Tectonic implication in the development of fluvial landforms,morphotectonic features,and surface processes in the main boundary thrust zone of Dehradun

Morphometric indices

To decipher the influence of tectonics, lithology in the region which is often reflected in the channel morphology, several morphometric indices and chi-transform (Table 1) have been computed using TopoToolbox tool in MATLAB R2024b environment (Schwanghart et al., 2013; Schwanghart and Scherler, 2014) and ArcGIS 10.5. In addition to this, Knick points that reflect transition in different lithology or tectonic activities, influencing the river profile, erosional pattern and sediment transport, have also been identified using Knickpointfinder function (Stolle et al., 2019). For these analyses, a high-resolution ALOS PALSAR digital elevation model (DEM) with a 12.5 m spatial resolution has been utilized. Complementary to the remote sensing and GIS-based analysis, detailed field investigations were undertaken to identify and validate tectonic imprints on fluvial landforms and morphotectonic features such as fault scarps, as well.

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Table 1.

The table describes the morphometric indices and their utility.

Morphometric indices	Description
Stream length gradient index (SL)	S L = ( Δ H Δ L ) × L
	Where, ΔH is the change in elevation between two points of a particular segment, ΔL is the length of the segment, and L is the total distance from the current segment to the source of the river
	• It quantifies changes in the slope of a river segment relative to its upstream segment and demonstrate variations in river gradients along a stream profile.
	• Under natural conditions, rivers tend to develop a gentle, concave graded profile over time. However, disturbances such as tectonic uplift, faulting, or the presence of resistant rock formations can disrupt this profile, leading to the formation of steep slopes or knickpoints (Berberian et al., 1992; Gomez and Marron, 1991; Hack, 1973; Marple and Talwani, 1993; Rhea, 1993).
	• For a stable, graded river, SL values remain relatively constant, while deviations/spikes in SL values typically indicate tectonic, structural, or lithological influences (Lifton and Chase, 1992; Troiani et al., 2017).
	• High SL values are often observed in zones of active uplift or faulting whereas zones with soft and easily erodible rocks demonstrate low SL values.
Normalized steepness index (Ksn)	S = K s A − θ
	Where S represents channel gradient, A is the upstream drainage area, Ks is the steepness index, and θ refers to the concavity.
	• It provides insights into the tectonic activity and erosion pattern in a river basin. It refers to local channel gradient in relation to the upstream drainage area.
	• The concavity in tectonically active landscapes is generally found to be restricted between a narrow range of 0.35-0.6 (Snyder et al., 2000; Whipple and Tucker, 1999). In the present study, median value of θ, that is, 0.45, has been used to compute normalized steepness index Ksn (Gailleton et al., 2021; Gururani et al., 2023; Kirby and Whipple, 2012).
	• High Ksn value indicates areas with active tectonics, fault zones, and erosion-resistant lithologies.
Chi integral (χ)	χ = ∫ x b x ( A 0 A ( x ) ⅆ x ) m n
	Where x is the observation point in the upstream direction, while xb is the base level; A0 is the reference area, and A(x) is the drainage area; m and n are constant
	• A robust analytical tool that allows the comparison of channel steepness and erosion rates across a drainage basin by transforming the longitudinal coordinate of the channel into a chi-elevation space (Perron and Royden, 2013).
	• The collinearity of the tributaries with main river in transformed river profile also known as chi-plot, reveals the steady state equilibrium state. Whereas the deviation from the linear profile help us understand the migration of basin-divide, since the basin divide migrates from low chi integral (aggressor) towards high integral (victim) in order to achieve equilibrium (Gururani et al., 2023; Joshi et al., 2021; Lin et al., 2021; Willett et al., 2014).

Stream length gradient index (SL)

The six sub-basins in the hanging wall of the MBT have been considered for the analysis (Figure 3). The SL index values of the sub-basins in the study area range from <250 to >1500 (Figure 3). High SL index values often indicate areas of recent or ongoing tectonic uplift, as the increased elevation requires streams to adjust their gradients accordingly. It has been observed that among the sub-basins studied, the elongated sub-basins such as SB-1, SB-2, and SB-4 exhibit relatively lower SL index values, reaching up to 1100 (Figure 3). This relatively lower SL index value can be attributed to their less concentrated stream networks, which facilitate smoother water flow and reduce localized erosion. In contrast, the less elongated sub-basins, including SB-3 near Kotidar and SB-5 and SB-6 near Mussoorie, display significantly higher SL values, around 1800 (Figure 3). The headward region of the sub-basins are characterized by steep slopes with steep stream gradient which represents the initial stage of drainage systems and has resulted in higher SL and knickpoints, making them more susceptible to landscape modifications.OPEN IN VIEWER

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Figure 3.

Longitudinal profiles and stream gradient index of streams flowing across the MBT.

There is a sudden gain in altitude across the MBT and consequently the steep stream gradient has higher SL values that resulted due to uplift of the hanging wall block. Differential erosive power of the tributaries streams and the main trunk, different lithological and structural conditions also resulted in the development of Knick points. The sudden drop of water increases the energy of the streams, causing more bedrock erosion and ultimately influencing the erosional dynamics of the river system (Scheingross et al., 2019; Shawez et al., 2025). As a result, rivers flowing down the steep slopes at the front of the mountains have rapid elevation changes over short distances, creating steeper gradients. Owing to these steeper slope, streams gain more force, causing higher SL index values as they erode the surface and carry sediment more intensely (Troiani et al., 2017).

Normalized steepness index (K_sn), knickpoint, and Chi Integral (χ)

In SB-1, lower values of K_sn and seven knickpoints are observed (Figure 4(a)). Two knickpoints are developed in the footwall of the MBT and observed along the tributaries channel, and the K_sn values are generally <150 with two sections of the stream having >150. Across the MBT, pronounced slope break is observed where the K_sn values are >150 with five knickpoints (Figure 4(a)). The best fit mn ratio for the basin is 0.540 (Figure 4(b)); and the χ plot diverges from linearity and has convexity (zone of uplift) and concavity (zone of erosion) in their longitudinal profile (Figure 4(c)). In SB-2, three knickpoints are developed downstream of the MBT and the K_sn values range from 1.109 to 268.4103 (Figure 4(d)). High K_sn value is associated with the knickpoints formed at the southern tips of the basin. Upstream of the MBT the K_sn values are mostly >150 and four knickpoints are developed three in the main trunk of the stream. The best fit m/n ratio for the basin is 0.5017 with R² being 0.9542 (Figure 4(e)). The χ plot diverges slightly from linearity marking zone of uplift and zone of erosion in their longitudinal profile (Figure 4(f)). In SB-3, a total of 17 knickpoints are developed, one knickpoint in the footwall portion of the MBT and the rest upstream of the MBT (Figure 4(g)). Downstream of the MBT, the K_sn values are generally <100 with few >150; while in the upstream the main trunk of the stream have K_sn values >200 and tributaries <100 Figure 4(g). The best fit mn ratio for the basin is 0.5086 (Figure 4(h)). Slight deviation from the steady state is observed in the χ plot (Figure 4(i)). In SB-4, all the knickpoints are formed in the hanging wall of the MBT and K_sn values in the footwall of the MBT are <100 with an abrupt rise in the value in a small stretch in the MBT zone (Figure 4(j)). Upstream of the MBT the K_sn values vary from low to the highest value. Clustering of knickpoints is observed. The best fit mn ratio for the basin is 0.6002 the R² being 0.8735 (Figure 4(k)). Convexity is more dominant as is observed in χ plot (Figure 4(l)). In SB-5, all the knickpoints are formed in the upstream of the MBT and the K_sn values range from 1.0887 to 435.331 (Figure 4(m)). Streams in the footwall of the MBT have K_sn values <150 while in the upstream maximum K_sn values are observed in the mid-section (Figure 4(m)). The best fit mn ratio for the basin is 0.0292 the maximum R² being 0.8735 (Figure 4(n)). Concavity is more dominant as is observed in the χ plot (Figure 4(o)). In SB-6, knickpoints are concentrated in the mid-section of the basin and seven knickpoints are observed (Figure 4(p)). K_sn values range from 1.3778 to 593.1657 and maximum values are observed in the mid-section of the stream (Figure 4(p)). Maximum values are observed in the main trunk of the stream. The best fit mn ratio for the basin is 0.2002 the maximum R² being 0.8735 (Figure 4(q)), and the χ plot indicates concavity in the downstream while upstream there is steady state (Figure 4(r)).OPEN IN VIEWER

Fluvial morphology

Streams flowing across the MBT particularly between Ganga and Giri rivers have been considered for fluvial morphologies analysis. In this section the MBT, Santaurgarh Thrust, and the Bhauwala Thrust are closely spaced (Figure 1), while Bansiwala Thrust lies further south. In the hanging wall of the MBT the Yamuna River and Tons river valleys are narrow with narrow to vertical valley slope with development of patchy terrace; this suddenly becomes very wide as it enters the Dun.

Shifting of the Yamuna River is observed between Hathiari in the footwall of the MBT to Ponta Sahib, and is represented by paleochannels, different levels of terraces, and scarps. Evidences drawn from the Topographical maps, DEM, and Google Earth Image, Asan, Tons and Yamuna rivers have migrated either way in the recent geological past (Figure 5(a)-(c)). Datta et al. (2024) and Sinha and Sinha (2016) have discussed about the river of migration of Giri, Bata and Yamuna rivers on the basis of fluvial terrace levels and abandoned river channels. The confluence points of Bata River with Giri and Yamuna River have shifted about 5 km SW ward (Datta et al., 2024). In the past as evident from extremely wide paleochannels and levels of terraces, Giri River migrated east ward while at present it is migrating southwest ward. Northwest ward migration of Yamuna River is observed at present in the Dak Pathar area (Figure 5(a)). The migration patterns and subsequent incisions are derived mainly from levels of terraces, paleochannels, and fan deposits. Singh et al., (2001) shows decreasing height of the terrace levels from SE to NW. Asan and Song-Suswa rivers migrated NW and NE ward, respectively, while Tons River migrated NW ward (Figure 5(b) and (c)). At Kalatha, a crescent shaped paleochannel is observed at 1070m asl while the present river bed is at 500m asl (Figure 5(d) and (e)). The width of the water gap or paleochannel measures about 40 m and measures about 0.7 km in length. A few kilometers away at Khadar, another paleochannel is observed of about 2 km in length with a maximum width of about 210 m at 640 m asl (Figure 5(d) and (f)), and is bounded by the MBT in the north and MBF in the south. At the Kalawar-Khadar area, unpaired levels of terraces are developed on the banks of the Tons River, three and two levels of terraces on each bank (Figure 5(i) and (j)). On the right bank three levels of terraces are developed at 541, 530 and 502 m asl while on the left bank at 584 and 530 m asl. In the adjacent Amlava Nadi at Kalsi five levels of terraces are developed on the right bank, 641, 630, 617, 608, and 580m asl, and only two terraces are well developed. Tons River and Amlava Nadi just before exiting into the Dehra Dun have very broad valley in the MBT-MBF zone (Figure 5(g) and (h)). The Tons valley measures approximately 2 km in width at Kalawar, but the valley becomes significantly narrower downstream from the MBF, contracting to under 150 m wide, due to litho-tectonic control. The Footwall of MBF is made up of massive sandstone while the hanging wall comprises of highly sheared Subathu shales. Similar morphology is also developed in the Amlava Valley where in the footwall section the valley width narrows down to about 50 m from about 650 m. A fault trace of about 1.5 km is also developed in the footwall of the MBF (Figure 5(f) and (g)). Asan River flows along the axis of the valley in almost west direction where it confluence with Yamuna River at Asan Barrage. Asan River have migrated northward by about 1.5 km as imaged from Google Earth (Figure 5(b) and (c)). In the eastern part Suswa-Song river also flows in the axis of the Dehradun valley with southwest flow direction, this river also shows northeast migration by about 2.0 km near Doiwala. In the upstream at Raiwala Suswa-Song river have migrated about 1.2 km northeast ward as evident from escarpment that trend NW-SE. The escarpment measures about 25 m in height and is formed in the Bhogpur Fan of Singh et al., (2001). Chandrabhaga Nadi takes a right-angle turn from south flowing to east flowing near Ohni. This stream has incised the fan by about 95m and has a very wide channel of about 700m wide. Semi-radial drainage pattern is observed in the Nagsidh Hill which is an anticlinal structure formed by thrusting along the Nagsidh Thrust (Figure 6(a)). Dulahani Nadi a tributary of Song River is deflected by the Nagsidh Anticline from south flowing to southeast flowing. Incised meandering of Dulahani Nadi is observed in the hanging wall of the Nagsidh Thrust (Figure 6(b)). NW-SE trending erosional escarpment of about 2 km long and 25 m in height (Figure 6(c)) is observed parallel to Dulahani Nadi. Song River and Sahastradhara Nadi flows almost along the NW-SE trending East Kalanga Fault for almost 10 km in length.OPEN IN VIEWER

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Figure 6.

