New Mexico Geological Society Annual Spring Meeting — Abstracts

The northeast trending Luna graben cuts diagonally across the center of the Luna quadrangle, is 3-5 miles wide, and is filled with volcaniclastic rocks of the Gila Group (aka Gila conglomerate) ~600 feet thick. The Luna graben separates Datil Group volcanic rocks, and Spears Group volcaniclastic sediments, of Eocene age, about 1000 feet thick, in the northwestern third of the quadrangle, from Mogollon Group volcanic rocks of Oligocene and Miocene age, 1000-1500 feet thick, in the San Francisco Mountains in the southeast third. Datil group rocks include thin (0-15 foot thick), discontinuous ignimbrites that represent distal outflow from Eocene calderas in the Socorro-Magdalena area. Mogollon Group rocks in the San Francisco Mountains block are mainly outflow ignimbrites (Bloodgood Canyon, Shelley Peak, and Davis Canyon tuffs) and lava flows (Bearwallow Mountain Andesite) from the Mogollon Mountains caldera complex, 40-50 miles southeast of Luna. Pliocene olivine basalt flow(s) of Trout Creek cut diagonally NW-SE across the Luna graben, and Luna Valley of the San Francisco River, and probably dammed an ancestral S.F.River, forcing the river to the south side of the graben, where it carved a steep-walled canyon that continues into the Dillon Mountain quadrangle to the east.

Narrow, E-W trending horsts and grabens in the S.F Mountains block appear to be bounded by tension fractures between the Spur Lake -Luna fault system and the San Francisco Mountains fault zone east of the San Francisco Mountains, and are interpreted to be formed by transtension related to a simple shear couple, resulting in an unknown amount of dextral strike slip on the bounding faults.