New constraints on the timing of partial melting and deformation along the Nyalam section (central Himalaya): implications for extrusion models
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New structural, U–Th/Pb and Ar/Ar data along the Nyalam section constrain the timing of partial melting, crystallization and deformation in the Greater Himalayan Sequence. Prograde metamorphism was followed by the onset of partial melting at c. 30 Ma. In the central Greater Himalayan Sequence, in situ melts crystallized between 24 and 18 Ma. Subsequent cooling was very fast (c. 200 °C Ma−1) and coeval with the emplacement of undeformed dykes that lasted until c. 15 Ma. In the upper Greater Himalayan Sequence, fast cooling continued until c. 13 Ma. Combined with published P–T and thermochronological data from the Langtang and Dudh Kosi valleys, these data imply that: (a) the partial melt zone thinned over time; (b) the end of melting preceded the end of motion on the Main Central Thrust and the South Tibetan Detachment by 6 and 2 Ma, respectively; (c) the South Tibetan Detachment possibly initiated at c. 25 Ma, probably reactivating a pre-existing thrust; and (d) the present-day topography has been established for <6 Ma and focused erosion on the present-day southern slopes of the Himalaya was not active at the time of the exhumation of the Greater Himalayan Sequence. These observations suggest that the Main Central Thrust/South Tibetan Detachment systems are not passive structures induced by focused erosion, as has been suggested previously by some lower crustal channel flow models.