry (FDI) technique used for tracking thin atmospheric layers. In the present paper, attention is drawn to the performances of this technique, discussed in light of comparisons between data sets collected in different radar configurations with the middle and upper atmosphere (MU) radar (Shigaraki, Japan, 34.85°N, 136.10°E). Measurements in FDI mode with two range resolutions (300 and 600 m) and two frequency differences (0.50 and 0.25 MHz) have been conducted, interleaved with measurements in Doppler‐beam‐swinging mode performed at a range resolution of 150 m. From the data comparisons it is found that the thickness of the echoing layers in FDI mode strongly depends on the initial range resolution used. Thus, even if the FDI mode is believed to give a better description of the stable layers, the FDI‐deduced thickness with an initial range resolution of 300 m may not be consistent with the true thickness of the atmospheric layers. It could then be misleading to use FDI observations in order to estimate the thickness of atmospheric layers. However, it is also shown that under some circumstances, FDI can provide reliable information on the characteristics of the atmospheric structures which cannot be available with “non‐FDI” techniques.
Mechanisms of low‐grazing‐angle scattering from spilling breaker water waves
Authors: Ja Shiou‐Jyh , West James C. , Qiao Haibing , Duncan James H.
Published: February 2004

The low‐grazing‐angle electromagnetic scattering from spilling breaker water waves has been examined using a numerical approach. A moment‐method‐based electromagnetic technique was used to find the instantaneous scattering from the crests of breakers generated mechanically in a wave tank in the absence of wind. A high‐speed imaging system carried on an instrument carriage traveling at the phase velocity of the breakers provided continuous measurements of the temporal evolution of the crest shape, in turn allowing a continuous calculation of the microwave backscatter from the crest. As the wave steepens and a bulge forms on the forward face of the crest, horizontal‐to‐vertical polarization backscattering ratios (HH/VV) as high as 0 dB are observed. Greatly reduced ratios are observed after breaking when no steep features remain on the wave. A time‐dependent Fourier analysis identifies “fast” scattering whose Doppler shifts are nearly equal to or greater than the phase velocity of the breaker, and “slow” scattering whose Doppler shifts are consistent with the orbital motion of the breaker. Comparison of the Doppler shifts with the measured wave profiles shows that the fast scattering identified is associated with very steep surface features that give specular (or nearly specular) reflection points on the surface, but the magnitude and HH/VV ratio of the response to a specific steep feature depends upon its electromagnetic size, thus giving a frequency dependence. The slow scattering identified (in the absence of wind) is correlated with the “turbulent scar” that remains after breaking.
A dual‐frequency compact microstrip patch antenna
Authors: Gao S. C. , Li L. W. , Yeo T. S. , Leong M. S.
Published: February 2004

In this paper, a new dual‐frequency, compact antenna, which uses an H‐shaped microstrip patch with a shorting pin, is described. Compared with the conventional rectangular patch antenna, this antenna can achieve both a significant reduction of antenna size and a dual‐frequency operation with a single feed. More freedoms for tuning the resonant frequencies, the frequency ratio, and the input impedance are available because of more design parameters. A detailed parameter study is performed, and the theoretical analysis is based on the finite difference time domain (FDTD) method. The FDTD programs are developed and validated by measurement results. The effects of several antenna parameters on two resonant frequencies, frequency ratio, and radiation pattern characteristics of the antenna are analyzed and compared. It is shown that various frequency ratios (1.91–4.23) can be obtained by varying the design parameters of this antenna. Several design curves are presented.
U‐Pb zircon ages from the Indian plate in northwest Pakistan and their significance to Himalayan and pre‐Himalayan geologic history
Authors: DiPietro Joseph A. , Isachsen Clark E.
Published: February 2004

