Katrol Hill Fault Research Articles

Delineation of Paleochannel and Groundwater Resources in the Khari River Basin, Kachchh using Transient Electromagnetics

Abstract The paleochannels are riverine geomorphic features, which are either buried or shifted due to tectonic, climate, and anthropogenic activities. In general, these channels serve as potential groundwater repositories, and their identification can help in developing potable groundwater sources. Here, the results of transient electromagnetic (TEM) study carried out near and around the Khari river basin of the tectonically active Kachchh intraplate region, western India are presented. The TEM investigations at 22 sites along three traverses have been carried out in the river basin that cut the present-day river channel and its presumed paleochannel. The geoelectric sections of the three profiles decipher information down to 300-350m and suggest a multilayer aquifer system. The sub-surface model infers a deeper confined aquifer at variable depths from 100-150m associated with the river as well as its paleochannel. The paleochannel aquifer is observed to be buried under 15-20m thick unconsolidated sediments that overlie a Mesozoic sandstone layer of the Bhuj Formation. The resistivity sections across the paleochannel suggest a relatively fresh aquifer layer with higher storage capacity. A vertical bedrock offset of 100-120m and the discontinuity of lithostratigraphic layers observed from the study suggest the dominant tectonic control in the formation of the paleochannel that correlated with the location of the reactivated splay of the Katrol Hill Fault.

Assessment of Groundwater Potential Zones across Katrol Hill Fault, Kachchh, Western India: A Remote Sensing and GIS Approach

The present study focuses on the use of remote sensing and geographical information system tools for morphometric and geomorphic analysis of major river basins across the Katrol Hill Fault, which makes drainage divide. It aims to find groundwater potential for the management and planning of groundwater resources. The study area consists of 6 major watersheds of major river systems namely Bhurud, Khari, and Pat flowing north of the major divide, while Rukmawati, Nagavanti, and Bhukhi are southerly flowing rivers. Based on linear, areal, and geomorphic aspects, a watershed with excellent groundwater potential was found. The highest order in the area is the 5th order stream. Appropriate drainage network characteristics, elongated shape, and permeable lithological formation with low relief among all the watersheds made WS3 be excellent potential for groundwater. The statistical analysis, where Cp value was computed, showed the potential groundwater zone to be in WS3 followed by WS2 and WS5. These results were even verified with field data, collected from well-inventory and that too favored WS3 as an excellent groundwater potential.

Geodetic characterization of active Katrol Hill Fault (KHF) of Central Mainland Kachchh, western India

After the M7.7 earthquake in 2001, the Kachchh rift basin became the focus of various geological and geophysical researches on the western Indian plate. As an essential technology, the Global Navigation Satellite System (GNSS) has been utilized to study the deformation pattern in the central mainland Kachchh. We select the east-west striking Katrol Hill Fault (KHF) as the study area and analyze the crustal deformation pattern using the datasets from 2014 to 2019. The geodetic results along the KHF reveal a mean deformation of about 2.1 mm per year, which is higher in the eastern part and lower in the central and western parts. The investigation of deformation and derived strain reveals the segmented behavior of KHF, while the dominance of compressive strain (maximum 22 nanostrain/yr) in the eastern segment makes it the most active segment of the KHF. A higher deformation rate along the eastern KHF can be considered significant in terms of seismic hazard for this part of the Indian plate.

Surface trace of the active Katrol Hill Fault and estimation of paleo-earthquake magnitude for seismic hazard, Western India

Estimation of seismogenic potential through geological studies of faults is an important step in seismic hazard evaluation and mitigation. We investigated the Katrol Hill Fault (KHF), an E-W trending and south dipping high angle reverse fault in the seismically active Kachchh Basin at the western continental margin of India. While several faults in the eastern part of the basin are presently seismically active, the KHF is the only fault in the entire basin that is shown to have produced three Late Quaternary surface faulting events. We illustrate here an integrated geological approach using field mapping, microscopic analysis (petrography and quartz surface textures using SEM) and shallow geophysical surveys using GPR for determining the precise surface trace of the active KHF. We also determined the displacement and slip-rate for each surface faulting event. Based on the Late Quaternary surface faulting parameters deduced, we calculated the moment magnitude (Mw) of the events using well established empirical relationships. These calculations yielded Mw values consistently ≥ 6.5. Our study shows that the KHF is capable of producing high magnitude earthquakes. We infer that these surface faulting events may be of relatively shallow focus as higher magnitude earthquakes in recent times along other faults in the basin, except the 1819 Allahbund earthquake, did not produce surface rupture. We recommend a revision of seismic hazard risk and mitigation strategies of the region as a consequence of our findings.

Magnetotelluric evidence for trapped fluids beneath the seismogenic zone of the Mw6.0 Anjar earthquake, Kachchh intraplate region, Northwest India

Lithospheric rheological heterogeneities across a rift play a significant role in intraplate deformation and seismic activity. The Kachchh rift in the northwestern part of the India has not only experienced lithospheric stretching but also witnessed large magnitude devastating earthquakes. Here, we present the results of a magnetotelluric (MT) investigation carried out in the southeastern part of the Kachchh intraplate region illuminating the seismogenic zone of the 1956 Anjar earthquake (Mw 6.0). The two-dimensional geoelectrical model derived from the MT data acquired at 21 sites along a NE-SW profile reveals assemblage of conductive and resistive zones in the upper and the mid crustal depths in the close vicinity of the major fault zones and the hypocentral depths. The results reveal a very high conductive zone near the Moho, affirming the presence of an upper mantle fluidized zone. Unlike the South Wagad Fault, the Kachchh Mainland Fault and the Katrol Hill Fault do not have a downward lower crustal extension, and terminate at the mid crustal depths. We suggest that the presence of trapped fluids in the brittle-ductile transition zone (12–15 km) and their seepage to the resistive plutons might be a possible mechanism for triggering the 1956 event as well as the current seismic activity in the region.

Late Quaternary drainage reorganization assisted by surface faulting: The example of the Katrol Hill Fault zone, Kachchh, western India

AbstractDrainage reorganization on restricted temporal and spatial scales is poorly‐documented. We attempt to decode the relatively complicated mechanism of drainage realignment involving two small rivers that show structurally controlled, highly anomalous channel networks. We provide geomorphic and shallow subsurface evidence using ground‐penetrating radar (GPR) for the presence of a buried paleo‐valley flowing northward through the wind gap and surface faulting along the range bounding Katrol Hill Fault (KHF) which correlates with the previously known three surface faulting events in last ~30 ka bp. Most of the present river channels and the KHF zone are occupied by aeolian miliolite (local name) which is stratigraphic and lithologic equivalent of the Late Quaternary carbonate rich aeolianite deposits occurring in several parts of the globe. The history of drainage evolution in the study area comprises pre‐miliolite, syn‐miliolite and post‐miliolite phases. Geomorphic evidences show that the paleo‐Gangeshwar River flowed north through the wind gap and paleo‐valley, while the short paleo‐Gunawari occupied the saddle zone to the east of Ler dome prior to and during the phase of miliolite deposition which ended by ~40 ka bp. Southward tilting of the Katrol Hill Range (KHR) due to surface faulting cut off the catchment of the paleo‐Gangeshwar River. The abandoned catchment stream extended its channel eastward along the strike through top‐down process while the paleo‐Gunawari River extended its course westward by headward erosion (bottom‐up process). As the channels advanced towards each other they joined to produce the “S”‐shaped bend which formed the capture point. We conclude that multiple surface faulting events along the KHF in the last ~30 ka bp, resulted in uplift and tilting of the KHR which caused drainage realignment by river diversion, beheading and river capture. Our study shows that the complexity of drainage reorganization processes is more explicit on shorter rather than longer timescales.

PS-InSAR derived deformation study in the Kachchh, Western India

The accumulation of crustal strain towards the western part of India, especially in the Kachchh Rift basin, is making one of the most seismically active parts of the Indian plate. Several strong to major earthquakes, including the recent 2001 (M7.7) Bhuj earthquake, were triggered in the Kachchh rift basin during the last two centuries. Therefore, in the present study, we have attempted to quantify crustal deformation towards the eastern part of mainland Kachchh using PSInSAR and GPS data from 2014 to 2019. The average LOS displacement of 4.3 ​mm/yr has been observed along the eastern segment of the Katrol Hill Fault (KHF). The deformation in this part can be correlated with the accumulation of strain along the hanging wall side of the south-dipping KHF. The accumulated strain is reflected in the form of seismic activity in this part and highlights the importance of the KHF zone for seismic hazard analysis. The PS-InSAR results are in good correlation with the GPS results of this part.

Modelling of earthquake locations and source parameters in Kachchh region to understand genesis of earthquakes

Modelling of earthquake source locations and parameters infers seismogenesis of earthquakes. In this study, we modelled the earthquake source locations through hypocenter location algorithm using the difference in arrival time of P and S waves and source parameters through the Levenberg–Marquardt non-linear inversion method using S-wave spectra. A total of 340 aftershocks of 2001 Bhuj mainshock ( $$ 1.8 \le M_{w} < 4.3 $$ ), which have occurred in Kachchh, Gujarat, India from January 2014 to January 2015, are located in this study. Out of 340 aftershocks, digital waveforms of 78 aftershocks ( $$ 2.2 \le M_{w} < 3.9 $$ ) are used for estimation of the earthquake source parameters. The results obtained from earthquake locations show two clusters of seismicity along the Kachchh Mainland Fault (KMF) and North Wagad Fault (NWF) and three felt events ( $$ M_{w} \ge 3.0 $$ ); one along the Katrol Hill Fault (KHF) ( $$ M_{w} = 3.3 $$ ), two along the Banni Fault (BF) ( $$ M_{w} = 3.0,3.2 $$ ). The generation of these three felt events is attributed to the triggering mechanisms caused by the migration of fluids or the stress pulse generated by the 20 MPa stress drop of the Mw 7.7 Bhuj earthquake. A marked concentration of events is noticed in 15–30 km depth range, which could be attributed to the presence of a mafic intrusive body, resulting in stress build-up for earthquake generation in this region. The results of source parameters; seismic moment (M0), source radius (r) and stress drop (Δσ) vary from 1.86 × 1012 to 3.2 × 1015 N m, 146–262 m and 0.04–5.73 MPa, respectively. The maximum stress drop value is estimated to be 5.73 MPa at 24 km depth for the largest studied event of $$ M_{w} $$ 3.9. Large stress drops are confined to the 8–33 km depth range, which indicates the probable existence of the base of the seismogenic layer in this depth range. This observed large stress drops could be attributed to stresses induced by crustal mafic intrusive bodies and the presence of aqueous fluids in the lower crust below the region.

Relative Assessment of Tectonic Activity along the Seismically Active Katrol Hill Fault, Kachchh, Western India

ABSTRACT The Katrol hill fault (KHF) is a 71 km long fault, striking E-W in the Kachchh intraplate region, western India which is moderately active, seismically, but exhibit high strain rate with considerable vertical deformation. It is dissected by several young transverse faults. Based on these transverse faults, KHF is longitudinally, divided into five segments. Even though there are evidences of its active nature during the Holocene Period, no studies have been carried out for quantifying the spatial variation in relative tectonic activity along the KHF, which is vital for assessing a more realistic seismic hazard potential of the fault. Quantitative geomorphic indices are employed to evaluate the ‘Relative Index of Tectonic Activity (RITA)’. It has been observed that the central part (segment 2 &amp; 3) is the most active segment, which covers an aerial extent of 38% of total KHF (class 1), compared to the eastern (segment 4 &amp; 5) and the western segments (segment 1), which are moderately active (class 2). Interestingly none of the segments of the KHF, corresponded to class 3 of RITA i.e. least active/inactive class. The study highlights the important role of transverse faults, which cut across the major E-W faults in the Kachchh, and may regulate the relative activity and the earthquake potential of an individual segment. The study thus, hints the KHF as an under-rated source for future seismic hazard for the Kachchh and western India region.

Magnetotelluric study to characterize Kachchh Mainland Fault (KMF) and Katrol Hill Fault (KHF) in the western part of Kachchh region of Gujarat, India

The Kachchh Mainland Fault (KMF) is a major E-W trending fault in the Kachchh region of Gujarat extending >150 km from Lakhpat village in the west to the Bhachau town in the east. The Katrol Hill Fault (KHF) is an E-W trending intrabasinal fault located in the central region of Kachchh Basin and the south of KMF. The western parts of both of the faults are characterized, and the sediment thickness has been estimated in the region using a Magnetotelluric (MT) survey at 17 sites along a 55 km long north-south profile with a site spacing of 2–3 km. The analysis reveals that the maximum sediment thickness is 2.3 km (Quaternary, Tertiary, and Mesozoic) in the region, out of which, the Mesozoic sediments feature a maximum thickness of 2 km. The estimated sediment thickness is found consistent with the thickness suggested by a deep borehole (depth approx. 2.5 km) drilled by Oil and Natural Gas Corporation (ONGC) at Nirona (Northern part of the study area). From 2-D inversion of the MT data, three conductive zones are identified from north to south. The first conductive zone is dipping nearly vertical down to 7–8 km depth. It becomes north-dipping below 8 km depth and is inferred as KMF. The second conductive zone is found steeply dipping into the southern limbs near Manjal village (28 km south of Nirona), which is inferred as the KHF. A vertical-dipping (down to 20 km depth) conductive zone has also been observed near Ulat village, located 16 km north of Manjal village and 12 km south of Nirona village. This conductive zone becomes listric north-dipping beyond 20 km depth. It is reported first time by a Geophysical survey in the region.

Estimation of strong motion parameters in the coastal region of gujarat using geotechnical data

Seven boreholes are drilled in the coastal region of Gujarat; three in Kachchh (at Jangi, Mandvi and Mundra), one in Saurashtra (Jodiya) and three in mainland Gujarat(Dahej, Kamboi and Dholera) to depth of 27–80m for estimating the surface strong ground motion parameters. These parameters are required for seismic resistant design of structures in the areas. The adopted methodology comprises of three parts: (i) soil modeling and estimation of depth to Engineering Bed Layer (EBL) (a prominent subsurface soil layer with shear wave velocity of 450m/s to 760m/s with SPT N value of more than 80) (ii) Estimation of the ground motion at EBL (iii) estimation of ground motion at the surface by 1D ground response analysis using SHAKE program. The soil models are prepared from borehole data and shear wave velocity/ N-values. The ground motion at EBL is estimated using Stochastic Finite Fault source Modeling Technique by considering scenario earthquakes along two major faults of Kachchh (Kachchh Mainland Fault (Mw7.6) and Katrol Hill Fault (Mw7.5)), one of Saurashtra (North Kathiawar Fault (Mw 6.5)) and one of Mainland Gujarat (West Cambay Fault (Mw 6.0)). The surface ground motion is estimated by passing ground motion simulated at EBL through prepared soil models of each site using SHAKE program. The surface peak ground acceleration from 0.076g to 0.396g and peak spectral acceleration from 0.24g to 1.4g are estimated at seven coastal sites. Spectral accelerations are found higher than Bureau of Indian Standard (BIS) suggested values at Jangi (between 0.1 and 0.3s & 0.8–1.4s), Jodiya (0.1–0.3s), Mundra (0.3–0.6s) and Dholera (0.1–0.4s) sites, where the importance factor of 1.5 needs to be considered. The study suggests taking BIS values at Dahej, Kamboi and Mandvi coastal sites.

A review and new data on neotectonic evolution of active faults in the Kachchh Basin, Western India: legacy of post-Deccan Trap tectonic inversion

