e-Journal Earth Science India, Vol.2 (IV), October, 2009, pp. 276 - 288 http://www.earthscienceindia.info/ ; ISSN: 0974 - 8350 Active Tectonics in the Outer Himalaya: Dating a Landslide Event in the Kumaun Sector Rameshwar Bali 1, A.R.Bhattacharya 1 and T.N.Singh 2 1Centre of Advanced Study in Geology, University of Lucknow, Lucknow 2Department of Earth Sciences, Indian Institute of Technology, Mumbai Email: [email protected] Abstract Of the entire Himalayan terrain, the Outer Himalaya is believed to show excellent signatures of active tectonics. The Main Boundary Thrust (MBT) that separates the Outer and Lesser Himalayas has a recorded history of tectonic activities in the recent past. The present study incorporates an additional example of a major landslide event, the Amiyan landslide, associated with the MBT that passes through the toe of this landslide. The Amiyan landslide is one of the biggest debris slides in the Central Himalayan region. Two prominent fault scarps running almost transverse to the MBT have developed during the last 15 years. The slide has been increasing in size at regular intervals from an earlier 0.02 sq km in 1968 to 0.05 sq km till 1992. Thereafter, the process of continuous reactivation of the MBT and the formation of fault scarps has resulted in about 12-fold increase of the slide. Such a topographical adjustment in response to active tectonics in this segment of the Himalaya suggests that the Outer Himalaya is possibly a major locale of present-day stress release in the Himalayan region. The results of this work have significant bearing on the seismotectonic, environmental, ground stability and the related aspects in the Himalayan domain. Introduction The Himalayan mountain chain has been geologically subdivided (Fig.1) into four major lithotectonic subdivisions (Gansser ,1964). These are, from south to north, (A) Outer Himalaya that is broadly constituted of the molassic Siwalik Supergroup of Mio-Lower Pleistocene ages) and is juxtaposed between two tectonic planes: Himalayan Frontal Thrust to the south and the Main Boundary Thrust to the north. (B) Lesser Himalaya that exposes a thick pile of folded Proterozoic sedimentary strata and includes a few outcrops of older crystalline rocks and is bound by the MBT in the south and Main Central Thrust (MCT) in the north. (C) Greater or Higher Himalaya constituted of a massive, north-dipping pile of crystalline-metamorphic rocks – the Central Crystalline Zone – and is demarcated by the MCT to its south and the Dar-Martoli Fault or the South Tibetan Detachment to the north. (D) Tethys Himalaya that includes a thick sedimentary pile of Cambrian to Lower Eocene ages. Of all these, the Outer Himalaya with a youthful and rugged topography is the youngest of all and constitutes an important unit of the foreland fold-and-thrust belt of the Himalaya. Active Tectonics in the Outer Himalaya: Dating a Landslide Event in the Kumaun Sector: Rameshwar Bali et al. Fig. 1: Lithotectonic subdivisions of the Himalaya (after Gansser, 1964). The area of study has been indicated. The mighty Himalayan mountain chain is believed to have formed as a consequence of collision of the Indian and the Asian plates during the Tertiary times. The development of the mountain was episodic in nature that ultimately gave rise to the present-day morphotectonic setup. The fact that the entire mountain belt is neotectonically active suggests that mountain building processes continue to be active even today and are often expressed in the form of earthquakes, landslides, subsidence and uplift of land, etc. Of the four major lithotectonic subdivisions, the Outer Himalaya is believed to be seismically and tectonically more active than others. An attempt has been made in this paper to precisely date a major active tectonic event in a part of the Outer Himalaya in the light of the active tectonics in this segment. This major event of landslide is associated with a major geotectonic element of the Himalaya, i.e. the Main Boundary Thrust (MBT). The event has significant bearing, amongst others, on the active tectonics of the Himalaya. As yet, information on precise dating of neotectonic events in the Himalayan region, in general, is meagre. The strong neotectonic behaviour of the Outer Himalaya (see Hodges et al., 2001) is recently brought out by numerous examples. Several workers ( e.g. Nakata, 1989; Yeats and Lillie, 1991; Yeats et al., 1992; Mukul, 2000; Valdiya, 2001; Malok et al., 2003; Joshi 2004; Philip and Virdi 2006) have shown that the major faults, especially those of the Outer Himalaya, are presently active. Although this region has experienced only a few major earthquakes, recurring seismicity of low to moderate magnitudes is a common phenomenon. Further, landslides, ground uplift and slope instability are very common features. All these processes are relatively more confined to the vicinity of the MBT, thus suggesting it to be neotectonically very active. In the Tista valley of northeast Himalaya, the Precambrian metamorphics of the Darjeeling hills have been shown to rest over the deformed Sub-Recent alluvial deposits (Heim and Gansser, 1939). Between Dehradun and Rishikesh, the Palaeozoic Chandpur phyllite overrides the Sub-Recent Dun gravel at several places (Jalote, 1966). The movement along the MBT has caused an uplift of the Dun deposits by 290 ± 76 m on the slopes of the Mussoorie hill (Nossin, 1971). Geodetic measurements carried out at Kalwar indicate that the Nahan Thrust separating the Lower Tertiary from the Siwalik is very active – the horizontal component of the movement is 0.902 cm/yr towards 132 0 E and the rate of strike-slip motion is 0.038 cm/yr (Sinhwal et al., 1973). Along the Lohit River, in the northeast Himalaya, the Mishmi Thrust juxtaposes, along the Mishmi Thrust, the Palaeozoic Mishmi metamorphics against the gravels and sands (Thakur and Jain 1974). Bali et al., 2003 and Bali, 2005 presented an outline of the neotectonic and morphotectonic evolution of a part of Garhwal Himalaya and northeastern Kumaun Himalaya respectively. e-Journal Earth Science India, Vol.2 (IV), October, 2009, pp. 276 - 288 http://www.earthscienceindia.info/ ; ISSN: 0974 - 8350 The present study brings into light an active tectonic event associated with the Amiyan landslide located near the MBT in the Kumaun region, and is located at about 4 km north of Kathgodam. The Amiyan Landslide Geological Setting: The Amiyan landslide is one of the biggest active slides of the Central Himalayan region. It is located in the Lower Siwalik Formation of the Outer Himalaya (Fig.2). The Main Boundary Thrust (MBT), that separates the Outer Himalaya from the Lesser Himalayan sedimentary sequence, follows the Gola River in this area and passes from the toe of the landslide. The Lesser Himalaya in the Amiyan area is characterized by the occurrence of a crystalline unit called as the Amritpur granite. The MBT thus separates the Siwalik rocks (Amiyan landslide zone) of the Outer Himalaya from the Amritpur granite of the Lesser Himalaya, and here the Gola River follows the MBT for at least 4-5 km (Fig. 2). However, in the adjoining as well as other parts of the Kumaun-Garhwal Himalaya, the MBT separates the Siwalik strata from the Krol Group (Bhattacharya, 1983). Fig. 2: False Colour Composite (FCC) showing the location of the Amiyan landslide. The dominant lithology of the Amiyan landslide area is sandstone (Fig. 3) occasionally interbedded with mudstone and siltstone (Pant and Luirei, 2005). The general dip of the rocks in the landslide area is 40°–50° towards NE while the slope of the ground is towards north. The relationship between the dominant dip of the rocks and the slope of the ground has been shown in the stereoplot (Fig.4). The rocks are affected by a number of joints, and at least four sets of joints striking 60 O, 40 0, 290 O and 80 O with different attitudes are present in the area. The northern face of the slide is rather steep and is represented by the slide overburden that includes heterogeneous debris constituted of blocks of weathered sandstones that mostly occur in cobble to pebble sizes, together with rock fragments occurring in a sandy to clayey matrix (Fig. 5). Active Tectonics in the Outer Himalaya: Dating a Landslide Event in the Kumaun Sector: Rameshwar Bali et al. Fig 3: Geological map of the Amiyan landslide area e-Journal Earth Science India, Vol.2 (IV), October, 2009, pp. 276 - 288 http://www.earthscienceindia.info/ ; ISSN: 0974 - 8350 Fig 4: Stereoplot showing the relationship between the dip of the dominant lithology and the slope of the ground surface of the Amiyan landslide Fig. 5: Photograph showing the general constitution of the debris material Geomorphological Features: The Amiyan landslide measures about 1.8 km from head to toe with a maximum width of about 0.7 km. The active part of the slide has a perimeter of 4.97 km, and covers an area of more than 0.62 sq km. The overall slope of the landslide from its head to toe is moderate. There is a fall in height of about 450 m within a distance of 1.7 km. Most of the slopes are covered by thick shrubs, forest cover and at places is being used for agricultural practices. The general topography of the slide zone is rugged with moderate to steep faces. Locally dip slopes (Fig. 6) are also developed. Steep ground slopes and river valleys with steep to sub-vertical walls are characteristic features of the terrain around the landslide. Active Tectonics in the Outer Himalaya: Dating a Landslide Event in the Kumaun Sector: Rameshwar Bali et al. Fig. 6: Photograph of the Amiyan landslide showing the dip-slope of the rocks of the slide area.