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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 30°20’55.1", E 77°57’09.3") 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. |
