ROLE OF INITIAL BASIN WIDTH IN PARTITIONING TOTAL SHORTENING IN THE LESSER HIMALAYAN FOLD-THRUST BELT: INSIGHTS FROM REGIONAL BALANCED CROSS SECTIONS

Previously published regional balanced cross sections constructed across various transects along the Himalayan fold-thrust belt (FTB) suggest that significant lateral variations exist in the total shortening, and in partitioning of the shortening among the various lithotectonic units in the FTB. Shortening is greatest in the western Nepal Himalaya and remains high in the east in the Darjeeling-Sikkim Himalaya; it progressively decreases toward the two syntaxes.

To evaluate the role of initial width of the Lesser Himalayan basin in controlling the lateral variation in total shortening in the mountain belt, we estimated the initial and final lengths of the Lesser Himalayan rocks from the published, admissible and viable, regional balanced cross sections. The width of the exposed Lesser Himalayan rocks increases eastward from the Kumaon-Garhwal Himalaya (~90km), becomes widest in the western Nepal Himalaya (~143km), and decreases towards the Sikkim-Bhutan Himalaya (~35-55km). Results from published balanced cross sections show that the shortening accommodated within the Lesser Himalayan sequence increases from the Kumaon Himalaya (~97-180km) eastward to western Nepal (~349-458 km) where the total shortening is also greatest. The Lesser Himalayan shortening decreases from western Nepal towards the east in Sikkim (~260km) – Bhutan (~178-266km); however, it remains higher in this region than in the Kumaon-Garhwal Himalaya in the west. In addition, the Lesser Himalayan shortening percentage remains approximately the same (~74-77%) in the middle of the Himalayan arc (from western Nepal to Sikkim). The restored cross sections suggest that the Lesser Himalayan basin was widest in western Nepal (~430-590 km), and narrower in Sikkim-Bhutan (~290-390 km). Therefore, at a first order, the greater width of the original basin in western Nepal possibly resulted in the largest amount of shortening being recorded in that region compared to other parts of the Himalaya, and thus the width of the basin possibly played an important role in partitioning the shortening in the Himalayan FTB.