Morphotectonic segmentation and spatial variability of neotectonic activity along the Narmada–Son Fault, Western India: Remote sensing and GIS analysis

Geomorphology 180–181 (2013) 292–306

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Morphotectonic segmentation and spatial variability of neotectonic activity along the Narmada–Son Fault, Western India: Remote sensing and GIS analysis Parul N. Joshi, D.M. Maurya ⁎, L.S. Chamyal Department of Geology, The M.S. University of Baroda, Vadodara-390002, Gujarat, India

a r t i c l e

i n f o

Article history: Received 12 May 2011 Received in revised form 11 October 2012 Accepted 23 October 2012 Available online 31 October 2012 Keywords: Geomorphic indices Neotectonics Fault segmentation Narmada–Son Fault (NSF) Western India

a b s t r a c t Morphotectonic analysis of landscape using remote sensing and GIS is an effective way of deducing the pattern and spatial variation of neotectonic activity along poorly investigated active faults. In this paper, we evaluate the neotectonic activity along a part of the seismically active but poorly understood Narmada–Son Fault (NSF), a more than 1000 km long ENE–WSW trending fault transecting through the central part of the Indian plate. The NSF in the study area is geomorphologically expressed as an ENE–WSW trending line of north facing scarps that delimit the rugged mountainous topography of the uplands to the south and the alluvial basin to the north which prominently slope away from the scarps. The scarps are developed in the south dipping basaltic flows belonging to the Deccan Trap Formation of the late Cretaceous–Eocene and continue westward into Tertiary rocks and further as a paleobank of the Narmada River. Field studies have revealed that the NSF in the study area is divisible into four morphotectonic segments (I to IV), which is attributed to the Tilakwada, Karjan, Madhumati and Rajpardi Faults cutting across the NSF. We characterize the neotectonic activity of the segments through DEM analysis, using geomorphic indices which include the mountain front sinuosity, hypsometric curves, hypsometric integral, asymmetry factor, stream length–gradient index and channel sinuosity. Interpretations of morphometric data are well supported by field data. Both the entire length of the NSF and the cross faults are found to be neotectonically active. We demonstrate that segment II has undergone the highest intensity of neotectonic activity followed by segments III, I and IV. This is corroborated by the highest elevation of scarps and the alluvial deposits, the deepest incision (~40 m depth) and steep northward slope of the alluvial plain in segment II. We infer that the NSF is characterized by differential uplift in recent past which agrees with the high angle reverse faulting with oblique-slip movements along the cross faults. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Tectonic activity along active fault lines exerts a significant effect on the geomorphological properties of affected landscapes (Gordon, 1998; Giamboni et al., 2005). Quantitative analyses of landscapes utilizing various geomorphic indices are useful in determining tectonic behavior. Geomorphic indices have been found to be useful in identifying areas experiencing tectonic activity because they facilitate rapid evaluation of large areas (Strahler, 1952; Bull and McFadden, 1977; Keller et al., 2000; Keller and Pinter, 2002). Further, active faults and growing folds commonly have topography that is useful in identifying different geomorphic or structural segments along the fault and estimating the most active segments (Azor et al., 2002; Font et al., 2010). Segments along a morphostructure may be outlined and identified to determine the relative intensity of tectonic activity along a fault by utilizing a detailed study of drainage anomalies coupled with geomorphic indices (Azor et al., 2002; Keller and ⁎ Corresponding author. Tel.: +91 9824438424; fax: +91 265 2795329. E-mail address: [email protected] (D.M. Maurya). 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2012.10.023

Pinter, 2002). Moreover, data on geomorphic indices can be obtained easily with the help of remote sensing data and GIS software (Troiani and Della Seta, 2008). For example, the combination of the Shuttle Radar Topography Mission data and GIS technique is an effective, fast, inexpensive and precise way to perform morphometric analyses (Farr and Kobrick, 2000; Grohmann, 2004; Grohmann et al., 2007). There have been many attempts to systematically investigate geomorphic responses to tectonism with the help of geomorphic indices in various tectonically active areas or fault zones, such as the Southern Rhine Graben in Central Europe (Giamboni et al., 2004, 2005), the Normandy intraplate area of NW France (Font et al., 2010), Central Italy (Troiani and Della Seta, 2008), the southwestern USA (Bull and McFadden, 1977), the Pacific coast of Costa Rica (Wells et al., 1988), the Mediterranean coast of Spain (Silva et al., 2003), the Midcontinent of the USA (Adams, 1980), the Ventura basin of southern California (Azor et al., 2002), the Marrakech High Atlas (MHA) of Morocco (Delcaillau et al., 2010) and the Central Range Fault of Eastern Taiwan (Bruce et al., 2006). Here, we present a detailed quantitative geomorphological analysis of the Narmada–Son Fault (NSF) in Gujarat State, located in central to western India, a well-known but poorly constrained

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seismically active fault ~1000 km long (Fig. 1A, B). Our specific objective is to study the relative level of late Quaternary tectonic activity along the western part of the NSF. 2. Geological setting The NSF, which divides the Indian plate into two halves has a long tectonic history dating back to the Archean (Ravishankar, 1991). The NSF trends ENE–WSW and is laterally traceable for more than 1000 km from central India to the west coast (Fig. 1B). The Narmada River, which arises in central India and empties into the Gulf of Cambay, flows in the NSF zone throughout its course (Fig. 1B). Geophysical studies in the central part of this zone reveal this to be a deep-seated fault zone (Reddy et al., 1995). The zone witnessed large-scale tectonothermal events associated with large granitic intrusions approximately 2.5–2.2 and 1.5–0.9 Ga ago (Acharyya and Roy, 2000). It was reactivated during the Deccan volcanic eruption during the Late Cretaceous–Paleocene (Agarwal et al., 1995). The entire NSF zone is presently characterized by high gravity anomalies, high-temperature gradients and heat flows, and anomalous geothermal regimes (Ravishankar, 1991).

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In Gujarat State, the NSF is expressed as a single deep-seated fault confirmed by deep seismic sounding studies (Kaila et al., 1981). Seismic reflection studies indicate that the NSF is a normal fault in the subsurface, becoming markedly reverse near the surface (Roy, 1990). The reactivation of the fault in the Late Cretaceous led to the formation of a depositional basin in which marine Bagh beds were deposited (Biswas, 1987). The NSF has remained tectonically active since that time with continuous subsidence of the northern block, accommodating 6–7 km thick Tertiary and Quaternary sediments (Biswas, 1987). The total displacement along the NSF exceeds 1 km within the Cenozoic section (Roy, 1990). However, the movements along this fault have not been unidirectional throughout its history. The general tendency of the basin to subside has been punctuated by phases of structural and tectonic inversion (Roy, 1990). Compressive stresses oriented in N–S direction during the early Quaternary folded the Tertiary sediments to the south of the NSF into several ENE–WSW trending anticlines with steep reverse faults along the same trend (Fig. 1C). Historical and instrumental records indicate that compressive stresses continue to accumulate along the NSF because of the continued northward movement of the Indian plate, as evidenced by the earthquakes at Broach (March 23, 1970) and

Fig. 1. Location and geological setting of the study area. A) Location map. B) Geological map of the Narmada basin along the NSF. The area of the present study is marked. C) Simplified geological map of the study area. Note that the fault controlled the linear contact of the alluvial plain with the pre-Quaternary lithologies that form upland terrain, and the strong ENE– WSW fabric of the landscape developed over basaltic and Tertiary rocks. I to IV are the morphotectonic segments delineated in the present study.

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Jabalpur (May 22, 1997), which suggests a thrusting movement (Gupta et al., 1972, 1997; Chandra, 1977; Acharyya et al., 1998). Investigations into the late Pleistocene alluvial sediments exposed along the cliff sections of the Narmada River have suggested uplifting movements under compression during the late Quaternary (Maurya et al., 1997, 1998, 2000; Chamyal et al., 2002). Studies in the Kim River basin to the south of the NSF also indicate reactivation of reverse faults within the Tertiary rocks during the late Quaternary (Mulchandani et al., 2007). The NSF is, however, poorly constrained in terms of its geomorphic characteristics and neotectonic history. 3. Geomorphological setting Our study is confined to the narrow alluvial tract between the NSF scarpline and the Narmada River. This area covers approximately 5500 km 2 that is drained by several north-flowing rivers cutting across the NSF to merge into the Narmada (Fig. 1C). The study area is divisible into two major geomorphological domains — the upland area to the south of the NSF and the alluvial plains to the north (Fig. 1C). The uplands comprise mountainous landscape over the south dipping basaltic flows and relatively gentler hummocky topography over the folded and faulted Tertiary rocks in the west. The NSF is geomorphologically expressed as a linear series of ENE–WSW trending prominent north facing scarps (Fig. 2). The scarps delimit the rugged mountainous terrain over the south dipping basaltic flows of the Deccan Trap formation in the south and the northward sloping alluvial terrain to the north (Fig. 3A, B). The scarps in the west are ENE–WSW trending folds formed in the northern limbs comprising Neogene rocks (Fig. 1C). In the westernmost part of the NSF, a continuous ENE–WSW trending straight scarp comprising late Pleistocene sediments occurs, which has been described as the paleobank of the Narmada River in its estuarine reach (Agarwal, 1986; Maurya et al., 2000; Chamyal et al., 2002). The fault is therefore geomorphologically traceable up to the coastal zone in the west. Several NNW–SSE trending faults are mapped in the upland area to the

