Neotectonism and reactivation of tectonic elements in eastern part of Shillong Plateau, India: constraints from morpho-tectonic analyses

ABSTRACT

The Shillong Plateau is the only elevated topography in the Himalayan foreland bounded and crisscrossed by several prominent faults and shear zones. The plateau is situated in a complex tectonic compressional set up, owing to which the fault systems both at the periphery and within tend to be neotectonically active. Due to the elevation difference of the plateau with the Brahmaputra and Meghna River valleys flanking it, coupled with the impervious lithologies present, an intricate network of drainage system has developed in the plateau. In this paper, morphometric and structural analyses of four selected drainage basins from the east-central part of Shillong Plateau have been carried out to comprehend the effects of neotectonic reactivation of fault systems and associated basin tilting which in turn controls the development of drainage pattern. This has led to the understanding of the effects of the ever-evolving fluvial landforms on the anthroposphere in this part of Indian sub-continent, where the agrarian-economy is completely dependent on the intricate fluvial systems.

KEYWORDS:

• Shillong plateau
• neotectonics
• morpho-structural analyses
• basin asymmetry

Introduction

The Shillong Plateau (SP) is a rigid cratonic block, situated in the eastern most fringe of the Indian Plate (Pascoe, 1950), separated from the peninsular India by the Garo Rajmahal depression (Bengal basin) (Ermenco et al., 1969). The plateau is situated in a very complex tectonic regime and is bounded by a set of fault systems along the periphery as well as within itself (Halder et al., 2022; Seeber et al., 1981; Tapponnier & Molnar, 1976) (Figure 1). The compounded effect of the peripheral faults linked with the Himalayan and Indo-Burmese orogenies uplifted the plateau, which is the only raised block in the entire Himalayan foreland (Najman et al., 2016), having elevation difference both with the Brahmaputra Valley in the north and Bangladesh plains in the south. This has developed an intricate system of structure-controlled drainage network in the plateau. As evolution of an area is the manifestation of a wide range of factors including tectonics, lithological disposition, and climate, geomorphic responses would provide valuable insights to understand the development of landscape in this complex tectonic regime, and explaining the neo-tectonism. A study of the drainage pattern and geomorphology produces an insight to the tectonic activity of the region (Joshi et al., 2022; Luirei et al., 2018, 2021; Misra et al., 2020). Due to the elevation difference with the Assam valley as well as the Bangladesh plains and due to the presence of a regional hitherto unnamed water divide (Halder et al., 2022), the streams of SP are either north bound flowing into Brahmaputra or south bound flowing into the Bangladesh plains (Mishra, 2019). The progressive modification of landforms by neotectonic activity (Burbank & Anderson, 2013; Jaiswara et al., 2021; Saber et al., 2019), in the study area, has been studied in corroboration with regional tectonics, to work out the regional neotectonic setup. The morphometric study of riverine landforms like drainage analyses and basin morphometry (Gailleton et al., 2019) is supplemented by morphostructural investigations, which can also elucidate the relationship between the landform geometry and active faults (Centamore et al., 1996; Raj, 2012). Globally drainage morphometric analyses have been used to interpret neotectonism (Han et al., 2017; Harel et al., 2016; Haviv et al., 2010; Kirby & Whipple, 2012; Mandal et al., 2015; Moodie et al., 2017), which in turn helps in studying the variation of relative displacements in regions affected by active faults (Azor et al., 2002; Kothyari, Rastogi, Murthekai, & Dumka, 2016, 2016b; Prizomwala et al., 2016).

Figure 1. Major tectonic lineaments and regional drainage pattern of northeastern part of the Indian subcontinent. The studied basins, situated in the east central part of the Indian state of Meghalaya, along with their trunk streams have also been demarcated. Magnitude classified location of the earthquake epicenters has been superposed (data source: Seismo tectonic atlas of India and its environs, GSI, 2000).

Several attempts have been made by earlier workers to comment on the geomorphology of SP using basin morphometry (Agrawal et al., 2022; Imsong et al., 2016, 2019; Mishra, 2019; K. Nath et al., 2023; P. K. Sarma et al., 2013; Sarmah et al., 2012). In this work, an attempt has been made to couple basin morphometry with the lineament analyses from the study area to understand the geomorphic evolution. The area selected for this particular study falls in the East Khasi Hills District of Meghalaya, which receives the maximum rainfall in the state; and the area is best suited for morphometric studies as it has an intricate and mature drainage system (K. Nath et al., 2023). Stream networks (Figure 2), concavity of the river profiles and the knick points of two north flowing rivers and two south flowing rivers from the study area have been worked out. Using the results, an attempt has also been made to establish the constraints of neotectonism and reactivation of tectonic elements on the development of drainage pattern and to understand the effects thereof on the anthroposphere herein.

Figure 2. Watersheds of the four studied basins and the network of streams extracted from DEM data. The stream ordering has been done after (Strahler, 1952). The 1^st order streams have been intentionally avoided from the figure to avoid cluttering.

