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

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 D2 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 decollement plane at an approximate depth of 10 km. Palinspastic bedlength 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. (C) 2003 Elsevier Ltd. All rights reserved.