Géomorphologie et géomorphogenèse comparées des bourrelets marginaux brésiliens et angolais

Abstract : This work tries to bring explanations to genesis and evolution of the reliefs bordering the South central Atlantic, in Angola and Brazil. According to this purpose, it finds back the major stages of the morphologie evolution for the three great unities and draws up a synoptic list of them, which allow: the following conclusions l -The Relief modifications on land and at sea have been absolutely synchronous during the Cretaceous, partially during the Tertiary, when both "swells" are only affected by the same strong beats. 2 -Marginal swells have been rhythmically heaved in Neocomian-Aptian, Turono-Senonian, Oligocene, Pleistocene-Holocene. The upheavals were unequal leading to under and over-raised parts. 3 -The terracing of erosion surfaces, present everywhere although unequally, arises from this rythmic upheaval as well as from the evolution of forms under a climate whose basic trend was -to the semi-arid, shaping pediplain. The morphologic evolutions are thus liable to be interpreted as balance-sheets 4 -Swells have been strongly standing out as major reliefs since the lower Cretaceous with certainty, that is to say, since the unquestionable opening of the South Central Atlantic. Topographic~ permanence, synchronism of evolution require explanations whose value is global. Under and over-raised parts of swells, morphologic dive:. fication demand elucidations of local, regional scope. -The topographic permanence arises from : thermal heritages, juxtaposition of oceanic and continental lithosphere. - The thermal heritage is twofold, resulting from the tectono-thermal event (late Precambrian) and from the splitting of the gondwanian continent (lower Cretaceous). The continental litho-sphere building the swells was thermically outbalanced, heated such is not the case with the one which is to bi found inland, denser. - The swells locate the contact between oceanic and continental lithosphere. The first one cools and subsides, whereas one speaks of thermal inertia for the second one. A distorsion arises from this difference, well marked by a strong flexuration. -The synchronism of evolution calls out compensating mechanisms in relation with the differences between lithostatic pressures, and thermal heritages. The differences are brough about on the very swell by the juxtaposition of both lithospheric types, whence compensation by' creep' of partially molten material from ocean to continent under a mini-mal depth of about 40 km. The extensional faults result of pressure differences at lesser depth, make easier the rising of material and explain the wealth of dykes, heating and lightening the continental crust, provoking isostatic upheaval. The mid-atlantic ridge pushes, resulting from the relief differences between a ridge and oceanic basins, which varies according to the rhythms, of expansion, increase the creep of material under the swell, adding to the balance of constraints an horizontal component, unsteady. Indeed, periods of dyke intrusions and periods during which the rhythms of expansion vary, are firmly synchronous. In the Tertiary, however, the synchronous upheaval could be explained by the delay required between heating of the lower part of the crust during, the cretaceous events, and conveyance of thermal energy by conduction. -Under and over-raised .parts of swells are primarily owed ta the passing of lithospheric plates over hot spots. Distribution of marks let back explains a very clear alternation of parts in Africa, whereas the brazilian situation appears quite unlike. The morphologie diversification of swells arises from a varying lithologic composition of the precambrian basement, too. The cratons, not concerned by the last precambrian event, contribute with their strong lithologie heterogeneity to the shaping of disordered forms . . Thus, the marginal swells stand out first as thermically inherited reliefs. This notion of heritage is easily to be deduced from the two basic elements which explain them : lithospheric justaposition and effects of hot spots.
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Patrick Pigeon. Géomorphologie et géomorphogenèse comparées des bourrelets marginaux brésiliens et angolais. Sciences de l'environnement. Université Paris-Sorbonne - Paris IV, 1984. Français. ⟨tel-00691634⟩



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