The Tibetan plateau is arguably the most important geological feature on Earth, yet its formation and evolution are poorly understood. This investigation utilizes Cretaceous sedimentary strata exposed in the Lhasa terrane of southern Tibet in order to constrain the paleogeography and tectonic setting of the region prior to the Indo-Asian collision. Lower Cretaceous strata consist of clastic sedimentary units that were deposited in shallow marine and fluvial environments. In northern Lhasa these sediments were deposited in a peripheral foreland basin that formed in response to the Lhasa-Qiangtang collision. The lower Cretaceous sediments in southern Lhasa are quartzose and were derived from cover strata exposed by local uplifts. A marine limestone of Aptian-Albian age overlies the lower Cretaceous clastic strata and was deposited in a shallow continental seaway. The paleogeography of the Lhasa terrane during deposition of the carbonate units was dominated by the effects of the Lhasa-Qiangtang collision, although other conditions, such as a high eustatic sea-level, influenced sedimentation as well. The Upper Cretaceous Takena Formation is composed of a basal member of marine limestone and an overlying member of fluvial red beds. The arkosic strata of the Takena Formation were deposited in a retro-arc foreland basin that formed to the north of the Gangdese magmatic arc. Collectively, the Cretaceous sedimentary strata indicate significant tectonic activity occurred in southern Tibet prior to the Indo-Asian collision. Moreover, the data suggest the crust of southern Tibet was thickened and possibly at high elevations before the Cenozoic.

Made available in DSpace on 2014-06-11T19:32:17Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-09-02Bitstream added on 2014-06-13T19:42:48Z : No. of bitstreams: 1 kuerten_s_dr_rcla.pdf: 2917154 bytes, checksum: 1fef4e00aa5e9313d624978f26a3f118 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O megaleque fluvial do Nabileque é um sistema deposicional que vem sendo construído pelo rio Paraguai na borda sudoeste do Pantanal Mato-Grossense desde tempos pleistocênicos. É um sistema aluvial peculiar, pois não está associado a rios oriundos de relevos altos situados nos planaltos adjacentes à planície. Trata-se de um megaleque fluvial construído pelo rio Paraguai, rio-tronco coletor das águas do todo sistema hidrográfico do Pantanal, na saída do rio para a planície do Chaco, onde coalesce com o megaleque do Pilcomayo. Com base em dados de sensores remotos e verificação de campo, foi realizada compartimentação e caracterização geomorfológica do megaleque, que ocupa área de aproximadamente 9100 km². Descrição e datação dos depósitos dos compartimentos identificados permitiram elaborar modelo evolutivo para a área, cuja maior parte é ocupada por vasta planície aluvial pleistocênica (idades LOE entre 57,8 e 12,8 Ka AP), marcada pela presença de intrincada rede de paleocanais distributários de baixa sinuosidade a meandrantes. Feições fluviais erosivas e redes de canais tributários superimpostos evidenciam que as áreas de ocorrência dos depósitos pleistocênicos se encontram em degradação, embora sujeitas a frequentes inundações, responsáveis pela deposição de delgadas e irregulares coberturas de sedimentos mais recentes. Um aspecto notável é a existência de dois cinturões de meandros formados no Holoceno, em vales incisos na planície de depósitos pleistocênicos. O rio Paraguai corre hoje em um destes vales, que corta longitudinalmente o megaleque, ajustado a padrões de fratura NE associados ao lineamento Transbrasiliano, com deflexão para SSE ao se encontrar com o rio Negro (Bolívia), com o qual compõe a drenagem periférica do leque. No cinturão de meandros abandonado... / The Nabileque fluvial megafan is depositional system that has been built by the Paraguay River at the southwestern border of the Pantanal wetland since Pleistocene times. With an area of approximately 9100 km², it is a peculiar fan system because it is not associated with rivers flowing from adjacent plateaus. The Nabileque megafan has been forming by Paraguay River at the exit of the Pantanal wetland, in lateral coalescence with the Pilcomayo megafan of the Chaco basin. It is important to emphasize that, although the name is also used to name a small river within the wetland, the name Nabileque was adopted to