Palaeomagnetic investigations on the Deccan Traps from Chincholi,Mysore State,India

Summary 38 oriented samples of Deccan Traps have been collected from the neighbourhood of Chincholi, Mysore State, India. The Natural Remanent Magnetisation of these rocks has been studied using an astatic magnetometer. It has been found that these rocks are magnetically reversed, the mean magnetic direction being N154°E in declination and 61° down in inclination. Thermoremanance studies conducted on four specimens showed that two specimens with weak NRM and a high secondary magnetisation have Curie temperatures around 560°C for the NRM and exhibited partial reversal of TRM at room temperature, while two specimens with high NRM and with little secondary magnetisation have Curie temperatures much lower than 560°C for the NRM.     

Fission-track ages of apatites from the Precambrian of Rwanda and Burundi: relationship to East African rift tectonics

Twenty-seven apatite samples recovered from Precambrian intrusive rocks of Rwanda and Burundi have been dated with the fission-track method. The resulting ages range from 75 to 423 m.y., which is much younger than the ages of emplacement or metamorphism of these rocks according to other radiometric methods. They even postdate the late Proterozoic (Pan-African) tectonic event which caused a partial isotopic rehomogenisation in the western part of the basement rocks around 650–700 m.y.From the regional geology and from the length ratios of spontaneous to induced tracks determined on 18 of the 27 samples it can be inferred that the fission-track ages are not mixed ages caused by some relatively recent thermal event, but that they essentially date the last cooling history of the studied massifs. This last cooling is interpreted to be mainly the result of a slow epirogenetic uplift (less than 100 m/10⁷ y) which affected the area during the major part of the Phanerozoic. In this way the large age variations can be ascribed to differential cooling caused by regional differences in the rate of epirogenetic uplift, by small (less than 1.5°C/100 m) but persistent spatial variations in the geothermal gradient or by a combined effect of both phenomena.The spatial distribution of the ages indicates that differential cooling took place along SW-NE to W-E directions which is nearly perpendicular to the general orientation of the rift and none of the ages can be related directly to late Tertiary and Quaternary rift tectonics. Uplift movements associated with rifting were probably responsible for the relatively younger ages found on samples from the high rift shoulders and the rift walls, when compared to the ages of nearby samples from the central plateau. However, it must be concluded that these movements were not strong enough to bring rocks to the surface yielding ages that would date this major tectonic event itself.     

On the African rift system,theoretical and experimental study

The East African rift system is a product of two separate activities. An ancient process (Precambrian?) resulted in brittle crustal fracture, a deep vertical tensional feature. This defined the principal fracture pattern of the continent. It was initiated by an upward diapiric stress and propagated laterally according to the principles of elastic fracture and bifurcation. Later (Jurassic-Recent) horizontal and vertical multi-stage forces caused tectonic displacements along the ancient fractures and the present rift pattern was derived. As opposed to previous models, the present concept is compatible with many conflicting opinions of previous workers (e.g. indications of tensional versus compressional or vertical forces), and is capable of explaining many moot questions.     

Potential application of remote sensing systems to the African rift system

Interdisciplinary interpretation of satellite and geologic-geophysical data can be applied to East Africa as a cost-effective means of regional tectonic evaluation. All available data sources: geologic and geophysical maps and remote sensing images should be used. Remote sensing methods are very efficient for the “synoptic overlook” necessary to pinpoint areas for more detailed investigation.The use of satellite imagery is a way of applying and testing structural frameworks for mineral potential as surface reflections of deep tectonic features can be recognized. Mineralization can be correlated with tectonic lineaments that are probably related to persistent discontinuities in the upper mantle. The outlining of these discontinuities would be of value in the search for the understanding of riftogenesis and in the search for mineralization.This paper is a review of such work with a specific example from Canada, and how the approach can be applied to Africa for continuing research on recent crustal movements.     

