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Sédiments et reliefs du front des systèmes chevauchants : modélisation et exemples du front andin et des Siwalik (Himalaya) à l'Holocène .

Abstract : Fold and thrust belts are characterised by tectonic activity, erosion and sedimentation which act simultaneously to create relief and syntectonic basins. The aim of this work is a better understanding and quantifying of those three phenomena control on 1) Reliefs morphology; 2) sedimentation localisation and kind of deposits; 3) tectonic activity localisation. To reach this aim, I developed a numerical model involving a simulation of the growing of fault propagation folds coupled with a diffusion law simulating superficial processes (erosion/sedimentation). It is presented in the first part of the work. It allows to study erosion and sedimentation distribution according to tectonic activity and to relations between folding and thrusting. The use of this numerical model also allows to analyse tbrust belts evolution. It use in the case of the Subandean zone of Bolivia shows that 1) the fold and thrust belt evolution and the development of the large piggy-back basins are controlled by the geometry of the initial sedimentary wedge and by the geometry of the basal decollement; 2) Superficial transports that allow the formation of wide basins influence deformation localisation by controlling the surface geometry of the wedge. ln the second part of tbis work, I applied this new methodology to the Siwalik foothills in Western Nepal. ln this area where the tbrust system is known better and better as numerous field works have been done, wide piggy-back basins are developing with an unequal distribution. Modellisations show that the dip of the basal decollement is the major parameter that determines the development of those basins. A steep basal decollement, which is coupled with thick tbrust sheets, favours the development of piggy-back basins. In the opposite case, that is a gentle dip of the basal decollement with thin tbrust sheets, the steep surface slope favours erosion and out-of sequence reactivations. Those results have been confirmed by balanced cross sections realised through the Siwalik wedge of western Nepal. Actually those sections show that piggy-back basins are weIl developed when the wedge is made of thick thrust sheets (5500 m), while when the thrust system is thinner (4000 m) out of sequence reactivations of an internaI thrust are observed. Modellisations also show that the lateral development of frontal fault propagation folds controls the syncline and the internai ridge morphologies. It is confirmed by a morpho structural study which bring out that the frontal structure in Siwalik, is composed of short segments which relay, and that the system morphology is controlled by those relays. ln Siwalik of west Nepal, there is a seismic gap, where instantaneous velocities are weak (2-7 mm/yr). The study of abandonment of alluvial terraces in syntectonic basins shows that 1) the Holocene infill in Deukhury valley depends on the tectonic activity localised along thrusts neighbouring the basin and forming tectonic dams; 2) the recording of climatic changes is masked by the importance of the tectonic activity; 3) uplift velocities of those terraces deduced from CI 4 dating, allow to calculate mean shortening velocities around 15-20 mm/yr during the Holocene period; 4) there is numerous out-of sequence reactivations, locaiised along an intermediate thrust named MDT. Weak actual velocities deduced from GPS measurements do not represent tectonic activity of this part of the Himalayan chain during long periods. Difference between actual velocities and mean velocities recorded in this Nepalese area, suggests a momentary slow down in which is in accord with an interseismic period of a seismic cycle. The present day would then be . Periods of tectonic quiescence could alternate with period with intense tectonic activity. Alternation of localisation of shortening along the frontal structure or along more internai thrusts would follow a cycle of several thousand years. One step of this cycle would be characterised by all the shortening localised along the frontal structure, the other step would be characterised by a shortening equally distributed between the frontal structure and an internai thrust.
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Submitted on : Friday, October 21, 2011 - 9:10:05 AM
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Pascale Leturmy. Sédiments et reliefs du front des systèmes chevauchants : modélisation et exemples du front andin et des Siwalik (Himalaya) à l'Holocène .. Tectonique. Université de Grenoble, 1997. Français. ⟨tel-00634396⟩



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