P-T-t-d history of the Greater Himalayan Sequence in the Zanskar Shear Zone, NW India

Emma N. Beck, Mary L. Leech , Matthew Coble, Laura E. Webb, Martin J. Grove

We present a synthesis of pressure-temperature-time-deformation (P-T-t-d) conditions that track the evolution of the Greater Himalaya Sequence (GHS) exhumed along the Zanskar Shear Zone (ZSZ), the western arm of the South Tibetan Detachment and that spans the entirety of the Himalayan range. The dominant mineralogy is Qz + Kfs + Pl + Bt + Grt + Ilm + Sil(Fi) ± Ms ± Ky ± St. P-T paths constructed from pseudosection models follow a 4-stage clockwise trajectory: (1) nearly isothermal burial; (2) temperature increase to peak P-T conditions, (3) decompression, and (4) exhumation. Each portion is constrained by mineralogical/microstructural equilibrium growth associations, garnet geochemistry, and Fe-Mg exchange thermometry. U-Pb SHRIMP dating of metamorphic and igneous zircon constrains the timing of ongoing regional metamorphism to ~22-20 Ma and anatexis to ~25-20 Ma. White mica 40Ar/39Ar cooling ages indicate the ZSZ was active from 28 to 18 Ma between Padum and Rangdum, while doming and granite crystallization continued until 17 to 15 Ma in the Suru dome area. Samples from Malung Tokpo become progressively sheared closer to the ZSZ and develop an S-C foliation with shear banding. Quartz CPOs suggest deformation on increasingly “high temperature” slip systems with increased distance from the ZSZ in agreement with the increasing metamorphic grade and recrystallization at lower strain rates at lower structural levels. Absolute deformation temperatures are not well constrained due to the variable deformation history and degree of foliation development. The evolution of the GHS in Zanskar includes burial to peak metamorphic conditions (c. 33-27 Ma), closely followed by leucogranite generation (c. 25-20 Ma), leading to a melt-weakened crust and relaxation-facilitated normal faulting and rapid exhumation (c. 24-19 Ma). This history is best described as a combination of the critical-taper wedge and the channel flow model.