bluuebunny/crossref_metadata_2025 huggingface dataset:
Lake Bonneville: Constraints on lithospheric thickness and upper mantle viscosity from isostatic warping of Bonneville, Provo, and Gilbert stage shorelines
Authors: Bills Bruce G. , May Glenn M.
Published: February 2008

The deformed shorelines of the late Pleistocene Lake Bonneville provide an excellent opportunity to analyze the response of the Earth to surface loads of relatively short wavelengths (≤ 400 km) and short duration (≤ 30 kyr). Recently revised estimates of the lake loading history and an expanded compilation of shoreline elevations for the Bonneville (∼1550 m), Provo (∼1440 m), and Gilbert (∼1300 m) shorelines have prompted us to reexamine the deflection. The earth model we use is identical with that employed by Nakiboglu and Lambeck, and consists of an elastic plate overlying a Maxwell viscoelastic channel and a rigid base. Our calculation differs from that of Nakiboglu and Lambeck in two respects: we used a significantly more detailed load model, and we compared our results with observations from three distinct shorelines. The lake elevation history spans 32 kyr in 18 piecewise linear segments. The water load is computed at each time step from a digital terrain model (4×4 km cell size) and a specified lake elevation. We find that the use of three separate shorelines significantly reduces the acceptable region of parameter space: the effective elastic lithospheric thickness is (23 ± 2) km, the mantle viscosity is (1.2 ± 0.2) 1020 Pa s, and the depth to a significant viscosity increase is no less than 300 km, though it might be much more. Examination of the residual deflections (differences between observed and computed shoreline elevations) reveals a regional tilt down to the NE of ∼6 10−5, which we attribute to collapse of the peripheral bulge produced by the Laurentide ice sheet.
A model of ocean basin crustal magnetization appropriate for satellite elevation anomalies
Authors: Thomas Herman H.
Published: February 2008

A model of ocean basin crustal magnetization measured at satellite altitudes is developed which will serve both as background to which anomalous magnetizations can be contrasted and as a beginning point for studies of tectonic modification of normal ocean crust. The model is based on published data concerned with the petrology and magnetization of the ocean crust and consists of viscous magnetization and induced magnetization estimated for individual crustal layers. Thermal remanent magnetization and chemical remanent magnetization are excluded from the model because seafloor spreading anomalies are too short in wavelength to be resolved at satellite altitudes. The exception to this generalization is found at the oceanic magnetic quiet zones where thermal remanent magnetization and chemical remanent magnetization must be considered along with viscous magnetization and induced magnetization.
A determination of the helium 4 content of near‐surface atmospheric air within the continental United States
Authors: Holland P. W. , Emerson D. E.
Published: February 2008

The Bureau of Mines, Helium Field Operations, in Amarillo, Texas, has determined the helium 4 content of near‐surface atmospheric air within the continental United States. Analysis of 40 atmospheric samples obtained from 12 locations during a 1‐year period showed the helium 4 content of dry air to be constant at 5.2204 ± 0.0041 ppm by volume (ppmv). The reported accuracy of ±0.0041 ppm is at the 68% confidence level. The mean helium concentration obtained is in agreement with the concentration of 5.222 ± 0.017 ppmv reported by Oliver et al. (1984) but is 0.36 ± 0.08% lower than the currently accepted value of 5.239 ± 0.004 ppm reported by Glueckauf (1946). A helium‐4‐tuned mass spectrometer having a precision of ±0.0011 ppm was used to analyze the air samples. Helium‐4‐in‐nitrogen gravimetric standards accurate to ± 0.003 ppm were used for calibration of the mass spectrometer.
Source parameters for 11 earthquakes in the Tien Shan, central Asia, determined by P and SH waveform inversion
Authors: Nelson Michael R. , McCaffrey Robert , Molnar Peter
Published: February 2008

The Tien Shan mountain belt of central Asia was formed in late Paleozoic time and reactivated in Tertiary time following the collision of India with Eurasia, more than 1500 km to the south. To study the style and distribution of faulting occurring today in the Tien Shan, we digitized long‐period World‐Wide Standard Seismograph Network P and SH waveforms of 11 of the largest Tien Shan earthquakes between 1965 and 1982 and then used a least squares inversion routine to constrain their fault plane solutions and depths. Four of the earthquakes occurred near the southern edge of the Tien Shan, and two occurred in the intermontane Fergana Basin. These earthquakes occurred at depths of 10–20 km and are associated with thrust faulting on east‐west to southwest‐northeast striking fault planes that dip moderately (35°–55°). The other five earthquakes, in the northern Tien Shan, were mostly deeper (15–44 km), and the fault plane solutions of four of them are similar to the events farther south. The exception, an event on June 2, 1973, probably occurred on a gently northward dipping, east‐west striking fault plane. All events occur within the basement at depths of 10 km or greater. Waveforms for earthquakes near the edges of the Dzungarian, Fergana, and Kucha basins are fit best using velocity structures with thick sediment layers, implying that sediments from these basins have been and are being underthrust beneath the neighboring mountains. We examined only the relatively small (mb = 5.5–6.2) events after 1965 and did not study any of the largest events (Ms > 8) that occurred in the Tien Shan earlier in this century. Still, we can conclude that north‐south shortening is presently occurring in the Tien Shan, with the formation of basement uplifts flanked by moderately dipping thrust faults. The present‐day tectonics of the Tien Shan seem to be analogous to those of the Rocky Mountains in Colorado, Wyoming, and Utah during the Laramide orogeny in Late Cretaceous and Early Tertiary time.
Mariana Trough lavas from 18°N: Implications for the origin of back arc basin basalts
Authors: Sinton John M. , Fryer Patricia
Published: February 2008