Geology and tectonics

The SP has ~35 km thick crust (Pascoe, 1950, Ermenco et al., 1969). The crust majorly comprises basement rocks designated as the Assam Meghalaya Gneissic Complex (AMGC) (Devi & Sarma, 2010; Gogoi, 1972; Khedkar, 1941; Medlicott, 1869; Oldham, 1859; Palmer, 1932; K. P. Sarma, 2014). The meta-sediments of Shillong Group (SG) form the cover sequence of the AMGC. The SG comprises a meta-volcano-sedimentary sequence (Naik et al., 2020) and covers an area of ~ 2500sq km in the eastern part of SP. It is considered equivalent to the other Purana basins of India (Nandy, 2001; Neogi et al., 2021; Pascoe, 1950). The SG and AMGC have been intruded by a set of basic sills and dykes. These intrusives are dominantly composed of amphibole and pyroxene, which lend them a greenish colour (GSI, 2009; Halder et al., 2022; Mallikharjuna Rao et al., 2009; K. Nath et al., 2021, 2023). These are of epidiorite or doleritic in composition (Medlicott, 1869). The earlier workers have designated these rocks as Khasi Greenstones (KG), which actually is a misnomer. Subsequently several granite plutons have intruded the AMGC (Rongjeng, Sindhuli, Nongpoh plutons) and SG (Mylliem, Kyrdem, Nartiang plutons) (GSI, 2009) during the Neoproterozoic time. The Sung Alkaline Complex (Srivastava and Sinha, 2004; Srivastava et al., 2005; GSI, 2009) and Sylhet Trap (Baksi, 1995; GSI, 2009; Ray et al., 2011) represent the intrusive and extrusive phases of Kerguelen plume-related magmatism of Cretaceous age. Transgression during Cretaceous age and thereafter produced huge sediment pile from both shelf as well as geosynclinal facies, represented by conglomerates, pebbly horizons, sandstones, siltstone, shale, marl, limestone and intercalations. These rocks of Cretaceous up to Tertiary age are exposed along a near east west trend towards the south of the plateau. At places, these sequences are fossiliferous and coal bearing (GSI, 2009). The selected study area is majorly covered with erosion-resistant Precambrian rocks comprising the gneisses of AMGC, SG metasediments and intrusive granite plutons (Figure 3).

Figure 3. Lithological map of the east central part of Shillong Plateau with the watershed boundary of the four studied basins. The trunk streams of each basin have been marked and also the three major lineaments are shown. The maximum lateral shifting of each river is marked with an arrow.

The SP is situated in the eastern most part of the Indian Plate. The tectonic set up is complex due to the collision of the Indian plate with the Eurasian Plate along the Himalaya (Diehl et al., 2017; Seeber et al., 1981; Tapponnier & Molnar, 1976) and the subduction of the Indian Plate under the Burmese Plate (Mitchell and McKerrow, 1975; Mukhopadhyay & Dasgupta, 1988; Ni & Barazangi, 1984; Verma et al., 1976; Gupta et al., 1984). The conjugate effect produces a compressional setup, resulting in the counter clockwise movement of the SP. Thus, the plateau is still moving in the northeast to north direction due to the intense compressional tectonism (Harijan et al., 2003). It has also been suggested that the eastern segment of the Indian Plate is undergoing rapid shortening and has higher convergent rates (Banerjee et al., 2008).

The resultant pop-up structure of Shillong Plateau is bounded by the Dawki Fault (Imsong et al., 2016; Nandy, 2001; Yin et al., 2010) in the south and towards the north, the plateau meets the Brahmaputra valley through a series of faults (Bilham & England, 2001; Islam et al., 2011). Yin et al., (2010) have opined that the Dawki Fault is a north dipping reverse fault. The uplift of SP is the result of Oldham Fault to be a back thrust to the Dawki Fault. Contrary to this, another school opines that the pop-up of SP is the result of differential movement along to reverse faults, i.e., north dipping Oldham Fault along the Brahmaputra and the south dipping Dawki Fault (Bilham & England, 2001; Islam et al., 2011). In the west, the plateau is bounded by the N – S trending Dhubri fault (Nandy & Das Gupta, 1986). The Dhubri Fault extends northward into Bhutan and is together known as the Dhubri-Chungthang fault zone (DCF Zone). The deformation front of the Himalayas is connected with the SP by the dextral DCF zone. Towards the east the plateau is delimited by the NW – SE trending right-slip Kopili fault (DasGupta & Nandy, 1982; Evans, 1964) and the NE – SW trending Haflong – Disang thrust (Evans, 1964). The Kopili fault is seismically very active and it separates the Mikir massif from the SP. It has also been postulated that this fault system extends towards the north to cut across the Himalayas, producing curvilinear structures and displacement Main Boundary Thrust and Main Central Thrust Zone (Kayal et al., 2012). The Haflong-Disang thrust system is a complex tectonically active zone, comprised of NE-SW trending overturned folds and reverse faults, NW-SE normal faults, and NNW-SSE and WNW-ESE strike-slip faults (Mathur and Evans, 1964). The plateau is also traversed by other faults and shear zones, viz., Oldham fault, Dapsi Fault, Nongcharam Fault, Kulsi Lineament and Barapani-Tyrsad Shear zone (Figure 1).

