bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of focal mechanism on peak accelerations of strong motions of the Whittier Narrows, California, earthquake and an aftershock
Authors: Vidale John E.
Published: February 2008

Focal mechanisms affect the pattern of the peak accelerations of the October 1, 1987, Whittier Narrows earthquake and its October 4 aftershock. The peak accelerations observed on 21 Strong Motion Instrumentation Program and 22 U.S. Geological Survey accelerograms correlate well with the ratio of shear wave amplitude computed from the thrust mechanism of the main shock and the strike‐slip mechanism of the aftershock. This correlation means that seismic energy is radiated from the fault with close to the standard double‐couple radiation pattern at the frequencies 3–6 Hz corresponding to the peak accelerations.
Late Holocene movement along the San Andreas Fault at Melendy Ranch: Implications for the distribution of fault slip in central California
Authors: Perkins James A. , Sims John D. , Sturgess Steven S.
Published: February 2008

An alluvial deposit of the San Benito River at Melendy Ranch, which is located along the creeping segment of the San Andreas fault in central California, records an 800‐year‐long average slip rate of 22−4+6 mm/yr. The rate is calculated from one radiocarbon age and two measures of displacement and is based on various sedimentologic and stratigraphic data. Numerous other radiocarbon ages were determined for detrital charcoal from the alluvium at Melendy Ranch. These radiocarbon ages appear to be at variance with the age of the alluvium on the basis of sedimentologic and stratigraphic data. The 22 mm/yr long‐term rate is similar to the 20–23 mm/yr range of slip rates that are recorded by geodetic instruments at Melendy Ranch and a 22 mm/yr slip rate recorded by an offset corral fence built in 1945. This suggests that the historic rate of movement along the San Andreas fault at Melendy Ranch is near its 800‐year average. The long‐term slip rate of about 22 mm/yr at Melendy Ranch is 12−5+7 mm/yr less than the long‐term rate found along the San Andreas fault south of the creeping segment, and it is greater than the minimum long‐term rate of about 12 mm/yr found along the fault north of the creeping segment on the San Francisco Peninsula. Thus the long‐term slip‐rate data suggest a ∼20 mm/yr northwestward decrease in slip rate along the San Andreas fault. The northwestward decrease in long‐term slip rate is attributed to a two‐part transfer of slip, south of Hollister, from the San Andreas fault to the Paicines and Calaveras faults. If the Paicines and Calaveras faults south of Hollister have a combined long‐term slip rate of about ∼20 mm/yr, then a similar rate may also represent slip across the Calaveras and Hayward faults north of Hollister.
Paleomagnetic study of the Eastern Klamath terrane, California, and implications for the tectonic history of the Klamath Mountains Province
Authors: Mankinen Edward A. , Irwin William P. , Grommé C. Sherman
Published: February 2008

Paleomagnetic study of Permian through Jurassic volcanic and sedimentary strata of the Eastern Klamath terrane has shown the remanent magnetization of many of these rocks to be prefolding and most likely primary. Similarities in magnetic declinations recorded by coeval strata over a broad area are consistent with the hypothesis that the terrane, in general, has behaved as a single rigid block. Paleomagnetic data indicate that the volcanic island arc represented by this terrane, the nucleus of the province, was facing toward the present southwest during late Paleozoic time, although its orientation during earlier periods is unknown. Whether the arc was separated from the North American craton by a small marginal basin or originated far offshore cannot be determined from paleomagnetic data. The declination anomalies for both Permian and Triassic strata are similar (average = 106° ± 12°), so we infer that clockwise rotation of the late Paleozoic arc did not begin until latest Triassic or earliest Jurassic time. The arc may have completed its initial rotation with respect to stable North America by Middle Jurassic time. After some retrograde motion, the arc was again facing west by the Late Jurassic, by which time some of the more westerly terranes of the province had become attached to the Eastern Klamath terrane. The composite Klamath Mountains terranes continued to rotate until the final 60° of clockwise rotation was nearly complete by the Early Cretaceous. Coincidence of the waning stages of rotation, at about 136 Ma, with the beginning of deposition of the basal Great Valley sequence onto the Klamath basement probably represents the completion of accretion of the Klamath Mountains terranes to the North American continent. Nearly all the rotation occurred while the Klamath Mountains terranes were part of a converging oceanic plate, with only about 20° of rotation in mid‐Tertiary time during Basin and Range extension. No data currently available show evidence for any significant latitudinal displacement of any Klamath Mountains terranes relative to cratonic North America.
Major element, volatile, and stable isotope geochemistry of Hawaiian submarine tholeiitic glasses
Authors: Garcia Michael O. , Muenow David W. , Aggrey Kwesi E. , O'Neil James R.
Published: February 2008

