bluuebunny/crossref_metadata_2025 huggingface dataset:
Oceanic crustal thickness from seismic measurements and rare earth element inversions
Authors: White Robert S. , McKenzie Dan , O'Nions R. Keith
Published: February 2008

Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot‐spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow‐spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow‐spreading ridges to the conductive heat loss that enables the mantle to cool as it rises beneath the rift.
Teleseismic b values; Or, much ado about 1.0
Authors: Frohlich Cliff , Davis Scott D.
Published: February 2008

In this paper we investigate the value of b in the Gutenberg‐Richter relation for four teleseismic catalogs of earthquakes: Abe's historical catalog, the Harvard Centroid Moment Tensor (CMT) catalog, the catalog of the International Seismological Centre (ISC), and the Blacknest catalog. An unfortunate result is that b differs by 30% or more when determined in different magnitude ranges, in different catalogs, or using different methods. For global catalogs separated into shallow, intermediate, and deep earthquake groups, all values determined for b lie between 0.72 and 1.34. We can identify no systematic global variation of b with depth. For teleseismic catalogs it is difficult to believe measured geographic variations in b because systematic errors cause problems of earthquake detection, earthquake location, aftershock identification, and magnitude determination. However, some variations in b are so persistent and large that they must be real. For deep earthquakes in Tonga‐Fiji, for example, various measurements of b He between 1.06 and 1.57, comparable to b for shallow earthquakes, whereas measurements of b for deep earthquakes in the rest of the world are much lower, between 0.53 and 0.96. For shallow earthquakes in the Harvard CMT catalog, earthquakes with thrust and strike slip focal mechanisms have significantly lower b values (0.86 and 0.77) than earthquakes with normal faulting mechanisms (1.06). When we separate the ISC catalog into primary events (mainshocks and earthquakes with no aftershocks or foreshocks) and secondary events (aftershocks and foreshocks), we observe that b for secondary events is nearly always significantly higher than b for mainshocks. However, we show that the difference has no physical significance, as it arises simply from the act of choosing mainshocks as the largest earthquake in a foreshock‐mainshock‐aftershock sequence. When we correct for this systematic effect by comparing the real catalogs to identical catalogs with randomly reassigned magnitudes, we find that b for secondary events in the real catalog is actually lower than expected. Thus among aftershocks large earthquakes are relatively more common than expected, perhaps because the mainshock rupture loads asperities in adjacent regions.
New age data for Mid‐Atlantic Ridge hydrothermal sites: TAG and Snakepit chronology revisited
Authors: Lalou Claude , Reyss Jean‐Louis , Brichet Evelyne , Arnold Maurice , Thompson Geoffrey , Fouquet Yves , Rona Peter A.
Published: February 2008

The chronologies of TAG and Snakepit hydrothermal fields have been established using 210Pb/Pb, 230Th/234U and 14C dating. At the TAG field, a Mn‐oxide record, indicative of low temperature events, began at least 125,000 years and possibly 140,000 years ago with maximum intensities at 15,000, 7000 and 4000 years before present. High temperature events, giving rise to sulfide deposits, began about 100,000 years ago and have been intermittent to the present day. A presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20,000 years. Decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000, 7000 and 9000 years ago. After a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago. The Snakepit field is much younger and no sulfides older than 4000 years have been recovered. Relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at TAG. Reactivation of Snakepit. took place about 80 years ago, and is presently concurrent with that of TAG. Comparison with hydrothermal sites on the East Pacific Rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones, while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits.
Fission track analysis of the Late Cenozoic vertical kinematics of continental pacific crust, South Island, New Zealand
Authors: Tippett J. Mark , Kamp Peter J. J.
Published: February 2008

