bluuebunny/crossref_metadata_2025 huggingface dataset:
A kinetic model for the pattern and amounts of hydrate precipitated from a gas steam: Application to the Bush Hill vent site, Green Canyon Block 185, Gulf of Mexico
Authors: Chen Duo Fu , Cathles Lawrence M.
Published: May 2003

We construct a linear kinetic model of hydrate crystallization from a gas stream. We use this model to predict the fraction of gas that crystallizes as hydrate in the subsurface of Bush Hill, and the depth profile of subsurface hydrate accumulation. This is possible because the Bush Hill vent is fed by reservoir gas from the nearby Jolliet field whose composition is known. On the average, ∼9% of the vent gas is precipitated as hydrate in the subsurface. Although other explanations are possible, the observed vent gas compositions and the greater range of hydrate gas compositions are consistent with a single source gas whose venting rate varies by a factor of at least 3 over periods of a few years or less. The predicted depth profile of hydrate accumulation and the hydrate content of the Bush Hill mound suggest that between ∼1.1 × 109 and 2.8 × 109 m3 (STP) of gas may have accumulated as hydrate between the seafloor and ∼614‐m depth. For the radiometrically and geologically suggested system age of 10,000 years, the time average venting rate is ∼106 m3/yr (0.7 × 106 kg/yr). If distributed evenly across the 600 m diameter mound, as suggested by echo sounder images, the methane flux is >3.2 kg/m2 yr. This is >103 times that inferred for hydrates associated with bottom‐simulating seismic reflectors. The subsurface hydrate accumulation and the cumulative methane venting are related. We show how both may be estimated from measurements of vent gas composition, bottom water temperature, and geothermal gradient.
Paleomagnetism and magnetic anisotropy of Cretaceous red beds from the Tarim basin, northwest China: Evidence for a rock magnetic cause of anomalously shallow paleomagnetic inclinations from central Asia
Authors: Tan Xiaodong , Kodama Kenneth P. , Chen Hanlin , Fang Dajun , Sun Dongjiang , Li Yongan
Published: May 2003

Anomalously shallow paleomagnetic inclinations from Tarim basin red beds have suggested more than 1000 km of northward translation of the Tarim block since the Cretaceous. This is in conflict with geologic observations that indicate only a few hundred kilometers of crustal shortening north of the Tarim basin. To determine whether a rock magnetic effect could be the cause of the shallow inclinations, samples were collected from the Cretaceous Kapusaliang Group red beds. Both thermal and chemical demagnetization were employed to isolate the characteristic remanence (ChRM). The ChRMs pass the reversals test, as well as a local fold test. The anisotropy of magnetic susceptibility of the ChRM‐bearing particles was isolated by chemical demagnetization and had an oblate fabric with minimum axes perpendicular to bedding and foliations of ∼1.035. A 10–20% remanent anisotropy was obtained by comparing the saturation isothermal remanent magnetization for subsamples drilled parallel and perpendicular to bedding planes. The correlation of AMS and remanent anisotropy parameters yielded a value for the individual particle magnetic susceptibility anisotropy between 1.05 and 1.62. A particle anisotropy of 1.0638 allowed the best fit between corrected data and theoretical correction curves. An inclination correction corrected the mean Kapusaliang direction from D = 16.3°, I = 29.0°, α95 = 7.4° to D = 14.1°, I = 61.5°, α95 = 6.4°. The inclination correction reduced the paleomagnetically predicted latitudinal offset from more than 1000 km to less than the mean direction's 95% confidence limits, suggesting that paleomagnetic inclination shallowing is the cause of low inclinations recorded by the red beds from the Tarim basin.
Eight thousand years of geomagnetic field intensity variations in the eastern Mediterranean
Authors: Genevey Agnès , Gallet Yves , Margueron Jean‐Claude
Published: May 2003

Twenty new intensity determinations of the ancient geomagnetic field have been obtained from groups of potsherds and brick fragments from Syria. These artifacts, archeologically well dated from ∼6000 B.C. to approximately A.D. 1200, have been analyzed using the Thellier and Thellier [1959] method as modified by Coe [1967]. Intensity values have been corrected for the effects of anisotropy of thermal remanent magnetization and cooling rate. Our results indicate that field intensities were moderate in Syria from ∼6000 B.C. to ∼3500 B.C., with values of ∼30–40 μT. There was a significant increase in intensity by a factor of 2 from ∼3500 B.C. to ∼700 B.C., which was interrupted by a moderate decrease between ∼2550 B.C. and ∼1750 B.C. During more recent periods, our results show an intensity minimum approximately A.D. 200 and a maximum around the tenth century. Comparison with different data sets from the eastern Mediterranean and central Asia shows that geomagnetic field intensity variations were consistent at this large regional scale, at least over the last 5 millennia.
Apparent break in earthquake scaling due to path and site effects on deep borehole recordings
Authors: Ide Satoshi , Beroza Gregory C. , Prejean Stephanie G. , Ellsworth William L.
Published: May 2003