Tholeiitic glasses were dredged from the submarine rift zones of the five volcanoes comprising the island of Hawaii and Loihi Seamount. The major element composition of the glasses follows a systematic trend that is related to the stage of evolution of the volcano. Glasses from Loihi Seamount (the youngest Hawaiian volcano) are enriched in Fe, Ca, Ti, Na, and K and depleted in Si and Al relative to the glasses from the other, older volcanoes. Kilauea is intermediate in age and its glasses are intermediate in composition between those from Loihi and Mauna Loa, the largest and oldest of the active Hawaiian tholeiitic volcanoes. The volatile contents (H20, CO2, S, F, Cl) of the glasses from these volcanoes follow the same trend (highest in Loihi; lowest in Mauna Loa). Glasses from Hualalai Volcano are similar in composition to those from Mauna Loa; those from Kohala Volcano are similar to Kilauea; Mauna Kea glasses range from Mauna Loa‐like to Kilauea‐like. The observed systematic variation in composition of Hawaiian tholeiites may be related to the progressive melting and depletion of the source of these volcanoes during their growth. Oxygen and hydrogen isotope analyses were made on many of the glasses from each volcano. The δ18O values of Hawaiian tholeiites are distinctly lower than those of mid‐ocean ridge basalt (MORB) (averages: 5.1 versus 5.7). These low values are probably a distinct feature of hot spot lavas. The δD values for these glasses (−88 to −61) are typical of mantle and MORB values. Thus the H2O in the Hawaiian glasses is probably of magmatic origin. Previous isotopic and trace element data indicate that the source of Hawaiian tholeiites contains two distinct source components. Based on the results of this study, the plume component in the source for Hawaiian tholeiites is characterized by moderate 87Sr/86Sr (0.7035–0.7037) and 206Pb/204Pb ratios (18.6–18.7), a low δ18O value (∼5.0), and greater contents of volatiles, Fe, Ca, Ti, Na and K relative to the MORB source.
Dynamical and chemical effects of melting a heterogeneous source
Authors: Richter Frank M. , Daly Stephen F.
Published: February 2008

The geochemical variability of mantle‐derived melts in both trace element and isotope ratios is such as to require a chemically heterogeneous source. This raises the need to understand the dynamics of melt segregation from a heterogeneous source and the associated effects of chemical fractionation and transport. A series of idealized numerical model problems, based on a two‐phase flow representation of a partially molten source region, are used to address how the physical and geochemical properties of local heterogeneities contribute to the overall segregating melts. We find that when the heterogeneities represent material that melts more easily than the background depleted mantle they will give rise to discrete traveling pulses of high melt fraction that can be understood in terms of two‐dimensional solitary waves, which we separately show are solutions to the governing equations used. The generation of solitary wave‐like pulses by heterogeneous melting depends on the scale of the melting anomaly compared to the natural compaction length of the partially molten system, being most effective when the region of excess melting is large. In terms of the geochemical transport by solitary waves we find that they leave behind any distinctive trace element concentration associated with the region of more rapid melting that produces the wave and therefore that the physical and chemical effects decouple. The chemical transport of incompatible trace elements by solitary waves is still anomalous in the sense that while the trace element concentration becomes no different than that of the lower melt fraction background there is locally much more of the enriched fluid (melt) moving with a large phase velocity. The general conclusion is that when a heterogeneous source partially melts it can give rise to fast moving melt fraction pulses or solitary waves, but these do not carry with them any chemical distinctiveness of the local heterogeneities that produce them. This suggests that the geochemical properties of mantle‐derived melts should be interpreted in terms of averages over length scales comparable to the region of partial melting, and not particularly biased by local anomalous components even when these are preferentially melted during upwelling.
Noble gas constraints on the evolution of the Earth's atmosphere
Authors: Zhang Youxue , Zindler Alan
Published: February 2008

A model for the degassing of Xe, Ar, and He from the Earth has been constructed. The Earth is divided into three idealized reservoirs in terms of their noble gas inventories: undegassed mantle, degassed mantle or mid‐ocean ridge basalt (MORB) mantle, and atmosphere plus continental crust. Degassing is assumed to occur via the partitioning of gases between a vapor phase and basaltic melt, in accord with He‐Ar isotope systematics which require that 3He be degassed at a slower rate than 36Ar on average. A total inversion technique is used to handle the large uncertainties associated with the input parameters. The model successfully reconciles presently available isotope ratios of 129Xe/130Xe, 40Ar/36Ar, and 4He/3He in the different Earth reservoirs, and accurately predicts the present degassing rates of He and Ar. This is the first time that noble gas isotope data have been shown to be consistent with a single degassing model. The success of this model demonstrates the feasibility of a solubility‐controlled degassing mechanism, and shows that there is a relationship between noble gas isotope ratios and the melt‐vapor partition coefficients for those gases, which has been obscured by the complicated radiogenic growth and degassing equations. The high 129Xe/130Xe ratios in MORB are shown to be consistent with the low solubility of Xe in silicate melts, which resulted in a faster degassing rate for Xe than for other gases in the context of this model. Therefore, the mean degassing time derived from Xe is not directly applicable to other gases. The degassed mantle, our end‐member degassed mantle reservoir, is found to be almost completely degassed for the noble gases, while the exact degree of degassing depends on the species under consideration, as well as the choice of isotope ratios to characterize this mantle segment. If extreme isotope ratios are used, the degree of degassing for the degassed mantle is: 99.92% for 130Xe, 99.7% for 36Ar, 97.5% for 3He, 82% for 40Ar and 88% for 4He. The mean time of degassing, measured from the formation of Earth, is 21±7 Ma for 130Xe, 56±19 Ma for 36Ar, 310±120 Ma for 3He, ∼1.5 Ga for 40Ar, and ∼0.8 Ga for 4He. Because the degassing is likely to be solubility controlled at some level, study of noble gases alone does not fully constrain the evolution of the atmosphere. However, the model can be extended to major gaseous species in the Earth's primary atmosphere if the relevant solubilities are known. Similarities between the solubilities of CO2, CO, and He, and between N2 and Ar, suggest a mean age for the total atmosphere on the order of 4.4 to 4.3 Ga. The formation of the oceans was a much later event because of the much higher solubility of H2O in silicate melt. If H2O has been the primary outgassing species for ocean water, the oceans have a mean age on the order of 2.7 Ga, suggesting that the volume ratio of continental crust to the oceans might have remained relatively constant during Earth history.
Heat flow and the thermal origin of hot spot swells: The Hawaiian Swell revisited
Authors: Von Herzen R. P. , Cordery M. J. , Detrick R. S. , Fang Changle
Published: February 2008