designate the megafan because the whole area is known as the Pantanal of Nabileque. The establishement of its geomorphologic evolution was based on remote sensing data interpretation with field ground truthing, sediment vibrocore-sampling and optical luminescence dating (OSL). Most of the area is covered by alluvial deposits, which dating has revealed ages spanning from 57.8 to 12.8 ka BP, but probably older deposits are present that were not reached by vibrocoring. Distributary, low-sinuosity to meandering paleochannels are visible as ancient depositional geoforms, but the Pleistocene alluvial surface is degradational and the alluvial deposits have being affected by pedogenesis and dissected by tributary small channels. Despite its degradational nature, the area is inundated during exceptional flooding events; this process is responsible for reworking and depositing of thin and irregular layer of Holocene sediments capping the oldest deposits. Two Holocene meander belts, confined in broad and shallow incised-valleys, are conspicuous features and record paleo-hydrologic changes. One of them is an abandoned N-S meander belt, in which runs the Nabileque River that has a channel much smaller... (Complete abstract click electronic access below)

Orientador: Mario Luis Assine / Coorientador: Edna Maria Facincani / Banca: Sandra Baptista da Cunha / Banca: Mauro Parolin / Banca: Edvard Elias de Souza Filho / Banca: José Cândido Stevaux / Resumo: O megaleque fluvial do Nabileque é um sistema deposicional que vem sendo construído pelo rio Paraguai na borda sudoeste do Pantanal Mato-Grossense desde tempos pleistocênicos. É um sistema aluvial peculiar, pois não está associado a rios oriundos de relevos altos situados nos planaltos adjacentes à planície. Trata-se de um megaleque fluvial construído pelo rio Paraguai, rio-tronco coletor das águas do todo sistema hidrográfico do Pantanal, na saída do rio para a planície do Chaco, onde coalesce com o megaleque do Pilcomayo. Com base em dados de sensores remotos e verificação de campo, foi realizada compartimentação e caracterização geomorfológica do megaleque, que ocupa área de aproximadamente 9100 km². Descrição e datação dos depósitos dos compartimentos identificados permitiram elaborar modelo evolutivo para a área, cuja maior parte é ocupada por vasta planície aluvial pleistocênica (idades LOE entre 57,8 e 12,8 Ka AP), marcada pela presença de intrincada rede de paleocanais distributários de baixa sinuosidade a meandrantes. Feições fluviais erosivas e redes de canais tributários superimpostos evidenciam que as áreas de ocorrência dos depósitos pleistocênicos se encontram em degradação, embora sujeitas a frequentes inundações, responsáveis pela deposição de delgadas e irregulares coberturas de sedimentos mais recentes. Um aspecto notável é a existência de dois cinturões de meandros formados no Holoceno, em vales incisos na planície de depósitos pleistocênicos. O rio Paraguai corre hoje em um destes vales, que corta longitudinalmente o megaleque, ajustado a padrões de fratura NE associados ao lineamento Transbrasiliano, com deflexão para SSE ao se encontrar com o rio Negro (Bolívia), com o qual compõe a drenagem periférica do leque. No cinturão de meandros abandonado... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The Nabileque fluvial megafan is depositional system that has been built by the Paraguay River at the southwestern border of the Pantanal wetland since Pleistocene times. With an area of approximately 9100 km², it is a peculiar fan system because it is not associated with rivers flowing from adjacent plateaus. The Nabileque megafan has been forming by Paraguay River at the exit of the Pantanal wetland, in lateral coalescence with the Pilcomayo megafan of the Chaco basin. It is important to emphasize that, although the name is also used to name a small river within the wetland, the name Nabileque was adopted to designate the megafan because the whole area is known as the Pantanal of Nabileque. The establishement of its geomorphologic evolution was based on remote sensing data interpretation with field ground truthing, sediment vibrocore-sampling and optical luminescence dating (OSL). Most of the area is covered by alluvial deposits, which dating has revealed ages spanning from 57.8 to 12.8 ka BP, but probably older deposits are present that were not reached by