U‐Pb zircon isotopic ages of seven rock samples are combined with field data to provide constraints on deposition, intrusion, and metamorphism in the northwestern part of the Indian plate (Pakistani) hinterland south of the Indus suture zone in Pakistan. The results suggest deformation, intrusion, and regional metamorphism at ca. 2174 Ma, an event that may correlate with granulite facies metamorphism in the Nanga Parbat area. This was followed by erosion and deposition prior to a second Proterozoic deformation at ca. 1850 Ma, which was associated with widespread intrusion and possibly with low‐grade regional metamorphism. Metasedimentary and intrusive rocks of this age are present within the Lesser Himalaya of Nepal but are apparently absent in the High Himalayan crystalline slab north of the MCT. This intrusive/deformational event was followed by erosion and Late Proterozoic (?) deposition with development, in the Cambrian, of an epicontinental, shallow marine shelf. Intrusion in Late Cambrian‐Middle Ordovician resulted only in minor uplift/erosion and development of a disconformity. Marine shelf conditions were reestablished in the Middle Paleozoic prior to a widespread intrusive event in the Carboniferous‐Permian and normal faulting, erosion, and syndeformational deposition and volcanism in the Late Permian. Marine shelf conditions were reestablished in the Triassic prior to the Himalayan orogeny. Zircons with a concordant age of 89 Ma coupled with field and published isotopic age data suggest that Himalayan deformation and metamorphism in the Pakistani hinterland began between 90 and 75 Ma due to subduction of the Indian plate beneath Indus ophiolitic melange and reached peak amphibolite facies conditions between 70 and 48 Ma. This metamorphism precedes Eocene (54–50 Ma) collision of India with the Kohistan arc complex. Field data suggest that the presently exposed Pakistani hinterland from Afghanistan to Babusar was never significantly overthrust or buried by the Kohistan arc. Rather than initiating the metamorphism, the collision of Kohistan with India resulted in uplift, exhumation, and cooling of the metamorphic pile.
Crustal thickening leading to exhumation of the Himalayan Metamorphic core of central Nepal: Insight from U‐Pb Geochronology and 40Ar/39Ar Thermochronology
Authors: Godin Laurent , Parrish Randall R. , Brown Richard L. , Hodges Kip V.
Published: February 2004

New and published U‐Pb geochronology and 40Ar/39Ar thermochronology from footwall and hanging wall rocks of a segment of the South Tibetan detachment system exposed in the Annapurna area of central Nepal Himalaya bring additional constraints on the timing of metamorphism, crustal thickening, and normal faulting resulting in exhumation of the Himalayan metamorphic core. Early Oligocene crustal thickening led to Eohimalayan kyanite‐grade metamorphism between 35 Ma and 32 Ma. The resulting thermal event affected the Early Ordovician augen gneiss (Formation III) and produced kyanite‐bearing leucosomes in the upper part of the metamorphic core. This event is linked with underthrusting of the Greater Himalayan metamorphic sequence below the Tethyan sedimentary sequence and the growth of an Oligocene fan structure that has thickened the Tethyan sedimentary sequence to 25 km, thus provoking kyanite‐grade melting at deeper structural levels. Early Paleozoic monazite and zircon populations indicate that part of the metamorphism affecting the Himalayan metamorphic core could be pre‐Cenozoic. Regional correlations indicate that the Annapurna detachment was active during early Miocene time. A weakly deformed leucogranitic dike intruding into the immediate hanging wall yielded reversely discordant monazite ages between 23 and 22.5 Ma, which suggest that the ductile strain in the Annapurna detachment zone terminated at ca. 22 Ma. On the basis of a 40Ar/39Ar muscovite age, renewed southwest verging deformation (D4) is interpreted to occur at ca. 18 Ma. Rapid exhumation resulting from extensional faulting cooled the entire metamorphic core through the muscovite Ar closure temperature (330°–430°C) between 15 and 13 Ma. Muscovites from the immediate hanging wall of the Annapurna detachment yielded slightly younger ages, between 13 and 11 Ma, testifying to late hydrothermal activity in the Annapurna detachment zone that could be linked with the initiation of brittle faulting associated with the late Neogene Thakkhola graben system.
SHRIMP U‐Pb zircon dating of Archean core complex formation and pancratonic strike‐slip deformation in the East Pilbara Granite‐Greenstone Terrain
Authors: Zegers T. E. , Nelson D. R. , Wijbrans J. R. , White S. H.
Published: February 2004

Sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb dating of zircons from granitic rocks in the East Pilbara Granite‐Greenstone Terrain has provided time constraints for main tectonic events in the Shaw Granitoid Complex and has shown that deformation was intricately related to granitoid intrusion. The earliest recognized deformation activity occurred within the Split Rock Shear Zone (SRSZ), probably between 3470 and 3420 Ma. The timing, geometry, and kinematics of this shear zone suggest that it was a midcrustal detachment in a core complex type extensional setting. It was active during the intrusion of synkinematic sheeted granodiorite of the North Shaw Suite and brittle extensional faulting within the upper crust. Samples of gray migmatic gneiss and a diorite that occur structurally below the SRSZ were dated at 3451±1 Ma and 3463±2 Ma, respectively, indicating that melting and metamorphism took place in the lower units during extension in the middle to upper crust. The 3470–3420 Ma extensional event is most consistent with extensional collapse, possibly as a result of delamination or convectional thinning of part of the lithosphere. Activity on the pancratonic strike‐slip Mulgandinnah Shear Zone (MSZ) was constrained by the crystallization age of a synkinematic granitic dyke to have occurred at 2934±2 Ma. The MSZ and activity on the Sholl Shear Zone of the western Pilbara at 2960 Ma [Smith et al., 1998] are consistent with a regional NW‐SE directed compressional stress field at this time. Steepening of greenstone belt stratigraphy has been attributed to diapirism, but probably occurred after a 3300–3200 Ma compressional event and following deposition of the youngest clastic sediments in the eastern Pilbara stratigraphic sequence. Diapiric processes did not play a major role during the early evolution of the eastern Pilbara Craton from 3470 to 3300 Ma.
Structure of the Altyn Tagh Fault and Daxue Shan from magnetotelluric surveys: Implications for faulting associated with the rise of the Tibetan Plateau
Authors: Bedrosian Paul A. , Unsworth Martyn J. , Wang Fei
Published: February 2004

Magnetotelluric measurements in the foreland of the Daxue Shan have imaged low‐angle (<20°) thrust faults that extend to a depth of 3 km. The surface projections of the faults at depth coincide with the mapped traces of the Chang Ma (M = 7.6, 1932) and North Hills thrusts. Minimum cumulative offsets of 7 km on the Chang Ma thrust and 14 km on the North Hills thrust are estimated from the horizontal extent of underthrust sediment. Assuming that regional thrusting began at 5–6 Ma, this corresponds to minimum convergence rates of 1.3 mm/yr and 2.5 mm/yr on the Chang Ma and North Hills thrusts, respectively. These slip rates correspond to ∼ 3 mm/yr of sinistral slip on the Altyn Tagh Fault between 96°E and 97°E. This is consistent with the 4 ± 2 mm/yr of slip estimated by geological studies. Assuming comparable slip rates and similar fault geometry in the neighboring ranges, this requires a minimum of 11 mm/yr of shortening parallel to the Altyn Tagh fault between the Dang He Nan Shan and Qilian Shan. Both the style of thrusting and rate of shortening are in agreement with geologic studies that favor a relatively high rate of slip on the Altyn Tagh fault. This, in turn, implies that lithospheric extrusion contributes significantly in accommodating the ongoing convergence between India and Asia. Farther west, the Altyn Tagh Fault is imaged on four magnetotelluric profiles as a vertical resistivity contrast extending to a depth of at least 8 km. Two strands of the North Altyn Tagh Fault are imaged east of the asperity near Subei (39°30′N, 95°E).
The recent (upper Miocene to Quaternary) and present tectonic stress distributions in the Iberian Peninsula
Authors: Herraiz M. , De Vicente G. , Lindo‐Ñaupari R. , Giner J. , Simón J. L. , González‐Casado J. M. , Vadillo O. , Rodríguez‐Pascua M. A. , Cicuéndez J. I. , Casas A. , Cabañas L. , Rincón P. , Cortés A. L. , Ramírez M. , Lucini M.
Published: February 2004

A general synthesis of the recent and present stress situation and evolution in the Iberian Peninsula was obtained from microstructural and seismological analysis. The stress evolution was deduced from (1) fault population analysis (FPA) from 409 sites distributed throughout the Iberian Peninsula, (2) paleostress indicators given by 324 stations taken from the bibliography, and (3) seismic data corresponding to 161 focal mechanisms evenly spread in the studied region. The application of FPA together with the determination of stress tensors and focal mechanisms for the whole Iberian microplate has provided two main results: (1) the Iberian Peninsula is undergoing a NW‐SE oriented compression, except for the northeastern part (Pyrenees, Ebro Basin, and Iberian Chain), where it is N‐S to NE‐SW, and the Gulf of Cádiz, where it seems to be E‐W, and (2) the main trends of the stress field have remained almost constant since the upper Miocene. The analysis performed by zones suggests the presence of local heterogeneities in the stress field.
Cretaceous to Neogene cooling and exhumation history of the Oetztal‐Stubai basement complex, eastern Alps: A structural and fission track study
Authors: Fügenschuh Bernhard , Mancktelow Neil S. , Seward Diane
Published: February 2004

The central Austroalpine Oetztal‐Stubai basement complex (OSK) of the eastern Alps together with its overlying sedimentary (Brenner Mesozoic (BMZ)) and tectonic units (upper Austroalpine nappes) underwent extension and exhumation during the Cretaceous before its incorporation into the hanging wall of the Neogene Brenner normal fault. During this Late Cretaceous to early Tertiary deformation the Brenner Mesozoic and the immediately underlying uppermost part of the OSK developed a strong stretching lineation within a near‐horizontal and often mylonitic foliation related to consistent top‐to‐ESE directed transport. Quartz and calcite behaved ductilely throughout the area, whereas dolomite shows a transition from brittle fracture in the north to plastic flow in the south, consistent with the known Cretaceous metamorphic temperature gradient. The transition to crystal plastic flow of dolomite occurs at ∼480°C. Quartz textures indicate an increase in the contribution of prism glide together with a decrease in basal glide toward the south, also consistent with the temperature gradient in the OSK and BMZ. Fission track analysis of both zircon and apatite along a N‐S profile, in combination with existing data from the literature, allows a continuous cooling path to be constructed for the OSK. Cooling of the eastern OSK started at 90 Ma (the thermal peak of metamorphism) and ended some 30 Myr later at near‐surface temperatures. At 60 Ma, the northern region had already cooled below the upper limit of the apatite partial annealing zone, whereas in the south, cooling related to this exhumation phase stopped at temperatures between 100° and 200°C. The ESE directed shearing is interpreted as Late Cretaceous to early Tertiary normal faulting that exhumed the OSK and brought the upper Austroalpine (Blaser and Steinacher) nappes into contact with the underlying Brenner Mesozoic. The pattern of Alpine metamorphic isograds established in the OSK directly reflects the geometry and kinematics of this Late Cretaceous/early Tertiary exhumation due to low‐angle normal faulting.
Tectonic implications of exposure of lower continental crust beneath the Iberia Abyssal Plain, Northeast Atlantic Ocean: Geophysical evidence
Authors: Whitmarsh R. B. , Dean S. M. , Minshull T. A. , Tompkins M.
Published: February 2004

