Main Trough Research Articles

Rift processes at the Valles Marineris, Mars: Constraints from gravity on necking and rate‐dependent strength evolution

Recent spherical harmonic representations of the gravity field of Mars have sufficient resolution to examine the regional crustal and lithospheric structure of the Valles Marineris and are used as constraints for rift modeling. While gravity signatures of individual troughs are not evident, a broad 190–260 mgal low indicates that the central chasmata as a whole are compensated at a depth of 30–80 km, representing the thickness of crust surrounding the troughs. Furthermore, at the time large‐scale relief was established, the effective thickness of the elastic lithosphere was <30 km, corresponding to heat flow >20 m W m−2. The calculated range of crustal thickness and heat flow can be used to test two semianalytic models of rift formation. In the first model, the distinction between single (“narrow rift”) and multiple (“wide rift”) troughs is determined by the wavelength of necking instabilities. In the second model, narrow‐versus‐wide morphology is controlled by the evolution of lithospheric strength during rifting. The large‐scale, parallel, multiple troughs of the central Valles Marineris are morphologically similar to terrestrial wide rifts, but a lack of distinct faulting in some of the chasmata could imply that the faulted troughs may have formed as isolated narrow rifts. Our results are only marginally consistent with necking leading to a wide rift; allowable combinations of crustal thickness and heat flow lead to decoupling within the lithosphere resulting in a second principal necking wavelength that is smaller than the main trough spacing, for which evidence is equivocal. At high heat flow (>40 m W m−2), however, necking of the strong upper crust alone can yield a shorter wavelength characteristic of a single trough, allowing narrow‐rift origins of faulted chasmata. In contrast, the inferred range of crustal thickness and heat flow poorly match the narrow‐rift regime of the strength‐evolution model but are in good agreement with its predictions for wide rifting. Furthermore, the distinction between wide rift and core complex in this model places an upper bound on heat flow of 70 m W m−2.

Detection of the Southern Hemisphere Main Trough, Effects of Auroral Activity and Nighttime Tec Enhancements with GPS

Total electron content (TEC) values are obtained from measurements of the Gobal Positioning System (GPS) satellite signals at the Australian and sub-Antarctic receiver stations situated at mid and high latitudes in the southern hemisphere. A technique was developed to derive GPS TECs from raw data files. The reduced data of 1995 and 1996 were used to investigate the diurnal and seasonal variations of the TEC in the region. Various features were also studied such as the mid-latitude trough, the effects of auroral activity on GPS recording and nighttime TEC enhancements. The latitudinal motion of the mid-latitude through was observed as well as the magnetic activity dependence of the geographic location of the auroral oval. The seasonal variation in the time of occurrence of nighttime TEC enhancements was also investigated. The GPS findings were compared with TEC from TOPEX/POSEIDON satellite and f0F2 data. Theoretical TECs were obtained by employing an ionospheric model called PIM (Parameterized Ionospheric Model), and results were compared with experimental values.

Two branches of day-time winter ionospheric trough according to Cosmos-900 data at F2-layer heights

With the large data set of the Cosmos-900 satellite it is shown that in the low-altitude day-time winter ionosphere, besides the high-latitude trough two other troughs are often observed: the “daytime main trough” and the “Muldrew trough”. The first trough is observed in the shadow region as a continuation of the nighttime main trough close but equatorwards of the auroral oval. The second trough is observed at sunlit longitudes. Developing in the stagnation region (at 17–18 MLT) it is then transported by the westward drift into the day-time sector of the auroral oval, but equatorwards of the day-side cusp. The variations of their characteristic features are different in the Northern and Southern hemispheres creating an asymmetry of the high-latitude ionospheres.

Main ionospheric trough as a channel for MF propagation in the magnetosphere

By interpretation of the long‐delayed echoes (L=3.2) observed on the frequency 1.8 MHz, the most probable mechanism of propagation was found to be wave guiding in the odd magnetospheric channel such as the main ionospheric trough. The method of numerical simulation of the middle wave propagation allowed us to formulate (1) the conditions of trapping and keeping waves in a channel and (2) the relations between the positions of the transmitter, the trough, and the plasmapause corresponding to both the echo appearance and the wave passing to the magneto‐conjugate region as far as the Earth's surface. Similar conditions and relations are satisfied in a wide range of frequencies and L shells of the transmitter and may be used for finding the position of the trough and the plasmapause.