Abstract The Kachchh Basin, located in Gujarat (India) at the western trailing edge of the Indian plate, comprises several east–west trending seismically active faults. The Kachchh Basin evolved in two major stages. The first is the rift stage, which correlates with the break-up and separation of the Indian plate in the Late Triassic–Early Cretaceous and synrift sedimentation. The second is the post-Deccan Trap inversion stage, when the basin was periodically uplifted along the existing east–west trending intrabasinal master faults: the Katrol Hill Fault, the Kachchh Mainland Fault, the South Wagad Fault, the Gedi Fault and the Island Belt Fault. The inversion of basin was initiated by the onset of a compressive stress regime in response to the collision of the Indian plate with the Eurasian plate in the far north during the Early Eocene. This is shown by the tilting of the Deccan Trap lava flows along with the underlying Mesozoic sequence and the associated intrusive bodies that occupy the core portions of domal and anticlinal flexures bounded by major fault lines. Seismotectonic data on the prolonged aftershock sequence after the 2001 Bhuj earthquake (M w 7.7) reveal continuous low-to-moderate seismic activity along multiple faults covering a large area, now identified as the Kachchh Seismic Zone. This article reviews the neotectonic perspective of the active faults with the prime objective of delineating the post-Deccan Trap inversion phase of the Kachchh Basin, with an emphasis on neotectonism with regard to modern seismic activity. The datasets presented are primarily field-based neotectonic data from active fault zones that cover aspects of the tectonic geomorphology, Quaternary stratigraphy, near-surface fault traces and the nature of the fault in the shallow subsurface based on ground-penetrating radar studies. We also attempt a comparative neotectonic evaluation of each active fault in the Kachchh Basin and discuss the constraints for evolving a viable neotectonic model of the basin.

Tectonic variability along the South Katrol Hill Fault, Kachchh, Western India: Insights from geomorphic indices

Tectonic variability along the South Katrol Hill Fault, Kachchh, Western India: Insights from geomorphic indices

Response of a dryland fluvial system to climate–tectonic perturbations during the Late Quaternary: Evidence from Rukmawati River basin, Kachchh, western India

Dryland rivers, dominated by short-lived, localised and highly variable flow due to discrete precipitation events, have characteristic preservation potential, which serves as suitable archives towards understanding the climate–tectonic coupling. In the present study, we have investigated the fluvial records of a major, southerly-draining river – the Rukmawati River in the dryland terrain of southern Kachchh, in western India. The sediment records along the bedrock rivers of Kachchh register imprints of the Indian summer monsoon (ISM), which is the major source of moisture to the fluvial system in western India. The Rukmawati River originates from the Katrol Hill Range in the north and flows towards the south, into the Gulf of Kachchh. The field stratigraphy, sedimentology, along with the optical chronology suggests that a braided-meandering system existed during 37 ka period due to an overall strengthened monsoon. A gradual decline in the monsoon strength with fluctuation facilitated the development of a braided channel system between 20 and 15 ka. A renewed phase of strengthened monsoon with seasonality after around 15 ka which persisted until around 11 ka, is implicated in the development of floodplain sequences. Two zones of relatively high bedrock uplift are identified based on the geomorphometry and morphology of the fluvial landform. These zones are located in the vicinity of the North Katrol Hill Fault (NKHF) and South Katrol Hill Fault (SKHF). Geomorphic expression of high bedrock uplift is manifested by the development of beveled bedrock prior to or around 20 ka during weak monsoon. The study suggests that the terrain in the vicinity of NKHF and SKHF is uplifting at around 0.8 and >0.3 mm/a, respectively. Simultaneously, the incision in the Rukmawati River basin, post 11 ka, is ascribed to have occurred due to lowered sea level during the LGM and early Holocene period.

Active Tectonics and Geomorphological Studies in India During 2012-2016

This article reviews the literature on tectonic geomorphological studies in India published in peer reviewed journals between 2012-2016. The northward drift of the Indian plate and its collision with the Eurasian plate results in compressive stresses in Indian subcontinent. Based on the broad morphotectonic setup the studies are categorised into the Himalaya and the Indian Peninsula. Studies in the Himalaya mainly demonstrate that most of the deformation during Holocene has occurred in the sub-Himalaya and especially along the Main Frontal Thrust (MFT). Evidences of active tectonics is also reported from several parts of the Himalaya which also includes the Main Boundary Thrust (MBT) zone. The impact of active tectonics on the geomorphology along the active faults is also investigated in several areas of the Himalaya. In the Indian Peninsula Narmada-Son fault, Tapti fault and Katrol hill fault are shown to be active on the basis of sedimentological and geomorphological investigations.

