Active tectonic deformation of the Shillong plateau, India: Inferences from river profiles and stream-gradients

Journal of Asian Earth Sciences 181 (2019) 103904

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Active tectonic deformation of the Shillong plateau, India: Inferences from river profiles and stream-gradients

T

Mukteshwar Nath Mishra Geological Survey of India Training Institute, GSI Complex, Bandlaguda, Hyderabad 500 068, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Shillong plateau Northeast India Morphotectonics Active tectonics Hack’s stream-gradient index Plateau uplift

The Shillong plateau in northeast India constitutes an actively deforming anticlinal basement uplift in the foreland of the Himalayas in the north and the Indo-Burman ranges in the east. The study aims to document zones of active crustal deformation in the Shillong plateau through the interpretation of the geometry and the quantitative analysis of stream profiles derived from 30-m SRTM DEM. Longitudinal profiles of the streams draining the plateau exhibit conspicuous convex upward reaches, mainly to the upstream of faults and in areas of deep river incision. Analysis of river profiles using Hack’s streamgradient index (SL-index) reveals perturbations in profiles and steep channel-gradients in the middle reaches of the streams. Spatial distribution of the SL-index brings out extensive zones of anomalously high stream-gradients along the monoclinal flexure at the southern margin of the plateau adjacent to the Dauki fault and along the northern margin, where multiple en echelon faults define step-like topography. In the western part of the plateau, spatial association of zones of high stream-gradients with well-defined fault-scarps is suggestive of active crustal deformation by differential vertical movement along faults. The observed spatial distribution of anomalously high stream-gradients is uncorrelated with the lithological variations and hence, perturbations in river profiles and abrupt changes in stream-gradients are suggestive of differential tectonic uplift of the plateau. The study suggests that the rate of active tectonic deformation in the plateau decreases from the east to the west.

1. Introduction Evolution of orogenic systems and associated landforms is controlled by a strong coupling between climate, tectonic deformation, and surficial processes. River systems serve as an efficient agent of redistribution of mass through erosion, sediment transport, and deposition and are sensitive indicators of differential uplift, folding, and faulting that cause channel incision, steepening of river gradients, and onset of accelerated erosion. As exemplified by the Himalayan river anticlines, rivers also influence the deformation and development of the orogen through focused rock uplift due to the isostatic rebound in response to significant differences between net erosion along major rivers and surrounding regions (Montgomery and Stolar, 2006; Robl et al., 2008). The complex interrelationship between active tectonic processes, climate, and landscape development can be well understood by the study of stream networks, which have regular, quantifiable geometric properties (Hack, 1973). Study of long profiles and analysis of channel gradients of different reaches of rivers provide an insight into the variable rock uplift rates in a region undergoing active tectonic

deformation if the effects of local lithologic and climatic variations, sediment load, and hill-slope processes can be accounted for (Hack, 1973; Kirby et al., 2003). Hence, the study of drainage systems and river profiles furnishes important clues for unraveling the nature and relative rate of the tectonic activity. The Shillong plateau, in northeast India, is an actively deforming basement uplift under the influence of compressive stress due to the collision tectonics along the Himalayas (Tapponnier and Molnar, 1976; Seeber et al., 1981) in the north and active subduction beneath the Indo-Burman ranges (Fitch, 1970; Verma et al., 1976) in the east (Fig. 1). The present paper constitutes the first attempt to the detailed study of the drainage network to decipher the signals of the ongoing tectonic activity in the plateau. The study examines the spatial relationship amongst nickpoints, faults, bedrock geology, and areas exhibiting longitudinal profile convexity and anomalous river gradients. The work primarily focuses on the geometry of longitudinal profiles of rivers and the analysis of steepness of the stream channels, computed from the Hack’s stream-gradient index (also known as SL-index; Hack, 1973), which constitutes an effective tool to detect the impact of

E-mail addresses: [email protected], [email protected]. https://doi.org/10.1016/j.jseaes.2019.103904 Received 22 October 2016; Received in revised form 22 June 2019; Accepted 25 June 2019 Available online 26 June 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Major morphotectonic features of northeastern India. 1. Dhubri fault, 2. Rangira thrust, 3. Fulbari fault, 4. Baladinggiri–Rongramgiri–Nagupara fault, 11. Dapsi thrust, 16. Chedrang fault, 18. Dudhnai fault, 19. Samin fault, 21. Dauki fault, 24. Kulsi fault, 28. Oldham fault, 29. Tyrshat–Barapani fault, 31. Um Ngot fault, 34. Kopili fault, 35. Main Foothill thrust, 36. Main Boundary thrust, 37. Main Central thrust, 38. Halflong–Disang thrust, 41. Brahmaputra fault.

active tectonic deformation in orogenic belts (Howard et al., 1994; Whipple and Tucker, 1999; Kirby et al., 2003; Ramsey et al., 2006; Clark and Bilham, 2008; Robl et al., 2008). Using stream length as a proxy for water discharge, Hack (1973) proposed the stream-gradient index or SL-index, which reflects the available stream power at a point along the river channel. The index at any given point in a river bed is the product of the channel slope at that point and the channel length measured along the longest river above the point. Variation in streamgradient index along a river is dependent upon (a) the underlying lithology and characteristics of the river bed in terms of particle size, (b) the presence of differential tectonic uplift causing changes in slopes, and (c) disequilibrium between two drainage systems close to their confluence. High SL-values are normally associated with lithologies resistant to erosion or where the river bed contains large-sized particles, which effectively protect the stream bed from erosion. Thus high SLvalues in a region are indicative of the presence of resistant rocks or where steepening of slopes have been caused by active deformation (Keller, 1986). Lack of correlation between anomalously high gradients and the stream bed lithology or where rocks of low or of uniform resistance show high stream-gradients, is indicative of active tectonic deformation. Hack’s stream-gradient index (SL-index), together with the geometry of longitudinal river profiles, has been employed as a reconnaissance tool to decipher zones of active uplift related to the ongoing tectonic activity (Seeber and Gornitz, 1983; Keller, 1986; Merritts and Vincent, 1989; Brookfield, 1998; Chen et al., 2003). Seeber and Gornitz (1983) and Brookfield (1998) attested that the anomalously high SL-index of the trans-Himalayan rivers were uncorrelated with rocks having differential resistance to erosion and that the high rivergradients were due to tectonic causes rather than to lithological changes. Through spatial analysis of SL-index, Keller (1986) correlated zones of anomalously high stream-gradient indices to areas of known active uplift in parts of the San Gabriel Mountains in southern California. At the Mendocino triple junction in northern California, analysis of SL-index provided the landscape response to the varying rates of active uplift in the region (Merritts and Vincent, 1989).

tectonic deformation. Based on the analytical results, the study documents the spatial variability of stream-gradient index (Hack, 1973) in the plateau to identify zones with differential rates of vertical tectonism in response to active tectonic deformation of the plateau due to the collision tectonics along the northern and active subduction accretion along the eastern margins of the Indian plate. In recent years, evaluation of the extent, nature, pattern, and the relative rates of active tectonic deformation in different tectonic settings, using a number of geomorphic indices, has been successfully accomplished by Strahler (1952), Schumm (1956), Bull and McFadden (1977), Rockwell et al. (1985), Merritts and Vincent (1989), Keller and Pinter (2002), Chen et al. (2003), Wobus et al. (2006), Cohen et al. (2008), Font et al. (2010), Kirby and Whipple (2012), Mahmood and Gloaguen (2012), and many others. In addition to the study of the drainage network, the present work makes use of a GIS-based quantitative analysis of a variety of geomorphic indices extracted from DEM to evaluate relative active tectonics in the plateau. The study demonstrates that the regional variations in relative tectonic activity interpreted from the Hack’s stream-gradient index (Hack, 1973) correspond well with the results obtained from the analysis of geomorphic indices computed from the drainage basins of the plateau. The study also attempts to discuss the morphological expressions of the major surficial and sub-surface morphotectonic features, which may have a bearing on active crustal deformation in the Shillong plateau. 2. Theoretical framework 2.1. The stream power law Central to the modeling of the variations in channel gradients and channel incision, caused by active tectonic deformation is the stream power law, which characterizes river incision as a function of contributing drainage area (a proxy for water discharge) and local channel gradient (Hack, 1957; Howard et al., 1994; Whipple and Tucker, 1999). This area-slope method has extensively been used to identify areas with varying rock uplift rates and to evaluate the nature, pattern, and style of 2

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Fig. 2. (a) Idealized longitudinal profile of a graded river; (b) Semi-logarithmic plot of the graded profile; (c) Methodology of computation of SL-index (modified after Hack, 1973).

where ΔΗ is the elevation difference between the top and the bottom of a reach, ΔL is the length of the reach and L is the total channel length of the longest stream from the water divide to the midpoint of the reach (Fig. 2c). The quantity (ΔΗ/ΔL) is, in effect, the average gradient of a reach of the river. It may be noted that the Eqs. (2) and (3) are equivalent and yield almost the same result. Once SL-index of a large number of streams in a region have been computed, the SL values may be interpolated to create a map showing the spatial distribution of stream-gradient index. Areas with unusually high SL values would indicate active uplift which would warrant further study.

2.2. River profiles and the stream-gradient index (SL-index) The longitudinal profile of a graded river in a tectonically stable region is typically concave upward (Fig. 2a). Departure of the river profile from this ideal smooth concave shape may reflect lithological variations in the river bed or variations in rock uplift rate of the stream bed (Burbank and Anderson, 2001, p. 186). In tectonically deformed areas, larger bedrock rivers, particularly in humid climate, have great erosive power and hence tend to obliterate the perturbations in the longitudinal profile quickly once tectonic uplift ceases. Hence, perturbations in river profiles, especially when not correlated to lithologic contrasts, may be due to ongoing tectonism (Burbank and Anderson, 2001). The semi-logarithmic plot of a graded river is nearly a straight line (Fig. 2b), represented by the equation:

H = C − k ln (L)

3. Material and methods 3.1. Analysis of long profile and Hack’s stream-gradient index

(1)

Study of longitudinal profile and analysis of stream-gradient index of the major rivers draining the Shillong plateau were carried out from the drainage network extracted from 1 arc-second (∼30-m) resolution SRTM DEM (data source: earthexplorer.usgs.gov). The original DEM was hydrologically conditioned by filling the pits, removing the spikes, and using the D8 method (Jenson and Domingue, 1988) for determining the flow directions on the topographic surface. The extracted drainage network was further processed to remove errors and inconsistencies. In all, 102 major streams (Fig. 7) of the plateau were selected for the extraction of longitudinal profiles and for computation of SLindex. Each of the 102 streams was split to ∼2-km segments, long enough to average out the sharp local changes in slope, such as those along water pools and rapids. Thus 2747 drainage segments were obtained and SL-index of each of these drainage segments was computed using equation (3).The individual segments were transformed to point features at their mid-points and the corresponding SL-value of each segment was attached to the points. The points with the attached SLvalues were smoothened and interpolated using the Ordinary Kriging algorithm (Isaaks and Srivastava, 1989) to obtain a map showing the spatial distribution of SL-index. Hack (1973) proposed a constant elevation difference (the contour

where H is the altitude at the point of the profile, L is the planimetric stream length from the drainage divide to the same point on the stream measured along the channel, and C and k are constants (Hack, 1973). The value of k, the slope of the graded profile, and called the “gradientindex” (Hack, 1973) can be computed by the formula

k = (H1 − H2)/(ln L2 − ln L1)

(2)

where H1 and H2 are elevations at each end of the reach measured, and L1 and L2 are distances from the source to each end of the reach. Quantification of nickpoints or perturbations in river profiles can be done by computing stream-gradient index or SL-Index (Hack, 1973) along the river course. The stream-gradient index is correlated to stream power and is expected to be constant along the river course if the river is graded. The SL-index is an important parameter because it gives an indication of the available stream power to transport the bed load. Variations in this index along the river course may be due to changes in bedrock lithology or active tectonism. The stream-gradient index (SL) has been defined as

SL = (ΔH /ΔL) ∗ L

(3) 3

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interval of the map) for the calculation of ΔΗ. This method has disadvantage in interpolation as most of the values are confined to areas where the contours are closely spaced due to steeper river gradients. In the lower reaches of rivers, where gradients are expected to be low, fewer points are available for interpolation. Hence, to obtain a uniform and equitable distribution of points for a meaningful geostatistical interpolation, stream-gradient (SL) index of drainage segments of constant length (≈2 km), as opposed to that of constant elevation difference, was computed. To evaluate the effects of bedrocks of variable resistance to erosion over the steepness of the river channels, geological contacts were plotted over the river profiles since SL-values are also influenced by the underlying geology. Apart from calculating SL-values of ∼2 km long reaches of rivers, the gradient-index k of each river for its entire length was also computed from Eq. (1) using the method of Seeber and Gornitz (1983). Accordingly, H2 and L2 were taken on the local base levels of the rivers at the alluvial plains, while H1 and L1 were taken at the end of the first segment, i.e., ∼2 km downstream from the source of the river. The latter was necessary because the development of the channel close to the water divide, at least for a distance of half-a mile (0.8 km), is not solely due to running water (Hack, 1973). Another index called the SLkindex was computed by normalizing the SL-values by the gradient-index k (Seeber and Gornitz, 1983; Pérez-Peña et al., 2009) for the entire river. The ratio SL/k thus quantifies the steepness of a short reach of a river against its overall steepness and also permits comparison of stream-gradients among different rivers. The SLk-index identifies the significantly steeper (SL/k ≥ 2) and much steeper (say, SL/k ≥ 5 or even SL/k ≥ 10) channel gradients. Maps showing spatial distribution of SL/k values are utilized to delineate areas where there is an anomalous departure of river gradients from their expected values. Such areas would indicate zones of active tectonic deformation (Seeber and Gornitz, 1983) where more detailed studies would be required.

were also utilized to study the geomorphic features of the terrain on either side of the fault. 4. Geological and tectonic setting The Shillong plateau is located in the northeastern India (Fig. 1), which is subject to complex tectonic stress patterns on account of its location in the proximity of the tri-junction of the Indian, the Eurasian, and the Burmese plates. The active north–south convergence along the Himalayas between the Indian and the Eurasian plates (Molnar et al., 1977; Seeber et al., 1981; Ni and Barazangi, 1984) and the east–west convergence, subduction of the oceanic crust, and deformation of the Indo-Burman ranges between the Indian and the Burmese plates (Nandy, 1980; Mukhopadhyay, 1984; Mukhopadhyay and Dasgupta, 1988; Alam et al., 2003; Hurukawa et al., 2012) impart geologic and tectonic complexity to the region. The immense loads of the Himalayan and the Indo-Burman thrust fold belts in the north, east, and the southeast, and the enormous weight of the 23 km thick sediments of the Bengal Basin (Bilham and England, 2001) and that of the Gangetic fan, further amplify the stresses in the Indian lithosphere in this region. As a consequence, the northeastern India is seismotectonically one of the most active regions of the earth. The eastern Himalayas, the Brahmaputra valley, the Indo-Burmese fold belt, the Bengal basin, and the Shillong plateau constitute the major seismotectonic units of the northeastern India. The Shillong plateau is a detached portion of the Indian peninsular shield across the Rajmahal–Garo gap and forms a basement uplift of dimensions 300 km × 100 km in the foreland of the eastern Himalayas in the north and the Indo-Burman ranges in the east. The Mikir Hills, a fragmented part of the Shillong plateau, is situated towards the east across the Kopili gap. The plateau, with Mikir Hills, constitutes a prominent, E–W trending, doubly plunging, and south-verging basementcored anticlinal fold (Clark and Bilham, 2008), the northern and the southern flanks of which have been truncated by major faults. The plateau is delimited by the N–S trending Dhubri fault (Nandy and Das Gupta, 1986) in the west, the E–W trending Dauki fault (Evans, 1964; Molnar, 1987) in the south, the NW–SE trending Kopili fault (Das Gupta and Nandy, 1982) and the NE–SW trending Haflong–Disang thrust (Evans, 1964) in the east, and the E–W to ENE–WSW trending Brahmaputra fault (Das Gupta and Nandy, 1982) in the north (Fig. 1). The plateau exposes Archaean–Proterozoic gneisses, schists, and migmatites as the basement, which is overlain by the Proterozoic sediments and greenstones of the Shillong Group, deposited in a NE–SW trending intracratonic basin. The gneisses and the rocks of the Shillong Group have been intruded by late tectonic Neoproterozoic – early Palaeozoic granite plutons and ultramafic and carbonatite bodies of early Cretaceous age. The basement of the Shillong plateau experienced peneplanation till Jurassic. After the break-up of the Gondwanaland, the Late Jurassic-Cretaceous Sylhet traps were extruded along the Raibah fault (fault no. 23; Fig. 3), a fault sympathetic to the Dauki fault (Nandy, 2001). Marine transgression during the Eocene contributed to the deposition of limestones and arenites under stable shelf conditions. The end of the Eocene and start of the Oligocene witnessed marine regression due to uplift of the landmass and formation of deltaic conditions in which Oligocene rocks were deposited. By the middle to late Miocene times, exhumation of the plateau started and deposition of feldspathic sandstones and mudstones took place in the southern part of the plateau. The kernel of the plateau consists of a smooth, almost un-dissected peneplained surface at elevations between 1500 and 1900 m above MSL. The elevation of the plateau decreases towards the south where the Precambrian basement and the overlying sedimentary strata display the development of a south-facing monocline (Fig. 5), truncated sharply by the approximately 300 km long, E–W trending Dauki fault, which also delimits the southern margin of the plateau. The Dauki fault presents a 1000 to 1200 m high scarp along the southern margin of the

3.2. Geomorphic indices Geomorphic indices viz. hypsometric integral (HI; Strahler, 1952), drainage asymmetry factor (AF; Hare and Gardner, 1985; Keller and Pinter, 2002), basin elongation ratio (Re; Schumm, 1956), basin shape index (Bs; Bull and McFadden, 1977), ratio of volume to surface area (Rva; Frankel and Pazzaglia, 2006), and ratio of valley-floor to valleyheight (Vf; Bull and McFadden, 1977) were utilized to evaluate the relative level of tectonic activity in the plateau and to corroborate the results obtained from the analysis of SL-index. These indices were computed for all the major drainage basins of the plateau having an area of more than 50 sq km. In all, 52 drainage basins of the plateau were analyzed. 3.3. Study of topography from DEM In recent years, extensive use of digital elevation models has been done to study the topographic features of the terrain with the view to decipher zones of active crustal deformation (Florinsky, 1996; Modenesi-Gauttieri et al., 2002; Ganas et al., 2005; Fodor et al., 2005; Jordan et al., 2005; Arrowsmith, 2006; Azañón et al., 2012; and many others). The present work utilizes topographic profiles, hill-shade images, and perspective viewing of DEM in 3-D environment to enhance and study the geomorphic and morphotectonic elements of the terrain. To study the abrupt changes in elevation across morphotectonic faults, longitudinal and transverse topographic sections of the Shillong plateau were constructed. Visualization of the terrain in 3-D environment facilitated the study of the monoclinal flexure at the southern margin of the plateau and the delineation of the morphotectonic faults (Figs. 4, 5). Topographic profiles showing abrupt change in the elevation of the terrain across prominent fractures and incision of drainage on the uplifted blocks indicated the presence of faults showing differential movement along them. High-resolution Google Earth images 4

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Fig. 3. (a) Morphotectonic faults and elevation values (m above MSL) on the Shillong plateau. (b) Topographic profile along A–B. Note the abrupt rise of the plateau along the Dauki fault (21) and topographic steps along faults numbered 30, 26, and 27 near the northern margin of the plateau. (c) Longitudinal profile along C–D showing step-faulting and the presence of horsts and grabens in the western part of the plateau. To the east of the Dudhnai fault (18), the plateau rises consistently to an elevation of ∼1900 m forming the highest planation surface. The elevation falls abruptly by 400 m across the Um Ngot fault (31). The main faults in (a, b, and c) are: 1. Dhubri fault, 2. Rangira thrust, 3. Fulbari fault, 4. Baladinggiri–Rongramgiri–Nagupara fault, 5. Chibra Agal–Urenggre–Balupara fault, 6. Tura fault, 7. Arbela–Songsak fault, 8. Mandalanggiri–Songsak fault, 9. Dadengiri–Manda fault, 10. Manda–Songsak fault, 11. Dapsi thrust, 12. Emangiri–Rewak fault, 13. Simsang south fault, 14. Simsang north fault, 15. Songsak–Chaphok fault, 16. Chedrang fault, 17. Darugiri fault, 18. Dudhnai fault, 19. Samin fault, 20. Sijobari–Borgap fault, 21. Dauki fault, 22. Pynursla fault, 23. Rhaiba fault. 24. Kulsi fault, 25. Chichra Githim–Nongkrem fault, 26. Jambalgiri–Mawthoh fault, 27. Gohanimara–Nangpoh fault, 28. Oldham fault, 29. Tyrshat–Barapani fault, 30. Nonpydem–Japung fault, 31. Um Ngot fault, 32. Tluh fault, 33. Umarangso fault, 39. Chachat–Karuba fault.

Shillong plateau (Clark and Bilham, 2008; Bilham and England, 2001; Johnson and Alam, 1991; Evans, 1964; Oldham, 1899). Recent geological studies suggest that the rise of the Shillong Plateau can be attributed to the slip of the plateau on Dauki fault, which is a northerly dipping high-angle reverse fault (Ferguson et al., 2012). Current models of the uplift and deformation of the Shillong Plateau envisage the presence of collisional tectonics along the Himalayas and subduction tectonics along the Indo-Burman ranges. Thermo-chronometric studies show that exhumation and the spectacular rise of the plateau initiated in middle to late Miocene times between 15 and 8 Ma ago (Biswas et al., 2007; Clark and Bilham, 2008). The rapid uplift of the plateau during this period has also influenced the development and evolution of the eastern Himalayas and the Indo-Burman fold-thrust belt since a substantial part of the N–S convergence between the Indian

plateau overlooking the low-lying Sylhet Plains of the Bengal Basin, having an elevation of only 10 to 15 m above the mean sea level (MSL). The fault has been variously described as a mega-scale strike-slip fault (Evans, 1964), as a north dipping low-angle thrust fault (Molnar, 1987; Johnson and Alam, 1991), and as a composite fault zone comprising three branching system of faults, each showing change in geometry from high-angle reverse fault to vertical fault and to high-angle normal fault along the strike (Nandy, 2001). The development of the monocline and a high degree of deformation in the sedimentary strata along the southern edge of the plateau suggests large-scale vertical uplift of the plateau to a height of nearly 2 km (Evans, 1964). Interpretation of gravity data and earthquake focal mechanism (Mitra et al., 2005; Chen and Molnar, 1990; Verma and Mukhopadhyay, 1977) support the existence of thrust or reverse faults beneath the southern part of the 5

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1987; Gahalaut and Chander, 1992). Bilham and England (2001) suggested that the great earthquake was caused by the violent, more than 11 m uplift of the northern edge of the Shillong plateau along a buried, 110 km long reverse fault dipping steeply at 57° towards the SSW from a depth of nearly 9 km to 45 km. They opined that the Shillong Plateau formed a ‘Pop-up’ structure between this “Oldham fault” and the northdipping Dauki fault at its northern and southern margins respectively. Apart from the major earthquakes, the plateau regularly experiences moderate to small, and micro-earthquakes (Mukhopadhyay et al., 1993; Kayal et al., 2006). High seismic and micro-seismic activity in the Shillong plateau has been attributed to a high rate of crustal deformation, uplift, and the presence of variable stress patterns beneath the plateau due to crustal heterogeneity (Khan and Chakraborty, 2007; Kayal and Jhao, 1998). Deep-seated (> 40 km depth) earthquakes (Khan and Chakraborty, 2007) and high gravity anomaly extending from the Shillong Plateau to the Upper Assam Valley (Verma and Mukhopadhyay, 1977), suggest active deformation and uplift of the mantle beneath the plateau in response to the N–S compression from the Himalayan collision and E–W compression due to the subduction beneath the Indo-Burman Ranges (Kayal and Jhao, 1998; Khan and Chakraborty, 2007).

Fig. 4. 3-D representation of the topography of the Garo Hills in the western part of the Shillong plateau. Note block faulting with abrupt changes in elevation across morphotectonic faults. Horst, graben, tilted blocks, and step-like topography from the west to the east are discernible. For names of the faults, refer to Fig. 3.

5. Results 5.1. Interpretation of morphotectonic features from DEM The major lineaments, structural trends, faults, and fracture systems of the Shillong plateau have been documented by many workers (Evans, 1964; Murthy et al., 1976; Mazumder, 1976, 1986; Gupta and Sen, 1988; Das et al., 1995; Srinivasan, 2003 ; Biswas and Grasemann, 2005, Duarah and Phukan, 2011; this study). The major faults in the plateau include the Dhubri, Dauki, Chedrang, Dudhnai, Kulsi, Kopili, and Um Ngot faults; the Dapsi thrust, and the Tyrshad–Barapani shear zone (Figs. 3–5). The present study contributes a few more hitherto unreported tectonic features that are morphotectonically important. All the faults have been numbered and are shown in Figs. 1 and 3. The main planation surface of the plateau forms a doubly plunging anticlinal structure (Clark and Bilham, 2008). The plateau forms a W–E trending highland with its elevation increasing from ∼250 m in the west to ∼2000 m in the east. From the west to east, the plateau consists of the Garo, the Khasi, and the Jaintia Hills, the boundary of each of which is delimited by N–S trending morphotectonic faults that cut through the entire width of the plateau (Figs. 1, 3). The Garo Hills lies between the Dhubri fault and the Dudhnai fault, the Khasi Hills occurs between the Dudhnai fault and the Um Ngot fault, whereas the Jaintia Hills occurs to the east of the Um Ngot fault (Figs. 1, 3).

Fig. 5. 3-D representation of the topography of the Khasi and Jaintia Hills. Note the prominent monoclinal flexure and canyon topography along the southern margin of the plateau. Sudden change in the elevation of the highest planation surface is seen across Um Gnot fault (31). Faults as seen in Fig. 3.

5.1.1. Regional Fracture, fault, and lineament patterns The Shillong plateau is traversed by E–W, N–S, NE–SW, and NW–SE trending sets of lineaments (Figs. 1, 3, 6) represented mainly by fractures, shear zones, and faults. The E–W trending set of lineaments, with slight deviations towards ENE–WSW and WNW–ESE, represents the oldest set and constitutes the major structural grain of the Precambrian terrain. This set parallels the Dauki fault in the south and the Brahmaputra River in the north. The up-arching of the Shillong plateau during the Mio–Pliocene and Pleistocene appears to have reactivated this set as has the collision tectonics along the northern edge of the Indian plate. The NE–SW and the NW–SE trending intersecting sets of lineaments have dismembered the terrain of the western part of the Shillong plateau in Garo Hills into a mosaic of rhombic blocks, which at places constitute horst, graben, and half-grabens across step-faults (Fig. 4).

and the Eurasian plates (Bilham and England, 2001; Jade et al., 2004; Mukul et al., 2010) and the E–W convergence between the Indian and the Burmese plates (Mukul et al., 2010) is being accommodated by the plateau through its deformation and uplift. Clark and Bilham (2008) propose that the plateau continues to rise in the form of a structural wedge bound by the Oldham fault in the north and an upward migrating, north-dipping fold axis, representing the Dauki fault that propagates from a blind fault at depth. 4.1. Seismicity The plateau is seismically highly active and has experienced large earthquakes (Mw > 7.0) in 1897, 1923, and 1930 (Kayal, 1987; Bilham and England, 2001). The great Assam earthquake (Mw = 8.1) of 12 June 1897 (Oldham, 1899), is believed to have originated along a gentle, north-dipping thrust beneath the western part of the Shillong Plateau (Oldham, 1899; Seeber et al., 1981; Molnar, 1987; Khattri,

5.1.2. The northern and the southern margins of the plateau The northern margin of the Shillong plateau comprises fractured and dismembered hill ranges, gneissic inselbergs, and erosional 6

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Fig. 6. Lineament fabric of the Shillong plateau.

embayments of rivers. Close to the northern margin, along a 10 to 12 km wide E–W trending zone between latitudes 25.70°N and 25.80°N and extending from the Dudhnai fault in the west to the Um Ngot fault in the east, the elevation of the plateau falls in a flight of topographic staircases, bound by prominent fault scarps or relatively steeper slope segments. These faults include (i) Chichra Githim (90.87°E, 25.75°N) – Nongkrem (91.47°E, 25.78°N) fault (fault no. 25, Fig. 3), (ii) Jambalgiri (90.85°E, 25.83°N) – Mawthoh (91.96°E, 25.82°N) fault (fault no. 26), and (iii) Gohanimara (91.08°E, 25.84°N) – Nangpoh (91.88°E, 25.91°N) fault (fault no. 27). The topographic front of the highest planation surface of the plateau in this area also shows the development of six en echelon, E-W and NW-SE trending, nearly 700 m high topographic scarps (shown as fault no. 30, Fig. 3) between Nongpydem (91.48°E, 25.80°N) and Japung (91.85°E, 25.65°N). Interestingly, these topographic scarps coincide with the southeastern part of the surface projection of the subsurface Oldham fault (fault no. 28) of Bilham and England (2001). Intense fracturing and river channel incision is also observed in this part of the terrain. High density of fractures and the presence of the three E–W trending sub-parallel faults and en echelon topographic scarps in this zone might suggest the presence of a master thrust or a fault which splays out at shallower crustal levels and reaches the surface. Topographic profile from the south to north (from 91.568°E, 25.143°N to 91.545°E, 26.107°N) brings out four topographic levels at the average elevations of 1800 m, 1200 m, 750 m, and 500 m along the northern margin of the plateau (Fig. 3b). It is likely that differential block movement along these splay faults has given rise to the topographic steps and has also controlled the uplift of the plateau along its northern margin during the seismic events. The southern edge of the plateau is sharply defined by the Dauki fault scarp along which it rises abruptly from the Sylhet plains at about 10 m above MSL to 1340 m above MSL near Cherrapunji. Large-scale uplift of the plateau along the Dauki fault has created vertical scarps, V-shaped deep canyons, and a series of waterfalls at the edge of the Cretaceous sandstones. From Cherrapunji, the terrain slopes gently towards the south for about 7 km conforming to the dip of the strata forming the monocline and then falls rapidly to the Sylhet plains as the beds start dipping at high angles along the Dauki fault (Fig. 5). Biswas et al. (2007) attribute a slight northerly tilt of the plateau to a higher rate of uplift (0.77 to 1.25 mm/yr) along the Dauki fault compared to that (0 to 0.68 mm/yr) along the Oldham fault.

lying tract at an average elevation of about 450 m above MSL and represents a block-faulted region, cut across by fractures and morphotectonic faults trending E–W, N–S, NW–SE, and NE–SW. Differential vertical movement with the formation of horsts, grabens, and halfgrabens is seen across some of these features (Fig. 4). Another distinctive feature of the terrain in Garo Hills is the presence of step-like topography from the west to east (Fig. 3c). East of the Dhubri fault (fault no. 1) at the western margin of the plateau, the terrain rises in the form of topographic steps, bound by four N-S trending faults: (a) Phulbari (90.051°E, 25.882°N) fault (fault no. 3), (b) Baladinggiri (90.304°E, 25.511°N) – Rongramgiri (90.307°E, 25.695°N) – Nagupara (90.312°E, 25.962°N) fault (fault no. 4), (c) Chedrang fault (fault no. 16) and (d) Dudhnai fault (fault no. 18, Fig. 3), with the successive down-thrown sides located to the west of each fault. All the faults present conspicuous westerly facing scarps or break in slopes and show abrupt change in the elevation of the terrain across them. In the Arbela Range (Fig. 3a), the elevation of the terrain changes abruptly from 450 m to 700 m across the Baladinggiri–Rongramgiri–Nagupara fault (fault no. 4). Southward thrusting of the Precambrian basement over the Tertiary sediments is also observed along the WNW-WSE trending Dapsi thrust (fault no. 11, Fig. 3), which forms an imposing 400 m high scarp in Garo Hills.

5.1.3. The Garo Hills The Garo Hills in the western part of the plateau, constitutes a low-

5.1.5. The Jaintia Hills The central upland zone of the Shillong Plateau in Khasi Hills

5.1.4. The Khasi Hills The Khasi Hills, delimited by the Dudhnai fault (fault no. 18) and the Um Ngot fault (fault no. 31), constitutes the highest central upland zone of the Shillong plateau at elevations in excess of 1500 m. The Shillong Peak (1964 m) forms the highest peak of the plateau. From the Dudhnai fault in the west, the terrain in the Khasi Hills rises from ∼500 m to ∼1900 m over a horizontal distance of about 65 km (Fig. 3c). The upland zone of main plateau is developed on the Precambrian tract as well as on the overlying sub-horizontal Cretaceous sandstones, which constitute a nearly horizontal structural platform towards the south. The plateau surface is being encroached upon by the gullies and canyons through extensive headward erosion. Spectacular rise of the plateau in relatively recent geological past, presence of nearly horizontal to low-dipping platform of shelf sediments, and extremely high rainfall of about 14000 mm/yr, has produced a highly dissected terrain along the southern margin (Figs. 3a, 5) of the Shillong plateau in the Khasi and the adjacent Jaintia Hills.

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Fig. 7. Drainage map of the Shillong plateau. The SL- and SLk-indices of all the rivers shown in the figure were computed and interpolated. The longitudinal profiles of the select 31 rivers are shown in Figs. 8–10. For the names of the faults please refer to Fig. 3.

profile upstream of the Samin fault (fault no. 19), which was associated with coseismic slip during the great Assam earthquake of 1897 (Oldham, 1899). The Chedrang River (Fig. 8: R_113) exhibits a totally convex profile with a high SL (370) and SLk-index of 5.9 in its lower reaches. The river is controlled by the well-known Chedrang fault, along which Oldham (1899) witnessed a vertical displacement of 11 m after the great earthquake. The Simsang River (Fig. 8: R_13499) in Garo Hills is the only river of Shillong Plateau which shows a typical concave upward profile. However, as the river flows over the uplifted Tura range as an antecedent stream, it exhibits nickpoints, entrenched meanders and higher stream-gradients (SL 620 to 1366; SLk 2.7 to 5.8) across Chachat–Koruba (fault no. 39) and Darugiri (fault no. 17) faults and Dapsi thrust (fault no. 11).

continues towards the east in Jaintia Hills but the elevation of the highest planation surface drops suddenly from ≈1900 m in the west to < 1500 m in the east across the N–S trending Um Ngot fault (Fig. 3c). Like Khasi Hills, the Jaintia Hills is also bordered by the highly dissected terrain in the north and the zone of the canyon topography in the south. The central part of the Jaintia Hills contains E–W trending faults and fractures which display slight differential block movement. 5.2. Description of river profiles The rivers draining the Shillong Plateau in the Garo, the Khasi, and the Jaintia Hills have distinct profile characteristics mainly due to varying degrees of dissection, terrain characteristics, climatic factors, and uplift rates in different parts.

