bluuebunny/crossref_metadata_2025 huggingface dataset:
Geomorphology and structural segmentation of the crest of the southern (Pacific‐Antarctic) East Pacific Rise
Authors: Lonsdale Peter
Published: February 2004

Geomorphology of the boundary between Pacific and Antarctic plates was mapped with a Sea Beam multibeam echosounder and a SeaMARC II bathymetric side scan sonar, from the southern end of Juan Fernandez microplate at 35°S to Heezen transform at 56°S. There are six spreading center systems separated by two large, left‐stepping nontransform offsets (at 36.5°S and 41.5°S), two right‐stepping transform‐fault systems, and a left‐stepping hybrid structure with equally long strike‐slip and nontransform offsets. Axial rift zones (spreading axes) are generally within 1° of normal to the relative motion predicted by current plate rotation models. Most axial rift zones crop out along the crests of typical East Pacific Rise (EPR) axial ridges which commonly have flat rather than humped long profiles. About 2% of this fast‐spreading (84–100 mm/yr) rise crest has an axial rift valley instead of an axial ridge. The longest rift‐valley segment is midway between Menard and Vacquier transforms near 51°S, where an anomalously deep rise crest may mark a zone of below‐average mantle upwelling. Nontransform offsets structurally similar to those on the tropical EPR, but mostly formed by differential asymmetric spreading, subdivide the four longest spreading center systems. On the northern, transform‐free two thirds of the rise, net rift propagation at migrating offsets has been into the faster Pacific plate. This causes right steps to migrate north, left steps to migrate south, and crust to be transferred to the east flank. This pattern has prevailed for several million years, judging from the oblique fracture zones (pseudofaults) mapped on the rise flanks with older marine data and Geosat altimetry. On this rise crest, the difference in stress on two plates with very different velocities may control the direction of offset migration.
The brittle‐plastic transition in experimentally deformed quartz aggregates
Authors: Hirth Greg , Tullis Jan
Published: February 2004

Deformation experiments have been conducted to provide constraints on the processes responsible for the brittle‐plastic transition in quartz aggregates. A correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle‐plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and/or pressure. First there is a transition from cataclastic faulting to semibrittle faulting; microstructural observations indicate that this transition occurs due to the activation of dislocations. In addition, faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide. Second, there is a transition from semibrittle faulting to semibrittle flow; this transition corresponds to a change from localized to distributed deformation. Microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime. We conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries. Third, there is a transition from semibrittle flow to dislocation creep. Microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature. In addition, microstructural observations indicate that a transition from dominantly mode I (axial) to mode II (shear) microcracking occurs with an increase in confining pressure from 0.4 to 0.8 GPa, regardless of temperature. The differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions. However, a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the Earth. The results of this study provide further evidence to indicate that the brittle‐plastic transition in the continental crust occurs over a relatively wide range of conditions.
Temporal variations of flexural deformation in Hawaii
Authors: Wessel Pål , Keating Barbara H.
Published: February 2004

We present a simple time‐progressive model for the Hawaiian Island chain bathymetry and the associated flexural response of the elastic oceanic lithosphere. The model is used to study the vertical tectonic history of Cross seamount, a Cretaceous guyot brought close to the Hawaiian hot spot by plate motion. Geological evidence indicates that Cross was truncated at sea level, but its flat top is presently 400 m below sea level. Our reconstruction suggests the seamount was elevated on the Hawaiian Swell and truncated about 3.2 m.y. ago. The post‐truncation subsidence is attributed to the flexural moat that developed around the growing Hawaiian Islands. Flexural isostatic adjustment explains the current depth of Cross seamount provided the elastic plate thickness is in the 30–35 km range.
A physical explanation for the positioning of the depth to the top of overpressure in shale‐dominated sequences in the Gulf Coast basin, United States
Authors: Mello Ulisses T. , Karner Garry D. , Anderson Roger N.
Published: February 2004

