bluuebunny/crossref_metadata_2025 huggingface dataset:
Early development of the southern Kerguelen Plateau (Indian Ocean) from shallow wide‐angle ocean bottom seismometer and multichannel seismic reflection data
Authors: Könnecke Lis K. , Coffin Millard F. , Charvis Philippe
Published: February 2004

We examine the early geological history of the southern Kerguelen Plateau (Indian Ocean) using ocean bottom seismometer (OBS), multichannel seismic (MCS), and Ocean Drilling Program data. Velocity‐depth models in the sedimentary Raggatt Basin are constrained by near‐range OBS data (refractions and reflections, including multiples and converted shear waves) and migrated MCS data. The models elucidate the significance of Lower Cretaceous lava flows, Albian to Coniacian/Santonian terrestrial and terrigenous sediment, and Maastrichtian and Paleocene seismic sequences. The Albian/Aptian basaltic basement complex consists of an upper and a lower series. The upper series is characterized by average Vps of 4.6–4.7 km/s, an upward increase of intercalated terrestrial sediment and altered flowtops, and a concomitant decrease in flow thickness; the lower series is marked by average Vps of 5.3–5.5 km/s, and contains thicker flows and less sediment. A volcanic center in the Raggatt Basin shows relatively low Vps (3.7–3.9 km/s for the upper series, 4.7–4.9 km/s for the lower series), and dipping reflections on the Raggatt Basin's flanks are also recorded as refractions. Terrestrial and terrigenous sediment of the Raggatt Basin, immediately overlying basement, is characterized by a seismic low‐velocity layer with Vps ranging from 2.2–2.9 km/s and a thickness of >1100 m in the central basin. Nearby source regions (e.g., Banzare Bank and other elevated areas south of the basin) account for a terrestrial and terrigenous sediment volume of ∼12,500 km3, deposited over ∼20 m.y. The depocenter of the Raggatt Basin began shifting in Santonian to early Maastrichtian time, and concluded by early late Paleocene time.
Modeling the Tonga slab: Can travel time data resolve a metastable olivine wedge?
Authors: Koper Keith D. , Wiens Douglas A. , Dorman LeRoy M. , Hildebrand John A. , Webb Spahr C.
Published: February 2004

We present the results of detailed modeling of the Tonga slab with the goals of determining whether high‐resolution travel time data (1) can be fit by simple thermal and petrological slab models and (2) can resolve a metastable olivine wedge at depths greater than the equilibrium olivine‐spinel phase boundary. We model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers (OBS) and island broadband seismic stations extending from the Lau backarc basin, across the Tonga trench and onto the Pacific plate. The data consist of 388 local, P wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3‐month OBS deployment in late 1994. We locate the events using both local and teleseismic arrival times, and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference Earth model. The modeling consists of grid searches using a three‐dimensional finite difference algorithm to compute local, first arriving travel times for equilibrium and metastable P wave velocity models constructed from thermal, mineralogical, and morphological constraints. The travel time anomalies are well fit by standard slab thermal models and P velocity temperature derivatives of −0.4 to −0.3 ms−1°C−1. Forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low‐velocity region. Wedge velocity models provide a slightly better fit to the data than equilibrium models, but F tests indicate the improvement is not significant at the 95% level. Our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary.
Strain accumulation rates in the San Francisco Bay area, 1972–1989
Authors: Savage J. C. , Simpson R. W. , Murray M. H.
Published: February 2004

Maps of the strain accumulation rate in the San Francisco Bay area have been constructed from trilateration observations extending from about 1972 until the Loma Prieta earthquake in late 1989. The observations were corrected to remove offsets imposed by shallow fault creep and by four M ∼6 earthquakes that occurred in the Bay area during that time interval. The Bay area was divided into 32 contiguous polygons, and the uniform (in both space and time) strain rates that best explain the changes in the corrected (earthquake and shallow fault creep offsets removed) distances within each polygon were calculated. In a coordinate system with the 1 axis directed N58°E and the 2 axis N32°W (perpendicular and parallel to the local tangent to the small circle drawn about the Pacific‐Sierra Nevada pole of rotation) the averages of these 32 strain rates (each weighted by the area of the polygon) are ε11=9.2±7.4, ε12=−160.7±4.6, and ε22=8.2±6.2 nanostrain/yr, where extension is reckoned positive and quoted uncertainties are standard deviations. As expected from the Pacific‐Sierra Nevada relative plate motion, the overall strain rate is predominantly right‐lateral shear across a vertical plane striking N32°W. The net increase in the 12,225 km2 area of the trilateration network is only 212±110 m2/yr, which arises from almost equal extensions in the N32°W and N58°E directions. Within the network the strain rates vary from polygon to polygon. Significant areal dilatation rates are observed in almost ⅓ of the individual polygons and the N32°W extension rates tend to be negative to the west of the Hayward‐Rodgers Creek fault trend and positive east of it. The N58E extension rate is positive in 22 out of 32 polygons, a proportion that is significantly larger than would be expected by chance if the N58°E extension rate were zero or negative. The pre‐1989 strain accumulation across the eventual site of the Loma Prieta rupture involves fault normal contraction as well as right‐lateral shear, consistent with the rupture mechanism.
Modeling Cenozoic sedimentation in the central equatorial Pacific and implications for true polar wander
Authors: Mitchell Neil C.
Published: February 2004

High productivity along the central Pacific equator has left thick deposits of calcareous and siliceous ooze. West of 102°W, these have deposited on the Pacific plate which has been drifting northwards and therefore the thickest deposits are displaced north of the equator. This study attempts to simulate the sediment distribution using information from Deep Sea Drilling Project (DSDP) cores and models for Pacific plate motion. The DSDP core data are used to account for accumulation rate variations with time, paleolatitude and paleodepth, accounting for variable carbonate dissolution. The model predicts the distribution on 35–50 Ma seafloor relatively well and demonstrates that the sedimentary bulge is distorted by variable dissolution over a regional bathymetry gradient due to the northern edge of the South Pacific Superswell. A westerly deepening of the carbonate compensation depth and sedimentary lysocline evolution are proposed, which improve the model for seafloor 30 Ma and older. The degree of match between model and data provides a test of the amount of northward drift of the Pacific plate and, assuming a model for Pacific hotspots motion, of true polar wander (TPW, motion of the hotspot frame relative to the Earth's spin axis). Assuming the Duncan and Clague [1985] model for Pacific hotspots motion, the sedimentary modeling favors TPW models which have little or no effect on the central Pacific paleoequator over the past 30 Ma. We also explore implications of interhotspot motion and the suggestion that the bend in the Hawaiian volcanic chain represents an abrupt change in Pacific hotspot motion within the last 5 Ma.
Seismic attenuation above 10 Hz in southern California from coda waves recorded in the Cajon Pass borehole
Authors: Adams David A. , Abercrombie Rachel E.
Published: February 2004

We estimate the attenuation around Cajon Pass, southern California, for frequencies above 10 Hz, and we find total Q to exhibit only weak frequency dependence, ranging from ∼800 at 10 Hz to ∼1500 at 100 Hz. The intrinsic attenuation is approximately twice the level of the scattering attenuation. Measurements are made using earthquake seismograms recorded at 0–3 km depth using the multiple lapse time window method. The results are not dependent on receiver depth and are consistent with previous estimates of Q made from direct waves recorded at 2.5 km depth. Our Q values are therefore thought to be representative of the seismogenic crust, and the technique used is uncontaminated by the highly attenuating near surface, at least above ∼10 Hz. We also calculate Q from surface data between 1 and 10 Hz and find a clear change in the frequency dependence of both intrinsic and scattering attenuation at ∼10 Hz. Q exhibits strong frequency dependence below 10 Hz ( ∝ƒ1.8), consistent with previous studies in active tectonic regions, and only weak frequency dependence at higher frequencies ( ∝ƒ0.34). This change in behavior renders it unwise to extrapolate Q measurements outside the frequency range from which they were derived, for example, in earthquake‐source studies. Possible factors responsible for the apparent change in frequency dependence of Q are considered. Further work is required, however, to resolve the causes of this change.
Okinawa trough backarc basin: Early tectonic and magmatic evolution
Authors: Sibuet Jean‐Claude , Deffontaines Benoît , Hsu Shu‐Kun , Thareau Nicolas , Le Formal Jean‐Pierre , Liu Char‐Shine
Published: February 2004

The Okinawa Trough, lying between Japan and Taiwan, is a backarc basin formed by extension within the continental lithosphere behind the Ryukyu trench‐arc system. Stress directions associated with the two last extensional phases in the southwestern Okinawa Trough have been deduced from a comparison with analog modeling: the direction of extension is N150° for the Pleistocene phase of extension (2–0.1 Ma) and N170° for the late Pleistocene‐Holocene phase of extension (0.1–0 Ma). The present‐day Ryukyu volcanic arc, a narrow continuous feature extending from Japan to Taiwan, is located on the eastern side of the Okinawa Trough, 80–100 km above the Wadati‐Benioff zone, the minimum depth for emplacement of arc magmatism. Scarce present‐day backarc volcanism appears in the middle and southern Okinawa Trough within linear en echelon bathymetric depressions. A N045° oriented seamount volcanic chain cuts across obliquely the southwestern Okinawa Trough and lies in the direct line of extension of the Gagua ridge, a N‐S linear volcanic feature of the Philippine Sea plate. Associated with this extension of the Gagua ridge, a large reentrant located at the base of the Ryukyu prism, the uplift of part of the Nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross‐backarc volcanism could be tied to the subduction of the Gagua ridge located there at a depth of 80–100 km beneath the backarc basin. A second area of anomalous volcanism has been identified in the middle Okinawa Trough in the ENE extension of the Daito ridge, a WNW‐ESE 400‐km‐long volcanic feature of the Philippine Sea plate. We suggest that the Gagua and Daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere, allowing magmas with arc affinities to erupt at the seafloor. This excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts. The Okinawa Trough is consequently still in an early stage of evolving from arc type to backarc activity.
Paleomagnetic data from Triassic strata, Zuni uplift, New Mexico: Further evidence of large‐magnitude Triassic apparent polar wander of North America
Authors: Garza Roberto S. Molina , Geissman John W. , Gomez Alfred , Horton Brian
Published: February 2004

