UNCONFORMABLE, CROSS-CUTTING AND THERMOCHRONOLOGIC EVIDENCE FOR MAJOR PRECAMBRIAN FAULTING ON THE PICURIS-PECOS FAULT SYSTEM, SOUTHERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO

What is the age of the Picuris-Pecos fault system (PPFS) of northern New Mexico? Its 37-km, dextral separation of Precambrian lithologic belts makes it the largest strike-slip system in the southern Rocky Mountains. A complicated history is evidenced by numerous studies that have relied on regional stratigraphic relationships, basin geometries and structural patterns to variously assign major strike slip on the PPFS to Precambrian, Ancestral Rocky Mountain and Laramide orogenies.

In Deer Creek Canyon, which contains the southern-most PPFS in the southern Sangre de Cristo Mountains, a resistant zone of brecciated, granitic gneiss up to 0.5 km in width is exposed adjacent to folded, but not brecciated, Paleozoic limestones and siliciclastic rocks. The well-indurated, granitic crush breccias contain unaltered microcline + chlorite +/- biotite assemblages and lack voids. 40Ar/39Ar dating of microcline (ca. 1.0 Ga) and biotite (ca. 1.35 Ga) from granite clasts and pegmatitic veins within the breccias yield complex spectra. However, the Neoproterozoic apparent ages coupled with a period of low-grade metamorphism support Precambrian brecciation.

Weathered shales and brecciated grus are locally exposed between crystalline breccias and overlying, minimally-deformed Mississippian and Pennsylvanian limestones. This apparent erosional nonconformity also supports the occurrence of major brecciation prior to the late Paleozoic. Furthermore, northwest-striking bands of limestone up to 24 m in length crosscut both the brecciated rocks and the overall trend of the PPFS. Compositional, faunal and textural similarities between the carbonate bands and the lowermost Mississippian carbonate rocks indicate that the bands are penecontemporaneous clastic dikes injected from above into coherent Precambrian crystalline breccias.

If the breccias in Deer Creek Canyon are related to the dextral strike-slip separations along the PPFS, then the displacements are largely Precambrian, not Phanerozoic. This conclusion contradicts hypotheses invoking large dextral strike-slip displacements on north-striking faults during either the Laramide or the Ancestral Rocky Mountain orogenies and reopens the possibility of major Proterozoic dextral strike-slip faulting in the southern Rocky Mountains.