La chaîne himalayenne est influencée par le climat global et régional et joue un rôle de barrière orographique pour les circulations atmosphériques. Son évolution tectonique et la mousson asiatique qui l’affecte en font un laboratoire idéal pour les études du lien entre la tectonique, le climat et l’érosion et leurs implications pour l’altération. Des variations latérales de taux d’exhumation ont été documentées et des études paléoclimatiques ont été réalisées dans les parties centrale et occidentale de la chaîne, mais l’initiation, l’évolution et les caractéristiques de la mousson sont encore débattues. Les intensités et les taux d’altération engendrés ont été peu étudiés, surtout dans la partie orientale de l’Himalaya.Cette thèse se focalise sur les variations latérales d’Ouest en Est de l’altération, de la végétation (évolution C3/C4) et du climat, le long de l’Himalaya depuis le Miocène à partir de l’enregistrement sédimentaire du bassin d‘avant-pays. Les données apportent des nouvelles avancées pour la compréhension de l’évolution de la mousson asiatique. Pour ce travail, j’ai étudié trois coupes pré-Siwalik et Siwalik dans l’ouest de la chaîne (les coupes Joginder Nagar, Jawalamukhi et Haripur Kolar à Himachal Pradesh, Inde), qui documentent un enregistrement sédimentaire de 20 Ma, et une coupe dans l’est (la coupe de la Kameng à Arunachal Pradesh), qui quant à elle documente 13 Ma.Les isotopes stable du carbone de la matière organique ont été utilisés pour reconstruire des changements de la végétation C3 à C4 et montrent des variations importantes entre l’ouest et l’est de la chaîne, avec une augmentation brutale des valeurs dans l’ouest à ~7 Ma, ce qui indique un changement de végétation des plantes C3 à C4. Au contraire, aucune variation n’est notée dans l’est, ce qui indique que la partie orientale caractérisée par une végétation C3 ne subit pas de changement majeur de végétation. Les variations indiquent un « asséchement » du climat et notamment une plus grande saisonnalité dans la partie occidentale de la paléoplaine. La partie orientale reste trop humide pour une évolution vers une végétation en C4, sans doute à cause de la proximité de source d’humidité du Golfe du Bengale.Le long de la coupe orientale de la Kameng, il a été nécessaire d’étudier les changements de provenance et la diagenèse, pour les éliminer, avant de reconstruire les paléo-régimes d’altération. L’analyse des minéraux lourds a permis de repérer les changements de source et de montrer que seule la partie basse de la coupe a été influencée par la diagenèse. L’évolution de la minéralogie des argiles et des éléments majeurs montre que l’altération augmente avec le temps avec une période remarquable à ~8 Ma.La compilation des trois coupes occidentales représente l’enregistrement le plus long du bassin d’avant-pays himalayen avec une durée de 20 Ma. L’évolution des argiles montre une tendance similaire entre les parties occidentale et l’orientale du bassin d’avant-pays, impliquant l’évolution vers un climat plus saisonnier à partir de ~8 Ma. Les éléments majeurs indiquent une augmentation du taux d’altération avec le temps, la partie occidentale étant généralement plus altérée que la partie orientale. Les résultats des éléments majeurs et des argiles sont cohérents avec les interprétations des isotopes stables du carbone, qui indiquent un climat plus saisonnier dans l’ouest.Enfin, une nouvelle approche permettant de reconstruire les taux d’altération à partir de la composition isotopique du lithium a été testée pour la première fois dans les sédiments miocènes du bassin d’avant-pays. Les résultats montrent à nouveau un changement dans l’ouest de la chaîne, où les valeurs isotopiques deviennent plus positives avec le temps, alors que les valeurs dans l’est restent stables. Dans l’ouest, le facteur limitant du système limitant devient l’altération, alors que dans l’est le facteur limitant est l’apport. / The Himalaya orogen has major impact on global and regional climate and acts as an orographic barrier for atmospheric circulations. The interplay of the Asian monsoon system and the tectonic evolution of the mountain belt make it an ideal laboratory to study interactions between tectonics, climate and erosion, and its implications on weathering and atmospheric CO2 drawdown. Lateral variations in exhumation rates have been observed and studies on paleoclimate have been conducted in the central and western Himalaya, but the onset, the evolution and the characteristics of the monsoonal climate are still debated. Paleo weathering rates and intensities are challenging to reconstruct and remain poorly studied, especially in the eastern part of the orogen.This thesis focuses on lateral variations in climate, weathering and vegetation along the Himalayan mountain range, on weathering regimes in the eastern Himalaya since Miocene times, and on the implications for the evolution of the Asian monsoon. The foreland basin sediments of the pre-Siwalik and Siwalik Groups contain a record of tectonics and paleoclimate. The approach focuses on a direct west-east comparison; we therefore sampled three previously dated sedimentary sections in the western Himalaya, namely the Joginder Nagar, Jawalamukhi and Haripur Kolar, which combine into a timespan of 20 Ma, and the Kameng river section in the east, which spans over the last 13 Ma.Stable carbon isotopes on organic matter are used to reconstruct changes in vegetation. Stable carbon isotopes show important lateral variations, with a change toward more positive values in the west at ~7 Ma and in contrast no change in the east, indicating a change in vegetation from C3 to C4 plant in the west but not in the east. These variations implicate a change towards a dryer and more seasonal climate in the western Himalaya, whereas the climate in the eastern part remained too humid for C4 plants to evolve, due to its proximity to moisture source (Bay of Bengal).In order to reconstruct paleo weathering regimes by analyzing foreland basin sediments, it is important to take into account changes in provenance and possible influences of burial diagenesis. Results of heavy-mineral and petrographic analyses of the Kameng section provide better insight into diagenesis and provenance, showing that the older part of the Kameng section is influenced by diagenesis. Changes in provenance do not correlate with changes in clay mineralogy and major elements, which are therefore indicating an overall increase in weathering over time, with a remarkable change at ~8 Ma.The compilation of the three sections in the west represent one of the longest sedimentary records in the Himalayas, spanning over 20 Ma. Clay minerals show similar trends in the west and the east, indicating the development of a more seasonal climate starting at ~8 Ma. Major elements show a trend toward stronger weathering over time in the west and the east, but the western Himalaya are generally more weathered than in the east, which is consistent with the interpretation of the stable carbon isotope data, suggesting the climate to be more humid in the east. More runoff and erosion inhibit extensive weathering of the sediments, whereas dry sea sons with little runoff allow sediments to weather.Lithium isotopic compositions were measured on bulk sediments as a new approach to reconstruct chemical weathering rates, applied for the first time on Siwalik sediments. Results show a change in weathering intensity in the west, where lithium isotopic values become more positive over time, whereas, they stay relatively constant in the east. More positive values in the west, suggest that the system becomes more weathering-limited and more incongruent.

Rivers sourced in the Himalayan mountains support more than 10% of the global population, where the majority of these people live downstream of the mountain front on the alluvial Indo-Gangetic Plain. Many of these rivers however, are also the source of devastating floods. The tendency of these rivers to flood is directly related to their large-scale morphology. In general, rivers that drain the east Indo-Gangetic Plain have channels that are perched at a higher elevation relative to their floodplain, leading to more frequent channel avulsion and flooding. In contrast, those further west have channels that are incised into the floodplain and are historically less prone to flooding. Understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the Plain. This thesis examines controls on river morphology across the central portion of the Indo-Gangetic Plain drained by the Ganga River (the Ganga Plain). Specifically, the relative roles of basin subsidence, sediment grain size and sediment flux have been explored in the context of large-scale alluvial river morphology over a range of timescales. Furthermore, this thesis has developed and tested techniques that can be utilised to help quantify these variables at catchment-wide scales. This analysis has been achieved through combining new sediment grain size, pebble lithology and cosmogenic radionuclide data with quantitative topographic and sedimentological analysis of the Ganga Plain. In the first part of this thesis, I examine the contrast in channel morphology between the east and west Ganga Plain. Using topographic analysis, basin subsidence rates and sediment grain size data, I propose that higher subsidence rates in the east Ganga Plain are responsible for a deeper basin, with perched low-gradient rivers systems that are relatively insensitive to climatically driven changes in base-level. In contrast, lower basin subsidence rates in the west are associated with a shallower basin with entrenched river systems that are capable of recording climatically induced lowering of river base-level during the Holocene. Through an analysis of fan geometry, sediment grain size and lithology, I then demonstrate that gravel flux from rivers draining the central Himalaya with contributing areas spanning three orders of magnitude is approximately constant. I show that the abrasion of gravel during fluvial transport can explain this observation, where gravel sourced from more than 100 km upstream is converted into sand by the time it reaches the Plain. I attribute the over-representation of quartzitic pebble lithologies in the Plain (relative to the proportion of the upstream catchment area likely to contribute quartzite pebbles) to the selective abrasion of weaker lithologies during transport in the mountainous catchment. This process places an upper limit on the amount of coarse sediment exported into the Indo-Gangetic Plain. Finally, I consider the use of cosmogenic 10Be derived erosion rates as a method to generate sediment flux estimates over timescales of 102-104 years. Cosmogenic radionuclide samples from modern channel and independently dated Holocene terrace and flood deposits in the Ganga River reveal a degree of natural variability in 10Be concentrations close to the mountain front. This is explored using a numerical analysis of processes which are likely to drive variability in catchment-averaged 10Be concentrations. I propose that the observed variability is explained by the nature of stochastic inputs of sediment (e.g. the dominant erosional process, surface production rates, depth of landsliding, degree of mixing), and secondly, by the evacuation timescales of individual sediment deposits which buffers their impact on catchment-averaged concentrations. In landscapes dominated by high topographic relief, spatially variable climate and multiple geomorphic process domains, the use of 10Be concentrations to generate sediment flux estimates may not be truly representative. The analysis presented here suggests that comparable mean catchment-averaged 10Be concentrations can be derived through different erosional processes. For a given 10Be concentration, volumetric sediment flux estimates may therefore differ.

The genus Heracleum (Apiaceae) includes 65-70 species, and is distributed across the Northern Hemisphere from North America to East Asia (Pimenov & Leonov, 1993). The Sino-Himalaya region (Nepal eastwards to SW China) is one of the most important distribution areas of Heracleum (with 10 species in the Himalaya and about 30 species in China). This thesis integrates a series of studies on the systematics of Sino-Himalayan Heracleum including allied taxa and a taxonomic revision of Nepalese Heracleum species. The taxonomic value of the various morphological characters used in Heracleum is generally discussed and potentially important fruit morphological characters investigated alongside. Several characters were found to be distinctive, constant out of taxonomic importance. These included extent of development of the lateral wings, presence/absence of epidermal hairs, hair types, and the transverse sectional arrangement of the mericarp, including position of sclerenchyma of vascular bundles, rib development, pattern of endosperm, distribution of vittae and thickening of wing margins. Based on these characters, Heracleum was divided into at least 5 types that are also supported by molecular data (ITS DNA sequences). In order to evaluate the monophyly of Sino-Himalayan Heracleum, and clarify the species relationships within this group, sequences of ITS, rps16 intron, and psbAtrnH spacer of Heracleum and related genera were studied. The resulting data matrix was analyzed using Maximum Parsimony and Bayesian analysis. Both ITS and rps16 intron regions show a good resolution of phylogenetic relationships down to species level. However, although psbA-trnH region has been successively tested in other plant families, it failed to resolve relationships among the major Heracleum clade. Only higher level resolution was successful. The results of our comparative phylogenetic study confirmed that the “Major Heracleum clade” (Sensu Downie) remains monophyletic. Within this clade at least six major lineages are defined: Heracleum, Malabaila-Pastinaca, Zosima, Semenovia, Tordylium, and Tordyliopsis. The Sino-Himalayan Heracleum lineage is seen to comprise three distinct sub-lineages (Heracleum group I, II and III) including several European Heracleum species. Heracleum is thus not monophyletic. Generic segregates of Heracleum proposed in past classifications,such as Tetrataenium, can be correlated with these subclades. These phylogenies also helped resolve the taxonomic placement of some enigmatic taxa such as Tordyliopsis brunonis and Heracleum millefolium. These molecular data provide valuable characters for inferring relationships within the Sino-Himalayan Heracleum. A taxonomic account of the species of Heracleum in Nepal is presented including a key to identification of the species is produced. A reinterpretation of the taxonomy of Sino-Himalaya Heracleum is presented based on these results including an account of the Nepalese species.

第23回名古屋大学年代測定総合研究センターシンポジウム平成22(2010)年度報告

In their comment on our publications on pervasive remagnetization of Jurassic-Paleogene carbonate rocks of the Tibetan Himalaya (Huang et al., 2017, Journal of Geophysical Research: Solid Earth, 122, doi: 10.1002/2016JB013662 and 122, doi: 10.1002/2017JB013987), Yi et al. (2017) questioned our fold tests applied to their published paleomagenetic results from the Paleogene Zongpu and latest Cretaceous Zongshan carbonate rocks (Patzelt et al., 1996, Tectonophysics, 259(4), 259-284; Yi et al., 2011, Earth and Planetary Science Letters, 309(1), 153-165). They argued that authigenic magnetite pseudomorphic after pyrite, which is the dominant magnetic carrier within these carbonate rocks as indicated by our thorough rock magnetic and petrographic investigations, was formed during early diagenesis and that the primary natural remanent magnetization (NRM) is retained by these carbonate rocks. However, their statement for the invalidity of our fold tests is based on unrealistic assumptions that these carbonate rocks carry primary NRM and that the upper Zongpu Formation was deposited on a 10 degrees primary dip. Their argument for immediate oxidization of pyrite to authigenic magnetite after carbonate deposition onto the continental passive margin ignores that sulfate-reducing conditions were prevailing during early diagenesis, it is also inconsistent with the timing of the secondary remanence acquisition in remagnetized carbonate rocks elsewhere. As previously demonstrated, and agreed upon by Yi et al. (2017), the Zongpu and Zongshan carbonate rocks in Gamba are remagnetized; here we argue that the timing of remagnetization cannot be demonstrated to shortly postdate sedimentation. These data should therefore not be used for tectonic reconstructions.

Abstract not available

The Yarlung (India-Asia) suture zone in southern Tibet records Middle Jurassic—Late Cretaceous development of the Lhasa terrane (Eurasian) convergent margin and subsequent India-Asia collision beginning in Paleocene time. This dissertation reports data from field-based geologic investigation of the Yarlung suture zone in the Lopu Range and Lazi Regions, ~600 and ~300 km west of the city of Lhasa, respectively. Field data were combined with new geochronology (detrital and igneous zircon U-Pb, garnet Lu-Hf), thermochronology (white mica Ar-Ar and zircon U-Th/He), and metamorphic petrology data to develop a tectonic model involving multiple episodes of shallow underthrusting, rollback, and breakoff of both oceanic and continental lithosphere. Switches between extensional and contractional deformation along the Yarlung suture zone appear to be controlled by changes in subduction dynamics. If this tectonic model is representative, then the tectonic process of inter-continental collision is responsible for much larger magnitudes of crustal recycling that previously thought. A hornblende-plagioclase-epidote paragneiss block in ophiolitic mélange, deposited during Middle Jurassic time, records Late Jurassic or Early Cretaceous subduction initiation along the Eurasian margin followed by Early Cretaceous forearc extension. Detrital zircons from Xigaze forearc basin strata deposited unconformably atop ophiolitic mélange produced a maximum depositional age of 97 ± 1 Ma, providing a minimum age for establishment of an arc-forearc-trench convergent margin along the southern Lhasa terrane. Metasedimentary rocks that were originally deposited along the Indian passive margin were subducted beneath the Lhasa terrane to upper-mantle depths, reaching high-pressure (HP), low-temperature conditions (≥1.4 GPa at T≤600 °C). Garnet Lu-Hf geochronology indicates that prograde metamorphism of the Indian metasedimentary rocks was ongoing at 40.4 ± 1.4 Ma while white mica Ar-Ar thermochronology indicates exhumation to mid-crustal depths between 39-34 Ma. Gangdese arc magmatism persisted after the onset of India-Asia collision, producing plutons that intruded sedimentary-matrix mélange of the southern Lhasa terrane subduction-accretion complex between 49-37 Ma. These data suggest steep subduction or southward trench retreat immediately prior to shutdown of arc magmatism along the Yarlung suture zone (37 Ma), shortly after the onset of high-pressure rock exhumation. We interpret that these data record a Paleocene—Eocene episode of southward rollback, breakoff, and underthrusting. During Oligocene—Miocene time, nonmarine strata were deposited along the Yarlung suture zone immediately prior to shortening across a system of out-of-sequence, top-north reverse faults. Based on our data and previous work, we interpret that sedimentation was driven by a second episode of rollback and breakoff of Indian continental lithosphere, whereas subsequent contractional deformation was driven by renewed shallow subduction. Compilation of regional thermochronological data and interpretation of seismic reflection data from previous investigations suggests that the top-north reverse faults comprise a foreland-dipping passive roof duplex above the leading edge of a structurally deeper, hinterland-dipping duplex beneath the southern Lhasa terrane. The Yarlung suture zone switched from north-south contraction to east-west extension by ~16 Ma based on a crosscutting relationship between a leucogranitic dike and a normal fault related to a larger horst structure in the Lopu Range region. Tectonic exhumation in the footwall block of the horst drove cooling through zircon (U-Th)/He closure temperature (~180 °C) between 12-6 Ma.

