Middle Eocene-Oligocene anatexis and exhumation of the Greater Himalayan Sequence in central Nepal

We present structural analysis and zircon and monazite geochronology of the Greater Himalayan Sequence (GHS) in the Kali Gandaki valley, central Nepal. Although the foliation is moderately NE-dipping, the upper GHS shows a top-to-WNW shear sense, whereas the lower GHS developed top-to-SW ductile shear zones. Quartz fabrics from mylonitic gneisses indicate deformation temperature from >650 degrees C to similar to 450 degrees C. Monazite from the GHS recorded anatexis at "near peak" conditions from similar to 43 to 28 Ma and exhumation near the sillimanite-kyanite transition at 25-21 Ma. Ages of zircon rims constrain the onset of exhumation. From the upper GHS to the Main Central Thrust zone, pervasive deformation and anticlockwise rotation of the lineation from the orogen-parallel to orogen-perpendicular direction suggest exhumation of the GHS by general shear from similar to 31 to 21 Ma. A two-stage exhumation model is proposed to explain the diffuse strain distribution and anatexis during early crustal thickening.