bluuebunny/crossref_metadata_2025 huggingface dataset:
Mid‐Cretaceous extensional fragmentation of a Jurassic‐Early Cretaceous Compressional Orogen, Alaska
Authors: Miller Elizabeth L. , Hudson Travis L.
Published: February 2008

Several key geologic relations of interior and northern Alaska suggest that the hinterland of the Brooks Range thrust belt, which formed part of a major Middle Jurassic to earliest Cretaceous compressional orogen, underwent significant regional extension in the mid‐Cretaceous. These geologic relations include (1) simultaneous development of depositional basins in both foreland and hinterland of the Brooks Range fold and thrust belt, (2) regional normal faults along the “root zone” for Brooks Range allochthons, (3) anomalously thin (10–20 km) crust in the hinterland of the Brooks Range developed as a consequence of extension, (4) missing source regions for arc‐derived clastic rocks of the foreland, perhaps removed by rifting, (5) preservation of high pressure/low temperature metamorphic rocks in the hinterland, possibly resulting from extensional denudation and rapid uplift, (6) omission rather than duplication of structural section along many regional terrane boundaries in the hinterland, (7) shallow dipping metamorphic foliations, stretching lineations, and high‐temperature cooling histories consistent with normal sense displacement of overlying allochthonous rocks, (8) widespread crustal anatexis in parts of the hinterland, and (9) emplacement of rift‐related alkaline and associated igneous rocks in the Yukon‐Koyukuk basin. Consideration of the amount of crustal thinning, tectonic denudation, and throw on normal faults implied by the above relations indicates that mid‐Cretaceous events could have been responsible for at least 100% stretching (ß=2) across most of interior Alaska. Evaluation of timing constraints suggests that extensional tectonism in interior Alaska spanned an interval from about 130 to 90 Ma. Review of known geologic relations indicates that extension postdated an important episode of crustal shortening in the Brooks Range related to closure of an ocean basin (now represented by the Angayucham terrane) during the approach and collision of a Jurassic and older arc system with the continental margin. Extension in interior Alaska was coeval with the inception of rifting and with rift‐related magmatism in the Canadian Arctic and likely occurred during the opening of the Canada Basin. This important period of extensional deformation fragmented the various components of an earlier Jurassic to earliest Cretaceous compressional orogen in Alaska, broadening the cross‐sectional width of the Cordilleran orogen at this latitude and drastically modifiying paleogeography with the formation of large marine basins in the Cretaceous. As such, this episode of extension has important implications for our understanding of the paleogeography and tectonic setting of earlier crustal shortening and our understanding of the tectonic and thermal controls on subsequent Cretaceous sedimentation across Alaska. This extension also plays a critical role in larger‐scale plate tectonic problems such as the pre‐Cretaceous reconstruction of the Arctic and opening of the Canada Basin.
Structural development of an Archean Orogen, Western Point Lake, Northwest Territories
Authors: Kusky Timothy M.
Published: February 2008

The Point Lake orogen in the central Archean Slave Province of northwestern Canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites. Fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening. In the western part of the orogen the basal decollement places mafic plutonic/volcanic rocks over an ancient tonalitic gneiss complex. Ten kilometers to the east in the Keskarrah Bay area, slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins. Conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel, levee, and overbank facies of this thrust‐related sedimentary fan system. The synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation, while other thrusts related to hinterland‐progressing imbrication displace this unconformity. Tightening of synorogenic depositional troughs resulted in the conglomerates' present localization in synclines to the west of associated thrust faults and steepening of structural dips. Eastern parts of the orogen consist of isoclinally folded graywackes composed largely of Mutti and Ricci‐Lucchi turbidite facies B, C, and D, interpreted as submarine fan deposits eroded from a distant volcanic arc. Thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter‐scale duplex structures, and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances. The sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms. Conditions of deformation along the thrusts parallel the regional metamorphic zonation: amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic crush zones in the region of emergent thrusts in Keskarrah Bay. Depth differences can account for only half of the metamorphic gradient; thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients. A tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen.
Structure of the collision zone between Bougainville Guyot and the accretionary wedge of the New Hebrides Island Arc, southwest Pacific
Authors: Fisher Michael A. , Collot Jean‐Yves , Geist Eric L.
Published: February 2008

Multichannel seismic reflection data show the structure that develops within an island arc‐guyot collision zone. The Bougainville guyot fills the New Hebrides trench, stands about 3 km above the abyssal ocean plain, and is capped by a broad platform that is underlain by a parallel bedded sequence, probably lagoon and reef rocks. A thick debris apron made up of unconsolidated sediment underlies the unsubducted part of the lower guyot flank and possibly the subducted flank as well. The contact zone between the arc and the north and east sides of the guyot is marked by discontinuous antiforms that include reflective, thrust faulted rocks that may have been derived from the guyot. The extent of collision deformation to the arc and guyot depends in part on the contrast in compressibility and viscosity between these features. We propose that the high‐drag, subcircular guyot evolves during collision into a more streamlined shape. Streamlining may be achieved by processes like: (1) bulk deformation of the guyot by thrust faulting and sediment compaction; (2) ripping away of projecting parts of the guyot and blunting sharp edges (tectonic erosion), like that formed by the limestone cap; and (3) formation of a boundary layer, between the guyot and the arc, that is made up of water‐rich, mobilized units of soft sediment. We draw an analogy between some features of glacial origin and the subducted part of a guyot. If the bulk of the guyot is low strength, especially in view of the thick debris apron, then the guyot may behave like till that was overridden by a glacier and forms a drumlin, an elongated, blunt‐faced, streamlined feature. Thrust faults evident in the guyot may indicate deformation that leads toward a streamlined shape and not necessarily to accretion of guyot rocks.
Late Proterozoic tectonic model for the Avalon Terrane in Maritime Canada
Authors: Keppie J. Duncan , Dostal J.
Published: February 2008

The Avalon Terrane in Maritime Canada extends from Cape Breton Island through northern Nova Scotia (Antigonish and Cobequid highlands) to southern New Brunswick. The terrane contains abundant late Proterozoic volcanic rocks (∼570–680 m.y. old). The geochemistry of the volcanic rocks of southern Cape Breton Island and southern New Brunswick indicates that they were erupted in a volcanic arc environment. In both regions the mafic rocks display a remarkably similar compositional zonation which resembles the across‐arc variations observed in modern volcanic arcs. The progressive compositional changes that include a transition from island arc tholeiites along the southeastern coasts to calc‐alkaline rocks inland to the northwest are probably due to a northwest dipping subduction zone with a trench located to the southeast. In the Antigonish and Cobequid highlands, continental tholeiites and calc‐alkaline volcanic rocks are interbedded and overlain by turbidites that have probably been deposited in a back arc or intra‐arc environment. The superimposed positive gravity and magnetic anomalies that may be traced discontinuously from the Grand Banks through Nova Scotia (Collector Anomaly) and along the northern side of the Gulf of Maine to near Boston, probably represent late Proterozoic ophiolitic remnants preserved by incomplete suturing along a late Proterozoic plate boundary. Northeast trending segments are inferred to mark subduction zones, and the E‐W portions could correspond to transform faults. Thus the present geographical distribution of the Late Proterozoic volcanic rocks is explained by a tectonic model involving a single subduction zone offset by two late Proterozoic transform faults located approximately along the Minas Fault and the Scatarie Ridge.
Collision propagation in Papua New Guinea and the Solomon Sea
Authors: Silver Eli A. , Abbott Lon D. , Kirchoff‐Stein Kimberly S. , Reed Donald L. , Bernstein‐Taylor Barrie , Hilyard David
Published: February 2008

The mountain system in Papua New Guinea (PNG) is growing by accretion of Australian margin strata to the front of the Papuan fold‐and‐thrust belt in the south and by accretion of exotic terranes along its NE margin. Presently, the Finisterre Terrane is being accreted along the NE margin of PNG, and the collision point is migrating eastward within the Solomon Sea. East of the collision point the New Britain Trench marks the tectonic front between the Solomon Sea and South Bismarck Sea plates, whereas to the west the tectonic front lies in the southern part of the Papuan fold‐and‐thrust belt. Present‐day sedimentary environments within the western Solomon Sea are inferred from interpretation of seismic reflection data. Similar facies are observed accreted to the front of the Finisterre Terrane in the Markham valley. The frontal Leron Formation is largely fluvial and shallow marine, and the Erap Complex contains elements of turbidites and pelagic deposits. There appears to be a smaller volume of coarse, proximal marine strata accreted to the front of the Finisterre Terrane than is expected from interpretation of reflection profiles in the western Solomon Sea. This difference may be attributed to a combination of submarine erosion in the Markham canyon, partial incorporation of these deposits into the Erap Complex, and a difference in depositional rates and products carried to the Solomon Sea in the past relative to what is observed today. Based on plate motions, alternative collision geometries, and the youngest dates available for the Erap Complex, we determine a rate of progression of the collision point of about 212 km/m.y. during the past 1 m.y.
Pre‐Mid‐Mesozoic tectonic evolution of the Yukon‐Tanana Terrane, Yukon and Alaska
Authors: Mortensen J. K.
Published: February 2008

Yukon‐Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid‐Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian‐Early Mississippian, mid‐Permian, and Late Triassic‐Early Jurassic time. From Late Devonian to mid‐Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid‐Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid‐Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent‐continent collision. Although the Triassic‐Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other “pericratonic” terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed by transcurrent faulting.
Structure of Tertiary and Pleistocene normal faults in Iceland
Authors: Forslund Trond , Gudmundsson Agust
Published: February 2008

Normal faults were measured in three selected areas of the late Tertiary and Pleistocene lava pile in Iceland. The arithmetic mean throw of 315 faults is 8.2 m, the minimum being 0.5 m and the maximum 150 m. About 93% of the faults have throws of less than 20 m. The mean strike of the faults is N31°E, but the mode is N45°E. The faults dip from 42° to 89°, nearly 80% dip between 65° and 79°, and the mean dip is 73°. Fault breccia is common and varies in thickness from 0.05 m to 2 m. There is a positive linear relation between breccia thickness and throw, the linear correlation coefficient being 0.78. It is proposed that the normal faults in Iceland form by two basic mechanisms. One is the development of faults from sets of inclined joints in the lava pile. The initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1° for every 150–170 m depth. The inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints. The other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures. The great majority of fractures within the active rift zone are tension fractures, but when they attain the length of several hundred meters they normally change into normal faults. It is concluded that normal fault development in Iceland commonly starts at depths of about 1 km in the crust.
Thrust tectonics of the Binalood Region, NE Iran
Authors: Alavi Mehdi
Published: February 2008

The Binalood region in NE Iran, which is an eastern segment of the Alborz Mountains, is affected by thin‐skinned tectonics. Lower Paleozoic, Jurassic, Cretaceous, and Cenozoic rock sequences as well as remnants of the Paleo‐Tethys oceanic realm are transported in a southwestward direction by several thrust faults which are of two generations: (1) Cimmerian and (2) Alpine. Structural units in this region form either single thrust sheets or duplexes. They have been displaced sequentially in a piggyback style from the hinterland (NE) to foreland (SW) under ductile, brittle‐ductile, and brittle conditions. Geometrically, they constitute an antiformal stack breached on its southwestern side by major and minor out‐of‐sequence thrusts.
Lithoprobe crustal reflection cross section of the southern Canadian Cordillera, 1, Foreland thrust and fold belt to Fraser River Fault
Authors: Cook Frederick A. , Varsek John L. , Clowes Ronald M. , Kanasewich Ernest R. , Spencer Carl S. , Parrish Randall R. , Brown Richard L. , Carr Sharon D. , Johnson Bradford J. , Price Raymond A.
Published: February 2008

Seismic reflection data from the south central Canadian Cordillera covering the interval from the easternmost metamorphic core complexes near Arrow Lakes to the Fraser River fault system along the Fraser River reveal a highly reflective and complex crust. The base of the crustal reflectivity, interpreted as the reflection Moho, is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12.0 s in the east to about 10.5 s in the west. This virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief. Some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust. Even though part of this area was subjected to large amounts of Eocene extension, the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the U.S. Cordillera. Three structural culminations, the Monashee complex, the Vernon antiform, and the Central Nicola horst, are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during Jurassic to Eocene compression and then to have been modified and exposed during early and middle Eocene extension. An example of a compressional structure observed on the profiles is the Monashee decollement, which can be traced from the surface westward into the lower crust. Extension is manifested along a variety of normal faults, including the regionally extensive low angle Okanagan Valley‐Eagle River fault system, moderately dipping faults such as the Columbia River and Slocan Lake faults, and high‐angle faults such as the Quilchena Creek and Coldwater faults. Both Jurassic to Eocene compressional shear zones and early to middle Eocene extensional shear zones are listric into the lower crust or Moho under the Intermontane belt.
Strain Partitioning between structural domains in the forearc of the Hikurangi Subduction Zone, New Zealand
Authors: Cashman Susan M. , Kelsey Harvey M. , Erdman Craig F. , Cutten Huntly N. C. , Berryman Kelvin R.
Published: February 2008

The Pacific plate obliquely converges with the Australian plate at latitude 39°50′S along the Hikurangi margin off the east coast of the North Island of New Zealand. An extensive and youthful subaerially exposed forearc on the east coast of the North Island in the Hawke's Bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting. Geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension, contraction, and strike‐slip. The domains are elongate and trend parallel to the margin. Measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface. Deformation rates calculated for the past 1–2 m.y. for structures in all six forearc domains account for 50–70% of the margin‐parallel motion required by Pacific‐Australian plate convergence and about 6% of the plate motion perpendicular to the plate boundary. At the surface in the forearc, this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike‐slip fault zone and an accompanying contractional fault‐and‐fold zone on the trenchward side. There are two such transcurrent faulting‐and‐contraction couplets, one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin. The small amount of shortening, relative to strike‐slip, in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the subduction zone megathrust.
Distinct element modeling of structures formed in sedimentary overburden by extensional reactivation of basement normal faults
Authors: Saltzer Sarah D. , Pollard David D.
Published: February 2008