This thesis deals with the perception and representation of spirits and landscape among Tibetan Buddhist laity in Ladakh, Himalayan India. It contrasts the conventions of Tibetan textual description of places with stories told by Ladakhi Buddhist laity, with a focus on the role played by local spirits and deities. It argues that while textual representations employing the unified and symmetrical imagery of the maṇḍala – a schematic representation of the palace of a divinity – depict the landscape as it might be known to a transcendent observer, stories about places and the spirits associated with them (lhande in Ladakhi) point to an indeterminate, fragmented and culturally unbounded world that has yet to be integrated within any single system of knowledge. This world is pieced together from multiple sources and truth claims, and from the imperfections inherent in ordinary perception; but the inconsistencies and uncertainties involved in this are not usually apparent, and are only made manifest in illness, experiences of disorder and encounters with spirits at night. These persistent uncertainties can be overcome in ritual contexts, in acts of writing or through the invocation of the faculty of divine vision: the palatial image of the maṇḍala is used to counteract the presence of the night outside. This thesis draws attention to the often overlooked role played by the limitations of perception and knowledge in understandings of landscape, and is intended to partly bridge a disciplinary divide by reconstructing the invisible context within which textual representations are created and employed.

Satellite image interpretation, geologic mapping, and paleoseismic trenching are used to investigate the Trans-Yamuna active fault system in the northwestern Doon Valley of the Indian Himalayan foothills. This east-west fault system is subparallel to and crosses the Main Boundary thrust near the structural transition from the Nahan salient to the Dehra Dun reentrant. The Trans-Yamuna active fault system may terminate to the east at a lateral ramp of the Main Boundary thrust. A south-side-up, relatively linear fault trace with variable fault dips suggests that the fault system is high-angle reverse with a component of strike-slip. It is subdivided into the Sirmurital, Dhamaun, and Bharli faults, which probably connect at depth. The Dhamaun fault is exposed where it cuts the late Holocene upper Bhatrog terrace deposit of the Giri River. A paleoseismic investigation of the Sirmurital fault at another Giri River terrace did not expose the fault, but it suggests that late Holocene terrace deposits there may be folded into a syncline parallel to fault strike. The fold axis of the syncline continues into bedrock to the west. Earthquakes in 1905, 1803, or perhaps earlier may have been the source of folding of the fine-grained sediments within this terrace deposit. The Trans-Yamuna active fault system is a secondary hangingwall fault that may accommodate some strain release above the decollement during large-magnitude earthquakes. Strike-slip motion may be related to the lateral translation of the Karakoram fault block and east-west extension of the southern Tibet block as a result of oblique convergence between the Indian and Eurasian plates in the northwest Himalaya. / Graduation date: 1999

第22回名古屋大学年代測定総合研究センターシンポジウム平成21(2009)年度報告