Detection of a weak late-stage deformation event in granitic gneiss through anisotropy of magnetic susceptibility: implications for tectonic evolution of the Bomdila Gneiss in the Arunachal Lesser Himalaya, Northeast India

Abstract

Outcrop-scale structures and magnetic fabric anisotropy of the Bomdila Gneiss (BG) that intruded the Lesser Himalayan Crystallines (LHC) of the Arunachal Lesser Himalaya are studied to understand the BG deformation history and tectonic evolution. Detailed analysis of structures reveals that the LHC have undergone three phases of deformation, D1, D2 and D3. The S2 foliation developed during the second phase of deformation (D2) is the most penetrative planar fabric in the studied rock, which shows a general ENE–WSW strike with moderate NW dip. Mesoscopic evidence of a later phase of deformation (D3) in the BG is lacking. Evidence of D3 deformation in the form of F3 folds is only observed in the adjacent metasedimentary rocks of the LHC. The magnetic foliations recorded from anisotropy of magnetic susceptibility (AMS) analysis of the BG are mostly striking NW–SE with a moderate dip towards the NE or SW, and magnetic lineation is mostly sub-horizontal and dominantly plunging towards the SE. Our study shows that the magnetic fabric of the BG does not correspond to any visible outcrop-scale mesoscale foliation. However, the magnetic foliation of the BG is parallel to the axial plane of the F3 folds of the adjacent metasedimentary rocks of the LHC. Integration of AMS and outcrop-scale structural analysis helps us envisage the superposed deformation history of the BG. Our study emphasizes the importance of AMS to detect late-stage or feeble deformation events that leave no visible outcrop-scale imprint and are difficult to discern through conventional geological means.

Brittle tectonics in the western Arunachal Himalayan frontal fold belt, northeast India: Change in stress regime from pre‐collisional extension to collisional compression

“…The MBT separates the Gondwana sequence from the pre‐Gondwana metasedimentary sequence in the entire eastern Himalaya (Acharyya, 1980; Figure 2). Along the MBT‐1, the Lesser Himalayan Bomdila Group of rocks are thrust over the Gondwana sequence in the western Arunachal Himalaya (Singh, Singh, Sen, & Sangode, 2017). The Gondwana rocks are thrust over the Neogene Siwalik sequence along the MBT‐2.…”

Tectonics of Sikkim and Eastern Himalaya

“…Field studies on the Bomdila Gneiss (Location 7 in Fig. 1), Arunachal Lesser Himalaya by Singh et al (2017b) have led to the demarcation of three deformation phases. The obtained magnetic fabric (SE plunging and sub-horizontal) does not match with any mesoscale foliation, but parallels the F 3 axial plane.…”

Mesoscopic, magnetic and petrofabric study of the High Himalayan gneisses and leucogranite along oblique and frontal ramps of the Vaikrita Thrust in Satluj and Bhagirathi valleys: Thrust locking and superposed folding

“…Earlier studies carried out by Aranguren et al (1996) and Tomezzolli et al (2003) have shown that magnetic fabric for granitic mylonites or granitic gneiss may represent an intermediate fabric between 'S' and 'C' fabrics or depict a weak fabric developed at later stages of deformation (Ono et al 2010). In the case of Himalaya, a complexly deformed terrene, comparison between the magnetic fabric and their corresponding mesoscopic foliation is important as it can reveal multiple deformation events (Jayangondaperumal et al 2010;Sen et al 2012;Tripathi et al 2012;Singh et al 2017).…”