We investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the Oman Ophiolite. Our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase-rich lithology typical of the lower crust and an olivine-dominated harzburgite. We constrain deformation conditions using field relationships, geothermometry, grain size piezometry, and electron backscatter diffraction. The viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite, consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws. These flow laws predict that a significant viscosity contrast exists at the crust-mantle boundary where the crustal lithology is dominated by plagioclase, or where deformation localizes within plagioclase-rich layers.