Fault and fold patterns in sedimentary rocks have been modeled using a computer code based on the distinct element method (DEM). This numerical method considers the relative motion of disconnected elastic elements governed by Newton's laws and frictional sliding criteria. Forces are transmitted between elements at contact points. By changing element size ratios, assemblages of elements can simulate different kinematic behavior from localized faulting to distributed flow. As an application of the method, we investigate folding and faulting patterns that develop in the sedimentary overburden above basement normal faults. The number, position, orientation, and sense of slip on faults as well as the fold geometry depend on the basement fault dip and basement rotation. The DEM model results have practical implications for predicting the evolving characteristics of structural traps and their petroleum potential.
Geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing: Example from the foreland thrust system of the southern Bolivian Subandean Zone
Authors: Baby Patrice , Hérail Gérard , Salinas René , Sempere Thierry
Published: February 2008

The Subandean Zone of Bolivia is a foreland fold and thrust belt which forms the eastern edge of the central Andes mountains. Between 19°S and 22°S latitude, the construction of five balanced cross sections shows that the N‐S trending Subandean Zone is characterized by the existence of passive roof duplexes. These complex structures can be distinguished by the lithotectonic unit within which duplexing occurs. The five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes. The sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes. Apparently, these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units. While a passive roof duplex was developing, the sole thrust stuck and the major horizontal displacements were then transferred either to out‐of‐sequence thrusts or to a new sole thrust, in a deeper detachment horizon. Therefore each passive roof duplex would correspond to the orogenic front of the Andean range at one very particular time in the history of the Subandean Zone of southern Bolivia. From south to north, the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front. Available data do not permit us to explain completely this phenomenom.
Use of carbon isotopes in paleosols as an indicator of the P(CO2) of the paleoatmosphere
Authors: Cerling Thure E.
Published: February 2008

Measurements of carbon isotopic composition of coexisting paleosol organic matter and carbonate results in improved estimates of the paleo‐P(CO2) content of the paleoatmosphere. Measurements from Tertiary paleosols indicate that the levels of CO2 in the early Eocene, middle and late Miocene, and Pliocene were less than 1000 ppmV. Early Cretaceous levels of P(CO2) were significantly higher, of the order of 1500 to 3000 ppmV.
Iron fertilization of the austral ocean—The Hamburg Model Assessment
Authors: Kurz Katharina D. , Maier‐Reimer Ernst
Published: February 2008

We have investigated the effect of an enhanced biological productivity, caused by an artificial iron fertilization, in the southern ocean with a three‐dimensional carbon cycle model, which is based on an ocean circulation obtained by an ocean general circulation model. The increase of the biological production did not result in a substantial increase of CO2 uptake by the ocean, as it has been proposed by Martin et al. (1990). We have found that—assuming that the basic structure of the particulate downward flux does not change when higher production occurs—the ocean circulation provides a higher return flux of remineralized products that compensates two thirds of the effect of the higher export production on the atmospheric pCO2. This finding is only slightly modified when assuming a much deeper penetration of particle flux due to the fertilization. Owing to the nonlinearity of the buffer factor the absolute value of CO2 reduction in the atmosphere is higher at high levels of pCO2. In view of the logarithmic dependency of the radiative forcing on CO2, the fertilization is more efficient at low CO2 levels. With our carbon cycle model we have, in essence, confirmed earlier three‐dimensional model studies with a simplified geochemical cycling of the Princeton group (Sarmiento and Orr, 1991).
Analysis of the borehole televiewer log from DSDP hole 395A: Results from the Dianaut Program
Authors: Morin Roger H. , Moos Daniel , Hess Alfred E.
Published: February 2008

