[en] The largest (M8+) known earthquakes in the Himalaya have ruptured the upper locked section of the Main Himalayan Thrust zone, offsetting the ground surface along the Main Frontal Thrust at the range front. However, out-of-sequence active structures have received less attention. One of the most impressive examples of such faults is the active fault that generally follows the surface trace of the Main Boundary Thrust (MBT). This fault has generated a clear geomorphological signature of recent deformation in eastern and western Nepal, as well as further west in India. We focus on western Nepal, between the municipalities of Surkhet and Gorahi where this fault is well expressed. Although the fault system as a whole is accommodating contraction, across most of its length, this particular fault appears geomorphologically as a normal fault, indicating crustal extension in the hanging wall of the MHT. We focus this study on the reactivation of the MBT along the Surkhet-Gorahi segment of the surface trace of the newly named Reactivated Boundary Fault, which is ~ 120 km long. We first generate a high-resolution Digital Elevation Model from triplets of high-resolution Pleiades images and use this to map the fault scarp and its geomorphological lateral variation. For most of its length, normal motion slip is observed with a dip varying between 20° and 60° and a maximum cumulative vertical offset of 27 m. We then present evidence for recent normal faulting in a trench located in the village of Sukhetal. Radiocarbon dating of detrital charcoals sampled in the hanging wall of the fault, including the main colluvial wedge and overlying sedimentary layers, suggest that the last event occurred in the early sixteenth century. This period saw the devastating 1505 earthquake, which produced ~ 23 m of slip on the Main Frontal Thrust. Linked or not, the ruptures on the MFT and MBT happened within a short time period compared to the centuries of quiescence of the faults that followed. We suggest that episodic normal-sense activity of the MBT could be related to large earthquakes rupturing the MFT, given its proximity, the sense of motion, and the large distance that separates the MBT from the downdip end of the locked fault zone of the MHT fault system. We discuss these results and their implications for the frontal Himalayan thrust system.
Disciplines :
Earth sciences & physical geography
Author, co-author :
Riesner, Magali; Earth Observatory of Singapore, NTU, Singapore, Singapore. magali.riesner@gmail.com ; CEA, DAM, DIF, 91297, Arpajon, France. magali.riesner@gmail.com
Bollinger, Laurent; CEA, DAM, DIF, 91297, Arpajon, France
Hubbard, Judith; Earth Observatory of Singapore, NTU, Singapore, Singapore
Guérin, Cyrielle; CEA, DAM, DIF, 91297, Arpajon, France
Lefevre, Marthe ; Université de Liège - ULiège > Sphères ; Institut de Physique du Globe de Paris, Paris, France
Vallage, Amaury; CEA, DAM, DIF, 91297, Arpajon, France
Basnet Shah, Chanda; Department of Mines and Geology, Kathmandu, Nepal
Kandel, Thakur Prasad; Department of Mines and Geology, Kathmandu, Nepal
Haines, Samuel; Earth Observatory of Singapore, NTU, Singapore, Singapore
Sapkota, Soma Nath; Department of Mines and Geology, Kathmandu, Nepal
Language :
English
Title :
Localized extension in megathrust hanging wall following great earthquakes in western Nepal.
Commissariat à l'Énergie Atomique et aux Énergies Alternatives Nanyang Technological University French embassy in Singapore
Funding text :
This work was funded by the Nanyang Technological University (NTU, Singapore) and the Commissariat à l’énergie atomique et aux énergies alternatives (CEA, France) as well as the French embassy in Singapore through the PHC-Merlion program (2018–2019). All radiocarbon dating analysis were performed by Beta analytics (Miami, USA).
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