Upper Carnian (∼225 Ma) strata of the Bluewater Creek Formation of the Chinle Group of the Colorado Plateau, western New Mexico, contain predominantly reverse polarity characteristic magnetizations of high coercivity and distributed unblocking temperature up to 695°C. Data from a syndepositional structure suggest acquisition of a stable remanence during or soon after deposition. The mean paleomagnetic pole for 13 virtual geomagnetic poles (VGP) accepted (of 17 sites collected) is located at 55.2°N, 87.5°E (A95 = 6.7°, K = 39.7). The angular distance between this pole and the mean of recently published middle and late Norian (∼210 Ma) poles from overlying strata is 12.5°, supporting the hypothesis that significant apparent polar wander occurred during Late Triassic time. Furthermore, the Bluewater Creek pole lies to the west (6.1°, angular distance) of the cratonic reference pole for Carnian time supporting the hypothesis of a small Colorado Plateau rotation since the early Mesozoic. Anisian strata of the Moenkopi Formation contain dual polarity magnetizations of high coercivity and distributed unblocking temperatures up to 690°C, The mean VGP of six sites yields is located at 53.1°N, 96.3°E (A95 = 6.4°, K = 109.2), which is similar to previously determined Middle Triassic poles. Triassic paleomagnetic poles from southwest North America from strata in continuous stratigraphic succession, both on and off the Colorado Plateau, are not consistent with the hypothesis of a Late Triassic stand still of the geomagnetic pole.
Carboniferous through Jurassic paleomagnetic data and their bearing on rotation of the Colorado plateau
Authors: Garza Roberto S. Molina , Acton Gary D. , Geissman John W.
Published: February 2004

Small, yet systematic, differences between paleomagnetic poles derived from strata on the Colorado plateau and paleopoles determined from rocks on the North America craton have been interpreted to support the hypothesis of modest post‐Late Cretaceous clockwise rotation of the plateau, as a quasi‐rigid body, with respect to the craton. Using an iterative search for the best fit Euler pole and rotation angle, comparison of the best quality Late Carboniferous through Late Jurassic paleomagnetic poles from the Colorado plateau and the North America craton gives a cumulative rotation estimate (based on a rotation pole at 34°N, 105°W) of 7.4°±3.8° (95% confidence limits). A similar comparison using subsets of the cratonic database from localities in (1) northeast North America and (2) the craton platform interior give larger (8.8°±3.6°) and smaller (5.1°±3.8°) estimates, respectively, reflecting the fact that poles from localities in northeast North America, in particular those from Triassic rift basins, indicate a larger rotation (as concluded in direct pole to pole comparisons). The Euler pole, as determined by the paleomagnetic data only, can lie anywhere within a relatively large area that encompasses locations in the western United States previously proposed from geological observations. Paleomagnetic data and geologic observations, together or independently, do not support the hypothesis of a large Colorado plateau rotation (of 11° to 15°). If geologically reasonable, previous estimates of significant (>∼20 km) dextral slip along the eastern margin of the plateau require a position for the Euler pole east of the 105°W meridian.
Structure and tectonics of the upper Cenozoic Puerto Rico‐Virgin Islands carbonate platform as determined from seismic reflection studies
Authors: van Gestel Jean‐Paul , Mann Paul , Dolan James F. , Grindlay Nancy R.
Published: February 2004

The Puerto Rico‐Virgin Islands carbonate platform was deposited over an area of 18,000 km2 from early Oligocene to Holocene on top of an inactive and subsiding Cretaceous‐earliest Oligocene island arc. Regional single‐channel and multichannel seismic reflection lines presented in this study provide the first information on the regional stratigraphy and structure of this platform that has previously been known mainly from onshore stratigraphic sections of a relatively small (2250 km2) portion of the platform exposed by late Neogene tectonic uplift along the north coast of Puerto Rico. Seismic reflection lines are used to map the thickness of the carbonate platform strata and to correlate this thickness with onshore outcrop and well data from northern and southern Puerto Rico, St. Croix (U.S. Virgin Islands), and the Saba Bank. Limestone thickness variations from a little over 2 km to almost zero are used to subdivide the Puerto Rico‐Virgin Islands platform into five distinct carbonate provinces: (1) north Puerto Rico area including the onshore exposures; (2) Virgin Islands area; (3) St. Croix and Saba Bank area; (4) south Puerto Rico area; and (5) Mona Passage area. Carbonate thickness and structural information from each area are used to test five previously proposed models for the deformation and vertical movements of the platform. The most prominent feature of the platform in the Puerto Rico‐Virgin Islands area is a large, east‐west trending arch. The northern limb of this arch exhibits a smoother, more uniform dip than the steeper, more abruptly faulted, southern limb. The core of the arch is responsible for the exposure of arc basement rocks on Puerto Rico. The origin of this arch, which occurs over a 300 km wide area, is best explained by north‐south shortening and arching, caused by interaction at depth of subducted slabs of the North America and Caribbean plates. Other important evidence for this model can be found in the Benioff zones observed in the earthquake profiles. Loading of the Caribbean plate results in downward flexing of the North America plate and causes the 4 km subsidence of the carbonate platform north of Puerto Rico.
Quasi‐Love phases between Tonga and Hawaii: Observations, simulations, and explanations
Authors: Levin Vadim , Park Jeffrey
Published: February 2004