We reexamine the scaling of stress drop and apparent stress, rigidity times the ratio between seismically radiated energy to seismic moment, with earthquake size for a set of microearthquakes recorded in a deep borehole in Long Valley, California. In the first set of calculations, we assume a constant Q and solve for the corner frequency and seismic moment. In the second set of calculations, we model the spectral ratio of nearby events to determine the same quantities. We find that the spectral ratio technique, which can account for path and site effects or nonconstant Q, yields higher stress drops, particularly for the smaller events in the data set. The measurements determined from spectral ratios indicate no departure from constant stress drop scaling down to the smallest events in our data set (Mw 0.8). Our results indicate that propagation effects can contaminate measurements of source parameters even in the relatively clean recording environment of a deep borehole, just as they do at the Earth's surface. The scaling of source properties of microearthquakes made from deep borehole recordings may need to be reevaluated.
Geochemical relationships between dikes and lavas at the Hess Deep Rift: Implications for magma eruptibility
Authors: Stewart M. A. , Klein E. M. , Karson J. A. , Brophy J. G.
Published: May 2003

Differences in composition and phenocryst assemblages between dikes and lavas collected from 1‐m.y.‐old oceanic crust exposed at the Hess Deep Rift have important implications for density filtering of magmas ascending through the upper crust in mid‐ocean ridge settings. The majority of crystalline lavas collected have lower FeOT and MgO, and elevated Al2O3 and CaO concentrations compared to crystalline samples of dikes. These chemical differences, as well as associated modal variations, can be attributed to the accumulation of plagioclase in the magmas that ultimately erupt as lavas on the seafloor. The accumulation of plagioclase combined with the fractionation of mafic phases lowers the magma density by more than 0.04 g/cm3 relative to most magmas represented by the dikes. We suggest that these lower‐density magmas are preferentially erupted because of their increased buoyancy, resulting in the predominance of this magma type as lavas. Conversely, the majority of dikes are crystallized from the higher‐density magma type, which is rarely represented by lavas. These relationships suggest that most dikes never reach the surface and erupt lava. Examination of phenocryst contents and modal abundances in mid‐ocean ridge basalts worldwide indicates that magma density variations and crustal heterogeneity may influence magma eruptibility across a range of spreading rates. Specifically, lavas from slow spreading ridges have a wider range of modal phenocryst proportions compared to lavas from intermediate or fast spreading ridges. Presumably, these lavas will have a greater range of densities and their eruption on the seafloor may reflect the heterogeneous upper crust of slow spreading ridges compared to that of fast spreading ridges.
Small‐scale ultralow‐velocity zone structure imaged by ScP
Authors: Rost Sebastian , Revenaugh Justin
Published: May 2003

Short‐period, teleseismic ScP phases from events in the Tonga‐Fiji region recorded at the short‐period, small‐aperture Warramunga Array in Australia are used to study structure at the core‐mantle boundary (CMB). We examine the data for evidence of precursors and postcursors to ScP. Such additional arrivals are generated by the interaction of ScP with structure at the reflection and conversion point at or near the CMB. We show precursors and postcursors to ScP, indicating an ultralow‐velocity zone (ULVZ) at the CMB in the region between Tonga‐Fiji and Australia. One‐dimensional forward modeling of the short‐period data leads to models of the ULVZ with thickness varying between 5 and 20 km, velocity reductions of 0–10% and 10–30% for P velocity and S velocity, respectively, and 0–50% density increase. Many events, however, show simple waveforms, consistent with a locally simple CMB. Models for proximal events can vary greatly, indicating very complex, three‐dimensional structure with scale lengths of a few tens of kilometers. These results point to complex chemical and physical processes in the lowermost mantle.
Equation of state of liquid Fe‐10 wt % S: Implications for the metallic cores of planetary bodies
Authors: Balog P. S. , Secco R. A. , Rubie D. C. , Frost D. J.
Published: May 2003

The equation of state (EOS) of molten Fe‐10 wt % S was investigated by means of static high‐pressure density measurements using a newly developed variation of the sink/float technique. The method involves the use of tailored density composite spheres made of Pt or WC cores and an Al2O3 (ruby/sapphire) mantle as bracketing markers for comparative density measurements of liquid Fe‐10 wt % S at pressures (P) up to 20 GPa and at temperatures (T) up to 2123 K. The results of isothermal compression data confirm the validity of extrapolating to 20 GPa an EOS for pressures up to 6 GPa obtained earlier by Sanloup et al. [2000] using an X‐ray absorption technique. The analysis of new data obtained in the 1773–2123 K interval up to 20 GPa resulted in new EOS parameters and also revealed the need for a well‐determined melt density value at 1 atm. Application of our results to the identification of composition of liquid metallic cores of Ganymede, Io, and Mars showed Fe‐10 wt % S to be a possible candidate. We used different EOS formulations to extrapolate our results to the P‐T conditions of the Earth's interior. For temperatures between 4773 K at the core‐mantle boundary and 5973 K at the inner core boundary the extrapolation made using the Vinet EOS resulted in density values closest to the density of the core suggested by the preliminary reference Earth model.
Båth's law and the self‐similarity of earthquakes
Authors: Console Rodolfo , Lombardi Anna Maria , Murru Maura , Rhoades David
Published: May 2003