(a) Sketch map of drainage pattern associated with Nagsidh Anticline. (b), (c) Google Earth images showing fluvial landforms associated with Nagsidh thrust and Nagsidh Anticline, incised meandering, escarpment, and deflected stream; Google Earth images showing various fluvial morphologies associated with tectonic activity along the MBT, Santaurgarh Thrust, and the Bhauwala Thrust. (d). Stream migrations, incised valleys, and stream piracies of Nun Nadi and Nimi Nadi with reference to tectonic activity along Bhauwala and Santaurgarh thrusts (PC – Paleochannel, CP capturing point of stream piracy). (e) Tectonically induced fluvial morphology of Sorna Khala, Syaru Khala, and Koti Nadi encompassed by Bhauwala and Santaurgarh thrusts (PC – Paleochannel, CP capturing point of stream piracy). (f) Fluvial morphology of Mauti Nadi, Gauna Nadi, Naro Nadi, Bin Nadi, and Barun Nadi sculptured by MBT, Bhauwala and Santaurgarh thrusts (PC – Paleochannel, CP capturing point of stream piracy).

Stream/river piracy

A number of wind gaps are observed in the satellite imageries, topographic maps and Google Earth images between Dehradun and Tons River. At Santaurgarh, Nun Nadi has migrated about 1.5 km eastward (Figure 6(d)). The width of the paleochannel measures 1.2 km, where the eastern portion has been incised by the stream, and the width of the valley floor measures about 130 m. The stream migrated eastward gradually and a number of paleochannels were developed. Within the wind gap, new smaller channels have developed, which may result in capturing of the stream and realignment of the new channel. At one stage of the stream migration of Nun Nadi and Nimi Nadi, they confluenced at Anwaliwala-Sailan area. Nimi Nadi, then, started migrating eastward followed by the migration of Nun Nadi (Figure 6(d)). Nimi Nadi have migrated eastward by 2.2 km from its former channel. The paleochannel of Nun Nadi at present time is incised by Nimi Nadi and its small tributary. Bhidoli area represents the paleochannel of Sorna Khala. Sorna Khala migrated westward and Dunga area represents the paleochannel (Figure 6(e)). Syaru Khala has shifted eastward over time and currently merges with Sorna Khala at a location approximately 3.5 km downstream from where it previously confluenced (Figure 6(e)). Koti Nadi migrated westward in phase wise though not all the paleochannels have been preserved, but three very prominent paleochannels are developed having widths 430 m, 202 m, and 185 m, respectively from east to west. Erosional scarps in the form of scoops are preserved supporting the paleochannels that measures about 10 m in height. Extremely wide paleochannel of Sorna Nadi is exposed at Barwa-Tikri area where the paleochannel is 1200 m in width. Few meters below two distinct paleochannels are developed with a width of 985 m and 650 m, respectively from northwest to southeast. The confluence point of Gauna Nadi and Mauti Nadi was at Langha, while at present the confluence is at Dabthala which is about 5.5 km southwest of Langha (Figure 6(f)). Landforms suggest that Gauna Nadi, Naro Nadi, and Bin Nadi represent different stages of stream migration and stream piracy. Initially Gauna Nadi was flowing through the Pasta, Langha, Tauli, and confluenced with Yamuna River at Katpathar. In the later phase the flow direction changes to Pasta, Langha, Dungakhet, and Dabthala (Figure 6(f)). The maximum width of paleochannel presently occupied by Naro Nadi is at Tauli where the width is about 960 m while at Langha the width is about 650 m. Along Gauni Nadi the maximum width is about 685 m. The present Bin Nadi channel is very narrow with two fans deposits observed at 922 m asl and 859m asl, respectively near Malwara and the present river bed is at 791m asl (Figure 6(f)). Northwest of Bin Nadi is the Barun Nadi, this stream has also incised the both the fan and bedrock and is characterized by very narrow valley. Fan deposits are observed in patches; both the bedrock and fan deposits have been incised by the stream. Bin Nadi, Barun Nadi, Naro Nadi, and its small tributary streams are deflected northwest ward between the MBT and Bhauwala Thrust in Hathiari-Kata Pathar area in the exit area of Yamuna River into the Dehradun Valley (Figure 6(f)).

