Short time scale landscape evolution and topographic signature of extreme climatic and tectonic events – TOPO-Extreme

- Field investigations will be carried out in two different settings to better decipher the signature of climatic and tectonic forcing. The first will be focused on the Cévennes region, which is particularly subject to flash floods and where no recent tectonic activity has been reported. The second setting is along the Himalayan front of Bhutan, which is affected by earthquakes and by intense flooding and landslides.
- Three-dimensional information about the surface shape will be obtained from multiple methods yielding very high resolution topographic data. Geometry of near-surface seismogenic fault will be image from a wide set of geophysical measurements and innovative inversion methods.
- Landscape dynamics under the influence of extreme events will be measured over a range of time-scales from annual floods events to several seismic cycles. We will focus on the interplay between climatic and tectonic extreme events and their impact on morphology by comparing the contrasting contexts of Bhutan and the Cévennes.
- All existing data including those acquired in this project will be investigated at the light of both experimental and numerical models, which account for the stochastic nature of extreme events and deformation associated with all aspects of the seismic cycle. Beyond this key but traditional approach, the novelty also comes from the comparison between both model types, which will use a similar nesting procedure to integrate a large variety of spatial and temporal scales.

The field mission carried out in 2019 demonstrated the MBT's activity in eastern Bhutan. The dating and the acquisition of a high-resolution topography of the Bangtar cone allow a detailed study of the role of the fault slip behavior (stick-slip or creep) on the shaping of the topography. The study of this geomorphological object is therefore key for this project.
The analysis of the hydrological stations (HYDRO Bank) on the Cévennes border shows a very strong gradient of variability of the discharge between the margin and the interior of the Massif, which confirms the ideal character of this site for the study of the long-term impact of this type of forcing. The 10Be denudation data (new data set of 35 samples) also shows a high variability in erosion rates, which is being analysed and compared with climatic forcing. The first results show a singularity of the Cevennes zone in terms of the relationship between denudation rates and morphological parameters, which could be associated with the very high variability of flows in this area.
The first results in analogical and numerical modelling suggest a 2nd order effect of the type of the fault slip on the morphology near an active fault escarpment.

Future work is partly conditioned by the possibility of being able to travel to Bhutan in 2021. Two scenarios are envisaged:
- Missions abroad are again possible, so we plan to maintain the initial objectives of the project with two multidisciplinary missions to Bhutan. One will aim to acquire geophysical data (seismic, electrical and gravimetric) along the Himalayan front where uplifted terraces are due to the combined effect of tectonic and climatic forcings. The other will be dedicated to the re-measurement of geodetic points installed in eastern Bhutan and the acquisition of geomorphological data for denudation and incision measurements.
- The crisis lasts until 2022, when our work will be refocused on a densification of the measurements made along the Cevennes rivers and the development of analog and digital models.

Publication
Sassolas-Serrayet, T., Cattin, R., Ferry, M., Godard, V., Simoes, M,, Estimating the disequilibrium in denudation rates due to divide migration at the scale of river basins, Earth Surface Dynamics, 7, 1041-1057, 2019
Stevens VL, De Risi R, Le Roux-Mallouf R, Drukpa D, Hetényi G (2020) Seismic hazard and risk in Bhutan. Nat Hazards 104:2339-2367. doi:10.1007/s11069-020-04275-3

International meteing
Sassolas-Serrayet, Simoes, Cattin, Le Roux-Mallouf, Ferry, Drukpa. Quantifying active tectonics in the case of dynamic and instable landscape : an example from the Bhutan Himalayas, EGU2020-8884, 2020.
Lasserre C, Marconato L, Sassolas-Serrayet T, De Zan F, Ansari H, Doin MP, Mazzotti S, Cattin R, Ferry MA. Lateral Variations of Interseismic Coupling along the Himalayan Arc in Bhutan: Clues from Time Series Analysis of Sentinel-1 InSAR Data?. T41E-0328, AGU, 2019
Caniven, Morgan, Blank, Cattin, DEM Simulations of Earthquake Cycles along Strike-Slip Faults : Controls on the Slow-Slip Nucleation of Earthquakes, AGU 2019.

It is now well established that extreme events such as storms, floods, landslides or earthquakes have a significant effect in shaping landscapes. River terraces, knickpoints, alluvial fans and other morphological markers are routinely used to date and quantify landscape response to changes in climate or tectonics over long time-scale (>10.000 yr). However, those markers are rarely used to assess the natural variability of extreme events’ frequency and magnitude because both the formation and the evolution of those markers in response to internal or external solicitations remains poorly studied.

Here, we propose to develop a new approach based on a multidisciplinary study of the small time-scale evolution of landscapes (from 0.1 to 10000 yr). We aim to investigate the feedbacks between river high-discharge events and earthquake cycle deformation, which are the elementary driving processes linking climate, tectonics and landscapes. This approach is complex and requires deciphering the signature of climatic and tectonic processes (1) from the detailed study of present-day landscape features, and (2) by considering their stochastic nature as well as their coupling and feedback. This requires exploring many fundamental but unresolved questions: What is the signature of the magnitude and frequency distribution of climatic and tectonic perturbations in the topography equilibrium? What is the time-scale over which abrupt changes due to extreme events remain visible? Is steady-state a purely theoretical concept or does it apply to natural landscapes subjected to extreme events?
We will address these issues through an integrated approach combining field observations, remote sensing data, and the development of a new generation of both experimental and numerical models:
- Field investigations will be carried out in two different settings to better decipher the signature of climatic and tectonic forcing. The first will be focused on the Cévennes region, which is particularly subject to flash floods and where no recent tectonic activity has been reported. The second setting is along the Himalayan front of Bhutan, which is affected by earthquakes and by intense flooding and landslides.
- Three-dimensional information about the surface shape will be obtained from multiple methods yielding very high resolution topographic data. Geometry of near-surface seismogenic fault will be image from a wide set of geophysical measurements and innovative inversion methods.
- Landscape dynamics under the influence of extreme events will be measured over a range of time-scales from annual floods events to several seismic cycles. We will focus on the interplay between climatic and tectonic extreme events and their impact on morphology by comparing the contrasting contexts of Bhutan and the Cévennes.
- All existing data including those acquired in this project will be investigated at the light of both experimental and numerical models, which account for the stochastic nature of extreme events and deformation associated with all aspects of the seismic cycle. Beyond this key but traditional approach, the novelty also comes from the comparison between both model types, which will use a similar nesting procedure to integrate a large variety of spatial and temporal scales.

Compared to previous studies that generally consider climatic and tectonic forcing separately, the proposed approach, which takes into account the stochastic nature of floods and earthquakes as well as the deformation associated with all earthquake cycle periods, represents a new milestone in landscape evolution understanding. Beyond the expected scientific results and technical benefits, this project by its generic nature will be useful to improve flood- and seismic-hazard assessment, in terms of predictability and mitigation, worldwide.