Upper mantle seismic anisotropy in the intra-continental Kachchh rift zone, Gujarat, India

Shear wave splitting study of 411 SKS/SKKS phases covering backazimuth range of 13° to 305° recorded by 12 broadband stations in the Kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements. The average vector mean of fast axis orientation (86 ± 14°) corresponds to the E–W axis of the Kachchh rift and the delay time (~ 1.6 s) is attributed to the ~ 184 km-thick upper mantle layer with 4% anisotropy. The anisotropic character observed for the Kachchh rift (KR) is comparable to other continental rifts and these are related to the high-temperature, lattice-preferred orientation fabric of olivine, inherited from the mantle flows. The source of the rift-axis parallel anisotropy is traced to the rift-parallel flows within the 76 ± 6 km-thick lithosphere. Additionally, the rift-parallel pockets of partial melts also induce anisotropy within the asthenosphere. Both these are inherited from the plume–lithosphere interaction during the Deccan/Reunion plume episode (~ 65 Ma).

Research Highlights
► The estimated mean fast orientations and SKS splitting times for the 118 good measurements range from 4° to 170° and 0.3 to 3.3 s, respectively.
► The average of vector means for all the stations is (φ, δt) = ((86 ± 14)o, (1.6 ± 0.8) s) suggesting a rift-axis parallel anisotropy.
► The observed large delay times (~ 1.6 s) depict a 184 km thick anisotropic layer with 4% anisotropy below the Kachchh region.
► The source of the rift-axis parallel anisotropy is traced to the rift-parallel flows within the 76 ± 6 km-thick lithosphere.
► Additionally, the rift-parallel pockets of partial melts also induce anisotropy within the asthenosphere.