bluuebunny/crossref_metadata_2025 huggingface dataset:
Velocity structure of the Kapuskasing Uplift, northern Ontario, from seismic refraction studies
Authors: Boland A. V. , Ellis R. M.
Published: February 2008

A crustal scale seismic refraction experiment was conducted over the Kapuskasing structural zone, northern Ontario, in 1984. The zone cuts obliquely across the east‐west structural grain of the Superior Province in the Canadian shield and has been proposed as a cross section of Archean crust exposed by thrust faulting along the Ivanhoe Lake cataclastic zone during early Proterozoic time. Five seismic refraction lines of 360–450 km were shot over the area. There were 18 profile shots and two fan shots with a recorder spacing of 2–5 km. We have modeled the travel times and amplitudes of the data from the profile lines and analyzed reflections from the crust‐mantle boundary on the fan shots. We have imaged a low‐velocity zone under the Abitibi greenstone belt ranging from 4–5 to 9–12 km depth that is underlain by a highly reflective zone. There is a considerable deepening of the Moho from 40–43 km to 50–53 km under and to the west of the southern end of the Kapuskasing structural zone. A high‐velocity anomaly of 6.6–6.7 km/s has been imaged in the upper crust down to 20 km depth beneath the Kapuskasing structure with a suggested dip of 15°±2° to the west. This corresponds well to the proposed location of a granulite zone thrust up from the middle or lower crust in the early Proterozoic.
Paleoconstraints during rifting of the northeast Atlantic passive margins
Authors: Sibuet Jean‐Claude
Published: February 2008

The Northeast Atlantic passive margins formed during the early Cretaceous phase of rifting. From Goban Spur to Galicia Bank, these margins are characterized by a tilted fault block geometry, well defined by numerous single‐channel and multichannel seismic data. Stress directions cannot be deduced from these structural data, but recently Withjack and Jamison (1986) have described the strain states and fault patterns produced by oblique rifting using experimental and analytical techniques. We have applied these techniques to the Northeast Atlantic continental margins. All types of extensional margins are represented there with pure extensional rifting (Goban Spur and West Galicia margins), oblique rifting (North Spanish and Celtic margins), and close to pure wrenching (Armorican margin). The rifting directions are parallel to the adjacent oceanic fracture zones or flow lines defined in the Northeast Atlantic and in the Bay of Biscay, except for the West Galicia margin where a 20° difference exists. This difference could be within the error range of the method or could be due to a slight relative motion between Galicia Bank and Iberia during rifting. Globally, the coherence between rifting and spreading directions is an argument in favor of the validity of the method. On the basis of kinematic arguments, the motion of Iberia with respect to Eurasia has an eastward component related to the west to east motion of Africa with respect to Eurasia, while Iberia was a small plate whose motion was roughly coupled to Africa. This is confirmed by our analysis. We conclude that during both the rifting and spreading episodes in the Bay of Biscay, the motion of Iberia with respect to Eurasia was in the southeastward direction. Recently, Boillot (1984) suggested that Iberia moved about 150 km southwestward with respect to Eurasia during the rifting phase and then moved in the southeastward direction during the spreading phase. We suggest that the Boillot hypothesis could be wrong.
Seismotectonics and relative plate motions in the Scotia Sea region
Authors: Pelayo Aristeo M. , Wiens Douglas A.
Published: February 2008

Source parameter inversion of P and SH waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the Scotia Sea region. The derived focal mechanisms show both thrust and strike‐slip faulting along the North Scotia Ridge and normal and strike‐slip faulting along the South Scotia Ridge. Two thrust faulting mechanisms south of Tierra del Fuego indicate diffuse convergence of the Antarctic plate along the western margin of the Scotia plate. Earthquakes are consistent with active subduction and back arc spreading along the South Shetland Trench and Bransfield Strait. The depths of two earthquakes (35 km and 55 km) and one shallow thrust faulting event suggest continued subduction along the South Shetland Trench, but the underthrusting is largely aseismic due to the young plate age and slow subduction rate. The magnitude and depth of normal faulting earthquakes along Bransfield Strait and the South Scotia Ridge are suggestive of diffuse extension rather than typical organized mid‐ocean spreading, which is generally associated with smaller and shallower earthquakes. Eight strike‐slip and thrust faulting events provide well‐constrained slip vectors showing Scotia plate motion relative to the surrounding larger plates. Using a four‐plate model (Scotia (SC), South Sandwich (SS), Antarctica (AN) and South America (SA)) and a fixed SA‐AN Euler vector, we inverted these data together with published centroid moment tensor slip vectors along the South Sandwich Trench and published marine magnetic data from the SS back arc spreading center to derive the first quantitative plate motion model for the Scotia region. The SC‐SA Euler pole is located in the South Atlantic and shows large latitudinal uncertainties. The SC‐AN pole is better constrained and located in the Weddell Sea. These results predict left‐lateral strike‐slip motion with a component of compression along the North Scotia Ridge, left‐lateral strike‐slip with a component of extension along the South Scotia Ridge, and east‐west compression in Drake Passage. Although no rate data have been published connecting the Scotia plate with the global plate system, Euler vector magnitudes are obtained through closure. The results, though poorly constrained, suggest relative motion rates of 0.5 cm/yr along the North Scotia Ridge and 1.0 cm/yr along the South Scotia Ridge and in Drake Passage.
Sediment compaction and fluid migration in the Makran Accretionary Prism
Authors: Minshull Tim , White Robert
Published: February 2008

