Reappraisal of emplacement models for Himalayan external crystalline nappes; the Jajarkot Klippe, western Nepal

Recent mapping in the Jajarkot klippe highlights the presence of the South Tibetan detachment in an external crystalline nappe south of the main Himalayan range and provides structural constraints on the emplacement mechanism of such nappes. The top-to-the-northeast South Tibetan detachment in the Jajarkot klippe separates lower-greenschist- to lower-amphibolite-facies metasedimentary rocks of the Greater Himalayan sequence in the footwall from weakly to nonmetamorphosed siliciclastic and carbonate rocks of the Tethyan sedimentary sequence in the hanging wall. The maximum depositional age of an arenite unit at the base of the Tethyan sedimentary sequence is constrained to the late Cambrian. The temperature of deformation increases structurally upward from approximately 450 degrees C at the base of the klippe to approximately 600 degrees C in the South Tibetan detachment and decreases sharply to 250-300 degrees C in its hanging wall. The existence of the South Tibetan detachment between the Greater Himalayan sequence and the Tethyan sedimentary sequence is only compatible with the one-thrust tectonic model for the emplacement of Himalayan external crystalline nappes. The minimum dip-slip displacement along the South Tibetan detachment is constrained to 160-185 km in western Nepal based on the restoration of this folded structure. The lack of evidence for partial melting in the footwall of the South Tibetan detachment in the Jajarkot klippe implies that the melt-weakened middle crust did not flow as far south as the klippe or that it flowed obliquely or parallel to the orogen on both sides of the klippe.