We revisit the issue of the so‐called Båth's law concerning the difference D1 between the magnitude of the main shock and the second largest shock in the same sequence. A mathematical formulation of the problem is developed with the only assumption being that all the events belong to the same self‐similar set of earthquakes following the Gutenberg–Richter magnitude distribution. This model shows a substantial dependence of D1 on the magnitude thresholds chosen for the main shocks and the aftershocks and in this way partly explains the large D1 values reported in the past. Analysis of the New Zealand and Preliminary Determination of Epicenters (PDE) catalogs of shallow earthquakes demonstrates a rough agreement between the average D1 values predicted by the theoretical model and those observed. Limiting our attention to the average D1 values, Båth's law does not seem to strongly contradict the Gutenberg–Richter law. Nevertheless, a detailed analysis of the D1 distribution shows that the Gutenberg–Richter hypothesis with a constant b‐value does not fully explain the experimental observations. The theoretical distribution has a larger proportion of low D1 values and a smaller proportion of high D1 values than the experimental observations. Thus, Båth's law and the Gutenberg–Richter law cannot be completely reconciled, although based on this analysis the mismatch is not as great as has sometimes been supposed.
Seismic evidence of Alpine crustal thickening and wedging from the western Pyrenees to the Cantabrian Mountains (north Iberia)
Authors: Pedreira D. , Pulgar J. A. , Gallart J. , Díaz J.
Published: May 2003

The Alpine collision between the Iberian and European plates resulted in a complex crustal structure beneath the northern Iberian Peninsula, as revealed from a new set of seismic refraction/wide‐angle reflection profiles. The study area is characterized by two major E‐W ranges, the Cantabrian Mountains and the Pyrenees, which are relayed to the south by the Iberian Chain. Important variations in crustal thickness and velocity distribution are found in a 560‐km‐long E‐W transect. In contrast to the typical 30‐ to 32‐km‐thick European Variscan crust of the western end of the profile, a continuous Alpine crustal root is evidenced from the Cantabrian Mountains to the central Pyrenees, with a Moho depth of 46–48 km, locally rising to ∼40 km depth in between, beneath the Basque‐Cantabrian Basin. Another outstanding feature is the inferred presence of high velocities of 6.40–6.75 km/s at midcrustal depths, which can be associated with portions of a lower crustal wedge from the northern (European) domain indenting the southern (Iberian) crust during the Alpine stage of compression. This indentation produces the delamination of the Iberian crust, with northward underthrusting of its lower half and the consequent crustal thickening. The indenting wedge has a discontinuous presence along the longitudinal section, as it was controlled and/or affected by N‐S to NE‐SW structures. Further evidence of the northward subduction of the Iberian plate is provided by another profile toward the Aquitaine Basin, while a N‐S profile across the Iberian Chain reveals an Alpine midcrustal thickening beneath this belt with Moho depths of ∼42 km.
Universal nucleation length for slip‐weakening rupture instability under nonuniform fault loading
Authors: Uenishi Koji , Rice James R.
Published: May 2003

We consider the nucleation of instability on a slip‐weakening fault subjected to a heterogeneous, locally peaked “loading” stress. That stress is assumed not only to gradually increase due to tectonic loading but also to retain its peaked character. The case of a linear stress versus slip law is considered in the framework of two‐dimensional quasi‐static elasticity for a planar fault. Slip initiates when the peak of the loading stress first reaches the strength level of the fault to start slip weakening. Then the size of the slipping region grows under increased loading stress until finally a critical nucleation length is reached, at which no further quasi‐static solution exists for additional increase of the loading. That marks the onset of a dynamically controlled instability. We prove that the nucleation length is independent of the shape of the loading stress distribution. Its universal value is proportional to an elastic modulus and inversely proportional to the slip‐weakening rate, and it is given by the solution to an eigenvalue problem. That is the same eigenvalue problem introduced by Campillo, Ionescu, and collaborators for dynamic slip nucleation under spatially uniform prestress on a fault segment of fixed length; the critical length that we derive is the same as in their case. To illustrate the nucleation process, and its universal feature, in specific examples, we consider cases for which the loading stress is peaked symmetrically or nonsymmetrically, and we employ a numerical approach based on a Chebyshev polynomial representation. Laboratory‐derived and earthquake‐inferred data are used to evaluate the nucleation size.
Tectonic plate coupling and elastic thickness derived from the inversion of a steady state viscoelastic model using geodetic data: Application to southern North Island, New Zealand
Authors: Cohen Steven C. , Darby Desmond J.
Published: May 2003

A steady state viscoelastic model of deformation at an oblique convergence zone is used to analyze crustal velocities deduced from Global Positioning System (GPS) observations in southern North Island, New Zealand. The model is physically more reasonable than elastic dislocation theory because the tectonic plates have finite elastic thicknesses. In an inversion that makes use of Green's functions derived from finite element calculations, we solve for depth‐dependent fault backslip rates. The associated chi‐squared goodness of fit parameter depends on the values of the elastic thicknesses of the overriding Australian and subducting Pacific Plates. These thicknesses are systematically varied in order to find the chi‐squared minimum. We find that: (1) the plates have coupling coefficient between 0.8 and 1.0 to a depth of about 22 km; (2) elastic dislocation theory appears to adequately fit the observations because the effects of viscoelastic flow are small; (3) viscoelastic results depend on the contrast between the elastic moduli of the plates, (4) the trench normal, rather than the trench parallel component of motion is more diagnostic for choosing between models with different parameters; (5) for the favored model (one with a weak continental crust), the estimated value of the Pacific Plate thickness is 40–60 km. Although the estimates of the plate thickness are not tightly constrained, those deduced from geodetic data tend to be larger than those deduced from geologic data, consistent with the idea that thickness estimates depend on the time scale of the loading process.
New age constraints on the timing of volcanism in central Afar, in the presence of propagating rifts
Authors: Lahitte Pierre , Gillot Pierre‐Yves , Kidane Tesfaye , Courtillot Vincent , Bekele Abebe
Published: May 2003

