bluuebunny/crossref_metadata_2025 huggingface dataset:
Variations in the Fine‐Scale Composition of a Central Pacific Ferromanganese Crust: Paleoceanographic Implications
Authors: Hein James R. , Bohrson Wendy A. , Schulz Marjorie S. , Noble Marlene , Clague David A.
Published: February 2008

A 47‐ to 60‐mm‐thick Fe‐Mn crust from Horizon Guyot (water depth 1800–1780 m), central Pacific, was used to evaluate the potential of crusts as recorders of Neogene paleoceanographic and paleoclimatic conditions. The chemical composition was determined by microprobe for 16 elements from a polished thin section. Three analyses were made per millimeter and averaged to give the composition of each millimeter. The age of the crust was determined by measuring the strontium isotope composition of the crust and comparing it with the Tertiary seawater curve. The crust represents 18.5 m.y. of growth of Fe and Mn oxyhydroxides. The crust is composed of alternating botryoidal and laminated layers. The botryoidal layers formed during the same time intervals that widespread Neogene deep‐sea hiatuses were forming in bottom sediments. The botryoidal layers represent growth during times of intensified deepwater flow, whereas the laminated intervals represent more quiescent conditions. The correspondence between the botryoidal layers and the Neogene hiatuses is so strong that we were able to choose a variable growth rate model over a constant growth rate model for the crust. Chemical changes in the crust take two forms. The first is represented by broad changes in the composition defined chiefly by fourth‐order polynomial fits to the chemical profiles of each element with depth in the crust. The second is high‐frequency changes in composition. The broader changes occurred primarily at about 15, 11.5, 7.4, 6.4, 5.2, and 4.6 Ma, which may correlate with major changes in paleoceanographic circulation and development of ice caps at the poles. The periods of the high‐frequency changes may reflect climatic changes that resulted from orbital forcing. These high‐frequency changes may correspond to the high‐order eccentricity periods of 3.47, 2.04, and 1.31 m.y.
Late Quaternary Surface Circulation of the Southern Indian Ocean and its Relationship to Orbital Variations
Authors: Howard William R. , Prell Warren L.
Published: February 2008

The paleoceanographic history of the Southern Indian Ocean is reflected by the movement of two prominent dynamical features of the Southern Ocean: the Subtropical Convergence (STC) and the Antarctic Polar Front (APF). These fronts, and their associated sea surface temperature (SST) signatures, are well delineated by planktonic foraminiferal faunas in surface sediments of the southern Indian Ocean. Using a transect of piston cores between 42°S and 48°S at about 90°E, we have reconstructed the latitudinal distribution of planktonic foraminiferal faunas over the past 500,000 years. These faunal variations imply changes in the paleolatitudes of the STC and APF and the surface isotherms associated with the fronts. Stratigraphic and chronologic control is provided by δ18O, %CaCO3, and biostratigraphy. Our reconstruction indicates that the STC has been equatorward of its present position (∼40°S) for most of the past 500,000 years and has been poleward of that position for only four relatively brief (∼10,000‐year) intervals during that time. We estimate six equatorward excursions of the APF over the same period, with a maximum total range of about six degrees of latitude. The average paleo‐position of the APF is about 46°S, about 4°north of its present position (∼50°S). Ice‐rafted debris, another indicator of APF position, occurs as far north as 45°S during glacial intervals. Latitudinal gradients in SST show little glacial‐interglacial change between 42° and 48°S, suggesting that surface isotherms were displaced uniformly or that compression of SST gradients occurred outside the transect. Time series analyses of the SST records in this transect reveal statistically significant concentrations of variance in the primary orbital frequency bands. SST variations in these bands are coherent with orbital variations and with changes in δ18O. SST in the subantarctic Indian Ocean slightly precedes changes in δ18O occurring in the eccentricity, obliquity, and precession bands, and lag orbital variations in the obliquity and precession bands. These results place important constraints on possible mechanisms of interhemispheric climatic timing. The similarities in temporal patterns and timing among Southern Ocean SST, atmospheric CO2, and relative flux of North Atlantic Deep Water implicate atmospheric carbon dioxide and deep water circulation as possible interhemispheric pacing mechanisms.
Seasonal Changes in the Isotopic Compositions and Sinking Fluxes of Euthecosomatous Pteropod Shells in the Sargasso Sea
Authors: Fabry V. J. , Deuser W. G.
Published: February 2008

Seasonal variations in the oxygen and carbon isotopic compositions and fluxes of five euthecosomatous pteropods were determined from a 14‐month series of sediment trap deployments in the Sargasso Sea. Medium and large shell sizes ofStyliola subula, Clio pyramidata, Limacina inflata, Creseis acicula, andCuvierina columnellawere collected throughout the sampling period. Comparisons of the δ18O of shell samples with the vertical and temporal variations in the calculated δ18O of aragonite in equilibrium with seawater suggest that these pteropods deposited the bulk of their shell mass at the following depths:S. subulaandL. inflataat 50 m,C. pyramidataat 75 m,C. aciculain the upper 25 m, andC. columnellaat 50–75 m. Although several of these species undergo diel vertical migration of several hundred meters in this region, the estimated depths of calcification match the upper parts of the species' vertical ranges, where the mean populations occur only at night. In all species, seasonal changes in the δ18O of shells were closely coupled to those of equilibrium δ18O for aragonite, suggesting that most of the shell mass of these individuals was formed within several months. Flux‐weighted, mean δ18O values for the species reveal that seasonal variations in the sinking fluxes of shells would not affect the isotopic compositions of shell accumulations in Bermuda Rise sediments. Carbon and oxygen isotopes were positively correlated in all species exceptC. columnella, which suggests that temperature may influence the δ13C of the shells of these species.
An introduction to the global positioning system and some geological applications
Authors: Dixon T. H.
Published: February 2008