Methodology and database

The findings of this study are based on the synthesis of open source Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) data 1 Arc-Second Global downloaded from United States Geological Survey (USGS) website, (https://earthexplorer.usgs.gov). These data have been used as they have low algorithmic errors & data gaps (Rawat et al., 2014; Reuter et al., 2007) and have a resolution of 1 arc-second (∼30 m). Along with the DEM data, contour data from Survey of India (SOI) topographical sheets nos. 78O/10, 78O/11, 78O/13, 78O/14, 78O/15, 83C/2, and 83C/3 has been utilized. The drainage pattern of the selected basins of SP has been prepared by processing the DEM data with the Hydrology tools extension of the ArcGIS 10.3 software (Figure 2). Initially the DEM has been hydrologically conditioned by filling of the pits and removal of the spikes, followed by determination of the flow direction on the surface using D8 method (Jenson & Domingue, 1988). Subsequently flow accumulation raster, raster calculation, stream ordering (Strahler, 1952), stream to features conversion and watershed delineation have been done. The raster calculation step had been repeated several times with condition ranging from 850 ≥ flow accumulation =<8 to generate several stream networks. For extraction of the drainage flow accumulation ≥ 100 has been used, as this produced the best results, which was confirmed by overlaying the extracted drainage on the georeferenced SOI Toposheets (geometrically rectified and georeferenced in Universal Transverse Mercator (UTM) projections using World Geodetic System (WGS) 1984 UTM Zone, 46 datum). The four river courses under consideration have been manually analysed from google earth imagery and the perpendicular lateral shifting of the trunk stream to ascertain the lateral shifting of rivers (Figure 3).

Brief methodologies for calculation of the different morphometric parameters for each of the four studied basins have been tabulated in Table 1, along with the references. In total, 32 morphometric parameters have been calculated for all the studied basins.

Table 1. Methodology adopted for computation of morphometric parameters.

Sl. No	Parameter	Formula	after
Linear parameters
1.	Stream order(U)	Hierarchical rank	Strahler (1952)
2.	Stream number (Nu) Stream number %	Nu = N1 + N2 + … + Nn Nu% = (Nu/∑Nu)*100	Horton (1945)
3.	Stream length (Lu) (km) Stream length %	Length of the stream Lu% = (Lu/∑Lu)*100	Strahler (1964)
4.	Mean stream length (Lsm)(km)	Lsm = Lu/Nu	Strahler (1964)
5.	Stream length ratio (Lur) Mean Stream length ratio (Lurm)	Lur =Lu/(Lu-1) Lurm =(Lur2+ Lur3…+ Lurn)/n	Strahler (1964)
6.	Bifurcation ratio (Rb) Mean bifurcation ratio (Rbm)	Rb = Nu/Nu + 1 Rbm=(Rb2 + Rb3 … .+Rbn)/n	Strahler (1964)
7.	Rho coefficient (r)	r = Lur/Rb	Horton (1945)
Areal parameters/Basin Geometry
8.	Perimeter (P) (km)	GIS analysis/DEM	Schumm (1956)
9.	Basin length (Lb) (km)	GIS analysis/DEM	Schumm (1956)
10.	Basin area (km2) (A)	GIS analysis/DEM	Schumm (1956)
11.	Main Channel Length (Cl)	GIS analysis/DEM
12.	Mean basin width (Wb)	Wb = A/Lb	Horton (1932)
13.	Lemniscate’s ratio (k)	k = Lb^2/A	Chorley (1957)
14.	Form factor ratio (Rf)	Rf = A/Lb^2	Horton (1932)
15.	Elongation ratio (Re)	Re = 2/Lb * (A/π)^0.5	Schumm (1956)
Drainage texture analysis
16.	Drainage density (Dd)	Dd = Lu/A	Horton (1932)
17.	Stream frequency (Fs)	Fs = Nu/A	Horton (1932)
18.	Drainage texture (Dt)	Dt = Nu/P	Horton (1945)
19.	Circularity ratio (Rc)	Rc = 12.57 * (A/P2)	Miller (1953)
20.	Constant of channel maintenance (C)	C = 1/Dd	Schumm (1956)
21.	Infiltration number (If)	If = Fs * Dd	Faniran (1968)
22.	Fitness ratio (Rf)	Rf=Cl/P	Melton (1957)
23.	Wandering ratio (Rw)	Rw= Cl/Lb	Smart and Surkan (1967)
24.	Compactness coefficient (Cc)	Cc = 0.2841 * P/A^0.5	Gravelius (1914)
25.	Length of overland flow (Lg) km	Lg= A/2 * Lu	Horton (1945)
Relief Analysis
26.	Height of basin mouth (z)	GIS analysis/DEM
27.	Maximum height of the basin (Z)	GIS analysis/DEM
28.	Total basin relief (H) (m)	H = Z – z	Strahler (1952)
29.	Relief ratio (Rh) (m)	Rh = H/Lb	Schumm (1956)
30.	Ruggedness number (Rn)	Rn = H* Dd	Rao (2016)
31.	Asymmetric number (An)	An=Area left half/area right half	Gardner et. al., (1987)
32.	Dissection index (Dis)	Dis=H/absolute relief	Singh and Dubey (1994)

The Ksn value (Figure 4) and knickpoint (Figures 5 and 7) for the four studied basins have been calculated using the Topotoolbox written in MATrix LABoratory (MATLAB) (Schwanghart & Scherler, 2014). The Topotoolbox algorithm uses the ChiProfiler to analyse the Ksn value on stream (Gallen & Wegmann, 2017) and KZ-Picker for automatic knickpoint detection (Neely et al., 2017). Using this toolbox, the knickpoints have been plotted on the slope map of the basins, prepared from DEM (Figure 5). The actual longitudinal profile of the rivers and the model profiles have also been prepared (Figure 6).

