title of the thesis - Department of Geology - Queen's University

Chapter 2
From cessation of south-directed mid-crust extrusion to onset of
orogen-parallel extension, NW Nepal Himalaya
2.1 Abstract
Field mapping and, structural, microstructural, and chronological analyses confirm the
existence of a segment of the Gurla-Mandhata-Humla fault, an orogen-parallel strike-slip
dominated shear zone in the upper Karnali valley of northwestern Nepal. This shear zone forms
the upper contact of, and cuts obliquely across the Greater Himalayan Sequence (GHS). Data
from this study reveal two phases of GHS deformation.
Phase 1 is characterized by U-Th-Pb monazite crystallization ages (~26–12 Ma, peak
~18–15 Ma), consistent with typical Neohimalayan metamorphic ages, and the final stages of
south-directed extrusion of the GHS.
Phase 2 is characterized by south-dipping high-strain foliations and intensely developed
ESE-WNW trending, shallowly plunging mineral elongation lineations, indicating orogen-parallel
extension. Thermochronology of muscovite defining these fabrics implies that the area was
cooling and experiencing orogen-parallel extension by ~15–9 Ma. Mineral deformation
mechanisms and quartz c-axis patterns of these fabrics record a rapid increase in temperature
from ~350°C along the shear zone, to ~650°C at ~2.5 structural km below the shear zone. Such
temperature gradients may be remnants of telescoped and/or flattened isotherms generated during
south-directed extrusion of the GHS.
Overprinting ESE-WNW fabrics record progressive deformation of the GHS at lower
temperatures. Progressive deformation included a significant component of pure shear, as
13