We present 150 new heat flow measurements obtained at eight sites along a 1230‐km‐long profile across the Hawaiian Swell about 700 km ESE of Midway Island. Most of the measurements include in situ thermal conductivity determinations, which helped to reduce the statistical uncertainties (95% confidence) at all sites to <±2.l mW m−2. Surprisingly, there is no systematic variation in heat flow across the axis of the swell. With one exception, the mean heat flow (corrected for sedimentation) at each site is within ±10% of the mean value of 57.7±4.3 (S.D.) mW m−2 for all sites. At the two sites at either end of the profile, clearly located off the swell, the observed heat flow is >59 mW m−2, about 20% higher than the predicted heat flow for 100 Ma seafloor based on simple lithospheric cooling models. Thus even though there is a small increase in heat flow with age along the swell, when compared with the off‐swell heat flux, the anomalous heat flow associated with the Hawaiian Swell is probably of the order of 5–10 mW m−2 and arguably may not exist at all. A previous investigation [Von Herzen et al., 1982] may have overestimated the magnitude of the heat flow anomaly associated with the Hawaiian Swell by comparing heat flux measurements on the swell with values expected for simple lithospheric cooling models. The heat flow anomalies associated with the Bermuda Rise and the Cape Verde Rise may also be smaller than previously estimated, given the uncertainties in the heat flux on the normal seafloor surrounding these swells. The lack of a significant heat flow anomaly associated with the bathymetric expression of the Hawaiian Swell is inconsistent with simple models of lithospheric reheating. Dynamic support, accompanied by modest temperature increases (<100°–200°C) confined to the lower lithosphere, and underlying asthenosphere appear to be required to explain both the height of the swell and its small heat flow anomaly. However, detailed modeling of the origin of midplate swells, and their associated heat flow anomalies, is hampered by our lack of understanding of the thermal evolution of old (>80 Ma) oceanic lithosphere.
Particle size distributions and the sequential fragmentation/transport theory applied to volcanic ash
Authors: Wohletz K. H. , Sheridan M. F. , Brown W. K.
Published: February 2008

The assumption that distributions of mass versus size interval for fragmented materials fit the log normal distribution is empirically based and has historical roots in the late 19th century. Other often used distributions (e.g., Rosin‐Rammler, Weibull) are also empirical and have the general form for mass per size interval:n(l) =klαexp (−lβ), wheren(l) represents the number of particles of diameterl,lis the normalized particle diameter, andk, α, and β are constants. We describe and extend the sequential fragmentation distribution to include transport effects upon observed volcanic ash size distributions. The sequential fragmentation/transport (SFT) distribution is also of the above mathematical form, but it has a physical basis rather than empirical. The SFT model applies to a particle‐mass distribution formed by a sequence of fragmentation (comminution) and transport (size sorting) events acting upon an initial massm′:n(x,m) =C∫∫n(x′,m′)p(ξ)dx′dm′, wherex′ denotes spatial location along a linear axis,Cis a constant, and integration is performed over distance from an origin to the sample location and mass limits from 0 tom. We show that the probability function that models the production of particles of different size from an initial mass and sorts that distribution,p(ξ), is related tomg, whereg(noted as γ for fragmentation processes) is a free parameter that determines the location, breadth, and skewness of the distribution;g(γ) must be greater than −1, and it increases from that value as the distribution matures with greater number of sequential steps in the fragmentation or transport process; γ is expected to be near −1 for “sudden” fragmentation mechanisms such as single‐event explosions and transport mechanisms that are functionally dependent upon particle mass. This free parameter will be more positive for evolved fragmentation mechanisms such as ball milling and complex transport processes such as saltation. The SFT provides better fits to many types of volcanic ash samples than does the log normal curve. Modeling of the SFT shows its similarity to the log normal curve on size frequency histograms; it differs by its variable skewness controlled by γ. Skewed distributions are typical of many volcanic ash samples, and characterization of them by the SFT allows interpretation of eruptive and transport mechanisms.
Tectonic setting of the Yukon‐Koyukuk Basin and its borderlands, western Alaska
Authors: Patton William W. , Box Stephen E.
Published: February 2008

The Yukon‐Koyukuk basin of western Alaska is composed of an arcuate belt of Jurassic and Early Cretaceous subduction‐related volcanic and plutonic rocks (Koyukuk terrane) flanked by deep subbasins filled with mid‐Cretaceous terrigenous sedimentary rocks. The basin is bordered on three sides by metamorphosed Proterozoic and Paleozoic continental rocks (Seward, Arctic Alaska, and Ruby terranes) and is separated from the metamorphic borderlands by a narrow, highly tectonized belt of oceanic crust and mantle rocks (composite Angayucham‐Tozitna terrane). The oceanic and mantle rocks, which dip inward beneath the basin and are thrust outward onto the borderlands, are divided into three separate thrust panels: (1) a structurally lowest panel (Slate Creek) composed of phyllite and metagraywacke of probable Devonian age, (2) a middle panel (Narvak) composed of imbricated basalt, chert, and gabbro of Devonian to Early Jurassic age, and (3) a structurally highest panel (Kanuti) composed of gabbro and peridotite of probable Middle and Late Jurassic age. The three thrust panels appear to represent a reversely stacked sequence that progresses from continental slope deposits in the lower panel to cumulus and mantle peridotites in the upper. Metamorphic mineral K‐Ar ages from garnet amphibolite on the sole of the upper panel suggest that the upper panel was emplaced on the middle panel in the Middle to Late Jurassic. Subsequent accretion of all three panels to the continental rocks of the borderlands occurred in the latest Jurassic and Early Cretaceous, synchronous with arc volcanism within the basin. Arc volcanism waned and the accretionary phase ended in the middle of Early Cretaceous time. Uplift and erosion of the metamorphic borderlands and the obducted oceanic rocks began in late Early Cretaceous and was accompanied by the rapid filling of two flanking subbasins with turbiditic sediments. In the latest Early Cretaceous and early Late Cretaceous, shallow marine and nonmarine conglomerates were deposited around the margins of the basin, and a prograding delta was built out from the southeast margin of the basin across the turbiditic subbasins and the remnant volcanic arc. In the Late Cretaceous, western Alaska was subjected to strong east‐west compression which severely deformed both the Yukon‐Koyukuk basin and the borderlands. Several widespread magmatic episodes in the mid‐ and Late Cretaceous and in early Tertiary transgress the basin boundaries and stitch together the accreted arc and oceanic terranes and the continental borderlands.
Axial and off‐axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35′N–12°51′N and 11°26′N–11°30′N
Authors: Hekinian Roger , Thompson Geoffrey , Bideau Daniel
Published: February 2008

Surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the EPR at 12°35′N–12°51′N and at 11°26′N–11°30′N and from four adjacent seamounts centered at less than 18 km from the rise axis. The basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition. There is no correlation between lava flow morphology, geological settings, and compositions. Based on mineral and compatible element composition, these basalts were classified into three types. The least evolved olivine‐basalts and highly phyric plagioclase‐basalts (type A) have high Fo87–89, high Mg # (66–70), and high Ni (>100) contents. The most evolved plagioclase‐olivine basalts with or without pyroxene (type B and C) have relatively low Fo78–86, low Mg # (55–65), and low Ni (60–100 ppm) contents. Simple crystal fractionation (3–18%) accounts for some of the observed compositional range. However, based on their incompatible element contents, the above basalts are divided into three groups (the various types A to C are found in each group): depleted basalts with low K/Ti (0.04–0.15) ratios, low Zr (<100 ppm) and low Nb (<0.4 ppm) contents, undepleted basalts with the highest K/Ti (0.25–0.46), Zr (150–170 ppm), and Nb (8–16 ppm) contents and the transitional basalts having compositions between these two extremes. These three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3–4 km in width near 12°43′N–12°51′N on the EPR. Similar volcanic diversity in a more limited area (<3 km2) is observed on off‐axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis. Depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle. The transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation. Two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting; we suggest that a lherzolitic mantle, with minerals having a variable behavior under partial melting, constitutes this type of composite mantle. Indeed, clinopyroxene, enriched in incompatible elements, will be consumed at an earlier stage of melting than the olivine plus orthopyroxene. Eruption of compositionally distinct magma batches in close proximity on axial and off‐axial structures implies periods of quiescence between successive magmatic stages and cycles.
Nonlinear generation of elastic waves in granite and sandstone: Continuous wave and travel time observations
Authors: Johnson Paul A. , Shankland Thomas J.
Published: February 2008

