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Estimation of driving pressure ratio and paleostresses from veins in the Pelling-Munsiari shear zone, Sikkim Himalayan Fold Thrust Belt

We analyze veins from the deepest exposure of the regionally folded Pelling-Munsiari thrust (PT), the roof thrust of the Lesser Himalayan duplex, in the Sikkim Himalaya. The PT is exposed as a discontinuous, ~970 m thick quartz-mica mylonite zone near Mangan (27°29′ N, 88°31′ E), and records progressive deformation path where shallow crustal deformation features overprint deeper crustal deformation structures. The mean mylonitic foliation is north easterly oriented in the studied location (mean ~31°, 042°). Based on the angular relationship with respect to the mylonitic foliation, we recognize three different fractureand associated vein-sets at the outcrop scale. These are low-angle set (<30° with respect to the mylonitic foliation), moderate-angle (30°-60°) and high-angle set (>60°).The high-angle fracture set overprints the mylonitic foliation and is the youngest set. These are also the most dominant fracture set (~58 %), followed by the moderate-angle (~32%) and low-angle (~10%) sets. Interestingly, the low-angle vein set (mean orientation ~ 29°, 054°) is the most dominant set (~61%), followed by the moderate-angle set (~26%; mean orientation ~ 19°, 055°), and the high-angle set (~13% ; mean ~23°, 340°).Field analysis indicates that ~95% of low-angle, ~71% of moderate-angle and ~ 40% of high-angle fracture-sets form veins. Some of the lowand moderate-angle veins are locally folded along with the mylonitic foliation. The co-efficient of variation (C v ) of spacing of both the fracture and vein sets are less than 1, indicating that these follow anti-clustered distribution. The poles to the veins indicate two distinct patterns. The lowand moderate-angle veins define girdle distribution, implying pore fluid pressure (P f ) exceeded intermediate principal stress axis (σ 2 ), whereas the highangle set shows a clustered distribution indicating σ 2 exceeded P f . A preliminary study reveals presence of blocky texture in the lowand moderate-angle veins with quartz growing at high angles with respect to the vein walls. The average thickness of the low-angle, moderate-angle, and high-angle veins, measured along appropriate scan-lines are ~ 0.92 cm, ~1.03 cm and ~0.64 cm respectively. As the lowand moderate-angle vein-sets are the most dominant sets and both show girdle distribution, we estimated a driving pressure ratio (R' ~0.35-0.6) and stress ratio (ɸ~0.251) for these veins. The estimated paleostresses from these veins are σ 1 (28°, 058°), σ 2 (2°, 327°), σ 3