R.K. Verma

An analysis of the gravity field in Northeastern India

Abstract Northeastern India comprises several major tectonic units including the Shillong Plateau, the Upper and Lower Brahmaputra (Assam) Valleys, the Northeastern Himalaya, the Naga Hills and the Bengal Basin. The area lies approximately between latitude 23–28°N, and longitude 88–96°E. A revised Bouguer anomaly map of the area with nearly 400 new observations is presented. A Pratt-Hayford isostatic anomaly map using all available information is also given. The whole area shows a large variation in gravity anomalies, Bouguer anomalies show a variation from +40 mGal over the Shillong Plateau to −250 mGal over the northwestern part of the Upper Assam Valley. Isostatic anomalies also show a variation, from +100 mGal over the Shillong Plateau to −125 mGal in the Assam Valley. A study of the Bouguer anomaly map shows that the gravity field is considerably influenced by low-density sediments overlying the Assam Valley as well as the Bengal Basin. A geological correction for these sediments was computed for a few selected profiles for which geological information was available from seismic and bore-hole data along the Upper Assam Valley and the Bengal Basin. The magnitude of the geological correction was found to be of the order of 50 to 100 mGal for the Assam Valley and 40 to 130 mGal for the Bengal Basin. Models for the crust and mantle underlying the Shillong Plateau, the Upper Assam Valley and the Bengal Basin were constructed considering the nature of geologically corrected Bouguer anomalies as well as isostatic anomalies. Gravity data suggest that the crust underlying the Shillong Plateau is probably denser as well as thicker than normal for its elevation. The Assam Valley may overlie a crust which is thicker than normal for its topography, and the crystalline solid crust underlying a large thickness of sediments of Bengal Basin could be denser as well as thinner than the normal continental crust.

Earthquake mechanisms and tectonics in the Assam-Burma region

Eleven new focal mechanisms from earthquakes in the Assam-Burma region have been determined using P-wave first-motion directions reported in the Bulletins of the International Seismological Centre (Edinburgh). Out of them, eight mechanisms indicate thrust faulting, two normal faultings and one strike-slip faulting. In the thrust type of mechanism solutions, sense of motion on the shallow dipping of the two nodal planes is consistent with underthrusting beneath the arc-like mountain ranges. Seismic slip vectors strike in almost northerly direction along the eastern Himalayas and in almost easterly direction along the Burmese arc. A predominance of thrust faulting is consistent with geological evidences of thrusting and uplift in the Himalayas and the Assam-Burma region.

Earthquake mechanisms and tectonic features of Northern Burma

Abstract A seismicity map of northeast India and Burma for the period 1900–1970 is given. A zone of intermediate-depth foci dipping to the east of Arakan Yoma is found to be present underneath the Burmese plains. The seismic zone appears to be of V shape. Six new focal-mechanism solutions of earthquakes in Burma have been determined. These mechanisms, in conjunction with earlier studies of Fitch (1970), Rastogi et al. (1973) and Tandon and Srivastava (1975), indicate that normal as well as thrust faulting takes place in Burma. The compressive stress axis is found to be more nearly vertical than horizontal. The results are discussed in the light of plate-tectonic theory as applied to the Indo-Burma regions.

Gravity field, structure and tectonics of the Eastern Ghats

Abstract The Eastern Ghats are a prominent topographic feature on the Indian Peninsula, stretching from the southern tip of the peninsula to near Bhubaneswar (20°N, 86°E) along the east coast. The belt is characterised by occurrences of high grade metamorphic rocks such as pyroxene granulites, sillimanite gneisses, charnockites and gabbro-anorthosite masses. The gravity field over the Eastern Ghats is appreciably positive as compared to the surrounding low grade gneissic terrain. Analysis of the gravity field along the coastal and southern granulite terrain comprising the Eastern Ghats shows that a large number of gravity highs are associated with charnockites of basic and intermediate nature as well as gabbro-anorthosite masses. The lows appear to be associated with acid charnockites, syenite masses or granitic intrusives. The boundary between the Eastern Ghats terrain and the adjoining Dharwar/Bastar cratons appears to be a faulted one. The crust underneath the Eastern Ghats is inferred to be of a higher density than that of the Dharwar/Bastar cratons to its west. The gravity field over the Eastern Ghats is compared to that of similar terrains in other parts of the world. It is inferred that the Eastern Ghats are characterised by a crust of higher than normal density.

Gravity interpretation of the Dharwar greenstone-gneiss-granite terrain in the southern Indian shield and its geological implications

Abstract The Dharwar craton in the southern Indian shield has a wide distribution of volcano-sedimentary sequences surrounded by a vast gneissic complex, both of which have been intruded by younger granites. A gravity anomaly map of this craton, compiled from all the available data, is analysed here to study the structures and depths of the greenstone belts, the mode of granite emplacements and the greenstone-gneiss-granite associations in general. The anomaly map is a mosaic of well-defined gravity highs and lows characterizing the dense volcano-sedimentary sequences and exposed and/or concealed granites respectively. Gravity modelling indicates that the Shimoga belt has a limited depth range of only 3–4 km while the Chitradurga and Sandur belts have greater depths of over 10 km. The structures inferred for the Dharwar formations are alternating bands of synclines, filled with dense schistose rocks, separated by anticlinal ridges of gneisses and granites.

Seismotectonics of the Hindukush and Baluchistan arc

Abstract A seismicity map of that part of the Pakistan-Afghanistan region lying between the latitudes 28° to 38°N and longitudes 66° to 75°E is given using all available data for the period 1890–1970. The earthquakes of magnitude 4.5 and above were considered in the preparation of this map. On the basis of this map, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent, mutually orthogonal, seismicity lineaments, namely the northvestern and the north-eastern trends, characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well-defined zones of intermontane seismicity exhibiting north-south alignment. Thirty-two new focal-mechanism solutions for the above-mentioned region have been determined. These, together with the results obtained by earlier workers, suggest the pre-dominance of strike-slip faulting in the area. The Hazara Mountains, the Sulaiman wrench zone and the Kirthar wrench zone, as well as the supposed extension of the Murray ridge up to the Karachi coast, appear to be mostly undergoing strike-slip movements. In the Hindukush region, thrust and strike-slip faulting are found to be equally prevalent. Almost all the thrust-type mechanisms belonging to the Hindukush area have both the nodal planes in the NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of P-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of the seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast of Uzbekistan on the northwest through Hindukush. Almost orthogonal to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting. The above results are discussed from the point of view of convergence of the Indian and Eurasian plates in the light of plate tectonics theory.

Gravity field over Singhbhum, its relationship to geology and tectonic history

Abstract The gravity field over Singhbhum and adjoining areas lying between latitude 21° to 23°15′N and longitude 84° to 87°30′E has been analysed. The region has a very complex Precambrian history dating as far back as 3200 m.y. and extending up to 850 m.y., during which time it experienced a number of orogenic cycles. The activity has left an imprint on the gravity field which is marked by prominent gravity highs and lows. The highs are invariably surrounding the Singhbhum granite batholith, whereas the lows are conspicuous over the granites and their margins. The regional gravity field has been removed using a 4th order polynomial surface. The residual anomalies over prominent highs and lows have been interpreted using two-dimensional and three-dimensional models for underlying rock formations. It is inferred that the gravity highs are generally related to synclinal structures filled with sedimentary or metasedimentary formations and volcanics, such as the Iron Ore Group, the Singhbhum group, Dhanjori and Simlipal basins. The gravity lows can be attributed to a considerable thickness of granites of varying density, which could have been formed during different metamorphic/orogenic cycles by melting of a preexisting sialic crust.