Beams generated by nonlinear interaction have proved useful as low‐frequency, highly directional sources in water and may ultimately find application in geophysical exploration. For nonlinear elastic waves to be used as seismic sources they must be detected as discrete arrivals; measuring travel times of pulsed, nonlinear beams is a necessary first step toward that end. In this paper we discuss two new observations of the interaction of nonlinear elastic waves in both crystalline rock and sandstone. We show that observed travel times of pulsed waves agreed with predicted times for the case where two P wave pulses interacted to produce an S wave pulse. The travel time measurement was difficult to obtain because conversion efficiency is low. In extending our continuous wave experiments to sandstone we also satisfied three criteria previously verified in crystalline quartz norite and in granite to demonstrate nonlinear interaction: (1) the frequency of the nonlinear‐generated signal equalled the difference frequency f1–f2 when two P waves interacted to produce an S wave, (2) the amplitude of the signal was proportional to the amplitude product of the primary waves, and (3) in the case of intersecting P waves the trajectory of the nonlinear beam matched that predicted by nonlinear elasticity theory.
Experimental equilibration of multicomponent pyroxenes in the spinel peridotite field: Implications for practical thermometers and a possible barometer
Authors: Sen Gautam , Jones Robert
Published: February 2008

Pyroxene equilibration experiments were carried out in a multivariate system at 10–15 kbar pressure and 925°–1150°C using PbO flux. Two starting mixes were used: one is a natural spinel lherzolite and the other is a 50:50 molar mix of synthetic diopside and enstatite crystals. Large (50–100 μm), homogeneous, euhedral crystals of pyroxenes were grown in the presence of a lead silicate melt in the interface region between the two starting mixes. These pyroxenes gave partitioning coefficients for En and Di exchange reactions that are slightly different from those obtained from CaO‐MgO‐SiO2 and CaO‐MgO‐Al2O3‐SiO2 systems. Two new practical thermometric equations were derived which gave similar temperatures as those obtained with the Bertrand and Mercier (1986) thermometer. All other thermometers gave higher values. The Mercier et al. (1984) spinel peridotite barometers reproduced the run pressures quite well but were unable to give reasonable pressure estimates for natural spinel peridotites from Hawaii. Fs component of clinopyroxene showed an apparent P dependence which needs to be verified with studies of simple systems containing iron.
A comparison of earthquake coda waves at surface versus subsurface seismometers
Authors: Blakeslee S. N. , Malin P. E.
Published: February 2008

The coda waves of 21 Parkfield earthquakes were recorded simultaneously at the surface and at a depth of 198 m. We characterize these codas by computing (1) the time dependence of the integral of squared particle velocity, (2) their frequency content, (3) time‐frequency coda decay planes, (4) the frequency dependence of root mean square ellipticity, and (5) the difference in their normalized decay rates. These five measures reveal significant differences in the early portion of the uphole versus downhole codas. Our analysis solves for coda Q as an explicit function of both time and frequency. Average apparent coda Q for the 21 events observed both at the surface and downhole is Qa(f, τ) = 39(f/f0)0.43 (τ/τ0)0.37, with the reference frequency and time defined at 1 Hz and 1 s, respectively. Coda Q exhibits a trend in its value over the 1‐month period of the study. The trend correlates with a spatial variation in source locations and is not produced by temporal variations of the Q of the medium. The spectral ratio between the uphole signal and the downhole signal varies with time through the coda. The early portion of the uphole coda experiences a pulse of low‐frequency energy not seen in the downhole coda. To explain this observation, we propose a model of the coda that includes body wave to surface wave scattering near the site. In this model, the near‐surface acts as both a filter and a scattering waveguide. The coda at the surface is the sum of a coda produced by scattering deep in the lithosphere and a coda produced by scattering in the near‐surface. The scattering in the near‐surface is made up of near‐vertically traveling multiples and horizontally travelling surface waves. The guided waves are recorded in the surface coda but are absent from the coda recorded at depth. The time‐frequency structure of the surface waves in the uphole coda is determined by the velocity, scattering, and attenuation characteristics of the site. By fitting the model to our data, we have found that the fractional conversion factor of body waves to surface waves travelling toward the receiver is 0.03 at the Vineyard Canyon site. The relatively rapid damping of these trapped waves can be modelled by assuming a quality factor of QT = 50. The principles of this model are general enough to be of use at other locations.
Metamorphic and stratigraphic constraints on the evolution of the Santa Catalina Mountains Metamorphic Core Complex, Arizona
Authors: Palais David G. , Peacock Simon M.
Published: February 2008

