Sinistral transpression along the Main Boundary Thrust in Amritpur area, Southeastern Kumaun Himalaya, India

Outcrop mapping and structural analyses in a 100 m thick fault damage zone elucidate geometry and kinematics of the Main Boundary Thrust in the study area. The north-dipping fault damage zone is sandwiched between the Proterozoic Amritpur granite in the hangingwall and the Neogene Siwalik sediments in the footwall. The Amritpur granite is overlain by the volcanic-sedimentary sequence along a top-to-the-south thrust. The footwall sediments are deformed into gentle upright and steeply-plunging folds. By contrast, the hangingwall granite is generally massive. The km-scale folds in the Sub Himalayan sediments are geometrically discordant to the folds in the Lesser Himalayan volcanic-sedimentary sequence.The damage zone contains both the Sub Himalayan sediments and the Lesser Himalayan amphibolite. It consists of an imbricate structure, tectonic wedges and pressure solution bands. The imbricate structure is made up of seven fault-bounded lithotectonic domains that record the imprints of coaxial refolding and subsequent modification of the folds into sheath-like structures. The geometry and kinematics of mesoscopic structures and the results of paleostress analysis reveal a sinistral wrench accompanied N-S to NNE-SSW directed subhorizontal shortening in the damage zone. The aforesaid evidences also help distinguish between pure contractional, pure wrench, contraction-dominated and wrench-dominated deformation in the different domains and the tectonic wedges. The sinistral transpression in the damage zone is ascribed to the occurrence of an oblique/lateral ramp along the Main Boundary Thrust in the study area.

Schematic diagram shows sinistral transpression and domainal strain partitioning in damage zone along the Main Boundary Thrust in Amriptur area, Southeast Kumaun Himalaya, India.Figure optionsDownload as PowerPoint slideHighlights
► The damage along the Main Boundary Thrust contains both the Sub Himalayan sediments and the Lesser Himalayan amphibolite in the study area.
► The subvertical dip angle and the sinistral transpression are attributed to the presence of an oblique/lateral ramp along the Main Boundary Thrust in the study area.
► Strain in transpression zone can be partitioned into contraction-dominated and wrench-dominated domains.
► Mesostructures in different domains record various stages of a progressive transpressional deformation in the damage zone.