We investigate the relationship between rift propagation and volcanism in the Afar Depression in the last 4 Myr. Potassium‐argon and thermoluminescence dating allow detailed reconstruction of the temporal evolution of volcanism. Volcanic activity is almost continuous since 3.5 Ma, with intervals characterized by more intense activity, especially around 2 Ma. Spatial distribution of ages reveals that Stratoid Series volcanism migrated northward along a 200‐km trend between 3 and 1 Ma, at about 10 cm/yr, linked to northward propagation of the Gulf of Aden Ridge, after it had cut across the Danakil horst at 4 Ma. Our work underlines the role of rhyolitic volcanism in initiation of rifting. Acid volcanoes, initially formed near the axes of extensional zones, have been subsequently dissected and are presently located on both sides of active rift segments. These lavas were the first to be erupted in areas of low extensional strain and were followed by basaltic lavas as extension increased. Differentiated volcanoes acted as zones of local weakness and guided localization of fractures, then leading to fissural magmatism. This regional‐scale, composite style of rifting, including volcanic and tectonic components, can be compared to the large‐scale continental breakup process itself. Deformation occurs through propagation of faults and fissures under a regional stress field. These become localized because of weakening of the crust (or lithosphere) due to emplacement of magmas, under the influence of a plume in the large‐scale case, or of silicic centers linked to magma chambers in the regional‐scale case.
Subduction and the depth of convection in the Mediterranean mantle
Authors: Faccenna Claudio , Jolivet Laurent , Piromallo Claudia , Morelli Andrea
Published: May 2003

We tie together geological data, paleotectonic reconstruction, plate motion, and tomographic analysis to unravel the history of subduction and back arc extension of the eastern and central Mediterranean. In these two regions, extensional processes started contemporaneously, around 30 Myr ago, but with marked differences. In the eastern region, the Aegean basin opened slowly (∼1 cm/yr) behind a shallow dipping slab (40–45°). The corresponding high‐velocity anomaly extends inside the upper mantle and can be also followed in the midmantle down to a depth of at least 1500 km. Its descent into the midmantle initiated most probably during the Late Cretaceous, and the trench moved northeastward, following the path of the Eurasian plate and under the persistent push of the African plate. Conversely, in the central Mediterranean region, subduction initiated later, and the motion of the subducting slab is confined to the upper mantle, causing punctuated and rapid episodes of back arc extension (Provençal and Tyrrhenian basins) behind a slab that dips steeply (75°). We explore the causes that control how the slab subducted and interacted with the lower, more viscous part of the mantle.
Signal‐to‐noise ratios of teleseismic receiver functions and effectiveness of stacking for their enhancement
Authors: Morozov Igor B. , Dueker Kenneth G.
Published: May 2003

We present a method for the measurement of spatially variable signal‐to‐noise (S/N) ratios in multichannel teleseismic receiver function (RF) images. The S/N ratio is defined as a measure of coherency of the final image, and the approach is applicable to any RF imaging technique that employs mapping of the records into depth followed by their summation as the final signal enhancement step. In such methods, all of the converted phases become horizontally aligned in the depth domain, and their coherent (signal) and incoherent (noise) components can be estimated by using stacking statistics. For 10 locations along two subarrays of the Continental Dynamics of Rocky Mountains Project (CD‐ROM) teleseismic array, after a limited RF editing, we apply our method to the image resulting from three‐dimensional (3‐D) prestack RF depth migration. The resulting amplitude S/N values to vary from 0.1 to 0.3 in the individual RFs and from 1 to 5 in the final image, with significant spatial variability. These moderate S/N values argue in favor of sampling redundancy achieved in array recording and multichannel RF processing. In order to further reduce noise in RF images, denser and longer deployments or additional signal enhancement techniques are advisable. Because of its ability to provide direct assessment of image quality in three dimensions, including its dependence on the frequency band and data coverage, our method can also be potentially used for real‐time array tuning and image focusing required for moving arrays proposed for the USArray program.
A 2.14‐Myr astronomically tuned record of relative geomagnetic paleointensity from the western Philippine Sea
Authors: Horng Chorng‐Shern , Roberts Andrew P. , Liang Wen‐Tzong
Published: May 2003

