Complex tectonic setting and deep crustal seismicity of the Sikkim Himalaya: An electrical resistivity perspective

Abstract

Tectonic setting of the Himalayan arc is dominated by thrust-type earthquakes mainly associated with rupture of the décollement surface, also called the Main Himalayan Thrust (MHT). However, the region encompassing eastern Nepal, Sikkim and western Bhutan departs from this general tectonic model of the Himalaya and experiences deep crustal seismicity of strike-slip nature with focal planes oblique to the convergence direction of the collision front. The 2011 Sikkim earthquake (Mw 6.9) is an example of such transverse tectonic deformation. Several geophysical studies have been carried out in Sikkim to understand the complex tectonics of this region. Among these, a crustal model derived from magnetotelluric (MT) data revealed contrasting crustal structure across the Main Central Thrust Zone (MCTZ) beside a small-scale MHT ramp. Here, we revisit the MT model by including long-period MT data (up to 10,000 s) at six sites along the profile, invert the distortion corrected impedance data, and analyse the results in conjunction with other published geophysical and structural information. Based on this study, we propose a tectonic model for the Sikkim Himalaya that reveals the complex nature of the lithospheric structure including two major contacts beneath the MCTZ and the Main Frontal Thrust (MFT), respectively. The contact beneath the MCTZ is a NW-SE trending lithospheric-scale seismogenic fault that separates two geologically and compositionally distinct blocks of the Indian plate underthrusting the Himalaya. The other contact beneath the MFT could be responsible for the necking of the Main Central Thrust (MCT) at this location.