Figure 4. Statistical analyses of the Ksn values of the four studied basins, calculated by stream length, chi-regression and chi-bins, using the Topotoolbox.

Figure 5. Elevation map prepared from the DEM data of each of the studied river basins. The locations of the knick points (found out using the Topotoolbox) have also been plotted in each of the Elevation Maps, with red dots.

Figure 6. a & b. the actual longitudinal profile of the rivers extracted from the DEM data using the Topotoolbox has been marked in pale blue lines. Based on the gradient data from DEM the model profile lines as predicted by the Topotoolbox have been plotted in red lines. The profile of the north bound and south bound rivers is shown in Fig. 6.A and 6.B respectively. The river profiles have been superimposed on the lithology and macro structural lineaments intersected along the course of the trunk stream.

Figure 7. a & b. the knick points found using the Topotoolbox has been plotted on the actual longitudinal profiles of the studied basins. The knick points of the north bound and south bound rivers have been marked in red dot in Fig. 7.A and 7.B respectively. The knick points have been superimposed on the lithology and macro structural lineaments intersected along the course of the trunk stream.

The lineaments within the basins have been manually extracted (Figure 8(a,b)) by visual interpretation of topographic contour patterns from SOI toposheets (scarp faces, topographic ridges, valleys), False Colour Composite (FCC) image prepared from Linear Imaging and Self Scanning Sensor (LISS) III data, hill shade map and also taking into account the extracted drainage pattern (viz. parallel, trellis, rectangular). Genetically the lineaments have been categorized into two classes, the geomorphic lineaments such as drainage parallel, ridge parallel lineaments and the structural lineament (fault or joint related lineaments) (Halder et al., 2022; Mahanta et al., 2012). These two classes of lineaments have been further categorized into micro and macro depending upon their length less than 3 km and greater than 3 km respectively (Halder et al., 2022; Mahanta et al., 2012). In order to have a statistical approach towards the lineament analyses, the number of classified lineaments has been expressed in percentage (Figure 8). Directional analyses of the categorized lineaments have been carried out by preparing rose diagrams (Figure 8(a)).

Figure 8. (a) Lineament map of the study area in parts of Meghalaya plateau, classified on the basis of genesis and length. Rose diagrams are showing trend wise distribution pattern of the lineaments, whereas pie charts give a statistical representation of the categorized lineaments. The lineaments and the trunk streams have been superimposed on the geology of each studied basin.(b) a seismotectonic map of the studied river basins has been prepared by plotting the earthquake epicenters and landslide occurrences. The prominent tectono-structural elements have also been marked.

To better understand the neotectonic reactivation of the structural elements of the study area, earthquake epicentre locations have also been plotted on the regional lineament map (Figures 1 and 8(b)). Coordinates of earthquake epicentre locations along with their magnitude have been downloaded from USGS (https://earthquake.usgs.gov/). Landslides are another natural phenomenon which is closely associated with neo-tectonic activity. The data of landslide investigation and susceptibility – hazard risk mapping, carried out by Geological Survey of India in Meghalaya, is available in BHUKOSH website (https://bhukosh.gsi.gov.in/Bhukosh/Public). This data has been downloaded and used to prepare a Landslide Inventory. Using the Landslide Inventory a classification of the landslide has been done (Figure 8(b)) & Table 2). This landslide classification has been used to depict the control of neo-tectonic activity in the study area. Asymmetric factor has been calculated by separately calculating the areas of the left and right half of the basins respectively. The left and right half of the basin is determined by looking down along the flow direction of the trunk stream.

Table 2. A summarized inventory of the 110 Landslides (LS) occurring in the study area.

LS Length	No	LS Area	No	Type of slide material	No	Movement	No
>50 m	10	>1000 sq m	14	Mixed Debris	57	Fall	4
>20 m	21	>100 sq m	68	Earth	20	Slide	105
<20 m	79	<100 sq m	28	Rock Mass	33	Subsidence	1

Results

The results of the morphometric analyses carried out separately for the four studied river basins have been tabulated in Figures 2(a-d).

The linear analyses (Table 3) indicate that only Umkhen Basin has streams upto 7^th order and the remaining basins have streams upto 6^th order. Similarly, Umkhen Basin has the maximum number of streams and Umngi has the least number of streams. The areal parameters tabulated in Table 4 indicate that Umkhen Basin is the largest among the studied basins. Leminscate’s ratio of Umiam Basin is 8.02 which is almost twice that of the other basins. The drainage texture analyses of the studied basins have been tabulated in Table 5. All the basins have similar drainage density and stream frequency. Umkhen Basin has the highest (16.30) drainage texture and Umngi has the lowest (8.80). Relief characterization of the basins have been tabulated in Table 6. Umiew Basin has the maximum relief (1753 m) and highest ruggedness number (4014.37). Umngi Basin has the least relief (1417 m) and lowest ruggedness number (3273.27). Geologically the studied basins largely drain through the Proterozoic Shillong Group of rocks and Neoproterozoic granite plutons (K. Nath et al., 2021) (Figure 8(a)), which in most part are impermeable to semi pervious in nature (Figure 3). The southward flowing rivers before reaching the periphery of the SP flow over the highly erodible Cretaceous and Tertiary rocks (GSI, 2009; K. Nath et al., 2021).

