Low-angle normal faults in the compressional Himalayan orogen; Evidence from the Annapurna–Dhaulagiri Himalaya, Nepal

Abstract

The Annapurna Detachment (AD) is a low-angle (∼20°–30° dip), north-dipping normal fault and ductile high-strain shear zone in calc-mylonites, and forms part of the South Tibetan Detachment (STD) that runs along 1800 km length of the Himalaya. The AD separates kyanite and sillimanite grade gneisses and tremolite + clinopyroxene ± hornblende-bearing marble–calc-silicates of the Greater Himalayan Sequence (GHS) below from unmetamorphosed Palaeozoic–Mesozoic sedimentary rocks of the Tethyan sedimentary zone above. It was active at ca. 22–18 Ma during south-vergent ductile extrusion (channel flow) and exhumation of the Himalayan mid-crust footwall. Restoration of the STD system suggests around 80–100 km of southward extrusion of the footwall gneisses relative to the Tethyan hanging-wall rocks. Folds in the hanging wall of the AD were formed prior to normal faulting, but axial planes are curved into alignment with the shear zone suggesting extrusion of the metamorphic footwall rocks beneath a passive roof fault. North-vergent recumbent backfolds in the Nilgiri–Tukuche peaks were enhanced by backsliding during footwall extrusion, although this does not indicate “orogenic collapse,” lowering of surface elevation, or decreasing crustal thickness because new material was continually being underthrust from the south. Axial planes of backfolds are curved and progressively rotate from subvertical in the north to subhorizontal immediately above the AD. Low-angle normal faults in the Himalaya were active during the Early Miocene, concomitantly with thrusting at deeper structural levels along the Main Central Thrust (MCT) zone. The passive normal faults and ductile shear zone were initiated at low angles aided by partial melting and ductile flow within the GHS. They do not indicate alternating periods of extension and compression but were active in a wholly compressional environment. The Channel Flow model for the Greater Himalaya and the passive roof fault model for the low-angle normal faults adequately explain all geological field structural and metamorphic criteria.