P-T-t-d history of the Greater Himalaya sequence in the Zanskar Shear zone, NW India

Abstract

Pressure-temperature-time-deformation (P-T-t-d) conditions describe the evolution of the Great Himalaya Sequence (GHS) deformed and exhumed along the Zanskar Shear Zone (ZSZ), the westernmost arm of the South Tibetan Detachment that spans the entire Himalayan range. Malung Tokpo samples are increasingly sheared nearing the ZSZ, developing an S-C foliation with shear banding. The dominant mineralogy is Qz + Kfs + PI + Bt + Grt + llm ± Sil(Fi) ± Ms ± Ky ± St. Perple_X-generated pseudosections yield a 4-stage clockwise P-T trajectory: (1) nearly isothermal burial; (2) progressive temperature increase to peak P-T conditions, (3) near isothermal decompression, and (4) final exhumation. U-Pb SHRIMP dating of metamorphic and igneous zircon demonstrates that peak regional metamorphism conditions were reach at c. 25- 20 Ma and that anatectic leucogranite crystallization continued to c. 23-18 Ma. Quartz CPOs suggest a progressive increase in deformation temperature with increased distance from the ZSZ, however absolute deformation temperatures are not designated due to the complex deformation history and degree of foliation development. The evolution of the GHS in Zanskar includes rapid burial (c. 33-27 Ma), closely followed by peak metamorphic conditions (c. 25-20 Ma), leading to a melt-weakened crust and relaxation-facilitated normal faulting through c. 21-18 Ma) and rapid exhumation. The P-T history of the GHS in Zanskar best matches channel flow model predictions but the modeled T-t histories for critical taper and channel flow require further thermochronological and local heat flow measurements for comparison to the results of this study.