Tectonic geomorphology

At Binhar in Mandarsu-Chamela-Kol area about 1.5 km long three almost parallel fault traces 200 m apart are observed at the ridge crest in the Google Earth image (Figure 7(a)). Extensional tectonics related open fractures are observed in alternation of phyllite and siltstone (Figure 7(b)). Traces of other faults are also observed but they are poorly preserved and are placed southward of the two fault traces (Figure 7(a) and (c)). The fault scarps dip toward the south, and triangular facets are observed (Figure 7(a), (c)–(e)). The width of the fault zone measures about 560 m and the fault traces trends SE to NW. In the out crop the fault zone are represented by highly sheared limestone, with thinly bedded slates and phyllite. The fault zones are measured to be more than 10 m in width and are best exposed along the road sections (Figure 7(f)). Amongst the fault traces the middle fault trace is the most prominent, and sag ponds are formed along the fault traces (Figure 7(c) and (d)). Along two of the fault traces, equal number of sag ponds of four each is developed. The ridge crest is relatively gentler than its surrounding ridge crest due to movement along the faults. The present topography suggests that the northern most fault trace has been much modified by the headward erosion of the north flowing streams. In the western extremity the two northern most fault traces merges into a single fault near Mandarsu. The largest sag pond is observed at Mandarsu, where a remnant of sag pond is observed. Linear depressions along the fault traces are one of the main tectonic landforms associated with the MBT and its fault splays. Normal faults are observed in thinly bedded bedrock. Linear depression characterized the trace of MBT in Bhitarli area that has become less distinct due to erosion with the passage of time. Triangular facets and fault scarps have also been degraded by erosions. Highly sheared Siwalik bedrocks are observed in the footwall of the MBT that dips steeply. Further west of Bhitarli at Thangaon linear depression measuring about 1.5 km is observed in the immediate hanging wall of the MBT. Here the hanging wall rock is made up of highly fractured quartzite and phyllite. The quartzite bedrock dips almost horizontal to gently dipping (Figure 7(h)). Brittle shear sense can be observed in the bedrock, while the overlying phyllite dips moderately (28° - 35°) towards northeast. The footwall is made up of steeply dipping to almost vertical conglomeratic boulders of the Upper Siwalik (Figure 7(i)). The boulders of the conglomeratic bed are highly sheared and individual boulders are marked by displacement along the faults (Figure 7(g)). The shear zones discernible within the boulders measures about 6 cm in width, while displacement measures from mm to 1 or 2 cm length. Trace of the MBT is depicted by slightly curved depression between Pasta and Koti that have degenerated by the action of landslides and erosion. At Pasta the conglomerate clasts of the Upper Siwalik constituting the footwall of the MBT are highly sheared and fractured, where a thick fault zone represented by fault gouge is seen along the road section. The highly sheared conglomerates are underlain by sandstone of the Middle Siwalik (Figure 7(k)-(m)).OPEN IN VIEWER

The MBT and MBF in the Ton River valley are only 1.5 km apart and the intervening valley is very wide. The bedrock in the MBT zone is highly sheared and fractured where the footwall is made up of marron colored shale of the Subathu Formation. The narrow valley in the hanging wall of the MBT is about 65 m wide of steep slopes with multiple landslides, while in the immediate footwall the valley width becomes about 1.7 km. In the MBT the Lesser Himalayan and sub-Himalayan rocks are clearly discernible by their color contrast. The MBT here is low dipping as indicated by almost horizontal bedding of the hanging wall and footwall rocks. The wide valley suddenly narrows down in the footwall of the MBF to about 100 m from 1.7 km wide. In the Khadar area the MBF is represented by a fault trace that measures about 1.5 km long depression, with scarp dipping towards north (Figure 5(f) and (g)). The footwall is made of massive Middle Siwalik rocks that dip 20 towards northwest. The wide intervening valley between the MBT and the MBF is characterized by strath and fill terraces (Figure 5(f)-(j)). Strath terrace is also observed at the top of the footwall of the MBF, where gently dipping Middle Siwalik sandstone is overlain by fluvial sediments.

