Holocene Hydrological Changes Inferred from Alluvial Stream Entrenchment in North Tian Shan (Northwestern China)

We analyze the possible contribution of climate change or tectonics on fluvial incision from the study of a case example along the northern flank of Tian Shan. The rivers that exit the high range fed large alluvial fans by the end of the last glacial period. They have since deeply entrenched the piedmont by as much as 300 m. We have surveyed several terraces that were cut and abandoned during river entrenchment, providing information on intermediate positions of the riverbed during downcutting. They suggest a gradual decline in river slope during a major phase of incision throughout the Holocene. Tectonic uplift affects only a zone about 5 km wide, corresponding to a growing anticline, and is shown to account for about 10% of total incision. Incision was therefore most probably driven by climate change. From observed fluvial incision, we estimate the water discharge in excess of that needed to carry the sediments supplied by hillslope erosion in the headwaters. We used a model based on a transport‐limited erosion law. The model predicts relaxation process with entrenchment in the upper reach, downstream progradation of the incision‐sedimentation line, and a progressive decrease of river slope during incision consistent with our observations. According to this model, river slope might be used as a proxy for specific discharge and then for volumetric discharge, provided that an assumption is made about river width variations. We conclude that river incision in the study area has resulted from dynamic adjustment of the hydrological system to the settlement of wetter conditions in the early Holocene, when water discharge might have been about three times as high as at present. Then, a rather arid climate with enhanced seasonality has likely prevailed from the mid‐Holocene (~6 ka B.P.) until now

Induced seismicity along a fault due to fluid circulation: conception and application

It is believed that the some seismicity is driven by the fluid circulation within fault zone and different rheology models have been proposed principally based on the Darcy's law, fluid flow in porous medium. Although it is very difficult to quantify such feature in natural seismicity (some aftershocks of large earthquakes, or seismicity in subduction), the direct application is the induced seismicity at the geothermal sites where micro-fracturing (seismicity) is necessary to allow fluid circulation between two wells and thus the assessment of such seismicity becomes also important. In this study, we construct a conceptual model for the simulators, taking into account of elastic and plastic porosity change (e.g. Segall and Rice, 1995) and fault width evolution (e.g. Yamashita, 1999), supposing first that the seismicity (fluid flow) expands dominantly along a plane. In fact, for an injection of about a few 10 l/s, pore pressure increases immediately (about 1 min) up to more than 10 MPa. This is much faster than the fluid circulation in general. This requires that the fracturing co-seismic process should play a dominant role for bringing the fluid circulation

Self-induced seismicity due to fluid circulation along faults

International audienceIn this article, we develop a system of equations describing fluid migration, fault rheology, fault thickness evolution and shear rupture during a seismic cycle, triggered either by tectonic loading or by fluid injection. Assuming that the phenomena predominantly take place on a single fault described as a finite permeable zone of variable width, we are able to project the equations within the volumetric fault core onto the 2D fault interface. From the basis of this "fault lubrication approximation", we simulate the evolution of seismicity when fluid is injected at one point along the fault to model induced seismicity during an injection test in a borehole that intercepts the fault. We perform several parametric studies to understand the basic behaviour of the system. Fluid transmissivity and fault rheology are key elements. The simulated seismicity generally tends to rapidly evolve after triggering, independently of the injection history and end when the stationary path of fluid flow is established at the outer boundary of the model. This self-induced seismicity takes place in the case where shear rupturing on a planar fault becomes dominant over the fluid migration process. On the contrary, if healing processes take place, so that the fluid mass is trapped along the fault, rupturing occurs continuously during the injection period. Seismicity and fluid migration are strongly influenced by the injection rate and the heterogeneity

Evolution of coastal zone vulnerability to marine inundation in a global change context. Application to Languedoc Roussillon (France)

The coastal system is likely to suffer increasing costal risk in a global change context. Its management implies to consider those risks in a holistic approach of the different vulnerability components of the coastal zone, by improving knowledge of hazard and exposure as well as analyzing and quantifying present day and future territory vulnerability. The ANR/VMC2007/MISEEVA project (2008-2011) has applied this approach on Languedoc Roussillon region in France. MISEEVA approach relies on several scenarios for 2030 and 2100, in terms of meteorology (driver of coastal hazard), sea level rise, and also considering further trends in demography and economy, and possible adaption strategies Hazard has been modeled (SWAN, MARS and SURFWB), on the base of the presentday situation, sea level rise hypotheses, and existing or modeled data, of extreme meteorological driving f. It allowed to assess the possible surges ranges and map coastal zone exposure to: - a permanent inundation (considering sea level rise in 2030 and 2100, - a recurrent inundation (considering sea level rise and extreme tidal range) - an exceptional inundation (adding extreme storm surge to sea level rise and tidal range). In 2030, exposure will be comparable to present day exposure. In 2100, extreme condition will affect a larger zone. Present days social and economic components of the coastal zone have been analyzed in terms of vulnerability and potential damaging. Adaptation capacity was approached by public inquiries and interviews of stakeholders and policy makers, based on existing planning documents The knowledge of the present day system is then compared to the possible management strategies that could be chosen in the future, so to imagine what would be the evolution of vulnerability to marine inundation, in regards to these possible strategies

Impact du climat et de la tectonique sur l'évolution géomorphologique d'un piémont - Exemple du piémont Nord du Tian Shan depuis la fin du Pléistocène

Rapporteurs : M. Philippe Davy, Mme Françoise Gasse Examinateurs : M. Christian France-Lanord, M. François Métivier Président du jury : M. Laurent LabeyrieA piedmont consists in the transition zone between a mountain range and its foreland basin. We studied the northern piedmont of Eastern Tian Shan (NW China), in order to describe the influence of climate and tectonics on this zone and to quantify observed surface processes. Tectonics induces a long-term signal by uplifting the range, giving rise to a sedimentary flow toward the piedmont, and through the development of a fold-and-thrust belt. Climate has a short-term influence because its evolution follows the main global climatic cycles with some local modifications. During glacial times, some material is eroded in the high range but it is difficult to export from there. Sediment transport becomes easy at the transition to the interglacial, so that large alluvial fans emplace in the piedmont around 12 ka BP. They are then deeply entrenched by the rivers with incision rates ranging from 1 to 2.5 cm/yr over the Holocene. During this phase, wide terraces are first abandoned. Then, valleys become narrows without stopping river incision. We propose a modeling for one of these rivers over the Holocene, and so intend to inverse the decreasing trend of riverbed gradient shown by topographic profiles of terraces. This approach suggests that river discharge was divided by 3 since the mid-Holocene. This study is supported by the analysis of the lake into which the piedmont rivers flow. The lake surroundings show evidence of ancient shorelines corresponding to Holocene lake-levels. The lake-level may have been 50 m higher than today, attesting that the lake area was 3 times larger than present one. We compute a lake hydrological balance for present and mid-Holocene conditions, respectively. This calculation shows that the river inflow was 2.6 higher at 6000 BP. From this two complementary studies, we conclude that Holocene climate was regionally much wetter than today, as paleoclimatic models had already shown but not quantified.Un piémont constitue la zone de transition entre une chaîne de montagnes et son bassin d'avant-pays. Tectonique et climat se partagent le contrôle de son évolution géomorphologique à toutes les échelles de temps. L'étude de l'exemple du piémont Nord du Tian Shan oriental (Nord-Ouest de la Chine) permet une description précise de ces influences, ainsi qu'une quantification des phénomènes majeurs observés. Le signal tectonique intervient sur le long terme, par le soulèvement actif de la chaîne qui induit un flux de matière vers le piémont, ainsi que par la migration de la déformation vers le bassin dzungar sous forme de plis d'avant-pays. Au contraire, le signal climatique se manifeste à court et moyen terme, suivant les grands cycles climatiques globaux plus ou moins nuancés à l'échelle régionale. En période glaciaire, le matériel érodé dans les bassins versants montagneux est difficilement exporté de la chaîne. Le transport est facilité à la transition vers l'interglaciaire, par l'augmentation brutale des flux d'eau qui provoque la mise en place dans le piémont de grands cônes alluviaux, abandonnés vers 12 \pm 2 ka BP. A partir de ce moment, les rivières s'encaissent profondément dans les cônes, à des taux d'incision moyens de 1 à 2.5~cm/an sur l'Holocène. Les terrasses d'abrasion abandonnées au cours de cette phase sont d'abord de largeurs plurikilométriques, puis les vallées se font de plus en plus étroites sans que les rivières ne cessent d'inciser. Nous modélisons l'évolution du profil d'une de ces rivières au cours de l'Holocène à partir de la théorie du transport de Bagnold, en cherchant à retrouver la décroissance des paléo-pentes du lit de la rivière observée à l'aide de la topographie actuelle des terrasses. Cette reconstruction nous mène à considérer que la largeur du lit varie peu, alors que sa sinuosité diminue pendant la progression de l'encaissement. Elle suggère en outre que le débit de la rivière a chuté d'un facteur 3 entre la mi-Holocène et l'actuel. Cette étude est complétée par l'exploitation de données de paléo-niveaux holocènes du lac exutoire du piémont. Des lignes de rivage repérées à plus de 50~m montrent en effet que le lac s'est étendu sur une surface triple de sa surface actuelle au début de l'Holocène. Le calcul de bilans hydrologiques pour les conditions actuelles et mi-holocènes atteste que les apports des rivières vers 6000~ans~BP étaient supérieurs d'un facteur 2.6 aux apports actuels. La synthèse de ces deux approches révéle que la région a connu pendant la première moitié de l'Holocène un climat beaucoup plus humide que l'actuel, dans une mesure jusqu'alors ignorée des reconstitutions paléoclimatiques

