Frontal and piggy-back seismic ruptures in the external thrust belt of western Nepal

Abstract This paper gives structural and morphological evidence for meter-scale episodic displacement pulses along the MFT and MDT during a complex sequence of thrusting in the sub-Himalayan fold and thrust belt. The studied cross-section is located in Western Nepal at a latitude of E 82° 20′. Along this section, the Main Dun Thrust (MDT) is in a piggy-back setting and comprises three splays that are spaced by less than 200 m. The splays display the following sequence of motion: (1) the medium splay was active; (2) the internal splay was active and had a dip-slip displacement of ∼3 m that could be the surface rupture of an earthquake; (3) a period of quiescence occurred for all the splays of the MDT; (4) the external splay is active. The motion along the medium and internal thrust predates ∼70 kyr whereas motion along the external splay postdates 5468–5214 yr before J.C. The Main Frontal Thrust (MFT) is the most external Himalayan thrust. Ten encased strath terraces are found at its hangingwall. Sudden ∼1.5 m uplift events could have induced their abandonment and could have been linked to ∼2 m slip events along the MFT, a value close to the lower bound inferred for co-seismic slip along the basal detachment (MHT) during M ∼8 earthquakes. From the offset of a terrace level, a displacement of ∼8 m is inferred between 1224–1280 yr after J.C. and 1828–1883 yr after J.C.; it could be linked to a succession of several seisms along the MFT. This study suggests that: (a) several thrusts, that branch off the basal decollement, are active faults; (b) great earthquakes, that occur along the outer part of the basal decollement of the Himalayan thrust belt, reach the surface at different location; (c) only one thrust moves during one earthquake; (d) the sequence of thrusting is successively out-of-sequence and in sequence and changes at an intermediate time-scale between earthquake cycle and finite geometry of the thrust system.