Variations of seismic velocities in the Kachchh rift zone, Gujarat, India, during 2001–2013

We herein study variations of seismic velocities in the main rupture zone (MRZ) of the Mw 7.7 2001 Bhuj earthquake for the time periods [2001–05, 2006–08, 2009–10 and 2011–13], by constructing dVp(%), dVs(%) and d(Vp/Vs)(%) tomograms using high-quality arrival times of 28,902 P- and 28,696 S-waves from 4644 precise JHD (joint hypocentral determination) relocations of local events. Differential tomograms for 2001–05 reveal a marked decrease in seismic velocities (low dVp, low dVs and high d(Vp/Vs)) in the MRZ (at 5–35km depths) during 2001–10, which is attributed to an increase in crack/fracture density (higher pore fluid pressure) resulted from the intense fracturing that occurred during the mainshock and post-seismic periods. While we observe a slight recovery or increase in seismic velocities 2011–13, this could be related to the healing process (lower pore fluid pressure due to sealing of cracks) of the causative fault zone of the 2001 Bhuj mainshock. The temporal reduction in seismic velocities is observed to be higher at deeper levels (more fluid enrichment under near-lithostatic pressure) than that at shallower levels. Fluid source for low velocity zone (LVZ) at 0–10km depths (with high d(Vp/Vs)) could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content, while fluid source for LVZ at 10–35km depths could be due to the presence of brine fluids (released from the metamorphic dewatering) and volatile CO2 (emanating from the crystallization of carbonatite melts in the asthenosphere), in fractures and pores. We also imaged two prominent LVZs associated with the Katrol Hill fault zone and Island Belt fault zone, extending from shallow upper-crust to sub-crustal depth, which might be facilitating the deeper circulation of metamorphic fluids/volatile CO2, thereby, the generation of lower crustal earthquakes occurring in the Kachchh rift zone.

Three-dimensional gravity modelling of Kutch region, India

Understanding the crustal architecture of Kutch rift region, India is important due to the presence of thick Mesozoic sediments and also due to occurrence of earthquakes. We present here three-dimensional gravity models. The major features of our shallow crustal model are: (1) thickness of Deccan Traps is almost negligible towards northern side of Vigodi Fault and Katrol Hill Fault (2) Mesozoic sediment thickness is about 3km on the southern side and (3) several undulations in the basement on the Wagad Uplift. The model is consistent with the geological observation that the basin slopes towards southwest. The observed Bouguer gravity anomalies are explained by ∼10km thick Granitic upper crust of 2.7g/cm3; ∼10km thick intermediate mid crust of 2.82g/cm3; ∼15km thick mafic lower crust of 2.91g/cm3 density. Crustal thickness is about 40–45km on Wagad Uplift and near Katrol Hill Fault. The modelled gravity data do not require presence of anomalous amount of fluids, melts and/or magma chambers in the deep crust of Kutch region.

Numerical modeling of seismicity and geodynamics of the Kachchh rift zone, Gujarat, India

The numerical block-and-fault model of lithosphere dynamics and seismicity (BAFD) is used to understand crustal motion and features of the observed seismicity in the Kachchh rift zone, Gujarat, Western India. The block-model allows simulating seismicity and geodynamics simultaneously unlike other modeling approaches for studying seismicity or geodynamics. The model structure of Kachchh rift zone is composed of seven major crustal blocks separated by fault planes. Based on the orientation of boundary crustal block movements, we develop a set of numerical experiments to analyze the spatial distribution of earthquakes, frequency-to-magnitude relationships, earthquake focal mechanisms, velocity field, and fault slip rates in the model. The main results of our modeling suggest that an NNW–SSE trending compression is a principal driving force in the Kachchh rift zone that explains basic features of the regional seismicity, direction of block motions, and the presence of an extensional stress regime associated with the Cambay rift zone. Large synthetic events occur on the fault segments associated with the Allah-Bund fault, Katrol hill fault and north Wagad fault which have been causative faults for the 1819 Mw7.7 Allah-Bund, 1956 Mw6.0 Anjar and 2001 Mw7.7 Bhuj earthquakes. The frequency–magnitude distribution for both synthetic seismicity and observed seismicity shows a similar slope. The focal mechanisms of the synthetic events are found to be consistent with those of earthquakes in the region. A special attention has been paid to study long-term and post-seismic deformations. Our results are in a qualitative agreement with the GPS post-seismic observations in the Kachchh rift zone. We infer that the observed seismicity and crustal block motions are a consequence of the dynamics of the entire regional fault and block system rather than that of a single causative fault only.

Electrical Conductance Map for the Kachchh Rift Basin: Constraint on Tectonic Evolution and Seismotectonic Implications

Geomagnetic field variations recorded by an array of magnetometers spread across the Kachchh Rift basin are reduced to a set of induction arrows as a diagnostic of lateral electrical conductivity variations. A non-uniform thin-sheet electrical conductance model is developed to account for the salient induction patterns. It indicates that the imaged conductivity anomalies can be related to the sediment-filled structural lows in between the fault bounded uplifts. It is suggested that sagging structural lows preserved the marine sediments deposited during the Mesozoic sea transgression and later developed into first order embayment basins for the deposition of sediments in association with Late Eocene transgression. Depth integrated electrical conductance helped in mapping two depo-centres: along the ENE-WSW trending Banni half-Graben bounded by the Kachchh Main fault on the south and, second, along the Vinjan depression formed in response to the subsidence between the Vigodi fault and westward extension of the Katrol Hill fault together with the westward bending of the Median High. Presence of metamorphosed graphite schist clasts in shale dominated Mesozoic sequence and/or thin films of carbon resulting from the thermal influence of Deccan activity on Carbonate-rich formations can account for the high electrical conductivity anomalies seen in the depo-centres of thick Mesozoic and Tertiary sediments. Additionally two high conductivity zones are imaged encompassing a block defined by the 2001 Bhuj earthquake and its aftershocks. In agreement with gravity, magnetic and seismic velocity signatures, aqueous fluids released by recrystallizing magmatic bodies intruded in association with Deccan trap activity account for mapped high conductivity zones. High fluid pressure in such a fractured domain, surrounding the intruded magmatic plugs, perturb the regional stress concentrations to produce frequent and low magnitude aftershocks in the shallow section of the epicentral track of the 2001 Bhuj earthquake.

Near surface hydrocarbon prospecting in Mesozoic Kutch sedimentary basin, Gujarat, Western India—A reconnaissance study using geochemical and isotopic approach

Abstract The Mesozoic Kutch sedimentary basin in the western continental margin of India has known accumulations of oil and gas. However, the multilayered thick Deccan Traps of Late Cretaceous overlie the Mesozoic targets, thereby imposing inherent problems in imaging sedimentary layers below the traps for exploratory studies. One of the unconventional techniques, the near surface geochemical prospecting study, has been carried out in the onland and offshore Kutch to understand the microseepage and sources of the light hydrocarbon gases (methane through butane) observed in the soils and sediments from the basin. The geochemical and isotopic compositions of the soil samples from the Kutch onland vary in the range of C1 (methane)=3–582 ppb, C2 (ethane)=0–39 ppb, C3 (propane)=0–15 ppb and the δ13C1 (methane)=−24.9‰ to −35.7‰. The offshore sediments showed the presence of butane along with the lower hydrocarbons and the concentration ranges as C1=36–475 ppb, C2=5–27 ppb, C3=5–27 ppb, iC4=2–6 ppb, nC4=0–4 ppb, and their δ13C values are characterized as δ13C1=−37.7‰ to −44‰, δ13C2=−24.6‰ to −30.5‰ and δ13C3=−26.1‰ to −34‰. The compositional ratios and carbon isotopic signatures of light gaseous hydrocarbons suggest their thermogenic origin. The δ13C1–δ13C2 signatures of samples from offshore Kutch indicate the gases to be non-associated, originating from sapropelic liptinitic organic matter. The concentration distribution pattern of light hydrocarbons indicates the areas Bhuj, Anjar, Mandavi and Mundra to have higher values for these gases. The anomalies are also coincident with the active faults, the Bhuj Fault (BF), the Katrol Hill Fault (KHF) and the Sandsra Dungar Fault (SDF) passing through the area, which may serve as a preferential migratory pathway for the light hydrocarbon gases. The results of this study support the generation, preservation, migration and the near surface manifestation of thermogenic hydrocarbons in Kutch basin, particularly in the northern and southern regions of the study area.