5.2.2. Khasi Hills The rivers of the Khasi Hills, exhibit steep river gradients with high SL- (max. 7882), k- (max. 665), and SL/k- (max. 18.2) indices. Originating at elevations from 1900 m to 1500 m, these rivers reach their local base level in the Brahmaputra plains (∼30 to 50 m above MSL) in the north or in the Sylhet plains in the south (∼10 to 20 m above MSL). The rivers are controlled by basement fractures and are typically un-incised in their upper reaches with low SL- and SLk-indices. Flowing through an interconnected network of fracture systems, the rivers descend to lower topographic levels with their profiles showing prominent steps (Fig. 9: R_103, R_104, R_107, R_119), knick points, and convex segments (e.g. R_121, R_10005, R_11625 in Fig. 9). Five to ten km from the source, the rivers start incision of the river bed, with the consequent development of entrenched meanders and narrow, Vshaped gorges and deep canyons. High stream-gradients, V-shaped gorges, and significant profile convexities are commonly seen across faults associated with abrupt changes in the elevation of the terrain. Convexity in the profile is mostly associated with the up-thrown blocks on the upstream side due to migration of knick points (Fig. 9: R_106, R_121, R_124, R_9707, R_10005). The rivers draining the Khasi Hills and flowing towards the south are characterized by very steep streamgradients (SL/k up to 18.2; SL up to 7882; Fig. 9: R_101, R_103, R_14777, R_15457) and exhibit the development of 1.2 to 1.5 km deep canyons along the Dauki fault zone. In the zone of canyon topography, the rivers display some degree of concavity in their profile, seemingly because the rate of the plateau uplift is, in part, compensated by high erosion rates along the canyons on account of the extreme rainfall of

5.2.1. Garo Hills In block-faulted Garo Hills, the rivers originating from the Tura and Arbela ranges (Fig. 3a) and flowing towards the west or northwest show incised meanders, and high SL- (up to 698) and SLk- (up to 7.0) indices over the relatively uplifted blocks. Nearly all the rivers of the Garo Hills display prominent nickpoints and convex reaches upstream of faults (Fig. 8: R_110, R_114, R_115, R_116, R_10439, R_11146, R_13027), high SLk-index (up to 9.4), and high SL- gradient (up to 1083) suggesting continuing differential uplift of the terrain across tectonic features. Convex reaches upstream of Dadengiri–Manda (fault no. 9), Samin (fault no. 19), and Fulbari (fault no. 3) faults and Rangira thrust (fault no. 2) suggest neotectonic adjustment across these features. Rivers draining the Tertiary sediments, which are less resistant to erosion, also show high stream-gradients and convexity in their profile near faults. For example, R_13027 (Fig. 8), while crossing the Emangiri–Rewak fault (fault no. 12), south of the Dapsi thrust exhibits convex profile and high SL- (758) and SLk- (5.5) indices, suggesting that this fault might be active. Rivers draining the Khasi Hills and entering the Garo Hills across the Dudhnai fault, show the development of waterfalls, deep gullies, and high SL-gradient (up to 3066) and high SLk-values (up to 13.2) upstream of this fault. The Krishnai River (Fig. 8: R_9842) shows the development of steps and prominent knickpoints (SLk up to 7.4; SL up to 1331) in the granitoid – gneiss terrain along its profile upstream of the Mandalngiri–Songsak (fault no. 8) and Dadengiri–Manda (fault no. 9) faults. The Dudhnai River (Fig. 8: R_114) displays conspicuous convexity in its 8

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Fig. 8. Longitudinal profiles of rivers draining the Garo Hills. Decimal numbers on profiles denote SLk-index suggesting anomalously steep (SL/k > 2.0) reaches; C = upward convex reaches; faults are represented by vertical lines with half-arrows, fault numbers are the same as in Fig. 3; Lithology: Gn = gneiss, Gr = granite, Shi = Shillong Group, T = Cretaceous–Tertiary sediments.

higher values of SL- (max. 7952), k- (max. 448), and SLk- (max. 22.2) indices than those of the Garo Hills. High SL- and SLk-values are seen along rivers in the in the eastern, northeastern, and the southern parts of Jaintia Hills, suggesting deformation and differential uplift of the plateau in these parts. Most stream profiles display significant convexity (Fig. 10: R_132, R_134, R_139, R_10056, R_10185, R_16609) in their overall shape or show well-defined steps (Fig. 10: R_131). The rivers draining the Jaintia Hills flow on diverse lithology comprising the Precambrian gneisses, schists, and granites; conglomerates, quartzite, sandstones, siltstones, and shales of the Shillong Group; and the Tertiary rocks consisting of conglomerates, sandstones, shales, mudstones, and limestones. Examination of the river profiles of the Jaintia Hills suggests that the SL-values are not significantly affected by the rocks of varying resistance to erosion (Fig. 10: R_131, R_132, R_10185, R_10056, R_16609). Even the rivers draining the softer Tertiary sediments exhibit high SLk (up to 11.0) value (Fig. 10: R_16609) in proximity with the Dauki fault. Convex reaches upstream of faults are seen in most of the profiles (Fig. 10: R_131, R_132, R_134, R_139, R_10056, R_10185,

10,000 to 15,000 mm/yr (Soja and Starkel, 2007) in this sector. The rivers draining the northern part of the Khasi Hills are characterized by high SL- (up to 3600) and high SLk- (up to 10.5) indices and exhibit deeply incised V-shaped gorges. Convexity in the river profiles (Fig. 9: R_121, R_124, R_9707, R_10005) upstream of the faults traversing the northern margin of the plateau, is suggestive of the continuing uplift of the latter along these faults. The rocks of the Precambrian Shillong Group contain varied lithology consisting of conglomerate, basic sill, quartzite, sandstone, siltstone, and shale. Rivers draining these rocks and flowing towards the Dauki fault region exhibit a series of nickpoints (Fig. 9: R_15627) with high SL- (up to 1759) and SLk- (up to 3.4) indices. Likewise, streams rising within and flowing on the Tertiary sequence close to the Dauki fault, show prominent nickpoints and convex reaches upstream of the fault and have large SL- (up to 936) and SLk- (up to 7.5) indices. 5.2.3. Jaintia Hills Like Khasi Hills, the rivers of the Jaintia Hills, display significantly 9

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Fig. 9. Longitudinal profiles of rivers draining the Khasi Hills. Decimal numbers on profiles denote SLk-index suggesting anomalously steep (SL/k > 2.0) reaches; C = upward convex reaches; faults are represented by vertical lines with half-arrows, fault numbers are the same as in Fig. 3; Lithology: Gn = gneiss, Gr = granite, Shi = Shillong Group, T = Cretaceous–Tertiary sediments.

the Khasi, and the Jaintia Hills are summarized in Table 1. The numbers represent the frequency of the observations in each class of SL and SLk in the hard rock terrain of the three regions of the plateau.

R_16609). Rejuvenation and uplift of the terrain is prominent in the southeastern part of the Jaintia Hills close to the junction of the Dauki fault and the Haflong–Disang thrust (Fig. 1). Variations in SL- and SLk-values in the hard rock terrain of the Garo, 10

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Fig. 10. Longitudinal profiles of rivers draining the Jaintia Hills. Decimal numbers on profiles denote SLk-index suggesting anomalously steep (SL/k > 2.0) reaches; C = upward convex reaches; faults are represented by vertical lines with half-arrows, fault numbers are the same as in Fig. 3; Lithology: Gn = gneiss, Gr = granite, Shi = Shillong Group, T = Cretaceous–Tertiary sediments.

Differences in the shapes of hypsometric curves and HI values in different drainage basins provide an insight into the stages of landscape evolution. Using the shapes of the hypsometric curves, Strahler (1952) classified drainage basins as young (with convex upward curve), mature (with S-shaped concave-convex upward curves) and monadnock (with concave upward curve). Hypsometric integral (HI) is a measure of the area below the hypsometric curve and thus indicates the volume of the basin that has not been eroded. It is especially useful in the comparison of several basins and to identify anomalous drainage basins in areas undergoing uplift on account of active deformation. High HI-value indicates that not much of the uplands have been eroded and hence the landform may be young, possibly produced by active tectonics. Low HIvalues may represent older, eroded landscape, less affected by recent tectonism. To understand the spatial variation in the uplift rate of the Shillong plateau, the hypsometric integral (HI) of all the basins (Area ≥ 50 sq. km) was computed using the equation

Table 1 Regional variations of SL- and SLk-indices in the hard rock terrain of the Garo, the Khasi, and the Jaintia Hills in the Shillong plateau. Indices

Garo Hills

Khasi Hills

Jaintia Hills

SL > 500 SL > 1000 SL > 2000 SL > 3000 SL/k ≥ 2 SL/k ≥ 5 SL/k ≥ 10 Total no. of observations

45 (10.2%) 12 (2.7%) 1 (0.2%) 1 (0.2%) 84 (19.0%) 17 (3.9%) 3 (0.7%) 441

313 (36.2%) 159 (18.4%) 50 (5.8%) 20 (2.3%) 245 (28.4%) 60 (6.9%) 10 (1.2%) 864

173 (34.7%) 92 (18.5%) 29 (5.8%) 11 (2.2%) 161 (32.3%) 38 (7.6%) 14 (2.8%) 498

5.3. Geomorphic indices 5.3.1. Hypsometric integral (HI) and hypsometric curves Hypsometry determines the relationship between elevation and area in a basin (Strahler, 1952; Keller and Pinter, 2002). Hypsometric curves and hypsometric integrals (HI) of drainage basins are important attributes that describe the relationship between erosion and deposition rates and active tectonism. Hypsometric curves permit the comparison of forms of basins of different sizes and elevations (Strahler, 1952).

HI = (Emean − Emin )/(Emax − Emin )

(4)

where Emean, Emin, and Emax respectively represent the mean, the minimum, and the maximum elevations of the individual basins. The equation is essentially a measure of the elevation – relief ratio. The HI-values in the drainage basins of the Shillong plateau range 11

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Fig. 11. Distribution of (a) SL- and (b) SLk-values in Shillong Plateau. Note well-defined zones of high SL- and SLk-indices along zones of faulting. Spatial association of high SL and SLk values with seismicity is also noticed. Thickness of rivers is proportional to SL and SLk values of individual reaches. Faults are the same as shown in Fig. 3.

adjacent to the zone of convergence of the Dauki fault and the Halflong–Disang thrust (Figs. 12, 13c). Langlai River (Basin ID 10388, HI = 0.42) and Dlyung River (Basin ID 15220, HI = 0.47) largely display convex upward hypsometric curves. The Langyen River basin (Basin ID 10193, HI = 0.26) presents a mature basin. The Kopili River basin (Basin ID 14296) at the eastern margin of the plateau displays pronounced convexity in the hypsometric curve in its central part and has a relatively high HI- value of 0.35, suggesting uplift of the landscape in its basin. The northerly flowing rivers of the Shillong plateau exhibit relatively low HI-values (0.14 to 0.36) but have complex hypsometric curves with straight to slightly convex and S-shaped curves. The Kulsi River Basin in Khasi Hills (Basin ID 10005, HI = 0.36) displays a

from 0.10 to 0.68 (Fig. 12). An E–W trending, nearly 200 km long zone, comprising basins with high HI-values (0.42 to 0.68) is seen along the southeastern and the southern part of the plateau in Jaintia and Khasi Hills adjacent to the Dauki fault. The zone is marked by the presence of narrow, elongated, deformed basins (Basin IDs: 14777, 15457, and 16609) with high HI values between 0.60 and 0.68. High HI-values in this zone indicate relatively rapid uplift of the plateau along the Dauki fault. The hypsometric curves of the basins in this zone are predominantly convex (Fig. 13a), suggesting young, recently uplifted terrain. Drainage basins along the eastern margin of the plateau in Jaintia Hills show HI-values ranging from 0.26 to 0.47 (Fig. 12). High HI-values with prominent convexity in hypsometric curves characterize basins

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Fig. 12. Hypsometric integral of the drainage basins of the Shillong plateau. Decimal numbers in squares denote the hypsometric integral of individual basins. Numbers in bold fonts denote Basin IDs. Numbers in circles are the same as shown in Fig. 3 and denote some important faults along the northern and the southern margins of the plateau.

