STRAIN PARTITIONING WITHIN THRUST SHEETS: INSIGHTS FROM FAR-EASTERN SIANG WINDOW, ARUNACHAL HIMALAYAN FOLD THRUST BELT

In a fold thrust belt (FTB), penetrative strain within thrust sheets vary in its magnitude, orientation and type. Addressing variation in magnitude and orientation of strain from major thrust sheets in an FTB, both along the transport direction and along-strike, enable us to understand the complexity of strain partitioning during orogeny. Tectonic windows provide an opportunity to understand the impact of footwall structures on finite strain geometry and orientations. In this study, we investigate how penetrative strain is partitioned from the internal to the external major thrust sheets in the Siang window in far-eastern Arunachal Himalayan FTB.

We conduct finite strain analysis on quartz grains using R_f-Φ, normalized Fry and Shape Matrix Eigenvector methods. The studied lithologies are gneiss for the internal Pelling-Munsiari thrust (PT) sheet, while quartzite and sandstone dominantly comprise the external Main Boundary thrust (MBT) and the Main Frontal thrust (MFT) sheets. The northernmost Main Central thrust (MCT) of the Himalayan FTB lies farther north of the study area and is not accessible. Results from this study indicate that all the studied rocks record an overall flattening strain. Magnitude of the finite penetrative strain decreases from the internal PT sheet to the external MBT, MFT sheets in the Siang window. The long axes of the finite strain ellipsoids (X) generally have a low plunge and a dominant bearing of N-S, irrespective of the structural positions of the different thrust sheets. Finite strain ellipses are folded along with the thrust sheets indicating that the penetrative strain developed prior to folding of the thrust sheets. The results also indicate that the footwall structures affect the strain geometry in the interior part of the Himalayan wedge. Preliminary results indicate modification of early layer parallel shortening strain in both internal and external thrust sheets in the Siang window.