In the southern Rio Grande rift, two extensional regimes of different origin (but transitional with each other through the Miocene) can be interpreted from structures and rocks formed within the past 28 to 29 m.y. The earlier regime, which began about 28 to 29 m.y. B.P., is characterized by emplacement of “basaltic andesite” flows with relatively high strontium isotope ratios; formation of broad, relatively deep, northwest-trending basins; and incipient uplift of some of the region's fault-block mountains. This regime appears to have developed in a back-arc setting, perhaps behind a rapidly steepening slab and a westward-sweeping arc system.
The younger episode seemingly represents a renewal or acceleration of block faulting and volcanism during the latest Miocene and Pliocene, 9 to 3 m.y. B.P., after a long transitional period during the early and mid-Miocene when volcanism was absent and tectonism was less vigorous. The latest Miocene-Pliocene episode produced the modern northerly-trending rift basins and uplifts, regional uplift of the rift 1 to 2 km above sea level, and renewal of volcanism, this time dominated by relatively primitive alkali-olivine basalt. New basalt dates reveal that in the southern rift, modern ranges and basins were almost fully developed and that near-modern drainage ways were established across uplifts into bolsons by about 5.0 m.y. B.P. An ancestral Rio Grande had extended itself southward into the southern rift by 3 to 4 m.y. B.P., and the river entrenched itself into its modern valley between 0.7 and 0.5 m.y. B.P.
Horst-graben development of the southern Basin and Range province, as well as associated basaltic volcanism, swept progressively eastward from southeastern California in the past 20 m.y., culminating in formation of the Rio Grande rift and other fault-block terrane in west Texas, New Mexico, and northern Chihuahua in the latest Miocene and Pliocene. Late Quaternary Basin and Range fault scarps increase in density eastward, which also suggests that more easterly parts of the province are youngest. These relationships support a previous model of an eastward-expanding, slab-free triangle (related to growth of the San Andreas transform), through which mantle upwelling triggers eastward-younging patterns of tectonism, volcanism, and uplift and promotes lithospheric thinning and increased heat flow. Across most of the southern Basin and Range and Rio Grande rift, the horst-graben structures related to growth of this triangle are superimposed on somewhat older (late Oligocene-middle Miocene) extensional terrane that appears to have formed in a back-arc or arc setting.