Structural and metamorphic features of the Main Central Thrust Zone and its contiguous domains in the eastern Nepalese Himalaya

Pietro Mosca, Chiara Groppo, and Franco Rolfo

In the Kanchenjunga area (far eastern Nepal), as well as in the whole Himalayan chain, the juxtaposition of the high-grade mid-crustal Higher Himalayan Crystallines (HHC) onto the low grade Lesser Himalayan Sequence (LHS) is structurally marked by the Main Central Thrust Zone (MCTZ). On the base of original field mapping and meso- and micro-structural data, the MCTZ has been here identified as a crustal-scale ductile to ductile-brittle shear zone roughly centred on the Inverted Metamorphic Sequence (IMS), in which different rocks are more pervasively deformed and sheared with respect to adjacent rocks. The boundaries of the MCTZ are not represented by single thrusts. The lower boundary is marked by phyllonites and mylonitic schists located at the uppermost portions of the LHS at the contact with strongly mylonitic augen gneisses of the IMS. The upper boundary of the MCTZ is roughly located at the base of Grt-Kfs-Ky-Sil anatectic gneisses in the lower portion of the HHC, being itself characterized by pervasive ductile shearing.

Structural field data combined with petrologic results clearly indicate that the MCTZ is internally imbricated, resulting in the juxtaposition of rock packages characterized by different P-T evolutions and T/depth gradients, separated by “metamorphic discontinuities” which do not always correspond to evident structural breaks. This is the case of the metamorphic discontinuity identified at the upper structural levels of the IMS and juxtaposing the upper IMS rocks, locally anatectic, characterized by higher T/depth gradients on the lower IMS rocks, characterized by lower T/depth gradients. A similar metamorphic discontinuity was previously reported westward (Milke Danda transect) thus suggesting that it could be of regional importance in the tectonometmaoprhic architecture of eastern Nepal Himalaya.