Upper mantle seismic anisotropy in the intra-continental Kachchh rift zone, Gujarat, India Prantik Mandal ⁎ National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad-500606, Andhra Pradesh, India abstract article info Article history: Received 6 November 2010 Received in revised form 25 April 2011 Accepted 31 May 2011 Available online 13 June 2011 Keywords: SKS/SKKS splitting Fast axis Delay times Kachchh rift Partial melt Rift axis parallel upper mantle anisotropy 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). © 2011 Elsevier B.V. All rights reserved. 1. Introduction Teleseismic shear-wave splitting is a widely adopted technique for detecting anisotropic structures in the Earth's crust and mantle (Barruol and Hoffmann, 1996; Silver and Chan, 1991; Vinnik et al., 1989). Certain regions preserve signatures of major geologic events that are observable in terms of its current crustal/upper mantle struc- tures, morphological characteristics, tectonics as well as seismic productivity (Barruol and Granet, 2002; Flesch et al., 2005; Fontaine et al., 2007; Gao et al., 1997; Kendall et al., 2006; Levin et al., 2004; Vauchez et al., 2000; Waite et al., 2005). The Mesozoic Kachchh rift in NW India is a unique tectonic region that displays the inheritance of a paleorift, through its mid/lower crustal mafic intrusives and fault systems that are occasionally activated through M N 7.5 earthquakes (Gupta et al., 2001; Mandal and Pujol, 2006; Rajendran and Rajendran, 2001). The large amount of data from seismic stations operating in this region since the 2001 Mw 7.6 earthquake provides an opportunity to examine the structure of the Kachchh rift. The east–west trending Kachchh rift, a prominent geotectonic feature of northwestern India, is thought to have opened up along major Precambrian trends during the early Jurassic period (Biswas, 1987; Roy, 2004). Post-Himalayan collision, the rifting ceased due to an inversion of the extensional stresses and the imbricate faults within the rift, most of which remain blind are now being reactivated in response to NS oriented compressive stresses. The numerous historical and recent earthquakes including the Mw 7.6, 2001, Bhuj earthquake originate on the nearly east–west oriented fault systems (Mandal et al., 2004; Mandal and Horton, 2007; Rastogi et al., 2001). The aftershock activity of the 2001 earthquake is still continuing and is being monitored by the National Geophysical Research Institute (NGRI), (Mandal et al., 2007). Analyses of the abundant data generated since 2001 have helped to image this rift and the seismogenic structures in great detail (Kayal et al., 2002; Mandal, 2006; Mandal, 2011; Mandal and Chadha, 2008; Mandal and Pandey, 2010; Mandal and Pujol, 2006; Mishra and Zhao, 2003). The availability of such a large data set and the understanding derived from the previous work, has motivated us to perform a detailed SKS/SKKS splitting study for this region, which has a unique geodynamic and history. This study would for the first time, use the data generated in the source zone of a large paleorift to understand the anisotropic characteristics of the underlying upper mantle. Some previous attempts to understand the state of anisotropy beneath the Indian continent have shown that consistent NNE-SSW fast axis orientations of anisotropy characterize the Indian lithosphere (Chen and Ozalaybey, 1998; Ramesh and Prakasam, 1995; Sandvol et al., 1997). More recent studies based on the SKS splitting study of only four events, have reported two different directions of anisotropy (N142 ° and N60 ° ) over the Indian shield (Kumar and Singh, 2008). They also reported evidence of fossilized anisotropy in the southern part of the Indian continent. On the other hand, lack of anisotropy in the eastern Himalayas and Tibet, which has been explained by some as apparent transverse anisotropy (Sandvol et al. 1997); underthrusting of the Tectonophysics 509 (2011) 81–92 ⁎ Tel.: +91 23434688; fax: +91 40 23434651. E-mail address: prantik@ngri.res.in. 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.05.013 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto