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

L. K. Das ¹, B. Das ², S. N. Chowdhury ³, D. C. Naskar ³, G. Karunakar ³, S. K. Dey ²

Affiliations
1 GSI, Marine Wing, DK 6, Salt Lake, Kolkata - 91, IN
2 GSI, seminary Hills, Nagpur, IN
3 GSI, CGD, 27 JLN Road, Kolkata - 16, IN

Abstract

Gravity-Magnetic mapping and resistivity sounding have been conducted in a grid pattern of 50 m × 100 m and 200 m × 200 m respectively over one square km area between Tokapal and Duganpal villages on Jagdalpur-Gidam road about 18-20 km from Jagdaipur township. The objective has been to map the eastern and the northern extension of an already known kimberlite sill in the area which is exposed in a pit and intersected in some drill holes at shallow depth to the west of the area surveyed.

The Bouguer anomaly (BA) map reveals a near circular peak marginal high of + 0.5 mGal around 0/N100 station over the background value of -59 mGaI in the area surrounded by an envelope of +0.3 mGal, almost in a N-S disposition between the traverses W350 and E100 stretching from the northern end down to S400 grid. Another such N-S feature, somewhat subdued (+0.3m Gal), is recorded in the BA map, in the northeastern part of the area bounded by traverses E200 and E400 and the base line. It is interesting to note that this sill like body is exposed around station E200/S300 where no BA high is detected. However, in the magnetic (VF) map, this sill body is picked up as a dipolar anomaly. Joining the axes of the dipolar lows and highs, the magnetic kimberlite body is yet again resolved into two near circular bodies as recorded in the gravity map. A gap in the continuity of the kimberlite sill around E200 is also indicated.

3D Euler's deconvolution solutions arrived from both the gravity and the magnetic (VF) data assuming a sill type body have been extremely useful in demarcating the geometry of the causative body at three different depth levels.The resistivity in a grid pattern shows the evidence of buried kimberlite in the area characterized by a marginal resistivity high (30-130 0hm.m). A 3D subsurface map of the kimberlite sill using ROCK WORKS has been prepared from the resistivity data.

Several 2-D G-sMe ctions have also been presented constraining these from the results of resistivity survey. Such sections will immensely help in targetting future drill holes in the area. The 3D Euler's deconvolution solutions and the 3-D configuration of the Kimberlite body have lent the third dimension in prospecting the covered kimberlites through geophysical mapping.

Keywords

Kimberlites, 3D Configuration, Gravity-Magnetic Mapping, Resistivity, Indravati Basin, Bastar District, Chhattisgarh.

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J. R. Kayal ¹, Sagina Ram ², O. P. Singh ¹, P. K. Chakraborty ¹, G. Karunakar ³

Affiliations
1 Gelogical Survey of India, Central Geophysics Division, 27, J.N. Road, Kolkata - 70001 6, IN
2 Geological Survey of India, Northern Region, Lucknow - 226 024, IN
3 Geological Survey of India, Eastern Region, Salt Lake, Kolkata - 700 094, IN

Abstract

The aftershock sequence of the Chamoli earthquake (m_b 6.3) of March 28, 1999 provides an opportunity to study the aftershock characteristics and the seisrnotectonic structure in the Garhwal Himalaya. Detailed analysis of the spatial variation, the frequency-magnitude distribution (b-value), the activity decay-rate (p-value) and fault-plane solutions of the aftershocks throw new light on the aftershock characteristics and seismotectonics of the region. The observations suggest that the aftershock generating process and the b- and p- values changed with time, space and magnitude-ranges. The main shock occurred on the Basement Thrust by a thrust-fault mechanism. The aftershocks, on the other hand, are triggered by the seismogenic faults to the south of the Main Central Thrust (MCT) by thrust as well as by strike-slip faulting, and all the events occurred above the plane of detachment.

Keywords

Main shock, Aftershocks, Seismicity, Fault-Plane Solution, Seismotectonics, Chamoli, Garhwal Himalaya.

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