Geometry of Nacimiento-Gallina fault system, northern New Mexico
— Lee A. Woodward, Michael C. Hultgren, David L. Crouse, and Margaret A. Merrick

The Nacimiento-Gallina fault system trends northerly for about 110 km, separating the San Juan Basin and the Colorado Plateau from the Rocky Mountain foreland to the east. From south to north the Pajarito, Nacimiento, Gallina and Tierra Montañosa faults comprise this system. The east-dipping Pajarito and Nacimiento faults bound the west side of the Nacimiento uplift and are characterized mainly by reverse separation. The Gallina and Tierra Montañosa faults are nearly vertical and define the west boundary of the Gallina-Archuleta arch. Movement on the Gallina fault had both dip-slip and strike-slip components and displacement on the Montañosa fault was essentially dip slip. This complicated eastern boundary of the Colorado Plateau has led to markedly different interpretations of its tectonic evolution, mainly because of previous lack of detailed geologic maps. Since the pioneering work of Vincent Kelley in the 1950s there has been general agreement that the Colorado Plateau underwent right shift with respect to the adjacent Rocky Mountain foreland in northern New Mexico prior to the Pajarito and Nacimiento faults rupturing the sedimentary cover. However, there has been considerable debate concerning the amount of right slip (as distinct from shift) along the Nacimiento- Gallina fault system. Right shift in Precambrian basement rocks among the eastern margin of the Colorado Plateau in late Paleocene to mid-Eocene time created northwest-trending echelon folds in the overlying Phanerozoic strata. This was followed by development of a west-facing monocline that was cut by the Pajarito and Nacimiento reverse faults. As the San Juan Basin subsided it was differentially folded relative to the Gallina- Archuleta arch, resulting in a component of strike slip along the Gallina fault. This differential folding resulted in variable offset along the Gallina fault with nonmatching folds on opposite sides of the fault. The axes of the folds do not predate fault movement and therefore cannot be used to determine the amount of strike slip.Lack of piercing points along the Pajarito, Nacimiento and Gallina faults precludes precise calculations of the strike-slip component of movement. These discontinuous faults with relatively short traces suggest that only minor amounts of right slip occurred along them. Thus, right shift of the Colorado Plateau dies out at the north end of the Gallina fault and displacement on the Tierra Montañosa fault is principally dip slip.