bluuebunny/crossref_metadata_2025 huggingface dataset:
Compressional sound velocity, equation of state, and constitutive response of shock‐compressed magnesium oxide
Authors: Duffy Thomas S. , Ahrens Thomas J.
Published: February 2004

Wave profile and equation of state (EOS) data are reported for low‐porosity polycrystalline magnesium oxide under shock compression. The Hugoniot equation of state between 14 and 133 GPa is US = 6.87(10) + 1.24(4)up, where the numbers in parentheses are one standard deviation uncertainties in the last digit(s). Reverse‐impact wave profiles constrain the compressional sound velocity, Vp, at 10–27 GPa to ±2%. Measured Vp values are consistent with ultrasonic data extrapolated from 3 GPa. By combining the Hugoniot results with ultrasonic data, the adiabatic bulk modulus and its first and second pressure derivatives at constant entropy are 162.5(2) GPa, 4.09(9), and −0.019(4) GPa−1. The shear modulus and its first and second pressure derivatives are 130.8(2) GPa, 2.5(1), −0.026(45) GPa−1. Polycrystalline MgO has a compressive yield strength of 1–1.5 GPa at the elastic limit which increases to 2.7(8) GPa along the Hugoniot and is similar at unloading. Wave profiles for MgO at 10–39 GPa are described using a modified elastic‐plastic model. There are significant differences in the dynamic response of single‐crystal and polycrystalline MgO.
Climate change on the Colorado Plateau of eastern Utah inferred from borehole temperatures
Authors: Harris Robert N. , Chapman David S.
Published: February 2004

Temperature profiles from boreholes on the Colorado Plateau of southeastern Utah have been examined for evidence of climate change. Because these boreholes penetrate layered sedimentary rocks with different thermal conductivities, Bullard plots (temperature versus integrated thermal resistance) are used to estimate background heat flow and surface temperature intercepts. Reduced temperatures, which represent departures from a constant heat flow condition, are inverted for a surface ground temperature history at each borehole site using a singular value decomposition algorithm. Singular value cutoffs are selected by analyzing the spectral energy and the standard deviation of the model fit to the data as a function of the number of eigenvalues; solutions are constructed from areas of large spectral energy and a cutoff where additional eigenvalues fail to improve the solution significantly. The solution is parameterized in terms of 13 time steps increasing in duration and going back 400 years. Eight of nine borehole sites indicate between 0.4 and 0.8°C (±0.2° C) warming over the past 200 years with some evidence for accelerated warming in this century; one borehole indicates local cooling over the same time period. The amplitude of the warming inferred from borehole temperatures is less than that deduced from analysis of 100‐year surface air temperature records at four of the five weather stations surrounding the borehole sites.
Dynamo theory and paleomagnetism
Authors: Merrill Ronald T. , McFadden Phillip L.
Published: February 2004

For more than 200 years the origin of Earth's magnetic field was attributed to permanent magnetization. Even today no single argument (e.g., that Earth's deep interior is too hot to sustain permanent magnetization) conclusively rules out the permanent magnetization hypothesis. Nevertheless, when all the evidence is considered, this hypothesis can be safely discarded and replaced with an electric current (dynamo) hypothesis. Surprisingly, this can be done even though there is no adequate dynamo model for Earth. The development of geodynamo models began with the disk dynamo of Larmor in 1919 and expanded to include many classes of models, such as αω, α2, α2ω, Taylor state, and Model Z dynamos. Because of mathematical difficulties associated with solving the many coupled partial differential equations of dynamo theory, numerous simplifying assumptions are made. The majority of numerical dynamo models assume a three‐dimensional velocity field in an inviscid fluid and use mean field theory to solve for axisymmetric magnetic fields. There is also an increasing number of intermediate and strong field models emerging, in which feedback from the magnetic field to the velocity field is permitted. Nevertheless, these models still require several simplifying assumptions and there are many additional problems. For instance, many core parameters are difficult to estimate; there is debate on whether the top of the core is stably stratified and on the effects such stratification might have; what effects the presence of an inner core have; and whether the coupling across the coremantle boundary significantly affects the geodynamo. Perhaps it is not surprising that dynamo theoreticians, faced with large difficulties in mathematics and many uncertainties in physics, essentially choose to ignore input from fields such as paleomagnetism. However, it is precisely because of such difficulties that paleomagnetism can provide valuable constraints to narrow the range of viable dynamo models. For example, paleomagnetism ultimately should provide constraints on the velocity and magnetic field symmetries of dynamos; determine whether the geodynamo is in the weak, intermediate, or strong field regime; determine if there is a fundamental difference in dynamo processes during superchrons when reversals of the magnetic field essentially cease; and provide valuable information on the growth of the inner core and its possible stabilizing effects on geodynamo processes.
Stability of coda wave attenuation during the Loma Prieta, California, earthquake sequence
Authors: Beroza Gregory C. , Cole Alex T. , Ellsworth William L.
Published: February 2004