During the late Cenozoic the Pacific plate has been converging obliquely with the Australia plate in South Island, New Zealand. A result of this convergence has been the growth of a major mountain range (the Southern Alps) at the leading edge of the Pacific plate. The results of fission track analysis of 140 samples from 13 transects across the Alps reported here establish the late Cenozoic vertical kinematics (amount, age, and rate of rock uplift) of the Pacific crust underlying the Alps. The late Cenozoic rock uplift of the Pacific crust is asymmetrical with respect to the Alpine fault, being a maximum (19 km) immediately east of the central part of the fault, with lesser values at the eastern (3 km), northern (10 km), and southern (8 km) extremities of the Alps. The age of the start of rock uplift varies spatially across the Southern Alps, the earliest indications from fission track analysis being at 8 Ma at the southern end of the Alps, decreasing to 5 Ma at the northern end and 3 Ma along the southeastern margin. This age variation reflects the longer time over which the southern parts of the Alps have been in collision. The rate of propagation of rock uplift southeastward into the Pacific plate has been 30 mm/yr, nearly 4 times the late Cenozoic average rate of convergence normal to the plate boundary. Late Cenozoic mean rock uplift rates range from a maximum of ∼2.8 mm/yr at the Alpine fault to a minimum of ∼1.0 mm/yr in the east and have been sustained for periods of 3–8 m.y. Accompanying denudation has exhumed amphibolite grade rocks immediately east of the Alpine fault. The rock uplift has been controlled by oblique‐slip displacement on the Alpine fault. A continental crustal section at least 19 km thick has been uplifted on the Alpine fault. Comparison of the late Cenozoic mean rock uplift rates with uplift rates derived from reset zircon data (2–10 mm/yr) near the Alpine fault shows that uplift has accelerated over time, but only significantly since 1.3 ± 0.3 Ma. The amount of Mesozoic uplift ranged from minimal amounts north of Arthur's Pass, to ∼3 km near Mount Cook, to 10 km in the south at Lake Wanaka.
Infrared image analysis of volcanic thermal features: Láscar Volcano, Chile, 1984–1992
Authors: Oppenheimer Clive , Francis Peter W. , Rothery David A. , Carlton Richard W. T. , Glaze Lori S.
Published: February 2008

Fifteen Landsat Thematic Mapper (TM) images of Láscar volcano (Chile), recorded between December 1984 and April 1992, document the evolution of a lava dome within the summit crater. Four of the scenes were acquired at night. In every image, the two short‐wavelength infrared bands, 5 and 7, have detected thermal radiation from the volcano. As a consequence of the Planck distribution function, the relative response of these two channels depends on the proportions of very hot (> 600°C) surfaces occupying tiny pixel areas and broader regions at moderate temperatures (< 280°C). Intercomparison of bands 5 and 7 thereby provides a means for interpreting TM thermal anomalies even in the absence of ground observations. Pronounced changes in the configuration and intensity of the Láscar anomaly suggest that the volcano has experienced at least two cycles of lava dome activity since 1984. The first of these progressed through a “cooling” period, possibly reflecting a reduced flux of magmatic volatiles at the surface, and culminated in an explosive eruption on September 16, 1986, which appears to have completely destroyed the inferred lava dome. The TM data indicate that a new dome had been emplaced by November 1987, more than 15 months before it was first discovered by local observers. Láscar's style of cyclical effusive and explosive activity is typical of many volcanoes, and the remote sensing techniques presented herein could be applied elsewhere.
Shear wave properties and seismic imaging of Mylonite Zones
Authors: McCaffree Carey L. , Christensen Nikolas I.
Published: February 2008

Major crustal fault zones have displayed high reflectivity on observed compressional reflection profiles and synthetic models of laboratory measurements. This high compressional wave reflectivity suggests that shear waves may also produce similar reflectivity. The shear wave properties and reflectivity of mylonites from the Brevard fault zone have been determined using measured shear velocities as a function of confining pressure. The measured shear wave velocities are strongly influenced by mineralogy, especially quartz and oriented micas. The strong oriented fabric of Brevard mylonites creates seismic anisotropy, which, for nonnormal incident propagation produces significant shear wave splitting. A wide range of Vp/Vs ratios within the Brevard fault zone originates from a complex combination of composition and anisotropy, making lithology determination difficult. The shear wave reflectivity of the Brevard fault zone was derived from one‐dimensional normal incidence modeling of detailed geologic fault zone models. The shear wave reflectivity of the Brevard fault zone, which originates from fine compositional layering, is moderate and can be directly correlated with individual geologic formations. Within the fault zone variable shear wave reflectivity, both horizontally and vertically, originates from geologic heterogeneity. The modeled shear wave reflectivity also depends on frequency. Frequencies below 20 Hz will resolve boundary structure, while higher frequencies image detailed geologic features within the fault zone.
Coseismic deformation and dislocation models of the 1989 Loma Prieta Earthquake derived From Global Positioning System measurements
Authors: Williams Chesley R. , Arnadottir Thora , Segall Paul
Published: February 2008

Global Positioning System (GPS) surveys were used to determine coseismic displacements within the epicentral region of the 1989 Loma Prieta earthquake near Santa Cruz. The California Highway Department (Caltrans) conducted the preearthquake survey in February and March 1989 using single‐frequency GPS receivers. In March 1990, we reoccupied eight of the Caltrans stations using dual‐frequency receivers. Relative displacements were computed by differencing the station coordinates determined from these two surveys. Displacements were determined relative to sites remote from the earthquake by incorporating GPS measurements from Loma Prieta (station LP1) to the Very Long Baseline Interferometry (VLBI) site at Fort Ord. Examination of repeated measurements in the single frequency preearthquake survey indicates precision is ∼10 mm in the horizontal, although it may be as low as ∼40 mm in the weakly connected part of the network. The precision in the vertical is generally 30–40 mm. In contrast, the largest observed coseismic displacement is 410 mm horizontal and 341 mm vertical. We use a quasi‐Newton method to estimate the parameters of the uniform rectangular dislocation source that best fits the observed displacements. The best fitting dislocation dips 68°southwest, extending from 3 to 16 km depth, strikes N66°W, and has 1.8 m of dip slip and 1.2 m of strike slip. The best fitting dislocation overlaps the aftershocks but is rotated 15°counterclockwise from the trend of the afterhock zone. We find that the data can be fit nearly as well with a dislocation that is constrained to lie in the plane of the aftershocks, In this case the fault extends from 5 to 15 km depth. The constrained model explains 96% of the data. Both models have a moment ofM0= 2.9 × 1019N m. We also use the quasi‐Newton method to estimate rectangular dislocation models from the leveling data of Marshall et al. (1991), and the trilateration and GPS data of it Lisowski et al. (1990). In contrast to the previous studies, we find that both of these data sets are independently best fit with dislocations embedded within the aftershock zone.
Attenuation due to partial melting: An experimental study on a model system, using the Lab Coda Method
Authors: Gao L. S. , Poirier J. P. , Aki K.
Published: February 2008

Partial melting at the grain boundaries of mantle rock is a source of attenuation often invoked in the interpretation of teleseismic data. However, related experimental studies are extremely difficult and hence are scarce. We have used a convenient metallurgical system to model a rock sample with a layer of fluid at grain boundaries: the analog of the rock sample is a cylinder of fine‐grained, pure aluminum (diameter is 6 cm; height is 6 cm), and the analog of the fluid is provided by a thin layer of aluminum‐gallium eutectic at the grain boundaries. The metal gallium, liquid at room temperature, diffuses extremely rapidly into the grain boundaries of aluminum where it forms a soft eutectic that melts at 28°C. The total amount of gallium in the sample represents about 0.2 wt %. Ultrasonic (1 MHz) pulses were transmitted through the sample by a transducer, and the full waveform of the reflected and scattered signals, including the coda, were recorded. Analyses of the decay parameter of the coda envelope yield values of Q−1. Preliminary results show that the thin film of aluminum‐gallium eutectic at the grain boundaries noticeably increases attenuation. For pure aluminum at room temperature, Q−1 = 0.095 × 10−3; after invasion of gallium into the grain boundaries, the value measured at 26°C (in solid state) was Q−1 = 0.62 × 10−3 at 28°C, when the eutectic became liquid, Q−1 = 1.01 × 10−3, an order of magnitude higher than for pure aluminum.
Tomographic image of the Mid‐Atlantic Plate Boundary in southwestern Iceland
Authors: Bjarnason Ingi Th. , Menke William , Flóvenz Ólafur G. , Caress David
Published: February 2008

The 170 km South Iceland Seismic Tomography (SIST) profile extends from the west and across the Mid‐Atlantic Ridge spreading center in the Western Volcanic Zone and continues obliquely through the transform zone (the South Iceland Seismic Zone) to the western edge of the Eastern Volcanic Zone. A total of 11 shot points and 210 receiver points were used, allowing precise travel times to be determined for 1050 crustal P wave rays and 180 wide‐angle reflections. The large amplitudes of the wide‐angle reflections and an apparent refractor velocity of 7.7 km/s are interpreted to be from a relatively sharp Moho at a depth of 20–24 km. This interpretation differs from the earlier models (based on data gathered in the 1960s and 1970s), of a 10–15 km thick crust underlain by a upper mantle with very slow velocity of 7.0–7.4 km/s. Nevertheless, these older data do not contradict our new interpretation. Implication of the new interpretation is that the lower crust and the crust‐mantle boundary are colder than previously assumed. A two‐dimensional tomographic inversion of the compressional travel times reveals the following structures in the crust: (1) a sharp increase in thickness of the upper crust (“layer 2A”) from northwest to southeast and (2) broad updoming of high velocity in the lower crust in the Western Volcanic Zone, (3) depth to the lower crust (“layer 3”) increases gradually from 3 km at the northwestern end of the profile to 7 km at the southeastern end of the profile, (4) a low‐velocity perturbation extends throughout the upper crust and midcrust into the lower crust in the area of the transform in south Iceland (South Iceland Seismic Zone), and (5) an upper crustal high‐velocity anomaly is associated with extinct central volcanos northwest of the Western Volcanic Zone. The travel time data do not support the existence of a large (> 0.5 km thick) crustal magma chamber in this part of the Western Volcanic Zone but do not exclude the possibility of a smaller one.
Spectra of seismic radiation from a tensile crack
Authors: Walter William R. , Brune James N.
Published: February 2008

A model for the average far‐field P and S wave amplitude spectra from a circular crack failing in tension is presented. The model explicitly avoids specifying the details of the rupture process; instead, the spectral amplitude is determined by applying physically realistic constraints to an idealized spectral shape for each wave type. The idealized spectral form is given by Ω0 /(1 + (ω/ωc,)2)ψ/2, where ωc is a parameter representing the corner frequency, Ω0 determines the overall scaling of the spectra, and ψ determines the falloff at high frequencies. For fixed ψ, the complete specification of the body wave spectra, P and S waves, requires the determination of four unknowns: Ω0P, Ω0S, ωcP, and ωcS. The P and S spectral models are constrained by fixing the low‐frequency level of each to be equivalent to a point source. They are further constrained by equating the total radiated energy with the available elastic energy through a seismic efficiency parameter η. A final constraint between the corner frequencies of the P and S spectra is needed to determine the four free parameters. For the type of model considered here: an equidimensional fault with greatest displacement in the center, we expect the P to S corner frequency ratio to range between 1 and 1.73 depending on the details of the rupture. For ψ = 2, this model gives an average S/P spectral amplitude ratio of about 2.1 at very low frequencies and between 2.1 and 0.7 at very high frequencies, depending on the P to S corner frequency ratio. Applying the same criteria to a circular shear crack gives an average S/P spectral amplitude of about 7.1 at very low frequencies and between 7.1 and 2.4 at very high frequencies, again depending on the P to S corner frequency ratio. The tensional crack thus has lower average S/P spectral amplitudes than the shear crack, and such low S/P spectral amplitudes may be an identifying characteristic of tensile or tensile equivalent rupture. Using the shear and tensional models, we construct composite spectra that have significantly smaller S/P spectral amplitude ratios than the shear cracks alone, even when the tensional events radiate just a fraction of the total seismic energy. The tensile crack average spectral model may be useful as a first approximation for modeling seismic sources with a volumetric component in them, such as slip on a nonplanar fault, or magma injection.
Experiments to investigate the origin of anomalously elevated unblocking temperatures
Authors: Halgedahl Susan L.
Published: February 2008

A fundamental problem in rock magnetism and paleomagnetism is the physical origin of anomalously elevated unblocking temperatures. To investigate this problem, we have studied viscous remanent magnetization (VRM) acquired at 225°C by 1–2 μm magnetite particles synthesized with the glass‐ceramic method. Experiments were performed with two primary goals: (1) to determine which particular conditions produce anomalously elevated unblocking temperatures, and (2) to compare various observed aspects of viscous behavior to predictions based on two different physical mechanisms that might raise the unblocking temperature: the reduction of magnetic relaxation time by the applied field (the Neel medium‐field mechanism), and directional ordering of defects within domains and domain walls. When compared to model predictions, the present results indicate that neither the Neel medium‐field mechanism nor the directional ordering mechanism satisfactorily accounts for anomalously high unblocking temperatures. Surprisingly, however, the unblocking temperature spectrum of VRM is critically sensitive to a sample's initial state. When samples first are thermally demagnetized from 580°C to 225°C, VRMs acquired subsequently at 225°C exhibit pronounced high‐temperature “tails” extending to 450°C. Yet when samples first are demagnetized in an alternating field at room temperature before acquisition at 225°C, their VRMs unblock by approximately 300°C. Despite these two very different thermal responses, VRM acquisition curves at 225°C are virtually identical for the two initial states. Furthermore, zero‐field storage prior to acquisition suppresses VRM acquisition coefficients by similar amounts for both initial states. Significantly, standard thermal demagnetization to room temperature greatly attenuates the high‐temperature tail of a VRM acquired subsequently at 225°C. We conclude that thermal response of domain structure, as determined by thermomagnetic history, plays a critical role in the thermal unblocking process.
The accretion of oceanic crust by episodic sill intrusion
Authors: Henstock Timothy J. , Woods Andrew W. , White Robert S.
Published: February 2008

Seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites, yet the characteristics of different spreading centers show great variation. This leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite. We have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust, as implied by recent geophysical studies of spreading centers. Using our model, we have calculated velocity and temperature fields in the crust, including the effects of hydrothermal cooling. We show that oscillations in the temperature field due to episodicities of 200–20000 years have an effect localized to the region within 50–500 m of the intrusion. By assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation, we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux. We have calculated seismic velocities associated with the model temperature field in the crust, and we show that these are consistent with results from both seismic refraction and reflection experiments. The model may be applied to both fast and slow spreading ridges, and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate, hence resolving the paradox. We also show that the implications of this model are consistent with geological observations of layering in ophiolites.
Radiative temperature measurements at Kupaianaha Lava Lake, Kilauea Volcano, Hawaii
Authors: Flynn Luke P. , Mouginis‐Mark Peter J. , Gradie Jonathan C. , Lucey Paul G.
Published: February 2008

Field spectroradiometer data in the wavelength range of 0.4–2.5 μm and spectral resolution of 1–5 nm have been used to compute the radiative temperature of the surface of Kupaianaha lava lake, Kilauea Volcano, Hawaii. Two sets of observations (a total of 120 spectra) were made on October 12, 1987, and January 23, 1988, when the lava lake was in a period of active overturning. The area of the surface for which temperatures were measured was ∼0.23–0.55 m2. Two numerical models of two and three components have been used to match the measured radiant flux ratios and to describe the surface of the lava pond in terms of radiant area and temperature. Three stages of activity on the lake surface are identified: Stage 1, characterized by magma fountaining and overturning events exhibited the hottest crustal temperatures (180–572°C) and the largest fractional hot areas (> 10−3). Stage 1 average flux densities were ∼2.2 × 104 W/m2, the highest recorded for the three stages of activity on either day. The largest radiative area of fresh magma was 29% at 1100°C, while cooling from magmatic temperatures to newly formed crust at 790°C took place in a matter of seconds. Stage 2, marked by rifting events between plates of crust, exhibited crustal temperatures between 100 and 340°C with fractional hot areas at least an order of magnitude lower than those found for stage 1. Average flux densities calculated for three examples of stage 2 activity were 5.3 × 103 W/m2. Stage 3, which was quiescent periods when the lake was covered by a thick crust, dominated the activity of the lake both temporally and spatially over 90% of the time. The characteristic crustal temperature of stage 3 was 80–345°C with most solutions near 200–300°C and fractional hot areas of ≤ 10−5 of the viewing area. Average flux densities for stage 3 were 4.9 × 103 W/m2. For many stage 3 examples, a two‐component model was sufficient to describe the spectral data; however, for almost all of the stage 1 and 2 examples and the remainder of the stage 3 examples a three‐component model was required. These determinations of lava temperature and radiant area have relevance for satellite and airborne measurements of the thermal characteristics of active volcanoes and indicate that temporal variability of the thermal output of lava lakes occurs on the time scale of seconds to minutes.
Structural, petrological and thermal evolution of a Tertiary ductile strike‐slip shear zone, Diancang Shan, Yunnan
Authors: Leloup P. H. , Harrison T. Mark , Ryerson F. J. , Wenji Chen , Qi Li , Tapponnier P. , Lacassin R.
Published: February 2008

The Diancang Shan, a horst massif within the Red River fault zone in Yunnan, People's Republic of China, preserves a structural, petrological, and thermal record of two distinct phases of tectonic activity: a left‐lateral ductile shear that terminated between 20 and 17 Ma and a ductile‐to‐brittle phase of normal faulting which began at 4.7 Ma and remains active. Mylonitic rocks in the core of the range display an early, steep, high‐temperature (HT), schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt. Kinematic indicators indicate that shear was left‐lateral. The complex shape of the HT schistosity at the southern termination of the massif likely results from a large‐scale, oblique, left‐lateral C' shear plane that dismembered the shear zone and separated the Ailao Shan and the Diancang Shan as left‐lateral deformation terminated. Thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event. Along the eastern edge of the Diancang Shan, the HT fabrics were overprinted by low‐temperature structures during activation of east dipping normal faults. Cooling associated with this normal/right‐lateral faulting along the Diancang Shan (and perhaps activation of the right‐lateral/normal movement on the Range Front fault farther south along the Ailao Shan) began at 4.7±0.1 Ma. These results tend to support the view that extrusion of Indochina occurred along the left‐lateral Red River shear zone between 35 and 19–17 Ma. Initiation of right‐lateral/normal slip during the late Miocene may relate to eastward extensional collapse of the thickened Tibetan crust or, more probably, to initiation of the second phase of extrusion.
Sequential thrusting model for mountain building: Constraints from geology and heat flow of Taiwan
Authors: Hwang Win‐Tsuang , Wang Chi‐yuen
Published: February 2008

In this study we analyze the thermal evolution of the Taiwan fold‐and‐thrust belt based on the available geological information. A finite element program is used to simulate the complex kinematics of the observed sequential thrusting and the interplay of uplift, erosion and sedimentation processes. The discrete sequential thrusting model successfully reproduces the observed decrease in heat flow at the deformation front in northern Taiwan. This reveals that the heat flow drop may be due to the recent stacking of cold thrust sheets at the deformation front. This inference is consistent with the high seismicity and dense Quaternary fault distribution in western Taiwan. The model also successfully reproduces the observed high heat flow in the Central Ranges, suggesting that the observed high heat flow may be a result of tectonic unroofing due to active underplating, uplift, and erosion. These conclusions are insensitive to uncertainties in the frictional coefficient on the thrust plane.
Deformation of the oceanic crust between the North American and South American Plates
Authors: Müller R. Dietmar , Smith Walter H. F.
Published: February 2008

Fracture zone trends and magnetic anomalies in the Atlantic Ocean indicate that the North American plate must have moved with respect to the South American plate during the opening of the Atlantic. A comparison of plate tectonic flow lines with fracture zones identified from Geosat and Seasat altimeter data suggests that the North American‐South American plate boundary migrated northward from die Guinea‐Demarara shear margin to the Vema Fracture Zone before chron 34 (84 Ma), to north of the Doldrums Fracture Zone before chron 22 (51.9 Ma), and to north of the Mercurius Fracture Zone between chron 32 (72.5 Ma) and chron 13 (35.5 Ma). The paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area, indicate that the triple junction may have been located between the Mercurius and the Fifteen‐Twenty fracture zones after 67 Ma (chron 30). Plate reconstructions indicate a Late Cretaceous phase of transtension, followed by transpression in the Tertiary for the Tiburon/Barracuda Ridge area south of the Fifteen‐Twenty Fracture Zone. The ocean floor in this area is characterized by a series of ridges and troughs with large Bouguer gravity anomalies (up to ∼135 mGal). We use smoothing spline estimation to invert Bouguer anomalies for crustal layer structure. Our model results suggest that the Moho is uplifted 2–4 km over short wavelengths (∼70 km) at the Barracuda and Tiburon ridges and imply large anelastic strains. The severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted. We propose that the North‐South American plate boundary migrated to the latitude of the Tiburon Ridge, bounded by the Vema and Marathon fracture zones, before chron 34 (84 Ma). Post‐chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the Vema, Marathon, Mercurius, and Fifteen‐Twenty fracture zone troughs, but reaching the Fifteen‐Twenty Fracture Zone and future Barracuda Ridge area only after chron 32 (72.5 Ma). This interpretation concurs with our crustal structural model, which shows stronger crustal thinning underneath the Tiburon Ridge than at the Barracuda Ridge. Subsequent transpression may have continued along the existing zones of weakness in the Tertiary, creating the presently observed crustal deformation and uplift of the Moho, accompanied by anelastic failure of the crust. Middle‐Eocene‐Upper Oligocene turbidites on the slope of the Tiburon Ridge, now located 800 m above the abyssal plain, suggest that most of its uplift occurred at post‐Oligocene times. The unusually shallow Moho underneath the Tiburon and Barracuda ridges represents an unstable density distribution, which may indicate that compressive stresses are still present to maintain these anomalies, and that the North American‐South American plate boundary may still be located in this area.
Seismic radiation by magma injection: An anomalous seismic event near Tori Shima, Japan
Authors: Kanamori Hiroo , Ekström Göran , Dziewonski Adam , Barker Jeffrey S. , Sipkin Stuart A.
Published: February 2008

The earthquake with a bodywave magnitude mb=5.5, which occurred near Tori Shima, Japan, on June 13, 1984 (origin time: 0229:25.3 UT, 31.448°N, 140.036°E, depth of 10 km, mb =5.5, MS=5.5) is anomalous because it generated tsunamis which are disproportionately large for the magnitude of the earthquake. At Hachijo Island, 150 km from the epicenter, tsunamis were visually observed with peak‐to‐peak amplitude of 130 to 150 cm. Long‐period seismic radiation is also anomalous. Love waves are almost absent, and Rayleigh waves are radiated with equal amplitude and phase in all directions. A simple double‐couple model cannot explain these observations. With the assumption of no net volume change at the source, these data can be best explained with a compensated linear vector dipole (CLVD) with the principal tensional dipole in the vertical direction. The scalar moment of this dipole is 4×1024 dyn cm. Moment tensor inversions of long‐period body waves and surface waves yield an almost identical solution. This CLVD source can be interpreted as horizontal fluid injection. The location of the event is in the Smith depression which is one of the nascent back arc basins just behind the Bonin arc. These basins are filled with thick sediments, and numerous young volcanoes are found near this site. Magmatic injection is most likely to occur in this tectonic environment. However, the time scale of the seismic event seems too short for magma injection to occur. A more likely mechanism involves water‐magma interaction. The injection may be viewed as hydrofracturing driven by supercritical water heated by injected magma. The estimated volume of injected water is about 0.018 km3 and that of basaltic magma is about 10% of this. This type of deformation is more efficient for tsunami generation than faulting with the same scalar moment.
A nonlinear mechanism for maintaining coastally trapped eastern boundary currents
Authors: McCreary Julian P. , Fukamachi Yasushi , Lu Peng
Published: February 2008

Several versions of a 2½ ‐layer model are used to investigate a nonlinear mechanism for maintaining wind‐driven currents trapped to an eastern ocean boundary. All the models neglect the advection terms in the momentum equations, retaining advection only in the continuity equations. Solutions are forced by a band of upwelling‐favorable wind without curl. A possible solution to this system is a state of rest in which the upper layer pressure gradient everywhere balances the wind stress (the Sverdrup balance). As expected, solutions to a linearized version of the model adjust to this equilibrium state by the westward radiation of both mode‐1 and mode‐2 Rossby waves. In contrast, when the wind is sufficiently strong, the nonlinear solutions retain a reasonable coastal circulation (with an equatorward surface jet and a poleward undercurrent) even after 40 years of integration. The basic nonlinear process that causes this marked difference in response is the following: an onshore geostrophic flow, which compensates for offshore Ekman drift, is established by the radiation of mode‐1 Rossby waves, and this current can be strong enough to reverse the propagation direction of mode‐2 Rossby waves. As a consequence, the resulting eastern boundary currents are dynamically similar to the frictional, western boundary currents that occur in other models. The equilibrium state of the nonlinear solutions is presumably also a state of rest, but the diffusive adjustment occurs at such a slow rate that we have not been able to confirm this conclusion numerically.
Relationship between wind speed and gas exchange over the ocean
Authors: Wanninkhof Rik
Published: February 2008

Relationships between wind speed and gas transfer, combined with knowledge of the partial pressure difference of CO2across the air‐sea interface are frequently used to determine the CO2flux between the ocean and the atmosphere. Little attention has been paid to the influence of variability in wind speed on the calculated gas transfer velocities and the possibility of chemical enhancement of CO2exchange at low wind speeds over the ocean. The effect of these parameters is illustrated using a quadratic dependence of gas exchange on wind speed which is fit through gas transfer velocities over the ocean determined by the natural‐14C disequilibrium and the bomb‐14C inventory methods. Some of the variability between different data sets can be accounted for by the suggested mechanisms, but much of the variation appears due to other causes. Possible causes for the large difference between two frequently used relationships between gas transfer and wind speed are discussed. To determine fluxes of gases other than CO2across the air‐water interface, the relevant expressions for gas transfer, and the temperature and salinity dependence of the Schmidt number and solubility of several gases of environmental interest are included in an appendix.
Field observations of the fluid‐granular boundary layer under near‐breaking waves
Authors: Conley Daniel C. , Inman Douglas L.
Published: February 2008

Numerous synchronized time series from video cameras, pressure sensors, current meters, and hot film anemometers on natural beaches show that boundary layer development under the crest of near‐breaking waves can be idealized as a process composed of three distinct regimes here referred to as streaking, roiling, and pluming. The roiling and pluming regimes fail to develop under the trough. As a consequence, there is a pronounced asymmetry in instantaneous sand transport and boundary layer phenomena between the wave crest and trough. However, laboratory waves with field scale periods and wave heights over thin sand beds do not exhibit this crest‐trough boundary layer asymmetry, indicating that a critical element of similitude is absent in laboratory experiments. We suggest that wave induced boundary ventilation is responsible.
A model for the deposition of hydrothermal manganese near ridge crests
Authors: Lavelle J. W. , Cowen J. P. , Massoth G. J.
Published: February 2008

A two‐stage scavenging model is used to describe the transport of hydrothermal Mn to the sediments adjacent to ridge crests. Dissolved Mn is hypothesized to be scavenged by slowly settling metal‐depositing capsuled bacteria which, in turn, are incorporated into rapidly settling macroaggregates. Upon reaching the seafloor, the Mn is subject to resuspension in particulate form and to remobilization within the sediment column and release back into the water column as dissolved Mn. Measured Mn distributions in the vicinity of the southern Juan de Fuca Ridge and estimated values of process rate constants are used to limit the range of possible model outcomes. The results present a picture of water column distributions and fluxes of dissolved, fine particulate, and large‐particle associated Mn in a plume advecting off axis. The model and best available parameter values suggest that more than 80% of the hydrothermal Mn is deposited within several hundred kilometers of the ridge crest, though dissolved Mn concentrations beyond that distance exceed background levels by many times. The residence time of hydrothermal Mn in the water column is of the order of several years. An off‐axis component of advection of the order of 0.1–0.3 cm/s is needed to make similar the model and measured distributions of Mn depositing in the sediments.
An assessment of Geosat wave height and wind speed measurements
Authors: Carter D. J. T. , Challenor P. G. , Srokosz M. A.
Published: February 2008

An assessment is made of the significant wave height and wind speed measurements obtained by the Geosat radar altimeter. In comparison with buoy data, it is found that Geosat underestimated the significant wave height by 13% throughout the range of measurements. An attempt to compare the wave measurements made by Seasat and Geosat reveals problems with both data sets. For the Geosat wind speed measurements, it is found that the smoothed‐Brown algorithm, usually employed to convert backscattered power to wind speed, does not give good results in comparison with buoy measurements. A simple linear relationship between backscattered power and wind speed is derived, which is in close agreement with the algorithm recently proposed by Witter and Chelton for wind speeds above 3 m s−1. The implications of these results for deriving wind and wave climatologies from altimeter data are discussed.
The response of the South Brazil Bight to the passage of wintertime cold fronts
Authors: Stech José L. , Lorenzzetti João A.
Published: February 2008

On the southeastern continental shelf of Brazil the wintertime subtidal variability of the circulation is highly dominated by the passage of cold fronts. Hydrographic data for the region reveal that during this (July, August, September) season only a weak vertical stratification is observed. In this paper the response of the region to cold fronts is studied by using a barotropic finite element model, forced by a conceptual cold front wind field derived from the analysis of coastal winds and satellite imagery.
Near‐inertial motion on the shelf‐slope front off northeast Spain
Authors: Salat Jordi , Tintore Joaquin , Font Jordi , Wang Dong‐Ping , Vieira Mario
Published: February 2008

Near‐inertial motion on the shelf‐slope front off northeast Spain was monitored using surface drifters and moored current meters. On the shelf, strong inertial currents were generated by a wind burst. The inertial current amplitude was about 70 cm/s at the surface, 30 cm/s at the base of the mixed layer, and 10 cm/s in the interior. The observed near‐inertial frequency on the shelf was about 10% lower than the local inertial frequency, suggesting that the near‐inertial motion was embedded in region of strong anticyclonic shear. Also, the phase of near‐inertial motion increased through the water column, indicating that the energy propagation was downward. By contrast, the surface inertial currents were only about 10 cm/s in the center of the shelf‐slope front. Indirect evidence suggests that the observed small surface inertial currents were the result of rapid downward transfer of near‐inertial energy in the front.
The oceanography of winter leads
Authors: Morison J. H. , McPhee M. G. , Curtin T. B. , Paulson C. A.
Published: February 2008

Leads in pack ice have long been considered important to the thermodynamics of the polar regions. A winter lead affects the ocean around it because it is a density source. As the surface freezes, salt is rejected and forms more dense water which sinks under the lead. This sets up a circulation with freshwater flowing in from the sides near the surface and dense water flowing away from the lead at the base of the mixed layer. If the mixed layer is fully turbulent, this pattern may not occur; rather, the salt rejected at the surface may simply mix into the surface boundary layer. In either event the instability produced at the surface of leads is the primary source of unstable buoyancy flux and, as such, exerts a strong influence on the mixed layer. Here as many as possible of the disparate and almost anecdotal observations of lead oceanography are assembled and combined with theoretical arguments to predict the form and scale of oceanographic disturbances caused by winter leads. The experimental data suggest the velocity disturbances associated with lead convection are about 1–5 cm s−1. These appear as jets near the surface and the base of the mixed layer when ice velocities across the lead are less than about 5 cm s−1. The salinity disturbances are about 0.01 to 0.05 psu. Scaling arguments suggest that the geostrophic currents set up by the lead density disturbances are also of the order of 1–5 cm s−1. The disturbances are most obvious when freezing is rapid and ice velocity is low because the salinity and velocity disturbances in the upper ocean are not smeared out by turbulence. In this vein, lead convection may be characterized at one extreme as free convection in which the density disturbance forces the circulation. At the other extreme, lead convection may be characterized as forced convection in which the density disturbance is mixed rapidly by boundary layer turbulence. The lead numberLo, which is the ratio of the pressure term to the turbulence term in the momentum equation, and the turbulent lead numberLot, which is the ratio of buoyant production to shear production in the turbulent kinetic energy equation, define the boundary between the free and forced regimes. ForLoandLotless than one, both the large‐scale circulation and the turbulence are forced by surface stress. ForLoandLotgreater than one, both the large‐scale circulation and the turbulence are forced by the buoyancy flux. The magnitudes of velocity and salinity disturbances from a model developed elsewhere, suitable to free convection, agree with what few observations we have. The results of a forced convection model, developed here, suggest salinity disturbances of the order of 0.01–0.02 practical salinity units, with the maximum occurring at the surface of the lead and decreasing substantially below 5–10 m. This unstable gradient is a unique characteristic of lead convection. Though the salinity disturbances may be small when ice velocities are large, the buoyancy flux in leads has a major effect on the boundary layer turbulence.