Three images showing the study area from different perspectives.
Study site of Hamahashi et al. [2022]: (a) shows the India-Eurasia collision zone and the location of the satellite image (b), showing the central Himalaya in Nepal and India. The white box shows the location of the study site, for which an interpretative block diagram is shown in (c). Red lines are traces of the Main Frontal Thrust (MFT) system, blue lines are previously collected seismic profiles, red dots with numbers show locations of boreholes of the current study. The Patu Thrust shows evidence for surface-breaking earthquakes, while the Bardibas Thrust is blind. Credit: Hamahashi et al. [2022], Figure 1
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Geochemistry, Geophysics, Geosystems

The Main Frontal Thrust (MFT) is an active surface-breaking megathrust at the front of the Himalaya. It marks the location of where the Main Himalayan Thrust, the plate interface along which the Indian plate underthrusts Tibet, comes to the surface. There is instrumental, historical, and paleo-seismological evidence for great (M > 8) earthquakes along the MFT, making it a significant source of seismic hazard for the population of the Himalaya and the Ganges plains.

Studies of active thrusting commonly use deformed fluvial terraces as markers of deformation, with the (either explicit or implicit) assumption that the system is in steady state, i.e., the baselevel of the river that produced these terraces does not change over time. Hamahashi et al. [2022] report detailed sedimentological data from the surroundings of the MFT in central Nepal, based on 10 new boreholes of up to 100 meters depth. In this area, an earlier seismic profile collected by the same group elucidated the structure of the MFT and showed that the above assumption could not hold. The MFT has two active branches in the study area, the northern surface-breaking Patu thrust and the southern blind Bardabas thrust. Although the southern ramp is clearly active, the hanging-wall anticline does not produce topography but is buried beneath Pleistocene-Holocene sediments, requiring a base-level rise of at least 100 meters.

The authors describe the sediments from this area in detail and develop a model for how base-level rise and fall may be related to phases on intensified or weaker monsoons. They convincingly show such climatic controls on sedimentation need to be taken into account to correctly interpret the geomorphic record of even this very active megathrust system, and that the assumption of a steady base level leads to a significant overestimate of shortening rate on the structure.

Citation: Hamahashi, M., Hubbard, J. A., Almeida, R. V., Haines, S. H., Owen, L. A., Mishra, S., & Sapkota, S. N. (2022). Fluvial sedimentary response to late Quaternary climate and tectonics at the Himalayan Frontal Thrust, central Nepal. Geochemistry, Geophysics, Geosystems, 23, e2022GC010366.

—Peter van der Beek, Editor, Geochemistry, Geophysics, Geosystems

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