Manoj Mukhopadhyay

Deep structure and tectonics of the burmese arc: constraints from earthquake and gravity data

Abstract Active subduction of the Indian plate is currently occurring beneath the Burmese arc along an east dipping Benioff zone which extends to a depth of about 180 km. The overriding Burma plate has an appearance of an inland seismic slab that is deflected downwards in the vicinity of the Benioff zone. A crustal seismic zone some 60–80 km east of the Benioff zone correlates to backarc activity. A triangular aseismic wedge in the top part of the crust outlines the Central Belt molasse basin east of the Burmese foldbdt. Fault plane solutions show that the Burmese Benioff zone is characterized by shallow angle thrusting at its upper edge whereas down-tip tensional events dominate its lower edge. Most of the backarc seismicity is accounted for by the Sagaing transform or by the activity of the Shan scarp normal fault zone at the margin of the Asian plate. A gravity anomaly pair with amplitude of 175 mGal coincides with the 1100-km long Burmese arc lying in a north-south direction. The gravity anomalies along a profile in central Burma and in adjacent areas of the Bengal basin are interpreted in terms of plate subduction as well as near-surface mass anomalies. This suggests that sediments below the Central Belt may have an average thickness of the order of 10 km but may be as thick as 15 km at the subduction zone. The oceanic crust underlying deeper parts of the Bengal basin experiences phase transition at about 30 km depth in a Benioff zone environment east of the Burmese foldbelt. Several thrust planes are present within the folded and deformed Cretaceous-Tertiary sediments of the fold belt; these are often associated with ophiolites and basic to ultrabasic rocks. A low density zone, at least 60 km wide, underlies the andesitic volcanic axis in the overriding plate.

Active transverse features in the central portion of the Himalaya

Abstract Many transverse lineaments or faults across the Himalaya are already known. Interpretation of satellite images of the central portion of the Himalaya given here adds to that number and suggests their continuity from the Himalaya to its foredeep. Several such faults appear to be seismically active, Their shear motion, recognized from geologic data and focal mechanism solutions, compares well with a classical stress model when applied to northward progress of India under the Himalayan collision front.

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.

Seismicity and plate deformation below the Andaman arc, northeastern Indian Ocean

Abstract The seismic activity originating below the Andaman arc-Sea region is generally discernible into fore- and back-arc seismic zones which are traceable for nearly 1500 km in a N-S direction at the junctures between the Indian, Burma and SE Asia plates. The fore-arc seismicity displays an east-dipping (40–55°) Benioff zone upto about 200 km focal depths. Details of the Benioff zone, in correspondence to the observed gravity field, are discussed in four N-S sectors, which suggest some significant variations in the configuration of the Benioff zone. The back-arc seismicity affects only the top 40–45 km of the lithosphere below the Andaman Sea, where the back-arc spreading ridge splits the volcanic arc. Stress distribution and faulting due to earthquakes below the Andaman-West Sunda arc are studied here using 68 focal mechanism solutions. Their most significant results are: low-angle thrust events occur along the upper edge of the descending Indian plate, downdip tensional events have steeply dipping ( ⩾ 60°) nodal planes, and normal faulting takes place in most parts of the Benioff zone along moderately dipping (30–45°) planes. Downdip compressional events (high-angle reverse fault, nodal plane dip > 60°) or reverse faulting along moderately dipping (30–45°) nodal planes also occur below the Andaman arc. The compressive earthquakes dominate the shallower level of the subducting slab, and the tensional stress observed locally in north part of the Andaman Sea may be an outcome of the weak coupling between the descending and overriding plates. Generally, a more or less complete sequence of faulting i.e., thrusting below the trench, normal faulting below the fore arc, and strike-slip motion along the inner edges of the fore arc characterize the Andaman-West Sunda arc. In the southern Andaman region, a rather oblique convergence between the Indian Ocean and the SE Asia plates is needed to explain the existence of a somewhat contorted Benioff zone, in which, compressional stress dominates in deeper lithosphere. Oceanward, the Ninetyeast Ridge also impinges on the subduction zone in this region. Left-lateral shear motion along the east margin of the Ninetyeast Ridge is further inferred by the results of focal mechanism solutions.

Seismotectonics of subduction and back-arc rifting under the Andaman Sea

Abstract Active underthrusting of the Indian plate below the Burma plate produces seismic activity under the Andaman and Burmese arcs. It is suggested that significant differences exist in the configuration of the Benioff zone underlying the two arcs. Several parallel and linearly extensive fault zones are outlined by epicentral distribution in the Andaman arc. The Andaman back-arc spreading ridge is defined by a relatively narrow shallow seismic zone that orients NNE to SSE in north and south parts of the Andaman Sea respectively. The spreading ridge is offset by active transform faults. On the far north the spreading ridge appears to connect to a major transform fault in Burma across the Burmese coast. Focal mechanism study provides evidence for tensional stress near the inner wall of the Andaman trench, dominant compressive deviatoric stress within the overriding Burma plate, and tensional stress under the back-arc spreading ridge.

