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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 1 Centre of Advanced Study in Geology, University of Lucknow, Lucknow 2 Department 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.

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Page 1: Active Tectonics in the Outer Himalaya: Dating a Landslide Event …earthscienceindia.info/pdfupload/tech_pdf-44.pdf · Active Tectonics in the Outer Himalaya: Dating a Landslide

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 Bali1, A.R.Bhattacharya1 and T.N.Singh2

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.

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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 1320 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.

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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 60O, 400, 290O and 80O 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).

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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

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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.

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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.

Wedge Failure:

A wedge failure is a rapid downward movement of wedge shaped rock block along

the line of intersection of the two discontinuous surfaces forming the wedge. A wedge failure occurs when the angle of inclination of the line of intersection is greater than the angle of internal friction but less than the slope face (Hoek and Bray 1981). During wedge failure, the line of intersection daylights within the slope surface (Waltham 2002). In the Amiyan slide area, two very prominent N-S trending scarp faces of about 40-50 m length are developed in the central part of the slide (Fig.7). These mark the presence of two fault planes running almost normal to the strike of the MBT. These fault scarp faces dipping towards each other have resulted in the formation of a wedge. The overburden material marked by thick vegetation cover, has been detached as a result of movement along these planes and has subsequently moved downslope along the line of intersection (Fig. 7).

Fig. 7: Photograph showing two prominent N-S trending scarp faces in the Amiyan landslide.

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e-Journal Earth Science India, Vol.2 (IV), October, 2009, pp. 276 - 288

http://www.earthscienceindia.info/; ISSN: 0974 - 8350

A prominent geomorphological feature of the slide is the presence of a prominent drainage or stream (locally called a Nala) that broadly bisects the slide zone (Fig. 3). The slopes on the eastern side are much steeper as compared to those on the western side of the Nala. The general profile section (Fig. 8) shows that the slope is characterized by breaks thus producing at least five distinct flats, marked here has A, B, C, D, E and F. Field investigation shows that there are two distinct zones that show the development of a number of tensional cracks running almost E-W, i.e. across the slide.

Fig. 8: General profile section of the Amiyan landslide showing at least five distinct flats named here as A, B, C, D, and E.

A B

Fig. 9: Terraces along the left bank of the Gola River. A- Distant view, B- A closer look.

Neotectonic Character:

The Main Boundary Thrust (MBT), passing through the toe of the slide is known for its neotectonic behaviour, mainly in the form of recurring seismicity, landslides, uplift/subsidence of lands, etc. (see also Nakata 1989; Valdiya 1986, 2001). It appears that the bulk of the neotectonic activity particularly in the Amiyan slide area is used up in the initiation, maintenance and recurrence of landslide. The neotectonic behaviour of the area is also attested by a number of signatures in the form of neotectonic markers and geomorphic features around the landslide area, some of which are highlighted below.

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Active Tectonics in the Outer Himalaya: Dating a Landslide Event in the Kumaun Sector: Rameshwar Bali et al.

(i) In the landslide area, the Gola River shows development of prominent terraces

especially along its left bank (Fig. 9). (ii) About 6 km upstream of the Amiyan village, a major tributary of the Gola River,

referred to as Lugar Gad, meets near Hairakhan. Here the younger fan deposits covering the Siwalik sandstone have been uplifted by about 40 m with respect to the Lesser Himalayan rocks, as evidenced by a prominent scarp face (Fig. 10) at the left bank of the Gola River at about 8 km downstream of the slide area. Their occurrence indicates that the area has been subjected to rejuvenation.

Fig. 10: The MBT vertically offsets the Recent fan deposits along the Gola River.

(iii) At Beluwakhan, near Jeolikot, the stream-bed deposit of the now abandoned

Saulia stream has been uplifted by 30 m as a consequence of reactivation of the MBT. The huge landslide fan at Suriyajala resting on the river terrace of recent origin has been dissected by the MBT such that the toe lying on the Siwaliks is raised 85 m higher across the MBT (see Valdiya, 1986, 1992, 2001).

Discussion: Dating the Amiyan Landslide and its Implications

The Amiyan landslide has preserved geomorphic signatures that enable precise dating of the major landslide event. A detailed geomorphologic analysis has been carried out mainly to ascertain its precise geomorphic setting. Liss III and PAN data have been analysed and merged together to get an overall digital view of the slide (Fig. 2). Systematic contouring has been carried out on 1:2000 to understand the precise and actual topographical setting of the slide (Fig. 8). The large sale surveyed topographical and geological maps show the precise location of breaks in slopes, scarp faces, extent and orientation of the tensional cracks, etc.

