Deformation mechanisms in shallow-crustal active fault zones: Implications from the Main Frontal Thrust of Himalayas

The nature of the shallow-crustal active faults has been widely studied to elucidate their stress accommodation mechanisms and response to the fault movement in terms of stress localizations and ultimately stress release by rupture or creep mechanisms. The Main Frontal Thrust (MFT) and its hinterland area are at present the most tectonically active zones within the Himalayan orogen accommodating northward convergence of India against the Eurasian plate with periodic release of accumulating strain energy through moderate–large earthquakes. This work is a first report on detailed microstructural and mineralogical studies from fault gouges of the Nahan Thrust (NT) occurring in the vicinity of MFT. The NT exhibits a ~150 m wide fault core-damage zone in the studied area, where the fault core consists of grey-green, red and black gouge layers. The intact rocks comprise alternations of arenitic and argillaceous sandstone, while the damage zone consists of crackle to mosaic and foliated breccia. The modal mineral analysis of the fault core rocks and the intact rocks determined in conjunction with SEM-BSE, EPMA, and image processing by ImageJ software (Schneider et al., 2012), suggests that the original rock of the grey-green and black gouge layers is the arenitic sandstone, while that of the red gouge layers is the argillaceous sandstone. Microstructures indicate dominance of stress localization and cataclasis in the grey-green gouge, while pressure solution creep in the red gouge. Additionally, the calcite vein fragments present in the red gouge show development of Type-I deformation twins indicating the overall temperature of deformation ~170℃ (Rowe and Rutter, 1990; Burkhard, 1993; Ferrill et al., 2004). However, the black gouge layer shows development of ultracataclasite-rich principal slip zones with degassing bubbles and clay clast aggregates (CCA), indicating localized frictional heating and possible seismic faulting (Boutareaud et al., 2008; Tesei et al., 2014). Hence, this study proposes that the arenitic sandstone suffered seismic rupture, while the argillaceous sandstone deformed aseismically, implying a strong lithological dependence on the seismic cycle within the Nahan thrust.