Wedge Structures Research Articles

Turbulent flow structure in a gravel‐bed river: Markov chain analysis of the fluctuating velocity profile

AbstractThree electromagnetic current meter probes were deployed in a Canadian gravel‐bed river to obtain simultaneous records at 10 Hz of streamwise (u) and vertical (v) velocity components at three heights above the bed. By looking at the positive and negative signs of the instantaneous fluctuations from the time‐average values of each velocity component at each height, the fluctuating velocity profile of u or v can be treated as a Markov chain with eight states and its statistical properties can be tested against null hypotheses based on the absence of spatial structure. We report results of this novel approach.The most common states of the u profile were those with either higher‐than‐average or lower‐than‐average velocities at all heights; these ‘high speed’ and ‘low speed’ states persisted for up to 3 s. The most common v profiles were all‐upwards or all‐downwards, but these persisted for shorter times than the high speed and low speed u profiles. Analysis of transition probabilities shows statistically significant tendencies for acceleration from the low speed u profile, and change from all‐upwards to all‐downwards v profile, to take place progressively from the uppermost probe downwards, in a sweep‐like way. Deceleration from the high speed to low speed u profile and change from all‐downwards to all‐upwards v profile (burst‐like behaviour) do not show such clear patterns.The results are interpreted in terms of the advection of inverted wedges of relatively high‐momentum fluid, followed by more chaotic structures. A separate set of flow visualization experiments over a mixed gravel bed in a flume supports the presence of advected wedge structures, the decelerating part of the sequence corresponding to irregular ejections of near‐bed fluid.

Magnetic and structural instabilities of ultrathin Fe(100) wedges (invited)

An overview is provided of recent efforts to explore magnetic and related structural issues for ultrathin Fe films grown epitaxially as wedge structures onto Ag(100) and Cu(100). Experiments were carried out utilizing the surface magneto-optic Kerr effect. Ordinary bcc Fe is lattice matched to the primitive unit cell of the Ag(100) surface. Fe wedges on Ag(100) can be fabricated whose thick end has in-plane magnetic easy axes due to the shape anisotropy, and whose thin end has perpendicular easy axes due to the surface magnetic anisotropy. A spin-reorientation transition can thus be studied in the center of the wedge where the competing anisotropies cancel. The goal is to test the Mermin–Wagner theorem which states that long-range order is lost at finite temperatures in an isotropic two-dimensional Heisenberg system. Fe wedges on Cu(100) can be studied in like manner, but the lattice matching permits fcc and tetragonally distorted fcc phases to provide structural complexity in addition to the interplay of competing magnetic anisotropies. The results of these studies are new phase identifications that help both to put previous work into perspective and to define issues to pursue in the future.

Antiferromagnetic versus ferromagnetic coupling in Fe/Cr(107) and Cr/Fe(107)

We have calculated the local magnetic moments and magnetic order for a Fe(Cr) monolayer adsorbed on a stepped Cr(Fe)(107) substrate. The electronic structure at T=0 K has been self-consistently determined within the unrestricted Hartree–Fock approximation of the Hubbard Hamiltonian in the framework of a real-space tight-binding method. In the Cr/Fe(107) system, two magnetic arrangements have been obtained, the more stable being the less frustrated as obtained in the case of V overlayers on vicinal substrates of Fe. An analysis of both solutions in terms of the total energy calculation and the different degree of frustration is presented. For Fe/Cr(107), a two-step periodicity is obtained. The sign of the magnetization at the Fe overlayer changes from step to step. This spin-flop transition is consistent with the two-layer period oscillation recently observed in Fe/Cr/Fe wedge structures, and with the total magnetization determined from in situ magnetometer measurements during growth of ultrathin Fe films on Cr(001).

Gloria Mosaic of the Gulf of Alaska and the British Columbia Margin: Deep-Sea Channels, Margin Deformation, and the Queen Charlotte Fault: ABSTRACT

GLORIA images collected from 1986 to 1989 show sea-floor morphology from the shelf break seaward to 400 km in the Gulf of Alaska and a 70-km-wide swath along British Columbia. Along the Aleutian convergent margin sediment is dominantly trapped in mid-slope basins, where few canyons reach the trench. Accretionary wedge structures range from highly discontinuous to long and continuous. The Yakutat transition margin is either extensively cut by dendritic drainages or, at sea-valley mouths, covered by glacially derived sediment. Young structures underlie the slope from Middleton Island to Pamplona Spur, but are absent from Pamplona Spur to Cross Sound. Along the southeast Alaska transform margin the Queen Charlotte fault is imaged as a narrow linear feature. The fault steps westward at Tuzo Wilson Knolls, which likely is a spreading ridge segment. Large anticlines lie seaward of and trend parallel to the fault. On the abyssal plain off the Shumagin margin inherited structural and bathymetric features trend parallel to magnetic anomalies, and trench parallel features reflect faulting as the ocean plate bends into the trench. To the north, three turbidite systems drain the margin. The Surveyor system begins between Pamplona Spur and Alsek Canyon and empties into the Aleutian Trench. Themore » Chirikof system arises near Cross Sound and ends in turbidite fans south of the Kodiak-Bowie Seamount chain, a relic Chirikov channel that once carried sediment westward to the Aleutian Trench. The Mukluk and Horizon channels start along southeast Alaska and end 1,000 km away on the Tufts abyssal plain.« less

Structural model for the Saxothuringian-Moldanubian suture in the Variscan basement of the Oberpfalz (Northeastern Bavaria, F.R.G.) interpreted from geophysical data

The Moldanubian-Saxothuringian boundary ∗ ∗ As defined by Kossmat. marks a prominent subduction zone in the internal parts of the European Variscides. In the Oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process. Seismic structures and related data from surface geology suggest that the Moldanubian-Saxothuringian collision consisted of two main phases. The first phase is marked by extensive NW-directed overthrusting of Moldanubian onto Saxothuringian crust along SE-dipping master décollements, some of which can be traced down to the Moho. During an advanced stage progressive overthrusting led to the development of wedge structures associated with SE-directed backthrusting and backfolding in the upper structural levels. Subsequently some of these wedges and their related décollements were folded to form the main NE-striking anticlines. The first phase of crustal imbrication correlates with a regional low-pressure-high-temperature metamorphism (at about 320 Ma) decreasing in grade northwestward. The second phase is characterized by a reorientation of the main direction of crustal shortening from NW-SE to SW-NE which is interpreted as the result of the N-directed indentation of the Vindelician terrane into the Moldanubian. This event is recorded by conjugate systems of overthrusting and subsequent strike-slip faulting in the Oberpfalz and the Bavarian Forest and the Black Forest-Vosges area. In the area of the southern German block between these basement outcrops the indentation front is marked by the arc-shaped axes of gravimetric and magnetic anomalies and Permo-Carboniferous troughs.

Quartäre Tektonik und ihre Auswirkungen auf die Erstreckung von Würgeböden und Keilstrukturen

Quartäre Tektonik und ihre Auswirkungen auf die Erstreckung von Würgeböden und Keilstrukturen

Alternative model of thrust-fault propagation

A widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory. We propose an alternative model that applies Gretener's beam-failure mechanism to a multilayered sequence. Invoking compatibility conditions, which demand that a thrust propagate both upsection and downsection, we suggest that ramps form first, at shallow levels, and are subsequently connected by flat faults. This hypothesis also explains the formation of many minor structures associated with thrusts, such as backthrusts, wedge structures, pop-ups, and duplexes, and provides a unified conceptual framework in which to evaluate field observations.

Precambrian permafrost horizons as indicators of palaeoclimate

Abstract Periglacial phenomena such as polygonal wedge structures and microrelief features commonly preserved in fossil permafrost horizons can provide valuable information on palaeoclimate, including indications of mean annual air temperature, seasonal temperature range, relative precipitation, and evidence for long-term (∼ 104 years) oscillations of climate. Such features are known from early and late Proterozoic rocks and indicate mean annual air temperatures as low as −12 to −20°C or lower, and strongly seasonal periglacial climates. These palaeoclimatic data, together with information on palaeolatitude and proximity to palaeo-sea level, may impose strong constraints on palaeoclimate models for Proterozoic glaciation.

Fossil Ice and Sand Wedges in South-Central Ulster, Northern Ireland

Spatial and temporal variations in the development of periglacial wedge structures in south-central Ulster. Northern Ireland can be related to climatic change during the earlier part of the late Glacial period. Ice wedges formed in the early stages of deglaciation (c.18,000 — 14,000 yrs B.P.) when cold and relatively humid periglacial conditions prevailed. Sand wedges formed during the later stages of deglaciation (c. 14,000 — 13,500 yrs B.P.?) and suggest a change to colder (?) and relatively drier climatic conditions.

Polygonal patterned ground in central New Jersey

Polygonal patterned ground and associated sediment-filled wedges occur in thin-bedded shale in central New Jersey. During a dry growing season, numerous areas of polygonal ground were visible owing to differential growth of vegetation over the sediment-filled fractures. Construction sites where material was removed from the surface prior to excavation also revealed areas of polygonal ground. Measurements of the patterns show networks of polygons with diameters ranging from about 3 m to over 30 m, with an average of approximately 20 m. The wedges examined in vertical exposures range in depth from 25 to 260 cm (average, 125 cm) and in width from 10 to 240 cm (average, 50 cm). The infilling material of the wedges is mostly a sandy loam, although this material is not homogeneous throughout the wedges. The presence of ventifacts and frosted sand grains within the wedges indicates eolian activity during the formation of these features. The polygonal ground and wedge structures exhibit several characteristics similar to those of ice-wedge casts, and available evidence suggests they may have originated as ice-wedge polygons during the last glacial maximum.

Late Quaternary fan gravels and slope deposits at Rocky Cape, northwestern Tasmania: their palaeoenvironmental significance

The paper describes a sequence of stratified quartzite slope deposits and alluvial fan gravels from a small catchment at Rocky Cape. A lower series of indurated slope deposits overlain by a podzol soil profile suggests that cold climatic conditions, which favoured frost fracturing of exposed 13 rock surfaces and gelifluction of detritus, occurred prior to the Last Glacial Stage. An upper series of unconsolidated slope deposits contains wedge structures. These deposits appear to have resulted from frost fracturing and to have been moved downslope by gelifluction processes. The wedge structures may have been produced by seasonal freezing of the ground surface during the period of maximum cold in the Last Glacial Stage (after 22,000 and before 10,000 BP). Radiocarbon dating of charcoal and wood from organic horizons and a buried soil in the lower part of a thick sequence of alluvial fan gravels shows that they began to accumulate mainly between about 33,000 and 22,000 BP, when strong but episodic erosion of the open Eucalyptus forested catchment occurred. The upper part of the alluvial fan gravels does not contain organic horizons but probably accumulated after 22,000 and before 10,000 BP. The gravels were largely derived from the upper series of slope deposits which were formed in a non-forested environment.

Periglacial structures in the Aberystwyth region of Central Wales

Examples of well-exposed involutions and associated vertical stones are described. The detail of the involutions suggests that they were not formed by a sorting of heterogeneous material, but by downward pressure developed in the active layer during regelation. It is doubted that they ever formed patterned ground. The wedge structures, as in many areas, are dissociated from involutions and show a lack of up-arching of the beds on either side. The distribution of all these structures does not show any breaks at previously suggested ice-front positions in the area.

Some Periglacial Features in Central Montana

Two types of structures represent the results of former intensive frost action in central Montana. Involutions, which consist of local deformation and interpenetration in stratified materials, are common throughout the area and occur in a variety of forms. They are believed to have developed by differential freezing and growth of ground-ice, in a seasonally thawed zone overlying perennially frozen ground. Wedge structures, found at a few localities, consist of wedge-shaped vertical fissures in weathered bedrock. The fissures form polygonal networks and have been filled by slumping of surface materials. They are believed to have been opened by ice veins developed when the ground became perennially frozen. A mean annual temperature at least 8° C. below that of the present would be required for the development of perennially frozen ground. This period of colder climate probably occurred at the time of the last glaciation of adjoining areas, which may belong to the Middle Wisconsin substage.