south of the scarps (Agarwal, 1986; Chamyal et al., 2002), which presumably extend up to the NSF and possibly beyond (Fig. 3). These include the NW–SE trending Tilakwada Fault along the Narmada River that appears to have displaced the scarpline by ~ 2.5 km. The other two comprise the NNW–SSE trending Karjan Fault and the Rajpardi Fault, which mark the tectonic contact between the basaltic flows and the Tertiary rocks (Figs. 1C and 3). During our field studies, we mapped another NNW–SSE trending fault at Tejpur (Figs. 2 and 4). This fault shows right lateral offset of the NSF of approximately 1 km. The presence of the fault is evidenced by the straight channel of the Madhumati River and the formation of a large, deeply incised and compressed meander in alluvium as the river emerges from the uplands (Fig. 2). The slickensides that are exposed along the fault plane in basaltic rocks on the left bank of the river suggest an oblique slip movement (Fig. 3C). Based on the transverse faults, we subdivide the NSF in the study area from east to west into four distinct morphotectonic segments (segments I to IV; Figs. 1, 2 and 4). Segment I is located between the Narmada and Karjan Rivers; segment II is between the Karjan and Madhumati Rivers, segment III is between the Madhumati River and the Rajpardi Fault; and beyond the Rajpardi Fault lies segment IV. The uplands, and particularly the basaltic uplands, show evidence of rejuvenation in the form of youthful topography, featuring narrow and deeply incised fluvial valleys with occasional gorges. The deformed Tertiary rocks forming the eastern part of the uplands comprise conglomerates, sandstone and limestone. This area shows comparatively subdued and structurally controlled hummocky topography. The rivers flow around the structural highs before swinging northwards to meet the Narmada. The alluvial plain to the north of the uplands that extends to the Narmada consists of unconsolidated fluvial sands, silts and gravels. In segment I, these deposits have been interpreted as small coalesced alluvial fans formed in front of the scarps (Bhandari et al., 2001). Recently, Joshi et al. (in press) has reported a late Pleistocene bajada formed by coalescing of several small alluvial fans in segment II based on sedimentological and

Fig. 2. Contour map of the study area with drainage and fault lines. Note the ENE–WSW trending north-facing scarps of the NSF and the gentle northward slope of the alluvial plain.

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Fig. 3. Pictures of the study area. A) South-viewing photomosaic showing the sharp physiographic contrast along the NSF. The scarps cut basaltic flows of the Deccan Trap Formation. The foreground is the incised alluvial plain. Arrows indicate downstream direction of the Nandikhadi River. B) View of a ~10 m high waterfall along the Nandikhadi River near the upland zone. South-dipping basaltic flows can be seen in the channel. C) Close view of the slickensided surface of the Madhumati Fault in basaltic rocks indicating oblique slip movement.

stratigraphical studies. The contour pattern in this segment (Fig. 2) conforms to the morphology of the bajada. Consistent with this, previous studies have indicated an age of the late Pleistocene for the fine grained alluvial deposits exposed along the Narmada River (Chamyal et al., 2002; Bhandari et al., 2005). A variety of geomorphological evidence, such as the youthful nature of the NSF scarps against which the alluvial sediments abut, the distinct northward slope of the alluvial plain, the deeply incised meandering rivers, the knickpoints, the waterfalls and the deep ravines in the vicinity of the river that were produced by extensive gully erosion, indicates the neotectonically active nature of the NSF

(Fig. 3). The role of neotectonic activity is further suggested by the fact that these geomorphic features do not fit with the ephemeral character of the rivers and the semi-arid to sub-humid climate with seasonal monsoon-controlled rainfall in the region. Our study focuses on the morphotectonic characterization of the alluvial plain to the north of the scarps to discuss the segmented nature and pattern of neotectonic activity along the NSF. The NSF zone shows dominantly transverse north-flowing parallel drainage lines (Fig. 3). Most of the rivers arise in the trappean uplands and incise through the alluvial plain before flowing into the Narmada. Several lower order streams arise from the Tertiary uplands to meet

Fig. 4. DEM-derived topographic image of the study area. Locations of topographic sections in Fig. 7 are shown.

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the major rivers coming from the trappean hills. The major drainage basins from east to west include the Shamlayakhadi, Karjan, Nandikhadi, Madhumati, Kaveri and Amravati basins (Fig. 2). The Madhumati River flows along a transverse fault trending N–S, then shifts westward upon entering the alluvial plain before turning north to northwest to meet the Narmada (Fig. 2). The Kaveri and Amravati Rivers originate in the western fringe of the trappean uplands and flow westward along structurally controlled courses composed of Tertiary rocks before swinging towards north to merge with the Narmada (Fig. 2). All the rivers are characterized by accelerated incision, asymmetrical catchments and anomalous diversions that may be used to evaluate recent tectonic activity (Cox, 1994; Jackson et al., 1998; Keller et al., 2000; Azor et al., 2002; Clark et al., 2004; Molin et al., 2004; Salvany, 2004; Schoenbohm et al., 2004). The drainage basins in the study area, including a few unnamed drainage basins, are numbered R1 to R10 from east to west (Fig. 2), and the basic morphometric parameters of the ten drainage basins are described in Table 1. Segment I includes three drainage basins, R1 (Shamlayakhadi), R2 and R3 (Karjan). R3 occupies the largest basin area (1525.9 km 2) with the highest stream order (7th). R1 is a 5th order basin and occupies the smallest drainage area, 7.5 km 2. Segment II also comprises three river basins, R4, R5 (Nandikhadi) and R6. The highest stream order is 5th (R4 and R6) and 7th (R5). The total areas of the R5 and R6 basins are almost equal (113.9 and 141.1 km2, respectively), with R4 occupying a smaller area (43.3 km2). Segment III is the smallest segment and includes the Madhumati basin (R7) that flows along the N–S trending transverse fault described above. R7 is a 6th order basin and occupies an area of 121.4 km2. The rivers of segments II and III show maximum incision in alluvial deposits (up to ~40 m) near the scarps, decreasing to 6–7 m in a straight distance of less than 3 km, which indicates the tectonically generated steep slope. The river basins of segment IV, R8, R9 (Kaveri) and R10 (Amravati), are controlled, for the most part, by structural features of the deformed Tertiary rocks. The highest stream orders for R8, R9 and R10 is 5th and the corresponding basin areas are 95.4, 106.2 and 322.0 km2, respectively. 4. Methodology Our study is fundamentally based on a detailed geomorphic analysis of the NSF zone in Gujarat using remote sensing and GIS. Additionally, extensive field observations have been an integral part of the study. This has helped in confirming, consolidating and interpreting the results obtained from the geomorphic analysis. The Shuttle Radar Topography Mission (SRTM) v.4 digital elevation model (DEM) with 90 m resolution was downloaded from CGIAR–CSI (Jarvis et al., 2008). The DEM is seamless because voids have been filled by interpolation techniques (Reuter et al., 2007), and is for regional scale geomorphic analysis (e.g., Grohmann et al., 2007; Rossetti and Valeriano, 2007; Rehak et al., 2008; Pedersen et al., 2010). The DEM was analyzed using GIS software, which is a fast