Seismograms of some shallow Tonga earthquakes observed at Hawaii contain SV‐polarized phases in the Love wave time window, most prominently on the vertical component. Given the geometry of the observations (Δ ≈ 40–45°), such phases may be explained either as body waves or as mode‐converted surface waves. Detailed synthetic seismogram modeling of representative events reveals several instances where the body wave explanation is inadequate, even when plausible uncertainties in the source mechanism are taken into account. The observed, SV‐polarized phase can instead be generated through Love‐Rayleigh scattering, which requires laterally varying seismic anisotropy along the Tonga‐Hawaii path. Trial‐and‐error forward modeling with simple structures based on the transversely isotropic mid‐Pacific velocity model PA5 of Gaherty et al [1996] obtains velocity structure that yields synthetic seismograms matching the observations. This model, while non unique, suggests first‐order constraints on the lateral variation in anisotropic properties, and associated mantle flow, along the Tonga‐Hawaii path. By examining trade‐offs in model parameters, we conclude that robust features of the model are: (1) a transition from radial to mixed radial and azimuthal anisotropy 3°–5° from Hawaii; (2) the NW‐SE alignment of the axis of azimuthal anisotropy; (3) higher degree of P anisotropy relative to S anisotropy; and (4) the presence of azimuthal anisotropy within upper 200–250 km of the mantle. Taken together, these features imply a disruption of mantle fabric by the processes forming Hawaii‐Emperor volcanic system. A model with anisotropic gradients in both the lithospheric lid and shallow asthenosphere is the simplest extension of our starting model. However, an equivalent data fit can be obtained if the azimuthal‐anisotropy gradients are restricted to line beneath the high‐velocity “lid” of model PA5, so that mantle hot spot flow need not penetrate the lithospheric lid.
Compaction‐corrected inclinations from southern California Cretaceous marine sedimentary rocks indicate no paleolatitudinal offset for the Peninsular Ranges terrane
Authors: Tan Xiaodong , Kodama K. P.
Published: February 2004

Paleomagnetic data have been used extensively to delineate the terrane displacement and accretion history of the western margin of North America. However, the anomalously shallow paleomagnetic inclinations used to indicate large‐scale northward translation might be alternatively interpreted as due to postmagnetization tilting of batholithic rocks and compaction of marine sediments. To understand the magnitude of burial compaction effects on the post‐Cretaceous motion history of the Peninsular Ranges‐Baja Borderlands terrane, a rock magnetic, compaction, and paleomagnetic study of the Ladd Formation and the Point Loma Formation from southern California was conducted. The anhysteretic anisotropy of remanence of the characteristic remanence‐carrying grains and individual magnetic grain anisotropy were used to correct the inclinations of each formation. Individual magnetic grain anisotropy was determined by both compaction experiments and redeposition of a magnetic separate in a DC magnetic field. Standard paleomagnetic studies of the units indicated that previous Ladd Formation results could be reproduced, and a correction was made at the sample level. We were unable to adequately reproduce earlier results of the Point Loma Formation, so the average remanence anisotropy was used to correct the previously reported mean direction. The mean inclination of the Ladd Formation was conected from 46° (α95 = 8°) to 58° (α95 = 4°), and the mean inclination of the Point Loma Formation was conected from 39.5°±5.4° (normal) and −36.4° ± 16.6° (reversed) to 56.0°±5.1° (normal) and −53.0°±16.7° (reversed). These results suggest that the Peninsular Ranges‐Baja Borderland terrane has been part of the western North America since the Late Cretaceous and that clay‐containing sedimentary rocks may typically experience from 10° to 15° of inclination shallowing due to burial compaction.
Detailed subduction structure across the eastern Nankai Trough obtained from ocean bottom seismographic profiles
Authors: Nakanishi Ayako , Shiobara Hajime , Hino Ryota , Kodaira Shuichi , Kanazawa Toshihiko , Shimamura Hideki
Published: February 2004

To investigate the deep crustal structure of the Philippine Sea Plate at its northern margin, we performed a seismic refraction and wide‐angle reflection survey around the eastern Nankai Trough by using an ocean bottom seismograph array with an air gun source. We derived a crustal structure model across the trough, from the Shikoku Basin to the continental slope, that explains not only our seismic data but also previously published gravity data. The origin of the Zenisu Ridge, a conspicuous topographic high along the oceanward slope of the Nankai Trough, is still a matter of argument. There has been some controversy as to whether the igneous activity of the Izu‐Ogasawara Arc or the seafloor spreading of the Shikoku Basin is responsible for the formation of the ridge. Although the crustal thickness beneath the ridge is between 8 and 11 km, slightly thicker than typical oceanic crust, our structure model clearly indicates that the Zenisu Ridge has an oceanic crust and its structure is very similar to that of the Shikoku Basin. Beneath the south flank of the Zenisu Ridge, the Moho shows an offset of 5 km in depth. This offset may correlate with the recently proposed nascent subduction boundary and subduction‐collision tectonics of this area. The velocity structure beneath the continental slope appears characteristic for a well‐developed accretionary wedge bounded by the continental upper crust of the Japan Island Arc to the northwest, and subducting oceanic crust which can be traced beneath the accretionary wedge and the continental upper crust.
Closing the gap between regional and global travel time tomography
Authors: Bijwaard Harmen , Spakman Wim , Engdahl E. Robert
Published: February 2004

