bluuebunny/crossref_metadata_2025 huggingface dataset:
Eocene dextral strike‐slip and extensional faulting in the Bridge River Terrane, southwest British Columbia
Authors: Coleman Margaret E. , Parrish Randall R.
Published: February 2008

The early Mesozoic Bridge River Terrane of oceanic affinity lies in the eastern Coast Mountains of southwestern British Columbia and is one of several terranes located near the Intermontane Superterrane‐Insular Superterrane boundary. Eocene strike‐slip faulting and related extensional faulting and plutonism are largely responsible for the present upper crustal configuration of the Bridge River Terrane. This paper presents U‐Pb geochronology and a detailed structural and geochronological analysis of Eocene deformation of the Bridge River Terrane near Lillooet, British Columbia. Evidence for Eocene magmatism and deformation within the Bridge River Terrane includes 48.5–46.5 Ma U‐Pb zircon ages from four deformed granitic rocks, two of which are from the Mission Ridge pluton. All are cut by the Mission Ridge normal fault, which juxtaposes low‐grade Bridge River Group in the hanging wall against medium‐grade Bridge River Schist and associated intrusions in the footwall. The Mission Ridge fault is a low‐angle east‐dipping brittle normal fault with at least 10 km of downdip displacement and a probable dextral strike‐slip component. In the Bridge River Schist, in the footwall of the Mission Ridge fault, penetrative ductile fabrics in northwest‐trending, steeply to shallowly‐dipping mylonites with northwest‐southeast trending subhorizontal stretching lineations record a dextral sense of shear. Development of these fabrics predates normal movement on the Mission Ridge fault and is attributed to dextral movement on the Yalakom fault system within the middle crust. Two Middle Eocene syntectonic foliated intrusions, dated by U‐Pb zircon methods, have the same kinematic signature as the previously described mylonites; these constrain timing of dextral movement on the Yalakom fault system to be earlier than, contemporaneous with, and in part younger than the intrusion of the circa 47 Ma Mission Ridge pluton. The Mission Ridge fault is correlated with the Petch Creek fault, the probable extension of the Ross Lake fault near the United States border. They were broken and offset by about 100 km of dextral movement on the north‐trending Fraser fault. The Petch Creek fault hanging wall consists of the Hozameen Group, which, like the Bridge River Group, is an assemblage of low‐grade oceanic rocks. The footwall of the Petch Creek fault, the Custer Gneiss, is higher grade than the Bridge River Schist in the footwall of the Mission Ridge fault but has the same lithological and kinematic signature. Mylonitic fabrics in both the Bridge River Schist and Custer Gneiss have subhorizontal stretching lineations that were generated prior to normal movement on the Mission Ridge‐Petch Creek fault system and are attributed to earlier dextral motion along the Yalakom fault system. The clear offset of these geologic features constrains about 100 km of movement on the Fraser fault to be younger than 46.5 Ma. The Fraser fault is cut by 34 Ma phases of the Chilliwack batholith.
The Manokwari Trough and the western end of the New Guinea Trench
Authors: Milsom John , Masson Doug , Nichols Gary , Sikumbang Nafrazal , Dwiyanto Bambang , Parson Lindsay , Kallagher Helen
Published: February 2008

The New Guinea Trench is a sparsely surveyed seafloor depression which runs parallel to the north coast of New Guinea for a distance of some 700 km. Its western end lies about 600 km east of the Philippine Trench; the intervening region is occupied by a complex series of north trending ridges and troughs, of which the Ayu Trough and the Tobi and Mapia ridges are the most prominent. It has been suggested and is now widely accepted that the trench marks a site of subduction, but the degree of present‐day activity is disputed. The western termination has recently been defined by combined single‐channel seismic and long range sidescan sonar surveys as lying at the ridge system which culminates in Mapia Island. The trench contains as much as 1 km of flat‐lying undisturbed sediments within the area surveyed, and the southern slopes are extensively channeled, suggesting a lack of recent deformation. The same survey has also shown that for a distance of some 400 km west of the western end of the trench, the north coast of New Guinea is flanked, at a distance of only a few tens of kilometers, by a deep trough. The sonar imagery of this “Manokwari Trough” suggests recent convergence as well as transcurrent movement. Both the existence of this trough and the abrupt termination of the New Guinea Trench are consequences of the seafloor spreading which took place in the Ayu Trough after subduction had ceased at the trench.
Stress magnitude, strain rate, and rheology of extended Middle Continental Crust inferred from quartz grain sizes in the Whipple Mountains, California
Authors: Hacker Bradley R. , Yin An , Christie John M. , Davis G. A.
Published: February 2008

Knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension. We analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the Whipple Mountains, California, using grain‐size piezometers and kinetic laws for grain growth. Mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 µm. Analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures ≤500°C, compatible with estimates from two‐feldspar thermometry. Quartz grain‐size piezometers suggest that the mylonitization occurred under differential stresses (σ1–σ3) of ∼40–150 MPa, or maximum shear stresses of 20–75 MPa. Extrapolation of quartzite flow laws to 500°C indicates that the mylonitization occurred at strain rates faster than 10−14s−1. These estimates suggest that the mylonitic zone within the Whipple Mountains had an effective viscosity of the order of 1018±4−1020±4Pa s. These low viscosities and rapid strain rates, combined with seismic reflection data showing that continental crust is layered, suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high‐viscosity regions separated by low‐viscosity zones.
Late Paleozoic Orogenies in western Africa and eastern North America: The diachronous closure of Theic Ocean
Authors: Piqué Alain , Skehan James W.
Published: February 2008

The Carboniferous evolution in western Morocco, southeastern Canada, and New England was dominated by intracontinental processes: the subsidence of pull‐apart sedimentary basins, and their subsequent deformation during middle Carboniferous to Early Permian times, with the development of autochthonous structures and mainly low‐ to medium‐grade metamorphism. On the other hand, the structural style of the late Carboniferous deformation in the southern Appalachians, and to a substantial degree also in New England, that is related to a continental collision is characterized by deep‐seated northwest‐vergent thrusts and medium‐ to high‐grade metamorphism. The Mauritanides structures are symmetrically east‐vergent thrusts. This pattern is explained by the diachronous closure of the Theic ocean that separated the Avalon and Carolina microcontinental blocks from paleo‐Gondwana. The closure of the Theic ocean occurred during Devonian times between the northern Appalachians and the northwestern edge of paleo‐Gondwana (Morocco), whereas it was achieved only at the end of Carboniferous times between the southern Appalachians and western Africa (Mauritania‐Senegal). The Late Devonian to early Carboniferous counterclockwise rotation of Gondwana induced the opening of the pull‐apart basins of Canada, New England, and Morocco as well as, later, the oblique collision in the southern Appalachians and the generalized late Paleozoic dextral transcurrent faulting along the northern edge of paleo‐Gondwana.
Quaternary blind thrusting in the southwestern Sacramento Valley, California
Authors: Unruh J. R. , Moores E. M.
Published: February 2008

Patterns of microearthquakes and Quaternary surface deformation suggest that the tectonic setting of the SW Sacramento Valley is similar to areas of the western San Joaquin Valley known to be underlain by seismogenic blind thrust faults. On the basis of previous work and analysis of geologic and seismic reflection data, the following late Cenozoic tectonic features and processes are identified: (1) uplift of the northern Coast Ranges beginning approximately 3.4 Ma, and eastward propagation of uplift into the southwestern Sacramento Valley by 1.0 Ma; (2) uplift and homoclinal flexure of Plio‐Pleistocene strata at the eastern Coast Ranges mountain front; (3) uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern Sacramento Valley. Similar associations of structures and processes have been observed in thrust belts in Pakistan, the Peruvian Andes, and the Canadian Cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone. By using other thrust belts as analogs, the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern Coast Ranges mountain front. Seismic reflection profiles reveal that (1) the triangle zone consists primarily of east‐vergent blind thrusts and (2) west‐vergent backthrusts exposed in the the eastern Coast Ranges and southwestern Sacramento Valley are rooted in the east‐vergent thrusts. Transfer of slip from the east‐vergent blind thrusts to the west‐vergent backthrusts occurs locally beneath the southwestern Sacramento Valley. Fault‐bend folding in the hanging walls of the backthrusts has created a north‐northwest striking chain of low hills approximately 15 km east of the mountain front. The folds deform 3.4–1.0 Ma fluvial sediments and thus are middle Pleistocene in age or younger. Local variations in strike suggest that the fold chain is segmented, like the New Idria‐Coalinga‐Kettleman Hills segmented fold chain in the southwestern San Joaquin Valley (Stein and Ekström, 1989). These data have implications for seismic hazard assessment. Anecdotal accounts indicate that two M = 6.0+ events of the 1892 Winters‐Vacaville earthquake sequence probably occurred beneath the eastern Coast Ranges (Dale, 1977; Toppozada et al., 1981). Ground cracking was observed following the main shocks along the mountain front in the southwestern Sacramento Valley. We propose that the earthquakes were generated by slip on a blind thrust beneath the Coast Ranges, and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone. The 1892 earthquake sequence suggests that blind thrusts beneath the southwestern Sacramento Valley are active and capable of generating moderate to large magnitude earthquakes.
The Transantarctic Mountains of southern Victoria Land: The application of apatite fission track analysis to a rift shoulder uplift
Authors: Fitzgerald Paul G.
Published: February 2008

A fission track study of the Transantarctic Mountains (TAM) in the Granite Harbour and Wilson Piedmont Glacier areas of southern Victoria Land reveals information on the timing of uplift, the amount of uplift and erosion, and the structure of the mountains, especially the onshore Transantarctic Mountain Front (TAM Front), which represents the boundary between East and West Antarctica. Apatite ages are < 175 Ma and represent a thermal regime established after heating accompanying Jurassic magmatism. An apatite age profile from Mount England records a break in slope indicating uplift began at ∼55 Ma. Horizontal sampling traverses, plus fieldwork, delineate the structure of the TAM Front as a zone of north‐south striking, steeply dipping normal faults, with displacements, dominantly down to the east, of 40–1000 m. The overall structure of the mountains in the area studied can be envisaged as a large tilt block or flexure. Its westerly limb dips gently under the ice cap, compared to its faulted eastern edge, the TAM Front. The bounding structure to the south is the Ferrar fault and to the north is a graben through which the Mackay Glacier drains the polar plateau. The edge of the flexure, or axis of maximum uplift, lies at Mount Termination, ∼30 km west of the McMurdo Sound coast. There has been ∼6 km of uplift since the early Cenozoic and 4.5–5 km of erosion along this axis. The amount of uplift decreases to the west at the same rate as the decrease in dip of the Kukri Peneplain, but the amount of erosion decreases more quickly as indicated by the increasing height of the mountains to the west. The axis of maximum uplift is traced north to Granite Harbour. The axis does not parallel the coast but has a more northerly trend. North‐south striking longitudinal faults that delineate the structure of the TAM Front lie at an acute angle to the axis, indicating a dextral component to the dominantly east‐west extension in the Ross Embayment. Architecture of the TAM typifies the features of an upper plate passive mountain range, whereas the Ross Embayment has the characteristics of a lower plate. The TAM Front represents an upper plate breakaway zone. Transfer faults may exist up major outlet glaciers that cut the TAM. The inflection point in the coastline at the southern end of McMurdo Sound may be due to the presence of a major transfer fault up or near the Skelton Glacier.
Structure and tectonic history of the Southern Kerguelen Plateau (Indian Ocean) deduced from seismic reflection data
Authors: Rotstein Yair , Schlich Roland , Munschy Marc , Coffin Millard F.
Published: February 2008

Early single‐channel and recent multichannel seismic reflection data together reveal major tectonic events in the history of the Southern Kerguelen Plateau (SKP). The SKP was created by Early Cretaceous volcanism which was mainly subaerial. We estimate the age of the adjacent ocean basins to be similar to that of the plateau. Following termination of the main volcanism, the SKP began to subside and to accumulate sediment. A lower sedimentary megasequence was deposited at this time, primarily in depressions on the plateau. At about 88 Ma, tectonism affected the eastern Raggatt Basin; this tectonism may be related to faulting which created the steep eastern margin of the SKP. At about 72 Ma, predominant tectonic activity, characterized by widespread extension and uplift forming the tilted block morphology, occurred over large areas of the SKP. The extension centered along several NW‐SE trending rift systems, in places creating well‐preserved axial rifts. The largest structure of this kind, the Central SKP Uplift, lies along the center of the SKP; its relief increases from the southernmost part of the plateau toward the NW. The northernmost part of this uplift, the Banzare Bank, was elevated during rifting above sea level and was eroded. A second extensional structure, the SW Uplift, lies in the SW corner of the plateau, and other, smaller structures may also be present. The extension appears to have culminated in the initiation of the Southeast Indian Ridge at 43 Ma, but, at least in the Raggatt Basin, the two events were not continuous. Tectonic subsidence that was associated with the extension corresponds to deposition of a second megasequence, estimated to be about 1000 m thick on Banzare Bank. The subparallel trend of the rift systems on the SKP to the Southeast Indian Ridge and their timing suggest that the rearrangement of spreading in the South Indian Ocean at 43 Ma was not solely the result of the collision of India with Asia; it started earlier in association with other plate motions in the area.
The Saint‐Daniel Melange: Evolution of an accretionary complex in the Dunnage Terrane of the Quebec Appalachians
Authors: Cousineau Pierre A. , St‐Julien Pierre
Published: February 2008

The Saint‐Daniel Mélange is part of a series of mélanges located along the Baie Verte‐Brompton line in the Northern Appalachians. This line marks the suture between rocks of oceanic affinities and those of the ancient passive margin of North America with which they collided during the Taconian (Middle to Late Ordovician) orogeny. The Saint‐Daniel Mélange contains a wide variety of lithologies including well‐bedded to dismembered sedimentary sequences, pebbly mudstone, olistostromes, and slivers of igneous and metamorphic rocks. Black shales with interbeds of green shale, calcareous siltstone, or sandstone are the dominant units. They exhibit various stages of mélange formation such as those present in shallow parts of an accretionary complex. Units of oceanic origin include sediments derived from the forearc basin and slivers of an ophiolite and of a magmatic arc. Units derived from sediments of the passive margin of North America are also present. The ratio between these various lithologies changes greatly within the mélange on a kilometric scale along strike. The Saint‐Daniel Mélange is a structural complex in which the various units were assigned a sequential order mimicking a stratigraphic order. The Saint‐Daniel Mélange is interpreted as the relict of an accretionary complex because of its actual structural position within the Northern Appalachians and because all its lithologies and their structural fabric can be found in modern accretionary complexes.
Seasonality of methane emissions from five lakes and associated wetlands of the Colorado Rockies
Authors: Smith Lesley K. , Lewis William M.
Published: February 2008

Methane emissions were measured over a 15‐month interval for five lakes and five associated wetland sites in the Rockies of Colorado at elevations between 2800 and 3600 m. Three of the five lakes accumulated dissolved methane under ice; accumulation was as high as 53‐fold above ice‐free water column concentrations in the shallowest lake. The combination of high dissolved oxygen and low dissolved inorganic nitrogen concentrations within the water column during ice breakup suggests that methane emissions rather than methane oxidation led to the substantial loss of dissolved methane from the shallowest lake at the time of spring thaw. A pulsed release of methane to the atmosphere at the time of ice breakup within lakes may be widespread at high latitudes and may play a role in the observed increase in tropospheric methane concentrations in the northern hemisphere. During the ice‐free season, the mean emission rate was 1.6 mmol m−2 d−1 over open water for the five lakes, but the mean was much higher (13 mmol m−2 d−1) over the Nuphar lutea beds located in one of the lakes. Open water emissions occurred primarily through diffusion rather than bubbling. For wetlands near the lakes, average lake emissions ranged from negligible to almost 6 mmol m−2 d−1; the average across all sites was 2.1 mmol m−2 d−1 during the warm season. There was no measurable emission during the winter months. Surface dissolved methane explains 40% of the variation in emissions from the open water sites; the combination of soil organic C content and soil temperature explains 40% of variation in emissions from the wetland sites. The data from the Southern Rockies and information that has accumulated on other lake types over the last 15 years indicate that lakes may be a larger source strength of methane than reported estimates.
A strategy for estimating the impact of CO2 fertilization on soil carbon storage
Authors: Harrison Kevin , Broecker Wallace , Bonani Georges
Published: February 2008

As soils are a likely candidate for the so called missing carbon sink, we explore the possible impact of CO2 fertilization on the global humus inventory. For any given greening‐induced enhancement of plant growth, the increase in soil carbon inventory will depend on the spectrum of turnover times with respect to oxidation. Here we develop estimates of carbon turnover rates based on soil radiocarbon measurements.
The Pinatubo eruption cloud observed by lidar at Garmisch‐Partenkirchen
Authors: Jäger H.
Published: February 2008

The spread of the stratospheric plume from the explosive eruption of the Philippine volcano Pinatubo in mid‐June 1991 has been observed at Garmisch by lidar since July 1, 1991. The layered structure of the volcanic debris can be related to three transport phases to northern mid‐latitudes. The aerosols centered at 15–17 km were transported to Garmisch in early July by the shortest possible route. The double layered aerosol structure observed in August coincided with the typical transport pattern at mid‐latitudes in summer with westerlies below 20 km and easterlies above. After the change to the winter wind regime with westerlies throughout the altitude range of stratospheric aerosols, one large layer was observed sice the beginning of October, extending from the tropopause to about 28 km. This transport pattern and the layered structure of aerosols is similar to that which had been observed at the same lidar site after the El Chichón eruption in 1982.
Mt. Pinatubo SO2 column measurements from Mauna Loa
Authors: Goldman A. , Murcray F. J. , Rinsland C. P. , Blatherwick R. D. , David S. J. , Murcray F. H. , Murcray D. G.
Published: February 2008

Absorption features of the v1 band of SO2 have been identified in high resolution infrared solar absorption spectra recorded from Mauna Loa, Hawaii, on July 9 and 12, 1991, shortly after the arrival of the first eruption plume from the Mt. Pinatubo volcano in the Phillipines. A total SO2 vertical column amount of (5.1 ± 0.5) × 1016 molecules cm−2 on July 9 has been retrieved based on nonlinear least‐squares spectral fittings of 9 selected SO2 absorption features with an updated set of SO2 spectral parameters. A SO2 total column upper limit of 0.9 × 1016 molecules cm−2 deduced from measurements on September 20–24, 1991, is consistent with the dispersion of the SO2 cloud and the rapid conversion of the SO2 vapor into volcanic aerosol particles.
A long‐term trend in the height of the atmospheric sodium layer: Possible evidence for global change
Authors: Clemesha B. R. , Simonich D. M. , Batista P. P.
Published: February 2008

An analysis of a long series of lidar measurements of the vertical distribution of atmospheric sodium shows the existence of a long‐term trend in the centroid height of the layer. After making allowance for the sampling effects of the mean seasonal and diurnal variations in centroid height, it is found that the height of the layer fell by approximately 700 meters between 1972 and 1987. A regression analysis indicates a mean rate of fall of 49 ±12 m yr−1, with a correlation coefficient of −0.33, significant at the 99.99% level. The observed change is consistent with long term trends in mesospheric temperatures detected by other techniques.
Slow initial phase of the P‐wave velocity pulse generated by microearthquakes
Authors: Iio Yoshihisa
Published: February 2008

It was found that the initial rise of the far‐field P‐wave velocity pulse generated by microearthquakes does not act as a ramp but gradually increases according to the function tn (2<n<4), where t is the time measured from the onset. This slow rise of the P‐wave velocity pulse was termed the slow initial phase. The slow initial phase can not be explained by theoretical source models which assume a constant kinematic friction and a constant rupture velocity but by models which predict slow slip velocities and/or rupture velocities immediately after the rupture is initiated, such as the slip‐weakening crack model.
He2+ in field‐aligned beams: ISEE results
Authors: Fuselier Stephen A. , Thomsen Michelle F.
Published: February 2008

He2+ concentrations are determined for 14 field‐aligned proton beam events observed in the ion foreshock region upstream from the Earth's bow shock by the ISEE spacecraft. For all 14 events, most under normal solar wind conditions, the He2+ concentrations were a factor of 10 or more below the H+ concentrations in the field‐aligned beams when normalized by the solar wind He2+ to H+ density ratio. These results indicate that field‐aligned beams do not provide the principal He2+ seed population for ion acceleration in the ion foreshock region.
Ion‐cyclotron waves at Jupiter: Possibility of detection by Ulysses
Authors: Mei Yi , Thorne Richard M. , Horne Richard B.
Published: February 2008

We evaluate the convective growth of ion‐cyclotron waves in the Io plasma torus using realistic plasma parameters. Significant wave amplification is restricted to two dominant frequency bands. As originally proposed by Thorne and Moses [1983,1985], waves between the O+ and H+ gyro‐frequencies may be excited at higher latitudes (λ >15°) in the region where H+ becomes the dominant ion. Strong cyclotron resonant damping should prevent wave propagation to lower latitude. Even under optimum conditions the path integrated gain of such waves is modest and extremely sensitive to the properties of both the thermal plasma and the cyclotron resonant energetic ions. Consequently, these waves should be confined to a limited region of the torus (6<L<9). The equatorial region of the torus can be unstable to L‐mode waves below the O+ gyro‐frequency. Rapid amplification should drive waves to non linear amplitudes for any reasonable choice of the plasma properties. While the orbit of Ulysses passes through the Io torus close to the optimum location (near L=8) to detect both the high frequency (10Hz) waves at λ >15° and the low frequency waves (<0.5Hz) near the equator, the sensitivity of the magnetometer and the lower frequency threshold of the plasma wave detector offer only a marginal possibility to clearly identify either of these important waves.
Ulysses plasma observations of coronal mass ejections near 2.5 AU
Authors: Phillips J. L. , Bame S. J. , Gosling J. T. , McComas D. J. , Goldstein B. E. , Smith E. J. , Balogh A. , Forsyth R. J.
Published: February 2008

The Ulysses solar wind plasma experiment observed a series of interplanetary shocks and coronal mass ejections (CMEs), the latter evidenced by counterstreaming electrons and a variety of ion signatures, during March and April 1991. A striking sequence was observed near 2.5 AU from March 23 through April 2, with the second of two counterstreaming events lasting 6.4 days. We summarize the plasma observations for these features, suggesting that the second counterstreaming period may be two juxtaposed CMEs. The relationship between the events observed at Ulysses, ∼60° east of Earth in ecliptic longitude, and those causing a geomagnetic storm on March 24, is unclear.
Reconstruction and geostatistical analysis of multiscale fracture apertures in a large block of welded tuff
Authors: Vickers B. C. , Neuman S. P. , Sully M. J. , Evans D. D.
Published: February 2008

Reconstruction of fracture apertures in a block of welded tuff has revealed two scales of autocorrelation, one on the order of millimeters, the other on the order of tens of centimeters. The spatial and frequency distributions of apertures changed drastically as a result of minor nondestructive shear.
The local properties of ocean surface waves by the phase‐Time method
Authors: Huang Norden E. , Long Steven R. , Tung Chi‐Chao , Donelan Mark A. , Yuan Yeli , Lai Ronald J.
Published: February 2008

Traditionally, investigation of statistical properties of ocean waves has been limited largely to global quantities related to elevation and amplitude such as the power spectral and various probability density functions. Although these properties give valuable information about the wave field, the results cannot be related directly to any portion of the time data from which it was derived. We present a new approach using phase information to view and study the properties of frequency modulation, wave group structures, and wave breaking. We apply the method here to ocean wave time series data and identify a new type of wave group (containing the large “rogue” waves), but the method also has the capability of broad applications in the analysis of time series data in general.
Ground temperature histories for central and eastern Canada from geothermal measurements: Little Ice Age signature
Authors: Beltrami Hugo , Mareschal Jean‐Claude
Published: February 2008

Deep borehole temperature profiles have been analyzed to determine ground temperature histories in central and eastern Canada. Clear signs of a cold period between 1500 and 1800 A.D., corresponding to the little ice age, have been found. A warming trend after 1800 A.D. was detected throughout eastern and central Canada. Temperature profiles from western Ontario are consistent and were inverted simultaneously to yield a regional ground temperature history. The recent warming appears to be correlated with the increase of atmospheric CO2 reported for a Greenland ice core.
Testing for causal relationships between large pyroclastic volcanic eruptions and mass extinctions
Authors: Erwin Douglas H. , Vogel Thomas A.
Published: February 2008

The Permo/Triassic mass extinction, the most extensive of all Phanerozoic extinctions [Erwin, 1990], has been correlated with both massive pyroclastic eruptions [Yang and Yin, 1987; Yin et al., 1989; Jinwen, 1989] and flood basalts [Rampino and Strothers, 1988]. More tentatively, the end‐Cretaceous mass extinction has also been linked with massive volcanic events [Officer et al, 1987; Rampino and Strothers, 1988]. However, the nature of the connection between volcanism and either mass extinction remains uncertain. Rather than study volcanic events at known extinction horizons, another approach is to identify extremely large (>1000 km3) pyroclastic eruptions with well‐constrained age determinations, and evaluate the fossil record for evidence of significant regional or global extinctions. We selected the largest, best constrained pyroclastic events known, (Toba Tuff, Huckleberry Ridge Tuff, Tuff of Blacktail and the Elkhorn Mountain volcanics); neither global marine diversity nor global or regional terrestrial vertebrate diversity exhibit any biological effect from these extremely massive pyroclastic eruptions. We conclude that massive pyroclastic eruptions are unlikely to have been an effective cause of mass extinctions.
On Arnol'd diffusion in a perturbed magnetic dipole field
Authors: Warren Harry P. , Bhattacharjee A. , Mauel Michael E.
Published: February 2008

For certain initial conditions, the motion of charged particles in a magnetic dipole field is well described by the hierarchy of adiabatic invariants: the magnetic moment μ, the longitudinal invariant J, and the magnetic flux ψ. Electrostatic waves that break the axisymmetry of the dipole field and resonate with the drift motion can generate large‐scale or so‐called thick‐layer chaos in ψ. This chaos will drive Arnol'd diffusion in μ and J, making the motion asymptotically unstable. Previous studies involving mappings [Tennyson et al., 1980; Kook and Meiss, 1989] have found the thick‐layer Arnol'd diffusion rate to be proportional to the square of the perturbtion amplitude, consistent with quasilinear theory. Here we present numerical evidence that for many cases of physical interest, such as particle motion in a perturbed dipole field, the thick‐layer diffusion rate is greatly attenuated from the quasilinear result.
Solar wind thermal electrons in the ecliptic plane between 1 and 4 AU: Preliminary results from the Ulysses radio receiver
Authors: Hoang S. , Meyer‐Vernet N. , Bougeret J.‐L. , Harvey C. C. , Lacombe C. , Mangeney A. , Moncuquet M. , Perche C. , Steinberg J.‐L. , MacDowall R. J. , Stone R. G.
Published: February 2008

The radio receiver of the Unified Radio and Plasma (URAP) experiment aboard the Ulysses spacecraft records spectra of the quasi‐thermal plasma noise. The interpretation of these spectra allows the determination of the total electron density Ne and of the cold (core) electron temperature Tc in the solar wind. A single power law does not fit the variations of Ne which result from the contribution from different solar wind structures. The distribution of the values of Tc suggests that, on the average, the solar wind is nearly isothermal.
Distribution of deformation around a fault in a non‐linear ductile medium
Authors: Barr Terence D. , Houseman Gregory A.
Published: February 2008

The deformation field surrounding a fault in a non‐linear ductile medium is characterized by two components: a singularity in the stress field at the fault tip which is required by the zero shear stress and discontinuous velocity boundary conditions on the fault, and a non‐singular component which is required to satisfy arbitrary stress or velocity boundary conditions on the external boundary. For regions closer to the fault tip than to the external boundary, the singular component dominates. Near the fault tip the strain rate components decay as a power‐law function of radius with an exponent of −n/(n+1) (where strain rate varies as deviatoric stress to the nth power), and the stress components decay as a power‐law function of radius with an exponent of −1/(n+1). The behavior of the mechanical energy dissipation, however, is independent of the rheology; it is inversely proportional to the radial distance from the fault tip for all values of n. These results are consistent with the prediction of Rice and Rosengren [1968] who obtained the corresponding power‐law relations for the analogous case of a traction free crack in a nonlinear elastic medium. From both the radial and angular dependences of the components of shear strain rate, a localized shear zone will develop along the immediate extension of the fault, and the degree of localization increases with increasing n.
The effect of post Pangea subduction on global mantle tomography and convection
Authors: Scrivner Craig , Anderson Don L.
Published: February 2008

There is an excellent correlation between tomographic patterns and global tectonics in the upper 200 km of the mantle. Below this depth there is little relationship with present tectonic provinces. Most of the power in the tomography is in the longest wavelengths (l=1–3) with l=2 generally dominant. This is quite different from expectations based on mantle convection models. The question then arises, what causes the long wavelength variations mapped by seismic techniques? We investigate the hypotheses that past locations of supercontinents and subduction may control temperature variations in the mantle. We look for correlations between Pangea and post‐Pangeatic subduction and seismic velocity variations at various depths. The best correlations with time integrated slab locations occur near the base of the uppermantle, suggesting that slabs bottom out in the mesosphere. The Pangea hemisphere has a colder than average uppermantle, probably due to circum‐ and intra‐Pangea subduction. Hotspots and ridges avoid regions cooled by subduction over the past 180 Ma. The presence of large continents at the surface and large areas of cold slab at depth explain the dominance of long wavelengths in the mantle's thermal structure, despite the high Rayleigh number.
Global emissions of helium‐3 by subaerial volcanism
Authors: Allard Patrick
Published: February 2008

The emission rate of helium‐3 by subaerial volcanism is re‐evaluated by coupling 3He/CO2 ratios in volcanic gases with new estimate of the flux of carbon dioxide from both arc and non‐arc volcanoes. Steady volcanic plume emissions are computed to supply 240–310 mole 3He/yr, most of which is due to non‐arc volcanoes. Major eruptions represent minor additions on average, whereas diffuse 3He emanations through volcanic piles may be important sources. Although not including this contribution, the above figure is equivalent to 20–40% of the 3He flux from mid‐ocean ridges and, thus, suggests that primordial 3He degassing from subaerial volcanic regions may be larger than expected.
New temporal constraints on the rotation of the Peruvian central Andes obtained from paleomagnetism
Authors: Macedo‐Sánchez O. , Surmont J. , Kissel C. , Laj C.
Published: February 2008

New paleomagnetic data from middle Eocene to middle Miocene volcanic and plutonic formations of the Western Cordillera of Central Peru document a counterclockwise rotation of 11°±7.4°. This rotation, similar to previous results reported for the northern Peruvian margin, is post middle‐Miocene in age. It is thus coeval with the acceleration of the Andean uplift and supports Isacks' model.
Effects of partial liquid/gas saturation on extensional wave attenuation in Berea sandstone
Authors: Yin C.‐S. , Batzle M. L. , Smith B. J.
Published: February 2008

Extensional wave attenuation measurements on Berea sandstone were made during increasing (imbibition) and decreasing (drainage) brine saturations. Measurements on samples with both open‐pore and closed‐pore surfaces were made using the resonant‐bar technique. The frequency dependence was examined using the forced‐deformation method. The attenuation was found to be dependent on saturation history as well as degree of saturation and boundary flow conditions. The sample with open‐pore surface had a larger attenuation which peaked at greater brine saturations than the sample with closed‐pore surface. During drainage, the attenuation reached a maximum at about 90% brine saturation as opposed to about 97% brine saturation during imbibition. The variation of the size and number of air pockets within the rock can account for this discrepancy. The magnitude of the attenuation peak value decreases substantially with decreasing frequency to the extent that no attenuation peak with saturation was apparent at seismic frequencies, say, below 100 Hz.
Large‐scale atmospheric controls on local precipitation in tropical Mexico
Authors: Hewitson Bruce C. , Crane Robert G.
Published: February 2008

Sixty‐five percent of the short term variability in southern Mexican precipitation is accounted for by the large‐scale circulation. Empirical relationships between sea level and 500 mb circulation fields, and the local precipitation in Chiapas, Mexico, are derived using a neural net. Although much of the rainfall is a result of convective processes, the neural net captures the onset of the precipitation season, and the phase of individual precipitation events. The analysis indicates that both of these aspects of the precipitation regime are controlled to a large extent by the atmospheric circulation.
Stacking in paleomagnetism: Application to marine sediments with weak NRM
Authors: Scheepers P. J. J. , Zijderveld J. D. A.
Published: February 2008

Plio‐Pleistocene marine marls and clays from southern Italy often have weak natural remanent magnetizations (NRM) and show poor thermal demagnetization results. Some random scatter appears after treatment at higher temperatures, due to the low remaining intensities and promoted by the acquisition of viscous magnetizations. Since these problems hamper a uniform analysis of the demagnetization results, a method is proposed to improve their representation and interpretation. A (simple) stacking routine reduces the amount of scatter considerably and makes it possible to separate different NRM components in a more consistent and reliable manner. In addition, the construction of NRM‐decay curves is more straightforward from stacked diagrams and thus a more accurate information is obtained about the remanence carrying minerals. The application to two sites from southern Italy serves as an illustration for the method.
Deformation in the Santa Barbara Channel from GPS measurements 1987–1991
Authors: Larson Kristine M. , Webb Frank H.
Published: February 2008

Geodesy is one of the few techniques that can be used to estimate strain rates across the submerged structures in the Santa Barbara Channel. By resolving relative surface velocities, the Global Positioning System (GPS) can be used to infer crustal deformation rates independent of geologic models. GPS measurements from the period 1987–1991 have been used to derive velocities for a 8 station network bounding the channel. These estimates are based on 10 GPS experiments each of several days in length over this 4.5 year period. Precision of individual determinations of intersite vectors range from 3 to 7 mm for the horizontal components and average 20 mm in the vertical. The GPS data indicate high rates of shortening in the eastern channel (6±1 mm/yr at N16±3E) and low rates of deformation in the western channel. Comparisons with previous geodetic studies suggest that deformation in the region has not been uniform over the last 100 years.
An evolving MHD vortex street model for quasi‐periodic solar wind fluctuations
Authors: Siregar Edouard , Roberts D. Aaron , Goldstein Melvyn L.
Published: February 2008

We use magnetohydrodynamic (MHD) simulation to provide a dynamical basis for the “vortex street” model of the quasi‐periodic meridional flow observed by Voyager 2 in the outer heliosphere. Various observations suggest that near the current sheet at solar minimum one can expect to find a vorticity distribution of two opposite shear layers with an antisymmetric staggered vorticity pattern due to structured high‐speed wind surrounding low‐speed equatorial flow. We show that this flow pattern leads to the formation of a highly stable vortex street through the nonlinear interaction of the two shear layers. Spatial profiles of various simulated parameters (velocity, density, meridional flow angle and the location of magnetic sector boundaries) and their relative locations in the quasi‐steady vortex street are generally in good agreement with the observations. A strong, flow‐aligned magnetic field, such as would occur in the inner heliosphere, inhibits the development of the street which would then be masked by the background interplanetary turbulence. The flow produced by the street induces a (relatively small) transport of plasma and magnetic flux as a result of the meridional flow away from the ecliptic region.
Earth's gravitational field: Seismic tomography resolves the enigma of the Laurentian Anomaly
Authors: Peltier W. R. , Forte A. M. , Mitrovica J. X. , Dziewonski A. M.
Published: February 2008

We demonstrate that the negative free air gravity anomaly over Hudson Bay is not due to glacial isostatic disequilibrium but rather to the influence of large scale mantle convection.
Thermal diffusivity of the lithosphere derived from altimetry and bathymetry profiles across the Southwest Indian Ridge
Authors: Doucouré C. Moctar , Patriat Philippe
Published: February 2008

In standard models for computing oceanic lithosphere thickness, the lithosphere's thermal diffusivity is one of the basic parameters required and present estimates have a substantial uncertainty. Extending the work of Crough [1979], Lister [1982] and Gibert and Courtillot [1990], we propose a new method for the determination of thermal diffusivity of oceanic lithosphere in the near‐ridge crest domain. The method requires geoid height and depth data, both of which reflect wavelengths associated with the lithospheric cooling process and expressed as a function of seafloor age. Over the Southwest Indian Ridge, the thermal diffusivity so found [ (0.55±0.17)×10−2cm2s−1] is consistent with data from the South Atlantic. Differences in diffusivity calculated on either side of the ridge and caused by asymmetrical subsidence are possibly linked to a differential thermal regime beneath the ridge and/or to the general motion of the ridge. Our findings are interpreted in terms of a lithospheric boundary layer less than 14age½ kmMyr−½ thick and initial temperature between 1050 and 1390°C.
Precipitation of suprathermal (100 EV) Electrons by oblique whistler waves
Authors: Jasna D. , Inan U. S. , Bell T. F.
Published: February 2008

Electron precipitation induced by oblique whistler waves is investigated using a new test particle simulation code based on gyro‐averaged equations of motion. Results indicate that highly oblique waves can efficiently pitch angle scatter suprathermal electrons (10–100 eV). At L = 3, for example, in a single encounter with a wave with power density S ∼ 8 pW/m2 and propagating at ψ = 60° with respect to the Earth magnetic field Bo, average pitch angle scattering of 100 eV electrons is ∼ 0.1°. In comparison, the average scattering of energetic (100 keV) electrons by a parallel propagating wave with the same S is ∼ 0.01°. Estimates indicate that the precipitated electron energy fluxes resulting from the interaction of ∼ 100 eV electrons with oblique waves can be up to 30 times larger than that due to the precipitation of 100 keV electrons by parallel propagating waves.
An experimental approach to thermochemical convection in the Earth's core
Authors: Cardin Philippe , Olson Peter
Published: February 2008

Both thermal and compositional buoyancy affect convection in the Earth's liquid outer core. We report preliminary experiments on the structure of thermochemical convection in a rotating sphere with the geometry of the Earth's core. Fully‐developed thermochemical convection in a rapidly rotating sphere is driven by buoyant ribbon‐shaped plumes concentrated in the equatorial region. Curtains of fluid aligned parallel to the rotation axis and extending across the sphere move with the plumes. The motion is turbulent in planes parallel to the equator. The convection generates a secondary, large scale zonal flow which is primarily retrograde (westward). The main difference between chemically‐dominated and thermally‐dominated convection is the smaller scales of motion present when compositional buoyancy drives the flow. In cases when both are present simultaneously, the probable situation in the outer core, we find the structure of the motion is similar to purely thermal convection.
The solar flux influence on quiet time conductances in the auroral ionosphere
Authors: Moen Jøran , Brekke Asgeir
Published: February 2008

Hall and Pedersen conductances derived from EISCAT Common Program 1 (CP‐1) data during half a solar cycle are presented. The data set consists of 9 quiet summer days with 10.7‐cm solar flux (Sa) ranging from solar minimum (Sa=74) to solar maximum (Sa=248) conditions. The solar flux and the solar zenith angle (χ) dependence of Hall (∑H) and Pedersen (∑P) conductances are well represented by respectively. The present results are in good agreement with the theoretical model calculations by Rasmussen et al. [1988] for solar minimum conditions. For higher solar activity levels, however, our model gives consistently higher conductance values than older models.
Time‐dependent three dimensional compressible convection with depth‐dependent properties
Authors: Balachandar S. , Yuen D. A. , Reuteler D.
Published: February 2008

Using a spectral code, we have studied the time‐dependent regime of three‐dimensional anelastic compressible convection with depth‐dependent thermal expansivity, viscosity and thermal conductivity in a wide box of size 5×5×1. Surface Rayleigh numbers up to 5 ×l06, have been considered. Very few cylindrical plumes are developed at the bottom but they join up collectively to form strong upwellings, which pulsate chaotically. Major descending flows occur in sheets which form rectangular planform at the top. The thermal and flow fields are dominated by large‐scale features. The bottom 20% of the convecting layer is found to be superadiabatic.
Evaluation of topographic effects on P and S‐waves of explosions at the northern Novaya Zemlya Test Site using 3‐D numerical simulations
Authors: Frankel Arthur , Leith William
Published: February 2008

Using a 3‐D finite difference simulation, we calculated the effects of surface topography on the amplitudes and waveforms of seismic waves generated by a hypothetical nuclear explosion at the northern Novaya Zemlya test site (Matochkin Shar). The simulation shows substantial azimuthal variations in the amplitude of the downgoing P waveform at shallow depths beneath the source, caused by variations in the amplitude of pP. However, these azimuthal amplitude variations diminish as the wavefront propagates deeper in the crust. Based on these results, we estimate that the topographic scarp considered here produces a maximum azimuthal variation of ±0.05 magnitude units for mb determined from teleseismic P‐waves. The sloping topography causes substantial SH motion for azimuths along the strike of the scarp.