We present a 2.14‐Myr astronomically tuned relative geomagnetic paleointensity record from the western Philippine Sea. Pseudosingle‐domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations. No significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization. This suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at Earth orbital periods. The paleointensity record is highly coherent with the Sint‐800 global paleointensity stack for the last 800 kyr and with a coeval record from the West Caroline Basin. Our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals, with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction. We do not observe an asymmetrical sawtooth form in our paleointensity record, which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts. Also, we do not observe a persistent 100‐kyr inclination periodicity, in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity. Good agreement between our paleointensity record and the coeval West Caroline Basin record provides the beginning of a detailed view of geomagnetic field behavior between 0.8 and 2.14 Ma.
Along‐strike variations in the shallow seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound
Authors: Calvert Andrew J. , Fisher Michael A. , Johnson Samuel Y.
Published: May 2003

Around 1100 years ago, the Seattle fault, which trends east‐west beneath Puget Sound and the greater Seattle metropolitan area, experienced a M > 7 earthquake. We present high‐resolution images of the shallow P wave velocity variation across the fault zone. These images were obtained by tomographic inversion of the first arrivals recorded along two north‐south oriented seismic reflection lines shot within Puget Sound near Seattle. Just beneath the seafloor, the fault zone includes uplifted Tertiary rocks with seismic velocities in the range of 2300 to 2600 m s−1. These velocities contrast markedly with values of ∼1600 m s−1 in shallow Holocene sediments. South of the Seattle fault zone volcanic rocks of the Crescent Formation, which exhibit velocities >3700 m s−1, are identified at depths of only 900 m. Seismic velocities of around 2600 m s−1, which represent Oligocene rocks, are found in the hanging wall of the Seattle fault beneath eastern Puget Sound. In the west, lower, 2300 m s−1 seismic velocities occur, probably due to the presence of Miocene rocks, which are not found in the east. Along‐strike velocity variations arise from the folding of Tertiary rocks and the presence of distinct fault splays, including a north striking tear fault characterized by depressed seismic velocities that was intersected by the eastern seismic line. Along‐strike differences in the uplift of Tertiary rocks beneath Puget Sound are likely associated with the existence of a segment boundary of the Seattle fault system.
Exhumation of the central Wasatch Mountains, Utah: 1. Patterns and timing of exhumation deduced from low‐temperature thermochronology data
Authors: Armstrong Phillip A. , Ehlers Todd A. , Chapman David S. , Farley Kenneth A. , Kamp Peter J. J.
Published: May 2003

The Wasatch Mountains are often cited as an example of normal fault growth and footwall flexure. They represent a tilted footwall at the edge of the Basin and Range extensional province, a major rift basin. Thus understanding the detailed spatial and elevation changes in coupled thermochronometer data, and how these changes can be interpreted, may aid in the analysis of thermochronometer data from other extensional regions around the world. We present a dense data set from the Cottonwood Intrusive Belt (CIB) of the Wasatch that includes apatite fission track (AFT), zircon fission track (ZFT), and apatite (U‐Th)/He ages. ZFT, AFT, and apatite (U‐Th)/He ages are 10, 5, and 3 Ma, respectively, adjacent to the Wasatch fault. AFT and (U‐Th)/He ages increase slightly with distance east of the fault until about 15–20 km, where a more abrupt increase in these ages occurs at or near the Silver Fork‐Superior fault zone. ZFT and AFT ages are concordant with 31–38 Ma pluton emplacement ages on the eastern side of range. Modeling of the data leads to the following interpretation: (1) Early cooling and ∼3–4 km of exhumation for the middle and eastern parts of the range occurred in the late Oligocene‐middle Miocene. (2) Beginning at 10–12 Ma, the locus of exhumation shifted westward toward the present range front, where the rocks cooled from >200°C in the last 10–12 Myr. Our data and interpretations are consistent with a model in which the locus of faulting and exhumation shifted opposite the direction of tilt, similar to that predicted by rolling‐hinge extensional models. However, this westward shift and rapid Miocene to recent exhumation may be a local effect superimposed on lower fault displacement and exhumation rates elsewhere along the Wasatch.
Direct dating of left‐lateral deformation along the Red River shear zone, China and Vietnam
Authors: Gilley Lisa D. , Harrison T. Mark , Leloup P. H. , Ryerson F. J. , Lovera Oscar M. , Wang Jiang‐Hai
Published: May 2003

Exposures of high‐grade, midcrustal rocks within the Red River shear zone (RRSZ), which separates the Indochina and South China blocks, exhibit clear evidence of left‐lateral, ductile deformation. Assuming that the South China Sea represents a pull‐apart basin formed at the southeastern termination of the RRSZ, it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the RRSZ between ∼32 and 17 Ma at a rate of ∼4 cm/yr. While 40Ar/39Ar thermochronometry indicates that left‐lateral slip occurred along the RRSZ between 25 and 17 Ma, the timing of earlier high‐temperature deformation has not been directly constrained. In situ Th‐Pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite‐grade metamorphism and synchronous left‐lateral shearing. Results from northern segments of the RRSZ in Yunnan, China, indicate that synkinematic garnet growth occurred between 34 and 21 Ma and are the first to document late Oligocene metamorphism and left‐lateral shearing. Data from the southern RRSZ within Vietnam are complicated by Tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the Indosinian orogeny (∼220 Ma). The period during which sinistral deformation is now constrained to have occurred along the RRSZ (i.e., 34–17 Ma) is essentially coincident with spreading of the South China seafloor (32–17 Ma). This temporal and kinematic link between left‐lateral shearing along the RRSZ and opening of the South China Sea supports the view that Indochina was extruded from Asia as a block along lithospheric‐scale strike‐slip faults.
Greenland glacial history, borehole constraints, and Eemian extent
Authors: Tarasov L. , Peltier W. Richard
Published: May 2003

