HR: 0830h
AN: T21C-0474    [PDF]
TI: A Topographic Analysis of the Colorado River Drainage: Insights into Interaction Between Topography and Incision History
AU: * Coblentz, D D
EM: coblentz@lanl.gov
AF: Los Alamos National Lab, MS D446, Los Alamos, NM 87545 United States
AU: Karlstrom, K
EM: kek1@unm.edu
AF: Dept, of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87185 United States
AB: Topography is a fundamental geophysical observable that reflects the geodynamic, geologic, and climatic history of a region. A number of recent studies have explored ways to extract tectonic information from topography and several provocative interpretations have been made about the topographic fabric in the Western U.S., including: 1) The topographic spectral power decreases systematically as the tectonic age of the underlying crust increases; 2) Local relief, mean elevation, and thermochronologically-determined exhumation history vary systematically across the major Paleozoic accretionary boundaries; 3) Neogene and ongoing rock- and possibly surface-uplift is driven by the creation of mantle buoyancy; and 4) there is a strong correlation between topography and lithospheric features such as mantle velocity structure, crustal thickness, and Precambrian crustal provinces. The Colorado River drainage provides a unique natural laboratory to evaluate to relationship between crustal-scale tectonic processes and the observed physiography. The drainage, extending more than 1400 km from the Gulf of California to the heart of the Rocky Mountains in Colorado, traverses the Southern Basin and Range, Colorado Plateau and Rocky Mountain provinces. A priori, we expect changes in the topographic character of the drainage to correspond to differences in bedrock geology, tectonic genesis, ongoing faulting, and the nature and position of province boundaries. Here we test this hypothesis through the use of a quantitative topographic analysis that extracts information about the topographic roughness organization, grain orientation, spectral power and fractal dimension. The analysis was applied to a 3-arc-second topographic DEM of the drainage. A particular topographic feature of interest is the change in river gradient at Lee's Ferry, just north of the Grand Canyon at the boundary between the Marble and Glen Canyons. This river knickpoint correlates with a change in topographic roughness and in standard deviation of the fractal dimension, both of which correspond with a peak in the spectral power at topographic wavelengths less than 10km. Strong peaks in the spectral power at very long wavelengths (40 to 100km) are spatially associated with structural features (East Kaibab monocline, Toroweap-Hurricane faults system, Grand Wash trough, Uncompahgre uplift) suggesting that these structural features influence topography. We note only a weak signal in the topographic roughness and organization values across the Basin and Range - Colorado Plateau boundary Preliminary conclusions include: 1) an observed spectral bulge in Rockies appears to correspond to the Aspen anomaly and is indicative of active doming due to mantle magmatism and resulting buoyancy; 2) spectral bulges in Grand canyon correspond to structures of different ages; 3) Changes in topography character (e.g., roughness) at river profile knickpoints is striking and offers a way to test whether these topographic features of Colorado River drainage are due to young tectonism or to more passive incision of a previously elevated plateau. In the latter case, topographic features are simply responses to bedrock changes and style of early structures; in the former case the river system is a sensitive gauge to differential tectonism in the western US.
DE: 8107 Continental neotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting