bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial and temporal relations between coronae and extensional belts, northern Lada Terra, Venus
Authors: Baer Gidon , Schubert Gerald , Bindschadler Duane L. , Stofan Ellen R.
Published: February 2004

Preliminary studies of the distribution of coronae and volcanic rises on Venus show that many of these features tend to cluster along zones of rifting and extension. The plains north of Lada Terra are crossed by two such extensional belts. Each belt is composed of grabens, ridges, faults, volcanic flows, coronae, and coronalike features. The longer and more prominent belt is the NW trending Alpha‐Lada extensional belt, which is over 6000 km long and 50–200 km wide, and includes the coronae Eve, Tamfana, Carpo, Selu, Derceto, Otygen, and an unnamed corona south of Otygen. The second belt is the NNE trending Derceto‐Quetzalpetlatl extensional belt, which is about 2000 km long and in places over 300 km wide, and includes the coronae Sarpanitum, Eithinoha, and Quetzalpetlatl. The two belts intersect at the 1600 × 600 km wide Derceto volcanic plateau. It is apparent that deformation along the two belts overlapped in time, though deformation along the Alpha‐Lada extensional belt probably continued after the deformation along the Derceto‐Quetzalpetlatl extensional belt terminated. In certain areas, volcanism originated in grabens within the extensional belts, whereas in other areas, such as in Eve, Selu, Derceto, and Quetzalpetlatl, volcanism originated in the coronae and flowed into the lower parts of the extensional belts. Regional extension has affected the evolution of all the coronae at some stage of their development. Regional deformation occurred before the initiation of Derceto and Eithinoha and after the initiation of Carpo, Tamfana, Otygen, and Sarpanitum. It is thus unlikely that coronae formation along the belts is solely a consequence of the regional extension, and it is also unlikely that regional extension has been caused solely by the coronae. No corona along the belts was formed subsequent to the cessation of the regional extension. We therefore suggest that the regional extension and the coronae are interrelated. Some of the coronae may have determined the location of the surface expression of the regional extension, whereas the locations of other coronae may have been influenced by the concentration of regional extensional stresses.
Silicate interactions with ammonia‐water fluids on early Titan
Authors: Engel Steffi , Lunine Jonathan I. , Norton Denis L.
Published: February 2004

Plausible models of the early history of Titan suggest that ammonia and water were present in liquid form at the surface. We show here by thermodynamic modeling that such an ocean could have reacted with silicates to put substantial quantities of sodium and potassium into solution. Following the formation of an ice crust by cooling, mantle ammonia‐water fluids enriched in potassium would have been brought to the surface through the cryogenic equivalent of volcanism. Later impacts would have released the 40Ar produced by decay of the 40K into the atmosphere. The abundance of atmospheric 40Ar, measurable by the Huygens probe gas chromatograph mass spectrometer, may be dominated by this source and hence gives a proxy indication of the volume of ammonia‐water resurfacing on Titan over geologic time.
Geochemical evidence for enhanced preservation of organic matter in the oxygen minimum zone of the continental margin of northern California during the Late Pleistocene
Authors: Dean Walter E. , Gardner James V. , Anderson Roger Y.
Published: February 2004

The present upper water mass of the northeastern Pacific Ocean off California has a well‐developed oxygen minimum zone between 600 and 1200 m wherein concentrations of dissolved oxygen are less than 0.5 mL/L. Even at such low concentrations of dissolved oxygen, benthic burrowing organisms are abundant enough to thoroughly bioturbate the surface and near‐surface sediments. These macro organisms, together with micro organisms, also consume large quantities of organic carbon produced by large seasonal stocks of plankton in the overlying surface waters, which are supported by high concentrations of nutrients within the California Current upwelling system. In contrast to modern conditions of bioturbation, laminated sediments are preserved in upper Pleistocene sections of cores collected on the continental slope at water depths within the present oxygen minimum zone from at least as far north as the California‐Oregon border and as far south as Point Conception. Comparison of sediment components in the laminae with those delivered to sediment traps as pelagic marine “snow” demonstrates that the dark‐light lamination couplets are indeed annual (varves). These upper Pleistocene varved sediments contain more abundant lipid‐rich “sapropelic” (type II) organic matter than the overlying bioturbated, oxidized Holocene sediments. The baseline of stable carbon isotopic composition of the organic matter in these slope cores does not change with time, indicating that the higher concentrations of type II organic matter in the varved sediments represent better preservation of organic matter rather than any change in the source of organic matter.
The Late Pleistocene history of surface water δ13C in the Sulu Sea: Possible relationship to Pacific Deepwater δ13C changes
Authors: Linsley Braddock K. , Dunbar Robert B.
Published: February 2004

