Penetrative Strain and Partitioning of Convergence‐Related Shallow Crustal Shortening, Across Scales, in the Lesser‐ and Sub‐Himalayan Thrusts: Insights From the Eastern Himalaya, Sikkim

摘要

Abstract Shortening estimates from regional balanced cross‐sections remain lower than the predicted shortening from paleomagnetism and area‐balancing methods in the Himalaya. To reconcile this shortening deficit, we evaluate the shortening contribution from the often‐ignored penetrative strain of quartz grains and integrate these results with two published regional balanced cross‐sections from Sikkim Himalaya. Penetrative strain progressively decreases from the internal to the external thrusts with changing deformation conditions. Strain increases toward the basal thrust zone within each thrust indicating strain localization. Strain ellipsoids are folded along with the thrusts indicating the strain initiated before thrusting and folding. The layer‐normal shortening (LNS) strain dominates the internal thrusts. A combination of LNS and layer‐parallel shortening (LPS) strain exist in the low‐strained external thrust sheets. Approximately 78–91?km of shortening is recorded as penetrative strain in the Lesser‐ and Sub‐Himalayan thrusts. Lateral structural variations in the thrusts are reflected in the thrust‐sheet scale shortening and in the strain ellipsoid orientations, but not in the grain‐scale shortening. Shortening is partitioned differently across scales; the duplex records the highest thrust‐sheet scale shortening, while the roof thrust toward the hinterland records the highest grain‐scale shortening. Due to the dominance of LNS strain, removing strain from thrusts will collectively decrease ～54–59?km of base lengths, thereby reducing the total minimum shortening of the Sikkim Himalaya by ～13%. Therefore, the penetrative strain does not contribute to the shortening deficit in the Sikkim Himalaya.