Deformed alluvial terraces record an excess of slip over the last few centuries on the Himalayan Topographic Frontal Thrust of central Bhutan

Abstract

Abstract Deformed alluvial terraces are ubiquitous markers of a fault’s recent activity and may help assess its slip rate and associated seismic hazard. They are often considered as a nearly flat surface translated and rotated along a planar or listric fault. The present study challenges these assumptions by revealing uneven terrace treads and verticalization of the Topographic Frontal Thrust (TFT) in south-central Bhutan. We model this finding as combined variability in both the aggradation and geometry of the TFT. We estimate a Holocene slip rate of 19.6 ± 4.1 mm.yr−1, which confirms that the TFT accommodates most of the shortening across the range. Contrary to previous studies, we find an excess of slip over the last few centuries, which implies a lower seismic hazard. These results highlight the importance of considering the non-planar component in terrace shape, shallow abrupt changes in fault geometry, and aggradation in future morphotectonic studies worldwide.