Trough Axis Research Articles

Structural and volcanic evolution of the Gregory Rift Valley

The East African Rift System extends southwards from the Red Sea for a distance of about 3000 km. Within East Africa it is represented by the Western Rift and the Eastern Rift, of which the Gregory Rift Valley is that part lying within Kenya and northern Tanzania. The Gregory Rift Valley was initiated in early Miocene times as a downwarp along the continental watershed on a land surface having considerable relief (up to 2000 m). Subsequent faulting produced a graben about 80 km wide and some 450 km in length, to the north and south of which the faults splay outwards over much broader zones. Volcanic activity was not confined to the Rift, but developed both to the west in the Miocene and to the east from Plio-Pleistocene to Recent times. Within the Rift itself volcanism commenced in the north and extended to the south in the Pliocene, while both tectonic and volcanic activity tended later to restriction within a central zone, which from Lake Hannington northwards formed a narrow axial trough. Episodes of volcanism and sedimentation largely or wholly infilled the downwarping trough, but only in the Pleistocene, by uplift of the shoulders and downfaulting of the floor, was the morphology of the present Rift Valley achieved. The altitude of the sub-volcanic surface averages about 1500 m along the shoulders of the Rift; its greatest depth below the floor may be southwards from near Lake Baringo, corresponding to an absolute displacement of some 4 km. About 8 km of crustal extension over about 8o km of rift width may have occurred, but there is no evidence for crustal separation.

Red Sea heat flow

Since 1970, the number of heat flow observations in the Red Sea has increased fivefold. A new heat flow map is presented together with a table of observations. The new data confirm that the whole of the Red Sea is associated with high heat flow. The observations within 5 km distance of the deepest water of the axial trough have a mean of 467 mWm-2 (i.e. about eight times the world mean) and the observations from 50 to 170 km from the axial deep water have a mean of 111 mWm−2 (i.e. about twice the world mean). The heat flow distribution is thus typical of that associated with an active spreading centre. The observations are difficult to assess and interpret in detail because of the complex sedimentary and tectonic environment. The high heat flow extends at least as far as the Red Sea coasts.

The age and extent of the Red Sea oceanic crust

THE evolution of the Red Sea depression and the extent within it of oceanic crust have been widely discussed1–5. It is generally accepted that the axial trough is floored by oceanic lithosphere formed at a constructive margin over the last 3–4 Myr by spreading at a rate of about 1.0 cm yr−1 per flank2,6. It is also clear from stratigraphic continuity and the trend of isofacial lines that the opening of the Red Sea started less than 45 Myr ago after a long period of tectonic and magmatic quiesence7,8. Two areas of contention exist. First, how much of the Red Sea on either side of the axial trough is underlain by oceanic crust, and second, when was this oceanic crust formed?

Some results of the surface wind and atmospheric pressure fields analysis near the Equator

Based on meteorological observations from five USSR research ships during Expedition TROPEX-72 in the Atlantic Ocean for the period from June to August 1972, the relationship between surface winds and pressure gradients near the Equator is analysed. For the narrow Equatorial zone (0–2.5° N) the nearly complete independence of the surface wind speed (rather strong 7–8 m/sec) and direction from the local pressure gradients was found and on this basis the conclusion is drawn concerning the important role of the air stream inertia in this zone. However, the estimates of different terms in the equation of motion for the near-Equatorial latitudes showed that, as in the middle latitudes, the inertia terms of the mean motion are here one order of magnitude smaller than the terms of pressure gradient. Having this in mind we believe that the only proposition which could be made concerns the important role of the inertia associated with turbulent rather than average motion in the dynamics of the Equatorial atmosphere. It is shown that the inertia of the air stream near the Equator can account for the asymmetry of the vorticity field around the axis of the Equatorial trough (cyclonic relative vorticity north of the trough axis and the anticyclonic one south of the axis in Northern Hemisphere). It is shown that the wind deflection angles become normal for the usual geostrophic relation over the sea only north of 11–13° N. DOI: 10.1111/j.2153-3490.1976.tb00699.x

Opening of the red sea with two poles of rotation

Previous studies have shown that the Red Sea was formed by two stages of sea-floor spreading, with a quiescent period in between. We suggest that these two phases have occurred in different directions. The shape of the central trough indicates that the present-day motion is almost E-W, whereas the total opening, deduced from the shape of the coastlines, is NE-SW. If the axial trough has opened in an E-W direction, the earlier stage of opening was in a direction which made the Dead Sea Rift fall along a small circle to the pole of early opening, and hence suggests that the Dead Sea Rift was a transform fault during this early stage. The later movement gives almost pure extension along the Dead Sea Rift, and this should be seen by normal faulting. Available first-motion studies are not precise enough to confirm or deny this hypothesis.

Some results of the surface wind and atmospheric pressure fields analysis near the Equator

Based on meteorological observations from five USSR research ships during Expedition TROPEX-72 in the Atlantic Ocean for the period from June to August 1972, the relationship between surface winds and pressure gradients near the Equator is analysed. For the narrow Equatorial zone (0–2.5° N) the nearly complete independence of the surface wind speed (rather strong 7–8 m/sec) and direction from the local pressure gradients was found and on this basis the conclusion is drawn concerning the important role of the air stream inertia in this zone. However, the estimates of different terms in the equation of motion for the near-Equatorial latitudes showed that, as in the middle latitudes, the inertia terms of the mean motion are here one order of magnitude smaller than the terms of pressure gradient. Having this in mind we believe that the only proposition which could be made concerns the important role of the inertia associated with turbulent rather than average motion in the dynamics of the Equatorial atmosphere. It is shown that the inertia of the air stream near the Equator can account for the asymmetry of the vorticity field around the axis of the Equatorial trough (cyclonic relative vorticity north of the trough axis and the anticyclonic one south of the axis in Northern Hemisphere). It is shown that the wind deflection angles become normal for the usual geostrophic relation over the sea only north of 11–13° N. DOI: 10.1111/j.2153-3490.1976.tb00699.x

A Geophysical Study of the Red Sea Axial Trough between 20*5 and 22 N

Summary New bathymetric, seismic reflexion, gravity and magnetic data have been obtained in a detailed survey of part of the axial trough in the central Red Sea. These data show that the trough is not continuous, but is broken into two sections separated by a shallow region, called the ‘inter-trough zone’. This zone has a thick sediment cover and is devoid of magnetic anomalies. It may represent a fracture zone into which salt and other sediments have flowed, or it may be a section of spreading axis which has remained covered with sediments for an unknown reason. The transverse magnetic anomalies, previously recognized by Allan, Phillips and other, have been fully mapped, and a further transverse lineation has been discovered. Using three-dimensional computations, a model has been developed which can account for these lineations as the magnetic end effects of a series of short spreading axes offset by closely-spaced transform faults. The development of such a configuration is discussed and can be seen as a natural consequence of the plate geometry in this region.

Penecontemporaneous Recumbent Folds in Trough Cross-Bedding of Pleistocene Sands in Saskatchewan, Canada

ABSTRACT Sets of trough cross-bedding in the glacio-fluvial sands and gravels of the Pleistocene Floral Formation near Saskatoon, Saskatchewan, are deformed into recumbent synclines by, the overturning of individual cross-strata. The direction of overturning of these folds is to the south and east, and is the same as the palaeocurrent direction indicated by the dips of cross-strata and the orientations of trough axes. These folds, which are exposed at several levels in the sequence have apical angles of from 30° to 60°, and approximately horizontal hinge lines. The trends of hinge lines vary greatly depending on position within an individual cross-bed set. Along the margins of troughs, hinge lines are at high angles to those near trough apices. All of the folds have upper contacts whic have been produced by erosion before deposition of the overlying layers. In general, the lower part of the folded bed is not deformed and the cross-stratification is tangential to the underlying layer. Each unit of deformed cross-bedding consists of one overturned fold, but in many places the hinge area and/or the upper limb of such a fold contains a series of smaller folds also with flat-lying axial planes. Considerations of kinematics of fold formation show that fold geometry is controlled largely by the shape of the shear profile, and by the initial inclination of the bedding. The distribution of the folds suggests that the process which caused the deformation occurred on several occasions during deposition of the Floral sands. The consistent pattern of the deformation and the lack of other types of deformed bedding suggest that liquefaction of the sediment was not common, that downslope slumping did not occur, and that ice-thrusting did not cause the deformation. It is believed that the folding was caused by frictional drag as a result of the passage of a mass of saturated sand over the surface of the cross-bedded sand. Such an interpretation is favoured also because this is the only mechanism which has produced overturning of cross-beds in laboratory experiments.

A Relationship Between Cloud Activity and the Low-Level Convergence as Observed in Reed-Recker’s Composite Easterly Waves

Abstract The method proposed by Ogura and Cho is applied to the averaged easterly wave disturbances in the western Pacific based on Reed and Recker’s data set to determine the cloud mass flux distribution in various weather conditions, ranging from disturbed conditions in the trough region to a relatively undisturbed condition in the ridge region. The results show that shallow clouds are present practically everywhere in the wave disturbances, even in the ridge region where the large-scale vertical motion is downward. In contrast, deep clouds penetrating into the layer above the 500-mb level are present in a significant amount only in the region near the trough axis (most intense in the region east of the trough axis). More quantitatively, while the total mass flux associated with all cumulus clouds at the cloud base level is not correlated with the low-level, large-scale convergence, the mass flux due to deep clouds is found to increase linearly with the increase of low-level, large-scale convergence. Th...

Volcanic and sedimentary processes in the Red Sea axial trough

Volcanic features observed in the axial trough of the Red Sea are typical of those seen in other sites of mid-ocean ridge volcanism. Extrusive volcanic features photographed include flows, pillow lavas and rock fragments. Many lavas appeared fresh and unweathered. Sedimentary cover is thin in areas of recent volcanic activity. Bottom currents and bioturbation are responsible for reworking of the sediments in the axial trough. Sediments within the Atlantis II hot brine area appear smooth in most places and thick enough to cover the volcanic topography. Sediments underlying the hot brines (about 60°C) may be reworked into ripples and dunes by internal waves where interfaces between brines of differing density intercept the sea floor. A group of moundbuilding organisms (polychaete worms?) similar in appearance to organisms photographed in other parts of the axial trough, apparently inhabit the transitional region between the overlying normal Red Sea Water and the hot brine waters, or the hot brine area proper.

Discussion of ‘Volcanic islands of the Red Sea’

D r R. W. G irdler writes: In their fascinating paper on the volcanic islands of the southern Red Sea, Gass et al. 1973 give the impression that I consider that only the ‘central median trough’ of the Red Sea is underlain by oceanic crust. This is not so; evidence has been accumulating for a considerable time indicating that much more than the axial trough is oceanic (e.g. Girdler 1966, 1970; Girdler & Darracott 1972). At the time of my 1958 paper (to which they refer) we had no idea that the thickness of evaporites forming the shelves of the Red Sea would be so enormous. In the 1960s, exploration boreholes showed thicknesses of salt (specific gravity 2.1) of the order of 4 km! It is clearly necessary to make corrections for these in the gravity interpretations and the positive Bouguer anomaly over the centre of the Red Sea then becomes much wider and hence the extent of the ‘intrusive zone’ (oceanic crust) is found to be much wider. Gass et al. also refer to the existence of large magnetic anomalies over the median trough and their ‘virtual absence elsewhere’. Unfortunately, there are few ships’ tracks over the shelves due to navigational and other difficulties but it is important to realise that absence of data does not imply absence of anomalies. There are aeromagnetic surveys which reveal the existence of anomalies over the shelves (Girdler 1970). For example, an aeromagnetic survey by the Gulf Oil Company showed the presence of sea floor

A Geomagnetic Variation Anomaly Across the Northern Gulf of California

Summary Variations in the geomagnetic field have been observed along a profile perpendicular to the northern Gulf of California. The profile extends from the Pacific Ocean (including two sea-floor stations off the coast) to Tucson, Arizona, and includes stations at San Felipe and Puerto Penasco on opposite sides of the Gulf. At these two stations there is a large anomaly in the vertical component variations in the frequency range 0.025–4 c hr-1 which amounts to almost complete reversal from one station to the other at the higher frequencies. Analysis of the variations demonstrates that the anomaly is due to induced currents flowing in or beneath the Gulf of California. Using a modelling technique developed by Greenhouse, Parker and White the anomaly may be interpreted either in terms of very high Gulf sediment conductivity or an uplifted zone of hot mantle material beneath the Wagner basin. A suitable combination of these models is compatible with the horizontal field observations. Both of these factors support the suggestion that the Wagner basin is the site of a spreading centre similar to those found at ocean ridges. Observations along a profile of stations across the Imperial Valley indicate that the upwelling of mantle material does not necessarily occur continuously along the Gulf of California—Salton Trough axis and is probably a local phenomenon associated only with such spreading centres. Because of the consequent lack of two-dimensionality in the actual situation, the predicted height of the upwelling will be too small. It is thought that a conductivity of 0.02 (Ωm)-1 could be present at 20–30 km depth in such areas.

