Tectonism and geomorphic response in the Sacramento Mountains, south-central New Mexico

The Alamogordo Fault bounds the western range-front of the Sacramento Mountains in south-central New Mexico. Previous studies have determined that uplift of the range initiated during Tertiary time and continues to present day. This study utilizes remote-sensing methods to observe relations between geomorphology—the bedrock fluvial network in particular—and tectonism in the Sacramento Mountains. Analysis of longitudinal stream profiles, stream slope-area data, and topographic relief reveal temporal and spatial variations in uplift rates along the Alamogordo Fault and support the hypothesis that the Alamogordo Fault consists of four segments. The geomorphic indices of concavity (Θ) and steepness (ks) are calculated from stream slope-area data. The concavity index is a measure of stream-profile curvature and is a function of channel substrate erosional properties. Concavity indices of the study area exhibit no consistent response to along-stream changes in sedimentary lithology of any type. This suggests that sedimentary bedrock of the Sacramento Mountains—though heterogeneous at small-scales (meters to 10’s of meters)—may be generally uniform at larger-scales. However, concavity indices do suggest that a strong lithologic contrast is apparent at the scale of this study between granitic rocks and the surrounding sedimentary rocks. Concavity index values of stream segments within a granitic pluton are typically low (Θ < 0.40); whereas respective downstream channel segments that flow over sedimentary/tuffaceous rock are typically high (Θ > 1.0). Theoretical considerations of the detachment-limited, shear-stress incision law predict a relation between stream channel steepness (ks) and a response to uplift rates experienced by bedrock channel beds. North-south and west-east trends in normalized steepness indices (ksn, for reference concavity (Θref) of 0.45) are observed. Values of ksn, averaged for each watershed, are lowest (ksn < 60) in watersheds that are coincident to the northernmost, Three Rivers segment of the Alamogordo Fault. Average ksn values increase to maximum values (ksn > 120) along the Deadman fault-segment. This north-south trend of average ksn values mimics the north-south trend of measured, present-day uplift rates. The west-east trend in ksn values is demarcated by knickpoints that separate downstream channel segments with higher steepness (ksn > 120) from upstream channel segments with lower steepness (ksn < 120). It is inferred that these trends reflect spatial variations of relative uplift rates of uplift along the Alamogordo Fault (north-south trend), and relative, temporal variations in uplift rates of the Sacramento Mountains (west-east trend).