Search

The Baringo Trachyte Fault System is located within the central Kenya Rift and forms part of a N–S-trending linked extensional fault network. This fault system bounds to the west the 8 km deep Baringo Basin which itself lies within the axial valley of the central Kenya Rift. It mainly affects a middle Pleistocene trachytic dome (510 ka), the so-called Baringo Trachyte (BT). A morphotectonic study of the 10 km long BT master fault and associated downthrow geometries provides constraints on the evolution of a magma-type rift fault system from an initial stage of crack opening through to propagation. A model of radial fault growth is proposed in order to account for the longitudinal segmentation of the main fault escarpment from the median part to the tips. The small-scale half-graben geometry developed in the median high-strain zone is progressively accommodated laterally by both flexure and related narrow compensation grabens. The resulting crack swarms are well-developed at the free southern tip zone. Both the spatial distribution of rock-breaking products and their relations to the immediate hangingwall provide further evidence for this hypothesis. Well-developed screes and other gravity-driven structures (slumps) preferentially occur along the median part of the Baringo Trachyte Fault Escarpment, probably as earthquake-induced features. The hangingwall fault zone shows an asymmetrical triangular-shape with a maximum width of about half the length of the main scarp. This zone of maximum deformation and subsidence appears to be laterally controlled by two major, conjugate, transverse basement discontinuities lying with a conjugate geometry. Its internal architecture is dominated by antithetic westerly-dipping normal faults bounding discrete half-grabens, locally infilled by syn-tectonic volcaniclastics. Chronological data on hydrothermal silica filling open cracks on the BT footwall suggest that the master fault evolution occurred from 345 to 198 ka, as the possible result of at least four major normal faulting earthquakes.

In the central sector of the Main Ethiopian Rift, the Ziway–Shala lake basin system includes four present-day residual lakes, from north to south, lakes Ziway, Langano, Abijata, and Shala. This region of East Africa is under the influence of the Intertropical Convergence Zone seasonal migration. Thus it has been designated as a potential core site by the ERICA Project (`Environmental Research for Intertropical Climate in Africa'). The four lakes have been subjected to strong changes in water level and water salinity at least during the Late Pleistocene. The purpose of this study is to produce a model of basin formation and sediment accumulation for this system of lakes, in order to separate the effects of climatic change from environmental variations induced by local or regional factors such as volcano-tectonic forcings. In addition to an exhaustive synthesis of available data, various investigations have been used to develop this model: 3D remote sensing, high-resolution seismics, coring, and structural, sedimentological, and hydrological field studies. New AMS radiocarbon dating helped to refine the pre-existing stratigraphic framework for this region, and basin age estimations were calculated using mean sediment accumulation rates. The history of the Ziway–Shala lake basin system has been reconstructed from the Late Pliocene–Early Pleistocene period (106 yr), mainly characterized by catastrophic explosive volcanic eruptions. The early-middle Pleistocene–Late Pleistocene period (104–106 yr) was marked by a regional volcano-tectonic paroxysm, resulting in major changes in the morphology of the area, with the formation of the Abijata, Ziway and Shala lake basins. From ∼0.20 Ma, the Ziway–Shala basin history is marked by the eastward migration of volcano-tectonic activity, resulting in the development of the youngest basin of the Ziway–Shala system, the Langano Basin. The joint history of sedimentation in the Ziway, Langano, Abijata, and Shala lake basins started during the early-Late Pleistocene period (101–104 yr) and is characterized from this period up to the present-day by a series of climatically controlled rises and falls of lake level.

The Mesozoic-Tertiary Aquitaine basin overlaps the Aquitaine block and the northern edge of the Iberian margin. Both units are situated in the southwesternmost part of the European Continent. The Aquitaine shelf is a stable platform overlying a relatively thin crustal segment; it underwent extensional block faulting and many of its folded structures are related to salt tectonism. The Iberian block is a thicker lithospheric unit that acted as a buttress. At the junction of these two structural domains the South Aquitaine or North Pyrenean area developed, where crustal stretching, rifting, wrenching, and orogenic compression were maximal from the Mesozoic to the Tertiary. The history of the basin has been a suite of rifting attempts, in the context of the progressive opening of the Bay of Biscay, which never completely succeeded. The relative plate motions and the later convergence led, nevertheless, to the formation of the Aquitaine basin and to the emergence of the Pyrenean fold belt. The Mesozoic-Tertiary sedimentary infill of the basin is mostly marine, with thick evaporites, stable platform carbonates, subsiding platform shaly-calcareous deposits, and a characteristic, diachronous turbiditic (flysch) and molasse complex. Events during the basin-forming, extensional, and translational periods contributed more to the generation of the hydrocarbon accumulations than did the effects of the subsequent compressive structural regime. The latter, however, may have enhanced thermal flow and, thus, maturation of hydrocarbon source rocks.

