Paleostress determinations from radiometrically dated dikes indicate ENE-WSW regional extension began by about 32 Ma in New Mexico, an perhaps earlier in the southern part of the state. The extension direction maintained this orientation until sometime after 23 Ma when it rotated clockwise to an E-W or WNW-ESE direction, which is the present orientation. The spreading direction in the Rio Grande rift, however, has been E-W since the beginning of rifting about 31 Ma and reflects control of spreading by preexisting structural grain. NE-SW extension occurred in the Grants-Pie Town area of the Colorado Plateau and in the roofs of batholiths in the Socorro and Questa areas concurrently with E-W spreading in the Rio Grande rift. Nearly orthogonal orientations of late Eocene-Oligocene (38-32 Ma) paleostress indicators in southern New Mexico indicate a nearly isotropic middle Tertiary stress field.

Both the western and eastern boundaries of the zone of E-W spreading along the Rio Grande rift in northern New Mexico shifted 60-200 km eastward sometime between 23 and 16 Ma, perhaps coeval with rotation of the direction of regional extension. This eastward shift also coincided approximately with the renewal of volcanism following an early Miocene (22-17 Ma) lull. During the late Miocene (9-5 Ma), the eastern boundary of E-W spreading shifted back to the present eastern margin of the Rio Grande rift. The N-S to NNE-SSW orientation of the least principal horizontal stress in the southern Great Plains province has persisted since at least 28 Ma. Stress province boundaries can be remarkably sharp with stress orientations changing by 90¿ within 30 km or less. The NE trending Jemez lineament transects the Colorado Plateau, Rio Grande rift (Basin and Range), and southern Great Plains stress providences, but except for a transition zone along the present southeastern boundary of the Colorado Plateau, both paleostress and contemporary stress reflect the respective stress provinces, not the lineament. Chemical analyses of dated dikes reveal highly variable compositions but indicate that true basalts are more common among early rift (>20Ma) volcanic rocks than is generally recognized. A histogram constructed from nearly 200 radiometric ages of igneous rocks in New Mexico between 104¿W and 108¿W longitude indicates that volcanism along the Rio Grande rift has been episodic with peaks at37-34, 31-27, 26-22, 17-13, 12-6, and 5-0 Ma.