Normal reactivation of the Main Central Thrust Zone in Sikkim Himalaya

Abstract

• The Main Central Thrust Zone, Sikkim , was reactivated as a normal shear zone. • The normal shear zone is 2.5 km wide with >5 km normal movement. • Normal movement followed thrusting marking the start of doming in footwall. • Doming caused steepening and destabilization of the MCTZ and normal slip. The Main Central Thrust Zone (MCTZ) controlled the Miocene development of the Himalaya. Its kinematic evolution is key to understanding the orogeny. The MCTZ accommodated shortening of the Himalayan orogen and thrusted hot rocks of the Greater Himalayan Sequence over cooler ones of the Lesser Himalayan Sequence, giving rise to an inverted metamorphic sequence. In Sikkim, NE India, the transport direction during thrusting describes a broad curved pattern indicating complex kinematics. In this paper we document further complexities in the motion of the MCTZ in the NW section of the Tista Dome in Sikkim, that reveals that SE-directed thrusting, at 45° to the orogenic front, was reactivated by top-to-NW normal movement. This produced a 2.5 km-wide ductile shear zone that accommodated >5 km of normal dip-slip movement, where the inverted metamorphic sequence is preserved with minor retrogression. This change from reverse to normal ductile movement likely occurred in the early stages of doming at ∼12 Ma, following peak metamorphism at 13.8 Ma. We argue that normal movement relates to the abandonment of the MCTZ and movement transfer to the Ramgarh Thrust and/or the Main Boundary Thrust, structurally deeper in the system. This new thrust gave rise to a duplex system responsible for the growth of the Tista Dome from ∼12 Ma onwards causing steepening and destabilization of the MCTZ leading to stress relaxation and normal movement reactivation. This must have occurred in the early stages of doming because the rocks were still sufficiently hot to respond in a ductile fashion.