bluuebunny/crossref_metadata_2025 huggingface dataset:
Terrestrial and planetary gravity fields
Authors: Nerem R. S.
Published: February 2004

The determination of the gravity field, together with knowledge of the surface topography, provides one of the primary means of inferring the density structure and dynamics of planetary interiors [Phillips and Lambeck, 1980;Hager, 1985]. After removing the topographic gravity signal, the distribution of internal density anomalies caused by thermal or compositional differences can be studied. Gravity field models can also be used to study the compensation of surface topography, which can provide information on the mechanical properties and state of stress of the lithosphere.

The determination of these models has been accomplished using a wide variety of different measurement types and solution techniques. The near‐Earth measurement types can be divided into satellite tracking measurements, surface gravity measurements, and satellite altimeter measurements. In both near‐Earth and interplanetary contexts, tracking data are used to measure gravitational perturbations affecting satellites, with the accuracy and spatial/temporal distribution of the data being the most important factors in the resulting field accuracy and resolution. Surface gravity data provide a more direct measurement of the gravity field, but acquiring data uniformly over the Earth has always been difficult. Satellite altimetry provides precise measurements of the marine gravity field, provided that satellite orbit errors and non‐geoidal sea surface height variations can be adequately modeled. Comprehensive gravity field solutions must incorporate these disparate data types in order to estimate mathematical parameters describing the gravity field, such as spherical harmonic coefficients. The proper combination of these data, along with the desire to acquire an accurate representation of the model errors, require that complex procedures be developed for computing these solutions.
Mineral physics of iron and of the core
Authors: Anderson Orson L.
Published: February 2004

Mineral physics is a field of materials physics and materials chemistry confined to the study of materials pertinent to the Earth and planetary interiors. In the last two decades, emphasis has been given to research on the values of physical properties in materials appropriate to the Earth's mantle and core.

Braginsky and Roberts[1995] listed the physical properties that play a key role in their analysis of convection of the Earth's core and the dynamo. These connect mineral physics to geomagnetism of the core. This review will be concerned primarily with progress made in the last four years in refining values of these properties.
On the interbasin‐scale thermohaline circulation
Authors: Schmitz William J.
Published: February 2004

The global‐scale circulation has long been one of oceanography's most challenging and exciting research topics. A few features of the abyssal (near bottom) and deep circulation of the Atlantic Ocean have been known for over 50 years, and in the past decade or so there has been a developing focus on the world oceans' thermohaline circulation. The term thermohaline circulation as used here applies not only to a direct response to atmospheric buoyancy fluxes but also in the general sense of water mass modification or conversion, where mechanisms may be associated with internal mixing processes and even wind forcing (i.e., wind‐induced upwelling or wind‐driven mixing). The thermohaline circulation components reviewed and summarized in the following are associated with water mass conversion processes that are involved with interbasin exchange. Updated summary maps of the volume transports (in sverdrups; 1Sv = 106 m³ s−1) for the interbasin‐scale pathways of the abyssal and deep thermohaline circulation and associated upper level compensating flows are developed for two to four vertical layers or potential density intervals, based primarily on a synthesis of published observational results. The cell(s) involving the largest worldwide exchange transport‐wise (53 Sv) are associated with an interaction between various deep and bottom water components via Circumpolar Deep Water (CDW). The first major conversion step in the replacement path for the renewal (14 Sv) of North Atlantic Deep Water (NADW) is taken to be primarily to CDW. Bottom water in the Indian Ocean originates as lower CDW which recirculates while also moving equatorward in deep western boundary currents with eventual conversion to both deep and intermediate layer flows. Some of the intermediate water so formed in the Indian Ocean moves through the Agulhas Current system (ACS) and may “leak” into the Benguela Current regime (BCR), although probably primarily flowing through the ACS into the Subantarctic Frontal Zone (SFZ). It is modified throughout its transit in the SFZ south of the Indian Ocean, south of Australia, and across the South Pacific. Up to 10 Sv of the least dense brand of intermediate water flows through the northern sector of Drake Passage, becomes involved in a Malvinas Current‐Brazil Current‐Subtropical Gyre interaction, and then joins the BCR after perhaps also interacting with the ACS again. This compensating flow is warmed and becomes more saline in the South Atlantic and is later further modified and upwelled in the equatorial Atlantic, crossing the equator and moving through the Gulf Stream system to replace NADW. There is also an NADW replacement path of secondary importance westward around the tip of Africa (∼4 out of 14 Sv) associated with an interbasin circulation pattern throughout the southern hemisphere oceans involving an O(10 Sv) Indonesian Throughflow.
Ore deposits
Authors: McKibben Michael A.
Published: February 2004

Metallic ore deposits constitute the largest geochemical anomalies within the crust. Their study has been critical to understanding the behavior of elements and isotopes in mineral‐ and rock‐ forming processes, as well as to deciphering the geochemical differentiation of the Earth through time. The study of ore deposits therefore influences and draws upon virtually every subdiscipline in the Earth Sciences. The four‐year research review presented here cannot be comprehensive, given the editor's limit of citing less than 100 research papers by U.S. authors or authors from U.S. institutions. Instead, the intent is to provide the reader with synopses of a representative spectrum of papers containing important advances in U.S. research on ore deposits. In cases where difficult citation choices had to be made the more recent papers on a particular subtopic are usually cited, because they lead the reader back to earlier papers within the 4‐year review period. Judicious use of the selected citations in conjunction with standard literature searching tools should allow any reader to quickly find most of the relevant literature on each subtopic.
Deep subsurface microbial processes
Authors: Lovley Derek R. , Chapelle Francis H.
Published: February 2004

Information on the microbiology of the deep subsurface is necessary in order to understand the factors controlling the rate and extent of the microbially catalyzed redox reactions that influence the geophysical properties of these environments. Furthermore, there is an increasing threat that deep aquifers, an important drinking water resource, may be contaminated by man's activities, and there is a need to predict the extent to which microbial activity may remediate such contamination. Metabolically active microorganisms can be recovered from a diversity of deep subsurface environments. The available evidence suggests that these microorganisms are responsible for catalyzing the oxidation of organic matter coupled to a variety of electron acceptors just as microorganisms do in surface sediments, but at much slower rates. The technical difficulties in aseptically sampling deep subsurface sediments and the fact that microbial processes in laboratory incubations of deep subsurface material often do not mimic in situ processes frequently necessitate that microbial activity in the deep subsurface be inferred through nonmicrobiological analyses of ground water. These approaches include measurements of dissolved H2, which can predict the predominant microbially catalyzed redox reactions in aquifers, as well as geochemical and groundwater flow modeling, which can be used to estimate the rates of microbial processes. Microorganisms recovered from the deep subsurface have the potential to affect the fate of toxic organics and inorganic contaminants in groundwater. Microbial activity also greatly influences the chemistry of many pristine groundwaters and contributes to such phenomena as porosity development in carbonate aquifers, accumulation of undesirably high concentrations of dissolved iron, and production of methane and hydrogen sulfide. Although the last decade has seen a dramatic increase in interest in deep subsurface microbiology, in comparison with the study of other habitats, the study of deep subsurface microbiology is still in its infancy.
Anelastic structure and evolution of the continental crust and upper mantle from seismic surface wave attenuation
Authors: Mitchell Brian J.
Published: February 2004

Regional variations of the intrinsic shear wave quality factor Qµ in both the upper crust and upper mantle of continents are large, with values in old, stable cratons exceeding those in tectonically active regions in both depth ranges by as much as an order of magnitude or more. Qµ depends upon frequency, at least near 1 Hz, and that frequency dependence also varies regionally in the upper crust. It is typically low in tectonically active regions and higher in stable regions. Because of the large variations in Qµ from region to region, it is easy to map regional variations of both upper crustal Qµ and Q estimated from the coda of Lg waves (QLgc), even though both measurements may be marked by large uncertainties. Although coda Q of direct body waves may be strongly affected by scattering, QLgc appears to be primarily governed by intrinsic Qµ in the upper crust. Both upper crustal Qµ and QLgc values correlate with the time that has elapsed since the most recent tectonic activity in continental regions. A tomographic image of the variation of QLgc values across Africa shows reduced Q values which correspond to recent tectonic activity in the East African rift system and other regions of Mesozoic or younger age. Reductions of QLgc that correlate with tectonic activity that occurred in the early Paleozoic during the coalescence of the cratons which formed that continent can also be detected. Qµ increases rapidly at midcrustal depths, in a range which appears to coincide with the transition to the plastic lower crust. In the lower crust and upper mantle, Qµ decreases with increasing depth, possibly by progressive unpinning of dislocations with increasing temperature. Observed regional variations in upper mantle Qµ at depths of about 150 km can be explained by differences in temperature alone, but those at crustal depths cannot. Regional variations of Qµ in the upper crust are most easily explained by differences in the density of fluid‐filled fractures in which fluids can move during the propagation of seismic waves. Studies of the regional variation of Qµ and QLgc indicate that crack density is greatest during and immediately following tectonic activity in a region and that it decreases with time. Permeability determinations in deep wells show that fluid movements in those cracks may be largely restricted to zones of crustal fracturing. That situation will produce widely differing values of Q in local studies, depending on the location of the study relative to the fractures. The fluid volume in cracks appears to decrease with time by loss to the surface or by retrograde metamorphism, causing a reduction in the number of open cracks and a concomitant increase in Qµ.
The dynamics of explosive volcanic eruptions
Authors: Woods Andrew W.
Published: February 2004

