<p>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&#176;29&#8242; N, 88&#176;31&#8242; 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&#176;, 042&#176;). &#160;Based on the angular relationship with respect to the mylonitic foliation, we recognize three different fracture- and associated vein-sets at the outcrop scale. These are low-angle set (<30&#176; with respect to the mylonitic foliation), moderate-angle (30&#176;-60&#176;) and high-angle set (>60&#176;).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&#176;, 054&#176;) is the most &#160;dominant set (~61%), followed by the moderate-angle set (~26%; mean orientation &#160;~ 19&#176;,&#160; 055&#176;), &#160;and the high-angle set (~13% ; mean ~23&#176;, 340&#176;).Field analysis indicates that ~95% of low-angle, ~71% of moderate-angle and ~ 40% of high-angle fracture-sets form veins. Some of the low- and moderate-angle veins are locally folded along with the mylonitic foliation. The co-efficient of variation (C<sub>v</sub>) 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 low- and moderate-angle veins define girdle distribution, implying pore fluid pressure (P<sub>f</sub>) exceeded intermediate principal stress axis (&#963;<sub>2</sub>), whereas the high-angle set shows a clustered distribution indicating &#963;<sub>2</sub> exceeded P<sub>f</sub>. A preliminary study reveals presence of blocky texture in the low- and 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 low- and 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 (&#632;~0.251) for these veins. &#160;The estimated paleostresses from these veins are &#963;<sub>1</sub> (28&#176;, 058&#176;), &#963;<sub>2</sub> (2&#176;, 327&#176;), &#963;<sub>3</sub> (62&#176;, 233&#176;).</p>
Hardware Assistance for Constrained Circle Constructions II: Cluster Merging Problems