The previous records show that the slide activity has increased rather very fast in the last few decades. The Survey of India toposheet of 1968 shows the slide area to be of 0.02 sq km that increased to 0.03 sq km in 1981 and further to 0.05 sq km in 1992 (Srivastava et al., 1996). This study shows that the present-day (2005) slide area is 0.62 sq km, i.e. it has increased by about 12 times during the last 13 years (Fig. 11 A). The progressive increase of the slide area with time has been plotted in a graph (Fig. 11 B). Significantly, the tensional cracks present in the landslide zone are not shown in the earlier toposheets and as such these cracks appear to have developed post-1992.

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e-Journal Earth Science India, Vol.2 (IV), October, 2009, pp. 276 - 288

http://www.earthscienceindia.info/; ISSN: 0974 - 8350

One possible interpretation of the graph is that there has been a gradual, but slow, buildup of stresses (as revealed by the slowly rising part of the graph) approximately from, say, 1968 or so till possibly 1992 when the stresses were released in the form of a major landslide or in the form of rejuvenation of some old landslide thus causing an increase of the slide area. Further, our field study clearly shows the presence of fault-scarp faces in the central part of the Amiyan slide. These fault-scarp faces have not been recorded in the Survey of India toposheet of 1968 as well as in the later studies carried out by the Geological Survey of India (Srivastava et al., 1992). All these evidences obviously imply that a major landslide or neotectonic activity may have taken place post-1992. It is however not possible to say whether this increase of the area (by about 12 times) has taken place in one single event or in pulses or in a progressive manner. But by all means, the period between 1992 and 2005 marks a major event of active tectonics in the area. This particular time period could thus be considered to be a major phase of rejuvenation of neotectonic activity in this part of the Himalaya.

Fig. 11: Quantitative representation of the progressive increase of the area of the Amiyan landslide during the last 13 years (1992-2005). A- Histograms showing the actual areas for the years measured. B- Graph showing the trend of increase of the slide area with time.

In the light of the above data, it appears that the present-day morphotectonic features of the Outer Himalaya may have developed post-1992 in response to some neotectonic event. The event has resulted in the subsidence of overburden material between two N-S striking gravity faults. The overburden material has been continuously moving down the slope under the action of gravity as well as due to the increase in pore water pressure during the heavy rainfalls of the monsoon seasons. The present-day Amiyan landslide with an aerial extant of 0.62 sq km thus appears to be one of the most glaring

examples of reactivation of mass movement activities in response to active tectonics in the

Outer Himalaya.

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Active Tectonics in the Outer Himalaya: Dating a Landslide Event in the Kumaun Sector: Rameshwar Bali et al.

From above, it appears that the Amiyan area is a major region of present-day stress

buildup possibly due to plate collision followed by release of these stresses as and when the strength of the rocks exceeds the external stresses. Recently, Bhattacharya (2005) and Bhattacharya and Agarwal (2008) have shown that the Siwalik rocks are characterized by complex thrust geometry, especially in the form of duplex, imricate fan systems, pop-up structures, snakehead anticlines and snakehead duplexes, fault propagation folds, antiformal stacks and a variety of related structures. These structures imply that these rocks are undergoing strong internal deformation that are a reflection of the internal stresses developed due to persistent plate collision. All these suggest that the Outer Himalaya is presently under the influence of strong internal stresses. Since the MBT is a prominent plane of anisotropy of regional scale, it is possible that the bulk or a major part of the tectonic stresses is being released along this tectonic plane. Precise dating of the Amiyan event, as presented in this work, thus suggests that the Outer Himalaya – particularly the Main Boundary Thrust - is possibly experiencing prominent tectonic reactivation and intense release of stresses during the last decade. Whether it is a precursor to any large-scale earthquake in the region is thus an open question!

Conclusions

• Landslides, in general, are initiated by several factors. In the Himalayan region,

majority of them are initiated due to excessive precipitation during the monsoons. The Amiyan landslide is, however, a glaring example of neotectonically induced landslide.

• In the Amiyan landslide, the two north-south trending fault scarp planes dipping towards each other have produced a prominent wedge. The overburden slumped wedge material has slipped northwards under the action of gravity.

• The previous records show that the slide initially had a very small slide area (0.05 sq km) along the Gola River and there were absence of fault scarps till 1992. However, the present study shows that there has been a sudden geomorphic change (increase) in the slide area with the appearance of two fault scarp planes post-1992. This sudden change in geomorphology has also been actually witnessed, confessed and acknowledged by the local people.

• The Amiyan slide zone thus constitutes one of the best documented areas where a large landslide has resulted as a consequence of active tectonics of the Himalaya. Its precise date of formation, i.e. 1992, is an addition of knowledge for the Himalayan region in general.

Acknowledgements: RB and ARB are thankful to Prof. N.L. Chhabra, Head, Centre of Advanced Study in Geology, University of Lucknow, for providing working facilities. Discussions with Dr. K.K. Agarwal have been fruitful. Financial assistance to two of us (RB and TNS) from the Department of Science & Technology, New Delhi, is thankfully acknowledged.

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