The Loma Prieta, California, earthquake occurred in a densely instrumented region with a history of microearthquake recording beginning more than a decade before the October 1989 mainshock. This affords an unprecedented opportunity to detect changes in seismic wave propagation in the Earth's crust associated with a major earthquake. In this study we use pairs of nearly identical earthquakes (doublets) to search for temporal changes of coda attenuation in the vicinity of the Loma Prieta earthquake. We analyze 21 earthquake doublets recorded from 1978 to 1991 that span the preseismic, coseismic, and postseismic intervals and measure the change in coda Q using a running window ratio of the doublet spectral amplitudes in three frequency bands from 2 to 15 Hz. This method provides an estimate of changes in coda Q that is insensitive to other factors that influence coda amplitudes. Our observations place an upper bound of about 5% on preseismic, coseismic, and postseismic changes of coda Q in the epicentral region of the Loma Prieta earthquake. Even at this low level, the changes are neither spatially coherent nor correlated between adjacent frequency bands. The only hint of a signal is in the preseismic data where there is a possible precursory increase in coda Q of approximately 5% in the two years before the mainshock. The stability of coda Q throughout the Loma Prieta sequence is in sharp contrast to other studies that have reported much larger precursory changes in coda Q for other earthquakes.
The 1946 Hispaniola earthquakes and the tectonics of the North America‐Caribbean plate boundary zone, northeastern Hispaniola
Authors: Russo R. M. , Villaseñor A.
Published: February 2004

We have determined focal mechanisms for the largest earthquake (Ms = 7.8) recorded instrumentally in the Caribbean Basin, the August 4, 1946, Hispaniola earthquake, and three of its large‐magnitude (Ms ≥ 6.1) aftershocks. We also relocated 63 aftershocks and one foreshock of the event series. The aftershock series is elongate, trends WNW, and is centered on the Samaná Peninsula of northeast Hispaniola. Shallow aftershocks are in a 75‐km‐wide linear zone, and intermediate depth (70 to 130 km) aftershocks apparently delineate a moderately south or SSW dipping slab. It is not clear, however, whether these events indicate active subduction of North American Atlantic Ocean lithosphere or are strike‐slip events on the interface between subducted but no longer sinking slab and Caribbean mantle. We constrained focal mechanisms of the main shock and three aftershocks by combining observed P and S polarities and amplitude ratios and also by waveform modeling. The two methods yield consistent results. The mechanisms include strike‐slip and thrust displacements on NW striking nodal planes. Fault dip is variable, NE or SW. The NW striking fault planes parallel mapped terrane boundaries and faults in the North America (NA)‐Caribbean (Ca) plate boundary zone and are also parallel to the aftershock series trend. We interpret the events to be motions on a WNW trending restraining bend segment of the NA‐Ca plate boundary in eastern Hispaniola. We have calculated magnitudes for eight of the earthquakes in the series; for the three events (including the main shock) for which data are available, our magnitudes are systematically less than the previously published magnitude estimates. Given the high magnitude and large aftershock area of the August 4, 1946, event, these earthquakes probably represent the true long‐term interplate motions between North America and the terranes in this portion of the plate boundary zone.
Seismic imaging of the enigmatic Sudbury Structure
Authors: Wu Jianjun , Milkereit Bernd , Boerner David E.
Published: February 2004

The Sudbury Structure on the southern margin of the Superior craton was created by a catastrophic explosion in 1.85 Ga and hosts one of the world's largest Ni‐Cu reserves. As a unique terrestrial geological feature, its genesis has been vigorously debated for more than a century. In an effort to optimize the image from seismic reflection data acquired across the center of the Sudbury Structure, we have developed a straight‐line common midpoint binning strategy and employed cross‐dip corrections. These pseudo‐three‐dimensional seismic processing techniques, coupled with standard processing methods, have overcome limitations associated with conventional two‐dimensional seismic data processing and have substantially enhanced the seismic image, warranting a more detailed and reliable structural interpretation. The new image has revealed a major, previously unrecognized, zone of imbricated NW thrusts. Interpretation of these thrusts provides critical timing constraints relating the Sudbury tectonic deformation to deposition of the Sudbury Basin sediments. The Onwatin argillites are penetrated by blind thrust faults, whereas the overlying Chelmsford turbidites are undeformed and most likely postdate the thrusting. Thus the uniform paleocurrent trends observed in the Chelmsford reflect subsequent deformation history and may be dismissed as evidence against an impact origin. Based on our interpretation of the seismic image, the original volume of the Sudbury Igneous Complex, likely an impact melt sheet, is ∼1 × 104 km3, supporting the claim that the Sudbury Structure represents the eroded and tectonized remanent of one of the largest known impact structure (∼200 km) on Earth.
Source time function complexity of the great 1989 Macquarie Ridge earthquake
Authors: Velasco Aaron A. , Ammon Charles J. , Lay Thorne
Published: February 2004