Contact metamorphic mineral assemblages in the Santa Catalina Mountains metamorphic core complex constrain the emplacement depth of the metaluminous Late Cretaceous Leatherwood quartz diorite to between 7.5 and 14 km (2–3.8 kbar). The peraluminous Eocene Wilderness granite is constrained to have been emplaced at depths of less than 14 km. Stratigraphic and tectonic considerations are consistent with emplacement of both the Leatherwood quartz diorite and Wilderness granite at upper crustal levels. Published igneous thermobarometry (Anderson et al., 1988), although in approximate agreement with the metamorphic and tectonostratigraphic estimates for the depth of emplacement of the Wilderness granite, suggests that the Leatherwood quartz diortite was emplaced at depths of ∼21 km (5.6 kbar) or greater. The discrepancy between the different emplacement estimates may reflect ambiguities associated with interpretations of the results from each technique. Igneous barometric assemblages may reflect conditions experienced during a period of crystallization during ascent of the magma through the crust (i.e., ponding of the magma). Alternatively, emplacement of the voluminous Wilderness granite and associated plutons may have entirely overprinted earlier Leatherwood‐related contact metamorphism. The proposed upper crustal emplacement of the Leatherwood quartz diorite is consistent with the observation that Lararrade‐age crustal thickening in southeast Arizona probably did not exceed 8 km.
Relationship between Mesozoic and Cenozoic tectonic features in west central Arizona and adjacent southeastern California
Authors: Spencer Jon E. , Reynolds Stephen J.
Published: February 2008

The Maria fold‐and‐thrust belt (MFTB) is a narrow belt of Mesozoic crustal shortening that trends east‐west across west central Arizona and adjacent southeastern California. It is characterized by generally south vergent folds and thrusts that commonly displace Proterozoic crystalline rocks over deformed and metamorphosed Paleozoic and Mesozoic strata. The MFTB is cut by a south to southeast trending belt of mid‐Tertiary extensional deformation. Extension was characterized by large displacements on low‐angle normal faults, known as detachment faults, and by isostatic uplift of mylonitic midcrustal rocks now exposed in metamorphic core complexes. The geometry and style of extensional deformation change along the extensional belt and reveal the influence of the MFTB. Several extensional features are spatially coincident with the root zone of MFTB thrusts: (1) an areally extensive west‐northwest trending belt of denuded Tertiary mylonitic fabrics in the Whipple and Harcuvar metamorphic core complexes, (2) a style of extension characterized by minimum extensional dismemberment of the upper plate and maximum denudation and uplift of deep‐seated lower plate rocks, (3) an abrupt bend in the belt of arched, uplifted rocks below detachment faults, and (4) an abrupt bend in the trend of the breakaway zone of the detachment faults and an associated lateral ramp in the detachment fault system. The first two features may be the result of isostatic uplift of a crustal root (downward protruding Moho bulge) inherited from crustal thickening within the MFTB, and the other two features may reflect, at the time of detachment fault initiation, the influence of stresses in the crust associated with flexural support of the buoyant Mesozoic crustal root.
Use of Landsat Thematic Mapper images in regional correlation of syntectonic strata, Colorado River Extensional Corridor, California and Arizona
Authors: Beratan K. K. , Blom R. G. , Nielson J. E. , Crippen R. E.
Published: February 2008

This study utilizes enhanced Landsat Thematic Mapper (TM) images in concert with field work to solve problems in regional correlation of Miocene rocks in the Colorado River extensional corridor of California and Arizona. These syntectonic volcanic and sedimentary deposits preserve a dateable record of crustal extension, although regional correlation and analysis of the deposits are difficult because of the small sizes of the sedimentary basins in which they accumulated and disruption by postdepositional faulting. Based on field investigations, four sequences of sedimentary and volcanic strata, distinguished by stratigraphic and structural relations, can be recognized in the Mohave Mountains, Arizona, and the eastern Whipple Mountains, California. Although the sequences are found in each of four structural regions, the relative volumes and organization of lithologies within the sequences differ considerably between regions. These sequences have distinctive appearances on the TM image. The volcanic component is particularly important in distinguishing between regions and sequences because differences in color and textural patterns in the image are usually related to variations in the proportion and composition of the volcanic material and in the geometry of the volcanic bodies. The recognition criteria derived from field mapping and image interpretation in the Mohave and Whipple Mountains were applied to an adjacent area in which stratigraphic affinities were less well known. Stratigraphic and structural relations suggested by the TM image were confirmed by subsequent field work.
Seismic structure of the continental crust based on rock velocity measurements from the Kapuskasing Uplift
Authors: Fountain David M. , Salisbury Matthew H. , Percival John
Published: February 2008

The Michipicoten greenstone belt (MGB), Wawa gneiss terrane (WGT), and Kapuskasing structural zone (KSZ) in Ontario, Canada, are regarded as a 25‐km‐thick partial cross section through the crust of the Superior Province. We measured compressional wave velocities (Vp) at confining pressures up to 600 MPa for representative rock samples from these terranes. Vp for MGB is variable (6.1–7 km/s) and depends on composition and anisotropy. Compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the WGT and the KSZ show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range, a trend corroborated by data for other high‐grade rocks. A model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics. The shallowest levels (MGB) are seismically heterogeneous with high average velocities (6.6 km/s at 100 MPa) because of the dominance of metabasalts. The balance of the upper crust (WGT) is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6.35 km/s at 100 MPa and 6.6 km/s at 600 MPa. The uppermost lower crust (KSZ) is layered, seismically heterogeneous and transversely isotropic. The average velocity at 600 MPa is 6.84 km/s. The boundary between WGT and KSZ is marked by a velocity increase of between 0.2 and 0.3 km/s, an increase similar to that commonly associated with midcrustal discontinuities (i.e., Conrad) in shield areas. Here, this boundary corresponds to the amphibolite‐granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust. Refraction results show an anomalous high‐velocity zone in the upper crust under the KSZ that, on the basis of laboratory velocity data, can be correlated with KSZ lithologies. These data indicate that KSZ rocks can be traced to the west to depths as great as 20 km. Levels deeper than 20 km, not exposed at the surface in the KSZ, are characterized by velocities greater than 7.0 km/s and may be dominated by mafic and anorthositic lithologies or by more garnet‐rich rocks. The velocity model of the Superior Province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the MGB from refraction experiments. The velocity structure of this model is also generally similar to the velocity structure of shield areas.
Winter frost at Viking Lander 2 site
Authors: Svitek Thomas , Murray Bruce
Published: February 2008

A key question in the study of Mars is water exchange between atmosphere and surface on daily, seasonal, and astronomical timescales. We believe that small‐scale processes are a key for enhanced understanding of the global water behavior of Mars. The principal data for this study of small scale properties of the Martian surface were collected by the second Viking lander (VL 2) and by both Viking orbiters. The annual deposition and retreat of the frost layer were observed in situ by VL 2. The frost is inferred to be H2O frost but with some properties suggesting a much thicker layer than would be expected from the simple mass balance calculation. Our original contribution is in considering the effect of cold trapping (frost redeposition) which has been previously neglected and which enables us to reconcile all the observations with environmental conditions. In addition, we believe that this study points to a more general phenomenon of cold trapping in the Martian environment. Our study of the VL 2 observations suggests that H2O frost occurs in two forms: (1) thin, almost continuous, early frost and (2) much thicker, patchy, later frost. Both frost forms contain essentially the same total water content, but they cover different fractions of the surface. The transition between the two frost forms occurs by recondensation at local cold traps when solar insolation sublimates the first frost but the atmosphere is still too cold to transport the resultant water vapor away. These cold traps are created by shadowing from the small‐scale surface roughness, rocks, troughs, etc. This hypothesis hinges on the disparity between local and long‐range transport of water vapor by the atmosphere. The local transport is driven by abundant insolation energy available at the time of transition. This results in a large fraction of surface frost being redistributed rapidly into locally thermodynamically preferable locations, cold traps. Long‐range transport is constrained by the atmospheric carrying capacity. At the time of transition, the atmosphere is still cold, not far from its winter minimum, and is almost saturated by residual water vapor (5–8 precipitable micrometers). Therefore it cannot carry much additional water vapor to lower latitudes. This disparity delays the global transport of water vapor by the atmosphere.
Measurements of radiated elastic wave energy from dynamic tensile cracks
Authors: Boler Frances M.
Published: February 2008

To determine the conditions under which elastic waves are radiated from crack sources, dynamic tensile cracks were propagated in glass samples in the double cantilever beam geometry. This geometry allows simple calculation of the strain energy release rate G at initiation from measured parameters of crack length, applied crack opening force, and crack opening displacement. Partial control over the strain energy state in the sample at fracture initiation, and hence G at initiation, was achieved by varying the geometry of the notch tip from which the fracture emanates. Elastic wave displacements were monitored with a broadband capacitance transducer with a pointlike probe. A single component of elastic wave displacement (parallel to the crack plane and perpendicular to the crack propagation direction) was measured. Two fracture configurations were investigated: (1) “primary fracture” in glass plates of dimension 305×102 ×12.7 mm and (2) “secondary fracture” in previously fractured glass plates of the same dimensions, bonded intermittently along the fracture plane. Primary fracture experiments afforded a means of investigating elastic wave radiation from mode I cracks in a highly brittle material, such that the strain energy released by the fracture is partitioned into fracture surface energy of the newly formed crack walls and radiated elastic wave energy; negligible energy is expended in ductile or frictional processes. Secondary fracture experiments afforded a means of investigating elastic wave radiation in the case of varying fracture surface energy along the crack path. For primary fracture, measurable elastic waves from the macrofracture were generated in 31% of the 16 dynamic fracture events monitored. The condition for radiation of measurable waves from these fractures appears to be a local abrupt change in the fracture path direction, such as occurs when the fracture intersects a surface flaw. For the five events with measurable elastic waves, the ratio of radiated elastic wave energy in the measured component to the fracture surface energy of the macrocrack was 0.0001–0.001. For secondary fracture, 100% of the 13 events monitored showed measurable elastic waves. The ratio of radiated elastic wave energy in the measured component to fracture surface energy was 0.001–0.01, or 10 times greater than for primary fracture. The observed value of G at crack initiation for both primary and secondary fracture ranged from 3 to 48 J m−2. When the time window for radiated elastic wave energy calculation was restricted to a few microseconds after the first arrival, a weak correlation of radiated elastic wave energy with initiation G value was observed for secondary fractures.
Biogenic magnetite and the magnetization of sediments
Authors: Stolz John F. , Lovley Derek R. , Haggerty Stephen E.
Published: February 2008