Explosive volcanic eruptions involve the ejection of dense mixtures of ash and gas from a volcanic vent at high speed and pressure. This mixture is generated as liquid magma rises from a crustal magma chamber and decompresses, exsolving water vapor. As gas is exsolved, the mixture inflates, accelerates, and becomes foam‐like. Once the liquid films around the bubbles are unable to spread as rapidly as the bubbles are expanding through decompression, the films rupture, and a fragmented mixture of ash and volatiles ascends to the volcanic vent. On eruption from the vent, the material decompresses, either into a volcanic crater or directly into the atmosphere. In the case of free decompression, the mixture typically has a high speed, while decompression in a crater can lead to either very low or very high eruption speeds. After decompression, the hot, dense mixture begins to entrain and heat ambient air, thereby lowering the mixture density, but it also decelerates under gravity. If the eruption velocity is sufficiently high, then the material can become buoyant and will generate a buoyant ash plume, called a Plinian eruption column, which rises above the vent. In contrast, if the eruption velocity is small or the mass flux is very large, then the material will typically collapse back toward the Earth and form a dense, laterally spreading flow. Buoyant eruption columns are able to transport the material high into the atmosphere, since they provide an efficient means of converting the initial thermal energy of the mixture into potential energy through entrainment and heating of ambient air. The height of rise of such eruption columns depends upon the eruption rate, the stratification of the atmosphere, the degree of thermal disequilibrium between the particles and the air, and the amount of water vapor in the atmosphere. Dense, hot ash flows, generated by collapsing fountains, transport ash and clasts laterally from the vent, sedimenting many of the larger clasts and entraining air en route. As a result, the density of the mixture may fall below that of the atmosphere, and the finer‐grained solid material may thereby become buoyant and rise from the flow. The distance it travels increases with both the cloud mass and the mean particle size. The ensuing buoyant ash plume, called a coignimbrite eruption column, may have a source several kilometers from the original volcanic vent. Once the thermal energy of an eruption column has become exhausted, the ash intrudes laterally into the atmosphere. Ultimately, the cloud is swept downwind, where sedimentation of ash leads to fall deposits over hundreds of kilometers from the volcano.
Indian MST radar 1. System description and sample vector wind measurements in ST mode
Authors: Rao P. B. , Jain A. R. , Kishore P. , Balamuralidhar P. , Damle S. H. , Viswanathan G.
Published: February 2004

An MST radar operating at 53 MHz with an average power aperture product of 7 × 108 W m2 has been established at Gadanki (13.5°N, 79.2°E), India. The radar development has been accomplished in two phases. In the first phase it was commissioned in ST mode using a partial system comprising one quarter (16 × 16)of the Yagi antenna array and 16 driver units of the transmitters providing an average power aperture product of 4.8 × 106 W m2. In this part we present the radar system description, including off‐line data processing, and some sample high‐resolution vector wind measurements made in ST mode operation.
Shapes of irregularities in the sporadic E layer producing quasi‐periodic scintillations
Authors: Maruyama Takashi
Published: February 2004

Field strengths of the beacon radio waves transmitted from the geostationary satellite ETS 2 are observed at a midlatitude station in Japan. Regular periodic variations in the field strength with a deep fade‐out at the central portion, called quasi‐periodic scintillations, are analyzed in terms of the minimum and maximum field strengths during each event. The variation in the field strengths is simulated with irregularity models for a variety of shapes, sizes, and electron densities. The numerical results are quantitatively compared with the observations and suggest that one type of quasi‐periodic scintillations is generated by linear irregularities that are elongated perpendicular to the drift direction, while the other type is generated by two‐dimensional, disk‐shaped irregularities.
Near‐grazing illumination and shadowing of rough surfaces
Authors: Barrick Donald E.
Published: February 2004

A simple and computationally efficient methodology is demonstrated for treating the interaction of radio waves near grazing above rough surfaces. We apply it to the sea from HF through the microwave. Results using actual finite seawater conductivity depart significantly from those of a perfectly conducting profile for vertical polarization; for horizontal polarization there is essentially no difference. Shadowing behind wave crests is surprisingly shallow for vertical polarization even when the occluded depth is tens of wavelengths. This methodology gives expressions for the effective surface impedance at grazing as modified by the roughness when height scales greatly exceed the radio wavelength, augmenting prior expressions valid at HF for the sea obtained from perturbation theory. These effective impedances can then be used in formulations for radiation and surface wave propagation/diffraction at and below the horizon.
Systematic errors in spaced antenna wind estimators under inhomogeneous flow conditions
Authors: Sürücü Fahri , Kudeki Erhan , Rastogi Prabhat K.
Published: February 2004

The influences of horizontal wind gradients on “apparent” and “true” spaced antenna wind estimators are examined within the scope of a linear wind model. It is found that apparent velocity estimators are highly sensitive to horizontal gradients in horizontal wind components. Systematic errors in true velocity estimates increase with the probing radar beam width. For a beam width of ∼6° and realistic values of horizontal wind gradients, systematic errors in true velocity estimates remain less than ∼4 m/s at mesospheric altitudes in the absence of gradients in the vertical wind component.
The effects of atmospheric circulation on line‐of‐sight microwave links
Authors: Silveira Reinaldo B. , Massambani Oswaldo , Costa Emanoel
Published: February 2004

