Upper mantle seismic anisotropy beneath the Deccan Volcanic Province and the adjacent Eastern Dharwar Craton in south Indian shield from shear wave splitting analysis

Abstract

We investigate the upper mantle seismic anisotropy beneath the Deccan Volcanic Province (DVP) and the adjacent Eastern Dharwar Craton (EDC) in the south Indian shield to provide evidence of their differentiated evolution. For this objective, we use the shear wave splitting analysis of the teleseismic SKS, SKKS, PKS and PKKS phases data recorded across a transect, with a linear array of fifteen broadband seismograph stations that spans these two predominant tectonic domains. The resulting splitting parameters (i.e. fast polarization orientation, Φ, and the delay time, δt) show an interesting pattern that is not easily explained by a simple source of anisotropy for the entire transect study region, such as ‘fossil’ (or frozen) anisotropy in the lithosphere or asthenospheric (or geologically recent) anisotropy. However, we observe distinct splitting patterns highlighting deeper geological structures, with which we are able to segregate the transect study region into two zones. In the first zone consisting of DVP, Φ is nearly NE-SW for most stations, along with an average (at each station) δt varying between ~0.5 s and ~1.2 s. In the second zone consisting of predominantly EDC and partly EGMB (Eastern Ghat Mobile Belt), our results show Φ predominantly in the NW-SE orientations for EDC and NE-SW orientations for EGMB, with δt varying between 1.0 and 1.4 s. Based on the splitting and predominant non-APM (Absolute Plate Motion) trending in the EDC, we infer that the observed splitting is related to ‘frozen’ anisotropy in a thick lithosphere, which is preserved between late Archean to Proterozoic, or the last major episode of tectonic and magmatic activity affecting the region ~2.6 Ga ca. In the DVP, the deformation seems to represent a predominant APM-trending asthenospheric anisotropy beneath a thinned lithosphere. This study suggests that the upper mantle fabric is possibly influenced by shear interactions from the geologically recent ~65 Ma Deccan plume event and the fast-moving Indian plate. At the edge of the EDC, encompassing the EGMB, our results show APM-trending anisotropy. This signature is possible from the ancient rifted margins and other lithospheric-scale heterogeneities. Our study uses a single-layer anisotropy model with a horizontal fast axis to approximate the first-order anisotropy beneath each station. Although multiple layers of anisotropy are not ruled out, it needs to be resolved with further studies having richer back-azimuthal coverage.