Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India-Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U-Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c. 800-1000 Ma, 1500-1700 Ma and 2300-2500 Ma and an epsilon(Nd(0)) signature of -18.3 to -12.1, and is intruded by c. 800 Ma and c. 500-600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c. 1800-1900 Ma, with older populations spanning 1900-3600 Ma, and an epsilon(Nd(0)) signature of -27.7 to -23.4, intruded by c. 1830 Ma granites. The data reveal a c. 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones.