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.
In the Indian Himalayan region (IHR), landslide-driven hazards have intensified over the past several decades primarily caused by the occurrence of heavy and extreme rainfall. However, little attention has been given to determining the cause of events triggered during pre-and post-Indian Summer Monsoon (ISM) seasons. In the present research, detailed geological, meteorological, and remote sensing investigations have been carried out on an extreme rainfall landslide event that occurred in Sadal village, Udhampur district, Jammu and Kashmir Himalaya, during September 2014. Toward the receding phase of the ISM (i.e., in the month of September 2014), an unusual rainfall event of ~488.2 mm rainfall in 24 h took place in Jammu and Kashmir Himalaya in contrast to the normal rainfall occurrence. Geological investigations suggest that a planar weakness in the affected region is caused by bedding planes that consist of an alternate sequence of hard, compact sandstone and weak claystone. During this extreme rainfall event, the Sadal village was completely buried under the rock slides, as failure occurred along the planar weakness that dips toward the valley slope. Rainfall data analysis from the Tropical Rainfall Measuring Mission (TRMM) for the preceding years homogeneous time series (July-September) indicates that the years 2005, 2009, 2011, 2012, and 2014 (i.e., closely spaced and clustering heavy rainfall events) received heavy rainfalls during the withdrawal of the ISM; whereas the heaviest rainfall was received in the years 2003 and 2013 at the onset of the ISM in the study region. This suggests that no characteristic cyclicity exists for extreme rainfall events. However, we observe that either toward the onset of the ISM or its retreat, the extreme rainfall facilitates landslides, rockfall, and slope failures in northwestern Himalaya. The spatiotemporal distribution of landslides caused by extreme rainfall events suggests its confinement toward the windward side of the Himalayan front.
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