Bulletin 34—Geology of the south Manzano Mountains, New Mexico

By J. T. Stark, 1956, 49 pp., 1 fig., 8 plates, 1 appendix, 1 index.

The South Manzano Mountains in central New Mexico form a unit of the easternmost of the Basin Ranges and are structurally continuous with the Los Pinos to the south and the North Manzano and Manzanita Mountains to the north. Precambrian metaclastics of the area consist of the basal Sais quartzite overlain by the Blue Springs schist and White Ridge quartzite in conformable sequence, followed by 5,000 ft of rhyolite flows and intercalated basic sills. A small outcrop of granite in the northwest part of the area is similar to and correlated with the Ojito stock of the North Manzano Mountains, which intruded metaclastics older than the Sais quartzite. In the south, the Priest granite and associated pegmatite and aplite dikes intrude all older formations. Vein quartz ranging from thin stringers to massive quartz reefs over 1,000 ft thick are prominent throughout the range and represent several periods of intrusion.

Three periods of Precambrian orogeny are recognized: (1) Early folding of the metaclastics and metarhyolite into an asymmetric syncline, its axis striking northeast across the north trending range and the steeper eastern limb overturned, with the axial plane dipping steeply to the southeast. At this time, regional cleavage was developed paralleling the axial plane, with small drag folds on the flanks of the syncline. (2) Later, cross-folding, faulting, and small crenulations were developed on the older schistosity. (3) Finally, granites were intruded, with associated dikes and quartz veins, across preexisting structure.

A later disturbance, probably Laramide, formed three major thrust-fault zones, the easternmost bringing Precambrian formations against Paleozoic sediments. A normal fault in Tertiary time is responsible for the block tilting of the range with respect to the Rio Grande Valley on the downthrown side. Small scarps in Quaternary gravels of the Tio Bartola pediment indicate recent normal faulting.