Ductile Shear as a Cause of Inverted Metamorphism: Example from the Nepal Himalaya

In many mountain belts metamorphic sequences occur in an inverted arrangement such that rocks of higher metamorphic grade overlie rocks of lower metamorphic grade. In some locations such as the Main Central Thrust (MCT) zone of the Himalaya, the metamorphic inversion is coincident with the zone of ductile thrust deformation. Although many explanations have been suggested for the metamorphic inversion of the Himalaya, none are consistent with all of the observed features. A new model based on observations from the eastern Nepal Himalaya consists of inversion of the metamorphic sequence due to ductile shearing of a pre-existing zone of right-way-up metamorphism. This simple structural explanation creates an inverted metamorphic sequence consistent with observations of: (1) a zone of distributed deformation; (2) metamorphic textures overprinted by deformation; and (3) metamorphic isograds sub-parallel to shear fabric. These observations are not all well accounted for by previously proposed models. In this structural model crystal-plastic processes are responsible for translating material in a thrust sense, thus re-ordering the metamorphic sequence but without necessarily ever physically inverting any package of rock. Such a structural process is responsible for the overall metamorphic inversion but could also be accompanied by other processes such as advection of heat during deformation or fluid migration, or creation of heat by friction. These other processes could lead to localized post-deformational metamorphism or re-equilibration of existing assemblages.