vibrocoring. Distributary, low-sinuosity to meandering paleochannels are visible as ancient depositional geoforms, but the Pleistocene alluvial surface is degradational and the alluvial deposits have being affected by pedogenesis and dissected by tributary small channels. Despite its degradational nature, the area is inundated during exceptional flooding events; this process is responsible for reworking and depositing of thin and irregular layer of Holocene sediments capping the oldest deposits. Two Holocene meander belts, confined in broad and shallow incised-valleys, are conspicuous features and record paleo-hydrologic changes. One of them is an abandoned N-S meander belt, in which runs the Nabileque River that has a channel much smaller... (Complete abstract click electronic access below) / Doutor

Orientador: Maria Luis Assine / Banca: Márcio de Morisson Valeriano / Banca: Archimedes Perez Filho / Resumo: Esta dissertação visa caracterizar mudanças morfológicas recentes no megaleque do Taquari, através de dados de Sensoriamento Remoto. As baixas declividades do terreno e pequenas amplitudes das formas deposicionais, que predominam nesta paisagem, demandaram o desenvolvimento de métodos específicos para a extração das informações. Dados SRTM e imagens multiespectrais foram utilizados no mapeamento dos paleocanais e confecção de modelos digitais de elevação. A drenagem relicta foi mapeada através de desenho digital, sobre mosaicos georreferenciados dos sensores ASTER, MODIS e CBERS2. Nos vetores dos paleocanais identificados foi realizado cálculo de densidade areal e criados histogramas direcionais (rosetas), que ressaltaram relações topológicas e permitiram a interpretação de seis lobos deposicionais. Modelos digitais de elevação foram gerados através da subtração dos dados SRTM originais com uma superfície topográfica simplificada pelo método de regressão polinomial cúbica. Este processamento ressaltou zonas deprimidas no megaleque, espaços potenciais para acúmulo de sedimentos, e zonas de sobrelevadas, devido à processos agradacionais contínuos em determinadas áreas. A identificação e caracterização destas formas é passo inicial para a reconstituição dos eventos geomórficos e antevisão de futuras mudanças no curso do rio Taquari. A dinâmica de construção e abandono dos lobos mostrou-se mais complexa do que o apresentado em trabalhos anteriores, sendo de aspecto fragmentado e de natureza fractal. / Abstract: This work aims characterize morphological changes in Taquari megafan, by remote sensing techniques. Gentle slopes of landscape and low amplitudes of geoforms demands the development of specific methods to extract valuable topographic information. SRTM data and multispectral images were used to map paleochannels and to build digital elevation models (DEM). Relict streams were mapped by computer-aided design in GIS environment, using georeferenced mosaics of ASTER, CBERS and MODIS sensors. These vectors were used to calculate drainage density and directional histograms, revealing topological aspects and lead to interpret 6 depositional lobes. DEM was obtained by subtracting the original SRTM data from a simplified surface generated by cubic polynomial regression. This product revealed depressed areas in the Taquari megafan, potential accommodation spaces for deposition, and geoforms with a higher amplitude than their neighbor, probably because of continuous gradational processes in these specifics sites. The recognition of these landforms is the initial step for figure out the evolution of geomorphic events and to support a well done planning in land occupation. The dynamics of Taquari megafan showed to be more complex than previous studies proposed. / Mestre

Made available in DSpace on 2014-06-11T19:26:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-10-18Bitstream added on 2014-06-13T20:54:16Z : No. of bitstreams: 1 zani_h_me_rcla.pdf: 8429664 bytes, checksum: 62c5b99b684e6da2a2faa22d20ae412e (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Esta dissertação visa caracterizar mudanças morfológicas recentes no megaleque do Taquari, através de dados de Sensoriamento Remoto. As baixas declividades do terreno e pequenas amplitudes das formas deposicionais, que predominam nesta paisagem, demandaram o desenvolvimento de métodos específicos para a extração das informações. Dados SRTM e imagens multiespectrais foram utilizados no mapeamento dos paleocanais e confecção de modelos digitais de elevação. A drenagem relicta foi mapeada através de desenho digital, sobre mosaicos georreferenciados dos sensores ASTER, MODIS e CBERS2. Nos vetores dos paleocanais identificados foi realizado cálculo de densidade areal e criados histogramas direcionais (rosetas), que ressaltaram relações topológicas e permitiram a interpretação de seis lobos deposicionais. Modelos digitais de elevação foram gerados através da subtração dos dados SRTM originais com uma superfície topográfica simplificada pelo método de regressão polinomial cúbica. Este processamento ressaltou zonas deprimidas no megaleque, espaços potenciais para acúmulo de sedimentos, e zonas de sobrelevadas, devido à processos agradacionais contínuos em determinadas áreas. A identificação e caracterização destas formas é passo inicial para a reconstituição dos eventos geomórficos e antevisão de futuras mudanças no curso do rio Taquari. A dinâmica de construção e abandono dos lobos mostrou-se mais complexa do que o apresentado em trabalhos anteriores, sendo de aspecto fragmentado e de natureza fractal. / This work aims characterize morphological changes in Taquari megafan, by remote sensing techniques. Gentle slopes of landscape and low amplitudes of geoforms demands the development of specific methods to extract valuable topographic information. SRTM data and multispectral images were used to map paleochannels and to build digital elevation models (DEM). Relict streams were mapped by computer-aided design in GIS environment, using georeferenced mosaics of ASTER, CBERS and MODIS sensors. These vectors were used to calculate drainage density and directional histograms, revealing topological aspects and lead to interpret 6 depositional lobes. DEM was obtained by subtracting the original SRTM data from a simplified surface generated by cubic polynomial regression. This product revealed depressed areas in the Taquari megafan, potential accommodation spaces for deposition, and geoforms with a higher amplitude than their neighbor, probably because of continuous gradational processes in these specifics sites. The recognition of these landforms is the initial step for figure out the evolution of geomorphic events and to support a well done planning in land occupation. The dynamics of Taquari megafan showed to be more complex than previous studies proposed.

This thesis aims at deciphering the respective roles of autogenic processes and allogenic forcing in the post-orogenic evolution of a coupled mountain catchment/foreland megafan system, with a focus on the Lannemezan megafan and its mountainous catchment in the northern Pyrenees (France). AFT data are consistent with previously published thermochronological data, showing (i) the main exhumation phase of the Axial Zone (AZ; ~50-30 Ma) with lateral variations in the exhumation rates, (ii) a later unroofing of the North-Pyrenean Zone (NPZ; since ~20 Ma), and (iii) a late (~20-15 Ma) exhumation phase only recorded in the western part of the range (west of the Neste River). The megafan was thus built at a late stage of the orogeny and its source catchment lays on the boundary between two tectonic domains. Petrographic data shows that the source area encompasses the NPZ and AZ but modern river sands yield more granitic material than the megafan sediments. This discrepancy suggest that (i) the foreland deposits are locally sourced, but this is not consistent with the definition of a megafan, (ii) the crystalline massifs may have been insufficiently unroofed, but this is inconsistent with the presence of granitic material in Eocene deposits of the south foreland, (iii) the main drainage divide migrated to the south around that period, but this seems inconsistent with the exhumation history of the range as inferred from thermochronological data. Comparison of my estimation of the sedimentary budget and AFT-derived exhumation rates suggests that the current watershed of the Neste is more than sufficient to provide the material for the building of the Lannemezan megafan. This implies that (i) the catchment boundaries have not necessarily changed since the Miocene and (ii) a significant portion of the sediments bypassed the megafan. I dated the abandonment of Quaternary surfaces using cosmogenic nuclides (10Be, 26Al). The abandonment of the megafan (~300 ka) and of the alluvial terraces (~100ka, ~20ka and ~1ka) suggests that incision episodes are triggered by cold-to-warm climatic shifts. The terrace slopes increase with time: I propose that the region was subjected to northward tilting by flexural isostatic rebound as a response to erosional unloading of the range during the Quaternary. From the morphometric analysis (χ-proxy, steepness index, concavity, long river profiles) I show (i) drainage reorganization near the megafan apex and (ii) no influence on the drainage network from active tectonic structures or base-level variations. The abandonment and of the Lannemezan megafan is thus linked to the capture of the Neste by the Garonne and its episodic incision is linked to Quaternary climatic cycles. Numerical modelling adequately reproduce the building of a large megafan in 15 Ma by progradation of the deposits carried by a central river that becomes dominant in the sediment routing. I suggest that, in the northern Pyrenean foreland, the Atlantic Ocean to the west and large, efficient rivers create the conditions needed for a river to be singled out and build a megafan. Negligible subsidence at during the Miocene may have encouraged progradation. I suggest that, in the northern Pyrenees, the main driving force for long-term entrenchment of the megafan is linked to autogenic processes but these are modulated by short-term climatic changes (and possibly isostatic movements). Numerical modelling also evidenced the strong coupling between the mountainous catchment and the basin and the characteristic response times (on the order of 10 ka). / Le but de cette thèse est de comprendre ce qui contrôle l’évolution géomorphologique post-orogénique d’un ensemble composé d’un bassin versant montagneux et de son piedmont à partir de l’exemple du méga-cône alluvial de Lannemezan et son bassin versant au flanc nord des Pyrénées. Les données de thermochronologie in-situ et détritiques confirment les phases d’exhumations : (1) une phase d’exhumation majeure (~50-30 Ma), enregistrée dans toute la ZA bien qu’avec des vitesses différentes à l’est (Ariège) qu’à l’ouest, (2) une excavation plus tardive de la ZNP (à partir de ~20 Ma), et (3) une phase tardive d’exhumation (vers 15-20 Ma) enregistrée seulement à l’ouest. Le méga-cône se forme donc à une période post-orogénique où la tectonique cesse de jouer un rôle dominant, mais le bassin versant d’alimentation du cône se situe à une position charnière entre deux zones qui s’exhume différemment. Les données de pétrographie des sédiments indiquent une zone source qui inclut la ZNP et la ZA. La proportion de matériel cristallin dans les sédiments Oligo-Miocène est plus faible que celle des sédiments des rivières actuellesce qui pourrait suggérer que (1) les massifs cristallins n’étaient pas suffisamment exhumés mais cela semble incohérent avec les études précédentes, (2) que la ligne de partage des eaux a migré vers le sud, mais ce n’est pas cohérent avec l’histoire d’exhumation de la chaîne. J’ai comparé le volume érodé dans le bassin pour produire le méga-cône et le flux sédimentaire estimé à partir des taux d’exhumation : il semble que le bassin versant actuel de la Neste est plus que suffisant pour produire les sédiments du cône, ce qui suggère que (1) la taille du bassin versant n’a pas nécessairement changé et (2) une partie des sédiments a été directement exportée vers le bassin marin. Les résultats des datations par nucléides cosmogéniques (10Be et 26Al) suggèrent que le cône est abandonné vers ~300 ka et les terrasses alluviales qui bordent les rivières du piémont (~100ka, ~20ka et ~1ka) pourraient suggérer un contrôle climatique sur l’incision (cycles glaciaires/interglaciaires). Pour expliquer la pente des terrasses je propose l’hypothèse d’un basculement de la région en lien avec un rebond isostatique au Quaternaire. L’analyse morphométrique du réseau de drainage montre (i) une réorganisation par capture dans la partie sud du cône et (ii) qu’il n’y a pas de structure tectonique active ou de variation du niveau marin. J’ai utilisé un modèle numérique pour comprendre les conditions de formation et d’incision d’un méga-cône et comparé les résultats avec les observations faites sur le cône de Lannemezan et son bassin versant. Les résultats suggèrent que le temps de construction d’un tel cône est de l’ordre de 15 Ma, par progradation des dépôts d’une rivière centrale qui domine les apports du piémont. La comparaison avec le cas de Lannemezan suggère que (1) l’Océan Atlantique à l’ouest et les grandes rivières, au transport efficace créent un environnement propice à la formation du méga-cône; (2) la subsidence négligeable du piémont pourrait avoir favorisé la construction du méga-cône; (3) les processus autogéniques sont suffisants pour produire au long terme une incision permanente du cône par sa rivière d’alimentation. Les forçages externes (tectoniques, climat) ne font que moduler l’incision autogénique. La réponse du paysage à un forçage climatique cyclique montre un temps caractéristique de l’ordre de 10 ka. Finalement, la modélisation a permis de mettre en évidence les interactions entre le piémont et la chaîne.

L'évolution d'une chaîne de montagne résulte des rétroactions entre le climat et la tectonique via l'érosion. La chaîne Himalayenne, soumise à des processus climatiques et tectoniques très actifs est un chantier d'étude idéal pour comprendre ces interactions. Dans cette étude nous nous sommes intéressés à l'évolution topographique de la région du Sikkim (Inde) ainsi qu'aux processus de formation et de développement du mégafan Quaternaire de la Tista, dépôt fluviatile situés au pieds de ces reliefs. Nos résultats indiquent qu'à l'échelle géologique, les taux d'exhumation (obtenus par les traces de fission sur apatite) les plus forts sont concentrés dans le Sud-Ouest du Sikkim (1.2 ± 0.6 mm/an) suggérant un contrôle tectonique lié à la formation du duplex du Moyen-Pays (« Rangit Duplex »). Les taux d'érosion à l'échelle millénaire (obtenus par 10Be) les plus forts (≈ 5 mm/an) sont enregistrés au niveau de la « Main Central Thrust Zone », mais les zones sources de ces échantillons semblent affectées par des glissements de terrain menant à une surestimation de ces taux. Des taux d'érosion importants sont également enregistrés au Nord du Sikkim (1 à 2 mm/an) et coïncident avec les fortes valeurs de l'indice ksn, localisées au niveau de l'altitude d'avancée maximale des glaciers durant le Dernier Maximum Glaciaire. Nous en concluons que l'érosion millénaire au Sikkim soit en partie liée à l'héritage géomorphologique glaciaire. Le mégafan de la Tista stocke les produits d'érosion du Sikkim et s'étend sur plus de 16 000 km2 depuis le front de la chaîne, où il est fortement incisé, jusqu'à la confluence de la Tista avec le Brahmapoutre. Nous présentons un scénario de développement du mégafan avec le développement successifs de deux lobes distaux en aval d'un lobe proximal commun, développé au front de la chaine depuis au moins 50 000 ans et incisé par la Tista depuis 4000+600/-400 ans. Le lobe distal occidental a été formé tôt dans l'histoire du mégafan, par des rivières drainant le Sikkim et se jettant dans le Gange, et a été abandonné au début de l'Holocène (10 000 – 11000 ans). Le lobe distal oriental, très peu incisé et plus récent (< 1000 ans), s'est construit quand le réseau de drainage principal du Sikkim a migré vers l'Est grâce à d'importantes avulsions. Les périodes d'aggradation et d'incision des rivières sont compatibles avec un guidage climatique, où les périodes de forte intensité de mousson et de forts flux sédimentaires associés coïncident avec des phases d'aggradation dans l'avant-pays. Les phases initiales et terminales de ces périodes de fortes moussons sont caractérisées par de l'incision dans l'avant pays. En revanche, le rôle de l'activité tectonique sur l'alluvionnement et l'incision du mégafan semble mineur. Les résultats pétrographiques et isotopiques (Sr et Nd) utilisés pour contraindre les sources des sédiments du mégafan indiquent une forte différence avec les sédiments actuellement transportés par la Tista qui pourrait s'expliquer par l'influence de roches sédimentaires téthysiennes crétacés exposées au nord du Sikkim, et actuellement drainées par la Kosi. Cela suggère que cette zone ait été récemment (4000 ans) capturée par la Kosi, provoquant l'incision récente et importante du mégafan, due au rééquilibrage du profil de la Tista. Cette hypothèse préliminaire permettrait d'expliquer également la taille disproportionnée du mégafan comparée à celle de son bassin versant actuel. Par ailleurs, nos résultats montrent que les variations de l'érosion et de l'altération chimique