Estimation of driving pressure ratio and paleostresses from veins in the Pelling-Munsiari shear zone, Sikkim Himalayan Fold Thrust Belt
Authors: Ray Deblina , Bhattacharyya Kathakali 
Published: March 2021

&lt;p&gt;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&amp;#176;29&amp;#8242; N, 88&amp;#176;31&amp;#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&amp;#176;, 042&amp;#176;). &amp;#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 (&lt;30&amp;#176; with respect to the mylonitic foliation), moderate-angle (30&amp;#176;-60&amp;#176;) and high-angle set (&gt;60&amp;#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&amp;#176;, 054&amp;#176;) is the most &amp;#160;dominant set (~61%), followed by the moderate-angle set (~26%; mean orientation &amp;#160;~ 19&amp;#176;,&amp;#160; 055&amp;#176;), &amp;#160;and the high-angle set (~13% ; mean ~23&amp;#176;, 340&amp;#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&lt;sub&gt;v&lt;/sub&gt;) 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&lt;sub&gt;f&lt;/sub&gt;) exceeded intermediate principal stress axis (&amp;#963;&lt;sub&gt;2&lt;/sub&gt;), whereas the high-angle set shows a clustered distribution indicating &amp;#963;&lt;sub&gt;2&lt;/sub&gt; exceeded P&lt;sub&gt;f&lt;/sub&gt;. 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 (&amp;#632;~0.251) for these veins. &amp;#160;The estimated paleostresses from these veins are &amp;#963;&lt;sub&gt;1&lt;/sub&gt; (28&amp;#176;, 058&amp;#176;), &amp;#963;&lt;sub&gt;2&lt;/sub&gt; (2&amp;#176;, 327&amp;#176;), &amp;#963;&lt;sub&gt;3&lt;/sub&gt; (62&amp;#176;, 233&amp;#176;).&lt;/p&gt;