Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal

Abstract

Abstract. We analyze geomorphic evidence of recent crustal deformation in the subHimalaya of central Nepal, south of the Kathmandu Basin. The Main Frontal Thrust fault (MFT), which marks the southern edge of the sub-Himalayan fold belt, is the only active structure in that area. Active fault bend folding at the MFT is quantified from structural geology and fluvial terraces along the Bagmati and Bakeya Rivers. Two major and two minor strath terraces are recognized and dated to be 9.2, 2.2, and 6.2, 3.7 calibrated (cal) kyr old, respectively. Rock uplift of up to 1.5 cm/yr is derived from river incision, accounting for sedimentation in the Gangetic plain and channel geometry changes. Rock uplift profiles are found to correlate with bedding dip angles, as expected in fault bend folding. It implies that thrusting along the MFT has absorbed 21 _+ 1.5 mm/yr of N-S shortening on average over the Holocene period. The +1.5 mm/yr defines the 68% confidence interval and accounts for uncertainties in age, elevation measurements, initial geometry of the deformed terraces, and seismic cycle. At the longitude of Kathmandu, localized thrusting along the Main Frontal Thrust fault must absorb most of the shortening across the Himalaya. By contrast, microseismicity and geodetic monitoring over the last decade suggest that interseismic strain is accumulating beneath the High Himalaya, 50-100 km north of the active fold zone, where the Main Himalayan Thrust (MHT) fault roots into a ductile d•collement beneath southern Tibet. In the interseismic period the MHT is locked, and elastic deformation accumulates until being released by large (M w > 8) earthquakes. These earthquakes break the MHT up to the near surface at the front of the Himalayan foothills and result in incremental activation of the MFT.

Recent fold growth and drainage development: The Janauri and Chandigarh anticlines in the Siwalik foothills, northwest India

“…In the foothills of frontal Himalaya, the growing anticlines and Himalayan Frontal Thrust (HFT) take up a significant part of the convergence (e.g. Delcaillau, 1992;Lavé and Avouac, 2000). Nevertheless, the PlioQuaternary evolution of the frontal Siwalik hills in NW India is poorly constrained, and the sense of movement in Quaternary times remains a matter of debate (Nakata, 1972;Nakata, 1989;Kumar et al, 2001;Valdiya, 2001).…”

Investigation of Coulomb stress changes in south Tibet (central Himalayas) due to the 25th April 2015 M W 7.8 Nepal earthquake using a Coulomb stress transfer model

“…The three thrusts partition the Himalayan range, which is the archetype of a compressive thrust belt, into the Siwalik Himalaya (SH), the Lesser Himalaya (LH), and the High Himalaya Crystalline (HHC) in terms of petrography . The MFT, which forms the boundary between Siwalik Himalaya and Quaternary deposits of the Ganges Plain (GP) (Liu et al 2012), seems to merge with the MBT into the MHT at a depth of *5 km Ma and Gao 2011;Lavé and Avouac 2000). North of the MFT, the décollement of the Indian basement is thought to extend with flat geometry beneath the Lesser Himalaya and to form a steeper ramp at the front of the High Himalaya (Lavé and Avouac 2000).…”

“…The MFT, which forms the boundary between Siwalik Himalaya and Quaternary deposits of the Ganges Plain (GP) (Liu et al 2012), seems to merge with the MBT into the MHT at a depth of *5 km Ma and Gao 2011;Lavé and Avouac 2000). North of the MFT, the décollement of the Indian basement is thought to extend with flat geometry beneath the Lesser Himalaya and to form a steeper ramp at the front of the High Himalaya (Lavé and Avouac 2000). The MBT is a series of thrusts that separate the Lesser Himalaya sediments from the Tertiary Siwalik sedimentary belt (e.g., Leloup et al 2010;Ni and Barazangi 1984).…”

“…Thus, we should pay attention to studying these normal faults in the future. In an interseismic period, the MHT is locked, and elastic deformation accumulates until it is released by large (M W [ 8) earthquakes (Lavé and Avouac 2000). These earthquakes break the MHT up to the near surface at the front of the Himalayan foothills and results in incremental activation of the MFT (ibid.).…”

“…Based on the focal mechanism information from the U.S. Geological Survey (USGS) and the finite fault inversion results by Gavin Hayes (USGS/NEIC, http://earthquake.usgs.gov/earth quakes/eventpage/us20002926#scientific_finitefault), it seems that the mainshock of the event in Nepal ruptured along a shallow-dipping reverse fault to the north of the Main Frontal Thrust (MFT) between the Indian and Eurasian plates. The MFT seems to merge into the Main Himalaya Thrust (MHT) at a depth of * 5 km Ma and Gao 2011;Lavé and Avouac 2000), and the MHT is inferred to be the rupture fault along which many large earthquakes have occurred (Ader et al 2012). Therefore, according to the inversion results from USGS finite faults, we set the source fault to be the MHT, where the M W 7.8 Nepal event occurred, and the rupture fault dips approximately 10°to the north near the surface to about 30 km depth under the Himalayan orogen.…”