COMPARATIVE STRATIGRAPHY OF THE DAKOTA SANDSTONE ACROSS THE PICURIS–PECOS FAULT SYSTEM SOUTH OF LAMY, NEW MEXICO: DEFINITIVE EVIDENCE OF LARAMIDE STRIKE-SLIP

The Picuris–Pecos fault of northern New Mexico, the largest fault in the Rocky Mountains, shows 37 km of dextral strike separation of Proterozoic lithotypes. The timing of slip is disputed. The Picuris-Pecos fault system strikes southward from Lamy, NM, as a zone of faults that cuts strata of Mesozoic age and is intruded by the unfaulted, 27 Ma Galisteo dike. On the San Cristobal Ranch, ~20 km south of Lamy, the fault system is characterized by numerous, steep NNE-striking faults that exhibit normal separation and form an en echelon array. Many of the fault blocks in this en echelon array include outcrops of the Dakota Ss (Upper Cretaceous, ~95 Ma), thus affording an opportunity to stratigraphically evaluate Laramide strike-slip across much of the Picuris–Pecos fault system. On the San Cristobal Ranch, eight measured stratigraphic sections of the Oak Canyon and Cubero Mbrs display significant stratigraphic differences between adjacent fault blocks. Comparison to six control sections measured in unfaulted areas west of the Picuris–Pecos fault indicates the across-fault stratigraphic differences observed on the San Cristobal Ranch are too great to be attributed simply to lateral facies variation, but instead require strike-slip juxtaposition of dissimilar Dakota Ss sections. This is indisputably displayed on two faults in the Hub Canyon area that exhibit only minor (<2 m) dip separation but divide markedly different, well-exposed Dakota Ss sections. Based on the length-scale of facies variation in the control sections, we estimate the minimum dextral Laramide slip on the Picuris–Pecos fault to be several kilometers; the upper slip limit has not yet been determined. Limited kinematic data for minor faults adjacent to the en echelon faults on the San Cristobal Ranch show mostly fault-normal deformation (approximately E-W extension), similar to Laramide fault-normal shortening or extension observed elsewhere on nearby segments of the Picuris-Pecos fault system (Erslev, 2001, GSA Bulletin). Reconciliation of stratigraphic evidence for significant strike slip between fault blocks with minor-fault evidence for fault-normal deformation within fault blocks indicates that Laramide dextral-oblique deformation was partitioned into strike- and dip-slip components. Minor-fault analysis alone thus cannot reliably determine the kinematics of obliquely deformed orogens.