Strain gradients in zones of ductile thrusting: Insights from the External Hellenides

New finite strain data from a thrust-sense shear zone, which extends for 600 km along-strike in the high-pressure belt of the External Hellenides (Greece), were used to describe the variation of ductile strain with structural distance (D) from the basal thrust. Sampling was carried out along four traverses across the central and southeastern parts of the zone. The strain ratio in XZ sections (RXZ) of 120 samples was obtained using the theta-curve, the mean object ellipse and the mean radial length methods. All methods give very consistent results. Based on these results, four strain profiles were constructed depicting a non-linear increase of RXZ values with proximity to the basal thrust. These profiles in combination with published strain data from the northwestern parts of the zone enabled us to obtain an empirical logarithmic function describing the relationship between RXZ and D. This function includes only one varying parameter, which represents the strain gradient in linear RXZ − lnD space. The value of strain gradient is directly correlated with the strain level in the thrust zone and provides information for strain localization at its base. The proposed function may also be used for quantifying the strain profiles of ductile to semi-ductile thrust zones in other orogenic belts such as the Alps, the Appalachians and Himalaya.