Fans and landslides

The width of the sub-Himalaya ranges from 0 to 80 km along the entire length from east to west (Karunakaran, 1976; Luirei et al., 2014; Powers et al., 1998; Raiverman, 2002). Apart from this section the entire stretch of the sub-Himalaya is either broad or narrow. In the northwestern Himalaya, During the Quaternary period the re-entrant structure of Dehradun characterized by wide synclinal valley has been filled by alluvial and fan sediments. The hanging wall block of the MBT is more susceptible to landslides and severe erosion due to highly sheared and crushed bedrock with steep topography than the low lying footwall block which are sedimented. The hanging wall rocks are more prone to landslide than the footwall rocks due to highly fractured nature of the bedrock and steeper slopes (Figure 8(a) and (b)). In the MBT zone at Hathiari, the gently sloping slopes in the Siwalik are overlain by fan deposits of varying thickness. At Thangaon the Upper Siwalik rocks are overlain by huge thickness of fan sediments. Multiple levels are developed in the higher elevations while fluvial terraces occupy lower elevations (Figure 8(c) and (d)). Singh et al. (2001) classified the fans in Dehradun valley into Donga Fan, Dehradun Fan and Bhogpur Fan, of which in the Donga and Dehradun fans the sediments indicates different energy conditions and environment; the proximal part dominated by sediments deposited by small streams by sediment gravity - flow deposits while middle part characterized by mostly mud flow deposits and minor braided streams (Singh et al., 2001). Deposition by larger braided stream is envisaged for the formation of the Bhogpur Fan (Singh et al., 2001). Along the Hatnala in the hanging wall block of the MBT a stretch of about 105 m long fan deposit is exposed as a result of incision by the stream. More than 2 m thick fan sediment is exposed which is composed of tilted laminated sediments overlain by unstratified fan deposits. The sediment section resembles numerous intruded dyke sediments. Here the dykes like sediment bodies still maintain the stratification while the top overlying sediments lacks stratifications (Figure 8(e)-(h)). The undeformed layers are lighter in colors where are the deformed layers are muddy in color illustrating two different deposits. At Thangaon the Lesser Himalayan rocks rides over highly sheared conglomerates (Figure 7(i) and (j)). The bedrock in the hanging of the MBT is made up of highly fractured gently dipping quartzite, while the footwall is made up of steeply to almost vertical beds of Upper Siwalik conglomerate. The landslide occurring in the hanging wall of MBT near Koti is active prior to 1985 as imaged from the Google Earth Image, while the landslide in the upstream of Koti Nadi initiated around 2016 (Figure 8(a) and (b)).OPEN IN VIEWER

The hanging wall of the MBT between Tons and Song Rivers is defined by number of old as well as active landslides. Tectonic, geomorphological, climatic, and anthropogenic factors are the main reason for the instability of the south facing slopes of the Mussoorie hills. Tectonic factor is the most important factor amongst all the factors as the bedrock is traversed by number of joints and faults. Hanging wall slope have been affected much more severely than the footwall slope as the footwall rocks have suffered less shearing and shattering in comparison with the hanging wall rocks. The width of fault gouge and fault breccias in the footwall is much less than those in hanging wall rocks. The continued deformations along the MBT and its splay thrusts have resulted in the development of number of joint sets, and the intersecting joints have formed wedges. The closely spaced open fractures and joints, bedding planes along with the wedges have initiated most of the landslides. In the limestone and shale closely spaced open fractures that are perpendicular to the bedding planes are developed resulting in complex slope movements. In the phyllite constituting the bottom of the MBT fault plane the bedrock are highly sheared in the form of fault gouge, and numbers of normal faults are also observed. Sliding type of movement is the main type of movement observed in phyllite and slates. The highly crushed zones are site of many landslides. The quartzite bedrock is traversed by numbers of joints and faults. The intersecting joints forms wedges towards the open slopes and wedge failure are very common in the quartzite.