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A geophysical field investigation, deploying Magnetotelluric (MT) and Deep Electrical Resistivity Sounding (DES), and analysis of the seismological data were carried out in the Shillong Plateau and Assam Valley area for deciphering the south dipping hidden Oldham fault at the plateau and valley boundary. Bilham and England (2001) reported that the great 1897 Shillong earthquake was caused by the pop-up tectonics of the Shillong Plateau between the two bounding faults, the south dipping hidden Oldham fault to the north and the well known north dipping Dauki fault to the south at the Plateau and Bengal basin boundary. The MT survey was carried out using SAMTEC-2 MT instrument, and the DES was conducted by unilateral equatorial arrays with 5-10 km separation between the transmitting and receiving dipoles. A total of 14 DES and 21 MT soundings were observed in the area. The DES delineated different subsurface layers down to a depth of about 1.5 km. It is observed that the depth of the high resistivity basement (granite gneiss/Archaea.n gneiss) increases towards north of Brahmaputra river; the depth varies from 150111 to 650 rn. To the south of Brahmputra, on the other hand, the depth of the basement is shalIower, it varies from 56m to 340 m. The MT soundings were conducted along three parallel north-south traverses in the area : Traverse I along Dauki-Kaurbaha, from Bangladesh border to Bhutan border, Traverse I1 dong Chherapunjee-Chhagaon and the Traverse I11 along Dainadubi-Dalgama, across the boundary of the ShilIong Plateau and Assam Valley, The traverses I1 and 111 were too shon due to local problems. Interpretation of the 21 MT soundings reveals a conductive horizon at a deeper lcvel (7-8 km) only at the Kulsi (Traverse 11) and at the,Dalgama (Traverse 111) stations, to the north of the proposed Oldham fault. These limited MT observations, however, do not resolve the south dipping hidden Oldham fault.

The seismological data recorded by the permanent network in the Shillong Plateau and Assarn Valley area during the last decade (1990-1997) are analysed. A intense seismic activity is observed in the Plateau region. A north-south depth section of the earthquakes across the Plate'au and the valley, suggests that the most intense seismic activity beneath the Plateau is bounded by two major tectonic features, the north dipping Dapsi thrust, western extension of the Dauki fault, and the south dipping Oldham fault. A detailed gravity survey and a deeper MT survey along longer traverses across the Oldham fault may shed more light on this hidden structure.

Keywords

Magnetotelluric, Deep Resistivity, Oldham Fault, Shillong Plateau, Assam Valley.

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