Unconformable and cross-cutting relationships indicate major Precambrian faulting on the Picuris-Pecos Fault System, Southern Sangre de Cristo Mountains, New Mexico

Abstract

A .—What is the age of the Picuris-Pecos fault system? Its 37 km dextral separation of Precambrian lithologic belts makes it the largest well-documented strike-slip system in the southern Rocky Mountains. Numerous studies focusing on regional stratigraphic relationships, basin geometries and structural patterns have variously assigned the major strike slip on the Picuris-Pecos fault system to Precambrian, Ancestral Rocky Mountain, and Laramide orogenies. New field and petrographic observations were collected from the southern Picuris-Pecos fault system in the southern Sangre de Cristo Mountains. In Deer Creek Canyon, an exceptionally broad and resistant zone of brecciated granitic gneiss is exposed adjacent to folded, but not brecciated, Paleozoic limestones and siliciclastic rocks. The induration of the crush breccias and protocataclasites along with their microcline + chlorite +/- biotite assemblages and lack of void space fillings suggest that brecciation occurred during Precambrian low-grade metamorphic conditions near the brittle-ductile transition. Phanerozoic deformation appears to be dominated by narrow zones of unconsolidated fault gouge cutting the crystalline rocks and associ ated folding of the Paleozoic strata. In addition, easily excavated shales and weathered granitic grus showing cataclastic breccia textures are locally exposed between crystalline breccias and overlying, minimally-deformed Mississippian and Pennsylvanian limestones. These apparent erosional nonconformities indicate that major brecciation occurred prior to the late Paleozoic. Furthermore, two northwest-striking bands of Mississippian limestone, 24 and 10 meters in length respectively, crosscut both the brecciated rocks and the overall trend of the Picuris-Pecos fault system. Compositional, faunal and textural similarities between several carbonate bands and the lowermost Mississippian carbonate rocks indicate that they are penecontemporaneous clastic dikes injected from above into coherent Precambrian crystalline breccias. If the spectacular breccias in Deer Creek Canyon are related to the dextral strike-slip displacements along the Picuris Pecos fault system, then these displacements are Precambrian, not Phanerozoic. This conclusion contradicts hypotheses invoking large dextral strike-slip displacements on north-striking faults during both Laramide and Ancestral Rocky Mountain orogenies and reopens the possibility of major Proterozoic dextral strike-slip faulting in the southern Rocky Mountains.