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Dynamics of giant landslides and their contribution to Himalayan erosion

Thesis subject - deadline for candidates June 20 2019

This is the English version of the original posting in French : Dynamique et contribution à l’érosion himalayenne des méga-glissements de terrain

Subject summary Mountain ranges are affected by intense landsliding activities. Landslides usually display extremely variable sizes and rates of evolution, and can potentially give rise to giant rockslides, i.e. catastrophic landslides mobilizing several cubic kilometers of rock. Because of their scarcity, the mechanics and kinetics of these giant landslides remain relatively unknown, especially in the Himalayas, despite the fact that they represent a significant source of natural hazards and a major contribution to long term mountain erosion (a recent study by Marc et al. (2019) suggests that they participate approximately half of the global budget of the Himalayan erosion).

The aims of the thesis will be to study giant landslides, whether they are very slow and of progressive nature, or rapid and of dynamic origin, in order to constrain their age and to characterize their recurrence in a given region, to better understand their causes and their evolution, and to bring constraints on the landslide hazards, as well as on their consequences on erosion of mountain ranges.

The thesis will include studies on natural objects and a numerical modeling exercise :

  • The first part will focus on the study of well-identified giant landslides, whether they are at an early stage in the form of large, slow-moving, DSGSD (Deep Seated Gravitationnal Slope Deformation), or that they have evolved toward catastrophic rockslides. In detail, escarpments indicating DGSD activity already identified in satellite imagery will be characterized using high resolution digital topographies and field work. The characterization will be complemented by the description of the structural setting as well as by the dating of the landslide activity using cosmogenic isotopes (CRN) or paleoseismic trench. In addition, the description and dating of some giant rockslide deposits (ultimate stage of slope destabilization) by 14C, CRN or OSL will be carried out in order to estimate their average frequency of occurrence as well as the mobilized volumes.
  • The numerical modeling exercise will be devoted to the implementation of mechanical models dedicated to study in a systematic and parametric way the conditions required to create, propagate and destabilize large slides along hillslopes. These models will be built using the open source code YADE DEM, based on the discrete element method whose main interest is its ability to explicitly describe progressive failure mechanisms (initiation, coalescence and propagation of fractures) at the origin of landslides. Understanding these processes will then help exploring the conditions required for the evolution of the giant landslides in the Himalayas.

Objectives During the Late Quaternary, the Himalayas have been affected by numerous major landslides. The occurrence of similar gravitational hazards these days would have strong societal, human and/or economic impact. The questions related to the major and giant slope instabilities are multiple, both with respect to the inventory of slopes currently unstable and subject to active DSGSD, or to the frequency of catastrophic rockslides giving rise to deposits of several cubic kilometers. One of the major questions concerns the conditions that make a slope slide evolves gradually (with potential gradually cannibalization by aggressive erosion of secondary landslides (Gallo and Lavé, 2014)), or, on the contrary, collapses catastrophically. In the case of the Himalayas, the physiographic location of the slopes seems to play a major role on the deformation processes : on the southern side, which is wetter and warmer, the progressive type of slides seem to prevail, whereas the catastrophic deposits have been described almost exclusively at high altitude, on the northern flank of the range. Is it a problem of preservation or rather a response to glacial erosion at high altitude ? Does some kind of coupling between deep fracturing and alteration in the southern flank induce more gradual sliding ? These questions are fundamental to properly establish the distribution of the gravitational hazard.

The aim of the PhD thesis is to provide answers to the above questions. For this, it is proposed :

  • to study major Himalayan DSGSDs in order to characterize the structural setting, the amplitude of the displacements on the fractures and the sliding rates.
  • to complete the existing catalog of giant rockslides (> 1 km3) in central Nepal with additional observations on previously identified deposits.
  • to carry out numerical simulations to study slope deformation processes and DSGSD more systematically, with, ultimately, the objective of integrating the effect of fluid circulations and deep alterations in the analyses in order to explain the apparent disparities between the north and south flanks of the high Himalayan range.

Methods and scientific procedure The thesis work will focus on two complementary axes.

Characterization of DSGSD and giant rockslide deposits

A detailed examination of central Nepal has already permitted to identify numerous DSGSD escarpments along the flanks and ridges of the high Himalayan range, including major fracture zones, of several kilometers long, with normal component motion. To constrain the amplitude and direction of movement on these sites, high-resolution tri-stereo digital elevation maps (DEMs) realized with Pleiades images (in collaboration with Yann Klinger (IPGP))will allow preliminary analysis of the escarpments and their evolution.

Two field trips on Fall 2019 and 2020 will be dedicated to study these escarpments by documenting their structural position in relation to major regional faults, their orientation with respect to foliation and families of fractures. They will also be dedicated to collecting samples for cosmogenic 10Be dating at potential fracture planes or on blocks sitting on top of morainic ridges displaced by active fractures. In addition, one to two trenches will be excavated to further constrain the chronology of movements on these fractures via a paleoseismological study.

The field trips will contribute to complete the existing catalog of giant landslides in central Nepal (5 slides between 1 and 15 km3 (Weidinger, 2006)) by observations on a few (2 or 3) giant rockslide deposits located on the northern flank. The field work will focus on the description of the breccia deposits, the mobilized volumes and the source area of the landslides (geochemical tracing - collaboration with C. France-Lanord at the CRPG). The dating of these landslides will be done according to the contexts and available outcrops via 14C on the organic matter buried at the base of rockslide deposits, by cosmogenic isotopes measured on the blocks on top the preserved deposits surfaces, or by OSL / IRSL dating of the frictionites (with Pierre Valla, Isterre). The samples of 10Be will be treated at the CRPG by a dedicated technician and the PhD student, while the measurement of 36Cl in the calcareous samples, if necessary, will be done in collaboration with L. Benedetti (Cerege).

Numeric modeling of landslide dynamics and DSGSD

The PhD student will learn first how to use the code YADE DEM (Scholtès and Donzé, 2012 _ https://yade-dem.org/) under the supervision of a specialist of this code (Luc Scholtès, GeoRessources). The choice of such a code is motivated by its ability to describe the appearance of new fractures as the deformation progresses, until the sudden collapse of the slope, as well as the possibility to follow the runout of the resulting brecciated products.

In order to progress in the understanding of the mechanical factors that underlie the destabilization of slopes, the student will first focus on simple 2D configurations to investigate the failure mechanisms and the role played by the structural features in the deformation processes. In particular, he will study the influence of the topographic height and of the foliation or families of pre-existing fractures, as well as the impact of glacial U-shaped valleys deepening. This will also include analyzing the conditions leading DSGSD to collapse.

In a second step, the student will build 3D simulations to extend the 2D analyses, but also to constrain the model in relation to the studied giant Himalayan rock slides and DSGSDs. Depending on the degree of advancement of the thesis, and on the motivation of the PhD student, it will be envisaged to integrate, in a simplified way, couplings with fluid circulations and deep weathering. Here again, the goal will be to understand if the these hydrological or chemical processes promotes or inhibits sudden rupture of DGSD and to explain the differences observed in the Himalayas between the arid and cold northern flanks, and warmer and wetter southern flanks.

Depending on the skills and interest of the PhD student, the emphasis on one of the two components may be preferred. Nevertheless, programming (Python and C++) and use of numerical models will be an important part of the doctoral work.

Scientific and technical skills required by the candidate

Candidates must hold a Master’s degree in the field of geosciences. A solid background in earth sciences, geophysics, or geomechanics is required, preferably with research experience in quantitative geomorphology, rock mechanics, or numerical modeling. An ability to communicate and write scientific articles in English is also expected.

Dates : 01/10/2020 to 30/09/2022 PhD supervisors : Jérôme Lavé (CRPG, University of Lorraine, France), Luc Scholtès (GéoRessources, University of Lorraine, France). Collaborations : Y. Klinger (IPGP), C. France Lanord (CRPG), L. Benedetti (Cerege), P. Valla (Isterre)

Employer : CNRS-CRPG France

To apply, please send an email to luc.scholtes@univ-lorraine.fr and jlave@crpg.cnrs-nancy.fr, with a CV and a letter of motivation.

publié mardi 3 juin 2014