The rupture process of the great 1989 Macquarie Ridge (Mw = 8.1) strike‐slip faulting earthquake is examined using an empirical Green function deconvolution method which provides broadband source time functions from both body and surface waves. Recordings from a nearby, much smaller (Mw = 6.4) strike‐slip event provide accurate broadband Green functions for PP, S, SS, Rayleigh, and Love waves, spanning the period range 20–150 s. Body wave deconvolutions indicate that the principle short‐period (<30 s) moment release occurred in the first 25 s of rupture with no resolvable directivity; only minor short‐period energy release occurred over the next 30 s. Surface wave deconvolutions for the usually neglected 20–150 s period range produce source time functions with slightly larger total moment than the body wave deconvolutions and no resolvable directivity from the onset to the peak of moment release in the first 20 s. However, azimuthally dependent broadening of the surface wave source functions, which extend to more than 50‐s duration, indicates a component of northeastward unilateral rupture extension requiring about 30–50 km asymmetry in the overall rupture, compatible with the northeastward asymmetry of the 1‐day aftershock zone. Spectral inversions of long‐period Love and Rayleigh waves indicate a centroid time shift of approximately 25±3 s (50±7s duration trapezoid source) in the period range of 150–280 s, compatible with the deconvolved surface wave source time functions. Deconvolutions using both empirical and theoretical Green functions are optimized to detect any long‐period precursory radiation for periods up to about 250 s, but no convincing evidence for precursory radiation in the passband is found. The long‐period surface wave phase data also do not require a precursor.
Numerical study of nucleation and growth of bubbles in viscous magmas
Authors: Toramaru Atsushi
Published: February 2004

The nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formulation which accounts for effects of viscosity, as well as diffusivity, interfacial tension, and decompression rate. The numerical solutions show two regimes in the nucleation and growth process, a diffusion‐controlled regime and a viscosity‐controlled regime, mainly depending on the decompression rate, initial saturation pressure and viscosity. The basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components. In the diffusion‐controlled regime, bubble growth is limited by the diffusion of degassing components, and the total number density of bubbles, N, is proportional to (decompression rate / diffusivity)3/2. In the viscosity‐controlled regime the bubble growth is limited by the expansion of gas bubbles, and In(const×N) is approximately proportional to a dimensionless parameter,
, where μ is viscosity,
is the decompression rate, and P0 is the initial saturation pressure. The transition between the two regimes dramatically occurs when the value of the above parameter decreases below about 2×l03. In the viscosity‐controlled regime abundant tiny bubbles (exceeding 1014 (m−3)) form with a relatively high internal pressure. Such a highly stressed melt is easily fragmented into fine ash by an external shock or other disturbance. In basaltic eruptions the vesiculation is essentially controlled by diffusion, and the viscosity‐controlled regime is limited to very high decompression rate and very small water content. When andesitic magma saturated by water at 10 MPa is decompressed through the propagation of rarefaction wave induced by a landslide, as took place in the Mount St. Helens 1980 eruption, the vesiculation is controlled by the viscosity up to 100 m depth. On the other hand, in a rhyolitic magma for the same situation, vesiculation is controlled by the viscosity over the whole depth of the magma column. In the viscosity‐controlled regime, the vesicularity may be 90% or less as seen in silicic pumice, whereas in the diffusion‐controlled regime the vesicularity equals or exceeds 98% such as in reticulite in Hawaiian basalt. An observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the SiO2 content can be attributed approximately to the dependence of diifusivity or viscosity on SiO2 content and temperature, assuming the apparent correlation between SiO2 content and temperature of magma.
Simulation of magnetic hysteresis in pseudo‐single‐domain grains of magnetite
Authors: Williams W. , Dunlop David J.
Published: February 2004

Magnetic hysteresis has been simulated in grains of magnetite for the size range 0.1–0.7 μm. This was achieved using an unconstrained three‐dimensional micromagnetic model of single grains of magnetite with cubic magnetocrystalline anisotropy. Hysteresis loops were obtained for fields applied along both the easy and hard magnetocrystalline axes. Both discrete (Barkhausen) jumps and gradual changes in the magnetic structure are seen, and the reversal of the magnetization can be followed from near saturation in a normal field to a similar state in a reverse field. Predictions of coercivity agree well with published experimental data for unstressed cubic grains of magnetite.
Hotspot melting generates both hotspot volcanism and a hotspot swell?
Authors: Phipps Morgan Jason , Morgan W. Jason , Price Evelyn
Published: February 2004

Two prominent features mark the passage of oceanic lithosphere over a hotspot. The first is the initiation of oceanic volcanism leading to a chain of islands or seamounts. The second is the generation of a ∼1‐km‐high, ∼1000‐km‐wide bathymetric swell around the volcanic island chain. Here we show that recent estimates for the volume of hotspot volcanism and the size of the swell suggest a shared origin: swell relief is created by the density reduction created by melting beneath the hotspot. This results in a seafloor age dependence to swell size and volcanism along the Hawaiian chain: beneath younger, thinner lithosphere the hotspot undergoes more decompression melting, resulting in both a larger swell volume and greater island building. For rapidly moving plates the swell root residue from hotspot melting is dragged away from the hotspot by the overriding lithosphere; its buoyancy induces further spreading and thinning of swell root material, producing, for example, the characteristic bow‐shaped form of the 0–5 Ma section of the Hawaiian swell. This post emplacement spreading and thinning of the swell root may be the reason for the ∼5 m.y. duration of late stage melting and volcanism along the Hawaiian hotspot chain. The ∼5 m.y. timescale for spreading of the swell root implies a characteristic viscosity of the depleted swell root of ∼ 1–3×1020 Pa s, which is less fluid than underlying, less melted asthenosphere. Melt extraction at the hotspot is our preferred mechanism for the increase in viscosity of the swell root relative to underlying asthenosphere.
Far‐field tilting of Laurentia during the Ordovician and constraints on the evolution of a slab under an ancient continent
Authors: Coakley Bernard , Gurnis Michael
Published: February 2004

During a brief period of 10 to 15 million years in the Middle‐Ordovician, the Michigan Basin departed from its bull's‐eye subsidence pattern and tilted toward the east, opening to the Appalachian basin. This tilting is observed in maps of tectonic subsidence estimated for the Black River and Trenton Formations and extends over 300 km across the Michigan Basin and into eastern Wisconsin. Contours of constant tectonic subsidence rate are approximately parallel to the inferred position of the Laurentian‐Iapetus convergent margin. The distance between the inferred position of the subduction zone to the limit of tilting is approximately 1000 km. Three alternative models for the tilting are tested, two relying on the rigidity of the continental lithosphere and a third on the viscous flow generated by a subducted slab. In the first elastic model we assume the edge of the elastic plate is simply loaded from above (by a fold and thrust sheet, volcanic pile, etc.). This model, however, cannot simultaneously satisfy the space of tectonic subsidence and subsidence rate, even for lithospheres which have a rigidity of 1028 Nm. In the second elastic model, the Laurentian continental margin descends into a trench of an eastward dipping slab, that is, a slab descending under the Iapetus ocean. This process cannot generate any significant far field displacements, even for extremely rigid plates, and must be rejected. For the third model we use finite element solutions of a negatively buoyant slab in a viscous medium with a faulted lithosphere. Such slabs can easily generate not only realistic trenches on the under thrusting plate but also significantly tilt the lithosphere as much as 1000 km from the plate margin. The magnitude and distribution of far‐field displacements depend on the age, length, and dip angle of the slab. In contrast to the elastic models, penetration of a west‐dipping slab beneath the continent can reproduce both the extent, magnitude, and rate of tectonic subsidence observed in the Trenton and Black River Formations. The observed data are best fit by an old slab (140 Ma), which initially descended at a moderate dip (20°–30°) for 10–15 m.y., which then steepened as the slab penetrated deeper into the mantle. At the position of the previous Ordovician plate margin, there are narrow, block faulted basins which underwent rapid subsidence ∼10 m.y. before the Michigan Basin tilted toward the east. We propose that these earlier subsidence events were caused by the initial descent of the slab under the preexisting Cambro‐Ordovician passive margin. The time lag of ∼10 m.y. may be due to the time it takes the slab to penetrate the upper mantle. This result is important for understanding the time evolution of mantle convection and mechanisms for the initiation of subduction.
Getting granite dikes out of the source region
Authors: Rubin Allan M.
Published: February 2004

Whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow. Various formulations are developed for a viscous fluid at temperature Tm intruding a growing crack in an elastic solid. The initial solid temperature equals Tm at the source and decreases linearly with distance from the source. If Tm is the unique freezing temperature of the fluid, dike growth is initially self‐similar and an essentially exact solution is obtained; if Tm is above the solidus temperature, the solution is approximate but is designed to overestimate the distance the dike may propagate. The ability of a dike to survive thermally depends primarily upon a single parameter that is a measure of the ratio of the dike frozen margin thickness to elastic thickness. Perhaps more intuitively, one may define a minimum distance from the essentially solid reservoir wall to the point at which the host rock temperature drops below the solidus, necessary for dikes to propagate far into subsolidus rock. It is concluded that for reasonable material properties and source conditions, most basalt dikes will have little difficulty leaving the source region, but most rhyolite dikes will be halted by freezing soon after the magma encounters rock at temperatures below the magma solidus. While these results can explain why granitic dikes are common near granitic plutons but rare elsewhere, the potentially large variation in magmatic systems makes it premature to rule out the possibility that most granites are transported through the crust in dikes. Nonetheless, these results highlight difficulties with such proposals and suggest that it may also be premature to rule out the possibility that most granite plutons ascend as more equidimensional bodies.
Reliability of travel time data computed from interpreted migrated events
Authors: Jannaud L. R.
Published: February 2004

In the Sequential Migration Aided Reflection Tomography method (Grau and Lailly, 1993), travel times used by reflection tomography are computed by tracing rays which propagate with the migration velocity and reflect from reflectors picked on migrated images. Because of limits of migration resolution, this picking involves inaccuracies, to which computed travel times are unfortunately very sensitive. The objective of this paper is to predict a priori the confidence we can have in emergence data, i.e., emergence point location and travel time, from the statistical information that describes the uncertainties of the reflectors. (These reflectors can be obtained by picking on migrated images as explained above or by any other method.) The proposed method relies on a linearization of each step of the ray computation, allowing one to deduce, from the statistical properties of reflector fluctuations, the statistical properties of ray‐tracing outputs. The computed confidences and correlations give access to a more realistic analysis of emergence data. Moreover, they can be used as inputs for reflection tomography to compute models that match travel times according to the confidence we have in the reflector. Applications on real data show that the uncertainties are generally large and, what is much more interesting, strongly varying from one ray to another. Taking them into account is therefore very important for both a better understanding of the kinematic information in the data and the computation of a model that matches these travel times.
Paleomagnetism of the Pennington Mountain terrane: A near‐Laurentian back arc basin in the Maine Appalachians
Authors: Potts Stephen S. , van der Pluijm Ben A. , Van der Voo Rob
Published: February 2004

Paleomagnetic studies of volcanic terranes in the Appalachians provide quantitative data on the Ordovician paleogeography of the Iapetus ocean. New paleomagnetic results from submarine volcanics of the Middle to Late Ordovician Winterville Formation of Maine further constrain the evolution of Iapetus. Ten sites yield a tilt corrected direction of D/I = 3277/−21 (α95 = 9.3, k = 27.9); the corresponding paleomagnetic pole calculated for the average site location (46.8°N, 291.3°E) is located at 26°N, 148°E (dp = 5°, dm = 10°, A95 = 7.7). A Silurian conglomerate (Frenchville Formation) test is inconclusive due to a strong present‐day field overprint which obscures any primary remanence. The presence of a positive tilt test, however, supports the conclusion that the characteristic magnetization of the Winterville Formation is a prefolding, primary magnetization. An Ordovician paleolatitude of 11±5° for the Pennington Mountain terrane of northern Maine is indistinguishable from that of the Laurentian margin (15–20°) during the Middle to Late Ordovician. The paleolatitude of the Pennington Mountain terrane is also similar to, but slightly more equatorial than that of the previously studied Bluffer Pond Formation of the nearby Munsungun terrane (18±9°). We conclude that the Ordovician Pennington Mountain and Munsungun volcanic terranes of Maine were formed and acquired their characteristic magnetization near the Laurentian margin. The paleolatitudes of these terranes are also similar to that obtained from the Stacyville volcanics of the more outboard Lunksoos terrane (20±8°), but they contrast strongly with the Ordovician Miramichi terrane of northern New Brunswick that yielded a paleolatitude of 51° (+21, −16°). These results from Maine support the presence of one or more backarc basins adjacent to the Laurentian margin during the Middle to Late Ordovician.
Crust and mantle structure across the Basin and Range‐Colorado Plateau boundary at 37°N latitude and implications for Cenozoic extensional mechanism
Authors: Zandt George , Myers Stephen C. , Wallace Terry C.
Published: February 2004

We present new evidence on the seismic velocity and density of the crust and upper mantle along a 200‐km‐long transect across the eastern Basin and Range and western Colorado Plateau at 37°N latitude. Receiver functions computed from the P waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations. The crust is 30–35 km thick within the eastern Basin and Range and increases over a distance of ∼100 km at the western edge of the Colorado Plateau, reaching a maximum of approximately 45 km east of the Hurricane fault. The timing of crustal multiples within the reciever functions were used to estimate the Vp/Vs of the crust along the profile, and we found that the western Colorado Plateau crust is characterized by a high Poisson's ratio (0.28–0.29) indicative of a crust with an average mafic composition. We estimated the upper mantle lid thickness along our profile based on teleseismic P wave travel times and constraints provided by gravity data. Our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the Basin and Range to 100 km beneath the western Colorado Plateau, although the Basin and Range lithosphere may have significant thickness variations. The thick, strong mafic crust and thicker mantle lid under the Colorado Plateau can account for the relative geologic stability and subdued magmatism of the plateau during Laramide compression and Cenozoic extension compared to surrounding regions. The crustal and lithospheric thinning across the tectonic boundary occurs over a short distance (∼100 km), suggesting it is a geologically young feature produced by a predominantly mechanical response to late Cenozoic extension. Our new lithosphere model at 37°N latitude is consistent with the existence, in early Cenozoic time, of a flat subducted slab at 100 km depth and a relict Sevier‐Laramide 50–60 km thick crustal welt, and 60–100% pure shear extension (β values of 1.6–2.0) during the late Cenozoic.
Simple mathematical model of a rough fracture
Authors: Brown Stephen R.
Published: February 2004

A simple mathematical model of rough‐walled fractures in rock is described which requires the specification of only three main parameters: the fractal dimension, the rms roughness at a reference length scale, and a length scale describing the degree of mismatch between the two fracture surfaces. Fractured samples, collected from natural joints and laboratory specimens, have been profiled to determine the range of these three parameters in nature. It is shown how this surface roughness model can be implemented on a computer, allowing future detailed study of the mechanical and transport properties of single fractures and the scale dependence of these properties.
Terrestrial and GPS measurements of deformation across the Taupo back arc and Hikurangi forearc regions in New Zealand
Authors: Darby Desmond J. , Meertens Charles M.
Published: February 2004

