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

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 décollement, are active faults; (b) great earthquakes, that occur along the outer part of the basal décollement 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.