Controls on alkylphenol occurrence and distribution in oils from lacustrine rift basins in East China

Oils from two lacustrine rift basins in east China are thoroughly investigated using geochemical method to understand controls on alkylphenol occurrence and distribution in oils. Oils in the Lujiapu Depression, Kailu Basin are derived from the Cretaceous source rocks, and those in the Dongying De- pression, Bohai Bay Basin, from the Tertiary source rocks. All oils are experienced relatively short distance of migration and have similar maturity in each basin. Differences in homologue distributions from different oilfields are most likely caused by organic facies variation of source rocks. The oils in the Lujiapu Depression are characterized by high proportion of C3 alkylphenols (prefixes refer to the number of alkylcarbons joined to the aromatic ring of the phenol molecule) and low proportion of cre- sols and C2 alkylphenols compared to oils from the Dongying Depression. Alkylphenol isomer distri- bution is possibly affected by depositional environment especially for C3 alkylphenols. Dysoxic fresh- water environment is favorable for the formation of propyl or isopropyl substituted C3 alkylphenols, while highly reducing saline water is more suitable for trimethyl substituted C3 alkylphenols. Variations in alkylphenol concentrations within a petroleum system are controlled mainly by secondary migration processes with alkylphenol concentrations decreasing along migration direction. Interestingly, coupled with geological factors, a subtle change of alkylphenol concentrations can be applied to differentiate carrier systems. When oil migrates through sandy beds, concentrations of total alkylphenols decrease dramatically with migration distance, while such change is less significant when oil migrates vertically along faults. However, most isomer ratios potentially related to migration distance are not as effective as those alkylcarbazoles in migration diagnosis due to complicated affecting factors.     

Seismic investigations in South West African river beds

Summary In sand-filled river channels in arid parts of South West Africa it is of importance to find methods of locating the rock surface under the sand, for purposes of water supply. This paper describes preliminary investigations in three such river beds, to test the applicability to this problem of the seismic refraction method, using the hammer-electronic seismic instrument previously developed by one of the writers for shallow refraction work. Both the method in general, and the instrument in particular, proved satisfactory. Examples are given of travel-time diagrams, interpretation is discussed, and channel profiles deduced from the work are shown. Depths ranged from a few feet to a few tens of feet. At two of the three sites, variations in velocity in the sand made standard calculations of dip and depth from two-way shooting inapplicable. In such situations depth could still be estimated reasonably reliably near the origin of the traverse. There was evidence of a superficial layer in which the velocity was less than that of sound in air.     

Membrane tectonics and the East African Rift

It is proposed that the East African Rift System is a propagating fracture system caused by membrane stresses in the lithosphere. The membrane stresses are due to the northward movement of the African plate on a non-spherical earth. The theory predicts that the interior of the African plate should be in tension and that the membrane stresses within it should be sufficient to fracture the lithosphere. The location of the rift system within the plate and the magnitude of the crustal extension observed along the rift are both consistent with such a hypothesis. The southward migration of the onset of rift volcanism with time is expected as the continent continues to change latitude and the tension fracture propagates southward. Once initiated volcanism along such fracture continues as long as the lithosphere stays in tension.     

Ray tracing through models of the East African Rift

In this paper seismic rays are traced through proposed models of the East African Rift and the predicted travel-time residuals are compared to those observed at a number of African seismic stations. The velocity models are based on published gravity models of the East African Rift and empirical velocity-density relationships. Searle's (1970) revision of the models first proposed by Girdler et al. (1969), comprising a low-density, low-velocity asthenolith that partly replaces and thins the continental lithosphere beneath East Africa, is found to be compatible with most of the observed travel-time residuals. Results from the ray tracing suggest that the model may be improved by increasing the volume of normal mantle material between the two branches of the rift. Some of the interesting travel-time residuals associated with anomalous material away from the rift are also discussed.     

The Proto-Indian Ocean and a probable paleozoic/mesozoic triradial rift system in East Africa

A revised Paleozoic/Mesozoic stratigraphy of coastal Kenya (including, in particular, the Karroo) based on current geological mapping near Mombasa is briefly described. This stratigraphy provides the geological framework for proposals concerning the Proto-Indian Ocean and the tectonic setting of the Karroo depositional basins.Recent geophysical evidence suggests that, within Gondwanaland, Madagascar was situated off East Africa near Kenya/Tanzania. The southern limits of the marine Lower Jurassic and southern limits of the marine Middle and Upper Jurassic are in similar positions in mainland Africa and Madagascar using the latter reconstruction. These paleogeographic limits also define the position, during the Jurassic, of an embayment from an ocean to the north. Regional geological similarities also support this reconstruction and are reinforced by paleocurrent data from the Karroo of Kenya indicating drainage north-northeast during the Permian and Triassic and possibly the Lower Jurassic. Marine connections during Karroo times appear to be of different ages in Kenya, Tanzania, Somalia, and Madagascar, probably reflecting physical limitations to marine access in fault-separated basins.The above embayment encroached across the Karroo depositional basins from northeast Kenya to southern Tanzania during the Lower and Middle Jurassic, i.e. from the direction towards which the Karroo drainage had been previously directed. Marine conditions remain to the present day so this embayment can be considered the Proto-Indian Ocean for East Africa. The marine incursion took place before the breakup of Gondwanaland suggesting that during the Jurassic the Proto-Indian Ocean in East Africa was an epicontinental sea and not a true ocean (i.e. floored by simatic crust). The epicontinental nature of this sea is confirmed by the lithologies of the associated sediments. Paleontological data indicate that this sea was an arm of Tethys. True oceanic conditions could not have been established until the displacement of Madagascar away from Africa, probably in the Cretaceous.Accepting the above northern position of Madagascar, the writers also postulate that in East Africa the fault-bounded Karroo depositional basins (troughs) were located within a major triradial rift system extending from Lake Malawi at least as far as eastern Kenya (some 1600 km). This rift system, if valid, was established within Gondwanaland over a period ～100 m.y. in the Paleozoic/Mesozoic (pre-breakup) in marked contrast to the East African Rift System (classical rift valleys) which is mainly a Cainozoic phenomenon (post-breakup). It is, therefore, considered that there is a fundamental difference in origin between the two rift systems.     

On the theory of the East African Low Level Jet Stream

Several theoretical models for the East African Low Level Jet Stream are described. They all share the notion that the northward advection of planetary vorticity across the equator, coupled with the presence of a north-south mountain barrier, leads to the formation of a low-level western boundary current (akin to the Gulf-Stream) along the equatorial east coast of Africa. They differ in the manner in which the planetary vorticity advection is balanced to obtain a quasi-steady state. A purely inertial model predicts the correct cross-stream scale of the jet, but does not reproduce the observed inner shear layer which reduces the jet velocity to zero inland near the highlands. The lateral friction model can produce a realistic jet profile if the horizontal eddy viscosity (appearing as a free parameter) is chosen appropriately. However this solution shows a recirculation, i.e., northerly flow, off the coast that has not yet been observed. Finally, a model that includes bottom friction over variable topography also can give realistic jet profiles. If one accepts that the mountains, the Beta effect, and some form of inertial or frictional acceleration act together to produce the cross-equatorial low level jet stream, then one can formulate the types of observations needed to distinguish between the various theories.     

Modelling catchment‐scale erosion patterns in the East African Highlands

Prompt location of areas exposed to high erosion is of the utmost importance for soil and water conservation planning. Erosion models can be useful tools to locate sources of sediment and areas of deposition within a catchment, but the reliability of model predictions of spatial patterns of erosion at catchment scale has seldom been validated against observations. This study aimed to evaluate the performance of a simple empirical model (Morgan, Morgan and Finney model, MMF) in predicting spatial patterns of erosion at two small catchments in the East African Highlands: Kwalei (Tanzania) and Gikuuri (Kenya). Erosion maps predicted by the MMF model were compared with erosion maps obtained by direct survey. In Kwalei, erosion features were especially frequent in fields of annual crops. In Gikuuri, slope was the critical erosion factor, with estimated erosion rates >10 kg m^?2 a^?1 on slopes >18 per cent. Predicted erosion rates were mainly transport‐limited and ranged from <0·01 to 13·50 kg m^?2 a^?1 in Kwalei and 9·29 kg m^?2 a^?1 in Gikuuri. The performance of the MMF model in predicting the spatial patterns of erosion was acceptable in Kwalei, but poor in Gikuuri. However, by excluding the elements at the valley bottoms in Gikuuri Catchment, the performance of the model improved dramatically. The spatial pattern of erosion predicted by the MMF model was driven by the accumulation of surface runoff, which did not consider the possibility of re‐infiltration along the slope. As a result, the MMF erosion patterns predicted by the model increased invariably from the ridges to the valley bottoms, hampering the model suitability for locating areas subjected to high and very high erosion. It is concluded that the model predictions could be substantially improved by introducing a more realistic hydrological component for the prediction of surface runoff along the hill‐slope. Copyright © 2005 John Wiley & Sons, Ltd.     