The French wireline re‐entry expedition known as DIANAUT culminated in the successful completion of a series of downhole measurements in three Deep Sea Drilling Project (DSDP) holes in the north‐central Atlantic Ocean. Among these measurements was a borehole televiewer (BHTV) log obtained in Hole 395A from bottom of casing at 112 meters below seafloor (mbsf) to a depth of 605 mbsf. In contrast with a BHTV survey previously conducted in this well during DSDP Leg 78B in 1981, televiewer data were not degraded by random cable oscillations due to ship heave and the quality of this log is excellent. These latest data were digitized and processed in terms of acoustic travel time and amplitude to compute hole size and shape, evaluate the structural integrity of the surrounding rock, and characterize intersecting fractures. The acoustic caliper and reflectivity logs correlate well with the lithologic column, particularly at the boundaries between major units which often are marked by breccias. Fractures in the lower part of the hole appear to be effectively sealed, in contrast with the open fractures identified in the upper sections. This supports a systematic pattern derived from complementary geophysical logs of increasing density, electrical resistivity, and elastic‐wave velocity, and decreasing in situ permeability as a function of depth. Fractures intersecting the well exhibit a wide range of orientations marked by some clustering of dip azimuths approximately east‐west. The presence of moderately dipping fractures striking subparallel to the ridge axis is consistent with both ridge‐parallel topographic elongations observed in this area and with azimuthal seismic anisotropy reported elsewhere, suggesting that fractures such as those identified from the BHTV record contribute to these effects.
The “Minihole” event on 6. Feb. 1990: Influence of Mie‐scattering on the evaluation of spectroscopic measurements
Authors: Fiedler M. , Frank H. , Gomer T. , Hausmann M. , Pfeilsticker K. , Platt U.
Published: February 2008

Groundbased differential optical absorption spectroscopic measurements of stratospheric trace gases are sensitive to stratospheric aerosol (PSC) layers as measured at Feb. 6, 1990, over northern Scandinavia. Mie extinction alters the intensity distribution of zenith scattered sunlight, in an extreme case presented here this can lead to 30%–40% changes of the airmass factor (AMF) for ozone. NO2 and OClO AMFs change less, O2 AMFs change dramatically. Ozone measurements at Kiruna, Sweden, for the time Feb. 4–8, 1990 are reported. No significant change in the O3 total column was observed during that period, when the effect of PSC's on the AMF was taken into account.
Groundbased spectroscopic measurements of stratospheric NO2 and OClO in Arctic winter 1989/90
Authors: Fiedler M. , Frank H. , Gomer T. , Hausmann M. , Pfeilsticker K. , Platt U.
Published: February 2008

During January and February 1990 groundbased UV/visible spectroscopic observations of direct moon light and zenith scattered sun light were performed near Kiruna, Sweden at 67.9°N, 21.1°E. From the measured differential absorption of NO2 and OClO the corresponding slant column abundances were determined and converted to vertical column densities using airmass factors evaluated by a ray tracing model. Upper limits for the stratospheric NO2 column of 0.4–0.9 · 1015/cm² were deduced and elevated levels of OClO could be detected during Feb. 6–8.
electron drift echoes in the inner magnetosphere
Authors: Vampola A. L. , Korth A.
Published: February 2008

A magnetospheric shock event on 24 March 1991 produced major effects in the energetic particle population in the inner magnetosphere. Electron drift echoes were observed in the range of L=2.6 to 2.1 by instrumentation on the CRRES satellite. It is inferred from the drift rate that the energy of the electrons which were observed was in the range of 12 to 25 MeV and perhaps higher.
Crust and upper mantle tomography in Tibet using surface waves
Authors: Bourjot Laurence , Romanowicz Barbara
Published: February 2008

We have obtained maps of lateral variations of structure in the crust and lithosphere in Tibet and surrounding regions by inversion of single station measurements of fundamental mode Rayleigh wave phase velocities in the period range 25 ‐ 100 sec, for earthquakes located in central Asia and observed at teleseismic digital stations in Eurasia. Phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries. The lateral variations of S velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the Chang Tang region (north central Tibet). This zone is bounded by rather steep velocity gradients to the south and east, about 250 and 400 km north and west of the frontal thrusts of the Himalayas and Lungmen Shan, respectively. The northern boundary of the zone crosses the northern limit of the Tibet plateau (Altyn Tagh and Kunlun) extending into the southwest Tarim and Qaidam basins. The low crustal velocities are consistent with a thick, hot crust in the central part of Tibet, consistent with Quaternary volcanism in Chang Tang. The steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south, and suggest that a similar situation might exist in the north.
Hybrid simulations of protons strongly accelerated by a parallel collisionless shock
Authors: Giacalone J. , Burgess D. , Schwartz S. J. , Ellison Donald C.
Published: February 2008

We present initial results from one‐dimensional hybrid simulations which directly address the problems of using such methods to simulate the acceleration of ions to high energy by parallel shocks. As particles are accelerated from the thermal population, they are repeatedly “split,” thereby ensuring statistically valid energy spectra covering a wide dynamic range. In order to model the complex foreshock, as expected if the simulation domain were large enough, and as seen in observations, we introduce a source of upstream turbulence. This turbulence produces an enhanced high energy tail in the upstream particle distribution extending to over a hundred times the plasma flow energy, and a prominent shoulder downstream.
A thermosphere/ionosphere general circulation model with coupled electrodynamics
Authors: Richmond A. D. , Ridley E. C. , Roble R. G.
Published: February 2008

A new simulation model of upper atmospheric dynamics is presented that includes self‐consistent electrodynamic interactions between the thermosphere and ionosphere. This model, which we call the National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamic general circulation model (NCAR/TIE‐GCM), calculates the dynamo effects of thermospheric winds, and uses the resultant electric fields and currents in calculating the neutral and plasma dynamics. A realistic geomagnetic field geometry is used. Sample simulations for solar maximum equinox conditions illustrate two previously predicted effects of the feedback. Near the magnetic equator, the afternoon uplift of the ionosphere by an eastward electric field reduces ion drag on the neutral wind, so that relatively strong eastward winds can occur in the evening. In addition, a vertical electric field is generated by the low‐latitude wind, which produces east‐west plasma drifts in the same direction as the wind, further reducing the ion drag and resulting in stronger zonal winds.
Daily estimates of the Earth's pole position with the global positioning system
Authors: Lindqwister Ulf J. , Freedman Adam P. , Blewitt Geoffrey
Published: February 2008

Daily estimates of the Earth's pole position have been obtained with measurements from a worldwide network of Global Positioning System (GPS) receivers, obtained during the three week GIG'91 experiment in January–February, 1991. For this short‐term study, the GPS based polar motion series agrees with the other space based geodetic techniques (Very Long Baseline Interferometry and Satellite Laser Ranging) to ∼0.4 mas rms, after the removal of mean biases of order 1–3 mas. The small error in day‐to‐day variability is not sensitive to the fiducial strategy used, nor are fiducial sites even necessary for monitoring high frequency pole position variability. The small biases indicate that the applied reference frames of the three geodetic techniques are nearly aligned, that the GPS fiducial errors are small, and that systematic errors in GPS are also small (of order 5 ppb). A well determined reference frame is necessary for monitoring the long‐term stability of polar motion and for separating it from other long‐term signals such as tectonic motion and internal systematic errors.
Plate motion and dragging of the upper mantle: Lateral variations of lithospheric thickness and their implications for intraplate deformation
Authors: Sabadini Roberto , Giunchi Carlo , Gasperini Paolo , Boschi Enzo
Published: February 2008

The impact of lateral variations in the thickness of the lithosphere on surface topography, horizontal intraplate deformation and stress accumulation is studied for plates that drift with respect to the highly viscous lower mantle and the transition zone. The lithosphere and upper mantle are described by a viscoelastic Maxwell rheology within the framework of a finite element scheme which allows the modeling of heterogeneous lithospheric structures in 2D vertical cross sections. The geophysical signatures are found to be extremely sensitive to lateral viscosity contrasts which interact with the upper mantle flow. This mechanism can contribute to a certain extent and in concert with the other driving forces of plate tectonics to the evolution of back‐arc basins, to the explanation of the largest angle of subduction in west‐dipping slabs and to the initiation of subduction of an oceanic lithosphere underneath a stable continental one.
Tectonic implications of the 1989 Afar Earthquake sequence
Authors: Sigmundsson Freysteinn
Published: February 2008

In August 1989 a sequence of mb 5–6 normal faulting earthquakes occurred in the area between the Ghoubbet Asal‐Manda Inakir and Manda Haroro‐Abhe Bad rifts in central Afar. Relocation of the events suggest they occurred on major faults in the Dobi graben, located midway between the principal rift zones. The earthquakes are evidence that extensional strain is still occurring in the area. The bookshelf tectonic model for central and southern Afar proposed by Tapponnier et al. [1990] is modified to account for ongoing extension in the area between the principal rifts as well as distributed shear.
Ion acceleration in cometary plasma tails
Authors: Ip W.‐H.
Published: February 2008

The curious bursts of hot ions in the inner coma of comet Halley detected by plasma instruments on the Giotto probe remain unexplained. In the present paper we discuss the possible contribution from current sheet acceleration in the near‐tail region of this comet. If the transformation of the tear‐drop shape of the magnetic field free boundary of the ionospheric contact surface to an elongated current sheet is accompanied by the formation of a magnetic neutral line, the introduction of a cross‐tail electric field could be effective in generating keV ions near the neutral line and the current sheet. The beaming of these accelerated ions towards the inner coma will likely result in the production of a secondary hot ion component with degraded energies from collisional thermalization. Such an “auroral” activity could play an important role in the energetics, ionization and dynamics of cometary ionospheres.
Early stratospheric effects of the Pinatubo Eruption
Authors: Gobbi Gian Paolo , Congeduti Fernando , Adriani Alberto
Published: February 2008

