bluuebunny/crossref_metadata_2025 huggingface dataset:
Gravity fields over mid‐ocean ridges from GEOSAT GM data: Variations as a function of spreading rate
Authors: Owens Robin , Parsons Barry
Published: February 2004

Mean gravity profiles have been obtained from gravity fields constructed using Geosat Geodetic Mission altimeter data for a number of areas over mid‐ocean ridges. The technique of averaging many profiles reduces the effects of along‐axis variability and allows systematic changes as a function of spreading rate to be seen more clearly. For spreading rates larger than about 60–65 mm/yr, an axial gravity high is observed with an almost constant amplitude (∼12 mgal) that narrows as the spreading rate increases. For spreading rates lower than this, an axial gravity low is observed whose amplitude decreases rapidly with spreading rate while the width reduces only slowly. The disappearance of a median valley occurs abruptly on any one ridge, but occurs at slightly different spreading rates on the Southeast Indian Ridge (68 mm/yr) and the Pacific‐Antarctic Ridge (60 mm/yr). These observations are compared with the predictions of current models of mid‐ocean ridge processes but are not fully explained by them.
Photoluminescent volumetric imaging: A technique for the exploration of multiphase flow and transport in porous media
Authors: Montemagno Carlo D. , Gray William G.
Published: February 2004

A new measurement technique, photoluminescent volumetric imaging (PVI), has been developed for the visualization and quantification of multiphase flows and interface behavior in porous media. For this technique, a transparent, multiphase porous‐medium system is constructed of optical‐quality quartz sand, and two immiscible fluids matched to the optical refractive index of fused silica. Doping the fluids with property‐selective fluorophores allows planar areas of the system to be successively excited with a shaped laser light to reveal the structure of the system. The resulting illumination is recorded by a high‐resolution, cooled, slow scan charged‐coupled device (CCD) camera. The planar digital images produced are then processed to generate a true three‐dimensional data set that allows for quantitative study of the distribution of phases and interfaces within the porous medium. Sample volumes up to 125 mm³ in size with a resolution of better than 1 µm have been measured. Because of the large sample size and the high resolution of the measured data, geometric, flow, and phase interface information can be visualized and extracted at both subpore and system scales.
The distribution of hydrogen, nitrogen, and chlorine radicals in the lower stratosphere: Implications for changes in O3 due to emission of NOy from supersonic aircraft
Authors: Salawitch R. J. , Wofsy S. C. , Wennberg P. O. , Cohen R. C. , Anderson J. G. , Fahey D. W. , Gao R. S. , Keim E. R. , Woodbridge E. L. , Stimpfle R. M. , Koplow J. P. , Kohn D. W. , Webster C. R. , May R. D. , Pfister L. , Gottlieb E. W. , Michelsen H. A. , Yue G. K. , Wilson J. C. , Brock C. A. , Jonsson H. H. , Dye J. E. , Baumgardner D. , Proffitt M. H. , Loewenstein M. , Podolske J. R. , Elkins J. W. , Dutton G. S. , Hintsa E. J. , Dessler A. E. , Weinstock E. M. , Kelly K. K. , Boering K. A. , Daube B. C. , Chan K. R. , Bowen S. W.
Published: February 2004

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N2O5 agree well with measured concentrations of NO and ClO, but concentrations of HO2 and OH are underestimated by 10 to 25%, concentrations of NO2 are overestimated by 10 to 30%, and concentrations of HCl are overestimated by a factor of 2. Discrepancies for [OH] and [HO2] are reduced if we allow for higher yields of O(1D) from O3 photolysis and for heterogeneous production of HNO2. The data suggest more efficient catalytic removal of O3 by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [O3] in the lower stratosphere may be larger in response to inputs of NOy from supersonic aircraft than estimated by current assessment models.
Total analysis of the 1993 Hokkaido Nansei‐Oki Earthquake using seismic wave, tsunami, and geodetic data
Authors: Tanioka Yuichiro , Satake Kenji , Ruff Larry
Published: February 2004

The fault geometry and slip distribution of the Hokkaido Nansei‐oki, Japan, earthquake of July 12, 1993 are estimated using seismic wave, tsunami, and geodetic data. The Moment Tensor Rate Function inversion from P waves shows one nodal plane shallowly dipping to the west and the other nodal plane steeply dipping to the east. The best depth is estimated as 10–15 km. The source time history consists of an initial pulse with a duration of 10 s and moment release of 2 × 1020 Nm, followed by a complex rupture for at least 40 s. The Centroid Moment Tensor (CMT) solution shows one nodal plane shallowly dipping to the east and the other steeply dipping to the west. The overall seismic moment is estimated as 5.5 × 1020 Nm (Mw 7.8). The joint inversion of geodetic data on Okushiri Island and tsunami waveforms in Japan and Korea shows that the largest slip, about 6 m, occurred at a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shallowly to the west. The slip on the northernmost fault, dipping to the east, is about 2 m. The slips on the southern faults, dipping steeply to the west, are more than 3 m. Total seismic moment of 4.9 × 1020 Nm, estimated from the slip distribution, is similar to the estimate from CMT inversion.
Overview: The stratospheric photochemistry aerosols and dynamics expedition (SPADE) and Airborne Arctic Stratospheric Expedition II (AASE‐II)
Authors: Wofsy S. C. , Cohen R. C. , Schmeltekopf A. L.
Published: February 2004

The Stratospheric Photochemistry, Aerosols, and Dynamics Expedition (SPADE) made in situ observations of the composition of the lower stratosphere from the NASA ER‐2 aircraft at latitudes from 15°N to 60°N, during November 1992 and April, May and October 1993. SPADE followed the Airborne Arctic Stratospheric Expedition II (AASE‐II, September 1991 to March 1992) by 8 months. Together the two missions provide a record of stratospheric trace species and aerosols at middle and high latitudes spanning the input and decay of debris from the eruption of Mt. Pinatubo. New instruments deployed for SPADE include sensors to measure OH, HO2, H2O, CO2, NO2 and the UV/visible radiation field, complementing sensors previously deployed on the ER‐2 (ClO, BrO, NO, NOy, N2O, O3, H2O, HCl, CH4, CFC‐11, CFC‐113, and aerosol number and size distribution). The data provide the first simultaneous in situ measurements of radicals and reservoir species to include representatives from all the important families of stratospheric reactants, observed as functions of time of day (at ∼18 km), latitude and altitude (15–20 km). The results place strong new constraints on models of stratospheric photochemistry. For example, measurements of the radicals HO2, NO2, ClO and BrO allow for a nearly completely empirical evaluation of local rates for photochemical removal of ozone (by known catalytic cycles).

Highly precise observations of the seasonal cycle and interannual changes in CO2, combined with data for N2O and other tracer species, provide new insights into rates for transport in the lower stratosphere. High resolution data obtained during SPADE by the ER‐2 (in its own wake) and in AASE‐II by the DC‐8 (in wakes from commercial aircraft) provide support for engineering models of NOx emissions from subsonic jet aircraft at cruise conditions. Observations from the DC‐8 during AASE‐II, defining global distributions of NOx and NOy near the tropopause, and of HF, HCl, ClNO3, and HNO3 column abundances, provide new information on the processes influencing polar ozone loss.
Flow‐induced mixing in the GRIP basal ice deduced from the CO2 and CH4 records
Authors: Souchez R. , Lemmens M. , Chappellaz J.
Published: February 2004

This paper documents a larger degree of mixing in ice near the bottom of an ice sheet than described, or suspected, previously. It shows, thanks to favourable circumstances due to CO2 and CH4 production underneath the ice, that flow‐induced mixing within the basal ice has taken place at the scale of a few centimeters in the GRIP core. Such a mechanism must be considered when interpreting the ice properties in the bottom part of ice sheets and must be taken into account as a potential process of layer disruption in the low levels of the Central Greenland ice cores.
Lidar observations of a lot of sporadic sodium layers in mid‐latitude
Authors: Nagasawa Chikao , Abo Makoto
Published: February 2004

We have routinely observed mesospheric Na layers since November 1991 with a Na ground‐based lidar above Tokyo Metropolitan University (TMU) at Hachioji, Tokyo (35.6°N, 139.4°E) in mid‐latitude. In the past, the sporadic Na layers have been observed commonly at low‐and high‐latitude lidar sites [Kwon et al., 1988; Batista et al., 1989; von Zahn et al., 1987] but rarely observed at mid‐latitude sites [Senft et al., 1989]. Contrary to expectations, we could observe more than 100 events of the sporadic Na layers for two years from November 1991 until October 1993 in this mid‐latitude lidar site, and the large sporadic Na layers had been observed especially during spring and summer. Most of these events were accompanied by sporadic E layers and the most enhancement of the sporadic E layers preceded that of the sporadic Na layers by 15 to 30 minutes.
First‐principles prediction of the high‐pressure phase transition and electronic structure of FeO: Implications for the chemistry of the lower mantle and core
Authors: Sherman David M. , Jansen Henri J. F.
Published: February 2004

Under shock‐wave compression, Fe1−xO undergoes a transition to a dense metallic phase at pressures near 70 GPa. The geochemical significance of this transition has been unclear. Here, first‐principles electronic structure calculations (using the FLAPW method and GGA exchange‐correlation) show that the shock‐wave discontinuity of FeO results from a RB1 (rhombohedrally distorted NaCl structure) to B8 (NiAs structure) transition. The metallic nature of the FeO (B8) phase is argued to result from a breakdown of the Mott insulating condition, rather than an Fe(3d)‐O(2p) gap closure. As such, the metallization of FeO is probably not a basis for invoking oxygen in the Earth's core. The stability of FeO(B8) over FeO (RB1) at high pressure is comparable to the ideal ‐TΔS of mixing of FeO in (Mg,Fe)O at mantle temperatures. Consequently, it is uncertain if FeO(B8) is present as a separate phase in the Earth's interior.
Localization of toroidal motion and shear heating in 3‐D high Rayleigh number convection with temperature‐dependent viscosity
Authors: Balachandar S. , Yuen D. A. , Reuteler D. M.
Published: February 2004