Imaging of electron density troughs by tomographic techniques

Troughs in the latitudinal distribution of electron density are a well‐known feature of the ionosphere from subauroral to polar latitudes. The location and depth of the trough minimum, the width of the feature, and the horizontal gradients in electron density associated with the trough walls are all quantities of interest or concern to practical applications of radio systems involving the ionosphere. In practice, the precise characteristics of trough‐like structures have been difficult to monitor using ground‐based methods. Ionospheric tomography represents a new development that is maturing into a technique ideally suited to the study of electron density troughs. Results are presented from a variety of observations made during tomographic campaigns in northern Europe. A long‐term investigation has been made of the main trough from a network of stations in the United Kingdom. The position of the trough minimum and the wall gradients have been studied on a diurnal basis using tomographic images reconstructed from measurements for a succession of passes of Navy Navigation Satellite System satellites. With stations deployed for more than 6 months, the average behavior has also been studied. Examples are shown of extreme behavior of the trough under very disturbed geomagnetic conditions, during which tomography continues to yield images while the limitations of ionosondes are exposed. Studies of narrow troughs with very steep gradients seen at auroral latitudes have been used to investigate some of the successes and limitations of the tomographic method. Measurements made in the polar cap show the depleted densities of the polar hole in the center of the dawn convection cell and illustrate the power of the tomographic method at high latitudes. Finally, the dayside trough at the high‐latitude boundary between corotating and counterstreaming flux tubes in the afternoon sector has been revealed in a tomographic image extending over some 30° latitude, made using a chain of six stations in Scandinavia.

Trough reconstruction in blast-furnace cast houses

Replacing the rammed carbonaceous lining of the main trough of a blast furnace is one of the most difficult and time-consuming operations in preparing for tapping of the furnace. The efficiency with which this operation is performed determines the number of taps that can be made and the overall efficiency of the furnace.

Labor productivity improvement at Kimitsu No. 2 blast furnace

Kimitsu No 2 blast furnace was relined with the objective of labor productivity improvement and was blown in on November 1994. The number of tapholes was reduced from 3 to 2 by changing the main trough from fixed type to changeable type. In addition, by introducing high-power hydraulic openers, a new monitoring system and remote controlled cast house machines, the number of workers in the cast house was reduced from 9 to 4 persons/shift, with better working conditions, and the continuation of stable operations.

Electron content measurements in the auroral zone using GPS: preliminary observations of the main trough and a survey of the degree of irregularity in summer

Ionospheric electron content was monitored from sites in or near Tromsø, Norway, for six months of 1993, using the transmissions from the satellites of the Global Positioning System (GPS). The data have been used for preliminary studies of two important phenomena of the high-latitude ionosphere: the main trough and the incidence of large irregularities. The latitude and motion of the trough were determined on several occasions during the spring period, and the results compared with previous data. Best agreement is with the formula of Collis and Haggstrom (1988). The incidence of large irregularities was surveyed during a four-month period, approximately from the summer solstice to the autumnal equinox, and the variation with time of day and magnetic activity has been determined. It was found that irregularities are considerably larger by night than by day, but that they are enhanced during both periods by increased magnetic activity. Statistical results are presented. It is suggested that these irregularities are the same as the “auroral blobs” previously studied by incoherent-scatter radar.

The formation of electron-temperature hot spots in the main ionospheric trough by the internal processes

Abstract. A mathematical model of the convecting high-latitude ionosphere is described which produces three-dimensional distributions of electron density, positive-ion velocity and electron and ion temperatures at the F-layer altitudes. The results of simulation of the behaviour of the high-latitude ionosphere, in particular, the heat regime of the F-layer, are presented and analysed. From our study, it was found that electron-temperature hot spots in the main ionospheric trough can arise owing to internal ionospheric processes, and not due to effects of any external causes. Three conditions, to be satisfied simultaneously, are necessary for the formation of the considered electron-temperature hot spots: first, low values of electron density; second, solar illumination of the upper F region and darkness of the lower F region; third, low values of neutral-component densities. These conditions are valid in the main ionospheric trough near the terminator on the nightside when the density of the neutral atmosphere is not high. The physical processes which lead to the formation of the electron-temperature hot spots are the heat transfer from the upper into the lower F region, the reduced heat capacity of electron gas and the weakened cooling of electron gas due to inelastic collisions with neutral atoms and molecules. Also investigated is the influence of seasonal and solar-activity variations on the efficiency of the identified mechanism responsible for the formation of the electron temperature peaks in the main ionospheric trough by the internal processes.

The formation of electron-temperature hot spots in the main ionospheric trough by the internal processes

The formation of electron-temperature hot spots in the main ionospheric trough by the internal processes

The Roermond earthquake of 13 April 1992, the Netherlands: geological aspects

ABSTRACTThe epicentre of the Roermond earthquake is located near the western boundary fault of the Roer valley trough, one of the deepest and the most active in the Quaternary part of the Lower Rhine graben. The Late Pleistocene and Holocene activity of the trough is manifested by offsets of the main (Mindel) and the lower (Riss and Late Pleistocene) terraces along the boundary faults.Surface fractures have been observed in an area of more than 50 km2: 2.5–3.5 km to the north of the town of Roermond, at 0.8 km to the south of the village of Herkenbosch and in the southeastern part of the village of Montfort. Three types of ruptures were differentiated: scarps up to 50 cm high along open fractures near the Maas River; open fractures (continued by scarps in some places) and open fractures with a liquefaction of the Quaternary alluvium sands. The last type is predominant. The location of the ruptures depends on the landscape and water‐table of the region. While they could be produced solely by hydraulic shock during the earthquake (increased by the wet spring season), the majority of the ruptures strike N50°W ± 10°, i.e. parallel to the main trough faults, or N55°E ± 10°, along ‘neotectonic lines’, parallel to the Maas valley and deduced from space imagery. Thus, the ruptures could be the secondary surficial effect of the earthquake, linked indirectly with the active tectonics of the region.

Medium-scale wave-like irregularities of electron density and electron temperature in the upper ionosphere at subauroral and high latitudes during different phases of the magnetic storm of January 1974

Data obtained aboard the INTERCOSMOS-10 satellite during different phases of the geomagnetic storm in the last week of January, 1974, are used for the investigation of medium-scale wave-like irregularities of electron density and electron temperature in the upper ionosphere (altitudes from about 800 to about 1400 km) for high and subauroral latitudes. Daytime and nighttime conditions are analysed in detail. It is shown that independent of local time and of the degree of magnetic disturbance the spectra of medium-scale electron-density and electron-temperature fluctuations reveal equal characteristic wavelengths of l ≈ 130–150, 150–200, 210–240, 250–320, 340–400 km and so on. During nighttime conditions in the region of the main ionospheric trough the fluctuations of electron density (at the equatorward wall of the trough) and of electron temperature at the bottom of the trough) are separated in space. Later on it is shown that the intensity of the fluctuations of electron density and electron temperature at high and subauroral latitudes is dependent on the phase of magnetic storm.

Seismic surveys in the Northern Red Sea: asymmetric crustal structure

Wide angle reflection and refraction seismic (WARRS) surveys at key sections of the Northern Red Sea and adjacent areas enabled a better definition of the crust and upper mantle structure. A fundamental asymmetry of the region was revealed by the results of profiles, which cover both flanks of the Red Sea main trough, the coastal plains and the bordering Precambrian shields. This asymmetry is clearly expressed by the presence of different crustal types on both flanks: the western flank shows oceanised crust within only 20 km off the African coast, while the whole eastern flank is underlain by a continental type of crust. Likewise, the shape of the continent-ocean transitions differs significantly on both flanks: the Arabian margin extends almost into the centre of the Red Sea and shows considerable stretching and attenuation of the continental crust, while the African margin, on the other hand, even thickens towards the escarpment and appears very abrupt. The influence of the Red Sea opening, as reflected by the decrease of seismic velocities and the increase of Poisson's ratio in the upper mantle due to partial melting, is strictly limited to the area seaward of the Egyptian escarpment. The asymmetry in the Northern Red Sea is regarded to be the expression of strike-slip processes in the initial stages of Red Sea evolution.

Miocene magmatism of sinai related to the opening of the red sea

Extension associated with the initial (early Miocene) stages of opening of the Red Sea resulted in intrusion of a widespread system of dikes and smaller bodies, primarily along the northeastern margin of the Red Sea/Gulf of Suez axis. Dikes, some up to more than 800 km in length, were emplaced dominantly parallel to the Red Sea/Gulf of Suez from the Sinai Peninsula to Yemen. Intrusive rocks are mainly basaltic in composition, but minor granophyres are also present. In the Sinai, this magmatic event is about 20 m.y. old. Dikes of the southern and southeastern Sinai crop out in Precambrian terrane. In the central and northern Sinai, they occur in Phanerozoic terrane, where they may be associated with linear grabens and cryptovolcanic features. Basaltic dikes of the Sinai underwent variable degrees of alteration. Alteration phases include seriate, quartz, smectite, chlorite, and serpentine. X-ray diffraction and petrographic analysis indicate that, in spite of extensive alteration, primary minerals still control the major-element compositions of the rocks. Basalts are tholeiitic in composition, with SiO 2 = 48.5 to 50%, and Na 2O + K 2O = 3 to 3.5%. Rb and Sr concentrations are probably dominated by alteration and by introduction of secondary material (primarily carbonate). Zr/TiO 2, which is relatively unchanged during alteration, substantiates that the basaltic rocks are subalkaline. Initial 87Sr/ 86Sr, determined after leaching of samples, is approximately 0.7043–0.7063, much higher than Red Sea axial trough basalts ( < 0.703). We infer that these Sinai basalts equilibrated at depths of 35–50 km, possibly within an enriched mantle. Whether the main trough of the Red Sea is underlain by extended and modified continental crust or by oceanic crust is controversial. Since Miocene dikes were intruded along both coasts (but primarily the Saudi Arabian coast) of the Red Sea/Gulf of Suez axis in a zone up to 150 km wide, they may also underlie the main trough of the Red Sea and result, at least partially, in the discontinuous magnetic and gravity anomalies observed there. Possibly the abundance of dikes beneath the main trough is much greater than adjacent to the Red Sea, giving rise to properties intermediate between those of continental and oceanic crust. Downwarping and widespread intrusion of dikes along the Red Sea 18–26 m.y. ago probably signified crustal dilation and the initiation of a continental rift. Because tholeiitic basalts of the Sinai are higher in 87Sr/ 86Sr than Red Sea axial trough basalts, we infer that the conditions of their origin must also be very different. We suggest that the Sinai basalts may have been generated within but near the base of subcrustal continental lithosphere , perhaps indicating thermal thinning of lithospheric mantle above upwelled asthenosphere associated with continental rifting. This inference, if correct, would indicate that the lithosphere was at least 35–50 km thick during the early Miocene phase of rifting but possibly not much thicker. Since dikes farthest from the Red Sea are apparently identical in composition to those closest, asthenospheric upwarping must have been broad enough that all magmas were derived from approximately the same depth. In the Yemen-Ethiopia areas, volcanism began earlier (about 30 m.y. ago) and was much more voluminous than elsewhere along the Red Sea/Gulf of Suez axis, suggesting the presence of underlying mantle upwelling accompanying rifting of the Red Sea. In spite of differences in age and volume, the presence of large volumes of tholeiites among the Miocene basalts of the Yemen-Ethiopia areas suggests that the lithosphere was similarly thinned along the entire length of the Red Sea.

Tropical Storm Formation in a Baroclinic Environment

Abstract An analysis is presented of the large-scale conditions associated with the initial development of Tropical Storm Diana (September 1984) in a baroclinic environment. Ordinary extratropical wave cyclogenesis began along an old frontal boundary east of Florida after 0000 UTC 7 September and culminated in tropical cyclogenesis 48 h later. Water-vapor satellite imagery showed that the initial cyclogenesis and incipient tropical storm formation was nearly indistinguishable from a classical midlatitude development. Cyclogenesis occurred in three stages. A large-scale cold trough and associated frontal system crossed the Atlantic coast, while a small potential vorticity maximum aloft fractured off the main trough and stalled over central Florida in the first stage. As the main trough sheared off eastward, cyclogenesis began along the southwestern end of the stalled frontal zone east of Florida. Anticyclogenesis to the north in the wake of the shearing trough allowed a surge of cooler and drier air to flo...

