THE CONTRIBUTION OF THE 15-0 MA LESSER HIMALAYAN-SUBHIMALAYAN THRUST BELT TO THICKENING, TAPER BUILDING, AND HIGH ELEVATIONS: INSIGHTS FROM ALONG-STRIKE COMPARISON OF CROSS SECTION GEOMETRY

The deformation processes that have been the most influential for building the Himalayan mountain belt are actively debated. We argue that the importance of deformation accommodated in the Lesser Himalayan-Subhimalayan (LH-SH) portion of the Himalayan thrust belt over the past ~15 Myr has long been overlooked. To quantify its contribution to Himalayan thickening, we measured parameters including structural elevation, accreted area, shortening, and wedge taper from 22 published cross sections that span the length of the orogen. The mean structural elevation accomplished by the LH-SH thrust belt increases from 10-15 km in the eastern half of the orogen to 15-23 km in the western half. An antiformal culmination built by LH duplexing is observed along the entire orogen and increases in N-S width and structural height (up to 15-20 km) moving westward. Construction of this culmination was the primary mechanism for building and maintaining wedge taper. The westward increase in culmination size is accompanied by doubling and tripling of LH-SH shortening and accreted area, respectively; when combined with a high orogen-wide modern taper angle (11±2°), this indicates that duplexing facilitated the growth of an overall larger orogenic wedge moving westward. This could be the result of a greater original N-S width of LH basins moving westward, limited outward growth due to eastward-increasing precipitation, and/or along-strike variation in convergence partitioning. The mean thickening accomplished by the LH-SH thrust belt (10-13 km in the east and 15-20 km in the west) demonstrates that LH duplexing was a primary thickening mechanism, which outweighed the contribution of emplacement of the 6-12 km-thick Greater Himalayan package in the western half of the orogen. Following the propagation of deformation into LH rocks at ~15-13 Ma, the Himalayan wedge has been characterized by stacking of multiple thin, small-displacement thrust sheets, which was likely facilitated by migration of the deformation front into comparatively weak LH rocks that required duplexing to maintain the taper necessary to drive forward propagation. The net result was the construction of the high-taper orogenic wedge observed today. Therefore, LH-SH deformation has been the primary driver of thickening and the attainment of high elevations since ~15 Ma.