This paper presents an investigation of a lake‐land breeze atmospheric circulation responsible for strong horizontal and vertical inhomogeneities of moisture distribution which affected the refractivity in the neighborhood of a microwave line‐of‐sight link, as observed during a multi‐instrument radiometeorological experiment carried out in São Paulo, Brazil. A boundary layer and mesoscale atmospheric model was used to simulate the local circulation within the link region and to compute the time evolution of the vertical structure of the atmospheric refractive index. A ray‐tracing model was also used to calculate the intensity of the received signals from the measured refractive index vertical profiles. The integrated simulation results were then compared with the measurements. The good agreement obtained between the measured and the simulated characteristics of the temperature and moisture advection, as well as between the measured and the calculated received signal levels, indicates that the lake‐land breeze circulation mechanism is a likely source of the propagation phenomena observed in the microwave link.
Overview of the singular behavior of electromagnetic fields at edges and tips in bi‐isotropic and special bianisotropic media
Authors: Olyslager Frank
Published: February 2004

An overview of some recent work concerning the singular behavior of electromagnetic fields at edges and tips in bi‐isotropic and some bianisotropic materials is presented. Special emphasis will be placed on the singular behavior at the tip of concentric metal cones embedded in a special bianisotropic material. Some new results for this case are presented. Finally, some general conclusions regarding the effect of chirality and anisotropy are discussed.
Equatorial F1 characteristics and the international reference ionosphere model
Authors: Adeniyi J. O.
Published: February 2004

Average values of the F1 critical frequency (f0F1) and the height of the F1 ledge (hmF1) for Ibadan (Latitude 7.4°N, Longitude 3.9°E) were used for this study. Well‐defined F1 characteristics are observed during winter at low solar activity. International reference ionosphere (IRI) does not predict F1 parameters during this season. Deviation of predicted F1 electron density (NF1) by the IRI model from observed values are less than 10% for all seasons of low solar activity, when IRI predicts NF1. Higher percentage deviations are observed during summers of high solar activity. IRI overestimates hmF1. Deviations from experimental values vary from 4 to 35%.
Method of modeling of VLF wave propagation in the Earth's magnetosphere and upper ionosphere
Authors: Molchanov O. A. , Maltseva O. A. , Hayakawa M. , Hobara Y.
Published: February 2004

A method of ray tracing (RT) with amplitude variations for simulating VLF wave propagation in a weakly inhomogeneous upper ionosphere and magnetosphere is discussed in detail. This ray tracing plus amplitude (RTA) technique is different from the existing ones in the selection of the most complete model with all its plasma parameters dependent on geomagnetic activity, allowance of thermal correction to the refractive index, and calculations of amplitude factors (coefficients of focusing‐defocusing, polarization, and linear amplification‐attenuation). The presented examples of calculations show that the introduction of the above new elements in the RT technique makes it possible to clarify the wave propagation properties and to account for numerous experimental phenomena.
Transient response of an electrodynamic tether system in the ionosphere: TSS‐1 first results
Authors: Bilén Sven G. , Gilchrist Brian E. , Bonifazi Carlo , Melchioni Enrico
Published: February 2004

The electrical transient response of an electrodynamic tether system in low earth orbit has been investigated experimentally and by computer simulation. Our experimental results come from the first Tethered Satellite System (TSS‐1) mission flown on the space shuttle in 1992. During this mission a tethered satellite was deployed to a distance of 267 m above the shuttle using a conducting, insulated cable. It is shown by computer simulation that the electrical transient response of the system is due to a combination of both the tether electrical circuit and the interaction of the tether system with the space plasma. In general, ground measurements showed that the TSS‐1 electrical circuit is approximately second order and underdamped. However, it was found that on‐orbit transient decay as well as the symmetry of transient oscillations could be dramatically altered, following closely with ionospheric plasma density variations. Combined electrical circuit and plasma interaction modeling shows that these differences can be explained by accounting for ion and electron current collection capabilities of the tether end connections with the space plasma. These results have implications for the use of electrodynamic tethers for high current switched applications including direct generation of low‐frequency electromagnetic signals and high electrical power generation as well as detection of natural electric field transient signatures.
A test of water vapor radiometer‐based troposphere calibration using very long baseline interferometry observations on a 21‐km baseline
Authors: Linfield R. P. , Keihm S. J. , Teitelbaum L. P. , Walter S. J. , Mahoney M. J. , Treuhaft R. N. , Skjerve L. J.
Published: February 2004

Simultaneous very long baseline interferometry (VLBI) and water vapor radiometer (WVR) measurements on a 21‐km baseline showed that calibration by WVRs removed a significant fraction of the effect of tropospheric delay fluctuations for these experiments. From comparison of the residual delay variations within scans and between scans, the total tropospheric contribution to the delay residuals for each of the three 5–20 hour sessions was estimated as 1%, 17%, and 10%, with the first value being very uncertain. The observed improvement in rms residual delay from WVR calibration during these three sessions was 4%, 16%, and 2%, respectively. The improvement is consistent with the estimated 2–3 mm path delay precision of current WVRs. The VLBI measurements of natural radio sources were conducted in April and May 1993 at Goldstone, California. Dual‐frequency (2.3 and 8.4 GHz) observations were employed to remove the effects of charged particles from the data. Measurements with copointed WVRs, located within 50 m of the axis of each antenna, were performed to test the ability of the WVRs to calibrate line‐of‐sight path delays. Factors which made WVR performance assessment difficult included the facts that (1) the level of tropospheric fluctuations was smaller than is typical for Goldstone during these experiments, and (2) VLBI delay variations on longer timescales (i.e., over multiple scans) contained uncalibrated instrumental effects (probably a result of slow temperature variations in the VLBI hardware) that were larger than the tropospheric effects.
Linear electro‐optic effect applied to a radio astronomy correlator
Authors: Price N. Ross , Kronberg Philipp P. , Iizuka Keigo , Freundorfer Alois P.
Published: February 2004

Studies of weak millimeter‐wave cosmic radio sources in the next generation of millimeter interferometers will, for optimal sensitivity to continuum radio sources, require continuum cross correlators with an input passband of approximately 10 GHz, in comparison to the 500 MHz devices presently installed. We have designed and tested an optoelectronic continuum correlator which can achieve such high bandwidths using integrated optic Mach‐Zehnder interferometric modulators (to convert the IF signals from the radio telescope receivers to optical signals) and photoreceivers (to convert the optical signals back to electrical signals). A prototype real‐channel continuum correlator of this type is described. It performs with an overall efficiency of 81 ± 3%, which compares favorably with typical electronic digital and analogue correlators currently used in radio astronomy.
On the potential applicability of the simplified ionospheric regional model to different midlatitude areas
Authors: Zolesi Bruno , Cander Ljiljana R. , De Franceschi Giorgiana
Published: February 2004

The simplified ionospheric regional model (SIRM) was originally developed for modeling the most relevant ionospheric characteristics over Europe for radiocommunication purposes. The model, based mainly on the Fourier expansion of a reference past data set, is here adapted and applied to different sets of dishomogeneous periods of observed data coming from a sparse network of ionospheric stations in midlatitude areas, that is, northeastern North America, southeastern South America, northeast Asia, and southeast Australia. Notwithstanding the simple SIRM formulation and the reduced number of numerical coefficients involved, a good agreement has been found between modeled and observed monthly median values of ƒ0F2 and M(3000)F2 under different heliogeophysical conditions.
Meso‐Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite fission track thermochronology
Authors: Rohrman Max , van der Beek Peter , Andriessen Paul , Cloetingh Sierd
Published: February 2004

Apatite fission track (AFT) thermochronology of Precambrian and Paleozoic basement samples from southern Norway reveals a post‐Paleozoic exhumation history, related to offshore Mesozoic and Cenozoic extensional basin development. The data indicate two major phases of rapid exhumation. A first Mesozoic phase started in the Triassic (∼220 Ma) in the east and south of the study area and migrated to the west where Jurassic (∼160 Ma) ages of exhumation predominate. A second event is indicated by thermal history modeling of AFT ages and track length distributions. It is inferred to be Neogene in age, initiated at about 30 Ma, and it produced a domal pattern of AFT isochrons which follow present‐day topographic elevation. Youngest AFT ages (∼100 Ma) are encountered at sealevel in the inner fjords near the areas of highest topography; ages increase radially outward to the mountain peaks and the coastlines. Forward modeling of age‐elevation patterns suggests that Mesozoic geothermal gradients were 10–15°C/km higher than the present value of 20°C/km. During the Triassic and Jurassic, a total of 1.3–3.5 km of overburden was removed from the study area, assuming a 30°C/km geothermal gradient for that period. We attribute this to rift margin erosion as a result of erosional base level lowering and flank uplift, as evidenced by thick continental clastic sequences deposited in Triassic‐Jurassic half grabens in the North Sea basins. We propose that 1.5–2.5 km of Neogene exhumation were a result of late stage domal uplift. This is supported by basinward dipping pre‐Neogene strata in the basins surrounding southern Norway and the infill of a 1‐ to 2‐km‐thick Neogene sediment wedge containing various internal unconformities. Domal uplift probably started in the Late Oligocene, may have been amplified in the Pliocene, and was overprinted by Plio‐Pleistocene glacial erosion. Maximum Neogene tectonic uplift is estimated at approximately 1–1.5 km, radially decreasing outward to a value <500 m near the shoreline. Neogene domal uplift is coincident with Oligocene and Pliocene plate reorganizations in the North Atlantic; similar Neogene domes are found around the Norwegian‐Greenland Sea (i.e., Svalbard and the Barents Sea, northern Norway, east Greenland), suggesting a regional tectonic cause. The onset of Neogene uplift postdates major volcanism and continental breakup by ∼25 m.y. and predates Plio‐Pleistocene glaciations. Its origin is possibly a combination of induced mantle convection, resulting in thermal erosion of the lithosphere, and the operation of intraplate stresses.
Intermediate‐depth seismicity in northern Colombia and western Venezuela and its relationship to Caribbean plate subduction
Authors: Malavé Gustavo , Suárez Gerardo
Published: February 2004

The recent intermediate‐depth seismicity in northern Colombia and western Venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern South America. The area included in this study is located to the north and east of the Bucaramanga nest, which is a particular region beneath Bucaramanga, northern Colombia, that presents a high concentration of intermediate‐depth earthquakes. To the north of the nest, the seismicity of the area is sparse, and most of the events are of low magnitude (mb≤5.l). Thus only 23 earthquakes were large enough to be investigated using teleseismic data. The focal parameters of the two largest events (mb≥5.4) were obtained from the formal inversion of long‐period body waves recorded at teleseismic distances. The focal mechanisms of 10 more events were determined from first‐motion data. In total, the focal mechanisms of 12 events were determined from both the inversion of P and S H waveforms and the polarities of first arrivals. For the smaller earthquakes, the focal depths were calculated by measuring the observed pP‐P interval, time and comparing it to the theoretical travel time tables. The isodepth curves reflect a slab striking in a NNE–SSW direction and dipping approximately at 25°–32° to the southeast. This observation is corroborated by the direction and plunge of the T axes of the focal mechanisms, which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters. These results indicate that the intermediate‐depth earthquakes in western Venezuela and northern Colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of Colombia. The two largest earthquakes, located at a significant distance from the Bucaramanga nest, present similar fault plane solutions. Moreover, they also agree with those of the two largest earthquakes reported inside the nest. This similarity suggests that the Bucaramanga nest lies on the same subducted slab where the other earthquakes occur. There is not enough shallow seismicity to define the location where the Caribbean lithosphere subducted beneath the South American plate. However, the extension of the slab toward the surface, inferred from the intermediate‐depth seismicity, suggests that the subducted lithosphere may still be attached to the Caribbean plate.
Pre‐Neogene evolution of the Western Carpathians: Constraints from the Bochnia‐Tatra Mountains section (Polish Western Carpathians)
Authors: Roca E. , Bessereau G. , Jawor E. , Kotarba M. , Roure F.
Published: February 2004

On the basis of field studies and subsurface data, this first balanced transect crosses most of the Carpathian foreland basin and the entire Polish Western Carpathian thrust units (Outer flysch nappes, Pieniny Klippen Belt, and Inner Carpathian Tatric thrust sheets). It shows that (1) me Outer Carpathians are made up of several flysch units whose internal structure is strongly controlled by initial thicknesses, whereas the thin flysch successions are strongly deformed by tight folds and include several imbricated thrust sheets; by contrast, the thick flysch successions are less deformed and accreted in slightly imbricated thrust sheets; and (2) the Inner Carpathians are structured in an antiformal stack of basement thrust sheets. The unfolding of this cross section shows that the post‐middle Oligocene shortening in the European foreland platform and Outer Carpathians exceeds 180 km. In the substratum, most of this shortening seems to be solved by basement consumption vertically beneath the suture zone of the Pieniny Klippen Belt. Additional basement shortening took place also (1) in the Inner Carpathians, where, during the Neogene, buried basement thrust sheets were piled up to form an antiformal stack, and (2) beneath the inner zones of the Outer Carpathians. Palinspastic restoration of the cross section reflects two well‐differentiated zones prior to the late Oligocene‐late Miocene compression: (1) the present Inner Carpathians and the Pieniny Klippen Belt, characterized by a Late Cretaceous thrust belt unconformably overlain by an undeformed Palaeogene cover, and (2) the Outer flysch Carpathians, in which pre‐upper Oligocene configuration comprised a typical foreland basin flexed toward the SSW, with a system of NW trending highs and troughs.
Formation and evolution of the Nova Scotian rifted margin: Evidence from deep seismic reflection data
Authors: Keen C. E. , Potter D. P.
Published: February 2004

A deep marine seismic reflection profile was obtained across the Mesozoic rifted continental margin off Nova Scotia, eastern Canada. This profile crosses the Seotian Basin, one of the deepest basins on the margin of eastern North America, and it complements other deep crustal seismic data on this margin. The seismic data have been interpreted in conjunction with gravity anomaly and subsidence data. They show significant thinning of the continental crust over a zone about 200 km wide. The mode of extensional deformation is probably a combination of pure and simple shear; there is evidence for simple shear in the crust. The continent‐ocean boundary lies near the seaward edge of synrift salt below the continental rise. A 100‐km‐wide zone of very thin (approximately 9 km or less) continental or transitional crust extends seaward from the outer shelf to this boundary. Reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction, perhaps modified by extensional faulting. This margin is nonvolcanic, and the transition to the volcanic margin off the eastern United States occurs about 500 km southwest of the seismic line. The width of the zone of crustal extension is much greater on this nonvolcanic margin segment than it is on the volcanic margin to the south. It seems likely that the prerift fabric of the continental lithosphere controls this width. A narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism. However, significantly narrower zones of crustal extension occur on other nonvolcanic margins, so factors in addition to rift width, such as the rate of rifting, may also be important.
Microseimicity and strain pattern in northwestern Greece
Authors: Hatzfeld Denis , Kassaras Iannis , Panagiotopoulos Dimitris , Amorese Daniel , Makropoulos Kostas , Karakaisis George , Coutant Olivier
Published: February 2004

