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

B. S. Gogte ¹, K. V. L. N. S. Sarma ¹, Y. V. Ramana ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Petrologically, the metabasites comprise of metavolcanics (metabasalt, spilitic basalt, metadolerite, spilitic diabase and trachybasalt), green schists, amphibolites and meta-greywackes, and of these the meta volcanics are the most predominant. The physical properties of these rocks are: density (2.77-3.00 g·cm^-3), porosity (0.2-1.9%), compressional velocity (5-7 km·sec^-1), shear velocity (3-4km·sec^-1), P-anisotropy (up to 7.6%), compressive strength (700-2800kg cm^-2), tensile strength (70-190 kg cm^-2), Young's modulus (6.5-11.5 × 10^-11 dynes·cm^-2), and Poisson's ratio (0.14-0.29). These variations in the physical parameters are due to (i) Metamorphic effects (low to moderate P and T conditions, green schist-amphibolite facies), (ii) The serpentinization of the phenocryst minerals, (iii) The glassy nature of the groundmass, and (iv) The differences in the groundmass/phenocryst ratios.

In general, the physical parameters of these crustal rocks from the Himalayas are higher than those of Deccan basalts even at ambient conditions. This is due to (i) Differences in their mineralogical characters, and (ii) The mode of their occurrence, the former being associated with highly folded, disturbed and tectonically affected eugeosynclinal zones, whereas the latter are of the fissure eruption type, relatively free from metamorphic effects and large scale tectonic activity.

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K. V. L. N. S. Sarma ¹, M. V. Ramana ¹, G. P. S. Murty ¹, V. Subrahmanyam ¹, K. S. Krishna ¹, A. K. Chaubey ¹, M. M. Malleswara Rao ², S. L. Narayana ²

Affiliations
1 National Institute of Oceanography, Donapaula, Goa - 403004, IN
2 National Institute of Oceanography, Regional Centre, Visakhapatnam-530023, IN

Abstract

Analysis of the total intensity magnetic data over the western Andaman shelf has been carried out for determination of depth to magnetic basement using the Analytical signal and Werner Deconvolution methods. The basement depths are refined with optimisation procedure of iteration modelling. The depths Derived from these methods match well with the acoustic basement mapped by the seismic reflection survey across the Andaman shelf. The interpretation by these methods demonstrates the rapid utility in virgin areas forderiving the first order basement configurationquickly. The gradual slope of the basement from west to east on the western Andaman shelf may be attributed to the ongoing process of the subduction of the older crust in the Sunda Trough.

Keywords

Marine Geophysics, Andaman Sea.

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M. V. Ramana ¹, V. Subrahmanyam ¹, K. V. L. N. S. Sarma ¹, B. T. V. Seshavataram ²

Affiliations
1 National Institute of Oceanography, Donapaula, Goa-403 004, IN
2 Oil and Natural Gas Commission, Sire Mansion, Mount Road, Madras-600 006, IN

Abstract

Close grid masine magnetic surveys in the vicinity of a drill well site PH 9-1 in Palk Ray revealed that the area is characterized by smooth magnetic field except for a local anomaly caused by a lost wellhead. The smooth magnetic field is attributed to the deep burial of Precambrian granitic basement devoid of any charnockite intrusions. The seismic reflection records of the study area show >3200 m thick sediments over the basement.

Keywords

Marine Geology, Geophysics, Palk Bay, Cauvery Basin.

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A. S. Subrahmanyam ¹, G. P. S. Murty ¹, K. V. L. N. S. Sarma ¹, K. Mohana Rao ¹, N. P. C. Reddy ¹, M. M. Malleswara Rao ¹, V. Subrahmanyam ¹, P. Suneetha Rani ¹, A. Anuradha ¹, K. S. R. Murthy ¹

Affiliations
1 National Institute of Oceanography, 176, Lawson's Bay Colony, Visakhapatnam- 530 017, IN

Abstract

Earth tremors of low to moderate intensity (3.0 to 5.5) recurring over last century in and around coastal and shelf regions of Visakhapatnam has prompted us to acquirc and study the geophysical data between coastal regions of Kakinada and Kalingapatnam in western part of Central Bay of Bengal (17°N - 18°30'N) for understanding the recent seismicity of the region. Preliminary analysis of magnetic and shallow seismic data reveals the presence of some prominent land-Ocean tectonic lineaments, reactivation of which is considered to be responsible for tremors in the region. Total field magnetic anomaly map of the study area shows two major anomaly lineations such as: (a) N-S trending medium amplitude (˜250 nT) lineation off Visakhapatnam which is considered to be offshore fragment of converging point of two onshore lineaments. Also the lineament is associated with- five earth tremors of magnitude 3.7 to 4.3 at 40 m water depth. Impact of tremors is also observed in the overlying sedimentary strata in the form of vertical faults. (b) A NE-SW high amplitude anomaly zone (˜820 nT) off Kalingapatnam trending NE-SW, which is in close proximity with the epicentral location of earthtremors of 5.5 magnitude. This lineament can be spatially correlated to onshore NW-SE trending structural lineament indicating the probable reactivation of offshore segment resulting in the earth tremors Off Vizianagaram, reactivation of preexisting structural lineaments associated with the NW-SE trending Kandivalasa river fault may be the source of tremors at 20 m water depth. Magnetic basement contour pattern changes from NE-SW to NW-SE off Visakhapatnam. It may be inferred that the basement is bounded by two fault controlled lineaments passing through epicenters of earthtremors off Visakhapatnam and Vizianagaram. Basement structure shows down faulted basement (2 to 3 km) underlying the epicentral location along the lineaments trending NW-SE in the innershelf of visakhapatnam. Shallow seismic data also indicate steepiy dipping sub bottom fault in the study area of Visakhapatnam. South of Visakhapatnam, shallow seismic records indicate vertical faults and shearing reflectors in the lower sedimentary layers. From the deformed lower sedimentary layers. It may be considered that the region might have undergone seismic activity in the recent past, but presently there are no reported tremors in this region. Even undeforrned top sedimentary layers suggest lack of recent tectonic activity between Visakhapatnam and Kakinada in coastal and shelf regions. It may be summarized from the preliminary analysis of the geophysical data that the earth tremors observed north of Visakhapatnam are caused mainly due to reactivation of tectonic lineaments whereas south of Visakhapatnam it is relatively free from any seismicity. Based on the felt reports, tremors are attributed to shallow focus origin. Detailed investigations from multidisciplinary approach like geological, geomorphological, geophysical and remote sensing observations coupled with seismic information in the study area will throw more light on the micro seismic activity and related recent tectonics of this coastal region of east coast of India.

Keywords

Tectonic Lineaments, Magnetic anomalies, Microseismic activity, East coast, Andhra Pradesh.

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