Late Cenozoic rift tectonics and mountain-front landforms of the Sangre de Cristo Mountains near Taos, northern New Mexico
Christopher M. Menges
Patterns and rates of faulting along the eastern margin of the Rio Grande rift in northern New Mexico are derived from geomorphic analysis of fault scarps, triangular facets, spur ridge crests, range crests and drainage basins on the Sangre de Cristo mountain front. Systematic variations in width, complexity and position of fault scarps and facets at the mountain-front base define a geometric fault segmentation pattern consisting of four primary segments 15 to 20 km in length, each with two to four, 5- to 10-km-long subsegments. Morphologic analysis of alluvial fault scarps suggests several late Holocene to latest Pleistocene surface ruptures that were complexly distributed among the segments and subsegments. Statistical analyses of map and field data on triangular facets indicate that variations in the geometry and/or rates of range-front faulting influence facet-slope gradient, height and slope area more than bedrock lithology of piedmont dissection. The steepest, highest, largest and least dissected basal facets occur above structurally simple, narrow zones in the central and southern parts of fault segments or subsegments. These sites commonly contain the youngest fault scarps, as well as the steepest, most convex, and least benched systems of faceted spurs on the range front. These patterns suggest a long-term persistence of fault segmentation through at least the Quaternary, further supported by lateral variations in the altitude of a prominent set of mid-escarpment benches. The amounts and rates of post-mid-Pliocene vertical displacements estimated from the bench heights vary systematically from 500 m (115 m/my) to 1124 m (261 m/my) with position along the range-bounding fault. The benches also define an abrupt change in the form and steepness of facet-spur ridge crests on the mountain front. They correspond in altitude to prominent knickpoints in the channel profiles of adjacent mountain-front drainage basins, and project into the top of distinct inner gorges in most canyons draining the interior of the Sangre de Cristo Mountains. The mid-Pliocene benches and related features appear to mark a major tectonic transition along this part of the rift, recorded in erosional landforms of the bordering range block.
- Menges, Christopher M., 1990, Late Cenozoic rift tectonics and mountain-front landforms of the Sangre de Cristo Mountains near Taos, northern New Mexico, in: Tectonic development of the southern Sangre de Cristo Mountains, New Mexico, Bauer, Paul W.; Lucas, Spencer G.; Mawer, Christopher K.; McIntosh, William C., New Mexico Geological Society, Guidebook, 41st Field Conference, pp. 113-122.