and inexpensive way to calculate morphometric parameters (Farr and Kobrick, 2000; Grohmann, 2004). Satellite image data from Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper+ (ETM +) were used to improve visual terrain interpretation. The basic processing steps were performed in ERDAS Imagine (v. 9.1), including functions for stacking, reprojection, mosaicking, and clipping. The Cartographic Reference System (CRS), Universal Transverse Mercator (UTM) zone 43 N and the WGS 1984 data were applied to all digital data to enable overlaying. For further analysis, we used ArcGIS (v.9.3). Ten drainage basins traversing the NSF zone were extracted from the DEM and contour maps were generated at 10 m interval for each basin. Subsequently, terrain analyses were conducted from the DEM and its derivatives, followed by calculation of geomorphic indices. 4.1. Terrain analysis Terrain analyses were conducted with the help of the DEM. This includes the analyses of the elevation map, slope map, aspect map, topographic profiles and longitudinal profiles of rivers. The slope map and aspect map were generated from the DEM in ArcGIS. DEM analysis is extremely useful for analyzing the elevation-related attributes of a fault zone area (Menges, 1990; Duncan et al., 2003). The tonal variation of an elevation map assists in the study of regional topography and structural features (e.g., Menges, 1990; Duncan et al., 2003; Hooper et al., 2003, Ganas et al., 2005). Topographic cross profiles aligning ENE–WSW and N–S were obtained from the DEM to visualize elevation characteristics in the upland and the alluvial zone. Longitudinal profiles of the trunk streams found in the 10 drainage basins located in the NSF study area were also obtained. River longitudinal profiles are one of the indicators of active tectonics because they reflect the balance between erosion and uplift (Schumm et al., 2000; Keller and Pinter, 2002; Molin and Fubelli, 2005; Menéndez et al., 2008; Bull, 2009). 4.2. Geomorphic indices Geomorphic indices sensitive to active tectonics (Keller and Pinter, 2002) were calculated, including the hypsometric curve, hypsometric integral (HI), mountain front sinuosity (Smf), asymmetry factor (AF), stream length–gradient index (SL) and river sinuosity (S). 4.2.1. Hypsometric curve and hypsometric integral The hypsometric curve and HI are useful in determining the drainage basin developmental stage (Strahler, 1952; Gardner et al., 1990). Convex-up hypsometric curves indicate a relatively young region, S-shaped curves characterize moderately eroded regions, and concave curves indicate a relatively old drainage basin (Strahler, 1957; Delcaillau et al., 1998; Keller and Pinter, 2002). The curve is created by plotting the proportion of the total basin height (h/H) against the proportion of the total basin area (a/A). We calculated these for individual drainage basins using the DEM in ArcGIS. HI is defined as the

Table 1 Basic properties of the drainage basins in the study area. Drainage basin

Basin area (km2)

Basin length (km)

Maximum elevation (m)

Mean elevation (m)

Minimum elevation (m)

Length of trunk stream (km)

Highest stream order (Strahler, 1964)

Asymmetry factor (AF%)

Hypsometric integral (HI)

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10

7.50 30.80 1525.91 43.37 113.93 141.06 121.46 95.41 109.20 322.02

9.99 8.51 64.02 13.90 18.57 18.48 28.09 16.40 23.01 41.10

783 406 849 348 425 433 397 158 220 311

341 180 396 166 209 189 188 77 93 129

29 17 13 15 13 3 6 3 3 1

14 13 100 18 31 25 42 27 36 75

V IV VII V VI V VI V V V

49 76 71 33 58 75 57 33 60 39

0.41 0.42 0.45 0.45 0.47 0.43 0.46 0.47 0.41 0.41

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relative area under the hypsometric curve that can be calculated as follows (Pike and Wilson, 1971; Mayer, 1990; Keller and Pinter, 2002):

