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

Publication year: 2011
Source: Tectonophysics, In Press, Accepted Manuscript, Available online 13 June 2011

Prantik, Mandal

Shear wave splitting study of 411 SKS/SKKS phases covering backazimuth range of 13o to 305o recorded by 12 broadband stations in the Kachchh rift has led to estimates of fast axes orientations and splitting times for 118 good measurements. The average vector mean of fast axes orientation (86±14o) correspond 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…

 Research highlights: ► A total of 411 measurements of SKS/SKKS phase splitting were obtained from twelve broadband seismic stations in Kachchh, Gujarat, over an event backazimuth range of 13o to 305o. The estimated mean fast orientations and splitting times for the 118 good measurements range from 4o to 170o and 0.3 to 3.3 sec, respectively. ► The average of vector means for all the stations is (φ, δt) = ((86±14)o, (1.6±0.8) s). Thus, the average fast axes direction is E-W in the Kachchh rift zone, which is parallel to the axis of the rift. ► Two-layer anisotropy models with horizontal symmetry axes are found to be inconclusive. ► The observed large delay times (~1.6 s) depict a 184 km thick anisotropic layer with 4% anisotropy below the Kachchh region. ► However, if we assume a range of anisotropy (3–5 %), then the calculated delay time, δt = 1.6 s would be corresponding to an anisotropic layer with a thickness varying from 147 to 245 km. ► 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).