Relative active tectonics evaluation using geomorphic and drainage indices, in Dadra and Nagar Haveli, western India

Nveen Kumr , Rkesh K. Dumk , Kpil Mohn , Sumer Chopr

a Institute of Seismological Research (ISR), Raysan, Gandhinagar, Gujarat, India

b Department of Earth Science, Gujarat University, Navrangpura, Ahmedabad, 380 009, India

ABSTRACT The present study area, Dadra and Nagar Haveli, contains several lineaments and traces of active faults.The various aspect of the geomorphic analysis,i.e.,stream length(SL)gradient,hypsometric integral(HI),basin shape (BS), valley floor (VF), have been applied to evaluate the relative index of active tectonics(RIAT) of the Damanganga watershed. The high and low zones of tectonic activity have been identified based on the geomorphic analysis of the watershed.After evaluation of all indices,three classes,class IIhigh (1.3 ≤RIAT ＜1.5), class III- moderate (1.5 ≤RIAT ＜1.8), and class IV- low (1.8 ≤RIAT), have been obtained to outline the degree/gradation of comparative tectonic activities in the study area. The appraised outcome of the RIAT dispersal is also well reinforced by the geomorphic evidence in the field.The collective outcomes of geomorphic evidence, such as stream deflection and analysis of lineament,deflection of streams, and geomorphic indices, conceal that the Damanganga watershed is affected by tectonic activity.© 2022 Editorial office of Geodesy and Geodynamics. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Active tectonics Geomorphic indices Relative index of active tectonics (RIAT)Drainage indices

1. Introduction

Tectonic geomorphology is a growing field in earth science due to the inclusion of unique tools like geodetic, geomorphology,geochronology,etc.Further,these tools support calculating a rate of deformation,upliftment,incision,erosion,and rates of slip on faults at changing timescales [1-11]. In a tectonically active area, the geomorphic indices like drainage pattern, stream deflection, river incision are most likely influenced [5]. They are vital signs and capable of interpreting the responses of landform to mobile deformation activities and are used broadly as an investigating tool to distinguish the areas deformed by tectonic activities [12-14].These geomorphic indices of active tectonics are actively used and established as a basic investigating tool to recognize whether an area is undergoing or influenced by rapid tectonic deformation[15]and provides elementary investigation tools to recognize tectonically active zones on a regional scale[16]with their level of tectonic activity and vulnerability to tectonic deformation [17].

In the present study,the geomorphic indices to assess the range of tectonic activities are computed.The present location is situated in the northern part of the Konkan coastal belt in Dadra and Nagar Haveli(U.T)(Fig.1)in western India.The Dadra and Nagar Haveli is a union territory developing as an industrial hub. The tectonic activity in the area is reflected by the presence of low magnitude(＜5)earthquakes on the lineaments as well as N-S striking Panvel flexure fault conceals the rejuvenation of these structures[18].The GPS measurements of 1996-2015 advise that the Panvel flexure fault zone moves at a rate of approximately 3±1 mm/year towards northward (near Mumbai region) [19] signifying that the Panvel flexure zone is tectonically active.Therefore,it is essential to mark the presence of neotectonic movement (if any) by applying basin shape index (Bs), hypsometric integral (HI) and curve, the stream length-gradient index (SL), and valley floor width to valley height ratio (Vf) in the area to evaluate the RIAT in the DNH region.

However, there are no significant studies related to the geomorphological evolution of the study area by using the geomorphic and drainage indices(Konkan coastal belt);hence,this is the first study in which the relative tectonic evolution of the area done.This study will be important to know the seismotectonics of the region to assess the seismic hazard specifically. Hence, the present paper is targeted to study(i)tectonic evolution of the area by using the geomorphic and drainage indices,(ii)to categorize the area in low and high zones of tectonic activities using geomorphic indices,(iii)to interpret the possible relationship between different indices and geomorphological features in the study area, (iv) to correlate these observations with regional tectonics.

Fig.1. Geological map of t he study area.

2. Geological setting and study area

The study area consists of Deccan trap basaltic terrain and a thick (～10-20 m) quaternary pile observed alongside the major river in the study area[18].Tectonically,the study area is confined by the Panvel flexure (PF) in the central part, the west coast fault(WCF)in the west,and the western Ghats escarpment(WGS)in the east [18] (Fig.1). The westerly dipping Deccan trap flows in south Gujarat are prevailing as a monocline flexure structure in the Kalyan and Panvel region of Mumbai and are commonly known as Panvel flexure,is faulted along the axis and having some hot water springs along with it [20]. The development of PF and WCF are linked with a tectonic episode of the basement during the early Tertiary [21-23].

