R7: Petrology, Mineralogy and Economic geology (Joint Session with Society of Resource Geology)

[R7-07] Fault-rock mineralogy and weakening mechanisms at deep- to shallow-crustal depths of Main Himalayan Thrust, NW India

「招待講演」

*Dyuti Prakash SARKAR¹, Takehiro Hirose¹, Jun-ichi Ando², Kaushik Das², Gautam Ghosh³ (1. Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2. Hiroshima University, 3. Presidency University)

Keywords:Main Himalayan Thrust, Main Central Thrust, Main Frontal Thrust, rock friction, phyllosilicates

Fault rock mineralogy and deformation microstructures of crustal-scale faults provide insights into the stress accommodation mechanisms in terms of fault weakening and possibilities of earthquake nucleation. The Main Himalayan Thrust is considered to be the crustal-scale collisional boundary along which the Indian plate subducts beneath the Asian plate. The near-surface exposure of the present active boundary of the subduction is considered the Main Frontal Thrust, while the Main Central Thrust at the northernmost boundary represents the older post-collisional paleo-thrust boundaries between Indian and Asian plates. Here we present a comparative study of our findings on the fault rocks from the Nahan Thrust and North Almora Thrust belonging to the Main Frontal Thrust and the Main Central Thrust respectively. The deformation zone from North Almora Thrust represents the deep crustal part of the Main Himalayan Thrust exhibiting granite mylonite-ultramylonite zone. Quartz microstructures indicate deformation temperatures of 450–550 ℃ with evidence of grain-size sensitive creep as the dominant weakening mechanism. Additionally, two-feldspar thermometry has been used to estimate temperatures from sheared myrmekitic feldspar in the mylonites and ultramylonites. Crystallographic preferred orientations of quartz and mica indicate a shift in partitioning of strain from quartz to mica with an increase in mica content. In comparison, the fault zone from the Nahan Thrust shows the dominance of brittle deformation structures consisting of highly brecciated sandstone and a wide gouge zone. Microstructures of the gouge zone indicate distributed deformation and progressive stress localization leading to fracture development, cataclasis, and frictional sliding. Especially the ultra-fine bands of the principle slip zone exhibit a possible fingerprint of frictional heating during the seismic slips. The frictional properties of the fault rocks estimated from the rotary-shear velocity step experiments also indicate a velocity weakening to strengthening behavior based on phyllosilicate content. Our results on fault rocks from deep- to shallow-crustal depth indicate that phyllosilicate content play a pivotal role in frictional behavior and weakening mechanisms along the subduction boundary.