Recent global travel time tomography studies by Zhou [1996] and van der Hilst et al. [1997] have been performed with cell parameterizations of the order of those frequently used in regional tomography studies (i.e., with cell sizes of 1°–2°). These new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations (5° cells). The inferred structures are, however, of larger scale than is usually obtained in regional models, and it is not clear where and if individual cells are actually resolved. This study aims at resolving lateral heterogeneity on scales as small as 0.6° in the upper mantle and 1.2°–3° in the lower mantle. This allows for the adequate mapping of expected small‐scale structures induced by, for example, lithosphere subduction, deep mantle upwellings, and mid‐ocean ridges. There are three major contributions that allow for this advancement. First, we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the Earth's mantle by seismic waves [Spakman and Bijwaard, 1998]. Second, we exploit the global data set of Engdahl et al. [1998], which is a reprocessed version of the global data set of the International Seismological Centre. Their reprocessing included hypocenter redetermination and phase reidentification. Finally, we combine all data used (P, pP, and pwP phases) into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal‐to‐noise ratio is improved. In the approximate solution of the huge inverse problem we obtain a variance reduction of 57.1%. Synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells (0.6° or 1.2°) in the shallow parts of subduction zones decreasing to approximately 2°–3° resolution in well‐sampled regions in the lower mantle. Vertical resolution can be worse (up to several hundreds of kilometers) in subduction zones with rays predominantly pointing along dip. Important features of the solution are as follows: 100–200 km thick high‐velocity slabs beneath all major subduction zones, sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle; large high‐velocity anomalies in the lower mantle that have been attributed to subduction of the Tethys ocean and the Farallon plate; and low‐velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under Iceland, east Africa, the Canary Islands, Yellowstone, and the Society Islands. Our findings corroborate that the 660 km boundary may resist but not prevent (present day) large‐scale mass transfer from upper to lower mantle or vice versa. This observation confirms the results of previous, global mantle studies that employed coarser parameterizations.
Heat flow in Oklahoma and the south central United States
Authors: Cranganu Constantin , Lee Youngmin , Deming David
Published: February 2004

We report 23 new heat flow values in the state of Oklahoma and combine these with previously reported values to create heat flow maps of Oklahoma and the central United States. In general, thermal gradients increase from southwestern Oklahoma (minimum, 14°C km−1) to northeastern Oklahoma (maximum, 42°C km−1); the mean thermal gradient in central Oklahoma is 31°C km−1. We made 1498 thermal‐conductivity measurements on drill cuttings from oil and gas wells. Heat flow in Oklahoma varies between 22 and 86 m W m−2; the average is about 50 m W m−2. Heat flow is relatively low (30–40 m W m−2) in southwest Oklahoma and is relatively high (70–80 m W m−2) in one section of east central Oklahoma. We also report 27 new estimations of radioactive heat production in basement rocks and combine these with 22 previously published data to create a map of basement heat production in Oklahoma. Heat production values range from 1.1 to 3.5 μW m−3, with an average of 2.5 μW m−3. Geologic evidence suggests that the present‐day thermal regime is not significantly affected by sedimentation, erosion, or metamorphic/igneous activity and is essentially in a near steady state condition. Heat flow in Oklahoma appears to be largely controlled by heat production in basement rocks, although the correlation between heat flow and heat production is not strong. An area of high heat flow (70–80 m W m−2) in east central Oklahoma may be related to regional groundwater flow through the Arbuckle‐Simpson aquifer.
Distribution of isolated volcanoes on the flanks of the East Pacific Rise, 15.3°S–20°S
Authors: White Scott M. , Macdonald Ken C. , Scheirer Daniel S. , Cormier Marie‐Helène
Published: February 2004

Volcanic constructions, not associated with seamount (or volcano) chains, are abundant on the flanks of the East Pacific Rise (EPR) but are rare along the axial high. The distribution of isolated volcanoes, based on multibeam bathymetric maps, is approximately symmetric about the EPR axis. This symmetry contrasts with the asymmetries in the distribution of volcano chains (more abundant on the west flank), the seafloor subsidence rates (slower on the west flank) and the distribution of plate‐motion‐parallel gravity lineaments (more prominent on the west flank). Most of the isolated volcanoes complete their growth within ∼14 km of the axis on crust younger than 0.2 Ma, while seamount chain volcanoes continue to be active on older crust. Volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities, suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities. The volume of isolated near‐axis volcanoes correlates with ridge axis cross‐sectional area, suggesting a link between the magma budget of the ridge and the eruption of near‐axis volcanoes. Within the study area, off‐axis volcanic edifices cover at least 6 % of the seafloor and contribute more than 0.2% to the volume of the crust. The inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the Mid‐Atlantic Ridge (<4 km), northern EPR (<20 km), and southern EPR(<28 km), so that isolated volcanoes form primarily on lithosphere younger than 0.2 Ma (<4–6 km brittle thickness), independent of spreading rate. This suggests some form of lithospheric control on the eruption of isolated off‐axis volcanoes due to brittle thickness, increased normal stresses across cracks impeding dike injection, or thermal stresses within the newly forming lithosphere.
Sedimentary record of two geomagnetic excursions within the last 15,000 years in Beijing, China
Authors: Zhu R. X. , Coe R. S. , Zhao X. X.
Published: February 2004

A fresh‐water sedimentary sequence from Haidian in the Beijing basin has been sampled in three parallel trenches up to 300 m apart for paleomagnetic study. Age control is provided by nine radiocarbon dates, indicating an average accumulation rate of 45 cm/ka. A total of 683 specimens were measured for paleomagnetic directions. Two short excursions of field direction are recorded at closely comparable depths in all three cores, occurring between 5,060 and 4,860±90a B.P. and 14,230 and 13,690±130 a B.P. interpolated ages. The paleomagnetic records for the individual trenches were adjusted to a common depth scale using stratigraphic markers and then merged. The record correlates quite well with a paleomagnetic record in Beijing‐basin sediments 55 km to the southwest (Changgou section), with regard to both the large excursions and also some of the smaller variations in direction. The excursions appear to reflect real geomagnetic phenomena, not artifacts of recording, but the spatial extent (regional or global) of the anomalous geomagnetic signals is not known.
Late Neogene exhumation patterns in Taranaki Basin (New Zealand): Evidence from offset porosity‐depth trends
Authors: Armstrong Phillip A. , Allis Richard G. , Funnell Robert H. , Chapman David S.
Published: February 2004

