New Mexico Geological Society Annual Spring Meeting — Abstracts

Seen from the south, the Rio Grande Rift appears as an arm of the Mexican Basin and Range province. In central Mexico, a Laramide fold-and-thrust belt was inundated by midTertiary volcanism, culminating in the Great Ignimbrite Flareup. As associated heat flow subsided and ductile extension gave way to brittle extension, Basin and Range fault blocks developed in the eastern part of the province. Its western part, the Sierra Madre Occidental, remained a relatively undisturbed volcanic plateau. In the SW corner of New Mexico, the central Mexican province split: As the main fold-and thrust belt bent northwest and continued into Arizona, it cut across the Sierra Madre Occidental, leaving the Mogollon-Datil volcanic field as a northern outlier. A lesser branch headed north, as the Rio Grande Rift system.

North of Hatch, the Rio Grande valley marks the major fracture zone separating the extending western third of the continent from its more stable interior. The degree to which a distinct rift developed depended on the absence of crust “softened” by residual heat from midTertiary volcanism. In the absence of volcanism, a distinct topographic and structural rift border developed on the east side of the Palomas Basin, along the Caballo-Fra Cristobal front. The “soft” west sides splintered into grabens around the Mogollon Plateau pluton, core of the Mogollon-Datil field. Along the San Augustin Plains, this graben system merged with the Socorro Accommodation Zone, which translated the Rio Grande Rift 50 km NE and remained a site of minor siliceous volcanism into the Neogene. The east border of the Albuquerque-Belen Basin remained rifted along the Manzano-Sandia front. On the “soft” west side, intra-rift basaltic volcanism was active into the Pleistocene. The Jemez volcano-tectonic lineament again translated the rift 50 km NE; its volcanism climaxed in the Pleistocene Valles Caldera. The Española Basin has distinct rift borders on both east (Sangre de Cristo) and west (Chusas) sides. Thick intra-rift basalts resemble a failed ocean ridge. A monocline on the “soft” west side of the San Luis Basin borders the mid-Tertiary San Juan volcanic field; the Sangre de Cristo block continues on the east side . In the Arkansas Valley the rift narrows into a fault zone buttressed on both sides by Rocky Mountain fault blocks.