Lateral variations along the Main Central Thrust in the central Nepal Himalayas: The evidence from chemical maps of garnet

Chemical maps of garnet are the key to understand the tectonometamorphic evolution of the metamorphic rocks. They provide useful information on element distribution, diffusion, and mechanical truncation of crystal. In the Himalayas, studies on ultra-high pressure rocks denudated along the Main Central Thrust (MCT) revealed the great displacement along this intracontinental shear zone. Pêcher (1989) suggested the difference in thickness of hangingwall as a result of large erosion along the thrust, and Macfarlane (1993) discussed the various thermal structures within the hangingwall. However, lateral variations in footwall have been poorly investigated although inverted thermal gradient has been often discussed. In order to clarify the variation of tectonometamophic events along the MCT, garnet crystals within pelitic rocks were investigated in detail. In the central Nepal Himalayas, Annapurna area and Kathmandu nappe are suitable to study these variations. Estimated metamorphic temperatures just above the MCT are different in two areas: 600-700 °C in Annapurna area (Kaneko et al. 1995) and 550-600 °C in Kathmandu nappe (Johnson et al. 2001). These variations are interpreted as different positions within hangingwall; Kathmandu nappe corresponds to frontal zone whereas Annapurna area to root zone.