Numerical modelling of historical landslide-generated tsunamis in the French Lesser Antilles

International audienceTwo historical landslide-induced tsunamis that reached the coasts of the French Lesser Antilles are studied. First, the Martinique coast was hit by a tsunami down the western flank of Montagne Pelée at the beginning of the big eruption of May 1902. More recently, the northeastern coast of Guadeloupe was affected by a tsunami that had been generated around Montserrat by pyroclastic flows entering the sea, during the July 2003 eruption of the Soufrière Hills volcano. We use a modified version of the GEOWAVE model to compute numerical simulations of both events. Two source hypotheses are considered for each tsunami. The comparison of the simulation results with reported tsunami height data helps to discriminate between the tested source decriptions. In the Martinique case, we obtain a better fit to data when considering three successive lahars entering the sea, as a simplified single source leads to an overstimation of the tsunami wave heights at the coast. In the Montserrat case, the best model uses a unique source which volume corresponds to published data concerning the peak volume flow. These findings emphasize the importance of an accurate description of the relevant volume as well as the timing sequence of the source event in landslide-generated tsunami modelling. They also show that considering far-field effects in addition to near-field effects may significantly improve tsunami modelling

Holocene hydrological changes inferred from alluvial stream entrenchment in North Tian Shan (Northwestern China)

International audienceWe analyze the possible contribution of climate change or tectonics on fluvial incision from the study of a case example along the northern flank of Tian Shan. The rivers that exit the high range fed large alluvial fans by the end of the last glacial period. They have since deeply entrenched the piedmont by as much as 300 m. We have surveyed several terraces that were cut and abandoned during river entrenchment, providing information on intermediate positions of the riverbed during downcutting. They suggest a gradual decline in river slope during a major phase of incision throughout the Holocene. Tectonic uplift affects only a zone about 5 km wide, corresponding to a growing anticline, and is shown to account for about 10% of total incision. Incision was therefore most probably driven by climate change. From observed fluvial incision, we estimate the water discharge in excess of that needed to carry the sediments supplied by hillslope erosion in the headwaters. We used a model based on a transport‐limited erosion law. The model predicts relaxation process with entrenchment in the upper reach, downstream progradation of the incision‐sedimentation line, and a progressive decrease of river slope during incision consistent with our observations. According to this model, river slope might be used as a proxy for specific discharge and then for volumetric discharge, provided that an assumption is made about river width variations. We conclude that river incision in the study area has resulted from dynamic adjustment of the hydrological system to the settlement of wetter conditions in the early Holocene, when water discharge might have been about three times as high as at present. Then, a rather arid climate with enhanced seasonality has likely prevailed from the mid‐Holocene (∼6 ka B.P.) until now

Tectonic interpretation of transient stage erosion rates at different spatial scales in an uplifting block

International audienceWe explore the extent to which it is possible to convert erosion rate data into uplift rate or erosion laws, using a landscape evolution model. Transient stages of topography and erosion rates of a block uplifting at a constant rate are investigated at different spatial scales, for a constant climate, and for various erosion laws and initial topographies. We identify three main model types for the evolution of the mountain‐scale mean erosion rate: “linear”‐type, “sigmoid”‐type and “exponential”‐type. Linear‐type models are obtained for topographies without drainage system reorganization, in which river incision rates never exceed the uplift rate and stepped river terraces converge upstream. In sigmoid‐type and exponential‐type models (typically detachment‐limited or transport‐limited models with a significant transport threshold), drainage growth lasts a long time, and correspond to more than linear transport laws in water discharge and slope. In exponential‐type models, the mean erosion rate passes through a maximum that is higher than the rock uplift rate. This happens when the time taken to connect the drainage network exceeds half the total response time to reach dynamic equilibrium. River incision rates can be much greater than the uplift rate in both cases. In the exponential‐type model, river terraces converge downstream. Observations of a mountain in the Gobi‐Altay range in Mongolia support the exponential‐type model. This suggests that the erosion of this mountain is either detachment‐limited or transport‐limited with a significant transport threshold. This study shows that drainage growth could explain differences in erosion rate measurements on different spatial scales in a catchment

CIDRE

Cidre is a landscape evolution model coupling the dynamics of erosion, sedimentation and topography over geological times on the one hand, and the transport of grains tracers with different properties like the calculus of mineral weathering or cosmogenic nuclide concentration on the other han