Geodetic measurements of deformation across the Taupo back arc and Hikurangi forearc regions of New Zealand are derived from Global Positioning System (GPS) measurements made in 1990 and 1991 and triangulation observations made in the 1920s, 1950s, and 1970s. The GPS horizontal coordinate differences have precisions of 5–6 mm and the triangulation observations have precisions of 0.7–1.2 arc sec. These different kinds of observation allow simultaneous estimation of strain parameters and single–epoch coordinates for a total of 184 stations, at 68 of which GPS observations were made. Under the assumption that no shift of the scale or orientation of the GPS reference frame is embodied in the data from 13 GPS stations within the Taupo Volcanic Zone (TVZ) common to the 1990 and 1991 surveys, the rotational, dilatational and shear components of the deformation rate tensor within the TVZ can be estimated for that 1‐year interval. The principal extension rate from 1990 to 1991 is 0.21±0.09 × 10−6/yr (68% confidence) at an azimuth of 124±13°, corresponding to 8±4 mm/yr extension over 40 km. Neither the orthogonal principal extension rate, 0.02±0.03 × 10−6/yr, nor the rotation rate, 0.06±0.04 μrad/yr, is significant. The dilatation rate of 0.23±0.09 × 10−6/yr is therefore produced by uniaxial extension. The data from 75 stations distributed from behind the back arc region and well into the forearc region, which were surveyed in at least two of the triangulation or GPS epochs between the 1920s and 1991, allow the spatial variation of the shear components of the deformation rate tensor to be estimated. The maximum engineering shear rates lie in the range 0.1–0.2 × 10−6/yr within the TVZ and maintain these values, with similar orientations, in the forearc region to the east. The western and southern margins of active extension are reasonably well determined by changes in orientation and magnitude of the shear component. These results provide confirmation of the previously less well‐determined deformation field, and in comparison with subduction models indicate that the surface deformation is reflecting variation from north to south of coupling of the subduction plate interface, and that both trench suction and gravitational collapse probably contribute to the extension in the forearc region.
Tertiary calc‐alkaline magmatism associated with lithospheric extension in the Pacific Northwest
Authors: Hooper P. R. , Bailey D. G. , Holder G. A. McCarley
Published: February 2004

The long‐standing assumption that calc‐alkaline magmatism requires contemporaneous subduction is critically reviewed. Two of many widely spread fields of Tertiary calc‐alkaline magmatism in the Pacific Northwest, one Eocene and one Miocene, provide clear examples of the association between calc‐alkaline magmatism and lithospheric extension. We suggest that the calc‐alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle, enriched by previous subduction events, than by contemporaneous Eocene subduction. This model proposes that extension will cause calc‐alkaline‐dominated magmatism in areas of continental crust with a history of previous subduction, as along the western margin of North America. Currently popular models for the tectonic/magmatic evolution of the Basin and Range province, in which subduction‐driven calc‐alkaline magmatism is followed by extension‐related bimodal magmatism, may need modification; much more of this magmatism may result from extension. In the Pacific Northwest, we suggest that subduction beneath the Idaho batholith ceased at the end of the Cretaceous, blocked by the accreted terranes, to be reactivated in its present position beneath the Cascades during the Eocene. The various isolated Eocene volcanic fields spread across the Pacific Northwest were due to lithospheric extension, not to the presence of a subducting plate immediately below.
Experimental generation of hybrid silicic melts by reaction of high‐Al basalt with metamorphic rocks
Authors: Patiño Douce Alberto E.
Published: February 2004

Melting and crystallization experiments (T = 1000°C; P = 0.5, 0.7, 1.0, 1.2, and 1.5 GPa) were performed on two hybrid bulk compositions consisting of 50% anhydrous high‐Al olivine tholeiite glass (HAOT) and 50% metamorphic rock (aluminosilicate‐bearing metapelite and aluminosilicate‐free biotite gneiss). The objective of the experiments was to determine phase equilibrium constraints attending bulk assimilation of crustal rocks by basaltic melts. Experiments generated 32–38 wt % of H2O‐undersaturated granitic melt (SiO2 > 70 wt %). Thermal and phase equilibrium arguments show that production of this amount of granitic melt at the chosen experimental conditions is a plausible model for bulk assimilation in nature. The compositions of the melts and of the coexisting crystalline assemblages are affected to a comparable extent by pressure and by composition of the crustal source. Reaction of HAOT with biotite gneiss at P ≤ 1.0 GPa produces calc‐alkaline melts in equilibrium with gabbronoritic cumulates (plag + opx + cpx). Reaction of HAOT with metapelite at P ≤ 0.7 GPa produces strongly peraluminous melts that resemble S‐type granites from the Lachlan Fold Belt, in equilibrium with noritic cumulates (plag + opx ± spi ± gar). At P > 1.2 GPa, both source compositions produce strongly peraluminous leucocratic melts (<2 wt % FeO + MgO + TiO2) in equilibrium with garnet‐rich and plagioclase‐poor residues (opx + cpx + gar + plag from the biotite gneiss, gar + plag from the metapelite). The experiments show that a wide spectrum of high‐SiO2 melts can be hybrids formed directly by reaction of basaltic melts with amphibolite‐facies metamorphic rocks. Accumulation of the complementary mafic crystalline assemblages in the deep crust will generate granulites which are neither restitic nor the products of subsolidus dehydration. Mafic granulites and granitic melts formed by reaction of basaltic melts with metamorphic rocks will share isotopic and trace element signatures reflecting inheritance from both crustal and mantle sources.
Simulation of the 1986–1987 El Niño and 1988 La Niña events with a free surface tropical Pacific Ocean general circulation model
Authors: Zhang Rong‐Hua , Endoh Masahiro
Published: February 2004

