Spatial distribution of SL, HI, AF, Smf, SI, Bs, RSS, RIAT classes.
The eastern part of Kachchh Rift basin was reactivated after 2001 Bhuj earthquake of Mw 7.7 and continuous seismicity has been recorded since then. The northern part of Wagad upland also experienced moderate earthquakes Mw ≥ 5.7 in February 2006 and March 2007. These moderate to major Intraplate earthquakes provide a unique opportunity to study the effects and linkage between brittle-ductile dynamics, surface processes and drainage evolution. We presented a geomorphological analysis of the Wagad highland providing new constraints on the evolution of river network. The shallow to deeper nature of fault and their response to development of hydrological networks has been analyzed using seismic tomography. Based on surface drainage offset and seismic structures several E-W oriented faults controlling fluvial dynamics are identified. From seismic structures and drainage offset it is clear that the fluvial dynamics is controlled by shallower to deeper faults. The estimated attributes are well supported with seismic structures and focal mechanisms solutions. Based on fluvial offset and seismic structure analysis a new tectonic model has been proposed for WH. The tectonic model shows that the faults WH are well connected at deeper level and generated negative flower structures and significantly controlling surface fluvial dynamics.
- fluvial dynamics
- seismic structure
- focal mechanisms
Spatial distributions of geomorphic landforms in active regions are the results of the complex interaction of shallow and deep earth processes . The imprints of these processes are reflected in the form of changes of local relief, drainage pattern, hypsometry, steepness, and channel slope relationship [2, 3, 4, 5, 6, 7, 8]. These parameters can be used to quantitatively characterize the relationship between shallow and deeper crustal structure, and geomorphic processes [1, 9]. The dry land fluvial systems of intraplate Kachchh rift basin, allow us to study the effects and linkage between brittle - ductile dynamics and surface processes on landscape evolution. The Kachchh basin evolved during the Early Jurassic, bound by Nagar Parkar Fault to the north and North Kathiawar Fault to the south (Figure 1A). The rifting was aborted by the trailing edge uplift during the Late Cretaceous pre-collision stage of the Indian plate, when the leading edge of the plate was slab-pulled towards the Tethyan trench [10, 14, 16]. Lateral motion during the drift stage of the plate induced horizontal stress and near vertical normal faults, which were reactivated as reverse faults during initiation of the inversion cycle, and became strike-slip faults involving divergent oblique-slip movement [10, 14, 17, 18].
Major structural features of the Kachchh region include east – west trending active faults [16, 19] (Figure 1A). The Wagad highland (WH) of Kachchh is bounded by the South Wagad Fault (SWF) in the south and Gedi Fault (GF) in the north comprises of Mesozoic sediments overlying a granitic basement . The initiation and steadiness of dynamics support beneath Kachchh basin have been explored in several studies . Earlier researchers argued that the impingement of a large intrusive body in the lower crust [19, 20, 21]. However, the fault adjacent to intrusive body at deeper depth gradually flattens close to magmatic body owing to listric nature of fault [20, 22]. The fault model proposed by  suggests that the GF is a sub-vertical fault and gradually changing listric nature in lower crust.
The chronometric and geomorphic attributes of the GF, suggests that the region is uplifting at the rate of 0.3–1.1 mm/y during the last 9 ka . The results of geomorphic processes and subsurface dynamics of GF zone can be explored by investigating how base level fall at the WH region propagates through the drainage network. The subsurface nature of faults in WH and their association with landscape evolution is still unknown. However, in present study an attempt has been made to understand brittle and ductile dynamics of fault system controlling surface landscape pattern. In this study first time we proposed an integrated sub surface model of WH by combining seismic structure (Tomograph) and surface geomorphology. In this connection we analyzed the 27 basins of WH. In particular, we investigated the general topographic features e.g. swath profiles, local relief and the river network (river longitudinal profiles). We employed a knickpoint celerity approach in order to provide a chronological framework to the evolution of the river network. Furthermore, we made an attempt to image subsurface fault pattern of the area using the seismological approaches such as tomoDD and focal mechanisms. The results permitted us to trace the long-term evolution of the WH, to confirm dynamic support and documenting its impact on the contrasting development of the drainage basins.
2. General geomorphology and seismicity of Wagad Kachchh
The WH is second largest uplifted block of Kachchh basin, after mainland of Kachchh, covering an area of ~2432 km2, and is bounded by GF to the north and SWF to the south [11, 16]. The area is drained by numerous ephemeral streams; flow direction regularly spaced around the upper planation surface [23, 24]. From north to south, the WH comprises of three E-W trending active faults, namely the GF between Deshalpar and Fatehgarh area, the North Wagad Fault (NWF) north of Bharudia and the SWF between Mai and east of Chitrod (Figure 1A).
Geomorphologically the WH is divided into 3 units; (i) the upper planation surface (Mesozoic) with juvenile streams, (ii) the middle incised slopes with piedmont (Tertiary), and (iii) the low-lying areas representing Quaternary deposits [23, 25]. The upper surface represents an early Quaternary erosional event, whereas the middle incised slopes with terraces were developed during late Quaternary . These two geomorphic units provide sediments to the lower peripheral areas. Suvai, Bhimguda, Narelawali, Dhadawali, Karaswali, Malan, Baniyo, and Dabhodanwari are six ephemeral rivers that flow northward and originate from the WH, following the regional slope and drain into the Great Rann of Kachchh . Along their longitudinal length, these rivers cross several E-W oriented faults (Figure 2).
The aftershocks of the January 26, 2001 Bhuj earthquake (Mw 7.7) are still continuing . Distribution of the hypocenter of these aftershocks suggests that they are distributed mainly towards NE and SW directions. It has been observed that the WH is pronounced activated after the 2001 mainshock [12, 15, 27]. It is testified by a large number of aftershocks (Mw ≥ 2.5) occurred in the WH with focal depths ≥10 km. A few moderate earthquakes also occurred along the GF. Among these earthquakes, the most recent are the February 2006 (Mw 5.0); February 2006 (Mw 4.8); March 2006 Mw 5.7 and April 2008 Mw 4.1  (Figure 1B). In the GF zone some 30 earthquakes (Mw 3.0–5.7) have been recorded at Seismic Network of Gujarat (SesisNetG) at shallow focal depths (≤ 20 km) during the period of 2006–2013 (Figure 1B).
3. Data and methodology
To investigate the relationship between brittle - ductile dynamics and surface processes in northern Wagad, we focused on topographic features (filtered topography, swath profiles, local relief), and hydrography (river longitudinal profiles), We used two elevation data sources: the CARTOSAT 2.5 m resolution digital elevation data (
|Basins||Area (km2)||Area %||AF||BS||SL class||Ks class||HI class||AF class||BS class||RIAT value||RIAT class|
3.1. Map of local relief
The map of local relief in the present study is produced from River-Tool by subtracting arithmetically a sub-envelope surface that describes the general pattern of valley bottoms elevations from an envelope surface (that connects peaks elevations) . We obtained such surfaces by smoothing the minimum and maximum topography of the SRTM DEM by a 20 km wide circular moving window (Figure 1C). We chose the value of 20 km since it is the average spacing of the main valleys (5th, 6th and 7th Strahler order with respect to a critical area of ~4 km2). This allowed us to remove small valleys, in effect operating like a low-pass filter that highlights the regional-scale features.
3.2. Swath profiles
We have considered five swath profiles across the study area to describe and quantify the topographic trend of the northern WH. The results show the trend of minimum, maximum and mean elevation into a single plot [29, 30] (Figure 1D). The statistical analyses such as maximum, minimum and mean elevations were calculated along each swath profile within a GIS platform. (Figure 1D). A rectangular swath of 300 m width was chosen to extract a series of parallel profiles that are separated by 1-cell (5 m). The width of the swath profile has been used to condense both elevated surfaces and streams. The higher elevation in swath depicts maximum elevation corresponds to the ridgelines; whereas, the lower elevation curve for the minimum elevation represents the valley floors. The Incision by river can be measured by the arithmetic difference between the maximum and minimum elevations within the longitudinal distance of the swath rectangle .
3.3. Stream length gradient index (SL)
The SL index is one of the quantitative geomorphic parameters included in morphotectonic assessment (Hack, ). This index will increase in value as rivers and streams flow over active uplifts and may have lesser values when flowing parallel to structures such as valleys made by strike-slip faulting . The SL index seems to be a valid tool to detect local uplift as well as the incipient local response to regional processes . Conventionally the SL index shows a quantitative approach to differential geomorphic studies related to erosion and depositional processes that include the river channel, long profile, and valley morphology as well as tectonically derived features such as fault scarps. This index was defined by  as:
where ΔH/ΔL is the local slope of the channel segment being evaluated, and L is the channel length from the divide to the midpoint of the channel reach. The SL index value increases as streams flow over active uplifts and areas with high rock resistance and may decrease with low rock resistance. The integrated plot of stream longitudinal profile and SL index of all the rivers are presented graphically on the x- and y-axis of the longitudinal profile of the main channel (Figure 3).