(after Pérez-Peña et al., 2009),

Mean elevation−Minimum elevation Maximum elevation−Minimum elevation

(after Summerfield, 1991), where C is the elevation at river head, hf is the elevation at the river mouth, Lt is the total length of the river, γ is the unit weight of water per unit length (9800 N m −3), Q is the discharge (m 3 s −1) and s is the slope of the water surface, which is generally approximated by the slope of the channel bed. As gauging data for the rivers in our study area do not exist, Q was calculated by applying the power law relation between A and Q, assuming a uniform climate (Finlayson and Montgomery, 2003; Jain et al., 2006):

ð1Þ

4.2.2. Mountain front sinuosity (Smf) Smf is an effective parameter to differentiate tectonically active fronts from inactive fronts based on their degree of youthfulness (Bull and McFadden, 1977; Keller and Pinter, 2002). A tectonically active front typically shows less sinuosity, whereas inactive mountain fronts are characterized by high sinuosity that is attributed to the dominance of erosional processes over tectonic activity. Smf values were measured for segments I, II and III from the Landsat TM images. Smf is defined as the ratio of the length of the foot of the mountain front (Lmf) to that of the straight line approximately parallel to the mountain front (Ls) (Bull and McFadden, 1977; Keller and Pinter, 2002): Smf ¼ Lmf =Ls

ð2Þ

ΩT ¼ γQ s

Q ¼ aA

b

ð7Þ

where a and b are positive constants. 4.2.5. Sinuosity (S) S is an important parameter for the rivers located in tectonically active areas (Gomez and Marron, 1991; Keller and Pinter, 2002) because it reflects the secondary effect of uplift (Adams, 1980): S ¼ C=V

4.2.3. Drainage basin asymmetry (AF) AF is a parameter used to evaluate ground tilting in response to tectonic activity or lithological control at the drainage-basin scale (Hare and Gardner, 1985; Keller and Pinter, 2002; Dehbozorgi et al., 2010; Pérez-Peña et al., 2010) using the following equation:

ð6Þ

ð8Þ

where C is the channel length and V is the valley length. All drainages in the study area show highly sinuous channels in the alluvial plain (Fig. 3). The channels were divided into specific reaches for sinuosity measurements. 5. Results

AF ¼ 100ðAr =At Þ

ð3Þ 5.1. DEM analysis

where Ar is the area of the basin to the right of the trunk stream (facing downstream), and At is the total drainage basin area. 4.2.4. Stream length–gradient index (SL) SL is used to infer stream power and rock erodibility (Hack, 1973) because it is sensitive to minute changes or perturbations in the channel slope (Burbank and Anderson, 2001; Harkins et al., 2005; Font et al., 2010).

SL ¼ ðΔH=ΔLÞL

ð4Þ

where ΔH / ΔL is the local channel slope or gradient and L is the length of the channel from the midpoint of a segment in which SL is calculated to the highest point in the channel. SL is used to identify recent tectonic activities. Uplift zones are indicated by anomalously high SL values with a specific rock type and within a particular drainage segment (Merritts and Vincent, 1989; Brookfield, 1998; Azor et al., 2002; Keller and Pinter, 2002; Chen et al., 2003; Troiani and Della Seta, 2008). In this analysis, the trunk stream of each drainage basin was segmented at a contour interval of 10 m. SL was calculated for elevations below 250 m to maintain the uniformity of the data along the NSF zone. Because we focused on neotectonic influences on the alluvial terrain, SL values below 100 m were interpreted for streams dissecting the alluvial plain to the north of the scarps. SL was analyzed with longitudinal profiles to infer the role of lithology and tectonic activity. Graphs were prepared to understand the SL distribution within the alluvial reaches. Pérez-Peña et al. (2009) explored the relation between graded river gradient (K), SL and total stream power (ΩT), and confirmed that SL and K have the identical formulation when applied to the entire river profile. The relationship between K and ΩT were calculated for each drainage basin by applying the following equations: K ¼ C−hf = lnLt

