Structural problems along the east side of the Socorro constriction, Rio Grande rift, New Mexico
— Clay T. Smith

The Socorro constriction was defined by Kelley (1952) in his dis-cussion of the tectonics of the Rio Grande depression. The constriction represented a narrow link between the wider expressions of the rift, the Albuquerque-Belen Basin to the north and the San Marcial Basin to the south. More recent work (Chapin and Seager, 1975; Kelley, 1979; Woodward, 1977) has emphasized the extensional nature of the Rio Grande rift and has recognized that the rift is much more complex to the south where it merges with the Basin and Range province of southern New Mexico. Kelley (1982) states that the Socorro constriction, while included in the Rio Grande rift, may be a part of a much older and broader structural belt which includes the Jornada del Muerto Basin, the San Andres Mountains, Tularosa Basin, and northern end of the Sacramento Mountains. Some of the relations exposed along the eastern edge of the Socorro constriction are older than the Rio Grande rift and certainly do not represent crustal extension.

Detailed mapping, although not continuous, has been completed in a patchwork pattern from latitude 33° 50' N to 34° 15' N and between longitude 106° 40' W and 106° 46' W. This rectangular area covers the majority of the eastern side of the Socorro constriction where Penn-sylvanian and Permian rocks are exposed (see Kelley, 1982, fig. 1). Theses by Rejas (1965), Maulsby (1981), Bauch (1982), and Fagrelius (1982) have documented a pattern of overturned folds, steep reverse faults, and decollement-like structures previously not emphasized along this part of the rift (see fig.1-15.2a, first day road log, this guidebook). Recent mapping by Rosen (oral commun., 1983) has demonstrated that the Tones Member of the Yeso Formation (Permian) rests at a very low angle upon Mancos Shale (upper Cretaceous) near the northeast corner of the rectangular area described above. These exposures suggest a possible Laramide age for some of the decollement structures, but do not date the overturned folds.