Observed atmospheric forcing fields over the period 1984–1989 force a free surface tropical Pacific Ocean general circulation model. Numerical simulation of the 1986–1987 El Niño and 1988 La Niña events is presented in the paper. Some quantitative comparisons between model time series and corresponding observations of sea level, and upper ocean current and temperature are made to verify the model performance. Diagnostic analyses of heat balance and available energy budget are given as well. The space‐time evolution of various model variables demonstrates that the model produces interannual variations with reasonable success. Beginning in mid‐1986, westerly wind over the western equatorial Pacific drives strong eastward surface currents which accomplish the massive transfer of warm surface water. The strong westerly wind in late 1986 excites the pronounced equatorial Kelvin waves, which propagate eastward toward the eastern and coastal Pacific where they depress the thermocline and raise sea level twice, and increase sea surface temperature. The eastern Pacific warming occurs primarily from the diminished cooling contribution of vertical advection, whereas in the central Pacific, eastward advection by anomalous zonal flows is the principal mechanism. The El Niño conditions in the eastern Pacific disappear in mid‐1987, whereas they remain in the central and western Pacific until early 1988. Subsequently, the tropical Pacific Ocean rebounds to significant La Niña conditions. Available energy (AE) has a good phase relationship with respect to other variables characterized by warm and cold conditions. AE is anomalously high prior to a warm event, accompanying conversion from kinetic energy (KE) to available potential energy (APE). During the development of El Niño, although relaxation of trade wind reduces input of wind energy, the appearance of westerly wind in the western Pacific leads to a sharp increase in KE. This excites excessive conversion from APE to KE through work done by the buoyancy force, corresponding to out‐of‐phase fluctuation between KE and APE. In the aftermath of the warm event, APE, as well as KE and available surface energy, is depleted and is extremely low. Development of La Niña is characterized by long and slow accumulation of APE with the transformation from KE to APE.
Evidence for eddy formation in the eastern Arabian Sea during the northeast monsoon
Authors: Bruce John G. , Johnson Donald R. , Kindle John C.
Published: February 2004

The seasonal formation of a large (500–800 km diameter) anticyclonic eddy in the upper 300–400 m of the eastern Arabian Sea during the northeast monsoon period (December‐April) is indicated from hydrographic and satellite altimetry sea level observations, as well as from numerical model experiments. The center of the eddy circulation is approximately 10°N, 70°E, just to the west of the north‐south Laccadive Island chain. In this paper the eddy is called the Laccadive High (LH). In some ways it is a mirrorlike counterpart to the Great Whirl, which develops during the southwest monsoon off the Somali coast (western Arabian Sea). The LH occurs at the same latitude but on the opposite side of the basin during the reversed monsoon. It is different from the Great Whirl, however, in its formation process, its intensity, and its decay. The hydrographic data obtained from surveys all during a single season give sufficiently close station spacing to allow reasonable contouring of the geopotential surfaces and of the properties within and around the LH region with minimum time aliasing. The Geosat altimeter record extends over 4 years, during which the seasonal variability of the LH indicates a dynamic relief of approximately 15–20 cm, which is in good agreement with the hydrographic observations. The altimetry time series also suggests a westward translation of the LH by January with a subsequent dissipation in midbasin. The model used is a wind‐forced three‐layer primitive equation model which depicts a LH in agreement with the timing, position, and amplitude of both the hydrographic and altimetric measurements. The numerical simulation includes a passive tracer located in the western Bay of Bengal; the western advection of the tracer around the south coasts of Sri Lanka and India in December and January is consistent with the appearance of low‐salinity water observed to extend into the Arabian Sea during this period. The modeling studies suggest that both local and remote forcing are important in formation of the LH.
Nonlinear wave interactions and high‐frequency seafloor pressure
Authors: Herbers T. H. C. , Guza R. T.
Published: February 2004

Linear wave theory predicts that pressure fluctuations induced by windgenerated surface gravity waves are maximum at the ocean surface and strongly attenuated at depths exceeding a horizontal wavelength. Although pressure fluctuations observed at the seafloor in deep water are indeed relatively weak at wind‐wave frequencies, the energy at double wind‐wave frequencies is frequently much higher than predicted by applying linear wave theory to near‐surface measurements. These double‐frequency waves can in theory be excited by nonlinear interactions between two surface wave components of about equal frequency, traveling in nearly opposing directions. Observations from a large aperture, 24‐element array of pressure sensors deployed in 13‐m depth are presented that quantitatively support this generation mechanism. As in previous studies, dramatic increases in the spectral levels of seafloor pressure at double wind‐wave frequencies (0.3–0.7 Hz) frequently occurred after a sudden veering in wind direction resulted in waves propagating obliquely to preexisting seas. The observed spectral levels and vector wavenumbers of these double‐frequency pressure fluctuations agree well with predictions obtained by applying second‐order nonlinear, finite depth wave theory (Hasselmann, 1962) to the observed directionally bimodal seas. High‐frequency seafloor pressure spectral levels also increased in response to directionally narrower but more energetic seas generated by strong, steady or slowly rotating winds. Bispectral analysis suggests that these pressure fluctuations are generated by nonlinear mechanisms similar to the veering wind cases.
Effluent from diffuse hydrothermal venting: 1. A simple model of plumes from diffuse hydrothermal sources
Authors: Trivett D. A.
Published: February 2004

This paper focuses on modeling the fate of effluent from diffuse seafloor hydrothermal activity after it has been vented into the water column. The model was formulated using a number of simplifying assumptions which permit direct application of this model to field measurements. I have limited the configurations to those where the hydrothermal outflow velocities are smaller than horizontal current. I assume that the entrainment of ambient seawater into the plume is constant over the length of the plume. This permits formulation of a first‐order relation for the rise height and dilution in a diffuse hydrothermal plume as a function of downstream distance. The analytic model is compared with a simple laboratory simulation of the hydrothermal flow. The results suggest that diffuse hydrothermal effluent will penetrate to a height in the water column that is proportional to the overall dimension of the diffuse vent patch, multiplied by a dimensionless plume intensity parameter. I also show relations for plume dilution which will be compared with field data in part 2 of this work.
Effluent from diffuse hydrothermal venting: 2. Measurement of plumes from diffuse hydrothermal vents at the southern Juan de Fuca Ridge
Authors: Trivett D. A. , Williams A. J.
Published: February 2004

An array of temperature and velocity sensors was deployed for two weeks in the bottom boundary layer at the Southern Juan de Fuca Ridge during the National Oceanic and Atmospheric Administration VENTS'90 field experiment. The instruments were placed close to the seafloor to detect effluent from diffuse hydrothermal sources located 1–2 km distant. Over the duration of the field deployment, we detected effluent from two vent sources. The plume from the larger of the two sources was sensed at all of the instrument locations, permitting estimates of dilution based on the field measurements. Our data also permitted estimation of energy flux from diffuse vents (approximately 125±75 MW from a source to the north of the instrument array, and 12±8 MW from a small source to the east). The largest source of error in the data was estimation of the plume width. We have described a sensor array that would reduce this error in future work. Such an array would be suitable for a long‐term vent activity monitor.
The influence of strongly varying topography on coastal‐trapped waves at the southern Great Barrier Reef
Authors: Merrifield Mark A. , Middleton Jason H.
Published: February 2004

Coastal‐trapped waves (CTWs) that propagate into a region of strongly varying topography are investigated using 6 months of moored current meter observations at the southern Great Barrier Reef (SGBR). The dominant topographic variations in this region include a large sand island that extends from the coast to the shelf break, and an abrupt doubling in shelf width over an alongshelf distance of approximately 100 km. Previous observational and numerical studies indicate that the SGBR topography scatters mode 1 CTW energy into higher wave modes. In the present study, a spatially coherent CTW signal is found in a narrow frequency band near 0.1 cpd. Adjusted sea level and wind stress measurements from south of Fraser Island show that the similar alongshelf propagation speeds of coastal winds at 10‐day timescales and mode 1 CTWs lead to the narrow‐banded CTW signal. A proxy for the incident CTW signal is derived to estimate the wave amplitude and phase in the alongshelf current records. The observed northward phase progression of the wave response is consistent with the scattering of mode 1 into mode 2 CTW energy by the SGBR topography. The strongest observed wave amplitudes are observed over the shelf slope at 300‐m depth below the East Australia Current, and near the shelf break at Fraser Island. The strong current response over the slope at the N‐line suggests a significant outgoing mode 2 wave.
Currents off the east coast of New Ireland, Papua New Guinea, and their relevance to regional undercurrents in the western equatorial Pacific Ocean
Authors: Butt Jeff , Lindstrom Eric
Published: February 2004

In the Western Equatorial Pacific Ocean a coastal undercurrent lying within 40 km of the east coast of New Ireland was in May 1988 found to be transporting high‐salinity, low dissolved oxygen, high‐nutrient thermocline waters northwestwards towards the equator. Acoustic doppler current profiler measurements showed the peak alongshore current speeds of approximately 60 cm s−1 occurred at 235 m depth and the volume transport between the 24.5 and 26.5 kg m3 isopycnals (approximately 176 to 320 m) was 2.0±0.1×106 m3 s−1. We suggest that this current be called the New Ireland Coastal Undercurrent (NICU). Just to the east of 149°E, the NICU was diverted to the north and forced to interact with the Equatorial Undercurrent (EUC). Transport budgets in the region bounded by 149°E, 153°E, 2°S and the equator, between the 24.5 and 26.5 kg m−3 isopycnals show that approximately 5×106 m3 s−1 of water originating from east of New Ireland, including the NICU was added to the EUC system. This accounted for approximately half of the southern hemisphere thermocline waters in the EUC system at 153°E.