Polyphase development of chocolate-tablet boudins in the SAT zone, Kumaun Lesser Himalaya, India

Abstract

A combination of two groups of boudins, exposed in orthogonal sections, is observed as chocolate tablet structures in the South Almora Thrust (SAT) zone, Kumaun Lesser Himalaya. Evidence from boudin associated veins, fractures, folds and microstructures suggest that the chocolate-tablet boudins were developed as a consequence of layer-parallel stretching in two orthogonal directions during two successive phases of ductile shearing in the SAT zone. The early phase of top-to-NW shearing is speculated as a pre-Himalayan deformation during which the early boudins were developed. This deformation was superposed by a late phase of top-to-SW Himalayan shearing and development of the attendant late boudins. It is concluded that the interference of two phases of successively developed boudins has resulted into development of the chocolate tablet structures in the SAT. Fluid inclusion and microstructures indicate a low temperature, about 300 °C, growth of the chocolate-tablet boudins. The finite strain analysis reveals that a maximum strain ratio (Rs) of 1.7 was accumulated during the pre-Himalayan phase of deformation whereas Rs was of the order of 2.1 in the Himalayan phase of deformation.