Geophysical investigations of the Olonium Roman site (Northern Como Lake)

The study area is located at S. Agata (Gera Lario), a small center at the northern end of Como Lake, near the junction of Valchiavenna and Valtellina Valleys. This site played a strategic role since ancient times, providing the control on the communications routes to both the Como Lake and the Spluga and Septimer alpine passes. Since the end of the last century archaeological findings are reported in literature, also supported, from the early XI century, by archival documents confirming the existence of the `Olonium' settlement, an administrative and fiscal center of primary importance, as well as a parish amongst the most influential in the Como Lake area. Within an area of 45,000 m² an electrical survey has been carried out in conjunction with magnetic and GPR investigations. These studies have indicated the presence of a number of sub-areas characterized by significant anomalies defined by the overlapping of the results obtained from two or more geophysical methods. In two of such sub-areas, excavation tests have been conducted, which have brought to light a number of archaeological findings of interest. In one of the two sub-areas, which is characterized by the superimposition of electrical and radar anomalies, a deposit of large pebbles has been found. The origin of this deposit has not been ascertained, whether it is of fluvial origin, related to the deviation of the Adda river in the Pian di Spagna region in Roman times, or it is part of reclamation works, still of Roman times, of paleolacustrine marshes. The overlapping stratigraphy, however, suggests the development of fluvial channels between Roman and Low-Medieval times. In the other sub-area, excavations were carried out on sites defined by electrical and radar anomalies, and confirmed by the results from magnetic survey. The excavations brought to light, below the fluvial deposits, a large medieval edifice, which could be identified as the S. Stefano church abandoned in 1444. The church is built on earlier structures, amongst which an apse with a single aisle has been interpreted as a paleochristian sacellum.     

新疆北天山中东段地区震源参数研究

本文首先利用经验格林函数方法证明了新疆北天山中东段地区的震源谱基本满足ω-2 模型。在此基础上 ,通过对S波观测谱中的噪音、仪器、路径、场地等影响的逐一消除 ,得到了研究区 10 5次ML2 5～ 5 7地震的震源谱 ,进而根据Brune的圆盘震源模型计算了地震矩、应力降、震源半径等震源参数。结果表明 ,这些地震的标量地震矩M0 与ML 震级线性相关较好 ,关系为Log10 M0 =1 10ML+ 17 2 0 ;震源半径在 10 0～ 15 0 0m之间 ;应力降介于 1～ 16 0bar之间 ,优势分布于 1～ 10 0bar。地震矩与拐角频率的关系呈现出两段趋势 ,可能表明了研究区震源的多重标度特征 :当M0 小于 4× 10 2 1dyne cm时 (相当于ML4 0地震 ) ,应力降显示出对M0 有依赖关系 ;当M0 大于 4× 10 2 1dyne cm时 ,地震的应力降不依赖于M0 。     

Evaluation of the LISEM soil erosion model in two catchments in the East African Highlands