A reconstruction of late Pleistocene surface water carbon isotopic (δ13C) variability is presented from Ocean Drilling Program (ODP) site 769 in the Sulu Sea in the western tropical Pacific. The Sulu Sea is a shallowly silled back arc basin with a maximum sill depth of 420 m. Site 769 was drilled on a bathymetric high in 3643 m of water and has average late Pleistocene sedimentation rates of 8.5 cm/kyr. The oxygen isotope record (δ18O) of Globigerinoides ruber at site 769 shows a strong correlation with the SPECMAP stacked δ18O record, attesting to the continuity of sediment archive at the site. Surface δ13C displays consistent glacial‐interglacial variability which averages ∼0.9‰ and has varied from 0.75 to 1.1‰ over the last 800 kyr. Comparison to surface water δ13C records in the South China Sea and western tropical Pacific suggests that the glacial‐interglacial surface δ13C variability is regional in scale. Planktonic δ13C data from ODP site 677 in the eastern Pacific is also coherent with the site 769. Additionally, we have found that the site 769 surface δ13C record is coherent at periods of 100 and 41 kyr with deepwater δ13C records from the Pacific. The highest correlation occurs with the deep eastern Pacific, where benthic δ13C data from cores RC13‐110 and ODP site 677 closely match the Sulu Sea surface water record. We evaluate several possible controls of surface water δ13C in the Sulu Sea that may explain the coherent timing with Pacific deepwater δ13C records. These include variations in terrestrial organic matter flux to the basin, the upwelling of subsurface water and productivity changes, and the influx of western Pacific intermediate water to the Sulu Sea. Our preferred explanation involves a region of upper intermediate water upwelling in the far western Pacific which has been shown to outgas CO2 from subsurface waters into surface waters. Upwelling also occurs in the area of Panama Basin site 677. These equatorial upwelling zones could potentially provide a route by which Pacific intermediate water can modulate the δ13C composition of certain Pacific surface water locations. Future reconstructions of late Pleistocene surface water δ13C variability in the western Pacific and Indonesian seas will be required to further evaluate the source of the glacial‐interglacial surface water δ13C change.
Changes in East Atlantic Deepwater Circulation over the last 30,000 years: Eight time slice reconstructions
Authors: Sarnthein Michael , Winn Kyaw , Jung Simon J. A. , Duplessy Jean‐Claude , Labeyrie Laurent , Erlenkeuser Helmut , Ganssen Gerald
Published: February 2004

Using 95 epibenthic δ13C records, eight time slices were reconstructed to trace the distribution of east Atlantic deepwater and intermediate water masses over the last 30,000 years. Our results show that there have been three distinct modes of deepwater circulation: Near the stage 3‐2 boundary, the origin of North Atlantic Deep Water (NADW) was similar to today (mode 1). However, after late stage 3 the source region of the NADW end‐member shifted from the Norwegian‐Greenland Sea to areas south of Iceland (mode 2). A reduced NADW flow persisted during the last glacial maximum, with constant preformed δ13C values. The nutrient content of NADW increased markedly near the Azores fracture zone from north to south, probably because of the mixing of upwelled Antarctic Bottom Water (AABW) from below, which then advected with much higher flux rates into the northeast Atlantic. Later, the spread of glacial meltwater over the North Atlantic led to a marked short‐term ventilation minimum below 1800 m about 13,500 14C years ago (mode 3). The formation of NADW recommenced abruptly north of Iceland 12,800–12,500 years ago and reached a volume approaching that of the Holocene, in the Younger Dryas (10,800–10,350 years B.P.). Another short‐term shutdown of deepwater formation followed between 10,200 and 9,600 years B.P., linked to a further major meltwater pulse into the Atlantic. Each renewal of deepwater formation led to a marked release of fossil CO2 from the ocean, the likely cause of the contemporaneous 14C plateaus. Over the last 9000 years, deepwater circulation varied little from today, apart from a slight increase in AABW about 7000 14C years ago. It is also shown that the oxygenated Mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30,000 years.
Ocean modeling in a global ocean observing system
Authors: Smith Neville R.
Published: February 2004

The oceanographic community is currently contemplating the design of a global ocean climate observing system to help monitor, describe, and understand the seasonal to decadal climate changes of the ocean and to provide the observations needed for climate prediction. This review attempts to define a role for modeling within that system, the central theme being that the observational and modeling elements must be developed in concert, with the presence of one enhancing the value of the other. Three distinct categories of model‐to‐data interface are identified. In the first class, models and data collection develop separately, being joined only by intermittent validation steps. In the second, and by far most important, class the model and data collection evolve together, either in a time‐space data assimilation and prediction system, or through the application of inverse methods. In the final category, model information feeds back to the observing system design, and vice versa, and the model assimilation system provides quality control on the data. The key role of (atmospheric) models in the determination of surface fluxes to drive ocean models is discussed. A nontrivial role is proposed for ocean models whereby they provide additional, and largely independent, constraints on atmospheric forecast system estimates. The role of ocean models in the analysis of surface and upper ocean fields needs to be developed, particularly with respect to salinity and nonphysical fields. The use of models in rationalizing the choice of observation platforms is discussed, together with some of the difficulties in interpreting such studies. The state of tropical ocean prediction is reviewed with particular emphasis on systems that assimilate subsurface temperature data. A range of thermocline models are also reviewed with the emphasis on subduction and the problem of initializing and constraining models that resolve mesoscale eddies. Some of the issues involved in matching the models to particular observational methods, and vice versa, are discussed with respect to tropical ocean and thermocline modeling. The current state of global ocean and coupled climate general circulation models and models for studying tracer circulation and coupled physical‐biological systems is evaluated. This prognostic path and the products and knowledge derived from integrations are contrasted with the inverse modeling approach which attempts to infer ocean circulation through a combination of observational and modeling constraints. Again we speculate on the model‐data interface and on the different measurement strategies and data requirements. The concept of community modeling and the need for substantial resources and international organization are discussed. A case for global ocean observing system centers with colocated modeling and data collection is made, but with model diversity and individuality emphasized.
Electromagnetic precursors to earthquakes in the Ulf band: A review of observations and mechanisms
Authors: Park Stephen K. , Johnston Malcolm J. S. , Madden Theodore R. , Morgan F. Dale , Morrison H. Frank
Published: February 2004

Despite over 2 decades of international and national monitoring of electrical signals with the hope of detecting precursors to earthquakes, the scientific community is no closer to understanding why precursors are observed only in some cases. Laboratory measurements have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed. However, field experiments have had much less success. Many purported observations of low‐frequency electrical precursors are much larger than expectations based on laboratory results. In some cases, no precursors occurred prior to earthquakes, or precursory signals were reported with no corresponding coseismic signals. Nonetheless, the field experiments are in approximate agreement with laboratory measurements. Maximum resistivity changes of a few percent have been observed prior to some earthquakes in China, but the mechanism causing those changes is still unknown. Anomalous electric and magnetic fields associated with fluid flow prior to earthquakes may have been observed. Finally, piezomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields.
Large igneous provinces: Crustal structure, dimensions, and external consequences
Authors: Coffin Millard F. , Eldholm Olav
Published: February 2004

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen‐Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0–7.6 km s−1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short‐lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid‐ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.
Transport of particles across continental shelves
Authors: Nittrouer Charles A. , Wright L. Donelson
Published: February 2004

Transport of participate material across continental shelves is well demonstrated by the distributions on the seabed and in the water column of geological, chemical, or biological components, whose sources are found farther landward or farther seaward. This paper addresses passive (incapable of swimming) particles and their transport across (not necessarily off) continental shelves during high stands of sea level. Among the general factors that influence across‐shelf transport are shelf geometry, latitudinal constraints, and the timescale of interest. Research studies have investigated the physical mechanisms of transport and have made quantitative estimates of mass flux across continental shelves. Important mechanisms include wind‐driven flows, internal waves, wave‐orbital flows, infragravity phenomena, buoyant plumes, and surf zone processes. Most particulate transport occurs in the portion of the water column closest to the seabed. Therefore physical processes are effective where and when they influence the bottom boundary layer, causing shear stresses sufficient to erode and transport particulate material. Biological and geological processes at the seabed play important roles within the boundary layer. The coupling of hydrodynamic forces from currents and surface gravity waves has a particularly strong influence on across‐shelf transport; during storm events, the combined effect can transport particles tens of kilometers seaward. Several important mechanisms can cause bidirectional (seaward and landward) transport, and estimates of the net flux are difficult to obtain. Also, measurements of across‐shelf transport are made difficult by the dominance of along‐shelf transport. Geological parameters are often the best indicators of net across‐shelf transport integrated over time scales longer than a month. For example, fluvially discharged particles with distinct composition commonly accumulate in the midshelf region. Across‐shelf transport of particulate material has important implications for basic and applied oceanographic research (e.g., dispersal of planktonic larvae and particle‐reactive pollutants). Continued research is needed to understand the salient mechanisms and to monitor them over a range of timescales.
Convection of polar cap patches observed at Qaanaaq, Greenland during the winter of 1989–1990
Authors: Fukui K. , Buchau J. , Valladares C. E.
Published: February 2004

