RESEARCH ARTICLES CURRENT SCIENCE, VOL. 109, NO. 10, 25 NOVEMBER 2015 1814 *For correspondence. (e-mail: sdbanj87@gmail.com) Overburden-induced flattening structure in the Himalaya: mechanism and implication Sayandeep Banerjee 1, *, Abdul Matin 1 and Malay Mukul 2 1 Department of Geology, University of Calcutta, Kolkata 700 019, India 2 Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai 400 076, India 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 spa- tial distribution, particularly in places where the folia- tion 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 sub- vertical 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. Keywords: Orogeny, overburden-induced flattening, recumbent fold, thrust sheet. THE geometry of mesoscopic and microscopic structures provides insights into the structural history of fold and thrust belts (FTBs) 1 . Small-scale structures and their tec- tonic implications are established from different FTBs like Alpine orogeny 2 , Caledonides 3 , Sevier 4 , Himalaya 5–9 and Zagros 10 , where small-scale folds and other tectonic structures are common. The genesis of these structures is related with the emplacement of the thrust sheets and are formed in successive stages of progressive deforma- tion during the formation of the FTB 8–11 . Bucher 12 and Gansser 13 described the presence of some apparently unrelated small-scale structures in the Alps and the Hima- laya, but the genesis of these structures is not fully under- stood. In the southern part of the Lesser Himalaya in Darjil- ing-Sikkim area (Figure 1) 14 , metre- to centimetre-scale recumbent to near-recumbent folds on regional foliation is developed in a narrow, strike-parallel, steeply dipping zone, mainly in the southern side of the Lesser Himala- yan Duplex (LHD) 15 . This type of fold is also observed in two other adjacent areas where regional foliation is steep- dipping. In this article, we present the geometry of the small-scale near-recumbent symmetrical folds in the Ramgarh Thrust (RT) sheet in the Darjiling–Sikkim Himalaya (DSH), and also a model is proposed for the possible genesis of the structure with respect to the oro- genic movement of the Himalaya in light of the role of gravity and overburden. The formation of these folds, how they are linked to the Himalayan deformation and their kinematic significance are not clear. Given that this type of fold is not discussed by earlier workers and their possible genesis has not been attempted so far in this area, the present study may be helpful to understand the genesis of the structure in the FTBs of the world in general and the Himalaya in particular. Geological setting The wedge-shaped fold-thrust belt of the Himalaya con- sists of several east-west trending south vergent thrust faults, such as the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal thrust (MFT), all of which sole into the Main Himalayan Thrust 16 . MCT carries the high-grade Greater Himalayan sequence in its hanging wall over the low-grade (green- schist–facies) Lesser Himalayan sequence. The Lesser Himalayan sequence is carried by the MBT over the syno- rogenic unmetamorphosed to mildly metamorphosed Tertiary Siwalik deposits of the foreland. The Siwalik deposits, in turn, are currently being thrusted over Quaternary deposits along the MFT. Recent studies from different parts of the Himalaya have recognized a re- gional thrust, named as Ramgarh Thrust (RT), which is now considered as a major regional Himalayan thrust, like other major thrusts (e.g. MCT, MBT, MFT) of the Himalayan fold–thrust belt, that transports upper Lesser Himalayan rocks (Pre-Cambrian sequence) over lower Lesser Himalayan rocks (Permian–Eocene– Miocene) 8,15,17–23 . The area of study encompasses a small part of the RT sheet in the Lesser Himalaya in the DSH (Figure 1). The frontal part of the Darjiling–Sikkim–Tibet (DaSiT) wedge has been studied by earlier workers 8,15,24–29 . In this area, the Tista half-window was formed as a result of ero- sion of folded thrust sheets by the Tista River (Figure 1).