[S-CG46] Rheology, fracture and friction in Earth and planetary sciences

[SCG46-P06] Role of lithology in deformation mechanisms of shallow-crustal active faults: Evidence from the Nahan thrust of Himalayas

*Dyuti Prakash Sarkar¹, Jun-ichi Ando¹, Kaushik Das¹, Gautam Ghosh², Prabir Dasgupta³ (1.Hiroshima University, 2.Presidency University, 3.Durgapur Govt. College)

Keywords:NT, Fault gouge, CCA, Calcite e-twins

Research on deformation nature and fault rock geochemistry of shallow-crustal active faults has been widely studied for elucidating the stress accommodation mechanisms and response to the fault movement in terms of stress localizations, fault weakening, and earthquake nucleation. The heterogeneous nature of the fault in response to tectonic stresses is also reported by few researchers. The current study is focused on Nahan Thrust (NT), a shallow-crustal thrust fault within the Main Frontal Thrust (MFT) sheet of the Himalayan Fold Thrust belt. In the present study, we have attempted to elucidate the nature of deformation mechanisms inferred from detailed field mapping of the fault zone, microstructural observation, and characterization of deformation mechanisms. In the studied area, NT has developed within the alternation of quartz-rich and phyllosilicate-rich siltstone of the Paleogene Dagshai Formation. It forms a wide brittle fault zone with a damage zone of 110 m and a fault core of 40 m. The damage zone of NT shows dominantly brittle deformation with an increasing degree of brecciation toward the fault core. The fault core shows several layers of chaotic breccia-grey gouge and foliated red gouge with a single layer of black gouge ~1 m (forming black gouge). Calcite vein fragments present with the damage zone rocks and fault core show the development of deformation twins, which suggests a deformation temperature of ~170 ℃. The modal mineral analysis of the intact and gouge rocks, determined in conjunction with SEM-BSE, EPMA, and image processing by ImageJ software, as well as the obtained XRD patterns from intact and gouge rocks indicate that the formation of different gouges within the fault core is dependent on the intact rocks. Based on the microstructural observations, the quartz-rich intact rock has dominantly deformed by fracturing and cataclasis with minimum effects of fluid alteration (if any) during the fault activity. The phyllosilicate-rich layer, however, has exhibited pervasive alteration and generation of finer phyllosilicates, with deformation dominantly by pressure solution mechanisms. The phyllosilicate-rich layers also act as sealing zones, localizing stress and subsequent stress buildup within the quartz-rich layers, thereby narrowing the active zone of the fault during subsequent fault activity. Additionally, grain size analysis of the gouge rocks by ImageJ, and their XRD patterns indicate the presence of amorphous material in the black gouge. The presence of amorphous materials is also evident from Transmission Electron Microscopy (TEM) analysis of the black gouge. The black gouge in the studied area is, therefore a product of co-seismic frictional heating, indicated by the presence of amorphous materials, gas escape structures, and clay clast aggregates.