bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactive ionosphere modeling: A comparison between TIGCM and ionosonde data
Authors: Codrescu M. V. , Roble R. G. , Forbes J. M.
Published: February 2008

Results from a time dependent geomagnetic storm simulation of the coupled thermosphere and ionosphere using the new interactive thermosphere‐ionosphere general circulation model (TIGCM) (Roble et al., 1988) of the National Center for Atmospheric Research are compared with F2‐ layer data obtained from a latitudinal chain of East Asian ionosonde stations situated close to the −165° magnetic meridian and separated by about 5° in magnetic latitude. This is among the first extended comparisons (10 days) between the TIGCM modeled ionosphere and data, where the effects of neutral dynamics on the ionosphere are studied using a global, fully interactive thermosphere‐ionosphere model. The ionosonde stations provide latitudinal coverage that extends from 15° to 50° magnetic north. Hourly values from both the simulation results and ionosonde data for hmF2, ƒoF2, and meridional neutral winds, for the period March 19–28, 1979, are fitted in latitude using Legendre polynomials, and variations from quiet‐time values are displayed in latitude‐UT coordinates. Color graphics for both the simulation and data are used to illustrate the equatorward penetration of ionospheric disturbances and their dependence on Kp, storm time, and local time. Observed effects are interpreted in terms of plausible electric field, neutral wind, and neutral composition changes during the storm period and where possible are related to TIGCM predictions. It is shown that the TIGCM ionosphere responds very well to varying geomagnetic conditions. The agreement between data and model for critical frequency ƒoF2, is very good at 45° magnetic north, but the agreement decreases equatorward. The agreement for critical height hmF2, however, is good through the whole region of comparison. A test of the general procedure used to derive winds from ionosonde data is also presented. Winds derived from the model's ionosphere hmF2 compare well with winds computed by the TIGCM. This agreement shows the strong coupling between meridional neutral winds and critical height at mid‐latitudes and gives some validity to the methods of computing disturbance winds from variations in F2 layer height.
Low‐latitude particle precipitation and associated local magnetic disturbances
Authors: Rassoul H. K. , Rohrbaugh R. P. , Tinsley B. A.
Published: February 2008

The time variations of optical emissions during low‐latitude auroral events have been shown to correlate well with those of magnetograms in the region where the aurorae are observed. Two events not previously reported are analyzed and are shown to confirm the nature of the correlations found for two earlier events. The maximum optical emissions at mid‐latitudes occur in concert with the maximum positive (northward) excursions in the H trace and with rapid fluctuations in the D trace of nearby magnetograms. The fluctuation in ΔD is usually from the east (positive) to the west (negative) in vicinity of the ΔH perturbation. The positive excursions in H at low‐latitude observatories at the time of the maximum optical emissions are associated with negative H excursions at higher‐latitude observatories in the same longitude sector. The source of the particles has been inferred to be the ring current, with precipitation occurring when the |Dst| index is large at the time of the large short term excursions in the local magnetic field. This result is consistent with the finding of Voss and Smith (1979), derived from a series of rocket measurements of precipitating heavy particles, that the flux correlates better with the product of |Dst| and the exponential of Kp than with either alone. In the present case it is shown that the product of |Dst| and the amplitude of the short term excursions in the horizontal component in local magnetograms has better time resolution and better correlation with the observed emission rates than the index using Kp.
Ion composition of the topside equatorial ionosphere during solar minimum
Authors: González S. A. , Fejer B. G. , Heelis R. A. , Hanson W. B.
Published: February 2008

We have used observations from both the Bennett ion mass spectrometer and the retarding potential analyzer on board the Atmosphere Explorer E satellite to study the longitudinally averaged O+, H+, and He+ concentrations from 150 to 1100 km in the equatorial ionosphere during the 1975–1976 solar minimum. Our results suggest that the ion mass spectrometer measurements need to be increased by a factor of 2.15 to agree with the densities from the retarding potential analyzer and with ground‐based measurements. The peak H+ concentrations are about 2.5×104 cm−3 during the day and 104 cm−3 at night and vary little with season. The O+/H+ transition altitude lies between 750 and 825 km during the day and between 550 and 600 km at night. He+ is a minor species at all altitudes; its concentration is highly variable with a maximum value of about 103 cm−3 during equinox daytime.
A quasi‐static magnetospheric convection model in two dimensions
Authors: Erickson G. M.
Published: February 2008

A self‐consistent, two‐dimensional model of subsonic, plasma sheet convection is presented. Specifically, time sequences of static equilibrium solutions for the two‐dimensional magnetospheric magnetic field are constructed consistent with adiabatic convection. This model self‐consistently includes a dipole field and a reasonable accounting for the effects of inner magnetospheric shielding. Starting from a relaxed magnetospheric equilibrium, the earthward convection of plasma sheet flux tubes results in the stretching of inner plasma sheet field lines, the development of a local minimum in the equatorial magnetic field Be in the near‐Earth plasma sheet, and an increasing lobe magnetic field. This evolution in time occurs generally, independent of the specific magnetopause or far‐tail boundary conditions, provided the plasma sheet is not at marginal interchange stability. This behavior results solely from the convection of flux tubes of increasing entropy into the near‐Earth plasma sheet. We discuss these results in the general context of earthward convection in Earth's plasma sheet and in the specific context of magnetospheric substorms.
Substorm intensifications and field line resonances in the nightside magnetosphere
Authors: Samson J. C. , Wallis D. D. , Hughes T. J. , Creutzberg F. , Ruohoniemi J. M. , Greenwald R. A.
Published: February 2008

