HR: 11:50h
AN: T52A-07    [Abstracts]
TI: Evidence For Tilting Of Two Fans Along The MBT In The Pinjaur Dun, Northwestern Himalayan Foothills.
AU: * Singh, V
EM: vimalgeo@india.com
AF: Department of Geology, Chattra Marg, University of Delhi, Delhi, 110007 India
AU: Tandon, S K
EM: sktand@nda.vsnl.net.in
AF: Department of Geology, Chattra Marg, University of Delhi, Delhi, 110007 India
AB: The Pinjaur dun is a NW-SE trending intermontane valley in the foothills belt of the Punjab sub-Himalaya. It has a width of 7km in the southeastern part that increases to about 18km towards the northwest. The northeastern boundary of the Pinjaur dun is marked by mountains ranging in elevation between 640-1240m, whereas the southwestern boundary is marked by hills with elevation ranging between 400-620m. The fill of the Pinjaur dun consisting of late Pleistocene and Holocene mudstone, sandstone, and conglomerate, overlies the Lower and Upper Siwalik sequences. Three faults marked by previous workers in the study area are: i) the Barsar Thrust that separates the Pinjaur dun from the mountain ranges in the northeast, ii) the Pinjaur Thrust that occurs within the dun and separates the older Lower Siwalik Subgroup from the Upper Siwalik Subgroup and iii) the Surajpur fault that separates the outermost Siwalik hills from the southwestern margin of the dun. Several streams drain the Pinjaur dun; they mostly originate either in the Lesser Himalayas or in the sub-Himalayas. Of these, the most prominent ones are the Balad nadi, Kiratpur nadi, and the Koshallia rivers. These rivers deposit their sediments in the dun resulting in the formation of either alluvial fans or piedmont aprons. The Balad and the Kiratpur rivers have formed fans in the dun. The Kiratpur fan and the Balad fan have an area of $\sim$28 and $\sim$21 km$^{2}$ respectively; the fans show a typical concave upward longitudinal profile with the surface slopes varying from $1\deg$ to $10\deg$ in their distal and proximal parts respectively. The axial rivers of both the fans are incised along their entire length; and due to this incision the surfaces of the fans do not receive any sediment from their axial streams. Using the method outlined by Keller and Pinter (1995), a tilt towards southwest for the Kiratpur and Balad fans of $0.83\deg$ and $0.57\deg$ respectively has been worked out. Based on these angles of tilt, an uplift of $\sim$41m and $\sim$46m at the fan head has been calculated for the Kiratpur and Balad fans respectively. The incision by the Kiratpur and Balad Rivers at the fan heads is also approximately 40m. The terraces in the proximal part of the Kiratpur fan are unpaired and are well developed on the northern bank supporting the inference of southwestern tilt of the fan. The geomorphic indices (mountain front sinuosity and valley floor width to height ratio) calculated for the mountain front in the dun show low values suggesting that the fault bounding the mountain front (Barsar Thrust) is very active. The stream orientation trends reveal a bimodal distribution for both the fans. The two major trends that the streams follow are $270\deg$ - $280\deg$ and $230\deg$ - $240\deg$ for Kiratpur fan and $260\deg$ - $270\deg$ and $230\deg$ - $240\deg$ for the Balad fan. The Balad and Kiratpur axial rivers flow in southwest direction probably in response to the tilt. Tectonic activity along the MBT and its splays has resulted in fan deposition, followed by SW tilting of the fans. The tilt of the fans has resulted in a vertical uplift of $\sim$40m near the fan head. Fan head incision is of a similar order, and may in part be related to episodic strengthening of monsoons in the Early Holocene.
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting