bluuebunny/crossref_metadata_2025 huggingface dataset:
A synthesis of magnetostratigraphic results from Pliocene‐Pleistocene sediments cored using the hydraulic piston corer
Authors: Clement Bradford M. , Kent Dennis V. , Opdyke Neil D.
Published: February 2004

We present a summary evaluation of the distribution and qualitative ranking of the Plio‐Pleistocene magnetostratigraphic results obtained to date from Deep Sea Drilling Project and Ocean Drilling Program piston‐cored sites. A review of the published magnetostratigraphic records provides insights into the important extrinsic and intrinsic factors which affect the quality of the paleomagnetic records. The extrinsic factors originate with drilling processes, such as core barrel remagnetization, and steps can be taken to reduce these effects and improve the data quality. The distribution of the high‐quality records correlates well both with areas of terrigenous sediment input as well as regions of moderate biological productivity. This suggests that important intrinsic factors include the origin of the original magnetic carrier in the sediment (lithogenic or biogenic) and the degree to which the magnetic carrier has been affected by reduction diagenesis.
Instability of bottom‐water redox conditions during accumulation of Quaternary sediment in the Japan Sea
Authors: Piper David Z. , Isaacs Caroline M.
Published: February 2004

The concentrations of Cd, Cr, Cu, Mo, Ni, Sb, U, V, and Zn were measured in early Quaternary sediment (1.32 to 1.08 Ma) from the Oki Ridge in the Japan Sea. The elements were partitioned between a detrital fraction, composed of terrigenous and volcaniclastic aluminosilicate debris, and a marine fraction, composed of biogenic and hydrogenous debris derived from seawater. The most important factors controlling minor‐element accumulation rates in the marine fraction were (1) primary productivity in the photic zone, which largely controlled the flux of particulate organic‐matter‐bound minor elements settling through the water column and onto the seafloor, and (2) bottom‐water redox, which determined the suite of elements that accumulated directly from seawater. This marine fraction of minor elements on Oki Ridge recorded six periods of high minor‐element abundance. Assuming a constant bulk sediment accumulation rate, each period lasted roughly 5,000 to 10,000 years with a 41,000‐year cycle. Accumulation rates of individual elements such as Cd, Mo, and U suggest sulfate‐reducing conditions were established in the bottom water during the 10,000‐year periods; accumulation rates of Cr and V during the intervening periods are indicative of less reducing, denitrifying conditions. Interelement ratios, for example, Cu:Mo, V:Cr, and Sb:Mo, further reflect bottom‐water instability, such that bottom‐water redox actually varied from sulfate reducing to denitrifying during the periods of highest minor‐element accumulation rates; it varied from denitrifying to oxidizing during the intervening periods. Sediment lithology supports these interpretations of the minor‐element distributions; the sediment is finely laminated for several of the periods represented by Cd, Mo, and U maxima and weakly laminated to bioturbated for the intervening periods. The geochemistry of this sediment demonstrates the unambiguous signal of Mo, principally, but of several other minor elements as well in recording sulfate‐reducing conditions in bottom water. The forcing function that altered their accumulation, that is, that altered primary productivity and bottom water redox conditions, is problematic. Currently held opinion suggests that O2 depletion was most strongly developed during glacial advances. Low sea level during such times is interpreted to have enhanced primary productivity and restricted bottom‐water advection.
The sun‐climate question: Is there a real connection?
Authors: Reid George C.
Published: February 2004

No one can doubt that the sun is the chief driving force for our terrestrial climate. The annual march of the seasons as the earth's axis of rotation tilts toward or away from the sun's direction is sufficient proof of that, while the presence of periodicities in glacial deposits matching those of known orbital variations has revealed the apparent sensitivity of global climate to relatively small changes in the distribution of sunlight. What has remained debatable and controversial, however, is the question of whether or not variations in the sun's radiative and plasma emissions occur that are capable of influencing the weather and climate at the earth's surface. The current status of the field and the implications for global change have recently been reviewed by the U.S. National Research Council [NRC, 1994].
Eat, breathe, and be wary: Mineralogy in environmental health
Authors: Guthrie George D.
Published: February 2004

With the growing public interest in environmental issues, the geological sciences face exciting new research opportunities. Some environmental problems are obviously geological in nature (e.g., the use of hydrology to understand the migration of contaminants). However, many environmental problems that can benefit from the application of geological principles are not so obvious. A prime example is the evaluation of the impact of minerals on human health, which would typically be viewed as a biological problem. This topic encompasses many possible issues (see, e.g., the series of articles in the January 1994 issue of Geotimes [Nuhfer, 1994; Ross and Skinner, 1994; Skinner and Ross, 1994; Smith, 1994]), but this review will focus on two that involve the interactions of minerals with biological systems: inhaled minerals and ingested minerals. Much of the review, in fact, focuses on the first of these two topics, not because it is more important or has been the focus of a greater number of studies but because it provides an excellent example of the wide range of possible research opportunities for mineralogists and geochemists. The exclusion of topics such as radon from this review is not meant to dismiss the potential impact geoscientists can have on these topics, rather it is simply not possible (within a short review) to cover all of the geoscience issues in the health sciences.
Tongues, bottles, and disconnected loops: The opening and closing of the interplanetary magnetic field
Authors: McComas David J.
Published: February 2004

For years the field of Space Physics has had a problem, a really big problem for it occurs on the largest spatial scales in Space Physics ‐ across the entire region under the Sun's influence, the heliosphere. The problem is that the Sun appears to keep opening new magnetic fields into interplanetary space with no obvious way to close them back off again. This state of affairs, without some method for closing the open interplanetary magnetic field (IMF), would lead to an ever growing IMF magnitude in interplanetary space ‐ a catastrophe that is clearly not observed.

