Résumé
Alluvial terraces are important markers of morpho-tectonic activity. Datable, these tectonically deformed structures are useful for assessing the slip rate of active faults and the associated potential seismic hazard. When studying these terraces in the field, we analyze their lithology, composed of alluvial deposits, and their shape, which is often assumed to be approximately planar over distances of several hundred meters. Focused on south-central Bhutan, the present study challenges this assumption by revealing uneven terrace treads through a high-resolution digital surface model. By using geophysical imaging and an original modelling approach, we interpret this finding as a result of lateral variation in aggradation and changes in the geometry of the frontal Himalayan thrust fault along dip and strike. We show that considering terrace rugosity helps us estimate Holocene slip consistently with central Bhutan geodetic measurements. At a shorter time scale, while the flat tread rigid block model indicates a slip deficit over the last few centuries, our approach suggests an excess of slip, leading to a lower seismic hazard in Bhutan. These results emphasize that the non-planar component in terrace shape is a crucial parameter that must be considered in future morphotectonic studies worldwide.