Microstructures of bedding-parallel faults under multistage deformation: Examples from the Southeast Basin of France

We conducted a microstructural analysis of bedding-parallel faults (BPFs) in Mesozoic clay-rich layers of the Southeast Basin of France. Various microstructures are recognized in thin sections under a petrographic microscope and by cathodoluminescence. The microscale observations are combined with outcrop observations from previous studies to provide insight into the origin of the BPFs and their evolution during successive phases of deformation in a basin that had a polyphase tectonic history. The BPFs slipped while normal faults were formed during the Oligocene extension. Then, another phase of slip occurred later during the basin inversion. These two phases of deformation are expressed by recurrent crack-seal veins, pull-apart veins and stylolites. In addition, calcite veins with an elongate blocky morphology suggest an opening normal to bedding before the reactivation. The BPFs initiated in clay layers that were shallow dipping. Such conditions may appear mechanically unfavourable for an opening normal to bedding or a shearing parallel to bedding. We suggest that the role of rock anisotropy is critical. This study furthermore demonstrates that BPFs can be long-lived brittle structures that may record successive tectonic events.