Geochemical data on glass separates from four dredges and two Deep Sea Drilling Project sites near 18°N in the Mariana Trough define 17 distinct chemical groups that probably represent individual magma batches. Much of the observed variation within the sample suite can be explained by low‐pressure fractionation, although effects of magma mixing and variations in melt‐generating processes are also apparent. Relative to mid‐ocean ridge basalts (MORB), Mariana Trough glasses have higher H2O and Al2O3 and lower FeO* at moderate degrees of differentiation. The high Al2O3 is a consequence of the displacement of the olivine‐plagioclase cotectic to lower MgO in the hydrous Mariana Trough magmas. Low FeO can be attributed to low T and P of melting of diapirs rising along cooler adiabats than those beneath normal mid‐ocean ridges. Extents of melting giving rise to Mariana Trough primary magmas were about 15%, within the range of melting extents for primary MORB. Relative to MORB, Mariana Trough lavas are enriched in Ba>Rb>K>light rare earth elements>Sr and depleted in Y. The degree of enrichment inversely correlates with solid/liquid distribution coefficients, suggesting control by a low‐degree melt component in the Mariana Trough source. Relative to submarine lavas of the Mariana arc, Mariana Trough volatiles are depleted in halogens. Melting of the upper part of the subducted crust could occur 125–175 km beneath the Mariana region by addition of volatiles derived from the breakdown of hydrous phases in the lower crust and upper mantle at depths of 150–210 km. Addition of such a melt component to the source region for Mariana Trough magmas will not quantitatively account for the incompatible element enrichments unless altered crust or crustal sediments are included in the slab being melted or open system hybridization reactions occur in the overlying mantle wedge.
Single‐crystal yield surface for trigonal lattices: Application to texture transitions in calcite polycrystals
Authors: Takeshita T. , Tomé C. , Wenk H.‐R. , Kocks U. F.
Published: February 2008

Assuming that deformation is caused solely by dislocation glide and mechanical twinning, with negligible strain rate sensitivity, the plastic behavior of crystals is described by a polyhedron in stress space, the single‐crystal yield surface (SCYS). Each facet of the SCYS represents the yielding condition on one slip or twinning system, and the distance from the origin to a facet is proportional to the critical resolved shear stress (CRSS) on that system. Changes in the various CRSS, for example, due to temperature variations, will be accompanied by topological shape transitions of the SCYS and by changes in the activity of the deformation systems. Thus the final polycrystal deformation texture depends on the particular topology of the SCYS. In this work we analyze the topology changes and the associated texture transitions for calcite based upon the Taylor‐Bishop‐Hill criterion, assuming homogeneous strain. Although the topological changes are very complex, only a few transitions are associated with sharp texture transitions related to major changes in the activity of the different deformation systems. The Taylor‐Bishop‐Hill model agrees with experimental plane strain and pure shear deformation textures at various temperatures even though the model neglects strain rate sensitive process and heterogeneous strain. By comparing simulations with experiments we can infer the relative importance of the different deformation systems as a function of the temperature.
Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots, and deep slab anomalies
Authors: Anderson Don L.
Published: February 2008

Pressure‐induced solid‐solid phase changes are responsible for most of the increase of density and seismic velocity with depth in the upper mantle. Lateral variations in temperature cause a similar effect; abrupt changes of density and seismic velocity due to phase changes are superposed on smaller changes associated with thermal expansion. Temperature‐induced isobaric phase changes are as important in explaining various recent geophysical data as are the more familar pressure‐induced phase changes. In cold slabs the equilibrium mineral assemblage contains high‐density, high‐velocity phases which are not stable in hotter mantle. In particular, the ilmenite form of MgSiO3 and the γ‐spinel form of Mg2SiO4 have broad stability fields in cold mantle which increase the density and velocity of deep slabs to values in excess of those which have been used in geoid and seismic travel time modeling. Recent arguments for slab penetration into the lower‐mantle and whole mantle convection are based on thermal models of the slab which ignore the large density and seismic velocity anomalies associated with temperature‐induced phase changes. When these effects are taken into account, the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and layered models of mantle convection. The seismicity cutoff and evidence for slab thickening at 670 km also favor this style of convection. Mantle seismic velocities between 200 and 400 km depth in tectonic and young oceanic regions are lower than in shield regions, and this is due to the presence of a melt phase and lower‐velocity, high‐temperature phase assemblages. Deep, > 200 km, long‐lived continental roots, differing in chemistry from “normal” mantle, are not required when isobaric phase changes are taken into account. High‐velocity subshield mantle is closer to normal subsolidus mantle than is suboceanic mantle which is affected by the presence of high‐temperature phase assemblages. The whole mantle convection, thick continental root, and deep slab penetration hypotheses are not supported by seismic and geoid data when isobaric phase changes are included in the analysis. Phase changes are more effective in changing density and seismic velocity than are lateral variations in temperature and composition. The lack of correlation of the geoid with ridges, shields, heat flow, and upper mantle velocity variations suggests a low geoid sensitivity to the upper mantle, consistent with layered convection.
Fusion curve slope and crystal/liquid buoyancy
Authors: Walker David , Agee Carl B. , Zhang Youxue
Published: February 2008

A general solution for the slope of a fusion curve is presented. It is not the Clausius‐Clapeyron equation which is a special case for one‐component systems. The connection between fusion curve slope and crystal/liquid buoyancy is obscure in multicomponent systems even though the slopes of fusion curves for any given composition are always well defined. The olivine flotation hypothesis at high pressure is poorly constrained by information derived from fusion curve analysis. Physical property measurements provide the best constraints on buoyancy questions.
Nd and Sr isotope systematics of clastic metasediments from Isua, West Greenland: Identification of pre‐3.8 Ga Differentiated Crustal Components
Authors: Jacobsen Stein B. , Dymek Robert F.
Published: February 2008

Recent field work on the Isua metasediments has established that there are two lithological sequences (A and B), which are divided into a number of distinct formations. Each clastic metasedimentary unit has a relatively unique trace element signature. The depleted mantle Sm‐Nd model ages of samples from all major units range from 3.59 to 3.98 Ga, whereas Rb‐Sr model ages cover a very wide range (1.9–4.6 Ga). The samples from the sequence A felsic gneiss formation (A6) yield a relatively good Rb‐Sr isochron age of T = 3.71±0.07 Ga. The sequence B samples do not yield a Rb‐Sr isochron; however, some of the samples plot close to the A6 Rb‐Sr isochron, and sequences A and B appear to have essentially the same deposition age. The Sm‐Nd data for both sequences yield initial εNd values in the range −1 to +3 for a deposition age TSTRAT = 3.64 Ga (if TSTRAT = 3.81 Ga, the range is 0–4). This range appears to reflect real variability of initial εNd values corresponding to the time of deposition. Data trends in εNd; versus 1/Nd diagrams suggest at least three distinct components in each of sequences A and B. Model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as ≈0.4 b.y. Other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition. The very high initial εNd values implied for early depleted mantle sources (+2 to +4), suggest a very large pre‐3.8 Ga continental crust. Alternatively, these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a ≈4.5 Ga magma ocean.
Gravity anomalies and flexure of the lithosphere at the Middle Amazon Basin, Brazil
Authors: Nunn Jeffrey A. , Aires Jose R.
Published: February 2008

The Middle Amazon Basin is a large Paleozoic sedimentary basin on the Amazonian craton in South America. It contains up to 7 km of mainly shallow water sediments. A chain of Bouguer gravity highs of approximately +40 to +90 mGals transects the basin roughly coincident with the axis of maximum thickness of sediment. The gravity highs are flanked on either side by gravity lows of approximately −40 mGals. The observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide. Within this region, the continental crust has been intruded/replaced by more dense material to more than half its original thickness of 45–50 km. The much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere. The observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15–20 km. Although this value is lower than expected for a stable cratonic region of Early Proterozoic age, it is within the accepted range of effective elastic thicknesses for the earth. Rapid subsidence during the late Paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere. The high‐density zone in the lower crust, as delineated by gravity and flexural modeling, has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin. The linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust.
Elastic properties of hedenbergite
Authors: Kandelin John , Weidner Donald J.
Published: February 2008

The single‐crystal elastic moduli of hedenbergite (CaFeSi2O6) hare been measured at 20°C and 1 bar using Brillouin spectroscopy. The moduli are (in gigapascals): C11 = 222, C22 = 176, C23 = 249, C44 = 55, C55 = 63, C66 = 60, C12 = 69, C13 = 79, C33, = 86, C15 = 12, C25 = 13, C35 = 26, C46 = −10. The comparison of elastic properties among Mg‐Fe‐Ca bearing pyroxenes, known as quadrilateral pyroxenes, reveals only weak variations with changes in composition. Of the four quadrilateral pyroxenes, orthoferrosilite has elastic properties distinctive from the others. The principal differences among these pyroxenes are due to subtle structural differences. In particular, the mechanical linkage between the M2 polyhedral chains in clinopyroxenes enhances the importance of the cation in this site. In contrast to the orthopyroxenes, the aggregate shear modulus μ of the calcium‐bearing clinopyroxenes (diopside and hedenbergite) exhibits no dependence on the amount of iron (Fe2+) present in the structure, while the ratio K/μ does. As a result, the compressional and shear acoustic velocities of the calcium‐bearing clinopyroxenes show a smaller dependency on iron content than do the orthopyroxenes.
Paleomagnetism of the Late PEnnsylvanian and Permian: A test of the rotation of the Colorado Plateau
Authors: Steiner Maureen B.
Published: February 2008

