STRUCTURAL ANALYSIS OF THE GAURISHANKAR LINEAMENT TO UNDERSTAND ITS ROLE AS A CROSS FAULT IN THE EAST-CENTRAL NEPAL HIMALAYA

This field study focuses on structural analysis of the Gaurishankar Lineament (GL) in east-central Nepal to understand if this topographic feature is a cross-fault (faults perpendicular to the regional E-W strike). The GL has been proposed to be a NNE -striking cross fault that may have blocked the lateral propagation of the thrust fault that caused the Mw 7.8 Gorkha earthquake in 2015. To understand this feature, we conducted a field study along ~E-W traverses south of Gaurishankar peak (Greater Himalayan Sequence (GHS) in Rolwaling) and between Charikot and Jiri (Lesser Himalaya Sequence: LHS), east of the Tamakosi river. Traverses, south of the peak Gaurishankar, did not reveal strong evidence of cross-faults. However, the outcrops possess numerous fracture zones that strike N-NE with an average strike range from N13E to N55E, and a dip angle greater than 50⁰. The zone is dominated by migmatitic orthogneiss primarily composed of Rolwaling-Khumbu Migmatite units (Formation II of GHS). The fracture zone could be the effect of basement activity of the downgoing Indian plate resulting in a blind cross-fault. Traverses near Jiri indicate a shear zone in the brittle NNE-striking fabric of black, graphitic phyllite in outcrops that may represent a cross fault system. Strikes average N32E with a dip angle of 17⁰ and change to N29W towards east. The observed lineation of mineral fiber at the slickenside is trending 52⁰ and plunging 5⁰. The shear zone observed from the outcrops possesses a top-to-north sense of shear. The western end of the Jiri traverse is dominated by the mylonitic augen gneiss of Melung-Salleri Augen Gneiss of LHS with a few lenses of metaquartzite. The eastern end is dominated by biotite-muscovite schist with phyllitic parting and intercalation of quartz veins of the Khare Phyllite of LHS. We interpret these observations of deformational fabric at a high angle to the general ~EW strikes to be evidence for a cross fault in this area.