SEGMENTATION AND OFFSET OF THE PICURIS-PECOS FAULT SYSTEM OF NORTHERN NEW MEXICO CONSTRAINS LATE LARAMIDE MODELS PROPOSING SIGNIFICANT RIGHT-LATERAL DISPLACEMENTS

The Laramide Orogeny in northern New Mexico is a controversial topic. The debate centers on the relative amount of slip on several N-trending basement faults including the Picuris Pecos Fault System (PPFS). The mapped trace of the PPFS extends over 80 km from near Taos, NM to near Santa Fe. Along the PPFS foliations in Precambrian metasediments are offset and deflected by the fault indicating 37 km’s of right-lateral separation. Several authors have proposed that all of its observed right-lateral displacement is the result of late Laramide (Latest Paleocene to Late Eocene) deformation. Regional analysis determined that the PPFS is actually made of up at least two different segments. A northern segment (Picuris) that trends 170-180, and a southern segment (Pecos) that trends 180-190. The junction between these two segments is found at the intersection with the NW-trending Pilar-Vadito Fault (PVF), which is covered by Tertiary and Quaternary sediments. Along the PVF, Precambrian supracrustal units are offset in a left-lateral sense over 2 km (Miller et al, 1963). Detailed mapping in the eastern Picuris Mountains showed that the PVF also offsets the PPFS in a left-lateral sense. Farther south the PPFS is also offset by the NW-trending Santa Barbara fault (SBF) (new name). Based on their orientation and on their association with several N-trending reverse faults, the PVF and the SBF are interpreted to be tear faults, which developed in the Early Laramide (Late Cretaceous to Late Paleocene). If the 2 km’s of left-lateral motion along the PVF were removed, then most of the rotation of the PPFS from the Picuris and the Pecos segments would be resolved. During the Late Laramide, σ₁ had rotated to NE (Erslev, 2001) and the result was minor right-lateral strike-slip motion on the PPFS. The cross-cutting NE-trending Embudo fault, to the north, and the NW-trending faults to the south of the Picuris segment, limited the amount of offset on the PPFS and caused limited counterclockwise rotation of the Picuris segment of the PPF relative to the Pecos segment contributing to the difference in orientation. As a result the significant right-lateral displacement proposed for the PPFS in the Late Laramide is not envisioned. The lack of offset of the Pilar-Vadito or Embudo faults is the most compelling evidence.