Coeval formations on both the North American craton and the Colorado Plateau have been sampled for the comparison of paleomagnetic pole positions. The latest Pennsylvanian‐earliest Permian Laborcita and the Early Permian Abo formations of eastern New Mexico, and the latest Pennsylvahian Wescogame Formation of the Supai Group of northwestern Arizona were studied. Thermal demagnetization demonstrated that the majority of the samples in all three formations were remarkably free of significant secondary overprinting. Computation of pole positions used only samples which exhibited univectorial decay to the origin over at least three or more temperature steps. The paleopole of the Wescogame Formation is 119.2°E, 47.3°N (N=45 samples, k=41.6, dp=1.7, and dm=3.4); the Laborcita Formation paeopole is 130.6°E, 41.8°N (N=114 samples, k=42.9, dp=1.0, and dm=2.0); and the overlying Abo Formation paleopole is 118.3°E, 49.7°N (N=84 samples, k=54.9, dp=l.l, and dm=2.1). Comparison of the Wescogame, Laborcita, Abo and other paleopoles of this age shows the plateau poles displaced clockwise, confirming a clockwise rotation of the Colorado Plateau with respect to stable North America. The new data could be interpreted to indicate a 7°±4° rotation of the plateau; however, the direction of displacement of Pennsylvanian and Permian paleopoles is significantly different from that exhibited by the Early and the Late Triassic poles. The current body of data is best fit by not one, but two, rotations and suggests that the Colorado Plateau rotated in Late Permian time, prior to a later, presumably Laramide, rotation. Separate calculations of the Late Permian rotation, from Late Pennsylvanian data and from Early Permian data, both indicate a rotation of about 9.5°±2.7° northward and 5.4°±2.7° westward. This study thus suggests that at least two rotations of the Colorado Plateau have occurred prior to the most recent rotation involving the opening of the Rio Grande Rift. This history of rotations implies a quasi‐independent plate motion behavior for the Colorado Plateau microplate.
Tectonic evolution of the North Pacific during the Cretaceous quiet period
Authors: Mammerickx J. , Sharman G. F.
Published: February 2008

The Cretaceous Quiet Period ended with a significant reorganization of the plate tectonics of the North Pacific. The Farallon plate broke into three primary pieces: (1) the Kula plate, (2) a large fragment transferred to the Pacific plate, and (3) the continued Farallon plate. The time of the rearrangement is constrained to just before magnetic anomaly 34 by its presence, contrasts in tectonic spreading fabric (TSF), the Emperor Trough, and the Chinook Trough. Our model is based on (1) a complete reevaluation of primary bathymetric and magnetic data, (2) the distribution of major troughs (Emperor, Chinook, Hokkaido, and Krusenstern), (3) mapping and analysis of TSF orientation and amplitude on Quiet Zone crust, and (4) available Deep Sea Drilling Project drill hole data. East‐west trending magnetic reversal lineations and TSF south of the Aleutian Trench are bounded on the west by the Buldir Fracture Zone at 175°E. The major troughs mark the abandonment and initiation of spreading in the reorganization; their absence elsewhere precludes other long‐distance boundary relocations. Emperor Trough is the abandoned Pacific‐Farallon spreading center. Rift initiation and early slow spreading have created the high amplitude topography of the Chinook Trough. We believe that the Hokkaido Trough is the initiation site of a failed oceanic rift. The incorporation of large amounts of Farallon plate into the Pacific plate obviates the need for the very rapid Pacific‐Farallon spreading rates of previous models and significantly alters the interpretation of mid‐Pacific geology.
Mineral compositions of plutonic rocks from the Lewis Hills Massif, Bay of Islands ophiolite
Authors: Smith Susan E. , Elthon Don
Published: February 2008

The Lewis Hills massif is interpreted as having been formed at an oceanic fracture zone (FZ) prior to its obduction. The compositions of minerals in the residual mantle and cumulate rocks of the massif are similar to those in other massifs of the Bay of Islands ophiolite and those in comparable rocks from the oceanic basins. The residual mantle rocks consist of olivine (Fo89.9–91.7, Ni0 = 0.37 ‐ 0.46%), orthopyroxene (100*Mg/[Mg+Fe] ranges from 90.6 to 91.8), spinel (100*Cr/[Cr+Al] = 43.7 ‐ 57.4 and 100*Mg/[Mg+Fe2+] = 51.2 ‐ 67.7), and rare clinopyroxene. These minerals have compositions similar to those of the most depleted abyssal peridotites. The cumulate units are mixtures of olivine (Fo88.8‐78.0, Ni0 = 0.32 ‐ 0.08), plagioclase (An88.4‐68.8), clinopyroxene (100*Mg/[Mg+Fe] = 93.0 ‐ 75.6) and Cr‐Al spinel (100*Cr/[Cr+Al] = 36.0 ‐ 8.8 and 100*Mg/[Mg+Fe2+] = 73.7 ‐ 50.0). The ultramafic cumulates appear to have crystallized at moderate to high pressures and the gabbroic rocks at low pressures. Differences between the magmatic processes recorded in the Lewis Hills massif during its formation adjacent to a FZ and those inferred for the North Arm Mountain massif while it formed more distantly from the FZ are (1) the more depleted nature of the mantle near the FZ, (2) the higher Ti02 contents of diabase dikes near the FZ, (3) the greater abundance of feldspathic ultramafic rocks near the FZ, and (4) the absence of orthopyroxene as a cumulate phase near the FZ.
A numerical model for the mechanical behavior of intraplate volcanoes
Authors: Chevallier Luc , Verwoerd Wilhelm J.
Published: February 2008

A new approach toward understanding the mechanical behavior of intraplate volcanoes is offered by first postulating a geological model of the internal structure and then developing an axisymmetric numerical simulation using the finite element method. This enables one to consider the types of fracturing at various depths in terms of state of stress under specific conditions of loading. The geological model is based on comparative field studies of three volcanically active oceanic islands (Marion, Tristan da Cunha, and Reunion) and a dormant one (Gough). The model contains a shallow (3 km deep) magma chamber with distinguishable floor, wall, and roof sections. A magma column rises from the chamber toward a central cone at the surface. Depending on the structural level at which they originate, different dyke and fracture patterns are generated. For the numerical simulation the medium is assumed to be linear elastic, continuous, and isotropic. Different loading combinations are applied, including hydrostatic pressure plus magmatic pressure in the chamber and external pressure that represents regional tectonic effects. It appears that the structural levels of the geological model can be adequately explained by variations in the distribution, intensity, and trajectories of principal stresses σ1, σ2, and σ3. It is concluded that the shape of the magma source is a major factor in the stress pattern. The effect of the driving pressure (Pmag‐ Pext) on the orientation of the principal stresses σ1and σ2is also considered. The sloped surface of the volcanic edifice is found to cause local rotation of the stress field. Finally, the evolution of the stress field with loading path is related to successive structural levels in a dynamic eruptive model.
Flexure and seismicity beneath the south flank of Kilauea Volcano and tectonic implications
Authors: Thurber Clifford H. , Gripp Alice E.
Published: February 2008

We present a refined model for the tectonic behavior of Kilauea volcano's south flank, including flexure calculations and microearthquake data supportive of the model. In our model the south flank moves seaward over the downwardly flexed ocean crust in a manner analogous to an accretionary prism in a subduction zone. The flank is driven seaward by the gravitational stresses inherent in its shape; this driving force is augmented by high‐density material in Kilauea's rift zones. Elastic flexure calculations predict a configuration for the downwardly flexed crust which agrees with previous seismic refraction and gravity modeling. This configuration in turn is consistent with the hypothesis that many earthquakes occur along the volcanic pile/ocean crust interface. Focal mechanisms for these events predominantly indicate southeastward directed overthrusting. We also propose a relationship between rift zone intrusion and south flank displacement, including a suggestion that the decollement configuration controls the relative activity of Kilauea's east and southwest rift zones.
Direct Rate Measurements of Eruption Plumes at Augustine Volcano: A Problem of Scaling and Uncontrolled Variables
Authors: Rose William I. , Heiken Grant , Wohletz Kenneth , Eppler Dean , Barr Sumner , Miller Theresa , Chuan Raymond L. , Symonds Robert B.
Published: February 2008

The March–April 1986 eruption of Augustine Volcano, Alaska, provided an opportunity to directly measure the flux of gas, aerosol, and ash particles during explosive eruption. Most previous direct measurements of volcanic emission rates are on plumes from fuming volcanoes or on very small eruption clouds. Direct measurements during explosive activity are needed to understand the scale relationships between passive degassing or small eruption plumes and highly explosive events. Conditions on April 3, 1986 were ideal: high winds, clear visibility, moderate activity. Three measurements were made: 1) an airborne correlation spectrometer (Cospec) provided mass flux rates of SO2; 2) treated filter samples chemically characterized the plume and 3) a quartz crystal microcascade impactor provided particle size distribution. Atmospheric conditions on April 3 caused the development of a lee wave plume, which allowed us to constrain a model of plume dispersion leading to a forecast map of concentrations of SO2 at greater distances from the vent. On April 3, 1986, the emission rate of SO2 at Augustine was 24,000 t/d, one of the largest direct volcanic rate measurements yet recorded with a Cospec. The results, coupled with analytical results from samples simultaneously collected on filters, allow us to estimate HCl emissions at 10,000 t/d and ash eruption rate at 1.5×106 t/d. Based on other data, this ash eruption rate is about 1/50 of the maximum rate during the March–April activity. Filter samples show that the gas:aerosol proportions for sulfur and chlorine are about 10:1 and 4:1, respectively. By contrast, measurements of Augustine's plume, together with ground‐based gas sampling in July 1986 when the volcano was in a posteruptive fuming state, are 380 t/d SO2 and approximately 8000 t/d HCl with no ash emission. The observations of large Cl releases at Augustine support the Cl abundance conclusions of Johnston (1980) based on study of melt inclusions in the 1976 lavas. The results reinforce the need for more measurements during eruptions and for better understanding of scaling of volcanic emissions of various eruptive components.
Saturated and subcooled hydrothermal boiling in groundwater flow channels as a source of harmonic tremor
Authors: Leet Robert C.
Published: February 2008

The potential of hydrothermal boiling in groundwater flow channels for generating harmonic tremor (a relatively monochromatic ground vibration associated with volcanic activity) is examined. We use simple “organ pipe” theory of normal‐mode fluid vibration and fundamental energy considerations to develop a first‐order analytical model of a hydrothermal‐boiling source of harmonic tremor. We use this model to estimate order‐of‐magnitude groundwater flow channel lengths and boiling heat transfer rates required to produce harmonic tremor with dominant frequencies in the range 0.5–5 Hz and surface wave reduced displacements of up to 100 cm2. Depending on groundwater sound speed, flow channel lengths of the order of 1–1000 m are required to produce fluid vibration eigenfrequencies in the range 0.5–5 Hz. The boiling heat transfer rate required to produce tremor with a given surface wave reduced displacement depends on the tremor frequency and on whether saturated boiling or subcooled boiling is the cause of the tremor. Saturated boiling produces groundwater vibration via steam bubble growth, whereas subcooled boiling produces groundwater vibration via steam bubble collapse. We find that subcooled hydrothermal boiling is from 102 to 104 times more efficient than saturated boiling in converting boiling “thermal” power to seismic power. For example, the boiling heat transfer rates required to produce 1‐Hz tremor with reduced displacements of up to 100 cm2 via subcooled boiling are generally less than a few thousand megawatts; for saturated boiling, the required boiling heat transfer rates are several orders of magnitude larger than this. The highest values of heat flow reported in the literature for volcanic crater lakes and terrestrial and ocean floor geothermal areas are of the order of 1000 MW. Taking this value as a first‐order estimate of an upper limit on possible boiling heat transfer rates in volcanic hydrothermal systems, our results suggest that saturated hydrothermal boiling is capable of generating only low‐amplitude harmonic tremor, with surface wave reduced displacements no higher than a few square centimeters. However, subcooled hydrothermal boiling could potentially generate high‐amplitude harmonic tremor, with reduced displacements as large as several hundred square centimeters. As a specific application of our model, we evaluate the potential of hydrothermal boiling for generating harmonic tremor at recently active Mount St. Helens and Nevado Del Ruiz volcanoes. We conclude that subcooled boiling likely could have produced the tremor episodes considered at both volcanoes. Saturated boiling also could explain the Nevado Del Ruiz tremor but probably not the more powerful Mount St. Helens tremor.
Paleomagnetic results from the Mogollon‐Datil Volcanic Field, southwestern New Mexico, and a refined Mid‐Tertiary Reference Pole for North America
Authors: Diehl J. F. , McClannahan K. M. , Bornhorst T. J.
Published: February 2008

Miocene‐Oligocene rocks of the Mogollon‐Datil volcanic field in southwestern New Mexico have yielded paleomagnetic data giving 61 site‐mean/unit VGPs and a paleomagnetic pole located at 81.9°N, 143.6°E (A95 = 4.5°; k = 17.6). This pole is indistinguishable from other mid‐Tertiary reference poles for North America. Combining the Mogollon‐Datil data with VGP data from other studies of mid‐Tertiary (20–40 Ma) rocks yields a paleomagnetic pole at 81.5°N, 147.3°E (N = 239; A95 = 2.4°; k = 15.4). When divided into normal and reversed polarity subsets the data yield paleomagnetic poles that are statistically different at the 95% confidence level [McFadden and Lowes, 1981]. The paleopole determined from reversely magnetized units, located at 78.6°N, 138.5°E (N = 125; A95 = 3.4°; k = 14.5), is 6.8° on the far side of the paleopole determined from normally magnetized units, positioned at 84.2°N, 166.4°E (N = 114; A95 = 3.3°; k = 17.0), when viewed from a common site location (i.e., the western United States). Similar pole positions an differences are also seen in 10‐Ma subsets of this data set. Incomplete removal of a normal overprint on the reversely magnetized units appears unlikely to explain these results. Rather, we suggest that the differences are consistent with the concept that differing harmonics may be associated with the two polarity states over the long term [Merrill and McElhinny, 1977; Merrill et al., 1979].
Implications of zoned garnets for the evolution of sheared lherzolites: Examples From Northern Lesotho and the Colorado Plateau
Authors: Smith Douglas
Published: February 2008

Zoned garnets and bulk rock compositions have been compared in order to examine relationships between the processes that created zoning and the compositional evolution of peridotite xenoliths with high equilibration temperatures and sheared textures. Most xenoliths discussed are from kimberlites in northern Lesotho and from the minette neck at The Thumb on the Colorado Plateau. Zoned garnets in sheared rocks from both suites have characteristic rim enrichment in Ti; rim enrichment in Fe and depletion in Cr are also present in garnets of some samples. Garnets in many peridotite xenoliths, including those typical of coarse‐textured rocks, contain too little Ti to be in equilibrium with normal mantle melts. Rim enrichment in Ti is best explained by exchange between garnets and percolating melt. Single episodes of melt‐rock interaction like those responsible for zoned garnets appear insufficient to derive compositions of sheared peridotites from those typical of coarse peridotites. Minimum effects of melt‐crystal exchange have been calculated from garnet zonation. Fe and Ti were relatively enriched by at least 10% and over 100%, respectively, in one rock, but the calculated changes are not sufficient to produce the present Fe‐Ti values of this rock from those typical of coarse peridotite. In addition, garnet zoning is not correlated with Fe, Ca and Al enrichment in host peridotites. Finally, even core compositions of garnets in most deformed rocks are not as low in Ti as are garnets of most coarse peridotites. Repeated episodes of melt percolation, and consequent exchange with and crystallization of infiltrated melt, are suggested to be responsible for the enrichment of the sheared peridotite xenoliths in Fe and Ti and perhaps in Ca, Al, and other elements. Evidence of multiple events has been recognized in zoning within garnets in a few xenoliths, despite the ephemeral nature of zoning at high temperature, and melt percolation may have affected sheared xenoliths repeatedly. Repeated enrichment processes like those documented by garnet zoning should be most effective in the uppermost asthenosphere and should lead to a compositional divergence of that region from the lithosphere. The suites from The Thumb and northern Lesotho have distinct differences, as in Mg/Si and Fe/Mg. In each suite, however, similar compositional characteristics, such as increased Ti and Fe/Mg, serve to distinguish most sheared from most coarse‐textured peridotites, despite the differences in host magmas, equilibration temperatures, and presumed ages of lithosphere stabilization. These distinctive characteristics of the sheared peridotites in both regions may have been imposed by melt‐rock interactions just below the continental lithosphere.
Hydrothermal brecciation in the Jemez fault zone, Valles Caldera, New Mexico: Results from continental Scientific Drilling Program core hole VC‐1
Authors: Hulen Jeffrey B. , Nielson Dennis L.
Published: February 2008

An unusual breccia sequence penetrated in the lower 30 m of Continental Scientific Drilling Program core hole VC‐1 (total depth 856 m) records a complex hydrothermal history culminating in hydraulic rock rupture and associated alteration at the edge of the Quaternary Valles caldera. The breccias, both tectonic and hydrothermal in origin, were formed in the Jemez fault zone, near the intersection of this major regional structure with the caldera's ring‐fracture margin. Tectonic breccias in the sequence are contorted, crushed, and sheared. Coexisting hydrothermal breccias lack such frictional textures but display matrix flow foliation and prominent clast rounding, features characteristic of fluidization. These hydrothermal breccias were intensely altered, during at least five major stages, to quartz‐illite‐phengite‐pyrite aggregates; traces of molybdenite occur locally. This assemblage indicates interaction with hydrothermal fluid at temperatures in excess of 200°C. The extrapolated present maximum temperature of 184°C in the breccia zone therefore represents considerable cooling since these phases were formed. Fluid inclusions in the breccias also preserve evidence of the prior passage of hotter fluids. The inclusions are principally two phase, liquid rich, secondary in origin, and concentrated in hydrothermal quartz. Older, high‐salinity inclusions, unrelated to brecciation, homogenize in the temperature range 189°–246°C. Younger inclusions, in part of interbreccia origin, are low‐salinity and homogenize in the temperature range 230°–283°C; locally coexisting liquid‐ and vapor‐rich inclusions document periodic boiling of the dilute fluids. These fluid‐inclusion data, along with the probable age of the hydrothermal breccias (<1.5 Ma), the assumed depth at which they developed (about 515 m), and the contemporaneous state of stress (extensional) can be combined to model hydrothermal brecciation at the VC‐1 site. The minimum fluid pressure (Pfr) required to hydrofracture these rocks was probably about 7.5 MPa (0.0146 MPa/m). A boiling point versus depth curve based on these Pfr values graphically defines the physical conditions prevailing when the breccias were formed. When fluid pressure at the assumed depth of brecciation exceeded that curve, in response to rapid release of confining pressure possibly augmented by renewed heating, flashing hydrothermal fluid fractured the enclosing rock. Large overpressures, most likely induced by sudden seismic cracking of a hydrothermally sealed portion of the Jemez fault zone, led to local fluidization of the resulting breccias. Late quartz veining, hydrothermal alteration, and molybdenite mineralization were probably produced by the fluids responsible for brecciation.
Restoration of compositional zonation in the Bandelier silicic magma chamber between two caldera‐forming eruptions: Geochemistry and origin of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico
Authors: Stix John , Goff Fraser , Gorton Michael P. , Heiken Grant , Garcia Sammy R.
Published: February 2008

The Cerro Toledo Rhyolite is a group of high‐silica rhyolite domes and tephras that range in age from 1.45 to 1.12 Ma. The unit crops out in the Jemez Mountains of northern New Mexico and lies stratigraphically between the compositionally zoned upper and lower members of the Bandelier Tuff. The Cerro Toledo Rhyolite provides an exceptional opportunity to study the origin of compositional zonation in silicic magma chambers because it allows us to follow the restoration of the zonation with time between these two large caldera‐forming ignimbrite eruptions. Based upon stratigraphic, geochronologic, and geochemical evidence, we have correlated different Cerro Toledo Rhyolite tephra units with groups of domes. Early Cerro Toledo Rhyolite domes appear to be located generally along the Toledo caldera ring fracture, whereas younger domes tend to cluster in the Toledo embayment. The Cerro Toledo Rhyolite appears to have tapped the most fractionated liquids at or near the top of the Bandelier magma chamber and records the restoration of compositional gradients over a period of 0.33 m.y. after the Lower Bandelier ignimbrite eruption. Cl, Rb, Cs, heavy rare earth elements, Y, Nb, Th, and U increase in concentration in progressively younger Cerro Toledo Rhyolite rocks. Because of depletions in K/Cs, Sr, Zr/Nb, and La/Yb over the same interval, we favor crystal fractionation of essentially quartz, alkali feldspar, zircon, and a light rare earth element enriched phase (probably allanite) as the primary mechanism by which the compositional gradients were reestablished. We believe diffusive processes did not play an important role because (1) certain cations of widely different valencies and diffusivities are not fractionated with respect to each other and (2) the observed chemical gradients conflict with those predicted by recent experimental Soret studies.
A kinematic model for deformation of the lithosphere at subduction zones
Authors: Sato Toshinori , Matsu'ura Mitsuhiro
Published: February 2008