We have applied spectral‐transform methods to study three‐dimensional thermal convection with temperature‐dependent viscosity. The viscosity varies exponentially with the form exp(‐BT), where B controls the viscosity contrast and T is temperature. Solutions for high Rayleigh numbers, up to an effective Ra of 6.25×106, have been obtained for an aspect‐ratio of 5×5×1 and a viscosity contrast of 25. Solutions show the localization of toroidal velocity fields with increasing vigor of convection to a coherent network of shear‐zones. Viscous dissipation increases with Rayleigh number and is particularly strong in regions of convergent flows and shear deformation. A time‐varying depth‐dependent mean‐flow is generated because of the correlation between laterally varying viscosity and velocity gradients.
Geomagnetic polarity reversal rate for the Phanerozoic
Authors: Johnson H. Paul , Van Patten Darcy , Tivey Maurice , Sager William W.
Published: February 2004

The recently updated Global Paleomagnetic Database provides an opportunity to construct a preliminary model of long‐term magnetic polarity reversal behavior for the last 570 My. After filtering for quality, a sub‐set of data from this global database is used to estimate relative reversal frequency, and the results show the expected long intervals of low reversal rate during the Cretaceous Normal and Permo‐Carboniferous Reversed Superchrons. In addition to the established superchrons, these new results indicate a long period of very low reversal rate in the Ordovician, which may be a previously unidentified superchron, and an additional short period of anomalously low reversals at the Jurassic/Triassic boundary. If the Ordovician reversal rate anomaly is a single polarity superchron, it represents the third major change in reversal rate within the Phanerozoic, with approximately 200 My between the CNS, the PCRS and the new Ordovician reversed interval.
Factors affecting the photochemical production of carbonyl sulfide in seawater
Authors: Zepp Richard G. , Andreae Meinrat O.
Published: February 2004

Our laboratory studies indicate that the photochemical production of carbonyl sulfide (COS) in seawater can result from the photosensitized reaction of organosulfur compounds. Because natural photosensitizers are more prevalent in coastal waters than the open ocean, these results help to explain why marine COS concentrations are highest in coastal and shelf regions. Wavelength studies of COS photoproduction in freshly collected seawater samples demonstrated that COS formation is induced primarily by the UV part of solar radiation. Studies with model organosulfur compounds in synthetic and coastal seawater indicated that compounds with bivalent sulfur atoms (thiols, sulfides) react most efficiently to produce COS. We propose that the initial step of the photosensitized reaction primarily involves attack on the sulfur atom or sulfhydryl H atom by reactive transients.
Tree‐ring evidence of the widespread effects of explosive volcanic eruptions
Authors: Jones P. D. , Briffa K. R. , Schweingruber F. H.
Published: February 2004

Tree‐ring evidence from 97 sites over North America and Europe are used to develop a chronology of widespread cool summers since 1600. Averaging annual maximum‐density information from the chronologies provides an index that correlates with Northern Hemisphere summer temperatures to 0.44 over 1851–1975. A number of the extreme low density years occur in both North America and Europe, suggesting a common response to high‐frequency forcing. The analysis was repeated with ring‐width information but the results are markedly less coherent. Of the five common extreme low‐density years (1601, 1641, 1669, 1699 and 1912) four are known to be coincident with the year or year following major volcanic eruptions. Other extreme low density indices in the continental series are also clearly coincident with known volcanoes. The density indices clearly demonstrate great potential for quantifying the climate effectiveness and perhaps establishing precise calendar dates of large explosive volcanoes.
Generation mechanism of tsunamis from the 1883 Krakatau Eruption
Authors: Nomanbhoy Nazli , Satake Kenji
Published: February 2004

The 1883 eruption of Krakatau generated a destructive tsunami which claimed about 34,000 lives. We compute the generation and propagation of tsunamis from three models previously proposed. They are 1) large‐scale caldera collapse of the northern part of Krakatau Island; 2) emplacement of pyroclastic flow deposits; 3) submarine explosion. We numerically solve the non‐linear shallow water equations on actual bathymetry. All three models displace the same volume (11.5 km³) of water in different ways. We compare the results of our computation with the observed tsunami waveforms at Batavia and tsunami wave heights along the coasts of Java and Sumatra. The comparison shows that the submarine explosion model of 1 to 5 min duration best explains the generation of the largest tsunami.
The 1994 Northridge Earthquake: 3‐D crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms
Authors: Zhao Dapeng , Kanamori Hiroo
Published: February 2004