Fig. 13. Hypsometric curves of the drainage basins of the Shillong plateau. The decimal numbers denote HI- values and IDs pertain to basin IDs (a) Hypsometric curves of the basins along the Dauki fault and the Haflong–Disang thrust. The curves are largely convex or inverted S-shaped. (b) Hypsometric curves of the basins along the northern margin of the plateau. The curves have convexity in their body or are straight. (c) Hypsometric curves of the basins of the Jaintia Hills, most of the basins display convexity in their hypsometric curves. (d) Hypsometric curves of the basins of the Garo Hills in the southwestern part of the plateau. Basins display concave up hypsometric curves with some convexity.

differential uplift along the Arbela–Songsak, Mandalanggiri–Sonsak, and Dadengiri–Manda faults (faults no. 7, 8, & 9; Fig. 3). In the southwest part of the plateau in Garo Hills, the southerly and the westerly flowing rivers show low HI-values and display curves typical of mature streams. The Simsang River (Basin ID 13499, HI = 0.31) has moderate hypsometric integral and exhibits a typical upwardly convex – concave hypsometric curve (Fig. 13d). The convexity in the upper part of the curve may represent the uplift of the Tura Range which forms the headwaters of the river. Examination of the hypsometric curves of the river basins of the Shillong plateau suggests that they depart from the typical S-shaped

straight hypsometric curve with slight upward convexity in its body (Fig. 13b). Relatively higher HI- value and straight to convex hypsometric curve might suggest uplift of the northern margin of the plateau along the Oldham (fault no. 28), ChichraGithim–Nongkrem (fault 25), Jambalgiri–Mawthoh (fault 26), Gohanimara–Nangpoh (fault 27), and the Nongpydem–Japung (fault 30) faults. The UM Khen (Basin ID 10056, HI = 0.29) and the Umiam (Basin ID 10185, HI = 0.28) Rivers, despite having rather low HI-values, show largely convex curves with prominent inflexion. The Krishnai basin in Garo Hills (Basin ID 9842, HI = 0.21) shows S-shaped curve (Fig. 13b) with prominent convexity in the upper part in the Arbela Range which may have undergone

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curves that are characterized by a concave-up “head” and a convex-up “toe”. S-shaped curves are indicative of a mature stage of drainage (Strahler, 1952). Most curves of the drainage basins of the Shillong plateau exhibit convex-up character in the head and concave-up form at the toe, resembling an inverted “S”. They are akin to those of the Type II landforms of Troeh (1965) that are typical of catchments where fluvial process is superimposed upon an older landscape formed by diffusive processes. Using a catchment evolution model, Willgoose and Hancock (1998) opine that convex-up region at the water divide requires a strongly slope-dependent process or diffusive process while to obtain the upward-concave region downstream requires a fluvial process that is dependent on both contributing area and slope. Predominance of diffusive processes in the headwaters of the rivers of the plateau provides indication of uplift of the landmass and accentuation of relief. In general, the drainage basins of the Khasi and the Jantia Hills display higher HI-values and more convexity in their hypsometric curves than the basins of the Garo Hills, suggesting spatial variation in uplift rates. Also, basins with high HI values and convex hypsometric curves correspond well with the zones of high SL- and SLk-indices.

display an easterly tilt. In Garo Hills, AF-values are generally low and the tilt directions are variously oriented according to the tilt of the crustal blocks. 5.3.3. Valley floor width-to-height ratio (Vf-ratio) Systematic analysis of cross valley profiles provides an insight into the relative rates of uplift, subsidence, erosion, and deposition in a region. Presence of U-shaped cross-valley profiles indicates low uplift rates and the dominance of surface processes whereas V-shaped profiles are indicative of areas undergoing rapid uplift and drainage incision. The shape of a cross valley profile is mathematically computed through Vf-ratio which is given by the equation

Vf =

(6)

where Vfw is the width of the valley floor, Eld and Erd are elevations of the left and the right valley divides, and Esc is the elevation of the valley floor (Bull and McFadden, 1977). Low values of Vf (Vf < 1) are associated with deeply incised V-shaped valleys in areas witnessing relatively rapid uplift. Computation of Vf-ratios along 182 profiles was done by placing the cross valley profile approximately one to two kilometer upstream of the mountain fronts, catchment outlets, or fault scarps (Fig. 15). Profiles were also computed at the plateau top where drainage incision was seen. The width of the valley floor was measured on high-resolution Google Earth images. Spatial variations in Vf-ratios have been modeled using Theissen polygons in Fig. 15. Majority (160 out of 182, 88%; Table 3) of the Vf-ratio values in Shillong plateau are less than one, suggesting the presence of deep V-shaped valleys associated with higher degree of tectonic activity. An E-W trending zone of extremely low (Vf < 0.01) Vf-ratios is seen in the zone of the canyon topography along the Dauki fault. Another NW–SE trending zone with Vf-values less than 0.01 occurs along the northern margin of the plateau following the trend of surface projection of the Oldham fault. Vf-ratios with values greater than one are mainly confined to the alluvial tract bordering the plateau where river embayments and softer lithologies are present.

5.3.2. Drainage basin asymmetry factor (AF) Drainage basin asymmetry factor (AF) is a geomorphic index (Keller and Pinter, 2002; Pérez-Peña et al., 2010) that aims to detect tilting of drainage basins on account of tectonic deformation. Asymmetry factor is especially sensitive to change in tilt perpendicular to the stream flow direction. The asymmetry factor of all the 52 river basins of the Shillong plateau was computed using the equation:

AF = ABS [(Ar /At ) × 100 − 50]

2Vfw (Eld − Esc ) + (Erd − Esc )

(5)

where Ar represents the area of the basin to the right of the trunk drainage, At is the total area of the basin and ABS stands for the absolute value of the expression within square brackets. If there is no tilting of the basin, the value of AF would be close to zero while highly tilted basins would have AF close to 50. The drainage basins of the Shillong plateau range in AF from 0.57 (nearly symmetric) to 32.76 (highly asymmetric) (Fig. 14, Table 2) with the direction of the tilt having been represented by arrows in Fig. 14. Large asymmetry in basins is observed in the central and the eastern part of the plateau in Khasi and Jaintia Hills where the Shillong plateau attains the highest elevations. In general, the basins on the highest part of the plateau

5.3.4. Ratio of volume to area (RVA) The ratio of volume to area (RVA) of a drainage basin has been used as a tool for deciphering the relative rate of tectonic activity (Frankel

Fig. 14. Drainage asymmetry factor (AF) map of the Shillong plateau. Decimal numbers represent the magnitude of the tilt of the basins and the arrows depict the direction of the tilt. 14

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Table 2 Morphometric parameters of the drainage basins of the Shillong plateau. Sl. No.

Basin_ID

Location

AF

HI_Integral

Basin shape index

Basin elongation ratio

RVA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

6859 7940 8239 8336 8941 8962 9152 9707 9827 9842 9852 9990 10005 10056 10185 10193 10388 10439 11146 11224 11256 11625 11874 12317 12565 12680 13027 13229 13499 13739 13939 14016 14053 14296 14425 14554 14777 15220 15258 15457 15627 15790 15852 16258 16356 16609 17012 17198 17293 17368 17436 17496

Jaintia Hills Khasi Hills Garo Hills Khasi Hills Khasi Hills Garo Hills Garo Hills Khasi Hills Jaintia Hills Garo Hills Garo Hills Garo Hills Khasi Hills Jaintia Hills Khasi Hills Jaintia Hills Jaintia Hills Garo Hills Khasi Hills Khasi Hills Khasi Hills Khasi Hills Garo Hills Garo Hills Garo Hills Garo Hills Garo Hills Garo Hills Garo Hills Garo Hills Khasi Hills Khasi Hills Khasi Hills Jaintia Hills Garo Hills Khasi Hills Khasi Hills Jaintia Hills Khasi Hills Khasi Hills Khasi Hills Khasi Hills Khasi Hills Khasi Hills Jaintia Hills Jaintia Hills Jaintia Hills Jaintia Hills Jaintia Hills Jaintia Hills Jaintia Hills Jaintia Hills

8.74 13.97 8.56 15.59 25.99 4.78 5.26 4.97 2.73 7.27 25.62 9.61 12.28 2.28 28.00 7.36 10.42 11.13 19.30 32.76 22.18 17.35 0.57 19.60 9.84 7.17 4.51 12.91 11.81 23.62 17.65 26.24 6.20 14.51 1.95 7.64 15.60 17.91 10.35 7.37 16.21 18.20 1.47 21.33 9.81 20.43 28.99 14.69 14.45 20.47 13.49 3.41

0.10 0.24 0.19 0.16 0.12 0.28 0.16 0.14 0.30 0.21 0.10 0.19 0.36 0.29 0.28 0.26 0.42 0.11 0.64 0.54 0.22 0.31 0.28 0.13 0.17 0.29 0.17 0.21 0.31 0.16 0.53 0.32 0.34 0.35 0.16 0.13 0.60 0.47 0.27 0.68 0.49 0.34 0.35 0.57 0.37 0.61 0.42 0.31 0.34 0.32 0.32 0.40

1.51 1.56 1.91 1.61 1.28 1.77 1.69 1.42 1.87 1.42 1.64 1.69 1.19 2.12 1.77 1.58 2.55 1.57 1.97 1.91 1.64 2.37 4.11 1.97 1.71 1.40 2.01 1.63 1.38 1.96 1.51 2.46 1.98 1.60 2.00 1.41 3.13 1.60 1.34 2.26 2.15 1.95 1.55 1.54 1.34 1.51 1.30 1.61 1.51 1.22 2.06 1.40

0.72 0.70 0.61 0.58 0.81 0.64 0.64 0.74 0.63 0.72 0.58 0.67 0.78 0.55 0.63 0.67 0.55 0.62 0.55 0.52 0.62 0.59 0.41 0.65 0.67 0.66 0.56 0.65 0.67 0.65 0.63 0.56 0.58 0.62 0.54 0.67 0.46 0.75 0.68 0.49 0.57 0.56 0.70 0.69 0.74 0.70 0.66 0.73 0.68 0.79 0.59 0.73

4.13 81.21 3.27 55.71 71.99 2.26 3.88 118.11 28.41 98.01 50.11 2.77 183.69 125.92 118.93 93.02 163.90 92.92 23.31 68.58 84.38 108.89 34.72 118.67 14.59 27.00 98.47 26.82 218.47 88.87 73.50 50.33 126.56 59.59 11.34 33.14 220.15 149.97 135.16 194.14 367.55 23.43 71.86 239.44 93.59 61.98 321.26 269.69 202.97 283.71 474.68 245.70

5.3.5. Basin shape index (Bs) Basin shape index (Bs) is a metric that measures the degree of elongation of a drainage basin (Ramirez-Herrera, 1998) and is given by the equation:

and Pazzaglia, 2006). Immature drainage basins have smaller area and low sediment yield, resulting in a low volume to area (RVA) ratio. As the river basin matures and tectonic uplift takes place, the volume of the basins increases significantly due to drainage incision while the basin area remains nearly the same, and hence higher (RVA) ratio results. As the volume of the basins is normalized to basin area, direct comparison of basins having different size can be done. The RVA-values in the study area exhibit marked spatial variability in the basins of the Garo, the Khasi, and the Jaintia Hills (Fig. 16, Table 2). The mean RVA-values for the basins of the Garo, the Khasi, and the Jaintia Hills are 56 (range 2.3–218.5), 117 (range 23.3–367.5), and 172 (range 4.1–474.7) respectively. The highest RVA-values are shown by the basins of the Khasi and the Jaintia Hills bordering the Dauki and the Um Ngot faults and in the region of the convergence of the Dauki fault and the Haflong–Disang thrust (Fig. 16). In Garo Hills, only the Simsang valley (Basin ID 13499, RVA = 218) and the Krishnai valley (Basin ID 9842, RVA = 98) exhibit higher RVA values.

Bs =

Bl Bw

(7)

where Bl is the length of the basin from its headwaters to its mouth and Bw is the width of the basin measured at its widest point. Basins in tectonically active areas are expected to be elongate in shape along the slope of the mountain front. If the tectonic activity reduces or ceases with time, the basin will acquire circularity as the stream starts widening the basin rather than down-cutting it. In areas of active uplifts, the Bs -values will be larger than those in areas having tectonic quiescence. The basin shape index (Bs ) values for the drainage basins of the Shillong plateau range from 1.19 to 4.11 with mean 1.78 and standard deviation 0.49. Elongate basins with higher basin shape index 15

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Fig. 15. Valley floor width-to-height (Vf) ratio map. Dots represent the location of measurement of Vf-ratio.

6. Discussions

are seen along the Dauki fault and along the zone of convergence of the Dauki fault and the Haflong–Disang thrust (see Fig. 17).

6.1. Stream profile characteristics and relation of SL-values with lithology and tectonics 5.3.6. Basin elongation ratio (Eb) Basin elongation ratio (Eb) has been defined as the ratio of diameter of a circle having the same area as the basin to the maximum length of the basin (Schumm, 1956; Bull and McFadden, 1977). This relationship can be expressed by the equation:

Eb = 2 √ (A/ π )/ Lb

Longitudinal profiles of the rivers draining the Shillong plateau exhibit prominent steps, nickpoints, and well-developed convex upward reaches on the upstream side of faults having their downthrown blocks downstream. Convexity in stream profiles is also commonly encountered where the rivers are deeply incised. In the absence of lithological controls, such features may be interpreted as propagation of the tectonically generated nickpoints upstream (Burbank and Anderson, 2001; p. 160) on account of the continuing or recent uplift of the terrain. Development of significant convexities upstream of faults in central Apennines of Italy has been regarded as the transient response of the river to faults associated with increase in uplift rates during the past (Whittaker et al., 2008). Hence, the presence of profile convexities upstream of most faults of the plateau is indicative of the accelerated uplift of the Shillong Plateau across active faults during the recent past. While SL-index is also influenced by lithological contrasts (Hack, 1973; Burbank and Anderson, 2001; Harkins et al., 2007) with more resistant lithologies having higher SL-values, the plot of lithological units on longitudinal profiles of the rivers (Figs. 8–10) does not show significant correlation between stream-gradients and lithology. Even the Tertiary rocks, comprising less resistant shale and sandstones at the southern margin of the plateau, show high SLk-values (up to 5.8) that are comparable with those on the resistant granitoids and gneisses. The Simsang River (Figs. 7, 8: R_13499) in Garo Hills, which originates in the gneissic terrain but flows on the Tertiary rocks for most part of its course, shows tectonically generated knickpoints and high SL- (up to 1352) and SLk- (up to 5.8) indices in its middle and lower reaches on Tertiary strata. Variations in SL-index, not related with lithological contrasts, have been considered to indicate tectonic deformation of a region (Burbank and Anderson, 2001). Seeber and Gornitz (1983) and Brookfield (1998) in the Himalayas and Chen et al. (2003) in the western foothills of Taiwan observed general lack of control of lithology over stream-gradients. They argued that the steepened river-gradients were caused by tectonic deformation along active faults and thrusts, rather than due to lithological variations. Thus higher SL values, both on the erosion-resistant Precambrian basement rocks as well as on the less resistant Tertiary strata, are indicative of active tectonic

(8)

where, A is the area of basin, π = 3.14, and Lb is the length of the basin measured from the headwaters to the mouth. Thus circular basins in tectonically inactive regions will have Eb values close to 1 while elongate basins in areas undergoing tectonic uplift are expected to have values significantly less than 1. The elongation ratio of the drainage basins of the Shillong plateau ranges from 0.41 to 0.81 with the mean of 0.64. Most of the elongate basins with Eb < 0.64 occur along the Dauki fault and along the zone of convergence of the Dauki fault and the Haflong–Disang thrust (See Fig. 18)

5.4. Index of relative tectonic activity (IRTA) Evaluation of active tectonic deformation was done using the geomorphic indices computed from the drainage basins of the plateau. Through spatial analysis and data integration, the average of the six measured geomorphic indices, viz., hypsometric integral (HI), drainage basin asymmetry factor (AF), valley floor width- to-height ratio (VfRatio), ratio of volume to area (RVA), basin shape index (Bs), and basin elongation ratio (Eb) was used to document the spatial distribution of the index of relative tectonic activity (IRTA) in the study area. The values of the IRTA were classified into five classes to define the degree of active tectonics in the plateau (Fig. 19). The distribution of the IRTA suggests that the plateau is undergoing active deformation along zones adjacent to the Dauki fault, Haflong–Disang thrust, and the faults bordering the northern margin of the plateau. The results obtained from the evaluation of the index of relative tectonic activity are broadly consistent with those from the analysis of SL- and SLk-indices.

16

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Table 3 (continued)

Table 3 Vf-ratios on the Shillong plateau. Latitude

Longitude

Vfw

Esc

Erd

Eld

Vf

25.6672 25.6600 25.5711 25.5707 25.3644 25.5462 25.5741 25.4892 25.4666 25.4634 25.3191 25.1996 25.1956 25.3769 25.3527 25.2137 25.2293 25.2262 25.3058 25.2046 25.2753 25.1921 25.1821 25.2135 25.1841 25.1953 25.2182 25.1948 25.2442 25.3288 25.2793 25.1705 25.2198 25.0929 25.2003 25.3375 25.3198 25.3290 25.2201 25.2084 25.1931 25.1807 25.1917 25.2189 25.3805 25.2442 25.6079 25.6433 25.6822 25.8809 25.9412 25.9111 25.7600 25.7408 25.6174 25.3846 25.4529 25.4890 25.4430 25.7060 25.6130 25.5445 25.4175 25.4306 25.9206 25.8334 25.7546 25.7677 25.6502 25.6651 25.7020 25.7411 25.8771 25.8738

90.0829 90.0943 90.2115 90.1822 90.3193 90.4021 90.5098 90.6652 90.6854 90.7686 90.6543 90.6542 90.4988 90.7334 91.0118 91.2264 91.3605 91.3601 91.4588 91.6372 91.6152 91.6375 91.7089 91.7880 91.7619 91.8829 91.8876 92.0088 92.0004 92.1954 91.9795 92.2626 92.2754 92.3520 92.4466 91.7478 91.6510 91.5530 91.5093 91.5007 91.4872 91.5118 91.5678 91.6339 91.7576 91.7917 91.8522 91.7335 91.7236 92.4419 92.5123 92.4963 92.3408 92.3511 92.1354 92.1114 92.1143 92.0108 92.0553 92.2272 92.7452 92.6492 92.4127 92.5367 91.7766 90.1316 90.2004 90.5687 90.4020 90.4852 90.4896 90.5164 90.6155 90.7836

33 25 30 10 18 56 46 65 62 65 188 281 126 54 76 220 106 84 30 113 40 161 32 115 359 43 43 104 60 74 72 25 79 68 33 24 39 52 57 22 70 447 19 211 20 42 40 29 41 72 42 123 45 26 38 36 174 20 27 78 207 30 23 41 15 41 47 31 15 15 32 123 1504 1614

76 130 272 285 69 375 284 209 234 134 38 21 23 70 207 14 37 42 253 40 234 36 63 55 24 60 143 45 109 838 175 75 214 47 213 1092 506 702 135 84 36 25 82 55 1329 158 1037 967 872 425 179 313 678 763 882 1016 1053 1244 1158 773 266 664 1087 804 224 60 139 109 448 223 148 77 55 67

155 201 438 455 121 631 334 284 392 178 276 97 196 560 366 455 548 366 1417 477 1337 190 218 1015 237 529 598 385 696 1197 1210 414 680 435 511 1648 1630 1554 593 345 124 115 225 853 1800 1248 1439 1006 973 628 470 595 909 833 967 1403 1459 1582 1522 822 384 733 1276 935 340 93 234 269 476 398 240 248 301 323

169 187 464 433 164 608 410 336 321 224 200 117 149 520 421 196 365 275 1065 588 1342 244 248 737 221 442 638 207 897 1200 892 480 712 628 912 1659 1604 1748 600 303 102 135 239 900 1800 1002 1425 1273 951 581 424 497 714 827 1033 1298 1494 1620 1500 815 600 735 1212 944 359 86 199 206 478 381 311 251 193 195

0.38 0.39 0.17 0.06 0.24 0.23 0.52 0.64 0.51 0.97 0.94 3.27 0.84 0.11 0.41 0.71 0.25 0.30 0.03 0.23 0.04 0.89 0.19 0.14 1.75 0.10 0.09 0.41 0.09 0.21 0.08 0.07 0.16 0.14 0.07 0.04 0.04 0.05 0.12 0.09 0.91 4.47 0.13 0.26 0.04 0.04 0.10 0.17 0.46 0.40 0.16 0.53 0.34 0.39 0.32 0.11 0.41 0.06 0.08 1.71 0.92 0.43 0.15 0.30 0.12 1.39 0.61 0.24 0.52 0.09 0.25 0.71 7.83 8.41

Latitude

Longitude

Vfw

Esc

Erd

Eld

Vf

25.7633 25.8894 25.8304 25.8017 25.7117 25.6953 25.7204 25.8013 25.7564 25.6032 25.7571 25.7384 25.6748 25.4371 25.3837 25.3314 25.4391 25.4366 25.6135 25.6644 25.6359 25.5995 25.8297 25.8295 25.8484 25.8290 25.8855 25.8815 25.7146 25.9072 25.8500 25.7493 25.7038 25.8950 25.9026 25.9227 25.8660 25.8131 25.7892 25.6384 25.6259 25.6020 25.5712 25.5224 25.6981 25.6785 25.5668 25.5801 25.5533 25.5216 25.5083 25.4640 25.3677 25.3924 25.4023 25.5044 25.3896 25.2103 25.1995 25.3809 25.2510 25.1951 25.1965 25.3928 25.3668 25.3643 25.3437 25.5495 25.5193 25.4804 25.4733 25.4559 25.5235 25.6596 25.2029

90.7911 91.3976 91.3492 91.3914 91.4588 91.4638 91.2672 91.3282 91.4357 91.4214 91.2779 91.3214 92.5184 91.1533 91.1364 91.1097 91.2008 91.2464 90.0335 90.0848 90.0947 90.1020 90.2191 90.2877 90.3234 90.3715 90.5055 90.6632 90.6184 90.7318 90.6735 90.6728 90.6809 90.8442 90.8842 90.9192 90.9588 90.9909 90.7470 90.9029 90.8493 90.8209 90.8233 90.8164 90.3209 90.3277 90.2553 90.2867 90.3514 90.2555 90.2004 90.2012 90.2658 90.3088 90.3369 90.2887 90.3888 90.3852 90.4183 90.4389 90.8849 90.8421 90.7777 90.7647 90.8015 90.8512 90.9422 90.8649 90.9208 90.9807 90.9989 91.0180 91.0364 90.4374 91.0166

19 503 230 79 81 43 20 41 62 51 70 30 27 21 79 104 31 49 14 16 10 27 100 50 54 40 1100 500 70 70 50 30 50 200 900 600 70 40 100 1000 40 60 140 56 40 165 60 109 191 20 100 600 250 30 40 25 30 27 25 30 25 27 460 50 45 42 50 50 45 35 70 55 75 50 30

249 57 74 113 515 557 727 203 227 1268 346 630 614 722 471 228 881 1204 122 103 245 271 62 71 150 120 65 65 185 80 80 230 270 97 75 70 115 388 248 555 530 418 422 275 310 455 418 468 500 575 185 108 52 160 200 800 355 60 64 220 190 40 22 135 178 235 288 572 562 480 380 625 720 318 37

476 245 419 495 1145 1143 1136 472 799 1535 662 876 868 1010 670 407 1201 1361 151 162 270 394 135 186 254 340 388 320 272 220 310 410 380 450 415 460 520 617 462 765 630 544 538 555 535 635 570 800 825 910 282 227 142 270 585 1160 480 195 220 432 670 400 187 330 405 390 495 670 665 650 680 630 915 485 250

411 246 232 472 1168 1298 1008 543 864 1539 753 850 858 1037 912 419 1211 1331 149 170 324 357 120 154 274 272 230 200 302 310 281 410 405 368 280 270 475 550 460 710 675 480 556 552 438 610 680 625 900 1200 262 170 100 330 380 1120 620 205 220 400 560 370 165 360 580 450 500 690 650 670 650 660 970 465 285

0.10 2.67 0.91 0.21 0.13 0.06 0.06 0.13 0.10 0.19 0.19 0.13 0.11 0.07 0.25 0.56 0.10 0.35 0.50 0.25 0.19 0.26 1.53 0.51 0.47 0.22 4.51 2.56 0.69 0.38 0.23 0.17 0.41 0.64 3.30 2.03 0.18 0.20 0.47 5.48 0.33 0.64 1.12 0.20 0.23 0.99 0.29 0.45 0.53 0.04 1.15 6.63 3.62 0.21 0.14 0.07 0.15 0.19 0.16 0.15 0.06 0.08 2.99 0.24 0.14 0.23 0.24 0.46 0.47 0.19 0.25 2.75 0.34 0.32 0.13

(continued on next page) 17

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anomalously high stream-gradients along the northern, southern, and northeastern margins of the plateau (Fig. 11a, b). The most prominent linear zone of high SL-values extends for a length of over 220 km along the southern margin of the plateau on the monoclinal flexure in the Precambrian basement and the overlying Cretaceous–Tertiary strata adjacent to the Dauki fault zone in Jaintia and Khasi Hills. The zone of steepened gradients may be interpreted as the result of the higher rate of vertical component of deformation and uplift of the plateau along the Dauki fault. It may also signify the growth of the asymmetric south verging anticlinal structure (Clark and Bilham, 2008) in the basement and the overlying sediments due to the continuing deformation of the plateau in response to its southerly convergence with the Indian Plate (Vernant et al., 2014). The zone of high stream-gradients also extends north-westward into the Garo Hills across the Dudhnai fault (fault no. 18). The N-S trending Dudhnai fault, a major dislocation in the plateau, presents an abrupt change in elevation from ∼400 m in the west to ∼500 m in the east with the development of a west-facing fault scarp. Rivers flowing from the Khasi Hills in the east to Garo Hills in the west have carved out deep canyons in the vicinity of the scarp. These observations are suggestive of uplift of the terrain to the east of the fault relative to that on the west. Rivers flowing across the Dudhnai fault exhibit high SLk- (2.0 to 10.5) and SL- (1126 to 3315) indices. A high degree of micro- seismicity associated with the Dudhnai fault (Kayal, 1987) further points to the active nature of this fault. Besides, in the adjoining area, the Simsang River (R_13499, Figs. 7, 8) shows high SL- (450–1120) and SLk(2.0–4.8) indices along entrenched meanders and across two sub-parallel, NNE–SSW trending Chachat–Karuba and Darugiri faults (faults no. 39 and 17 respectively) that together give rise to step-like topography in the area to the west of the Dudhnai fault. The zone of high stream-gradients also encompasses areas with prominent drainage incision and block uplift along the Samin, Chedrang, Arbela–Songsak, Mandalanggiri–Songsak, and Dadengiri–Manda faults (Fig. 11a, b). In Garo Hills, the SL-values are, in general, lower than those in Khasi and Jaintia Hills, but a strong spatial association of high SL-values with Dapsi thrust and the Dudhnai, Samin, Chedrang, and Baladinggiri–Rongramgiri–Nagupara faults is observed. The SL- and SLk-index maps (Fig. 11a, b) show well-defined zones of high stream-gradients in the faulted landscape of Garo Hills. Spatial association of zones of high SLand SLk- gradients with uplifted horsts and faults showing well-defined scarps suggests that much of the deformation of the Garo Hills occurs

Table 3 (continued) Latitude

Longitude

Vfw

Esc

Erd

Eld

Vf

25.1956 25.2091 25.2115 25.2044 25.2020 25.2872 25.3612 25.1931 25.8055 25.8310 25.7412 25.6652 25.7158 25.7282 25.7356 25.6481 25.6245 25.5247 25.3678 25.3197 25.3464 25.2088 25.2726 25.8068 25.7576 25.6890 25.9821 25.5569 25.6794 25.4613 25.3831 25.3219 25.1154

91.0285 91.0593 91.0789 91.1107 91.1624 91.2875 91.3647 90.9724 91.0792 91.3289 91.3978 91.5860 91.5162 91.5400 91.5719 91.6833 91.7131 92.0338 91.9145 92.0773 91.6947 92.1009 92.2261 92.2514 92.2522 92.2923 92.4859 92.4167 92.7805 92.9024 92.8745 92.9676 92.9118

15 20 72 40 20 50 25 70 43 320 32 30 15 10 20 15 25 30 30 35 28 35 17 40 15 18 22 40 80 22 15 35 1300

30 165 138 75 50 225 1020 15 160 85 465 1228 1050 900 910 1218 1185 1200 410 380 1000 168 712 840 720 830 112 900 157 182 385 239 205

250 250 242 240 188 1050 1520 155 660 310 880 1450 1200 1150 1170 1470 1470 1535 1600 1420 1762 850 1064 1025 910 932 530 1260 272 624 736 566 1270

175 247 345 220 212 900 1560 135 510 322 820 1450 1195 1220 1120 1470 1440 1432 1650 1050 1650 500 960 1100 965 940 620 1120 330 560 673 591 1236

0.08 0.24 0.46 0.26 0.13 0.07 0.05 0.54 0.10 1.39 0.08 0.14 0.10 0.04 0.09 0.06 0.09 0.11 0.02 0.04 0.04 0.07 0.06 0.18 0.07 0.17 0.05 0.14 0.56 0.05 0.05 0.10 1.24

deformation and differential uplift of the plateau.

6.2. Spatial distribution of SL- and SLk-indices The SL- and the SLk-indices along the rivers of the Shillong plateau have low values in the headwaters and at the local base level in the Brahmaputra or Sylhet plains. The middle reaches of the rivers show steep gradients with high SL- and SLk-values. Interpolation of SL- and SLk-indices brings out extensive, 20–25 km wide zones with

Fig. 16. Ratio of drainage basin volume to drainage basin area (RVA) map of the Shillong plateau. RVA-values are shown in bold decimal numbers. 18

Journal of Asian Earth Sciences 181 (2019) 103904

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Fig. 17. Basin shape index map of the Shillong plateau.

Oldham fault and the faults bordering the northern margin of the plateau. Along the northeastern margin of the plateau in Jaintia Hills, high values of SL- and SLk-indices are spatially associated with shear zones and faults, showing differential vertical displacements. High SL- and SLk-indices are also observed along zones of channel incision and at the confluence of rivers showing hydrologic disequilibrium caused by differential uplift of the drainage basins. A 33-km long, N–S trending fault (fault no. 33, Umarangso fault; Fig. 3a) with an easterly throw of ∼200 m, causes hydrologic disequilibrium and the development of high SL- and SLk-indices in the easterly flowing rivers of the Jaintia Hills. High stream-gradients across the Tyrshat–Barapani shear zone (fault no. 29) and the Um Ngot fault (fault no. 31) suggest neotectonic activity and rejuvenation of the terrain. The rivers, crossing the N–S trending Um Ngot fault (fault no. 31, Fig. 11a & b), are characterized by high SLk- (3.0 to 22.2) and high SL- (490 to 7952) values at its extremities suggesting continuing deformation along this fault. High SLand SLk-indices at both the ends of the Um Ngot fault, a slight eastward

across faults showing differential vertical movement. The northern margin of the Shillong Plateau in Khasi Hills shows the development of topographic breaks and steps across E–W trending faults (faults numbered 25, 26, & 27; Fig. 11a & b). While the surface manifestation of the Oldham fault (Bilham and England, 2001) is absent in this sector (Rajendran et al., 2004), the topographic front of the highest planation surface of the plateau shows multiple prominent E-W and NW-SE trending en echelon, nearly 700 m high topographic scarps that occur at the south-eastern end of the surface projection of the subsurface Oldham fault. The hanging wall block of the Oldham fault displays deep incision of drainage, extreme fracturing, and bulging. This part of the terrain is characterized by a well-defined, WNW–ESE trending zone of high SL- and SLk-indices that extends for nearly 125 km and forms an important tectonically derived nickzone. The zone of anomalously high SL- and SLk-indices extends further in an eastsoutheasterly direction beyond the suggested extent of the Oldham fault. Deeply incised drainage and steepened river-gradients in this zone are suggestive of deformation and uplift of the terrain along the

Fig. 18. Basin elongation ratio map of the Shillong plateau. 19

Journal of Asian Earth Sciences 181 (2019) 103904

M.N. Mishra

Fig. 19. Spatial distribution of index of relative tectonic activity (IRTA) in Shillong plateau.

fault, displays a totally convex longitudinal profile (Fig. 8: R_113) possibly due to the fact that the eastern block along this fault registered an uplift of 10 m during the 1897 great earthquake (Oldham, 1899) and that the river is still trying to adjust to this uplift. Spatial association of linear patterns of earthquake epicenters with zones showing high SLand SLk-values is suggestive of continuing crustal deformation. A prominent linear pattern comprising nine earthquake epicenters (M = 3.8 to M = 5.2, depth from 25 km to 61 km, data source: USGS) of earthquakes between 31st August 1982 and 27th February 2005 is seen at latitude 25.40°N between longitudes 91.38°E and 92.10°E (Fig. 11a, b). The pattern parallels the Dauki fault and is at a distance of about 18 to 25 km to the north of it. A WNW–ESE pattern of earthquake epicenters from the southern part of the Jaintia Hills to the northwestern part of Garo Hills (Fig. 11a, b) shows strong correspondence with the zone of high stream gradient-indices, suggesting deformation of the plateau.

swerve in its strike to the north of the Sung Valley alkaline–carbonatite complex (Srivastava et al., 2005), and dislocation of the highest planation surface of the Shillong Plateau from an elevation of ∼1900 m to < 1500 m across this fault (Fig. 3c) in the east can be interpreted as deformation along this morphotectonic fault under the prevailing stress conditions related to the southerly convergence and clock-wise block rotation of the plateau (Vernant et al., 2014). The distribution of SLk-values (Fig. 11b) exhibits well-defined zones of high SL/k values (SL/k ≥ 2 to SL/k ≥ 15) along the northern, northeastern and southern margins of the Shillong plateau. Anomalously steep reaches (SL/k ≥ 2) of rivers have strong association with morphotectonic faults, zones of channel incision, and convex segments of river profiles. Anomalously high SL/k values are seen on the hanging wall of the Oldham fault throughout its length (Fig. 11b). High SL/k values are also seen along Dapsi thrust, and the Um Ngot, Dudhnai, Samin, Chedrang, and Baladinggiri–Rongramgiri–Nagupara faults. Extremely high SL/k values are also seen on the monoclinal flexure along the Dauki fault. Most of the tract in Garo Hills is characterized by anomalously steep gradients (SL/k ≥ 2) along streams suggesting deformation and differential uplift of the terrain across faults. It is observed that the rivers of the Khasi and the Jaintia Hills show significantly higher SL and SLk values than those of the Garo Hills. High stream-gradients in these two regions may be attributed to higher rates of uplift (hence higher relief) and vertical component of tectonic deformation. The rivers of the Jaintia Hills show more steepening of reaches (32% of the reaches have SL/k ≥ 2) than those of the Khasi Hills (28% of the reaches have SL/k ≥ 2). Though the rivers of the Garo Hills exhibit low SL values in absolute terms, a significant proportion (∼19%) of the SL/k values show significantly steeper gradients (SL/ k ≥ 2) than expected along rivers that flow on the block-faulted terrain.

