DEFORMATION TEMPERATURE GRADIENTS AND KINEMATIC EVOLUTION OF A DISTRIBUTED MID-CRUSTAL SHEAR ZONE, LHAGOI KANGRI RANGE, SOUTHERN TIBET

The Lhagoi Kangri gneiss dome, located ~100 km NE of Mt. Everest, features a domed core of orthogneiss and granite mantled by sillimanite-grade metasedimentary rocks that decrease in metamorphic grade up structural section to unmetamorphosed sediments. At the resolution of our field study we did not observe discrete structural breaks within this lithologic succession. Instead, deformation is characterized by a distributed shear zone that encompasses ≥ 600 m of orthogneiss and ~1500 m of the overlying metasedimentary rocks (2-2.5 km total thickness).

On the north side of the dome, a down-section transition from phyllite to garnet-chloritoid schist accompanies a transition in structural fabric from pencil cleavage to a pervasive crenulation cleavage and finally to a weakly mylonitic foliation. The transition from pencil cleavage to crenulation cleavage marks the approximate upper boundary of the shear zone, below which the foliation (S2) dips moderately to gently toward the NNE. As a result of doming, S2 dips gently east on the eastern side of the dome and gently to moderately south on the south side of the dome. An accompanying stretching lineation, L2, is present, but not ubiquitous, throughout the shear zone and has an average trend of 010° on the north, south, and east sides of the dome. High temperature kinematic indicators such as porphyroblast strain shadows, mica fish, and asymmetric myrmekite lobes on K-feldspar record mixed shear sense throughout the shear zone. In addition to the mineral assemblages present, quartz, feldspar, and calcite microstructures indicate deformation temperatures between 200–300 °C at the uppermost structural levels to ≥ 650 °C near the base of the section with no indication of a lower temperature deformation overprint. These results are in agreement with quartz [c] axis opening angle thermometry and inferred quartz slip systems. Collectively, these data define an elevated thermal field gradient (100–225 °C/km) indicating a component of shortening across the shear zone. Simple telescoping of a set of contemporaneous isotherms does not adequately explain all microstructural observations, suggesting that the calculated field gradient records both spatial and temporal changes in thermal structure within the shear zone.