A detailed 3‐D P‐wave velocity structure of the crust in the epicentral area of the 17 January, 1994 Northridge earthquake is determined by using 104,709 arrival times from 1673 Northridge aftershocks and 2948 other local earthquakes. A test performed using the data from the nearby portable stations suggests that the aftershock hypocenters relocated with the obtained 3‐D model are accurate to about 2 km. We found that regions with high aftershock activity are generally associated with faster P‐wave velocities. The velocity is high around the main south‐dipping fault of the 1994 Northridge earthquake and the north‐dipping fault of the 1971 San Fernando earthquake. A linear distribution of strike‐slip aftershocks was found along a NE‐SW boundary between high‐velocity and low‐velocity structures. To the west of this boundary a cluster of large shallow aftershocks with mixed mechanisms occurred in or near the border of a low‐velocity area, while to the east aftershocks with thrust mechanisms occurred in a high‐velocity area. These observations suggest that lateral variations of crustal properties are closely related to the fault segmentation in the Transverse Ranges. A better understanding of these features is important for long‐term seismic hazard assessment in the Los Angeles area.
Anisotropy beneath the Pyrenees Range from teleseismic shear wave splitting: Results from a test experiment
Authors: Barruol Guilhem , Souriau Annie
Published: February 2004

We have investigated the seismic anisotropy from SKS and SKKS wave splitting along a N‐S profile perpendicular to the Pyrenean mountain belt. The line crossed the North Pyrenean Fault (NPF), which probably represents the plate boundary between the European and the Iberian plates. We observed large delay times δt (1.3 to 1.5 s) and uniform fast polarization directions ϕ (N100E) at three stations located on the Iberian plate. On the other hand, δt are smaller (0.5–1.0 s) and the ϕ scattered at stations located north of the NPF. The small scale variability, the relation with surface geology and the correlation of the δt with lithospheric thickness suggest that the anisotropy could be predominantly of lithospheric origin. However, an asthenospheric contribution cannot be ruled out. In the North Pyrenean Zone, an important thermal event before the Pyrenean compression has likely erased the former mantle fabric. The observed anisotropy is then likely related to the Pyrenean orogeny. On the Iberian plate, our splitting parameters are similar to those obtained at two stations located on Hercynian structures in Central Spain. This suggests that two contributions are likely superposed to that of the Pyrenean built up: one related to the Hercynian orogeny and the other one related to the rotation of the Iberian plate.
Impact did not trigger Deccan volcanism: Evidence from Anjar K/T Boundary intertrappean sediments
Authors: Bhandari N. , Shukla P. N. , Ghevariya Z. G. , Sundaram S. M.
Published: February 2004

Many hypotheses including asteroidal and cometary impacts, Deccan volcanism, impact induced volcanism and coincidental impact and volcanism have been put forth to explain the observed enhancement of iridium and mass extinction at the K/T boundary (KTB). The identification of KTB layer within the Deccan intertrappean sediments at Anjar, about half way between Flow III and Flow IV provides new constraints on some of these hypotheses. The chemical characteristics of this layer show high concentrations of Ir, Os and other siderophiles accompanied by enrichment of chalcophiles and depletion of lithophiles. The Os/Ir ˜1.1, close to the meteoritic value and other chemical and stratigraphic criteria indicate that it may be the ejecta fallout layer, resulting from a bolide impact at the KTB. Presence of three basalt flows below this layer implies that the volcanism was already active when this layer was deposited and impact of the K/T bolide did not trigger Deccan volcanism.
Multichannel seismic image of the crustal thinning at the NE Iberian Margin combining normal and wide angle reflection data
Authors: Gallart J. , Vidal N. , Dañobeitia J. J.
Published: February 2004

The seismic image of the Moho at the transition from the NE Iberian Peninsula to the Western Mediterranean Sea is investigated using coincident steep and wide‐angle reflection data merged into a single stacked and migrated section. The multichannel analysis of onshore/offshore large‐aperture data provides new insight on the deep structure in areas where the steep sections lack penetration, and reveals that the crust undergoes a strong but continuous thinning along the flanks of the Valencia trough. The combined seismic image indicates that almost half of the continental crust (14–15 km) is lost in less than 60 km horizontal distance.
Thermal dependence of magnetocrystalline anisotropy and magnetostriction constants of single crystal Fe2,4Ti0.61O4
Authors: Sahu Sanghamitra , Moskowitz Bruce M.
Published: February 2004

The temperature dependence of the magnetocrystalline anisotropy (K′) and magnetostriction (λ) constants of single crystal Fe2.4Ti0.61O4 (TM61) is measured using torque magnetometry and strain gage techniques in the temperature range between 300 K and Tc. At all temperatures, K′ is positive making the <100> instead of <111> crystal directions the easy axes of magnetization in TM61. The thermal dependence of K′(T) varies as (1‐T/Tc)2.2. In terms of the saturation magnetization, K′(T)∝Ms(T)n where n≈6, a much weaker dependence than that found for magnetite (n=8–10). We also observed that K′∝λ1–2, consistent with K′∝Ms6. These results show that the magnetoelastic contribution to the magnetocrystalline anisotropy dominates the anisotropy constant of TM61 at elevated temperature and that the effects of stress cannot be neglected in theories of thermoremanence and micromagnetic models for intermediate titanomagnetites commonly found in oceanic basalts.
Mantle shear structure beneath the Americas and surrounding oceans
Authors: Grand Stephen P.
Published: February 2004

Maps of lateral variation in shear velocity within the mantle beneath North and South America, their surrounding oceans, and parts of Africa and Eurasia are produced from inversion of travel times of horizontally polarized shear body waves. The data consist of S and ScS waves as well as multibounce phases SS, SSS and SSSS. Waves that bottom within the upper mantle are modeled using synthetic seismograms in order to estimate travel times for each of the multiple arrivals caused by velocity discontinuities near 400 and 660 km depth. The model consists of blocks with uniform slowness anomalies relative to a one‐dimensional starting model and extends from the surface to the core‐mantle boundary. The blocks have horizontal dimensions of roughly 275 by 275 km and vary in the vertical dimension from 75 to 150 km. The data are inverted using a simultaneous iterative reconstruction technique algorithm. The upper 400 km of the model is dominated by lateral variations that correspond to surface tectonic environments. Three shields on three separate continents have higher than average velocities down to between 320 and 400 km depth. Young tectonically active regions are very slow in the upper 250 km. The transition zone from 400 to 660 km depth is the most poorly resolved region. High velocity beneath western South America in the transition zone is probably associated with subducting slab. The transition zone velocity beneath the western and central part of North America also appears to be slightly faster than average. The lower mantle is dominated by large‐scale sheets of higher than average velocity and more equidimensional regions of slow velocity. From South America to Siberia, sheet‐like high‐velocity anomalies are observed from 750 km depth to the core‐mantle boundary. Another lower mantle high‐velocity anomaly is seen beneath southern Eurasia. The high‐velocity lower mantle anomalies seem to be associated with subduction during the last 150 Ma. Comparing the location of past subduction with the location of lower mantle anomalies, the identification of lower mantle anomalies with old subducted slabs suggests slow sinking of slabs in the lower mantle (about 1 to 2 cm/yr). If this interpretation is correct then high velocity in the deepest mantle off the west coast of South America through the western United States requires significant subduction from 120 to 150 Ma a few thousand kilometers off the coast of the Americas. The slowest deep region found in this study is at the base of the mantle beneath the eastern Atlantic Ocean and may be associated with hotspots in that region. Other hotspots do not appear to be associated with slow lower mantle velocity.
Low S velocities under the Tornquist‐Teisseyre zone: Evidence for water injection into the transition zone by subduction
Authors: Nolet Guust , Zielhuis Alet
Published: February 2004

A strong low S velocity anomaly at 300–500 km depth coincides with the western boundary of the Russian Platform. The anomaly is too large to be explained by a simple temperature anomaly or by compositional variations, nor is it an artifact induced by seismic anisotropy. We present a model that explains the anomaly through the injection of water at the time of closure of the Tomquist Ocean that separated the continents of Avalonia and Baltica in the early Paleozoic. Dense hydrous magnesium phases are the most likely agents for transporting water to the transition zone, but an important role may also be played by nominally anhydrous clinopyroxene. When these minerals are brought out of their stability field, water is released. It may accumulate in β‐spinel or K‐amphibole and may be released when the mantle warms up after subduction halts. The effect of this water is to induce weakening of the shear modulus or even creation of a heavy melt. By a conservative estimate, a subduction episode lasting 85 m.y. would inject enough water to saturate a large volume of mantle rock with 0.3% H2O. Low velocities or low Q anomalies have also been observed in the transition zones near currently active slabs.
An empirical thermal history of the Earth's upper mantle
Authors: Abbott Dallas , Burgess Lee , Longhi John , Smith Walter H. F.
Published: February 2004

We have compiled petrological and geochemical data from 71 ophiolite suites and greenstone belts, which range in age from 15 to 3760 Ma. We have selected those rocks whose compositions indicate that they are either normal mid‐ocean ridge basalts (MORBs) or hotspot‐type MORBs. Then we used the data base to calculate the most primitive liquidus temperature for each rock suite. The results show that the liquidus temperature of the Phanerozoic ophiolites ranges from a low of 1212°C to a high of 1417°C. Using these data and two exponential curves bracketing the maximum and minimum temperatures versus time, we infer that the Phanerozoic suites had a mean liquidus temperature of 1272±7°C and a mean temperature range of 1218° to 1425°C. The liquidus temperatures of Archean MORBlike greenstones range from 1305° to 1576°C. Using these data and two exponential curves bracketing the maximum and minimum temperatures versus time, we infer that Archean melts at 2.8 Ga had a mean liquidus temperature of 1399±13°C and a temperature range from 1301° to 1533°C. Using two different methods, we show that the change in the mean liquidus temperature since the late Archean is from 96±13°C (from temperature ranges) to 127±20°C (from temperature means). When we convert these liquidus temperatures to potential temperature of the mantle, we find that the change in the mean upper mantle potential temperature since the late Archean is from 137±8°C (from temperature ranges) to 187±42°C (from temperature means). This change is less than that which was previously thought to have occurred. We compared the liquidus temperatures calculated from our data set with an independent data set from the modern day Pacific plate. The resulting histograms have the same shape and the same temperature range, showing that our method for calculating mantle temperatures from MORBlike rocks in ophiolite suites is valid. When our calculated liquidus temperatures for all time intervals are plotted in histograms, the resulting distributions are not bimodal, but skewed unimodal. That is, the distributions show a high‐T tail which results from the presence of hotspot magmas in the data set. The Archean temperature distribution is also skewed unimodal, and the high‐temperature Archean rocks, such as komatiites, plot in the hotspot area of the distribution. This strongly supports the contention that komatiites do not represent “normal” Archean mantle but rather were probably erupted by hotspots. Our data suggest that the relative proportion of hotspot magmas in oceanic lithosphere has remained nearly constant over geologic time.
Model studies of advective heat flow associated with compaction and dehydration in accretionary prisms
Authors: Shi Yaolin , Wang Chi‐Yuen
Published: February 2004

