The Structural Evolution of Shear Clay Smear in Normal Faults: Insights from Watersaturated Model Experiments

This study reports physical models of clay smear formation along faults. The main factors influencing clay smear are the competence and thickness of the clay layer， and fault throw. Although previous experimental studies acknowledge the impact of competence， to manipulate competence these studies altered the clay layer instead of the sand layer， leaving the influence of sand unknown. We investigated the competence of shear clay smear in normal faults through water-saturated sandbox analogue modeling experiments， aiming to identify factors influencing the structural evolution of clay smear in normal faults. We used Particle Image Velocimetry (PIV) to track deformation as it offers non-invasive， full-field measurements with high spatial resolution， enabling comprehensive analysis of deformation patterns. In experiments with more competent (strong) of sand layers， steeper branch faults developed as fault throw increased. The resulting clay smear is concentrated along the main fault plane， which was inclined at 60°. In experiments with less competent (weak) sand layers， two extensional overlapping fault segments interact and deform progressively， leading to the formation of continuous clay smear. Clay smear continuity increases as the competence of the sand layer decreases under otherwise identical conditions. Shear clay smear in normal faults is also influenced by the thickness of the clay layer and fault throw. During early stages of faulting， the clay layer undergoes gentle rotation without notable thinning， as the shear stress remains below the threshold required to initiate fault slip. However， during middle and late stages， continuous clay smear forms through shear localization of faults as distribution becomes more concentrated. In cases with constant clay layer competence and varying sand layer competence， increasing difference in competence between sand and clay leads to lower continuity of shear clay smear. Therefore， to accurately assess clay smear continuity in normal faults， it is essential to account for the competence contrast between sand and clay layers. Our results show that greater differences in competence reduce the likelihood of continuous smear formation， whereas weaker sand layers facilitate the development of broad， connected clay smear zones.