The Pinatubo eruptions of June 1991 introduced large plumes into the local stratosphere. On several occasions, volcanic gases and particles reached altitudes of about 30 km, quickly spreading to the west. Twenty days after the first eruption, the volcanic aerosol cloud was detected by lidar 14 km over Frascati, Italy. The upper portion of the cloud was observed for the first time on September 4, 1991, at an altitude of 23 km. Vertical and temporal evolution of the cloud, as observed from Frascati, are in agreement with trajectories estimated by means of northern hemisphere, stratospheric wind maps. Temperature records and characteristics of the cloud during the first 6 months following the eruption are reported. These characteristics are also compared to the ones of El Chichon, whose eruption, in 1982, rated amongst the largest in the century. This first analysis shows that, three and a half months after the eruption, the aerosol perturbation generated by Pinatubo reached and exceeded the maximum loads, recorded 11 months after the El Chichon event. However, by the end of the year, the aerosol columns of the two events tend towards comparable magnitudes.
Possible effect of the tropospheric temperature on the variations in the middle atmosphere temperature
Authors: Shibata Kiyotaka
Published: February 2008

Radiatively determined temperatures (RDTs) are calculated to investigate the possible effect of the tropospheric temperatures through terrestrial radiation on the variations in the middle atmosphere temperatures. The effect is obtained as RDT anomalies corresponding to the observed tropospheric temperatures for 12 years. The RDT anomalies relative to the observed variations are found to be less 10 % in the upper stratophere, but 20–40 % in the middle and lower stratosphere.
On chaotic conductivity in the magnetotail
Authors: Holland Daniel L. , Chen James
Published: February 2008

The concept of chaotic conductivity and the acceleration of particles due to a constant dawn dusk electric field are studied in a magnetotail‐like magnetic field. A test particle simulation is used including the full nonlinear dynamics. It is found that the acceleration process can be understood without invoking chaos and that the cross tail current is determined by the particle dynamics and distributions. We conclude that in general there is no simple relationship between the electric field and the current, i.e. J ≠ σ·E.
Viscously driven plasma flows in the deep geomagnetic tail
Authors: Owen C. J. , Slavin J. A.
Published: February 2008

We present an analysis, based on the principles of stress balance in a 1‐dimensional current sheet, which considers the problem of closed magnetic flux transport into the deep tail by a “viscous”‐like interaction between the solar wind and the magnetosphere. We illustrate our analysis with an example of ISEE‐3 data showing strong tailward plasma sheet flows on apparently closed field lines in the deep tail. Apart from narrow regions adjacent to the magnetopause, these flows are not driven by the scattering of magnetosheath plasma into the magnetosphere. We estimate the fraction of the magnetosheath momentum flux needed to be anomalously transferred into the plasma sheet to drive the flows. In our example this is ∼6%. No previously suggested mechanism (e.g., the Kelvin‐Helmholtz Instability) has been shown capable of providing anomalous momentum transport of this magnitude. Our current understanding of the “viscous” interaction between the solar wind and magnetosphere is thus insufficient to explain these observations.
Kinetic properties of mirror waves in magnetosheath plasmas
Authors: McKean M. E. , Winske D. , Gary S. P.
Published: February 2008

Linear and nonlinear properties of waves excited by the mirror instability in high beta, low anisotropy (β⟂ > β‖ ≥ 1) plasmas characteristic of the magnetosheath are investigated using linear theory and one‐dimensional hybrid simulations. The mechanisms for wave growth and saturation at low amplitudes are discussed. A new method is considered for generating the large amplitude mirror waves observed in the magnetosheath based on external compression of magnetic flux tubes. Simulations in which the anisotropy is maintained by recycling the ions shows this process can inhibit the growth of ion cyclotron waves and enhances the growth of mirror waves.
Practical geological comparison of some seafloor survey instruments
Authors: Kleinrock Martin C. , Hey R. N. , Theberge A. E.
Published: February 2008

Seafloor survey instruments are integral to the study of marine geology. Because understanding their resolution and limitations is critical, we compare how different survey systems represent the seafloor. Coincident data collected at the Galapagos propagator (GLORIA, SeaMARC II, Sea Beam, Deep‐Tow, camera sled, and Alvin) allow comparisons of how well seafloor features (e.g., faults and volcanoes) observed and characterized in high resolution data are represented in lower resolution, coarser‐scale data sets. Our reported values for the minimum sizes of detected and well‐represented features show that practical geological resolutions are generally ∼2‐10 times lower than theoretical resolutions; care must be taken in evaluating which system to use to address a particular problem.
Atmospheric lifetimes and ozone depletion potentials of methyl bromide (CH3Br) and dibromomethane (CH2Br2)
Authors: Mellouki A. , Talukdar Ranajit K. , Schmoltner Anne‐Marie , Gierczak Tomasz , Mills Michael J. , Solomon Susan , Ravishankara A. R.
Published: February 2008

