The mechanism through which the Appalachian Mountains have maintained moderate relief some ~300 Myr after the cessation of mountain building has long puzzled geomorphologists. As recent studies have shown that Appalachian exhumation has occurred at slow rates consistent with isostatic rebound of thickened crust, the driving forces behind localized episodes of accelerated incision and the associated rugged topography have been difficult to explain given the absence of tectonic uplift. This study uses previously undocumented relict fluvial gravels and knickpoint location to confirm the role of drainage rearrangement in producing local base level drop and subsequent basin-scale transient incision in the southern Appalachians. This process is fundamentally driven by the high potential energy of streams flowing across the elevated, slowly eroding Blue Ridge Plateau relative to the present Atlantic and landward interior base levels. Gravel deposits confirm that repeated capture of landward-draining Plateau streams by Atlantic basin streams, whose immediate base level is 250-300 m lower, forces episodic rapid incision and overall erosional retreat of the Blue Ridge Escarpment along the Plateau margin. The distribution of knickpoints, bedrock gorges, and relict surfaces in the interior of the Plateau indicate that the New River, which drains to the continental interior, is actively incising the low-relief Plateau surface due to episodic drops in landward base level. The origin of landward base level perturbation is unclear, but it may be the result of glacially-driven shortening and steepening of the lower New River during the Pleistocene. Collectively, these data indicate that rapid base level drop through drainage reorganization can energize streams in otherwise stable landscapes and accelerate fluvial incision and relief production without uplift of the land surface. This process is likely quite significant in post-orogenic settings, where inherited drainage patterns may not reflect the most direct, and thus energetically appropriate, path to present base level. Passive margins may therefore never achieve a topographic steady-state, despite uniformly slow and constant uplift due to isostatic rebound. / Ph. D.

[pt] A análise dos sistemas fluviais são de extrema importância na compreensão da evolução geomorfológica da paisagem, pois possuem características, seja no seu arranjo ou distribuição dos canais, que permitem elaborar os episódios que ocorreram ao longo do tempo. A quebra do continente Gondwana e seus múltiplos eventos até a consolidação do Rift Continental do Sudeste (RCS) implicou em uma mudança significativa no relevo sudeste e, consequentemente, nos sistemas de drenagens. Isto promoveu a reorganização dos sistemas de drenagens e uma nova dinâmica hidro-erosiva pelo rebaixado do nível de base na porção leste do continente com a consolidação do RCS. Diante do exposto, selecionou-se como estudo de caso a bacia do Rio Grande no Rio de Janeiro com o objetivo principal de entender o processo de evolução geomorfológica da bacia, a partir da organização da rede de drenagem e feições elementares da bacia frente aos eventos geológicos de formação e estruturação do substrato rochoso. A metodologia utilizada se baseia em trabalhos de gabinete, de campo e posterior análise dos dados gerados. Estes procedimentos visaram compreender a evolução do relevo na bacia do Rio Grande com base na correlação entre aspectos geológicos e geomorfológicos na organização do seu sistema fluvial. Pode-se inferir que a evolução geomorfológica da bacia do Rio Grande foi influenciada pelos múltiplos eventos tectônicos até a consolidação do RCS. Os dados demonstram forte controle do substrato rochoso na morfologia do relevo e na orientação da rede de drenagem. Os resultados das orientações dos fluxos de drenagem demonstram grande quantidade de rios nas orientações NE e NW. Acredita-se que a região correspondeu a um extenso planalto de altitude mais elevada que a atual, e sistema fluvial com direção predominante para noroeste/norte. A evolução da bacia do Rio Grande se deu através do avanço das drenagens atlânticas sobre o antigo Planalto Sudeste, através da erosão remontante e controladas pelo nível de base. / [en] Analysis of river systems are very important in understanding the geomorphological evolution of the landscape because they have characteristics, either in your arrangement or distribution channels, which enable the development of the episodes that have occurred over time. The break of the Gondwana Continent and its many events to the consolidation of Southeast Continental Rift resulted in a significant change in Southeast relief and hence the drainage systems. This promoted the reorganization of drainage systems and a new hydro-erosive dynamic by lowered the base level in the eastern portion of the continent with the consolidation of the Cenozoic RCS. Given the above, it was selected as a case study the basin of the Grande River in Rio de Janeiro with the main objective to understand the process of geomorphological evolution of the basin, from the drainage network organization and basic features of the front basin to events geological formation and structure of the bedrock. The methodology is based on office work, field and subsequent analysis of the data generated. These procedures were aimed at understanding the evolution of relief in the Rio Grande basin based on the correlation between geological and geomorphological aspects of the organization of its river system. It can be inferred that the geomorphological evolution of the Grande River basin was influenced by multiple tectonic events to the consolidation of RCS. The data demonstrate strong control of bedrock in relief morphology and orientation of the drainage system. The results of the guidelines of drainage flows demonstrate large number of rivers in the NE and NW directions. It is believed that the region corresponded to an extensive plateau higher than the current altitude, and river system with predominant direction north-west / north. The evolution of the Rio Grande basin was through the advancement of the Atlantic drainages on the old Plateau Southeast, through remontant erosion and controlled by the basic level.