6.4. Coupling between climate, erosion, and tectonics The Shillong plateau constitutes an orographic barrier for the Indian summer monsoon with the zone of maximum elevation (> 1500 m) and relief (∼1.5 km) on the plateau coinciding with the zone of extremely high average annual precipitation (∼12,000 mm), drainage incision, and intense erosion along the southern margin. The tectonic aneurysm model proposed by Zeitler et al. (2001) explains the interplay between erosion and tectonics. The model advocates that the zones of anomalous uplift and localized mountain building in contractional tectonic regime result from fluvial incision and rapid erosion, causing perturbations in isotherms and upward advection of heat. This results in thermal weakening and flow of the crust, focused deformation, and rapid uplift, which in turn, produces formation of an orographic barrier causing another cycle of more precipitation, erosion, and uplift. Low-temperature thermochronometric studies (Biswas et al., 2007; Clark and Bilham, 2008) suggest that rapid exhumation, initiation of faulting, uplift of the plateau, and accelerated erosion took place between 14 and 8 Ma. Despite the onset of the summer monsoon nearly 10 million years ago (Biswas et al., 2007; Clark and Bilham, 2008), the erosion rate (0.1–0.4 mm/yr; Clark and Bilham, 2008), has not kept pace with faulting and uplift rate of 0.7–1.3 mm/yr (Biswas et al., 2007; Clark and Bilham, 2008). Thus tectonic deformation caused by active convergence and slips along fault planes must have played a more dominant role in exhumation, and uplift of the plateau rather than

6.3. Correspondence between seismicity and zones of high stream gradients A number of faults and fractures traversing the Shillong Plateau appear to be seismogenic in nature as they exhibit linear patterns of micro (Mukhopadhyay et al., 1993) and major (M > 3.5) earthquake epicenters along them (Fig. 11a, b). Development of coseismic faults, permanent changes in landforms, and reactivation of the Chedrang, the Dudhnai, and the Samin faults in Garo Hills during the great Assam earthquake (M = 8.7, Mw 8.1) of the 12th June 1897 was noticed by Oldham (1899).The Chedrang River, which flows along the Chedrang 20

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300 m to 1000 m, have been interpreted (Sharma et al., 2012). In most areas, uplift of the magnetic basement corresponds to the topographically higher areas. Presence of ultramafic and alkaline intrusives along the faults cited above further corroborates their deep-seated nature.

accelerated erosion. 6.5. Regional variations in plateau uplift 6.5.1. Geomorphic evidence The morphology of the terrain to the east of the Dudhnai fault indicates that the whole of the Shillong Plateau has not witnessed uplift as a single rigid mass. The N-S trending Dudhnai fault, traversing the entire width of the plateau, separates two contrasting types of terrains having differential uplift rates and degree and style of deformation. Geomorphic study of the plateau suggests that the maximum uplift of the plateau is confined to the central and the eastern parts in Khasi and Jaintia Hills respectively. To the east of the Dudhnai fault (at 90.8°E longitude), the terrain, represented by a smooth planation surface, climbs consistently from an elevation of ∼500 m to ∼1900 m above MSL over a planimetric distance of 65 km before attaining horizontality to the east of 91.4°E longitude in the central part of the Khasi Hills (Fig. 3c). The rise of the planation surface, over such a long distance, is remarkably uniform, without any break in slope. The westerly slope of this tilted planation surface furnishes geomorphic evidence for a rather rapid rate of uplift of the eastern Khasi Hills as compared to the western Khasi Hills and the Garo Hills. The high elevation of the eastern part of the plateau has been attributed to the higher rate of convergence (∼7mm/yr) of the southeastern part of the plateau with the Indian Plate in the south as compared to its western part, which converges at the rate of ∼3 mm/yr (Vernant et al., 2014). A high degree of southerly tilting and warping of the planation surface to the south and southwest of Nongstoin (91.28°E, 25.47°N) corroborates spatial variations in surface uplift. Extremely high SL-index along the rivers draining the Khasi Hills reflects the rapid uplift of the plateau with the concomitant steeping of the slopes. In contrast, the terrain in the Garo Hills is lowlying and block-faulted, with the development of horsts and grabens. The elevation of the terrain is mostly less than 500 m except along the Tura Range (maximum elevation 1413 m), which is thrust over the Tertiary strata along the WNW–ESE trending Dapsi thrust. The highly microseismic Dudhnai fault (Mukhopadhyay et al., 1993; Kayal, 1987; Kayal and De, 1991), which contains earthquakes with focal depths of > 30 km (Mukhopadhyay et al., 1993) and contains dyke swarms of mafic and ultramafic – alkaline – carbonatite composition (Nambiar and Golani, 1985; Srivastava and Sinha, 2004), appears to be a deepseated fault (Golani, 1991) affecting the relatively thin continental crust. The fault may have a decoupling effect on the terrain to the east of the fault, which has witnessed large-scale uplift to elevations of nearly 2000 m in contrast to the terrain in Garo Hills that lies at ∼450 m. Step-faulting in Garo Hills and the presence of Lower Gondwana (Permian) sediments in its western part at Singrimari (89° 44′ E: 25° 44′ N), emplacement of mafic dyke swarms and plugs of ultramafic – alkaline – carbonatite rocks along tensional fractures and faults, such as, the Dudhnai fault (fault no. 18; Nambiar and Golani, 1985; Golani, 1991), the Tyrshat–Barapani shear zone (fault no. 29, Fig. 3), and the Um Ngot fault (fault no. 31, Fig. 3; Srivastava and Sinha, 2004; Srivastava et al., 2005), and effusion of Sylhet traps (early Creatceous) along the southern margin of the Shillong plateau, are manifestations of lithospheric stretching and crustal thinning during rifting and break-up of the Gondwanaland (Mukhopadhyay et al., 1986; Khan and Rahman, 1992; Nandy, 2001). Faulting and differential uplift of terrain across faults, interpreted through digital techniques in this work, is consistent with the structural interpretation of aeromagnetic data and magnetic basement-depth modeling (Rama Rao, 1999; Sharma et al., 2012). Interpretation of aeromagnetic data brings out a good correspondence between topographic and magnetic lineaments and faults. The northern part of the Tyrshad–Barapani shear zone (fault no. 29), the Dapsi thrust (fault no. 11), parts of the Um Ngot fault (fault no. 31), and the Darugiri fault (fault no. 17) are deeper crustal fractures along which faulting and differential uplift of the magnetic basement, with throws varying from

6.5.2. Evidence from geomorphic indices The presence of well-defined, extensive zones of anomalously high SL- and SLk-indices along the northern, southern, south-eastern, and north-eastern margins of the plateau indicates regional variations in the rate of plateau uplift. Zones of high SL- and SLk-indices correspond well with the zones defined by drainage basins with high HI-values and convex hypsometric curves along the southern, southeastern, and the eastern margin of the plateau. Regional variations in values of the geomorphic indices, e.g., drainage basin asymmetry factor (AF), ratio of volume to area (RVA), basin shape index (Bs), valley floor to valley height (Vf-ratio) and basin elongation ratio (Be) are suggestive of differential uplift of the plateau. Spatial analysis and integration of spatial distribution of these geomorphic indices (Fig. 19) suggest high rate of active tectonic deformation and plateau uplift along well defined zones along the northern, southern, south-eastern, and north-eastern margins of the plateau. The maximum amount of tectonic activity is inferred in the Jaintia Hills and in the central and the eastern part of the Khasi Hills in zones adjacent to the Um Ngot and Dauki faults and the Haflong–Disang thrust. Geomorphic indices along the northern margin of the Shillong plateau reveal uplift and tectonic deformation of the plateau along the Chichra Githim–Nongkrem, Jambalgiri–Mawthoh, Gohanimara–Nangpoh, Nongpydem–Japung, and the Oldham faults. The relative tectonic activity along the northern margin of the plateau is however less than that along the Dauki fault and the Haflong–Disang thrust. Towards the west, tectonic activity is less pronounced in the western Khasi Hills and Garo Hills. However, the Tura and the Arbela ranges in the Garo Hills display evidence of uplift along the Dapsi thrust and the Arbela–Songsak, Mandalanggiri–Songsak, and Dadengiri–Manda faults. High stream gradients across faults in Garo Hills suggest tectonic deformation of the terrain along these morphotectonic faults. 7. Conclusions Analysis of longitudinal profiles of rivers and evaluation of geomorphic indices provide valuable information to assess differential uplift of the Shillong plateau on account of active tectonic deformation. Study of topography and landforms from digital elevation model brings out a number of morphotectonic faults in Shillong plateau that show geomorphic evidence for active tectonics. The present study reveals that the SL- and SLk-indices constitute excellent tool for identifying zones of regional crustal deformation. As the SL- and SLk-values on the Shillong plateau are not significantly influenced by lithologic variations, zones of high SL- and SLk-indices may be interpreted as an evidence for tectonic forcing, active crustal deformation, and differential uplift of the flexed Precambrian basement in a contractionary tectonic milieu, in the foreland of the Himalayan and the Indo-Burman ranges. Convexity in river profiles and high values of SL- and SLk-indices upstream of the faults bordering the northern margin and along the Dauki fault is suggestive of the continuing uplift of the Shillong Plateau along these tectonic features. The study also corroborates that the SLindex is sensitive enough to characterize the rate of differential uplift in different parts of the Shillong Plateau. Relatively rapid rate of uplift of the Khasi and Jaintia Hills is reflected in higher fluvial incision rates and very large SL- and HI-values in these areas, while smaller values of these geomorphic indices in Garo Hills indicate that the rate of differential uplift is slow though crustal deformation is continuing across tectonic features. Evaluation of active tectonic deformation and differential uplift of the Shillong plateau from the analysis of the SL- and SLk-indices is corroborated by the results obtained from drainage basin 21

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morphometric indices like hypsometric integral (HI), drainage basin asymmetry factor (AF), ratio of volume to area (RVA), basin shape index (Bs), Valley floor width-to-height ratio (Vf-Ratio) and basin elongation ratio (Be). The plateau is undergoing active tectonic deformation and differential uplift along morphotectonic features, such as, faults and the monoclinal fold axis. Spatial analysis and integration of spatial distribution of indices of relative tectonic activity computed from various geomorphic indices suggest high rate of active tectonic deformation and plateau uplift along well defined zones along the northern, southern, south-eastern, and north-eastern margins of the plateau. The maximum amount of tectonic activity is inferred in the Jaintia Hills and in the central and the eastern part of the Khasi Hills in zones adjacent to the Um Ngot and Dauki faults and the Haflong–Disang thrust. High seismicity of the plateau is the manifestation of its continuing uplift and deformation. In its tectonic setting, the Shillong Plateau has been proximal to the Mesozoic continental margin of eastern India, and hence the faults and fractures traversing the plateau bear imprints of rifting and continental break-up. The plateau exhibits the superimposition of the present compressional tectonics over an older extensional tectonics, the former causing reactivation of the older extensional faults. It may be suggested that the Dauki fault and its west-northwesterly splay, the Dapsi thrust, and the Brahmaputra fault that delimits the northern margin of the plateau, must have originated as normal faults during the continental break-up and have later changed into high-angle reverse fault and thrust respectively in response to the collision tectonics along the Himalayas. These tectonic features constitute an active tectonic regime that facilitates differential movement of crustal blocks.

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