GEOLOGICAL CHARACTERISTICS OF THE MAIN CENTRAL THRUST (MCI) ZONE BETWEEN HELANG AND JOSHIMATH, ALAKNANDA VALLEY, GARHWAL HIMALAYA

Abstract

The Himalayan Main Central Thrust (MCT) has been a source of controversy since it was first defined by Heim and Gansser in 1939. There has been a lot of confusion regarding the exact position and definition of the MCT. A major problem in the structural geology of the Himalayas has been that different workers have mapped the MCT along various different structural levels. In the Alaknanda - Dhauliganga section of the Garhwal Himalaya, where Heim and Gansser first defined the Munsiari thrust at 1-lelang as the Main Central Thrust in 1939, we decided to do a detailed structural mapping of the area to provide some clarification on its position and definition. Based on the structural studies done in the Helang-Joshimath section, we suggest that, there is a Main Central Thrust Zone (MCT Zone) which is a high-strain zone and starts at the first mylonite exposure at RM-37. It is bound on the top by the Vaikrita Thrust (MCT-ll) and at the bottom by the Munsiari Thrust (MCT-1). The shearing and strain in the MCT Zone is penetrative (at millimeter - decimeter scale) and is relatively higher compared to the surrounding rocks which show non-penetrative shearing IV in discrete zones. The Higher Himalayan Crystalline (HHC) sequence can be considered as a thick ductile shear zone which shows discrete zones of non-penetrative shearing and these discrete shear zones are so extensively distributed throughout the traverse, that it is not possible to demarcate and identify each individual zone. The MCT Zone is a zone of penetrative shearing within this thick ductile shear zone (HHC). iv S Based on the geoclironological studies done in this section in terms of Sm - Nd depleted mantle model ages and rNd values, we arrive at the conclusion that majority of samples show trends similar to those mentioned by Ahrnad et al. (2000). However, three samples, namely RM - 121, RM - 69 and H - 44, show different trends extending the possibility that we should not blindly use isotopic criteria to define the Vaikrita Thrust as the Main Central Thrust and a major terrain boundaiy. Also, a thrust is a structural feature, which can cut across different stratigraphies; SO we cannot use isotopic criteria, independently, to identif' a major thrust. We suggest that the Munsiari Thrust which was originally recognized as a major terrain boundary in the Garhwal Himalaya, should be called the Main Central Thrust.