Structural evolution of the Tethyan sedimentary sequence in the Annapurna area, central Nepal Himalaya

Abstract

The Tethyan sedimentary sequence of central Nepal has been affected by three phases of folding (D 1, D 2, and D 4) with contrasting vergence, interspersed by two major extensional events (D 3 and D 5). The north-verging second phase folds (F 2) are the dominant deformation features preserved in the studied area. These D 2 structures indicate a vertical structural zonation that is interpreted to mark a preserved D 2 strain gradient increasing with depth, to a presumed north-verging décollement plane at an approximate depth of 10 km. Palinspastic bed-length restoration of the sequence implies a minimum 43% shortening, and a minimum 150% vertical thickening due to D 2 deformation. The D 4 event overprints the D 3 Annapurna detachment, and manifests itself as post-peak metamorphic south-verging kink folds and spaced crenulation cleavage. In the structurally lowest level observed, D 4 is associated with thrust-sense meter-scale fault zones. The youngest extensional event (D 5) marks the initiation and eventual southward propagation of the Thakkhola graben. A newly named northeast-dipping normal fault, the Lupra fault, is described. In the early stages of extension, the Lupra fault may have acted as a southern termination of the graben system, dropping down Cretaceous rocks and high-level structural features in its hanging wall. It is suggested that, as the extension progressed southward, the Lupra fault was itself down-faulted in the graben, and the extent of the Thakkhola graben reached the present position of the Annapurna detachment.