Finite strain analysis of a natural ductile shear zone in limestones: insights into 3-D coaxial vs. non-coaxial deformation partitioning

In order to obtain new insights into the process of strain localization, a ca. 50 m thick shear zone has been investigated. The shear zone, located in the footwall to a major thrust, involves limestones and pelites. Calc-mylonites are characterized by a stretching lineation orthogonal to the shear direction and a foliation at an angle of 45-10 degrees to the thrust fault. Strain analysis and mean foliation analysis have been carried Out on 42 samples. Strain gradients and ellipsoid shape across the shear zone indicate, respectively, an increase of strain intensity approaching the thrust surface and overall oblate strain. Apparent flattening increases significantly for intensely deformed rocks. Theoretical models are proposed to explain the recorded finite strain configuration with a main deformation characterized by simultaneous simple shear and localized pure shear involving lengthening normal to the shear direction. A model of asymmetric lateral extrusion is adopted, as is consistent with measured horizontal strain gradients. This model also takes into account regional geological constraints such as the occurrence of accommodation space along the present-day Tyrrhenian extensional margin to the SW, and of a massive body of platform carbonates-forming a rigid buttress-to the NE.