Structural Evolution and Deformation Pattern of the Lumla Window, Tawang District, Western Arunachal Himalaya

The Lumla window, situated in western Arunachal Pradesh near Tawang, represents a significant tectonic window, exposing medium-grade Lesser Himalayan metasediments of the Lumla Formation, overlain by gneisses of the Sela Group equivalent to the Higher Himalayan Crystalline (HHC) gneisses along the Main Central Thrust (MCT). The Lumla Formation comprises of crystalline limestone, garnet-bearing quartzbiotite-muscovite schist, and micaceous quartzite in ascending order, with minor carbonaceous phyllite and amphibolite sills. Mylonitic augen gneiss occupies the core of the Lumla window, separated from the overlying metasediments of the Lumla Formation by the Lumla Thrust. Lumla window shows a domal structural pattern with four distinct deformation phases. Imprints of Himalayan orogeny are represented by tight, asymmetric to overturned, ENE-WSW trending, south verging F2 folds and ENE-WSW trending, gently plunging, nearly upright crenulations (F3) followed by late, orogen transverse, NNW-SSE trending, gently plunging upright crenulations (F4). Frequent occurrence of asymmetric boudins in both strike-parallel and perpendicular sections, along with low to moderate pitch of stretching lineation on S1/mylonitic foliation, indicates partitioning of simple shear into strike and dip-slip components with simultaneous compression, akin to inclined transpression. Pitch of stretching lineation in the N-S trending arms of the Lumla window becomes nearly strike parallel, indicating strike-slip dominated deformation along the possible traces of lateral ramps. In the southern arm of the Lumla window, north-verging asymmetric folds and C/S fabric indicate top-towards-north movement, contradictory to the general top-towards-south thrust transport along the Himalaya. These findings support a ductile deformation model, dominated by buckle folds, inclined transpression, and lateral ramp development, contributing new insights into the structural evolution and deformation mechanisms of Himalayan tectonic windows.