Receivers equipped to measure dual frequency carrier phase signals from satellites of the Global Positioning System (GPS) have been capable, under special conditions, of determining relative horizontal positions among stations separated by one to a few hundred kilometers with a precision of one to several millimeters since the early 1980s. The major obstacles to making this capability routine, extending it to all parts of the globe, and extending it to longer station separations, have been equipment cost, limitations in the GPS satellite constellation, arduous data analysis, uncertainties in satellite orbits, uncertainties in propagation delays associated with variable tropospheric water vapor, and difficulties in resolving carrier phase cycle ambiguities. Recent improvements have occurred in all these areas. The increasing ease and reduced cost of GPS data acquisition and analysis are having a significant impact on studies of near‐fault crustal deformation and earthquake processes, until recently the province of conventional terrestrial geodetic techniques. The enhanced satellite constellation, improved models, and establishment of global tracking networks have extended several millimeters horizontal positioning capability to station separations of 1000 km or more in virtually all parts of the world. This enables study of new classes of tectonic problems that previously were difficult to attack with any geodetic technique. Examples include a complete kinematic description of ongoing crustal deformation in broad, complex continental plate boundary zones, and measurement of relative plate motion at convergent boundaries where global models may be poorly constrained.
High‐temperature elastic constant data on minerals relevant to geophysics
Authors: Anderson Orson L. , Isaak Donald , Oda Hitoshi
Published: February 2008

The high‐temperature measurements of elastic constants and related temperature derivatives of nine minerals of interest to geophysical and geochemical theories of the Earth's interior are reviewed and discussed. A number of correlations between these parameters, which have application to geophysical problems, are also presented. Of especial interest is α, the volume coefficient of thermal expansion, and a section is devoted to this physical property. Here we show how α can be estimated at very high temperatures and how it varies with density. An estimate of α for Mg‐perovskite at deep‐mantle conditions is made. The formula for the Grüneisen ratio γ as a function of V and T is presented, including plots of the numerical values of γ over a wide T and V range. An example calculation of γ for MgO is made. The high‐T‐high‐P values of γ calculated here agree well with results from the ab initio method of calculation for MgO. The use of the thermoelastic parameters is reviewed, showing application to the understanding of thermal pressure, thermal expansivity, enthalpy, and entropy. We review an extrapolation formula to determine Ks, the adiabatic bulk modulus, at very high T. We show that the thermal pressure is quite linear with T up to high temperatures (∼1800 K), and, as a consequence, the anharmonic contribution to the Helmholtz free energy is sufficiently small, so that it can and should be ignored in thermodynamic calculations for mantle conditions.
Electromagnetic field of dipoles and patch antennas on microstrip
Authors: King Ronold W. P.
Published: February 2008

The formulas for the complete electromagnetic field of a unit horizontal electric dipole on the surface of a dielectric‐coated conductor (microstrip) as derived elsewhere are written down and used to derive expressions for the far field. This includes the space wave and the surface wave. The radiated power and radiation resistance are derived. The corresponding formulas for the far field of an antiresonant rectangular patch antenna with an assumed sinusoidal current distribution are derived.
NICARE I HF propagation experiment results and interpretation
Authors: Argo P. , Fitzgerald T. Joseph , Carlos Robert
Published: February 2008

The NICARE I chemical release created a large electron density depletion in the nighttime F region of the ionosphere at an altitude of approximately 300 km which persisted for at least 30 min. A three‐dimensional ray‐tracing code (TRACKER) predicted that the lenslike refractive perturbation would create new off‐axis modes in long‐distance HF radio paths which reflected in the ionosphere near the depletion. Channel measurements during this experiment bear this out. The multipath created by these new modes induced an increased fading rate on the received signals. Analysis of the time history of the direction of arrival of the new modes indicates that after the initial expansion of the release the western edge continued to move westward while the eastward edge remained stable in location. This behavior is consistent with model predictions of the behavior of the depletion.
The effect of multiple scattering on the aspect sensitivity and polarization of radio auroral echoes
Authors: Donovan E. F. , Moorcroft D. R.
Published: February 2008

A Monte Carlo model of radio wave scattering in the auroral electrojet has been developed to investigate multiple scattering of radio auroral echoes. Using this model, predictions of the aspect angle behavior of first‐, second‐, and third‐order scattered power have been made. The results indicate that multiple scattering may be an important effect for VHF radars which observe the auroral E region at large magnetic aspect angles. The model shows that linearly polarized radio waves can become depolarized because of multiple scattering if the radio transmitter is horizontally polarized but not if the radio transmitter is vertically polarized.
Crustal kinematics and deep structure of the northern Rhine Graben, Germany
Authors: Meier Lars , Eisbacher Gerhard H.
Published: February 2008