Seismotectonics of the himalaya, and the continental plate convergence

Abstract The relationship between seismicity and tectonics of the northwestern, central and part of the eastern Himalaya lying between longitudes 74–88°E and latitudes 26–35°N has been studied. A seismicity map of the area for the period 1890–1970 is presented. A strain-energy release map of the Himalaya was prepared following the method discussed by Allen et al. (1965). On the basis of this map it is suggested that the Himalaya does not behave as a single unit so far as the seismic activity is concerned. It could be divided appropriately into three different units; 1. (1) the Panjab Himalaya; 2. (2) the Kumaon Himalaya; and 3. (3) the Nepal Himalaya. It is suggested that the extension of Aravalli structures into the Himalayan regions has played a role in the tectonics of the Kumaon Himalaya, and probably is the cause of the complex nature of seismicity of the region. In the Kumaon Himalaya the maximum strain-energy release is related to the Main Central Thrust (MCT), whereas in the Panjab as well as in Nepal Himalaya it is related to the activity of the Main Boundary Fault (MBF). The strain-energy release characteristics of the two thrusts show that their mechanisms of storage and release of energy are different. Strain-energy release through the MCT seems to be more uniform as compared to the MBF along which the release of energy has been mostly abrupt, through large magnitude earthquakes. Focal-mechanism solutions of Himalayan earthquakes located north as well as south of the Indus Suture Zone indicate that the Indian plate is underthrusting the Tibetan plate towards the north, wehereas the latter is underthrusting the Indian plate towards the south. P-axes lying to the north as well as south of the Indus Suture Line are shallow dipping. This confirms the view that the present-day seismicity of the area is a result of continental-continental collision and not of lithospheric subduction.

Gravity field and structures of the Rajmahal Hills: Example of the Paleo-Mesozoic continental margin in eastern India

Abstract A narrow strip of Gondwana basins separates the Rajmahal traps from the peninsular shield in eastern India. This part of the shield margin is associated with a conspicuous gravity high of 100 km wavelength and 48 mGal amplitude over an area of 25,000 km 2 . Second order residual anomalies due to Gondwana sediments and traps are superposed on this wider gravity high. Gravity interpretation, partly constrained by seismic data, suggests that the wider high is caused by a denser metamorphic layer (amphibolite and granulite) up to 3.5 km thick. The metamorphic layer also extends below the eastern Rajmahal hills where the Gondwanas, traps and younger sediments have covered it. The Gondwanas are downfaulted against the shield edge and are preserved over an irregular basin floor whose deepest part underlies the eastern flank of the Rajmahal hills adjacent to the Bengal basin. It is inferred that the Gondwanas were deposited over a rifted and highly faulted shield margin that was intruded by the Rajmahal traps nearly 100 m.y. ago. High-grade metamorphism along the shield edge presumably preceded the continental rifting, perhaps occurring in the Precambrian as a part of the Eastern Ghats orogeny, along the east coast of India.

Seismicity and tectonics in south China and Burma

Abstract A seismicity map of southwest-south China, eastern Burma and adjoining areas lying between latitudes 19° and 35°N, and between longitudes 91° and 108° E for the period 1900–1975 (except the period 1952–1954) is presented using all available data. The relationship between seismicity and tectonics of the area has been studied. On the basis of analysis it is suggested that the seismicity in the region is well-defined by several arcuate fracture belts surrounding the Assam wedge. Intense seismic activity is observed along the Burmese arc, Assam wedge, Hengtuan ranges, Kang Ting and Red River faults, Kang Tien ranges and western Szechwan province. Twenty-five new focal mechanism solutions for earthquakes in south China and the adjoining areas have been determined. These, in conjunction with the results obtained through earlier studies, suggest the dominance of thrust and normal faulting in the Burmese arc as well as in the Himalayan region, while normal faulting predominates in the mountainous regions in south China. The Kang Ting and Red River faults are mostly associated with strike-slip faulting. The results indicate that in Burma, the seismic slip vectors are directed towards the east from the Arakan-Yoma side and towards the west from the Shan plateau side, thus creating a V-shaped seismic zone underlying the Irrawaddy plains. Focal mechanism solutions of Himalayan earthquakes located north as well as south of the Indus suture zone indicate that the Indian plate is underthrusting the Tibetan plateau towards the north, whereas the latter is underthrusting the Indian plate towards the south. It further appears that following the convergence of the Indian and the Eurasian plates, the Tibetan and surrounding landmass is moving east to southeast. This, in turn, seems to create strike-slip faulting along giant faults in eastern Tibet and south China.