Results in Earth Sciences

The Kutch Rift Basin (KRB) is of great attention for its possible hydrocarbon reserve. In this context, geomor-phologic and structural geologic studies of the onland basin area are crucial. Application of geomorphic indicators viz., compactness coefficient (Cc), lemniscate coefficient (k), form ratio (Rf), basin shape index (Bs), basin asymmetry factor (AF), hypsometric integral (HI), elongation ratio (Re) and circularity ratio (Rc) have been utilized to calculate the Index of Active Tectonics (IAT) for a part of KRB. Watersheds 5 (the Bhuj area), 9, 10, 11, 17, 18 and 20 belong to class 1 as per the IAT. This indicates that these watersheds are very highly tectonically active at present. Calculated valley floor width height ratio (V f ) and mountain front sinuosity (Smf) quantify the active valley incision in the Quaternary that uplifted the Bhuj area. This uplift was accompanied by a high-degree of drainage rejuvenation due to neotectonics and formation of a canyon in the Khari river. Paleostress analyses revealed NNE and NW-oriented extensions around Bhuj that can correspond to NW and NNE-trending normal fault planes, respectively.


Introduction
Tectonic geomorphology and structural geologic analyses of rift basins (e.g., Mukherjee, 2017, 2019;Dasgupta et al., 2022) are of global attention. Such geoscientific studies attain more importance when the basin is petroliferous. The Kutch/ Kachchh basin in the Indian western offshore region has been well known for a long time for its possible hydrocarbon prospect (e.g., review in Dwivedi, 2016) and therefore has been studied intensely geophysically and geologically (e.g., Sen et al., 2019). Recent discovery of hydrocarbon from the Kutch onshore area has promoted geoscientists to re-look at the terrain (internet ref 1, 2). Interestingly, geochemical studies from sub-surface rocks samples proved that the Bhuj area (along with the locations Anjar, Mandavi and Mundra) can have thermogenic hydrocarbon at some depth (Patil et al., 2013). Thakkar et al. (1999) reported the geomorphology around Bhuj and provided a four-fold geomorphic classification of the Kutch region. The authors referred to meandering to river patterns around the location of the Bhuj. Patidar et al. (2007) reported an abnormally deep and unique incised river channel near Bhuj, which is uncommon in the nearby areas. Sandstone crops out as a uniform lithology around Bhuj, and the typical geomorphology-gentle slopes, low relief, dendritic drainage. Tilting due to morpho-dynamics have been worked out quantitatively (Alapati et al., 2019).
While some amount of geomorphic information from places around Bhuj is available, meso-scale structural data is especially sparse. For example, only Morino et al. (2008) identified ∼ NE-trending Bhuj Fault based on fieldwork and remote sensing images. Omid et al. (2021) documented three or more generations of quartz veins from the Bhuj sandstone. Lohani et al. (2022) presented a structural geologic field guide from the Bhuj and nearby areas, mostly covering the Katrol Hill Range Fault Zone.
This article presents (i) morpho-tectonic analyses of geomorphic features from the Kutch, and (ii) detailed paleostress analyses from the sandstone exposures around Bhuj. Performing smaller-scale paleostress studies in the regional geoscientific context is a standard practice in geoscience (e.g., Shaikh et al., 2018). (Mukherjee, 2018). The recent earthquake history around Bhuj (and Saurashtra) also suggests an active tectonic regime (e.g., Vanik et al., 2018;Shaikh et al. 2022). Uplift rate of the KRB ranges from 0.8 up to 2.8 mm yr −1 (Jani et al. 2021;Kothyari et al. 2022).
The tectonic landscape of the KRB is characterized by the Mainland uplift, Wagad uplift, Desalpar uplift and the Island Belt uplift. In the Cretaceous Period, the basin was inverted, whereby prior normal faults activated as reverse faults (review in Chandrasekhar et al., 2012), and formed the intra-basinal uplifts with respect to the structural lows (Biswas and Khattri, 2002). During post-Cretaceous inversion, flexing further modified the topography. Later, a thin Quaternary sediment layer was deposited on Katrol Hill (Chowksey et al., 2011a(Chowksey et al., , 2011b. The entire area is drained by rivers and their tributaries flowing S-N, N-S, E-W and W-E originating from the W-E anticlinal ridge forming meso to micro-scale watersheds. These watersheds modified slopes due to tectonics (Singh, 2014;Withanage et al., 2014). With time, drainage system  Here P B is the perimeter of the basin and A B is the area of the same basin. A tectonically active basin will have a higher Cc value and vice-versa. e.g. Iqbal and Sajjad (2014)