Surface and subsurface geological data, unpublished information gleaned from industrial seismic surveys, and results from a deep reflection seismic profile across the Tertiary northern Rhine Graben indicate that crustal extension in excess of 6 km was achieved by asymmetric detachment of the upper crust at a depth of about 15 km. Comparison of a simple geometric‐kinematic cross‐sectional model and its depth extrapolation with the DEKORP‐9‐N deep crustal reflection data suggests that extension below the graben was achieved by slip along a gently westward dipping shear zone which truncated and offset lower crustal planar fabrics and which continues into the uppermost mantle. The planar lower crustal reflectors probably originated mainly during late Variscan (Permocarboniferous) crustal extension and/or magmatic underplating.
Paleomagnetic results from the Waipapa Terrane, Northland Peninsula, North Island, New Zealand: TEctonic implications from widespread remagnetizations
Authors: Haston Roger B. , Luyendyk Bruce P.
Published: February 2008

Paleomagnetic samples were collected from a wide variety of rock types of the Waipapa terrane, which is exposed along the eastern coast of the Northland peninsula. Twelve sites yield stable magnetic directions, which fail both a fold and regional attitude test at the 99% confidence level, and indicate that widespread remagnetization has occurred over a large (<200 km) area. Rock magnetic experiments suggest that the mechanism for remagnetization was through the formation of fine‐grained magnetite and hematite and was probably controlled by local variation in fluid migration. The mean in situ direction for the 12 sites is I = −70.2°, D=14.4° (α95=7.1°, kappa=38.6), which is significantly different from the predicted axial dipole field directions at the 95% confidence level. The age of the remagnetization is unclear. In an effort to date the remagnetization, a synthetic apparent polar wander (APW) path was constructed for New Zealand using Australian data, and the apparent south pole from the remagnetized Waipapa sites (latitude=69.7°S, longitude= 150.7°E, α95=11.2°) was compared to this APW path. Using this method, the age of the remagnetization is constrained to be between ≈12 and 58 Ma. A major change in the tectonic regime occurred at ≈25 Ma, which is evidenced by obduction of the Northland allochthon and initiation of arc volcanism and subduction. Fluid migration and/or elevated temperatures driven by the initiation of subduction and arc volcanism, along with the obduction of the Northland allochthon seem likely mechanisms for the remagnetization.
Geology and tectonic evolution of the Bear Mountains Fault Zone, Foothills Terrane, central Sierra Nevada, California
Authors: Miller Robert B. , Paterson Scott R.
Published: February 2008

The Late Jurassic‐Early Cretaceous Bear Mountains fault zone (BMFZ) is the westernmost strand of the Foothills fault system in the Western Metamorphic belt of the Sierra Nevada. The tectonic significance of this 300‐km‐long fault zone has been downplayed in the past, but we contend that it is a major discontinuity within the Foothills terrane. The BMFZ is an ˜ 5‐km‐wide shear zone consisting of slate‐metagraywacke‐matrix melange that experienced intense polyphase deformation and metamorphism. Small‐scale structures indicate reverse slip along most of the extent of this steeply east dipping shear zone. A host of lithologically diverse tectonic blocks are enclosed in the BMFZ. Blocks of sandstone, metavolcanic rocks, and plutonic rocks of intermediate composition have counterparts in the adjacent eastern and western zones of the Foothills terrane, whereas other block types are exotic. Exotic blocks include ultramafites that locally contain pods of gabbro, garnet amphibolite, greenschist, sedimentary breccia, and volcaniclastic rocks. The breccia is composed primarily of clasts of amphibolite and minor garnet‐bearing impure quartzite (metachert?); it probably accumulated at the base of fault scarps. The exotic blocks comprise a broadly ophiolitic assemblage similar to that inferred to form basement to the Foothills terrane, and they record a deformation prior to incorporation in the slate‐metagraywacke matrix. Amphibolite blocks and quartzite preserved as clasts within the breccia probably were deformed during overthrusting of oceanic lithosphere in a setting similar to that beneath the Tuolumne ophiolite east of the BMFZ. The presence of exotic blocks implies large‐scale mixing and large displacements in the BMFZ, as do differences in stratigraphy across the fault zone. Metamorphic grade does not vary significantly across the BMFZ, however, suggesting that dip slip did not exceed several tens of kilometers if the fault zone originally dipped gently eastward as we propose. There is no direct evidence for major strike slip along the BMFZ as has been proposed by others; any such slip must have occurred before the reverse‐slip related structures of the zone. We thus interpret the BMFZ as an intra‐arc reverse fault of moderate to large displacement, with possible earlier strike slip, that incorporates ophiolitic basement showing an older, complicated history.
Metamorphic evidence for tilt of the Spuzzum Pluton: Diminished basis for the “Baja British Columbia” Concept
Authors: Brown E. H. , Burmester R. F.
Published: February 2008