The Makran continental margin in the Gulf of Oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland. A recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region. Velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge, as seen in previous sonobuoy wide‐angle data. Accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism; this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment. A prominent “bottom simulating reflector” appears 500–800 m beneath the sea bed. Several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas, which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile. The hydrate reflector appears to shallow in the region of some faults, suggesting a temperature anomaly due to the presence of warm pore fluids. A heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens. Simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly. The inferred presence of overpressured pore fluids in the Makran suggests that accreted sediments have a low permeability. The seismic evidence suggests a two‐stage compaction process, with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones.
Spinel‐lherzolite‐bearing quaternary volcanic centers in San Luis Potosí, Mexico: 2. SR and ND Isotopic Systematics
Authors: Pier Jean G. , Podosek Frank A. , Luhr James F. , Brannon Joyce C. , Aranda‐Gomez J. Jorge
Published: February 2008

We present Sr and Nd isotopic data for 18 basanites, and chemical and isotopic data for five associated kaersutites, from the Ventura and Santo Domingo volcanic fields in the Mexican state of San Luis Potosí. All isotopic data fall in or near the “mantle array.” The kaersutites have the most depleted isotopic compositions (low 87Sr/86Sr and high ε143) but are less depleted than mid‐ocean ridge basalts (MORB). Basanite isotopic compositions are distributed bimodally, with most samples in the more‐depleted mode, which is near the high ε143 and low 87Sr/86Sr end of the compositional range observed for ocean island basalts (OIBs). In a companion paper, Luhr et al. [1989] find chemical variations in these volcanic rocks that suggest partial melting could be responsible for the principal chemical trends, but the variability of isotopic compositions precludes any simple one‐component model. 87Sr/86Sr, but not ε143, is found to correlate with these chemical trends. SiO2 is lowest and normative ne + lc, incompatible element concentrations and light versus heavy rare Earth element ratios are highest in samples with the lowest 87Sr/86Sr values. Crustal contamination may be significant for a few of these samples, but we conclude that the principal isotopic trends arise in mixing of at least three mantle reservoirs: a depleted component analogous to the MORB reservoir, a St. Helena type component, and a third component which affects Sr composition substantially more than Nd composition. The third component carries relatively radiogenic Sr and appears to correlate with degree of partial melting, suggestive of a hydrous component, possibly derived from a subducted slab, such as has previously been invoked in studies of island arcs.
Spectral characterization of igneous rocks in the 8‐ to 12‐μm region
Authors: Walter Louis S. , Salisbury John W.
Published: February 2008

Variation in the spectral response of silicates in the thermal infrared (TIR) from 8 to 12 μm is the foundation for potentially important applications in the lithologic characterization of terrestrial and extraterrestrial surfaces. The purpose of this study is to elucidate the crystallo‐chemical basis for the variation in spectral behavior in terms which may be useful in remote sensing. This has led to the definition of a new parameter, SCFM, which is defined as the ratio SiO2/(SiO2 + CaO + FeO + MgO) in minerals and rocks. This parameter reflects the degree of depolymerization of the silica tetrahedra and is therefore highly correlated with the structure of TIR reflectance spectra of major silicate minerals in both fine‐ and coarse‐grained igneous rocks. It is also a good numerical descriptor of igneous rock composition. Thus SCFM is proposed as a very useful determinant for spectral identification of igneous rocks. The SCFM parameter was used to assess the effect of variations in location, number, and width of bands in discriminating rock composition. (An unfortunate ambiguity exists in the use of the term “band” which in spectroscopy indicates a feature in a spectrum and in remote sensing signifies a specific wavelength increment or channel. The latter definition is used in this paper.) Spectra of coarse‐particulate mineral and solid rock samples were acquired in the laboratory. A regression analysis of bands varying in width from 0.2 to 1.4 μm versus SCFM results in coefficients of determination (correlation coefficients), R2, ranging from 0.88 (for two 1.2‐μm‐wide bands) to 0.97 (for seven 0.2‐μm‐wide bands). Deletion of bands in the ozone absorption region (9.4–10.2 μm) results in a decrease in R2 by only 0.01. Stimulation of system noise by addition of ±5% random counts to the bands decreases R2 by only 0.03. Because spectra structure is modified by weathering, alteration, and texture of the samples, the validity of these results is limited to fresh igneous rocks. With this constraint, it is concluded that accurate spectral identification of igneous rock composition is not greatly affected by bandwidth or system noise. It is the overall spectral envelope rather than the fine structure in the TIR which contains most of the information diagnostic of igneous rock composition. This permits design of remote sensing systems with broad bands and corresponding higher signal‐to‐noise, which can be traded off against the important requirement for higher spatial resolution.
High‐pressure equation of state of molten anorthite and diopside
Authors: Rigden Sally M. , Ahrens Thomas J. , Stolper E. M.
Published: February 2008