Biogenic magnetites are produced through the reduction of ferric iron by both biologically induced (extracellular) and biologically controlled (intracellular) processes. With few exceptions, all are ultra‐fine‐grained, single‐domain magnetite. Biogenic magnetites formed by magnetotactic bacteria (biologically controlled) have been shown to contribute significantly to the natural remanent magnetization of carbonates and limestones, hemipelagic and deep‐sea marine sediments. The input into sediments of ultra‐fine‐grained magnetite produced by dissimilatory iron reducing bacteria (biologically induced) has yet to be firmly established but may be even more significant. Whether either type of authigenic biomagnetite is preserved is determined by postdepositional factors including oxidation, reduction by substitution, and dissolution. Unconsolidated and lithified sediments can be screened for putative biogenic magnetite by rock magnetic techniques. It is not generally appreciated that magnetotactic bacteria and their magnetofossils can be identified by the unusual, and in some cases unique, morphologies, size range, and composition of the magnetite crystals. Magnetite produced by dissimilatory iron reducing bacteria have a distinctive morphology and size range, but it is currently controversial as to whether these can be distinguished from certain chemically precipitated magnetites. The presence of dissimilatory iron reducing bacteria, however, can be detected using microbiological techniques and sediment geochemistry. Biogenic magnetites are trace fossils and potentially useful environmental indicators and are considered to have significant input to the magnetization of most sediments, both modern and ancient.
Processing and inversion of refraction and wide‐angle reflection data from the 1986 Nevada Passcal Experiment
Authors: Hawman Robert B. , Colburn Robert H. , Walker David A. , Smithson Scott B.
Published: February 2008

We derive averaged, one‐dimensional velocity models for the crust beneath the Basin and Range in western Nevada by inverting τ(p) composites extracted from slant stacks of refraction and wide‐angle reflection data. A combination of high‐cut filtering in the frequency domain with coherency filtering in the (p,τ) domain has proved effective in smoothing severe static shifts due to lateral heterogeneities and suppressing artifacts due to spatial aliasing. We supplement the low‐frequency information provided by the refraction records with higher‐frequency arrivals picked from the normal incidence and wide‐angle reflection sections. Generalized least squares inversion of data from eight shots for P and S wave velocity structure beneath two perpendicular profiles yields consistent estimates for total crustal thickness (P wave: 30–31 km; S wave: 30–32 km) and average crustal velocity (6.2 km/s; 3.4–3.5 km/s). Average Poisson's ratio beneath the N‐S profile is 0.27–0.28. P wave velocities near the base of the crust (7.4±0.3 km/s) are estimated by inverting precritical reflections from the Moho. Standard errors in depth for these models range from 1 to 3 km; extremal depth bounds, in general, are about twice as large. Uncertainties in velocity, estimated from resolving kernels, range from 0.1 to 0.3 km/s. Estimates of Poisson1s ratio as a function of depth are characterized by large standard errors, reflecting the poor resolution of shear wave velocities. The combination of low average P and S wave velocities with a high average Poisson's ratio in the upper half of the crust, however, is consistent with the presence of fluid‐filled cracks. These results highlight the need for three‐component recording to better resolve shear wave velocity structure.
Shear wave splitting and anisotropy from the aftershocks of the Nahanni, Northwest Territories, Earthquakes
Authors: Buchbinder Goetz G. R.
Published: February 2008

Two large earthquakes (Ms = 6.6, 6.9) occurred in 1985 near the North Nahanni River in the Northwest Territories, Canada, the first on October 5 and the second on December 23. The earthquakes were followed by long aftershock sequences. Aftershocks between M = 1.0 and 3.9 were recorded by five to eight three‐component stations during three field experiments in October 1985 and in January and September 1986. Two distinct shear waves were generally observed. In September 1986, the plane of polarization of the first arriving shear wave was well defined and averaged 156°±8° measured clockwise from north. The stress direction was inferred from the initial shear wave polarizations recorded at one station from a widespread aftershock distribution but was limited to no more than 10 km around it. The delay between first and second arriving shear waves was up to 190 ms. Assuming that the two shear waves result from anisotropy and that the entire ray paths, varying in length from 7 to 23 km, contributed, the maximum velocity anisotropy is 7%. However, the distribution of the anisotropy is not uniform. The pressure axes determined from fault plane solutions for the two main shocks are nearly E‐W, normal to the compressive direction found in the September 1986 field survey from shear wave splitting data. Four out of seven of the aftershocks for which focal mechanism orientations could be determined showed pressure axis orientations that agreed with the stress direction inferred from the initial shear wave polarizations from the field survey.