3.4. Steepness index (Ks)
The normalized channel steepness index could be expected to vary with rock uplift rate (relative to base level), lithology, and climate [34, 35]. The method used for evolution of steepness index in the study area is based on the empirical power law equation of [36, 37] that relates the local slope (S) to the upstream contributing drainage basin area (A) [37, 38], However in present paper similar methodology has been:
where ks is the steepness index and Ѳ is concavity. Given Flint’s law, the relationship between drainage area and distance downstream often described with Hack’s law strongly influences the rate of change in channel gradient with distance downstream, which of course defines the concavity of river profiles. If there is no differential uplift, the value of Ks should remain constant. In the case of the river basin is undergoing differential uplift, Ks may change from one segment to another .
3.5. Hypsometric integral (HI)
The hypsometric integral is an index that describes the distribution of elevation of a given area of a landscape . The integral is generally derived for a particular drainage basin and is an index that is independent of basin area. The index is defined as the area below the hypsometric curve and thus expresses the volume of a basin that has not been eroded. The simple equation that may be used to calculate the index [32, 40] is:
The hypsometric curve indicates degree of dissection of the basin, i.e., erosional stage of the basin. Concave profiles represent long-term equilibrium between uplift and erosion rates. Concave – convex profiles with erosion steps in the middle reaches indicate long-term predominance of erosional processes. Convex profiles are characteristic of areas where uplift (active tectonics) is dominant  The area below the hypsometric curve is known as the hypsometric integral (HI). The value of HI varies from 0 to 1 [41, 42, 43]. These profiles are drawn by projecting rivers onto a theoretical pre-incision surface that is obtained by interpolating the altitudes from present-day lateral divides of the basins (Figure 4). The values of elevation necessary for the calculation are obtained from a digital elevation mode.
3.6. Basin asymmetry (AF)
where Ar is the area of the basin to the right (facing downstream) of the trunk stream and At is the total area of the drainage basin. If a basin has developed under stable conditions with little or no tilting, the Af factor is close to 50. The index is sensitive to change in inclination perpendicular to the channel direction. An AF factor above or below 50 may result from basin tilting, resulting either from active tectonics or lithologic, structural control, differential erosion. The AF value ranges from 18 to 85. The absolute difference (AF-50) has been calculated and the obtained values are grouped into four classes: class-1 (AF ≤ 5; symmetric basins), class-2 (6 ≤ AF ≤ 15; gently asymmetric basins), class-3 (16 ≤ AF ≤ 25; moderately asymmetric basins), and class-4 (AF ≥ 26; strongly asymmetric basins) (Table 1). The area of these drainage basins corresponding to asymmetric values is shown on the map (Figure 5A).
3.7. Basin shape (Bs)
The horizontal projection of a basin may be described by the basin shape index or the elongation ratio, Bs :
where Bl is the length of a basin measured from the highest point, and Bw is the width of a basin measured at its widest point. Relatively young drainage basins in tectonically active areas tend to be elongated in shape, normal to the topographic slope of a mountain [41, 43]. Therefore, Bs may reflect the rate of active tectonics.
4. Relationship between fault displacements and drainage offset
Conventionally, quantitative displacement in a fault zone has been demonstrated by an offset of the river channel [25, 44, 45]. Usually, it has been widely observed that the maximum displacement occurred in the central part of the fault zone  and that the displacement decreases with increasing length. Decrease of river offset towards the west is possibly, westward propagation of the faults. These streams were taken into account to calculate offset ratio:
where D is the amount of stream offset along fault and L is upstream length of the displaced stream . A relationship between long-term slip rate (S) along the lateral slip of a fault and offset ratio (a = D/L) has been roughly calculated as S (m/1000 years), i.e., 10 a [25, 44, 45].
We investigated the major to north flowing rivers such as the Suvai, Bhimguda, Narelawali, Dhadawali, Karaswali, Malan, Baniyo, and Dabhodanwari rivers and adjoining streams to estimate river offset along the E-W faults F1–F4 (GF). However, 17 south flowing rivers such as 1–8, Khalwa River (9), 10, 11, Sharan Nadi (12), and rivers 13–17 have been analyzed to estimate offset along faults F6–F8. Based on offset of river channel we estimated net displacement along F2–F10 respectively.
5. Relative index of active tectonics (RIAT)
Several remote sensing studies have used the geomorphic indices to obtain index of relative tectonic activity [48, 49, 50]. To understand the RIAT of the fault segments, the results of all calculated geomorphic indices were synthesized. In this study we used geomorphic indices of active tectonics to obtained RIAT for the (NWF) North Wagad Fault, (GF) Gedi Fault, Bharudia Fault and (IBF) Island Belt Fault. Based on these classes we have considered each basin class average and combined to obtain the RIAT. The values of RIAT is assigned into five classes as Class- VI (RIAT >4), Class V: (> 3.5 RIAT <4), Class IV: (>3 RIAT <3.5), Class III: (>2.5 RIAT <3), Class II (>2 RIAT <2.5), and Class I (RIAT <2), (Supplementary Table 1; Figure 5B). Depending on the relative index of active tectonics classes, Western part of Wagad uplift covering basin 11, and Easterly covering 15 and 16 comes in highly active zone and shows offsetting drainage pattern (Class 5). Basin 1, 3, 4, 7 and 10 in Bela island, basin 13 and 18 in Wagad uplift are tectonically active and comes in (Class 4) zone and also delimiting by transverse fault between Basin 5, 6 and 9 in Bela Island 12, 14 and 17 in Wagad uplift are moderately active zones (Class 3) (Figure 5B). Basin 2 is intermediately active and comes in (Class 4) and Basin 8 is low active region.
6. Seismological approaches
The evidences of geomorphological in the Kachchh Rift Basin (KRB) is also supported by available high resolution seismic structures, fault plane solutions and recorded seismicity at SeisNetG [13, 15, 51]. Theses integrated approaches provide a clue to understand what instigate to generating a negative flower structure of drainage patterns and its role in the seismicity in the regions. The snapshot of the seismic images play important role to understand brittle-ductile dynamics. It also shed light about hidden causative faults and the drainage patterns that can dictate the degree of damage through shaking (Figure 1B). Several faults in the Kachchh areas are mapped on the surface but still there are many among the existing factory of faults which still remains undiagnosed and not mapped on the surface [15, 51]. That is why we have determined almost all geological evidences and crustal heterogeneities parameters in the present study to understand the nature and extent of underlain structures controlling drainage network. The available 16 fault-plane solutions of the inferred fault planes of the events (Mw ≥ 3.5) recorded during 2007–2014 [13, 15, 51] (Figure 1B). The results obtained from morphotectonic analysis of fluvial networks and seismological approaches are jointly analyzed in the next section.
7. Imprints of active tectonics
The rivers in WH containing the knickpoints over the Mesozoic formations; are related to active geological structures. The maps of local relief, swath, stream length gradient index, and channel steepness (Figures 1D and 2) represents two major topographic zones. The high elevated zones are located between 28 and 228 m. The high relief locations represent remnants of a relict landscape that was preserved at high elevations caused by erosionally balanced rapid Late Quaternary uplift . The low relief zones are marked by Banni and Rann surfaces (Figure 1C). The DEM generated swath profiles shows incision into the alluvial surfaces. This incision is associated with multiple phases of tectonic events and intensified climate forcing during early to mid-Holocene . The sedimentation in lower reaches of river valleys is predominantly controlled by processes that act in response to tectonically triggered and climatically enhanced events . The uplift involve the Bela and WH that shows abrupt and anomalous variation in elevation as noticed in the swath profiles (Figure 1D). The swath profiles shows maximum elevation correspond to uplifted region and low elevation correspond to valley floor. The swath analysis of each profile shows asymmetrical nature of valley shape. The results of the SL index are shown in Figure 2A. Integrated longitudinal river profiles with SL and Ks longitudinal profiles of all rivers are illustrated in Figure 3. The SL values range from 5 to 2300 which are grouped into five classes: class-1 (low activity: 5 ≤ SL ≤ 100), class-2 (intermediate activity: 101 ≤ SL ≤500), class-3 (moderate activity: 501 ≤ SL ≤ 1000), class-4 (active: 1001 ≤ SL ≤ 1500), class-5 (very active: SL ≥ 1501) (Figure 2A). The high and moderate classes of the SL values correspond to significant faults except for the Bela zone, may be caused by the high rock resistance prevailed in that area. The Ks and Ɵ were estimated from the log–log plot of S vs. A. The Ks values range from 5 to 125. To evaluate segmental tectonic activity; these values were also grouped into five classes: class-1 (low activity: 5 ≤ Ks ≤ 25), class-2 (intermediate activity: 26 ≤ KS ≤ 50), class-3 (moderate activity: 51 ≤ KS ≤ 75), class-4 (active: 76 ≤ Ks ≤ 100), and class-5 (very active: Ks >101) (Table 1; Figure 2B). It is observed that the Ks values are high close to the E-W faults.