The rate coefficients for the reactions of OH radical with CH3Br and CH2Br2 were measured as functions of temperature using the laser photolysis ‐ laser induced fluorescence method. This data was incorporated into a semi‐empirical model [Solomon et al., 1992] and a 2‐D model to calculate the steady ‐ state Ozone Depletion Potentials (ODP) and atmospheric lifetimes, τ, with greatly improved accuracy as compared to earlier studies. The calculated ODPs and τ are 0.65 and 1.7 years and 0.17 and 0.41 years for CH3Br and CH2Br2, respectively, using the semi‐empirical model. These lifetimes agree well with those calculated using a 2‐D model. This study better quantifies the ODPs and τ of these species which are needed inputs for discussion of possible regulation of human emissions currently under international considerations.
VLF heating of the lower ionosphere: Variation with magnetic latitude and electron density profile
Authors: Barr Richard , Stubbe Peter
Published: February 2008

Recent results on the comparative efficiencies of VLF versus HF heating of a typical nocturnal midlatitude ionosphere have been extended. The effects of varying the magnetic latitude and electron density profile of the ionosphere are investigated. Our results indicate that the superior heating efficiency of VLF vs HF radiation and the resulting perturbation on VLF propagation is a latitude‐dependent phenomenon, the effects being accentuated at low magnetic latitude and reversed at polar latitudes. The marked superiority of VLF heating is also found to be a feature of nocturnal ionospheres. VLF and HF radiation produce similar heating effects in ionospheres typical of summer daytime conditions. The optimum design of experiments for the study of the perturbation of VLF propagation by VLF heating transmitters is discussed.
A structural approach to estimate seismic stacking velocity and zero‐Offset time
Authors: Li Fu
Published: February 2008

In this paper, we present a new approach to simultaneously estimate the stacking velocity and zero‐offset time of seismic wave propagation. The new approach includes the following steps: preprocessing, structure extraction, and estimation of stacking velocity and zero‐offset time. The advantages of the proposed approach are higher resolution and less computation in comparison with the traditional semblance approach. Simulations show good performance of the new approach.
Ionospheric evidence of hot oxygen in the upper atmosphere of Venus
Authors: Mahajan K. K. , Ghosh S. , Sethi N. K. , Kohli R.
Published: February 2008

Model calculations, as well as optical observations, have shown that hot oxygen is the dominant neutral constituent up to about 3000 km in the upper atmosphere of Venus. Consequently O+ is expected to be the dominant ion up to about 3000 km in the upper ionosphere of Venus. Under normal circumstances, it is difficult to verify this result, since the solar wind interaction terminates the Venus ionosphere at altitudes below about 500 km for subsolar conditions. However, the upper boundary of the Venus ionosphere can move up to very high altitudes during some rare events when the solar wind dynamic pressure is extremely low. An analysis of Pioneer Venus measurements during such events indeed shows O+ to be the dominant ion at altitudes even as high as 2800 km. This result provides ionospheric evidence of hot oxygen in the upper atmosphere of Venus.
Measurements of dynamic topography during synop utilizing a Geosat synthetic geoid
Authors: Porter David L. , Dobson Ella B. , Glenn Scott M.
Published: February 2008

Dynamic topography estimates were obtained for the Gulf Stream region employing a previously derived synthetic geoid and Geosat radar altimeter measurements. Ascending tracks passed through the central and eastern arrays of the Synoptic Ocean Prediction experiment. By utilizing the geoid estimate instead of the mean altimetric signal, the axis position and velocity can be computed. A time series of the dynamic height profiles crossing the Gulf Stream and a table of the positions of the axis of the Stream along two tracks are given. Time series of the maximum velocity perpendicular to the ground track were computed for both arrays. It was found that there was a seasonal cycle in this time series with a maximum in November.
Submillimeterwave heterodyne measurements of stratospheric ClO, HCl, O3, and HO2: First results
Authors: Stachnik R. A. , Hardy J. C. , Tarsala J. A. , Waters J. W. , Erickson N. R.
Published: February 2008

The vertical profiles of ClO, HCl, HO2 and O3 were determined from thermal emission measurements by a balloon‐borne spectrometer operating near 640 GHz. Results are in broad agreement with previous measurements and models, but differ significantly with model photochemistry for the ratio of ClO to HCl.