Magnetometer and HF radar data often indicate the presence of magnetohydrodynamic, field line resonances in the nightside magnetosphere. These resonances have frequencies of about 1.3, 1.9, 2.6, and 3.4 mHz and are due to cavity modes or waveguide modes which form between the magnetopause and turning points on dipolelike magnetic shells. Energy from these cavity modes tunnels to the field line resonances which are seen in the F region by the HF radar and on the ground by the magnetometers. The presence of these field line resonances gives us an excellent diagnostic tool for determining the position of the mechanism leading to the energetic electrons and field‐aligned currents associated with substorm intensifications and auroral brightening. Using data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, riometers, and bistatic auroral radars and data from the Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay in Canada, we have identified a number of intervals in which substorm intensifications occurred during times when field line resonances existed in the region of the magnetosphere where the intensification occurred. In the events that we have analyzed in detail, the ionospheric signatures of the substorm intensification began equatorward (earthward) of existing field line resonances. These observations give very strong evidence indicating that at least one component of the substorm mechanism must be active very close to the Earth, probably on dipolelike field lines in regions with trapped and quasi‐trapped energetic particles. Furthermore, the auroral intensifications started near the position of one of the equatorward resonances, indicating that the field line resonances may play a role in triggering or producing the substorm intensifications. One possible scenario is mode conversion to kinetic Alfvén waves in the resonance.
An ash flow caldera in cross section: Ongoing field and geochemical studies of the Mid‐Tertiary Turkey Creek Caldera, Chiricahua Mountains, SE Arizona
Authors: du Bray Edward A. , Pallister John S.
Published: February 2008

Volcanic and shallow plutonic (hypabyssal) levels of the Turkey Creek caldera, located in southeast Arizona, are exposed as a consequence of uplift and erosion. The 40Ar/39Ar geochronology and paleomagnetic data indicate that the caldera cycle was relatively short lived and occurred at about 26.9 Ma, coincident with early phases of ductile extension in the southern Basin and Range. The caldera is a transitional calc‐alkali to alkalic magmatic system and is similar to other relatively small volcanic‐plutonic centers that formed after the main pulse of compressional calc‐alkalic magmatism in the Cordillera. Trace element ratios and elemental distribution patterns for Turkey Creek rocks are consistent with origin in a transitional subduction to within‐plate extensional setting. Field relations also suggest synextensional magmatism; regional northwest trending high‐angle faults offset early caldera rocks but are buried by late moat rhyolite. Caldera collapse accompanied eruption of more than 500 km3 of high‐silica rhyolite tuff (the Rhyolite Canyon Tuff). Eruption of the tuff was followed immediately by emplacement of a dacite porphyry intrusion, probably a thick laccolith, into the caldera fill and by extrusion of the dacite porphyry from ring‐fracture‐hosted feeders. Intracaldera tuff at the roof of the intrusion was metamorphosed and brecciated to produce low‐pressure shock‐metamorphic effects and was locally melted. Interpretation of the intrusion as an intracaldera laccolith readily explains a lack of floor rocks within the caldera and the absence of intracaldera equivalents of most of the outflow tuff. Intrusion of intracaldera laccoliths may represent a relatively common, though rarely recognized process within calderas. Stratigraphic relations, rare mingled rocks, and overlapping 40Ar/39Ar ages indicate that both high‐silica rhyolite magma (Rhyolite Canyon Tuff) and dacite porphyry magma were present in the source reservoir (rhyolite above dacite) and were separated by a sharp interface. Dacite porphyry magma from beneath the interface was drawn up into and erupted from vents that previously fed ash flow eruptions; some dacite porphyry was trapped in and beneath the vents and solidified to form a ring dike at depth. Following an erosional hiatus of ≤0.3 m.y., rhyolite was again erupted, filling the caldera moat with ∼135 km3 of mainly aphyric high‐silica rhyolite. Gradational contacts with underlying densely welded tuff, relatively large volumes, planiform aspect ratios, and superliquidus temperatures suggest that some of the laminated rhyolites are rheomorphic tuff. Eruption of moat rhyolites records generation of a voluminous new batch of mainly high‐silica rhyolite with a distinct geochemical signature.
Modelling plastic deformation of peridotite with the self‐consistent theory
Authors: Wenk H.‐R. , Bennett K. , Canova G. R. , Molinari A.
Published: February 2008

Theories for deformation of polycrystals have been substantially refined, enabling us to model deformation of metals and minerals with considerable sophistication. So far, most modelling has been confined to single‐phase aggregates such as quartzite and limestone. We present the first results for a polyphase aggregate, peridotite, consisting of 70% olivine and 30% enstatite. The problem is approached with a viscoplastic self‐consistent theory satisfying stress equilibrium and strain compatibility for the average polycrystal and taking account of anisotropic neighbor interactions. It is assumed that olivine deforms by (010)[100], (001)[100], and (010)[001] slip and enstatite deforms by (100)[001] slip. Simulated textures for olivine and enstatite in peridotite resemble simulated textures in the pure phases, indicating that for this system and for these volume fractions there is little influence of the different phases upon each other. In our model the harder mineral enstatite deforms at a slower rate than olivine. Interaction between neighboring grains appears to be minimal, which may be due to model assumptions. Predicted pole figures with olivine (010) axes and enstatite (100) axes aligning with the direction of shortening are in good agreement with preferred orientations in naturally and experimentally deformed peridotites.
An analysis of spectral differences between Nevada Test Site and Shagan River nuclear explosions
Authors: Taylor Steven R. , Denny Marvin D.
Published: February 2008

Spectral ratio discrimination studies carried out on events located in the western United States and Soviet Union (S.U.) illustrate that pronounced differences in radiated explosion‐source spectra relative to nearby earthquakes exist between the two regions. Nevada Test Site (NTS) explosions are characterized by the existence of more low‐frequency and/or less high‐frequency energy (greater low‐to high‐frequency spectral ratios) than western U.S. earthquakes. The opposite pattern is observed in the S.U. with nuclear explosions appearing to have more high‐frequency (and/or less low‐frequency) energy than earthquakes. These observations may be caused by at least two principal effects that are probably acting in parallel: (1) variations in depth‐dependent effects of attenuation acting between the shallow explosions and deeper earthquakes and (2) differences in the dynamic response of the near‐source geology to the passing explosion shock wave. Anelastic synthetic seismogram calculations illustrate that depth‐dependent attenuation effects may explain the spectral observations. However, a number of observations using near‐ and far‐field data from NTS explosions suggest that near‐source effects are the dominant factor. A quasi‐empirical explosion source model is proposed that simultaneously fits the spectral ratio data from both the U.S. and S.U. relative to earthquakes in each of the respective regions. Additionally, the model fits the trends of the spectral ratios observed as a function of magnitude. The key to the model is the shape of the pressure time history acting at the elastic radius. For explosions detonated in weak, porous rock, the radiated shock wave divides into a two‐wave system consisting of an elastic precursor followed by a plastic wave. The generation of this two‐wave system introduces a rise time into the pressure time history. In the frequency domain a second corner frequency is established in a third‐order model (with an ω−3 high‐frequency decay) whose value is inversely proportional to the time separation of the two waves. In higher‐strength, saturated rocks (or for overburied explosions) the effective rise time is short, and a second‐order model is appropriate (with an ω−2 high‐frequency decay). The second‐order model provides a good fit to the S.U. data. In contrast, a hybrid model is required to fit the NTS data with an ω−3 high‐frequency decay for shallow explosions detonated in unsaturated tuff that evolves to an ω−2 decay as depth of burials reach higher‐strength, saturated rocks below the water table.
Coherence of seismic body waves from local events as measured by a small‐aperture array
Authors: Vernon Frank L. , Fletcher Jon , Carroll Linda , Chave Alan , Sembera Eugene
Published: February 2008

Eight local earthquakes were recorded during the operation of a small‐aperture seismic array at Pinyon Flat, California. The site was chosen for its homogeneous granitic geology and its planar topography. Amplitude spectral ratios for the same signal measured at different stations had average values of less than 2 and maximum values of 7. Magnitude‐squared coherences were estimated for all station pairs. These estimates were highest for the P wave arrivals on the vertical component and lowest for the P wave recorded on the transverse component. At 500 m station separation the P and S waves were incoherent above 15 Hz and 10 Hz, respectively. Coherence for both the P and S waves decrease as frequency increases and as distance increases. The coherence of signals from borehole sensors located at 300 and 150 m depth displays higher average coherence than equally spaced sites located on the surface. The results here suggest that even for sites that appear to be very similar, that is, those which are located on a planar surface within a few meters of granite bedrock, the measured seismic wavefield can be distorted substantially over scale lengths of 500 m. Coherence properties were calculated from synthetic seismograms which were computed for velocity models with exponential and self similar distribution perturbations. Standard deviations of 10% are not sufficient for the random velocity distributions to approximate the results from the small‐aperture array.
Basin and range crustal and upper mantle structure, northwest to central Nevada
Authors: Catchings R. D. , Mooney W. D.
Published: February 2008

