STRAIN PARTITIONING WITHIN THRUST SHEETS OF TECTONIC WINDOWS: INSIGHTS FROM EASTERN HIMALAYA

JYOTI PRASAD DAS*, KATHAKALI BHATTACHARYYA *Email: [email protected] Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata (IISERK), Mohanpur, Nadia-741246, West Bengal, India Introduction 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 Arunachal strain partitioning during orogeny. Also, these aspects Sikkim Bhutan of strain may evolve as deformation progresses from internal to external part of the tectonic wedge. Tectonic windows provide an opportunity to understand the impact of footwall structures on finite Regional map of the Himalayan FTB showing major thrusts and the litho-tectonic units (modified after Yin, 2006). Red box shows the current study area, eastern-Arunachal Siang window. strain geometry and orientations. In this study, we focus on one of the remotest and least studied tectonic windows, the Siang window, from far-eastern Arunachal Himalayan FTB that exposes the Lesser Himalayan sequence (LHS) and Sub Himalayan sequence (SHS), bounded by the Pelling-Munsiari thrust (PT; Basa et al., 2019). We investigate how the penetrative strain is partitioned among five exposed thrust sheets that comprise of gneisses, quartzites and sandstones in this segment of the Himalayan orogenic wedge. We analyze both 2D and 3D strains and study their magnitudes and orientations. We also compare these results with similar thrust sheets from tectonic windows in the Sikkim (Parui and Bhattacharyya, 2019; Ghosh et al., 2020) and Bhutan Himalayan FTB (Long et al., 2011).

Regional map of the Siang window showing the major thrust sheets(modified after Acharyya, 1994; Ahmed and Bhattacharyya, 2018; Basa et al., 2019). The Lesser and Sub-Himalayan rocks are repeated along imbricates in the region forming the Siang window (Ahmed and Bhattacharyya, 2018; Basa et al., 2019). The strain ellipses are projected along which section on the map. Objectives

• To estimate 2D and 3D finite strain from five exposed thrust sheets in the Siang window in far-eastern Arunachal Himalayan FTB

• To address strain partitioning in this segment of the Himalayan wedge

• To compare the strain results from this study with similar tectonic windows exposed in Sikkim –Bhutan Himalaya Methodology

(Parui et al., 2020) Best-fit strain ellipse were estimated using Rf-phi, enhanced-fry (Erslev and Ge, 1990), shape matrix eigenvector (Shimamoto and Ikeda, 1976)

3D finite strain ellipsoid via Mathematica program (Mookerjee and Nickleach, 2011) Flinn’s plot (Fossen, 2010) Results

Stereoplots showing the maximum (X) and minimum (Z) principal strain axes of the finite strain ellipsoids

Transport parallel regional balanced cross-section from Arunachal Himalaya (modified after Ahmed and Bhattacharyya, 2018) with finite strain ellipsoids, strain ellipses and strain ratio (Rxz) for different thrust sheets Results

Flinn plot using quartz grains from Siwalik sandstones, Yambung quartzites, Buxa quartzites and recrystallized quartz grains from Paro gneisses in Arunachal. Comparison with Sikkim (Bhattacharyya et al., 2015; Parui and Bhattacharyya, 2019; Ghosh et al., 2020) and Bhutan (Long et al., 2011) Himalayan FTB Results

Plot of Rs[X/Z]) vs. q’, the angle between the long axis of the strain ellipse and the apparent dip of bedding/foliation in the cut plane indicating modified LPS (modified after Sanderson, 1982; Yonkee, 2005; Long et al., 2011; Ghosh et al., 2020) from Arunachal and Sikkim Himalayan FTB Conclusions

• The penetrative strain decreases from the Paro gneiss of the internal PT sheet to the Yambung and Buxa quartzite of the MBT and siwalik sandstone of the MFT sheet • Finite strain ellipses are folded along with the thrust sheets indicating that the penetrative strain developed prior to folding of the thrust sheets • Orientations of X and Z do not have a uniform bearing, with respect to the structural positions on different thrust sheets • The rocks from PT, MBT and MFT thrust sheet record an overall flattening strain • High q’ in frontal MBT sheet indicates early Layer Parallel Shortening (LPS) while low q’ in PT sheet indicates a modified LPS • The strain magnitude is lower in the thrust sheets of Arunachal Himalaya as compared to the corresponding thrust sheets from Sikkim-Bhutan Himalaya References

• Acharyya, S.K., 1994. The Cenozoic foreland basin and tectonics of the eastern sub-Himalaya: problems and prospects. Himalayan Geology, 15, 3– 21. • Acharyya, S.K., 2007. Evolution of the Himalayan Paleogene foreland basin, influence of its litho-packet on the formation of thrust-related domes and windows in the Eastern Himalayas–A review. Journal of Asian Earth Sciences, 31(1), pp.1-17. • Ahmed, F. and Bhattacharyya, K., 2018, November. Structural geometry and kinematic evolution of far-eastern lesser Himalayan fold thrust belt: insights from Siang window, Arunachal Pradesh, India. In GSA Annual Meeting in Indianapolis, Indiana, USA-2018. GSA. • Basa, A., Ahmed, F., Bhattacharyya, K. and Roy, A., 2019. Evolution and characterization of fracture patterns: Insights from multi-scale analysis of the Buxa dolomite in the Siang Valley, Arunachal Lesser Himalayan fold-thrust belt. Journal of Structural Geology, 123, pp.54-66. • Das, J.P., Bhattacharyya, K., Mookerjee, M. and Ghosh, P. (2016). Kinematic analyses of orogen-parallel L-tectonites from Pelling-Munsiari thrust of Sikkim Himalayan fold thrust belt: Insights from multiple, incremental strain markers. Journal of Structural Geology, 90, pp.61-75. • Erslev, E.A. and Ge, H., 1990. Least-squares center-to-center and mean object ellipse fabric analysis. Journal of Structural Geology, 12(8), pp.1047- 1059.Haakon, F., 2010. Structural geology. Cambridge, UK. • Ghosh, P., Bhattacharyya, K., Parui, C., 2020. Tracking Progressive Deformation of an orogenic wedge through two successive internal thrusts: Insights from Structural, Kinematic Evolution and Deformation Profiles of the Main Central thrust (MCT) and the Pelling Munsiari thrust (PT), Sikkim Himalayan fold thrust belt. Journal of Structural Geology, 140, 104-120. • Long, S., McQuarrie, N., Tobgay, T. and Hawthorne, J., 2011. Quantifying internal strain and deformation temperature in the eastern Himalaya, Bhutan: Implications for the evolution of strain in thrust sheets. Journal of Structural Geology, 33(4), pp.579-608. • Parui, C. and Bhattacharyya, K., 2018. Duplex and along-strike structural variation: A case study from Sikkim Himalayan fold thrust belt. Journal of Structural Geology. • Parui, C. and Bhattacharyya, K., 2019, December. Penetrative strain variations across Sikkim Himalayan fold thrust belt (FTB) and its contribution to the FTB shortening budget. In AGU Fall Meeting 2019. AGU. of the Eastern Himalaya. Geol. Soc. Am. Bull. 122, 360–395. • Sanderson, D.J., 1982. Models of strain variation in nappes and thrust sheets: a review. Tectonophysics, 88(3-4), pp.201-233. • Shimamoto, T. and Ikeda, Y., 1976. A simple algebraic method for strain estimation from deformed ellipsoidal objects. 1. Basic theory. Tectonophysics, 36(4), pp.315-337. • Yin, A., 2006. Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation. Earth-Science Reviews, 76(1), pp.1-131. • Yonkee, A., 2005. Strain patterns within part of the Willard thrust sheet, Idaho–Utah–Wyoming thrust belt. Journal of Structural Geology, 27(7), pp.1315-1343. Acknowledgements

• IISER Kolkata Academic Research Fund (ARF) to Kathakali Bhattacharyya • CSIR-SRF to Jyoti Prasad Das • Farzan Ahmed, Swastik Suman Behera for field assistance and Chirantan Parui for discussions • Rupan Rakshit for preparing thin sections