The tectonic transition between Laramide crustal shortening and mid-Tertiary extension occurred during Datil Group (upper Eocene-lower Oligocene) volcanism in the northern Mogollon-Datil field, New Mexico. Radiometrically constrained stratigraphic evidence for (1) displacement reversal across intrabasin fault zones and (2) initial foundering and burial of Laramide uplifts that would later form basins of the Rio Grande rift indicates that the tectonic transition occurred about 36 Ma. The change from Laramide shortening to mid-Tertiary extension coincides temporally and spatially with the end of early intermediate- composition volcanism (lower Datil Group) and the beginning of bimodal volcanism (upper Datil Group and younger rocks) in the northern Mogollon-Datil field.

Lithospheric stress regimes provided a common link between tectonism and volcanism during Datil time. Tectonic compression during Laramide crustal shortening prevented dike propagation on a regional scale. Diapirism was the only major mechanism available for ascent of early Datil magmas. Because diapirs derive buoyancy from density contrast over a short depth interval, basaltic melts must evolve to at least andesitic composition to penetrate low-density continental crust.

Tectonic tension began about 36 Ma in west-central New Mexico, allowing widespread dike propagation. Upper Datil basaltic andesite melts, which were more dense than parts of the continental crust, were able to ascend via dike propagation because dikes have sufficient vertical extent to derive buoyancy from "roots" in the dense upper mantle or lower crust. Crustal melting caused by widespread intrusion of mafic dikes may have produced the silicic mode of late Datil volcanism.

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