New Mexico Geological Society Annual Spring Meeting — Abstracts

Among the most striking physiographic features of the southern Rocky Mountains is the remarkably linear mountain front of the Sierra Nacimiento. This mountain front is just a portion of a much larger linear feature that extends for about 450 km from Truth or Consequences, New Mexico to the San Juan Mountains of Colorado. The steep western escarpment of the Sierra Nacimiento in northern New Mexico is bounded by a linear range front, narrow valleys, and steep frontal slopes. When trying to asses the relative activity of a mountain front, tectonic geomorphologists evaluate the effects of two competing processes: 1) uplift that tends to straighten and steepen a mountain front and, 2) stream erosion that cuts embayments into the front, widens valleys and hillslope processes that reduce the frontal slope. This study uses traditional and new geomorphic techniques to ascribe the relative roles of active tectonism and erosional exhumation in shaping the Sierra Nacimiento landscape.

Sierra Nacimiento lies at the juxtaposition of the Colorado Plateau, the Rio Grande rift, and the Jemez Lineament; each of these tectonic regimes have likely imparted a distinct topographic signature to the modern landscape. Although the Sierra Nacimiento has been traditionally viewed as a New Mexico version of a classic Laramide-style uplift, the striking linearity of the mountain front led us to investigate the role that post-Laramide deformation, including Quaternary tectonism, has played in shaping this mountain front. Detailed surficial mapping was completed in the southern Sierra Nacimiento from Arroyo Penasco in the west, around the southern portion of the range, and east to the town of Canones in the Jemez River drainage basin. Based on this mapping and correlation of post-Bandelier Tuff terrace deposits in the Jemez River drainage basin, a Quaternary stratigraphy was established with relative ages for surficial deposits using limited J4C radiometric ages, the occurrence of Lava Creek Bash (620 ka), uranium-series dates of terrace deposits and carbonate stage morphology of soil profiles.

There is clear evidence for Quaternary deformation throughout the southern Sierra Nacimiento; however, the rates of fault offset (0.02 mm/yr) are an order of magnitude slower than calculated rates of fluvial incision (0.17 mm/yr). Therefore, the alternative process of regional, large-scale exhumation appears to be the critical factor in shaping the modern landscape. Tectonism associated with the Rio Grande rift and thermally-caused inflation from high heat flow associated with the Valles caldera and the Jemez Lineament may be important in creating accommodation space in the Jemez River valley; however, these effects are outweighed by high rates of denudation.

GIS-based analysis of digital topography (DEM) of tectonically active and inactive mountain ranges; the Taos range, the Colorado Front range and the SierTa Nacimiento, allowed a test of the exhumation hypothesis and an estimation of the relative residence time of topographic forms in the Southern Rocky Mountain landscape. The results of this study suggest that accelerating denudation rates, hastened in the middle to late Pleistocene by drainage integration in less-resistant strata, has played the dominant role in defining the characteristic relief and geomorphic expression of the southern Sierra Nacimiento.