Eocene simple shear and plio‐quaternary pure‐shear folding in the Central‐Eastern Algerian Atlas

Abstract

Tertiary folds of the central and eastern part of the Algerian Atlas were studied in order to assess their kinematics and regimes of deformation. Folds developed following two main phases of deformation during the Eocene and the Pliocene. The NE-SW trending Eocene folds show a clockwise rotation of fold axis with depth, noncylindrical geometry and a right-stepping en echelon configuration. Early secondary structures inside each fold are also rotated clockwise with respect to the younger folds. These data are in agreement with progressive 'simple shear' deformation. Pliocene folds strike E-W and display a cylindrical geometry. Associated Plio-Quaternary brittle structures show no rotational path. These observations are compatible with 'pure shear' style of deformation. These results, in combination with focal mechanism solutions, have important implications for the understanding of the kinematic evolution of the western segment of the African-European plate boundary. For Algeria they imply that transcurrent motion during the Paleogene was followed by coaxial shortening with a NNW direction during Neogene-Quaternary times. Comparison with published data for Morocco and Tunisia suggests that this transcurrent regime developed from west to east continuing from the Middle Jurassic through to the Miocene. Terra Nova, 2,653-660

Active inversion tectonics, simple shear folding and back-thrusting at Rioni Basin, Georgia

“…The paleostress instead are much more complex showing different orientations with time. We suggest that these different orientations do not correspond to changes in the principal stress direction with time, but they are the effect of rotation of the folds (or at least of some portions) under a simple shear regime, similarly to what has been found for example during the development of en-échelon folds in the Algerian Atlas (Ferrari et al, 1990).…”