Himalayan Geology (Journal), Dehradun...........

Abstract: Geomorphic features and neotectonic evidences of the upper Spiti valley (31-33° N; 78-70° E) and mineral magnetic proxies of a palaeolake section (Seko-Nasung) in NW Himalayas were studied to generate a tectono-climatic history during the late Quaternary in this region. The OSL chronology brackets the lacustrine sedimentation between 12-8 ka BP and is underlain and overlain by a fluvial phases at Seko-Nasung and Kioto sections. Our results indicate two major tectonic/climatic events at ~12 and ~7 ka BP (OSL ages of the palaeoseismic structures associated with the lacustrine deposits) that were responsible for formation of lake. The mineral magnetic parameters of the Seko-Nasung section were studies and compared to the results of Kioto section previously studied by Sangode & Mazari (2007). The results show that during the late Quaternary, prior to 12 ka fluvial phase dominated the upper part of the Spiti valley, with the enhanced monsoon and land sliding activity around 12 ka the river was blocked and two major lake systems came into existence. Their sedimentary records show several warm and cold climatic phases with 4 warm spells, being prominent (between 11-12 ka; around 10 ka and two minor warm phases between 8.4-9.2 ka BP) which are comparable to the W-1 to W-4 warm spells reported by Sangode & Mazari (2007) in the Kioto palaeolake section. The lakes were terminated at ~7 ka with the recurrence of the tectonic/seismic activity and gave way to a fluvial phase which exists till toady. These lake sediments have been uplifted, eroded and incised by the river and are preserved only as patches today at a considerable height (~10-30 m) above the present day river bed.