New Hugoniot equation of state data for molten diopside (at 1773 K) and molten anorthite (at 1923 K) are reported to 38 and 35 GPa, respectively. The diopside data (initial density, 2.61 Mg/m3) are described by a straight‐line fit to the shock velocity‐particle velocity results of US = 3.30 + 1.44 UP km s−1, and our preferred fit to the anorthite data (initial density, 2.55 Mg/m3) is given by US = 2.68 + 1.42 UP km s−1. Reduction of the data to a third‐order Birch‐Murnaghan isentrope assuming the Gruneisen ratio times the density is a constant, and the Mie‐Gruneisen equation of state gives K0S = 22.4 GPa and K′S = 6.9 for diopside. For anorthite we calculate K0S = 17.9 GPa and K′S = 5.3. The present data for diopside are used to calculate the diopside solidus at high pressures. We expect the solidus to be shallow above ∼10 GPa, but the lack of data on the variation of either the Gruneisen parameters of the liquid and crystal or the heat capacity and thermal expansion at elevated pressures makes extrapolation of fusion curves uncertain. Solidus temperatures of 2400–2500 K and 2560–2705 K for diopside are calculated at 10 and 20 GPa, respectively. The new data are combined with those of Rigden et al. [1988] for the Di0.64A0.36 eutectic composition to examine the degree to which such liquids mix ideally with respect to volume up to ∼25 GPa. For the eutectic composition at 1400°C we calculate the volumes of the An and Di mix nearly ideally to 25 GPa. We find that the ratio of the partial molar volumes of the oxides in silicate melts to that of the crystal oxides at 1673 K and 1 atm is 1.0±0.1 for a wide range of oxide components. For the low‐pressure tetrahedrally coordinated oxides (e.g., SiO2, Al2O3, Fe2O3) the ratio is >1.3 with respect to oxides such as stishovite, corundum, and hematite in which the cations are octahedrally coordinated by oxygens. If changes in coordination of Al and Si from tetrahedral at low pressures to octahedral at high pressures occur in melts, they do so gradually over an interval of ∼40 GPa. Although 1 atm bulk moduli for a wide compositional range of silicate melts are similar, the differences in integrated compression to mixed oxide‐like high‐pressure configurations are reflected mainly by variations in KT′. KT′ is found to vary inversely with fraction of network forming initially tetrahedrally coordinated cations (e.g., Al3+, Si4+). Thus KT′, which may be uncertain by ±1.5, is estimated to vary from ≲7 for molten anorthite, enstatite, and diopside to ∼8 for molten ferrosilite to ∼10 for molten forsterite and to ∼11 for molten fayalite.
Analysis of regional broadband recordings of the 1987 Whittier Narrows, California, earthquake
Authors: Bolt B. A. , Lomax A. , Uhrhammer R. A.
Published: February 2008

Recordings of the Whittier Narrows earthquake of October 1, 1987 (BRK ML = 6.0), and the large aftershock of October 4 (BRK ML = 5.5) from broadband seismographs (10 mHz to 10 Hz) at six stations in the Berkeley Seismograph Network in northern California at 430 < Δ < 825 km demonstrate the range of information provided on source mechanisms in such circumstances. Analysis of the waveforms shows that estimation of source mechanisms in this case requires incorporation of the significantly different velocity structures along the Coast Ranges and the Sierra Nevada indicated by the broadband signals. Inversion of the transverse component waveforms, after allowance for regional wave dispersion, provides the following source estimations for the main shock: seismic moment M0 of (1.0±0.06) × 1025; preferred fault plane (strike, rake, and dip) of (259°±33°, 128°±16°, 11°±15°). The main shock mechanism is interpreted as a shallow north dipping reverse fault with the hanging wall moving southeasterly relative to the footwall. Sources of error and of differences with the more robust solution from locally recorded first motions include wave mixing and depth bias at the source. The broadband recordings indicate clearly that the mechanism of the aftershock (M0 of (1.5±0.11) × 1024 dyn cm), although not well resolved by these regional measurements, is quite different from the main shock mechanism.
Petrogenesis of Valle Grande Member rhyolites, Valles Caldera, New Mexico: Implications for evolution of the Jemez Mountains Mgmatic System
Authors: Spell Terry L. , Kyle Philip R.
Published: February 2008

