Electrical anisotropy of asthenosphere in a region of window to mantle underneath Eastern Indian Craton

Abstract

We attempt to resolve electrical anisotropy in the mantle underneath the Eastern Indian Craton (EIC) (∼3.3 Gyr) with relatively short-period (<2000 s) magnetotelluric (MT) measurements. The phase-sensitive regional strike is frequency dependent. The average strike changes from N45°W for frequency >1 Hz to N25°W for frequency <0.1 Hz. This, along with negligible skew, suggests a two-dimensional (2-D) structure. A 2-D geoelectric model shows a rare example of highly resistive continental lower crust (CLC) of 1 S conductance instead of the generally observed highly conductive CLC. The presence of a resistive CLC provides unambiguous determination of the Moho. Two distinct phase-sensitive regional strikes for frequencies less than 0.1 Hz and >1 Hz indicate that the conductivity structure in the crust does not coincide with the mantle structure. This, along with the resistive CLC, provides a window to the mantle, helping in resolving the mantle structure. The phase data showing consistent phase splitting and negligible magnitude of induction arrow suggest an anisotropic mantle. The 1-D modeling shows that the anisotropic mantle is at a depth of about 175 km. The phase-sensitive strike at this depth deviates from the direction of the present-day Indian Plate (moving at a minimum speed of 3.7 ± 0.2 cm/year) by a minimum of 43°. This discrepancy could be due to either resistance to the mantle flow by the present-day plate motion or complex convection of mantle or the combined effect of these.