During a 7‐week microearthquake experiment conducted in Epirus, Akarnania, and the Ionian islands of western Greece, we located approximately 600 earthquakes with magnitudes between 2 and 4.2. No event was deeper than 40 km. The seismicity cannot be clearly associated with any single fault except the Lixourion right‐lateral fault located west of the Ionian islands. Focal mechanisms of about 100 earthquakes show, for a narrow band of earthquakes located along the coast, ENE–WSW shortening consistent with the surface tectonics. Farther east, focal mechanisms show NNW–SSE extension beneath the foothills of the Pindus mountains, which is unrelated to surface faulting but is consistent with the presently subsiding basins. This strain pattern is seen far north and south of the Lixourion fault and is similar to the one observed in the Peloponnese. It suggests that a large‐scale mechanism is responsible for the recent geodynamics of both the northwestern and southwestern Aegean
U‐Pb and 40Ar/39Ar geochronology of the Symvolon granodiorite: Implications for the thermal and structural evolution of the Rhodope metamorphic core complex, northeastern Greece
Authors: Dinter David A. , Macfarlane Allison , Hames Willis , Isachsen Clark , Bowring Samuel , Royden Leigh
Published: February 2004

North Aegean continental lithosphere was thickened by southwest vergent thrusting and continental subduction within the Alpine collisional orogen but has subsequently been greatly extended on a northeast‐southwest axis in the back arc of the Hellenic subduction zone. Crosscutting relationships with two granodiorite bodies emplaced at ∼31–33 Ma, the Xanthi and eastern Vrondou plutons, constrain a pre‐mid‐Oligocene origin of Alpine convergent structures in northeastern Greece. Post‐Alpine thinning of the north Aegean nappe pile began in earliest Miocene time and has been accommodated by a succession of distinct structural systems. The earliest of these, the “Symvolon shear zone”, appears to represent a midcrustal, coaxial rupture of the Falakron marble series, a carbonate platform >5000 m thick that was subducted northeastward beneath high‐grade rocks of the Rhodope metamorphic province in late Alpine time. Zircon and titanite U‐Pb dates and hornblende 40Ar/39Ar dates obtained in this study constrain the intrusion and incipient mylonitization of the Symvolon or “Kavala” granodiorite within the Symvolon shear zone at ∼21–22 Ma. Following its emplacement, the Symvolon body resided at temperatures between 300°C and 500°C for 5–7 m.y., during which coaxial deformation may have continued within a widening Symvolon rupture. The Strymon Valley detachment, a regionally south‐west dipping low‐angle normal fault, succeeded the Symvolon shear zone in middle Miocene time. Southwestward displacement of the Serbo‐Macedonian gneiss complex by as much as 80 km in the hanging wall of this detachment facilitated the unroofing of the Rhodope metamorphic core complex, including the Falakron marble series and several Tertiary plutons, in its footwall. Biotite and K‐feldspar 40Ar/39Ar dates from 11.1 ± 0.2 Ma to 15.5 ± 0.3 Ma yielded by Symvolon granodiorite samples document the cooling of the Rhodope core complex below 150°C–300°C during its southwestward‐progressive exhumation in the footwall of the Strymon Valley detachment.
Late Jurassic blueschist facies pebbles from the Western Carpathian orogenic wedge and paleostructural implications for Western Tethys evolution
Authors: Piaz Giorgio V. Dal , Martin Silvana , Villa Igor M. , Gosso Guido , Marschalko Robert
Published: February 2004

In spite of the absence of ophiolitic slices at the surface, some traces of the lost Tethys ocean are recorded along the Pieniny Klippen Belt (PKB), a narrow décollement thrust system sutured at the transpressive boundary between the Outer and Inner Carpathians. The enigmatic precollisional evolution of Western Carpathians can be deciphered from some late Albian to Campanian flysch conglomerates which display chrome spinel grains, ophiolitic detritus and pebbles of blueschist facies tholeiitic metabasalts yielding a 40Ar/39Ar plateau age of 155.4±0.6 Ma. Other detrital components are represented by extrabasinal pebbles of limestones, arc volcanics, and igneous to metamorphic basement rocks from southern sources. Our results suggest a markedly northward extension of the sublongitudinal Triassic Vardar (Meliata) Ocean and its subduction since the late Middle Jurassic, supposedly balanced westward by coeval spreading in the Ligurian‐Piedmont basin of the Apennine‐Western Alpine Tethys. A lateral kinematic connection between these diachronous and roughly parallel Tethys branches was provided on the north by a left‐lateral east‐west trending shear zone running from the Swiss‐Austrian Penninic domain to the Northern Carpathians. This reconstruction replaces the classic model of two paired North Penninic and South Penninic oceanic basins and eastern homologues with the Briançonnais‐Hochstegen and Czorstin microcontinents in between. The Late Jurassic‐Early Cretaceous evolution of the Carpathian active margin was characterized by subduction metamorphism and accretion of a wide orogenic wedge; in this time, the shallowing to deeply subsiding basins inferred from facies analyses on the sedimentary units of the PKB were likely floored by individual sections of the growing wedge. Later, some exhuming blueschist ophiolitic units of the wedge were uplifted to the surface and functioned in the Albian‐Campanian as an “exotic ridge” supplying clasts to the forearc basin. Finally, the colliding wedge became a cryptic paleostructure when, since the latest Cretaceous, it disappeared beneath the Inner Carpathian orogenic lid and was incorporated within the eastward moving infrastructure of the Carpathian orocline.
A deep crustal shear zone exposed in western Fiordland, New Zealand
Authors: Hill Evelyn June
Published: February 2004

An approximately 180‐km‐long, deep crustal shear zone in western Fiordland, New Zealand, has been shown to have formed during continental extension between 116 and 100 Ma. The western Fiordland shear zone forms a major tectonic boundary between two sets of gneisses with unrelated deformation histories. A detailed study of the northern part of the shear zone has revealed that the shear zone formed at depths of ∼40 km (approximately 12 kbar and 680°C) and was originally very gently dipping or subhorizontal prior to subsequent upright folding. The current thickness (postfolding) of the shear zone is ∼3–4 km. Abundant asymmetric structures in the shear zone indicate noncoaxial deformation and a top‐to‐the‐NE sense of shear. Unlike ductile shear zones in metamorphic core complexes from elsewhere in the world, there is no evidence that decompression of the shear zone rocks or foot wall rocks occurred during the active life of the shear zone, suggesting that displacement along the shear zone by noncoaxial deformation must have been largely horizontal. This shear zone provides important evidence for the nature of deep crustal deformation occurring during continental extension.
Geological evidence against large‐scale pre‐Holocene offsets along the Karakoram Fault: Implications for the limited extrusion of the Tibetan plateau
Authors: Searle M. P.
Published: February 2004

Two end‐member models proposed to accommodate the convergence between India and Asia north of the Himalaya are (1) homogeneous crustal thickening of the Tibetan plateau and (2) continental escape, or extrusion, of Tibet and southeast Asia, away from the indenting Indian plate. Foremost among the arguments supporting the latter would be large‐scale (∼1000‐km) offsets and high present‐day slip rates along the major strike‐slip faults bounding the postulated extruding crust, notably the Altyn Tagh Fault along the northern margin of Tibet and the Karakoram Fault along the SW margin. Satellite photographic interpretation and field mapping in the Karakoram mountains in Pakistan, the Nubra‐Siachen area of north Ladakh, and the Pamirs in Xinjiang show that although the Karakoram Fault is extremely active today, geological offsets along the right‐lateral fault are probably less than 120 km. The 21±0.5 Ma Baltoro monzogranite‐leucogranite batholith has been rotated clockwise about a vertical axis 35°–40° into NW‐SE alignment, parallel with the Karakoram Fault, east of the Siachen glacier, with a maximum offset of 90 km across the fault. The Bangong‐Shyok suture zone similarly has a dextral offset of 85 km. The course of the Indus River, which was antecedent to the rise of the Ladakh, Karakoram, and Himalayan ranges, has been offset dextrally by 120 km south of Pangong Lake. If present‐day slip rates (approximately 32 mm/yr) (Avouac and Tapponnier, 1993) are correct, only 4 Ma are required to obtain a 120‐km offset. There is no geological evidence for any larger‐scale pre‐Holocene offsets, and it is suggested that the Karakoram Fault cannot have accommodated major eastward lateral motion of Tibetan crust. The fault has also exerted little or no influence on surface topographic uplift, cutting obliquely across the highest peaks of the Karakoram. Dextral motion along the central part of the Karakoram Fault has been transferred in the north to the Rangkul, Murghab, and Karasu transpressional faults in the central Pamir. North of Tashkurgan, the Karakoram Fault shows mainly normal motion around the Kongur and Mustagh Ata gneiss domes (metamorphic core complexes) and the extensional Muji graben. Minor dextral displacement has occurred along the Shiquanhe Fault, but this motion cannot be linked to the Jiale Fault of east Tibet. The southern end of the Karakoram Fault merges with the Indus (Yarlung) suture zone near Mount Kailas and does not cut across the Himalaya. The lack of large‐scale geological offsets along the Karakoram Fault, together with its very recent initiation (?5 or 4 Ma), suggests that it is related to the Pliocene‐Quaternary northward indentation of the Pamir, and not to any long‐term extrusion of Tibetan crust following the Indian collision.