New Mexico Geological Society Annual Spring Meeting — Abstracts

The Montosa Fault (MF) zone bounds the Los Pinos and southern Manzano mountain ranges on their eastern margins. The MF, formerly mapped as a Laramide thrust fault, displays dextral-oblique motion by an anastomozing zone of conjugate Riedel shears and by slickensides which rake the fault plane from 26°S to 75°S. The MF zone (Fig. 1) follows foliation in the Proterozoic Sais Quartzite, which dips 55° -75° →N70° W, for several miles from Cerro Montosa in the Los Pinos Mountains to the Abo Pass area of Socorro and Valencia counties. It is then deflected to the east around a chevron fold in the Sais Quartzite and the southern margin of the Proterozoic, quartz monzonitic Priest Pluton (Fig. 1). This deflection causes a right-stepping, releasing (transtensional) bend. This releasing bend is characterized by a 1-km-long, by 1/3-km-wide depression flanked by normal faults and an extensional syncline. The west-flowing Abo Arroyo turns NNE for 1/2 km through this bend east of the Sais Quarry of the Santa Fe Railroad, before resuming its westward drainage. North of Abo Arroyo, the dip of the MF varies from 50° E (dextral normal-slip) to 70° W (dextral reverse-slip), and resumes the NNE trend. The variable-slip geometry of this fault zone indicates that a complex dextral transpressional tectonic regime was responsible for the late Laramide uplift of the Manzano and Los Pinos mountain ranges. Previously published fission-track dates suggest that uplift occurred from Paleocene time in the Los Pinos Mountains to Late Eocene time in the Manzano Mountains.