RETHINKING AN ARCHETYPE OF RIFT HALF-GRABEN FORMATION—A REVISED MODEL OF BASIN GEOMETRY FOR THE ALBUQUERQUE SEGMENT OF THE RIO GRANDE RIFT, NEW MEXICO

The Albuquerque basin, composing the Rio Grande rift in central New Mexico, is commonly cited as one of the archetypes of a continental rift basin that is segmented into alternating, oppositely tilted half-grabens separated by a scissor-like transfer zone. The practice originates from a structural model of the Albuquerque basin developed primarily from oil-industry seismic-reflection data and published in the early 1990s. Key elements of this model are an east-tilted half-graben on the north separated from a west-tilted half-graben on the south by a southwest-trending transfer zone. However, this model is conceptually inconsistent with current geologic evidence and gravity modeling for the Albuquerque basin. Problems with the earlier model probably stem from inadequate seismic and well data on the east side of the basin.

A model of basin-fill thickness constructed from gravity data supports a different picture of basin geometry, primarily for the southern half of the Albuquerque basin. The thickness model is constrained by well information, allows for variable density, matches new structural cross-sections, and is supported by limited re-interpretation of seismic-reflection data. In the new model, the northern sub-basin generally consists of an ENE-facing half-graben, differing primarily from the earlier model by its farther extent to the east. A northwest-trending structural high rather than a southwest-trending transfer zone separates the northern and southern parts of the Albuquerque basin. Dominating the southern half of the basin is a sub-basin that generally deepens to the east, modified by several north-trending structural highs. In sharp contrast to the earlier structural model, the areas of west-tilted strata are confined to a 15- to 30-km wide zone along the southwestern margin of the basin. The divide between the oppositely tilted regions is a crooked, south- to southeast-trending structural high. The new model suggests that the entire Albuquerque basin predominantly tilts eastward, with only a marginal region of westward tilting and no scissor-like transfer zone. The restricted region of westward tilting may have resulted from a brief change in subsidence history, the influence of pre-rift structure, or accommodation that occurs as a double-sided hinge rather than as a scissor-like slip zone.