[S-CG49] Rheology, fracture and friction in Earth and planetary sciences

[SCG49-11] Deformation mechanisms in the brittle-ductile transition zone of crustal-scale faults: Evidence from the Main Boundary Thrust of Himalayas

*Dyuti Prakash Sarkar¹, Jun-ichi Ando¹, Kaushik Das¹, Gautam Ghosh² (1.Hiroshima University, 2.Presidency University)

Keywords:MBT, Calcite e-twins, Calcite Clumped Isotope Thermometry, U-Pb Dating, Felxural Slip

The increase of pressure and temperature with depth result in transition of brittle upper crust to viscous flow at depth. The study of crustal-scale faults can elucidate the deformation mechanisms facilitating this transition. The current study is focused on Main Boundary Thrust (MBT) of the crustal-scale Himalayan Fold Thrust belt and is locally known as Krol Thrust. In the present study, we have attempted to elucidate the nature of deformation mechanisms in the hanging wall and footwall blocks after identification of the fault boundary. The fault boundary in the study area has been inferred from litho-structural mapping and U-Pb detrital zircon geochronology of the lithological units. Additionally, the calcite twin morphology and calcite clumped isotope thermometry provide information of the ambient temperature of deformation that can be correlated to deformation depth. The litho-structural mapping indicates the repetition of lithological units and large-scale folding in the area. The folds exhibit asymmetrical geometry with steeply dipping towards the inferred fault boundary. U-Pb detrital geochronology using LA-ICP-MS from the sandstone units of the steeply dipping beds indicate a sharp difference of depositional age, the hanging wall rocks being ~600 Ma while that footwall rocks being ~61 Ma. This fault boundary comprising of Neo-Proterozoic rocks in hanging wall and Cenozoic rocks in footwall is part of the regional thrust known as Main Boundary Thrust (MBT) of Himalayas. The clumped isotope thermometry and calcite e-twin morphology indicates the ambient temperature of hanging wall to be ~269±30℃ indicating a depth of about ~11–12 km. Deformation microstructures in the hanging wall include occurrences of grain-scale fractures and deformation band in the sandstone while the argillaceous sandstone exhibit asymmetrically rotated quartz clasts in phyllosilicate rich layers. Footwall deformation microstructures include dominantly fractures and detached calcite veins. Additionally, pervasive veins of calcite in the footwall rocks exhibit Type-I deformation e-twins while veins close to the fault boundary exhibit Type-II and Type-III deformation e-twins. Deformation microstructures of the hanging wall indicate the activation of flexural slip in phyllosilicate rich layers of argillaceous sandstone, and calcite e-twins and pressure solution seams in stromatolitic limestone indicate ductile mechanisms. Additionally, brittle mechanisms in sandstone are also observed in form of deformation bands. The deformation depth and mechanisms indicate the features of brittle-ductile transition in the Main Boundary Thrust.