Table 3. Linear aspect of the river basins from the study area.

(U)	(Nu)	Nu%	(Lu) (km)	(Lu) (%)	(Lsm)(km)	(Lur)	(Rb)	(r)
Umiam Basin
I	2219	78.41	978.78	55.67	0.44
II	471	16.64	379.71	21.60	0.81	0.55	4.71	0.12
III	106	3.75	189.54	10.78	1.79	0.45	4.44	0.10
IV	26	0.92	77.99	4.44	3.00	0.60	4.08	0.15
V	7	0.25	42.05	2.39	6.01	0.50	3.71	0.13
VI	1	0.04	90.02	5.12	90.02	0.07	7.00	0.01
	∑Nu = 2830	∑Nu% = 100	∑Lu = 1758.10	∑Lu% = 100		Lurm = 0.43	Rbm = 4.79
Umkhen Basin
I	3521	79.77	1499.07	55.32	0.43
II	718	16.27	580.59	21.42	0.81	0.53	4.90	0.11
III	132	2.99	309.83	11.43	2.35	0.35	5.44	0.06
IV	32	0.73	162.25	5.99	5.07	0.46	4.13	0.11
V	8	0.18	74.53	2.75	9.32	0.54	4.00	0.14
VI	2	0.05	24.42	0.90	12.21	0.76	4.00	0.19
VII	1	0.02	59.31	2.19	59.31	0.21	2.00	0.10
	∑Nu = 4414	∑Nu% = 100	∑Lu = 2710.00	∑Lu% = 100		Lurm = 0.48	Rbm = 4.08
Umiew Basin
I	1169	79.69	501.56	55.17	0.43
II	235	16.02	190.92	21.00	0.81	0.53	4.97	0.11
III	46	3.14	114.24	12.57	2.48	0.33	5.11	0.06
IV	14	0.95	48.13	5.29	3.44	0.72	3.29	0.22
V	2	0.14	15.80	1.74	7.90	0.44	7.00	0.06
VI	1	0.07	38.56	4.24	38.56	0.21	2.00	0.11
	∑Nu = 1467	∑Nu% = 100	∑Lu = 909.21	∑Lu% = 100		Lurm = 0.44	Rbm = 4.47
Umngi Basin
I	866	78.37	374.18	54.40	0.43
II	190	17.20	162.53	23.63	0.86	0.51	4.56	0.11
III	37	3.35	67.12	9.76	1.82	0.47	5.14	0.09
IV	9	0.82	35.39	5.15	3.93	0.46	4.11	0.11
V	2	0.18	23.90	3.48	11.95	0.33	4.50	0.07
VI	1	0.09	24.70	3.60	24.70	0.48	2.00	0.24
	∑Nu = 1105	∑Nu% = 100	∑Lu = 687.83	∑Lu% = 100		Lurm = 0.45	Rbm = 4.06

Table 4. Areal parameters of the river basins from the study area.

	Umiam Basin	Umkhen Basin	Umiew Basin	Umngi Basin
Perimeter (P) (km)	252.67	270.83	127.40	125.50
Basin length (Lb) (km)	75.45	74.97	42.37	35.12
Basin area (km2) (A)	709.70	1141.95	396.90	297.90
Main channel length (Cl) (km)	110	102	62	56
Mean basin width (Wb)	9.41	15.23	9.37	8.48
Lemniscate’s (k)	8.02	4.92	4.52	4.14
Form factor ratio (Rf)	0.13	0.20	0.22	0.24
Elongation ratio (Re)	0.40	0.51	0.53	0.55

Table 5. Drainage texture analysis of the river basins from the study area.

Drainage density (Dd)	2.48	2.37	2.29	2.31
Stream frequency (Fs)	3.99	3.87	3.70	3.71
Drainage texture (Dt)	11.20	16.30	11.51	8.80
Circularity ratio (Rc)	0.14	0.20	0.31	0.24
Constant of channel maintenance (C)	0.40	0.42	0.44	0.43
Infiltration number (If)	9.88	9.17	8.47	8.56
Fitness ratio (Rf)	0.44	0.38	0.49	0.45
Wandering ratio (Rw)	1.46	1.36	1.46	1.59
Compactness coefficient (Cc)	8.26	8.89	5.56	6.02
Length of overland flow (Lg) km	0.20	0.21	0.22	0.22

Table 6. Relief characterization of the river basins from the study area.

Height of basin mouth (z)(m)	263	307	125	364
Maximum height of the basin (Z)(m)	1799	1836	1878	1781
Total basin relief (H) (m)	1536	1529	1753	1417
Relief ratio (Rh) (m)	20.36	20.39	41.37	40.35
Ruggedness number (Rn)	3809.28	3623.73	4014.37	3273.27
Asymmetric number (An)	55.5	42.68	65.8	61.9
Dissection Index (Di)	0.85	0.83	0.93	0.80

The detailed classifications of the extracted lineaments from the basins under consideration have been tabulated in Table 7. Geomorphic macro lineaments are the most dominant in all the studied basins and structural micro lineaments are the least abundant (Table 7).

Table 7. Basin wise categorization of the extracted lineaments.