Surface heat flow across the deformation front of subduction zones shows complex patterns, and fluid flow is often invoked to interpret these patterns. Here we estimate the maximum possible amount of fluid flow due to sediment compaction and dehydration in subduction zones, and compare these estimates with the required fluid flow to produce significant thermal disturbances across faults and vents in two‐ and three‐dimensions. The results show that lateral heat loss becomes significant for faults or vents with thickness or diameter less than about 1 km. For two‐dimensional channel flow in fault zones or permeable strata, subduction‐induced compaction alone can hardly provide enough steady state flow to cause surface heat flow anomalies such as that observed in the Barbados accretionary prism [Foucher et al., 1990; Langseth et al., 1990], Fluid flow in three‐dimensional pipes (vents or vent fields) of a few hundred meters to ∼1 km in diameter is efficient in producing thermal disturbances; but steady state flow in small isolated pipes of a few meters in diameter is incapable of producing measurable heat flow anomalies. Small‐scale heat flow anomalies which are observed in some accretionary prisms are probably best explained by intermittent, transient flow.
Rise and fall of the southern Santa Cruz Mountains, California, from fission tracks, geomorphology, and geodesy
Authors: Bürgmann Roland , Arrowsmith Ramón , Dumitru Trevor , McLaughlin Robert
Published: February 2004

The Santa Cruz Mountains are closely associated with a left bend along the right‐lateral San Andreas fault. The Loma Prieta area on the northeast side of the San Andreas consists of fault‐bounded blocks that rise along active, deeply rooted, reverse and oblique‐slip faults. Six samples from a transect across this area yield concordant apatite fission track ages averaging 4.6±0.5 Ma. These ages date the time of cooling below ∼110°C and suggest that about 3 km of unroofing has occurred over the last 4.6 m.y. Allowing for current elevations of about 1 km, this suggests an average uplift rate of the order of 0.8 mm/yr over the last 4.6 m.y. To further define the extent and distribution of this young uplift, we used morphometric analyses of the youthful topography of the area. Steep drainage slopes and high local relief indicate that the area northeast of the San Andreas forms a well‐defined zone of high uplift. In contrast, the region on the southwest side of the San Andreas is characterized by broad upwarping and folding, more subdued topography, old fission track ages, and mean Quaternary uplift rates of 0.1–0.4 mm/yr. Geodetic data (1906 San Francisco earthquake and subsequent strain transients, 20 years of interseismic deformation, 1989 Loma Prieta earthquake, and 2 years of post‐Loma Prieta earthquake strain) show that the Southern Santa Cruz Mountains repeatedly rise and subside through a complex sequence of bay area deformation events. An additional deformation element that involves reverse slip averaging 2–3 mm/yr along the Foothills thrust system must occur to explain the longer‐term uplift pattern in the Loma Prieta area since the late Pliocene. The asymmetry in deformation on opposite sides of the San Andreas probably reflects the contrasting rock types on opposite sides of the fault, the influence of preexisting structures, and the interaction with neighboring faults of the San Andreas system.
Tectonic and climatic significance of a late Eocene low‐relief, high‐level geomorphic surface, Colorado
Authors: Gregory Kathryn M. , Chase Clement G.
Published: February 2004

New paleobotanical data suggest that in the late Eocene the erosion surface which capped the Front Range, Colorado was 2.2–3.3 km in elevation, which is similar to the 2.5‐km present elevation of surface remnants. This estimated elevation casts doubt on the conventional belief that the low‐relief geomorphic surface was formed by lateral planation of streams to a base level not much higher than sea level and that the present deeply incised canyons must represent Neogene uplift of Colorado. Description of the surface, calculations of sediment volume, and isostatic balance and fluvial landsculpting models demonstrate that while the high elevation of the erosion surface was due to tectonic forces, its smoothness was mostly a result of climatic factors. A sediment balance calculated for the Front Range suggests that from 2 to 4 km of material were eroded by the late Eocene, consistent with fission track ages. This amount of erosion would remove a significant portion of the 7 km of Laramide upper crustal thickening. Isostatic modeling implies that the 2.2–3.3 km elevation was most likely created by lower crustal thickening during the Laramide. A numerical model of fluvial erosion and deposition suggests a way that a late Eocene surface could have formed at this high elevation without incision. A humid climate with a preponderance of small storm events will diffusively smooth topography and is a possible mechanism for formation of low‐relief, high‐level surfaces. Paleoclimate models suggest a lack of large storm events in the late Eocene because of cool sea surface temperatures in the equatorial region. Return to a drier but stormier climate post‐Eocene could have caused the incision of the surface by young canyons. By this interpretation, regional erosion surfaces may represent regional climatic rather than tectonic conditions.
Stacking gravity tide measurements and nutation observations in order to determine the complex eigenfrequency of the nearly diurnal free wobble
Authors: Defraigne P. , Dehant V. , Hinderer J.
Published: February 2004

