Orientations of dikes, veins, faults, and slickenlines reveal the evolution of stress during Eocene to Miocene magmatism in the southern Cordillera. Where most thoroughly studied by us in Trans-Pecos Texas, magmatism began at about 48 Ma shortly after the cessation of Laramide folding. Dikes and veins that formed from then until about 32 Ma strike dominantly east-northeast. This indicates that the least principal stress (&sgr;3) was north-northwest; additional data suggest that the maximum principal stress (&sgr;1) was east-northeast. The stress field changed to &sgr;1 vertical and &sgr;3 east-northeast (i.e., east-northeast extension) at least by 28 Ma and probably by 31 Ma. Dikes and veins that formed between 31 and 17 Ma, when all magmatism ceased in Texas, strike north-northwest. This change marks the beginning of regional, Basin and Range extension; however, major normal faulting, exclusively of high-angle type, did not begin until about 24 Ma. A similar stress change, marked by a similar change in dike and vein orientations, occurred throughout the southwestern United States and northern Mexico. The time of change is not well constrained in Texas, but available information allows it to have occurred at the same time throughout the southern Cordillera.

We suggest the earlier stress field is related to east-northeast convergence between the Farallon and North American plates. The change in stress is approximately coincident with collision of the East Pacific Rise and paleotrench. Extension may be related to the change from a convergent to a transform margin along the western edge of North America. The changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in Texas. Contemporaneity of the changes in stress and magmatism indicates that they are related. Combined with regional age patterns, paleostress and geochemical data indicate that pre-31 Ma magmatism in the southern Cordillera occurred in a subduction-related, continental volcanic arc. Subsequent magmatism occurred in an environment of intraplate extension of the Basin and Range province. ¿1991 American Geophysical Union