Conventionally the hypsometric integrals reveal complex interactions between erosion and tectonics [39, 52, 53]. Hypsometric integrals are thought to be affected by basin parameters such as geometry, area and rapid lowering of basin elevations [53, 54]. The hypsometric integral of each basin has been computed based on drainage area and basin geometry. Hypsometric integrals are thought to be affected by basin parameters such as geometry, area and rapid lowering of basin elevations [53, 54]. The HI of each basin has been computed based on drainage area and basin geometry. However, we deployed conventional statistical technique for the entire basin as well as the computation is implemented to individual squire where high and low values can be obtained together. The contour map shows spatial distribution of high and low values, imply that the WH experiencing rapid changes in elevation and incision; owing to tectonic and climatic variations . The higher values of HI clustered around the uplifted regions however the lower values representing low lying areas (Figure 2C). In the analysis of HI, it is considered whether the curve is convex in its upper portion, convex to concave, or convex in the lower portion. The HI curves of all basins are given in Figure 4. It is assumed that if part of the hypsometric integral is convex in the lower portion, it could be associated with uplift along a fault or associated with recent folding.
In present study 10 north flowing and 17 south flowing rivers of WH were analyzed in order to estimate river offset and tectonic control by the faults that cut across the area (Figure 2A,B). The estimated values of offset along F2 ranges between 1.1 and 3.9 km, along F3, 0.49–1.3 km, along F4 0.63–4.3 km, along F5 0.8–1.8 km, along F6 0.79–3.1 km, along F7 0.3–0.9 km, along F8 0.4–1 km, along F9 0.8–0.9 km, and along 10 the offset ranges between 0.3 and 2.2 km respectively (Figure 2D). The stream offset along the F2–10 is comparatively less westward. However, the computed values of offset ratio of about 0.34 km for F2, 0.11 km for F3, 0.14 km for F4, 0.09 km for F5, 0.07 km for F6, 0.08 km for F7, 0.06 km for F8, 0.14 km for F9 and 0.31 km for F10 respectively (Table 2).
|Equation||References||Fault||(SRL km)||(RLD km)||Average RLD||Coefficient||MD (km)||AD (km)||Offset ratio (m)||Magnitude (M)||Slip rate (mm/y)|
|M = A + B * log (SRL)|
M = A + B * log (RA)
M = A + B * log (MD)
M = A + B*log (RLD)
|Mw = 1.36*log SRL+ 4.67||Johnston ||F9||21.17||19||7.2||1.36||4.67||0.8||0.9||0.14||5.6-6.2-6.7||1.4|
8. Geomorphic evidences controlling fluvial network
We identified various geomorphic features associated with active movement along F1 such as block tilting, fault scarps, co-seismic uplift and drainage offset, are well characterized by . At a place the Mesozoic sandstone ridded over Quaternary deposits, resulted ~2 m high active fault scarp (Figure 6A–E). The Mesozoic strata are tilted by 8° southward. We identified secondary surface deformation ~10 km along the strike of the F2 (Figure 6A). The co-seismic movement has developed tensional fractures along the strike of F2 (Figure 6D–E). The north dipping tensional fractures near F2 reflect “en echelon” pattern having step-overs with strike-slip component; probably developed during the northward movement of the hanging wall followed by surface bending (Figure 6D and E) .
Further Kothyari et al.  documented remarkable geomorphic features such as (a) E-W trending fault across gently folded Mesozoic strata, (b) steeply dipping strata with south facing active fault scarp at Deshalpar, (c) wide tensional in E-W direction, (d) highly fractured and sheared litho-units within faulted blocks, (e) subsidence of ground, and drainage offset along the structures (Figure 6F). The tectonic movement in this region resulted changes in surface elevation by forming approximately 9 m high fault scarp (Figure 6G). A significant gradient change in the valley floor causes development of 3 m high knickpoint (Figure 6H). The litho-units are tightly folded within the fault zone. The southern limb of the fold is steeply (~75°) dipping towards south. These steeply dipping beds are characterized by presence of slickensides parallel to the strike direction (E-W) of the F5 (Figure 6I). Kothyari et al.  believe that these features are results of middle to late Holocene tectonic reactivation of GF. Furthermore, the significant amount of changes have been observed within the hydrological network between 8 to 4 ka . The north flowing Karaswali River takes 90o turn and flows in west direction, parallel to GF (F4). East–west offsetting of the two rivers is also observed south of the Gedi village (Figure 6F).