ð5Þ

Various geomorphic features were observed in the study area, such as the continuous ENE–WSW trending NSF scarp (Fig. 4). In front of the scarp, the highly rugged surface represents an alluvial plain which is superimposed upon by north-flowing parallel drainages (Fig. 4). It is clear that the drainages of segment II are deeply incised compared to the drainages of the other segments. The prominent straight courses of the Karjan and Madhumati Rivers near the scarp are attributed to the Karjan and Madhumati Faults, respectively, which cut across the NSF. Another prominent ENE–WSW trending linear feature in the western part of the NSF is the alluvial scarp that represents the paleobank of the Narmada River (Fig. 4). In the slope map (Fig. 5), the abrupt change in slope across the landscape corresponding to the ENE–WSW scarpline of the NSF stands out. In segments I, II and III, the slope break is 19.4° in the vicinity of the front scarp, as manifested in the tonal transition in Fig. 5; whereas, in segment IV, the slope break drops to 5.4°. In addition, the slope changes significantly across the channel reach in alluvium (3°– 4°). The ravines along the Narmada in the north also present a visible change in slope (2°–3°). The aspect map (Fig. 6) shows that the majority of pixels in the vicinity of the scarp evidence a strong northward slope. In the uplands, southerly dipping pixels verify the southward tilting of the basaltic flows (Fig. 6). 5.2. Analysis of topographic profiles The highly variable relief in these ENE–WSW and N–S topographic profiles corresponds closely to the structural framework. The ENE– WSW topographic profile of Fig. 7A shows sharp contrasts in elevation among the segments. The scarp attains maximum elevation in segment II between the Madhumati and Karjan Rivers, whereas it is relatively subdued in segment I between the Karjan and Narmada Rivers, and in segment III between the Madhumati River and Rajpardi Fault. The ENE–WSW topographic profile for the alluvial plain to the

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Fig. 5. Slope map of the study area. Slope values are in degrees. Arrows indicate the steep northward slopes formed in alluvial plain close to the NSF scarps.

north of the scarps (Fig. 7B) shows that the maximum elevation occurs in segment II between the Madhumati and Karjan Rivers. By contrast, the alluvial plain in other segments is at a lower elevation. The convex-up shape in the topography of segment II appears to be a typical alluvial fan, and the fan-shape is also indicated by the shape of the contours (Fig. 2). The N–S topographic profiles clearly show the northward tilt of the alluvial plain (Fig. 7C). The elevation of the alluvial plain is ~40 m near the Narmada, rapidly increasing to ~80 m near the NSF scarp. The northward slope of the alluvial plain is visibly

greatest between the Madhumati and Karjan Rivers (segment II) compared to the other segments (Fig. 7C). 5.3. Analysis of river longitudinal profiles In general, the longitudinal river profiles show steeper gradients in the uplands reach and lower gradients in the alluvial reach. Most of the streams exhibit knickpoints in the resistant rocks of the uplands (Deccan basalt and Tertiary rocks) and concave-up shapes

Fig. 6. Aspect map of the study area. Note the dark pink colored pixels along the NSF zone indicating northward slope.

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Fig. 7. Topographic profiles. A) Slightly to the south of the NSF. B) Alluvial terrain to the north of the NSF. C) N–S oriented profiles. Locations of the profiles are shown in Fig. 4.

with straight reaches in the less- or non-resistant alluvial lithology (Fig. 8). The knickpoints are located close to the intersection of channels and the NSF. Pronounced variations in the morphology of the channel profiles are observed in different segments of the NSF (Fig. 8). The longitudinal profiles of R1, R2 and R3 in segment I show steep gradients in the upland reaches, which contrasts with the graded, but steeper profiles in the alluvial reach. The longitudinal profile of R3 shows a higher degree of grading because it is a higher order stream that also forms the largest drainage basin in the area. The profiles show good correspondence with the weaker incision in segment I compared to the other segments. In segment II, the longitudinal profiles of R4 to R7 show distinct convex-up morphology in the alluvial plain. Additionally, in segment IV, the convex-up morphology is observed in the longitudinal profiles of R8 to R10 (Fig. 8). 5.4. Hypsometric curve and hypsometric integral The hypsometric curves show dominantly concave-up shapes. However, distinct convex-up shapes appear in at least in one basin in each segment, such as R1 in segment I, R5 in segment II and R9 in segment IV (Fig. 9). The convex parts correspond to the younger stage of erosion. The mean values of HI of segments I, II and IV are 0.42, 0.45 and 0.43, respectively. The highest mean HI value for

segment II is associated with convex hypsometric curves that present prominently in R5 (Fig. 9). 5.5. Mountain front sinuosity The relatively young age of the fault-generated scarps is quantified by analyzing Smf. The range bounding NSF scarps have yielded low Smf values that classify the entire front as class I, which is the highest class of tectonic activity, as defined by Bull and McFadden (1977). Smf values measured from Landsat TM images are 1.04, 1.03 and 1.17 for segments I, II and III, respectively (Fig. 10). 5.6. Drainage basin asymmetry Based on the AF values for the ten drainage basins (Table 1), the directions of ground tilting are inferred (Fig. 10). In segment I, the R1 basin is nearly symmetrical, whereas R2 and R3 are strongly asymmetrical, suggesting westward ground tilting (Fig. 10). In segment II, R4 basin is inferred to tilt eastward, whereas R5 and R6 seem to tilt westward (Fig. 10). The AF for R7 basin in segment III suggests westward tilting, which is in continuity with the adjacent part of segment II. In segment IV, R8 is strongly asymmetric with suggested eastward tilting, while R9 shows westward tilting (Fig. 10).

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Fig. 8. Longitudinal profiles of the trunk streams of drainage basins R1 to R10. Note the steepened reach of the profiles in the vicinity of the NSF. Circles show the locations of anomalous high values of the stream length–gradient index (SL) shown in Table 2.

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Fig. 9. Hypsometric curves of the drainage basins R1 to R10 traversing the NSF. The values of hypsometric integral (HI) are also shown.

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Fig. 10. Map of drainage basins R1 to R10 and the direction of tilting inferred from drainage basin asymmetry (indicated by black arrows). Mountain front sinuosity (Smf) values for the respective segments are also given. Values of drainage basin asymmetry are shown in Table 1.

Fig. 11. SL map for drainage basins R1 to R10. Dashed lines show inferred different sinuosity zones.

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Fig. 12. Variation in SL values in the alluvial plain. X: elevation (m). Y: SL.

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5.7. Stream length–gradient index 5.7.1. Segment I In segment I, R1 shows relatively high SL values throughout its alluvial reach, with two prominent channel segments of 60 to 50 m and 90 to 80 m in elevation, with anomalously high SL values of class 3 (100–500; Figs. 11 and 12). These high values correspond to the steep channel gradient developed within the alluvial reach, as the longitudinal profile shows (Fig. 8). In R2, classes 1 (10–50) and 2 (50–100) have low SL values (Table 2). However, relatively high SL values occur at the channel segment of the 80–90 m elevation that coincides with the trap–alluvium contact, which also can be observed in the longitudinal profile (Figs. 8 and 11). R3, the largest drainage basin of segment I, shows a significant contrast in SL with the trappean and alluvial reaches. The channel segments of the trappean reaches are characterized by low SL values of classes 1 and 2, whereas, within the alluvial reach, they present high SL values of class 5 (>1000; Table 2). The drastic increase in SL occurs at the channel segment of 80 to 90 m elevation at the interface of the trap and alluvium (Figs. 8, 11, and 12). 5.7.2. Segment II In segment II, drainage basin R4 has low to intermediate SL values of classes 1, 2 and 3 throughout the reach (Table 2). There are two channel segments of 90 to 80 m and 50 to 40 m elevation within the alluvial reach in which sudden increase of SL can be observed. The high SL value of the former coincides with the trap to alluvium transition, while the steep channel gradient in the longitudinal profile coincides with the latter (Figs. 8 and 12). Similarly, drainage basin R5 shows two segments with anomalously high SL values of classes 5 and 3 for 100 to 90 m and 50 to 40 m elevations, respectively (Figs. 11 and 12). The SL value of class 5 coincides with knickpoint located at the trap to alluvium transition and the second highest value of class 3 overlaps with the steep channel gradient in the alluvium and visible in the longitudinal profile (Fig. 8). R6 is the smallest drainage basin of the study area that shows low SL values of class 1 throughout the reach (Table 2). However, a relatively high SL value is noted at 50 to 40 m elevation, where it increases abruptly from 18.03 to 41.29. This increase in SL coincides with the steep convexity of the longitudinal profile within the alluvial reach (Figs. 8 and 12). 5.7.3. Segment III In R7, channel segments of the trappean reach have predominantly low to intermediate SL values of classes 1 and 2, increasing to Table 2 SL values calculated for the drainage basins in the study area. Elevation range (m)

R1

R2

R3

R4

R5

R6

R7

R8

R9

R10

240–250 230–240 220–230 210–220 200–210 190–200 180–190 170–180 160–170 150–160 140–150 130–140 120–130 110–120 100–110 90–100 80–90 70–80 60–70 50–60 40–50

245 645 154 106 61 165 81 160 88 669 117 238 84 173 171 131 164 100 115 141 111

113 117 48 89 170 184 199 213 246 106 79 60 211 127 46 63 93 49 38 39 38

39 34 40 67 67 49 45 49 64 93 129 203 60 145 21,314 6225 21,146 20,514 18,168 15,626 13,926

10 17 24 31 47 121 173 167 60 51 137 112 43 55 55 42 69 54 92 46 67

106 47 23 53 40 55 54 281 75 51 235 159 191 1426 122 1260 53 73 83 84 109

– – – – – – – – – – – – – – – 17 23 20 28 18 41

10 13 14 14 37 71 36 36 196 66 162 36 1867 469 81 100 187 255 176 767 1710

– – – – – – – – – – – – – – 11 16 28 40 40 44 37

– – – – – – – – – – 27 22 20 16 24 33 24 26 32 42 49

28 28 28 23 30 24 28 30 29 61 46 60 52 56 47 75 80 50 52 904 714

classes 3, 4 and 5 in its alluvial reaches (Fig. 11). Within the alluvial reach, two distinct segments of high SL values occur at elevations of 80 to 70 m and 50 to 40 m, coinciding with the steep channel gradient in the longitudinal profile (Figs. 8 and 12). 5.7.4. Segment IV R8 is the smallest drainage basin of this segment and has low SL values of class 1 throughout the reach (Fig. 11). However, there is a gradual increase in SL values from the upper to lower reaches, and an abrupt increase occurs at a channel segment at 60 to 50 m elevation. This abrupt increase coincides with the steep channel gradient in the longitudinal profile where its course is influenced by the Rajpardi Fault and the Jhagadia anticline developed in the Tertiary rocks (Figs. 1C and 11). A similar distribution of SL is also observed in R9 and R10. SL is generally low, but sometimes locally high within the alluvial reach (Fig. 12). 5.8. Sinuosity The measurements reveal alternating zones of relatively high and low sinuosity, roughly trending E–W, in the alluvial terrain to the north of the NSF (Fig. 11). In segment I, the zone of lower sinuosity is located close to the scarpline, whereas the higher sinuosity zone is located close to the Narmada. In segment II, the alteration of sinuosity is also clear, with a narrow zone of lower sinuosity near the scarpline. Notably, the two zones of low sinuosity in segment II correspond to the zones of high SL values. This suggests a negative correlation between sinuosity and SL. In segment IV, in spite of the extremely narrow alluvial plain between the Tertiary uplands and the Narmada River, zones of low and high sinuosity are found (Fig. 11). 6. Discussion Remote sensing and GIS-based morphometric analyses suggest a dominant but spatially variable intensity of neotectonic activity along the NSF. The abrupt change in slope corresponding to the NSF and northward slope of the alluvial surface are indicators of active faulting (Fig. 5). The variable morphology of the longitudinal profiles also reflects tectonic control. For example, the long profiles of the rivers in segment IV show convex-up morphology as much of their courses fall within the deformed Tertiary rocks (Fig. 8). The sigmoidal shape of hypsometric curves suggests a moderately eroded landscape and an early mature stage of erosion (Fig. 9); convex features indicate a relatively young stage related to neotectonic rejuvenation. The highest mean value of HI in segment II confirms a higher level of tectonic activity (Fig. 9). Smf values indicate the young age of the NSF scarp. However, variations in the Smf values of various segments correspond to spatial variation in the magnitude of neotectonic activity along the NSF. Low

Fig. 13. Relationship between total stream power (ΩT) and graded river gradient index (K) for basins R1 to R10.

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Smf values for segments I and II are interpreted to indicate the most tectonically active segments of the NSF (Fig. 10). The drainages in the alluvial plain are strongly asymmetric, which conforms to the morphotectonic setting. For example, in segment II, eastward tilting in the eastern part and westward tilting in the western part correlate with the contour pattern and relief of the alluvial surface (Figs. 2, 7B and 10). Westward tilting in segment III conforms to the westward swing of the course of the Madhumati River (Fig. 2). Anomalous tilt directions in segment IV may be attributed to the complexly deformed Tertiary rocks (Fig. 10). Spatial variations in SL along the NSF suggest varying intensities of neotectonic activity. Lithological control is ruled out because of the nearly homogeneous composition of unconsolidated alluvial sediments in all the segments. The consistently high SL values at the trap–alluvium interface possibly reflect the probable location of the NSF in the shallow subsurface (Figs. 11 and 12). The linear correlation between K and ΩT (Fig. 13) may reflect the effects of A and Q. If A increases then Q, K and ΩT also increase. However, in the study area, some basins with low A and ΩT have high K, leading to scatters in Fig. 13. We infer neotectonic activity as the reason for such an anomaly. The lower sinuosity values near the NSF are the result of the steeper alluvial surfaces generated by neotectonic activity. By contrast, the highest sinuosity values are observed near the Narmada River (Fig. 11), because the rivers have attained equilibrium by forming high-sinuosity channels. We believe that the negative correlation between the sinuosity and the SL may be the result of the tendency of rivers to flow straight in response to an increase in gradient of the downstream direction before adjusting and settling down to a stable sinuous course. Overall, the various landscape parameters and the deeply incised drainages indicate the uplift of the alluvial plain with prominent tilting of the alluvial surface away from the NSF. We attribute this to a differential uplift along the NSF with reverse faulting under compressive stresses. The data presented shows that highest level of neotectonic activity occurred in segment II, followed by segments III, I and IV. 7. Conclusions The quantitative geomorphic approach has revealed spatial variations in neotectonic activity in various segments of the NSF. The results of our study indicate that segment II has undergone the highest intensity of neotectonic activity, followed by segments III, I and IV. We conclude that late Quaternary tectonic activity along the NSF was dominated by vertical uplifting, while the cross faults have undergone oblique slip movements, leading to segmentation of the NSF and the development of morphotectonic zones with different geomorphic characteristics. The style of neotectonic activity along the NSF is, therefore, more complex than previously thought. Acknowledgments The study is a part of the research project MoES/P.O.(Seismo)/ 23(638)/2007 funded by the Ministry of Earth Sciences, Government of India. Parul Joshi acknowledges a summer research fellowship awarded by the Science Academies of India. Help rendered by Dr. P.S. Roy and Dr. S.K. Srivastava in carrying out this study is gratefully acknowledged. Constructive reviews by Prof. T. Oguchi and two anonymous reviewers were useful for improving the quality of the paper. References Acharyya, S.K., Roy, A., 2000. Tectonothermal history of the Central Indian tectonic zone and reactivation of major faults/shear zones. Journal of the Geological Society of India 55, 239–256. Acharyya, S.K., Kayal, J.R., Roy, A., Chaturvedi, R.K., 1998. Jabalpur earthquake of May 22, 1997: constraint from a aftershock study. Journal of Geological Society of India 51, 295–304.

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