Cenozoic structural development of the Taos area, New Mexico
P. W. Bauer and K. I. Kelson
New, detailed geologic maps of Precambrian and Paleozoic bedrock, Tertiary basin fill, Quaternary surficial deposits, and the faults that cut them all, provide important controls on the geometry and kinematics of tectonism in the Taos area, from Laramide shortening to modern rifting. The three major fault systems that intersect in the Taos area are much more complex than previously described. The north-striking, 8-km-wide, strike-slip Picuris-Pecos fault system cuts deposits younger than about 5 Ma. The Picuris-Pecos fault is truncated by the Embudo fault, a major sinistral, antithetic rift transfer zone between the San Luis and Española rift basins. The Embudo fault is a complex system of left-oblique, north-down, strike-slip fault strands that is over 2 km wide. Pliocene basalt offset by the Embudo fault southwest of Pilar suggests a post-3-Ma minimum throw rate of 35 m/m.y. and a minimum average net slip rate of 130 m/m.y. The northeastern terminus of the Embudo fault is at the Rio Grande del Rancho drainage, where the east-striking Embudo fault system swings northward and smoothly merges with the dominantly dip-slip Cañon section of the rift-bounding Sangre de Cristo fault. The transition zone from the Embudo fault to the Sangre de Cristo fault is coincident with the Picuris-Pecos fault system and the Miranda graben. The Picuris-Pecos fault projects northward across the Taos embayment and aligns with the Questa section of the Sangre de Cristo fault. In the San Luis Basin directly north of the Embudo fault, is the buried, north-trending, 13-km-wide, 5000-m-deep Taos graben. We speculate that this graben may be the sinistrally displaced equivalent of the Miranda and Rio Grande del Rancho grabens. Our mapping also yields information on the timing of tectonism in the southeastern San Luis Basin. Substantial strike-slip faulting occurred in the Taos area at least until 18 million years ago, well past the 27 Ma onset of extension in the San Luis Basin. During early extension, prior to development of the Embudo fault, the Picuris-Pecos fault system, and its now-buried northward extension, may have represented the eastern rift margin. The transition from Laramide shortening to rift extension is indistinct, and at least some Laramide transverse faults probably evolved into rift normal faults in a transtensional(?) setting. The change to pure extension may have coincided with a middle Miocene change in crustal extensional strain rate, and may have also triggered the initial development of the modern Embudo fault.
- Bauer, P. W.; Kelson, K. I., 2004, Cenozoic structural development of the Taos area, New Mexico, in: Geology of the Taos Region, Brister, Brian S.; Bauer, Paul W.; Read, AdamS.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 55th Field Conference, pp. 129-146.