Asymmetric development of the Himalaya: quantitative evidence from strain analysis in Kimin‑Ziro‑Tamen area, Arunachal Lesser Himalaya, India

摘要

The eastern segment of the Himalaya makes a quarter of the mountain belt. While its plate tectonic setting is well understood, the intensity of deformation along the thrusts is not well constrained. This knowledge gap has restricted a quantitative comparison of deformation and crustal shortening in the Eastern Lesser Himalaya with the Central and Western Lesser Himalaya. In this study field evidence, together with microstructural investigations, and two-dimensional (2D) finite strain analysis are used to decipher the deformation pattern in the Kimin-Ziro-Tamen area of the Eastern Lesser Himalaya. Present investigations reveal that micro-to-outcrop-scale structures such as plastically deformed quartz grains, crenulation cleavages, S-C fabric, pull-apart grains, pinch-n-swell structures, boudins and faults confirm that the rocks have been subjected to ductile and brittle-ductile phases of deformation with a regional top-to-south shear sense. The 2D finite strain estimated here is comparable to that in other parts of Eastern Lesser Himalaya, such as the Sikkim and Bhutan Himalaya. However, the strain estimated here is higher than in the Central and Western Lesser Himalaya, such as in the Central Nepal Himalaya, Almora Crystalline Zone, the Garhwal Lesser Himalaya and the Hazara-Kashmir Syntaxis in Pakistan. This provides quantitative proofs of greater deformation and crustal shortening in the Eastern Lesser Himalaya than the Central and Western Himalaya, and of the asymmetric development of Himalaya.