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Himalayan Geology
(Journal)

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Abstract:

Late Pleistocene activity of intrabasinal Bhauwala Thrust (BT), Dehra Dun, NW Himalaya

R. JAYANGONDAPERUMAL1*, SENTHIL KUMAR2, WESNOUSKY, STEVEN G.3, A.K. MAHAJAN1, VIKRAM GUPTA1, B.R. ARORA1, SURESH, N.1 1Wadia Institute of Himalayan Geology, 33, GMS Road, Dehra Dun-248001, India, 2Centre for Earth Sciences, Indian Institute of Science, Bangalore 560012, India 3Center for Neotectonic Studies, University of Nevada, Reno, MS 169, Reno, NV 89557, USA
* Presently at Centre for GeoTechnology, M.S. University, Tirunelveli, Tamilnadu, India Email: ramperu.jayan@gmail.com

The intra-basinal faults located within the ‘duns’ (duns=valley) in NW Himalaya, such as Dehra Dun, Pinjore, and Soan Dun in Himalaya have been largely ignored during evaluation of the Himalayan seismic hazard (Sahni & Khan 1963; Nakata 1972; Malik & Nakata, 2003; Suresh et. al. 2007). To assess the hazard associated with the intra-basinal faults, we conducted studies along a section of ~19 km long Bhauwala thrust (BT) located within the Dehra Dun valley using field observations, shallow seismic profiling, and trenching. Nakata (1972) recognized three late Quaternary Dun surfaces (Higher, Middle and Lower Dun surface) and three fluvial terraces based on
their elevation, degree of dissection, and soil development. Previously mapped Bhauwala thrust (BT) has shown to have cut and displaced the Middle Dun surface of varying age and height preserved within the Dehra Dun valley.

 The BT is well expressed and preserved along its entire length and potential site for shallow seismic profiling and trenching campaign is chosen in combination with field mapping and SRTM (Shuttle Radar Topographic Mission) data analysis. We acquired shallow seismic profiles across a relatively less diffused ~10 m scarp identified near the village of Khanduali (N 3020’55.1", E 7757’09.3") using Multichannel Analysis of surface wave technique with roll along method. Processed shallow seismic profiles show displacement and thrust sense of motion at around ~10 m depth along the down dip section of low angle thrust fault (~30o), however, up dip section of the reflection data record a simple monoclinal warping with no discernible displacement. We placed a trench close to shallow seismic profiles to understand style and recency of earthquake responsible for the deformation observed on the surface. A ~
4 m shallow trench exposure show broad warping of the Middle Dun surface with no causative fault seen cutting through the late Quaternary alluvial gravels. Structural relationship and associated stratigraphy exposed in the trench agree well with the interpreted shallow seismic reflection data pertaining to top ~4 m. A radiocarbon sample obtained from the tilted and warped sediments in the trench yields an uncalibrated radiometric age of 25770 + 220 yrs.

An Optically Stimulated Luminescence (OSL) age obtained from the Middle Dun surface sitting on top of the bedrock strath near the eastern extension of BT yields an age of 26.19 + 2.75 kybp, which corresponds to age obtained using radiometric method. The OSL and AMS ages overlap within their respective uncertainty and constrain the age of the last activity and deformation to have occurred post ~26 kybp. Dividing the elevation of the bedrock (40 m) from present river grade by OSL age (26.19+2.75 kybp) yields a maximum vertical uplift rate of 1.4 to 1.7 mm/yr. Other mapped and unmapped intra-basinal faults within Dehra Dun may have contributed towards upliftment of the Dun surfaces and hence the vertical uplift rate inferred from the elevation of the bedrock need to be treated as maximum value. Using the geometry of fault (30o) inferred from shallow seismic profile, we estimate a maximum slip rate ranging from 2.8 to 3.4
mm/yr for this fault.

References

Nakata, T. 1972. Geomorphic History and Crustal movements of the Foot-Hills of the Himalaya, Sci. Reports Tohoku Uni., 7th Series (Geography), 22(1), 40-177. Malik, J. N., Nakata, T. 2003. Active faults and related Late Quaternary deformation along the northwestern Himalayan Frontal Zone, India. Ann. Geophys., 46, 917– 936. Sahni, M.R., Khan, E. 1963. Tectonic features of the area around Pinjaur. Current Science, 32, 77-79. Suresh, N., Bagati, T.N., Kumar, R., Thakur, V.C. 2007. Evolution of Quaternary alluvial fans and terraces in intermontane Pinjaur dun, Sub Himalaya, NW India. Interaction between tectonics and climate change. Sedimentology, 54 (4), doi: 10.111/J.1365 –3091.2007. 00861.X, 809-833.