Electrical anisotropy in the Main Central Thrust Zone of the Sikkim Himalaya: Inference from anomalous MT phase G. Pavan Kumar, A. Manglik ⇑ CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India article info Article history: Received 24 December 2011 Received in revised form 6 June 2012 Accepted 24 June 2012 Available online 4 July 2012 Keywords: Sikkim Himalaya Electrical anisotropy Main Central Thrust Zone MT phase abstract The Sikkim Himalaya in the eastern sector of the Himalayan collision belt shows some distinct features, such as the occurrence of the Main Central Thrust as a several-km-wide zone (MCTZ) of intense ductile deformation and its deviation from the general east–west strike of the Himalayan thrusts, suggesting that the tectonics of this region could be different from the regional Himalayan tectonics. We have carried out a MT survey across MCTZ to understand the tectonics of the region. The broadband MT data of the site at the northern boundary of the zone reveals anomalous phase exceeding 90° for frequencies smaller than 1 Hz whereas other sites further north of this zone show normal behavior. Such anomalous phase behav- ior has been explained earlier in terms of anisotropy in the medium and special configuration of local conductors. We have analyzed the observed anomalous phase behavior in terms of anisotropy based on geological consideration that alternating bands of graphitic schists are present within high-grade crys- talline gneisses at the northern boundary of MCTZ. Among various models analyzed, a block model of the crustal structure with strong anisotropy contrast between the block and the underlying anisotropic layer provides good fit to the observed data. We, thus, infer that the presence of a localized strongly anisotropic block at the northern boundary of MCTZ could be a viable explanation for the observed anomalous phase behavior. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The Sikkim Himalaya in the eastern sector of about 2,500 km long Himalayan continent–continent collision belt shows some spectacular deviations from the general tectonic setup of the Himalayan belt. In this region, the Main Boundary Thrust (MBT) follows an east–west trend whereas the Main Central Thrust (MCT) takes a sinusoidal shape with a narrow neck at the southern end, encompassing a dome-shaped structure, the core of which ex- poses a vast expanse of pre-Tertiary rocks arranged in a pile of thrust sheets (Dasgupta et al., 2004; Ray, 2000; Mitra et al., 2010). The MCT in this region is also not a well-demarcated sharp boundary. It occurs as a several-km-wide ductile deformation zone, often referred as the Main Central Thrust Zone (MCTZ), and consists of schists and gneisses representing both the Lesser and the Higher Himalaya (Dasgupta et al., 2004; Sinha-Roy, 1982). In addition to MCT and MBT, several approximately N–S trending gravity faults are also present in this region (Nath et al., 2005). The region is also traversed by NNW-SSE trending sub-parallel Gangtok and Tista lineaments (Narula et al., 2000). These linea- ments, along with NE–SW tending Kanchenjunga lineament west of Sikkim, appear to have significant influence on the tectonics of the region. Satellite imageries also reveal oblique-to-transverse lineaments in the Nepal and Sikkim Himalaya, strikes of which vary from NW to NE (Dasgupta et al., 1987). Thus, the Sikkim Himalaya presents a complex tectonic setup. Although several geophysical investigations have been carried out in the western sector of the Himalaya and in the Tibetan re- gion, very little geophysical information is available for the Sikkim Himalayan region despite its seismic vulnerability and complex tectonic setting. Most of the work carried out in this region has dealt with seismological studies for identification of seismically ac- tive faults and hazard potential. The Sikkim Himalaya falls in the zone IV of the seismic zonation map of India and is well known to be seismically active, the most recent earthquake being the 6.8 magnitude 18.09.2011 earthquake. The seismic activity is mainly clustered to the north of MBT with earthquakes occurring up to the depth of 50 km indicating a dominant control of MBT on the tectonics of the region (De and Kayal, 2004). A significant inference from various micro-seismic surveys carried out in the Sikkim Himalaya (De and Kayal, 2004; Nath et al., 2005; Hazarika et al., 2010) is that MBT is a seismogenic fault and it does not con- verge with the plane of detachment unlike in the western Hima- laya but possibly traverses across the entire crust. Further, composite focal mechanisms show strike-slip nature of faulting 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.06.017 ⇑ Corresponding author. Tel.: +91 40 23434684; fax: +91 40 23434651. E-mail addresses: ajay@ngri.res.in, amngri@gmail.com (A. Manglik). Journal of Asian Earth Sciences 57 (2012) 120–127 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes