LIMITS TO LARAMIDE STRIKE-SLIP DISPLACEMENTS IN THE SOUTHERN ROCKY MOUNTAINS, USA: IMPLICATIONS OF PRECAMBRIAN PINNING LINES AND PRECAMBRIAN FAULTING ON THE PICURIS-PECOS FAULT

The hypothesis of 100+ kilometers of dextral Laramide displacement on N-striking faults re-energized Rocky Mountain researchers in the 1980s. Indications of large Rocky Mountain dextral slip are clearest in northern New Mexico where the Picuris-Pecos fault system displays 37 kilometers of dextral separation between distinctive Precambrian metamorphic belts. Offset aeromagnetic anomalies reveal several parallel zones of equivalent slip which sum to over 100 kilometers of dextral strike slip. Attempts to determine the age of these displacements using Phanerozoic stratigraphic markers have had varied results, with some investigators finding abundant evidence for major Laramide dextral slip and others finding little evidence for any Phanerozoic lateral slip. This suggests the possibility of a Precambrian age for the dextral slip, consistent with the interpretations of earlier field investigators.

The extrapolation of hypothesized Laramide dextral slip northward is problematic. In basement exposures of the northern Colorado Front Range, the continuity of Precambrian shear zones and dikes preclude major post-Laramide strike-slip displacements. To the north and west, the Cheyenne belt’s continuity eliminates the possibility of major strike-slip extending into Wyoming between the Front Range-Medicine Bow and Park Range-Sierra Madre arches. Farther west, the sinuosity of the Grand Hogback rules out major dextral strike-slip displacements.

Re-examination of the southern Picuris-Pecos fault system in New Mexico has revealed well-lithified crush breccias of granitic basement adjoining folded, but not brecciated, Paleozoic limestones. The breccias’ induration, combined with their low grade metamorphic assemblages and lack of open space fillings, indicates that their post-cataclasis temperatures exceeded temperatures in adjacent Paleozoic rocks. A Precambrian age of brecciation is indicated by Precambrian Ar-Ar microcline ages as well as by Mississippian limestones that both unconformably overlie the breccias and inject them as clastic dikes. If the Picuris-Pecos fault system’s 37 km of dextral slip generated the thick granitic breccias, then the major strike-slip event in northern New Mexico was Precambrian in age, consistent with the lack of lateral offsets of Precambrian units to the north.