Under increasing population pressure, soil erosion has become a threat in the East African Highlands, and erosion modelling can be useful to quantify this threat. To test its applicability for this region, the LISEM soil erosion model was applied to two small catchments, one in the Usumbara Mountains, Tanzania, and the other on the slopes of Mount Kenya. Input data for the model were collected in both catchments, as were data on runoff and erosion that were used for calibration and validation of the model. LISEM was first calibrated on catchment outlet data, and afterwards simulated spatial patterns of erosion were compared to available erosion data. The results showed that LISEM can, after calibration, give good discharge predictions for some events, but not for all. However, LISEM generally overpredicted soil loss from the catchments. Comparison with observed erosion patterns did not show overprediction, but according to the model, erosion was more widespread than was observed. There are several reasons for these discrepancies. First, it is difficult to obtain enough accurate data to run the model, such as accurate maps, rainfall data and soil and plant characteristics. Second, it is also difficult to obtain accurate data to evaluate the performance of the model, either for the catchment outlet or spatially, therefore observed erosion rates are also uncertain. Third, the model could not deal correctly with complex events, i.e. those having double rainfall peaks, and might also have difficulties with catchment characteristics such as soil type and the complexity of land use. Finally, LISEM could not deal with events in which throughflow or baseflow played a role, which was to be expected since those processes are not simulated by LISEM. Nevertheless, LISEM could be calibrated to give good discharge predictions for some events, and also gave reasonable results when compared to data obtained from erosion plots. Furthermore, only complex, distributed, storm‐based models such as LISEM can give spatial predictions for single storms. Therefore, it is concluded that if the aim is spatial prediction on an event basis, there is no alternative to complex erosion models such as LISEM, but if the aim is to predict average annual erosion, the data‐demanding, physically based LISEM erosion model may not be the most appropriate model. Copyright © 2005 John Wiley & Sons, Ltd.     

Racemization of amino acids in fossil bones and teeth from the Olduvai Gorge region,Tanzania, East Africa

Investigation of amino acid racemization in fossil bones and teeth from the Olduvai Gorge region, Tanzania, indicates that aspartic acid racemization can be used to date samples which are less than ～80,000–100,000 years old in this area. In older samples, significant secondary aspartic acid is apparently present and thus these samples have lower than expectedD/L aspartic acid ratios. Isoleucine in older samples, however, is apparently syngenetic with the samples, so the epimerization of isoleucine to alloisoleucine can be used with certain limitations to date fossil bones and teeth from the older stratigraphic sections in the Olduvai region.     

Experiments on rift zone evolution in unstable volcanic edifices

Large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses. Flank deformation is often caused by slip along a décollement within or underneath the volcanic edifice. We studied how such a stressed volcanic flank may bias the rift zone development. The influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence; here our analog experiments provide new insights. We injected colored water into gelatin cones and found systematic orientations of hydro-fractures (dikes) propagating through the cones. At the base of the cone, diverse friction conditions were simulated. By variation of the basal creep conditions we modeled radial dike swarms, collinear rift zones and three-armed rift systems. It is illustrated that a single outward-creeping flank is sufficient to modify the entire rift architecture of a volcano. The experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano’s architecture.     

Salt-bearing fumarole deposits in the summit crater of Oldoinyo Lengai, Northern Tanzania: interactions between natrocarbonatite lava and meteoric water

Oldoinyo Lengai in the Northern Tanzania rift is the only active nephelinite–carbonatite stratovolcano. We report the discovery of thermonatrite, aphthitalite, halite and sylvite fumarole deposits on recent natrocarbonatite lava flows erupted in the summit crater during the wet season. These salt deposits occur as delicate, concave fringes or tubes that line the cooling cracks in the lava flows and consist of intergrowths of euhedral crystals. The presence of a dark altered zone, depleted in halides and alkalies, adjacent to cooling cracks and observations of steam fumaroles emanating from the fractures suggest that the salts are formed by sublimation from saturated vapours generated by the extrusion of lavas over meteoric water. The crystallisation sequence recorded in the salts suggests that mixing between meteoric steam and magmatic CO₂ and H₂S occurs at high temperatures resulting in the sublimation of carbonates and sulphates. At lower temperatures the vapours are dominated by meteoric steam and sublimate halides. The high solubility of the fumarole salts within meteoric water and their formation only during the wet season implies that these are ephemeral deposits that are unlikely to be preserved in the geological record.     

青藏高原“亚东—东巧—葫芦湖”大陆裂谷带形成的深层动力过程

正确理解亚东—东巧—葫芦湖构造带的形成与属性对深化认识青藏高原的地壳形变、物质运动的行为与轨迹和深层动力过程极为重要.通过较系统的多元要素分析和研究发现：（1）基于壳、幔结构的空间展布特征表明,这是一条在EW向拉张力系作用下的陆内裂谷带;（2）强烈地震的活动与发生、大地热流异常值展布和地幔对流应力场研究证明,它是一条现今活动的大陆裂谷带;（3）该裂谷带的形成与演化乃地球内部物质与能量强烈交换的产物.