Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Manoj Mukhopadhyay ¹

Affiliations
1 Department of Applied Geophysics, lndian School of Mines, Dhanbad - 826 004, IN

Abstract

The West Bengal basin (WBB) which developed at the margin of the eastern Indian shield in response to Gondwana rifting underlies a deep Gondwana trough covered by the Rajmahal (Cretaceous) basalts and younger sediments. Available crustal seismic data for four profiles in southern half of the basin, as well as other available seismic, drill-hole and gravity data, provide valuable information about the deep crustal structure and evolution of the WBB. The derived crustal model suggests significant mass anomalies in this part of the Indian palaeocontinental margin. The main results of the model are: (a) the Indian shield crust undergoes almost 33% thinning (36 to 24 km) across the WBB within a short distance of 130 km; (b) anomalous crustal layers appear in the top and middle parts of the crust below the shield-margin and aIso below the basin; (c) the top crust (8-12 km depth) at the shieldrnargin is less dense by an estimated amount of -0.14 g/cm³, corresponding to a low velocity layer as detected by the crustals eismic data, while its underlying crust (12-36 km depth) is significantly denser but (d) further east, a wider low-density crust occupies the central part of the basin between midcrustal depth (16 km) and the crustal base. Rapid crustal thinning occurs along its eastern flank, where a denser lower crustal block is located adjacent to the Eocene 'hinge zone' that separates the WBB from deeper parts of the Bengal delta. The intense zone of crustal necking below the WBB further bears a spatial relationship to the anomalous crustal layers and their attendant faults. The origin of these features is ascribed to the Gondwana rifting episode, resulting in the formation of a highly stretched crust at the Indian shield margin, where the Rajmahal volcanism eventually erupted.

Keywords

Crustal Structure, Gravity, Deep Seismic Sounding, West Bengal Basin.

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Manoj Mukhopadhyay ¹

Affiliations
1 Department of Geophysics, Indian School of Mines, Dhanbad 826004, Bihar, IN

Abstract

The Bengal anorthosite is the only known major anorthosite occurrence in India outside the Eastern Ghats province. The characteristic Eastern Ghats gravity 'high', however, continues northward across the Bengal anothosite. Recent gravity mapping shows that this anorthosite is associated with a Significant positive anomaly of 20 mgal amplitude and 25 km width. The gravity high also extends far another 100 km westward into the Chhotanagpur gneissic terrain. Gravity interpretation suggests a more mafic phase of the anorthosite underlying its southern part, the mafic mass being of gabbroic composition. The mafic mass is inferred to have the form of an inverted cone, at least 3.3 km thick, below a thin veneer of anorthositic rocks, but its thickness reduces westward inside the Chhotanagpur gneisses where the mafic mass assumes a more tabular form. Two large granitic stocks, upto 1.8 km thick and 20km wide, are inferred to have intruded the country rocks to the immediate south of the anorthosite suite. The granites are supposedly derived by palingenesis of in situ leptynites.

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Manoj Mukhopadhyay ¹, S. K. Arora ²

Affiliations
1 Department of Applied Geophysics, Indian School of Mines, Dhanbad - 826 004, IN
2 Seismology Division, Bhaba Atomic Research Centre, Chembur, Mumbai - 400 085, IN

Abstract

The Western Indian shield is seismically active as shown by both historical seismicity records and recently acquired instrumental data. Moderate to large earthquakes have occurred in the region but they do not clearly correlate with known tectonic features or faults. Current seismicity for this part of the shield for the period 1977-97. as evidenced from Gauribidanur Seismic Array (GBA) data, is found to be largely restricted to an area of about 80 km radius which accounts for almost 85 percent of the activity. The area is referred to as the Surat-Daman Seismic Zone (SDSZ) which has a general correspondence to the regional pattern of seismicity over the Western Indian shield established from both instrumental and non-instrumental data for a longer time span. The Indian shield crust is known to exhibit a sharp reduction in crustal thickness from 36 km below the Cambay Rift to about 20 km below the SDSZ within a distance of about 70 km. The thin shield crust is dissected into several faulted blocks that arc deformed to various degrees; some of these crustal blocks are seismically active. These features suggest that seismic activity in the region possibly relates to the fossil trace of the Khambat Plume along the Western continental margin near the Gulf of Cambay. The fossil plume appears to be located at least 100 km eastward onland. Bouguer anomalies over the fossil plume are largely positive with amplitudes upto 130 mgal, rendering further support to the presence of anomalous crust in the region. Available stress data from focal mechanism solutions indicate compressive stress in east-west to NNE-WSW direction. This suggests that stresses responsible for rift tectonics are not significant at the present stage; rather, the plume tectonics seems to be regionally predominant.

Keywords

Seismicity, Tectonics, Fossil Plume, Gulf of Cambay.

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Manoj Mukhopadhyay ¹

Affiliations
1 Indian School of Mines, Dhanbad 826 004, IN

Abstract

No Abstract.

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Kamal Kant Sharma ¹, Ritesh Purohit ¹, Manoj Mukhopadhyay ²

Affiliations
1 Government College, Sirohi - 307 001, Rajasthan, IN
2 Indian School of Mines , Dhanbad- 526 004, Jharkhand, IN

Abstract

No Abstract.

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Basab Mukhopadhyay ¹, Anshuman Acharyya ¹, Manoj Mukhopadhyay ², Sujit Dasgupta ¹

Affiliations
1 Geological Survey of India, Central Headquarters, 27 J. L. Nehru Road, Kolkata - 700016, IN
2 Department of Geology & Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451, SA

Abstract

An extraordinarily strong and persistent earthquake swarm (Andaman swarm 2005) originated in the Andaman back-arc following the aftershock sequences of the 26 December 2004 Sumatra earthquake. The swarm (n = 651, mb_max= 5.9) came mainly in two phases: January 26-31 and Feb. - Aug. 2005, in an area of size 90 x 40 km², at the centre of which lies a broad bathymetric depression and high gravity zone. The swarm demonstrates a complex faulting series, initially the strike-slip motion followed by normal faulting in repetitive sequences, whose representative fault planes orient at high angle to the regional faults. The swarm character as well as the distribution of stress-axes and their correlation to tectonic features lends speculation for formation of a nascent rift segment in NW-SE direction at the doorstep of the Sewell Seamount. The swarm has given rise to 21 episodes of rifting activities of variable time extent within 26 -31 January 2005. The r-t plots corresponding to the swarm data, modelled with variable hydraulic diffusivity (D) values 4, 6, 8 and 10 m²/s, suggest for excess pressure front from ascending magmatic fluid. This eventually heralded the rifting; causing pore pressure perturbations that propagated in accordance with known diffusion parabolic equations.

Keywords

Andaman Swarm 2005, Focal Mechanisms, Pore Pressure, R-T Plot, Nascent Rift.

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Sujit Dasgupta ¹, Manoj Mukhopadhyay ²

Affiliations
1 Geological Survey of India, 27, J.L. Nehru Road, Calcutta - 700 016, IN
2 Department of Applied Geophysics, Indian School of Mines, Dhanbad - 826 004, IN

Abstract

The Barren Island volcano in central Andaman Sea erupted afresh on March 29, 1991, since its last eruptions in 1789 and 1832. Tectonic framework of the Barren Island volcano is delineated by a set of seismically mapped faults, including the West Andaman Fault, which. connects to the Semangko Fault in Sumatra. The morphology of Andaman Benioff zone as seen on a vertical section reveals a seismic gap at 90-110 km depth. Spatially it relates to the zone of current volcanism of the Barren and its neighbouring Alcock Seamount in a structurally depressed area. The seismic gap in the Benioff zone is interpreted as a zone of partial melting in the descending Indian Ocean lithosphere which prevents stress accumulation owing to the decreased viscosity. Best double couple solutions for the Barren Island earthquakes of October 10 and December 25, 1990, which preceded the volcanism suggest that rupturing through normal dip-slip faulting prevailed in the overriding Burma plate prior to the recent volcanism. Pronounced back-arc seismicity at shallower depths is seen further to the east and southeast of the Alcock Seamount; this relates to back-arc rifting under the Andaman Sea.

Keywords

Seismicity, Andaman, Barren Island, Volcanism.

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Basab Mukhopadhyay ¹, Manoj Mukhopadhyay ², Sujit Dasgupta ³

Affiliations
1 Geological Survey of India, Central Headquarters, 27, J.L. Nehru Road, Kolkata - 700 016, IN
2 Department of Geology and Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451, SA
3 Geological Survey of India, Central Headquarters, Kolkata, IN

Abstract

The plate margin features defining the Arabian Sea Triple Junction (ASTJ) are: the Aden Ridge (AR), Sheba Ridge (SR) with their intervening Alula-Fartak Transform (AFT), Carlsberg Ridge (CR) and Owen Fracture Zone (OFZ). Exact nature of ASTJ is presently debated: whether it is RRF (ridge-ridge-fault) or RRR (ridge-ridge-ridge) type. A revised seismicity map for ASTJ is given here using data for a period little more than a century. "Point density spatial statistical criterion" is applied to short-listed 742 earthquakes (mb≥4.3), 10 numbers of spatio-temporal seismic clusters are identified for ASTJ and its arms. Relocated hypocentres help better constraining the cluster identification wherever such data exist. Seismic clusters actually diagnose the most intense zones of strain accumulation due to far field as well as the local stress operating at ASTJ. An earthquake swarm emanating from a prominent seismic cluster below SR provides an opportunity to investigate the pore pressure diffusion process (due to the active source) by means of "r-t plot". Stress and faulting pattern in the active zones are deduced from 43 CMT solutions. While normal or lateral faulting is characteristic for these arms, an anomalous thrust earthquake occurs in the triangular 'Wheatley Deep' deformation zone proximal to ASTJ. The latter appears to have formed due to a shift of the deformational front from OFZ towards a transform that offsets SR. Though ASTJ is still in the process of evolution, available data favour that this RRF triple junction may eventually be converted to a more stable RRR type.

Keywords

Revised Seismicity Map, Foreshock-Mainshock-Aftershock Sequence, Aspect Ratio and B-Values, Seismic Clusters, CMT Solutions, R-T Plot, RRR Plate Margin Kinematics.

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Manoj Mukhopadhyay ¹

Affiliations
1 Indian School of Mines, Dhanbad - 826 004, IN

Abstract

Northern Maharashtra and Southern Gujarat (NMSG) bordering the Gulf of Cambay and their immediate surroundings are seismically active as known from both non-instrumental and instrumental data. An intense seismic zone occupying an area of 80 km radius in this region is now delineated by the Gauribidanur Seismic Array (GBA) data covering a period of 20 years (1977 - 1997). This accounts for nearly 85% of the recorded activity, which is geographically delineated between Surat on the north, Daman to the west and Nasik to the east. The intense seismic zonc in NMSG does not clearly correlate to known tectonic features or faults. Here the crust of the.Indian shield is abnormally thin (as thin as 20 km) as demonstrated by available deep seismic sounding data. Contrasting tectonic trends of the West Coast fault, Narmada-Tapti faults and Cambay graben converge in this region. The Bouguer gravity anomaly varies by more than 120 mgal within NMSG, displaying a steep gravity gradient by 1.06 mgal/km at its centre. Nature of seismic activity, space-time diagram of earthquake ruptures since 18 19 as correlated to crustal fabric of the region, and available geophysical evidences suggest that current seismic activity of NMSG may bc an outcome of plume tectonics bordering the Gulf of Cambay. Further studies are warranted for detailing the seismic zone and the resultant crustal deformation.

Keywords

Seismicity, Plume tectonics, Maharashtra, Gujarat.

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Sujit Dasgupta ¹, Basab Mukhopadhyay ², Manoj Mukhopadhyay ³

Affiliations
1 Ex Geological Survey of India, Kolkata, IN
2 Geological Survey of India, Central Headquarters, 27 J.L. Nehru Road, Kolkata-700016, IN
3 Department of Geology & Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451, SA

Abstract

The Burmese Arc seismic activity is not uniform for its ~ 1100 km length; only the Northern Burmese Arc (NBA) is intensely active. Six large earthquakes in the magnitude range 6.1 - 7.4 have originated from the NBA Benioff zone between 1954 -2011, within an area of 200 × 300 km² where the Indian plate subducts eastward to depths beyond 200 km below the Burma plate. An analysis on seismogenesis of this interplate region suggests that while the subducting lithosphere is characterized by profuse seismicity, seismicity in the overriding plate is rather few. Large earthquakes occurring in the overriding plate are associated with the backarc Shan-Sagaing Fault (SSF) further east. The forecasting performance of the Benioff zone earthquakes in NBA as forerunner is analysed here by: (i) spatial earthquake clustering, (ii) seismic cycles and their temporal quiescence and (iii) the characteristic temporal b-value changes. Three such clusters (C1- C3) are identified from NBA Benioff Zones I&II that are capable of generating earthquakes in the magnitude ranges of 7.38 to 7.93. Seismic cycles evidenced for the Zone I displayed distinct quiescence (Q_1, Q₂ and Q₃) prior to the 6^th August 1988 (M 6.6) earthquake. Similar cycles were used to forecast an earthquake (Dasgupta et al. 2010) to come from the Zone I (cluster C1); which, actually struck on 4 February 2011 (M 6.3). The preparatory activity for an event has already been set in the Zone II and we speculate its occurrence as a large event (M > 6.0) possibly within the year 2012, somewhere close to cluster C3. Temporal analysis of b-value indicates a rise before an ensuing large earthquake.

Keywords

Northern Burmese Arc (NBA), Precursor, Seismic Clusters, Seismic Cycles, Temporal Quiescence, Temporal B-Value.

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