New insights into the position and geometry of the main central thrust from Sikkim, Eastern Himalaya

Chakraborty, S. and Mukul, M.

Volume: 127 Pages: 289-322
DOI: 10.1086/702566
Published: 2019

The Main Central Thrust (MCT) features prominently in the Cenozoic evolution of the Himalaya, but no consensus exists on its definition and position. The MCT is best defined by a protolith boundary-structural definition: a high-strain zone with thrust-sense transporting the Greater Himalayan Sequence (GHS) rocks over the Lesser Himalayan Sequence (LHS) rocks. Protolith signatures have proved useful in distinguishing the GHS and LHS, but delineating the structural break of the MCT is still challenging. We have used the conceptual framework of shear zones to delineate the structural break of the MCT at different structural levels in Sikkim Himalaya, India, and identified rock units on either of its sides by available protolith signatures. Previous workers placed the MCT at different locations in Sikkim, varying up to ∼12 km structural distance, without providing any insights on its geometry. Our study shows that thickness and geometry of the MCT vary spatially along and across strike. In the relatively thicker exposures (∼2.5– 5.4 km), the MCT shows “island-channel geometry” with mylonites anastomosing around relatively undeformed rocks, transporting the GHS over the LHS and straddling both units. In the thinner exposures (∼1 km), the MCT shows three-dimensional zone-type geometry with a core of highly deformed mylonites flanked by relatively less-deformed proto-mylonite zones and has a minor portion of the GHS in its footwall. We define the MCT in Sikkim as a mappable shear zone that transported the GHS over the LHS, straddling both units in the thicker exposures, but has a minor part of the GHS in the footwall of the thinner exposures. Our shear zone framework–based approach can be used with protolith signatures along the Himalayan arc to map and study the MCT in detail. © 2019 by The University of Chicago.

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