2.
Form ratio(Rf) Here Form ratio (R f ), ratio of the basin area (A ) B to the square of its length (L b ) e.g., Paul and Biswas (2019); Kale et al., (2014)

3.
Lemniscate Here, A B is the area of basin and L b is the length of the basin.

4.
Basin Shape Index (Bs) Here Length (L b ) to the width of the basin at its wisest part (W b ). Chorley (1957) e.g. Anand and Pradhan (2019)

= × Basin Asymmetry Ratio 100 Ar At
Here A r is the area of basin to the right of the stream flowing downstream and A t is the total area of the entire basin. When this value is nearer to 50, the basin is supposed to be tectonically inactive, while as the value go away from 50 (0-35 and 65-100

8.
Circularity ratio(Rc) = Rc A P 4 / 2 Here, A= Area of the basin P = Perimeter of the basin e.g. Anand and Pradhan (2019)

Channel Length
Valley length Here, Straight channel values < 1.05 Straight to Sinuous values between > 1.05 -< 1.50 Meandering channel indicates > 1.50 Brice (1964), e.g. Biswas and Dhara, (2019) adjusted to local structures such as faults and lineaments and acquired typical valley incision geometry, basin shape, asymmetry, stream length gradient, sinuosity etc. (Prakash et al., 2017;Biswas and Paul, 2021). The Kutch region is characterized by four geomorphologic zones: (i) Great Rann in the north and Little Rann in the western part, (ii) Banni-Plains or Rann segment, (iii) rocky upland in the Kutch Mainland, and (iv) southern fringe of curved coastal littoral section (Malik et al., (2001)). Anticline, monocline flexure, and W-E striking Kutch Mainland Fault reoriented the drainage pattern in response to the fluvial interaction.
This study section of the KRB is a part of the rocky upland. This article uses the terms 'watersheds' and 'river basins', which are the same in the context of their linear and spatial analyses (e.g., Biswas et al., 2022;Dasgupta et al., 2022). This article uses the parameters "basin shape index", and "basin asymmetry" on the defined watersheds for which it was necessary to clarify this point.

Morphotectonic parameters
Integrated morpho-tectonic analyses were performed and data were computed using Digital Elevation Model (DEM). The DEM data consist of SRTM 30 m resolution of Tile Downloader in EOSDIS Earth data of National Aeronautics and Space Administration (NASA). It is a zipped SRTMHGT file at 1-arcsecond resolution (3601 ×3601 pixels) in a latitude/longitude projection (EPSG: 4326).
Using the fill DEM to flow accumulation through stream feature class criteria, twenty-one watersheds have been identified in the ARC GIS 3.4 platform (2016). The extracted surface terrain model calculates slope, creates aspect maps and helps to delineate the watersheds. Drainage lines are enhanced to calculate the parameters of the individual watersheds. Q-GIS 3.12 platform has been used to prepare the 3D layout of the area on which major faults (Kutch Mainland Fault, Katrol Hill Fault, Naira River Fault, Sandra Dungar Fault, Bhujpur Fault and Godpar Fakirwari Fault) were overlaid from the literature. Maps are georeferenced and faults are plotted on the 3D terrain map. Draping of data was avoided in the DEM. We have used an advanced technique (Kumar et al., 2018) that gives results of different considered morphotectonic parameters. We can compare tectonic set-up, lithology, geomorphic features and channel characteristics.
For the critical analysis of these parameters, mountain front sinuosity (Smf) and valley floor width to height ratio (V f ) were calculated and classified into four classes by separately considering the same spatial entity. Further, we compared the Index of Active Tectonics (IAT) in response of uplift rate and existence of fault lines. This leads to tectonically relevant conclusions (as in Kumar et al., 2018).
The tectonically active watersheds are assessed using the IAT. Computed IATs provide a reliable semi-quantitative measure of tectonic activeness in a relative sense (e.g., Bull and McFadden, 1977;Gupta and Biswas, 2022). We evaluate relative active tectonics based on seven measures such as compactness coefficient (Cc), lemniscate coefficient (k), form ratio (Rf), basin shape index (Bs), basin asymmetry factor (AF), hypsometric integral (HI), elongation ratio (Re) and circularity ratio (Rc) ( Table 1). All the values of the considered parameters have been classified into four classes (1−4). These parameters taken together enable computing the IAT. This led to categorizing the watersheds with low, moderate, high and very high tectonic activities.
To assess the relative tectonic activity, the Smf index has frequently been used (Bull and McFadden, 1977;El Hamdouni et al., 2008). This parameter indicates equilibrium in tectonic uplift, which helps to produce irregular fronts. Smf is defined as follows (Bull and McFadden, 1977;Bull, 2007): Here Lmf: overall planimetric length of a mountain front along the mountain and piedmont junction, and Ls: length of this front's straight line.
The V f ratio is defined as (Bull and McFadden, 1977): Here V f w is the width of the valley floor, Eld and Erd are the elevations of the divide on the left and right side of the valley, respectively. Esc is the average elevation of the valley.
This ratio is connected to an incision and indicates active tectonics due to uplift. Low values of V f related to V-shaped valleys may be related to tectonic uplift-induced vertical incision (Sahu and Mohanty, 2021). High values of V f for the U-shaped valley connote lateral erosion primarily due to dormant relative tectonic. In this study, 63 points from 21 watersheds have been considered and sequentially two points are taken-one from upstream mountain front and the other from the downstream portion.

