Precambrian geology and tectonics of the southern Manzano Mountains, central New Mexico
— Paul W. Bauer

The Manzano Mountains of central New Mexico consist of a north- south elongate, eastward-tilted fault block on the eastern flank of the Rio Grande rift. The Manzanos extend northward to the Manzanita and Sandia Mountains and southward to the Los Pinos Mountains. Erosion in the southern Manzanos has removed all Phanerozoic cover from the Precambrian metaigneous, metasedimentary, and granitic rocks. Pre-cambrian rocks are separated from Paleozoic sediments on the east by a high-angle, Laramide-style reverse fault, and are covered on the west by Cenozoic rift-fill sediments. The area which this report discusses is bounded on the north by Monte Largo Canyon and on the south by latitude 34°13' (fig. I). Exposure is excellent, showing well-developed tectonic fabrics and the common presence of metamorphic index minerals.

Precambrian rocks within this area have been divided into five major formations by Stark (1956). Stark recognized an older sequence of quartzites, mica schists, metasiltstones, and phyllites composing the Sais, Blue Springs, and White Ridge formations, and a younger succes-sion of metarhyolite, amphibolite, and basic schist units of the Sevilleta and basic schist formations. Diverse lithology is present in most of these formations, although Stark's reconnaissance map at a scale of 1:48,00 does not reveal this. This area later was mapped by Myers and McKay (1976) at a scale of 1:24,000, and Precambrian lithology and structure were generalized. The present study of the southern Manzanos consisted of structural analysis and detailed lithologic mapping at a scale of 1:6,000 (fig. 2).

The southern Manzanos are characterized by a strong N15°E-to-N40°E trending structural fabric consisting of compositional layering (So/S,) and superimposed S-tectonite surfaces. Local incongruous areas of minor folding are present throughout. Extensive transposition masks original structure and stratigraphy in many areas.

Three ages of Precambrian folds are present in the area: two early episodes of tight to isoclinal northeast-trending folding (F, and F2), and a later northwest-oriented open fold set with an associated crenulation cleavage (F3). The only large fold in the Sandia—Manzanita—Manzano-Los Pinos chain is a tight, upright F2 synform in the southern Manzanos. It trends N30°E with a nearly horizontal fold axis (Condie and Budding, 1979). The wavelength is at least 8 km, with an almost vertical axial surface; the axial trace approximately follows the center of the Sevilleta formation. The White Ridge formation is appreciably thinner on the western limb of this fold. Minor folds range in size from less than a centimeter to more than a kilometer in wavelength.

A high-angle, northeast-trending Phanerozoic fault, which generally parallels compositional layering, cuts Precambrian rocks (fig. 2). The only other significant fault is a high-angle, westward-dipping, northeast- trending, Laramide-style reverse fault along the eastern map boundary (fig. 2).

Regional metamorphism of the metasediments has been low to mod-erate (upper greenschist to amphibolite facies). A thermal aureole around the intrusive Priest quartz monzonite is characterized by increased grain size and the presence of abundant sillimanite in schists.