During weak Bz or Bz < 0 conditions, antisunward convection dominates the central polar cap as shown by Digisonde drift measurements. During these conditions, polar cap F layer patches are observed routinely to drift antisunward within the overall plasma convection. A study is presented which compares patch motion derived from 630.0‐nm all‐sky intensified camera (ASIC) images taken at Qaanaaq, Greenland (87° CGL) with simultaneously obtained Digisonde drift measurements. During four periods of the winter 1989/1990 43 630.0‐nm patches were identified and followed in their motion across the ASIC field of view. The shape, velocity, and drift direction of the individual patches were determined. The patch motion was then compared with the Digisonde drift measurements. The drift direction is generally antisunward (±25°); both data sets are in excellent agreement, with a typical ±20° scatter around a common central value. The velocity magnitudes from both measurements show considerable variability, but both measurements generally cover the same velocity range. The variations and variability of the velocity magnitude are discussed in the context of simultaneous IMF measurements. Rapid changes in velocity were traced to changes in Bz. Deviations from antisunward of the azimuths of the optical patch drift and the Digisonde drift measurements were controlled by IMF By in agreement with published By control of convection.
Chaotic echoes on ionograms: Underdense scattering or total reflections?
Authors: Wright J. W. , Argo P. E.
Published: February 2004

Apart from spread F and some forms of sporadic E, little attention is usually paid to the variable occurrence of “chaotic” or disordered “scatter” in ionosonde measurements. In contrast to the echoes of broad radio bandwidth and high amplitude from the plane‐stratified ionosphere, for which total internal reflection is unmistakably applicable, the processes responsible for occasional scattering are not obvious. With the multiparameter information of a digital ionosonde it should be possible to develop criteria by which the echoing processes can be decided, if not on an echo‐by‐echo basis, at least for groups of echoes that bear similar properties. The distinction is important since these chaotic echoes may be identified with significant physical processes, whereupon the ionosonde becomes one of their more powerful diagnostics. Three examples illustrate these points; they arise in consequence of (1) the (magnetic) equatorial electrojet; (2) some suggested neutral‐atmosphere interactions near sporadic E at mid latitudes; and (3) the auroral electrojet, respectively.
HF radio field strength and total propagation invariants
Authors: Tsedilina E. E.
Published: February 2004

This paper investigates the relationship between measured field strengths, observed over two midlatitude long‐distance paths, and total adiabatic invariants calculated for all possible propagation channels, for equinoxes and for low and high solar activity. Communication channel invariants or channel volumes of all types of channels have been calculated for a frequency of 15 MHz using the EMI‐81 ionospheric model for both simple channels (F, E, FE) made up of conventional hop trajectories (Fh, Eh) refracted by ionospheric layers, and ricochet, or chordal, trajectories (Fr, Er, FE) which propagate in stratification ducts within the ionospheric layers, or combinations of these channels. It is shown that under night and twilight (day‐night) conditions the field strength, in general, is proportional to the total channel volume: E ≈ (IΣ) n, where n = 0.5 to 2. This indicates the strong influence of multiple scattering by irregularities on the processes of capture, loss, and radio wave propagation in ionospheric waveguide channels. This is in accordance with the results of using ray diffusion theory and adiabatic approximation, where the horizontal character of the waveguide channel varies slowly in relation to the oscillation of the ray within the duct. Greater field strengths observed during sunset, when the terminator was moving along the path, are explained by the influence of the larger wave channel volumes at this time in comparison with other periods.
Resolution degradation parameters of ionospheric tomography
Authors: Na Helen R. , Lee Hua
Published: February 2004

Tomographie reconstruction of the electron density structure in the ionosphere has generated much interest due to the possibility of high‐resolution two‐dimensional image reconstruction using total electron content (TEC) data. An algorithm based upon the filtered backprojection algorithm of X ray tomography that provides such two‐dimensional reconstructions has been developed; however, the resolving capability is limited by the nature of the ionospheric system. In ionospheric tomography, each piece of data contains information similar to a sample on a projection in X ray tomography. Collectively, however, the information content of ionospheric data sets is significantly different from a set of tomographic projections. These differences are the result of characteristics of the data acquisition system and are directly related to the resolution limits of the reconstructed images. In this paper the data acquisition system is analyzed to determine the sources of this degradation. Two key parameters of nonuniform sample spacing and angular reindexing error are analyzed to determine their effect upon the reconstruction process. A method of evaluating sampling schemes to measure the information content of the resulting projections is presented. Finally, a relationship between the angular reindexing error and the point spread function of the imaging system is derived.
Ionospheric refraction effects in slant range profiles of auroral HF coherent echoes
Authors: Uspensky M. V. , Kustov A. V. , Sofko G. J. , Koehler J. A. , Villain J. P. , Hanuise C. , Ruohoniemi J. M. , Williams P. J. S.
Published: February 2004