Taranaki Basin, New Zealand, is located adjacent to the Australian‐Pacific Plate boundary where the tectonic regime changes from dominantly subduction‐related to the north to transpression‐related along the Alpine Fault to the south. During the Neogene, burial and exhumation varied extensively, in both time and space, in response to subsidence and uplift along this evolving plate boundary zone. The basin can be divided into two regions: (1) the Western Platform outside the plate boundary deformation zone where no uplift nor exhumation has occurred and (2) the Eastern Mobile Belt, which lies inside the deformation zone and has been variably uplifted and exhumed. Exhumation magnitudes for sedimentary deposits of the basin are estimated from offset porosity‐depth trends. The analysis is based on correlating sonic log travel times with compensated density logs, permitting sonic travel time to be used as a proxy for porosity. Twelve Western Platform wells are used to define a standard exponential porosity‐depth trend with an extrapolated surface porosity of 50% and an exponential decay factor of 2265 m, valid for mudstone/shale sections spanning a depth range of 300–3000 m. This curve is a calibration curve against which offset porosity‐depth trends from wells in exhumed regions can be compared to determine apparent exhumation amounts. Porosity‐depth trends for 43 Eastern Mobile Belt wells are offset 0–2800 m shallower relative to the Western Platform trend but generally are parallel to it. In the southern region of the Eastern Mobile Belt, net exhumation amounts, which are the sum of the porosity‐depth trend offset and depth to unconformity, range from 850 to 3000 m; most of this exhumation occurred on discrete contractional structures in late Miocene to early Pliocene time, probably associated with an increased rate of convergence across the Alpine Fault system to the south. In the eastern region of the Eastern Mobile Belt, net exhumation progressively increases from 0 m in the south to 1800 m in the northeast part of the basin, consistent with the 2°–4°SW structural tilt of onshore strata. The regional and consistent pattern of exhumation in the eastern region contrasts with the discrete structure‐related exhumation in the southern region and is consistent with models of thermal and flexural uplift.
Tertiary remagnetization of Paleozoic rocks from the Eastern Cordillera and sub‐Andean Belt of Bolivia
Authors: Libarkin J. C. , Butler R. F. , Richards D. R. , Sempere T.
Published: February 2004

Paleomagnetic samples were collected from 98 sedimentary horizons in eight different Devonian to Permian sedimentary units at eight localities in the Eastern Cordillera and the sub‐Andean Belt of Bolivia. For 77 sites, thermal demagnetization allowed determination of a characteristic magnetization (ChRM) with site‐mean 95% confidence limit, α95, ≤15°. The ChRM is carried predominantly or entirely by hematite. Fold and reversal tests from two of the sampled localities indicate that the characteristic magnetization is synfolding, likely acquired during the earliest stages of deformation. Additionally, a modified conglomerate test at one locality and the nearly uniform direction of ChRM across the Devonian to Permian age units clearly reveals the secondary nature of the characteristic magnetization. Finally, the ChRM directions are discordant from any expected Paleozoic directions. Paleomagnetic poles calculated from the ChRM directions fall near the Cenozoic portion of the apparent polar wander path for South America. We interpret these observations to indicate widespread chemical remagnetization of these Paleozoic strata during, but prior to completion of, Cenozoic Andean folding.
Inversion of source parameters from near‐ and far‐field observations: An application to the 1915 Fucino earthquake, central Apennines, Italy
Authors: Amoruso Antonella , Crescentini Luca , Scarpa Roberto
Published: February 2004

We use a nonlinear inversion approach for determining source parameters of the Fucino Ms=6.9 earthquake that occurred along central Apennines, Italy, in 1915, taking into account both near‐field surface deformations and far‐field first‐motion polarities. This event was one of the most destructive in Italy this century, causing over 30,000 casualties. Altitude and leveling measurements were performed before (1862) and after (1917) the event, along a circular route consisting of 18 benchmarks. Polarities of first arrivals do not constrain a unique fault‐plane solution, while the one obtained by inverting geodetic data, with relevant constraints provided by neotectonic data, disagrees with P wave polarities. Our joint inversion, based on a robust fitting technique, has resulted in only one well‐defined solution, satisfying geophysical data as well as surface‐faulting distribution and paleoseismological information.
Efficacy of mefloquine and mefloquine–artesunate for the treatment of uncomplicated Plasmodium falciparum malaria in the Amazon region of Bolivia
Authors: Avila Juan Carlos , Villaroel Rodolfo , Marquiño Wilmer , Zegarra Jorge , Mollinedo René , Ruebush Trenton K.
Published: February 2004

We assessed the efficacy of mefloquine monotherapy and mefloquine–artesunate (MQ–AS) combination therapy for the treatment of Plasmodium falciparum malaria at four sites in the Bolivian Amazon region. Patients with uncomplicated P. falciparum infections between 5 and 60 years of age were randomly assigned to be treated with either MQ (15 mg/kg in a single oral dose) or MQ (15 mg/kg) plus AS (4 mg/kg daily for 3 days). A total of 143 patients were enrolled and followed for 28 days. None of the 73 patients who received MQ alone or the 70 patients who received MQ–AS combination therapy had recurrences of parasitaemia during the 28‐day follow‐up period. Asexual parasite densities fell significantly more rapidly and the proportion of patients with gametocytes was significantly lower on days 7–28 in patients treated with MQ–AS than in those treated with MQ alone. All patients tolerated the medications well. After this study, the Bolivian Ministry of Public Health changed its treatment policy for uncomplicated P. falciparum malaria in the Amazon region to combination therapy with MQ–AS to slow or prevent the development of resistance.
Association between the occurrence of the anticardiolipin IgM and mite allergen‐specific IgE antibodies in children with extrinsic type of atopic eczema/dermatitis syndrome
Authors: Szakos E. , Lakos G. , Aleksza M. , Gyimesi E. , Páll G. , Fodor B. , Hunyadi J. , Sólyom E. , Sipka S.
Published: February 2004

Background:  As the literature has only controversial data on the role of nonallergen‐specific antibodies in atopic eczema dermatitis syndrome, the authors investigated the link between the occurrence of the antiphospholipid [anticardiolipin (ACL), anti‐β2‐glycoprotein I] and allergen‐specific immunoglobulin E (IgE) antibodies in 72 children with atopic eczema/dermatitis syndrome (AEDS).

Methods:  The measurement of antiphospholipid antibodies was carried out by enzyme‐linked immunosorbent assay (ELISA), serum total IgE by nephelometry, and allergen‐specific IgE by immunoblotting assay. The statistical analysis was carried out by Fisher's exact test and odds ratio was calculated.

Results:  Thirteen of 72 children with AEDS (mean age 8.3 years) had elevated serum levels of ACL, and eight anti‐β2‐glycoprotein I antibodies. The presence of allergen‐specific IgE against inhalant allergens and nutritive allergens was among eight of 13 and three of eight in the cases with elevated ACL. The ratio of patients with highly increased severity scoring of atopic dermatitis (SCORAD) index (>75) was significantly higher in the group with elevated (4/13) than in those with the normal ACL levels (2/59). There was a significant association between the appearance of mite (Dermatophagoides pteronyssinus, D. farinae)‐specific IgE and ACL IgM antibodies (6/13).

Conclusion:  These findings show that there are significant linkage and association between the appearance of ACL IgM or the production of allergen‐specific IgE against inhalant (mainly mite) allergens in children with atopic eczema/dermatitis syndrome.
OBSERVATION OF SiC WITH Si(111) - 7 SURFACE STRUCTURE USING HIGH-ENERGY ELECTRON DIFFRACTION
Authors: Henderson R. C. , Polito W. J. , Simpson J.
Published: February 2004

Using grazing-angle high-energy electron diffraction (HEED) epitaxial crystallites of β-Sic have been observed on Si surfaces simultaneously with the silicon fractional-order surface structures. SiC has not been observed with the LEED technique under comparable experimental conditions. The surfaces containing SiC were prepared by iodine desorption. Surfaces prepared by oxide decomposition at 1200 °C or by in situ pyrolysis of SiH4 were carbide free. The experiments showed the carbide was due to decomposition of a carbon adsorbate.
INTENSITIES OF SPIN-WAVE RESONANCE MODES IN THIN FILMS
Authors: Weber R. , Tannenwald P. E. , Bajorek C. H.
Published: February 2004

Accurate measurements of spin-wave resonance absorption peaks from 5 to 70 GHz show that the deviation from a 1/p2 falloff is a strong function of frequency. It is pointed out that any intensity model must incorporate such frequency dependence.
LASER EMISSION FROM METAL-SEMICONDUCTOR BARRIERS ON PbTe AND Pb0.8Sn0.2Te
Authors: Nill K. W. , Calawa A. R. , Harman T. C. , Walpole J. N.
Published: February 2004

Laser emission is obtained from forward biased evaporated metal barriers on degenerate p-PbTe and p-Pb0.8Sn0.2Te at T=4.2°K. Metals with small work functions such as In, Pb, and Zn produce a degenerate inverted n-type surface region on p-type samples without chemical doping. Low-threshold laser emission has been obtained from these barriers on p-PbTe at λ = 6.4μ and from Pb barriers on p-Pb0.8Sn0.2Te at λ = 15μ.
HIGH-FIELD PHOTOCONDUCTIVITY OF AMORPHOUS GeTe and GeSe FILMS
Authors: Stiles P. J. , Chang L. L. , Esaki L. , Tsu R.
Published: February 2004

The photoconductivity edge in amorphous films of GeTe or GeSe is found to shift toward lower photon energies with the application of high electric fields. Electrical measurements indicate that the shift is a bulk effect, not due to junctions at the contact-semiconductor interface.
COHERENT SCATTERING OF HOT ELECTRONS IN THIN GOLD FILMS
Authors: Gould R. D. , Collins R. A.
Published: February 2004

Hot-electron emission has been observed through the top surface of the Au electrode in Al–SiO–Au cold cathodes. Previous investigations have indicated that electrons which have undergone coherent scattering in the Au electrode may only be emitted from pinhole edges. The present measurements are found to be consistent with the existing theory.
OPTIMUM DEFLECTION ANGLE FOR CYLINDRICAL AND SPHERICAL ELECTROSTATIC SPECTROMETER
Authors: Roy D. , Carette J. -D.
Published: February 2004