a UMR CNRS 6538 ‘‘Domaines Oceaniques’’, Institut Universaire Europeen de la Mer, Universite de Bretagne Occidentale, place Nicolas Copernic, 29280 Plouzane, France b EP 1748 CNRS, Equipe Hydrologie, Paleohydrologie et Paleoenvironnement, Universite Paris-Sud, Bâtiment 504, 91405 Orsay Cedex, France c Laboratoire d’Hydrogeologie, Universite d’Avignon et des Pays du Vaucluse, 33 rue Louis Pasteur, 8400 Avignon Cedex, France d 3 Rue des Ajoncs, 64160 Morlaas, France e CEREGE, B.P. 80, 13545 Aix-en-Provence Cedex 04, France f Laboratoire ‘‘Environnements sedimentaires et Stratigraphie’’, Universite de Perpignan, 52 avenue de Villeneuve, 66860 Perpignan, France g Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia h Royal Belgian Institute of Natural Sciences, Freshwater Biology, Vautierstraat 29, B-1000 Brussels, Belgium i Department of Earth Sciences, Via Universita 4, 41100 Modena, Italy

Das nordpyrenäische Gebiet, westlich von Pau, besitzt eine Schüsselstellung zwischen Béarn und Baskenland. Der Oberkreide-Flysch besteht aus mächtigen Schichten mit engen Spezialfalten die wegen unzusammenhängenden Aufschlüssen nur geringe Erstreckung zeigen. Eine detaillierte tektonische Analyse vom mikroskopischen bis zum Hundertmeter-Massstab führt zur Klärung der Geometrie des kinematischen und zeitlichen Deformationsablaufs. Vergleichselemente wurden in den nördlichen Kalkzügen sowie in der paläozoischen Zentralzone gefunden. Die gegenwärtige Struktur hat sich im Laufe von zwei Faltungsstadien derselben Hauptfaltungsphase gebildet. Zuerst kam es zu einer N-S Einengung mit Falten und regionaler Axialflä-chenschieferung. Danach folgte eine Wiederbelebung alter Richtungen mit Bildung von Querverschiebungen. Je nach den Gebieten führten sie zu Rotationen oder eingezwickten Falten sowie zu einer lokalen zweiten Schieferung., Структурный анализ шарнирной зоны между Беарн (Bèarn) и Баскиской стороны. В секторе Северные Пиренеи на западе от По, шарнирная зона Беарнских Пи-реней и Баскиских Пиреней, мощные серии флиша верхнего Мела подвержены интенсивным микроскладкам, которые наблюдаются в небольших и прерывистых выходах на поверхность. Структурный анализ подробностей позволил, благодаря наблюдениям сделанным от микроскопического масштаба до гектометрического масштаба, уточнить геометрию структур и приступить к кинематике их относительного расположения и хронологии их образований . Поиски элементов сравнения производились в известковых цепочках в первоначальной осевой зоне. Настоящая структура была приобретена благодаря двум периодам возникновения складок в середине единственной главной тектонической фазы. Первый период со-отвествует эпизоду сжатия в северно-восточних направлениях. Эти сжатия образо¬ вали складки и местное осланцевание в осевом плане. Второй период привел к приведению в движение прежних направлений, которые материализуются случаями скольжения с наклонными перемещениями. Эти случаи образуют согласно секторам вращения или интерференцию складок или осланцевание местного характера., In the Northern Pyrenean area west of Pau, a key zone between the Beam -and Basques Pyrenees, the tick series of Upper Cretaceous Flysch are intensely folded, as may be seen on small and discontinuous exposures. From observations at microscopic to hectometric scale, detailed structural analysis has allowed to refine the geometry of structures and to approach the kinematics of their pattern and the chronology of deformations. Comparable examples have been searched for into the calcareous chain and the axial primary zone. The actuel structure has been determined by two folding stades within a single major tectonic phase. The first stade corresponds to a compressive episode along a N-S direction, which induces folds and a regional axial plane schistosity. The second stade involves a reworking of older direction and may be seen by slipping accidents with an oblique displacement. According to the different areas, they produce, either rotations, or fold interferences and a local secondary schistosity., Dans le secteur nord-pyrénéen à l'W de Pau, zone charnière entre les Pyrénées Béarnaises et les Pyrénées Basques, les épaisses séries du Flysch crétacé supérieur sont affectées d'intenses plissotements qu'on observe sur des affleurements réduits et discontinus. L'analyse structurale de détail a permis, à partir d'observations faites de l'échelle microscopique à hectométrique, de préciser la géométrie des structures, d'aborder la cinématique de leur agencement et la chronologie des déformations. Des éléments de comparaison ont été recherchés dans les chaînons calcaires et dans la zone primaire axiale. La structure actuelle a été acquise à la suite de deux stades de plissement au sein d'une même phase tectonique majeure. Le premier stade correspond à un épisode de serrage suivant des directions N-S, produisant des plis et une schistosité régionale de plan axial. Le second stade amène la remise en mouvement de directions anciennes et se matérialise par des accidents de coulissage à déplacement oblique. Ils produisent suivant les secteurs des rotations ou des interférences de plis et une schistosité secondaire locale., Richert Jean-Paul. Analyse structurale de la zone charnière entre le Béarn et le Pays Basque. In: Bulletin du Service de la carte géologique d'Alsace et de Lorraine, tome 21, n°3, 1968. Géologie structurale, pétrographie. pp. 137-164.

Henry Jacques, Richert Jean-Paul, Ruhland Michel. Les apports de l'analyse structurale à la connaissance des Pyrénées Occidentales. In: Bulletin du Service de la carte géologique d'Alsace et de Lorraine, tome 21, n°3, 1968. Géologie structurale, pétrographie. pp. 135-136.