Overburden-Induced Flattening Structure in the Himalaya: Mechanism and Implication

Small-scale structures in fold-thrust belt are mainly formed in response to the emplacement of thrust sheets. However, some small-scale structures may not be developed directly in response to the emplacement of thrust sheets, but might be genetically tied with the orogenic process. Metre- to centimetre-scale late-stage folds on foliation in phyllite with near-recumbent fold geometry are selectively developed with a specific spatial distribution, particularly in places where the foliation is steeply dipping, in the Ramgarh thrust sheet in the Darjiling-Sikkim Himalaya. The recumbent-fold structures appear to have been formed in response to overburden-induced vertical compressive deformation on steep dipping foliation, especially in the subvertical southern limb of the antiformal structure of the Lesser Himalayan Duplex in the Darjiling-Sikkim Himalaya. The role of gravity and overburden in the formation of these structures from worldwide orogenic belts may be considered to validate their genesis.