Abstract
The Himalayan arc is one of the most of active intra-continental orogen in the world. Geological explorations of the Himalayas since the late 19th century have emphasized a 2500-km-long roughly cylindrical structure, with striking continuity of main units and thrust faults. However recent geophysical and geological investigations have revealed lateral variations (present-day convergence rates, gravimetry, seismicity, morphology, exhumation rates, and crustal structures).Over the las centuries, several major earthquakes (Mw > 7.5) have struck the arc and the dramatic effects of the Gorkha earthquake sequence in 2015 point to the important societal dimension. However, both the maximum size and the occurence probability of such earthquakes are still debated. While the kingdom of Bhutan is a key area in the understanding of the whole Himaayan arc, the country is still poorly documented. Long considered as a 350-km-long seismic gap, a single historical account reports a devastating earthquake in AD 1713.The first part of this thesis focuses on the quantification of vertical deformation along a N-S transect, from the southern-most thrust (The Main Frontal Thrust) to the High Himalaya. In the past three years we have carried out three fieldwork campains during which we sampled different deformation features such as (1) alluvial terraces along the front in order to quantify and the spatial and temporal variation of the Holocene uplift, (2) watershed basins in order to quantify the short term denudation rate (< 20 ky) derived from cosmonuclides 10Be and (3) alluvial terraces hinterland within the Higher Himalaya to quantify the incision rate at longer time scale (< 50 ky).The second part focuses on the several paleoseismic study along the west and central bhutanese front. Six sites were investigated during three fieldwork campains. Detritic charcoals sampling and modelisation have lead to highlight theoccurence of at least five surface-rupturing earthquakes during the last 2700 years. These results demonstrate that the present-day low seismicity rate observed in Bhutan is not representative of the seismic activity at longer time scale. At regional scale, they also take part of a broader discussion on the probability of occurence of a magnitude 9 earthquake along the Himalayan arc.