Characterizing the sub-surface geometry of the Main Frontal Thrust in the Bardibas area of central Nepal

Abstract:

The Main Frontal Thrust (MFT) represents the most frontal surface expression of the Himalayan orogen. It is contained within the Miocene-Holocene age Siwalik Group. To better constrain the sub-surface geometry and slip history of the MFT, we conducted seismic reflection surveys around the town of Bardibas in central Nepal. Previous surveys in the area have used surface data and shallow trenches to identify the surface rupture of the great 1934 earthquake. The seismic surveys are intended to (1) confirm that the rupture identified at the surface is a through-going feature at depth; (2) image the geometry and infer the kinematics of the fault to assess the behaviour of the fault at longer timescales, and (3) evaluate how the many surface fault strands interact at depth. The surveys were conducted using a 6300 kg vibroseis minibuggy during January-March, 2014. 34 km of data were acquired in 3 long transects across the Main Frontal Thrust. The data were acquired using 264 channels with 5 m spacing. Vertical stacks of 6-12 sweeps were done at each station to improve the signal to noise ratio of the data. The seismic lines follow several of the dry riverbeds in the area and are generally orthogonal to the range front. The three seismic lines encompass a ~5 km southward step in the deformation front, with the front extending further south on the eastern side. We acquired one line on each side of the step (~2-3 km away) and the central line at the step. The Siwalik Group is strongly folded in this region. Measurements of bedding attitudes were done throughout the area to complement the seismic data. The bedding planes are generally tilted to the NE with a NW-SE strike. The faults at the surface are mostly ENE-WSW. This discrepancy seems to be spatially correlated with the southward step. Folding near the outcropping faults is asymmetric, typically with steep, narrow forelimbs and shallow, wide back-limbs. The river terraces, however, are sub-horizontal and uplifted over large areas, although in a few locations they are also gently folded. This difference suggests a transition from fault-propagation folding to translation along the fault sometime before the formation of the terraces. We expect to use the cross-sections obtained from the seismic data to generate a pseudo-3D view of the fault system in the area to further constrain this transition.