The Valles Caldera, located near the center of the Jemez Mountains Volcanic Field in north‐central New Mexico, was formed at ∼1.12 Ma upon eruption of the upper Bandelier Tuff. The Valle Grande Member (VGM) consists of 8 rhyolite domes or dome complexes erupted from vents which define the ring fracture within the caldera. They range in age from the time of caldera formation to ∼0.45 Ma and are volumetrically and temporally the dominant post‐caldera eruptives. VGM rhyolites are high‐silica (>75% SiO2 anhydrous), high‐K, are characterized by low abundances of Fe, Mg, Mn, Ti, and Ca, and are metaluminous to weakly peraluminous. Phenocryst assemblages include sanidine (Or44–61) + quartz + plagioclase (An7–20) + biotite + hornblende + Fe‐Ti oxides ± zircon ± allanite ± apatite ± clinopyroxene ± orthopyroxene. Many phenocryst phases show progressive changes in composition, size, and abundance with decreasing age. Whole rock chemistry, phenocryst chemistry, and isotopic dating indicate that VGM rhyolites can be divided into 3 groups: group 1 ranging in age from 1.18 to 0.71 Ma, group 2 from 0.55 to 0.51 Ma, and group 3 at 0.45 Ma. Groups 1 and 2 define differentiation trends in which trace elements such as Rb, Cs, Y, Nb, HREE, Ta, Th, and U increase with decreasing age, whereas Sr, Ba, Zr, LREE, and Eu decrease. Abundances of trace elements may increase or decrease 2–3 fold within eruptive groups; major elements remain relatively constant. The observed differentiation trends can be accounted for by crystal‐liquid fractionation at a minimum or eutectic using modal phenocryst phases. Fractionation may have occurred by roof and side wall crystallization with the collection of bouyant differentiated liquids at the roof of the magma chamber. Two‐feldspar geothermometry, geobarometry using hornblende rim Al contents, and plots of normative compositions on the Q‐Ab‐Or diagram indicate that fractionation took place at depths of 2.5–7.5 km with pre‐eruptive temperatures ranging from ∼720–810°C. Rhyolites erupted in the Jemez Mountains over the last 2–3 m.y. appear to be the products of many separate magma batches and were not derived from one large long‐lived magma chamber. VGM rhyolites represent 3 separate magma batches (groups 1, 2, and 3), two of which remained closed systems for sufficient periods of time to generate highly evolved differentiates (groups 1 and 2).
The Magellan seamounts: Early Cretaceous record of the South Pacific isotopic and thermal anomaly
Authors: Smith Walter H. F. , Staudigel Hubert , Watts Anthony B. , Pringle Malcolm S.
Published: February 2008

We present geophysical, geochemical, and geochronological data from two Early Cretaceous seamounts in the western Pacific Ocean which formed at melt sources in what is now French Polynesia. One seamount has an 40Ar/39Ar mineral age of 119.6(±0.6, 1 σ) Ma, is flexurally compensated with an elastic plate thickness Te of 10(±2) km, and is isotopically similar to the island of Mangaia, or toward the HIMU mantle end‐member. The second has an age of 100.1(±0.5, 1 σ) Ma, is compensated with a Te of 15(±3) km, and is isotopically similar to Samoa, toward the EM‐II “Dupal” mantle end‐member. Geologic evidence suggests that the summits of these seamounts formed in shallow water, indicating that the underlying Jurassic seafloor was unusually shallow during the Cretaceous. Te values from these Cretaceous seamounts and localities in French Polynesia depart systematically from the simple square root of age law which otherwise applies throughout the ocean basins. A thermal rejuvenation of the lithosphere beneath these seamounts during or prior to their emplacement may explain their unusually low Te values and subsidence history. These two Early Cretaceous seamounts exhibit the isotope, Te, and depth anomalies which have been previously described at young volcanoes in French Polynesia, and we infer that a South Pacific Isotopic and Thermal Anomaly has existed for at least 120 m.y.
Structure of young oceanic crust at 13°N on the East Pacific Rise from expanding spread profiles
Authors: Harding A. J. , Orcutt J. A. , Kappus M. E. , Vera E. E. , Mutter J. C. , Buhl P. , Detrick R. S. , Brocher T. M.
Published: February 2008

We present the results of the analysis of expanding spread profiles (ESPs) collected on and near the axis of the East Pacific Rise at 13°N. These profiles were collected at 0, 1.1, 2.1, 3.6, and 9.5 km from the rise axis, and all but the most distant profile show a distinct low‐velocity zone (LVZ) located within layer 3 of the oceanic crust. At the ridge crest, the top of the magma chamber is at the base of layer 2, while 3.6 km off axis, the roof of the LVZ is 1.1 km below the top of layer 3. The profile farthest from the ridge could possibly have a residual LVZ confined to the lower 1.5 km of the crust. The total width of the LVZ, as determined from the ESP data, is at least 6 km, and possibly much greater. This wide LVZ apparently contradicts multichannel seismic data which show cross‐axis reflections from the magma chamber with a width of <5 km. We suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt. The ESP data are sensitive to this larger region, while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast. We identify the presence of a layer at the top of the oceanic crust with initial P wave velocities between 2.35 and 2.6 km/s, while the S wave velocity is estimated as being ≤0.8 km/s. The layer thickness lies between 100 and 200 m. These velocities are consistent with previous estimates for the Pacific and recent results for the Atlantic. The thickness of this layer is consistent with that of layer 2A determined from geophysical measurements at Deep Sea Drilling Project hole 504B.
Magnetic evidence for the nature of the crust beneath the southern Red Sea
Authors: Hall Stuart A.
Published: February 2008