The resonance due to the nearly diurnal free wobble, observed in the diurnal tesseral tides and in the associated nutations, is used to determine the frequency and the damping of this wobble. The data are stacked to provide a least squares estimation of the resonance parameters: frequency, quality factor, and complex strengths. Some previous papers presented the same kind of analysis using either tidal or nutation data sets; in both cases, the frequency was about −(1 + 1/435) cycle/sidereal day, but the quality factor was larger for nutation data than for tidal data. In this paper we analyze simultaneously both sets of data, and we focus our attention on the determination of the free core nutation period and quality factor. We have analyzed a series of tidal data sets provided by the International Center for Earth Tides (ICET), and nutation data sets gathered from published matter. Our final result is obtained from a global stacking using four independent nutation data sets and three superconducting gravimeter data sets together. The values of the fitted parameters are found to be λFCN = −(1 + 1/(434.1 ± 0.9)) cycle/sidereal day and Q ≃ 54,000 (36,000; 109,000).
Seismicity and stress in the vicinity of Mount Spurr volcano, south central Alaska
Authors: Jolly Arthur D. , Page Robert A. , Power John A.
Published: February 2004

Focal mechanism solutions and hypocenters for earthquakes near Mount Spurr volcano in south central Alaska reveal spatial perturbations in the regional stress field and in the maximum depth of seismicity. At the volcano, practically all the shocks during the dormant decade 1981–1991 have depths shallower than 5 km and are characterized by nearly pure normal slip. In contrast, earthquakes located outboard of the volcano mostly concentrate in the depth range 3–18 km and exhibit predominantly strike slip, oblique reverse slip, or reverse slip. The regional stress field is characterized by subhorizontal maximum principal stress directed N30°W, concordant with the direction of convergence between the North American and Pacific plates, whereas the maximum principal stress is probably more nearly vertical beneath the volcano. We suggest that the shoaling of seismicity beneath the volcano reflects localized elevation of the depth to the midcrustal transition from brittle failure to plastic flow. We attribute this localized elevation to relict magmatic heat associated with past volcanic eruptions. The available data are not sufficient to determine the cause of the suggested rotation of the maximum principal stress axis beneath the volcano; possible mechanisms include an increase in the vertical stress from the weight of the Mount Spurr massif and a decrease in the maximum horizontal stress associated with doming of the shallow crust from magmatic processes.
In situ determination of thermal properties of sediments using a friction‐heated probe source
Authors: Lee Tien‐Chang , Von Herzen Richard P.
Published: February 2004

Marine heat flow measurements usually require inserting temperature sensor probes into seafloor sediments. The probe entry displaces the sediments and causes frictional heating along the probe‐sediment interface. Since the probe cools at a rate dependent primarily upon the sediment's thermal conductivity and heat capacity, analysis of the cooling data can yield not only ambient equilibrium temperature but also thermal properties. This paper describes a study which simulates the cooling process with a finite element method. Solutions are given for seven model‐defining parameters using a least squares nonlinear inversion algorithm. Advective mass‐heat transfer is incorporated by introducing a heat partition factor into the initial condition. The simulation provides a means of estimating frictional heat that can be considered as a heat pulse to determine thermal conductivity from an asymptotic relation between temperature rise and inverse time. Our simulation inversion is iterated until the simulated conductivity matches the “asymptotic” conductivity. Field tests indicate that the conductivity and equilibrium temperature can be satisfactorily determined from 5‐min long recordings of temperature for 3 mm‐diameter probes if the cooling is purely radial. For probes with sensors near their tips, conductivity determination is rendered uncertain because of more complex two‐dimensional geometry and heating process, but the equilibrium temperature can still be determined.
Quantitative detection of methane hydrate through high‐resolution seismic velocity analysis
Authors: Wood Warren T. , Stoffa Paul L. , Shipley Thomas H.
Published: February 2004

A laterally extensive, high‐resolution travel time velocity analysis and acoustic wave form, inversion were used to quantitatively determine methane hydrate content in deep water sediments of the Blake Ridge off the southeast U.S. coast. The interval acoustic velocity (Vp) analyses were performed in the τ‐p domain by interactively picking the τ‐p trajectories of prominent reflections in each of 50 plane wave‐decomposed common midpoint gathers. The reflections correspond to seismic stratigraphic boundaries so that lateral Vp changes due to lithology changes are mitigated, and Vp changes due to changing hydrate content are enhanced. Two separate interval Vp analyses were performed, one with thick (∼0.4 km) layers which yielded lower uncertainty but also lower resolution, and one with thinner layers (∼0.1 km), yielding higher resolution but slightly larger uncertainties. Results show no correlation between low‐sediment reflectivity and Vp. However, in the areas exhibiting a bottom simulating reflector (BSR) a high Vp interval (∼2.0 km/s and 0.15 km thick) is seen immediately above the BSR. Where the BSR is strongest a 256‐layer, least squares acoustic wave form inversion reveals the BSR to be caused by a Vp decrease from ∼2.0 to ∼1.5 km/s, with little or no change in density. The inversion also reveals a thin (0.025 km) layer of anomalously low Vp lying immediately below the BSR. Two models of methane hydrate distribution are tested, each indicating that the volume of methane hydrate in the intervals of elevated Vp is up to ∼25% of the total volume.
Seismic anisotropy in the shallow crust of the Loma Prieta segment of the San Andreas Fault System
Authors: Zhang Zhi , Schwartz Susan Y.
Published: February 2004

We study seismic anisotropy beneath the Santa Cruz Mountains using digital recordings of S waves from 60 aftershocks of the 1989 Loma Prieta earthquake (Mω = 7.0) recorded at 18 local stations. Three‐component particle motion covariance matrix decomposition combined with a cross‐correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves. Results from this procedure are compared with those obtained through visual inspection of horizontal particle motions. Inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of S to P converted phases from our study. Uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20° and 10 ms, respectively. About half of the S wave records examined show clear evidence of shear wave splitting. Two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events; 52% of the polarizations are parallel to the strike of the San Andreas fault (NW‐SE), and 24% are parallel to the probable direction of local maximum horizontal compressive stress (NE‐SW). The fault‐parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary. The fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the San Andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy (EDA), where parallel alignment of fluid‐filled fractures produces the anisotropy. The predominance of fault‐parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results. The splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth. This suggests that the seismic anisotropy is no deeper than 2 km, the depth of the shallowest earthquakes analyzed. For an anisotropic zone 2 km thick, the average velocity anisotropy is about 5% in our study region.
Self‐propping and fluid flow in slightly offset joints at high effective pressures
Authors: Durham W. B. , Bonner B. P.
Published: February 2004

We have made laboratory measurements of joint closure and permeability of both well‐ and poorly mated, laboratory‐produced joints in Westerly granite under joint normal stresses to 160 MPa, appropriate to midcrustal depths. We have also made detailed topographic measurements of the joint surfaces, both before and after pressure testing. The purpose of the study was to characterize and understand the hydraulic behavior of joints under conditions where fluid migration under natural settings is known to occur but where physical conditions, namely high lithostatic load, suggest that the presence of open, fluid‐conducting joints might be improbable. Normal stress was applied by hydrostatic confinement of cylindrical samples, each containing a single axial tensile fracture. Experiments were performed at room temperature under conditions designed to minimize chemical effects of dissolution or precipitation. We found that while a mated joint can be completely closed by the application of high normal stresses, the same joint laterally offset by 0.53 mm remains open and many orders of magnitude more hydraulically conductive than the mated joint even at highest stresses. Detailed topographic observations are consistent with self‐propping of the offset joint. Permanent damage to the surfaces of the offset joint is not widespread but occurs at isolated locales covering about 10% of the surface. Damage is confined to regions of negative (convex upward) curvature, such as summits and the edges of plateaus. Approximately 20% of the joint surface is in wall‐to‐wall contact at 160 MPa, and the flow tortuosity thus induced is probably the cause of the breakdown of the parallel plate approximation for fluid flow in the offset joint.
A 12,000‐year record of vertical deformation across the Yellowstone caldera margin: The shorelines of Yellowstone Lake
Authors: Locke William W. , Meyer Grant A.
Published: February 2004

The 600 ka Yellowstone caldera exhibits several signs of unrest, the most evident of which is historic ground deformation including both uplift and subsidence. We document deformation in the area of the southeastern caldera across ∼12,000 years using the postglacial shoreline terraces of Yellowstone Lake. Raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces, with an accuracy of +/−0.5 m. Of about 11 recognizable terraces, the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns. Shoreline ages are based on minimum‐ and maximum‐limiting radiocarbon and obsidian‐hydration dates. Each terrace is interpreted as representing an episode of uplift (∼1 kyr−1) of the caldera interior and subsequent subsidence, with little net volume change. This cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization, and/or episodic trapping and release of magmatic fluids (evolved during cooling and crystallization) in a self‐sealing reservoir, as hypothesized by Dzurisin et al. (1990). Early postglacial shoreline deformation reveals substantial intracaldera subsidence, possibly reflecting cooling and loss of trapped hydrothermal fluids. Extension along north‐south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin, including downwarping and local faulting of hot, weak intracaldera crust. Net deformation over the past ∼3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim, relative to the extracaldera region. This uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode. Subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence. The cause of this trend is undetermined but is most likely related to the effects of regional extension and long‐term cooling within the Yellowstone caldera.