We examine the extent to which observations from the Greenland ice sheet combined with three‐dimensional dynamical ice sheet models and semi‐Lagrangian tracer methods can be used to constrain inferences of the Eemian evolution of the ice sheet, of the extent and frequency of summit migration during the 100 kyr ice age cycle, and of the deep geothermal flux of heat from the Earth into the base of the ice sheet. Relative sea level, present‐day surface geometry, basal temperature, and age and temperature profiles from the Greenland Ice Project (GRIP) are imposed as constraints to tune ice sheet model and climate forcing parameters. Despite the paucity of observations, model‐based inferences suggest a significant northeast gradient in geothermal heat flux. Our analyses also suggest that during the glacial cycle, the contemporaneous summit only occupied the present‐day location during interglacial periods. On the basis of the development and use of a high‐resolution semi‐Lagrangian tracer analysis methodology for δ18O, we rule out isotropic flow disturbances due to summit migration as a possible source of the high Eemian variability of the GRIP δ18O record. Finally, in contrast with results obtained in some recent attempts to infer the extent to which Greenland may have contributed to the anomalous highstand of Eemian sea level, we find that conservative bounds for this contribution are 2–5.2 m, with a more likely range of 2.7–4.5 m.
Pahoehoe transport as a correlated random walk
Authors: Baloga S. M. , Glaze L. S.
Published: May 2003

Transport and emplacement of pahoehoe lava is often dominated by random influences, such as budding, small‐scale variations in topography, degree of inflation, and variations in how the skin cools, stretches, and cracks. A random walk model is used to describe the transport of small pahoehoe lobes when random effects are a significant influence on emplacement. We derive the stochastic formalism for random movements on a two‐dimensional grid. Our formulation features correlation and bias that determine the degree of randomness in the random walk of each parcel of lava. Qualitative and quantitative predictions of the random walk are compared with observations and data for pahoehoe lobes in Hawaii. The correlated random walk quantitatively describes lobe thickness transects, whereas the uncorrelated random walk only reproduces the meandering of small filaments of a few toes. Correlation expresses momentum effects and a chain of memory from the flow front to the source of lava supply. The correlated random walk features a channel‐forming regime that decays with distance from the source of lava supply. The theory can also be interpreted in the context of a time‐dependent planar flow. When many parcels of lava participate, the overall shape of a one‐dimensional lobe has the appearance of a continuum wave. The types of processes contained in this basic random walk model explain the multitude of concurrent or overriding features found in broad fields of toes. The formalism can thus be extended to simulate large fields of emplaced pahoehoe in either terrestrial or planetary settings.
A description of tides near the Chesapeake Bay entrance using in situ data with an adjoint model
Authors: Hallock Z. R. , Pistek P. , Book J. W. , Miller J. L. , Shay L. K. , Perkins H. T.
Published: May 2003

Time series of surface elevation and current velocity in the vicinity of the Chesapeake Bay entrance are assimilated into a linear, barotropic model using an iterative adjoint method. Tide surface elevations, NOAA‐NOS tidal data, moored acoustic Doppler current profiler (ADCP), and ocean surface current radar (OSCR) data are included in the assimilation. Data are analyzed for three tidal constituents (M2, S2, O1); tidal series are constructed from these constituents and then used in the model assimilations. Statistics of predicted currents compare favorably with those calculated from data not used in the assimilation. An error analysis of the data distribution is performed for the M2 tidal component, showing lower errors near assimilation data series but large errors, particularly for velocity, at some locations. Addition of a hypothetical current mooring dramatically reduces errors over most of the domain. We believe these results give the most demonstrably accurate description now available of tidal currents for this region of strong currents and heavy shipping.
Laboratory studies of thermally and/or salinity driven flows with partial mixing 1. Stommel transitions and multiple flow states
Authors: Whitehead J. A. , Timmermans M. L. E. , Lawson W. Gregory , Bulgakov Sergey N. , Zatarain Alejandro Martinez , Medina Jaime F. Almaguer , Salzig John
Published: May 2003

Laboratory studies with flows driven by both temperature and salinity differences illustrate the role of partial mixing in such flows. The results are compared to fundamental box model results. In the classic Stommel experiment, an experimental chamber is heated from below and exposed to a steady flux of salt water from above. The chamber is laterally connected to a reservoir of fresh isothermal water. This duplicates in part the well‐known box models of deep ocean and estuarine circulation where both temperature and salinity determine the density in the chamber that drives a steady exchange flow between chamber and reservoir. Two locally stable modes of circulation are found within a well‐known range of the temperature and salinity forcing: one mode is primarily temperature driven (the T‐mode) and one mode is primarily salinity driven (the S‐mode), with hysteresis and Stommel transitions between the two modes. New experiments with limited turbulent mixing and a narrow vertical slot in the sidewall between a laboratory test chamber and the reservoir produce a three‐layer structure for the S‐mode and a two‐layer structure for the T‐mode. Hysteresis and the Stommel transitions are considerably smaller than in the fully mixed box models. Results are consistent with the notion that the Arctic Ocean is presently in the layered S‐mode.
Basin‐scale internal wave dynamics during a winter cooling period in a large lake
Authors: Rueda Francisco J. , Schladow S. Geoffrey , Pálmarsson Sveinn Ó.
Published: May 2003

Basin‐scale internal waves are examined using a combined field observation and 3‐D numerical approach. The field site, Lake Tahoe (CA‐NV) during winter, is characterized by the presence of a weak density stratification and the passage of several storm fronts of varying character. Under these conditions internal waves with periods greater than 5 days and displacements of 30 m are possible. The model used is shown to have negligible numerical dissipation and sufficiently small phase lag over the time periods needed to represent basin‐scale waves. Both the numerical results and the observations identify the presence of three Kelvin modes and one Poincaré mode. While the wave periods are consistent with theoretical predictions, the spatial characteristics are far more complex than theory can predict. The spatial features are revealed through analysis of the integrated potential and kinetic energy fields yielded by the model. This shows that the basin topography exerts an influence on both Kelvin and Poincaré waves. The effect of the variable storms and, in particular, the phasing of the storms with respect to the internal waves is shown to be responsible for the amplification and annihilation of the basin‐scale waves.
Observations of cross‐channel structure of flow in an energetic tidal channel
Authors: Cáceres Mario , Valle‐Levinson Arnoldo , Atkinson Larry
Published: May 2003

Measurements of velocity and density profiles were made to describe the transverse structure of flow in Chacao Channel, Southern Chile (41.75°S), where typical tidal velocities are ∼4 m/s. Current profiles were obtained with a 307.2 kHz Acoustic Doppler Current Profiler (ADCP) over 25 repetitions of a cross‐channel transect during one semidiurnal tidal cycle. The 2.2 km long transect ran northeast/southwest across the channel. A northern channel (120 m deep) and a southern channel (85 m deep) were separated by Remolinos Rock, a pinnacle that rises to 20 m depth at ∼0.7 km from the southern side. Density measurements to depths of ∼50 m were obtained with a Conductivity, Temperature, and Depth (CTD) recorder at the north and south ends of each transect repetition. One CTD profile was also taken in the middle of the northern channel. The mean flow exhibited weak vertical structure because of strong vertical mixing. The predominant lateral structure consisted of mean outflow (toward the ocean) in the channels and mean inflow (toward Gulf of Ancud) over the pinnacle and the sides of the channel. This lateral structure pattern was consistent with the mean flow pattern expected from tidal rectification, as robust overtides were generated throughout the transect. The contributions to flow divergence and vorticity by the lateral variations of the lateral flow (∂v/∂y) and by the lateral shears of the along‐channel flow (∂u/∂y), respectively, were both of the order of 10−3 s−1. This caused advective and frictional forces (both horizontal and vertical) to be dominant in the across‐channel momentum balance, as they were more than twenty times the Coriolis acceleration. The present work then represents one of the few examples reported where lateral friction (proportional to ∂2v/∂y2) appears relevant to the transverse momentum balance.
On vertical advection truncation errors in terrain‐following numerical models: Comparison to a laboratory model for upwelling over submarine canyons
Authors: Allen S. E. , Dinniman M. S. , Klinck J. M. , Gorby D. D. , Hewett A. J. , Hickey B. M.
Published: May 2003

Submarine canyons which indent the continental shelf are frequently regions of steep (up to 45°), three‐dimensional topography. Recent observations have delineated the flow over several submarine canyons during 2–4 day long upwelling episodes. Thus upwelling episodes over submarine canyons provide an excellent flow regime for evaluating numerical and physical models. Here we compare a physical and numerical model simulation of an upwelling event over a simplified submarine canyon. The numerical model being evaluated is a version of the S‐Coordinate Rutgers University Model (SCRUM). Careful matching between the models is necessary for a stringent comparison. Results show a poor comparison for the homogeneous case due to nonhydrostatic effects in the laboratory model. Results for the stratified case are better but show a systematic difference between the numerical results and laboratory results. This difference is shown not to be due to nonhydrostatic effects. Rather, the difference is due to truncation errors in the calculation of the vertical advection of density in the numerical model. The calculation is inaccurate due to the terrain‐following coordinates combined with a strong vertical gradient in density, vertical shear in the horizontal velocity and topography with strong curvature.
Variational assimilation of SSH variability from TOPEX/POSEIDON and ERS1 into an eddy‐permitting model of the North Atlantic
Authors: Köhl Armin , Willebrand Jürgen
Published: May 2003

A first step for improving the climatological state of high‐resolution general circulation models by means of data assimilation is presented. A method developed for the assimilation of statistical characteristics into chaotic ocean models is applied to assimilate SSH variability from TOPEX/POSEIDON and ERS1 in association with temperature and salinity from the World Ocean Atlas 1997 in order to estimate the underlying mean circulation. The method requires a parameterization of SSH variability which derives from the approach of Green and Stone. By estimating initial conditions for temperature and salinity, a mean state is achieved which, although not fully consistent with the altimetric and climatological data, is markedly improved on time scales of one year in comparison to the control run. The assimilation of SSH variability data introduces complementary information about the main frontal structures consistent with climatological observations. The state is however not an equilibrium state and returns back to the first guess quasi‐equilibrium state for longer integration periods.
Nearshore oblique sand bars
Authors: Ribas F. , Falqués A. , Montoto A.
Published: May 2003

The coupling between hydrodynamics and the evolving topography in the surf zone has been theoretically examined for oblique wave incidence. It is shown that positive feedback can lead to the initial growth of several types of rhythmic systems of sand bars. The bars can be down‐current oriented or up‐current oriented, which means that the offshore end of the bar is shifted down‐current or up‐current with respect to the shore attachment. In the limit of strong current compared to wave orbital motion, very oblique down‐current oriented b ars are obtained with a spacing of several times the surf zone width. When wave orbital motions are dominant, systems of up‐current oriented bars and crescentic/down‐current oriented bars appear with spacings of the order of the surf zone width. The latter feature consists of alternating shoals and troughs at both sides of the break line with the inner shoals being bar‐shaped and oblique to the coast. The growth (e‐folding) time of the bars ranges from a few hours to a few days and it is favored by constant wave conditions. The range of model parameters leading to growth corresponds to intermediate beach states in between the fully dissipative and the fully reflective situations. Preliminary comparison with field observations shows qualitative agreement.
Wave–current interactions in an idealized tidal estuary
Authors: Lin Ray Q. , Perrie Will
Published: May 2003

We extended a recently developed model for wave–current interactions by Lin and Huang [1996a, 1996b] to simulate the impact of topography, currents, and slanting coastlines on sea state estimates. Our formulation uses the action conservation equation and the nonlinear dispersion relation, resulting in additional flux terms, ∂(cωA)/∂ω, A∂cgλ/∂λ, A∂(cgϕcos ϕ)/∂ϕ, and A∂cgθ/∂θ, compared to “standard” wave models such as WAM, where A is spectral action density, cω is phase velocity, and cgλ, cgϕ, and cgθ are group velocities. For large‐scale motions in deep water without varying currents, these effects may be neglected. However, for shallow estuary waters with varying currents, we show that these effects can cause as much as 25% variation in wave height estimates during moderate wind conditions. This phenomenon is consistent with observations and theory, for example, regarding isolated topography such as seamounts.
SeaWinds validation with research vessels
Authors: Bourassa Mark A. , Legler David M. , O'Brien James J. , Smith Shawn R.
Published: May 2003

The accuracy of the SeaWinds scatterometer's vector winds is assessed through comparison with research vessel observations. Factors that contribute to uncertainty in scatterometer winds are isolated and examined as functions of wind speed. For SeaWinds on QuikSCAT, ambiguity selection is found to be near perfect for surface wind speed (w) > 8 m s−1; however, ambiguity selection errors cause directional uncertainty to exceed 20° for w < ∼5 m s−1. These average uncertainties for wind speed and direction are found to be 0.45 m s−1 and 5° for the QSCAT‐1 model function and 0.3 m s−1 and 3° for the Ku‐2000 model function. The QuikSCAT winds are examined as vectors through two new approaches. The first is a method for determining vector correlations that considers uncertainty in the comparison data set. The second approach is a wind speed‐dependent model for the uncertainty in the magnitude of vector errors. For the QSCAT‐1 (Ku‐2000) model function this approach shows ambiguity selection dominates uncertainty for 2.5 < w < 5.5 m s−1 (0.6 < w < 5.5 m s−1), uncertainty in wind speed dominates for w < 2.5 m s−1 and 5.5 < w < 7.5 m s−1 (w < 0.6 m s−1 and 5.5 < w < 18 m s−1), and uncertainty in wind direction (for correctly selected ambiguities) dominates for w > 7.5 m s−1 (w > 18 m s−1). This approach also shows that spatial variability in the wind direction, related to inexact spatial co‐location, is likely to dominate rms differences between scatterometer wind vectors and in situ comparison measurements for w > 4.5 m s−1. The techniques used herein are applicable to any validation effort with uncertainty in the comparison data set or with inexact co‐location.
Response of an eddy‐permitting ocean model to the assimilation of sparse in situ data
Authors: Li Jian‐Guo , Killworth Peter D. , Smeed David A.
Published: May 2003

The response of an eddy‐permitting ocean model to changes introduced by data assimilation is studied when the available in situ data are sparse in both space and time (typical for the majority of the ocean). Temperature and salinity (T&S) profiles from the WOCE upper ocean thermal data set were assimilated into a primitive equation ocean model over the North Atlantic, using a simple nudging scheme with a time window of about 2 days and a horizontal spatial radius of about 1°. When data are sparse the model returns to its unassimilated behavior, locally “forgetting” or rejecting the assimilation, on timescales determined by the local advection and diffusion. Increasing the spatial weighting radius effectively reduces both processes and hence lengthens the model restoring time (and with it, the impact of assimilation). Increasing the nudging factor enhances the assimilation effect but has little effect on the model restoring time.