Stratigraphic Expression and Slip Rate of the Main Frontal Thrust in Central Nepal: A 50,000-year record in the Bardibas area
Abstract
Published geodetically derived estimates of shortening across the Nepal Himalaya average 15.5 +/- 3.6 mm/yr. This shortening likely reaches the surface along the Main Frontal Thrust (MFT), and other intra-Siwalik thrust faults south of the Main Boundary Thrust. Paleoseismic studies have suggested that the frontal MFT accommodated most of the Himalayan shortening budget. By contrast, recent seismic lines across the Bardibas thrust, the surface expression of the MFT near Bardibas in eastern Nepal, reveal an erosionally beveled fault propagation anticline buried by ~100 m of recent sediment. This field relationship paradoxically indicates relative inactivity across a major seismogenic plate-bounding fault.
To constrain age relationships and thus the slip rate on this section of the MFT, we cored near-surface sediments in 2017 and 2018 in 3 sections along the Lakshmi, Bhabsi and Ratu rivers across the MFT. All holes penetrated 50-100 m, with variable core recovery. The Lakshmi (P1, P2) and Bhabsi river sections (P3-P5) are in the hanging wall of the Bardibas thrust, while the Ratu river section (P6-P10) crosses the blind thrust tip. We dated recovered sediments by 14C and OSL to constrain both slip rates along the Bardibas thrust and sedimentation patterns. All sections have an upper 12 - 31 m thick gravel that is <12 ka. Below this gravel package, sedimentation is variable, with interbeds of gravels, sandstone and extensively bioturbated siltstones (paleosols). 14C ages from these sediments range from 12 to 48 ka; OSL ages are similar. Along the Bhabsi river profile, 35-42 ka 14C ages in each core define a time horizon that increases in depth from upstream, at the crest of the buried anticline (P3), to downstream (P5), mimicking the shape of the anticline and suggesting tilting of the time horizon by the fault below. An area balance on this time horizon suggests ~70-80 m of shortening on the detachment since 35 ka, with a resulting shortening rate of ~2 mm/yr. Our age constraints from these cores suggest that fluvial fans have buried the Bardibas reach of the MFT for most of the last 40 ka, with low shortening rates of ~2 mm/yr since deposition; this suggests that much of the present-day shortening rate of 15 mm/year in central Nepal must instead be accommodated on other intra-Siwalik thrusts, such as the Patu and Kamala thrusts to the north.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T41E0329H
- Keywords:
-
- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGY;
- 8123 Dynamics: seismotectonics;
- TECTONOPHYSICS