M. M. Dixit

Structure and tectonics of the Proterozoic Aravalli-Delhi Fold Belt in northwestern India from deep seismic reflection studies

Abstract Seismic imaging of the crust along a 400-km-long deep seismic reflection profile across the Palaeo/Mesoproterozoic Aravalli-Delhi Fold Belt, in the northwestern Indian Shield, brings into focus its complex structure and provides clues to understand the geological processes involved in the evolution of this belt. The reflectivity pattern varies considerably for different crustal units along the profile. The deep-crustal reflection data image two sets of oppositely dipping strong reflection bands, from upper- to lower-crustal levels. These are identified as the signatures of the collision corresponding to Aravalli and Delhi orogeny. The data also exhibit a clear Moho and strong lower-crustal reflections near the collision boundaries. A stack of dipping reflections from the top of the Moho to the surface is identified as a major thrust fault indicating that the Proterozoic collision and deformation were primarily thick-skinned in nature

A crustal density model across the Cambay basin, India, and its relationship with the Aravallis

Abstract The Cambay rift basin in the northwest Indian platform shows high Bouguer gravity anomaly values as compared to the values outside the basin. The seismic results show the presence of a large thickness of sediments, which should have produced a gravity low of about 50–60 mGal. The deep-seismic sounding results along the strike of the basin show the presence of a velocity of 7.3 km/s at 23–25 km depth, followed by the Moho at a depth of 31–33 km. Two-dimensional density modelling of the seismic structure across the Cambay basin shows that the crust is thinner underneath the basin, and is associated with a high-density lower crust. A comparison of the gravity anomaly across the Cambay basin with that across the Proterozoic Aravalli/Delhi trends suggests crustal thinning in both the regions, on either side of which the Moho deepens rapidly.

Seismicity, faulting, and structure of the Koyna‐Warna seismic region, Western India from local earthquake tomography and hypocenter locations

Although seismicity near Koyna Reservoir (India) has persisted for ~50 years and includes the largest induced earthquake (M 6.3) reported worldwide, the seismotectonic framework of the area is not well understood. We recorded ~1800 earthquakes from 6 January 2010 to 28 May 2010 and located a subset of 343 of the highest-quality earthquakes using the tomoDD code of Zhang and Thurber (2003) to better understand the framework. We also inverted first arrivals for 3-D Vp, Vs, and Vp/Vs and Poissons ratio tomography models of the upper 12 km of the crust. Epicenters for the recorded earthquakes are located south of the Koyna River, including a high-density cluster that coincides with a shallow depth (<1.5 km) zone of relatively high Vp and low Vs (also high Vp/Vs and Poissons ratios) near Warna Reservoir. This anomalous zone, which extends near vertically to at least 8 km depth and laterally northward at least 15 km, is likely a water-saturated zone of faults under high pore pressures. Because many of the earthquakes occur on the periphery of the fault zone, rather than near its center, the observed seismicity-velocity correlations are consistent with the concept that many of the earthquakes nucleate in fractures adjacent to the main fault zone due to high pore pressure. We interpret our velocity images as showing a series of northwest trending faults locally near the central part of Warna Reservoir and a major northward trending fault zone north of Warna Reservoir.

Relationship of the Cambay rift basin to the Deccan volcanism

Abstract The Cambay rift basin in northwestern India is one of the three basins that originated between the early Jurassic and Tertiary, during Indias drift after the break-up of Gondwanaland. The thickness of the Quaternary and Tertiary sediments in some parts of the basin varies between 3000 and 5500 m. These sediments are underlain by the Deccan trap volcanics which form the basement of the Tertiary sediments. The presence of a high velocity lower crust and crustal thinning across the Cambay basin are in agreement with the geochronological model of volcanic accretion for the Deccan traps. The large thickness of volcanics in the Cambay basin, estimated to be between 1000 and 3200 m, and their rapid eruption suggest that the Cambay rift originated in late Cretaceous. Large post-volcanic subsidence of the rift is evidenced by the presence of up to 4000 m thick Tertiary sediments which were deposited in various phases of basin development. The first phase of basin subsidence appears to be associated to the rapid withdrawal of the magma from the Reunion mantle plume due to the proximity of the Cambay rift to the axis of the plume.