No description available.

Cette étude s'inscrit dans un contexte de demande croissante en ressources en eau, conduisant à mieux contraindre et estimer la distribution des aquifères karstifiés qui représentent une ressource importante, notamment sur le pourtour Méditerranéen. Le but de ce travail est d'élaborer une méthode prédictive de la distribution de la karstification et de l'épaisseur de massifs carbonatés karstifiés, notamment pour les karsts profonds. La méthodologie est développée à partir de l'étude des karsts de la plateforme carbonatée téthysienne du Languedoc montpelliérain; région qui a subi une histoire tectonique complexe et polyphasée, entrainant des phases de karstification successives. Elle est basée sur la compréhension des relations entre eustatisme, géodynamique et niveau de base, enregistrées par les paléokarsts de la région. Le remplissage sédimentaire des paléokarsts permet d'identifier les épisodes de karstification et de les dater ; c'est un marqueur des variations -que l'on cherche à quantifier- du niveau de base local responsable de la formation et du comblement des karsts. On analyse 3 paléokarsts d'extension régionale dont on peut étudier : le remplissage karstique, la distribution le long de profils amont-aval et l'extension verticale indicatrice de l'amplitude de l'abaissement du niveau de base dont il est issu. On en déduit les causes géodynamiques à l'origine de leur formation: - Le « Paléokarst des bauxites » à remplissage caractéristique, résulte de l'évolution en 2 phases de « l'isthme Durancien » : altération bauxitique de la couverture Néocomienne sur le sud du Massif Central, liée à une inversion au Barrémien, suivi d'une surrection ≥ 1600m à l'Aptien-Albien, causant l'érosion des altérites en amont et le dépôt des bauxites détritiques en aval..- Le « Paléokarst à remplissage marin Paléocène inférieur » indiquent des variations du niveau de base local ≥350m dans la zone amont sud-cévenole. On propose que ces variations de niveau de base d'amplitude très supérieures aux variations eustatiques et dans une zone faiblement déformée, résultent d'assèchements et d'ennoiements d'un bassin endoréique à seuil.- Le « paléokarst à remplissage détritique d'origine cévenole », enregistre la chute du niveau de base au Serravalien-Tortonien, qui se traduit par l'incision de canyons dans l'arrière-pays de la marge du Golfe du Lion. On détermine une surrection de plus de 400 mètres à l'amont, près de 250m dans les blocs faillés intermédiaires et quelques dizaines de mètres dans la zone aval. En parallèle, on établit la géométrie et la distribution actuelles des massifs carbonatés dans la zone d'étude (y compris sous couverture), en construisant un modèle géologique 3D, des Cévennes à la Méditerranée. On intègre les données de sismique réflexion, de forages et les données gravimétriques. La restauration tectonique séquentielle d'une coupe Nord-Sud extraite du modèle géologique permet d'appréhender la position des paléo-niveaux de base par rapport aux massifs carbonatés, pour chaque époque de karstification. On déduit l'organisation 3D des paléo-écoulements à travers la géométrie complexe des massifs carbonatés languedociens, notamment pendant l'évènement Messinien. En terme d'évaluation de la ressource en eau, on déduit que: - Les surrections de l'arrière-pays (Crétacé « moyen », Miocène) produisent des karsts de grande amplitude, mais ne constituent pas de réservoirs significatifs.- La karstification à l'origine de réservoirs exploitables importants et profonds est créée par l'assèchement de bassins endoréiques, qui ont affecté la région au Paléocène inférieur et au Messinien. / The present study addresses the question of increasing need in water supply. One line of research consists in investigating karst aquifers that represent significant water supplies for the areas around the Mediterranean. This work aims at developing a methodology of predictive evaluation of karst distribution and karst thickness within the carbonate massifs. It is based on the study of the Tethysian carbonate platform from the area of Montpellier, South of France. This area underwent a polyphased, complex, tectonic evolution, involving successive karst forming events. The approach is based on the relationships between eustacy and geodynamics, on one hand, and base-level variation recorded within palaeokarsts, on the other hand. Sedimentary fill of palaeokarsts allows to identify distinct karstification phases and sometimes to date them. It is a marker of base-level variations, which we attempt to quantify, that is responsible for karst formation and karst filling and sealing. For three palaeokarsts of regional extent, we analyze : karst sedimentary-fill, the distribution along upstream-downstream profile, the vertical extension of palaeokarsts, indicative of the base-level lowering.- The « Bauxite palaeokarst » results from the two-stage evolution of the « Isthme Durancien » : i) Bauxitic weathering of the Neocomian cover over the southern Massif Central , related to an inversion during Barremian ; ii) ≥1600 m uplift and exhumation during Aptian-Albian, inducing erosion of alterites, upstream, and deposition of detrital bauxites, downstream.- The « Early Paleocene palaeokarsts with marine sedimentary-fill » indicate base level variations of more than 350m in the upstream area. It is suggested here that such base-level variations, exceeding eustacy and recorded in tabular undeformed series, result from desiccation-flooding events in an endoreic silled basin.- The « Miocene palaeokarst with Cevennes-sourced sediment-fill » records a Serravalian-Tortonian base-level drop, depicted by canyon incision in the hinterland of the Gulf of Lion Margin. Uplift values of ≥ 400m upstream, about 250m in an intermediate area and 10's of m downstream, are measured. The present 3D geometry and distribution of carbonate massifs (exposed and covered) are modeled in the study area, from the Cevennes to the Mediterranean. The 3D geological model integrates seismic reflection, borehole and gravimetric data. Tectonic restoration of a N-S section, extracted from the 3D model, allows to reconstruct paleo-base-levels with respect with the carbonate massifs, at different stages of deformation and successive phases of karstification. The 3D paleo-flows through carbonates across the study area, are thus constrained, especially during the Messinian desiccation stage. Application of the results to hydrogeology leads to the following :- Uplift of upstream hinterland (Cretaceous, Miocene) produces large amplitude karsts, but they do not make significant reservoirs.- Karstification responsible for the development of important and deep karst reservoirs is related to desiccation of endoreic basins (early Paleocene and Messinian).- Theses two events have produced two main deep karst reservoirs, with a gravimetric signature : (1) Upstream of Montpellier Thrust, between Pic St Loup, Les Matelles Fault and Hérault Basin, a deep karsts developed over 900m thickness in the Malm limestone, during Paleocene. (2) Downstream (south) of Montpellier Thrust, the coastal karsts extend offshore over more than 10km, they are sealed by a thick Miocene to Quaternary cover, and they result from the Messinian event.