répondent à des variations climatiques globales et régionales. Les périodes de fortes intensités de précipitations de mousson se traduisent au Sikkim par une plus forte pénétration des précipitations dans la haute chaîne et une plus forte altération chimique des silicates. Depuis le dernier maximum glaciaire, la région du Sikkim est caractérisée par une augmentation de l'érosion et de l'altération chimique. / Understanding the relative strengths of tectonic and climatic forcing, as well as the interactions between these controls, at different spatial and temporal scales is important to understand the evolution of orogenic topography. The Himalayas, subjected to both tectonic and climatic phenomena very active, is one of the most popular sites for the study of these interactions. The focus of this project is to understand (1) the evolution of orogenic topography of Sikkim (India), (2)the processus of formation and development of th quaternary Tista megafan. Our results show that long-term exhumation rates obtained by thermochronology (apatite fission tracks) do not correlate with any geomorphic or climatic parameters. We suggest a tectonic control where high rates in southwest Sikkim (1.2±0.6 mm/yr) may be linked to the building of the lesser Himalaya Rangit Duplex. The highest erosion rates recorded at millennial timescales (10Be ∼5 mm/yr) occur in catchments spanning the Main Central Thrust Zone but appear to be strongly influenced by recent landsliding. High erosion rates (1-2 mm/yr) also occur in north Sikkim and may be linked to a strong glacial inheritance on the landscape, as attested by high channel-steepness values close to the maximum extent of glaciers during the Last Glacial Maximum. In contrast, variations in precipitation rate do not seem to strongly influence either short-term erosion or long-term exhumation rates in Sikkim. The Tista megafan spreads over more than 16 000 km2 form the mountain front, where it is strongly incised, to the confluence of the Tista River with the Brahmaputra River, and stores sediments produced in the Sikkim Himalaya. We propose a scenario for the formation and abandonment of the Tista megafan based on new Infra-Red Stimulated Luminescence (IRSL) and 10Be cosmogenic age constraints. We suggest that two distal lobes developed successively downstream from a common proximal lobe. Deposition took place since ~50 ka assuming a constant sedimentation rate of 0.88±0.10 mm.yr-1 and incision began at 〖4.0〗_(-0.4)^(+0.6) ka at an average rate of 〖10.5〗_(-1.8)^(+0.6) mm yr-1. The western distal lobe of the megafan was deposited early in the history of the megafan, possibly when the Sikkim Himalaya catchments were drained by rivers that where tributaries of the Ganga River, and was abandoned in the early Holocene (10-11 ka). The eastern poorly incised and recent (<1ka) may have been built when the main Tista drainage system had shifted eastward through possible nodal avulsions. Tectonic processes seem to play a minor role in guiding the Tista River course and driving incision of the megafan deposits. Abandonment and incision of megafan surfaces and hinterland terraces appears associated to both the onset and the ending of phases of strong monsoon precipitation, during which the balance between water and sediment discharge changes rapidly. Petrographic ans isotopic (Nd and Sr) used to constrain sources of the megafan deposits display a significant difference with sediments actually transported by the Tista River, which could be explained by the influenced of tethyan sedimentary rocks exposed North to Sikkim, and actually drained by the Kosi River. This suggests that this area has been recently (4000 years) captured by the Kosi River, inducing a recent and important incision of the megafan in its proximal part due to the balancing of the river profile. This preliminary hypothesis could also explain the higher size of the megafan comparing to the one of its catchment. Otherwise, our results show that the variations of erosion and chemical alteration respond to global and regional climatic variations. Periods of strong summer monsoon and heavy rainfall are traduced in Sikkim by the more penetration of precipitation into the relief and a strong chemical weathering. Since the Last Glacial maximum, the Sikkim Himalaya is characterised by an augmentation of erosion and chemical weathering.