We present new seismic data from a basement high beneath the southern Iberia Abyssal Plain that appears, from the latest Ocean Drilling Program (ODP) results and indirectly from various seismic observations, to consist of lower continental crustal material. This is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks. We infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting. Our results lead to the following new observations: low‐angle detachment faults, previously reported along the east‐west Lusigal‐12 profile across the southern Iberia Abyssal Plain, penetrate both the 3–10 km thick continental crust and, unusually, the uppermost mantle. Processing of new multichannel seismic reflection profiles across the basement high on which ODP Sites 900, 1067, and 1068 are located reveals that the high is bounded by a previously unsuspected steep, landward dipping normal fault on its east flank. Basement cores from the above ODP sites also reveal that the high is not capped by the previously predicted early syn‐rift sediment but by rocks from the lower continental crust. These unexpected observations are incorporated into a new tectonic model of the development of this part of the west Iberia margin which is also consistent with other geophysical observations. A novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3–6 km by a currently poorly understood process. The model implies three stages of deformation: (1) lithospheric extension, principally by symmetric pure shear, which leads to a ∼10 km thick crust in which the lower crust is largely absent over the axial zone and the crust‐mantle boundary forms a shear zone, (2) further thinning and then dissection of the most distal continental crust by seaward dipping, low‐angle normal faults, (3) inception of a high‐angle, landward dipping normal fault that offsets the tectonized crust‐mantle boundary and uplifts the lower crust to the crest of the basement high on which ODP Sites 900, 1067, and 1068 were drilled.
Mediterranean extension and the Africa‐Eurasia collision
Authors: Jolivet Laurent , Faccenna Claudio
Published: February 2004

A number of tectonic events occurred contemporaneously in the Mediterranean region and the Middle East 30–25 Myr ago. These events are contemporaneous to or immediately followed a strong reduction of the northward absolute motion of Africa. Geological observations in the Neogene extensional basins of the Mediterranean region reveal that extension started synchronously from west to east 30–25 Myr ago. In the western Mediterranean it started in the Gulf of Lion, Valencia trough, and Alboran Sea as well as between the Maures massif and Corsica between 33 and 27 Ma ago. It then propagated eastward and southward to form to Liguro‐Provençal basin and the Tyrrhenian Sea. In the eastern Mediterranean, extension started in the Aegean Sea before the deposition of marine sediments onto the collapsed Hellenides in the Aquitanian and before the cooling of high‐temperature metamorphic core complexes between 20 and 25 Ma. Foundering of the inner zones of the Carpathians and extension in the Panonnian basin also started in the late Oligocene‐early Miocene. The body of the Afro‐Arabian plate first collided with Eurasia in the eastern Mediterranean region progressively from the Eocene to the Oligocene. Extensional tectonics was first recorded in the Gulf of Aden, Afar triple junction, and Red Sea region also in the Oligocene. A general magmatic surge occurred above all African hot spots, especially the Afar one. We explore the possibility that these drastic changes in the stress regime of the Mediterranean region and Middle East and the contemporaneous volcanic event were triggerred by the Africa/Arabia‐Eurasia collision, which slowed down the motion of Africa. The present‐day Mediterranean Sea was then locked between two collision zones, and the velocity of retreat of the African slab increased and became larger than the velocity of convergence leading to backarc extension. East of the Caucasus and northern Zagros collision zone the Afro‐Arabian plate was still pulled by the slab pull force in the Zagros subduction zone, which created extensional stresses in the northeast corner of the Afro‐Arabian plate. The Arabian plate was formed by propagation of a crack from the Carlsberg ridge westward toward the weak part of the African lithosphere above the Afar plume.
Oligo‐Miocene midcrustal subhorizontal shear zone in Indochina
Authors: Jolivet Laurent , Beyssac Olivier , Goffé Bruno , Avigad Dov , Lepvrier Claude , Maluski Henri , Thang Ta Trong
Published: February 2004

New structural observations of Oligo‐Miocene deformation in north Vietnam along the Red River Shear Zone suggest that left‐lateral strike‐slip shear was restricted to the upper and middle crust above a horizontal shear zone. Left‐lateral shear deformation is associated with low‐pressure‐low‐temperature parageneses. High‐temperature deformation is restricted to zones where the foliation has a low dip in the core of the Dai Nui Con Voi antiformal dome. These observations complete those made earlier in the Bu Khang extensional dome farther south. Above the horizontal shear zone left‐lateral transpression was active during the first stage and changed to transtension some 33 Myr ago. Purely extensional metamorphic domes (Bu Khang) or transtensional domes (Dai Nui Con Voi) were exhumed during this younger stage. Our observations plead for caution when interpreting strike‐slip structures at lithospheric scale.
Tectonic analysis of the Husavik‐Flatey Fault (northern Iceland) and mechanisms of an oceanic transform zone, the Tjörnes Fracture Zone.
Authors: Bergerat Françoise , Angelier Jacques , Homberg Catherine
Published: February 2004

The inversion of ∼ 1000 fault slip data collected in the Flateyjarskagi Peninsula allows reconstruction of four main tectonic regimes. These include normal and strike‐slip faulting modes and are related to the general behavior of the Husavik‐Flatey Fault (HFF), a major structure of the Tjörnes Fracture Zone connecting the Kolbeinsey Ridge and the North Icelandic Rift. The two most important regimes (E‐W and NE‐SW extensions), consistent with the right‐lateral motion along the Husavik‐Flatey Fault, constitute the main tectonic group. The two others (NW‐SE and N‐S extensions), forming the subordinate tectonic group, are incompatible and result from drastic stress permutations. The relationships between these stress regimes imply not only σ1/σ2 and σ2/σ3, stress permutations but also σ1/σ3 reversals. A critical review of other data available, such as lava bedding, dike, and major fault attitudes, allows us to complete the structural pattern of the Flateyjarskagi peninsula and to highlight the mechanism of the transform zone. The complex pattern of dikes and faults in the northern part of Flateyjarskagi can be explained by the superposition of several processes: (1) a transform‐perpendicular extension (E‐W to ESE‐WNW trends), (2) a simple shear (NNE‐SSW to NE‐SW trends), and (3) a stress perturbation due to the transform motion (NW‐SE trends). An important factor controlling the transform mechanism is the variation of coupling along the HFF. The obliquity between the direction of transform motion and the trend of extension for the two main regimes may vary between 20° and 90°, reflecting repeated changes of the coefficient of friction along the HFF. Such change from very low mechanical coupling (weak fault) to moderate friction may occur very rapidly since it takes place several times in a few years, as shown by focal mechanisms of earthquake analysis.
Architecture and seismotectonics of a regional low‐angle normal fault zone in central Italy
Authors: Boncio Paolo , Brozzetti Francesco , Lavecchia Giusy
Published: February 2004

Information from surface geology, subsurface geology (boreholes, seismic reflection, and refraction profiles), and seismicity are used to depict the geometry and the possible seismogenic role of the Altotiberina Fault (AF), a low‐angle normal fault in central Italy. The AF extends along the inner Umbria region, for a length of ∼70 km, with an average dip of ∼30° and an horizontal displacement up to 5 km. It emerges west of the inner border of the Tiber basin and deepens beneath the Umbria‐Marche carbonate fold‐and‐thrust belt to a depth of 12–14 km. Close to the AF surface trace, low‐angle synthetic east dipping normal faults extensively outcrop, whereas high‐angle antithetic west dipping normal faults prevail farther east. Integrating geological and seismologic information, it can be stated that the AF behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the Umbria‐Marche region. The AF belongs to a regional NE dipping low‐angle normal fault system (Etrurian Fault System (EFS)), which extends for ∼350 km from northwestern Tuscany to southern Umbria. Early preliminary considerations suggest that the EFS may play an important role in controlling active extension and related seismicity in northern central Italy.