To address the question of tilt versus translation as the mechanism responsible for discordance between paleomagnetic directions of Cretaceous plutons in the British Columbia Coast Plutonic Complex and the North American reference direction, metamorphic pressures around the margin of the Spuzzum pluton have been determined. Pressures are derived from microprobe analyses and evaluation of exchange equilibria in the assemblage garnet‐biotite‐plagioclase‐aluminum silicate‐quartz. Samples studied come from eight localities in the contact aureole around the pluton and encompass the area of a previous paleomagnetic study. The analyzed samples are coarse grained and exhibit textural features indicative of equilibrium crystallization following emplacement of the Spuzzum pluton. Results of this study indicate a complicated tilt history for the Spuzzum pluton, with tilt first to the northeast then to the southwest. The southwest tilt can fully explain the discordance of the paleomagnetic direction in the Spuzzum but does not preclude translation. However, in view of the tilt history of this pluton, the paleomagnetic data derived from it should not be used in an analysis of north‐south translation.
The block structure and Quaternary strike‐slip block rotation of central Japan
Authors: Kanaori Yuji , Kawakami Shin‐ichi , Yairi Kenji
Published: February 2008

Central Japan is situated on the inflection point of the bow‐shaped Japanese islands. Numerous NW‐SE trending active faults, arranged in parallel at intervals of 20 to 80 km are found in this area. These active faults are more than 30 km long with shattered zones from 30 to 300 m wide. Several active faults constitute a given block boundary, which serves as the dividing line for one of the four blocks that make up central Japan. The block boundaries require careful study since numerous historical earth‐quakes have occurred along these lines. Offset measurements of basement rocks, created during the Quaternary period due to left‐lateral faulting, amount to 1 to 7 km. Gravity lineaments, which link points of sudden change and saddles of Bouguer anomalies, are clearly found along the block boundaries. The NW‐SE trending active faults appearing on the ground surface are associated with motions of the block boundaries. Block rotational movement, caused by left‐lateral faulting, plays an important role in the crustal deformation of central Japan. Rotational angles of the blocks calculated from the amount of displacement of basement rocks, initiated during the Quaternary period, are estimated to be 3° to 7° in a clockwise manner.
Tectonic architecture of the mountain front‐foreland basin transition, South Island, New Zealand, assessed by fission track analysis
Authors: Kamp Peter J. J. , Green Paul F. , Tippett J. Mark
Published: February 2008

Oblique continental convergence across the Alpine fault since the mid‐Miocene has loaded the Australia plate with the leading edge of the Pacific plate and formed a foreland basin. The basin occurs mostly offshore beneath the continental shelf, but remnants of the marine basin fill overlie Ordovician basement in a narrow coastal strip 10–20 km wide in Westland, between the Tasman Sea and the Alpine fault. The results of fission track analyses of apatite and zircon separates from this basement, integrated with the regional structure and stratigraphy, indicate that the foreland basin formerly extended across the whole of Westland and that its succession increased in thickness eastward toward the Alpine fault, reaching a maximum of about 4 km. Continuing covergence across the main thrust (Alpine fault) has caused its footwall to thicken by reverse fauting in a transitional zone 10–20 km wide immediately west of the mountain front (Southern Alps). Consequently, the inner margin of the foreland basin has been inverted and the succession almost completely eroded. In the southern part of Westland, upper crustal shortening in the Australia plate amounted to about 2 km; associated uplift started about 5 m.y. ago and involved only one major fault‐bounded block. In northern Westland, upper crustal shortening amounted to about 12 km; two major fault‐bounded blocks were involved, and uplift was underway by 10 Ma. Assuming a preuplift geothermal gradient of 25°C/km, the long‐term rate of uplift ranged between 0.8 and 2.0 mm/yr in southern Westland and between 0.4 and 1.2 mm/yr in different parts of northern Westland.
Evolution of the Southern San Joaquin Basin and mid‐Tertiary “transitional” tectonics, central California
Authors: Goodman Emery D. , Malin Peter E.
Published: February 2008

A Cenozoic tectonic and sedimentary history is proposed for the Southern San Joaquin Basin (SSJB) and Tehachapi Mountains that evolved adjacent to the plate margin off central California. Seismic reflection, borehole, field, biostratigraphic, and paleomagnetic data are integrated into geologic and fault structure maps, cross sections, and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence, timing, and distribution of complex, tectonically linked events. The largely buried structures and strata in the SSJB preserve an unusually complete record of the mid‐Tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the Pliocene. Significant structural relief, existing across both extensional and contractile features, is preserved in the subsurface and an active fold‐thrust belt propagates basinward along the margin of the U‐shaped Tejon embayment The Cenozoic evolution of the SSJB reflects the regional deformation of central California as different tectonic events followed each other along the adjacent North American plate margin. Five Oligocene‐Miocene basin phases are identified in the SSJB: (1) late Oligocene/early Miocene extensional subsidence, with high‐ and low‐angle normal faulting, accompanied by volcanism and deposition of coarse syntectonic conglomerates; (2) middle Miocene uplift; (3) later mid‐Miocene transtensional subsidence to lower bathyal depths; (4) alternating subsidence and uplift until the late Miocene; and (5) flexural subsidence due to Pliocene to Recent contraction. Reconstructions of mid‐Tertiary California place the southern San Joaquin/Tehachapi extensional terrane as a paleotectonic block located between the Western Mojave terrane (then to the east) and the Western California terrane (then to the south and west). Regional extension occurred during a long transition period between convergent and transform boundaries along the North American plate margin. Significant slip along the San Andreas transform postdates this extensional event. Its origin apparently coincided with a regional middle Miocene uplift event, indicating that the San Andreas fault is younger than previously supposed. The Tertiary SSJB has subsided due to extension/transtension, crustal tilting, and thrust‐related loading. The Maricopa Subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the Sierra Nevada and locally has subsided beyond 12 km. In contrast, the relatively stable Tejon embayment, underlain by Sierran crystalline crust, achieved its maximum subsidence in the Miocene. Since Pliocene time, the SSJB has filled and continued to deepen after the basin was tectonically shut off from the Pacific Ocean.
Temporal variations in latest Quaternary slip across the Australian‐Pacific Plate Boundary, northeastern South Island, New Zealand
Authors: Knuepfer Peter L. K.
Published: February 2008

