Multifold seismic reflection data provide the basis for recognition of an offshore Late Triassic-Early Jurassic half graben complex beneath the DeSoto Canyon salt basin (DSCSB) along the northeastern Gulf of Mexico margin. The base of salt or equivalent (BSE) surface is a prominent unconformity recognized throughout the DSCSB that is mostly overlain and onlapped by extensive Middle Jurassic (Callovian age?) premarine evaporites (Louann salt and equivalents) and younger sedimentary rocks. Widespread dipping subsalt reflectors, truncated by the BSE, are the seismic expression of an interpreted thick section of synrift strata deposited within a half graben. The half graben probably overlies older prerift Paleozoic (and Precambrian?) sedimentary, igneous, and metamorphic rocks and widens basinward about a NE-SW axis. Stratigraphic onlap relationships of subsalt reflectors close to the BSE surface suggest a lacustrine sequence may be present in the uppermost section of the rift fill; similar deposits occur within synrift strata below Aptian salt along the rifted West African margin. A NE-ENE striking normal slip boundary fault beneath the Mississippi-Alabama-Florida (MAFLA) shelf area and the inferred NW-SE oriented Florida-Bahamas transfer fault along the eastern margin of the DSCSB approximate the updip limit of rift fill. Trends of these structural lineaments and overall half graben morphology are similar to those of the South Georgia basin, a buried onshore early Mesozoic graben complex in northern Florida and southern Georgia, and the trend of exposed Triassic-Jurassic continental rift basins along eastern North America. Structural architecture of the DSCSB half graben is consistent with NW-SE rift phase extension during the early Mesozoic opening of the Gulf of Mexico. ¿ American Geophysical Union 1995