A subduction model that interprets the whole pattern of topography and gravity anomalies across the island arc‐trench system has been constructed on the basis of dislocation theory. We model the lithosphere‐asthenosphere system by a gravitating, stratified half‐space, which consists of an elastic surface layer, an intervening layer with Maxwell viscoelasticily, and an elastic substratum. The steady subduction of the oceanic plate beneath the continental plate is represented by uniform slip at a constant rate over the plate interface which divides the elastic‐viscoelastic half‐space into two blocks. Using the limiting value theorem of the Laplace transform, we can show that the steady slip on the interface deeper than the lithosphere‐asthenosphere boundary does not contribute to surface deformation in the steady state. Accordingly, the pattern of surface deformation associated with the steady state subduction depends only on the thickness of the lithosphere and the geometry of the plate interface shallower than the lithosphere‐asthenosphere boundary. Numerical results show that the pattern of vertical motion at subduction zones is characterized by steep uplift on the continental side, sharp subsidence at the plate boundary, and gentle uplift on the oceanic side. This pattern bears a striking resemblance to the general pattern of topography across island arc‐trench systems. The whole pattern of topography and gravity anomalies across the southern Kurile arc is well matched by the subduction model with a lithosphere thickness of 50 km and a dip angle of 30° at depths.
Degassing of rhyolitic magma during ascent and emplacement
Authors: Westrich H. R. , Stockman H. W. , Eichelberger J. C.
Published: February 2008

The degassing history of a rhyolitic igneous system was documented from analyses of drill core samples through the extrusive and intrusive portions of Obsidian Dome and of surface samples of associated tephra. The initial volatile composition of the Inyo magma was estimated to be 4.0 wt % H2O, 500 ppm F, 800 ppm Cl, and 80 ppm S. Retained volatile contents of glassy and crystalline samples reflect the effects of decompression and second boiling. Decompression is rapid and involves loss of water‐rich fluid until a close approach to lithostatic equilibrium is achieved. Second boiling is a slower process and produces a chlorine‐rich fluid, some of which can be trapped during development of extremely fine crystallization textures. Nearly complete dewatering during decompression of surface‐extruded magma strongly undercools the system (ΔT ≈ 175°C), suppressing crystallization and yielding glassy rhyolitic lava. Partial degassing of shallowly intruded magma permits pervasive crystallization even at high cooling rates. The subvolcanic intrusive regime is the zone of maximum volatile release because second boiling is incomplete in extrusives, and volatile‐bearing crystalline phases are stable in magma crystallized at greater depth.
Correlation of interannual length‐of‐day variation with El Niño/Southern Oscillation, 1972–1986
Authors: Chao B. Fong
Published: February 2008

The El Niño/Southern Oscillation (ENSO) is the most prominent interannual fluctuation in the atmosphere and ocean. The associated motion and mass redistribution cause variations in the (solid) Earth's rotation. This paper studies the correlation of ENSO with the length of day (LOD) variations by means of cross correlation in the time domain and complex coherence in the frequency domain. A Monte Carlo simulation is conducted to obtain estimates for the coherence confidence thresholds that are needed in the study. ENSO is represented by the sea level (air) pressure difference between Tahiti and Darwin, Australia, similar to the Southern Oscillation Index. The cross correlation between this ENSO representation and LOD variation on the interannual time scale, roughly from 1 to 10 years, is found to have a maximum value of 0.68 at the LOD phase lag of 2 months during the studied period of 1972–1986. We conclude the following. (1) Most of the interannual LOD variation is caused by ENSO. This extends the atmosphere‐LOD relationship into the interannual time scale. (2) The transfer of ENSO's axial angular momentum to the solid Earth lags behind the (Tahiti‐Darwin) pressure variation by about 2 months. This places the angular momentum transfer in its proper timing with respect to the evolution of ENSO, providing gross constraints on the modeling of ENSO. The corresponding coherence spectrum, in addition, shows minimum correlation around biennial periods, suggesting the influence of the stratospheric quasi‐biennial oscillation on LOD variations.
Modeling surface deformations at complex strike‐slip plate boundaries
Authors: Li Victor C. , Lim Hun Seng
Published: February 2008

Two new versions of the physical base‐traction model of Li and Rice (1987) have been developed to study the loading processes and surface deformation near plate boundaries with geometric complexities, including a shallow creeping fault segment and two subparallel faults. Predictions of surface velocity and surface slip rate for the region between the San Andreas fault and the Pacific coast at the latitude of the Parkfield‐Cholame segment using model parameters based on geologic, geodetic, and seismic considerations were found to be in good agreement with contemporary geodetic field data (not used in constraining model parameters). Also, predicted surface velocity for the Salton Sea‐Coachella Valley area were fit to recently derived contemporary geodetic data and very large baseline interferometry data, although here the model parameters are not well constrained.