9. Seismic tomography and fault plane solutions
The estimated velocity images and fault-plane solutions are shed new light in understanding the fluvial network and geomorphic development in the intraplate region of Western India . The epicenters of the relocated earthquakes show that the majority of the events are confined along F1–F4 (Figure 1B) during the recorded period. It is also noticed that some of small to moderate events are located north of F4 and north of F1 (Figure 1B). Interestingly, epicenters of small to moderate earthquakes are associated with the offset zone of fluvial network. Theses fluvial network are plotted over seismic tomography images at 5.0 km, 10.0 km, 15.0 km, 20.0 km, 25.0 km, and 30.0 km depths to understand brittle - ductile dynamics (Figure 7A). Based on significant perturbations in Vp anomalies, several shallow and deep fault controlling surface fluvial network are identified. Conventionally the low velocity of Vp anomalies at shallower to deeper depths represents presence of fluid, unconsolidated rocks fault gauge, cracks and fractured basement [15, 51, 58, 59]. Based on surface fluvial offset and velocity perturbation in depth sections of tomography data, the slow velocity is interpreted as a subsurface fracture pattern or strong heterogeneities. The high velocity zones at deeper depth are associated with the surface offset of drainage network .
The depth slices of tomography images are critically examined to identify subsurface fault pattern (Figure 7A). Both combined seismological and geological results clearly show that the zone between Bharudia/NWF and IBF is controlled by eight (F1–F8) E-W oriented parallel faults. Seismic tomograms at depths 5–15 km shows NW-SE and NE-SE oriented transverse faults. The Fault F4 is a vertical fault, which is well defined by velocity perturbations sections down to the depth of 30 km depth. Faults F1, F2, F3, F5, and F6 are connected with the F4 at 25 km deeper level (Figure 7A, B). However, the fault F7 is connected with F5 at 15 km depth, whereas the F8 joints with F7 down to the depth of 10 km. Figure 7B represents inferred depth geometric relationship of all these faults. Geometrically all these faults are converge at depth along a single sub-vertical fault (F4), making an E-W oriented negative flower structure, where all branches of faults are interacting at different depth resulting rhomb shape graben structure (Figure 7B). From the experimental model of  it is clear that the strike–slip fault zone is generally composed of several branches that join together at depth into a single vertical plane. As a consequence, bulk displacement accommodated at depth on the basement fault is distributed towards the surface among several faults whose tectonic activity evolves through time. Some branches remain inactive during a certain period, and then they are reactivated later when their geometry becomes compatible again with the evolving strain field in the wrench zone . The model shows that the local relief apparently has a clear influence on subsurface fault geometries (Figure 7A). In the regions of low topography or where strong river incision traverses the wrench zone, a narrow releasing bend generally develops when the fault trace is deviated towards topographic lows. In regions of higher relief, compression is often associated with shearing, and the fault zone appears much more deformed and segmented . From the earthquake fault plane solution data, it is clear that all these faults are strike slip pattern in nature (Figures 1B,6 and 7A). However, a few solutions show thrust motions may be due to local tectonic adjustment between segmented fault blocks.
10. Combined interpretation and discussion
Globally, the rift basins are controlled by extensional tectonic forces. Changes of stress regime may cause inversion of such system from normal strike slip and reverse Coward , Scisciani  or by development of new faults between the basement rocks and overlying sedimentary succession [62, 63]. The wrench zone of a strike slip fault system shows complex arrays of structures in brittle upper crust in which fault splays are oblique to the principal fault trend and generate a wide deformation zone [60, 64, 65]. The depth section of such faults tend to be steep at deeper level and to splay upwards, forming characteristic flower structures; have reverse (positive flower) or normal (negative flower) components [66, 67]. In such tectonic environment the shape of fault pattern is influenced by degradation and aggradation surface processes . Conventionally in a negative flower zone the faults splays become listric as a result of synchronous deformation and sedimentation . However, in a positive flower setting, faults tend to become steeper towards the surface as a result of aggradation in the footwall and degradation in the hanging wall [70, 71, 72, 73, 74, 75].
A wide zone of fault rupture pattern is investigated in WH of Kachchh peninsula. The seismic structures, seismicity and fault plane solution investigation show that the GF (F4) is a nearly vertical fault (Figure 7B, C) having dominant strike-slip deformation [15, 18, 51]. Using double difference tomography, we identified several shallower and deeper faults (F1–10); these faults are well connected with sub-vertical south dipping GF (F4) at different level (Figure 7B and C). Geometrical relationship inferred from seismic structures show all these faults are converge at certain depth along GF and making E-W oriented negative flower structure. All fault branches are interacting at different depth level and generating rhomb shape graben structure, which is well imaged in the seismic structures (Figure 7B and C). Depth section of tomography suggested that the faults F1, F2, F3, F5 and F8 are deeper faults. However, the F6 and F7 are imaged at shallower depth. It is also clear from the seismic structures that the fault F1 is a north dipping fault plane and connected with the F4 at 27 km depth level. The north dipping faults F2 and F3 and the south dipping faults F5 and F8 are connected with F4 at 25 km depth level. On the other hand the fault F6 is a southward dipping splay of F5 and connected with F5 at 15 km depth. The F7 is a small south dipping subsidiary branch of F6. The fault is connected with F6 at 12 km depth.
Conventionally, an extensional overstep zone of a strike–slip fault several branches that join together at depth into a single vertical plane . As a consequence, bulk displacement accommodated at depth on the basement fault is distributed towards the surface among several faults whose tectonic activity evolves through time . Some branches remain inactive during a certain period, and then they are reactivated later when their geometry becomes compatible again with the evolving strain field in the wrench zone . A few researchers argued that KRB regions show dominantly strike-slip with the slightly reverse faulting natures [13, 51, 76]. However, a few solution shows reverse type of motion could be associated with the local tectonic adjustment between segmented fault blocks.
The archeological records from the WH experience several damaging earthquakes of magnitude 6.0–7.8; that have occurred between 2900 BC and 1300 BC (Table 3). The archeological evidences observed from 40 km west of study area (e.g. Dholavira) suggested that the ancient town was damaged by several major earthquakes between 2900BC and 1300BC [63, 77, 78, 79]. Presence of geomorphic and paleoseismic features within the WH, studied by previous workers are correlated with these historical earthquakes [23, 25, 63]. Further, based on trench investigation and optical chronology identified three earthquake events during last 7000 years . The displaced fluvial sediment and optical chronology in the area suggested that the SWF reactivated during Middle to Late Holocene period i.e., between 3 ka and 1 ka . Similarly other geomorphic studies suggest that the F4 reactivated during Middle Holocene around 4 ka .
|1||2900BC||Damage in Dholavira, dislocation of walls, subsidence of floors and crushed bricks|||
|2||2700BC||Damage in Dholavira, 4 m wide fissured wall, collapse of southern features and multiple cracks running E-W|||
|3||2100BC||Damage in Dholavira; tremendous damage to the gates of castle, tilting and arching of thick inner walls and collapse of outer walls at north gate|||
|6||1900BC||Abandonment of settlements|||
In-SAR measurements show current deformation rate as ~16 mm/y in the western part and 7–16 mm/y in the southern part of the GF zone [14, 23, 80]. Based on trench investigation  estimated uplift rate of 0.5 ± 0.05 mm/y, horizontal shortening rate of 1.1 ± 0.12 mm/y along SWF. However based of fluvial offset  estimated slip rate 2.2 mm/y along the SWF zone which is compatible with the slip rate 1.19 ± 0.13 mm/y observed by . Based on strath terraces  estimated the uplift rates 0.3 to 1.1 mm/y along F4, during the last 9 ka. Further, based on fluvial offset, we estimated slip rates for faults (F2–F10) using the imperial relationship given by . The estimated slip rate along F2 of about 3.4 mm/y, along F3 of about 1.1 mm/y, along F4 of about 1.4 mm/y. along F5 of about 0.9 mm/y, along F6 of about 0.71 mm/y, along F7 of about 0.84 mm/y, along F8 of about 0.67 mm/y, along F9 of about 1.4 mm/y, and along F10 of about 3.1 mm/y (Table 2). The observed slip rate of F2–F10 are well corroborated with the uplift rate 0.3 to1.1 mm/y during the last 9 ka as estimated from the OSL dates. The slip rates obtained from drainage analysis are well correlated with the published results of GPS from the WH (ISR technical report by . Four sites located in the southern part of study area shows variations in localized deformation (Figure 7B). The site Ekal shows deformation rate 2.5 ± 0.12 mm/y, Suvai is deforming at the rate of 1.9 ± 0.03 mm/yr., Badargadh is deformation at the rate of 1.8 ± 0.13 mm/y, and the site located near Rapar shows deformation rate of 2.1 ± 0.11 mm/y . To sites Deshalpar and Fatehgadh shows deformation rates of 1.0 ± 0.04 mm/y and 1.7 ± 0.04 mm/y (Figure 7B). However, a site located in the northern part of study area shows crustal deformation rate 3.8 ± 0.14 mm/y . The estimated slip rates and GPS driven deformation rates together suggests that the landform development in the area is controlled by localized crustal deformation.
The future earthquake along active faults can be evaluated from estimates of fault rupture parameter in turn, released to earthquake magnitude . Active fault studies require an assessment of seismic hazard analysis for the future potential earthquakes . More specifically to estimate the size of earthquake that might be generated by a particular fault may be correlated with rupture parameter such as length, strike and displacement [55, 82, 83]. Moreover, the timing of the past earthquake and size of magnitude can be estimated with the help of geomorphic and paleoseismic records [84, 85]. We used regression analysis proposed by Wells and Coppersmith  and by Johnston  and Johnston and Kanter  for intraplate region to estimate moment magnitude. The detail regression analysis is expressed in Table 2. The regression analysis proposed by Wells and Coppersmith  and Johnston  shows strong correlation between surface rupture length and magnitude of earthquakes. However the regression analysis shows that probability of occurrence of earthquake magnitude range between 6.3 and 7.3) along F1, magnitude (M 5.4–M 7.3) along F2, magnitude (M 6.1–M 7.3) along F3, magnitude (M 6.7–M 7.3) along F4, magnitude (M 6.0–M 6.6) along F5, magnitude (M 6.7–M 7.3) along F6, magnitude (M 6.7–M 7.2) along F7, magnitude (M 6.2–M 7.2) along F8, magnitude (M 5.6- M 6.7) along F9, magnitude (M 5.6–M 7.0) along F10 respectively (Table 2). From the regression analysis it is clear that the faults (F1–F10) passes through the WH are capable for generating earthquake magnitude M 5.4 and M 7.3. Occurrence of February 2006M-5.6 along F9 validates our results and significant level.
Geomorphic indices are widely used to obtain index of active tectonics [25, 48, 49, 50]. Here we synthesized conventional geomorphic indices of active tectonics to calculate relative index of active tectonic (RIAT) distribution along the all fault segments (F1–F10). The RIAT classes show that the deformation is higher along the offset zone of fault segment (Figure 5B). Conventionally the SL and KS values show abnormal increase of gradient within normal and reverse faults. Minor changes of SL and KS observed within the zone of strike slip fault. At few places the SL and KS values do not shows any variation, which is compared with the strike slip motion of fault. The strike slip motion is well corroborated with the results of seismic tomography and fault plane solution (Figure 7B). The existing studies suggest that the terrain close to the GF (F4) experienced occurrence of moderate earthquakes during recent time [23, 27]. The strike slip motion observed from focal mechanism is well correlated with drainage offset along E-W oriented faults within the north and south flowing river system.
Further the observations gathered from the SL, KS and RIAT distribution are corroborated with the geomorphological studies carried out by . The geomorphic expressions such as active fault scarp, shifting and offset of channels indicates that the area is controlled by E-W oriented strike slip faults. Further, steepening of river gradient as observed from tectonic proxy (SL and KS) and uplift of ground further support that the area is tectonically active. Previous studies by  suggests that the uplifts in GF zone were associated with interlinking of strike slip fault segments. Further major two phase of tectonic uplift have been identified by  based on uplifted fluvial strath terraces. The study shows that the first phase of enhanced uplift took place round 8.0 ± 0.9 ka; however, the second phase of uplift is began after 4 ka and continued till today . In present study, several E-W oriented knickpoints were identified across north and south flowing drainage basins of WH. At several locations these knick points have migrated primarily due to river response to sudden base level fall and secondly incision owing to vertical tectonic forces. As sudden base-level fall can be triggered by tectonic upheaval, climatic change, river capture, or channel incision [87, 88, 89]. The incision by river and the formation of knickpoints are well correlated with tectonic upheaval along F1–F8 (Figure 2A).
The study of the topographic configuration morphometric analysis and the subsurface imaging of the fluvial network from WH permitted us to develop a model for the long-term evolution of the drainage basin which is influence by deep tectonic processes. Based on present observations the following conclusions have been drawn.
Based on analysis of fluvial offset and subsurface velocity images major 10 traces of active fault (F1–F10) controlling hydraulic network have been identified.
The surface geomorphology, velocity imaging and fault plane solution data together suggest that the subsurface structure of WH is controlled by strike slip faulting.
The fault geometry inferred from seismic images show presence of negative flower structure at a deeper level.
The depth wise images show that the GF (F4) is steeply dipping towards south. The fault F1, F2 and F3 are dipping towards north, whereas, the fault F5–F8 are dipping towards south. All these faults are connected with GF (F4) at different depth level, which is well imaged in the seismic structures.
The authors are thankful to Ministry of Earth Science, Government of India (MoES/P.O.(Seismo)/1(270)/AFM/2015) for financial support under the active fault mapping program. We are thankful to Dr. M. Ravikumar, Director General and Dr. Sumer Chopra, Director, Institute of Seismological Research, Gandhinagar for fruitful discussion.
Conflict of interest
There is no conflict of interest between all co-authors.
All the authors have made equal contribution in this manuscript.