Deformation styles in fold-and-thrust belts: detachment-driven thin-skinned tectonics and passive-roof duplex structures in the Tunisian imbricated zone

Abstract

This study refines the understanding of deformation styles within the Tunisian imbricated zone through an integrated mapping approach combining detailed field observations with high-resolution Google Earth satellite imagery. The widespread Ypresian carbonate outcrops are interpreted as emergent components of a thin tectonic stack, structurally confined by Cretaceous ridges to the northwest and southeast. Detachment faulting, thin-skinned tectonics and passive-roof duplex structures represent fundamental mechanisms for accommodating shortening processes in fold-and-thrust belts. During the Late Eocene Pyrenean shortening, a thin-skinned tectonic regime developed, characterized by thin skinned folds rooted in a basal detachment within the ductile El Haria Formation. Since the Tortonian Atlassic orogeny, progressive collisional stresses have promoted the evolution of southeast-verging imbricate fan systems, interlayer shear zones, tear faults, and northwest-verging passive-roof structures. Regionally, the study area lies within a NW-trending triangle zone: its northwestern part defined by low-angle, foreland-verging thrust sheets, whereas its southeastern sector is dominated by duplexes and hinterland-verging passive-roof structures, here documented for the first time. The hinterland-verging back-thrusts likely reflect deep-seated tectonic processes associated with basal detachment, which played a key role in the emplacement of the upper passive roof. Mechanical contrast between lithological units controls the formation of internal interlayer shear zones and deep shear structures when weaker rocks come into contact. Simultaneously, anticline hinges within hanging-wall blocks localized extensional deformation, triggering raft tectonics and gravitational collapse.