A balanced cross section across the Himalayan foreland belt, the Punjab and Himachal foothills: A reinterpretation of structural styles and evolution

The Siwaliks in the foothills of the Himalayas, containing molasse sediment s derived from the rising mountain front, represent a foreland fold-thrust belt which was deformed during the continued northward convergence of the I ndian plate following the continent-continent collision. In this contributi on we present balanced and restored cross sections along a line from Adampu r through Jawalamukhi to Palampur in the foothills of the Punjab and Himach al Himalayas using published surface/subsurface data. The cross section inc orporates all the rock units of the Sub-Himalaya Zone as well as that of th e northern Lesser Himalaya Zone. The structural geometry of the fold-thrust belt in this section is largely controlled by three buried thrusts within the Sundernagar Formation of the Lesser Himalaya Zone. Two of these buried thrusts splay from the basal detachment and delineate a buried horse. Three thrusts towards foreland, including the Main Frontal Thrust (inferred to b e a blind thrust in this sector), splay from these buried thrusts. In the h interland, an anticlinal fault-bend fold was breached by a sequence of brea k-back thrusts, one of which is the Main Boundary Thrust. A foreland propag ating thrust system is inadequate to explain the evolution of the fold-thru st-belt in this section. We show that a "synchronous thrusting" model in wh ich in-sequence initiation of thrusts at depth combined with continued moti on on all the thrusts leading to out-of-sequence imbrication at the upper s tructural levels better explains the evolution of the fold-thrust belt in t he Jawalamukhi section. The estimated shortening between the two chosen pin lines is about 36% (about 72 km).