Structural and paleostress studies
Fault planes and slickenlines were documented from five locations around Bhuj within the Bhuj Sandstones. Other meso-scale faults were noted and many of them were not incorporated into paleostress studies because of the lack of development of tectonic lineations over them. The collected data from each sector is presented in the Repository Tables 1-5. Those were identified based on the consistent attitudes of Table 2 A: Calculation of each parameter and classification in to four classes to evaluate IAT. B: Obtained classes according to the watersheds and calculated IAT. C: Extracted four classes of IAT as very high, high, moderate and low.
(continued on next page) few other faults in the nearby area and each having slickenlines with consistent attitudes. Slickenlines are visible only when observed closely. No displaced marker layers were found. Paleostress analysis was performed on the collected attitudes of faults and lineations developed on them. We use T-TECTO (Studio X5) and Win_Tensor (4.0) software for paleostress analyses. The working principles of the software are available elsewhere (e.g., Vanik et al., 2018;Shaikh et al., 2020, Maurya et al., 2021Goswami et al., 2022). We used two software programmes to cross-check results. This makes our analyses more reliable. We follow the convention, σ1 > σ2 > σ3, regarding the choice of symbols for maximum, intermediate and least principal stress axes, respectively.

Morphotectonic analysis
The morphology of the study area is characterized by fluvial activities with distinct tectonic episodes. The 21 watersheds ( Fig. 2A) are located in the hilly ranges of steep north-facing escarpment and gently south-dipping Mesozoic strata. The aspect slope map (Fig. 1A of Repository 1) displays the multi-slope directions followed by the rivers from the E-W elongated arc-shaped hilly patch dissected by several drainages. The average slope of the section varies from ∼ 1 to > 12.6 0 ( Fig. 1B of Repository 1).
A dendritic drainage pattern is well developed in watersheds 1 and 2. Regular high-angle joints and/or faults e.g., Banni Fault (NNW-SSE), Kutch Mainland Fault (W-E), Wagad Fault (E-W) and lineaments (probably) produced rectangular drainage patterns, which are distinct in watersheds 1, 3, 6 and 7. The complex structure with deformation of the Mid Jurassic-Lower Cretaceous uplift is accompanied by flexures and a second uplift that affected the Tertiary to Quaternary successions (Patil et al., 2013). It indicates major tectonic episodes resulting in a trellis drainage pattern (Flugel et al., 2015) near the Katrol Hill Fault (KHF) in the upper parts of watersheds 8-18 and 21. In watersheds 19 and 20, sub-parallel drainage was identified where channels joins ∼ 40º around a recent sandy deposit (Fig. 2B). Rivers originated from the hilly elongated domeshaped terrain with maximum elevation ( Fig. 2A of Repository 1) and flow towards N, S, W, SW and NE. Compactness coefficient (Cc), form ratio (R f ) and lemniscate coefficient (k) denote the classes that include the watersheds (Fig. 2B-D of Repository 1).
The spatial distribution values specify the watersheds 5, 9, 10, 11, 17, 18 and 20 as highly active under class 1; watersheds 12-16 and 19 under high tectonic activity (class 2); watersheds 4, 6 and 8 under Fig. 3. Field locations E, K, M and N around Bhuj, on Google Earth Image. T-Tecto and Win_Tensor results at those locations are also presented. Great circles: input faults; three principal stress axes-yellow circles, the biggest intermediate and the smallest ones as σ 1 , σ 2 and σ 3 , respectively. Hollow squares: principle strain directions: biggest, intermediate and the smallest ones being ε 1 , ε 2 and ε 3 , respectively; blue patches: extensional field; red patches: compressive fields; Red and blue arrows: direction of compressive and tensile stresses, respectively. Black inward and outward arrows: direction of strain tensors. These tensors can be multidirectional representing several phases of faulting. There are three main channels: the Khari, Pat and the Pur Rivers (Fig. 2D) in watershed 5. All flow towards the north and are characterized by deep vertical incision in the Khari river (Fig. 2E). The canyon (Fig. 2F) in the Khari River indicates the active tectonics near Bhuj. Fig. 2F1 represents recent activities of the Kutch Mainland Fault (Valdiya and Sanwal, 2017) that imposes more incision of the channel.
The calculated values of Smf and V f have been used to classify the degree of active tectonics. A comparison with IAT ranges of different sectors discloses variable degrees of tectonic activities. The computed classes 1 and 2 have Smf within 1.6-1.53. Watersheds 4, 5 and 8-18 have Smf = 0.54-1.89. This indicates (very) active tectonic regime. These Smf values are strongly related to the IAT ranges where watersheds 4, 9-18 and 20 are under the very high and high ranges. The class 1 and 2 ranges of V f are 0-0.6 and 0.61-1.21, respectively, and include the watersheds 4, 5 and 10-21. This reflects active tectonics associated with the KMF, Naira river Fault, KHF, Godpar Fakirwari Fault, Bhujpur Fault, Sandsra Dungar Fault etc. (plotted in Fig. 2c) created relatively narrow and deep valleys.
Formation of river terraces and their incision rates help to determine the uplift rate of the area. Along the central and eastern segment of KMF the rates of incision are 2-9 mm yr −1 and 2.3-10 mm yr −1 (Kothyari et al., 2022) that signify the low Smf, V f and IAT values (under class1 and 2) of watershed 4,5 and 6. Near the Katrol hill zone, the average uplift rate is 0.8 mm yr −1 (Kothyari et al., 2022). Watersheds 9-20 are under the very high and high classes of IAT, V f (watersheds10-21) and Smf (watersheds 8-18). The uplift rate is 3.1 mm yr −1 (Kothyari et al., 2022) in Khari watershed (watershed 5). In this watershed, Smf = 1.3, V f = 0.56 and IAT= 1.8. All these values are under class 1 (very high tectonic activity) (Table 3).

Paleostress studies
Structural data were collected mostly from the rock exposures aside the Kodki road, Kalo Dungar road and the Airport Road passing through Bhuj. Fig. 3 in insets and Repository Tables 2-5 present the data collected for the paleostress analyses from different field locations. Brittle fault planes crop out in several sites. We are unable to map these faults since their trends and styles vary significantly. A single direction of extension is incompatible with the different fault-slip data that were recorded in the field. For a plausible explanation of such data, two directions of extension directions would be needed. The fault planes strike ∼ N-S at locations K ( Fig. 1 in Repository 2), and at location E ( Fig. 2 in Repository 2) and ∼ ENE at location N ( Fig. 3 in Repository 2).

Win_Tensor4.0 and comparison with T-Tecto
Principal stress axes σ 1 , σ 2 and σ 3 are denoted by circle, triangle and square, respectively, in Win_Tensor. The planes of probable P, B and T axes deduced from the computed inversion analysis were also plotted.
The Win_Tensor results indicate that the stress regime is pure and extensive in areas E and M, whereas it is extensive in areas K and N. The σ 1 direction for areas E and K was ∼ east and for areas M and N it was ∼NNE. The σ 3 direction for area E was ∼ west and for areas K, M and N it was ∼ south. A dominantly extensional tectonic regime was revealed from the study locations with NW-and NNE-oriented extension directions, respectively. This means that in and around the Bhuj area, the Wto NE-oriented normal faults did not undergo reverse reactivation under current compressional stress regimes. Evidence of ∼ N-S compression related to the India-Eurasia collision since ∼ 55 Ma has been sparse from the study area ( Fig. 3-8 in Repository 2).

Geomorphic issue
Existence of faults were manifested as low Smf and V f values that are comparable with low IAT range of watersheds 5, 9 and 10. The faults induced high uplift and straight mountain front. The low V f values indicate vertical incision that justify the low IAT values connoting tectonic activeness.
The entire area is drained by rivers and their tributaries flowing S-N, N-S E-W and W-E from the anticline ridge forming a number of mesoscale watersheds. The SI value of tectonically active three watersheds indicate straight channel patterns, which are associated with vertical, valley side steep cutting corresponding to the least lateral erosion. Tectonic episodes reversed the stream gradients by bulging of the ground (Valdiya and Sanwal, 2017). Such uplifted section is accompanied by the high degree of drainage rejuvenation due to neotectonics (Valdiya and Sanwal, 2017). The main consequent streams in watersheds 2 and 3 near Bhuj show complex barbed pattern where tributaries join at hook-like junctions. It is accompanied by the E-W KHF, which is Table 4 R-values define stress regime, as per Delvaux et al. (1997  intersected by N-S, NNE-SSW, and NNE-SSW lineaments/active faults (Valdiya and Sanwal, 2017). With time drainage system were modified by several faults and reframed the river course. These indicators justify the active tectonics and uplift of the area along with the neotectonic evidences e.g. strath terraces, knick points, laterally displaced crests. The study section of the KRB is characterized by several climatic as well as tectonic forces in the Late Quaternary (Maurya et al., 2003) under which topography got configured. This Quaternary tectonics was responsible for the 1821 and 1956 seismicities (M > 5) with the Kutch Mainland Fault Zone near Bhuj.

Paleostress analyses
The T-Tecto and Win-Tensor results from all the sectors match well. Most of the results show rather limited variations in plunge of the principal stress axes within 10 0 and a trend within ∼ 20-25 0 . These discrepancies arose due to the different algorithms used in the two software (Dutta et al., 2019;Simon, 2019). For example, while T-Tecto uses the Gauss weighted right dihedral method, WinTensor applies the method the PBT axes. Limited variation in the paleosstress axes' attitudes deduced by different approaches give more confidence that the paleostress analyses were run correctly. Over a few km distance, principal paleostress axes orientations vary (e.g., the σ 3 direction in areas E, K and M; Fig. 3), and this is a common observation from many other terrains (e.g., Vanik et al., 2018;Kumar et al., 2022;Shaikh et al., 2022) (Table 5). Pealeostress directions do not necessarily link with the present-day stress system. This is true in terrains where a previously extensional regime is under compression, such as the KRB.

Conclusions
The IAT map of 21 watersheds in and around the Bhuj denotes that the watersheds 5, 9, 10, 11, 17, 18 and 20 are tectonically very active and come under class 1. It is accompanied by complex barbed and trellis drainage patterns, which indicate the area's tectonic interferences.
Channels in some of these watersheds partially display complex barbed and trellis drainage patterns, which indicate a strong tectonic control.
Smf and V f analyses disclose the active tectonic interferences both in the front area indicating uplift. The valley incision mechanism is characteristic in watershed 5. The active fault area between Dhrang and Khirsaia was the epicentre for the 2001 Bhuj earthquake (M ∼ 7.7) and it belongs to the tectonically very active watershed 5.
Paleostress analyses indicate that NNE and NW-oriented extension acted around Bhuj producing normal faults. Sub-surface seismic image and ground penetrating radar (GPR) profile interpretation for hydrocarbon exploration in the area need to address the structures related to the paleostress regimes deciphered in this study.

Data Availability
Data will be made available on request.

Declaration of Competing Interest
We have strong conflict of interest with Arkprobho Biswas (Editorial board member on Results in Earth Sciences). Please do not give this article to him to handle ort review.