Magnetic data over the southern Red Sea are used to examine various models for the evolution of the region. The linear nature of the magnetic anomalies over the coastal margins and adjacent portions of the main trough together with their decrease in amplitude with elevation is shown to be compatible with deep, thin sources that are strongly magnetized. The general lack of correspondence between gravity anomalies and deskewed magnetic anomalies indicates that the anomalies are due to intrabasement bodies rather than basement relief associated with density contrasts. Across‐axis correlations of individual anomalies indicate a symmetrical disposition of magnetic sources about a line close to the present deep water axis. The regular, symmetrical pattern of linear anomalies strongly suggests a seafloor spreading origin. The combined geologic and geophysical evidence, therefore, supports a model in which the margins are underlain by material more typically oceanic in character than continental. The magnetic evidence further implies that this oceanic crust formed during an earlier phase of spreading in the Red Sea. Although the age of this crust is unknown, anomaly correlations suggest spreading may have taken place during the Late Oligocene‐Early Miocene.
Historical seismicity and implications for diffuse plate convergence in the northeast Indian Ocean
Authors: Petroy David E. , Wiens Douglas A.
Published: February 2008

Large historical earthquakes provide constraints on the geographical extent and kinematics of the slow, diffuse plate boundary extending from the Central Indian Ridge to the Sumatra Trench. The fault parameters of pre‐1963 earthquakes are derived using a grid search for the best fit between observed and synthetic body waveforms, surface wave spectral amplitudes, and polarity and polarization constraints, and location uncertainties are determined using a bootstrap resampling method. The March 9, 1928 Ninetyeast Ridge (M 7.7) event shows left lateral strike‐slip faulting along a plane approximately coincident with the orientation of the Ninetyeast Ridge. Two small events which occurred at the location of the 1928 event in 1980 show focal mechanisms nearly identical to that determined for the 1928 earthquake and apparently reactivated the same fault. The 1928 waveforms can also be fit by a thrust faulting solution with ENE striking fault planes, but a variety of evidence, including the recent strike‐slip earthquakes at the same location, suggest the strike‐slip solution. These earthquakes are located along the western scarp of the ridge and do not represent reactivation of the Ninetyeast Fracture Zone to the east of the edifice, which was a major plate boundary in the early Cenozoic. The January 23, 1949 (M 6.75) and March 22, 1955 (M 7.0) Wharton Basin earthquakes, although only 350 km apart, show different strike‐slip and thrust faulting mechanisms. All earthquakes show a regionally consistent pattern of horizontal compressional stress, which changes from NW‐SE in the Wharton Basin to N‐S in the Central Indian Basin. The compressional axes are oriented perpendicular to linear gravity anomalies associated with lithospheric flexure, indicating little change in stress orientation since the onset of folding. Seismic deformation along the diffuse boundary consists of sinistral strike‐slip faulting associated with N‐S features such as the Ninetyeast Ridge and the 86°E fracture zone, and thrust or reverse faulting in regions between fracture zones. Assuming transform motion, the approximate slip rate along the northern Ninetyeast Ridge calculated from the summed moments of earthquakes is 2.8 mm/yr. This value is an order of magnitude less than previous estimates, and suggests that at most only a small fraction of the current relative plate motion (∼7 mm/yr) occurs along the ridge. The geographic distribution of seismicity suggests that the Wharton Basin east of the Ninetyeast Ridge is nearly as seismically active as the Central Indian Basin to the west. This result, along with observed long wavelength gravity anomalies in the Wharton Basin, indicates that the India‐Australia plate boundary intersects the Sumatra Trench along a diffusely deforming region several thousand kilometers long, rather than along a discrete transform zone.
Hydrologic effects on water level changes associated with episodic fault creep near Parkfield, California
Authors: Roeloffs Evelyn A. , Burford Sandra Schulz , Riley Francis S. , Records Andrew W.
Published: February 2008

As part of the Parkfield, California, earthquake prediction experiment, water level is monitored in a well 460 m from the main trace of the San Andreas fault on Middle Mountain, in the preparation zone of the anticipated Parkfield earthquake. The well configuration allows water level to be monitored in two fluid reservoirs at depths of 85 and 250 m below land surface. During 1987, water level changes were recorded during 12 of the 18 episodes of accelerated fault creep detected by a creep meter spanning the fault trace 750 m northwest of the well. The creep‐related water level changes in the shallow reservoir have durations of less than 1 day, whereas in the deeper reservoir the changes persist for as long as 2 months. These data suggest that the transient nature of the water level changes in the shallow interval is due to vertical flow to the water table and is not evidence that creep events propagate past the well. Phase leads of earth tidal constituents in the water level data from the shallow interval relative to the same constituents in the local volume strain tide support the interpretation of significant flow to the water table at periods of 1 day or less. The form of the water level changes in the deep interval is affected by horizontal flow to the well bore. This effect can be removed from the water level records using a theoretical response curve constrained by the phases of earth tidal constituents in the deep interval relative to the local volume strain tide. For the events where the signal in the shallow interval has been large enough to measure, the sizes of the simultaneous water level changes in the two reservoirs are consistent with the same amounts of volume strain occurring at both depths.
Synfolding magnetization in the Jurassic Preuss Sandstone, Wyoming‐Idaho‐Utah Thrust Belt
Authors: Hudson Mark R. , Reynolds Richard L. , Fishman Neil S.
Published: February 2008