Recent Rainfall Trends in Africa, the Middle East and India

IN the Middle East and Africa north of the Sahara the wet season extends from October to May; small amounts of rain also fall in Pakistan and north-west India at that time of year. Winter–spring rainfall in these areas is controlled primarily by divergence patterns in pressure troughs in the mid- and upper-tropospheric circumpolar westerlies. Rainfall is associated with the widespread ascent of warm, moist air from the tropics on the eastern side of the troughs, and also with deep convective instability in the cold air at, and slightly to the rear of, the trough axes, especially over coasts and mountains. It is in winter that the increase in the latitudinal temperature gradient in the northern hemisphere causes the westerly circulation to increase in strength and the westerly troughs to penetrate into sub-tropical latitudes. In summer, the latitudinal temperature gradient is at a minimum, the general circulation is relatively weak, and the westerly troughs are of small amplitude and restricted to middle and high latitudes.

Volcanic islands of the Red Sea

The Recent volcanic islands of the Red Sea are (1) Jebel at Tair, a single small volcano of tholeiitic basalt lava; (2) the Zubair Islands with pyroclastic cones and flows intermediate between tholeiite and alkali basalt and with picrite basalt and trachybasalt blocks in the agglomerates; (3) the Hanish-Zukur Islands with alkali basalts accompanied by trachybasalts, trachyandesites and trachytes together with pyroclastic rocks. The chemistry of 46 lava specimens indicates that a gradational series exists between the sea-floor basalts (K-poor tholeiites) and the alkali basalts of Hanish-Zukur, with the rocks of Jebel at Tair and Zubair representing intermediate stages. Two alternative petrogenetic models are discussed to account for this gradational behaviour. One derives the parental magma from successively greater depths, the other considers derivation by successively greater fractionation on route to the surface. The relationship of the volcanoes to the opening of the Red Sea is discussed. Possibly, eruptive activity was initiated at the southern end and is migrating northwards in response to the anticlockwise rotation of Arabia relative to Africa. The Red Sea axial trough may die out southwards owing to vocanic fill from the Hanish-Zukur volcanoes.

Orientation Analysis of Trough Cross Stratification in Upper Cambrian Sandstones of Western Wisconsin

ABSTRACT Trough cross stratification is dominant in Upper Cambrian Sandstones in western Wisconsin. Paleocurrent analysis of such stratification is difficult because of the geometric complexity, but trough-axis orientations have been shown to provide less dispersion than cross-set orientations. Heretofore, trough-axis orientations have been used only sparingly as an index of sediment transport direction, so a study was undertaken in a region where extensive cross-set data already existed in order to further test the superiority of the trough-axis approach and to reevaluate regional Late Cambrian sediment dispersal for three formations. Regional vector analysis of trough-axis orientations in western Wisconsin yielded sediment dispersal directions toward the south-southwest with much local variation (toward 232° for the Galesville Sandstone, 245° for the Tunnel City Group, and toward 180° for the Jordan Sandstone). The orientation data presented here are comparable with earlier-published results, respective formation mean directions for trough axes falling within at most a 90° arc of means for cross sets. But, with one exception, the trough-axis data is much less dispersed than previously reported cross-set data. The data collected from the Tunnel City Group sandstones are statistically nonsignificant probably due to poor exposures and the variety of environments represented within this unit in west-ce tral Wisconsin. A southerly average sediment transport direction inferred from cross stratification is independently confirmed by a train of fine pebbles of red quartzite in Cambrian sandstones, which extends south from its source in the Baraboo district. An island complex 30 km long by 12 km wide existed in the Late Cambrian sea in south-central Wisconsin, where Precambrian Baraboo Quartzite hills projected above sea level. Cambrian sandstones within 5 or 6 km of the old islands show apparent longshore drift and lee-side circulation eddy influences. These anomalies produce greater dispersion for mean trough-axis orientations in the immediate Baraboo area than for a somewhat larger surrounding region, but other influences produced about the same amount of regional dispersion for the much larger wes ern Wisconsin region. Therefore, the presence of unknown islands could not be demonstrated from cross bedding analysis alone. The inferred regional dispersal direction is an index of the bottom-paleocurrent direction, which, in shallow seas, is influenced primarily by winds, tides, and topographic obstructions. The persistent regional pattern for the Wisconsin Cambrian sandstones is fully consistent with previously-postulated tradewind dominated marine circulation at a low paleolatitude interrupted by occasional brief tropical storms, whose effects are clearly recorded in coarse conglomerates adjacent to the Baraboo islands.

Authigenic Dolomites from Red Sea: ABSTRACT

End_Page 807------------------------------Deep-Sea Drilling Project Sites 225 and 227 were located in the main trough of the Red Sea, 10 and 3 mi, respectively, east of the axial trough in the vicinity of the Atlantis II Deep. Coring was continuous. The sedimentary section included 3 units. Unit 1 (0-175 m) is carbonate and nannofossil ooze and chalk, with minor variable detrital admixture. It is of early Pliocene to Holocene age. Unit 2 (175-195 m) is claystone, rich in dolomite and pyrite, and is earliest Pliocene. Unit 3 (195-? m) is layered anhydrite and halite, with interbeds of black shale. Dolomite is present in the shale and between anhydrite nodules. Unit 3 is of late Miocene age. Scanning electron micrographs show the dolomites in Units 2 and 3 to have euhedral planar crystal faces and to show penetration twinning, indicating them to be authigenic. The overall percentage of dolomite increases from less than 10% at the top of Unit 2 to, locally, more than 80% within the evaporite. Geochemical signatures differ between the dolomites of Units 2 and 3. The dolomites in Unit 2 contain excess calcium. The dolomites of Unit 3, generally of finer crystal size, achieve stoichiometric composition and show better ordering. Both types of dolomite are interpreted as early diagenetic, with the calcium-magnesium ratios being indicative of paleosalinity conditions, analogous to the dolomites of the Zechstein and the Triassic Keuper Formation. Stable carbon and oxygen isotope data show the environments of deposition of dolomite of both units to have been restricted, but to differing degrees. The situation is directly analogous to the dolomites of the Mediterranean basin. End_of_Article - Last_Page 808------------

Numerical Simulation of the Development of the Intermediate-Scale Cyclone in a Moist Model Atmosphere

Abstract The genesis and development of an intermediate-scale cyclone in middle latitudes was simulated numerically and its three-dimensional structure analysed. A six-level moist primitive equation model was run for a channel flow with an initial weak baroclinicity. The motion was initiated in the flow by introducing wave motion with wavelength of 6000 km. As the initial disturbance was intensified, an extended front developed and, after several days in real time, an intermediate-scale disturbance was formed on the extended front. The analysis indicates that this secondary cyclone has a longitudinal wavelength of about 1500 km and a latitudinal half-wavelength of about 600 km. The depression is observed most strongly below 700 mb with a vertical or slightly eastward tilted trough axis. A sharp concentration of isotherms is situated in the northern part of the cyclone. In the lower troposphere, the warm air is observed on the cast side of the low center and the cold air on the west side. This thermal cont...

New K/Ar Determinations on Rift Volcanics of S. Kenya and their Bearing on Age of Rift Faulting

Isotopic ages of lava from the rift province of southern Kenya provide age limits for some episodes of rift faulting. A temporal correlation between the age of faulting and the time of formation of the deep axial trough of the Red Sea appears to exist.

Analysis of Cross-Stratification Orientation in the St. Peter Sandstone in Southwestern Wisconsin

The orientation of cross-bedding in the St. Peter Sandstone in southwestern Wisconsin has been known to be very complex and possibly even random. Special attention to cross-set trough-axis plunge azimuths, together with the recent reinterpretation of large-scale cross-stratification in the St. Peter as relict internal structures of large submarine sand dunes, waves, and ridges, provides added insight for a refined analysis of cross-set orientations and, thus, for inference of paleocurrent directions. In the hierarchical array of cross-sets west of Madison, Wisconsin, smaller scale sets show no significant orientation by vectorial and variance analyses. The large sets, however, have a significant preferred orientation toward the southwest (260° ± 85° standard deviation), and trough axes show a significant plunge toward the northwest (281° ± 82° standard deviation). Only by recognition of the complexity of the cross-stratification and by careful selection of particular geometric features to be measured can any meaningful index of paleo-currents be discerned. At least for the limited area of study (700 sq mi), the net sand transport direction was toward the west rather than south-southwest as previously postulated from limited data for the upper Mississippi Valley region.

Concurrent association between 700-mb 5.day mean contour patterns and 5-day rainfall anomaly over India during July

Utilising pentad charts for July for the years 1957-1966, broadscale features of 5-day mean contour patterns associated with widespread abnormal and subnormal rainfall over five large areas, viz., northeast, northwest, central, western and southern India have been examined. The association between monsoon trough axis location and rainfall anomaly over these large areas has been brought out, Association between monsoon trough axis location and rainfall has also been investigated for all the sub-divisions of India. The study reveals that for most of the sub-divisions of India the concurrent association between location of monsoon trough axis and occurrence of non-subnormal and non-abnormal rain is high, In conjunction with forecast 5-day mean 700-mb contour chart, these relationships could be used to forecast non-subnormal and non-abnormal rainfall over most sub-divisions of India.