We present an interpretation of the crustal and uppermost mantle structure of the Basin and Range of northwestern Nevada based on seismic refraction/wide‐angle reflection, near‐vertical reflection, and gravity data. In comparison to most previous estimates, we find that the crust is somewhat thicker (32–36 km versus 22–30 km), and the uppermost mantle velocity is somewhat higher (8.0 km/s versus 7.3–7.9 km/s). Along our transects, the crust is thinnest (32 km) in the Carson Sink‐Buena Vista Valley region and increases by 2–4 km to the west and east, respectively. There is considerable complexity throughout the crust where velocities range from of 2.5 km/s at the surface to 7.4 km/s in the lowermost crust. Variations in velocity and structure of the upper crustal layers reveal apparent basement velocity depressions (areas of lower velocities extending up to 10 km in depth) that underlie some surface ranges as well as the basins. The middle crust rises from about 20 km beneath central Nevada to within 12 km of the surface beneath the area of thinnest crust and is characterized by a modest (∼0.1 km/s) change in velocity and low‐velocity gradients. These midcrustal layers mark the onset of high crustal reflectivity and the apparent limiting depth to which Basin and Range faults can be traced in near‐vertical reflection profiles, suggesting that these midcrustal layers represent the transition between the brittle and ductile zones of the crust. The lower crust is more structurally complex, with layers thickening and thinning in a systematic manner with the upper crustal layers; generally, where there are velocity depressions in the upper crust, the lower crust is thickest and shallowest. The geometry of these lower crustal layers (derived from refraction modeling) coincides with changes in the crustal reflectivity, determined from the Consortium of Continental Reflection Profiling reflection data. The lower crustal layer is unusually high in velocity (7.4 km/s) and is likely the layer identified as mantle in some previous studies. We do not identify the 7.4 km/s layer as mantle because (1) there is an underlying layer with a velocity (8.0 km/s) that is more consistent with the worldwide average velocity for the upper mantle, and (2) the 7.4 km/s layer does not correspond to the “reflection” Moho. Gravity modeling and comparison to existing seismic models show a general consensus in many aspects with respect to crustal structure. This new model forms the basis for speculation on some of the processes associated with rifting of the Basin and Range Province. One such process, lithospheric magmatism, is inferred from the strong attenuation of transmitted seismic waves, which occurs at the same interface at which high‐amplitude, bright spot reflections originate. Unlike previous models, the overall structure and velocity of the crust and uppermost mantle of our new model are similar to other regions worldwide which have undergone high degrees of extension.
Paleomagnetism of the Chinle and Kayenta Formations, New Mexico and Arizona
Authors: Bazard David R. , Butler Robert F.
Published: February 2008

Paleomagnetic data were obtained from 22 sites (6–10 samples/site) in the Upper Shale Member of the Chinle Formation, 43 sites in the Owl Rock Member of the Chinle Formation, and 35 sites in the Kayenta Formation. Thermal demagnetization and data analyses indicate that within‐site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization (ChRM). Site‐mean directions define at least four antipodal polarity zones within each member/formation, suggesting the ChRM was acquired soon after deposition. Fifteen site‐mean virtual geomagnetic poles (VGPs) from the Upper Shale Member of the Chinle Formation yield an early Norian paleomagnetic pole position of 57.4°N, 87.8°E (K = 60, A95 = 5.0°). Eighteen site‐mean VGPs from the Owl Rock Member of the Chinle Formation yield a middle Norian paleomagnetic pole position of 56.5°N, 66.4°E (K = 183, A95 = 2.6°). Twenty‐three site‐mean VGPs from the Kayenta Formation yield a Pliensbachian pole position of 59.0°N, 66.6°E (K = 155, A95 = 2.4°). Combined with paleomagnetic poles from the Moenave Formation and the Shinarump Member of the Chinle Formation, these data record ∼30 m.y. of North American apparent polar wander (APW) within a regional stratigraphic succession. During the Camian and Norian stages of the Late Triassic, Chinle poles progress westward. During the Hettangian through Pliensbachian stages of the Early Jurassic, the pattern of APW changed to an eastward progression. Even after correction for 4° clockwise rotation of the Colorado Plateau, a sharp comer in the APW path (J1 cusp) is resolved near the pole from the Hettangian/Sinemurian (∼200 Ma) Moenave Formation (59.4°N, 59.2°E). Amongst other implications, the sharp change in the APW path at the J1 cusp implies an abrupt change from counterclockwise rotation of Pangea prior to 200 Ma to clockwise rotation thereafter.
Paleomagnetic and petrographic evidence for early magnetizations in successive Terra Rosa Paleosols, Lower Pennsylvanian black prince limestone, Arizona
Authors: Nick Kevin , Xia Kedong , Elmore R. D.
Published: February 2008

An early, synsedimentary, chemical remanent magnetization (CRM) is found in and below terra rosa paleosols which cap regressive carbonate cycles in the Pennsylvanian (Morrowan) Black Prince Limestone in southeastern Arizona. Authigenic hematite, occurring in fractures between soil nodules, in Liesegang bands, and replacing allochems, is abundant in specific horizons and is related to soil‐forming processes. Paleomagnetic analysis of many specimens from the paleosol horizons reveals both northwesterly and shallow and southeasterly and shallow directions. These unidirectional, northwesterly and southeasterly magnetizations are interpreted to be antipodal equivalents and show a stratigraphic distribution with at least three reversals. The mean direction (declination 308°, inclination 9°) corresponds to a pole position at latitude 34°N, longitude 143°E similar to other pre‐Kiaman, Carboniferous poles. Some specimens also contain multicomponent magnetizations (those with dual polarities or with curved thermal demagnetization trajectories) and are interpreted to be the result of remagnetization after a reversal in the Earth's field. Demagnetization and rock magnetic results for the reddened rocks confirm that hematite is the primary magnetic carrier. In contrast to the paleosol zones, the magnetization in the interbedded, gray marine limestones resides in magnetite and does not exhibit stable decay. Terra rosa paleosols are likely to have preserved the hematite and associated magnetizations that formed as a result of soil forming processes. These paleosols, therefore, may contain data that are useful in determining pole positions and validating polarity zonations.
Dyke intrusion model for the 1989 seismovolcanic activity off Ito, central Japan
Authors: Okada Yoshimitsu , Yamamoto Eiji
Published: February 2008