The morphology and structure of the West O’Gorman Fracture Zone

The West O’Gorman Fracture Zone is an unusual feature that lies between the Mathematician Ridge and the East Pacific Rise on crust generated on the East Pacific Rise between 4 and 9 million years ago. We made a reconnaissance gravity, magnetic and Sea Beam study of the zone with particular emphasis on its eastern (youngest) portion. That region is characterized by an elongate main trough, a prominent median ridge and other, smaller ridges and troughs. The structure has the appearance of large-offset fracture zone, possibly in a slow spreading environment. However, magnetic anomalies indicate that the offset, if any, is quite small, and the spreading rate during formation was fast. In addition, the magnetic profiles do not support earlier models for a difference in spreading rate north and south of the fracture. The morphology of the fracture zone suggests that flexure may be responsible for some of the topography; but gravity studies indicate some of the most prominent features of the fracture zone are at least partially compensated. The main trough is underlain by a thin crust (or high density body), similar to large-offset fracture zones in the Atlantic, while the median ridge is underlain by a thickened crust. Sea Beam data does not unambiguously resolve between volcanism or serpentinization of the upper mantle as a mechanism for isostatic compensation.

The Geology and Geochemistry of the Red Sea, Saudi Arabia, and Its Relation to the Pacific Region: ABSTRACT

Geological, geochemical, and comparative studies were carried out on the Red Sea, part of the multirift Circum-Pacific region and other oceanic crust areas represented by MORB-type basalts. The Red Sea geochemical data indicate four magma groups related to the volcanic ridges and the rift floor. This area has different ages and assorted rock compositions ranging from calc-alkaline to sub-alkaline affinities. The Red Sea is formed by the fastest spreading rate and the rotation of Arabia away from Africa in four phases affecting the Indian plate and the Bitils/Zagros sutures. This recent developed ocean consists of shallow continental shelves, a wide main trough (600-1,000 m depth), and a narrow (4-5 km wide) axial trough (2,000 m depth) that is formed by seafloor spreading currently active for plate separation. This axial trough is related to some of the erupted low temperature lava flows on the Afro-Arabian shields. The Red Sea inner floor is occupied by hot points, upwelling areas, and pillowed volcanoes forming elongated hills. The 15 km crustal thickness of the Red Sea shelf with a metamorphic and thick sedimentary basin that is salt-filled suffers major and minor structures of tilted, faulted, foliated, and sheared zones with general NW-SE strikes. Eightmore » m.y. ago, 75 % of this sea was opened, the Gulf of Suez graben remained essentially stagnant, and the Gulf of Agaba-Levant became active and extended to the Dead Sea Arava Rift.« less

Magnetic evidence for the nature of the crust beneath the southern Red Sea

Magnetic data over the southern Red Sea are used to examine various models for the evolution of the region. The linear nature of the magnetic anomalies over the coastal margins and adjacent portions of the main trough together with their decrease in amplitude with elevation is shown to be compatible with deep, thin sources that are strongly magnetized. The general lack of correspondence between gravity anomalies and deskewed magnetic anomalies indicates that the anomalies are due to intrabasement bodies rather than basement relief associated with density contrasts. Across‐axis correlations of individual anomalies indicate a symmetrical disposition of magnetic sources about a line close to the present deep water axis. The regular, symmetrical pattern of linear anomalies strongly suggests a seafloor spreading origin. The combined geologic and geophysical evidence, therefore, supports a model in which the margins are underlain by material more typically oceanic in character than continental. The magnetic evidence further implies that this oceanic crust formed during an earlier phase of spreading in the Red Sea. Although the age of this crust is unknown, anomaly correlations suggest spreading may have taken place during the Late Oligocene‐Early Miocene.

High resolution measurements of the main ionospheric trough using EISCAT

Measurements from the EISCAT incoherent scatter radar are used to investigate ionospheric variations in the vicinity of the main trough using experiments with good temporal- and spatial-resolution. For the two cases reported, the F-region density depletion indicative of the trough is associated with increased electric field and F-region ion temperature, elevated F-region electron temperature, increased electron density in the E region, and a positive magnetic bay. These qualities are determined by comparing measurements from trough and ‘non-trough’ days. No small-scale ionospheric changes were detected, but the motion and orientation of the trough could be determined by cross-correlating measurements from four closely-spaced volumes.

How to prevent zinc from entering a blast furnace

A white deposit sometimes formed on the main trough of furnaces Nos. 1-4 at the "Krivorozhstal'" combine during the first half of 1986. Analysis of the material of the deposit showed that it n contained 93% Zn. The character of distribution of zinc in the blast furnace can be studied only after the furnace is blown out for an overhaul. Such studies were conducted on furnaces Nos. 3 and 7, with useful volumes of 1386 and 2002 m 3, respectively.