Study of fold and thrust belt deformation of Almora nappe using field observations, microstructural studies and strain analysis

Abstract

The Almora Crystalline Zone (ACZ) is one of the Lesser Himalaya's largest thrust nappes. The ACZ has a unique boat-shaped synclinal structure, which is a concave-up fold shaped like a trough or a boat. The ACZ syncline is bounded on its northern side by the North Almora Thrust (NAT), and on its southern side by the South Almora Thrust (SAT). Despite being a repository of tectonic deformation spanning from the Precambrian to the present, the Almora Crystalline Zone (ACZ) has received little attention. Therefore, this study aims to fill this gap by conducting a thorough investigation of the fold and thrust induced structural deformation in ACZ. This is done through a combination of field observations, microstructural studies, and 2D strain analysis. The study seeks to provide a detailed description of the structural features of ACZ and improve the understanding of the tectonic evolution of the region. Four distinct types of folds and a faults having NE thrusting have been identified from field study along Patiya river section. F1 folds are gently plunging isoclinal folds with recumbent geometry and dipping towards east. The F1 fold and S1 foliations are refolded by F2 folds having similar geometry. The F1 folds are rotated in Sirar village and the axial plane here is dipping due north. The axial plane of the F3 folds is dipping in NNE direction and plunging towards NNE direction having sub-vertical axial planes. The F4 folds are open type and are dipping towards the south. Thin section analysis of samples of cataclastic rocks taken from Sirar and Sunderpur revels static recrystallization with BLG and SGR recrystallization. The deformation temperature is calculated to be within 390-490°C from both recrystallized grain size data as well as from microstructural analysis. The value of 2D strain analysis on cataclastic samples varies within the range of 1.41-1.56 at both sites. The stress experienced in the area acted in an NNE-SSW direction. This study provides valuable insights into the deformation history of the ACZ, which may have broader implications for understanding the tectonic evolution of the Lesser Himalayas as well as MCT in general.