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 and deep structure of the Bengal Fan and its surrounding continental margins, northeast Indian Ocean

Abstract A revised gravity anomaly map for the northeast Indian Ocean shows that the shelf edge underlying the eastern continental margin of India is a rather narrow but extensively linear gravity low (minimum free-air = −149 mGal). The Bengal Fan seaward of the shelf has a depressed gravity field (average free-air = −20 to −30 mGal) in spite of the enormous thickness of sediments of as much as 10–15 km. The two buried ridges below the Bengal Fan—the 85° East and 90° East Ridges—have a large negative (−75 mgal) and a substantial positive (40 mGal) free-air anomaly, respectively. The Andaman and Burmese arcs lying along the east margin of the Bengal Fan are active subduction areas which have typical bipolar gravity signatures with a maximum amplitude of 300 mGal. Gravity interpretation for three regional traverses across the central and northern parts of the Bengal Fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposes the transitional crust under the eastern continental slope of India; the 85° East Ridge, that was created when the Indian Ocean lithosphere was very juvenile, appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal block consisting of the ridge material embedded in the upper lithosphere; while the 90° East Ridge submarine topography/buried load below the Bengal Fan is probably isostatically compensated by a low-density mass acting as a cushion at the base of the crust. The Bengal Fan crust, with its thick sediment layer, is carried down the Andaman subduction zone to a depth of about 27 km where, possibly, phase transition takes place under higher pressure. The maximum sediment thickness at the Andaman-Burmese subduction zone is of the order of 10–12 km. The gravity model predicts a low density zone about 60 km wide below the Andaman-Burmese volcanic arc, penetrating from crustal to subcrustal depths in the overriding Burma plate. A more complex density distribution is however, envisaged for the Andaman volcanic arc that is split by the Neogene back arc spreading ridge. The ocean-continent crustal transition possibly occurs farther east of the volcanic arc; below the Shan plateau margin in Burma or below the Mergui terrace at the Malayan continental margin east of the Andaman Sea.

Gravity anomalies and deep structure of the Andaman arc

The Andaman arc is associated with a major Free-air anomaly pair of mean amplitude 180 mgal. Two-dimensional gravity interpretation suggests significant mass anomalies below the arc that presumably have resulted due to subduction of the Indian plate below the Burma plate. It is inferred that the Andaman trench is of asymmetric V-shape containing about 7 km sediments. An outer bathymetric rise seaward of the trench possibly corresponds to a lithospheric flexure by 500 m. The Cretaceous-Tertiary sediments constituting the Andaman sedimentary arc attain their maximum thickness of about 13 km under the Nicobar. Deep at the subduction zone. At this location a mafic mass is emplaced within the sedimentary section. The underlying oceanic crust apparently experiences phase transition at about 27 km depth in a Benioff zone environment. The Andaman volcanic arc underlies a low density zone that is at least 60 km wide. Along the east margin of the Andaman Sea, cuustal transition presumably occurs below the Mergui Terrace at the Malayan coast.

Seismotectonics of transverse lineaments in the eastern Himalaya and its foredeep

Abstract The Himalayan collision zone and the Burmese subduction zone lie in rather close vicinity across northeast India. Their possible interaction produces complex tectonics and a high level of seismicity in the region. Several prominent transverse lineaments across the eastern Himalaya and its foredeep appear to be seismically active. Quite a few of the active lineaments are regionally extensive, even transgressing the Bengal basin. A focal mechanism study indicates that active lineaments are either normal or strike-slip faults. For the highly active Shillong-Mikir massif (a fragmented portion of the Indian shield) this correlation is less clear, although it appears that some of the activity may be associated with the northeasterly lineaments crossing the massif into the eastern Himalayan foredeep. This presumably results due to drag experienced by the Indian lithosphere near its margins under the Himalayan and Burmese arcs. The Assam Valley, which is the common foredeep for both the arcs, is remarkably aseismic though it is surrounded by active regions. Mainly thrusting mechanisms characterize the earthquakes which originate from the Himalayan and Burmese arcs adjoining the Valley. A model of basement reactivation below the Valley is proposed in order to explain the style of tectonic deformation and current seismicity in the Himalayan and Burmese orogens adjoining the Valley.

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.