Decoding Stress Partitioning and Kinematics in Strike-Slip Systems: Hierarchical Fault Architecture in the La Sarraz-Mormont Fault Zone

This study unravels the intricate relationship between fault geometry, stress fields, in the La Sarraz-Mormont Fault Zone - a key dextral strike-slip structure at the Jura Mountains–Molasse Basin transition (Switzerland). Through integrated kinematic analysis of 518 fault-striation pairs across 17 sites and paleostress modeling, we demonstrate that this fault zone operates as a multi-order Riedel shear system with remarkable self-similarity. Three hierarchical strands of imbricated faults generate complex, scale-invariant deformation patterns, rotating local stress orientations by >90° despite a consistent regional NW-SE compression. The fault hierarchy dictates stress rotation, not polyphase tectonics. Field evidence from exceptional exposures in Eclépens Quarry reveals how fluid overpressure critically enables fault slip: calcite-filled veins and breccias permeate damage zones, reducing friction and facilitating lateral displacements of 80–500 m. Paradoxically, the main La Sarraz – Mormont fault zone aligns parallel to the modern maximum horizontal stress (SH ≈ N116°), yet accommodates significant shear through probably fluid-assisted weakening—resolving the apparent mechanical enigma.