Ductile deformation and rheology of sub-continental mantle in a hot collisional orogeny: Example from the Bohemian Massif

Abstract

Fabric patterns of strongly serpentinized peridotite were determined using eigenvector analysis and eigenvalue classification of lattice preferred orientation of olivine and orthopyroxene. This approach has been applied to a rootless fold-shaped body of mylonitized spinel to garnet peridotite surrounded by fine-grained and partially retrogressed ky–kfs granulite. The EBSD data show either axial [010] or [100](0kl) pattern, both characteristic for ‘dry’ slip systems. The former pattern occurs predominantly along the inner margin and southern limb while the latter is mainly developed in the hinge of the fold shaped body. Foliations and lineations deciphered from the LPO data suggest that the [100](0kl) pattern reflects constrictional deformation (prolate strain ellipsoid) in the hinge of the peridotite fold while the axial [010] pattern reflects pure flattening (oblate strain ellipsoid) inherited from the period of emplacement of the peridotite sheet in the crust. Similarity in finite strain pattern of peridotite and surrounding granulites indicates their common thermal and mechanical evolution during folding. The petrology and structural data result in a model of burial of peridotite below thickened crustal root, its exhumation and folding. The burial stage is associated with prograde metamorphism resulting in a coarse-grained microstructure and development of spinel and garnet zones. The emplacement of peridotite into lower crustal granulites occurred along a shear zone associated with grain size reduction in both peridotite and granulite and rapid cooling of mylonitized peridotite to the ambient temperatures of lower crust. Further ascent to mid-crustal levels occurred within the vertical granulite channel. Final fold shape of the peridotite developed during subsequent indentation of the weak vertically anisotropic crust by the adjacent continental promontory. The degree of mechanical coupling between folded peridotite and granulite in mid-crustal levels is estimated using comparison of studied microstructures with experimental data.