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Description: We examine 3-D seismic reflection data from the Santos Basin, offshore Brazil to determine how, where and when do radial faults grow near a sub-circular salt diapir (stock). We show roof stretching alone cannot account for the large heights and lengths of the kilometre-scale radial faults, suggesting stock widening (‘stem push’), a mechanism implied in numerical models but not yet documented in natural examples, played a pivotal role in radial fault formation. We suggest that, when a diapir is covered by a roof, radial faults form in its overburden due to roof stretching, extending no further than the limit of the drape folding. The roof may then be shouldered aside and the faults buried along the stock flanks, exposing these strata to stem push-related stresses that may then reactivate pre-existing or form new radial faults. Radial faults, irrespective of how they formed, may dip-link with or offset one-another as salt continues to rise. We suggest the causal mechanism for radial fault formation will likely change as roof thickness varies during diapirism, with this reflecting the ratio between sedimentation rate and salt volumetric flux. Our findings are likely applicable to other diapirs, helping us not only to interpret the paleo-stress state of salt-bearing sedimentary basins, but also advancing our understanding of fracture distributions, potential fluid flow pathways, and reservoir compartmentalization around salt diapirs in basins where seismic reflection imaging is poor.