METAMORPHISM AND METASOMATISM OF GREATER AND LESSER HIMALAYAN ROCKS IN THE MODI KHOLA TRANSECT, CENTRAL NEPALESE HIMALAYA

Thermobarometric estimates for Greater and Lesser Himalayan rocks in the Modi Khola valley combined with detailed structural mapping show that the spatial pattern of exposed metamorphic rocks is controlled by large-displacement normal and thrust faults. Individual fault-bounded domains contain rocks that record the same peak metamorphic conditions. Structurally higher thrust sheets carry higher-grade rocks. This spatial pattern of exposed metamorphic rocks results from the geometry of thrust faults. A previously unrecognized normal fault entirely within Greater Himalayan rocks juxtaposes hanging wall rocks that equilibrated at ~9.5 kilobars and 690 °C with footwall rocks that equilibrated at ~14 kilobars and 720 °C. Large-displacement normal faults occur at the structural top, within, and just below the base of the Greater Himalayan series. Retrograde metasomatism is spatially associated with large normal faults, suggesting that these faults controlled retrograde fluid flow through Lesser and Greater Himalayan rocks. Only one episode of metamorphism to garnet or higher grades is recorded by the major mineral assemblages in these samples.