Rates of latest Quaternary slip from stream terraces and moraines displaced by faults of the Alpine shear system in NE South Island, New Zealand, vary in space and time. Detailed histories of fault slip are obtained from combining displacement data with estimates of the age of the surface from weathering characteristics. Precision in surface ages is 5–20% using rock weathering rinds and 15–50% using soil properties. The oldest surfaces examined are 15–20 ka and have right‐lateral fault offsets up to 400–600 m. The main faults of the in the NE South Island (the Wairau, Awatere, Clarence, Hope, Kekerengu, and Porters Pass faults) have average latest Quaternary right‐lateral slip rates of 3–10, 5–10, 7–10, 25–40, 5–7, and 3–4 mm/yr respectively. Every fault has undergone a substantial decrease in lateral slip rate in the last 3–5 kyr. Summed across the plate boundary, the average latest Quaternary slip rates are comparable to long‐term rates of horizontal slip across the Australian‐Pacific plate boundary (around 40 mm/yr parallel to the boundary) and rates of geodetic strain and seismic moment release over the last 50–100 years (approximately the same). However, sums of lateral fault‐slip rates over the interval from 15 to 5 ka exceed the plate motions, whereas late Holocene lateral fault‐slip rates are less than half the long‐term average. The best explanation of these variations is slip across the plate boundary is episodic, varying over perhaps 5‐kyr intervals. This implies that 15–20 kyr is the time interval necessary to average out shorter, 5‐kyr episodic variations in plate boundary motions.
Role of melt in the formation of a deep‐crustal compressive shear zone: The MaClaren Glacier Metamorphic Belt, south central Alaska
Authors: Davidson Cameron , Hollister L. S. , Schmid S. M.
Published: February 2008

The Maclaren Glacier metamorphic belt is an exhumed portion of a deep‐crustal shear zone where hot, upper amphibolite facies rocks were emplaced over cooler, lower grade rocks. It is located in south central Alaska within the collision zone between terranes previously accreted to North America and the Wrangellia superterrane. Crosscutting relationships and orientations of thin granitoid sills within the hanging wall show that melt was repeatedly intruded into the shear zone during overall compression. In addition, a 1 km thick tonalite sill (the Valdez Creek tonalite) was emplaced into the shear zone while it was active; this conclusion is based on the presence of a well‐developed foliation within the sill which is concordant to the fabric of the shear zone, the alignment and tiling of plagioclase laths, the presence of highly strained mafic enclaves within a matrix which shows no evidence of any subsolidus deformation, and the preservation of a delicate magmatic texture which indicates that a magmatic fluid was still present after deformation in the sill had ceased. A one‐dimensional thermal model for sill‐shaped plutons of varying thicknesses, which are emplaced into country rocks at different initial temperatures, indicates that melt can be present for long periods of time in the deep crust (>1 m.y.). An important factor controlling the length of time that a sill remains molten is the temperature of the surrounding rocks which, in the lower crust, can be as high as the solidus temperature of granitoid melts. If the amount of time for melt to crystallize is sufficiently long, potentially large amounts of strain will be accommodated by zones containing melt due to the low strength of melt compared to rock. In the Maclaren Glacier metamorphic belt, the amount of time needed for the Valdez Creek tonalite to fully crystallize is calculated to be about 90,000 years. This is a minimum value since the original thickness of the sill is not known. Nevertheless, using the 90,000 year value and assuming that the bulk of convergence between North America and Wrangellia was concentrated within the Valdez Creek tonalite, a displacement of at least 10 km could have been accommodated across the sill while melt was present.
A dynamic model of extension in the Baltimore Canyon Trough Region
Authors: Harry Dennis L. , Sawyer Dale S.
Published: February 2008

A dynamic model of extension in the Baltimore Canyon trough region of the central North Atlantic indicates that rifting proceeded in two stages and was controlled by laterally offset preexisting weaknesses within the middle crust and upper mantle beneath the Appalachian orogen. The crustal weakness is hypothesized to result from thin‐skinned thrust faulting in the Appalachian foreland, and the upper mantle weakness results from crustal thickening beneath the Appalachian hinterland. During the early stages of rifting (225–185 Ma), the loci of extension in the crust and mantle lie within the respective preweakened regions and are connected by a shear zone in the lower crust in a manner similar to whole lithosphere simple shear. Extension and thinning of the upper crust at this time results in the formation of Late Triassic‐Early Jurassic rift basins in the Appalachian Piedmont and in the development of a horizontal pressure gradient which drives widespread ductile flow in the lower crust. The pressure gradient may also have influenced magma transport, moving melt generated beneath the eastern Baltimore Canyon trough into the more westerly Triassic basin province. During the late stages of rifting (185–175 Ma), extension in the crust shifts to a position overlying the preexisting upper mantle weakness as a result of thermal weakening by the upwelling asthenosphere in this region. The loci of extension in the crust and mantle are then coincident, and the style of rifting becomes similar to whole lithosphere pure shear. The shift in the locus of crustal extension is accompanied by the cessation of subsidence and volcanism in the Triassic basins, the onset of rapid subsidence in the Baltimore Canyon trough, and the formation of a basement hinge zone separating the highly thinned crust beneath the Baltimore Canyon trough from moderately thinned crust beneath the Triassic basin province. Extension becomes increasingly focused beneath the eastern edge of the Baltimore Canyon trough until the onset of seafloor spreading, approximately 50 m.y. after rifting began. The resulting conjugate margins are asymmetric, with most of the region encompassing the Triassic basins stranded on the North American plate.
The Northeast Brazilian Rift System
Authors: de Matos Renato M. Darros
Published: February 2008

The northeast Brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the South Atlantic. These basins represent the locus of intersection of the Southern and Equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases. Similar data are not found in its counterpart in Africa, especially for the Neocomian. These Early Cretaceous rift basins of northeast Brazil illustrate key three‐dimensional geometries of intracontinental rift systems, mainly controlled by the basement structural framework. During the main rift phase (Syn‐rift II, Neocomian‐early Barremian) extensional deformation was distributed over three main rift axes: (1) the Gabon‐Sergipe Alagoas (GSA) trend, (2) the Recôncavo‐Tucano‐Jatobá (RTJ) trend and (3) the Cariri‐Potiguar (CP) trend. During this phase, extensional deformation jumped west from the easternmost basins (GSA trend) to a series of NE trending intracratonic basins (RTJ and CP trends), characterized by a set of asymmetric half grabens separated by basement highs, transfer faults, and/or accommodation zones. These basins are typically a few tens of kilometers wide and trend NE‐SW, roughly perpendicular to the main extension direction during the Neocomian. Preexisting upper crustal weakness zones, like the dominantly NE‐SW trending shear zones of the Brazilian/Pan‐African orogeny, controlled the development of intracrustal listric normal faults. Internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework. The megashear zones of Pernambuco (Brazil) and Ngaundere (Africa) seem to have behaved like a huge accommodation zone, accommodating extensional deformation along the RTJ/GSA trends with simultaneous extension along the CP trend. During the late Barremian (Syn‐rift phase III), a significant change in rifting kinematics occurred, when the CP trend was aborted and major rifting initiated at the Equatorial branch. During the Aptian, while the Equatorial branch and Benue trough (Africa) experienced the main rift phase, the RTJ trend was aborted and the GSA trend developed a transitional phase between the rift and drift stage. The GSA trend and the offshore Potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode.
Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO2 levels
Authors: Freeman Katherine H. , Hayes J. M.
Published: February 2008

Reports of the 13C content of marine particulate organic carbon are compiled and on the basis of GEOSECS data and temperatures, concentrations, and isotopic compositions of dissolved CO2 in the waters in which the related phytoplankton grew are estimated. In this way, the fractionation of carbon isotopes during photosynthetic fixation of CO2 is found to be significantly correlated with concentrations of dissolved CO2. Because ancient carbon isotopic fractionations have been determined from analyses of sedimentary porphyrins [Popp et al., 1989], the relationship between isotopic fractionation and concentrations of dissolved CO2 developed here can be employed to estimate concentrations of CO2 dissolved in ancient oceans and, in turn, partial pressures of CO2 in ancient atmospheres. The calculations take into account the temperature dependence of chemical and isotopic equilibria in the dissolved‐inorganic‐carbon system and of air‐sea equilibria. Paleoenvironmental temperatures for each sample are estimated from re‐constructions of paleogeography, latitudinal temper‐ ature gradients, and secular changes in low‐latitude sea surface temperature. It is estimated that atmospheric partial pressures of CO2 were over 1000 μatm 160–100 Ma ago, then declined to values near 300 μatm during the next 100 Ma. Analysis of a high‐resolution record of carbon isotopic fractionation at the Cenomanian‐Turonian boundary suggests that the partial pressure of CO2 in the atmosphere was drawn down from values near 840 μatm to values near 700 μatm during the anoxic event.
Interactions between carbon and nitrogen dynamics in estimating net primary productivity for potential vegetation in North America
Authors: McGuire A. D. , Melillo J. M. , Joyce L. A. , Kicklighter D. W. , Grace A. L. , Moore B. , Vorosmarty C. J.
Published: February 2008

