Titolo della tesi: Time-dependent rock-mass deformations, geological aging and landscape evolution as predisposing factors for large rock landslide triggering
The slope time-dependent deformations are usually related to Mass Rock Creep (MRC) process that acts on a large time-space scale through a continuous and non-linear variation of the tensile-deformational state of entire portions of slopes. Such processes are related to large gravitational stresses acting on steep mountain slopes but might also be accelerated by rapid fluvial downcutting, glacial debuttressing (the exposure of glacially steepened rock walls, due to the down wastage and retreat of glacier ice), and seismic activity. In this regard, it is also strongly conditioned by the tectonics and the evolution of the drainage network. Indeed, both uplift and the morphoevolution kinematically release portions of slopes isolating a rock mass carapace that starts to deform by the MRC process until collapse. The limit of these theoretical schemes is represented by the difficulty of estimating accurately the starting time of the process, discriminating the distinct phases, as well as determining the viscosity of the rocky matrix, one of the most important component in the system. In this regard, the general objective of my research is to isolate the contribute of geological aging (intended as the time-evolution of morphostructures in the direction of propagation of the orogen from the hinterland to the foreland), and of landscape evolution in the development of MRC-driven time-dependent deformations. However, the specific objective is to demonstrate the relationships among the aforementioned factors through a methodological test along a transept oriented parallel to direction of the morphostructures in Lorestan, in the Zagros Mountains (Iran) from NE (older geological age) to SW (younger geological age).
The thesis is structured as follows:
1) Introduction: MRC-driven instabilities and conditioning factors:
Bibliographic study of predisposing, preparatory and triggering factors that influence the deformational process with particular regard to landscape evolution, geological aging and stratigraphy highlighting the goal of the research: a transpositive analysis of MRC current processes in space and time along a transept oriented in the propagation direction of the Zagros chain (NE-SW).
2)Geological aging and deformation style of Lorestan region (Zagros Mountains):
Geological setting of the Zagros orogen and validation of the starting ergodic model through the reconstruction of the tectonic evolution and of the deformation style of the Lorestan folds as well as of its geomorphic response in the landscape obtained by applying terrain analyses.
3) Landslide Inventory and sensitivity analysis:
In order to evaluate the sensitivity of the slopes to the MRC conditioning factors and to calibrate the landscape evolution models, I built an inventory of landslides, in which I distinguished the occurred and ongoing instabilities in shear driven landslides (rockfalls, earth/rock slides, and earth/debris flows), and creep-driven ones (viscous slidings, flexural topplings, rock avalanches, slope-scale deformations, and lateral spreadings).
4) Case studies:
4.1) Loumar creep driven slide:
This research focused on the Loumar instability that affects the NE slope of the Gavar anticline whose evolution is strictly related to the vertical and lateral growth of the fold and to the evolution of the Seymareh river drainage system. In this regard, I inferred the Quaternary tectonic and landscape evolution of the Gavar fold, as well as the chronology of the events that led to the gravitational deformation, through linear temporal conversion of river longitudinal profiles, geomorphometric and remote analysis, field surveying, and OSL dating.
4.2) The giant Seymareh rock avalanche:
It was used optically stimulated luminescence (OSL) to date lacustrine and fluvial terrace sediments, whose plano-altimetric distribution has been correlated to the detectable knickpoints along the Seymareh River longitudinal profile, allowing the reconstruction of the evolutionary model of the fluvial valley. I infer that the knickpoint migration along the main river and the erosion wave propagation upstream through the whole drainage network caused the stress release and the ultimate failure of the rock mass involved in the landslide.
4.3) Siah-kuh spreading deformation\\
The ongoing gravitational deformation affects the SE slope of the Siah-kuh anticline in its SE periclinal tip and is strictly related to the evolution of the of Dowairij River drainage system. River incision originated a stress release at the bottom of the slope which likely caused the initiation of the deformational process. In this regard, I present an integrated study, based on InSAR techniques and quantitative geomorphic analyses to assess the present-day landscaping processes, in terms of tectonic, erosion and ground displacement rates due to landsliding.
5) Landscape Evolution Modelling:
Based on the time constraints to the pre-failure morpho-evolutionary reconstruction provided by the aforementioned works on the 3 case studies, a Landscape Evolution Modelling (LEM) was implemented in order to calibrate the model by back-analysis individuating the exact temporal step in which a critical morpho-evolutive condition was reached originating the MRC process responsible for unelastic strains within the involved rock mass.
6) Strain evolution modelling and creep history reconstruction:
The strain rate computation, and then subsequently the displacement rate, was performed, first, on the real landslide profiles to evaluate the possible strain rate value range in the case studies through a sensitivity analysis on the viscosity parameter. After that, it was also implemented in the LEMs to define the deformation history linked to the MRC process of the simulated slopes.
Here, the main resulted predisposing and preparatory factors of large rock slope deformations, highlighting the geological, tectonic, and morpho-evolutionary contributes, are highlighted and discussed according to the occurrence of triggering factors (e.g., earthquakes).
I can conclude that the landscape evolution modelling, temporally constrained by the morpho-evolutionary analysis, allows reconstructing the creep history of slope-valley systems. The presented multi-modelling approach will be continued by the stress-strain numerical modelling to calibrate the rock mass rheology by further back analysis allowing assessing a time-dependent risk.