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2009. Morpho-sedimentary records at the Brahmaputra River exit, NE Himalaya: climate-tectonic interplay during the Late Pleistocene-Holocene.ABSTRACT: Morphological and sedimentary records at the exit of Brahmaputra River at Pasighat in the NE Himalaya inform about the climate-tectonic interplay during the past ca. 15 ka. The geomorphology of the area comprises (1) fan terrace T 3 , (2) a high-angle fan (3) terrace T 2 , (4) terrace T 1 and (5) a low-angle fan. Geomorphic consideration suggests that the fan terrace T 3 and high-angle fans are the oldest units and were coeval. The low-angle fan is the youngest geomorphic unit. Sedimentological studies and optically stimulated luminescence chronology suggest that (i) fan terrace T 3 formed between 13 and 10.5 ka and comprised multiple events of debris flows separated by the aggradation as channel bars in a braided river environment; (ii) the high-angle fan formed during 15-10 ka and comprises channel bar aggradation in braided river conditions; (iii) terrace T 2 formed during 10-8 ka due to aggradation in a braided channel environment with lesser events of debris flows; (iv) terrace T 1 formed during <7 and 3 ka took place as bars of the braided river. Sudden coarsening of the sediment indicated a tectonic rejuvenation in the provenance region between 7 and 3 ka; and (v) the low-angle fans dated to <3 ka formed due to aggradation in a small tributary joining the Brahmaputra River. This implies a phase when the main channel of the Brahmaputra did not flood regularly and the tributaries were actively aggrading. The sedimentation style and incision of these geomorphic units responded to contemporary climatic changes and uplift in the Siwalik range along the Himalayan Frontal Fault.

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Landforms developed across terrain defining boundary the Main Boundary Thrust (MBT) have imprints of recent tectonic activity. Depositional landforms such as colluvial fan bear signatures of later phases of tectonic activity in the form of faulting of colluvial fan deposits and development of fault scarps. Tectonic geomorphology applied to the MBT zone suggests recent subsurface activity along the MBT and its splay thrusts. Present day tectonic activity of MBT is indicated by ground creeping, thrusting of Lower Siwalik rocks over recent colluvial fan deposit, aligning of series of lakes along splay faults and laterally along a fault, deflected streams, fault scarps and waterfalls. In the present paper we are addressing the tectonic situation in the foothill region of southeastern Kumaun Sub-Himalaya, India based on detailed field work carried out in the region which brought forward some outstanding morphotectonic evidence of neotectonic activities in the MBT zone.

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