Contractional, monoclinal kink bands that dip to the south and southwest deform northdipping slaty cleavage in Permian rocks of the Somport area (central Pyrenees). These bands are strongly curved such that the poles (normals) to individual kink bands define E-W-striking great-circle girdles in stereographic nets. The kink bands form an anastomosing network that isolates lozenge-shaped, undeformed domains. Cross-cutting relations between intersecting kink bands indicate that all the kink bands in the Somport slate formed penecontemporaneously during a single deformation event. Total strain accommodated by the kink bands at Somport is small. Bulk shortening has been calculated assuming that slip during kinking was perpendicular to local kink fold axes. Bulk shortening of 3% and less than 1% have been calculated in the subvertical and E-W-striking subhorizontal directions respectively, indicative of constrictonal strain. We propose the unusual geometry of the strongly curved kink bands at Somport is partly the product of non-plane-strain deformation. A model is developed that predicts the formation of up to four sets of kink bands during coaxial, non-plane-strain deformation, while two non-conjugate sets form during non-coaxial, non-plane-strain deformation such as occurred at Somport. This model is a natural extension of models proposed for the geometry and kinematics of brittle faults formed during non-plane-strain deformation. The kink bands at Somport probably formed before the cessation of Pyrenean convergence and were related to a modest gravity-induced spreading in the hinterland of the orogenic belt.
|Publication status||Published - 30 Sep 1996|