Figure 1 displays a composite picture of the Sun's outermost atmosphere or corona from 30 June 1973. The outer portion was taken in ordinary white light from the ground during a solar eclipse. The superposed soft X‐ray image of the denser, near‐Sun corona was taken from Skylab on the same day. The structure evident in the images is a consequence of the solar magnetic field that permeates the corona; more and less populated magnetic fields are mapped out as density structures in these images. Coronal holes are low‐density regions where the magnetic fields open out into interplanetary space; a large coronal hole is displayed as the dark region near the top of Figure 1. In contrast, bright loop‐like structures indicate closed field regions on the Sun that contain high‐density plasma. The centers of helmet streamers (closed field loops overlaid by nearly radial high density streamer structures), such as those seen extending outwards to the left and right sides in Figure 1, map out to large reversals of the magnetic field or current sheets in interplanetary space. The solar wind flow from streamers is slower and higher density than it is from coronal holes [e.g., Borrini et al., 1981]. Coronagraph and soft X‐ray observations over the past several decades have shown that the solar corona is highly dynamic with open and closed regions evolving over time scales as short as minutes and as long as the 22‐year solar cycle.
Recent advances in geostatistical inference on hydrogeological variables
Authors: Kitanidis Peter K.
Published: February 2004

Among the challenges facing hydrogeologists are to (a) predict where the contaminants are and where they are going and (b) to design effective remediation schemes, To meet these challenges, they must analyze data in order to estimate the parameters of geologic formations that affect the flow of water or the transport of chemicals. However, because these parameters vary in complicated and insufficiently understood ways, estimates are usually approximate and hard to obtain. Statistical methods are helpful in selecting the “best” possible estimate given the information and in evaluating the confidence bounds associated with this estimate. Furthermore, they can be used to evaluate the prediction uncertainty and are thus valuable in assessing management strategies and monitoring plans.
Air‐sea interaction: Connecting the ocean and atmosphere
Authors: Rogers David P.
Published: February 2004

The large‐scale dynamics of the ocean and the atmosphere are closely related. Energy is transferred from the atmosphere to the ocean surface mixed layer driving the circulation of the upper ocean. In turn, energy from the ocean is fed back to the atmosphere affecting the atmospheric circulation, the weather and the climate. The concept is deceptively simple, but as we explore the coupled earth system, we are frequently limited by our lack of understanding of the interchanges between the atmosphere and ocean. AsBrown[1990] pointed out, there are two gaps in our knowledge of the air‐sea interface: the theory is inadequate and the data are sparse. More specifically,Kraus and Businger[1994] highlighted several areas that continue to require more attention, the interaction of the wind and surface waves, for example, the parameterization of subgrid scale processes in large‐scale circulation models, and the transfer of gases across the air‐sea interface, among others.
Magnetic records of climate change
Authors: Reynolds Richard L. , King John W.
Published: February 2004

Sediments deposited on the sea floor, on lake bottoms, as well as on land as accumulations of windblown silt (loess) and as ancient soils (paleosols) formed in the loess possess many different properties that record past changes in climate. Geologically recoverable information on paleoclimate includes changes in global temperature as it may affect global ice volume and thus temperature of sea water, net precipitation, storm intensity (wind speed) and frequency, as well as wind direction. Such information further contributes to interpretations about changes in oceanic and atmospheric circulation patterns. As such, the understanding of past climatic changes can provide baselines of natural climatic fluctuations as related to those that may be induced or influenced by human activities.
Earthquake prediction, societal implications
Authors: Aki Keiiti
Published: February 2004

“If I were a brilliant scientist, I would be working on earthquake prediction.” This is a statement from a Los Angeles radio talk show I heard just after the Northridge earthquake of January 17, 1994. Five weeks later, at a monthly meeting of the Southern California Earthquake Center (SCEC), where more than two hundred scientists and engineers gathered to exchange notes on the earthquake, a distinguished French geologist who works on earthquake faults in China envied me for working now in southern California. This place is like northeastern China 20 years ago, when high seismicity and research activities led to the successful prediction of the Haicheng earthquake of February 4, 1975 with magnitude 7.3. A difficult question still haunting us [Aki, 1989] is whether the Haicheng prediction was founded on the physical reality of precursory phenomena or on the wishful thinking of observers subjected to the political pressure which encouraged precursor reporting. It is, however, true that a successful life‐saving prediction like the Haicheng prediction can only be carried out by the coordinated efforts of decision makers and physical scientists.
Interstellar pickup ions: Not just theory anymore
Authors: Isenberg Philip A.
Published: February 2004

Although the far‐flung Pioneer and Voyager spacecraft will not leave the heliosphere (the region of particles and fields emitted by the Sun) for at least another few quadrennia, an important component of the surrounding interstellar medium is already under study in the form of interstellar pickup ions. In the last four years, there have been great advances in the study of these particles, primarily motivated by the new observations from instruments on the Ulysses spacecraft. This article will review the exciting work on interstellar pickup ions which took place during the last quadrennium, concentrating on the efforts of the U.S. scientists and their collaborators.

As the Sun moves through interstellar space, the heliospheric magnetic field prevents the ionized portion of the surrounding interstellar gas from entering the solar system, but the population of interstellar neutral atoms flows unimpeded through the essentially collisionless plasma. When these atoms travel close enough to the Sun, solar radiation (in the form of both photons and solar wind particles) acts to ionize them and the new ions must suddenly respond to the electromagnetic fields of the supersonic solar wind. In the reference frame moving with the solar wind, this response consists of an immediate gyration of the new ions about the magnetic field, followed by a rapid isotropization by ambient or self‐generated low‐frequency fluctuations in the plasma. This isotropization in the solar wind frame means that the ions are now convecting with the solar wind ‐ they have been “picked up”. Since the inflowing interstellar atoms move very slowly with respect to the Sun (20 – 25 km/s), they have supersonic speeds in the solar wind frame, and the isotropization of the ions in this frame results in a distinct energetic population of ions with origins in the interstellar medium.
Recent advances and themes in hydrology
Authors: Vogel Richard M.
Published: February 2004

The twenty‐three review articles which follow provide a compendium of recent advances over the last quadrennium, 1991–94, which relate to some of the scientific, technologic, and policy aspects of the field of Hydrology. Due to space and time limitations, neither the titles of the review articles chosen for this report nor the review articles themselves are exhaustive, instead they are only intended to provide an overview of some recent advances in hydrology. A more exhaustive review of research agendas relating to the scientific aspects of hydrology may be found in a recent National Research Council report [NRC, 1991]. An exhaustive review of the current state of hydrologic practice may be found in the newHandbook of Hydrology[Maidment, 1993].
Comparison of plasma flow velocities determined by the ionosonde Doppler drift technique, SuperDARN radars, and patch motion
Authors: Grant I. F. , MacDougall J. W. , Ruohoniemi J. M. , Bristow W. A. , Sofko G. J. , Koehler J. A. , Danskin D. , André D.
Published: February 2004

We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3‐hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.
Polarimetric scattering from layered media with multiple species of scatterers
Authors: Nghiem S. V. , Kwok R. , Yueh S. H. , Kong J. A. , Hsu C. C. , Tassoudji M. A. , Shin R. T.
Published: February 2004

Geophysical media are usually heterogeneous and contain multiple species of scatterers. In this paper a model is presented to calculate effective permittivities and polarimetric backscattering coefficients of multispecies‐layered media. The same physical description is consistently used in the derivation of both permittivities and scattering coefficients. The strong permittivity fluctuation theory is extended to account for the multiple species of scatterers with a general ellipsoidal shape whose orientations are randomly distributed. Under the distorted Born approximation, polarimetric scattering coefficients are obtained. These calculations are applicable to the special cases of spheroidal and spherical scatterers. The model is used to study effects of scatterer shapes and multispecies mixtures on polarimetric signatures of heterogeneous media. The multispecies model accounts for moisture content in scattering media such as snowpack in an ice sheet. The results indicate a high sensitivity of backscatter to moisture with a stronger dependence for drier snow and ice grain size is important to the backscatter. For frost‐covered saline ice, model results for bare ice are compared with measured data at C band and then the frost flower formation is simulated with a layer of fanlike ice crystals including brine infiltration over a rough interface. The results with the frost cover suggest a significant increase in scattering coefficients and a polarimetric signature closer to isotropic characteristics compared to the thin saline ice case.
On the true velocity in full correlation analysis
Authors: Woodman Ronald F.
Published: February 2004

The usual assumption made of elliptical symmetry in the shape of the correlation functions for deriving algorithms to determine the “true” velocity of a time‐changing random pattern is relaxed. We start, instead, by stating that in the frame of reference moving with the medium, the space‐time autocorrelation function of any state parameter of the medium is invariant under an r = −r transformation, where r is the displacement vector. With no further assumptions, a simple formula for the velocity of the medium is found for statistically isotropic patterns and for anisotropic patterns when the sensors are aligned along the axis of symmetry. The simplicity of the derivation and the formula contrast with previous derivations under more restrictive conditions. For the case of unknown elongation direction, previously known algorithms for determining the velocity under the assumption that the locus of equal correlation in r space are elliptical are revised. They are derived as Taylor expansion approximations of the functions specifying the shape of equal correlation shells. Furthermore, the assumption of fixed elliptical elongation parameters is relaxed. Three‐dimensional cases are considered throughout, with one‐ and two‐dimensional applications included as special cases.
Fractional lags in alternating codes: Improving incoherent scatter measurements by using lag estimates at noninteger multiples of baud length
Authors: Huuskonen A. , Lehtinen M. S. , Pirttilä J.
Published: February 2004

Alternating codes are nowadays a standard measurement technique in the incoherent scatter observations. In most conditions they give the best accuracy for the plasma autocorrelation function estimates in the E and F region measurements. We explain here how oversampling can be used to further increase the speed of the experiments. In an oversampled alternating code the sampling interval and the length of the postdetection filter are a fraction of the baud length, and autocorrelation function estimates are calculated both for integer lag values, which are multiples of the baud length, and for the intermediate values, which are called fractional lags. We show that alternating codes with fractional lags give data for multiple range resolutions and that the experiments are always more efficient than the standard alternating code experiments. Finally, we study how the oversampling affects the signal processing.
Extension of stable continental lithosphere and the initiation of lithospheric scale faults
Authors: Govers R. , Wortel M. J. R.
Published: February 2004

We address the physical conditions which control the style of continental extension. Geological evidence suggests that once lithospheric scale zones of localized deformation have been formed, they strongly affect continental deformation. It is the purpose of this paper to investigate mechanisms which may cause lithospheric scale faults to initiate in stable continental lithosphere which is laterally fairly homogeneous. Faults and shear zones cutting strong layers in the lithosphere will have a very significant influence on the evolution during extension. Based upon experimental flow laws, a strength maximum can be expected in the mantle directly beneath the Moho. Strain localization in the shallow upper mantle is therefore expected to have a very pronounced effect on the evolution of the extending lithosphere. Low viscosities in the lower crust decouple the crust mechanically from the upper mantle. Therefore causes for strain localization in the sub‐Moho mantle must be found in the mantle itself. Two potential causes satisfying this requirement are boudinageing and strain weakening. We use thermal‐mechanical finite element models which incorporate the elastic, visco‐plastic, and viscous response of lithospheric rocks. The results of our model experiments suggest that boudins do not evolve during extension of continental lithosphere in most situations; only when the extension rate is fast relative to thermal reequilibration may homogeneous boudinage result. By its very nature, however, this type of boudinage cannot produce lithospheric scale faults. Our model results suggest that shear zones may evolve after strain weakening. However, the style of extension on the scale of the lithosphere is pure shear like because the shear deformation in localized zones is balanced; in most cases the shear zones occur in conjugate pairs. Initiation of lithospheric scale faults is concluded to be unlikely in stable and homogeneous lithosphere in interior parts of continental plates.
Holocene paleoseismicity of the Tunka fault, Baikal rift, Russia
Authors: McCalpin James P. , Khromovskikh Vladimir S.
Published: February 2004

The Tunka fault is a major normal‐oblique transform fault within the NNE trending Baikal rift that displays geomorphic evidence of recurrent Quaternary movement. A flight of six fanhead terraces of the Kyngarga River is displaced by several parallel faults and has scarps up to 32 m high at Arshan. The main fault zone is exposed in an abandoned gravel quarry about 1 km east of the Kyngarga River. Thirteen radiocarbon dates from the quarry, a shallow trench in a graben, and natural streamcuts constrain the timing of the latest two or three Holocene paleoearthquakes. The latest earthquake is bracketed by the 1024–1315 calendar years (cal. year) B.P. age of an unfaulted terrace and by the 1947–2179 cal. year B.P. age of a soil buried by scarp colluvium in the graben trench. The penultimate earthquake is also relatively well constrained, assuming that the displacement event slightly younger than 7091–7867 cal. year B.P. in the upper quarry exposure is the same as the fissuring event slightly older than 6733–7385 cal. year B.P. in the lower quarry exposure. Evidence for earlier event(s) between 9.2–12.7 ka depends on ambiguous stratigraphic evidence in the lower quarry exposure. On the basis of only the two well‐dated earthquakes, the recurrence interval at Arshan may range from 2.9 to 6.8 ka for earthquake displacements of at least 1.3 m or a slip rate of 0.19–0.44 mm/yr. Intermediate‐term (100 ka) and long‐term (circa 500 ka) slip rates computed from terrace age estimates and fault scarp heights are 0.08–0.16 mm/yr and 0.07–0.11 mm/yr, respectively, rates that are considerably lower than the late Holocene rate and the approximately 0.5 mm/yr that might be inferred from the tectonic geomorphology of the Tunka Range front.
Constraints on present‐day Basin and Range deformation from space geodesy
Authors: Dixon Timothy H. , Robaudo Stefano , Lee Jeffrey , Reheis Marith C.
Published: February 2004

We use new space geodetic data from very long baseline interferometry and satellite laser ranging combined with other geodetic and geologic data to study contemporary deformation in the Basin and Range province of the western United States. Northwest motion of the central Sierra Nevada block relative to stable North America, a measure of integrated Basin and Range deformation, is 12.1±1.2 mm/yr oriented N38°W±5° (one standard error), in agreement with previous geological estimates within uncertainties. This velocity reflects both east‐west extension concentrated in the eastern Basin and Range and north‐northwest directed right lateral shear concentrated in the western Basin and Range. Ely, Nevada is moving west at 4.9±1.3 mm/yr relative to stable North America, consistent with dip‐slip motion on the north striking Wasatch fault and other north striking normal faults. Comparison with ground‐based geodetic data suggests that most of this motion is accommodated within ∼50 km of the Wasatch fault zone. Paleoseismic data for the Wasatch fault zone and slip rates based on seismic energy release in the region both suggest much lower slip rates. The discrepancy may be explained by some combination of additional deformation away from the Wasatch fault itself, aseismic slip, or a seismic rate that is anomalously low with respect to longer time averages. Deformation in the western Basin and Range province is also largely confined to a relatively narrow boundary zone and in our study area is partitioned into the eastern California shear zone, accommodating 10.7±1.6 mm/yr of north‐northwest directed right‐lateral shear, and a small component (∼1 mm/yr) of west‐southwest ‐ east‐northeast extension. A slip rate budget for major strike‐slip faults in our study area based on a combination of local geodetic or late Quaternary geologic data and the regional space geodetic data suggests the following rates of right‐lateral slip: Owens Valley fault zone, 3.9±1.1 mm/yr; Death Valley‐Furnace Creek fault zone, 3.3±2.2 mm/yr; White Mountains fault zone in northern Owens Valley, 3.4±1.2 mm/yr; Fish Lake Valley fault zone, 6.2±2.3 mm/yr. In the last few million years the locus of right‐lateral shear in the region has shifted west and become more north trending as slip on the northwest striking Death Valley‐Furnace Creek fault zone has decreased and is increasingly accommodated on the north‐northwest striking Owens Valley fault zone.
Plate motions in the North Pacific: The 43 Ma nonevent
Authors: Norton Ian O.
Published: February 2004

The Hawaiian‐Emperor seamount chain in the North Pacific Ocean is commonly considered to have been produced by motion of the Pacific plate over a hotspot. If the hotspot is assumed to have remained fixed in a mantle frame of reference, the 60° change in trend between the Hawaiian and Emperor portions of the chain results from a change in motion direction of the Pacific plate relative to the mantle at 43 Ma, during Middle Eocene time. Such a large and abrupt direction change should result in significant plate reorganizations and tectonic events in the Pacific and surrounding plates, but no evidence for these events can be found. In addition, in this paper it is shown that there were no changes in relative plate motions between Pacific‐area plates that can be associated with the Hawaiian‐Emperor bend. This is done by calculating motion of the Pacific, Farallon, and Kula plates relative to Asia and North America using the plate circuit Farallon‐Pacific‐Antarctica‐Africa‐North America‐Asia. With no major regional tectonic event at the time of the bend, it is suggested that there was no plate reorganization at 43 Ma and that the Emperor portion of the seamount chain was formed by a nonstationary hotspot.
The Butte Valley and Layton Well Thrusts of eastern California: Distribution and regional significance
Authors: Wrucke Chester T. , Stevens Calvin H. , Wooden Joseph L.
Published: February 2004

The Butte Valley and Layton Well Thrusts are major structural features in two adjacent mountain ranges west of southern Death Valley. The Butte Valley Thrust in the southern Panamint Range underlies most of the range and emplaces Proterozoic rocks over strata as young as Jurassic(?) in age. The Layton Well Thrust to the southwest in the Slate Range has been interpreted to have Proterozoic rocks juxtaposed on rocks as young as Jurassic, suggesting that the Butte Valley Thrust and the Layton Well Thrust might be correlative. New information indicates that the allochthonous rocks of the Layton Well Thrust are Mesozoic in age and are not likely part of the same allochthon as that above the Butte Valley Thrust. In addition, the Butte Valley Thrust cuts sharply downward to the north and west across lower plate Paleozoic strata, suggesting that the fault roots beneath the Layton Well Thrust. The Layton Well Thrust probably belongs to the East Sierran thrust system and thus would be in the upper plate of the Butte Valley Thrust.
Why is it downhill from Tonopah to Las Vegas?: A case for mantle plume support of the high northern Basin and Range
Authors: Saltus R. W. , Thompson G. A.
Published: February 2004

A major, abrupt, southward decline in elevation (∼800 m) with an accompanying increase in Bouguer gravity anomaly (∼80 mGal) crosses a seismically active region of southern Nevada at about 37°N. The steepness of the gravity gradient requires significant crustal density contrast, some of which is probably caused by the plutonic roots of voluminous Tertiary volcanism to the north, but the amplitude of the anomaly requires additional contrast within the mantle. The topographic step probably developed in mid‐Miocene time, coincident with the arrival of the starting head of the Yellowstone thermal plume at the base of the lithosphere. A plausible combination of crustal and deep buoyancy sources, related to the heat and melt input of an anomalously hot asthenospheric source, is consistent with gravity, seismic, heat flow, and isotopic observations and explains the origin of the topographic step.
Seismic reflection images of high‐angle faults and linked detachments in the Trans‐Hudson Orogen
Authors: Hajnal Z. , Lucas S. , White D. , Lewry J. , Bezdan S. , Stauffer M. R. , Thomas M. D.
Published: February 2004

Postcollisional (1.8–1.7 Ga) intracontinental deformation in the Trans‐Hudson Orogen (Canada) produced a series of orogen‐parallel high‐angle faults and folds. In seismic reflection profiles, the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4–8 s (12–24 km). The folded and faulted upper part of the crust is underlain by laterally coherent shallow‐dipping reflections that are locally bounded by discrete, highly reflective zones. These zones are interpreted as detachments (shear zones) and can be traced from the upper to lower crust, where some of them appear to pass into laterally continuous reflections that define the Moho. Two distinct regimes of postcollisional crustal deformation are inferred from the seismic images: high‐angle faulting and lateral block extrusion in the upper crust and low‐angle ductile shearing in the mid/lower crust. The surface geology indicates that the faults resulted in southwest (orogen‐parallel) extrusion of the orogen's internal zone relative to the bounding Archean Hearne and Superior cratons. Faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction. The seismic images suggest that the high‐angle fold/fault structures are kinematically linked to low‐angle detachments represented by laterally coherent, highly reflective zones. The detachment shear zones are inferred to have a top‐to‐the‐southwest sense of shear associated with a subhorizontal, northeast‐southwest extension direction, parallel to those observed for 1.83–1.80 Ga collisional shear zones exposed in major postcollisional fold culminations. Long‐lived orogen‐parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the Superior and Hearne margins.
Structural expressions and tectonic implications of general noncoaxial flow in the midcrust of a collisional orogen: The northern Scandinavian Caledonides
Authors: Northrup C. J.
Published: February 2004

Mapping and strain analysis in the northern Scandinavian Caledonides indicate that one way in which contemporaneous upper crustal extension and deeper thrusting within collisional orogens may be linked in a kinematically compatible manner is through the ductile flow of rocks in the midcrust of these mountain belts. In the Ofoten‐Efjorden region of north Norway, amphibolite facies metamorphism and strong penetrative strain occurred at deep levels of the Caledonian nappe stack as it was thrust eastward onto the western margin of Baltica. Kinematic analysis of deformation in the lower portion of the Caledonian nappe stack indicates that the rock mass underwent heterogeneous noncoaxial flow, with components of foreland directed simple shear and subvertically shortened pure shear. General noncoaxial flow (simultaneous simple shear + pure shear) can accomplish foreland‐directed tectonic transport, subvertical thinning, and transport‐parallel elongation at deep levels of a nappe stack. This pattern of flow is significant technically for two reasons: first, it suggests spreading of the Caledonian allochthon during thrust emplacement, and second, it can form a kinematic link between normal faulting at shallower structural levels and thrusting at the base of the nappes. By analogy with the Caledonides, general noncoaxial flow of midcrustal rocks may represent an important process in other collisional and contractional orogens, particularly in those that have undergone upper crustal extension while thrusting continued in the foreland.
Deformational history of the central Brooks Range, Alaska: Results from fission‐track and 40Ar/39Ar analyses
Authors: Blythe Ann E. , Bird John M. , Omar Gomaa I.
Published: February 2004

The Brooks Range is the northernmost orogenic belt in Alaska. From south to north it consists of a thin belt of oceanic basalt and chert, followed by two belts of high‐pressure/low‐temperature metamorphic rocks (the schist and central belts), a basement‐cored anticlinorium (the Doonerak Window), a fold and thrust belt, and a foreland basin (the Colville Basin). We have used apatite and zircon fission‐track (FT) and 40Ar/39Ar white mica analyses of a N‐S transect of the central Brooks Range to study the cooling history. These data are used with a balanced cross section to constrain the timing of deformational events in northern Alaska. The oldest cooling ages from this study were clusters of ∼185 Ma and ∼135 Ma zircon FT ages from the fold and thrust belt. Based on the similarity of the 185 Ma ages with the age of crystallization of the western Brooks Range ophiolites [Wirth et al., 1993], we suggest that the Brooks Range orogeny in northern Alaska had begun prior to this time. Cooling rates in the fold and thrust belt were very slow from 185 to 135 Ma, suggesting little or no deformation was occurring. Apatite and zircon FT analyses from the fold and thrust belt indicate that a substantial amount of cooling (caused by uplift and erosion) occurred from 135 to 95 Ma. We suggest that an increase in the rate of cooling at ∼135 Ma was caused by contractional deformation associated with renewed subduction with the opening of the Canada Basin. The 40Ar/39Ar white mica cooling ages of 130 to 120 Ma from the schist belt rocks of the southern Brooks Range indicate that peak metamorphism must have occurred prior to 130 Ma. An 40Ar/39Ar muscovite age of 113 Ma from the southernmost Brooks Range is interpreted to be the age of movement on normal faults juxtaposing lower‐grade phyllite belt rocks against schist belt rocks. At the same time, rapid deposition of Brooks Range‐derived sedimentary sequences began to the north, in the Colville foreland basin, and to the south, in the Yukon‐Koyukuk basin. Unstrained high‐level granitic plutons, which intruded the Yukon‐Koyukuk basin and Ruby terrane from 107 to 95 Ma, indicate the end of collision with the Yukon‐Koyukuk arc. By 95 Ma, cooling rates in the Brooks Range had slowed and little or no deformation was occurring. At 60 Ma, a major episode of rapid cooling occurred throughout northern Alaska, when the active plate margin in Alaska was more than 500 km to the south. In the Colville Basin, cooling was caused by thrust faulting and folding, and hence erosion, within the Albian‐age and younger sedimentary rocks. Within the interior of the Brooks Range, a large‐scale basement‐involved anticlinorium (the Doonerak Window) became active at this time. At 25 Ma, another episode of rapid cooling occurred within the Doonerak Window region. Both of these events may be related to shallow subduction of the Kula plate from the southern Alaskan plate margin.
Two‐Phase Flow Visualization and Relative Permeability Measurement in Natural Rough‐Walled Rock Fractures
Authors: Persoff P. , Pruess K.
Published: February 2004

A laboratory flow apparatus was used to visualize and measure two‐phase gas‐liquid flows in natural rough‐walled rock fractures. Experiments at carefully controlled flow rate and pressure conditions have been performed using a natural fracture and three transparent fracture replicas. Two‐phase flow exhibited persistent instabilities with cyclic pressure and flow rate variations even under conditions of constant applied boundary conditions. Visual observations of changes in pore occupancy showed that the instabilities could be explained as resulting from an interplay between capillary effects and pressure drop due to viscous flow. Measurements of relative permeabilities indicated strong phase interference, with relative permeabilities reduced to very small values at intermediate saturations for both wetting and nonwetting phases. These results run counter to a conventional view of fracture relative permeabilities that assumes that the relative permeability of each phase is equal to its saturation, but the results are consistent with recent models that view fractures as two‐dimensional heterogeneous porous media.
Chemical Evolution of Groundwater Near a Sinkhole Lake, Northern Florida: 2. Chemical Patterns, Mass Transfer Modeling, and Rates of Mass Transfer Reactions
Authors: Katz Brian G. , Plummer L. Niel , Busenberg Eurybiades , Revesz Kinga M. , Jones Blair F. , Lee Terrie M.
Published: February 2004

Chemical patterns along evolutionary groundwater flow paths in silicate and carbonate aquifers were interpreted using solute tracers, carbon and sulfur isotopes, and mass balance reaction modeling for a complex hydrologic system involving groundwater inflow to and outflow from a sinkhole lake in northern Florida. Rates of dominant reactions along defined flow paths were estimated from modeled mass transfer and ages obtained from CFC‐modeled recharge dates. Groundwater upgradient from Lake Barco remains oxic as it moves downward, reacting with silicate minerals in a system open to carbon dioxide (CO2), producing only small increases in dissolved species. Beneath and downgradient of Lake Barco the oxic groundwater mixes with lake water leakage in a highly reducing, silicate‐carbonate mineral environment. A mixing model, developed for anoxic groundwater downgradient from the lake, accounted for the observed chemical and isotopic composition by combining different proportions of lake water leakage and infiltrating meteoric water. The evolution of major ion chemistry and the 13C isotopic composition of dissolved carbon species in groundwater downgradient from the lake can be explained by the aerobic oxidation of organic matter in the lake, anaerobic microbial oxidation of organic carbon, and incongruent dissolution of smectite minerals to kaolinite. The dominant process for the generation of methane was by the CO2 reduction pathway based on the isotopic composition of hydrogen (δ2H(CH4) = −186 to −234‰) and carbon (δ13C(CH4) = −65.7 to −72.3‰). Rates of microbial metabolism of organic matter, estimated from the mass transfer reaction models, ranged from 0.0047 to 0.039 mmol L−1 yr−1 for groundwater downgradient from the lake.
Chemical Evolution of Groundwater Near a Sinkhole Lake, Northern Florida: 1. Flow Patterns, Age of Groundwater, and Influence of Lake Water Leakage
Authors: Katz Brian G. , Lee Terrie M. , Plummer L. Niel , Busenberg Eurybiades
Published: February 2004

Leakage from sinkhole lakes significantly influences recharge to the Upper Floridan aquifer in poorly confined sediments in northern Florida. Environmental isotopes (oxygen 18, deuterium, and tritium), chlorofluorocarbons (CFCs: CFC‐11, CCl3F; CFC‐12, CCl2F2; and CFC‐113, C2Cl3F3), and solute tracers were used to investigate groundwater flow patterns near Lake Barco, a seepage lake in a mantled karst setting in northern Florida. Stable isotope data indicated that the groundwater downgradient from the lake contained 11–67% lake water leakage, with a limit of detection of lake water in groundwater of 4.3%. The mixing fractions of lake water leakage, which passed through organic‐rich sediments in the lake bottom, were directly proportional to the observed methane concentrations and increased with depth in the groundwater flow system. In aerobic groundwater upgradient from Lake Barco, CFC‐modeled recharge dates ranged from 1987 near the water table to the mid 1970s for water collected at a depth of 30 m below the water table. CFC‐modeled recharge dates (based on CFC‐12) for anaerobic groundwater downgradient from the lake ranged from the late 1950s to the mid 1970s and were consistent with tritium data. CFC‐modeled recharge dates based on CFC‐11 indicated preferential microbial degradation in anoxic waters. Vertical hydraulic conductivities, calculated using CFC‐12 modeled recharge dates and Darcy's law, were 0.17, 0.033, and 0.019 m/d for the surficial aquifer, intermediate confining unit, and lake sediments, respectively. These conductivities agreed closely with those used in the calibration of a three‐dimensional groundwater flow model for transient and steady state flow conditions.
Similarity in Catchment Response: 2. Moving Rainstorms
Authors: Ogden Fred L. , Richardson Jerry R. , Julien Pierre Y.
Published: February 2004

The influence of storm motion on runoff is explored, with a focus on dimensionless hydrologic similarity parameters. One‐ and two‐dimensional physically based runoff models are subjected to moving rainstorms. A dimensionless storm speed parameterUte/Lp, where U is the storm speed, te is the runoff plane kinematic time to equilibrium, and Lp is the length of the runoff plane, is identified as a similarity condition. Storm motion effects on the peak discharge are greatest when the storm is traversing a one‐dimensional runoff plane in the downslope direction at a dimensionless speed of Ute/Lp = 0.5. This conclusion holds for all values of the dimensionless storm sizes Ls/Lp where Ls is the length of the storm in the direction of motion. Simulations with a two‐dimensional rainfall‐runoff model confirm the applicability of this similarity parameter on natural watershed topography. Results indicate that the detailed simulation of storm motion is necessary when the storm is moving near the velocity of maximum effect, which is considerably slower than typical storm velocities.
Steady State Flow in an Unsaturated, Two‐Dimensional, Macroscopically Homogeneous, Miller‐Similar Medium
Authors: Roth Kurt
Published: February 2004

The influence of microscopic heterogeneity on flow through a macroscopically homogeneous, variably saturated medium is studied by numerical simulations in a two‐dimensional, Miller‐similar medium. As a consequence of the local heterogeneity there appears a complex network of flow channels with interspersed islands of low‐flux domains. This hydraulic structure of the medium is characterized by two states which are separated by a critical point at which the hydraulic structure practically disappears. Transitions between the states are governed by the degree of saturation. The flow channels of the two states are complementary in that high‐flux regions of one state become low‐flux regions of the other one, and vice versa. This variable hydraulic structure clearly has far‐reaching practical and theoretical consequences for the transport of dissolved chemicals. The simulations further indicate that the autocovariance model of the microscopic heterogeneity is of minor importance for the global structure of the flow field.
Analysis of Early‐Time and Steady State Single‐Ring Infiltration Under Falling Head Conditions
Authors: Elrick David E. , Parkin Gary W. , Reynolds W. Daniel , Fallow David J.
Published: February 2004

The prediction of infiltration of water into field soils requires measurements of the field‐saturated hydraulic conductivity (Kfs) and the matric flux potential corresponding to field saturation (ϕm). We develop analytical solutions for (1) early‐time, one‐dimensional infiltration into slowly permeable soils such as clay liners under the conditions of an initial constant head followed by a falling head phase and (2) steady state, three‐dimensional infiltration into permeable soils under the condition of steady state flow at a constant head followed by falling head conditions. Kfs and ϕm govern the shape of both solutions. The parameters are simultaneously identifiable, unique, and stable within the early‐time solution; however, there is a limited range in which the parameters of the steady state solution are unique. Laboratory and field experiments based on both solutions gave physically realistic best fit parameter values using the solutions in inversion procedures. Field measurements gave an excellent correlation between Kfs from steady state constant head and falling head techniques. The correlation between ϕm values was somewhat lower.
Effects of Column Conditions on the First‐Order Rate Modeling of Nonequilibrium Solute Breakthrough
Authors: Young Dirk F. , Ball William P.
Published: February 2004

First‐order mass transfer models are commonly used as a means of interpreting sorption‐related mass transfer in laboratory columns, often with the intent of approximating diffusion‐based processes. We have fitted first‐order model parameters to computer‐simulated breakthrough curves from hypothetical column experiments in which Fickian diffusion into spherical particles limited the rate of sorption and desorption. Using both step and pulse inputs, we show that the fitted first‐order coefficient is a function not only of the intrinsic diffusion rate, but also of the column length, the step experiment's duration, the input pulse width, the fluid velocity, and the solute retardation factor. For a range of typical column run conditions and a given diffusion rate, we show that the fitted first‐order coefficient varies over three orders of magnitude in a manner roughly predictable through proper definition of a dimensionless timescale. In general, step inputs (as opposed to pulse inputs) provide a more consistent and predictable relationship between fitted coefficients and underlying diffusion rates. For either type of input, we recommend cautious use of the first‐order model, since many observed variations in fitted rate constants are not the result of mechanistic phenomena.