The electron-energy distribution for the idealized cylindrical (127°) and spherical (180°) electrostatic electron spectrometer have been computed in second-order approximation. It is shown that one can improve these spectrometers by injecting at the entrance slit of the spectrometer only that fraction of incident electrons that have positive radial velocity components and at the same time setting the angle between entrance and exit slit plane to a proper smaller value. A method to build an electron spectrometer operating this way is suggested.
IONIC SPECIES PRESENT IN CESIUM DISCHARGES
Authors: Bergman R. S. , Chanin L. M.
Published: February 2004

Measurements have been made of the positive ion composition of cesium plasmas for normalized pressures from 10−4 to 10−1 Torr using ion-sampling techniques. At low pressure (≈ 10−4 Torr), Cs+ is dominant; however, at approximately 10−1 Torr, the ion signals of Cs+ and Cs2+ are comparable. The ion-current ratio I(Cs3+)/I(Cs+) increases rapidly with pressure.
CRITICAL FIELD AND CRITICAL CURRENT OF Pb–Bi ALLOYS IN POROUS GLASS
Authors: Watson J. H. P.
Published: February 2004

Critical fields in excess of 100 kOe can be achieved with Pb–Bi alloys in porous glass at 4. 2°K.
SURFACE WAVE COUPLING THROUGH A FLUID INTERFACE
Authors: Cho Frederick Y. , Hunsinger Bill J. , Lawson Ronald L.
Published: February 2004

A method to determine the coupling length for optimum transfer of surface wave energy from one substrate through an interface to a second substrate is presented. Less than 10 dB of energy loss is measured for coupling surface waves (20 MHz) on Y cut Z propagation (YZ) lithium niobate substrates through a water interface.
OPTICAL MIXING IN CADMIUM TELLURIDE USING THE PULSED WATER VAPOR LASER
Authors: McArthur D. A. , McFarlane R. A.
Published: February 2004

We report here the first observation of second harmonic generation and sum frequency mixing with fundamental wavelengths in the 26 to 33μ range, using the pulsed water vapor laser and CdTe as a nonlinear material. We obtain an approximate value of d36(λ1 = 27.97μ)=(9 ± 6) × 10−7 esu in CdTe, about twice as large as the value at 10.6μ, which we discuss in terms of Garrett's model of the dispersion of d36.
TWO KINDS OF CdS HIGH-FIELD DOMAIN MODES
Authors: Furukawa Seijiro , Hata Tomonobu , Motosugi Tsuneharu
Published: February 2004

This paper presents the experimental and theoretical results about CdS high-field domain modes in case of C ∥ E. It is observed by use of direct probe measurement of two-dimensional potential distributions on the sample surface that there are two kinds of domain modes: In one of the modes which is observed for a considerably high-impurity sample, the off-axis angle θ increases with the increase of electric field. In the other mode which is observed for a low-impurity sample, the angle θ tends to 30°. The dependence of θ on the applied field for the latter mode can be theoretically explained by use of the assumption that the domain front travels to the direction where the small signal gain becomes the maximum value.
HIGH-EFFICIENCY RED-EMITTING GaP DIODES GROWN BY SINGLE EPITAXY ON SOLUTION-GROWN (η≃6%) AND CZOCHRALSKI (η≃2%) SUBSTRATES
Authors: Hackett W. H. , Saul R. H. , Verleur H. W. , Bass S. J.
Published: February 2004

High external quantum efficiencies have been obtained for GaP red-emitting diodes fabricated by a single liquid-phase epitaxy, in which Zn, O-doped p-type layers were grown directly on Te-doped n-type solution-grown (SG) and liquid-encapsulated Czochralski (LEC) substrates. For SG single-epitaxy diodes, the quantum efficiencies (η≃6%) and the other electrical and electroluminescent characteristics are nearly identical to those observed in the previously reported double-epitaxy diodes, demonstrating that the double-epitaxy structure is not essential for high efficiencies. For LEC single-epitaxy diodes, the quantum efficiencies (η≃2%) establish for the first time the present utility and future potential of LEC crystals in fabricating efficient diodes by single epitaxy.
ELECTRICAL FIELD DISTRIBUTION ASSOCIATED WITH DYNAMIC SCATTERING IN NEMATIC LIQUID CRYSTALS
Authors: Lu Sun , Jones Derick
Published: February 2004

A small probe was used to measure the electric field distribution in a 1-cm-thick cell containing a nematic liquid crystal. It is found that apart from distances of about 100μ from each electrode, the electric field is quite uniform. The field at the anode is at least ten times higher than the average field. This indicates the existance of space-charge carriers. The cathode also shows the above effect, but to a much smaller extent. The voltage applied to the cell was varied from 1 ∼ 8 kV. Identical results were obtained for the material in its isotropic state. These measurements give support to the model proposed by Heilmeier to explain dynamic scattering in nematic liquid crystals.
PHOTOINDUCED REFRACTIVE INDEX INCREASE IN POLY(METHYLMETHACRYLATE) AND ITS APPLICATIONS
Authors: Tomlinson W. J. , Kaminow I. P. , Chandross E. A. , Fork R. L. , Silfvast W. T.
Published: February 2004

Ultraviolet laser radiation at 0.325μ m has been found to produce refractive index increases of up to 3×10−3 in poly(methylmethacrylate) and to resolve at least 5000 lines/mm. The index change is tentatively attributed to localized density increases arising from photoinduced cross linking. Applications to optical waveguides and thick dielectric holograms and gratings are described.