The fault-bounded Cianzo basin represents a Cenozoic intermontane depocenter between the Puna plateau and Eastern Cordillera of the central Andean fold-thrust belt in northern Argentina. New characterizations of fold-thrust structure, nonmarine sedimentation, and sediment provenance for the shortening-induced Cianzo basin at 23°S help constrain the origin, interconnectedness, and subsequent uplift and exhumation of the basin, which may serve as an analogue for other intermontane hinterland basins in the Andes. Structural mapping of the Cianzo basin reveals SW and NE-plunging synclines within the >6000 m-thick, upsection coarsening Cenozoic clastic succession in the shared footwall of the N-striking, E-directed Cianzo thrust fault and transverse, NE-striking Hornocal fault. Growth stratal relationships within upper Miocene levels of the succession indicate syncontractional sedimentation directly adjacent to the Hornocal fault. Measured stratigraphic sections and clastic sedimentary lithofacies of Cenozoic basin-fill deposits show upsection changes from (1) a distal fluvial system recorded by vi fine-grained, paleosol-rich, heavily bioturbated sandstones and mudstones (Paleocene‒Eocene Santa Bárbara Subgroup, ~400 m), to (2) a braided fluvial system represented by cross-stratified sandstones and interbedded mudstones with 0.3 to 8 m upsection-fining sequences (Upper Eocene–Oligocene Casa Grande Formation, ~1400 m), to (3) a distributary fluvial system in the distal sectors of a distributary fluvial megafan represented by structureless sheetflood sandstones, stratified pebble conglomerates and sandstones, and interbedded overbank mudstones (Miocene Río Grande Formation, ~3300 m), to (4) a proximal alluvial fan system with thick conglomerates interbedded with thin discontinuous sandstone lenses (upper Miocene Pisungo Formation, ~1600 m). New 40Ar/39Ar geochronological results for five interbedded volcanic tuffs indicate distributary fluvial deposition of the uppermost Río Grande Formation from 16.31 ± 0.6 Ma to 9.69 ± 0.05 Ma. Sandstone petrographic results show distinct upsection trends in lithic and feldspar content in the Casa Grande, Río Grande, and Pisungo formations, potentially distinguishing western magmatic arc (Western Cordillera) sediment sources from evolving eastern thrust-belt sources (Puna‒Eastern Cordillera). In addition to growth stratal relationships and 40Ar/39Ar constraints, conglomerate clast compositions reflect distinct lithologic differences, constraining the activation of the Cianzo thrust and coeval movement on the reactivated Hornocal fault. Finally, U-Pb geochronological analyses of sandstone detrital zircon populations in conjunction with paleocurrent data and depositional facies patterns help distinguish localized sources from more distal sources west of the basin, revealing a systematic eastward advance of Eocene to Miocene fold-thrust deformation in the central Andes of northern Argentina. / text