Polyphase or time-dependent kinematics and quaternary reactivation of thrust bounding Baijnath Klippe: western Kumaun Himalaya, India

Abstract

The morphotectonic elements of the Baijnath Klippe (BK) have been examined to understand the kinematics of the Inner Kumaun Lesser Himalaya from Tertiary to Late Quaternary. The BK is bounded by two thrusts: the Askot Thrust (AT), to the north and the Baijnath Thrust (BjT), to the south. The older (20 Ma) hanging wall rocks (mylonites and phyllites) are underlain and surrounded by quartzites (equivalent to Nagthat Formation of Outer Lesser Himalaya). The kinematics study and strain analysis along Dewal–Narayanbagarh transect provides the varied intensity of deformation (D1, D2, and D3) within the zone of BK. The D1 event is characterized by isoclinal tight folds in phyllite and mylonite; the D2 phase is characterized by open folding in mylonite. However, the youngest event D3 phase is observed as a regional synclinal folding of BK and associated rocks. The presence of 1–3-km-thick mylonite zone parallel to the thrust planes is related to the compressional event of Miocene age. The geomorphic pattern and chronologically constraints (OSL ages) with spatial variability in the incision/uplift rate suggests that the terrain is undergoing differential crustal deformation during Late Quaternary. Kinematic indicators such as streaky foliations, S–C foliation, mantled porphyroclasts, micro-shears with bookshelf gliding, and mica fish found in mylonites (macroscopic as well as microscopic) indicate ductile deformation in the area. We used Flinn plot (Rf/ϕ) and Fry (Tectonophysics 60:89–105, 1979) analysis to delineate the finite strain pattern in 21 oriented samples collected across the BK bound thrusts (BjT and AT). The strain analysis revealed an increasing trend of strained quartz grains, which presents triaxialoblate strain geometry (flattening i.e., 0 < k < 1) towards the structural discontinuity. The textural (chessboard, myrmekite, undulose extinction and fracturing in quartz and feldspar) and mineralogical properties (non-flaky) of quartz and feldspar indicate the temperature of deformation and mylonitization ranging 300–500 °C.