2:30 PM - 2:45 PM
[SCG61-04] Observation of geological structure and rock microstructure associated with strain release along a plate boundary fault
Keywords:Bedding plane slip, Main boundary fault, Microstructure observation, Frictional heating, Shear plane, Structural geology
Introduction: The Himalayan region was formed by the collision of the Indian subcontinent with the Asian continent, which resulted in the development of three major faults (from north to south): the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Main Frontal Thrust (MFT). Even today, the Indian subcontinent continues to move northwards, resulting in earthquakes occur in the Himalayan region. The study by Bilham (2019) suggests that the strain accumulated by the ongoing subduction has not been fully released by the earthquakes to date, and there is a strong possibility that one or two Mw = 8.6 level earthquakes will occur in the future. On the other hand, it has not been clarified what kind of geological phenomena occur at plate boundaries during plate subduction. The present study involves geological surveys and observations of rock microstructures to elucidate the nature of deformation that occurred at the brittle deformation zone during the subduction process.
Study area: The study area is a 3 km x 2 km area flanked by MBTs exposed at Sabathu, Himachal Pradesh, India. The hanging wall side of the MBT is thought to expose rocks deformed at a depth equivalent to a temperature of 220℃, while the footwall side exposes rocks deformed at a depth equivalent to temperature of 180℃ below the surface (Sarkar et al., 2021). The hanging wall side of the MBT consists mainly composed of Precambrian sandstone beds. The thickness of the individual sandstone bets ranges from 5 cm to 30 cm. Alternate layers of mudstone (thickness of a single layer is about 2 cm) can also be observed. The footwall side primarily consists of Neogene calcareous sandstone, and calcite veins are widely distributed.
Field survey: The following were revealed. 1) On the hanging wall side of the MBT, numerous bedding plane slips and associated kink bands and duplex structures were observed throughout the entire study area. 2) The damage zones and the well-ordered stratum can be identified. The damage zone is defined as the presence of outcrop-scale folds and faults. The strike direction of the damage zones and the well-ordered stratum are almost the same but differ in the dip direction by 60°to an acute angle. 3) From the slicken lines and kink bands developed on the bedding planes, the directions of the slip and main compression axes were determined, respectively. As a result, it is clear that many of them are consistent with MBT activities. 4) A 5 cm-10 cm white stripe parallel to the slip line was observed on the slip surface.
Microstructural observation: 5) Most of the bedding plane slip develops along multiple shear planes with 1 mm-10 mm thick parallel to the bedding plane within the sandstone monolayer. 6) Each shear plane records small to large strain of bets. As the strain increases, quartz and feldspar reacted with the fluid to form fine grains of 5μm and muscovite crystallizes. 7) Within the slip surface (shear zone), the quartz grains of the sandstone bets are about 100 μm in size, elongate in the shear direction, and show the undulose extinction. This strongly suggests that the quartz grains in the sandstone bets have been plastically deformed by frictional heat generated by shearing. Quartz veins are formed from such plastically deformed quartz. This quartz vein corresponds to the white stripe in 4). 8) The crystal orientation of the plastically deformed quartz grains was measured by EBSD. This confirmed the CPO formed by dislocation creep due to basal slip. This CPO is known to be dominant at 300-400°C. The grain size of dynamically recrystallized quartz in the quartz vein is about 2 μm. We estimated the differential stress using a piezometer based on the recrystallized grain size, and temperature. The results showed a differential stress of 925 MPa, a strain rate of 10-10 to 10-8 /s, and a slip rate of 10-12 to 10-10 m/s. These slip velocities correspond to the Slow Slip Event slip velocities (Rowe et al., 2015).
These results suggest that some of the strain energy resulting from plate subduction is released by “bedding plane slip motion” and “frictional heating due to slip”.
Currently, computer simulations are being conducted to confirm whether a temperature increase of 80°C or more can occur under the conditions estimated from these results due to frictional heating.
Study area: The study area is a 3 km x 2 km area flanked by MBTs exposed at Sabathu, Himachal Pradesh, India. The hanging wall side of the MBT is thought to expose rocks deformed at a depth equivalent to a temperature of 220℃, while the footwall side exposes rocks deformed at a depth equivalent to temperature of 180℃ below the surface (Sarkar et al., 2021). The hanging wall side of the MBT consists mainly composed of Precambrian sandstone beds. The thickness of the individual sandstone bets ranges from 5 cm to 30 cm. Alternate layers of mudstone (thickness of a single layer is about 2 cm) can also be observed. The footwall side primarily consists of Neogene calcareous sandstone, and calcite veins are widely distributed.
Field survey: The following were revealed. 1) On the hanging wall side of the MBT, numerous bedding plane slips and associated kink bands and duplex structures were observed throughout the entire study area. 2) The damage zones and the well-ordered stratum can be identified. The damage zone is defined as the presence of outcrop-scale folds and faults. The strike direction of the damage zones and the well-ordered stratum are almost the same but differ in the dip direction by 60°to an acute angle. 3) From the slicken lines and kink bands developed on the bedding planes, the directions of the slip and main compression axes were determined, respectively. As a result, it is clear that many of them are consistent with MBT activities. 4) A 5 cm-10 cm white stripe parallel to the slip line was observed on the slip surface.
Microstructural observation: 5) Most of the bedding plane slip develops along multiple shear planes with 1 mm-10 mm thick parallel to the bedding plane within the sandstone monolayer. 6) Each shear plane records small to large strain of bets. As the strain increases, quartz and feldspar reacted with the fluid to form fine grains of 5μm and muscovite crystallizes. 7) Within the slip surface (shear zone), the quartz grains of the sandstone bets are about 100 μm in size, elongate in the shear direction, and show the undulose extinction. This strongly suggests that the quartz grains in the sandstone bets have been plastically deformed by frictional heat generated by shearing. Quartz veins are formed from such plastically deformed quartz. This quartz vein corresponds to the white stripe in 4). 8) The crystal orientation of the plastically deformed quartz grains was measured by EBSD. This confirmed the CPO formed by dislocation creep due to basal slip. This CPO is known to be dominant at 300-400°C. The grain size of dynamically recrystallized quartz in the quartz vein is about 2 μm. We estimated the differential stress using a piezometer based on the recrystallized grain size, and temperature. The results showed a differential stress of 925 MPa, a strain rate of 10-10 to 10-8 /s, and a slip rate of 10-12 to 10-10 m/s. These slip velocities correspond to the Slow Slip Event slip velocities (Rowe et al., 2015).
These results suggest that some of the strain energy resulting from plate subduction is released by “bedding plane slip motion” and “frictional heating due to slip”.
Currently, computer simulations are being conducted to confirm whether a temperature increase of 80°C or more can occur under the conditions estimated from these results due to frictional heating.