The theory of auroral coherent echoes developed for VHF scattering by Uspensky et al. (1988, 1989) is applied to the interpretation of intensity and Doppler velocity slant range profiles of HF radar aurora. The theoretical model includes the effects of irregularity aspect sensitivity, ionospheric refraction of the radar beam, and the reception of signals from different heights. The predicted profiles of HF radar aurora are compared with Schefferville HF radar observations in the frequency interval of 9–18 MHz. Satisfactory agreement is found between theory and experiment for the intensity profiles. However, there are significant discrepancies for the Doppler velocity profiles. We discuss this lack of agreement in light of other recent observations.
Structure, metamorphism, and geochronology of the Cosmos Hills and Ruby Ridge, Brooks Range schist belt, Alaska
Authors: Christiansen Peter P. , Snee Lawrence W.
Published: February 2004

The boundary of the internal zones of the Brooks Range orogenic belt (the schist belt) is a fault contact that dips toward the hinterland (the Yukon‐Koyukuk province). This fault, here referred to as the Cosmos Hills fault zone, juxtaposes oceanic rocks and unmetamorphosed sedimentary rocks structurally above blueschist‐to‐greenschist facies metamorphic rocks of the schist belt. Near the fault contact, schist belt rocks are increasingly affected by a prominent, subhorizontal transposition foliation that is locally mylonitic in the fault zone. Structural and petrologic observations combined with 40Ar/39Ar incremental‐release geochronology give evidence for a polyphase metamorphic and deformational history beginning in the Middle Jurassic and continuing until the Late Cretaceous. Our 40Ar/39Ar cooling age for Jurassic metamorphism is consistent with stratigraphic and other evidence for the onset of Brooks Range orogenesis. Jurassic metamorphism is nearly everywhere overprinted by a regional greenschist‐facies event dated at 130–125 Ma. Near the contact with the Cosmos Hills fault zone, the schist belt is increasingly affected by a younger greenschist metamorphism that is texturally related to a prominent foliation that folds and transposes an older fabric. The 40Ar/39Ar results on phengite and fuchsite that define this younger fabric give recrystallization ages ranging from 103 to less than 90 Ma. We conclude that metamorphism that formed the transposition fabric peaked around 100 Ma and may have continued until well after 90 Ma. This age for greenschist metamorphism is broadly synchronous with the depositional age of locally derived, shallow‐marine clastic sedimentary strata in the hanging wall of the fault zone and thus substantiates the interpretation that the fault zone accommodated extension in the Late Cretaceous. This extension unroofed and exhumed the schist belt during relative subsidence of the Yukon‐Koyukuk province.
Reevaluation of Holocene faulting at the Kaysville site, Weber segment of the Wasatch fault zone, Utah
Authors: McCalpin James P. , Forman Steven L. , Lowe Mike
Published: February 2004

The 1978 Kaysville, Utah, trench excavated by Swan and others (1980) across a large graben of the Weber segment of the Wasatch fault zone was reexcavated in 1988 to reevaluate the timing and nature of Holocene faulting. Relogging of the trench reveals evidence for five or six faulting events younger than the Provo phase of Lake Bonneville (circa 13,000 14C years B.P.). Geometric reconstruction of net vertical offset in the last three events suggests a variation in coseismic vertical displacement at this site, ranging from a net of 1.4–3.4 m per event. The three latest faulting events occurred at shortly before 0.6–0.8 ka, 2.8 ± 0.7 ka, and circa 3.8–7.9 ka. Earlier events cannot be directly dated because older graben‐fill sediments yielded thermoluminescence ages older than the time of deposition, and some scarp‐derived colluvial wedges beneath the trench floor were not exposed. The two younger faulting events we recognize at Kaysville correlate reasonably well with faulting events on the same segment 25 km north near East Ogden, Utah, at circa 0.8–1.2 ka and 2.5–3.0 ka (Forman and others, 1991), whereas the earlier Kaysville event is significantly older than the earliest (3.5–4.0 ka) event dated at East Ogden. The 3.5–4.0 ka ground rupture recognized at East Ogden may have died out at a subsegment boundary between the two trench sites within the 61‐km‐long Weber segment.
Lower crustal thinning in the Rhinegraben: Implications for recent rifting
Authors: Echtler Helmut Peter , Lüschen Ewald , Mayer Gunter
Published: February 2004

Seismic near‐vertical and undershooting experiments carried out in the southern upper Rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties. Significant differences in the apparent thickness of the reflective lower crust and the transparent upper crust appear to be related to different structural settings. A 12‐ to 14‐km‐thick reflective lower crust beneath a 15‐km‐thick transparent upper crust of the eastern graben shoulder (Black Forest) probably originated during Permo‐Carboniferous reequilibration of thickened Variscan orogenic crust. Thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder (the Dinkelberg block) is interpreted to be related to Cenozoic extensional faulting. A discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupling at depth during extension. Congruent modification of other physical properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust. In the transitional Dinkelberg block, such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity; it is interpreted to be the presently active zone of decoupling which in time shifted from the rift axis to the eastern transition area.
Basin genesis associated with strike‐slip faulting in the Basin and Range, southeastern Nevada
Authors: Campagna David J. , Aydin Atilla
Published: February 2004

The structural nature of two prominent Basin and Range strike‐slip systems, the Lake Mead fault system and the Las Vegas Valley shear zone, are examined using both geological field mapping and geophysical surveys at various scales. The intent of this study is to document deformation directly attributable to strike‐slip faulting with an emphasis on basin genesis. Our field study of the Lake Mead fault system reveals a major left step in the system. The stepover results in the genesis of a 2‐km‐deep pull‐apart, the Overton Arm Basin, consistent with the kinematics and mechanics of strike‐slip faulting. Having established the origin of the Overton Arm basin as a strike‐slip‐related pull‐apart basin, the geometry of poorly exposed strike‐slip systems can be revealed by understanding the related basins along their length. One such system, the Las Vegas Valley shear zone, is shown with the aid of geophysical data to be geographically related to three distinct en echelon basins. When these basins are interpreted as pull‐apart basins, the geometry of the Las Vegas Valley shear zone becomes apparent for the first time. We assert that strike‐slip faulting in southeastern Nevada is a deformational mechanism that contributes locally to the regional extensional tectonics. Such an interpretation modifies extant models that treat strike‐slip faults in rifted terrains as passive lateral boundaries that simply accommodate extension without contributing to the process.
Shortening of analogue models of the continental lithosphere: New hypothesis for the formation of the Tibetan plateau
Authors: Burg J.‐P. , Davy Philippe , Martinod Joseph
Published: February 2004

Initial stages of compression produce periodic buckling of models analogous to a four‐layer continental lithosphere. With further shortening, amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle. The rescaled results suggest that a continental lithosphere under compression will buckle with a first‐order wavelength of about 200 km and a second‐order wavelength of 20–30 km. Continued shortening results in the amplification of undulations. Troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle, thus giving rise to an irregular topography of the Moho. Fifty percent shortening results in a 60‐km‐thick continental crust. We argue that this model history may explain the elevation and deep crustal structure of the Tibetan plateau.
Precambrian basement structure and Laramide deformation revealed by seismic reflection profiling in the Laramie Mountains, Wyoming
Authors: Speece M. A. , Frost B. R. , Smithson S. B.
Published: February 2004

The University of Wyoming conducted seismic reflection profiling in the Laramie Mountains of Wyoming to investigate the nature of Precambrian crustal structure and Laramide deformation in the region. Here the imaging of layering in the Laramie Anorthosite complex confirms that layered intrusions can be reflective and establishes an analog for interpreting events in other seismic reflection data sets where layered intrusions could be the cause of multicyclic events. Moreover, the presence of prominent primary layering in the Complex supports a conclusion that it formed in situ and not as the result of the emplacement of a diapiric crystal mush. Imaging of Laramide thrusts in the Laramie Mountains adds credence to the idea that brittle faults can be reflective in some situations. Migrated apparent dips on these faults are 30° westward. Basement‐involved compressive structures, consisting of monoclinal folding and a fault triple junction, demonstrate that compression controlled the style of deformation during the Laramide orogen in Wyoming. West dipping Precambrian structures imaged in these data have a similar attitude to the Laramide structures, suggesting that Laramide deformation was influenced by Precambrian features. These Precambrian structures may be subthrust slices of exotic Proterozoic terranes.
The influence of preexisting structure on the evolution of the Cenozoic Malawi rift (East African rift system)
Authors: Ring Uwe
Published: February 2004

This paper analyzes the importance of preexisting structure for the evolution of the Cenozoic Malawi rift, which constitutes the southernmost part of the western branch of the East African rift system. Kinematic analyses demonstrate that the regional extension direction rotated clockwise from ENE to SE during rifting. Cenozoic rift faults (of dip‐, oblique‐, and strike‐slip character) rejuvenated crustal structures whenever those were properly oriented with respect to the extension directions. Depending on these boundary conditions the rift attempts to find the easiest pathway in order to develop in a mechanically modest way. Proterozoic basement structures represent the basic anisotropy which already influenced pre‐Cenozoic rifts and also the Cenozoic Malawi rift. Amongst these basement structures, Pan‐African retrograde to cataclastic, strike‐slip shear zones have the strongest influence on later rift structures. Rift‐related transfer faults appear as differently striking passive structures, and their orientation seems to be controlled by a variety of steep basement structures (retrograde shear zones, foliation planes, and fracture arrays).
Postmylonitic deformation in the Raft River metamorphic core complex, northwestern Utah: Evidence of a rolling hinge
Authors: Manning Andrew H. , Bartley John M.
Published: February 2004

Much of the recent debate over low‐angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model. The model predicts tilting of seismogenic high‐angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation. This shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex. The predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms. Two end‐members have been proposed: subvertical simple shear and flexural failure. Each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction. Subvertical simple shear (SVSS) should generate subvertical faults and kink bands with a shear sense antithetic to the detachment. For an SVSS hinge, the hinge‐related strain magnitude should depend only on initial fault dip; rolling hinge structures should shorten the mylonitic foliation by >13% for an initial fault dip of >30°. In flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses. Resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation. Extensional sets could predominate as a result of superposition of far‐field and flexural stresses. Strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam. Postmylonitic structures were examined in the footwall of the Raft River metamorphic core complex in northwestern Utah to test these predictions. Observed structures strike perpendicular to the regional extension direction and include joints, normal faults, tension‐gash arrays, and both extensional and contractional kink bands. Aside from the subvertical joints, the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets. Range‐wide, the extensional structures accomplish about 4% elongation of the mylonitic foliation. Contractional structures dip steeply, mainly record shear antithetic to the detachment, and accomplish <1% contraction of the foliation. These observations are consistent with the presence of a rolling hinge in the Raft River Mountains, but a rolling hinge that reoriented a high‐angle normal fault by SVSS is excluded. The pattern and magnitudes of strain favor hinge‐related deformation mainly by flexural failure with a subordinate component of SVSS.
The Falkland Islands: A key element in Gondwana paleogeography
Authors: Marshall J. E. A.
Published: February 2004

Stratigraphic and sedimentological fieldwork on the Falkland Islands confirms that they were originally part of South Africa. East and West Falkland have contrasting geological and deformation histories and are separated by the Falkland Sound fault, which marks a significant deformation edge to the Cape Fold Belt. The degree of structural deformation and metamorphism in West Falkland is substantially less than that of East Falkland, adjustments to differential shortening between the two islands being reflected by structures developed along the Falkland Sound fault. Prior to fragmentation of Gondwana, the Falkland Platform and Maurice Ewing Bank were positioned in the Natal Valley, adjacent to the east coast of South Africa. This predrift reconstruction shows that the Falkland Islands have subsequently been rotated by 180° relative to South Africa and complements recent paleomagnetic data. There was a complete overlap of the Maurice Ewing Bank and Falkland Platform to the exclusion of the Falkland Plateau Basin which is a small oceanic basin formed during the Jurassic fragmentation of Gondwana. Extension in this basin was accommodated by movements along the Aghulas Falkland Fracture Zone and the Gastre Fault System. Predrift restoration of these faults shows that Patagonia was positioned closer to both Africa and Antarctica. The Falkland Plateau Basin was an integral component of the West Gondwana Jurassic microplate system and provides an invaluable insight into the evolution of its western margin. At some time prior to the main opening of the South Atlantic, the Falkland Platform was rotated clockwise by 120° against the Agulhas Falkland Fracture Zone, such that the Falkland Islands were translated some 300 km to the south. The driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the Falkland Plateau Basin as the Gastre Fault System, within continental crust, became unable to accommodate the continuous dextral shear in the Agulhas Falkland Fracture Zone. The Falkland Islands represent an outboard extension of the Eastern Cape and are significant for understanding the paleogeography of both the Cape and Karoo supergroups. East Falkland is closely similar to South Africa in its geology, while West Falkland shows significant contrasts with both the Devonian and Permian successions being more proximal in character but still demonstrate the eastward continuation of the Cape and Karoo basins.
Fold‐fault relationships in low‐angle detachment systems
Authors: Mancktelow Neil S. , Pavlis Terry L.
Published: February 2004

A close kinematic and timing relationship between low‐angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well‐exposed Neogene detachment systems, in the Simplon region of the Alps and in the Death Valley region of California. Fold amplitude is largest in the oldest mylonitic foliation of the footwall. These folds are transected by younger mylonitic foliation(s) and cataclastic detachment faults which are themselves folded about similar axes. In general, the amplitude decreases and the wavelength increases for progressively younger fold structures, consistent with a history of progressive folding during faulting. The interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes. This strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning.
Late Neogene kinematics of intra‐arc oblique shear zones: The Petilia‐Rizzuto Fault Zone (Calabrian Arc, Central Mediterranean)
Authors: Van Dijk J. P.
Published: February 2004

The kinematics of intra‐arc shear zones play a key role in the secondary shaping of orogenic arcs such as the Calabrian Arc (central Mediterranean). Comparison of the Neogene structural development of the Petilia‐Rizzuto Fault Zone and the basement structure of the bordering Sila massif reveals that the fault zone is the surface expression of a deep NW–SE trending sinistral crustal oblique shear zone. This shear zone continues over a length of more than 130 km across the northern segment of the Calabrian Arc and shows a post‐Eocene sinistral displacement of about 50 km. The late Neogene forearc basin development and syndepositional tectonics along the fault zone are reconstructed in great detail by analyzing the middle Miocene‐Recent tectonic sequence stratigraphy. A strike‐slip cycle can be recognized whereby the subsequent activity of Riedel shears, tensional faults, and P shears, positive flower structures and principle displacement wrench faults, can accurately be traced in time. Observed phenomena are discussed in terms of the activity of a conjugate system of oblique thrust zones within the growing accretionary complex. The evolution of special types of thrust belt basins is illustrated. These include oblique thin‐skinned pull‐apart basins, oblique rhomboidal “harmonica” basins, and “detached slab” basins (new terms introduced here), evolving one into the other. A new feature illustrated is the recurrent basin inversion which generated passive roof duplexes through back‐shear motion and out‐of‐sequence thrusting along the wedge. The fault patterns and the style of inversion tectonics imply an E–W directed axis of effective compressive stress in this part of the arc. This resulted from an interaction of (1) local E–W directed compression related to a differential displacement of two parallel segments of the arc (generated by the migration to the southeast of the Calabrian Arc and opening of the Tyrrhenian backarc basin); (2) alternating NW–SE directed compression and extension (related to pulsating thrust wedge dynamics with phases of accretion and underthrusting respectively) and (3) regional, compressive interplate stress (middle Messinian‐middle Pliocene). All structures are overprinted by post middle Pleistocene extensional faulting (related to rapid uplift of intra‐arc massifs) and reversal along thrust planes and transcurrent faults. This extensional collapse reflects isostatic adjustments in response to plate rupture which was provoked by regional compressive stress.
Brunswick subduction complex in the Canadian Appalachians: Record of the Late Ordovician to Late Silurian collision between Laurentia and the Gander margin of Avalon
Authors: van Staal Cees R.
Published: February 2004

The Brunswick subduction complex in the New Brunswick part of the Canadian Appalachians records the Late Ordovician to Late Silurian collision between Laurentia and the Gander margin of Avalon. The Brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the Quebec reentrant of the Laurentian margin. This part of the margin experienced less underthrusting and exhumation and overprinting by orogen‐parallel faulting than the adjacent promontories where collision started earlier. The early, southeast to east vergent thrust‐related structures represent a progressive D1 deformation that formed in response to northwestward subduction of the previously extended Gander margin and subsequent tectonic unroofing of the subduction complex. The original, shallow northwestward dipping envelope to S1 was deformed in the Late Silurian by D2 upright folds and associated shear zones into a steep belt during terminal collision. The D2 structures probably formed in response to sinistral transpression. Together, D1 and D2 indicate that convergence was oblique and sinistral. Most rocks incorporated in the subduction complex formed in the Tetagouche back arc basin that evolved from rifting of an Arenig magmatic arc built on the Gander margin into a wide marginal basin. Subduction was initiated in the Late Ordovician (≈455 Ma) in the back arc basin following collision of the Middle Ordovician Popelogan arc with Laurentia in the Caradoc, shortly after its opening in the late Arenig (≈473 Ma). The closure history of the Iapetus Ocean involved more than one subduction zone and arc‐continent collision and rivals the southwestern Pacific Ocean in its complexities.