Zusammenfassung
Das ostafrikanische Rift-System dient hier als Grundlage für ein Studium der Natur und des Alters taphrogener Lineamente in kontinentalen Platten. Besondere Betonung ist auf jene Segmente gelegt, in denen die Infrastruktur des Alt-Präkambriums durch fortgesetzte Hebung und tiefe Abtragung bloßgestellt ist. Einige Befunde deuten augenscheinlich auf eine genetische Beziehung zwischen Grabenverwerfungen im Neogen und Dislokationszonen hin, die ihren Ursprung im Präkambrium haben und bis in die tiefe Lithosphäre greifen. Diese Proto-Riftdislokationen sind im Laufe aufeinanderfolgender tektono-thermischer Zyklen wiederholt verjüngt worden und enthalten der tektonischen Tiefe gemäß Blastomylonite, Flasergneise und Migmatite und kulminieren in einer spröden Verwerfung der Oberfläche. Da die Lineamente jüngere Faltengürtel überqueren, ist der zwischen den taphrogenen und orogenen Prozessen bestehende fundamentale Unterschied besonders betont.
Geophysikalische und andere Resultate aus Afrika bestätigen jene, die im Rheingraben und im Baikalsee erzielt worden sind und zeigen, daß sich unter den kontinentalen Riftsystemen Zonen erstrecken, in denen hohe Kerntemperaturen und Einschaltungen von Erdmantelgestein mit unregelmäßig geringem spezifischem Gewicht zur Entwicklung von Strukturen führen (durchdringende Konvektion), die mit jenen unter den mittelozeanischen Dorsalen liegenden vergleichbar sind. Geologische Kartierung zeigt minimale Dilation und kontinentalen Riftvulkanismus, der sich mit einigen Ausnahmen auf den sich südlich vom Afar-Dreieck in Äthiopien bis nach Kenya erstreckenden Sektor beschränkt. Möglicherweise ist ein Verbreiten des Meeresbodens in die afrikanischen Rifte hinein durch ihre geotektonische Stellung verhindert worden sowie auch durch den Druck, dem der Kontinent durch die ihn an drei Seiten umschließenden aktiven mittelozeanischen Dorsalen ausgesetzt war.
Gewisse Befunde aus Ostafrika, dem Westafrikanischen Schild sowie vom GuianaSchild deuten darauf hin, daß sich kontinentale Lineamente bis in die ursprünglichen Rifte der Meeresbodenverbreiterung erstreckten. Daraus kann gefolgert werden, daß in der Lithosphäre über der Zone geringer Geschwindigkeit gekreuzte, ursprüngliche Lineamente existieren, von denen einige als Dislokationszonen und Kanäle für durchdringende Konvektion im Präkambrium dienten und auf diese Weise auf eine zeitweilige, tektono-thermale Verjüngung vorbereitet wurden. Einige Segmente dieser Kanäle wurden von den sich stark hebenden und mit der Meeresbodenverbreiterung verbundenen Dorsalen des Mantels gewählt und spielen darum eine Rolle in der Plattentektonik.
Abstract
The Rift System of eastern Africa is taken as a basis for an enquiry into the nature and age of taphrogenic lineaments in continental platforms. Special emphasis is placed on those segments in which the early Precambrian infrastructure has been exposed by continued uplift and deep erosion, and the evidence points to a genetic association of the Neogene graben faults with dislocation zones originating in the Precambrian and affecting the deep lithosphere. These proto-rift dislocations were repeatedly rejuvenated during successive tectonothermal cycles, showing blasto-mylonites, flaser gneisses and migmatites according to tectonic depth, and culminating in brittle surface faulting. A fundamental difference between taphrogenic and orogenic processes is emphasized by the oblique crossing of younger fold belts by the lineaments.
Geophysical and other evidence from Africa confirms data from the Rhinegraben and Lake Baikal showing that continental rift systems are underlain by zones of high heat flow and intrusions of anomalous low density mantle rock (penetrative convection), producing structures comparable with those beneath mid-ocean ridges. Geological mapping, however, proves that dilation is minimal and that rift volcanism is of continental type and, with small exceptions, is limited to a sector extending south from the Afar triangle in Ethiopia through Kenya. It is suggested that sea-floor spreading in the African rifts was prevented by their geotectonic position, and that the continent was under pressure from the active mid-ocean ridges surrounding it on three sides.
Evidence is described from East Africa, and from the West African and Guiana Shields, indicating that continental lineaments extended into the ancestral rifts of sea-floor spreading. It is concluded that in the lithosphere, above the mantle lowvelocity zone, deep criss-crossing lineaments of ancient origin exist, of which some served in Precambrian times as dislocation zones and channels for penetrative convection and were thus pre-conditioned for periodic tectonothermal rejuvenation. Certain segments of these channels have been selected by the powerful rising mantle dorsals associated with sea-floor spreading, and thus play a part in plate tectonics.
Résumé
Le système des grandes cassures de l'Afrique Orientale est pris comme exemple dans une recherche sur la nature et l'âge des linéaments taphrogéniques recoupant les plateformes continentales. Cette étude insiste spécialement sur ceux de ces segments qui font apparaître les infrastructures du Précambrien ancien par suite d'un soulèvement continu et d'une érosion profonde. Les observations géologiques montrent une relation génétique entre les fossés néogènes et les zones de dislocation remontant au Précambrien et ayant leur origine dans la lithosphère profonde. Ces dislocations «proto-rift» ont subi des rajeunissements réitérés lors de chacun des cycles «thermotectoniques» successifs, mettant à jour des blastomylonites, des flaser-gneiss et des migmatites selon la profondeur tectonique et aboutissant en surface à des failles cassantes. Une différence fondamentale entre les processus taphrogéniques et orogéniques est démontrée par le fait que les linéaments recoupent obliquement des zones mobiles.
Les résultats géophysiques et autres en Afrique corroborent ceux qui sont obtenues dans le fossé du Rhin et au Lac Baikal; ils semblent prouver que les systèmes continentaux de fossés surmontent des zones de flux calorifique élevé accompagnées de montées intrusives de roches du manteau à densité anormalement faible (convection pénétrative). Ces structures ressemblent donc aux soubassements des dorsales médioocéaniques. La cartographie géologique montre cependant que la dilatation est minime, et que le volcanisme associé aux grabens est de type continental se limitant, à peu d'exceptions près, à un segment situé entre le triangle d'Afar et le Kenya. Ces faits semblent suggérer que la position géotectonique des cassures africaines entrave leur dilatation, car le continent subit des pressions qui sont la suite des élargissements des dorsales océaniques entourant le continent dans trois directions.
Aussi bien en Afrique Orientale qu'en Afrique Occidentale et dans le bouclier de la Guyane on trouve des témoignages montrant que les linéaments continentaux s'étendaient jusque dans les disjonctions ancestrales des plaques. On arrive ainsi à la conclusion que la lithosphère au-dessus de la zone de Gutenberg est traversée par des systèmes de linéaments très anciens. Quelques-uns ont déjà servi au Précambrian localisant les dislocations, en déterminant des zones de passage pour la «convection pénétrative», créant ainsi les conditions pour un rajeunissement périodique «thermotectonique». Quelques tronçons ont servi à la montée des roches du manteau associées aux dorsales océaniques et ils jouent ainsi un rôle dans la tectonique des plaques.
Краткое содержание
На восточно-африканс кой рифтовой зоне про вели изучение природы и возраста тафрогенны х линеаментов матери ковых щитов. Упор сделали на те участки структур, в ко торых инфраструктур а древнего докембрия оказалась обнажённой в результ ате непрекращающего ся подъема и глубоко идущего снос а. Некоторые факты указ ывают генетическую с вязь между сбросами грабенов в неогене и зонами смещ ения, зародившимися е ще в докембрии и захватив шими глубины литосферы. Эт и до-рифтовые дислока ции в течение последующих тектоно-термических циклов все “омолажива лись” они содержат в тектониче ской глубине бластомилон иты, флазер-гнейсы и ми гматиты; их развитие завершаетс я поверхностными сбро сами. Т, к. линеаменты п ересекают пояса складок, то особенно подчеркнут ы фундаментальные ра зличия между тафрогенными и ороге нными процессами.
Как геофизические, та к и другие исследован ия Африки подтвердили данные, полученные на Рейнск ом грабене и Байкальс ком щите, которые показали, что под системой материк овых рифтов располаг аются зоны, в которых высокая температура ядер и вк лючений пород мантии с нерегулярно низким у дельным весом ведет к развити ю структур просачива ющейся конвекции, которые мо жно сравнить с зонами, зал егающими под среднеокеаническим и дорзалами. Картиров анием установлена незначи тельная дилатия и континентальный вул канизм рифтов, который за неб ольшим исключением органичивает на юг от треугольника Афар в Эфиопии до Кени и. Возможно, что расшир ение морского дна в зону африканских риф тов приостановили им енно геотектоническое по ложение последних и давление, которое испытывал ма терик от охватывющих его с тре х сторон активных сред неокеанических дорз алов. Известные находки в в осточной Африке,
Западно-африканском щите и щите Гвианы указывают на т о, что материковые лин еаменты простирались даже в исходные рифты распр остранения морского дна. Отсюда можно сделать вывод, что в литосфере над зо ной с небольшой скоро стью, когда-то существовал и линеаменты, часть кот орых служила зонами д ислокаций и каналов для конвекции в докембри и и т. о. подготовляли п ериоды тектоно-термическог о омоложения. Некоторы е участки этих канало в оказались путями для дорзалей мантии, резко поднявш ихся и связанных с распространением мо рского дна. Поэтому им отводят ва жную роль в тектонике платформ.
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McConnell, R.B. Evolution of taphrogenic lineaments in continental platforms. Geol Rundsch 63, 389–430 (1974). https://doi.org/10.1007/BF01820822
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DOI: https://doi.org/10.1007/BF01820822