In the last 10 years, extensive earthquake swarms have occurred intermittently in the region east of Izu Peninsula, central Japan. In July 1989 an active swarm again took place off Ito City, and among all past activities, it was nearest to land and shallowest. This activity eventually became a weak submarine eruption, the first of its kind in this region. Associated with this event, a borehole tiltmeter at 92 m depth, installed several kilometers from the source area, detected distinct continuous tilt change with the following features: (1) Tilt of 20 μrad was observed within 1 week preceding the eruption. (2) Tilt change showed a good correlation to swarm activity. (3) No obvious coeruption tilt change was observed. On the basis of the tilt record and swarm distribution, we constructed a dislocation model to interpret the seismovolcanic activity. The model consists of two tensile faults, each of which corresponds to magma intrusions in May and July, and a right‐lateral reverse fault representing the largest earthquake (M5.5) at 1109 LT on July 9. This model can simulate not only the above tilt data but also the record of a nearby dilatometer, the results from daily laser distance measurements and daily Global Positioning System monitoring, as well as results from other conventional distance surveys and leveling surveys in the surrounding region. From this model, we conclude that the intersection of the shear fault with the tensile fault caused magma to come up into soft sedimentary layers at the seabottom, creating a lava dome. Later an instability brought about a magma‐phreatic explosion which did not generate any substantial crustal deformations at deeper levels.
The role of silica precipitation kinetics in determining the rate of quartz pressure solution
Authors: Mullis Andrew M.
Published: February 2008

Traditionally, models of intergranular pressure solution have assumed that grain boundary diffusion, probably through an adsorbed water layer, is the rate‐limiting process (Weyl, 1959; Rutter, 1976). However, observations by Heald (1956, 1965) and experiments by Cecil and Heald (1971) show that the presence of clays may strongly inhibit the growth of secondary quartz. Under these circumstances, silica precipitation kinetics may also be important in determining the pressure solution rate. A model of quartz pressure solution including the rate of silica precipitation kinetics is developed, and strain rate equations are derived for a compacting sediment. Laboratory data for the precipitation constant k_, for quartz, indicate that precipitation kinetics should not be important in determining the rate of pressure solution in clean quartz sand. However, an estimate of the in situ value of k_ for the Dogger Beta formation, made from the data of Füchtbauer (1983), suggests that in a geological setting, k_ may be much smaller than under laboratory conditions. In consequence, it is asserted that silica precipitation kinetics may influence the rate of quartz pressure solution when secondary quartz growth is inhibited.
Distributed deformation and block rotation in three dimensions
Authors: Scotti Oona , Nur Amos , Estevez Raul
Published: February 2008

The focus of this paper is to understand distributed deformation, in particular the relationship between fault slip and rotation of faults and blocks in a three dimensional stress field. Regions of distributed deformation, such as Southern California, are organized in complex arrays of contemporaneously active block‐faulted domains. We believe that the present day orientation of faults in many domains is due to the contemporaneous slip and rotation of the faults and of the blocks they bound. Traditional friction models cannot explain active unfavorably oriented faults and do not consider how faults become unfavorably oriented. To solve this problem, we propose a three dimensional block rotation model that tracks the orientation of blocks and their bounding faults during rotation. Mechanically, we consider Coulomb criteria for rock fracture as an upper bound, and slippage on a frictional surface as a lower bound. The key parameter in our model is the value of ϕ = (σ2 – σ3)/(σ1 – σ3). Principal stress directions are assumed irrotational through time. This model predicts up to 75° of vertical axis rotation along a single set of faults. During rotation, fault slip may change, sometimes dramatically, giving rise to mixed vertical as well as horizontal axis of rotation of blocks and faults. For very unfavorably oriented faults the model predicts rotations about a vertical axis in both the normal and reverse stress regimes, and about a horizontal axis in the strike‐slip stress regime. Therefore paleomagnetically inferred rotations may not always be directly related back to a specific stress regime. Combining frictional constraints of the block rotation model with paleomagnetic, structural and geological data, we show how only one set of faults, preexisting and rotating in an irrotational strike‐slip stress field, can account for the three major phases of deformation observed in the Western Transverse Range domain, Southern California: preexisting north‐northeast faults were reactivated as normal faults, rotated and became strike‐slip, and subsequent rotations of faults resulted in their present east‐west high angle reverse orientation. This example demonstrates that it is not necessary to invoke complex regional and local changes in the stress regime or erratic changes in plate motion to account for alternate periods of compression and extension.
Evidence for a frozen bed, Byrd Glacier, Antarctica
Authors: Scofield John P. , Fastook James L. , Hughes Terence J.
Published: February 2008

Ice thickness, computed within the fjord region of Byrd Glacier on the assumptions that Byrd Glacier is in mass‐balance equilibrium and that ice velocity is entirely due to basal sliding, are on average 400 m less than measured ice thicknesses along a radio‐echo profile. We consider four explanations for these differences: (1) active glacier ice is separated from a zone of stagnant ice near the base of the glacier by a shear zone at depth; (2) basal melting rates are some 8 m/yr; (3) internal shear occurs with no basal sliding in much of the region above the grounding zone; or (4) internal creep and basal sliding contribute to the flow velocity in varying proportions above the grounding zone. Large gradients of surface strain rate seem to invalidate the first explanation. Computed values of basal shear stress (140 to 200 kPa) provide insufficient factional heat to melt the ice demanded by the second explanation. Both the third and fourth explanations were examined by making simplifying assumptions that prevented a truly quantitative evaluation of their merit. Nevertheless, there is no escaping the qualitative conclusion that internal shear contributes strongly to surface velocities measured on Byrd Glacier, as is postulated in both these explanations.
Regional gravity studies in southeastern California, western Arizona, and southern Nevada
Authors: Mickus Kevin L. , James W. C.
Published: February 2008

Bouguer gravity anomaly, low‐pass, and band‐pass maps of southeastern California, western Arizona, and southern Nevada are analyzed to determine regional crustal structure. Based on 100–250 km band‐pass wavelength filtering of the Bouguer anomaly map, a low‐amplitude gravity maximum is associated with the highly extended terrane along the Colorado River region of California, Arizona, and Nevada. Two crustal models, constructed using Bouguer gravity anomaly data and constrained by geological mapping, aeromagnetic data, and seismic refraction data, are consistent with the dome‐shaped, midcrustal lens of probable dioritic composition centered under the Whipple Mountains proposed by McCarthy et al. (1989), although other models are not precluded. This midcrustal lens extends from the western edge of the Colorado River extensional corridor to east of the Buckskin and Hualapai mountains in Arizona. Dioritic material is exposed in various mountain ranges in the Colorado River extensional corridor and may be part of the source of the gravity maximum that is associated with each range.
Natural convection in the subarctic snow cover
Authors: Sturm M. , Johnson J. B.
Published: February 2008

The purpose of this study was to determine if air convects in a natural snow cover. To detect convection, the temperature field in the subarctic snow cover in Fairbanks, Alaska, was measured hourly during three winters (1984–1987) using an array of thermistors which were suspended on threads and allowed to be buried by snowfall. The results indicate that convection occurred sporadically in 1984–1985 and almost continuously in 1985–1986 and 1986–1987. The evidence was (1) simultaneous warming and cooling at different locations in a horizontal plane in the snow, and (2) horizontal temperature gradients of up to 16°C m−1 During the winter, warm and cold zones developed in the snow and remained relatively fixed in space. We interpret these zones to be the result of a diffuse plumelike convection pattern linked to spatial variations in the temperature of the snow‐soil interface. Air flow was inferred to have been primarily horizontal near the base of the snow and vertical elsewhere. Calculated flow speeds were of the order of 0.2 mm s−1, with a maximum value of 2 mm s−1 The convective circulation was time‐dependent, with perturbations such as high wind or rapid changes in air temperature triggering periods when horizontal temperature gradients were strongest, suggesting that these were also periods when the air flow was fastest. The coincidence of depth hoar crystals with horizontal c axes and the horizontal flow lines at the base of the snow suggests that convection may have affected crystal growth directions.
The Elkhorn Mountains revisited: New data for the Late Cretaceous paleomagnetic field of North America
Authors: Diehl J. F.
Published: February 2008

The Late Cretaceous portion of North America's apparent polar wander (APW) path is poorly known due to a dearth of suitable post‐85‐Ma reference poles. A paleomagnetic re‐investigation of Elkhorn Mountains Volcanics (80 Ma) was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of APW during this time interval correlates with known Late Cretaceous tectonic events. A total of 249 paleomagnetic samples (30 sites) were collected from three separate areas within the Elkhorn Mountains. Using both alternating field and thermal demagnetization, a characteristic direction of magnetization was isolate at 15 sites (a95 < 15°) which passes a fold test at the 95% confidence level. The mean pole calculated from site virtual geomagnetic poles is located at 80.3°N, 189.5°E (N = 15; A95 = 9.6°; k = 16.8). Interestingly, the Elkhorn Mountains pole is not concordant with the Cretaceous stillstand poles but is nearly coincident with the Paleocene reference pole and the recently published pole for the Adel Mountains dated at 76 Ma. These new data suggest the onset of APW which brought the pole from its Cretaceous position to near its early Tertiary position occurred prior to 80 Ma and the transition was exceedingly rapid. The onset of this rapid APW now appears to coincide more closely in time with many of the significant Late Cretaceous tectonic events, e.g., the Laramide orogeny. However, since the Elkhorn Mountains Volcanics are floored by a major east‐directed thrust and have undergone crustal scale deformation, the data should be viewed with caution until more results are available for this time interval.
Heat flow and structure of the lithosphere in the Eastern Canadian Shield
Authors: Pinet Christophe , Jaupart Claude , Mareschal Jean‐Claude , Gariepy Clément , Bienfait Gérard , Lapointe Raynald
Published: February 2008

Twenty‐two new determinations of heat flow and radiogenic heat production in the Superior and Grenville provinces of the Canadian Shield are presented. The new data and previously published measurements strongly constrain the thermal structure of the eastern Canadian Shield. In the Abitibi greenstone belt, heat flow gradually increases from 29 mW m−2 near the Grenville Front to 44 mW m−2 east of the Kapuskasing uplift. This heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1.1 μW m−3. This interpretation, based on estimated heat production of major rock types in the region, is consistent with crustal models derived from recent seismic reflection and refraction studies. It also leads to an estimate of about 12 mW m−2 for the mantle heat flow beneath the area. The average heat flow in the Grenville Province, 41 ± 10 mW m−2, is the same as that of the Superior Province. This similarity and the lack of significant variation of heat flow across the Grenville Front indicate that the crust on both sides of the front has similar heat production and thus composition. In the western part of the Grenville Province, heat flow reaches high values in the vicinity of the boundary between the Allochtonous Polycyclic and Monocyclic belts where enriched granitic plutons are found. In the crystalline terranes in the central part of the Grenville Province, heat flow and heat production are related to each other. The parameters of the linear heat flow‐heat production relationship (Qr = 30 ± 2 in mW m−2 and D = 7.1 ± 1.7 km) are close to those of the much younger Appalachian Province, implying that the higher Appalachian heat flow is due solely to higher heat production in the upper crust. The data provide no evidence for variation of mantle heat flow between the Superior, Grenville, and Appalachian provinces, whose tectonic ages range between 2700 and 400 Ma. The small value of the mantle heat flow, about 12 mW m−2, implies that the depth to the 450°C isotherm, which controls the effective elastic thickness of the lithosphere, is very sensitive to crustal heat production.
The equation of state of a molten komatiite: 1 Shock wave compression to 36 GPa
Authors: Miller Gregory H. , Stolper Edward M. , Ahrens Thomas J.
Published: February 2008

The equation of state (EOS) of an initially molten (1550°C) komatiite (27 wt % MgO) was determined in the 5–36 GPa pressure range via shock wave compression. Shock wave velocityUsand particle velocityUp(kilometers/second) follow the linear relationshipUs= 3.13(±0.03) + 1.47(±0.03)Up. Based on a calculated density at 1550°C, 0 bar of 2.745±0.005 g/cm3, thisUs‐Uprelationship gives the isentropic bulk modulusKS= 27.0 ± 0.6 GPa, and its first and second isentropic pressure derivatives,K′S= 4.9 ±0.1 andK″S= −0.109 ± 0.003 GPa−1. The calculated liquidus compression curve agrees within error with the static compression results of Agee and Walker (1988) to 6 GPa but is less dense than their extrapolated values at higher pressures. We determine that olivine (Fo94) will be neutrally buoyant in komatiitic melt of the composition that we studied near 8.2 GPa. Clinopyroxene would also be neutrally buoyant near this pressure. Liquidus garnet‐majorite may be less dense than this komatiitic liquid in the 20–24 GPa interval; however, pyropic‐garnet and perovskite phases are denser than this komatiitic liquid in their respective liquidus pressure intervals to 36 GPa. Liquidus perovskite may be neutrally buoyant near 70 GPa. At 40 GPa, the density of shock‐compressed molten komatiite would be approximately equal to the calculated density of an equivalent mixture of dense solid oxide components. This observation supports the model of Rigden et al. (1989) for compressibilities of liquid oxide components. Using their theoretical EOS for liquid forsterite and fayalite, we calculate the densities of a spectrum of melts from basaltic through peridotitic that are related to the experimentally studied komatiitic liquid by addition or subtraction of olivine. At low pressure, olivine fractionation lowers the density of basic magmas, but above 13–14 GPa this trend is reversed. All of these basic to ultrabasic liquids are predicted to have similar densities at 13–14 GPa, and this density is approximately equal to the density of the bulk (preliminary reference Earth model) mantle in this pressure range. This suggests that melts derived from a peridotitic mantle may be inhibited from ascending from depths greater than 400 km.
Strain accumulation in western Washington
Authors: Savage J. C. , Lisowski M. , Prescott W. H.
Published: February 2008

The Juan de Fuca plate is subducted beneath the North American plate off the coast of Washington at a rate of about 40 mm/yr N68°E. The average principal strain rates (extension reckoned positive) measured in northwestern Washington are as follows: Olympic peninsula 25 km south of Port Angeles from 1982 through 1990,
and
and near Seattle from 1972 through 1985,
and
. Both strain measurements are consistent with uniaxial contraction in the direction of plate convergence. Uplift rates inferred from tide gage recordings are about 4 mm/yr on the Pacific coast and near 0 mm/yr farther inland near Seattle. These deformation rates are consistent with a model of the Cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward. The limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth (∼20 km) of the isotherm (∼450°C) that defines the brittle‐ductile transition. Small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths. The principal strain rates measured from 1972 through 1983 in the back arc region near Richland, Washington, are
and
.
Spectral analysis and inversion of codas
Authors: Jannaud L. R. , Adler P. M. , Jacquin C. G.
Published: February 2008

A systematic investigation of codas in media with Gaussian and exponential correlation function is presented. By means of various tools such as the wavelet decomposition, it is shown that the spectral analysis of codas has to be limited to a restricted time interval whose boundaries are given. When this condition is fulfilled and when the fluctuations of the sound velocity are smaller than 4%, the average codas power spectrum is indeed proportional to the backscattering coefficient as predicted by the linear theory developed by Aki and Chouet. This coefficient can be successfully inverted to retrieve the statistical properties of the velocity distribution. Various numerical and practical recommendations are suggested.
Geochemical complexities preserved in the volcanic rocks of the Zambales Ophiolite, Philippines
Authors: Evans Cynthia A. , Casteneda Gerry , Franco Helen
Published: February 2008

The geochemical characteristics of the volcanic rocks from the Zambales Ophiolite, Luzon, Philippines, indicate that the oceanic crust now preserved in the ophiolite must have been generated in an oceanic subduction zone environment. The volcanic rocks and dikes range in composition from magnesian basalt (including mid‐ocean ridge basalt (MORB), island arc tholeiite, and boninite‐like lavas) to andesite and dacite and have trace element abundances which suggest derivation from a mantle source which was heterogeneous on both regional and local scales. the sources are mixtures of components variably depleted with respect to a MORB source. Differences exist between respective exposures of the ophiolite. From north to south the following geochemical patterns in the basaltic rocks emerge: Sm/Nd decreases with corresponding decreases in 143Nd/144Nd. Rare earth element (REE) patterns range from extremely light REE (LREE) depleted with generally low abundances of all REE (1–5X chondritic) to patterns which are slightly LREE depleted, similar to MORB patterns. Throughout the range, Sr isotopic abundances are relatively high (87Sr/86Sr = 0.7035–0.7040). Perhaps more significant than the regional geochemical trends, however, is that all volcanic provinces display ranges of rock compositions, and the spatial distribution of basalts with such variable geochemical affinities indicate superposition, interfingering, and possible mixing of different lavas. We suggest that the array of geochemical data from the Zambales ophiolite can be explained in terms of processes observed in present‐day convergent plate margins, such as the Marianas or Lau Basin in the western Pacific. Complicated plate boundaries which have existed for long periods of time, including closely opposing and changing subduction systems, the rifting of arcs, and the formation of backarc basins may result in the superposition of one lava type on another or may produce many small domains in the upper mantle sources for subduction‐related lavas, some of which become extremely depleted or secondarily enriched. Magmas derived from such a heterogeneous mantle will display ranges in geochemical characteristics, possibly similar to those observed in the Zambales ophiolite. Indeed, recent studies from present‐day backarc systems suggest that young arc volcanism is superimposed on MORB‐like crust and that basalt compositions of certain backarc basins are zoned inward from arc‐like to MORB‐like. These studies indicate that relationships between arc and MORB‐like crust in young subduction complexes is extremely complex (Hawkins and Melchior, 1985; Volpe et al. 1987, 1990). We extrapolate arguments from studies of zoned backarc systems to the Zambales Ophiolite and, in turn, use data from the Zambales Ophiolite as a ruler for defining the scale of variation in these complex supra‐subduction zone systems.
Magnetic property zonation in a thick lava flow
Authors: Audunsson Haraldur , Levi Shaul , Hodges Floyd
Published: February 2008

In this study, grain size and composition‐dependent magnetic properties of titanomagnetite minerals are used as indicators of intraflow structures and magmatic evolution in an extensive and thick (30–60 m) basaltic lava flow. Similar zonation occurs in this flow at three localities separated by tens of kilometers. The magnetic properties subdivide the flow to three zones. The upper layer, representing the top 1/3 of the lava (≤ 20 m), has higher magnetic stability due to smaller and more deuterically oxidized titanomagnetite grains, approaching pure magnetite. The central layer in the underlying 2/3 of the flow (≤ 35 m) has larger, magnetically less stable, and less oxidized grains with relatively uniform magnetic properties. The basal layer, the bottom 1/10 of the flow (≤ 5 m), has near primary, least oxidized titanomagnetites (Ulv68Mag32). The upper intraflow boundary of the magnetic properties appears to coincide with the transition from entablature (above) to colonnade (below), distinguishing between regions of faster and slower cooling. Microprobe data indicate that the intraflow oxidation state (Fe3+/Fe2+) of the initially precipitated primary titanomagnetites increases with falling equilibrium temperature from the flow margins to a maximum near the center, the position of lowest equilibrium temperature. In contrast, Curie temperature measurements indicate that titanomagnetite oxidation increases with height in the flow. Modification of the initially symmetric equilibrium titanomagnetite compositions was caused by subsolidus high‐temperature oxidation possibly due to hydrogen loss produced by dissociation of magmatic water, as well as unknown contributions of circulating air and percolating water from above. The titanomagnetites of the basal layer of the flow remain essentially unaltered.