COOLING AGE RECORD OF DOMAL UPLIFT IN THE CORE OF THE HIGHER HIMALAYAN CRYSTALLINES (HHC), SOUTHWEST ZANSKAR, INDIA

The cooling and tectonic history of the Higher Himalayan Crystallines (HHC) in southwest Zanskar (along the Kishtwar-Padam traverse) is cons trained by K-Ar biotite and fission-track (FT) apatite and zircon ages . A total of nine biotite samples yields ages in the range of 14-24 Ma , indicating the post-metamorphic cooling of these rocks through simil ar to 300 degrees C in the Miocene. Overall, the ages become younger a way from the Zanskar Shear Zone (ZSZ), which marks the basement-cover detachment fault between the HHC and the Tethyan sedimentary zone, tow ards the core of the HHC. The same pattern is also observed for the FT apatite ages, which record the cooling of the rocks through similar t o 120 degrees C. The apatite ages ranges from 11 Ma in the vicinity of the ZSZ to 4 Ma at the granitic core of the HHC. This pattern of disc ordant cooling ages across the HHC in southwest Zanskar reveals an inv ersion of isotherms due to fast uplift-denudation (hence cooling) of t he HHC core, which is, in turn, related to domal uplift within the HHC . The Chisoti granite gneiss is the exposed domal structure along the studied traverse. Cooling history of two granite gneisses at the core of the HHC is also quantified with the help of the biotite, zircon and apatite ages; the time-temperatures thus obtained indicate a rapid pu lse of cooling at similar to 6 Ma, related to accelerated uplift-denud ation of the HHC core at this time. Long-term denudation rates of 0.5- 0.7 mm/yr are estimated for the high-grade rocks of the Higher Himalay a in southwest Zanskar over the past 4.0-5.5 m.yr.