Paleoseismological findings along the identified back thrust in the Eastern Himalayan foothills near the India-Bhutan border

The tectonic landscape of the Himalayas is mainly depicted by the E-W trending major regional thrusts, the southernmost being the Himalayan frontal Thrust (HFT) or the Main Frontal Thrust (MFT. But there are also out of sequence transverse faults and back thrusts that play important role in strain adjustment.

The map traces of thrust faults make cuspate-lobate patterns suggesting differential fault growth. These orogen-scale curvatures at an intermediate scale are expressed as salients and recesses. Salients are normally associated with mountain fronts defined by frontal imbricate faults, whereas recesses are open to the foreland. Himalayan salients, recesses, and associated cross-structures help in determining the deformation kinematics along the length of the Himalayan arc over space and time.

In the Eastern Himalaya, east of the Tista River, the sequential and out-of-sequence structures are well observed in the Jaldhaka recess. Here the splay on the HFT is marked by southerly sloping Chalsa and Matiali scarps whereas the northerly sloping Thaljhora scarp represents the Frontal Back Thrust (FBT).

In this study, we are presenting the geometry and structural detail of the back thrust below the Thaljhora scarp. The attitude of the thrust plane, folding of the bedding, and displacement is evident from an excavated trench perpendicular to the strike of the fault scarp. The folded beds join against the thrust plane to form a piggyback structure. The thrust plane dips 20⁰ → S. The maximum displacement of the bed is recorded at 4.5cm along the thrust plane. There are liquefaction structures, convolute laminations and flame structures within the deformed sediments. The attitude of the gentler limb of the fold is about 40⁰→S and that of the steeper climb is around 55-60 degrees towards North.

From earlier works (Guha et al. 2010, Singh et al, 2016, Goswami et al., 2019) the age of deposition of different sediments of this area varies from 70ka to 22ka. The oldest sediment here from the north bank of Thaljhora River, below the deformed boulder bed, is around 70 ka., eastward from the same bank from an upper stratum, comprising of black sandy clay dated around 27ka, a black clay around 6m high from the river bed, on the Thaljhora scarp itself dated as around 37 ka whereas from somewhere within that scarp dated as around 22ka. From the present study, the sediments which are deformed and displaced gives the depositional dates varying from 14 to 17ka. So, it can be said that the faulting or thrusting which has formed the scarp is at least as young as 14ka.

The movement on the splay of HFT in the adjacent Matiali fan started earlier than 70 ka and the major upliftment forming the T2 terrace was around 20ka.

The movement along the Thaljhora fault started somewhere between 20-30ka. This movement may have started to adjust the stress along the northerly dipping fault. These two northerly and southerly dipping thrust systems may be interpreted as a conjugate thrust which maybe adjust the stress in this particular area.