Welds form due to tectonically induced thinning and/or dissolution of salt, with their composition and completeness thought to at least partly reflect their structural position within the salt-tectonic system. Despite their importance as seals or migration pathways for accumulations of hydrocarbons and CO₂, we have relatively few published examples of drilled subsurface welds; such examples would allow us to improve our understanding of the processes and products of welding and to test analytical models of the underlying mechanics. In this study, we integrate three-dimensional seismic reflection and borehole data from the Green Canyon area of the northern Gulf of Mexico, United States, to characterize the geophysical and geological expression of a tertiary weld, as well as its broader salt-tectonic context. These data show that although appearing complete in seismic reflection data, the weld contains 124 ft (38 m) of relatively pure halite. This thickness is consistent with the predictions of analytical models and with observations from other natural examples of subsurface welds. Our observations also support a model whereby compositional fractionation of salt occurs as the salt-tectonic system evolves; in this model, less mobile and/or denser units, if originally present, are typically stranded within the deeper, autochthonous level trapped in primary welds or near the basal root of diapirs, whereas less viscous and/or less dense units form the cores of these diapirs and potentially, genetically related allochthonous sheets and canopies. We also show that shearing of the weld during downslope translation of the overlying minibasin did not lead to complete welding.