HR: 16:45h
AN: T22E-04    [PDF]
TI: Using Digital Topography to Differentiate Erosionally Exhumed and Tectonically Active Mountains Fronts
AU: * Frankel, K L
EM: kfrankel@usc.edu
AF: University of Southern California, Department of Earth Sciences 3651 Trousdale Parkway, Los Angeles, CA 90089-0740 United States
AU: Pazzaglia, F J
EM: fjp3@lehigh.edu
AF: Lehigh University, Department of Earth and Environmental Sciences 31 Williams, Bethlehem, PA 18015 United States
AB: Mountain ranges in the southern Rocky Mountains have departed on unique landscape evolutionary pathways in the late Cenozoic that are directly dependent upon the degree of post-orogenic tectonic activity they have experienced. The topography of Sierra Nacimiento, a Laramide uplift in west-central New Mexico lacking an active range-front fault, is shaped primarily by erosional exhumation that is continuous, but not steady, being driven by distal base level fall from Rio Grande incision and resultant south to north knickpoint migration. In contrast, the topography of the Taos Range, a rift flank uplift in north-central New Mexico is shaped by contrasting active stream incision and aggradation astride an active range front normal fault. The distinction between exhumation-dominated and tectonically-dominated mountain fronts is best quantified by analyses of a new metric we call the drainage basin volume to drainage basin area ratio (V-A ratio) as well as the gradients of first-order streams. Drainage basin volume and area are calculated by constructing topographic envelope maps from 10 m resolution digital elevation models (DEM). The envelope maps are pinned by the watershed divide and cover the maximum elevations in each drainage basin. Subtracting the original DEM from the maximum elevation envelope map produces a topographic residual map from which area and volume data can be obtained. The erosionally exhumed Sierra Nacimiento has a mean V-A ratio of 88 m while the tectonically active Taos Range has a mean V-A ratio of 140 m. Similarly, there are systematic differences in the gradients of first order streams measured both in the range block and approximately 5 km of adjacent piedmont. Streams were defined and subsequently Strahler ordered by a flow accumulation threshold of 250 water-equivalent grid cell units. First order stream channel long profiles were extracted from the DEM at 30 meter increments and gradients were calculated by a FORTRAN program. Gradients of first order streams in the exhumation-dominated Sierra Nacimiento have a mode of 6.8 degrees, significantly less than the 17.7 degrees for Taos Range first order streams. Furthermore, in the Taos Range first-order stream gradients steepen with increasing activity on the range-front fault. The distinct V-A ratio and stream gradient populations hint at an important change in the processes shaping hillslopes and low-order channels that is supported by the lack of slope-clearing landslides in the Sierra Nacimiento landscape and the presence of such landslides in the Taos Range. Slopes on Sierra Nacimiento are not steep enough to landslide and here, creep processes following a linear diffusion law dominate. In contrast, landsliding is present in the Taos Range where creep processes following a non-linear diffusion law are dominant. The signatures of distal base level fall are low V-A ratios accompanied by low modal channel gradients. Tectonically active mountain fronts have both high V-A ratios and high modal channel gradients.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 8107 Continental neotectonics
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting