Unravelling the evolution of regional-scale shear zones with the aid of microstructures and textural analysis: an example from the central Himalaya

Abstract

Microstructure and texture data on the Main Central Thrust zone (MCTz) in central Himalaya, on two nearly N-S oriented valleys in the Manaslu range (Western Nepal), are provided. Kinematic indicators at the meso and microscale support a south-directed flow. From south to north, greenschist facies paragneiss in the garnet-zone pass to amphibolite facies rocks in the kyanite-zone, suggesting an up-section metamorphic temperature increase. Combining microstructures and data from two different texture analysers (X-ray Texture goniometry and crystal-fabric microanalyzer) allowed to link the temperature path (from 460°C to 600°C up-section) and kinematic vorticity estimates. The "orientation of oblique grain shape fabrics" method was applied, pointing out a general shear flow (40-56% of pure shear) at the MCTz top. Results are in line with a "decelerating strain-path" model, as relatively high simple shear components are recorded at deeper structural levels. This work highlights that the combination of X-ray texture goniometry and crystal-fabric microanalyzer techniques is a good strategy to ensure quartz full-fabric.

Constraining the evolution of shear zones in the Himalayan mid crust in Central–Western Nepal: implications for the tectonic evolution of the Himalayan metamorphic core

“…2b). The main differences to the Marsyangdi valley are the virtual absence of calcsilicate and marble of Unit II (Colchen et al 1986) and the occurrence of carbonate-bearing lithologies within the MCTz (Colchen et al 1986; Nania, 2021). A ductile shear zone within Unit I, 1 km south of the village of Philim (Fig.…”