3. Seismotectonics of the study area

The Dadra and Nagar Haveli region has been experiencing earthquake events since 1856. Two historical events (M5 in 1935 and M5.7 in 1856) were reported from the southern part of the study area [24-26]. The present-day seismicity in the study area and surrounding confirms the active nature of the existing tectonic structures;the earthquake epicenters are concentrated along with these tectonic structures [18] (Fig. 2). A significant number of earthquakes between M1 to 5.7[24,25]were recorded in the study area (Table 1). The tectonic disturbance is already evident by numerous earthquakes in western India [26-38]. The geodetic studies also recognize a significant level of deformation occurrence to the western part of the Indian plate, although the magnitude of deformation is very low as compared to the plate boundary region of the Indian plate [39-51].

Fig. 2. (a) Seismotectonic map of western India (b) Seismotectonic map of the study area.

4. Methodology

The technique of remote sensing (RS) and geographic information systems (GIS) are used to understand geomorphology,tectonics, and structural studies. Stream network and the watershed boundaries were demarcated using SOI toposheet no. 46D/16 and 46H/4 (1:50,000) and SRTM DEM (30 m) (https://earthexplorer.usgs.gov/) under the GIS environment. The linear feature recognition, image processing, generation of the false colour composite image,and shaded relief maps preparation were done on a GIS platform.The geomorphic indices,i.e.,SL,HI,Vf,Bs,were calculated using DEM and Google Earth imagery by the formulas(Table 2).The plate interior of the Indian plate comprises complex tectonics and lithological framework developed in the long geological past [4-7,34,36]. The active tectonics plays an important role in the development and modification of these features. It is now established fact that the morphotectonic indices i.e., SL, HI, BS, VF, are continuously applied to calculate relative index of active tectonics(RIAT)[7,34,36].After evaluating all indices,the sub-watersheds were categorized into three classes based on the index value. All the indices were combined and divided into the number of indices to classify every sub-watershed according to the relative index of active tectonics. Stream deflection parameters,lineament/fault-map/field evidence were used to back the result.

5. Result and discussion

The indices including SL, Vf, HI, and Bs have been analyzed and combined to produce the relative index of tectonic activity (RIAT).

5.1. SL index

The SL index is a significant tool to evaluate the comparative tectonic activity of an area. The tectonic, lithological, or climatic factors may be responsible for the deviation of the river profile from its steady-state[51].The soft bedrock with a high value of SL is the sign of tectonic activities in recent.The low values of SL in the area represents strike-slip tectonic motion, and streams are flowing through it maybe due to tectonically activities [16]. The rocks of constant resistance with a high SL index value or if there is the flux of SL values also indicate that,the area is active concerning tectonic[17]. The value of the SL index has been calculated for the main streams(some examples are shown in Fig.3)of all sub-basins using google earth imagery and ASTER DEM with a spatial resolution of 30 m. The values obtained were categorized into three; Class 1(SL ≥600),Class 2(300 ＜SL ＜600),Class 3(SL ≤300)(Fig.3).The sub-basins 1,4,21,38,40,52 and 54 falls in class-I,the sub-basins 2,3,5,16,17,18,19,35,39 and 51 falls in class-II and sub-basins 6 to 15,20,22 to 34,36,37,41 to 50 and 53 falls in class-III(Fig.3).The comparative analysis reveals the existence of moderate and high activity in the eastern and northern parts,respectively.The western and central part is comparatively tectonically less active with a relatively high SL value (Fig. 4).

Table 1 Earthquakes with magnitude ＞3 in the study area from 1856 to 2019.

Table 2 The formulas used to measure the different parameters of geomorphic indices in the present study.

Fig. 3. SL index value for all sub-basins.

5.2. Vf index

Fig. 4. SL anomalies map of the study area.

The Vf index is considered to distinguish between V-shaped valleys formed in reaction to active upliftment and U-shaped flatfloored wide valleys developed as a response to the base level stability[8,52].The episode of uplift is related to incision by rivers,whereas low Vf is related to advanced uplift and incision rate.The Vf value of ＜1 is linked to the V-shaped valleys, continuous down cutting,linear-streams feature with an exposed active uplift.The Vf value of ＞1 is related to flat or U valleys, which shows the attainment of base-level erosion mostly in reaction to comparativetectonic dormancy [13,16]. The master streams of sub-basins were used to calculate the Vf in the present study after portraying a cross-section depicting in the DEM. Here also, three classes were categorized; Class 1(Vf ≤0.5), Class 2 (0.5 ＜Vf ＜1.0), Class 3(Vf ≥1.0).The results of our analysis indicate that most of the part falls under Class 1 with V- shape and hence reveals a high rate of tectonic activity (Fig. 5).

5.3. HI index

The HI is an index that is independent of the basin area and is normally derived for a specific drainage basin. The HI generally defines the elevational distribution of a certain area of land, predominantly a drainage basin [53]. The HI index is alike to SL in which rock strength affects the value. The elevated HI is probably associated with recent tectonic activity, while the low values denote mature landscapes that have been more eroded and less affected by current active tectonics [53] (Fig. 6). We calculated HI for each sub-basins (using the equation given in Table 1). After careful observation of the hypsometric curve in terms of the convexity and concavity, the HI can be classified into three classes,Class 1(HI ≥0.5)with concave curve;Class 2(0.4 ＜HI ＜0.5)with concavo-convex curve;Class 3(HI ≤0.4)with the shape of convex curve(Fig. 7).

5.4. Bs index

The elongated shape of drainage basins specifies comparatively it to young in regions of active tectonic.Through tectonic activities moderates with the period and with constant topographic development,the elongation becomes circular shape basins[2,15].The value of the width of sub-basins differs from one location to other therefore the average value was considered to evaluate the shape of the basin. The results show that the high values of Bs are related to the basins with elongated shapes, usually linked with comparatively advanced tectonic activity, and low values of Bs designate to basins with a circular shape mostly linked with the low tectonic activity [54]. The basin shape index (Bs) includes three classes: (Class 1) Elongate basin with Bs ≥4; (Class 2) Semielongate basin with 3≤Bs ＜4; and (Class 3) Circular basin with Bs ＜3 [66] (Fig. 8).

5.5. Discussion on relative index of tectonic activity (RIAT)

The Deccan flood basalt exploded at ～65 Ma and concealed＞500, 000 km[23,24,30,31]. The previous studies attributed the witnessed inconsistencies basically to climate-change, geomorphology, and fluvial systems, sea-level fluctuation, and only dedicated to Deccan upland region with involvement from neotectonic movements [55-61]. In the current study, we made an effort to appraise RIAT (relative active tectonics). The average values of the computation are combined to calculate RIAT and then evaluated the spatial range and distribution of tectonic activities of the area. The range of RIAT obtained by combining all the indices is clustered into three classes to explain the degree of relative tectonic activities in the area, which are as follows: Class II,high(1.3 ≤RIAT ＜1.5);class III,moderate(1.57-1.86);and class IV, low (2.0-2.33) relative tectonic activity respectively. The dispersal of the four RIAT classes is shown in (Fig. 9). The subbasins 2, 21, 42 and 44 belong to class II (high activity); the subbasins 1,3,4,8,43,and 52 belong to class III(moderate activity);and the remaining all sub-basins belong to class IV(low activity).The RIAT tends to be high along the upper Godavari fault (UGF),along the Western Ghats escarpment,new faults,and lineaments mapped in the study area (Fig. 9).

Fig. 5. Classes of Vf and its distribution for the studied basins.

Fig. 6. The hypsometric curve of four different HI values. (a) HI = 0.562, (b) HI = 0.539, (c) HI = 0.485, and (d) HI = 0.248.

Fig. 7. HI classes for the studied sub-basins.

Fig. 8. Bs classes for the studied sub-basins.

Fig. 9. Distribution of relative index of active tectonics (RIAT) in the Darda and Nagar Haveli and surroundings.

6. Interpretation of field evidence

The river valleys of the study area are extremely narrow in the central and eastern portion,near to the WGE,and their V-shaped presence often gives the imprint of their being fluvial in origin.To develop such narrow,deep-floored valleys(0.088 ＜Vf ＜5.10),it is of extreme significance that the base level of the area has been disturbed in a short time interval. As suggested by [61], the western upland region of the Deccan trap has undergone differential upliftment during the Quaternary period. If the western upland region has experienced the constant and prolonged uplift during Neogene and Quaternary, then the results of the current study in the study area also suggest evidence of uplift. The engraved bed-rock meandering with a series of gorge-headknick-points are reported in the Damanganga river basin. The carved or engrained-meanders, as well as knick-points, are a common phenomenon or a good indicator of tectonic disturbance[62]. Some distinguished knick-points, internal channels, waterfalls, and potholes are found in the Damanganga river/tributaries(Fig. 10). The occurrence of waterfall with the gorge-head indicates the migration of upstream migration of knick-points and may be related with the vicinity of uplift.The major waterfall with an elevational difference of 12-36 m are shown in Fig.10.

Numerous marks of vertical displacement and several rock deformations were witnessed in the study area. However, the chronology of tectonic is hard due to the absence of dates, as the area is concealed by Basaltic flows of the Deccan trap along with the scanty patches of Quaternary and recent deposits along the river course(Damanganga river and its tributaries)[63].Although the presence of soft sedimentary deformational structures(SSDS),seismites indicate the signature of the past earthquake [18]. The presence of seismites in the Quaternary sediments suggests that the study area has been affected by neotectonic activities [18,64].The slickenside bearing fault planes, brittle shear zones, extensional features,and deformed dykes has been reported by[64].On the basis of their origin,proximity with the major structural trend,presence of water spring,linear valleys and presence of seismicity reveals that the study area has experienced the neotectonic activities and is still seismically active. The field evidence like faulting, river deflection, presence of palaeochannels and presence of waterfalls/ponding in the course of streams have been reported from the study area. The palaeochannel along the Damanganga River is well preserved, and is easily identifiable on the ground (Fig. 11a). It is possible to trace the channel over a distance of 3.0 km in length and ～1.0 km in width.The sediments underlying the palaeochannel are sandy gravels, sandy silt, and silty clay;there is uniformity in sediment deposition(Fig.11 b,c,d,and e).An N-S profile(Fig.11 g)across the palaeochannel and the current channel revealed a steep elevation difference of 12-15 m between the palaeochannel and the current river channel.During geological investigations conducted in the riverbed, an E-W trending normal fault had been inferred (Fig.11f).

The fault surface was marked by the remarkable development of slickensides(Fig.11 i).The slickensides were steeply inclined(68)in the S-SW direction on the fault surface(Fig.11 i).The smoothness of the fault surface indicates downward motion in the block.However,due to bedrock homogeneity,the amount of downthrow is not determinable.

Fig.10. Major knick points and river meandering in Dmanaganga River basin, (a) Barundi river meander and (b) view of ～20 m high from waterfall on Barundi river, (c) view of～12 m high from Rayate waterfall, (d) view of ～36 m high from Dabhosa waterfall.

Fig.11. (a) Palaeochannel and present-day channel of River Damanganga with associated faults and lineaments, (b)stratified Palaeochannel sedimentary deposits exposed on the right limb of fossil river, (c) the stratigraphy of alluvial deposits exposed in the right limb of Palaeochannel of Damanganga River, (d) stratified Palaeochannel sediment deposits exposed on the left limb of fossil river, (e) the stratigraphy of alluvial deposits exposed in the left limb of Palaeochannel of Damanganga River, (f) E-W geological cross sections along Palaeochannels, (g) N-S geological cross sections along Palaeochannels and Damanganga River, (h) an E-W longitudinal profile of river, (i) close view of the slickensided surface of fault in Basaltic rock type indicating downslip movement.

Abrupt deviation of streamflow course is a stream anomaly that occurs maybe because of tectonic instability/deformation like faulting or folding or may be due to sub-surface structure or change in lithology.Although the tectonic instability/deformation rate within the intra-plate region is very low, some recent geodetic studies revealed 2-3 mm/yr of deformation towards the western part of the Indian plate and concluded that the rate of deformation is significant[8-11].These geodetic results infer the occurrence of deformation in the present study area located towards the western part of the Indian plate. The present-day seismicity record of this part is also in concurrence with the geodetic results [24-38]. Therefore, the analysis of stream networks becomes an essential technique to understand the tectonic geomorphology in the study area [65]. The deflection of the river channel takes place while it flows through fault because of the differential erosion which took place in a fault zone. The areas is marked in the circle present anomalies i.e., river channel deflection (Fig.12).

7. Conclusion

The evaluation of geomorphic indices is a dominant tool for appraising the effect of active tectonics.This technique is useful for the Darda and Nagar Haveli and its neighboring regions, where relatively no study on active tectonics is not available. The assessment of geomorphic indices reveals the spatial distribution of the neotectonic activities in the study area. In the present study, we estimated the relative active tectonics from drainage network and geomorphic indices using the DEM;HI,SL,Bs,Vf,and a combination of all these indices,RIAT,identified three classes indicating a degree of various categories of tectonic activity. After evaluation of all indices,three classes were obtained to define the degree of relative tectonic activity in the study area,Class II- high (1.3 ≤RIAT ＜1.5),Class III-moderate(1.5 ≤RIAT ＜1.8),and Class IV-low(1.8 ≤RIAT).Therefore,based on the results of the present study,we summarize that the area is affected by episodic tectonic activities.Moreover,it is confirmed by the observation of RS as well as field shreds of evidence in the form of stream deflection, presence of paleochannels, the abrupt change in stream gradient (waterfalls), and deformed landforms.The high values of Bs,HI,SL were found to be associated with the major faults and low Vf highlight the formation of narrow/steep-deep valleys with high rate of uplift.

Conflicts of interest

The authors declare that there is no conflicts of interest.

Acknowledgments

The authors are thankful to DG ISR and DST(GoG)for providing required support. The earthquake locations as presented in the manuscript were taken from the monitoring program of USGS(www.earthquake.usgs.gov), IMD (www.seismo.gov.in) and ISR(www.isr.gujarat.gov.in). We acknowledge the DMC Dadra and Nagar Haveli(DNH/1255) for providing financial support.