The Jurassic Preuss Sandstone, exposed in five thrust plates of the Wyoming‐Idaho‐Utah thrust belt, carries directions of remanent magnetization that group most tightly after only partial unfolding. Field, petrographic, and rock magnetic evidence indicates that the carrier of this magnetization is detrital, low‐Ti titanomagnetite. The detrital titanomagnetite was remagnetized at low temperatures (75°–150°C) probably completely during folding. Anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain. We suggest that low‐temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization, the acquisition of which was enhanced by stress, or a piezoremanent magnetization that involved stress‐induced movement of domain walls during intracrystalline strain, or was a combination of the two mechanisms. Stress may promote remagnetization at temperatures much lower than predicted by current theoretical models. Other mechanisms, such as acquisition of chemical remanent magnetization during folding, deflection of a prefolding magnetization by internal strain, or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the Preuss. The locus of acquisition of synfolding magnetization in the Preuss migrated in conjunction with deformation in the thrust belt. A model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps. A lack of reverse‐polarity directions remains a puzzling feature of the remanence. The remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in Late Cretaceous and early Tertiary (?) periods of predominantly normal polarity.
Magnetism and transmission electron microscopy of Fe‐Ti oxides and pyroxenes in a granulite from Lofoten, Norway
Authors: Schlinger Charles M. , Veblen David R.
Published: February 2008

Granulites from southwest Lofoten, Norway, often carry intense and stable components of natural remanent magnetization (NRM). Coercivities in excess of 100 mT and blocking temperatures up to 580°–645°C are common. Several phases in these rocks can carry stable components of NRM. Clinopyroxenes (augite with exsolved pigeonite and orthopyroxene) usually contain abundant Fe‐Ti oxides, the majority of which are too small to be studied conclusively with visible light. Additionally, these rocks typically contain coarse‐grained (∼100–1000 μm) exsolution intergrowths of rhombohedral Fe‐Ti oxides and coarse‐grained magnetite. We have studied the magnetism and mineralogy of the titanohematite and the inclusion‐bearing augites in one granulite sample using transmission electron microscopy (TEM) and magnetic experiments to understand better the remanence properties of naturally occurring Fe‐Ti oxides and their relationships to silicate minerals. The coarse‐grained titanohematite has a coercivity of 350 mT and a laboratory saturation remanence that unblocks at 570°–600°C. The hysteresis characteristics of this titanohematite are typical of much smaller ∼10μm‐sized single domains, which apparently reflects the presence of exsolved ilmenite lamellae, as small as <10 nm in thickness. These lamellae presumably inhibit (1) domain wall nucleation and (2) movement of walls that manage to nucleate. The inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560°C. Magnetite, hematite, and ilmenite are present in the augite, and intergrowth of the rhombohedral phases, revealed by TEM, persists down to the 20‐nm scale. The distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure, while that of the rhombohedral Fe‐Ti oxides is strongly controlled by silicate microstructure, with intergrowths of these oxides commonly decorating lamellae of pigeonite, orthopyroxene, and clinoamphibole within the augite. The NRM of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m.y. ago. Paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event. In addition to the included oxides, the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites: pigeonite lamellae are not parallel to the b‐crystallographic axis, pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur. Rare earth elements are concentrated within inclusions that reside in the augite.
Simultaneous stochastic inversion for geomagnetic main field and secular variation: 2. 1820–1980
Authors: Bloxham Jeremy , Jackson Andrew
Published: February 2008

We apply the method of simultaneous stochastic inversion for the geomagnetic main field and secular variation to data from the period 1820–1980, producing two time‐dependent field models, one for the period 1900–1980, the other for 1820–1900. We introduce several adaptations of our original formalism of the inverse problem: we choose to represent the time dependency of the field by an expansion in Chebychev polynomials; we apply an alternative regularization condition in the time domain; and we adopt a different method from that used in our earlier study for solving the resulting system of equations. Whereas our earlier study used only twentieth century observatory data, we consider almost all the available data from the time interval 1820–1980. We examine the effects of the crustal field on our observations and find that existing methods of accounting for these fields as sources of random noise are inadequate in two circumstances. The first is sequences of measurements made at one particular site (for example, at permanent magnetic observatories), in which case the crustal field produces temporal correlation of the errors in the observations: we devise a method for accounting for these correlations. The other is measurements made at satellite altitude, the crustal field producing spatial correlation of the errors. We show how, using a very simple statistical model of the crustal field, these correlations can be calculated. Rather than attempting to account directly for these correlations (which would require, without further assumptions, massive computation) we instead limit our data selection to points sufficiently well separated in space that the effect of the crustal field is minimal. We are able to fit the data to within our estimates of the noise in the data, yet the resulting models have lower power in the secular variation than previously postulated by some authors. Many of the features in the field at the core‐mantle boundary described in recent work by Bloxham and Gubbins are seen most clearly; indeed this time‐varying representation of the field reinforces many of their earlier conclusions.
Geologic implications of topographic, gravity, and aeromagnetic data in the northern Yukon‐Koyukuk Province and its borderlands, Alaska
Authors: Cady John W.
Published: February 2008

The northern Yukon‐Koyukuk province is characterized by low elevation and high Bouguer gravity and aeromagnetic anomalies in contrast to the adjacent Brooks Range and Ruby geanticline. Using newly compiled digital topographic, gravity, and aeromagnetic maps, I have divided the province into three geophysical domains. The Koyukuk domain, which is nearly equivalent to the Koyukuk lithotectonic terrane, is a horseshoe‐shaped area, open to the south, of low topography, high gravity, and high‐amplitude magnetic anomalies caused by an intraoceanic magmatic arc. The Angayucham and Kanuti domains are geophysical subdivisions of the Angayucham lithotectonic terrane that occur along the northern and southeastern margins of the Yukon‐Koyukuk province, where oceanic rocks have been thrust over continental rocks of the Brooks Range and Ruby geanticline. Basalt of the Angayucham domain causes strong gravity highs and weak magnetic highs. The Kanuti domain is distinguished from the Angayucham domain by intense magnetic highs caused by cumulus mafic and ultramafic plutonic rocks, abundant ultramafic mantle tectonites, and magnetic syenite and monzonite. Long‐wavelength, low‐intensity magnetic highs and undulating gravity anomalies indicate an undulating basement surface of varied lithology beneath the Kobuk‐Koyukuk and Lower Yukon basins. Modeling of gravity and magnetic anomalies shows that oceanic rocks of the Angayucham and Kanuti domains dip inward beneath the Kobuk‐Koyukuk basin. The modeling supports, but does not prove, the hypothesis that the crust of the Kobuk‐Koyukuk basin is 32–35 km thick, consisting of a tectonically thickened section of Cretaceous volcanic and sedimentary rocks and older oceanic crust. Plutons of the Brooks Range and the southern Ruby geanticline are nonmagnetic, ilmenite series, S‐type granites that cause magnetic lows. Plutons of the northern Ruby geanticline are variable in their magnetic properties and cause both highs and lows. Plutons of both the eastern and western Yukon‐Koyukuk province are variable in their magnetic expression but commonly cause magnetic lows in contrast to andesite.
Sequence and mechanisms of deformation around the Hellas and Isidis Impact Basins on Mars
Authors: Wichman R. W. , Schultz P. H.
Published: February 2008

The distribution and age of early volcanic and tectonic features surrounding the Hellas and Isidis impact basins are shown to fit a four‐stage sequence. First, concentric “canyons” form outside the basin boundary scarp at or near the time of basin formation, followed shortly thereafter by radial troughs extending beyond the boundary scarp. After a hiatus, concentric graben develop inside the basin within the massif ring. Finally, soon after graben formation, widespread volcanic plains are emplaced between the massif ring and the boundary scarp to one side of the basin. Three models of basin‐centered deformation are compared to these observed deformation events: elastic flexure due to loading, elastic flexure due to uplift, and impact fracture. A central load of basin fill can account for concentric fractures by elastic flexure around the load. Formation of the distant concentric canyons, however, requires lithospheric thicknesses a factor of 5 greater than those indicated by planetary thermal history models and an increase in feature width by an order of magnitude relative to flexural graben identified elsewhere on Mars and the Moon. Although inclusion of loading by basin ejecta reduces the required lithospheric thickness to acceptable values, flexure still fails to account for the large canyon widths observed. Impact fracture from a rapid inward flow of asthenosphere during transient cavity collapse, however, is consistent with the three key observational constraints for the distant canyons: their age, width, and distance from the basin. As the basin then undergoes isostatic uplift soon after impact, elastic flexure induces a radial pattern of failure consistent with the observed extent and timing of the radial troughs. Later, after a long period of basin infilling, elastic failure due to loading of the central basin region can account for the ring grabens in the massif ring. The localization of volcanism into a single ridged plains unit on the basin rim indicates a thermal source offset from the basin which is not included in these models and perhaps reflects an additional mechanism unique to Mars.
Three‐dimensional inversion of the magnetic field over the Easter‐Nazca propagating rift near 25°S, 112°25′W
Authors: Sempere Jean‐Christophe , Gee Jeff , Naar David F. , Hey Richard N.
Published: February 2008

The Easter microplate boundary configuration is being reorganized by rift propagation. A Sea Beam survey of the Easter‐Nazca spreading center, which forms the eastern boundary of the microplate, has revealed the presence of a young propagating rift growing northward (Naar and Hey, 1986). The tip of the propagating rift is associated with a high‐amplitude positive magnetic anomaly. We have performed a three‐dimensional inversion of the magnetic field over the propagating rift tip area. The magnetization solution suggests that the western and eastern pseudofaults strike 014° and 338°, respectively, and converge near the rift tip. These orientations yield a propagation to spreading rate ratio of 1.5, slightly higher than the estimate of Naar and Hey (1986). Using the revised estimate of the full spreading rate along the Easter‐Nazca spreading center near 25°S (80 mm/yr) (D. F. Naar and R. N. Hey, unpublished manuscript, 1989), we obtain a propagation rate of 120 mm/yr. Within 27–30 km of the rift tip, the propagating rift curves by about 15° to the east toward the failing rift, probably as a result of the interaction between the two offset spreading centers. As at the Galapagos propagating rift, rift propagation appears to be a very orderly process along the Easter‐Nazca spreading center. The magnetization distribution that we obtain exhibits a high at the propagating rift tip. At other large ridge axis discontinuities, similar magnetization highs have been interpreted as being the result of the eruption of highly differentiated basalts enriched in iron. The origin of the high magnetization zone in the case of the Easter‐Nazca propagating rift near 25°S may be more complex. Preliminary rock magnetic measurements of basalts recovered in the vicinity of the propagating rift confirm the presence of highly magnetized basalts but suggest that the relationship between high magnetization intensities and high Fe content is not straightforward.
Late Cretaceous paleomagnetism and clockwise rotation of the Silver Bell Mountains, south central Arizona
Authors: Hagstrum Jonathan T. , Sawyer David A.
Published: February 2008

Late Cretaceous ash flow volcanism in the Silver Bell Mountains of southern Arizona (32.3°N, 248.5°E) was associated with caldera formation and porphyry copper mineralization. Oriented samples from 34 sites in volcanic, volcaniclastic, and intrusive units related to this episode of igneous activity (73–69 Ma) yield a mean paleomagnetic direction of I = 61.2°, D = 24.0°, α95 = 7.6°. Primary remanent magnetizations are indicated by the presence of both normal and reversed polarities and by the significantly improved grouping (95% confidence level) of site‐mean directions for a widespread tuff unit after structural correction. A comparison of this direction with a direction at Silver Bell calculated from contemporaneous units for stable North America (I = 58.8°, D = 342.3°, α95 = 7.7°) indicates a Late Cretaceous paleolatitude anomaly of −2.4°±7.6° (not significant at the 95% confidence level) and a declination anomaly of 41.7°±14.3° for the Silver Bell Mountains. Previously determined paleomagnetic data for southeastern Arizona suggest that this apparent clockwise rotation in the Silver Bell Mountains is a local phenomenon. Although preliminary, the average paleomagnetic direction for Oligocene and lower Miocene rocks in the Silver Bell area (I = 43.8°, D = 357.3°, α95 = 16.5°) is similar to that calculated for stable North America (I = 50.2°, D = 352.2°, α95 = 3.9°), implying that the observed rotation in the Silver Bell Mountains occurred before 26 Ma and was most likely associated with dextral strike‐slip movement along the Ragged Top and related WNW trending faults bounding the Silver Bell Mountain block. These data, in conjunction with plate reconstructions and other paleomagnetic data from southwestern North America, imply that WNW trending strike‐slip faults may have played an important role during Late Cretaceous to early Tertiary deformation in southern Arizona.
Viscoplastic modeling of texture development in quartzite
Authors: Wenk H.‐R. , Canova G. , Molinari A. , Kocks U. F.
Published: February 2008

Polycrystal plasticity theory has been successfully used to simulate development of preferred orientation in rocks. In particular, G. Lister and coworkers have done a comprehensive study, applying the Taylor theory to quartzite. In these calculations it was assumed that deformation is homogeneous; that is, all grains deform at the same rate, that critical resolved shear stresses (CRSS) of slip systems remain constant, and that deformation is rigid‐plastic; that is, dislocations move only when the CRSS has been reached and then with indeterminate velocity. We have been investigating the influence of work hardening and of strain rate dependence on texture development. In particular, modification of the viscoplastic Taylor theory suggests that textures are greatly dependent on the rate sensitivity of flow stress when this stress exponent is small, such as in quartz where it is near 3. The influence of work hardening is less critical in the case of quartz. Results from Taylor simulations are also compared with those from a self‐consistent theory. The latter sacrifices local strain continuity for better stress equilibrium. In the self‐consistent scheme, grains which are favorably oriented for slip are allowed to deform at a faster rate. Texture patterns obtained with the two theories are moderately different. In self‐consistent deformation a C axis maximum in the intermediate strain direction (Y) is generated which is absent in Taylor deformation but is a common feature of many natural quartz fabrics. Grains associated with this maximum are most strongly deformed. Deformation modeling with more realistic boundary conditions adds complexities but appears to be necessary in the case of anisotropic and rate sensitive rocks.