We use the terrestrial ecosystem model (TEM), a process‐based model, to investigate how interactions between carbon (C) and nitrogen (N) dynamics affect predictions of net primary productivity (NPP) for potential vegetation in North America. Data on pool sizes and fluxes of C and N from intensively studied field sites are used to calibrate the model for each of 17 non‐wetland vegetation types. We use information on climate, soils, and vegetation to make estimates for each of 11,299 non‐wetland, 0.5° latitude × 0.5° longitude, grid cells in North America. The potential annual NPP and net N mineralization (NETNMIN) of North America are estimated to be 7.032 × 1015 g C yr−1 and 104.6 × 1012 g N yr−1, respectively. Both NPP and NETNMIN increase along gradients of increasing temperature and moisture in northern and temperate regions of the continent, respectively. Nitrogen limitation of productivity is weak in tropical forests, increasingly stronger in temperate and boreal forests, and very strong in tundra ecosystems. The degree to which productivity is limited by the availability of N also varies within ecosystems. Thus spatial resolution in estimating exchanges of C between the atmosphere and the terrestrial biosphere is improved by modeling the linkage between C and N dynamics. We also perform a factorial experiment with TEM on temperate mixed forest in North America to evaluate the importance of considering interactions between C and N dynamics in the response of NPP to an elevated temperature of 2°C. With the C cycle uncoupled from the N cycle, NPP decreases primarily because of higher plant respiration. However, with the C and N cycles coupled, NPP increases because productivity that is due to increased N availability more than offsets the higher costs of plant respiration. Thus, to investigate how global change will affect biosphere‐atmosphere interactions, process‐based models need to consider linkages between the C and N cycles.
Infrared emissions arising from the reactions of FAST O/O +with N2
Authors: Upschulte B. L. , Oakes D. B. , Caledonia G. E. , Blumberg W. A. M.
Published: February 2008

We have performed a laboratory investigation of the infrared emissions arising from the interaction of a fast beam of O/O+with N2 over the velocity range of 8 to 12 km/s. Emissions from the ground state vibration‐rotational bands of both NO and NO+ are observed with apparent excitation cross sections exceeding 10−17 cm2. The relationship of these observations to recent measurements of the infrared shuttle glow will be discussed.
Infrared spectral measurement of space shuttle glow
Authors: Ahmadjian M. , Jennings D. E. , Mumma M. J. , Espenak F. , Rice C. J. , Russell R. W. , Green B. D.
Published: February 2008

The U.S. Air Force and NASA successfully conducted infrared spectral measurements of the space shuttle glow during STS‐39. Preliminary analysis indicates that NO, NO+, OH, and CO produce infrared glow during quiescent orbiter conditions. During orbiter thruster firings the glow intensities in the infrared are enhanced by factors of 10X to 100X with significant changes in spectral distribution. These measurements were obtained with the Spacecraft Kinetic Infrared Test (SKIRT) payload which included a cryogenic infrared circular variable filter infrared spectrometer covering the 0.7 to 5.4 μm wavelength region. Approximately 14,000 spectra of shuttle glow, airglow, aurora, and the orbiter environment were obtained during the eight day mission. The STS‐39 Space Shuttle Discovery was launched from the NASA Kennedy Space Center on 28 April 1991 into a 57 deg inclination circular orbit at an altitude of 260 km.
The technology for wireline re‐entry of deep ocean boreholes employed for the Dianaut Program
Authors: Floury Luc , Gable Robert
Published: February 2008

More than 850 holes have been drilled in the world's oceans over the past 24 years, in connection with the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). At least 45 of these holes were fitted with a re‐entry cone and many were cased through the sedimentary and unconsolidated layers, allowing hole re‐entry with a drill string from a drilling vessel and wireline logging to the drill stem. Re‐entry of seafloor drill holes with a wireline logger positioned on the seabed was first successfully accomplished during the FARE program. The DIANAUT program was a scientific application of seafloor wireline re‐entry utilizing the logging shuttle ‘Nadia’ in combination with the manned submersible ‘Nautile’. The Nadia re‐entry logging system was successfully seated in drill re‐entry cones of three boreholes in the Atlantic Ocean (water depths of 1666, 4485, and, 4976 m) and logging operations with different probes (temperature, heat‐pulse flowmeter, BHTV, fluid sampler, magnetometer) were performed from the seafloor with full logging control and data acquisition from the Nautile. In the near future the logging shuttle ‘Nadia’ could be developed into a support facility for long‐term downhole measurements and later into an unmanned seafloor logging system.
Counter‐streaming magnetosheath ions in the dayside low latitude boundary layer
Authors: Fuselier S. A. , Klumpar D. M. , Shelley E. G.
Published: February 2008

Observations in the subsolar low latitude boundary layer (LLBL) during reconnection show counter‐streaming suprathermal ions on rare occasions. Composition measurements for one event presented here indicate that the ions are from the magnetosheath although no similar equal energy per charge suprathermal He2+ and H+ populations were observed there. Acceleration of magnetosheath ions either across the magnetopause or in the LLBL followed by magnetic mirroring of the initially parallel suprathermal ions best explains these observations.
A model of convection and corotation in Jupiter's magnetosphere: Ulysses predictions
Authors: Cheng A. F.
Published: February 2008

The Ulysses Jupiter encounter will include the first spacecraft pass through the dusk magnetosphere and will allow new tests of Jovian convection and corotation models. The Cheng and Krimigis [1989] model is extended to suggest that corotation lag occurs principally within the magnetodisk proper, which extends out to ∼60–70 RJ. It is predicted that outside this radius, in the pre‐midnight sector, spin‐up to near corotation may occur owing to a local reduction in the outward mass transport rate. The mass loss may occur mainly in the dawn sector, to the magnetospheric wind and the dawn magnetosheath.
The large normal‐faulting Mariana Earthquake of April 5, 1990 in uncoupled subduction zone
Authors: Yoshida Yasuhiro , Satake Kenji , Abe Katsuyuki
Published: February 2008

A large, Ms = 7.5, shallow earthquake occurred beneath the Mariana trench on April 5, 1990. From the relocated aftershock distribution, the fault area is estimated to be 70 × 40 km2. A tsunami observed on the Japanese islands verifies that the depth of the main shock is shallow. For waveform analysis, we use long‐period surface waves and body waves recorded at global networks of GDSN, IRIS, GEOSCOPE and ERIOS. The centroid moment tensor (CMT) solution from surface waves indicates normal faulting on a fault whose strike is parallel to the local axis of the Mariana trench, with the tension axis perpendicular to it. The seismic moment is 1.4 × 1020 Nm (× 1027 dyn.cm) which gives Mw = 7.3. Far‐field P and SH waves from 13 stations are used to determine the source time function. Since the sea around the epicentral region is about 5 km deep, body waveforms are contaminated with water reverberations. The inversion results in a source time function with a predominantly single event with a duration of 10 sec, a seismic moment of 2.1 × 1020 Nm, and a focal mechanism given by strike = 198°, dip = 48°, slip = 90°. The short duration indicates a small area of the rupture. The location of the main shock with respect to the aftershock area suggests that the nodal plane dipping to the west is preferred for the fault plane. The local stress drop of the single subevent is estimated to be 150 MPa (1.5 Kbars). The Mariana earthquake is considered to have occurred in an uncoupled region, in response to the gravitational pull caused by the downgoing Pacific plate.
A simple fracture‐mechanical model for the evolution of seismicity
Authors: Henderson Jeremy , Main Ian
Published: February 2008

A simple model is presented in which a fault is represented by a one‐dimensional array of elements to which a remote stress is applied. The fracture toughnesses of the fault elements are initially generated by a random walk function, but the fracture system quickly becomes organized into a fractal set characterised by a (box‐counting) fractal dimension. The slope, b, of the frequency‐magnitude distribution and the fractal dimension of the crack population are measured at each step, and found to vary in a similar manner to the seismic b‐value and fractal dimension of earthquake epicentres. The model also predicts the observed variation of b‐value of acoustic emissions in laboratory experiments.
The atomic oxygen 3P → 1D electron‐excitation cross section near threshold
Authors: Doering J. P.
Published: February 2008

Preliminary results of a new measurement of the differential electron excitation cross section of the O I (3P→ 1D) transition near threshold confirm the presence of a large peak in the inelastic cross section at 5 to 6 eV. The new results are compared with previous measurements and theoretical calculations. The near‐threshold shape of the O I (3P→ 1D) cross section has interesting implications for the aeronomy of the thermosphere.
Plasma heating of Io's atmosphere
Authors: Pospieszalska M. K. , Johnson R. E.
Published: February 2008

A Monte‐Carlo, molecule‐tracking program was constructed to describe the structure of Io's atmosphere in the region penetrated by ions from the plasma torus. This region is shown to exhibit high temperatures, consistent with corona observations, independent of whether significant UV heating also occurs. The atmospheric structure is determined near the exobase, which is the region responsible for the supply of the Io torus.
the CIV processes in the CRIT experiments
Authors: Papadopoulos K.
Published: February 2008

Recent in situ observations during Critical Ionization Velocity (CIV) experiments under the CRIT project produced puzzling results, apparently at odds with current CIV theories. It is shown that incorporation in the theory of lower hybrid wave (LHW) instabilities of strong turbulence and finite size effects results in reconciliation of the theory with the observations.
Geometry and state of stress of the downgoing Cocos Plate in the Isthmus of Tehuantepec, Mexico
Authors: Ponce Lautaro , Gaulon Roland , Suárez Gerardo , Lomas Elías
Published: February 2008

The seismicity in the Isthmus of Tehuantepec, Mexico is anomalously active compared with other parts of the Middle American subduction zone. The results of a microearthquake study conducted in this region show that the slab changes dip from an almost subhorizontal geometry west of longitude 96° W, to a dip of about 45° to 50° to the east of this longitude. The change in dip occurs gradually in a broad flexure of the subducting slab that takes place over 150 km. This geometry of the slab is reminiscent of that in southern Peru; the change in dip, however, takes place more gently in Tehuantepec. In addition to the change in the dip of the subducted Cocos plate, a rapid increase is observed in the maximum depth extent of the subducted slab. In the region of subhorizontal subduction, focal depths are consistently shallower than about 80 km, whereas earthquakes as deep as 200 km are found where the subducted slab dips at a steeper angle. Both the change in the dip of the slab and of the maximum depth of intermediate‐depth earthquakes appear to correlate with the presence of the Tehuantepec Fracture Zone which separates two distinct provinces of the oceanic Cocos plate. The age and the crustal thickness of the Cocos plate increase to the southeast of the Tehuantepec Fracture Zone relative to its northwestern counterpart. In Tehuantepec, the dip and maximum depth of the subducted slab appears to be correlated with the age of the oceanic plate, as has been observed in other subduction zones of the world.