New constraints on the timing of partial melting and deformation along the Nyalam section (central Himalaya): implications for extrusion models

<p>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 <em>c.</em> 30 Ma. In the central Greater Himalayan Sequence, <em>in situ</em> melts crystallized between 24 and 18 Ma. Subsequent cooling was very fast (<em>c.</em> 200 °C Ma⁻¹) and coeval with the emplacement of undeformed dykes that lasted until <em>c.</em> 15 Ma. In the upper Greater Himalayan Sequence, fast cooling continued until <em>c.</em> 13 Ma. Combined with published <em>P</em>–<em>T</em> 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 <em>c.</em> 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. </p>