A one‐dimensional model of fluid pressure and porosity evolution is used to investigate the physical processes that control the development and maintenance of overpressure in a compacting sedimentary basin. We show that for shale‐dominated sequences the variation of the hydraulic diffusivity in both space and time is such that it produces a minimum between 2 and 4 km depth, consistent with observations from the Gulf Coast basin. This minimum inhibits the upward flow of fluid by acting as a “bottleneck” and thus determines the shallowest position of the depth to the top of overpressure. Above this region of bottleneck, overpressure does not develop because the porosity is sufficiently large to maintain high values of hydraulic diffusivity that are conducive to the rapid dissipation of excess fluid pressure. Within the overpressured shales, compaction propagates downward through the section, releasing fluids from the upper part of the section while continuing to restrain the upward flow of fluids from deeper within the section. As such, overpressures are predicted to be maintained within the deeper regions of a basin for tens to hundreds of millions of years. Further, fluid viscosity plays an important role in defining the depth behavior of hydraulic diffusivity as a function of time. Assuming a temperature‐dependent fluid viscosity guarantees that the hydraulic diffusivity minimum will always exist during the development of the basin. On the basis of our results, we find that the depth at which the porosity equals 14±4% correlates with the depth to the local hydraulic diffusivity minimum and thus the depth to the top of overpressure. Moreover, we interpret that the 14±4% represents the threshold porosity for which a shale actually begins to act as a seal. Within the Gulf Coast basin, the gross sediment facies consists of lower massive shales across which deltaic systems have prograded allowing the deposition of an alternating series of sandstones and shales that grade vertically into massive sandstones. The massive sandstones are highly permeable and are connected hydrologically to the surface. We conclude that these sandstones play little role in the development of overpressure because of their high permeability except to the extent that the base of the massive sandstones marks the minimum depth possible for the top of overpressure. In contrast, overpressuring is observed to develop within either the shale‐dominated sequence or the region of interspersed/interfingering sands and clays. The clay‐encompassed sands play only a passive role in the development and maintenance of overpressure because it is the low‐permeability clays that control the movement of fluids into and out of the sands.
Oceanic crustal thickness and seismic character along a central Pacific transect
Authors: Eittreim Stephen L. , Gnibidenko Helios , Helsley Charles E. , Sliter Ray , Mann Dennis , Ragozin Nikita
Published: February 2004

Seismic reflection data along a flow line of crustal generation in the central Pacific that spans ages of zero to 85 Ma and spreading half rates of 30 to 100 km m.y.−1 shows a nearly constant travel time of 2 s through igneous crust to reflection Moho. The highest‐amplitude and most laterally continuous Moho reflections were recorded over 20–30 Ma crust that was emplaced at “superfast” (75–95 km m.y.−1) spreading rates. The superfast spread portion also records the lowest scatter about the 2‐s average travel time to reflection Moho. Seismic images show that lower crustal reflectors dip consistently eastward toward the ridge crest. These dipping reflectors are truncated by the reflection Moho.
Global gravity field recovery from the ARISTOTELES satellite mission
Authors: Visser P. N. A. M. , Wakker K. F. , Ambrosius B. A. C.
Published: February 2004

One of the primary objectives of the future ARISTOTELES satellite mission is to map Earth's gravity field with high resolution and accuracy. In order to achieve this objective, the ARISTOTELES satellite will be equipped with a gravity gradiometer and a Global Positioning System (GPS) receiver. Global gravity field error analyses have been performed for several combinations of gradiometer and GPS observations. These analyses indicated that the bandwidth limitation of the gradiometer prevents a stable high‐accuracy, high‐resolution gravity solution if no additional information is available. However, with the addition of high‐accuracy GPS observations, a stable gravity field solution can be obtained. A combination of the measurements acquired by the high‐quality GPS receiver and the bandwidth‐limited gradiometer on board ARISTOTELES will yield a global gravity field model with a resolution of less than 100 km and with an accuracy of better than 5 mGal for gravity anomalies and 10 cm for geoid undulations.
Active tectonics of the Pamirs and Karakorum
Authors: Fan Guangwei , Ni James F. , Wallace Terry C.
Published: February 2004

The source parameters of moderate‐ to large‐sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three‐dimensional configuration of the subducted lithosphere in the Pamir‐Karakorum intracontinental convergence zone. The continental collision has resulted in intense deformation in the upper crust. The deformation is distributed over a broad area and is absorbed by thrust and strike‐slip faulting. Along the northern margin of the Pamirs, where the deformation due to continental collision is most intense, the Northern Pamir Thrust and several major fault zones mark the present boundary. Most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region. Along the Pamir Front, thrust faulting dominates, while on the western and eastern edges of the Pamirs the style of deformation is characterized by oblique thrusting with a component of strike‐slip motion. Thrust‐type events beneath the Tadjik Depression indicate that both the sedimentary rocks and the basement are involved in shortening. Right‐lateral strike‐slip motion is observed on the eastern edge of the Pamirs where they border the Tarim Basin and in the Talas‐Fergana‐Kun Lun fault zone. A southward dipping seismic zone beneath the Pamirs and Karakorum indicates that the Asian lithosphere has been subducted along the Pamir Front to a depth of at least 200 km. This interpretation is consistent with published geological and geophysical data. Focal mechanism solutions of some intermediate‐depth events beneath the Pamirs show strike‐slip faulting with approximately N‐S horizontally oriented P axes, indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation. A 90‐km‐deep event beneath the Karakorum is interpreted as occurring in the subducted Asian lithosphere; the fact that the P axis of this event is oriented parallel to the descending plate suggests downdip compression within the Asian lithosphere. The overall tectonics of the region is interpreted as a consequence of the underthrusting Asian lithosphere being impinged upon by the shallow northward underthrusting Indian lithosphere.
Volcanic tremor: Nonlinear excitation by fluid flow
Authors: Julian Bruce R.
Published: February 2004

A nonlinear process analogous to the excitation mechanism of musical wind instruments and human vocal cords can explain many characteristics of volcanic tremor, including (1) periodic and “chaotic” oscillations, with peaked and irregular spectra respectively, (2) rapid pulsations in eruptions occurring at the same frequency as tremor, (3) systematic changes in tremor amplitude as channel geometry evolves during an eruption, (4) the period doubling reported for Hawaiian deep tremor, and (5) the fact that the onset of tremor can be either gradual or abrupt. Volcanic “long‐period” earthquakes can be explained as oscillations excited by transient disturbances produced by nearby earthquakes, fluid heterogeneity, or changes in channel geometry, when the magma flow rate is too low to excite continuous tremor. A simple lumped‐parameter tremor model involving the flow of an incompressible viscous fluid through a channel with movable elastic walls leads to a third‐order system of nonlinear ordinary differential equations. For different driving fluid pressures, numerical solutions exhibit steady flow, simple limit‐cycle oscillations, a cascade of period‐doubling subharmonic bifurcations, and chaotic oscillations controlled by a strange attractor of Rössler type. In this model, tremor occurs most easily at local constrictions, and fluid discharge is lower than would occur in unstable steady flow.
Late Cenozoic flexural deformation of the middle U.S. Atlantic passive margin
Authors: Pazzaglia Frank J. , Gardner Thomas W.
Published: February 2004

Despite the century‐long recognition of regional epeirogeny along the middle Atlantic passive margin, relatively few studies have focused on understanding postrift uplift mechanisms. Here, we demonstrate that epeirogenic uplift of the central Appalachian Piedmont and subsidence of the Salisbury Embayment represent first‐order, flexural isostatic processes driven by continental denudation and offshore deposition. Our results show that regional epeirogenic processes, present on all Atlantic‐type passive margins, are best resolved by specific stratigraphic and geomorphic relationships, rather than topography. A simple one‐dimensional geodynamic model, constrained by well‐dated Baltimore Canyon trough, Coastal Plain, and lower Susquehanna River (Piedmont) stratigraphy, simulates flexural deformation of the U.S. Atlantic margin. The model represents the passive margin lithosphere as a uniformly thick elastic plate, without horizontal compressive stresses, that deforms flexurally under the stress of strike‐averaged, vertically applied line loads. Model results illustrate a complex interaction among margin stratigraphy and geomorphology, the isostatic response to denudational and depositional processes, and the modulating influence of exogenic forces such as eustasy. The current elevation, with respect to modern sea level, of fluvial terraces and correlative Coastal Plain deposits or unconformities is successfully predicted through the synthesis of paleotopography, eustatic change, and margin flexure. Results suggest that the middle U.S. Atlantic margin landward of East Coast Magnetic Anomaly is underlain by lithosphere with an average elastic thickness of 40 km (flexural rigidity, D = 4 × 1023 N m), the margin experiences an average, long‐term denudation rate of approximately 10 m/m.y., and the Piedmont has been flexurally upwarped between 35 and 130 meters in the last 15 m.y. Long‐term isostatic continental uplift resulting from denudation and basin subsidence resulting from sediment loading are accommodated primarily by a convex‐up flexural hinge, physiographically represented by the Fall Zone. Our results elucidate an inherent danger in using topography alone to constrain late‐stage passive margin deformation mechanisms. Only through careful synthesis of field stratigraphic and geomorphic elements such as fluvial terraces, Coastal Plain deposits, and offshore stratigraphy can age control be extended from the offshore depositional setting to the erosionally dominated continent. This study demonstrates that despite a relatively subdued topography, the middle U.S. Atlantic margin experiences progressive flexural isostatic deformation similar to that proposed for high‐relief margins characterized by great escarpments. Thus margin topographic diversity remains a function of other factors, such as lithospheric composition and/or structure, supracrustal stratigraphy and structure, degree of drainage integration, drainage divide migration, and climate.
Topographic form of the Coast Ranges of the Cascadia Margin in relation to coastal uplift rates and plate subduction
Authors: Kelsey Harvey M. , Engebretson David C. , Mitchell Clifton E. , Ticknor Robert L.
Published: February 2004

The Coast Ranges of the Cascadia margin are overriding the subducted Juan de Fuca/Gorda plate. We investigate the extent to which the latitudinal trend in average topography of the Coast Ranges is a function of the latitudinal change in attributes related to the subduction process. These attributes include the variable age of the subducted slab that underlies the Coast Ranges and average vertical crustal velocities of the western margin of the Coast Ranges for two markedly different time periods, the last 45 years and the last 100 kyr. These vertical crustal velocities are computed from the resurveying of highway bench marks and from the present elevation of shore platforms that have been uplifted in the late Quaternary, respectively. Topography of the Coast Ranges is in part a function of the age and buoyancy of the underlying subducted plate. This is evident in the fact that the two highest topographic elements of the Coast Ranges, the Klamath Mountains and the Olympic Mountains, are underlain by youngest subducted oceanic crust. The subducted Blanco Fracture Zone in southernmost Oregon is responsible for an age discontinuity of subducted crust under the Klamath Mountains. The northern terminus of the topographically higher Klamaths is offset to the north relative to the position of the underlying Blanco Fracture Zone, the offset being in the direction of migration of the fracture zone, as dictated by relative plate motions. Vertical crustal velocities at the coast, derived from bench mark surveys, are as much as an order of magnitude greater than vertical crustal velocities derived from uplifted shore platforms. This uplift rate discrepancy indicates that strain is accumulating on the plate margin, to be released during the next interplate earthquake. In a latitudinal sense, average Coast Range topography is relatively high where bench mark‐derived, short‐term vertical crustal velocites are highest. Because the shore platform vertical crustal velocites reflect longer‐term, permanent uplift, we infer that a small percentage of the interseismic strain that accumulates as rapid short‐term uplift is not recovered by subduction earthquakes but rather contributes to rock uplift of the Coast Ranges. The conjecture that permanent rock uplift is related to interseismic uplift is consistent with the observation that those segments of the subduction zone subject to greater interseismic uplift rates are at approximately the same latitudes as those segments of the Coast Ranges that have higher magnitudes of rock uplift over the long term.
Lithospheric structure and compensation mechanisms of the Galápagos Archipelago
Authors: Feighner Mark A. , Richards Mark A.
Published: February 2004

Volcanic islands of the Galápagos Archipelago are the most recent subaerial expression of the Galápagos hotspot. These islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young (<10 m.y.) oceanic lithosphere just south of the active Galápagos Spreading Center. The 91°W fracture zone crosses the platform and creates an estimated 5‐m.y. age discontinuity in the lithosphere. Major tectonic features of the Galápagos include an unusually broad distribution of volcanic centers, pronounced structural trends such as the NW‐SE Wolf‐Darwin Lineament (WDL), and a steep escarpment along the western and southern margins of the archipelago. We use shipboard gravity and bathymetry data along with Geosat geoid data to explain the tectonic and structural evolution of the Galápagos region. We model the gravity anomalies using a variety of compensation models, including Airy isostasy, continuous elastic flexure of the lithosphere, and an elastic plate with embedded weaknesses, and we infer significant lithospheric strength variations across the archipelago. The outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of ∼12 km. This area includes the large shield volcanoes of Fenandina and Isabela Islands, where the lithosphere regionally supports these volcanic loads. The central platform is weaker, with an elastic thickness of 6 km or less, and close to Airy isostasy. The greatest depths to the Moho are located beneath eastern Isabela Island and the central platform. Thinner lithosphere in this region may account for the broad distribution of volcanoes, the extended period of eruption of the central volcanoes, and their reduced size. The transition from strong to weak lithosphere along the southern escarpment appears to be abrupt, within the resolution of our models, and can be best represented by a free end or faultlike discontinuity. Also, modeling the WDL as a lithospheric fault increases the match to the observed gravity anomalies. The primary feature is the weak central platform, whose existence is a logical convergence of several factors: the 91°W transform/age offset, the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping, and the formation of the central platform at a spreading ridge.
The apparent friction of granular fault gouge in sheared layers
Authors: Scott David R. , Marone Chris J. , Sammis Charles G.
Published: February 2004

Data are presented from a series of experiments on layers of granular quartz gouge in the double‐direct‐shear geometry at a normal stress of 25 MPa. The apparent friction of a layer, defined as the ratio of the applied shear and normal stresses, shows considerable variability depending on the thickness of the layer and the particle size distribution of the gouge. Measurements of layer thickness during the experiments also show that the layers thin as shearing proceeds. Layer thinning is attributed to the processes of extrusion and densification, and by analyzing these processes the observed variations in apparent friction are reconciled with a constant coefficient of internal friction for quartz gouge. For thinning due only to extrusion, the prinicipal assumption made is that the deviatoric stress and plastic strain rate are coaxial. When densification is also admitted, a simple flow law with one adjustable parameter is required to relate the volumetric and shear strain rates. The results and analysis show that (1) extrusion and densification must be considered when interpreting the measured frictional properties of gouge layers; (2) shear localization is inhibited in these experiments; and (3) the apparent friction of a natural fault zone may be less than sin ø, where tan ø is the coefficient of internal friction of gouge in the fault zone.
Kinematic models of dewatering accretionary prisms
Authors: Wang Kelin
Published: February 2004

Steady state two‐dimensional analytical solutions are derived for sediment velocity and rate of fluid expulsion in a thickening accretionary prism with smooth upper and lower surfaces. Two different kinematic assumptions lead to two similar sets of solutions. In one case, the sediment flow lines are assumed to diverge uniformly landward. In the other case, the horizontal velocity of any given sediment column is assumed uniform. By further assuming zero horizontal Darcy velocity in the prism, expressions can be obtained for the approximate vertical fluid velocity and pore fluid pressure fields. Results from applying these solutions to the Cascadia accretionary prism off Vancouver Island show the importance of modeling the effects of lateral as well as vertical porosity variation on the hydrogeology of the prism. These solutions provide a means to estimate fluid flow and pore pressure distribution for prisms with realistic geometry. The rate of fluid expulsion can also be incorporated into numerical models as the fluid source term.
On electric and magnetic galvanic distortion tensor decompositions
Authors: Chave Alan D. , Smith J. Torquil
Published: February 2004

The physics governing galvanic distortion of natural source electromagnetic induction measurements is reexamined beginning from first principles. The conditions under which a decomposition of measured magnetotelluric response tensors and magnetic transfer functions is applicable are described, and the form of the decomposition describing distortion of the electric and magnetic fields is derived directly from the integral equation defining the scattering of electric and magnetic fields by surface heterogeneities. The inclusion of magnetic field galvanic distortion leads to indeterminacy of the regional magnetotelluric response in the form of scaling by frequency‐dependent, complex factors controlled by two unknown real constants. This is a generalization of the well‐known static shift effect from electric field galvanic distortion and can in principal be removed if the magnitude and phase of the regional response are known at some frequency. Distortion of the magnetic transfer function is shown to be even more indeterminate, containing a term proportional to one of the regional magnetotelluric responses which is inseparably additive to the regional magnetic transfer function, as well as the complex scaling seen for magnetotellurics. A set of simultaneous nonlinear equations describing the full electric and magnetic field galvanic distortion decomposition of the magnetotelluric response tensor and magnetic transfer function is derived, and methods for their solution are described, including implementation of jackknife error estimates. The full magnetotelluric decomposition is applied to severely distorted data from the Canadian shield and seafloor data from the EMSLAB experiment. In both cases, magnetic field galvanic distortion is important at periods under a few thousand seconds. This suggests that greater attention to galvanic distortion of the magnetic field is needed during magnetotelluric surveys.
Effects of lithology and depth on the permeability of core samples from the Kola and KTB drill holes
Authors: Morrow C. , Lockner D. , Hickman S. , Rusanov M. , Röckel T.
Published: February 2004

Permeability measurements were conducted on intact core samples from the Kola drill hole in Russia and the KTB drill hole in Germany. Samples included granodiorite gneisses, basalts and amphibolites from depths up to 11 km. The tests were intended to determine the pressure sensitivity of permeability and to compare the effects of stress relief and thermal microcracking on the matrix permeability of different rock types and similar samples from different depths. The pore pressure Pp was fixed at the estimated in situ pressure assuming a normal hydrostatic gradient; the confining pressure Pc was varied to produce effective pressures (Pe = Pc ‐ Pp) of 5 to 300 MPa. The permeability of the basaltic samples was the lowest and most sensitive to pressure, ranging from 10−20 to 10−23 m2 as effective pressure increased from 5 to only 60 MPa. In contrast, the granodiorite gneiss samples were more permeable and less sensitive to pressure, with permeability values ranging from 10−17 to 10−22 m2 as effective pressures increased to 300 MPa. Amphibolites displayed intermediate behavior. There was an abundance of microfractures in the quartz‐rich rocks, but a relative paucity of cracks in the mafic rocks, suggesting that the observed differences in permeability are based on rock type and depth, and that stress relief/thermal‐cracking damage is correlated with quartz content. By applying the equivalent channel model of Walsh and Brace [1984] to the permeability data of the quartz‐rich samples, we can estimate the closure pressure of the stress‐relief cracks and thereby place bounds on the in situ effective pressure. This method may be useful for drill holes where the fluid pressure is not well constrained, such as at the Kola well. However, the use of crack closure to estimate in situ pressure was not appropriate for the basalt and amphibolite samples, because they are relatively crack‐free in situ and remain so even after core retrieval. As a result, their permeability is near or below the measurable lower limit of our apparatus at the estimated in situ pressures of the rocks.
Periodic instabilities during compression of the lithosphere: 2. Analogue experiments
Authors: Martinod Joseph , Davy Philippe
Published: February 2004

We have modeled the behavior of the continental and oceanic lithospheres under compression, using materials with analogous properties in laboratory experiments, to study the development of lithospheric buckling. Periodic instabilities, which are a major deformation process during the compression of the lithosphere, have already been described by several authors using an analytical perturbation method. At small strains, analogue experiments corroborate most of the results obtained by the perturbation method: (1) the deformation modes (geometrical relationships of interfaces and related wavelengths) are mainly dependent on the spatial distribution of the brittle layer(s), and (2) the amplitude of buckling is an exponential function of the horizontal strain. Some departure from the perturbation method occurs when there are two instabilities growing concurrently. The breakdown of the exponential growth occurs for strains of about 5%, and is concomitant with the appearance of thrust faults. In experiments including one brittle layer, which model the compression of the oceanic lithosphere, faults are regularly located at the inflection points of the folds. In experiments including two brittle layers, which model continental lithosphere, faults form more complicated patterns with an asymmetrical deep thrust overlain by a fan‐shaped symmetrical thrust system in the upper brittle layer. Such fault geometries give some new highlights on typical compressive geological structures such as those encountered in Central Asia.
Extraction of marginal ice zone thickness using gravity wave imagery
Authors: Shuchman Robert A. , Rufenach Clifford L. , Johannessen Ola M.
Published: February 2004

Synthetic aperture radar (SAR) imagery of gravity waves in the marginal ice zone (MIZ) was acquired during 1987 and 1989 using aircraft flying over the Greenland Sea. The dominant gravity wavelengths acquired during 1987 exhibit a systematic lengthening with penetration distance into the MIZ for three different regions examined. The extracted dominant wavelengths were analyzed using a linear least squares fit with penetration distance and a flexure‐gravity wave model to estimate mean ice thickness. The extracted mean thickness varies from about 1±0.4 m near the ice edge to approximately 2±0.4m near the deepest detectable penetration of the waves (about 20 km). These thicknesses are in agreement with 31 in situ measurements taken in the Fram Strait MIZ within 2 weeks of the SAR measurements. We also acquired wave imagery across a 150‐km section of newly forming ice called the Odden, a tongue‐shaped region, during 1989. These SAR images at L, C, and X bands display small wave modulation signatures at the northern edge of the Odden which we interpret as arising from rapidly growing new ice.
The 1982–1984 El Niño in the Gulf of California as seen in coastal zone color scanner imagery
Authors: Santamaría‐del‐Angel Eduardo , Alvarez‐Borrego Saúl , Müller‐Karger Frank E.
Published: February 2004

The 1982–1984 El Niño was well documented for the Gulf of California, primarily with sea level anomalies. However, owing to strong tidal and atmospheric forcing, the general effects on chlorophyll concentrations ([Chl]) and phytoplankton productivity (PP) are not clear. The literature reports low [Chl] and PP for the mouth region and high values for both variables in the central gulf and in the region of the midrift islands. No data were collected in situ in the northern gulf. Using monthly composites of coastal zone color scanner (CZCS) satellite imagery, we generated time series for 33 locations throughout the gulf. We calculated pigment concentration anomalies and derived a relative power index that provides a measure of the relationship between negative anomalies and El Niño episodes. Our analyses show that during 1978–1986, variability in phytoplankton biomass in the Gulf of California was not dominated by El Niño events. Rather, strong tidal mixing and upwelling in the interior of the gulf masked the effect of El Niño 1982–1984 that otherwise showed so clearly in other coastal ecosystems of the eastern Pacific. Lower pigment concentrations were in fact detected with the CZCS at and near the entrance of the gulf, where vertical mixing is not as strong. Locations in the central and northern gulf showed either a weak effect or no effect of El Niño. Ballenas Channel showed an exceptional behavior, with an increase of pigment concentrations toward the end of 1983. Despite the Ballenas Channel having the highest surface nutrient concentrations, pigment maxima there are not in general the highest of the gulf. The excess vertical turbulence in the channel leads to light limitation of phytoplankton. Reduced mixing at the end of 1983 promoted the increase of phytoplankton biomass in Ballenas Channel. Strong non‐El Niño conditions in 1984 caused lower phytoplankton biomass in Ballenas Channel than during other years of the period analyzed.
Interpretation of the coastal zone color scanner signature of the Orinoco River plume
Authors: Hochman Herschel T. , Müller‐Karger F. E. , Walsh John J.
Published: February 2004

The Caribbean Sea is an area that traditionally has been considered oligotrophic, even though the Orinoco River contributes large quantities of fresh water, nutrients, and other dissolved materials to this region during the wet boreal (fall) season. Little is known about the impact of this seasonal river plume, which extends from Venezuela to Puerto Rico shortly after maximum discharge. Here we present results from a study of the bio‐optical characteristics of the Orinoco River plume during the rainy season. The objective was to determine whether the coastal zone color scanner (CZCS) and the follow‐on sea‐viewing wide‐field‐of‐view sensor (SeaWiFS) satellite instrument can be used to assess the concentrations of substances in large river plumes. Recent in situ shipboard measurements were compared to values from representative historical CZCS images using established bio‐optical models. Our goal was to deconvolve the signatures of colored dissolved organic carbon and phytoplankton pigments within satellite images of the Orinoco River plume. We conclude that the models may be used for case II waters and that as much as 50% of the remotely sensed chlorophyll biomass within the plume is an artifact due to the presence of dissolved organic carbon. Dissolved organic carbon originates from a number of sources, including decay of dead organisms, humic materials from the soil, and gelbstoff.
The structure and propagation of a Gulf Stream frontal eddy along the North Carolina Shelf Break
Authors: Glenn Scott M. , Ebbesmeyer Curtis C.
Published: February 2004

The Frontal Eddy Dynamics (FRED) experiment, conducted offshore North Carolina between Cape Fear and Cape Hatteras, included a multiplatform intensive survey phase in May 1987. The intensive phase data set includes satellite infrared imagery, aircraft‐ and shipboard‐deployed expendable bathythermographs, moored current meters and thermistors, and satellite‐tracked drifting buoys. These observations are used to examine the structure and propagation of Eddy Abbott, the most extensively surveyed frontal eddy in this study. The analysis includes the use of a frontal eddy feature model to calculate the surface circulation and propagation characteristics of Eddy Abbott's cold dome from the buoy trajectories. The observations and analyses presented are unique in that Eddy Abbott is the first Gulf Stream frontal eddy seeded with tracking buoys to be followed beyond Cape Hatteras. Previously, frontal eddies were assumed to be sheared apart as they approached the confining topography of Cape Hatteras, leaving the near surface nutrient rich waters of the cold dome stranded on the North Carolina shelf. The in situ tracking data provided by the buoys indicate that rather than being stranded, Eddy Abbott's cold dome accelerates, propagating out of the South Atlantic Bight beyond Cape Hatteras.
Observations of Gulf Stream frontal eddies in the vicinity of Cape Hatteras
Authors: Glenn Scott M. , Ebbesmeyer Curtis C.
Published: February 2004

The Frontal Eddy Dynamics (FRED) experiment was conducted offshore North Carolina between Cape Fear and Cape Hatteras from May through November 1987. Frontal eddies propagating northward along the North Carolina shelf break were observed during a 3‐week intensive shipboard and aircraft survey phase, followed by a six‐month (long‐term) mooring and remote sensing monitoring phase. Using observations of a frontal eddy named Abbott from the intensive phase, it has been established that a frontal eddy cold dome can propagate out of the South Atlantic Bight past Cape Hatteras. The focus here therefore is on frontal eddy observations in the long‐term data set in the immediate vicinity of Cape Hatteras. The long‐term moorings indicate that 39 northward propagating frontal eddies were observed immediately south of Cape Hatteras, giving a recurrence interval of once every 4–5 days. Two types of frontal eddies are identified, dependent on the bimodal character of the amplitude of the parent Gulf Stream meanders. During the small‐meander mode, the mean Gulf Stream front is approximately aligned with the 100‐m isobath, and the small warm filaments rarely penetrate beyond mid‐shelf. During the large‐meander mode, the mean Gulf Stream front is displaced offshore, but the larger warm filaments often penetrate onto the inner shelf. The duration of each meander mode at Cape Hatteras is 2–3 months, with rapid (several day) transitions between the modes. The transition to the large‐meander mode appears to be related to local interactions with cold core rings in the Sargasso Sea rather than the offshore deflections of the Gulf Stream at the Charleston Bump. The intense observations of Eddy Abbott occur during the small‐meander mode, demonstrating that even when the Gulf Stream is closer to the confining topography of the continental shelf, frontal eddy cold domes can propagate beyond Cape Hatteras and form new warm filaments in the Middle Atlantic Bight.
Water masses and circulation in the Eurasian Basin: Results from the Oden 91 expedition
Authors: Anderson L. G. , Björk G. , Holby O. , Jones E. P. , Kattner G. , Koltermann K. P. , Liljeblad B. , Lindegren R. , Rudels B. , Swift J.
Published: February 2004

The Oden 91 North Pole expedition obtained Oceanographic measurements on four sections in the Nansen and Amundsen basins of the Eurasian Basin and in the Makarov Basin of the Canadian Basin, thereby proving the feasibility of carrying out a typical Oceanographic program using an icebreaker in the Arctic Ocean. The data show greater spatial variability in water structure and circulation than was apparent from previous data. The results show that a clear front exists between the Eurasian and Canadian basins such that upper halocline water in the Canadian Basin is almost absent from the Eurasian Basin. The lower halocline water produced in the Barents‐Kara Sea region permeates much of the Eurasian Basin and flows along the continental slope into the Canadian Basin. The deeper circulation is strongly influenced by topography. Three return flows of the Atlantic layer are identified, one over the Nansen‐Gakkel Ridge, one over the Lomonosov Ridge, and a third flowing from the Canadian Basin. The slight differences observed in salinity and temperature characteristics of the deeper waters of the Nansen and Amundsen basins do not lead to an obvious explanation of their origin or flow pattern.
Determination of thorium and particulate matter cycling parameters at station P: A reanalysis and comparison of least squares techniques
Authors: Murnane Richard J.
Published: February 2004

This reanalysis of data from weather station P in the North Pacific provides least squares estimates of thorium and particle cycling rate constants and of the concentration of sinking large particles. The estimated rate constants are for a particle model that includes a suspended small‐particle class and a sinking large‐particle class and for an analogous thorium model that includes a dissolved class, a suspended small‐particle class, and a sinking large‐particle class. Steady state, one‐dimensional conditions are usually considered reasonable approximations for the deep North Pacific. These assumptions lead to estimated rate constants and concentrations of large particles (PL) that are inconsistent with independent observations. A solution consistent with all observations is found by using lognormal probability distributions for the initial estimates of the rates and PL, by using a cost function that includes all imprecisely known terms, and by allowing for deviations from steady state, one‐dimensional conditions. Estimates of the mean and standard deviation for the rates and PL are k1 = 0.7 ± 0.4 y−1 for thorium adsorption, k−1 = 1.7 ± 0.9 y−1 for thorium desorption, β2 = 0.8 ± 0.9 y−1 for particle aggregation, β−2 = 400 ± 10000 y−1 for particle disaggregation, and PL = 2 ± 60 × 10−6 kg m−3. All the means and standard deviations are for lognormal probability distributions. The results suggest that estimated PL and particle aggregation and disaggregation rate constants have the greatest consistency when deviations from steady state and one‐dimensional conditions, positivity constraints (based on lognormal probability distributions), and measurement uncertainty are incorporated into a solution.
Estimation of the ocean wave–radar modulation transfer function from synthetic aperture radar imagery
Authors: Brüning C. , Schmidt R. , Alpers W.
Published: February 2004

Estimates of the ocean wave‐radar modulation transfer function (MTF) are derived from synthetic aperture radar (SAR) imagery acquired by the American Naval Air Development Center airborne three‐frequency SAR over the North Sea during the U.S./German SAR and X Band Ocean Nonlinearities‐Forschungsplattform Nordsee experiment in November 1990. This is achieved by comparing measured and simulated SAR image spectra. The simulated SAR image spectra are computed from ocean wave height spectra measured by a pitch and roll buoy and by applying the generalized velocity‐bunching model. First, SAR simulations are carried out by using the theoretical MTF which contains the tilt and range‐bunching MTFs as well as the hydrodynamic MTF calculated from the relaxation time model. Second, SAR simulations are carried out by taking the modulus and phase of the MTF as free parameters. For waves traveling toward the radar antenna, best agreement is achieved when using the following values for the modulus |M0| and phase η of the nondimensional MTF defined by (5) and (10): |M0| = 8 ‐ 13 for VV polarization and |M0| = 12 ‐ 15 for HH polarization; η = 60° ‐ 90° past the long‐wave crest when the wind is blowing downwave, and η = 0° ‐ 60° past the long‐wave crest when the wind is blowing upwave. The values derived for the modulus of the MTF are in good agreement with values obtained from tower‐based radar backscatter measurements. However, for the downwave case the phase of the MTF disagrees with the phase obtained from tower‐based measurements, where usually values between 20° and 60° are found.
North Brazil Current retroflection eddies
Authors: Richardson P. L. , Hufford G. E. , Limeburner R. , Brown W. S.
Published: February 2004

During 1989–1992, six different anticyclonic eddies were observed to translate up the coast of South America between 7°N and 12°N. These eddies, which are similar to those recently observed with coastal zone color scanner images, current meters, and altimetry, are inferred to have formed from pieces of the North Brazil Current, which retroflects or veers offshore near 7°N to flow eastward. The maximum near‐surface diameter of the eddies when newly formed is estimated to be 400 km. The retroflection eddies were identified in trajectories of four surface drifters and three SOFAR floats at depths from 900–1200 m that looped as they translated northwestward with a mean velocity of 9 cm/s. The two longest trajectories were (1) by a 900‐m float beginning near 7°N in August 1989 that looped 14 times over 151 days with a maximum diameter of 140 km and (2) by a surface drifter beginning near 7°N in October 1990 that looped 9 times over 116 days with a maximum diameter of 250 km and maximum swirl speed of 80 cm/s at that diameter. The mean rotation periods of these two eddies were 11 and 13 days, respectively, with the rotation period of small loops at 900 m around 7 days. Three of the eddies were observed during the period August 1989 to April 1990. Retroflection eddies are considered to provide significant northward volume transport, ∼3 Sv, along the western boundary even during the months when the North Brazil Current has been observed to retroflect into the countercurrent.
Water circulation and a cyclonic ring in the Gulf Stream splitting region in May–June 1990
Authors: Koshlyakov Mikhail N. , Sazhina Tatyana G.
Published: February 2004

Results of two hydrographical surveys of the area southeast of Newfoundland, where the Gulf Stream branches into the North Atlantic Current and the Azores Current, are presented. The surveys were carried out by the R/Vs Vityaz and Professor Shtokman in May–June, 1990, as a part of the Atlantex‐90 experiment. The process of detachment of a cyclonic meander and the appearance of a new, extremely strong cyclonic ring are described. Thermohaline structure and local dynamics of the ring and the origin of its water mass are explored. Integral parameters describing the heat and salt content of the ring are defined, and fluxes of heat and salt transferred across the Subarctic Front by such rings are estimated. Volume transport variations of the Gulf Stream are coupled with the appearance of the ring. Structure and variability of individual jets of the Azores Current and the North Atlantic Current and their interaction with the synoptic scale cyclones and anticyclones are described. From the analysis of the thermohaline structure and parameters of these eddies their origin can be determined. The hydrological structure of the Subarctic, Labrador Current, and Azores Fronts is described.