Upper mantle seismic anisotropy beneath the Kachchh rift zone, Gujarat, India, from shear wave splitting analysis

Abstract

Unravelling the anisotropic behaviour of the upper mantle helps to shed light on its present and past deformation processes. In this study, we attempt to explore the seismic anisotropy prevailing within the upper mantle beneath the Kachchh rift zone through shear wave splitting analysis. We have measured the splitting parameters (e.g., fast axis orientation ( $$ \varPhi $$ ) and delay time ( $$ \delta t $$ )) using SKS/SKKS core refracted phases from 112 teleseismic events recorded at NGRI network in the Kachchh region, during 2006–2009 and 2013–2016. The ‘ $$ \varPhi $$ ’ and ‘ $$ \delta t $$ ’ estimates vary from N34° to N73°E and 0.80 to 1.5 s, respectively. The average vector means of ‘ $$ \varPhi $$ ’ and ‘ $$ \delta t $$ ’ for all the stations are found to be $$ {\text{N}}\left( {58 \pm 10} \right) $$ °E and (0.99 ± 0.19) s, respectively. Measurements of 59 good SKS/SKKS splitting parameters from 112 earthquakes reveal that the upper mantle is highly anisotropic beneath the Kachchh rift zone with an average fast axis orientation of $$ {\text{N}}\left( {58 \pm 10} \right) $$ °E, which is deviated nearly $$ (\sim{\text{N}}18 $$ °E) from the absolute plate motion (APM) direction (~N40°E) of the Indian plate in a no-net-rotation reference frame. This deviation of fast axis orientation from APM direction may be attributed to the effect of Kachchh rift zone as well as the presence of structural imprints of the 65 Ma Deccan mantle plume in the study region. And the average delay time of (0.99 ± 0.19) s is consistent with the global average (1 s) for continents. Furthermore, the modelled seismic layer thicknesses reveal that anisotropic sources beneath study region are associated with both the lithospheric deformation processes (e.g., 184 Ma African rifting, 88 Ma Madagascar rifting, 65 Ma Deccan mantle plume) as well as asthenospheric flows.