Structure of the Koyna‐Warna Seismic Zone, Maharashtra, India: A possible model for large induced earthquakes elsewhere

The Koyna-Warna area of India is one of the best worldwide examples of reservoir-induced seismicity, with the distinction of having generated the largest known induced earthquake (M6.3 on 10 December 1967) and persistent moderate-magnitude (>M5) events for nearly 50 years. Yet, the fault structure and tectonic setting that has accommodated the induced seismicity is poorly known, in part because the seismic events occur beneath a thick sequence of basalt layers. On the basis of the alignment of earthquake epicenters over an ~50 year period, lateral variations in focal mechanisms, upper-crustal tomographic velocity images, geophysical data (aeromagnetic, gravity, and magnetotelluric), geomorphic data, and correlation with similar structures elsewhere, we suggest that the Koyna-Warna area lies within a right step between northwest trending, right-lateral faults. The sub-basalt basement may form a local structural depression (pull-apart basin) caused by extension within the step-over zone between the right-lateral faults. Our postulated model accounts for the observed pattern of normal faulting in a region that is dominated by north-south directed compression. The right-lateral faults extend well beyond the immediate Koyna-Warna area, possibly suggesting a more extensive zone of seismic hazards for the central India area. Induced seismic events have been observed many places worldwide, but relatively large-magnitude induced events are less common because critically stressed, preexisting structures are a necessary component. We suggest that releasing bends and fault step-overs like those we postulate for the Koyna-Warna area may serve as an ideal tectonic environment for generating moderate- to large- magnitude induced (reservoir, injection, etc.) earthquakes.

Crustal velocity structure along the Nagaur–Rian sector of the Aravalli fold belt, India, using reflection data

Abstract Deep reflection data was collected along the Nagaur–Nandsi profile, Rajasthan (across the Aravalli fold belt), India, to study the tectonic pattern and evolutionary history of the Northwestern Indian Shield. However, as there was no coincident wide-angle/refraction input/well data input, no velocity picture could be derived to supplement the reflection results. In the present study, as velocity information is very important for a realistic analysis of the reflection time sections, we have used the interval velocity derived from stack velocities (Hubral, P. and Krey, T., 1980. In: Larner, K.I. (Ed.), Interval Velocities from Seismic Reflection Time Measurements. SEG, Tulsa, OK), to arrive at a possible velocity-depth section, along the 90-km Nagaur-Rian sector. This was followed by dynamic forward modeling, using the interval velocity model as the starting model. The resultant velocity depth section has a four-layer structure. One of the layers (third) is a low-velocity layer. The lateral extension of this layer vis-a-vis with other layers is presented, for a better understanding of the structure and tectonics of the Aravalli-Delhi fold belt.

Three dimensional velocity structure of the Koyna-Warna region using local earthquake tomography

A dense seismic network (∼100 stations) was operated in the Koyna-Warna region from January 2010 to May 2010, that allow us to collect 400 high-quality local earthquake data of magnitude less than 4. In this region, the fault structure and tectonic setting that accommodate the induced seismicity is not well understood. To investigate the seismotectonics of the region, we have inverted 7826 P- and 7047 S-P arrival times for 3-D Vp and Vp/Vs tomographic models along with hypocenters parameters in the region. Although, Dixit et al. (2014) have performed 3-D local earthquake tomography with double-difference tomography code using catalog differential time data. In this paper, Simulps14 code on the same data set is applied. For better approach P arrival time and S-P travel times are inverted directly for Vp, Vp/Vs variations and earthquake locations. High Vp ∼5.9 to 6.5 and low to high Vp/Vs ∼1.69-1.74 imaged in the hypocenter region. These features interpreted as a fluid bearing rock mass under high pore pressure. It is also observed that below the trap basement form a local topography depression between the Koyna and Warna Reservoirs. To the South of the Warna reservoir, intense seismic activity defines a major cluster of ∼ 5 km width at 3 to7 km deep, located under the trap, where the basement is deepening. Such regions are inferred to be associated with the seismically active faults zones. The obtain velocity anomalies are reliable down to a depth of 10 km. This is also confirmed by the analysis of three resolution parameters viz. Hit count, Derivative Weight sum (DWS) and Resolution Diagonal Elements (RDE).