Cette thèse examine les impacts sur la morphologie des tributaires du fleuve Saint-Laurent des changements dans leur débit et leur niveau de base engendrés par les changements climatiques prévus pour la période 2010–2099. Les tributaires sélectionnés (rivières Batiscan, Richelieu, Saint-Maurice, Saint-François et Yamachiche) ont été choisis en raison de leurs différences de taille, de débit et de contexte morphologique. Non seulement ces tributaires subissent-ils un régime hydrologique modifié en raison des changements climatiques, mais leur niveau de base (niveau d’eau du fleuve Saint-Laurent) sera aussi affecté. Le modèle morphodynamique en une dimension (1D) SEDROUT, à l’origine développé pour des rivières graveleuses en mode d’aggradation, a été adapté pour le contexte spécifique des tributaires des basses-terres du Saint-Laurent afin de simuler des rivières sablonneuses avec un débit quotidien variable et des fluctuations du niveau d’eau à l’aval. Un module pour simuler le partage des sédiments autour d’îles a aussi été ajouté au modèle. Le modèle ainsi amélioré (SEDROUT4-M), qui a été testé à l’aide de simulations à petite échelle et avec les conditions actuelles d’écoulement et de transport de sédiments dans quatre tributaires du fleuve Saint-Laurent, peut maintenant simuler une gamme de problèmes morphodynamiques de rivières. Les changements d’élévation du lit et d’apport en sédiments au fleuve Saint-Laurent pour la période 2010–2099 ont été simulés avec SEDROUT4-M pour les rivières Batiscan, Richelieu et Saint-François pour toutes les combinaisons de sept régimes hydrologiques (conditions actuelles et celles prédites par trois modèles de climat globaux (MCG) et deux scénarios de gaz à effet de serre) et de trois scénarios de changements du niveau de base du fleuve Saint-Laurent (aucun changement, baisse graduelle, baisse abrupte). Les impacts sur l’apport de sédiments et l’élévation du lit diffèrent entre les MCG et semblent reliés au statut des cours d’eau (selon qu’ils soient en état d’aggradation, de dégradation ou d’équilibre), ce qui illustre l’importance d’examiner plusieurs rivières avec différents modèles climatiques afin d’établir des tendances dans les effets des changements climatiques. Malgré le fait que le débit journalier moyen et le débit annuel moyen demeurent près de leur valeur actuelle dans les trois scénarios de MCG, des changements importants dans les taux de transport de sédiments simulés pour chaque tributaire sont observés. Ceci est dû à l’impact important de fortes crues plus fréquentes dans un climat futur de même qu’à l’arrivée plus hâtive de la crue printanière, ce qui résulte en une variabilité accrue dans les taux de transport en charge de fond. Certaines complications avec l’approche de modélisation en 1D pour représenter la géométrie complexe des rivières Saint-Maurice et Saint-François suggèrent qu’une approche bi-dimensionnelle (2D) devrait être sérieusement considérée afin de simuler de façon plus exacte la répartition des débits aux bifurcations autour des îles. La rivière Saint-François est utilisée comme étude de cas pour le modèle 2D H2D2, qui performe bien d’un point de vue hydraulique, mais qui requiert des ajustements pour être en mesure de pleinement simuler les ajustements morphologiques des cours d’eau. / This thesis investigates the impacts of climate-induced changes in discharge and base level on the morphology of Saint-Lawrence River tributaries for the period 2010–2099. The selected tributaries (Batiscan, Richelieu, Saint-Maurice, Saint-François and Yamachiche rivers) were chosen because of their differences in size, flow regime and morphological setting. Not only will these tributaries experience an altered hydrological regime as a consequence of climate change, but their base level (Saint-Lawrence River water level) will also change. A one-dimensional (1D) morphodynamic model (SEDROUT), originally developed for aggrading gravel-bed rivers, was adapted for the specific context of the Saint-Lawrence lowland tributaries in order to simulate sand-bed rivers with variable daily discharge and downstream water level fluctuations. A module to deal with sediment routing in channels with islands was also added to the model. The enhanced model (SEDROUT4-M), which was tested with small-scale simulations and present-day conditions in four tributaries of the Saint-Lawrence River, can now simulate a very wide range of river morphodynamic problems. Changes in bed elevation and bed-material delivery to the Saint-Lawrence River over the 2010–2099 period were simulated with SEDROUT4-M for the Batiscan, Richelieu and Saint-François rivers for all combinations of seven tributary hydrological regimes (present-day and those predicted using three global climate models (GCM) and two greenhouse gas emission scenarios) and three scenarios of how the base level provided by the Saint-Lawrence River will alter (no change, gradual decrease, step decrease). The effects on mean annual sediment delivery and bed elevation differ between GCM and seem to be related to whether the river is currently aggrading, degrading or in equilibrium, which highlights the importance of investigating several rivers using several climate models in order to determine trends in climate change impacts. Despite the fact that mean daily discharge and mean annual maximum discharge remain close to their current values in the three GCM scenarios for daily discharge, marked changes occur in the mean annual sediment transport rates in each simulated tributary. This is due to the important effect of more frequent large individual flood events under future climate as well as a shift of peak annual discharge from the spring towards the winter, which results in increased variability of bed-material transport rates. Some complications with the 1D modelling approach to capture the complex geometry of the Saint-Maurice and Saint-François rivers suggest that the use of a two-dimensional (2D) approach should be seriously considered to accurately simulate the discharge distribution at bifurcations around islands. The Saint-François River is used as a test case for the 2D model H2D2, which performs well from a hydraulics point of view but which needs to be adapted to fully simulate morphological adjustments in the channel.

No description available.