Mid-crustal ramping of the Main Himalayan Thrust in Nepal to Bhutan Himalaya: New insights from analogue and numerical experiments

The mid-crustal ramp on the Main Himalayan Thrust (MHT) in the Lesser Himalaya Sequence (LHS) is the most critical geometrical asperity to trigger major seismic events in the Himalaya, including the 2015 Mw 7.8 Gorkha, Nepal earthquake. However, it is still not well understood what caused the MHT to ramp up at laterally varying locations in central to eastern Himalaya during the southward propagation of the Himalayan tectonic wedge. Based on laboratory and numerical model experiments, approximated to the central (Nepal) and eastern (Bhutan) Himalayan geological settings, we show that the coal-bearing Gondwana horizon in the stratigraphic sequence has played a crucial role in producing the mid-crustal ramp on the MHT. During the southward propagation of the Himalayan wedge, the mechanically weak Gondwana sequence resulted in strain localization at its northern edge, and it eventually caused the MHT to ramp from a depth of >20 km to ~6–7 km. Combining our experimental and field observations with the geophysical sections, we interpret that a strike-wise non-uniform occurrence of the Gondwana horizon is responsible for the MHT to ramp at different locations, separated by N-S trending natural barriers, like tear faults. The weak-zone model also explains the rock uplift versus erosion pattern recorded in central Nepal. The ramp-induced surface uplift rates, calculated from our model, are in good agreement with those reported from central Nepal (U* ~ 0.28, normalized to the convergent rate).