Name of Basin	Total no. of lineaments	Geomorphic Micro Lineaments	Geomorphic Macro Lineaments	Structural Micro Lineaments	Structural Macro Lineaments
Umkhen	265	136 (51%)	90 (34%)	16 (06%)	23 (09%)
Umiam	210	138 (66%)	59 (28%)	0	13 (06%)
Umiew	120	68 (57%)	45 (37%)	0	7 (06%)
Umngi	95	60 (63%)	23 (24%)	3 (03%)	9 (10%)

The directional analyses of the extracted lineaments have been presented in the rose diagrams in Figure 8. It is observed that in Umiam and Umkhen basins there is a dominance of the NE-SW trending lineaments. The Umngi basin is dominated by N-S trending lineaments. The Umiew basin has a bimodal, NNE-SSW and ENE-WSW orienting lineament distribution patterns.

The study area is situated in a seismically active zone (Kothyari et al., 2022). A total of about 185 incidences of earthquakes have occurred in this part of the Indian subcontinent from 1950 to 2020 (Baro & Kumar, 2015; Fitch, 1970; Kayal, 1987). Seismological studies conducted by USGS have confirmed that out of these earthquakes, 26 earthquakes had magnitude greater than or equal to 5 (Figure 1). Many of the earthquake epicenters are located along the major lineaments or have occurred within some river/stream courses. Several earthquakes have occurred in the Brahmaputra Valley which is sandwiched between the Shillong Plateau [in the south] and the Himalayas [in the north]. Within the four studied basins seven earthquake incidences have occurred. Only one of the earthquakes has magnitude more than 5.

A total of 110 nos. of landslides have been reported from the four studied basins. The longest being 120 m in length. The details of the landslides have been summarized in Table 5. Geomorphologically the landslides in the area occur in colluvial foot slopes, highly to lowly dissected hills, hilltop, pediment, ridges, rolling plains and transportation mid slopes. The major geological reasons for the occurrences of landslides are formation of wedge due to presence of joints, highly jointed and weathered rock mass, highly weathered rock material lying on steep slopes, in-situ soil overlying highly weathered rock mass, toe cutting by rivulets, water percolation along fractures and joints, water logging in weathered rock mass, presence of shear zones and other neo-tectonically active structural elements. However, there are some anthropological factors contributing to occurrences of landslides as well, viz., unscientific slope cutting for construction, deforestation, reactivation of older debri material due to human activity and water logging due to restriction of natural path of surface and sub-surface water due to anthropogenic activity. Rainfall and seismic activities are the most dominant triggers for landslides in the area.

The topographic profile along the trunk stream of each of the studied river/stream courses has been analyzed (Figures 6 and 7). Along the stream course each of the river profiles shows steep loss of elevation at points, where it intersects the lineaments or there is a change in lithology. The loss of elevation in the north bound rivers is much steeper than the south bound rivers. At the steepest instance the elevation difference along Umiam river is almost 400 m (Figures 6 and 7). The Ksn value is highest for Umiew and the lowest for Umkhen river basins respectively. Umiew Basin has a maximum of 32 and Umngi Basin has a minimum of 15 numbers of knick points in the study area. Umiam and Umkhen basins have 22 and 27 knick points, respectively (Figures 5 and 7). The longitudinal profiles of all the studied basins show steep fall associated with intersection of structural lineaments (Figures 6 and 7). These steep falls are associated with the several faults crisscrossing the Meghalaya Plateau (Halder et al., 2022) (Figure 1).

The north bound streams i.e., Umkhen and Umiam, are near symmetrical in nature without any major tilting as is evident from the calculated asymmetric factor (Table 6) (Figure 9). Both the south bound rivers show a leftward tilt (Table 6) (Figure 9). Analyses of all the four river basins show that there is a left lateral shifting of the river course. The maximum river course shifting is around 8.5 km in the Umiam River (Figure 3). The maximum course shift of the remaining rivers ranges from 5.0 km to 6.12 km (Figure 3). Detailed point to point analysis of each of the studied river courses has shown that river course shifting of the order of 1 km to 4 km is very common (Figure 3).

Figure 9. Asymmetric tilting of the four river basins as indicated by the trunk streams in the basins, (i)Umiam, (ii)Umkhen, (iii)Umngi and (iv)Umiew river basins. The halves of the basins have been designated dextral and sinistral facing down- stream for each basin.

Discussions

Morphometric analyses of the studied basins show a dominance of structure-controlled trellis drainage pattern. The high Stream Number % (Nu%) (Table 3) and high order truck stream (6^th order) indicate that the basins have developed in a tectonically controlled dissected mountainous region (K. Nath et al., 2023) (Table 3). The overall trellis pattern drainage indicates that the stream networks are structurally controlled. The stream numbers show variability among all the basins, but when expressed in percentage the stream number (N%) for the different orders is comparable among all the studied basins. With an increase in stream order there is a steady decrease in the stream number, in all the basins. This also indicates that all the basins have developed in similar terrain conditions. The higher number of 1^st order streams indicate tectonic control (Imsong et al., 2016) over the stream network developed in an impervious to semi-pervious rock strata (Patton & Baker, 1976; Reddy et al., 2004). The high bifurcation ratio and rho coefficient are indicating a matured drainage system developed in a structure controlled terrain, which is evolving to this day. The tectonic elements responsible for the upliftment of the Meghalaya Plateau largely control the geomorphology of the plateau. This in turn controls the development of the structure controlled matured drainage pattern in the studied basins.

According to basin geometry, all the studied basins are elongated in nature. Leminscate’s ratio is high for all the basins and Umiam Basin has the highest among all (Table 4). This is possible if the basin shape is attenuated, due to some tectonic element e.g., the Barapani-Tyrsad Shear Zone and Oldham Fault present in the study area. The compactness coefficient and circularity ratio (Table 5) show that the basins are still evolving. The drainage texture analyses (Table 5) of the studied basins indicate that the basins are matured and evolving at the same time. This also indicates the runoff potential and the amount of landscape dissected by the rivers. The high values of stream frequency and drainage density are indicative of steep slopes and underlying impervious rock strata.

From the calculated relief ratios, it is understood that the slope of the plateau towards the Brahmaputra valley from the central part is gentler compared to the slope towards the Bangladesh plains. In the north bound rivers, the average gradient is about 8°-9° and in the south bound rivers the average gradient is around 17°-18°. The steeper gradient in the south bound rivers is the result of deep valley incision in the Cretateous-Tertiary cover sequences present towards the southern periphery of the plateau (Figure 3). The high relief ratio and steep gradient complemented by the impervious to semi-pervious strata constituting the basins, result in very high-water discharge. The higher ruggedness number and dissection index of the basins also indicates both high erodibility and high-water discharge. These factors coupled with the highly erodible rock strata, explains the higher river valley deepening towards the south compared to the rivers flowing towards Brahmaputra.

The stream long profiles of the different sectors of the studied basins are presented in Figures 6 and 7. The actual profiles are overlain with the model profile predicted by Topotoolbox. All the streams exhibit a concave up morphology and there are several knick points along the trunk stream as well as the tributary streams. The knick points present in the upstream must have resulted in fall of base level related to plateau uplift. The presence of knick points downstream in the south flowing rivers may be attributed to the change in lithology from the impervious metasediments of Shillong Group to the highly erodible Cretaceous-Tertiary rock strata. The steep falls of height at the intersection of the river profiles with the macro-structural lineaments indicate that in all probability the lineaments are neo-tectonically activated fault zones. This can be linked to the rapid shortening of the eastern part of the Indian Plate, due to higher convergent rate (Banerjee et al., 2008).

The topographical profiles, of the two rivers flowing towards Brahmaputra valley i.e., Umkhen and Umiam, show an abrupt slope break of about 200 m to 250 m and those have been identified as knick points along the river course (Figure 7). It is observed that, if the NW-SE trending Oldham fault is extended and made to intersect with the channel profiles, it matches with the locale of the slope breaks in both the river profiles. These corroborates with the presence of NW-SE trending lineaments identified in the Umiam and Umkhen basins (Figures 8(a), 6 (a) and 7(a)). Fault zones are zones of weakness, thus in an all probability when the river courses cross over these zones, they produce greater valley incision. This sudden valley incision produces the knickpoints along the course of Umkhen and Umiam rivers. The trellis drainage pattern of the north bound river basins indicates the development of the drainage network as structure/lineament controlled. The dominance of NE-SW trending lineaments both in Umiam and Umkhen basin can be attributed to the dominant foliation of SG (K. Nath et al., 2021). The N-S trending macro lineament, extending from Umkhen basin into the Umiam basin can be attributed as an extension of the Umngot lineament (Halder et al., 2022) (Figure 8(a)). The location of the knick points and the modeled river profile have been overlain on the geology along the course of the north bound rivers (Figures 6(a) and 7(a)). It is evident that though the sharp descents are controlled by tectono-structural elements the, change in lithologies also contributes to the creation of knick points.

Sharp decent of 350–375 m has been observed along the topographic profiles of the south bound rivers flowing into the Bangladesh plains (Figure 6). These abrupt slope breaks similar to the north bound rivers could be attributed to some tectonic element. It is postulated that the Raibah Fault (Ali & Duarah, 2022) could be responsible for these slope breaks, which can be substantiated by the presence of near E-W lineaments observed in the Umiew and Umngi basins (Figure 8). Neotectonic movement or slip along the preexisting fault planes result in the abrupt steepness along the river profiles (Imsong et al., 2016). The dominance of near N-S lineaments in the Umngi basin could be attributed as the extension of the Kulsi Lineament (Duarah & Phukan, 2011). Corroborating the geological information with the river profiles of the south bound rivers depict that change in lithology along with the tecno-structural elements contribute to the sharpest descent in a conjugate fashion. In this case actually the lithological contact is induced by tectonics related to Dawki Fault (B. Nath et al., 2022).

The high Ksn values of the streams suggest high incision rates. In the north bound rivers in the absence of any erodible rock strata this valley incision can be attributed to migration of knickzones or “bottom up process” (Bishop, 2007). In the south bound rivers however the increase in Ksn value down stream is majorly attributed to change in erodible lithology (Figure 3).

The high relief of the rivers and the abrupt slope breaks result Tyrsad in the significant head difference. Huge surface run off and significant relief difference produces an adequate potential, for generating hydroelectricity. Shillong Plaeau in this regard has immense scope for generation of green energy. The huge untamed surface runoff results in devastating floods every year in Assam and Bangladesh. Hydropower projects can be conceived not only to generate green electricity but also to mitigate the flood hazards in this part of the sub-continent. The hydro projects can be a boon to the irrigation system also. Meghalaya Energy Corporation Ltd. the state-owned power generation company in Meghalaya is solely dependent on hydroelectric power generation from different run of the river and dams as detailed in Table 8, however, the state has many hydroelectric power projects in pipe line.

Table 8. Details of hydroelectricity generating dams in the state of Meghalaya.

	Name	Purpose	River	District
1	Khandong Dam	Hydroelectric	Kopili	Jaintia Hills
2	Kyrdemkulai (Umiam st-III) Dam	Hydroelectric, Irrigation, Drinking/Water Supply	Umtru	West Khasi Hills
3	Mawphlang Dam	Hydroelectric, Irrigation, Drinking/Water Supply	Umiew	East Khasi Hills
4	Myntdu-Leshka Dam	Hydroelectric	Myntdu	Jaintia Hills
5	Nongkhyllem Dam	Hydroelectric	Umtru	Ri Bhoi
6	Umiam Dam	Hydroelectric	Umiam	Ri Bhoi
7	Umtru Dam	Hydroelectric, Irrigation, Drinking/Water Supply	Umtru	Ri Bhoi

Landslides are predominantly identified in the road cut sections of national and state highways, minor roads, settlement areas, and quarry zones, in the studied area. Field surveys reveal that in regions characterized by hard rock terrain, landslides primarily result from alterations in slopes during construction and excavation activities. Failure is often influenced by factors such as weathered rock and the thickness of overburden, with rainfall and earthquakes serving as major triggering factors. The presence of landslides having length as long as 120 m and area as high as 13,500 m² indicate that these are controlled by regional tectonic activity.

Asymmetric factors calculated for the basins are also a very important indicator of neotectonic activity in the plateau. The north bound rivers have very less asymmetricity and the south bound rivers show higher asymmetry (Table 6). Incidentally both the south bound rivers have tilted towards the left (Fig.9). In all probabilities the tilting of both the basins has occurred due to reactivation of N-S trending cross faults (Bilham & England, 2001) of the Dawki fault system. This also indicates that the southern part of SP is neotectonically active.

The analysis of sinuosity of the rivers shows that the north bound rivers have significantly shifted laterally towards the left along their entire course (Figure 3). The maximum shift of about 8.5 km and 6.0 km has been measured along the course of the Umiam and Umkhen rivers respectively (Figure 3). This lateral shifting of the river course is a clear indication of neotectonic activity in the SP. The lateral shifting of the rivers of the study area can also be manifested by the presence of unpaired terraces (Yin et al., 2010). It is opined that the shifting of the river courses occurred due to the reactivation of the lateral faults developed oblique to the Barapani-Tyrsad shear zone.

Several earthquakes have affected the SP and the Brahmaputra Valley in the recent past and entire zone has been classified as intensity 4/5 seismic zone. When the epicenter locations of the recent earthquakes are plotted with the regional tectonic elements; it has been observed that majority of the epicenters overlap with one or the other regional tectonic elements (Figure 1). Similarly, some of the earthquake epicenters overlap the course of the streams, which are structure-controlled trellis pattern in nature (Figure 1). The occurrence of earthquake epicenters invariably across the Brahmaputra Valley and Shillong Plateau also indicates that the area is neotectonically active.

Conclusions

The SP has undergone several phases of deformation. The continuity of the Oldham fault towards the southeast direction in the SP can be proved from the slope breaks of the north bound river profiles, location of the knick points and Ksn value on stream. In the absence of robust surface manifestation, the existence of the Raibah fault has been debated by many workers. However, the analyses of the south bound river profiles, in similar manner, indicate the existence of the Raibha fault. The studied river systems have been affected by several fault systems controlled by the regional tectonic setup of the area, which are related to the pop up of the plateau. Morphometric analyses of the studied basins suggest that though the river basins are matured, they are evolving to this day. This is a conclusive evidence of neotectonic reactivation of the tectonic elements of SP.

Neotectonism moulds the structurally controlled drainage pattern of Shillong Plateau into its present shape. The high drainage texture, relief parameters of the analyzed basins, earthquake data, sinuosity assessment and lineament analyses establish that the structural elements both within and at the marginal part of the plateau are prone to be reactivated. However, the typical river profiles of the analyzed rivers lend them immense potential for generation of hydroelectricity, which is yet to be tapped to its fullest extent. Further, already commissioned dams are to be maintained and the ones in pipe line are to be conceived considering the regional tectonic elements which could be reactivated at any point of time. Thus, there can be reactivation of faults leading to terrain modification and shifting of the river courses synchronous or subsequent to the earthquake events. It is significant that lineaments are manifestations of regional stress regime and should be analyzed from structural engineering point of view.

The ever evolving landforms in any area have an effective control on the anthropological activities therein. The landform in Meghalaya is controlled by the regional tectonic elements which are active. These active structural elements should be considered for successful urbanization, effective natural hazard management, planning of mitigation measures to evade floods, landslides and development projects as well.

Acknowledgments

KN, SH, DG and BNM thanks the Addl. Director General, Northeastern Region, Geological Survey of India, Shillong, Meghalaya for providing all the logistics and infrastructural support to carry out the work. TKG and RKS are thankful to the HOD, Applied Geology, Dibrugarh University for providing permission and other exigencies. Gratitude is extended towards K. Paul, Research Scholar, Department of Earth Sciences, IIT Kanpur for helping the authors work with the Topotoolbox. Acknowledgements are due to the esteemed reviewers and handling editor for their critical review and constructive suggestions

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