A geophysical study of basement structure in northeastern New Mexico
— A. S. Suleiman and G. R. Keller

The Precambrian basement in northeastern New Mexico (Foster et al., 1972; Roberts et al., 1976) is dominated by structures associated with the ancestral Rocky Mountains which are widely believed to be the result of the same tectonic event that produced the Ouachita—Marathon orogenic belt (e.g., Kluth and Coney, 1981). Although there is much disagreement regarding the nature of Late Paleozoic plate interactions along the southern margin of North America which resulted in the development of this orogenic belt (e.g., Keller and Cebull, 1973; Wickham et al., 1976; Dickinson, 1981; Lindell and Dewey, 1982), activity along an irregular continental margin can explain many of the observed variations in timing (Kluth and Coney, 1981).

In the study area (Fig. 1), this Pennsylvanian and Permian tectonic activity produced positive areas, the ancestral Rocky Mountains (Apishapa uplift, Sierra Grande uplift, Bravo dome, Pedernal uplift, Central Basin platform), separated by troughs in which large quantities of sediments accumulated. Late in Early Pennsylvanian time, deformation spread northwestward from the eastern portion of the Ouachita—Marathon orogenic belt, and by Middle Pennsylvanian time the ancestral Rocky Mountains reached their greatest extent. Late in the Middle Pennsylvanian, tectonic activity began to spread southward in New Mexico, and, as a result, the Pedernal uplift was raised to a level sufficient to provide feldspar-rich sediments to the adjoining basins. Structural activity continued at a gradually slower rate throughout Late Pennsylvanian and Early Permian time. In latest Pennsylvanian or Early Permian time, the Central Basin platform and the Diablo platform in west Texas were rapidly uplifted, the Pedregosa basin rapidly subsided, and the Delaware and Orogrande basins reached maximum development (Kluth and Coney, 1981).

The Laramide orogeny was another major phase of tectonic activity which affected the Precambrian basement of northeastern New Mexico. This orogenic activity is thought to have been confined to latest Cretaceous through latest Eocene time (80 to 40 m.y. ago; Coney, 1976). The major Laramide uplifts and basins of the Rocky Mountain foreland were developed in a stress field of regional compression while the North American plate was drifting westward over an eastward-dipping subduction zone (Woodward, 1976). The major effect of this orogeny in northeastern New Mexico was to form uplifts with at least a minor amount of eastward-directed overthrusting (e.g., Sangre de Cristo Mountains).

Middle and late Cenozoic activity in the area has been confined to uplift (e.g., Sangre de Cristo Mountains, Sacramento uplift), basin formation (e.g., Tularosa basin), volcanism associated with the Rio Grande rift (e.g., Chapin, 1971; Kelley and Duncan, 1985) and volcanism along the Jemez lineament.

Working with colleagues at the U.S. Geological Survey and various universities (most notably the University of Texas at Dallas and the University of Wyoming), we have been collecting gravity data in New Mexico for nine years. These efforts have resulted in published complete Bouguer anomaly maps of the Rio Grande rift (Cordell et al., 1982) and of New Mexico (Keller and Cordell, 1983). For this study of the area shown in Figure 1, we have added to the data base used to produce  these maps, resulting in the gravity-station distribution shown in Figure 2.